Multi-feedstock lignocellulosic biorefineries based on biological processes: An overview

Highlights

• Lignocellulosic biomass (LCB): a key and complex renewable source of fuels and chemicals.

• Recycling ethanol-water pretreatment: a low-cost and efficient biomass pretreatment.

• LCB enzymatic saccharification: an interplay of dynamic and kinetic phenomena.

• The biorefinery as an opportunity to valorise biomass-waste.

• Biorefinery biological end processes needs for a techno-scientific approach.

Abstract

The evolution of lignocellulosic biorefineries increasing the process complexity and integration through the processing of multiple raw biomass-based materials into several products is described. This critical overview deals with available lignocellulosic feedstocks, pretreatment operations, enzymatic saccharification to monosaccharides and their final transformation into bioethanol and other bio-based products. Aspects as process operating conditions, modes of operation, underlying physical and chemical phenomena, and mathematical modelling are addressed.

Pretreatment stage is a key step for any further processing, specially when using multifeedstock. The use of ethanol-water mixtures for biomass fractionation is a promising treatment able to compete with the nowadays more employed pretreatment such as steam explotion and diluted acid processes. Enzymatic saccharification of pretreated biomass has been analysed considering its complexity and selectivity. Its mathematical modelling is described in detail, together with the underlying phenomena, highlighting the importance of mass transfer in a biphasic system. An adequate operation unit design, scale-up and control, and process design and integration can be ensured in this way. Finally, biotransformations of monosaccharides to biofuels and platform chemicals is analysed as they are becoming key processes within bioeconomy, in the framework of integrated biorefinery to fuels, chemicals and materials.

Introduction

Nowadays, there is a wide consensus within scientists that anthropogenic greenhouse gas (GHG) emissions are being the primary cause of global warming since the Industrial Revolution; CO₂ concentration in the atmosphere has increased from 275 ppm to around 400 ppm since the middle of the 18th century, being the temperature increase from this time around 1 °C. At the current emission rate, the world will be 2 °C warmer than in pre-industrial times by 2050–2070, depending on the energy policy; this increase in temperature must be the limit in order to avoid hard consequences (O’Neill et al., 2017). Among the GHG sources, probably transportation is the activity more contributing to these emissions with over 30 % of total CO₂ world emissions (O’Neill et al., 2017). Some partial solutions are being considered, such as improvements to reduce emissions and fuel consumption, the development of new technologies, such as the electric car or the use of hydrogen-fuel cells, and the development of processes for obtaining biofuels from renewable sources (Fulton et al., 2015).

The substitution of non-renewable fossil and raw materials from petroleum has focused attention on the biomass, a feedstock with elementary composition based on carbon, hydrogen and oxygen (similar to fossil resources) with the advantage of being renewable, ubiquitous and abundant. Indeed, biomass has historically been used as fuel until it was replaced by fossil fuels as coal, oil or natural gas. Recently, the lack and dependence of fossil raw materials and the increase of the atmospheric CO₂ associated to their use have boosted the search of new and renewable feedstocks to produce fuels, energy and the variety of products now produced from fosces (Cherubini, 2010). In this context, some concepts are emerging; for example, bioeconomy, as define by the European Commission, includes biotechnological approaches, biomass as the key feedstock and integrated use of such resource, as a whole, to face most of the needs of Humanity (Manzanares, 2020). This is also the concept of an integrated biorefinery based on biocatalysis, just highlighting the technological component, instead of the socioeconomical one. Moreover, these two concepts are key tools in circular economy, whose aim is to minimize resource extraction and waste production, increasing cyclic mass and energy flows in transformation-reutilization processes (Manzanares, 2020). In the end, sustainability is the idea behind the creation of these concepts.

Biomass is very abundant and renewable (more than 2.0.10¹¹ ton. yr⁻¹) and is the best sustainable source of fuels and chemical products (Mussatto and Dragone, 2016). Between 70 and 95 % of the total biomass (at least 1.6 .10¹¹ ton. yr⁻¹) is lignocellulosic biomass (LCB) (De Lasa et al., 2011), residue from many sources, such as wood, branches, bark, pruning, cereal straw, etc., which are produced in abundance, continuously, in many cases concentrated in one place and at a time of the year, which presents obvious advantages for processing (Bhutto et al., 2017). There are two strategies for the LCB valorisation: the thermochemical pathways and the processes based on separation or fractionation of the main components (cellulose, hemicellulose and lignin), as schematized in Fig. 1. The first one uses combustion (only for energy production: heat and power), gasification, liquefaction, hydrogenation and pyrolysis processes, which originate a complex mixture of products (Menon and Rao, 2012); the second includes different physical, chemical and biological processes that have received much attention in recent years, beyond its use to obtain bioethanol (Wettstein et al., 2012). Recent reviews have been published both on the thermal and catalytic transformation and on the biologic transformation of biomass in several products once the sugars are obtained (Deng and Amarasekara, 2021; Kumar and Verma, 2020).

