Centre for Remote Sensing and Geo-Informatics, Sathyabama University, Chennai 3

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5 BIOMASS TO FUEL: CONVERSION TECHNIQUES 8 Chapter Alok Singh 1, Kishan Singh Rawat* 2, O.P. Nautiyal 3, and Tilak V. Chavdal 4 1,4 College of Agril Engineering and Technology, Navsari Agricultural University Navsari, Dediapada, Gujarat 2 Centre for Remote Sensing and Geo-Informatics, Sathyabama University, Chennai 3 Uttarakhand Science Education and Research, Centre (USERC), Dehradun ksr.krishan@gmail.com Abstract There are many biomass conversion routes to prepare energy-efficient biofuels. The conversion routes are broadly divided in 4 categories. The methods are Physical, Agrochemical, Thermochemical and Biochemical. In physical method of conversion, biomass is densified into solid briquettes while in agrochemical route of conversion, fuel is extracted from freshly cut plants. Thermochemical process of conversion consists of combustion, pyrolysis, gasification and anaerobic digestion to methane. Biochemical route of biomass conversion to fuel consists of ethanol fermentation. These methods are described in this chapter in detail. Introduction Ability to do work is Energy. Basically energy can be classified into two types: (i) Potential Energy, it is stored energy and the energy of position (gravitational). It exists in various forms and (ii). Kinetic Energy, it is energy in motion - the motion of waves, electrons, atoms, molecules and substances. It exists in various forms e.g. Chemical Energy, Nuclear Energy, Stored Mechanical Energy, Gravitational Energy, Radiant Energy, Thermal Energy, Electrical Energy, Motion Energy, Sound Energy, Light Energy and Nuclear Energy etc. The various sources of energy can be conveniently grouped as (i) Non-renewable sources of energy : Commercial primary energy resources or non-renewable sources of energy or conventional sources of energy are being accumulated in nature for a very long time and can t be replaced if exhausted. Nature gifted resources which are consumed can t be replaced. Eg: coal,

6 156 Energy Resources: Development, Harvesting and Management petroleum, natural gas, thermal power, hydro power and nuclear power are the main conventional sources of energy. (ii) Renewable sources of energy : Energy sources, which are continuously and freely produced in the nature and are not exhaustible, are known as the renewable sources of energy. Eg: solar energy, solar photovoltaic, biomass and wood energy, geo thermal energy, hydrogen fuel cell, wind energy, tidal energy and ocean energy. (iii) New sources of energy : The new sources of energy are available for local use. Most prominent new sources of energy are tidal energy, ocean waves, OTEC, peat, tar sand, oil shale, coal tar, geo thermal energy, draught animals, agricultural residues etc. Renewable Energy Sources Energy has been an important component to meet the day to day need of human beings. The degree of civilization is measured by the energy utilization for human advancement or needs. Energy has been defined as the capacity to do work or capability to produce an effort. It is expressed in N-m or Joules. The energy density is expressed as J\kg. Renewable energy sources derive their energy from existing flows of energy from on-going natural processes, such as sunshine, wind, flowing water, biological processes, and geothermal heat flows [1,2]. 1. Solar energy Energy comes to the earth from the sun. This energy keeps the temperature of the earth above that in colder space, causes current in the atmosphere and in ocean, causes the water cycle and generates photosynthesis in plants. The solar power where sun hits atmosphere is watts, whereas the solar power on earth s surface is watts. Total world-wide power demand of all needs of civilization is watts. Therefore, the sun gives us 1000 times more power than we need. If we can use 5% of this energy, it will be 50 times what the world will require. The energy radiated by the sun on a bright sunny day is approximately 1 kw/m². The applications of solar energy: (1) Heating and cooling of residential building; (2) Solar water heating; (3) Solar drying of agricultural and animal products; (4) Solar distillation on a small community scale; (5) Salt production by evaporation of seawater or inland brines; (6) solar cookers; (7) solar engines for water pumping; (8) Food refrigeration; (9) Bio conversion and wind energy, which are indirect source of solar energy; (10) Solar furnaces; (11) Solar electric power generation by solar

7 Energy Resources: Development, Harvesting and Management 157 ponds, steam generators heated by rotating reflectors and (12) solar photovoltaic cells, which can be used for conversion of solar energy directly into electricity or for water pumping in rural agricultural purposes [3]. 2. Wind energy Wind energy is a clean and renewable source of power. Energy from wind can be economically used for the generation of electrical energy. Wind is an indirect source of solar energy and is caused by the uneven heating of the atmosphere by the Sun, the irregularities of the Earth s surface, and rotation of the Earth. The amount and speed of wind depends on the Earth s terrain and other factors. The wind turbines use the kinetic energy of the wind and convert that energy into mechanical energy, which in turn can be converted into electricity by means of a generator. The energy available Fig. 1: Components of a Wind mill. in the winds over the earth s surface is estimated to be MW. There are essentially two types of wind turbines: The horizontal-axis variety, and the vertical-axis design. The main disadvantage to wind power is that it is unreliable. Wind does not blow at a constant rate, and it does not always blow when energy is needed. Furthermore, the windiest locations are often in remote locations, far away from big cities where the electricity is needed. Just like with any other energy plant, people oppose it because of aesthetic reasons. The rotor noise produced by the rotor blades is another reason for opposition [3]. 3. Hydroelectric Power Harvesting energy from water is possible due to the gravitational potential energy stored in water. As water flows from a high potential energy (high ground) to lower potential energy (lower ground), the potential energy difference thereby created can be partially converted into kinetic, and in this case electric energy through the use of a generator. There are essentially two major designs in use that utilize water to produce electricity: the hydroelectric dam, and the pumped-storage plant. The principle is simple: the force of the water being released from the reservoir through the penstock of the dam spins the blades of a turbine. The turbine is connected to the generator

8 158 Energy Resources: Development, Harvesting and Management that produces electricity. After passing through the turbine, the water re-enters the river on the downstream side of the dam [4]. Fig. 2: Hydroelectric Dam. 4. Geothermal energy Geothermal energy is one of the only renewable energy sources not dependent on the Sun. Instead, it relies on heat produced under the surface of the Earth. There are two main applications of geothermal energy, which include producing electricity at specialized power plants, and direct-heating, which puts to direct use the temperature Fig. 3: Extraction of Geothermal Energy.

