Biomass as a Source of Energy

In many developing countries wood has been used as a fuel. The situation is growing so desperate that wood is poached from forest reserves. As a result the ecosystem is degrading deplorably. So, in order to protect the natural environment, there is every necessity of producing alternative source of energy for the needs of the people. Biomass is the best alternative as it is available in plenty and production of energy from biomass is also less costly. 

In India, the concept of energy as "Shakti" has been at the focus of philosphic, scientific and metaphysical thought from time immemorial. The conventional energy sources like fossil fuels, crude oil, natural gas etc. are dwindling fast. The world stock of non-renewable natural sources indeed have decreased. There is every necessity of going for renewable alternative resources for energy. The energy crisis of 1973 left scientists to accelerate the renewable energy programmes. The important renewable energy sources are sun, wind, tides, waves, biomass, hydro-power (from water) charcoal, peat, fuelwood, geothermal energy etc. The pattern of energy consumption in India shows that 56.5 % of total energy is from the commercial sources like coal, oil & electricity and remaining 43.5% is non-commercial energy. Fire wood, charcoal, agricultural residues, vegetable wastes, cow dung, urban and industrial wastes, forest residues are the main sources of this non-commercial energy. The most efficient utilization of these resources comes when they are converted to biomass by appropriate technologies. The non-commercial biomass fuels are the main sources of energy available in the rural areas. The 80% of our population resides in villages are dependent on this non-commercial biomass fuels.

Concept of Biomass

The term biomass refers to all organic matter generated through photosynthesis and other biological processes. The ultimate source of this renewable biomass is the inexhaustible solar energy which is captured by plants through photosynthesis. It includes both terrestrial as well as aquatic matter such as wood, herbaceous plants, algae, aquatic plants and residues, like straw, husks, corncobs, cow dung, saw-dust, wood shavings and other wastes like disposable garbage, night soil, sewage solids, industrial refuse etc. In spite of all these biomass resources available in India, they are not being properly utilized. In fact, a large amount of it is disposed off by burning in open fields causing serious air pollution.

In order to utilise these resources properly, biomass should be converted to energy which can meet a sizeable percentage of the country's demands for fuel as well as energy. Three main approaches can be adopted for generation and proper utilization.

1. Collection of agricultural and forest residues to produce fuels, organic manures and chemical feed stock.

2. Collection of urban and industrial wastes as fuel in boilers and as a feedstock for producing methane and some liquid fuels.

3. Growth of some specific energy plants for use as energy feed stock and cultivation of commercial forestry, aquatic and marine plants for different products.

By a number of processes, the collected wastes can be converted into solid, liquid and gaseous fuels. The technologies include thermal, thermo-chemical and bio-chemical conversions. The actual processes in these technologies are combustion, pyrolysis, gasification, alcoholic fermentation, liquefaction etc.

The main products of conversion technologies are energy (thermal, steam, electricity), solid fuels (charcoal, combustibles) and synthetic fuels (methanol, methane, hydrogen gas etc.). These can be used for different purposes like cooking, lighting, heating, water pumping, electricity generation and as industrial and transport fuels.

Types of Biomass 

Depending on the nature and availability of these wastes and organic residues they can be utilized in different manners as described here. 

1. Fuel biomass

By some processes and procedures, biomass products like fuel gas, liquid fuels, gaseous fuels etc. are obtained, which are given here 

a. Biomass from plants or animal origin are directly burnt for cooking and other purposes. Municipal and sewage wastes, industrial wastes and agricultural wastes are converted to energy which can meet the demand for energy in rural sector. 

b. Paddy straw and rice husk can be profitably converted to fuel gas by thermal decomposition (Combustion) 

c. Ethanol, which is used as a liquid fuel can be produced from carbohydrates by alcoholic fermentation. 

d. When wood and agricultural residues are heated in the absence of air (pyrolysis), charcoal is the resultant product which can be used as a fuel more advantageously than wood. 

e. By the process of gasification, gas is evolved which can be used as a fuel for engines.

f. Biogas, which is popular in rural areas is produced by anaerobic fermentation from farm wastes.

2. Feed biomass

Conventionally, crop residues are used as cattle-feed. However, some of them with high percentage of lignin or non-digestible constituents need certain treatments such as soaking in water, alkali/alcohol to make their use as a fuel. The oil-cakes of various crop seed like cotton, rubber, tobacco etc. can also be used as a feed after extraction of toxic materials.

3. Organic fertilizer biomass

Dry fermented slurry can be used as a direct organic fertilizer for crop land.

4. Fibre biomass

The fibrous agricultural wastes and residues are being profitably utilised for making pulp for cheap grade paper.

