GB2460689A

GB2460689A - Method for heating and temperature regulation of a CO clean up reactor

Info

Publication number: GB2460689A
Application number: GB0810308A
Authority: GB
Inventors: Martin Weinberger
Original assignee: Voller Energy Ltd
Current assignee: Voller Energy Ltd
Priority date: 2008-06-05
Filing date: 2008-06-05
Publication date: 2009-12-09
Also published as: GB0810308D0

Abstract

A method of performing carbon monoxide (CO) clean up in a fuel processing system is described (e.g. to produce, via a water gas shift reaction, a hydrogen-rich gas suitable to be fed to a fuel cell module). In the fuel processing system (100), fuel is processed in a reformer (3) to generate fuel reformat. The fuel reformat is processed in a CO clean up reactor (6) to generate Co purified reformat. The CO purified reformat is then provided to a fuel cell (8). In an embodiment, during start up operation of the CO clean up reactor (6), exhaust gas from the reformer (3) is used to heat up the CO clean up reactor (6) to a predetermined operating temperature. During steady state operation of the CO clean up reactor, exhaust gas from the reformer (3) is used to maintain the operating temperature of the CO clean up reactor.

Description

Method For Heating and Temperature Regulation of a CO Clean Up Reactor

Technical Field

The present invention relates to a fuel processing system for generating fuel cell suitable gas for a fuel cell module and more particularly to the temperature control of a chemical reactor.

Background Art

A fuel processing system (FPS) converts a hydrocarbon fuel into a hydrogen rich gas suitable to be fed into a fuel cell module (FCM). The FPS and the FCM together provide a fuel cell power system which generates electric power from the hydrocarbon fuel.

Depending on the type, the FPS may comprise different stages. In one example illustrated in Figure 1, three main stages are provided comprising of a reformer, a water gas shift stage and a final carbon monoxide (CO) clean up stage being either methanation or preferential oxidation (PROX). In a first stage, hydrocarbon fuel is converted/refonned by a reformer into a reformat (e.g. hydrogen and carbon monoxide rich gas, also known as synthesis gas, syngas). For most fuel cells, the carbon monoxide concentration needs to be lowered to levels of several percents or even down to levels of several parts per million (ppm). The CO clean up is accomplished with one or more subsequent reactor stages.

For example, a known method for a second stage in a FPS is to oxidise the carbon monoxide with water according to Equation 1, which is hereinafter referred to as water gas shift reaction: CO + H20->C02 + H2 L HR = -41 kJ/mole Equation 1: Chemical equation and reaction enthalpy per mole CO for the water gas reaction In such a second stage, a serially connected water gas shift reactor may be used to reduce the carbon monoxide concentration. While this level of carbon monoxide clean up could be sufficient for certain types of fuel cells, it is still not adequate for other, such as low temperature proton exchange membrane (PEM) fuel cells. Accordingly, additional steps must be taken to ensure that the concentration of carbon monoxide in the reformat is further reduced.

Two common approaches exist for achieving the exceptionally low carbon monoxide concentrations necessary for proper PEM operation. In one method, a third stage is provided where carbon monoxide is reacted with hydrogen, typically in the presence of a catalyst, to produce methane and water according to Equation 2: CO+3H2->CH4+H20 zHR=-206kJ/mole Equation 2: Reaction equation of CO and H2 forming methane and water with reaction enthalpy (t 111�=J per mole CO, termed as methanation reaction.

In a second method, the third stage for a final CO clean up involves the preferential oxidation (also known as selective oxidation) of the carbon monoxide in the presence of a catalyst according to Equation 3: CO + 1/2 02 -> CO2 HR 283 kJ/mole Equation 3: Reaction equation for the oxidation of CO with 02 with the reaction enthalpy H per mole CO. termed as preferential oxidation The reaction according to Equation I, Equation 2 and Equation 3 are generally catalytically enhanced and occur at elevated temperature. The catalyst, and more precisely the catalyst substrate, and the reactor chamber including manifolds and fittings have a certain heat capacity.

