Research on Large Scale Power Plant Boiler Burning Shenhua Coal Cheng Yinbiao

Beijing Guohua Electric Power Corporation，Beijing 100025

[Abstract] Shenfu Dongsheng coalfield is developed by Shenhua Group. The coalfield has an area of 3481 square kilometer, a possible ore of 2.236×1011t, and a workable reserve of 35.4×109t. In 2004, the total sales volume of Shenfu Dongsheng coal reaches 0.96×109t, among which 0.49×109 t was sold to domestic power coal market. With the increase of the yield of Shenhua coal, it will be utilized in more and more power plants. Shenhua coal, with a good combustibility and low pollutant emission, is a kind of high quality coal used for electricity generation. However Shenhua coal is easy to melt under low temperature and has a high ash-fouling tendency. So the security use of the Shenhua coal has attract much attention of the power sector. In this article, we made a proposal of the security and economical use of Shenhua coal according to the research on its characteristic and the experience to burn Shenhua coal in a 600MW tangentially subcritical boiler operated by Guohua Power Electricity Corporation. It will be a good reference for the building and safe operation of the boiler which burns low-ash-fusion coal.

Key Words: Shenhua coal; combustion performance; large capacity utility boiler; resistance to slagging

1、Overview of Shenhua Coal 1.1 Distribution and reserves Shenfu Dongsheng coalfield is developed by Shenhua Company. The coalfield is located in Yulin area of Shanxi Province and Eerduosi of Mongolia municipality. It has an area of 31172 square kilometer, a possible ore of 2.236×1011t, which is the biggest coalfield in China and is known as one of the eight biggest coalfields in the world. The camp area now under development is 3481 square kilometer, and the workable reserve of the coalfield is 35.4×109t.[1] 1.2 Exploitation and marketing

Shenhua coalfield is suitable to be built into a super modern mine due to its own advantage such as simple architectonic; shallow coal seam; litter dislocation; stable adjoining rock; low content of gasses and easy exploitation. Now Shendong coalfield has 16 mines, and Daliuta mine, Bulianta mine and Yujialiang mine have over 0.200×109t production capacity, being the super-scale mines in china.. In 2004, the coal yield of Shendong coalfield was 0.8584×109t.

Shenhua Group has built Shenmu-Suzhou-Huanghua railway and Huanghua harbor to transport coal from west to east. The railway transportation ability is near 100 million tons. The harbor capacity runs up to 65 million tons and will be extended to 100 million tons. 1.3 Application

Shenhua coal is a kind of high quality coal with low ash, low sulfur, low phosphorus, low chlorine and medium-high thermal value, used for power generation, metallurgy, coal gasification and liquefactions.

Shenhua coal is mainly sold to power plant, steel mill and chemical plant in North China, East China and South China. The running power plants designed to burn Shenhua coal include Huaneng Beijing Thermal Power Plant, Huaneng Yangliuqing Power Plant, Huaneng Shidongkou

second Power Plant, Shanghai Wujin Thermal Power Plant, Huaneng Taichang Power Plant, Zhejiang Jiaxing Power Plant, Jiangsu Yangzhou second Power Plant, Shanghai Waigaoqiao Power Plant, Fujian Housi Power Plant, Huaneng Shantou Power Plant, Guangdong Shajiao C Power Plant, Guangdong Zhuhai Power Plant, Guangzhou Zhujiang Power Plant, Guohua Taishan Power Plant and Dingzhou Power Plant, etc.. Also, a lot of power plant under construction including some supercritical units and Ultra supercritical units are designed to burn Shenhua coal. In addition, Shenhua coal is used in Shanghai Baoshan Steel Company, Capital Steel Company and Anshan steel company for blast furnace injection. Shenhua coal is also sold to Japan, Korean, India, Philippines, Europe, Hong Kong and Taiwan. In recent years the sales volume is increasing rapidly. In 2004, the total sales volume is 0.96Mt; among which 0.26Mt was exported and the coal for electricity generation occupied 95%. 2、The essential characteristic of Shenhua coal 2.1 Basic characteristic of coal quality

Due to its low ash (8%), special low sulfur (<0.5%), special low phosphorus, special low chlorine, high volatile compound and high thermal value (lower calorific value 23.45~25.12MJ/kg), Shenhua coal is widely used in power plant. But its ash fusibility temperature is low. We can divided Shenhua coal ( fig 1) according to GB/T7562-1998 “Technical condition of coal used for pulverized coal-fired boiler for power generation”，and then we can choose the kind of boiler suitable for burning of Shenhua coal.

