An introduction to biofuels, foods, livestock, and the environment

An introduction to biofuels, foods, livestock, and the environment

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Yaser Dahman, Cherilyn Dignan, Asma Fiayaz, and Ahmad Chaudhry Department of Chemical Engineering, Ryerson University, Toronto, ON, Canada

13.1

Introduction

Present global conditions have led to a collective consciousness that recognizes that a modernization of the world’s energy system must be on the horizon. Climate change and energy security are at the forefront of nations worldwide, influencing social, economic, and political decisions. As such, there has been a recent necessity to research and develop innovative energy alternatives that are strong and viable contenders that have the potential to mitigate the issues of climate change and energy security. Since the industrial revolution, energy demands of the world have been primarily fulfilled through the combustion of fossil fuels. Consequently, this has contributed to the elevation of greenhouse gas (GHG) emissions, thereby resulting in a series of climate change issues specific to disruptions in weather systems, increased sea levels and coastal flooding, biodiversity loss, and human health impacts. Coupled with issues pertaining to climate change, strict attention has also been paid to the overwhelming evidence that indicates that current conventional energy reserves are depleting. Rising fossil fuel and food prices, and increasing international pressure on climate change mitigation, have intensified the search for a renewable source of energy [1]. With the theoretical knowledge and practical evidence of the implications, renewable energies have received significant attention in the past decade. However, due to the extent of climate-related events and the developing concern over finite energy sources, there is boundless global urgency to continue to develop innovative ways to address these issues. Furthermore, present renewable energies have not been sufficient solely to reduce GHG emissions and substantially reduce the dependence on fossil fuel energy. Alternative fuels derived from natural, sustainable sources have the potential to assist in these twin crises. The greenhouse gas effect explains that the Earth’s atmosphere is comprised of specific gases that have the purpose of increasing the Earth’s surface temperature by 20 C 34 C, thus making the planet habitable with a mean global temperature of 14 C [2]. These gases include carbon dioxide (CO2), nitrous oxide (NO2), ozone (O3), and methane

Biomass, Biopolymer-Based Materials, and Bioenergy. DOI: https://doi.org/10.1016/B978-0-08-102426-3.00013-8 © 2019 Elsevier Ltd. All rights reserved.

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(CH4). Due to the specific properties and chemical composition of these gases they have the capacity to emit and absorb infrared radiation. Without the presence of these gases within the atmosphere, the Earth’s surface temperature would drastically drop to 19 C, thereby inhibiting various natural processes and the existence of human and nonhuman life [3]. However, due to the rapid expansion of industrial practices specific to fossil fuel combustion, an exorbitant level of GHG has been emitted into the atmosphere, thereby resulting in a series of undesirable impacts upon the Earth. With the industrial revolution came the understanding that fossil fuels could be the answer to the world’s energy needs. Fossil fuels are comprised of products such as petroleum, coal, bitumen, natural gas, oil shales, and tar sands. Procuring these products has allowed generations of human-kind to meet energy demands specific to electricity, transportation, and heating. Furthermore, the fossil fuel industry has also supported the development and progression of nations worldwide. Economic and political support of the fossil fuel industry has also been a significant driver for its vast expansion over the course of several decades. However, due to recent scientific evidence correlating environmental degradation with fossil fuel combustion, the industry has succumbed to a global controversy. Comprehensive scientific knowledge has indicated that approximately 98% of greenhouse gas emissions released into the atmosphere are a direct product of fossil fuel combustion [2]. Another rampant concern of the fossil fuel industry is that such sources are nonrenewable, thus indicating their inevitable depletion in the near future. With global population expected to experience a growth, the demand to meet energy requirements will evidently increase. From a global perspective, heavy fossil fuel dependence has been strongly correlated with the energy sector and transportation industry. As indicated by the Intergovernmental Panel on Climate Change (IPCC), 69% of CO2 emissions are a direct product of the energy sector and 14% of CO2 emissions are a direct product of the transportation industry [4]. Canadian data conclude that 45% of emissions correspond to the stationary combustion sources and 28% of emissions correspond to the transportation industry [5]. Recognizing the primary GHG emitters, both at a global and national scale, can assist in developing the necessary mitigation strategies. Pressing concerns of atmospheric emissions have led to nations worldwide developing stringent targets to assist in GHG emission reductions. Climate targets, specifically those of the Paris Agreement, have bound numerous nations, including Canada, to reduce emissions by a certain percentage by a designated year. Since 2016, the Canadian government has pledged to reduce emissions to 30% below 2005 levels by the year 2030 [6]. The presence of such climate targets can push nations into the direction of reducing their GHG emissions. Placing nations in a binding agreement to reduce emissions will stimulate the energy and transportation industries to further develop sustainable alternatives to reduce their environmental burden via GHG emissions. From a global perspective, biofuels have proven to be a viable contender in reducing GHG emissions and addressing issues specific to energy demand.

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13.1.1 Biofuel impact on the future of food stocks A biofuel is a type of renewable and green fuel whose energy is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels like ethanol, soybean oil, and various biogases. Biofuels are being looked upon as not only an effective alternative energy source but also as a means of lowering GHG emissions [7,8]. Unlike fossil-derived fuels, biofuels are produced from natural sources such as biomass including plants, crops, and agricultural residue. Based on the specific biomass source, biofuels can be categorized into three different groups; first generation, second generation, and third generation. First-generation biofuels are those that are produced from food crops that have the capacity to directly provide sugar, starch, and vegetable oil. Second-generation biofuels are distinguishable from firstgeneration biofuels because they are produced from nonfood crops such as waste biomass, agricultural residue, and stalks of corn and wheat. Lastly, third-generation biofuels are considered a relatively new form of alternative fuel and are comprised of algae. With influential factors such as population growth and increasing energy demand coupled with the urgency to meet climate targets, it is crucial that practical, sustainable alternatives be researched, developed, and implemented. As recent biofuel research can be indicative of algae, a third-generation biofuel has received significant attention over the past few years. Relative to first- and second-generation biofuels, algal biofuels have presented a variety of promising advantages. Using algal biomass as a feedstock source has the capacity to produce a variety of biobased products including biodiesel, bioethanol, biobutanol, and biogas. Furthermore, algal biofuels also present an enticing option because of their low growth requirements, potential to improve air quality, and carbon sequestration potential [9]. Algal biomass can be considered a superior alternative relative to first- and second-generation biofuels as it eliminates many of their drawbacks. Furthermore, recent research has indicated that algae possess favorable properties, including low viscosity and low density, rendering them a more suitable lignocellulosic material [10]. Additionally, algae have the capacity to yield greater biomass yield per hectare and grow at high rates (1 3 doublings/day) [10]. However, it is important to note that continued research within the scope of algal biofuel is necessary to improve costs and energy outputs such that this specific alternative be a viable and cost-effective alternative compared to traditional fossil fuel sources.

13.2

Biofuels

With world energy consumption increasing, primary attention has been directed toward the development of carbon-neutral energy and sustainable sources to meet future needs. Biofuels are an attractive substitute to current petroleum-based fuels because they can be utilized as transportation fuels with diminutive change to

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current technologies; they also have significant potential to improve sustainability and reduce greenhouse gas emissions. Biofuels can be described as an alternative fuel that is developed from biological, natural, and renewable sources [11]. The production of biofuels relies upon sources such as plants, crops, and agricultural residue. Primary biofuels are organic material burned directly to produce energy [12]. Wood and other unprocessed plant matter fall into this category. This material is impractical for transport fuel purposes for a couple of reasons. First, burning wood, for example, in any transport vehicle today would be logistically unwieldly. Second, burning unprocessed biomass has a reputation of creating air pollution due to incomplete combustion in areas of high population density where large amounts of energy are required [13]. Based on the biomass used, biofuels can be categorized into three different groups.

13.2.1 First-generation biofuels First-generation biofuels are produced from food crops such as corn and wheat [14]. First-generation biofuels make up the majority of the biofuels used today. First-generation biodiesel and ethanol biofuels produced today also can use vegetable oils (e.g., corn oil) and animal fats as their source feedstock [15]. Therefore, there is a need to move away from relying on first-generation biofuels because their feedstock would otherwise be human food [16]. With a growing population, it is more reasonable to use human food feedstock byproducts, known as second-generation feedstock, to produce second-generation biofuels [16].

13.2.2 Second-generation biofuels Second-generation biofuels are produced from nonfood crops including the waste from food crops, agricultural residue, wood chips, and waste cooking oil [14]. Second-generation biofuel feedstock is the nonedible byproduct of food crops. For example, wheat straw from wheat production and corn husks from corn cultivation are second-generation feedstock [17]. There are advantages to using the inevitable byproduct of the agricultural industry for biofuel production; no additional fertilizer, water, or land are required to grow this feedstock. Industry does use some of this nonedible byproduct to produce animal feed, however there is a substantial amount that could also be used for biofuel production [18]. Expensive processes arguments against biofuel production from second-generation feedstock plague this biofuel pathway [19]. Regardless, second-generation biofuel research and policy has the potential to develop this biofuel pathway into a productive source of biofuel [20,17].

