External combustion air preheating

Abstract

A system and process for external combustion air preheating to supply preheated combustion air to a furnace. The convection heating section of the furnace system includes multiple heating coils for waste heat recovery. The heating coils may be used to preheat the feed (feed preheat coils), heat boiler feed water, superheat steam, or heat or superheat a feed stream before the feed is supplied to the radiant coils. Waste heat in the combustion gases is also used to heat a heat transfer fluid, which may be used to preheat the combustion air or for other purposes within the plant.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments of the present disclosure relate generally to fired tubular furnaces. More specifically, embodiments herein relate to pyrolysis furnaces for the thermal cracking of hydrocarbons. Even more specifically, embodiments herein relate to an arrangement for efficiently preheating air and enabling reduced carbon dioxide emissions from fired tubular furnaces. [Background technology]

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

[0003] WO2021052642 (Wijnja and Oud) also describes a three-stage feed preheating scheme, in which a combustion air preheater is placed downstream of the convection section, preferably downstream of all other convection section banks. Heat for the pyrolysis reaction is provided by "high nitrogen depleted oxygen" in the presence of fuel gas, combustion air, and optionally recirculated exhaust gas. In the limit where all nitrogen is replaced by recirculated exhaust gas, the system can be considered as oxygen-fired combustion, with the advantage that the CO2 in the exhaust gas is highly concentrated, suitable for the CO2 capture process, as well as eliminating the problem of NOx formation. However, the oxygen provided must be separated from the atmosphere, introducing significant capital costs and the energy required for separation must be factored into the operating costs.

[0004] Albano et al (1992) describe the preheating of combustion air using hot exhaust gas from a gas turbine (GT) with a low oxygen content (15-17 mole % compared to about 21 mole % for air). Co-production of electricity (30-60 MW) is possible, greatly impacting plant economics. The use of turbine exhaust gas (TEG) for high levels of air preheating reduces the fuel requirements of the furnace. However, unlike air preheating systems, the total mass flow rate of the exhaust gas through the furnace is increased due to the low oxygen content of the TEG. This results in increased steam production in the convection (heat recovery) section of the pyrolysis module.

[0005] US 4,617,109 (Wells and Petterson) describes an air preheating system in which the combustion air is preheated by indirect heat exchange with high pressure steam produced in the hottest section of the furnace and expanded through a steam turbine. As in the previous example, the radiant efficiency is increased, but the combustion air does not directly exchange heat with the exhaust gases in the convection section.

[0006] US4044820 (Nobles) teaches the use of a heat transfer fluid to recover heat from the exhaust gas to the combustion air stream via a continuously flowing heat transfer fluid with high heat capacity (product of mass flow rate and specific heat capacity). The proposed method includes a two-chamber system in which a first chamber (for exhaust gas cooling) is separated from a second chamber (for air heating) by a common wall. The high circulation rate of the heat transfer fluid ensures that the temperature of the heat transfer surface remains above the acid dew point of the exhaust gas, thereby preventing condensation and the associated risk of corrosion.

[0007] US3623549 (Smith) presents a liquid-gas heat exchanger capable of transferring heat from a relatively hot gas at one location to a relatively cold gas at a separate location. Multiple heat transfer fluids with different phase change properties are used to cover a wider temperature range than would be possible by using a single heat transfer fluid such as water. Such a system is presented as an alternative to recuperative heat exchangers, where hot gas is separated from cold gas by a heat transfer surface, or to regenerative heat exchangers, where hot and cold gases are cyclically circulated through a thermal mass.

[0008] US20170363288 (Lee and Sadler) describes a combustion furnace system with a heat pipe preheater. A heat pipe is a sealed tube containing a working fluid that transfers heat from a hot heat source to a cooler sink. The heat pipe is used to transfer heat from hot exhaust gases to the combustion air without the need to introduce combustion air into the stack. Instead, a separate air preheater duct is attached to the stack and the heat pipe penetrates the stack wall. This arrangement avoids the need to incorporate an air preheater in the convection section of the furnace, but does not address the structural costs associated with mounting the device in the stack. The working fluid must be carefully selected as several constraints must be simultaneously met for the heat pipe system to function properly, such as viscosity, sonic, capillary, entrainment, and nucleate boiling levels. This system does not allow flexibility in the location of the air preheater other than that it is outside the stack.

