External combustion air preheating

Abstract

Systems and processes for external combustion air preheating for providing preheated combustion air to a furnace. The furnace system convective heating section includes a plurality of heating coils for waste heat recovery. The heating coil may be used to preheat a feed (feed preheating coil), to heat boiler feed water, to superheat steam, or to heat or superheat a feed stream prior to feeding the feed to a radiant coil. Waste heat in the combustion gases is also used to heat a heat transfer fluid that can be used to preheat the combustion air or for other purposes within a plant.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to fired tube furnaces. More specifically, embodiments herein relate to pyrolysis furnaces for thermal cracking of hydrocarbons. Even more specifically, embodiments herein relate to an arrangement for efficiently preheating air, thereby allowing for reduced carbon dioxide emissions from fired tube furnaces. Background Art

[0002] US2020 / 0172814 (Oud) describes a heat recovery system for improving radiant efficiency and reducing emissions, wherein heat is recovered from cracked gas products to preheat a hydrocarbon feed. Several embodiments are described whereby preheated oxidant can be obtained by indirect heat exchange between the combustion flue gases and the oxidant. In one embodiment, the oxidant preheater is located downstream of the convection section (i.e., where the flue gases are coldest).

[0003] WO2021052642 (Wijnja and Oud) similarly describes a three-step feed preheating scheme, in which a combustion air preheater is located on the downstream side of the convection section and preferably downstream of all other convection section groups. The heat of the pyrolysis reaction is provided by fuel gas, combustion air and "highly nitrogen-depleted oxygen", optionally with the presence of recycled flue gas. To the extent that all nitrogen is replaced by recycled flue gas, the system can be considered as oxygen combustion, with the advantage that the CO2 in the flue gas is very concentrated and suitable for the CO2 capture process, and eliminates the problem of NOx formation. However, the oxygen provided must be separated from the atmosphere, which can bring huge capital costs, and the energy required for the separation must be included in the operating costs.

[0004] Albano et al. (1992) describe the use of hot exhaust gases from a gas turbine (GT) for combustion air preheating with a reduced oxygen content (15 to 17 mol%, compared to about 21 mol% for air). Co-production of electricity (30 MW to 60 MW) is possible, which has a significant impact on the economics of the power plant. Using turbine exhaust gas (TEG) for high levels of air preheating reduces the fuel requirement of the heater. However, unlike air preheating systems, the total mass flow of flue gas through the furnace is increased due to the reduced oxygen content of the TEG. This results in an increase in steam production in the convection (heat recovery) section of the pyrolysis module.

[0005] US4617109 (Wells and Petterson) describes an air preheating scheme whereby the combustion air is preheated by indirect heat exchange with steam which is expanded by high pressure steam generated by a steam turbine from the hottest section of the furnace. As in the previous example, the radiative efficiency is improved, however, the combustion air is not directly heat exchanged with the flue gases in the convection section.

[0006] US4044820 (Nobles) teaches the use of a heat transfer fluid to recover heat from flue gases to a combustion air stream via a continuous flow of a heat transfer fluid having a high heat capacity (product of mass flow rate and specific heat capacity). The proposed method involves a dual chamber system in which a first chamber (for cooling the flue gases) is separated from a second chamber (for heating the air) by a common wall. The high circulation rate of the heat transfer fluid ensures that the temperature of the heat transfer surface is maintained above the acid dew point of the flue gases, thereby preventing condensation and the associated corrosion risks.

[0007] US3623549 (Smith) proposes a liquid to gas heat exchange apparatus having the function of transferring heat from a relatively hot gas in one location to a relatively cold gas in a separate location. Multiple heat transfer fluids with different phase change properties are used to cover a wider temperature range than using a single heat transfer fluid (such as water). The system is proposed as an alternative to a recovery heat exchanger, in which hot gas is separated from cold gas by a heat transfer surface, or as a regenerative heat exchanger, in which hot and cold gases are periodically circulated through a thermal mass.

[0008] US20170363288 (Lee and Sadler) describes a fired heater system with a hot pipe preheater. A hot pipe is a sealed tube containing a working fluid that transfers heat from a heat source to a cooler heat sink. The hot pipe is used to transfer heat from hot flue gases to combustion air without introducing the combustion air into the chimney. Instead, a separate air preheater duct is connected to the chimney and the hot pipe penetrates the chimney wall. While this arrangement avoids the need to install an air preheater in the convection section of the furnace, it does not address the structural costs associated with mounting the device on the chimney. The working fluid should be carefully selected because it must simultaneously meet several constraints, such as viscosity, sonic velocity, capillary, entrainment, and nucleate boiling levels, in order for the hot pipe system to work properly. The system does not allow flexibility in the location of the air preheater, which can only be placed outside the chimney.

[0009] In many industrial applications, heat transfer fluid (HTF) is used to provide the required heat for multiple heat requirements from a single heat source. For example, it is known to use biomass heat source to provide high temperature heat application, thereby using heat transfer fluid as an alternative to steam generation. The appropriate combination of fired heater and process makes the overall reduction of energy demand and capital investment. However, in large-scale special furnaces (such as steam cracking furnaces for the production of bulk chemicals such as ethylene, propylene, butenes and aromatic hydrocarbons), non-process heat recovery is carried out via the generation of steam, and steam is used to drive other processes in compressors and downstream partition sections. Therefore, the energy of downstream processing actually comes from the fuel burned in the steam cracking furnace. Summary of the invention