13.2.3 Third-generation biofuels Third-generation biofuels are those that are considered relatively new in the field and they primarily include algae as well as fast-growing trees [14]. Thirdgeneration biofuels are for the most part in their development stage as there are

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very few commercial-scale plants. Biofuels have been regarded as a sustainable alternative as they have the capacity to reduce GHG emissions by up to 90%, depending on the biomass [11]. Additionally, biofuels also have the added benefit of being blended with conventional fuels or being used entirely on their own [11]. Third-generation biofuel production methods, like second-generation biofuel production methods, are still more expensive than fossil fuel production methods. Consequently, the production of third-generation biofuels, using algal feedstock, remains today predominantly at the pilot scale [21,22]. However, algae’s ability to sequester carbon dioxide (CO2), produce relative large amounts of lipids, grow in variable conditions, and grow orders of magnitude faster than all terrestrial plants (including second-generation feedstock) make it an ideal biomass source for biofuels [23,24]. Several researchers have indicated that algae have a significant role to play in future liquid biofuel development [25,21]. Due to escalating issues such as climate change and depleting fossil fuels, biofuels have become of great importance globally within the past decade. Biofuels offer a wide variety of advantages and uses. Encompassed within their benefits is their prime capacity to reduce dependence on fossil-based fuels and to reduce GHG emissions. According to the International Energy Agency (IEA), biofuels provide the gateway for a low-carbon, petroleum-free, transport sector [26]. Moreover, biofuels also improve global energy security, reduce dependency on oil imports, and promote domestic and rural sources of income [26]. Simultaneously, biofuels also offer extensive applications for the transportation, industrial, and energy sectors. Because of the extended range of biomass available for production, numerous products can be developed through sustainable and natural means. Through a comprehensive biorefinery process, an array of biologically based products can be created to provide alternative fuel options for transport and electrification purposes. Furthermore, value-added industrial products and solvents can also be developed through the respective biorefinery process. Although biofuels have received significant attention in recent years, they have however played an important part in the past. The concept of utilizing biologically and naturally occurring products found in nature began in the early 1700s when lamps relied upon vegetable oils, whale oil, and lard oil [14]. Following this, ethanol blended with turpentine was utilized for illumination. Subsequently, ethanol began to be of interest within various nations worldwide including Germany, the United States, and the United Kingdom. Research and development initiatives in Germany with respect to ethanol-fueled trucks and automobiles began in 1899 and subsequently further promotion of ethanol-fuel household appliances in 1902 [14]. Consequently, this spread a global interest in biofuels, specific to that of ethanol, in other nations including the United States, France, Italy, and Spain. Due to the scarcity of oil resources, the interest in biofuels began to experience growth during the early 1900s, particularly in the United Kingdom and France [14]. During the 1920s and 1930s, the United States began to incorporate biologically based products, such as ethanol, in their transportation industry. Henry Ford provided the necessary momentum to facilitate the promotion of chemurgy, which ultimately focused on crop utilization to produce industrial materials [14]. With

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this concept, by 1943, 77% of synthetic rubber produced in the United States incorporated ethanol [14]. On-going research and development initiatives have made it possible to not only bring ethanol fuel to a larger scale but to also explore various other alternatives. As such, alternatives such as biodiesel have also experienced significant growth and utilization. For instance, between 2008 and 2011, the United States’ production of ethanol coupled with biodiesel increased more than 40% [14]. Additionally, the strong theoretical knowledge to support the growth and progression of the biofuels industry provided the set-up and implementation of second-generation biofuels as the first cellulosic ethanol commercial-scale plant was fully operational in 2013 [14]. Through successive scientific research and industrial practices, a wide variety of biologically based products have come into play. These include bioethanol, biodiesel, biogas, and biobutanol. Although current biofuel markets have relied upon first- and second-generation biofuels, shifting from first- and second-generation biofuels has been on the horizon. Escalating issues of climate change and energy security issues are constantly urging the transition to a more sustainable and economically viable biofuel format. While once recognized as an evolutionary technological development, firstgeneration biofuels are the focal point of many controversies to date, including the ever-pressing food vs. fuel debate. The ethical issues surrounding first-generation biofuels along with environmental issues pertaining to land use changes paved the way for second-generation biofuels. Despite the notion that second-generation biofuels have overcome the primary challenges associated with first-generation biofuels they have however presented some difficulties. Second-generation biofuels still require a vast amount of land for production, which is accompanied with economic impacts in terms of high agricultural costs. Furthermore, the cellulosic material comprised in second-generation feedstocks is not consistent and, in many cases, insufficient to produce adequate supplies to become a viable competitor in the market [27]. Therefore, third-generation biofuels in the form of algal biofuels can have the potential to fill in the many inadequacies of previous-generation biofuels.

13.2.4 Biobutanol Butanol is an alcohol comprised of a four-carbon structure with the chemical formula C4H10O. It has been primarily used as a solvent found in paints and can also be utilized as a fuel. Butanol formed from plant material is often referred to as biobutanol. It is chemically similar to butanol produced from petroleum. Butanol, because of its longer hydrocarbon chain, has 30% more energy content than ethanol and is closer to gasoline in properties. Butanol, in its pure form, can be blended in any concentration with gasoline, unlike ethanol which can only be blended up to 85% [28]. Initial production of biologically based butanol, referred to as biobutanol, began in the late 1800s and early 1900s. Louis Pasteur discovered the acetone butanol ethanol (ABE) pathway in which butanol is produced. Significant reliance upon biobutanol commenced during prohibition in the United States as there was an amyl

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alcohol scarcity for the paint industry [28]. Biobutanol served as a replacement for the amyl alcohol and subsequently numerous ABE fermentation plants were created to provide a biobutanol supply to the pain industry. Although having a strong reliance upon biobutanol until 1950, the paint industry then turned to a cheaper alternative, butanol produced from chemical synthetic processes relying about petrochemical feedstocks [29]. Although the interest and subsequent production of biobutanol experienced a lag following competitive and cheaper alternatives, it is now of great interest and high priority due to merging issues of climate change and energy crisis. The introduction of butanol into existing car engines as a combustible fuel requires minimal modification to the engine technology. Biobutanol has been demonstrated to work in some vehicles designed for use with gasoline without any modification [30]. The product’s lower vapor pressure makes it safer to handle. Butanol is not hygroscopic (water absorbent), which allows blending with gasoline at a refinery way ahead of storage and distribution. This is in stark contrast to ethanol, which requires blending to occur shortly before distribution due to its hygroscopic nature [28].

13.2.5 Biofuel policies and canadian government goals Mainstream society used biofuels as the primary fuel source before the 1950s rise of the petrochemical industry. Thus, an example of such an industry, on a smaller scale, is not a foreign concept and a lack of technology is not the reason for biofuel production impediment. North America was aware that anthropogenic carbon dioxide emissions would induce global warming in the 1960s [31]. Thus, both the scientific and political community were aware in the 1970s that modifying the trajectory of the petrochemical industry and distribution system was the main impediment to a biofuel industry [32]. Consider, in the 1940s and 1950s, however, just after World War Two, that many large petrochemical companies of today, such as KOCH, were just beginning business development [33]. The lack of political will to alter the economy and dissuade small business development is likely the cause of the political community’s choice to overlook biofuels. This choice, in hindsight was irresponsible, but would have seemed completely understandable at the time. Similar public feelings associated with fostering small local business development are prominent today, and political figures are aware of public opinion. Unfortunately, today, production chains that support traditional fuel production also support the production of several other chemicals, thus making the conglomerate very difficult to modify now without significant investment and overall system modification. For example, the major objective of American refineries in the last 50 years has been to increase the octane rating of gasoline [34]. This objective gave rise to a multitude of different processes, including oligomerization, alkylation, and catalytic reforming [35]. These processes are not only used in consort with crude oil distillation and refining but with several other product developments. Naphtha, one of the products of the crude oil distillation process, is subject to catalytic reforming to produce benzene, toluene, and xylenes [35]. Naphtha is also subject to anaerobic steam cracking to produce olefins such as propylene and ethylene [35].

