Dr. P. K. Jena* in Bhubaneswar, May 14, 2022: With rapid increase in population, transport system, industrialization, urbanization and adoption of modern agricultural practices in different parts of the world, demand for energy mostly from fossil fuels, is increasing very fast in recent years. As a result, large amounts of green house gases namely carbon dioxide, methane, oxides of sulphur and nitrogen, particulate matters etc., are being released to the atmosphere causing hazardous global warming and climate change.

To address the situation, governments of different countries are giving importance to use of renewable energy from sun, wind, biomass, and small hydro power as well as from nuclear source. In the present scenario, biomass is considered by scientists and technologists as potential material for producing renewable energy and can be effective to replace the fossil fuels to a considerable extent.

Biomass has always been a major source of energy particularly for cooking and providing heat for mankind since the dawn of civilization. Large quantities of biomass can be obtained through cultivation of various energy crops, plants and bushes, perennial grasses, harvesting of leaves and branches of trees from forest and also from organic wastes of agricultural, industrial and domestic origin including urban and industrial sewage.

Biomass is considered to be carbon neutral on the assumption that, carbon as carbon dioxide released from biomass during its burning, will be reabsorbed in course of growth of the trees. On this assumption and the availability of biomass, it is felt necessary to use increasing amounts of renewable energy from this source in Industrial, domestic and transport sectors to minimize the use of fossil fuels.

Biomass includes all organic materials that stems from plants such as algae, trees, crops including all land and aquatic vegetation and also organic wastes such as agricultural and domestic wastes, animals dung, human excreta etc. Biomass is the plant material produced through reaction of carbon dioxide present in air with water, moisture and sunlight. This process is known as Photosynthesis. Generally Photosynthesis converts <1% of sunlight to be stored as chemical energy in biomass.

Biomass as an energy and heat source has been in use since the dawn of human civilization. Most of the countries particularly the developing ones use biomass for cooking and producing heat in their houses. However, due to fast depletion and polluting nature of non- renewable fuel resources like coal, oil and natural gas, it has been essential to use a renewable energy source like biomass for meeting the fast increasing need for energy in different sectors of human society.

For harnessing energy the typical biomass resources available on this planet include vegetation and forests, agricultural wastes, urban and rural organic wastes, waste from wood processing and agro industries. If the biomass is burnt efficiently oxygen in the atmosphere will combine with carbon present in the biomass to produce carbon dioxide and water.

This carbon dioxide and water through photosynthesis will produce new biomass. It is accepted that, biomass absorbs the same amount of carbon dioxide while growing that it releases when it is burnt. Assuming this in the out set, we may say that biomass as a fuel will add negligible amount of carbon dioxide to the atmosphere.

In recent time, a lot of developments have taken place to extract energy and heat from various types of biomass including organic wastes. In order to choose any thermo-chemical or bio-chemical process, it is necessary to know the type and the quantity of biomass feed stock available. The thermo-chemical conversion of biomass can be carried out in four different ways namely, Combustion, Gasification, Pyrolysis and Bio chemical conversion.

Bio-chemical conversion can be carried out in two different ways namely, anaerobic digestion to produce methane along with carbon dioxide and fermentation to produce methanol.

(a) Combustion

Combustion of biomass to produce heat for cooking food and heating purposes is a common practice in rural areas of most of the developing world. The quality of biomass depends on the amount of moisture present in it. In practice, biomass containing less than 50 pct. of moisture can be used through its combustion under a proper condition resulting in a temperature around 8000 – 10000C. However, the high moisture content of biomass is better suited for biological conversion processes.

The size of combustion plant ranges from very small scale (e.g. for domestic purpose) up to large-scale industrial plants producing energy in the range of 100–3000 MW of electricity. Co-combustion of biomass in coal-fired power plants is an especially attractive option because of high conversion efficiency of these plants in addition to producing less amount of carbon dioxide and particulate matters. It is reported that, the combustion efficiency of biomass based power plants ranges from 20-40 pct. The higher efficiency can be achieved by scaling up the combustion system over 100 MWe.

