What is Biomass Energy?

Biomass is a renewable energy source because the growth of new plants and trees replenishes it. Biomass is the term used for the mass derived from carbonaceous waste of various human and natural activities. It includes agricultural and forest residues, animal waste, household waste and by products of wood and discarded material from food processing plants.

Biomass is basically a hydrocarbon containing carbon, hydrogen and oxygen. Chemical formula for biomass is C_6n(H₂O)_5n.

Solar energy is stored in plants by the process of photosynthesis, absorbing carbon dioxide (CO₂) from the atmosphere and converting into carbohydrates such as sugar, starch and cellulose.

Biomass does not add carbon dioxide (CO₂) to the atmosphere, as it aborbs the same amount of carbon in growing as it releases when consumed as a fuel. So, energy produced from biomass is carbon-cycle neutral.

Biomass can be used in the form of solid, liquid and gaseous fuels. Biomass fuels used in India contribute about 30% of total fuel used at present. It is an important fuel used in over 90% of the rural households and 15% in urban area. The potential of biomass energy in India is over 1,10,000 MW, and a capacity of over 4400 MW has already been commissioned.

TYPES OF BIOMASS AND THEIR APPLICATION

India produces about 55 crore tonnes of agricultural and industrial waste every year. 29 crore cattle population produces about 44 crore tonnes of dung annually. The main types of biomass are described below in brief:

1. Energy crops: Energy crops include fast growing plants which supply wood, vegetable oil and alcohol. These plants are grown on degraded or wasteland. It is called energy farming because the crops are used to produce power.

2. Woody biomass: Woody biomass is obtained from trees. The calorific value of soft wood is 4600 kcal/kg and that of hard wood is 5000 kcal/kg. Woody biomass is mainly used for our energy needs in addition to household and agricultural purposes. In rural areas woody biomass is an important fuel used for cooking and heating purposes. In urban areas, woody biomass is converted into charcoal and used for cooking and other purposes.

3. Crop residues: Crop residues consist of rice husk, wheat straw, corn cobs, cotton sticks, sugar cane bagasse, ground nut and coconut shells. Some of these are converted to briquettes and used as fuel. Bagasse is obtained from sugar mills and used in cogeneration plants. Similarly, rice husk is used in rice mills as cogeneration plants.

4. Animal waste: It is an organic material and is rich source of fuel. Animal dung is mainly used in rural area for cooking and heating purposes in the form of dung cakes. Animal dung is the main raw material of biogas plant and the waste slury obtained from the biogas plant has high nitrogen content and used as manure in agriculture. Biogas has a number of applications like heating, cooking, lighting, engine fuel and power generation.

5. Urban waste: It consists of municipal solid and liquid waste from domestic sewage and effluent from institutional activities. About 4 crore tonnes of solid waste, 600 crore cubic metres of liquid waste are generated in urban area every year. Both the sources are useful for biogass production.

6. Industrial waste: It comprises of paper and pulp industries, starch and glucose industry waste, palm oil industry waste, distillary waste and tannery waste. Industrial waste can be used for power generation by adopting different methods and technology.

ENERGY CONTENT IN BIOMASS

Energy content of biomass depends upon its ingredients. The main ingredients of biomass are carbon (C), hydrogen (H₂), oxygen (0₂). moisture and ash, in addition to some traces of sulphur (S) and nitrogen (N). Carbon is the main source of heat in the biomass. Moisture content of biomass is the quantity of water present in the material and is expressed as percentage of its weight. Moisture content adversely affects the value of biomass as a fuel. Ash is the organic matter left out after complete combustion of biomass. It is also expressed in percentage of its weight.

The energy content of biomass fuel mainly depends upon the carbon, moisture and ash content. High moisture and high ash contents have low energy value and vice versa. For example, woody biomass has low ash content and high energy value in comparison to crop residue which has high ash content and low energy value.

The energy content of wood with 1% ash content and 13% moisture content is 16 MJ/kg. A typical crop residue with 10 % ash and 13% moisture has energy content of 13.5 MJ/kg. Animal dung with 20% ash and 13% moisture has energy content of 14.5 MJ/kg.

BIOMASS BASED FUELS

Biomass is one of the main energy sources, used in the form of solid, liquid and gaseous fuels called biofuels. These are obtained by different conversion processes.

1. Charcoal: It is a smokeless dry solid fuel. It has high energy content.

Charcoal is produced by heating woody biomass at a temperature of 170° to 500°C in the absence of air (pyrolysis).

Charcoal is widely used in domestic as well as industrial applications, The calorific value of charcoal having 30% carbon content is 23 MJ/kg which is obtained at a temperature of PC. At higher temperatures carbon content will be more and will have a higher calorific value.

2. Producer Gas: It is obtained by gasification conversion process of biomass, which takes place at a temperature range of 500 to 1000°C in the presence of air. The main constituents of producer gas are carbon monoxide (CO), hydrogen (H), nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), methane (CH₂). This gaseous fuel is widely used for heating, running engines and power generation purposes. Calorific value of producer gas is 4.35 MJ/ m³ for a fuel content of 35% CO and 65% N₂.

3. Biogas: It is obtained from animal waste, like cattle dung by anaerobic fermentation taken place in a sealed digester at a temperature 35 to 55°C. It is mainly composed of methane (60%) and carbon dioxide (35%) alongwith some traces of other gases. In rural areas it is used for cooking, heating and lighting. Biogass can also be used for power generation purposes.

