The influence of biomass additions on the pyrolysis behaviour of an inertinite rich South African coal

Abstract

The influences of selected biomasses (hardwood chips, softwood chips, pinewood chips and sugarcane bagasse) on the degradation of an inertinite-rich South African coal were studied. The pyrolysis process is important since it is a precursor for most thermal processes (such as combustion). The coal and the four types of biomass were pulverised to a particle size of less than 75 µm. A thermogravimetric analyser coupled to a quadruple mass spectrometer (TG-MS) was used to investigate the devolatilisation behaviour of the coal, biomass and their blends (0, 20, 40, 60, 80 and 100% wt./wt.) in a temperatures range of ambient temperature to 1100 ˚C. The biomasses consist of ligno-cellulosic components, which contain 27.8-39.9 wt. % cellulose, 17.4-21.1 wt. % hemi-cellulose and 8.0-32.4 wt. % lignin, which were in the range of reported values in literature. The TGA results indicated that biomass samples reacted at lower temperatures compared to coal, as cellulose and lignin decomposed between 152-660 ˚C and 113-900 ˚C respectively. Chars were characterised using different analytical methods (for instance proximate and mineral analyses). The sugarcane bagasse and blend of sugarcane bagasse in coal samples indicated a significant large content of ash in comparison to other samples. This may be attributed to the particle size selection of <75 µm and environmental or harvesting conditions used to maintain or harvest the crops. The results obtained from XRF indicated significant contents of silica (SiO2) and aluminium (Al2O3) for the parent and blended chars, where these oxides confirm the presence of the clay minerals. From the XRD carbon crystallite results, it was observed that at higher temperatures the chars undergo structural ordering. The carbon aromaticity of the chars increased from biomasses (pinewood chips had the lowest aromaticity value) to coal, and the blended chars showed carbon aromaticity values in between that of the parent materials. As expected, these results showed biomasses to have a larger reactivity than coal alone, which can be attributed to the biomasses low thermal resistance and higher surface areas. The parent and blended samples from wood origin have reacted at lower temperatures than these from herbaceous origin. This behaviour can be attributed to the composition of the biomass as well as the biomass' ash composition. The possible synergistic or inhibiting effects that may arise during the pyrolysis process were investigated by comparing the calculated weighted averages of the mass loss data with the experimentally determined results. The calculated weighted blend thermal behaviour (mass loss curves) of the 20, 40, 60 and 80% wt./wt. coal-biomass blends indicated that the pyrolysis behaviour of the biomass is not influenced to any meaningful extend by the presence of the coal and vice versa, as a maximum average deviation of only 2.8% is observed. These results may be attributed to the experimental conditions (i.e. low heating and high flow rates) applied. The MS results indicated the release of the main gases (compounds containing the ions of H2 +, CH3 +, H2O and CO2 +) during the pyrolysis process. The hydrogen evolution of all the samples was mainly above 650 ˚C, which was in the last stage of the pyrolysis process. The second CO2 MS peaks (due to the degradation of the coal) and the DTG results were compared to evaluate the influence of the biomass on the coal reactivity with respect to CO2 evolution. The results indicated a small increase in the coal's reactivity due to the presence of the biomass samples. The largest decrease in the temperature at maximum rate of CO2 evolution was observed for the pinewood chip-coal sample (-43 °C), followed by hardwood chip-coal (-36 °C), softwood chip-coal (-31 °C) and sugarcane bagasse-coal blends (-32 °C). The decrease in temperature at maximum rate of CO2 evolution (thus the specific reaction steps involving CO2) seems to be due to a catalytic effect of inorganic compounds in the biomass samples and/or secondary reactions between biomass thermal products and the coal at higher temperatures.