Numerical simulation of pulverized coal combustion in a 660MW tangentially fired boiler with fuel variation

To study the effects of various coal properties on the combustion characteristics during actual operation, and on the high temperature corrosion of the water wall. The paper presents a numerical simulation of a 660MW boiler using ANSYS FLUENT software. Three cases were investigated with various fuel properties: (1) the designed coal NO.1, (2) the non-designed coal NO.2 with high ash, high sulphur content and low volatile matter, (3) the blended fuel of coal NO.3with similar coal quality to the coal NO.1 and NO.4 with high sulphur content. Simulation results indicate that blended coal feeding can improve the combustion characteristics of poor-quality coals. It provides a theoretical reference for the actual operation of power plants with non-designed coals.


Introduction
In recent years, China's social and economic development has led to higher demands for living standards and an increasing demand for electricity.China has abundant coal reserves and proven technical development, and coal-fired thermal power generation is still the main form of power generation.As a result of the double carbon policy, China's energy structure has changed and the proportion of new energy sources such as wind and solar power has increased [1,2].The large-scale grid connection of wind and solar power has increased the difference between peak and valley year by year, resulting in the great challenge of the power grid's safe operation.Therefore efficient and clean coal-fired power generation is necessary, especially operating under lower load and greater load variation, as a peaking regulation power source to calm down the power fluctuations [ 3,4,5].But the coal used in actual operation is normally variable, sometimes deviates far from the design coal type, because of the coal production, transportation capacity, changes in purchase price and other uncontrollable factors.Under the original air distribution method, the use of worse quality coal that deviates from the design coal type can easily cause problems such as delayed ignition and difficulty in stable combustion [6].
In order to better investigate the specific situation of the heating surface during the combustion process, actual experimental measurements on site or computational fluid dynamics (CFD) can be adopted.Due to a combination of factors such as rapid load changes, limited instrumental measurements, and high measurement costs, it is difficult to obtain sufficient data from actual measurements.CFD method can be used to calculate the combustion situation in the furnace under different conditions with lower cost and convenient load adjustment, and to observe visually the distributions of velocity, temperature and species in the furnace.Many CFD studies undertaken of coal combustion relating to tangentially fired furnace [7][8][9][10], which have shown that CFD provides an accurate tool for predict the behavior of coal combustion.
The objective of this study is to predict the combustion behavior of various fuels in a tangential furnace using CFD model.We selected three different feeding coal properties at full load for the combustion simulation.The temperature distributions, CO distributions and H 2 S distributions near the water walls are analyzed and compared respectively to provide a theoretical reference for the actual combustion.

Model description
A 660MW domestic tangentially fired pulverized coal boiler is studied.The furnace cross section is 18816×18816mm, with six layers of burners.According to the structural parameters of the boiler, a three-dimension model is established by Solidworks, and is shown in Figure 1.Due to the coal used in the actual operation of the power plant is usually not the designed type, we often need considerable coal production, policy requirements and other uncontrollable factors.In this study, we introduce the non-designed coal type, and another type with two coals of different proportions blended which is more in line with the actual combustion conditions of the power plant.Table 1 shows the industrial analysis of the different coal properties.The primary air velocity is 24.2 m/s at 350 K and the secondary air and SOFA velocity is 56.8 m/s at 608 K.The blended coal with coal NO.3 and NO.4 is fed into the furnace in a 5:1 mass ratio, and the higher sulphur coal NO.4 is fed in the B burner and coal NO.3 in the ACDEF burner.
The model is solved by ANSYS Fluent.Meshing is done in ANSYS meshing, and were done with approx.2,590,000 elements.