Table1 the quality plot of Shenhua coal (coal for motive force)

Parameter

Volatile compoun

d

Thermal value

Ash Water

content Sulfur content

Ash fusion point

Grind ability

Symbol

V Q A M S ST HGI

Level 4～5 1～2 1 2～3 1～2 1～2 1～2 Performanc

e

middle～high

middle～high

low middle～

middle high

special low～low

low～

middle hard～

low

The main parameters of coal quality of Shenhua coal are listed in the following table. Table 2 the main parameters of coal quality of Shenhua coal

Parameters Symbol Unit Value Total water content Mt % 12-17

Ash Ad % 6-10 Volatile compound Vdaf % 31-40

Thermal value Qnet,ar MJ/kg 23.0-24.5 Total sulfur St,d % 0.4-0.7

Ash fusion point ST ℃ 1150-1300 Hardgrove HGI 50-67

According to the analysis of coal quality, the combustibility of Shenhua coal is good. But while burning Shenhua coal, it is easy to form local high temperature zone due to its high thermal value and it is easy to slag due to it low ash fusion point. The Hardgrove grindability index of Shenhua coal is between 50 and 60, belonging to medium-hard grindability coal. The high thermal

value and less impurity of Shenhua coal can low the milling electric cost. With high volatile compound, Shenhua coal appears spontaneous combustion easily, so it will increase the risk of explosion during milling. 2.2 Fire behavior

Thermogravimetric analysis combustion curve (Fig 1) given by Xian Thermal power Research Institute (TPRI) shows that the combustibility of Shenhua coal is between that of lignite which is easy to burn and typical bituminous coal (Dating high quality blend coal). The main parameter of Shenhua coal’s combustion curve is listed in table 3.

Fig 1 Comparison of the combustion curve betweenShenhua coal,typical lignite and typical bituminous

coal

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400 500 600 700

T ℃

burn

ing

rate mg/min

Yunnan lignite

Shenhua coal

Datong blendcoal

Table 3 the main parameter of Shenhua coal combustion curve

Coal sample Shenhua coal Yunnan lignite Datong high quality blend coal

Beginning temperature ℃ 260 210 275 Ending temperature ℃ 535 608 675 Maximal reaction rate

mg/min 13.75 15.64 10.74

corresponding temperature for Maximal reaction rate

℃ 317 253 483

Index of response ℃ 221 210 230 Burnout indices 1.01 1.76 3.37

From the experience and analysis, we can come to such conclusion: The kindling temperature of Shenhua coal is lower than other coal so it should not adopt

excess measure to intensify the ignition process. Good fire behavior makes low oxygen combustion possible. Under same condition, compared with other coal, while burning Shenhua coal, the performance of the boiler at low load efficiency is better. It is helpful for power plant to improve the peak shaving ability of units under economical operation. 2.3 Combustion characteristics

Shenhua coal has high volatile component content, low ignition point and a high combustion

reaction rate in boiler. While burning Shenhua coal, it is easy to reach high combustion rate under low temperature, and appear high intensity combustion in the combustion zone. Therefore the combustion temperature will reach a peak temperature, which is much higher than lignite and typical bituminous coal. During real combustion process, low temperature combustion is the main combustion method of this kind of coal[2]. 2.4 Burnout characteristic