Biorefineries, the industry associated with the sustainable transformation of biomass into fuel, energy and products, have experienced a transformation, becoming more complex in the last few decades. The first generation (1 G) biorefineries use crops rich in free sugars (sugar cane, sugar beet) or starch (corn, wheat, among others) to produce ethanol by fermentation. Similarly, other 1 G biorefineries employs oily crops as sunflower or soy to extract the oil, further transformed into biodiesel for automotive. The main drawback of the 1 G biorefineries is the competition with food production for arable land. The most effective 1 G biorefineries are those based on sugarcane, only factible in tropical climates (Cherubini, 2010). For these reasons, the development of second-generation (2 G) biorefineries that can use any type of LCB, mainly wastes and residues, was proposed. Biofuels from LCB are most promising (Parajuli et al., 2015) although their economic profitability depends on obtaining other products with higher added value. Accordingly, the concept of an “integrated biorefinery” was developed, a type of biorefinery based on several raw materials and processes whose aim is to obtain very diverse products (Agrawal and Sikdar, 2012). Therefore, there is a need, and a trend, to implement multi-feedstock industrial facilities focused on the manufacturation of various final products that include processes based on chemical and biological processing to transform LCB, the so-called integrated biorefineries (IntBR) (Ajao et al., 2018; Maity, 2015). Although LCB has different origins (forestry, agriculture, industries as papermaking or sawmills, mainly), in all cases the main components are cellulose, hemicelluloses and lignin. Cellulose chains are disposed in micro-fibrils that alternate crystalline and amorphous regions. They are embedded into a hemicelluloses-lignin matrix; the resulting structure imparts enough strength and stiffness to the material and make the LCB resistant to microorganisms and chemical decomposition reactions.

In the last decades, bioethanol derived from corn starch (USA) or sugarcane (Brazil) has been the most widely produced biofuel in the world (Fulton et al., 2015). However, it is well-known that ethanol presents several disadvantages to be employed as gasoline substitute, such as low energy density (70 % of gasoline), high vapor pressure, hygroscopicity and corrosivity, thus the need of using bioethanol-gasoline blends, usually at 15 % (E15). Other biofuels, “advanced biofuels”, mainly higher C atom alcohols are being proposed and studied. These biofuels are attractive because of their higher energy density, lower vapor pressure and corrosivity to metals compared to ethanol. The first higher alcohols first under study were 4C atoms alcohols, mainly butanol and isobutanol. Recently, also 5C atoms alcohols, such as isopentanol and 2-methyl-1-butanol, have deserved attention, as their functional properties are more similar to those of gasoline (Monroe et al., 2020; Sherkhanov et al., 2020; Zhang et al., 2019). The most promising alcohol, as engine-fuel, is isobutanol: it can be mixed with gasoline in blends up to 90 % w/w. This biofuel has a energetic density similar to gasoline, with increasing octane number and reduced copper corrosiveness in comparison to ethanol (Acedos et al., 2018a). Moreover, in latter years, research and development efforts are undertaken to substitute oil as raw material in the chemical industry, looking for the production of the so-called “building blocks” and “platform chemicals”, chemicals that are the basis of a myriad products, including materials, chemicals and food and feed ingredients (Bozell and Petersen, 2010). The impact of using LCB to produce biofuels, bioenergy and bioproducts via a integrated biorefinery has been studied by Manzanares (2020), who highlighted the social and economic impacts, consequence of the industrial valorization of local biological resources. They presented the biorefineries as a strategic pillar in the development of the bioeconomy although also emphasized the need of improving and developing tools to assure the sustainability of the full value chain, especially the socio-economic impacts of the biorefinery-based biofuels and bioproducts.

