3.1. Conventional/slow pyrolysis
Slow pyrolysis has been employed for thousands of years to produce charcoal. Production and charcoal property knowledge accumulated over the past 38 millenia have been reviewed (Antal and Grønli, 2003). Biomass is heated slowly to about 500 °C in absence of air. Vapor residence times vary from 5 to 30 min. Vapors in conventional pyrolysis do not escape rapidly unlike in fast pyrolysis.
3.2. Fast pyrolysis
Fast pyrolysis requires dry feedstock (<10 wt% moisture), rapid heat transfer, fast temperature increase by heating small biomass particles (1–2 mm) to 400–500 °C and vapor residence times of 1 s (maximum 5 s) (Lima et al., 2010). Fast pyrolysis differs fundamentally from slow pyrolysis.
3.3. Gasification
Gasification produces a gas fuel that can be used for direct heat generation or electricity generation. Gasification is a partial combustion of solid. The product mix (gas, liquid and solid) is controlled by altering temperature, particle size, residence time, pressure, gas composition under which the biomass is treated and the catalyst’s identity if one is used. Brewer et al. characterized the chars from fast pyrolysis and gasification of switchgrass and corn stover (Brewer et al., 2009). Higher char aromaticity was obtained in slow pyrolysis than in fast pyrolysis or gasification. Fused aromatic ring compound sizes in fast and slow pyrolysis chars were similar (∼7–8 rings per compound) while gasification char was more highly condensed (∼17 rings per compound) (Brewer et al., 2009).
3.4. Hydrochar
It is important to differentiate biochars from hydrochars. Hydrochars form by hydrothermal carbonization (HTC) of biomass at high temperature and pressure in water, producing a char–water–slurry (Libra et al., 2011). The solid char is easily separated. Its chemical and physical properties differ significantly from the starting biomass. Conversely, biochars are produced by pyrolysis (slow or fast) or gasification at various temperatures, pressures and carbonization times. Hydrochars from agricultural residues were characterized and used for water and soil remediation (Wiedner et al., 2013). Chemical properties of biochars from gasification or pyrolysis were compared to hydrochars from HTC (Wiedner et al., 2013). Hydrochars are less stable (dominated by alkyl moieties) than biochars (dominated by aromatics). Hydrochars are not included in the “European Biochar Certificate” (EBC) standardization due to their different chemical properties (Wiedner et al., 2013).
3.5. Torrefaction and retification
Wood “retification” takes place at ∼230–250 °C. The wood color becomes reddish brown/chocolate. This product is considered to be resistant towards biological attack. Similarly, torrefaction occurs between ∼250–280 °C at low heating rates, and creates a brown or black product with little strength. Torrefied wood yields range from 67% to 84% with energy yields of ∼77–90% (Antal and Grønli, 2003). Torrefaction increases the biomass energy density, greatly reduces weight, enhances hydrophobic nature and improves wood’s commercial use for energy production by reducing transportation costs. Typically, torrefaction and retification do not create adsorbent chars since only partial biomass decomposition occurs to prevent rot and induce some water loss.
4. Development of biochars
Different feeds and reactors are employed to make biochars by pyrolysis (slow or fast), gasification or HTC. Important feedstocks and operating conditions are listed in the Supplementary material (Table SM1). Commonly used reactors include auger (Mohan et al., 2014, Mohan et al., 2007, Mohan et al., 2011 and Mohan et al., 2012), well-swept fixed-bed, bench-scale fixed bed (Ates and Un, 2013), vertical tubular (Hameed and El-Khaiary, 2008), and fluidized bed types (Han et al., 2013b). Feed stock pyrolyses into biochars have been conducted on debarked loblolly pine (Pinus taeda) chips ( Park et al., 2013) oak wood, pine wood, oak bark and pine bark ( Mohan et al., 2007b), pine needles ( Ahmad et al., 2013a), corn stover and apple wood ( Sun et al., 2012) among many others.
5. Characterization of biochars
Biochar physical, chemical and mechanical properties can vary with production conditions, making it challenging to engineer biochars that are simultaneously optimized for carbon sequestration, nutrient storage, water-holding capacity and adsorption (Sun et al., 2012). The IBI has graded biochar into three classes based on carbon content (http://www.biochar-international.org/sites/default/files/Guidelines_for_Biochar_That_Is_Used_in_Soil_Final.pdf). These include, Class 1 biochar (contains 60% carbon or more), Class 2 biochar (between 30 and 60% carbon) and Class 3 biochar (between 10% and 30% carbon). Thermal conversion of charcoal, derived from fluidized-bed fast pyrolysis oil production of switchgrass, has been reported (Boateng, 2007). Biochars pyrolyzed under several highly controlled conditions from corn stover and apple wood were characterized. Particle size, pyrolysis heating rate, and residence time had significant effects on biochar chemical composition, aromaticity, and pore structure (Sun et al., 2012). In fast pyrolysis, the shorter the vapor residence time, less important are the secondary reactions of vapor with the developing char. Thus, reactions in the vapor and aerosol phases mainly influence chemical composition of the biooil liquid phase and the gases produced. In slow pyrolysis, the vapors are confined and react extensively with the solid phase. More mass ends up as char. Increasing both pyrolysis time and temperature increases the %C in the char and decreases both the %O and surface hydrophilicity. The effect of these parameters on adsorption is discussed in subsequent paragraphs.
6. Applications of biochars in water remediation
Biochar applications have been divided into subsections on organics (6.1) and inorganics remediation (6.2). Organics are divided into (a) dyes (b) phenolics, pesticides, polynucelar aromatics and antibiotics removal. Inorganics are divided into (a) cations and (b) anions removal. Table 2 briefly summarizes example adsorptions of inorganic species on various biochars while a comprehensive listing is given in Supplementary Material (Table SM2). The first column of Table SM2 lists the biochar type employed while column four gives the adsorbed species. This organization is somewhat arbitrary because the same general biochar types were prepared in different ways by different authors. Also, several different adsorbates were targeted for adsorption by the same biochar types made by different research groups.
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