Wood fuel ash is a resource that should be used for nutrient recycling to forest land andwhich also has the potential to be used for small-scale upgrading of biogas into CNG foruse as vehicle fuel. In the biogas upgrading process, carbon dioxide is fixed through acarbonation process. The carbonation process is also an important part of theconventional ash recycling process, since ash to be returned to forest is usually stabilizedby storing in a pile for a significant period of time to allow the carbon dioxide of the airto act on the ash. This project has explored the possibilities of developing a technicalsystem and business models that can lead to ash filter technology being used to processbiogas into vehicle fuel while at the same time contributing to more wood fuel ash beingreturn to forest land. Work has shown that the preconditions are good for the ashproducers existing infrastructure to be well suited for use in a future system where thebiogas plant replaces the role of the ash terminal for stabilizing the ash. Desirableproperties for ash used for biogas upgrading is that it has a high content of CaO and anability to hold water without creating backpressure in the ash bed, and that the biogasstabilized ash meets the limit values for heavy metals and nutrients for return to forest.Within the project tests were carried out with 10 tonnes of moistened ash involving shortterm storage of fresh ash, ash stabilization in biogas upgrading filters and subsequentreturn to forest land. The biogas stabilized ash had a very low conductivity in relation tothe limit value, showed a lowering of the pH value from close to 13 to below 10 and metthe limit values for heavy metals and plant nutrients for spreading on forest land. Thespreading trail with biogas stabilized ash to forest land showed an acceptable distributionpattern and did not cause any damage to the trees. A slightly higher moisture contentprobably would have further improved the distribution pattern. The tests were successfuland showed that there is good potential for biogas stabilized ashes to be spread with ashrecycling technology currently in use.

In a system where biogas upgrading with ash filter technology is integrated into the ashrecycling chain, the biogas plant will act as a micro-terminal, where ash is handled closerto the ash producer and the distribution site compared to a conventional terminal. Inorder for this to be effective, one partner must be able to coordinate transportation ofash and ensure the ash quality, which in many cases can be an ash contractor. It is alsoof the utmost importance that forest operators and landowners are involved to secureend-users for the stabilized wood fuel ash. The economic calculations show that the costfor ash producers and forest owners would be in the same order of magnitude as for thecurrent ash recycling system. However, there is a potential that ash filter technology cancreate a product of a more uniform and higher quality while at the same time upgradingthe biogas to vehicle gas quality. The system will also contribute to local production ofvehicle fuel and an increased supply of biofuel in rural areas. Revenues from theupgraded biogas are expected to cover a large part of the costs incurred at the biogasplant linked to ash management. However, the cost of handling ash at a biogas plant isdependent on local conditions such as whether the ash is supplied dry or moistened andwhat carbon dioxide uptake capacity it has.

In order to be able to handle ashes from smaller biomass energy plants and other ashproducers that currently deliver dry ash to end-users, it would be desirable to continuework on cost-effective methods for dust-free reception at biogas plants. Furthermore,there is a need for continued work linked to the storage of fresh ash. From a logisticalperspective there is a need to store the ash for shorter periods to get more efficienttransport and to be able to store ash from the winter season for use during the summer.For a long-term successful implementation of the developed system, it is important tocontinue to address the challenge linked to the forest owners’ interest in spreading ashin the future. For a smaller biogas plant that handles 500 tonnes / year of dry ash, acollaboration with up to 200-300 forest owners may be needed to find the distributionarea for the ash over time. The challenge of finding end users for the stabilized ash isshared by other players in the ash value chain and the project group sees opportunitiesthat local use of ash for production of vehicle gas to the community could provide apositive local connection that will aid in the work for increased ash recycling.