Fuel thermodynamics

See also Aerodynamics Diesel Cycle Engines Diesel Fuel Engines Environmental Problems and Energy Use Nuclear Fission Fuel Thermodynamics Transportation, Evolution of Energy Use and Turbines, Gas Turbines, Steam Waves. 

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

Power plants based on the Rankine thermodynamic cycle have served the majority of the world s electric power generation needs in the twentieth century. The most common heat sources employed by Rankine cycle power plants are either fossil fuel-fired or nuclear steam generators. The former are the most widely used. 

Physica.1 Properties. Carbonyl sulfide [463-58-1] (carbon oxysulfide), COS, is a colorless gas that is odorless when pure however, it has been described as having a foul odor. Physical constants and thermodynamic properties are Hsted ia Table 1 (17,18). The vapor pressure has been fitted to an equation, and a detailed study has been made of the phase equiUbria of the carbonyl sulfide—propane system, which is important ia the purification of propane fuel (19,20). Carbonyl sulfide can be adsorbed on molecular sieves (qv) as a means for removal from propane (21). This approach has been compared to the use of various solvents and reagents (22). 

When the partial pressures of the radicals become high, their homogeneous recombination reactions become fast, the heat evolution exceeds heat losses, and the temperature rise accelerates the consumption of any remaining fuel to produce more radicals. Around the maximum temperature, recombination reactions exhaust the radical supply and the heat evolution rate may not compensate for radiation losses. Thus the final approach to thermodynamic equiUbrium by recombination of OH, H, and O, at concentrations still many times the equiUbrium value, is often observed to occur over many milliseconds after the maximum temperature is attained, especially in the products of combustion at relatively low (<2000 K) temperatures. 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

Thermodynamics. The first law of thermodynamics, which states that energy can neither be created nor destroyed, dictates that the total energy entering an industrial plant equals the total of all of the energy that exits. Eeedstock, fuel, and electricity count equally, and a plant should always be able to close its energy balance to within 10%. If the energy balance does not close, there probably is a big opportunity for saving. 

In order to see why, we need to look at our car in a bit more detail (Fig. 5.2). We start by assuming that it is surrounded by a large and thermally insulated environment kept at constant thermodynamic temperature Tq and absolute pressure po (assumptions that are valid for most structural changes in the earth s atmosphere). We define our system as (the automobile -1- the air needed for burning the fuel -1- the exhaust gases... 

Usually, a gas turbine plant operates on open circuit , with internal combustion (Fig. 1.3). Air and fuel pass across the single control surface into the compressor and combustion chamber, respectively, and the combustion products leave the control surface after expansion through the turbine. The open circuit plant cannot be said to operate on a thermodynamic cycle however, its performance is often assessed by treating it as equivalent to a closed cyclic power plant, but care must be taken in such an approach. 

In both cases heat is taken from the exhaust gases to feed the reaction process, enhancing the heating value of the resulting modified fuel, which is then fed to the combustion chamber. But the main thermodynamic feature is that the exergy loss in the final exhaust gas is thus reduced and the efficiency increased. 

Determine the flash fraction of fuel on the basis of actual thermodynamic data. Equation (7.1) provides a method of estimating the flash fraction. 

Although biomass used directly for heating and cooking is the thermodynamically most efficient use, followed by use for electricity generation, the economics are much more favorable to convert to a liquid fuel. Economic considerations outweigh thermodynamics as an electricity generator, biomass must compete with relatively low-priced coal, but as a liquid fuel the competition is higher-priced oil. 

Cogeneration encompasses several distinct thermodynamic processes of simultaneous heat and power production. One utilizes air as a medium, another steam, a third employs heat rejected from a separate combustion process, such as an internal-combustion engine, and a fourth utilizes a thermochemical process such as found in a fuel cell. Although each process is distinct, they are often combined together to inaxiniize the energy production in a single thermodynamic system. 

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