[0010] In one aspect, embodiments of the present invention relate to a furnace system for cracking hydrocarbons. The furnace system includes a furnace having a radiant heating section and a convection heating section, wherein the radiant heating section includes one or more burners configured to burn a fuel with a preheated air flow to produce flue gas fed to the convection heating section. A heat transfer fluid heating coil is disposed within the convection heating section of the furnace. The furnace system also includes a feed preheating coil configured to receive and preheat a hydrocarbon feed to produce a preheated hydrocarbon feed. A radiant process coil is disposed within the radiant heating section and is configured to receive the preheated hydrocarbon feed and produce a cracked hydrocarbon product. The furnace system further includes an air preheater having an air inlet, a preheated air outlet, a heat transfer fluid inlet, a heat transfer fluid outlet, and one or more heat exchange surfaces for indirectly heating air received via the air inlet by indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet to produce a preheated air stream recovered via the preheated air outlet and a cooled heat transfer fluid recovered via the heat transfer fluid outlet. One or more flow lines connect the preheated air outlet to the one or more burner fluids for supplying the preheated air stream to the one or more burners. A heat transfer fluid circulation system is also included, which has a flow line for circulating the cooled heat transfer fluid from the heat transfer fluid outlet to the inlet of the heat transfer fluid heating coil disposed in the convection heating section of the furnace, and a flow line for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet.

[0011] In some embodiments, the furnace system includes a selective catalytic reduction catalyst bed disposed within the convection heating section of the furnace. In some of such embodiments, the heat transfer fluid heating coil may have an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil, wherein the selective catalytic reduction catalyst bed is disposed within the convection heating section of the furnace above the location of the lower heat transfer fluid heating coil and below the location of the upper heat transfer fluid heating coil.

[0012] In another aspect, embodiments of the present invention relate to a method for cracking hydrocarbons using the above system. The method includes: heating a heat transfer fluid in the heat transfer fluid heating coil to produce the hot heat transfer fluid; heating air via indirect heat exchange with the hot heat transfer fluid in the air preheater to produce preheated air; and mixing the preheated air with a fuel to form a combustion mixture and combusting the combustion mixture in the one or more burners.

[0013] In some embodiments, such as those including a selective catalytic reduction catalyst bed, the method may include controlling the temperature of the flue gas entering the selective catalytic reduction catalyst bed.

[0014] In another aspect, embodiments herein relate to a method of retrofitting a cracking furnace having a steam superheating coil, a boiler feed water heating coil, and a feed preheating coil, each coil comprising a plurality of tubes connected to a feed header and an exhaust header. The retrofit method comprises: disconnecting one or more tubes of the boiler feed water heating coil from a boiler feed water circulation system; fluidly connecting the one or more tubes to a heat transfer fluid circulation system; and fluidly connecting an air preheater to the heat transfer fluid circulation system and to a burner of the cracking furnace.

[0015] In yet another aspect, embodiments of the present invention relate to a heater system. The heater system includes: a furnace having a radiant heating section and a convection heating section, wherein the radiant heating section includes one or more burners configured to burn fuel using a preheated air stream. A heat transfer fluid heating coil is disposed in the convection heating section of the furnace. The heater system includes an air preheater having an air inlet, a preheated air outlet, a heat transfer fluid inlet, a heat transfer fluid outlet, and one or more heat exchange surfaces, the one or more heat exchange surfaces being used to indirectly heat the air received via the air inlet via indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet to produce the preheated air stream recovered via the preheated air outlet and the cooled heat transfer fluid recovered via the heat transfer fluid outlet. One or more flow lines connect the preheated air outlet to the one or more burner fluids for supplying the preheated air stream to the one or more burners. The heater system further includes a heat transfer fluid circulation system including a flow line for circulating the cooling heat transfer fluid from the heat transfer fluid outlet to the inlet of the heat transfer fluid heating coil disposed in the convection heating section of the furnace, and a flow line for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet. In various embodiments, the heat transfer fluid heating coil is configured as a plurality of heating coil sections separated by one or more additional heat recovery coils configured to heat one or more of hydrocarbon feed, water, steam, or mixtures thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 and Figure 1A A prior art combustion air preheating method is shown.

[0017] Figure 2 and FIG. 2A to FIG. 2E A system for heat recovery and air preheating utilizing intermediate heat transfer fluid heat recovery according to embodiments herein is shown. DETAILED DESCRIPTION

[0018] As described above, existing practice typically involves expensive modifications to the convection section intended to improve the combustor radiative efficiency and thereby reduce fuel consumption by increasing the heat available for process heating (feed gas or oxidant) at the expense of heat available for non-process heating (e.g., steam generation).

[0019] Heat exchange between the flue gases and the oxidant (such as in the case of Ord or Vigna and Ord) occurs downstream of the convection section (ie, exchanging heat with the coolest flue gases).

[0020] Heating the air by indirect heat exchange with the expanding steam can be performed outside the convection section, but does not address the fact that fuel (and associated emissions) must be consumed to generate the steam in the first place.

[0021] It has been practiced to use hot duct heat exchangers, multi-flow loops, or dual chamber systems to recover heat from flue gases without introducing air into the convection section, but these methods require extensive structural changes (in the case of retrofits) or expensive convection section designs to attach to the heater convection section.

[0022] In many industrial applications, heat transfer fluid (HTF) is used to provide required heat to multiple heat requirements from a single heat source. For example, it is known to use biomass heat sources to provide high temperature heat applications, thereby using heat transfer fluids as an alternative to steam generation. The appropriate combination of fired heaters and processes reduces energy demand and capital investment overall. This is particularly relevant to the global ethylene industry because its installation base is very large. Due to the limitations of structural steel and foundation loads, it is impractical or extremely expensive to apply the methods of the prior art to such large installation bases. A simple, inexpensive method is needed to reduce CO2 emissions, and the installation base of ethylene steam cracking furnaces can be quickly applied.

