WO2020221708A1

WO2020221708A1 - Method for operating a fluidized bed boiler

Info

Publication number: WO2020221708A1
Application number: PCT/EP2020/061672
Authority: WO
Inventors: Bengt-Åke Andersson; Angelica GYLLÉN
Original assignee: Improbed Ab
Priority date: 2019-04-29
Filing date: 2020-04-28
Publication date: 2020-11-05
Also published as: EP3963261A1

Abstract

The invention relates to a method for operating a fluidized bed boiler, comprising the combustion of solid fuel, and carrying out the combustion process with a fluidized bed comprising ilmenite particles, characterized in that the bed temperature of the fluidized bed is between 900°C and 1,000°C.

Description

Method for operating a fluidized bed boiler

The invention relates to a method for operating a fluidized bed boiler, comprising carrying out the combustion process with a fluidized bed comprising ilmenite particles. Fluidized bed combustion is a well-known technique, wherein the fuel is suspended in a hot fluidized bed of solid par ticulate material, typically silica sand and/or fuel ash. Other bed materials are also possible. In this technique, a fluidizing gas is passed with a specific fluidization ve- locity through a solid particulate bed material. The bed material serves as a mass and heat carrier to promote rapid mass and heat transfer. At very low gas velocities the bed remains static. Once the velocity of the fluidization gas rises above the minimum fluidization velocity, at which the force of the fluidization gas balances the gravity force acting on the particles, the solid bed material behaves in many ways similarly to a fluid and the bed is said to be fluidized. In bubbling fluidized bed (BFB) boilers, the fluidization gas is passed through the bed material to form bubbles in the bed, facilitating the transport of the gas through the bed material and allowing for a better control of the combustion conditions (better temperature and mixing control) when compared with grate combustion. In circulat ing fluidized bed (CFB) boilers the fluidization gas is passed through the bed material at a fluidization velocity where the majority of the particles are carried away by the fluidization gas stream. The particles are then separated from the gas stream, e.g., by means of a cyclone, and re circulated back into the furnace, usually via a loop seal. Usually oxygen containing gas, typically air or a mixture of air and recirculated flue gas, is used as the fluidizing gas (so called primary oxygen containing gas or primary air) and passed from below the bed, or from a lower part of the bed, through the bed material, thereby acting as a source of oxygen required for combustion. A fraction of the bed material fed to the combustor escapes from the boiler with the various ash streams leaving the boiler, in partic ular with the bottom ash. Removal of bottom ash, i.e. ash in the bed bottom, is generally a continuous process, which is carried out to remove alkali metals (Na, K) and coarse inorganic particles/lumps from the bed and any agglomerates formed during boiler operation, and to keep the differen tial pressure over the bed sufficient. In a typical bed management cycle, bed material lost with the various ash streams is replenished with fresh bed material.

From the prior art it is known to replace a fraction or all of the silica sand bed material with ilmenite particles in the CFB process (H. Thunman et al . , Fuel 113 (2013) 300- 309) . Ilmenite is a naturally occurring mineral which con sists mainly of iron titanium oxide (FeTiCU) and can be re peatedly oxidized and reduced. Due to the reducing/oxidiz- ing feature of ilmenite, the material can be used as oxygen carrier in fluidized bed combustion. The combustion process can be carried out at lower air-to-fuel ratios with the bed comprising ilmenite particles as compared with non-active bed materials, e.g., 100 wt.-% of silica sand or fuel ash particles . EP 3 153 776 A1 relates to a bed management cycle for a fluidized bed boiler and to a corresponding arrangement for carrying out fluidized bed combustion.

EP 2 178 629 B1 relates to a fluidized bed reactor for the chemical and/or physical treatment of fluidizable sub stances and to a process therefor.

US 2010/074805 A1 discloses a fluidized bed method for the heat treatment of solids containing titanium.

