EP2156500A1

EP2156500A1 - Fuel cell for use in electricity generation from a solid carbonaceous substrate

Info

Publication number: EP2156500A1
Application number: EP08750259A
Authority: EP
Inventors: Geoffrey Howard Kelshall; Klaus Hellgardt
Original assignee: Imperial Innovations Ltd
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2007-05-14
Filing date: 2008-05-13
Publication date: 2010-02-24
Also published as: GB0709244D0; WO2008138923A1

Abstract

The present invention provides an electrochemical fuel cell for generating electricity from a solid carbonaceous substrate, comprising an electrochemical reactor for the preparation of dissolved oxygen and an oxidation reactor for reaction of said dissolved oxygen with the solid carbonaceous substrate. The invention further provides a method of generating electricity from a solid carbonaceous substrate.

Description

Fuel cell for use in electricity generation from a solid carbonaceous substrate

The present invention relates to an electrochemical fuel cell for generating electricity from a solid carbonaceous substrate, comprising an electrochemical reactor and an oxidation reactor and a method of generating electricity from a solid carbonaceous substrate.

The use of fuel cells to convert the chemical energy of carbonaceous materials (e.g. coal, polymeric waste etc.) and atmospheric oxygen directly into electrical energy have been investigated in the art. In particular, it is understood that the combustion of carbonaceous materials with oxygen in air can be separated into the component half cell reactions in an electrochemical reactor, to generate electrical (with the generation of some thermal) energy, rather than merely thermal energy, as in conventional combustion processes. The direct carbon fuel cell systems involve reduction of atmospheric oxygen at a cathode (for example a ₂Fe₀₈O₃ cathode)

La₀₆Sr₀₄Co₀₂Fe_{0 8}O₃ cathode: O₂ + 4e^~ > 2O^2~ (electrolyte ) ( 1 )

The resulting oxide ions are transported under an electric potential and oxygen concentration gradients, through an ionically-conducting electrolyte (e.g. ZrO₂- Y₂O₃ yttrium- stabilised zirconia (YSZ) or Ce₀₉Gd_{0 1}O_{1 95} (CGO)) 10s of μm thick, to an electronically conducting anode, at which those ions oxidise carbon particles, caused to impinge from a molten anolyte onto the anode, evolving CO₂.

Depending on the temperature, the overall reaction and corresponding Gibbs energy change ( ΔG° ), is predicted to be: T < 972 K: C + O₂^→ CO₂ (2)

ΔG° _cθ2 / J InOl^"1 = -394383 + 0.84r ; AE° I V =1.0219 -2.17xl0^~6r (3)

T > 972 K: 2C + O_{2 (} > 2CO (4)

ΔG° _CO / J mor¹ = -223500 -1757 ; Δ£^ / V = 0.5791 + 4.5xl0^"4r (5) As the entropy change of reaction (2) is near zero and of reaction (4) is positive, so the temperature dependences of the thermodynamic reversible cell voltages (ΔE°) are predicted (by ΔG° = -nFAE° ) to be near zero (equation (3)) or positive (equation (5)). Outputs of ca. 0.8 V at 1-2 kA m^"2 have recently been reported from direct carbon fuel cells, using a modified molten carbonate type fuel cell design. Despite CO being predicted thermodynamically to be more stable than CO₂ at > 972 K, the latter is the kinetically preferred product, even at ca. 1270 K.

The use of carbonaceous materials, such as coal, provides a number of problems when used with direct carbon fuel cells. In particular, the use of sulphur containing fuels, such as coal particles can cause corrosion of the

(usually Ni) anode material, leading to the "poisoning" of the fuel cells.

Furthermore, while the oxidation of expensive materials such as graphite and carbonised charcoal can occur in the fuel cell, materials such as coal particles are insufficiently electronically conducting to be oxidised by contact with an anode.

Attempts to overcome these problems have involved the theoretical exploration of the use of molten iron as a solvent for carbon. The dissolved carbon would be oxidised at the electrolyte / molten iron interface by oxide ions. However, the required temperatures to obtain the molten iron are too high (> 1426 K) for practical operation of a fuel cell and the design of the fuel cell is compromised by the lack of materials available to be used in a practical reactor. There is therefore a need in the art for an improved method of oxidising carbonaceous materials.

