Numerical Simulation Of Combustion In Biomass Grate Furnaces

As a renewable energy sources, biomass plays an increasingly important roleworldwide. Grate firing is one of the main technologies that are currently used inbiomass combustion for heat and power production. Grate-fired boilers can fire awide range of fuels of varying moisture content and show great potential in biomasscombustion.Computational fluid dynamics (CFD) software platform such as PHOENICSand FLUENT are used to simulate biomass grate-firing, include modeling ofbiomass conversion in the fuel bed on the grate and CFD modeling of mixing andcombustion in the freeboard. Calculation of biomass conversion in the fuel bed onthe grate are brought out by developing a1-D bed model, providing the inletconditions (e.g., distribution of gas species concentration, velocity, and temperaturealong the grate) for freeboard simulation; while the freeboard simulation returns theheat flux released from the flame and furnace walls onto the fuel layer to the bedmodel. The two simulations iterate again and again, until convergence criteria arereached.Module which can forecast the release of the NOxprecursors from the fuel bedis added in the bed model, obtaining the distribution of the NOxprecursors such asNO and HCN along the fuel bed, which are used as the grate inlet boundaryconditions. NOxpostprocessor included in FLUENT is used for the CFD modelingof NOxformation in the freeboard. Results indicates that NOxmainly formed in thefront-bottom part; the advanced air staged combustion process which formsappropriate local air-rich environment and anaerobic environment, can suppress thechemical process during which HCN is oxidized to NO, and promote the de-NOxprocess in which HCN can react with NO to form N2. The advanced air stagedcombustion generates a considerable improvement in a big NOxreduction.Biomass grate-firing combined model is used to design the geometric structureand the air supply of a25MW biomass boiler with water-cooled reciprocatingstokers, obtaining the distribution of temperature, velocity and flue gas componentsin the furnace. Numerical simulation results show that the momentum-synthesismethod is unsuitable for the geometric design of the biomass grate boiler. It is appropriate for biomass grate boiler to use shorter and higher rear arch. The angle ofthe rear arch ranges from40°to50°makes little difference to the burning conditionin the furnace. When the angle between the second air jets located on the rear wall inthe lower furnace and horizontal level (counter clock wise) is set at20°, the centerof the flame is right in the middle of the furnace; recirculation zone in the rearbottom of the furnace is larger, prolongs the paths of the volatiles and fly ash, helpsstable the combustion on the last section of the grate and reduces the incompletelyburned char in fly ash and the bottom ash; a larger local anaerobic environment isformed, which reduces NOxformation. Using advanced OFA air supply such asforming a double rotating flow on the horizontal cross-sections in the burnout zone,can distribute the secondary air inside the boiler more evenly and make theaerodynamic configuration more effective for reduction of pollutant emissions.Adding flue gas recirculation in the primary combustion zone leads to a lowertemperature level there, which mitigates deposit formation on furnace walls in theprimary combustion zone, especially in the throat zone. Flue gas recirculation has aconsiderable potential to optimize the mixing and reduces the incomplete burninglosses. Selective flue gas recirculation radio makes an additional reduction of NOemissions. The location of flue gas recirculation nozzles is important for combustion.Because the location of the second air jets located on the rear wall also has thebiggest thermal load of the water wall, flue gas recirculation located near the secondair jets on the rear wall can slow down the deposit formation and protect the nozzlesfrom being overheated.The sensitivities of biomass properties (e.g., moisture content, ash content) inbiomass grate-firing are also studied, and the simulation results showed that eventhough moisture content and ash content have effects on the combustion conditionsin grate boiler, the one designed in this thesis has some adaptability.