Vijay et al . , Preoxidation and hydrogen reduction of ilmen- ite in a fluidized bed reactor, Metallurgical and Materials transactions A: Physical Metallurgy & Materials Science,

ASM International, Materials Park, vol. 27B, no. 5, 1 Octo ber 1996, p. 731-738, relates to studies on preoxidation and hydrogen reduction of Quilon-grade ilmenite carried out in a lab-scale fluidized bed reactor.

The invention is concerned with the problem of improved op eration of a fluidized bed boiler, such as, e.g., a circu lating fluidized bed boiler or a bubbling fluidized bed boiler .

This problem is solved by the features of the independent claims. Advantageous embodiments are defined in the depend ent claims.

First, several terms are explained in the context of the invention .

The invention is directed to a method for operating a flu idized bed boiler comprising the combustion of solid fuel, and carrying out the combustion process with a fluidized bed comprising ilmenite particles, wherein the bed tempera ture of the fluidized bed is between 900°C and 1000°C. The invention is based on the surprising finding that the reac tivity of the ilmenite comprising bed significantly in creases at temperatures of 900°C and above. Reactivity is defined as the ability to oxidize carbon monoxide (CO) , when feeding syn-gas (50 mole-% CO and 50 mole-% ¾, apply ing a standard cycle procedure used for determining the ef ficiency of an oxygen carrier, commonly used for Chemical Looping Combustion (CLC) .

A fluidized bed boiler used in the method of the present invention is a device having the purpose of continuously producing thermal energy through the combustion of solid fuel. Continuously means that the process of combustion and producing thermal energy is carried out continuously for hours, days, weeks or longer. It further means that there is no operation in batches and no separate designated reac tion zones e.g. for oxidation and reduction as in chemical looping combustion (CLC) known from the prior art. Prefera bly, the combustion process is carried out with air as flu idizing gas for fluidizing the bed material.

Solid fuel is any solid combustible material, e.g. coal, wood, other solid biomaterial, or waste.

Preferably, the claimed method operates the boiler at an excess air ratio (l) below 1.3. The excess air ratio l is a common parameter in the operation of BFB boilers and is de fined as the mass ratio of air to fuel (MR_air/fUei = m_air/m_fuei) actually present in the combustion process divided by the stoichiometric mass ratio of air to fuel. That is, l = (MRair/fuel) actual/ (MRair/fuel) stoichiometric· ThS ItiaSS ratlO Of 3111 to fuel actually present in the boiler is determined by the amount of fuel and air supplied to the boiler. The stoichi ometric mass ratio of air to fuel is the mass ratio re- quired by stoichiometry for complete combustion of the pro vided fuel and can be calculated for any given fuel compo sition .

In preferred embodiments, l is 1.25 or less, more prefera- bly 1.2 or less, more preferably 1.1 or less, most prefera bly between 1.05 and 1.1. Preferably, for the combustion of waste-based fuel, l is 1.23 or less, more preferably 1.1 or less, more preferably between 1.05 and 1.23, most prefera bly between 1.05 and 1.1. For the combustion of biomass fuel, l preferably is 1.19 or less, more preferably 1.1 or less, more preferably between 1.05 and 1.19, most prefera bly between 1.05 and 1.1.

In the claimed process, the fluidizing bed comprising il- menite is a means for aiding the combustion of the solid fuel. This is in contrast to fluidized bed reactors wherein the bed material is treated or otherwise manipulated to modify this bed material and produce a desired modified ma terial .

The claimed method is not directed to the operation of la boratory-scale reactors, this being preferably excluded from the claimed method. Laboratory-scale reactors have the main purpose of research, they do not have the main purpose of producing thermal energy on a commercial scale. Typi cally, laboratory-scale reactors have a thermal output of less than 1 MW, if any. A laboratory-scale reactor as dis closed in EP 3 153 776 A1 does not carry out combustion of a solid fuel at all, instead, it is designed to test reac tivity of a bed material under the influence of different gas compositions. Such lab-scale reactors are not to be un derstood as fluidized bed boilers in the sense of the pre- sent invention.

