Temperature control system for air preheater equipment

Abstract

The air preheater (APH) temperature control system comprises at least a first APH or combustion / secondary air bypass duct in metered communication with a combustion air inlet duct and a secondary air duct, the first APH or combustion / secondary air bypass duct adapted in use to bleed a portion of the combustion air from the air inlet duct upstream of the APH 100 as a secondary air bypass and reintroduce it into the downstream secondary air duct, and a flow control device for metering or controlling the volumetric flow rate of the secondary air bypass and regulating the primary air flow, the flow control device being adapted in use alone or in combination with other regulating means to regulate the T10 MIN ~T10 MAX and a flow control device operable to maintain the flue gas outlet temperature at or above a desired minimum predetermined temperature for the incoming flue gas volumetric flow rate exiting the APH to maintain the mill outlet temperature within a safe range of .

Description

Technical Field

[0001] Cross - reference to Related Applications Cross - reference to Related Applications This application is a non - provisional application of U.S. Provisional Patent Application No. 63 / 148,689, filed on February 12, 2021, claims the priority thereof, and the entire content thereof is incorporated herein by reference.

[0002] The present invention relates to a method and apparatus for controlling the temperature of the gas exiting an air preheater for a fossil fuel combustion boiler, and more particularly, to improving the operating efficiency of downstream pollution control equipment by retrofitting an installed air preheater in a coal - fired power plant.

Background Art

[0003] Steam generation systems are used in the generation of electricity and chemical processing plants. The energy for steam generation may be provided by the combustion of fossil fuels such as coal pulverized in a mill, natural gas, or fuel oil. A coal - fired steam generation system generally includes a boiler (which evaporates water into steam) having a furnace where pulverized coal is burned, an air preheater ("APH"), and an outlet gas for flue gas ("flue gas"). The flue gas passes through a plurality of pollution control devices designed to reduce gaseous and particulate pollutants before exiting from the APH through a chimney into the atmosphere.

[0004] Typically, the APH of a rotary regenerative heat exchanger utilizes profile steel heat exchange elements to capture heat, called "waste heat," from the flue gas exiting the boiler and releases that heat to the combustion air supplied to the boiler, thereby improving boiler efficiency (often partially through one or more coal mills for transporting pulverized coal to the boiler furnace), and one such is the three - sector APH1 schematically shown in FIG. 1.

[0005] The third sector APH1 is located at its upper end ("high temperature end") and is arranged circumferentially around its rotor 2. The flue gas inlet duct D1, primary air outlet duct D4, and secondary air outlet duct D6 are axially aligned with the flue gas outlet duct D2, primary air inlet duct D3, and secondary air inlet duct D5, respectively, which are similarly located at the lower or low temperature end of the rotor 2. For the sake of illustration and functional explanation, the primary and secondary air ducts are shown arranged radially, but the true arrangement and configuration are well known in the art and are shown in the applicant's commercially available publications relating to the Ljungstrom® Trisector Air Preheater. Rotor 2 houses a basket of heat exchange elements as it rotates to preheat the combustion air supplied to the boiler as a mixed flow of primary air V2 (supplied through one or more coal mills to transport pulverized coal to the boiler furnace) flowing between ducts D3 and D4 and secondary air V3 (supplied directly to the boiler furnace via a wind box mounted on the outside of the boiler furnace) flowing between ducts D5 and D6. The heat exchange elements are typically of two types: one is an upper ("high-temperature end") layer for the high-temperature end of rotor 2, adjacent and typically positioned above to maximize heat exchange; and the other is a lower ("low-temperature end") layer to minimize fouling (occurring at lower temperatures received at the low-temperature end of rotor 2). In combination, the heat exchange elements are designed to transfer the maximum amount of preheat while still operating at temperatures that suppress or avoid, for example, fouling, oxidation, and / or acid corrosion. Typically, primary air V2 accounts for about 20% of the combustion air and contains about 10% of the heat captured from flue gas V1, while secondary air V3 accounts for about 80% of the combustion air and contains 90% of the captured heat.

[0006] Looking at the rotor 2 shown in FIG. 1, it will be understood that at any given time there is a "flue gas" portion between the flue gas ducts D1 and D2, a "primary air" portion between the primary air ducts D3 and D4, and a "secondary air" portion between the secondary air ducts D5 and D6. The flue gas V1 having a volumetric flow rate V1 exits the inlet duct D1 at the flue gas inlet temperature T1 adjacent to the hot end of the "flue gas" sector of the rotor 2 and enters the outlet duct D2 at a lower flue gas outlet temperature T2 adjacent to the cold end of that sector. Conversely, the primary air V2 having a volumetric flow rate V2 exits the inlet duct D3 at an air inlet temperature T3 (typically near the general ambient air temperature) adjacent to the cold end of the "primary air" sector of the rotor 2 and enters the outlet duct D4 at a primary air outlet temperature T4 adjacent to the cold end of that sector. Similarly, the secondary air V3 having a volumetric flow rate V3 exits its inlet duct D5 at approximately the same air inlet temperature T3 and enters its outlet duct D6 at a secondary air outlet temperature T5 adjacent to the cold end of that sector. The temperature of the rotor 2 itself varies between a high hot end metal temperature T6 at its hot end when immediately adjacent to the flue gas inlet duct D1 and a low cold end metal temperature T7 at its cold end when immediately adjacent to the secondary air outlet duct D6. These temperatures can be measured or estimated using local sensors or other known methods.

[0007] In a typical three-sector APH configuration, these ducts are arranged in a "G-P-S" configuration, a "forward" rotating APH, such that in the direction of rotation of the rotor 2, the primary air duct P follows the flue gas duct G, followed by the secondary air duct S, i.e., any given radial portion of the rotor 2 rotates between the flue gas ducts D1-D2, then the primary air ducts D3-D4, and finally the secondary air ducts D5-D6, with the result that the primary air is preheated before the secondary air, and this process is repeated at a rate of approximately one rotation per minute. In the "G-S-P" configuration (or "reverse" rotating APH), it will be readily understood that the secondary air is preheated before the primary air.

[0008] During normal operation, the volumetric flow rates of V1, V2, and V3 each vary between their respective minimum values ​​when the boiler is operating at low load and their maximum values ​​when the boiler is operating under full load conditions. These volumetric flow rates are also partially controlled by the operation of fans (e.g., fan 600) in various upstream and / or downstream ducts of the APH, particularly the primary and secondary air fans. Fan operation may be influenced and controlled to increase and / or maintain flow rates, for example, to compensate for pressure drops inherent in ducts and / or pollution control equipment such as the APH. Furthermore, the operation of the primary and secondary air fans is differentially controlled with respect to primary air V2 and secondary air V3, so that the former is increased to a higher pressure, for example, to facilitate the transport of pulverized coal from the mill to the furnace. Typically, secondary air V3 has a pressure of 10–30 inches wg and primary air V2 has a pressure of 40–60 inches wg. One consequence of such higher pressures is that the primary air V2 has a higher heat of compression compared to the secondary air V3, which is partially observable as T4 being typically higher than T5.

[0009] Generally speaking, except as described below, the air inlet temperature T3S remains essentially constant at or near the dominant ambient and / or local air temperature T3, while the primary air inlet temperature T3P is somewhat higher than the secondary air inlet temperature T3S (e.g., only about 20 degrees Fahrenheit) due to additional compression caused by the primary air fan or primary air booster fan (if the latter is used with a secondary air fan to blow primary air through the APH). However, other temperatures T vary between their respective minimum and maximum values, depending on the boiler load and associated volumetric flow rate. The mean low-temperature end temperature T7 has a practical minimum value, i.e., the mean low-temperature end temperature T7 must be above industry-established standards based on the properties of the boiler fuel to mitigate acid corrosion, for example. The mean low-temperature end temperature (ACET) is the average of the air inlet temperature and the gas outlet temperature. This average can be achieved even if the actual low-metal temperature T7 during rotation is actually below the acid dew point, although it will be understood that the ACET is generally better above the dew point.

[0010] Furthermore, there is a practical maximum value for the primary air outlet temperature T4, because the primary air must have a mill inlet temperature T8 (not shown) lower than a given maximum value (the temperature at which spontaneous combustion of coal particles may occur outside the furnace, potentially leading to a fire in the coal mill and the operation of fire suppression equipment). That said, there is also a practical minimum value for the mill inlet temperature T8, particularly related to the moisture content of pulverized coal associated with high-moisture lignite or brown coal. It will be understood that coal particles cannot burn efficiently if they are still wet and must be dried to some extent. This can be done in the furnace, but since this reduces the overall efficiency of the furnace, it is preferable that the wet particles be substantially dried by primary air, i.e., by preheating from flue gas or by fan operation (e.g., fan 600), before being supplied to the furnace. This is one reason why it is desirable for T3P to be higher than T3S.

[0011] Normally, T8 is not measured, but instead the primary air outlet temperature T10 (not shown) is controlled, and the primary airflow is T10 MIN (To facilitate proper coal drying) and T10 MAX The temperature is adjusted to have a temperature between (above) and (above) (not ensuring the spontaneous combustion of coal in the mill). Various means of primary air conditioning (i.e., changing the temperature of the air) are disclosed in U.S. Patent No. 3,373,520 (Hottenstine) and U.S. Patent No. 4,442,783 (Pajonas et al.). Air temperature conditioning includes, but is not limited to, adjusting or changing the temperature of the air by temperature conditioning means, i.e., cooling or heating, including, for example, fans, baths, cooling systems, heating systems, and the addition of air at different temperatures.

[0012] It will be understood that the primary objective of any APH is to improve boiler efficiency by removing heat from the flue gas to preheat the combustion air, thus reducing the amount of waste heat discharged into the atmosphere and effectively reducing the amount of fuel required to heat the furnace. Therefore, a generally accepted goal of APH design is to improve its own heat transfer efficiency by maximizing the heat captured for secondary combustion air preheating, and consequently minimizing the flue gas outlet temperature, e.g., T2, while keeping the low metal temperature T7 higher than the minimum required.

