Mercury removal method

Abstract

To provide a mercury removal method which can simply and efficiently remove mercury from exhaust gas without accumulating an adsorbent adsorbing mercury in an electric dust collector and lowering dust collection capability.SOLUTION: A mercury removal method includes the steps of: charging an adsorbent of mercury to a gas route 5 for feeding exhaust gas G to an electric dust collector 4 from a preheater 2 constituting a cement firing facility 1; recovering the adsorbent from the electric dust collector 4; and measuring a voltage and / or current between a dust collecting electrode and a discharge electrode of the electric dust collector 4, wherein when the measurement value of the voltage and / or current exceeds a numeric value range and fluctuates, the method stops charging of the adsorbent to the gas route 5.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a mercury removal method for efficiently removing mercury from exhaust gas generated in a cement burning facility. [Background technology]

[0002] Cement, one of the materials used in the construction of buildings, has established a firm position as an indispensable material in our daily lives. Cement has a long history, and its manufacturing process is known to be roughly divided into three steps: the raw material process, the firing process, and the finishing process. In the first process, the raw material process, a number of raw materials such as limestone, clay, silica stone, and iron raw materials are mixed in a predetermined ratio and then crushed in a raw material mill. The mixed raw materials crushed in the raw material mill are then air-transported in a cyclone and separated from the gas used for transportation. The separated mixed raw materials are sent to a blending silo where they are homogeneously blended and then stored in a raw material storage silo.

[0003] In the subsequent firing process, the raw materials crushed and mixed in the raw materials process are fired in a rotary kiln. In order to increase the firing efficiency, the raw materials are sent to a preheater with multiple cyclones before being fed into the rotary kiln, where they are preheated. The preheated raw materials are then fired at high temperatures while moving through the rotary kiln. The raw materials fired in the rotary kiln in this way become lumps, which are rapidly cooled by air at the outlet side of the rotary kiln. These quenched lumps are clinker, the intermediate product of cement.

[0004] Then, in the final finishing process, the clinker with gypsum added is sent to a preliminary crusher where it is pre-crushed, and then it is put into a finishing mill with steel balls where it is finely ground to an average particle size of about 10 to 20 μm. The resulting fine powder is either used as is as Portland cement, or mixed with fly ash and slag powder in a mixer to be used as blended cement.

[0005] Of the three steps that make up the cement manufacturing process described above, the air used to cool the clinker in the calcination process passes through the inside of the rotary kiln or preheater, is consumed in combustion, and then flows to the electrostatic precipitator, where the dust contained therein is collected and then released into the atmosphere. In the present invention, the gas that has left the rotary kiln or preheater is referred to as "exhaust gas," even if it is gas that has not yet been released into the atmosphere and is used for preheating, etc. It is known that raw materials for cement contain small amounts of mercury originating from limestone, etc. This mercury vaporizes in the preheater or rotary kiln, where the temperature is raised to above its boiling point, and flows into the electrostatic precipitator together with the exhaust gas.

[0006] The mercury vaporized in the preheater or rotary kiln is not released directly into the atmosphere with the exhaust gas, but is adsorbed to dust present in the exhaust gas as it travels to the electrostatic precipitator and fixed onto the dust. In this way, the mercury is captured by the electrostatic precipitator together with the dust. Due to this mechanism, mercury in cement raw materials has never been released into the atmosphere in concentrations exceeding the regulatory limit.

[0007] However, with the recent rise in interest in protecting the global environment, there is a possibility that mercury emission management will become stricter in the future, and cement manufacturers need to take measures immediately. In addition, depending on the quality of the cement raw materials used and changes in the mix (for example, increasing the loading of recycled resources such as soot containing mercury), it is possible that cement may be produced using raw materials containing a high mercury content, even if it is a very small amount, and there was concern that if such raw materials were used, the surplus mercury that was not adsorbed by the dust could be inadvertently released into the atmosphere. For this reason, there was an urgent need to establish technology that could efficiently remove mercury even if the amount of mercury in exhaust gas increased for some reason. In this context, the following four inventions, for example, have been recently disclosed as prior applications relating to mercury removal technology.

[0008] Patent Document 1, entitled "Cement Kiln Exhaust Gas Treatment System and Its Operating Method," discloses an invention relating to technology for treating mercury and organic pollutants in exhaust gas from a kiln. The treatment system, which is the invention disclosed in Patent Document 1, will be described using the same symbols as in Figure 1 in the document, and is characterized by comprising a gas adsorbent addition device 7 that adds gas adsorbent A to exhaust gas G2 discharged from a preheater 3 of a cement burning facility, an electric dust collector 8 that collects dust D contained in the exhaust gas G2 from the preheater 3, and a mercury recovery device 9 that recovers mercury contained in the collected dust D. According to the invention disclosed in Patent Document 1 having the above configuration, the concentrations of mercury and organic pollutants discharged from a cement burning facility are measured, and depending on the concentrations, the collected dust D is reintroduced into the exhaust gas G2 for mercury adsorption, or new gas adsorbent A is added, thereby comprehensively and efficiently reducing mercury and organic pollutants.

