Types of Fuels

Uranium dioxide, UO2, is the compound of choice in many nuclear reactors despite its relatively poor heat conduction properties. This is due to its chemical stability, its high melting point, and the ease of production of well-characterized morphological and physical properties. The complete characterization is described in Chapter 2. Uranium metal and particularly uranium alloys like U-Al, U-Zr, U-Si, and U-Mo are also used as fuel. Their heat conduction is superior to that of uranium oxide but the metal and alloys are less stable chemically. 

Another promising uranium compound that can be used in nuclear fuels is uranium carbide that has a high melting point and better thermal conductivity than the oxide and in addition does not form oxygen when radiolyzed. Uranium nitride can also be used, but formation of from N could be problematic. In addition, other uranium compounds that can be used as a fuel in a nuclear reactor, ranging from aqueous solutions to molten salts that are brought to a high temperature in order to keep them in a molten state. MOX of uranium and plutonium also serve as a nuclear fuel in some reactors. 

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Desulfurize the fuel. Most types of fuel can be desulfurized. However, as we go from gaseous to liquid to solid fuels, the desulfurization process becomes increasingly difficult. 

Fuel-bound NO. is formed at low as well as high temperatures. However, part of the fuel nitrogen is directly reacted to N2. Moreover, N2O and N2O4 are also formed in various reactions and add to the complexity of the formation. It is virtually impossible to calculate a precise value for the NO, emitted by a real combustion device. NO, emissions depend not only on the type of combustion technology but also on its size and the type of fuel used. 

Note that the RVP is a relative pressure that is a difference compared to the atmospheric pressure. The RVPs for gasolines are generally between 350 and 1000 millibar. The level corresponding to European specifications are shown in Table 5.6 the fuel must be simultaneously within minimum and maximum limits, identical for each type of fuel, gasoline and premium, but... 

Table 5.9 shows the octane specifications for the different types of fuels distributed in France. 

The potential advantages of LPG concern essentially the environmental aspects. LPG s are simple mixtures of 3- and 4-carbon-atom hydrocarbons with few contaminants (very low sulfur content). LPG s contain no noxious additives such as lead and their exhaust emissions have little or no toxicity because aromatics are absent. This type of fuel also benefits often enough from a lower taxation. In spite of that, the use of LPG motor fuel remains static in France, if not on a slightly downward trend. There are several reasons for this situation little interest from automobile manufacturers, reluctance on the part of automobile customers, competition in the refining industry for other uses of and fractions, (alkylation, etherification, direct addition into the gasoline pool). However, in 1993 this subject seems to have received more interest (Hublin et al., 1993). 

Another even more significant use of methyl alcohol can be as a fuel in its own right in fuel cells. In recent years, in cooperation with Caltech s Jet Propulsion Laboratory (JPL), we have developed an efficient new type of fuel cell that uses methyl alcohol directly to produce electricity without the need to first catalytically convert it to produce hydrogen. 

Another important potential appHcation for fuel cells is in transportation (qv). Buses and cars powered by fuel cells or fuel cell—battery hybrids are being developed in North America and in Europe to meet 2ero-emission legislation introduced in California. The most promising type of fuel cell for this appHcation is the SPEC, which uses platinum-on-carbon electrodes attached to a soHd polymeric electrolyte. 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

Steam-Generating Facilities These form the second largest investment item for chemical-plant auxiliaiy equipment. Variations in capacity, location indoors or outdoors, the type of fuel used, pressure and temperature levels, and the type of process served have an important effect on actual cost as well as on cost relative to other auxiliaiy items. Package boiler instaUations can be purchased as shop-built units which are assembled, piped, and wired ready to be erected on the owner s foundations. They are available in units up to about 136,000 kg/h (300,000 Ib/h), although units larger than about 45,360... 

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

The hfe of a gas turbine depends heavily on the type of fuel used. An inherent fuel flexibility is the gas turbines major advantage. Gaseous fuels traditionaUy include natural gas, process gas, and low-Btu gas... 

Reliability of a plant depends on many parameters, sueh as the type of fuel, the preventive maintenanee programs, the operating mode, the eontrol systems, and the firing temperatures. 

Other important parameters that govern the selection and location of the plant are distance from transmission lines, location from fuel port or pipe lines, and type of fuel availability. Site configuration is generally not a constraint. Periodically, sites are encountered where one plant configuration or another is best suited. 

Plant Type. The determination to have an aero-derivative type gas turbine or a frame-type gas turbine is the plant location. In most cases if the plant is located off-shore on a platform then an aero-derivative plant is required. On most on-shore applications, if the size of the plant exceeds 100 MW then the frame type is best suited for the gas turbine. In smaller plants between 2-20 MW, the industrial type small turbines best suit the application, and in plants between 20-100 MW, both aero-derivative or frame types can apply. Aero-derivatives have lower maintenance and have high heat-recovery capabilities. In many cases, the type of fuel and service facilities may be the determination. Natural gas or diesel no. 2 would be suited for aero-derivative gas turbines, but heavy fuels would require a frame type gas turbine. 

The type of fuel is one of the most important aspects that govern the selection of a gas turbine. Chapter 12 handles the type of fuels and their effect in detail. Natural gas would be the choice of most operators if natural gas was available since its effects on pollution is minimal and maintenance cost would also be the lowest. Table 4-1 shows how the maintenance cost would increase from natural gas to the heavy oils. 

Typical Gas Turbine Maintenance Cost Based on Type of Fuel... 

Type of Fuel Expected Actual Maintenance Cost Relative Maintenance Cost Factor... 

Gas turbines may be designed to burn either gaseous or liquid fuels, or both with or without changeover while under load. This standard covers both types of fuel. 

Liquid fuels require atomization and treatment to inhibit sodium and vanadium content. Liquid fuels can drastically reduce the life of a unit if not properly treated. A typical fuel system is shown in Figure 4-7. The effect of fuels on gas turbines and the details of types of fuel handling systems is given in Chapter 12. 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

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