Effect of additives on particulate matter formation of solid biofuel blends from wood and straw

The combustion of solid biofuels is characterized by the formation of particulate matter emissions harmful to humans and the environment. Inorganic elements which are volatile under high temperatures (∼600–1200 °C) are emitted as vapor, then cooling down in the flue gas, and are re-sublimated and emitted as solid particulate matter emissions (fly ash). Thus, the objective of this paper is it to summarize the current knowledge of the forming mechanism of these particulate matter emissions taking place during the combustion of wood and straw, and bring the elements in particulate matter with the help of additive in a stable solid phase, so they stay in the bottom ash and are not emitted. Here, two different additives are tested, based on Al, Mg and Ca. Important in this respect is the knowledge of the chemical formations during the combustion process of the different ash-forming elements contained within solid biofuels. Therefore, the chemical binding forms of the ash-forming elements (Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S, and Cl) within the solid biofuel are presented. Based on this, possible conversion products are discussed including theoretical calculated intermediates; this includes the chemical conversion pathway. On this basis, additives are identified based on the difference of the elemental composition of wood/straw blends to wood and the results of the formation of particulate matter during combustion are assessed in lab scale. The additives are composed of Al₂O₃, CaHPO₄, and CaCO₃ or Al₂O₃, MgHPO₄, and MgCO₃. The results from the characterization techniques TGA, AAS, IC, and XRD show that the formation of particulate matter during a complete thermo-chemical conversion can be suppressed to certain extend.