[0023] However, in large dedicated furnaces (such as steam crackers used to produce commodity chemicals such as ethylene, propylene, butenes and aromatic hydrocarbons), non-process heat recovery is performed via the generation of steam, which in turn is used to drive compressors and other processes in downstream compartmentalized sections. Thus, in effect, the energy for downstream processing comes from the fuel burned in the steam cracker.

[0024] Embodiments herein relate to alternative heat recovery methods that do not require the conversion of the chemical energy of the fuel into thermal energy and functionality via the generation of steam (and associated emissions), and can significantly reduce the fuel consumption of existing furnaces by at least 5% and up to 30%. Such embodiments described herein are also inexpensive and do not require significant additional structural changes to the convection section. In addition, the heat recovery of the embodiments herein maintains the desired heat distribution in the convection section, has high efficiency and allows flexibility in feed preheating. Finally, this heat recovery method can be applied to existing furnaces without major modifications to the heating surface, structure, and base load. The embodiments herein meet one or more of these goals, and in some embodiments meet all of these goals.

[0025] Embodiments herein relate to furnace systems for processing hydrocarbons. More particularly, embodiments herein relate to furnace systems and the arrangement of heat recovery coils within a convection section of such furnaces.

[0026] The furnace system according to the embodiments of the present invention has a radiation section and a convection heating section. The radiation section, also known as a radiation zone or a fired heater box, includes one or more burners, such as a floor burner and / or a wall burner for burning fuel, generating radiant heat and combustion gases. The combustion gases are transferred from the radiation section to the convection heating section before being discharged into the atmosphere as flue gas. One or more radiation process coils are disposed in the radiation section, and the process coils are heated by the radiant heat from the burners so that the processed fluid is heated and / or converted.

[0027] The convection heating section includes a number of heating coils for waste heat recovery. The heating coils can be used to preheat the feed (feed preheat coils), heat boiler feed water, superheat steam, or heat or superheat the feed stream before feeding it to the radiant coils. Waste heat in the combustion gases is also used to heat a heat transfer fluid that can be used to preheat the combustion air or for other purposes within the plant.

[0028] Embodiments herein allow heat to be extracted from the combustion flue gases into a heat exchange medium (heat transfer fluid) at any section of the heater as desired. Some embodiments extract heat from multiple sections. In contrast, in the prior art, heat extraction for air preheating is typically limited to the coldest portion of the convection section. When heat is extracted from multiple sections into a heat transfer fluid according to embodiments herein, the sections may be connected in series or in parallel or in a combination of series and parallel.

[0029] According to embodiments of the present invention, heat extraction can be performed using a heat transfer fluid. The heat transfer fluid can be one type of fluid or different types of fluids to accommodate different usage temperatures. The heat extracted from the heater using the heat transfer fluid can then be used to heat the combustion oxidant or other process fluids.

[0030] Embodiments herein may also use heat extraction of the heat transfer fluid to adjust the flue gas temperature profile in the convection section. The ability to adjust the flue gas temperature profile may improve heater process performance and reduce emissions, such as by controlling the flue gas temperature at the selective catalytic reduction (SCR) catalyst bed to achieve optimal NOx reduction.

[0031] Heat extracted from the selected heater (such as heat used to heat the combustion air or other fluid) can be indirectly applied back to the same heater or other heat transfer equipment. In addition, embodiments herein may provide the option of using flue gas to dilute the preheated oxidant (e.g., external flue gas recirculation to the burner to reduce NOx).

[0032] exist Figure 1 and Figure 1A A prior art combustion air preheating method is shown in FIG. 1 . For simplicity, the flow lines described are not shown. The hydrocarbon feed is preheated in the feed preheating coil FP in the convection section, mixed with dilution steam, and optionally further preheated in an external heat exchanger (STLE, not shown) for quenched cracked gas products. The hydrocarbon plus dilution steam mixture is further preheated in the mixing preheating coil MP with the hot combustion gases in the convection section and fed to the radiation box, where the energy required for thermal cracking of hydrocarbons to produce cracked gas products is supplied by burning fuel in an oxidant stream to produce hot flue gas 5. Between the two feed preheating sections FP, MP of the convection section, steam is generated by continuously heating boiler feed water in the boiler feed water coil BFW, feeding the heated boiler feed water to a steam drum (not shown), which generates steam by quenching the pyrolysis gases leaving the radiant section, and further heating the high pressure steam in the convection section to produce superheated steam in the upper steam superheat section USSH and the lower steam superheat section LSSH. In the coldest section of the convection section, an air preheater (APH) is introduced, which transfers heat indirectly from the flue gas to the combustion air 10 before the resulting hot air 12 is mixed with the fuel in the burner. The addition of the APH, whether in the Figure 1 In the same convection section shown in Figure 1A The individual heat recovery zones shown enable improved radiant efficiency while maintaining high overall thermal efficiency.

[0033] Radiative efficiency is the heat transferred by the radiant section divided by the total energy supplied. The heat absorbed by the radiant coil plus the heat losses from the radiant section equals the heat given off by the flue gases from the adiabatic flame temperature to the temperature at which they leave the radiant section. Radiative efficiency is affected by the radiative heat transfer to the convection section, which in turn depends on the burner location, as burners located higher in the combustion chamber will radiate more heat to the convection section, which will not be transferred to the reactions in the combustion chamber.

