Fuel fossil characteristics

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

Direct use of coal as a primary fuel is often the most efficient and economic method of utilizing this important energy resource. In many cases, however, certain undesirable properties of coal make direct utilization difficult. Coal is a solid and requires more effort to handle, measure and control than gases or liquids. Coal is usually contaminated with ash and other undesirable components and has widely variable chemical and physical properties. As a result, there is often a need to convert coal into more convenient and cleaner forms of energy and products. Before considering the basic principles of coal conversion, some important characteristics of fossil fuels will be reviewed. 

In a very fast changing world, trends are not only characteristic for fashion and political standpoints also, science is not free from hypes and buzzwords. The use of renewable resources is again more important nowadays however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about for how many years we will still be able to use fossil fuels, opinions ranging from 50 to 500 years, they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources. 

Cement made with hazardous waste fuels contains essentially the same amount of metals as cement made using traditional fossil fuels, such as coal, coke, or oil. Also, tests show cement made with hazardous waste fuels has essentially identical leaching characteristics as those of cement produced solely with traditional fuels. This means the metals are no more likely to leach out of the cement made using hazardous waste fuels than if it were made using coal, coke, or oil. 

While quantifying the sources and sinks of CH4 has been difficult, isotopic measurements of l3CH4 and CH,D4, x are promising. Various sources have characteristic isotopic signatures e.g., as mentioned previously, fossil fuel derived CH4 is depleted in l4C (Lowe et al., 1988, 1994 Wahlen et al., 1989). The sinks of CH4, e.g., reaction with OH, reaction with Cl, and uptake by soil bacteria, also exhibit kinetic isotope effects and these have been used to probe the causes of the observed recent changes in CH4 growth rates (e.g., see Gupta et al., 1996, 1997). Measurements of isotopic... 

Combustion is an oxidation-reduction reaction between a nonmetallic material and molecular oxygen. Combustion reactions are characteristically exothermic (energy releasing). A violent combustion reaction is the formation of water from hydrogen and oxygen. As discussed in Section 9.5, the energy from this reaction is used to power rockets into space. More common examples of combustion include the burning of wood and fossil fuels. The combustion of these and other carbon-based chemicals forms carbon dioxide and water. Consider, for example, the combustion of methane, the major component of natural gas ... 

Rankine Cycle. The sleam-Rankine cycle employing steam turbines has been the mainstay of utility thermal electric power generation for many years. The cycle, as developed over the years, is sophisticated and efficient. The equipment is dependable and readily available. A typical cycle (Fig. 21) uses superheat, reheat, and regeneration. Heat exchange between flue gas and inlet air adds several percentage points to boiler efficiency in fossil-fueled plants. Modern steam Rankine systems operate at a cycle top temperature of about 800 K with efficiencies of about 40%. All characteristics of this cycle are well suited to use in solar plants. 

Figure 13.4b emphasizes the finite nature and strong irreversibility of an economic system. The stock of energy and resources will eventually run out and so will the absorptive capacity of the environment for waste. An obvious extension of Figure 13.4b, therefore, is the one represented by Figure 13.4c. Just like in nature, waste has to be recycled. In nature, there is no real waste. Every form of waste is a resource for a living system. This living system is very small and called a microbe. Microbes make sure that all matter recycles in nature. Man needs to assume this humble but valuable and important role of microbes in the economic system and make sure that the material cycles get closed. Therefore, energy (or rather work) is required. But obviously this work should not be supplied from a nonrenewable source, like fossil fuels, but rather from a renewable source like the sun. Figure 13.4c therefore seems to be characteristic for a sustainable economic system and agrees remarkably with the definition of sustainability from biological systems A...

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

Another molecular ratio that has been proposed to differentiate the source of PAHs is fluoranthene to fluoranthene plus pyrene (Fl/Fl + Pyr) with 0.40 being defined as the petroleum/combustion transition point (Yunker et al., 2002). Most petroleum samples have Fl/Fl + Pyr ratio below 0.40 while those of most combustion generated PAHs are above 0.40. In addition, the ratio is further divided into two regions a ratio between 0.40 and 0.50 is more characteristic of liquid fossil fuel (vehicle and crude oil) combustion whereas a ratio greater than 0.50 is characteristic of grass, wood, or coal combustion (Zhang et al., 2004a). 

Milliman J. D. and Muller J. (1977) Characteristics and genesis of shallow-water and deep-water limestones. In The Fate of Fossil Fuel CO2 in the Oceans (eds. N.R. Anderson and A. Malahoff), pp. 655-672. Plenum Press, New York. 

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