Ilmenite is a naturally occurring mineral which consists mainly of iron titanium oxide (FeTiCh) . Ilmenite can be re peatedly oxidized and reduced and has been used as a redox material in chemical looping combustion (CLC) . From the prior art it is known to replace a fraction of the silica sand bed material with ilmenite particles in the CFB pro cess (H. Thunman et al . , Fuel 113 (2013) 300-309) . Due to the reducing/oxidizing feature of ilmenite, the material can be used as oxygen carrier in fluidized bed combustion. The combustion process can be carried out at lower air-to- fuel ratios with the bed comprising ilmenite particles as compared with non-active bed materials, e.g., 100 wt.-% of silica sand or fuel ash particles. The ilmenite particles used in the invention can for example be ilmenite sand or crushed ilmenite.

After having experienced an initial activation phase, il menite particles undergo chemical aging as they are sub- jected to repeated redox-conditions during combustion in fluidized bed boilers and the physical interactions with the boiler structures and other fluidized particles induce mechanical wear on the ilmenite particles. The invention has recognized that even after extended use as bed material in a fluidized bed boiler at the claimed elevated temperatures, ilmenite still shows very good oxy gen-carrying properties and reactivity towards oxidizing carbon monoxide (CO) into carbon dioxide (CO2) , so called "gas conversion".

A preferred bed temperature range of the fluidized bed is between 920 and 980°C, further preferably between 930 and 970°C, further preferred between 940 and 960°C.

Preferably the ilmenite content of the fluidized bed mate rial is at least 30 wt.-%, preferably at least 50 wt.-%, preferably at least 70 wt.-%, further preferred at least 90 wt . - %

In a preferred embodiment the average residence time of the ilmenite particles in the boiler is at least 100 hours, preferably at least 120 hours, further preferably at least 150 hours, further preferably at least 200 hours, further preferably at least 250 hours, further preferably at least 290 hours, most preferably at least 300 hours. In a preferred embodiment the average residence time of the ilmenite particles in the boiler is less than 600 hours, preferably less than 500 hours, further preferably less than 400 hours, further preferably less than 350 hours. The invention allows for average residence times of the il menite particles in the boiler which are at least a factor of 2.5 higher than typical residence times of bed material in conventional fluidized bed boilers while maintaining high gas conversion rates at elevated bed temperatures. Setting the average residence time of the ilmenite parti cles to such long values in turn significantly reduces the overall consumption of the natural resource ilmenite and makes the combustion process more environmentally friendly and more economical.

The attrition rate of the ilmenite particles decreases af ter an extended residence time in the boiler and the me chanical strength is still very good after the ilmenite has been utilized as bed material for an extended period of time .

According to a preferred embodiment the invention contem plates bed material recirculation comprising the steps: a) removing at least one ash stream comprising ilmenite particles from the boiler; b) magnetically separating ilmenite particles from the at least one ash stream. c) recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

A bottom ash removal device is known in the art and removes boiler bottom ash together with bed material. The bottom ash removal device may be part of an existing system for bottom ash recirculation.

The purpose is to improve separation of reusable ilmenite bed material from the bottom ash so as to allow effective recirculation/recycling of removed ilmenite bed material back into the boiler.

Ilmenite particles can be conveniently separated from the boiler ash and even after extended use as bed material in a fluidized bed boiler ilmenite still shows very good oxygen carrying properties and reactivity towards oxidizing carbon monoxide (CO) into carbon dioxide (CO₂₎ , so called "gas conversion" and good mechanical strength. Ilmenite parti cles, after having experienced an initial activation phase, undergo chemical aging as they are subjected to repeated redox-conditions during combustion in fluidized bed boilers and the physical interactions with the boiler structures induce mechanical wear on the ilmenite particles. It was therefore expected that the oxygen-carrying capacity of il menite particles and their attrition resistance rapidly de teriorate during the combustion process in a fluidized bed boiler at elevated bed temperatures. It was therefore sur prising that the attrition rate of the ilmenite particles decreases after an extended residence time in the boiler and the mechanical strength is still very good after the ilmenite has been utilized as bed material for an extended period of time at the claimed high temperatures.

