Energy Fuels 2009, 23, 5467–5473 Published on Web 09/11/2009
: DOI:10.1021/ef900615h
Characteristics of Alkaline Wastewater Neutralization for CO2 Capture from Landfill Gas (LFG) Ankur Gaur,† Jin-Won Park,*,† Jung-Hwa Jang,† Sanjeev Maken,‡ Joonjae Lee,† and Ho-Jun Song† †
Department of Chemical and Bio-molecular Engineering, Yonsei University, Seoul 120-749, South Korea, and ‡Department of Chemistry, Deenbandhu Chhotu Ram University of Science and Technology, Murthal-131039, Haryana, India Received June 17, 2009. Revised Manuscript Received August 24, 2009
In this study, a lab-scale apparatus is used for removal of carbon dioxide (CO2) from a landfill gas (LFG) mixture using a chemical absorption technique. Alkaline wastewater obtained from a nearby industry is used as a chemical absorbent, and a comparison is performed with liquid ammonia solution and sodium hydroxide (NaOH) solutions. The absorptive capacity of the liquid solvents is obtained by the respective “equilibrium breakthrough curves”. The maximum CO2 loading for 0.1 M NaOH, 0.2 M NaOH, wastewater, and 2, 5, and 10 wt % ammonia are 0.41, 0.6, 1.7, 0.7, 1.6, and 2.6 mol of CO2/L, respectively. Alkaline wastewater proved to be a highly effective CO2 absorbent because of its high alkalinity, which occurred from the presence of ammonia. Raman spectroscopy was conducted to confirm that the reactions occurring inside the wastewater followed a trend similar to aqueous ammonia. Methane purified from this process can be further used for different applications. Hypothetical prices are calculated when wastewater absorption is applied to the purification of LFG. LFG sites of 1 million and 5 million tons are considered, and the cost of pipeline gas generated (won/mmBTU) is estimated using simple economical relations.
pressure over membrane and is highly dependent upon the permeability of the component in the membrane material. For high methane purity, permeability must be high. The maximum pressure that membrane can withstand must be taken into consideration.9 The most popular process is separation of CO2 using chemical absorbents, such as monoethanol amine (MEA) and diethanol amine (DEA), which involves the formation of reversible chemical bonds between the solute and solvent. For regeneration of solvents, we need to break these bonds and, thus, need a high-energy input.11-15 Commercial adsorbents, such as silica, alumina, activated carbon, or silicates, can be used to remove CO2 and other impurities from LFG, but adsorption is generally accomplished at low temperature and high pressure. It is simple in design and easy to operate but a costly process with high-pressure drops and elevated heat requirement.16 A large amount of alkaline wastewater is generated in numerous industries, such as the beverage industry, dairy, slaughterhouses, and meat-processing plants, baking and confectionary industry, electroplating industry, paper and pulp industry, leather industry, textile industry, laundry and dyeing plants, cement and concrete industry, petrochemical industries, etc. There are numerous ways of treatment of these alkaline wastewaters, such as the
1. Introduction Landfill gas (LFG) consists of mainly CH4 and CO2, along with numerous toxic trace components. In LFG, CH4 is around 55-60% and CO2 is around 45% by volume. Separation of CO2 from LFG is important to increase the calorific value of LFG and to remove CO2 because it contributes to global warming, which is mentioned in previous works by Allen and Braithwaite,1 Brousseau and Heitz,2 and Themelis and Ulloa.3 LFG is burnt instead of being used as a fuel to decrease the impact of LFG on global warming. Numerous ways can be used to separate CO2 from LFG, such as absorption, adsorption, and membrane separation, presented previously by Kapdi et al.,4 Chakma,5 Yeh and Pennline,6 and Penny and Ritter.7 Each process has its own advantages and disadvantages. Physical absorption is one of the cheapest and easiest methods, which involves the use of pressurized water as an absorbent. The water scrubbing method is popular for CO2 removal from sewage-sludge-based biogas plants in Sweden, France, and the U.S.A. A number of experiments using the water scrubbing method for purification of biogas have been performed by Rasi et al.8 In membrane separation, transportation of each component is based on the difference in partial *To whom the correspondence should be addressed. Telephone: þ822-364-1807. Fax: þ82-2-312-6001. E-mail:
