21
Biochar in thermal and thermochemical biorefineries—production of biochar as a coproduct
Abstract
Biochar offers an effective way of removing carbon from the atmosphere and presents numerous opportunities for synergies in different areas of biomass production and its utilization. Biofuel and biochemical production is one such area where implementation of the biochar concept is potentially attractive, reducing the carbon footprint, and improving sustainability of biomass sourcing. This is due to the fact that in biofuel plants and biorefineries based around thermochemical biomass conversion, eg, fast pyrolysis, gasification, biochar is a natural coproduct, which is currently most often combusted for process heat. Where higher-value uses (agriculture, horticulture, environmental management, etc.) can be developed, this biochar could be beneficially utilized, rather than combusted. Although biochemical conversion processes do not directly produce biochar, they yield residues that are well suited to production of biochar (eg, lignin).
In thermochemical biorefineries, where biochar is a coproduct, the production process is optimized for production of biofuels or chemicals, and therefore not necessarily set up to yield biochar with specific properties. Although this may reduce the perceived value of such biochar, using knowledge of interactions between biochar properties and different applications, it is possible to match biochar from different processes to specific applications. Where biochar is produced in a dedicated process, eg, utilizing residues from biochemical conversion, the production conditions can be tuned to yield biochar with properties matching specific applications.
Keywords
Biochar; Biorefinery; Hydrochar; Polycyclic aromatic hydrocarbons (PAHs); Pyrolysis; Thermochemical21.1. Introduction
Biochar is a solid carbonaceous material produced as a coproduct of thermochemical conversion (pyrolysis, gasification, or hydrothermal carbonization) of biomass. It is purposefully produced for application to soil or other storage media with the intention to serve as a carbon sink and/or soil conditioner. Therefore its properties need to be designed to match this purpose. Depending on the feedstock and production conditions, the carbon content of biochar and its stability (resistance to decomposition) can be varied, together with other parameters, such as physical (porosity, strength, density, etc.) and chemical (composition, active surface groups, etc.) properties. The resulting set of properties determines the suitability of any particular biochar for different applications.
The concept of biochar as a method for carbon sequestration is fairly new, despite numerous historical examples of widespread use of carbonized biomass in agricultural practices around the globe as a soil amendment medium (Ogawa and Okimori, 2010; Young, 1804). The concept relies on diverting a portion of carbon away from the global carbon cycle (60 Gt of carbon per year) into a much slower cycle, or a carbon sink in the form of stable carbon. Due to the scale of the carbon cycle, diverting even a small fraction into a stable form can contribute considerably to offset anthropogenic emissions of greenhouse gases (GHGs) (7 Gt of carbon per year). As a result, widespread deployment of biochar can slow down increases in atmospheric GHG concentrations and eventually lead to their reduction, once stabilization of emissions from fossil fuels is achieved.
In the context of biofuels and bioenergy, biochar can help to achieve their improved environmental performance and move these technologies along a carbon intensity axis from close to carbon neutral to carbon negative. Therefore, there are several possible motivations for integrating biochar as a concept into biofuels, biorefinery, and bioenergy concepts; climate change mitigation, agricultural productivity, and waste management. Often more than one of these would apply in any given scenario, as important synergies can be achieved.
21.1.1. Biochar for climate change mitigation
Turning organic matter into biochar, with organic carbon stabilized in a recalcitrant carbonized matrix, diverts carbon from a relatively rapid biological cycle, with average turnaround time of at most a few decades (Prentice et al., 2001), into a slow carbon cycle, with turnaround time in the order of centuries to millennia (Kuhlbusch, 1998). This use and function of biochar depends critically on its environmental stability, ie, recalcitrance to decomposition and release of carbon back into the atmosphere. Besides high stability, a second condition must also be met for biochar to play a valid role in carbon sequestration. As the actual removal of carbon from the atmosphere is the result of photosynthesis, which converts atmospheric CO2 into organic carbon compounds in plant matter that can be used as feedstock for biochar production, it is necessary that new biomass is grown at least at the same rate at which it is being turned into biochar.
