ON-LINE QUALITY CONTROL OF SOLID FUELS · PDF fileFUELS DETECTION AND SEPARATION TECHNIQUES ....

VATTENFALL RESEARCH AND DEVELOPMENT AB Repor t Number

2007-08-22

Mats Norberg

ON-LINE QUALITY CONTROL OF SOLID FUELS

DETECTION AND SEPARATION TECHNIQUES

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On-line quality control of solid fuels, detection and separation techniques

From Date Serial No.

Vattenfall Research and Development AB, Com-bustion and process control

2008-03-26

Author/s Access Project No.

Mats Norberg Full Access

Customer Reviewed by

Marcus Svanberg

Issuing authorized by

Magnus Berg

Key Word No. of pages Appending pages

Fuel quality, Separation, Detection 51 15

Summary

A project to investigate techniques and systems for fuel quality control was started due to in-

creasing problems with polluted solid fuel (coal, peat and bio-fuels) fired in power plants

owned and operated by Vattenfall. The project was carried out at Vattenfall Research and de-

velopment as a master’s thesis the year 2007. A background study of fuels and the different

fuel preparation processes was performed to gain knowledge about the cause to problems with

polluted fuels. This shows that the fuel handling is responsible for much of the contamination.

The first literature study of both old and new detection and separation techniques gave some

interesting facts, which pointed out some interesting techniques to investigate further. All in-

vestigated techniques have been described in the report, which were equipment for magnetic

separation (suspension magnets and magnetic drums), non-ferrous material separation (eddy

current separators), induction separation systems (electromagnetic sensors) and X-ray systems.

Additional research and laboratory tests resulted in conclusions that electromagnetic sensors

and dual energy x-ray transmission systems that are not used commercial in the fuel quality

control today have a potential to increase the fuel quality. These systems are used in industry

environments much like coal handling for example, which would make it easy to implement

these systems into the fuel quality control. Three suggestions for fuel quality control systems

are purposed. The systems differ depending on which contaminants are removed. Two systems

for metal contaminants and two systems for contaminants with non-metal basis where the effi-

ciency was considered for a standard sized Swedish power plant. As a future suggestion, per-

forming a full-scale test with a complete fuel quality control system to gain knowledge about

the system accuracy and its effectiveness is recommended.

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Distributionlist Company Department Name Number of

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Table of Contents Page

LIST OF ABBREVIATIONS VII

1 INTRODUCTION 1

1.1 Background 1 1.2 Purpose of the project 2 1.3 Material & methods 2

2 REVIEW ON FUELS AND THE PRE-TREATMENT 3

2.1 Fuel contaminants 3 2.2 Coal 4 2.3 Coal preparation 5 2.4 Peat 5 2.5 Peat preparation 6 2.6 Wood 6 2.7 Straw 7 2.8 Refuse-derived fuel – RDF 7 2.9 Pellets production 8 2.10 Briquettes production 10

3 METHODS FOR REMOVAL OF CONTAMINANTS 11

3.1 Ferrous metals 11 3.1.1 Suspension magnets 11 3.1.2 Magnetics drums 12

3.2 Non-ferrous metals 13 3.2.1 Eddy Current separators 13 3.2.2 Metal detectors 16 3.2.3 Electromagnetic separation systems EMS and PEMS 17

3.3 Non-metals (rocks, plastics, glass, pottery and fibrous material) 18 3.3.1 Dual energy x-ray transmission 18 3.3.2 X-ray Fluorescence (XRF) 21 3.3.3 Prompt Gamma Neutron Activation Analysis (PGNAA) 22 3.3.4 Colour sorting 23

3.4 Summary 24

4 RESEARCH, TESTS AND RESULTS 28

4.1 Introduction 28 4.2 De-stoning of coal using an EMS system 28

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4.2.1 Experimental setup 28 4.2.2 Results 29 4.2.3 Analysis of results 31

4.3 Processing ROM and residual mine reject with X-ray technology 31 4.3.1 Experimental setup 32 4.3.2 Results 32 4.3.3 Analysis of results 34

4.4 Practical fuel test in cooperation with Steinert 34 4.4.1 Introduction/background 34 4.4.2 Method/experimental procedure 35 4.4.3 Results 38 4.4.4 Analysis of results 38

5 DESIGN OF THE FUEL QUALITY CONTROL SYSTEM 39

5.1 Reference power plants 39 5.2 Performance, capacity and investments costs 40 5.3 Design suggestions for quality control systems 41 5.4 Different ejection devices for EMS and DE -XRT systems 45

6 DISCUSSION 46

6.1 Studied techniques 46 6.2 EMS and DE -XRT technique in automatic separation systems 46 6.3 Uncertainties 47

7 CONCLUSIONS 48

8 FUTURE WORK 49

9 REFERENCES 50

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Appendices Number of Pages

APPENDIX A1 – Eddy Current measurement theory

A2 – X-ray transmission thickness independency theory

A3 – Report of the results from Steinert

A4 – Drawings of dual energy X-ray transmission system

A5 – Drawings of electromagnetic sensor system

A6 – Drawings of eddy current separation system

A7 – Drawings of suspension magnets

A8 – Drawings of magnetic drums

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2

2

2

2

2

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List of abbreviations AC – Alternative current DE-XRT – Dual Energy X-ray Transmission EC – Eddy Current ECS – Eddy Current separator EMS – Electromagnetic sensors MSW – Municipal solid waste RDF – Refuse derived fuel ROM – Run of mine A – Amplitude B – Magnetic flux density H – Magnetic field strength φ - Magnetic flux σ – Electric conductivity µ - Magnetic permeability δ - Depth of penetration

Vattenfall Research and Development AB

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1 Introduction

1.1 Background

It is a general fact that solid fuels used for heat- and electric production may contain high concentrations of different types of pollutions, and that these pollutants can gen-erate chemical, physical and environmental problems. Physical problems are usually connected to some sort of solid metals, for example nails. Today there are certain limi-tations in the fuel quality control systems. It is only possible to detect objects on the metal basis. Other non-metal objects (rocks, glass, concrete and plastics) pass through the system undetected. Chemical and environmental effects are often connected with high concentrations of alkaline metal, phosphor, chlorine and arsenic. Co-firing coal with biomass provide additional undesired effects especially if both fuels are prepared (pulverized) in the same process. As an example mill problem like fires caused by sparks from metal to metal contact could ignite the biomass, which is not the case with pure coal. Several reports of mill problems have come up the last couple of years. A fire caused by sparks, excessive high wear from metal and hard objects and sticky threads from fibrous materials that rolls up around wheels and axels causes hold-ups that result in energy production losses with consequences of lower profits and higher expenses in maintenance. Excessive wear must be minimized specially in expensive equipment. Fuel quality control will become even more important as the restrictions around land filling of waste increase. The use of waste fuels for power generation, e.g. MSW (mu-nicipal solid waste) and RDF (refuse-derived fuel), is therefore expected to increase. RDF exists compressed as pellets and bricks, which later can be grinded into powder, and this feature makes RDF fuels interesting as a secondary fuel used in pulverized fuel boilers, fired separately or co-fired with coal on a regular basis. This argument indicates that the demand on fuel quality control will increase further.


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1.2 Purpose of the project

The overall purpose with the project is to find technical ways to improve the present conventional systems for quality control of solid fuels used in pulverized fuel fired boilers owned and operated by Vattenfall. The focus should be on modern techniques that can handle both metal and non-metal contaminants. The project goals can be di-vided into the following parts:

• Make a review of conventional systems for fuel quality control as well as the latest development within this field focusing on their pros and cons and tech-nical performance.

• Make a more detailed evaluation of the technical performance of the most in-teresting techniques by performing practical tests as well as a more thorough literature study.

• As a desktop study, present a design of a new fuel quality control system ca-pable of detecting and separating a broad spectrum of contaminants, including an indication of its technical performance and a rough economical estimate of the necessary investment costs of such a system.

1.3 Material & methods

To achieve the goals of the project the following strategy has been used. • Literature study using different scientific databases and technical journals for

the collection of information about existing techniques and earlier research re-sults.

• Contacts with leading manufacturers, research teams at European universities and fuel sales agents.

• Practical testing with three different solid fuels has been performed in labora-torial scale in cooperation with a manufacturer of specific separation devices.


