Fluidized Bed Combustion Boiler Technology For Cogeneration

Transcription

1 Technical Study Report on B I O M A S S F I R E D Fluidized Bed Combustion Boiler Technology For Cogeneration

2 II About the Technical Study Report Cleaner Production (CP) and Energy Efficiency (EE) are established and powerful strategies helping enterprises throughout the world to reduce costs, generate profits by reducing waste and mitigate climate change. Integration of CP and EE provides synergies that broaden the scope of their individual application and give more effective results both environmental and economic. Implementing CP-EE projects in industries also requires efficient and environment-friendly technology interventions. Co-generation through fluidized bed combustion (FBC) boiler using biomass (such as rice husk, straw etc.) is one such proven technology which could help in mitigation of green house gases emissions. UNEP-DTIE's Energy Branch is planning to develop a series of technical study reports covering various specific technologies that can be adopted by the industries all over the world as a part of their CP-EE initiatives. This is first such technical study report that documents the various techno-economical and managerial aspects of biomass-based FBC technology for practical use by the industries in the regions where large amounts of biomass are available. The study report provides an overview of FBC technology, co-generation system and practical aspects of implementing such a system in an industry. A detailed case study provides insights to the technical specifications of the various equipments, systems and cost economics. It also provides list of technology providers and suppliers worldwide. All in all, this technical report is a comprehensive and complete documentation for implementation of biomass based FBC boiler for co-generation. The technical study report is targeted to the decision makers, technical personnel in the industry, academia, consultants as well as government agencies. Specifically, it is very useful for the technical managers in the industries who would like to implement biomass based co-generation systems in their facilities.

3 III Contents About the Technical Report...ii Contents...iii List of Tables...v List of Figures...vi Abbreviations and Acronyms Used...vii 1.0 Introduction Cleaner Production & Energy Efficiency Biomass as a Fuel Biomass Energy Conversion Technologies FBC Boiler & Cogeneration Systems FBC Boilers Cogeneration (Combined Heat & Power) Biomass-based FBC and Co-generation Technology Overview of the Technology Areas of Application Issues in Implementation of Biomass-based Cogeneration Systems Environmental Benefits of Biomass based cogeneration Systems Social Benefits of Biomass based cogeneration Systems Implementing Biomass Cogeneration Technology Raw material, Energy Resource requirement Infrastructure Requirement Supporting Technologies Waste Disposal Human Resources Demand Equipment Suppliers Case Study Introduction...49

4 IV 5.2 Manufacturing Process Baseline Energy Scenario Implementation of Rice Husk based Cogeneration System Further Suggestions Power Generation using bio-mass in FBC Boiler Power Generation through Biomass Gasifier...59 Annex 1 Block Diagram of Kraft Paper...61 Annex 2: Block Diagram of White Duplux Board...62 Annex 3: Technical Specification of Key Equipment/Components...64

5 V List of Tables Table 1 Global Biomass-fuel based Electricity Generation Capacity, Table 2 Crop Residue from 4 Major Crops in EJ (1987)...11 Table 3 : Global Bagasse Residues...12 Table 4 Comparison of Different Types of Biomass Conversion Technologies 16 Table 5: Heat to Power ratios and other parameters of cogeneration systems 31 Table 6 : Typical heat to Power ratio for Certain Energy intensive Industries 32 Table 7 : Fuels and their typical calorific values...43 Table 8 : External Infrastructure Requirements...43 Table 9 : Area requirements for different components of a typical cogeneration system...44 Table 10 : Supporting Technologies for Cogeneration Systems 44 Table 11 : Waste Generated in Cogeneration Plant...46 Table 12 : Suppliers for Steam Turbine and FBC Boiler...47 Table 13 : Specifications of the DG sets installed for captive power generative 50 Table 14 : (A) Preliminary & Preoperative Expenses...53 Table 15: (B) Cost Involved for procuring Land & Site Development 53 Table 16 (C): Cost of Civil Works Required...53 Table 17 : (D) Cost of Plant & Machinery Required...54 Table 18: (E.)Repair & Maintenance Cost for Building, Plant & Machinery 54 Table 19: (F) Additional Manpower required for Co-generation project 54 Table 20 : Summary of Costs (From A to E)...55 Table 21: Cost Analysis Before and After Implementation of Cogeneration Scheme...55 Table 22 : Greenhouse Gases Emissions Reduction due to Cogeneration

