METHOD FOR OPERATING A FLUIDIZED BED BOILER

Abstract

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, wherein the average residence time of the ilmenite particles in the boiler is at least 75 hours. The invention further relates to ilmenite particles obtainable by a corresponding method and the use of said ilmenite particles as oxygen-carrying material.

Claims

1. A method for operating a fluidized bed boiler, comprising carrying out the combustion process with a fluidized bed comprising ilmenite particles, wherein the average residence time of the ilmenite particles in the boiler is at least 75 hours.

2. The method of claim 1, characterized in that 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.

3. The method of claim 1 or claim 2, 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.

4. The method of any one of claims 1-3, further comprising: a) removing at least one ash stream comprising ilmenite particles from the boiler; b) separating ilmenite particles from the at least one ash stream.

5. The method of claim 4, characterized in that the ilmenite particles are magnetically separated from the at least one ash stream.

6. The method of claim 4, characterized in that the ilmenite particles are electrically separated from the at least one ash stream, preferably by means of an electrostatic separator.

7. The method of any one of claims 4-6, characterized in that steps a) and b) are carried out multiple times.

8. The method of any one of claims 4 to 7, characterized in that it further comprises a pre-selection step, in which the particles in the at least one ash stream are pre-selected before separating the ilmenite particles from the ash stream; wherein preferably the pre-selection comprises mechanical particle separation and/or fluid driven particle separation, more preferably sieving and/or gas driven particle separation.

9. The method of any one of claims 4 to 8, characterized in that the at least one ash stream is 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.

10. The method of any one of claims 4 to 9, further comprising c) recirculating separated ilmenite particles into the bed of the fluidized bed boiler; wherein preferably steps a), b) and c) are carried out multiple times.

11. The method of any one of claims 1-10, further comprising feeding fresh ilmenite particles to the boiler at a rate compensating for ilmenite lost with the removal of an ash stream from the boiler; wherein preferably the removed ash stream comprises fly ash and/or bottom ash.

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

13. Ilmenite particles, obtainable by a method comprising: a) providing fresh ilmenite particles as bed material to a fluidized bed boiler, preferably a bubbling fluidized bed (BFB) boiler or a circulating fluidized bed (CFB) boiler; b) carrying out a combustion process with the fluidized bed boiler; wherein the average residence time of the ilmenite particles in the boiler is at least 75 hours; c) removing ilmenite particles from the boiler.

14. The ilmenite particles of claim 13, 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 preferably at least 200 hours, further preferably at least 250 hours, further preferably at least 290 hours, most preferably at least 300 hours and/or 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.

15. Use of ilmenite particles according to claim 13 or claim 14 as oxygen-carrying material.

Description

[0044] In the following, advantageous embodiments will be explained by way of example.

[0045] It is shown in:

[0046] FIG. 1: a schematic illustration of the outward diffusion of Fe and the formation of Fe-shell around ilmenite particles exposed to combustion conditions in a fluidized bed boiler;

[0047] FIG. 2; a schematic picture of the boiler and gasifier system at Chalmers University of Technology;

[0048] FIG. 3: a schematic picture of the procedure for magnetic separation of ilmenite particles from ashes using bottom bed samples from a commercial fluidized bed boiler;

[0049] FIG. 4: a schematic picture of the lab scale reactor system employed for ilmenite tests;

[0050] FIG. 5: equipment for determining attrition rate of particles;

[0051] FIG. 6: average gas conversion of CO to CO.sub.2 at 850, 900 and 950 C., for bed materials used within the Chalmers boiler and samples after 28 hours of operation, 107 hours of operation and 296 hours of operation and for fresh ilmenite particles activated in the lab reactor;

[0052] FIG. 7: average oxygen carrier mass-based conversion at 850, 900 and 950 C., for bed materials used within the Chalmers boiler and sampled after 28 hours of operation, 107 hours of operation and 296 hours of operation and for fresh ilmenite activated in the lab reactor;

[0053] FIG. 8: performance parameters used for mechanical strength evaluation for the bed materials used within the Chalmers boiler and sampled after 28 hours of operation, 107 hours of operation and 296 hours of operation;

[0054] FIG. 9: electron micrographs of fresh ilmenite particles (left) and ilmenite particles that have been used as bed material in a CFB boiler after 24 h of operation (right);

[0055] FIG. 10: electron micrographs of ilmenite particles before (left) and after exposure in a lab scale fluidized bed reactor (right); and

[0056] FIG. 11: a schematic exemplary fluidized bed combustion system;

[0057] FIG. 12: another schematic exemplary fluidized bed combustion system;

[0058] FIG. 13: a phase diagram from FactSage computer calculations;

[0059] FIG. 14: a phase diagram from FactSage computer calculations;

[0060] FIG. 15: a phase diagram from FactSage computer calculations.

