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

5B2. NOx Reduction Technology
Technology overview

1. Background
Nitrogen oxide emissions are regulated, with acceptablegas denitration equipment was commercialized in 1977, andconcentration levels set according to the type of fuel and the size

ongoing efforts have been made to improve the durability ofof the boiler. However, with the recent stiffening of regulations,

DeNOx catalysts as well as to reduce costs. The Selective

some regions are subject to a "total amount control," whichCatalytic Reduction (SCR) process is used to decompose

provides for a region-wide overall emissions level as is the casenitrogen oxides, mainly through the use of ammonia.

with sulfur oxide emissions. To comply with these regulations, flue

2. Technology(1) Selective catalytic reduction process

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

process is also designed to limit NH3 leaks from the reactor toOverview

clogs the piping when separated out by an air pre-heater.

In this process, ammonia (NH3) is blown into exhaust gas,

The NOx removal efficiency is around 80-90% for pulverized

allowing the ammonia (NH3) to selectively react with nitrogencoal-fired thermal power plants. On the other hand, measures to5ppm or less. If a significant quantity should leak, it will reactwith the SO3 in the exhaust gas, producing NH4HSO4, which

oxides NOx (NO, NO2), and decompose them into water (H2O)

equally disperse and mix the NH3 with the exhaust gas as welland Nitrogen (N2). In the DeNOx reactor, since soot and dust are

as to create greater uniformity of the exhaust gas flows to copepresent in the exhaust gas, a grid- or plate-like catalyst is mainlywith growing boiler sizes have been developed. These include

used, as shown in Figure 1. The catalysts, as shown in Photo 1

placing a current plate, called a "guide vane" at the gas inlet, orand Photo 2, are installed in the reactor to react with the NH3

dividing the gas inlet into grids, each to be equipped with anblown into the catalyst layer from its inlet, allowing NOx (NO,NH3 injection nozzle.

NO2) to breakdown into water vapor (H2O) and nitrogen (N2). Thecatalyst is mainly composed of TiO2, to which vanadium (V),

Smokestacktungsten (W), and the like are added as active ingredients.
4NO + 4NH3 + O2

4N2 + 6H2O

The temperature at which the catalyst attains optimal

Catalyst
layer

performance is 350oC. At a temperature lower than this, SO3 inthe exhaust gas reacts with NH3, producing ammonium hydrogensulfate (NH4HSO4) that covers the surface of the catalyst,
thereby reducing the ability to remove NOx. At a temperature
higher than 350oC, the NH4HSO4 decomposes, improving theExhaustgas

removal of NOx regardless of the SO3 concentration. At aReactorotemperature above 400 C, NH3 is oxidized and its volume
decreases, thereby reducing its ability to remove NOx. TheFig. 1 Selective contact reduction method denitration process

Photo 1 Grid-like catalyst

Photo 2 Plate-like catalyst

87

Clean Coal Technologies in Japan(2) Selective non-catalytic reduction (SNCR) process

Research and development: Chubu Electric Power Co., Inc.; Mitsubishi Heavy Industries, Ltd.
Project type: VoluntaryOverview
SNCR is a process to, as shown in Figure 2, blow NH3 into theboiler section where the exhaust gas temperature is 850-950oC
and to breakdown NOx into N2 and H2O without the use of aInjection
nozzlecatalyst. Despite the advantages of not requiring a catalyst andBoiler water for nozzle-coolingits lower installation costs, the NOx removal efficiency is as low
as 40% at an NH3/NOx molar ratio of 1.5. Because of this, it isused in regions or equipment where there is no need for a highNOx removal efficiency. More NH3 is also leaked than with theselective contact reduction method, requiring measures to copewith NH4HSO4 precipitation in the event of high SO3

AHAmmonia

concentrations in the exhaust gas.

SAHMixer

This technology is mainly used at small commercial boilers andAir for dilution

refuse incinerators. With respect to thermal power plant

FDF

Booster fan

applications, this technology has only been installed at Chubu
Electric Power’s Chita thermal power plant No. 2 unit (375kw), inFig. 2 Selective non-catalytic denitration NOx removal process

1977.

(3) Radical injection method

Research and development: Japan Coal Energy Center
Project type: Subsidized coal production/utilization technology promotion
Development period: 1999-2002
Overview
In the radical injection method, as shown in Figure 3, argon
plasma is injected into NH3, generating NH2 plasma and otherplasmas, which are then blown into the boiler to decompose NOx
into N2 and H2O. The target for this technology is to attain anNOx concentration of 10ppm or less.
At present, basic research is underway at the Japan Coal Energy
Center, with commercialization expected around 2010.

Fig. 3 Overview of radical injection methodReferences1) Masahiro Ozawa et al., "Latest Flue Gas Denitrator Technology," Ishikawajima-Harima Technical Journal, Vol. 39, No. 6, 1999.
2) Tadamasa Sengoku et al., "Selective Non-Catalytic Denitration Method for Boilers," Thermal/Nuclear Power Generation,
Vol. 29, No. 5, 1978.
3) Coal Utilization Next-Generation Technology Development Survey; Environment-friendly coal combustion technology field(advanced flue gas treatment technology)/NEDO results report, March 2002.
4) Masayuki Hirano, "New Flue Gas Denitrator Technologies for Large Boilers/Gas Turbines," Thermal/Electronic Power Generation,
Vol. 50, No. 8, 1999.

88