[0009] Patent Document 2, entitled “Method and system for removing mercury from exhaust gas from a cement kiln,” discloses an invention relating to a technology for removing mercury from exhaust gas emitted from a kiln. The mercury removal system, which is the invention disclosed in Patent Document 2, will be described using the same symbols as in Figure 1 in the document. It is characterized by comprising a dust collector 8 that collects and treats exhaust gas G1, a measuring device 9 that measures the unburned carbon content of collected dust D from the dust collector 8, and an addition amount adjustment device 12 that adjusts the amount of mercury adsorbent A to be added to the cement burning equipment including the kiln 2 based on the measurement value of the measuring device 9. According to the invention disclosed in Patent Document 2 having the above configuration, the amount of mercury adsorbent A to be added to the cement burning equipment including the kiln 2 can be adjusted based on the measured unburned carbon content of the collected dust D, thereby making it possible to appropriately maintain the amount of mercury adsorbent A in the cement burning equipment and to efficiently remove mercury from the exhaust gas G1 discharged from the kiln 2 at low cost.

[0010] Patent Document 3, entitled "Exhaust Gas Treatment System and Exhaust Gas Treatment Method," discloses an invention relating to an exhaust gas treatment system that can accurately determine the optimal supply amount of activated carbon for the mercury concentration. The mercury removal system, which is the invention disclosed in Patent Document 3, is described using the same symbols as in Figure 1 in the document, and is characterized by comprising a dust collector 2 that removes harmful substances contained in exhaust gas, a mercury meter 3 that is arranged upstream of the dust collector 2 in the flow direction of the exhaust gas and measures the concentration of mercury contained in the exhaust gas introduced into the dust collector 2, an activated carbon supplying device 4 that supplies activated carbon to the exhaust gas introduced into the dust collector 2, and a control device 5 that controls the amount of activated carbon supplied from the activated carbon supplying device 4.

[0011] According to the invention disclosed in Patent Document 3 having the above configuration, the control device 5 calculates the average value of the mercury concentration measured by the mercury meter 3 using a moving average to determine the amount of activated carbon to be supplied, so that only the amount of activated carbon required for the mercury concentration can be supplied into the exhaust gas. As a result, activated carbon that does not contribute to mercury adsorption is not supplied, which can lead to a reduction in the cost of exhaust gas treatment.

[0012] Patent Document 4, titled “Exhaust Gas Treatment Device and Exhaust Gas Treatment Method,” discloses an invention related to technology that can suppress the amount of mercury in exhaust gas emitted from clinker manufacturing facilities to a stable, low level. The invention disclosed in Patent Document 4 will be described using the same reference numerals as in FIG. 3 of the document. The invention is characterized by comprising a crusher 12 that crushes cement raw materials M1, a cement kiln 10 that produces cement clinker M5 by burning the cement raw materials M4 crushed by the crusher 12 and supplies combustion gas G1 generated inside by the burning to the crusher 12, a dust collector 16 that collects powder and granular material contained in the crusher exhaust gas G2 discharged from the crusher 12, a raw material measuring device 14 that measures the amount of the cement raw materials M1 introduced into the crusher 12, a feeder 26 that treats the dust collector exhaust gas G3b with an adsorbent Ad that adsorbs mercury contained in the dust collector exhaust gas G3 discharged from the dust collector 16, and a controller 28 that controls the amount of contact of the dust collector exhaust gas G3b with the adsorbent Ad based on the measurement result of the raw material measuring device 14.

[0013] According to the invention disclosed in Patent Document 4 having the above configuration, the controller 28 appropriately controls the contact amount of the dust collector exhaust gas G3b with the adsorbent Ad based on the measurement results of the raw material measuring instrument 14, so that the mercury concentration in the dust collector exhaust gas G3b can be reduced even if the contact amount is not always maintained at a high level. As a result, the amount of the adsorbent Ad used is appropriately controlled, so that it is possible to suppress the running costs of mercury removal while suppressing the amount of mercury emissions to a stable and low level. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2013-112579 A [Patent Document 2] JP 2018-135248 A [Patent Document 3] JP 2020-146643 A [Patent Document 4] JP 2019-51485 A Summary of the Invention [Problem to be solved by the invention]

[0015] According to the invention disclosed in the above-mentioned Patent Document 1, the collected dust D can be introduced into the flue gas G2 in accordance with the mercury concentration in the flue gas G2, and the mercury in the flue gas G2 can be efficiently removed. However, if the flue gas G2 contains an excessive amount of mercury, there is a risk that the dust D will be introduced excessively into the flue gas G2, and if the dust D is introduced excessively, the dust collection capacity of the electrostatic precipitator 8 will be exceeded, which may require the device to be stopped for maintenance. Furthermore, if the electrostatic precipitator 8 is stopped, there is also a problem that it will be necessary to stop the operation of the cement burning facility itself and reconsider the production plan itself. Furthermore, the invention disclosed in Patent Document 1 requires a measuring device for measuring the mercury concentration and piping for each circulation route in order to select one of multiple circulation routes for dust D based on the mercury concentration. Therefore, there is a problem that application of this invention to conventionally used facilities is not easy in terms of cost and construction time.

