Numerical Simulation On Co-combustion Of Pulverized Coal With Sludge In A 100MW Coal-fired Boiler

With the development of China in recent years, the production of urban sewage sludge, especially industrial sludge has increased rapidly. Now the environmental standards are increasingly stringent that leads to the fact that sludge treatment has become an important research subject. In the numerous methods for the disposal of sludge, the burning method has many advantages such as fast processing speed, short storage time and transportation distances, outstanding capacity reduction effect, effective disinfection and calorific value recovery. Thus, there are broad prospects for development of burning treatment for sludge.In this paper, on the basis of laboratory research, the study for co-combustion of sludge and coal with different moisture contents and ratio on a 100 MW coal-fired boiler was carried out by using numerical simulation method. Filed test has also been performed base on the results of simulation as a contrast. Further more, the NOx decreasing effort has also been carried out by air distribution optimization.The laboratory research showed that the fouling index of blended sample was similar to coal in small ratio of sludge, and it would not bring serious contamination problems. The numerical simulations using PDF(Probability Density Function) transport model and Eddy-Dissipation model(EDM) had been carried out to evaluate the characteristics of coal combustion and coal combined firing with sludge. Results showed: two models could both simulate successfully the temperature and velocity fields of the boiler for coal fired case. But the effect of moisture absorption of heat was more accurately simulated for combustion characteristic of the boiler when using EDM, and NOx emission was better simulated. Thus for high moisture sludge co-combustion, EDM had more advantages than PDF model on simulations of flow, combustion and NOx emissions characteristics.Numerical simulation studies had also shown that keeping blending ratio invariant, reducing moisture content of sludge into the furnace, the NOx emission would increase slightly. If the sludge moisture content kept unchanged, the difference of combustion and NOx emission characteristics was quiet small by comparison with single coal combustion when the blending ratio of sludge less than 20%. However, if the ratio was over 20%, boiler combustion performance would become worse with a significant increasing of NOx emission. When the total air volume kept unchanged, adding the ratio of over firing air or reducing the ratio of secondary air could decrease the air amount in main combustion zone of the boiler, and it would let down the NOx emission while unburned carbon in flue dust increased little.The field test showed that the content of moisture in the furnace increased with the increasing ratio of sludge which led to the decreasing of temperature in main combustion zone and the increasing of unburned carbon fly ash. Nitrogen content of the sludge was obviously higher than that of coal, which caused the increasing of NOx emission co-combustion with coal. The simulation result was consistent with the field test results which indicated co-combustion of coal and sludge was feasible with the ratio of sludge smaller than 10% of sludge with moisture content lower than 56%. Under these conditions, the influence on the normal operation of the boiler was ignorable.