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2 Review on fuels and the pre-treatment

2.1 Fuel contaminants

Today’s fuel market is spread worldwide. Within Vattenfall many different fuels are used and they are produced and imported from different areas of the world. The fuels being used are coal (both lignite and hard coal) and biomass as peat, wood (br iquettes and pellets), demolition wood and straw (pellets). Also coal and peat can be bought as pellets or briquettes. It is interesting to look into the different fuel production processes to see if there are any measures taken to minimize contaminants such as metal, rocks and other foreign materials in the raw material to increase the quality of the outgoing fuel. One aspect that could motivate a fuel supplier to these kinds of actions is probably if there are expensive equipments that can take harm in the preparation process. Customer de-mands are not expected to have the same influence if the competition of fuels is large and hence it is probably easy to find other buyers. There are many different materials considered as contaminants. The definition is al-most the same for every fuel with some difference depending on how the pre-treatment is done and what equipment that has been used. It also depends of which fuel that is considered as polluted. Metals, rocks, brick, concrete, plastics and rubber objects is always considered as contaminants while wood is a contaminate depending in which fuel it occur (pressure impregnated wood is always a contaminant). Contaminant in fuel doesn’t have to come from the fuel raw material, e.g. the mining of coal, the harvesting of peat and crops, the forest industry or sawmills. A known example is transportations with boats where coal is transported one way and scrap metals are loaded for transport the other way, where lack of accuracy when unloading the scrap metal before loading new coal results in mixing scrap metal with coal. An-other known example is when containers dedicated for fuel transport are left open and unsupervised, it is a chance for some people to get rid of their junk. In the forest in-dustry tools and metal parts splitting loose from cutters in the chipping process can come into the wood. Sawdust and wood chips can be contaminated when it is col-lected from the factory floor. Figure 2.1-1 shows some contaminants that med ejected by the fixed suspension mag-net at pulverized power plant owned and operated by Vattenfall in Uppsala.


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Figure 2.1-1: Rejected metal contaminants at Uppsala KVV

2.2 Coal

China is the largest producer of coal in the world, while the United States contains the world's largest coal reserves. China and the US are also among the largest coal consumers. Other important coal producing countries include: Australia, India, South Africa, and Russia. Figure 2.2-2 shows the top ten coal producing and consuming countries. 67 % of the world coal is located in United States, Russia, China and India [1]. Other countries with significant coal reserves are New Zealand and Canada. There is also smaller coal reserves spread wide in Asia, Africa, Central- and South America. Even in Europe there are smaller amounts of coal. Poland is one example. Vattenfall import coal mainly to power plants in Germany, Denmark, while Poland uses mostly domestic coal.


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2.3 Coal preparation

Raw coals usually go through a physical cleaning process to decrease the amount of sulphur and other non-combustible mineral matter. Major physical cleaning processes are based on differences in density, where hydro-cyclones are used. Chemical cleaning processes are under development, but their performance due to the costs is discussable at this stage. It is also common to prepare coal as briquettes where the cleaning proc-esses already are included. Hydro-cyclones are based on the use of upward currents of pulses of a fluid such as water or sand to fluidize a bed of crushed coal and pollutants. The lighter coal parti-cles rise to the surface and are removed from the top of the bed. The heavier particles sinks and are removed from the bottom. Afterwards the coal must be dried to prevent freezing problems and to increase the heat content if water was used, which is usually done by using hot steam or other hot gases [9]. Dry cleaning with sand is not used in the commercial industry scale yet.

2.4 Peat

Peat deposits are found at many locations around the world, notably in Russia, Belarus, Ireland, Finland, Estonia , Scotland, Poland, northern Germany, the Netherlands and Scandinavia , and in North America, principally in Canada, Michigan,

Figure 2.2-1: World coal production and consumpsion [1]


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the Florida Everglades, and California 's Sacramento-San Joaquin River Delta. Approximately 60 % of the world's wetlands are peat. Peatlands cover a total of around 3 % of global land mass or 3,850,000 to 4,100,000 km². About 7 % of this total has been exploited for agriculture and forestry, with significant environmental repercussions. Under proper conditions, peat will turn into lignite coal over geologic periods of time [2]. There are basically two kinds of peat used as commercial fuels, sod peat and cutter peat. Pellets or briquettes are produced from cutter peat. Vattenfall imports peat briquettes to Swedish power plants mainly from Belarus.

2.5 Peat preparation

There are basically two different methods for peat digging, one to get sod peat and another for cutter peat. Sod peat is taken from the ground at about 1 meters depth. A scraper with a line of cylindrical tubes is put into the ground. As the scraper is drawn forward the peat is pressed through the tubes and forms pistons. Usually there are no further pre-treatment except for drying of sod peat before it is finished fuel. Cutter peat is produced from the top peat layer where a cutter is used to gather peat. The peat is turned around and left to dry for some time before it is collected and transported for further pre-treatment in some cases before it is used as fuel [2]. Cutter peat can be turned into briquettes or pe llets, but it can also directly be used as fuel without any pre-treatment except for drying [8].

2.6 Wood

There are different types of wood fuel such as pellets, briquettes, chips and sawdust. These fuels can be produced wherever there are forests or field land where grains can grow. It is common that wood fuels are consumed near to the production site due to expensive transportation costs. Only pellets have the economics for long distances transports since the heating value is high enough. There is a large trade of pellets in and near to Europe as shown in Figure 2.6-1. In Sweden Vattenfall uses demolition wood mostly produced locally but also signif i-cant amounts are imported from Norway, Finland, Estonia and Latvia. Wood pellets are imported to Denmark from Latvia and Lithuania. Short distance transport over the Baltic Sea is possible with smaller vessels and is not as expensive compared to long distance transports with large vessels [3].


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Figure 2.6-1: Flow of pellets in Europe [4]

2.7 Straw

The use of straw as a carbon-neutral energy source is increasing rapidly. Straw is a good alternative to wood as raw material for pellets production when the demand for wood increases. Straw grows fast and is relatively easy to handle. In Denmark (Köge), Vattenfall produces their own pellets from straw for use in their near located power plants.

2.8 Refuse-derived fuel – RDF

Refuse-derived fuel or solid recovered fuel is a fuel processed from municipal solid waste (MSW). It is processed by either shredding MSW or by steam pressure treat-ment. The fraction of which RDF is processed is the organic components of MSW such as plastics and degradable bio waste. Noncombustible materials are removed during pre-treatment with mechanical separation processing. The RDF fraction can be sold directly after shredding or it can be compressed into pellets, bricks or logs.


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The production of RDF may involve some of the following steps: • Preliminary liberation • Size screening • Separation of magnetic content • Coarse shredding • Refined separation • Processing for easier handling (pelletizing)

RDF can be used in a variety of ways for energy production. It can be used as a com-plement to traditional fuel sources, coal power plants is one example. RDF can also be used in gasification and pyrolysis plants to produce synthetic gases [12]. RDF is co-fired with lignite in the Vattenfall power plant Jänschwalde in Germany.

2.9 Pellets production

Pellets for firing purposes are usually produced from wood, straw and peat but can be produced from many different raw materials. Figure 2.9-1 shows how pellets look like.

Wood pellets are the most common pellets, generally made from compacted sawdust that usually are produced as a byproduct of sawmilling and other wood shredding activities. The pellets are extremely dense and can be produced with a low humidity content (below 10 %, depending on which raw material that is used) that gives them high energy content. Further, their regular geometry and small size allow automatic feeding with very fine calibration. They can be fed to a burner by auger feeding or by pneumatic conveying.

Their high density also permits compact storage and rational transport over long distance. They can be conveniently blown from a tanker to a storage bunker or silo. As the price of heating with fossil fuels increases, more capacity for pellet heating has been installed. A large number of models of pellet stoves, central heating furnaces and other heating appliances have been developed and marketed since about 1999. With the surge in the price of fossil fuels in 2005, the demand has increased all over Europe and a sizable industry is emerging [5].

Pellets are produced by compressing the material after it passed through a hammer mill to provide a uniform dough-like mass which is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press increases the temperature of the material greatly, in wood the lignine plasifies slightly forming a natural “glue” that holds the


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pellet together as it cools [6]. If other materials without lignine are used some extra glue has to be added.

To protect the pellet press from contaminants such as metal, rocks and other hard material, the first thing to do is to screen the raw material with a sifter device. Stone traps for rocks [7] and magnet for the removal of metal [6]. Foreign objects can be separated by density in the drying process before the grinding take place if an air cyclone is used [8]. The removal efficiency of foreign objects depends on which raw material that is used. For example, coal is heavier than wood chips, sawdust or grass thus it is harder to use density separation on coal. Pellets normally have less than 10% water content, are uniform in density and have low dust and ash content.

