Keywords

1 Introduction

Presently, we are almost fully dependent upon exhaustible supplies of petroleum, coal and other fossil fuels for meeting the day to day demands of transportation fuels, energy, commodity chemicals and various other products. Due to rapid and ever-increasing consumption of energy and commodity chemicals, search for sustainable supplies of carbon-neutral feedstock and/or resources as well as production processes is inevitable (Maity 2015). In biorefinery, production of biofuels, renewable energy/power, and various chemicals from plant biomass resources is carried out based upon the integration of various production methods as well as the machinery (Luo et al. 2010). The concept of biorefinery can be considered equivalent to that of petrochemical refinery in a way that the biomass feedstock is processed to obtain a multitude of products, such as biofuels, chemicals, biomaterials, biomolecules, etc. (Moncada et al. 2015).

A variety of physico-chemical, biochemical/microbial processes are used in biorefinery operations for production of various products to be used in transportation, pharmaceutical/health, food and other sectors, with zero or minimal waste generation. Complete utilisation of biomass for large volume production of low commercial value product simultaneously with one or more high-value products will enhance the competitiveness of biorefinery operation (https://www.aber.ac.uk/en/media/departmental/ibers/pdf/innovations/07/07ch8.pdf accessed on 20th Dec 2017).

The production of transportation fuels and chemicals from each and every fraction and/or process wastes during biomass processing is thus an essential requirement of an integrated biorefinery and has the potential to reduce dependence upon non-renewable fossil-based resources and decline global warming as well. Sustainability of the biorefineries requires that the production of multi-products in total should be economic, energy efficient, environmentally safer and carbon-neutral (or preferably carbon negative). Biorefinery is expected to produce a variety of products both in terms of their chemical/biochemical properties and their economic or commercial value. To be economic, it is desirable that the volumes of the low-value products such as bioethanol, biodiesel, etc. should be high; however, the low-cost chemicals and biomaterials obtained in even low volumes can give similar or higher economic advantage (Moncada et al. 2015). Although integrated biorefinery concept is gaining worldwide publicity and acceptance and has enormous potential as far as the sustainable and green manufacture of bio-based products is concerned, this area is still in its infancy and comparatively fewer reports have focused on all of its aspects. Moreover, due to newer scientific as well as technological advances in this ever-growing area (Kamm and Kamm 2004; Maity 2015) the concepts of biorefinery are evolving and being redefined very fast. Therefore, there is a need to understand various concepts of biorefinery in more detail that will be helpful in developing a techno-socio-economically viable biorefinery system. This chapter deals with the concept and practice of integrated lignocellulosic biorefinery for sustainable development, different products of integrated lignocellulosic biorefinery and process technology in lignocellulosic biorefinery with important considerations. In the end, a short survey of the biorefinery industry followed by challenges, opportunities and future prospects is provided.

2 Biorefinery Concept

The perception of biorefinery developed towards the end of the twentieth century to address the challenges of declining petroleum-based transportation fuels and outlook for cheaper and environmentally friendly synthesis of valuable materials and products from renewable biomass (Kamm and Kamm 1997; Kamm et al. 2007). The concept of biorefinery is depicted in Fig. 1.

Fig. 1
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The concept of biorefinery

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The concept of biorefinery is similar to petrochemical/refinery industry in a manner that both types of refineries are involved in production of a wide array of chemicals as well as fuels (Maity 2015). Globally, revenue generation by bio-based products for chemical industry is estimated at USD 10–15 billion. Many papers, reviews and reports have addressed the biomass potential for chemical and polymer production in much detail (de Jong et al. 2012). Some comparable aspects of biorefinery petroleum refinery are engineering aspects, including feedstock fractionation, multiple products (both platform and end use), process integration and flexibility as depicted in Fig. 2. Two major differences are the raw material and complexity in application of technologies. Some of the products produced in the biorefinery cannot be obtained after refining petroleum, e.g. some food products (Moncada et al. 2016).

