BED MANAGEMENT CYCLE FOR A FLUIDIZED BED BOILER AND CORRESPONDING ARRANGEMENT

Abstract

The invention relates to abed management cycle for a fluidized bed boiler, comprising the steps of: a) providing fresh ilmenite particles as bed material to the fluidized bed boiler; b) carrying out a fluidized bed combustion process; c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler; d) separating ilmenite particles from the at least one ash stream; e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler. The invention also relates to a corresponding arrangement for carrying out fluidized bed combustion, comprising a fluidized bed boiler comprising ilmenite particles as bed material; and a system for removing ash from the fluidized bed boiler; wherein the arrangement further comprises a separator for separating ilmenite particles from the re-moved ash; and means for recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

Claims

1. A bed management cycle for a fluidized bed boiler, comprising the steps of: a) providing fresh ilmenite particles as bed material to the fluidized bed boiler; b) carrying out a fluidized bed combustion process; c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler; d) separating ilmenite particles from the at least one ash stream; e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

2. The bed management cycle of claim 1, characterized in that the ilmenite particles are separated by magnetic separation and/or electric separation, wherein preferably electric separation comprises electrostatic separation.

3. The bed management cycle of claim 1 or claim 2, characterized in that steps c), d) and e) are carried out multiple times, preferably to provide for a continuous recirculation of separated ilmenite particles into the boiler.

4. The bed management cycle of any one of claims 1-3, characterized in that the ilmenite particles are i) separated from the at least one ash stream; and/or ii) recirculated into the bed of the fluidized bed boiler; based on their degree of activation.

5. The bed management cycle of any one of claims 1-4, characterized in that all separated ilmenite particles are recirculated into the bed of the fluidized bed boiler.

6. The bed management cycle of any one of claims 1-4, characterized in that a first fraction of the separated ilmenite particles is recirculated into the bed of the fluidized bed boiler, wherein preferably a second fraction of the separated ilmenite particles is discharged; wherein further preferably the first and second fractions are determined based on the degree of activation and/or the particle size of the ilmenite particles.

7. The bed management cycle of any one of claims 1-6, characterized by one or more of the following features: 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; the fluidized bed boiler is a bubbling fluidized bed boiler or a circulating fluidized bed boiler.

8. The bed management cycle of any one of claims 1-7, 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 bed management cycle of any one of claims 1 to 8, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler is at least 75 hours, preferably at least 100 hours, further preferably at least 120 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 preferably less than 600 hours, further preferably less than 500 hours, further preferably less than 400 hours, further preferably less than 350 hours.

10. The bed management cycle of any one of claims 1-9, characterized in that the feeding rate of fresh ilmenite particles is decoupled from the ash removal rate, preferably from the bottom ash removal rate.

11. The bed management cycle of any one of claims 1-10, characterized in that it comprises controlling the ilmenite concentration in the bed; wherein preferably the ilmenite concentration is kept within a predetermined range; wherein the ilmenite concentration range in the bed is preferably 10 wt. % ato 95 wt %, more preferably 50 wt.-% to 95 wt. %, most preferably 75 wt.-% to 95 wt.-%.

12. An arrangement for carrying out fluidized bed combustion, comprising a fluidized bed boiler comprising ilmenite particles as bed material; and a system for removing ash from the fluidized bed boiler; characterized in that the arrangement further comprises a) a separator for separating ilmenite particles from the removed ash; and b) means for recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

13. The arrangement of claim 12, characterized by one or more of the following features: the separator comprises a magnetic separator and/or an electric separator, wherein preferably the electric separator is an electrostatic separator; the system for removing ash from the fluidized bed boiler is configured to remove bottom ash and/or fly ash and/or boiler ash and/or filter ash; the means for recirculating ilmenite particles are selected from the group consisting of pneumatic recirculation systems, mechanical recirculation systems and magnetic recirculation systems; the fluidized bed boiler is a bubbling fluidized bed boiler or a circulating fluidized bed boiler.

