The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.