Pressurized internal circulating fluidized-bed boiler

Abstract

A pressurized internal circulating fluidized-bed boiler is incorporated in a combined-cycle electric generating system in which a fuel such as coal, petro coke or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine. The pressurized internal circulating fluidized-bed boiler includes a pressure vessel, a combustor disposed in the pressure vessel and a main fluidized bed combustion chamber provided with an air diffusion device. A thermal energy recovery chamber is partitioned from the main combustion chamber by an inclined partition wall. A fluidized medium flows into and out of the main combustion chamber and the thermal energy recovery chamber. A free board is provided integrally above the main combustion chamber and the thermal energy recovery chamber so that combustion gas from the main combustion chamber and the thermal energy recovery chamber is mixed in the free board.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a pressurized internal circulating fluidized-bed boiler, and more particularly to a pressurized internal circulating fluidized-bed boiler for use in a pressurized fluidized-bed combined-cycle electric generating system in which a fuel such as coal, petro coke, or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine.2. Description of the Prior ArtEfforts to reduce the emission of carbon dioxide from various sources are important in view of environmental damages that are being caused by air pollution which appears to be more and more serious on the earth. It is conjectured that coal will have to be relied upon as a major energy resource because greater dependency on nuclear and oil energies is not favorable at present. To suppress carbon dioxide emission and provide a substitute for oil and nuclear power, there has been...

AI Technical Summary

Benefits of technology

The pressurized bubbling type fluidized-bed electric generating system is controlled to meet a load imposed thereon by varying the height of the fluidized bed in the combustor. More specifically, the fluidized medium is drawn from the combustor into the storage container to change heat transfer area of the heat transfer tube, thereby controlling the steam generation to meet the load. When the heat transfer surfaces of the heat transfer tube are exposed to the gas, the heat transfer coefficient thereof is lowered, and hence the amount of heat recovered is lowered. Since the exhaust gas emitted from the fluidized bed is cooled by the exposed heat transfer surfaces, the temperature of the exhaust gas supplied to the gas turbine is lowered, thus reducing the output energy of the gas turbine.

However, the above control process is disadvantageous in that the bed material storage container is necessary to withdraw and return the high-temperature fluidized medium from and into the combustor, it is not easy to withdraw and return the fluidized medium at high temperature and pressure, and agglomeration tends to occur when the fluidized medium of high heat capacity are taken into and out of the bed material storage container.

Furthermore, since the pressurized fluidized-bed boiler is under pressure, the heat transfer tube in a splash zone of the fluidized bed is more subject to erode than that in the atmospheric fluidized-bed boilers (AFBC). Another problem is that an large amount of carbon monoxide is produced because an exhaust gas emitted from the fluidized bed is cooled by the heat transfer tube and the exhaust gas remains in the fluidized bed for a short period of time as the height of the fluidized bed is reduced in the time of low load.

conventionally, the pressurized bubbling type fluidizedbed boiler comprises square combustors 146 accommodated in a circular pressure vessel 145 in a plan view as shown in FIG. 14. Therefore, a useless space is defined between the combustors 146 and the pressure vessel 145, resulting in a large-sized pressure vessel and increasing the construction cost of the boiler.

It is therefore an object of the present invention to provide a pressurized internal circulating fluidized-bed boiler for a combined-cycle electric generating system, which can be controlled to meet a load without varying the height of a fluidized bed, prevents agglomeration, minimize the emission of carbon monoxide and nitrogen oxide, and can increase a limestone utilization ratio

Problems solved by technology

It is conjectured that coal will have to be relied upon as a major energy resource because greater dependency on nuclear and oil energies is not favorable at present.

To meet such a demand, atmospheric fluidized-bed boilers (AFBC) capable of burning coals of different kinds for electric generation have been developed because a stable energy supply cannot be achieved by pulverized coal boilers which pose a limitation on available coal types.

However, the atmospheric fluidized-bed boilers (AFBC) fail to perform the functions that have been expected.

In addition, since only steam turbines can be combined with the atmospheric fluidized-bed boilers, there are certain limitations on attempts to increase the efficiency and energy output of the atmospheric fluidized-bed boilers.

(A) Disadvantage in load control

Since the exhaust gas emitted from the fluidized bed is cooled by the exposed heat transfer surfaces, the temperature of the exhaust gas supplied to the gas turbine is lowered, thus reducing the output energy of the gas turbine.

However, the above control process is disadvantageous in that the bed material storage container is necessary to withdraw and return the high-temperature fluidized medium from and into the combustor, it is not easy to withdraw and return the fluidized medium at high temperature and pressure, and agglomeration tends to occur when the fluidized medium of high heat capacity are taken into and out of the bed material storage container.

Furthermore, since the pressurized fluidized-bed boiler is under pressure, the heat transfer tube in a splash zone of the fluidized bed is more subject to erode than that in the atmospheric fluidized-bed boilers (AFBC).

Another problem is that an large amount of carbon monoxide is produced because an exhaust gas emitted from the fluidized bed is cooled by the heat transfer tube and the exhaust gas remains in the fluidized bed for a short period of time as the height of the fluidized bed is reduced in the time of low load.

Therefore, a useless space is defined between the combustors 146 and the pressure vessel 145, resulting in a large-sized pressure vessel and increasing the construction cost of the boiler.

The reason for the above structure is that arrangement of heat transfer tubes is complicated in the pressurized bubbling type fluidized-bed boiler having a cylindrical combustor.

Further, since the bed material storage container and pipes are necessary to withdraw and return the high-temperature fluidized medium from and into the combustor, housing of bed material storage container and the pipes inside the pressure vessel makes the pressure vessel large.

In the conventional pressurized bubbling type fluidized-bed boiler, fuel such as coal is insufficiently dispersed horizontally in the fluidized bed.

In order to avoid nonuniform combustion, many fuel feeding pipes must be installed in the boiler, resulting in a complicated fuel supplying system.

Further, it is difficult to supply fuel such as coal to each of the fuel feeding pipes uniformly.

Unbalanced supply of fuel causes nonuniform combustion and generates agglomeration, resulting in shutdown of the boiler.

However, the limestone wears rapidly, and is scattered as ash from the dust collector without sufficiently contributing to the desulfurizing action.

The conventional pressurized fluidized-bed electric generating system fails to achieve a high desulfurization rate that are required by power plants.

The conventional pressurized bubbling type fluidized-bed requires plenty of desulfurizing agent in order to obtain high desulfurization rate, and then produces a vast amount of ashes.

Further, a fixed-bed gasifier is disadvantageous in that coal tar remains in the fixed bed, and an entrained flow gasifier is disadvantageous in that ash-sticks occurs because of high temperature reaction.

However, the bubbling type fluidized-bed gasifier has the same disadvantages as enumerated in (A)-(D).

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