[0034] There are some variations on this prior art convective heat transfer scheme, but generally speaking, in addition to preheating the hydrocarbon feed and hydrocarbon feed plus dilution steam, the energy of the hot flue gas is used to preheat boiler feed water, generate steam, and superheat steam. Energy is saved in the convection section by preheating the hydrocarbon feed and dilution steam; by using utility preheating, i.e., boiler feed water preheating, dilution steam superheating, high pressure steam superheating, or preheating combustion air. Some convection sections may also include an integrated catalyst system for removing NOx by selective catalytic reduction (SCR) technology, whereby NOx present in the flue gas reacts with ammonia.

[0035] Embodiments herein relate to furnace systems, such as furnace systems for cracking hydrocarbons. Figure 2 As shown, such a furnace includes a radiant heating section 50 and a convection heating section 52, wherein the radiant heating section includes one or more burners 54 configured to combust a fuel 56 with the preheated air stream 12 to produce a flue gas 60 that is fed to the convection heating section. Radiant process coils 62 are disposed within the radiant heating section and receive a preheated process fluid, such as hydrocarbons, to further heat and crack the hydrocarbons.

[0036] Embodiments herein further relate to an arrangement of heating coils disposed within a convection heating section (e.g. FIG. 2A to FIG. 2E 1 and further described below). The embodiments herein include: a heat transfer fluid heating coil 22 disposed within a convection heating section 52 of the furnace; one or more steam heating coils (see FIG. 2A to FIG. 2E ); and one or more hydrocarbon feed preheating coils (see FIG. 2A to FIG. 2E ), which is configured to receive and preheat a hydrocarbon feed to produce a preheated hydrocarbon feed. The radiant process coil 62 disposed in the radiant heating section is configured to receive the preheated hydrocarbon feed and produce a cracked hydrocarbon product.

[0037] The air preheater 32 has an air inlet 70, a preheated air outlet 72, a heat transfer fluid inlet 74, a heat transfer fluid outlet 76, and one or more heat exchange surfaces (not shown) configured to indirectly heat the air 10 received via the air inlet 70 by indirect heat exchange with the hot heat transfer fluid received via the heat transfer fluid inlet 74 to produce a preheated air stream 12. The preheated air stream 12 is recovered via the preheated air outlet 72, and the cooling heat transfer fluid is recovered via the heat transfer fluid outlet 76. One or more flow lines (not shown) fluidly connect the preheated air outlet with one or more burners for supplying the preheated air stream 12 to the one or more burners. In addition, the heat transfer fluid circulation system 24 includes a flow line 28 for circulating the cooling heat transfer fluid from the heat transfer fluid outlet 76 to the inlet of the heat transfer fluid heating coil 22 disposed in the convection heating section 52 of the furnace, and a flow line 30 for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet 74. A pump 26 may be used to transport the heat transfer fluid within the heat transfer fluid circulation system 24 .

[0038] FIG. 2A to FIG. 2E A system for preheating combustion air using intermediate heat transfer fluid heat recovery according to embodiments herein is shown. Common features of these embodiments include: (i) circulating heat transfer fluid; (ii) (one or more) heat recovery sections 22 (heat transfer fluid heating coils) located inside the convection section; (iii) a heat transfer fluid circulation loop 24 for circulating the heat transfer fluid; and (iv) an air preheater APH.

[0039] Heat transfer fluids used at high temperatures up to 400°C can be divided into synthetic materials, thermal oils and inorganic materials (including silicones). In processes requiring high temperatures in the liquid state, suitable synthetic heat transfer fluids are usually eutectic mixtures of biphenyl and diphenyl ether, commonly known as biphenyl or HTF (heat transfer fluid). Such fluids can be found on the market, for example, under the brand names DOWTHERM-A and THERMINOL VP-1. Such fluids can be used at temperatures up to 400°C. Other heat transfer fluids can be selected based on their properties and stability under operating conditions. Silicone-based heat transfer fluids provided under trade names such as SYLTHERM 800 are also suitable.

[0040] For systems utilizing high temperature process waste heat, the HTF heater represents the highest local fluid film temperature as well as the highest bulk fluid temperature. The HTF velocity on the heat transfer surface should be maximized as this will result in lower skin temperatures. Dead flow zones resulting in high residence times should be avoided. Therefore, configurations containing the HTF within the tube are preferred, with heat transfer fluid velocities in the range of 2 m / s to 3 m / s or higher.

[0041] The heat transfer fluid circulation loop according to embodiments herein includes piping from an expansion tank (not shown) to a pump 26, piping 28 from the pump to one or more heat transfer fluid heating coils 22, and piping 30 from the heat transfer fluid heating coils to an external heat exchanger 32. FIG. 2A to FIG. 2E In the embodiment of the present invention, the external heat exchanger is an air preheater for preheating the oxidant mixture (such as air) used to burn the fuel in the radiant combustor. The heat transfer loop can be non-pressurized as long as the operating temperature is below the normal boiling point of the heat transfer fluid. If the operating temperature of the heat transfer fluid is above its normal boiling range, a pressurized liquid phase system may be required, by which an inert gas (usually N2) is applied through an expansion tank to keep the heat transfer fluid as a liquid.

[0042] A pressurized gas phase system for the synthesis of diphenyl ether / biphenyl type fluids is also possible. The gas phase system can be designed to gravity return the condensate from the radiator (air preheater) to the vaporizer (convection section coil) using a pump or hydrostatic pressure. More complex systems require a flash tank, condensate return tank, and condensate return pump. The increased capital equipment cost and complexity of the gas phase system is offset by the increase in heat transport per unit mass of the heat transfer fluid.