Fig. 2
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Analogy between petroleum-based refinery and biorefinery

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Need for biorefinery in current scenario is threefold: (1) the depletion of petroleum resources, (2) concerns about global climate change and (3) energy security issues. Additionally, there are other reasons for the need of biorefineries, such as avoiding over-dependence on petroleum-based products; need of economic products and chemicals; strengthening bio-based circular economy; protection of natural environment and ecosystem; and increasing employability in rural regions and stimulate the sustainable development of regional areas (Langeveld et al. 2012; McCormick and Kautto 2013). There are a number of merits of biorefinery-based production of bioenergy, biofuel, biochemicals and materials as shown in Fig. 3.

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Merits and demerits of lignocellulosic biorefineries

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3 Type of Biorefineries

In general, biorefineries are categorised into three different classes depending upon the utilisation of the substrate: first-generation (starch and sugar feedstock) biorefineries, second-generation (plant biomass feedstock) biorefineries and third-generation (algal feedstock) biorefineries (Hossain et al. 2016). These are discussed below in more detail, and the examples of each type of these biorefineries and respective products are listed in Table 1.

Table 1 Examples of types of biorefineries and their products
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3.1 First-Generation Biorefineries

Such biorefineries are utilising sugary, oil-based and starch-containing substrates as feedstocks to produce fuels and commodity chemicals. Currently, almost all biofuels including bioethanol and biodiesel, and biochemicals are produced by first-generation biorefinery. The main drawback of these biorefineries is competition of raw substrate with food demand and deterioration of soil.

3.2 Second-Generation Biorefineries

In these, the feedstock is lignocellulose-containing plant or forestry-based biomass such as agricultural residue, forestry and urban waste. The complexity of chemical composition of lignocellulosic biomass is the main reason for its usefulness in manufacturing of various chemicals as well as fuels (Maity 2015). The biomass can be processed through thermochemical as well as biological routes. The abundance, diversity and non-competence with food crops of lignocellulosic biomass make it superior to first-generation biorefineries.

3.3 Third-Generation Biorefineries

Microalgae, as third-generation substrate, have vast potential for the sustainable production of commodity products. Such biorefineries can provide cleaner energy (biodiesel and bioethanol), value-added products including cosmetics, therapeutics, animal feed and food, and technical solution to waste management concerns. The major advantage of using microalgae as a substrate is their ability to grow very fast within a shorter span of time.

4 Lignocellulosic Biorefineries

Due to the issues of economic sustainability and environmental concerns, the global research interest for production of various chemicals, fuels, energy and other materials has been shifted towards renewable sources as substitutes to petroleum-derived products. Lignocellulose biomass, the most abundantly available organic carbon source, can be a sustainable alternative to petroleum-dependent fuels and petrochemicals and will surely emerge as an important source of biomass and be widely available at moderate costs showing less competition with food and feed production.

4.1 Feedstock and Products

Lignocellulosic biorefinery under different above said categories are mentioned in Table 2. Lignocellulosic biomass is the most abundant biomass with vast potential for production of a wide range of bio-products and biofuels (Amidon and Liu 2009; Liu et al. 2012; Menon and Rao 2012; Maity 2015). The annual production of lignocellulosic biomass has been reported to be approximately 150–170 × 109 tonnes. However, despite its abundance and low cost, the conversion of lignocellulosic biomass to value-added products and their selective recovery remain a bottleneck due to the lack of economic viability, and this has become the active area for extensive research across the globe to address this concern worldwide (Cherubini and Ulgiati 2010; Sarma et al. 2017).

