Patent Description:
Fluidized bed combustion is a well-known technique, wherein the fuel is suspended in a hot fluidized bed of solid particulate 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 velocity 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 circulating 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 recirculated 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 particular 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 differential 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 (<NPL>). Ilmenite is a naturally occurring mineral which consists mainly of iron titanium oxide (FeTiO<NUM>) and can be repeatedly oxidized and reduced. 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., <NUM> wt. -% of silica sand or fuel ash particles.

<CIT> relates to a bed management cycle for a fluidized bed boiler and to a corresponding arrangement for carrying out fluidized bed combustion.

<CIT> relates to a fluidized bed reactor for the chemical and/or physical treatment of fluidizable substances and to a process therefor.

<CIT> discloses a fluidized bed method for the heat treatment of solids containing titanium.

<CIT> and <CIT> both relate to a fluidized bed reactor of solid hydrocarbon feedstocks.

<NPL>, 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 operation of a fluidized bed boiler, such as, e.g., a circulating fluidized bed boiler or a bubbling fluidized bed boiler.

This problem is solved by the features of the independent claim.

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

The invention is directed 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, wherein the bed temperature of the fluidized bed is between <NUM> and <NUM>. The invention is based on the surprising finding that the reactivity of the ilmenite comprising bed significantly increases at temperatures of <NUM> and above. Reactivity is defined as the ability to oxidize carbon monoxide (CO), when feeding syn-gas (<NUM> mole-% CO and <NUM> mole-% H<NUM>, applying a standard cycle procedure used for determining the efficiency 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 reaction zones e.g. for oxidation and reduction as in chemical looping combustion (CLC) known from the prior art. Preferably, the combustion process is carried out with air as fluidizing gas for fluidizing the bed material.

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

The claimed method operates the boiler at an excess air ratio (λ) below <NUM>. The excess air ratio λ is a common parameter in the operation of BFB boilers and is defined as the mass ratio of air to fuel (MRair/fuel = mair/mfuel) actually present in the combustion process divided by the stoichiometric mass ratio of air to fuel. That is, λ = (MRair/fuel) actual/ (MRair/fuel) stoichiometric. The mass ratio of air to fuel actually present in the boiler is determined by the amount of fuel and air supplied to the boiler. The stoichiometric mass ratio of air to fuel is the mass ratio required by stoichiometry for complete combustion of the provided fuel and can be calculated for any given fuel composition.

In preferred embodiments, λ is <NUM> or less, more preferably <NUM> or less, more preferably <NUM> or less, most preferably between <NUM> and <NUM>. Preferably, for the combustion of waste-based fuel, λ is <NUM> or less, more preferably <NUM> or less, more preferably between <NUM> and <NUM>, most preferably between <NUM> and <NUM>. For the combustion of biomass fuel, λ preferably is <NUM> or less, more preferably <NUM> or less, more preferably between <NUM> and <NUM>, most preferably between <NUM> and <NUM>.

In the claimed process, the fluidizing bed comprising ilmenite 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 material.

The claimed method is not directed to the operation of laboratory-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. Typically, laboratory-scale reactors have a thermal output of less than <NUM> MW, if any. A laboratory-scale reactor as disclosed in <CIT> does not carry out combustion of a solid fuel at all, instead, it is designed to test reactivity of a bed material under the influence of different gas compositions. Such lab-scale reactors are not to be understood as fluidized bed boilers in the sense of the present invention.

Ilmenite is a naturally occurring mineral which consists mainly of iron titanium oxide (FeTiO<NUM>). Ilmenite can be repeatedly 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 process (<NPL>). 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., <NUM> 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, ilmenite particles 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 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 oxygen-carrying properties and reactivity towards oxidizing carbon monoxide (CO) into carbon dioxide (CO<NUM>), so called "gas conversion".

According to the invention, the bed temperature range of the fluidized bed is between <NUM> and <NUM>, further preferably between <NUM> and <NUM>, further preferred between <NUM> and <NUM>. Preferably the ilmenite content of the fluidized bed material is at least <NUM> wt. -%, preferably at least <NUM> wt. -%, preferably at least <NUM> wt. -%, further preferred at least <NUM> wt.

