BIOMASS Energy

Biomass

It is the organic matter in trees, agricultural crops  and other living plant material. It is made up of carbohydrates-organic compounds that are formed in growing plant life. Biomass is solar energy stored in organic matter.

As trees and plants grow, the process of photosynthesis uses energy from the sun to convert carbon dioxide into carbohydrates (sugars, starches and cellulose). Carbohydrates are organic compounds that make up biomass. When plants die, the process of decay releases the energy stored in carbohydrates and discharges carbon dioxide back into the atmosphere. Biomass is a renewable energy source because the growth of new plants and trees replenishes the supply.

The biomass are generally termed for the following:-

• Land and water based vegetation 
• Organic wastes
• Photosynthetic organisms

The primary step in the buildup of biomass is photosynthesis. In photosynthesis, sunlight is absorbed by chlorophyll in the chloroplasts of green plant cells and is utilized by the plant to produce carbohydrates from water and carbon dioxide.

Biomass can be categorized broadly as woody, non-woody and animal wastes. It is also characterized as natural and derived resources. We will briefly discuss them.

Woody Biomass: All the biomass from forests, agro industrial plantations, bush trees, urban trees and farm trees are termed as woody biomass. It is generally a high valued product because it has diverse uses such as timber, raw material for pulp and paper, pencil and matchstick industries and cooking fuel.

Non-Woody Biomass: It is referred to as crop residues like straw, leaves and plant stem (Agro wastes), processing residues like sawdust, bagasse, nutshells and husks and domestic wastes (food, rubbish, sewage). Many of these are harvested at the village level and are essentially used either as fodder or cooking fuel.

Animal Wastes: It constitute the wastes from the animal husbandry. Animal dung is a potentially large biomass resource and dried dung  has almost the same energy content as wood. 

The efficiency is only about 10% when the dung is burnt to produce heat. The efficiency of conversion of animal residues could be raised to about 60% by digesting anaerobically (to produce biogas).

Natural Biomass Resources: Biomass resources include wood and wood wastes, agriculture crops and their waste byproducts.

Agricultural Waste: Begasse, soya husk, cotton straw, rice husk, arhar stalk, wheat straw and peanuts, peanut stalk etc.

Derived Biomass Resources: Municipal solid waste, animal wastes, waste from food processing and aquatic plants etc.

Forest Products: Wood, logging, residues, trees, shrubs and wood residues, sawdust, bark etc. from forest cleanings.

Energy Crops: Energy crops are short rotation woody crops, herbaceous woody crops, grasses, starch crops (corn, wheat and barley), sugar crops (cane and beet), oilseed crops (soya bean, sunflower, safflower). These are the biomass materials, which can be used during the pyrolysis process and to produce the bio-oil.

ENERGY CONTENT OF BIOMASS:-

The energy content of biomass that can be obtained after transformation is an important characteristic of biomass. The moisture content is measured as the heating value. For woody biomass resources, the moisture content of the wood is the main determinant of the available energy. For non-woody biomass, the ash content and the moisture content affect its energy value.

The moisture content is variable and depends on the extent to which the wood is dried. That is why the energy content of fuel varies from 10.9 - 21.3 GJ per ton, with an average of about 16.9 GJ for oven - dried wood (moisture content of 0 percent). One tonne of air-dried wood (average 20% moisture content) has an energy value of about 13.5 GJ.

One of the interesting aspects of wood is that it can be used for fuel purposes without any treatment or modification except that of being cut into small pieces. This because of its high volatility, high char reactivity and low sulphur and ash content.

Lignin is more abundant and has a higher degree of polymerization in softwoods  in hard woods. Woods having higher lignin content and plenty of extractives will have a higher heating value. Cellulose and hemi-cellulose contain only around 17.5 MJ/kg, while lignin has about 26.5 MJ/kg and extractives can approach 35 MJ/kg.

It can be seen that woody biomass can be converted to charcoal, liquid fuels like methanol and ethanol and producer gas (CO and nitrogen).

Energy Content of BIOMASS

Characterisation of biomass

Since biomass differs greatly in their chemical, physical and morphological properties, they make different demands on the method of their utilization and consequently require different types of technologies.

Most important properties which may affect the gasification/pyrolysis, are calorific value, moisture content, ash content, volatile matter, fixed carbon as melting point and bulk density. The moisture content, ash content, volatile matter and fixed carbon content of a particular fuel can be determined by proximate analysis of the fuel.

The method of gradation takes into account the comparative effect of different properties of the fuel in the gasification process.

1). Effect of Calorific Value:-

As the calorific value increases, the amount of gas produced at the normal temperature and pressure increases. This simply means that with the increase in calorific value, the material consumption also decreases.

2). Effect of Moisture Content:-

With increasing moisture content, the calorific value of wood decreases and correspondingly the amount of gas produced at normal temperature and pressure (NTP) and fraction of the
useful components decrease.

3) Effect of Ash Content:-

With increasing ash content, the removal of the ash from the gasifier becomes more power consuming and fraction of useful component also decrease. If the as content is very high, the biomass can be taken as unsuitable for gasification because due to removal of large amount of ash, material flow problems in the gasifier can arise.

4). Effect of Volatile Matter:-

The volatile matter helps in the complex reactions of gasification but if the volatile matter content is high, tar formation is also high. Tar is one of the products of thermal decomposition of solid fuels. The tar yield is therefore related to the volatile matter in a fuel.

5). Effect of Fixed Carbon:-

With increasing fixed carbon, the amount of Gas produced and Calorific Value of the producer gas so obtained increase. The conversion of Carbon  to CO which is one of the main components of producer gas is the result of many complex reactions taking place during gasification.

6). Effect of As Melting Point:-

The ash melting points is essential to determine to maximum temperature that can be achieved at any zone in the gasifier exceeds the ash melting temperature, there are chances of clinker formation. The clinkers once formed can  cause breakdown of the ash removal mechanism of the gasifier.

7). Effect of Bulk Density:-

Bulk density is one of the important factors for designing of a gasifier to obtain proper material flow. Biomass with very low bulk density have problem of proper fall through different zones of gasifier. The bulk density varies significantly with moisture content and particle size of the fuel.

Video Regarding:- What is Biomass?

Environmental Effects of Biomass:-

The use of biomass energy provides a number of environmental benefits. Some of them are:-

• It can help mitigate climate change
• It can reduce acid rain
• It can prevent soil erosion and water pollution
• It can minimize pressure on landfills
• It can provide wildlife habitat
• It can help maintain forest health through better management.

The use of biomass will greatly reduce the greenhouse gas emissions. Biomass releases carbon dioxide as it burns but the plants also need carbon dioxide to grow. This creates a closed carbon cycle. All the carbon dioxide released during the combustion of biomass materials is recaptured by the growth  of these same materials. Thus, with biomass combustion there is no net increase in carbon dioxide released into the atmosphere, In addition, substantial quantaties of carbon can be captured in the soil through biomass root structures, creating a net carbon sink.

Biomass has other environmental benefits. The waste land may be utilized to grow biomass crops thar will retore soil carbon, reduce erosion and chemical runoff and enhance wildlife habitat.

GAURAV SARASWAT

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