Table 2 Biomass feedstock for lignocellulosic biorefineries
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Lignocellulosic biomass broadly can be categorised into agriculture waste, forest and industrial waste, aquatic waste and municipal waste. Included among the first category are various crop wastes such as straws (wheat, rice), stalks (cotton, mustard) and bagasse (cane, sweet sorghum) that often are burned to prepare the agricultural fields for sowing of next crop. Such feedstock does not compete with food and is widely available. The main challenge in their exploitation in a biorefinery is their transportation cost in view of their low density and unavailability at a single place (Kamm and Kamm 2004). Biomass in the second category also has a similar composition to agricultural crop wastes, except that these have comparatively lower cellulose and more lignin (Kim et al. 2006; Speight 2014) and the biomass is not affected much by seasonal variations and the main reasons for biomass variations are location and forest type. Algal (aquatic) lignocellulosic biomass also does not compete with the food and is benefitted by much higher production rate under cheaper conditions (Talebian-Kiakalaieh et al. 2014), utilising either open ponds or photobioreactors (Nakamura and Whited 2003; Maity 2015). Potential biomass feedstocks for lignocellulosic biorefinery under different above said categories are mentioned in Table 2.

Lignocellulose mainly contains three complex structural polymer entities including cellulosic, hemicellulosic and lignin fractions (Fig. 4), which can be utilised for synthesis of various useful chemicals and products using enzymatic/biochemical or chemical platform after their conversion to simpler sugars. Lignin is another polymer of high economic importance, which can be utilised for cogeneration; synthesis of phenolic components and other chemicals. (de Bhowmick et al. 2018). Different products obtained from various factions of lignocellulosic biomass are depicted in Fig. 5 and these products are categorised into five different types, i.e. biofuels, bioenergy, food products, biochemical and biomaterials in Table 3.

Fig. 4
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Structural components of lignocellulosic biomass. H: p-coumaryl alcohol; G: coniferyl alcohol and S: sinapyl alcohol

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Fig. 5
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Various commodity products derived from lignocellulosic components

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Table 3 Different categories of lignocellulosic biorefinery products (Moncada et al. 2016)
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Interestingly, via lignocellulosic biorefinery-based approach, various platform chemicals can be formed by direct fermentation of sugars [ethanol (C2); propanol (C3); butanol (C4)], syngas transformation of propylene (C-3), dimerization of ethylene (produced from dehydration of ethanol) to butenes (C-4), etc. Moreover, approaches such as ABE fermentation can also be used to produce multiple products acetone, butanol and ethanol (BREW 2006; Bos and Sanders 2013; Yao and Tang 2013; Kajaste 2014). A list of examples of C1-C6 platform chemicals derived from lignocellulosic biomass through microbial fermentation is shown in Table 4.

Table 4 Typical chemicals from biomass produced via microbial fermentation
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4.2 Lignocellulose Conversion Processes

Depending upon the diversity of biomass and their compositional variability, a variety of conversion processes such as thermal and chemical (e.g. combustion, liquefaction, fast pyrolysis, etc.), chemical [viz. aqueous phase dehydration/hydrogenation (APD/H)], or biological (e.g. fermentation, digestion, microbial processing, etc.) can be used in biorefinery operations.

4.2.1 Thermochemical Processes

Such conversions include gasification process, fast pyrolysis and liquefaction processes (Balat 2008). The process of gasification generates various intermediates, for synthesis of various chemicals and biomaterials, as well as liquid biofuels, electricity and heat (Fahlén and Ahlgren 2009). In pyrolysis, the chemical reaction involves short interval high-temperature treatment of biological feedstock under anaerobic conditions. At low temperature, the process converts biomass to a liquid biocrude, which can subsequently be used to generate liquid biofuels (Balat 2008). Chemical reactions under liquefaction process are carried out under an environment of H2O and CO/H2 (Leduc et al. 2008; Wetterlund and Soderstrom 2010), for generation of bio-crudes and other important products (Palmqvist and Hahn-Hägerdahl 2000; Hamelinck et al. 2005).

4.2.2 Biochemical Processes

Such process consists of depolymerisation of structural polymers viz. cellulose and hemicellulose into monomeric sugars and further fermentations or enzymatic reactions for synthesis of useful products, such as bioethanol, biobutanol, ETBE, MTBE, acids, etc. The major drawback of thermochemical conversion of lignocellulosic biomass is high process cost (especially enzymes) and recalcitrance of biomass. However, biorefinery approach can compensate the cost by production of high-value products.