As those skilled in the art will appreciate, thermal energy is needed during a start up operating phase in order to get the chemical reactor up to its required operating temperature. One common way is to provide the source of thermal energy from energy stored in a battery. The chemical reactors can be heated up to its operating temperature by running the electric heating elements with power from the battery. Alternatively, the electric energy could also come from the grid if the availability is given. Another way for heating is to use the hot flue gas of an extra burner and heat up the chemical reactor from outside. The extra burner would have the function as a start up burner and would be switched off as soon as the appropriate temperature is reached.

Another method to heat up the CO clean up reactors according to Equation 1, Equation 2 and Equation 3 is to heat them up from inside by passing through hot reformat which is generated in the reformer upstream the CO clean up reactors. However, as those skilled in the art will appreciate that this may lead to carbon formation and deposition on the catalyst which may lead to deactivation and clogging up the CO clean up reactors.

While the aforementioned methods for preheating the reactors are capable of achieving the required temperature, their inclusion results in added weight, volume and complexity to the FPS or even bear the risk of degrading the catalyst during start up.

Furthermore, in operation the temperature of the reactor needs to be regulated as the reactions are exothermic (as A HR is negative). This imposes another technical challenge to the design of the chemical reactor.

Accordingly, there exists a need to heat up and maintain the temperature of the clean up stages downstream the reformer which avoids these and other drawbacks.

Statements of Invention

The present invention has been devised to provide for a faster and/or more energy efficient start up of the first and/or second CO clean up reactor stages down stream the reformer for fuel cell application. Electric heater or additional burner are not needed. In addition, the invention provides temperature regulation of the reactors during operation without changing the configuration (i.e. by the usage of valves).

The invention solves the problem by the use of hot combusting gases from the combustion section of a reformer in a specially designed heat exchanger/reactor assembly. The design of the reactor allows preheating during start up and cooling at steady state in particular for exothermic reactions as described with Equation I, Equation 2 and Equation 3.

According to one aspect, the present invention provides a method of heating and temperature regulation of a CO clean up reactor for processing fuel reformat to generate purified reformat in a fuel processing system by receiving heated effluent from a component in the fuel processing system upstream from the CO clean up reactor, heating up the CO clean up reactor to a predetermined operating temperature during a start up operation using the received heated effluent, and maintaining the operating temperature of the CO clean up reactor during a steady state operation using the received heated effluent.

According to another aspect, the present invention provides a fuel processing system comprising a CO clean up reactor for processing fuel reformat from a reformer to generate purified reformat, wherein the system is arranged to provide a heated effluent to the CO clean up reactor to heat up the CO clean up reactor to a predetermined operating temperature during a start up operation using the heated effluent and to maintain the operating temperature of the CO clean up reactor during a steady state operation using the heated effluent.

Iii this way, the present invention advantageously provides a CO clean up chemical reactor which is heated up and maintained at temperature by the effluent of another reactor without the need to switch hot gases between different pathways even if the chemical reactor dissipates heat when it is in operation.

Description of Drawings

Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram illustrating the three main stages of an example fuel processor; Figure 2 is a schematic diagram of the major components of a standalone integrated fuel cell system used in an embodiment of the current invention; Figure 3 is a schematic diagram illustrating the working principle of the CO clean up reactor during start-up operation; Figure 4 is a schematic diagram illustrating the working principle of the CO clean up reactor during steady state operation; Figure 5 is a graph plotting experimental data achieved with the application of the present invention to the third stage (methanation reactor); Figure 6 is an isometric view of an example CO clean up reactor for use in the embodiment of Figure 2; and Figure 7, which comprises Figures 7a to 7c, is a schematic diagram of the example CO clean up reactor of Figure 6 in more detail.

Detailed Description of Invention

System overview Figure 2 shows an example standalone integrated fuel cell system 100.

Liquefied Petroleum Gas, or LPG, from an external LPG supply 1, along with air from an air supply 13 and water from a water supply 11 is piped into a reformer 3. h this embodiment, the reformer 3 is a steam reformer and contains a burner chamber 4 for heating a reformat chamber 5 where the chemical reaction of LPG and water (steam) occurs. A central control unit 20 may be provided to control the supply of LPG, water and air from the respective supplies into the reformer 3 after a start-up operation is complete, as will be discussed in detail below.

