Fuel Cell
A fuel cell is an electrochemical device in which the energy of a chemical reaction is converted directly into electricity. Unlike an electric cell or battery, a fuel cell does not run down or require recharging; it operates as long as the fuel and an oxidizer are supplied continuously from outside the cell.
A fuel cell consists of an anode to which fuel, commonly hydrogen, or methanol, is supplied; and a cathode-to which an oxidant, commonly air or oxygen, is supplied. The two electrodes of a fuel cell are separated by an ionic conductor electrolyte. In the case of a hydrogen-oxygen fuel cell with an alkali metal hydroxide electrolyte, the anode reaction is 2H2+4OH−→4H2O+4e and the cathode reaction is O2+2H2O+4e→4OH−. The electrons generated at the anode move through an external circuit containing the load and pass to the cathode. The OH— ions generated at the cathode are conducted by the electrolyte to the anode, where they form water by combining with hydrogen. The water produced at the anode is removed continuously in order to avoid flooding the cell. Hydrogen-oxygen fuel cells using ion exchange membranes or immobilized phosphoric acid electrolytes found early use in the Gemini and Apollo space programs, respectively.
Hydrogen Fuel Cell
The hydrogen fuel cell is an attractive replacement alternative to the internal combustion engine for producing electricity because it is both highly efficient and weakly polluting. The hydrogen fuel cell has existed for a long time and could produce an estimated 40% of the total energy demand within the next fifty years.
A conventional hydrogen fuel cell identified with numeral reference 10 is shown in FIG. 1. As shown, a hydrogen fuel cell mainly consists of two electrodes, an anode (2) and a cathode (4), separated by an electrolyte (6). At the anode (2), gaseous hydrogen liberates an electron and a positively charged ion. The electron transits through an external circuit (8) producing an electrical current while the ion diffuses through the electrolyte. At the cathode (4), the electrons combine with the hydrogen ions and oxygen to form water a non-polluting waste product that can be reused. A catalyst is used in order to accelerate this oxidizing reaction.
Hydrogen fuel cells are classified according to operating temperature and electrolyte type. There are five different categories of hydrogen fuel cells according to the type of electrolyte used: phosphoric acid; molten carbonate; solid oxide; proton-exchange membrane; and alkaline.
In order to implement hydrogen fuel cells in a day-to-day life, two technological obstacles must be overcome. The first concerns the cell itself and the second concerns the production, storage and transport of the hydrogen that is the energy vector of the process. Up to now, the development of the hydrogen fuel cell has leaped ahead while the hydrogen storage technology lags behind. This technological delay could very well be a determining factor in the marketability of hydrogen fuel cells.
Although hydrogen is a relatively widely present element in nature, it rarely presents itself as a readily available molecularly free gas. Hydrogen is therefore not considered as a source of energy per se, but rather as an energy vector. Various methods are available in order to produce hydrogen. Among these, there is the electrolysis of water or pyrol. However, the only profitable method at the present time is the production of hydrogen from fossil fuels.
Once produced, hydrogen may be stored as a liquid at extremely low temperatures or alternatively, under extremely high pressure. However, the high cost of this storing approach favours the use of a technology where the hydrogen is produced in place and at a production rate governed by its consumption rate. Hence, instead of storing liquid hydrogen, a liquid fuel containing hydrogen, for instance liquid hydrocarbons, is transported in place to be transformed in a later stage, and as needed, into hydrogen.
The presently available hydrogen fuel cell technology typically uses a liquid fuel containing hydrogen, for example diesel, gasoline, methanol, natural gas, etc. Such liquid fuel is converted by a reformer into a gaseous mixture of hydrogen and carbon dioxide. Then, the hydrogen is extracted from the gaseous mixture and supplied to the hydrogen fuel cell.
Methanol, for example, which is in liquid form at room temperature, contains a large quantity of hydrogen that may be chemically extracted. The process used to convert hydrocarbons such as methanol into hydrogen and carbon is internal or external reforming. Partial oxidizing and autothermic reforming are other physico-chemical processes that allow for separation of hydrogen from hydrocarbons. Vapour reforming of hydrocarbons (crude or refined oil, natural gas or gasified coal) yields hydrogen-rich mixtures that must be treated further in order to remove carbon monoxide and carbon dioxide which hinder the operation of certain hydrogen fuel cells.