EXAMPLE 1

[0061] By way of example, FIG. 11 shows a schematic diagram of a preferred fluidized bed boiler set-up.

[0062] The boiler is operated by carrying out the combustion process with a fluidized bed comprising ilmenite particles. The average residence time of the ilmenite particles in the boiler is set to at least 75 hours, preferably to at least 100 hours, further 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.

[0063] Furthermore, the average residence time of the ilmenite particles in the boiler can preferably be set to less than 600 hours, further preferably less than 500 hours, further preferably less than 400 hours, further preferably less than 350 hours.

[0064] Preferably, the bottom ash comprising ilmenite particles is removed from the boiler (typically via a bottom ash removal system).

[0065] Further preferably, the bottom ash stream can optionally be pre-treated to select particles in the ash stream based on their size, preferably by fluid-mechanical sieving. This pre-selection step is advantageous when the fluidized bed boiler is operated with a fuel type, such as, e.g., waste, which leads to a high ash content, e.g. 20-30 wt-% ash with respect to the total weight of the fuel. Pre-selection is optional and FIG. 12 shows a schematic diagram of a preferred fluidized bed boiler set-up without this step.

[0066] Further preferably, the flue gas is also cleaned to remove fly ash which comprises ilmenite particles. Preferably, ilmenite particles are separated from the bottom ash and fly ash streams by means of magnetic separators. Another preferred option for separation of ilmenite particles from the ash stream is the use of electrostatic separators.

[0067] FIGS. 11 and 12 diagrammatically show a preferred location of the magnetic separators in a fluidized bed combustion set-up along with a preferred location for the optional pre-selection device.

[0068] Preferably, the steps of removal of the ash streams from the boiler and separation of the ilmenite particles from the ash streams are carried out multiple times to provide a continuous stream of separated ilmenite particles.

[0069] Preferably, the separated ilmenite particles are recirculated into the bed of the fluidized bed boiler as indicated in FIG. 11 and FIG. 12. Route B in FIGS. 11 and 12 indicates a preferred recirculation route into the boiler of ilmenite particles separated magnetically from the bottom ash stream, preferably after having undergone optional fluid-mechanical sieving (FIG. 11).

[0070] Route A shown in FIG. 11 indicates a possible recirculation route into the boiler of bed material separated only by fluid-mechanical sieving from the bottom ash stream.

[0071] Preferably, the average residence time of the ilmenite particles in the boiler is set by adjusting the feeding rate of fresh ilmenite and the recirculation rate of separated ilmenite.

[0072] Another preferred option is to discharge all or a fraction of the separated ilmenite particles for use in further activities as diagrammatically indicated in FIG. 11 and FIG. 12 by route C. In addition to the routes for the bottom ash stream, FIGS. 11 and 12 also indicate a preferred removal of a fly ash stream in the flue gas cleaning plant and subsequent magnetic separation of the ilmenite particles from the fly ash. Preferably, the ilmenite particles separated from the fly ash, due to their small size, are not recirculated into the boiler but discharged via Route C for use in other applications.

EXAMPLE 2

[0073] The Chalmers 12 MW.sub.th CFB-boiler is shown in FIG. 2. Reference numerals denote: [0074] 10 furnace [0075] 11 fuel feeding (furnace) [0076] 12 wind box [0077] 13 cyclone [0078] 14 convection path [0079] 15 secondary cyclone [0080] 16 textile filter [0081] 17 fluegas fan [0082] 18 particle distributor [0083] 19 particle cooler [0084] 20 gasifier [0085] 21 particle seal 1 [0086] 22 particle seal 2 [0087] 23 fuel feeding (gasifier) [0088] 24 fuel hopper (gasifier) [0089] 25 hopper [0090] 26 fuel hopper 1 [0091] 27 fuel hopper 2 [0092] 28 fuel hopper 3 [0093] 29 sludge pump [0094] 30 hopper [0095] 31 ash removal [0096] 32 measurement ports

