Part 2 CCT Overview
Environmental Protection Technologies (Flue Gas Treatment and Gas Cleaning Technologies)

5B1. SOx Reduction Technology
Technology overview

1. BackgroundSulfur oxides (SOx, mainly SO2) are regulated by a "K-value"and lower costs have been ongoing. At present, most pulverizedcontrol, which uses exhaust heights and regional coefficients tocoal-fired thermal power plants are equipped with wet limestone

limit SOx emissions. "Total amount control" is a term used inand gypsum-based desulfurization systems. Furthermore, a wet

Japan to define the total emissions from the whole of a region.

desulfurization process requiring no wastewater treatment is now

To comply with these regulations, flue gas desulfurizers wereunder development.

commercialized in 1973. Efforts to improve their performance

2. Technology(1) Wet limestone-gypsum process

The overall reaction is as follows:

Research and development: Mitsubishi Heavy Industries, Ltd.;
Babcock Hitachi K.K.; Ishikawajima-Harima Heavy Industries Co., Ltd.;
Chiyoda Corporation; Kawasaki Heavy Industries, Ltd.; others

CaSO3-0.5H2O+0.5O2+1.5H2O

CaCO3+SO2+0.5H2O

CaSO3-0.5H2O+CO2
CaSO4-2H2O

There are two types of absorption towers, as shown in Figure 1: aOverview

CaSO3-0.5H2O separate tower oxidation system and a

There are two limestone-gypsum processes; a soot-separation

comprehensive single tower oxidation system. At present, single

process, in which a dust (cooling) tower is installed upstream for

tower oxidation systems are less expensive to install and operate

dust collection and HCI/HF removal and cooling, and a soot-and their use is increasing annually. There are several methods tomixed process without such a dust (cooling) tower. The soot-have recycled absorption liquid come into contact with SO2 in theseparation process is used when high-purity gypsum containing

absorption tower section; the "spray method," which sprays theno soot or dust is desired. At present, however, more and moreabsorption liquid, the "grid method," which spreads absorption

systems employing the soot-mixed process, which is lessliquid on the surface of a grid-like pad, the "jet-bubbling method,"

expensive to install, are being installed since high-performance

which blows exhaust gas into the absorption liquid, and thedust collection devices, such as an advanced low-temperature"water-column method," in which absorption liquid flows in theelectrostatic precipitator, which lowers soot/dust concentrations,absorption tower.

have been developed.

For developing countries, a simple desulfurizer installable in the

In the absorption tower, on the other hand, a water-mixed

flue gas duct or at the lower part of a smoke stack has also beenlimestone slurry is reacted with SO2 within the exhaust gas forcommercialized.

the recovery of sulfur contents as gypsum (CaSO4-2H2O).

Oxidation tower-separated typeIn-absorption tower oxidation type

Absorption tower
Sulfuricacid

Absorption tower
Oxidation
tower

Basic flow
GypsumGasGasAir
Limestone

Limestone

Absorption tankGypsumAbsorption tower tank

Air

Fig. 1 Limestone-gypsum process-based desulfurizers

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Clean Coal Technologies in Japan(2) Coal ash-based dry desulfurization process

approximately 20% and a dust collection efficiency of 96% orResearch and development: Hokkaido Electric Power Co., Inc.;
Hitachi, Ltd.; Babcock Hitachi K.K.
Project type: Subsidized development of oil-alternative
energy-related technology for commercialization
Development period: 1986-1989

more. By 2003, the technology had been installed at Hokkaido
Electric Power’s Tomatoh-Atsuma Thermal power plant No. 1
unit (350MW), where the equipment is being used to treat one-Spentdesulfurization agent

Boiler Exhaust gas

Water
Kneader

Drier
Hot air

is a process to produce a new absorbent from limestone,

Conveyorcalcium hydroxide (Ca(OH)2) and spent absorbent. By using theMixer

ExtruderConveyor

newly produced absorbent, SOx is removed from the flue gas.
Figure 2 shows an outline of this process, which also includes aScalestage to manufacture the absorption agent. The desulfurization

Cleangas
Desulfurization
fanScale

SteamPrimary absorption
agent

technologies developed to make effective use of coal ash. This

Pre-absorption agent

The coal ash-based dry desulfurization process was one of theDesulfurization
silo

Overview

Slack lime

Coal ashhalf of the flue gas.

