Ash
The non-combustible content of biomass is referred to as ash. High ash content leads to fouling problems, especially if the ash is high in metal halides (e.g., potassium). Unfortunately, biomass fuels, especially agricultural crops/residues tend to have a high ash with high potassium content. As a result, the ash melts at lower temperatures, resulting in "clinkers" that can jam furnace elements (Figure 3). Alternately, slagging and fouling occur when ash is vapourized and condensed in the boiler, resulting in the production of hard formations on the heat transfer surfaces (Figure 4).
Wood (core, no bark) has less than 1% ash. Bark can have up to 3% ash. Agricultural crops have higher ash content, from 3% and higher (Figure 5). Some boilers/stoves cannot handle fuels with high ash content. More ash means more maintenance.