WO2013087428A1

WO2013087428A1 - System and method for operating solid oxide fuel cells

Info

Publication number: WO2013087428A1
Application number: PCT/EP2012/074104
Authority: WO
Inventors: Thomas Pfeifer; Laura NOUSCH
Original assignee: Fraunhofer-Gesellschaft Zur Förderung Der Angewandten Forschung
Priority date: 2011-12-15
Filing date: 2012-11-30
Publication date: 2013-06-20
Also published as: DE102011121691A1; DE102011121691B4

Abstract

The invention relates to a system and to a method for operating solid oxide fuel cells. The aim of the invention is to specify possibilities for a variable operation of solid oxide fuel cells, by means of which the respective current demand for electrical power and/or usable waste heat can be taken into account with simple means and low additional overheads. In the system according to the invention, fuel and/or reformate is supplied to a stack of solid oxide fuel cells on the anode side via a reformer. The reformer can be heated by means of hot exhaust gas from the solid oxide fuel cells. Moreover, at least one heat exchanger is present in the exhaust gas line for external use of the waste heat of the solid oxide fuel cells. In addition to the supplied fuel, an oxidant, preferably air, is supplied in a regulated form via a line to the reformer.

Description

System and method for operating solid oxide fuel cells

The invention relates to a system and a method for operating solid oxide fuel cells. This type of fuel cell achieved because of the high operating temperatures at which the waste heat can be used in addition, a high efficiency. They can therefore be operated on the principle of combined heat and power (CHP) and preferably used in isolated applications.

When using such systems, however, in terms of time, frequently either an increased proportion of usable electrical power or an increased proportion of usable heat must be taken into account, so that for the respective current requirement an adaptation of the operating parameters of the solid oxide fuel cells, hereinafter also referred to as SOFC be called required. However, it must be noted that a certain operating temperature is required in order to be able to tap an electric power from the SOFC. In SOFCs with special configurations, such as anode-supported fuel cells (anode-supported cells, ASC), their limited RedOx stability causes problems. Therefore, an inert or reducing atmosphere must be maintained during heating and cooling at temperatures above about 300 ° C at the anodes. So far, it is customary to supply additional inert or forming gas, which requires increased effort and additional sources, such as corresponding tanks or containers.

Furthermore, it is known to carry out an endothermic reforming of gaseous hydrocarbons within a stack of SOFC to increase the electrical efficiency, which is also referred to as internal reforming. However, unfavorable shifts occur within the temperature profile within the fuel cells, which lead to electrical power losses and high mechanical stresses. For the setting of favorable process conditions during all possible operating conditions, the realization of a controllable internal reforming is required.

Because of the temporarily given asynchrony of each need to be covered at different times needs of electrical power and heat efficiency is particularly determined by the possibility of operation under partial load conditions. In conventional embodiments of such SOFC systems, the ratio of net electrical power to useful heat provided, also referred to as a power factor, can not be changed arbitrarily. Therefore, only one mode of operation, which takes into account either the electric power or the heat demand, is possible. To cover the needs not covered by the selected operation, additional system components are required. These can be, for example, hot water peak load boilers, backup batteries or an additional connection to a public power grid.

Thus, it is known from EP 1 770 812 A1 to provide for an internal reforming a recirculation of exhaust gas from the anodes to provide water vapor in the reforming reaction. The composition and the However, the temperature of the exhaust gas from the anodes are not constant in the various operating states and at different times, so that the actual exhaust gas conditions that are actually relevant at the respective time point either have to be taken into account with great effort or are disregarded. Both are of course disadvantageous.

It is therefore an object of the invention to provide options for a variable operation of solid oxide fuel cells, in which the respective current need for electrical power and / or usable waste heat, with simple means and little additional effort, can be considered.

According to the invention, this object is achieved with a system having the features of claim 1. The system can be operated by a method according to claim 5. Advantageous embodiments and further developments of the invention can be realized with technical features described in the subordinate claims.

