Thermochemical conversion

Hydrogen can be obtained by direct electrolysis, by direct thermal conversion, thermochemically, photochemically, photoelectrochemically, and biochemically from water. 

Work in the mid-1970s demonstrated that the vitamin K-dependent step in prothrombin synthesis was the conversion of glutamyl residues to y-carboxyglutamyl residues. Subsequent studies more cleady defined the role of vitamin K in this conversion and have led to the current theory that the vitamin K-dependent carboxylation reaction is essentially a two-step process which first involves generation of a carbanion at the y-position of the glutamyl (Gla) residue. This event is coupled with the epoxidation of the reduced form of vitamin K and in a subsequent step, the carbanion is carboxylated (77—80). Studies have provided thermochemical confirmation for the mechanism of vitamin K and have shown the oxidation of vitamin KH2 (15) can produce a base of sufficient strength to deprotonate the y-position of the glutamate (81—83). 

Thermodynamic calculations for reactions forming carbon disulfide from the elements are compHcated by the existence of several known molecular species of sulfur vapor (23,24). Thermochemical data have been reported (12). Although carbon disulfide is thermodynamically unstable at room temperature, the equiHbtium constant of formation increases with temperature and reaches a maximum corresponding to 91% conversion to carbon disulfide at about 700°C. Carbon disulfide decomposes extremely slowly at room temperature in the absence of oxidizing agents. 

The thermochemical equation allows us to relate the enthalpy change to amounts of reactants and products, leading to conversion factors such as... 

The only quantity considered here is the enthalpy of formation, A fH°, at 298.15 K. Data are given in units of kJmol-1. The conversion factor 1 thermochemical calorie = 4.1840 joules was used. 

In order to have a consistent basis for comparing different reactions and to permit the tabulation of thermochemical data for various reaction systems, it is convenient to define enthalpy and Gibbs free energy changes for standard reaction conditions. These conditions involve the use of stoichiometric amounts of the various reactants (each in its standard state at some temperature T). The reaction proceeds by some unspecified path to end up with complete conversion of reactants to the various products (each in its standard state at the same temperature T). 

When the reactivity functions are applied to 8 in Fig. 33, the two reactions most favored by the reactivity function lead back to 47 and 48. Both reactions are endothermic and thus unfavorable in the direction leading to 8. In synthesis design, endothermic retroreaetions should be preferred. Therefore, in searching for a synthesis of 8, the compounds 47 and 48 are attractive precursors based on considerations of electronic effects, favorable reaction mechanisms for the conversion of 47 or 48 to 8 can be established. Furthermore, these conversions are thermochemically favorable. 

De Maria, G. et al., Thermochemical conversion of solar energy by steam reforming of methane, Energy, 11, 805, 1986. 

When implementing a biomass-based thermochemical conversion system, it is important to critically evaluate the feedstock characteristics such as cost, distribution, mass, and physical and chemical properties. The feedstock qualities must be considered when matching feedstocks with a proper conversion technology. 

This review limits itself to the treatment of high-temperature thermochemical biomass conversion technologies. There are very good overviews of biological conversion technologies for hydrogen production, for example, Ni et al.13 and Zaborsky.29... 

Ethanol can be derived from biomass by means of acidic/enzymatic hydrolysis or also by thermochemical conversion and subsequent enzymatic ethanol formation. Likewise for methanol, hydrogen can be produced from ethanol with the ease of storage/transportation and an additional advantage of its nontoxicity. Apart from thermodynamic studies on hydrogen from ethanol steam reforming,117-119 catalytic reaction studies were also performed on this reaction using Ni-Cu-Cr catalysts,120 Ni-Cu-K alumina-supported catalysts,121 Cu-Zn alumina-supported catalysts,122,123 Ca-Zn alumina-supported catalysts,122 and Ni-Cu silica-supported catalysts.123... 

Baker, E. Mudge, L. Wilcox, W. A., Catalysis of gas phase reactions in steam gasification of biomass. In Fundamentals of Thermochemical Biomass Conversion, Overend, R. P. et al., Ed., Elsevier Applied Science, London, 1985, pp. 1194-1208. 

Mudge, L. K. Baker, E. G. Brown, M. Wilcox, W., Catalytic destruction of tars in biomass-derived gases. In Research in Thermochemical Biomass Conversion, Bridgwater, A. V. Kuester, J. L., Eds., Elsevier Applied Science, London, 1988, pp. 1141-1155. 

Biollaz, S. Sturzenegger, M. Stucki, S., Redox Process for the production of clean hydrogen from biomass. In Progress in Thermochemical Biomass Conversion, Seefeld, Tirol, 2000. 

Tables C. 1-C.4 provide conversion factors from a.u. to SI units and a variety of practical (thermochemical, crystallographic, spectroscopic) non-SI units in common usage. Numerical values are quoted to six-digit precision (though many are known to higher accuracy) in an abbreviated exponential notation, whereby 6.022 14(23) means 6.022 14 x 1023. In this book we follow a current tendency of the quantum chemical literature by expressing relative energies in thermochemical units (kcal mol-1), structural parameters in crystallographic Angstrom units (A), vibrational frequencies in common spectroscopic units (cm-1), and so forth. These choices, although inconsistent according to SI orthodoxy, seem better able to serve effective communication between theoreticians and experimentalists.

For the gas-phase, second-order reaction C2H4 + C4H6 - CgHio (or A + B - C) carried out adiabatically in a 2-liter experimental CSTR at steady-state, what should the temperature (T/K) be to achieve 40% conversion, if the (total) pressure (P) is 1.2 bar (assume constant), the feed rate (q0) is 20 cm3 s-1, and.. the reactants are equimolar in the feed. The Arrhenius parameters are EA = 115,000 J mol-1 and A =3.0x 107L mol-1s-1 (Rowley and Steiner, 1951 see Example 4-8). Thermochemical data are as follows (from Stull et al., 1969) ... 

Baxter, L.L., T Gale, S. Sinquefield, and G. Sclippa, 1997a. Influence of Ash Deposit Chemistry and Structure on Deposit Physical and Transport Properties. Developments in Thermochemical Biomass Conversion, A.V. Bridgwater and D.G.B. Boocock, eds., Blackie. Academic and Professional Press, London, pp. 1247-1262. 

Chaudhari, S.T., Ferdous, D., Dalai, A.K, Bej, S.K, Thring, R.W., and Bakhshi, N.N. (2000). Pyrolysis and Steam Gasification of Westvaco Kraft Lignin for the Production of Hydrogen and Medium Btu Gas, Abstracts Progress in Thermochemical Biomass Conversion, Tyrol, Austria, 17-22 September. 

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