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BIOMASS CONVERSION TECHNOLOGIES

ACHIEVEMENTS AND PROSPECTS FOR HEAT AND POWER

GENERATION

t*r* EUR 18029 EN

EUROPEAN COMMISSION Edith CRESSON, Member of the Commission responsible for research, innovation, education, training and youth

DG Xll/F JOULE programme

European Commission

BIOMASS CONVERSION TECHNOLOGIES

ACHIEVEMENTS AND PROSPECTS FOR HEAT AND POWER GENERATION

Directorate-General Science, Research and Development

November 1998 EUR 18029 EN

LEGAL NOTICE Neither the European Commission nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information

A great deal of additional information on the European Union is available on the Internet, it can be accessed through the Europa server (http://europa.eu.int).

Cataloguing data can be found at the end of this publication.

Luxembourg: Office for Official Publications of the European Communities, 1999

ISBN 92-828-5368-3

European Communities, 1999

Reproduction is authorised provided the source is acknowledged.

Printed in Belgium

PRINTED ON WHITE CHLORINE-FREE PAPER

FOREWORD The aim of this publication is to give an overview of the state-of-the-art and future prospects in technology and implementation issues for energy production through thermochemical conversion of solid biomass in the EU and the way and extent to which European Community funded RTD activities have contributed to these developments. Biomass is the energy source with the largest single contribution among renewable energy sources and it is hoped that it will provide up to 9% of the total energy consumption in the EU by 2010.

With reference to the contribution made by the Community in the area of biomass use, it should be noted that:

it has extended to all the parts of the process chain, - from crop production to end use applications

practically all technologies of thermochemical and biological conversion have been covered throughout the years. Important dissemination activities (some of them outside the scope of R&D programmes but within various policies such as environment, structural funds, agricultural policy, etc.) have also taken place

as a result an amount of knowledge and know-how has been accumulated and is being continually developed. Examples are:

operational problems and emission levels in large scale co-combustion tar removal prime movers in small and medium scale gasification co-gasification pilot gasification plants extended research in flash pyrolysis liquid bio-fuels biochemical processes

networks and working links have been established between researchers and industrial partners which are valuable not only from the purely scientific/technical point of view but will also be instrumental in any future standardisation and/or harmonisation activities

hardware has been created, not least in areas of high technological risk (pyrolysis plants, the current targeted projects for gasification, to mention only some of them)

the financial weight of this support is very important, - for a number of major players in the field Community projects represent a substantial percentage of their turnover.

M. Poireau, Advisor to the Director

ACKNOWLEDGEMENTS:

Prof. L.Sjunnesson

Dr J.Spitzer

Prof.J.Hustad

Prof. A. Schmidt

Dr P.Girard

Sydkraft/S weden

Joanneum Institute of Energy Research/Austria

Universitet Trondheim/Norway

TU Wien/Austria

CIRAD-Fort/France

Mr. M. Papadoyannakis

Mr. P. Doyle

Md. A. Segerborg-Fick

Mr. K. Maniatis

DGXII/F.4

DGXII/F.4

DGXII/E.2

DGXVII/D.3

IV

TABLE OF CONTENTS:

1 SUMMARY

2 INTRODUCTION 2.1

2.2

2.3

2.4

2.5

Background

Objectives

Biomass fuels

Introduction of biomass to the market

Technical options

SMALL SCALE COMBUSTION 3.1

3.1.1

3.1.2.

3.1.3

3.1.4

3.1.5.

3.2.

3.2.1

State of the art and fuel specifications

Biomass fuels Wood logs Wood chips Wood briquettes Wood pellets Woody industrial residues Non woody biomass fuels Wood log and wood briquettes furnaces Wood fire places Stoves and chemines Wood log furnaces Tiled stoves Wood chip furnaces Understoker furnaces Horizontal stoker furnaces Down stream furnaces

Wood pellet furnaces Furnaces for wood pellets only Furnaces for combined use of wood pellets and wood logs/briquettes Medium scale heat and power production

Technical barriers

Fuel related technical barriers Transportation and feeding Storage Variable properties of biomass fuels Safety of fuel storage

5

5

6

7

8

10 10

10 10 11 11 12 13 13 13 13 14 14 15 16 16 17 17

18 18 19 19

21

21 21 21 21 22

3.2.2 Ash related technical barriers 22 Ash removal 22 Slag 23 Disposal of ash 23

3.2.3 Emission related technical barriers 23

3.3 Environmental issues 24

3.3.1 Emissions in the flue gas 24

3.3.2 Harmful substances in ash and particulates 25

3.4 Economic aspects 26

3.4.1 Cost components 26 Investment costs 26 Fuel costs 26 Maintenance costs 27

3.4.2 Specific heat production costs 28

3.5 European Commission projects achievements 29 Combustion optimisation in grate furnaces 29 Combustion optimisation in fiuidised bed furnaces 30 Heat and power generation using an indirectly fired gas turbine 31 Heat and power generation using a biomass gasifier with a gas Otto engine 32

3.6. Future prospects 32

3.6.1 Future R&D requirements within the scope of the 5lh FP 32

3.6.2 Future requirements within the scope

of a liberalised energy market 32

LARGE SCALE COMBUSTION 34

4.1 State of the art 34

4.1.1 Fuel specifications for large scale boilers 34

4.1.2 Combustion and boiler technology 37 Combustion process 37 Combustion technology 39

4.1.3 Energy recovery and efficiencies 51

4.2 Technical barriers 54

4.2.1 Fuel specification, pre-treatment and feeding 54

4.2.2 Combustion and boiler technology/fouling/deposits/corrosion 54

4.2.3 Flue gas cleaning systems 56

VI

4.3

4.3.1

4.3.2

4.4

4.4.1

4.4.2

4.4.3

4.4.4

4.5

4.6

Environmental issues

Emissions to air

Emissions in slag/filter dust

Economic aspects

Fuel costs

Investment costs

O&M costs

Cost of electricity (COE) compared to coal,

natural gas and MSW

European Commission projects achievements

Future prospects

56 56 59 60 60 60 61

61 62 63

References 66

BIOMASS GASIFICATION 69 5.1 Objectives of biomass gasification 69

5.2 Biochemical gasification (Anaerobic digestion) - state of the art 69

5.2.1 Raw materials 71

5.2.2 Digesters 71

5.2.3 Utilisation of biogas and of the gasification residue 72

5.2.4 Economics of biochemical gasification 73

5.2.5 Future developments 74

5.3 Thermochemical gasification-state of the art 74

5.3.1 Principles of thermal gasification 75

5.3.2 Feed materials pre-treatment 76

5.3.3 Fixed bed gasification 77

5.3.4 Fiuidised bed gasification 79

5.3.5 Entrained flow gasification 82

5.3.6 Pressurised gasification 82

VII

5.3.7 Gas treatment 83 Particulates 84 Tar removal 84 Removal of trace impurities 85 Carbon monoxide/methane conversion, CO removal 86 Hydrogen separation 87

5.3.8 Ash disposal and waste water treatment 87

5.4 Gas utilisation 87

5.4.1 Direct combustion and conventional power production 88

5.4.2 Gas motors 89

5.4.3 Gas turbines 89

5.4.4 Fuel cells 90

5.4.5 Synthesis of natural gas and liquid fuels 91

5.5 Environmental and safety issues 92

5.6 Economic aspects 93

5.6.1 Investment costs 94

5.6.2 Costs of production 95 Basis of the calculation: 95

5.7 European Commission projects achievements 96

5.8 Future prospects 98

5.8.1 Comparison of gasification technologies 98

5.8.2 Improvement of gasification technologies 101

5.9 Conclusions 103

PYROLYSIS TECHNOLOGIES 116 6.1 Pyrolysis mechanisms and pathways 117

6.1.1 Products quality 120 Characteristics of pyrolysis end-products 120 Solid 120 Liquid 121 Gas 123

6.1.2 Feedstock requirements 124 Feedstock quality 124 Feedstock availability 124

VIII

6.1.3 Process characteristics and requirements 125 Process for liquids 126 Process for solid 127

6.2 State of the art 127

6.2.1 Pyrolysis processes 127 North America 127 Ensyn - Canada 128 Resource Transforms International ltd. (RTI, Canada) 128 Pyrovac (Canada) 128 National Renewable Energy Laboratory (NREL, USA) 129 European Union 129

6.2.2 Pyrolysis liquids quality 132 Solid and ash content 133 Water content 133 Viscosity 135 Stability 135 Bio-oil characterisation and analysis standards 135 Upgrading 136 Catalytic upgrading 136 Physical upgrading 137

6.2.3 Applications for combined heat and power 138 Combustion 138 Diesel- Engine 140 Gas turbine 140 Chemicals 141

6.3 Technical Barriers 142

6.3.1 Feedstock limits 142

6.3.2 Process problems 143

6.3.3 End product problems 143

6.4 Environmental issues 144

6.4.1 Impacts on environment 144 Pyrolysis process level 144 Application level 145

6.4.2 Impact on human health 147 Acute toxicity 148 Chronic toxicity 148

6.5 Economic aspects 149

6.6 European Commission projects achievements 152

IX

6.7 Future Prospects 152 Feedstock related needs 152 Conversion technology 153 Product quality 155 Applications 156

CONCLUSIONS AND RECOMMENDATIONS 165 7.1 Overall assessment of biomass in the energy area 165

