1.5. Methods of Processing Biomass
1.5.2. Bio-chemical conversion technologies
Bio-chemical conversion of biomass involves the use of bacteria, microorganisms, and enzymes to breakdown biomass into gaseous or liquid fuels, such as biogas or bioethanol. The most popular bio-chemical conversion technologies are anaerobic digestion (or biomethanation),
Introduction Chapter-1 hydrolysis and fermentation. The three major steps involving the bio-chemical conversion technique are pretreatment, enzymatic hydrolysis, anaerobic digestion followed by fermentation.
The details of the process are discussed here in detail.
1.5.2.1. Hydrolysis
Hydrolysis is the process of converting biomass polymers to fermentable sugars (Verardi et al., 2012). The process of hydrolysis involves cleaving the polymers of cellulose and hemicellulose into their monomers (Taherzadeh and Karimi, 2007). Complete hydrolysis of cellulose leads to the formation of glucose, whereas the hemicellulose gives rise to several pentoses and hexoses.
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The hydrolysis of biogas to produce bio-oil can be chemical hydrolysis and enzymatic hydrolysis. The details of these two processes are depicted in Figure 1.6.
1.5.2.2. Chemical Hydrolysis
Chemical hydrolysis involves the exposure of lignocellulose materials to chemical for a period of time at a specific temperature, resulting in sugar monomers from cellulose and hemicellulose polymers. The acids which are predominant for the chemical hydrolysis process are sulfuric acid (Harris et al., 1945), and HCl (Hasheem and Rashad, 1993). In general, the hydrolysis by dilute acids occur at high temperatures and pressures with a short residence time, resulting the degradation of hemicellulose and cellulose. Thereby the glucose yield is low due to the higher decomposition rate of glucose and high formation rate of undesirable products such as
Introduction Chapter-1 levulinic acid, acetic acid, uronic acid, vanillin, phenol, formaldehyde, etc. (Larsson et al., 2000, Taherzadeh, 1999). Due to the severity of acid together with high temperature and pressure, chemical hydrolysis lead to high utility cost with additional burden of downstream neutralization.
1.5.2.3. Enzymatic hydrolysis
Enzymatic hydrolysis in which enzymes facilitate to depolymerize the lignocellulose biomass into biofuels and bio-chemicals. It is naturally a very slow process because of the structural parameters of the substrate (e.g. lignin and hemicellulose), surface area, and cellulose crystallinity. The process of conversion of cellulose to glucose is carried out by the cellulose enzymes that are highly specific catalysts. The hydrolysis is performed at mild conditions of temperature between 40oC -50oC and pH ranging between 4.5 - 5.0. This reduces the corrosion problems, low utility consumption, low toxicity of hydrolysates, etc. The process of cellulose degradation to glucose majorly depends on the three major classes of enzymes namely endo- glucanases, exo-glucanases, and β-glucosidases. In addition to this, enzymatic hydrolysis is an environmental friendly process, whereas it has a disadvantage of longer hydrolysis time, however, enzymes are more expensive than acids and the end product inhibition may occur.
1.5.2.4. Fermentation
A Group of chemical reactions induced by microorganisms or enzymes that split complex organic compounds into relatively simple substances, especially the anaerobic conversion of sugar to carbon dioxide and alcohol. The common biomass that is used in the fermentation process is rich in sugar compounds majorly lignocellulose biomass (sugarcane, sweet potato and corn) consisting of fermentable sugars namely cellulose and hemicellulose along with non-
Introduction Chapter-1 fermentable sugars like lignin. The lignin fraction is high in energy and can be utilized for the generation of heat and electricity. Various product outputs are targeted through fermentation process depending on the type of feedstock majorly ethyl alcohol from lignocellulose biomass.
Figure 1.6: Bio-chemical conversion process a) hydrolysis b) fermentation (Source: www.arkenol.com/Arkenol.Inc/tech01.html)
The yeast or the enzyme that is commonly used in the fermentation to breakdown glucose into fructose and sucrose for the ethanol production are schizosaccharomyces pombe, saccharomyces cerevisivae. The ethanol produced is distilled and dehydrated to get a higher concentration of alcohol to attain to the required immaculateness (higher concentration) for the utilization as automotive fuel. The solid residue from the fermentation process is utilized as a cattle feed and
Introduction Chapter-1 in the case of sugarcane; bagasse is used as a fuel for boilers. Although decomposition of the material into fermentable sugars is complicated, the processing steps of dehydration, distillation, fermentation processes are interlinked and are identical for either agricultural crops or lignocellulose biomass.
1.5.2.5. Anaerobic Digestion
This process not only decreases the greenhouse gases but also reduce dependence on fossil fuels for energy requirements. It is a microbiological conversion of organic matter to methane in the absence of oxygen. The decomposition is mainly caused by the natural bacterial action in various stages. It takes place in variety of natural anaerobic environments including water sediments, water logged soils, natural hot springs, ocean thermal vents and stomach of various animals. The process generates three main products.
Biogas: A mixture of carbon dioxide and methane which is used to generate heat and electricity.
Fiber: It is used as a nutrient rich soil conditioner
Liquor: It can be used as a simple liquid fertilizer.
There are two types of anaerobic digestion processes
Mesophilic digestion
Thermophilic digestion
Mesophilic digestion is most commonly used process for anaerobic digestion, in particular waste sludge treatment. Decomposition of volatile waste suspended solids is around 40% over a retention time of 15 - 40 days at a temperature of 30oC – 40oC, that requires larger digestion tanks. Whereas thermophilic digestion is less common and not as mature a technology as
Introduction Chapter-1 Mesophilic digestion. The digester is heated to 55oC and held for a period of 12-14 days. This technology provides higher biogas production, faster throughput and improved pathogen and kills virus, but the technology is more expensive more energy is needed and it is necessary to have more sophisticated control and instrumentation.
Anaerobic digestion Process
Figure 1.7 presents an anaerobic digestion system for converting biomass feedstock to bio-oil. The initial stage of this process involves shredder where the feed streams are passed into it and the particles are shredded into 12 mm size as per the regulations of EU and UK and less than the biogas and the liquor/fiber, known as digestate. The biogas generated from the digestion tank finally will be sent to gas storage.10mm as per Indian standard. From the shredder, particles are transferred to input buffer which is simply a sealed tank that holds the shredded material.
Introduction Chapter-1 Figure 1.7: Anaerobic digestion system (bio-chemical method) for processing biomass feed stock to bio oil. (Source: Energy systems research unit- University of Strathclyde;
www.esru.strath.ac.uk/EandE/Web_sites/03-04/biomass/background info8.html).
This allows controlling the flow to further processing steps. It also allows excess storage in case of any unforeseen amount of feed stream offered. Then feedstock is allowed to pass to pasteurizer where the temperature is maintained at 70oC for at least one hour. This temperature is enough to destroy the pathogens in the feedstock which prevents the bacterial competition in the digestion stage. To allow a constant supply of feed to digestion stage three pasteurizers are used.
Further the material is transferred to digestion tank where the majority of bio gas is produced.
The anaerobic bacteria convert a quantity of organic matter into biogas in a sealed container.
This is continuously stirred and heated to 35oC and the average retention time is approximately 18 days. This allows a significant percentage of organic solids to be converted to biogas. The outflows of biogas are in two forms, the biogas and the liquor/fiber, known as digestate. The biogas generated from the digestion tank finally will be sent to gas storage.