biom1-x.txt
biom1-x.txt - - - - meth805\txt\pnt whyreT SOLAR BIOMASS WIND HYDRO UNITS
3 BIOMASS 3.1 INTRODUCTION
Biomass as the solar energy stored in chemical form in plant and animal materials, most precious and versatile resources on earth. provides not only food but also energy, building materials, paper, fabrics, medicines and chemicals. Biomass has been used for energy purposes ever since man discovered fire. biomass fuels can be used for tasks ranging from heating the house to fuelling an engine or electricity.
The chemical mix of biomass: varies among species, but plants consists of about 25% lignin and 75% carbohydrates or sugars. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two big carbohydrate categories that have noted value are cellulose and hemi-cellulose. The lignin fraction are of non-sugar type molecules. Nature uses the long cellulose polymers to build the fibers that give a plant its strength. The lignin fraction acts like a ?glue? that holds the cellulose fibers together.
WHERE DOES BIOMASS COME FROM?
Carbon dioxide from the air and water from the earth are combined in the photosynthetic process to make carbohydrates (sugars) that form the building blocks of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the structural part of biomass. If we burn biomass up (extract the energy stored in the chemical bonds) oxygen from the atmosphere combines with the carbon in plants to produce carbon dioxide and water. The process is circles, carbon dioxide is then available to produce new biomass.
In addition to the aesthetic value of the planet?s flora, biomass represents a useful and valuable resource to man. For millennia humans have exploited the solar energy stored in the chemical bonds by burning biomass as fuel and eating plants for the nutritional energy of their sugar and starch content. More recently, in the last few hundred years, humans have exploited fossilized biomass in the form of coal. This fossil fuel is the result of very slow chemical transformations that convert the sugar polymer fraction into a chemical composition that resembles the lignin fraction. Thus, the additional chemical bonds in coal represent a more concentrated source of energy as fuel. All of the fossil fuels we consume - coal, oil and natural gas - are simply ancient biomass. Over millions of years, the earth has buried ages-old plant material and converted it into these valuable fuels. But while fossil fuels contain the same constituents - hydrogen and carbon - as those found in fresh biomass, they are not considered renewable because they take such a long time to create.
Environmental impacts pose another significant distinction between biomass and fossil fuels. When a plant decays, it releases most of its chemical matter back into the atmosphere. In contrast, fossil fuels are locked away deep in the ground and do not affect the earth?s atmosphere unless they are burned.
Wood may be the best-known example of biomass. When burned, the wood releases the energy the tree captured from the sun?s rays. But wood is just one example of biomass. Various biomass resources such as agricultural residues (e.g. bagasse from sugarcane, corn fiber, rice straw and hulls, and nutshells), wood waste (e.g. sawdust, timber slash, and mill scrap), the paper trash and urban yard clippings in municipal waste, energy crops (fast growing trees like poplars, willows, and grasses like switchgrass or elephant grass), and the methane captured from landfills, municipal waste water treatment, and manure from cattle or poultry, can also be used.
Biomass is considered to be one of the key renewable resources of the future at both small- and large-scale levels. It already supplies 14 % of the world?s primary energy consumption. But for three quarters of the world?s population biomass is the most important source of energy. Sweden has plans
BIOMASS - SOME BASIC DATA
* Total mass of living matter (including moisture) - 2000 billion tonnes
* Total mass in land plants - 1800 billion tonnes
* Total mass in forests -1600 billion tonnes
* Per capita terrestrial biomass - 400 tonnes
* Energy stored in terrestrial biomass 25 000 EJ
* Net annual production of terrestrial biomass - 400 000 million tonnes
* Rate of energy storage by land biomass - 3000 EJ/y (95 TW)
* Total consumption of all forms of energy - 400 EJ/y (12 TW)
* Biomass energy consumption - 55 EJ/y ( 1. 7 TW)
BIOMASS IN DEVELOPING COUNTRIES
Despite its wide use in developing countries, biomass energy is usually used so inefficiently that only a small percentage of its useful energy is obtained. The overall efficiency in traditional use is only about 5-15 per cent, and biomass is often less convenient to use compared with fossil fuels. It can also be a health hazard in some circumstances, for example, cooking stoves can release particulates, CO, NOx formaldehyde, and other organic compounds in poorly ventilated homes, often far exceeding recommended WHO levels. Furthermore, the traditional uses of biomass, i.e., burning of wood is often associated with the increasing scarcity of hand-gathered wood, nutrient depletion, and the problems of deforestation and desertification. biomass into modern, easy-to-use carriers (such as electricity, liquid or gaseous fuels, or processed solid fuels).much more useful energy could be extracted from biomass than at present.
FOOD OR FUEL?
agricultural and export policy and the politics of food availability are factors of far greater importance. The argument should be analysed against the background of the world?s (or an individual country?s or region?s)so competition between food and crops is not significant. Furthermore, crop rotation in sugarcane areas has led to an increase in certain food crops, while some byproducts such as hydrolyzed bagasse and dry yeast are used as animal feed. potential for producing food in conjunction with sugarcane appears to be larger than expected and should be explored further,?.