[0043] The air preheater may be a single heat exchanger for each heater, or multiple heat exchangers for each heater, or a single preheater for multiple heaters, the variation of which depends on the size and capacity of the individual units. The air preheater may be a plate or tube heat exchanger, and may include, for example, an extended heating surface, such as a stud or fin. Since the heat transfer coefficient of low-pressure air is quite low compared to the heat transfer fluid, the appropriate design is that the heat transfer fluid flows inside the tube and the air flows outside the tube. An extended surface in the form of a heat transfer fin may be provided on the outside of the tube to increase the heat transfer surface area and compensate for the much lower heat transfer coefficient on the air side. The tube may be a round or oval steel tube. The fin may be applied to the tube in the form of a spiral fin or a helical fin, which is continuously wound around each individual tube and may be edge-wound, bonded to the tube by tension, L-shaped or embedded in a groove on the tube surface. Alternatively, many tubes may be incorporated into a plate fin structure by inserting the tube into a plate fin group and mechanically expanding the tube into the plate fin group. In either design, the air will flow cross-flow relative to the heat transfer fluid and the heat transfer tube side may have multiple channels.

[0044] In addition to the heat transfer fluid heating coil 22, one or more additional heating coils may be disposed within the convection heating section and may be used as described above. FIG. 2A to FIG. 2ESuch additional heating coils may include steam superheat coils (SSH). Other heating coils may include feed preheat coils (FP) for heating hydrocarbon feed, mixed feed preheat coils (MP) for mixing hydrocarbon and steam mixtures, and boiler feed water (BFW) heating coils. The relative position of a particular type of coil within the convection section is indicated by upper (U), middle (M), or lower (L), for example, USSH is an upper steam superheat coil.

[0045] Various embodiments herein include a feed preheat coil (FP) disposed above the location of the heat transfer fluid heating coil 22 within the convection heating section of the furnace. As will be appreciated by those skilled in the art, the temperature of the flue gas at a particular coil location decreases with distance from the radiant heating section flue gas outlet, in part due to heat extraction via multiple heating coils. The above may be restated as the feed preheat coil is at a lower flue gas temperature than the heat transfer fluid heating coil.

[0046] Additionally, embodiments herein include a boiler feedwater heating coil and one or more steam superheat coils disposed within the convection heating section of the furnace. A heat transfer fluid heating coil may be disposed below the location of the boiler feedwater heating coil and above the location of the steam superheat coil.

[0047] Some embodiments herein may also include a selective catalytic reduction catalyst bed disposed within the convection heating section of the furnace. Some embodiments may also include a plurality of heat transfer fluid heating coils, such as an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil. In some arrangements, the heat transfer fluid heating coils include an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil, wherein the selective catalytic reduction catalyst bed is disposed within the convection section of the furnace above the location of the lower heat transfer fluid heating coil and below the location of the upper heat transfer fluid heating coil.

[0048] In yet other embodiments, an external feed and / or mixed feed preheat section may be disposed external to the convective heat transfer section.

[0049] References below FIG. 2A to FIG. 2E The various configurations described above are specifically shown and described.

[0050] Figure 2AAn embodiment for recovering heat from flue gases according to embodiments herein is shown. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, farthest from the combustion box outlet): mixing preheat coil (MP); lower steam superheat coil (LSSH); upper steam superheat coil (USSH), heat transfer fluid heating coil 22, boiler feed water heating coil (BFW), and feed preheat coil (FP). As described above, the heat transfer fluid heating coil 22 receives cold heat transfer fluid from the circulation pump 26, heats the heat transfer fluid, and the heated heat transfer fluid is circulated via flow line 30 to the air preheat coil APH, which may include one or more of the above-mentioned fin tubes or other heat transfer surfaces.

[0051] For modification such as Figure 1 In order to achieve the following, the embodiment of the heat recovery system shown in FIG. Figure 2A The heat recovery system shown is modified so that a portion of the piping previously used to preheat the BFW is reused to instead heat the heat transfer fluid. In some embodiments, the upper steam superheat coil is also modified to account for the smaller amount of steam generated. Reducing steam generation is a goal of some embodiments herein.

[0052] Figure 2B An embodiment for recovering heat from flue gas according to the embodiments herein is shown. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, farthest from the combustion box outlet): mixing preheating coil (MP); lower steam superheating coil (LSSH); upper steam superheating coil (USSH); heat transfer fluid heating coil 22, which includes a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, and the middle of the lower heat transfer fluid heating coil and the upper heat transfer fluid heating coil is a selective catalytic reduction catalyst bed (SCR); boiler feed water heating coil (BFW); and feed preheating coil (FP). As described above, the heat transfer fluid heating coil 22 receives cold heat transfer fluid from the circulation pump 26, heats the heat transfer fluid, and circulates the heated heat transfer fluid via flow line 30 to the air preheat coil APH, which may include one or more of the above-mentioned fin tubes or other heat transfer surfaces.

[0053] exist Figure 2B In the heat recovery section Figure 2A A similar modification was made, however, both the BFW coils above and below the catalyst bed for NOx removal were reused to heat the heat transfer fluid. In this case, the heat transfer fluid was heated in two sections in series.

[0054] Figure 2C An embodiment for recovering heat from flue gases according to embodiments herein is shown. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the firebox) to highest (lowest flue gas temperature, farthest from the firebox outlet): mixing preheat coil (MP); lower steam superheat coil (LSSH); intermediate steam superheat coil (MSSH); upper steam superheat coil (USSH); heat transfer fluid heating coil 22; boiler feed water heating coil (BFW); and feed preheat coil (FP).

[0055] Figure 2C and Figure 2A The arrangement is similar to that of the USSH coil, except that instead of eliminating some tubes in the USSH coil, the fin density is modified to reduce steam generation.

