Effects of Injection Strategy on Combustion and Emission Characteristics in a Direct Injection Spark Ignition Engine

이호승 한양대학교 대학원 2025 국내박사

To reduce greenhouse gas emissions, a direct injection (DI) system was introduced to increase performance and reduce exhaust emissions in a downsized engine compared with a port fuel injection (PFI) system when fossil fuel conditions are the same. However, the DI system emits more ultrafine particles than the PFI system, which has led to regulations and research to reduce particulate emissions. This study focuses on the injection characteristics including entrained lubricant and spray interaction, dual-fuel system, oxygenated fuels, and nozzle hole layout that affect particulate formation factors on combustion characteristics and exhaust emissions in a DISI engine under various engine operating conditions. An instrumented combustion single-cylinder DISI engine was used to measure the combustion characteristics and gaseous exhaust emissions, and single- and two-cylinder optical DISI engines were used to capture the spray and flame images. To identify the effect of entrained lubricant and spray interaction, fuel blends composed of lubricant and gasoline in different ratios were used and experimented with various injection pressures and timings. The combustion characteristics and gaseous exhaust emissions indicated that the lubricant did not influence the combustion performance and mixture homogeneity but had an impact on the unburned fuels. The spray behavior and flame image showed that the lubricant did not influence the spray but influenced the wall film formation during the injection period, which is a major factor affecting particulate matter generation. Particulate emissions indicated that the lubricant included in the wall film significantly affected PN emissions depending on the injection conditions. Additionally, the influence of the lubricant on the wall film affects the overall particle size and its distribution. The effect of utilizing methane was identified by injecting methane using a PFI injector and injecting gasoline using a DI injector at different injection ratios, which were conducted with various injection pressures and timings. Combustion characteristics showed that at G70M30, an increase in the intake pressure and spray momentum from gasoline injection compensated for the slow flame propagation speed of methane, leading to an increase in IMEP; however, a further increase in the methane ratio resulted in the dominance of the burning velocity of methane, reducing IMEP. The gaseous emissions indicated that the addition of methane increased the mixture homogeneity, lowered the combustion temperature, and reduced the carbon-hydrogen ratio. An increase in the methane ratio decreased the wall film caused by gasoline injection and increased mixture homogeneity, resulting in a reduction in PN emissions. However, gasoline injection is still required to aid air/fuel mixture formation by facilitating in-cylinder flow, which results in lower PN emissions compared to methane PFI injection. To observe the effect of oxygenated fuel, methanol was blended with gasoline at different ratios and investigated at various injection pressures and timings. The combustion results showed that methanol’s fast burning velocity, high latent heat of evaporation, and oxygen content increased the combustion speed at early injection timing, combustion completeness, and reduction of compression work at late injection timing, all leading to an increase in IMEP under all injection conditions. The gaseous exhaust emissions indicated that blending methanol could lower the carbon mass fraction, enhance mixture homogeneity, and lower the combustion temperature under all injection conditions. The lower boiling point and oxygen content of methanol reduced the wall film, enhanced the mixture homogeneity, and stimulated soot oxidation, leading to a reduction in PN emissions at lower injection pressure; however, the difference in PN emissions between fuel blends was reduced owing to better atomization and increased in-cylinder flow at high injection pressures. Particulate emission reduction targeted multi-hole DI injectors with one injector having a part of the injector nozzle enlarged and positioned at the middle of the nozzle holes, and another injector with a partially enlarged nozzle hole at the bottom were used and compared by changing one of the engine operating parameters, including engine speed, air-fuel ratio, and injection timing. Changing the engine speed and air-fuel ratio did not affect the combustion characteristics and gaseous exhaust emissions because the