The vapour reforming of natural gas involves the catalysed endothermic conversion of light hydrocarbons (including methane through gasoline) with water vapour. The industrial process usually occurs at 850° C., under 2.5 mPa of pressure, according to the following equation:CnHm+nH2OnCO+(n+m/2)H2  (I)                where n is an integer equal or greater than 1;m=2n+2.        
The catalyzed exothermic conversion (shift reaction) of the carbon monoxide by-product produces hydrogen according to the following equation:CO+H2OCO2+H2  (II)
The CO2 is separated from the gas mixture by absorption processes or membrane separation, and the hydrogen, before being transferred to the hydrogen fuel cell, is treated a second time in order to remove unwanted compounds. Presently, and despite the important costs linked to its extraction, the carbon dioxide is generally released into the atmosphere, thereby contributing to the increase of the greenhouse effect.
The partial oxidizing of heavy hydrocarbons involves the exothermic or autothermic conversion of heavy hydrocarbons in the presence of oxygen and water vapour. The following equation describes the phenomenon:CH1,4+0,3H2O+0,4O20,9CO+0,1CO2+H2  (III)
The partial oxidizing of coal, except for the initial preparation of coal, follows the same process as the one used for the gasification of heavy hydrocarbons. By-products also include CO2.
Although hydrogen fuel cell technologies presently pollute less than internal combustion engines, these systems when combined with hydrocarbon reforming for example, still produce CO2, the greatest contributor in terms of volume to the greenhouse effect. The growing concerns over climatic changes add to the pressures aimed at reducing such emissions which contribute to the greenhouse effect. The aforementioned need for decarbonization of hydrocarbons during reforming is but one example that warrants the use of a carbon dioxide separation and sequestration technology in order to avoid CO2 dumping into the environment. Also, the presence of all the non-hydrogenated contaminants in the fuel decreases the hydrogen fuel cell's power and efficiency.
There already exists, in the prior art, a large number of processes for the production of electricity which are based on the hydrogen fuel cell concept. Among these, many rely on a typically less polluting process than the internal combustion engine but still dump gaseous CO2 back into the atmosphere (JP2001-229946; JP11-229167; WO 01/23302; U.S. Pat. Nos. 6,299,744; 5,746,985) or into the water (U.S. Pat. No. 5,141,823).
Literature abounds with a substantial number of systems integrating a treatment intended to reduce CO2 emissions by these types of cells. The United States government already uses this technology for certain space applications and intends of providing such a system for a future Mars mission (U.S. Pat. No. 5,005,787). The carbon dioxide produced by the crew and the hydrogen fuel cell would be introduced into a greenhouse and in return, the oxygen-charged air would be recirculated back into the living quarters and cabins. Japanese documents JP2000-287546, JP11-275965, and JP06-333589 also propose a similar solution in order to definitely remedy the CO2 emission problem stemming from hydrogen fuel cell operation.
CO2 recycling is also proposed in other applications, often depending on the type of cell used. CA 2,326,024 and JP2001-219271, for example, suggest the recycling of CO2 and its use as a shielding gas during an arc welding process. The molten carbonate cells are a particular case in which CO2 is used in order to enrich the oxidant at the cathode. A number of these technologies aim to convert CO into CO2 first (JP2000-340247; JP2000-302405; JP2000-251918; JP11-199202; TW 432,741), and then to recycle this CO2 toward the hydrogen fuel cell's cathode (JP2000-331698; JP2000-260446; JP11-162488; JP11-067251; JP06-275291; JP02-301968; CA 2,002,227/Absorption).
Some designers wager on a slightly different know-how, namely the methanation of CO2 (JP04-190831; JP08-069808; WO 01/04045; U.S. Pat. Nos. 5,997,594; 6,221,117). Thus reconversion of carbon dioxide into CH4 permits the recycling of carbon and hydrogen by conventional hydrogen fuel cells. U.S. Pat. No. 6,187,465 puts forward a means for recirculating CO2 for a hydrogen fuel cell without any unwanted emissions.