The stability of biochar is a complex topic, and its detailed discussion is beyond the scope of this chapter. It is an area of intensive ongoing research (Cross and Sohi, 2013; Lehmann et al., 2009; Naisse et al., 2013; Spokas, 2010), owing to its importance. The main issues arise because of the long timescales involved, and the difficulty of reliable experimental determination of biochar carbon resistance to decomposition on such long timescales. This is further complicated by the sensitivity of stability to environmental conditions, reflecting different climatic and geographic regions. Although, determination of absolute biochar carbon stability is still not possible, different approaches to assessment of relative stability have been proposed. Among these, proximate analysis (ASTM D1762-84(2013)), O:C or H:C molar ratios (Enders et al., 2012; Spokas, 2010), chemical oxidation (Cross and Sohi, 2013), and thermal oxidation (Harvey et al., 2012) are among those most commonly used. Analyses using these methods show strong dependence of biochar stability on production conditions and feedstock. However, the carbon sequestration potential, calculated as the product of biochar carbon stability and its yield, appears to be much less sensitive to production conditions (Crombie et al., 2013; Mašek et al., 2011; Zhao et al., 2013).
In addition to this direct way of carbon sequestration, evidence suggests that biochar can also have an indirect effect on GHG emissions from agricultural activities. This is due to its potential to reduce the need for primary inputs (water, fertilizer, etc.) and associated energy consumption, as well as reduced emissions of CH4 and N2O from cultivated soils (Karhua et al., 2011; Yanai et al., 2007).
21.1.2. Biochar for soil conditioning
There is growing evidence that the addition of biochar to agricultural soils, at least under certain circumstances (ie, where it helps to address existing constraints), leads to improved crop yield and therefore enhanced productivity (Sohi et al., 2010; Spokas et al., 2012; Verheijen et al., 2009, 2010). There are a number of potential reasons for this beneficial effect and these can be categorized as physical (change in soil texture, bulk density, water-holding capacity, etc.) or chemical (change of soil pH, addition of nutrients from leachable ash, promotion of microbial activity, etc.) (Atkinson et al., 2010). Often more important than direct addition of macronutrients to soil in the form of P and N is its ability to alter their availability to plants (Gundale and DeLuca, 2006a,b). Some of these effects are likely to be only short-term while others are likely to be longer-lasting (over a number of years). Current research suggests that addition of biochar can bring the biggest benefits to poor soils by influencing carbon content and water-holding capacity (Shackley and Sohi, 2010).
21.1.3. Biochar for waste management
As biochar can be produced from nearly any organic matter, it can, with certain caveats, also be produced from wastes and nonvirgin biomass. In fact, pyrolysis has been used extensively for waste management, as it can significantly reduce the volume and weight of waste material. As a result, significant experience exists with associated material handling and processing (Heermann et al., 2002), and the industry has accumulated a great deal of knowledge and experience with pyrolysis technology.
However, production of biochar from waste requires a cautious approach to avoid risks associated with potential biochar contamination. This is despite the fact that pyrolysis destroys many potential pathogens and contaminants found in wastes that would otherwise usually pose a challenge to their application to soil (Bicudo and Goyal, 2003; Westrell et al., 2004). There are two main types of contamination (inorganic and organic) and associated sources. The first potential source is the feedstock itself. Most heavy metals contained in the feed would be retained in the biochar after pyrolysis, and therefore feedstock with a high content of heavy metals yields contaminated biochar. In addition, the relative concentrations of heavy metals in biochar are higher than in the feedstock, due to the loss of volatile matter. The second source is the pyrolysis (or other thermochemical conversion) process, which can contaminate biochar with organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), furans, dioxins, and other potentially toxic compounds (Buss et al., 2015b; Buss and Mašek, 2014; Hale et al., 2012). While in reality the content of heavy metals in biochar can only be controlled by choice of feedstock, the content of organic contaminants can be successfully minimized by suitable design and operation of the conversion process (Buss et al., 2015a), or by additional post-treatment of contaminated biochar (Kołtowski and Oleszczuk, 2015).