In light of the good attrition resistance the surprisingly good oxygen-carrying properties of used ilmenite particles can be exploited by recirculating the separated ilmenite particles into the boiler bed. This reduces the need to feed fresh ilmenite to the boiler which in turn signifi cantly reduces the overall consumption of the natural re source ilmenite and makes the combustion process more envi ronmentally friendly and more economical. In addition, the separation of ilmenite from the ash and recirculation into the boiler allows for the control of the ilmenite concen tration in the bed and eases operation. Furthermore, the inventive bed management cycle further increases the fuel flexibility by allowing to decouple the feeding rate of fresh ilmenite from the ash removal rate, in particular the bottom ash removal rate. Thus changes in the amount of ash within the fuel become less prominent since a higher bottom bed regeneration rate can be applied without the loss of ilmenite from the system.

The invention preferably combines a first mechanical clas sification using a mesh size from 200 to 1,000 pm and a subsequent magnetic separation of the fine particle size fraction to retrieve ilmenite to be recirculated into the boiler .

The majority of ilmenite in the bottom ash comprises a par ticle size of 500 pm or lower so that the mechanical clas sifier provides a fine particle size fraction having a more homogenous size distribution while still comprising the ma jority of the ilmenite particles. The magnetic separation in the second step can be carried out more efficiently.

The initial mechanical classification in particular serves three purposes. First, it contributes to protect the mag netic separator from large ferromagnetic objects such as nails which could otherwise damage the magnetic separator or its parts. Second, it reduces the load on the magnetic separator by reducing the mass flow. Third, it enables sim pler operation of the magnetic separator as it generates a narrower particle size distribution.

Preferably the mechanical classifier comprises a mesh size from 300 to 800 pm, preferably 400 to 600 pm. A typical preferred mesh size is 500 pm. This is sufficient to remove the bulk of the coarse bottom ash species. In a particularly preferred embodiment, the mechanical classifier comprises a rotary sieve which has been found effective to pre-classify the bottom ash to remove coarse particles .

In one embodiment the mechanical classifier further com prises a primary sieve prior to the mechanical classifier having the mesh size as defined above (e.g. the rotary sieve) to separate coarse particles having a particle size of 2 cm or greater, e.g. coarse particle agglomerates of golf ball size.

In another embodiment the system may comprise a primary classifier separating very fine particles and recirculating those fine particles into the boiler prior to the mechani cal classifier of feature b. of the main claim and the mag netic separator. This primary classifier may comprise an air classifier to retrieve the very fine particle fraction.

The system may comprise a device for separating elongate ferromagnetic objects from the ash stream prior to the mag netic separator. The mechanical classifier can comprise a slot mesh to remove small pieces of thin metal wire or nails that tend to plug mesh holes and also affect the mag netic separation in the subsequent step.

Preferably the magnetic separator comprises a field inten sity of 2,000 Gauss or more, preferably 4,500 Gauss or more on the surface of the transport means of the bed material. This has been found effective to separate ilmenite from ash and other nonmagnetic particles in the particle stream. It is also possible to utilize the magnetic separator hav ing the field intensity or field strength as indicated above without prior mechanical classification or mechanical sieving .

Preferably the magnetic separator comprises a rare earth roll (RER) or rare earth drum (RED) magnet. Corresponding magnetic separators are known in the art per se and are e.g. available from Eriez Manufacturing Co.