[0013] In practice, for any given boiler load, there exists a constant volumetric flow rate (and sometimes mass flow rate) required for the combustion air, e.g., V2 + V3, and therefore, at moderate loads, only an amount of preheating is required that results in a flue gas outlet temperature T2 higher than what would be achievable with the inherently increased flue gas flow during peak load conditions. In such situations, the achieved flue gas outlet temperature T2 is found to be too high for the efficient operation of an electrostatic precipitator (ESP) (not shown) located immediately downstream of the APH to capture particulate contaminants from the flue gas, such as fly ash. To overcome this, U.S. Patent No. 6,089,023 proposes oversupplying the APH with more air than is required as combustion air, and then removing the “undesirable” excess air upstream of the APH, for example, from a secondary air outlet duct D6 (for other uses or simply released into the atmosphere), thereby providing an acceptable lower flue gas outlet temperature, e.g., T2 (the so-called “oversupply technique”). It will be understood that undesirable excess air, when discharged into the atmosphere while maintaining a desirable low flue gas outlet temperature, e.g., T2, is actually waste heat and therefore reduces the operating efficiency of the APH and boiler.

[0014] Recently, the need has arisen to operate boilers with zero wastewater to avoid the need for water treatment plants to remove contaminants before discharging wastewater into the environment. It is known that evaporation systems such as spray dry absorbers (SDAs) are used. Other such evaporation systems, as will be discussed later, include, for example, circulating dry scrubbers (CDSs), all of which operate to evaporate wastewater to reduce or eliminate wastewater. These systems are supplied with flue gas from the APH either directly or further downstream of other pollution control equipment. This is not a problem at high loads, as the flue gas outlet temperature, e.g., T2, and volumetric flow rate V1 at such design loads are high enough to promote complete evaporation in, for example, the SDA; i.e., T2 is at the temperature T9 required for a particular volumetric flow rate V1 to promote near 100% evaporation. It will be understood that T2 ≥ T9 must be maintained; otherwise, the SDA will not be sufficiently effective for zero wastewater, and the remaining water will need to be treated by the very wastewater treatment equipment that the SDA is intended to replace. It will be understood that SDAs and CDS each evaporate water in known ways, which are similar in many respects.

[0015] Historically, coal-fired power plants were operated at high load / output even during grid-based load conditions, and the APH was optimized for such conditions, essentially having T2≧T9, and therefore ideal for SDA operation. During moderate load conditions above 50% of total load, the full-load flue gas outlet temperature T2 may be essentially too low for efficient SDA operation, and therefore it needs to be raised to achieve zero water emissions. It is known that the desired increase of T2≧T9 can be achieved by heating the secondary air using a steam coil before the secondary air enters the APH, resulting in the APH air inlet temperature being sufficiently higher than the ambient temperature, thereby increasing the flue gas temperature T2 exiting the APH1. However, such systems still cannot achieve T2≧T9 itself under normal load conditions, and especially not under low load conditions, typically below 50% load, e.g., 33% design load (the so-called "steam coil technique"). Regardless of its effectiveness, steam coil technology will be understood to divert heat from a boiler, which could otherwise be used for power generation, with the accompanying reduction of boiler efficiency.

[0016] With the emergence of renewable energy in today's energy market, coal-fired steam generators (boilers) are often required to operate for extended periods at low loads (less than 50% of full load, e.g., typically 33% design or full load). Many coal-fired boilers are not designed to operate in this mode, and installed APH low-load operation often results in low or suboptimal flue gas outlet temperatures T2, as well as low primary air outlet temperatures T4 and secondary air outlet temperatures T5. Under such conditions, T2 is well below the T9 value required for efficient SDA operation; that is, to achieve typically 100% or near-100% wastewater evaporation, the temperature of the flue gas leaving the APH needs to be increased, and as mentioned earlier, the use of steam coil technology is insufficient on its own and completely ineffective under low-load conditions. Therefore, to achieve a flue gas temperature ≥ T9 for any given volumetric flow rate V1, it was necessary to heat the flue gas above the temperature leaving the APH using a secondary heat source such as a natural gas burner or oil burner (the so-called "secondary burner technology"). It will be understood that heating flue gas downstream of the APH using any secondary burner technology will negatively impact boiler efficiency and require capital, fuel, and maintenance costs to operate.

[0017] The applicant investigated whether it was possible to achieve an increase in flue gas outlet temperature T2 using the overfeed technology described above, in an attempt to avoid the additional costs associated with using secondary burner technology under such low-load conditions (typically less than 50% of the total load, e.g., 33% of the design load), and found that it was not possible. From the above written evaluation of these technologies, it will be understood that the only obvious solution from the latest technology is, for example, to upgrade the currently used steam coil technology with one that is much larger than a typical steam coil. The applicant believes that the present invention described below addresses the problems associated with the prior art, overcomes the problems arising from obvious solutions, and provides a novel and non-obvious solution. [Overview of the Initiative]

[0018] The object of the present invention is to provide an APH temperature control system used to facilitate the efficient operation of pollution control equipment and to provide flue gas at a temperature that eliminates the need to use secondary burner technology and / or steam coil technology under low boiler load conditions. In one embodiment, the pollution control equipment may include a water evaporation system such as a spray dry absorber (SDA), a circulating dry scrubber (CDS), or a wet scrubber (e.g., a wet FGD). The present invention may allow more reagents to be injected into the pollution control equipment, which improves removal efficiency. The pollution control equipment can benefit from higher gas inlet temperatures at low loads, which improves water balance and reduces water treatment requirements.

[0019] The present invention is further intended to address problems related to the minimum operating temperature. Furthermore, the present invention addresses problems related to acid dew point and corrosion. The present invention relates to sulfur dioxide ( SO2 The aim is to address issues related to the removal efficiency, reagent utilization rate, and water balance of )

[0020] In some embodiments of the present invention, the invention is intended to reduce the mass available for heat transfer and increase the flue gas outlet temperature. The increase in flue gas temperature to pollution control equipment has advantages including, but not limited to, an increase in the amount of process water and a reduction in the minimum load required for operation, depending on the system used. In some embodiments of the present invention, the invention is further intended to increase the primary air temperature to the mill, which increases the coal drying and mill outlet temperatures. In some embodiments of the present invention, the invention is intended to increase the APH gas outlet temperature and raise the mean low-temperature edge temperature. In some embodiments of the present invention, a secondary bypass is intended to result in an increase in flue gas outlet temperature, which increases the APH gas outlet temperature and raises the temperature approaching the acid dew point.

[0021] The present invention includes a temperature control system for an air preheater. In one embodiment, the temperature control system is a two-sector air preheater (100), comprising: a flue gas inlet duct (D1) configured to supply flue gas from a boiler to the two-sector air preheater (100); a flue gas outlet duct (D2) configured to discharge flue gas from the two-sector air preheater (100); a combustion air inlet duct (D50) configured to transport air to the two-sector air preheater (100); and a combustion air outlet duct (D1) configured to discharge air from the two-sector air preheater (100). 00) The combustion air outlet duct (D100) is in fluid communication with the primary air duct (D40) and the secondary air duct (D60), the secondary air duct (D60) is located downstream of the primary air duct (D40), the primary air duct (D40) is configured to supply a first amount of primary air (V20) to the boiler via at least one grinding mill, and the secondary air duct (D60) is configured to supply a second amount of secondary air (V30) directly to the boiler, and the combustion air outlet duct (D100) is located close to the flue gas inlet duct (D1) A heat exchange rotor (20) having a high-temperature end and a low-temperature end adjacent to a combustion air inlet duct (D50), wherein the amount of secondary air (V30) is greater than the amount of primary air (V20), and an air preheater bypass duct (D70) communicating with the combustion air inlet duct (D50) and the secondary air duct (D60), wherein the air preheater bypass duct extracts a portion of the combustion air (V300) as a secondary air bypass (V40) from the air inlet duct (D50) upstream of the two-sector air preheater (100) and supplies air downstream. An air preheater bypass duct (D70) configured to be reintroduced into a secondary air duct (D60), and a flow control device (DV10) for controlling the volumetric flow rate of the secondary air bypass (V40) and adjusting the primary airflow (V20), wherein the primary airflow (V20), alone or in conjunction with other adjusting means, maintains the flue gas outlet temperature (T2) above a first predetermined minimum temperature (T9) for the flue gas volumetric flow rate (V1) leaving the two-sector air preheater (100), thereby maintaining the minimum temperature (T10) necessary to promote coal drying in the grinding mill. MIN The primary air outlet temperature (T10) within the safe range of the grinding mill (T10) MAXThe system includes a two-sector air preheater, which is configured to maintain the temperature at which the coal inside the preheater will spontaneously ignite.

[0022] In one embodiment, the temperature control system is a three-sector air preheater (1), comprising: a flue gas inlet duct (D1) configured to supply flue gas from a boiler to the three-sector air preheater (1); a flue gas outlet duct (D2) configured to discharge flue gas from the three-sector air preheater (1); a primary air inlet duct (D3) configured to supply primary air (V2) to the three-sector air preheater (1); a primary air outlet duct (D4) configured to supply a first amount of primary air (V2) to the boiler via at least one grinding mill; a secondary air inlet duct (D5) configured to supply secondary air (V3) to the three-sector air preheater (1); a secondary air outlet duct (D6) configured to supply a second amount of secondary air (V3) directly to the boiler; a high-temperature end adjacent to the gas inlet duct (D1); and the secondary air inlet duct (D5 A heat exchange rotor (2) having a low-temperature end adjacent to the 3-sector air preheater (1), and at least a secondary air bypass duct (D7) communicating with a secondary air inlet duct (D5) and a secondary air outlet duct (D6), wherein the secondary air bypass duct D7 is configured to extract a portion of the secondary air (V3) as a secondary air bypass (V4) from the secondary air inlet duct (D5) upstream of the 3-sector air preheater (1) and reintroduce it to the downstream secondary air outlet duct (D6), comprising a first secondary air bypass duct (D7), and a flow rate control device (DV1) for controlling the volumetric flow rate of the secondary air bypass (V4) and adjusting the primary airflow (V2), wherein the primary airflow (V2) maintains the flue gas outlet temperature (T2) above a second minimum temperature (T9) relative to the flue gas volumetric flow rate (V1) leaving the 3-sector air preheater (1), thereby maintaining the minimum temperature (T10) necessary to promote coal drying in the grinding mill. MIN The primary air outlet temperature (T10) within the safe range of the grinding mill (T10) MAX It includes a three-sector air preheater, which is configured to maintain the temperature at which the coal inside the preheater will spontaneously ignite.

