Capture Of Carbon Dioxide From Simulated Power Plant Flue Gas Based On Clathrate Hydrate Technology

There are increasing concerns for greenhouse gas recently, especially for carbon dioxide emissions mitigation of the influence of global warming and climate change on human activities. The emissions of carbon dioxide primarily result from the combustion of fossil fuels and the main approach for CO2 emissions is carbon capture and sequestration. Hydrate-based capture of CO2 is a new separation technology, which is based on the differences of hydrate phase equilibrium conditions of CO2 and N2 and selectively engaged CO2 into the cages of hydrate. The technology is of high efficiency, simpleness and conveniency and so on. However, hydrate-based technology with. THF as promoter is being at the initial stage, thus there are many problems need to be investigated, such as rigorous temperature and pressure, low reaction rate and high energy consumption. In this study, tetra-n-butyl ammonium bromide (TBAB), tetra-n-butyl ammonium fluoride (TBAF) and cyclopentane were chosen as additives for hydrate formation. The phase equilibrium conditions of CO2, simulate flue gas (CO2/N2) hydrate were determined with TBAB and TBAF, respectively. The dynamics character and separation efficiency of CO2/N2 mixture hydrate were investigated with TBAB, TBAF and cyclopentane for developing high efficient and low energy consumption hydrate-based capture of CO2.The dynamics character of simulate flue gas (CO2/N2) hydrate were studied with TBAB or TBAF at 4.5℃, such as, the effects of the feed pressure, gas liquid volume ratio, TBAB aqueous concentration and CO2 concentration in the feed on the hydrate formation. The higher feed pressure, CO2 or TBAB aqueous concentration, or the lower gas liquid volume ratio were set, the bigger driving force of hydrate formation and the rate of pressure drop could be reached. The hydrate rate constant and space velocity of reactor were calculated and compared with TBAB and TBAF, respectively. The hydrate formation rate constant reached the maximum of 2.82×10-7 mol2/(s·J) with TBAB while it was 8.26×10-7 mol2/(s·J) with TBAF, and the maximal space velocity of hydrate reactor was 14.36 h'1 with TBAB while it was 25.96 h-1 with TBAF.The process of CO2 capture from simulate flue gas (CO2/N2) by formation of TBAB and TBAF hydrate were studied at 4.5℃, and the effects of the feed pressure, temperature, TBAB or TBAF concentration and CO2 concentration in the feed on the CO2 separation efficiency were also investigated. The results showed that the maximum of CO2 separation factor was 9.65 with TBAB and that was 36.98 with TBAF. CO2 could be enriched from 16.60 mol% to-90.15 mol% under the medium feed pressure by three stages hydrate formation process with TBAB, meanwhile CO2 separation factor could be increased with the addition of a little methanol or acetone. CO2 could be enriched from 16.60 mol% to 90.40 mol% under the low feed pressure by only two stages hydrate formation process with TBAF. The process of CO2 capture,via hydrate formation could be carried out at ambient temperature (20.1℃) by increasing the TBAF aqueous concentration.The CO2 capture via hydrate formation with cyclopentane as an additive was also studied. The hydrate formation rate and separation efficiency were investigated at 8.1℃in the presence of the mixture of cyclopentane and water or cyclopentane in water emulsion. The hydrate formation rate was represented by the temperature rise due to the exothermic nature of the hydrate formation. It was found that the hydrate formation rates with cyclopentane in water emulsion were at greater rates than those without emulsion. While CO2 separation factors with cyclopentane in water emulsion were lower than those without emulsion under the same conditions.The economic analysis of CO2 capture from simulate flue gas via two stages TBAF hydrate formation was made. The energy consumption of CO2 capture was estimated and was compared with those for membrane separation and monoethanolamine (MEA) absorption. Based on Aspen flowsheeting simulation and estimation, as for a 1000 MW power plant the energy consumption of capture of CO2 via hydrate was 0.57 kW-h/(kg-CO2) or 2.05 MJ/(kg-CO2). The energy consumption of CO2 capture was mainly caused by the compression process which was 66.3% of entire process. Then the results were compared with two conventional separation processes:MEA amine based absorption separation and membrane separation processes. It was concluded that hydrate separation technology is competitive comparing with the chemical absorption for CO2 capture.