Here is a compilation of essays on ‘Biomass’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Biomass’ especially written for school and college students.
Essay on Biomass
Essay Contents:
- Essay on the Introduction to Biomass
- Essay on the Availability of Biomass
- Essay on the Biomass Conversion
- Essay on the Fluidised Bed Combustion of Biomass
- Essay on Steam Turbine Cycle
- Essay on Gas Turbine Cycles
- Essay on Biomass Gasification
- Essay on Energy Plantation
Essay # 1. Introduction to Biomass:
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All organic materials such as plants, trees and crops are potential sources of energy and are collectively called biomass. The plants may be grown on land (terrestrial plants) or grown on water (aquatic plants). Biomass also includes forest crops and residues, crops grown especially for their energy content on “energy farms”, animal manure, wood waste and bagasse.
Coal, oil and natural gas may take millions of years to form, but biomass can be considered renewable energy source because plant life renews and adds to itself every year.
It can also be considered a form of solar energy as the latter is used indirectly to grow these plants by photosynthesis by the following reaction:
6CO2 + 6H2O + Light (Minimum 8 photons) → C6H12O6 (glucose) + 6O2
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The biomass sources are highly dispersed and bulky and contain large amounts of water (50 to 90 percent). Thus, it is not economical to transport them over long distances, and conversion into usable energy must take place close to source, which is limited to particular regions. However, biomass can be converted to liquid or gaseous fuels, thereby increasing its energy density and making transportation feasible over long distances.
Essay #
2. Availability of Biomass:
The total terrestrial crop alone is about 2 × 1012 metric tonnes.
This includes:
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i. Sugar crops such as sugarcane and sweet sorghum;
ii. Herbaceous crops, which are non-woody plants that are easily converted into liquid or gaseous fuels; and
iii. Silviculture (forestry) plants such as cultured hybrid poplar, sycamore, sweet gum, alder, eucalyptus, and other hardwoods.
The terrestrial crops have an energy potential of 3 × 1022 joules. The efficiency of solar energy utilization in natural photosynthesis is only 0.1 to 2%. At present only 1% of world biomass is used for energy conversion.
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Current research focuses on the screening and identification of species that are suitable for short-rotation growing and on the optimum techniques for planting, fertilization, harvesting, and conversion. Fast growing trees, sugar, starch and oil containing plants can be cultivated which have about 5% efficiency of solar energy utilization.
The estimated production of agricultural residue in India is 200 million tonnes per year and that of wood is 130 million tonnes. At an average heating value of 18 MJ/kg db, a total potential of 6 × 1012 MJ/year or approximately 75 × 107 MJ/hour exists. At a power conversion rate of 35%, total useful potential is about 75,000 MW. This can supply all our villages with power at a rate of 30,000 kWh per day per village against the present meager consumption of only 150 kWh per day per village.
With the electrification of the irrigation pumps, there will be a boost in agricultural production resulting in availability of more biomass for energy recovery and hence the process is self-adjusting. Aquatic crops are grown in fresh, sea, and brackish waters both submerged and emergent plants. These include seaweeds, marine algae, etc.
Animal and human waste are an indirect terrestrial crop from which methane for combustion and ethylene can be produced while retaining the fertilizer value of the manure. The daily produce of cow-dung is 13.5 kg per cattle which can be used to produce 0.46 m3 of bio-gas in a Gobar Gas plant. This gas is sufficient to produce 1 kWh of electricity in a bio-gas engine. In India, there are sufficient numbers of catties in each village; the animal waste can be used to meet the total energy requirements via above technology.
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The human waste can be also be used for production of bio-gas. Community latrines can be planned in the villages for collection of night soil for feeding to biogas plants. Wastes of 200 persons can be used to produce about 5m3 of gas per day to extract 12 kWh of equivalent energy by running a biogas engine.
Essay # 3. Biomass Conversion:
Biomass can either be utilized directly as a fuel, or can be converted into liquid or gaseous fuels, which can also be used as feedstock for industries. Most biomass in dry state can be burned directly to produce heat, steam or electricity. On the other hand biological conversion technologies utilize natural anaerobic decay processes to produce high quality fuels from biomass.
Various possible conversion technologies for getting different products from biomass are shown in Fig. 6.1.
These technologies can be grouped as:
1. Direct combustion, such as wood waste and baggage,
2. Thermo chemical conversion, and
3. Biochemical conversion.
1. Combustion of Biomass:
Various combustion techniques are available for burning biomass, the most suitable depending upon actual application. The selection of most suitable combustion equipment will depend upon properties, such as chemical analysis, volatile content, calorific value, particle size and ash characteristics, as well as the specific application. However, the versatility of the fluidised bed combustion makes it a strong candidate in many cases.
