Development and performance investigation of gas separation processes for the reduction of greenhouse gas emission: CO2 absorption and biogas purification

Abstract

The Intergovernmental Panel on Climate Change (IPCC) has warned that environmental pollution and climate change due to energy consumption behavior without consideration of indiscriminate fossil fuel consumption and energy circulation for high growth has reached a serious level. These environmental changes are caused not only by simple temperature changes but also by direct economic losses caused by natural disasters and weather changes caused by global warming.

In order to respond effectively to climate change, the international community has been actively debating climate change response systems since the IPCC First Assessment Report (FAR) was released in 1990. The New Climate Economy Report, published by the Global Commission on the Economy and Climate, shows that the use of oil, coal and gas is 30 And a long-term plan including a 31% increase in low-carbon infrastructure including renewable energy and CCS. As interest in low carbon green growth grows in South Korea, interest in renewable energy and emission control of carbon dioxide, a representative greenhouse gas, is high. Accordingly, from 2008, the Korean government has started to make efforts to secure carbon emission reduction technology to propose 'Low Carbon - Green Growth Strategy' as a new policy direction and actively cope with climate change problem domestically and internationally. In addition, it announced that it plans to increase the proportion of renewable energy generation to 20% by 2030 through the "renewable energy 3020" policy announced in 2017.

Greenhouse gas reduction technologies are normally classified into post-combustion capture technology and pre-combustion capture technology. Typical post-combustion capture techniques include pressure swing adsorption (PSA), and MEA absorption, which collects carbon dioxide from the mixed gas using an MEA sorbent. PSA is difficult to increase in facility size, and there is a problem that the operating and maintenance cost is high because a lot of electricity is used. The MEA absorption method is capable of treating a large amount of flue gas, but it has a problem in that it absorbs a small amount of CO2, deteriorates facilities due to the degradation of absorbent and requiring of high heat energy during regeneration. Therefore, this study proposed a CO2 capture technique using aqueous ammonia to overcome the disadvantages of MEA absorption and PSA. Aqueous ammonia-based CO2 capture is attractive because of its low cost, low heat energy requirement, high CO2 absorption capacity, low degradation rate, and ability to capture multiple pollutants simultaneously. The most serious problem of a carbon dioxide capture process that uses aqueous ammonia is a phenomenon involving ammonia vaporization known as ammonia slip. In this study, therefore, it is suggested that chilled ammonia process (CAP) in which washing tower is installed and low-temperature aqueous ammonia used to absorbent. The chilled ammonia process is an improved ammonia-based CO2 capture technology which involves a system operated at a low temperature to minimize the ammonia loss due to evaporation.

In this study, the performance of the carbon dioxide capture process using the aqueous ammonia was analyzed by fabricating a lab-scale absorption process equipment with a treatment capacity of 1.5 Nm3/h. The effects of process parameters on CO2 removal rate and ammonia loss rate were investigated in CO2 absorption process using aqueous ammonia at room temperature. As the L/G ratio, which is the ratio of absorbent flow rate and the feed gas flow rate, were higher, the absorption rate of CO2 in the absorption process tended to increase. In particular, the absorption rate of 95% was relatively high at a L/G of 7.50 and the ammonia loss was 17%. However, the ammonia loss rate at L/G of 7.14 was 14%, which is lower than the ammonia loss rate at L/G of 7.50. Therefore, it was confirmed that the ratio of the flow rate of absorbent and feed gas in various operating parameters is an important factor for predicting the performance of the absorption process and the ammonia loss rate. Also, as the concentration of containing ammonia in absorbent increased, the CO2 absorption rate increased. The CO2 absorption rate and ammonia loss increased from 30% to 97% and from 12% to 25%, respectively, as the concentration of containing ammonia in absorbent increased from 2 wt% to 12 wt%. The concentration of ammonia is too low, the absorption capacity of the absorbent becomes too low to reduce the CO2 absorption rate. Conversely, if the concentration of ammonia is increased, the absorption rate of CO2 can be increased. However, as the ammonia partial pressure at the top of the absorber increases, the evaporation amount of ammonia increases, resulting in an increase in the ammonia loss. Consequently, the absorption process design must determine the ammonia concentration in consideration of both the CO2 absorption rate and the ammonia loss and featured that they are in a trade-off relationship with each other.

