Journal of Environmental Protec tion, 2013, 4, 1-4
doi:10.4236/jep.2013.41b001 Published Online January 2013 (http://www.SciRP.org/journal/jep)
1
Cause Analysis and Countermeasure of Gypsum Rain in
Coal-fired Power Plants
Qizhen Liu, Yanjing Sun, Yi Sun
Shanghai Environmental Monitoring Centre, 200030.
Received 2013
ABSTRACT
Focusing on the phenomenon of gypsum rain while wet desulphurization(WFGD) were adopted in coal fired power
plant without GGH, the paper studied and put forward the solutions: 1) desulfurization facilities related equipment
modification; 2) optimal opera tion of existin g de sulfurization facilities.
Keywords: Coal Fired Power Plant; Gypsum Rain; GGH; Wet Desulphuriza tion
1. Introduction
Currently, we have already completed the inspection of
desulfurization units of coal-fired power plants with 14.24
million kilowatts (about 96% of the total capacity of in-
stalled coal-fired units). The Desulfurization units are
mainly based on limestone/gypsum wet FGD, onl y 8 set s
are installed GGH, which accounts for 15% of total ca-
pacity.
The des ulp huri za tion s ys te m wit hout G GH ha s a lo wer
exhaust temperature because of the lack of stack reheat-
ing measures, so that flue gas cannot quickly dissipate,
going with the inefficient uplift and spread, resulted by
the net smoke with saturated water of absorption tower
expo rt par tly co ndensing to form droplets in the discharge
process. Especially for adverse weather conditions like low
temperature and low pressure, the gas will form con-
densed into drops when leaving chimney, falli ng d own to
the ground around the chimney like rain, leaving solid
white residues after landing and polluting the surrounding
environment of power plant. That is the so-called “gyp-
sum r ain”.(F igure 1)
2. Investigation and Cause Analysis of
“Gypsum r ain”
Thr ough the i n-depth investigation of six power plants in
Shanghai, it is found that “gypsum rain” phenomenon is
mainly affected by the following factors.
2.1. External Causes
The g yp su m ra i n is no t conduc ive to sprea d in stead y state
conditions. When the temperature is low, it is easy to
form droplets and the probability of particle coagulation
also increases.
2.2. Internal Reasons
The internal reason includes excessive high ga s flo w and
speed; low efficiency of defogger; and the low te mpera-
ture of ex haust flue gas.
(a)
(b)
Figure 1. I nstal lation GGH exterior effect c ontrast diagra m.
(a) With GGH- “a smoke-free look”; (b) No GGH –“white

Cause Analysis and Countermeasure of Gypsum Rain in Coal-fired Power Plants
2
smoke billowing”.
● Excessive high speed of flue gas when passing the
desulfurizat ion precipita tor
When the unit is running in the high load operation,
the excessive high gas flow results in the increasing ca-
pacity of carrying gypsum slurry.
● The uneve n distribution of imported gas
The smoke distributes unevenly when coming out from
the absorb er, which re sul ts i n the u neve n flo w in the defog-
ger and then increases partly defogger clogging. In th at c as e,
flue gas flow channel becomes smaller, smoke-speed in-
creases, and the ability to carry gypsum slurry is further
strengthened.
● The low temperature of flue gas at desulfurization
export
The flue ga s tempe ratur e goi ng int o the ab sorp tion to wer
is 100˚C - 130˚C while the temperature out after through
the desulfurization absorber is 60˚C - 70˚C. The tem-
perature will be further decreased to about 50˚C after
water was hin g in t he 2-le vel d efog ger , whic h will shorter
the climbing distance after smoke leaving the chimney
and then make it condense into droplets rapidly and fall
to the ground.
Except the above-mentioned three main factors, fac-
tors such as absorber liquid level, limestone slurry pH,
original content o f PM in the flue gas will also affect the
car rying amount of liquid amo ng flue ga s [1].
3. Preliminary Analysis of the Composition
of “Gypsum Rain”
Consid ering that the traditional drop bucket method of col-
lecting gypsum rain can onl y get few sample for composi-
tion analysis. Experts of desulfurization technology suggest
that: as par t of the flue gas condensing and back-flowing
on the wall of the chimney, and then fo rming a backflow
consisted with the same co m p o sitio n as “gypsum Rain”
is, we collected the backflow fluid on Shanghai power
plant A’s chimney wall and analyzed its co mposition.