early injection timing did not affect the in-cylinder flow and mixture homogeneity. PN emissions decreased as the nozzle hole was partially enlarged and positioned at the bottom due to the spray from the enlarged nozzle hole, hindering the in-cylinder flow moving towards the exhaust side cylinder liner, which causes the spray to move in that direction. However, when the injection timing was retarded, the enlarged nozzle hole reduced spray momentum and kept the spray from reaching the exhaust side cylinder wall, but hindered the in-cylinder flow, resulting in a reduction in the combustion performance and an increase in the gaseous exhaust emissions. In conclusion, the lubricant-gasoline blend combustion and exhaust emission results showed that the physical properties of the lubricant affected wall film formation, causing a substantial amount of PN emissions, even with a small amount. Methane PFI-gasoline DI experiment showed that dual fuel system alleviated particulate formation factors which reduced exhaust emissions while minimizing engine performance degradation through increase in intake air flow rate. The methanol-gasoline blend experiment indicated that the physical and chemical properties of methanol can not only reduce PN emissions, but also increase engine performance and reduce exhaust emissions. The nozzle hole layout experiment showed that nozzle hole size and position can reduce PN emissions under various engine operating conditions but can affect combustion characteristics and exhaust emissions owing to the spray pattern affecting the in-cylinder flow. Therefore, combustion characteristics and exhaust emissions are important because they aid in analyzing the effect on particulate formation factors and engine performance, which is a crucial factor for downsized DISI engines. 온실가스 배출량을 감소시키기 위하여, 다운사이징 된 엔진에서 동일 연료 조건 아래 연소 성능을 향상하게 시키고 배기 배출량을 포트 분사(PFI) 방식에 비해 감소시키기 위하여 직분사(DI) 방식이 도입되었지만, 직분사 방식은 포트 분사 방식에 비하여 입자상 물질을 더 많이 배출하기 때문에 이를 줄이기 위한 규제와 연구가 진행되고 있다. 본 연구에서는, 다양한 엔진 운전 조건에서 유입된 윤활유와 분무 상호작용, 이중 연료 시스템, 함산소 연료, 그리고 인젝터 노즐 형상을 포함하는 입자상 물질 형성 요인에 영향을 주는 분사 특성이 직분사식 불꽃 점화 엔진의 연소 성능 및 배기 배출에 미치는 영향에 대하여 분석하였다. 이를 위하여 단기통 직분사 엔진을 사용하여 연소 특성과 가스상 배기 배출물을 측정하였고 단기통 및 2기통 직분사 엔진을 사용하여 분무 및 화염 이미지를 촬영하였다. 유입된 윤활유와 분무 상호작용에 대한 영향을 분석하기 위하여, 다양한 비율로 윤활유와 가솔린을 혼합하여 다양한 분사 조건에서 실험을 진행하였다. 연소 특성과 가스상 배기 배출물 결과를 통하여 윤활유는 연소 성능과 혼합기 형성에는 영향을 미치지 않았지만, 미연소 연료에는 영향을 미치는 것을 확인하였다. 분무 및 화염 가시화 결과를 통하여 윤활유가 분무 형상에는 영향을 주지 않지만, wall film 형성에는 영향을 주는 것을 확인하였다. 입자상 물질 개수를 확인한 결과, wall film에 포함된 윤활유로 인하여 분사 조건에 따라 입자상 물질 개수에 유의미한 영향을 미치며, wall film에 포함된 윤활유로 인하여 입자상 물질 크기와 분포도에도 영향을 미치는 것을 확인하였다. 메탄을 이용한 효과는 다양한 비율로 포트 분사 인젝터를 사용하여 메탄을 분사하고 직분사 인젝터를 사용하여 가솔린을 분사하여 확인하였으며, 실험은 다양한 분사 조건으로 진행하였다. 연소 특성을 분석한 결과, G70M30에서 증가한 흡기 압력과 가솔린의 분무 모멘텀으로 인하여 메탄의 느린 화염전파속도를 보상하여 IMEP가 증가하였으나, 그 이상으로 메탄이 분사되었을 경우 메탄의 화염전파속도의 영향이 더 지배적으로 되어 IMEP가 감소하였다. 가스상 배기 배출물 결과를 통하여 메탄 분사는 혼합기를 균질하게 하고, 연소 온도를 낮추며, 탄소 비율을 감소시켜 주는 것을 확인할 수 있었다. 메탄 분사 비율이 증가할수록 입자상 물질 개수가 혼합기 균질도 향상 및 가솔린으로 인한 wall film 감소로 인하여 줄어들지만, 메탄만 분사했을 경우에는 가솔린 분사로 인한 연소실 내부 유동이 없어 있는 경우에 비하여 혼합기 형성이 덜 되어 입자상 물질 개수가 증가하는 것을 확인하였다. 함산소 연료가 미치는 영향에 대하여 확인하기 위하여, 함산소 연료 중 하나인 메탄올을 가솔린과 다양한 비율로 혼합하고, 다양한 분사 조건에서 실험을 진행하였다. 연소 결과를 통하여 메탄올의 빠른 연소 속도, 높은 증발 잠열, 그리고 산소 함량이 이른 분사 시기에서 연소 속도를 증가시키고, 늦은 분사 시기에서 연소 완정성과 압축 손실을 줄여 모든 분사 조건에서 IMEP를 증가시켜 주는 것을 확인하였다. 이런 메탄올의 특성은 가스상 배기 배출물에도 영향을 주어 혼합기 균질도 향상, 연소 온도 감소, 그리고 탄소 함량 감소로 가스상 배기 배출물을 감소시켜 주는 것을 확인하였다. 메탄올의 낮은 끓는점과 산소 함량은 wall film 형성 감소, 혼합기 균질도 향상, 그리고 soot 산화를 가져와 낮은 분사 압력에서는 입자상 물질 개수를 감소시켜 주었으나, 높은 분사 압력에서는 분무 미립화 및 내부 유동 강화로 인하여 혼합물 간의 차이가 없는 것을 확인하였다. 인젝터 중 하나는 인젝터 중앙부에 위치한 노즐들의 직경을 키우고 나머지 하나는 하단부에 위치한 노즐들의 직경을 키운 다공 직분사 인젝터를 사용하여 엔진 운전 조건(엔진 속도, 공연비, 분사 시기) 중 하나를 변경하여 인젝터 간의 차이를 비교하였다. 엔진 속도 및 공연비를 변경하였을 경우에는 이른 분사 시기가 혼합기 형성 및 연소실 내부 유동에 영향을 주지 않아 연소 성능 및 가스상 배기 배출물에 영향을 주지 않았으나 노즐 홀의 직경이 커지고 아래쪽으로 이동할수록 입자상 물질 개수가 감소하였는데, 이는 직경이 커진 노즐에서 나온 분무가 흡기에서 배기 방향으로 이동하는 유동을 막아 분사된 연료가 배기 쪽 벽면에 닿는 것을 막기 때문이라는 것을 확인할 수 있었다. 하지만, 분사 시기가 지각될 경우에는 직경이 커진 노즐에서 나온 분무의 모멘텀이 줄어들어 배기 쪽 벽면에 분무가 닿는 것을 막아주지만, 동시에 연소실 내부 유동을 방해하여 연소 성능이 감소하고 배기 배출물이 증가하게 만든다는 것을 확인할 수 있었다. 결론적으로, 윤활유-가솔린 혼합 연료 연소 특성 및 배기 배출물 결과는 윤활유의 물리적 특성이 wall film 형성에 영향을 미쳐 소량에도 불구하고 많은 양의 PN 배출을 초래하는 것을 보여주었으며, 메탄 포트 분사-가솔린 직분사 실험은 이중 연료 시스템이 입자상 물질 형성 요인을 완화해 배기 배출물을 감소시켰으며 흡기 유량 증가를 통하여 연소 성능 하락을 최소화하였다. 메탄올-가솔린 혼합 실험은 메탄올의 물리적 및 화학적 특성이 PN 배출량을 감소시켜 주었을 뿐만 아니라 엔진 성능을 향상하게 시키고 배기 배출물을 감소시켰다. 노즐 홀 형상 변경 실험의 경우 노즐 홀 크기 및 위치는 다양한 엔진 운전 조건에서 PN 배출량을 감소시켜 줄 수 있지만 분무 패턴이 연소실 유동에 영향을 미치는 조건에서는 연소 성능 및 배기 배출물에 영향을 미칠 수 있다. 따라서, 연소 특성 및 배기 배출물을 분석하는 것은 입자상 형성 물질 요인을 분석하는 데 도움을 줄 뿐만 아니라 이는 다운사이징 된 직분사식 불꽃 점화 엔진에서 중요한 요소인 엔진 성능에 영향을 미치기 때문에 중요하다.