6 VI List of Figures Figure 1: Electricity Generation by Source...10 Figure 2 Regional Distribution of various Sources of Biomass & its use in EJ / annum...11 Figure 3 Global Agricultural Residues, Figure 4 : Principles of Fluidization...19 Figure 5 A View of AFBC Boiler...20 Figure 6 : A Detailed View of Different Components of AFBC Boiler 21 Figure 7 : A CFBC Boiler...23 Figure 8 : Energy Balance of a Typical Thermal Power Plant in India 26 Figure 9: Configurations of different types of turbine systems27 Figure 10: Different Configurations of Back Pressure Turbine28 Figure 11: Configuration of Extraction cum condensing turbine 28 Figure 12 : Elements of a Biomass Based Cogeneration System using FBC Boiler...31 Figure 13: Chipping Machine for Cajurina branches & coconut fronds at Varam Power, India...36 Figure 14 : Collection & Baling Machine for sugarcane trash at GMR technologies, India...36 Figure 15 : Example on estimation of fuel requirement for co-generation 42 Figure 16 Annual Production Trend...49 Figure 17 : Electrical Power requirements trends- Baseline values 51 Figure 18: Steam requirements trends- Baseline values...51 Figure 19: Electrical Power Requirements after Installing the Cogeneration System...52 Figure 20: Steam Requirements after Installing the Cogeneration System 52 Figure 21 : Schematics of the Cogeneration System...53 Figure 22 : Various Biomasses based power plants and their numbers in India 59 Figure 23 : Biomass Gassifier in Operation...59

7 VII Abbreviations and Acronyms Used BBCS CHP CP CPEE DG sets EJ ESP FBC GWh H.T. KVA KWth KWe MNRE MW 0C Biomass based Cogeneration System Combined Heat & Power Cleaner Production Cleaner Production Energy Efficiency Diesel Generator Set Exa-Jourles (IEJ = 1 x 1018 Joules) Electro static Precipitators Fluidized Bed Combustion Giga Watt hour High Tension Kilo Volt Ampere Kilo Watts Thermal Kilo Watts electrical Ministry of New & Renewable Energy, India Mega Watt Degree Centigrade

8 8 1.0 Introduction 1.1 Cleaner Production & Energy Efficiency For decades UNEP has been championing the concepts and practices of Cleaner Production (CP) and Energy Efficiency (EE) in a systematic manner. Recognizing the immense benefits that can be realized by the end-users, UNEP has recently developed guidelines for integration of CP and EE. The idea of this integrated approach is to incorporate the energy management principles into the resource efficiency approach that lies at the heart of CP. These guidelines have been presented in a form of a manual popularly known as the CP-EE Manual. This guidance manual is primarily used by facility personnel for conducting in-house assessments as well as by external consultants. While managers gain insights into the role they can play in instigating and supporting an ongoing, cost-effective process for continual improvement leading to both economic and environmental advantages, CP professionals and consultants (who may not necessarily be energy specialists) find such guidance on incorporating energy issues into their CP assessments at industrial or other facilities immensely valuable. The integrated methodology is derived from the basic principles of the Deming s Cycle of Plan Do Check Act popularly acronymed as PDCA Cycle. Moreover, it addresses eight different categories for identifying the options for resource conservation: 1. Good housekeeping 2. Process Optimization 3. Operation Practices/management 4. Raw Material Substitutions 5. New technology 6. New product design 7. Onsite recycle and reuse 8. Recovery of useful by products As seen from the list above, New Technology is one of the most important categories amongst the CP-EE options. For this, rapid technological advancements in the current times require the professionals to remain updated about the new technologies that are continuously evolving in response to the various environmental challenges. Global warming and Climate change is one of the most pressing and burning issues that needs urgent global action at all levels. The key to address this problem is by mitigation of carbon dioxide and other greenhouse gases produced by combustion of various fuels (both fossil and non fossil). Various new technological solutions are being tried and tested around the world to address this serious problem for the humankind. This study report highlights the use of one such proven technology viz. Biomass based Fluidized Bed Combustion Boilers for Combined Heat and Power Applications which could possibly help in addressing the issue of global warming and climate change.

9 9 1.2 Biomass as a Fuel 1 Biomass, the oldest form of renewable energy, has been used for thousands of years. However, with the emergence of fossil fuels, its relative share of use has declined over past years. Currently some 13% of the world s primary energy supply is from biomass, though there are strong regional differences. Developed countries source around 3% of their energy from biomass while, in Africa it ranges between 70-90%. With adverse environmental effects on the environment such as climate change coming to the forefront, people everywhere are rediscovering the advantages of biomass. Potential benefits of biomass: Reducing carbon emissions if managed (produced, transported, used) in a sustainable manner Enhancing energy security by diversifying energy sources & utilizing local resources Reduced problem of biomass waste management Possible additional revenues for the agricultural and forestry sectors Until the industrial revolution, humankind relied almost exclusively on biomass for their energy needs. Most of the biomass is burnt to provide heat for cooking or warmth. Some is used for small industrial applications (For instance, Charcoal is used in steelmaking in countries like Brazil, which have no major coal reserves). A small percentage of biomass is also used to generate electricity. Total biomass consumption at the beginning of the twenty-first century was 55 exa- Joules or 55EJ 2 out of total global energy consumption of around 400EJ. Estimates of the total quantities of biomass available vary widely but could represent up to 100EJ of energy. Biomass energy accounts for around 14% of total primary energy consumption. This bold figure hides a major disparity between the developed and the developing world. Estimates of the amount of energy that can be supplied from biomass too vary widely, but according to some estimates, by 2050 it could provide as much as 50% of global primary energy supply. Generating electricity from biomass is perhaps one very attractive and easy option to make use of this valuable resource. It uses exactly the same technology that has become common in the power generation industry - furnaces to burn coal, boilers to raise steam from the heat produced and steam turbines to turn the steam into electricity. Table 1 represents the electricity generation capacity of the world using biomass as fuel. Table 1 Global Biomass-fuel based Electricity Generation Capacity, 2004 Region Approx. Installed Capacity (MW) Europe 8000 US 7000 ASEAN region 2000 Australia 300 Indonesia 300 Philippines 20 Thailand Biomass, Issue Brief Energy and Climate Change, World Business Council for Sustainable Development 2 1EJ = 1x1018 Joules

