Thermochemical H2O decomposition

The concept of thermochemical H2 production from H2O was first given in the 1960s. In thermochemical H2O decomposition, H2O splits into H2 and O2 using a series of chemical reactions. AU chemical intermediates are recycled internally within the process so that H2O is the only raw material and H2 and O2 are the only products (Orhana, Dincer, Rosen, Kanoglu, 2012 Rosen, 2010). 

After carrying out a detailed literature search of all published thermochemical cycles (>100 cycles from 800 references), the most promising thermochemical H2O decomposition cycles were identified for efficient, cost-effective, large-scale production of H2 using high-temperature heat from an advanced nuclear power station (Brown et al., 2002 Rosen, 2010). According to literature, the adiabatic UT-3 cycle and the S—1 cycle were considered by researchers to have the greatest potential, and the S—I cycle was selected for further development and recommended as a suitable thermochemical process for H2O demonstration plant (Rosen, 2010). 

Several studies of H2 production by thermochemical processes have been presented recently, including reports of several cycle statues such as sulfur—iodine (S—I) cycle, ISPRA Mark 9 cycle, hybrid sulfur cycle, Ca—Br cycle, Cu—Cl cycle, and adiabatic UT-3 cycle (Rosen, 2010). Many of these cycles are driven by nuclear or solar energy sources. H2O thermal decomposition generally holds three distinct steps production of H2, production of O2, and material regeneration. In recent decades, thermochemical cycles have been used for H2O decomposition, because they allow appreciable amounts of H2 and O2 to be attained at lower temperatures (usually less than 1000 °C) than are needed for one-step fliermochemical H2O decomposition (Rosen, 2008, Rosen, 2010). 

The Hybrid Sulfur (HyS) thermochemical cycle task addresses the key technology issues involved in the development of a hybrid sulfur hydrogen production system - including the SO2 - H2O electrolyzer design, SO2/O2 separation, and the unique materials and process issues associated with the acid decomposition section. An electrolyser is being developed that can be used in conjunction with the sulfuric acid decomposition section being developed for the S-I cycle in a Hybrid Sulfur Integrated Laboratory-Scale Experiment. 

M. Roth and K.F. Knoche, Thermochemical water splitting through direct HI decomposition from H2O-HI 12 solutions , Int. J. Hydrogen Energy 14 545-9 (1989). 

For the sake of illustration we have calculated the equilibrium composition of a mixture, formed by decomposition of methane with steam at an initial ratio of 1 2 moles, 900 K and in the 10 to 1000 atm range. Thermochemical data for CH4, H2O, H2, CO will be found in Example 10, for CO2 in Example 11. The calculation was performed according to relations (6.96) — (6.102), constants of the Beattie-Bridgman equation are given in Appendix 11 for the individual pure constituents. The results are plotted in Fig. 24. It will be seen from the plot, that for all constituents 900 K is a high enough temperature for deviations from ideal behaviour to become apparent only at elevated pressure. 

Devolatilization (or pyrolysis) This step consists of the thermochemical decomposition in the absence of oxidizing agents of the cellulose, hemicellulose, and lignin compounds present in the original biomass with production of volatiles and char (Beneroso et al., 2014 Liu et al., 2009). Volatiles are produced until temperature reaches approximately 350—400° C, and include H2O (v), CO2, oxygenated vapor species, and primary oxygenated liquids. 

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