(www.eriez.com) . Rare earth roll magnetic separators are high intensity, high gradient, permanent magnetic separa tors for the separation of magnetic and weakly magnetic iron particles from dry products. The bottom ash stream is transported on a belt which runs around a roll or drum com prising rare earth permanent magnets. While being trans ported around the roll ilmenite remains attracted to the belt whereas the nonmagnetic particle fracture falls off. Mechanical separator separates these two particle frac tions .

In one embodiment the magnetic field is axial, i.e. paral lel to the rotational axis of the drum or roll. An axial magnetic field with the magnets having a fixed direction causes strongly magnetic material to tumble as it passes from north to south poles, releasing any entrapped nonmag netic or paramagnetic materials.

In another embodiment the magnetic field is radial, i.e. comprising radial orientation relative to the rotational axis. Generally, a radial orientation has the advantage of providing a higher recovery rate of all weakly magnetic ma terial which can come at the cost of less purity due to en trapped nonmagnetic material. It is also possible to use a two-stage magnetic separation with a first step using axial orientation thereby helping to release entrapped nonmagnetic material and the second step using radial orientation to increase the recovery rate .

Preferably the separation efficiency of the system for il- menite bed material is at least 0.5 by mass, preferably at least 0.7 by mass. That means that at least 50 or 70 wt . % of ilmenite comprised in the bottom ash stream can be sepa rated from the bottom ash and recirculated into the boiler.

Preferably the average syngas reactivity of the recircu- lated ilmenite particles is 0.5 or higher, preferably 0.6 or higher, preferably 0.7 or higher, more preferred 0.8 or higher .

The recirculated ilmenite particles preferably comprise the magnetic accept fraction and optionally additionally part of the magnetic reject fraction.

The at least one ash stream may be selected from the group consisting of bottom ash stream, fly ash stream, boiler ash stream and filter ash stream, preferably from the group consisting of bottom ash stream and fly ash stream.

The average syngas reactivity of the bed material prefera bly is 0.5 or higher, preferably 0.6 or higher, preferably 0.7 or higher, more preferred 0.8 or higher. The ratio of secondary air to primary fluidizing air in the boiler preferably is controlled dependent on the average syngas reactivity of the bed material.

The fluidized bed boiler preferably is a bubbling fluidized bed (BFB) boiler or a circulating fluidized bed (CFB) boiler .

Preferably the fluidized bed boiler operated according to the method of the present invention comprises a nominal thermal power of 1 MW or more. The nominal thermal power is the thermal power the boiler is designed to deliver at full load .

The following examples illustrate the invention.

Fig.l: A schematic drawing of a 12 MW_th CFB-boiler used for CFB experiments;

Figure 2: Example on a cycle in the lab-scale reactivity reactor;

Figure 3: The reactivity of ilmenite bed material, measured as CO conversion y_COz , versus exposure time, at different bed temperatures in the Chalmers 12 MWth CFB boiler;

Figure 4: The reactivity of ilmenite bed material, measured as CO conversion y_COz , versus temperature, at different ex posure times;

Figure 5: The local distribution of species (Fe, Ti, Ca, K, S, P, Si and Mg) in a magnetic particle in the bed ash, separated (accepted) by a RER magnet and analyzed by

SEM/EDX ;

Figure 6: The local distribution of species (Fe, Ti, Ca, K, S, P, Si and 0) in a non-magnetic particle in the bed ash, separated (rejected) by a RER magnet and analyzed by

SEM/EDX.

A schematic drawing of a 12 MW_th CFB-boiler located on the premises of Chalmers University is shown in Fig. 1. Refer ence numerals denote:

10 furnace

11 fuel feeding (furnace)

12 wind box

13 cyclone

14 convection path

15 secondary cyclone

16 textile filter

17 fluegas fan

18 particle distributor

19 particle cooler

20 gasifier

21 particle seal 1

22 particle seal 2

23 fuel feeding (gasifier)

24 fuel hopper (gasifier)

25 hopper

26 fuel hopper 1

27 fuel hopper 2

28 fuel hopper 3

29 sludge pump

30 hopper 31 ash removal

32 measurement ports

Experiments with rock-ilmenite as bed material were carried out in the Chalmers 12 MW_th CFB boiler (CTH) and in a 115 MW_th CFB boiler (KR) . The 115 MW_th CFB boiler is a biomass- fired CHP cycle boiler located at the Ortofta plant of Kraftringen . Samples of the bed material from the Chalmers boiler were taken at different times during a period of 320 hours of operation. No bed ash was extracted. Some bed material was elutriated by the flue gas and lost from the bed. New bed material was added to compensate for this loss.

