• Energy Research

The integrated generation of solid fuel and biogas from biomass (IFBB) system is an innovative approach to maximising energy conversion from low input high diversity (LIHD) biomass. In this system water pre-treated and ensiled LIHD biomass is pressed. The press fluid is anaerobically digested to produce methane that is used to power the process. The fibrous fraction is densified and then sold as a combustion fuel. Two process options designed to concentrate the press fluid were assessed to ascertain their influence on productivity in an IFBB like system: sedimentation and the omission of pre-treatment water. By concentrating press fluid and not adding water during processing, energy production from methane was increased by 75% per unit time and solid fuel productivity increased by 80% per unit of fluid produced. The additional energy requirements for pressing more biomass in order to generate equal volumes of feedstock were accounted for in these calculations.

Changes in agricultural practices and land abandonment across less favoured areas have led to an increase in land management for nature conservation. Substantial areas of vegetation are cut annually for habitat management, but the conservation biomass generated is generally discarded. Samples of two types of conservation biomass harvested from marginal sites dominated by rushes (Juncus spp) or bracken (Pteridium aquilinum) were washed and pressed to generate fluid and fibrous process streams using the Integrated Generation of Solid Fuel and Biogas from Biomass (IFBB) process. Previous work established the fluid from the IFBB process could be anaerobically digested to generate enough energy for the washing and pressing stages. The current study focussed on the fibrous process stream, subjecting material to hydrothermal conversion and investigated the extent to which i) vegetation type, ii) the impact of pre-treatment by hot water washing and pressing (partial demineralisation) and iii) hydrothermal conversion route (hydrothermal carbonisation or hydrothermal liquefaction) affected the yields, relative proportions, and characteristics of products generated. Feedstock source had substantially more effect on product chemistry than product yields. The most effective process route for combustion fuel production was based on hydrothermal carbonisation of pre-processed feedstock. However, if bio-oil production was to be the priority product in a biorefinery, then biomass pre-processing would not be required.

Waste biomass is generated during the conservation management of semi-natural habitats, and represents an unused resource and potential bioenergy feedstock that does not compete with food production. Thermogravimetric analysis was used to characterise a representative range of biomass generated during conservation management in Wales. Of the biomass types assessed, those dominated by rush (Juncus effuses) and bracken (Pteridium aquilinum) exhibited the highest and lowest volatile compositions respectively and were selected for bench scale conversion via fast pyrolysis. Each biomass type was ensiled and a sub-sample of silage was washed and pressed. Demineralization of conservation biomass through washing and pressing was associated with higher oil yields following fast pyrolysis. The oil yields were within the published range established for the dedicated energy crops miscanthus and willow. In order to examine the potential a multiple output energy system was developed with gross power production estimates following valorisation of the press fluid, char and oil. If used in multi fuel industrial burners the char and oil alone would displace 3.9 × 105 tonnes per year of No. 2 light oil using Welsh biomass from conservation management. Bioenergy and product development using these feedstocks could simultaneously support biodiversity management and displace fossil fuels, thereby reducing GHG emissions. Gross power generation predictions show good potential.