[0009] In many industrial applications, heat transfer fluids (HTFs) are used to provide the required amount of heat for multiple heat demands from a single heat source. For example, it is known to use biomass heat sources and heat transfer fluids as an alternative to steam generation to provide high temperature hot utilities. Proper integration of combustion furnaces with the process leads to an overall reduction in capital investments as well as energy requirements. However, in large specialty furnaces (e.g., steam crackers producing bulk chemicals such as ethylene, propylene, butylenes, and aromatics), non-process heat recovery is achieved via the production of steam, which is then used to drive compressors and other processes in downstream separations. Thus, in effect, the energy for downstream processing is obtained by burning fuel in the steam cracker. Summary of the Invention

[0010] In one aspect, embodiments herein are directed to a furnace system for cracking hydrocarbons. The furnace system includes a furnace having a radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated airflow to generate an exhaust gas that is fed to the convective heating section. A heat transfer fluid heating coil is disposed within the convective heating section of the furnace. The furnace system also includes a feed preheat coil configured to receive and preheat a hydrocarbon feed to generate a preheated hydrocarbon feed. A radiant process coil is disposed within the radiant heating section and configured to receive the preheated hydrocarbon feed and generate 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 through the air inlet by indirect heat exchange with a hot heat transfer fluid received through the heat transfer fluid inlet to produce a preheated air stream that is recovered through the preheated air outlet and a cooled heat transfer fluid that is recovered through the heat transfer fluid outlet. One or more flow lines fluidly connect the preheated air outlet to the one or more burners to supply the preheated air stream to the one or more burners. Also included is a heat transfer fluid circulation system having flow lines for circulating the cooled heat transfer fluid from the heat transfer fluid outlet to an inlet of a heat transfer fluid heating coil disposed within the convection heating section of the furnace, and flow lines for circulating the hot heat transfer fluid from an 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 such embodiments, the heat transfer fluid heating coil may include an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil, and 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 herein are directed to a method of cracking hydrocarbons using the aforementioned system, the method including heating a heat transfer fluid in a heat transfer fluid heating coil to produce a hot heat transfer fluid, heating air via indirect heat exchange with the hot heat transfer fluid in an air preheater to produce preheated air, mixing the preheated air with a fuel to form a combustion mixture, and combusting the combustion mixture in 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 exhaust gas entering the selective catalytic reduction catalyst bed.

[0014] In a further aspect, embodiments herein are directed to a method of retrofitting a cracking furnace having a steam superheat coil, a boiler feedwater heating coil, and a feed preheat coil, each coil including a plurality of tubes connected to a feed header and a discharge header. The retrofitting method includes disconnecting one or more tubes of the boiler feedwater heating coil from a boiler feedwater 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 herein are directed to a furnace system. The furnace system includes a furnace having a radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated air flow. A heat transfer fluid heating coil is disposed in the convective heating section of the furnace. The furnace 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 for indirectly heating air received via the air inlet through indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet to generate a preheated air flow recovered via the preheated air outlet and a cooled heat transfer fluid recovered via the heat transfer fluid outlet. One or more streamlines fluidly connect the preheated air outlet to the one or more burners to supply the preheated air flow to the one or more burners. The furnace system further includes a heat transfer fluid circulation system including flow lines for circulating cooled heat transfer fluid from a heat transfer fluid outlet to an inlet of a heat transfer fluid heating coil disposed within the convection heating section of the furnace, and flow lines for circulating 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 multiple heating coil sections separated by one or more additional heat recovery coils configured to heat one or more of the hydrocarbon feed, water, steam, or mixtures thereof. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 illustrates a prior art method for preheating combustion air. [Figure 1A] FIG. 1 illustrates a prior art method for preheating combustion air.

[0017] [Diagram 2] FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. [Figure 2A]FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. [Figure 2B] FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. [Figure 2C] FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. [Figure 2D] FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. [Figure 2E] FIG. 1 illustrates a system for heat recovery and air preheating according to an embodiment herein utilizing heat recovery of an intermediate heat transfer fluid. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Prior implementations, as mentioned above, often involve expensive modifications to the convection section aimed at increasing the radiative efficiency of the firebox, thereby reducing fuel consumption by increasing the amount of heat used for process heating (of the feed gas or oxidant) at the expense of heat used for non-process heating (such as steam generation).

[0019] As in Oud or Oud and Wijnja, the heat exchange between the exhaust gas and the oxidant takes place downstream of the convection section (ie, heat is exchanged with the coldest exhaust gas).

[0020] Heating the air by indirect heat exchange with the expanding steam may be done 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] The use of heat pipe heat exchangers, multiple flow loops, or two chamber systems have been implemented to recover heat from the exhaust gas without introducing air into the convection section, but these methods require a significant amount of structural modification (in the case of a retrofit) or expensive convection section designs to be installed in the furnace convection section.