In many cases, pyrolysis and biochar can be an attractive solution to waste management of organic residues, as it diverts organic material from disposal routes such as landfill or incineration. Conversion (upcycling) of such waste materials into a useful product offers several benefits, as material for agricultural applications as well as a method for carbon sequestration. At the same time, diverting the organic material from landfill and stabilizing it in the form of biochar prevents a release of methane (a potent GHG) that would otherwise occur during its decomposition.
Besides management of wastes, pyrolysis can also be used to manage coproducts of different processes, producing further useful products. Application of this concept to biorefineries will be discussed in Section 21.3.
21.2. Biochar as a coproduct in biofuels and bioenergy production
Most thermochemical conversion technologies used for production of biofuels and biochemicals yield solid residues as a byproduct. Depending on the technology, this residue contains more or less carbon, and can therefore be considered biochar in its own right, or a biochar precursor. The key relevant technologies, fast pyrolysis, gasification, and hydrothermal carbonization are discussed individually in the following sections.
The properties of biochar produced as a coproduct of biofuel and biochemical production depend greatly on the feedstock and conversion technology selected. As the primary objective of these processes is not the production of biochar, it is unlikely that the process conditions would be set up to yield biochar with properties tuned for specific applications, so-called “bespoke” or “engineered” biochar. Although this may reduce the perceived value of these biochars, it does not necessarily preclude their use. Based on understanding of interactions between biochar properties and requirements of different applications, such as soil amendment, environmental management, carbon sequestration, it is possible to match biochar from different production units to a suitable application. In addition, in some cases further modification/upgrading of biochar can be justified to improve its properties, and increase its value.
21.2.1. Fast pyrolysis
Fast pyrolysis is a thermochemical conversion process capable of producing liquid biofuels (bio-oil) (Balat et al., 2009; Bridgwater, 2000, 2012; Czernik and Bridgwater, 2004) from a range of lignocellulosic feedstock. Typical fast pyrolysis conditions are characterized by: moderate pyrolysis treatment temperatures (400–600°C), rapid heating rates of biomass particles (>100°C/min), combined with short residence times of the biomass particles and pyrolysis vapors (0.5–2 s) at high temperatures (Demirbas, 2004). Combination of medium pyrolysis temperature with short vapor residence time ensures high yield of good-quality pyrolysis liquids (up to 70–75 wt%, expressed on dry biomass feedstock basis), while keeping the char and gas yields to a minimum, 12 and 13 wt%, respectively (Bridgwater, 2012; Nachenius et al., 2013). To achieve the high heating rate needed, the process requires intensive heat transfer from heat source to biomass particle. For this reason, small particle sizes (1–2 mm) are required, due to low thermal conductivity of biomass. As a result, considerable physical pretreatment of biomass is necessary before fast pyrolysis.
A key distinguishing feature of fast pyrolysis technology is the need to keep the vapor residence time in hot zone to the minimum (below a few seconds), to achieve good bio-oil quality. This can be achieved by ensuring rapid quenching or cooling of the vapors. In this way, unwanted secondary vapor phase decomposition reactions, which would give rise to additional char and noncondensable gases, at the expense of bio-oil yield, are avoided. In this respect, fast pyrolysis differs from slow pyrolysis, where the latter process aims at achieving a maximum yield of char (up to 35 wt%) by employing long particle and vapor residence times and slower heating rates (Bridgwater and Peacocke, 2000; Brown et al., 2015; Lu et al., 2009).
