Biomass Conversion

Biomass can be converted into energy by the following ways :

The Non-Biological Process (Thermo-chemical Process)
There are different non-biological routes for biomass conversion into energy viz. direct combustion, gasification, pyrolysis and liquefaction.

Direct Combustion
Biomass from plants (wood, agricultural wastes) or animal (cow dung) origin are directly burnt for cooking and other purposes. In recent years "hog fuel" production technology has been developed which is being utilized for generation of electricity.

Now-a-days municipal, agricultural and light industrial wastes are used for conversion into energy by direct burning in refuse fired energy systems (Ghosh and Bisaria, 1981).

In India, short term rotation plantation and social forestry are being boosted up to meet energy demand for cooking in rural sector (see Energy plantations) as the industrial wastes and urban garbage are used for generation of electricity and engine in industry.

Hog Fuel : The mixture of wood and bark waste burnt directly is collectively termed as 'hog fuel' (Jahn, 1982). Hog fuel combustion technology has been developed recently in the U.S.A. This fuel is produced in large sized boilers made up of steel. Boilers are designed time to time to develop a good control system of combustion. In the U.S.A. a boiler has been modified by increasing the size from 15,000 lb/h to 500,000 lb/h with certain other improvements. In the U.S.A. a cogeneration technology has been developed to generate electricity from hog fuel, and to use the exhaust heat in the form of process steam for manufacturing operations.

This is done by burning hog fuel to make high pressure in the hog fuel boiler (600-1,200 lb/inch²), passing this steam through back pressure or extraction turbine driving a generator, and then using the steam exhausted from the turbine at low pressure (50-300 lb/inch²) for process heat. When wood containing 55 per cent moisture is used, the power generation has about 60 per cent efficiency compared to that 39 per cent from fossil fuels and 33 per cent from nuclear fuel. In order to economically deploy cogeneration systems, steam usage should be more than 70,000 - 120,000 lb/ h which is equivalent to 3-5 MV of back pressure (Jahn, 1982).

Pyrolysis
Pyrolysis is defined as the destructive distillation or decomposition of organic matter, for example, solid residues, wastes (saw dust, wood chips, wood pieces) in an oxygen-deficient atmosphere or in absence of oxygen at high temperature (200-500°C or rarely 900°C). Products of pyrolysis are gases, organic liquids and chars, depending on the pyrolysis process and temperature of reaction. The condensable liquids separate into aqueous (pyroligneous acid), oil and tar fraction (if the substrate is wood). The composition of gas is carbon monoxide (28-33 per cent), methane (3-5-18 per cent), higher hydrocarbons (1-3 per cent) and hydrogen (1-3 per cent). During pyrolysis, hydrogen content of gas increases with increasing the temperature (Jahn, 1982).

After pyrolysis, the amount of different products varies with the nature of wood, type of equipment and systems employed. For example, low temperature favors liquids and char; low heating rates favor gas and char and short gas residence time favors liquids. In contrast high heating rate, favors liquids and the long gas-residence time favors gas (Goldstein, 1980). Thus, . the liquid is obtained before the solid is completely burnt to yield gases. The liquid is very useful for high energy fuel.

Wilson et al, (1978) have described a mobile system for pyrolysis. The unit can move from one place to another and process the waste materials generated from various sources. This system will reduce the transportation cost of wastes/residues. Energy transported as coal and oil would be about 2.8 times greater than transporting the wet wood waste.

Pyrolysis has been employed to produce charcoal for the last few decades. Charcoal is a smokeless and low sulphur fuel used mostly for cooking purpose. Besides wood, other wastes/ residues used in pyrolysis are cotton, bagasse, ground nut shell, etc.

Gasification
Gasification is a process of thermal degradation of carbonaceous material under controlled amount of air or pure oxygen, and high temperature up to around l,000°C. As a result of gasification, high amount of gases is produced. Gasification of biomass is done in a gasifier designed in various ways. Success for gasification process is based on its designing.

Therefore, the design of a gasifier is an important factor in controlling gas quality. Gas is used in a controlled manner for irrigation, pumping and electricity generation. The advantages of gasification of wood over coal are: (i) much low oxygen requirements, (ii) practically no steam requirements, (iii) low cost for changing H₂/CO₂ ratios which are high in wood gas, and (iv) no or very little desulfurization cost (Goldstein, 1980).

When gasification of farm wastes (manure) takes place, the phenomenon is known as hydro-gasification, because gasification of organic wastes occurs in the presence of hydrogen at 500-600°C.

A seminar on biomass gasification technology was held in September, 1990 in Bombay, which was organized by the Department of Non-conventional Energy Sources (DNES). It was hoped that the sustained ability of biomass would go a long way in helping to use for irrigation in the country, with the "thermal biomass gasification plants". At present there are 400 gasifiers in the country, based on using waste wood. A few pilot gasifier plants, using non-wood biomass, have also been designed. For gasification purpose, wood wastes from agriculture, Pharmaceuticals, coconut coir, saw dust and tree-trimings are used. Efficiency of thermal conversion of wood to gases is 60-80 per cent (Jahn, 1982).

