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.
