Influence of Ammonium Phosphates on Gaseous Potassium Release and Ash-Forming Characteristics during Combustion of Biomass

The influence of ammonium phosphates, i.e., ammonium dihydrogen phosphate (NH4H2PO4) and ammonium monohydric phosphate [(NH4)(2)HPO4], on gaseous potassium (K) release was studied in a tube furnace during the combustion of maize straw (MS), cotton stalk (CS), and rice straw (RS). The gaseous potassium was absorbed by deionized water and measured by atomic absorption spectroscopy (AAS). The formed ashes were investigated using a combination of powder X-ray diffraction (XRD) and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEMEDS) analyses after visual evaluation. The results show that the addition of NH4H2PO4 and (NH4)(2)HPO4 both decrease the potassium release ratio and show the same trends. NH4H2PO4 and (NH4)(2)HPO4 can be more effective with a higher initial potassium content. The release ratios of RS with NH4H2PO4 and (NH4)2HPO4 are 12.1 and 13.2% at 1000 degrees C, respectively, which both decrease most compared to the ratio of RS. The release ratio decreases as the PO43-/K ratio increases from 0.5:1 to 4:1 at every temperature. The appropriate molar ratios of PO43-/K are between 1:1 and 2:1. Surface melting and sintering are inhibited by the addition of NH4H2PO4 and (NH4)(2)HPO4. Ammonium phosphates can react with potassium to form potassium phosphates, and potassium calcium phosphates bound potassium in a KCaP ternary system. A significant effect of NH4H2PO4 on the microcosmic structure is noticed, and the ashes of the mixture show quite different morphologies because of the formation of phosphorus compounds. These phosphates, including K2CaP2O7, K3PO4, and KCaPO4, are high-melting-point compounds contributed to retaining potassium and preventing sintering of the formed ash.