Mechanism Research And Application On Biomass Combustion And NOx Reduction By Biomass Co-Firing

With the deterioration in the greenhouse effects, more and more attention has been attractedby biomass which is considered as a CO₂-neutral fuel. However, because the content ofalkali and chlorine in biomass is high, combustion of100%biomass will result in severeproblems of slagging and corrosion, and this can be effectively avoided by biomass co-firingwith coal. Meanwhile, due to the high content of volatile, biomass co-firing with coal as acertain method can also achieve a high efficiency in NOx reduction. Aiming at biomasscombustion and biomass co-firing technology, the basic combustion characteristics and thekey problems during the industry application are investigated, and the investigation on thereductions of NOx by biomass char and gas are also presented. The main conclusions aresummerized as follows:Firstly, thermogravimetry （TG） is adopted to study the biomass combustion characteristicsand the synergistic effect during biomass co-firing. Results show that the ignition of biomassis delayed under oxygen-combustion conditioin, and the delay effect is more significant forchar combustion. The increasing of oxygen concentration improves the ignition of coalsignificantly but affects the ignition of biomass less. Tests on the combustion of biomasschar show that the iginition temperature of straw char is significantly lower than that ofwood char and coal char, and the ignition temperature of wood char is even much higherthan that of coal. Tests on biomass co-firing show that there is a positive synergistic effect toimprove the general iginition characteristics of biomass and coal, and the synergistic effect ismore significant for biomass of straw, for coal of worse iginition characteristic and for thecondition of O₂/N₂than that of O₂/CO₂.Secondly, the ash deposition and slagging on the heating surface are simulated in a drop tubefurnace of laboratory scale, and comapred with the samples from biomass-fired power plant.Results from laboratory investigation show that the initial deposition layer appear asponge-like structure of aerosols with a diameter scale of10¹00nm, which should bedeveloped by the accumulation of nano-sulfate particles. There is no chlorine detected in theinitial layer, and the main elements are K, S, O and Na, and the results are correspondingwith that from the samples of biomass-fired power plant. Tests on biomass co-firing showthat biomass co-firing will not affect the slagging characteristics when the co-firing mass ratio is lower than20%. Based on the results of laboratory and industrial furnace, amechanism on the core role of sulfate aerosols in biomass ash deposition is proposed.Finally, mold biomass pellets have been utilized on a300MW pulverized coal-fired furnacein China for the first time. Biomass was ground and transported using the existing millsystem without using any additional equipment, which achieves the simplification ofbiomass co-firing in a large scale in the practical engineering application. Experimentalresults show that the upper limit ratio of biomass heat input for this coal-fired unit is16.1%,under which, biomass co-firing will not affect the quality of the fly ash to be used in thecement industry. Meanwhile, it also obtains the effect of biomass co-firing on temperatureprofile in furnace, pollution emission and combustion efficiency. Because of the successfuloperation of this experiment, this biomass co-firing technology has been sustainably appliedin a power plant under the support of Shaanxi Provincial Development and ReformCommission.Before the investigation on the reduction of NOx by biomass gas and biomass char, thevolatile emission and residual char properties during biomass pyrolysis are comprehensivelyanalyzed. It demonstrates the transformation rules of nitrogen, sulfur and chlorine, and theeffect of pyrolysis temperature on the properties of biomasss char. Results show that thecontents of chlorine and potassium in biomass char have greatly decreased at the processingtemperature800℃, and there is no chlorine in the residual biomass char when the processingtemperature is higher than1000℃. For biomass char of straw with high content of potassiumand chlorine, there is a optimal pyrolysis temperature800℃, under this temperature, thebiomass char holds the most developed pore structures, the largest surface area and the bestcombustion activity.Experiments of NOx reduction by biomass char are conducted in a fixed bed system. Resultsshow that for the reaction between biomass char and NO, there is a general transitiontemperature region （800⁹00℃） from dynamic-control to diffusion-control, and the effect oftemperature of char preparing on the apparent activation energy is not significant. Theapparent activation energy for the reaction between NO and straw char (89.78⁹5.41kJ.mol^-1) is lower than that for wood char (115.22¹22.79kJ.mol^-1) and that for coal char(108.59¹17.63kJ.mol^-1), and the reaction order for all the three kinds of char is around0.85.Meanwhile, for the reaction between char and N₂O, the apparent activation energy is74.64kJ.mol^-1,122.06kJ.mol^-1and127.01kJ.mol^-1for straw char, wood char and coal charrespectively, and the reaction order for all the three kinds of char is around1.Investigation on the reduction of NOx by biomass gas is performed by dynamic calculation.A detailed mechanism of55species and382steps is constructed to accurately predict thereduction of NO and N₂O by biomass gas, and NH3transformation in SNCR process. Thedetailed mechanism is used to systematically analyze the potential optimal value of excess air coefficient for the maximum NO reduction rate during biomass gas reburning. The effectsof reaction temperature, initial NO concentration, residual time, H2O and CO₂concentrationare also considered. Finally, a skeletal mechanism （32species and179steps） and reducedmechanism （25species and21steps） is developed from the detailed mechanism. Bycomparing the results calculated using skeletal mechanism and reduced mechanism with theresults from experiments and calculation using detailed mechanism, it is verified that boththe skeletal mechanism and reduced mechanism can be used to well predict the iginitiondelay, flame propagation speed, NH3transformation, reduction of NO and N₂O during thecombustion of biomass gas.