In most pyrolysis systems, the operating temperatures are fairly modest. It is commonly found at laboratory and rig scale that the inherent mineral material in biomass tends to be retained within char, and is not released into gas or vapour phase in sufficient quantities to cause ash deposition or other operational problems within the reactor or in the gas collection equipment.
Very little work has been carried out on the distribution and stability of heavy metals in biochar. High mineral-ash biochars (especially chicken manure biochar and activated carbon) are known to adsorb heavy metals.
Very little has been published on the distribution of mineral ash within different type of biochar. Of the inorganic elements that comprise mineral ash, most are believed to occur as discrete phases separate from the carbonaceous matrix. In some biochars, however, K and Ca are distributed throughout the matrix where they may form phenoxides (K, Ca) or simply be intercalated between grapheme sheets (K).
Minerals found in biochars include sylvite (KCl), quartz (SiO2), amorphous silica, calcite (CaCO3), hydroxyapatite (Ca10(PO4)6(OH)2), and other minor phases such as Ca phosphates, anhydrite (CaSO4), various nitrates, and oxides and hydroxides of Ca, Mg, alumunium (Al), titanium (Ti), Mn, zinc (Zn) or Fe. Amorphous silica is of particular interest as it typically is in the form of phytoliths that contain and protect plant C from degradation. Crystalline silica is also of interest because it has been found in some biochars where it poses a very high level respiratory risk. Microprobe analysis of these biochars indicates that there is a large variation of mineral content even within each particle.