Among domestic coal used for motive force, Shenhua coal is one of the bituminous coal with best burnout characteristic. Its unbruned carbon in fly ash in boiler is close to that in wet bottom boiler. The thermal power can be fully used and boiler efficiency can be improved obviously. According to the surveyed data, comparing to Yunzhou coal and Datong high quality blend coal, the burnout ratio of Shenhua coal is 1~2% higher, unburned carbon in fly ash is 2~4% lower. Accordingly, the coal consumption can be 4~8g/(kW.h) lower and the unit’s economic efficiency can be improved significantly. The excellent burnout characteristic of Shenhua coal has been proved by various power plants. The unbruned carbon in fly ash in 300MW unit boiler is below 0.5~1%, much better than lignite. The thermogravimetric analysis of TPRI also proved this. The conclusion above is accordant with the real operation of power plant and the situation in one-dimensional flame furnace (a coal characteristic testing method with high precision) of TPRI. 2.5 Fouling characteristic

Slagging and fouling are resulted form the integrated function of coal ash characteristic and the temperature in boiler. While identify the coal slagging character, we can use received basic net calorific value as an auxiliary index except for ST [3]. According to the coal analysis, Shenhua coal belongs to the “Highly Slagging coal”. Although the combustion character of Shenhua coal is better than that of domestic typical bituminous coal, it is easy to form local high temperature zone。 In addition, the thermal value of Shenhua coal is high, so the peak temperature in high temperature zone may be much higher than that of lignite and typical bituminous coal. This is disadvantage to preventing slagging.

The slagging and fouling mechanism differs in different boiler heat surface while the ash component and structure is also different. Zhejiang University has studied the slagging character of Shenhua coal. Ash particle samples were taken from boiler cavity, boiler walls and caudal region. A comparison is investigated the result is showed in table 4.

Table 4 the chemical component of the three kinds ash particle of Shenhua coal （/%）

Name SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 Slagging of boiler

wall 60.01 17.64 13.29 5.01 1.32 0.76 1.02 0.87

Particle sample near the boiler wall

55.02 19.42 10.91 7.26 1.96 0.85 1.36 2.68

Ash 55.58 23.88 6.49 6.91 1.63 1.01 1.56 2.99 Table 4 lists the chemical component of the three kinds of ash particles. The Fe2O3 content in

particles of boiler and boiler wall is higher than that of ash. Fe concentrates in the particles of boiler wall, while the Na2O content of ash is the highest. At the same time, Si/Al ratio is biggest in particles of boiler wall as showed in table 4. The Si/Al index shows that the ash slagging tendency in boiler backpass is medium, which is weaker than the ash slagging tendency close to the boiler walls. And fouling index (judgment) shows that the particle fouling near boiler walls is heavier than those of slag on the walls and the fly ash in backpass.

2.6 Pollutant emission characteristic The sulfur content of Shenhua coal is very low（<0.5%），while the alkaline earth content is

high. The Self-DESULFURIZATION ratio can reach 30%~40%, 20% higher than the other coal. While burning Shenhua coal, the coal consumption is relative low, so the SOx emission is definitely low.

Shenhua coal has high content of volatile component and is easy to burn out. We can adopt low oxygen or staged combustion. The nitrogen content of the coal is low, so the average NOx

emission is also low. For example Huaneng Beijing cent company combust Shenhua coal in wet bottom boiler. The combustion temperature is high, but while adopting low excess air ratio ( the excess air ratio in the outlet of burner is between 0.9～1.0)， the NOx emission is under 500mg/m3, which is far lower than not only of the other wet bottom boiler, but also the state emission standard of dry bottom boiler. By burning Shenhua coal, the NOx emission in Wujing Power Plant is lowered to 410mg/m3. 2.7 Economical efficiency

Shenhua coal is one of the coals that has larges proportion in coal for motive force, so many enterprisers of thermal power station pay more attention to its economical efficiency. According to laboratorial research and the application of Shenhua coal in power boiler, compared with the other coal, burning Shenhua coal will gain huge economical and social benefit. This can be reflected in the following aspects: low unbruned carbon in ash, high combustion efficiency; less dusty gas emission, high environmental benefit; low electric power consumption, low coal consumption for power supply; less abrasion of the facility, long longevity; low indication of high/low temperature corrosion, large scope that the funnel temperature can be adjusted to and better potential economic benefit.