[0022] In many industrial applications, heat transfer fluids (HTF) are used to provide the required amount of heat for multiple heat demands from a single heat source. For example, it is known to use a biomass heat source and use the heat transfer fluid as an alternative to steam generation to provide high temperature hot utilities. Proper integration of a combustion furnace with the process leads to an overall reduction in capital investment along with energy requirements. This is particularly relevant for the global ethylene industry because of the exceptionally large installed base. Applying prior art methods to this large installed base is impractical or prohibitively expensive due to structural steel and foundation load limitations. A simple and inexpensive method to reduce CO2 emissions that can be rapidly applied to the installed base of ethylene steam cracking furnaces is needed.

[0023] However, in large specialty furnaces (e.g., steam crackers producing bulk chemicals such as ethylene, propylene, butylenes, and aromatics), non-process heat recovery occurs through the production of steam, which is then used to drive compressors and other processes in the downstream separation section. Thus, in effect, energy for downstream processing is obtained by burning fuel in the steam cracker.

[0024] The embodiments herein are directed to an alternative heat recovery method that does not require the conversion of chemical energy of fuel to thermal and work energy via steam generation (and associated emissions) and can significantly reduce fuel consumption of existing furnaces by at least 5% and up to 30%. The embodiments as described herein are also inexpensive and do not require significant additional structural changes to the convection section. Furthermore, the heat recovery of the embodiments herein maintains a desired thermal profile in the convection section, is highly efficient, and allows flexibility in feed preheating. Finally, such heat recovery methods can be applied to existing furnaces without significant changes to heating surfaces, structure, and foundation loads. The embodiments herein meet one or more of these objectives, and in some embodiments, all of these objectives.

[0025] SUMMARY OF THE DISCLOSURE Embodiments herein are directed to furnace systems used in the processing of hydrocarbons, and more particularly, to the placement of heat recovery coils within furnace systems and the convection section of such furnaces.

[0026] Furnace systems according to embodiments herein include a radiant section and a convective heating section. The radiant section, also referred to as the radiant zone or the combustion furnace box, includes one or more burners (e.g., floor and / or wall burners for combusting a fuel and generating radiant heat and combustion gases). The combustion gases proceed from the radiant section to the convective heating section before being exhausted to the atmosphere as exhaust gases. One or more radiant process coils are disposed within the radiant section and are heated by radiant heat from the burners, resulting in heating and / or transformation of the fluid being treated.

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

[0028] The embodiments herein allow for heat extraction from the flue gas to the heat exchange medium (heat transfer fluid) to occur in any section of the furnace as needed. In some embodiments, heat is extracted from multiple sections. In contrast, in the prior art, heat extraction for air preheating is typically limited to the coldest part of the convection section. When embodiments herein extract heat from multiple sections to the heat transfer fluid, the sections can be in series, or in parallel, or a combination of series and parallel.

[0029] Heat extraction may be performed according to embodiments herein using a heat transfer fluid, which may be one or different types of fluid for various operating temperatures, and the heat extracted from the furnace using the heat transfer fluid may be used to warm a combustion oxidant or other process fluid(s).

[0030] In embodiments herein, heat extraction by a heat transfer fluid may be used to adjust the exhaust gas temperature profile in the convection section. The ability to adjust the exhaust gas temperature profile may result in improved furnace process performance and emission reduction, such as by controlling the exhaust gas temperature in a selective catalytic reduction (SCR) catalyst bed for optimal NOx reduction.

[0031] Heat extracted from a selected furnace (e.g., to heat combustion air or other fluids) can be indirectly returned to the same furnace or other heat transfer device. Additionally, embodiments herein may provide the option to dilute the pre-heated oxidant with exhaust gas (e.g., external exhaust gas recirculation to the burner for NOx reduction).

[0032] A prior art method for preheating combustion air is shown in Figures 1 and 1A. For simplicity, the flow lines as described are not shown. The hydrocarbon feed is preheated in the feed preheat coil FP in the convection section, mixed with dilution steam and optionally further preheated in an external heat exchanger (STLE, not shown) against the quenched cracked gas products. The mixture of hydrocarbon plus dilution steam is further preheated in the mix preheat coil MP against the hot combustion gases in the convection section and fed to the radiant box. In the radiant box, the energy required for pyrolyzing the hydrocarbons to produce the cracked gas products is provided by burning a fuel in the oxidant stream to produce hot exhaust gas 5. Steam is generated between the two feed preheat sections FP, MP in the convection section. This is done 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), producing steam by quenching the hot cracked gases leaving the radiant section, and further heating the high pressure steam in the convection section to produce superheated steam in the upper steam superheater USSH and lower steam superheater LSSH. In the coldest part of the convection section, an air preheater (APH) is introduced. The air preheater indirectly transfers heat from the exhaust gas to the combustion air 10 before the resulting hot air 12 is mixed with fuel in the burner. The addition of an APH, whether in the same convection section as shown in Figure 1 or in a separate heat recovery zone as shown in Figure 1A, can increase the radiant efficiency while keeping the overall thermal efficiency high.