7.2 Analysis of the introduction of biomass 168

7.3 Achievements in projects funded by the European Commission 170

7.4 Analysis of technical barriers 170 Large-scale combustion 173 Gasification 174 Pyrolysis 174

7.5 Conclusions 175 Costs 176 Techniques 176 Demonstration and dissemination 177 Country by country approach 177

7.6 Recommendations 178

SUMMARY

The main objective of this work is to identify technical barriers and obstacles that impede the rapid development of the biomass market within the EU Member States. On the basis of the estimated opportunities open to bioenergy on a commercial market, the work is aimed at assessing the role that bioenergy technologies has so far achieved in the European Union through actions by the Member States and by the Commission in the past four to five years. It will identify the remaining difficulties and/or obstacles, from a technical point of view, to continued introduction of biomass, and analyse the fundamental technical questions that are of significance in enabling bioenergy to play an important role in the future supply of energy in Europe. In parallel with this, it is also essential to shed light on the extent to which other important factors affect the introduction of biomass into the energy system, such as non-technical factors related to the environment, socio-economics, public acceptance, etc. However, these issues are not discussed more than superficially in this study but will be covered in other studies.

Although several countries in Europe, mainly Sweden, Finland and Austria, are putting biomass to extensive use, most European countries are not utilising it to a sufficient extent to enable development to progress as rapidly as had been expected. However, this is not entirely clear since, according to statistics, electricity generation from biomass in the EU Member States has increased by around 50% between 1989 and 1994.

On the other hand, the statistics are unclear on certain issues, including the definition of what biomass actually is. There are different driving forces for this development, the most important appears to be environmental awareness, which reflects the view that biomass may be less harmful to the environment than many other energy sources. At the same time, however, the main obstacle to introduction on a large scale is the high cost of employing biomass compared to other energy sources, mainly fossil fuels.

Several studies have described biomass as the most important source of renewable energy for the future. In the long term, biomass will undoubtedly play a significant role in the supply of energy in many countries. However, it has become obvious in recent years that biomass may be unable to play as important a role in the coming years as had previously been expected. The main reason appears to be, as with most other renewable energy sources, the inability of biomass to compete with the energy sources in use today.

Opportunities are available for improving the competitiveness of biomass by lowering the cost of its utilisation. The two most important factors in this respect are the cost of the biomass fuel itself and the development of less costly biomass utilisation techniques.

This development has been in progressing a long time and has enabled knowledge

to be accumulated on matters such as the technical obstacles that must be overcome before biomass can be expected to make a significant contribution to the total energy supply. However, without any major break-through, the lead-time for a significant introduction seems long.

A number of biomass fuels are considered in this study. Some of these are in use today (forest residues, straw, etc.), others may be introduced more broadly at a later date (energy crops, etc). Although forestry residues are the main biomass resource used so far, various types of waste products and a growing proportion of energy crops are also employed and are therefore included in this study. However, municipal solid waste (MSW), contaminated wood, black liquor and liquid bio-fuels are only marginally addressed in this study.

Many of the techniques employed for exploiting biomass have been used for a number of years (e.g. stokers for combustion), while others are only just being tested and demonstrated (e.g. gasification). Others appear to have good potential for becoming conceivable future techniques, although they have not yet been fully developed (e.g. pyrolysis).

Biomass utilisation technologies are available for many applications and are widely used, although only within a few countries. It must be emphasised that, even if any technology can be improved further by research and development, the lack of appropriate technologies is not the only reason for the limited use of bioenergy in the European Union. Lack of competitiveness compared to fossil fuels is more likely to be the most important reason. Bioenergy plants use an expensive fuel and involve high investment costs.

The techniques of greatest current interest are:

direct combustion in small and large boilers (for heat, power and combined heat and power (CHP) production) advanced thermal conversion of biomass into a secondary fuel by thermal gasification or pyrolysis, followed by use of the fuel in an engine or turbine biological conversion into methane by anaerobic bacterial digestion and use of the bio-gas in engines and boilers chemical or biochemical conversion of organic materials into hydrogen, methanol, ethanol or diesel fuel.

Their use is currently limited to niches in which their competitive disadvantages can be avoided, such as:

straw combustion in medium and large-scale boilers for district heat (e.g. in Austria) and power (e.g. Denmark). Small-scale straw combustion is technically not yet developed,

the use of industrial wood residues, such as sawdust and bark, for conversion into energy within the plant

the use of municipal waste for power generation

In all other cases, bio-fuel must still be subsidised to achieve competitive energy costs. The form of these subsidies may vary from country to country, including general energy taxes (from which bioenergy is excluded), carbon dioxide taxes, subsidies on investments for bioenergy, and also undertakings in some cases by electricity utilities to use a certain proportion of renewable energy sources in their production.

Although the utilisation of renewable energy sources, especially bio-energy, is expected to assist in solving the problems of reliability of supply and environmental impact of energy generation from fossil fuels, the possible advantages of bioenergy will have to be compared to the additional cost incurred by its use. This means that the objective of all bioenergy research and development efforts must be focused on lowering the cost of bioenergy feedstock and the associated investment cost, including the removal of technical barriers. The achievements so far in the work by the European Commission go in this direction, but further effort is still required.

The various technical factors that affect the cost of bioenergy are of vital importance to its future utilisation. Some of the more important obstacles that are similar to all biomass conversion technologies and also to all users can be summarised in the following areas.

Specifications and standards for biomass fuels Fuel pre-treatment Feeding of fuel, especially in pressurised systems Storage of fuel Ash removal and handling

There are specific technical obstacles for different conversion technologies but these obstacles, together with those mentioned above, may be overcome leaving the final cost of the generated energy as the main obstacle to the introduction of bioenergy. In view of this, the following guidelines will have to be observed in the development of the conversion of biomass into useful energy.

It appears reasonable to develop small-scale (< lOOkW fuel input power) combustion systems, - although low-cost, high-quality fuels would have to be developed. To make the combustion system attractive, it is vitally important to develop plants with high availability and reliability. In all cases, the main commodity produced is heat, and whether or not it is combined with electricity generation is determined on a case to case basis.

Large-scale (> lOOkW and up to several MW fuel input power) biomass combustion systems are much easier to develop than small-scale systems. On the other hand, competition with fossil-fuelled plants is intense.

CHP plants are preferable to pure power generation plants. However, the

capacity of CHP plants is limited by the local demand for low-temperature heat. Due to their longer annual operating times, industrial applications are preferable to district heating systems. Power generation from biomass by means of conventional steam processes has a maximum energy efficiency of 35%, while combined cycle plants employing biomass gasification and gas turbines could achieve 40 - 50% or even higher. However, high plant costs and high fuel costs result in the electricity generated being more expensive than that from fossil fuelled power stations. Since the fixed cost of biomass gasification plants is the main cost element for such plants, technical developments should be concentrated on simplification of the plant by avoiding pre-drying of biomass. Tar formation in the gasifier should also be minimised by improving the gasifier design, in order to avoid elaborate gas purification and wastewater treatment systems. If possible, gasification with air should be avoided, since the nitrogen in the air dilutes the gas generated and necessitates bulkier equipment and larger piping. However, the production of oxygen is expensive and will probably be limited to large-scale gasification. The technical development of biochemical gasification of biomass by anaerobic fermentation is well advanced. The use of this system, especially for the gasification of biomass with high moisture content, has reached fully developed status and is restricted only by economic considerations. The gasification of biomass waste and the gasification of biomass jointly with waste materials, and with coal, have the potential to improve the economics and should be investigated further. Fast pyrolysis is a relatively new technology compared to other thermochemical conversion methods and offers the opportunity to separate the biomass conversion stage from its final use. However, R&D is still necessary to bring the technology to maturity. Attention should be focused mainly on process engineering to assist the development stage. In terms of applications for pyrolysis, power generation and co-generation with engines seem to offer the most promising options. Existing technologies for biomass utilisation are perceived, to some extent, as being unreliable. Demonstration has therefore a vital role to play in the development of the bioenergy utilisation technique and must be evaluated further. It is particularly important to concentrate effort on measures that have a good chance of success and can produce results within a relatively short period of time. The technologies now being demonstrated appear to have long lead times before they will be established on the market. This is particularly true of very new techniques, such as IGCC (Integrated Gasification Combined Cycle) which has not yet been demonstrated in more than one application (Vrnamo in Sweden).

