Sorption of Plant Nutrients and Other Pollutants

The potential benefits of biochar to improving soil health through nutrient addition, and consequent im­provements in fertilizer use efficiency have been well recognized through glasshouse and field trials (Sohi et al., 2010; Verheijen et al., 2010). These studies have shown that biochar and biochar-amended soil help retain plant nutrients and this is one of the means by which biochar application to soils is known to improve soil quality and increase crop yields in some cases. How­ever, it is important to recognize that if the nutrients are retained by biochar particles in soil to a degree that plants are unable to take up the nutrients, this could impact productivity (Kookana et al., 2011).

Studies have shown that retention of nitrogen (N) (Hollister et al., 2013; Liu et al., 2013), as well as phos­phorus (P) and potassium (K) (Schnell et al., 2012), is achievable through incorporation of biochar, demon­strating potential agronomic and wider environmental benefits. Within the context of an agroecosystem, plant nutrients are essential and often the limiting factor

determining the extent of crop growth. However, once N or P leave an agroecosystem, either through overland flow or leaching, they could potentially pose a threat to surface waters, with P being the limiting nutrient govern­ing eutrophication of fresh water and N being the limiting nutrient governing the eutrophication of estuarine and ocean systems (Brady and Weil, 2008). Similar to gaseous C emissions, aqueous N and P losses from agricultural soil have global effects. While the majority of biochar research focuses on short-term impacts of its application, more long-term field research focused on net C sequestra­tion, net GHG emissions, microbial community dy­namics, nutrient use efficiency, and water use efficiency is needed (Ippolito et al., 2012). Furthermore, an increased fundamental understanding of the mechanisms underly­ing the interactions between biochar and soil in order to optimize agricultural systems and protect the environ­ment should be a further focus (Spokas et al., 2012b).

While applied plant nutrients outside of an agricul­tural context represent a threat to the environment that biochar has demonstrated potential to address, it is possible to retain other pollutants (e. g. heavy metals and pesticides) using biochar, as well. When biochar is applied to cadmium (Cd)-, copper (Cu)-, and lead (Pb)-contaminated soils, these metals have been observed to become immobilized, decreasing phytotox­icity and bioavailability, and vastly improved crop pro­duction (Park et al., 2011). Other studies have shown biochar to exhibit strong sorption and degradation
inhibition of pesticide residues, leading to potential con­cerns regarding long-term accumulation in biochar amended soils treated with pesticides (Kookana, 2010). Some biochars have also been shown to retain estrogenic steroid hormones on dairy farm soils (Sarmah et al., 2010). While the potential for soil organic and inorganic contaminants (e. g. metal remediation, pesticide accumu­lation, and hormone retention) remediation are valid, it is important to consider that the enormous heterogene­ity of biochars with respect to their chemical qualities and resultant effects on soil pollutants remain largely uninvestigated (Kookana et al., 2011). Research on potential agronomic and environmental applications of biochar is currently in its infancy and it is through the establishment and monitoring of additional long-term field trials that its full potential could be realized (Sarmah, 2009).


Hydrolysis is generally defined as the depolymeriza­tion of a substance via hydration. An aqueous acid’s ions act to cleave long polymers like cellulose, hemicellulose and lignin into smaller chains. Pretreating LB to undergo hydrolysis or converting polysaccharides into monosac­charides will enhance later fermentation by improving
the ability of anaerobic organisms to digest the resultant, simpler sugars. Hydrolysis requires extended residence time. Unfortunately, monosaccharides degrade into other nonsugar molecules when subjected for extended times to relatively high temperatures and acid condi­tions (Hsu, 1996; Wyman et al., 2005). The hydrolysis reaction rate accelerates when either a chemical or an enzymatic catalyst is used and when the material to by hydrolyzed is concentrated.

