Experimental studies on cooling/compressing of carbon dioxide captured from oxy combustion

The importance of coal in energy generation is becoming highly essential. Due to its broad availability, safe and secure supply, low cost, coal is very popular. Among fossil fuels, the world's coal reserves remain the largest in terms of energy content. One of the most important challenge in environmental protection is global warming, which is caused by large amounts of greenhouse gas emissions, especially carbon dioxide. As known, carbon dioxide amount in the atmosphere is increasing rapidly as a result of use of primary fossil fuels in various fields. Precautions must be taken to reduce carbon dioxide emissions and consequently prevent global warming. Therefore, carbon capture and storage (CCS) is considered to be one of the most significant methods in carbon dioxide reduction, since it is reported that 90% of carbon dioxide emissions are generated by the combustion of fossil fuels. There are three main technology options exist for carbon dioxide capture: post-combustion, pre-combustion and oxy-combustion. Among post and pre-combustion carbon dioxide capture from industrial processes, chemical or physical sorption and membrane separation are prevailing fields of research. However, oxy-combustion is starting to take attentions currently for carbon capture. Oxy-combustion process includes the removal of nitrogen from the air with an air separation unit (ASU). In oxy-combustion, the fossil fuel is combusted with near-pure oxygen using recycled flue gas stream that is highly enriched in carbon dioxide to control the combustion temperature and to ensure proper heat transfer. In the oxy-fuel combustion system, high concentration of carbon dioxide (80–90%, dry basis) in the flue gas makes it easy and economical to capture it with the flue gas compression train. It also emits some major pollutants (SOx, NOx, trace metals including mercury and particulate matter) as well as conventional air-fired combustion. Carbon dioxide capture with cryogenic separation is a unique separation method which separates carbon dioxide under low temperatures. Separation of carbon dioxide from flue gas of oxy-fuel combustion by liquefaction method also condenses the carbon dioxide from the flue gas but uses higher pressures and temperatures than cryogenic distillation. According to the carbon dioxide phase transition properties, by manipulating both temperature and pressure, liquid carbon dioxide could be easily obtained. Therefore, liquefied carbon dioxide could be separated from other non-condensable gases such as oxygen and nitrogen. Once carbon dioxide is captured, with the high pressures it can be transported to the storage area by pipeline, ship, rail or road. The choice of transport depends on the quantity of carbon dioxide that needs to be transported, the distance and terrain to be travelled. Transporting carbon dioxide via pipeline is the most cost effective method of transport. Transporting by ship may be more economical if carbon dioxide needs to be transported over very large distances (>1000 km) while transporting by rail or road is expected to be economical for moving carbon dioxide on a small scale. Transporting carbon dioxide as a solid is not currently cost-effective method. Carbon dioxide can be stored into deep saline aquifers or depleted oil and gas reservoirs. Also, it can be used to enhance recovery of valuable fossil fuels, such as oil, natural gas, and coalbed methane. The use of oil or gas reservoirs, whether producing or depleted, has an economic advantage if injection can enhance hydrocarbon recovery. Once geologic storage may not be an optimal solution, carbon dioxide utilization efforts novel approaches for reducing carbon dioxide emissions. Carbon dioxide can be used in applications that could generate significant benefits. Captured carbon dioxide can be used or converted to useful products such chemicals, cements, or plastics. The aim of this work is to develop a novel lab-scale experimental set-up for separation carbon dioxide from flue gas of oxy-fuel combustion by liquefaction method which is based on pressurizing. In the second and third chapter, in order to understand the process completely, several studies about oxy fuel combustion and carbon capture and storage have been investigated in detail. Based on the studies which had been carried out and recorded the literature, designing of process forms the beginning of the fourth chapter of the study. Fourth chapter also refers manufacturing of experimental set up, encountered problems, modifications on the units. Experimental setup installed in the laboratory consists of five main units: Simulating flue gas composition, separation of water from highly enriched carbon dioxide simulated flue gas, carbon dioxide capture and storoge, PLC system and gas chromatography system. In the fifth chapter of the study, impact of the impurities on liquefaction of carbon dioxide are investigated and their results are presented. Nitrogen and oxygen are designated as the main impurities in the flue gas. Experiments based on cooling and compressing of the flue gas are designed in five groups. The behaviour of liquefied pure carbon dioxide in nitrogen and oxygen environment, the effects of nitrogen and oxygen on liquefaction of carbon dioxide separately and together and the performance of condensing water vapor fed to the system were tested. For using liquefaction method in our study, a specific compressor is designed and utilized to increase the pressure of simulated flue gas in three steps up to 150 bar. One active carbon and one demister column in addition to three heat exchangers have been used to remove the steam from the line. Owing to the experiments in our system, liquefaction of carbon dioxide have been successfully and noticeably carried out at 10°C and 46 bar at the final separation tank. The tank's temperature has been kept constant (about 10°C) by soaking in a waterbath. The water of waterbath is cooled by chiller continuously. Carbon dioxide-nitrogen-oxygen mixture, as well as steam, has been used as simulated flue gas which is common in oxy-fuel combustion. Generally, some impurities such as nitrogen and oxygen dissolve in the liquid carbon dioxide separated under high pressure. With higher separation pressures, more impurities could dissolve so that, the separation ratio can decrease. In order to investigate separation process deeply, the effect of impurities on the amount of "carbon dioxide separation ratio" have been studied. According to the experiments, by increasing the ratio of oxygen to nitrogen in the mixture, the carbon dioxide separation process seems to be getting harder than the case of using just nitrogen as impurity. It could be related to the different physical properties behavior of "oxygen with carbon dioxide" in comparison to "nitrogen with carbon dioxide". With the modifications in "Laboratory Scale Carbon Dioxide Capture and Storage Experimental Set-up" carbon dioxide can be liquefied from highly enriched flue gas successfully. Up to about 85% separation ratio has been recorded in single carbon dioxide separation unit for carbon dioxide-nitrogen mixture at an initial carbon dioxide concentration of 95%. Highest carbon dioxide separation ratios for both carbon dioxide-nitrogen and carbon dioxide-oxygen mixtures are obtained at same pressures for the same initial compositions. The flue gas composition of oxy-fuel combustion test results which have been received from TUBITAK-MAM Energy Institute were used in experiments (85% CO2 / 8 N2 /7 O2). Test results determine that more than 67% of carbon dioxide is separated at 90 bar. With lower carbon dioxide concentration at the same pressure, separation efficiency of carbon dioxide has fallen to 52% while with the higher the concentration of carbon dioxide efficiency has increased to 77% at 65 bars. Therefore, it can be said that increased carbon dioxide concentration in the flue gas also increases the separation efficiency. Experiment which is conducted with water shows that more than 95% of steam has been condensed in the first water separation tank.