2 INTRODUCTION

2.1 Background

Biomass has long been widely used as a fiiel for generating heat by direct combustion. Compared to traditional gaseous and liquid fossil fuels, bio-fuel, however, suffers drawbacks such as lower storage density, lower calorific value, handling difficulties and wide variations in properties. These are the main reasons why bioenergy is not used more extensively in industrialised countries. Although several countries in Europe, mainly Sweden, Finland and Austria, are putting biomass to extensive use, most European countries are not utilizing it to a sufficient extent to enable development to progress as rapidly as had been expected, i.e. an increase in utilization of biomass (as a percentage of the total energy consumption) from - approximately - the existing 3% to 9% by the year 2010. However, it is not entirely clear-cut, since statistics show that electricity generation from biomass in EU Member States has increased by about 50 percent between 1989 and 1994. On the other hand, the statistical information is unclear on certain issues, such as the definition of what actually is biomass. In the short term, the most important driving force appears to be environmental awareness, which reflects the view that biomass may be less harmful to the environment than many other energy sources. At the same time, however, the main obstacle to large-scale introduction is the high cost of utilizing biomass compared to other energy sources, mainly fossil fuels. Several studies, e.g. the White Paper by the Commission, have described biomass as the most important source of renewable energy for the future. In the long term, biomass will undoubtedly play a significant role in the supply of energy in many countries. However, it has become obvious in recent years that biomass may be unable to play as important a role in coming years as had previously been expected. As stated above, the main reason appears to be the inability of biomass to compete with energy sources used today. Opportunities are available for improving the competitiveness of biomass by lowering the cost of its utilization. The two most important factors in this respect are the cost of biomass fuel and the development of less costly biomass utilization techniques. This development has long been in progress and has enabled knowledge to be accumulated on matters such as the technical obstacles that must be overcome before biomass can be expected to make a significant contribution to the total energy supply. However, the lead-time for significant introduction appears to be long.

2.2 Objectives

The main objective of this work is to identify technical barriers and obstacles that

impede rapid development of the biomass market. On the basis of the estimated opportunities open to bioenergy in a commercial market, the work is aimed at assessing the role that bioenergy has so far achieved in the European Union through action taken by the Member States and by the Commission in the past four to five years. It will also identify the remaining difficulties and/or obstacles to continued introduction, and analyse the fundamental technical questions that are of significance in enabling bioenergy to play a significant role in the future supply of energy in Europe.

In parallel with this, it is also essential to shed light on the extent to which other important factors affect the introduction of biomass into the energy system, such as non-technical factors related to the environment, socio-economics, public acceptance, etc. These matters will become increasingly important following the introduction of any new energy technology in the future, and particularly energy sources that affect the individual to a greater extent than existing energy sources. However, this is not the prime objective of this work, and will be considered in other studies.

2.3 Biomass fuels

Biomass is a general term that describes fuels based on organic matter. Biomass includes a number of organic raw materials that are of varying significance in different countries. The term biomass used here thus covers all biological matter.

A number of fuels may be covered by this definition. Some of these are in use today (forest residues, straw, etc.), others may be introduced more broadly at a later date (energy crops, etc.).

Although forestry residues are the main biomass resource used so far, various types of waste products and a growing proportion of energy crops are also employed. The only exceptions not covered by this study are municipal solid waste (MSW), contaminated wood, black liquor and liquid bio-fuels.

Biomass in various forms can be used in conjunction with various techniques in a number of combinations. Within the framework of this particular work, biomass can be employed, either directly or in converted form, in two main applications, i.e. as fuel for generating electricity and/or heat and, principally in converted form, as transport fuel. However, transport fuels are not covered by this study.

Used as an energy raw material, biomass has a number of benefits to offer compared to other energy raw materials, the main benefit in this context being that biomass is carbon dioxide neutral. However, a number of circumstances in the entire life cycle chain of bioenergy must be strictly evaluated from the environmental aspect. Biomass gives rise directly to a number of environmental problems of varying types, including emissions of environmentally harmful

substances during combustion. Biomass also causes a number of indirect environmental problems, mainly due to transport requirements. Viewed on a global perspective, the latter problems are undoubtedly of little significance, although they must still be considered. This particular point has not been fully considered in this work, although it has been analysed to some extent.

2.4 Introduction of biomass to the market

The introduction of biomass technology, particularly in the short term and in some regions, seems to progress slowly and it is therefore of vital importance to identify the reasons. The entire area of biomass needs to be thoroughly reviewed, not only from technical and economic viewpoints, but also from environmental and socio-economic aspects. It is extremely important to achieve an in-depth understanding of the behaviour of various factors in the field of biomass and what role the various parameters play in developments within the field. These aspects relate to matters such as the impact on the landscape and its cultural geography, the dependence of the population on transport and infrastructure if energy raw materials are unevenly distributed, and anxiety about using techniques that have not yet been demonstrated or even those that have already been demonstrated but which arouse apprehension just for that reason.

As in many other cases, the main question in the introduction of biomass techniques is undoubtedly the cost of generation to the various utilities when using given raw materials and different conversion techniques. The cost of biomass is one of the most important elements in this area. The reason being the high proportion of the fuel cost in the total generation cost (depending on the technology and the commodity produced it could be up to 40 percent of the final cost), and also the difficulty in predicting stable price levels. Moreover, biomass is far more bulky than other fuels used today, which may make it necessary to expand the existing infrastructure. The associated costs may sometimes be substantial. As the final cost of generating useful energy from biomass also depends on the technology used, this factor must be subjected to particularly close scrutiny. All of these issues are of great importance when evaluating the reason for biomass not being introduced to the extent that has been expected in recent years.

The market in which biomass can be utilized may differ from country to country. However, in general terms, biomass is expected to find applications in all areas in which coal, oil and natural gas are now used. Development is driven by many factors which are different in different countries. One vital factor in this context is the task of introducing the relevant technique on various markets, and how important these markets are for ensuring dynamic development. As biomass is still used to such a modest extent, all of the benefits are unlikely to have been taken into account and are given only some consideration in this study.

2.5 Technical options

As stated above, the main objective of this work is to concentrate on identifying technical barriers and obstacles to the introduction of biomass. If biomass is to be used more extensively, not only existing techniques have to be further developed but also a number of new techniques must be introduced. Several aspects of this implementation process must be identified and analysed. These aspects obviously vary in magnitude and significance from country to country, mainly because of historical and traditional factors related to biomass in the various countries. Many of the techniques employed for exploiting biomass have been used for a number of years (e.g. stokers for combustion), while others are only just being tested and demonstrated (e.g. gasification). Others appear to have good potential for becoming conceivable future techniques, although they have not yet been fully tested (e.g. pyrolysis). The techniques of greatest current interest are:

direct combustion in boilers advanced thermal conversion of biomass into a secondary fuel by thermal gasification or pyrolysis, followed by use of the fuel in an engine or turbine biological conversion into methane by anaerobic bacterial digestion chemical or biochemical conversion of organic materials into hydrogen, methanol, ethanol or diesel fuel.

In summary, different technologies can be applied to biomass to generate the commodities tabulated below (examples are given):

PROCESS

Combustion

Gasification

Pyrolysis

PRODUCT

Hot exhaust gas

Fuel gas

Synthesis gas

Gas (fuel) Liquids (fuel oil) Char (solid fuel)

APPLICATIONS

Boiler/Steam engine

Boiler, Gas engine Gas turbine, Fuel cell Synthetic natural gas Liquid motor fuels Chemicals Engine Boiler Engine

Space heating, Process heat Hot water, Power / heat Heat Power / heat Heat Transport

Power / heat Power / heat Transport

Table 2.1 Biomass utilisation scheme

Although all of these technologies are in use in Europe, not all of them have reached full maturity. A number of obstacles must still be surmounted before the technologies are developed to a stage in which they can be regarded as fully commercialized.

In the following chapters, these obstacles will be analysed and considered further.

3 SMALL SCALE COMBUSTION

The differentiation between "small scale" and "large scale" biomass combustion is not possible by any sharp definition. Here "small scale" generally means combustion plants for room heating in smaller buildings (one- or two-family houses). A heat power range below 100 kW may be assumed for small-scale combustion.

Another criterion for differentiation could be the operation mode: "large-scale" plants normally are professionally (commercially) operated, while "small-scale" plants are normally privately operated.

The level of technology may also be different in the two cases: Large scale plants often are equipped with advanced measurement and control systems, which lead to optimized combustion with low emissions and high efficiency. For small-scale plants such systems usually are too expensive.

3.1 STATE OF THE ART AND FUEL SPECIFICATIONS

3.1.1 Biomass fuels

In this section an overview of the properties of biomass fuels for small-scale biomass furnaces is given. Detailed specifications of woody biomass fuels are given in Standard Sheets issued in some countries and are under preparation by CEN.

Wood logs

Wood logs (Figure 3.1) are the oldest form of fuel for biomass combustion. For small-scale biomass combustion wood logs are wood pieces with a length between 15 and 50 cm and a diameter between 5 and 15 cm. The usual preparation procedure of such wood logs comprises the following steps:

Felling of the trees and cutting them in pieces of a length of 1 meter. Cleaving of the 1-meter pieces with a cleaver, - often powered by a tractor.

Figure 3.1 Wood logs for use in small-scale biomass furnaces

10

Storage of the cleaved 1-meter pieces (covered) in the open air typically at the location of harvesting for one or two years.

