Influencing factors of slurry bubbling overflow in
desulfurization absorption tower of 1000 MW unit
Introduction
Limestone-gypsum
wet desulfurization is the most mature and common flue gas desulfurization
method at present, and is widely used in 1000MW coal-fired units. With the
continuous improvement of China's atmospheric emission standards, the
implementation of the "Ultra-low emission reduction plan for coal-fired
power plants" and the increasingly serious environmental problems, the safe
and stable operation of the flue gas desulfurization system is particularly
important, so the refined and professional management of the desulfurization
system is also Future Trends. However, the problem of foaming and overflow of
the desulfurization absorption tower slurry has become a thorny problem for its
safe operation. The foaming of the slurry often causes false liquid level,
overflow of the absorption tower, pollution of the environment, increase of
energy consumption, and problems such as cavitation of the pump; In severe
cases, it even affects the safe operation of the induced draft fan, resulting
in a deterioration of the stability of the entire unit. Researchers generally
believe that the foaming of the slurry is affected by a variety of factors. The
foaming of the slurry is often accompanied by the overflow of the absorption
tower. However, the current slurry bubbling overflow still lacks certain
analysis and monitoring methods.
1．
Mechanism and influencing
factors of foam production
The foam is a
porous film-like polydispersion system in which bubbles are dispersed in a
liquid and separated from each other by a liquid film. In general, the foam is
a thermodynamically unstable system, and the foam in the liquid can
automatically escape due to the action of gravity. There are three main reasons
for the foaming of the solution:
(1) A
surfactant-containing component such as iso-facillin;
(2) Generating gas
in the solution or entering air, such as blasting of oxidizing air;
(3) Mechanical
disturbances, such as pumping disturbances.
Studies have shown
that the factors affecting foam stability are: surface tension, solution
surface mobility, solution foam double gas-liquid interface structure liquid
film elasticity, gas diffusion through a gas-liquid interface structure, double
gas-liquid interface structure The influence of surface charge, the influence
of the molecular structure of impurities in the solution, and the like. Among
them, the two main factors are the surface tension of the bubble and the viscosity
of the surface of the solution. The smaller the surface tension and the higher
the viscosity of the surface of the solution, the easier it is to form a foam.
Therefore, to break the foam, it is necessary to increase the surface tension
and reduce the surface. Viscosity.
2. Analysis of the causes of bubbling overflow of
absorption tower slurry
The stability of
the foam is the main reason that affects the foaming of the absorption tower
slurry. It can be analyzed from two aspects:
Ø The substances entering the absorption
tower mainly include limestone, process water and flue gas;
Ø The second is equipment disturbance,
including the slurry circulation pump, the disturbance of the start and stop of
the oxidation fan, and the influence of the tray or choke
device. In this paper, a 1 000 MW coal-fired units desulfurization system for a
long track and monitor the entry of the flue gas, process water, limestone
indicators laboratory analysis, combined fan oxide slurry circulation pumps and
other equipment disturbing effect on the slurry, a detailed analysis The
influencing factor of the bubble overflow of the absorption tower slurry.
2.1 Entrance smoke
The original flue
gas at the inlet has a direct impact on the absorption tower slurry. Excessive
standards such as soot and heavy metals may cause slurry poisoning and foaming.
For example, when
the boiler is oiled or the combustion is insufficient, the flue gas may contain
unburned carbon particles into the absorption tower, so that the organic matter
content in the absorption tower slurry is increased to cause foaming. Poor
operation of the boiler precipitator will increase the concentration of dust in
the flue gas. The heavy metal substances, Al2O3 and Fe203 will increase the
viscosity of the absorption tower slurry, and even saponification reaction will
form an oil film on the surface of the slurry, causing the absorption tower
slurry to foam. Increase the stability of the foam. In January 2017, the
desulfurization absorption tower slurry of the unit developed a serious foaming
overflow phenomenon. During the period, the limestone quality was qualified,
the process water quality was stable, and the absorption tower was foamed and
overflowed. The absorption tower liquid level was reduced by 2 m and then
overflowed. The phenomenon is weakened but there is still more foam.
Analysis of the
reasons, found that during this period of time under high load (more than
900MW), the oxygen content of the FGD inlet is only about 3%, and the coal
powder may not be completely burned. FGD inlet dust concentration of about 9.14
~ 17. 89mg / Nm3, while the dust outlet FGD about 1.87 ~ 3. 45mg / Nm3, visible
absorber slurry trapped a large amount of smoke. Under high load, the soot
contains more unburned carbon particles, Al2O3 and Fe203, among which Al2O3 and
Fe203 are non-crystalline fine particles. After passing through the gypsum
cyclone, most of the particles return to the absorption tower body, so that The
smoke in the main body of the absorption tower is continuously gathered, which
greatly increases the viscosity of the slurry, enhances the stability of the
foam, and finally causes the foaming overflow of the absorption tower slurry.