4.2.3 Chemical Processes

Several chemical processes including acid hydrolysis, can be used to intricate in lignocellulosic biorefinery. These can lie may be in pretreatment step or in downstream processing. Under controlled conditions, acid hydrolysis can convert lignocellulosic biomass into xylan, xylose and monomer sugars and could fractionate cellulose and lignin components (Palmqvist and Hahn-Hägerdal 2000). For chemical transformation of lignocellulosic biomass to syngas (CO + H2), Fischer–Tropsch process can also be employed. Moreover, methyl alcohol production, hydro-formylation and methane synthesis can be carried out by using synthesis gases (Balat 2008).

4.3 Examples of Lignocellulosic Biorefineries

4.3.1 Bioethanol-Based Biorefinery

Cellulosic ethanol production involves various processes including its enzymatic after physico-chemical treatment followed by fermentation of hydrolysate and ethanol separation. Lignocellulose due to its structural complexity are very recalcitrant for its bioconversion and therefore, a prior physico-chemical processing or treatment step is carried out to remove biomass recalcitrance and make it amenable for enzymatic and microbial attack (Zheng et al. 2009). The pretreated substrate is then subjected to enzymatic depolymerisation, which is the most cost-intensive process due to high cost of enzymes. The hydrolysate thus obtained has both five-carbon and six-carbon sugars, which can be fermented to yield bioethanol. In nature, pentose fermenting microbes are very few and have relatively low yields of ethanol than C-6 fermenting microorganisms. In the final stage, ethanol thus produced after fermentation is harvested and concentrated by distilling the medium and/or by membrane separation. All these concerns led to a urge in developing bioethanol-based biorefinery so as to compensate the cost of bioethanol from the high-value additional products. Utilising ethanol yielding fraction such as hemicellulose and lignin to other value-added products could be a cost-effective approach to be considered. The solid residual unreacted products such as lignin, cellulose, hemicelluloses, enzymes and microorganisms are recovered after final ethanol recovery step and processed into other fuels (Mosier et al. 2005). Usually, solid residuals are dried to 10% mc and fired in a boiler or a gasifier to produce methane. A comparison of cost economic of bioethanol plant and bioethanol-based biorefinery is shown in Table 5.

Table 5 Comparative economics of lignocellulosic biorefinery and bioethanol plant (adapted from Luo et al. 2010)
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4.3.2 Biomethane-Based Biorefinery

A number of crop residues including waste from maize, wheat, rye, etc. can be used as substrate for the production of biomethane. It is estimated that the annual maize and cereal crop waste has the potential to produce 2000–4500 MT of methane per hectare (Kumar et al. 2008). Similar to bioethanol production, biomethanation is also a multistep process. Methane fermentation from lignocellulosic biomass involves hydrolysis, acidogenesis, acetogenesis and methanation steps. The diversity of microorganisms required for each step varies from each other. Microorganisms through various phases, finally, hydrolyse the undissolved complex structural polymers of lignocellulose such as cellulose, proteins and fats into monomers. The monomeric sugars thus formed after hydrolysis are further exploited by other organisms to produce various C1–C5 molecules, alcohols, short chains of organic acids, hydrogen and CO2 (Chandra et al. 2012). In the acetogenic phase, the organisms convert organic acids and alcohols into acetate. Finally, under the obligate anaerobes ferment these carbon sources (CO2, formate, methanol and acetate, etc.) to methane in the methanogenesis. A variety of products formed in between can be recovered from the process and can be used to compensate the process cost. Moreover, the leftover biomass could be used as compost. Moreover, the process also offers a potential solution to the waste management.