This technology may be used for the efficient combustion of forestry and agricultural waste material such as sawdust, wood chips, hog fuel, rice husks, straws nutshells and chips. It offers interesting possibilities in efficient energy recovery. The fluidised bed may be used in an air heater, liquid phase heater or steam generator. When fired directly into a boiler, steam can be generated for process or power. The ash is clean and inert and may be used as landfill or in concrete.
2. Thermo-Chemical Conversion:
There are two forms of thermochemical conversion- gasification and liquefaction. Gasification takes place by heating, the biomass with limited oxygen to produce low-heat gas or by reacting it with steam and oxygen at high pressure and temperature to produce medium-heat gas.
The latter may be used as fuel directly or used in liquefaction by converting it to methanol (methyl alcohol, CH3OH) or ethanol (ethyl alcohol, CH3 CH2 OH), or it may be converted to high-heat gas. Because the production of high-heat gas is more complex and expensive than low-heat gas, it is intended for use in lieu of natural gas in domestic and industrial applications. Low and medium-gases are considered for use as utility fuels.
3. Biochemical Conversion:
There are two types of biochemical conversion:
i. Anaerobic digestion and
ii. Fermentation
i. Anaerobic Digestion:
An anaerobe is a microorganism that can live and grow without air or oxygen. It gets its oxygen by the decomposition of matter containing it. Anaerobic digestion therefore, involves the microbial digestion of biomass. It has been used on animal manure (cow-dung) and can be extended to other biomass. The process takes place at low temperatures up to 65°C, and requires a moisture content of at least 80 per cent.
It generates a gas consisting mostly of carbon dioxide and methane with minimal impurities such as hydrogen sulphide. The gas can be burned directly or upgraded to synthetic natural gas by removing the CO2 and the impurities. The residue may consist of protein-rich sludge that can be used as animal feed and liquid effluents that are biologically treated by standard techniques and returned to the soil.
ii. Fermentation:
It is the breakdown of complex molecules in organic compounds under the influence of ferment such as yeast, enzymes, etc. Fermentation is a well- established and widely used technology for the conversion of grains and sugar crops into ethanol. It can be mixed with gasoline to produce gasohol 90% gasoline, 10% ethanol). Gasohal is used as fuel in the internal combustion engines.
Essay # 4. Fluidised Bed Combustion of Biomass:
Biomass is fed into a bed of hot inert particles, such as sand kept in fluidised state with air at sufficient velocity from below. The operating temperature is normally controlled within the range 750-950°C. Ideally it is kept as high as possible in order to maximize the rate of combustion and heat transfer but low enough to avoid the problem of sintering of the bed particles. The rapid mixing and turbulence within the fluidised bed enables efficient combustion to be achieved with high heat releases, as well as effective transfer, than in a conventional boiler. This can result in more compact boiler with less tubing.
A major advantage of fluidised bed combustion is that, because of the low temperatures involved, there is great potential for reducing and controlling atmospheric pollution. Nitrogen oxides are not formed from the nitrogen in the air and emission of the many of the trace elements associated with fuels are less than obtained using alternate combustion techniques.
Fluidised bed combustion technology has the ability to burn a wide range of material and can burn a combination of fuels or wastes in one unit. This versatility arises from the turbulent motion and large thermal inertia of the fluidised bed which enables the combustor to withstand the initial cooling action of fuels, particularly the high in ash and moisture as they enter the bed. This is typical characteristic of biomass. Agricultural wastes can be fired directly without any preparation with the exception of nutshells, which should be crushed.
i. Design of System:
Properties of some biomass fuels are given in Table 6.1. The chemical composition and physical form of the biomass will influence the design of the fluidised bed combustion system. As compared to coal, the oxygen content of biomass is very high (about 40%) and contains much more volatiles and less fixed carbon. The stoichiometric air requirements will be quite different.
The design of the fuel storage facilities, feeding arrangements and combustion equipment has to suit the higher volumetric feed rates necessary for biomass because of its generally low calorific value and density and to accommodate many shapes and sizes of the biomass.
The high moisture content of biomass (up to 50% as received) and volatiles (up to 80% dry basis) give rise to some operational problems connected with controlling bed temperatures and avoiding excessive smoke production. The water in the fuel evaporates as it enters the bed and this requires heat from the bed.
The volatile as released from the fuel tend to burn above the bed and do not provide heat for the bed itself. Therefore, relatively high fuel feed are needed to maintain bed temperature. However, with these high rates, incomplete combustion takes place with the limited oxygen provided by the fluidising air resulting in excessive smoke formation. This problem is aggravated with larger beds because of relatively poor lateral distribution of the fuel.
ii. Types of Fluidised Beds:
Various stages of fluidised beds are shown in Fig. 6.2. The pressure drop through the bed as a function of air velocity is shown in Fig. 6.3.