[0056] Figure 2D One embodiment for recovering heat from flue gases according to embodiments herein is shown. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the firebox) to highest (lowest flue gas temperature, farthest from the firebox outlet): mixed feed preheating coil; lower steam superheating coil; intermediate steam superheating coil; upper steam superheating coil; heat transfer fluid heating coil; and boiler feed water heating coil. Figure 2D The furnace system further includes a chimney 40 connected to the gas turbine exhaust (not shown). The chimney 40 includes a feed preheating coil (FP) and a second mixed feed preheating coil (SMP) which is fluidly connected to the mixed feed preheating coil (MP).

[0057] Modification to achieve Figure 2D In the arrangement of the convective section, the feed preheating (FP) coil and a portion of the USSH coil and LSSH coil are bypassed, and a portion of the BFW coil is used for the heat transfer fluid heating section. The gas turbine exhaust (GTE) flow is also used to preheat the feed (FP) and partially preheat the mixed feed (MP), a task previously performed in the convection section. By moving the feed preheating and mixed feed preheating to a new section with GTE heating, maximum heat recovery from the GTE can be achieved, and more load is provided for the heat transfer fluid recovery section as well as the air preheating section.

[0058] Figure 2EAnother embodiment for recovering heat from flue gas according to the embodiments herein is shown. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest flue gas temperature, closest to the combustion box) to highest (lowest flue gas temperature, farthest from the combustion box outlet): lower mixing preheating coil; lower steam superheating coil; intermediate steam superheating coil; upper steam superheating coil; heat transfer fluid heating coil, which includes a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, intermediate the lower heat transfer fluid heating coil and the upper heat transfer fluid heating coil from lowest to highest is the upper mixing feed preheating coil, the dilution steam superheating coil, and the lower feed preheating coil; and the feed preheating coil.

[0059] Modification to achieve Figure 2E In the embodiment of the present invention, the BFW heating coil is replaced by a heat transfer fluid section. A portion of the USSH section is also replaced by a heat transfer fluid heating coil.

[0060] For each of the above embodiments, thermal calculations were performed based on rearranging a pre-existing convection layout with a convection section employing a heat transfer fluid according to embodiments herein. The results are summarized in Table 1. The radiant efficiency was improved by 3% to 10% compared to an arrangement without a heat transfer fluid heating coil and an external heat transfer fluid air preheater. The resulting CO2 emission reductions were in the range of 8% to 20%.

[0061] Table 1 - Radiative efficiency and CO2 emissions

[0062]

[0063] Although described above with respect to a single furnace and heat transfer fluid circulation system, embodiments herein contemplate fluidly connecting the heat transfer fluid circulation system to multiple furnaces. For example, embodiments herein may include a second furnace having a radiant heating section and a convection heating section, wherein the radiant heating section includes one or more burners configured to combust a fuel using a preheated air flow to produce flue gas that is fed to the convection heating section, wherein the second furnace includes an air preheater fluidly coupled to the heat transfer fluid circulation system.

[0064] The heating surface in the fired heater may be in the form of a tube or any other type of surface. Heat may be extracted from the heater or any other type of heat carrying flow reactor.

[0065] Embodiments herein may also be used to input heat rather than extract heat (e.g., to heat the flue gas to a service temperature window of a downstream heat receiving stream, including an SCR application). For example, rather than extracting heat, excess heat available in a heat transfer fluid circulation system may be used to provide heat to a heat transfer fluid cooling coil disposed upstream of an SCR catalyst bed. The flow rate of the heat transfer fluid through the cooling coil may be controlled to produce a desired flue gas temperature at the inlet of the SCR catalyst bed.

[0066] In various embodiments, external air preheating can be combined with heat transfer from other streams containing heat for heating the combustion oxidant. For example, process or utility streams within the plant can be used to preheat the combustion air upstream of the air preheater (APH) or to further heat the combustion air downstream of the air preheater (APH). Alternatively, excess heat from the heat transfer fluid can be used to heat other streams within the plant, where this additionally provided heat can help further reduce CO2 emissions for the entire plant.

[0067] A single air preheater may be used per furnace.Alternatively, the heat transfer fluid may be divided into multiple circuits, including embodiments where the combustion air is heated via a preheater unit directly coupled to each individual burner.

[0068] Either liquid phase heat transfer fluid or vapor phase heat transfer fluid may be used.

[0069] In addition to heat recovery from the convection section, the heat transfer fluid circulation loop may also include other supplemental waste heat sources. For example, other waste heat sources may be used to preheat or further heat the heat transfer fluid within the circulation loop.

[0070] The prior art arrangements are typically used to heat the combustion air directly using combustion flue gases or external waste heat. In addition, extracting heat from flue gases using the prior art arrangements will require a large amount of duct work for the flue gases or heated air or both, and add significant loads to structural steel, foundations and / or site space. In addition, extracting heat from waste heat from other sources using the prior art arrangements will not be able to adjust the flue gas temperature in the convection section of the heater. If the process raw materials change or vary, the feed preheating section may require different heat inputs. The only way to maintain the thermal efficiency of the heater is to adjust the combustion rate. In contrast, the embodiments of the present invention allow the temperature or flow rate of the hot fluid to change, so the heat transfer in the section where heat is recovered from the flue gas can change independently of any other plant system. Therefore, the embodiments of the present invention apply an intermediate heat transfer fluid to heat the combustion oxidant, which allows compact design and flexibility for various heating schemes.

[0071] The embodiments herein will have minimal impact on the heater structural steel, foundation loads and require minimal site space.In contrast, adding an air preheater to the convection section involves significant structural changes.

[0072] In addition to the advantages noted above, embodiments herein conserve floor space and provide less congestion in the heater area which could interfere with heater operation and maintenance.

