Renewable Energy Sources – Biomass
What Is Biomass?
Biomass is a biological material derived from plants and other living organisms. Biomass is one of the primary renewable, eco-friendly, and abundantly available energy sources. Biomass can be directly used as a sole fuel or converted into biofuels such as biogas or biodiesel. The estimation of bioenergy potential is the assessment of the amount of energy available in the biomass sources. The utilization of bioenergy could supplement fossil fuels and reduce global greenhouse gas emissions. But the question is how much bioenergy could help. Research to assess bioenergy potential can provide a complete understanding of biomass types and quantities, and potential bioenergy distribution. Hence, such data will also help to identify how much bioenergy will substitute for fossil fuel and fulfill energy demands (Long et al. 2013).
The European Commission defines biomass types as agriculture residues, crop residues, forestry, and municipal solid waste. Crop residues are widely accepted biomass sources and are used for energy production (Erb, Haberl, and Plutzar 2012). A great deal of work has been carried out on alternative biofuel crops to generate biodiesel worldwide (Kumar and Sharma 2011). But choosing a low-cost feedstock is a critical problem to minimize the overall production cost. The feedstock should be selected based on availability and raw material cost (Atabani et al. 2012). The land availability, soil conditions, and local climate determine the feedstock availability (Silitonga et al. 2013). Biodiesel feedstocks are categorized into food-based feedstock and nonedible feedstock.
However, researchers are focusing on the non-edible crops for energy production due to food versus fuel and land degradation issues. The non-edible feedstock, waste frying oil, and animal fat are considered second-generation feedstock. Many researchers have studied algae as non-edible feedstock, but few researchers have categorized it as a separate feedstock (No 2011). However, it has to considered as an emerging source to produce biodiesel because it has more oil content than vegetable oil feedstock and it can be cultivated on a large scale in a short time (Bhuiya et al. 2016).
The main source of biodiesel production is edible vegetable oil. At present, over 95% of the world’s biodiesel is made from edible vegetable oils. Among these, rapeseed is the predominant source for biodiesel; it accounted for 84%, followed by sunflower oil (13%), palm oil (1%), soybean oil, and others (2%). Nevertheless, the utilization of edible oils is related to important habitat issues such as deforestation, depletion of soil resources, and the use of certain agricultural land. However, the price of edible oil has increased significantly, which will eventually affect the sustainability of edible oil-based fuels. At the beginning of the biodiesel era the use of edible feedstock for biodiesel production was very widespread. The key benefits of first-generation feedstocks are the quality of crops and the comparatively simple conversion process (Singh et al. 2020). The risk of food supply constraint is the major drawback to the use of such feedstocks, which raises the cost of food products (Aransiola et al. 2014). Moreover, edible oils are not produced in adequate amounts in many countries of the world to satisfy local human food needs and thus must be imported from other places (Bhuiya et al. 2016). Barriers to the production of biodiesel from edible feedstock also include adaptability to environmental conditions, high costs, and restricted area of cultivation. These shortcomings have forced consumers to turn to alternative sources for the production of biodiesel (Tariq, Ali, and Khalid 2012).
Non-edible oils are promising potential alternative sources of energy to meet future energy demand. Researchers are focusing on non-edible oil resources because they are readily available in the world and can be cultivated on wasteland. Moreover, they are not suitable for human use, and by-products are more eco-friendly and more economical compared to edible oils (Demirbas 2009). All over the world, an enormous variety of non-edible plant trees are available. Azam, Waris, and Nahar (2005) found that 75 non-edible feedstocks have more than 30% oil content. Among these, 26 feedstocks are identified as a promising source to produce biodiesel.
The use of non-edible vegetable oils instead of edible oils is the best choice for biodiesel production because it is not suitable for human consumption, owing to the presence of toxins (Bankovic-Ilic, Stamenkovic, and Veljkovic 2012). The potential of non-edible feedstocks can be utilized according to their availability to produce biodiesel. Widely used non-edible feedstock sources are jatropha, pongamia, castor, mahua, rubber seed, Calophyllum inophyllum, neem, Simarouba glauca, jojoba, Thevetia peruviana Merrill, etc. These feedstocks are abundantly available all over the world and are low-cost as compared to edible feedstocks (Atabani et al. 2013). Detailed descriptions and biological aspects of a few non-edible feedstocks are presented below.