Feeding of fuel directly into the bed, operating the bed at high temperatures, using deep beds, using fluidising velocities designed to promote good mixing and distribution of fuel throughout the bed will help combustion of volatiles in the bed. Provision of secondary air just above the bed can help in reducing smoke emissions. Because biomass fuels have low densities, it is preferable to feed them as relatively large-sized pieces like corn, cobs wood chips, etc., into a bubbling bed.
iii. Feeding of Biomass:
The fuel is first metered through a rotary valve or screw feeder before being dropped onto the fluidised bed at only a few points, the number of points depending on the overall bed area. Bed turbulence distributes the fuel throughout the bed. The ash content of these fuels is low with ash being very fine and easily elutriated from the fluidised bed.
iv. Circulating Fluidised Bed:
The finer feed stocks of biomass like rice husk and saw dust are difficult to burn satisfactorily in bubbling fluidised bed because of excessive elutriation at the fluidised velocities needed for good mixing of the fuel in bed. It is better to employ a circulating fluidised bed combustion system, for very finely divided forms of biomass.
This circulating bed is characterised by high fluidising velocities in excess of 3m/s with entrainment of majority of bed particles. These are collected in a cyclone separator and returned to the combustor. The primary combustion air is fed through the main fluidising air distributor at the bottom while secondary air is introduced further up the reactor. Fine grains of sand are used as the bed material.
The fuel is screw-fed into the combustor which operates between 700°C and 1000°C depending on the fuel being burned. Because the circulating material is in a highly turbulent state and mixes rapidly, bed temperature remains very uniform. Boiler tubes are situated in the upper part of the combustor and downstream after the cyclone. It is a good system for burning biomass fuels with low densities and small size which can be difficult to burn in more conventional “bubbling” type of fluidised bed combustor.
Essay # 5. Steam Turbine Cycle:
Atmospheric fluidised bed (AFB) system has been developed for combustion of biomass. This AFB is used in conjunction with a steam turbine cycle as shown in Fig. 6.5.
Heat is transferred to water by heat transfer surfaces within the bed, in the water-cooled walls, and in the convective space above the bed. A fuel conversion efficiency of 71% has been claimed.
Essay # 6. Gas Turbine Cycles:
i. Indirect-Fired Open-Cycle Gas Turbine System:
An indirect-fired open-cycle system is shown in Fig. 6.6.
The gas turbine combustor is replaced or operated in conjunction with a system for adding heat to the compressed air by air heat exchange tubes subjected to external firing in the fluidised bed unit.
The system consists of five components:
i. A fluidised-bed combustor,
ii. A heat exchanger,
iii. A gas turbine generator set,
iv. Cyclone separator, and
v. Emission control equipment (bag-house).
The system can be provided with a steam bottoming plant for efficient operation. The exhaust from the fluidised bed is joined to the exhaust from gas turbine. The combined stream is used to generate high pressure, high temperature steam in a heat recovery boiler. The bottoming steam plant works in the normal manner.
A primary gas clean-up system is installed after the fluidised bed to catch the larger ash particles before going to the boiler. Final clean-up can take place in the boiler by soot blowing and outside the boiler by cyclones and bag-house.
A separate air source is used for fluidisation to assure clean hot air for gas turbine operation. If some portion of compressor air is used for fluidisation, thorough clean-up is required prior to joining the main air stream for introduction into gas turbine.
Heat transfer tubes used with fluidised-bed system are both over the bed and immersed in the bed. Air from the compressor enters the over-bed tubes and proceeds down to the in-bed tubes, which operate at the highest temperature. Tube material is selected for both corrosion resistance on the outside surface and high temperature. Ceramic tubes are available with external extended surface rings.
The control of the system is achieved by control of fuel as well as air supply .The air should never be less than that required keeping the bed in fluid state.
ii. Closed Cycle Gas Turbine System:
In the closed system, the working fluid (gas) flows under pressure through a compressor, heat exchanger, fluidised-bed gas heater, gas turbine, again through the heat exchanger and then through the pre-cooler back to the compressor inlet. The schematic diagram is shown in Fig. 6.7.
The gas may be air, nitrogen or helium, which is not in direct contact with the products of combustion. The heat rejected by intercooler and pre-cooler can be recovered by a secondary heat transfer loop to generate low pressure steam to activate an absorption chiller for a cold storage or to produce additional power through an organic bottoming cycle. The cycle efficiency of a closed-loop gas turbine system is well above 42%.
