WO2004086544A1

WO2004086544A1 - Method and system for recovering the chemical energy of anode gases exiting from a fuel cell

Info

Publication number: WO2004086544A1
Application number: PCT/FR2004/050117
Authority: WO
Inventors: Didier Grouset; Philippe Marty; Jean-Christophe Hoguet
Original assignee: N Ghy
Priority date: 2003-03-20
Filing date: 2004-03-19
Publication date: 2004-10-07
Also published as: EP1609202A1; FR2852737A1

Abstract

The invention relates to a method and system for recovering the chemical energy of anode gases exiting from a fuel cell (1) for the production of hydrogen consumed in a fuel cell (1). Hydrogen is produced by steam reforming, partial oxidation, autothermal reforming or hybrid reforming of reagents in a hydrogen generator (2). The method comprises the following steps: preheating of anode gases exiting from the anode compartment (3) of the fuel cell (1) using the surplus calorific energy of hydrogen-rich gases produced in the hydrogen generator (2) in a first heat exchanger (4); provision of high-temperature anode gases by oxidation of residual combustible fractions in a post-combustion chamber (5); extraction of the calorific energy of the anode gases at high temperature in a second heat exchanger (6) in order to bring at least one fraction of the heat necessary for endothermic hydrogen generation reactions to the required temperatures and/or preheating of the incoming reagents in the hydrogen generator (2).

Description

METHOD AND SYSTEM FOR RECOVERING THE CHEMICAL ENERGY OF ANODIC GASES COMING OUT OF A FUEL CELL

Preamble to the description Area concerned, problem posed

1 / invention relates to a method and a system for recovering the chemical energy of anode gases leaving a fuel cell.

Fuel cell technologies are currently experiencing major developments for various future applications such as the electric vehicle or the cogeneration of heat and electricity in the tertiary or residential sectors. Most fuel cell technologies are powered by hydrogen. This hydrogen is produced: either centrally and then distributed to local dealers and users

- Either locally, by reforming a hydrocarbon, just upstream of the fuel cell, and for immediate use by the latter.

For local and decentralized applications, the need concerns the production of electricity, and possibly heat, from fuels available locally: butane, commercial propane, natural gas, domestic fuel oil or from common fuels: petrol, diesel. ... the sub reformer system, which produces hydrogen, is coupled to the fuel cell subsystem, which oxidizes hydrogen by producing electricity, and it is the assembly which must be optimized in terms of efficiency, compactness, reliability, cost ... The two subsystems use fuels (hydrogen in the fuel cell, more common fuel or fuel in the reformer) and heat exchanges at different temperature levels. It is interesting to couple the two subsystems in order to optimize heat transfers and overall energy efficiency.

The reformer operates at a temperature varying between 250 and 1400 ° C depending on the technique used and the fuel to be reformed: catalytic steam reforming (at 250 ° C for methanol, at 850 ° C for methane), partial catalytic oxidation ( around 900 ° C) or non-catalytic (1200 ° C), autothermal reforming, hybrid reforming. Preheating the reagents by recovering heat available in the system is essential to achieve good efficiency: this is one of the key points of reforming. The fuel cell operates at a temperature depending on the technology used: 60/90 ° C for PEMFCs

(Proton Exchange Fuel Cell: proton-permeable membrane cell) from, at 600/900 ° C for SOFC (Solid Oxide Fuel Cell: solid oxide cell) through 500/600 ° C for MCFC (Molten Carbonate Fuel Cell: molten carbonate cell). It releases gases at higher or lower temperatures whose sensitive energy can be harnessed. Furthermore, these gases at the outlet of the cell can contain residual fuels: H2, but also CO or CH4 or other light hydrocarbons whose calorific value can be upgraded to optimize the efficiency of the system.

