Novel energy and resource-efficient value chains through co-combustion of straw fuels and sludge

The project aimed to develop attractive, energy- and resource-efficient value chains based on co-briquetting and co-combustion of straw fuels and sewage sludge within existing plant infrastructure, with potential for both national and global applications. The value chain studied included handling and pretreatment of sludge and straw, such as briquetting, co-combustion, ash management, and the characterization and use of bottom ash for producing fast-growing biomass.

The project developed solutions to further advance sludge hygienization/handling, co-briquetting and co-combustion, and the optimization of ash nutrient content. Barriers and opportunities for co-combustion of straw fuels and sewage sludge, and the subsequent application of recycled bottom ash on agricultural land/peat extraction sites, were identified from environmental, legal, techno-economic, and greenhouse gas emission perspectives. Possible collaboration models were also identified, in collaboration with stakeholders, across the entire value chain.

The results show that co-combustion of straw and sewage sludge through campaign-based combustion in existing grate firing facilities can constitute a resource-efficient and competitive value chain for local and regional sewage sludge treatment and the use of straw fuels for energy purposes. Co-combustion
may be competitive with or cheaper than other sewage sludge treatment methods with phosphorus recovery, but access to cost-effective straw fuels may be decisive. A larger demonstration project, in which all parts of the value chain can be evaluated locally/regionally at larger scale, is required for full-scale implementation. Once the actual materials are available on site, it will be possible to determine how they fit within material classification and utilization frameworks, as well as which permits are required to enable co-combustion across all types of existing combustion facilities.