It is important to mention that developing countries are facing
LAND AVAILABILITY
Biomass differs from other forms of fuels since it requires land to grow on and also ignored the expertise of the local farmers who know the local conditions. This has resulted in many biomass project failures in the past. The ?multi-uses? approach asks how land can best be used for sustainable development, and considers what mixture of land use and cropping patterns. the identification of fast-growing species, breeding successes and multipl. High yields are also feasible with herbaceous (non-woody) crops where the agro- ecological conditions are suitable. For example, in Brazil, the average yield of sugarcane has risen from 47 to 65 t/ha (harvested weight) with nitrogen fixing Albizia trees (De Bell et al, 1989).
3.2 ENERGY VALUE
Biomass (when considering its energy potential) refers to all forms of plant-derived material that can be used for energy: wood, herbaceous plants, crop and forest residues, animal wastes etc. Because biomass is a solid fuel it can be compared to coal. On a dry-weight basis, heating values range from 17,5 GJ per tonne for various herbaceous crops like wheat straw, sugarcane bagasse to about 20 GJ/tonne for wood. The corresponding values for bituminous coals and lignite are 30 GJ/tonne and 20 GJ/tonne respectively (see tables at the end). At the time of its harvest biomass contains considerable amount of moisture, ranging from 8 to 20 % for wheat straw, to 30 to 60 % for woods, to 75 to 90 % for animal manure, and to 95 % for water hyacinth. In contrast the moisture content of the most bituminous coals ranges from 2 to 12 %. Thus the energy density for the biomass at the point of production are lower than those for coal. On the other side chemical attributes make it superior in many ways. The ash content of biomass is much lower than for coals, and the ash is generally free of the toxic metals and other contaminants and can be used as soil fertiliser.
Biomass is generally and wrongly regarded as a low-status fuel, and in many countries rarely finds way.biomass is not a net emitter of CO2 to the atmosphere when it is produced and used sustainably. It also has lower sulphur and NOx emissions and can help rehabilitate degraded lands.
Energy contents comparison table.
Content of water% MJ/kg KW/kg
Oak- tree 20 14,1 3,9
Pine-tree 20 13,8 3,8
Straw 15 14,3 3,9
Grain 15 14,2 3,9
Rape oil - 37,1 10,3
Hard coal 4 30,0-35,0 8,3
Brown coal 20 10,0-20,0 5,5
Heating oil - 42,7 11,9
Bio methanol - 19,5 5,4
MJ/Nm3 KWh/Nm3
Sewer gas 16,0 4,4
Wood gas 5,0 1,4
Biogas 22,0 6,1 from cattle dung
Natural gas 31,7 8,8
Hydrogen 10,8 3,0
3.3 BENEFITS OF BIOMASS AS ENERGY SOURCE
New energy crops may be more economically competitive than crops in surplus production.
3.4 ENVIRONMENTAL BENEFITS
The use of biomass energy has many unique qualities that provide environmental benefits. It can help mitigate climate change, reduce acid rain, soil erosion, water pollution and pressure on landfills, provide wildlife habitat, and help maintain forest health through better management.
3.4.1 CLIMATE CHANGE
Climate change is a growing concern world-wide. Human acts:the combustion of fossil fuels, has released hundreds of millions of tons will change the Earth?s climate, disrupting the entire biosphere which supports life as we know it. methane and carbon dioxide pose significant threats, CH4 is 20 times more potent (though shorter-lived in the atmosphere) than CO2. Capturing methane from landfills, wastewater treatment, and manure lagoons prevents the methane from being vented to the atmosphere.All crops, including biomass energy crops, sequester carbon in the plant and roots while they grow, providing a carbon sink. In other words, the carbon dioxide released while burning biomass is absorbed by the next crop growing.
3.4.2 ACID RAIN
sulphur and nitrogen oxides from the combustion of fuels. killing of lakes, humans and wildlife. biomass has no sulphur content, and easily mixes with coal, ?co-firing? is a very simple way of reducing sulphur emissions and thus, reduce acid rain. ?Co-firing? refers to burning biomass jointly with coal in a traditionally coal-fired power plant or heating plant.
3.4.3 SOIL EROSION & WATER POLLUTION
Biomass crops can be grown on more marginal lands, in floodplains, and in between normal crops. stabilize the soil,reduce nutrient run-off, which protects animals fish. do not have to be planted every year.reduces water pollution is by capturing the methane, through anaerobic digestion, from manure lagoons on cattle, hog and poultry farms polluting rivers.can reduce odour, capture the methane for energy, and create either liquid or semi-solid soil fertilisers which can be used on-site or sold.
3.5 BIOMASS FUELS
Plants are the most common source of biomass. They have been used in the form of wood, peat and straw for thousands of years. a lot of biomass quickly . These could be trees (e.g. willows or Eucalyptus) or other high growth rate plants (such as sugar cane or maize or soybean).
3.5.1 WOOD RESIDUES
At present these are often left to rot on site - even in countries with fuelwood shortages. They can be collected, dried and used as fuel by nearby. high water content makes transporting them for wider use uneconomicl. on-site kilns can reduce transport costs. wood chips which can be handled, dried and burned easily in chip-fired boilers. The use of forest residues to produce steam for heating and/or power generation is now a growing business. . Dry sawdust and waste produced during the processing of cut timber make very good fuel. sludge left after alcohol production (known as vinasse) can produce flammable gas. Other useful waste products include, waste from food processing and fluff from the cotton and textiles industry.
