Co-combustion of straw and waste rubber thermolysis char in a moving grate boiler

Abstract

The study investigates the co-combustion of straw with waste rubber thermolysis char and the addition of 4% lime in a small-scale pellet-fired boiler. Char content in the fuel was set at 10, 15, 25, and 49%. During combustion tests, flame visual were inspected, gas and particulate emissions were measured, individual losses and boiler efficiency were determined, and the amount and chemical composition of particulate depositions on boiler surfaces were estimated. Results indicate that burning fuel with char requires burner design modification to increase primary air relative to secondary air. Blends with up to 25% char content can be combusted while maintaining high boiler efficiency of approximately 87%, comparable to straw-only combustion. Significant fly ash deposits on boiler walls were observed, with K, Zn, Ca, and Si being predominant in the composition. Although there are risks of slagging and corrosion during co-combustion, these risks decrease with a higher char proportion. SO${}_2$ was detected in the flue gases, with high levels of particulate emissions, while NOx emissions stayed within permissible limits for small boilers. A marked increase in unburned carbon in bottom ash suggests the need for grate modifications and an extended combustion zone when co-combusting biomass with waste rubber thermolysis char.