[0033] Radiation efficiency is the amount of heat transferred in the radiant section divided by the total energy delivered. The heat absorbed by the radiant coil plus the heat loss in the radiant section is equal to the heat donated by the exhaust gases from the adiabatic flame temperature to the temperature leaving the radiant section. Radiation efficiency is affected by the radiant heat transfer to the convection section, which is dependent on the burner position, because the higher the burner is placed in the firebox, the more heat it will radiate to the convection section that is not transferred to the reaction in the firebox.

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

[0035] Embodiments herein relate to a furnace system such as that used to crack hydrocarbons. Such a furnace includes a radiant heating section 50 and a convective heating section 52, as shown in FIG. 2. The radiant heating section includes one or more burners 54 configured to combust a fuel 56 with the preheated air flow 12 to generate exhaust gases 60 that are fed to the convective heating section. A radiant process coil 62 is disposed within the radiant heating section and receives the preheated process fluid (such as a hydrocarbon) for further heating and cracking of the hydrocarbons.

[0036] Embodiments herein are further directed to the arrangement of heating coils (shown in FIGS. 2A-2E and described further below) disposed within the convection heating section. Embodiments herein include: a heat transfer fluid heating coil 22 disposed within the convection heating section 52 of the furnace; one or more steam heating coils (see FIGS. 2A-2E); one or more hydrocarbon feed preheat coils (see FIGS. 2A-2E) configured to receive and preheat a hydrocarbon feed and produce a preheated hydrocarbon feed. A radiant process coil 62 is disposed within the radiant heating section and 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). The air preheater 32 is configured to indirectly heat air 10 received via the air inlet 70 by indirect heat exchange with a hot heat transfer fluid received via the heat transfer fluid inlet 74 to generate a preheated air stream 12. The preheated air stream 12 is collected via the preheated air outlet 72, and the cooled heat transfer fluid is collected via the heat transfer fluid outlet 76. One or more flow lines (not shown) fluidly connect the preheated air outlet to one or more burners to supply the preheated air stream 12 to the one or more burners. Additionally, the heat transfer fluid circulation system 24 includes a flow line 28 for circulating cooled heat transfer fluid from a heat transfer fluid outlet 76 to an inlet of the heat transfer fluid heating coil 22 disposed within the convection heating section 52 of the furnace, and a flow line 30 for circulating 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] 2A-2E show systems for preheating combustion air with heat recovery of an intermediate heat transfer fluid, according to embodiments herein. Common features of these embodiments include: (i) a circulating heat transfer fluid, (ii) heat recovery section(s) 22 (heat transfer fluid heating coil(s)) 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 for use at high temperatures up to 400°C can be classified as synthetic, high temperature oils, and inorganic (including silicones). Synthetic heat transfer fluids suitable for processes requiring high temperatures in the liquid state are usually eutectic mixtures of biphenyl and diphenyl oxide, commonly referred to as biphenyl or HTF (heat transfer fluids). This fluid can be found, for example, commercially available under the trade names DOWTHERM- and THERMINOL VP-1. Such fluids can be used 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 offered under trade names such as SYLTHERM 800 are also suitable.

[0040] For systems utilizing high temperature process waste heat, the HTF furnace represents the highest local fluid film temperature as well as the highest fluid bulk temperature. HTF velocity over the heat transfer surface should be maximized to reduce skin temperatures. Dead flow zones with long residence times should be avoided. For this reason, HTF contained within tubes configurations are preferred, with heat transfer fluid velocities in the range of 2-3 m / s or higher.

[0041] A 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. In Figures 2A-2E, the external heat exchanger is an air preheater used to preheat the oxidant mixture (such as air) used to combust the fuel in the radiant firebox. The heat transfer loop may be unpressurized as long as the operating temperature is below the normal boiling point of the heat transfer fluid. A pressurized liquid phase system may be required where an inert gas (usually N2) is added through an expansion tank if the operating temperature of the heat transfer fluid is above its normal boiling range to maintain the heat transfer fluid as a liquid.

[0042] Pressurized gas phase systems for synthetic oxygenated diphenyl / biphenyl type fluids are also possible. Gas phase systems can be designed to gravity return the condensate from the heat sink (air preheater) to the vaporizer (convection coil) using pumps or hydrostatic pressure. More complex systems require flash tanks, condensate recovery tanks, and condensate recovery pumps. The added capital equipment cost and complexity of a gas phase system is offset by the increased heat delivery per unit mass of heat transfer fluid.

