Influence of minerals and added calcium on the pyrolysis and co-pyrolysis of coal and biomass

The pyrolysis behaviour was studied of two types of biomass (pine and wheat) and a Polish lignite (Turow) in the presence of minerals and ion-exchanged calcium using a variety of laboratory-scale methods including pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) and thermogravimetric analysis coupled to a FTIR spectrometer (TGA-FTIR). Ion-exchange worked well for all samples, but particularly for the biomass, and the wt% Ca introduced followed the order wheat (4-17 wt%) > pine (1.74 wt%) > coal (0-80 wt%). The degree of catalysis displayed by the calcium during pyrolysis of the ion-exchanged samples varied with the fuel used. Significant catalysis by calcium was observed in pyrolysis of pine, but only a small effect was seen for wheat and very little for coal. The inherent minerals also play a catalytic role in all the samples studied, but potassium is by far the most important in the pyrolysis of the raw wheat straw. The gases and light volatiles are influenced by the presence of catalytic metal. For catalysed pyrolysis of pine, a shift was seen in the gas evolution profiles of all monitored species (H2O, CO, CO2, methane, formaldehyde, acetaldehyde, formic acid and acetic acid) and the shape of the profiles of the volatile organics and methane also changed. In addition, new peaks were seen in the pyrolysis-GC-MS of the ion-exchanged pine, indicative of a different pyrolysis mechanism. In TGA experiments of co-pyrolysis, a lower char yield was measured from pine/coal mixtures than expected on an additive basis. However, the presence of calcium in one fuel did not appear to catalyse pyrolysis in the other fuel.