FBC Boiler Technology for on

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Technical Study Report on

B I O M A S S F I R E D

Fluidized Bed Combustion Boiler Technology

For Cogeneration

http://www.uneptie.org/energy


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IITechnical Study Report: Biomass Fired FBC Boiler for Cogeneration

UNEP-DTIE Energy Branch http://www.unep.fr/energy

About t he Technic a l Study Report

Cleaner Production (CP) and Energy Efficiency (EE) are established and powerful strategieshelping enterprises throughout the world to reduce costs, generate profits by reducingwaste 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 environmentaland economic.

Implementing CP-EE projects in industries also requires efficient and environment-friendlytechnology interventions. Co-generation through fluidized bed combustion (FBC) boilerusing biomass (such as rice husk, straw etc.) is one such proven technology which could helpin mitigation of green house gases emissions.

UNEP-DTIE's Energy Branch is planning to develop a series of technical study reportscovering various specific technologies that can be adopted by the industries all over theworld as a part of their CP-EE initiatives.

This is first such technical study report that documents the various techno-economical andmanagerial aspects of biomass-based FBC technology for practical use by the industries inthe regions where large amounts of biomass are available.

The study report provides an overview of FBC technology, co-generation system andpractical aspects of implementing such a system in an industry. A detailed case studyprovides insights to the technical specifications of the various equipments, systems and costeconomics. It also provides list of technology providers and suppliers worldwide.

All in all, this technical report is a comprehensive and complete documentation forimplementation of biomass based FBC boiler for co-generation. The technical study report istargeted to the decision makers, technical personnel in the industry, academia, consultantsas well as government agencies. Specifically, it is very useful for the technical managers in theindustries who would like to implement biomass based co-generation systems in theirfacilities.


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IIITechnical Study Report: Biomass Fired FBC Boiler for Cogeneration


Contents

About the Technical Report ..........................................................................................................ii

Contents..............................................................................................................................................iii

List of Tables.................................................................................................................................................................................v

List of Figures...............................................................................................................................................................................vi

Abbreviations and Acronyms Used....................................................................................................................................vii

1.0 Introduction ................................................................................................................................8

1.1 Cleaner Production & Energy Efficiency......................................................................................................................8

1.2 Biomass as a Fuel..................................................................................................................................................................9

1.3 Biomass Energy Conversion Technologies..............................................................................................................13

2.0 FBC Boiler & Cogeneration Systems................................................................................18

2.1 FBC Boilers ......................................................................................................................................................................... 18

2.2 Cogeneration (Combined Heat & Power)..............................................................................................................26

3.0 Biomass-based FBC and Co-generation Technology..................................................30

3.1 Overview of the Technology........................................................................................................................................30

3.2 Areas of Application ........................................................................................................................................................31

3.3 Issues in Implementation of Biomass-based Cogeneration Systems..............................................................32

3.4 Environmental Benefits of Biomass based cogeneration Systems...................................................................39

3.5 Social Benefits of Biomass based cogeneration Systems.....................................................................................39

4.0 Implementing Biomass Cogeneration Technology.......................................................41

4.1 Raw material, Energy Resource requirement.........................................................................................................41

4.2 Infrastructure Requirement ..........................................................................................................................................43

4.3 Supporting Technologies................................................................................................................................................44

4.2 Waste Disposal..................................................................................................................................................................46

4.5 Human Resources Demand..........................................................................................................................................46

4.6 Equipment Suppliers.........................................................................................................................................................47

5.0 Case Study.................................................................................................................................49

5.1 Introduction........................................................................................................................................................................49


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IVTechnical Study Report: Biomass Fired FBC Boiler for Cogeneration


5.2 Manufacturing Process.................................................................................................................................................... 49

5.3 Baseline Energy Scenario................................................................................................................................................50

5.4 Implementation of Rice Husk based Cogeneration System..............................................................................51

6.0 Further Suggestions ................................................................................................................58

6.1 Power Generation using bio-mass in FBC Boiler..................................................................................................58

6.2 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


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VTechnical Study Report: Biomass Fired FBC Boiler for Cogeneration


L is t o f Tab les

Table 1 Global Biomass-fuel based Electricity Generation Capacity, 2004 9

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 systems31

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 generative50

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 2004-05.............................................................................................................................57


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VITechnical Study Report: Biomass Fired FBC Boiler for Cogeneration


List o f F igures

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, 1987 ...................................11

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


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VIITechnical Study Report: Biomass Fired FBC Boiler for Cogeneration


Abbrev ia t ions and Ac ronyms Used

BBCS Biomass based Cogeneration System

CHP Combined Heat & Power

CP Cleaner Production

CPEE Cleaner Production Energy Efficiency

DG sets Diesel Generator Set

EJ Exa-Jourles (IEJ = 1 x 1018 Joules)

ESP Electro static Precipitators

FBC Fluidized Bed Combustion

GWh Giga Watt hour

H.T. High Tension

KVA Kilo Volt Ampere

KWth Kilo Watts Thermal

KWe Kilo Watts electrical

MNRE Ministry of New & Renewable Energy, India

MW Mega Watt

0C Degree Centigrade


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8Technical Study Report: Biomass Fired FBC Boiler for Cogeneration


1.0 In t roduc t ion

1.1 Cleaner Product ion & 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 theimmense benefits that can be realized by the end-users, UNEP has recently developedguidelines for integration of CP and EE.

The idea of this integrated approach is to incorporate the energy management principlesinto 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-EEManual. This guidance manual is primarily used by facility personnel for conducting in-houseassessments as well as by external consultants.

While managers gain insights into the role they can play in instigating and supporting anongoing, cost-effective process for continual improvement leading to both economic andenvironmental advantages, CP professionals and consultants (who may not necessarily be

energy specialists) find such guidance on incorporating energy issues into their CPassessments at industrial or other facilities immensely valuable.

The integrated methodology is derived from thebasic principles of the Demings Cycle of Plan Do Check Act popularly acronymed asPDCA Cycle. Moreover, it addresses eightdifferent categories for identifying the options forresource 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 categoriesamongst the CP-EE options. For this, rapid technological advancements in the current timesrequire the professionals to remain updated about the new technologies that arecontinuously evolving in response to the various environmental challenges.

Global warming and Climate change is one of the most pressing and burning issues thatneeds urgent global action at all levels. The key to address this problem is by mitigation ofcarbon dioxide and other greenhouse gases produced by combustion of various fuels (bothfossil and non fossil). Various new technological solutions are being tried and tested aroundthe world to address this serious problem for the humankind.