[0043] The air preheater can be one heat exchanger per furnace, or multiple heat exchangers per furnace, or one preheater for multiple furnaces, the variations depending on the size and capacity of one unit. The air preheater can be a plate or tubular type heat exchanger and can include extended heating surfaces (e.g., studs or fins, etc.). Since the heat transfer coefficient of low pressure air is much lower compared to the heat transfer fluid, a suitable design would be such that the heat transfer fluid flows inside the tube and the air flows outside the tube. Extended surfaces in the form of heat transfer fins can be provided on the outside of the tube to increase the heat transfer area and compensate for the much lower heat transfer coefficient on the air side. The tubes can be round or elliptical steel tubes. The fins can be applied to the tubes in the form of spiral or helical fins that are wound continuously around each individual tube, or they can be edge wound, bonded to the tube by tension, L-legged, or embedded in grooves on the tube surface. Alternatively, many tubes can be incorporated into a plate fin structure by inserting the tubes into the plate fin pack and mechanically expanding the tubes within the plate pack. In any of the above designs, the air cross-flows against the heat transfer fluid and the heat transfer tube side may have multiple passes.

[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 arranged as shown in Figures 2A-2E. Such additional heating coils may include steam superheat coils (SSH). Other heating coils may include feed preheat coils (FP) used to heat the hydrocarbon feed, mixed feed preheat coils (MP) used to mix the hydrocarbon and steam mixture, as well as boiler feed water (BFW) heating coils. The relative location of a particular type of coil within the convection section is designated by upper (U), middle (M), or lower (L), e.g., a USSH is an upper steam superheat coil.

[0045] Various embodiments herein include a feed preheat coil (FP) located in the convection heating section of the furnace above the location of the heat transfer fluid heating coils 22. As one skilled in the art will appreciate, the temperature of the exhaust gas at a particular coil location decreases the further away from the exhaust gas outlet of the radiant heating section due in part to heat extraction through the multiple heating coils. Thus, the above may be rephrased as the feed preheat coil is at a lower exhaust gas temperature than is the case with the heat transfer fluid heating coils.

[0046] Further, embodiments herein include a boiler feedwater heating coil and one or more steam superheat coils disposed within the convection heating section of the furnace. The 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 multiple 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, and 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, the external feed and / or mixed feed preheat section may be external to the convective heat transfer section.

[0049] The various configurations outlined above are specifically illustrated and described with respect to Figures 2A-2E below.

[0050] 2A shows one embodiment for recovering heat from exhaust gas according to embodiments herein. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest exhaust gas temperature, closest to the combustion box) to highest (lowest exhaust gas temperature, furthest from the exit of the combustion box): mixed 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 previously described, the heat transfer fluid heating coil 22 receives cooled heat transfer fluid from the circulation pump 26 and heats the heat transfer fluid, which circulates via flow lines 30 to the air preheat coil APH. The air preheat coil APH may include one or more finned tubes or other heat transfer surfaces as previously described.

[0051] In an embodiment of retrofitting a heat recovery system such as that shown in Figure 1, to arrive at a heat recovery system such as that shown in Figure 2A, the heat recovery system is modified so that some of the tubing previously used to preheat the BFW is reused to heat the heat transfer fluid instead. In some embodiments, the upper steam superheat coil is also modified to account for the lower amount of steam produced. Reducing steam production is an objective of some embodiments herein.

[0052] 2B shows an embodiment for recovering heat from exhaust gas according to embodiments herein. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest exhaust gas temperature, closest to the combustion box) to highest (lowest exhaust gas temperature, furthest from the exit of the combustion box): mixed preheat coil (MP), lower steam superheat coil (LSSH), upper steam superheat coil (USSH), heat transfer fluid heating coil 22 (including lower heat transfer fluid heating coil and upper heat transfer fluid heating coil, with selective catalytic reduction catalyst bed (SCR) in between), boiler feed water heating coil (BFW), and feed preheat coil (FP). As previously described, the heat transfer fluid heating coil 22 receives cooled heat transfer fluid from the circulation pump 26 and heats the heat transfer fluid, which circulates via flow lines 30 to the air preheat coil APH, which may include one or more finned tubes or other heat transfer surfaces as previously described.

[0053] In Figure 2B, the heat recovery section is modified as in Figure 2A, but the BFW coils above and below the NOx removal catalyst bed are both reused to heat the heat transfer fluid, which is now heated in series in the two sections.

[0054] 2C illustrates one embodiment for recovering heat from the exhaust gas according to embodiments herein. The coils contained within the convection heating section of the furnace are arranged in the following order from lowest (highest exhaust gas temperature, closest to the combustion box) to highest (lowest exhaust gas temperature, furthest from the exit of the combustion box): mixed 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 feedwater heating coil (BFW), and feed preheat coil (FP).