Statistics show that the pH of “Gypsum rain” filtrate is
about 2.6 which means a strong acid ity. All kinds of io-
nic compone nts analysis show that sulfate concentra - tion
in the filtrate reached a maximum of 774 mg/L, fol-
lowed by sodium, 125 mg/L, while the calcium compo-
nent is only 14.0 mg/L. The particle is mainly made up of
oxidation silicon, followed by about 23.7% calcium oxide
and 13.0% sulfur. Therefore, it can be seen that the acid-
ity of “Gypsum rain” is relatively strong, which affects
the surroundings seriously and causes severe corrosion.
The main pa r ticle s of “Gypsum rain” are small dust and
smoke particles. At the same time, “Gypsum rain” also
contains much limestone or gypsum.(Figure 2)
4. The Method to Solve “Gypsum Rain”
There are few reports about “Gypsum Rain” abroad. In
early German regulations, the temperature of discharging
flue gas should be higher than 72˚C. Therefore, all desul-
furization facilities have installed the GGH. In 2002,
German adopted the European Union standards and can-
celled the temperature limitation of discharging flue gas.
Then more coal fired power plant would discharge gas
through the cooling tower. U.S. environmental standards
has no requirements on smoke temperature of chimney
exit, but some American power plant still installed clean-
burning-fuel burner at the bottom of the chimney. Japan
has taken high temperature smoke emissions to enhance
the diffusion ability of flue gas in order to reduce the
native po llution of; in that case, all the FGD devices in-
stalled the GGH in J a pan [2-4].
Although there are “gypsum rain” phenomenon reported
in China, in-depth study is lacked. According to the ma-
terial researched and desulfurization experts, the method
to solve “gypsum rain” is divided into two categories: 1)
Equipment modification related to desulfurizatio n ; 2)
optimal ope ration of existing desulfurization facilities.
4.1. Equipment Modification Related to
Desulfurization
● Secondary air heating boiler
We added boiler secondary air bypass at the e xit of the
chimney and introduced 300˚C hot air into the second
boiler. The stack mixed by bypass flue and chimney one
was let out after heate d.
(a)
(b)

Cause Analysis and Countermeasure of Gypsum Rain in Coal-fired Power Plants
3
Figture 2. the impact of “Gy psum R ai n ” on v ehicles ar ound
power plants. (a) “Wh ite” glass ; (b) Vehicles with proteciton.
● Defogger transformation
1) Addition layer of demisting facilities
At present, most desulfurization facilities use one-layer
or two-layer demisting facilities. It is suggested that one
layer of de misting facilities s hould be a dded in the space
location allowed.
2) Addition uniform guide plate on the demister inlet
horizontal flue section
According to the high speed of flue gas nea r t he chimney
wall, a guide plate should be set up in the corner outside
the flue to reduce its velocity and eli minate flue gas dis-
turb ance of sprayer entrance. It makes smoke even into
demisting pr e c ipitator and reduces local con gestion.
3) Mist eliminator washing sy stem transformation
At p resent, co ld -water-rinse method is adop ted in mist
eliminator wishing. The method could greatly reduce the
temperature of flue gas and increase “gypsum rai n” phe-
nomenon. It is suggested t hat vapor source should be a dded
into the original mist eliminator washing system and a
water desuperheating device be set up on pipeline, ad-
justin g t he s tea m te mpera tur e, in order to prevent defogger
base metal from excessive steam temperature. The unit
adopts the method of water washing in conjunction with
st e am fl u sh in normal operation. With the steam auxiliary
steam source and contro lling pressure through a valve, a
variable pressure flushing can be achieved.
● Addition flue gas heater
In American, considering that a unit without GGH would
make the temperature too low and produce a negative im-
pact on t he surro unding e nviro nment, so me power plants
install clean burning fuel burner at the bottom of the
chimney to temporarily heat the gas after desulfurizatio n
in adverse weather conditions. This method can protect
the environment with low investment and running costs.