[0023] In some embodiments, when the air preheater is a three-sector air preheater (1), the control system further includes a primary air bypass duct (D8) communicating with a primary air inlet duct (D3) and a primary air outlet duct (D4), wherein the primary air bypass duct (D8) is configured to extract a portion of primary air (V2) as a primary air bypass (V5) from the primary air inlet duct (D3) upstream of the three-sector air preheater (1) and reintroduce it to the downstream primary air outlet duct (D4), and a second flow control device (DV2) for controlling the volumetric flow rate of the primary air bypass (V5).

[0024] In some embodiments, when the air preheater is a two-sector air preheater (100), the secondary air bypass (V40) under low-load conditions is 5-15% of the secondary air (V30) supplied to the two-sector air preheater (100).

[0025] In some embodiments, when the air preheater is a 3-sector air preheater (1), the secondary air bypass (V4) under low load conditions is 5-15% of the secondary air (V3) supplied to the 3-sector air preheater (1).

[0026] In some embodiments, when the air preheater is a three-sector air preheater (1), the primary air bypass (V5) under low-load conditions is 15-20% of the primary air (V2) supplied to the three-sector air preheater (1).

[0027] In some embodiments, when the air preheater is a 3-sector air preheater (1), under low load conditions, the secondary air bypass (V4) is 10% and the primary air bypass (V5) is 20%.

[0028] In some embodiments, if the air preheater is a two-sector air preheater (100), a first predetermined minimum temperature (T9) is configured to facilitate the efficient operation of pollution control equipment located downstream of the two-sector air preheater (100) to receive the flue gas flow (V1) exiting the flue gas outlet duct (D2).

[0029] In some embodiments, if the air preheater is a three-sector air preheater (1), a second predetermined minimum temperature (T9) is configured to facilitate the efficient operation of pollution control equipment located downstream of the three-sector air preheater (1) to receive the flue gas flow (V1) exiting the flue gas outlet duct (D2).

[0030] In some embodiments, the pollution control equipment is a spray dry absorber (SDA), a circulating dry scrubber (CDS), or a wet flue gas desulfurization system ("FGD"). In some embodiments, the pollution control equipment is further positioned to receive at least a first portion of the flue gas flow (V1) immediately downstream of a three-sector air preheater (1) or a two-sector air preheater (100), thereby resulting in increased wastewater evaporation.

[0031] In some embodiments, when the flue gas volumetric flow rate (V1) is above a predetermined minimum temperature (T9) under low load conditions, at least a first portion of the flue gas flow (V1) evaporates wastewater supplied to the pollution control system.

[0032] In some embodiments, at least one of the secondary air bypasses (V4) is pressure-dropped, and the primary air bypass (V5) is pressure-dropped. As those skilled in the art will recognize, in some embodiments disclosed herein, the pressure-dropped assistance is provided, for example, by an opening such as a duct or passage, to move a certain volume of air.

[0033] In some embodiments, the grinding mill (400) includes a grinder inlet for receiving fuel, a grinder outlet for discharging the ground fuel therefrom, and a boiler (500) in communication with the grinder outlet and configured to burn the fuel. In some embodiments, each of the two-sector air preheaters (100) and the three-sector air preheater (1) includes a rotor (20) mounted to rotate on a spindle and a plurality of heat transfer elements arranged on the rotor, the heat transfer elements being configured to transfer heat to the gas flowing through them, and each of the two-sector air preheaters (100) and the three-sector air preheater (1) defines a high-temperature end (20H) and a low-temperature end (20C). In some embodiments, the combustion air outlet duct (D100) is branched into a primary air duct (D40) and a secondary air duct (D60), with the primary air duct (D40) being smaller than the secondary air duct (D60).

[0034] The present invention relates to a method for controlling the temperature of gas exiting an air preheater using a temperature control system described in any of the embodiments described above.

[0035] The present invention relates to a method for modifying an air preheater system with a temperature control system as described in any of the embodiments described above, wherein the air preheater is a 3-sector air preheater (1). In some embodiments, the method includes utilizing an existing primary air bypass duct (D8) that is restricted by an existing damper valve (DV2), and installing a secondary air bypass duct (D7) of appropriate size that can be closed by a damper valve (DV1), wherein during low-load operation, when the damper valve (DV1) is open and the damper valve (DV2) is fully open, the primary air outlet temperature (T4) is reduced to at least the minimum primary air outlet temperature (T10). MIN It is raised by a predetermined amount sufficient to raise it to ).

[0036] In some embodiments of the modification method, 10-20% of the primary air flowing into the 3-sector air preheater (1) is bypassed, and the secondary air bypass duct (D7) can bypass approximately 0-10% of the secondary air flowing into the 3-sector air preheater (1).

[0037] The embodiments described above and others are described in more detail herein. [Brief explanation of the drawing]

[0038] The following drawings illustrate specific aspects and embodiments of the present invention, but the present invention is not limited to those shown and described in the drawings. [Figure 1] A schematic diagram of a conventional 3-sector air preheater is shown. [Figure 2] A schematic diagram of a 3-sector air preheater is shown along with a temperature control system according to one embodiment of the present invention. [Figure 3] Figure 2 shows a schematic diagram of a three-sector air preheater in a "reverse" rotation GSP configuration, with a typical temperature achievable during operation of one particular design of its embodiment. [Figure 4] A schematic diagram of a two-sector air preheater according to a further embodiment of the present invention is shown. [Modes for carrying out the invention]

[0039] Figure 2 shows an APH1 similar to that in Figure 1, with similar features indicated by the same or similar alphanumeric notation. In Figure 2, APH1 is a three-sector APH for use with a coal-fired boiler (not shown) that uses flue gas V1 to preheat the combustion air (V2+V3) supplied to the boiler. Part of the combustion air is primary air V2, which first passes through at least one coal mill or crusher (not shown) to carry coal particles into the boiler furnace. The remainder of the combustion air is supplied directly to the furnace from APH1 as secondary air V3. Furthermore, the flue gas V1 leaving APH1 is supplied directly to pollution control equipment (not shown). In one embodiment, the pollution control equipment is a water evaporation system. The water evaporation system includes a spray dry absorber ("SDA"), a circulating dry scrubber ("CDS"), or a wet flue gas desulfurization system ("FGD") that evaporates wastewater to reduce or eliminate water discharge. Hereinafter, for ease of explanation, the evaporation system will be referred to as an SDA. Other pollution control equipment may be used.

[0040] For ease of understanding, various gas flows are shown as V1 to V5, and these gas flows are synonymous with volumetric flow rate and mass flow rate. While it is easy to consider various volumes of gas flows, their mass composition is important when calculating the actual dominant flow and its thermal and / or combustion support characteristics.

[0041] The third sector APH1 in Figure 2 is located at its upper / hot end and is arranged "circumferentially" around its rotor 2. The flue gas inlet duct D1, primary air outlet duct D4, and secondary air outlet duct D6 are axially aligned with the flue gas outlet duct D2, primary air inlet duct D3, and secondary air inlet duct D5, respectively, which are similarly located at its lower / cold end around the rotor 2. For the sake of illustration and functional explanation, the primary and secondary air ducts are shown as being arranged radially.

[0042] Rotor 2 houses a basket of elemental profiles designed to rotate to extract and release heat from flue gas V1 exiting inlet duct D1, thereby preheating the combustion air entering primary air outlet duct D4 and secondary air outlet duct D6, respectively. This is achieved by heating the “flue gas” portion of rotor 2 with the hot flue gas passing through it, thereby preheating the combustion air for the boiler furnace, which passes through the “primary air” portion of rotor 2 as primary air and the “secondary air” portion as secondary air. Not shown are various electrical and / or mechanical control systems, fans / blowers, external ducts, pressure transducers and thermometers, dampers typically used to operate the APH, and the boiler furnace and its wind box. Such auxiliary equipment is well known in the art. The functional and operational limitations of such auxiliary equipment, including inherent problems with leakage, are well known, and therefore the usual design, engineering, installation, and operation of the aforementioned known embodiments of the 3-sector APH are not described in detail to those skilled in the art.

[0043] Flue gas (flue gas V1) with a volumetric flow rate V1 exits inlet duct D1 at a flue gas inlet temperature T1 adjacent to the hot end of rotor 2 and enters outlet duct D2 at a lower flue gas outlet temperature T2 adjacent to the cold end of rotor 2. Conversely, primary air (primary air V2) with a volumetric flow rate V2 exits inlet duct D3 adjacent to the cold end of rotor 2 at a primary air inlet temperature T3P and exits outlet duct D4 at a primary air outlet temperature T4. Similarly, secondary air (secondary air V3) with a volumetric flow rate V3 exits inlet duct D5 at a secondary air inlet temperature T3S and exits outlet duct D6 adjacent to the cold end of its sector at a secondary air outlet temperature T5. The temperature of rotor 2 itself varies between the hot end metal temperature T6 achieved at its hot end and the cold end metal temperature T7 achieved at its cold end.

