Production of Very Pure Hydrogen with Simultaneous Capture of Carbon Dioxide using the Redox Reactions of Iron Oxides in Packed Beds

Abstract

A chemical looping process, which uses a packed bed of the various oxides of iron, has been formulated to produce separate, pure streams of H2 and CO2 from syngas. The process has the following stages: (1) Reduction of Fe2O3 to Fe0.947O in the syngas from gasifying coal or biomass. This stage generates pure CO2, once the water has been condensed. (2) Subsequent oxidation of Fe0.947O to Fe3O4 using steam, to simultaneously produce H2. (3) Further oxidation of Fe3O4 to Fe2O3 using air to return the oxide to step 1. Step 1 was studied here using a mixture of CO + CO2 + N2 as the feed to a packed bed of iron oxide particles, while measuring the concentrations of CO and CO2 in the off-gas; step 2 was investigated by passing steam in N2 through the packed bed and measuring the quantity of H2 produced. The third step simply involved passing air through the bed. Reduction to Fe, rather than Fe0.947O, in step 1 gave low levels of H2 in step 2 after 10 cycles of reduction and oxidation and led to the deposition of carbon at lower temperature. Step 3, i.e. reoxidizing the particles in air to Fe2O3, led to no deterioration of the hydrogen yield in step 2 and benefited the process by (i) increasing the heat produced in each redox cycle and (ii) preventing the slip of CO from the bed in step 1. The proposed process is exothermic overall and very usefully generates separate streams of very pure H2 and CO2 without complicated separation units.