The first aspect of the invention therefore provides an electrochemical fuel cell for generating electricity from a solid carbonaceous substrate, comprising an electrochemical reactor and an oxidation reactor; wherein the electrochemical reactor comprises: a cathode for forming oxygen ions from an oxygen source; an anode for facilitating the dissolution of the oxygen into a liquid; and an ionically conducting electrolyte for transporting the dissolved oxygen to the anode; and wherein the oxidation reactor comprises: an inlet for introduction of the dissolved oxygen from the electrochemical reactor; a bed or slurry of solid carbonaceous substrate; an outlet for removal of the oxidised carbon species generated by oxidation of the carbonaceous substrate by the dissolved oxygen; and a second outlet for re-introduction of the deplenished liquid to the electrochemical reactor.

A particular feature of the first aspect of the invention is the use of a second separate reactor for the oxidation of the carbonaceous fuel. The dissolved oxygen and liquid are therefore transported from the electrochemical reactor to the oxidation reactor and contacted with the carbonaceous substrate. The dissolved oxygen oxidises the carbonaceous substrate resulting in the depletion of the dissolved oxygen from the liquid, evolving oxidised carbon species, in particular CO₂ or other carbon containing gases which can be further modified and/or are removed from the oxidation reactor and can be treated subsequently. The electrochemical reactor further comprises an inlet for the introduction of a source of oxygen. The source of oxygen can be any oxygen containing gas including O₂ and air. The source of oxygen can be compressed and provided in a supercritical state (i.e. >50 bar).

As none of the carbonaceous substrate, the resulting carbon dioxide or the byproducts of the oxidation (such as ash) are in contact with the components of the electrochemical reactor, the lifespan and efficiency of the electrodes is extended in the fuel cell of the present application. The range of substrates available for use in the fuel cell is therefore wider than that available for use in conventional fuel cells, which usually use gaseous (e.g. hydrogen, methane, etc) or liquid (e.g. methanol) fuels. The fuel cell of the present application for example allows the use of carbonaceous substrates which comprise heteroatoms and which could not be used in conventional fuel cells. It will be appreciated that the apparatus of the present invention facilitates the subsequent treatment of the resulting CO₂ product by sequestration and sulphur removal.

During oxidation of the carbonaceous substrate, by-products such as ash can be formed in the oxidation reactor due to the reaction of FeS₂ with oxygen and carbon. FeS₂ is present at (mean) 0.8 wt.% in UK coal. Reaction of FeS₂ in the oxidation reactor results in the formation of a solid metal oxide: e.g. 3FeS₂ +10[O]₁ + 6C > Fe₃O₄ + 6COS (6)

As such solid metal oxides are insoluble in the molten metal, their separation is facile. Furthermore, the formation of the solid metal oxide in the oxidation reactor rather than in the electrochemical reactor prevents voltage losses and other problems.

It will be appreciated that the oxidation of the carbonaceous substrate will result in its conversion to carbon dioxide or other carbon containing gases, which can be removed from the oxidation reactor. The oxidation reactor therefore provides an inlet for the carbonaceous substrate. The carbonaceous substrate can be provided continuously while the fuel cell is in use or in a batch wise manner. The carbonaceous substrate can be provided to the oxidation reactor continuously in a co-current or counter current flow with the dissolved oxygen. Preferably, the carbonaceous substrate is provided to the oxidation reactor continuously in a counter current flow with the dissolved oxygen.

The chemical process occurring in the fuel cell of the first aspect of the invention is as summarised below:

Electrochemical reactor

CaIhOdC (La₀₆SrO₄COo₂Fe_{0 8}O₃): O₂ (g) + 4e^~ >2O^2~ (7) Electrolyte (e.g. YSZ): 2O^2~ (cathode) >2O^2~ (anode) (8)

Anode: 4e^" + 2[O]₁ < 2O^2~ (9)

Overall: (10)

Oxidation reactor

Overall Process Reaction:

O₂ (g) + C(s) >CO₂ (g) (12)

By separating the electrochemical and oxidation reactions into separate reactors, it is possible to optimise the conditions for the separate reactions. In particular, the temperature of the electrochemical reactor and the oxidation reactor can be separately controlled and optimised. It will be appreciated that the optimum temperature of the reactors will depend on the melting point of the liquid and the integrity of the reactor compartments.

For the purposes of this invention, liquid preferably exists in a liquid form at a temperature above 400⁰C. The liquid is preferably a molten metal or alloy in which the oxygen is dissolved. In particular, where the molten metal is a mixture of two or more metals, the molten metal is preferably provided as a eutectic material. The molten metal can be selected from one or more of Sn, Pb or Pb-Bi. A particular example of a eutectic material for the present invention is Lead-Bismuth Eutectic (LBE). In a particular feature of the first aspect, the molten metal can be molten tin, more particularly molten tin with a melting temperature of T_M = 231.9 C. For the purposes of this invention, the anode is a liquid, particularly a molten metal as defined above.