This study report highlights the use of one such proven technology viz. Biomass basedFluidized Bed Combustion Boilers for Combined Heat and Power Applicationswhichcould possibly help in addressing the issue of global warming and climate change.


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1.2 Biomass as a Fuel1

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 pastyears. Currently some 13% of the worlds primary energy supply is from biomass, thoughthere 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 tothe forefront, people everywhere are rediscovering the advantages of biomass.

Potential benefits of biomass:

Reducing carbon emissions if managed (produced, transported, used) in a sustainablemanner

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 theirenergy needs. Most of the biomass is burnt to provide heat for cooking or warmth. Some isused for small industrial applications (For instance, Charcoal is used in steelmaking in countrieslike Brazil, which have no major coal reserves). A small percentage of biomass is also used togenerate electricity.

Total biomass consumption at the beginning of the twenty-first century was 55 exa-Joules or 55EJ2out of total global energy consumption of around 400EJ.

Estimates of the total quantities of biomass available vary widely but could represent upto 100EJ of energy.

Biomass energy accounts for around 14% of total primary energy consumption. This boldfigure hides a major disparity between the developed and the developing world. Estimates ofthe amount of energy that can be supplied from biomass too vary widely, but according tosome 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 makeuse of this valuable resource. It uses exactly the same technology that has become commonin the power generation industry - furnaces to burn coal, boilers to raise steam from theheat produced and steam turbines to turn the steam into electricity. Table 1 represents theelectricity 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 1200

1Biomass, Issue Brief Energy and Climate Change, World Business Council for Sustainable Development

21EJ = 1x1018 Joules


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In 2000, biomass was the largest renewable energy source for electricity generation - otherthan hydro - generating around 1% of the worlds electricity or 167 TWh. However, itsshare 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 neutralbecause plants and trees removecarbon dioxide (CO2) from the atmosphere and store it while they grow. Burning biomass inhomes, industrial processes, energy generation, or for transport activities returns thissequestered CO2 to the atmosphere. At the same time, new plant or tree growth keeps theatmospheres carbon cycle in balance by recapturing CO2.

This net-zero or carbon neutralcycle can be repeated indefinitely, as long as biomass is re-grown in the next management cycle and harvested for use. The sustainable managementof 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 carbonneutral because they release CO2 which has been stored for millions of years, and do nothave any storage or sequestration capacity.

1.2.1 Sources of Biom ass as fuel3

There are a variety of biomass residues available around the world. The most important ofthese are crop residues but there are significant quantities of forestry residues and livestockresidues 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 traditionallybeen 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 canbe conveniently and easily converted into energy. (Recent legislation has made strawburning 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 ofthis needs to be returned to the soil but when the harvested maize is stripped from itscob the latter remains, more biomass which can easily be converted into energy on-site.

3Business Insights, The Future of Global Biomass Power Generation: The technology, economics and impact of

biomass power generation By Paul Breeze, 2004


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Sugar cane harvesting leaves harvest 'trash' in the fields while processing producesfibrous bagasse. The latter is a valuable source of energy.

Harvesting and processing of coconuts produces quantities of shell and fibre that can beutilized.

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 isshown in Table 2 where the total residue from the four major crops listed is equivalent to32EJ. 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 couldgenerate 38EJ.

Table 2 Crop Residue from 4 Major Crops in EJ (1987)

REGION MAIZE STRAW WHEAT STRAW RICE STRAW BAGASSE TOTAL

Africa 0.48 0.25 0.20 0.54 1.47

US & Canada 2.95 1.93 0.13 0.19 5.20

Latin America 0.71 0.38 0.29 3.58 4.94

Asia 1.74 3.65 8.96 3.19 17.54

Europe 0.61 2.39 0.04 0.00 3.04

Oceania 0.23 2.26 0.06 0.22 2.77

Total 6.72 10.86 9.68 7.72 31.98


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Varying Estimates

A major problem when estimating the quantity of residues that might be used for energyproduction is to determine how much of each is required for other purposes. At least, partof 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 Agricultureconcluded that crop residues alone could provide electricity equivalent to 5% of USconsumption in 2003. Though local factors make direct comparisons with other regionsdifficult, 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 beable 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 agriculturalresidues and there is potential across all the continents. However, mere availability of theresidue does not guarantee its use.

From the perspective of electricity generation, the cost of collection of the residue becomesthe key factor in determining its viability. Wheat straw can be baled, making collection moreefficient. 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 theplants to either have a large storage facility or alternative sources of fuel. Fuels such as ricehusks and maize cobs are produced during processing of these crops. This takes place afterharvesting of the crop, so the waste is already concentrated at a point and is an easilyexploitable source of energy - particularly if it can be utilized on site to provide heat andpower.

Sugar cane bagasse is another valuable source of fuel and one that can be exploited easilybecause it, too, is generated during the processing of the cane. Table 3 provides abreakdown of global bagasse potential from the World Energy Council.

Table 3 : Global Bagasse Residues

REGION QUNATITY OF BAGASSE (,000 MT)Africa 26025

North America 55279

South America 88881

Asia 131197

Europe 502

Middle east 914

Oceania 19358

Total 322156

The bagasse figures in Table 3 represent only part of the biomass generated during sugarcane farming. The 'trash' which is left in the fields represents about 55% of the total, and thisis 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 generateprocess heat which is all used on-site. Modern combined heat and power plants can producemore energy than is required by the plant itself. According to one estimate, the amount ofsurplus electricity that sugar processing plants could generate and export to their local gridscould, by 2025, account for 15%- 20% of the total demand in the developing countries.


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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 generatedto 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 electricityfrom biomass today. It is also generally rather inefficient, though new technologies willbe able to improve efficiency significantly.

A simple, direct-fired biomass power plant can either produce electricity alone or it canoperate as a combined heat and power unit, producing both electricity and heat. Thislatter is common in the textile, food processing, chemical and paper industries wherethe heat is used in the processing plant. The electricity generated is used by the plant

Bioma ss Ava ilab ility in Ind ia and Potent ial for Co-generation

Biomass is the traditional fuel in India, used for cooking, and even today, mosthouseholds, in rural India, use it as cooking fuel. This biomass, mostly consists ofagricultural farm residues (e.g. paddy straw, sugar cane trash etc), agro-industrialresidues (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, providesthe 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 Ground nut shells

Cotton Stalk Rice Husk

Maize Stems Saw dust

Mango residues

Mustard Stalk

Palm leaf

Prosopis

Rai Stems

Sugar Cane Trash

Tamarind husk

Til stems

Casurina branches & fruit

Indian Ministry of New and Renewable Energys Annual Report for 2005-06 indicatessurplus 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 factorof 70%, the estimated potential for power generation in India alone is 13,000 MW fromvarious biomass based sources.