9 Energy Resources: Development, Harvesting and Management 159 of water piped under the earth s surface. Direct-heating provides immediate, usable energy. This type of energy can heat individual buildings or entire areas. It can also cool buildings by pumping water underground where the temperature remains relatively stable near 60 degrees Fahrenheit, and then into buildings, where the water absorbs heat, thus helping to air condition the building [5]. 5. Energy from biomass and biogas Biomass is produced in nature through photosynthesis achieved by solar energy conversion. Biomass means organic matter. In simplest form the reaction is the process of photosynthesis in the presence of solar radiation, can be represented as follows : H 2 O + CO 2 Solar energy CH 2 O + O 2 Biomass can be converted into fuels through a number of different processes, including solid fuel combustion, digestion, pyrolysis, and fermentation and catalyzed reactions. Electricity is generated in many places through solid fuel combustion. Biomass resources fall into three categories: Biomass in its traditional solid mass (Wood and agricultural residue: It can burn directly and get the energy) Biomass in non-traditional form (converted into liquid fuels: the biomass can be converted into ethanol and methanol to be used as liquid fuels in engines.) To ferment the biomass anaerobically to obtain a gaseous fuel called biogas(in biogas = 55 to 65 % Methane, 30 to 40% CO 2 and rest impurities i.e. H 2, H 2 S and some N 2 ) The main source for production of biogas is wet cow dung or wet livestock (and even human) waste, to produce biogas. Biogas can be captured from marshes, from landfill or wastes such as sewage, and burned to produce electricity. It can also be generated intentionally through anaerobic composting. When refined, it can be used to power vehicles directly. Biogas can be produced from the decomposition of animal, plant and human waste. It is a clean but slow burning gas and usually has a calorific value between 5000 to 5500 kcal/kg. It can be used directly in cooking, reducing the demand for firewood. Moreover, the material from which the biogas is produced retains its value as a fertilizer and can be returned to the soil. Biogas has been popular on the name, "Gobar Gas" mainly because cow dung has been the material for its production, hitherto. It is not only the excreta of the cattle, but also the piggery waste as well as poultry droppings are very effectively used for biogas generation. A few other materials through which biogas can be generated are algae, crop residues (agro-wastes), garbage kitchen wastes, paper wastes, sea wood, human waste, waste

10 160 Energy Resources: Development, Harvesting and Management from sugarcane refinery, water hyacinth etc. Any cellulosic organic material of animal or plant origin, which is easily biodegradable, is a potential raw material suitable for biogas production [6]. 6. Ethanol Fuel-quality ethanol is beneficial for car-owners, the economy and the environment. This growing technology is looking to be an immediate part of the solution to the forthcoming energy crisis. Ethanol, also known as ethyl alcohol or grain alcohol, is a colourless, clear liquid. The chemical formula is CH 3 CH 2 OH. Ethanol is not used by itself to fuel cars. Instead, it s mixed with gasoline. The two most common blends are E10 and E85. The number refers to the percentage of ethanol in the blend. E10 is a blend of 10% ethanol and 90% gasoline. E85, the most mainstream alternative fuel, is 85% ethanol and 15% gasoline. 7. Ocean Energy This is an indirect method of utilizing solar energy. A large amount of solar energy is collected and stored in tropical oceans. Nearly 70% of the Earth s surface is covered by oceans, which have the potential to supply humans with an enormous amount of renewable energy. Humans have exploited the vast energy potential of Earth s oceans by taking advantage of wave movement, tides, ocean currents, and ocean thermal energy. 1 km² of ocean interspersed with the devices would produce about 30 MW of electricity, which could power 20,000 homes. The heat contained in the oceans, could be converted into electricity by utilizing the fact that the temperature difference between the warm surface waters of the tropical oceans and the colder waters in the depths is about K. Utilization of this energy, with its associated temperature difference and its conversion into work, forms the basis of ocean thermal energy conversion (OTEC) systems. Characterization of Biomass The characterization of the conversion processes of lignocellulosic biomass to biofuels requires a large array of methods and analytical systems to extract the meaningful parameters necessary to describe the solid materials and the conversion liquors. The Biofuel Research Laboratory has been equipped with analytical systems [6, 7]. These systems include HPLCs for fluorescence, UV-VIS and Refractive Index (RI) detection, Liquid Chromatography coupled with a Mass Spectrometer (LC-MS) for metabolites characterization and quantification, UV-Vis and fluorescence plate readers, Fourier Transform Infrared and Near-Infrared spectrometers, gas chromatograph, UV-Vis spectrometers, automated protein purification system (FPLC).

11 Energy Resources: Development, Harvesting and Management 161 Typical biomass components (a) Cellulose A polysaccharide in which D-glucose is linked uniformly by β-glucosidic bonds. Its molecular formula is (C 6 H 12 O 6 ) n. The degree of polymerization, indicated by n, is broad, ranging from several thousand to several tens of thousands. Total hydrolysis of cellulose yields D-glucose (a monosaccharide), but partial hydrolysis yields a disaccharide (cellobiose) and polysaccharides in which n is in the order of 3 to 10. Cellulose has a crystalline structure and great resistance to acids and alkalis. (b) Hemicellulose A polysaccharide whose units are 5-carbon monosaccharides including D-xylose and D-arabinose, and 6-carbon monosaccharides including D-mannose, D-galactose, and D-glucose. The 5-carbon monosaccharides outnumber the 6-carbon monosaccharides, and the average molecular formula is (C 5 H 8 O 4 ) n. Because the degree of polymerization n is 50 to 200, which is smaller than that of cellulose, it breaks down more easily than cellulose, and many hemicelluloses are soluble in alkaline solutions. A common hemicellulose is xylan, which consists of xylose with 1,4 bonds. Other hemicelluloses include glucomannan, but all hemicelluloses vary in amounts depending on tree species and the part of the plant. (c) Lignin A compound whose constituent units, phenylpropane and its derivatives, are bonded 3-dimensionally. Its structure is complex and not yet fully understood. Its complex 3-dimensional structure is decomposed with difficulty by microorganisms and chemicals, and its function is therefore thought to be conferring mechanical strength and protection. Cellulose, hemicellulose, and lignin are universally found in many kinds of biomass, and are the most plentiful natural carbon resources on Earth. (d) Starch Like cellulose, starch is a polysaccharide whose constituent units are D-glucose, but they are linked by α-glycosidic bonds. Owing to the difference in the bond structures, cellulose is not water-soluble, while part of starch is soluble in hot water (amylose, with amolecular weight of about 10,000 to 50,000, accounting for 10% 20% of starch) and part is not soluble (amylopectin, with a molecular weight of about 50,000 to 100,000, accounting for 80% 90% of starch). Starch is found in seeds, tubers (roots), and stems, and has a very high value as food.

