Pyrolusite–CO reduction kinetics

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Pyrolusite is an inexpensive and nontoxic manganese ore with exceptional oxygen transfer capacity that makes it a potential candidate for chemical looping combustion. We evaluate the oxygen transfer capacity and kinetics of pyrolusite, an inexpensive and nontoxic manganese ore. We reduced it with carbon monoxide and a combination of carbon monoxide, methane and hydrogen. At 960°C, carbon monoxide and hydrogen reduce the calcined pyrolusite 86% more than does a pure CO stream. We derived a mechanistic kinetic model based on the effluent species concentration during reduction with a combination of methane, carbon monoxide and hydrogen from 810°C to 960°C. The direct reduction of surface oxygen sites by carbon monoxide, in equilibrium with the reverse CO2 reoxidation, in addition to the reoxidation of the surface by oxygen atoms from the bulk of the lattice, explains more than 95% of the variance of the data. We estimated surface parameters by comparing the carbon monoxide and helium washout curves and by analyzing the initial response to a step change in the inlet concentration. Adsorbed CO increased with temperature, up to 910°C, where it peaked at 55μmolm−2, corresponding to a 70% covered surface. The estimated oxygen surface site density followed an asymptote at 40μmolm−2, in agreement with measured values.