Since the time immemorial, the human civilization is dependent on the energy to sustain themselves. In the dawn of human evolution the energy requirement was limited to mere food. During the development, early human conserved the body heat by using the animal hide as clothes. The use of fire as the energy source has completely changed the course of human evolution and development. Since then till today, human flourished and its energy demand has increased. The development was made by using any energy source which could had been used like forest wood and fossil fuels. This caused a huge damaged to the environment as such energy sources are not only caused the negative impact on environment during extraction, but also caused the environmental pollution. For instance, the forest wood is obtained by cutting down the forest, thus it causes the damage of natural habitat of animals and bio-geological disturbance, while burning the wood cause the production of smoke and various gases which is not favorable for the atmosphere. Similarly, fossil fuel like coal and petroleum product also damages the environment. The rat race of the development lead to the destruction of millions of hectares of the forests and seafloor and overlooked the damage to the environment. According to an environmentalist’s point of view there are several problems which the biosphere is dealing with, in recent times. They are enlisted as follows:
- Natural resources depletion
- Deforestation and reduction in biodiversity
- Waste disposal
- Global warming and ozone layer depletion
- Public health issues
- Aesthetic losses
Due to the growing environmental concerns like global warming, pollution there are different reclamation measure are being taken. The European Union (EU) is very prompt in making the guidelines on the current situation towards the sustainable environment. EU have also formulated some objectives for the natural conservation (SEA 2015):
a. Reduction in the greenhouse gases emission by 20%
b. Improve the energy efficiency and reduce energy dissipation by 20%
c. Use of renewable energy up to 20% of all energy demand and 10% share in transport fuel usage
Sweden is one of the leading countries which is rapidly increasing the share of renewable energy for its overall energy demand. It has also forged a very ambitious goal to transform its transport sector into a fossil fuel independent medium by the year 2030 (SEA 2015). Generally the above mentioned problems are created by the use of non-renewable energy sources. Non-renewable energy is the energy harvested from sources like wood, coal, fossil fuels, nuclear energy et cetera. The damaged caused by the industrialization in the last century was obvious enough that it compelled to find the alternative for non-renewable energy. This notion motivated to exploit the everlasting energy available in the form of wind, water, sun, biological wastes et cetera by using different specific processes. Aforesaid form of energy which is readily replaced or recycled by the natural processes and can be used again, are referred as renewable energy (Sharma et al. 2017). With reference to the biological point of view the renewable energy gained by the use of biological entity is known as Bioenergy. Bioenergy or Biomass-based energy is produced from the materials of biological origin. Bioenergy is the form of energy which is derived directly or indirectly by the use of biomass. Biomass is any organic material which is biodegradable, is derived from plants, animals or any other biological forms and considered as renewable in the environment. Biomass includes wood, agricultural crops, household waste, biodegradable plastic, agricultural waste, compost and manure et cetera (Bridgwater 1999; Sharma et al. 2017).
The transformation of energy – Biomass to Bioenergy
Every biological material (non-living) contains simple and complex biomolecules made up of carbohydrates, proteins and lipids. Therefore, they stores the energy which can be utilized by other organisms like microorganisms or the biological material can be taken under some processing for the transformation of stored energy to the usable form of energy for us. For instance, the crop residue stores energy in the form of lignocellulose and structural proteins, but bioethanol produced from such sources can be used for running the biofuel based vehicles. There are several methods by which such energy transformation can be achieved, and these methods are represented in Figure 1.
For the maximum gain of the energy from the biomass, the bio-based materials are treated using different methods and techniques. This process makes the energy transformation easier and efficient. For a sense of understanding, the biofuels can be classified as solid (wood chips, pellets, charcoal), liquid (biodiesel, bioethanol) and gaseous (biogas, syngas, hydrogen) biofuels (Bauen et al 2009; Kwant & Buckley 2015). Different conversion technologies have been developed for the processing of biomass based on the physical and chemical properties on the biological material. The conversion technologies might be based on the thermochemical processes (chemical degradation at high temperature), physicochemical processes (mechanical application and biochemical processing) or biological conversions (fermentation) et cetera (Bauen et al 2009). On the basis of the physical state, the resulting biofuels can be classified into three classes i.e. solid biofuels, liquid biofuels and gaseous biofuels.
1. Solid Biofuels
The European technical committee for standardization (CEN) drafted a standard (CEN/TS 14961) which describe 27 technical specifications for the solid biofuels (Grammelis 2011). The two most important technical specifications according to European standard (EN) are classification and specification (EN 14961) and quality assurance (EN 15234) (E. Alakangas 2011; Grammelis 2011). On the basis of their origin and source, solid biofuels are classified into four classes: 1) woody biomass, 2) herbaceous biomass, 3) fruit biomass and 4) blends and mixtures (E. Alakangas 2011). Quality assurance (ISO 9001) aims to provide confidence that quality is monitored and maintained according to the European standards (EN 15234) and customer requirements.