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too, with any surplus exported to the grid. Simplicity is the key feature of direct firingtype of application.

A more advanced approach is biomass gasification. This employs a partial combustionprocess to convert biomass into a combustible gas. The gas has a lower energy contentthan natural gas. Nevertheless, it can be used in the same way as natural gas. Inparticular 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 forfuture biomass-based generation.

An intermediate option for exploiting biomass is to mix it with coal and burn it in acoal fired power station. In the short term this may offer the cheapest and most efficientmeans of exploiting biomass. Finally there are number of specialized methods of turningbiomass wastes into energy. These include digesters, which can convert dairy farm wasteinto a useful fuel gas, and power stations that utilize chicken farm litter, which they burnto 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

1.3.1 Pile Combust ion

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 thegrate where it is burned in air, which passes up through the grate (called under-fire air). Thegrate of a pile burner is within what is known as the primary combustion chamber wherethe bulk of the combustion process takes place.

Combustion at this stage is normally incomplete - there may be significant quantities of bothunburned carbon and combustible carbon monoxide remaining - so further air (called over-

fire air) is introduced into a secondary combustion chamber above the first - wherecombustion is completed.

The boiler for raising steam is positioned above this second combustion chamber so that itcan absorb the heat generated during combustion. The heat warms, and eventually boilswater in the boiler tubes, providing steam to drive a steam turbine. From the steam turbinethe steam is condensed and then returned to the boiler so that it can be cycled through thesystem again. (In a combined heat and power system, steam will be taken from the steamturbine outlet to provide heat energy first.)

Wood fuel is normally introduced from above the grate, though sometimes there is a morecomplicated arrangement, which feeds fuel from under the grate. The pile burner is capableof handling wet and dirty fuels but it is extremely inefficient. Boiler efficiencies are typically50%-60%.

There is no means to remove the ash from a pile burner except by shutting down thefurnace. Thus the power plant cannot be operated continuously. Pile burners are alsoconsidered difficult to control and they are slow to respond to changes in energy input. Thismeans 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 passsteam turbine generator operating at a relatively low steam temperature and pressure. Thisadds to the relatively low efficiency of the power plant, which can operate, with an overallefficiency as low as 20%.


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1.3.2 Stoker Combust ion

The pile burner represents the traditional method of burning wood. However, its basicoperation can be improved by introducing a moving grate or stoker. This allows continuousremoval of ash so that the plant can be operated continuously. Fuel can also be spread morethinly on the grate, encouraging more efficient combustion.

The first US stoker grate for wood combustion was introduced by the Detroit Stoker Co. inthe 1940s. In this type of furnace, combustion air still enters below the grate of a stokerburner. This flow of air into the combustion chamber helps cool the grate. The air flow andconsequent grate temperature determines the maximum operating temperature of thecombustor. This, in turn, determines the maximum moisture content allowable in the woodfuel if combustion is to proceed spontaneously.

There are refinements of the basic stoker grate such as inclined grates and water-cooledgrates, both of which can help improve overall performance and make the operation lesssensitive to fuel moisture. Nevertheless stoker combustors are still relatively inefficient, withboiler efficiencies of 65%-75% and overall efficiencies of 20%-25%.

1.3.3 Suspension Combust ion

Most modern coal-fired power stations burn pulverized coal, which is blown into thecombustion chamber of a power plant through a specially designed burner. The burnermixes air with the powdered coal, which then burns in a flame in the body of thecombustion chamber. This is suspension combustion and in this type of plant there is nograte. 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 thebiomass particle size has to be less than 15mm. Suspension firing results in boiler efficiencyof 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 thatthe fuel is of the right consistency. It also demands special furnace burners. A small numberof plants designed to burn biomass in this way have been built. The technology is also ofgreat interest as the basis for the co-firing of wood or other biomass with coal in pulverizedcoal plants.

1.3.4 Fluidized Bed Combust ion

Aside from suspension firing of wood, the most efficient method of directly burning biomassis in a fluidized bed combustor (FBC). This is also the most versatile since the system cancope 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 throughwhich air is blown from below. The air is pumped through the bed in sufficient volume andat a high enough pressure to entrain the small particles of the bed material so that theybehave much like a fluid.

The combustion chamber of a fluidized bed plant is shaped so that above a certain height theair velocity drops below that necessary to entrain the particles. This helps retain the bulk ofthe 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 conventionalfurnace. The proportion of combustible material such as biomass within the bed is normallyonly around 5%.

There are different designs of FBC system which involve variations around this principle. Themost common for biomass combustion is the circulating fluidized bed which incorporates a


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cyclone filter to separate solid material from the hot flue gases which leave the exhaust ofthe 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 abilityto burn a variety of different fuels without affecting performance. Second is the ability tointroduce chemical reactants into the fluidized bed to remove possible pollutants. In FBCplants 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 thancoal 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 applicationis 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 theseplants use FBCs.

Of the four different types of combustion technologies discussed above, the FBC technologyis 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% andcan be used for a wide variety of fuels.

1.3.4 Comparison of Different Types of B iomass Conversion Technologies

Table 4 below compiles a quick Comparison of Different Types of Biomass ConversionTechnologies commonly used worldwide.

Table 4 Comparison of Different Types of Biomass Conversion Technologies

Parameter Pile Combustion Stoker Combustion Suspension

Combustion

Fluidized Bed

Combustion

Grate Fixed / Stationary Grate Fixed or moving grate No grate or movinggrate

No grate

Fuel Size Uniform size of the fuelin the range of range 60to 75 mm is desired &% fines should not bemore than 20%

Uneven fuel size can beused

Preferable for high %of fins in the fuel

Uniform size fuel inthe range of 1 to 10mm.

Combustion Difficult to maintaingood combustion dueto :

Air fuel mixing is notproperBed height is instationary conditionresulting in clinkerformationDifficult to avoid airchannelingDue to intermittentash removal system itis difficult to maintaingood combustion

The combustion isbetter & an improvedversion of pilecombustion. Sincemost of the fuel isburnt in suspension theheavier size mass fallson the grate. If thesystem has a movinggrate the ash isremoved on acontinuous basis &therefore the chancesof clinker formationare less.

