TECHNICAL FIELD

The present invention relates to a plant for combustion of solid particulate fuel in a fluidized bed which is supplied with an absorbent with a varying grain size, wherein coarser and finer fractions are supplied separately to the bed.

BACKGROUND ART

During combustion of solid fuel, preferably coal, the presence of certain impurities, preferably sulphur, in the combustion gases generated during combustion of the fuel must be taken into consideration, which entails environmental drawbacks. One advantage with the combustion of the fuel taking place in a fluidized bed is that the material of the bed may be supplied with an absorbing substance, an absorbent, for example lime¬ stone or dolomite, which reacts with and binds the above- mentioned impurities such that these, in the form of harmless grain, powder or sludge, may be deposited.

When feeding absorbent into a fluidized bed, the following conditions, among other things, must be taken into consideration.

- When feeding particulate absorbent material into the bed, the absorbent is heated, whereby carbon dioxide, C02, escapes from the absorbent. The reason for the departure of the carbon dioxide is that the partial pressure of the carbon dioxide in the lower part of the bed is low and that the temperature is high (800-900°C) . This results in bursting of the absorbent grains. Also thermal stresses, which arise when the grains are heated, contribute to the bursting of the grains. That is to say, a rapid heating of the coarser absorbent particles entails an increased risk of bursting of the grains.

- The degree of utilization of fed-in absorbent depends partly on the particle diameter, partly on the residence time of the particles in the bed. This can be illustrated in a diagram of the degree of utilization of the absorbent in an approximate relationship according to Figure 1. The figure shows that the utilization of the absorbent in the bed becomes optimal if the grain sizes of the absorbent described through their diameter are less than 50 μm or amount to 0.4 mm and immedia¬ tely thereabove. In accordance therewith, it is desirable to have control of the particle size distribution in the bed. Further, it is desirable to prevent spontaneous cracking or bursting of the coarser particles. When the particles are split up, inter alia grain sizes with diameters in the inter¬ val 50 μm to 0.4 mm occur in the bed, which, in turn, entails the occurrence in the bed of particles which may be utilized to a smaller degree according to Figure 1. Smaller particles have a lower residence time in the bed, whereby all the lime in the core of the particle does not have time to be sulphated before the particle leaves the bed either via the flue gases or by constantly discharging bed material.

From, for example, the patent specification SE 434 087 (US 4,421,036), it is known to disintegrate the absorbent into both a coarser and a finer fraction, which may each be sepa- rately fed into the bed. When feeding absorbent according to this known technique, the two fractions are brought to the lower part of the bed. Since coarser particles of the absor¬ bent are thus fed in and are subjected to a great increase in temperature when entering the bed, while at the same time the C02 content at the low bed level is low, the coarser particles are split up into smaller particles which, according to the curve in Figure 1, shows that the degree of utilization of these split-up particles becomes considerably less favourable.

The absorbent may be fed into the bed in dry state. The feeding of absorbent mixed into a paste, which consists of

fuel and a liquid, is also frequently used in prior art plants.

In known devices the absorbent is frequently fed into a fluidized bed by means of pneumatic transport. A pneumatic method of transport is energy-demanding in, for example, a pressurized fluidized bed since the consumption of transport gas is substantial.

SUMMARY OF THE INVENTION

The present invention relates to a method and a device for combustion of a particulate fuel in a fluidized bed enclosed in a combustor, wherein impurities in the combustion gases generated during the combustion are absorbed by an absorbent which is supplied in particle shape to the combustor. The method comprises

- crushing absorbent particles with a diameter smaller than the order of size of 0.4 mm into a fine fraction with a grain size where the diameter of the grains is smaller than the order of size of 50 μm,

- feeding the fine fraction into the lower part of the bed, and

- feeding absorbent particles exceeding the order of size of 0.4 mm, the coarse fraction, into the uppermost part of the bed, near the surface of the bed.