[email protected]. (1) Allen, M. R.; Braithwaite, A.; Hills, C. C. Environ. Sci. Technol. 1997, 31, 1054–1061. (2) Brousseau, J.; Heitz, M. Atmos. Environ. 1994, 28, 285–293. (3) Themelis, N. J.; Ulloa, P. A. Renewable Energy 2007, 32, 1243– 1257. (4) Kapdi, S. S.; Vijay, V. K.; Rajesh, S. K.; Prasad, R. Renewable Energy 2005, 30, 1195–1202. (5) Chakma, A. Energy Convers. Manage. 1997, 38, 851–856. (6) Yeh, J. T.; Pennline, H. W. Energy Fuels 2001, 15, 274–278. (7) Penny, D. E.; Ritter, T. J. J. Chem. Soc., Faraday Trans. 1983, 79, 2103–2109. (8) Rasi, S.; Lantela, J.; Vejianen, A.; Rintala, J. Waste Manage. 2007, 28, 1528–1534. r 2009 American Chemical Society
(9) Glub, J. C.; Diaz, L. F. Biogas Purification Process. Biogas and Alcohol Fuels Production; The Jp Press, Inc.: West Palm Beach, FL, 1991; Vol. 2. (10) Addicks, J.; Owren, G. A. J. Chem. Eng. Data 2002, 47, 855–860. (11) Lee, S.; Song, H. J.; Maken, S.; Park, J. W. Ind. Eng. Chem. Res. 2007, 46, 1578–1583. (12) Lee, S.; Choi, S. I.; Maken, S.; Song, H. J.; Shin, H. C.; Park, J. W.; Jang, K. R.; Kim, J. H. J. Chem. Eng. Data 2005, 50, 1773–1776. (13) Lee, S.; Song, H. J.; Maken, S.; Shin, H. C.; Song, H. C.; Park, J. W. J. Chem. Eng. Data 2006, 51, 504–507. (14) Lee, S.; Song, H. J.; Maken, S.; Yoo, S. K.; Park, J. W.; Kim, S.; Shim, J. G.; Jang, K. R. Korean J. Chem. Eng. 2008, 25, 1–6. (15) Song, H. J.; Lee, S.; Maken, S.; Park, J. J.; Park, J. W. Fluid Phase Equilib. 2006, 246, 1–5. (16) Pandey, D. R.; Fabian, C. Sep. Purif. Technol. 1989, 3, 143–147.
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: DOI:10.1021/ef900615h
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introduction of acids, especially sulfuric acid, hydrochloric acid, or nitric acid. The most common material for neutralization, such as sulfuric acid, lime, and limestone, has been used as industrially important materials. However, these materials are hard to handle because of their poisonous nature, storage problem, secondary contaminant availability, and salt formation in water, as well as the high lime or limestone content can affect the pump selection and increase the maintenance cost.17 To avoid these problems, this study selected CO2 for neutralization of alkaline wastewater. Neutralization using CO2 has advantages because of its low toxicity, no role in excessive acidification, low cost, thermal stablility, and water friendly substance.18 As mentioned earlier, LFG consists of around 45% CO2, which makes it an attractive CO2 resource. Thus, passing LFG through alkaline wastewater helps to remove large amounts of CO2 and, at the same time, neutralize the wastewater. Nowadays, scientists are showing an increasing interest in environment technology toward “pollution prevention” techniques rather than “end of pipe” solutions. The wastewater used in this study contains ammonia between 2 and 5 wt % and has a very high alkalinity compared to other wastewaters that were used in previous studies by Yeh and Bai.19,20 The absorption of CO2 gas in an alkaline solution is strongly affected by the pH of the solution. Thus, controlling the pH of the solution is important in controlling the absorption rates of gases. To demonstrate the function of the system and to put data into perspective, CO2 absorption by various sodium hydroxide solutions is included. This process is also quite economical because wastewater is readily available and can be neutralized at room temperature. In this work, the properties of the wastewater have been carefully studied before and after conducting the experiment in comparison to standard NaOH and ammonia solutions. Because the wastewater contains some amount of ammonia, the experiment is performed using ammonia as an absorbent. Three different ammonia concentrations of 2, 5, and 10 wt % are tested. The absorptive capacity of several ammonia solutions is investigated to make different comparisons. Ammonia is easily found in various raw or treated wastewaters. Ammonia is a better CO2 absorbent and displays a much higher CO2 loading as compared to regular CO2 absorbents, such as MEA and DEA.