Enzyme hydrolysis is highly specific and relatively fast. Using an enzyme to act on its target polysaccharide will convert it rapidly into its component monomers. Additionally, this will convert the insoluble polysaccha­ride into a soluble monomer. Enzymatic hydrolysis is best applied after other pretreatment methods that leave cellulose as a major component. The most common method of saccharification is enzymatic hydrolysis following acid hydrolysis (Harun, 2010).

The enzyme cellulase converts cellulose into glucose. Cellulases are so specific that they only affect cellulose and do not treat hemicelluloses in the LB (Wang et al.,

2012) . There are five general types of cellulases. They are classified by the reactions they catalyze. These five cellulases are endocellulases, exocellulases, cellobiases, oxidative cellulases and cellulose phosphorylases (Bayer et al., 1998). Table 27.6 summarizes the effectiveness on the hydrolysis of wheat straw of Cellulase, alpha — Glucosidase and Xylananse from T. reesei, A. niger, and T. longibrochiatum after various pre-treatments. The high yields and mild conditions are attractive for commercial applications.

These enzyme structures are complex and can be found in various bacteria as organized supramolecular complexes called cellulosomes (Bayer et al., 1998). These enzymes are commonly found in fungi such as Tricho — derma reesei and Aspergillus niger and in bacteria such as Clostridium cellulovorans (Arai et al., 2006). These source organisms are either aerobic or anaerobic and

are either mesophilic or thermophilic. Commercial pro­duction of cellulase is focused on fungal sources because bacterial sources tend to be anaerobic and thus are slow to grow (Duff and Murray, 1996).

It appears that at least three classes of enzymes act together, synergistically, to hydrolyze cellulose: endocel — lulase, exocellulase, and cellobiase. Endocellulase (EC randomly breaks internal (b-D-1,4) bonds at amorphous sites that create new chain ends. Exocellu- lase (EC cleaves two to four units from the ends of the exposed chains produced by the endocellu — lase and results in tetrasaccharides or disaccharides. Lastly, the cellobiase (EC, otherwise known as b-glucosidase, hydrolyzes the exocellulase products into individual monosaccharides (Coughlan and Ljung — dahl, 1988; Galbe and Zacchi, 2002; Rabinovich et al., 2002; Zhang et al., 2006).

The cellulase action occurs in three steps. The first is adsorption of cellulase onto the surface of the cellulose. The second is biodegradation of cellulose into ferment­able sugars. Lastly, desorption of cellulase occurs completing the catalytic cycle.

Enzyme activity is affected by a variety of environ­mental and substantive conditions. Temperature and pH are known to affect enzyme activity. Most cellulose enzymes show an optimum activity at temperatures in the range of 45—55 °C and at pH values between 4 and 5 (Galbe and Zacchi, 2002). For LB applications, the optimum pH is shifted upward to between 5 and 6.5 due to the presence of lignin in the system (Lucas et al., 2012). These are mild operational conditions. These mild conditions lower the overall operational costs compared to purely chemical hydrolysis methods.

Additionally, substrate concentration, product con­centration, activators, inhibitors and cellulose structure are also significant determiners of enzyme effectiveness (Detroy and Julian, 1982).

Cellobiase is itself an inhibitor to endo — and exocellu — lases. Thus, the b-glucosidase activity is crucial for the efficiency of the hydrolysis process (Coughlan and Ljungdahl, 1988; Galbe and Zacchi, 2002; Rabinovich et al., 2002; Zhang et al., 2006).

The structure of cellulose affects the rate of hydroly­sis. The cellulose features known to affect the rate of hy­drolysis include (1) molecular structure of cellulose, (2) crystallinity of cellulose, (3) surface area of cellulose fiber, (4) degree of swelling of cellulose fiber, (5) DP, and (6) associated lignin or other materials (Det­roy and Julian, 1982). The purer and more refined the cellulose is, the more ideal the cellulase activity will be. Higher enzyme activity lowers the enzyme load and cost for the enzymatic hydrolysis process.