10 10 In 2000, biomass was the largest renewable energy source for electricity generation - other than hydro - generating around 1% of the world s electricity or 167 TWh. However, its share is and will remain small in comparison to fossil-based sources (see Figure 1). Figure 1: Electricity Generation by Source Biomass as a Carbon Neutral Fuel Use of biomass as a fuel is considered to be carbon neutral because plants and trees remove carbon dioxide (CO 2) from the atmosphere and store it while they grow. Burning biomass in homes, industrial processes, energy generation, or for transport activities returns this sequestered CO 2 to the atmosphere. At the same time, new plant or tree growth keeps the atmosphere s carbon cycle in balance by recapturing CO 2. This net-zero or carbon neutral cycle can be repeated indefinitely, as long as biomass is regrown in the next management cycle and harvested for use. The sustainable management of the biomass source is thus critical to ensuring that the carbon cycle is not interrupted. In contrast to biomass, fossil fuels such as gas, oil and coal are not regarded as carbon neutral because they release CO 2 which has been stored for millions of years, and do not have any storage or sequestration capacity Sources of Biomass as fuel3 There are a variety of biomass residues available around the world. The most important of these are crop residues but there are significant quantities of forestry residues and livestock residues as well, which can also be used for energy production. Most of the world's crops generate biomass residues that can be used for energy production. Wheat, barley and oats all produce copious amounts of straw, which have traditionally been burned (approx. 1-2 Billion T of crop residues may be burned annually). Rice produces both straw in the fields and rice husks at the processing plant which can be conveniently and easily converted into energy. (Recent legislation has made straw burning illegal in some parts of the world. Since the straw must still be removed from fields, such legislation could make it cost effective to convert these residues into energy. ) When Maize is harvested significant quantities of biomass remain in the field. Much of this needs to be returned to the soil but when the harvested maize is stripped from its cob the latter remains, more biomass which can easily be converted into energy on-site. 3 Business Insights, The Future of Global Biomass Power Generation: The technology, economics and impact of biomass power generation By Paul Breeze, 2004

11 11 Sugar cane harvesting leaves harvest 'trash' in the fields while processing produces fibrous bagasse. The latter is a valuable source of energy. Harvesting and processing of coconuts produces quantities of shell and fibre that can be utilized. Peanuts leave shells, which is a great source of biomass energy. Figure 2 Regional Distribution of various Sources of Biomass & its use in EJ / annum Figure 3 Global Agricultural Residues, 1987 Putting figures on the quantities of each of these crops is rather difficult. One estimate is shown in Table 2 where the total residue from the four major crops listed is equivalent to 32EJ. Another estimate puts the total of crop residues at 65EJ7 while yet another, from 1993, suggested that utilizing only 25% of the waste from the world's main agricultural crops could generate 38EJ. Table 2 Crop Residue from 4 Major Crops in EJ (1987) REGION MAIZE STRAW WHEAT STRAW RICE STRAW BAGASSE TOTAL Africa US & Canada Latin America Asia Europe Oceania Total

12 12 Varying Estimates A major problem when estimating the quantity of residues that might be used for energy production is to determine how much of each is required for other purposes. At least, part of many crop residues must be returned to the soil to maintain soil quality. Similarly, livestock residues need to be returned to pastures as manure. Taking this into account, a recent exercise carried out by US Department of Agriculture concluded that crop residues alone could provide electricity equivalent to 5% of US consumption in Though local factors make direct comparisons with other regions difficult, a similar contribution might be expected in other parts of the developed world. Given the high per capita electricity use in the US, developing countries might expect to be able to find a greater proportion of their electricity in this way. The figures in Table 2 also suggest that Asia produces the largest quantities of agricultural residues and there is potential across all the continents. However, mere availability of the residue does not guarantee its use. From the perspective of electricity generation, the cost of collection of the residue becomes the key factor in determining its viability. Wheat straw can be baled, making collection more efficient. Several European projects have demonstrated that power plants based on straw can become cost effective when the straw cannot be burned in the fields where it is cut. Another aspect to consider is the seasonal nature of the harvest, which necessitates the plants to either have a large storage facility or alternative sources of fuel. Fuels such as rice husks and maize cobs are produced during processing of these crops. This takes place after harvesting of the crop, so the waste is already concentrated at a point and is an easily exploitable source of energy - particularly if it can be utilized on site to provide heat and power. Sugar cane bagasse is another valuable source of fuel and one that can be exploited easily because it, too, is generated during the processing of the cane. Table 3 provides a breakdown of global bagasse potential from the World Energy Council. Table 3 : Global Bagasse Residues REGION QUNATITY OF BAGASSE (,000 MT) Africa North America South America Asia Europe 502 Middle east 914 Oceania Total The bagasse figures in Table 3 represent only part of the biomass generated during sugar cane farming. The 'trash' which is left in the fields represents about 55% of the total, and this is often burned. With efficient collection methods, this could provide a further rich source of energy, provided minimum required amount is returned to the soil to maintain fertility. Sugar processing plants have traditionally burned this fuel, generally inefficiently, to generate process heat which is all used on-site. Modern combined heat and power plants can produce more energy than is required by the plant itself. According to one estimate, the amount of surplus electricity that sugar processing plants could generate and export to their local grids could, by 2025, account for 15%- 20% of the total demand in the developing countries.

