The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions

Bituminous chars with a strongly non-uniform porous structure have not been accounted for in existing pore models used to predict reactivity. A new approach has been developed to predict a particle structure parameter used in the random pore model. The reaction rate has been correlated with the carbon burn-off. Six bituminous coal chars were produced by pyrolysis in a pressurised drop tube furnace and their reaction rates were measured under various conditions using a pressurised thermogravimetric analyser. The char morphology and the porous structure of the char particles were obtained using image analysis. The porous structure of char particles was also characterised using mercury porosimetry. It is shown that after high-pressure pyrolysis, most of the char particles are of a high porosity, low surface area and a non-uniform porous structure with a large central void surrounded by a thin shell which will influence apparent char reactivity and ash formation. Image analysis also confirmed that the maceral composition of the coals and the pyrolysis pressure influence char morphology. A maximum reaction rate for each char was observed at a carbon conversion of 10–20 wt% during both char combustion and gasification. The particle structure parameters were predicted based on the measured porosity and surface area of macropores and micropores in the particle, and show reasonable agreement with those derived from experimental measurements. The predicted reaction rate agrees well with experimental measurements obtained under various conditions.