12 162 Energy Resources: Development, Harvesting and Management (e) Proteins These are macromolecular compounds in which amino acids are polymerized to a high degree. Properties differ depending on the kinds and ratios of constituent amino acids, and the degree of polymerization. Proteins are not a primary component of biomass, and account for a lower proportion than do the previous three components. (f) Other components (organic and inorganic) The amounts of the other organic components vary widely depending on specie, but there are also organic components with high value, such as glycerides (representative examples include rapeseed oil, palm oil, and other vegetable oils) and sucrose in sugarcane and sugar beet. Other examples are alkaloids, pigments, terpenes, and waxes. Although these are found in small amounts, they have very high added value as pharmaceutical ingredients. Biomass comprises organic macromolecular compounds, but it also contains inorganic substances (ash) in trace amounts. The primary metal elements include Ca, K, P, Mg, Si, Al, Fe, and Na. Substances and their amounts differ according to the feedstock type. Several of the industry standard tests for characterizing biomass are described below: Total Solids : A way to determine the moisture content within the sample. Ash Determination : The amount of inorganic or mineral material present in the sample. Exhaustive Ethanol and Water Extractable : The removal of non-structural material from the biomass sample to prevent interferences during other analyses, as well as free sugar determination. Structural Carbohydrates : The determination of glucose, xylose, galactose, arabinose and mannose concentrations in the sample; used to determine cellulose and hemicellulose concentrations in the biomass. Acetyl Content : Acetic acid concentration in the sample, may also include formic and levulinic acid content depending on the feedstock. Lignin : Determination of the structural plant material that does not contribute to the sugar content in the sample. Starch Content : Represents the readily available source of sugar within some feedstock. Ethanol Content : Analysis of fermentation broths using gas chromatography. Bomb Calorimeter : The determination of the sample s calorific value

13 Energy Resources: Development, Harvesting and Management 163 Types, Construction, Working Principle, Uses and Safety/Environmental Aspects of Gasifiers Plant matter created by the process of photosynthesis is called biomass (or) all organic materials such as plants, trees and crops are potential sources of energy and are collectively called biomass. Photosynthesis is a naturally occurring process which derives its energy requirement from solar radiation. The plants may be grown on land (terrestrial plants) or grown on water (aquatic plants). Biomass also includes forest crops and residues after processing. The residues include crop residues (such as straw, stalks, leaves, roots etc.,) and agro-processing residues (such as oilseed shells, groundnut shells, husk, bagasse, molasses, coconut shells, saw dust, wood chips etc.). The term biomass is also generally understood to include human waste, and organic fractions of sewage sludge, industrial effluents and household wastes. The biomass sources are highly dispersed and bulky and contain large amounts of water (50 to 90%). Biomass means organic matter and photo-chemical approach to harness solar energy means harnessing of solar energy by photosynthesis. Availability of biomass (Sources of Biomass) Biomass resources fall into three categories: (i) Biomass in its traditional solid mass (wood and agriculture residue) In this, it is to burn directly and gets the energy. (ii) Biomass in non-traditional form (converted into liquid fuels) The biomass is converted into ethanol (ethyl alcohol) and methanol (methyl alcohol) (iii) It is to ferment the biomass anaerobically to obtain a gaseous fuel called biogas. Terrestrial crops include (1) sugar crops such as sugarcane and sweet sorghum; (2) herbaceous crops, which are non-woody plants that are easily converted into liquid or gaseous fuels; and (3) silviculture (forestry) plants such as cultured hybrid poplar, sycamore, sweet gum, alder, eucalyptus, and other hard woods. Animal and human waste are indirect crops from which methane for combustion and ethylene (used in the plastic industry) can be produced while training the fertilizer value of the manure. Aquatic crops are grown in fresh, sea and brackish waters. E.g. Sea weeds, marine algae. Calorific value The heat of combustion (calorific value) is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. It is

14 164 Energy Resources: Development, Harvesting and Management measured in units of energy per unit of the substance, usually mass, such as: kj/kg, kj/mol, kcal/kg, Btu/lb. Heating value is commonly determined by use of a bomb calorimeter. High calorific value The quantity known as higher calorific value (HCV) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapour produced. Such measurements often use a standard temperature of 15 C. This is the same as the thermodynamic heat of combustion since the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is liquid. The higher heating value takes into account the latent heat of vaporization of water in the combustion products, and is useful in calculating heating values for fuels where condensation of the reaction products is practical (e.g., in a gas-fired boiler used for space heat). In other words, HCV assumes the entire water component is in liquid state at the end of combustion and that heat below 150 C can be put to use. Low calorific value The quantity known as lower calorific value (LCV) is determined by subtracting the heat of vaporization of the water vapour from the higher heating value. This treats any H 2 O formed as a vapour. The energy required to vaporize the water therefore is not released as heat [8]. The LCV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered. It is useful in comparing fuels where condensation of the combustion products is impractical, or heat at a temperature below 150 C cannot be put to use. Biomass characteristics Ultimate & Proximate analysis Proximate Analysis Proximate analysis is used for calculation of chemical composition of the residue including Moisture content, Ash content, volatile matter & fixed carbon. Moisture content is one of the important property of biomass, over which its heating value depends. The moisture content is determined by drying the weighed amount of sample in an open crucible kept at 110 C in an oven for one hour by using standard oven dry method. The biomass sample is first ground to form fine powder, then this powdered sample is kept for determination of proximate analysis.

15 Energy Resources: Development, Harvesting and Management 165 Standard method for moisture determination involves heating of 1 gm biomass sample in a hot air oven to 110 C using the following equation. Moisture (% M) = (W 1 -W 2 )/W W 1 = Weight of the crucible & the air dried sample (g), W 2 = Weight of the crucible & oven dried sample (g), W 3 = Weight of the air dried sample taken (g) The experimentation of moisture content determination is extended for measurement of ash content of biomass. The sample, so obtained after determination of moisture content, is then heated to 750 C in a muffle furnace and is kept there for two hours or more till constant weight is recorded. The weight of reside represents the ash content of the biomass. Ash is defined as the weight of the residue remained after complete burning of 1gm of the biomass at 750 C. Ash (% A) = (W 4 -W 5 )/W W 4 = Weight of the crucible & the oven dried sample (g), W 5 = Weight of the crucible & residue (g), W 6 = Weight of oven dried sample taken (g) Volatile matter (% VM) is determined by keeping the dried sample in a closed crucible at 600 C for 6 minute and then at 900 C for another 6 minute. The difference in the weight due to the loss of volatiles is taken as the total volatile matter present in the biomass. It is termed as the weight loss due to heating of 1gm of biomass at 900 C in furnace for 6 minutes. Weight loss due to VM = Total loss of weight- loss due to moisture. Fixed carbon (FC) content is found by applying the mass balance for the biomass sample. The carbon content determined through this method is not the actual carbon content present in biomass but only the non-volatile part of carbon content, as some of carbon present in biomass also escape along with the volatiles. The content of fixed carbon is determined by subtracting the sum of A %, VM & % M from total of 100 % composition. FC= 100- (% A + % VM + % M) Ultimate Analysis This analysis is important for determining the elemental composition (C, N 2, H 2, S, O 2 etc.) of the biomass fuels & is also useful for calculating their heating value. The carbon and H 2 contents are determined by C-H-O analyser by standard method.