5. Chemical biomass

Highly siliconous agricultural residues like rice husk and rice straw can be converted into useful chemicals like morphous silicon, silicate products and solar grade silicon. Furfural an another chemical can be produced from biogases, cotton seed hulls, corn-cobs, flax fibres, oat hulls etc., which is used as a solvent for some petroleum products.

Sources of Biomass 

Important sources of biomass and their characteristics are given here. 

a. Crop residue and farm wastes

The straw of cereals and pulses, stalks and seed coats of oil seeds, stalks and sticks of fibre crops, pulp and wastes of plantation crops, peelings, pulp and stalks of fruits and vegetables and other wastes like sugarcane trash, rice husk, molasses, coconut shells etc. comes under this category. 

Most of the crop residues have a higher ash content and mainly constitutes carbon, oxygen and hydrogen. Volatile matter content is 60-75%. The agricultural residues are hygroscopic in nature. Ash content varies from 0.5 to 2.8 per cent.

b. Industrial wastes 

These wastes include wastes from paper mills, chemical mills etc. for eg., paper wastes, plastic wastes, textile wastes, gas, oil, paraffins, cotton seeds and fibres, bagasse etc. Plastic and rubber wastes have good calorific value.

c. Forest wastes

Logs, chips bark and leaves together constitute forest wastes. Sawdust is the forest based industry waste. Forest products are also used as a. domestic fuel in many developing countries.

d. Logging residues

Tree tops, small stems and roots removed from a standard logging operation and broken debris generally considered as logging residues. It contains 40-50% moisture, 50% carbon, 40% oxygen and mtrogen 5%.

e. Residues of wood product industries

Bark, knots, sawdust etc. are obtained from wood product industry. Moisture content of these residues is around 20% with 67% volatile matter and 11 % organic carbon.

f. Residues from pulp and paper industries

The bark and black liquor produced in pulp and paper factories can be used as major source of energy in the paper industry. Moisture content varies from 5-10% with organic. carbon 8-11 per cent.

g. Municipal solid wastes

Generally municipal solid wastes refer to a mixture of domestic, small construction and demolition wastes left out within a community. Composition of municipal solid wastes is given in a Table:

1. It shows the heterogenic nature of these waste mixture

Table 1 : Composition of municipal solid wastes

h. Municipal sewage sludges

The sludges contain 95% water, and 5% organic matter and nutrients as the main constituents. These can be utilized for the production of methane through anaerobic digestion.

I. Animal Wastes

The moisture content of the manures ranges from 60 to 85 percent. The nitrogen varies from 0.3 to 0.9 %, phosphorus 0.05-0.1 % and potassium 0.12 to 0.8% Available statistics indicates production of 1300 million tonnes of dung annually from all types of animals. Of the total produced, 84% is of cow and buffalo dung and 13% goat and sheep droppings. Dung is used as a fuel in the form of cakes and biogas.

Availability of biomass resources in India along with their coal equivalent is shown in Table 2.

Table 2. Major biomass resources in India

The different processes that are followed in our country to make these wastes into a useful fuel are discussed.

Combustion

Agricultural residues-fired thermal power plants of small capacity are becoming popular with agro-processing industries. Thus, sugar mills use bagasse (bi-product, wastes of sugar mills) for meeting their energy needs and rice mills bum husks. Combustion involves burning of biomass fuels in the presence of air to generate heat energy. Main products of combustion are ash and fuel gases. Usually fuels are burnt on hot furnace. The heat generated is used to drive steam engines and also for irrigation & thrashing purposes.

Pyrolysis

When wood and agricultural residues are heated in the absence of air they breakdown into a complex mixture of liquids involves heating organic matter at a high temperature of 540-1100oC in the absence of air for several hours. The earthen kilns or pit kilns or brick kilns or portable steel kilns are used for this process of which portable steel kilns make good charcoal.

Charcoal has a high calorific value, easy to transport, store and distribute and more efficient in burning and has a characteristic feature of creating less pollution. From eight tonnes of wood, around one tonne of charcoal can be made. The gases that are produced during the process of pyrolysis can be converted or synthesized into methanol and liquids which are used as fuels. Depending on the temperature, the degradation stages of pyrolysis are given in Table 3.

Table 3 : Different stages of degradation in pyrolysis

Destructive Distillation 

The destructive distillation is carried out in long steel retorts. Only the wood wastes such as branches, trunks of trees are used as raw materials. The process involves decomposition of wood at high temperatures in the absence of air. At an initial temperature of 230°C, the moisture is evaporated and then the temperature is raised to 370°C and maintained I for 6 hours. At the end of this period, wood is converted to charcoal which is cooled in the absence of air for 48 hours. After cooling, they are spread in open sheds, two days for drying and is ready for supply to consumers. The vapours of the volatile matters that are formed during the distillation process subsequently condensed to tar, methanol, acetic acid, methyl acetate, oil and gas. The uses of these distillation \ products are detailed here. 

