| Today, the ultra-supercritical power generation technology, which shows high power efficiency and good environmental protection performance, has become one of the main technologies in the field of clean coal power generation. In China, the lignite resource is rich, and in recent years, the proportion of lignite in power coal is on the increase, but at home and abroad the experience of1000MW level boiler burning Chinese old lignite is lack. Therefore, there is important practical significance and broad market prospect to exploit and develop the1000MW ultra-supercritical boiler which is suitable for Chinese lignite burning. The Eleventh Five-year Plan863project has carried out " Research on key technologies of1000MW ultra-supercritical lignite boiler ".1000MW ultra-supercritical boilers widely adopt the dual circle tangential firing combustion method, which causes the formation of "cold and hot corners". The melting point of lignite ash is low, so slagging could easily occur in the hot corner area. On the other hand, the volume heat load of lignite boiler is low, and the furnace size is larger, which will increase the gas thermal deviation at furnace outlet. Based on these two problems, this paper carried out research work by the means of theoretical analysis, cold single-phase flow experiments and hot numerical simulation, and then proposed solution methods.The thermal deviation at the furnace outlet of1000MW ultra-supercritical lignite boiler which adopts the dual circle tangential firing combustion method has been studied by method of numerical simulation. The causes and characteristics of the thermal deviation on all sorts of heating surfaces in the upper furnace have been investigated. The radiative heat load distribution of the platen heating surface is related with the cross section dimensions of the furnace, it shows bimodal distribution for the object of the study, and the heat loads of the panels at the central positions of the two furnace halves are highest; for the radiative-convective heating surfaces, due to the superposition of the gas velocity in the middle region, large area of gas recirculation region is formed here, and the maximums of gas temperature and velocity both appear in the locations which are in the middle-down of horizontal flue, close to the two sides; Heat load distribution on the convective heating surfaces is affected by the velocity distribution of the airflow field to the largest extent, the maximum temperatures also appear near the two side walls, but the cross section temperature distribution is more uniform. For these heating surfaces, the gas velocity has great influence on the heat load distribution, and the thermal deviations on the heating surfaces are relational with the gas residual rotation at the furnace outlet.A cold single-phase scale model of the boiler has been established. Cold experiments and hot numerical simulation on the1000MW ultra-supercritical lignite boiler were combined to study the over fire air parameters. Influence of over fire air velocity, over fire air horizontal swing angles and part over fire air vertical swing angles on the aerodynamic field in furnace, the pulverized coal combustion, the gas components distribution and the thermal deviation at the furnace outlet has been investigated. The results show that the larger over fire air proportion is beneficial to the effect of the air staged combustion, but at the same time, it can also increase the gas velocity peak at the furnace outlet, which is not conducive to the uniformity of gas parameters distribution. The best over fire air velocity is60m/s. Proper horizontal swing angles of over fire air could enhance the turbulence degree of the airflow field, weaken the scouring of airflow on the front wall, substantially reduce the rotating momentum of the airflow field, and effectively decrease the gas thermal deviation. The optimal over fire air horizontal swing angle is-10°. Part over fire air vertically swing downward could reduce the velocity maximum on each section of furnace outlet, and change the location of high velocity area. But it could not effectively reduce the thermal deviation in the whole horizontal flue. The best vertical swing angle is-10°.Slagging tendency on the water wall in the primary zone has been studied. The areas where slagging occurs easily are the area which is scoured by the jet on the same side, the area between the upper and the lower groups of burners, the area between the upper group of burners and the over fire air, and the hot corner area on the front wall. While the hot corner area on the rear wall shows no obvious slagging phenomenon. The main reason that slagging occurs easily in the hot corner area on the front wall is the airflow field in furnace shows oblique ellipse, which results in the scouring on the front wall by high temperature jets. While, by the horizontal swing of the primary air nozzles on cold corner could not effectively reduce the slagging problem.Research results show that the basic reason of the formation of the oblique elliptic airflow field in the dual circle tangential firing boiler is the rectangular arrangement of the burners and the different deviation degrees of the jets. By theoretical analysis, the empirical formula and the key factors that have effects on the jet deflection have been obtained. Experimental verification of the effect of the velocity ratio, the impacting position and the impacting angle on the jet deviation has been carried out. According to these factors, some optimization and modification measures in engineering practice have been put forward.The half-wall-half-corner burner arrangement form has been brought forward, and a cold model was built to measure the aerodynamic field in furnace, then the experimental results were compared with wall type burner layout form. At the same time, by the method of numerical simulation, the combustion process in furnace and the slagging tendency on the water wall in the two sorts of boilers were compared. Comparison results show that by using the half-wall-half-corner burner arrangement form, the problems of the oblique elliptic airflow field and the slagging on the hot corner of front wall have been basically solved. But the airflow field centers in this sort of boiler incline to the side walls, and the scouring to part of the water walls by the jets has been intensified, the rotating momentum of the whole airflow field increases, which leads to the increace of the thermal deviation at the furnace outlet.By wholly adjusting the jet angles, the airflow field location in the furnace of half-wall-half-corner burner arrangement type boiler has been optimized. More reasonable airflow field distribution has been obtained, and the slagging on each furnace wall has been reduced more effectively. By changing the over fire air horizontal swing angles, the thermal deviation at the furnace outlet in this sort of boiler has been decreased. Because of the large rotating momentum of the main airflow, the anti-cyclonic phenomenon was not formed in the furnace even though the over fire air horizontal reverse swing angle is big. The optimal over fire air horizontal swing angle is-10°.The study in this paper has solved the problems of the thermal deviation at the furnace outlet and the slagging on the hot corner in primary combustion zone in the1000MW dual circle tangential firing ultra-supercritical lignite boiler. The results have important guiding significance to the engineering design of this sort of boiler. |
PDF Full Text Request