Investigation of potassium transformation characteristics and the influence of additives during biochar briquette combustion

Low energy density and ash fusion problem were problems that limit biomass widespread utilization. To avoid these, maize straw (MS) was blended with two additive -NH4H2PO4 (ADP) and Ca(H2PO4)2 (CPM)-respectively, and the production chain of mixing additive-briquetting-pyrolysis was put forward to produce MS char briquettes (MSC, MSC-ADP and MSC-CPM). Ash fusion characteristics of MS char briquettes after combustion was investigated by XRF, XRD, SEM-EDS and simulation. Results show that the softening temperature increased with the enhanced ratio of P2O5/K2O until 0.2, and CaO/K2O until 0.9. The higher amount of CaO and P2O5, the lower amount of K2O, the better ash fusion characteristics. The P2O5 had larger influence on softening temperature. The surfaces of MSC ashes formed at 1000 °C were fusion and smooth in micro observation, while surfaces of MSC-ADP and MSC-CPM ashes were coarse and the particles were much looser. Both SEM-EDS and XRD results illustrated that phosphorus in additives combined with potassium to produce high melting temperature potassium phosphates rather than low melting temperature potassium silicates. The potassium fixation ability of ADP was better than that of CPM. The calcium silicates and calcium phosphates produced by CPM could improve ash characteristics.