Numerical Modeling Of The Combustion In Coal-fired Travelling Grate Boilers And The Optimization Of Arch And Air Distribution

Coal-fired travelling grate boilers are important thermal facilities in China with a huge number and wide distribution. The efficiency, however, is relatively low. The"3T+E"combustion optimization factors, i.e., time, temperature, turbulence and excess air in travelling grate boilers are mainly adjusted by arch in the furnace and air distribution under the grate. Optimization of arch and air distribution is the common method to improve the combustion efficiency.However, the design of arches, either conventional arch or novel arches such as double-herringbone arch, are based on experience. Air distribution is also mainly determined by the experience of stokers. These empirical methods are greatly blindfolded, which usually leads to low improvement of combustion efficiency, even unclear puffing combustion in the furnace.In consideration of the lack of prediction method in the combustion optimization, this paper aims to develop a coal-fired travelling grate boiler combustion numerical model using coal analysis and air supply as the input conditions. The model comprises three parts, i.e., bed model, furnace model and their coupling model. In the bed model, a non-isothermal two-dimensional method considering the temperature gradient and ash diffusion resistance in particles is established in view of the special features of domestic coals (large-diameter and high-ash). Then the bed model is combined with the furnace model based on an integrated grid. After the couplings between the furnace and the bed, such as flow, radiation and convection are added, the integrated travelling grate boiler combustion model is achieved.In order to verify the model, the gas components along the grate length in a 20t/h boiler are measured by a gas chromatograph and the data are compared with those predicted by the model. The result shows that the model is in reasonable agreement with the experiment. Additionally, the precision is improved compared with the conventional isothermal model. To further verify the model ability for different coals, a lab-scale furnace is set up. The gas components at the bed surface when burning three coals of different volatile level are measured by an infrared gas analyzer. The data are used for the verification and the result shows that the model has a reasonable precision on the whole, which is satisfied for the design of arches and air distribution. On this base, some influencing factors, such as grate velocity, bed thickness are analyzed and influencing mechanisms are revealed.Next, the influences and mechanisms of different air distribution modes on the beds of various volatile level coals are studied. For the novel double-herringbone arch, the working principle, suitable air distribution and arch shape are analyzed. Later, the arch and air distribution in a 20t/h boiler are improved with the aid of the numerical model. The efficiency measurement shows a good energy-saving result. Finally, a pulsating micro-pressure test rig of volatile combustion is set up to study the mechanism of non-resistance puffing combustion led by inappropriate arch design. A dynamic pressure transducer is used to measure the pressure fluctuations in the furnace under different air distribution modes. Large eddy simulation is performed to compare with the experiment and analyze the combustion features in the furnace. The result shows that non-resistance puffing combustion is led by the abnormal volatile combustion induced by periodic vortex shedding. The arch design scheme to avoid the non-resistance puffing combustion is suggested based on the mechanism.