JP2963488B2 - Fluid bed combustion boiler exhaust gas treatment method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas treatment method and an exhaust gas treatment apparatus for reducing the concentration of sulfur dioxide (SO ₂ ) contained in exhaust gas from a coal-fired fluidized bed combustion boiler. .

[Conventional technology]

Conventional coal-fired boilers include pulverized coal combustion type and fluidized bed combustion type. Among these, the fluidized bed combustion system includes a pressurized fluidized bed combustion boiler (PEBC) and a normal pressure fluidized bed combustion boiler (AFBC).

And PFBC and AFBC are made of coal grains (for example,
m) together with a desulfurizing agent such as limestone (CaCO ₃ ) and quick lime (CaO) as a fluidizing medium to absorb and fix sulfur dioxide (SO ₂ ) generated during coal combustion by the following reaction in the desulfurizing agent Since it has the feature of being able to perform desulfurization, a fluidized bed combustion system is currently widely used.

CaCO ₃ + SO ₂ + 1 / 2O ₂ → CaSO ₄ + CO ₂ CaO + SO ₂ + 1/1 / 2O ₂ → CaSO _{4 By} such a reaction, it is finally discharged in the form of gypsum (CaSO ₄ ).

[Problems to be solved by the invention]

In the above desulfurization reaction, diffusion of SO ₂ into the desulfurizing agent is rate-determining. To obtain a high desulfurization rate only by furnace desulfurization, FIG. 2 (CaO in the furnace when CaO is used as a desulfurizing agent) As shown in the graph of the relationship between the molar ratio of / SO ₂ and the desulfurization rate, when the desulfurization rate in the furnace is sufficient to be 60-70%, it is sufficient to maintain the molar ratio in the range of 1-2. When a desulfurization rate in the furnace of 90% or more is required, the molar ratio must be maintained at 3 or more, and a large amount of desulfurizing agent must be added as a fluid medium.

Therefore, adding a large amount of desulfurizing agent due to the need to increase the molar ratio as described above requires the following: (a) A large amount of unreacted calcium source is inevitably discharged from the boiler together with the coal ash, and the operating cost increases. .

(B) The height of the fluidized bed becomes too high, making stable fluidized bed combustion in the boiler difficult (the bed height of PFBC is about 4 m and the bed height of AFBC is about 1 m is optimal).

There is such a problem.

Such a problem is particularly prominent when, for example, low-grade coal having a calorific value of 3000 Kcal / kg or less and high sulfur coal having a sulfur content of 5.5 to 7.0% is burned.

In addition, a method has been proposed in which the exhaust gas desulfurized in the furnace is guided to a desulfurization device of an activated carbon adsorption type to perform secondary desulfurization to reduce the amount of a desulfurizing agent used in furnace desulfurization (primary desulfurization).
Consuming process, because the SO ₂ concentration in the exhaust gas after the primary desulfurization is higher in the case of high sulfur coal in the low-grade, since the wear of the activated carbon requires Silurian properly, a large amount of activated carbon, arrows tension There is a problem in terms of operating costs.

The present invention has been made in view of the above problems, and utilizes the features of furnace-desulfurizable PFBC and AFBC to reduce the Ca / S molar ratio, and provide a high desulfurization rate exhaust gas treatment method. It is intended to provide the device.

Means for Solving the Problems In order to achieve the above object, the present invention provides an exhaust gas treatment method comprising: a primary desulfurization step of fluidizing a small amount of a desulfurizing agent together with coal particles in a coal-fired fluidized bed combustion boiler. And the soot and dust discharged from the boiler after the primary desulfurization treatment and coal ash and the like extracted from the boiler are collected in a spray dryer type desulfurization device, and the unreacted calcium source contained in the soot and dust in the desulfurization device And a secondary desulfurization step utilizing the above.

Further, the amount of the desulfurizing agent in the primary desulfurization step is set at a molar ratio of Ca / S of 2 to 3 so that the primary desulfurization rate is 50 to 60%, and the molar ratio of Ca / S in the secondary desulfurization step is 1.5 to 2%. To make the secondary desulfurization rate 80% or more.

Then, the exhaust gas treatment device is a fluidized bed combustion boiler that fluidizes a small amount of a desulfurizing agent together with coal particles and performs primary desulfurization in a furnace, and passes exhaust gas from the boiler to separate solids in the exhaust gas to form A multi-cyclone to be returned to the boiler, an air heater for exchanging heat through exhaust gas from the multi-cyclone, dust and soot coming out of the air heater and coal ash extracted and slurryed from the boiler are introduced, respectively, and contained in the dust and coal ash. And a spray dryer type desulfurization device for performing secondary desulfurization with an unreacted calcium source.

(Example) The exhaust gas treatment method of the present invention will be described based on the flow sheet of the embodiment shown in FIG.

