Application of iron-Based adsorbent (feooh) to remove hydrogen sulfide (h2s) from biogas - Do Kha Uan

Journal of Science and Technology 54 (2A) (2016) 35-41 APPLICATION OF IRON-BASED ADSORBENT (FEOOH) TO REMOVE HYDROGEN SULFIDE (H2S) FROM BIOGAS Do Khac Uan 1, * , Nghiem Trung Dung 1 , Shin-Dong Kim 2 , Nguyen Thi Thu Hien 1 , Ly Bich Thuy 1 , Tran Dac Chi 1 1 School of Environmental Science and Technology, Hanoi University of Science and Technology,No 1 Dai Co Viet Road, Hanoi, Viet Nam 2 Environment & Chemistry Solution Corporation, 762 Deokpung-dong, Hanam-Si, Gyeonggi-do, 465-736, Korea * Email: uan.dokhac@hust.edu.vn Received: May 10, 2016; Accepted for Publication: June 15, 2016 ABSTRACT Biogas generated during the anaerobic digestion of piggery wastes is considered as one of the renewable energy sources. This biogas is methane-rich, based on a typical composition of 60 - 70 % methane (CH4). However, trace amounts of undesirable compounds, such as hydrogen sulfide (H2S) is also present, which hinder their use as it is very toxic and corrode the equipment. In this study, a lab-scale adsorption system was conducted to removal H2S from biogas collected at Thanh Hung pig farm (Thanh Oai district, Hanoi). The initial biogas contained high CH4 concentration of 72 %, and the H2S concentration was about 1995 ppm. The iron-based adsorbent (FeOOH) with the particle size of 0.50-1.18 mm was used for H2S purification. The system was operated continuously at various biogas flow rates from 0.5 to 3 L/min. Breakpoint of FeOOH appeared after 1,038 min. The adsorption capacity was estimated up to 0.18 g H2S/g FeOOH. A longer empty bed contact time increased the amount of H2S that was adsorbed up until the time of breakpoint. During operation, the temperature in the air and in biogas was varied insignificantly in the range of 29 to 32 o C. However, the humidity was much different between in the ambient air (56 %) and in biogas (87 %). In conclusion, FeOOH has high capacity for H2S purification to produce high quality purified biogas with H2S concentration of below 100 ppm which could be used for electrical generation. Keywords: adsorbent, biogas, hydrogen sulfide, methane, purification. 1. INTRODUCTION Biogas produced from the animal farm manure anaerobic fermentation is primarily composed of methane (CH4) and carbon dioxide (CO2), with smaller amounts of ammonia (NH3), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), saturated water or halogenated Do Khac Uan, et al. 36 carbohydrates [1, 2]. Biogas has a high heating value of between 15 and 30 MJ/Nm 3 so it is an attractive source of energy [3]. However, biogas contains significant quantities of undesirable compound, such as hydrogen sulfide (H2S) [4]. H2S is corrosive, toxic, and odorous. So it can significantly damage mechanical and electrical equipment used for process control, energy generation, and heat recovery. The combustion of H2S results in the release of sulfur dioxide, which is a problematic environmental gas emission [5, 6]. Boilers, which generate heat from biogas, do not have a high gas quality requirement, although it is recommended that H2S concentrations be kept below 1,000 ppm. Internal combustion engines, used for electricity generation, have required biogas with H2S concentrations below 100 ppm [7]. Biogas can also be utilized as a vehicle fuel. However, it must be upgraded because vehicles need a much higher gas quality. H2S can be controlled using a variety of methods which can be either physical- chemical or biological removal processes [8]. Activated carbons are frequently used for biogas adsorption because of their high surface area, porosity, and surface chemistry where H2S can be physically and chemically adsorbed. Much of the research has focused on how the physical and chemical properties of various activated carbons affect the breakthrough capacity of H2S. Activated carbon can be impregnated with potassium hydroxide (KOH) or sodium hydroxide (NaOH), which act as catalysts to remove H2S [4, 9]. Much research has focused on mechanisms of H2S removal using activated carbon. The studies have focused on hydrogen sulfide adsorption on activated carbons as it relates to surface properties, surface chemistry, temperature, concentration of H2S gas, addition of cations, moisture of gas stream, and pH. These experiments have used both biogas from real processes and laboratory produced gases of controlled composition [9, 10, 11]. NaOH impregnated activated carbon was also tested for H2S removal