[0016] On the other hand, the invention disclosed in Patent Document 2 can use collected dust D, which has a high content of unburned carbon, as a mercury adsorbent, and therefore is economical in that it can reduce the cement production cost compared to using only the expensive mercury adsorbent A. However, the invention disclosed in Patent Document 2 has the object of reducing the cement production cost by replacing collected dust D with mercury adsorbent A as described above, and has the problem that it does not anticipate adjusting the supply amount of mercury adsorbent A so as not to reduce the dust collection capacity of the electric dust collector 8 due to accumulation of collected dust D and mercury adsorbent A in the electric dust collector 8.

[0017] Furthermore, the invention disclosed in Patent Document 3 makes it possible to accurately supply an optimal amount of activated carbon for the mercury concentration in the exhaust gas even if the concentration fluctuates, and does not unnecessarily increase the cost of exhaust gas treatment. However, there is a problem in that no consideration is given to adjusting the amount of activated carbon supplied so that the dust and activated carbon contained in the exhaust gas do not accumulate excessively in the dust collector 2, thereby reducing the dust collection capacity of the dust collector 2.

[0018] In addition, the invention disclosed in Patent Document 4 is believed to reduce the running costs of mercury removal because the supply amount of the adsorbent Ad is controlled according to the input cement raw material M1. However, there is a problem in that no consideration is given to measures to be taken when dust and activated carbon are excessively accumulated in the dust collector 16, causing a decrease in the dust collection capacity of the dust collector 16.

[0019] The present invention has been made in response to such conventional circumstances, and its object is to provide a mercury removal method that can simply and efficiently remove mercury from exhaust gas without reducing the dust collection capacity of the electrostatic precipitator due to excessive accumulation of adsorbent added thereto. [Means for solving the problem]

[0020] The mercury removal method, which is a first invention for solving the above-mentioned problems, removes mercury contained in exhaust gas generated in a cement manufacturing process, and includes the steps of: introducing a mercury adsorbent into a gas pathway for sending exhaust gas from a preheater that constitutes a cement firing equipment to an electrostatic precipitator; recovering the adsorbent from the electrostatic precipitator; and measuring the voltage and / or current between the collecting electrode and discharge electrode of the electrostatic precipitator. The method is characterized in that introduction of the adsorbent into the gas pathway is stopped when the measured voltage and / or current fluctuates beyond a predetermined numerical range.

[0021] In the first invention having the above configuration, in the process of introducing the adsorbent, the adsorbent is introduced into the exhaust gas in the gas path, thereby having the effect of bringing mercury present in the exhaust gas into contact with the surface of the adsorbent and causing it to adsorb. In the process of recovering the adsorbent that has adsorbed mercury using an electrostatic precipitator, the adsorbent is passed between a collecting electrode (which acts as a positive electrode) and a discharge electrode (which acts as a negative electrode) that are applied with high voltage inside the electrostatic precipitator. This brings the adsorbent into contact with gas molecules that have become negative ions due to corona discharge, and the negatively charged adsorbent is attracted to the collecting electrode and collected.

[0022] The structure of an electric dust collector having such an effect will be described in detail with reference to FIG. FIG. 3 is a schematic diagram showing how the adsorbent is collected by the electrostatic precipitator. In the explanation of FIG. 3, reference symbols (collection electrode 4a, discharge electrode 4b, adsorbent 11) that are not used in the explanation of other figures are used. In addition, in FIG. 3, reference symbols are attached to only one adsorbent, and reference symbols are omitted for the others, in order to avoid the figure becoming complicated. In addition, even in a state in which no adsorbent is added, dust accompanying the production of clinker always flows into the electrostatic precipitator, but this is also omitted. The behavior of the dust is virtually the same as that of the adsorbent. 3, in the electric dust collector 4, a discharge electrode 4b made of a piano wire or the like is installed between a pair of parallel-arranged dust collecting electrodes 4a, 4a. The electric dust collector 4 is structured so that a DC voltage of, for example, 40 to 60 kV is applied between the dust collecting electrodes 4a, 4a and the discharge electrode 4b so that the dust collecting electrodes 4a, 4a are positively charged and the discharge electrode 4b is negatively charged.

