Conversion of biomass feedstock, either thermochemically or biochemically, results in a solid residue. In themochemical processing via combustion in air, the residue consists solely of ash. For biochemical processing, it contains both ash and other unconverted material, especially lignin. The bioprocess residue can be further processed thermochemically to yield ash as the final solid residue. The ash content negatively affects the energy density of the feedstock. Ash can also pose operational problems in thermochemical processing, such as slagging in which the ash melts and fuses together. Relatively low-cost control measures, such as leaching the raw feedstock with water and using different mineral additives (e. g. kaolinite, clinochlore, ankerite), can be used to reduce negative effects [27]. Potential end uses of ash include mineral agricultural fertilizer [28] and construction material additive [29]. Representative ash content values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1. As can be seen from the table, herbaceous feedstocks tend to have higher ash contents (e. g. >5%) than woody feedstocks (e. g. <2%).