13 13 Biomass Availability in India and Potential for Co-generation Biomass is the traditional fuel in India, used for cooking, and even today, most households, in rural India, use it as cooking fuel. This biomass, mostly consists of agricultural farm residues (e.g. paddy straw, sugar cane trash etc), agro-industrial residues (e.g. paddy husk, coffee husk etc), forests & social forests residues and energy plantations, which (i.e. energy plantation) is just picking up. The following Table, provides the different types of biomass, that are presently being used in India Biomass varieties presently used in India for Co-generation Agro and farm Biomass Agro-Industrial Biomass Forest Residues & plantations Babul Stems Coffee Husk Fire Wood Chilly stalks Bagasse Forest residues Coconut husk De oiled bran Julie Flora Coconut Pith Ground nut husk Other woody biomass chips Corn cobs Cotton Stalk Maize Stems Mango residues Mustard Stalk Palm leaf Prosopis Rai Stems Sugar Cane Trash Tamarind husk Til stems Casurina branches & fruit Ground nut shells Rice Husk Saw dust Indian Ministry of New and Renewable Energy s Annual Report for indicates surplus agro & forest residues of 60 Million MT available for power generation. Further, the report also projected an availability of 40 million MT of woody biomass annually, from energy plantation, on 4 million hectares of wasteland. Considering plant load factor of 70%, the estimated potential for power generation in India alone is 13,000 MW from various biomass based sources. 1.3 Biomass Energy Conversion Technologies There are a number of ways for converting biomass into electricity. The simplest approach is to burn the biomass in a furnace, exploiting the heat generated to produce steam in a boiler, which is then used to drive a steam turbine. This approach, often called direct firing, is the most widespread means of deriving heat and electricity from biomass today. It is also generally rather inefficient, though new technologies will be able to improve efficiency significantly. A simple, direct-fired biomass power plant can either produce electricity alone or it can operate as a combined heat and power unit, producing both electricity and heat. This latter is common in the textile, food processing, chemical and paper industries where the heat is used in the processing plant. The electricity generated is used by the plant

14 14 too, with any surplus exported to the grid. Simplicity is the key feature of direct firing type of application. A more advanced approach is biomass gasification. This employs a partial combustion process to convert biomass into a combustible gas. The gas has a lower energy content than natural gas. Nevertheless, it can be used in the same way as natural gas. In particular it can provide fuel for gas turbines and fuel cells. Biomass gasification is still in the development stage but it promises high efficiency and may offer the best option for future biomass-based generation. An intermediate option for exploiting biomass is to mix it with coal and burn it in a coal fired power station. In the short term this may offer the cheapest and most efficient means of exploiting biomass. Finally there are number of specialized methods of turning biomass wastes into energy. These include digesters, which can convert dairy farm waste into a useful fuel gas, and power stations that utilize chicken farm litter, which they burn to generate electricity. In terms of conversion technologies, following technologies are commonly used: 1. Pile Combustion 2. Stoker Combustion 3. Suspension Combustion 4. Fluidized Bed Combustion Pile Combustion The simplest form of direct firing involves a pile burner. This type of burner has a furnace, which contains a fixed grate inside a combustion chamber. Wood is fed (piled) onto the grate where it is burned in air, which passes up through the grate (called under-fire air). The grate of a pile burner is within what is known as the primary combustion chamber where the bulk of the combustion process takes place. Combustion at this stage is normally incomplete - there may be significant quantities of both unburned carbon and combustible carbon monoxide remaining - so further air (called overfire air) is introduced into a secondary combustion chamber above the first - where combustion is completed. The boiler for raising steam is positioned above this second combustion chamber so that it can absorb the heat generated during combustion. The heat warms, and eventually boils water in the boiler tubes, providing steam to drive a steam turbine. From the steam turbine the steam is condensed and then returned to the boiler so that it can be cycled through the system again. (In a combined heat and power system, steam will be taken from the steam turbine outlet to provide heat energy first.) Wood fuel is normally introduced from above the grate, though sometimes there is a more complicated arrangement, which feeds fuel from under the grate. The pile burner is capable of handling wet and dirty fuels but it is extremely inefficient. Boiler efficiencies are typically 50%-60%. There is no means to remove the ash from a pile burner except by shutting down the furnace. Thus the power plant cannot be operated continuously. Pile burners are also considered difficult to control and they are slow to respond to changes in energy input. This means that electricity output cannot easily be changed in response to changes in demand. Power generation in a pile-burner based power station will usually involve a single pass steam turbine generator operating at a relatively low steam temperature and pressure. This adds to the relatively low efficiency of the power plant, which can operate, with an overall efficiency as low as 20%.