When the biomass is in solid physical state, it can be used directly as the fuel (wood). However, if the biomass is not suitable for the direct use (sawdust, agricultural residues), pre-treatment or upgrading process is carried out to transform the biological material in the more usable form. This is generally carried out by pelletisation, pyrolysis/hydrothermal and upgrading torrefaction (Bauen et al 2009). Pellets are prepared by the compression of small particles and extrusion from an opening so that the resulting mass is a pellet which is easier to transport and handle as compared to its previous form (sawdust). Pyrolysis is the controlled thermal decomposition of the biomass in the absence of oxygen which produces Liquid bio-oil, syngas and bio-charcol. Torrefaction is an efficient process carried out at 200-300˚C and biomass is chemically upgraded into a dry, coal like product (Bridgwater 1999; Rosendahl 2013; Bauen et al 2009; Kwant & Buckley 2015).
2. Liquid biofuels
Liquid biofuels are being used since the late 1970s, however, recent development in the research aided the wide spread use of liquid biofuels as a supplement for the conventional fossil fuels like diesel, petrol et cetera. Bioethanol and biodiesel (80% and 20% of the market share, respectively), accounts for the around 3% of the global demand (Iea-Etsap 2013). The liquid biofuels are primarily classified into first generation and second generation biofuels on the basis of the feedstock used to produce them (Nigam & Singh 2011). First generation liquid biofuels are produced from fermenting sugar or grain/seed, and are relatively easier in production (Pathak et al. 2013). Second generation liquid biofuels are refined using different processing technologies i.e. biological and thermochemical methods on the lignocellulosic biomass (Nigam & Singh 2011). In addition to the two generations, third generation of liquid biofuels is an emerging alternative energy resource to the fossil fuels. Recent researches are focused on the use of agricultural residues and waste vegetable oils as a substrate for the microbes and microalgae (Nigam & Singh 2011). The advantage of liquid biofuels is that they are helpful in significant reduction of the greenhouse gas (GHG) up to (70%). The international energy agency (IEA) have estimated the use of liquid biofuels up to 9.3% by 2030 and 27% of the market share by 2050 (Iea-Etsap 2013).
3. Gaseous biofuels
Gaseous biofuels can not only be used as the means to produce electricity and heating, but also can be used as a good alternative to fossil fuel in transportation. Methane/biogas, hydrogen and dimethyl ether (DME) are most common used gaseous biofuels (Scragg 2009) (Figure 2). Biogas is a comprehensive biofuel produced from the renewable resources like crop residue, household waste, community waste or waste water is an excellent alternative to the fossil fuels. The upgraded biogas (methane content >98%) can be directly used as a fuel, provided small changes in the engine of vehicles. Methane rich biogas (biomethane) can also be used as a hydrocarbon source for the production of chemicals in industries (Weiland 2010). Biogas production is a very old method (1000-2000 years) primarily used for the sanitization of the communal wastes (Schnürer 2016). Later, biogas production was used for heating houses and cooking. In recent years, biogas is produced on the industrial scale and its becoming an integrated part of the biological waste treatment. Biogas is produced by the anaerobic digestion of the biomass in an oxygen free environment (Bauen et al 2009; Weiland 2010). Anaerobic digestion of the organic material by a cooperation/collaboration of microorganisms of different phylogeny is considered as most energy efficient and environmental friendly method of biofuel production (Scragg 2009; Weiland 2010).
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Figure 2 : Production of gaseous biofuels from biomass of different origin. Biogas, dimethyl ether and hydrogen are the 1st, 2nd and 3rd generation gaseous biofuels, respectively (Scragg 2009).
The main and inflammable constituent of biogas is methane. Methane production is a complex process which includes four main phases 1) hydrolysis, 2) acidogenesis, 3) acetogenesis and 49 methanogenesis (Schnürer 2016; Scragg 2009) (Figure 3). In hydrolysis and acidogenesis process complex biomolecules /polymers are converted into monomers or oligomers. Further, proton-reducing acidogenic organisms oxidize acids and alcohol produced in the acidogenesis process to hydrogen, carbon dioxide and acetate. Methane is produced in the last step of the anaerobic digestion process by a group of obligate anaerobes, which have very strict environmental requirements. These anaerobes are slow growing organisms and are sensitive towards the oxygen, temperature and pH (Schnürer 2016; Energigas Sverige 2011).
Biomass and biogas production
To maintain optimum population diversity of the microorganisms in biogas reactors, a good balance of macro and micronutrients are required. Most often the co-digestion (mixture of different substrate) reactors are the better choice as they are superior in maintaining the water and nutrient content and possible dilution of the inhibitors and harmful substances. The biochemical composition of the substrate is a critical parameter for the volume of methane produced, biomass degradation and reaction kinetics (Schnürer 2016). The protein rich materials like slaughterhouse waste yields high methane and the digestate is an excellent manure for the farmers (Westerholm et al. 2016). Reactors running on the substrate with high nitrogen content must be maintained monitored continuously because high nitrogen can result in the high ammonia concentrations which is inhibitory for the further process involving methanogens that are responsible for the biogas/methane production (Westerholm et al. 2016; Schnürer 2016). The biomass rich in the fat content is also a high methane producing substrate. Nevertheless, the degradation of lipids results in the increase content of short and long chain fatty acids which might cause the decreased pH, foaming and inhibits the methanogens. (Schnürer 2016).
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- Quote paper
- M.Sc. Abhijeet Singh (Author), 2017, Bioenergy and Biomass processing. An overview, Munich, GRIN Verlag, https://www.grin.com/document/372220