The jatropha plant is widely available in India, South America, Southeast Asia, and Africa. It is capable of growing in semi-arid and low-rainfall regions (Kibazohi and Sangwan 2011). The productivity data for these plant species are, to some extent, limited and indefinite. However, the seed productivity of the jatropha plant varies from 0.1 to 15 t/ha/year (Kumar and Sharma 2011). Many researchers have tried to increase jatropha productivity under various excess salt and drought conditions (Achten et al. 2010; Kumar, Sharma, and Mishra 2010). The maximum oil content that can be extracted from the seed is 40-60%. Seed oil is used in many applications such as making soap, lighting, biodiesel production, and lubricants. Jatropha oil has attracted attention in the alternative energy field due to its higher hydrocarbon levels. Jatropha seed oil is largely comprised of oleic acid (42%), lin- oleic acid (35%), palmitic acid (14%), and stearic acid (6%). The percentage of the chemical composition may vary with region and soil conditions (Kumar and Sharma 2008).
The pongamia tree is a short tree with a curved trunk and a wide crown of expanding or hanging branches. The plant is native to India and Myanmar but it also grows in Sri Lanka, Pakistan, Nepal, Bangladesh, and Southeast Asia (Atabani et al. 2013). The seed productivity of a single tree is 9-90 kg. The pongamia plant is considered as a nitrogen-fixing tree (Karmee and Chadha 2005). The seed contains 30-40 wt.% of oil and is of 0.77 to 1.11 cm in length and 0.69 to 0.92 cm in width (Mukta, Murthy, and Sripal 2009). This seed oil has many medicinal properties. Moreover, it has been getting attention in the biodiesel industry. The chemical composition of pongamia oil/feedstock may vary by region (Kumar and Sharma 2011).
Calophyllum inophyllum, also known as polanga, is an ornamental, evergreen tree from Indian, Malaysian, Indonesian, and Philippine tropical regions (Pinzi et al. 2009). It grows to a height of 25 m and produces a slightly toxic fruit containing a single, large seed. A shell (endocarp) and a thin, 3- to 5-mm layer of pulp covers the single, large seed. Oil yields were recorded at 2000 kg/ha per unit land area (Hathurusingha, Ashwath, and Subedi 2011; Venkanna and Venkataramana Reddy 2009). The seed productivity of a single tree is 100-200 fruits/kg. The seed is not suitable for human use because of its high acidity, i.e., 44 mg KOH/g of oil. The seed oil is mainly comprised of unsaturated fatty acids such as 38.3 wt.% of linoleic acid and 34.1 wt.% of oleic acids (No 2011).
Simmondsia (also known as jojoba) is a perennial shrub, native to Mexico, California, and Arizona’s Mojave and Sonoran Deserts. Jojoba seed has a peculiar lipid content in the seeds which varies between 45 and 55 wt.%, as opposed to triglycerides in the form of long-chain esters of fatty acids and alcohols (wax esters) (Canoira et al. 2006). Because of the peculiar structure of jojoba wax, methanolysis produces a product comprised of a mixture of fatty acid methyl ester and long-chain alcohols, since removing such materials is troublesome. A few physiochemical properties of jojoba biodiesel are poor as compared with biodiesel prepared from other feedstocks because the kinematic viscosity is 11.82 mm2/s at 40°C and the cold filter plugging point value is 4°C. The cold filter plugging point value can be improved to -14°C by removing the alcohol content (Kumar and Sharma 2011; Bouaid et al. 2007).
Mahua is a 20 m-tall tropical tree primarily found in the northern and eastern plains and forests of India (Demirbas 2009; Jena et al. 2010). It is fast growing and has evergreen or semi-eternal leaves and is suited for tropical areas. In India, indica and longifolia are the two main species of the genus Mahua. Mahua indica is one of India’s non-edible, forest-based feedstocks with a high annual production capacity of almost 60 million metric tons. Mahua fruit comprises one or two kernels, which can be harvested from a 4- to 7-year matured tree, and the trees can live for up to 60 years. The Mahua indica seed yield differs from 5 to 200 kg/tree based on the size and age of the tree. Seventy percent of the seed is in the kernel and it has 50% oil content (Atabani et al. 2013). The chemical composition is 17.8 wt.% of palmitic acid, 14 wt.% of stearic acid, 46.3 wt.% of oleic acid, and 17.9 wt.% linoleic acid. Hence, it has a higher percentage of saturated fatty acids as compared to other non-edible oils. Accordingly, this seed oil has poor cold flow properties. Mahua oil contains around 20% of free fatty acid and it requires a large amount of acid catalyst to produce biodiesel from oil (Kumar and Sharma 2011).
