Principal Research Results

Development of Technology for Reducing both Emissions of NOx andUnburned Carbon using In-Furnace Blending Method
BackgroundIn pulverized coal fired power plants, it is important to reduce emissions of both NOx and unburned carbon from theviewpoint of environmental protection. Also, considering the security of fuel supply and the cost of fuel, it will become increasingly
desirable for power plants to use different types of fuel. One method of reducing these emissions is to be fired with a high volatile
content coal ＊ 1. When two types of coal are fired in a boiler, two different blending methods can be utilized. One is the line blending
method, in which the two types of coal are stored in a bunker on the mill. The other is the in-furnace blending method, in which eachtype of coal is fired by each burner without prior blending in the bunker (Fig.1). In general, the line blending method is utilized inJapanese coal fired power plants. On the other hand, the in-furnace blending method is expected to reduce emissions of both NOx andunburned carbon. However, the effect of the in-furnace blending method on these emissions has not yet been clarified.

Objectives
The effect of the coal properties on the emissions of both NOx and unburned carbon by the in-furnace blending method is
clarified using a coal combustion test furnace (MARINE furnace) with three staged burners.

Principal Results
1. Combustion of blended bituminous coals
In the case of the in-furnace blending method, which means that high volatile content coal is fired by the upper burner andlow volatile content coal is fired by the middle and lower burners, the emissions of both NOx and unburned carbon are the lowest
among other types of in-furnace blending method and the line blending method (Fig.2). The reason for the reduction of theseemissions is considered to be that a large amount of NOx formed in the region of the middle and lower burners is decomposed by thereduction matter in high volatile content coal in the region of the upper burner. On the other hand, the reason for the decrease inunburned carbon is considered to be that the combustion time of low volatile content coal in the furnace is able to be lengthened byfiring from the middle and lower burners.
Furthermore, from the viewpoint of the blending combination between two types of coal, the effect of the in-furnace blending method on reducing emissions of both NOx and unburned carbon becomes high using lower volatile content coal, which exhaustsa large amount of NOx. Also, as the volatile matter content of higher volatile content coal is high, this reduction becomes considerable.
2. Combustion of sub-bituminous coal blended with bituminous coal
When the in-furnace blending method is utilized for the combustion of blended sub-bituminous coal, sub-bituminous coal isfired from the upper burner instead of bituminous coal with high volatile content. Then, NOx emission at the exit of the furnace wasdecreased compared with that for the line blending method (Fig.3).
For the line blending combustion of sub-bituminous coal, unburned carbon concentration in fly ash becomes much higher as
the moisture content of the sub-bituminous coal increases. The reason for this is considered to be that the combustibility of bituminous
coal is inhibited because of moisture vaporized from the sub-bituminous coal. In the case of the in-furnace blending method, theincrease in unburned carbon at high moisture content in sub-bituminous coal was able to be reduced compared with that for the line
blending method. The reason was considered to be that the combustion flames of bituminous coal and sub-bituminous coal wereseparated (Fig.4).
Main Researcher: Michitaka Ikeda, Ph. D.,
Research Scientist, Plant Engineering Sector, Energy Engineering Research Laboratory
Reference
M. Ikeda et al., 2008, “Development of Technology for Reducing NOx Emission and Unburned Carbon Concentration in Fly Ashusing In-Furnace Blending Method”, CRIEPI Report M07007 (in Japanese)＊1：Emissions of both NOx and unburned carbon decrease as the volatile content in coal becomes high. (Makino et al., JIE Jarrnal, 74, 906(1994)

104(a) Line Blend

25010NOx Uc

Air ratio :1. 24
OFA : 30 %In-Furnace Blend
Line Blend200▲▲■■

98715065100(b) In-Furnace Blend

4350

2
10

In-Furnace
Blend(LowerBurner)

In-Furnace
Blend(MiddleBurner)

In-Furnace
Blend(UpperBurner)

LineBlend(ThreeBurners)

Unburned Carbon Concentration in fly ash, Uc [%]2NOx emission at the exit of furnace (O 6%) [ppm]6. Fossil Fuel Power Generation

0Blending method(Burner for high volatile content coal)Line Blend: Two types of coal are stored in the bunker
on the mill
In-Furnace Blend : Each coal is fired by each burner
without prior blending in the bunker

In the case of in-furnace blending method, in which highvolatile content coal is fired by upper burner and low
volatile content coal is fired by middle and lower
burners, emissions of both NOxand unburned carbon arelower than these emissions on the line blending method

2Unburned carbon concentration in fly ash [%]Fig.2 Influence of blending method onemissions of NOx and unburned carbon
during blending combustion between
two types of coalNOx emission at the exit of furnace (O 6%) [ppm]Fig.1 Concept of two types of blending method

200190Air ratio : 1.24
OFA : 30 %

180170160150140130120110100

Line In-furnace
Blend Blend

Line In-furnace
Blend Blend

Moisture content
in sub-bituminous
coal : 5 %Moisture content
in sub-bituminous
coal : 24 %10Air ratio : 1.24
OFA : 30 %650

Line In-furnace
Blend Blend

Line In-furnace
Blend Blend

Moisture content
in sub-bituminous
coal : 5 %Moisture content
in sub-bituminous
coal : 24 %Fig.3 Relation between NOxemission at the exitof furnace and blending combustion of
sub-bituminous coalFig.4 Relation between unburned carbon
concentration in fly ash and blending
combustion of sub-bituminous coal

When the in-furnace blending method is
utilized for blending combustion of subbituminous coal, NOx emission at the exit offurnace is decreased compared with that for
the line blending method

In the case of in-furnace blending combustion of
sub-bituminous coal, the increase in unburned
carbon is able to be reduced compared with thatfor the line blending method105