Biochar particle size distribution

    Initially, particle size distribution in biochar is influenced mainly by the nature of the biomass feedstock and the pyrolysis conditions (Cetin et al., 2004). Shrinkage and attrition of the organic material occur during processing, thereby generating a range of particle sizes of the final product. The intensity of such processes is dependent on the pyrolysis technology (Cetin et al., 2004).

Biochar production (raw material characterization) 

Source: O. Varela Milla.

Hand application: the application of biochar by hand is well known, but cannot be considered viable at scale due to labor intensity and human health concerns due to prolonged contact with airborne biochar particulates. 

Source: http://www.biocharapplication.com/methods-to-date.html

 

     Particle size distribution in biochar also has implications for determining the suitability of each biochar product for a specific application (Downie et al., 2009), as well as for the choice of the most adequate application method. In addition, health and safety issues relating to handling, storage and transport of biochar are also largely determined by its particle size distribution.

    The influence of the type of feedstock on particle size distribution was discussed by Sohi et al. (2009), among others. Wood-based feedstocks generate biochars that are coarser and predominantly xylemic in nature, whereas biochars from crop residues (e.g. rye, or maize) and manures offer a finer and more brittle structure (Sohi et al., 2009). Downie et al. (2009) have further provided evidence of the influence of feedstock and processing conditions on particle size distribution in biochar.


Influence of Feedstock and Pyrolysis Temperature of Biochar Amendments on Transport of Escherichia coli in Saturated and Unsaturated Soil

The effects of biochar feedstock, pyrolysis temperature, and application rate (1 and 2%) on the transport of two Escherichia coli isolates through a fine sand soil under water-saturated and partially saturated conditions were investigated in column experiments. Biochars from two feedstocks (poultry litter and pine chips) and pyrolyzed at two temperatures (350 and 700 °C) were evaluated. Both biochars pyrolyzed at 700 °C resulted in significant reductions in E. coli transport, with greater reductions observed with the pine chip biochars. For the low temperature biochars, increased transport was observed for the poultry litter biochar whereas reduced transport was observed for the pine chip biochar. In general, the effect of biochar application on E. coli transport was more pronounced in the unsaturated soils and for the 2% application rates. Large differences were also observed between the two isolates indicating that bacterial surface properties play a role in how biochar affects E. coli transport. Source: http://pubs.acs.org/doi/abs/10.1021/es300797z

   

   The operating conditions during pyrolysis (e.g. heating rate, high treatment temperature-HTT, residence time, pressure, flow rate of the inert gas, reactor type and shape) and pre- (e.g. drying, chemical activation) and post- (e.g. sieving, activation) treatments can greatly affect biochar physical structure (Gonzalez et al., 2009; Antal and Gronli, 2003; Cetin et al., 2004; Lua et al., 2004). Such observations were derived mainly from studies involving activated carbon produced from a variety of feedstocks, including maize hulls, nut shells (Lua et al., 2004; Gonzaléz et al., 2009) and olive stones (Gonzaléz et al., 2009). Similarly, heating rate, residence time and pressure during processing were shown to be determinant factors for the generation of finer biochar particles, independently of the original material (Cetin et al., 2004).

Volatile matter, fixed carbon and ash content of the solid product resulted from pyrolysis
processes of the cherry sawdust. Source: GHEORGHE et al., 20 U.P.B. Sci. Bull., Series C, Vol. 72, Iss. 1, 2010 ISSN 1454-234x

Plots for the temperature effect on yield of char produced from wooden biomass samples. Source: GHEORGHE et al., 20 U.P.B. Sci. Bull., Series C, Vol. 72, Iss. 1, 2010 ISSN 1454-234x

     For instance, for higher heating rates (e.g. up to 105-500ºC sec-1) and shorter residence times, finer feedstock particles (50-2000 μm) are required in order to facilitate heat and mass transfer reactions, resulting in finer biochar material (Cetin et al., 2004). In contrast, slow pyrolysis (heating rates of 5-30ºC min-1) can use larger feedstock particles, thereby producing coarser biochars (Downie et al., 2009). Increasing the proportion of larger biochar particles can also be obtained by increasing the pressure (from atmospheric to 5, 10 and 20 bars) during processing, which was explained by both particle swelling and clustering, as a result of melting (i.e. plastic deformation) followed by fusion (Cetin et al., 2004).

