Article pubs.acs.org/EF
Engineering Operation Performance of Catalytic Deoxygenation Equipment for Landfill Gas Upgrading Zezhi Chen,*,† Huijuan Gong,*,†,‡,∥ Yanchu Bao,‡ and Weili Wu§ †
State Key Laboratory of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing, 210023, PR China ‡ Center of Materials Analysis, Nanjing University, 210093, Nanjing, PR China § Nanjing Carbon Recycle Bio-Energy Cooperated Limited Company, Nanjing, 210046, PR China ∥ Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing, 210093, PR China ABSTRACT: Catalytic methane (CH4) oxidation was proven to be a potential deoxygenation technique for upgrading landfill gas (LFG) as an alternative for natural gas or vehicle fuel. In this paper, a catalytic deoxygenation unit with 5000 N m3/h feed gas treatment capacity was installed in an LFG upgrading engineering project. This catalytic equipment has shown excellent deoxygenation performance. The catalytic oxidation reaction could be completely lit off in half an hour, and the full light-off temperature in the center of the catalytic bed was generally less than 300 °C. Complete deoxygenation was achieved once the reaction was lit off during the entire running period. The deoxygenation performance was not affected by the fluctuation of operating conditions such as gas flow rate and inlet oxygen content. The catalyst also showed satisfactory durability. After running continuously for more than one year the catalyst did not lose any deoxygenation reactivity. It is a cost-effective technology from the viewpoint of engineering application. To produce 1 m3 compressed natural gas (CNG), only about RMB 0.047−0.057 yuan was needed for deoxygenation using this equipment. Based on successful application of the deoxygenation equipment in this work, a conclusion could be drawn that catalytic methane oxidation is a feasible deoxygenation technology to be utilized widely in LFG upgrading projects.
1. INTRODUCTION Upgrading landfill gas (LFG) to produce high-quality fuel gas is an attractive approach for recovery and utilization of LFG, which is rich in methane (CH4) and generated from the organic waste anaerobic degradation process in landfills.1,2 By now, the main way to utilize LFG is by generating power through combustion in gas engines, and this has been widely put into practice and financially supported through providing subsidies in many countries including China.3 Compared with power generation, upgrading LFG to produce an alternative commercial fuel gas such as compressed natural gas (CNG) has attracted increasing interest for its flexible applications. In other words, it could not only be injected into city natural gas grid but also be used as a vehicle fuel or a gas fuel consumed in factories. Since the upgraded LFG is used as an alternative commercial gas fuel, its quality should meet the corresponding standards. Therefore, the LFG upgrade is a process of removing diverse harmful and invalid components, like carbon dioxide (CO2), oxygen (O2), water vapor (H2O), hydrogen sulfur (H2S), siloxanes, and so on.4 The product gas is a high-quality gas fuel which is almost entirely composed of methane. O2 is an inevitable component in LFG. As it is impossible to keep the landfill completely tight, a small amount of air is liable to be leaked into the collected LFG during the process of gas collection. O2 content determines the air−fuel ratio of the combustion process. If O2 content in gas fuel is higher than the requirement, it would cause some engine operation problems such as difficulty starting up, sudden shutdown, low combustion efficiency, and so on, due to the inappropriate © 2017 American Chemical Society
air−fuel ratio. Generally, O2 content monitored in the raw LFG is in range of 0.5−3.0 vol %. Methane enrichment (removing CO2) as an essential step in LFG upgrading will lead to O2 concentration almost doubling in the product gas. In China, according to the Standards of Compressed Natural Gas as Vehicle Fuel (GB 18047−2000) the oxygen content should not exceed 0.5%.5 Therefore, to utilize LFG as an alternative fuel gas, the deoxygenation is an indispensable procedure for LFG upgrading. Technologies to remove such components as CO2, H2S, and siloxanes have been intensively studied for LFG upgrading.6−14 However, research on O2 removal from LFG is quite limited, and almost no open publications could be consulted. As O2 content is not very high in the raw LFG, it is difficult to remove it cost-effectively through conventional techniques such as adsorption, membrane separation, absorption, and so on. Catalytic oxidation of CH4 has been successfully employed in eliminating small amounts of CH4 to achieve CH4 emission abatement.15−19 This CH4 abatement method is based on the principle that CH4 could be oxidized by O2 to produce CO2 or/ and CO under different conditions. Since LFG contains a large amount of CH4 and a little O2, these two components would react with each other under appropriate catalytic conditions to achieve LFG deoxygenation with only a little CH4 consumed, and the undesired products CO2 and H2O could be easily removed through decarbonization and treatment to remove Received: February 13, 2017 Revised: March 19, 2017 Published: March 21, 2017 4565
DOI: 10.1021/acs.energyfuels.7b00421 Energy Fuels 2017, 31, 4565−4570
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Figure 1. Flowsheet of the catalytic deoxygenation equipment.