[0023] When gas containing the adsorbent 11 is flowed inside the electrostatic precipitator 4 with a DC voltage applied between the collecting electrodes 4a, 4a and the discharge electrode 4b as shown by the wide arrow, a corona discharge occurs between the collecting electrodes 4a, 4a and the discharge electrode 4b, and countless gas molecules are negatively ionized. These ionized gas molecules move from the discharge electrode 4b with a low potential to the collecting electrodes 4a, 4a with a high potential, but these gas molecules also collide with and bond with the adsorbent 11, negatively charging the adsorbent 11. As a result, the negatively charged adsorbent 11 is attracted to the high potential collecting electrode 4a, and the adsorbent 11 that comes into contact with the collecting electrode 4a loses its negative charge, adheres to the surface of the collecting electrode 4a and is collected, and accumulates over time to cover the collecting electrode 4a. The accumulated adsorbent 11 (and other dust) is peeled off from the surface of the dust collecting electrode 4a and falls off when the dust collecting electrode 4a is vibrated by a vibrator or the like. The vibration by the vibrator or the like is usually performed periodically under the control of a timer or the like.

[0024] Next, in the step of measuring the voltage and / or current between the collecting electrode and the discharge electrode of the electric dust collector in the mercury removal method of the present invention, by measuring the voltage and / or current between the collecting electrode and the discharge electrode, it is possible to grasp the degree of accumulation and the charged state of the adsorbent and dust in the exhaust gas collected by the collecting electrode. Here, the voltage and current between the collecting electrode and the discharge electrode depend on the state between the two electrodes, and as the adsorbent and dust accumulate inside the electrostatic precipitator, the space between the collecting electrode and the discharge electrode gradually narrows. As this space narrows, the impedance of the discharge space between the collecting electrode and the discharge electrode changes, causing the voltage to decrease and the current to increase. In other words, the voltage and current are related to the degree of accumulation. Furthermore, such fluctuations in voltage and current also affect the charging of the adsorbent and dust, and if accumulation progresses to a certain extent for some reason, it becomes difficult to charge, and it is believed that the electrostatic precipitator will no longer be able to collect the adsorbent, etc. Therefore, the present invention uses the voltage and current as indicators for determining the degree of accumulation and charging state of the adsorbent and dust in the electrostatic precipitator.

[0025] In addition, if the accumulation of adsorbent and dust in the exhaust gas in the electrostatic precipitator progresses and the measured voltage and / or current values ​​fluctuate beyond a predetermined numerical range, the introduction of adsorbent into the gas path is stopped, thereby preventing further accumulation that could lead to blockage of the air passage inside the electrostatic precipitator, and preventing a decrease in dust collection performance due to poor charging. Note that the "predetermined numerical range" will vary depending on the specifications of the electrostatic precipitator and the user's method of operating the equipment, and so no specific numerical values ​​are specified; however, it must be at least less than the voltage fluctuation range when the electrostatic precipitator must be stopped due to the progression of deposition as described above, and also less than the current fluctuation range when it must be stopped.

[0026] The mercury removal method of the second invention is characterized in that in the step of introducing the adsorbent, the temperature of the exhaust gas when the adsorbent is introduced is 350°C or higher. The second invention having the above configuration has the same effect as the first invention described above. Furthermore, in the step of introducing the adsorbent, the temperature of the exhaust gas into which the adsorbent is introduced is set to 350°C or higher, particularly the temperature at which mercury vaporizes, so that the mercury in the exhaust gas is in a gaseous state and therefore comes into contact with the adsorbent more frequently. Note that the boiling point of mercury is about 360°C under normal pressure, but the reason why the exhaust gas temperature is set to 350°C or higher here is because a measurement error of about ±10°C is assumed when measuring the exhaust gas temperature in an actual facility.

[0027] The mercury removal method according to a third aspect of the present invention is the method according to the first or second aspect of the present invention, characterized in that the adsorbent is activated carbon. The third invention having the above configuration has the same effect as the first or second invention described above. Furthermore, the third invention having the above configuration has an effect that mercury present in the form of gas in the exhaust gas is brought into contact with the activated carbon and adsorbed thereon. Here, activated carbon has been used as an adsorbent for the purpose of adsorbing and removing impurities, and is also used to remove mercury from exhaust gas as described in the present invention. There are various grades of activated carbon used as an adsorbent, depending on the raw material, particle size, etc.

[0028] In general, activated carbon is a material having micropores with diameters of 10 to 200 Å. These micropores are arranged in a network shape inside the activated carbon, and the surface area of ​​the inner walls of the micropores is 500 to 2500 m. 2 / g, which is very large. Activated carbon is widely used as an adsorbent because it can adsorb various substances inside the micropores with such a large surface area. Activated carbon is produced by crushing various raw materials such as peat, wood, lignocellulosic materials, biomass, waste, tires, olive pits, peach pits, corn husks, rice husks, petroleum coke, lignite, brown coal, anthracite, bituminous coal, subbituminous coal, coconut shells, pecan shells, and walnut shells using a fine grinder, crusher, jet grinder, roll grinder, ball grinder, hammer grinder, classifier, etc.

