Objective
To generate 1 tonne of steam for electricity production out of 1 tonne of mine wastes, of which Belgium has a reserve of 850 million tonnes. To minimise costs the plant used for this purpose is an existing one which used to treat
1 million tonnes of low calorific value fuel (920 kcal/kg of dry material) and which has been adapted to the needs of the day.
Apart from the intrinsic savings in energy the following points have been taken in consideration :
- increased steam production by immerging hairpin boiling tubes inside the fluidized bed.
- improved thermal efficiency
- treatment of even lower calorific value fuels than in the past
- capture of SO-X liberated due to combustion
- exhaust gas scrubbing
- testing of other potential fuels.
The plant was started up in February 1982 and tests have given rise to the following observation:
The production of steam in the bed has to be limited by the capacity of the existing boiler drum, and therefore in order to prevent the melting of ash resulting from too high combustion temperature, excess of inlet air must be tolerated (up to 25%). Due to the air exchangers placed downstream of the boiler the exhaust gases entering the chimney are at 110 C (instead of 230); heat losses are reduced by a factor of 2. To avoid corrosion due to the reduction in this temperature the SO-X capture had to be improved not only within the bed but also at the exit of the boiler by injecting lime. Filters are then used to remove dust. Different types of product are burnt in the plant including lignite and the tests showed that it was possible to maintain the bed in fluidized state, and to establish design parameters.
The plant has been operated well from the start and it has been seen that fuelof very low calorific value (up to 500 kcal/kg of dry material) can be burnt. The technical problems are being resolved.
Immerged boiling tubes in the bed do not show after about 8.000 operating hours any trace of erosion or abrasion dammage.
It was a valuable demonstration which could be used elsewhere and at least 2 licences have been sold.
However the economical results of the demonstration do not appear as good as forecast mainly due to improved energy conservation in the zinc factory which drastically reduced its steam process needs (3.5 bars).
The demonstration plant was therefore only run during the winter, and remained still in summer: the ash sales for the construction industry was produced when the client did not need it. Therefore the necessity of an intermediate storage of large quantities of light products, and double handling operations, arose.
The plant in question is made up of:
a) a vertical fluidized bed with charging equipment, apparatus for the extraction and cooling of calcinated ash and for the introduction of air under a grill which is also the base of the bed.
b) a steam boiler which uses the combustion gases as a source of heat and comprises
- heat exchangers down stream from the boiler;
- dust filters;
- injection equipment for pulverised lime used to extract SO-X and reduce the exhaust gas dew point;
- control room;
- fluidized bed components;
- calcinated ash silos.
The boiler is characterised by a 7 m2 cross section at the grid and a free height of 4.6 m above the 1 m high fluidized bed.
The mine tip shales charged laterally and having a water content of 10-12% are sized between 0 and 12 mm and come directly from the washing plant of the Waterschei mine.
The ash is continually extracted mechanically from the grid thereby avoiding the accumulation of large particles at the base of the bed.
The fluidizationair is divided into two circuits, each one receiving about one half of the total flow. Each circuit contains an independant heat recuperator; one downstream of the boiler to recover heat in the exhaust gases before their leaving by a chimney and the second upstream taking heat from the ash. This latter heat recovery is particularly important considering the high concentration of inert material in the fuel.
A Lamont boiler and hairpin-tubes are installed in the bed and the steam, produced at 42 bar and 410/415 deg.C goes to a counter pressure turbine of
3.3 MW capacity.
The outlet steam, still at 3.5 bar supplies space heating in an adjacent zinc factory.
Programme(s)
Topic(s)
Call for proposal
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DEM - Demonstration contractsCoordinator
2710 HOBOKEN
Belgium