Lastly, even under ideal conditions, the activity of the cellulase enzyme is affected by the age of the enzyme it­self. The overall activity of the enzyme decreases rapidly and slows the rate of enzymatic hydrolysis. There is currently much research devoted to improving the over­all yield and maintaining a high rate of hydrolysis (Sun and Cheng, 2002).

Supplementing cellulase enzymes with other en­zymes is another area of current focus. Conjugating the action of cellulases and hemicellulases is known to increase the rate of enzymatic hydrolysis and result in an overall higher sugar yield. Cellulose is a homopoly­saccharide, hemicelluloses are heteropolysaccharides. To obtain a more complete hydrolysis of LB one must consider a multiple-enzyme system and reap the yield of the combined activities.

Can Biochar Be a Cost-effective Fertilizer Substitute?

An analysis by McHenry (2012a, b) quantified the po­tential of using biochar as a soil amendment to displace annual applications of single superphosphate (SSP)

(0% N, 8.8% P, 0% K, 11% S) in wheat cropping systems in WA. The analysis assumed two biochar applications over a 15-year period, applied in year zero, and year eight. The analysis ignored all production inputs and outputs, and only calculated the difference between us­ing an average "full rate" of SSP (90 kg/ha), and a "half rate" of SSP with deep banded biochar equivalent to 1 t/ha. The 45 kg/ha year half-rate SSP application is approximately equivalent to an annual application of 4 kg of P/ha. The simplified analysis assumed that the use of either method would achieve an identical wheat yield, negating the requirement to model wheat prices. The application cost of deep banding the biochar (tons per hectare per application) was assumed to be $110. The annual application costs of both the rates of SSP were assumed to be $20/ha, goods and services tax (GST)1 inclusive. A range of biochar prices (deliv­ered to farm, per ton) was analyzed: $0, $50, $100, $150, $200, $250, $300, $350, $400, and $450/t. Similarly, a range of SSP costs (delivered to the farm, per ton) were calculated: $250, $300, $350, $1250. A carbon price was included in the analysis, and was analyzed at intervals of $5 tCO2-e, between $0 and $100 tCO2-e. The analysis assumed a 0.8 carbon fraction recalcitrance. A real dis­count rate of 8% p. a. was used, and all capital and main­tenance costs were based on average current prices and were GST inclusive. In summary, the results showed that without a carbon value the "half rate" of SSP (45 kg/ ha year) and biochar (1 t/ha application) were only cost competitive with the full rate of SSP (90 kg/ha year) when the biochar purchase price was unreasonably low (< ~ $20). At 2012 prices of SSP (generally between $200 and $450/t), the choice of using half SSP application rates with biochar additions at the above application rate assumptions were not an attractive option unless

the biochar purchase price was practically zero. The net cost was also calculated assuming the carbon in the biochar was eligible in soil carbon markets, and the various potential prices of carbon were subtracted from the gross biochar purchase price. While the intro­duction of a carbon price would effectively subsidize biochar costs, very high carbon prices (>$100/t) were required for the sequestration value of biochar to simply equal the purchase price and cover costs of soil application (McHenry, 2012a). The low SSP price, the high market prices for biochar, and the high bio­char soil application cost of deep banding relative to conventional broadcasting, all resulted in the option of halving SSP applications by using biochar an unat­tractive practice.

Organosolv Pretreatment

Take an organic or aqueous organic solvent such as formic acid, acetic acid, methanol, ethanol, acetone, ethylene glycol, oxalic acid, triethylene glycol or tetrahy — drofurfuryl alcohol and combine it with an inorganic acid catalyst such as hydrochloric acid or sulfuric acid and one can eliminate the internal lignin and hemicellu — lose bonds. This is known as an organosolv process (Pan et al., 2006; Sarkanen, 1980; Thring et al., 1990). Alterna­tively, an organic acid, such as oxalic acid, acetylsalicylic acid and salicylic acid may be substituted for the inor­ganic catalyst.