14. The arrangement of claim 12 or claim 13, characterized in that it further comprises means for discharging separated ilmenite particles.

15. The arrangement of any one of claims 12-14, characterized in that it comprises at least one selector for pre-selecting particles in the at least one ash stream before passing the ash stream to the separator; wherein preferably the at least one selector is a mechanical particle selector, preferably a sieve and/or a fluid driven particle selector, preferably a gas driven particle selector.

Description

[0044] It is shown in:

[0045] 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;

[0046] FIG. 2; a schematic picture of the boiler and gasifier system at Chalmers University of Technology;

[0047] FIG. 3: a schematic picture of the procedure for separating ilmenite particles from ashes using bottom bed samples from a commercial fluidized bed boiler;

[0048] FIG. 4: a schematic picture of the lab scale reactor system employed for ilmenite tests;

[0049] FIG. 5: equipment for determining attrition rate of particles;

[0050] 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;

[0051] 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;

[0052] FIG. 8: performance parameters used for mechanical strength evaluation for fresh ilmenite and the bed materials used within the Chalmers boiler and sampled after 28 hours of operation, 107 hours of operation and 296 hours of operation;

[0053] 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);

[0054] FIG. 10: electron micrographs of ilmenite particles before (left) and after exposure exposure in a lab scale fluidized bed reactor (right); and

[0055] FIG. 11: a schematic exemplary bed management cycle and corresponding arrangement;

[0056] FIG. 12: another schematic exemplary bed management cycle and corresponding arrangement;

[0057] FIG. 13: a phase diagram from FactSage computer calculations;

[0058] FIG. 14: a phase diagram from FactSage computer calculations;

[0059] FIG. 15: a phase diagram from FactSage computer calculations.

EXAMPLE 1

[0060] By way of example, FIGS. 11 and 12 show a schematic arrangement for carrying out fluidized bed combustion, wherein the arrangement is shown with an optional pre-selector (FIG. 11) and without an optional pre-selector (FIG. 12). The arrangement can be utilized for implementing the bed management cycle described herein.

[0061] The arrangement comprises a fluidized bed boiler, which may be, e.g. a BFB boiler or a CFB boiler. The boiler may be fed with fresh ilmenite particles as bed material. The arrangement further comprises a system for removing ash from the fluidized bed boiler, which is configured to remove bottom ash (via a bottom ash removal system) and fly ash (via a flue gas cleaning plant) as indicated. Furthermore, the arrangement comprises a magnetic separator for separating ilmenite particles from the removed bottom ash and a magnetic separator for removing ilmenite from the fly ash. Furthermore, the system comprises means (not shown) for recirculating ilmenite particles separated from the bottom ash into the bed of the fluidized bed boiler via Route B as indicated by the arrows. Preferably, the means for recirculating ilmenite particles comprise pneumatic recirculation systems, mechanical recirculation systems and/or magnetic recirculation systems. The exemplary arrangement further comprises means (not shown) for discharging separated ilmenite particles (via Route C indicated by the arrows), preferably for use in downstream applications where the need for activated ilmenite particles arises.

[0062] The arrangement also comprises an optional selector for pre-selecting particles using fluid-mechanical sieving, wherein pre-selection can be preferably based on particle size and/or mass. Route A (not according to the invention) indicates a potential recirculation path for bed material that has passed the pre-selector but is not fed to the (magnetic) separator and does not provide the benefits of the invention.

[0063] The arrangement may be utilized for implementing the bed management cycle described above. In particular, the bed management cycle may comprise the steps of: [0064] a) providing fresh ilmenite particles as bed material to the fluidized bed boiler; [0065] b) carrying out a fluidized bed combustion process; [0066] c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler; [0067] d) separating ilmenite particles from the at least one ash stream; [0068] e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler.

