## 1 Introduction  
### Fluidized Bed Combustion in Sweden  
Biomass is an important source of energy and materials in the Nordic countries, especially in Sweden and Finland. The forestry industry, including the pulping industry, is of major importance in these countries. This has contributed to many decades of experience using biomass as fuel. When utilized in a sustainable manner, biomass can be considered a renewable energy source, making it a viable alternative to fossil fuels. According to Energilagget, the national energy status report published annually by the Swedish Energy Agency (Energimyndigheten), 105 TWh of the primary energy generated in Sweden 2018 was from wood fuels, most of which is produced in Sweden [1]. This corresponds to 19% of the total primary energy input to Sweden's energy system. Another large energy source in the Nordic countries is waste (Waste-to-Energy; WtE), which allows for simultaneous energy production and waste management. Waste is a broad term that includes both municipal solid waste (MSW) as well as other streams like recycled waste wood, and industrial wastes. Thus, it covers fuel fractions with very different chemical compositions, moisture, heating value, ash, etc., and can consist of both renewable and non-renewablematerials. Also, when it comes to biomass fuels, the composition and fuel quality differ with biomass type. During combustion, both wood fuels and waste fuels present considerable challenges related to ash chemistry in the boiler. Ash in woody biomasses contains mainly K, Ca, and Si. Forest residues have a higher ash content with similar elemental composition [2]. Ashes in fast-growing biomass like energy crops and agricultural wastes generally have a higher content of P, K, and Cl. K and Cl contribute the most to slagging, fouling, and severe corrosion during combustion, especially at high temperatures [2,3]. The discussion about waste ash composition becomes more complicated due to its large variations, but the same type of challenges can be expected [4]. To summarize, high ash reactivity, high moisture content, and potentially large feed composition variations are all challenging properties of waste and biomass fuels. Consequently, large-scale combustion of such fuels calls for specific boiler requirements. Fluidized bed combustion (FBC) is widely used in Sweden (and some other countries such as Finland) since it has several advantages that make it suitable specifically for this purpose. Some 120 industrial facilities in Sweden are currently using FBC, out of which 1/3 are circulating fluidized-bed boilers (CFB). The remaining 2/3 are bubbling fluidized bed boilers (BFB). Compared to other boiler types, like grate fired boilers and rotary kilns, FBC has the following advantages [4, 5, 6, 7]  
* High thermal inertia, i.e., good at handling sudden variations in fuel feed composition and heating value. * The bed material can adsorb volatile alkali components, which reduces the risk of high-temperature corrosion of heat transferring equipment. * Relatively low temperature levels; possibly low NOx formation. * Co-combustion of different fuels is possible. Historically, FBC has also had the advantage over grate-fired boilers of also being efficient at relatively low air-to-fuel ratios. However, nowadays, the difference between FBC and modern grate-fired boilers is low due to sophisticated air feeding systems [4]. The main disadvantage with FBC is that fuel processing is sometimes required. Fuel processing might include crushing, milling, sorting out incombustible components, etc., and can amount to large costs and maintenance work. In the following sections, \"bed material\" is defined as actively added minerals such as silica sand, while \"bed\" includes also fuel minerals and fuel. FBC was originally developed for coal combustion and has later been adapted for other fuels. Compared to biomass, coal contains more ash. Further, coal ash has typically larger concentrations of S, Al, and Fe, and lower concentrations of Si, K, and Cl [3].