Adsorption is a <<classic>> which invariably sustains interest. Zeolites (JP08-069808; EP 0 700 107) occupy a substantial part of this consideration, but other systems also attract attention such as JP09-204925 which uses an ionic resin. The separation of gaseous hydrogen by way of a porous membrane or surface (JP2001-139304/zirconate; CA 2,322,871) allows for the extraction of CO2 which may then be stocked (WO 01/83364), recycled or discarded (U.S. Pat. No. 5,759,712). There is also the PSA (Pressure Swing Adsorption) described for example in JP10-027621; JP62-274561; and U.S. Pat. No. 6,299,994.
Obviously, the separation and sequestration of CO2 remain a largely explored area covering a realm of possibilities. In view of the fact that carbon dioxide reduces the performance of hydrogen fuel cells, numerous processes include either a CO2 <<separation unit>> (JP62-170171; U.S. Pat. No. 5,248,567; CA 2,332,079), a CO2 <<separation apparatus>> (JP06-044993; JP07-169482; JP04-337252; WO 00/03126), or what is known as a <<scrubbing device>> (WO 01/25140; EP 0 677 883; U.S. Pat. No. 4,537,839; JP59-184468). Finally, JP11-162495 refers to a <<decarbonation system>>.
Absorption of exhaust fumes is another method which sustains keen interest among many inventors as a number of patents/patent applications demonstrate (JP02-206689; JP60-035470; JP60-241673; JP2001-106503/aqueous sodium hydroxide; JP2001-023677/water or alkaline solution; EP 0 376 219) (JP2000-297656; JP04-190831; JP04-051464; WO 01/04045; WO 01/17896; WO 01/23302; WO 01/65621; CA 1,291,787). Although different means have been employed, the cornerstone of innovation in this field remains the improvement of the solubilization of the gas in question.
Also known in the prior art, there are processes aiming at reducing CO2 emissions in general. Examples of those processes are given in U.S. Pat. Nos. 5,514,378; 6,143,556; CA 2,222,030; 6,258,335; and EP 0 991 462.
U.S. Pat. No. 5,514,378 discloses a process for the reduction of CO2 emissions in an enzymatic photobioreactor using carbonic anhydrase. Carbonic anhydrase is a very reactive enzyme that is common to most animal and plant species and hence, readily available. Trachtenberg (U.S. Pat. No. 6,143,556; CA 2,222,030) describes a system for the treatment of exhaust fumes with an enzyme that may be carbonic anhydrase, but does not suggest any specific application with respect to hydrogen fuel cells. The University of Michigan presents a photobioreactor for the treatment of CO2 with carbonic anhydrase for a medical application as an artificial lung (WO 92/00380; U.S. Pat. No. 5,614,378).
U.S. Pat. No. 6,258,335 discloses a catalytic process for the removal of exhaust CO2 through chemical fixation. This process comprises the optional use of carbonic anhydrase without any particular consideration for reforming exhaust. Materials Technology Corporation (U.S. Pat. No. 4,743,545) presents a bioreactor with a catalyst, that may be carbonic anhydrase, included inside of a hollow bead.
EP 0 991 462 in the name of the applicant discloses a counter-current-packed-column for CO2 treatment. In this process, carbonic anhydrase is used in free or immobilized states.
U.S. Pat. No. 6,110,370 discloses a process where carbonic anhydrase is used to treat water and U.S. Pat. Nos. 4,602,987; 4,761,209; CA 1,294,758 in the name of Aquanautics Technology Corporation disclose a method for extracting and using oxygen from fluids. This system includes a step for the separation of CO2 by using the carbonic anhydrase enzyme and an electrochemical cell.
Although a lot of efforts have been put forward in the development and improvement of hydrogen fuel cell in terms of production and storage of the hydrogen fuel, and in terms of reduction of CO2 emissions, there is still place for improvements in this field. More particularly, there is still a need for a process or an apparatus that would provide a simple and affordable way of producing the required fuel, hydrogen; and that without further polluting the atmosphere with CO2 emissions.