The Global Environmental Engineers, 2015, 2, 23-28 23 The Regeneration of Ammonia Solution by Calcium Hydroxide for Carbon Dioxide Absorption from the Flue Gas Hsin Ta Hsueh1, Tsung Wen Chien1, Tzu Hsing Ko2, Teng Te Huang1 and Hsin Chu1,* 1Department of Environmental Engineering and Sustainable Environment Research Laboratories, National Cheng Kung University, Tainan, Taiwan 701, Republic of China 2Department of Tourism Affairs, and Dean of Academic Affairs, Tzu-Hui Institute of Technology, Pingtung 926, Taiwan Abstract: The tests of this study were conducted by the semi-continuous flow experiments to absorb the carbon dioxide (CO2) gas in a bench-scale spraying column reactor. The absorption capacity and the regeneration efficiency of the absorbed ammonia (NH3) solution were determined. In this study, the maximum regeneration efficiency is 68.4% as the concentration of NH3 solution is 1% and the ratio of calcium hydroxide (Ca(OH)2) and CO2 is 1 as well. Furthermore, the absorption capacity of the NH3 solution decreases from 1.67 to 0.27 kg-CO2/kg-NH3 after regenerating four times due to the 30-38% loss each time. Regarding the regeneration kinetics between absorbed products and Ca(OH)2, the comparative degree of covered Ca(OH)2 during the reaction was recognized after fitting by a surface coverage model. Finally, the ammonium bicarbonate (NH4HCO3) reduced by Ca(OH)2 to calcium carbonate (CaCO3) solid and NH3 solution in the regeneration reactions was observed by scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS) pictures and X-Ray Diffraction (XRD) analyses. Although the NH3 solution can be regenerated by Ca(OH)2 effectively in this study, the overall benefit of this process should be estimated further in the energy aspect. Keywords: Carbon dioxide, absorption, regeneration, ammonium solution, calcium hydroxide. 1. INTRODUCTION regeneration of reacted ammonia solution is one of important topics for promoting cost-down and avoiding Chemical absorption of carbon dioxide (CO2) by secondary pollutions. The regeneration reactions in this ammonia (NH3) solution is proven as an effective study were conducted as equation from (3) and (5) method for CO2 elimination [1, 2]. Wang et al. (2007) [7, 8]. concluded that NH3 solution scrubbing is an effective way to reduce the anthropogenic CO2 emissions via life Ca(OH )2(S) + (NH 4 )2 CO3(S)(aq) ! cycle assessment [3]. The reactions between CO2 and (3) CaCO + 2NH + 2H O NH3 solution have been conducted as equations (1) 3(S) 3(aq) 2 (l) and (2) mainly [1, 2]. Several operating parameters Ca(OH )2(S) + (NH 4 )HCO3(S)(aq) ! were tested to determine the absorption kinetics of (4) CaCO NH 2H O these two reactants [4]. The reaction between NH3 3(S) + 3(aq) + 2 (l) solution and CO2 from fossil fuel revealed more Ca(OH ) + CO ! CaCO + 2H O (5) significant environmental beneficial than amine system 2(S) 2(g) 3(S) 2 (l) while NH HCO as synthetic N-fertilizer [5]. 4 3 The target of this study is to investigate both Furthermore, NH solution could be obtained from NH 3 3 absorption and regeneration capacities of NH solution. rich wastewaters and taken as a useful and free 3 In addition, the kinetic model and products for absorption solvent to capturing of CO for upgrading 2 regeneration of NH solution by Ca(OH) were also biogas [6]. 3 2 estimated. CO + 2NH + H O ! (NH ) CO (1) 2(g) 3(aq) 2 (l) 4 2 3(S)(aq) 2. MATERIALS AND METHODS CO + NH + H O ! (NH )HCO (2) 2(g) 3(aq) 2 (l) 4 3(S)(aq) All tests of this study were carried out in a bench- Although ammonia bicarbonate is one of important scale spraying column system shown in Figure 1. The synthetic N-fertilizer sources, however, the uncertain system was divided into three parts, including a flue gas simulation system, an absorption reactor, and a impurities of NH4HCO3 product is worthy to be gas sampling and analyzing system (Figure 2). Flow concerned while the CO2 comes from flue gas and/or rates of pure N and pure CO (San Fu) were controlled NH3 solution comes from wastewater. Therefore, the 2 2 by mass flow meters. CO2 was diluted by N2 in a plug flow mixer and further diluted by the mass flow- *Address correspondence to this author at the Department of Environmental Engineering and Sustainable Environment Research Laboratories, National controlled compressed air in another plug flow mixer to Cheng Kung University, Tainan, Taiwan 701, Republic of China; Tel: 886-6- the desired concentrations and O2 contents. The 2080108; Fax: 886-6-2752790; E-mail; [email protected] E-ISSN: 2410-3624/15 © 2015 Avanti Publishers 24 The Global Environmental Engineers, 2015, Vol. 2, No. 1 Hsueh et al. HOOD GAS OUTLET WATER SATURATED TANK LIQUID INLET COOLER MIXER GAS INLET CO2/CO/O2 LIQUID OUT LET ANALYZER N2 CO2 