Different types of fast pyrolysis reactors have been proposed, developed, and scaled up to pilot or even commercial scale level (Bridgwater, 2000; Venderbosch and Prins, 2010) over several decades of development, each with its strengths and weaknesses. However, due to their scalability, well-understood design, process control, and favorable parameters, such as feedstock flexibility, high heat and mass-transfer rates, possibility of use of catalysts, fluidized beds have become the preferred technology for industrial bio-oil production (Ringer et al., 2006; Venderbosch and Prins, 2010). In a fluidized bed, a preheated solid material (heat carrier, often sand or catalyst) is suspended in a stream of hot, inert gas, although mechanical fluidization is also possible. The resulting vigorous motion of biomass and bed-material particles ensures optimal mixing behavior and high heat and mass-transfer rates. Downstream of the pyrolysis reactor, cyclones separate the vapors from the char/heat carrier, followed by collection of bio-oil using electrostatic precipitators or spray condensers. For detailed discussion of fast pyrolysis and upgrading of bio-oil to biofuels, see chapters “catalytic fast pyrolysis for improved liquid quality” and “Production of biofuels via bio-oil upgrading & refining.”
In terms of biochar production, fast pyrolysis yields only approximately 12–15% of char, and therefore it is not the most suitable technology for dedicated biochar production. However, the solid residues resulting from bio-oil production can still be used as biochar if they possess the right properties. One potential disadvantage of fast pyrolysis biochar is the physical size of char particles (very fine powder), which makes its handling and field application challenging (Husk and Major, 2010). This can, however, be overcame by pelleting or granulation of biochar to obtain biochar in a more suitable physical form. In addition, it is also important to note that the primary decomposition of biomass during fast pyrolysis is an endothermic process (Venderbosch and Prins, 2010) and thus (in commercial-scale units) the required process heat is usually obtained by complete or partial combustion of the noncondensable gases or char. Therefore, to allow for valorization of fast pyrolysis biochar, other low-value sources of heat would need to be utilized.
Given the specifics of the fast pyrolysis process in terms of feedstock requirements and process conditions, ie, fast heating and short residence time in reactor, it can be expected to yield biochar with a different set of properties compared to other conversion processes, such as slow pyrolysis or gasification. The short residence time can lead to incomplete charring of the biomass particle, as observed by Bruun et al. (2011, 2012). This in turn leads to lower environmental stability of biochar, and therefore lower carbon sequestration potential. This is the case even when the biomass conversion during pyrolysis is apparently complete, as reported in Brewer et al. (2009). These authors observed lower stability of fast pyrolysis biochar, assessed based on fixed carbon content and aromaticity, compared to slow pyrolysis and gasification biochar produced from the same feedstock.
Besides stability, reflecting chemical composition, the physical properties of fast pyrolysis biochar also differ from those of slow pyrolysis biochar. Research by Bruun et al. (2012) showed somewhat higher surface area of fast pyrolysis biochar, and its higher pH, which will affect the effect of such biochar on soil. The fine powder nature of fast pyrolysis biochar may also make it more susceptible to microbial attack, due to larger surface:volume ratio (Zimmerman, 2010), resulting in higher carbon turnover. On the other hand, fine biochar particles are also likely to benefit from physical protection against degradation by interactions with minerals or by encapsulation (Brodowski et al., 2006).
In terms of integration with biofuel production, utilization of fast pyrolysis biochar instead of its combustion for process heat has been shown to positively affect the lifecycle GHG emissions of biofuel production (Zaimes et al., 2015), and offer a potentially attractive option for carbon sequestration and transportation fuel production (Brown et al., 2011).