메탄엔진 축소형 연소기의 연소특성: 분사기 설계 및 작동 조건의 영향

강철웅 충북대학교 일반대학원 2026 국내박사

This study experimentally investigated the effects of injector design variables and operating conditions on combustion characteristics in a 500 N sub-scale thrust chamber. For this study, cold-flow tests using simulated propellants, high-pressure hot-firing tests using liquid oxygen/gaseous methane, and hot-firing tests varying the temperature of the propellant injection were conducted. Additionally, research was conducted on the laser ignition characteristics of a LOX/LNG thrust chamber equipped with a porous injector using a laser igniter which is the next-generation ignition method. To investigate the spray characteristics of shear coaxial and swirl-coaxial injectors based on recess length and taper presence, measurements of injection pressure drop, discharge coefficient, breakup length, and spray angle were conducted using cold-flow tests with water and air as simulated propellants. The fuel injector exhibited less friction loss and a smaller injection pressure drop as the recess length increased. In the shear coaxial injector, the presence of the recess and taper tended to promote propellant breakup, resulting in a shorter breakup length. Conversely, the swirl coaxial injector showed a longer breakup length and a larger spray angle due to the presence of the recess and taper. Based on the results for breakup length and spray angle, the swirl coaxial injector is judged to have better atomization performance than the shear coaxial injector and is also expected to have improved combustion performance. The effects of the recess length, presence of a taper, and oxidizer swirl on combustion characteristics in a sub-scale thrust chamber were verified through hot-firing tests. Hot-firing tests were conducted using liquid oxygen/gaseous methane as the propellant, under various combustion chamber pressures and mixture ratios based on the design point. Injectors with long recess lengths exhibited improved propellant atomization and mixing within the recess region, leading to increased combustion efficiency and heat flux. The presence of a taper reduced the liquid oxygen velocity at the oxidizer injector exit. This increased the momentum flux ratio with the gaseous methane, promoting propellant breakup and enhancing combustion efficiency. In the swirl coaxial injector, the presence of the taper suppressed low-frequency combustion instabilities. Finally, the combustion efficiency and heat flux results for the swirl coaxial injector were better than those for the shear coaxial injector. In an expander cycle methane engine, fuel is injected into the combustion chamber in a high-temperature gaseous state. Therefore, hot-firing tests were conducted to change the fuel injection temperature, confirming the effect of fuel temperature on combustion characteristics. Additionally, the combustion characteristics were investigated with liquid oxygen injection temperature. The average characteristic velocity efficiency was approximately 1.33% higher when the liquid oxygen injection temperature was 130 K compared to 155 K. This is attributed to reduced atomization performance caused by two-phase flow at higher oxidizer temperatures. Lower oxidizer temperatures increase density, reducing injection pressure drop and potentially increasing susceptibility to low-frequency combustion instability. The high injection temperature of gaseous methane accelerated the methane injection velocity and increased the momentum flux ratio. This improved the propellant atomization performance, resulting in a 6.78% increase in the average combustion efficiency. Furthermore, it was confirmed that the increased injection pressure drop due to the higher methane temperature can suppress low-frequency combustion instability. Laser ignition tests were conducted in a porous injector combustor using LOX/LNG as propellants to analyze the effect of propellant valve sequencing on hard start and ignition delay. Under oxidizer-lead conditions, the increased cumulative oxidizer mass prior to ignition led to higher instantaneous pressure in the combustion chamber at ignition, leading to an increased risk of hard start. The high fuel injection velocity and low propellant mixture ratio at the fuel-lead and short oxidizer-lead conditions prevented the flame kernel from anchoring to the injector, causing ignition delays and ignition failures. The sequence where the oxidizer is pre-supplied for approximately 0.005 seconds is presented as the most suitable ignition condition in this study. The results of this study provide experimental evidence and design guidelines for the design of a methane engine coaxial injector, the influence of propellant temperature, and the laser ignition sequence configuration. This can provide useful fundamental data for the future development of methane-based liquid rocket engines. 본 연구는 3 톤급 메탄엔진을 모델로 설계된 500 N급 단일분사기급 연소기를 대상으로 연소특성을 실험적으로 규명하고, 분사기 설계변수와 작동 조건이 연소특성에 미치는 영향을 분석하였다. 이를 위해 모사 추진제를 활용한 상압 수류실험, 액체산소/기체메탄을 사용한 고압 연소시험, 그리고 추진제 분사의 온도를 변화시킨 연소시험을 수행하였다. 추가적으로 최근 차세대 점화 방법인 레이저 점화기를 활용한 LOX/LNG 연소기의 레이저 점화특성 연구를 수행하였다. 전단동축 분사기와 와류동축 분사기의 리세스 길이와 테이퍼 유무에 따른 수류특성을 확인하기 위해 물과 공기를 모사 추진제로 사용하는 상압 수류실험을 통해 분사차압, 유량계수, 분열길이, 분무각을 측정하였다. 연료 분사기는 리세스 길이가 길어지면 마찰 손실이 적어 분사차압이 작았다. 전단동축 분사기에서 리세스와 테이퍼의 존재는 추진제의 분열을 촉진시켜 분열길이가 짧아지는 경향을 보였다. 반면에 와류동축 분사기는 리세스와 테이퍼의 존재로 인하여 분열길이가 길어지고, 분무각이 커졌다. 분열길이와 분무각 결과를 통해 와류동축 분사기가 전단동축 분사기에 비해 미립화 성능이 우수하며, 연소성능도 우수할 것으로 판단된다. 단일분사기급 연소기에서 전단동축 분사기와 와류동축 분사기의 리세스 길이와 테이퍼 유무가 연소특성에 미치는 영향을 연소시험을 통해 확인하였다. 액체산소/기체메탄을 추진제로 사용하며, 설계점을 기준으로 다양한 연소실 압력과 혼합비 조건에서 연소시험이 수행되었다. 긴 리세스 길이를 갖는 분사기는 리세스 영역에서 추진제의 미립화와 혼합이 이루어져 연소효율과 열유속이 증가하였다. 테이퍼의 존재는 산화제 분사기 출구에서의 액체산소의 분사 속도를 감소시켜 기체메탄과의 모멘텀 플럭스 비가 커져 추진제의 분열이 촉진되어 연소효율이 증가하였다. 와류동축 분사기에서 테이퍼의 존재는 저주파 연소불안정을 억제하는 효과를 보였다. 마지막으로 와류동축 분사기의 연소효율과 열유속 결과가 전단동축 분사기보다 더 나은 결과를 보였다. 팽창식 사이클 메탄엔진에서 연료는 고온의 기체 상태로 연소실에 분사된다. 따라서 연료의 분사 온도 변경 연소시험을 통해 연료 온도가 연소특성에 미치는 영향을 확인하였으며, 추가적으로 액체산소의 분사 온도에 대해서도 확인하였다. 액체산소의 분사 온도가 130 K인 경우가 155 K인 경우보다 평균 특성속도 효율이 약 1.33% 높게 나타났으며, 이는 높은 산화제 온도에서 이상 유동으로 인한 분무 성능 감소에 기인한 것이다. 산화제 온도가 낮을수록 밀도는 증가하여 분사차압이 감소해 저주파 연소불안정에 취약해질 수 있다. 기체메탄의 분사 온도 증가는 분사속도와 모멘텀 플럭스 비를 증가시켜 연소효율이 향상되었으며, 분사차압 증가로 저주파 연소불안정을 억제할 수 있음을 확인하였다. LOX/LNG를 추진제로 사용하는 다공성 분사기 연소기에서 레이저 점화시험을 수행하여 추진제 밸브 시퀀스가 하드 스타트 및 점화 지연에 미치는 영향을 분석하였다. 산화제 리드 조건에서 점화 전 누적 산화제 질량이 증가할수록 점화 순간 연소실의 순간 압력이 커져 하드 스타트 위험이 증가하였으며, 연료 리드 및 짧은 산화제 리드 조건에서는 높은 연료 분사 속도와 낮은 추진제 혼합비로 인해 화염 핵이 분사기에 안착되지 못해 점화 지연이 발생하였다. 본 연구의 조건에서는 산화제가 약 0.005초 짧게 선 공급되는 시퀀스가 가장 적절한 점화 조건으로 제시된다. 본 연구 결과는 메탄엔진 동축 분사기 설계, 추진제 온도의 영향, 그리고 레이저 점화 시퀀스 설정에 대한 실험적 근거와 설계 지침을 제공하여 향후 메탄 기반 액체로켓엔진 개발에 유용한 기초 자료로 활용될 수 있다.