The pyrolysis char yields versus residence time. Source: GHEORGHE et al., 20 U.P.B. Sci. Bull., Series C, Vol. 72, Iss. 1, 2010 ISSN 1454-234x

Influence of the heating rate on char yields. Source: GHEORGHE et al., 20 U.P.B. Sci. Bull., Series C, Vol. 72, Iss. 1, 2010 ISSN 1454-234x

Pore size distribution and connectivity

   Biomass feedstock and the processing conditions are the main factors determining pore size distribution in biochar, and therefore its total surface area (Downie et al., 2009). During thermal decomposition of biomass, mass loss occurs mostly in the form of organic volatiles, leaving behind voids, which form an extensive pore network. Biochar pores are classified into three categories (Downie et al., 2009), according to their internal diameters (ID): macropores (ID >50 nm), mesopores (2 nm< ID <50 nm) and micropores (ID <2 nm).

Surface areas and volumes of different sizes of biochar pores

Source: Biochar for Environmental Management: Science and Technology.

     The elementary porosity and structure of the biomass feedstock is retained in the biochar product formed (Downie et al., 2009). The vascular structure of the original plant material, for example, is likely to contribute for the occurrence of macropores in biochar, as demonstrated for activated carbon from coal and wood precursors. In contrast, micropores are mainly formed during processing of the parent material. While macropores have been were identified as a ‘feeder’ to smaller pores (Martinez et al., 2006), micropores effectively account for the characteristically large surface area in charcoals (Brown, 2009). Among those operating parameters, HTT is thought to be the most significant factor for the resulting pore distribution in charcoals (Lua et al., 2004), as the physical changes undergone by the biomass feedstock during processing are often temperature-dependent (Antal and Grønli, 2003).

    The development of microporosity in biochar, which is linked to an increase in structural and organizational order, has been showed to be favored by higher HTT and retention times, as previously demonstrated for activated carbon (Lua et al., 2004). For example, increasing pyrolysis temperature from 250oC to 500oC enhanced the development of micropores in chars derived from pistachio-nut shells, due to increased evolution of volatiles. For subsequent increases in temperature (>800oC), a reduction of the overall surface area of the char was observed and was attributed to partial melting of the char structure (Lua et al., 2004). 

     For turbostratic arrangements, the successive layer planes are disposed approximately parallel and equidistant, but rotated more or less randomly with respect to each other (see letter “b” in Figure 1) (Emmerich et al., 1987). Similarly, heating rate and pressure during processing have also been found to influence the mass transfer of volatiles produced at any given temperature range, and are therefore regarded as key contributing parameters influencing pore size distribution (Antal and Grønli, 2003).  It is important to stress, however, that the relative influence of each processing parameter on the final microporosity in biochar is determined by the type of feedstock, as noted from the above studies (Cetin et al., 2004; Lua et al., 2004; Pastor-Villegas et al., 2006; Gonzaléz et al., 2009). In particular, the lignocellulosic composition of the parent material largely determines the rate of its thermal decomposition, and therefore, the development of porosity (Gonzaléz et al., 2009).

Structure of lignocellulosic plant biomass.
Source: Ratanakhanokchai et al., InTech, DOI: 10.5772/51820. Available from: http://www.intechopen.com/books/biomass-now-cultivation-and-utilization/paenibacillus curdlanolyticus-strain-b-6-multienzyme-complex-a-novel-system-for-biomass-utilization

    The pyrolysis of all biomass C will finally yield graphite when heated to 3500°C; however, some feedstocks graphitize at HTTs of less than 2000°C (Setton et al., 2002). The surface of non-graphitized C, such as wood biochars, consists of both the faces and edges of ordered sheets (Boehm, 1994 and 2002). The turbostratic linkage of these crystallites leaves random interstices (pores of various sizes). A further possible cause of micropores is from voids (holes) within hexagonal planes (Bourke et al., 2007). Heteroatoms, in particular oxygen (O), are predominantly located on the edges of ordered sheets as components of various functional groups (Boehm, 1994; Boehm, 2002). 

    The interplanar distance of graphite (0.335nm) is probably not achieved under typical pyrolysis conditions (<1000°C) due to the formation of O functional groups at the sheet edges, which through steric or electronic effects prevent the close packing of the sheets (Laine and Yunes, 1992). Pores, of whatever origin, may become filled with tars (condensed volatiles) and other amorphous decomposition products, which may partially block the microporosity created (Bansal et al., 1988).

Figure 1.  Ideal biochar structure development with highest treatment temperature. (HTT): (a) increased proportion of aromatic C, highly disordered in amorphous mass; (b) growing sheets of conjugated aromatic carbon, turbostratically arranged; (c) structure becomes graphitic with order in the third dimension (Emmerich et al., 1987).

The tars created from thermal biomass C decomposition impede the continuity of pores at low temperatures and these pores become increasingly accessible as the temperatures increase and tar components are volatilized (Pulido-Novicio et al., 2001). Mineral matter may also become occluded in the pores or exposed at the surface of the biochar particles.