16 166 Energy Resources: Development, Harvesting and Management Further, knowing the ash content, O 2 is determined by difference. However, the samples must be dried prior to analysis. Nitrogen and sulphur are normally negligible. C-H-O analyser is consisting of an electric furnace, a sample column and absorbent column. The dry matter s powdered and weighed (w 1 ) before putting it in the sample column. The absorbent column is filled with a weighed quantity (w 2 ) of calcium hydroxide [9]. Subsequently the furnace is started and O 2 from a separate O 2 cylinder is supplied to the sample column at a pressure of 4 PSL. A temperature of more than 1400 C is maintained for about 20 min. Then furnace is switched off and the fused sample is taken out and weighed (w 3 ). The calcium hydroxide from the absorbent column is also taken out and reweighed (w 4 ). From these observations the carbon content of the sample can be determined using the following relationship. The difference (w 4 w 2 ) will give carbon dioxide formed. Carbon in absorbent (w 5 ) = (w 4 w 2 ) 12 / w 3 % carbon in the sample = w / w 1 Biomass Conversion Biomass can either be utilized directly as a fuel, or can be converted into liquid or gaseous fuels, which can also be 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 [10, 11]. Various possible conversion technologies for getting different products from biomass is broadly classified into three groups, viz. (i) thermo-chemical conversion, (ii) bio-chemical conversion and (iii) oil extraction. These alternative technologies for biomass conversion offer sound and alternative options for meeting the future fuels, chemicals, food and feed requirements. Three main approaches can be adopted for generation and utilization of biomass: (i) Collection of urban and industrial wastes as supplementary fuel in boilers and as a feed stock for producing methane and some liquid fuels. (ii) Collection of agricultural and forest residues to produce fuels, organic manures and chemical feed stock. (iii) Growth of some specific energy plants for use as energy feedstock and cultivation of commercial forestry, aquatic and marine plants for different products.

17 Energy Resources: Development, Harvesting and Management 167 Thermo-chemical conversion includes processes like combustion, gasification and pyrolysis. Combustion refers to the conversion of biomass to heat and power by directly burning it, as occurs in boilers. Gasification is the process of converting solid biomass with a limited quantity of air into producer gas, while pyrolysis is the thermal decomposition of biomass in the absence of oxygen. The products of pyrolysis are charcoal, condensable liquid and gaseous products. Biochemical conversion includes anaerobic digestion to produce biogas and fermentation to obtain alcohol fuels, The third approach is oil extraction. Edible and non-edible oils can be extracted from a variety of grains and seeds. They can be directly used as fuels by transesterification process to produce bio-diesel, which is a good substitute for conventional diesel oil. Thermal conversion processes for biomass involve some or all of the following processes: Pyrolysis: Biomass + heat charcoal, gas and oil Gasification: Biomass + limited oxygen fuel gas Combustion: Biomass + stoichiometric O 2 hot combustion products Principles of combustion In general, the term combustion refers to the process of release of heat by the exothermic heat of reaction for the oxidation of the combustible constituents of the fuel. Practically the combustion process is an interaction amongst fuel, energy and the environment [11]. Fuel may be defined as a combustible substance available in bulk, which on burning in presence of atmospheric air generates heat that can be economically utilized for domestic and industrial purposes. The common fuels are compounds of carbon and hydrogen; in addition variable percentages of oxygen and small percentages of sulphur and nitrogen are also present. Biomass fuels are normally thermally degradable solids. Combustion of organic materials not only generates natural components of air such as carbon dioxide and water but also produces carbonaceous residues, smoke and tar and gases of carbonyl derivatives, and carbon monoxide. The important parameters affecting combustion are moisture, organic compounds and minerals (ash). Principles of pyrolysis The pyrolysis of solid wastes strictly refers to the thermal decomposition of the wastes in an inert atmosphere. In this process, a mixture of gaseous products, tars, water insoluble oils, and an aqueous solution of acetic acid, methanol and other organic

18 168 Energy Resources: Development, Harvesting and Management compounds is evolved and a solid residue composed of the inert content of the waste and a char is produced. The amounts of the various products generated are dependent upon the rate of heating and the final temperature to which the wastes are subjected. In general, the higher the heating rate and higher the final temperature, greater the fraction of the initial wastes that is converted into the gaseous and liquid products [11]. The yield of gaseous products is highly variable but is about 25% of the refuse on dry ash-free basis. The yield of char is about 15 to 25% by weight of the refuse. Pyrolysis or charring of a biomass fuel has three main objectives: (i) production of a less smoky, clean burning fuel without generation of any tar; (ii) production of a fuel with a high calorific value than that of the initial feed material; (iii) production of a more reactive fuel. As the biomass is subjected to thermal treatment, it decomposes and volatilizes some of the volatile matters, leaving a carbonaceous residue containing the mineral components. The volatile products consists of a gaseous fraction containing CO, CO 2, some hydrocarbons and H 2 ; a considerable fraction containing water and organic compounds of lower molecular weights such as acids, alcohols, aldehydes, and ketones, and a tar fraction. Fine airborne particles of tar and charred materials constitute smoke. The amounts of volatiles matters formed, the residue left, and the weight loss occurred can be determined by the thermogravimetric analysis (TGA) and its derivative is called differential thermogravimetry (DTG). The change in enthalpy, H can be measured by differential thermal analysis (DTA). All these analysis are called thermal analysis. The energy released during pyrolysis and combustion can be measured as a function of time or temperature by thermal evolution analysis (TEA). Principles of gasification The word gasification implies converting a solid or liquid into a gaseous fuel without leaving any solid carbonaceous residue. The equivalence ratio, (ER) is defined as the ratio of the actual air supplied to the theoretical air required. ER = Air used in reaction / Stoichiometric air required Pyrolysis, ER = Below 0.2 Effective gasification, ER = 0.2 to 0.4 Combustion, ER = 0.4

19 Energy Resources: Development, Harvesting and Management 169 The efficiency, η of a gasifier is defined as the ratio of chemical energy output in the dry producer gas at 15 C to the energy input from the biomass. The total energy in the gaseous phase increases with the increase of equivalence ratio. The important parameters affecting the fixed bed gasification are: (a) Shape and size of the biomass fuel and fuel bed structure, (b) Moisture content (c) Volatile matter content (d) Ash content (e) Ash composition, its moisture content and (f) Energy content In the course of gasification, a number of thermo-chemical reactions take place. The quality of the fuel gas is dependent upon the equilibrium constants of the reactions. In gasification, the quantity of air that is supplied to the gasifier is always sub-stoichiometric. A gasification process that produces pyrolytic oil and char can achieve an overall thermal efficiency in excess of 70%. It may be noted that the products of combustion are generally CO 2, H 2 O, N 2 and excess O 2 and those of gasification are CO 2, CO, H 2, CH 4, C 2 H 4, C 3 H 6, NH 3, H 2 S, N 2, H 2 O and tar vapours and low molecular weight organic liquids [12]. Three stages gasification Prevailing chemical reactions are listed in Table-1, wherein the following main three gasification stages are described. Stage I. Gasification process starts as auto-thermal heating of the reaction mixture. The necessary heat for this process is covered by the initial oxidation exothermic reactions by combustion of a part of the fuel. Stage II. In the second - pyrolysis stage, being passed through a bed of fuel at high temperature pyrolyzes combustion gases. Heavier biomass molecules distillate into medium weight organic molecules and CO 2. In this stage, tar and char are also produced. Stage III. Initial products of combustion carbon dioxide (CO 2 ) and (H 2 O) are reconverted by reduction reaction to carbon monoxide (CO), hydrogen (H 2 ) and methane (CH 4 ). These are the main combustible components of producer gas. These reactions, not necessarily related to reduction, occur at high temperature. Gasification reactions, most important for the final quality (heating value) of syngas, take place in the reduction zone of the gasifier. Heat consumption prevails in this stage and the gas temperature will therefore decrease. Tar is mainly gasified, while char, depending upon the technology used, can be significantly "burned", reducing the concentration of particulates in the product [12, 13].