[0097] A 300 hour long combustion experiment using rock ilmenite as bed material was conducted in the Chalmers 12 MW.sub.th CFB boiler, FIG. 2. The boiler was operated using wood-chips as fuel and the temperature in the boiler was kept around 830-880 C. during the experiment. No discharge of the ilmenite in the form of bottom bed regeneration was carried out during the whole experiment, this is different compared to operation with ordinary silica sand where around 10-15 wt. % of the bed is discharged and replaced with fresh silica sand on a daily basis.

[0098] Fresh ilmenite was fed only to compensate for the fly ash losses. Samples of the bed material were collected in location H2 by using a water-cooled bed sampling probe, after 28, 107 and 296 hours. These samples were further evaluated in a lab-scale fluidized bed reactor system (see example 3).

EXAMPLE 3

[0099] Three samples of bottom bed from the Chalmers boiler (see Example 2) were chosen for the evaluation. The samples were collected in the combustor after 28, 107 and 296 hours of operation. All samples were tested separately in a lab-scale fluidized bed reactor in a cyclic mode according to the below-described principle of altering the environment between oxidizing and reducing environment. In addition to the three samples from the Chalmers boiler, fresh ilmenite particles from the same mine (Titania A/S) were tested as a reference. In this case, the activation of the ilmenite was conducted within the lab-scale reactor and the time period represents around 20 cycles. In the lab-scale reactor system the exposure time for the ilmenite is referred to as cycles meanwhile the exposer time with in a combustor would be referred to as minutes or hours. A rather harsh and conservative correlation between the cycles in the lab-scale reactor system and the residence time would be that 20 cycles within the reactor system corresponds to 1 hour of operation in a conventional FBC boiler.

[0100] With regards to the chemical impact and the chemical aging of ilmenite, the oxygen-carrying properties of the ilmenite and its reactivity towards oxidizing carbon monoxide (CO) into carbon dioxide (CO.sub.2) have been examined.

[0101] The evaluation of the reactivity and oxygen transfer is based on experimental tests performed in a lab-scale fluidized reactor system, shown schematically in FIG. 4. All experiments are carried out in a fluidized bed quartz glass reactor with an inner diameter of 22 mm and an overall length of 870 mm. A porous quartz plate is mounted in the centre of the reactor and serves as gas distributor. The sample is weighed before the experiment and placed on the quartz plate at ambient conditions. 10-15 g of material with a particle size fraction of 125-180 m is used.

[0102] Temperatures of 850, 900 and 950 C. have been investigated in the present study. The temperature is measured by a type K CrAl/NiAl thermocouple. The tip of the thermocouple is located about 25 mm above the porous plate to make sure that it is in contact with the bed when fluidization occurs. The thermocouple is covered by a quartz glass cover, protecting it from abrasion and the corrosive environment. The reactor is heated by an external electrical oven.

[0103] During heating and oxidation, the particles are exposed to a gas consisting of 21 vol. % O.sub.2 diluted with nitrogen (N.sub.2). After the desired temperature has been reached, the gas atmosphere is shifted from oxidizing to reducing conditions by changing the ingoing gas. In order to prevent combustion of fuel by oxygen from the oxidation phase as well as to prevent reduction gas in the beginning of the oxidation phase, both phases are separated by a 180 s inert period. During the inert period the reactor is flushed with pure nitrogen. The fuel gases as well as synthetic air are taken from gas bottles whereas the nitrogen (N.sub.2) is supplied from a centralized tank. The fluidizing gas enters the reactor from the bottom. The gas composition is controlled by mass flow controllers and magnetic valves. The water content in the off gas is condensed in a cooler before the concentrations of CO, CO.sub.2, CH.sub.4, H.sub.2 and O.sub.2 are measured downstream in a gas analyser (Rosemount NGA 2000).

[0104] The reactivity of the materials as oxygen carriers were assessed through two main performance parametersthe oxygen carrier conversion () and the resulting gas conversion (y.sub.).