[0055] Figure 2C shows a similar arrangement to Figure 2A, except that instead of eliminating some of the tubes in the USSH coil, the fin density is altered to allow for reduced steam production.

[0056] FIG. 2D illustrates one embodiment for recovering heat from exhaust gas according to embodiments herein. The coils included in the convection heating section of the furnace are arranged in the following order from lowest (highest exhaust gas temperature, closest to the combustion box) to highest (lowest exhaust gas temperature, furthest from the combustion box exit): mixed feed preheat coil, lower steam superheat coil, intermediate steam superheat coil, upper steam superheat coil, heat transfer fluid heating coil, and boiler feed water heating coil. The furnace system of FIG. 2D further includes an exhaust stack 40 connected to the gas turbine exhaust (not shown). The exhaust stack 40 includes a feed preheat coil (FP) and a second mixed feed preheat coil (SMP), the second mixed feed preheat coil being fluidly connected to the mixed feed preheat coil (MP).

[0057] In retrofitting to arrive at the arrangement of Figure 2D, the feed preheat (FP) coil and a portion of the USSH and LSSH coils 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 further used for feed (FP) preheating and partial preheating of the mixed feed (MP), duties previously performed in the convection section. By moving the feed preheating and mixed feed preheating to a new section with GTE heating, it is possible to maximize the heat recovery from the GTE and increase the duty available for the heat transfer fluid recovery section and, consequently, the air preheating section.

[0058] 2E illustrates another embodiment for recovering heat from exhaust gas according to embodiments herein. The coils included within the convection heating section of the furnace are arranged in the following order from lowest (highest exhaust gas temperature, closest to the burn box) to highest (lowest exhaust gas temperature, furthest from the exit of the burn box): lower mixed preheat coil, lower steam superheat coil, intermediate steam superheat coil, upper steam superheat coil, heat transfer fluid heating coil (including lower heat transfer fluid heating coil and upper heat transfer fluid heating coil, between which are, from lowest to highest, upper mixed feed preheat coil, dilution steam superheat coil, and lower feed preheat coil), feed preheat coil.

[0059] In modifying to arrive at the embodiment of Figure 2E, the BFW heating coil is replaced with a heat transfer fluid section, and part of the USSH section is replaced with a heat transfer fluid heating coil.

[0060] For each of the previously described embodiments, thermal calculations were performed based on rearranging an existing convection layout with a convection section using heat transfer fluid according to an embodiment herein. The results are summarized in Table 1. The improvement in radiative efficiency was in the range of 3-10% compared to an arrangement without the heat transfer fluid heating coil and external heat transfer fluid air preheater. The resulting reduction in CO2 emissions was in the range of 8-20%. [Table 1]

[0061] While 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 furnace having a second radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated air flow to generate exhaust gases that are fed to the convective heating section, and the second furnace including an air preheater fluidly connected to the heat transfer fluid circulation system.

[0062] The heating surface in a combustion furnace can be tubular or any other type of surface shape. Heat extraction can be from the furnace or any other type of reactor of the heat carrying stream.

[0063] Embodiments herein can also be used to input heat instead of extracting heat (e.g., warming exhaust gas to a service temperature window for a downstream heat receiving stream, including SCR applications). For example, instead of extracting heat, excess heat available in a heat transfer fluid circulation system may be used to supply heat to a heat transfer fluid cooling coil located upstream of the SCR catalyst bed. The flow rate of heat transfer fluid through the cooling coil may be controlled to provide a desired exhaust gas temperature at the inlet to the SCR catalyst bed.

[0064] In various embodiments, the preheating of the external air can be combined with heat transfer from other streams that contain heat for warming the combustion oxidant. For example, process or utility streams within the plant may 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 may be used to heat other streams within the plant, and such additional heat may help to further reduce CO2 emissions from the overall plant.

[0065] One air preheater may be used per furnace. Alternatively, the heat transfer fluid may be split into multiple circuits, including embodiments in which the combustion air is heated through a preheater unit directly coupled to each individual burner.

[0066] Heat transfer fluids in either the liquid or vapor phase may be used.

[0067] The heat transfer fluid circulation loop may include other supplemental waste heat sources in addition to heat recovery from the convection section, for example, other waste heat sources may be used to preheat or further heat the heat transfer fluid in the circulation loop.