It has been observed that approximately 72% of xylose in untreated wood, in both its monomeric and oligomeric forms, could be recovered using an organo — solv pretreatment process (Pan et al., 2006). Pan et al. (2006) also investigated a bioconversion of hybrid pop­lar to ethanol at 180 °C, for 60 min, with 1.25% H2SO4, and 60% ethanol. They observed that nearly 74% of the lignin was removed as a precipitate in the ethanol extraction.

Raising the operational temperature above 185 °C eliminates the need for a catalyst, either for an inorganic acid or for an organic acid. At this condition, the amount of delignification is quite satisfactory. Adding acid yields a high quantity of xylose.

Since the organic solvents inhibit downstream biolog­ical processes, such as organism growth and enzymatic hydrolysis, it is necessary to remove these solvents from the system. This is quite difficult as some quantity of solvent is likely to reside in the system even after ef­forts to remove them. Organic solvents tend to evapo­rate into the atmosphere and are hazardous to the environment and one’s health. Containing the solvent is another challenge. Given these challenges, an organo — solv pretreatment is not necessarily ideal for large-scale or commercial operations.

Soil Greenhouse Gas Emissions

In addition to the potential of biochar to partially offset anthropogenic C emissions through C sequestra­tion, biochar has been observed to inhibit the release of GHGs from soil, thereby reducing net emissions of GHGs as a side effect of C sequestration. Decreased car­bon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions from soils have been observed following biochar applications (Spokas et al., 2010), although increased emissions of N2O have been

FIGURE 25.3 (a) Biochar enhancement using clay. Adapted from Joseph et al., 2013. (b) Direct and indirect application of biochar to soil.

observed in some cases (Clough et al., 2010). N2O, a GHG with a global warming potential hundreds of times greater than CO2, emissions have been shown to be reduced by a variety of biochars, yet the mechanisms responsible vary depending on soil moisture (Saarnio et al., 2013) and N content and forms (Kammann et al., 2012; van Zwieten et al., 2010). Additionally, bio­char has been demonstrated to specifically reduce earthworm-derived CO2 and N2O emissions (Augusten — borg et al., 2012). While there is an abundance of short­term field and laboratory derived data to indicate the N2O emissions reduction potential of biochar, there is a dearth of information on the long-term field studies on this topic (Ussiri and Lal, 2013).

Biochar is becoming increasingly broad in its contex­tual definition. At least one study has been conducted to evaluate the potential of biochar to reduce landfill — derived CH4 emissions. A recent study reported that when landfill cover soil was mixed with 20% biochar, nearly 200% more CH4 was adsorbed compared to con­trol soil, and 100% biochar was found to adsorb over 10-fold more CH4 than control soil (Yaghoubi, 2011). Bio­char has also been shown to effectively decrease cattle enteric CH4 emissions while increasing animal growth, by 22% and 25%, respectively, when biochar was mixed with animal feed (Leng et al., 2012).


Following hydrolysis, converting the resultant sugars to products is the next step. Fermentation is a bio­logical option and is the focus of this section. Both chemical/catalytic and biochemical conversions are common.

At this point, the pretreatment and hydrolysis activ­ities were designed and executed all with the intent of optimizing and preparing for fermentation, the capstone process of the bioconversion (Gamage et al., 2010). Fermentation is referred to as anaerobic digestion. Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms to produce ethanol or other alcohols, lactic acid, lactose, and hydrogen (Chandel et al., 2007; Wheals et al., 1999).

