Conventional fuel characteristics

A comparison of the characteristics associated with propellant burning, explosive detonation, and the performance of conventional fuels (see Coal Gas, NATURAL Petroleum) is shown ia Table 1. The most notable difference is the rate at which energy is evolved. The energy Hberated by explosives and propellants depends on the thermochemical properties of the reactants. As a rough rule of thumb, these materials yield about 1000 cm of gas and 4.2 kj (1000 cal) of heat per gram of material. 

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. 

SUMMARY OF SYNFUEL CHARACTERISTICS RELATIVE TO CONVENTIONAL FUELS... 

Synthetic liquid fuels derived from coal and shale will differ in some characteristics from conventional fuels derived from petroleum. For example, liquid synfuels are expected to contain significantly higher levels of aromatic hydrocarbons, especially for coal-derived fuels, and higher levels of bound nitrogen. These differences can affect the combustion system accepting such fuels in important ways. In continuous combustors, i.e. gas turbines, the increased aromatics content of coal-derived fuels is expected to promote the formation of soot which, in turn, will increase radiation to the combustor liner, raise liner temperature, and possibly result in shortened service life. Deposit formation and the emission of smoke are other potential effects which are cause for concern. Higher nitrogen levels in synfuels are expected to show up as increased emissions of N0X (NO+NO2) An earlier paper presented results of an experimental study on the effect of aromatics and combustor... 

The properties that negatively affect bio-oil fuel qualify are foremost low heating value, incompatibility with conventional fuels, solids content, high viscosity, and chemical instability. The heating value can be significantly increased, but it requires extensive changes to the chemical structure of bio-oils, which is technically feasible but not economic. The other undesired characteristics can be improved using simpler, physical methods. Both options are reviewed below. 

The use of AF in cemenf rofary kilns is widespread either in Europe or United States with increased usage in other parts of the word such as South-East Asia, Japan, or Australia. The cement rotary kiln can be fired on conventional fuels such as pulverized fossil coal, HFO or natural gas and in combination with AF in co-firing mode. The gasification of the solid waste fuels has been infroduced only at a limited number of cement plants. The individual cement rotary kiln characteristics should always be considered when the utilization of AF is foreseen. There is always an impact on kiln production and emissions as well as on the ring and builds up formation, fhen on fhe kiln running factor. 

Catalytic combustion has received an increasing interest as an ultra-low emission technology for gas turbines. The main advantage with catalytic combustion is the possibility to decrease, and for some fuels to increase, the fuel-to-air ratio beyond the limits of flammability. Hence the combustion process and the temperature evolution in a gas turbine combustion chamber can be better controlled and stabilized. This flexibility gives the possibility to reduce emissions of unwanted pollutants compared with conventional flame combustion. Depending on fuel characteristics, emissions such as thermal and fuel-NOx, CO and hydrocarbons as well as soot can be reduced to ultra-low levels. Other advantages are lower noise levels and less vibrations. 

The reference fuel characteristics are similar to those of conventional French PWRs. The reduction of the average linear power density (by about 25 percent compared with current PWRs) improves the thermal margin and provides operational flexibility, enabling longer fuel cycles and increases in overall plant capacity. The low core power density may allow use of alternative fuel cycles, e g. MOX fuel, advanced fuels with increased burn-up, etc. 

In fact a fuel cell is a mini chemical plant with a number of pumps, valves and recirculation loops. Obviously it is hard to miniaturize it to a pocket size system. Miniaturizing fuel cells is not a simple matter of reducing physical dimensions as macro-sized (conventional) fuel cell components are limited by characteristic fabrication constraints (Cha et al., 2004, Lee et al., 2002). The materials and the manufacturing processes used in the conventional fuel cells have restrictions on dimensions and scalability. The machining of flow structures, for example, is constrained by the brittleness of graphite, and molding is limited in deep narrow... 

The measurement error for conventional motor fuels is around 0.3 points and 0.7 points for the RON and the MON respectively. The RON is the characteristic more often used and more widespread than the MON moreover, when the octane number is used without reference either procedure, it is taken to be the RON. 

The high C/H ratio for heavy fuels and their high levels of contaminants such as sulfur, water, and sediment, tend to reduce their NHV which can reach as low as 40,000 kJ/kg by comparison to the 42,500 kJ/kg for a conventional home-heating oil. This characteristic is not found in the specifications, but it is a main factor in price negotiations for fuels in terms of cost per ton. Therefore it is subject to frequent verification. 

Dielectric dryers have not as yet found a wide field of application. Their fundamental characteristic of generating heat within the solid indicates potentialities for diying massive geometrical objects such as wood, sponge-rubber shapes, and ceramics. Power costs may range to 10 times the fuel costs of conventional methods. 

With respect to the future availability of nuclear fuels, among the above concepts, fast breeder designs are of particular interest. In the following, their characteristics as compared with conventional reactor designs are described. For a better understanding, the nuclear fuel options are addressed first. 

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