Biofuels from Lignocellulosic Biomass

Lignocellulose is a generic term for describing the main constituents in most plants, namely cellulose, hemicelluloses, and lignin. Lignocellulose is a complex matrix, comprising many different polysaccharides, phenolic polymers and proteins. Cellulose, the major component of cell walls of land plants, is a glucan polysaccharide containing large reservoirs of energy that provide real potential for conversion into biofuels. Lignocellulosic biomass consists of a variety of materials with distinctive physical and chemical characteristics. It is the non-starch based fibrous part of plant material.

First-generation biofuels (produced primarily from food crops such as grains, sugar beet and oil seeds) are limited in their ability to achieve targets for oil-product substitution, climate change mitigation, and economic growth. Their sustainable production is under scanner, as is the possibility of creating undue competition for land and water used for food and fibre production.

The cumulative impacts of these concerns have increased the interest in developing biofuels produced from non-food biomass. Feedstocks from ligno-cellulosic materials include cereal straw, bagasse, forest residues, and purpose-grown energy crops such as vegetative grasses and short rotation forests. These second-generation biofuels could avoid many of the concerns facing first-generation biofuels and potentially offer greater cost reduction potential in the longer term.

The largest potential feedstock for ethanol is lignocellulosic biomass, which includes materials such as agricultural residues (corn stover, crop straws and bagasse), herbaceous crops (alfalfa, switchgrass), short rotation woody crops, forestry residues, waste paper and other wastes (municipal and industrial). Bioethanol production from these feedstocks could be an attractive alternative for disposal of these residues. Importantlylignocellulosic feedstocks do not interfere with food security. Moreover, bioethanol is very important for both rural and urban areas in terms of energy security reason, environmental concern, employment opportunities, agricultural development, foreign exchange saving, socioeconomic issues etc.

Lignocellulosic biomass consists mainly of lignin and the polysaccharides cellulose and hemicellulose. Compared with the production of ethanol from first-generation feedstocks, the use of lignocellulosic biomass is more complicated because the polysaccharides are more stable and the pentose sugars are not readily fermentable by Saccharomyces cerevisiae. In order to convert lignocellulosic biomass to biofuels the polysaccharides must first be hydrolysed, or broken down, into simple sugars using either acid or enzymes. Several biotechnology-based approaches are being used to overcome such problems, including the development of strains of Saccharomyces cerevisiae that can ferment pentose sugars, the use of alternative yeast species that naturally ferment pentose sugars, and the engineering of enzymes that are able to break down cellulose and hemicellulose into simple sugars.

Lignocellulosic processing pilot plants have been established in the EU, in Denmark, Spain and Sweden. The world’s largest demonstration facility of lignocellulose ethanol (from wheat, barley straw and corn stover), with a capacity of 2.5 Ml, was first established by Iogen Corporation in Ottawa, Canada. Many other processing facilities are now in operation or planning throughout the world.

Economically, lignocellulosic biomass has an advantage over other agriculturally important biofuels feedstocks such as corn starch, soybeans, and sugar cane, because it can be produced quickly and at significantly lower cost than food crops. Lignocellulosic biomass is an important component of the major food crops; it is the non-edible portion of the plant, which is currently underutilized, but could be used for biofuel production. In short, lignocellulosic biomass holds the key to supplying society’s basic needs for sustainable production of liquid transportation fuels without impacting the nation’s food supply.

Ethanol Production via Biochemical Route

Ethanol from lignocellulosic biomass is produced mainly via biochemical routes. The three major steps involved are pretreatment, enzymatic hydrolysis, and fermentation. Biomass is pretreated to improve the accessibility of enzymes. After pretreatment, biomass undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. Subsequently, sugars are fermented to ethanol by the use of different microorganisms.

Pretreated biomass can directly be converted to ethanol by using the process called simultaneous saccharification and cofermentation (SSCF). Pretreatment is a critical step which enhances the enzymatic hydrolysis of biomass. Basically, it alters the physical and chemical properties of biomass and improves the enzyme access and effectiveness which may also lead to a change in crystallinity and degree of polymerization of cellulose. The internal surface area and pore volume of pretreated biomass are increased which facilitates substantial improvement in accessibility of enzymes. The process also helps in enhancing the rate and yield of monomeric sugars during enzymatic hydrolysis steps.