capacity. The results showed that with increasing amounts of NaOH added, the H2S removal capacity of the activated carbons increases [10]. Zeolites are especially effective at removing polar compounds, such as water and H2S, from non-polar gas streams, such as methane [1]. The use of zeolite-NaX and zeolite-KX as catalysts for removing H2S from biogas streams has been performed. The study found a yield of 86 % of elemental sulfur on the zeolites over a period of 40 hours [12]. When sludge undergoes pyrolysis, a material is obtained with a mesoporous structure and an active surface area with chemistry that may promote the oxidation of H2S to elemental sulfur. Samples with higher content of sewage sludge pyrolyzed at higher temperatures (800 °C and 950 °C) had the best adsorption capacity [10]. Metal oxides have been tested for hydrogen sulfide adsorption capacities. Iron oxide is often used for H2S removal. It can remove H2S by forming insoluble iron sulfides. Iron oxide can be used in either a batch system or a continuous system. In a continuous system, air is continuously added to the gas stream so that the iron oxide is regenerated simultaneously. In a batch mode operation, where the iron oxide is used until it is completely spent and then replaced, it has been found that the theoretical efficiency is approximately 85 % [13]. This study was aimed to find operational conditions that would maximize the amount of H2S removed from biogas in order to allow for systematic sizing of biogas purification system using the iron-based (FeOOH) adsorbent. Using the biogas produced by the anaerobic digesters at the Thanh Hung pig farm (Thanh Oai district, Hanoi), various conditions were tested to determine the optimal design and operational conditions for H2S removal from the biogas. The following conditions were tested such as empty bed contact time and mass of iron-based adsorbent used in the media bed. Application of iron-based adsorbent (FeOOH) to remove hydrogen sulfide (H2S) from biogas 37 2. MATERIALS AND METHODS 2.1. Lab-scale adsorption system The system was set-up at the Thanh Hung pig farm (Thanh Oai district, Hanoi). 123 m3 Purified biogas Biogas volume meterF lo w m e te r Blower Gauss pressure Gauss pressure A d s o rb e n t c o lu m n Initial Biogas Analyzing Point Biogas from anaerobic covered pond Figure 1. Schematic of adsorption system for biogas purification. Figure 2. The actual set-up of the experimental system at Thanh Hung pig farm. The schematic of the adsorption system is shown in Fig. 1. The actual set-up of the experimental system is shown in Fig. 2. The biogas from the anaerobic digesters at the Thanh Hung pig-farm entered the system. The biogas was then either sampled to measure the inlet H2S concentration if the inlet sampling valve opened, or continued to pass through a rotameter and into the bottom of the adsorbent column (inside diameter of 45 mm, height of 90 mm). The rotameter was used to control the flow rate of biogas through the system. Once the biogas entered the adsorber column, it passed through the media bed. There was also a gas sample port located in the outlet of the adsorption column for measuring the purified biogas compositions. In this study, the iron-based adsorbent (FeOOH, called as hydrated goethite or iron oxyhydroxide) developed and provided by E & Chem Solution Co. (Korea) was used for testing. It has a particle size ranging from 0.5 mm to 1.18 mm with the bulk density of 0.67 g/mL. The amount of adsorbent used were 73.4 g and 17.5 g for testing the effect of adsorbent mass on H2S removal. The biogas flow rates were controlled at various conditions from 0.5 L/min; 1 L/min; 2 L/min; and 3 L/min for changing the empty bed contact time. 2.2. Analytical methods A rotameter (Yinhuan Co., Taiwan) was used to control the biogas flow rate. It had the capability of measuring flows between 0.5 to 5 L/min. Besides, a biogas volume meter (Model G1.6, Shinhan Co., Korea) was used to record the total biogas which was purified. Temperatures and humidities in the inlet and outlet were measured by the digital temperature/humidity meters (Wellink HL101, Taiwan). The pressure difference in the adsorption column could be read from the pressure gausses (KK Gause, Taiwan) located both at the inlet and outlet sides of the scrubber. Samples of the inlet and outlet (purified) biogas were taken during experimental tests. The compositions of these samples (CH4, CO2, H2S, O2) were determined using an Optima 7 Biogas Analyzer (MRU