AIR PUMP FILTER Figure 1: Diagram of bench-scale experimental setup. simulated flue gas was then heated to 50°C by an CO2/CO/O2 analyzer (California Analytical Instrument, electrical heating tape before entering into the Inc., Model 300 type). absorption reactor. The NH3(aq) (Merck, purity 25%) were used to )*+$420./0 prepare the scrubbing liquor. The Ca(OH)2 (J.T. Baker, purity >98%) and CaO (J.T. Baker, purity >98%) were .,12,3$,-./0 added into CO2-saturated scrubbing liquor to regenerate the ammonia. +67*8,-)$-499./ The reactor that performed the scrubbing liquor & % regeneration tests was the same as the apparatus $ ( " # ' used in our previous study [4]. The crystalline solids in )*+$,-./0 the solutions for absorbent regeneration experiments were determined by scanning electron microscope & % $ # (SEM), Energy-dispersive X-ray spectroscopy (EDS) 5"#$%& " ! pictures and X-Ray Diffraction (XRD) analyses [1]. The operating conditions are listed in Table 1 and Table 2 in details. .,12,3$420./0 Table 1: Operating Conditions of Absorption Experiments Parameters Values CO2 concentration in the simulated flue gas (%) 15 Figure 2: Bench-scale absorption reactor. The flowrate of simulated flue gas (L/min) 4 The heated simulation flue gas entered a saturator The temperature of simulated flue gas (°C) 50 first to make sure it was saturated with water vapor. NH3 (aq) concentration (%) 1, 3, 5 Then the simulated flue gases were flow through the The flowrate of scrubbing liquor (mL/min) 200 absorption reactor or bypass to be analyzed by the The temperature of scrubbing liquor (°C) 35 The Regeneration of Ammonia Solution by Calcium Hydroxide The Global Environmental Engineers, 2015, Vol. 2, No. 1 25 Table 2: Operating Conditions of Regeneration Experiments Set CO2 (%) NH3(aq) Stoichiometric ratio Regeneration time 1 15 1 1/1 100 sec ~ 100 min. 2 15 1, 3, 5 1/1 20 min. 3 15 1 0.75/1, 1/1, 1.5/1 20 min. 4 15 1 1/1 Regeneration cycles (1~4) 3. RESULTS AND DISCUSSION + " CO2(g) + 2NH 3(aq) ! NH 4 + NH 2COO(aq) (9) The definitions of absorption capacity (AC), In fact, ammonium carbamate is approved for use regeneration efficiency (RE), and relative regeneration as an inert ingredient in aluminum phosphide pesticide efficiency (RRE) are listed as below. formulations and used in the aluminum phosphide CO absorbed formulation to reduce the potential fire hazard of the AC (kg CO /kg NH ) = 2 (6) phosphine in US Environmental Protection Agency 2 3 NH added in the absorption solution 3 (EPA) [10]. This approval induces ammonium CO Absorption Capacity of regenerated absorbent carbamate reusable if it can be purified further. RE (%) = 2 (7) CO Absorption Capacity of fresh absorbent 2 3.2. Effect of Ca(OH)2/CO2 Stoichiometric Ratio AC RRE (%) = i+1 i = 0, 1, 2, 3 (8) As shown in Figure 4, the regeneration efficiency is AC i better as Ca(OH)2/CO2 is 1 than 0.75. However, the 3.1. Effect of Ammonium Concentration regeneration efficiency decreases as Ca(OH)2/CO2 increases from 1 to 1.5. It probably results from the As shown in Figure 3, the regeneration efficiency increase of masked reaction surface and limitation of decreases when ammonia concentration increases. It mass transfer after adding excessive calcium may result from the irreversible reactions exist and hydroxide. Furthermore, the amounts of calcium ammonia volatilizes from absorption solution in the hydroxide added into the solutions increase as the duration of absorption and regeneration tests. Except ammonium concentration increases from 1% to 5% if the ammonium carbonate and ammonium bicarbonate, Ca(OH)2/CO2 ratio is the same, the mask effect also the reaction between CO2 and ammonium solutions becomes more significant due to the limitation of mass also may produce ammonium carbamate transfer. In addition, the existence of irreversible (NH2COONH4) according to Eq. (9). Song et al. (2014) reactants is probably another barrier for generation at indicated that the concentration of ammonium higher Ca(OH)2/CO2 Stoichiometric Ratio (ex. 1.5). carbamate increases as the ammonia concentration Consequently, the overall effect of irreversible increases at a given inlet CO2 concentration [9]. If reactants, masked reaction surface and limitation of ammonium carbamate cannot be regenerated by mass transfer results in the regeneration performance. Ca(OH)2 to ammonia, the regenerated ammonium 100 solutions will lose some CO2 absorption capacity. NH3=1% NH3=3% 100 80 NH3=5% 90 CO2 = 15% 60 NH3 = 1%, 3%, 5% 80 L/G = 50 mL/L Ca(OH)2/CO2 = 1/1 40 70 60 Regeneration efficiency (%) 20 50 Regeneration efficiency (%) 0 40 0.0 0.5 1.0 1.5 2.0 Stoichiometric ratio 30 1% 3% 5% Figure 4: Effect of stoichiometric ratio on the regeneration Ammonia concentration efficiency.