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Commercial Experience and Actual-Plant-Scale Test Facility

of MHI Single Tower FGD

Paper #33

Yoshio Nakayama

Tetsu Ushiku

Plant and Transportation Systems Engineering & Construction Center,

Mitsubishi Heavy Industries, Ltd.

3-3-1, Minatomirai, Nishi-Ku, Yokohama-City, Kanagawa 220-8401, Japan

Takeo Shinoda

FGD division, Mitsubishi Heavy Industries America, Ltd.

600 Carondelet St., New Orleans, LA 70130

ABSTRACT

MHI’s next generation scrubber, called the "Double-Contact-Flow-Scrubber" (DCFS), is unique

technology that provides many features such as higher availability, less power consumption with

high SO2removal efficiency, etc. This technology was recently selected by TVA for their Paradise

Fossil Plant Unit 3 FGD plant that will be started up on early 2007. This paper will report on

several of the latest outstanding commercial operating successes with this DCFS system including

super high desulfurization performance (i.e., 99.9%) with a single absorber vessel, high gas

velocity compact design with effective mist separation, and others.

The DCFS includes a single spray header located bottom of the scrubbing zone. Special nozzles

made of SiC create a spray fountain pointing upward into the scrubbing zone. Flue gas is

introduced below the spray header and turns upward 90 degrees and contacts the absorbent liquid

in the scrubbing zone. Since absorbent liquid is sprayed upward and then falls by gravity, the

sectional liquid density is twice that of conventional spray and/or grid packed system. This is why

MHI calls it the “Double-Contact€35 scrubber. The Internal gas velocity is selected to optimize plant

economics, but typically the gas velocity is between 10-15 fps which provides about 30% less foot

print than a conventional system. MHI already has over 20 domestic and international operating

installations of this DCFS technology, and each of these perform with greater than 99%

availability.

In this paper, performance test data for the latest single-tower coal-fired application (a 600MW

module) that started commercial operation on July 12, 2004 are reported. In addition, super high

SO2removal efficiency (99-99.9%) under high inlet SO2 conditions (2,000-3,000ppm) achieved

by a single DCFS module and an extraordinary space-saving design related to its compactness

feature are also introduced here.
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INTRODUCTION

Mitsubishi Heavy Industry Ltd. (MHI)’s first FGD system was introduced in 1964, and today we

have worldwide 163 installations, for a total of 55,000 MWe installed FGD capacity. Through

continuous improvement and development in pilot and demonstration facilities, MHI’s FGD

system design has evolved over time from a design using a grid packed absorber tower to the

current Double Contact Flow Scrubber (DCFS) that uses the highly efficient and reliable,

fountain-type spray header design.

The DCFS has three configurations. The first is the co-current type DCFS in which the flue gas is

introduced from the top of absorber and flows downward through the scrubbing zone. A second

configuration is the counter-current type DCFS which introduces flue gas from bottom of

scrubbing zone and directs it upward though the scrubbing zone. The third configuration is the

combination of the co- and counter-current DCFS integrated on a single absorber tank. Typically,

a co-current tower is selected when super high particulate removal efficiency (such as more than

90% or 10mg/m3N(0.01lb/MMBtu) at outlet of absorber) is required . In this case, the flue gas

velocity could be a maximum of 32 fps in order to achieve the required performance. Countercurrent design is the most popular design for standard FGD requirement such as 95% to 97% SO2

removal performance with medium- to high-sulfur coal. The twin tower design, which is the

combination of co- and counter-current designs, is selected when both high particulate removal

and extremely high desulfurization performance requirement (98% and over) for high sulfur coal

are required. This twin tower design also can provide very stable operation in situations where

quick load changes and rapid sulfur level changes can occur.

The twin tower design is always more reliable and efficient for stringent emissions requirement,

but MHI has been making improvements to achieve higher performance in a single counter-current

DCFS, which provides less initial investment (i.e., less foot print) with nearly zero SO2 emissions.

In the past 2 or 3 years, we have been successful in achieving ultra-high SO2removal efficiency

(e.g., 99.9%) with a high inlet SO2 using a single tower DCFS. This paper focuses on these new

applications of single tower DCFS, and its development facilities.

Figure. 1 ; Comparison between single and twin
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SINGLE TOWER DCFS

Description of Double Contact Flow Scrubber (DCFS)

The flue gas enters the absorber from the side of the absorber tower through the wet-dry section at

the breach of the inlet duct. The wet-dry zone is washed by fresh water at regular intervals. The

flue gas is immediately quenched by the slurry spray as it enters the absorber.

Absorber recirculation pumps are typically located

beside the absorber on the opposite side from the flue

gas inlet. The external header goes up to the spray

level and connects with the internal spray headers.