21.2.2. Gasification
Gasification is a thermochemical conversion process in which solid carbonaceous materials are converted into product gas, containing H2, CO, CH4, CO2, etc. This is achieved by reacting the solid fuel with steam, CO2, O2, or air at relatively high temperatures (McKendry, 2002) in an atmosphere with a low concentration of oxygen. The main product of gasification is a flammable gas (product gas), that can be used as a feedstock for production of liquid fuels and/or chemicals, or as fuel for power generation (using combustion engines, gas turbines, or fuel cells). With carbon conversion levels in most large-scale commercial biomass gasification systems in the order of 94–99%, there is only a relatively small yield of coproducts (char/ash and tar). For more detailed discussion of gasification technology for biofuel production see chapters “Production of bio-syngas and bio-hydrogen via gasification,” “Production of bioalcohols via gasification,” and “Production of biofuels via Fischer-Tropsch synthesis: biomass-to-liquids.”
In gasification units with high carbon conversion efficiency the solid residue, ie, gasification ash/char, contains only a small amount of carbon, and is therefore less suitable for use as biochar compared to biochar from pyrolysis, at least from the carbon sequestration point of view (Leiva et al., 2007). One possible exception could be fly-ash from fluidized bed gasification, as its carbon content can be up to 70% (Pels et al., 2005). However, in this case, close attention would need to be paid to contaminant content in this material, as harmful organic and inorganic contaminants can be present in high concentrations.
At the other end of the scale, small and medium-size gasification units, suitable for distributed biomass utilization, do not generally achieve such high carbon conversion efficiencies, and therefore more carbon is retained “lost” in the solid residue. However, instead of this being a disadvantage, it can present an opportunity for valorization of the solid residue, rather than dealing with its disposal. In addition, sequestering of the unconverted carbon in this way can improve the carbon balance of the gasification process in terms of GHG emission (Shackley et al., 2012a).
It has been shown in a number of studies that gasification char can have positive effects on soil and plants (Deal et al., 2012), and could therefore be beneficially used in agricultural, horticultural, and environmental applications. Muter et al. (2014) carried out glasshouse plant tests, using biochar obtained from a large-scale (500 MWth) wood gasifier, mixed with peat-sand substrate. The results showed a significant increase in substrate pH with addition of biochar, and stimulation effect on the biometric indices of the aboveground plant part and roots. Deal et al. (2012) showed that gasification biochar can be a suitable amendment for acidic soils, due to its positive effect on soil pH, as a result of high ash content. In this work improved plant (maize) growth (plant height, stem volume, leaf area, and cut dry weight) was reported when gasification char produced from eucalyptus and maize cobs was added to soil. On the other hand, Rogovska et al. (2012) reported inhibition of corn seedling growth when treated with water extract of high-temperature (750–850°C) gasification chars produced from switchgrass and corn fiber. The presence of inhibiting compounds, such as polycyclic aromatic hydrocarbons (PAHs) in these extracts was suspected as a potential cause of the observed inhibition effect. Besides PAHs, other potential compounds could be responsible for the effect (Buss et al., 2015b; Buss and Mašek, 2014).
One concern with gasification biochar is the potential for a high concentration of organic and inorganic contaminants. The concentration of inorganic contaminants, such as heavy metals, in biochar is mainly dictated by their concentrations in the starting feedstock, although attrition of steel parts (mainly stainless steel) of equipment can also contribute (Buss et al., 2015a). Therefore this parameter can be well controlled by suitable feedstock selection. It is important to note that due to the high burnout of organic matter during gasification, the concentrations of inorganic matter increase dramatically. However, the typically high concentration of metals and minerals in gasification char itself does not present a problem, as long as potential contaminants are presented in stable form, preventing their leaching. In fact, in some cases, high mineral content can be a source of nutrients, as shown for example by Kuligowski et al. (2010), Mozaffari et al. (2002), and Müller-Stöver et al. (2012).