It is similar to stokercombustion, but sincethe fuel sizes is small &even the combustionefficiency is improvedas maximum amountof fuel is combustedduring suspension.

Best combustion takesplace in comparisonwith the other typessince the fuel particlesare in fluidized state &there is adequatemixing of fuel & air.

Bed

temperature

1250- 1350 C 1000- 1200 C 1250- 1350 C 800- 850 C

Moisture High moisture leads tobed choking & difficultcombustion conditions

Combustion conditionnot very muchdisturbed with 4-5 %increase in moisture

Same as StokerCombustion

It can handle fuels withhigh moisturecondition up to 45-50% but high moisturein the fuels is notdesirable, & adequateprecautions are to betaken up in the designstage itself.


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Parameter Pile Combustion Stoker Combustion SuspensionCombustion

Fluidized BedCombustion

Draft

Conditions

Natural Draft / ForcedDraft/ Balance Draft

Forced Draft / Balancedraft

Balance draft Balance draft

Maintenance Not much maintenanceproblems

Frequent problemsdue to moving grate

Variation in fines infuel leads to delayedcombustion therebyaffecting the boiler

tubes

Erosion of boiler tubesembedded in the bedis quite often


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2.0 FBC Boi ler & Cogenerat ion System s

2.1 FBC Boilers4

2.1.1 Introduc tion t o FBC Boilers

The traditional grate fuel firing systems have several limitations and hence are techno-economically unviable to meet the challenges of the future. FBC has emerged as a viablealternative as it has significant advantages over conventional firing system.

FBC offers multiple benefits, such as: compact boiler design, flexibility with fuel used, highercombustion efficiency and reduced emissions of noxious pollutants such as SOx and NOx.The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and otheragricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over100 T/hr.

2.1.2 Mechanism of Fluidized Bed Combust ion

When an evenly distributed air or gas is passed upward through a finely divided bed of solidparticles such as sand supported on a fine mesh, the particles remain undisturbed at lowvelocities. As the air velocity is gradually increased, a stage is reached when the individualparticles are suspended in the air stream and the bed is called fluidized.

With further increase in air velocity, there is bubble formation, vigorous turbulence, rapidmixing and formation of dense defined bed surface. The bed of solid particles exhibits theproperties 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 calledas 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 velocityincreases at a slower rate than does the gas velocity. The difference between the mean solidvelocity and mean gas velocity is called as slip velocity. Maximum slip velocity between thesolids and the gas is desirable for good heat transfer and intimate contact. If sand particles influidized 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 auniform temperature.

The fluidized bed combustion (FBC) takes place at about 840C to 950C. Since thistemperature is much below the ash fusion temperature, melting of ash and associatedproblems are avoided. The lower combustion temperature is achieved because of highcoefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction ofheat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocityis maintained between minimum fluidization velocity and particle entrainment velocity. Thisensures a stable operation of the bed and avoids particle entrainment in the gas stream.

4Energy Efficiency in Thermal Utilities, A Guide Book for Energy Managers and Auditors, Bureau of Energy

Efficiency, Ministry of Power, Government of India, 2005


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Fixing, Bubbling & Fast Fluidized Beds: As the velocity of a gas flowing through a bed of particlesincreases, a value is reaches when the bed fluidizes and bubbles form as in a boiling liquid. At highervelocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled tomaintain a stable system.

Figure 4 : Principles of Fluidization

Any combustion process requires three Ts - that is Time, Temperature and Turbulence. InFBC, turbulence is promoted by fluidization. Improved mixing generates evenly distributedheat at lower temperature. Residence time is many times higher than conventional gratefiring. Thus an FBC system releases heat more efficiently at lower temperatures. Sincelimestone 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 anyadditional control equipment. This is one of the major advantages over conventional boilers.

2.1.3 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).


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2.1.3.1 AFBC / Bub b ling 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 typeof boiler find use in industries where there is a possibility of having a combined heat andpower 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 feedingsystem and is fed into the combustion chamber.

The atmospheric air, which acts as both the fluidization air and combustion air, is deliveredat a pressure and flows through the bed after being preheated by the exhaust flue gases. Thevelocity of fluidizing air is in the range of 1.2 to 3.7 m /sec. The rate at which air is blownthrough 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 beddepth is usually 0.9 m to 1.5 m deep and the pressure drop averages about 1 inch of waterper inch of bed depth. Very little material leaves the bubbling bed only about 2 to 4 kg ofsolids is recycled per ton of fuel burned. Typical fluidized bed combustors of this type areshown 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 theeconomizer, the dust collectors and the air pre-heaters before being exhausted toatmosphere. The main special feature of atmospheric fluidized bed combustion is theconstraint imposed by the relatively narrow temperature range within which the bed mustbe operated. With coal, there is risk of clinker formation in the bed if the temperatureexceeds 950C and loss of combustion efficiency if the temperature falls below 800C. Forefficient sulphur retention, the temperature should be in the range of 800C to 850C.

General Arrangements of AFBC Boiler

AFBC boilers comprise of following systems:

Fuel feeding system

Air distributor


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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 arefollowed: under bed pneumatic feeding and over-bed feeding.

Under Bed Pneumatic Feeding

If the fuel is coal, it is crushed to 16 mm size and pneumatically transported from feedhopper to the combustor through a feed pipe piercing the distributor. Based on thecapacity of the boiler, the number of feed points is increased, as it is necessary todistribute the fuel into the bed uniformly.

Over-Bed Feeding

The crushed coal, 610 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. Thistype 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 otheragricultural residues Over bed feeding system is quite prominent and economical. Someof 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 crosssection thereby keeping the solid particles in constant motion, and preventing the formationof de-fluidization zones within the bed. The distributor, which forms the furnace floor, is


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normally constructed from metal plate with a number of perforations in a definite geometricpattern. 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 sizeof about 1 mm. Depending on the bed height these are of two types: shallow bed anddeep bed. At the same fluidizing velocity, the two ends fluidize differently, thus affectingthe heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lowerbed resistance and hence a lower pressure drop and lower fan power consumption. Inthe case of deep bed, the pressure drop is more and this increases the effective gasvelocity and also the fan power.