[0068] Prior art arrangements generally use flue gas or external waste heat to directly heat the combustion air. Furthermore, prior art arrangements extract heat from the flue gas, requiring large ductwork for either the flue gas or the heated air, or both, adding significant loads to structural steel, foundations, and / or compartment space. Furthermore, prior art arrangements extract heat from waste heat of other heat sources, without the ability to adjust the flue gas temperature in the furnace convection section. If the process feedstock is changed or altered, a different heat input may be required in the feed preheat section. The only way to maintain the thermal efficiency of the furnace would be to adjust the firing rate. In contrast, embodiments herein allow for varying the temperature or flow rate of the heat fluid, thereby allowing for varying the heat transfer in the flue gas heat recovery section independent of any other plant systems. Thus, embodiments herein apply an intermediate heat transfer fluid to warm the combustion oxidant, allowing for a compact design and flexibility in various heating schemes.

[0069] The embodiments herein have minimal impact on furnace structural steel, foundation loads, and require minimal bay space. In contrast, adding an air preheater to the convection section involves significant structural modifications.

[0070] In addition to the advantages identified above, the embodiments herein conserve compartment space and reduce congestion in the furnace area that can interfere with furnace operation and maintenance.

[0071] Embodiments herein also relate to a method of cracking hydrocarbons using the aforementioned system, such as that shown in one or more of Figures 2A-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 combusting the combustion mixture in one or more burners.

[0072] For embodiments including an SCR catalyst bed, embodiments herein may further include controlling the temperature of the exhaust 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 exhaust gas temperature at the inlet of the SCR catalyst bed.

[0073] Additionally, embodiments herein may be used to retrofit existing convection heat recovery systems. For example, when retrofitting a cracking furnace having a steam superheat coil, a boiler feedwater heating coil, and a feed preheat 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 feedwater heating coil from the boiler feedwater circulation system, fluidly connecting the one or more tubes to the heat transfer fluid circulation system, and fluidly connecting an air preheater to the heat transfer fluid circulation system and to a burner of the furnace. Retrofitting may also include decommissioning one or more tubes of the steam superheat coil and / or changing the density of the tubes of the steam superheat coil. Retrofitting may allow optimization of heat transfer surfaces or meet new process design requirements (e.g., material upgrades, adjustments of extended surfaces (such as fins or studs), changes in the circular diameter of the tubes and fin tips, changes in process flow direction, and number of flow streams or parallel paths). Additionally, existing furnaces may have spare rows of tubes in the convection section, so the spare rows may be used to heat the heat transfer fluid, or the spare rows may be combined with a portion of the existing heating surface to form a new coil to heat the heat transfer fluid.

[0074] As used herein, coils are referenced (named) based on the material being treated, and specific coils referred to in this specification and claims herein are defined based on the coil function (e.g., a "boiler feedwater heating" coil). A system defined by and including such a coil necessarily includes fluid connections from a feed supply system (e.g., a water tank) and to an effluent treatment system (e.g., a boiler), along with associated tanks, pumps, valves, controls, etc. of the feed supply system and effluent treatment system.

[0075] Unless otherwise defined, 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, devices, methods, processes, and compositions belong.

[0076] The singular forms "a," "an," and "the" include plural referents unless the context clearly indicates otherwise.

[0077] As used in this specification and the appended claims, the terms "comprise," "has," and "include," and all grammatical variations thereof, are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0078] "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 it does not occur.

[0079] When the term "approximately" or "about" is used, the term can mean that there can be a variation in value of 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%.

[0080] Ranges may be expressed as from about one particular value to about another particular value, including the endpoints. When such a range is expressed, it is understood that another embodiment is from the one particular value to the other particular value, and also includes all particular values ​​within the range, and combinations thereof.

[0081] While the embodiments contained in this disclosure are limited in number, those skilled in the art having the benefit of this disclosure will appreciate that other embodiments may be devised that do not depart from the scope of the present disclosure, which should therefore be limited only by the scope of the appended claims.

Claims

1. 1. A furnace system for cracking hydrocarbons, comprising: a furnace having a radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated air flow to generate exhaust gases that are fed to the convective heating section; a heat transfer fluid heating coil disposed within the convection heating section of the 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, the radiant process coil configured to receive the preheated hydrocarbon feed and produce cracked hydrocarbon products; 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 through the air inlet by indirect heat exchange with hot heat transfer fluid received through the heat transfer fluid inlet to produce a preheated air stream that is recovered through the preheated air outlet and a cooled heat transfer fluid that is recovered through 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; a heat transfer fluid circulation system including flow lines for circulating the cooled heat transfer fluid from the heat transfer fluid outlet to an inlet of the heat transfer fluid heating coil disposed within the convection heating section of the furnace, and flow lines for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet.

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

3. 3. The system of claim 1 or claim 2, comprising a boiler feedwater heating coil and one or more steam superheat coils disposed within the convection heating section of the furnace, the heat transfer fluid heating coil being disposed below the position of the boiler feedwater heating coil and above the position of the one or more steam superheat coils.