After the main machine adjusts the electric dust removal operation condition,
the inlet soot is in the range of 4.34 to 5.47 mg/Nm3, and the phenomenon of the
foaming overflow of the absorption tower slurry is alleviated. This fully
demonstrates that the flue gas composition is an important factor affecting the
foaming of the absorption tower slurry. In the incident of absorbing foam in
the absorption tower in the country, the phenomenon of excessive accumulation
of heavy metals and particulate matter caused by the poor effect of the
precipitator is common.
2. 2 Process water
Process water is
the most important factor in maintaining the water balance of the
desulfurization absorption tower. The water quality affects the slurry quality
of the absorption tower to some extent. In February 2017, the unit's
desulfurization absorber tower experienced a more serious slurry blister
overflow after the start-up (shown in Figure 1). During this period, the liquid
level of the absorption tower is lower than the normal operating liquid level
by about 2m. The limestone content of the desulfurization system meets the
design value requirements. The unit load and coal combustion are stable and the
dust collector is good. The reason for the analysis shows that the process
water of the desulfurization system is almost all from the circulating cooling
water. Since the initial cooling water in the initial stage of the start-up is
added with more corrosion inhibitors and bactericides, the main component of
the fungicide is isothiazolinone and its derivatives. An organic substance such
as a substance that acts as a surfactant and can provide a higher COD
equivalent. The concentration of iso- ketone in the water of the test process
reached 199.34±8.23 mg/L, and the chemical COD in the desulfurization
wastewater also reached 169.6±5.6 mg/L. On the other hand, the low-temperature
economizer flushing water with unqualified water quality enters the absorption
tower from the trench. During the foaming, the addition of defoaming agent can
only weaken the foaming for about 2 hours, and the replacement slurry has no
obvious effect. Afterwards, the water quality of the process has improved, and
the foaming of the absorption tower has gradually weakened. It can be judged
that the foaming overflow of the absorption tower slurry is mainly caused by
the quality of the process water. Literature studies have shown that the
isothiazolinone fungicide in the circulating water acts as a surfactant after
entering the absorption tower slurry, which
2.3 Limestone
In addition to
process water and flue gas, one of the most important substances entering the
desulfurization absorber tower is limestone slurry. The quality of the
limestone slurry not only affects the desulfurization efficiency of the
absorption tower, but also greatly affects the slurry quality and gypsum
quality of the absorption tower. The inert components in limestone and the
metal elements (Cd, Ni, etc.) from the wear of the steel balls in the wet ball
mill are likely to cause the deterioration of the absorption tower slurry
quality to some extent, and may even cause the absorption tower to overflow.
Excessive Al2O3 and trace metal content in limestone will also accumulate in
the absorption tower, which will play the same role as excessive smoke. In
addition, the MgO content in limestone exceeds the standard, which often causes
the slurry to foam. Since the solubility of magnesium ions is higher than that
of calcium ions, the appropriate Mg0 in the slurry will be beneficial to
improve the desulfurization efficiency. However, the excessive MgO content in
the limestone not only affects the crystallization and dehydration, but also
reacts with S042-, resulting in an increase in dissolved salts in the filtrate.
Improve the elasticity of the foam formation and enhance the stability of the
foam. In March 2017, the absorption tower foamed again. There are two reasons
for this: on the one hand, the slurry with poor quality of the accident slurry
tank enters the absorption tower too much; on the other hand, the quality of
the limestone is unqualified. The limestone was tested and found to have a MgO
content of 4.61% to 5.77% in the limestone. The limestone color was observed to
be greenish, while the normal Mg0 test value was between 0.25% and 1.17%. At
the same time, the Mg0 content in the slurry was also more common. More than,
reaching 3.29% to 5.31%, it can be seen that the foaming is mainly caused by
the quality of limestone slurry. In June 2012, a phenomenon of slurry foaming
overflow occurred in a power plant. The reason for the analysis was that the
MgO content in the limestone was too large, reaching a maximum of 5.83%. The
literature also found that the primary bubbling overflow of a slurry in a power
plant in Shandong Province was also found in the study. The reason is that the
limestone quality is unqualified, the MgO content in the limestone is
3.53%~4.11%, and the acid insoluble matter is 19.00%. The literature indicates
that the main reason for the foaming overflow of the absorption tower slurry in
a power plant in Zhejiang is The MgO content in the limestone is excessive
(5.45%). Based on the actual site and related literature, it is known that the
poor quality of limestone, especially the high MgO content, is an important
factor causing foaming of the absorption tower slurry.
