Thermal degradation of cellulose between 250ºC and 350ºC results in considerable mass loss in the form of volatiles, leaving behind a rigid amorphous C matrix. As the pyrolysis temperature increases, so thus the proportion of aromatic carbon in the biochar, due to the relative increase in the loss of volatile matter (initially water, followed by hydrocarbons, tarry vapors, H2, CO and CO2), and the conversion of alkyl and O-alkyl C to aryl C (Baldock and Smernik, 2002; Demirbas, 2004).
Around 330ºC, polyaromatic graphene sheets begin to grow laterally, at the expense of the amorphous C phase, and eventually coalesce. Above 600ºC, carbonization becomes the dominant process. Carbonization is marked by the removal of most remaining non-C atoms and consequent relative increase of the C content, which can be up to 90% (by weight) in biochars from woody feedstocks (Antal and Gronli, 2003; Demirbas, 2004).
Each layer consists of hexagonal rings of carbon atoms linked together in a planar lattice, the formula is Cn where n is a very large number BUT a graphene layer is just a single atom in thickness.
It is commonly accepted that each biochar particle comprises of two main structural fractions: stacked crystalline graphene sheets and randomly ordered amorphous aromatic structures (Figure 1). Hydrogen, O, N, P and S are found predominantly incorporated within the aromatic rings as heteroatoms (Bourke et al., 2007). The presence of heteroatoms is thought to be a great contribution to the highly heterogeneous surface chemistry and reactivity of biochar.
Composition -C/99.45% H/0.55%
Figure 1. Structure of charcoal. A model of a microcrystalline graphitic structure is shown on the left and an aromatic structure containing oxygen and carbon free radicals on the right (Bourke et al., 2007).
Chemical composition and surface chemistry
Biochar composition is highly heterogeneous, containing both stable and labile components (Sohi et al., 2009). Carbon, volatile matter, mineral matter (ash) and moisture are generally regarded as its major constituents (Antal and Gronli, 2003). Table 1 summarizes their relative proportion ranges in biochar as commonly found for a variety of source materials and pyrolysis conditions (Antal and Gronli, 2003; Brown, 2009).
Table 1. Relative proportion range of the four main components of biochar. Weight percentage as commonly found for a variety of source materials and pyrolysis conditions
The relative proportion of biochar components determines the chemical and physical behavior and function of biochar as a whole (Brown, 2009), which in turn determines its suitability for a site-specific application, as well as transport and fate in the environment (Downie et al., 2009). Moisture is another critical component of biochar (Antal and Gronli, 2003), as higher moisture contents increase the costs of biochar production and transportation for unit of biochar produced.
Biochar for a two hectare block of zucchinis, cucumber and watermelons.
Keeping the moisture content up to 10% (by weight) appears to be desirable. In order for this to be achieved, pre-drying the biomass feedstock may be a necessity in biochar production. Despite the feasibility of biochar being produced from a wide range of feedstocks under different pyrolysis conditions, its high carbon content and strongly aromatic structure are constant features (Sohi et al., 2009). According to Sohi et al. (2009), these features largely account for its chemical stability. Similarly, pH shows little variability between biochars, and is typically >7 (Table 2).
Table 2. Summary of total elemental composition
(C, N, C:N, P, K, available P and mineral N) and pH ranges and means of biochars from a variety of feedstocks (wood, green wastes, crop residues, sewage sludge, litter, nut shells) and pyrolysis conditions (350ºC-500ºC) used in various studies (Chan and Xu, 2009).
Total carbon content in biochar was found to range between 172 to 905 g kg-1, although OC often accounts for < 500 g kg-1, as reviewed by Chan and Xu (2009) for a variety of source materials. Total N varied between 1.8 and 56.4 g kg-1, depending on the feedstock (Chan and Xu, 2009). Despite seemingly high, biochar total N content may not be necessarily beneficial to crops, since N is mostly present in an unavailable form (mineral N contents < 2 mg k-1; Chan and Xu, 2009).
Effect of temperature on carbon and oxygen contents in bio-char from bio-waste. Particle size: 1.5–2.5 mm.
Nuclear magnetic resonance (NMR) spectroscopy has shown that aromatic and heterocyclic N-containing structures in biochar occur as a result of biomass heating, converting labile structures into more recalcitrant forms (Almendros et al., 2003). C:N (carbon to nitrogen) ratio in biochar has been found to vary widely between 7 and 500 (Chan and Xu, 2009), with implications for nutrient retention in soils. C:N ratio has been commonly used as an indicator of the capacity of organic substrates to release inorganic N when incorporated into soils. Total P and total K in biochar were found to range broadly according to feedstock, with values between 2.7 - 480 and 1.0 - 58.0 g kg-1, respectively (Chan and Xu, 2009).
Interestingly, total ranges of N, P and K in biochar are wider than those reported in the literature for typical organic fertilizers. Most minerals within the ash fraction of biochar are thought to occur as discrete associations independent of the carbon matrix, with the exception of K and Ca (Amonette and Joseph, 2009). Typically, each mineral association comprises more than one type of mineral. Joseph et al. (2009) emphasize that our current understanding of the role of high-mineral ash biochars is yet limited, as we face the lack of available data on their long-term effect on soil properties.
The review of relevant literature has indicated that the full knowledge on the composition of biochar as a soil amendment, and the way it is influenced by those parameters, as well as the implications for soil functioning, is still scarce. Partially, this can be explained by the fact that most characterization work has involved charcoals with high carbon and low ash content, as required by the increasingly demanding market for activated carbon. Another factor is the wide variety of processing conditions and feedstocks available. Nevertheless, the current sparsity of biochar standards is largely reflected on the poor understanding of the link between biochar composition and its behavior and function in soil.