(b) Gasification

Gasification is the conversion of biomass into a combustible gas mixture by partial oxidation of biomass at high temperatures, typically in the range of 8000 to 9000C. The low calorific value (CV) gas produced (about 4–6 MJ/N m3) can be subjected to burning directly or used as a fuel for gas engines and gas turbines. The product gas also can be used as a feedstock for production of chemicals (e.g. methanol). During gasification, the biomass is converted into a gaseous fuel by heating in a suitable reactor in presence of air or steam or both. The oxidation of biomass is carried out in one step where as in gasification first the biomass is converted into a combustible gas. In the gasification process, the product consists of mixture of carbon monoxide, carbon dioxide, methane, hydrogen and water vapour.

(c) Pyrolysis

Pyrolysis is heating the biomass in absence of air to around 5000C in order to convert the biomass to liquid, solid and gaseous fractions. Pyrolysis can be used to produce predominantly bio-oil, enabling the conversion of biomass to bio-crude with an efficiency up to 80%. The bio-oil can be used in engines and turbines

Through pyrolysis, it is possible to get fuel gas with 80% yield (flash pyrolysis, at low temperature), char coal up to 35% yield (carbonization at slow pyrolysis) and bio oil up to 80% yield (flash pyrolysis at low temperature).

(d) Bio-chemical conversion

In the biological process the biomass can be subjected to fermentation or anaerobic digestion (AD).

(i) Fermentation

Fermentation of biomass has been commercially used in a number of countries to produce ethanol from bit sugar, starch crops like maize, wheat etc. The biomass is ground down to very small size and the starch is converted by enzymes to sugars, with yeast followed by conversion of the sugars to ethanol. The solid residue from the fermentation process can be used as cattle-feed and in case of sugar cane, the bagasse can be used as a fuel for boilers or for subsequent gasification. The conversion of lignocellulosic biomass (such as wood and grasses) is more complex and requires acid or enzymatic hydrolysis before the resulting sugars can be fermented to ethanol. Such hydrolysis techniques are currently at the pre-pilot stage.

(ii) Anaerobic Digestion (AD)

Anaerobic digestion of the finally divided biomass is carried out in aqueous slurry at about 350C in presence of bacteria and absence of oxygen to produce a gas mixture containing mainly methane and carbon dioxide. The biomass is converted by bacteria in an anaerobic environment, producing the biogas. The biogas can be used directly as a fuel and also in gas turbines. The biogas can be upgraded to the natural gas quality by removal of carbon dioxide. The overall efficiency in conversion of biomass to electricity is about 10 – 16%. Anaerobic Digestion process produces biogas and manure.

Biogas from Organic Landfill wastes

The municipal and industrial organic wastes which are used for landfill can also be used to produce bio-energy. Due to biomass decay the efficiency of production of biogas depends on the waste composition, moisture content of the landfill, and temperature. Biogas released from landfill is commonly called land- filled gas. A typical biogas from this route contains around 50% methane, 45% carbon dioxide and 5% other gases like hydrogen sulphide, nitrogen etc.

Theoretically a good landfill site should yield 150-300 m3 gas per ton of waste which is equivalent to 5-6 GJ of energy, but in practice gas yields are much less. The gas is collected by an array of interconnected perforated pipes buried at depths up to 20 metres in the waste. A landfill must be at least 12m deep and have about a million ton of wastes in place for landfill gas collection and power production to be techno- economically viable.

In new sites, this pipe system is constructed before the wastes start to arrive, and in a large well established landfill there can be several km lengths of pipes, with as much as 1000 m3 of gas per hour being pumped out. The gas typically fuels a boiler to produce heat (steam, hot water, etc.) or gas turbine to produce electricity.

It is suggested that, the waste organic biomass like waste from wood processing industry, agricultural waste, urban wood waste, cow dung, human excreta etc. can be utilized to produce bio-energy and the residues can be utilized as manure.

In the present world, most of the energy required in domestic, industrial, transport and similar other sectors are largely derived from fossil fuels releasing millions of tons of green house gases and particulate matters to the environment causing global warming and climate change.

In view of this, serious efforts are being made all over the world to utilize renewable energy a source of which biomass is a promising one. Biomass in the form of trees and plants and those which are generated as organic wastes in domestic, agriculture and industrial sectors can be utilized with the help of modern science and technology to produce bio-energy which is considered as carbon neutral.

• Former Director General, Council of Scientific & Industrial Research, India