阅读说明：本技术 用于操作流化床锅炉的方法 (Method for operating a fluidized bed boiler ) 是由 本特-奥克·安德松 于 2018-04-18 设计创作，主要内容包括：本发明涉及一种操作流化床锅炉(6)的方法,包括步骤：a)将具有0.8或以下的形状因子的新鲜钛铁矿颗粒作为床材料提供至流化床锅炉(6)；b)进行流化床燃烧过程；c)从流化床锅炉除去至少一个包含钛铁矿颗粒的灰流；d)从所述至少一个灰流中分离钛铁矿颗粒,其中分离包括使用包含2,000高斯或以上的场强的磁分离器(12)的步骤；e)将分离的钛铁矿颗粒再循环到流化床锅炉的床中；其中,钛铁矿颗粒在流化床中的平均停留时间为100小时或以上。(The invention relates to a method of operating a fluidized bed boiler (6), comprising the steps of: a) providing fresh ilmenite particles having a shape factor of 0.8 or less as bed material to a fluidized bed boiler (6); 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, wherein separating comprises the step of using a magnetic separator (12) comprising a field strength of 2,000 gauss or greater; e) recycling the separated ilmenite particles to the bed of the fluidized bed boiler; wherein the mean residence time of the ilmenite particles in the fluidized bed is 100 hours or more.)

1. A method for operating a fluidized bed boiler (6), comprising the steps of:

a) Providing fresh ilmenite particles having a shape factor of 0.8 or less as bed material to the fluidized bed boiler (6);

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, wherein the separating comprises the step of using a magnetic separator (12) comprising a field strength of 2,000 gauss or greater;

e) Recycling the separated ilmenite particles to the bed of the fluidized bed boiler;

Wherein the mean residence time of the ilmenite particles in the fluidized bed is 100 hours or more.

2. The process according to claim 1, characterized in that the fresh ilmenite particles are petro-mineral ilmenite.

3. The method according to claim 1 or 2, characterized in that the fresh ilmenite particles comprise a particle size distribution of at most 100 to 400 μ ι η, preferably 150 to 300 μ ι η.

4. The method according to any one of claims 1 to 3, wherein the at least one ash stream is selected from the group consisting of bottom ash streams and fly ash streams.

5. A process according to any one of claims 1 to 4, further comprising a pre-classification step, wherein particles in the at least one ash stream are pre-classified prior to magnetic separation of the ilmenite particles from the ash stream; wherein preferably said pre-classification comprises mechanical particle classification and/or fluid driven particle classification, more preferably sieving and/or gas driven particle classification.

6. the method according to claim 5, characterized in that the mechanical particle classification comprises the use of sieving with a mesh size of 200 to 1,000 μm, preferably 300 to 800 μm, further preferably 400 to 600 μm; among them, a rotary screen is preferably used.

7. The method of any one of claims 1 to 6, wherein the separation comprises the step of using a magnetic separator (12) comprising a field strength of 4,500 Gauss or more.

8. The method according to any one of claims 1 to 7, wherein the magnetic separator (12) comprises Rare Earth Rolls (RER) or Rare Earth Drum (RED) magnets.

9. the method of any one of claims 1 to 8, wherein the magnetic field is axial.

10. The method of any one of claims 1 to 8, wherein the magnetic field is radial.

11. A method according to any one of claims 1 to 10, characterised in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is at least 120 hours, further preferably at least 200 hours, further preferably at least 300 hours.

12. The method according to any of the claims 1 to 11, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is less than 600 hours, further preferred less than 500 hours, further preferred less than 400 hours, further preferred less than 350 hours.

13. The method according to any of the claims 1 to 12, characterized in that the boiler (6) is a circulating fluidized bed boiler (CFB).

14. The process according to any one of claims 1 to 13, characterized in that the separation efficiency of step d) on ilmenite is at least 0.5 by mass, preferably at least 0.7 by mass.

15. A process according to any one of claims 1 to 14, characterised in that the proportion of ilmenite in the bed material is 25 wt.% or more, preferably 30 wt.% or more.

Technical Field

The present invention relates to a method of operating a fluidized bed boiler, such as a circulating fluidized bed boiler or a bubbling fluidized bed boiler, in the environment of a bed management cycle of the fluidized bed boiler.