In a preferred embodiment the average residence time of the ilmenite particles in the boiler is at least <NUM> hours, preferably at least <NUM> hours, further preferably at least <NUM> hours, further preferably at least <NUM> hours, further preferably at least <NUM> hours, further preferably at least <NUM> hours, most preferably at least <NUM> hours.

In a preferred embodiment the average residence time of the ilmenite particles in the boiler is less than <NUM> hours, preferably less than <NUM> hours, further preferably less than <NUM> hours, further preferably less than <NUM> hours.

The invention allows for average residence times of the ilmenite particles in the boiler which are at least a factor of <NUM> 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 particles 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 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.

According to a preferred embodiment the invention contemplates bed material recirculation comprising the steps:.

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 (CO<NUM>), so called "gas conversion" and good mechanical strength. Ilmenite particles, 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 ilmenite particles and their attrition resistance rapidly deteriorate during the combustion process in a fluidized bed boiler at elevated bed temperatures. It was therefore surprising 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 significantly reduces the overall consumption of the natural resource ilmenite and makes the combustion process more environmentally 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 concentration 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 classification using a mesh size from <NUM> to <NUM>,<NUM> 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 particle size of <NUM> or lower so that the mechanical classifier provides a fine particle size fraction having a more homogenous size distribution while still comprising the majority 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 magnetic 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 simpler operation of the magnetic separator as it generates a narrower particle size distribution.

Preferably the mechanical classifier comprises a mesh size from <NUM> to <NUM>, preferably <NUM> to <NUM>. A typical preferred mesh size is <NUM>. 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 comprises 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 <NUM> 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 mechanical classifier of feature b. of the main claim and the magnetic 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 magnetic 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 magnetic separation in the subsequent step.

Preferably the magnetic separator comprises a field intensity of <NUM>,<NUM> Gauss or more, preferably <NUM>,<NUM> 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 having 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. Rare earth roll magnetic separators are high intensity, high gradient, permanent magnetic separators 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 comprising rare earth permanent magnets. While being transported around the roll ilmenite remains attracted to the belt whereas the nonmagnetic particle fracture falls off. Mechanical separator separates these two particle fractions.

In one embodiment the magnetic field is axial, i.e. parallel 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 nonmagnetic 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 material which can come at the cost of less purity due to entrapped 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 ilmenite bed material is at least <NUM> by mass, preferably at least <NUM> by mass. That means that at least <NUM> or <NUM> wt. % of ilmenite comprised in the bottom ash stream can be separated from the bottom ash and recirculated into the boiler.

Preferably the average syngas reactivity of the recirculated ilmenite particles is <NUM> or higher, preferably <NUM> or higher, preferably <NUM> or higher, more preferred <NUM> 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 preferably is <NUM> or higher, preferably <NUM> or higher, preferably <NUM> or higher, more preferred <NUM> 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 <NUM> 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.

A schematic drawing of a <NUM> MWth CFB-boiler located on the premises of Chalmers University is shown in <FIG>. Reference numerals denote:.

Experiments with rock-ilmenite as bed material were carried out in the Chalmers <NUM> MWth CFB boiler (CTH) and in a <NUM> MWth CFB boiler (KR). The <NUM> MWth CFB boiler is a biomass-fired CHP cycle boiler located at the Örtofta plant of Kraftringen.

Samples of the bed material from the Chalmers boiler were taken at different times during a period of <NUM> 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 <NUM> 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 continuously but at different rates during the test. The average residence time is roughly estimated to be around <NUM> hours.

The bed material samples were analyzed in a separate laboratory-scale fluidized bed reactor to measure the reactivity, i.e. its ability to oxidize the carbon monoxide (CO), when feeding syn-gas (<NUM> mole-% CO and <NUM> mole-% H<NUM>, applying a standard cycle procedure used for determining the efficiency of an oxygen carrier, commonly used for Chemical Looping Combustion (CLC)).

The cycle is shown in <FIG>. The abbreviation OCAC stands for oxygen carrier aided combustion. The sequence was as follows:.

