Release of Alkalis and Chlorine from Combustion of Waste Pinewood in a Fixed Bed

Combustion of biomass in a boiler releases alkali metals and chlorine which, together with silicon and sulfur, are responsible for slagging, fouling, corrosion, and particulate emissions. This research investigated the effects of the primary (under-fire) air flow rate, (m)Over dot(air), and its preheating temperature on the ignition and burning rates of pinewood chips in a laboratory fixed bed furnace and on the release of alkali metals and alkali earth metals (potassium (K), sodium (Na), calcium (Ca), magnesium (Mg)) and chlorine. The air flow rate, (m)Over dot(air), through the bed was varied in the range of 0.085-0.237 kg/(m(2) s), resulting in an overall excess primary air coefficient lambda varying from 0.56 to 1.1. Air was also preheated in the range of 20-135 degrees C. Results showed that increasing either (m)Over dot(air) or the air preheat temperature increased the flame propagation rate (ignition rate) and the mass burning rate of the fuel. Moreover, the release of Cl was nearly complete (>99%) in all examined cases, whereas the release of alkalis was only partial. Calcium was the most predominant alkali in the pinewood; however, potassium was the predominant alkali in the released gases. The mass fraction of Na in the pinewood was much lower than that of K but it was released more comprehensively. Increasing the air flow rate enhanced the release of K and Na significantly, whereas it enhanced the release of Ca and Mg only slightly. Preheating the primary air preferentially increased the migration of K to the gas phase, whereas Na, Ca, and Mg were affected only mildly. The preheated air promoted the transfer of chlorine to HCl. Overall, moderately high primary air flow rates generate globally fuel lean conditions and mildly preheated air can enhance the mass burning rate of pinewood and its conversion to fully and partially oxidized gases. However, they result in enhanced gasification of the alkalis in the biomass. In the case of pinewood, this may be a minor concern, as the absolute values of such emissions are low relative to other biomass fuels.