15 Stoker Combustion The pile burner represents the traditional method of burning wood. However, its basic operation can be improved by introducing a moving grate or stoker. This allows continuous removal of ash so that the plant can be operated continuously. Fuel can also be spread more thinly on the grate, encouraging more efficient combustion. The first US stoker grate for wood combustion was introduced by the Detroit Stoker Co. in the 1940s. In this type of furnace, combustion air still enters below the grate of a stoker burner. This flow of air into the combustion chamber helps cool the grate. The air flow and consequent grate temperature determines the maximum operating temperature of the combustor. This, in turn, determines the maximum moisture content allowable in the wood fuel if combustion is to proceed spontaneously. There are refinements of the basic stoker grate such as inclined grates and water-cooled grates, both of which can help improve overall performance and make the operation less sensitive to fuel moisture. Nevertheless stoker combustors are still relatively inefficient, with boiler efficiencies of 65%-75% and overall efficiencies of 20%-25% Suspension Combustion Most modern coal-fired power stations burn pulverized coal, which is blown into the combustion chamber of a power plant through a specially designed burner. The burner mixes air with the powdered coal, which then burns in a flame in the body of the combustion chamber. This is suspension combustion and in this type of plant there is no grate. Finely ground wood, rice husk, bagasse, or sawdust can be burned in a similar way. Suspension firing requires a special furnace. The size and moisture content of the biomass (wood) must also be carefully controlled. Moisture content should be below 15% and the biomass particle size has to be less than 15mm. Suspension firing results in boiler efficiency of up to 80% and allows a smaller sized furnace for a given heat output. However it also requires extensive biomass drying and processing facilities to ensure that the fuel is of the right consistency. It also demands special furnace burners. A small number of plants designed to burn biomass in this way have been built. The technology is also of great interest as the basis for the co-firing of wood or other biomass with coal in pulverized coal plants Fluidized Bed Combustion Aside from suspension firing of wood, the most efficient method of directly burning biomass is in a fluidized bed combustor (FBC). This is also the most versatile since the system can cope with a wide range of fuels and a range of moisture contents. The basis for a FBC system is a bed of an inert mineral such as sand or limestone through which air is blown from below. The air is pumped through the bed in sufficient volume and at a high enough pressure to entrain the small particles of the bed material so that they behave much like a fluid. The combustion chamber of a fluidized bed plant is shaped so that above a certain height the air velocity drops below that necessary to entrain the particles. This helps retain the bulk of the entrained bed material towards the bottom of the chamber. Once the bed becomes hot, combustible material introduced into it will burn, generating heat as in a more conventional furnace. The proportion of combustible material such as biomass within the bed is normally only around 5%. There are different designs of FBC system which involve variations around this principle. The most common for biomass combustion is the circulating fluidized bed which incorporates a

16 16 cyclone filter to separate solid material from the hot flue gases which leave the exhaust of the furnace. The solids from the filter are re-circulated into the bed, hence the name. The fluidized bed has two distinct advantages for biomass combustion: First, it is the ability to burn a variety of different fuels without affecting performance. Second is the ability to introduce chemical reactants into the fluidized bed to remove possible pollutants. In FBC plants burning coal, for example, limestone can be added to capture sulphur and prevent its release to the atmosphere as sulphur dioxide. Biomass tends to contain less sulphur than coal so this strategy may not be necessary in a biomass plant. A fluidized bed boiler can burn wood with up to 55% moisture. One specialized application is in plants designed to burn chicken litter, the refuse from the intensive farming of poultry. Power stations have been built that are devoted specifically to this fuel source and these plants use FBCs. Of the four different types of combustion technologies discussed above, the FBC technology is best suited for a range of small and medium scale operation for combined heat and power. With technological advancements the FBC boilers give efficiency of as high as 80-82% and can be used for a wide variety of fuels Comparison of Different Types of Biomass Conversion Technologies Table 4 below compiles a quick Comparison of Different Types of Biomass Conversion Technologies commonly used worldwide. Table 4 Comparison of Different Types of Biomass Conversion Technologies Parameter Pile Combustion Stoker Combustion Suspension Combustion Grate Fixed / Stationary Grate Fixed or moving grate No grate or moving grate Fuel Size Uniform size of the fuel Uneven fuel size can be Preferable for high % in the range of range 60 used of fins in the fuel to 75 mm is desired & % fines should not be more than 20% Combustion Bed temperature Moisture Difficult to maintain good combustion due to : Air fuel mixing is not proper Bed height is in stationary condition resulting in clinker formation Difficult to avoid air channeling Due to intermittent ash removal system it is difficult to maintain good combustion The combustion is better & an improved version of pile combustion. Since most of the fuel is burnt in suspension the heavier size mass falls on the grate. If the system has a moving grate the ash is removed on a continuous basis & therefore the chances of clinker formation are less. It is similar to stoker combustion, but since the fuel sizes is small & even the combustion efficiency is improved as maximum amount of fuel is combusted during suspension. Fluidized Bed Combustion No grate Uniform size fuel in the range of 1 to 10 mm. Best combustion takes place in comparison with the other types since the fuel particles are in fluidized state & there is adequate mixing of fuel & air ºC ºC ºC ºC High moisture leads to bed choking & difficult combustion conditions Combustion condition not very much disturbed with 4-5 % increase in moisture Same as Stoker Combustion It can handle fuels with high moisture condition up to 45-50% but high moisture in the fuels is not desirable, & adequate precautions are to be taken up in the design stage itself.