[0029] As described above, activated carbon manufactured using various raw materials has a very large surface area due to the presence of micropores, so it is possible to increase the frequency with which mercury comes into contact with its surface even in an environment in which the frequency of contact between the adsorbent and mercury in exhaust gas is very low. As a result, gaseous mercury present in exhaust gas is efficiently adsorbed and removed by the activated carbon, and the exhaust gas can be purified to a level that is acceptable for release into the atmosphere. On the other hand, since activated carbon is mainly composed of carbon and has electrical conductivity, when activated carbon is deposited in an electrostatic precipitator, the effect of the amount of deposition on the voltage or current is greater than with other adsorbents. In other words, when activated carbon is used, even if the amount of activated carbon added is the same as with other adsorbents, the frequency of large fluctuations in voltage and current may increase. Therefore, it is more important than with other adsorbents to measure the voltage and / or current in order to stop the addition of activated carbon at the appropriate time and prevent problems with the electrostatic precipitator. Effect of the Invention

[0030] In the first invention described above, mercury in the exhaust gas is adsorbed by the adsorbent that is added, and can be collected together with the adsorbent by the electrostatic precipitator. Furthermore, the voltage and / or current between the collecting electrode and the discharge electrode of the electrostatic precipitator can be measured to determine the state of accumulation of the adsorbent and dust in the electrostatic precipitator. As a result, when the voltage and current of the adsorbent and dust accumulated on the collecting electrode reach a level that is determined to exceed the allowable amount, the addition of the adsorbent can be immediately stopped to stop further accumulation. This can suppress deterioration of dust collection performance due to poor charging, reducing the frequency of stopping the electrostatic precipitator for repairs, etc., and making it easier to continue operating the cement burning facility.

[0031] The second invention described above has the same effect as the first invention. Furthermore, in the case of the second invention, since the mercury in the exhaust gas is in a vaporized state (highly mobile state), the frequency of contact between the mercury and the adsorbent increases, making it easier for the mercury to be adsorbed by the adsorbent. As a result, the amount of mercury adsorbed by the adsorbent increases, and a high mercury removal effect can be achieved even with a small amount of the adsorbent.

[0032] The third invention described above has the same effect as the first or second invention. Furthermore, in the case of the third invention, by using activated carbon that is porous and has a large specific surface area as the adsorbent, the amount of mercury adsorbed by the adsorbent per unit mass is more likely to increase compared to other adsorbents, and the mercury removal capacity of the equipment utilizing the mercury removal method of the present invention can be further improved. [Brief description of the drawings]

[0033] [Figure 1] 1 is a schematic diagram showing an example of a cement firing facility to which a mercury removal method according to an embodiment of the present invention is applied. [Diagram 2] 1 is a flowchart illustrating a mercury removal method according to an embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram for explaining the structure of an electric dust collector. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] A mercury removal method according to an embodiment of the present invention will be described in detail with reference to FIGS.

[0035] First, a cement burning facility to which a mercury removal method according to an embodiment of the present invention is applied will be described with reference to FIG. 1 is a schematic diagram showing an example of a cement firing facility to which a mercury removal method according to an embodiment of the present invention is applied. The dashed arrows in the figure indicate a gas path 5 through which exhaust gas G passes, and the direction of the arrows indicates the direction in which the exhaust gas G flows. Meanwhile, the solid arrows indicate the flow of the adsorbent and dust, as will be described in detail later (the arrows from the adsorbent feeder 6 indicate only the flow of the adsorbent). The cement firing equipment 1 to which the mercury removal method according to the embodiment of the present invention is applied is not limited to the configuration shown in FIG. 1, and may be modified by the user as long as the function and effect of the present invention are achieved.

[0036] A cement calcination facility 1 to which a mercury removal method according to an embodiment of the present invention is applied includes a preheater 2 that preheats cement raw materials (not shown) that have been charged for calcination, a rotary kiln 3 that calcines the cement raw materials preheated in the preheater 2, and a gas path 5 that sends gas including combustion gas generated in the rotary kiln 3 and the like as flue gas G via the preheater 2 to an electric dust collector 4. The flue gas G that has been subjected to dust collection by the electric dust collector 4 passes through the gas path 5 and is released into the atmosphere from a chimney 8.

[0037] Furthermore, the cement burning equipment 1 is provided with an adsorbent feeder 6 between the preheater 2 and the electrostatic precipitator 4, which feeds a fixed amount of adsorbent for adsorbing mercury into the flue gas G per hour. There are many types of adsorbent available on the market, and no particular adsorbent is specified, allowing the user to use one of their choice. Examples of such adsorbents include sand, stone particles, ceramics, glass beans, quartz, and activated carbon. Although not shown in Fig. 1, the exhaust gas G discharged from the preheater 2 has a high temperature, and is therefore generally used to preheat the blended raw materials or the individual raw materials before being sent to the electrostatic precipitator 4. In other words, a raw material dryer (not shown) or the like is often present along the gas path 5. For this reason, since the temperature of the exhaust gas G is often maintained at 350°C or higher, it is preferable to install the adsorbent feeder 6 upstream of these raw material dryers.

[0038] The adsorbent feeding device 6 is connected to a measuring device 7 that measures the voltage and / or current between a collecting electrode (not shown) and a discharge electrode (not shown) installed in the electrostatic precipitator 4. If the electrostatic precipitator 4 itself is capable of measuring the voltage and / or current between the collecting electrode and the discharge electrode and outputting the data, the adsorbent feeding device 6 may be directly connected to the electrostatic precipitator 4. In this case, there is no need to install the measuring device 7, making it possible to reduce the cost required for capital investment.