[0044] To facilitate highly efficient SDA operation, i.e., 100% wastewater evaporation or near-100% SDA operation, at any given time during typical operation under any given load conditions, the flue gas V1 exiting the flue gas outlet duct D2 must maintain a temperature T2 ≥ T9 for the reasons stated above. To facilitate this, the three-sector air preheater 1 is installed together with a temperature control system 200 according to one embodiment of the present invention. The temperature control system 200 includes a secondary air bypass duct D7 that extends between the secondary air inlet duct D5 and the secondary air outlet duct D6 and is in fluid communication with them. The secondary air bypass duct D7 is configured to bypass the volumetric flow rate of the secondary air V3 to a desired percentage under low load conditions.

[0045] In one embodiment, the temperature control system 200 includes a first flow control device DV1. The first flow control device DV1 may be a first damper valve that is electrically or pneumatically operated, a louver control damper of a known design, an electric controller, or a pneumatic controller that is appropriately linked with the aforementioned auxiliary equipment. In one embodiment, under low load conditions, the secondary air bypass duct D7 is configured to facilitate secondary air bypass of a desired percentage of the volumetric flow rate or possibly mass flow rate of the secondary air V3, preferably 5% to 20%, more preferably 10% to 15%.

[0046] The APH combustion air bypass according to the present invention reduces the overall efficiency of the APH, and the secondary air bypass increases the flue gas outlet temperature T2 when the secondary air is bypassed under low load conditions. However, somewhat unexpectedly and counterintuitively, the decrease in the secondary air outlet temperature T5 is accompanied by an increase in the primary air outlet temperature T4, which indicates that there is an increase in the relative heat transfer to the primary air flowing through it despite the decrease in the APH efficiency.

[0047] In some embodiments, to ensure that the primary air outlet temperature T4 is maintained within a range that ensures that the primary air outlet temperature T10 is between a minimum value T10 (required to promote proper coal drying) MIN and a maximum value T10 (which could lead to spontaneous ignition of the coal in the mill), MAX the primary air exiting the 3-sector APH1 is adjusted, for example, using known primary air bypass techniques or otherwise using overfeed techniques. Adjusting the temperature of the air includes, for example, adjusting or changing the temperature of the air by temperature adjustment means including fans, buses, cooling systems, heating systems, addition of air at different temperatures, i.e., cooling or heating, but is not limited thereto.

[0048] The embodiment illustrated and described in FIG. 2 is contemplated to be part of a retrofit project where existing primary air bypass technology has already been installed on the 3-sector APH1. In another embodiment, the embodiment shown and described in FIG. 2 is contemplated to be part of a new project that includes the 3-sector APH1, and the present invention is not limited in this regard.

[0049] In the embodiment shown in Figure 2, the temperature control system 200 of the present invention further includes a primary air bypass duct D8 extending between a primary air inlet duct D3 and a primary air outlet duct D4 and in fluid communication with them, and a second flow control device DV2. The second flow control device DV2 is an electrically actuated damper valve or a pneumatically actuated damper valve. The primary air bypass duct D8 is adapted to bypass 10% of the primary airflow V3 under design load conditions (e.g., when V3 is calculated from a mass flow rate of 791450 lb / hr at 111 degrees Fahrenheit) and 20% under 33% lower load conditions (e.g., when V3 is calculated from a mass flow rate of 450510 lb / hr at 121 degrees Fahrenheit). Under pre-modification conditions, the primary air outlet temperature T10 under lower load conditions is T10 MIN This is less than the required amount, resulting in inefficient coal drying before combustion; that is, some necessary drying occurs inside the furnace, reducing the furnace's efficiency.

[0050] The inlet and outlet ducts D1 and D2, D3 and D4, and D5 and D6 are shown as the respective cowlings of the three-sector APH1 attached to their housings. However, it will be understood that these ducts, as functionally described, are intended to encompass not only such cowlings of APH1 but also the associated ducts extending therefrom. Thus, the inlet ends of bypass ducts D7 and D8 communicate with their respective inlet ducts D5 and D3 via either the associated cowling of the three-sector APH1 or the ducts extending upstream therefrom, and the outlet ends of bypass ducts D7 and D8 communicate with their respective outlet ducts D6 and D4 via either the associated cowling of the three-sector APH1 or the ducts extending downstream therefrom.

[0051] It will be understood that, because heat transfer elements are packed in, there is an unavoidable pressure drop across rotor 2 through which both flue gas and combustion air pass axially. This is partly why auxiliary equipment includes, for example, auxiliary fans to maintain the required volumetric flow rate. The secondary air bypass duct D7 and primary air bypass duct D8, when open, allow for a nearly uninterrupted gas flow, resulting in a much lower pressure drop than the pressure drop caused across rotor 2 packed with heat transfer elements intended to cause an interrupted flow, and are also often soiled by use. Consequently, these bypass ducts, when open, provide a preferred path for combustion air to flow, resulting in what may be called a pressure-drop-assisted bypass flow.

[0052] The air outlet end of the secondary air bypass duct D7 is preferably located close to APH1, but it will be understood that it can be installed at any suitable location midway between APH1 and the boiler wind box (not shown). The material and size of the secondary air bypass duct D7 are generally a matter of design choice, but as an example, a wear-resistant duct approximately 48 inches in diameter and 40 feet in length, adapted to handle the volumetric flow rate of V4 during use, is provided. The present invention is not limited in this respect, and other materials and sizes are contemplated.

[0053] The above describes an ideal scenario where no leakage occurs from the "flue gas" path to the "combustion air" path within APH1, but this is not the case in reality, and therefore APH seal design remains critical. In one installation, for example, the pressure drop across the "flue gas" sector of rotor 2 may vary between 0.8 inches wg at low loads and 3.6 inches wg at design loads (depending on the configuration of the elements packed within rotor 2). Similarly, the pressure drop across the "primary air" sector may vary between 0.7 and 2.15 inches wg, and the pressure drop across the "secondary air" sector may vary between 0.5 and 2.8 inches wg. In all cases, a larger pressure drop in the "flue gas" sector results in leakage into the combustion air flowing through rotor 2. Therefore, V1 IN <V1 OUT and V3 OUT >V3 IN This results in leakage that fluctuates between, for example, 8.56% at low loads and 4.81% at design loads.

[0054] The amount of secondary air bypass can be controlled not only by the size and length of ducts D7 and D8, but also by the proper operation of damper valves DV1 and DV2. While the exact form of these valves is not critical, it should be understood that, where appropriate, these valves can be replaced with fan assist devices to provide the necessary bypass flow control function, working in conjunction with their inherent pressure drop assistance.

[0055] During high-load operation of the boiler (typically more than 50% of full load), sector 3 APH1 operates with dampers DV1 and DV2 in the closed position, i.e., without secondary air bypass exceeding the bypass associated with valve leakage, but the primary air outlet temperature T10 is within the safe range T10. MIN ~T10 MAXEnable a 10% primary air bypass necessary to maintain within. In this high load state, most of the secondary air V3 (also known as "combustion air") exiting the secondary air outlet duct D7, apart from the inherent leakage within the 3 - sector APH1, is the same as the secondary air V3 entering the secondary air inlet duct D5. The volumetric flow rate S1 of the secondary air (secondary air S1) entering the secondary air section of the rotor 2 is the same as the secondary air exiting the duct D6, i.e., S1 = V3. Similarly, the primary air V2 exiting the primary air outlet duct D4 is substantially about 10% flowing through the primary air bypass duct D8 and entering the primary air inlet duct D3. The 3 - sector APH1 is configured to operate in this way without a secondary air bypass during high load operation, providing a desired high - efficiency waste water evaporation at the flue gas outlet temperature T2≧T9 and SDA, but having a sufficient primary air bypass to ensure that the primary air outlet temperature T10 remains within the desired safety range.

[0056] In a low - load operation where the damper DV1 is closed and the damper DV2 is closed, i.e., there is no secondary air bypass (typically less than 50% of full load, e.g., 33% of the design load), the APH1 operates at an output temperature T2 < T9 of the flue gas V1, i.e., it is insufficient for the high - efficiency operation of the SDA. However, by opening the damper DV1, a part V4 of the air V3 entering the inlet duct D5 flows through the secondary air bypass duct D7, thus bypassing the rotor 2. More importantly, it reduces the volumetric flow rate of the secondary air S1 passing through the rotor 2. When the flow rate of the secondary air S1 passing through the rotor 2 decreases, the heat transfer due to the installed element profile decreases, and the flue gas outlet temperature T2 increases. It will be understood that with an appropriate configuration of the secondary air bypass duct D7 and / or the operation of the damper valve DV1, the low - load operation of the APH1 with the secondary air bypass duct D7 at least mainly or actually fully open can result in the desired efficient waste water evaporation at T2≧T9 and SDA. In any case, the secondary combustion air V3 = S1+V4.

[0057] During low-load operation using only the secondary air bypass V4, i.e., when damper DV1 is open and damper DV2 is closed, and less waste heat is extracted from flue gas V1, the low-temperature end metal temperature T7 of rotor 2 "rises" (compared to similar operation with damper DV1 closed), and the primary air outlet temperature T10 at the high-temperature end of the primary air section of rotor 2 also "rises". As a result, the primary air V2 exiting duct D4 has a higher outlet temperature T4 than when the secondary air bypass duct D7 is closed. To some extent this may depend in part on whether the three-sector APH in question is a "forward" or "reverse" APH, and the exact configuration of the heat transfer elements housed within rotor 2. The exact mechanisms and thermodynamics of such are assumed to be readily understood by those skilled in the art and will not be discussed further here.

[0058] As mentioned above, if T4 is too high, the primary air outlet temperature T10 of the coal mill will be the fire safety limit T10. MAX This could lead to the temperature approaching or exceeding the limit. In such situations, it is necessary to lower T10 / T4 below the fire safety limit by fully opening DV2 to allow for a maximum of approximately 20% primary air bypass. However, secondary air bypass can inadvertently raise the primary air outlet temperature T4, and the amount of such bypass, under low load conditions, can cause the primary air outlet temperature T10 to rise to T10. MIN It can be adjusted to exceed this value, that is, to provide efficient coal drying that was not previously available before the low-load modification. By appropriately limiting the secondary air bypass, the desired increase in T4 at low loads can be achieved.