3.5.3 SHORT ROTATION PLANTS
Biomass can be also be produced by so-called short-rotation plantation of trees and other plants like grasses (sorghum, sugarcane, switchgrass). All these plants can be used as fuels like wood with the main advantage of their For some grasses harvesting is taking place every six to 12 months. dry matter production of different tree species varies over a wide range depending on soil types and climate, certain species stand out. For Eucalyptus species, yields of up to 65 t/ha/y have been reported, compared to 30 and 43 t/ha/y in Salix and Populus species respectively.
3.6 BIOMASS FUELS IN DEVELOPING COUNTRIES
3.6.1 Fuelwood
The term fuelwood describe all types of fuels derived from forestry and plantation.about 20 % of all used in Asia.More than half of the total wood harvested in the world is used as fuelwood. is dwindling rapidly, leading to scarcity of and environmental degradation. It is estimated that, for more than a third of the world population, the real crisis is the daily scramble to obtain fuelwood to meet domestic use.
3.6.2 Charcoal
The main expansion in the use of charcoal in Europe came with the industrial revolution in England in the 17th and 18th centuries. In Sweden, charcoal consumption for iron making grew through most of the 19th century,, according to some estimates (Williams 1989) amount of electricity that can be produced from cane residues could be up to 44 times the on-site needs of the sugar factory or alcohol distillery. For each litre of alcohol produced a BIG/STIG unit would be able to produce more than 11 kWh of electricity in excess of the distillery?s needs (about 820 kWh/t). Another estimate of bagasse in condensing-extraction
In India alone, electricity production from sugarcane residues by the year 2030 could be up to 550 TWh/year (the total electricity production from all sources in 1987 was less than 220 TWh
3.7 METHODS OF GENERATING ENERGY FROM BIOMASS
Nearly all types of raw biomass decompose rather quickly, order them by the complexity of the processes involved:
* Direct combustion of biomass.
* Thermochemical processing to upgrade the biofuel. Processes in this category include pyrolysis, gasification and liquefaction.
* Biological processing. Natural processes such as anaerobic digestion and fermentation which lead to a useful gaseous or liquid fuel.
The immediate ?product, of some of these processes is heat - normally used at place of production or at not too great a distance, for chemical processing or district heating, or to generate steam for power production. For other processes the product is a solid, liquid or gaseous fuel: charcoal, liquid fuel as a petrol substitute or additive, gas for sale or for power generation using either steam or gas turbines.
3.7.1 COMBUSTION
The technology of direct combustion as the most obvious way of extracting energy from biomass is well understood,include space and water heating, industrial processing and electricity generation. its very low efficiency. open fire most of the heat is wasted and is not used to cook or whatever.
Combustion of wood can be divided into four phases:
* Water inside the wood boils off. Even wood that has been dried for ages has as much as 15 to 20% of water in its cell structure.
* Gas content is freed from the wood. It is vital that these gases should burn and not just disappear up the chimney.
* The gases emitted mix with atmospheric air and burn at a high temperature.
* The rest of the wood (mostly carbon) burns. In perfect combustion the entire energy is utilised and all that is left is a little pile of ashes.
Three things are needed for effective burning:
* high enough temperatures;
* enough air, and
* enough time for full combustion.
If not enough air gets in, combustion is incomplete and the smoke is black from the unburned carbon. It smells terrible, and you get soot deposited in the chimney, with the risk of fire. If too much air gets in the temperature drops and the gases escape unburned, taking the heat with them. The right amount of air gives the best utilisation of fuel. No smell, no smoke, and very little risk of chimney fires. Regulation of the air supply depends largely on the chimney and the draught it can put up.
Direct combustion is the simplest and most common method of capturing the energy contained within biomass. Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient, as a little elementary calculation reveals.
The energy content of a cubic metre dry wood is 10 GJ, which is ten million kJ. To raise the temperature of a litre of water by 1 degree Celsius requires 4,2 kJ of heat energy. Bringing a litre to the boil should therefore require rather less than 400 kJ, equivalent to 40 cubic centimetres of wood - one small stick, perhaps. In practice, with a simple open fire we might need at least fifty times this amount: a conversion efficiency no better than 2%.
Designing a stove or boiler which will make rather better use of valuable fuel requires an understanding of the processes involved in the combustion of a solid fuel. The first is one which consumes rather than produces energy: the evaporation of any water in the fuel. With reasonably dry fuel, however, this uses only a few percent of the total energy. In the combustion process itself there are always two stages, because any solid fuel contains two combustible constituents. The volatile matter is released as a mixture of vapours or vaporised tars and oils by the fuel as its temperature rises. The combustion of these produces the little spurts of pyrolysis.
Modern combustion facilities (boilers) usually produce heat, steam (used in industrial process) or electricity. Direct combustion systems vary considerably in their design. The fuel choice makes a difference in the design and efficiency of the combustion system. Direct combustion technology using biomass as the fuel is very similar to that used for coal. Biomass and coal can be handled and burned in essentially the same fashion. In fact, biomass can be ?co-fired? with coal in small percentages in existing boilers. The biomass which is co-fired are usually low-cost feedstocks, like wood or agricultural waste, which also help to reduce the emissions typically associated with coal. Coal is simply fossilized biomass heated and compressed over millions of years. The process which coal undergoes as it is heated and compressed deep within the earth, adds elements like sulphur and mercury to the coal. Burning coal for heat or electricity releases these elements, which biomass does not contain.