Xian Thermal Power Research Institute made a wide survey on the application of Shenhua coal in the unit between 300MW and 600MW. The main purpose is to investigate Shenhua coal’s economical efficiency. The survey shows that while burning Shenhua coal in 300MW unit, the coal consumption for power supply can reach326g/kW.h, about 10g/ kW.h lower compared with burning the other coal, which means a 3.38% decrease. In 600MW unit, the coal consumption for power supply can be decreased to 20 g/kW.h, which means a 5.76% decrease. [2]. 3、The design characteristics of Shenhua coal boiler

It has not been a long time for the application of Shenhua Coal in power plant boiler. Shenhua coal was not adopted as design coal for 300MW boiler until 1990s. The first 300MW boiler burning Shenhua coal was put into service in 1992. During the next ten years, with the large scale exploration of Shenfu Dongsheng coalfield, Shenhua coal has been the main power plant coal.

Shenhua coal is easy to ignite and burn out. So we do not need to pay much attention on its combustion economical efficiency and stability. But this kink of coal has severe tendency of slagging, so great effort should be put into preventing or eliminating the slagging[4]Coal slagging is in contradiction to its ignition performance. But when the coal has a good ignition performance, it is easier to eliminate the slagging.

Furnace parameters include two parts: dimension parameter and thermal load parameters (or thermal intensity parameters). These parameters have close connection with boiler operational performance, such as combustion stability, breeze burnout, slagging, high temperature corrosion,

and the formation of NOx. Only that these parameters can match with coal combustion and slagging performance, can the boiler run well. It has not been a long time for the application of Shenhua coal in large utility boiler, and the advanced research is insufficiency. So there are some un reasonable parameter and method in boiler designation, reforming and operation. We will discuss some main problems and recommend some solutions as reference. 3.1 Adopt low furnace thermal load

During design, we can choose a higher furnace dimension and smaller tangential circle diameter. Lower furnace thermal load can decrease flame temperature, and avoid the peak temperature. And also it can prolong the dwell time of the airflow in furnace, raise the burnout extent and decrease furnace outlet temperature. Thus it can avoid fouling in heating surface near the outlet, decrease combustion intensity in furnace section, and lower temperature near burner zone. It is effective for eliminating slagging and preventing burner melting loss.

Zhuhai Power Plant adopts a lower furnace thermal load (capacity thermal load: 94.72 kW/m3, section thermal load: 4.28 MW/m2) in a 700MW boiler, and there is no slagging while burning Shenhua coal. There is lower tendency of slagging in furnace, the and emission is reprehensively 657mg/m3 and 311mg/m3, the unbruned carbon in fly ash is only 0.6%, the boiler efficiency reaches 94.2%. So a low furnace thermal load is important for burning Shenhua coal, and it is feasible. 3.2 Burner configuartion At present, there are many configuration manners for burners. The burner requirement for Shenhua coal is avoiding the flame clinging to the walls and reducing the erosion of the inner walls by coal particles. For example, for four corner tangential boiler, concentric fired system II (CFS-II) can be adopted like Wujing power plant 300MW boiler. 3.3 Choose lower second air velocity

Choose suitable primary/second air velocity. Primary air velocity should not be too low. Higher primary air velocity can delay pulverized coal ignition and reduce flue gas recirculation. Raise primary air velocity can lower thermal load effectively and decrease slagging. Under prerequisite of stable combustion, raising second air velocity and improving the mixing of primary/second air are main method to enhance combustion and improve combustion efficiency. The combustion characteristic of Shenhua coal is between lignite and bituminous coal, so second air velocity should not be too high. It can be lower than or equal to the minimal second air velocity of boiler burning bituminous coal. 3.4 Adopt low oxygen combustion

This measure is aimed at the performance adjusting. It is usually considered that low oxygen combustion will lead to reducing environment near the burner area, which is against lowering the slagging tendency. Therefore, part of the power plants adopt large excessive air ratio. But the effect of large excessive air ratio is to lower the boiler’s temperature. And little change in the environment near the primary air area. Besides, this measure is usual realized by increasing the secondary airflow rate. For the boilers whose burners configuration is set, higher secondary air velocity than designed value leads to higher operational secondary air velocity, which in turn leads to heavier slagging. As to four corner tangential boiler, high secondary air velocity will lead to primary air clinging to the walls. So, when boiler is operated with high excessive air, we should balance between the decreasing furnace temperature and the impact on the secondary air to avoid severer slagging problem [5]。