The present invention provides a technique for coupling these subsystems, applicable, whatever their type, in most fuel cell systems: PEMFC, SOFC, MCFC ... and with a hydrogen production reactor : catalytic, autothermal reforming, partial oxidation, catalytic or non-catalytic ... Prior art

I. Reforming techniques I. A. Steam reforming

The reforming of a fuel consists in converting it into a mixture of hydrogen and carbon monoxide, by usually reacting it with water vapor:

CnHmOp + (n -p) H20> nCO + (m / 2 + n-p) H2 This reaction is essential for the production of hydrogen, that it is then used for energy purposes

(combustion in a gas engine or fuel cell) or chemical (hydrogenation of petroleum fractions in refineries for example). Reforming is a perfectly well controlled reaction in petrochemicals where the production of hydrogen from natural gas is common. It requires a nickel-based catalyst, suitable for the molecules to be reformed (methane and light hydrocarbons). It is carried out at a temperature of 850 to 950 ° C under pressures of 15 to 25 bar and H20 / HC ratios between 2 and 4 (IFP). These endothermic reactions are carried out in large ovens (KTI, TOPSOE) where bundles of parallel tubes, packed with catalysts, and heated externally (mainly by radiation), are traversed by the mixture to be reformed.

For other hydrocarbons, the conditions and catalysts are different. For heavier hydrocarbons than methane, the temperatures are lower than for methane (850/950 ° C). Methanol is easier to reform: temperatures of 250 ° C are sufficient and the catalyst is based on Cu / Zn / Al. Hydrocarbons containing sulfur require a preliminary desulfurization because the catalyst would be poisoned by sulfur.

The reforming therefore takes place under conditions of temperature and pressure adapted to the fuel and which can be calculate by the laws of thermodynamics involving chemical equilibria. It is always a slow reaction and that is why reforming is systematically catalytic.

I. B Partial oxidation An alternative to steam reforming for the production of hydrogen is the partial oxidation of the fuel:

CnHmOp + (n - p) / 2 02> nCO + m / 2 H2

This reaction is exothermic but produces less hydrogen and moreover it tends to produce solid carbon which can foul and clog the tubes and exchangers.

For example, for diesel, it is run between 950 and

1200 ° C (Texaco-Shell burners).

I. C Autothermal reforming The combination of reforming and partial oxidation allows, by adjusting the quantities of air and water injected, to produce a generally athermal reaction. This is autothermal reforming (ex: Haldor Topsoe on Nickel at 25 bar and 950 ° C). These technologies have been adapted to small powers by EPYX (Arthur D Little, now Nuvera) on the one hand, and Johnson Matthey on the other hand, with the HOTSPOT single reactor technology, initially developed for methanol and which can be extended other fuels.

I. D Hybrid reforming Hybrid reforming is similar to autothermal reforming. However, in this process, the quantity of air (the air factor) can be modified around the value corresponding to the autothermal reforming so as to oxidize a more or less large fraction of the fuel and thus to make the reaction overall exothermic or endothermic according to the needs and the state, cold or hot, of the system and therefore according to the possibilities of heat recovery.

II. Interactions between the two subsystems and the possibilities of coupling In some systems, lacking a hydrogen permeable membrane, the entire flow leaving the hydrogen production subsystem is introduced into the fuel cell. In most cases, the residual fuels leaving the anode circuit of the cell are burned, possibly at low temperature in a catalytic burner, and the heat released is valued:

- to slightly preheat the reforming water and produce, if necessary steam at low temperature, - or to heat the water of the user circuits in the event of cogeneration.

The reactants of the reformer (or, more generally, of the hydrogen production subsystem) are preheated before introduction into the reforming reactor by heat recovery from the products of the reforming reaction, using a recovering exchanger for the systems operating at low or medium temperature and using a regenerative system for high temperature systems. The recovery system involves a specific design of the flows of hot fluids, with returns or changes in direction similar to those of self-recovering burners. The regenerative system involves alternating or discontinuous operation of the system, with a change in direction of flow managed by a system of valves. Solution

The invention relates to a process for upgrading the chemical energy of anode gases leaving a fuel cell for the production of hydrogen consumed by the fuel cell. The production of hydrogen is carried out by steam reforming, partial oxidation, autothermal reforming or hybrid reforming of reactants in a hydrogen generator. The process includes the following steps:

