Thermochemical properties of fuels

We need to understand the molecular origins of the energy content of biological fuels, the carbohydrates, fats, and proteins. 

We saw in Case study 1.1 that photosynthesis and the oxidation of organic molecules are the most important processes that supply energy to organisms. In this section we begin our quantitative study of biological energy conversion by assessing the thermochemical properties of fuels. 

The consumption of a fuel in a furnace or tui engine is the result of a combustion. An example is the combustion of methcuie in a natural gas flame  

The standard enthalpy of combustion, A H-, is the standard chtmge in enthalpy per mole of combustible molecules. In this extunple, we would write AcH-(CH4, g) = -890 kJ molSome typical values tffe given in Table 1.5. Note that AcH- is a molar quantity and is obtained from the value of AH- by dividing by the amount of organic reactant consumed (in this case, by 1 mol CH4). 

According to the discussion in Sections 1.5 and 1.6, and the relation AU = qv, the energy transferred as heat at constant volume is equal to the change in internal energy, A [/, not AH. To convert from A1/ to AH, we need to note that the molar enthalpy of a substance is related to its molar internal energy by H = U + pV (eqn 1.12a). For condensed phases, pUm is so small that it maybe ignored. For example, the molar volume of liquid water is 18 cm mol , and at 1.0 bar 

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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. 

Now it is important to stress that, whereas the laminar flame speed is a unique thermochemical property of a fuel-oxidizer mixture ratio, a turbulent flame speed is a function not only of the fuel-oxidizer mixture ratio, but also of the flow characteristics and experimental configuration. Thus, one encounters great difficulty in correlating the experimental data of various investigators. In a sense, there is no flame speed in a turbulent stream. Essentially, as a flow field is made turbulent for a given experimental configuration, the mass consumption rate (and hence the rate of energy release) of the fuel-oxidizer mixture increases. Therefore, some researchers have found it convenient to define a turbulent flame speed, S T as the mean mass flux per unit area (in a... 

A core aspect of the model is the determination of the chemical conversion of gasifier SG to hydrogen by the RP. For conditions where steam availability is not limiting, the chemical conversion relates to the difference between the initial and final combustion/fuel ratio of the fuel gas. The initial CP/SG ratio is determined by the gasifier and the biomass feedstock. The final CP/SG ratio is determined by the thermochemical properties of the metal oxide material. Ideally the difference between the initial (CP/SG),and final (CP/SG)j , ratios should be as large as possible. In reality the availability of steam for the re-oxidation of the metal oxide is limiting for conditions where the difference in the CP/SG ratios are large. 

Now it is important to stress that, whereas the laminar flame speed is a unique thermochemical property of a fuel-oxidizer mixture ratio, a turbulent flame speed is a function not only of the fuel-oxidizer mixture ratio, but also of the flow characteristics and experimental configuration. Thus, one encounters great difficulty... 

Table 1 Thermochemical properties of various bioenergy feedstocks and fuels...

Jenkins, B.M., Ebeling, J.M., 1985. Thermochemical properties of biomass fuels. CaUforaia Agriculture 14—16. 

All these fuels belong to a group of high-energy-density fuels with compact molecular structure rendered by the presence of pentacyclic cages. They are stable and nonvolatile at room temperature and pressure. Three formulations are solid and the fourth is a viscous liquid. Their S3mthesis and molecular structure analysis that uses X-ray crystallographic methods have been described by Marchand [5, 6]. Their molecular structure and physical properties are presented briefly below. Measured thermophysical and thermochemical properties follow. 

A new class of pentacyclic HED fuels was analyzed and some thermophysical and thermochemical properties have been measured. The main findings were as follows ... 

Between 1963 and 1967, the International Atomic Energy Agencj (IAEA) in Vienna published three teehnieal reports with thermochemical assessments of the nuclear fuel systems U-C andPu-C [1963IAE], UO2 and related phases [19651AE], and Pu-0 and U-Pu-0 [1967IAE]. These reports were followed by nine speeial issues of Atomic Energy Review with eompilations of physicochemical properties of compounds... 

As a consequence of the better heat transfer and heat transport properties of liquids compared with gases, the normal peak operating fuel temperature in an AHTR is expected to be lower than in helium-cooled reactors for heat delivered at the same temperatures to the power cycle or thermochemical hydrogen production plant. There are four effects. 

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