Under the condition of reasonable burner configuration, Shenhua coal can be burned with low oxygen. With 2% excessive air ratio, Shenhua coal is burned at a 300MW boiler of Wujing power plant and a 700MW boiler of Zhuhai power plant. No severe slagging problem is observed and flue gas thermal loss and NOx emission is lowered to some extent. 3.5 Appropriate pulverized coal fineness

In the aspect of coal mining, attention should be paid to choose the appropriate pulverized coal fineness. Coarse pulverized coal can lower the peak boiler temperature and mean flame temperature. At the same time, it can lower the combustion efficiency. Therefore, it’s necessary to choose a appropriate pulverized coal fineness considering the two aspects mentioned above. Thanks to the excellent combustion characteristic of Shenhua coal, higher pulverized coal fineness than the conventional one can be chose to soften the slagging problem. The appropriate pulverized coal fineness varies between various power plant apparatus and should be determined by combustion adjustment experiments. Optimizing the coal distribution between the millers helps to prevent the slagging in the hopper area. 3.6 The else

The mentioned measures are mainly aimed at the practice problems of burning Shenhua coal. Otherwise, attentions should be paid to adjusting the burner’s tilt angle, lowering the flame center and monitoring the Furnace Exit Gas Temperature, The FEGT should not be over the melting point of Shenhua coal ash. The FEGT should not be over designed value when the burner’s tilt angle is adjusted to optimize the steam temperature. Other than the preventing measures for slagging, soot-blowing systems should be optimized. Enhanced soot blowing of heating surfaces can prevent fouling and slagging, but lead to the potential ash agglomeration in the backpass. The frequency of soot blowing should be optimized. Consistent distribution of air/coal among the burners is another important exercise.

In a word, thanks to the excellent combustion characteristic of Shenhua coal, it’s possible for preventing the slagging problem. By adopting the appropriate methods and hardware, Shenhua coal can be burned safely and economically.

4、Application of Shenhua coal in Guohua Dingzhou Power Plant and Taishan Power Plant

Guohua Dingzhou Power Plant’s and Taishan Power Plant’s 600MW unit of the first stage construction is used Shenhua coal as designed coal. The boiler is sub-critical single reheat controlled circulated boiler manufactured by Shanghai Boiler Plant. It is a four corner, tangentially fired boiler with tilting burners and pressure direct processing system of pulverized fuel. It has single furnace, Π type open air arrangement, dry bottom, full steel skeleton and balanced draft. The furnace is 19558mm wide, 16940.5mm deep and 73000mm high. The top of the furnace is equipped with separating pendant, back pendant and pendant reheater. The front wall and sidewalls are equipped with wall type radiation reheater. The convection pass is 8548mm deep and composed of the extensions of the water wall and the backpass. Final superheater and final reheater are placed in the convection pass. Low temperature reheater and coal economizer are placed in the backpass. There are three low head boiler circulation pumps and two tri-sectional air preheaters. The boiler has a pressure direct system of pulverized fuel and six middle speed coal pulverizer. Five of them will work under MCR, the other one is a backup. Each pulverizer is connected with a nozzle in the same corner and level. 4.1 summary of the operating condition

（1）Better boiler anti-slagging performance The operating condition shows that the Taishan Power Plant and Dingzhou Power Plant have

made great achievement in preventing boiler slagging. It will better for the boiler which has middle section thermal load and capacity thermal load (representatively 4.68MW/m2、87.6kW/m3) due to the burner design character. The primary air nozzles are opposed located and the secondary air nozzles follow the opposing tangential circle direction, which is named “opposite, concentric, opposing tangential circle direct ion”sysem. (Shanghai Boiler Works LTD- Concentric Fired System). This system is installed in Wujing 2, Taishan and Dingzhou Power plant. Such kind of burners have the following advantages: (a) air-surrounding-fuel can be realized in the bottom of burner zone;(b) the intensity of swirl and combustion in the bottom of burner zone are weak, so the peak temperature can be avoided;(3) adopt blunt body to realize quick ignition;(4) the second air and OFA are deflected to the opposite direction of the flow rotation direction in the furnace in order to weaken swirl intensity in the upper burner zone. So that the boiler temperature deviation can be controlled effectively. （2）Poor coal fineness and uniformity

We tested the coal fineness of Taishan Power Plant and Dingzhou Power Plant. The results, listed in table 4,show that TaiShan Power Plant should improve coal fineness uniformity and Dingzhou Power Plant should reduce coal fineness.