17 17 Parameter Pile Combustion Stoker Combustion Suspension Combustion Draft Natural Draft / Forced Forced Draft / Balance Balance draft Conditions Draft/ Balance Draft draft Maintenance Not much maintenance Frequent problems Variation in fines in problems due to moving grate fuel leads to delayed combustion thereby affecting the boiler tubes Fluidized Bed Combustion Balance draft Erosion of boiler tubes embedded in the bed is quite often

18 FBC Boiler & Cogeneration Systems 2.1 FBC Boilers Introduction to FBC Boilers The traditional grate fuel firing systems have several limitations and hence are technoeconomically unviable to meet the challenges of the future. FBC has emerged as a viable alternative as it has significant advantages over conventional firing system. FBC offers multiple benefits, such as: compact boiler design, flexibility with fuel used, higher combustion efficiency and reduced emissions of noxious pollutants such as SO x and NO x. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr Mechanism of Fluidized Bed Combustion When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles remain undisturbed at low velocities. As the air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream and the bed is called fluidized. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid bubbling fluidized bed. At higher velocities, bubbles disappear, and particles are blown out of the bed. Therefore, some amounts of particles have to be re-circulated to maintain a stable system and is called as circulating fluidized bed". This principle of fluidization is illustrated in Figure 4. Fluidization depends largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the gas velocity. The difference between the mean solid velocity and mean gas velocity is called as slip velocity. Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact. If sand particles in fluidized state are heated to the ignition temperatures of fuel (rice husk, coal or bagasse), and fuel is injected continuously into the bed, the fuel will burn rapidly and the bed attains a uniform temperature. The fluidized bed combustion (FBC) takes place at about 840 C to 950 C. Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided. The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between minimum fluidization velocity and particle entrainment velocity. This ensures a stable operation of the bed and avoids particle entrainment in the gas stream. 4 Energy Efficiency in Thermal Utilities, A Guide Book for Energy Managers and Auditors, Bureau of Energy Efficiency, Ministry of Power, Government of India, 2005

19 19 Fixing, Bubbling & Fast Fluidized Beds: As the velocity of a gas flowing through a bed of particles increases, a value is reaches when the bed fluidizes and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintain a stable system. Figure 4 : Principles of Fluidization Any combustion process requires three T s - that is Time, Temperature and Turbulence. In FBC, turbulence is promoted by fluidization. Improved mixing generates evenly distributed heat at lower temperature. Residence time is many times higher than conventional grate firing. Thus an FBC system releases heat more efficiently at lower temperatures. Since limestone can also be used as particle bed (in case the fuel with sulphur content is used), control of SOx and NOx emissions in the combustion chamber is achieved without any additional control equipment. This is one of the major advantages over conventional boilers Types of Fluidized Bed Combustion Boilers There are three basic types of fluidized bed combustion boilers: 1. Atmospheric Fluidized Bed Combustion System (AFBC) 2. Atmospheric circulating (fast) Fluidized Bed Combustion system (CFBC) 3. Pressurized Fluidized Bed Combustion System (PFBC).

20 AFBC / Bubbling Bed AFBC is one of the most important types of FBC boilers as it can be used for variety of fuels - such as agricultural residues like rice husk or bagasse and even low quality coal. This type of boiler find use in industries where there is a possibility of having a combined heat and power generation application. In AFBC boilers the fuel is sized depending on the type of fuel (in case of coal, the coal is crushed to a size of 1 10 mm depending on the grade of coal) and the type of fuel feeding system and is fed into the combustion chamber. The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust flue gases. The velocity of fluidizing air is in the range of 1.2 to 3.7 m /sec. The rate at which air is blown through the bed determines the amount of fuel that can be reacted. Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed depth is usually 0.9 m to 1.5 m deep and the pressure drop averages about 1 inch of water per inch of bed depth. Very little material leaves the bubbling bed only about 2 to 4 kg of solids is recycled per ton of fuel burned. Typical fluidized bed combustors of this type are shown in Figures 5 and 6. Figure 5 A View of AFBC Boiler The combustion gases pass over the super heater sections of the boiler, flow past the economizer, the dust collectors and the air pre-heaters before being exhausted to atmosphere. The main special feature of atmospheric fluidized bed combustion is the constraint imposed by the relatively narrow temperature range within which the bed must be operated. With coal, there is risk of clinker formation in the bed if the temperature exceeds 950 C and loss of combustion efficiency if the temperature falls below 800 C. For efficient sulphur retention, the temperature should be in the range of 800 C to 850 C. General Arrangements of AFBC Boiler AFBC boilers comprise of following systems: Fuel feeding system Air distributor