The neem tree can adjust to a wide temperature range (0-49°C). The neem tree is found in a variety of regions, including in South America, Africa, and Asia. The neem tree grows in almost all soil types, like clay, sandy, alkaline, acidic, stony, and shallow soils and even solid soils with heavy calcareous soil (No 2011). A well- grown tree can yield 30-50 kg of fruit, and tree life is up to 150-200 years. So, it can produce 540,000,107,000 and 425,000 metric tons of seed, oil, and cake, respectively. The neem seed oil is dark brown and has a bitter taste. It could be a promising alternative source of energy to replace other fossil fuel sources. This oil is primarily used in cosmetic, ayurvedic, and biopesticide preparations. The raw neem oil has a higher free fatty acid of 21.6 mg of KOH/g of oil and it requires more acid catalysts to convert oil into biodiesel (Atabani et al. 2013).
The rubber seed tree, referred to as Hevea brasiliensis, is part of the Euphorbiaceae family. This rubber tree is native to the Amazon (Brazil). The rubber tree is the predominant natural rubber source, supplying 99% of natural rubber in the world. It is also possible to obtain and use the sap-like extract of the tree (known as latex) for multiple purposes. It is found primarily in India, Indonesia, Sarawak, Malaysia, Sri Lanka, Thailand, and Liberia. The tree can grow up to a height of 34 m; the plant requires heavy rainfall and yields seeds ranging from 2 to 4 g, which are not used for commercial purposes nowadays (Tomes, Lakshmanan, and Songstad 2011).
Typically, 37% of the seed weight is the shell and the rest is the kernel. Rubber seed oil is a non-edible oil having 50-60 wt.% oil and 40-50 wt.% of brown-colored oil in the kernel. The seed oil has a higher percentage of unsaturated fatty acids such as
36.3 wt.% of linoleic acid, 24.6 wt.% of oleic acid, and 16.3 wt.% of linolenic acid (Kumar and Sharma 2011).
Lignocellulosic Biomass Waste
Lignocellulosic biomass is the expression used for the combination of lignin, cellulose, and hemicellulose polymers interlinked in a heterogeneous matrix as biomass from woody or fibrous plant material. The total mass of cellulose and hemicellulose content may vary with plants, but usually the biomass consists of 30% of lignin and 50-70% of cellulose and hemicellulose combined. Via a sequence of thermochemical and biological processes, cellulose and hemicellulose can be processed into sugars and finally fermented to bioethanol. Lignocellulosic biomass is a readily available and sustainable resource that consists of cereal, straw, wheat chaff, rice husks, maize cobs, corn stove, sugarcane bagasse, nut shells, forest harvest, residues from wood processing, and marginal and degraded land energy crops. According to an EIA report (International Energy Agency 2010), there will be more than 1500 EJ of technological capacity for bioenergy by 2050. The estimated potential of biofuel and bioenergy from agricultural and forestry residues would be 85 EJ in 2050 (Hoogwijk et al. 2005).
Lignocellulosic feedstocks are typically categorized into three types: agricultural residues (e.g., crop residues, sugarcane bagasse), forest residues, and crops with herbaceous and woody biomass.
A wide variety of agricultural residues, namely corn stove, grass, wheat grass, bagasse, etc., could be used to produce second-generation biofuels. These fuels are usually regarded as renewable because they are produced from food crop waste materials and do not compete for land with food crops. The crop residues can be converted into value-added products using different conversion technologies. Ethanol can be produced from agricultural residues through biochemical processes such as anaerobic digestion and fermentation. Also, gasification can be used to produce syngas from residues; subsequently, syngas can be converted to liquid fuels with the aid of catalysts.
The second-generation biofuels are mostly produced from two main types of forest residues. The residues of wood cutting, such as branches, leaves, roots, etc., can be used for biofuel production. The demand for cut wood includes the difference between the average amount of wood that can be harvested and the actual amount of wood required to satisfy the demand. The thermochemical route is the one of the best ways to convert residues from forests into biofuels. In Sweden, the world’s first BioDME plant was started, using black liquor from forest residues through gasification.
In order to provide feedstocks for the production of biofuels, there are a number of energy crops that could be cultivated on land which is not appropriate for agricultural farming. Hence, new energy crops, especially perennial grasses such as mis- canthus, switch grass, prairie grass, and forest species, namely eucalyptus, poplar, and Robitiia, can be used for biofuel production. Drawbacks associated with forest residues are that they are used to produce many other products, and that energy production on waste lands is problematic due to high energy demands and the low ability to adapt the new crops to these soil conditions (Ullah et al. 2015).