Table 5 coal fineness Coal fineness Dingzhou Power plant Dingzhou Power plant

R200 1.75 1.71 R90 14.90 22.21 R75 20.89 29.41

Uniformity index 0.958 1.279 （3）The superheat steam temperature is lower than needed, the reheat steam is overtemperature

We carried out baseline tests of Taishan Power Plant and Dingzhou Power Plant and the results are listed in table 5 [6].

Table 6 the main operational performance parameter

Dingzhou Power plant Dingzhou Power plant

November 7 November 7November

28 November

28 Parameter Unit

Baseline test1

Baseline test 2

Baseline test 1

Baseline test 2

Unit load MW 600

Coal capacity for burning t/h 229 230 228 227 Feed water flow t/h 2054 1957 1883 1914

Feed water temperature ℃ 272 274 276 276

Burner in operation

The lowest level burner

out of operation

The top level burner out of

operation

The top level burner out of

operation

The 5th level burner out of

operation

Tilt angle of burner ° 0 15 7 8 Stea Temperature A/B ℃ 539／539 533／535 538/538 536／536

Pressure A/B MPa 16.8／16.8 16.1／16.2 16.7/16.6 16.8／16.8 First level

desuperheating water quantity A/B

t/h 0／0 0／0 7/2 10／10

m gas

Second level desuperheating

water quantity A/B t/h 0／0 0／0 24/21 15／15

Temperature A/B ℃ 542／539 538／538 540／537 536／533 Pressure A/B MPa 3.4／3.4 3.2／3.2 3.2／3.2 3.2／3.2

Reheat

steam

Desuperheating water quantity A/B

t/h 36／55 24／41 4／23 13／22

Primary/secondary cool air temperature

℃ 24/16 22／15 24／22 30／26

Primary/secondary heated air temperature

℃ -/322 －／315 298／318 301／320

Maximum temperature

℃ 1220 1170 1070 1118 Separating pendant

in the front wall

Mean temperature

℃ 1168 1127 1042 1086

Gas temperature in the inlet of air preheater

℃ 392 373 367

Oxygen level in the inlet of air preheater

％ 3.8 4.6 5.0 4.5

Flue gas temperature ℃ 137 137 133 136 Flue gas Oxygen level ％ 4.5 5.2 5.5 5.2

It can be seen from table 5 that, the measured main and reheat steam temperatures are near or have reached the designed values. But this can only be obtained by burners tilting upwards. At the same time, in Taishan unit 2, the injection water flow rates for reheater and super heater are 30～35t/h and about 50t/h respectively. In Dingzhou unit 2, the injection water flow rates for reheater and super heater are90t/h and about 50t/h respectively. It is a severe damage of the performance economic. （4）High slagging tendency in separating pendant

Burning Shenhua coal will lead to severe slagging in separating pendant due to the above operational character and the short coal resident time in furnace. 4.2 Improvement

As a solution to the pendant reheater slagging and undertemperature of the main steam, Guohua, Xian Thermal power Research Institute and the Shanghai Boiler Plant proposal the following improvements: （1）Increase the low temperature pendant reheater surface by 3215m2.

It is a feasible to increasing the low temperature reheater. The major aim is to make the top-level miller backup. .