ScreenSteam generator

reaction allows Ca(OH)2 to remove SO2. Temperatures aremaintained between 100-200oC, where an SOx removal

Spent desulfurization agent
Breakerefficiency of 90% or greater can be attained. The process canFig. 2 Coal ash-based dry desulfurization process

also remove dust and NOx, with a NOx removal efficiency of(3) Spray dryer processtake place simultaneously, giving a particle mixture of gypsum

Research and development: The Electric Power Development
Company; Mitsubishi Heavy Industries, Ltd.;
Hokkaido Electric Power Co., Inc.
Project type: Voluntary under the Green Aid Plan

particles are recovered at a down-stream precipitator. Since thisOverview

waste. J-POWER, as part of the "Green Aid Plan," installed a spray

The spray dryer process is a so-called "semi-dry process," wheredryer desulfurizer with an exhaust gas treatment capacity ofwater is added to burned lime (CaO) to make a slack lime

300Km3N/h (one-half of total exhaust) at the Huangdao power(CaSO4-H2O) and calcium sulfite (CaSO3-0.5H2O). These
process is not adequate for good-quality gypsum and, moreover,ash remains in the mix, desulfurized particles are disposed of as(Ca(OH)2) slurry, which is sprayed into a spray dryer, causing theplant No. 4 unit (210MW) in Qingdao, China. It is currently in

SO2 in the flue gas to react with Ca(OH)2, and to be removed.

operation and attained a 70% desulfurization rate in verification

Within the dryer, the desulfurization reaction and limestone drying

tests (Oct. 1994 through Oct. 1997).

(4) Furnace desulfurization process

remove SO2 at a furnace temperature of 760-860oC:

Research and development: Hokkaido Electric Power Co., Inc.;
Kyushu Electric Power Co., Inc.; J-POWER;
Chugoku Electric Power Co., Inc.; Mitsubishi Heavy Industries, Ltd.;
Ishikawajima-Harima Heavy Industries Co., Ltd.;
Kawasaki Heavy Industries, Ltd.
Project type: Voluntary

CaCO3

CaO+CO2 CaO+SO2

CaSO3

At present, this process is employed at J-POWER’s Takehara
thermal power plant No. 2 unit normal-pressure fluidized-bed
boilers as well as the pressurized fluidized-bed boilers of
Hokkaido Electric Power’s Tomatoh-Atsuma thermal power
plant, Kyushu Electric Power’s New Kanda thermal power plant,
and Chugoku Electric Power’s Osaki thermal power plant. InOverview

pressurized

The furnace desulfurization process is used for fluidized-bed

breakdown into CaO due to the high partial pressure of CO2,

fluidized-bedboilers,

limestone

doesboilers. Limestone to be used for desulfurization is mixed andbut SO2 is removed in accordance with the following reaction:

combusted with the coal, causing the following reaction toCaCO3+SO2

CaSO3+CO2

References

1) "Introductory Course: Environmental Preservation Technology/Equipment for Thermal Power Plants IV Desulfurization Equipment,"
Thermal/Electronic Power Generation Vol. 41 No. 7, 911, 1990.
2) Kudo et al., "Coal Ash-Based Dry Desulfurizer Development," Thermal/Electronic Power Generation, Vol. 41, No. 7, 911, 1990.
3) "Coal Ash-Based Dry Flue Gas Desulfurizer" pamphlet, Hokkaido Electric Power.
4) General View of Thermal Power Generation, Institute of Electric Engineers, 2002.

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