Experiments Study On The Removal Of CO₂from Flue Gas Of Coal Fired Power Plants Via Membrane Absorption

As we all know, the greenhouse effect cause global warming is the main environmentalproblems that mankind confronts, CO₂is the main greenhouse gas that contributes togreenhouse effect. With the development economy and the increase of demand of Power, theamount of CO₂emission becomes larger. As the main focus on emissions source, the amountof CO₂emission from plants using fossil fuels especially coal fire accounts for80%of all theemission caused by human activity. So it is very important that study on the separation of CO₂coming from coal-fired flue gas to control the global warming and greenhouse effect.Membrane absorption is the new technology to separate CO₂. And it possesses the benefitequipment compactness related to membrane separation and the benefit of process selectivelyrelated to the chemical absorption process. It indicates that it has a great potential for CO₂separation.First, the solutions with polypropylene membrane contact angle was measured andevaluated, The performance of five kinds of single absorbing solutions methyldiethanolamine（MDEA）, diethanolamine（DEA）, ethanolamine（MEA）, sodium glycinate（SG）, potassiumglycinate（PG） and three kinds of hybrid absorption solutions MDEA+MEA, MDEA+PG,MDEA+SG were studied to evaluate the performance of each absorbing solution matchingpolypropylene membrane. The results showed that the performance of single absorbingsolutions matching polypropylene membrane was as follows: PG, MEA, DEA, SG, MDEA;and the performance of hybrid absorbing solutions matching polypropylene membrane was asfollows: MDEA+PG, MDEA+MEA, MDEA+SG.A bench-scale hollow fiber membrane contactor experimental for CO₂separation fromsimulated flue gas, which used the central distribution pipe, was designed and built up.Twokinds of gas and liquid flow chat were contrasted. The results showed that the removalefficiency of flue gas flowing in the membrane lumen is10%³0%higher than removalefficiency of flue gas flowing out of the hollow membrane. Based on that, Using aqueousMDEA, MEA, DEA, SG and PG as absorption solutions, the separation experiments weretested. Effects of the species and concentration of absorbents, flow rate, the flow rate of fluegas and the content of CO₂in the flue gas, as well as the solution temperature on the removalefficiency and mass transfer rate were studied. It is found that species of absorbents has greatinfluence up on the mass transfer velocity and the removal efficiency. As the liquid velocityincreases the mass transfer and the removal efficiency increases. As the gas velocity andcontent of CO₂increase, the mass transfer increases, while the removal efficiency has decreased much.2mol/L was the appropriate concentration and40℃was the propertemperature. At last the results showed that under the same operating conditions, theabsorption performance of each single absorbent was as follows:PG>SG>MEA>DEA>MDEA.On the base of previous experiments, the CO₂absorption of the hybrid absorbents whichadding MEA, SG or PG to aqueous MDEA as addition agents were tested. Effects of thespecies and concentration of absorbents, flow rate, the flow rate of flue gas and the content ofCO₂in the flue gas on the removal efficiency and mass transfer rate were studied. It is alsonoticed that species and concentration of addition agents has great influence up on the masstransfer velocity and the removal efficiency. However, when addition agent concentrationswere given，the absorption of CO₂did not increase with the increase of MDEAconcentration.As the liquid velocity increases the mass transfer and the removal efficiency increase. As thegas velocity and content of CO₂increase, the mass transfer increases, while the removalefficiency has decreased much. Putting little MEA, SG or PG greatly increases the CO₂removal efficiency. It means that the absorption performance of blended absorption solutionsis great better than single aqueous MDEA. At last the results showed that under the sameoperating conditions, the absorption performance of each hybrid absorbents was as follows:MDEA+PG> MDEA+MEA> MDEA+SG.At the end of the thesis，the singular cycle experiments of all the single absorbents andhybrid absorbents were tested. Experimental results showed that under the same operatingconditions（the liquid and gas temperature was in30℃, the flow rate of flue gas was0.287m/sand the liquid flow rate was0.035m/s,the liquid volume of cyclic was6L and the desorptiontime was about30minutes）, the absorption performance of above-mentioned five singleabsorbent was as follows: PG>SG>MEA>DEA>MDEA. When under the conditions of2mol/L PG concentration, at the given flow rate （gas flow rate was0.144m/s, liquid flow ratewas0.035m/s）, the temperature of liquid was30℃, the removal efficiency could reach99.49%. MDEA can maintain a metastable removal efficiency in a long runtime because ofit’s large absorption capacity. It also showed that under the same operating conditions（theliquid and gas temperature was in35℃, the flow rate of flue gas was0.287m/s and the liquidflow rate was0.035m/s,the liquid volume of cyclic was6L and the desorption time was about30minutes）, the absorption performance of above-mentioned three hybrid absorbents whichbased on MDEA was as follows: MDEA+PG> MDEA+MEA> MDEA+SG. And whenunder the conditions of1mol/L MDEA+0.5mol/L PG combination concentration, at the givenflow rate （gas flow rate was0.144m/s, liquid flow rate was0.035m/s）, the temperature ofliquid was35℃, the removal efficiency could reach97.22%.