On the internal spray headers, DCFS nozzles are

placed pointing upward as shown in Figure 2. Since

the nozzles are placed at one or a maximum of two

levels, nozzle installation and maintenance work is

extremely easy and quick. These nozzles have about

1 -1/2’opening so clogging of the nozzle is very

unlikely, and, even if it happens, checking for nozzle

pluggage is easy since nozzles are pointed upward.

At the top of fountain column, slurry is spread by the rising flue gas and forms well-mixed gasliquid contact zone. This fountain or liquid column in the absorber contacts the flue gas as it

proceeds co-current to the liquid spray and again counter-current as liquid returns to the reaction

tank by gravity. That is why we called this system “Double Contact Flow Scrubber (DCFS)”.

The DCFS nozzles create an upward fountain, so no impingement of spray pipes and walls occurs.

Also, as the spray is directed away from the inlet duct, no buildup in the inlet duct is experienced.

The reaction tank operates at 30 wt% solids, which increases solid retention time in the tank and

promotes gypsum crystal growth, which results in significant reduction of gypsum scaling on the

tank wall. Also, the high level of suspended solids in the reaction tank provides an elevated

alkalinity level that enhances SO2 absorption and makes the system more stable to sudden changes

in inlet SO2loading.

Fig.2 DCFS spray and nozzles
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SINGLE TOWER DCFS EXPERIENCE

Overview of Single Tower DCFS Experience

MHI has 11 single DCFS installations with a wide range of operating conditions as shown in Table

1. As one design advantage of the single tower DCFS, an alternate system that eliminates the air

compressor/blower along with associated piping and side-entry agitators is introduced here. This

proprietary design, called the Jet Air Sparger or“JAS,”is MHI newly developed simple oxidation

system, and is already installed and operated at some of these single tower DCFS.

 

For coal fired boilers, mercury emissions will be an important issue. Most recent measurements

confirm oxidized mercury removal efficiency across the single tower DCFS to be over 90%.

Table 1. Experience table for Single tower DCFS
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Latest Coal-Fired Experience of Single tower DCFS

The latest FGD module provided is on the 600 MW

coal-fired generation unit of Hirono thermal power

station is. This is one of the largest single tower DCFS

modules, and commercial operation started July 2004.

Notably, MHI designed and supplied all equipment from

boiler to stack through FGD . The system also includes

direct feed type (no hydroclone) gypsum dewatering

equipment and a wastewater treatment system designed

to meet stringent Japanese regulations, and MHI

optimized the total configuration of all of this

equipment.

Figure 4; Hirono #5 Unit Overall Layout

Corresponding to the client’s requirement of over than 96% desulfurization efficiency, MHI

applied a single tower design to maintain a small footprint and simple configuration. As a result,

the liquid-to-gas ratio is set at a relatively low value and results in lower power consumption.

Moreover, the limestone stoichiometric ratio is also a relatively low value resulting in 94.2%

gypsum purity.

Because of the 10,000mg/l chloride content in absorbent slurry, a resin lined carbon steel vessel

was supplied instead of 6% molybdenum alloy to reduce initial investment (common practice in

Japan). FRP internal spray pipes were supplied because of the no impingement characteristic of

the DCFS spray. The external spray header was made of rubber lined carbon steel.

Utility Area

WWT Area

Dewatering Area

Stack

FGD

ESP BEoSPiler

GGH

Figure. 3; Absorber tower outlook
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This FGD facility was turned over to the client on July 12, 2004 after successful trial operations

during the August 2003 to July 2004 period. This coal-fired FGD project demonstrated that single

tower technology is applicable and fully reliable for large coal-fired boilers.

Performance test results of Hirono Power Station’s FGD facility are shown in Table 2. As can be

seen, the actual desulfurization efficiency and particulate removal efficiency are 98.3% and 79.3%

respectively.

Table 2. Hirono FGD Facility Performance Test Results

Unit Experienced value

Desulfurization efficiency % 98.3

Particulate removal efficiency % 79.3

Gypsum Purity wt% 94.2

Ultra-High SO2 Removal Experience of Single Tower DCFS

COSMO oil Yokkaichi is an outstanding example of high

SO2removal by a single counter current DCFS.

Commercial operation at COSMO began in 2003, and the

FGD system has achieved a cumulative availability of 100

percent since startup. The system is designed at 99.5% and

operates at 99.9% SO2removal efficiency from VR fired

flue gas (measured SO2 concentration was 2,670 ppm).