The situation is more complicated when it comes to organic contaminants, as these can be either introduced with the feedstock, or produced during the gasification process. Concerns about the use of gasification char in agricultural, horticultural, or environmental applications include the presence of PAHs (Buss et al., 2015b; Kloss et al., 2012) and tars. The extent to which such organic contaminants are present in the resulting char varies a lot with feedstock and process. PAH concentrations ranging from less than one to above 100 mg/kg (Hansen et al., 2015; Shackley et al., 2012b; Wiedner et al., 2013) were reported. For comparison, guideline values for priority 16 US EPA PAHs content for biochar are 12 mg/kg (4 mg/kg for premium grade) according to European Biochar Certificate guidelines (“‘European Biochar Certificate – Guidelines for a Sustainable Production of Biochar.’ European Biochar Foundation (EBC),” 2015) and 20 mg/kg according to International Biochar Initiative (“IBI. Standardized produce definition and product testing guidelines for biochar that is used in soil,” 2013) guidelines. The reasons for such a large variation in PAH concentrations are the different potential mechanisms of contamination. These include: PAH formation within and outwith biochar particles, PAH release and potential PAH deposition onto biochar particles. Depending on the conditions and environment surrounding biochar particles during their stay in the gasification chamber and in separators, where biochar is separated from syngas, different mechanisms will be enhanced and suppressed. In addition, in some systems, water is used to cool down biochar discharged from the gasification chamber, and this water is often recycled and used in tar scrubbers, thus contaminating biochar (Shackley et al., 2012b). Therefore, if gasification biochar is to be used for soil application, the production system needs to be set up to minimize the risk of contamination.
21.2.3. Hydrothermal carbonization
Hydrothermal carbonization (HTC) is a thermal treatment process that can achieve a very high conversion of biomass carbon into solid carbonaceous residue. During the HTC process, organic substances are treated in an aqueous environment at temperatures between 150 and 350°C, under autogenous pressure, to form condensed carbonaceous solid product, also called hydrochar or HTC solid, and a liquid stream rich in organic compounds. Laboratory tests have shown that carbon retention may be as high as 80% in the solid residues (Sevilla and Fuertes, 2009), which gives the process excellent emission characteristics. The HTC process releases about one-third of the energy content of the feedstock (Titirici et al., 2007), with the rest remaining in the solid product. As no external source of energy and no feedstock dewatering and drying is required, this energy can be used, for example, for generating steam and power, for heating the process and for drying the produced hydrochar. These characteristics and the fact that wet starting material is not only acceptable, but required, make the HTC process potentially very suitable for processing of various organic residues with high moisture content.
Besides converting wet biomass into hydrochar, the HTC process is also capable of coproducing chemicals, which include phenolic compounds, 2,5-HMF, and aldehydes (acetic, lactic, propenoic, levulinic, and formic acids) that can potentially be used in biorefineries (Axelsson et al., 2012; Hoekman et al., 2012; Oladeji et al., 2015). The formation and concentration of these chemicals can be controlled by adjusting the HTC process conditions (Libra et al., 2011; Xiao et al., 2012), such as temperature, pressure, and residence time. Therefore the HTC process could be a useful part of biological and thermochemical biorefineries, processing wet residues, and coproducing chemicals and hydrochar.
It has been reported that unlike biochar, with highly aromatized carbon structure, hydrochar is mostly composed of aliphatic hydrocarbons (Fuertes et al., 2010; Schimmelpfennig and Glaser, 2011; Wiedner et al., 2013). This means that the environmental stability of hydrochar is considerably lower (Abel et al., 2013; Sun et al., 2014) than that of biochar with a higher degree of carbonization and higher aromaticity. The high content of labile/easily degradable carbon results in different effects of application of hydrochar to soil, compared to biochar. For example, hydrochar greatly affects composition of soil microorganisms, and causes a dramatic bacterial and archaeal community shift (Andert and Mumme, 2015). Besides this, the presence of phenolic and organic acid compounds on hydrochar surface, which cause negative plant and microbial response, makes effective direct use of hydrochar in environmental and agricultural applications complicated (Bargmann et al., 2013; Titirici et al., 2012), although this does not have to always be the case (Wiedner et al., 2013). However, it appears that these unfavorable properties, from the perspective of soil application, can be ameliorated by post-treatment of hydrochar. Biological post-treatment, such as composting of hydrochar with other organic materials can reduce its toxicity (Busch et al., 2013). Other biological treatments, such as anaerobic digestion could also reduce hydrochar toxicity and make it suitable for soil application. Another alternative to these biological post-treatment routes is thermal post-treatment, such as pyrolysis. The advantage of pyrolysis of hydrochar is not only the fact that it removes toxic compounds, making hydrochar suitable for soil application, but also the fact that it stabilizes it, due to development of aromatic carbon structures (Zhu et al., 2015). These changes during thermal post-treatment also lead to structural changes of hydrochar, developing its porosity and increasing ash content, thus further increasing its suitability for soil application.