[0059] In the specific design described, it is found that a secondary air bypass of X = 20%, along with a 20% primary air bypass, increases T4 by Y = 10 degrees. Thus, it is possible to achieve the desired rise Y of the flue gas outlet temperature T2 and the desired rise T4 of the primary air outlet temperature with an appropriate secondary air bypass ratio X. The amount of secondary air bypass V4 achievable without additional fan pressure or equipment is a function of the size, length, etc., of the duct D7 and valve DV1, and it will be understood that designing accordingly for any given new or modified installation is within the capabilities of those skilled in the art.

[0060] It will be understood that by utilizing these embodiments, it becomes easier for the 3-sector APH1 to maintain the desired gas outlet temperature even at low loads. An example of indicated temperatures is shown in Figure 3 relating to one specific design for the installation of the APH in a “reverse” rotation GSP configuration, but is not intended to illustrate what is seen in other installations. Temperatures in parentheses are the prevailing temperatures in such installations prior to modifications according to embodiments of the present invention, and represent the temperatures achieved when the secondary air bypass is installed but not operating, i.e., when damper DV1 is closed, for example, during high load conditions. Temperatures in Fahrenheit (without parentheses) are predictively modeled to represent temperatures achievable at low loads when both the secondary and primary air bypasses are installed and dampers DV1 and DV2 are fully open and operational, i.e., in this case, when there is a desirable rise in primary air temperature and the necessary rise in flue gas outlet temperature required for more efficient water evaporation system operation. Advantageously, the primary air outlet temperature T10 is set to a desired range ≥ T10. MIN There is also an incidental increase of 10°F in the primary air temperature. It will be understood that dampers DV1 and DV2 can be partially opened and closed in response to the flue gas outlet temperature through the flue gas volume flow rate V1 and APH1 to achieve a temperature between the values ​​in parentheses and the values ​​outside the parentheses as desired or as needed. As described above, the operation of the combustion air fan results in a higher heat of compression of the primary air compared to the secondary air, thus resulting in indicated temperatures of 574°F and 570°C (approximately 301°C and 298°C), respectively.

[0061] By using any of these embodiments, the need to utilize the aforementioned secondary burner technology and / or pressurized air technology is eliminated, thus saving fuel, capex, and / or maintenance costs for the auxiliary equipment they require. In fact, if one of these embodiments is not used, it would be necessary to use a less efficient APH to provide the flue gas outlet temperature T2 required under low load conditions, and / or to reheat the flue gas to a temperature that enters the SDA at T9 or higher. Such reheating could be achieved using secondary burner technology and / or by using a larger steam coil when steam coil technology is utilized.

[0062] The aforementioned embodiments described in relation to zero water discharge from wastewater evaporation systems such as SDAs will be seen as potentially more technically attractive than achievable, particularly in relation to existing boiler facilities. Thus, while avoiding the need for a water treatment plant to remove contaminants before discharging wastewater into the environment is a desirable goal, some wastewater may still be discharged from the system. If zero water discharge is not achieved, the cost of water treatment will be seen as a function of the volume of wastewater treated. Therefore, even if the present invention facilitates a reduction in discharged water, the wastewater treatment cost will be reduced in roughly proportion to the evaporation of excess water in the evaporation system facilitated by its use.

[0063] During low-load operation of the APH1, the secondary air bypass V4 and / or primary air bypass V5 inevitably result in less heat recovery from the APH, lower coal utilization efficiency, and consequently higher coal costs. To mitigate the decrease in low-load boiler efficiency, the APH1 can be made to have an improved air preheater design, such as its advanced profile element design and its AdvX® Technologies, as described in various current patent applications of the applicant, i.e., it can operate with higher heat exchange efficiency under both low-load and high-load conditions.

[0064] The present invention has been described in relation to adjusting the flue gas outlet temperature T2 leaving the APH1 using a secondary air bypass V4 to ensure more efficient operation of a water evaporator immediately downstream, such as an SDA. In practice, it will be readily apparent that the flue gas temperature entering the water evaporator is important. Indeed, the present invention can be used in connection with other pollution control equipment downstream of the APH to adjust the flue gas inlet temperature to that equipment. There may also be equipment between the APH and the pollution control equipment where the present invention is utilized to adjust the flue gas inlet temperature. Therefore, the foregoing description of embodiments is illustrative of, not limiting, the uses of the present invention, and the adjusted temperature T9 is sufficient to maintain the flue gas inlet temperature to the pollution control equipment at a level necessary to facilitate its more efficient operation.

[0065] As an example, embodiments of the present invention can be installed when modifying boiler equipment while leaving existing steam coils operational in place just upstream of the APH. The coal cost of the secondary air bypass V4 may actually be greater than the steam cost associated with using steam coils already installed to preheat the combustion air flowing into the APH, for example, thereby increasing the temperature of the flue gas V1 flowing out of the APH. Thus, in a further embodiment of the present invention (not shown), a steam coil is provided between the combustion air fan and the APH, which is upstream of the APH and increases the temperature of the flue gas flow V1 by increasing the temperature of the combustion air V3. In such embodiments, the critical temperature T9 required for efficient evaporation system operation should be measured at the steam coil in use, and it will be understood that the secondary air bypass V4 can thus be effectively operated to promote an increase in the temperature T2 of the flue gas V1 downstream of the APH, in addition to the temperature promoted by the steam coil operating at or near its maximum value.

[0066] One modification example is modeled, and the predicted performance at full load (i.e., secondary air bypass is inoperable) is detailed in Table 1. Predicted performance at low load (i.e., 33% design load) is detailed in Table 2, which includes the column for the 3-sector APH1 before modification with secondary air bypass, and the applicant's latest generation of advanced element profiles TF4™ (disclosed in PCT / US2016 / 069186) and DN8™ (disclosed in PCT / US2016 / 056209). The operational features of the present invention are generally described using the information inferred from Tables 1 and 2, and it will be understood that more detailed information can be extracted therefrom from both the claimed description of the invention and / or technical limitations. In addition, Tables 1 and 2 may be annotated with reference numbers in the drawings or read in conjunction with them, merely as illustrative examples, without limiting their actual predicted benefits or significance.

[0067] In another embodiment of the present invention, the two-sector APH100 ​​shown in Figure 4 has a temperature control system 200 with a combustion air bypass configured to minimize its influence on the mill inlet temperature T8. It will be understood that the two-sector APH100 ​​has only two gas paths, namely, a high-temperature flue gas path (as in the three-sector APH1) that flows in reverse adjacent to the second combustion air preheating path. Downstream of the two-sector APH100, the preheated combustion air flow branches into a primary air duct and a secondary air duct at the same temperature. The properties, conditions, use, and operational constraints of the various gas flows are the same for the two-sector APH100 ​​as for the three-sector APH1. Accordingly, in Figure 4, features similar to those of the three-sector APH1 shown in Figures 1 to 3 are indicated by the same numbers, and similar features are indicated by their decimal numbers.

[0068] The two-sector APH100 ​​has a rotor 20 having a flue gas path with a volumetric flow rate V1 and a combustion air preheating path with a combustion air inlet volumetric flow rate V300. The two-sector APH100 ​​has a flue gas inlet duct D1, a flue gas outlet duct D2, a combustion air inlet duct D50, and a combustion air outlet duct D100. The flue gas flows through the rotor 20, releasing waste heat to the rotor 20 between the flue gas ducts D1 and D2, and the combustion air is preheated by the rotor 20 as it flows between the ducts D50 and D100. Downstream of sector APH100, the combustion air outlet duct D100 splits into a smaller branch or primary air duct D40 that supplies a smaller portion of the combustion air as primary air V20 to a coal mill (not shown), and a larger branch or secondary air duct D60 that supplies a larger portion of the combustion air as secondary air V30 directly to a boiler (not shown). It will be understood that the combustion air flowing into primary air duct D40 and secondary air duct D60 is at the same temperature and pressure, which is a function of the efficiency of APH100 ​​and the boiler load. Furthermore, the primary air outlet temperature T10 is set to the minimum temperature (T10) required to promote the drying of coal in the grinding mill. MIN Within the safe range of ) the maximum temperature (T10) that causes the coal in the grinding mill to spontaneously ignite. MAX It is desirable to maintain the temperature up to ). Under low load conditions, it is desirable to maintain the flue gas outlet temperature T2 at or above T9. As a result, according to the present invention, the second sector APH100 ​​is provided with a secondary air bypass duct D70 (also referred to herein as "air preheater bypass duct D70") which fluidly connects the combustion air inlet duct D50 to a secondary air duct D60 having a damper valve DV10 that can be operated to enable a secondary air bypass flow V40 of a size similar to that described above with respect to the third sector APH1.

[0069] As explained earlier, any combustion air bypass reduces the efficiency of the second sector APH100 ​​and incidentally increases the flue gas outlet temperature T2, but reduces combustion air preheating. By placing the combustion air bypass V40 into the secondary air outlet duct D60 (and not simply into the combustion air outlet duct D100), the primary airflow V20 is effectively isolated from the temperature drop effect associated with mixing the low-temperature bypassed combustion air with the preheated secondary air. Therefore, with the proper operation of the damper valve DV10, the primary air outlet temperature T10 is kept within a safe range T10. MIN From T10 MAX To maintain within, or at least better than if such separation were not present, T10 MIN To maintain it very close to the target, the primary air V20 temperature can be substantially adjusted. Although a simple embodiment of the 2-sector APH100 ​​has been described, it will be understood that other primary air adjustment (i.e., air temperature adjustment) means can be used in addition to, or instead of, the adjustments brought about by the aforementioned separation in order to achieve the target of a suitable primary air outlet temperature T10.