Figure 2. Deoxygenation equipment in the LFG upgrading project. consists of a deoxygenation reactor, an electric heater, a heat exchanger, and an after cooler as shown in Figure 1. The inlet gas was preheated by the electric heater to a proper temperature (generally around 280−310 °C) based on the flow rate of feed gas and the oxygen content in it, so that the temperature inside the catalyst bed could be kept at a suitable level needed by the deoxygenation reaction. In order to save electricity consumption, a heat exchanger was installed before the electric heater to recover part of the heat generated from the deoxygenation reaction. The after cooler was used to cool down the outlet gas to around 40−50 °C which would be further treated in the next purification unit (the decarbonization unit to remove CO2). The catalyst was prepared through impregnating cylindrical γ-Al2O3 grains with diameter of 2.0 mm and length of 10.0 mm in aqueous H2PtCl6·6H2O solution of 1.0 wt % for 1 h, then drying at 120 °C for 12 h, and finally calcining at 550 °C for 4 h. The ultimate loadings of platinum (Pt) on γ-Al2O3 grains was 1.5 wt %. The deoxygenation reactor is a columnar reactor, which contains a catalyst fixed bed with a diameter of 0.8 m and a height of 2.0 m. The reactor was wrapped by heat insulating material to remain adiabatic. As shown in Figure 1, four thermocouples were arranged along the axis of the columnar reactor, which gave temperature information in different places of the reactor. The distance of each of them is 0.7 m. The maximum overall gas hourly space velocity (GHSV) through the catalyst bed was set at 5000 h−1 during the deoxygenation treatment. This deoxygenation equipment was installed in an LFG upgrading project for producing alternative CNG. The project was constructed in Hei Mifeng landfill which is located in Changsha city of Hunan province in China. The maximum raw LFG treatment capacity of this project was 5000 N m3/h, and about 3000 N m3/h product gas could be obtained. The deoxygenation equipment was set in the middle of
water. This method is named catalytic deoxygenation for LFG upgrading. Catalytic deoxygenation has been successfully utilized in a small-scale experimental installation with Pt/γAl2O3 and Pt−Rh/γ-Al2O3 coated on metallic substrate as the deoxygenation catalysts.20,21 Results showed that both deoxygenation performance and catalyst durability were satisfactory. Therefore, the catalytic methane oxidation method exhibited the potential to be applied in a practical deoxygenation procedure in LFG upgrading projects. Based on the previous pilot experiment study,20,21 a set of catalytic deoxygenation equipment with 5000 N m3/h feed gas treatment capacity was developed and installed in an LFG upgrading project in China. In this work, full-scale industrial experiments were performed on the equipment to investigate its deoxygenation performances under different running conditions in real engineering applications. The corresponding features of temperature distribution inside the catalytic deoxygenation reactor were summarized. Durability of the catalyst through consistent operation and cost of the operation were also analyzed. The experimental results of this work would help to design deoxygenation equipment in similar LFG upgrade engineering projects as well as optimize operation parameters.
2. EQUIPMENT AND METHODS A set of catalytic deoxygenation equipment which was designed to treat a maximum flow rate of 5000 N m3/h feed gas was developed. It 4566
DOI: 10.1021/acs.energyfuels.7b00421 Energy Fuels 2017, 31, 4565−4570
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Figure 3. Typical deoxygenation process started from the cold stage to the warm stage.
