¡¡
¡¡¡ô Normal stack
gas desulphurization technology
| Fire coal¡¯s
stack FGD is most widely used and efficient technology in
the desulphurization. To the heat-engine plants, in coming
long years, FGD is the main way to control the discharge of
SO2. The trend of the FGD of heat-engine plants is: efficient
desulphurization, large capacity, advanced technology, low
investment, small size, low running cost, high-grade automation
and reliability. |
1£®3£®1 Dry stack
gas desulphurization technology
| The technology
came into use in early 1980s. Compared with the normal and
wet FGD, it has the following advantages: low investment,
dry desulphurization output and mixed with fly ash, no need
of mist eliminator and reheater, anti-corrosive, anti-fouling
and anti-blocking. Its disadvantages are that its absorbing
agent¡¯s efficiency is lower than that of dry FGD; its economic
efficiency is when it¡¯s used for sulfur coal; the mixture
fly ash and desulphurization output may affect the comprehensive
efficiency and it has higher requirement in the control of
dry process. |
| (1) Spray stack
gas desulphurization technology (shorted as dry FGD) was first
developed by American Joy Company and Danish Niro Atomier
Company. The technology was developed in mid 1970s and widely
used in the electric power industry. The technology is that
the hydrated lime mist contacts the gas in the spray dry tower.
The hydrated lime mist reacts with SO2 and produces a dry
solid reactant and is lastly collected by the dust collector.
China has gained some experience in her experiments of the
rotary spray dry FGD in the Baima Electric Power Plant in
Sichuan Province. These experience offer the basis to optimize
the design of the rotary spray dry FGD in 200~300MW generating
unit. |
(2)
Fly ash dry FGD technology. Japan began its research of using
fly ash as the desulphurizer in 1985. By the end of 1988,
Japan finished the industrial experiment of the technology.
In 1991, the first fly ash dry FGD equipment began to work.
The processing gas volume reaches 644,000Nm3/h. Its desulphurization
reached 60% and it worked stably and reached the standard
of the normal wet FGD. The cost of desulphurizer is low. The
use of water was little so there was no need of water discharge
treatment and smoke discharge reheating. The price of the
equipment was 1/4 less that that of the wet FGD. The fly ash
desulphurizer can be reused. As there was no pasty stock,
the maintenance is easy. The equipment system is simple and
reliable.
1£®3£®2¡¡Wet stack gas desulphurization technology |
| All
the wet FGDs in the world are almost the same in the technology,
layout and the theory. The technology uses CaCo3, CaO or Na2Co3
pasty stock as the detergent to cleanse the gas in the reaction
tower and thus eliminates the So2 in the gas. The technology
has more than 50 year¡¯s history. With the continuous improvement,
the technology is now mature. Its desulphurization rate can
be as high as 90%~98%. The technology has large capacity and
can treat different kinds of coals. It also has the advantages
of low running cost and the subsidiary product is easy to
deal with. According to the statistics by the USA EPA, 39.6%
of the heat-engine factories use wet CaO; 47.4% use CaCo3.
These two methods takes up 87% of the total. 4.1% use A2 and
3.1% use Na2Co3. In other countries, like German and Japan,
90% of the large heat-engine plants use the wet CaO/ CaCo3
--- the calcium sulphate FGD technology. |
¡¡¡¡The chemical theory of CaO/ CaCo3 is
as below:
¡¡¡¡CaO method: SO2£«CaO£«1£¯2H2O
¡¡¡¡¡¡¡¡¡¡¡¡CaSO3.1£¯2H2O
¡¡¡¡CaCo3 method:SO2£«CaCO3£«1£¯2H2O
¡¡¡¡¡¡¡¡¡¡¡¡CaSO3.1£¯2H2O£«CO2
| The advantages of the
method are that its absorbing agent is rich and cheap, and
the offscum can either be discarded or be retrieved as the
commercial plaster. The CaO/ CaCo3 method is the most widely
used FGD technology. The desulphurization rate can be over
90% for the sulphr coal and 95% for low-sulphur coal. |
| The traditional CaO/
CaCo3 technology also has its disadvantages like the fouling,
blocking, corrosion and abrasion of the equipments. In order
to solve these problems, the equipment manufacturers use different
means to develop the second and third generation CaO/ CaCo3
desulphurization technology.Other mature wet FGD technologies
include magnesium hydroxide method, sodium hydroxide method,
Wellman-Lord FGD by Davy Mckee of USA and ammonia process.
|
| In the wet technologies,
the gas¡¯s reheating directly affects the investment of the
FGD technology. Because the temperature of gas after FGD is
usually low (45¡æ) and below the dew point. If the gas is not
reheated, it will form the acid mist in the chimney and thus
corrode the chimney and is not good for the irradiation of
the gas. So the wet FGD usually has the gas reheating system.
The mostly used technology is regeneration gas heating transfer
(GGH). The GGH is expensive and takes quite a portion of the
FGD processing investment. In recent years, Mitsubishi developed
a non-leaking GGH. The non-leaking GGH well solves the problem
of the gas leaking but it¡¯s still expensive. SHU Company of
Pre Germany developed a new technology which saved the GGH
and chimney. The technology installs the whole FGD equipment
in the cooling tower of the heat-engine plant. The technology
makes full use of the remaining heat of the recycle water
to heat the gas. The technology seems working quite well.
It¡¯s a quite promising method. |
¡¡¡ô Plasma stack
gas desulphurization technology
¡¡¡¡The research of plasma stack gas desulphurization technology
started early 1970s. Two methods are now widely researched.
¡¡¡¡(1) Electron beam irradiation (EB)¡¡
¡¡¡¡When irradiating the gas with steam, the electron beam will
excite and crack the molecules of O2, H2O and thus generate the
free radicals of strong oxidizing property like O, OH, HO2 and
O3. These free radicals will oxidize turn SO2 and NO in the gas
into SO3, NO2 and the related acid. If there is ammonia, stable
and solid thiamin and amidpulver will be generated. And they will
be collected by the dust catcher. Thus the desulphurization is
done.
¡¡¡¡(2) Pulse corona (PPCP)
¡¡¡¡The basic theory of PPCP and EB are almost the same. A lot of
countries in the world conducted many experiments. And the big
pilot tests are also conducted. But there are still a lot problems
needed to be solved.
¡¡¡ô Seawater stack gas desulphurization
technology
| Seawater is usually alkaline.
Its natural alkalinity is about 1.2~2.5 mmol/L. So the sea
water has the natural acid and alkaline cushioning capacity
and the capability of absorbing SO2. Some foreign countries
use its characteristics and successfully develop the technology
of clean the SO2 in the gas by using the sea water. |
| This technology includes
gas system, sea water supply and discharge system and sea
water recovery system. |
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