Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)55

AQCS (Air Quality Control System) for
Thermal Power Plants Capable of Responding to
Wide Range of Coal Properties and Regulations
NARUHITO OMINE*1

TATSUTO NAGAYASU*2

HIROSHI ISHIZAKA*3

KAZUAKI MIYAKE*4

KEIGO ORITA*5

SEIJI KAGAWA*6

Generally coals used in coal-fired power plants in India and China have high ash contents,
and along with the strengthening of PM (particulate matter) control regulations, high PM removal
performance is required. To demonstrate the Mitsubishi Hitachi Power Systems (MHPS) Group's
high-performance PM removal system, the PM removal performance under high PM concentration
conditions was evaluated at the 1.5MW pilot test facility, and the system was applied to the retrofit
project at Zouxian Power Station in China (1,000MW) and achieved the PM concentration of
5mg/m3N or less at the stack outlet. In addition, our proprietary high-performance Moving
Electrode type Electrostatic Precipitator (MEEP®) was applied to the retrofit project of the dry
Eloctrostatic Precipitator (ESP) at Rihand Super Thermal Power Station in India (500MW x 2),
and through modification of the existing space, the PM concentration at the ESP outlet was
reduced from 500 to 600mg/m3N to 50mg/m3N or less.

|1. Introduction
In recent years, in developing countries with soaring electric power demand, particularly in
India and China, the increase in the PM concentration in the atmosphere has been a problem, andthe control of PM emitted from thermal power plants has been increasingly strengthened.
Generally, the coal used in thermal power plants in India and China has an ash content of about
30% to 45%, which is very high compared with the coal generally used in Japan, Europe andAmerica (which has an ash content of around 10%). Therefore, high dust removal performance isrequired. At thermal power plants in India and China, general precipitators for PM removal havealready been installed, and to cope with the strengthening of regulations, many customers want tomodify the existing precipitators in their limited site areas.
The promising technologies for meeting these needs are the MHPS Group's proprietary
technologies: (1) high-performance PM removal system which combines a heat exchanger and the
ESP to adjust the flue gas temperature, thereby achieving high particulate collection efficiency, and(2) an ESP equipped with MEEP®. These technologies were field-proven in many projects both
inside and outside Japan, but there was no track record in using them with high ash content coals
such as Indian coals and Chinese coals. This report introduces an overview of the two
aforementioned high-performance PM removal technologies and the results of their application tohigh ash content type coals.

|2. High-performance PM removal technologies
2.1 High-performance PM removal system
Figure 1 shows the flow of the high-performance PM removal system and the conventional
system. In the conventional system, a gas cooler is installed between the ESP and the flue gas
desulfurization equipment (FGD), and the flue gas temperature at the ESP is in the range of 130°C
to 160°C. On the other hand, in the high-performance PM removal system, a non-leak type andfinned-tube gas cooler is installed in front of the ESP, and the flue gas temperature at the ESP is*1 Kure Air Quality Control System Technology Development Department, Air Quality Control Systems Technology Division, Mitsubishi Hitachi
Power Systems, Ltd.
*2 General Manager, Air Quality Control Systems Technology Division, Mitsubishi Hitachi Power Systems, Ltd.
*3 Chief Staff Manager, DeNOx Technology Department, Air Quality Control Systems Technology Division, Mitsubishi Hitachi Power Systems, Ltd.
*4 General Manager, Engineering and Development Department, Technology and Innovation Headquarters, Mitsubishi Hitachi Power Systems
Environmental Solutions, Ltd.
*5 Engineering and Development Department, Technology and Innovation Headquarters, Mitsubishi Hitachi Power Systems Environmental Solutions, Ltd*6 Chief Staff Manager, Research & Innovation Center., Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)56

lowered to 90°C to 100°C. The ESP uses electrostatic force for particulate collection, and PM
which is charged with negative ions emitted from the discharge electrode is moved to the collecting
electrode on the positive side and is collected, and then a shock is imparted to the PM byhammering for detachment and collection.

Figure 1 Configurations of the conventional system and the highperformance PM removal system
In the high-performance PM removal system, a gas cooler is installed at the front stageof the ESP and the flue gas temperature at the ESP is 90°C to 100°C.

Figure 2 Relationship between the flue gas temperatureand the electric resistivity of the ash
The electric resistivity of coal ash fluctuates with the flue gas
temperature. In the temperature range of the high-performance PM
removal system, the electric resistivity becomes lower than that of
the conventional system.