2.4 Equipment disturbance
Disturbance caused
by the start-stop of the absorption tower related equipment tends to exacerbate
the overflow of the foaming slurry.
The slurry at the
top of the absorption tower is mainly in the gas-liquid-solid three-phase
state, and the gas phase partial pressure is large when the slurry is foamed.
The blasting of the oxidizing wind greatly increases the upper slurry
disturbance, which not only causes the slurry to form more foam, but also makes
the upper part The gas phase partial pressure in the slurry is drastically
increased, resulting in an increase in the measured value of the upper pressure
transmitter and an increase in the liquid level. At the same time, due to the
lower density of the upper slurry, the increased pressure is sufficient to
cause the overflow of the slurry in the upper part of the absorption tower
where the "false level" is generated.
3 Prevention and control measures
(1)
Prevention of absorption tower slurry foaming is mainly monitored and
controlled from the substances entering the absorption tower. Strictly
control the water quality of the process entering the absorption tower,
focusing on its COD and organic content; if all circulating water is used,
communicate with the host in time to understand the use of the scale inhibitor
fungicide, and replace the process water if necessary. Strengthen the quality
supervision of purchased limestone and strictly control the content of MgO,
Al2O3 and Fe203 in limestone content. Strengthen the control of the inlet flue
gas parameters. When the parameters are abnormal, contact the main engine to
adjust the coal burning and dust removal conditions. Strengthen the control of
oil, rain, waste paper, wood blocks, wood chips, etc. in the desulfurization
zone that may enter the absorption tower through the trench, and avoid entering
the absorption tower as much as possible. The liquid phase and solid phase
components of the absorption tower are regularly monitored in strict accordance
with the norms and procedures. The operating personnel should strengthen the
monitoring. At present, most of the absorption towers use double liquid level.
It is found that the abnormal liquid level difference increases, which means
that the slurry may have been foamed. It is necessary to report the slurry
analysis and testing immediately.
(2)
If the slurry is found to be bubbling during the operation and has overflowed,
the defoaming agent can be added at first. However, the
defoaming agent can only alleviate the foaming, can not eliminate the harmful
substances in the slurry in the tower, and the defoaming agent also has a side
effect on the desulfurization efficiency, and the catalyst can be added at the
same time. Secondly, it can reduce the overflow of the slurry by means of
low-load stop or reduce the amount of oxidizing air, reduce the start and stop
of the slurry circulating pump, reduce the liquid level, etc., analyze the
cause of foaming in time, and strengthen the slurry replacement. Increase the
discharge of wastewater and reduce the concentration of substances in the
absorption tower slurry, such as fly ash, inert substances, chloride salts,
sulfates, etc., which contribute to foam stability.
(3)
Renovate the relevant equipment, re-calculate the pressure loss and the amount
of oxidizing air, such as increasing the opening degree of the tray, adjusting
the spacing of the turbulent tube, etc., to ensure that the flue gas is between
the tray and the top of the slurry in the tower under high load. The positive
pressure in the area is within the proper range. Since the absorption tower is
mostly a positive pressure absorption tower, the height of the slurry in the
overflow pipe is higher than the actual liquid level of the absorption tower.
Therefore, it is necessary to adjust the overflow height of the absorption
tower, increase the diameter of the exhaust pipe of the overflow pipe, and
increase the flushing water of the exhaust pipe. Wait.
(4)
Improve the absorption tower liquid level and density measuring device,
especially pay attention to determine the slurry density at the top of the
absorption tower. Since the slurry at the top of the
absorption tower is in a gas-liquid-solid three-phase state, the density is
much smaller than the bottom, so the top liquid level tends to be much smaller
than the actual value. In the case of slurry blistering, the deviation is
greater. In a 1000 MW power plant, the liquid level display value at the top of
the absorption tower was found to be 3. 02 m lower than the actual value, and
the upper actual density was 484 kg/m3 lower than the density meter.
The
results of this study indicate that the sorption bubbling overflow of the
slurry is often caused by a combination of factors.
The
unqualified dust, process water quality and limestone quality of the flue gas
at the inlet of the absorption tower will greatly promote the foaming of the
absorption tower slurry.
Slurry
circulation pump, oxidation fan and other absorption tower related equipment
start and stop disturbance to the slurry will affect the overflow of the
absorption tower foaming slurry, especially the start of the slurry circulation
pump will directly increase the overflow of the foaming slurry.
By
adding antifoaming agent, lowering the absorption tower liquid level, stopping
the slurry circulating pump and oxidizing fan, etc., the slurry bubbling
overflow can be slowed down to a certain extent, but to solve the problem of
slurry bubbling overflow, it still needs to be absorbed. Control and prevent
the substances entering the absorption tower, such as the dust content of the
tower entrance, the quality of the process water, and the quality of the limestone.
评论
发表评论