Pretreatment methods can be broadly classified into four groups – physical, chemical, physio-chemical and biological. Physical pretreatment processes employ the mechanical comminution or irradiation processes to change only the physical characteristics of biomass. The physio-chemical process utilizes steam or steam and gases, like SO2 and CO2. The chemical processes employs acids (H2SO4, HCl, organic acids etc) or alkalis (NaOH, Na2CO3, Ca(OH)2, NH3 etc). The acid treatment typically shows the selectivity towards hydrolyzing the hemicelluloses components, whereas alkalis have better selectivity for the lignin. The fractionation of biomass components after such processes help in improving the enzymes accessibility which is also important to the efficient utilization of enzymes.

The pretreated biomass is subjected to enzymatic hydrolysis using cellulase enzymes to convert the cellulose to fermentable sugars. Cellulase refers to a class of enzymes produced chiefly by fungi and bacteria which catalyzes the hydrolysis of cellulose by attacking the glycosidic linkages. Cellulase is mixture of mainly three different functional protein groups: exo-glucanase (Exo-G), endo-glucanase(Endo-G) and ?-glucosidase (?-G). The functional proteins work synergistically in hydrolyzing the cellulose into the glucose. These sugars are further fermented using microorganism and are converted to ethanol. The microorganisms are selected based on their efficiency for ethanol productivity and higher product and inhibitors tolerance. Yeast Saccharomyces cerevisiae is used commercially to produce the ethanol from starch and sucrose.

Escherichia coli strain has also been developed recently for ethanol production by the first successful application of metabolic engineering. E. coli can consume variety of sugars and does not require the complex growth media but has very narrow operable range of pH. E. coli has higher optimal temperature than other known strains of bacteria.

Lower GHG emissions and empowerment of rural economy are major benefits associated with bioethanol

The major cost components in bioethanol production from lignocellulosic biomass are the pretreatment and the enzymatic hydrolysis steps. In fact, these two process are someway interrelated too where an efficient pretreatment strategy can save substantial enzyme consumption. Pretreatment step can also affect the cost of other operations such as size reduction prior to pretreatment. Therefore, optimization of these two important steps, which collectively contributes about 70% of the total processing cost, are the major challenges in the commercialization of bioethanol from 2nd generation feedstock.

Enzyme cost is the prime concern in full scale commercialization. The trend in enzyme cost is encouraging because of enormous research focus in this area and the cost is expected to go downward in future, which will make bioethanol an attractive option considering the benefits derived its lower greenhouse gas emissions and the empowerment of rural economy.

A Glance at Drop-in Biofuels

drop-in-biofuelsBiofuel commercialization has proved to be costly and lingering than expected due to its high production cost and modification to flexibility in engines. Drop-in fuels are alternatives to existing liquid fuels without any significant modification in engines and infrastructures. According to IEA, “Drop-in biofuels are liquid bio-hydrocarbons that are functionally equivalent to petroleum fuels and are fully compatible with existing petroleum infrastructure”.

What are Drop-in Biofuels

Drop-in biofuels are can be produced from oilseeds via trans-esterification, lignocellulosic biomass via thermochemical process, sugars and alcohol via biochemical conversion or by hybrids of the above methods. Drop-in fuels encompass high hydrogen to carbon ratio with no/low sulfur and oxygen content, low water solubility and high carbon bond saturation. In short drop-in fuel is a modified fuel with close functional resemblance to fossil fuel.

Existing biofuels – bioethanol and biodiesel – have wide variation from fossil fuels in their blend wall properties – high oxygen content, hydrophilicity, energy density and mainly compatibility in existing engines and infrastructures. Oxygenated groups in biofuel have a domino effect such as reduction in the energy density, production of impurities which are highly undesirable to transportation components, instability during storage etc.

Major advantages of drop-in fuels over existing fuels are as follows:

  • Reduced sulphur oxide emissions by ultra low sulphur content.
  • Reduced ignition delay by high cetane value
  • Reduced hydrocarbons and nitrogen oxides emissions
  • Low aromatic content
  • Low olefin content, presence of olefin compounds undergo auto-oxidation leading to surface depositions.
  • High saturates, therefore leaving minimum residues
  • Low particulate emissions
  • No oxygenates therefore has high stability.

Potential Biomass Feedstock

Drop-in biofuels can be produced from various biomass sources- lipids (vegetable oils, animal fats, greases, and algae) and lignocellulosic material (such as crop residues, woody biomass, and dedicated energy crops). The prominent technologies for biomass conversion to drop-in fuel are the thermochemical and the biochemical process.

The major factor playing role in selection of biomass for thermochemical methods is the energy content or heating value of the material, which is correlated with ash content. Wood, wood chips accounts for less than 1% ash content, which is favorable thermal processing than biochemical process, whereas straws, husks, and majority of the other biomass have ash content ranging up to 25% of dry mass.

