Simulation on Soot Emission Control in Stoker-Fired Boiler by Secondary Air

Qingcheng Wang

Institute of Thermal and Power Engineering, Shanghai Institute of Technology, Shanghai 200235, China

Received: March 21, 2011 / Accepted: May 16, 2011 / Published: January 20, 2012.

Abstract: A work on soot emission control simulation in stoker-fired boiler by secondary air has been done. Some models such as k-ε, combustion, radiation, and soot Khan-Greeves have been adopted. Soot production and emission has been reduced by secondary air; the highest mass concentration is reduced from 7.46 × 10-14 to 6.94 × 10-15; mass concentration of soot is decreased from 1.12 ×10-15 to 9.25 × 10-32 in the upper areas.

Key words: Soot, stoker-fired boiler, secondary air, simulation.

1. Introduction

The soot particles derived from coal combustion affect the environmental quality and people’s health, which are paid attention to by more and more people. Soot particles in air relate to people’s acute and chronic diseases [1, 2]. Up to now, many works focus on the soot formation derived form simply gas hydrocarbons, liquid fuels and pulverized coals [3]. Few works has been mentioned for soot formation derived from lump-coal combustion in stoker-fired boiler. There are more than 0.5 millions industrial boilers and the total installed capacity of the industrial boilers is more than 1.2 millions tons per hour; more than 400 million tons of coals are consumed in China; and the stoker-fired boiler is the dominating combustion style [4]. The characters of this boiler style are low in efficiency and high in pollutants emission. It is significant to simulate soot generation in order to reduce the soot emission and increase the efficiency of stoker-fired boiler.

2. Simulation Method

Computation Model

Computation model is constituted with several sub-models as below:

(1) k-ε Model

In these equations:

A-constant in the Magnussen model;

YOX, Yfuel-mass fractions of oxidizer and fuel;-mass stoichiometries for soot and fuel combustion.

3. Computation Method

The height and width of computation area are 1,000 mm and 200 mm respectively, which are divided into 108,365 meshes. The computation method is Semi-Implicit Method for Pressure Linked Equation Consistent (SIMPLE). Inlet boundary conditions of fuel refer to testing result of volatiles in stoker-fired boiler. Methane, acetylene, and benzene, are related to soot formation. Inlet air velocity is 0.175-0.788 m/s. Excess air factor is 1.05. Secondary air velocity is 0.98 m/s.

4. Simulation Result

4.1 Soot Concentration without Secondary Air

Soot mass concentration is expressed as Fig. 1. The highest mass concentration is 7.46 × 10-15. Soot mass concentration is high in the area where the volatile concentration is high because volatile has not been burned out in time and formed soot by nucleation, surface growth, and polymerization process. Soot mass concentration is high in the area where the temperature is high. Some soot is formed with furnace wall increase, and the mass concentration of soot is about 1.12 × 10-15. In the upper middle area in the furnace, mass concentration of soot is 8.47 × 10-32, because temperature is high and volatile is mixed by air.

4.2 Soot Concentration with Secondary Air

Soot mass concentration is expressed as Fig. 2. The highest mass concentration is 6.94 × 10-15, which is lower than that without secondary air. Soot mass concentration is high in the area where the volatile concentration is high because volatile has not been burned out in time and formed soot by nucleation, surface growth, and polymerization process. The secondary air may strengthen the mixture of gases in the furnace, reinforce the reaction between volatile and air. In the upper area in the furnace, mass concentration of soot is decreased from1.12 × 10-15 to 9.25 × 10-32.

Comparing to the experimental results that have been finished by author, reduction of soot emission by secondary air have same tendency.

5. Conclusions

Based on the simulation which include k-ε, combustion, radiation, and soot Khan-Greeves models, these conclusions can be drawn: the secondary air may reduce soot production and emission; the highest mass concentration is reduced from 7.46 × 10-14 to 6.94 ×10-15; mass concentration of soot is decreased from 1.12 × 10-15 to 9.25 × 10-32 in the upper areas.

References

[1] B.L. He, Q. Song, C.H. Chen, X.C. Xu, Investigations on mechanism of soot formation during combustion and control of soot emission, in: The 5th International Symposium on Combustion, China, 2003, pp. 1-5.

[2] J.L. Ma, T.H. Fletcher, B.W. Webb, Effect of flame environment on soot formation in coal combustion, in: International Conference on Coal Science, 1995, pp. 869-872.

[3] H.F. Zhang, Nitrogen evolution and soot formation during secondary coal pyrolysis, Ph.D. Thesis, Department of Chemical Engineering, Brigham, Young University, 2001.

[4] M.J. Tan, J.X. Mao, The advance technologies of coal-fired industrial boiler in China, in: The Conference of the China-America Advanced Technologies of Industrial Boiler, Beijing, 2004, pp. 1-17.

[5] I.M. Khan, G. Greeves, A Method for Calculating the Formation and Combustion of Soot in Diesel Engines, Heat Transfer in Flames, Scripta, Washington DC, 1974, Chapter 25.