Fuel and chemicals from wet lignocellulosic biomass waste streams by hydrothermal carbonization

Most valorization processes for biomass waste require dry raw material or at least a relatively low amount of residual humidity. In contrast, hydrothermal carbonization (HTC) is a valorization process for lignocellulosic biomass which uses water as a reaction medium. The product, hydrochar, can be used as dry solid fuel making the post-process drying procedure much more energy-efficient. Herein, three lignocellulosic biomass waste feedstocks, i.e. the organic fraction of municipal solid waste (OFMSW), orange peel waste (OPW) and the residues of a pepper plantation, were processed by HTC on a ton scale and the product evaluated as solid fuel in form of pellets for domestic use (EN ISO 17225). A critical property of the product is the ash content which has to be adjusted by post-treatment. Ash content below the established limit was achieved by acid treatment with sulfuric acid. An implementation of the treatment into the pilot plant is straightforward. An organic liquid fraction was obtained as an additional effluent in the pilot plant depending on the biomass feedstock. For instance, limonene in a mixture with other monoterpenes was separated when orange peel waste was processed, constituting approximately 3 wt% of the dry matter. It is further shown at laboratory scale that the monoterpene mixture can be directly used or can easily be transformed into para-cymene, a fragrance compound, by catalytic dehydrogenation. Therefore, the HTC process can be considered as a source for valuable apolar platform molecules derived from lignocellulosic biomass waste in addition to the production of hydrochar.