Transportation of the 1-meter pieces to the heating installation, cutting in two to four parts, and further cleaving to reduce the diameter if necessary.

Wood log shaped fuel may also be produced from sawmill residues. Wood logs normally can not be used in automatically fed furnaces. The furnace has to be fuelled manually at least once a day. Reasons to use wood logs as fuel can be Wood log furnaces generally are the cheapest way of producing heat from

biomass. Wood logs can be produced without expensive machines, even by hand, as it

was done for hundreds of years.

Wood chips Wood chips (Figure 3.2) are pieces of chopped wood with dimensions between 2 and 10 cm. Wood chips can be transported automatically using screw or belt conveyors. Therefore an automatic feeding system for the furnaces is possible, enabling operation without manual help for several days. If an automatic ash disposal system is also installed the operation period can be further increased up to several weeks.

WMX. Figure 3.2 Wood chips for use in small-scale biomass furnaces

The production of wood chips is highly automated with special machinery (chipper). To minimize manipulation and transportation efforts, the chips should be produced at the harvesting site. An important advantage of wood chips is their homogeneous physical properties. Thus the use of automatic control systems for power level and combustion quality in the furnace is possible. This ensures low emissions of harmful substances and high efficiency (see Chapter 3.3.1).

Wood briquettes Wood briquettes (Figure 3.3) are cylindrical pieces of extruded wood chips and/or

11

Figure 3.3 Wood briquettes for use in small-scale biomass furnaces

powder with a diameter of approximately 10 cm and a length between 10 and 30 cm. Generally they are produced from wood manufacturing residues (shavings, saw dust, grinding dust). Wood briquettes have properties similar to wood logs. No automatic transportation systems for wood briquettes are available for small-scale furnaces.

Advantages of wood briquettes as fuel are for example:

high density and therefore high heat content related to the storage volume low water content (approx. 4 %) and therefore high combustion efficiency homogeneous physical and chemical properties

Wood pellets

Wood pellets (Figure 3.4) are cylindrical pieces of extruded wood particles with a diameter of approximately 1 cm and a length between 1 and 2 cm. They have the same advantages as wood briquettes:

high density and therefore high heat value related to the storage volume

low water content (approx. 4 %) and therefore high combustion efficiency homogeneous physical and chemical properties.

However, the most important additional property of wood pellets is that they can be fed by screw feeders and even by pneumatic equipment. Thus, the transportation of wood pellets is similar to the transportation of a liquid fuel:

Wood pellets can be sucked in by a special tank lorry and, after transportation,

Figure 3.4 Wood pellets for use in small-scale biomass furnaces

12

can by blown into a storage room at the location of the user without any manual help.

From the storage room, they can be easily transported to and into the combustion chamber by screw feeders or by pneumatic transportation systems.

Thus, wood pellet furnaces may be operated in a manner almost comparable to oil furnaces.

Automatic wood pellet furnaces have been developed only recently. R&D efforts are based on the vision that wood pellets will become the most important standard biomass fuel.

Woody industrial residues

Woody industrial residues are of importance as fuel for small biomass furnaces only when prepared as described in the previous paragraphs (as wood logs, wood chips, wood briquettes, or as wood pellets). Therefore no technical descriptions have to be given here. Unprocessed woody industrial residues, like bark, are used in large-scale combustion systems only.

Non woody biomass fuels

From a technical viewpoint (process engineering, handling, transportation, emissions) many different biomass fuels may be used for heat production in small-scale biomass furnaces. Not only solid non-woody biomass fractions (converted to briquettes or pellets) but also liquid biomass fuel (rapeseed oil, "Bio-diesel", pyrolysis liquid) could be used, but in general these applications are not economic for small-scale combustion. Therefore non-woody biomass fuels normally are used in large-scale biomass furnaces only, e. g. straw from agriculture.

3.1.2. Wood log and wood briquettes furnaces

In this section, small-scale combustion systems operated manually are described. They range from stoves and fire places to boilers for central heating systems.

Wood fire places

Wood fireplaces (Figure 3.5) have a long tradition in several countries, especially in North America and in the UK. The handling of course is difficult and dirty but, on the other hand, wood fireplaces offer to the user the pleasure to observe the open fire in his living room.

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Wood fireplaces normally are not optimized with respect to emissions and efficiency. Due to the open combustion chamber, they usually have too much combustion air and therefore a very low combustion efficiency with high emission of harmful substances.

Stoves and chemines

Figure 3.5 Wood fireplace

Stoves (Figure 3.6) are closed furnaces installed in the room to be heated. Often stoves are equipped with a glass window (chemines, Figure 3.7) so the fire can be observed as is possible with a wood fireplace. The glass furthermore leads to a certain heat transfer by radiation which, as well-known, leads to a comfortable feeling in the room. The combustion process in chemines can be controlled to some extent by primary and secondary air staging. However the combustion temperature sometimes is too low because of heat losses through the glass window.

Figure 3.6 Wood stove Figure 3.7 Wood chemine

Wood log furnaces

Wood log furnaces (Figure 3.8) are combustion plants for solid fuels with a combustion chamber and a boiler for a water heating system. They are normally fuelled manually by an opening in the combustion chamber. In modern wood log

14

furnaces the combustion chamber is divided into storage and combustion zones. Combustion in the storage zone is prevented by leading the combustion air through this zone in a co-current flow.

The storage volume of the combustion chamber is normally dimensioned for 8 full load hours. Therefore wood log furnaces are to be fuelled up to 3 times a day.

Wood log furnaces are usually fairly well developed with respect to combustion optimization, sometimes equipped with control systems for combustion air based on the measurement of the oxygen concentration in the flue gas ("Lambda Control"). To avoid partial load conditions, they are often equipped with a buffer store. The store is charged under optimal full power conditions and is discharged according to the actual (partial load) demand of the heating system.

Figure 3.8 Wood log furnace

Tiled stoves

Tiled stoves (Figure 3.9) are a traditional form of wood stove, mainly used in alpine regions and in Finland.

The combustion chamber of a tiled stove is surrounded by massive walls. The walls store the heat produced in the combustion chamber and transfer it into the room to be heated. Thus the daily heat requirement is produced by operating the furnace under optimal full power condition for a limited time period, while the heat flow to the room corresponds to the much lower actual heat requirement. Handling effort can thus be limited to only one fuelling procedure a day and the combustion process can be controlled and optimized for full load

Figure 3.9 Tiled wood stove

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conditions. However, in this operation made there is no possibility to control the heat flow to the room. Usually the outer surface of the stove is covered with tiles, offering the possibility to realize many varied designs. Therefore tiled stoves are often installed not only for heat production but for interior decoration too. The tiles also represent an additional heat store and enhance heat transfer by radiation.

Emissions of tiled stoves have been reduced in recent years by several technical improvements, including flue gas fan blowers and electronic control systems.

3.1.3 Wood chip furnaces

Wood chip furnaces are automatic combustion plants with a combustion chamber and a water boiler connected to a water heating system. In small scale wood chip furnaces the wood chips usually are fed by screw conveyors into the furnace at a rate depending on the actual heat demand (Figure 3.10). Control systems, often based on the measurement of certain emission components (oxygen, carbon monoxide, and others), ensure low emissions and high efficiency.

While no major manipulation effort is necessary for the furnace operation, some manipulation concerning fuel storage and feeding is needed.

Of course the installation of a large storage tank with automatic feeding systems is possible, but this leads to higher space requirement and costs. Therefore wood chip furnaces often are installed for heating buildings in rural areas, where some infrastructure for mechanical fuel handling and storage (tractors, barns) is available.

Wood chip furnaces can be subdivided into three types with different operating modes, as described below.

Figure 3.10 Wood chip furnace

Understoker furnaces

In an understoker furnace the wood chips are fed by a screw conveyor through an

16

opening at the bottom of the combustion chamber, generally into a panlike burner, in which a fuel mound is formed with a glowing layer on its surface. The advantages of such a feeding mode are:

it is simple and therefore cheap back-burning into the feeding system normally does not occur.

However some disadvantages of an understoker furnace have also to be mentioned:

No drying zone is formed in the fuel stock under the glowing layer, therefore wood chips with low water content (less than 30 %) can be used only. When using wood chips with higher water content, incomplete combustion will occur with high emissions of carbon monoxide and hydrocarbons, and the combustion process may even come to a halt. the inlet of primary air through the glowing layer is difficult.

ash may accumulate in the burner and is difficult to remove.

Horizontal stoker furnaces

Some of the disadvantages of understoker furnaces can be avoided in horizontal stoker furnaces. In this system the wood chips are fed into the combustion chamber horizontally onto a grate, normally by a screw conveyor.

The fuel stock moving horizontally on the grate is passing through a drying zone before burning. Therefore the use of wood chips with higher water content (up to 50 %) is possible without increasing emissions of harmful substances.

However back burning may not be avoided totally by the design of the feeding system and therefore fire safety systems (e.g. a sprinkler facility) have to be installed at the fuel inlet.