The output from the reformat chamber 5, i.e. the refonnat stream, is then piped directly into a carbon monoxide (CO) clean up unit 6 for performing further gas processing stages, which will be referred to as the CO clean up reactor stages. In the system 100 shown in Figure 2, the CO clean up stages performed by the CO clean up unit 6 are provided as two separate serially connected CO clean up reactors 15 and 16, which are discussed in more detail below. The output from the CO clean up unit 6 can then be sent to a fuel cell stack 8 which generates electricity for internal batteries 14 and external batteries 9. Anode off gas from the fuel cell stack 8 may be fed back into the burner chamber 4 of the reformer 3.

Further as shown in Figure 2, the sulphur in the LPG may be removed prior to entering the reformer by a de-suiphurizer 10. Water may also be fed into the CO clean up reactor 15 from the water supply 11, for the Water Gas Shift reaction. Jii addition, air inlets to the burner chamber 4 may be provided for receiving air generated by the air supply 13, such as an electric reformer blower 13, with the air flow rate being controlled by the control unit 20.

In the system shown in Figure 2, there are two outlets from the reformer unit 3: the reformate stream 22 and the heated effluent (hot flue gas) 21. As will be described in more detail below, in this embodiment, the flue gas 21 is passed to the CO clean up reactors 15 and 16 to facilitate heating and temperature regulation depending on the operating mode of the CO clean up reactor IS and 16. The reformate stream 22 is also piped into the CO clean up reactor 15 and 16 that is set to remove most of the carbon monoxide in the reformate stream by water gas shift reaction, methanation or preferential oxidation as discussed above.

The integrated fuel cell system 100 is described in more detail in the Applicant's co-pending GB application No. 0803955.4.

CO clean up reactor The heating and temperature regulation of the CO clean up reactors 15 and 16 according to an embodiment of the present invention will now be described with reference to Figures 3 and 4.

Figure 3 schematically illustrates the working principle of the CO clean up reactors 15 and 16 during start-up operation to bring the temperature of the CO clean up reactors 15 and 16 to a predefined operating temperature or within a predefined operating temperature range. During the start up

S

operation, the hot effluent (flue gas) 21 from the burner chamber 4 of the reformer 3 enters the CO clean up reactors 1 5 and 16, which in this embodiment a water gas shift reaction stage and a methanation or PROX reaction stage. The hot effluent 21 enters the flue gas chamber 1 5a of the water gas shift reactor 15, where heat 25 is transferred to the corresponding reactor chamber I 5b, provided that the reactor chamber I 5b is of a lower temperature than the temperature of the flue gas chamber 1 5a. After having transferred heat, the cooled down effluent 21' leaves the flue gas chamber I 5a and is passed to the methanation or PROX reactor 16. During this start up operation, the reforming process is not initiated and consequently reformat is not passed through at this point in order to avoid the before mentioned problems associated with reformat being passed through a cold catalytic reactor (coking effects). Received hot effluent 21 from the burner chamber 4 continues to heat up the reactor chamber I Sb to an associated operating temperature. Table I sets out preferred operating temperature ranges for components of the CO clean up reactor unit 6 according to embodiments of the present invention.

Method Preferred temperature range High temperature water gas shift reaction 250 °C. . .400 °C Low temperature water gas shift reaction 180 °C. . .300 °C Methanation reaction 200 °C. . .300 °C Preferential Oxidation (PROX) 90 °C.. .200 °C Table 1: Operating temperatures for CO clean up reactions As shown in Figure 3, the effluent 21' from the flue gas chamber I 5a of the water gas shift reactor 15 is passed to the flue gas chamber I 6a of the methanation or PROX reactor 16, where heat 26 is transferred to the reactor chamber 1 6b of the methanation or PROX reactor 16, provided that the reactor chamber I 6b is of a lower temperature than the temperature of the flue gas chamber 16a. After having transferred heat, the further cooled down effluent 21' leaves the flue gas chamber I 6a and is released to ambient.

As soon as the reactor chambers I Sb and I 6b of the water gas shift reactor 15 and the methanation or PROX reactor 16 have reached their respective operating temperatures, the control unit 20 controls the supply of LPG and water to the reformer 3 to begin the reforming process. The reformat 22 from the reformer 3 is then passed through the respective reactor chambers I 5b and I 6b in order to achieve further CO reduction and output as purified reformat 22'.