Free sugar generating plants such as sugarcane and sweet sorghum, are desirable feedstock for Acetone-Butanol-Ethanol fermentation and have been widely implemented. Presently there is a focus to exploit lignocellulosic residues, rich in hydrocarbon, for fuel production. However, this biomass requires harsh pretreatment to remove lignin and to transform holocellulose (cellulose & hemicelluloses) into fermentable products.

The lignocellulose transformation technology must be circumspectly chosen by its life cycle assessment, as it resists any changes in their structural integrity owing to its complexity. Lignocellulosic biomass, when deoxygenated, has better flexibility to turn to drop-in fuels. This is because, in its native state of the feedstock, each oxygen atom consumes two hydrogen atoms during combustion which in turn reduces effective H: C ratio. Biomass feedstock is characterized with oxygen up to 40%, and higher the oxygen content higher it has to be deoxygenated.

Thermochemical Route

Thermochemical methods adopted for biomass are pyrolysis and gasification, on thermolysis of biomass produce intermediate gas (syngas) and liquid (bio crude) serving as precursors for drop-in fuel. Biomass when exposed to temperature of 500oC-600oC in absence of oxygen (pyrolysis) produce bio-oil, which constitutes a considerable percentage of oxygen. After down streaming by hydroprocessing (hydrotreating and hydrocracking) the rich hydrocarbon tar (bio-oil) can be converted to an efficient precursor for drop-in fuel.

At a higher temperature, above 700, under controlled oxygen, biomass can be converted to liquid fuel via gas phase by the process, gasification. Syngas produced is converted to liquid fuel by Fischer-Tropsch with the help of ‘water gas shift’ for hydroprocessing. Hydroprocessing after the thermochemical method is however costly and complex process in case of pyrolysis and inefficient biomass to fuel yield with gasification process.

Biochemical Pathway

The advanced biocatalytic processes can divert the conventional sugar-ethanol pathway and convert sugars to fatty acids. Modified microbial strain with engineered cellular machineries, can reroute the pathway to free fatty acid that can be transformed into butanol or drop-in fuel with necessary processing.

Schematic for the preparation of jet fuel from biomass

Schematic for the preparation of jet fuel from biomass

Biological processing requires operation under the stressful conditions on the organisms to reroute the pathways, in additional to lowering NADPH (hydrogen) consumption. Other value added products like carboxylic acid, polyols, and alcohol in the same biological routes with lower operational requirements have higher market demands and commercial success. Therefore little attention is given by chemical manufacturers to the biological pathways for drop-in fuel production.

The mechanisms of utilization of lignocellulosic biomass to fuel by biological pathway rely heavily on the availability of monomeric C5 and C6 sugars during fermentation. Ethanol is perhaps the best-known and commercially successful alcohol from ABE fermentation. However, butanol has various significant advantages over ethanol- in the perception of energy content, feasibility to existing infrastructures, zero blend wall, safety and clean aspects. Although butanol is a closer drop-in replacement, existing biofuel ethanol, is a major commercial competitor. Low yield from fermentation due to the toxicity of butanol and complexity in down streaming are the vital reasons that hamper successful large scale butanol production.

Challenges to Overcome

Zero oxygen and sulphur content mark major challenges for production of drop-in fuels from conventional biomass. This demands high hydrogen input on the conventional biomass, with H: C ratio below 0.5, like sugar, starch, cellulose, lignocellulose to meet the effective hydrogen to carbon ratio of 2 as in drop-in fuel. This characterizes most of the existing biomass feedstock as a low-quality input for drop-in fuels. However oleochemicals like fats, oils, and lipids have closer H: C ratio to diesel, gasoline and drop-in fuels, thus easier to conversion. Oleochemical feedstock has been commercially successful, but to prolong in the platform will be a major challenge. Lipid feedstock is generally availed from crop-based vegetable oil, which is used in food sectors. Therefore availability, food security concerns, and economics are the major constraints to sustaining the raw material. Consequently switching to lignocellulosic biomass feedstock for drop-in holds on.

Conclusions

Despite the hurdles on biomass characteristics and process technology for drop-in fuel, it is a vital requirement to switch to better replacement fuel for fossil fuel, considering environmental and economic benefits. Understanding its concepts and features, drop-in fuel, can solve existing greenhouse emission debate on current biofuels. Through crucial ambiguities existing on future of alternative fuels, drop-in fuel has a substantial potential to repute itself as an efficient sustainable eco-friendly fuel in the naear future.

References

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