The heating calculations for the worse performance condition with increasing reheater surface show that When the lowest level burners(miller F) are out of operation and burners are tilting upwards(run 5), the gas inlet temperature in the major pendant is 15.2℃lower than the

present condition and the parameters of the steam can still satisfy normal operation. The heating calculations for the better performance condition with increasing reheater surface show that When the top level burners (miller A) are out of operation and burners are tilting downwards(run 3), the gas inlet temperature in the major pendant is 37.7℃lower than the present condition, reheater water injection rate is zero and superheater water injection vary slightly, Flue gas temperature is 3.3℃ lower. The economical efficiency is high and can satisfy long-term operation requirement. So, the miller A is spared and the slagging in the pendant area is reduced. (2) The improvement on burners configuration

The schemes on improving the burners configuration can be seen at table 6: change the secondary air’s direction from opposing tangential circle direction(AA, AB are 25°、20°anti-clockwise ) to following tangential circle direction(both 4.5°clockwise, ±15°tiltable in horizontal direction ). Table 7 adjusting the deflection angle of secondary air（clockwise “+”；counterclockwise

“-”） The number of primary/

secondary air Original design Improved design

OFA -25° -25° AA -25° Adjust to 4.5° A ～0° ～0°

AB -20° Adjust to 4.5° B ～0° ～0°

BC 4.5° 4.5° C ～0° ～0°

CD 4.5° 4.5° D ～0° ～0°

DE 15° 15°/10° E ～0° ～0°

EF 4.5° 4.5° F ～0° ～0°

FF ～0° ～0° The aim of the improvement is to enhance the primary/secondary air mixing in the burner

area and increases the boiler resident time of pulverized coal. The above analysis of the performance conditions of the burners shows that there is a large difference between the deflection angles of primary and secondary air, leading to poor mixing of primary/secondary air mixing in the burner area and therefore a long flame. Besides, due to the early reduce-swirl-air, the boiler resident time of pulverized coal is shorten. Change the reduce-swirl-air into enhance-swirl-air（+4.5°）, can increase the resident time of pulverized coal from miller B by over 2 s, with the other parameters remaining the same. From the burners to the OFA level, the resident time of pulverized coal is 1~1.5s. However, this scheme will possibly lead to at the exit of the furnace. It should be compromised that the OFA remain counterclockwise (reduce the swirl). (3) Improvement on the classifier of the miller

Due the advantages of the fine coal: quick burnout, high radiation and easy to be quenched, it can be use to control the slagging problem in the pendant area. It’s proved by field tests and by experiments carried out in the 1MW furnace and the 1D furnace in Thermal power Research

Institute. Improved coal fineness(R90=8～10%) can apparently help to reduce the slagging in the pendant area.

Comparison of the Taishan unit 3 furnace temperature between R90=15% and R90=8% by the Canadian CFD model computation shows that this improvement can lower the inlet temperature of the pendant by 10℃, almost the same effect as deepen the furnace of Taishan unit 3 by 0.5m. By adjusting the primary air, primary air nozzle damage and slagging in the burners area can be prevented.

5、Conclusion

Shenhua coal is high quality coal for electricity generation with fine combustion performance and low pollutant emission. But its exploitation and application as coal for motive force have not been too long and there is litter experience about the design of boiler using Shenhua coal. And the research is not perfect. So if there is any error in designation or operation, problems may exist in coal storage, coal pulverizing, boil combustion and ash and slagging handling system. And the following situations may happen: (1) slagging in radiation heating surface;(2) deposition in convection surface;(3) block in dry ash handling system;(4) high content free calcium oxide in dry ash. So it is necessary to establish a database on the coal quality, combustion, slagging and pollution of Shenhua coal from all coalfields. It is also a heavy task to make full investigation in the application of Shenhua coal in power plant so as to accumulate experience on designation and reforming of boiler burning Shenhua coal.

In a word, thanks to the excellent combustion characteristic of Shenhua coal, it’s possible for preventing the slagging problem. By adopting the appropriate methods and hardware, Shenhua coal can be burned safely and economically.

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China electric power technology, 2003, 5:45~46 [4] He Wangfei. Analysis and Research on Reducing Boiler Slagging [J], Thermal Power

Generation, 2003, 11:32~34 [5] Lei Junzhi, Ma Qiliang. Research on Prediction Method of Boiler Slagging [J], Boiler

Technology, 2001, 11:7~11 [6] Xian Thermal Power Research Institute, <<Guohua 600MW unit field testing

report>>,2004.12