Removal of Co2 from Flue Gas with Ammonia Solution in a Packed Tower

Ammonia was used for the major absorbent while sodium hydroxide was used as a reference in either a bubble column (semi-batch reaction) or a packed tower (continuous reaction) to absorb CO2 in this study. In the bubble column, bubbling time as a function of temperature, pH, and dissolved inorganic carbon concentration in the solution was measured to see the characteristics of the reaction and the capacity of ammonia with regard to CO2 absorption. Its CO2 removal efficiency (RE) and/or absorbent utilization (AU) in the packed tower were quantified as a function of packing type, concentration/pH of absorbent, concentration of simulated flue gas (CO2, O2, SO2, NO), gas flow rate, and liquid/gas ratio. Additionally, an empirical formula obtained, via the analysis of a multiple regression, was used to relate CO2 RE and the major operation parameters. In the bubble column, the capacity of ammonia on CO2 absorption is 1.4 kg CO2 kg -1 NH3 in an exothermic reaction. In the packed tower, the optimum number of packing layers is 21 layers in an orderly arrangement with a reaction length of 220 mm. In addition, the optimum CO2 RE and AU are 93 and 19%, respectively, at a given condition. After a multiple regression, the empirical formula indicates four major parameters responsible for CO2 removal, namely ammonia concentration, CO2 concentration, gas flow rate, and initial pH of ammonia solution. The results of this work are feasible for practical application with regard to CO2 absorption with ammonia in a packed tower. *Corresponding author Email: [email protected] INTRODUCTION The atmospheric concentration of CO2 has increased from 280 ppm for the years 1000-1750 to 368 ppm in the year 2000, while the global mean surface temperature increased by 0.6 ± 0.2 oC over the 20th century, as described in the IPCC third assessment report [1]. It is thus necessary to reduce greenhouse gas emissions, and a number of technologies to mitigate of CO2 emission have been developed [2]. For major industrial CO2 emission sources, such as fossil fuel power plant stacks, these include absorption by liquids, adsorption by solids, extraction by refrigeration, advanced power cycle and the use of alternative energy sources. The removal of CO2 from a gas mixture by washing with alkaline solutions is one of the most widely practiced industrial gas-absorption processes [3-5], and the performance and capacity of ammonia have been reported to be better than those of monoethanolamine (MEA) in a batch study [3]. Several possible pathways of CO2 absorption with ammonia have also been proposed as follows [6]: CO2(g) + 2NH3(g) ↔ NH2COONH4(s) (1) NH2COONH4(s) + H2O(g) ↔ (NH4)2CO3(s) (2) CO2(g) + 2NH3(aq) ↔ CO(NH2)2(s) + H2O(g) (3) CO2(g) + 2NH3(aq) → NH4(aq) + NH2COO(aq) (4) CO2(g) + 2NH3(aq) + H2O(g) ↔ (NH4)2CO3(s) (5) CO2(g) + NH3(aq) + H2O(g) ↔ NH4HCO3(s) (6) CO2(g) + 2NH3(aq) + H2O(l) ↔ (NH4)2CO3(s)(aq) (7) CO2(g) + NH3(aq) + H2O(l) ↔ NH4HCO3(s)(aq) (8) According to the above reactions, the complex mechanisms of CO2 absorption with ammonia are due to several reactants in the three-phase process. Therefore, it is difficult to analyze the recovery of carbon and nitrogen due to the multiple reactions, including volatilization (NH3(g)), dissolution, and condensation (NH2COONH4(s), (NH4)2CO3(s), CO(NH2)2(s), and 2 J. Environ. Eng. Manage., 20(1), 1-7 (2010)