Classification of Waste Biomass and Biogenic Materials: Comparison
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Please note this is a comparison between Version 2 by Peter Tang and Version 1 by Sonil Nanda.

Waste biomasses such as oil seed crops, lignocellulosic materials (e.g., agricultural and forest residues), microalgae, energy crops, manure, food waste and organic fraction of municipal solid waste have a huge potential to provide energy and value-added products via different conversion technologies.

  • biofuels
  • pyrolysis
  • torrefaction
  • liquefaction
  • gasification

1. Lignocellulosic Biomass

Lignocellulosic biomass is a sustainable, cost-effective and carbon-neutral feedstock that characteristically contains cellulose (40–60 wt.%), hemicellulose (20–40 wt.%) and lignin (10–24 wt.%). Some of the sources of lignocellulosic biomass are agricultural crop residues, forestry biomass and energy crops. Agricultural wastes are the residues produced by the harvesting and processing of agricultural vegetative crops. Crop residues are generally divided into two types such as field residues and agro-food processing residues. Since these agro-residues are non-edible, they pose no competition to the food supply or fertile arable lands. Regardless of the environmental, societal, and profitability of woody biomass in different valuable products for household, industrial, construction and power sectors, most of the forestry biomass remain underutilized. Some barricades that restrain the utilization of agricultural and forest biomass for the production of bioenergy are associated with their seasonal, geographical and climatic variations, which determine their availability and cost.
Research interest in energy crops has augmented worldwide due to their diversity, rapid growth, high production rate, the potential to fix CO2 during photosynthesis, cost-effectiveness and resilience to grow on marginal quality soils [8][1]. Therefore, their cultivation can encompass a substantial probability to biofuel industries to fulfill the clean energy necessity. Some common energy crops include switchgrass, elephant grass, timothy grass, Miscanthus and hybrid poplar.

2. Oilseed Crops

Plant-based oils have traditionally been a significant agrarian commodity. Several species of plants contain oil in the form of fatty acids, lipids, triglycerides and triacylglycerol. These components are stored in plant seeds and cells as reserves of carbon and energy for the improvement of seedlings. The structural similarity of triacylglycerol with long-chain hydrocarbons forms a foundation for a viable alternative to hydrocarbon-based products. Moreover, owing to the physicochemical properties of fatty acids, the non-edible plant-based oils are used to produce biodiesel and components as an integral part of paints, coatings, lubricants and inks. There is a great demand for plant oils in agricultural, nutraceuticals, cosmeceutical, pharmaceutical, food and biorefining industries.
Many plant-based oils are consumed as edible oils in cooking and food processing such as canola oil, vegetable oil, mustard oil, olive oil, sunflower oil, soybean oil, corn oil, almond oil, grapeseed oil and coconut oil. Non-edible plant-based oils are extracted from microalgae, Ailanthus altissima (heaven tree), Azadirachta indica (Neem), Jatropha, Linum usitatissimum (flax), Madhuca longifolia (Mahua), Pongamia pinnata (Karanja), Ricinus communis (Castor), Sapindus mukorossi (Soapnut), Toona sinensis (Juss), Vernicia fordii (Tung), rubber seed, silk cotton tree, etc. [9][2]. The fraction of oil fluctuates noticeably in different oilseed crops. However, it can be inferred that there is scope for genetically engineering the oilseed plants to generate a greater concentration of oils. Using oil seeds for biofuels and biodiesel production is an excellent way to replace petroleum-based sources.

3. Municipal Solid Waste

The waste materials (e.g., garbage, recyclable and non-recyclable residues) obtained from municipalities including households are known as municipal solid waste (MSW). MSW could also include Industrial, Commercial and Institutional (ICI) waste such as those discarded residues from businesses, large industries, hospitals, institutions, schools, colleges and universities. It should be mentioned that the composition of MSW and ICI varies depending on the origin, production patterns, household income and geographical location [10][3]. MSW contains items such as kitchen and household materials, plastic and rubber, metals, paper and cardboards, inert materials, electronic wastes, etc. [11][4]. MSW also consists of biodegradable and non-biodegradable fractions. It should be mentioned that about 15% of the total waste generated in municipalities is recycled while the remaining is disposed of in landfills or dumped in open sites [12][5].
The worldwide surge in MSW generation is primarily due to the rise in population, urbanization and industrialization. The global production of MSW has surpassed 1.2 billion tonnes per year with a projection to exceed 2 billion tonnes annually by 2025 [13][6]. It is projected that in some developing countries and other parts of the world, the MSW generation could reach or exceed that of developed nations without proper regulations and provisions for landfilling and waste recycling facilities [14][7]. The growth in the rate of generation of MSW is also related to the change in food habits, consumption patterns, consumer behavior and standards of living in rural and urban areas.
MSW generation creates severe environmental pollution when unmanaged. Moreover, its conversion into value-added products could provide a solution to the challenges of energy shortage and sustainable waste management. Various ways by which MSW can be disposed of, recycled or valorized into energy resources are through landfilling, composting, anaerobic digestion, pyrolysis and gasification [11][4]. The incineration of 1 ton of MSW could emit 1.3 tons of CO2 equivalent emissions, which is similar to the amount of CO2 emissions from petroleum-based power plants [15][8]. In addition, incineration of MSW emits a considerable amount of pollutants such as particulate matter and fly ash into the atmosphere, making it an unsustainable waste management practice. On the other hand, the fly and bottom ash produced from the incineration of MSW has been proven to contain heavy metals posing risks to ecosystems [16][9].
The disposal of MSW in landfills is preferred by many municipalities globally for the burial of non-recyclable wastes. Although promising, MSW disposal in landfills faces challenges such as groundwater pollution from landfill leachate and methane gas emissions. MSW landfill leachate exhibits chronic and acute toxicity and often permeates into groundwater biomagnification. Moreover, leachate could also contaminate the flow of water streams [17][10]. Energy production from MSW helps in minimizing pollution and could facilitate the economic development of a nation in terms of waste management and strengthening energy security.