Down stream furnaces

The most advanced combustion technology can be realized in down stream furnaces in which the wood chips are fed with a screw conveyor through an opening in the upper region of the combustion chamber and are falling onto the glow bed. This has several important advantages with respect to an optimized combustion process:

each wood chip is dropping directly into a fully burning environment. With a design leading to a dispersion of the chips, the combustion conditions are not influenced negatively by fresh (wet) material at the point of impact. In the hot burning environment, the drying process of the chips is happening nearly immediately. the glowing layer is free from mechanical disturbances, as caused by the screw conveyor feeding or by a sporadic gliding down the mound in an understoker

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furnace. Thus emission peaks resulting from such disturbances are avoided. The wood chips (normally in the size of not more than 10 cm) are too light to cause such disturbances in the glowing layer.

3.1.4 Wood pellet furnaces

Efforts are underway aiming at the development of wood pellet furnaces offering the same comfort as liquid fuel furnaces. Therefore, as mentioned in chapter 3.1.1, wood pellets have a good chance to become a standard biomass fuel to be used in furnaces in small-scale biomass combustion. Wood pellet furnaces are available also for combined operation with other solid fuels (wood briquettes, wood logs).

Furnaces for wood pellets only

Automatic furnaces for wood pellets only are designed similar to wood chip furnaces. The same control systems are used and therefore combustion quality and efficiency are very high. Often, the combustion chamber of a wood log boiler is equipped with specific installations (burning pans) for the combustion of wood pellets. The homogeneity of the fuel makes it possible to control the furnace near the optimal operating point (oxygen concentration in the flue gas of less than 4 %). Due to the low water content of the fuel this leads to flame temperatures of up to 1.600C. Thus high temperature resistant materials have to be used in the burn out zone. Wood pellets may also be used in manually fuelled stoves (Figure 3.11).

Figure 3.11 Stove operated with wood pellets

Disadvantages of furnaces for wood pellets only are:

the market of wood pellets is not yet established. Therefore the availability of wood pellets is not really guaranteed. the price of wood pellets is not stable and generally higher than the price of wood chips.

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Furnaces for combined use of wood pellets and wood lops/briquettes

By combining a wood pellet burner with a wood log boiler the problem of pellet availability can be avoided (Figure 3.12). The furnace can be operated by manual fueling of wood logs (or other solid fuels) or automatically by using the wood pellet burner. In such a combined "wood log and pellet burner" the following measures for reaching a maximum of operational comfort have been realized:

Figure 3.12 Wood pellet furnace (right) combined with wood log briquette boiler

the pellet burner can be put into operation automatically with an electric hot air ignition system. if wood logs are used, they will be ignited by the flame of the wood pellet burner. Depending on the heat demand fraction covered by the wood logs, the wood pellet burner adjusts its load automatically. After the wood logs have been burned, the heat demand will be covered by the wood pellet burner only.

So the operator of the furnace can feed the stove with wood logs only whenever he is motivated to do so or if wood pellets are not available or have become too expensive. The wood pellet burner in this model can also be switched to a special discontinuous partial load operation mode for heating water during the summer.

3.1.5. Medium scale heat and power production

Power production from biomass generally will only be economical in large-scale combustion systems, e.g. in the power range above 10MWC (see Chapter 4). However, in special cases, combined heat and power (CHP) production may also be advantageous in medium scale systems. Thus the power range in this chapter is extended to medium scale systems, e.g. up to approx. 10MW fuel power. From a technical viewpoint, steam technologies (steam engines and steam turbines) can in principle be used for medium scale CHP production. Plants ranging between lOOkW and 10MW are installed and successfully operated. However, special developments for biomass CHP systems have also been started in recent years - the

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so-called "hot-air turbine" process is under development and may be applicable in the future. Via a gasification process gas engines may also be used (see Chapter 5). In the lower power range, e.g. 50 -500kWe, Stirling engines will be of practical relevance for CHP production in biomass furnaces. The advantage of both the hot-air turbine and the Stirling engine is that they may be operated with hot unfiltered combustion gas from a standard biomass furnace thus avoiding expensive steam technology and operational requirements associated with this technology. Both systems have a relatively low electric efficiency (in the range of 5 -15%). But this is no major disadvantage since the flue gas energy leaving the power unit is used for heating.

The key component of the "hot-air" or "indirect fired" gas turbine is a high temperature heat exchanger transferring the energy from the combustion gas of the biomass furnace (or biomass gasifier) to the compressed air of the turbocharger-like turbine system (Figure 3.13). The power produced in the turbine is partly used to generate electricity and partly to operate the air compressor. Prototypes for a power range between 500kWc and 1.5MWC are under development.

Figure 3.13 Prototype biomass fuelled hot air turbine

Figure 3.14 Prototype biomass fuelled Stirling engine - 4kW mechanical

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In the last 10 years efforts to develop a Stirling engine for CHP production with biomass have been undertaken and some prototypes in the subject power range are available (Figure 3.14). In these developments the heat transfer from the biomass flue gas through the heat exchanger has appeared as the main problem, not the engine itself. Thus heat transfer accounting for pressure losses (inside of the heat transfer pipes) and fouling of the heat exchanger by condensing particulates (outside of the heat transfer pipes) is the main area requiring improvement.

3.2. Technical barriers

3.2.1 Fuel related technical barriers

Transportation and feeding The energy content of wood chips related to the volume is in the range of 3.000 to 4.500 MJ/m'. Thus compared to fuel oil with the specific energy content of 35.700 MJ/nr almost ten times the volume has to be transported. This of course leads to very high transportation costs, which increase the price of the fuel on the market. Feeding manipulation requirements are high with wood logs, because no automatic feeding system for wood logs is available. Manipulation is much easier when using wood chips. Screw conveyors, as well as side rods can be used for automatic transportation. As mentioned in chapter 3.1.1 furnaces operated with wood pellets require less manipulation effort since wood pellets can be transported in special tank lorries and fed into the furnace by pneumatic systems and screw conveyors.

Storage Due to the low specific energy content of wood, the required storage volume capacity for wood fuels is much higher than for oil. A typical one-family house in central Europe for example may have an energy demand of 125 GJ corresponding to 3.5m3 oil per year. The equivalent volume of wood chips is 30 to 40m3. A further problem with storing biomass is that it has to be dried before it may be stored in a silo. Wet wood, especially wet wood chips, can be-come moldy, which can lead to health problems. Also aerobic decay may occur resulting in a loss of energy. Thus wood chip stores often are de-signed as roofed open places allowing drying during storage.

Variable properties of biomass fuels Variable properties of biomass fuels (water content, particle size, etc.) can be a

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significant technical barrier especially in automatic combustion plants. The feeding system must be designed for certain fuel specifications. Deviations from these specifications cause operational problems. Some combustion systems can only be operated in a very small range of fuel properties (e.g. understoker furnaces with respect to the water content). The size of wood chips can vary, depending on the chipping technology. In small scale wood chip furnaces normally wood chips of more uniform quality are used. Wood chips from industrial wood residues, which often contain bigger pieces or even metal parts should be used in large-scale wood chip furnaces only, which normally have adequate screening systems. Varying fuel properties can also lead to control problems. The combustion air demand of wet wood for example is significantly higher than the combustion air demand of dry wood. Advanced control systems are able to adapt the combustion airflow rate adequately. But they are not available in all furnaces and therefore efforts should be undertaken to assure homogeneous fuel quality. A special problem related to the quality of wood pellets was observed: During transportation a certain fraction of the pellets can disintegrate into wood particulates which have different combustion properties influencing the combustion process.

Safety of fuel storage In case of fuel storage beside a furnace, fire safety has to be assured. This especially concerns automatic wood chip or wood pellet furnaces which are directly connected to the storage by the feeding system. To prevent back burning star feeders outside of the combustion chamber have to be installed. Additionally, temperature activated sprinkler systems have to be installed. When storing very fine wood particles (shavings, sawdust) a certain danger of dust explosion exists, even without a nearby installed combustion plant as a possible ignition source.

3.2.2 Ash related technical barriers

Ash removal For cost reasons small-scale biomass furnaces are usually not equipped with automatic ash removal systems. Therefore the operator has to remove the ash manually. When complete combustion of the fuel is achieved, the ash mass is less than 2% of the fuel mass. The volumetric weight of ash is in the range of 0.08 kg/dm3. With a

22

heating value of the fuel in the range of 15.000 kJ/kg, 5.8 kg/d of fuel is needed per kW of power. Under these conditions 0.115 kg/d/kW or 1.4 dm3/d/kW of ash are to be expected. But a certain amount of ash is normally removed from the flue gas in the form of particulates, so the ash volume is lower than the value calculated above. However, regular manual ash removal is necessary. Furthermore manual ash removal always leads to a certain pollution in the vicinity of the furnace and in the case of installation of a furnace in a living room, this can be seen as a problem.