Figure 1. Schematic diagram of the adsorption and absorption experimental apparatus.
structure, high adsorption capacity, and high degree of surface reactivity and is cheaply available. Granular activated carbon is used in this study, whose apparent density is around 0.4-0.43 g/ cm3 and the particle size is around 1.0-1.41 mm. The average pore diameter is between 14 and 18 A˚, and Brunauer-EmmettTeller (BET) surface area ranges from 1100 to 1200 m2/g. Activated carbon was obtained from Samchonli Ltd. Alkaline wastewater displayed a very high alkalinity and a great buffering capacity. Ammonia solutions of different concentrations were prepared using 25 wt % standard solution using simple stoichiometric calculations. The standard ammonia solution was obtained from Duksan Pure Chemical Co. Ltd. NaOH solutions (0.1 and 0.2 M) were prepared using the high-purity NaOH beads obtained from Sigma Aldrich. 2.3. Analytical Method. Raman spectroscopy measurements were performed in backscattering geometry with a JY LabRam fitted with a liquid-nitrogen-cooled detector. The spectra were collected under ambient conditions using the 514.5 nm line of argon-ion laser to compare the reaction similarity between ammonia and wastewater. Alkalinity of liquid samples was measured volumetrically using standardized sulfuric acid solution using end-point indicators (methyl green) and/or a pH meter. The chemical oxygen demand (COD) of the wastewater was calculated using a COD-digestion unit according to the open reflux method to know the quantity of oxidant species present in the wastewater. The conductivity was measured using the conductivity meter obtained from Hanna Instrument. The pH was measured using a standard pH meter obtained from Thompson Scientific. The total organic carbon (TOC) and inorganic carbon content of wastewater were measured using a TOC analyzer (Shimazu TOC5000A). 2.4. Apparatus and Procedure. The LFG was passed through the activated carbon to remove any gaseous contaminants and then first through a saturator and then through a reactor. The experimental setup is shown in Figure 1. The reactor contained 1 L of solution of different absorbents. A porous ceramic bubble sparger was used in the reactor to distribute the gas evenly in the solution. The gas flow rate of LFG was controlled by a gas flow meter obtained from MKP Industries. During the experiment, the reactor and saturator were placed in the water bath to maintain room temperature at around 293 K. The CO2 concentration in the outlet gas mixture was constantly analyzed by an infrared (IR) analyzer (KINSCO). A silica gel trap was used to remove the humidity from the gas phase and to protect the instrument. The pH of the liquid solvent was continuously measured using a pH meter (Orion 420 Aþ). The flow rate of LFG is kept at 1.5 L/min, controlled by mass flow controller obtained from MKP. In a typical experiment, 1 L of the liquid solvent was placed in the reservoir vessel and the LFG mixture consisting of a certain
2. Experimental Section 2.1. Raw LFG. The LFG was collected, stored, and transported to the laboratory from the Sudokwon landfill site, which is located in the west coast of Inchon and is the largest waste treatment site in Korea. As main components, the concentration of CH4 and CO2 are 47-55 and 45-52 vol %, respectively, and some poisonous compounds were present. LFG was compressed to the gas holder from the extraction well. The static pressure of the extraction well is around 0.2-0.3 kgf/cm2. The raw LFG was passed through the compressor, which was 3 horsepower, and compressed up to 50 kgf/cm2 in the gas holder. The moisture content of raw LFG was removed using the cyclone-type dehydrator. 2.2. Chemical Absorbents. Among various adsorbent materials, activated carbon was selected because it is a very versatile adsorbent, has a very large surface area, good microporous (17) Wastech Controls and Engineering, Inc. http://www.wastechengineering.com/papers/neutralization_chemicals.htm. (18) Linde Gas. http://www.lindegas.com/International/Web/LG/ COM/likelgcom30.nsf/repositorybyalias/pdf_neutralisation/6file/NeutralisationWasteWater_e.pdf. (19) Yeh, A. C.; Bai, H. U. Sci. Total Environ. 1999, 228, 121–133. (20) Bai, H.; Yeh, A. C. Ind. Eng. Chem. Res. 1997, 36, 2490–2493.
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Figure 3. Equilibrium breakthrough curve of 0.1 M NaOH, 0.2 M NaOH, and wastewater.
Figure 2. Change in pH with time for 0.1 M NaOH, 0.2 M NaOH, and wastewater because of CO2 absorption.
CO2 concentration was fed to the system. During the initial start of the experiment, the liquid absorbent was completely absorbing the CO2 from the fed mixture. As the absorption process progressed, the CO2 was continuously accumulated in the liquid absorbent, and after some time, CO2 started to evolve in the outlet stream. At the end of each run, the solution became completely saturated with CO2 and the concentration of CO2 in the outlet was equal to the inlet. One cycle of experiment took almost 80 min.
