Compositional constraints on slag formation and potassium volatilization from rice straw blended wood fuel

Experimental melting of biomass ash blends demonstrates that the addition of rice straw to a dominantly wood-based fuel causes a marked freezing point depression in the liquidus temperature of the inorganic slag from well above 2000 °C to a minimum of about 1260 °C. The minimum temperature is achieved for ash blends with about 30% rice straw ash. The melting interval (liquidus to solidus) for the ash blends is typically 100–200 °C. The solidus shows a systematic decrease from about 1350 °C to as low as 800 °C for pure rice straw ash. Potassium is completely lost from slag for blends with less than 30% rice straw ash content. The addition of more than 30% rice straw ash results in an enhanced retention of potassium in the solid slag. Potassium loss for fuel blends with above 30% rice straw ash is further positively correlated with melting temperature. As the temperature approaches the solidus, potassium is increasingly bound in the melt as well as in potassium–aluminum silicate minerals (leucite) and, therefore, partially retained in the slag. There are indications that melting temperatures above the ‘true’ liquidus for rice straw-rich blends cause partial potassium loss and consequently a rise in the liquidus. This will result in an apparent extending of the melting interval for blends with above 30% rice straw ash. The liquidus silicate mineralogy of the slag changes as a function of increasing rice straw ash from larnite, to åkermanite, wollastonite, and diopside. This mineralogical sequence reflects an increase in the Si/Ca ratio and polymerization of the melt. The experimental slag shows favorable similarities to the mineralogy and composition of slag formed in commercial biomass-fueled boilers, suggesting that the simplified conditions of the experimental melting study can be used to predict combustion conditions in commercial biomass-fueled boilers. Thus, small additions of straw to a predominantly wood fuel should have the effect of lowering slag melting temperature and relatively reducing potassium loss to the flue gas. If combustion temperature can be controlled to within, or below, the melting interval of the ash (< 1260 °C), the relatively loss of potassium can be minimized. Boiler operation below the minimum solidus temperature (∼ 1050 °C) will further strongly restrict loss of potassium.