In-Bed Heat Transfer SurfaceIn a fluidized in-bed heat transfer process, it is necessary to transfer heat between thebed 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 pressuredrop point of view, a horizontal bundle in a shallow bed is more attractive than a verticalbundle in a deep bed. Also, the heat transfer in the bed depends on number ofparameters 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 30 40 % of the total ash, the

rest being the fly ash. The bed ash is removed by continuous over flow to maintain bedheight and also by intermittent flow from the bottom to remove over size particles,avoid accumulation and consequent defluidization. While firing high ash coal such aswashery rejects, the bed ash overflow drain quantity is considerable so special care hasto be taken.

ii) Fly Ash Removal

The amount of fly ash to be handled in FBC boiler is relatively very high, compared toconventional boilers. This is due to elutriation of particles at high velocities. Fly ashcarried 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 isremoved in dust collectors.

The types of dust collectors used are cyclone, bag filters, electrostatic precipitators

(ESPs) or some combination of all of these. To increase the combustion efficiency,recycling of fly ash is practiced in some units.

2.1.3.2 Circu lat ing Fluid ized Bed Co mb ustion (CFBC)

Circulating Fluidized Bed Combustion (CFBC) technology has evolved from conventionalbubbling bed combustion as a means to overcome some of the drawbacks associated withconventional bubbling bed combustion (see Figure 7).


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Figure 7 : A CFBC Boiler

CFBC technology utilizes the fluidized bed principle in which crushed (6 12 mm size) fueland limestone are injected into the furnace or combustor. The particles are suspended in astream of upwardly flowing air (60-70% of the total air), which enters the bottom of thefurnace through air distribution nozzles. The fluidizing velocity in circulating beds rangesfrom 3.7 to 9 m/sec. The balance of combustion air is admitted above the bottom of thefurnace as secondary air. The combustion takes place at 840-900 C, and the fine particles(


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CFBC boilers are said to achieve better calcium to sulphur utilization 1.5 to 1 vs. 3.2 to 1for the AFBC boilers, although the furnace temperatures are almost the same.

CFBC requires huge mechanical cyclones to capture and recycle the large amount of bedmaterial, which requires a tall boiler. A CFBC could be good choice if the followingconditions 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 870C is reasonably constant throughout the processbecause of the high turbulence and circulation of solids. The low combustiontemperature also results in minimal NOx formation.

Sulphur present in the fuel is retained in the circulating solids in the form of calciumsulphate and removed in solid form. The use of limestone or dolomite adsorbentsallows a higher sulfur retention rate, and limestone requirements have beendemonstrated 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) ratherthan 35 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 thesurface is parallel to the flow. In a bubbling bed system, the surface generally isperpendicular to the flow.

CFBC boilers are generally claimed to be more economical than AFBC boilersfor industrial application requiring more than 75 - 100 T/hr of steam, therefore

this type of boilers is beyond the scope of the document.

2.1.3.3 Pressurized Fluid Bed Co mbustion Boiler

Pressurized Fluidized Bed Combustion (PFBC) is a variation of FBC technology that is meantfor large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure upto 16 ata ( 16 kg/cm2). The off-gas from the FBC drives the gas turbine. The steam turbine isdriven by steam raised in tubes immersed in the fluidized bed. The condensate from thesteam turbine is pre-heated using waste heat from gas turbine exhaust and is then taken asfeed water for steam generation.

The PFBC system can be used for cogeneration or combined cycle power generation. Bycombining 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

2.1.4 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 areaimmersed in the bed results in overall size reduction for the boiler.


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3. Fuel Flexibility: FBC boilers can be operated efficiently with a variety of fuels. Evenfuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These canbe 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 withan inferior quality fuel. The boilers can fire coals with ash content as high as 62% andhaving 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: SO2 formation can be greatly minimized by addition of limestoneor dolomite for high sulphur coals (3% limestone is required for every 1% sulphur in thecoal feed). Low combustion temperature eliminates NOx formation.

7. Low Corrosion and Erosion: The corrosion and erosion effects are less due to lowercombustion temperature, softness of ash and low particle velocity (around 1 m/sec).

8. Easier Ash Removal No Clinker Formation: Since the temperature of thefurnace is in the range of 750 900 C in FBC boilers, even coal of low ash fusiontemperature 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 ashhandling.

9. Less Excess Air Higher CO2 in Flue Gas: The CO2 in the flue gases will be of theorder 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 quickstart up and shut down. Full automation of start up and operation using reliableequipment is possible.

11.Fast Response to Load Fluctuations: Inherent high thermal storage characteristicscan easily absorb fluctuation in fuel feed rates. Response to changing load is comparableto that of oil fired boilers.

12.No Slagging in the Furnace No Soot Blowing: In FBC boilers, volatilization ofalkali components in ash does not take place and the ash is non sticky. This means thatthere is no slagging or soot blowing.

13.Provisions of Automatic Coal and Ash Handling System: Automatic systems forcoal and ash handling can be incorporated, making the plant easy to operate comparableto oil or gas fired installations.

14.Provision of Automatic Ignition System: Control systems using micro-processorsand automatic ignition equipment give excellent control with minimum supervision.

15.High Reliability: The absence of moving parts in the combustion zone results in a highdegree 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 heatdemands more easily than stoker fired systems. This makes it very suitable forapplications such as thermal fluid heaters, which require rapid responses.

18.High Efficiency of Power Generation: By operating the fluidized bed at elevatedpressures, it can be used to generate hot pressurized gases to power a gas turbine. This canbe combined with a conventional steam turbine to improve the efficiency of electricitygeneration resulting in a potential fuel savings of at least 4%.


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2.2 Cogeneration (Combined Heat & Power)

Cogeneration or Combined Heat and Power (CHP) is defined as the sequential generationof two different forms of useful energy - typically mechanical energy and thermal energy -from a single primary energy source.

Mechanical energy may be used to drive an alternator for producing electricity, or rotating

equipment such as motor, compressor, pump or fan for delivering various services. Thermalenergy can be used either for direct process applications or for indirectly producing steam,hot water, hot air for dryer or chilled water for process cooling.

Cogeneration provides a wide range of technologies for application in various domains ofeconomic activities. The overall efficiency of energy use in cogeneration mode can be up to85 per cent - and even above in some cases. Along with the saving of fossil fuels,cogeneration also helps reducing the emissions of greenhouse gases (particularly CO2emission).

