Effects of Thermo-Biological Pretreatments on the Combustion Properties of Wheat Straw in a Cascaded Biorefinery Concept

In this study, the effects of different thermo-biological pretreatments in combination with a washing process on the combustion properties of wheat straw were investigated in order to optimize its use as solid fuel. An essential part of the biological pretreatment for fermentative applications is the use of green waste compost. The compost provides the necessary microorganism to break down the lignocellulose contained in the straw and as consequence influence the combustion properties as well as making the dissolved sugar polymers usable. However, the use of compost as an additive in biomass combustion applications has been scarcely studied. In the following analysis, the effects on ash content, calorific value and ash melting behavior were verified and an analysis of variance (ANOVA) was performed. The results show that thermal treatment by autoclave with biological treatment by anaerobic incubation in combination with the washing process are promising pretreatment procedures for wheat straw for pellet production, improving the above-mentioned combustion characteristics of the straw and hence meeting the requirements of ISO 17225-6 for non-woody pellets. For the straw–compost mixtures, the applied combined pretreatment is a possible solution to the problems related to low ash melting temperatures, as the shrinkage temperatures were increased over 659 ∘C compared to raw straw. Adjustments in process design have to be made as well as the compost content has to be reduced to ensure that the ash content as well as the calorific value meet the requirements of ISO 17225-6. The potential of compost is promising and can be considered as a possible additive for pellet production from wheat straw. This study also showed that the cascading use of straw–compost mixtures increases the value-added potential of a biorefinery, since solid fuels with optimized combustion properties can be provided in addition to liquid or gaseous fuels.