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These products all play a role in the production of not only octane-enhancing additives but in the production of plastics (e.g., polyethylene and polypropylene), synthetic fiber precursors (e.g., acrylonitrile), and industrial chemicals (e.g., glycols) [34,35]. Products, such as gasoline, plastics, detergents, fibers, pesticides, tires, shampoo, and sunscreen are based on seven raw materials derived from petroleum and natural gas: ethylene, propylene, C4 olefins (i.e., butenes and butadienes), benzene, toluene, xylenes, and methane, all using well-established production processes [36]. Ninety five percent of the 500 billion pounds of chemical products produced each year come from the processes based on the production of these seven raw materials [37]. Government policy has a large and significant impact on product development [37]. Neat RD is produced internationally and available in California because California’s LCFS (Low Carbon Fuel Standard) guidelines, initiated in 2007, support and mandate an increase in sustainable fuels [38]. First-generation biofuels have seen incredible growth in the last decade, largely due to policy goals [39]. In Canada, current biofuel production and consumption are highest in regions where government has played a large role in initiating and fostering relationships with the renewable energy industry [40]. The Current Canadian Federal government renewable fuel mandate, implemented in 2010 and modified in 2011, requires 5% renewable content in gasoline and 2% renewable content in biodiesel sold in Canada [41]. On top of these federal regulations, provinces have created additional regulation to assist in the production and use of renewable fuels. For example, the Ontario Environmental Protection Act requires 4% total bio-based volume in fossil diesel, and requires a 70% reduction in emissions associated with the production of fossil diesel blends by 2017 [42,43]. Even though these regulations are a step in the right direction toward reducing the GHG impact of the Canadian transport system, key exemptions, such as fuel for aircraft, competition vehicles, trains, heating oil, and military use, as well as exemptions for some provinces make the federal and provincial regulations not as effective [44]. For example, Alberta’s regulations do not apply to fuel produced and consumed within industrial operations, such as volumes of diesel fuels used in oil sand operations [44]. The exemptions have the potential to make policy GHG targets difficult to achieve. Since the cost of producing or purchasing renewable content for both gasoline and petrol diesel (e.g., bioethanol and BD) is more than the cost of petrol diesel production, federal programs such as ecoEnergy of Biofuels, Next Gen Biofuels Fund, and AAFC Growing Forward have been developed. These programs provide subsidies to Canadian biorefineries and suppliers to facilitate the renewable fuel mandate. Some of these federal subsidies include production tax credits to support Canadian manufacture of BD, interest free loans, and grants. The subsidies also target both the energy and agricultural sectors (small facilities) to support local ownership. The overall effectiveness of these specific subsidies mentioned in the previous paragraph is still vague. Programs initiating these subsidies were developed in 2006 [45]. Most of the programs listed above have ended in 2017 or are finishing in

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2018. However, one way of measuring the cost-effectiveness of these subsidies is by comparing the cost of the subsidies intended to prevent the use of carbon dioxide emissions with the amount it cost to purchase carbon offsets. The cost per ton of carbon dioxide equivalent offset is between $4.23 US and $33.83 US on the Chicago and European Climate Exchanges [45]. From an economic perspective, it is thus eight to 137 times more expensive to avoid carbon dioxide emissions by producing biofuel [45]. Comparatively, the way to measure a nation’s overall GHG reduction effectiveness is by tracking GHG emissions and material use. A study completed by Moorhouse and Wolinetz [44] for Clean Energy Canada, found that Canada had increasingly reduced GHG emissions from 2011 to 2014, thus achieving a total of 4.3 MT of CO2e in 2014. This reduction in GHG emissions was calculated by using the 3.9 million cubic meters of renewable fuel Canadians used instead of fossil fuels that year (5% of the total fuel use in Canada) [44]. Ontario, the Canadian province with the largest population, has also created policies to reduce GHG emissions. More recently, Ontario passed the Climate Change Mitigation and Low Carbon Economy Act in 2016 and developed a Climate Change Action Plan (CCAP) adopted in 2017 that focuses on fighting climate change, reducing GHG emissions, and transitioning to a low-carbon economy [46]. The cap and trade program, introduced in 2017 as part of the CCAP, puts a price on carbon. It is too early to determine the impact of the cap and trade program. It has been suggested that there could be some benefits to Ontario leaving the Western Climate Initiative (WCI) cap and trade program with California and opting for a carbon tax similar to British Columbia [47]. However, even if the Ontario government were to implement a carbon tax similar to the carbon tax in BC, Seraj [39] indicates that unless this tax far exceeded BC’s threshold tax rate, the tax would not create the incentive required to reduce fossil fuel use. Seraj [39] indicates that the level of carbon tax required to perpetuate a shift in practice is around $2,000/tCO2e. British Columbia’s carbon tax is currently at its highest rate at $30/ tCO2e [39]. Seraj [39] argued that $30/tCO2e does not dissuade the use of fossil fuels nor does it make biofuel ventures economical. Presently, the focus of Ontario’s February 2018 CleanTech strategy as part of the CCAP prioritizes (1) energy generation and storage, (2) energy infrastructure, (3) bio-products and bio-chemicals, and (4) water and wastewater. The purpose of the strategy is to push Ontario’s technology sector to develop more clean technology (i.e., sustainable technology); this includes less GHG-intensive processes. By definition, clean technology is any process, product, or service that reduces environmental impacts by reducing pollution (e.g., reducing GHG), more efficiently using natural resources and/or the use of significantly less energy than current industry standard. Therefore, companies classified as clean technology companies need to fulfill at least one of these requirements. In order to foster the clean technology sector, the Ontario strategy focuses on developing programs and regulations that allow potential green companies to develop and thrive. The government of Ontario plans to leverage existing programs and develop new ones in order to improve information flow to new clean technology companies, provide access to needed capital, and facilitate more efficient regulatory frameworks.

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Current renewable fuel regulations, implemented by the Canadian government in 2010, play a key role in supporting the current Canadian government’s overall renewable fuel strategy [41]. The current Canadian government’s sustainable technology development strategy supports the overall objective of reducing GHG emissions and this includes the development and use of biofuels. For example, Sustainable Development Technology Canada (SDTC) is a re-established organization funded by the Government of Canada to support CleanTech projects and coach fledging companies that meet CleanTech standards [41]. This organization would financially and socially support a company’s project plans if this plan proves it reduces the GHG impact compared to the process used to produce a pre-existing redundant function. The above-mentioned regulations, strategy, and programs support the Canadian 2015 goal of a reduction of 17% GHG emissions from 2005 levels by 2020. Canadian 2005 total GHG emissions were 738 MT CO2e [46]. Increases in GHG emissions between 1990 and 2015, according to Environment and Climate Change Canada, were due to an increase in mining, upstream oil and gas production, and transport. Hence, Canadian 2015 GHG emissions were not that much less at 722 MT CO2e. A decrease in GHG emissions here was attributed to a reduction in public electricity and heat production utilities [46]. Thus, transport has not yet played a significant role in the reduction of Canada’s GHG impact. If Canada is going to reach 17% of the 2005 GHG level (125 MT CO2e) by 2020, a reduction of more than 16 MT CO2e/year (given 738 722) is required and the transport sector should play a role in this reduction. Canada also joined the Asia Pacific Partnership in 2005 with the intent of working with other nations and private sector companies to meet national goals for energy security, air pollution reduction, and climate change reduction [48]. The focus of the partnership is to expand investment in and trade of sustainable technologies, specifically energy technologies including biofuels. Moorhouse and Wolinetz [44] indicated that in order for Canada to achieve substantial decarbonization in support of international partnerships, biofuels would need to account for 20% of fuel use in Canada by 2030 and 90% by 2050.

13.3

Biomass for biofuel production

Biofuel production is reliant upon a variety of materials that are renewable and naturally occurring in the environment. Such materials are referred to as biomass and they are described as plant-based materials. According to the United Nations Framework Convention on Climate Change (UNFCC), biomass is recognized as a nonfossilized and biodegradable organic substance that stems from plants as well as animals and microorganisms. Further analysis of biomass has rendered two distinct categories: virgin and waste. Virgin biomass refers to biomass incorporated through terrestrial and aquatic means such as forest biomass, grasses, energy crops, and algae [49]. In contrast waste biomass refers to biomass acquired through municipal,

