Effects of preheating primary air and fuel size on the combustion characteristics of blended pinewood and corn straw in a fixed bed

Abstract

This research investigated the effects of preheating primary air and fuel size on combustion behaviors of pinewood and corn straw (50% ratio) in a fixed bed. Primary air temperature (Tair) was ranged in 20–130 °C, and the fuel size was varied in the range of 3–8 cm in length, which affected the whole combustion efficiency, emissions of hazardous pollutants and alkali metals and chloride. Results showed that by increasing Tair the ignition delay time first decreased by 40% and then increased, and both burning and ignition rates increased by 60%; however, the emissions of CO2, CO and CH4 increased first, and then decreased. At the Tair = 85 °C the smallest unburned carbon was left in the ash; the mostly generated N-bearing species was NO followed by HCN, and then by a small of NH3. The higher Tair increased the releases of K (by 16%) to the gas phase and promoted the transfer of Cl to HCl, while mildly effects on Ca. Also, increasing fuel size to 5 cm enhanced the combustion stability and decreased the emissions of NO. Accordingly, mildly increase the fuel size and Tair were found to be well-operating condition, which provide guidance for selections in large-grate boiler during blend combustion.