Effect of Co/Ni ratio and Ce amount in cobalt nickel mixed oxide catalyst for methane combustion

임태환 서울대학교 대학원 2015 국내석사

In view of the low emissions of nitrogen oxides, carbon monoxide and toxic hydrocarbons, natural gas vehicles (NGVs) have been attracted considerable attention as one of the substitutes for gasoline and diesel vehicles. However, the emission of unburned methane from NGVs heavily contributes to greenhouse effect since the global warming potential of methane is 21 times higher than that of carbon dioxide. This study aimed at investigating the effect of Co/Ni ratio of cobalt nickel mixed oxides and cerium doping on CoNi (50:50) catalyst for methane combustion to effectively reduce unburned methane. A series of cobalt nickel mixed oxide catalysts were prepared by co-precipitation method with the Cobalt oxide, Nickel oxide, CoNi (75:25), CoNi (67:33), CoNi (50:50) and CoNi (33:67). The notation of CoNi (X:Y) was used to designate the catalyst with the ratio of Co:Ni = X:Y. In addition, a series of cerium doped CoNi (50:50) catalysts were prepared by wet impregnation method with the cerium loading of 5 wt%, 10 wt% and 20 wt%. The catalytic combustion of methane was performed among all prepared catalysts. Various characterizations were performed such as N2 adsorption-desorption with BET method, ICP-AES, XRD, EXAFS, XPS and H2 TPR. It was found that CoNi (50:50) and CoNi (67:33) catalyst exhibit the superior activity for methane combustion. Both catalysts contain NiCo2O4 spinel structure in largely distorted form. Such structure disorder contributes to improvement for the adsorption of surface oxygen species and reducibility of NiCo2O4. In addition, cerium doped catalyst demonstrates the enhancement of the activity. Especially, 10 wt% cerium doped catalyst indicates the highest activity among the catalysts. It implies that cerium doping as well as structure disorder plays an important role for the methane combustion. Both characterization and reaction results lead us to the conclusion that not only optimized cobalt nickel mixed oxide but also proper amount of cerium doping on CoNi (50:50) can improve the activity of methane combustion.

