Two-step thermochemical

Roeb, M., et al. (2008), Development and Verification of a Two-step Thermochemical Process for Solar Hydrogen Production from Water , 14th SolarPACES Biannual Symposium, T. Mancini (Ed.), Las Vegas, USA, November. 

Sonoyama, N., et al. (2006), Synthesis of carbon nanotubes on carbon fibers by means of two-step thermochemical vapor deposition, Carbon, 44,1754-1761. 

Solar Fuel Production via Non-Stoichiometric CexZryHfz02-5 Based Two-Step Thermochemical Redox Cycle... 

When this reaction is combined with reactions (37.6) or (37.7) that take place in reactor 1, then the process formed is the thermochemical water or carbon dioxide splitting, provided the necessary heat for the reaction is supplied by solar energy. In that sense, the two-step thermochemical water or carbon dioxide splitting by redox oxide pairs, although as an idea was pubhshed and... 

Yang, C.K., Yamazaki, Y., Aydin, A., and Haile, S.M. (2014) Thermodynamic and kinetic assessments of strontium-doped lanthanum manganite perovskites for two-step thermochemical water splitting. 

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). 

Direct production of hydrogen from gasification is the simplest route. Gasification is a two-step process in which the solid feedstock is thermochemically converted to a low- or medium-energy-content gas. Natural gas contains 35 MJ/Nm3. Air-blown biomass gasification results in approximately 5 MJ/m3 oxygen-blown in 15 MJ/m3. 

Based on thermochemical data, evidence has been presented for a two-step mechanism that finally yields a protonated carboxylic acid and the radical. However, while the final products are well described, the second step has not fully been elucidated [100,101]... 

However, even these two-step routes require temperatures on the order of 1273 K or more. Water cannot be decomposed in one or two thermochemical steps when the available temperature is below 1273 K. 

Finally, the novel part of the three-step model is the identification of a separate unit operation (subsystem) in a PBC system, that is, the thermochemical conversion of the fuel bed, which by logical consequence requires the introduction of a third subsystem referred to as the conversion system. Commonly, PBC systems are modelled with two steps, that is, a two-step model [3,15], see Figure 7. In the two-step model the thermochemical conversion of solid fuels and the gas-phase combustion are lumped together. Several new concepts are deduced in the scope of the three-step model in general and the conversion system in particular, for example the conversion gas, conversion concept, conversion zone, conversion efficiency, which are all explained later in this summary. 

Steinfeld A (2002) Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. Int J Hydrogen Energy 27 611-619... 

The atomisation enthalpies of the lanthanides as metallic elemental substances exhibit very different trends. From La to Eu, we see a steady decrease, followed by an abrupt increase at Gd. The atomisation enthalpies then decrease (not quite monotonically) to Yb, then increase at Lu. These trends may be rationalised as follows. According to magnetic studies, the lanthanide atoms in the elemental substances have the electronic configurations 6s25d14f" Eu and Yb are exceptions, discussed further below. The band structure is evidently complex and will not be described in detail. The atomisation enthalpy can be broken down for thermochemical purposes into two steps ... 

Galvez M. E. Halmann M. Steinfeld A. Ammonia production via a two-step A1203/A1N thermochemical cycle. 1. Thermodynamic, environmental, and economic analyses. Ind. 

Table V shows the efficient organization of this reaction chemistry into five reaction families. Bond fission, for example, is the elementary step that creates two free radicals from a parent molecule. In chain processes this will often be the initiation step. Thermochemical estimates often show that the logarithm of the Arrhenius A factor (logioA) is of the order 14-17, whereas the activation energy is essentially equivalent to the bond dissociation energy (19,42). This equality is the result of the essentially unactivated reverse reaction step, radical recombination.

Early studies performed on H2O splitting thermochemical cycles were mostly cha racterized by the use of process heat at temperatures below about 1200 K, available from nuclear and other thermal sources. These cycles required multiple steps (more than two) and had inherent inefficiencies associated with heat transfer and product separation at each step. An overview of indirect thermochemical processes for hy drogen generation using more than two steps has been presented by Funk,4 and sev eral of these cycles are summarized in Table 3. An example includes cycle No. 2 in Table 3, which utilizes the following reaction steps ... 

Steinfeld, A., Solar Hydrogen Production via Two-Step Water Splitting Thermochemical Cycle Based on Zn/ZnO Redox Reaction, Int. J. 

From preliminary efficiency estimates and proof of principle experiments, Simpson et al. [4] have recently proposed a hybrid process based on the reverse Deacon cycle as a promising moderate temperature thermochemical process to produce hydrogen. The basic reactions involved are shown in the three steps in Table 3. As can be seen from the equations given in Table 3, the two-step sequence involving magnesium chloride hydrolysis (Step 1) followed by magnesium oxide chlorination (Step 3) reduces to the Reverse Deacon Reaction. The moderate temperatures involved in these reactions would... 

Simpson et al. [4] propose to carryout the two-step Deacon reaction through supported magnesium compounds on a zeolite such as Silicalite. Though they have addressed many of the development requirements of this process, some still need considerable effort for assessment of the commercial viability of the overall process. Due to the lower temperature requirements of this process (compared to high temperature thermochemical processes), the less stringent demand on materials, and the well-established status of the electrolysis step, this process deserves further evaluation. 

The Hybrid Sulfur (HyS) Process is one of the two baseline thermochemical cycles identified for development in the NHI program. (The sulfur-iodine cycle is the other). HyS is an all-fluids, two-step hybrid thermochemical cycle, involving a single thermochemical reaction and a single electrochemical reaction. The chemical reactions are shown below ... 

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