In a system according to the invention for operating solid oxide fuel cells, fuel and / or reformate is fed via an anode side reformer to a stack of solid oxide fuel cells. The reformer is heated with hot exhaust gas from the solid oxide fuel cells. In addition, at least one heat exchanger is provided for external use of the waste heat of the solid oxide fuel cells in the exhaust gas system. The reformer is supplied via a line in addition to the supplied fuel, an oxidizing agent, preferably air, in controlled form. Due to the respective proportions of supplied fuel and oxidizing agent, as a result of the exothermic oxidation reaction which can take place in the reformer in addition to the reforming, a share of up to 100% of the heat demand for the endothermic reforming reaction can be covered directly in the reformer. As a result, the heat flow supplied from outside (eg via separate heat transfer surfaces) is reduced, as a result of which an increased energy content remains in the SOFC waste gas. With 100% coverage of the heat demand for the reforming reaction by the exothermic partial oxidation of fuel in the reformer is called an autothermal operation of the reformer. In this case, the largest possible amount of waste heat from the SOFC waste gas can be used in the form of useful heat. The heat requirement for the reforming and, if appropriate, evaporation can thereby be at least partially covered by this oxidation of the supplied fuel in the reformer. For the heating of the reformer then at least less energy from the exhaust gas of the SOFC is required, so that the exhaust gas, which can be used externally as heat, has a higher temperature and consequently the removable usable amount of heat is increased. The upper limit of the supplied oxidant volume flow should be kept at the air ratio (λ value) at which an autothermal operation of the reformer occurs.

As a result, an adaptation to the current demand for electrical power and / or the usable waste heat or the respective efficiencies is possible.

In addition, in a system according to the invention by a variable distribution of supplied fuel, which is optionally supplied to the reformer and / or directly to the anodes of solid oxide fuel cells via a mixing point, allow an internal reforming in controlled form. The temperature-dependent chemical equilibrium can be established before entry into the anodes of the SOFCs.

By directly supplying fuel to the anodes of the SOFCs, the internal reforming can be carried out in a controlled manner. In this case, the electrical efficiency can be increased because a smaller power for a

Blower, which serves the cooling air promotion, is required.

The fuel should be supplied in gaseous form, so that a liquid fuel should be vaporized beforehand.

With a supply of methane as fuel and a reforming temperature of 650 ° C., the proportion of methane in the mixture fed to the anodes can be in the range from 8% by volume to 33% by volume. By influencing this proportion, it is possible to regulate the internal reforming in the anodes of the SOFC. Thus, with an increase in the proportion of fuel directly supplied to the anodes of the SOFCs, the electrical efficiency is increased. The overall efficiency increases even more than the electric power.

At the same time an influence of the reforming temperature can be achieved by an admixture of cooling air into the line, is removed with the hot exhaust gas from the SOFC and thereby used for heating the reformer. In / on the reformer can be provided for a heat exchanger can be performed by the hot exhaust gas either alone or mixed with cooling air.

The change in the tapped electrical power can be achieved by the correspondingly changed volume flow of fuel. At a constant volumetric flow rate and constant utilization of the fuel in the SOFC, the net electrical power and the heat which can be used on a heat exchanger are in an at least approximately constant relationship to one another. The power code is correspondingly constant.

In addition, there is the possibility in the invention, in addition to supply the solid oxide fuel cells fuel in a controlled manner on the anode side via another line, bypassing the reformer. In this case, this line should preferably be formed as a bypass line to the reformer. As a result, the temperature within the SOFCs and, accordingly, the cooling air requirement can be additionally influenced. It can be done in the SOFC an endothermic internal reforming, which in turn can affect the respective electrical power and electrical efficiency.