[0105] The conversion of the oxygen carrier is described by its mass-based conversion , according to

[00001]$=\frac{m}{m_{\mathrm{ox}}}$

where m denotes the actual mass of the oxygen carrier and m.sub.ox is the mass of the oxidized oxygen carrier. It is assumed that the changes in the mass of the oxygen carrier originate only from the exchange of oxygen.

[0106] The oxygen carrier mass-based conversion is calculated as a function of time t from the mass balance of oxygen over the reactor:

[00002]$\mathrm{syngas}.Math.\text{:}.Math..Math.{}_t={}_{t-1}-{}_{t-1}^t.Math.\frac{\stackrel{.}{n}-M_O}{m_{\mathrm{ox}}}.Math.\left(2.Math.y_{\stackrel{\_}{C}.Math.O.Math..Math.2}+y_{\stackrel{\_}{C}.Math.O}-y_{\stackrel{\_}{H}.Math..Math.2}+2.Math.y_{\stackrel{\_}{O}.Math..Math.2}\right).Math..Math.\mathrm{dt}$

{dot over (n)}.sup. is the molar flow rate at the reactor outlet and M.sub.O the molar mass of oxygen.

[0107] The gas conversion .sub.CO for syngas is defined as follows:

[00003]${}_{\mathrm{CO}}=\frac{y_{\stackrel{\_}{C}.Math.O.Math..Math.2}}{y_{\stackrel{\_}{C}.Math.O.Math..Math.2}+y_{\stackrel{\_}{C}.Math.O}}$

y.sub. is the molar fraction of the components in the effluent gas stream. In order for ilmenite to reach its maximum performance it needs to be activated through several consecutive redox cycles. Therefore, the number of cycles needed for activation was also used as a performance parameter for choice of material as this number is indicative for the time point when the oxygen carrier reaches its full potential. In a CFB boiler the activation occurs naturally since the particles meet alternating reducing/oxidizing environments while circulating in the CFB loop.

[0108] FIG. 6 show the gas conversion of CO into CO.sub.2 for three temperatures for the lab-scale experiments using the three bottom bed samples from the Chalmers boiler (Example 2) and for two temperatures for fresh ilmenite that was activated in the lab-scale reactor.

[0109] The lower line in FIG. 6 represents the experiments with the fresh ilmenite. The experiments using the three bottom bed samples collected at different times in the Chalmers give much higher gas conversion of CO to CO.sub.2 than what was expected. In fact, the gas conversion for these samples are 15%-units higher than the one with the fresh ilmenite used as reference. The relatively good agreement in gas conversion between the three samples from the Chalmers boiler clearly highlights the effects initiated from long term operation in a FBC-boiler.

[0110] Overall, these data show the surprising result that the ilmenite could be used for at least 300 hours in a combustor. As the gas conversion is still much higher than for fresh particles after 300 hours the results indicate that it is possible to extend the residence time of the ilmenite particles significantly longer.

[0111] FIG. 7 shows the average oxygen carrier mass-based conversion for three temperatures for the lab-scale experiments using the three bottom bed samples from the Chalmers boiler (Example 2) and for two temperatures for the fresh ilmenite that was activated in the lab-scale reactor.

[0112] Again, the lower line in FIG. 7 represents the experiments with the fresh ilmenite. The Omega number for the three bottom bed samples from the Chalmers boiler is much higher than expected. The discovery in increased gas conversion agrees well with the increase in oxygen transfer and the omega number and the gas conversion is therefore supporting each other.

[0113] These experiments provide evidence that the ilmenite particles can be used as oxygen-carrier even after having been exposed to boiler conditions for an extended period of time, ranging up to at least 300 hours.

EXAMPLE 4A

[0114] The samples from the Chalmers boiler obtained in Example 2 and the fresh ilmenite were also tested in an attrition rig as described below.

[0115] Attrition index was measured in an attrition rig that consists of a 39 mm high conical cup with an inner diameter of 13 mm in the bottom and 25 mm in the top, see FIG. 5. At the bottom of the cup through a nozzle with an inner diameter of 1.5 mm (located at the bottom of the cup) air is added at a velocity of 10 l/min. Prior to the experiments the filter is removed and weighed. The cup is then dismantled and filled with 5 g of particles. Both parts are then reattached and the air flow is turned on for 1 hour. In order to get the development of fines during the attrition tests the air flow is stopped at chosen intervals and the filter is removed and weighed.

