Biomass: Meaning, Sources and the Future of Biomass!
The capacity to do work is referred to as energy. Energy may be considered as a form of matter which is inter-convertible. The modern man is mostly dependent on three sources for his energy needs—coal, natural gas and oils, collectively referred to as fossil fuels or fossil energy sources.
The fear of depletion of global fossil fuels has forced man to look for suitable alternative energy sources such as solar, hydro, tidal and wind power, and more recently nuclear energy. In addition to these, advances in biotechnology have helped to fruitfully utilize the energy from biological systems.
Biomass:
Biomass is the total cellular and organic mass, produced by the living organisms. It is the primary product of photosynthesis and is a good source of energy i.e. bioenergy. Broadly speaking, biomass represents all forms of matter derived from biological activities. These include plants and agricultural products, microorganisms, animal wastes and manure.
The term biomass is also used to collectively describe the waste materials produced in food and agricultural industries. Besides being a good source of energy, biomass is important for the production of several commercially important products. Thus, biomass is appropriately regarded as a renewable source of energy which can be directly converted to energy or energy carrier compounds by various means.
In most developed countries, biomass is utilized for the production of industrial and commercial products (ethanol, oils, methane, and single – cell protein). In contrast, in the developing countries (India, Latin America, Africa), a major part of the biomass is directly used as a source of energy (as firewood).
It is estimated that the annual net yield of plant biomass is around 175 billion tons of dry matter (125 billion tons on land and 50 billion tons on oceans). Forests significantly contribute to the production of land based biomass (around 45%). Agricultural crops on the cultivated land account for about 6% of the plant biomass.
The agricultural biomass products (cereals, pulses, oils, animal feed etc.) adequately meet requirements of foods for humans and animals, besides other basic needs (fuels, chemicals etc.). From the chemical point of view, about 50% of the land produced biomass is in the complex form of lignocellulose.
Fossil fuels-derivatives of biomass:
The modern society is dependent of on the nonrenewable sources of energy namely oil, gas and coal. These fossil fuels are actually derivatives of ancient biomass. It took millions of years for the fossil fuels to be deposited beneath the earth and oceans.
However, in just within a century of exploration, the major fuel reserves (particularly gas and oil) are depleted, and at the present rate, they are not likely to last long. As such, there exists an energy crisis throughout the world. Consequently, researchers continue their search for alternate and renewable sources of energy.
Photosynthesis-the ultimate source of energy:
Photosynthetic organisms are the ultimate sources for trapping the solar energy. In the presence of photosynthetic pigment chlorophyll, carbon dioxide is converted into complex carbohydrates with the evolution of oxygen.
In the reactions that follow later, solar energy is trapped into molecules such as fat and proteins, besides other complex carbohydrates (cellulose, hemicellulose, and lignin). Photosynthetic organisms are the true solar energy converters. It is estimated that at present more than 10 times more energy is generated by photosynthesis annually than consumed by the world’s population.
Unfortunately, the role of photosynthesis is not well recognized to solve the present day problem of energy crisis. This, despite the fact that it is only the photo synthetically produced biomass that is available today in the form of fossil fuels. The biomass produced by photosynthesis can be appropriately utilized for the production of fuels (alcohol, methane) and various other commercial products.
Chemical nature of biomass:
The plant biomass is mainly composed of cellulose, hemicellulose, lignin, starch, proteins, water soluble (sugars, amino acids) and fat soluble (oils, pigments) compounds. In fact, majority of these constituents are present in the plant cell walls.
There is a wide variation in the chemical composition of the biomass, depending on the source. For instance, the biomass obtained from sugar cane and beet sugar is rich in sugars while the biomass of potato and topioca is rich in starch. On the other hand, cotton has high content of cellulose. The chemical nature of biomass derived from industrial and municipal wastes is highly variable which mostly depends on the sources that contribute to the biomass.
Sources and Utilization of Biomass:
The major sources of biomass are natural vegetation, energy crops, and agricultural, industrial and urban organic wastes (Fig. 31.1). Their production in turn is dependent on the solar energy.
