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Biomass : A Renewable Source of Energy

 
     
 

Biomass as a Source of Energy
The peculiar feature of plants is that they possess various photosynthetic pigments in thylakoids present in cells either in free state or in chloroplasts. The photosynthetic pigments are chlorophyll a, chlorophyll b, chlorophyll c, xanthophylls and carotenoids. The presence of these pigments varies with group of the plants. In prokaryotic photoautotrophs where chloroplasts lack, photosynthetic pigments are present in thylakoids. The different pigments absorb light of different wavelength. During photosynthesis in plant cells, in the presence of chlorophyll CO2 is converted into complex carbohydrates with the evolution of oxygen. The reaction is shown below:

 
light
 
CO2+H2O Arrow (CH2O)n+O2
 
chlorophyll
 

During photosynthesis, solar energy is trapped into light harvesting molecules in the chloroplasts by reduction of CO2 into carbohydrates, fats and proteins. Radiant energy stored in plant is known as primary production which later on creates plant biomass or biomaterial. The rate of storage of photosynthetic products is known as 'primary productivity'. The energy remaining as organic matter (after respiration) is called as net primary productivity (NPP). N.P.P. is expressed as KCal or g/m2/yr. NPP accumulates over time as plant biomass. It is expressed as dry weight of organic matter per unit area (g/m2). Biomass differs from production, which is the rate of organic matter production by photosynthesis. N.P.P. and biomass of some world ecosystems are given in Table 19.2. The biomass is all forms of matter derived from biological activities and present on the surface of soil or at different depth of vast body of water, lakes, rivers, sea and ocean.

 

Content

Energy sources : A general account 

 

Nuclear energy

 

Fossil fuel energy

 

Non-fossil and non-nuclear energy

Biomass as source of energy

 

Composition of biomass

 

 

Cellulose

 

 

Hemicellulose

 

 

Lignin

 

Terrestrial biomass

 

Aquatic biomass

 

 

Salvinia

 

 

Water hyacinth

 

Wastes as renewable source of energy

 

 

Composition of wastes

 

 

Sources of wastes (Industries, agriculture, forestry, municipal sources)

Biomass conversion

 

Non-biological process

 

 

Direct combustion-hog fuel

 

 

Pyrolysis

 

 

Gasification

 

 

Liquefaction

 

Biological process

 

 

Enzymatic digestion

 

 

Anaerobic digestion

 

 

Aerobic digestion

Biomass includes wood, crops, herbaceous plants, residues from agricultural and forest products, manure, fresh water and marine plants and microorganisms as well. Besides plant material, biomass also includes all animal waste, manure, etc. because in essence, the latter are basically plant based (Khashoo, 1988). Different types of biomass of various sources are given in Table 19.3.

Table 19.2. Net primary productivity and plant biomass of world ecosystems (source: Whittakar and Likens, 1973).
Ecosystems (in order of productivity)
Mean net primary production per unit area (g/m2/yr)
Mean biomass per unit area (kg/m2)
Tropical forests
Rain
2,000.0
44.00
Seasonal
1,500.0
36.00
Temperate forests
Evergreen
1,300.0
36.00
Deciduous
1,200.0
20.00
Boreal forest
800.0
20.00
Savanna
700.0
4.00
Wood land and shrub land
600.0
4.00
Tundra and alpine meadows
144.0
0.67
Desert shrub
71.0
0.67


The route of photosynthesis differs in certain group of plants. In C3 plants, where the first photosynthetic product is 3 carbon compound, CO2 combines with a 5-carbon compound (ribulose biphosphate, RuBP) to form phosphoglyceric acid (a 3 carbon compound). However, in C4 Plants, where the first photosynthetic product is a 4 carbon compound, CO2 combines with phosphoenolpyruvate (a 3 carbon compound), instead of RuBP and produces oxaloacetic acid, a 4 carbon compound.

Moreover, photosynthetic efficiency affects the accumulation of biomass as it depends on plant efficiency and light intensity. We are fortunate enough to have abundant sun-shine in our country. Total energy received in India is about 60 x l013 MWH, with 250-300 days of useful sun shine per year in most part of the country (Anonymous, 1981).

Table 19.3. Biomass as the source of energy.
Sources of biomass Forms of biomass Conversion process Forms of energy
A. Plantations :
Silviculture Fire wood Combustion Heat (fire)
(Energy plantations) Fuel wood Destructive distillation Charcoal
Agriculture Carbohydrate Fermentation Ethanol
(Energy crops) Hydrocarbon Fermentation Fuel oil
Aquatic biomass Aquaculture Fermentation Methanol
Weeds Whole plant body Fermentation Methane
B. Residues/wastes/weeds :
Rural/urban/industrial

wastes

Wastes Combustion Fire/fuel
Pyrolysis Fuel oil
Fermentation Methan and ethanol
Forestry wastes Wastes Combustion Fire/fuel
Pyrolysis Oil gas
Gasification ,Gas
Fermentation Methane, ethanol
Agricultural wastes Wastes Fermentation Methane
Weeds and aquatic biomass   Fermentation Methane
Cattle dung Combustion Fire/fuel
Fermentation Methane (Biogas)


Net primary productivity and biomass accumulation are high due to the high rate of CO2 assimilation and low rate of photo-respiration between 25-30°C. Photo-respiration accounts for 50 per cent reduction in efficiency of the process, as it has a rapid rate of photo-respiration and low rate of CO2 fixation.

Photo-respiration differs from the normal respiration. The later takes place in dark and is associated with oxidation of compounds produced during photosynthesis, which occurs in peroxisomes. Photo-respiratory CO2 arises from glycolate pathway, not from glycolysis. In this pathway, 4 molecules of glycolates are converted into one molecule of glucose and 2 molecules of CO2; CO2 is released when glycine is converted into serine. Glycolate is oxidized to glycolate by enzyme glycolate oxidase.

In recent years, attempts have been made to convert C3 plants into C4 ones by introduction of characteristics of C4 plants in C3 ones. Regulation of photo-respiration by chemical or genetical manipulation offers a possibility of increasing crop-productivity. Some metabolic inhibitors are reported which catalyze photo-respiration. In 1974, Zelitsch at the Connecticut Agricultural Research Station, New Haven (USA) demonstrated that an analogue of glycolate and glyoxalate, inhibited glycolate synthesis and photo-respiration by 50 per cent, and increased 14 CO2 assimilation by 50 per cent. In 1973, Peter Carlson at the Brookhaven National Laboratory obtained the mutants of tobacco cells by growing them on nutrient medium supplemented with sulphoxymine. The mutant cells displayed a low photorespiration rate.


 
     
 
 
     



     
 
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