[0073] The embodiments of the present invention also relate to a method for using the above system (eg FIG. 2A to FIG. 2E The method of cracking hydrocarbons may include heating a heat transfer fluid in a heat transfer fluid heating coil to produce a hot heat transfer fluid. The method also includes heating air via indirect heat exchange with the hot heat transfer fluid in an air preheater. And, the method includes mixing the preheated air with a fuel to form a combustion mixture and burning the combustion mixture in one or more burners.

[0074] For embodiments including an SCR catalyst bed, embodiments herein may further include controlling the temperature of the flue gas entering the selective catalytic reduction catalyst bed. For example, the flow rate of the heat transfer fluid in the heat transfer fluid circulation system may be used to adjust the flue gas temperature at the inlet of the SCR catalyst bed.

[0075] In addition, the embodiments of the present invention can be used to retrofit existing convection section heat recovery systems. For example, when retrofitting a cracking furnace having a steam superheater coil, a boiler feed water heating coil, and a feed preheater coil, each coil including a plurality of tubes connected to a feed header and an exhaust header, the method may include: disconnecting one or more tubes of the boiler feed water heating coil from the boiler feed water circulation system; connecting the one or more tubes fluidly to a heat transfer fluid circulation system; and connecting an air preheater fluidly to the heat transfer fluid circulation system and to the burner of the furnace. The retrofit may also include decommissioning one or more tubes of the steam superheater coil, and / or modifying the density of the tubes of the steam superheater coil. The retrofit may allow the heat transfer surface to be optimized or meet new process design requirements, including material upgrades, extended surface adjustments (fins or studs, etc.), changes in tube and fin tip diameters, changes in process flow direction, and the number of flow streams or parallel channels. Furthermore, since the existing heater may have spare tube banks in the convection section, the spare tube banks may be used to heat the heat transfer fluid, or the spare banks may be combined with a portion of the existing heating surface to form a new coil to heat the heat transfer fluid.

[0076] As used herein, coils are referred to (named) based on the material being processed, and specific coils referred to in the specification and claims herein are defined based on the function of the coil (e.g., a "boiler feed water heating" coil). A system defined by and including such coils necessarily includes a fluid connection from a feed supply system (e.g., a water tank) to an emissions treatment system (e.g., a boiler), and associated feed tanks, pumps, valves, controllers, etc. of the feed supply and emissions treatment systems.

[0077] Unless defined otherwise, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which these systems, apparatuses, methods, processes, and compositions belong.

[0078] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0079] As used herein and in the appended claims, the words "comprising," "having," and "including" and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0080] "Optionally" means that the subsequently described event or circumstance may or may not occur. The description includes instances where the event or circumstance occurs and instances where the event or circumstance does not occur.

[0081] When the word "about" or "approximately" is used, the term may mean that the value may vary by up to ±10%, up to 5%, up to 2%, up to 1%, up to 0.5%, up to 0.1%, or up to 0.01%.

[0082] Ranges can be expressed as from about one particular value to about another particular value, inclusive. When such a range is expressed, it should be understood that another embodiment is from the one particular value to the other particular value, as well as all the particular values ​​and combinations thereof within the range.

[0083] While the present disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the appended claims.

Claims

1. A furnace system for cracking hydrocarbons, comprising: a furnace having a radiant heating section and a convection heating section, wherein the radiant heating section includes one or more burners configured to combust a fuel with a preheated air stream to produce flue gas that is fed to the convection heating section; a heat transfer fluid heating coil disposed within said convection heating section of said furnace; a feed preheat coil configured to receive and preheat a hydrocarbon feed to produce a preheated hydrocarbon feed; a radiant process coil disposed within the radiant heating section and configured to receive the preheated hydrocarbon feed and produce a cracked hydrocarbon product; an air preheater having an air inlet, a preheated air outlet, a heat transfer fluid inlet, a heat transfer fluid outlet, and one or more heat exchange surfaces for indirectly heating air received via the air inlet by indirect heat exchange with hot heat transfer fluid received via the heat transfer fluid inlet to produce the preheated air stream recovered via the preheated air outlet and cooled heat transfer fluid recovered via the heat transfer fluid outlet; one or more flow lines fluidly connecting the preheated air outlet with the one or more burners for supplying the preheated air flow to the one or more burners; as well as a heat transfer fluid circulation system comprising a flow line for circulating the cooling heat transfer fluid from the heat transfer fluid outlet to the inlet of the heat transfer fluid heating coil disposed in the convection heating section of the furnace, and a flow line for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet.

2. The system of claim 1 wherein the feed preheat coil is disposed within the convection heating section of the furnace above the location of the heat transfer fluid heating coil.

3. A system according to claim 1 or claim 2, comprising a boiler feed water heating coil and one or more steam superheat coils arranged in the convection heating section of the furnace, wherein the heat transfer fluid heating coil is arranged below the location of the boiler feed water heating coil and above the location of the one or more steam superheat coils.

4. The system of claim 3, comprising a selective catalytic reduction catalyst bed disposed within the convection heating section of the furnace, and wherein the heat transfer fluid heating coil comprises an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil, wherein the selective catalytic reduction catalyst bed is disposed within the convection heating section of the furnace above the location of the lower heat transfer fluid heating coil and below the location of the upper heat transfer fluid heating coil.

5. The system of claim 1, wherein the feed preheat coil comprises an upper feed preheat coil and a lower feed preheat coil, and wherein the heat transfer fluid heating coil comprises an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil, wherein: The upper feed preheat coil is disposed in the convection heating section of the furnace above the location of the lower feed preheat coil; said upper heat transfer fluid heating coil being disposed within said convection heating section of said furnace above the location of said lower heat transfer fluid heating coil; said upper feed preheat coil being disposed within said convection heating section of said furnace above the location of said upper heat transfer fluid heating coil; and The lower feed preheat coil is disposed within the convection heating section of the furnace above the location of the lower heat transfer fluid heating coil.

