Biomass combustion has a huge potential to produce power and heat in a sustainable way. However, some biomass fuels have high potassium contents, which react with other ash forming elements (i.e. Cl, Si, P and S) and lead to different ash related operational problems. Utilization of additives to abate these problems have been studied and tested for several decades. This work reviews current knowledge and studies about properties and effects of additives reported in the literature. Various additives can mitigate ash related issues by the following possible mechanisms: 1) capturing problematic ash species via chemical adsorption and reactions, 2) physical adsorption and elutriating troublesome ash species from combustion facilities, 3) increasing the biomass ash melting temperature by enhancing inert elements/compounds in ash residues, and 4) restraining biomass ash sintering by diluting and powdering effects from the additives. Additives are grouped according to the contained reactive compounds, including Al-silicates based additives, sulphur based additives, calcium based additives, and phosphorous based additives. Additives with strong chemical adsorption and reaction capacities can minimize K related ash sintering, deposition and slagging during biomass combustion processes. The effective chemical reaction mechanisms are closely related to K-Al-Si, K-Ca-Si and K-Ca-P systems. The capacities of additives to reduce ash related problems are heavily influenced by mass/molar ratios between the reactive components in the additives and the problematic elements in the biomass ash, as well as the reaction atmosphere and combustion technology. More detailed studies on high temperature reactions between additives and ashes from biomass combustion are needed

Torrefaction has been recognized as a promising strategy to improve handling and storage properties of wood-based pellets, thus producing a uniform-quality commodity with high energy density and hydrophobicity. In this work, pellets produced from spruce stem wood, bark, and forest residues were torrefied in a bench-scale tubular reactor at 225 and 275 °C with two residence times (30 and 60 min). The effects of torrefaction on general properties, grindability, mechanical properties, hydrophobicity, and microstructure of the studied pellets were investigated. The increase of torrefaction severity reduced mass yields, but the heating values and the fixed carbon content of the torrefied pellets considerably increased. The grindability of raw pellets was substantially improved after torrefaction treatment. The energy required for grinding torrefied pellets is less than 50% of the energy needed for grinding the untreated pellets. In comparison to untreated pellets, the particles from ground torrefied pellets have clearly smaller sizes in a narrower size range. The increase of torrefaction severity improved hydrophobicity of the pellets, which have high resistance to water uptake and maintain their integrity after immersion testing. Upon torrefaction treatment, the durability and tensile strength of the pellets slightly decreased. Scanning electron microscopy analysis results show that particles from wood pellets torrefied at 275 °C lost their fibrous structure with an evident decrease of length/diameter ratios compared to untreated wood pellets. The particles from ground torrefied pellets are more uniform in terms of shape and size. Torrefaction can considerably improve grindability and uniformity of wood-based pellets and make them more acceptable in pulverized fuel applications.