20 170 Energy Resources: Development, Harvesting and Management Table-1: Biomass gasification chemical reactions Gasification Stage Reaction formula Reaction heat Stage 1: C+1/2O 2 CO Exothermal Oxidation and Other exothermic Reaction Stage II : Pyrolysis Stage III : Gasification (Reduction) CO + 1/2O 2 CO 2 C + O 2 CO 2 C 6 H 10 O 5 xco 2 + yh 2 O H 2 + 1/2O 2 H 2 O CO + H 2 O CO 2 + H 2 CO + 3H 2 CH 4 + H 2 O C 6 H 10 O 5 C x H z + CO C 6 H 10 O 5 C n H m O y C + H 2 O CO + H 2 C + CO 2 2CO CO 2 + H 2 CO + H 2 O C + 2H 2 CH 4 Endothermic Endothermic Exothermic (Source: J.B. Jones & C.A. Hawkins. Engineering Thermodynamics, 1986,P. 456) The net product of air gasification can be found by summing of the partial reactions, as follows: Carbohydrate matter (C 6 H 10 O 5 ) + O 2 C X H Y + C L H M O N +CO +H 2 +Heat Reactions labelled in Table as exothermic means that chemical energy is converted to sensible heat and reactions labelled as endothermic means that heat is consumed in favour of chemical energy. Gasifiers Gasifier is equipment which can gasify a variety of biomass such as wood waste, agricultural waste like stalks, and roots of various crops, maize cobs etc. Biomass gets dried, heated, pyrolysed, partially oxidised and reduced, as it flows through it. Biomass gasification is basically the conversion of solid biomass such as wood, agricultural residues etc., into a combustible gas mixture normally called producer gas (or Low Btu gas). The solid biomass is partially burnt in the presence of air or oxygen to produce a low or medium calorific value gas. Partial combustion process occurs when air supply is less than adequate for combustion of biomass to be completed. Given that biomass contains carbon, hydrogen and oxygen molecules, complete combustion would produce carbon dioxide and water vapour. Partial combustion produces carbon monoxide as well as hydrogen which are both combustible gases [13].

21 Energy Resources: Development, Harvesting and Management 171 The gas produced in the gasifier is a clean burning fuel having heating value of about 950 to 1200 kcal/m 3, Hydrogen (18 20%), and carbon monoxide (18 24%) are the main constituents of the gas. The advantages of gasifier are: 1. It is very easy to operate the gasifier 2. Its maintenance is easy 3. It is sturdy in construction 4. Reliable in operation The volumetric composition of biomass based producer gas is follows: CO : 20 22%, H 2 : 15 18%, CH 4 : 2 4%, CO 2 : 9 11%, and N 2 : 50 54%. The gas also contains measurable amounts of particulate material and tar. The heating value of the producer gas ranges from 4000 to 5000 kj/m 3. However, some important points which should be taken into consideration while undertaking any biomass gasification system: 1) A gasifier itself is of little use. It is used either (a) to generate a combustible gas to provide heat or (b) to generate a fuel gas which can be used in an internal combustion engine as a petroleum oil substitute. 2) Some of the gaseous, liquid and solid products of combustion are not only harmful to engines and burners, but also to human beings. That is why these gases are not used as cooking gas. 3) A gasifier must have an effective gas cleaning train if the gas is to be used for internal combustion engines. A maximum limit of 5-15 mg solids and tar per kg of gas may be allowed for the use of the gas in an internal combustion engine. 4) A gasification system may not be of much advantage to generate a combustible gas, as far as fossil fuel savings, economies and ease of operation are concerned. Types of gasifiers Design of gasifier depends upon type of fuel used and whether gasifier is portable or stationary. Gasifiers are classified according to how the air blast introduced in the fuel column. The fixed bed gasifier has been the traditional process used for gasification, operated at temperatures around 1000 C. The most commonly built gasifiers are classified as: 1) Fixed bed gasifiers 2) Fluidized bed gasifiers. Fixed Bed Gasifier : A fixed bed gasifier is generally a vertical reactor (furnace). The gasifier is fed either from the top or from the side at a certain height. Inside the

22 172 Energy Resources: Development, Harvesting and Management gasifier the feedstock is supported either on a fixed grate or on a sand bottom. The fixed bed gasifiers may further be divided into updraft, downdraft and cross draft or cross flow units. 1. Updraft Gasifier (Counter current) In an updraft gasifier, the feed materials descend from the top to the bottom and the air ascends from the bottom to the top, while air is being blown upward through the grate. The oxidation zone lies at its bottom and gasification occurs through zones of decreasing temperatures as the gas rises through the reactor-fuel bed. As the reaction gases flow counter to the path of the incoming cool Fig. 4: Schematic diagram of updraft gasifier. feedstock and exit at a relatively low temperature, the fuel gas produced by an updraft gasifier has high tar content. Hence this gasifier is suitable for tar free fuels like charcoal, especially in stationary engines. The height to diameter ratio is usually kept at 3:1. 2. Downdraft Gasifier (Co-current) A schematic diagram of downdraft gasifier is shown in Fig. It is a vertical cylindrical vessel of varying cross section. The biomass is fed at the top at regular intervals of time and is converted through a series of processes into producer gas and ash as it moves down. The first zone is the drying zone, in which the moisture content in the upper layers of the biomass is removed by evaporation. The Fig. 5: Schematic diagram of downdraft gasifier. temperature in this zone is about 120 C. This temperature is acquired by heat transfer from the lower zones which are at much higher temperatures. The dried biomass moves down to the second zone