[0070] Further embodiments of the present invention (not shown) provide two APHs, a smaller primary APH (supplying primary air to the mill) and a larger secondary APH (supplying secondary air directly to the boiler furnace). Yet another embodiment suitable for oil-fired boilers is a single large APH that supplies preheated combustion air directly to the furnace. In both embodiments, the combustion or secondary air bypass as described above is provided, and the initially installed APH is modified with an improved heating element profile having heat transfer characteristics equivalent to or better than the applicant's DN8® and / or TF4® elements. Compared to the pre-installed APH, the modified APH using the overall or partially improved profile exhibits reduced efficiency and a desired increase in flue outlet temperature when the secondary or combustion air bypass is operating under low-load conditions. Advantageously, there is also an effective increase in ACET compared to the pre-installed APH, which results in the unexpected benefit of not exacerbating the low-temperature end corrosion that is prone to occur in the pre-installed APH under low-load conditions. Under the high-load or design load conditions described above, the improved APH operates more efficiently in terms of heat transfer than the pre-installed APH. Somewhat counterintuitively, despite the expected decrease in APH efficiency when using combustion or secondary air bypass at low loads, the overall energy recovery facilitated by improved APH operation or circulation between high and low load conditions is actually greater than that which could be facilitated by an improved APH operating without air bypass under similar circumstances.

[0071] Further embodiments are described with reference to secondary airflow information from Tables 1 and 2, which are modeled for a 3-sector APH with primary airflow, but also apply to a 2-sector APH with only secondary air, or a 2-sector APH used with an oil combustion boiler that preheats all combustion air (primary air is not needed as there is no coal mill). These tables contain embedded information regarding compensation calculations for low-load penalties, high-load gains, and recovery. Low load penalty = specific heat of air * Secondary airflow at low load *(Mixed secondary air temperature with bypass - secondary air temperature without bypass) Secondary air flow rate at low load = SA LO Mixed secondary air temperature with bypass (improved or not improved by using a heating element) = T 混合 Secondary air temperature without improved elements = T バイパスなし . Specific heat of air = cp AIR Low load penalty = cp 空気 * SA 低 * (T 混合 -T バイパスなし ) High load gain = specific heat of air * Secondary airflow under high load * (Leave secondary air temperature without improved elements - Leave secondary air temperature with improved elements). Secondary air flow rate under high load = SA 高 Secondary air temperature without improved elements = T 標準 Secondary air temperature with improved elements = T 改善 Specific heat of air = CD 空気 High load gain = CD 空気 * SA 高 * (T 改善 -T 標準 ) Recovery coefficient = Low load penalty / High load gain Recovery coefficient = CD 空気 * SA 低 * (T 混合 -T バイパスなし ) / CD 空気 * SA 高 * (T 改善 -T 標準 ) Recovery coefficient = SA 低 / SA 高 * (T混合 -T バイパスなし ) / (T 改善 -T 標準 )

[0072] In summary, a re-examination of compensation calculations demonstrates that upgrading heat transfer elements to more efficient profiles can reduce the time required to recover the SA bypass penalty compared to no element upgrade. With a 10% bypass, there is a penalty of 9.34 MMBtu / hr for the time the bypass is open. When the bypass is closed, that energy is not recovered and is lost permanently. With the applicant's TF4 elements in low-temperature end or CE substitution, not only is the penalty reduced to 9.11 MMBtu / hr, but when the bypass is closed, 1.77 MMBtu / hr is recovered. Therefore, to recover the loss, it is necessary to operate under a full-load no-bypass condition for 5.14 × low-load 10% bypass condition. With both CE substitution of TF4 and high-temperature end or HE substitution of the applicant's DN8 elements, the penalty is further reduced to 8.41 MMBtu / hr. When the SA bypass is closed under high load, 4.73 MMBtu / hr is recovered. Therefore, to recover the losses, it is necessary to operate under full load no-bypass conditions compared to 1.78 × low load 10% bypass conditions. The nominal impact of the DN8 / TF4 upgrade on flue gas outlet temperature is negligible.

[0073] In a typical scenario, a boiler can operate at low load and design or high load simultaneously with the normal demand fluctuations of the power grid it supplies. That is, demand typically peaks in the afternoon and evening and declines at other times. In this embodiment of the present invention, compared to an APH with an SA bypass without the improved elements, under such fluctuations, it is achievable to recover the low-load penalty if the boiler is operated for a duration equal to a recovery factor obtained by multiplying the duration of low-load operation by the duration of high-load operation. For example, the two scenarios shown in Table 3 below illustrate the high-load and low-load APH operating times used to calculate the gains and penalties of this embodiment of the present invention. As shown in the 50 / 50 low-load and high-load operations, the present invention improves the penalty by 21% and 61% with different levels of element upgrades (T4F only or DN8+TF4, respectively). In the 25 / 75 low-load and high-load operations, the present invention improves the penalty by 59% and 162% with different levels of element upgrades (T4F only or DN8+TF4, respectively).

[0074] [Table 1]

[0075] Predictive performance of APH1 under high load or design load

[0076] [Table 2]

[0077] APH prediction performance under low load

[0078] [Table 3]

[0079] [Table 4]

[0080] The following sections listed as items represent further examples in addition to the other examples described herein.

[0081] Item 1 - Air preheater (APH) temperature control system for boiler sector 2 APH100, the boiler sector 2 APH100 ​​comprising flue gas inlet duct D1, flue gas outlet duct D2, combustion air inlet duct D50, and combustion air outlet duct D100, wherein the combustion air outlet duct D100 is in fluid communication with primary air ducts D40 and secondary air ducts D60 downstream of the combustion air outlet duct D100 to supply a first smaller amount of combustion air or primary air V20 to the boiler via at least one grinding mill during use, and a second larger amount of combustion air or secondary air V30 directly to the boiler, and a high temperature end and combustion air inlet duct D1 adjacent to the gas inlet duct D1 APH100 ​​comprises a heat exchange rotor 20 having a low-temperature end adjacent to 50, and further comprises at least a first APH or combustion / secondary air bypass duct D70 which is metered to a combustion air inlet duct D50 and a secondary air duct D60, which is adapted to extract a portion of the combustion air V300 from the air inlet duct D50 upstream of APH100 ​​as a secondary air bypass V40 and reintroduce it downstream to the secondary air duct D60, and a flow control device DV10 for controlling the volumetric flow rate of the secondary air bypass V40 and adjusting the primary airflow V20, wherein the primary airflow V20 is adjusted, on its own or in conjunction with other adjustment means, to bring the primary air outlet temperature T10 within a safe range T10 MIN ~T10 MAX An air preheater (APH) temperature control system comprising a flow control device DV10 that is capable of operating to maintain the flue gas outlet temperature T2 at or above a desired minimum T9 with respect to the injectable flue gas volume flow rate V1 coming out of APH100, while maintaining it within the limits.

[0082] Item 2 - Air preheater (APH) temperature control system for the third sector APH1 of a boiler, wherein the second sector APH1 of the boiler comprises a flue gas inlet duct D1, a flue gas outlet duct D2, primary air inlet ducts D3 and D4 configured to supply a first amount of primary air V2 to the boiler via at least one grinding mill, secondary air inlet ducts D5 and D6 configured to supply a second amount of secondary air V3 directly to the boiler, and having a high-temperature end adjacent to the gas inlet duct D1 and a low-temperature end adjacent to the combustion air inlet duct D5. The APH1 comprises a heat exchange rotor 2, and further comprises at least a first APH or secondary air bypass duct D7 that is in metering communication with an air inlet duct D5 and an air outlet duct D6, which is adapted to extract a portion of the secondary air V3 from the air inlet duct D5 upstream of the APH1 as a secondary air bypass V4 and reintroduce it downstream to the air outlet duct D6, and comprises a flow control device DV1 for measuring or controlling the volumetric flow rate of the secondary air bypass V4, and a device for adjusting the primary airflow V2, which, when in use, sets the mill outlet temperature T10 within a safe range T10 MIN ~T10 MAX An air preheater (APH) temperature control system that can operate to maintain the flue gas outlet temperature T2 at or above a desired minimum T9 relative to the inject flue gas volume flow rate V1 exiting from APH1, while maintaining it within a certain range.

[0083] Item 3 - The APH temperature control system according to Item 2, further comprising: a second APH or primary air bypass duct D8 that is metered and in communication with a primary air inlet duct D3 and a primary air outlet duct D4, the second APH or primary air bypass duct D8 being adapted to extract a portion of primary air V2 as primary air bypass V5 from the primary air inlet duct D3 upstream of APH1 and reintroduce it to the primary air outlet duct D4 downstream; and a second flow control device DV2 for metering or controlling the volumetric flow rate of primary air bypass V5.

[0084] Item 4 - The flow control device DV1 or DV2 is an electrically and / or pneumatically controlled damper valve, as described in Item 2 or Item 3 of the APH temperature control system.

[0085] Item 5 - The APH temperature control system as described in Item 1 or 2, wherein the secondary air bypass V4 or V40 under low load conditions is within the range of 5-15% of the secondary air V3 supplied to APH1.

[0086] Item 6 - The primary air bypass V5 under low load conditions is within the range of 15-20% of the primary air V2 supplied to APH1, as described in Item 3 of the APH temperature control system.

[0087] Item 7 - Under low-load conditions, the secondary air bypass V4 is approximately 10%, and the primary air bypass is approximately 20%, as described in Item 5 or Item 6 of the APH temperature control system.

[0088] Item 8-T9 is an APH temperature control system as described in any one of the above items, which facilitates the efficient operation of pollution control equipment located downstream of the APH to receive the flue gas flow V1 exiting the flue gas outlet duct D2.

[0089] Item 9 - Pollution control equipment is a water evaporation system, as described in Item 8, an APH temperature control system.