It is known to crush absorbent into a coarser and a finer fraction, but according to the invention advantages are achieved by feeding the coarser fraction of the absorbent at the upper part of the bed. In this way, the coarser grains will first enter the bed to a part of the bed where the C02 content is high, which entails a reduced risk of grain bursting by the departure of C02- A reduced risk of grain bursting in the upper part of the bed also reduces the risk

that absorbent particles burst into finer grains blow off the bed into a freeboard above the bed and escape via flue gases to dust cleaners, thus increasing the load thereon.

The degree of utilization of the coarse fraction of the absor¬ bent becomes higher since the grains are large and remain long in the bed (3-8 hours) , whereby the absorbent grains have time to sulphate in full before leaving the bed.

Since the absorbent grains in the coarse fraction are large and remain long in the bed, it is sufficient to arrange one feed point for the coarse fraction. This, in turn, means that an uneconomical pneumatic feeding of the coarse fraction may be omitted.

The long residence time for the coarse grains of the absorbent further means that a continuous feeding of the coarse fraction may be omitted, whereby, for example, lock-hopper devices and similar mechanical means may be omitted. Very suited for feed- ing of the coarse fraction of absorbent in a pressurized com¬ bustor is an open absorbent container with a particle column standing under the container and opening out at the bed sur¬ face. Such a device completely without mechanical propulsion members for the particulate material is described in the patent application SE 9200696-4 (PCT/SE93/00102) .

Since the feeding of the coarse fraction of the absorbent according to the invention takes place to the upper part of the bed, the coarse fraction may be supplied via a non- insulated supply conduit which has a long extension through the freeboard of the combustor. This results in the advantage of the absorbent grains being preheated during their residence time in the supply conduit, the heating of the grains thus being performed less abruptly than when the prior art absor- bent is fed a short distance directly into the hot bed. Also

this more slow heating of the absorbent particles contributes to a reduced risk of grain bursting according to the above.

Feeding of the coarse fraction of the absorbent into the upper part of the bed further entails advantages also when feeding the fraction when the combustion in the bed is performed under low-load conditions. During operation at low load, the bed surface is usually lowered while at the same time a low tem¬ perature and a high Cθ2 content prevail at the bed surface, both of these conditions being favourable for avoiding grain bursting.

According to the invention, the fine fraction of the absorbent material must be well distributed in the lower part of the bed. When feeding the fuel in the form of a paste into the bed, the fine fraction of the absorbent may be mixed into the paste, thus obtaining a good distribution of the fine fraction of the absorbent. If dry feeding of the fuel is utilized, mixing the absorbent with the fuel provides an optimum result. In the latter case, in addition, the fine fraction of the absorbent also serves as a drying agent for the fuel (coal) . In addition, such mixing reduces the risk of explosion when handling the fuel.

The outlet nozzle for the coarse fraction of the absorbent grains is arranged at the bed surface and, for example, con¬ sists of an L valve of a known kind, where the flow of absor¬ bent may be controlled by a pulsed air or gas flow through the L valve. It is possible to dispose the nozzle also above the bed surface. The large grains fall down into the bed because of their weight.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a diagram of the degree of utilization of absorbent as a function of the diameter of the absorbent grains.

Figure 2 shows a schematic view of a PFBC combustor with a surrounding pressure vessel, where feeding of the coarse fraction of absorbent according to the invention is applied.

Figure 3 shows a variant of the feeding of absorbent to a PFBC combustor, where the coarse fraction is fed in via feeding means which do not comprise any mechanical device for taking up pressure differences.

Figure 4 shows an L valve at the orifice of the feed pipe for the absorbent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A number of embodiments of the invention will be described with reference to the accompanying drawings. Feeding of absorbent into a fluidized bed is exemplified with reference to a PFBC power plant schematically represented in the figures.