The liquid, e.g. the molten metal is provided as a solvent for the oxygen. The amount of oxygen dissolved in the molten metal should preferably not exceed the oxygen solubility of the molten metal. If the dissolved oxygen solubility is exceeded, a metal oxide will form which can be used to oxidise the carbonaceous substrate. For example, if the dissolved oxygen solubility of the molten tin is exceeded then SnO₂ will form which itself could oxidise carbon. The Sn-O phase diagram predicts that two liquids (Sn + SnO₂) should co-exist at > 1318 K. After depletion of the dissolved oxygen in the oxidation reaction, the liquid, e.g. the molten metal will then be returned to the electrochemical reactor for further oxygenation.

The electrolyte is preferably selected from one or more of ZrO₂-Y₂O₃, yttrium stabilised zirconia and Ce₀₉Gd_O iOi ₉₅. The cathode is preferably La₀₆Sr₀₄Co₀₂Fe_{0 8}O₃ or La₀₆Sr₀₄MnO₃ (LSM).

The solid carbonaceous substrate can be any material comprising carbon. In particular, the solid carbonaceous substrate can be selected from one or more of coal, petroleum derived fuels, petroleum coke, natural gas, naphtha, low/high sulphur fuel oil, hydrocarbons including long chain hydrocarbons, such as methane, ethane, propane etc, acetylene black, furnace black, carbon black, carbon aerogels, graphite, charcoal, carbon-based polymers including plastics such as polyethylene, biomass, pitch, tar, asphalt, wood, waste carbon products etc.

When waste polymers, biomass etc. are used as the carbonaceous substrate in the fuel cell of the present application, the hydrogen content is oxidised to water and at higher temperatures (» 972 K) CO, rather than CO₂. The carbon monoxide in combination with the H₂O can be reformed to syngas (CO + H₂).

This endothermic reforming reaction provides a means of controlling autothermicity in the oxidation reactor.

The second aspect of the invention provides a method of generating electricity from a solid carbonaceous substrate, comprising

1) forming oxygen ions from an oxygen source at a cathode;

2) transporting the oxygen ions to an anode via an ionically conducting electrolyte;

3) dissolving the oxygen into a liquid at the anode characterised in that the carbonaceous fuel is contacted with the dissolved oxygen such that the carbonaceous fuel is oxidised to an oxidised carbon species.

In a particular embodiment of the second aspect of the invention, the steps of forming the oxygen ions at a cathode, transporting the oxygen ions to an anode and dissolving the oxygen in a molten metal and the step of oxidising the carbonaceous fuel are carried out in separate reaction vessels. The present invention therefore provides a method of generating electricity from a solid carbonaceous substrate, comprising

1) forming oxygen ions from an oxygen source at a cathode;

2) transporting the oxygen ions to an anode via an ionically conducting electrolyte; 3) dissolving the oxygen into a liquid at the anode; and

4) contacting the carbonaceous fuel with the dissolved oxygen such that the carbonaceous fuel is oxidised to an oxidised carbon species wherein steps 1), 2) and 3) are carried out in a first reaction vessel and step 4) is carried out in a second reaction vessel.

In particular, steps 1), 2) and 3) are carried out in an electrochemical reactor and step 4) is carried out in an oxidation reactor as defined in the first aspect of the invention.

Preferably, step 3) of the method of the second aspect involves dissolving the oxygen into a liquid anode, wherein said anode is preferably a molten metal or alloy. The present invention therefore provides a method of generating electricity from a solid carbonaceous substrate, comprising 1) forming oxygen ions from an oxygen source at a cathode; 2) transporting the oxygen ions to an anode via an ionically conducting electrolyte;

3) dissolving the oxygen into a liquid anode; and

4) contacting the carbonaceous fuel with the dissolved oxygen such that the carbonaceous fuel is oxidised to an oxidised carbon species.

As discussed above, steps 1), 2) and 3) are preferably carried out in a first reaction vessel and step 4) is preferably carried out in a second reaction vessel.

The resulting oxidised carbon species is preferably a carbon containing gas or a mixture of carbon containing gases, more preferably carbon dioxide.

