GB2106936A - Catalyzed electrochemical gasification of carbonaceous materials at the anode of an electrolysis cell - Google Patents

Abstract

The electrochemical gasification reaction of carbonaceous materials in an aqueous acidic electrolyte by anodic oxidation in an aqueous acidic electrolyte at the anode of an electrolysis cell is catalyzed by the use of iron in its elemental, <+>2 or <+>3 valence state. The reaction may be used to produce hydrogen at the cathode or to effect electrodeposition of a metallic element present as a cationic component of the electrolyte.

Description

SPFCIFICATION Catalyzed electrochemical gasification of car bonaceous materials at the anode of an electrolysis cell This invention relates to the use of an iron catalyst in the electrochemical gasification of carbonaceous materials in an aqueous acidic electrolyte.

It is known in the art that carbonaceous materials when mixed with an aqueous acidic electrolyte in an electrochemical cell through which a direct current passes electrochemically react oxidizing the car bonaceous material to oxides of carbon atthe anode. The electrochemical oxidation of the car bonaceous material at the anode occurs regardless of the nature of the simultaneous cathodic reaction which, for example, can be the production of hyd rogen, the electrodeposition of metals, the hydroge nation of unsaturated organic compounds or any other electrochemical reduction process, or any other process which may occur at the cathode, as well as in any other compartment of the electrolysis cell other than at the anode, and which would be generally known to one skilled in the art.

U.S. Patent No. 4,268,363 teaches the electrochemical gasification of carbonaceous materials by anodic oxidation which produces oxides of carbon at the anode and hydrogen, metallic elements, orthe hydrogenation of coal at the cathode of an electrolysis cell.

U.S. Patent No. 4,279,710 teaches the electrochemical oxidation of carbonaceous materials at the anode and the attendant formation of hydrogen at the cathode as well as the production of electric power by the use of said hydrogen as a fuel for a fuel cell.

U.S. Patent No. 4,226,683 teaches the method of producing hydrogen by reacting coal or carbon dust with hot water retained as water by superatmospheric pressure. The pressure is controlled by the use of an inert dielectric liquid which washes the electrodes and, while doing so, depolarizes them by absorption of the gases.

U.S. Patent No. 4,233,132 teaches a method wherein electrodes are immersed within oil which forms a layer over a quantity of water. When current is passed between the electrodes, water is caused to undergo electrodecomposition. Gaseous hydrogen is collected in the sealed space above the oil-water layers, and the oxygen is believed to react with the constituents in the oil layer.

As acknowledged in U.S. Patent No. 4,226,683, the principal problem in the past use of this principle for commercial production of hydrogen, was the slow rate of the electrochemical reaction of coal or carbon and water. It has now been found that iron, when added to an aqueous acidic electrolyte containing the carbonaceous material, and preferably iron in the +3 valence state, catalyzes the rate of reaction and assists in obtaining more complete oxidation for the electrochemical oxidation of the carbonaceous material at the anode thus making such processes as the commercial production of hydrogen or method of electrowinning commercially feasible.

As described above, it is well known that car bonaceous material such as coal can be oxidized at the anode of an electrochemical cell containing an aqueous acidic electrolyte with the simultaneous production of oxides of carbon at the anode and that this anodic half-cell reaction may be used in combi nation with the cathodic half-cell reactions such as the electrodeposition of a metal M from an aqueous solution of its ions Mm+ or the production of hydrogen.For example, in the case of the electrodeposition of a metal, focusing on the carbon in coal and representing it by C, this anodic reaction can be written according to the stoichiometry: C(5)+2H2O(()#CO2(g)+4H+ + 4e (I) in combination with the simultaneous cathodic reaction Mm+ + me < M (Il) The net reaction, that is the sum of equations (I) and (11) [ for case m = 1 ] is: C)5) + 2H2O(1) + 4M+ < CO2(9) + 4M + 4H+ (III) In the case of copper, for example, the coalassisted electrodeposition of copper would take the form:: C)5) + 2H2O (|) + 2Cu2+ < CO2(g) + 2 Cu(s) + 4W In the case of the production of hydrogen, focusing on the carbon in coal and representing it by C, this anodic reaction can be written according to the stoichiometry: C(5)+ 2H2O)I)#COS)g)+4W +4e- #= 0.21 volt (IV) in combination with the simultaneous cathodic reaction 4W + 4e ~ 2H2(9aE = 0.0 volt (V) The net reaction, that is the sum of equations (IV) and (V) is:: 2H2O(1)+ C)s)#CO2(g)+2H2)5)E0= 0.21 volt (vl) in these equations, the symbols (g), (s) and (I) symbolize the gaseous, solid and liquid states, respectively. Equations (I) and (IV), the reaction between coal and water, caused by impressing an appropriate potential on a suitable electrochemical cell, is what is referred to in U.S. Patent Nos.