4.3.3 Biohydrogen-Based Biorefinery

A number of lignocellulosic feedstocks including agriculture waste, stillage, industrial waste, fibre waste, kitchen waste, etc. are good feedstocks when seeking H2 generation. Out of the available processes for biohydrogen production, the most cost-competitive process is usually the one involving only single stage. Usually, the lignocellulosic biomass is pretreated and hydrolysed followed by dark fermentation of the hydrolysate for hydrogen production. However, pretreatments may have drawbacks of generating undesirable by-product that could threat the fermentability of the hydrolyzates (Cheng et al. 2011; Quéméneur et al. 2012). Various interventions in this regards are undergoing and a major shift of research interest has been made recently. Cheng et al. (2011) have developed a novel process of biohydrogen production involving two stages comprising alternate light and dark phases using phototrophic microalgal strains. The process also underlines the requirement of integrating all the techniques to produce multiple products at a time.

5 Sustainability Aspects of Lignocellulosic Biorefineries

The initial thrust to the concept of ‘sustainability’ in relation to the environment was derived from ‘The Brundtland report’ of WCED as ‘development that can meet the needs of the present generations without compromising abilities of future generations to meet their own demands’ (Hofer and Bigorra 2008). In this context, through sustainable development we can preserve the quality of life for our coming generations. ‘Sustainability’ and ‘sustainable development’ are the two broader terms whose exact meaning and definitions are highly reliant on the milieu, specific goals and solicited use and may be considered multidimensional. In actual terms, ‘sustainability’ and ‘sustainable development’ can be considered to be associated with the balance of three important as well as interdependent aspects, i.e. econo-, enviro- and societal aspects, so that the well-being of our and our coming generations is preserved (Kemp and Martens 2007; Posada and Osseweijer 2016; Parada et al. 2017). The overall impacts of any biorefinery project can be realised in a real sense by wise combination of above-mentioned sustainability aspects after avoiding overlapping aspects and putting proper weightage to various indicators, subcategories and impacts categories (Santoyo-Castelazo and Azapagic 2014).

5.1 Economic Sustainability

This refers to the expenditures involved in each and every stage involved in biomass production, collection, processing, product formation, recovery, commercialization, etc. Therefore, economic sustainability takes into account the cost-competitiveness by combining the technical and economic aspects jointly. If the products are not cost-competitive, then most likely they will not have any market despite derived from renewable feedstocks (Posada and Osseweijer 2016). Economic indicators can be categorised into three classes associated with the cost, benefit and value of investment. The important economic indicators in the first category include capital cost, total savings, operating cost, production cost, transportation cost and actual sequestration cost, with production cost being the most critical indicator. The second category associated with benefit includes margins and profit related to operation among which latter one is the most important and most frequently used indicator. The third class of economic indicators includes indicators related to the investment value of a biorefinery and consists of return on investment, duration for payback, total economic value, NPV, stakeholder value and minimum selling price. Net present value is the most critical as well globally used indicator in this category followed by minimum selling price (Seider et al. 2010; Tan et al. 2016; Parada et al. 2017).

5.2 Environmental Sustainability

This aspect of sustainability of biorefineries helps in minimising the potential environmental hazards of biorefinery while producing the desired product in optimum quantity and without affecting the economic sustainability.

A systematic set of procedures for compiling and examining the inputs and outputs of materials and energy and the associated environmental impacts directly attributable to the functioning of a product or service system throughout its life cycle. Various phases of product formation right from derivation or synthesis from its source up to its final use/consumption constitutes its life cycle and evaluation of the product’s life cycle for its impact on the environment are known as the life cycle assessment or LCA (ISO 2006). Additional efficient methodologies for environmental assessment may be based on evaluation of minimum impact, (Stefanis et al. 1995), minimum waste generation (Young and Cabezas 1999), risk to environment (Shonnard and Hiew 2000), thermodynamic analysis method (Bakshi 2002) and atmospheric hazards index (Gunasekara and Edwards 2003). Strategy for impact assessment may involve application of tools such as ReCiPe, CML, etc. based upon various factors related to several aspects of environment. However, modelling of some categories on basis of one geographic region may be inadequate for other geographical locations (Institute for Environment and Sustainability 2010). CO2 emission has direct impact on environmental sustainability of biorefinery and has been mainly attributed to the production, utility generation and transportation in biorefinery applications (Parada et al. 2017). Other GHG emissions are also equally relevant in all stages of any biorefinery, including agricultural practices also (Fan et al. 2013).