In a general figure (Fig. 2) the central units of a PFBC power plant are represented, wherein a combustor 1 is accommodated in a pressure vessel 2. Air from a compressor (not shown) is supplied to the pressure vessel 2 at the air inlet 3 for pressurization of the pressure vessel 2 and hence also of the combustor 1. The compressed air 4 is supplied to the combustor 1 via fluidization nozzles 5 at the bottom of the combustor for fluidization of a bed 6 enclosed in the combustor. The bed consists of bed material, particulate absorbent and a parti¬ culate fuel which is burnt in the fluidizing air 4 supplied to the bed 6. Combustion gases from the bed 6 pass through a freeboard 7 above the bed surface 8 and are forwarded via the

outlet 9 for cleaning in dust cleaners, whereafter the com¬ bustion gases are expanded in a gas turbine (not shown) , where the energy contents in the gases are transformed into useful energy.

In the fluidized bed 6 there is also shown a tube bundle 10 which is completely immersed in the bed at full-load opera¬ tion. Water is brought to the tube bundle 10 at 11 for genera¬ tion of steam in the tubes in the tube bundle. The steam is passed on at 12 to a steam turbine (not shown) in a steam cycle in the plant.

A container 15 for absorbent, open at the top, which consists of a coarser crushed fraction of absorbent, for example lime- stone or dolomite, is shown in Figure 2. In an embodiment according to Figure 2, the coarse fraction may be fed further to a closed container 16, which is pressurized via a valve 18 in a connection conduit between the pressure vessel 2 and the container 16. The absorbent is passed out from the pressurized container 16 via a feed conduit 17 to a region in the free¬ board 7 near the bed surface 8.

The outlet 19 from the feed conduit 17 may be formed as an L valve according to Figure 4 with the orifice 20 of the valve located immediately above, at or immediately below the surface 8 of the bed. In the L valve a standing particle column 21 of absorbent grains will form. By supplying air/gas at an inlet pipe 22, the flow of fed-in absorbent may be guided in a known manner. If the orifice 20 is located above the bed surface 8, the coarse absorbent grains fall down by their own weight into the bed 6. Supply of air or gas to the inlet pipe 22 may, for example, be performed by discharging air via a channel which puts the inlet pipe 22 in connection with the pressure vessel 2, the air quantity thus being controlled by means of a valve 22a in the channel.

Figure 3 describes another preferred feeding of absorbent, wherein the above-mentioned advantages of a long extension of the feed pipe 17 is utilized. In this plant, the combustor 1 is provided with a freeboard 7' which is extended in the vertical direction via a duct 25. By disposing the feed pipe 17 for the coarse fraction of absorbent inside and along this extended freeboard 7' , the advantage of an extra long distance is obtained under which the absorbent in the feed pipe is pre¬ heated before it reaches the hot bed 6.

In this latter described embodiment of a combustor there is advantageously used a method of feeding as described in Swedish patent application SE 9200696-4 (PCT/SE93/00102) . According to this method, a container 30 which is under a lower pressure is utilized, for example an open container which is hence subjected to atmospheric pressure, for storage of the coarse fraction of absorbent. In this case, the con¬ tainer 30 is provided at its lower part with a decreasing cross-section area and is transformed at its narrowest part into the feed pipe 17. A particle column 31 formed in the container 30 and the pipe 17 is capable of taking up the pressure difference between the pressure in the combustor 1 and the pressure prevailing across the absorbent surface in the container 30.

A fine fraction of the absorbent, which according to the invention is crushed into grain sizes smaller than 50 μ , is fed in via the same conduits through which the fuel is fed into the bed 6, in the figures symbolized by fuel conduits 35 leading to the lower part of the bed 6.

The fuel which is fed into the combustor in a plant according to the invention may be fed in in arbitrary shape. As an example, the fuel may be crushed or dried, in which case pneu- matic feeding is used. Fuel may also be fed in in the form of a paste where the liquid fraction consists of water or oil.

Also combinations of these methods may occur (see, for example, patent specification SE 90003542) .