- the step of preheating the anode gases leaving the anode compartment of the fuel cell while cooling and by recovering the excess heat energy from the hydrogen-rich gases produced in the hydrogen generator,

the step of bringing the anodic gases, thus preheated, to a high temperature, by oxidation of the residual fuel fractions by means of an oxygenated flow,

the step of preheating, by heat recovery from the hydrogen-rich gases leaving the hydrogen generator, the oxygen flow necessary for the oxidation of the anode gases. The method further comprises the step of extracting, by heat exchange, the heat energy of the anode gases at high temperature, for:

- bring at the appropriate temperatures for the implementation of the steam reforming, autothermal reforming or hybrid reforming at least a fraction of the heat necessary for the endothermic reactions of hydrogen generation, and / or

- preheat the reagents entering the hydrogen generator to temperatures appropriate for the steam reforming, partial oxidation, autothermal reforming or hybrid reforming implemented.

The method is such that it further comprises the step of controlling the additional heat necessary to implement at the appropriate temperatures the processes of steam reforming, autothermal reforming or hybrid reforming, depending on the heat energy provided by the anode gases.

Preferably according to the invention, the method is such that the oxidation of the anode gases is carried out in a stepwise fashion in order to limit the formation of nitrogen oxides.

The invention also relates to a system for upgrading the chemical energy of anode gases leaving a fuel cell for the production of hydrogen consumed by the fuel cell. The production of hydrogen is carried out by steam reforming, partial oxidation, autothermal reforming or hybrid reforming of reactants in a hydrogen generator. The system includes: at least a first heat exchanger for preheating the anode gases leaving the anode compartment of the fuel cell by recovering the excess heat energy from the hydrogen-rich gases produced in the hydrogen generator,

- A post-combustion chamber for bringing the anode gases, thus preheated, to a high temperature, by oxidation of the residual fuel fractions by means of an oxygen flow. The system also comprises at least a second heat exchanger for extracting the heat energy from the anode gases at high temperature and:

• bring at the appropriate temperatures for the implementation of the steam reforming, autothermal reforming or hybrid reforming at least a fraction of the heat necessary for the endothermic reactions of hydrogen generation, and / or

• preheat the reagents entering the hydrogen generator to temperatures suitable for the steam reforming, partial oxidation, autothermal reforming or hybrid reforming implemented.

The system further includes a third heat exchanger for preheating, by heat recovery from the hydrogen-rich gases exiting the hydrogen generator, the oxygen flow necessary for the oxidation of the anode gases. The system also comprises control means for controlling the additional heat necessary for the implementation at the appropriate temperatures of the steam reforming, autothermal reforming or hybrid reforming processes, as a function of the heat energy provided by the anode gases.

Preferably according to the invention, the system is such that the oxidation of the anode gases is carried out in a staggered manner in the post-combustion chamber in order to limit the formation of nitrogen oxides. Preferably according to an alternative embodiment of the invention, the system is such that the first heat exchanger and second heat exchanger are of the recovery type.

Preferably according to another alternative embodiment of the invention, the system is such that the first heat exchanger and second heat exchanger are of the regenerative type.

detailed description

Other characteristics and advantages of the invention will appear on reading the description of alternative embodiments of the invention given by way of non-limiting example, and from the

- Figure 1 which represents a system implementing recuperative exchangers for the high temperature recovery of the energy of anode gases for the generation of hydrogen, - Figure la which represents an evolution of temperatures in a system shown in Figure 1 ,

- Figure 2 which a configuration of an alternative embodiment of the invention with exchangers 4, 6 and 7 of regenerative type and alternating circulation in the high and low branches through a system of valves.

The present invention mainly relates to the recovery of fuels, H2 and possibly CH4, CO, present in the anode gas at the outlet of the cell. The proposed solution consists in coupling the flow leaving the anode compartment of the cell with that leaving the hydrogen production reactor. The coupling is carried out by heat exchange without material exchange, the flows remain separate. This avoids any dilution of the reformate as would be the case if the oxygen necessary for the hydrogen generator was recycled from cathode gases.