1. Charcoal -Apart from its use as a fuel, charcoal is used as a source of carbon in making carbon-di-sulphide.

2. Methanol -It is used as a solvent and an antifreeze for automobiles.

3. Acetic acid -It is used as a raw material for the manufacture of acetic anhydride, sodium acetate, cellulose acetate, ethyl acetate, butyl acetate etc.

4. Methyl acetate -is a solvent used in paint industry.

5. Tar -A product of tar known as "pitch" is used as rubber softener.

6. Oils -They are used as solvent and insecticide.

7. Gases -Gases are used as fuel for heating wood distillation and fuel for boilers.

Gasification 

Gasification is a process of degradation of carbonaceous material (wood wastes) under controlled air or pure oxygen at a high temperature of l000°C. As a result of gasification high amount of gases are produced. Biomass gasification is done in gasifiers designed m various ways. Types of gasifiers are generally classified based on the physical conditions of the feed stocks in the gasifiers. Fixed bed gasifier, stired bed gasifier, tumbling bed gasifier, fluidized bed gasifier etc. are in use. 

In course of gasification, a number of chemical reactions takes place. As soon as the biomass is ignited four distinct zones are set-up in the gasifier-unit. Biomass when introduced into the gasifier, it enters into drying zone where the temperature is 200-400°C. The products of this zone are vapours of tar, organo-chemicals, and liquid oils. After drying zone it enters into pyrolysis zone, temperature of 400- 750°C. Pyrolysis results in char, organic liquids and some gases. At 750-l000°C, mainly gases like carbon-di-oxide, carbon mon- oxide, hydrogen, methane etc. are produced which is called as gasification zone. The oxidation zone which is at l000-l400oC also produces gases like nitrogen, carbon-di-oxide, hydrogen etc. When the process of oxidation is over, with the steam treatment, ash, an inert material is formed. The whole process is depicted in Fig. l.

Carbon monoxide -20-22%, Hydrogen -15-18%, Methan -2-4%, Carbon-di-oxide -9-11%, Nitrogen -50-53%. 

Producer gas somewhat burnt like a natural gas which Ican be used as a fuel for engines.

Anaerobic Bio-gasification 

Degradation of organic matter in the absence of air to methane and carbon-di-oxide is called anerobic biogasification. In villages, cattle manure is used as a fuel for cooking purpose. So, preparation of biogas has become popular among rural people. Biogas can be utilized for cooking, lighting, operating diesel engines, water pumps etc. As cattle dung is the main raw material for the production of biogas, this method has been given importance. The main advantages of the biogasification are given here.

A. Advantages

The biogas (also called as gober gas) production can be started by constructing a permanent structures (tanks) at a convenient place at home. It can be manufactured with least maintenance. Initial investment is also cheap. Gober gas contains 60% methane and 40% carbon-di-oxide. The digested manure contains 1.5-2% nitrogen and other soil nutrients, which can be used as an organic fertilizer. The biogas can be easily purified to methane enriched fuel gas of high calorific value. The process of conversion of raw animal dung as well as some agricultural residues to biogas is detailed as follows.

B. Process

The conversion carbon-di-oxide and methane takes place by bacteria. The biogas from cattle dung requires an optimum temperature of 35°C. The biomass is kept in closed tanks where the temperature is 35-38°C and the conversion process is carried out anaerobically (in the absence of air). The digestion time varies from 15-30 days. On an average biogas generated from cattle dung is composed of 60% methane and 40% carbon-di-oxide. The basic structure of the biogas plant is given in Figure no. 2.

When the cattle dung is used as feed stock, the biogas plant is 10 be filled with a homogenous slurry made from a mixture of fresh dung and water in a ratio of 1:1. Subsequent digestion changes results in the production and accumulation of gas in the empty space at the upper portion of the tank. The gas accumulated presses the slurry digester and displaces it into the outlet displacement chamber. In other words, the slurry level in the digester falls where as in the outlet chamber, it starts raising. The slurry from the outlet displacement chamber can be used as organic fertiliser. The gas outlet pipe will be connected to the gas utiliser points by pipelines.