The chemical process of the method of the second aspect of the invention is set out below:

Step l

Cathode O₂ (g) + Ae^~ >2O^2~ (13)

(e.g. La₀6Sr₀₄Co₀2FeOgO₃):

Electrolyte (e.g. YSZ): 2O^2~ (cathode) >2O^2~ (anode) (14) Anode: Ae^~ + 2[O\ < 2O^2~ (15)

Overall: (16)

Step 2

Overall Process Reaction:

O₂ (g) + C(s) >CO₂ (g) (18)

The method of the second aspect of the invention can be carried out at a temperature of 400 to 2000⁰C, preferably 400 to 1200⁰C. Preferably, the electrochemical reaction can be carried out at a temperature of 400 to 1200⁰C, depending on the combination of electrolyte, cathode and anode employed. The oxidation reaction can preferably be carried out at a temperature of 400 to 1200⁰C, depending on the desired product gas composition.

As discussed for the first aspect of the invention, the solid carbonaceous substrate can be any material comprising carbon. Improved oxidation of the carbonaceous substrate can be obtained when the substrate is provided as solid particulate matter with a high surface area. The carbonaceous substrate can therefore be pretreated prior to oxidation to form particles of the substrate by for example, pyrolysis, partial oxidation, grinding, mechanical size reduction, etc. The method of the second aspect therefore provides the step of converting the carbonaceous substrate into particulate material prior to its contact with the dissolved oxygen. It will be appreciated that the smaller the size of the particles, the more surface area is available for reaction with the dissolved oxygen. The particles of the solid carbonaceous substrate are therefore preferably from 1 to 100,000 micrometres, more preferably 10 to 10,000 micrometres, alternatively 10 to 1000 micrometres or 100 to 1000 micrometres in diameter.

The solid carbonaceous substrate can additionally or alternatively be pretreated to modify the carbon/hydrogen ratio of the substrate, for example using high temperature pyrolysis. This pre-treatment will allow manipulation of the composition of the resulting oxidised carbon species. Hydrogen generated during the high temperature pyrolysis step can be collected and used for example as a fuel for fuel cells.

The CO₂ produced by the method of the second aspect can be further treated for example by sequestration and/or sulphur removal. Sequestration can be carried out by compression and condensation (wherein the pressure can be derived from the continuous evolution of CO₂). Alternatively, the CO₂ may be dissolved for example in DEA and separated through absorption.

The fuel cell and method of the present application provides a number of advantages over the use of fuel cells currently known in the art. In particular, the fuel cell and method of the present invention provides improved efficiency (i.e. less CO₂ emitted per kW) compared with electrical power generation by convention fossil fuel combustion. The generation of electricity from substrates such as coal and/or waste carbon products overcomes problems associated with the security and reliability of energy conversion as it allows the use of indigenous coal and/or waste carbon products. The fuel cell and method of the present application further obviates problems of gasification and gaseous fuel purification associated with conventional fuel cells.

The third aspect of the invention provides a use of an electrochemical fuel cell of the first aspect of the invention for generating electricity from a carbonaceous substrate.

All preferred features of each of the aspects of the invention apply to all other aspects mutatis mutandis.

The invention may be put into practice in various ways and a number of specific embodiments will be described by way of example to illustrate the invention with reference to the accompanying drawings in which:

Figure. 1. shows a schematic of carbon-air fuel cell, coupled to a combustion reactor in which carbon particles are oxidised to CO₂ by dissolved oxygen in molten metal or alloy;

Figure 2. shows the predicted temperature dependences of Gibbs energy and enthalpy changes (line (a)), and reversible cell voltage (line (b)), for molten tin oxidation by oxygen, assuming the worst case of SnO₂ as the (unwanted) product, as the oxygen solubility (predicted as ca. 0.32 at.% at 1170 K and 1.6 at.% at 1320 K) in molten tin is exceeded; and

Figure 3. shows a further schematic of a fuel cell of the invention comprising an electrochemical reactor (1) and an oxidation reactor (6). The electrochemical reactor comprises a cathode (2) and an anode (4) linked via an electrolyte (3). Oxygen is provided to the electrochemical reactor via inlet (5). The dissolved oxygen in the liquid anode is transported to the oxidation reactor (6) via an outlet (7) and passes over a bed (8). Carbonaceous substrate is supplied to the bed via inlet (9) to flow counter current to the dissolved oxygen flow. The resulting carbon dioxide is removed from the oxidation reactor via outlet (11) and the deplenished liquid anode is recycled to the electrochemical reactor and more specifically to the anode via outlet (10). By-products, including ash are removed from the oxidation reactor via outlet (12).