2.2.1 Need for Cogeneration

Thermal power plants are a major source of electricity worldwide. The conventionalmethod of power generation and supply to the customer is wasteful in the sense that only

about a third of the primary energy fed into the power plant is actually made available to theuser in the form of electricity (Figure 8).

Figure 8 : Energy Balance of a Typical Thermal Power Plant in India

The major source of loss in the conversion process is the heat rejected to the surrounding wateror air due to the inherent constraints of the different thermodynamic cycles employed in powergeneration. Also further losses of around 1015% are associated with the transmission and


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distribution of electricity in the electrical grid. In cogeneration, the production of electricity beingon-site, the burden on the utility network is reduced and the transmission line losses eliminated.

Cogeneration therefore makes sense from both macro and micro perspectives. At themacro level, it allows a part of the financial burden of the national power utility to be sharedby the private sector; in addition, indigenous energy sources are conserved. At the microlevel, the overall energy bill of the users can be reduced, particularly when there is a

simultaneous need for both power and heat at the site, and a rational energy tariff can bepracticed in the country.

2.2.2 Steam Turbines

Steam turbines are the most commonly employed prime movers for cogenerationapplications. In the steam turbine, the incoming high pressure steam is expanded to a lowerpressure level, converting the thermal energy of high pressure steam to kinetic energythrough nozzles and then to mechanical power through rotating blades. The different typesof steam turbine include extraction cum condensing type and back pressure steam turbines.

Figure 9: Configurations of different types of turbine systems

2.2.2.1 Bac k Pressure Turb ine

In this type of turbines, steam enters the turbine chamber at high pressure and expands tolow or medium pressure. Enthalpy difference is used for generating power/work. Dependingon the pressure (or temperature) levels at which process steam is required, backpressuresteam turbines can have different configurations as shown in Figure 10.

In extraction and double extraction backpressure turbines, some amount of steam isextracted from the turbine after being expanded to a certain pressure level. The extractedsteam meets the heat demands at pressure levels higher than the exhaust pressure of thesteam turbine.

The efficiency of a backpressure steam turbine cogeneration system is the highest. In caseswhere 100 per cent backpressure exhaust steam is used, the only inefficiencies are gear

drive and electric generator losses, and the inefficiency of steam generation. Therefore, withan efficient boiler, the overall thermal efficiency of the system could reach as much as 90 percent.

Fuel

Cooling Water

Steam

Boiler

Process

(ii) Extraction -Condensing Turbine

TurbineFuel

Steam

Boiler

Process

(i) Back-Pressure Turbine

TurbineFuel

Steam

Boiler

Process

(i) Back-

Turbine

Condenser

Fuel

Cooling Water

Steam

Boiler

Process

(ii) Extraction -Condensing Turbine

TurbineFuel

Steam

Boiler

Process

(i) Back-Pressure Turbine

TurbineFuel

Steam

Boiler

Process

(i) Back-

Turbine

Condenser


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(I) Simple Back Pressure

Exhaust SteamExtracted SteamHigh Pressure Steam

(II) Extaction Back Pressure (III) Double ExtractionBack Pressure

Figure 10: Different Configurations of Back Pressure Turbine

2.2.2.2 Extrac tion Co ndensing Turbine

In this type, steam entering at high / medium pressure is extracted at an intermediate

pressure in the turbine for process use while the remaining steam continues to expand andcondenses in a surface condenser and work is done till it reaches the condensing pressure(vacuum).

In extraction-cum-condensing steam turbine as shown in figure 11, high pressure steamenters the turbine and passes out from the turbine chamber in stages. In the process of two-stage extraction cum condensing turbine MP steam and LP steam pass out to meet theprocess needs. Balance quantity condenses in the surface condenser. The energy differenceis used for generating power. This configuration meets the heat-power requirement of theprocess.

Figure 11: Configuration of Extraction cum condensing turbine

The extraction condensing turbines have higher power to heat ratio in comparison withback pressure turbines. Although condensing systems need more auxiliary equipment such

as the condenser and cooling towers, better matching of electrical power and heat demandcan be obtained where electricity demand is much higher than the steam demand and theload patterns are highly fluctuating.

The overall thermal efficiency of an extraction condensing turbine cogeneration system islower than that of back pressure turbine system, basically because the exhaust heat cannotbe utilized (it is normally lost in the cooling water circuit). However, extraction condensingcogeneration systems have higher electricity generation efficiencies.

Steam

Generator

Steam

Turbine GG

CondenserFeed WaterPump

QH


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2.2.3 Factors Influenc ing Cogeneration Choice

The selection and operating scheme of a cogeneration system is very much site-specific anddepends on several factors, as described below:

2.2.3.1 Base Electrical Load Matching

In this configuration, the cogeneration plant is sized to meet the minimum electricity demandof the site based on the historical demand curve. The rest of the needed power is purchasedfrom the utility grid. The thermal energy requirement of the site could be met by thecogeneration system alone or by additional boilers. If the thermal energy generated with thebase electrical load exceeds the plants demand and if the situation permits, excess thermalenergy can be exported to neighboring customers.

2.2.3.2 Base Therma l Loa d Ma tc hing

Here, the cogeneration system is sized to supply the minimum thermal energy requirementof the site. Stand-by boilers or burners are operated during periods when the demand forheat is higher. The prime mover installed operates at full load at all times. If the electricitydemand of the site exceeds that which can be provided by the prime mover, then theremaining amount can be purchased from the grid. Likewise, if local laws permit, the excesselectricity can be sold to the power utility.

2.2.3.3 Elec trical Loa d Ma tc hing

In this operating scheme, the facility is totally independent of the power utility grid. All thepower requirements of the site, including the reserves needed during scheduled andunscheduled maintenance, are to be taken into account while sizing the system. This is alsoreferred to as a stand-alone system. If the thermal energy demand of the site is higher thanthat generated by the cogeneration system, auxiliary boilers are used. On the other hand,when the thermal energy demand is low, some thermal energy is wasted. If there is apossibility, excess thermal energy can be exported to neighboring facilities.

2.2.3.4 Thermal Loa d Matc hing

The cogeneration system is designed to meet the thermal energy requirement of the site atany time. The prime movers are operated following the thermal demand. During the periodwhen the electricity demand exceeds the generation capacity, the deficit can becompensated by power purchased from the grid. Similarly, if the local legislation permits,electricity produced in excess at any time may be sold to the utility.


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3.0 Biomass-based FBC and Co-generation Technology

This technical study report is on the use of rice husk as a fuel in an FBC boiler togenerate medium to high pressure steam and using this steam to generate electricity by a

steam turbine and also use part of the steam in the manufacturing process in the industry.