Optimization of geometric configuration and fuel flow rate in a microchannel reactor for steam reforming of methane

전승원 Graduate School, Korea University 2014 국내박사

There are some limitations in a heat exchanger integrated microchannel reactor for steam reforming of methane that hot spot formation is inevitable because of the imbalance between the generated and absorbed heat, and a lot of time and energy are required for reactor start-up. Therefore, it is necessary to optimize geometric configuration and fuel flow rate in a microchannel reactor for steam reforming of methane to minimize hot spot and achieve rapid start-up. First of all, a stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by a response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 145 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.94 and 2.37, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 6.0% less than that of the uniform stripe catalyst layer. Secondly, hydrogen assisted catalytic combustion was applied for rapid start-up. Hydrogen assisted catalytic combustion does not require preheating because the catalytic combustion of hydrogen occurs at room temperature. After start-up by hydrogen catalytic combustion, fuels of hydrogen and methane were changed to methane. The geometric configuration of the counter-flow reactor was optimized by the simulation model under steady state condition. The hydrogen flow rate in the counter-flow reactor was also optimized by transient simulations using the response surface methodology. As a result, the counter-flow reactor showed extremely short start-up time because of the optimized configuration and the optimized hydrogen flow rate. Hot spots were avoided because of the hydrogen shut-off after start-up. The operating characteristics of the counter-flow reactor were compared with those of the co-flow reactor.

A Catalytic Study on the Effective Utilization and Conversions of C1 a Materials : Methane-Dry Reforming, Methane Combustion,and Preferential Oxidation of CO in H_(2)-rich Reformate

이성호 Graduate School Korea University 2002 국내박사

For the effective utilization and conversion of C1 material, three different catalytic systems were considered in this thesis; methane-dry reforming, catalytic combustion of methane, and preferential CO oxidation. Currently, the issue for the methane-dry reforming catalyst is to overcome the coking, which deactivates the catalyst during the reaction. Therefore, it is tried to solve the problem developing the anti-coke catalysts with various types of Labased perovskites. Perovskite-type oxides suc h as La_(1-x)Sr_(x)CoO_(3), La_(1-x)Sr_(x)NiO_(3) and La_(2-x)Sr_(x)NiO_(4) catalysts were tested for the methane-dry reforming. They were transformed during methane-dry reforming to a stable and highly effective catalytic system of well-dispersed Ni over metal oxides. The transformation was subjected to the type of perovskite and the amount of Sr-substitution, which resulted from the removal of lattice oxygen by Sr-substitution promoted by the reducing atmosphere of the reaction (CH_(4)/CO_(2) = 1). When La_(0.9)Sr_(0.1)NiO_(3) and La_(1.8)Sr_(0.2)NiO_(4) were used as starting catalyst, the highest catalytic activity was obtained without coke formation. The reaction over these catalysts is considered to proceed by the catalytic reaction of CO_(2) adsorbed on La_(2)O_(3) and activated CH_(4) over well dispersed Ni sites. Also, the perovskite-type catalysts were inspected for the methane combustion. The focal point of the catalyst development was to establish the preparation methods of catalysts with high surface area, which can be adapted to the wide temperature range. At first, Using the citrate precursor and spray-freezing/freezedrying, LaCoO_(3) catalyst with high surface area, 20.0 m^(2)/g, could be prepared at 800℃ calcination temperature. Its surface area is the highest value among LaCoO_(3) catalysts reported up to date and also the catalyst showed the highest activity for the methane combustion among the catalysts prepared in this study. Therefore, it is thought that the preparation method of perovskite for the middle temperature application is chieved. Secondly, using the malate precursor and evaporation drying, LaCoO_(3) catalyst with high surface area, 4.6 m^(2)/g, could be prepared at 1200℃ calcination temperature. The surface area is the highest value among the perovskite containing Co calcined at 1200℃ reported up to now and also the catalyst showed the highest activity for the methane ombustion among the catalysts calcined at 1200℃ in this study. Therefore, it is also thought that the preparation method of perovskite for the high temperature application is achieved. For the last results, using citrate and spray- freezing/freeze-drying method, the calcination temperature for a single phase of perovskite, LaCoO_(3) could be downed to 500℃ and the product had high surface area, 24 m^(2)/g, which can be applied for the low catalytic combustion. It was confirmed by several analyses of dried precursor that LaCoO_(3) with high surface area could be prepared by the synergy of the physical and chemical contributions of spray- freezing/freeze-drying to the homogeneity of the dried precursor. Differently from the preliminary studies, which deal with the sorts of the effective utilization of methane, the last study was performed for the catalytic purification of CO. Main role of fuel processor for PEMFC is to generate hydrogen by reforming hydrocarbon. However, the reformed gas contains some amount of CO that can poison severely the anode of PEMFC. Therefore, the catalyst and reactor developments were performed to reduce 1~2% of CO in reformed gas to under 20 ppm, especially for fuel cell vehicle (FCV). Pt-Ru catalysts with various atomic ratios of Ru/Pt were screened and the optimum ratio was determined. Using the catalyst, the reactors for 1kWe and 10 kWe PEMFC were designed as compact as possible for vehicle application, and the performances were evaluated. In 1 kWe application, the outlet CO concentration was under 10 ppm in both steady state and transient conditions consuming 0.7% of hydrogen. The specific volume [L/kWe] of catalyst in 10 kWe PROX was almost a half of that of 1 kWe. Nevertheless, it showed good performances that the outlet CO concentration was under 20 ppm at steady state and under 30 ppm in load transient condition, consuming 1.8% of hydrogen. The volume [0.25 L/kWe] of 10 kWe PROX was only 19% of the fuel processor volume (1.33 L/kWe) which is corresponding to power density (0.75 kWe/L) for 2004 PNGV goals. Therefore, we can conclude that 10 kWe PROX system with high performance and compactness was developed. Also, it is thought that the good performances of the reactors could be achieved by the catalyst with high activity and the reactor design with low pressure drop, even where small size of catalyst was loaded.

