Application of fuel design to mitigate ash-related problems during combustion of biomass

Abstract: The energy supply of today is, through the use of fossil energy carriers,contributing to increased net emissions of greenhouse gases. This hasseveral negative effects on our environment and our climate. In order toreduce the impact of this, and possibly to reverse some of the effects, allrenewable energy sources must be used. Biomass is the renewable energycarrier that has the greatest potential to reduce net greenhouse gasemissions, but the transition from fossil fuels to biofuels is challenging.The combustion of biomass is associated with various technical andenvironmental problems such as slagging, corrosion, and emissions ofparticles, soot, or harmful chemical compounds. Most of these problemsare linked to ash chemical reactions involving alkali metals. Therefore, toreduce the risk of operational and environmental problems, it is importantto understand and control the ash transformation reactions involvingalkali metals.The research presented in this thesis has focused on the development oftools, such as models and indices, for predicting the behaviour of variousbiofuels during combustion, and on the development of the concept of fueldesign and implementation of the same during industrial combustion ofbiomass. The development of easy-to-use tools for predicting problematicash behaviour is crucial in order to make it possible to increase the use ofbiomass as an alternative to fossil fuels. The tools presented here are basedon theoretical and empirical knowledge and can be used to predictchallenges concerning the fuel ash composition and to propose relevantfuel design measures.The purpose of fuel design, as used here, is to broaden the fuel feedstockand to increase the usability of biomass in the global energy system. Thisis achieved through measures to change the ash chemical composition inorder to enhance beneficial properties, or reduce problematic properties,via the use of additives or blending of two or more different fuels.The present thesis extends the foundation of knowledge regarding fuel ashtransformation reactions and their implications for operational problemsthrough in-depth laboratory studies and analyses. Furthermore, thefeasibility of applying this extended knowledge in the medium and largescaleindustrial combustion of biomass is demonstrated and validated. More specifically, a slagging index has been developed using the results ofseveral years of combustion experiments. Fuel designs based on the indexwas demonstrated during normal operation in local and district heatingplants. Furthermore, a model was developed for predicting slaggingproblems that take into account both the chemical composition of the fueland the burner technology.Several studies have also been performed on different fuel designs basedon the same foundation as the index and the model. Additives to supply forexample calcium and sulphur, as well as the clay kaolin, have been used toreduce both technical and environmental problems.The conclusion is that fuel design, based on ash chemistry, is a possiblepath for increased fuel flexibility and a broader feedstock for bioenergy.