Effect of used edible oils in coal fluidised bed gasification

Edible oil wastes (EOW) are generally used in the production of soap and/or disposed in waste water treatment stations. In Europe, their use for energy is almost exclusively for the production of biodiesel. However, the nature and quality of EOW may turn their use for biodiesel not always suitable. Therefore, for environmental reasons, it is important to investigate other options like co-gasification with coal. Gasification is generally considered more environmentally friendly and its impact is considerably less polluting than other thermochemical processes. Co-gasification requires that synergy exists between coal and EOW to take profit of their complementary advantages. Co-gasification runs were undertaken on a bench-scale atmospheric fluidised bed gasifier, using both steam and air-steam mixtures as gasification medium. Operating conditions like gasification temperature, steam/air ratio and oil content in feedstock blends were varied to check their effect with the aim of optimising the gasification process. Some difficulties in feeding the blend of coal with EOW were observed when the oil content increased, which also raised hydrocarbons content in the gas produced. Both the rise of temperature and of air flow rate allowed lower tars and gaseous hydrocarbons concentrations. Higher gas yields were obtained at higher gasification temperatures, the gas being richer in hydrogen content at the expense of hydrocarbons. Solid residues (ashes and char) produced by gasification of blends of coal and EOW were also analysed to understand their nature and to evaluate their impact on the environment.