[0039] In addition, as will be described in detail later, the adsorbent feeding device 6 is controlled to stop when the measured voltage and / or current value between the dust collecting electrode and the discharge electrode fluctuates beyond a predetermined range. Furthermore, in combination with the mercury removal method of the present invention, a mercury recovery device 9 may be provided in tandem with the electrostatic precipitator 4, as shown in Fig. 1. As such a mercury recovery device 9, a known device that recovers mercury from the adsorbent and dust collected by the electrostatic precipitator 4 can be used, and an example of such a mercury recovery device is a reduction heating type device that employs a method of recovering mercury by heating the adsorbent and dust in an oxygen-deficient reducing atmosphere.

[0040] Next, the method for removing mercury in the cement burning facility 1 shown in FIG. 1 will be described in detail. The mercury removal method according to this embodiment is applied to, for example, cement burning equipment 1 shown in Fig. 1. The mercury removal method here comprises an "adsorbent introduction step" and, after the "adsorbent introduction step", a "measurement step of measuring the voltage and / or current between the dust collecting electrode and the discharge electrode of the electric precipitator 4" in order to determine whether or not to continue introduction of the adsorbent.

[0041] The "step of adding an adsorbent" is a step in which the adsorbent adding device 6 adds a fixed amount of adsorbent every hour to the flue gas G discharged from the preheater 2, and the "step of measuring the voltage and / or current between the dust collecting electrode and the discharge electrode of the electrostatic precipitator 4" is a step of measuring the voltage and / or current in order to monitor the accumulation of dust and the like and the presence or absence of charging defects in the electrostatic precipitator 4. In the mercury removal method according to this embodiment, while performing these two steps as described above, the adsorbent and the like added to the flue gas G are collected by the electrostatic precipitator 4, and mercury is recovered. It should be noted that the amount of adsorbent to be added is related to the type of adsorbent added (adsorption capacity), the specifications of the adsorbent addition device 6 used, the dust collection capacity of the electric dust collector 4, and the amount of exhaust gas G, so no specific specification is given and the user can set the amount to be added as appropriate.

[0042] In the mercury removal method according to this embodiment, as described above, while deposition and charging failures in the electrostatic precipitator 4 are monitored, the mercury in the flue gas G that is adsorbed to the adsorbent that has been introduced is stably captured and recovered together with the adsorbent by the electrostatic precipitator 4 without causing charging failures. Then, the flue gas G from which the mercury has been recovered and which meets the specified exhaust gas standards is released into the atmosphere.

[0043] Next, the mercury removal method according to this embodiment, in which the above-mentioned "step of adding an adsorbent" and "step of measuring the voltage and / or current between the collecting electrode and the discharge electrode of the electric dust collector 4" are performed in coordination, will be described in more detail with reference to FIG. 2. 2 is a flow chart showing a mercury removal method according to an embodiment of the present invention. In the mercury removal method 10, the adsorbent is introduced into the flue gas G while proceeding along the steps shown in FIG.

[0044] When the mercury removal method 10 is started, first, the count value N of a counter (not shown) installed in the adsorbent feeding device 6 becomes 1 (step S1), and the adsorbent feeding device 6 starts feeding the above-mentioned predetermined amount of adsorbent (step S2). Next, the measuring device 7 reads the voltage and / or current value between the collecting electrode and the discharge electrode of the electrostatic precipitator 4 (step S3). Note that the measuring device 7 may be operated when the mercury removal method 10 is started, or may be operated continuously to monitor the status of the electrostatic precipitator 4.

[0045] Following step S3, the measuring device 7 determines whether or not the voltage and / or current value between the collecting electrode and the discharge electrode of the electrostatic precipitator 4 exceeds a "predetermined numerical range" (step S4). At this time, if the voltage and / or current value is not within the numerical range, the supply of the adsorbent from the adsorbent supplying device 6 is stopped (step S5). On the other hand, if the voltage and / or current value does not exceed the numerical range, the process proceeds to step S1, where the counter value N is reset to 1, and the process proceeds again to step S2, where the supply of the adsorbent is continued. The determination in step S4 is performed while checking the voltage and / or current on a monitor at a predetermined cycle.