5.3 Social Sustainability

This aspect determines the usefulness and implications of any biorefinery product, process or service. This dimension of sustainability has not been considered much frequently in biorefinery projects due to the scarcity of the tools and methodologies for assessment and evaluation of social aspects at present and the historically long detachment of the social sciences from the natural and engineering sciences (Lehmann et al. 2011). The indicators of social sustainability can be categorised as stand-alone social (sub-categorised into energy and food security; latter one being assessed by food price increase and sustainability factor), socio-economic, i.e. employment (including both generation of employment and requirement of labour), and socio-environmental (which include health as assessed by human exposure risk) (Parada et al. 2017). Food security is an important issue for the bioeconomy and in turn dependents upon land (Souza et al. 2015).

6 Guidelines for Sustainable Biorefinery

Guidelines for sustainable biorefineries can be proposed by adopting the similar strategies from other disciplines (such as chemical and engineering sciences) as detailed in Table 3. Such principles in chemical sciences can be stated as ‘the design of chemicals and processes that reduce or eliminate the use or generation of hazardous substances’ and green engineering concept as ‘the design, commercialization, and use of processes and products in a way that minimises pollution, promotes sustainability, and protects human health without sacrificing economic viability and efficiency’ (Gallego et al. 2011). These guidelines emphasise the sustainability element in biorefinery operations and may include several considerations for the development of a sustainable biorefinery as listed in Table 6.

Table 6 Some important considerations for the development of sustainable lignocellulosic biorefinery
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7 Current Challenges and Future Prospects

Currently, biorefineries are being developed worldwide for sustainable synthesis of products and materials for various industrial sectors such as energy, transport, food, chemical, health, pharmaceutical, etc. One of the major future challenges for biorefinery is maintenance of socio-econo-environmental sustainability. For maximum utilisation of the complexity of lignocellulose, not a single technology or single biomass or production of a single product will be sufficient and obviously integration of different unit operations will be of utmost importance. The concept of biorefinery is already gaining popularity and few bio-based industries are currently operational or under demonstration stage. Recently, India has also mandated to establish various biorefineries in different parts of the country depending upon various technologies and feedstocks. A brief overview of few lignocellulosic biorefineries currently operational with their scale of operation is provided in Table 7.

Table 7 Some operational lignocellulosic biorefinery industries
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Currently, biorefinery industries face several challenges which can be broadly categorised into the biomass related and the process related challenges; whereas some are miscellaneous as they are common to both. Major challenges in each category are shown in Fig. 6. An overall challenging task is the commercial viability or economic sustainability of biorefineries. Cutting-edge research and state-of-the-art technologies need to be developed and implemented at all levels, i.e. lab-, pilot- and industrial production levels, which may require significant investments from government, academia and private industries. Consistent and dedicated research and development efforts are needed, especially for evaluation and validation of technologies being developed. There is a clear need for proper modelling, assessing and evaluating sustainability impacts on a life cycle-based analysis (https://www1.eere.energy.gov/bioenergy/pdfs/ibr_portfolio_overview.pdf; Agler et al. 2011).

Fig. 6
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Major current and emerging challenges for lignocellulosic biorefineries

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8 Conclusion

In order to keep the bio-based economy sustainable, it is important to shift focus from the concept of single product from lignocellulosic feedstock to development of multitude of products. More technologies need to be developed, a range of products and co-products need to be enhanced and multiple feedstocks need to be utilised for better realisation of lignocellulosic biorefineries. Integration of various processes for conversion of various fractions of lignocellulosic biomass to different products is needed. In conclusion, there is an urgent need for proper and more systematic improvement of feedstock, processes and the microbial and/or enzymatic performances for integration of biorefinery operations in a sustainable manner.