The samples from the KR boiler were taken after 408 hours of operation with supply of fresh ilmenite. The residence time of the ilmenite particles in the KR sample varied from zero to very long time, since ilmenite was feed continu- ously but at different rates during the test. The average residence time is roughly estimated to be around 202 hours.

The bed material samples were analyzed in a separate labor atory-scale fluidized bed reactor to measure the reactiv- ity, i.e. its ability to oxidize the carbon monoxide (CO), when feeding syn-gas (50 mole-% CO and 50 mole-% ¾, apply ing a standard cycle procedure used for determining the ef ficiency of an oxygen carrier, commonly used for Chemical Looping Combustion (CLC) ) .

The cycle is shown in Fig. 2. The abbreviation OCAC stands for oxygen carrier aided combustion. The sequence was as follows : Inert: 180s

Reduction: 20s

Inert: 180s

Oxidation: 900s

The gases used were as follows:

Inert: 100% N2, lOOOml/min

Reduction: Syngas (50/50 CO and ¾ ), 900ml/min

Oxidation: 5% O2 in N2, lOOOml/min

During the experiments, gas concentrations, temperatures and pressure were logged during the entire operation every 2 s. The obtained data was then evaluated to facilitate analysis of the results. To quantify the amount of con^¬ verted gas, the gas yield of CO2 (Yco2 ) was used. The CO2 yield is defined as the fraction of CO2 in the outgoing gas divided by the sum of the fractions of carbon containing gases. Thus, a g value of 0 corresponds to no conversion while 1 corresponds to total conversion of the fuel.

xi is the fraction of component i in the outgoing gases measured after water has been removed.

To illustrate the difference in reactivity between quartz sand and used ilmenite, gas data from one cycle of each is presented in Fig. 2. The cycles were carried out at 950°C according to the phase specification above. In Fig. 2, the phases are depicted as 1) Inert phase, 2) Reducing phase,

3) Inert phase and 4) Oxidizing phase. The most elevated peak in the gas analysis seen in the re ducing phase (shown as phase 2), for quartz sand is that of CO, which is a result of unconverted fuel due to insuffi- cient oxygen in the provided gas.

This is an expected phenomenon as the quartz sand does not possess oxygen transporting ability as fresh material. Un der the combustion process, CO leaves the reactor uncon- verted. As it can be seen from Fig. 2 oxygen returns to its levels immediately when the oxidizing phase is initiated. For the same cycle with the oxygen carrier, the observed peak in the reducing phase is that of CO2, which is a re sult of fully converted fuel. As the oxidizing phase (4) is initiated, the oxygen response is delayed approximately 150 s for the cycle. The response of CO2 and simultaneous ab sence of CO is a result of the oxygen being supplied effi ciently by the oxygen carrier. Furthermore, the delay in oxygen response is explained by the re-oxidation of the ox- ygen carrier.

The results are shown in Figure 4. It is observed that the reactivity of the ilmenite varies significantly over time and is dependent on the bed temperature, see Figure 3. The reactivity at the first sampling, at 2 hours, is in the range of 0.53 to 0.70 depending on the bed temperature. The value 1 represents complete conversion of CO to C02. After an initial increase during 30 to 50 hours the reactivity levels off at the reactivity between 0.93 and 0.99. At around 150 hours the reactivity starts to decrease slowly. After 320 hours the reactivity has decreased to 0.84-0.92. Hence, the ilmenite has a high reactivity for a long time, with the maximum between 50 and 150 hours of exposure. The initial rise in reactivity is due to the activation of the ilmenite. Quartz sand included for comparison shows no re activity, as expected.