By an additional controlled supply of liquid water in the reformer, a very large can be achieved up to the largest possible from the exhaust gas removable and usable heat flow. By the aforementioned controlled addition of oxidizing agent into the reformer, a change in the power factor of the system can be achieved with a slight deterioration in the electrical efficiency of the SOFC. Thus, for example, in a system designed for an electrical power of about 1 kW, the electrical power can be changed in a range between 100% and 35%. The ratio of electrical power to Useful heat (power code) can be changed independently between 2 and 1.

In a required shutdown of a system according to the invention may be required during the cooling of the SOFC's of the respective stack

Time the anodes a small volume flow of fuel are supplied, so that at the anodes reducing conditions can be met in order to avoid their RedOx-related performance degradation. In a system according to the invention, the reformer can be kept small in size. This allows a controlled cooling of the entire system with low energy consumption up to a low temperature level. In the invention, the reformer may be made smaller in relation to the rated load of the system. It can be designed so that the maximum amount of fuel that can be converted in the reformer is smaller than the fuel quantity required for electrical power generation at the rated load point, for which the active area of the SOFCs is dimensioned. For example, the reformer may be designed for 50% of the amount of fuel required in nominal load operation. The remaining amount of fuel required can then be fed directly to the SOFCs, for example via a bypass line bypassing the reformer. Fuel and reformate can be passed through a mixing point to the anodes of the SOFCs.

In the system according to the invention, further elements may be present which are at least partly also used in the prior art. These may include a starting burner, an afterburner, a heat exchanger for preheating oxidant, a heat exchanger for preheating and / or evaporation of water fed to the reformer, a control valve for the supply of fuel to the reformer and the anodes of the solid oxide fuel cells, a control valve for the direct cathode side Supply and indirect supply of oxidant via the heat exchanger, a control element for controlling the volume flow of oxidant, a control element for controlling the volume flow of fuel, a control element for controlling the volume flow of oxidant to the reformer, a Re- gel element for controlling the volume flow of oxidizing agent to be present in / on the reformer heat exchanger and / or a control element for controlling the volume flow of the fuel can be supplied to the starting burner.

The invention will be explained in more detail by way of example in the following. Showing:

Figure 1 in schematic form the structure of an example of a system according to the invention.

In the example shown in FIG. 1, the system is formed with a stack 1 of electrically series-connected SOFC, preceded by a reformer 2. The reformer 2 is supplied with fuel via the line 18. In line 18, a control element 14, with which the volume flow can be changed, and a control valve 11 are present. With the control valve 11 of the reformer 2 and the anodes of the SOFC via the line 5 supplied volume flow of fuel can be changed to the tappable electrical power, which is indicated by the arrow marked "electricity", and the heat exchanger 3 removable and to be able to influence usable heat within limits.

The anodes of the solid oxide fuel cells of the stack 1 can be supplied from the reformer 2 via the line 23 reformate and optionally the reformer 2 supplied non-oxidized fuel.

The reformer 2 can be additionally supplied via the line 19 water. With the control valve 20, the flow rate can be influenced. In addition, it can also be the volume flow of water, which is supplied to the heat exchanger 10 and from this the reformer 2, are regulated. In extreme cases, water can be supplied to the reformer 2 exclusively directly, ie cold or exclusively via the detour in the heat exchanger 9 in the form of hot water or steam. In the latter case, additional heat is extracted from the exhaust gas in the heat exchanger 10, so that the heat ultimately used on the heat exchanger 3 is thereby reduced. In the example shown can also be air as the oxidant by means of the control device 13 and the control valve 12 to the cathode of the SOFC directly so cold, fed via the lines 21 and 21.1. However, it is also possible to supply at least part of the air volume flow via the line 21.2 to the heat exchanger 9 and then to lead the heated air to the anodes. With the control valve 12, a mixture of cold and heated air, the cathodes of the SOFC with different, according to the current requirements adapted proportions are supplied to the anodes.

Via the line 4 and by means of the control element 15, a volumetric flow of air, as an oxidizing agent can also be supplied directly to the reformer 2 in a controlled manner. As a result, an oxidative steam reforming with the supplied fuel in the reformer 2 can be carried out.

