Feedstock is the term conventionally used for the type of biomass that is pyrolyzed and turned into biochar. In principle, any organic feedstock can be pyrolyzed, although the yield of solid residue (char) respective to liquid and gas yield varies greatly along with physicochemical properties of the resulting biochar. Feedstock is, along with pyrolysis conditions, the most important factor controlling the properties of the resulting biochar.
Firstly, the chemical and structural composition of the biomass feedstock relates to the chemical and structural composition of the resulting biochar and, therefore, is reflected in its behavior, function and fate in soils.
Secondly, the extents of the physical and chemical alterations undergone by the biomass during pyrolysis (e.g. attrition, cracking, microstructural rearrangements) are dependent on the processing conditions (mainly temperature and residence times).
Table 1 provides a summary of some of the key components in representative biochar feedstock. Cellulose and lignin undergo thermal degradation at temperatures ranging between 240-350ºC and 280-500ºC, respectively (Demirbas, 2004).
Table 1. Summary of key components (by weight) in biochar feedstock
The relative proportion of each component will, therefore, determine the extent to which the biomass structure is retained during pyrolysis, at any given temperature. For example, pyrolysis of wood-based feedstock generates coarser and more resistant biochars with carbon contents of up to 80%, as the rigid ligninolytic nature of the source material is retained in the biochar residue (Winsley, 2007).
Biomass with high lignin contents (e.g. olive husks) have shown to produce some of the highest biochar yields, given the stability of lignin to thermal degradation, as demonstrated by Demirbas (2004). Therefore, for comparable temperatures and residence times, lignin loss is typically less than half of cellulose loss (Demirbas, 2004).
The mineral content of the feedstock is largely retained in the resulting biochar, where it concentrates due to the gradual loss of C, hydrogen (H) and oxygen (O) during processing (Demirbas, 2004).
The mineral ash content of the feedstock can vary widely and evidence seems to suggest a relationship between that and biochar yield (Amonette and Joseph, 2009). Table 2 provides an example of the elemental composition of representative feedstock.
Table 2. Examples of the proportions of nutrients (g kg-1) in feedstock
In the plant, Ca occurs mainly within cell walls, where it is bound to organic acids, while Mg and P are bound to complex organic compounds within the cell Potassium which is the most abundant cation in higher plants and is involved in plant nutrition, growth and osmoregulation Nitrogen, Mn and Fe also occur associated to a number of organic and inorganic forms.
During thermal degradation of the biomass, potassium (K), chlorine (Cl) and N vaporize at relatively low temperatures, while calcium (Ca), magnesium (Mg), phosphorus (P) and sulphur (S), due to increased stability, vaporise at temperatures that are considerably higher (Amonette and Joseph, 2009).
The Thermal Degradation Spectrum
Other relevant minerals can occur in the biomass, such as silicon (Si), which occurs in the cell walls, mostly in the form of silica (SiO2). Many different materials have been proposed as biomass feedstocks for biochar, including wood, grain husks, nut shells, manure and crop residues, while those with the highest carbon contents (e.g. wood, nut shells), abundance and lower associated costs are currently used for the production of activated carbon (Lua et al., 2004; Martinez et al., 2006; Gonzaléz et al., 2009).
Regarding the characteristics of some plant feedstocks, even within a biomass feedstock type, different composition may arise from distinct growing environmental conditions (e.g. soil type, temperature and moisture content) and those relating to the time of harvest. In corroboration, (Chan and Xu, 2009) have shown that the adsorbing properties of a charcoal for copper ions can be improved 3-fold by carefully selecting the growth conditions of the plant biomass (in this case, stinging nettles). Even within the same plant material, compositional heterogeneity has also been found to occur among different parts of the same plant (e.g. maize cob and maize stalk, Table 2).
Nature of feedstock
In addition to plant biomass, an entire range of organic materials, including waste materials such as poultry litter and sewage sludge can be converted to biochars using pyrolysis. Recently, conversion of these other materials to biochars has been promoted as an alternative way of managing a range of organic wastes (Bridle and Pritchard, 2004; Shinogi, 2004; Hospido et al., 2005; Lima and Marshall, 2005). Given the vast differences in the properties of the potential feedstocks, biochars can have very different nutrient contents and availability, as discussed earlier.