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agricultural, forestry, and industrial applications [49]. Biomass is considered to be the oldest source for fuel energy. Heating and energy demands until the 19th century were primarily fulfilled through biomass in the form of firewood and charcoal [50]. With technological changes and subsequent advancements, fossil-based alternatives, such as coal and oil, became the alterative choice. However, within the past decade, significant attention has been placed on the expansion of biomass production for the purpose of fuel energy. Rampant concerns of climate change and energy security issues have led to the prospect of utilizing biomass for the production of transport fuels. Due to the diverse availability of biomass, a variety of different biofuels can be rendered possible. Through a series of biomass conversion pathways, both virgin and waste biomass can yield different forms of biofuels; ranging from biodiesel, bioethanol, and biogas [51]. As already stated, first-generation biofuels are derived from conventional technologies as they contain simple constituents that can be broken down easily. In contrast, second- and third-generation biofuels are derived from complex materials which require specific pretreatment to break down the biologically available compounds. Biomasses utilized to produce second- and thirdgeneration biofuels stem from lignocellulosic material. Such material is comprised of lignin, cellulose, and hemicellulose. Within this material, the three main components are bound strongly to each other as a result of noncovalent forces and covalent crosslinks which ultimately lead to its complex structure [52]. As such, bioavailability and subsequent production of biofuels are highly dependent on the fragmentation and disruption of the lignocellulosic composition. Cellulose has been defined as one of the most abundant polysaccharides on Earth [53]. Its physical composition is representative of a glucose polymer linked by B-1,4 glycosidic bonds and its basic building block is cellobiose [54]. Further characteristics of cellulose are correlated with the degree of polymerization, which is essentially the number of glucose units that comprise one polymer molecule [55]. The range of this degree is most commonly within 800 10,000 units, however the degree can extend upwards of 17,000 units in wood pulp [55]. In simple terms, cellulose contains carbohydrate chains which can be appropriately broken to render possible sugar conversion to develop alcoholic fuel. In scientific terms, such chains are developed as a result of B-1,4 glucosidic bonds [55]. Upon the exterior of such bonds are hydroxides which subsequently enable strong intermolecular hydrogen bonds between hydroxyl groups of adjacent molecules situated upon parallel chains [56]. The structure of cellulose can either be crystalline or noncrystalline. The crystalline structure arises due to the fusion of several polymer chains, which in turn builds microfibers [55]. Hemicellulose is recognized as a homopolymer and is a form of heterogeneous polysaccharide [57]. This particular polysaccharide is the linking material between the cellulose and lignin. Additionally, what sets hemicellulose apart from cellulose is that it consists of monosaccharides. Physical components of hemicellulose incorporate short, highly branched polymers of six-carbon sugars (glucose, mannose, and galactose) and five-carbon sugars (xylose, arabinose, and glucose) [51]. Such short branches are also amorphous, which ultimately provides hemicellulose with the

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capacity to be partially soluble in water [55]. Moreover, physical characteristics of hemicellulose also indicate that its backbone is comprised of a homopolymer or a heteropolymer; generally consisting of single sugars or a mixture of different sugars; respectively [55]. Xylose is considered to be the most important sugar found in hemicellulose. Amongst hardwood terrestrial plants, xylose is the main constituent of the backbone chain which provides the lack of crystalline structure [53]. Lignin can be simply described as the binding material which keeps cellulose and hemicellulose in a compressed structure [58]. Furthermore, upon examination of its physical structure it is quite different than cellulose and hemicellulose. It is a three-dimensional polyphenolic network comprised of dimethoxylated, monomethoxylated, and nonmethoxylated phenylpropanoid units [55]. Lignin is also hydrophobic and highly impervious toward chemical and biological degradation. Moreover, it is located central to the lamella and acts as the binding material for the cell wall and plant cells [55]. Subsequently, lignin develops an amorphous mix embedded within it hemicellulose and cellulose fibrils. Lignin content and composition vary amongst different plant groups, rendering possible simple to complex pretreatment options. Successful lignin fragmentation is key to accessing biologically active compounds of cellulose and hemicellulose in order for adequate sugar conversion and subsequently production of biofuels. Pretreatment of lignocellulosic material can remove lignin, which can be converted to value-added products such as dyes, pesticides, and industrial plastics [58]. Following the initial stages of pretreatment is the saccharification stage. During saccharification, the polysaccharide structure of the carbohydrates is broken down into soluble sugars through the process of hydrolysis [14,59]. After this, the fermentation and distillation processes begin. During fermentation, bacteria, such as yeast, provide the necessary enzymes to assist in breaking down the soluble sugars and converting them to alcohol [14,55]. Lastly, distillation is the process in which the desired products are extracted from the alcohol [14,55]. This process is widely utilized in the industry. However, a modification of the process incorporates the combination of the saccharification and fermentation stage into a single step known as simultaneous saccharification and fermentation (SSF). This modification of combining the saccharification and fermentation processes into a single step has demonstrated improvement in productivity and a greater yield in energy output [59].

13.3.1 Pretreatment of biomass As a simplified approach to producing biofuels states, the pretreatment stage is one of the initial stages. However, it is deemed a highly significant stage as it directly corresponds to the overall yield of energy output. Successful disintegration and fragmentation of lignin exposes the cellulose and hemicellulose which can be converted into the simple sugars utilized for alcoholic fuel production. The greater rate of lignin fragmentation and exposure of cellulose and hemicellulose directly results in greater energy output. During the pretreatment stage, the biomass is subject to a variety of tailored methods incorporating chemical, physical, and thermal methods to break down the structure of the lignocellulosic material [58].

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Although pretreatment is considered an essential aspect of the entire biorefinery process as it is directly correlated with higher extraction yields, it is however a costly process. However, advancements in pretreatment processes to minimize financial costs are a strong element of biofuel research. Subsequently, a variety of pretreatment methods exist to date that all serve the purpose of simultaneously improving extraction yield and the bioavailability of biologically active compounds within lignocellulosic material. Pretreatment techniques can be categorized into the following four groups: thermal, mechanical, chemical, and biological. Thermal pretreatments can be further categorized into the following: thermal, hydrothermal, and thermal treatment with steam explosion. Broadly put, all forms of thermal treatments incorporate the solubilization of algal biomass through the application of heat [60]. Mechanical treatments can also be further categorized into the following groups: ultrasound and microwave. Such treatments of algal biomass involve the disruption of cell walls through active physical force [60]. Chemical treatments can be categorized as acidic or alkali. Chemical treatments utilize chemicals to achieve the solubilization of algal biomass [60]. Lastly, biological treatments incorporate the utilization of enzymes to disrupt the cell wall structure of algal biomass. Thermal pretreatments of biomass involve the utilization of heat to disrupt the cell wall structure. The application of heat for pretreatment of algae as well as other forms of lignocellulosic material has been widely used in literature and in industrial practices. Thermal pretreatments incorporate high temperatures within the range of 50 C 270 C [61]. It has been addressed that choosing the appropriate temperature and subsequent form of thermal pretreatment is subject to the substrate in use and its associated characteristics. However, further studies have indicated optimal temperatures within the range of 150 C 180 C and temperatures above 250 C should be strictly avoided as cell wall damage is likely to occur [60]. The application of heat has been considered an effective method to improve the bioavailability of the lignocellulosic material of biomass as it increases the solubilization of particulate organic fractions, thus allowing the partial hydrolysis of polymeric organic molecules [62]. Additionally, heat can also play an integral role in also disrupting the lignin and hemicellulose structures of algal biomass. The heat actively interrupts the hydrogen bonds, which are situated in the crystalline structure of cellulose and lignocellulose complexes, which subsequently causes the algal biomass to swell [62]. Thermal pretreatments are very commonly used as they also provide a means for sanitization of the respective feedstock to remove any pathogens. Furthermore, thermal pretreatments are also utilized in conjunction with chemical treatments to enhance the effectiveness. Thermal pretreatments can be distinguished from the other two forms of heat-based pretreatments because of their relatively lower temperatures. Such pretreatments are defined as those that are below 100 C and under atmospheric pressure [60,62]. Due to this, this form of heat-based pretreatment if often referred to as low-temperature pretreatment. With the requirement for low temperatures, this form of pretreatment is advantageous because it requires a lower energy demand relative to high-temperature pretreatments. Upon examination, it has been addressed that the relative performance of thermal pretreatment is subject to temperature and exposure time [60]. Furthermore,

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when examining different sources of algal biomass, it was suggested that algae grown in wastewater resulted in higher solubilization of lignocellulosic material with temperatures within the range of 75 C 95 C as oppose to lower temperatures of 55 C. Moreover, a longer exposure time of 15 hours as oppose to 10 hours was also correlated with higher solubilization of lignocellulosic material. Contrary to the above-mentioned thermal pretreatments, hydrothermal pretreatments incorporate temperatures above 100 C with a gradual increase in atmospheric pressure [60]. Hydrothermal pretreatments also involve generally shorter exposure times within the range of 15 30 minutes [60]. Within hydrothermal pretreatments the cell walls of algal biomass are broken down and the starch is gelatinized [63]. This form of heat-based pretreatment is widely used in the biorefinery processes of numerous lignocellulosic materials, such as algae, as well as wheat straw, sugarcane, and softwoods [63]. Hydrothermal pretreatments are subject to reaction catalysts which ultimately speed up the rate of lignocellulosic solubilization. Acid, alkaline, or water reaction catalysts can be applied. Utilizing water as a reaction catalyzer has been regarded as a promising technique as it promotes a more sustainable process. Relative to chemical reaction catalysts, water is an environmentally friendly process that results in no toxic waste [63]. Although the utilization of water as a reaction catalyst adheres to a more environmentally friendly process, it does however require significantly higher temperatures and pressures which can only be achieved through the additional use of advanced, costly technologies. Consequently, this increases production costs. It has been indicated that achieving higher temperatures and pressures is strongly correlated with higher levels of gelatinization of starch [63]. However, exceeding temperature and pressure thresholds can result in the degradation of starch granules [63]. Heat-based pretreatment with steam explosion is a method that incorporates the increasing of pressure with temperature typically above 160 C [60]. Following pretreatment, the pressure can be gradually or rapidly decreased to ambient air pressure, which is defined as steam explosion [60]. Steam explosion is a commonly used pretreatment technique in the industry and is referred to as thermal hydrolysis. During this technique, the respective biomass, such as algae, is placed in a vessel and following this the vessel is subject to high temperatures (.160 C) and pressures (B6 bars) for an exposure time of within 10 30 minutes. Following this application, the pressure is decreased either gradually or suddenly to achieve ambient air pressure and subsequently rendering possible steam explosion. The so-called steam explosion results in the disintegration of the cell wall and biomass [60,64]. Although thermal hydrolysis is a common technique used in the industry, it is still however only under investigation at the laboratory scale for algal biomass. Research has indicated that optimal parameters of thermal hydrolysis incorporate high temperatures in the range of 140 C 180 C and 3 10 bars of pressures [60]. The mechanical pretreatment of algal biomass involves the disruption of cell walls through active physical force. The purpose of mechanical pretreatment methods is to reduce particle size and crystallinity [62]. Consequently, this renders possible an increase in the surface-to-volume ratio that can ultimately shorten and progress the solubilization process [62]. A variety of mechanical pretreatment

