Firing with a particulate fuel in a fluidized bed requires a good distribution of a fuel fed into the bed for a complete combustion of the particles to take place. The fuel in the bed comprises one gaseous part and one part of particles which are both capable of being blown off and not capable of being blown off.
If the number of feed points for fuel is too small, an accumulation of fuel takes place in a plume-shaped region above each individual fuel feed nozzle. In this region, all available oxygen in the fluidization air is rapidly consumed, in which case particles which have not been burnt out completely of their fuel content reach the bed surface and are blown off to a freeboard above the bed. From the freeboard the flue gases are passed on from the combustion to ash cleaners, so-called cyclones.
A problem which may arise when particles containing unburnt residues of fuel occur is that fires may arise at other locations in the plant. For example, it is known that insufficiently burnt-out particles may burn at undesired points, such as in the freeboard or in the cyclones. Solid particles and gas with still combustible constituents at these points may encounter fluidization air which still contains oxygen residues since air may have passed regions with a lower concentration of fuel, that is, between the above-mentioned plume-shaped combustion regions, during the flow upwards through the bed, this air not being depleted of oxygen by combustion.
Fires at the above-mentioned points are unacceptable and entail harmful thermal stresses on the construction material used, since an after-combustion after the bed increases the temperature of the flue gases. In addition, the composition of emissions in the flue gases may be changed in an undesired and uncontrolled manner.
Emission of nitrogen oxides may increase since the use of ammonia for reducing these oxides is rendered difficult when ammonia is burning in zones with coal combustion. Also the emission of sulphur may increase since the use of a sorbent, fed into the bed, for reducing sulphur is rendered difficult if the plume-shaped regions occur in the bed. This is due partly to the compound formed during the sulphur reduction not being stable at low oxygen contents, and partly to combustion taking place in zones where the sorbent content is lower than in the bed. Further, of course, the effective burn out of the fuel content in the particles is reduced, which reduces the efficiency of the plant.
The bed normally accomodates a tube bundle for steam generation above the fuel feed nozzles. Another problem which may arise is that fluidization air which is completely depleted of oxygen gives an increased corrosion on the tube material, which in turn leads to a rapid material wear in the tube bundle. A better distribution of the fuel leads to a reduced risk of corrosion in the plume-shaped regions since regions with a permanently low oxygen content in the fluidization air are eliminated.
A good distribution of the fuel in the bed can be achieved by the particulate fuel being supplied to the bed at a large number of feed points in the form of well distributed fuel feed nozzles, as disclosed, for example, in Swedish patent application 9000354-2.
In case of a large number of fuel feed points, an even distribution is obtained in the fluidized bed between the fuel and the fluidization air. This makes it possible for the gaseous part of the fuel and that part of the fuel which constitutes particles capable of being blown off to burn out completely before this part in the form of burnt out particles reaches the surface of the bed and is blown off.
A design with a large number of fuel feed points is costly and difficult to realize because of a complicated network of distribution conduits to the fuel feed nozzles, as well as problems if the different conduits are clogged.
Another possible solution to the mentioned problems is to arrange some form of distributor of the fuel in the bed after feeding of the fuel. The patent GB 1 326 651, for example, shows a distributor in the form of a rotating wheel provided with blades and located above a fuel feed opening. Upon rotation of the wheel, the fuel is distributed in the bed.
Using some type of movable parts to achieve a good fuel distribution in the bed is, however, not suitable. The movable parts are influenced by greatly destructive forces, such as, for example, the high temperature. In addition, the availability for service of any devices is greatly limited or disturbing for the operation of the plant.