4,268,363 and 4,279,710 as the electrochemical gasification of coal.

A problem with these prior art methods is the relatively slow rate of reaction and the incomplete combustion of the carbonaceous material at the anode.

It has now been found that the addition of a sufficient amount of iron in the elemental, +2 or +3 valonce state or mixtures thereof, to the carbonaceous material undergoing oxidation in an aqueous acidic electorlyte at the anode will increase the rate of reaction of the oxidation process. The iron catalyst assists the oxidation of carbonaceous material in going to completion and increases the amount of current produced at the anode per given operating voltage.

Thus in accordance with the present invention, there is provided a method of electrochemical gasification of a carbonaceous material in an aqueous acidic electrolyte by anodic oxidation at the anode of an electrolysis cell, characterised in that an amount of iron in its elemental, +2 or +3 valence state, or a mixture thereof, is added to the electrolyte containing the carbonaceous material sufficient to increase the rate of oxidation of the carbonaceous material at the anode.

The carbonaceous materials suitable for use in accordance with the present invention include a wide variety of material such as bituminous coal, chars made from coal, lignite, peat, active carbons, coke, carbon blacks, graphite; wood or other lignocellulosic materials including forest products, such as wood waste, wood chips, sawdust, wood dust, bark, shavings, wood pellets; including various biomass materials as land or marine vegetation or its waste after other processing, including grasses, various cuttings, crops and crop wastes, coffee grounds, leaves, straw, pits, hulls, shells, stems, husks, cobs and waste materials including animal manure; sewage sludge resulting from municipal treatment plants, and plastics or the scraps or wastes formed in the production of plastic such as polyethylene, cellulose acetate and the like.Thus, it is seen that substantially any fuel or waste material whether a liquid, such as oil, a gas, such as methane or other hydrocarbon or waste material containing carbon, with the exception of CO2, provides a suitable source of carbonaceous material for use according to this invention.

The particular apparatus used to carry out the electrolytic oxidation of carbonaceous materials is not critical. Substantially, the same apparatus and tech niquesthatare utilized in the electrolytic deposition of metals and the electrolytic decomposition of water as well as those described in U.S. Patent Nos.

4,268,363, 4,279,710, 4,226,683 and 4,233,132, can be used with the method ofthis invention. Any selection of appropriate changes in use of materials andlortechnique is well within the skill of those versed in the artto which this invention applies.

The cells described in U.S. Patent Nos. 4,268,363 and 4,279,710 including the use of acidic aqueous electrolytes, selection of anode and cathode materials and the optional but preferred use of cell membranes to keep the carbonaceous material on the anode side are most preferred.

While the electrode materials described in U.S.

Patent Nos. 4,268,363 and 4,279,710 are suitable for use in the method of this invention, anode materials which were found to perform especially well include a mixture of RuOJTiO2 on a Ti substrate and a mixture of lrO2/TiO2 on a Ti substrate, which anodes are both commercially available.

The preferred acidic aqueous electrolytes that can be employed have a pH of less than 3 and include solutions of strong acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid and the like.

While temperatures from above the freezing point of water and greater may be used, temperatures of from about 25 Cto 350 C are preferred. Temperatures of from 120"C to 3000C are most preferred especially when using solid carbonaceous materials such as coal. At temperatures below 140"C, the reactivity of solid carbonaceous materials such as coal steadily decreases as the electrochemical oxidation proceeds. This decreased reactivity is believed to be caused by surface oxides building on the surface of the coal which hinders further sustained reactivity of the coal. At temperatures of about 140 C and greater, the reactiveity of the coal is sustained and no substantial decrease is observed.