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options have been used in the literature and in industry. Such techniques can be categorized into those that rely upon relatively simple techniques and those that rely upon more advanced technologies such as ultrasound and microwave pretreatment methods. All forms of mechanical pretreatments can be utilized independently or can be combined with other pretreatment options to enhance the overall pretreatment process. Simple mechanical pretreatment options incorporate chipping, milling, shredding, grinding, and cutting. Upon examination, chipping is the most widely used form of mechanical pretreatment method for larger lignocellulosic biomass [62]. In contrast, grinding and milling are methods that are commonly used for biomass that is compact and rely upon tools such as shredders, knives, scissors, and hammers [62]. Milling is a common mechanical pretreatment method used for algal biomass [60]. This particular mechanical pretreatment method can be categorized into for different forms: ball milling, knife milling, disc milling, and hammer milling. With respect to ball milling, the crystalline structure of cellulose is reduced along with the polymerization degree of cellulose and particle size [62]. Moreover, the bulk density also increases, which allows for the pretreatment of more concentrated biomass and subsequently reduces the reactor volume [62]. A knife mill can be defined as a rotary piece of equipment comprised of four to six knives that are posted on a rotor. The rotor moves at a continuous speed of 500 600 rpm, whilst simultaneously cutting the feedstock until it is at an appropriate size to pass through a sizing screen [62]. A disc mill is a mechanical pretreatment method in which the biomass is supplied through an orifice coaxially with a rotation axis. A moving disc is pressed against a stationary one which brings in the feedstock to be crushed through pressure and fictional forces [62]. Lastly, the hammer mill is comprised of a rotor that possesses a set of attached hammers that simultaneously push the material in the appropriate plate to be crushed and shredded [62]. Complex mechanical pretreatment methods involve ultrasound and microwave technologies. Ultrasound mechanical pretreatment involves the utilization of rapid compressions and decompressions of sonic waves [60]. Ultrasound can be defined as acoustic sound energy that resonates in the form of waves which happen to be above a frequency of the human range [62]. This form of advanced mechanical pretreatment method calls for the continuous cycles of rapid compressions and decompressions which generate cavitation. Cavitation is known as the formation of specified regions with liquid vapor inside the cell wall which can also be known as microbubbles [60,62]. With the appropriate ultrasound intensity, these microbubbles are minimized in size and subsequently implode, thereby damaging the cell wall [60,62]. Through this process, heat, free radicals, high pressure, and shockwaves are produced [60,62]. Parameters of ultrasound pretreatment methods can be defined according to the substrate being used. Acoustic sound energy has been widely used within the realm of organic chemistry to further facilitate chemical reactions and to increase the bioavailability of biologically active compounds [63]. Considering this, a variety of parameters have been tested to determine the effects upon the solubilization of lignocellulosic material. Ultrasound frequencies can be applied at the lower threshold of less than 50 kHz and at the higher threshold of

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greater than 50 kHz [60]. Each threshold results in specified desirable characteristics. For instance, the lower threshold of less than 50 kHz renders mechanical effects, whereas the higher threshold of greater than 50 kHz renders the formation of free radicals [60]. In addition to the frequency, other parameters that are incorporated in ultrasound pretreatment are the output power and exposure time. The output power is correlated with the biomass concentration. Microwave-assisted mechanical pretreatment methods are also actively used to disrupt the cell walls of algal biomass. This form of mechanical pretreatment utilizes short waves of electromagnetic energy with frequencies in the range of 300 MHz to 300 GHz [60,62]. Such frequencies are a direct result of the rapidly oscillating electric field of a polar or dielectric material [60,62,65]. Subsequently, these frequencies are responsible for an increase in kinetic energy that results in the boiling state for water [62,65]. This process corresponds with the polarization of macromolecules, which subsequently changes the structure of protein and more importantly rapidly generates heat and pressure [62,65]. This rapid change in heat and pressure ultimately causes swelling of the biomass and forces the biologically active compounds out of their stationary state [62,65]. Similar to ultrasound, microwave-assisted pretreatments can be tailored according to the substrate used in the biorefinery processes. Output power and exposure times, along with frequency, are the main controllable parameters of this mechanical pretreatment process. Furthermore, with the optimal parameters, the native crystallinity of the starch is vanished and subsequently an entirely amorphous material is produced [63,66]. Additionally, microwave pretreatments have also been successfully utilized in conjunction with other pretreatment methods such as enzymatic pretreatments. Microwave-assisted pretreatments actively promote starch digestibility, which ultimately enhances the accessibility of enzymes to reach the respective substrate [63,67]. Chemical pretreatment methods integrate the utilization of chemical agents to facilitate cell wall disruption and subsequent solubilization of lignocellulosic material. A variety of chemical agents have been used to date to render possible lignin disruption and exposure of cellulose and hemicellulose. Despite this, when compared with thermal and mechanical pretreatment methods, chemical pretreatments have been used less. Chemical agents can be categorized into acidic or alkali (basic) groups. In both cases, the chemical agents used serve the purposes of solubilizing polymers, thus allowing enzyme attack in the subsequent biorefinery process [60]. Upon examination, it can be concluded that alkali pretreatment is best suited for lignin removal, whereas acidic pretreatment works to solubilize cellulose and hemicellulose [62]. To further enhance the effects of chemical pretreatments it is generally used in conjunction with thermal pretreatments. Within chemical pretreatment methods, the chemical utilization of alkali methods involves sodium, ammonium, calcium, and potassium hydroxides [62]. Such chemical agents have proven to be more effective if the lignin content of the algal biomass is relatively higher than cellulose and hemicellulose content. The role of alkaline chemical agents during the pretreatment process is the saponification and cleavage of lignin carbohydrate linkages, which directly corresponds to

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increased porosity and surface area [62]. Subsequently, this decreases the degree of polymerization and crystallinity of the algal biomass [62]. A beneficial aspect of alkali pretreatment is that it has the capacity to minimize the drop in pH during acidogenesis [62]. Acidic pretreatment options are also utilized within chemical pretreatments of algal biomass. Amongst acidic pretreatment options the most common chemical agent is sulfuric acid, however other acids have been used including nitric acid, hydrochloric acid, and acetic acid [62]. Dilute and concentrated acids can be utilized for the pretreatment of algal biomass. One of the primary benefits of utilizing acidic pretreatment is that it has the capacity to penetrate through the lignin without need for prior pretreatment of the biomass [68]. Furthermore, this pretreatment subsequently can break down the cellulose and hemicellulose polymers to produce individual sugar molecules [68]. When comparing dilute and concentrated acidic pretreatments, dilute (2% 3%) options are preferred because they require less acid, thus rendering less toxic waste. However, with dilute acidic pretreatment, thermal assistance is required to enhance the solubilization of the lignocellulosic material. Higher temperatures can facilitate acceptable rates of cellulose conversion to simple sugars [68]. In contrast, concentrated acidic pretreatments (10% 30%) require low temperatures, which subsequently produce high hydrolysis yields of cellulose [68]. However, upon comparison with dilute treatment options, acidic treatment options hinder the possibility of an environmentally friendly process. With greater amounts of acid required, corrosion problems can arise [68]. The biological pretreatment of algal biomass relies upon specific enzymes. Such enzymes have the ultimate purpose of breaking down the compounds of the cell wall into smaller compounds with a lower molecular weight [60]. Subsequently, this can facilitate anaerobic digestion of the biologically active compounds through bacteria [60]. A variety of parameters influence the effectiveness of the enzymatic hydrolysis of algal biomass. These include enzyme dose, exposure time, and temperature [60]. Considering the associated high costs of enzymes, enzymatic hydrolysis is regarded as one of the drawbacks of pursuing this at a larger scale for algal biomass in the biorefinery process. Different enzymes can work to breakdown the cell wall structure. Among these are cellulase, xylanase, and β-glucosidase. Cellulase is an enzyme that targets cellulose and facilitates its conversion to glucose [69]. Xylanase is an enzyme that works to target the polysaccharide structure of xylose, which happens to be a component of hemicellulose [69]. Lastly, the enzyme regarded as β-glucosidase further facilitates the breakdown of cellulase in the lignocellulosic material of algae [69]. Enzymatic pretreatments can be combined with different pretreatment methods including mechanical, thermal, and chemical. It has been indicated that with regards to the Rhizoclonium sp. of algae, methane yield increased by 20% when mechanical pretreatment was combined with enzymatic pretreatments [60]. Deploying enzymatic pretreatments has also been regarded as environmentally friendly as they require low energy, result in a higher yield of fermentable sugars, operate under relatively light conditions, and result in no corrosive problems [70]. Upon choosing

