Study On CO₂ Chemical Absorption Technology

Research on the CO₂ emission control has been a worldwide topic.Chemical absorption technology is thought as one of the most prospective CO₂ capture method for application.In this paper,effect of various factors(absorber types,liquid/gas fluid dynamics conditions,equipment conditions and the impact of SO₂) were studied.At the mean time,engineering design and economic analysis of CO₂ capture system was done.First,typical single absorbents(MEA,PZ and MEDA) for CO₂ capture were tested in experimental packed tower.It was found that fluid phase mass transfer resistance is the controlling factor,and absorbent consistency,temperature as well as liquid-gas ratio are critical factors;It also found that gas phase mass transfer resistance has less impact on absorption than fluid phase,the removal efficiency will be decreased by increasing CO₂ consistency or empty tower velocity;Besides,as increasing gas-fluid contact time and area can accelerate mass transfer,so removal efficiency increase with the increasing of height-diameter ratio or packing specific area.In order to improve absorption efficiency and reduce regeneration energy consumption, three typical blended absorbents(MEA/MDEA,MEDA/PZ and DEA/AMP) for CO₂ absorption were tested.The research focused on blended concentration and optimal operating parameters of the blended absorbents.Results showed that absorption capacity of blended absorbents obey the following rule:10%MEA+2%MDEA solution＞10%MDEA+4%PZ solution＞10%DEA+2%AMP solution;the activation of addition absorbents keep these rules:there is a negative correlation between the concentration of MDEA and MEA/MDEA(Best concentration 2%);PZ has a positive correlation with MDEA/PZ;AMP and DEA/AMP are related,but the chemical enhancer is bellow the average value.For MEA/MDEA and DEA/AMP,the best temperature of CO₂ absorption is about 50℃,while MDEA/PZ solution is insensitive to temperature(＜1.4%),so the better performance temperature is about 40℃.Three blended absorbents are all related to main concentration,liquid-gas ratio,height-diameter ratio and packing specific area.As a novel and effective method,removing CO₂ by ammonia scrubbing has drawn much attention.The experiment using ammonia for CO₂ absorption was also tried.Compared to other absorbents,it demonstrated the best performance,even in lower concentration it could also get better removal results.As ammonia was sensitive to the variety of temperature,its best effect emerged at 35℃;When liquid-gas ratio below 8.3～12.5L/m³,10 percent of ammonia absorbed most,while above 30L/m³,the removal efficiency decreased,equivalent to the performance of 10%MEA.The result showed that increasing liquid-gas ratio will accelerate the evaporation of ammonia;the absorption efficiency of ammonia has a positive correlation with height-diameter ratio and packing specific area,and the better height-diameter ratio is about 22.5.This article also focused on the effect of SO₂ concentration in flue gas on absorbents(MEA, ammonia,MEA/MDEA,MEDA/PZ and DEA/AMP).It was found that absorption of CO₂ is influenced more with SO₂ concentration increasing.SO₂ has the minimize impact on the effect of MEA absorbent than the others,the tolerance of the SO₂ is showed as follows:MEA absorbent＞ammonia absorbent＞DEA/AMP absorbent＞MDEA/PZ absorbent＞MEA/MEDA absorbent.As the concentration of SO₂ was small,the disturbance it brought can be ignored; Compared to other aqueous absorbents,SO₂ influences ammonia least,so SO₂ and CO₂ removal simultaneously should be realized.In addition,the impact of SO₂ can be reduced while increasing absorbent concentration or liquid-gas ratio,when the MEA concentration was 30 percent or liquid-gas ratio was16.7L/m³,the impact of SO₂ can be neglected(＜0.5%).At the end of the thesis,the designing and economic analysis was made with different CO₂ removal technology,chemical absorption and membrane absorption technology.A 75t/h coal-fired plant was chosen as the reference.The results showed that total investment of chemical absorption technology was lower but hard to maintain;the operational cost of membrane absorption was lower but was influenced by its critical life to some degree while CO₂ recovery cost will be higher.Steam price fluctuating would make both technology extremely competitive.