23 Energy Resources: Development, Harvesting and Management 173 called the pyrolysis zone, which is at temperatures ranging from 200 to 600 C from top to bottom. Throughout this zone, the biomass loses its volatiles [10]. In addition, in the lower part of this zone, when the temperature reaches 400 C, an exothermic reaction takes place in which the structure of biomass breaks down. As a result, water vapour, methanol, acetic acid and significant amounts of hydrocarbon tar evolved. The remaining solid is called char (carbon). The third zone is called the oxidation (or combustion) zone. A predetermined quantity of air is drawn into this zone through nozzles and temperatures ranging from 900 to 1200 C are attained. In this zone, a portion of char and pyrolysed gases coming from the second zone are burnt. The principal reactions are exothermic and oxidizing in nature, and the resultant products are carbon dioxide and water vapour. These products pass on to the fourth and last zone called the reduction zone, along with un-burnt pyrolysis gases and char. This zone is at temperatures ranging from 900 to 600 C, the highest temperature being near the oxidation zone. These reactions are endothermic and consequently the temperatures of the zone progressively decrease. At the end, the char is fully consumed and the final products are producer gas and ash. These gasifiers are suitable for fuels like wood and agricultural wastes. They may be used for power generation upto above 150 kw. They are cheap and easy to make. In a downdraft gasifier, the air is blown through a single duct or a number of equally spaced nozzles around the furnace. The air is blown towards the bottom of the gasifier. As a reaction gases also pass through the higher temperature zones at the bottom, the downdraft gasifier produces cleaner gas with relatively less amount of tars, compared to the updraft or cross-draft gasifiers. Usually larger amounts of tar and volatile matters are thermally cracked, while passing through the higher temperature zones of the bottom. As the gas also passes through the solid char bed, the carrying fly ash and dirt are trapped and the gas is cleaned. 3. Cross-Draft Gasifier The reactions in the cross- draft gasifier are similar to the downdraft gasifier. The gas produced passes upwards in the annual space around the gasifier that is filled with charcoal. The charcoal acts as an insulator and a dust filter. They are usually suitable for power generation upto 50 kw. Air enters through a water cooled Fig. 6: Schematic diagram of cross draft gasifier.

24 174 Energy Resources: Development, Harvesting and Management nozzle mounted on one side of the firebox. The gas is produced in the horizontal zone in front of the nozzle and passes through a vertical grate into the hot gas port on the opposite side. Because of the short path length for the gasification reactions, this type of gas producer responds most rapidly for changes in gas production. However, this gasifier is not commonly used. Fluidised bed gasifier In general, the fluidized bed gasifier is a tall refractory lined unit. The inert materials like sand or ash may be used as a fluidizing medium. At the bottom of the reactor, sand or other inert material is supported on a perforated plate with a grid network, which ensures uniform distribution of the fluidizing medium (Fig.7). Fluidization of the sand is achieved by supplying a controlled flow of air or oxygen at a fluidization velocity through the perforated supporting plate. As a result the whole bed including sand will be Fig. 7: Schematic diagram of fluidised bed gasifier. kept in suspension and separated. In a fluidized bed reactor, the temperature is typically uniform throughout the bed. Simultaneous oxidation and gasification occur rapidly at constant fluidized bed medium. Because of the high turbulence and thorough mixing of the fluidized bed, its capacity is generally dependent upon the volume of the fluid bed. Generally the height to diameter ratio of a fluidized bed reactor is 10:1. The bed temperature should be lower than ºC, in order to prevent slagging of the residues, but it should be higher than 850ºC to promote partial oxidation of the feed material. The output of a biomass gasifier can be used for a variety of direct thermal applications such as cooking, drying, heating water, and generating steam. It can also be used as a fuel for I.C. Engines to obtain mechanical shaft power or electrical power. If the producer gas is used in IC engines, it has to be cleaned for complete removal of particulate material and tar. The conversion efficiency of a gasifier is defined as the ratio of the heat content in the producer gas to the heat content in the biomass supplied and is usually around 75%.

25 Energy Resources: Development, Harvesting and Management 175 Application of gasification (Gasifier) 1. Small size gasifiers (upto 10 kw) It is used in rural areas, especially for providing shaft line power to agricultural pumps, processing machinery and agricultural-processing machineries like Thrashers, straw choppers, etc. 2. Medium size gasifiers (10 kw 50 kw) This gasifier can easily meet the shaft line power requirements of various rural industries like saw mills, carpentry workshops, mechanical fabrication shops as well as small rice mills. This can also be used as a decentralized source of electrical energy in milk chilling centres, primary health coverage centres and for rural electrification. To provide the vehicular power or for combinations applications like vehicular power or for shaft line power to vehicle mounted equipment. 3. Large size gasifiers (50 kw and above) It is used in rural areas as well as urban industries, besides being a source of decentralised electrification. This category of gasifiers can meet the shaft power requirements directly or indirectly of various industries like dairy, oil mill, mineral processing, brick manufacturing, ceramics and pottery industries etc. These gasifiers can also be used in mining operations; forest based wood processing units, well drilling etc. These gasifiers can be used for total electrification of small and medium size villages. Direct Thermal applications The primary application for direct heat gasifiers is in industries where fuel oil or coal/ lignite is being used to generate process heat or run furnaces and kilns. Industries that commonly use fuel oil or coal/lignite for supplying heat include the following: Cement manufacture, glass making, brick making, ceramics and pottery, rubber manufacture, food processing, brewing, crop drying, fertilizer production and chemicals. Types, Construction, Working Principle, Uses and Safety/Environmental Aspects of Biogas Plants Biogas, a mixture containing 55-65% Methane, 30-40% CO 2 and the rest being the impurities, can be produced from the decomposition of animal, plant and human waste. It is produced by digestion, pyrolysis of hydro-gasification. Digestion is a biological process that occurs in the absence of oxygen and in the presence of

26 176 Energy Resources: Development, Harvesting and Management anaerobic organisms at ambient pressures and temperatures of C. The container in which this digestion takes place is known as the digester [13, 14]. Methane gas is the main ingredient of natural gas. Smelly stuff, like rotting garbage, and agricultural and human waste, release methane gas - also called "landfill gas" or "biogas." Crops like corn and sugar cane can be fermented to produce the transportation fuel, ethanol. Biodiesel, another transportation fuel, can be produced from left-over food products like vegetable oils and animal fats. Raw materials for biogas generation Biogas is produced mainly from Cow dung, Sewage, Crop residues, Vegetable wastes, Water hyacinth, Poultry droppings, Pig manure Properties of Biogas Biogas is a mixture of different components and the composition varies depending upon the characteristics of feed materials, amount of degradation, etc. Methane is a combustible gas. The energy content of biogas depends on the amount of methane it contains. The composition and the properties of the biogas are given in the following tables. Name of the gas Composition in biogas (%) Methane (CH 4 ) Carbon dioxide (CO 2 ) Hydrogen (H 2 ) 5-10 Nitrogen (N 2 ) 1-2 Water vapour (H 2 O) 0.3 Hydrogen sulphide (H 2 S) Traces Properties Range Net calorific value (MJ/m 3 ) (5000 to 5500 Kcal/Kg) Air required for combustion (m 3 /m 3 ) 5.7 Ignition temperature ( 0 C) 700 Density (kg/m 3 ) 0.94 Microbiology of Biogas Production The production of biogas from organic material under anaerobic condition involves sequence of microbial reactions. During the process complex organic molecule present in the biomass are broken down to sugar, alcohols, pesticides and amino acids by acid producing bacteria. The resultant products are then used to produce methane by another category of bacteria. The biogas production process involves three stages namely: (1) Enzymatic hydrolysis; (2) Acid formation and (3) Methane formation.