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the appropriate enzyme or mixture of enzymes to target compounds of the cell wall, the following must be considered: (1) the compounds that comprise the algae cell wall; (2) the optimal ranges for temperature, pH, exposure time, and the substrate enzyme relationship as per enzyme activation; and (3) the associated costs of utilizing enzymes [70].

13.3.2 Pretreatment and hydrolysis of cellulosic feedstock The complex intertwined structure associated with the lignocellulosic biomass possesses a technological challenge in processing of lignin, cellulose, and hemicellulose. The process of breaking the bonds between the lignin and other carbohydrates using a chemical or thermochemical operation is called pretreatment. Pretreatment is an important step to increase the bioavailability of sugar monomers from cellulosic biomass. Several pretreatment methods like steam explosion, ammonia fiber explosion, dilute acid hydrolysis, lime treatment, carbon dioxide explosion, alkaline hydrolysis, oxidative delignification, pulsed-electric-field pretreatment, biological pretreatment, and many others have been employed in past studies [71]. If pretreatment was not performed, the hydrolysis enzymes would face difficulty accessing the polysaccharide chains embedded within the lignin polymer. Steam pretreatment coupled with dilute acid constitutes the best strategy to convert all hemicellulose into monosaccharides and oligosaccharides. While the wet-oxidation and alkaline methods are relatively more effective in solubilizing lignin, they leave behind insoluble hemicellulose in polymeric form [72]. It is possible that during acid pretreatment, a complex mixture of bacterial inhibitors, such as furfural and hydroxymethyl furfural, would be generated; however, the inhibitors could be substrate-specific [73]. Furfural is not an inhibitor to Clostridium beijerinckii, but it affects the growth of other microorganisms and the biobutanol yield [74]. Earlier experiments in the lab determined that furfural inhibitor production during the SSF was quantified to be in the range of 0.5 0.6 g/L for all strains [18]. These concentrations were significantly lower than the inhibitory levels of 1 g/L, above which furfural activity is known to negatively affect fermentation [75]. The low concentration of furfural can be explained by the lower temperature of 120 C at which pretreatment was performed and due to the use of dilute acid [76]. Studies have shown that the concentration of inhibitors increases with an increase in pretreatment temperature [77]. Therefore, this study focused on the effect of other inhibitors like butanol while neglecting the effect of insignificant concentrations of furfural.

13.3.3 Consolidation of bioprocess Biomass processing schemes involving enzymatic or microbial hydrolysis commonly involve four biologically mediated transformations: the production of saccharolytic enzymes (cellulases and hemicellulases); the hydrolysis of carbohydrate components present in pretreated biomass to sugars; the fermentation of hexose sugars (glucose, mannose, and galactose); and the fermentation of pentose sugars (xylose and arabinose) [78]. Usually, after pretreatment, enzymatic hydrolysis is

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used to convert the cellulose and hemicellulose into monomeric sugars. The sugars are then fermented to solvents (ethanol, biobutanol, and others) using microbes (yeast or bacteria). When enzymatic hydrolysis and fermentation are performed sequentially, the process is referred to as separate hydrolysis and fermentation (SHF). However, the two process steps can be performed simultaneously, that is, simultaneous saccharification and fermentation (SSF). In a variant of the SSF process, simultaneous saccharification and cofermentation (SSCF), the hydrolyzed hemicellulose and the solid cellulose are not separated after pretreatment, allowing the hemicellulose sugars to be converted to ethanol (EtOH) concurrently with SSF of the cellulose [79]. Kumar et al. [71] have proposed an interesting way to integrate the feedstock transport with the ethanol production facility and the saccharification process named simultaneous transport and saccharification. The authors consider that the enzymatic hydrolysis of corn stover can be carried out in pipelines during its transport; the hydrolyzed corn stover could directly enter the ethanol fermentation plant, saving about 0.2 US cents/L EtOH. This could be a potential method of transport for biobutanol production as well. Processes in which cellulosic biomass is fermented to desired products in one step without adding externally produced enzymes are of obvious appeal. Consolidated bioprocessing (CBP) is widely recognized as the ultimate configuration for low-cost hydrolysis and fermentation of cellulosic biomass [80]. However, the critical problem with CBP (SSF/SSCF) is the difference in the optimum temperatures of enzymatic hydrolysis and the fermentation [78].

13.4

Agricultural biomass of feedstock

Recent technologies for agricultural biomass conversion (through their hydrolysate solutions) have proven that agricultural wastes have enough carbon sources to produce value-added bio-based products [81]. Agriculture biomass is composed mainly of three bio-based chemicals called cellulose (35 48% dry wt), hemicellulose (22 48% dry wt), and lignin (15 27% dry wt) [82,83]. Together, they are called lignocellulose, a composite material of rigid cellulose fibers embedded in a crosslinked matrix of lignin and hemicellulose that bind the fibers. Lignocellulose material is by necessity resistant to physical, chemical, and biological attack, but it is of interest to bio-refining because the cellulose and hemicellulose can be broken down through a hydrolysis process to produce fermentable, simple sugars. Cellulose is a very large polymer molecule composed of many hundreds or thousands of glucose molecules (polysaccharide). Unlike starch, the glucose monomers of cellulose are linked together through bonds resulting in tightly packed and highly crystalline structures that are resistant to hydrolysis. Therefore, pretreatment of lignocellulosic biomass before enzymatic hydrolysis is a vital step. Hemicellulose consists of short, highly branched, chains of sugars. It contains five-carbon sugars (usually D-xylose and L-arabinose) and six-carbon sugars (D-galactose, D-glucose,

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and D-mannose) and uronic acid. Hemicellulose is amorphous and relatively easy to hydrolyze to its constituent sugars. While cellulose and hemicellulose contribute to the amount of fermentable sugars for ethanol production, products of lignin degradation are recognized as a potential source of microbial inhibitors [74].

13.4.1 Hemp as lignocellulosic feedstock Throughout history, cannabis has been used for food, medicine, and fiber. Cannabis can be categorized into two different groups [84]. One is known as marijuana and the other is known as hemp. The difference between the two groups is the amount of tetrahydrocannabinol (THC) and cannabidiol (CBD) contained within the product [84]. Hemp is mainly made up of CBD. In North America, hemp can only be considered as “hemp” when the concentration of THC is below 0.3% [84]. Hemp is the strongest available natural fiber available. It is found in over 25,000 different products including medicinal compounds and textiles [84]. Hemp is known be a good source of protein, starch, and oil [84]. The plant itself can adapt and grow easily in different geographic climates as illustrated in Fig. 13.1 [85]. Canada, China, and North Korea are among the largest producers of hemp [84]. Other important advantages include the ability to resistance pest and diseases [86]. Hemp is made up a woody core and bast fibers [84]. In 1998, a study conducted by Garcia [87] investigated the individual components of hemp bast fibers and woody core. The study found that bast fibers provided a higher amount of cellulose, hemicellulose, pectin, and lignin compared to woody core [87]. Cellulose provides structural stability (strength and stiffness) to the fiber [88]. A general structure of hemp fibers is provided in Fig. 13.2. Fig. 13.2 illustrates that cellulose is mainly located towards the core of the fiber (bottom layer), while hemicellulose and lignin cover the surface of the hemp bast fibers (top layer) [88]. There are a few differences between hemicellulose, lignin, and cellulose. Hemicellulose is characterized as having amorphous regions [88]. The amorphous areas are made up hydroxyl groups weakly bonded with the fiber structure [88].

Figure 13.1 Countries that are currently producing hemp. Source: Adapted from Schluttenhofer C, Yuan L. Challenges towards revitalizing hemp: a multifaceted crop. Trends Plant Sci 2017;22:917 929. doi: 10.1016/j.tplants.2017.08.004.