3.7.2 PYROLYSIS
Pyrolysis is the simplest and almost certainly the oldest method of processing one fuel in order to produce a better one. A wide range of energy-rich fuels can be produced by roasting dry wood or even the straw. The process has been used for centuries to produce charcoal. Conventional pyrolysis involves heating the original material (which is often pulverised or shredded then fed into a reactor vessel) in the near-absence of air, typically at 300 - 500 °C, until the volatile matter has been driven off. The residue is then the char - more commonly known as charcoal - a fuel which has about twice the energy density of the original and burns at a much higher temperature. For many centuries, and in much of the world still today, charcoal is produced by pyrolysis of wood. Depending on the moisture content and the efficiency of the process, 4-10 tonnes of wood are required to produce one tonne of charcoal, and if no attempt is made to collect the volatile matter, the charcoal is obtained at the cost of perhaps two-thirds of the original energy content.
Pyrolysis can also be carried out in the presence of a small quantity of oxygen (?gasification?), water (?steam gasification?) or hydrogen (?hydrogenation?). One of the most useful products is methane, which is a suitable fuel for electricity generation using high-efficiency gas turbines.
With more sophisticated pyrolysis techniques, the volatiles can be collected, and careful choice of the temperature at which the process takes place allows control of their composition. The liquid product has potential as fuel oil, but is contaminated with acids and must be treated before use. Fast pyrolysis of plant material, such as wood or nutshells, at temperatures of 800-900 degrees Celsius leaves as little as 10% of the material as solid char and converts some 60% into a gas rich in hydrogen and carbon monoxide. This makes fast pyrolysis a competitor with conventional gasification methods (see bellow), but like the latter, it has yet to be developed as a treatment for biomass on a commercial scale.
At present, conventional pyrolysis is considered the more attractive technology. The relatively low temperatures mean that fewer potential pollutants are emitted than in full combustion, giving pyrolysis an environmental advantage in dealing with certain wastes. There have been some trials with small- scale pyrolysis plants treating wastes from the plastics industry and also used tyres - a disposal problem of increasingly urgent concern.
3.7.3 GASIFICATION
The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the ?energy crisis? of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
Gasification based on wood as a fuel produces a flammable gas mixture of hydrogen, carbon monoxide, methane and other non flammable by products. This is done by partially burning and partially heating the biomass (using the heat from the limited burning) in the presence of charcoal (a natural by-product of burning biomass). The gas can be used instead of petrol and reduces the power output of the car by 40%. It is also possible that in the future this fuel could be a major source of energy for power stations.
SYNTHETIC FUELS
A gasifier which uses oxygen rather than air can produce a gas consisting mainly of H2, CO and C02, and the interesting potential of this lies in the fact that removal of the C02 leaves the mixture called synthesis gas, from which almost any hydrocarbon compound may be synthesised. Reacting the H2 and CO is one way to produce pure methane. Another possible product is methanol (CH3OH), a liquid hydrocarbon with an energy density of 23 GJ per tonne. Producing methanol in this way involves a series of sophisticated chemical processes with high temperatures and pressures and expensive plant, and one might wonder why it is of interest. The answer lies in the product: methanol is that valuable commodity, a liquid fuel which is a direct substitute for gasoline. At present the production of methanol using synthesis gas from biomass is not a commercial proposition, but the technology already exists, having been developed for use with coal as feedstock - as a precaution by coal-rich countries at times when their oil supplies were threatened.
3.7.4 FERMENTATION
Fermentation of sugar solution is the way how ethanol (ethyl alcohol) can be produced. Ethanol is a very high liquid energy fuel which can be used as the substitute for gasoline in cars. This fuel is used successfully in Brazil. Suitable feedstocks include crushed sugar beet or fruit. Sugars can also be manufactured from vegetable starches and cellulose by pulping and cooking, or from cellulose by milling and treatment with hot acid. After about 30 hours of fermentation, the brew contains 6-10 per cent alcohol, which can be removed by distillation as a fuel.
Fermentation is an anaerobic biological process in which sugars are converted to alcohol by the action of micro-organisms, usually yeast. The resulting alcohol is ethanol (C2H3OH) rather than methanol (CH3OH), but it too can be used in internal combustion engines, either directly in suitably modified engines or as a gasoline extender in gasohol: gasoline (petrol) containing up to 20% ethanol.
The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted to sugars. The best known source of ethanol is sugar-cane - or the molasses remaining after the cane juice has been extracted. Other plants whose main carbohydrate is starch (potatoes, corn and other grains) require processing to convert the starch to sugar. This is commonly carried out, as in the production of some alcoholic drinks, by enzymes in malts. Even wood can act as feedstock, but its carbohydrate, cellulose, is resistant to breakdown into sugars by acid or enzymes (even in finely divided forms such as sawdust), adding further complication to the process.
The liquid resulting from fermentation contains only about 10% ethanol, which must be distilled off before it can be used as fuel. The energy content of the final product is about 30 GJ/t, or 24 GJ/m3. The complete process requires a considerable amount of heat, which is usually supplied by crop residues (e.g. sugar cane bagasse or maize stalks and cobs). The energy loss in fermentation is substantial, but this may be compensated for by the convenience and transportability of the liquid fuel, and by the comparatively low cost and familiarity of the technology.