Figure 2 presents the relationship between the electric resistivity (ρ) of PM emitted fromcoal firing and the flue gas temperature. ρ is the factor that most greatly affects the particulate
collection performance of the ESP. In the conventional system which operates at a temperature
ranging from 130°C to 160°C, ρ is in a high region, while in the high-performance PM removal
system which operates at 90°C to 100°C, ρ is lowered. The higher ρ becomes, the stronger theadhesion force of PM to the collecting electrode becomes, which makes it difficult to shake off PM
by hammering. This causes a back corona in which positive ions with the reverse polarity aredischarged from the inside of the layer of PM accumulated at the collecting electrode into theparticulate collection area, resulting in a rapid decline of the particulate collection performance of
the ESP. Therefore, to maintain the high particulate collection performance, it is necessary to takecountermeasures for the back corona caused by PM with high electric resistivity. In thehigh-performance PM removal system in which the ESP operates at a temperature ranging from
90°C to 100°C, ρ is lowered to the range where PM is easily detached by hammering, resulting in
the improvement of the particulate collection performance compared with the conventional system.
This allows the ESP equipment to be made compact, and the decrease of the inflow of PM into the
FGD improves the quality (purity) of by-product gypsum from the FGD.
So far, it was demonstrated in the tests using the pilot test facility and actual units that theMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)57

high-performance PM removal system is more effective than the conventional system in thesubstantial removal of not only PM, but also toxic trace elements such as mercury (Hg) andselenium (Se), as well as sulfur trioxide (SO3), which causes bluish plumes or acid rain.1 However,to apply the high-performance PM removal system to high ash content type Indian and Chinese
coal, it is necessary to demonstrate that the gas cooler which is a key unit in the high-performance
PM removal system can operate without problem under the condition with a high PM
concentration. For that, the relationship between the state of the ash accumulated in the gas cooler,
which has a high PM concentration atmosphere, and the heat transfer characteristics, as well as thewearing speed of the finned tube, are evaluated.

2.2 Moving Electrode type Electrostatic Precipitator (MEEP®)
In a conventional ESP, which is comprised of only general fixed electrodes, fine PM isentrained in the flue gas and re-emitted during hammering for the detachment and collection of PM
accumulated at the collecting electrode, resulting in an increase of the concentration of PM emitted
from the stack. In addition, PM with high electric resistivity has a strong adhesion force to thecollecting electrode and a strong cohesive force between particles, and it is difficult to detach by
the impact of hammering, resulting in degradation of the performance of the electrode due tocontamination over time.
In our ESP, as shown in Figure 3, the general fixed electrodes are installed at the front stage
to roughly remove PM of several tens of thousands mg/m3N floating in flue gas to a certain degree,
and our proprietary MEEP® technology is installed at the rear stage to remove the remaining fine
PM and high electric resistivity PM. As a result, the concentration of PM at the ESP outlet can be
reduced to several tens of mg/m3N. Figure 4 shows the structure of MEEP®. While the collecting
electrode element is rotated by the driving chain, the electrostatically-collected PM on the element
surface is scraped off using a brush installed in the hopper in which no flue gas flows. Therefore,
MEEP® exerts a superior particulate collection performance for high electric resistivity PM at thegeneral fixed electrode which is difficult to detach by hammering or for fine PM which is entrainedin the gas flow and re-emitted by hammering. Accordingly, when MEEP® is applied to a retrofit
project for the purpose of enhancing the capability of the existing ESP, the performance can be
improved without increasing the ESP installation area, and the use of MEEP® is a very effective
measure to cope with the recent strengthening of flue gas regulations in various countries aroundthe world..

Figure 3

Structure of our ESP (one example)

Figure 4

Structure of MEEP®

One example of the structure of our ESP equipped with By scraping adhered ash with a brush when the
MEEP®
collecting electrode is driven, high PM collection
performance is maintained.

|3. Demonstration of high-performance PM removal system andexample of application to actual units in China
3.1 Demonstration of high-performance PM removal system using high ash content
coal at our pilot test facility
Using the 1.5MW pilot test facility introduced in a previous report1, the particulate collection
performance of the ESP, the heat transfer characteristics of the gas cooler and the wearing speed of
the finned tube during firing of high ash content Indian and Chinese coal were evaluated.2 AsMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)58

shown in Figure 5, this facility is, as with an actual coal-fired thermal power plant, thoroughly
equipped with a furnace, heat exchanger, NOx reduction catalyst, gas cooler, gas heater, ESP (afabric filter can also be used by bypass) and wet-FGD, and the operating conditions of each device
can be arbitrarily changed, allowing various kinds of tests. Therefore, we used the facility from thedevelopment stage of the high- performance PM removal system.

Figure 5

Flow diagram of our 1.5MW pilot test facility

As with the actual equipment, our test facility has all general devices such as a furnace through stack..

Indian coal and Chinese coal were used in the conventional system and the high-performance
PM removal system to compare the PM concentration at the ESP outlet, and the results are shownin Figure 6. As with the previous findings, in the high-performance PM removal system, evenwhen high ash content coal is used, the ESP temperature is lowered, and it exhibited an improved
particulate collection performance. Concerning the electric resistivity of Indian coal ash andChinese coal ash (Figure 7), it was found that the electric resistivity of the ash was lowered in thetreatment flue gas temperature range of the ESP in the high-performance PM removal system.