[0090] Item 10 - The water evaporation system is either a spray dry absorber (SDA) or a circulating dry scrubber (CDS) positioned to receive at least a first portion of the flue gas flow V1 immediately downstream of the APH1, thereby increasing wastewater evaporation from there and reducing wastewater treatment costs associated with the inefficient operation of the evaporation system, as described in Item 9 of the APH temperature control system.

[0091] Item 11 - An APH temperature control system according to Item 9 or 10, in which, under low-load conditions, a flue gas volumetric flow rate V1 at a temperature T9 or higher is sufficient to provide at least a first portion for evaporating all or nearly all of the wastewater supplied to the water evaporation system.

[0092] Item 12 - An APH temperature control system as described in any one of items 9-11, in which, under low-load conditions, the flue gas volumetric flow rate V1 at a temperature T9 or higher is sufficient to evaporate all or nearly all of the wastewater supplied to the evaporation system, in conjunction with the rise in flue gas temperature facilitated by secondary air preheating provided by a steam coil between the secondary air fan and the APH.

[0093] Item 13 - Secondary air bypass is a pressure drop assist type APH temperature control system as described in any one of the above items.

[0094] Item 14 - Primary air bypass is a pressure drop-assisted APH temperature control system as described in any one of items 2-13.

[0095] Item 15 - A method for controlling the temperature of the gas exiting the APH using the APH temperature control system described in any one of the above items.

[0096] Item 16 - A method for modifying a boiler APH installation with an APH temperature control system as described in any one of Items 3-14, wherein, during low-load operation, the secondary air bypass valve DV1 is opened and the primary air bypass valve DV2 is fully opened, thereby setting the primary air outlet temperature T4 to the mill outlet temperature T10 to a desired T10. MIN A method for raising a certain amount by a predetermined amount sufficient to raise it to a nearby or higher level.

[0097] Item 17 - Modification method as described in Item 16, wherein the specified amount is approximately 10°F, and the secondary air bypass duct is made to an appropriate size when working in conjunction with the existing primary air bypass duct DV7 which is fully open, in order to achieve that temperature rise when DV1 is fully open.

[0098] Item 18 - Modification method as described in Item 18, during operation, the primary air bypass duct D8 can bypass approximately 10-20% of the primary air flowing into APH1, and the secondary air bypass duct D7 can bypass approximately 0-10% of the secondary air flowing into APH1.

[0099] Item 19 - The overall operating efficiency of the installed APH1 is improved without modifying the rotor 2 by modifying the basket to include a more efficient heat transfer element that compensates for the reduced efficiency of the APH1 when the secondary air bypass is operational under low load conditions, as described in any one of items 17-19.

[0100] Item 20 - A method for improving a pre-installed combustion or secondary two-sector APH100, wherein an existing rotor 20 is modified with a basket having more efficient heat transfer elements and an APH temperature control system is installed, the APH100 ​​comprising a flue gas inlet duct D1, a flue gas outlet duct D2, a combustion air inlet duct D50, a combustion air outlet duct D100 for directly supplying combustion air or secondary air V30 to the boiler, and a heat exchange rotor 20 having a high-temperature end adjacent to the gas inlet duct D1 and a low-temperature end adjacent to the combustion air inlet duct D50, and the APH 100 is a first APH or combustion / secondary air bypass duct D70 that is metered and communicates with a combustion air outlet duct D100, and is adapted to extract a portion of the combustion air V300 from the air inlet duct D50 upstream of APH100 ​​as combustion or secondary air bypass V40 during use and reintroduce it to the downstream combustion or secondary air duct D60, and a flow control device DV that meters or controls the volumetric flow rate of the secondary air bypass V40 and adjusts the primary airflow V20. A method further comprising: a flow control device DV10, which, during use, is operable to maintain the flue gas outlet temperature T2 at or above a desired minimum T9 with respect to the incident flue gas volume flow rate V1 leaving the APH100, while increasing the mean low-temperature end temperature or ACET compared to a pre-installed APH, and is also operable to extract substantially more heat from the flue gas through similar conditions than a pre-installed APH cycle when cycling between high-load and low-load conditions during use.

[0101] Item 22 - A more efficient element is a combination that has an increase in heat transfer efficiency by at least weight under design load conditions, such as by using at least one of the DN8 (trademark) and TF4 (trademark) elements or a combination thereof, as described in Item 19 or Item 20.

[0102] Item 23 - Temperature control system for a steam generation system, comprising: a pulverizer (400) for pulverizing fuel, having a pulverizer inlet for receiving fuel and a pulverizer outlet for discharging the pulverized fuel therefrom; a boiler (500) communicating with the pulverizer outlet, the boiler being configured to burn fuel; and an air preheater (APH) having a rotor (20) mounted to rotate on a spindle, the air preheater having a plurality of heat transfer elements disposed therein, the heat transfer elements thereby flow The air preheater is configured to transfer heat to the gas and defines a high-temperature end (20H) and a low-temperature end (20C). The air preheater comprises an air preheater, a flue gas inlet duct (D1) in fluid communication with the high-temperature end (20H), and a flue gas outlet duct (D2) in fluid communication with the high-temperature end (20H), wherein the flue gas is configured to flow into the high-temperature end (20H) through the flue gas inlet duct (D1) and discharged from the high-temperature end (20H) to the flue gas outlet duct (D2), and a combustion air inlet duct in fluid communication with the low-temperature end (20C). The combustion air outlet duct (D100) is in fluid communication with the low temperature end (20C), and the air preheater (APH) is configured such that combustion air flows into the low temperature end (20C) through the combustion air inlet duct (D50) and is discharged from the combustion air outlet duct (D100) through the low temperature end (20C), and the combustion air outlet duct (D100) has a primary air branch section (D40) in fluid communication with the crusher inlet and a secondary air branch section (D60) in direct fluid communication with the boiler, and the combustion air inlet duct (D50) and combustion A bypass system that is in fluid communication with an air outlet duct (D100), and which has a flow regulator (DV10) therein for variably controlling the flow of combustion air through the bypass system from a combustion air inlet duct (D50) to a combustion air outlet duct (D100); a control system comprising a first sensor system configured to measure the flue gas outlet temperature (T2) coming out of a high-temperature end (20H); and a second sensor system configured to measure the air temperature (T10) downstream of a primary air branch (D40), wherein the control system isA temperature control system comprising: a control system having a control unit that controls a flow regulator (DV10) based on temperature inputs from a first sensor system and a second sensor system, thereby maintaining the flue gas outlet temperature (T2) above a predetermined minimum temperature (T9) and maintaining the air temperature (T10) downstream of the primary air branch (D40) below a predetermined maximum temperature.

[0103] As will be apparent to those skilled in the art, various modifications, adaptations, and variations of the aforementioned specific disclosures can be made without departing from the scope of the claimed invention. Various features, elements, and embodiments of the invention described herein may be combined in ways different from the specific examples described or claimed herein without departing from the scope of the invention. In other words, any element or feature may be combined with any other element or feature in different embodiments, unless there is an obvious or inherent incompatibility between the two, or unless such incompatibility is specifically excluded.

[0104] References in this specification such as "one embodiment" or "one embodiment" indicate that the embodiments described may include certain aspects, features, structures, or characteristics, but not all embodiments necessarily include such aspects, features, structures, or characteristics. Furthermore, such phrases may refer to the same embodiments, though not necessarily, as referred to in other parts of this specification. Moreover, where certain aspects, features, structures, or characteristics are described in relation to an embodiment, it is within the knowledge of those skilled in the art that such aspects, features, structures, or characteristics affect or connect to other embodiments, whether or not they are explicitly described.

[0105] Unless the context clearly specifies otherwise, the singular forms “a,” “an,” and “the” include multiple referents. Therefore, for example, a reference to “plant” includes multiple such plants. It should be further noted that claims may be drafted to exclude optional elements. Thus, this statement is intended to serve as an antecedent for the use of exclusive terms such as “solely” and “only” in relation to the enumeration of elements of the claims or the use of “negative” limitations. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that the item, condition, or step mentioned is an optional (non-essential) feature of the invention.

[0106] The term "and / or" means any one of the items to which this term is associated, any combination of items, or all of the items. The phrase "one or more" will be readily understood by those skilled in the art, especially when read in the context of its usage.

[0107] Each numerical or measured value in this specification is modified by the term “approximately.” The term “approximately” can refer to a variation of ±1%, ±5%, ±10%, ±20%, or ±25% of a given value. For example, “approximately 50” percent may have a variation of 45–55 percent in some embodiments. With respect to integer ranges, the term “approximately” can include one or two integers greater than and / or less than the integers enumerated at each end of the range. Unless otherwise indicated herein, the term “approximately” is intended to include values ​​and ranges close to the enumerated ranges that are equivalent in terms of the functionality of the composition or embodiment.

[0108] As will be understood by those skilled in the art, all numbers, including those representing the quantity of reagents or components, their properties such as molecular weight, and reaction conditions, are approximations and, in all cases, are optionally modified by the term "about." These values ​​may vary depending on the desired properties that those skilled in the art seek to obtain using the teachings of this specification. It will also be understood that such values ​​inherently include variability that inevitably arises from the standard deviation found in each of their test measurements.

[0109] As will be understood by those skilled in the art, for all purposes, and especially with regard to providing written explanations, all ranges enumerated herein also include all possible subranges and combinations thereof, as well as the individual values ​​constituting the ranges, in particular integer values. Enumerated ranges (e.g., weight percentages or carbon groups) include each specific value, integer, decimal, or identity within that range. Any enumerated range can be readily recognized as one that fully explains and enables that the same range can be decomposed into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can readily be decomposed into a lower third, a middle third, an upper third, and so on.

[0110] Furthermore, as will be understood by those skilled in the art, all terms such as “up to,” “at least,” “greater than,” “less than,” “more than,” and “or more” include the stated number, and such terms refer to a range that may later be broken down into subranges as discussed above. Similarly, all proportions listed herein also include all subproportions that fall within a broader proportion. Therefore, the specific values ​​stated for radicals, substituents, and ranges are for illustrative purposes only. They do not exclude other defined values ​​or other values ​​within the defined range for radicals and substituents.