Temperature distribution
The temperature distributions of the central section of the C-layer burner and the central section of the furnace are given in Figure 2 and Figure 3.The flame filling in the furnace is good, and the burners support each other.As can be seen from the distribution diagram of the center section of the furnace: in the main combustion zone, the flue gas temperature is low, and the high temperature flue gas is concentrated near the wall, which facilitates the heat exchange of the water wall.The temperature in the center of the furnace gradually builds up over the height of the furnace increasing.In the SOFA area, the flue gas temperature near the wall decreases due to the injection of a lower temperature secondary air stream.Because the boiler adopts the air classification combustion technology, the air volume in the main combustion zone is low and it is in an under-oxygenated combustion state.pulverized coal is not burned sufficiently, and the unburned pulverized coal enters the combustion air zone to burn.

CO distribution
The reducing atmosphere of the flue gas in the near-wall area of the water wall is an important reason for high-temperature corrosion, and the reducing gas in the near-wall area cause corrosion by redox reactions with the tube wall under high-temperature conditions.CO gas is a typical reducing gas, and it can be judged by the CO volume fraction in the furnace.

H 2 S distribution near the water walls
The water wall near the wall area flue gas composition contains a large number of reducing gas is an important cause of high temperature corrosion of the water wall.In high temperature conditions, H 2 S and water wall surface metal reaction led to corrosion.Therefore, in this paper, a longitudinal section of 300 mm from the front wall, back wall, left wall and right wall in the furnace was chosen as the object of study to compare the H 2 S generation characteristics of three different coal properties near the wall surface.
Figures 6 to 8 show the distribution of H 2 S on the near wall surface of three different coals respectively.The most significant type of high temperature corrosion is sulphide corrosion.After the pulverized coal spray enters the furnace, the sulphur in the coal is released into the flue gas and generates H 2 S gas due to incomplete combustion without oxygen.The areas of high H 2 S concentration near the wall are mainly concentrated at the end of the burner nozzle in the main combustion zone.When the pulverised coal stream leaves the burner nozzle, a large amount of pulverised coal rapidly consumes oxygen and the excess air coefficient is less than 1.H 2 S emerges in large volumes.Compared the H 2 S concentration distribution of the three coals, the coal NO.2 has a high sulphur content, so the high H 2 S concentration area is large and can cause serious high temperature corrosion in the water wall in a high temperature and high pressure environment.When comparing the blended coal combustion, even though the sulphur content in coal NO.4 is high, because of the blending of coal NO.3, relatively high H 2 S concentration distribution only occurs near the B-layer burner, with the highest concentration being below 100 ppm.

Conclusion
The combustion condition in the furnace is good and the temperature field is more evenly distributed when burning the designed coal.When burning the non-designed coal, the temperature in the main combustion zone decreases, the high temperature zone moves back and appears in the SOFA air region, and the CO concentration increases.When using blended coals, the temperature and the distribution of the components in the furnace are similar to those of the designed coals, indicating a good blending effect.
The non-designed coal has a large area of high H 2 S concentration near the wall due to its high sulphur content.The blended coal which is a coal with a high sulphur content blended with a coal of similar quality to the designed coal, so the temperature and the distribution of the components in the furnace are similar to those of the designed coal, and the concentration of H 2 S is relatively low.It indicates that blended combustion can improve the combustion characteristics of poor-quality coals.It provides a theoretical reference for the actual combustion of non-designed coals in power plants.

Figure 1 .
Figure 1.The geometry of the boiler.

Figure 2 .Figure 3 .
Figure 2. Temperature distribution in the central section of a C-layer

Figure 4
and 5 show the CO distribution in the central section of the C-layer burner and in the central section of the furnace.The distribution of CO in the furnace can reflect the mixing effect of the air and coal to a certain extent.It can be seen that the CO volume fraction is small at the burner outlet and larger at the end of the stream at the four corners.a) Coal NO.1 b) Coal NO.2 c) Blended coal

Figure 4 .Figure 5 .
Figure 4. CO distribution in the central section of a C-layer

Figure 6 .Figure 7 .Figure 8 .
Figure 6.Distribution of H 2 S in the near-wall zone of the coal NO.1 to be burned at 100% load