3.7.5 ANAEROBIC DIGESTION -end-
3 BIOMASS 3.1 INTRODUCTION
Biomass as the solar energy stored in chemical form in plant and animal materials, most precious and versatile resources on earth. provides not only food but also energy, building materials, paper, fabrics, medicines and chemicals. Biomass has been used for energy purposes ever since man discovered fire. biomass fuels can be used for tasks ranging from heating the house to fuelling an engine or electricity.
The chemical mix of biomass: varies among species, but plants consists of about 25% lignin and 75% carbohydrates or sugars. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two big carbohydrate categories that have noted value are cellulose and hemi-cellulose. The lignin fraction are of non-sugar type molecules. Nature uses the long cellulose polymers to build the fibers that give a plant its strength. The lignin fraction acts like a ?glue? that holds the cellulose fibers together.
WHERE DOES BIOMASS COME FROM?
Carbon dioxide from the air and water from the earth are combined in the photosynthetic process to make carbohydrates (sugars) that form the building blocks of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the structural part of biomass. If we burn biomass up (extract the energy stored in the chemical bonds) oxygen from the atmosphere combines with the carbon in plants to produce carbon dioxide and water. The process is circles, carbon dioxide is then available to produce new biomass.
In addition to the aesthetic value of the planet?s flora, biomass represents a useful and valuable resource to man. For millennia humans have exploited the solar energy stored in the chemical bonds by burning biomass as fuel and eating plants for the nutritional energy of their sugar and starch content. More recently, in the last few hundred years, humans have exploited fossilized biomass in the form of coal. This fossil fuel is the result of very slow chemical transformations that convert the sugar polymer fraction into a chemical composition that resembles the lignin fraction. Thus, the additional chemical bonds in coal represent a more concentrated source of energy as fuel. All of the fossil fuels we consume - coal, oil and natural gas - are simply ancient biomass. Over millions of years, the earth has buried ages-old plant material and converted it into these valuable fuels. But while fossil fuels contain the same constituents - hydrogen and carbon - as those found in fresh biomass, they are not considered renewable because they take such a long time to create.
Environmental impacts pose another significant distinction between biomass and fossil fuels. When a plant decays, it releases most of its chemical matter back into the atmosphere. In contrast, fossil fuels are locked away deep in the ground and do not affect the earth?s atmosphere unless they are burned.
Wood may be the best-known example of biomass. When burned, the wood releases the energy the tree captured from the sun?s rays. But wood is just one example of biomass. Various biomass resources such as agricultural residues (e.g. bagasse from sugarcane, corn fiber, rice straw and hulls, and nutshells), wood waste (e.g. sawdust, timber slash, and mill scrap), the paper trash and urban yard clippings in municipal waste, energy crops (fast growing trees like poplars, willows, and grasses like switchgrass or elephant grass), and the methane captured from landfills, municipal waste water treatment, and manure from cattle or poultry, can also be used.
Biomass is considered to be one of the key renewable resources of the future at both small- and large-scale levels. It already supplies 14 % of the world?s primary energy consumption. But for three quarters of the world?s population biomass is the most important source of energy. Sweden has plans
BIOMASS - SOME BASIC DATA
* Total mass of living matter (including moisture) - 2000 billion tonnes
* Total mass in land plants - 1800 billion tonnes
* Total mass in forests -1600 billion tonnes
* Per capita terrestrial biomass - 400 tonnes
* Energy stored in terrestrial biomass 25 000 EJ
* Net annual production of terrestrial biomass - 400 000 million tonnes
* Rate of energy storage by land biomass - 3000 EJ/y (95 TW)
* Total consumption of all forms of energy - 400 EJ/y (12 TW)
* Biomass energy consumption - 55 EJ/y ( 1. 7 TW)
BIOMASS IN DEVELOPING COUNTRIES
Despite its wide use in developing countries, biomass energy is usually used so inefficiently that only a small percentage of its useful energy is obtained. The overall efficiency in traditional use is only about 5-15 per cent, and biomass is often less convenient to use compared with fossil fuels. It can also be a health hazard in some circumstances, for example, cooking stoves can release particulates, CO, NOx formaldehyde, and other organic compounds in poorly ventilated homes, often far exceeding recommended WHO levels. Furthermore, the traditional uses of biomass, i.e., burning of wood is often associated with the increasing scarcity of hand-gathered wood, nutrient depletion, and the problems of deforestation and desertification. biomass into modern, easy-to-use carriers (such as electricity, liquid or gaseous fuels, or processed solid fuels).much more useful energy could be extracted from biomass than at present.
FOOD OR FUEL?
agricultural and export policy and the politics of food availability are factors of far greater importance. The argument should be analysed against the background of the world?s (or an individual country?s or region?s)so competition between food and crops is not significant. Furthermore, crop rotation in sugarcane areas has led to an increase in certain food crops, while some byproducts such as hydrolyzed bagasse and dry yeast are used as animal feed. potential for producing food in conjunction with sugarcane appears to be larger than expected and should be explored further,?.
