Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Reexamination Certificate
1998-08-26
2001-06-12
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
C423SDIG001
Reexamination Certificate
active
06245314
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a calcium sulfide oxidation method and apparatus for oxidizing calcium sulfide (CaS) generated at a power plant, etc., to thereby obtain calcium sulfate (CaSO
4
).
2. Description of the Prior Art
One example of a prior art oxidation apparatus for oxidizing CaS generated at a power plant, etc., into CaSO
4
is shown in FIG.
10
. In
FIG. 10
, numeral
1
designates an oxidation apparatus, numeral
1
A designates a fluidized bed formed therein and numeral
1
A designates a plenum. Numeral
6
designates a heat exchanger disposed in the oxidation apparatus
1
, numeral
7
designates a cyclone and numeral
8
designates a particle distributor.
Numeral
9
designates a distributor plate disposed at a bottom portion of the oxidation apparatus
1
. On this distributor plate
9
, the fluidized bed
1
A is formed, and limestone particles
100
containing char and CaS are supplied into the fluidized bed
1
A through a nozzle
2
. A mixture gas
101
of oxygen, steam and nitrogen is supplied into the plenum
1
D through a nozzle
3
. The mixture gas
101
is supplied into the fluidized bed IA via the distributor plate
9
to vigorously effect a mixture combustion of the particles
100
in the fluidized bed
1
A.
The oxygen concentration of a combustion gas
103
coming out of the fluidized bed
1
A is set to 3 to 4% or more. Unless the oxygen concentration of 3 to 4% or more is maintained, it will be difficult to burn the char constantly. In the fluidized bed
1
A, there occurs a reaction of CaS+
2
O
2
→CaSO
4
between CaS and oxygen in the gas. While a large proportion of CaS is converted to CaSO
4
as a whole in the fluidized bed
1
A, CaS remains still within the particles.
The heat exchanger
6
is disposed in the fluidized bed
1
A so that heat of the particles in the fluidized bed
1
A is collected and a heating medium fluid
107
flowing in the heat exchanger
6
is heated. Combustion gas
108
coming out of the oxidation apparatus
1
enters the cyclone
7
to be separated into a dedusted combustion gas
109
and collected particles
110
. The
30
collected particles
110
are distributed by the particle distributor
8
into fine powder particles
111
to be extracted outside the system and coarse particles
112
to be returned into the fluidized bed
1
A.
The coarse particles
112
are supplied into the fluidized bed
1
A via a nozzle
5
.
Coarse particles
102
, an ash content of the char, which are not pulverized in the fluidized bed
1
A, but remain there so as not to be elutriated by the gas
103
, are extracted outside the system via a nozzle
4
which is fitted to the distributor plate
9
.
In the prior art apparatus as described above, there is contained in the particles
111
and
102
extracted outside the system a high concentration of CaS which has not been converted into CaSO
4
. This high concentration CaS contained in the particles
111
and
102
is gradually decomposed in the air to generate H
2
S, which results in the problem of an unfavorable influence being given to the environment.
Two reasons are considered why CaS remains in the particles discharged from the prior art oxidation apparatus. Firstly, CaSO
4
generated on a particle surface at an initial stage of reaction forms a dense shell, so that oxygen is not supplied into the interior of the particle and CaS therein cannot react with oxygen. CaSO
4
, as compared with CaS, has a molecular volume of 1.8 times as larger, and as the reaction proceeds from CaS into CaSO
4
, gas diffusion pores existing in the particle clog, and oxygen cannot be supplied into the interior of the particle.
Secondly, a fine powder begins to entrain from the fluidized bed before ensuring sufficient reaction time required for complete oxidation of CaS, is discharged outside the oxidation apparatus as CaS contained in the fine powder, and is not yet completely oxidized.
Also, in case the fuel supply rate varies, because it is necessary to maintain the temperature and gas flow velocity in the fluidized bed within an appropriate range, it is preferable to change the heat transfer rate of heat transferred to the heating medium through the heat exchanger in the fluidized bed corresponding to the fuel supply rate.
In the prior art, however, it has been difficult to greatly change the heat transfer rate unless the height of the fluidized bed is changed. Further, in changing the fluidized bed height, it is necessary to put in or take out fluid medium to or from the fluidized bed, which work requires a great amount of time, and there has been a problem in that variations in the fuel supply rate cannot be followed well.
SUMMARY OF THE INVENTION
In view of the shortcomings in the prior art, as described with respect to the apparatus shown in
FIG. 10
, it is an object of the present invention to provide a CaS oxidation method and apparatus for oxidizing CaS into CaSO
4
by which CaS particles can be oxidized into CaSO
4
completely, as far as to the interior of the particle.
It is also an object of the present invention to provide an operation method of the CaS oxidation apparatus according to the present invention by which CaS can be efficiently oxidized into CaSO
4
.
In order to attain the object, the present invention provides the following oxidation method using an oxidation apparatus forming therein a first fluidized bed, a second fluidized bed on an outer side of the first fluidized bed and a space portion above the two fluidized beds.
That is, CaS-containing particles fluidized by a gas flow in the first fluidized bed are caused to collide violently with a heat exchanger or a baffle plate, disposed in the oxidation apparatus so as to traverse the gas flow. Accordingly, a dense shell of CaSO
4
generated on a surface of the particle is abraded and oxygen is spread as far as to the interior of the particle to thereby accelerate an oxidation reaction of CaS into CaSO
4
. The baffle plate has no heat exchange function and has a surface coating applied thereto made of a material of a hardness higher than that of a fluid medium.
Further, a flow velocity in the space portion above the first fluidized bed is set to a terminal flow velocity or less of a fine powder entraining from the first fluidized bed to thereby cause the entraining fine powder soaring into the freeboard portion from the first fluidized bed to fall down into the second fluidized bed disposed on the outer side of the first fluidized bed.
Also, a gas flow velocity in the second fluidized bed is set lower than that in the first fluidized bed so that the fine powder that has fallen down into the second fluidized bed may not be entrained again. A volume of the second fluidized bed is set such that a particle residence time in the second fluidized bed becomes the value (or more) as calculated to a time required for complete oxidation of the fine powder, and the fine powder, containing CaS, which has been supplied from outside of the oxidation apparatus is supplied into the second fluidized bed.
Furthermore, an abrasion rate of the shell of CaSO
4
in the first fluidized bed is controlled by a gas flow velocity in the first fluidized bed and an in-bed fill of the heat exchanger and baffle plate.
According to the CaS oxidation method described above, CaS is supplied into the second fluidized bed to be oxidized and is then sent to the first fluidized bed. In the first fluidized bed, CaS particles collide with the heat exchanger or baffle plate and the shell of CaSO
4
generated on the surface of CaS particle is pulverized and abraded so that CaS particles are accelerated to be oxidized into CaSO
4
as far as to the interior of the particle.
CaS particles so accelerated to be oxidized in the first fluidized bed soar into the space portion from the first fluidized bed and then fall down into the second fluidized bed, so that the CaS particles are oxidized into CaSO
4
completely with a lower gas flow velocity and with sufficient time.
According to the CaS oxidation method of the present
Fujioka Yuichi
Ishigami Shigeyasu
Kobayashi Yoshinori
Setoguchi Toshihiko
Shinoda Katsuhiko
Griffin Steven P.
Mitsubishi Heavy Industries Ltd.
Vanoy Timothy C
Wenderoth , Lind & Ponack, L.L.P.
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