Two gas flows are thus obtained. A first flow (A, figure la) is formed by the reactants (water (a), air (c), fuels (b)) then by the reformate resulting from their reaction, the other flow (B, figure la) is consisting of anode gases (e) and oxygenated gas (d) necessary for the oxidation of anode gases. These two flows have balanced heat flows and circulate in a practically linear arrangement of exchanger-reactors. For reasons of convenience and to facilitate understanding of the circuits, FIG. 1 shows separate exchangers and reactors connected by connecting elements. In fact, the quasi-linear arrangement makes it possible to integrate the exchangers and the reactors in the same enclosure while avoiding connecting elements transferring the fluids at high temperatures. The thermal coupling according to the invention is strong in the sense that it allows a high preheating temperature level for each of the flows by heat recovery from the other flow. In addition, the coupling is amplified by the generation of heat, at the highest temperature level, during post combustion. The evolution of the temperatures of the two flows (A) and (B) is shown in the figure through the exchangers (6, 4 and 7) and the reactors (2 and 5).

The flow leaving the hydrogen production reactor 2 enters the anode compartment 3 of the fuel cell 1. It emerges there after modification of its composition by the cell 1: in all types of cell, it has been depleted in hydrogen : in the case of a cell with a proton exchange membrane (PEMFC), the hydrogen has migrated through the membrane; in the case of a solid oxide cell (SOFC), the hydrogen was oxidized to water and the CO was oxidized to C02. The other components of the flow: N2, C02, H2O, residual light hydrocarbons (CH4 for example) present at the inlet of the anode compartment 3 do not react. Hydrogen not consumed by cell 1 also remains present in this stream. This gas flow has a temperature of the order of 60/90 ° at the outlet of PEMFC and 600/900 ° C at the outlet of SOFC.

The hydrogen production reactor 2 always operates at a higher temperature than that of the PEMFC to which it is coupled. The hydrogen production reactor most often operates at a higher temperature than the SOFC to which it is coupled. This is the case for reactors producing non-catalytic hydrogen by partial oxidation or hybrid reforming of any fuel: reactor 2 operates at 900/1200 ° C or even sometimes more (1400/1500 ° C). This is also the case with steam catalytic reforming or methane autothermal: the hydrogen production reactor 2 operates at 850/950 ° C. In all these cases, the flow leaving the anode compartment 3 can therefore be reheated by heat recovery in a first heat exchanger 4 from the high temperature gas flow leaving the hydrogen production reactor 2.

Brought to high temperature by this reheating, the residual fuels in this stream (H2, CH4, CO ...) react easily in an afterburner chamber 5 with oxygen, preferably also preheated in a third heat exchanger 7, introduced in slight excess relative to the stoichiometric quantity in this post-combustion reactor 5.

By this exothermic oxidation reaction, the temperature of this flow further increases and exceeds the temperature of the gases inside and at the outlet of the hydrogen production reactor 2. This flow can then be cooled by yielding its heat to the production of hydrogen which is the site of endothermic reactions and / or of the reactants supplying this hydrogen production reactor (fuel, water or water vapor, air possibly) in order to preheat them at high temperature in a second heat exchanger 6.

This exchange configuration is possible with recuperative exchangers, as illustrated in FIG. 1, and also with regenerative heat exchangers as specified in FIG. 2.

This configuration is interesting because: It allows in 3 stages coupling the two flows (preheating / reaction / recovery for the flow of the hydrogen production reactor and preheating / post-combustion / recovery on the other flow, leaving the anode compartment) to recover at high temperature the heat of the products leaving the hydrogen production reactor 2, the calorific value of the residual fuels leaving the fuel cell 1, and the heat of the gases leaving the fuel cell. Thus, the recovery of the calorific value of residual fuel is done at the highest possible temperature level. The energy efficiency of the whole is optimized.

