Research On CO₂ Chemical Absorption Technology And System Modification Based On Novel Membrane Contactor

Reduction of CO2 emission is an important means to deal with global warming.The technology of chemical absorption from the flue gas of coal-fired power plant is considered to be the best choice for post-combustion capture.The main reason why the chemical absorption is difficult to apply on a large scale at present is that the equipment investment cost and the operating cost is too high.Therefore,reducing the equipment cost and operating cost is a bottleneck problem that needs to be solved to develop and promote this technology.In this thesis,the following aspects were explored and studied systematically,including solvent selection for membrane absorption,membrane surface modification for high hydrophobicity,recovery of volatile solvent,and system modification for reducing energy consumption,for the purpose of reducing the CO2 capture cost and energy consumption.Aiming to solve the problem of membrane wetting during long-term operation,amino acid salts with higher surface tension were applied for membrane absorption process to improve the system stability.Six kinds of amino acid salts were selected among twenty common amino acid salts according to the solubility in water and then tested for the membrane absorption and heat regeneration experiments.The results showed that solution of L-proline potassium,glycine potassium and L-arginine potassium had faster absorption rate than MEA,while L-arginine potassium,L-serine potassium,L-alanine potassium and sarcosine potassium had faster regeneration rate than MEA solution.For the overall performance evaluation,L-arginine potassium,L-alanine potassium,L-serine potassium and sarcosine potassium got higher scores than MEA solution.In order to improve the membrane hydrophobicity and avoid membrane wetting,commercial PVDF membrane was modified by grafting organosilanes.The effects of alkali treatment time,grafting time and organosilane kinds on the membrane surface contact angle were investigated.SEM and MIP results showed that there was no significant change in the pore structure.FTIR results showed Si-O-Si peak after surface modification.The comparison of CO2 membrane absorption performance using the original and modified membrane was conducted.The CO2 absorption rate changed little,while the modified membrane contactor exhibited better stability and durability for long-term running.Aiming to reduce solvent loss due to volatility,vacuum membrane distillation(VMD)system was developed to regenerate ammonia from ammonia wash water.The key operation parameters were systematically investigated.The experimental results showed that increasing liquid flow rate,solution temperature and reducing pressure in the permeate side could help to improve total transmembrane flux and overall mass transfer coefficient.The increase of CO2 loading in feed ammonia solution would decrease the ammonia removal rate.The ammonia recovery could be up to 95.6%with a 120 minutes continuous circulation time,which showed that VMD system had potential to recover ammonia and solve the solvent loss problem.In terms of energy reduction,a membrane evaporation system based on PBI film was developed for recovering latent heat from the hot lean CO2 solution.The effects of key operational parameters(i.e.evaporation temperature,sweeping gas flow rate and liquid flow rate)were systematically investigated.The results showed that both vapor flux and recovered heat flux had exponential increases when the evaporation temperature increased.Sweeping gas flow rate and liquid flow rate had limited effects on both mass and heat transfer in membrane evaporation process.In order to evaluate the feasibility of the membrane evaporation system,the CO2 chemical absorption process with capture scale of 1 million ton CO2/year(including membrane evaporation system)was simulated with the help of Aspen Plus.The results showed the system with membrane evaporation could reduce reboiler duty,but increase the power of compressor for the evaporated steam.The reduction of generating efficient due to the installation of carbon capture system would be reduced with the help of membrane evaporation system.From the economic analysis,the addition of membrane evaporation system would increase the total investment cost a bit,but reduce the annual operating cost.The CO2 capture cost could be reduced from the basic scheme of 237 yuan/t CO2 to a minimum of 218.8 yuan/t CO2,with the drop of 7.7%.