Slag

In small scale wood log furnaces slagging normally does not occur, be-cause the combustion temperatures do not reach the ash melting point. But in wood chip furnaces and even more so in wood pellet furnaces, the combustion temperatures can become high enough to produce slag. In principle, slag problems can be decreased by reducing the temperature in the glow bed through controlling the primary air inlet flow to a value not higher than absolutely necessary for gasification of the fuel. Combustion of the gaseous components should be achieved by secondary air entering the combustion chamber above the glow bed. This measure also leads to low NOx emissions as described in Chapter 3.3.1. However, it requires a control system for the control of the glowbed height to guarantee the desired heat power under different combustion conditions.

Disposal of ash In small-scale biomass combustion ash usually is not seen as a problem with respect to contamination with harmful substances (heavy metals etc.). Therefore ash from small-scale furnaces is normally disposed ofin a garden, in an agricultural field or in the public waste. Thus, in agricultural regions no problems are to be expected at all. In urban regions, problems may be caused by limited volume of the dustbins, disposal costs, etc.

3.2.3 Emission related technical barriers

In some European countries small-scale biomass furnaces have to meet emission limits (CO, CxHy, NOx and particulates) which have to be demonstrated in a certification test. Due to these limits manufacturers are forced to optimize their products and even to develop new emission reducing technologies. These technologies mainly concern the installation of advanced combustion control systems (down-scaled from large-scale combustion technology) and filters (see Chapter 3.3.1).

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3.3 Environmental issues

3.3.1 Emissions in the flue gas

The emissions from biomass combustion, in general, are -carbon monoxide (CO) hydrocarbons (CxHy), including polycyclic aromatic hydrocarbons, (PAH) and dioxins, nitrogen oxides (NOx), and particulates.

CO and CxHy are products of incomplete combustion which can be minimized by special combustion chamber design and advanced combustion control. In this area progress has been achieved in recent years in medium and large-scale furnaces. Avoiding emission peaks caused by combustion fluctuation is the goal of the installation of control systems which are working on the measurement of the concentration of CO or other burnable flue gas components. With such control systems CO emissions can by decreased to a level lower than 10mg/Nm3 (= 3.2mg/MJ) and CxHy emissions can be decreased to a level lower than 2mg/Nm3 (= 0.64mg/MJ) even in partial load and discontinuous operating conditions. Thus variable load operation is possible in a wide range (down to 10% of fll load). Using a grate with very small openings (e. g. drilled chamotte plates) the retention time of the fuel particles in the combustion zone can be increased substantially. This ensures full combustion in the solid phase with low emissions of unburned fuel particles and high combustion efficiency. Measures for reducing NOx have also been developed and successfully tested, e.g. through primary air control. Decreasing the primary air input leads to increased height of the glow bed up to a new balance level. It has been observed that the resulting bigger glow bed volume (which means a bigger zone with under-stochiometric combustion conditions) can significantly decrease the emissions of NOx. Specific control systems allow the glowbed volume to be held as high as possible and therefore ensure very low NOx emissions in all combustion conditions (down to 50% of the previous level). Such measures are being developed for medium and large-scale furnaces in a JOULE project. However further research and development efforts are necessary for the application of these results to small-scale furnaces. NOx emissions also strongly depend on the nitrogen content of the fuel, whereby chipboard residues (chipboard pieces, chipboard chips, and chipboard sawdust) are the most problematic materials. NOx emissions when using natural wood as fuel are much lower (in the range of less than 100mg/Nm3 resp. 30mg/MJ) than in industrial biomass plants using fuel with high nitrogen content (in the range of up to more than 400mg/Nm' resp. 130mg/MJ). However, since typical fuels for small

24

scale furnaces, wood briquettes and wood pellets, can be produced from nitrogen containing wood residues, the development of nitrogen reducing techniques to be used economically for small scale furnaces should have high priority.

Particulates of small-scale biomass furnaces can be avoided to a certain extent by advanced combustion chamber design. Secondary measures like filters may be necessary to meet emission limits (typically below I50mg/ Nm3 resp. 47mg/MJ) but they are not usually installed due to their high costs.

As described above most of the emission problems have been solved for medium and large scale biomass furnaces. In these furnaces the following emission values can be reached with advanced control systems:

Carbon monoxide: Less than 16mg/MJ Hydrocarbons: Less than 1.6mg/MJ Nitrogen oxides: Less than 64mg/MJ Particulates: Less than 16mg/MJ

However further research and development efforts are necessary for the application of these results to small-scale furnaces.

3.3.2 Harmful substances in ash and particulates

Harmful substances in ash and particulates can be:

heavy metals and carcinogenic organic substances.

The amount of heavy metals in the ash of course depends on the amount of heavy metals in the fuel which again depends on the regional heavy metal impact in the area of fuel production. Wood chips produced from plants grown along highways for example can be contaminated significantly with heavy metals.

Investigations on medium and large-scale biomass furnaces have shown that it is possible to retain particles in the furnace by specific combustion chamber design. Thus it is possible to collect a considerable amount of heavy metals in the ash. The contaminated ash of course has to be disposed of correctly as special waste.

Organic substances resulting from incomplete combustion, like PAH, may be attached to ash particles. To the extent complete combustion is achieved, this does not represent a problem.

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3.4 Economie aspects

In this chapter the costs of heat production from biomass in small-scale furnaces are analysed based on the cost components (investment, fuel and maintenance) and compared to the cost of heat from oil-fired and gas-fired boilers. All cost figures refer to consumer prices including tax.

3.4.1 Cost components

Investment costs

As described in Chapter 3.1 a large variety of systems are available for small-scale biomass combustion. Consequently the investment costs also show a large variation.

An overview of investment costs of small-scale biomass furnaces is given in Table 3.1.

Furnaces

Wood log Mid wood briquettes furnaces

- Wood fire places (3 - 5 kW) - Stoves (3 - 8 kW)

- Chemines ( 3 - 8 kW) 1) . Woodlogboilers(15kW)

- T i l c d S t o v e s ( 3 - 8 k W ) 1)

Wood chip furnaces - 15 kW

. 30 kW

- 60 kW

100 kW

Wood pellet furnaces

- Furnaces for wood pellets only 1 5 kW

2 5 kW 3 5 kW

4 5 kW - Furnaces for combined use of

wood pellets and wood logs 1 5 kW

2 5 kW

3 5 kW 4 5 kW

- Oil furnaces

- Gas furnaces

Investment costs (ECU. April 1998) Furnace

Low levd

1.000

1.000 1.700

2.500

3.000

7.000

8.000

11.000

15.000

5.000

6.500

7.000

8.000

7.000

7.300

7.900 8.800

5.000 5.000

High level

2.000

2.000 3.000

3.500 10.000

10.000

12.000

15.000

18.000

7.000

8.500

9.000

10.000

10.400

10.900

11.700

13.200

5.000 5.000

Storage tank

Low level

5.000 8.000

10.000

15.000

5.000

8.000 10.000

15.000

5.000

8.000

10.000 15.000

4.000 2.500

High level

6.000

9.000

12.000

17.000

6.000

9.000 12.000

17.000

6.000

9.000

12.000 17.000

5.000 3.000

" A large variation of investment costs is possible depending 01 the design. Table 3.1 Investment biomass furnaces

costs of small-scale

Fuel costs

As described in Chapter 3.1 many different fuels are used in small-scale biomass combustion. Depending on the degree of mechanisation and on the cost of the equipment for fuel production the fuel costs show a large variation. An overview of wood fuel costs is given in Table 3.2. These costs are based on an annual fuel demand of about 10 - 50m3.

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Fuel

Wood logs (Beech), commercial price 2)

Wood logs (Beech), reduced price 3) Wood briquettes

Wood chips

Wood pellets Fuel oil

Natural gas

Current average consumer price

ECU/m3 loose

68,8

47,1

94,2

24,6

137,7

(April 1998) ECU/kg

0,197

0,135

0,157

0,077

0,212

ECU/kWh 1)

0,049

0,034

0,033

0,021 0,045

0,033 0,036

" Specific primary energy costs. 2) Commercial market price of chopped and dried wood logs (1/3 m) including delivery 31 "Private" market price of wood logs in rural areas without delivery.

Table 3.2 Fuel costs for small-scale biomass furnaces

Maintenance costs

Maintenance costs include the mandatory cleaning of the chimney and the service and repair requirements in the case of automatic furnaces. An overview of maintenance costs is given in Table 3.3.

Table 3.3 Maintenance costs of small scale biomass furnaces

Furnace

Manually fueled furnaces

- Wood lire places (3 - S kW)

- Stoves (3 - 8 kW)

- Chemines ( 3 - 8 kW)

- Wood log boilers (15 kW)

- Tiled Sloves ( 3 - 8 kW)

Wood chip furnaces

- 15 kW

- 30 kW

- 60 kW

- 100 kW

Wood pellet furnaces

- Furnaces for wood pcllcls only

15 kW

2 5 kW

3 5 kW

4 5 kW

- Furnaces for combined use of

wood pellets and wood logs/briquetU

15 kW

2 5 kW

3 5 kW

45 kW

- Oil furnaces

- Gas furnaces

Maintenance costs (ECU/a)

Chimney cleaning

100 100 100 100 100

100 100 100 100

100 100 100 100

100 100 100 100

100 100

Service and repair

100 200 300 500

100 150 200 250

100 150 200 250

100 100

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3.4.2 Specific heat production costs

Using the investment, fuel and maintenance costs given in the previous chapter, the specific energy costs may be calculated for a depreciation period of the investment costs of 15 years and a bank rate of 8% ("full cost calculation"). Table 3.4 shows the results.