Instruments, Inc., Germany). Do Khac Uan, et al. 38 3. RESULTS AND DISCUSSION 3.1. Characteristics of biogas used in this study Thanh Hung pig farm (at Thanh Oai district, Hanoi) has about 4500 pigs per year. The average wastewater was about 300 m 3 /d, and the solid wastes was 1500 - 2000 kg/d. The wastewater containing solids wastes was collected and introduced into a cover anaerobic lagoon with its dimensions of L × W × H = 36.5 × 14.5 × 4.0 m. In which, the biogas storage part including the height of about 2.5 m, coresponding of biogas storage volume varied from 490 to over 800 m 3 , depending on the amount of wastewater level and amount of biogas used. The initial biogas characteristics were analyzed. As a results, CH4 was about 72 %, showing that the biogas at Thanh Hung pig farm has high CH4 concentration and it could be good for using. However, the H2S was too high at about 2000 ppm. Due to the low quality of biogas, currently, biogas was used only for cooking. Sometimes, it was discharged directly to the atmosphere. 3.2. Flow rate and cumulated volume during experiment The flow rate of biogas during operation of the system was recorded. For example, Fig. 3 presents the flow rate controlled at about 1 L/min. After 374 min of operation, over 350 L of biogas was purified. During operation, the variation of temperature and humidity (moisture) (Fig. 4). The temperature in the air and in biogas was slightly fluctuated in the range of 29 o C - 32 o C. However, the humidity was much different between in the ambient air and in biogas. The humidity in the ambient air was around 56 %, whereas it was about 87 % in biogas. During operation, occasionally, condensation in some of the plastic tubing was present. This was usually due to a humidity difference in the biogas and the ambient air. The tubes containing condensation were drained during the experiment to ensure that the water was not causing blockages. 0 50 100 150 200 250 300 350 400 0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 250 300 350 400 C u m u la te d v o lu m e , L Fl o w ar at e , L /m in Experimental time, min Flowrate, L/min Cumulated volume, L 0 10 20 30 40 50 60 70 80 90 100 20 25 30 35 40 45 50 55 60 0 50 100 150 200 250 300 350 400 M o is tu re , % Te m p e ra tu re , o C Experimental time, min Outside Temp., oC Inside temp, before, oC Inside temp, after, oC Outside moisture, % Inside moisture, before, % Inside moisture, after, % Figure 3. Flow rate and cumulative volume of biogas during operation. Figure 4. Fluctuation of temperature and moisture in biogas before and after purification. 3.3. Pressure change during operation The flow of biogas through the media bed caused a pressure drop across the bed. Fig. 5 shows the pressure drop over the depth of the media bed versus the flow rate of biogas through the system. As can be seen in Fig. 5, it appears as if increasing the flow rate of biogas from 0.5 to 3.0 L/min through the system caused a slight increase in pressure change, only about 2 cmHg. Application of iron-based adsorbent (FeOOH) to remove hydrogen sulfide (H2S) from biogas 39 It should be noted that in H2S removal using iron oxide-impregnated wood chips, the pressure recommended for consistent operation should be over 10 cmHg which cause very high resistance [4]. Therefore, it seems that the iron-adsorbent (FeOOH) with the particle size of 0.5 - 1.18 mm caused a low resistance resulted in operation easily. 3.4. Breakpoint curve of H2S removal The iron-based adsorbent was tested at a single empty bed contact time, and mass. A typical breakpoint curve from this study is shown in Fig. 6. Breakpoint occurred when the H2S concentration in the outlet biogas began to increase upto 100 ppm that required for the internal combustion electricity generation engine. In this section, the time period is referred to the time from the start of the experiment until the point of breakthrough. Breakpoint of iron-based adsorbent (FeOOH) (lower than 100 ppm H2S, 5% of inlet H2S conc.) appeared after 1,038 min. Fig. 7 shows the change of adsorbent color during operation. It was clear that during operation, the adsorbent color was changed from brown to black. The black layer of adsorbent material increased by time. After 1000 min of operation (after the breakpoint), most of adsorbent was black. The black reflects the reaction between FeOOH with H2S to form FeS. It should be noted that H2S in biogas electric generator engine should be less than 100 ppm [13]. Based on the breakpoint curve, it could be estimated the adsorption capacity of about 0.18 g of H2S/g of FeOOH. This value is a litle higher than the H2S adsorption capacity for impregnated activated carbons (0.15 g H2S/g of activated carbon). A typical H2S adsorption capacity for unimpregnated activated carbons is only 0.02 mg H2S/g of activated carbon [14] which is lower than about 9 times compared with the H2S adsorption capacity of FeOOH. y = 0.052x2 - 0.594x - 0.496 R² = 0.996 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 P re ss u re c h an ge , c m H g Flowrate, L/min Pressure, cm Hg Figure 5. Pressure change due to flow of biogas through the system. 