[0116] FIG. 8 shows the results from the attrition experiments for the experiments using the three bottom bed samples from the Chalmers boiler (see Example 2) and fresh ilmenite. FIG. 8 shows the surprising result that after an extended residence time of the particles in the boiler the rate of attrition for the particles decreases. This suggests that the mechanical strength of the particles is sufficient for recycling even after 296 hours in a fluidized bed boiler.

EXAMPLE 4B

[0117] FIG. 9, which shows electron micrographs of fresh rock ilmenite particles and rock ilmenite particles that have been exposed to a redox environment in the Chalmers CFB boiler for 24 hours.

[0118] The exposed rock ilmenite particles have smoother edges and are likely to produce less fines. Without wishing to be bound by theory, it is contemplated that this phenomenon is likely coupled to the particles being exposed to friction in between particles and boiler walls resulting in a much smoother and round surface than the fresh particles. The increased roundness leads to a less erosive surface which is less abrasive to the walls of the boiler.

EXAMPLE 5

[0119] FIG. 10 shows electron micrographs of ilmenite particles before and after exposure in a lab scale fluidized bed reactor, an overview of the cross-section and elemental maps of Iron (Fe) and Titanium (Ti) are shown for both cases. The overview of the particles (top) shows once again that the exposed particles become less sharp. From the micrographs (center) it can also be confirmed that the porosity of the particles increases with exposure, with some of the particles having multiple cracks in their structure. The elemental mapping (bottom, right) shows that the Fe and the Ti fraction is homogeneously spread within the fresh ilmenite particles. In comparison to the fresh particles the exposed ones (bottom, left) clearly indicate that the Fe is migrating towards the surface of the ilmenite particles while the Ti fraction is more homogeneously spread in the particle. The iron migration is schematically indicated in FIG. 1 and a desired mechanism since the invention has recognized that this increases the possibilities for efficient separation of the ilmenite particles by a magnetic process.

EXAMPLE 6

[0120] Magnetic separation was evaluated using bottom bed samples from an industrial scaled boiler operated with ilmenite as bed material. The 75 MW.sub.th municipal solid waste fired boiler was operated using ilmenite as bed material during more than 5 months. Several bottom bed samples were collected during this operating time. The fuel that is fed to this boiler commonly comprises 20-25 wt. % non-combustibles in the form of ash and the regeneration of the bottom bed is thereby a continuous process 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.

[0121] The potential of separating the ilmenite from the ash fraction was investigated for six arbitrary samples collected during the operation of the boiler. A 1 meter long half pipe made from a steel plate was used together with a magnet as indicated in FIG. 3. The magnet was placed on the backside of the halfpipe and the halfpipe was tilted in a45 angel with the bottom end resting in a metal vessel (1). (i), A portion of the sample, roughly 10-15 g, was poured into the halfpipe and the material was allowed to flow across the metal surface by gravity. When the material flowed across the surface where the magnet was acting on the steel plate, the ilmenite was captured and the ash fraction passed by and was captured in the metal vessel (1). (ii), The half pipe was moved to the metal vessel (2) and the magnet was removed and the ilmenite fraction was captured in the vessel (2).

[0122] Furthermore, magnetic separation of ilmenite particles and ash has been successfully tested for rock and sand ilmenite with the Chalmers boiler.

EXAMPLE 7

[0123] FIGS. 13, 14 and 15 show phase diagrams from FactSage calculations. Such diagrams show which compounds and phases of the compounds are stable under the conditions given in the calculation. FIG. 13 shows the composition versus the gaseous oxygen concentration at the temperature 1173 K, which is the normal combustion temperature in FB boilers. FIG. 14 shows the stable compounds and phases of Fe, Ti and O versus the concentration of Fe and Ti, also at 1173 K. FIG. 15 shows the stable compounds and phases between the pure oxides; FO, TiO.sub.2, and Fe.sub.2O.sub.3. For example, at high concentration of oxygen and no Ti, the stable compound is Fe.sub.2O.sub.3. At reducing condition (=low oxygen concentration) and no Ti, the stable compound is FeO.