During the experiments, gas concentrations, temperatures and pressure were logged during the entire operation every <NUM>. The obtained data was then evaluated to facilitate analysis of the results. To quantify the amount of converted gas, the gas yield of CO<NUM> (γCO2 ) was used. The CO<NUM> yield is defined as the fraction of CO<NUM> in the outgoing gas divided by the sum of the fractions of carbon containing gases. Thus, a γ value of <NUM> corresponds to no conversion while <NUM> corresponds to total conversion of the fuel. <MAT> 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>. The cycles were carried out at <NUM> according to the phase specification above. In <FIG>, the phases are depicted as <NUM>) Inert phase, <NUM>) Reducing phase, <NUM>) Inert phase and <NUM>) Oxidizing phase.

The most elevated peak in the gas analysis seen in the reducing phase (shown as phase <NUM>), for quartz sand is that of CO, which is a result of unconverted fuel due to insufficient oxygen in the provided gas.

This is an expected phenomenon as the quartz sand does not possess oxygen transporting ability as fresh material. Under the combustion process, CO leaves the reactor unconverted. As it can be seen from <FIG> 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 CO<NUM>, which is a result of fully converted fuel. As the oxidizing phase (<NUM>) is initiated, the oxygen response is delayed approximately <NUM> for the cycle. The response of CO<NUM> and simultaneous absence of CO is a result of the oxygen being supplied efficiently by the oxygen carrier. Furthermore, the delay in oxygen response is explained by the re-oxidation of the oxygen carrier.

The results are shown in <FIG>. It is observed that the reactivity of the ilmenite varies significantly over time and is dependent on the bed temperature, see <FIG>. The reactivity at the first sampling, at <NUM> hours, is in the range of <NUM> to <NUM> depending on the bed temperature. The value <NUM> represents complete conversion of CO to CO2. After an initial increase during <NUM> to <NUM> hours the reactivity levels off at the reactivity between <NUM> and <NUM>. At around <NUM> hours the reactivity starts to decrease slowly. After <NUM> hours the reactivity has decreased to <NUM>-<NUM>. Hence, the ilmenite has a high reactivity for a long time, with the maximum between <NUM> and <NUM> hours of exposure. The initial rise in reactivity is due to the activation of the ilmenite. Quartz sand included for comparison shows no reactivity, 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 registers when designing new boilers.

The bed temperature was varied between <NUM> to <NUM>, the range of <NUM> to below <NUM> 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 <NUM> and between <NUM> and <NUM> hours of exposure. This provides valuable input to the economical 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 addition to the boiler and consequently lower cost of ash deposition given its lower flow rate. A decreasing reactivity would mean loss of economic value from the boiler operation. Also, a higher reactivity, due to the increased bed temperature, enables a lower excess air ratio, which increases 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 without 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 included in <FIG> as dotted lines. It was expected that the magnet would separate magnetic particles from non-magnetic 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 previously unknown.

The accept had a significantly higher reactivity than the reject, in the range <NUM>-<NUM>. This is a bit lower than the reactivity of the (non-separated) bed material from the CTH boiler at <NUM> hours of operation, <FIG>. 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 (<NUM>-<NUM>) is significant but lower than of the accept and of the initial (<NUM> hour) values in the Chalmers boiler at the respective bed temperatures (<NUM>-<NUM>). Apparently, the reject contains both degraded ilmenite particles and ash particles from the fuel. This quantitative information is important in the design and operation 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 content 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 accepted by the magnet was <NUM> %, indicating that <NUM>% of the bed material was ilmenite. The reason for this lower content is that the three-week test with ilmenite was accomplished 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 separation.

The accept particle shown in <FIG> 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 typical ash components; sulphur, phosphorous, silica and magnesium. It is obvious why it ended up in the accept.

The reject particle shown in <FIG> is hollow as seen in the electron image. It is rich in calcium, oxygen and silica but low in iron and titanium. It has a layer with sulphur and phosphorous. Potassium is found just below the surface. This particle is probably an old ash particle belonging to the non-magnetic fraction of the reject. It is obvious why it ended up in the reject.

Claim 1:
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, wherein the bed temperature of the fluidized bed is between <NUM> and <NUM>, wherein the combustion is carried out with an excess air ratio (λ) below <NUM>.