As the CO reduction of the reformat according to Equation 1, Equation 2 and Equation 3 are exothermic reactions, the temperature in the CO reactor chambers 1 Sb and 1 6b rises and eventually exceeds the flue gas temperature 21. At a point where the reactor chambers 1 Sb and I 6b have a higher temperature than the respective flue gas chambers 1 5a and 1 6a, the heat flux 25 and 26 goes in reverse, transferring heat from the CO reactor chamber 1 Sb and 1 6b into the respective flue gas chamber I Sa and 1 6a. The flue gas 21 takes up heat and leaves the flue gas chambers I 5a and I 6a as an effluent 21'. This is illustrated in Figure 4 which schematically illustrates the working principle of the CO clean up reactor unit 6 during steady state operation. As a result the temperatures of the reactor chambers I Sb and I 6b are effectively maintained within their respective operating temperature range.

In this way, the catalysts are prevented from operating outside their required temperature range.

It will be appreciated by those skilled in the art that the flue gas temperature of the combustion device of the reformer is defined by the type, the design and its operating conditions. Thus, the flue gas temperature determines the maximum achievable temperature to what a subsequent reactor can be heated up. However, due to the fact that the combustion device needs to be operated at about 850 °C to allow the reforming reaction occurring and the fact that the preheating of the reactants for the combustion device with the effluent (flue gas) is limited due to practical reasons the flue gas is normally well above the temperatures of the reactions in Table 1 discussed above. In order to adjust the flue gas to the right temperature which is within the operating temperature range of a reactor, allowing a fast heat up and providing enough cooling capacity during the operation of the reactor, the flue gas can be cooled with simple means, i.e. cooling with ambient air. This applies to both the adjustment of the flue gas temperature before it enters reactor 15 and reactor 16.

It will also be appreciated by those skilled in the art that the present invention particularly benefits from a reformer with a burner which produces a hotter flue gas during start up (where the thermal energy/heat is needed) than during normal operation (where temperature regulation is needed). The experimental data presented in the graph of Figure 5 are achieved with such type of reformer. The graph of Figure 5 shows the temperature 23 of the flue gas chamber section 1 6a and the temperature 24 of the reactor chamber 1 6b of a methanation reactor 16. As can be seen in Figure 5, there is a cross over point between the temperature 23 of the flue gas chamber section 1 6a and the temperature 24 the reactor chamber I 6b shortly after the completion of the heat up (which is at about 3350 seconds on the time axis). After this point the heat flux is reversed, which means that the flue gas 21 acts a temperature regulating media. The effect is enhanced through the fact that the combustion device of the reformer 3 used for this experiment initially produces a hotter flue gas 21 followed by a lower flue gas temperature in reforming mode (also referred to as flame mode and FLOX mode respectively, reference being made to the product specification of the Cl reformer from WS Reformer GmbH, Germany).

Figures 6 and 7 show different views of one example of a component of the CO clean up reactor unit 6 which can be employed as the water gas shift reactor 15 or methanation or PROX reactor 16 in the embodiment described above with reference to Figure 2. Figure 6 shows an isometric view of a CO clean up reactor 15;16. As shown in Figure 6, the CO clean up reactor 15;16 component is formed of an elongated tubular housing 41 including a flue gas inlet and outlet 43a and 43b, and a reformat inlet and outlet 45a and 45b. Figure 7, which comprises Figures 7a to 7c, schematically illustrates a CO clean up reactor 15;16 in more detail. Figure 7a schematically illustrates the CO clean up reactor I 5;16 of Figure 6 from a side view with the tubular housing 41 shown in dashed lines to allow an elongated tubular reformat chamber 47 housed within the tubular housing 41 to be visible. Figure 7b schematically illustrates a CO clean up reactor 15; 16 from a top view showing a reformat inlet 45a (or outlet depending on the direction of flow). Figure 7c schematically illustrates a side view of a CO clean up reactor 1 5;l 6 through section A-A indicated in Figure 7b. As illustrated in Figure 7, the flue gas inlet and outlet 43a and 43b provide flue gas to a helical tube 49 attached to and transversing along and around the outer surface of the elongated reformat chamber 47. In this way, the flue gas can heat or maintain the temperature of reformat present in the reformat chamber 47 as discussed in the embodiment above.

