Combustion Dynamics of Swirling Flame at Thermochemical Conversion of Biomass

Development of the swirling flame dynamics at thermo chemical conversion of biomass (wood pellets) has been investigated experimentally at a low swirl number of the swirling flame flow (S < 0.6), different rates of biomass thermal decomposition and swirling air supply with the aim to estimate key factors determining the formation of the flame dynamics and local variations of the flame temperature, composition and processes of heat energy production for such type of swirling flame flow. The experimental study is carried out using a small-scale pilot device with an integrated wood biomass gasifier and combustor. The limited primary axial air supply below the layer of wood pellets (α < 0.5) is used to support biomass gasification, and the secondary swirling air at air excess supply (α > 1) is used to provide mixing of the axial flow of volatiles with an air swirl and to support the combustion of volatiles with the formation of a downstream reaction zone. It is shown that the development of combustion dynamics is influenced by the competitive processes of endothermic thermal decomposition of biomass and exothermal combustion of volatiles. The enhanced thermal decomposition of biomass by increasing the primary air supply and the axial flow of volatiles results in an intensive heat energy consumption with a correlating increase of the air excess ratio at the bottom of the combustor and in ignition delay. This determines the incomplete combustion of the volatiles at the primary stage of flame formation and restricts the development of combustion dynamics. The combustion conditions can be improved by decreasing the secondary air supply and air excess ratio at the bottom of the combustor, which leads to enhanced ignition with a faster and more complete combustion of volatiles