One of the most significant factors in fermentation is the choice of organism or modification to an organism to acquire a desired product. Some organisms only metab­olize hexoses while others may metabolize both hexoses and pentoses. Saccharomyces cerevisiae is an old and very popular strain of yeast used throughout the food and fuel industries. When added to a batch of material, it will metabolize the glucose component, almost exclu­sively, into ethanol and carbon dioxide. It will generally follow the Embden-Myerhof pathway under anaerobic conditions when the temperature is controlled around 30 °C (Limayem and Ricke, 2012). S. cerevisiae grows optimally at this temperature and it also resists high os­motic pressure and it is tolerant of pH as low as 4.0 and it is tolerant of many inhibitory products (Hahn-Hagerdal et al., 2007). S. cerevisiae remains popular because of its high ethanol yield from hexose sugars; it generates 12.0—17.0% w/v, which is 90% of the theoretical maximum (Bayrock and Ingledew, 2001; Claassen et al., 1999).

Despite all its great characteristics, S. cerevisiae cannot metabolize both hexoses and pentoses and thus it is not a great organism for converting LB. In

LB, there is a significant portion of the hydrolysate containing hemicelluloses, pentose sugars such as D-xylose, which may potentially enhance yields (Martin et al., 2002). Identifying and employing an optimal organism is a great opportunity in fermenta­tion. The optimal organism ought to be high yielding, able to metabolize both hexose and pentose sugars, tolerant to high ethanol concentration and tolerant to chemical inhibitors left over from pretreatment and hydrolysis. There are numerous naturally occurring organisms that possess a subset of these characteristics, but none are ideal. To develop a more advantageous organism one might have to genetically modify an organism to achieve one’s goals. Table 27.7 lists several naturally occurring organisms and their features and liabilities (Limayem and Ricke, 2012).

Reducing operating costs and product inhibition is another important goal. There are strategies that combine hydrolysis and fermentation together. Simul­taneous saccharification and fermentation (SSF) is one strategy that has just that in mind. The needed en — zyme(s) and the corresponding organisms are added together so that enzymatic saccharification of cellulose and subsequent fermentation of the resultant sugars takes place at the same time in the same reactor (Dowe and McMillan, 2008). However, SSF requires an overall compromise between saccharification and fermentation, usually resulting in a less optimum oper­ation. Another strategy is to employ an organism that is capable of making its own enzymes for hydrolysis and of fermenting the resultant sugars. Consolidated bioprocessing lowers the cost of bioconversion by reducing enzymatic saccharification and fermentation into a single step and eliminates the need for cellulase enzymes (Ladisch et al., 2010; Lynd et al., 2005).

Despite the number of prokaryotic and eukaryotic microorganisms that convert sugars to ethanol, most remain limited in terms of cofermentation, ethanol yields, and tolerance to chemical inhibitors, high temperatures and ethanol.

Can Biochar Be a Cost-Effective Approach to Increase Grain Crop Primary Productivity?

For comparison, a further analysis by McHenry (2012a, b) was undertaken of the value of applying bio­char at 1 t/ha with the full rate of SSP as described above. This analysis was undertaken to explore the rela­tive impact of using biochar to increase yield, as opposed to increasing fertilizer use efficiency. The anal­ysis assumed that using the full rate of SSP (90 kg/ha year) with a 1 t/ha year application of biochar increased wheat yields by 15% on average over the 15 years rela­tive to full SSP applications only, in the southwest of WA2. The baseline yield used for the scenario was 1.75 t/ha, an approximate average wheat yield for WA. The assumptions of the model, including a total area wheat return increase of $71.75/ha, were based on an increased production of an additional 15% wheat yield from the 1.75 t/ha at a constant value of $350/t over

the 15 years using the 8% real discount rate. The scenario did not include additional harvesting or transport costs for the additional wheat yield. The results indicated that the required carbon prices to recoup biochar purchase price costs were lower when biochar is used to increase yield, rather than reduce fertilizer use. When biochar purchase prices were below $250/t, the application of biochar was attractive without any carbon price, assuming the 15% yield is achieved (McHenry, 2012a). Therefore, these relatively simple analyses suggest that the most cost-effective on-farm use for biochar is to simply increase the wheat yield. The results confirm pre­vious assertions that agricultural biomass production for the sole purpose of producing biochar for soil carbon sequestration may not be economically feasible (Lehmann et al., 2006).

