## 3 Materials and Production  
### Current Bed Material Consumption in Fluidized Bed Boiler  
When considering the development of OCAC and CLC, it is of interest to begin with examining what large-scale FBC looks like presently. Current bed material handling and quantities can give an idea of how the transition to novel bed materials could look like. \"Bed material\" is here defined as actively added minerals such as silica sand, while \"bed\" includes also fuel minerals and fuel. The material balance of minerals over the boiler can be explained as follows. Minerals enter the boiler in two forms; as bed material, and as minerals present in the fuel. The minerals then leave the boiler either as fly ash following the blue gas, or as actively extracted bottom ash. Both the fly ash and the bottom ash contain minerals that originate from both the fuel and the bed material. The material balance looks different for coal and biomass/WtE, which is illustrated in Figure 3. The amount of minerals that enter the boiler with coal typically exceeds the amount of fly ash. Thus, bottom ash extraction is primarily performed to control bed growth. In biomass or waste combustion, on the other hand, the amount of fly ash typically exceeds the amount of minerals entering with the fuel. Thus, theoretically, the bed height would decrease without a continuous addition of bed material. In practice, however, bottom ash extraction is performed to avoid an accumulation of unwanted species in the bed and downstream in the boiler. In practice, many factors affect the operation of a boiler, and variations are large. Different boiler strategies are in the end based on economical evaluations and practical experience about the specific boiler [5]. The bed extraction rate is determined based on the risk for agglomeration, which can result in defluidization of the bed, or other operational difficulties. Defluidization of the bed requires unplanned shutdowns which is very costly for the plant owner. Agglomeration is furthermore difficult to predict, which is an incentive for plants to operate with large safety margins. A rule of thumb for describing continuous sand replacement is that 3 kg/MWh of sand is used in biomass combustion and 6 kg/MWh of sand is used in WtE [54]. However, three examples from wood-fired plants were provided in Section 1.1, and those correspond to approximately 0.5-10 kg/MWh. The amount of sand currently consumed in FBC units in Sweden has been estimated as follows. The basis for the calculation is a collection of data on FBC units done in 2016, including their location, fuel type burned, and capacity (Electrical capacity, steam capacity, and heat capacity). The data is taken from the Chalmers Power Plant Database [61; 62]. Figure 3: Schematic representation of the mineral balance over coal and biomass-fired FBC boilers. For the calculation, assumptions have been made regarding the annual operating hours (h/yr), load, and replacement of bed material in the boilers. The assumptions used in the calculation are presented in Table 2 (for both CBF and BFB plants) and are discussed here. * Autoproducers (such as pulp mills) and WtE plants are assumed to be operating as much as possible, leaving downtime only for unplanned and planned stops for maintenance and repairs. * The utilization rate for biomass combustion plants producing primarily heat and power is more complicated to estimate. Biomass as fuel can be either cheap or expensive depending on the type, and the incentives for using it vary in different plants, due to several factors. The local energy mix varies throughout Sweden, resulting in a low or high threshold for including biomass in the active energy mix. Thus, three different assumptions of annual operating hours are used in the calculations. * The bed inventory of the BFB plants was estimated by collecting data about the bed inventory of a few plants (data from [19; 59; 63]) and assuming a linear relationship between bed material inventory and steam capacity.