Background

Fluidized bed combustion is a well known technique in which 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 through a bed of solid particulate material at a specific fluidizing velocity. The bed material acts 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 fluidizing gas rises above the minimum fluidizing velocity at which the force of the fluidizing gas balances the gravity acting on the particles, the solid bed material behaves in many ways like a fluid and the bed is said to be fluidized. In a Bubbling Fluidized Bed (BFB) boiler, the fluidizing gas forms bubbles in the bed through the bed material, facilitating gas transport through the bed material and allowing better control of the combustion conditions (better temperature and mixing control) compared to grate combustion. In a Circulating Fluidized Bed (CFB) boiler, a fluidizing gas is passed through the bed material at a fluidizing velocity, wherein a majority of the particles are entrained by a fluidizing gas flow. The particles are then separated from the gas stream (e.g., by a cyclone separator) and recycled back into the furnace, typically by an annular seal. An oxygen-containing gas, usually air or a mixture of air and recirculated flue gas, is usually used as fluidizing gas (so-called primary oxygen-containing gas or primary air) and passed through the bed material from below the bed or from below the bed, thereby serving as a source of oxygen for the combustion. A portion of the bed material fed to the burner escapes from the boiler while various ash streams leave the boiler, particularly the bottom ash. Removing bottom ash (i.e. dust in the bottom of the bed) is typically a continuous process that is used to remove alkali metals (Na, K) and coarse inorganic particles/lumps and any agglomerates formed during boiler operation from the bed, and to keep the pressure differential over the bed sufficient. In a typical bed management cycle, fresh bed material is used to replenish the bed material lost with the various ash streams.

From the prior art it is known to replace part or all of the silica sand bed material with ilmenite particles during CFB (h.thunman et al, Fuel 113(2013) 300-. Ilmenite is a naturally occurring mineral, consisting mainly of iron-titanium oxide (FeTiO)₃) And can be repeatedly oxidized and reduced. Due to the reduction/oxidation characteristics of ilmenite, this material can be used as an oxygen carrier in fluidized bed combustion. Using a bed comprising ilmenite particles, the combustion process may be carried out at a lower air-fuel ratio than a non-active bed material (e.g. 100 wt.% silica sand or fuel ash particles).

Disclosure of Invention

The problem to be solved by the present invention is to provide an improved method or process for a titaniferous iron ore containing bed material as mentioned in the preamble.

The inventive method for operating a fluidized bed boiler comprises the following steps:

a) Supplying fresh ilmenite particles having a form factor of 0.8 or less to the fluidized bed boiler as bed material;

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, wherein the separating comprises the step of using a magnetic separator comprising a field strength of 2,000 gauss or greater;

e) Recycling the separated ilmenite particles to the bed of the fluidized bed boiler;

Wherein the mean residence time of the ilmenite particles in the fluidized bed is 100 hours or more.

First, a number of terms are explained in the context of the present invention.

Fluidized bed boilers are well known terms in the art. The invention is particularly useful in Bubbling Fluidized Bed (BFB) boilers and Circulating Fluidized Bed (CFB) boilers. Preferably a CFB boiler.

the shape factor or sphericity of a particle is defined as the surface area of the particle divided by the surface area of a sphere of the same volume. The petrographic ilmenite particles described below have a sphericity (shape factor) of < 0.8. Typical sphericity values for the petrographic ilmenite are about 0.7. In the context of the present invention, a shape factor of 0.75 or less is preferred.

The field strength of the magnetic separator is preferably determined on the surface of the transport device of the bed material for carrying out the magnetic separation.

In the context of the present invention, the mean residence time (T) of ilmenite particles in the boiler_{Dwell, ilmenite}) Defined as the total mass (M) of ilmenite in the bed inventory_Ilmenite) Feed rate (R) relative to fresh ilmenite_{Feed, ilmenite}) Production rate (R) of boiler_{production of}) Ratio of the product of (a):

T_{Dwell, ilmenite}＝M_Ilmenite/(R_{Feed, ilmenite}×R_{Production of})

for example, if the total mass of ilmenite in the boiler is 25 tons, the feed rate of fresh ilmenite is 3kg/MWh and the production rate is 75MW, which gives the average residence time T_{Dwell, ilmenite}25/(3 × 75/1000) h 111 h. Recycling of the separated ilmenite particles is a convenient way to extend the average residence time of the ilmenite particles in the boiler, since the feed rate of fresh ilmenite can be reduced.

The present inventors have realized that ilmenite particles can be conveniently separated from boiler ash using magnetic separation as defined in the claims, and that ilmenite with a defined shape factor shows very good oxygen carrying properties and oxidation of carbon monoxide (CO) to carbon dioxide (CO), even after long-term use as bed material in a fluidized bed boiler₂) Reactivity (so-called "gas conversion") and good mechanical strength. In particular, the present invention has realized that the rate of loss of ilmenite particles is surprisingly reduced after prolonged residence time in the boiler, and that the mechanical strength is still very good after the ilmenite has been used as bed material for a long time. This is surprising because ilmenite particles undergo chemical ageing after undergoing an initial activation stage, because they are subjected to repeated redox conditions during combustion in a fluidized bed boiler and are physically associated with the boiler structureThe interaction causes mechanical wear of the ilmenite particles. Therefore, it is expected that the oxygen carrying capacity of ilmenite particles and the wear resistance thereof deteriorate rapidly during combustion in a fluidized bed boiler.