[0046] Here, the "predetermined numerical range" is not particularly specified as described above, but can be arbitrarily determined according to the state and specifications of the device to be used. The method of determination may be, for example, by gradually increasing the amount of adsorbent added into the exhaust gas G, and determining the fluctuation range of the voltage and current values ​​when deposition begins to occur in the electrostatic precipitator 4. In the case of voltage, for example, if the steady-state voltage is 60 kV and the voltage fluctuation range when excessive deposition occurs and the electrostatic precipitator 4 is recognized as having a problem is plus or minus 9 kV, the above range can be greater than 51 kV and less than 69 kV. In addition, considering that there is a time lag from the adsorbent introduction point until it reaches the electrostatic precipitator 4 even if the introduction of the adsorbent is stopped, it is preferable to set the range to 55 kV to 65 kV with a margin in the above case. As a rough guideline, the fluctuation range compared to the steady operation can be less than 15%, more preferably within 10%. As another guideline, a fluctuation of about 2 / 3 of the fluctuation range recognized as a problem (if a 15% fluctuation is recognized as a problem, 10% of that 2 / 3), more preferably about 1 / 2, can be used as a guideline with a margin. Also, the causes of trouble are often different when the voltage and current values ​​rise and fall. Therefore, the range of fluctuation when the above values ​​rise and fall do not need to be the same. For example, the above values ​​may be set to different values, such as up to 15% when they rise and up to 10% when they fall. Generally, when excessive dust accumulation occurs, the voltage will fall, so when voltage rises, there are many cases where the cause is something other than excessive dust accumulation.

[0047] Then, when the supply of the adsorbent is stopped as a result of the determination in step S4 (step S5), the count value N of the counter is incremented by 1 (step S6). Next, it is determined whether the count value N incremented by 1 in step S6 is smaller than a preset allowable number of fluctuations (hereinafter referred to as the allowable number of fluctuations X) (step S7). Note that this allowable number of fluctuations X is a value that can be arbitrarily determined in accordance with the specifications of the equipment owned by the user (for example, the measurement period of the voltage and / or current), and can also be said to be the limit value of the allowable waiting time until the above-mentioned voltage and / or current values ​​return to a predetermined numerical range.

[0048] In step S7, if the count value N is equal to or less than the permissible number of fluctuations X, the process returns to step S3 again to measure the current and / or voltage, and then proceeds to step S4 in which it is determined whether the current and / or voltage are within a predetermined numerical range. If the current and / or voltage return to the predetermined numerical range, it is determined that the adsorbent and dust deposition state on the dust collecting electrode has improved, and the process proceeds to step S1, where the counter count value N is reset to 1, and the subsequent steps are performed. On the other hand, if the current and / or voltage do not return to the predetermined numerical range in step S4 and the count value N gradually increases and finally exceeds the permissible number of fluctuations X, it is determined that the adsorbent and dust deposition state on the dust collecting electrode in the electrostatic precipitator 4 has exceeded the permissible range or some kind of failure has occurred, and the electrostatic precipitator 4 is cleaned, repaired, or otherwise processed as required (step S8).

[0049] Such a mercury removal method 10 is believed to have the following effects. That is, by measuring the voltage and / or current between the collecting electrode and the discharge electrode, it is possible to grasp the degree of accumulation and the charged state of the adsorbent and dust in the flue gas G collected by the collecting electrode of the electrostatic precipitator 4. Furthermore, by introducing the adsorbent, the adsorbent is diffused into the flue gas G in the gas path 5, and the mercury in the flue gas G comes into contact with and is adsorbed on the surface of the adsorbent. Then, by bringing the adsorbent that has adsorbed mercury into contact with gas molecules that are negative ions in the electrostatic precipitator 4, the adsorbent is negatively charged and is attracted to the collecting electrode (positive electrode) and collected.

[0050] Furthermore, in the case where the adsorbent and dust accumulate in the electrostatic precipitator 4 and the measured voltage and / or current value fluctuates beyond a predetermined range, the supply of the adsorbent to the gas path 5 is stopped until the measured voltage and / or current value returns to within the predetermined range. This makes it possible to prevent clogging of the electrostatic precipitator 4 due to further accumulation of the adsorbent and dust, and a decrease in dust collection capacity due to insufficient charging.

[0051] By the above-mentioned action, in the mercury removal method 10, the mercury in the exhaust gas G is adsorbed to the adsorbent that has been introduced, and is captured and recovered together with the adsorbent by the electrostatic precipitator 4. Furthermore, by measuring the voltage and / or current between the collecting electrode and the discharge electrode of the electrostatic precipitator 4, it is possible to determine the state of accumulation of the adsorbent and dust inside the electrostatic precipitator 4. Therefore, when a change in the voltage and current values ​​is observed that indicates that accumulation has occurred inside the electrostatic precipitator 4 (limited to when the change exceeds a predetermined numerical range), it is possible to stop the supply of the adsorbent and prevent further accumulation and a decrease in dust collection capacity due to poor charging. As a result, it is possible to prevent excessive accumulation of the adsorbent and dust, and the dust collection capacity is restored when the accumulated adsorbent and dust peel off and fall off due to vibration applied by a vibrator or the like at a predetermined cycle, making it possible to continue operating the cement burning equipment 1 without completely stopping the electrostatic precipitator 4. As a result, there is no need to stop the entire cement manufacturing facility, and no associated losses occur.

