Method for Improving Utilization of Sulphur-Absorbent Containing Calcium in a Power Plant and Equipment for Performing the Method

The present invention relates to a method for improving the utilization of sulphur-absorbent containing calcium, in a power plant burning fuel in a fluidized bed containing particulate material consisting at least partially of sulphur-absorbent. The invention also relates to a power plant including equipment for calcinating sulphur-absorbent for performing the method.

When removing sulphur during combustion of liquid or solid fuel containing sulphur, large quantities of preferably limestone or dolomite are consumed, calcium being the substance utilized. An efficient way of removing sulphur is to allow combustion to take place in a fluidized bed consisting at least partially of lime or dolomite.

The cost of procuring absorbent material and disposing of consumed absorbent material substantially increases the costs involved in the production of heat and electricity. Disposal of the consumed material also entails environmental problems. It is thus important, both from the expense and the environmental aspects, that the absorbent material is thoroughly utilized, preferably to almost 100%.

In what is known as a bubbling, fluidized bed, the gas speed is generally between 0.5 and 2.0 m/sec, and the particulate material in the bed behaves more or less like a liquid. Particles smaller than a certain size accompany the combustion gases leaving the bed and they therefore remain in the bed for only a short while. The duration of stay is also affected to a certain extent by the shape of the particles. With a gas speed of 1 m/sec, particles smaller than about 0.3-0.5 mm are blown away from the bed so quickly that their absorption capacity cannot be satisfactorily utilized. This means that the fraction of absorbent crushed to a size smaller than 0.3-0.5 mm is poorly utilized since its duration of stay in the bed is so short.

According to the invention, when fresh absorbent is prepared it is crushed and divided into a coarse and a fine fraction. The coarse fraction is supplied directly to the bed in a combustion chamber, either separately or together with fuel. The fine fraction, on the other hand, is calcinated before being supplied to the bed, so that calcium oxide CaO is formed. This calcinated absorbent is mixed with fuel and then acts as drying agent for the fuel. The use of CaO as drying agent for fuel is known through Swedish patent 8403665-6, for instance.

Upon contact with fuel particles, the CaO combines with moisture in the fuel in accordance with the following formula:

The calcium hydroxide (Ca(OH).sub.2) formed in this reaction is a dry, fine-particled powder. When it is returned to the bed and heated, the reaction takes place in the opposite direction and fine-particled calcium oxide CaO is formed. The small particle size offers favorable reaction conditions and an extremely quick reaction is obtained with sulphur dioxide, SO.sub.2. Supplying the mixture of fuel and the now extremely fine-particled absorbent, thus gives a very high utilization of the absorbent despite its short duration of stay in the bed.

The invention is described in more detail with reference to the accompanying drawings.

FIG. 1 illustrates a partially utilized absorbent particle,

FIG. 2 the relation between particle size and absorption efficiency, and

FIG. 3 shows a power plant with pressurized fluidized bed including equipment for calcinating the fine-particled absorbent fraction.

The above situation causes the efficiency of the absorbent to vary with the particle size as indicated by curve C in FIG. 2. Very small particles react almost instantaneously with SO.sub.2 and are thus fully utilized before having time to leave the bed. With increasing particle size, the time required for the particle to be fully utilized increases. This means that some of the particles within the size range from the .eta..sub.s axis to the line A--A, which are carried away quickly with the combustion gases, will be incompletely utilized. The degree of utilization within the range B--B to C--C will be totally unsatisfactory. The location of A--A is dependent, inter alia, on the speed of the gas in the bed. With increasing gas speed, the line A--A will be shifted to the right. By sorting out particles below a certain size when preparing fresh bed material by crushing, said particles disappearing rapidly from the fluidized bed, and calcinating the fine-particled fraction sorted out, enables better utilization of this fraction. It can be mixed with moist fuel, thus drying at least the surface of the fuel particles, thus improving the transport properties of the fuel. The calcium oxide, CaO, formed at calcination is combined with moisture in the fuel, forming calcium hydroxide, Ca(OH).sub.2, and disintegrates into fine particles. The particles in the sorted fraction acquire a size making them efficient from the absorption aspect, despite the short duration of stay in the bed.

Calcination can be performed in a power plant with an atmospheric bed or a pressurized bed of the type shown in FIG. 3. In the figure, 10 designates a pressure vessel and 11 a combustion chamber with fluidized bed 12 where a fuel is burned. The combustion gases formed are collected in the free-board 13, cleaned in the cleaning plant symbolized by cyclone 14, and then supplied to the gas turbine 15. This turbine 15 drives a compressor 16 which compresses combustion air. The space 18 is supplied with this compressed combustion air via pipe 17. The combustion air is supplied to the combustion chamber 11 via shaft 20 and nozzles 21 in the combustion chamber bottom 22.

After crushing in crusher 50, the fine-particled fraction 23 of absorbent is supplied via conduit 51 to container 24 and from container 24 via cell feeder 25 to transport pipe 26. The heavy fraction is supplied from crusher 50 via conduit 52 to conduit 53 to the combustion chamber and/or conduit 54 to container 42. The absorbent is carried by transport air from compressor 27 through the pipe system 28 in the bed portion of the combustion chamber, where it is heated and thereby fully or partially calcinated. From the pipe system 28 the material is conducted via pipe 30 to pipe system 31 in shaft 20. Here the material is cooled by combustion air on its way to the combustion chamber 11. These pipe systems 28 and 31 may be constructed like the transport system described in EP 0 108 505. An auxiliary combustion chamber 32 may be inserted in pipe 31, in which additional heat can be supplied to the absorbent material if the supply of heat in the pipe system 28 is insufficient to achieve calcination. Thanks to the heat supplied and a longer duration of stay in the calcination equipment, the degree of calcination increases. Fuel 34 from container 35 is supplied to the combustion chamber 32 via cell feeder 33. From the cooler 31 the calcinated absorbent is carried to the collection container 36 via pipe 37. The transport air is removed through dust filter 38. Calcinated absorbent material 40 and coal 41, or a mixture of coal and the coarse absorbent fraction, is supplied to the mixer 43 from container 36 or container 42, respectively, via cell feeder 44 or 45, respectively, and thence via pipe 46 to nozzles 47 in the combustion chamber 11. Transport gas is obtained from the compressor 48.