As a kind of practic al solutions, it is worth usi ng for r ef-
erence.
● Installing the GG H
Installing the GGH will be able to better solve the “gyp-
sum rain” phenomenon; it is also conducive to long-dis-
tance gas transportation. T hough it can so lve local po llu-
tion problems, the investment costs are very high on one
hand and , o n t he o the r hand , p a rt s o f the power plants are
limited by space. Besides, GGH blockage reduces the
rate of desulfurizatio n facilities into operation dra wbacks.
Therefore, we recommended that it could be considered
that coal-fired units could compromise GGH while in-
stalling desulfurization devices [2-4].
● Transformati on of the inner chimney wall
As t he lower temperature of the inner chimney wall, it is
easy for slurry to condense int o dr ops han ging o n t he wal l
and be carried out of the c himne y by flue gas in the form
of droplets. It is recommended that a circular blocking
device should be set in the chimne y to reduce the slurry
carried on the chimney wall.
4.2. Optimal Operation of Existing
Desulfuriz at ion Fa ci lities
● Paying attention to the pressure gap of defoggers
and reducing defogger jam
Highe r s l urry de n si t y, pH a nd liquid-gas ratio will ca use
defogger jam, increase the pressure of defogger and reduce
the efficienc y. Ther efor e, p aying a tte ntio n to the pre ssure
gap of defogge rs an d controll ing the slurry density, pH and
liquid-gas ratio within reasonable ranges will decrease
defogger jam and the probability to form “gypsum Rain”.
● Optimizing defogger washing and reducing the
amount of slurry carried
Taking the washi ng o rd er as “upper parts of secondary
defogger, lower parts of secondary defogger, primary de-
fogger upper, primary defogger lower, seco ndar y defogger
upper and secondary lower parts again”. It means firstly
washing the upper part of the secondary defogger, then
washing the lower part to make the secondary defogger’s
channel unobstructed and ease the congestion with a
sudden increase in concentration of gypsum droplets into
the secondary defogger. After the pressure of the second-
dary defogger reduced, washing the upper and lower part
of the primary defogger and then washing the secondary
defogger again to remove the gypsum droplets from the
primary defogger, avoiding being carried away. At the
same time, pay attention to the rule of power flow. Be-
fore unit load increases, washing defogger ahead will
greatly reduce carrying capacity of gypsum slurry during
high load [6].
● Appropriately reducing the air flow and the flow
rate of flue gas
“Gypsum rain” phenomenon is often seen during high
load operations of boilers, which is rela ted to the flue gas
flow. When with a large load, under the premise of en-
sure the oxygen of daily boiler combustion, reducing the
air flow appropriately and controlling hearth negative
pressure and boostering fan pressure to reduce the gas
flow and veloc ity.
5. Conclusions and Recommendations
For that mo st local coal -fired power plants adopt limestone /
gypsum wet FGD without installing the GGH, “gypsum
rain” is easily induced b y the low-temperature stac k under
the adverse condition of weather, polluting the environ-
men t and the surroundings of power plants. In combination
with the actual circumstances of coal-fired power plant,
we put forward the following recommendatio ns.
● All the local co al-fired power plants facilities which
adopted wet desulphurization without GGH should
strengthen the optimized operation and management

Cause Analysis and Countermeasure of Gypsum Rain in Coal-fired Power Plants
4
so as to reduce the influence of the gypsum rain.
● For the exit coal-fired u nits without GGH i n nearb y
areas, a check should be done to the Desulphuriza-
tion facilities defogger. In the mean time, carrying
out a technological transformation on the desulfur i-
zation fa ciliti es whose effect of defogger is not idea l.
Taking measures like adding a gas heater or hot air
into the boiler equipment for reconstruction project
in or der to lift the tempera ture of flue gas and cut from
the source “gypsum Rain” production.
● While modifying the desulphurization facilities of
the coal-fired units, an effective control should be
taken t o “gyp sum Rain” as o ne of the ke ys of pollu-
tion controllin g. It is required that a GGH and other
gas heating facilities should be installed simu ltane-
ously. All the abo ve-mentioned measures are aimed
to avoid "gypsum rain" phenomenon.
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