Reference numeral 1 denotes a coal-fired fluidized bed combustion boiler (PFBC or AFBC-hereinafter referred to as a boiler). In the boiler 1, coal particles (for example, a particle size of about 1 cm) and desulfurization supplied as a bed material on an air distribution plate 2 Agents (eg, CaCO ₃ , CaO, etc.)
Is fluidized, and a part of SO ₂ generated by burning of the coal is fixed to the desulfurizing agent (primary desulfurization).

Therefore, in the above furnace desulfurization, low quality (calorific value)
3000 kcal / kg) and high sulfur coal (sulfur content 5.5-7.0%), set the Ca / S molar ratio to 2-3 and increase the desulfurization rate.
50 to 60%.

The exhaust gas that has been subjected to primary desulfurization in the boiler 1 is sent to a multi-cyclone (M / C) 3 where unburned coal and the like contained in the exhaust gas are separated and collected, and returned to a reburning chamber (CBC) 4 in the boiler 1.

The dust (approximately 850 ° C.) leaving the multicyclone 3 is sent to an air heater (A / H) 5 where heat is exchanged, and then sent to a spray dryer type desulfurizer 6.

The soot and dust leaving the multicyclone 3 contains calcium ash such as coal ash and unreacted CaSO ₄ and CaO during the primary desulfurization treatment.

It is withdrawn from the bottom of the boiler 1 and sent to a slurry manufacturing apparatus 7 where coal ash (including unreacted CaSO ₄ and CaO) slurried in 20% is blown into the spray dryer type desulfurization apparatus 6 in a spray form. Then, SO ₂ is adsorbed and fixed to unreacted calcium sources such as CaSO ₄ and CaO contained in the dust from the air heater 5 and in the coal ash slurried at 20%, and the secondary desulfurization treatment is performed.

Thus, the desulfurization rate of the secondary desulfurization is expected to be 80% or more, and the molar ratio of Ca / S in the spray dryer type desulfurization apparatus 6 is 1.5 to
Set to 2.

The amount of spray water in the spray dryer type desulfurization unit 6 is determined by the amount of dust (150 ° C.) emitted from the air heater 5.
Control to evaporate with flash heat.

Exhaust gas having an overall desulfurization rate of 80% or more in the secondary desulfurization treatment is sent to the chimney 9 via the back filter 8.

In addition, a well-known thing is applied to a spray dryer type desulfurization apparatus and a slurry production apparatus.

〔The invention's effect〕

The present invention recovers the exhaust gas after the primary desulfurization treatment in the furnace and the coal ash extracted from the boiler after the primary desulfurization treatment into a spray dryer type desulfurization device, and performs secondary desulfurization treatment in the device. (A) The amount of the desulfurizing agent in the primary desulfurization treatment can be reduced, and the combustion in the PFBC and AFBC is stabilized.

(B) A high desulfurization rate can be obtained by effectively utilizing the unreacted calcium source during the primary desulfurization treatment.

(C) Since the unreacted calcium source is used for the secondary desulfurization treatment instead of the conventional activated carbon adsorption method, the operating cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a flow sheet for implementing the exhaust gas treatment method of the present invention. FIG. 2 is a graph showing the relationship between the molar ratio of CaO / SO ₂ and the desulfurization rate. 1 ... coal-fired fluidized bed combustion boiler, 3 ... multi cyclone, 4 ... reburning chamber, 5 ... air heater, 6 ... spray-dry desulfurization device, 7 ... slurry production device.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B01D 53/50 B01D 53/80-53/83

Claims (3)

(57) [Claims] 1. A primary desulfurization step of fluidizing a small amount of a desulfurizing agent together with coal particles in a coal-fired fluidized bed combustion boiler; and removing dust and dust discharged from the boiler after the primary desulfurization treatment and coal ash extracted from the boiler. A secondary desulfurization step using an unreacted calcium source contained in dust and coal ash in the desulfurization apparatus, wherein the exhaust gas from the fluidized bed combustion boiler is provided. Processing method. 2. The amount of the desulfurizing agent in the primary desulfurization step is set at a molar ratio of Ca / S of 2 to 3 so that the primary desulfurization rate is 50 to 60%.
2. The exhaust gas treatment method for a fluidized bed combustion boiler according to claim 1, wherein the molar ratio of Ca / S in the secondary desulfurization step is set to 1.5 to 2 to make the secondary desulfurization rate 80% or more. 3. A fluidized bed combustion boiler in which a small amount of a desulfurizing agent is fluidized together with coal particles to perform primary desulfurization in a furnace, and an exhaust gas from the boiler is passed to separate solids in the exhaust gas into the boiler. A multi-cyclone to be returned, an air heater for exchanging heat through exhaust gas from the multi-cyclone, dust and dust from the air heater and coal ash extracted and slurryed from the boiler are introduced, respectively, and the soot and dust contained in the dust and coal ash are introduced. An exhaust gas treatment device for a fluidized bed combustion boiler, comprising: a spray dryer type desulfurization device for performing secondary desulfurization using a reactive calcium source.