3. Results and Discussion As displayed in Figure 1, before passing the LFG through the absorption apparatus, it is passed through the activated carbon flask to remove the small amount of poisonous substances, such as chlorinated and aromatic components. The moisture and sulfur compounds were removed at the LFG site before compressing it. LFG entering the absorption beaker consists of methane and CO2. The methane content is 55%, and the CO2 content is 45%. The experiment is started using NaOH solution of different molarity. The pH of the solution decreased rapidly with time in the case of NaOH. The 0.1 M NaOH solution is neutralized by LFG in less than 7 min, and the 0.2 M NaOH solution is neutralized in less than 18 min, as seen in Figure 2. Two solutions of 0.1 and 0.2 M are used. Increasing the molarity of the NaOH solution also increased the amount of CO2 absorbed in the solution, as displayed by results in Figures 3 and 4. The CO2 reaction with NaOH is quite simple and proceeds in two steps as displayed below CO2 þ 2OH- f CO3 2- þ H2 O
ð1Þ
CO2 þ CO3 2- þ H2 O f 2HCO3 -
ð2Þ
Figure 4. CO2 loading for 0.1 M NaOH, 0.2 M NaOH, and wastewater. Table 1. Wastewater Characteristicsa parameters
value
pH temperature (K) total alkalinity (mg of CaCO3/L) ammonia alkalinity (mg of CaCO3/L) hydroxide alkalinity (mg of CaCO3/L) conductivity (mS) COD (ppm) TOC (ppm) IC (ppm)
10.41 293 34000 20000 14000 200.9 3.45 1.91 27.61
a Standard methods were applied to calculate the above characteristics.
Initially, CO2 reacts with hydroxyl ions (OH-) according to reaction 1, while as the course of the experiment progresses, the reaction with carbonate anions (reaction 2) takes over. The aqueous solutions are neutralized in the end of the experiments (pH 7-8). Wastewater used has a very high alkalinity and pH. It is important to decrease the pH of wastewater for subsequent biological treatment or for discharge to the water body. The different properties of wastewater are listed in Table 1. Wastewater used has a very high alkalinity and pH. It is important to decrease the pH of wastewater for subsequent biological treatment or for discharge to the water body. The different properties of wastewater are listed in Table 1.
The total alkalinity of wastewater is around 34 000 mg of CaCO3/L. This alkalinity is the sum of hydroxide alkalinity and NH3 alkalinity. The NH3 alkalinity is around 20 000 mg of CaCO3/L, and the hydroxide alkalinity is around 14 000 mg of CaCO3/L. The alkalinity of 0.1 and 0.2 M NaOH solutions is 5000 and 10 000 mg of CaCO3/L, respectively. Wastewater has a low COD value because of the low presence of oxidant species, and the presence of ammonia and its derivatives are usually not oxidized easily, which are present in a large amount in the wastewater under study. When LFG is passed through the wastewater, there is a linear decrease in pH, which 5469
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: DOI:10.1021/ef900615h
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Figure 7. Equilibrium breakthrough curve of 2, 5, and 10 wt % ammonia and wastewater.
Figure 5. Reaction rate during absorption for 0.1 M NaOH, 0.2 M NaOH, and wastewater.
The above reactions proceed at different temperature and operating conditions. Ammonium carbamate (NH2COONH4) is formed by the reaction of CO2 and NH3 in dry conditions according to eq 3 at room temperature and a pressure of 1 atm. Ammonium carbanate is quite soluble in water, and the formation of ammonium carbonate takes place according to reaction 4. The reactions 5-6789 occur at room temperature and atmospheric pressure. The formation of ammonium (NH4þ) and carbamate (NH2COO-) is very rapid and irreversible. The last four reactions 6-789 are reversible. The forward reaction occurs at room temperature, whereas the backward reaction occurs at higher temperature mentioned by Bai and Yeh.20 In some previous studies by Rao et al.21 and Lei et al.,22 different type of wastewaters are used for CO2 absorption, such as dye bath effluent and anaerobic digestion effluent. The CO2 removal capability in both previous cases reported is exceptional, and the results are very positive. The wastewater used in the previous study23 showed a high CO2 loading of around 4.5 L of CO2/L of solvent. The CO2 inlet concentration was 10% in the previous study compared to 50% in our study. Moreover, the gas mixture of N2 and CO2 was used, but in the present study, LFG is used. The wastewater from the catalyst industry is used in this study. The maximum CO2 loading for 0.1 M NaOH, 0.2 M NaOH, and wastewater are 0.41, 0.6, and 1.7 mol of CO2/L, respectively (Figure 4). The maximum CO2 loading for 2, 5, and 10 wt % ammonia solutions are 0.7, 1.6, and 2.6 mol of CO2/L (Figure 8). The CO2 loading follows the order 0.1 M NaOH < 0.2 M NaOH < 2 wt % ammonia < wastewater