As expected in the experiment using low temperature solution, the ammonia loss rate is lower due to the reduced evaporation of absorbent in the lower temperature range, but CO2 removal efficiency decreases significantly. According to the experimental result of ammonia solution at 2°C, CO2 removal efficiency is 80%, which is lower than commercialized standard efficiency (85%), but the ammonia loss rate is lower than 1%, and its level can operate the absorber column without the washing column. The influence of feed gas temperature on ammonia loss is very small; therefore, it is not feasible to cool the feed gas at low temperature. However, in the case of liquid temperature, the CO2 removal efficiency was increased by increasing the absorbent temperature, though at the same time ammonia loss was also increased. Therefore, the absorber should operate at optimal operating conditions. In our experimental results, the optimal inlet temperature of the solution is 7°C, whereas the optimal feed gas temperature is 10°C.

Renewable energy generally refers to energy collected from renewable resources that are naturally supplemented in human time zones, such as solar, bio, wind, hydro, ocean, waste, and geothermal. Biogas is a biofuel, an alternative energy source that includes methane, carbon dioxide, sulfur, and siloxane compounds that are generated during anaerobic digestion of organic matter. However, the sulfur compounds and siloxane compounds contained in the biogas are harmful gases that cause abnormal operation of the fuel source and deterioration of efficiency. Therefore, it is essential to separate and remove the carbon dioxide, sulfur and siloxane compounds in the biogas to solidify the methane. In this dissertation, therefore, the possibility of removing impurities in biogas by using multi-stage adsorption purification process and membrane gas separation process using polysulfone hollow fiber membrane was investigated through experiments and mathematical analysis.

The removal of sulfur compounds and siloxanes, which are major impurities in the biogas produced from the anaerobic digestion of sewage sludge, was studied using a bench-scale adsorptive gas purification experimental setup. The main impurities are hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The commercially available adsorbents iron oxide (IO), iron oxide hydroxides (IH, IHS), activated carbon (AC), impregnated activated carbon (IAC), silica gels (A2 and NS10) and molecular sieves (5A and 13X) were first extensively characterized using scanning electron microscopy (SEM), X-ray fluorescence (XRF), and BET surface area measurements. IHS, comprising mainly 42% iron oxide hydroxide, 11% silica gel, and 10% activated carbon, exhibited the best adsorption capacities for H2S (539 mg/g) and COS (32 mg/g) among the adsorbents studied, as well as relatively good adsorption capacities for siloxanes D4 and D5. AC and IAC showed the greatest CS2 removal efficiency. A2 demonstrated extremely high adsorption capacities for siloxanes D4 and D5, namely 1055 and 1968 mg/g, respectively.

An experimental investigation was performed to suggest and demonstrate a multi-stage adsorption process that can simultaneously and more effectively remove major impurities (e.g., H2S, COS, CS2, and siloxanes D4 and D5) from methane- and carbon dioxide-rich biogas, using commercially available adsorbents including iron oxide (IO), iron oxide hydroxides (IH and IHS), activated carbon (AC), impregnated activated carbon (IAC), silica gels (A2 and NS10), and zeolites (5A and 13X). Five candidate adsorbents were first selected by dynamic adsorption analysis of each individual adsorbent for binary gas mixtures containing a trace impurity gas in a nitrogen balance; subsequently, three adsorbents were selected as the most promising candidates for the multi-stage adsorption process via dynamic breakthrough measurements using a simulated biogas mixture. Furthermore, using dynamic breakthrough tests on a series of configurations based on a tandem arrangement of the three adsorbents using the simulated biogas mixture, it was demonstrated that the optimal packing configuration for the multi-stage adsorptive purification process, in which the maximum increase in breakthrough time was achieved, consisted of AC, A2, and IHS along the gas-flow direction. Furthermore, the performance of gas separation by PSF-A hollow fiber membrane was evaluated to design a hybrid biogas purification process integrate adsorption and membrane systems. A bench-scale experimental setup was designed to analysis the performance evaluation of hollow fiber membranes using binary gas mixtures and simulated biogas mixture (CH4, CO2, H2S, COS, and CS2). The purity and recovery of CH4 and the removal efficiency of sulfur compounds (H2S, COS, and CS2) were measured for the pressure of feed gas flow rate and the various stage cut, respectively.

In this dissertation, the performance of the carbon dioxide capture process using the room temperature and low temperature aqueous ammonia, the multi-stage adsorption process and the biogas purification process using the hollow fiber membrane were evaluated. The high capture efficiency of the CO2 capture process using the aqueous ammonia experimentally demonstrated the possibility of commercialization of the process, and it is expected to be a more valuable CO2 capture technology if an inexpensive cooling source such as deep seawater is to be obtained in the future. In addition, studies on the multi-stage adsorption process and the biogas purification performance of PSF-A hollow fiber membranes are expected to contribute to the reduction of the maintenance cost by improving the purification performance through the effective process configuration and simplifying the purification process.