To solve the ash related problems during biomass combustion, the combustion process, potassium fixation and ash fusion temperatures of two agricultural biomass briquettes (maize straws and cotton stalks) with phosphorus-based additives (NH4H2PO4 and Ca(H2PO4)(2)) were investigated. The briquetting process were performed in a lab-scale densification system. Results showed that the ash of maize straw briquette extends to agglomeration at 900 degrees C, and fully melted at 1000 degrees C and 1100 degrees C. The melted products were dark solid glaze shape, adhered to the porcelain boat. Additives enhanced potassium fixed ratios by different levels. At 1100 degrees C, the potassium fixed ratio changing rate of NH4H2PO4 was 52.87% -64.89%, while that of Ca(H2PO4)(2) was 35.90%-41.74%. So the influence of NH4H2PO4 on potassium fixation ability was better than that of Ca(H2PO4)(2). High melting point calcium potassium phosphates were produced by adding additives from XRD. SEM-EDS showed that the additives attached on fibers physically, reacting with potassium chemically, preventing the shrinkage and melt. Both NH4H2PO4 and Ca(H2PO4)(2) increased ash fusion temperature of biomass briquettes by 55-277 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.