Physisorbents like zeolites, activated carbons, silica, and alumina gels have been traditionally used for separation of bulk or trace carbon dioxide from a gas mixture using a pressure or a thermal swing adsorption process. These processes are generally operated at near ambient or at moderate temperatures (say <100 °C) because the equilibrium sorption capacity and selectivity of sorption of CO 2 on the physi-sorbents rapidly decrease at higher temperatures. Recently, a variety of reversible CO 2 chemisorbents have been developed which offer decent sorption capacity and high selectivity for CO 2 at relatively higher temperatures (∼150 to 500 °C). Some of these chemisorbents even exhibit high selectivity of CO 2 sorption in the presence of H 2 O which is mechanistically impossible for a physisorbent. Consequently, these chemisorbents can be used in (a) sorption enhanced reaction (SER) concepts for production of fuel-cell grade H 2 from natural gas by low temperature steam-methane reformation reaction at ca. 400-500 °C or from synthesis gas by water-gas shift reaction at ca. 200-400 °C, and (b) removal and recovery of CO 2 from a flue gas at a temperature of 150-200 °C without precooling, predrying, and precompression. A comprehensive review of this subject is presented.

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Hydrogen production by steam reforming of natural gas is a well-established technology. The possibility of using hydrogen, a nonpolluting fuel, in fuel cells has brought new interest in developing small, efficient, fuel-cell grade hydrogen production units for residential or industrial use. A novel, step-out, low-temperature, steam−methane reforming (SMR) process concept called “thermal-swing sorption-enhanced reaction” (TSSER) is described. The concept simultaneously carries out the SMR reactions at 490−590 °C and removes the byproduct CO2 from the reaction zone in a single unit operation, thereby (a) circumventing the thermodynamic limitations of the SMR reactions and (b) directly producing a fuel-cell grade H2 product with very high CH4-to-H2 conversion. A K2CO3 promoted hydrotalcite is used as the CO2 selective chemisorbent in the reactor, which is periodically regenerated by steam purge at 590 °C. Model simulations of the TSSER process using recently measured CO2 chemisorption characteristics of the promoted hydrotalcite indicate that a very compact H2 generation unit can be designed that requires relatively low amounts of steam for regeneration. New CO2 desorption data from the chemisorbent and its thermal stability are reported.

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A simplified model for diffusion and reaction in the boundary layer surrounding a burning carbon particle is considered. The model accounts for the homogeneous combustion of carbon monoxide and the heterogeneous reaction of carbon with oxygen and with carbon dioxide, the latter two reactions appearing in the model as nonlinear boundary conditions. It provides an insight into the double and single film models, proposed by others, as well as of the distribution of the products in the combustion of carbon.