액체메탄-액체산소 소형 로켓 연소실의 연소특성에 관한 수치적 연구

이태윤 국립부경대학교 대학원 2026 국내석사

With the increasing demand for reusable and cost-effective space launch vehicles, liquid methane (LCH4) has emerged as a promising propellant owing to its high specific impulse, non-toxicity, in-situ resource utilization (ISRU) compatibility, and minimal coking characteristics suitable for reusable engines. However, the combustion of LCH4-LOx involves complex multi-physics phenomena, including cryogenic liquid injection, atomization, vaporization, turbulent mixing, and chemical reactions. Therefore, developing a reliable and efficient numerical methodology is essential for optimal engine design and performance prediction. In this study, three-dimensional computational fluid dynamics (CFD) simulations were conducted to investigate the combustion characteristics and flow structures of a small LCH4-LOx rocket combustor. The computational domain was extended from the injector recess through the combustion chamber and converging-diverging nozzle to the external plume region, enabling simultaneous observation of internal combustion phenomena and external supersonic flow characteristics including shock diamond structures. To accurately capture the turbulent reacting flow, the Reynolds-Averaged Navier-Stokes (RANS) equations were solved with the k-ω SST turbulence model, and the Eddy Dissipation Concept (EDC) model was employed for turbulence-chemistry interactions. A detailed chemical mechanism (DRM-19) consisting of 21 species and 84 reactions was implemented to resolve the methane-oxygen combustion process. Advanced acceleration techniques including Chemistry Agglomeration, Dynamic Mechanism Reduction (DMR), and In-Situ Adaptive Tabulation (ISAT) were introduced to mitigate the computational expense associated with detailed chemistry. The multiphase flow characteristics of cryogenic propellants were simulated using the Eulerian-Lagrangian approach, where the continuous gas phase was solved with the Eulerian framework and the discrete liquid droplets were tracked in a Lagrangian manner. The Wave breakup model was applied to simulate secondary atomization processes. As a results, the temperature field analysis revealed distinct swirl-induced combustion structures characterized by a low-temperature core along the centerline and high-temperature zones near the chamber walls. As the equivalence ratio increased, the high-temperature region extended upstream and distributed more continuously throughout the combustion chamber. Species distribution analysis showed that increasing equivalence ratio enhanced fuel-oxidizer mixing efficiency, resulting in improved characteristic velocity. At the lowest equivalence ratio of 1.20, significant amounts of unreacted oxygen were observed even at the nozzle exit, indicating incomplete mixing. In contrast, the highest equivalence ratio of 2.06 exhibited the widest and most intense flame zone, as indicated by OH radical distribution, effectively utilizing the combustor volume. Flow field analysis identified strong recirculation zones at the upstream corner regions, which contributed to flame stabilization and promoted droplet atomization and mixing. The strongest recirculation structure was observed at equivalence ratio 2.06, forming a clear concentric vortex pattern. In the external flow region, shock diamond patterns including Mach disks were clearly captured, demonstrating the capability of the numerical method to resolve complex supersonic expansion flows. This study presents a validated and efficient combustion analysis framework incorporating detailed chemical kinetics for small LCH4-LOx rocket engines. The developed methodology successfully captures the complex interactions among turbulent flow, spray dynamics, and finite-rate chemistry with reasonable computational cost. The findings provide valuable insights into the effects of equivalence ratio on mixing efficiency, combustion intensity, and thrust performance. The proposed numerical approach can serve as a practical design tool for injector geometry optimization, equivalence ratio selection, and thermal management in the development of next-generation methane-fueled propulsion systems.

Flame Structure and Combustion Dynamics of GOX/GCH4 Diffusion Flames in a Pintle Injector and the Effects of the Pintle Tip Angle

