Gasification of fermentation residue in a fluidised-bed gasifier

Biogas production through anaerobic digestion in biogas stations is currently prevalent in the Czech Republic. Along with the increasing installed capacity of biogas stations and the amount of gas produced, the need for handling the solid residue of this process – digestate – is increasing. The digestate from pure input raw materials is most often used as a fertiliser; however, the regulations limit its use, hence creating a surplus of unused material. Moreover, digestate obtained from contaminated biomass or bio-waste is unsuitable for agricultural application. A natural demand for other means of utilisation is thus created, and since digestate cannot be directly recycled, the only feasible way is thermal disposal. To get a better understanding of its thermochemical transformation to synthetic gas, a series of experiments were carried out on an atmospheric fluidised-bed gasifier with a 100kWt output. Two digestate samples were chosen, one from wet and the other from dry fermentation, and are compared to results obtained from wood gasification. The lower heating value of the gas generated from digestate, with air as the gasifying agent, was found to be between 4 and 5,5 MJ/Nm3 with an efficiency of about 70%. The syngas quality and purity of both fermentation residues are comparable to the referential wood-gas. The results thus confirm the feasibility of digestate as a fuel for industrial-scale fluidised-bed gasification.