Alternative Embodiments It will be understood that embodiments of the present invention are described herein by way of example only, and that various changes and modifications may be made without departing from the scope of the invention.

For example, in the embodiment described above, the CO clean up unit includes two separate CO clean up reactor stages: a water gas shift reaction stage and a methanation or preferential oxidation reaction stage. As those skilled in the art will appreciate, it is not essential to include both stages and in an alternative, only one of the stages may be used as a CO clean up stage. For example, the CO clean up unit may simply comprise a water gas shift reactor. in yet another alternative, the CO clean up unit may comprise a different combination of reaction stages, such as a low temperature water gas shift reaction stage followed by a high temperature water gas shift reaction stage.

Additionally, in the embodiment above, both the water gas shift reactor and the methanation or preferential oxidation reactor are heated and temperature regulated by effluent from the reformer. As an alternative, only one of the water gas shift reactor and the methanation or preferential oxidation reactor may be heated and temperature regulated by hot effluent from the reformer. The other of the water gas shift reactor and the methanation or preferential oxidation reactor may be heated and temperature regulated by thermal energy from another source, such as a battery. In such an alternative, the arrangement will still provide the above mentioned advantages because at least one of the CO clean up reactor stages is being heated and temperature regulated by effluent from a preceding component in the process.

As yet another alternative, when both CO clean up stages are heated and temperature regulated by flue gas, the flue gas may be provided in parallel to each reactor in the CO clean up reactor unit instead of serially as illustrated in Figures 2 to 4. In such an alternative, the reformat would still be provided serially to each CO clean up reactor for sequential CO clean up in the manner discussed above.

In the embodiment and alternatives described above, the reformer is provided as a steam reformer comprising separate burner chamber and reformate chamber. As those skilled in the art will appreciate, the present invention is not restricted to steam reformers and may instead be provided as another type of reformer, such as a partial oxidation (POX) reformer or an autotherrnal reformer (AIR). Those skilled in the art will further appreciate that if an AIR or POX is used as the reformer in the fuel processing system, there would be no exhaust flue gas output from the AIR or POX and the effluent from the reformer would be the hot reformat itself. In this case, excess heat from the hot reformat as the effluent would be used to heat and temperature regulate the CO clean up reactor unit as described above.

As yet another alternative, an additional separate burner or other thermal heat source may be provided in the fuel processing system to supply a hot effluent to the CO clean up reactor unit instead of or in addition to the effluent from the refonier.

In the embodiment described above, a co-current flow of flue gas and reformate is illustrated in Figures 3 and 4. As those skilled in the art will appreciate, the flue gas and reformate streams may instead be arranged in a counter-current flow.