[0052] Meanwhile, as shown in Fig. 1, in a cement calcination facility 1 to which a mercury removal method 10 according to an embodiment of the present invention is applied, an adsorbent dosing device 6 doses an adsorbent into flue gas G from a gas path 5 connecting a preheater 2 and an electrostatic precipitator 4. The temperature of the flue gas G discharged from the preheater 2 is generally about 400°C or higher, which exceeds the boiling point of mercury. However, as the flue gas G flows toward the electrostatic precipitator 4, it is gradually cooled by natural cooling, and it is known that the temperature of the flue gas G drops to about 100°C in the electrostatic precipitator 4. For such flue gas G, in the above-mentioned "step of dosing the adsorbent", it is preferable to provide the adsorbent dosing device 6 at a position where the gas temperature is as high as possible. Specifically, the adsorbent may be dosed into flue gas G at a temperature of 350°C or higher.

[0053] By setting the introduction position of the adsorbent introduction device 6 on the gas path 5 in this manner, in addition to the effects of the mercury removal method 10 described above, the temperature of the exhaust gas G into which the adsorbent is introduced becomes 350°C or higher (particularly above the boiling point of mercury), and since the mercury in the exhaust gas G exists as a gas, the frequency of contact with the adsorbent is further increased due to its large surface area and high mobility. Generally, the boiling point of mercury is about 360°C under normal pressure, but the exhaust gas temperature here is set to 350°C or higher because, as mentioned above, a measurement error of about ±10°C is taken into account when measuring the exhaust gas temperature in the actual gas route.

[0054] In addition to the same effects as those of the above-mentioned mercury removal method 10, the above-mentioned action increases the frequency of contact between the mercury and the adsorbent because the mercury in the exhaust gas G into which the adsorbent is introduced is in a vaporized state, making it easier for the mercury to be adsorbed by the adsorbent. As a result, the amount of mercury adsorbed by the adsorbent increases, and a high mercury removal effect can be achieved even with a small amount of adsorbent.

[0055] Alternatively, the adsorbent used in the mercury removal method 10 may be activated carbon. The activated carbon used herein may be in either granular or powder form, as well as unmodified activated carbon and activated carbon that has been chemically treated, such as activated carbon impregnated with bromine or sulfur. As mentioned above, activated carbon, which comes in a variety of types, is porous and has a large surface area, meaning that it can adsorb a large amount of mercury per unit mass. The smaller the particle size, the larger the surface area for the same weight, and so the greater the amount of mercury that can be adsorbed. However, the finer the powder, the higher the unit price, so care must be taken when trying to remove mercury inexpensively. In order to reduce the cost of mercury removal, it is also possible to use a blend of activated carbon and pulverized coal, which is used as fuel for cement kilns, in a specified ratio.

[0056] By using the activated carbon described above, in addition to having the same effect as mercury removal method 10 (including the case where the temperature of the exhaust gas G into which the adsorbent is introduced is 350°C or higher), the method has the effect of adsorbing mercury present in the form of gas in the exhaust gas G by coming into contact with the activated carbon, which has a large surface area. Due to the above-mentioned effects, activated carbon increases the amount of mercury adsorbed per unit mass of the adsorbent, further enhancing the mercury removal capacity, and even a small amount of adsorbent can efficiently adsorb a large amount of mercury. As a result, the amount of adsorbent introduced into the flue gas G can be reduced, reducing the frequency of accumulation of the adsorbent and dust in the electrostatic precipitator 4. In other words, this leads to a reduction in the frequency of stopping the introduction of the adsorbent, making it possible to extend the period during which the adsorbent is continuously introduced into the flue gas, and making it possible to collect the mercury present in the flue gas G without releasing it to the outside. [Industrial Applicability]

[0057] INDUSTRIAL APPLICABILITY As described above, the present invention provides a mercury removal method for removing mercury from exhaust gas without reducing the dust collection capacity of an electrostatic precipitator, and can be used in technical fields related to the treatment of mercury in exhaust gas. [Explanation of symbols]

[0058] Reference Signs List 1...Cement burning equipment 2...Preheater 3...Rotary kiln 4...Electrostatic precipitator 4a...Dust collecting electrode 4b...Discharge electrode 5...Gas path 6...Adsorbent feeding device 7...Measuring device 8...Chimney 9...Mercury recovery device 10...Mercury removal method 11...Adsorbent G...Exhaust gas N...Count value X...Allowable number of fluctuations

Claims

1. A mercury removal method for removing mercury contained in exhaust gas generated in a cement manufacturing process, comprising the steps of: introducing the mercury adsorbent into a gas path for sending the exhaust gas from a preheater constituting a cement burning facility to an electric dust collector; recovering the adsorbent from the electrostatic precipitator; measuring a voltage and / or a current between the collecting electrode and the discharge electrode of the electrostatic precipitator; In the step of introducing the adsorbent, the temperature of the exhaust gas when the adsorbent is introduced is 350° C. or higher, A mercury removal method comprising: stopping supply of the adsorbent into the gas path when the measured value of the voltage and / or the current fluctuates beyond a predetermined numerical range.

2. 2. The method for removing mercury according to claim 1, wherein the adsorbent is activated carbon.

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