6. The furnace system of claim 1, comprising coils arranged in the convection heating section of the furnace from lowest to highest in the following order: Mixing preheating coil; Lower steam superheat coil; Upper steam superheat coil; The heat transfer fluid heats the coil; Boiler feed water heating coils; and The feed preheating coil.

7. The furnace system of claim 1, comprising coils arranged in the convection heating section of the furnace from lowest to highest in the following order: Mixing preheating coil; Lower steam superheat coil; Upper steam superheat coil; The heat transfer fluid heating coil comprises a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, wherein a selective catalytic reduction catalyst bed is disposed between the lower heat transfer fluid heating coil and the upper heat transfer fluid heating coil; Boiler feed water heating coils; and The feed preheating coil.

8. The furnace system of claim 1, comprising coils arranged in the convection heating section of the furnace from lowest to highest in the following order: Mixing preheating coil; Lower steam superheat coil; Intermediate steam superheat coil; Upper steam superheat coil; The heat transfer fluid heats the coil; Boiler feed water heating coils; and The feed preheating coil.

9. The furnace system of claim 1, comprising coils arranged in the convection heating section of the furnace from lowest to highest in the following order: Lower mixing preheating coil; Lower steam superheat coil; Intermediate steam superheat coil; Upper steam superheat coil; The heat transfer fluid heating coil comprises a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, wherein between the lower heat transfer fluid heating coil and the upper heat transfer fluid heating coil are, from lowest to highest, an upper mixed feed preheating coil, a dilution steam superheating coil, and a lower feed preheating coil; and The feed preheating coil.

10. The furnace system of claim 1, comprising coils arranged in the convection heating section of the furnace in the following order from lowest to highest: Mixed feed preheating coil; Lower steam superheat coil; Intermediate steam superheat coil; Upper steam superheat coil; The heat transfer fluid heats the coil; and Boiler feed water heating coils.

11. The furnace system of claim 10, comprising a chimney connected to the gas turbine exhaust, the chimney comprising a feed preheat coil and a second mixed feed preheat coil fluidly connected to the mixed feed preheat coil.

12. The furnace system of any one of claims 1 to 11, comprising a second furnace having a radiant heating section and a convection heating section, wherein the radiant heating section comprises one or more burners configured to combust a fuel with a preheated air flow to produce flue gas that is fed to the convection heating section, wherein the second furnace comprises an air preheater fluidly coupled to the heat transfer fluid circulation system.

13. A method for cracking hydrocarbons using the system according to any one of claims 1 to 11, comprising: heating a heat transfer fluid in the heat transfer fluid heating coil to produce the hot heat transfer fluid; heating air via indirect heat exchange with the hot heat transfer fluid in the air preheater to produce preheated air; as well as The preheated air is mixed with fuel to form a combustion mixture and the combustion mixture is combusted in the one or more burners.

14. A method of cracking hydrocarbons using the system of claim 4, comprising: heating a heat transfer fluid in the heat transfer fluid heating coil to produce the hot heat transfer fluid; heating air via indirect heat exchange with the hot heat transfer fluid in the air preheater to produce preheated air; mixing the preheated air with a fuel to form a combustion mixture and combusting the combustion mixture in the one or more burners; as well as The temperature of the flue gas entering the selective catalytic reduction catalyst bed is controlled.

15. A method according to claim 14, comprising mixing a portion of the flue gas with an air feed to provide a dilution air feed which is fed when the air is heated in the air preheater.

16. A method of retrofitting a cracking furnace having a steam superheat coil, a boiler feedwater heating coil, and a feed preheat coil, each coil comprising a plurality of tubes connected to a feed header and a effluent header, the method comprising: disconnecting one or more tubes of the boiler feed water heating coil from the boiler feed water circulation system; fluidly connecting the one or more tubes to a heat transfer fluid circulation system; as well as An air preheater is fluidly connected to the heat transfer fluid circulation system and to the burners of the cracking furnace.

17. The method of claim 16, comprising decommissioning one or more tubes of the steam superheat coil.

18. A method according to claim 16 or claim 17, comprising modifying the fin density of the tubes of the steam superheat coil.

19. A heater system comprising: a furnace having a radiant heating section and a convection heating section, wherein the radiant heating section includes one or more burners configured to combust a fuel using a preheated air flow; a heat transfer fluid heating coil disposed within said convection heating section of said furnace; an air preheater having an air inlet, a preheated air outlet, a heat transfer fluid inlet, a heat transfer fluid outlet, and one or more heat exchange surfaces for indirectly heating air received via the air inlet via indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet to produce the preheated air stream recovered via the preheated air outlet and a cooled heat transfer fluid recovered via the heat transfer fluid outlet; one or more flow lines fluidly connecting the preheated air outlet with the one or more burners for supplying the preheated air flow to the one or more burners; as well as a heat transfer fluid circulation system comprising a flow line for circulating the cooling heat transfer fluid from the heat transfer fluid outlet to the inlet of the heat transfer fluid heating coil disposed in the convection heating section of the furnace, and a flow line for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet.

20. The heater system of claim 19, wherein the heat transfer fluid heating coil is configured as a plurality of heating coil sections, the plurality of heating coil sections being separated by one or more additional heat recovery coils, the one or more additional heat recovery coils being configured to heat one or more of hydrocarbon feed, water, steam, or a mixture thereof.

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