Multicycle Performance of a Single-Step Process for H₂ Production

Abstract

Combining the chemical reaction and product separation steps in a single processing vessel is currently of great interest. In this study, reaction and separation are combined by carrying out the water-gas shift reaction in the presence of a calcium-based CO₂ acceptor. The continuous removal of CO₂ from the gas phase alters the shift reaction equilibrium and permits almost complete CO conversion and CO₂ removal. The reaction temperature is significantly higher than employed in the traditional shift process, and, as a consequence, no shift catalyst is required.

Previously reported results showed that greater than 99% removal of total carbon oxides could be achieved over a range of experimental conditions with greater than 99.9% removal achieved at the most favorable conditions. Total carbon oxide content of the product gas in the latter case was approximately 30 ppmv (dry basis). However, for the process to be economical, it is necessary that the CO₂ acceptor retain its activity and capacity through a number of carbonation-calcination cycles. Results of multicycle tests in which dolomite precursor was cycled between the fully calcined (CaO + MgO) and half-calcined (CaCO₃ + MgO) forms are presented in this paper. Tests were carried out to investigate the effects of temperature, gas composition, and space velocity on multicycle performance. Greater than 99% carbon oxide removal was achieved in each cycle of an eleven-cycle test with only moderate deterioration in acceptor performance.