Claims

1. An electrochemical fuel cell for generating electricity from a solid carbonaceous substrate, comprising an electrochemical reactor and an oxidation reactor; wherein the electrochemical reactor comprises: a cathode for forming oxygen ions from an oxygen source; an anode for facilitating the dissolution of the oxygen into a liquid; and an ionically conducting electrolyte for transporting the oxygen ions to the anode; and wherein the oxidation reactor comprises: an inlet for introduction of the dissolved oxygen from the electrochemical reactor; a bed or slurry of solid carbonaceous substrate; an outlet for removal of the oxidised carbon species generated by oxidation of the carbonaceous substrate by the dissolved oxygen; and a second outlet for re-introduction of the deplenished liquid to the electrochemical reactor.

2. The electrochemical fuel cell as claimed in claim 1 further comprising an inlet for the introduction of a source of oxygen.

3. The electrochemical fuel cell as claimed in claim 1 or claim 2 wherein the carbonaceous substrate is provided to the oxidation reactor continuously or in a batch wise manner.

4. The electrochemical fuel cell as claimed in claim 3 wherein the carbonaceous substrate is provided to the oxidation reactor continuously in counter current flow with the dissolved oxygen.

5. The electrochemical fuel cell as claimed in any one of claims 1 to 4 wherein the liquid is a molten metal or alloy.

6. The electrochemical fuel cell as claimed in any one of claims 1 to 5 wherein the molten metal is selected from one or more of Sn, Pb and Pb-Bi.

7. The electrochemical fuel cell as claimed in any one of claims 1 to 6 wherein the electrochemical reactor is maintained at a temperature of 400 to 1200⁰C.

8. The electrochemical fuel cell as claimed in any one of claims 1 to 7 wherein the oxidation reactor is maintained at a temperature of 400 to 1200⁰C.

9. The electrochemical fuel cell as claimed in any one of claims 1 to 8 wherein the liquid is the anode.

10. The electrochemical fuel cell as claimed in any one of claims 1 to 9 wherein the electrolyte is selected from one or more of ZrO₂-YiOs, yttrium stabilised zirconia and Ce₀₉Gd₀ Λ ₉₅.

11. The electrochemical fuel cell as claimed in any one of claims 1 to 10 wherein the cathode is La₀₆Sr₀₄Co₀₂Fe_{0 8}O₃ or La₀₆Sr₀₄MnO₃.

12. The electrochemical fuel cell as claimed in any one of claims 1 to 11 wherein the solid carbonaceous substrate is selected from one or more of coal, petroleum derived fuels, petroleum coke, natural gas, naphtha, low sulphur fuel oil, hydrocarbons including long chain hydrocarbons, acetylene black, furnace black, carbon black, carbon aerogels, graphite, charcoal, carbon-based polymers including plastics, pitch, tar, asphalt, wood, biomass and/or waste carbon products.

13. A method of generating electricity from a solid carbonaceous substrate, comprising 1) forming oxygen ions from an oxygen source at a cathode;

14. The method as claimed in claim 13 comprising

1) forming oxygen ions from an oxygen source at a cathode; 2) transporting the oxygen ions to an anode via an ionically conducting electrolyte;

3) dissolving the oxygen into a liquid at the anode; and

15. The method as claimed in claim 14 wherein steps 1), 2) and 3) are carried out in an electrochemical reactor and step 4) is carried out in an oxidation reactor as defined in any one of claims 1 to 12.

16. The method as claimed in any one of claims 13 to 15 wherein the method is carried out at a temperature of 400 to 1200⁰C. o

17. The method as claimed in any one of claims 13 to 16 wherein the carbonaceous substrate is converted into particulate material prior to its contact with the dissolved oxygen.

18. The method as claimed in claim 17 wherein the carbonaceous substrate has a particle diameter of from 10 to 1000 micrometres.

19. The method as claimed in any one of claims 13 to 18 wherein the resulting carbon dioxide is further treated by sequestration and/or sulphur removal.

20. A use of an electrochemical fuel cell as claimed in any one of claims 1 to 12 for generating electricity from a carbonaceous substrate.

21. An electrochemical fuel cell as substantially described herein with reference to one or more of the figures.

22. A method as substantially described herein with reference to one or more of the figures.

23. A use as substantially described herein with reference to one or more of the figures.