Control of air can be accomplished by passing air from the compressor discharge around the regenerator and to the turbine exhaust. While compressor power remains almost constant the turbine output decreases as a result of simultaneous decrease in turbine mass flow, expansion ratio and efficiency. The fluidised bed combustion unit will have both bed-immersed tubes and over-bed tubes.
The fluidised bed unit can work in conjunction with various cycle configurations using steam turbines, gas turbines, and combined cycles with and without cogeneration of thermal loads. The preferred system for burning many types of biomass is fluidised bed combustion with circulating system and bubbling beds more suitable for coarser sized biomass.
Essay # 7.
Biomass Gasification:
Gasifier is essentially a thermochemical reactor in which the biomass undergoes partial oxidation and producer gas is obtained. The agricultural and forest residues can first be dried in solar driers to restrict the moisture content below 15 per cent.
These are pressed in compaction briquetting machines to prepare feed size of 50 – 75mm for the gasifier. The biomass briquets are ignited in the gasifier and air flow is controlled to ensure partial combustion. The producer gas is reduced to CO and H2 by carbon present in the burnt lower layers. The main constituents of fuel gas are CO, H2 and CH4.
The gas produced by the gasifier is hot and contains tar, vapours and soot particles. To make it engine worthy, it is cooled to near ambient conditions and cleaned to remove tar and dust by cross current water scrubbers. Air and clean gas are supplied to the engine in pre-specified proportions. On an average 1 kg of biomass produces 2.5 nm3 of fuel gas and 1 kWh of shaft energy to drive the irrigation pump.
The diesel engine utilizes the gas as a supplementary fuel operating on dual-fuel mode. The solar driers, briquetting machines and gasifiers are now made in India. The total cost of a biomass gasifier with gas cooler and scrubber of 15m3/hr capacity, 5 kW/1500 rpm diesel engine and 100 × 100 mm centrifugal pump with necessary controls and accessories is about Rs. 37000.
There are a variety of types of biomass gasifiers:
1. Fixed Bed Gasifiers:
The three main designs of fixed bed gasifiers are up-draught, down-draught and cross draught gasifiers depending upon the direction of flow of air.
2. Fluidised bed gasifiers which have many advantages over fixed bed gasifiers.
(i) Quick start up,
(ii) High combustion efficiency,
(iii) High output rate,
(iv) Consistent rate of combustion,
(v) Usage of fuel with high moisture content,
(vi) Rapid response to fuel input changes,
(vii) Fuel flexibility,
(viii) Good heat storage capacity,
(ix) Compact size,
(x) Reduced emission of harmful nitrogen oxides,
(xi) Simple operation.
Fig. 6.9 gives the flow diagram of oxygen donor gasifier commercial plant along with various applications of fuel gas.
Chemistry of Gasification Process:
The reactions take place in three zones of the gasifier bed- oxidation, reduction and distillation.
Biomass (C) + O2 + 3.79N2 = 3.79N2 + CO2 + Heat
The oxidation reaction is exothermic and 395,000 k/kg atom of carbon in biomass heat is produced.
The carbon dioxide formed is reduced in the presence of carbon. Over 90% of CO2 is reduced to CO at 900°C.
C + CO2 + 3.79N2 + Heat = 3.79N2 + 2CO
It is an endothermic reaction and 172,000 kJ/kg atom of carbon heat is absorbed in the reaction.
The oxygen requirement is 0.25 – 0.3 kg/kg of biomass.
The dry gas has the following composition by volume:
H2: 36.2%
CO: 44.4%
CO2: 19.1%
CH4: 0.3%
Essay # 8. Energy Plantation:
Biomass energy concepts under study are resulting in the cultivation of large forests in areas not suitable for food production. Energy plantation may yield 10 to 20 tons/acre per year. The energy plantation would be perhaps 125 to 500 km^ in land area. The trees are to be harvested by automated means, then chipped and pulverized for burning in a power plant that would be located in the middle of the energy plantation.
The choices of plants to be cultivated in India are Casuarina, Eucalyptus and Sorghums, etc. The properties of common species recommended for energy plantations are given in Table 6.3.
Other schemes envision aquatic farms growing algae, tropical grasses, floating kelp, water hyacinth and others. In controlled environments, they could yield several hundred tons/acre year. One interesting idea is to use hot condenser cooling water from a power plant to grow algae in large quantities or increase the yield of other crops.
Fast growing trees, sugar, starch and oil containing plants can also be cultivated for bio-energy. The sugar beet and cane have about 5% efficiency of the solar energy utilization. Special studies are required to increase the efficiency of solar energy use in the growth of crops to 10—11%.