the present inventors have realized that in view of good wear resistance, the surprisingly good oxygen carrying properties of the ilmenite particles used can be exploited by recycling the separated ilmenite particles to the boiler bed. This reduces the need to supply fresh ilmenite to the boiler, which in turn significantly reduces the overall consumption of natural resource ilmenite and makes the combustion process more environmentally friendly and economical. Furthermore, separating ilmenite from ash and recycling to the boiler can control ilmenite concentration in the bed and simplify operation. In addition, the bed management cycle of the present invention further increases the flexibility of the fuel by allowing the feed rate of fresh ilmenite to be decoupled from the ash removal rate (especially bottom ash removal rate). Thus, the change in the amount of ash in the fuel becomes less pronounced because higher bottom bed regeneration rates can be applied without losing ilmenite from the system.

The fresh ilmenite particles are preferably rock ilmenite particles.

Hard rock or huge ilmenite is obtained in igneous rock deposits, for example in canada, norway and china. TiO in rock-mineral type ilmenite₂The content of (A) is rather low (usually 30 to 50 mass%), but the iron content thereof is relatively high (usually 30 to 50 mass%). The petrographic ilmenite is mined and upgraded by crushing and impurity separation. This makes the sphericity of the petrographic ilmenite lower than that of, for example, natural silica sand. The shape factor of norwegian type ilmenite (provided by titanium dioxide a/S) is about 0.7.

Ilmenite sand (not preferred according to the invention) can be found in, for example, heavy mineral sand deposits produced in south africa, australia, north america and asia. Typically, the sand ilmenite is from a weathered rock deposit (rock deposit). Weathering results in a reduction of iron content with simultaneous increase of TiO₂The concentration of (c). Also known as altered ilmenite, TiO due to oxidation and dissolution of natural iron₂The content can be as high as 90 wt.%. Shape factor of sand ilmeniteWithin the range of 0.8-1, the average factor value is about 0.9.

Preferably, the fresh ilmenite particles have a particle size distribution of at most 100 to 400, more preferably 150 to 300 μm.

To determine the particle size distribution, a sieve with the appropriate mesh size sequence was used. Sieve plates of the following mesh sizes may be used: 355 μm, 250 μm, 180 μm, 125 μm, 90 μm, and a bottom plate for the sub-90 μm fraction.

Preferably, the at least one ash stream is selected from the group consisting of a bottom ash stream and a fly ash stream. Most preferably, at least one ash stream is a bottom ash stream. In an advantageous embodiment of the bed management cycle of the present invention, any combination of two or more ash streams is possible. Bottom ash is one of the main causes of loss of bed material in fluidized bed boilers, and in a particularly preferred embodiment, at least one ash stream is a bottom ash stream. Fly ash is the portion of ash that is entrained by the gas from the fluidized bed and flies out of the furnace with the gas.

Preferably, the method further comprises a pre-classification step, wherein particles in at least one ash stream are pre-classified prior to magnetically separating ilmenite particles from the ash stream; wherein preferably the pre-classification comprises mechanical particle classification and/or fluid driven particle classification, more preferably sieving and/or gas driven particle classification. In fluid-driven particle separation, particles are separated based on their hydrodynamic behavior. A particularly preferred variant for fluid-driven separation comprises gas-driven particle separation.

Preferably, the mechanical particle classification comprises the use of sieving with a mesh size of 200 to 1,000 μm, preferably 300 to 800 μm, further preferably 400 to 600 μm.

The present invention has found that the majority of the ilmenite in the bottom ash comprises a particle size of 500 μm or less, so that the mechanical classifier provides a fine particle size fraction with a more uniform size distribution, while still comprising a majority of the ilmenite particles. The magnetic separation in the second step can be performed more efficiently.

the initial mechanical classification has three particular purposes. First, it helps to protect the magnetic separator from damage that large ferromagnetic objects, such as nails, may cause to the magnetic separator or its components. Second, it reduces the load on the magnetic separator by reducing the mass flow. Again, it makes the operation of the magnetic separator simpler, as it results in a narrower particle size distribution.

In a particularly preferred embodiment, the mechanical classifier comprises a rotating screen, which has been found to be effective in pre-classifying the bottom ash to remove coarse particles.

In one embodiment of the invention, the mechanical classifier further comprises a main screen before the mechanical classifier (e.g. rotary screen) having a mesh size as defined above, to separate coarse particles having a particle size of 2cm or more, e.g. coarse particle agglomerates of golf ball size.