It is important to mention that developing countries are facing
LAND AVAILABILITY
Biomass differs from other forms of fuels since it requires land to grow on and also ignored the expertise of the local farmers who know the local conditions. This has resulted in many biomass project failures in the past. The ?multi-uses? approach asks how land can best be used for sustainable development, and considers what mixture of land use and cropping patterns. the identification of fast-growing species, breeding successes and multipl. High yields are also feasible with herbaceous (non-woody) crops where the agro- ecological conditions are suitable. For example, in Brazil, the average yield of sugarcane has risen from 47 to 65 t/ha (harvested weight) with nitrogen fixing Albizia trees (De Bell et al, 1989).
3.2 ENERGY VALUE
Biomass (when considering its energy potential) refers to all forms of plant-derived material that can be used for energy: wood, herbaceous plants, crop and forest residues, animal wastes etc. Because biomass is a solid fuel it can be compared to coal. On a dry-weight basis, heating values range from 17,5 GJ per tonne for various herbaceous crops like wheat straw, sugarcane bagasse to about 20 GJ/tonne for wood. The corresponding values for bituminous coals and lignite are 30 GJ/tonne and 20 GJ/tonne respectively (see tables at the end). At the time of its harvest biomass contains considerable amount of moisture, ranging from 8 to 20 % for wheat straw, to 30 to 60 % for woods, to 75 to 90 % for animal manure, and to 95 % for water hyacinth. In contrast the moisture content of the most bituminous coals ranges from 2 to 12 %. Thus the energy density for the biomass at the point of production are lower than those for coal. On the other side chemical attributes make it superior in many ways. The ash content of biomass is much lower than for coals, and the ash is generally free of the toxic metals and other contaminants and can be used as soil fertiliser.
Biomass is generally and wrongly regarded as a low-status fuel, and in many countries rarely finds way.biomass is not a net emitter of CO2 to the atmosphere when it is produced and used sustainably. It also has lower sulphur and NOx emissions and can help rehabilitate degraded lands.
Energy contents comparison table.
Content of water% MJ/kg KW/kg
Oak- tree 20 14,1 3,9
Pine-tree 20 13,8 3,8
Straw 15 14,3 3,9
Grain 15 14,2 3,9
Rape oil - 37,1 10,3
Hard coal 4 30,0-35,0 8,3
Brown coal 20 10,0-20,0 5,5
Heating oil - 42,7 11,9
Bio methanol - 19,5 5,4
MJ/Nm3 KWh/Nm3
Sewer gas 16,0 4,4
Wood gas 5,0 1,4
Biogas 22,0 6,1 from cattle dung
Natural gas 31,7 8,8
Hydrogen 10,8 3,0
3.3 BENEFITS OF BIOMASS AS ENERGY SOURCE
New energy crops may be more economically competitive than crops in surplus production.
3.4 ENVIRONMENTAL BENEFITS
The use of biomass energy has many unique qualities that provide environmental benefits. It can help mitigate climate change, reduce acid rain, soil erosion, water pollution and pressure on landfills, provide wildlife habitat, and help maintain forest health through better management.
3.4.1 CLIMATE CHANGE
Climate change is a growing concern world-wide. Human acts:the combustion of fossil fuels, has released hundreds of millions of tons will change the Earth?s climate, disrupting the entire biosphere which supports life as we know it. methane and carbon dioxide pose significant threats, CH4 is 20 times more potent (though shorter-lived in the atmosphere) than CO2. Capturing methane from landfills, wastewater treatment, and manure lagoons prevents the methane from being vented to the atmosphere.All crops, including biomass energy crops, sequester carbon in the plant and roots while they grow, providing a carbon sink. In other words, the carbon dioxide released while burning biomass is absorbed by the next crop growing.
3.4.2 ACID RAIN
sulphur and nitrogen oxides from the combustion of fuels. killing of lakes, humans and wildlife. biomass has no sulphur content, and easily mixes with coal, ?co-firing? is a very simple way of reducing sulphur emissions and thus, reduce acid rain. ?Co-firing? refers to burning biomass jointly with coal in a traditionally coal-fired power plant or heating plant.
3.4.3 SOIL EROSION & WATER POLLUTION
Biomass crops can be grown on more marginal lands, in floodplains, and in between normal crops. stabilize the soil,reduce nutrient run-off, which protects animals fish. do not have to be planted every year.reduces water pollution is by capturing the methane, through anaerobic digestion, from manure lagoons on cattle, hog and poultry farms polluting rivers.can reduce odour, capture the methane for energy, and create either liquid or semi-solid soil fertilisers which can be used on-site or sold.
3.5 BIOMASS FUELS
Plants are the most common source of biomass. They have been used in the form of wood, peat and straw for thousands of years. a lot of biomass quickly . These could be trees (e.g. willows or Eucalyptus) or other high growth rate plants (such as sugar cane or maize or soybean).
3.5.1 WOOD RESIDUES
At present these are often left to rot on site - even in countries with fuelwood shortages. They can be collected, dried and used as fuel by nearby. high water content makes transporting them for wider use uneconomicl. on-site kilns can reduce transport costs. wood chips which can be handled, dried and burned easily in chip-fired boilers. The use of forest residues to produce steam for heating and/or power generation is now a growing business. . Dry sawdust and waste produced during the processing of cut timber make very good fuel. sludge left after alcohol production (known as vinasse) can produce flammable gas. Other useful waste products include, waste from food processing and fluff from the cotton and textiles industry.