4. Food Waste

Food waste, a component of the organic fraction of MSW, refers to the organic and biodegradable waste produced from various sources including food processing plants, restaurants, kitchens and households. A large amount of food waste is produced annually due to food processing and consumption. Food waste could also be generated because of overproduction, damage to food items including fruits and vegetables by microorganisms, pests and insects, overwhelming purchases and delayed consumption [18][11]. Approximately 1.3 billion tonnes of food such as processed meat, dairy products, fruits, and vegetable are lost along the food supply chain every year [19][12].
Landfilling and incineration are not feasible waste management practices for food waste. Landfilling of food waste can lead to the emissions of methane, a more potent GHG than CO2. On the other hand, incineration or combustion is suitable for dry biomass. Hence, high-moisture containing food waste may result in greater energy requirements for the incinerator leading to high operating costs. In such a scenario, anaerobic digestion of food waste is a reasonable alternative to producing biogas (or biomethane) through biomethanation by methanogenic bacteria [20][13].
Food waste consists of carbohydrates, organic acids, fatty acids, lipids, proteins and cellulose. Moreover, the carbohydrates present in food waste could undergo hydrolysis to produce oligosaccharides and monosaccharides suitable for biological conversion [19][12]. Owing to its organic composition, food waste can also be a crucial resource to produce bioethanol [21][14], biobutanol [22][15] and biohydrogen [23][16] through fermentation. As biohydrogen is gaining popularity, food waste can prove to be an eco-friendly and cost-effective feedstock for its production through photo/dark fermentation and gasification. Thermochemically, food waste can also be converted to produce bio-oil and biochar by pyrolysis [24][17] and hydrogen-rich syngas by hydrothermal gasification [18][11]. Food waste has also shown promising results for biochar and activated carbon production [25][18]. The parameters such as the composition of food waste, pretreatment methods and processing parameters influence the production of biofuels.

5. Animal Manure

Animal manure refers to the metabolic and waste by-products of livestock and poultry farming. Manure is a valuable material containing organic matter and nutrients essential for the cultivation of crops. Moreover, animal manure could also contain different types of pathogens posing ecological risks. Typically, animal manure contains metabolic waste or feces, waste feed and waste feedwater. Despite being a major source of agricultural nutrients, livestock manure can also contribute to the emission of GHGs such as CH4 by microbial decomposition [26][19]. It should be mentioned that greenhouse gas emission from animal manure accounts for 10% of the overall emissions from agricultural production [27][20].
Some other traditional manure treatment includes composting and vermicomposting [27][20]. These treatments are very common in developing countries because of their simplicity and cost-effectiveness. Furthermore, composting also ensures the availability of nutrients to plants. Composting also leads to an increase in the aeration and water infiltration of clay soils. Animal manure could be valorized by several methods such as anaerobic digestion [28][21], dark fermentation [29][22], fermentation [30][23], pyrolysis [31][24], hydrothermal liquefaction [32][25], hydrothermal gasification [33][26] and torrefaction [34][27] to produce biomethane, biohydrogen, bioethanol, bio-oil, bio-crude oil, syngas and torrefied biomass, respectively. The digestate left behind after the anaerobic digestion of manure could be used as feedstock for biochar, bio-oil and syngas production through pyrolysis, liquefaction and gasification, respectively.
Animal manure has shown significant potential for biofuels and biochemicals production via thermochemical and biological conversion processes. Nanda et al. [33][26] showed that horse manure is an effective feedstock for hydrogen production via hydrothermal gasification. In another study, chicken manure was used as feedstock for bioethanol production via co-anaerobic digestion with ethanol plant effluent [35][28]. A mixture of poultry manure and Eucalyptus wood was used for hydrogen production via catalytic hydrothermal gasification [36][29].

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