The method may include the step of separating the elongated ferromagnetic objects from the ash stream prior to the magnetic separator. Mechanical classifiers may include slotted meshes (slots mesh) to remove small pieces of fine wire or nails that tend to plug the mesh and also affect magnetic separation in subsequent steps.

The magnetic separator comprises a field strength 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 to effectively separate ilmenite from ash and other non-magnetic particles in the particle stream.

Preferably, the magnetic separator comprises Rare Earth Roll (RER) or Rare Earth Drum (RED) magnets. Corresponding magnetic separators are known per se in the art and are available, for example, from Eriez Manufacturing Co. (www.eriez.com). The rare earth roll magnetic separator is a high-strength, high-gradient, permanent magnetic separator for separating magnetic and weakly magnetic iron-containing particles from a dried product. The ash stream is conveyed on a belt that runs around a roll or drum containing rare earth permanent magnets. When transported around the rollers, the ilmenite remains attracted to the belt, while the non-magnetic particle fraction falls off. Mechanical separator blades assist in separating the two particle fractions.

In one embodiment of the invention, the magnetic field is axial, i.e. parallel to the axis of rotation of the drum or roller. The axial magnetic field of the magnet with a fixed orientation causes the ferromagnetic material to tumble as it passes from the north pole to the south pole, releasing any entrained non-magnetic or paramagnetic material.

In another embodiment of the invention, the magnetic field is radial, i.e. comprises a radial orientation with respect to the axis of rotation. Generally, radial orientation has the advantage of providing higher recovery of all weakly magnetic materials at the expense of lower purity due to entrained non-magnetic materials.

Two-stage magnetic separation may also be used, with an axial orientation being used in the first step to help release entrained non-magnetic material and a radial orientation being used in the second step to improve recovery.

Preferably, the average residence time of the ilmenite particles in the fluidized bed boiler is at least 120 hours, further preferably at least 200 hours, further preferably at least 300 hours. Surprisingly, it was found in the present invention that ilmenite particles still show very good oxygen carrying properties, gas conversion and mechanical strength, even when operated continuously in a fluidized bed boiler for about 300 hours, clearly indicating that higher residence times can be achieved.

In a preferred embodiment, the average residence time of the ilmenite particles may be less than 600 hours, further preferred less than 500 hours, further preferred less than 400 hours, further preferred less than 350 hours. All combinations of lower and higher values of average residence time are possible within the scope of the invention and are expressly disclosed herein.

Preferably, the boiler is a circulating fluidized bed boiler (CFB).

preferably, the separation efficiency of the process for the ilmenite deposit material is at least 0.5 by mass, preferably at least 0.7 by mass. This means that at least 50 or 70 wt% of the ilmenite contained in the ash stream can be separated from the ash and recycled to the boiler. In the context of the present invention, the term wt.% is used as a synonym for mass%.

The recycle capacity and separation efficiency are also affected by the ash stream temperature, where there is a tradeoff between separation efficiency and ash stream temperature. Higher temperatures can reduce the efficiency of magnetic separation and result in the use of more expensive refractory materials in the system used to practice the method of the invention. By taking the measure of cooling the ash stream, the negative effects on the separation efficiency and the requirements of the high temperature materials can be eliminated. The system may also be equipped with temperature sensors and ash flow diverters to redirect flow and bypass the separation system in the event of a temporary high temperature.

In operation of the boiler, the proportion of ilmenite in the bed material may be kept at 25 wt.% or more, preferably 30 wt.% or more. In another embodiment of the invention, the preferred ilmenite concentration in the bed is from 10 wt.% to 95 wt.%, more preferably from 50 wt.% to 95 wt.%, more preferably from 75 wt.% to 95 wt.%.

Drawings

Embodiments of the present invention will now be shown by way of example with reference to the accompanying drawings.

It shows that:

FIG. 1: a schematic diagram of a system for practicing the invention;

FIG. 2: schematic of a magnetic drum separator;

FIG. 3: a schematic diagram illustrating mass flow in one embodiment of a method according to the present invention;

FIG. 4: SEM micrographs of ilmenite particles used as bed material during the experiment: a) sand ilmenite; b) a rock-ore type ilmenite ore;

FIG. 5: SEM micrographs of cross sections of the ilmenite particles extracted after 2 and 15 days of exposure, wherein a) and b) are sand ilmenite, and c) and d) are petrographic ilmenite;

FIG. 6: a screening curve obtained by screening sand ilmenite and rock-ore type ilmenite;

FIG. 7: cumulative wear measured on placer ilmenite and petrographic ilmenite;

FIG. 8: cumulative wear of rock-ilmenite and sand-ilmenite changes over time.

Detailed Description