This revision focuses on the biologic valorisation of LCB wastes based on their fractionation, an activity that requires the following stages (Cai et al., 2017):

• Logistics: collection, handling, storage and transport of raw materials.

• Conditioning: drying, grinding and/or densification of the biomass.

• Fractionation: necessary to isolate the main components of the LCB -cellulose, hemicellulose and lignin- for the valorization of these fractions.

• Enzymatic hydrolysis of the polysaccharides, cellulose and hemicellulose, separated from lignin, which are hydrolyzed to obtain sugars containing 5 and 6 carbon atoms.

• Biological transformation of the sugars into many products, nowadays mainly bioethanol, but also into other compounds, new biofuels and platform chemicals (Dessie et al., 2020).

A search of papers related with the valorisation of LCB as raw materials shows a continuous increase of research in the last 20 years. Other authors have found a similar evolution in the number of published papers (Wenger and Stern, 2019); although this number varies considerably depending on the database consulted or even the selected keywords. Although the use of biomass at large scale had already been proposed throughout the 20th century (Lipinsky, 1981) and the term biorefinery first appears at the end of the’ 70 decade as a consequence of the petroleum crises, the widespread use of the concept “biorefinery” as the substitution of oil as main source of fuels and chemicals is more recent. As can be seen in Fig. 2, for any kind of LCB, the published papers are concentrated in the last 15 years, when the concept of biorefinery became an important subject of interest. The papers published for any raw material are less than 500 before 2005, while the number is more than 100,000 after this date.

The LCB more employed as raw materials are three agricultural residues: straw, corn stover and sugarcane bagasse. They are the most studied, comprising almost half (49 %) of the works published in the period 2000–2020 within the selected raw materials in this study (Fig. 2a). Wheat and corn, in addition to rice, are the most cultivated cereals in the world; thus the notable interest in the valorisation of their residues; while sugarcane bagasse is associated to one of the most common crops in areas as Central and South America regions. This residue of the production of sugar has firstly been used as fuel in the sugarcane industry, secondly in the production of paper (Bhardwaj et al., 2019) and, more recently, it has been proposed to produce bioethanol in 2 G biorefineries (Ntimbani et al., 2021; Oliveira et al., 2014; Cardona et al., 2010). Another third (32 %) of the papers corresponds to the valorisation of wood species of genera eucalyptus, pine and poplar, frequently used in cellulose pulp production (Fig. 2b). The consideration of a pulp mill as a type of biorefinery has focused the attention on how to improve the value of parts of the tree, as the bark, or fractions of wood, as lignin, that are burned to generate the energy of the global process (Kazzaz and Fatehi, 2020; Van Heiningen, 2006). Lignin valorisation to high-value products, as phenols or carbon fibres, or new and non-paper uses for the cellulose with innovative applications (Bajwa et al., 2019; Kim et al., 2015) can increase the economic and environmental sustainability of the pulp industry. Other agricultural crops as giant reed (Arundo donax) or cardoon (Cynara cardunculus) and agricultural residues as vine shoots and olive tree pruning have also been studied as sources of biomass especially in Europe (Fig. 2c). Giant reed is a promising crop for energy production because it offers high biomass productivity with low fertilisation requirements (Angelini et al., 2009). The cardoon is a special case, with relevance in arid zones, as the Mediterranean region, where few crops offer good productivity (Gominho et al., 2018; Lourenço et al., 2017; Vergara et al., 2018a). Olive tree pruning is an abundant agricultural residue, concentrated in Mediterranean countries as Spain and Italy, which have been proposed as source of sugars and bio-ethanol (Cara et al., 2007; Fernandes-Klajn et al., 2018). A similar interest has received the use of vine pruning in biorefineries, due to the extension of wine production around the world (Dávila et al., 2017).

According with the previous comments, the aim and scope of the present work is to overview the state of the art of the LCB valorisation taking into account only biological technologies to transform pretreated LCB, not with a classical review orientation based on the compilation and ordering of information, but aiming at giving some guidelines regarding the selection of key technical processes. The main LCB raw materials are described, together with the analytical methods to determine their composition; the main pre-treatments trying to isolate the different constituents: cellulose, hemicellulose and lignin; the subsequent enzymatic hydrolysis of the polysaccharides and the transformation of sugars into alcohols and other chemicals by biological processes are also approached. This critical perspective will address the concept of integrated biorefineries with a particular look at biological routes to new biofuels and chemicals that are being considered as new platform chemicals.