Figure 4. Temperature variations and the deoxygenation performance in the light-off stage. H2S and siloxanes. After the deoxygenation unit, CO2 was removed in an amine liquid absorption tower and then a temperature swing adsorption (TSA) unit was used to remove water vapor. Installation of
the whole LFG upgrading engineering devices. In front of the deoxygenation unit, two tanks were installed in series, which were respectively filled with sponge ferric oxide and active carbon to remove 4567
DOI: 10.1021/acs.energyfuels.7b00421 Energy Fuels 2017, 31, 4565−4570
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Figure 5. Oscillating temperatures inside the deoxygenation reactor in the warm stage.
3.2. Temperature Features Inside the Catalytic Deoxygenation Reactor. As shown in Figure 1, four thermocouples were installed along the axis of the deoxygenation reactor to record the temperature of the feed gas as well as those respectively in the front, the middle, and the rear of the catalytic bed. During the light-off stage, the electric heater was turned on to preheat the feed gas; the corresponding temperature curves of TA, TB, TC, and TD with different flow rates of the gas in this stage as well as O2 contents in the inlet and outlet gas are displayed in Figure 4a,b,c,d. It was observed that each temperature inside the reactor rose along with the running time, and the corresponding oxygen content in the outlet gas decreased as well. From Figure 4a,b,c,d, it was found that the temperature curves of TA and TB were almost concurrent, while the temperatures of TC and TD increased faster than those of TA and TB. Once the catalyst bed was fully lit off (100% deoxygenation efficiency attained), whatever the feed flow rate, the temperature of TC was the highest among the four temperatures of TA, TB, TC, and TD at the same time. As the catalytic oxidation reaction is exothermic, heat released from the reaction would increase the temperature of the catalyst. Therefore, temperature could reflect the degree of the oxidation reaction. Conclusions could be drawn that the catalytic oxidation reaction mainly occurred in the middle and posterior parts of the catalytic bed at this period of operation, and the anterior part of the catalytic bed played a small role. This is due to the oxygen content in LFG being less than 1.0 vol %; the heat released from the reaction in this part was relatively small to light off the catalyst in the anterior part. As the catalytic oxidation reaction occurring at point C was the most intense among the four points, TC was selected as the indicating temperature to control the electric heater switching on or off in the warm stage. The temperature at which 100% oxygen removal was attained during the light-off stage is named the full light-off temperature. According to Figure 4a,b,c,d, it is obvious that when the flow rate of the gas is lower, the corresponding period of the light-off stage is shorter, and the full light-off temperature at the same point is also lower. For example, when oxygen content in the feed gas was around 0.8%, the flow rates of the feed gas at 1200, 2200, 3200, and 4200 N m3/h correspond to the periods of light-off stage as 6, 6.5, 18, and 28 min and the full light-off temperatures of TC at 246, 250, 257, and 290 °C, respectively. Thus, lower gas flow rate is favorable for shortening the duration of the light-off stage and decreasing the light-off temperature.
the deoxygenation equipment in the LFG upgrading project is shown in Figure 2. This LFG upgrading project has been operated for over ten months, and pressure inside the deoxygenation reactor was controlled by a pressure regulating valve at 5 bar, while other conditions such as temperature and gas flow rate were not controlled deliberately; they both depended on the physical truth. The contents of gas components of CH4, CO2, and O2 in the inlet and outlet gas were monitored by gas detecting sensors from Germany ADOS Company, and the measurement precision of each sensor was 0.01%.Working performances of the deoxygenation equipment are illustrated in the following text.
