Research On Heat Transfer Characteristics And Enhanced Condensation For Steam With High Concentration CO₂

Climate change caused by greenhouse gases has become a critical issue ofworldwide concern. Among greenhouse gases, CO2accounts for a high proportion ofthe amount in the atmosphere, and is responsible for70%of the global warmingeffect.Although renewable energy resources have been exploited, the world willremain largely dependent on fossil fuels for the next decades. In this case, CO2capture and storage (CCS) has been proposed as a sustainable technology to mitigategreenhouse gases by the Inter-governmental Panel on Climate Change (IPCC). Tomeet the burial requirement, the CO2must be separated and purified after capturing.Whether the well developed amine absorption method or the low-costcalcium-looping method, the gas mixture containing CO2must be further condensatedfor storage. Therefore, the research of steam condensation heat transfer with highfractional CO2gas has important application value for CO2purification.In this paper, the experimental apparatus was designed and set up for the filmcondensation heat transfer of CO2-steam mixture on a horizontal tube and a verticalplate with different CO2concentration and vapor velocity. The results of vertical plateexperiments showed that the heat transfer coefficient and heat flux decrease with theincrease of CO2mass fraction. There was a critical concentration at different vaporvelocities under which the heat transfer characteristics changed greatly, while above itthe variation of results became flat. This critical value was64.2%for vapor velocityof0.4m/s,75%for0.8m/s and1.2m/s. Based on the basic equations for laminar filmcondensation of binary vapor from Fuijii, the surface subcooling-ratio of gas/liquidfilm resistance curves and the surface subcooling-interface temperature curves wereused to explain the experimental phenomenon.The experimental relations of CO2concentration and surface sub-cooling wereproposed basing on21experimental data can improve prediction uncertainty less than±20%. The heat transfer coefficient howas inversely proportional to the ΔT0.55at vaporvelocity of0.43m/s, ΔT0.47at0.74m/s and ΔT0.43at1.2m/s. By comparing the resultsof horizontal tube and vertical plate, there was a cross point where the heat transfercoefficient of horizontal tube was higher than vertical plate at small surfacesub-cooling, but lower at high surface sub-cooling. This surface sub-coolingintersection decreased with the increase of CO2concentration and vapor velocity. The condensation heat transfer of CO2-steam mixture on V-shaped stainless steelvertical fluted surface was investigated. It is shown that the heat transfercharacteristics of two V-shaped vertical fluted surfaces were superior to flat platewithin the CO2concentration range of80%to94%. The heat transfer coefficient of1*2mm fluted surface increased8.51%~15.4%than flat surface. The heat transfercharacteristics of1*2mm fluted surface were superior to the0.5*1mm within CO2concentration range of20%~94%, so the proper increase of the fin spacing and heightcan improve the heat transfer process.Through the prepared titania-fluoroalkysilane coating on stainless steel surface,and the super-hydrophobic surface made from copper plate with carbon chains byelectrodeposition method, the condensing surface shown superhydrophobicity anddropwise condensation character. But this condensation mode was not steady due tothe monolayer was more affected by the higher temperature and vapor velocity. Thecondensation experiments with high concentration CO2are conducted on gold platedsurface. The results reveal that the heat transfer characteristics were related tocondensation patterns. Parts of the condensing surface were covered by liquid film asa result of drop accumulation under larger condensed water, and the heat transfercoefficient raised about15%than filmwise condensation. As the CO2concentrationincreased and the surface sub-cooling decreased, the droplet distributed randomly andthe heat transfer corfficient increased. The movement of droplet growing,congregating and falling was more obvious when CO2concentration over87%. Theaverage heat transfer coefficient raised31%than filmwise condensation.