Since it is desired to maintain the reaction in a liquid phase, it is of course necessary, that at elevated temperatures, the reaction be carried out at elevated pressure. Generally, pressures of from about 2 to 400 atmospheres are satisfactory.

It has also been found that the addition of the iron catalyst to a solid carbonaceous material such as coal will sustain the activity of the coal longer at temperatures below 120 C as compared to systems not containing the iron catalyst. Also, the catalytic effects of the iron catalyst are more pronounced at the highertemperatures.

While iron may be used in its elemental state, iron in its +2 and +3 valence, i.e., ferrous and ferric states, respectively, are preferred. Most preferred is the use of iron in the +3 valence state. Thus, inorganic iron compounds such as iron oxides, iron carbonate, iron silicates, iron sulfide, iron oxide, iron hydroxide, iron halides, iron sulfate, iron nitrate, and the like, may be used. Also, various organic iron salts and complexes, such as salts of carboxylic acids, e.g., iron acetates, iron citrates, iron formates, iron glyconates, and the like, iron cyanide, iron chelate compounds, such as chelates with diketones as 2,4pentanedione, iron ethylene- diaminetetracetic acid, iron oxalates, and the like.

While the iron catalyst may be used at a concentration up to the saturation point in the aqueous electrolyte, the preferred range of iron catalyst is in the range of from 0.04to 0.5 molar and most preferably from 0.05 to 0.1 molar concentration.

While certain carbonaceous materials, such as coal, may contain iron as an impurity, an ironcontaining catalyst from an external source is generally required in order to increase the rate of reaction, at least initially to acceptable levels for commercial use. The iron catalyst can conceivably be generated in situ by oxidizing sufficient iron-containing coal to generate an effective amount of iron catalyst in the electrolyte.

Of course, essentially iron-free carbonaceous materials, such as carbon black, requires an iron catalyst to be added from an external source.

Thus, in one embodiment of this invention, sufficient iron catalyst is added from an external source in orderto supply the preferred range of catalyst, namely, 0.04to 0.5 molar.

In a second embodiment, an effective amount of iron catalyst can be generated in situ by oxidizing sufficient iron-containing coal, albeit initially at a slower rate, to supply the preferred range of catalyst.

The catalyst generated would then be freed from the coal and be able to function in a similar manner as externally-supplied iron catalyst.

In a third embodiment, a combination of externally-supplied iron catalyst and in situ generated catalyst can be used to supply the preferred range of catalyst, i.e., 0.04to 0.5 molar.

The concentration or amount of carbonaceous material present in the electrolyte may vary over a wide range depending upon whether it is a liquid, solid or gas and depending on particle size, however, the preferred range is from about 0.1 gm to 0.7 gm per ml.

As in the case described in U.S. Patent No.

4,268,363, it is possible to electrowin, electroplate or electrodeposit any element that can be cathodically reduced from aqueous solution with simultaneous electrochemical anodic oxidation of carbonaceous material. Typical metallic elements often deposited in practice from aqueous electrolyte include, Cr, Mn, Co, Ni, Pb, Cu, Sn, Zn, Ga, Hg, Cd, Ir, Au, Ag, Os, Rh, Ru, Ir, Pd and Pt. Preferably the metallic elements are Cu, Zn, Ag, Ni, and Pb.

The following examples will serve to illustrate the invention, Examples 1,2, 4 and 5 being by way of comparison. It is not intended that this invention be limited to the specific examples or modifications which have been given merely for the sake of illustration, nor unnecessarily by any theory asto the mechanism of the operation of the invention.

EXAMPLES Example I Electrodeposition of Cu was conducted at constant voltage in a cell with a Nafion membrane and catholyte and anolyte solutions being pumped.

The anolyte in this first experiment contained no coal and no added iron and was pumped through an external circulation system. The catholyte was pumped through a system similar to the anolyte.

Aqueous electrolyte was 0.5M H2SO4 with the cathode also containing 0.5M in CuSO4. Total volume was 500 ml; temperature was run at 95 C and 120 C; anode was 55cm2 of iridium oxide/titanium dioxide (TIR) coated titanium; cathode was copper sheet 950C 120 C Cell Cell Potentiallvl Current (mA) Potential (V) Current (mA) 1.25 370 1.22 600.