Oxidation Pretreatment

Oxidation is a pretreatment option whereby an oxi­dizing agent, such as hydrogen peroxide or peracetic acid, is applied to LB. The result is the removal of hemi — cellulose and lignin and thus, an increased accessibility to cellulose to enzymatic hydrolysis. This result is the culmination of several reactions: electrophilic substitu­tion, displacement of side chains, cleavage of alkyl aryl ether linkages, or the oxidative cleavage of aromatic nuclei (Hon and Shiraishi, 2001). Often the oxidative agent is not selective and a significant loss of hemicellu — lose and cellulose may occur. Additionally, there is a high risk of forming downstream inhibitors as soluble aro­matic compounds are formed while the lignin oxidizes.

When using hydrogen peroxide as the oxidative agent on sugarcane bagasse, the rate of enzymatic hy­drolysis improves. In one study, hemicelluloses and approximately 50% lignin were solubilized by 2% hydrogen peroxide at 30 ° C over 8 h. This was followed by enzymatic hydrolysis, or saccharification, using cellulase at 45 °C within 24 h. The result was 95% effi­ciency in glucose production (Azzam, 1989).

In another study, peracetic acid was applied at ambient temperatures to a hybrid poplar and sugarcane bagasse mixture (Teixeira et al., 2000). It was determined that peracetic acid was very selective for lignin and in some cases, no significant carbohydrate was lost. When peracetic acid was applied at 21%, the enzymatic hydrolysis of cellulose increased from 6.8% for un­treated biomass to 98% in the peracetic acid-pretreated biomass.

Soil-Specific Biochar Design

Published data on biochar and its interactions with soil are increasingly detailed in relation to feedstocks and production conditions (Sohi et al., 2010). As this body of literature grows, it will enable biochar pro­ducers to more predictably custom design biochars to
alleviate the organic matter constraints of a given soil, using a procedure similar in effect to those used by chemical fertilizer companies to determine the macro — and micronutrient requirements of a given soil (Joseph et al., 2013). While some researchers aim to produce improved biochars by simply blending multiple bio­chars together (Novak and Busscher, 2013), truly novel biochars are being produced by pyrolyzing an organic feedstock mixed with clay and other minerals, in order to produce a biochar mineral complex (BMC), which may represent the current forefront of custom biochar design. This BMC production process (Figure 25.3(a)) enables the production and customization of biochars through blending of materials before pyrolysis, followed by subsequent blending of materials with desirable chemical qualities, which are then subjected to torrefac — tion treatments. This can facilitate the loading of the bio­char surfaces with additional plant-available nutrients and enhanced CEC, representing a clear progression from biochar as a soil conditioner toward biochar as an organic fertilizer (Lin et al., 2012a, b).


The process of converting LB to products using pri­marily heat as the engine of conversion is thermochem­ical conversion. Thermochemical processing appears more promising than bioconversion of the lignin frac­tion of the LB in that it serves as a source of process energy and the coproducts have benefits in a life-cycle context; however, it has a detrimental effect on enzy­matic hydrolysis (Lynd et al., 1999, 2005; Lynd and Wang, 2004). This method differentiates on how much air is supplied to the conversion, as shown in Figure 27.7. If LB is heated in the presence of excessive amounts of air, specifically oxygen, then the biomass will combust. If the amount of air or elements of air is limited then gasification will occur. Lastly, if no air is allowed then pyrolysis or hydrothermal liquefaction is the outcome.


Combustion is a result of a complicated network of exothermic chemical reactions. The reaction generates copious amounts of heat and radiation. The reaction tends to be self-perpetuating and continues spontane­ously due to the large amount of heat generated by the reaction. Specifically, combustion is when carbon, hydrogen, combustible sulfur, and nitrogen in LB react with oxygen. The process includes fusion, evaporation, pyrolysis, a gas phase, and surface reactions.