Figure 2.9-1: Wood pellets [6]


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2.10 Briquettes production

Briquettes can easily be described as larger pellets normally 50 to 200 mm in diameter. They can be produced from wood, peat, coal and many other materials. Mostly the material is just crushed into large fractions and no milling is performed. The material is put into press machines to become briquettes. If wood is used there is no need for any “extra glue” to hold the briquettes together because of the lignite that provides the same effect as in pellet production [6]. It is usual to have a combined pellet and briquette production process where the larger fractions becomes briquettes which will decrease demand of only fine fractions from the mill and less regrinding is necessary [6]. Same measures as in the pelle ts process are usually taken to protect the press equipment. The heating value for 1 kg of briquettes is almost the same as for 1 kg of pellets when the same material is used but briquettes have lower density and are hence more expensive to transport. Figure 2.10-1 shows one type of briquettes.

Figure 2.10-1:Wood briquettes [Google pictures]


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3 Methods for removal of contaminants Many detection and separation techniques are known and available on the market. The difference is in the matter of what kind of contaminants each technique is able to de-tect and also how the separation of pollutants is done. Generally the techniques could be divided into the following categories:

• Ferrous metals (metals responding to magnetism). • Non-ferrous metals (metals low responding to magnetism). • Materials that are non-metallic with have low response to magnetism and are

low conductive. For ferrous metals there are basically two methods that can be applied on a running conveyor belt, suspension magnets and magnetic drums. In the second category it is sometimes possible to use Eddy Current separators or more commonly metal detectors connected to detached separation equipment. In the third category there are not any existing industry scale techniques that have been proven to work for every material considered as contaminants. There are some techniques measuring with x-ray that are used in other industries to detect and sort different materials and additional informa-tion can be found in the waste recycling industry.

3.1 Ferrous metals

This chapter will describe suspension magnets and magnetic drums. If magnetic sepa-rators are installed in the fuel feed system in a power plant it can be worth to try to tune the magnets or in an existing installation upgrade them. The development of magnetic separators has resulted in much stronger magnets compared to 10 or 20 years ago.

3.1.1 Suspension magnets

Application areas: • Removing ferrous metals from a flow of material

A suspension magnet is either an electric or permanent magnet. Suspension magnets usually are mounted at a fixed working distance above and sometimes also across a conventional belt conveyor. Iron in the material conveyed is attracted by the magnet and so removed from the flow of conveyed material. Self-cleaning suspension magnets (UM) remove the separated iron on a moving conveyor belt, which of the magnet is placed inside. The pure suspension magnets (AM) are manually cleaned at defined intervals. These magnets are used where the amount of iron involved is small, while self-cleaning suspension magnets are better


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suited to higher concentrations of iron components. If suspension magnets are arranged longitudinally, above the belt discharge, then the discharge is more reliable and can, in some cases, be achieved with the use of smaller magnets. If the separator is arranged transversely (across) above the conveyor belt, the conveyed material remains unaffected. A larger magnet is required in these circumstances. The flow of material can, however, often is improved. Figure 3.1-1 illustrates how the suspension magnet can be placed above the conveyor belt. This technique is most suitable for larger magnetic particles. The surface of small particles is too small to be attracted by the magnetic force. This is a well-known technique that is applied in many industries for removal of magnetic materials from different bulk flows where MSW is one example.

Figure 3.1-1: Setup of a suspension magnet [17]

3.1.2 Magnetics drums

Application areas: • Removing ferrous metals from a flow of material

A magnetic drum is mainly a rotating cylinder with a fixed permanent or electrical magnet on the inside. One half of the drum is magnetised while the other half is neu-tral. The setup is usually that the bulk material drops down from a conveyor belt onto the drum and then further down to another conveyor belt. Magnetic particles sticks to the drum until the drum have rotated enough and reached the neutral side where it falls off into some sort of dump. Figure 3.1-2 shows the setup with a magnetic drum. This technique is suitable even for separation of smaller magnetic particles from non-magnetic materials.


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Figure 3.1-2: Setup of a magnetic drum [17]

3.2 Non-ferrous metals

This chapter will describe the principle of the Eddy Current (EC) and how it is used. Systems using the EC principle are Eddy Current separators (ECS), metal detectors and electromagnetic sensors (EMS). The physics behind electromagnetic measurement is explained in Appendix 1.

3.2.1 Eddy Current separators

Application areas: • Metal cleaning of scrap wood • To recover metal packaging from municipal waste • Other areas of application include compost, glass, and paper processing and

the recycling of refuse incineration ash – for both the maximum recovery of valuable metals and for metal-free products.

The principle of the Eddy Current separator is basically a rotor comprised of magnetic blocks (standard ferrite ceramic or more powerful earth magnets depending on appli-cation) spun at high revolutions (over 3000 rpm) to produce an “eddy current” from the induction of a magnetic field in conducting materials. This eddy current can only exist if the magnetic field changes with time or if the conducting material moves rela-tive the field. A repelling force called “Lorentz force” is created between the eddy current and the charged particle. The strength depends on the particle charge, which is affected of the ratio between the specific mass and resistivity of the particle. High resistivity and low specific mass increases the charge and hence the force which makes the separation easier. This is confirmed by equation 3.2-1 that is used to calcu-late the Lorentz force. The ratio between resistivity and specific mass is also a separa-


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tion factor of how easy a specific material can be removed. It is used to compare the possibility to remove different conductive materials, usually metals. Aluminium should be easier to remove compared to lead as is shown by the separation factors in Table 3.2-1.

( )BvQF ×= Equation 3.2-3.2-1 ( )BvBvQF ,sin=

:F Lorentz force Q: electric charge of the material (i.e. the material properties) :v velocity vector, i.e. the vector that the mate rial moves with through

the magnetic field :B magnetic field vector ( )Bv,sin : is the sinus of the angle between the

magnetic field and the Velocity

Table 3.2-1: Material properties for some non-ferrous metals

Metal type Electrical resistivity ? [10-6 1/(? cm)]

Density ? [g/cm3]

Ratio ?/? [10-4 m3/(? cm kg)]

Aluminium 0,35 2,7 13 Copper 0,59 8,9 6,7 Silver 0,63 10,5 6,0 Zinc 0,17 7,1 2,4 Brass 0,14 8,5 1,7 Tin 0,09 7,3 1,2 Lead 0,05 11,3 0,4 353 MA (Stainless steel)

0,01 7,9 0,13

In the setup this rotor is placed inside the last rotating drum at the end of the conveyor belt. Conducting materials will be pushed away from the bulk flow. How far they will be thrown depends on the speed of the magnetic rotor and the material separation fac-tor, which also make it possible to sort the waste into different valves (splitters). The setup is shown in Figure 3.2-1. Rotors can be mounted in two ways inside the drum, concentric or eccentric, see Figure 3.2-2. The difference is the angle in which the magnetic field will work. Con-centric placement gives a wider angel in which the magnetic field will act. The risk that magnetic particles follow with the drum by sticking to it and hence causes dam-ages on the inside of conveyer belt increases. With eccentric placement the angel within whom the magnetic field acts is smaller compared with concentric placement but manufacturers say that the separation will be more precise [11]. Periodic adjust-


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ment of the rotor placement could be necessary with eccentric rotor placement. The risk for particles sticking to the drum due to magnetism is lower. In both cases it is an advantage to remove ferrous (magnetic) metals before the eddy current separator due to the heating that can occur in ferrous metals when they are exposed to a magnetic field. Consequences can be damages to the conveyor belt. There is a demand for thin and even materia l distribution on the conveyor belt for good separation. Other factors that have influence on the separation efficiency are listed in Table 3.2-2.

Table 3.2-2: Factors with influence on the separation efficiency

Material properties System settings Particle shape Speed on the conveyor Particle size Rotational speed of the magnet Particle distribution in flow Material flow, the thickness of the layer Electrical conductivity Feeding device Density The setup of splitters Moisture content Magnetic field strength Stickiness Proportion of metal in flow

Figure 3.2-1: Setup of an eddy current separator [17]


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Figure 3.2-2: Setup with concentric and eccentric placement of the rotor [11]

3.2.2 Metal detectors

Application areas: • Used to secure a metal free bulk material

A metal detector works with induction of a magnetic field from one or more inductor coils. A pulsing current is applied to each coil, which then induces a magnetic field that interacts with metallic elements beneath. When metal get induced with electric currents, it creates its own magnetic field, which generate an opposite current back into the coils. This current is the signal that indicates the presence of metal. The illus-tration of the detection of a coin is shown in Figure 3.2-3. Some positive and negative aspects are that all kinds of metals in the bulk flow can be detected but there is no automatic separation.

