Recirculation of char from biomass gasification: Effects on gasifier performance and end-char properties

Char and tars are the main compounds restraining practical applications of biomass gasifiers. While tars have to be minimized to avoid operational problems, char could have alternative uses because of its properties. In this work, recirculation of the char (primary char) into the gasifier (85% wt. biomass/15% wt. char mixtures) has been analysed in a lab-scale drop tube gasifier for evaluating the improvements on the performance and on the characteristics of the end-char. Two gasifying agents have been studied, air and an air/steam mixture (55/45% by wt.). Both the Cold Gas Efficiency and the Lower Heating Value of the producer gas significantly increase when recirculating the char, this effect being more noticeable for air (a rise around 42% for CGE and 29% for LHV has been obtained). This enhancement seems to be due to the catalytic effect of the metals species of the recycled char together with the additional loss of volatiles. When compared to the primary char, the surface area, metals and carbon content and density of the end-char make it more suitable for soil treatment (surface area around 5–6 times greater than that of primary char) as well as to satisfy the criteria of the European Biochar Foundation.