27 Energy Resources: Development, Harvesting and Management 177 The efficiency of the digestion depends how far the digestion happens in these three stages. Better the digestion, shorter the retention time and efficient gas production. Enzymatic hydrolysis : The complex organic molecules like fats, starches and proteins which are water insoluble contained in cellulosic biomass are broken down into simple compounds with the help of enzymes secreted by bacteria. This stage is also known as polymer breakdown stage (polymer to monomer). The major end product is glucose which is a simple product. Acid formation : The resultant product (monomers) obtained in hydrolysis stage serve as input for acid formation stage bacteria. Products produced in previous stage are fermented under anaerobic conditions to form different acids. The major products produced at the end of this stage are acetic acid, propionic acid, butyric acid and ethanol. Methane formation : The acetic acid produced in the previous stages is converted into methane and carbon dioxide by a group of microorganism called Methanogens. In other words, it is process of production of methane by methanogens. They are obligatory anaerobic and very sensitive to environmental changes. Methanogens utilise the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. It is these components that make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low ph s and occurs between ph 6.5 and ph 8. Major reactions occurring in this stage is given below: CH 3 COOH CH 4 + CO 2 Acetic acid Methane Carbon dioxide 2CH 3 CH 2 OH + CO 2 CH 4 + 2CH 3 COOH Ethanol Carbon dioxide Methane Acetic acid CO H 2 CH 4 + 2H 2 O Carbon dioxide Hydrogen Methane Water The process of biogas formation through different stages is depicted in figure. Proteins Carbohydrates Fats Stage I Stage II Stage III Acetic acid Acids Acetic acids Alcohol Biogas (CH 4 & CO 2 ) Fermentative Acetogenic Methanogenic

28 178 Energy Resources: Development, Harvesting and Management Stages of biogas formation Uses of Biogas Biogas can be used for production of power, for cooking, lighting, etc. Figure explains the flow chart of different applications of biogas. Fig. 8: Applications of biogas. Cooking and lighting The primary domestic uses of biogas are cooking and lighting. Because biogas has different properties from other commonly used gases, such as propane and butane, and is only available at low pressures (4-8 cm water), stoves capable of burning biogas efficiently must be specially designed. Biogas burns with blue flame and without any soot and odour which is considered to be one of the major advantage compared to traditional cooking fuel like firewood and cow dung cake. Lighting can be provided by means of a gas mantle, or by generating electricity. Biogas mantle lamps consume 2-3 cft per hour having illumination capacity equivalent to 40 W electric bulbs at 220 volts. This application is predominant in rural and unelectrified areas. Biogas as an Engine Fuel Biogas can be used as a fuel in stationary and mobile engines, to supply motive power, pump water, drive machinery (e.g., threshers, grinders) or generate electricity. It can be used to operate four stroke diesels and spark ignition engines. Electricity generation using biogas is a commercially available and proven technology. Typical installations use spark-ignited propane engines that have been modified to operate on biogas. Biogas-fueled engines could also be used for

29 Energy Resources: Development, Harvesting and Management 179 other on-farm applications. As discussed below, diesel or gasoline engines can be modified to use biogas. IC engines (typically used for electricity generation) can be converted to burn treated biogas by modifying carburetion to accommodate the lower volumetric heating value of the biogas into the engine and by adjusting the timing on the spark to accommodate the slower flame velocity of biogas ignition systems. When biogas is used to fuel such engines, it may be necessary to reduce the hydrogen sulphide content if it is more than 2 percent otherwise the presence will lead to corrosion of engine parts. In terms of electricity production, small internal combustion engines with generator can be used to produce electricity in the rural areas with clustered dwellings thus promoting decentralized form of electricity avoiding grid losses. Use of biogas as vehicular fuel Biogas is suitable as a fuel for most purposes, without processing. If it is to be used to power vehicles, however, the presence of CO 2 is unsatisfactory, for a number of reasons. It lowers the power output from the engine, takes up space in the storage cylinders (thereby reducing the range of the vehicle), and it can cause problems of freezing at valves and metering points, where the compressed gasexpands, during running, refuelling, as well as in the compression and storage procedure. All, or most, of the CO 2 must therefore be removed from the raw biogas, to prepare it for use as fuel for vehicles, in addition to the compression of the gas into high-pressure cylinders, carried by the vehicle. Uses of Biodigested Slurry The slurry after the digestion will be washed out of the digester which is rich in various plant nutrients such as nitrogen, phosphorous and potash. Well-fermented biogas slurry improves the physical, chemical and biological properties of the soil resulting qualitative as well as quantitative yield of food crops. Slurry from the biogas plant is more than a soil conditioner, which builds good soil texture, provides and releases plant nutrients. Since there are no more parasites and pathogens in the slurry, it is highly recommended for use in farming. The economic value of the slurry shows that investment can be gained back in three to four year's time if slurry is properly used. The cow dung slurry after digestion inside the digester comes out with following characteristics and has following advantages: When fully digested, effluent is odourless and does not attract insects or flies in the open condition.

30 180 Energy Resources: Development, Harvesting and Management The effluent repels termites whereas raw dung attracts them and they can harm plants fertilised with farmyard manure (FYM). Effluent used as fertiliser reduces weed growth with about 50%. When FYM is used the undigested weed seeds cause an increased weed growth. It has a greater fertilising value than FYM or fresh dung. The form in which nitrogen available can be easily assimilated by the crops. Densification of Biomass-Briquetting Briquetting - The concept and background Briquetting of biomass is an age old technology and one of the several techniques which are broadly classified as densification technology. This process, especially in India, is as old as time. Cow dung cakes, fuel balls made from coal-dust as well as hand compressed special chulhas using wood shavings are some of the prime examples of this process being used in India for centuries. The process of briquetting consists of applying pressure to a mass of particles with or without binder and converting it into a compact agro-mate. The products obtained could be in a solid geometrical form or in the form of hollow cylinders. Briquetting, as a technology was invented in early 19 th century. Utilization of biomass in the form of leaves, bark, wood, cakes etc., for cooking food and warming the space in winter is known to human civilization for many centuries. The easiest and simplest way of utilizing the heat content of these biomass resources has been practiced till recent past, is to burn them as it is. However, now it has been realized that such burning of biomass has certain disadvantages as mentioned below: 1) Most of the agro-forestry biomass contains high moisture (18-20%). The heat generated by burning of high moisture biomass is consumed in drying the biomass itself, thus depriving the quality. 2) Most of the biomass is bulky and many of them are available in powder form, creating dust and polluting the environment. Also it is very difficult to handle and transport, and storage expensive. 3) Biomass has poor combustion properties. 4) It produces plenty of smoke. 5) Expensive and sophisticated furnace is needed. 6) Complete draft and exhaust system gets blocked due to ash carry over and the emission pollutes the environment. 7) Ash may insulate the heating area, thereby reducing the energy efficiency of the system.