Furnace Manually fueled furnace - Wood fire places - Stoves - Chemines - Wood log boilers - Tiled Stoves

Wood chip furnaces - 15 kW - 30 kW - 60 kW - 100 kW

Wood pellet furnaces - Furnaces for

wood pellets only * 15 kW *25kW *35kW *45kW

- Furnaces for combined use of wood pellets and wood logs/briquetts

* 15 kW *25kW *35kW *45kW

- Oil furnaces - Gas furnaces

Heat production costs with different fuels (ECU/kWh) Wood logs

normal price s 0,171 -0,197 0,130-0,156 0,148 - 0,182 0,088 - 0,093 0,170- 0,352

0,128 - 0,149 0,114 -0,126 0,106-0,117 0,106-0,116

red. price

0,132- 0,158 0,104- 0,130 0,122- 0,156 0,065-0,071 0,148- 0,330

0,108- 0,128 0,093 - 0,105 0.085 - 0,096 0,086 - 0,096

Wood briquettes

0,132- 0,158 0,104- 0,130 0,122- 0,156 0,065-0,070 0,148 - 0,330

0,107 - 0,128 0,092 - 0,105 0,085 - 0,096 0,085 - 0,095

Wood chips

0,100- 0,126 0,083 - 0,109 0,101 - 0,135 0,047 - 0,052 0,130- 0,312

0,091 - 0,109 0,070 - 0,107 0,056 - 0,072 0,052 - 0,057

Wood pellets

0,161 - 0,187 0,123 - 0,149 0,141 - 0,175 0,082 - 0,087 0,164 - 0,346

0,123 -0,141 0,103 - 0,139 0,088 - 0,104 0,084 - 0,090

0,114 - 0,127 0,106-0,114 0,099 - 0,106 0,100-0,106

0,123 - 0,143 0,108- 0,121 0,100-0,112 0,101 - 0,111

0,090 - 0,095 0,087 - 0,089

Shaded numbers: biomass furnaces competitive with oil and gas furnaces

Table 3.4 Specific energy costs of small-scale biomass furnaces

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From these figures it may be concluded, that in a full cost calculation heat from biomass is competitive with oil-fired and gas-fired boilers only in the case of wood log boilers and wood chip furnaces in the power range above 30 kW fossil fuel systems in a few cases only. However, most of the small-scale biomass furnaces are operated by persons who do not make full cost calculations, i.e., do not consider capital costs of the investment. They typically invest their own labour to produce fuel at lower cost compared to the figures in Table 3.2. Thus the relatively large share of biomass used for heating in small-scale furnaces in some European countries may be explained.

3.5 European Commission projects achievements

Within the 4th Framework Programme the European Commission supports work leading to improvements in biomass combustion. While most of them are not focussed on systems in the small-scale power range, many of the results expected or already achieved are applicable to these systems. However, to fully utilize these results it will be necessary to define specific follow-up projects aimed at the power range below 100 kW. A description of the content, the objectives, and the achievements of relevant projects follows.

Combustion optimisation in grate furnaces

Some projects have the challenging aim to improve the combustion behavior of small and medium-scale wood combustion systems such as stoves and boilers in a range from 6 kW up to 500 kW in order to:

reduce products of incomplete combustion such as CO, CxHy and particulates, increase the thermal efficiency,

define guidelines for low emissions for small scale stoves and boilers firing untreated wood,

develop and manufacture these low emission combustion units, and develop catalytic flue gas cleaning systems of a type suitable for domestic heating appliances.

The projects are focused on the combustion of untreated wood fuels in systems operating with batch-wise and quasi-continuous feeding. The results will be the basis for guidelines which will be worked out for low-emission wood-fired stoves and boilers for prototypes manufactured within the project.

The development of wood chip grate furnaces with NO* emissions that are substantially lower than the emissions of current state-of-the-art designs is the aim of another JOULE project. The work program contains fundamental research to develop methods to minimize NOx emission, investigation of measures to reduce

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emission by experiments on test furnaces, and prototype experiments to realize and to test the developed methods in practical operation. The project will develop and use mathematical models supported by appropriate laboratory experiments to identify influences on NOx formation mechanisms. The work is split into activities concerning the development of primary and secondary NOx reducing measures. The primary measures aim at minimizing the NOx emissions by fuel and combustion air control. The secondary measures to be investigated are the selective catalytic reduction process (SCR), and the selective noncatalytic reduction process (SNCR). To carry out prototype experiments pilot plants of LowNOx furnaces will be built and tested. The results of the project will consist of methods and measures to reduce NOx emissions from wood chip grate furnaces which have been developed and tested in laboratory and prototype furnaces. The results will be directly applicable to the design of improved wood chip furnaces. The increased competitiveness of lowNOx furnaces will be a strong incentive for the practical application of the results. The project will be completed in April 1999. Preliminary results have shown a possible NOx reduction of more than 50 %.

Combustion optimisation in fiuidised bed furnaces The development of an optimized fluidized bed furnace is the objective of another JOULE project. NOx and CO emissions are to be minimized in a new fluidized bed combustor which is already being commercialized in a first version in the domestic and industrial sectors, up to 2000kWu,. In the first stage, and in order to obtain a knowledge of the physical and chemical phenomena occurring in the combustor, a computer model of the combustion process will be developed. The model will include a submodel to simulate the fluid dynamics and thermodynamics of the combustor, as well as a submodel for the kinetics and chemistry of the process. In addition, a computer program to predict NOx formation in the combustor will also be developed and installed in both submodels. Once developed, the whole modeling system will be run under different process conditions and will be validated, and eventually improved, by comparing the predicted combustion results with those obtained in the experimental tests of a cold laboratory model and two combustore of lOkWu, and 500kWu, operated under similar process conditions. The validated model will be utilized as a tool to define the appropriate combustor design modifications to achieve the primary objective of emissions reduction. In the second stage a 160kWm combustor incorporating the defined design modifications will be constructed and operated with biomass of different characteristics in terms of water content, heating value, density etc, under real operating conditions to supply heat to a building. The performance of the new combustor will be compared, both in efficiency and emission levels, with that of a

30

120kWih boiler of first generation already commercially operated for domestic heating.

The experimental setup will help to determine the efficiency of the new prototype with different biomass fuels and conclude whether the expected emission levels have been achieved. It is expected to reduce the CO and NOx emission levels of the present combustion equipment by at least 30% and to achieve combustion efficiencies of about 90% with a large variety of granular biomass, also including those of high water content. The results of the research will be directly utilized to develop a second generation of improved combustors.

Heat and power generation using an indirectly fired gas turbine

One FAIR project aims at the development of a new heat and power generation plant in the range below 500kWc using a turbo charger unit indirectly fired by combustion gases ("hot-air turbine"). A two-stage combustor and a high temperature gas-to-gas heat exchanger were developed and specified. Research activities were directed towards the development and design of the heat exchanger since problems occurring in directly fired gas turbine cycles are transferred to fouling and corrosion problems in the heat exchanger. Therefore a detailed review was made on corrosion aspects of materials exposed to flue gas from biomass combustion. Based on this analysis five different heat exchanger materials were selected and tested in the laboratory: - three of them passed the tests successfully. Furthermore an optimal control strategy for the total system has been designed resulting in stable power production keeping voltage and frequency within the limits allowed for the grid.

The aim of another combined heat and power JOULE project was to establish a complete biomass fired co-generation plant (2MWe), which can be used for decentralised heat and power production. An electrical efficiency larger than 30% should have been reached by using an innovative 3-stage indirectly fired hot air process, however, due to several technical problems, the project could not be finished successfully.

A third THERMIE project demonstrates a combined heat and power plant, based on a "hot air turbine" (500kWe) fuelled by product gas from a biomass fluidized bed gasifier. The fluidized bed gasifier is designed for sawdust. Some natural gas topping combustion will be included to overcome the temperature limit of the metallic heater which is about 850C. Water injection in the air heater is included to enhance power output and to allow flexible power to heat ratios. Target maximum performances for the demonstration plant are 70% total and 24% electrical efficiency. When injecting water, peak electrical efficiencies approaching 30% have been observed.

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Heat and power generation using a biomass gasifier with a gas Otto engine

A demonstration plant of a biomass gasifier equipped with a 200kWe gas Otto engine has been realized in a further FAIR project. The gasifier has operated up to now for several days and further test runs will be carried out with different kinds of biomass. The plant is designed to use all sizes of biomass particles below 40 mm with variable water content.

3.6. Future prospects

3.6.1 Future R&D requirements within the scope of the 5' FP

Based on the problems mentioned in the previous chapters, future R&D requirements in the field of small-scale biomass combustion are as follows:

Advanced combustion air control systems, developed and success-fully used for medium and large scale biomass furnaces, should be down-scaled and optimised, for use with small scale furnaces with respect to reduction of costs.

