Numerical Simulation Of Combustion In Natural Gas Boiler

At present, the major energy of China is coal , of which about half is used as fuel in utility boiler. About 60% of nitrogen oxides emissions in the atmosphere are from coal-fired.The industrialization and urbanization have aggravated the the contradiction between supply and demand and the situation of environmental pollution. Compared to coal, oil and other energy , pollutants which come from the combustion process of natural gas can affect the health of human's respiratory is little. With the using of natural gas, the boiler can reach the national emission standards for NOx,which is a significant reduction in NOx pollutants in the atmosphere.Using natural gas instead of coal as a boiler fuel can not only greatly reduce the pollution of the environment,but also act in concert with the "West-East Gas Pipeline", "South of Russia for gas" and other projects. This can increase the use of natural gas in China and expand the use of natural gas, to achieve sustainable development strategy. The implementation of the project shows that China has entered the natural gas era, which has accelerated the rapid development of gas-fired boiler, and has shown extremely broad prospects for development of nature gas.However, the combustion of natural gas is a very complicated physical and chemical processes. In order to know the working process in the furnace, establishing a reasonable turbulent flow model, heat transfer model and combustion model for combustion simulation is necessary . Experimental research is necessary,but such experimental study needs a long time and is costly. As a result of a variety of objective reasons, the data often can not be fully satisfied if we carry out experimental research simply. Due to the complexity of the process, as well as the diversity of the boiler fuel, to design and operation of a reasonable adjustment of various types of boiler, combining the methods of numerical simulation and experimental study is offen used.This results of a more reasonable and reliable result in order to reduce research costs and shorten the research cycle . Therefore,the establishment of the turbulent flow model, heat transfer model and combustion model in the furnace to a comprehensive simulation is necessary. With development of modern computer technology and numerical methods, the development of natural gas combustion NOx in the process of numerical simulation technology is matures day after day. You can very well predict the generation of NOx, and can carry out the study of a wide range of varying parameters in the short term. In this paper, CFD simulation software is used to build the three-dimensional model of natural gas boiler which is used to study the following aspects: The Simulation object in this paper is a 0.7MW gas-fired boiler as well as a low-pressure swirl-type gas burner, respectively, with the simulation condition of no load and full load. A variety of mathematical models and numerical calculation methods are chosen according to the simulation content. In this paper,the turbulence model selection isκ?εhigh-Reynolds model.Combustion model is non-adiabatic PPDF turbulent combustion model.Gas specific heat and viscosity calculated use polynomial interpolation.Gas density according to the calculation is ideal gas equation of state.Simulation for the generation of NOx is the thermal NOx based on the use of the PDF transport model, neglecting the fast-type NOx and combustion-based NOx,and use the mixture fraction probability density function for NOx generated by solving the coupling. The SIMPLE algorithm is used for the coupling of velocity and pressure. Considering the radiation in this paper, selection of discrete ordinate exchanger radiation (DO) model is used.Firstly use the software of CATIA to do the three-dimensional geometric modeling. Then generate a file with stl suffix which can be transferred to STAR-CD software. The mesh generation is one of the key technologies for engineering applications of CFD as an effective tool for confronting.This paper has coordinated the conflict between computing speed and the quality of grids,and choosed the unstructured grid technology.The establishment of a dense burner grids and the gradient furnace grids have achieved a good coupling, resulting in faster computing speed and good convergence.The low-NOx burners can not only ensure the needs of ignition and combustion of natural gas,but also effectively inhibit the formation of NOx. Therefore,the simulation and researchment of the burner is also very necessary. In the cold simulation, the main analysis of the furnace and boiler burner are the velocity fields, pressure fields and the fields of the turbulent kinetic energy rate and its dissipation rate . Simulation results show that the burner of the internal flow field is a three-dimensional,asymmetric,and swirl flow. The gas in the burner flows with a high speed ,and reaches the highest near the holes in the eight jet,at nearly 50m / s. In addition,the speed near the exit of the burner is also very high,at nearly 40m / s. The air is rotating into the burner,and the nature gas is emiting with high-speed from the eight nozzles,and mix the air with a certain point of view. Finally part of the premixed nature gas and air flow through the burner to the outflow end of rotation.For the later period of the burn,this can increase the flame strengthly,and enhance the stability of the flame. Furnace outlet pressure is about 1000Pa,the pressure is imported from the air and gas of the burner available,so the pressure is larger in the burner. The maximum pressure in the burner reaches more than 2700 Pa. Pressure field inside the furnace is very uniform,with very small changes.This is because the speed of gas flow inside the boiler is very low,and can be considered as incompressible internal flows. The of change turbulent kinetic energy and turbulent kinetic energy dissipation rate in the heater is uniform,and where the larger the turbulent disturbance is,the larger of the turbulent kinetic energy k and turbulent kinetic energy dissipation rateεare. The simulation results can be seen in line with the actual theory, this paper also proves that choice of the calculation model is reasonable.In the thermal simulation,the focus is on the temperature change in the furnace with two parameters: excess air ratio and air preheat temperature.Firstly fix the simulation in the intake air temperature to 293K at a constant, and change in excess air ratio.Then make a constant excess air ratio at 1.05, and change the temperature of intake air. Consider the radiative heat transfer and take the wall as a blackbody, its blackness forε= 1; and to maintain constant temperature of the wall, the temperature T is 353K. Simulation results show that natural gas combustion in the boiler has combustion flame which is long and turbulence. In different conditions, the average temperature of the furnace cross-section along the furnace height of the overall trends are the same. When the excess air factor is 1.0, the combustion temperature gradient distribution of the smallest, that is the most uniform temperature distribution throughout the boiler furnace ,and the combustion average maximum temperature is the smallest; when the excess air ratio deviated from 1.0, the combustion temperature and the average temperature has fallen. Combustion temperature with excess air coefficient of variation compared with the theoretical value, the overall trend is consistent, but the combustion temperature is lower than the theoretical combustion temperature. In addition, the higher the air preheating temperature, the smaller the furnace temperature gradient, the greater the high temperature zone, the more uniform temperature distribution, the higher average temperature furnace. These results show that simulation results are also consistent with the theory and practice situation.In the thermal simulation, we also analyse the changes in the concentration of NOx generated when we change the excess air ratio and air preheat temperature.Comparing the results, we can see that NOx concentration reach the highest value in the excess air factor of 1.0. When excess air ratio is greater than or less than 1.0, the combustion temperature will fall.And the more the excess air ratio deviated from 1.0, the sooner the temperature will down.,and also will the volume of NOx concentration decreased gradually. This is because the combustion temperature reach the maximum value at the coefficient of excess air in the vicinity of equal to 1.0. The NOx from the combustion of natural gas in the boiler is thermal NOx, and thermal NOx generated by the main influencing factors of temperature,oxygen concentration and the duration of high temperature zone, of which the temperature is the greatest impact. In addition, the higher the air temperature preheat, the smaller NOx concentration gradient distribution is, that is, the more uniform of NOx be distributed,and the higher the average concentration of NOx is. According to simulation studies, this paper has got the further method to reduce NOx emissions. The accurate prediction of the NOx changes in trends could reduce the number of tests to facilitate the design of studies, to shorten the design cycle and reduce product development costs.