Study On Mechanism And Control Of Flue Gas Pollutants In The CO₂ Chemical Absorption System Of Coalfired Power Plants

Currently,chemical absorption is the most potential CO2 capture technology for industrial applications.While in this process,some amines solvent and their degradation products are discharged with the flue gas,causing a large amount of secondary environmental pollution.There are mainly two emission forms of flue gas pollutants in the chemical absorption system of coal-fired power plants,vapor and aerosol.This paper focused on these two types of emissions,improves the vapor emission model,and analyzes the changes in aerosol emissions.The study covers from bench scale to industrial demonstrations.We hope these could guide the emission and control of pollutants in industrial plants.To solve the low prediction accuracy of amine vapor equilibrium partial pressure models,this paper improves the Aspen thermodynamic models for piperazine(PZ),monoethanolamine(MEA),and 2-amino-2-methyl-1-propanol(AMP)systems.The model screens the latest VLE data and simplifies the NRTL regression parameters to ensure data quality and prevent overfitting.The optimized model has good applicability and accuracy.The final model can be applied in amine concentration of 0.1-9.0 m PZ/ 15 wt.%-45 wt.% MEA / 0.1-4.8 m AMP.The model average prediction error of CO2 solubility is 16.6%-29.8%.The reconstructed Aspen model can predict the volatility of amines.The model shows that the equilibrium partial pressures of 2.9m PZ,MEA,and AMP are 1.46 Pa,2.40 Pa,and 11.54 Pa at 40℃,respectively.The volatility increases 2-3 times for every 10 ℃ increase in temperature.In order to study the phenomenon of 1000mg/Nm3 level of amine emissions reported in the literature,this article conducted a study on MEA chemical absorption system with high condensation nuclei flue gas(3.43 × 107 cm-3).It was found that the number concentration of aerosol particles decreased to 1/10 of the original after passing through the absorber.While the particle size has increased by more than 10 times.The MEA emissions after absorber were up to 1519 mg/Nm3,and that after water wash was249 mg/Nm3.The growth of aerosol leads to a rapid increase in the aerosol particle mass.No matter how the conditions in the absorber change,the change of aerosol particle mass and total amine emissions have the same trend.In the blends MEA/AMP bench study,this paper first investigated amine emission and influence factors under different flue gas nuclei scenarios.When the flue gas nuclei were low(1.05 × 105 cm-3),the MEA and AMP emissions after the absorber were 38mg/Nm3 and 206 mg/Nm3.The formation of mist was observed in the flue gas outlet in intermediate nuclei(1.27 × 106 cm-3).In the high nuclei scenario(3.51 × 107 cm-3),the emissions were 401mg/Nm3 and 1022 mg/Nm3 respectively.The water wash removal rates of MEA and AMP emissions were 86.7% and 90.9% respectively in the high nuclei scenario.They both increased to more than 98% in the low nuclei scenario.The assessment showed that under the intermediate nuclei,the CO2 concentration and the lean solvent CO2 loading have enough impact on both aerosol and vapor emissions.While the influence of lean solvent temperature and liquid-gas ratio on aerosol growth is very small.This paper further studied MEA/AMP blends in the 300Nm3/h CO2 chemical absorption system.Under typical operating conditions,the total amine emission after the absorber is 360 mg/Nm3,and 60 mg/Nm3 after water wash.The inlet flue gas is between intermediate and high nuclei.The increase in flow rate and the decrease in temperature of the water wash can suppress amine emissions.Although the inter-stage cooling process could effectively reduce the amine emissions after the absorber,it had little impact on the final emission after the water wash.Except for the different effects of lean solvent temperature,the amines emission patterns in the bench scale and pilot scale are consistent.Based on the thermodynamic model above and experimental data fitting empirical formulas,this paper first achieves industrial scale amine emissions assessment containing aerosol.The flue gas nuclei of the planned 200t/a CO2 capture system in Shengli Power Plant were 3.33×106 cm-3.When the MA-1 was chosen as the solvent,the aerosol form amine emissions after the absorber would be 1078 mg/Nm3 according to the fitting formula.The control methods of combining water wash and dry bed were evaluated in the Aspen Plus.The simulation result showed the return water of water wash flowing through the dry bed is the best process.The control method of using a 3m water wash combined with a 2 m dry bed(Mellapak 250 Y filler)is proposed,resulting in a final vapor emission of 0.82 mg/Nm3 of amine.The dry bed and water wash promote the growth of aerosol,it cooperates with the wire mesh demister can control most of the aerosol droplets.