The multiple use of five H.P gasifier programme was also emphasized in the seminar. The gasifier used for pumping water for irrigation can also be used to light house-lamps and street lamps (with a single gasifier).
Three to five kilograms of biomass per hour is needed for generating power for an hour. In a year it has to run for at least 15,000 hours for which 6 tonnes of biomass is required. Therefore, it is very essential to make sure that the biomass is continuously available. Recently, United Nations Department Programme (UNDP) has recognized the India's five H.P. gasifiers as the best ones.

Liquefaction
Liquefaction involves the production of oils for energy from wood or agriculture and carbon residues by reacting them with carbon monoxide and water/steam at high pressure (4,000 lb/in²) and temperature (350-400°C) in the presence of catalysts. By this method about 40-50 per cent oil can be obtained from wood. This oil serves a good source of fuel (Jahn 1982).

The Biological Process (Bioconversion)
Bioconversion involves the conversion of organic materials into energy, fertilizer, food and chemicals through biological agency. The term biological agents means the microorganisms i.e. bacteria, actinomycetes, fungi and algae. In broad sense bioconversion involves 2 steps : photosynthetic production of biomass, and its subsequent conversion into more useful energy forms (gaseous, liquid or solid fuel; heat and electricity) (Khoshoo, 1988).

Ghosh (1980) has estimated that the average production of waste materials in India is about 1,540 x 10⁶ tonnes/year. Process of bioconversion of biomass into various utilizable forms are briefly discussed.

Enzymatic Digestion
This process involves the conversion of cellulosic and lignocellulosic materials into alcohols, acids and animal feeds by using microbial enzyme e.g. cellulose, hemicellulase, amylase, pectinase, etc.

(A) Degradation of Cellulose : It is clear that cellulose is a polymer of 6- 1,4 linked anhydrous glucose units, comprising of 40-60 per cent of cell wall materials of plants. Microorganisms, which produce cellulases and other enzymes in high amount are given in Table 17.1. In recent years, Cellulomonas, Trichoderma reesei, T. viride and other microorganisms are used for the production of cellulases in high amount.


There are 3 enzyme components of cellulase : b-1, 4-endoglucanase, b-1, 4-exoglucanase and b-1, 4-glucosidase. b-1, 4-endoglucanase randomly attacks along cellulose chain; b-1, 4-exogiucanase splits from non-reducing end of cellulose and b-1, 4-glucosidase i.e. cellobiase cleaves 2 molecules of glucose from cellobiose (Fig. 19.6). Ramasamy (1980) has presented the following sequence of cellulose degradation.




(B) Degradation of Hemicellulose : Sugars constituting hemicellulose are discussed under Composition of biomass. An analogous system of enzymes is involved in the degradation of hemicellulose. This enzyme-system consists of 3 enzymes : exoxylanase endoxylanase and b-xylosidase (which split xylose and other short chain xylobioses). Steps of hemicellulose break down are given in Fig. 19.7.

Anaerobic Digestion
An aerobic digestion is a partial conversion by microorganisms of organic substrates into gases in the absence of air. The gases produced are collectively known as 'biogas'.
Anaerobic digestion is accomplished in 3 stages: solubilization (of complex substrates by enzymes into simple forms i.e. fatty acid, sugars, amino acids), fermentation (of hydrolyzed organic substrates into simplest forms e.g. organic acids) and methanogenesis (production of methane from simple substrates by methanogenic bacteria under anaerobic conditions). Anaerobic digestion is carried out in a digester, which is a brick-lined or concrete-lined chamber covered completely to prevent the entry of air. A detailed account of biogas production is given elsewhere (see Alcohols : the liquid fuel).

In contrast, a lagoon is a pond, lined with concrete or other water-proof material and open to the atmosphere. Waste materials move slowly into the lagoon and solid matter settles at bottom. Lagoonification is the anaerobic digestion process, used for treatment of high moisture material, as a result of evolution of methane and CO₂. Microbial reactions evolving gases are same as in anaerobic digestion (Fig. 19.8). Microbial activity is high at suitable temperature (29--35°C) and, therefore, the rate of evolution of gases is also high as compared with low and high extremes of temperature.

Aerobic Digestion
Aerobic digestion involves the conversion of organic substrates by microorganisms into utilizable forms in the presence of air, for example composting (biological decomposition of organic wastes/residues under controlled conditions to result in release of C, N, P, K, etc.) and oxidation systems (of sewage in oxidation ponds by bacteria and algae) to produce gases, single cell protein, fertilizers, etc, (see Advantages of producing microbial protein).