Figure 6 Changes in the overall coefficient of heat transfer ofthe gas cooler in the high-performance PM removal system
By lowering the flue gas temperature of the ESP for both Indian andChinese coal with a high ash content, the particulate collection
performance was improved.

Figure 8 shows the structure of the gas cooler at the pilot test facility. It is a non-leak typeand finned-tube gas cooler, which is the same as the actual unit. Ash adhered to the finned tube canbe removed by soot blow. The change of the overall coefficient of heat transfer α, which indicatesthe heat exchange performance of the gas cooler when Indian coal is fired in the high-performance
PM removal system is shown in Figure 9. The PM concentration of the flue gas at the inlet of thegas cooler is high, 30g/m3N, and the overall coefficient of heat transfer α tended to be lowered overtime. Through the periodical soot blow, however, the accumulated ash could be easily removed,
and it was found that the overall coefficient of heat transfer α could be recovered to the initialvalue. In addition, a carbon steel test piece, which has the same material as the finned tube, wasinstalled at the inlet of the gas cooler, and the wearing caused by PM was evaluated as the thinningspeed. As a result, it was found that the thinning speed of the conventional system and that of thehigh-performance PM removal system were the same, and it was confirmed that there was noproblem in applying the high-performance PM removal system to high ash content type coal.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)59

Figure 7 Temperature characteristics of electric resistivityof coal ash with a high ash content
The factor for the improvement of the particulate collection
performance in the high-performance PM removal system is the
reduction of the electric resistivity.

Figure 8

Structure of gas cooler

The gas cooler cools the flue gas by passing the coolant through thefinned tubes arranged inside. Ash adhered to the tubes is removed by
the soot blower.

Figure 9 Changes in the overall coefficient of heat transfer of the gas cooler
in the high-performance PM removal system

Even under the condition where the PM concentration in flue gas is high, for example, when
Indian coal is used, ash accumulated on the finned tubes of the gas cooler can be removedby the soot blower and the overall coefficient of heat transfer is restored to the initial value.

3.2 Application example to the PM removal performance improvement project at
Zouxian Power Station in China (1,000MW)
In China, the control of toxic substance emissions from thermal power plants has beentightened, and in particular the reduction of sulfur dioxide (SO2) and PM emissions are required.
Therefore, retrofit projects for existing flue gas treatment systems for the enhancement ofcapability have been increasing. This is one such retrofit project. In 2014, the ESP and the FGD
were modified for the enhancement of capability by another manufacturer. To cope with the further

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)60

tightening of regulations, Zhejiang Feida MHPS High Efficiency Flue Gas Cleaning Systems
Engineering Co., Ltd. (FMH), which is part of the MHPS Group, received a retrofit order for the
first time since its establishment in 2015.
Figure 10 illustrates the system flow starting at the ESP inlet before and after modification.
Major improvements include the incorporation of a high-performance PM removal system by therotating GGH (gas-gas heater) being removed, and the non-leak type GGH heat recovery device
and the re-heating device being installed at the front stage of the ESP and the rear stage of theFGD, respectively. Other major improvements include an increase of the SO2 removal performanceand the PM removal performance of the FGD itself through the renewal of auxiliaries and internal
parts – such as recirculation pumps, spray headers, nozzles and agitators – of the FGD.

Figure 10

Overview of modification for reduction of PM emissions in China

Using our advantage as a supplier of entire AQCS, we satisfied the customer's desired values through theintroduction of the high-performance PM removal system and an increase of the particulate collection
performance and the SO2 removal performance of the FGD.

Table 1 shows the result values after modification relative to the customer's desired values.
The desired value for PM concentration has been set at 5mg/m3N or less in view of future PM
regulations, which is equivalent to the recent desired value for the concentration of PM emissions
at thermal power plants in Japan. Moreover, the existing equipment that was installed by anothermanufacturer was the target equipment in the retrofit project, and therefore, a close preliminary
investigation was needed. As such, this project required a high level of work, but leveraging ouradvantage as a supplier of furnace-to-stack air quality control systems, we modified the equipment
as a comprehensive system and achieved the customer's desired values for both the particulate
collection performance and the SO2 removal performance.
Table 1Results of modification of actual unit in China (1000MW) for PM emissions reduction

The customer's desired values were achieved by the optimization of the whole system.
Beforemodification

Customer's
desired value

After
modification

PM concentration at ESP inlet [mg/m3N]

25000

―

―

PM concentration at ESP outlet [mg/m3N]

30

≦30

10

PM concentration at the FGD outlet [mg/m3N]

