Direct oxidation fuel cells based on other fuels such as ethanol, formaldehyde, formic acid, DME, dimethoxymethane, and trimethoxymethane, have been studied in laboratories worldwide. However, none of these has shown so far the promise of either the H2-PEM fuel cell or DMFC, although application of fuel mixes is feasible.
Biofuel cells use biocatalysts for the conversion of chemical energy into electrical energy. As most organic materials undergo combustion with the evolution of energy, the biocatalyzed oxidation of organic substances by oxygen or other oxidizers at two-electrode interfaces provides a means for the conversion of chemical to electrical energy. Abundant organic raw materials such as ethanol, hydrogen sulfide, organic acids or glucose can be used as substrates for these oxidation process, while molecular oxygen or H2O2 can be reduced. Intermediate formation of hydrogen as a potential fuel is also possible. Biofuel cells can use biocatalysts, enzymes or even whole-cell organisms. The power produced in such devices are miniscule (microwatt to nanowatt range), although such devices have potential uses as chemical and biological sensors.
A regenerative fuel cell concept based on methanol/formic acid fuel cells has also been proposed (Fig. 11.10) . The key to the success of such an approach is efficient capture of CO2 and its electrochemical reduction to either HCOOH or CH3OH in high current efficiencies. Intense research to achieve efficient electrochemical reduction of CO2 is currently under way in many laboratories.
Electric Power on demand
Liquid-Feed Fuel Cell c02 + 2h20" or c02 + 2 h20 "
Figure 11.10 Regenerative fuel cell system based on CO2.
Transportation and other mobile applications are not the only areas where methanol can be used as a fuel; indeed, it is also an attractive fuel for static applications. It can be used directly as a fuel in gas turbines to generate electric power. Gas turbines use typically either natural gas or light petroleum distillate fractions as fuels. Compared to these fuels, tests conducted by many institutions beginning in the 1970s, have shown that methanol can achieve higher power output and lower NOx emissions due to lower flame temperatures. Since methanol does not contain sulfur, SO2 emissions are also eliminated [159, 160]. Operation on methanol offers the same flexibility as on natural gas and distillate fuels, including the ability to start, stop, accelerate and decelerate rapidly, following the electric power needs. Existing turbines, designed originally for natural gas and other fossil fuels, can be relatively easily and inexpensively modified to run on methanol. For this application, fuel-grade methanol with lower production costs than higher purity chemical-grade methanol can be used. Considering increasing natural gas prices, methanol produced at low-cost from remote natural gas resources in the Middle-East or other regions and shipped much more easily and less expensively than LNG, offers also an alternative for power generation in large consuming centers such as North America, Europe, or Japan.
For static uses, the size and weight of fuel cells are of lesser importance compared to mobile applications. Besides PEM fuel cells and DMFC, phosphoric acid, molten carbonate and solid oxide fuel cells (PAFC, MCFC and SOFC), all of which are ill-suited for automobiles, can also be used for static power and heat generation. As described in Chapter 9, these fuel cells are already being used in the production of electricity in facilities sensitive to power outages such as airports, hospitals, military complexes, and banks. Whereas the present cost of these installations is still high, their price is expected to decrease with further development and the number of units produced. General Electric, for example, is developing a fuel cell unit called the HomeGen 7000 with a 7-kW capacity, and which is intended for the production of electricity for the residential home market . For such static applications, liquid methanol, which is easy to handle, deliver and store, would be the fuel of choice.
In developing countries, methanol has been proposed as a substitute cooking fuel in place of wood and expensive and inconvenient kerosene. The consumption of large quantities of wood for cooking purposes by more than 2.5 billion people is in fact one of the major causes of deforestation and all the ecological (desertification, excessive erosion, land-slides, etc.) and socio-economical problems associated with it in the developing areas of the world. Wood-burning stoves in use in these countries are generally also very inefficient, producing also much smoke, fumes and soot, all of which are serious health hazards. In eliminating these drawbacks, stoves designed specifically for methanol have been developed [162, 163].
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