useful biochemical metabolites like starch, oils and hydrocolloids that land up into algal biomass. Aquatic biomass produces more biomass per hectare compared to terrestrial crops as well as they are not main food crops, therefore, considered as a best possible raw material for the third-generation biofuel production. However, current techniques employed in biomass cultivation, processing and fuel separation are non-economical, leading to increase the cost of biofuel production. Therefore, many techno-economical challenges must be overcome to use aquatic biomass derived biofuel to be commercially hit in near future.
3.3.4 Animal and Human Waste Biomass
The remnants of animal such as bone, organic waste from slaughter house and fishery animal dung, and human dung are classified as animal waste biomass [49]. Earlier, animal dung was used mainly as fertilizer or kitchen fuel. However, a huge amount of waste used to be simply spread on fertile land which was the reason for many environmental issues and later crafted into waste-energy conversion. Biogas obtained from anaerobic digestion of above organic waste is an attractive alternative of kitchen fuel or converting gas energy into electric energy.
3.3.5 Biomass Mixtures and Municipal Biomass
In some cases, when several substrates belonging to different classes, mentioned above, are in mixed form and are classified in this category [52]. Municipal generated biomass is organic waste rich in carbohydrates that have great potential to be utilized as renewable energy source. It ranges from food waste to paper or any material collected from residential or commercial place. Broadly, it is classified into three major categories such as municipal solid waste, municipal sewage, and urban wood biomass [53]. The characteristics of municipal biomass differ a lot in developing and developed countries. It is available in relatively huge amount in urban areas and need proper waste management program. Through direct combustion or anaerobic digestion in landfills or processing plants, municipal waste can convert into energy. Similarly, waste from sewage which is also substantial source of biomass can be subjected to energy production using anaerobic digestion, pyrolysis or drying and incineration (direct combustion).
The physio-chemical properties of biomass determine the selection of conversion method and its underlying processing difficulty. The main properties are moisture content, calorific value, ash/residue content, alkali metal content, cellulose/lignin ratio and proportions of fixed carbon and volatiles. The soil and other detritus contaminants present in biomass affect ash and alkali metal content of material. Depending on the conversion process undertaken, particular property of material becomes important. In case of wet biomass conversion, properties like moisture content and cellulose/lignin ratio predominates. Whereas, if conversion process is subjected to dry biomass, then, all properties are of interest except cellulose/lignin ration which is not of much concern. The low moisture containing woody and herbaceous biomass is most suitable for thermal conversions like gasification, pyrolysis/combustion. The feedstocks high in moisture content are economically more fitted for fermentation derived biological conversions producing. The higher cellulose proportions compared to lignin determine the rate of biodegradability of material and act as major factor in selecting type of biomass for lignocellulosic bioconversions.
Aqueous processing reactions are used for those material for which moisture content is such that, energy input for drying process is more than energy of product to be formed [47].
3.4 Sustainability of Biofuels
The world’s major bioethanol renewable source is lignocellulosic biomass, therefore, it is a promising material in terms of sustainable biofuels. It is synthesized from sunlight, nutrients and carbon dioxide through capture by plants. The raw materials of lignocellulosic biomass mainly include woody substrate, agricultural residues, marine algae and municipal solid waste. These feedstock materials are renewable and can be used to produce biofuels. It was estimated that lignocellulosic biomass could produce about 80–90 billion gallons of biofuel, which could replace the huge burden of world’s fuel consumption [54, 55]. Additionally, it also offers various advantages compared to fossil fuels regarding its production, usage and products. Therefore, lignocellulosic biomass is considered sustainable in terms of its environmental safety, resource renewability and low cost in a long run. It has higher oxygen content of 10–45% compared to fossil fuels which makes it more sustainable in terms of lesser CO2 emissions [56]. Its production doesn’t involve any infrastructural changes compared to traditional fuels and with the help of thermochemical methods and biological methods, it can be easily converted into liquid and gaseous biofuels, therefore, it is cost effective and eco-friendly.
Biofuels have some advantages and disadvantages in terms of environmental, economic and social sustainability. It has advantages in terms of carbon emission reduction, greenhouse gas reduction, energy safety, and rural development. However, it has some disadvantages related to increasing food price values, risk of increase in greenhouse gas emission by land use change for production of biofuels feedstocks, degradation of forests, land, water resources and ecosystem. The first generation biofuels obtained from feedstocks such as corn will compete with food production due to which agricultural land will be diverted into fuel production land. It will also produce risks of increase in deforestation and use of fertilizers and pesticides will cause negative effects on environments. While in second generation biofuels, economic viability is another concern. In third generation biofuels, the production of microalgae is energy intensive [57]. Therefore, to encourage sustainability of biofuels, regulatory policies like the Renewable Energy Directive (RED) have to specify various sustainability criteria for biofuels. The RED already have stipulated in 2015 that biofuels should have to reduce the greenhouse gas emissions to 50% compared to their fossil fuels, which have been raised in 2021 and now biofuels should have to lower the emissions to 65% according to the European Commission, 2018 [58]. The impact of climate change in terms of greenhouse gas emissions should be evaluated on a life cycle assessment (LCA). During production of biofuel processes, various co-products like animal feed, electricity, heat and biochemicals are produced and impact of biofuels and its co-products should be allocated. In LCA cycle of biofuels, system expansion and allocation of energy approaches are used.
Furthermore, land-use change directly transforms uncultivated land (grassland, forests) into croplands for biofuel production. Additionally, land use change indirectly induces displacement of food and feed crop production to new land areas which previously were not used for cultivation. The LCA studies accounts land use change in only 25% of their studies and include other environmental impacts by acidification, eutrophication, photochemical smog, human or eco-toxicity [57]. These environmental impacts are discussed in the following section.
3.5 Environmental Impacts
1) Global warming
LCA studies suggest some advantages and disadvantages of the same type of feedstocks due to difference in allocation method, sources, land use change, etc. The feedstocks like corn, wheat, molasses and sugar beet do not meet the requirement of reduction in emission of 60%. However, the bioethanol obtained from sugarcane can reduce 60% reduction in greenhouse gases relative to fossil fuels. It is due to lower inputs of agrochemicals and more yield of sugarcane compared to other crops like corn. Additionally, the electricity can also be produced, which is a co-product in biorefinery. Moreover, the palm oil, soya bean based biofuel are found to be effective for 60% reduction of emissions compared to diesel. However, if land use change is involved then bioethanol could not meet the 60% reduction emission of greenhouse gases despite of the type of feedstock. The increasing demand of bioethanol from sugarcane leads to continuous increase of land for cultivation and cause deforestation. Moreover, soyabean cultivation is driving both direct and indirect land use change in South America [59]. Similarly, palm cultivation in Malaysia and Indonesia causes drainage of peat lands as well as deforestation and results in 3–40 times higher greenhouse gas emissions compared to diesel [60].
While for second generation biofuels,