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

5. the feed preheat coils include an upper feed preheat coil and a lower feed preheat coil, and the heat transfer fluid heating coils include an upper heat transfer fluid heating coil and a lower heat transfer fluid heating coil; the upper feed preheat coil is positioned within the convection heating section of the furnace above the location of the lower feed preheat coil; the upper heat transfer fluid heating coil is positioned within the convection heating section of the furnace above the location of the lower heat transfer fluid heating coil; the upper feed preheat coil is positioned within the convection heating section of the furnace above the location of the upper heat transfer fluid heating coil; 2. The system of claim 1, wherein the lower feed preheat coil is positioned within the convection heating section of the furnace above the location of the lower heat transfer fluid heating coil.

6. Within the convection heating section of the furnace, from lowest to highest, Mixture preheat coil, Lower steam superheat coil, Upper steam superheat coil, the heat transfer fluid heating coil; Boiler feedwater heating coil, 2. The furnace system of claim 1, further comprising coils arranged in the order:

7. Within the convection heating section of the furnace, from lowest to highest, Mixture preheat coil, Lower steam superheat coil, Upper steam superheat coil, the heat transfer fluid heating coils including a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, between which is a selective catalytic reduction catalyst bed; Boiler feedwater heating coil, 2. The furnace system of claim 1, further comprising coils arranged in the order:

8. Within the convection heating section of the furnace, from lowest to highest, Mixture preheat coil, Lower steam superheat coil, Intermediate steam superheat coil, Upper steam superheat coil, the heat transfer fluid heating coil; Boiler feedwater heating coil, 2. The furnace system of claim 1, further comprising coils arranged in the order:

9. Within the convection heating section of the furnace, from lowest to highest, Lower mix preheat coil, Lower steam superheat coil, Intermediate steam superheat coil, Upper steam superheat coil, the heat transfer fluid heating coils including a lower heat transfer fluid heating coil and an upper heat transfer fluid heating coil, intermediate therebetween, from lowest to highest, an upper mixed feed preheat coil, a dilution steam superheat coil, and a lower feed preheat coil; 2. The furnace system of claim 1, further comprising coils arranged in the order:

10. Within the convection heating section of the furnace, from lowest to highest, mixed feed preheat coil; Lower steam superheat coil, Intermediate steam superheat coil, Upper steam superheat coil, the heat transfer fluid heating coil; 2. The furnace system of claim 1, further comprising a coil arranged in the order: a boiler feedwater heating coil.

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

12. 12. The furnace system of claim 1, further comprising a second furnace having a radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated air flow to generate exhaust gases that are supplied to the convective heating section, the second furnace including an air preheater fluidly connected 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 the steps of: 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.

14. 5. A method for 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; and controlling a temperature of the exhaust gas entering the selective catalytic reduction catalyst bed.

15. 15. The method of claim 14, comprising mixing a portion of the exhaust gas with an air feed to provide a dilute air feed that is supplied as the air to be heated in the air preheater.

16. 1. A method of retrofitting a cracking furnace having a steam superheat coil, a boiler feedwater heating coil, and a feed preheat coil, each coil including a plurality of tubes connected to a feed header and a discharge header, the method comprising: isolating one or more tubes of the boiler feedwater heating coil from a boiler feedwater circulation system; fluidly connecting the one or more tubes to a heat transfer fluid circulation system; and fluidly connecting an air preheater to said heat transfer fluid circulation system and to a burner of said cracking furnace.

17. 17. The method of claim 16, further comprising taking one or more tubes of the steam superheat coil out of service.

18. 18. The method of claim 16 or claim 17, comprising varying fin density of tubes of the steam superheat coil.

19. 1. A furnace system comprising: A furnace having a radiant heating section and a convective heating section, the radiant heating section including one or more burners configured to combust a fuel with a preheated air flow; a heat transfer fluid heating coil disposed within the convection heating section of the 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 through the air inlet through indirect heat exchange with hot heat transfer fluid received through the heat transfer fluid inlet to produce the preheated air stream that is recovered through the preheated air outlet and a cooled heat transfer fluid that is recovered through 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; a heat transfer fluid circulation system including flow lines for circulating the cooled heat transfer fluid from the heat transfer fluid outlet to an inlet of the heat transfer fluid heating coil disposed within the convection heating section of the furnace, and flow lines for circulating the hot heat transfer fluid from the outlet of the heat transfer fluid heating coil to the heat transfer fluid inlet.

20. 20. The furnace system of claim 19, wherein the heat transfer fluid heating coils are configured as multiple heating coil sections separated by one or more additional heat recovery coils configured to heat one or more of a hydrocarbon feed, water, steam, or mixtures thereof.

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