The finding how the reactivity initially increases to a maximum followed by a gradual decline can be utilized for re-distribution over time of the fraction of air supplied to the various air inlets in the boiler, e.g. the primary air, the secondary air, the tertiary air and any other air inlet registers. At increasing reactivity, the distribution of air in the vertical direction, between the various air registers, can be altered so that a greater fraction of the total air is directed to the lower registers. This moves the combustion up-streams towards the bed. Moreover, this finding can be utilized in the design of the air inlet reg isters when designing new boilers.

The bed temperature was varied between 800 to 950°C, the range of 800 to below 900°C being included for comparative purposes. The reactivity increases with a linear dependence on increasing temperature in the range tested. The maximum reactivity is achieved at 950°C and between 50 and 150 hours of exposure. This provides valuable input to the eco nomical optimization of the concept of using ilmenite in fluidized bed combustors, especially in those applications where magnetic separation is applied for recirculation of the magnetic ilmenite particles to the fluidized bed. It is then possible to control and optimize the residence time of the ilmenite particles in the bed. A longer residence time will mean lower cost for fresh ilmenite bed material addi tion to the boiler and consequently lower cost of ash depo sition given its lower flow rate. A decreasing reactivity would mean loss of economic value from the boiler opera tion. Also, a higher reactivity, due to the increased bed temperature, enables a lower excess air ratio, which in creases the boiler efficiency.

The new finding of the temperature dependence is important for the operation of existing boilers given the finding that the softening and melting temperatures of ilmenite are high enough to withstand higher temperatures than normally is used with the traditional silica sand bed material with out forming agglomerates deteriorating the fluidization. Also, ilmenite mechanically withstands a longer residence time in the bed thanks to its mechanical strength.

Samples of the ilmenite bed material from the KR boiler were separated by a magnet into a magnetic accept fraction and a "non"-magnetic reject fraction. The results are in cluded in Figure 4 as dotted lines. It was expected that the magnet would separate magnetic particles from non-mag- netic particles. The aim in this experiment though was to measure the reactivity of the two fractions, to reveal any correlation between magnetic and reactivity, which was pre viously unknown.

The accept had a significantly higher reactivity than the reject, in the range 0.68-0.92. This is a bit lower than the reactivity of the (non-separated) bed material from the CTH boiler at 36 hours of operation, Figure 4. Thus, the results obtained from the "pilot scale" CTH boiler seem to be reasonably representative for larger scale CFB boilers.

The finding that the magnetic particles are very reactive and that the non-magnetic particles are still reactive but on a lower level is valuable input to the optimization of the operation of the magnetic separation and the recovery of the used ilmenite bed particles.

The reactivity of the reject (0.20-0.37) is significant but lower than of the accept and of the initial (4 hour) values in the Chalmers boiler at the respective bed temperatures (0.40-0.61) . Apparently, the reject contains both degraded ilmenite particles and ash particles from the fuel. This quantitative information is important in the design and op eration of the system with magnetic separation and return of the accept fraction to the bed for further use to secure an optimal reactivity of the bed material and an optimal consumption of fresh bed material.

The bed material in the CTH boiler consisted of ilmenite and some ash from the biomass fuel fed. Since the ash con tent in biomass is comparatively small, only a few percent of the fuel, it can be assumed that the bed consisted of nearly only ilmenite. In the KR boiler the fraction ac cepted by the magnet was 70 %, indicating that 70% of the bed material was ilmenite. The reason for this lower con tent is that the three-week test with ilmenite was accom plished by stopping the sand feeding and starting ilmenite feeding. Thus, the bed successively changed from being a sand bed to becoming an ilmenite bed.