21 21 Bed & In-bed heat transfer surface Ash handling system. Many of these are common to all types of FBC boilers. Figure 6 : A Detailed View of Different Components of AFBC Boiler a) Fuel Feeding System For feeding fuel and adsorbents like limestone or dolomite, usually two methods are followed: under bed pneumatic feeding and over-bed feeding. Under Bed Pneumatic Feeding If the fuel is coal, it is crushed to 1 6 mm size and pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. Based on the capacity of the boiler, the number of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly. Over-Bed Feeding The crushed coal, 6 10 mm size is conveyed from coal bunker to a spreader by a screw conveyor. The spreader distributes the coal over the surface of the bed uniformly. This type of fuel feeding system accepts over size fuel also and eliminates transport lines, when compared to under-bed feeding system. Now a days for rise husk and other agricultural residues Over bed feeding system is quite prominent and economical. Some of the boilers are so designed that they have both types of feeding systems. b) Air Distributor The purpose of the distributor is to introduce the fluidizing air evenly through the bed cross section thereby keeping the solid particles in constant motion, and preventing the formation of de-fluidization zones within the bed. The distributor, which forms the furnace floor, is

22 22 normally constructed from metal plate with a number of perforations in a definite geometric pattern. The perforations may be located in simple nozzles or nozzles with bubble caps, which serve to prevent solid particles from flowing back into the space below the distributor. The distributor plate is protected from high temperature of the furnace by: Refractory Lining A Static Layer of the Bed Material or Water Cooled Tubes. c) Bed & In-Bed Heat Transfer Surface: Bed The bed material can be sand, ash, crushed refractory or limestone, with an average size of about 1 mm. Depending on the bed height these are of two types: shallow bed and deep bed. At the same fluidizing velocity, the two ends fluidize differently, thus affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lower bed resistance and hence a lower pressure drop and lower fan power consumption. In the case of deep bed, the pressure drop is more and this increases the effective gas velocity and also the fan power. In-Bed Heat Transfer Surface In a fluidized in-bed heat transfer process, it is necessary to transfer heat between the bed material and an immersed surface, which could be that of a tube bundle, or a coil. The heat exchanger orientation can be horizontal, vertical or inclined. From a pressure drop point of view, a horizontal bundle in a shallow bed is more attractive than a vertical bundle in a deep bed. Also, the heat transfer in the bed depends on number of parameters like (i) bed pressure (ii) bed temperature (iii) superficial gas velocity (iv) particle size (v) Heat exchanger design and (vi) gas distributor plate design. d) Ash Handling System i) Bottom Ash Removal In the FBC boilers, the bottom ash constitutes roughly % of the total ash, the rest being the fly ash. The bed ash is removed by continuous over flow to maintain bed height and also by intermittent flow from the bottom to remove over size particles, avoid accumulation and consequent defluidization. While firing high ash coal such as washery rejects, the bed ash overflow drain quantity is considerable so special care has to be taken. ii) Fly Ash Removal The amount of fly ash to be handled in FBC boiler is relatively very high, compared to conventional boilers. This is due to elutriation of particles at high velocities. Fly ash carried away by the flue gas is removed in number of stages; firstly in convection section, then from the bottom of air pre-heater/economizer and finally a major portion is removed in dust collectors. The types of dust collectors used are cyclone, bag filters, electrostatic precipitators (ESP s) or some combination of all of these. To increase the combustion efficiency, recycling of fly ash is practiced in some units Circulating Fluidized Bed Combustion (CFBC) Circulating Fluidized Bed Combustion (CFBC) technology has evolved from conventional bubbling bed combustion as a means to overcome some of the drawbacks associated with conventional bubbling bed combustion (see Figure 7).

23 23 Figure 7 : A CFBC Boiler CFBC technology utilizes the fluidized bed principle in which crushed (6 12 mm size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution nozzles. The fluidizing velocity in circulating beds ranges from 3.7 to 9 m/sec. The balance of combustion air is admitted above the bottom of the furnace as secondary air. The combustion takes place at C, and the fine particles (<450 microns) are elutriated out of the furnace with flue gas velocity of 4 6 m/s. The particles are then collected by the solids separators and circulated back into the furnace. Solid recycle is about 50 to 100 kg per kg of fuel burnt. There are no steam generation tubes immersed in the bed. The circulating bed is designed to move a lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion zone convection section, water walls, and at the exit of the riser. Some circulating bed units even have external heat exchanges. The particles circulation provides efficient heat transfer to the furnace walls and longer residence time for carbon and limestone utilization. The controlling parameters in the CFB combustion process are temperature, residence time and turbulence. For large units, the taller furnace characteristics of CFBC boiler offers better space utilization, greater fuel particle and adsorbent residence time for efficient combustion and SO 2 capture, and easier application of staged combustion techniques for NOx control than AFBC generators.