31 Energy Resources: Development, Harvesting and Management 181 8) Most of the biomass is not successfully used in simple gasifiers and small boilers. 9) Some of the biomass need an expensive and sophisticated equipment like fluidized bed boilers to achieve maximum efficiency. The simplest solution of all above problems (low bulk density, high volume and expensive handling, transportation and storage) lies in briquetting of biomass. The process of compaction of biomass into a product of higher density than the original raw material is known as densification or briquetting. It has compression ratio of approximately 7:1, the loose biomass to form briquettes. The compacted fuels, known as briquettes is more or less similar to coal and has potential to replace conventional solid fuels and even diesel to meet the local needs of various sectors. Briquettes can be produced with the density of 1.2 to 1.4 g/cm 3 from loose agro residues with a bulk density of 0.1 to 0.2 g/cm 3. Advantages in briquetting of biomass 1) The process helps to solve the problem of loose waste / residues of agricultural forestry and agro-industrial processing so as to check environmental pollution. 2) The process increases the net calorific value per unit volume. 3) The fuel produced is uniform in size and quality. 4) No toxic gas and sulphur emission, even no odour during combustion. 5) Densified product is easy to transport and store. Bulk density of briquettes (1000 kg /m 3 ) is higher than agro-wastes (50 kg /m 3 ). 6) Fire risk in loose storage of biomass is minimized. 7) The process produces high quality fuel with very low ash content (2-5 %) compared to 30-40% in case of coal. 8) The briquettes are easy to burn, as briquettes have lower ignition temperature compared to coal. 9) It produces gas during burning which accelerates burning efficiencies and inhales CO 2 and releases oxygen to the atmosphere. Disadvantages in briquetting of biomass 1) High investment cost and energy consumption input to the process 2) Undesirable combustion characteristics often observed, e.g., poor ignitability, smoking, etc. 3) Tendency of briquettes to loosen when exposed to water or even high humidity weather

32 182 Energy Resources: Development, Harvesting and Management Raw materials for briquetting Agro residues such as saw dust, rice husk, tapioca waste, groundnut shell, cotton stalks, pigeon pea stalks, soybean stalks, coir pith, mustard stalks, sugar cane bagasse, wood chips, tamarind pod, castor husk, coffee husk, dried tapioca stick, coconut shell powder are the commonly used raw materials for briquetting in India. All these residues can be briquetted individually and in combination with or without using binders. The factors that mainly influence on the selection of raw materials are moisture content, ash content, flow characteristics, flow characteristics, particle size and availability in the locality. Moisture content in the range of 10-15% is preferred because high moisture content will pose problems in grinding and more energy is required for drying. The ash content of biomass affects its slagging behaviour together with the operating conditions and mineral composition of ash. Biomass feedstock having up to 4% of ash content is preferred for briquetting. The granular homogeneous materials which can flow easily in conveyers, bunkers and storage silos are suitable for briquetting. Method of briquetting Briquetting is a technological method of compressing and densifying the bulky raw material, thereby reducing its volume-weight ratio and making it usable for various purposes. The vital requirement of briquette formation from woody biomass is the destruction of the elasticity of the wood, which could be done either by previous heat treatment or by a high pressure or by a combination of both. There are two processes of briquetting biomass, namely direct compaction and compaction after pyrolysis or carbonization as mentioned below: Table-4: Method of briquetting Direct compaction Binderless process With binder process Briquetting Pyrolysis / carbonization and extrusion / compaction Direct compaction There are two technologies for the manufacture of briquettes by directly compacting the biomass without previous heat treatment. (i) Binderless process : The process involves two steps (a) Semi-fluidizing the biomass : Biomass is semi-fluidized through the application of high pressure in the range of kg/cm², at which

33 Energy Resources: Development, Harvesting and Management 183 conditioned biomass gets heated to a temperature of about 182 C and the lignin present in biomass begins to flow and act as binder, provides mechanical support and repels water. (b) Extracting the densified material : The semi-fluidized biomass is densified through electrically operated briquetting machines available in the range of kg/h, The cost of such briquetting units depend upon its capacity and is in between Rs. 3 lakh to 20 lakhs. (ii) With binder process : In this process, the biomass requires addition of some external binding materials like molasses, dung slurry, lignasulphonate, sodium silicate etc. The briquetting machines operate at lower pressure range of kg/ cm² and are powered by electricity. Such machines are available in the capacity range of 100 to 400 kg/h. Unit operations of Briquetting Process The series of steps involved in the briquetting process are 1. Collection of raw materials 2. Preparation of raw materials 3. Compaction 4. Cooling and Storage [A] Collection of raw materials In general, any material that will burn, but is not in a convenient shape, size or form to be readily usable as fuel is a good candidate for briquetting. Fig. 9: Collection of raw material.

34 184 Energy Resources: Development, Harvesting and Management [B] Preparation of raw materials The preparation of raw materials includes drying, size reduction, mixing of raw materials in correct proportion, mixing of raw materials with binder etc. Drying The raw materials are available in higher moisture contents than what required for briquetting. Drying can be done in open air (sun), in solar driers, with a heater or with hot air. Fig. 10: drying and Size reduction of biomass. The raw material is first reduced in size by shredding, chopping, crushing, breaking, rolling, hammering, milling, grinding, cutting etc. until it reaches a suitably small and uniform size (1 to 10 mm). For some materials which are available in the size range of 1 to 10mm need not be size reduced. Since the size reduction process consumes a good deal of energy, this should be as short as possible [C] Briquetting Technologies Briquetting technologies used in the briquetting of the agro residues are divided into three categories. They are (i) high pressure or high compaction technology, (ii) Medium pressure technology and (iii) low pressure technology. In high pressure briquetting machines, the pressure reaches the value of 100 MPa. This type is suitable for the residues of high lignin content. At this high pressure the temperature rises to about o C, which is sufficient to fuse the lignin content of the residue, which acts as a binder and so, no need of any additional binding material. In medium pressure type of machines, the pressure developed will be in the range of 5 MPa and 100MPa which results in lower heat generation. This type of machines requires additional heating to melt the lignin content of the agro residues which eliminates the use of an additional binder material. The third type of machine called the low pressure machines works at a pressure less than 5 MPa and room temperature. This type of machines requires addition of

35 Energy Resources: Development, Harvesting and Management 185 binding materials. This type of machines is applicable for the carbonized materials due to the lack of the lignin material. The high pressure compaction technology for briquetting of agro residues can be differentiated in to two types (i) hydraulic piston press type and (ii) screw press type. Mostly cylindrical shaped briquettes with 30 mm to 90 mm diameter were produced. All the commercial firms involved in briquette making produces 60 mm and 90 mm diameter briquettes. A scheme of a hydraulic piston press briquetting technology [D] Cooling and Storage of briquettes Briquettes extruding out of the machines are hot with temperatures exceeding 100 o C. They have to be cooled and stored in dry place. Fig. 11: A Scheme of a Hydraulic Piston Press Briquetting Technology. Fig. 12: Stored Briquettes.