In the preceding sections different types of FBC boilers, steam turbines and theirconfigurations have been discussed in detail to develop a thorough understanding of theequipments used in the process. The contexts in the other sections are with reference tothis specific technology only. Although numerous configurations are possible, but for smalland medium scale of operations the following four are the main configurations

i) Steam generation using FBC boiler and no electricity generation

ii) Steam generation using FBC boiler and electricity generation using Backpressure type ofturbine

iii) Steam generation using FBC boiler and electricity generation using Extraction cumcondensing type of turbine

iv) Steam generation using FBC boiler and electricity generation using condensing type ofturbine with no steam used in process

The fourth case is rarely used by the industries and is more applicable to the thermal powerplants which use biomass as a fuel and FBC boilers for steam generation. The configurationsof system and the design of the boiler and the turbine are wholly dependent on the sitespecific requirements and a detailed feasibility analysis needs to be conducted to determinethe correct configuration and the design parameters. Beside this, the choice is also governedby other factors like, economic feasibility, fuel availability, electricity availability, etc. Forexample if the cost and availability of the grid electricity supply is satisfactory, industriesrarely go for co-generation systems and just settle for steam generation by a FBC boiler(Case i).

The most important parameters which helps us to determine the choice of technologyimplementation between Case ii and iii are, the steam quantity and steam pressurerequirements in the process house. Beside this a choice has to be made as per section 2.2.3

3.1 Overview of the Technology

The overall working of the technology with major process steps and equipments with inputsand outputs is depicted in Figure 12. The process steps may vary from site to site dependingon the nature and quality of Biomass, the type of system and the local environmentalregulations.


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BiomassfromField

Baling ofBiomass at

Site

Storage ofBiomass at

Site

Sizing ofBiomass ifrequired

Mixing ofBiomass ifrequired

F B C

BOI LER

ESP

Flue

Gases

Fly Ash

Bottom Ash

Clean Flue Gases toChimney

Water

TreatmentPlant (DM)

Raw WaterWater Fu

el

High

PressureSteam

G

ElectricityMedium

Pressur e St eamto Process

Low Pressure Steam

Condenser

Condensate to Boiler

TURBI NE

BiomassfromField

Baling ofBiomass at

Site

Storage ofBiomass at

Site

Sizing ofBiomass ifrequired

Mixing ofBiomass ifrequired

F B C

BOI LER

ESP

Flue

Gases

Fly Ash

Bottom Ash

Clean Flue Gases toChimney

Water

TreatmentPlant (DM)

Raw WaterWater Fu

el

High

PressureSteam

G

ElectricityMedium

Pressur e St eamto Process

Low Pressure Steam

Condenser

Condensate to Boiler

TURBI NE

Figure 12 : Elements of a Biomass Based Cogeneration System using FBC Boiler

3.2 Areas of Applicat ion

The cogeneration technology can be adopted in various industrial sectors such as textile,pulp and paper, brewery, food processing etc.). The first and basic requirement forimplementation of cogeneration system is that the industry must require both steam andelectrical power in its operations.

The ratio of the heat value of the steam required to the electricity required is known as

heat to power ratio and is one of the most important factor which helps to decide the typeand configuration of the cogeneration systems to be installed.

Heat to Power Ratio is defined as the ratio of thermal energy to electricity required by theenergy consuming facility. It can be expressed in different units such as Btu/kWh, kcal/kWh,lb./hr/kW, etc. The heat-to-power ratio of a facility should match with the characteristics ofthe cogeneration system to be installed. Basic heat-to-power ratios of the differentcogeneration systems are shown in Table 5 along with other technical parameters. Thesteam turbine cogeneration system can offer a large range of heat-to- power ratios.

Table 5: Heat to Power ratios and other parameters of cogeneration systems

Cogeneration System Heat-to-power ratio

(kWth / kWe)

Power output (as per

cent of fuel input)

Overall efficiency

(per cent)

Back-pressure steamturbine

4.0-14.3 14 - 28 84 92

Extraction- CondensingTurbine

2.0- 10.0 22 40 60 - 80

Cogeneration is likely to be most attractive under the following circumstances:

The demand for both steam and power is balanced i.e. consistent with the range ofsteam: power output ratios that can be obtained from a suitable cogeneration plant.


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A single plant or group of plants has sufficient demand for steam and power topermit economies of scale to be achieved.

Peaks and troughs in demand can be managed or, in the case of electricity, adequatebackup supplies can be obtained from the utility company.

The ratio of heat to power required by a site may vary during different times of the day andseasons of the year. Importing power from the grid can make up a shortfall in electrical

output from the cogeneration unit and firing standby boilers can satisfy additional heatdemand. Many large cogeneration units utilize supplementary or boost firing of the exhaustgases in order to modify the Heat to Power Ratio of the system to match site loads.

The proportions of heat and power needed (heat: power ratio) vary from site to site, so thetype of plant must be selected carefully and appropriate operating schemes must beestablished to match demands as closely as possible. The plant may therefore be set up tosupply part or all of the site heat and electricity loads, or an excess of either may beexported if a suitable customer is available. The following Table 6 shows typical heat: powerratios for certain energy intensive industries:

Table 6 : Typical heat to Power ratio for Certain Energy intensive Industries

Industry Minimum Maximum Average

Breweries 1.1 4.5 3.1

Pharmaceuticals 1.5 2.5 2.0

Fertilizers 0.8 3.0 2.0

Food 0.8 2.5 1.2

Paper 1.5 2.5 1.9

3.3 Issues in Implem entat ion of Biomass-based Cogeneration Systems

The key issues in implementation of a biomass based cogeneration systems (BBCS) arebroadly classified as: technical and economical, environmental and social issues and are

discussed in the following sections.

3.3.1 Technic al Issues and Barriers

Biomass based cogeneration is faced with some technical barriers, which not only have adirect impact on day-to-day operations, but also on overall viability of the project. Theseissues are sometimes stand-alone issues and some are more complex and interrelated. In thefollowing sections, these issues and problems have been discussed in detail.

It may be noted that, some of the issues/problems are interconnected and complement eachother and thus add to the complexities in the overall scenario. However, for reasons ofclarity, these problems have been presented as stand - alone issues.