Figure 3.2-3: The magnetic fields of a metal detector that detects a coin [20]


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3.2.3 Electromagnetic separation systems EMS and PEMS

Application areas: • Recycling of demolition wood • Recycling of scrap metal • Automatic cleaning of glass from metal

In an electromagnetic separation system (EMS) it is the output voltage from the re-ceiver at a specific frequency that is used to identify the measured material. If the ma-terial is classified as a contaminant a signal is used to activate the separation device. There are two electromagnetic systems, the EMS using an alternating magnetic field and the PEMS in which a pulsed magnetic field is applied. The main difference is that the PEMS has the advantage of better identification of low conductive materials [11]. Furthermore the signal of an electromagnetic sensor is insensitive to moisture, dust and other surface contamination. The separation device setup can look different; hatches for dumping material or for example air nozzles like the setup in Figure 3.2-4. Separation with air nozzles can be done selectively with the use of many parallel air nozzles to cover the whole width of material on the conveyor. Air nozzles can represent loss of fuel and problem with dust formation. The amount of nozzles is directly connected to the amount of lost fuel dur-ing separation. Selective separation is possible if the receiver consist of an array of detector coils. A computer able to handle the signals will know where on the width of the conveyor belt the contaminated particles appear. The speed of the conveyor together with the parti-cle position makes the separation possible with good precision. The fuel needs to be pre-treated (crushed) before it goes through scanning. Some variation in the fraction size is allowed but with increasing variation in fraction size the separation accuracy decreases. It is harder to optimize the air nozzles for a wider fraction interval. There is also a demand for thin and even material distribution on the belt to reach optimal working conditions.


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Figure 3.2-4: The setup of an EMS system using air nozzles for separation [11]

3.3 Non-metals (rocks, plastics, glass, pottery and fibrous material)

It is hard to detect non-metal contaminants because of non-magnetic characteristics, low resistivity, and low conductivity. Measurement techniques must be based on dif-ference in density, colour and other particular characteristics. This chapter describes the Dual energy X-ray transmission (DE-XRT) method, which is based on absorption of x-rays, X-ray fluorescence and the prompt neutron gamma activation analysis method. Colour sorting is also described.

3.3.1 Dual energy x-ray transmission

Application areas: • Recycling and cleaning of crushed glass • Recycling of material with a specific material composition • Mineral processing • Sorting of scrap metal (heavy and light fractions)

The X-ray sorting system greatly increases the range of possibilities for sorting mate-rial mixtures. It can “see” through the materials, recognizing different material dens i-ties, components containing halogens, and organic components. Composite materials and internal joins can also be recognised. This enables light metals to be sorted from heavy metals, for instance, or PVC from plastics, scrap wood from stone, aluminium castings from wrought alloys, and ore-bearing mineral blocks from country rock. It handles grain sizes from 5mm and normally up to 200mm but sometimes even larger grains.


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Material measurement based on the atomic density, which is determined with x-ray technique is a relative new method. X-ray transmission produced from a radiation source is irradiated through the material. The non-absorbed radiation through the ma-terial is measured with a line sensor on the other side of the material. Measurement setup is shown in Figure 3.3-1. Distinction of waste material from good material in the bulk flow is based on analysing the material dependent absorption coefficient. The method is independent of the material thickness since x-ray transmission from two radiation sources at different energy stages is used (this is explained in Appendix 2). Signal data obtained from the sensor output is evaluated in real time by a high-powered computer. The results are transmitted as the separation decision that can be used in external separation equipment. Good equipment for the separation part is usu-ally some sort of pneumatic devices, an array with air nozzles where each nozzle can be activated selectively as described in section 3.2.3. One manufacture setup is shown in Figure 3.3-2. Advantage with x-ray technique is that organic compounds can be distinguished from inorganic compounds. Non-metal contaminants can be detected; for example rocks, glass and PVC plastics. Additional advantages are that the DE-XRT system has the potential to perform on-line data regarding composition (ash content) and particle size distribution. A disadvantage is that this measuring technique is a newly developed method that has been adopted by the industry only 1.5 years ago. References from known and tested applications are sparse. There is little information about what kind of contaminants this technique can detect in different materials. Varying results would be expected depending of which bulk material is used. The sensors capacity in scanned material per hour is not high compared to the fuel consumption of large power plants. The effectiveness depends on the particle size. It is probably not possi-ble to detect thin materials like plastic bags etc. because of low radiation absorption [11] & [14].


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Figure 3.3-1: Setup with dual energy x-ray sources and line se nsors [18]

Figure 3.3-2: X-ray measurement with pneumatic separation [21]


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3.3.2 X-ray Fluorescence (XRF)

Application areas: • Sorting of scrap metal, glass, plastic, electronic waste and wood • Waste to energy sorting (reduce the part of MSW for land filling) • To identify/find valuable ores in the mining industry

In X-ray Fluorescence (XRF) Spectrometry, high-energy primary X-ray photons are emitted from a source (X-ray tube), which strikes the sample. The primary photons from the X-ray tube have enough energy to knock electrons out of the innermost, K or L, orbitals shown in Figure 3.3-3. When this occurs, the atoms become ions, which are unstable. An electron from an outer orbital, L or M, will move into the newly vacant space at the inner orbital to regain stability. As the electron from the outer orbital moves into the inner orbital space, it emits an energy known as a secondary X-ray photon. This phenomenon is called fluorescence. The secondary X-ray produced is characteristic of a specific element. The energy (E) of the emitted fluorescent X-ray photon is determined by the difference in energies between the init ial and final orbitals of the individual transitions. This is described by the formula in equation 3.3-1.

1−= λhcE Equation 3.3-1,

where h is Planck's constant; c is the velocity of light; and ? is the characteris-tic wavelength of the photon.

Energies are inversely proportional to the wavelengths; they are characteristic for each element. For example the Ka energy for Iron (Fe) is about 6.4keV. Typical spectra for EDXRF Spectrometry appear as a plot of Energy (E) versus the Intensity (I).

Figure 3.3-3: The nuclear with its electrons and orbital’s [23]


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XRF Spectrometry is the choice of many analysts for elemental analysis. XRF Spectrometry easily and quickly identifies and quantifies elements over a wide dynamic concentration range, from PPM levels up to virtually 100% by weight. XRF Spectrometry does not destroy the sample and requires little, if any, sample preparation. It has a very fast overall analysis turnaround time. These factors lead to a significant reduction in the per sample analytical cost when compared to other elemental analysis techniques [23] & [24].

The biggest disadvantages is that XRF can only measure the top layer and therefore the thickness of the bulk flow must be thin, otherwise particles in the bottom will not be measured. Too receive a high throughput the layer thickness must be compensated with a wider conveyor width compared to X-ray transmission for example.

However XRF is used in the industry for scrap sorting of metal, plastics, glass, wood and waste to energy sorting where the fractions of plastic, metal and glass material is removed from MSW [23].

3.3.3 Prompt Gamma Neutron Activation Analysis (PGNAA)

Application areas: • Determine the elements and ash content in bulk materials • Elemental analysis in mineral processing (mining)

In chemistry, neutron activation analysis is a technique used to very accurately determine the concentrations of elements in a sample. The particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts. The sample is introduced into the intense radiation field of a nuclear reactor. The sample is thus bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. Neutron Activation Analysis is a sensitive multi-element analytical technique used for both qualitative and quantitative analysis of major, minor, trace and rare elements. NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions. To carry out an NAA


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analysis the specimen is placed into a suitable irradiation facility and bombarded with neutrons, this creates artificial radioisotopes of the elements present. Following irra-diation the artificial radioisotopes decay via the emission of particles or more impor-tantly gamma-rays, which are characteristic of the element from which they were emitted [25]. This course of events is shown in Figure 3.3-4.

Figure 3.3-4: The course of event in neutron activation analysis [27]

PGNAA exist as on-line measuring method for elemental analysis in the industry. It is applied on both coal and other minerals such as limestone and iron ore. The disadvan-tage with this method of a fuel quality control view is that PGNAA measures an aver-age of the material composition and is unable to identify single contaminants. It can for example measure the average ash content in coal that passed the last 5 minutes [26].

