A model of biomass char gasification describing the change in catalytic activity of ash

A comprehensive description of catalytic effects during char gasification under various conditions relevant to biomass gasification was made. A three-parallel reaction model was proposed to describe the dynamic change in catalytic activity of ash during gasification of biomass char particles. Three different regimes of conversion were identified by analyzing char reactivity experiments conducted in a vertical TGA with nine biomasses under a wide range of operating conditions (temperature: 1023-1123 K, pressure: 0.1-3.0 MPa and gasification mixtures of CO2-CO-H2O-H-2): (1) catalytic char gasification with deactivation of catalyst, (2) non-catalytic char gasification, and (3) catalytic char gasification with small amount of stable ash, without suffering deactivation. A model including the three regimes was developed and the measurements were used to fit the kinetic coefficients. It is shown that the model accurately predicts the reactivity of biomass char in CO2-CO mixtures during the whole range of conversion. It was detected that char gasification maintains the catalytic activity during the entire conversion process when: (i) biomasses having small amount of silicon was used, and (ii) steam is used as part of the gasification agent. The model is still useful as predicting tool for these two conditions but its physical significance is contestable on the light of the model developed. For the conditions where the model is valid, it is shown that the model is a useful tool as sub-model in reactor simulations, predicting the conversion rate of single particles fast and accurately at different stages of conversion. The aspects that need to be further investigated for expanding the applicability of the model were identified. (C) 2012 Elsevier B.V. All rights reserved.