21.3. Biochar from biorefinery residues
Besides production of biochar as an intended or unintended coproduct during conversion of biomass to biofuels and chemicals in thermochemical biorefineries, biochemical biorefineries also provide opportunities for production of biochar as a standalone process, valorizing residues resulting from biological treatments. Various residues can be left behind after enzymatic, microbial, and other treatments in biorefineries, mainly constituting recalcitrant fractions high in lignin and mineral matter. Both of these streams can be used in biochar production, as discussed below.
For dedicated production of biochar from biorefinery residues, such as lignin, slow pyrolysis provides a viable choice, due to its flexibility in terms of feedstock, its technical maturity, and level of control. Slow pyrolysis is in many aspects the opposite of fast pyrolysis. It is characterized by slow heating and relatively long residence times of biomass in the reactor, in the order of tens of minutes or even several hours. Due to the relatively slow heating, the feedstock particle size requirements are not as tight as for fast pyrolysis, and depending on the technology used feedstock in form ranging from small chips to cord wood can be successfully used. As a result of these markedly different production conditions in slow pyrolysis, the product distribution and properties are very different from those in fast pyrolysis (see Table 16.1). The yield of biochar in slow pyrolysis is considerably higher and subsequently the yields of liquid and gaseous products are lower.
Due to its high biochar yield, slow pyrolysis is a very suitable technology for biochar production. Unlike other more recent biomass conversion technologies, eg, fast pyrolysis, hydrothermal carbonization (HTC), and to some extent gasification, slow pyrolysis has been used extensively for production of charcoal and chemicals for thousands of years (Domac et al., 2008) and could therefore be considered well understood. Although this may be true for charcoal production processes, there are many uncertainties and unknowns when it comes to production of specified/bespoke biochar. The key challenges are related to production of biochar with desirable properties and stability, as an integral part of systems for coproduction of biochar, energy, and/or chemicals. Historically, charcoal production was focused on converting a relatively limited range of feedstock, mostly woody biomass, to fuel-grade charcoal, which did not require a high degree of specification, ie, relatively simple quality control could be used. On the other hand, to make biochar a useful material for agriculture or for other specific purposes such as cleaning-up waste water requires detailed understanding of the relationship between feedstock properties, conversion process parameters, and the end use of the resulting biochar. Perhaps the only comparably demanding historic application for charred biomass was in the production of gun powder, where specific properties of char were required, to ensure good product quality, however this was typically achieved by use of a limited range of carefully selected feedstock. To some extent, activated carbon industry may also be a useful analogue. Therefore, future research and development needs to focus on production of biochar with properties specifically suited for its application, whether carbon storage, soil amendment, etc., in a way that is environmentally sustainable and economically viable.
Two examples of modern slow pyrolysis systems are the rotary drum and screw pyrolyzer. Rotary drum pyrolyzers move feedstock through an externally heated horizontal (or slightly inclined) cylindrical reactor by rotation of the reactor or by action of paddles moving inside a stationary cylindrical shell. Both of these technologies can achieve long residence times in the order of tens of minutes and good control of product quality and distribution (Klose and Wiest, 1999). In addition, rotary drum pyrolyzers can operate at a wide range of scales up to tens of thousands tons per year capacity. Screw pyrolyzers use a rotating auger to move material through a tubular reactor that can be heated either externally or internally by introduction of heat carrier material such as sand or metal balls. Screw pyrolyzers also offer good control of the pyrolysis process and are well suited for small to medium-scale applications, but unlike rotary drum pyrolyzers are not suitable for large-scale applications.