3.3.1.1 Tec hno logy Sourc ing fo r Bio- Ma ss Pow er Ge nera tion

In a typical thermal power station, the basic fuel is prepared to the specific size, according tothe technical requirements of the boiler furnace in order to ensure efficient combustion. Insuch cases, the boiler furnaces are specifically designed to suit the characteristics andparameters of the fuel (say, coal or gas) on which the system is proposed to run. Theavailability of this specific fuel is ensured by the user well in advance through techno-legalagreements with fuel suppliers, for guaranteed supply of the fuel in the specified quality andquantity.


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Ironically, in case of biomass projects, no such agreements exist as biomass fuel market isunorganized and rural based. The supply position of any particular type of fuel is neverassured, and the biomass based projects are forced to fend for themselves in the best waythey can against the whims, fancies and the vagaries of the biomass supply chain.

Making the right technology choice for biomass-based FBC boiler therefore is a key elementfor the success of such projects.

3.3.1.2 Viab le Ava ilab ility o f Biom ass Fuel

There have been innumerable instances, where the supplier have taken undue advantage ofdemand supply gap and wrested very high prices from reluctant but helpless biomass basedcogeneration projects.

Even assuming that the biomass based cogeneration project is lucky enough to strike acordial deal with the suppliers, more critical issue of the wide variation in the sizes of thebiomass as it is received, poses another bottleneck. This calls for an additional process ofappropriate sizing of the bio mass.

If it were the case of a particular biomass, the situation would perhaps have beencomparatively simple. But considering the wide variation and seasonality in the availability ofthe bio mass, and their basic characteristics, (size, shape, texture, moisture content, volatilematter, Calorific Values, etc.) make effective preparation of biomass to suit the boilertechnical requirements, a very complex exercise.

It is but natural that the efficiencies of the boilers would be low as compared to a boileroperating with a single fuel, for which the basic operating parameters can be set once & forall, needing only periodic adjustments. This is very difficult with multi-fuels scenario withfrequently changing mix.

Apparently, this seems to be one of the reasons, for several biomass based cogenerationprojects, to have opted for higher heating surface area, compared to the well establishedfossil fuel based power plants (of equivalent rating).

Following is a summary of various factors related to the availability of biomass, which cangreatly affect the viability of the cogeneration projects;

A) Types of biomass used in biomass based cogeneration projects

In any country there could be several varieties of biomass which are in use, dependingupon the geographic regions, geo-climatic conditions, agricultural practices, growingpatterns, season and their commercial availability. Considering wide variety of biomassbeing used, with different moisture content, volatiles, unknown chemical compositionand external impurities like mud, clay, sand etc. It is easy to appreciate the technicaldifficulties of collecting, preparing and combusting them in an efficient manner.

B) Availability of Biomass:It is a well known fact that, biomass availability is highly influenced by crop patterns of aregion, climate, weather and seasons, added to these factors is the diffused availability ofbiomass, which makes the collection and transport logistics a difficult and costly task.These factors impose constrains on the total quantity of biomass that can be madeeconomically available at the project site.

C) Fuel Collection & Logistics:

When biomass power generation was conceived in the mid 1990s in India andentrepreneurs came up with project proposals with rice husk as biomass, the earlybiomass plants did not face any problems in collecting their main biomass fuel (i.e., ricehusk), since rice husk was available in plenty at rice mills. In fact, the rice millers weremore than happy to give away the rice husk at very nominal rates (some times free ofcost) since that would solve their disposal problem. The plants only had to engagetransporters to bring in the rice husk from rice mills.


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However, with installation of more and more biomass based cogeneration plants, thesituation changed, to an extent that, today, biomass based cogeneration plants arelooking for any agro or forest residue (woody biomass) that could be burned in theirboilers.

Accordingly, the spectrum of biomass fuels broadened from one or two main fuels to 5to 10 different types of biomass being used in a single cogeneration plant. This has led to

several technical and financial problems for these plants:More number of biomass types necessitated different types of collection andhandling equipment. Since most of these biomass fuels, such as Cajurina branches,cotton stalks, husks of different pulses, sugar cane trash, spent coffee waste, coconutfronts & shells, jute waste, marind husk, red chilly waste etc., were new to theseplants, there were no readily available equipment/machines to suit the newrequirements. Hence, all biomass cogeneration plants were forced to spend a sizableamount of money in sourcing such equipment from domestic / international vendorsor developing these machines indigenously. There are also cases of in-house design& development to manufacture collection equipment resulting additional capitalinvestment and operating cost.

Many of the agro residues need to be collected manually, baled and transported to

cogeneration plants. Since this is a highly labor intensive activity and biomass isavailable in distributed quantities, some small and some large, the fuel contractorswould only be interested to supply biomass that is available in large quantities at asingle location. Thus biomass available in smaller lots would be ignored.

The transport of biomass from rice mills / other places of availability is effected bytransport contractors. Sometimes, transport contractors also become fuel supplycontractors. Depending on type, biomass is transported in lorries/trucks, tractortrolleys, bullock carts etc.,

The major problem is, the high bulk density of biomass fuels, which results in lowertonnage per vehicle, spillage due to light weight when transported in open trucks,and thus higher transportation cost. The transportation cost (including loading andunloading cost) constitutes a significant portion of the landed cost of the biomass.

For example, rice husk in India costs around INR800 (US $ 20) to INR1,000 (US $25) per ton at the rice mill, whereas transportation costs are an additional INR300(US $8) to INR400 (US $10) per ton. For biomass fuels which have to be collecteddirectly from the fields (such as sugar cane trash, coconut fronds, forest residuesetc), and which do not have a centralized collection point, the cost of logistics(collection, loading, transport and unloading) further increases.

3.3.1.3 Fue l PricingThe biomass fuel, presently an unregulated commodity and available in the open market,makes its price very dynamic and varies extensively from region to region. The price isinfluenced by several factors; such as: supply-demand gap (fierce competition amongentrepreneurs), seasonality, distance to be transported, quantities available in single lots etc.

Depending on the price, the cost of fuel constitutes a major portion in total generation cost.The cost of fuel ranges between 50 to 70% of electricity generation cost, in case onlyelectricity is produced.

3.3.1.4 Fuel Sto rage Hand ling a nd Preparat ion

A) Problems in biomass storageIt is observed that many of the cogeneration plants have no sheltered storage spacewherein different types of (degradable) biomass could be safely stored, protected fromthe vagaries of the weather. The propensity of biomass fuel to decay/decompose with


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time, when exposed in open yards,

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