Effect of the pyrolysis process on the physicochemical and mechanical properties of smokeless fuel briquettes

A greater understanding of the physics and chemistry of lignite–biomass briquetting could lead to better briquette performances and cost-effectiveness making these fuels more attractive to both producers and consumers. With this aim, chars obtained from different low rank coals and biomasses (sawdust, straw, olive stone and almond shell) were used to prepare smokeless fuel briquettes and their physicochemical and mechanical properties were studied depending on the pyrolysis conditions. Coal was pyrolysed at temperatures between 500 °C and 700 °C and the temperature chosen to carry out pyrolysis was 600 °C due to the lowest content of sulphur per thermie in the pyrolized material. In order to study the influence of the pyrolysis process on the properties of the briquettes, biomasses were pyrolysed separately at 400 °C and 600 °C and together with the coal at 600 °C of temperature. The materials pyrolysed at 600 °C showed a lower content of volatile matter and a higher calorific value than the standard levels reported in the literature for materials to prepare smokeless briquettes. The briquettes were prepared by mixing the pyrolysed materials with humates as binder and Ca(OH)₂ as sulphur sorbent. The briquetting process was followed by FT-IR, scanning electron microscopy (SEM), CO₂ adsorption and the mechanical properties were tested evaluating their impact resistance, water resistance and compression strength. The best briquettes with respect to the mechanical properties were those prepared with coal and biomasses co-pyrolysed at 600 °C although some of them fixed a higher percentage of sulphur during pyrolysis due to the metal content of the biomasses.