By oxidation of the fuel, the resulting heat can also be used for heating the stack 1 of the SOFC to the required operating temperature. During this phase of operation, only exhaust gas can be supplied to the heat exchanger of the reformer 2 in order to shorten this phase of the heating.

For this purpose, a start burner 8 is present, which is supplied with fuel and air as the oxidant via line 22.

With the afterburner 7 through which the entire exhaust gas of the SOFC is guided, a complete oxidation of the not yet oxidized constituents of the exhaust gas can be achieved. The thus treated exhaust gas can be released to the environment and it has because of the exothermic oxidation reaction to a further elevated temperature, which contributes to an increased usable heat.

The following table illustrates how a partial oxidation in the reformer affects the electrical power, the usable waste heat and the power factor. In this case, a constant amount of methane is supplied as fuel to the reformer and a water to fuel ratio (S / C) of 3 to 2 are maintained.

FuelBurning ReformerElectr. Useful heat Fuel supply to air ratio λ Electricity supply feeder Reformer

direct to

SOFC

50% 50% 0 45.6% 20.5% 2.22

50% 50% 0 40.0% 30.0% 1.33

50% 50% 0.5 36.1% 37.1% 0.98

Claims

claims

System for operating solid oxide fuel cells, wherein the fuel and / or reformate via a reformer (2) on the anode side a stack (1) of solid oxide fuel cells can be fed and the reformer (2) is heated with hot exhaust gas of the solid oxide fuel cells; in addition, at least one heat exchanger (3) is provided for external use of the waste heat of the solid oxide fuel cells in the exhaust gas line (6);

characterized in that the reformer (2) via a line (4) in addition to the supplied fuel, an oxidizing agent, preferably air, can be supplied in controlled form.

System according to claim 1, characterized in that on the anode side via a further line (5) bypassing the reformer (2) the solid oxide fuel cells fuel is supplied in controlled form; wherein the conduit (5) is preferably formed as a bypass line to the reformer (2).

System according to claim 1 or 2, characterized in that the reformer (2) is smaller in relation to the nominal load of the system.

System according to one of the preceding claims, characterized in that at least one of the following elements is part of the system:

- a starting burner (8)

- an afterburner (7)

- A heat exchanger (9) for preheating oxidant

- A heat exchanger (10) for preheating of the reformer (2) feedable water

- A control valve (ll) for the supply of fuel to the reformer (2) and the anodes of the solid oxide fuel cells - A control valve (12) for the direct cathode-side supply and an indirect supply of Oxidatiuonsmittel via the heat exchanger (9)

- A control element (13) for controlling the volume flow of Oxidati- onsmittel

- A control element (14) for controlling the volume flow of fuel

- A control element (15) for controlling the volume flow of oxidizing onsmittel to the reformer (2)

- A control element (16) for controlling the volume flow of Oxidati- onsmittel to a in / on the reformer (2) existing heat exchanger and

- A control element (17) for controlling the volume flow of the starting burner (8) Zuführbarem fuel.

Method for operating a system according to one of the preceding claims, characterized in that the anodes of the solid oxide fuel cell fuel in controlled form directly bypassing the reformer (2) is supplied, so that an adaptation to the current need for electrical power and / or the usable waste heat is achieved.

A method according to claim 5, characterized in that the anodes of the solid oxide fuel cells fuel in bypassing the reformer (2) and reformate from the reformer (2) are supplied in a variable ratio, whereby the proportion of the fuel formed by internal reforming in the solid oxide fuel cells is being influenced.

A method according to claim 5 or 6, characterized in that the reformer (2) oxidant is supplied in controlled form for a targeted influencing of the power code.

Method according to one of claims 5 to 7, characterized in that the reformer (2) water is supplied in controlled form.

9. The method according to any one of claims 5 to 8, characterized in that the reformer (2) is heated with exhaust gas of the solid oxide fuel cells.