3.5.3 SHORT ROTATION PLANTS
Biomass can be also be produced by so-called short-rotation plantation of trees and other plants like grasses (sorghum, sugarcane, switchgrass). All these plants can be used as fuels like wood with the main advantage of their For some grasses harvesting is taking place every six to 12 months. dry matter production of different tree species varies over a wide range depending on soil types and climate, certain species stand out. For Eucalyptus species, yields of up to 65 t/ha/y have been reported, compared to 30 and 43 t/ha/y in Salix and Populus species respectively.
3.6 BIOMASS FUELS IN DEVELOPING COUNTRIES
3.6.1 Fuelwood
The term fuelwood describe all types of fuels derived from forestry and plantation.about 20 % of all used in Asia.More than half of the total wood harvested in the world is used as fuelwood. is dwindling rapidly, leading to scarcity of and environmental degradation. It is estimated that, for more than a third of the world population, the real crisis is the daily scramble to obtain fuelwood to meet domestic use.
3.6.2 Charcoal
The main expansion in the use of charcoal in Europe came with the industrial revolution in England in the 17th and 18th centuries. In Sweden, charcoal consumption for iron making grew through most of the 19th century,, according to some estimates (Williams 1989) amount of electricity that can be produced from cane residues could be up to 44 times the on-site needs of the sugar factory or alcohol distillery. For each litre of alcohol produced a BIG/STIG unit would be able to produce more than 11 kWh of electricity in excess of the distillery?s needs (about 820 kWh/t). Another estimate of bagasse in condensing-extraction
In India alone, electricity production from sugarcane residues by the year 2030 could be up to 550 TWh/year (the total electricity production from all sources in 1987 was less than 220 TWh
3.7 METHODS OF GENERATING ENERGY FROM BIOMASS
Nearly all types of raw biomass decompose rather quickly, order them by the complexity of the processes involved:
* Direct combustion of biomass.
* Thermochemical processing to upgrade the biofuel. Processes in this category include pyrolysis, gasification and liquefaction.
* Biological processing. Natural processes such as anaerobic digestion and fermentation which lead to a useful gaseous or liquid fuel.
The immediate ?product, of some of these processes is heat - normally used at place of production or at not too great a distance, for chemical processing or district heating, or to generate steam for power production. For other processes the product is a solid, liquid or gaseous fuel: charcoal, liquid fuel as a petrol substitute or additive, gas for sale or for power generation using either steam or gas turbines.
3.7.1 COMBUSTION
The technology of direct combustion as the most obvious way of extracting energy from biomass is well understood,include space and water heating, industrial processing and electricity generation. its very low efficiency. open fire most of the heat is wasted and is not used to cook or whatever.
Combustion of wood can be divided into four phases:
* Water inside the wood boils off. Even wood that has been dried for ages has as much as 15 to 20% of water in its cell structure.
* Gas content is freed from the wood. It is vital that these gases should burn and not just disappear up the chimney.
* The gases emitted mix with atmospheric air and burn at a high temperature.
* The rest of the wood (mostly carbon) burns. In perfect combustion the entire energy is utilised and all that is left is a little pile of ashes.
Three things are needed for effective burning:
* high enough temperatures;
* enough air, and
* enough time for full combustion.
If not enough air gets in, combustion is incomplete and the smoke is black from the unburned carbon. It smells terrible, and you get soot deposited in the chimney, with the risk of fire. If too much air gets in the temperature drops and the gases escape unburned, taking the heat with them. The right amount of air gives the best utilisation of fuel. No smell, no smoke, and very little risk of chimney fires. Regulation of the air supply depends largely on the chimney and the draught it can put up.
Direct combustion is the simplest and most common method of capturing the energy contained within biomass. Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient, as a little elementary calculation reveals.
The energy content of a cubic metre dry wood is 10 GJ, which is ten million kJ. To raise the temperature of a litre of water by 1 degree Celsius requires 4,2 kJ of heat energy. Bringing a litre to the boil should therefore require rather less than 400 kJ, equivalent to 40 cubic centimetres of wood - one small stick, perhaps. In practice, with a simple open fire we might need at least fifty times this amount: a conversion efficiency no better than 2%.
Designing a stove or boiler which will make rather better use of valuable fuel requires an understanding of the processes involved in the combustion of a solid fuel. The first is one which consumes rather than produces energy: the evaporation of any water in the fuel. With reasonably dry fuel, however, this uses only a few percent of the total energy. In the combustion process itself there are always two stages, because any solid fuel contains two combustible constituents. The volatile matter is released as a mixture of vapours or vaporised tars and oils by the fuel as its temperature rises. The combustion of these produces the little spurts of pyrolysis.
Modern combustion facilities (boilers) usually produce heat, steam (used in industrial process) or electricity. Direct combustion systems vary considerably in their design. The fuel choice makes a difference in the design and efficiency of the combustion system. Direct combustion technology using biomass as the fuel is very similar to that used for coal. Biomass and coal can be handled and burned in essentially the same fashion. In fact, biomass can be ?co-fired? with coal in small percentages in existing boilers. The biomass which is co-fired are usually low-cost feedstocks, like wood or agricultural waste, which also help to reduce the emissions typically associated with coal. Coal is simply fossilized biomass heated and compressed over millions of years. The process which coal undergoes as it is heated and compressed deep within the earth, adds elements like sulphur and mercury to the coal. Burning coal for heat or electricity releases these elements, which biomass does not contain.