1.00 9 1.00 35 Example 2 Apparatus and conditions were the same as those in Example 1 except that the pumped anolyte contained 0.5 g/cm3 of coal (WOW 3932). The results were as follows: 950C 1200C Cell Cell Potential (V) Current (mA) Potential (V) Current (mA) 0.40 103 0.40 170 0.60 330 0.60 520 1.00 370 1.00 627 Example 3 In the third experiment, the conditions and apparatus were the same as in Example 2 except that the anolyte was made 0.04M in Fe3+ by the addition of Fe2(SO4)3.

950C 120 C Cell Cell Potential (V) Current(mAJ Potential (V) CurrentlmAJ 0.40 180 0.40 370 0.60 530 0.60 1140 1.00 620 0.65 1310 At 120"C the Fe3+ concentration was increased to O. 1M with all other conditions and apparatus remaining constant.

The resuls were: 120 Cell Potential (V) Current (mA) 0.35 260 0.40 418 0.60 1230 Example 4 Electro production of hydrogen was conducted at constant voltage in a cell with a Nafion membrane and catholyte and anolyte solutions being stirred with magnetic stirrers.

The aqueous electrolyte was 0.5M H2SO4 for both catholyte and anolyte solutions. Total cell volume was 500 ml; temperature was 180 C; anode was 98 cm2 of iridium oxide/titanium dioxide coated titanium; cathode was platinum. The first experiment was conducted with no coal added to the anolyte. Results: 180 C Cell Potential (V) Current (A) 1.00 0.05 1.30 0.15 1.70 1.00 Example 5 Apparatus and conditions were the same as those in Example 4 except that the anolyte contained either 0.17 g/cm2 of coal (WOW 3932) or coke (El Segundo).

Coal Coke Cell Cell Potential {VJ Current (A) Potential (V) Current (A) 1.00 0.48 1.00 0.21 1.30 1.50 1.30 0.79 Example 6 The conditions and apparatus were identical to those in Example 5 except the anolyte was made 0.07M in Fe3+ by adding Fe2(SO4)3.

Coal Coke Cell Cell Potential {VJ Current (A) Potential {VJ Current (A) 1.00 1.09 1.00 1.24 1.30 3.16 1.30 3.17

Claims (13)

1. A method of electrochemical gasification of a carbonaceous material in an aqueous acidic electrolyte by anodic oxidation at the anode of an electrolysis cell, characterised in that an amount of iron in its elemental, +2 or +3 valence state, of a mixture thereof, is added to the electrolyte containing the carbonaceous material sufficient to increase the rate of oxidation of the carbonaceous material at the anode.

2. A method according to Claim 1, wherein the electrolysis cell has a cathode electrode and an anode electrode and hydrogen is produced at said cathode of the electrolysis cell.

3. A method according to Claim 1, wherein the aqueous acidic electrolyte contains a metallic element as a cationic component and the metallic element is deposited at the cathode of the electrolysis cell.

4. A method according to Claim 3, wherein said metallic element is selected from Cr, Mn, Co, Ni, Pb, Cu, Sn, Zn, Ga, Hg, Cd, Ir, Au, Ag, Os, Rh, Ru, Ir, Pd and Pt.

5. A method according to Claim 4, wherein said metallic element is copper, zinc, silver, nickel or lead.

6. A method according to any preceding claim, wherein the iron is present as a salt in a +2 or +3 oxidation state.

7. A method according to any preceding claim, wherein the electrochemical process is conducted at a temperature in the range from 25 Cto 350 C.

8. A method according to Claim 7, wherein the temperature is in the range from 120 C to 300 C.

9. A method according to any preceding claim, wherein the iron catalyst is present at a concentration of from 0.04to 0.5 molar.

10. A method according to Claim 9, wherein the iron catalyst is present at a concentration of from 0.05 to 0.1 molar.

11. A method according to any preceding claim, wherein said carbonaceous material is selected from coal, lignite, peat, char, coke, charcoal, soot, carbon black, activated carbon, asphalt, graphite wood, biomass materials and sewage sludge.

12. A method according to any preceding claim, wherein the aqueous electrolyte is acidic having a pH of 3 or less.

13. A method of electrochemical gasification of a carbonaceous material, substantially as described in the foregoing Example 3 or 6.