Overall efficiency Direct conversion

In contrast to the operation of vehicles, electricity and heat for stationary applications can be generated by the combustion of solid biomass without upstream biomass conversion to pure hydrogen (or methanol, BTL or DME). The efficiency of the direct use of solid biomass is generally higher. The overall efficiency of a solid-biomass-fuelled heat and power (CHP) plant is typically about 70% to 80% direct combustion of solid biomass (e.g., wood chips, wood pellets) in suitable boilers for heat generation only can reach an efficiency of more than 90%. 

Stationary hydrogen demand has not been considered. Stationary fuel cells are not necessarily a market for hydrogen, because natural gas from the gas mains can easily be used directly a conversion of natural gas or biogas to hydrogen would only reduce the overall efficiency (see Chapter 13). 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

Reasons for interest in the catalyzed reactions of NO, CO, and COz are many and varied. Nitric oxide, for example, is an odd electron, hetero-nuclear diatomic which is the parent member of the environmentally hazardous oxides of nitrogen. Its decomposition and reduction reactions, which occur only in the presence of catalysts, provide a stimulus to research in nitrosyl chemistry. From a different perspective, the catalyzed reactions of CO and COz have attracted attention because of the need to develop hydrocarbon sources that are alternatives to petroleum. Carbon dioxide is one of the most abundant sources of carbon available, but its utilization will require a cheap and plentiful source of hydrogen for reduction, and the development of catalysts that will permit reduction to take place under mild conditions. The use of carbon monoxide in the development of alternative hydrocarbon sources is better defined at this time, being directly linked to coal utilization. The conversion of coal to substitute natural gas (SNG), hydrocarbons, and organic chemicals is based on the hydrogen reduction of CO via methanation and the Fischer-Tropsch synthesis. Notable successes using heterogeneous catalysts have been achieved in this area, but most mechanistic proposals remain unproven, and overall efficiencies can still be improved. 

Well-established anode materials are Ni cermets such as Ni/YSZ composites. The presence of the second phase increases the contact area and prevents the catalytically active Ni particles from aggregating. The use of the composite becomes problematic if hydrocarbons are to be directly converted Ni catalyzes cracking, and the resulting carbon deposition deactivates the fuel cells. Therefore either pure H2 has to be used or the fuel has to be externally reformed. A third way is internal conversion of CHV with H20 to synthesis gas. The necessary steam addition, however, reduces the overall efficiency. Another problem of Ni cermets, if they are to be used at lower temperatures, is a potential oxidation of the Ni. Alternatives are Cu/Ce02 cermets in which Cu essentially provides the electronic conductivity and Ce02 the catalytic activity. Note that an efficient current collecting property of the electrode presupposes a metal concentration above the percolation threshold. 

The first report of direct photoconversion of water into dihydrogen and dioxygen was studied on Ti02 (n-type) electrodes by Fujishima and Honda [64]. Although the overall efficiency of the system was low, their results initiated the interest in photoelectrochemical cells for conversion of solar energy to chemical energy (or to electrical energy). 

With areal power outputs only 20-30% that of a PEFC and an energy-conversion efficiency of 30% near peak power versus 50% in the case of the PEFC, the DMFC remains of great interest because of the attractive properties of methanol fuel, a liquid of high energy density under ambient conditions, and because the DMFC enables direct conversion of this liquid carbonaceous fuel to electric power. Particularly in portable applications, these features help minimize the overall dimensions of the power system (fuel + fuel cell + auxiliaries) and achieve high system energy density. 

It has been pointed out (83,96,97) that the direct conversion of syngases of low H2 to CO ratio does not only save the investment of the shift reactor but reduces additionally operation costs due to savings in steam demand. Hence, the overall thermal efficiency can be improved. ... 

More recently, many efforts to develop hybrid power plants combining fuel cells and gas turbines were made. While the high-temperature fuel cells, such as SOFGs and MGFGs, produce electrical power, the gas turbines produce additional electrical power from the heat produced by the fuel cells operation. At the same time, the gas turbines compress the air fed into the fuel cells. The expected overall efficiency for the direct conversion of chemical energy to electrical energy is up to 80 percent. 

It was recognized early that the overall thermodynamic efficiency of steam engines is only about 15%. The efficiency of modem electrical generators is about 20-50%, whereas the efficiency of the fuel cell (in which there is direct conversion of chemical energy into electrical energy) does not have any thermodynamic limitation. Theoretically, the efficiency of the fuel cell can approach 100%, and in practice, efficiency of over 80% can be achieved. 

Calculations show that such plants would have an overall efficiency for the direct conversion of chemical energy (natural gas) to electrical energy [referred to the lower heating value (LHV)] of 62% in the near term, 67% in the medium term, and possibly as much as 74.6% in the long term, when the gas turbines will have been further improved. Disregarding the turbine, the efficiency of the fuel cells alone would be 50% in the near term and 57% in the medium term. 

---