In both cases, ie, the rotary drum and screw pyrolyzer, the heat needed to drive the conversion process is most often provided by combustion of the gaseous and liquid byproducts of the pyrolysis process. The heat thus released can then be directed to different parts of the pyrolyzer to achieve the desired operating conditions. Any excess heat can be used in external applications (eg, heating, feedstock drying). In some cases, part of the gaseous and liquid stream can be used for combined heat and power (CHP) generation, where the gases are burned in a combustion engine driving a generator and the heat contained in the flue gases is used for heating applications. On the other hand, in certain cases, the heat released by combustion of byproducts is not sufficient to drive the pyrolysis process (particularly where feedstock with high moisture content is used) and additional fuel needs to be provided.
Due to the wide range of processing conditions (temperature and residence time) available and feedstock options (woody biomass, agricultural biomass, and diverse organic residues) that can be processed in slow pyrolysis units, the yield and properties of biochar can vary widely. This provides an opportunity to optimize the production to yield biochar with properties matching its application. In their research Ronsse et al. (2013) and Zhao et al. (2013) showed that certain biochar properties are primarily affected by processing conditions (eg, surface area, pH, carbon sequestration potential), while others are mainly feedstock-dependent (eg, content of total organic carbon, ash, and fixed carbon). Based on such information and knowledge, it is possible to design biochar (Crombie et al., 2014; Novak et al., 2009) to address specific requirements, whether it is soil improvement, carbon sequestration, or environmental functions. A thorough review of different biochars and their uses is beyond the scope of this chapter, more information can be found in the relevant reviews (Qian et al., 2015; Spokas et al., 2012; Verheijen et al., 2009; Xu et al., 2012).
In addition to biochar as the main product (in terms of yield), the slow pyrolysis process also yields gases and liquids. The pyrolysis gases are of relatively low heating value (2–12 MJ/kg), and generally contain less than one-quarter of the energy contained in all coproducts (Crombie and Mašek, 2015). Pyrolysis gases therefore have a low value for external applications, but can be used as a source of heat for the pyrolysis process itself or potentially for feedstock drying. In terms of energy content, pyrolysis liquids, without water separation, have also a low heating value (HHV of 5–9 MJ/kg), and contain approximately 20% of energy in the coproducts (Crombie and Mašek, 2015). The heating value is low mainly due to the high water content, which also causes the liquids to separate into several phases. This issue can be at least partially overcome by fractional condensation of the pyrolysis vapors, to obtain water and organic fractions separately (Tumbalam Gooty et al., 2014; Westerhof et al., 2011).
Although the energetic content of pyrolysis liquids is relatively low, due to the low calorific value (CV) and low yield of water-free fraction, the composition makes it a potentially interesting source of chemicals in the biorefinery context. The type and concentration of chemicals that can be obtained from pyrolysis liquid depends mainly on the feedstock composition. A number of key value-added chemicals have been identified in recent studies (Bozell et al., 2007; Werpy and Petersen, 2004). For example, furfural and levoglucosan can be obtained from hemicellulose and cellulose fractions of biomass, while BTX chemicals and phenols can be obtained from lignin. One obstacle in the commercialization of production of these chemicals from pyrolysis liquids has been their complex nature, and also the relatively low concentration of most of these chemicals, typically below 1%. To overcome this, the so-called staged pyrolysis (De Wild et al., 2009) in combination with fractional condensation (Tumbalam Gooty et al., 2014; Westerhof et al., 2011) can be used to increase the content of these chemicals in resulting liquids, and make their recovery more profitable.
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