Claims

CLAIMS: 1. A method of heating and temperature regulation of a CO clean up reactor for processing fuel reformat to generate CO purified reformat in a fuel processing system, the method comprising the steps of: receiving heated effluent from a component in the fuel processing system upstream from the CO clean up reactor; heating up the CO clean up reactor to a predetermined operating temperature during a start up operation using the received heated effluent; and maintaining the operating temperature of the CO clean up reactor during a steady state operation using the received heated effluent.
2. The method of claim 1, wherein the heated effluent is received from a reformer upstream from the CO clean up reactor.
3. The method of claim 2, wherein the heated effluent is exhaust gas received from the reformer upstream from the CO clean up reactor.
4. The method of claim 3, wherein the reformer is a steam reformer.
5. The method of claim 2, wherein the heated effluent is reformat received from the reformer upstream from the CO clean up reactor.
6. The method of claim 5, wherein the reformer is an auto-thermal reformer.
7. The method of claim 5, wherein the reformer is a partial oxidation reformer.
8. The method of claim 1, wherein the heated effluent is received from a thermal source other than the refonner.
9. The method of any preceding claim, wherein CO clean up reactor includes a water gas shift reactor to generate partially CO purified reformat and a further chemical reactor to process the partially CO purified reformat, and wherein the step of heating up the CO clean up reactor to a predetermined operating temperature during start up operation comprises heating up at least one of the water gas shift reactor and the further chemical reactor.
10. The method of claim 9, wherein the step of maintaining the operating temperature of the CO clean up reactor during steady state operation comprises maintaining the operating temperature of at least one of the water gas shift reactor and the further chemical reactor.
11. The method of claim 10, wherein the heated effluent is passed to the water gas shift reactor and subsequently to the further chemical reactor.
12. The method of claim 10, wherein the heated effluent is passed to the water gas shift reactor and the further chemical reactor in parallel.
13. The method of any one of claims 9 to 12, wherein the operating temperature of the water gas shift reactor is between 250 °C and 400 °C.
14. The method of any one of claims 9 to 12, wherein the operating temperature of the water gas shift reactor is between 180 °C and 300 °C.
15. The method any one of claims 9 to 12, wherein the further chemical reactor is a methanation reactor.
16. The method of claim 1 5, wherein the operating temperature of the inethanation reactor is between 200 °C and 300 °C.
17. The method any one of claims 9 to 12, wherein the further chemical reactor is a preferential oxidation reactor.
18. The method of claim 17, wherein the operating temperature of the preferential oxidation reactor is between 90 °C and 200 °C.
19. A method of performing carbon monoxide (CO) clean up in a fuel processing system, comprising: processing fuel in a reformer to generate fuel reformat; processing the fuel reformat in a CO clean up reactor to generate CO purified reformat; providing the CO purified reformat to a fuel cell; and performing heating and temperature regulation of the CO clean up reactor as set out in any one of claims Ito 18.
20. A fuel processing system comprising a CO clean up reactor for processing fuel reformat from a reformer to generate CO purified reformat, wherein the system is arranged to provide a heated effluent to the CO clean up reactor to: heat up the CO clean up reactor to a predetermined operating temperature during a start up operation using the heated effluent; and maintain the operating temperature of the CO clean up reactor during a steady state operation using the heated effluent.
21. The system of claim 20, wherein the system is arranged to provide the heated effluent from the reformer upstream from the CO clean up reactor.
22. The system of claim 21, wherein the heated effluent is exhaust gas received froi-n the reformer upstream from the CO clean up reactor.
23. The system of claim 22, wherein the reformer is a steam reformer.
24. The system of claim 2!, wherein the heated effluent is reformat received from the reformer upstream from the CO clean up reactor.
25. The system of claim 24, wherein the reformer is an auto-thermal reformer.
26. The system of claim 24, wherein the reformer is a partial oxidation reformer.
27. The system of claim 20, wherein the heated effluent is received from a thermal source other than the reformer.
28. The system of any one of claims 20 to 27, wherein the CO clean up reactor comprises: a water gas shift reactor for processing the fuel reformat to generate partially CO purified reformat; and a further chemical reactor for processing the partially CO purified reformat to further remove CO from the reformat; wherein the system is arranged to heat up at least one of the water gas shift reactor and the further chemical reactor to a predetermined operating temperature during start up operation.
29. The system of claim 28, wherein the system is arranged to maintain the operating temperature of at least one of the water gas shift reactor and the further chemical reactor during steady state operation.S
30. The system of claim 29, wherein the system is arranged to pass the heated effluent to the water gas shift reactor and subsequently to the further chemical reactor.
31. The system of claim 29, wherein the system is arranged to pass the heated effluent the water gas shift reactor and the further chemical reactor in parallel.
32. The system of any one of claims 28 to 31, wherein the operating temperature of the water gas shift reactor is between 250 °C and 400 °C.
33. The system of any one of claims 28 to 31, wherein the operating temperature of the water gas shift reactor is between 180 °C and 300 °C.
34. The system of any one of claims 28 to 31, wherein the further chemical reactor is a methanation reactor.
35. The system of claim 34, wherein the operating temperature of the methanation reactor is between 200 °C and 300 °C.
36. The system of any one of claims 28 to 31, wherein the further chemical reactor is a preferential oxidation reactor.
37. The system of claim 36, wherein the operating temperature of the preferential oxidation reactor is between 90 °C and 200 °C.
38. A carbon monoxide (CO) clean up reactor substantially as herein described with reference to the accompanying drawings.
39. A method of performing carbon monoxide (CO) clean up substantially as herein described with reference to the accompanying drawings.I
40. A fuel processing system substantially as herein described with reference to the accompanying drawings.