Combustion exhaust gas treatment system

Details Combustion exhaust gas treatment system Combustion exhaust gas treatment system

1999-09-27

2003-01-28

Johnson, Jerry D. (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S168000, C422S139000, C422S140000

Reexamination Certificate

active

06511636

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ARTS
The present invention relates to a combustion exhaust gas treatment system, more particularly to a combustion exhaust gas treatment system capable of easily removing dust and selenium (Se) in combustion exhaust gas containing dust and Se components, and making harmless.
Hitherto, as the combustion exhaust gas treatment system installed in a thermal power plant or the like, a combustion exhaust gas treatment system comprising a dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and a combustion exhaust gas desulfurization apparatus for absorbing sulfurous acid in the combustion exhaust gas is generally employed.
Moreover, conventionally, as the combustion exhaust gas treatment system installed in a thermal power plant or the like, a combustion exhaust gas treatment system comprising a dry dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and a wet combustion exhaust gas desulfurization apparatus for absorbing sulfurous gas in the combustion exhaust gas by contacting with an absorbent slurry (for example, slurry containing calcium compound) in the absorption column, and separating and recovering gypsum as byproduct from the slurry in the absorption column is generally employed.
Recently, handling of harmful impurities contained in the combustion exhaust gas aside from sulfur oxides is posing a problem. In particular, in the combustion exhaust gas treatment system for coal fired boiler, the harmfulness of selenium (Se) contained at a maximum level of about 10 mg/kg in coal is a problem lately, and its harmless treatment is demanded.
Meanwhile, Se exists as tetravalent Se (main form: selenious acid SeO
3
2−
) which is easy to treat by making insoluble by treating agent, and hexavalent Se (main form: selenic acid SeO
4
2−
) which is hard to treat by making insoluble, and in particular the hexavalent Se is high in solubility (solubility at 20° C. is 95%) and is easy to elute. Besides, this Se has a toxicity similar to that of arsenic compound, and disaster cases and emission regulations are known overseas, and it is newly added to the list of regulated items also in Japan, and is controlled by the environmental standard (0.01 mg/liter), discharge standard (0.1 mg/liter), and elution standard in landfill (0.3 mg/liter).
FIG. 24
shows an example of prior art of combustion exhaust gas treatment system of this type (an example of combustion exhaust gas treatment system for coal fired boiler). In FIG.
24
and
FIG. 25
, the combustion exhaust gas A emitted from a coal fired boiler
1
is sent into a denitration apparatus
2
installed downstream of the boiler
1
to be rid of nitrogen oxides (NOx), and passes through an air heater
3
and a heat recovery unit
4
of gas-gas heater (GGH), and is introduced into an electrostatic precipitator (EP)
5
, in which fly ash and dust are removed. In succession, the combustion exhaust gas is guided into a wet combustion exhaust gas desulfurization apparatus
7
by a fan
6
, and sulfurous gas is removed in this desulfurization apparatus
7
, and after passing through a reheater
8
of the gas-gas heater (GGH), it is led into a stack
10
by a fan
9
, and is released into the atmosphere through the stack
10
(FIG.
25
).
On the other hand, fly ash and dust removed in the electrostatic precipitator
5
are discharged from plural hoppers
5
a
(dust recovery units) formed in the electrostatic precipitator
5
, and are conveyed and collected in batch by a conveyor
11
. Thus collected dust B is either recycled as cement material or the like, or discarded in an ash disposal yard (FIG.
24
).
Herein, the desulfurization apparatus
7
comprises an absorption column, for example, in which combustion exhaust gas is introduced, and by contact of combustion exhaust gas with absorbent slurry (usually slurry containing calcium compound) in this absorption column, the sulfurous acid in the combustion exhaust gas is absorbed in wet process, and usually from the slurry in the absorption column, gypsum is separated and collected as byproduct.
Incidentally, the heat recovery unit
4
of the gas-gas heater (GGH) may be also disposed immediately before the desulfurization apparatus
7
as shown in FIG.
26
.
In these combustion exhaust gas treatment systems, most of Se in coal (Se in combustion exhaust gas) is condensed at the downstream side of the air heater
3
(that is, the position before introduction into the electrostatic precipitator
5
), and is removed by the electrostatic precipitator
5
in a state being contained in the dust in combustion exhaust gas, and is directly mixed in the refuse in the ash disposal yard or in the cement material. To render Se harmless by conforming to the elution standard, it requires a complicated and costly aftertreatment of, for example, diluting the ash removed by the electrostatic precipitator
5
in a huge volume of water.
OBJECTS AND SUMMARY OF THE INVENTIONS
In the light of the prior arts, it is an object of the invention to present a combustion exhaust gas treatment system capable of easily removing and making harmless the Se components contained in combustion exhaust gas.
To achieve the object, five inventions are proposed.
A. First Invention:
(1) A combustion exhaust gas treatment system for treating combustion exhaust gas containing dust and Se components, comprising means for cooling combustion exhaust gas to 350° C. or less, dust collecting means for separating dust in the combustion exhaust gas, and Se treating means for transforming the existent form of Se in the dust into an insoluble compound by adding water and treating agent to the dust separated by the dust collecting means.
(2) A combustion exhaust gas treatment system for treating combustion exhaust gas containing dust and Se components, comprising means for cooling combustion exhaust gas to 350° C. or less, dust collecting means for separating dust in the combustion exhaust gas, Se treating means for transforming the existent form of Se in the dust into an insoluble compound by adding water and treating agent to the dust separated by the dust collecting means, and solid-liquid separating means for separating the slurry containing insoluble Se compound discharged from the Se treating means into solid and liquid.
Preferred embodiments of the first invention includes the following combustion exhaust gas treatment systems (3) to (6).
(3) A combustion exhaust gas treatment system of (1) or (2), wherein the dust collecting means is a dust collecting apparatus constituted by forming a plurality of recovery units for separating and collecting dust from the inlet side to the outlet side of the combustion exhaust gas, separating and collecting the dust collected from the recovery unit at the inlet side of the combustion exhaust gas and the dust collected at the outlet side separately, and introducing only the dust collected at the outlet side into the Se treating means.
(4) A combustion exhaust gas treatment system of (1) or (2), further comprising sorting means for sorting the dust separated by the dust collecting means into large particle size group and small particle size group, wherein only the small particle size dust sorted by the sorting means is introduced into the Se treating means to make Se insoluble.
(5) A combustion exhaust gas treatment system of (3) or (4), further comprising means for mixing the dust making Se insoluble by the Se treating means, and the remaining dust not making Se insoluble, so that the moisture content may be 20% or less.
(6) A combustion exhaust gas treatment system of (5), further comprising means for forming the dust mixed by the mixing means into scale.
In the combustion exhaust gas treatment system of the first invention, the method of making Se insoluble may be realized in various forms, and representative forms include a method of forming slurry by adding water to the dust containing Se components separated and recovered by

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