3.3.4 Colour sorting

Application areas: • Sorting of crushed glass

Colour sorting systems bases their sorting criteria on differences in colour, brightness, particle size and shape. Sensors in colour sorting systems are much more sensitive to differences in colour than the human eye. The material to be sorted is transported on a conveyor belt passing through a sensor (camera) where the material is illuminated from above with LED’s mounted parallel to the camera (it is important that the light is homogeneous and stable). The camera measures reflected light and sends a signal to a computer for analysis to determine


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which material that pass by. Unwanted material can be rejected with a selective sepa-ration system. Figure 3.3-5 illustrates the setup of a colour sorting system. One advan-tage is that the system is usually easy to maintain and it can be installed with an auto-matic cleaning system for the camera lenses. The camera optics is located in an area with slightly increased air pressure. Airflow is used like a curtain to prevent any depo-sition of dust or moisture. However a disadvantage is that dirty minerals have to be washed before scanning is possible. Sorting of small particles in the range of 3 –5 mm has a very low capacity. This technique is not suited for use in quality control of coal and is not expected to give any good results.

Figure 3.3-5: Colour sorting system [19]

3.4 Summary

The market research for detection and separation techniques resulted in the discovery of many interesting techniques. Some techniques are more conventional than others, and most industries are familiar with suspension magnets and magnetic drums, and even sometimes eddy current separators. So far there is no widespread knowledge and experience about EMS and DE-XRT systems used for quality control of solid fuels fired in power plants. These techniques seem to have large potential to detect both metal and non-metal materials, and based on this feature a deeper study on these tech-niques was performed and the results are presented in chapters 4.2 – 4.3. The XRF and


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the technique are left in this study because of the fact that it can’t be applied on bulk flows with thick material layers. PGNAA technique is unsuitable for identification of single contaminants. Camera sorting is not suitable for dirty bulk materials, which are a huge limit. Table 3.4-1 summarises the strength and weaknesses of each technique in the review.


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Table 3.4-1 Summary

Equipment Advantages Disadvantages Suspension magnets The equipment is cheap and easy to install. The need of

maintenance is low. It can handle large objects. The separa-tion process is done automatically.

Only ferrous metal can be removed.

Magnetic drums The equipment is cheap and easy to install. It is suitable for removal of small ferrous objects. The separation process is done automatically.

Only ferrous metal can be removed.

Eddy Current separator The separation process is done automatically. Some of the non-ferrous metals for example aluminium can be sepa-rated. The investment cost can be really attractive compared with other more advanced systems, e.g. EMS and X-ray systems. The installation is easy. The throughput of bulk is high.

Non-ferrous metals with high density and low resistivity such as lead are hard to separate. Magnetic metals should be removed first due to the development of heat. Non-metal objects are hard to remove. The layer of the bulk material must be thin. Enclosed metal is hard to remove.

Metal detector It can detect all types of metal, even enclosed metal. No automatic separation. Separation with air nozzles involves material losses and sometime dust formation. The fuel needs to be crushed in smaller fractions and it should also be homogenous divided.

X-Ray transmission detec-tion

Except for metal, organic material can be detected and separated from inorganic material, which includes contami-nants like rocks, glass and thicker plastics (PVC).

There is less knowledge about how good this method works for different bulk materials. There is no known reference installations applied for fuel quality control, hence more testing is necessary. Thin materials such as plastics bags might not absorb enough x-rays to be detected. The throughput of material in equipment manufactured today is low compared with large power plants fuel consumption.


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X-Ray fluorescence It is possible to detect material based on the elemental con-

tent. Other contaminants than metal can be identified. The thickness of the bulk material must be thin (single layer).

Prompt gamma neutron activation analysis

Elemental analysis of the material is possible. It is not possible to identify single contaminants. Can only measure an average over time.

Camera sorting Low maintenance with automatic cleaning system. Easy to reach for maintenance.

The method cannot be applied on dirty bulk materials. Low throughput when sorting small particles.


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4 Research, tests and results Chapter 4 describes relevant facts from experiments that been carried out by other scientists at other universities (chapter 4.2-4.3) but also information about experiments that been carried out in cooperation between Vattenfall R&D and Steinert Elektro-magnetbau GmbH (chapter 4.4). The information from the literature study and the experiments is used to evaluate the most interesting techniques for fuel quality control within Vattenfall.

4.1 Introduction

A deeper study of the EMS and DE-XRT systems was performed with the main aim of presenting detailed information about the technical performance and ratings of these two techniques with respect to the detection and separation of common contaminants in different fuels. The approach was divided into two parts: (1) a deeper literature study about dry-coal cleaning and recovery of coal from mining waste, with the results from a PhD thesis made at the technical university of Delft (NL) in 2003 as the main source of literature and (2) practical laboratory scale tests at the German manufacturer Steinert Elektromagnetbau GmbH in Cologne (DE).

4.2 De-stoning of coal using an EMS system

The experiment was performed at the Delft University of technology as a part of a PhD thesis and published by Take de Jong in December 2003. The purpose was to investigate the possibility to use the EMS technique for dry-cleaning (de-stoning) of coal. Today run-of-mine (ROM) coal having up to 50 % ash content is no exception, of which in many cases the main part consists of free shale (rocks/stones). Some min-erals and rocks show variable electric conductivity and magnetic interaction, which means that such minerals could be identified according to measurements of their elec-tric and magnetic properties.

4.2.1 Experimental setup

In research performed earlier by de Jong and others at the Delft University of technol-ogy, it has been shown that differences in magnetic properties between coal and shale can be used for distinction and hence it should be a good separation parameter [13]. The material, in this case a coal and shale mixture is scanned by an electromagnetic sensor system where the transmitter sends out a signal (an output voltage), which is picked by a receiver unit. The measured voltage is converted from an analogue signal to a digital signal in a signal converter. The digital signal is analyzed in a computer. The difference in the magnetic properties of the material has different effect on the


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signal on which the material is distinguished on the ash content basis [13]. The per-formance could be expected to be near the automatic glass recycling or almost the same if it is applied on coal according to de Jong. In automatic glass recycling the removal of metal is produce with a purity and recovery greater than 90 % [11]. Nor-mally glass is sorted by their colour, which is performed by optical sensors. The coal and shale mixtures used for the experiment consisted of different types of ROM from several of German and UK coal mines with size ranges between 20 to 50 mm. Particles were split into different density fractions; 1.2-1.3, 1.3-1.4, 1.4-1.5, 1.6-1.7, 1.7-1.8, 1.8-2.0, 2.0-2.4, and 2.4-2.8 g/cm3.

4.2.2 Results

The output voltage (U) relative the specific density from the weighted and scanned particles of the mixed coal sample is shown in Figure 4.2-1. In the figure it can be read-out that the particle density of 2.6 g/cm3 corresponds to 2000 mV and that parti-cle densities lower than 2 g/cm3 gives output voltages lower than 1000 mV. The determined ash content from total oxidation of the mixed coal sample is shown in Figure 4.2-2 for the different particle fractions. From the figure it can be read-out that ash content lower than 50% corresponds to voltage output lower than 1000 mV. Density partition curves for two voltage amplitudes are shown in Figure 4.2-3, where certain thresholds for a particle density can be read-out for certain percentage of coal contents at different amplitudes. The coal and shale distinction is ne ither affected by moisture and it is the same for dry material and material with adhered moisture ac-cording to the test [13].


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Figure 4.2-1 Measured voltage amplitude U versus different density fractions [13]

Figure 4.2-2 Measured voltage amplitude U versus particle ash content [13]


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Figure 4.2-3 Partition density curves at amplitudes of 500 and 750 mV [13]

4.2.3 Analysis of results

As was proved in previous work done by de Jong and others, an electromagnetic sig-nal can give information from which the particle density and/or the ash content can be determined. The observed variation in output voltage is smaller in the lower density classes and larger in the larger density classes. A reason could be the increased width in density in the higher classes. Most coal is valued mainly on the basis of ash content and it seems like the higher density fractions are correlated with higher ash content when Figure 4.2-1 is compared with Figure 4.2-2. The determined density partition curves show that a certain density cut-point for the particle coal content or ash content can be selected at any measured output voltage amplitude. The cut-point can be used as a threshold for particle rejection.

4.3 Processing ROM and residual mine reject with X-ray technology

The experimental purpose was to investigate if X-ray sorting equipment could be used to recover coal from old mine reject with low coal content and if the X-ray method can be used to process run of mine (ROM) coal directly as a dry-cleaning process instead of a complement to the usual wet-cleaning. ROM is the raw mined material as it is delivered directly from the mine prior to treatment of any sort. Tako de Jong performed the experiments at Delft University of technology in coopera-tion with German manufacturer s of X-ray sorters.


