The Research On Cold-state Flow Field In The Furnace Of 6MW Biomass Generation Units

Based on the national policy of energy saving and pollutant emission reduction, National Development and Reform Commission of China(NDRCC) proposed to reform power industry structure, and required to close or rebuild small thermal power plants. The No.2 electricity generation unit with 6MW power output and equipped with a 35t/h coal burned boiler with spreader stokers and traveling grate stokers was renovated by using biomass fuel to replace Huainan Datong bituminous coal fuel. In this paper, the objectives are determining the boiler combustion system renovation program based on the characteristics of the biomass fuel and performing the cold test after the implementation of the renovation program.The characteristic parameters of the original coal and the biomass fuel are calculated and compared. The results show that the difference of the combustion features between the two fuels is significant, which means that the combustion system, especially the air distribution system, should be reconstructed. Without changing the boiler heating surface and retaining the original traveling grate stoker structure and the furnace structure, the renovation plan is determined as follows. The three original spreader stokers are removed , the spreader stoker tuyere is changed to primary air tuyere, and the secondary air flow rate is increased by increasing the number of secondary air vents and changing the type of the secondary air blower.Fluent software is used to simulate the air flow field in the furnace in the case that the spreader stoker tuyere is changed to primary air tuyere and the secondary air flow is increased. The results show that the airstream impacts the back wall and the flow field of the lower part of the back wall is detrimental to the particle settlement to the grate.In order to prevent water-cooled wall slagging and improve the biomass fuel settlement share on the grate, the angle of secondary air jet is adjusted. The flow field after the adjustment is simulated using Fluent software. The comparison shows the flow field is reasonable when the frontal wall secondary air is horizontal and the back wall secondary air jet angle is 45 degree up. After the secondary air jet angle is determined, the flow field in the furnace with different air flow rate conditions is simulated and the optimum air flow rate ratio is found. Through the above work, the boiler air distribution system is determined.The flow field of the new system is measured after the boiler air distribution system is built. The numerical air velocities at the inlets of the air tuyere are used as the reference data. In the experiments, the air velocities are adjusted to be the same as the reference data. The velocity distribution at three cross sections is measured and the real flow field is visualized by using spark tracing technique. The visualization results, the test results and the numerical simulation results agree very well, which shows that the numerical simulation results are reliable and the boiler transformation program is feasible.The numerical simulation and the cold test show that the position of the secondary air intercepting the primary air is reasonable after the renovation, the furnace is filled with the airstream, the particle settlement is uniform and to some extent, the flame impact on the wall is abated. The primary air in the furnace is normal without obvious deviation and the exit velocity of the gas is uniform, which helps improve the combustion efficiency and prevent the slagging on the water-cooled wall and the superheater surface.