Nitrogen oxide reducing measures, developed and successfully used for medium and large scale biomass furnaces, should be down-scaled and optimised for small scale furnaces with respect to reduction of costs. Low cost particulate filters for small-scale biomass furnaces should be developed.

Specifications of standard fuels for small-scale biomass combustion should be developed as a basis for new high quality bio-fuels for small-scale furnaces.

3.6.2 Future requirements within the scope of a liberalised energy market

As a general consequence of the liberalization of the energy market it is to be expected that consumer prices of the traditional energy carriers ("conventional" electricity, natural gas, fuel oil) will decrease. After all, this was the intention of the process. Thus it is obvious that the gap between the cost of "consumer generated" renewable energy and traditional energy will in-crease. Therefore, in order to support the goal set by the "White Book on Renewable Energy", namely to double the current share of renewables in the European Union, R&D efforts directed towards decreasing the cost of renewable energy sources have to be increased.

Regarding small-scale biomass combustion the direct consequences of a liberalized energy market may be as follows:

The liberalization introduced in the European electricity market is expected to lead to lower prices for industrial customers. There is some expectation that the price for private consumers will not decrease since they will not be able to take

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advantage of the new opportunities. Prices for private users might even increase since some utility companies might be forced to raise the price for private customers to compensate for possible losses in the industrial sector. Thus increased competition from electric energy is not to be expected for small-scale biomass furnaces.

A liberalized market for natural gas is currently in discussion within the European Commission. The consequences will probably be similar to what is expected for the electricity market.

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4 LARGE SCALE COMBUSTION

4.1 State of the art

4.1.1 Fuel specifications for large scale boilers

Biomass resources can be grouped into various categories such as: Wood residues (wood chips, forest residues, bark, black liquor)

Urban wood waste/demolition wood Agricultural residues (from crops, food processing and animals) Dedicated energy crops (Short Rotation Coppice-SRC)

Large volumes of wood residues are generated annually by the forest management and wood product industry including paper mills, sawmills and furniture manufacturing. This industry currently uses much of its own residue, but still there is a substantial amount available on the free market. Urban wood waste and demolition wood include such materials as tree trimmings, wood packing, waste wood from construction and demolition. Agricultural residue is another important biomass resource and straw in particular is a major contributor to biomass combustion in larger plants. Dedicated energy crops are the future largest source of biomass energy including both short-rotation woody crops and herbaceous crops. Table 4.1 gives an overview of composition properties of the most important biomass resources of interest in large-scale combustion systems. From Table 4.1 it can be seen that the proximate analysis varies a lot for the different biomass resources. The moisture content varies from 20% on average for demolition wood to 60% for grass. In addition there can be a great seasonal and geographical variation for all resources. As can be seen, the ash content also varies by a factor of 10. The higher heating value also varies by 25 to 30% between the different resources. The variation in the three major chemical components in the ultimate analysis (Carbon, Oxygen and Hydrogen) is in the order of 10% only, but the variation in nitrogen content can be more than a factor of 10, sulphur by a factor of approximately 30 and chlorine by a factor of up to 50. These great variations in moisture content, ash and chemical composition, especially nitrogen, sulphur and chlorine, give rise to large variations in combustion behaviour and pollutant emissions from combustion systems. This must be carefully considered in the design and operation of the furnaces and boilers in addition to the implementation of proper cleaning systems for a particular plant.

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Moisture Ash Flxed-C HHV LHV

Wood and wood residues Clean wood

50 1.3

13.2 19.2 15.4

Composition C H O N S CI

49.1 6.00 4 4 3 0.48 0.01 0.10

Bark from spruce 55-65 2.34

22.46 19.83 18.54

51.1 6.04 42.4 0.41 0.03 0.03

Agricultural residues Straw Grass

from wheat Reed canary 55 60

4.71 B.B5 17.59 17.65 18.94 18.37 17.65 17.13

4 9 6 49.4 6.16 6.25 43.5 42.7 0.61 1.54 0.07 0.15 0.18 0.07

Waste-/demolition wood 15-20

0.9

15.4 13.9

48.8 5.25 45.6 0.15 0.03 0.08

Energy crops Salix

50 1.18

18.92 19.75 18.42

50.3 6.17 43.1 0.40 0.03

0.004

Unit

wt% of wet fuel wt% of dry fuel

MJ/kg MJ/kg wt% (daf)

Table 4.1 Proximate and ultimate analysis of typical biomass fuels for large-scale applications [1,2].

Viewing the chemical composition of biomass compared to other solid fuels in a Van Krevelen diagram, the biomass fuels are high in the O/C - and H/C - ratios compared to peat and coals (Figure 4.1). These high ratios are responsible for the biomass fuels being more volatile than coals and peat, and high O/C - ratios give lower heating values for the biomass fuels.

0.4 0.6 Atomic O : C Ratio

Figure 4.1 Van Krevelen diagram for various solid fuels [3],

Table 4.2 gives an overview of the properties of biomass ashes with special emphasis on erosion and corrosion potential of these ashes. Fuels having the ratio (Na2 + K2OVSO2 above 2 require special precautions to avoid fouling problems. If the ratio is below 0.2, then precautions must be taken to avoid erosion which can occur from the high silica content. The range in ash content and composition for

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bituminous coal is also given in the table for comparison. When co-firing, differences in the slagging and fouling characteristics for the blend of biomass and coal must be considered in addition to the composition of the ash for use as a by-product in concrete applications.

Potential erosion a

Rice husk Bagasse, Hawaii Construction wood Wheat straw Rice straw Pine bark

Demolition wood Whole tree average Manure Western hog fuel Softwood bark Jack pine Eastern hemlock Almond shells Oat straw Tree prinsings Walnut shells

Potential fouling a

Cotton stalks Hardwood bark Oak Sunflower seed husks Cotton gin trash b

Annual ryegrass b

Bituminous coal

AshC)

23.4 3.5 3.4 8.9

13.4 3.0

4.9 1.3

34.9 0.5 2.0 2.1 2.5 4.8 4.0 2.0 0.6

4.6 3.4 1.6 4.2 9.4 5.0

5-13

Fe203

0.10 14.80 4.23 1.50 0.30 3.00

6.22 3.99 1.88 4.41 5.00 5.00 1.30 3.77 0.50 1.94 2.40

0.50 0.60 3.40 0.60 3.30 0.20 5-35

Acids Al203 P205 T i0 2

2.00 15.30 3.50 12.55 1.35 0.59 2.00 3.30

14.00

6.03 0.88 8.87 3.03 6.06 5.52 0.25 2.31 0.01 6.30 6.30 0.20 2.10

0.80

0.80 0.50

0.10 5.30

10-35

CaO

0.20 1.92

10.87 5.00 2.00

25.50

15.96 23.43 13.05 25.37 57.00 51.60

5.60 12.27 12.30 19.90 7.00

16.40 77.00 26.00

9.20 16.10

3.80 1-20

MgO

0.20 2.21 2.69 3.60 2.80 6.50

3.76 4.59 4.45 7.62 5.50 5.50

13.10 2.49 3.00 8.30 3.65

5.20 1.90 2.90 7.20 8.00 1.90

0.3-4

Bases Na20

0.10 0.86 4.71 0.30 1.10 1.30

3.87 1.62 4.85 5.64 3.10 3.10 1.10 508 0.30 1.48 1.08

2.00 3.90 1.60 0.40

4.10 1^1

K20

1.20 3.52 5.55 6.60 8.00 6.00

2.41 10.48 12.29 9.26 4.10 4.10 4.60

14.14 40.30 12.66 21.50

30.00 7.20

42.00 39.30 11.30 14.00

S i0 2

95.60 54.00 53.56 78.20 79.80 39.00

41.21 34.81 41.99 35.18 16.00 16.00 10.00 22.60 37.30

9.95 13.60

8.40 1.50 5.50 1.70

40.70

Ratio '

0.01 0.08 0.09 0.09 0.11 0.19

0.23 0.35 0.41 0.42 0.45 0.45 0.57 0.85 1.09 1.42 1.66

3.81 7.4

7.93 23.35

20-60 Fuel types of a ratio [(Na>0 + KO)/SiO.>] above 2 require special precautions to avoid fouling problems. If the ratio is below 0.2, the erosion may occur unless precautions are taken. Insufficient information to calculate the ratio.

Table 4.2 Biomass ash characteristics [4].

Biomass resources have normally high bulk volumes compared to oil and coal which limit the transport distances for these fuels. Table 4.3 gives an overview of bulk volumes and bulk densities for various biomass re-sources. The table shows that there is even a large variation between the various biomass resources as well, up to a factor of 50. Based on biomass properties like particle size, moisture content and bulk volumes, various feeding systems can be used. Different combustion technologies can, to various degrees, handle the high moisture content and be flexible regarding particle size. Typically, feeding systems include conveyor belts, drag chain conveyor, hydraulic pistons, screw feeders and sluice feeders.

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Bulk volume Bulk density (m3Aon daf) (kg/m3)

Wood Har