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Figure 13.2 General physical structure of hemp fibers. Source: Adapted from Kabir M, Wang H, Lau K, Cardona F. Effects of chemical treatments on hemp fibre structure. Appl Surf Sci 2013;276:13 23. doi: 10.1016/j.apsusc.2013.02.086.

Since these amorphous regions are towards the surface of the hemp bast fibers, they are more easily available to interact with [88]. Cellulose, on the other hand, is made up of both crystalline and amorphous regions [88]. The crystalline regions are covered by the amorphous regions, making it difficult to interact with the cellulose. The hydroxyl groups in the hemicellulose and lignin provide an access point for chemicals to penetrate the fiber surface [88]. Pretreatment is used to separate the lignin and hemicellulose from the rest of the fiber. This allows cellulose to be exposed and easily interacted with for future use [88]. This is done by reducing the number of hydroxyl groups that exist in the amorphous region [88].

13.5

Algae as biomass feedstock

Third-generation biofuels, predominately algae, bring new and desirable characteristics, whilst also eliminating the drawbacks associated with their predecessors. Only being in its infancy stage, algal biomass has been the focal point of numerous scientific studies pertaining to biofuel production [89]. Relative to other biomass feedstocks, algae have proven to possess unwavering characteristics that make them an enticing alternative. Furthermore, algal biomass is rich in carbohydrates, protein, and lipids, which ultimately provide biologically based products including biodiesel, bioethanol, biogas, and biobutanol [89]. Algae can also be considered an attractive alternative as they have low growth requirements, sequester carbon, and improve air quality [9]. When comparing algae with second-generation biofuels,

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primarily consisting of terrestrial plants, algae provide an array of benefits. For one, algal biomass provides a higher biomass yield per hectare and grow at higher rates relative to terrestrial plants [9,90]. Furthermore, algae are known for their low growth requirements and are not restricted to seasonal production [9,90]. With great pressure to increase GHG reduction potential and to develop a more sustainable biorefinery process, it is also equally important to note that algae also offer a higher CO2 sequestration rate. Moreover, algal biomass growth also assists in wastewater bioremediation as it utilizes phosphorous and nitrogen from wastewater sources [90].

13.5.1 Microalgae versus macroalgae There are an estimated 300,000 species of algae classified into two main groups: microalgae and macroalgae [16]. Microalgae are unicellular eukaryotic microphytes (i.e., aquatic plants) and by virtue of their name, contain membrane-bound organelles with plastids that contain chlorophyll in order to carry out photosynthesis [16]. Fig. 13.3 presents the basic organelles found in eukaryotic microalgae. Microalgae live in both fresh water and marine ecosystems [16] and their size ranges between 3 and 30 μm [91]. Macroalgae, also known as seaweed, are multicellular eukaryotic macrophytes that are also photosynthetically inclined and live in both freshwater and marine ecosystems [16]. The cell walls and structural components that keep organelles in place in each cell contain most of the algae’s lignocellulosic material.

Figure 13.3 Basic components (i.e., organelles) of eukaryotic microalgae Edwards M. Algae 101 part six: algal classification. Algae Industry Magazine 2010. http://www.algaeindustrymagazine. com/part-six-algal-classification/.

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Figure 13.4 Macroalgae from plant to cellular level. Source: Adapted from Quiroz-Castan˜eda R, Folch-Mallol J. Hydrolysis of biomass mediated by cellulases for the production of sugars. Sustain degrad lignocell biomass Tech Applicat Commer 2013. doi:10.5772/53719 [95].

Both micro- and macroalgae have a much higher growth rate and CO2 fixation than any other plant on Earth [93]. The photosynthetic efficiency of algae is on average 13%, whereas terrestrial plants have photosynthetic efficiencies of between 1% and 2% [14]. Both micro- and macroalgae can also accumulate high levels of lipids. As biodiesel predominantly consists of mono-alkyl esters of long-chain fatty acids, both macro- and microalgae are ideal for biodiesel feedstock [23]. There are limited reports concerning the production of biofuels from macroalgae [16]. It is suspected that higher production costs and harvesting difficulties make using macroalgae less appealing in general and less appealing than microalgae [94]. Suganya et al. [16] indicate that growing macroalgae is suited to a limited area, such as coastline in relatively stagnant waters. Harvesting a reliable quantity is also difficult because of climatic variability [16]. There is also the possibility that microalgae have been studied more because they have been grown in small capacities for niche markets (i.e., pharmaceutical and aquaculture) [16] (Fig. 13.4).

13.5.2 Microalgal physical makeup Microalgae contain carbohydrates, lipids, and proteins. Carbohydrates, also called “lignocellulosic biomass,” make up most of microalgae’s physical structure. Lignocellulosic biomass in microalgae consists of cellulose, hemicellulose, and lignin, as seen in Fig. 13.5. For biofuel production, cellulosic and hemicellulosic biomasses are broken down into monomers and fermented to produce alcohols [96,97]. Fatty acids are carboxylic acids that have long hydrocarbon chains (normally between 4 and 22 carbons in length) with a carboxylic acid group at one end [99]. The basic form of a fatty acid has a hydrocarbon chain completely saturated with hydrogen (i.e., no double bonds). When double bonds are present in the fatty acid hydrocarbon chain, either one or more, these fatty acids are called monounsaturated

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Figure 13.5 Schematic of the location and structure of lignin (web), hemicellulose (casing), and cellulose (inside hemicellulose) in lignocellulosic biomass. Source: Adapted from Liu W-J, Jiang H, Yu H-Q. Thermochemical conversion of lignin to functional materials: a review and future directions. Green Chem 2015;17:4888 4907. doi:10.1039/c5gc01054c [98].

fatty acids (MUFAs) or polyunsaturated fatty acids (PUFAs), respectively. Saturated fatty acids make superior biodiesel, thus algal species for use as biomass for biodiesel production are selected accordingly [100]. The most abundant saturated fatty acid in microalgae is palmitic acid (16:0), followed by stearic acid (18:0), while of the monounsaturated types, oleic acid proves most abundant (18:1 n 9) [101].

13.5.3 Advantages of algae Algal biofuel has the potential to be the fuel of the future and meet energy demands. Algal biomass is an ideal feedstock due to its attractive properties like high productivity, reduced land use, etc., which are explained in detail further in this section. 1. High productivity As mentioned earlier, algae have high growth rates. Algae are the fastestgrowing plants in the world. They don’t require herbicides and fungicides either, providing a cleaner growing environment and fewer emissions [94]. 2. Feedstock and land use One of the main drawbacks of first-generation fuels is the use of food crops for energy production. Using corn and other food products as a feedstock takes away

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from the amount of food available for consumption. Algae do not require the use of fertile land which could otherwise be growing food crops, and therefore has no impact on the food supply or food costs. In fact, depending on the cultivation methods, algae could be grown on land which does not serve any other purpose, such as brownfields, thereby increasing the carbon density of the land upon which it is grown [102]. 3. Water use Another great advantage of algae is that it doesn’t require huge amounts of fresh water to grow in [103]. In fact, it can be grown in seawater and even wastewater. Algae can be especially grown in wastewater to remove pollutants like nitrogen and phosphorus which are hard to remove [21]. This not only satisfies the nutritional needs of the algae but cleans the water as well. 4. Waste to energy There are a number of industries that create large quantities of CO2 as a byproduct of operation. Efforts have been made to integrate the algae biofuel creation process into these industries to stop the CO2 emissions at the source. “The microalgae Botryococcus braunii 765 is one strain which has shown that it is able to thrive in flue gas CO2 concentrations ranging from 2% to 20%” [104]. This makes it feasible for use at industrial plants. For example, Pond Biofuels has established an operation that utilizes the CO2 generated during the concrete manufacturing process as feedstock for algal growth. As for power plants fueled by coal, the literature states that conjoined algal biofuel production could lead to a net greenhouse gas avoidance of 26.3% [105]. Even though algae have these advantages, the main barrier to commercialization of algal biofuel remains the cost of cultivation. However, algae produce a variety of different products and a good approach is to use the algae left over from fuel generation for coproducts. The biomass left over after the oil has been extracted could also be used for applications such as livestock feed, fertilizer, or electricity production via direct burning or digester gas methane combustion [105]. Also, after extraction of lipids, the biomass can be used to produce biobutanol as another source of biofuels.