0 500 1000 1500 2000 2500 0 200 400 600 800 1000 1200 1400 H 2S c o n ce n tr at io n , p p m Time, min H2S Inlet H2S Outlet Breakpoint: H2S outlet > 100 ppm B e fo re t e s ti n g In it ia l b re a k a t th e b o tt o m P ro g re s s o n b re a k in g A ft e r b re a k p o in t Figure 6. A breakpoint curve from this study. Figure 7. Change of adsorbent color during operation. Do Khac Uan, et al. 40 3.5. Effect of empty bed contact time on H2S removal capacity Empty bed contact time was varied by adjusting flow to the adsorber column using a rotameter equipped with a needle valve. A longer contact time was found to slightly increase the amount of H2S removed up until the time of observed breakpoint. Figurre 8 shows the empty bed contact time on H2S removal capacity. From the figure, it could be seen that the amount of H2S removed was dependent slightly on the contact time. This suggests that there was a maximum amount of H2S that could be removed by the FeOOH particles throughout the duration of the experiment. A longer contact time resulted in greater H2S removal prior to reaching maximum removal capacity. 3.6. Effect of the mass of FeOOH on H2S removal Reducing the mass of the FeOOH media bed decreased the amount of H2S that was removed on a mass of H2S per mass of the FeOOH basis. This process is shown in Fig. 9, where the flow rate was the same for both the full FeOOH in the adsorbent column (which contains 73.4 g FeOOH) and a quarter of the adsorbent column (which contains 17.5 g FeOOH) trials and the density was maintained at its uncompacted density. A full FeOOH in the adsorbent column had an average mass of H2S removed approximately 13.21 g, and a quarter of FeOOH the adsorbent column had approximately 3.15 g, respectively. Figurre 9 shows that the amount of H2S removed from the biogas was dependent on the mass of FeOOH present. It was found that the higher the H2S loading from the inlet biogas on the media, the more H2S was adsorbed. The change of removal capacity in the quarter bed may have been due to a decrease in the contact time, and therefore, related to the adsorption reaction time. y = 0.003x + 0.136 R² = 0.632 0 0.05 0.1 0.15 0.2 0.25 0 5 10 15 20 g H 2 S/ g Fe O O H Empty bed contact time, s Adsorbent capacity, g H2S/g FeOOH Linear (Adsorbent capacity, g H2S/g FeOOH) 0 2 4 6 8 10 12 14 16 0 10 20 30 40 50 60 70 80 H 2S r e m o ve d , g Mass of FeOOH, g Flowrate, 0.5 L/min Flowrate, 1 L/min Flowrate, 2 L/min Flowrate, 3 L/min Figure 8. Effect of empty bed contact time on H2S removal capacity. Figure 9. Effect of the mass of the media bed on the amount of H2S removed. 4. CONCLUSIONS In Vietnam, there are about 17,000 pig farms with over 500 pigs and less than 0.3 percent of them have a biogas facility. Biogas production from this size is about 50,000 m 3 /year. Therefore, developing the biogas purification technology plays an important role in biogas market in Vietnam. In this study, iron-based adsorbent (FeOOH) were tested at various conditions for the removal of hydrogen sulfide from biogas at the Thanh Hung pig farm (Thanh Oai district, Hanoi). A longer empty bed contact time increased the amount of H2S that was adsorbed up until the time of breakpoint. Based on the results obtained from the study, it could be seen that FeOOH has high capacity for H2S purification from biogas. However, there are still some issues need to be addressed before this product can be commercially used. It is important that materials currently on the market for H2S adsorption (such as activated carbon) have the ability to be regenerated. In this study, the Application of iron-based adsorbent (FeOOH) to remove hydrogen sulfide (H2S) from biogas 41 desorption experiments has not been performed. Another interesting future study could determine the effects of other components in the biogas on H2S adsorption. 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