SEM/EDX analysis was performed on the accept and reject particles to reveal origin of the particles studied and the presence and distribution of the key species in them, with the aim to understand the results of the magnetic separa tion . The accept particle shown in Figure 5 is a typical active ilmenite particle, characterized by its content of iron and titanium throughout the particle. The typical deposition of calcium from the fuel ash is observed on the surface of the particle. The absorption of potassium inside the particle is clearly seen. In the ash layer is also found some typi cal ash components; sulphur, phosphorous, silica and magne sium. It is obvious why it ended up in the accept. The reject particle shown in Figure 6 is hollow as seen in the electron image. It is rich in calcium, oxygen and sil ica but low in iron and titanium. It has a layer with sul phur and phosphorous. Potassium is found just below the surface. This particle is probably an old ash particle be- longing to the non-magnetic fraction of the reject. It is obvious why it ended up in the reject.

Thus, the observations from Figures 5 and 6 explain the re sults from the magnetic separation.

Claims

1 A method for operating a fluidized bed boiler, com

prising the combustion of solid fuel, and carrying out the combustion process with a fluidized bed comprising ilmenite particles, characterized in that the bed tem perature of the fluidized bed is between 900°C and 1000°C.

2 The method of claim 1, wherein the bed temperature of the fluidized bed is between 920 and 980°C, preferably between 930 and 970°C, further preferred between 940 and 960 °C .

3 The method of claim 1 or 2, wherein the ilmenite con tent of the fluidized bed material is at least 30 wt . - %, preferably at least 50 wt.-%, preferably at least 70 wt.-%, further preferred at least 90 wt.-%.

4 The method of any of claims 1-3, wherein the average residence time of the ilmenite particles in the boiler is at least 100 hours, preferably at least 120 hours, further preferably at least 150 hours, further prefer ably at least 200 hours, further preferably at least 250 hours, further preferably at least 290 hours, most preferably at least 300 hours.

5 The method of any of claims 1-4, wherein the average residence time of the ilmenite particles in the boiler is less than 600 hours, preferably less than 500 hours, further preferably less than 400 hours, further preferably less than 350 hours.

6 The method of any of claims 1-5, further comprising: a) removing at least one ash stream comprising ilmen- ite particles from the boiler; b) magnetically separating ilmenite particles from the at least one ash stream. c) recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

7. The method of claim 6, wherein the recirculated ilmen ite particles comprise the magnetic accept fraction and optionally additionally part of the magnetic re ject fraction.

8. The method of 6 or 7, wherein the at least one ash

stream is selected from the group consisting of bottom ash stream, fly ash stream, boiler ash stream and fil ter ash stream, preferably from the group consisting of bottom ash stream and fly ash stream.

9. The method of any one of claims 1-8, wherein the aver age syngas reactivity y_COz of the recirculated ilmenite particles is 0.5 or higher, preferably 0.6 or higher, preferably 0.7 or higher, more preferred 0.8 or higher .

10. The method of any one of claims 1-9, wherein the aver age syngas reactivity y_Co₂ of the bed material is 0.5 or higher, preferably 0.6 or higher, preferably 0.7 or higher, more preferred 0.8 or higher.

11. The method of any one of claims 1-10, wherein the ra tio of secondary air to primary fluidizing air is con trolled dependent on the average syngas reactivity of the bed material.

12. The method of any one of claims 1-11, wherein the flu idized bed boiler is a bubbling fluidized bed (BFB) boiler or a circulating fluidized bed (CFB) boiler.

13. The method of any one of claims 1-12, wherein the flu idized bed boiler comprises a nominal thermal power of 1 MW or more.

14. The method of any one of claims 1-13, wherein the com bustion is carried out with an excess air ratio (l) below 1.3.

15. The method of claim 14, wherein l is 1.25 or less,

more preferably 1.2 or less, more preferably 1.1 or less, most preferably between 1.05 and 1.1.