The natural vegetation (growing natural forests and aquatic weeds) significantly contributes to biomass. Wood-rich plants are grown in many countries (particularly developing countries) to generate fire for cooking and other purposes. In recent years, well planned and organized plantations are carried out in some countries to produce biomass to meet energy demands. For instance, sugar cane and cassava plantations in Brazil and Australia are used for ethanol production. Plants rich in lignocellulose are grown in America and Sweden which are useful for the production of liquid fuels (ethanol, methanol).
Agricultural, industrial and municipal wastes were earlier considered as useless and discarded. But in the recent past, many countries have developed methods for converting these wastes into biofuels and commercially important products. The successfully used agricultural wastes include straw, bagasse, bran, cotton wastes. Among the industrial wastes, molasses, whey, distillery wastes and sewage are the important ones. The biomass is utilized for the production of biofuels and various other compounds. The technique mainly depends on the chemical nature and moisture content of biomass.
Combustion:
Low moisture containing biomass (wood, straw, bran) can be directly burnt by a process referred to as combustion to generate electricity.
Dry chemical processes:
The biomass with little moisture content can be subjected to various dry chemical processes-pyrolysis, gasification to produce methanol, oil and ammonia biomass.
Aqueous processes:
The biomass with high water content is used in aqueous processes such as fermentation to produce ethanol, oils and methane. An overview of the sources and utilization of biomass is depicted in Fig. 31.1.
Production of Alcohol from Biomass:
Alcohol, chemically ethanol (C2H5OH) has been produced by fermentation for thousands of years. Although the developed countries these days prefer to manufacture ethanol by chemical means, the developing countries continue to produce it by microbial fermentation. Alcohol is the liquid fuel which is mostly produced from the biomass. The raw materials (biomass) used for alcohol production include starchy materials (wheat, rice, maize, potato) and cellulosic materials (wood, agricultural wastes).
Energy-Rich Crops:
Some of the plants are very efficient in converting CO2 into biomass and such plants are collectively referred to as energy-rich crops.
Sugar and starch crops:
Certain plants like sugar cane, sugar beet, cereals and tuber crops produce high quantities of starch and fermentation sugars. These crops supply energy-rich foods and feeds. Such plants are useful for production of biofuels, particularly ethanol often referred to as bioethanol.
Wood-rich plants:
Some plants grow very fast and they serve as good suppliers of wood. E.g. Eucalyptus, Butea, Melia, Casurina. These plants are important sources of firewood. It is estimated that approximately 50% of the total wood harvested annually is utilized for the purpose of firewood. Wood is also useful for the supply of pulp for paper manufacture.
Petroleum plants:
There are certain plants which can accumulate high molecular weight hydrocarbons. They are referred to as petro-crops or gasoline plantations. The products of these hydrocarbon-rich plants can serve as good substitutes of conventional petroleum and petroleum products.
The rubber plant (Hevea rubber), grown in South-East Asia is the principal source of rubber. Rubber is collected in the form of latex from the stems of trees. This plant meets about one third of the total world’s demand of rubber.
However, the rubber produced from petroleum is preferred for use in automobiles and planes, due to low-cost and high elasticity. Besides Hevea rubber, there are some other plants for the production of natural rubber e.g. Parthenium agrentatum (guayule) Taraxacum koksaghyz (Russian dandelion) grown in Mexico and some parts of USA.
Euphorbia lathyrus and E. terucalli contain high contents of terpenoids (complex hydrocarbons) that can be directly converted to gasoline/petrol. It is estimated that E. terucelli can yield about 5-10 barrels of oil/acre/year.
Aak plant (Calotropis procera) secretes latex which is very rich in hydrocarbons. These hydrocarbons, and the yield are comparable to Euphorbia lathyrus, and they also serve as good substitutes of petroleum.
For obvious reasons, the cultivation of petroleum plants is encouraged throughout the world.
Besides its utility for the generation biofuels (alcohol, methane), biomass is also used for the production of butanol, acetone, single-cell protein and many other products. As such, the contribution of biomass to the world’s requirements of energy is very low. It is around 5% in the U.S.A., and may be a little higher in the developing countries. However, being a renewable source of energy, biomass will have immense value in future. This is particularly true as the world’s non-renewable fuels (gas and oil) get depleted.
There is a growing realization on the fuel value of biomass. In the coming years, biomass production and utilization strategies will be fully exploited. In addition, further improvements in the biotechnological processes for better management and utilization of industrial, agricultural and domestic wastes will also solve the problem of world energy crisis.