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4.3.1 Experimental setup

Samples of ROM and residual mine reject from different coal mines were used, both from former mines and mines that are in operation today. The Dual energy X-ray sorter used in the experiment was manufactured by CommoDaS GmbH (Wedel, Ger-many) [14] & [15]. The X-ray sorter has to be calibrated to recognise what is pure coal and pure shale. When the calibration is done a relationship based on the absorption is put together to predict the ash content of coal-shale particles. Final the material was scanned in the X-ray sorter, where each sample was scanned individually.

4.3.2 Results

Results of the coal recovery from the four samples of mine reject are shown in Table 4.3-1 where the results are given per size fraction. For each sample the results of the size fractions are added proportionally to their distribution and can be read-out in col-umn 2. This gives the coal grade and recovery of a process that would sort all investi-gated size fractions in parallel. The second column gives the distribution of the material over the indicated size ranges of the fractions that were actually sorted. Fractions outside the size ranges be-ing sorted, was either too small (< 5 mm) or too large (> 40 mm or > 80 mm). It is not a machine limitation but caused by insufficient sample amounts for the larger material according to de Jong. Also it was visually observed that the coal content of the largest fractions was considerably lower than the smaller fractions. The coal content from each sample and its different fractions can be read-out from column 7 named “coal”. De Jong used the R-factor, which is defined as the minimum percentage of coal in a scanned particle needed in order to not be rejected, as a sorting criterion for the coal recovery process. This means that the lower the R-factor is the higher the ash content of coal. In the column at the far right the recovery of the combustible content (C-recovery) in the sorted coal is given (only present for sample 4). Table 4.3-2 shows the results composed by weighted summation of the size fractions. The last column shows that from sample 4 50.4 % coal were recovered with ash con-tent less than 50%. The shaded entries of sample 1 are estimations being the average of the values at sample 2 and 3.


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Table 4.3-1: Results of the coal recovery from mine reject per size fraction [14].

Table 4.3-2: Results of the sorting experiments composed by weighted summation of the

size fractions [14].

The result from the experiment for dry-cleaning of run of mine (ROM) coal is shown

in Table 4.3-3. The “OC” column indicates the percentage of combustible content that

was recovered from the cleaned rune of mine coal. From the field in the last row and

the last column it can be read-out that an average of 88 % of combustible coal was

extracted from the run of mine.


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Table 4.3-3: Results of the experiment to dry-clean ROM coal [15]


From the recovered mine reject between 5 % and 10 % of coal was extracted from the 5x40mm range of fractions. In the experiment to dry-clean the run of mine coal, the recovery of coal concentrations with ash content beneath 10 % for larger fractions and beneath 20 % for smaller fractions was possible. According to de Jong, this informa-tion confirms that the DE-XRT sorting system is useful to distinguish coal from stone. The sorting efficiency also seems to be independent of particle size in the 4x40 mm range from the experiment with mine rejects. Even particles sized as small as 5 mm can be effectively sorted.

4.4 Practical fuel test in cooperation with Steinert

4.4.1 Introduction/background

To test and eva luate some of the industrial technologies for detection and separation of solid materials it was decided to do practical tests in cooperation with one of the lead-ing manufacturers of detection and separation systems in Europe. There were some manufacturers to choose between especially on the German market. It was decided to do the test with the German manufacturer Steinert, a company specialized on auto-matic sorting processes for recycling of solid materials, which also had a Swedish contact person at Svenska lyft AB. Another important thing was that this manufacturer provides equipments from simple suspension magnets to complex X-ray systems, which made it possible to test many combinations of equipments. This kind of knowl-edge could be used later when the fuel quality control system should be designed.


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4.4.2 Method/experimental procedure

The test fuels (coal, peat and demolition wood) were deliberately contaminated with ferrous metal (iron), non-ferrous metal (lead, brass, aluminium, stainless steel, cup-per), rocks, glass, different plastics, PVC, rubber and concrete. Some of the contami-nants are shown in Figure 4.4-1. Each fuel was wrapped into eight different bags marked with numbers, where each bag contained different contaminants. The bags were shipped in boxes by truck to Germany. At the laboratory of Steinert, each fuel type was scanned by the different equipments in a certain order, first for removal of ferrous metals, second for non-ferrous metal and third for other contaminants. The rejected material and used equipment was docu-mented after every scan. The fuels were scanned with the same equipment several times, and the rejected material was put back into to fuel after each scan to determine the accuracy of the equipment. The tested equipment was a combination of magnet separators, eddy current separator, EMS system and DE-XRT system. The laboratory personnel at Steinert documented the results in a test result protocol. Some of the equipment that been used is illustrated by pictures where Figure 4.4-2 shows the bench scale magnetic drum, Figure 4.4-3 the bench scale eddy current sepa-rator and Figure 4.4-4 shows the air nozzles used in the industry scale EMS system. Figure 4.4-5 show the size of a stone that is possible to eject without any difficulties by the air nozzles.


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Figure 4.4-1: Contaminants that been put into the fuel

Figure 4.4-2: A laboratorial scale magnetic drum


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Figure 4.4-3: A laboratorial scale eddy current separator

Figure 4.4-4: The array with air nozzles in the industry scale EMS system


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Figure 4.4-5: Shows the size of a stone that can be ejected by air nozzles

4.4.3 Results

Results have not arrived yet (They was expected in December 2007).



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5 Design of the fuel quality control system Chapter 5 describes 3 suggestions for fuel quality control systems with capacity based on reference data from Uppsala KVV. The Uppsala KVV is described as the reference power plant where capacity data been fetched. Information about performance, capac-ity and approximated investment costs for the different components in the suggested systems is described.

5.1 Reference power plants

In this project the pulverized fuel fired boiler in Uppsala (SE), hereafter called Upp-sala KVV, and was used as reference plant. Data from the present fuel handling sys-tem regarding fuel feed capacity were used as reference data when designing the sug-gestion for a new system. The fuel feed system is built for coal and peat. Two roller mills are installed for crush-ing of fuel briquettes. Today Uppsala KVV has a fixed suspension magnet for removal of metal and a metal detector as a secondary control system on the conveyor, where peat is transported into the mill building. If metal is detected the conveyor belt has to be stopped and cleaned manually. The coal feed line has only a suspension magnet mounted over the conveyor and no metal detector. Where the two separate fuel feed lines are connected, just before the fuel reaches the mills the fuel is fed through a sieve, which is installed to secure the fraction size into the mills. Too large fractions are crushed in a crusher. Because of the type of mill installed in Uppsala, no other fuels than fuels pre-processed as briquettes and pellets can be grinded. It is desired to install a hammer mill to be able to use untreated raw fuels. If this installation is realized it would be necessary with a good fuel quality control system.


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5.2 Performance, capacity and investments costs

The capacity of a fuel quality control system for a power plant like Uppsala KVV should be at least 50 tonnes of fuel per hour. The investment cost must be calculated from the demand of performance and for each single component involved in the sys-tem. The component costs listed beneath is approximated costs for components with a capacity of least 50 t/h [11], [16] & [22]. The cost for the conveyor between compo-nents is hard to estimate other than a price per meter conveyor belt. Costs for prepara-tion of the ground for buildings etc. is not considered, neither costs for buildings or installation work. However, it should be clear that the cost for such activities is a large part of the total investment cost and it is not the same for different power plants. Approximate costs system components:

• Suspension magnet 25 000 euros (100 000 euros complete installation)

• Sieve No information • Crusher No information • Magnetic drum No information • Eddy Current separators 60 000 euros • EMS system 100 000 - 200 000 euros • DE-XRT system 300 000 – 500 000 euros • Metal detector 10 000 euros • Conveyor belt approximate 160 – 200 euros per meter

(Without any fittings, only for the belt) Technical papers with drawings of components are attached as appendices. DE-XRT in appendix 4, EMS in appendix 5, eddy current in appendix 6, suspension magnets in appendix 7, magnetic drums in appendix 8.

To be able to obtain exact investment costs it is necessary to invite a tender for the

complete fuel quality control system specified for a specific power plant where all

necessary data about fuel capacity, available space and other limitations are specified.


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5.3 Design suggestions for quality control systems

The size of the system and the involved components (equipment) included in the fuel quality control system must be chosen depending of which contaminants that ordinar-ily occur in the fuel. The size of the system is more dependent on the system capacity while the choice of type of equipment depends more on what kind of contaminants that should be removed. The first system in Figure 5.3-1 includes almost the same components as the fuel feed system in Uppsala KVV. The thing that been added is the hatch which make it possi-ble to eject fuel after the metal detector, which is indicated as number 2 in Figure 5.3-1. The possibility to eject fuel after the metal detector would be an improvement, which could reduce the unnecessary stops for metal cleaning. Still the system only handles contaminants on the metal basis. This suggestion would be the cheapest alter-native to implement in Uppsala KVV.

