Liquid heaters and vaporizers – Miscellaneous
Patent
1992-10-08
1994-08-09
Favors, Edward G.
Liquid heaters and vaporizers
Miscellaneous
60 39464, 16510416, F22B 3100
Patent
active
053356306
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the control of the power output during combustion of fuel in a fluidized bed in which energy is recovered with heat transfer surfaces, arranged close to the fluidized bed, which are traversed by a liquid or gaseous heat transfer medium.
The invention is particularly valuable in power plants with combustion in a pressurized fluidized bed, PFBC (Pressurized Fluidized Bed Combustion) plants, where energy is recovered with a gas turbine and a steam turbine in combination. In such a plant the invention makes possible a more rapid change of the power output, an increase of the gas temperature at partial load, and control of the power balance between gas turbine and steam turbine during operation.
BACKGROUND OF THE INVENTION
During combustion of fuels in a fluidized bed, the power output is usually controlled by changing the bed depth. With conventional fluidized bed boilers for the production and superheating of steam, which is expanded in a steam turbine, the control of the bed depth in case of changes of the power output entails the transportation of large quantities of bed material back and forth to the fluidized bed. These large material flows require a complicated transport systems involving, among others, intermediate storage containers for bed material. In addition, the considerable bed depth adjustments and the associated material flows as well as the exposure of large heat transfer surfaces result in a slow control and in the temperature of the flue gases falling in case of partial load, which is reflected in inferior environmental performance.
If the fluidized bed is pressurized, that is, is included in a plant for combustion in a pressurized fluidized bed, a PFBC (Pressurized Fluidized Bed Combustion) plant, in which energy is recovered also from the hot pressurized flue gases by means of gas turbines, conventional control of the power output with bed depth adjustments also results in the efficiency of the plant decreasing with decreasing flue gas temperature as well as in considerable difficulties in adjusting the power balance between the steam and gas turbines during operation.
SUMMARY OF THE INVENTION
The heat transfer surfaces included in the fluidized bed boiler are arranged in a high pressure section, comprising an evaporator and a possible superheater, as well as a low pressure section, with one or more intermediate superheaters. In the fluidized bed, the heat transfer surfaces are arranged, according to the invention, such that, in case of a change of the bed depth, substantially heat transfer surfaces included in the evaporator are exposed from or covered by the fluidized bed whereas heat transfer surfaces included in the intermediate superheater are substantially located in the fluidized bed irrespective of the power output. A fluidized bed boiler designed and arranged according to the invention and with evaporation and superheating of steam carried out according to the invention requires a considerably smaller change of the bed depth in case of a corresponding change of the power output than a conventionally arranged fluidized bed boiler. In addition, with a fluidized bed boiler according to the invention, a direct control of the evaporation power is obtained by changes of the bed depth.
By concentrating the evaporator substantially to the upper part of the fluidized bed in this way, a direct and rapid control of the evaporation power upon a change of the bed depth is obtained. In addition, at a given change of the power output, a considerably smaller change of the bed depth is needed in a plant with the evaporator arranged according to the invention, than in a plant with a conventionally arranged evaporator. In addition, the flue gas temperature is not changed to the same extent as a result of changes in the power output.
A fluidized bed boiler arranged such that the flow of a heat transfer medium through the boiler is varied to control the mean temperature difference between the fluidized bed and the heat transfer medium, and
REFERENCES:
patent: 4748940 (1988-06-01), Honig
patent: 4909163 (1990-03-01), Hjalmarsson
patent: 5163384 (1992-11-01), Brannstrom et al.
ABB Stal AB
Favors Edward G.
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