[0111] Those skilled in the art will readily recognize that, when members are grouped together in a general manner, as in the Markush groups, the present invention encompasses not only the entire enumerated group as a whole, but also each member of the individual groups and all possible subgroups of the principal groups. Furthermore, for all purposes, the present invention encompasses not only the principal groups but also principal groups in which one or more of the group members are absent. Accordingly, the present invention anticipates the explicit exclusion of one or more members of the enumerated groups. Accordingly, the proviso may apply to any of the disclosed categories or embodiments, thereby excluding one or more of the enumerated elements, species, or embodiments from such categories or embodiments, for example, as used in an explicit negative limitation.

Claims

1. A temperature control system for an air preheater, wherein the temperature control system is (a) A two-sector air preheater (100), A flue gas inlet duct (D1) is configured to supply flue gas from the boiler to the two-sector air preheater (100), A flue gas outlet duct (D2) configured to discharge flue gas from the two-sector air preheater (100), A combustion air inlet duct (D50) configured to transport air to the two-sector air preheater (100), A combustion air outlet duct (D100) configured to discharge air from the two-sector air preheater (100), wherein the combustion air outlet duct (D100) is in fluid communication with a primary air duct (D40) and a secondary air duct (D60), the secondary air duct (D60) is located downstream of the primary air duct (D40), the primary air duct (D40) supplies a first amount of primary air (V20) to the boiler via at least one grinding mill, and the secondary air duct (D60) supplies a second amount of secondary air (V30) directly to the boiler, configured such that the amount of secondary air (V30) is greater than the amount of primary air (V20), and A heat exchange rotor (20) having a high-temperature end adjacent to the flue gas inlet duct (D1) and a low-temperature end adjacent to the combustion air inlet duct (D50), An air preheater bypass duct (D70) communicating with the combustion air inlet duct (D50) and the secondary air duct (D60), wherein the air preheater bypass duct (D70) is configured to draw a portion of the combustion air (V300) as a secondary air bypass (V40) from the air inlet duct (D50) upstream of the two-sector air preheater (100) and reintroduce it to the downstream secondary air duct (D60), A flow control device (DV10) for controlling the volumetric flow rate of the secondary air bypass (V40) and adjusting the primary airflow (V20), wherein the primary airflow (V20), alone or in conjunction with other adjusting means, maintains the flue gas outlet temperature (T2) above a first predetermined minimum temperature (T9) for the flue gas flow (V1) exiting the two-sector air preheater (100), thereby setting the primary air outlet temperature (T10) to the minimum temperature (T10) necessary to promote coal drying in the grinding mill. ＭＩＮ ) and the grinding mill (T10 ＭＡＸ A flow control device (DV10) configured to maintain the temperature within a safe range below the maximum temperature that would cause the coal inside to spontaneously ignite, and a two-sector air preheater (100), or (b) A three-sector air preheater (1), A flue gas inlet duct (D1) configured to supply flue gas from the boiler to the three-sector air preheater (1), A flue gas outlet duct (D2) configured to discharge flue gas from the three-sector air preheater (1), A primary air inlet duct (D3) is configured to supply primary air (V2) to the three-sector air preheater (1), A primary air outlet duct (D4) configured to supply a first amount of primary air (V2) to the boiler via at least one grinding mill, A secondary air inlet duct (D5) configured to supply secondary air (V3) to the three-sector air preheater (1), A secondary air outlet duct (D6) is configured to supply a second amount of secondary air (V3) directly to the boiler, A heat exchange rotor (2) having a high-temperature end adjacent to the gas inlet duct (D1) and a low-temperature end adjacent to the secondary air inlet duct (D5), A secondary air bypass duct (D7) communicating with the secondary air inlet duct (D5) and the secondary air outlet duct (D6), wherein the secondary air bypass duct D7 is configured to extract a portion of the secondary air (V3) as a secondary air bypass (V4) from the secondary air inlet duct (D5) upstream of the three-sector air preheater (1) and reintroduce it to the downstream secondary air outlet duct (D6), and is a first secondary air bypass duct (D7), A flow control device (DV1) for controlling the volumetric flow rate of the secondary air bypass (V4) and adjusting the primary airflow (V2), wherein the primary airflow (V2) maintains the flue gas outlet temperature (T2) above a second minimum temperature (T9) for the flue gas flow (V1) exiting the three-sector air preheater (1), thereby setting the primary air outlet temperature (T10) to the minimum temperature (T10) necessary to promote coal drying in the grinding mill. ＭＩＮ ) and the grinding mill (T10 ＭＡＸ A three-sector air preheater (1) comprises a flow control device (DV1) configured to maintain the temperature within a safe range below the maximum temperature that would cause the coal inside to spontaneously ignite, A temperature control system equipped with the following features.

2. The air preheater is the three-sector air preheater (1), and the control system is A primary air bypass duct (D8) communicating with the primary air inlet duct (D3) and the primary air outlet duct (D4), wherein the primary air bypass duct (D8) is configured to extract a portion of the primary air (V2) as a primary air bypass (V5) from the primary air inlet duct (D3) upstream of the three-sector air preheater (1) and reintroduce it to the primary air outlet duct (D4) downstream. The temperature control system according to claim 1, further comprising a second flow control device (DV2) for controlling the volumetric flow rate of the primary air bypass (V5).

3. (a) When the air preheater is the two-sector air preheater (100), the amount of air passing through the secondary air bypass (V40) under low load conditions of less than 50% of the total load is 5 to 15% of the amount of air supplied to the two-sector air preheater (100), (b) The temperature control system according to claim 1, wherein the air preheater is the three-sector air preheater (1), and the amount of air passing through the secondary air bypass (V4) under low-load conditions of less than 50% of the total load is 5 to 15% of the amount of secondary air (V3) supplied to the three-sector air preheater (1).

4. The temperature control system according to claim 3, wherein the air preheater is the three-sector air preheater (1), and the amount of air passing through the primary air bypass (V5) under low-load conditions of less than 50% of the total load is 15 to 20% of the amount of primary air (V2) supplied to the three-sector air preheater (1).

5. The temperature control system according to claim 2, wherein the air preheater is the three-sector air preheater (1), and under low-load conditions of less than 50% of the total load, the amount of air passing through the secondary air bypass (V4) is 10% of the amount of secondary air (V3) supplied to the three-sector air preheater (1), and the amount of primary air bypass (V5) is 20% of the amount of primary air (V2) supplied to the three-sector air preheater (1).

6. (a) When the air preheater is the two-sector air preheater (100), the first predetermined minimum temperature (T9) is set so that the pollution control equipment located downstream of the two-sector air preheater (100) can receive the flue gas flow (V1) coming out of the flue gas outlet duct (D2), (b) The temperature control system according to claim 1, wherein, if the air preheater is the three-sector air preheater (1), the second predetermined minimum temperature (T9) is set so that a pollution control device located downstream of the three-sector air preheater (1) can receive the flue gas flow (V1) coming out of the flue gas outlet duct (D2).

7. The aforementioned pollution control equipment is a spray dry absorber (SDA), a circulating dry scrubber (CDS), or a wet flue gas desulfurization system (FGD). Furthermore, the pollution control device is positioned to receive at least a first portion of the flue gas flow (V1) immediately downstream of the three-sector air preheater (1) or the two-sector air preheater (100), and the amount of wastewater evaporation is increased compared to the operating conditions when the pollution control device is not immediately downstream of the three-sector air preheater (1) or the two-sector air preheater (100), according to claim 6.

8. The temperature control system according to claim 7, wherein when the temperature of the flue gas flow (V1) is above the predetermined minimum temperature (T9) under low-load conditions where the temperature is 50% of the total load, at least the first portion of the flue gas flow (V1) evaporates the wastewater supplied to the pollution control system.

9. The temperature control system according to claim 2, wherein at least one of the secondary air bypass (V4) and the primary air bypass (V5) is a pressure drop support bypass.

10. The aforementioned grinding mill (400) includes a grinding mill inlet for receiving fuel and a grinding mill outlet for discharging the ground fuel from the grinding mill. The boiler (500) is in communication with the outlet of the crusher, and the boiler is configured to burn the fuel. The temperature control system according to claim 1, wherein each of the two-sector air preheater (100) and the three-sector air preheater (1) comprises a rotor (20) mounted to rotate on a spindle and a plurality of heat transfer elements arranged on the rotor, wherein the heat transfer elements are configured to transfer heat to the gas flowing through them, and each of the two-sector air preheater (100) and the three-sector air preheater (1) defines a high-temperature end (20H) and a low-temperature end (20C).

11. A method for controlling the temperature of the gas exiting an air preheater using the temperature control system described in claim 1.

12. A method for modifying an existing air preheater system to an air preheater system equipped with the temperature control system described in Claim 1, wherein the air preheater is the three-sector air preheater (1), and the method is By utilizing the existing primary air bypass duct (D8) that is restricted by the existing damper valve (DV2), This includes installing a secondary air bypass duct (D7) of appropriate size that can be closed with a damper valve (DV1), During low-load operation, which is less than 50% of the full load, when the damper valve (DV1) opens and the damper valve (DV2) is fully open, the primary air outlet temperature (T4) will rise to at least the minimum primary air outlet temperature (T10). ＭＩＮ A method of raising by a predetermined amount sufficient to raise to ).

13. The modification method according to claim 12, wherein 10 to 20% of the amount of primary air flowing into the three-sector air preheater (1) is bypassed, and the secondary air bypass duct (D7) can bypass 0 to 10% of the amount of secondary air flowing into the three-sector air preheater (1).

14. The temperature control system according to claim 1, wherein the combustion air outlet duct (D100) branches into the primary air duct (D40) and the secondary air duct (D60).

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