24 24 CFBC boilers are said to achieve better calcium to sulphur utilization 1.5 to 1 vs. 3.2 to 1 for the AFBC boilers, although the furnace temperatures are almost the same. CFBC requires huge mechanical cyclones to capture and recycle the large amount of bed material, which requires a tall boiler. A CFBC could be good choice if the following conditions are met. Capacity of boiler is large to medium Sulphur emission and NOx control is important The boiler is required to fire low-grade fuel or fuel with highly fluctuating fuel quality. Major performance features of the CFBC system are as follows: It has a high processing capacity because of the high gas velocity through the system. The temperature of about 870 C is reasonably constant throughout the process because of the high turbulence and circulation of solids. The low combustion temperature also results in minimal NOx formation. Sulphur present in the fuel is retained in the circulating solids in the form of calcium sulphate and removed in solid form. The use of limestone or dolomite adsorbents allows a higher sulfur retention rate, and limestone requirements have been demonstrated to be substantially less than with bubbling bed combustor. The combustion air is supplied at 1.5 to 2 psig (pounds per square inch gauge) rather than 3 5 psig as required by bubbling bed combustors. It has high combustion efficiency. It has a better turndown ratio than bubbling bed systems. Erosion of the heat transfer surface in the combustion chamber is reduced, since the surface is parallel to the flow. In a bubbling bed system, the surface generally is perpendicular to the flow. CFBC boilers are generally claimed to be more economical than AFBC boilers for industrial application requiring more than T/hr of steam, therefore this type of boilers is beyond the scope of the document Pressurized Fluid Bed Combustion Boiler Pressurized Fluidized Bed Combustion (PFBC) is a variation of FBC technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure up to 16 ata ( 16 kg/cm2). The off-gas from the FBC drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidized bed. The condensate from the steam turbine is pre-heated using waste heat from gas turbine exhaust and is then taken as feed water for steam generation. The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in conventional system. The overall conversion efficiency is higher by 5% to 8%. PFBC Boiler is beyond the scope of this document Advantages of FBC Boilers 1. High Efficiency: FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. FBC boilers can operate with overall efficiency of 84% (±2%). 2. Reduction in Boiler Size: High heat transfer rate over a small heat transfer area immersed in the bed results in overall size reduction for the boiler.

25 25 3. Fuel Flexibility: FBC boilers can be operated efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These can be fed either independently or in combination with coal into the same furnace. 4. Ability to Burn Low Grade Fuel: FBC boilers would give the rated output even with an inferior quality fuel. The boilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500 kcal/kg. Even carbon content of only 1% by weight can sustain the fluidized bed combustion. 5. Ability to Burn Fines: Coal containing fines below 6 mm can be burnt efficiently in FBC boiler, which is very difficult to achieve in conventional firing system. 6. Pollution Control: SO 2 formation can be greatly minimized by addition of limestone or dolomite for high sulphur coals (3% limestone is required for every 1% sulphur in the coal feed). Low combustion temperature eliminates NOx formation. 7. Low Corrosion and Erosion: The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (around 1 m/sec). 8. Easier Ash Removal No Clinker Formation: Since the temperature of the furnace is in the range of C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation. Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling. 9. Less Excess Air Higher CO 2 in Flue Gas: The CO 2 in the flue gases will be of the order of 14 15% at full load. Hence, the FBC boiler can operate at low excess air - only 20-25%. 10. Simple Operation, Quick Start-Up: High turbulence of the bed facilitates quick start up and shut down. Full automation of start up and operation using reliable equipment is possible. 11. Fast Response to Load Fluctuations: Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates. Response to changing load is comparable to that of oil fired boilers. 12. No Slagging in the Furnace No Soot Blowing: In FBC boilers, volatilization of alkali components in ash does not take place and the ash is non sticky. This means that there is no slagging or soot blowing. 13. Provisions of Automatic Coal and Ash Handling System: Automatic systems for coal and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas fired installations. 14. Provision of Automatic Ignition System: Control systems using micro-processors and automatic ignition equipment give excellent control with minimum supervision. 15. High Reliability: The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs. 16. Reduced Maintenance: Routine overhauls are infrequent and high efficiency is maintained for long periods. 17. Quick Responses to Changing Demand: FBC can respond to changing heat demands more easily than stoker fired systems. This makes it very suitable for applications such as thermal fluid heaters, which require rapid responses. 18. High Efficiency of Power Generation: By operating the fluidized bed at elevated pressures, it can be used to generate hot pressurized gases to power a gas turbine. This can be combined with a conventional steam turbine to improve the efficiency of electricity generation resulting in a potential fuel savings of at least 4%.