14

≦5

2

70-330

≦35

10-23

3

SO2 concentration at the FGD outlet [mg/m N]|4. Application example of MEEP® to high concentration PM case
4.1 Overview of the retrofit project of the existing ESP in India
As an application example of MEEP®, the retrofit project of the existing ESP at Rihand
coal-fired power plant owned by India's National Thermal Power Corp. (NTPC) is introduced here.
In this project, the internal components of four ESPs installed in each of two 500MW boiler units
shown in Figure 11 were upgraded. The six-section type fixed electrodes had been in use for 25
years after the delivery. As shown in Figure 12, the front four sections were modified into fixed
electrodes with our specifications, and the rear two sections were modified into MEEP®.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)61

Figure 11 (Stage-I) Flow of the equipment at Rihand Super
Thermal Power Station
Flow chart of ESP (Stage-I) equipment for which we received the orderFigure 12

Renewal of the inside of ESP

The front four sections were modified into fixed electrodes with our
specifications, and the rear two sections were modified into MEEP®.

The ESP planning conditions before and after modification are shown in Table 2. Since theash content of the Indian coal used was about 40%, the PM concentration at the ESP inlet was high.
In addition, as a result of a prior study, it was estimated that the PM emitted from this coal had avery high electric resistivity, and if the PM continued accumulating on the collecting electrode, theperformance would be significantly reduced. It was estimated that if only fixed electrodes wereused to meet the customer's strict performance requirement (outlet PM concentration of 50mg/m3Nor less), about twice the collecting area would be needed and a large-scale modification including
the replacement of ducts would be necessary. On the other hand, MEEP® allows the collecting
electrode to keep clean and exerts high PM collection performance for Indian coal that emits a highelectric resistivity PM, enabling the ESP to be made compact. Therefore, in this project, MEEP®
was applied to the rear two sections, and using only the existing space for modification, thecustomer's desired value could be satisfied.
Table 2

Flue gas conditions before and after modification of ESP

The flue gas conditions of the ESP before modification (customer's
presented values) and after modification (planned values)

Inlet gas volume [m3/s]
Inlet gas temperature [°C]
Inlet PM concentration [mg/m3N]
Outlet PM concentration [mg/m3N]

Before modification(Customer's presented value)
1050
160
44000
500-600

After modification(Planned values)
1050
160
56000
≦50

4.2 MEEP® application results
The performance test was completed in March 2017, and the modified system is in operation.
It was found during the test operation that PM collected by MEEP® was finer and had a higher
electric resistivity than estimated in the plan, and the operating conditions of MEEP® werereviewed. As a result, in the performance test, both units achieved the PM concentration at the ESP
outlet of 50mg/m3N or less that was required by the customer, and the PM emissions wereMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)62

significantly reduced compared with the value before modification (outlet PM concentration: 500
to 600mg/m3N). Figure 13 shows the states of flue gas from the stack before and after
modification. It is obvious that the performance was improved through modification with MEEP®,
and it was demonstrated that the superiority of MEEP® could be exerted even at coal-fired power
plants in India, which require the treatment of high concentration, high electric resistivity and finePM.

Figure 13

States of gas exhausted from the stack

Photos of the flue gas from the stack before and after
modification of the ESP, by which we can observe that the PMconcentration exhausted from the stack was obviously reduced.

|5. Conclusion
In recent years, various environmental measures have been promoted all over the world, andunder these circumstances, the importance of environmental technologies has been increasing. So
far, we developed AQCS from the basic stage and demonstrated through pilot tests and tests using
actual units that AQCS could remove PM and toxic trace elements in flue gas to a high degree, andcould also be applied to high ash content coal. Accordingly, we applied AQCS to retrofit projects
for actual units. In the future, as a supplier of furnace-to-stack air quality control systems, we willcontinue offering optimal systems corresponding to coal properties and regulations in thedestination country or the state of the plant, and will develop technologies for reducing toxic
substances not only in flue gas, but also in solid emissions (collected PM, FGD gypsum) and inwastewater as well as fixation technologies, so that environmental load can be reduced for entire
thermal power plants. Finally, we are deeply grateful to the Japan Coal Energy Center (JCOAL),
which cooperated with the demonstration test for Indian coal at the 1.5MW pilot test facility.
Note: MHPS Environmental Solutions, Ltd. took over all the precipitator businesses including
MEEP® from Hitachi Plant Construction, Ltd. in October 2015.

References
1.

2.

Kikkawa et al., Highly-Efficient Removal of Toxic Trace Elements and Particulate Matter in Flue Gas
Emitted from Coal-fired Power Plants by Air Quality Control System (AQCS), Mitsubishi Heavy
Industries Technical Review Vol. 52 No. 2 (2015)
Nagarajan, S. et al., Experimental results of AQCS for Indian coal, Power-Gen India & Central Asia 2017