Figure 5.3-1: Quality control system for metal contaminants


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The second suggestion is a more complex system, still only suited too handle metal contaminants. The system removes the different metal contaminants in several steps would give a better performance and probably a cleaner fuel compared to the first suggestion. The suggestion for the system is shown in Figure 5.3-2. Six different steps are used to remove the different type of metal contaminants. First a suspension magnet removes large object of ferrous metal. Second the material is sized with a sieve and third the large fraction is crushed. Next in the fourth step smaller objects of ferrous metal is removed by a magnetic drum. Non-ferrous metal is removed with an eddy current separator in the fifth step. The last step is a metal detec-tor used to secure that no metal remains in the fuel, which have the possibility to eject the fuel if metal occur. This system compared to suggestion one would be more ex-pensive to build in Uppsala KVV due to the large system change. If investing in a totally new system were the case this system compared to system one would be a bet-ter choice due to higher accuracy, which means a cleaner fuel.

Figure 5.3-2: Quality control system for metal contaminants


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The third suggestion is a fuel quality control system, which is equipped with compo-nents to detect and separate the most common material defined as contaminants (metal, rocks, glass, concrete, etc) is shown in Figure 5.3-3. Compared with the sys-tem in Figure 5.3-2 this system performs the quality control in several steps, which will probably increase the cleaning effectiveness of the fuel (to take care of the differ-ent type of contaminants by using many steps are a recommendation from the manu-facturers). It will also make it easier to take care of and sort out the rejected material, which will make it possible to get some money from selling scrap metal for example. The system components are:

1. Suspension magnet for removal of large magnetic particles. 2. Sieve for control of size fractions 3. Crusher used to crush the larger fraction 4. Magnetic Drum for removal of small magnetic particles 5. Eddy Current separator for removal of non ferrous particles 6. EMS or DE-XRT system for removal of non metal based contami-

nants 7. Metal detector connected to a hatch for ejecting any remaining con-

taminated fuel

If the metal detector 7 is moved to pos ition 5 and equipped with a sensor which is able to measure material based on ash content besides material on the metal basis it will be possible to feed the fuel directly to the mill or the boiler as shown in Figure 5.3-4. When contaminants occur in the fuel the fuel is ejected and cleaned in a separate line, which includes an eddy current separator or an EMS/DE-XRT system or both. Some-times it is enough to install one of the systems, which depends on which contaminants that is most common. A sieve and a crusher is necessary to secure a maximum fraction size that the EMS or DE-XRT system is designed to handle. It will also simplify the separation through the whole system. The choice of an EMS or a DE-XRT unit depends on the type of con-taminants that should be removed. A system called Varisort [16] that can be equipped with a combined EMS and DE-XRT sensor system could be a good choice. This sys-tem is the most expensive system of the three suggestions.


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Figure 5.3-3: A fuel quality control system build to handle multiple types of contaminants

Figure 5.3-4:


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5.4 Different ejection devices for EMS and DE-XRT systems

The ejection device presented earlier in chapter 3 is not optimal to apply on fuels where the dust formation is of high sense. In Figure 5.4-1 (a) the air nozzle array is seen from the front together with the conveyor. This can be mounted in two ways, either under as in (b) or above as in the (c) part of the figure. Above mounting of the air nozzles array is preferred to minimize the spread of dust and to make it easier to eject light materials with large area like metal (aluminium) foil. Another ejection device would be the setup of parallel rods, which is shown in (d) where each rod can be turned selectively to eject polluted fuel as is illustrated in the (e) part. The grey line means that a rod is open while the black line illustrates when the rod is closed. The fuel could either slide over the rods or fall above them without any contact except for when a pollutant is ejected. When all rods are closed there is no gap between them. This is not a conventional product on today’s market.

Figure 5.4-1: Different ejection devices


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6 Discussion

6.1 Studied techniques

Among all the studied techniques, there are some old proven and some new promising but un-tested techniques. Old separation techniques such as suspension magnets and magnetic drums should not be disregarded by the new techniques. Stronger magnets have been developed the last years, which cannot be compared with old magnets. This fact is important to consider when upgrading an old fuel quality control system. If the biggest problem or the only problem is magnetic contaminants it could be enough to upgrade old magnets to newer stronger ones. Even some times tuning could be enough. Due to the increase of the use of bio-fuels more unfamiliar contaminants have started to occur, which the old fuel handling systems are not suited for. There are new de-mands on the systems and new techniques that can handle new different problems are necessary. The systems also have to be flexible and be able to handle many different fuels with the same equipment. Interesting separation systems exists on the market, where the most interesting systems for this project are the EMS and the DE-XRT sys-tem. Other interesting techniques are separation based on density (air separators, air classi-fiers and fluidized sand beds) and X-ray systems based on X-ray fluorescence. In the end the time was not enough to investigate them all in this project.

6.2 EMS and DE-XRT technique in automatic separation systems

The EMS system is widely applied for glass recycling and metal scrap sorting. This industrial environment could be comparable with a coal preparation process, which would enable rapid introduction of the automatic sorting system. If the performance could be expected to be near the automatic glass recycling or almost the same if it is applied on coal, it would facilitate the introduction of the system. The coal and shale distinction is neither affected by moisture and is the same for dry material and material with adhered moisture according to tests, which is a necessary property if it should be used as a measuring method for fuels with varying moisture content. The efficiency of the ejection device with air nozzles can be discussed, other systems would probably be better to avoid problems with dust formation. To blow out the reject material from above instead from under would probably facilitate the ejection of heavy and large materials. It would also decrease the spread of dust.


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The DE-XRT system has shown to be useful as a dry-cleaning process for run of mine (ROM) coals and to recover high concentrations of coal in old mine rejects. The sys-tem works well in the recycling industry of glass and scrap metal. It is also used worldwide in the mining industry to concentrate different types of ores. This system is actually over-qualified and could be considered as too expensive to be used in the fuel quality control process for power generation because of the low concentration of con-taminants (compared to ROM material). However it seems to be the superior system, which can handle many different contaminants. The combination of a system using both an EMS and a DE-XRT sensor could make it possible to sort the removed material fractions in a recycling purpose. The EMS and DE-XRT technique is important and useful for Vattenfall because of the possibility to distinguish fuel and non-metal based contaminants, to remove stones and non-ferrous metals. This would be very useful in fuels like coal, peat and probably also demolition wood.

6.3 Uncertainties

Because of the absence of reference system where the EMS and the DE-XRT systems are installed for fuel quality control of solid fuels it is hard to give a strong indication about the technical performance of these systems. Although some interesting results have been presented at the Delft University of Technology, but it is recommended that further experiences from full-scale or pilot-scale testing is done to obtain more data about the performance of these systems. We hope to get more information from the tests performed at Steinert, which will make it possible to recommend further work. The price on different devices would be easier to approximate on a specified system for a certain power plant. With exact input data to give the manufacturer it would be possible to estimate a more exact price on the investment of a system. Only a specified tender could put an exact price on the investment cost.


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7 Conclusions • An automatic sorting system using the EMS sensor system could be used for

fuel quality control as a multi metal separator but it is able to detect other con-taminants than only metal materials. The system is cost effective compared to other sensors (x-ray). Compared to hand picking the system has a much higher capacity at a lower cost. The system has shown good efficiency in the recy-cling of demolition wood, scrap metal sorting and glass cleaning. It is offered as a commercial system for these purposes.

• The DE-XRT system has potential to be used in the fuel quality control, which

makes it possible to detect other contaminants than contaminants on the metal basis. Because of the fact that the system is used in industry environments much like the fuel treatment the system have potential to be rapidly introduced in the fuel quality control system. Additional advantages are that except for sorting the DE-XRT system is able to perform on-line data regarding composition (ash content) and particle size distribution. This information could be useful in the control of the combus-tion.

• To be able to give more specified data about the detection and the separation

efficiency of a combined sensor/sorting system full-scale test with a complete system is necessary, which should be constructed based on which fuel and which contaminants that should be removed.

• Better fuel qualities could be reached through discussion with the fuel suppli-

ers. Longer contracts with the same suppliers could give good relations, which would make it easier to set higher demands and discuss the problems with contaminated fuel. A central buying organisation could set higher demands due to larger purchase.