3. RESULTS AND DISCUSSION 3.1. Deoxygenation Performance of the Equipment. During the running period, the contents of CH4, CO2, and O2 detected in the raw LFG varied in the ranges 54−58%, 36− 38%, and 0.2−1.2%, respectively. From the viewpoint of catalyst temperature, the deoxygenation process can be divided into three stages: first, the cold stage when the deoxygenation equipment has just started up, while the catalyst is still in a cold state, and the deoxygenation reaction has not yet begun; then, the light-off stage during which the feed gas has been gradually preheated by the electric heater, which results in the increase of both catalyst bed temperature and deoxygenation efficiency; and finally, the warm stage in which the temperature inside the catalyst bed is kept at a high level steadily, and stable high deoxygenation efficiency is attained. Figure 3 displays a typical deoxygenation process started from the cold stage to the warm stage. It is shown that after heating for about half an hour by the electric heater, catalyst in the deoxygenation reactor was fully lit off and it demonstrated such excellent deoxygenation performance that complete oxygen removal was achieved in the outlet gas. Besides, full deoxygenation efficiency was maintained in the entire warm stage of the running period despite the sharp fluctuations of the gas flow rate and the inlet O2 content as illustrated in Figure 3, which means the deoxygenation equipment can endure a wide range of gas flow rate and oxygen content in the inlet gas. After more than ten months of operation, it was observed that the catalytic oxidation reaction could be lit off completely below 300 °C easily under methane rich atmosphere, and then complete deoxygenation efficiency of the catalyst was maintained until the equipment was shut down. During the operation period, the monitored temperature in the center of the catalytic bed (TC) was very moderate in the range of 280− 340 °C. When the GHSV was lower than 5000 h−1, as long as the inlet gas temperature (TA) was kept over 285 °C, complete deoxygenation could be reliably obtained. 4568
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could be reduced. As a result the GHSV of the catalyst bed could be further increased. The optimization of the structure of the reactor will be tried in future work. 3.3. Durability of the Catalyst. The catalytic deoxygenation equipment was put into use at the end of 2015 and it has been operating continuously for more than one year. Until now, the catalyst has been demonstrating excellent deoxygenation performance in the actual engineering operation, and no deterioration tendency has been observed; thus the durability of the catalyst is satisfactory and can meet the requirement of engineering applications. It is worth emphasizing that two points need special attention to prolong service life of the catalyst. One is that the temperature inside the deoxygenation reactor should be controlled strictly. As the oxygen content in LFG is generally no more than 2.0 vol %, the catalytic oxidation reaction is mild; thus the heat released from the reaction usually would not lead to excessive heating of the catalytic bed. If only the heating rate and temperature range to switch the electric heater are set properly, the catalyst would not fail due to sintering. In this work the monitored temperature of TC was lower than 350 °C; thus such a moderate reaction temperature is quite favorable to protect the catalyst from sintering during long-term operation. The other point is to guarantee sufficient removal of H2S and siloxanes in LFG to attain an acceptable concentration level, i.e., H2S lower than 5 mg/Nm3 and total siloxanes lower than 0.2 mg/Nm3, so that the catalyst would not be deactivated by poisoning. 3.4. Investment and Operating Cost. The total investment of the equipment with 5000 N m3/h feed gas treatment capacity was about RMB 1.2 million yuan (approximately equivalent to US $ 0.18 million), in which the deoxygenation catalyst accounted for RMB 1.0 million yuan (US $0.15 million). The estimated service life of the catalyst was more than 3 years.20 For example, if the annual operation time of the equipment is 7800 h and the feed gas flow rate is in the range of 3000−5000 N m3/h, the depreciation cost of the deoxygenation catalyst is RMB 0.008−0.014 yuan to treat 1 m3 of LFG. The main operating cost of the equipment comes from electricity consumption of the electric heater. After ten months of operation, the cost of electricity consumption was RMB 0.547 million yuan, and the treated total amount of raw LFG was 27.36 million m3; therefore the operating cost is about RMB 0.020 yuan to treat 1 m3 of LFG. From this analysis, the total cost including catalyst depreciation and electricity consumption is about RMB 0.028−0.034 yuan per cubic meter feed gas. Generally, 1 m3 raw LFG could produce about 0.6 m3 product CNG; thus the cost of operating the deoxygenation unit to produce 1 m3 CNG is about RMB 0.047−0.057 yuan. At present the sale price of the alternative CNG from LFG is in the range of approximately RMB 2−3 yuan per cubic meter in different cities of China, which means the cost of deoxygenation treatment is lower than 5% of the sale price, and accordingly it is acceptable in the engineering application.