4. Ethanol (C₂H₅OH): It is a liquid fuel obtained from biomass containing sugar by fermentation process. It can be blended with petrol and diesel which enhances their octane number rating. Also blending of 10% of ethanol with this fuel will save a lot of foreign exchange. The calorific value of ethanol is 30 MJ/kg.

5. Methanol (CH₃OH): The purified producer gas is subjected to liquefaction process over a zine chromium catalyst and is converted into methanol. It is widely used as a liquid fuel having calorific value of 23 M.J/kg. This can be used as biofuel and can be blended with petrol and diesel.

BIOGAS PLANT

Basic Working Principle

Biogas comprises of 60-65% Methane (CH₂), 35-40% Carbon-dioxide (CO₂), 0.5 to 1.0% hydrogen sulphide (H₂S) and rest is water vapours, etc. Biogas is lighter than air. The ignition temperature of biogas is in the range of 650-750°C. It is a colourless and odourless gas, which burns with clear blue flame and has calorific value of 20 Maga Joules (MJ) per cubic meter.

Biogas is produced by anaerobic digestion of animal waste. This conversion process is also known as anaerobic fermentation or biodigestion.

The process takes place in air tight tank called digester in the absence of air. It involves three steps as described below: 

(a) Hydrolysis: In this step of conversion process, all the complex organic matter are broken down into simple water soluble compounds. This process takes about one day at 25°C.

(b) Acid formation: The simple organic materials are decomposed by acetogenic bacteria and converted into acetic acid (CH₃COOH). This process also takes about one day at 25°C. (c) Methane formation: The acetic acid (CH₃COOH) is decomposed into methane (CH₄) and carbon dioxide (CO₂) with the help of methanogenic bacteria.

The whole process of decomposition of biomass into biogass requires many weeks and the total period is called retention period. It depends on feedtstock used and the retention period is 50 days for cowdung; 30 days for night soil and 20 days for pig dung feed stock.

Basic Requirements of a Biogas Plant Site

For selecting a suitable site for a biogas plant following points should taken into consideration: 

(i) The land should be levelled and at a higher elevation than the surrounding to avoid runoff water.

(ii) The soil should not be too loose and should be of good bearing strength. (iii) It should be nearer to place of gas utilization to minimise gas pipeline expenses. 

(iv) It should be near to cattle shed/stable for easy handling of cattle dung.

(v) There should be sufficient supply of water at the plant site. 

(vi) The plant should get clear sunshine during most part of the day.

(vii) The plant should be well ventilated because methane (CH₂) when mixed with oxygen (O₂) is very explosive. (viii) It should be away from any water source used for drinking purposes.

Structure of Biogas Plant

The physical structure of the digester which converts organic matter into biogas is called a biogas plant. They are mainly of two types:

1. Floating Drum Type (Constant pressure, variable volume). 

2. Fixed Dome Type (Constant volume, variable pressure).

(1) Floating Drum Type Biogas Plant

This biogas plant design was approved in the year 1962 by the Khadi and Village Industries Commission (KVIC), and hence is popularly known as KVIC model.

Because of the floating drum structure of the plant constant pressure of the gas is maintained, while the volume varies. In this design the digester chamber is made of brick masonry in cement mortar. A steel or iron drum placed on the top of the digester to collect the biogas produced from the digester. This drum floats over the slurry and moves up when the gas is accumulated in the tank and moves down when the gas is discharged from the tank. There are two separate structures, digester for gas production by anaerobic fermentation of slurry and gas holder for gas collection.

Dung is mixed with water and fed into the digester from the inlet tank. Digester holds the slurry where anaerobic fermentation takes place and biogas is produced. A partition wall is provided in the digester for better circulation and necessary fermentation. Digested slurry is taken out from the digester to an outlet tank.

(2) Fixed Dome Type Biogas Plant

A fixed dome biogas plant consists of an enclosed digester, which is combined with a dome shaped gas holder. It is an economical design and is made of brick, cement and masonary. It has no moving part, so working life of the plan is longer.

Dung and water is mixed in inlet tank, and the slurry is fed into the digester through the inlet gate. The gas is produced in the digester by the inanaerobic fermentation of slurry and is collected in the dome shaped upper part of the digester. When gas is produced pressure in the dome increases. The gas is collected from the gas holder by a gas pipe. Digested slurry flows out to the outlet displacement tank through outlet gate.

Gas pressure increases with the volume of the gas stored therefore the volume of the digester should not exceed 20 m³.

If the gas is required at constant pressure a gas pressure regulator provided with the gas pipe.

ADVANTAGES OF BIOMASS ENERGY

1. It is a renewable source of energy.

2. It is indegenous source.

3. The by-product of biogas production is manure, rich in nitrogen (N), phosphorous (P), potassium (K), which have the capacity to improve the soil fertility, food production and reduce import of chemical fertilizer which currently drains a large amount of foreign exchange (forex).

4. It has inbuilt storage of energy.

5. Biomass is carbon neutral so net air pollution due to carbon dioxide (CO₂) is practically nil, as it releases the same amount of CO₂ as it absorbss during its formation by photosynthesis.

6. Biomass energy also provides economic development in rural areas.

7. There is no problem of waste disposal.

8. Biogas is clean gas which improves health and hygiene.

DISADVANTAGES OF BIOMASS ENERGY

1. The sites for biogas plants are not suitable at all locations.

2. It involves intensive labour and costs for collecting large quantity of biomass required for commercial applications.

3. It has low energy density.

4. It is a dispersed source of energy.

5. Collection of biomass is not reliable.