C. Feasibility

Many developing countries are encouraging for installation of biogas plants to meet the demand for fuel. India is one of the pioneer country in biogas technology where biogas research and plant construction has been caused out over the past 30 years. In most of biogas plant, cattle dung is used for gas production. Though, biogas is used mainly for cooking and lighting purposes, there are many other advantages also. It can be used in internal combustion engines to power water pump and electric generators. The most economical benefits are minimizing environmental pollution and meeting the demand of energy for various purposes. In general, five cattle produces enough dung to generate about 2m3 biogas to meet the demand of cooking and lighting for a family of four-five people.

Alcoholic Fermentation 

Transformation of sugars into alcohols by fermentation is a common practice. Fermentation is carried out by a group of living organisms: yeasts or bacteria. Ethyl alcohol, commonly known as alcohol is one of the most important and popular industrial fermented products. It is a liquid fuel, and can be used as an alternative to automobile fuels. The sugar enriched materials like cane sugar, beet sugar, fruit sugar, potato, com, rice or any other crop of high sugar contents can be used as substrates mainly, along with starchy and ligno-cellulosic materials.

Materials for fermentation

Following are the types of substrates used for alcehot production.

1. Sugary materials

Examples of sugary materials like sugarcane and its biproducts (bagasse, molasses) and sugarbeet, fruit juice, sweet sorghum, sweet-potatoes etc. Sugarcane molasses is largely being used in many countries for alcohol production. The yield of ethanol is directly proportional to the amount of sugar present.

2. Starchy materials

Tapioca, maize, wheat, barley, oat, sorghum, rice and potatoes are the starchy materials that are used in ethanol production. It has been estimated that 11.7 kg of corn starch can be converted into about 7 liters of ethanol.

3. Lignocellulosic materials

The sources of cellulose and lignocellulosic materials are the agricultural wastes and wood. On the basis of technology available today, about 409 litres of ethanol can be produced from one tonne of lignocellulose.

Process of fermentation

Micro-organisms can grow on carbohydrates such as sugars, under anaerobic conditions. During anaerobic growth, sugars or other fermentable carbohydrates are oxidized and ethyl alcohol is produced as a product of the fermentation process. Optimum temperature requirement for alcohol production is 30-38°C and optimum pH ranges from 3 to 8. Proper balance of macro (nitrogen, phosphorus, potassium, sulphur etc.) and micro (iron, zinc, manganese, copper etc.) nutrients are required for the growth of micro-organisms.

When fermentation is over, which normally takes 4-10 days, the resultant product consists of a mixture of alcohols and water. Ethanol (ethyl alcohol) can be recovered by distillation process i.e. vaporization of alcohol-water mixture.

Production of Biomass

Biomass production can be studied under these sub-heads. 

1. Energy plantations

Energy plantation is the practice of planting trees, purely for their use as fuel. Terrestrial biomass i.e. the wood plants has been used since long time to generate fire for cooking and other purpose. In recent years, to meet the demand for energy, plantation of energy plants has been re-emphasized. Government has started many plans like afforestation, agri-horticultural practices etc.. It has been estimated that at present, only l/7th of the world's total energy comes from biomass and a large amount of it remains untapped. In view of getting maximum biomass afforestation, forest management systems will have to be developed. These must include social forestry, silvi-culture tree-use systems, drought tolerant, salt tolerant, pollutant resistant plantations and high density energy plantations.

Annual plants in energy plantations should have fast growth, stress resistance, less palatable to cattle and other animals, high calorific value, absence of deleterious volatiles when smoke comes out, high yield of biomass and should be disease and pest free.

2. Social forestry

Plantation through social forestry has been much emphasized by the government of India to meet the demand of fuel and fodder in rural areas. This includes planting trees along road sides, canals, railway lines and wastelands in village. Some of the important plants are Acacia nilotica, Albizia lebbek, Cassia fistula, Eucalpytus globulus, Ficus glome rata, Morus alba, Ferminalia arjuna etc.

3. Silvi-culture energy farms

These are known as short rotation forestry. These silviculture farms employ techniques more similar to agriculture than forestry. The chief objective of energy plantation is to produce biomass from the selected trees and shrub species in the shortest possible time (generally 5-10 years) and at the minimum cost, so as to satisfy local energy needs. This would certainly relieve the pressure on the consumption of fossil fuel like kerosene and prevent the destruction of plant cover, which is one of the primary component of life support system. Due to its high yield per unit area, smaller land requirements for biomass output, shorter life span, increased labour efficiency are the advantageous to adopt silviculture.

Different sources of biomass, their forms, their conversion process to respective forms of. energy are given in Table no. 4.

Table 4:  Biomass as a source of energy​

Thus production of biomass helps in developing cheaper source of energy from unutilized agricultural residues, wastes, forest wastes, plantations etc. which is an alternative source of energy for fossil fuel energies.