An experimental investigation on high temperature CO2 capture by hydrotalcite sorbent Aamir Hanif Soumen Dasgupta Anshu Nanoti and M O Garg CSIR Indian Institute of Petroleum Dehradun 248005 Abstract Rising levels of CO2 in the atmosphere due to burning of fossil fuel have been recognized to be the main contributor of global warming and associated climate change phenomenon 1 Fossil fuel combustion for power generation is the major source of increased CO2 levels in the atmosphere 2 3 There are two strategies for the CO2 capture from the power plants viz post combustion capture and pre combustion capture Post combustion is an end of pipe treatment of the flue gases for removal of the CO2 present prior to discharge through the stack 4 CO2 levels are generally in the range of 5 to 15 depending on the type of fuel undergoing combustion and the CO2 must be removed from mixtures with N2 O2 moisture and SOx NOx if present In pre combustion approach gasification of feed stock Coal produces a hot multi component gas stream containing acidic gases like H2S CO2 along with H2 Conventional solvent based technology for removal of acid gases operates at low temperatures hence the gas cleanup train requires cyclic heating and cooling steps to produce clean H2
These temperature swings lead to over all lower thermal efficiency of the process 5 Developing an alternative adsorption based technologies using adsorbents which can work at high temperature for CO2 and H2S removal will be a step change towards increasing the thermal efficiency of the process Various materials such as calcium oxide 6 7 lithium zirconates 8 9 other metal oxides 10 and hydrotalcites 11 13 have been tried as adsorbents in precombustion approach Hydrotalcites a class of anionic clays have been evolved as the best choice due to most of the desirable properties such as lower energy of regeneration retention of capacity after multiple cycles and suitable kinetics of carbon dioxide adsorption 14 15 The hydrotalcites however have lower carbon dioxide capacity compared to other high temperature sorbents but continuous research efforts have shown that carbon dioxide capture capacity can be improved by various manipulations in the chemical composition 16 17 In this paper we present experimental work on development of high temperature CO2 selective hydrotalcite adsorbents for the removal of CO2 from a gas feeds representing pre combustion process streams Effect of different synthesis techniques such as microwave and ultrasonic for hydrotalcite adsorbent on CO2 adsorption capacities and results are reported and compared with hydrotalcite synthesised through conventional co precipitation method The promotional effect of the potassium carbonate impregnated hydrotalcite is also studied on CO2 loading capacities at different adsorption temperatures Hydrotalcites synthesised by different routes were characterised by surface area and pore size distribution X Ray Diffraction XRD and Scanning Electron Microscopy SEM These adsorbents were further evaluated by measuring their equilibrium loading capacities in a gravimetric microbalance and column dynamic studies such as breakthrough measurements and
Pressure Temperature Swing Adsorption PTSA cycle studies were carried out in a single column micro adsorber unit The results indicate that the equilibrium carbon dioxide capacity of unimpregnated and potassium promoted hydrotalcite synthesised through microwave method is highest compared to hydrotalcites synthesised by co precipitation and ultrasonic method Breakthrough carbon dioxide capacities observed are also higher with microwave synthesised hydrotalcite The dynamic CO2 capacity values obtained with microwave synthesised hydrotalcites are comparable with the best literature reports for HTLC based sorbents for similar process application The observed results have been explained on the basis of difference in crystallinity BET Surface area and basic site strength of the samples synthesised by the different methods PTSA experiments show that the hydrotalcites exhibits very good cyclic adsorption and desorption capacity over multiple cycles and can be regenerated at slightly higher temperature than adsorption temperature thus avoiding large temperature swing as typically experienced in solvent based process for CO2 capture in pre combustion approach References 1
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