13.5.4 Algae cultivation Although not specific to biofuel production from algae, it is important to understand the basics of algae cultivation systems as this forms an important consideration in respect to algae costs. Cultivation of algae is considered a highly significant step in producing a promising and renewable hydrocarbon feedstock for biofuel production. Compared with the growth of other biomass for biofuel production, such as a variety of terrestrial plants, algae have the capacity to yield higher energy values, eliminate the reliance upon arable land, and have the capability of using marginal sources of water that are not specifically designated for irrigation or drinking purposes [106]. Despite the limited resurgence of algal biofuel, there are a variety of methods to cultivate algae. The systems to cultivate algal growth can range from open ponds, closed photobioreactors, as well as hybrid systems comprised of

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different elements of various systems [90]. Systems which use artificial light increase the energy demand, thus reducing the ERoI gained; only systems using natural light are considered useful for large-scale algae growth for commercial use. Seaweed has historically been harvested from natural populations or collected after washing up on shore. To a much lesser extent, a few microalgae have also been harvested from natural lakes by indigenous populations. However, these practices are unlikely to contribute significantly to algal feedstock supply. Artificial cultivation systems are mainly of three types: open, closed, and sea-based cultivation systems.

13.5.4.1 Open cultivation system Cultivation of algae in open ponds has been extensively studied. Open ponds can be categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds. One of the major advantages of open ponds is that they are easier to construct and operate than most closed systems. However, major limitations in open ponds include poor light utilization by the cells, evaporative losses, diffusion of CO2 to the atmosphere, and requirement of large areas of land. Furthermore, contamination by predators and other fast-growing heterotrophs has restricted the commercial production of algae in open culture systems to only those organisms that can grow under extreme conditions. Also, due to inefficient stirring mechanisms in open cultivation systems, their mass transfer rates are very poor, resulting in low biomass productivity [107]. The ponds in which the algae are cultivated are usually what are called the “raceway ponds.” In these ponds, the algae, water, and nutrients circulate around a racetrack. With paddlewheels providing the flow, algae are kept suspended in the water, and are circulated back to the surface on a regular frequency. The ponds are usually kept shallow because the algae need to be exposed to sunlight, and sunlight can only penetrate the pond water to a limited depth. The ponds are operated in a continuous manner, with CO2 and nutrients being constantly fed to the ponds, while algae-containing water is removed at the other end. The biggest advantage of these open ponds is their simplicity, resulting in low production costs and low operating costs. While this is indeed the simplest of all the growing techniques, it has some drawbacks owing to the fact that the environment in and around the pond is not completely under control. Bad weather can stunt algae growth. Contamination from strains of bacteria or other outside organisms often results in undesirable species taking over the desired algae growing in the pond. The water in which the algae grow also has to be kept at a certain temperature, which can be difficult to maintain. Another drawback is the uneven light intensity and distribution within the pond [108].

13.5.4.2 Closed cultivation systems Many of the issues with an open cultivation system can be resolved by using a closed system. In a closed system, the configuration usually consists of transparent

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containers/tubes through which the culture medium flows and, since they are transparent, light can be provided to the algae, to provide ideal growing conditions. Also, CO2 can be supplied from various different sources like cement factories, etc. A photobioreactor is defined as a closed reactor which is utilized for the inside growth of prototroph or photobiological reactions to occur. In contrast to open ponds, photobioreactors have better control over the growing environment, leading to higher yields [25]. Photobioreactors have various reactor geometries, that is, tubular reactors can be vertical or horizontal and they can also be inclined. Usually flat type photobioreactors are preferred because of low-energy consumption, high mass transfer capacity, reduction of oxygen increases, and high photosynthetic efficiency compared to other bioreactors. Flat plate bioreactors have illuminated surfaces and are made out of transparent materials so they utilize solid light to a maximum degree. Photobioreactors can run in a batch or continuous process. For an industrial approach, continuous bioreactors are preferred because they provide more control, growth rates can be maintained, for longer periods [109].

13.5.4.3 Sea-based cultivation system Microalgae cultivation has been discussed in the above two sections, however algae can be grown in seawater, which is the traditional mode of growing seaweed. Due to the availability of large tracts of seawater, cultivation of seaweed for various byproducts could be very valuable. Seaweed should be produced in floating cultivation systems across hundreds of hectares. Most seaweed needs a support to hook to, which in practice means that the cultivation system must contain a network of support, usually ropes. The amount of construction material could be drastically reduced when free-floating seaweed (like some Sargassum species) is cultivated, a structure to contain the colony would then be needed. Sea-based systems are less well developed than land-based systems, although currently R&D initiatives have been undertaken. The system for seaweed cultivation around the world, such as in China, Chile (a major exporter of seaweed), etc. has not changed much, hence there is scope for research and development there and options for modernization have been identified [110].

13.5.5 Algae-based bioenergy products There are numerous fuel options that can be produced from algae. Over the years, algae have become an increasingly popular feedstock choice not only for its sustainable characteristics but its overall capacity to produce a wide variety of biologically based products. Allowing such biomass to undergo different energy conversion paths can yield possible different products. The energy conversion pathways usually consist of two different categories, biochemical conversion and thermochemical conversion. The most popular fuel produced is biodiesel, which is made from esterification of lipids. Algae as a source of biodiesel have gained a lot of popularity since they can contain potentially over 80% total lipids (while rapeseed plants, for instance, contain about 6% lipids) [111]. Under normal growth conditions the lipid

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concentration is lower (,40%) and a high oil content is always associated with very low yields. Production of the various lipids can be stimulated under stress conditions, for example, insufficient nitrogen availability. Stress conditions enhance lipid production but also decrease the biomass content, restricting the use for other purposes. Some of the bioenergy products are explained in greater detail here: 1. Hydrocarbons: Botryococcus species of algae do not produce lipids as other species do, making them unsuitable for biodiesel production, but they do produce long-chain hydrocarbons. These can be processed and refined in the same manner as conventional petroleum [112]. The disadvantage of this species is the extremely slow growth rate. 2. Ethanol: ethanol can be produced from starch-containing feedstock as well as the cellulosic and hemicellulosic components of algae, present mainly in the cell wall. Algae contain low levels of cellulose and hemicellulose as compared to other feedstocks, however, they also contain very low amounts of lignin. Also, algae are easier to breakdown than other cellulosic feedstocks, reducing the energy required for the process [113]. 3. Biobutanol: cellulosic biomass and starch present in algae can also be converted to biobutanol using the acetone butanol ethanol pathway. A recent study in this field has shown this to be a promising product produced from algae growing mostly in wastewater [114]. 4. Biogas: anaerobic digestion converts organic material into biogas that contains about 60% 70% biomethane, while the rest is mainly CO2, which can be fed back to the algae. A main advantage is that this process can use wet biomass, reducing the need for drying. Another advantage is that the nutrients contained in the digested biomass can be recovered from the liquid and solid phase [115]. However, the high cost of feedstock makes this not commercially viable, but using algae from wastewater can mitigate this. 5. Hydrogen: some green algae can be manipulated to photosynthetically generate H2 gas. This is done by a two-stage photosynthesis process in a closed sulfurdeprived environment. The addition of ferrous hydrogenase caused anaerobiosis in the growth medium, a condition that automatically stimulated H2 production by the algae Chlymadomonas reinhardtii. However, this is extremely expensive and not enough yield is shown to be considered for commercial use [116].

13.5.6 Biorefinery of algae Although algae possess many potential advantages, such as the ability to produce petroleum fuel substitutes without the need for fresh water or arable land [97], profitable production has yet to be realized. This is mainly due to issues of low energy return on investment (ERoI) and huge resource requirements for large-scale production facilities [117]. The ERoI of biodiesel from algae has been calculated to be around 0.22 [118]. This value is very low and leads to more energy being used than produced, making it a very inefficient source of energy. Thus, the low ERoI of algal fuel makes it unable to compete with other fuels [118].

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Continued research and development could yield innovations to address these challenges. We propose the use of oil waste, the biomass left after extraction of oil, as feedstock for the development of transportation fuels, such as butanol. The biomass contains cellulose and hemicellulose and can be used by microorganisms to produce other products, thus increasing the ERoI and eventually making the process economically viable [119]. This is a great use of the biomass, which is normally wasted from the extraction of algal oil. The present work studies the carbohydrate concentrations left in the algal biomass after extraction of oil and its feasibility to generate transportation fuel, especially biobutanol.

13.6 Conclusion With growing issues of climate change and energy security, it is critical that current energy resources be revisited, and greater lengths be taken to achieve stronger, more viable, renewable and alternative fuels. Biofuels have certainly been a growing factor within the transport industry. Their unwavering capacity to deliver sustainable outputs, especially in the midst of rampant anthropogenic climate change issues and depleting fossil fuels, has allowed them to become an extremely attractive opportunity. However, as the current global biofuel market can be indicative of, the capacity of biofuels to a feasible competitor against fossil-based fuels has fallen short. Ethical and environmental issues coupled with economic viability have raised many concerns for the future of first- and second-generation biofuels. However, third-generation biofuels, comprised of algae, indeed hold a promising future for biofuels. Furthermore, as this report has discussed, algae have great potential for revitalizing the world’s energy outlook in a highly economic and sustainable manner.

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