It does not introduce any change in direction in the gas flows: the flow relating to the hydrogen production reactor circulates in a single direction and against the current of the flow leaving the anode compartment of the fuel cell. Rectilinear and compact assemblies can be designed, without loss of space for returns of hot fluids at the head of the exchanger, or technological difficulty relating to the control of these hot fluids. All reagent flow rates can be controlled at the relatively cold inlets of the two reagent streams in the compact assembly.

In the case where the hydrogen production reactor 2 is globally endothermic (in the case of steam reforming or of a hybrid reforming reactor supplied with air), the hydrogen residues at the outlet of cell 1 are best used at the level of hydrogen production and it is then useless to seek an optimum operation of the cell by greater depletion of the hydrogen therein (by recycling at the level of the anode compartment 3 for example).

This configuration lends itself particularly well to the flexible and regulated operation of a hybrid reforming reactor: this reactor can be controlled by the control means 8 to be over or under supplied with oxygen or air, which leads to a reactor for producing 'hydrogen 2 globally exo or endo thermal, depending on whether the energy available in the flow of heated gas from cell 1, at the desired temperature level, is in deficit or in excess.

Claims

REVENDICATIONSProcédé

1. Process for upgrading the chemical energy of anode gases leaving a fuel cell (1) for the production of the hydrogen consumed by said fuel cell

(1); said hydrogen production being carried out by steam reforming, partial oxidation, autothermal reforming or hybrid reforming of reactants in a hydrogen generator

(2); said method comprising the following steps:

the step of preheating said anode gases leaving the anode compartment (3) of said fuel cell (1) by cooling and recovering the excess heat energy of the hydrogen-rich gases produced in said hydrogen generator (2),

the step of bringing said anodic gases, thus preheated, to a high temperature, by oxidation of the residual fuel fractions by means of an oxygenated flow,

the step of preheating, by heat recovery from the gases rich in hydrogen leaving the hydrogen generator, said oxygenated flow necessary for said oxidation of said anode gases,

the step of extracting, by heat exchange, the calorific energy of said anodic gases at high temperature, for:

• preheating said reagents entering said hydrogen generator (2) at temperatures suitable for the steam reforming processes, partial oxidation, autothermal reforming or hybrid reforming implemented, the step of controlling the additional heat necessary to bring about carries out at the appropriate temperatures the said processes of steam reforming, autothermal reforming or hybrid reforming, depending on the heat energy provided by said anode gases.

2. Method according to claim 1; said process being such that said oxidation of said anode gases is carried out in a stepwise fashion in order to limit the formation of nitrogen oxides.

System

3. Valorization system for the chemical energy of anode gases leaving a fuel cell (1) for the production of the hydrogen consumed by said fuel cell

(2); said system comprising:

- at least a first heat exchanger (4) for preheating said anode gases leaving the anode compartment (3) of said fuel cell (1) by recovering the excess heat energy from the hydrogen-rich gases produced in said hydrogen generator ( 2)

- a post-combustion chamber (5) for bringing said anode gases, thus preheated, to a high temperature, by oxidation of the residual fuel fractions by means of an oxygen flow,

- at least a second heat exchanger (6) for extracting the heat energy from said anode gases at high temperature and:

• preheating said reagents entering said hydrogen generator (2) at temperatures suitable for the steam reforming, partial oxidation, autothermal reforming or hybrid reforming implemented, a third heat exchanger (7) for preheating, by heat recovery from the gases rich in hydrogen leaving the hydrogen generator, said oxygenated flow necessary for said oxidation of said anode gases, control means (8) for controlling the complement of heat necessary for the implementation at the appropriate temperatures of said steam reforming, autothermal reforming or hybrid reforming processes, depending on the heat energy provided by said anode gases.

4. System according to claim 3; said system being such that said oxidation of said anode gases is carried out in a staggered manner in said post-combustion chamber (5) in order to limit the formation of nitrogen oxides.

5. System according to any one of claims 3 or 4; said system being such that said first heat exchanger (4) and second heat exchanger (6) are of the recovery type.

6. System according to any one of claims 3 or 4; said system being such that said first heat exchanger (4) and second heat exchanger (6) are of the regenerative type.