Combustion of solids can take place in many forms including evaporation combustion, decomposition com­bustion, surface combustion, and smoldering combus­tion. Evaporation combustion is when fuel, with a relatively low fusing point, fuses and evaporates by heating, and reacts with gaseous oxygen and burns. When gasses such as H2, CO, CmHn, H2O, and CO2 are produced by thermal decomposition and react with O2, it is called decomposition combustion. A common by-product of evaporation and decomposition combus­tion is char. Char burns by surface combustion. Surface combustion occurs when the material only contains car­bon and small amounts of volatile compounds such as charcoal, oxygen, carbon dioxide, or steam. When these compounds diffuse into pores inside or on the surface of the solid they burn in a reaction of the surface of the ma­terial. Lastly, smoldering combustion is a slower and lower temperature reaction. It is a form of thermal decomposition that takes place at a temperature below the ignition temperature of the volatile components of the LB. If the temperature is forced up, smoke might be produced or the reaction may ignite. If it ignites the reaction is referred to as flammable combustion. Indus­trial LB conversion processes commonly employ decom­position or surface combustion.


Gasification is the conversion of LB from its solid form to fuel gas or syngas. Syngas is simply a chemical feedstock in its gas phase. These might be CO, H2, CH4, CO2 and N2 as well as char (Balat, 2008b; Demirbas, 2004; Li and Suzuki, 2009).

Gasification is a broad treatment method and pro­duces a variety of different results depending on how it is controlled. Manipulating pressure, temperature, heating method, and conversion agent produces specific results. Pressure is usually controlled for either normal pressure (0.1—0.12 MPa) or high-pressure conditions






Saccharomyces cerevisiae

Facultative anaerobic yeast

Naturally adapted to ethanol

Not able to ferment xylose

(Gamage et al., 2010; Hahn-


and arabinose sugars

Hagerdal et al., 2007; Jorgensen,

High alcohol yield (90%)

Not able to survive high

2009; McMillan, 1994; Rogers

High tolerance to ethanol (up

temperature of enzyme

et al., 2007; Talebnia et al., 2010)

to 10% v/v) and chemical inhibitors


Amenability to genetic modifications

Candida shehatae

Microaerophilic yeast

Ferment xylose

Low tolerance to ethanol

(Banerjee et al., 2009; Ligthelm

Low yield of ethanol

et al., 1988; McMillan, 1994; Sun

Require microaerophilic conditions

et al., 2011; Zaldivar et al., 2001)

Does not ferment xylose at low pH

Zymomonas mobilis

Ethanologenic gram-negative

Ethanol yield surpasses

Not able to ferment xylose

(Balat and Balat, 2008; Herrero,


S. cerevisiae (97% of the


1983; Liu et al., 2010; McMillan,


Low tolerance to inhibitors


High ethanol tolerance (up to 14% v/v)

Neutral pH range

High ethanol productivity (fivefold more than S. cerevisiae volumetric productivity)

Amenability to genetic modification

Does not require additional oxygen

Pichia stipites

Facultative anaerobic yeast

Best performance xylose

Intolerant to a high

(Jeffries et al., 2007; McMillan,


concentration of ethanol

1994; Nigam, 2001; Shupe and

Ethanol yield (82%)

above 40 g/L

Liu, 2012; Zaldivar et al., 2001)








Able to ferment most of

Does not ferment xylose

cellulosic-material sugars

at low pH

including glucose, galactose,

Sensitive to chemical inhibitors

and cellobiose

Requires microaerophilic

Possess cellulase enzymes

conditions to reach peak

favorable to SSF process


Reassimilates formed ethanol

Pachysolen tannophilus

Aerobic fungus

Ferment xylose

Low yield of ethanol

(Zaldivar et al., 2001; Zayed and

Require microaerophilic conditions

Does not ferment xylose at low pH

Meyer, 1996)