3.7.2 PYROLYSIS
Pyrolysis is the simplest and almost certainly the oldest method of processing one fuel in order to produce a better one. A wide range of energy-rich fuels can be produced by roasting dry wood or even the straw. The process has been used for centuries to produce charcoal. Conventional pyrolysis involves heating the original material (which is often pulverised or shredded then fed into a reactor vessel) in the near-absence of air, typically at 300 - 500 °C, until the volatile matter has been driven off. The residue is then the char - more commonly known as charcoal - a fuel which has about twice the energy density of the original and burns at a much higher temperature. For many centuries, and in much of the world still today, charcoal is produced by pyrolysis of wood. Depending on the moisture content and the efficiency of the process, 4-10 tonnes of wood are required to produce one tonne of charcoal, and if no attempt is made to collect the volatile matter, the charcoal is obtained at the cost of perhaps two-thirds of the original energy content.
Pyrolysis can also be carried out in the presence of a small quantity of oxygen (?gasification?), water (?steam gasification?) or hydrogen (?hydrogenation?). One of the most useful products is methane, which is a suitable fuel for electricity generation using high-efficiency gas turbines.
With more sophisticated pyrolysis techniques, the volatiles can be collected, and careful choice of the temperature at which the process takes place allows control of their composition. The liquid product has potential as fuel oil, but is contaminated with acids and must be treated before use. Fast pyrolysis of plant material, such as wood or nutshells, at temperatures of 800-900 degrees Celsius leaves as little as 10% of the material as solid char and converts some 60% into a gas rich in hydrogen and carbon monoxide. This makes fast pyrolysis a competitor with conventional gasification methods (see bellow), but like the latter, it has yet to be developed as a treatment for biomass on a commercial scale.
At present, conventional pyrolysis is considered the more attractive technology. The relatively low temperatures mean that fewer potential pollutants are emitted than in full combustion, giving pyrolysis an environmental advantage in dealing with certain wastes. There have been some trials with small- scale pyrolysis plants treating wastes from the plastics industry and also used tyres - a disposal problem of increasingly urgent concern.
3.7.3 GASIFICATION
The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the ?energy crisis? of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
Gasification based on wood as a fuel produces a flammable gas mixture of hydrogen, carbon monoxide, methane and other non flammable by products. This is done by partially burning and partially heating the biomass (using the heat from the limited burning) in the presence of charcoal (a natural by-product of burning biomass). The gas can be used instead of petrol and reduces the power output of the car by 40%. It is also possible that in the future this fuel could be a major source of energy for power stations.
SYNTHETIC FUELS
A gasifier which uses oxygen rather than air can produce a gas consisting mainly of H2, CO and C02, and the interesting potential of this lies in the fact that removal of the C02 leaves the mixture called synthesis gas, from which almost any hydrocarbon compound may be synthesised. Reacting the H2 and CO is one way to produce pure methane. Another possible product is methanol (CH3OH), a liquid hydrocarbon with an energy density of 23 GJ per tonne. Producing methanol in this way involves a series of sophisticated chemical processes with high temperatures and pressures and expensive plant, and one might wonder why it is of interest. The answer lies in the product: methanol is that valuable commodity, a liquid fuel which is a direct substitute for gasoline. At present the production of methanol using synthesis gas from biomass is not a commercial proposition, but the technology already exists, having been developed for use with coal as feedstock - as a precaution by coal-rich countries at times when their oil supplies were threatened.
3.7.4 FERMENTATION
Fermentation of sugar solution is the way how ethanol (ethyl alcohol) can be produced. Ethanol is a very high liquid energy fuel which can be used as the substitute for gasoline in cars. This fuel is used successfully in Brazil. Suitable feedstocks include crushed sugar beet or fruit. Sugars can also be manufactured from vegetable starches and cellulose by pulping and cooking, or from cellulose by milling and treatment with hot acid. After about 30 hours of fermentation, the brew contains 6-10 per cent alcohol, which can be removed by distillation as a fuel.
Fermentation is an anaerobic biological process in which sugars are converted to alcohol by the action of micro-organisms, usually yeast. The resulting alcohol is ethanol (C2H3OH) rather than methanol (CH3OH), but it too can be used in internal combustion engines, either directly in suitably modified engines or as a gasoline extender in gasohol: gasoline (petrol) containing up to 20% ethanol.
The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted to sugars. The best known source of ethanol is sugar-cane - or the molasses remaining after the cane juice has been extracted. Other plants whose main carbohydrate is starch (potatoes, corn and other grains) require processing to convert the starch to sugar. This is commonly carried out, as in the production of some alcoholic drinks, by enzymes in malts. Even wood can act as feedstock, but its carbohydrate, cellulose, is resistant to breakdown into sugars by acid or enzymes (even in finely divided forms such as sawdust), adding further complication to the process.
The liquid resulting from fermentation contains only about 10% ethanol, which must be distilled off before it can be used as fuel. The energy content of the final product is about 30 GJ/t, or 24 GJ/m3. The complete process requires a considerable amount of heat, which is usually supplied by crop residues (e.g. sugar cane bagasse or maize stalks and cobs). The energy loss in fermentation is substantial, but this may be compensated for by the convenience and transportability of the liquid fuel, and by the comparatively low cost and familiarity of the technology.
3.7.5 ANAEROBIC DIGESTION -end-
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