김대환 서울대학교 대학원 2025 국내박사

국제 발사체 시장의 급속한 성장은 SpaceX의 Falcon 시리즈 발사체로부터 시작된 급격한 발사 비용 감소로부터 촉발되었습니다. Falcon 시리즈 발사체의 Merlin 엔진이 핀틀 분사기를 사용했기 때문에, 비록 수십년 전에 개발된 후로 잊혔지만 최근에 다시 연구 주제로서 주목받게 되었습니다. 핀틀 분사기의 기본 특성에 관한 연구는 많지 않은데, 이는 다양한 설계 변수들이 비독립적이고 조절이 어렵기 때문이기도 하지만 문헌 등에서 제시되는 핀틀 분사기의 설계 방침들이 정확한 실험결과로 뒷받침되지 않았기 때문이기도 합니다. 대략 지난 10년간 핀틀 분사기의 기본 특성에 관한 다양한 연구가 이루어졌지만, 연소 동특성에 관한 연구는 고압 환경에서의 압력 섭동 세기 등을 측정한 몇 가지 연구를 제외하고는 거의 없었습니다. 핀틀 분사기의 연소 동특성에 관한 이해는 로켓 연소기 설계에 있어 매우 중요하며, 일부 실험 결과 들에서 연소 안정성에 관한 의견이 상충되는 바가 발견되었기에, 이러한 점들을 고려하여 본 학위논문은 핀틀 분사기의 기본적인 연소 동특성에 관하여 연구하는 것을 목표로 했습니다. 일부 문헌 들에서 핀틀 연소기 내부의 맨틀과 중심 재순환 구조로 이루어진 독특한 유동장 구조가 강건한 연소 안정성을 제공하는 주된 원인으로 꼽혔으며, 특히 맨틀 재순환 영역의 크기가 연소 동특성에 강한 영향을 끼친다고 예상되어 이를 확인하는 연구를 진행하고자 했습니다. 우선, 기존 선행연구에서 핀틀 분사기에 대한 연소기 경계조건이 부적절했기에, 수평형 연소기에 있어 새롭게 설계되어야 하는 경계조건들에 관해 확인하는 절차를 거쳤습니다. 핀틀 분사기는 단일 연소기에 단 한개의 분사기만이 사용되어 경계조건들이 화염 특성에 끼치는 영향이 특히 더 크다고 예상되기에 꼼꼼하게 확인한 결과를 바탕으로 연구 내에서 다음 단계의 실험들을 설계할 수 있었습니다. 이후, 일반적인 핀틀 분사기와 기울어진 핀틀 분사기의 화염을 형태학적 관점에서 주로 분석했습니다. 반경방향 유동을 축 방향으로 꺾어주는 각도의 크기를 뜻하는 '핀틀 팁 각도'는 총 네 단계로 준비되어 핀틀 팁 각도를 조정함으로써 화염각을 줄이고 맨틀 재순환 영역의 크기는 증가시킬 수 있도록 했습니다. 다양한 혼합비와 유량 그리고 기하학적 형태에 관하여 그 형태를 분석한 결과, 동특성 분석을 진행할 세부 후보 케이스들을 선정할 수 있었습니다. 핀틀 분사기의 화염 동특성은 기존의 (또는 공개된) 결과가 없기 때문에, 가장 널리 인정받는 방법으로 동특성 중에서도 화염의 고유 진동수를 확인하고자 했습니다. 다양한 혼합비 범위에서 가변 길이의 연소기에서 발생하는 자발적인 불안정 주파수를 측정했고, 실험 결과를 바탕으로 다양한 핀틀 팁 각도가 화염의 동특성에 끼치는 영향을 비교 평가할 수 있었습니다. 핀틀 팁 각도에 의해 결정되는 화염각과 연소 조건에 해당하는 혼합비가 자기유도 불안정 주파수와 갖는 관계를 실험식으로 보였으며, 구체적으로 어떤 과정을 통해 맨틀 재순환 영역 내에서 섭동이 전파되는지를 시간지연모델을 활용해 설명했습니다. 본 연구는 수평방향의 대기압 모델 핀틀 연소기를 이용해 핀틀 분사기의 연소 특성을 연구했으며, 관련 연구를 위한 체계적인 실험 방법과 그 기초적인 결과를 제공합니다. 산소 중심의 핀틀 분사기는 상대적으로 잘 연구되지 않는 역확산 화염을 연구했다는 지점에서 의미가 있으며, 또한 여러 실험을 통해 그간 실험적 근거가 부족했던 추천 화염각(또는 분무각) 설계 범위에 관한 실험적 근거를 일부 마련했다는 데 의의가 있습니다. 실험 결과로부터 핀틀 분사기가 연소불안정에 있어 완벽하게 안전하지는 않음을 확인했으나, 전반적인 안전성에 관한 평가를 내리기에는 추가적인 연구가 필요할 것으로 보입니다. The rapid growth of the global launch vehicle market is heavily due to the dramatic decrease in the launch cost triggered by the famous Falcon series of SpaceX. Since the pintle injector was applied in the Merlin engine of the Falcon series, it has been revisited as a research topic from then. However, despite the long period of its initial development, there was not much previous work on the fundamental characteristics of the pintle injector. Several tips and rules about designing the pintle injector in the literature were not clearly supported by precise experimental results. From about a decade ago, numerous academic researches about the fundamental characteristics of the pintle injector were done. However, the combustion dynamic characteristics are nearly unvisited topics until now, except for a couple of previous researches measuring the pressure perturbation during high-pressure hot tests of the model rocket engine. Considering some controversial results and the importance of understanding the dynamic combustion characteristics of the pintle injector in designing the rocket engine, this thesis aims to investigate the fundamental dynamic combustion characteristics of the pintle injector. Since the unique flow field consists of mantle and core recirculation zone is thought to be the main reason for the strong combustion stability of the pintle injector in some literature, in this thesis the effect of the mantle recirculation zone (which is the major component of the unique flow field) size on the dynamic combustion characteristics will be studied. This thesis starts by questioning whether the boundary conditions of the pintle injector combustor in the previous research were appropriate. Since a single pintle injector is used per engine, appropriate boundary conditions must be critical in determining the flame shape and thus the combustion characteristics of the flame. Each boundary condition including the combustor end, wall, and injector head is proposed and confirmed for further research. The diameter ratio between the combustor and pintle injector turned out to be the crucial factor in designing the atmospheric pintle injector model combustor. Then, the flame of normal and canted pintle injectors are analyzed in a morphological perspective. The canted pintle injectors with the pintle tip angle are designed to deflect the radial flow towards the axial direction which increases the size of the mantle recirculation zone by decreasing the flame angle. Various flames with different geometrical designs under a wide experimental range are classified with major morphological characteristics. From the stability map obtained through numerous experiments, specific experimental conditions are selected for analyzing the dynamic combustion characteristics of the flame. Since there is no existing (or public) experimental result about the fundamental dynamic characteristics of the pintle injector, the most widely accepted method is applied to find the natural frequency of the pintle injector flame under a wide range of mixture ratio – self-induced instability test with a length-variable combustor. The experimental results of different pintle tip angles are compared to assess its effect on the combustion dynamics. By interpreting the experimental results from the perspective of time-delay, the main procedure of onset of instability is proposed. This study provides a well-structured experimental guideline and background for studying the pintle injector’s combustion characteristics through a horizontal atmospheric model combustor. The oxygen-centered pintle injector forms an inverse-diffusion flame which is comparatively rarely investigated in combustion. Plus, during several experiments, it was also possible to build an experimental background of the recommended flame angle design range. This recommended design range for spray or flame angle lacked the experimental support, which is supplemented by this study. From experimental results, it is also confirmed that the pintle injector is not perfectly safe from combustion instability, however further analysis is required to assess overall stability.