Escherichia coli

Mesophilic gram-negative

Ability to use both pentose and

Repression catabolism

(Gamage et al., 2010; Liu et al.,


hexose sugars

interfere to cofermentation

2010; Weber and Boles, 2010;

Amenability for genetic

Limited ethanol tolerance

Zayed and Meyer, 1996)


Narrow pH and temperature growth range

Production of organic acids Genetic stability not proved yet

Low tolerance to inhibitors

and ethanol

Kluveromyces marxianus

Thermophilic yeast

Able to grow at a high

Excess of sugars affect its

(Banat et al., 1992; Kumar et al.,

temperature above 52 ° C

alcohol yield

2009b; Weber et al., 2010)

Suitable for SSF/CBP process

Low ethanol tolerance

Reduces cooling cost

Fermentation of xylose is poor

Reduces contamination

and leads mainly to the

Ferments a broad spectrum of sugars

formation of xylitol

Amenability to genetic modifications

Thermophilic bacteria

Extreme anaerobic bacteria

Resistance to an extremely

Low tolerance to ethanol

(Georgieva et al., 2008; Kumar

• Thermoanaerobacterium

high temperature of 70 °C

et al., 2009b; Lynd et al., 2002;


Suitable for SSCombF/CBP

Shaw et al., 2008; Zeikus et al.,

• Thermoanaerobacter ethanolicus



• Clostridium thermocellum

Ferment a variety of sugars

Display cellulolytic activity

Amenability to genetic modification

CBP, consolidated bioprocessing.


(0.5—2.5 MPa). Temperatures are usually either operated under low-temperature (<700 °C) or high-temperature (>700 °C) conditions. High-temperature decomposition may occur at the ash fusion point or above. Indirect gasification occurs when heating the raw material and gasification agent using an external heat source. Direct gasification occurs when heat generated from the reaction of partial gasification of raw material and oxygen is used as the heating source. The gasification agent is another variable with significant influence on the product. An agent is any combination of air, oxygen, or steam. Additionally, carbon dioxide maybe used for a set period of time to affect the product.

There are a variety of gasifiers in use today. Fixed bed, flow bed, circulating flow bed, entrained bed, mixing bed, rotary kiln, twin tower, and molten furnace are examples of industrial gasifiers (Yokoyama, 2008).

Another method, supercritical water gasification (SCWG), is interesting because water under supercritical conditions has properties that facilitate the transport processes of compounds while remaining a benign me­dia for processing. It even acts as a catalyst for acid/ base reactions under these conditions (Calzavara et al., 2005; Nolen et al., 2003; Savage et al., 1995). Of note in SCWG is that it takes place in either high — or low — temperature conditions (Matsumura et al., 2005). How­ever, if one adds an alkali catalyst to the processing at low temperatures, it increases the conversion into oil and gas. Additionally, the catalyst lowers the tempera­ture at which the cellulose decomposes, or the onset of the gasification process (Minowa et al., 1997, 1995, 1998a, b, Murakami et al., 2012).


The conversion of LB by heating is pyrolysis (Balat, 2008a; Bridgwater, 2003; Mohan et al., 2006). Depending on the desired product, one chooses the operational con­ditions for pyrolysis. Factors such as heating rate, reactor temperature, residence time and composition of the feedstock determine the product. The goal of py­rolysis is to execute the process in the absence of oxygen and thus avoid the burning of the feedstock and instead
break down the lignocellulosic bonds and crystalline structure. By doing so under these conditions, new com­pounds are formed. Compounds such as char, bio-oil, and gasses are produced (Thuijl et al., 2003). The bio­oil formed by pyrolysis is not easily stored because it is unstable and requires additional processing prior to long-term storage (Adam, 2005).

There are three categories of pyrolysis: conventional, fast and flash. Conventional pyrolysis produces solids, liquids and gasses. Fast and flash pyrolysis produces primarily liquid and gaseous products (Demirbas, 2005).