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8 Future work It would be interesting to build up a full-scale system and to run tests with contami-nated fuel to determine the exact detection and separation accuracy. Cooperating with other manufacturers than Steinert is good because of their speciali-zation towards the recycling industry of waste material. It could be better to work with a manufacture specialized in the mining (mineral processing) business where the bulk materials are more like the fuels used in the power plants. To look at the other not investigated techniques as density based separation and mate-rial identification with X-ray fluorescence. A company named INNOVX SYSTEMS supplies equipment for sorting waste based on X-ray fluorescence.


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9 References [1] http://en.wikipedia.org/wiki/Major_coal_producing_regions

Wikipedia the free encyclopedia, 071120. [2] http://sv.wikipedia.org/wiki/Torv

Wikipedia the free encyclopedia, 071120. [3] “International trade of biomass”, Monika Bubholz, Vattenfall august

2007.

[4] ”Biomass fuel trade in Europe – Summary Report”,Alakangas, E. et.al. VTT, Jyväskylä, March 2007.

[5] http://en.wikipedia.org/wiki/Wood_pellets

Wikipedia the free encyclopedia, 071120. [4] http://www.afabinfo.com

Allt inom bioenergi & förbränningsteknik, 071120. [7] Köge pellet processing, Denmark [8] Skellefteå kraft, Daniel Byström 071101. [9]

http://www.cartage.org.lb/en/themes/sciences/earthscience/Geology/Coal/Physicalcoal/Physicalcoal.htm 071119.

[10] “Sensor for quality control of materials, products and processes”

M.B. Mesina, 2005. [11] http://www.steinert.de/index.php?id=1&L=1

Steinert Elektromagnetbau GmbH

[12] http://www.waste-technology.co.uk/RDF/rdf.html Refuse-derived fuel, 071212.

[13] “Electromagnetic de-shaling of coal”, T.P.R. de Jong, M.B. Mesina and

W. Kuilman, January 2004.

[14] “Coal recovery from mine reject by means of x-ray sorting”, Tako de Jong, Erwin Bakker.


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[15] “DE-XRT sorting of coal”, Tako de Jong, published 2006. [16] Mobergs processkontroll, Sweden. [17] “Funktion av allmetallseparatorer för avfallsbränslen – Etapp 1”, Jurgen

Jacoby, Lars Wrangensten, april 2004. [18] “A method and apparatus for analyzing and sorting a flow of material”,

27 June 2002. [19] http://www.mogensen.de, Mogensen GmbH, 071011. [20] http://www.thomasathomas.com/Metal_detectors_work.htm, How a

metal detector works, 080114. [21] http://www.commodas.de/holding/homepage/main.html, CommoDas

website, 080114. [22] Vattenfall Uppsala KVV, Sven Johansson, 071217.

[23] http://www.innov-x-sys.com, Innovx systems, 080114.

[24] http://en.wikipedia.org/wiki/X-ray_fluorescence, 080114.

[25] http://en.wikipedia.org/wiki/Neutron_activation_analysis, 080114.

[26] http://scantech.com, 080114.

[27] http://archaeometry.missouri.edu/naa_overview.html, 080114.


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Appendix 1

Measurement with the Eddy Currents

If a magnetic field varies with time relative to any conduction medium and EC are induced. EC are closed loops of inducted currents in planes perpendicular to the mag-netic flux. The EC moves parallel coils windings and only exists within the area of the inducting magnetic field. Measurement of the inducted EC to the material gives knowledge about the materials magnetic and electrical properties. Most common method for measurement of inducted EC is the transmitter-receiver configuration. It can be divided into four steps: - Creation of signal (transmitter); - Interaction with the material; - Signal pickup (receiver); - Signal analysis and presentation. The transmitter coil is connected to an alternative current (AC) while the receiver or receiver’s coils are connected to a voltmeter. A primary magnetic field (Hp) is pro-duced from the transmitter that couples to the receiver and exciting an electromotive force (voltage). The measured voltage will remain constant assuming constant AC to the transmitter and fixed setup locations. When a conducting material appears near the receiver the magnetic field will change and hence also the magnetic flux (φ) due to the inducted current, which will be reflected in the measured voltage. Just as a normal current this current creates a secondary magnetic field directed opposite to the primary magnetic field. The field monitored by the receiver is the total field. Compared to the initial state this change is the change in amplitude (A) given by the measured material, which gives information about the inspected material. The electrical conductivity (σ) of the material is the main parameter affecting the measured voltage. Figure 0-1 shows the setup for the measurement with EC. Equations for calculation of magnetic flux density B, the magnetic flux φ and the volt-age output U from the receiver comes from the Maxwell’s equations:

HB ∗= µ Equation 0-1

∫ ∗= danBφ Equation 0-2

BfDNU S )2)(4/( 2 ππ= Equation 0-3


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where µ is the magnetic permeability of the material, n is the normal component of B which is the relevant surface for integration. Ns are the number of windings of the receiver coil, D its diameter and f the frequency of the magnetic field.

Figure 0-1 Setup for EC measurement [1]

The depth of the EC penetration (the density of the eddy currents) into the material in

x-direction as shown in Figure 0-2 gives information about the electrical conductivity

σ and hence information about the material. The density of the EC’s (J) in the z-

direction can be solved from the Maxwell’s equation, providing that they vary with the

x-direction (depth in the conductive material) [1]. The relation between z- and x-

direction is given by:

∗−

∗−= xtxJxJ

2cos

2exp/)( 0

ωµσω

ωµσ Equation 0-4

Where J(x)/J0 is the current density ratio between depth x and the surface and ω is the

angular frequency (rad/s). Relative to J0 the exponential term in Equation 4 is the fac-

tor of which the EC reduces with increasing depth. The standard depth of penetration

(δ) is defined to be the depth at which the strength has decreased to exp (-1) =1/e or 37

% of the currents strength at the surface J0 [1].


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Figure 0-2 Co-ordinates of the depth of penetration

If the exponential term and the current density ratio is defined as 1/e and δ is inserted

as x in Equation 4 the following equation is given forδ:

fπµσωµσδ

1

2

1== Equation 0-5

For easier calculations the effective depth (δe) has been generalized as 3δ. This gives

the following relation [1]:

fe

µσδ

1150= Equation 0-6

The effective depth δe is different for each material at a specific frequency, f as a func-

tion of differences in electric conductivity, σ and magnetic properties. It is important

to know that f has significant influence on δe. The maximum penetration depth is gen-

eral higher at lower f and decreases when f increases [1]. It is important to find an

optimal frequency to get a large variation in δe between different materials, which will

simplify the identification of material type.

When measuring materials it is the voltage amplitude and signal phase (ϕ) relative the

reference signal, which is analyzed as shown in Figure 0-3. Difference in signal output

between materials is affected of the chosen signal frequency. It is larger at low fre-

quencies for high conductive materials and lager at high frequencies for low conduc-

tive materials.


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Figure 0-3

References

[1] “Sensor for quality control of materials, products and processes” M.B. Mesina, 2005.


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Appendix 2

Thickness independency with dual x-ray transmission measurement

Thickness independency can be explained by considering basic x-ray transmission theory. The Lambert law governs the absorption at a specific wavelength λ.

dρλµ )(-0det eI I = Equation 0-1

Idet is the detected intensity, I0 the intensity of the undisturbed beam, µ (λ) the mass absorption coefficient, ρ the solid density, and d the thickness of the irradiated sample. For a given λ the effective µ of material with mixed elemental composition is calcu-lated from

∑ ==

n

i iieff f1

µµ ´ Equation 0-2 where f i is the mass fraction of element i and µ i the according µ . µ eff is independent of the materials phase or state. Then consider the dimensionless detected intensities I1=Idet1/I01 and I2=Idet2/I02 at two different wavelengts, Lambert’s law gives the following relationship between I1 and I2:

dm

dd CII

=== ∆∆ )e(e --

2

1 µρµρ Equation 0-3

Cm represents a constant that only depends on the chosen wavelength and the material properties. If Cm of two materials is different than they can be distinguished from each other. I1 relative to I2 determines Cm and hence the material. The absolute value of I1 or I2 determines the thickness d.

References

[1] “Automatic sorting and control in solid fuel possessing: opportunities in European perspective”, Tako P.R. de Jong, J.A van Houwelingen & W. Kuilman, 2004.


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Appendix 3


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Appendix 4


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Appendix 5


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Appendix 6


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Appendix 7


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Appendix 8


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