When the deoxygenation catalytic reaction is fully lit off, the deoxygenation process enters the warm stage. In order to keep the temperature of the catalyst bed high enough to guarantee the deoxygenation efficiency, the electric heater was turned on or off automatically according to the indicating value of the TC. In other words, if the temperature of TC was 10 °C lower than the full light-off temperature, the electric heater would be switched on; while the TC reached 10 °C higher than the full light-off temperature, the electric heater would be turned off. The temperature variation curves of TA, TB, TC, and TD during the warm stage in the span of several operation hours and 20 min are displayed in Figure 5a and b, respectively. From Figure 5a,b, the variations of TA and TB were almost synchronous and the amplitudes were nearly the same, which means very little reaction was taking place in the front segment (before the position of point B) of the catalyst bed. The temperatures of TC and TD were generally much higher than those of TA and TB during most of the operation time; thus it proved that the deoxygenation reaction took place mostly in the middle and posterior zones of the catalytic bed. According to Figure 5a, it was also observed that during the deoxygenation process, the range of the temperature variation inside the reactor (refer to TC and TD) was relatively wide. This was due to the change of O2 content in the feed gas. From the analysis above, as the deoxygenation reaction is exothermic, a higher content of oxygen might lead to more heat released, which would result in the temperature of the catalyst bed becoming higher. On the other hand, oxygen content in the LFG is generally lower than 3.0 vol %; therefore, the temperature in the catalyst bed would not increase too much to damage the catalyst due to the high content of O2. The temperature of TC as well as the corresponding inlet oxygen content in 4 h operation is displayed in Figure 6. As shown, the
Figure 6. Curves of temperature TC and the inlet oxygen content.
variation tendency of the temperature curve of TC coincided well with that of the inlet oxygen content, which indicated that the catalytic deoxygenation reaction under methane-rich atmosphere is a kind of fast dynamics. Thus, temperature TC could also reflect the oxygen content in the feed gas. As discussed earlier, the deoxygenation reaction mainly took place in the middle and posterior zones of the catalytic bed, and the catalyst grains that were laid above point B played a small role in the deoxygenation process. Therefore, the catalyst in the inactive area of the catalyst bed could be replaced by inert materials with large heat capacity, so that the cost of the catalyst
4. CONCLUSIONS A set of catalytic deoxygenation equipment with 5000 N m3/h feed gas treatment capacity was developed and put into usage in an LFG upgrading project to produce alternative CNG fuel gas. This equipment has been operated for over ten months. It was observed that the conditions of the catalytic oxidation reaction 4569
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were quite moderate. The full light-off temperature in the center of the catalytic bed was below 300 °C, and the operating temperature during the warm stage of the running period was monitored in the range of 280−340 °C. The equipment showed such excellent deoxygenation performance that complete oxygen elimination was maintained during the entire warm stage of the operation period. Besides, the catalytic deoxygenation reaction could be fully lit off from the cold stage to the warm stage in less than half an hour. Moreover, the catalytic deoxygenation equipment could endure a wide range of gas flow rate and oxygen content in the inlet gas without losing deoxygenation efficiency. As oxygen content in LFG is fluctuating, an electric heater is essential in the deoxygenation equipment to provide extra heat for the catalyst in case the heat released from the reaction is not enough to maintain the catalytic oxidation process. The total cost of the deoxygenation equipment including catalyst depreciation and electricity consumption is calculated to be about RMB 0.028−0.034 yuan to treat 1 m3 raw LFG. After ten months of operation, the catalyst did not show any deterioration tendency, which proved that the catalyst’s durability is quite satisfactory. All these operation features demonstrated that the technology of catalytic deoxygenation is competitive for practical engineering applications, as it could meet both the technique and economic requirements. Therefore, this deoxygenation technology could be widely applied in LFG upgrading projects.
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AUTHOR INFORMATION
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Zezhi Chen: 0000-0003-0455-4329 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Sciences Foundation of China (No. 21577060, 21107044), Science and Technology Ministry of Jiangsu Province (BE 2016172, BK 20141315), and Analysis & Test Fund of Nanjing University.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.7b00421 Energy Fuels 2017, 31, 4565−4570