[0069] In this example, the removal of the bottom ash stream and the fly ash stream is shown, as well as magnetic separation of ilmenite particles from the two ash streams. Step e) is carried out on ilmenite particles removed from the bottom ash stream, wherein it is possible to recirculate a first fraction of the separated ilmenite particles into the boiler via route B and to discharge a second fraction of the separated ilmenite particles via route C. Separation and or recirculation of the ilmenite particles may be carried out based on the degree of activation of the ilmenite particles, by using the magnetic susceptibility of the ilmenite particles as a proxy for the degree of activation and setting the appropriate magnetic threshold levels, accordingly.

[0070] The bed management cycle may further comprise an optional pre-selection step, in which the particles in the bottom ash stream are pre-selected using fluid-mechanical sieving before magnetically separating the ilmenite particles from the ash stream.

[0071] The average residence time of the ilmenite particles in the fluidized bed boiler may preferably be set to at least 75 hours, further preferably at least 100 hours, further preferably at least 120 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 preferably less than 600 hours, further preferably less than 500 hours, further preferably less than 400 hours, further preferably less than 350 hours.

[0072] Preferably, the feeding rate of fresh ilmenite particles is decoupled from the ash removal rate, preferably from the bottom ash removal rate.

[0073] The exemplary bed management cycle may further comprise controlling the ilmenite concentration in the bed; wherein preferably the ilmenite concentration is kept within a predetermined range; wherein the ilmenite concentration range in the bed is preferably 10 wt. % ato 95 wt %, more preferably 50 wt.-% to 95 wt. %, most preferably 75 wt.-% to 95 wt.-%.

EXAMPLE 2

[0074] The Chalmers 12 MW.sub.th CFB-boiler is shown in FIG. 2. Reference numerals denote: [0075] 10 furnace [0076] 11 fuel feeding (furnace) [0077] 12 wind box [0078] 13 cyclone [0079] 14 convection path [0080] 15 secondary cyclone [0081] 16 textile filter [0082] 17 fluegas fan [0083] 18 particle distributor [0084] 19 particle cooler [0085] 20 gasifier [0086] 21 particle seal 1 [0087] 22 particle seal 2 [0088] 23 fuel feeding (gasifier) [0089] 24 fuel hopper (gasifier) [0090] 25 hopper [0091] 26 fuel hopper 1 [0092] 27 fuel hopper 2 [0093] 28 fuel hopper 3 [0094] 29 sludge pump [0095] 30 hopper [0096] 31 ash removal [0097] 32 measurement ports

[0098] 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.

[0099] 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

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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).

[0105] The reactivity of the materials as oxygen carriers were assessed through two main performance parametersthe oxygen carrier conversion)() and the resulting gas conversion(.sub.i.sup.).

[0106] 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.

[0107] The oxygen carrier mass-based conversion is calculated as a function of time t from the mass balance of oxygen over the reactor:

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

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

[0108] The gas conversion .sub.CO for syngas is defined as follows:

[00003]${}_{\mathrm{CO}}=\frac{y_{\mathrm{CO}.Math..Math.2}^{-}}{y_{\mathrm{CO}.Math..Math.2}^{-}+y_{\mathrm{CO}}^{-}}$

.sub.i.sup. 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] These experiments show 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. The data further provide evidence that it is possible to recirculate used ilmenite particles into the boiler multiple times for an extended period of time as the recirculated ilmenite particles will still have very good oxygen-carrying properties.

EXAMPLE 4A

[0115] The samples from the Chalmers boiler obtained in Example 2 and the fresh ilmenite were also tested in an attrition rig as described below.

[0116] 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.

[0117] 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

[0118] FIG. 9 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.

[0119] 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

[0120] 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

[0121] 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 keep the differential pressure over the bed sufficient.

[0122] 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 a 45 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).

[0123] Furthermore, magnetic separation of ilmenite particles and ash has been successfully tested for rock and sand ilmenite with the Chalmers boiler.

EXAMPLE 7

[0124] 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; FeO, 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.