## 1 Introduction
### Fluidized Bed Combustion in Sweden
The bed in coal FBC to a large extent consists of the coal ash and the coal itself. This, however, is not feasible for biomass- and WtE-FBC, where ash interactions with the bed material and the boiler walls play an important role. The ash content in these fuels might also be too low, or form particles of sizes not suitable for fluidization. Beyond causing corrosion and fouling, ash reactions also increase the risk of bed agglomeration. Agglomeration has been studied experimentally both on lab scale [8, 9, 10] and on industrial scale [6, 11, 12, 13] to gain an understanding of the formation mechanisms and possible remedies. Agglomeration is mainly caused by the formation of K-components which melt and become sticky already at relatively low temperatures. The sticky components originate from the fuel or forms by interactions between the ash and the bed material. Thus, agglomeration is a function of bed material properties, ash composition, temperature, etc. Agglomeration negatively affects the bed by interfering with the fluidization, but alkali absorption in the bed can also reduce alkali-induced problems downstream of the boiler. This can be enhanced by adding alkali-capturing components to the bed. Certain minerals have shown to be resistant towards agglomeration, and this is used in a few plants by using Al-, Ca- or Mg-rich additives, like kaolin [3,5]. Another way to inhibit the harmful K-interactions is to add sulfur to the bed, which forms the stable compound K\({}_{2}\)SO\({}_{4}\). This was done historically through co-combustion with coal or peat, but these fossil fuels are barely used any more in Sweden. Nowadays, elemental sulfur is sometimes used as additive. However, the main measure taken to prevent defluidization is to have a high rate of replacement of the bed itself. Silica sand is the commonly used bed material in FBC. Silica sand (or quartz sand) is extracted at three locations in Sweden. It is a primary product that risks becoming scarce in the future. Silica sand and gravel deposits are furthermore important for ground-water production by removing impurities from the water through filtration. Therefore, protecting natural deposits of gravel is a subgoal of the Good-quality groundwater objective, which is one of Sweden's environmental objectives [14; 15]. An evaluation in 2019 by the Geological Survey of Sweden found that the subgoal is not fulfilled and that approximately 10 Mton of natural gravel was still being extracted in the year 2016 [16; 17]. The trend is that natural gravel is being replaced by crushed rock in many applications such as construction material, but this is: (a) not always technically feasible, which is the case with bed material sand in FBC and (b) risks shifting the problem over to exploiting another non-renewable resource (rock). The production of quartz sand in Sweden was between 579 and 783 kton during the years between 2008 and 2018 [18]. To put FBC in perspective, the silica sand consumption in FBC can be illustrated by some examples [19]: The bed material consumption in three wood-fired BFB plants with 30, 50 and 80 MW\({}_{\text{th}}\) output was 0.4, 2-5, and 15-20 metric tons per day, respectively. This shows that the regeneration rate of bed material is not only high but also differs a lot between plants of similar type. The main reason for replacing bed material was stated to be to reduce the risk for bed agglomeration. Sand consumption and other practical aspects of agglomeration prevention in FBC will be discussed in greater detail in Section 3.2. One more negative aspect of using silica sand as bed material needs to be highlighted, which is that long-time exposure to silica sand can cause silicosis in workers, a lung disease that can lead to cancer [20]. Apart from the negative environmental effects of sand extraction, high sand consumption in the boiler is also a significant monetary cost. This cost can be divided into two parts: the cost of new bed material, and the cost of disposing of the replaced bed. Spent bed material ends up in both bottom- and fly ash.