Measured outlet SO2 concentration at the stack was only 3

ppm. In the past, such a high desulfurization capability has

been considered impossible in a single tower design. This

experience proves that the single DCFS can achieve over

99% performance as can the twin tower DCFS. The major

reason for this high performance is the enhanced liquid-togas contact created and the high reagent reactivity from the

30wt% slurry concentration.

Figure. 5; Outlook of COSMO FGD
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Space Saving Design of Single tower DCFS

FGD systems for a petroleum refining company in Japan demonstrate a clearly differentiating

space-saving advantage of our single tower, stack integrated DCFS design. In this design, the

absorber tower is combined at the bottom of the stack and the absorber outlet is directly connected

to the stack through the mist eliminator. For this stack integrated design, the FGD system performs

like a part of the stack itself, and high reliability is mandatory. Single DCFS technology meets

such a requirement and no by-pass design is selected as a result. In the site arrangement, other than

the stack integrated absorber itself, the only required additional area is for limestone preparation

and gypsum dewatering. Thus, the stack integrated design saves considerable space compared to a

traditional flue gas treatment system. These compact single DCFS modules also show ultra-high

removal efficiency (99% over) from a high SO2-laden flue gas (over 2,500ppm) (see table 3 for

measured performance).

Table 3. Measured performance of stack integrated FGD

Plant name

Boiler

Capacity

Fuel Flue gas flow rate Inlet SO2 Outlet

SO2

Desulfuriza

tion eff.

Plant A 149 VR

+pet.coke 610,000Nm3

/h 2,850ppm 2.8ppm 99.9%

(99.4%)

Plant B 99 VR 438,200Nm3/h 2,590ppm 11ppm 99.6%

(98.3%)

(Design value of SO2removal efficiency)
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Outstanding compact layout and outlook of these stack-integrated Single DCFS is shown at Figure

6 and 7.

ESP

Stack and

Absorber

Limestone

silo

View

Figure. 6 ; Layout plan and outlook of Plant A

ESP

Stack and

Absorber

Limestone

silo

View

Figure.7; Layout plan and outlook of Plant B
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FURTHER EXPERIMENTAL ACTIVITY

As shown above, the single tower DCFS configuration is effective in compacting and simplifying

FGD design. As a result, MHI continues to develop the future technology of single DCFS and has

constructed a 400 MW equivalent full-scale single DCFS test plant. MHI’s objective in building

this test equipment is to speed up the development of next generation SO2removal devices, and,

additionally, to respond to future regulations that the SO2 absorber tower may have to meet, such as

mercury, PM2.5, etc.

The MHI Mihara works is the home of MHI’s full-scale FGD fluid dynamics test rig, shown in

Figures 8 and 9.

This full scale wet FGD test facility is a pilot plant treating approximately 1,200,000 Nm3/h-w flue

gas using a single tower DCFS module.

The pilot plant includes a flue gas recirculation system with SO2injection that allows flue gas

dynamic distribution testing and SO2removal performance testing, respectively. The pilot plant is

also equipped with mist eliminator and related limestone preparation / gypsum dewatering systems.

Hence, it is able to simulate the total FGD system under various conditions.

Figure. 8. Full scale FGD model. Figure. 9. Inside of full scale model
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CONCLUSION

The DCFS has been developed as a highly reliable absorber to replace the conventional gridpacked absorber. The single tower DCFS design was applied in Japan for high SO2

removal

design and on coal-fired boilers because of its simplicity and compactness.

As the latest coal fired single tower application, the Hirono Power station FGD module started

commercial operation on July 12, 2004. MHI supplied all equipment from boiler to stack and

optimized it as a total system. Regarding desulfurization system performance, over 98% SO2

removal performance and over 94% gypsum purity is achieved simultaneously by a single tower

DCFS module. This experience should be firm proof of the single DCFS’s applicability to achieve

high SO2removal on a coal-fired boiler.

Further, at the Yokkaichi plant and, two FGD facilities of a petroleum refinery company with a

simple stack integrated type design achieved over 99% SO2removal performance under high inlet

SO2 concentration conditions (2,000-3,000ppm). These performance proves not only the single

DCFS’s high flexible arrangement but its high performance capability as well.

From these latest operation results, we have a firm belief that these single tower DCFS type

absorbers will largely contribute to further development of the desulfurization technology.

REFERENCES

[1] T. Shimizu, Y. Nakayama, H. Nagayasu “Operation Experience of The latest Mitsubishi FGD System”

1999 Environmental Technology Forum

[2]J. Klingspor, K.Okazoe, G.Munson, “High Efficiency Double Contact Flow Scrubber for the U.S. FGD

Market€35 , 2003 MEGA symposium.

KEY WORD LIST

Mitsubishi

DCFS

SO2

Removal

Single tower