Low-temperature methane combustion over Co and Pd-based catalysts

유상범 서울대학교 대학원 2023 국내박사

천연가스는 지구상에서 가장 풍부한 에너지원 중 하나이며 발전, 난방, 운송에 사용될 수 있다. 천연가스의 주요 성분은 메탄이며, 기존의 석유 기반 연료보다 연소 중에 오염을 덜 발생시킨다. 따라서, 질소산화물, 황산화물 및 탄화수소의 배출을 줄이기 위해 가솔린 및 디젤 엔진이 천연가스 엔진으로 대거 대체되고 있다. 하지만, 일부 미연소된 메탄이 천연 가스 차량에서 배출된다. 메탄의 지구 온난화 지수가 이산화탄소보다 25배 이상 높기 때문에 환경 보호를 위해 메탄 배출 규제가 적용될 것으로 전망된다. 촉매를 사용한 메탄 연소 반응은 (CH4+2O2 → CO2+2H2O) 천연 가스 엔진의 메탄 배출을 감소시키는 가장 효과적인 방법이다. 그러나 메탄은 강한 C-H 결합을 가지고 있어 매우 안정한 물질이기 때문에 메탄 산화 반응은 높은 온도를 요구하고 에너지 소모가 매우 크다. 따라서, 이 과정의 에너지 소비를 줄이기 위해서는 저온 메탄 산화 반응을 위한 촉매가 필요하다. Pd 기반 귀금속 촉매가 메탄 연소 반응에 가장 널리 사용되며 가장 활성이 우수한 촉매이다. 그러나 귀금속의 불안정한 가격때문에 실제 산업으로의 적용이 어렵다. Pd 촉매의 대체재로, 페로브스카이트 또는 스피넬 산화물과 같은 전이 금속 산화물 촉매가 있다. 이러한 전이금속 산화물 중 스피넬 형태의 코발트 산화물이 산화 반응에 널리 사용되고 있다. 그러나, Pd 촉매에 비해 코발트 산화물의 반응 활성이 매우 낮기 때문에 저온 메탄 산화 반응에 곧바로 적용하기 어렵다. 제 2장에서는 벌크 코발트 산화물 촉매의 낮은 반응 활성을 두 가지 다른 방법으로 향상시켰다. 첫째, 중형기공의 도입으로 벌크 촉매의 낮은 표면적을 개선하였다. 둘째, 합성된 중형기공성 코발트 산화물 촉매를 질산으로 처리하여 표면 구조를 개선하였다. 제조된 촉매를 사용하여 메탄 연소 반응을 진행했으며, 산처리된 촉매는 기존 촉매에 비해 반응 활성이 크게 향상된 것을 확인하였다. 다양한 분석 기법을 사용하여 산처리된 촉매의 특성 분석을 진행하였다. 투과 및 주사전자현미경 이미지는 산처리된 촉매의 표면이 거칠어지는 것을 보여주었다. 한편, 수소 승온 환원 결과를 통해, 산처리 후 코발트 산화물의 환원성이 향상되는 것을 확인할 수 있었다. 또한, 산소 승온 탈착 및 X선 광전자 분광 분석 결과, 산처리된 촉매가 기존 촉매보다 표면에 화학 흡착된 산소를 더 많이 보유한 것으로 밝혀졌으며, 이를 통해 산처리된 촉매의 향상된 환원성 및 산화 반응 활성을 설명할 수 있다. 즉, 산처리는 메탄 연소 반응을 위한 높은 활성을 가지는 벌크 산화물 촉매의 합성에 유용한 방법이라고 결론을 내릴 수 있다. 산 처리에 의해 코발트 산화물 촉매의 메탄 산화 활성을 크게 증가시킬 수 있었지만, 촉매 활성은 여전히 귀금속 촉매에 비해 낮았다. 귀금속 촉매, 특히 Pd를 포함하는 촉매는 다양한 산화 반응에 대한 우수한 활성으로 인해 상당한 주목을 받고 있다. 귀금속 촉매는 높은 가격 때문에 일반적으로 알루미나, 세리아, 제올라이트 등의 지지체에 소량의 귀금속을 분산시켜 제조된다. 하지만 Pd 촉매는 배기가스 내의 물에 취약한데, 물은 촉매의 초기 활성을 심각하게 저해하고 장기 운전시 비활성화를 일으키는 것으로 알려져 있다. 물에 내성이 있는 촉매를 합성하기 위한 가능한 해결책 중 하나는 소수성 지지체를 사용하는 것이다. 다양한 지지체 중 제올라이트는 Si/Al2 비율을 증가시켜 소수성을 향상시킬 수 있기 때문에 메탄 산화 촉매를 위한 효과적인 지지체가 될 수 있다. 제올라이트는 고온에서 증기 처리에 의해 쉽게 탈알루미늄화될 수 있으며, 이는 종종 수열 처리라고 불린다. 제 3장에서는 탈알루미늄화가 Pd/SSZ-13 촉매의 메탄 연소 반응 활성에 미치는 영향을 조사하였다. Pd/SSZ-13 촉매를 수열 처리하여 제조한 Pd(1)/SSZ-13(HTA)는 매우 낮은 촉매 활성을 나타내었다. 반면에, SSZ-13 지지체를 수열 처리한 후에 Pd를 담지하여 제조한 Pd(1)/SSZ-13(DeAl) 촉매가 가장 높은 촉매 활성을 보였다. 제올라이트 소수성, Pd 위치 및 상태 등의 촉매 특성을 다양한 분석 기법을 이용하여 조사하였다. Pd/SSZ-13(HTA) 촉매에 포함된 대부분의 Pd 종은 Pd 이온으로 존재하였으며, Pd 이온은 메탄 산화 반응 활성이 거의 없었다. 반면에, Pd(1)/SSZ-13(DeAl) 촉매에 포함된 대부분의 Pd 종들은 외부 표면에 PdO 나노입자로 존재하여 메탄 산화 반응 활성이 높았다. 결론적으로, 메탄 연소 반응에 높은 활성을 보이는 Pd/SSZ-13 촉매를 합성하기 위해서는 외부 표면에 존재하는 PdO 나노입자의 양을 극대화해야 하는데, 이는 Pd 담지전에 SSZ-13 지지체의 탈알루미늄화를 진행하면 가능하다. 앞선 연구에서 제올라이트 지지체의 소수성이 촉매 활성에 큰 영향을 끼치지 않는 것을 확인했기 때문에 다른 지지체를 사용해보았다. 세리아는 일반적으로 Ce4+/Ce3+ 사이클에서 발생하는 산화 환원 특성, 담지된 금속과의 강한 상호 작용 및 높은 산소 저장 용량으로 인해 산화 반응에 자주 활용된다. 귀금속의 매장량이 제한적이기 때문에, 낮은 금속 함량으로 높은 활성을 보이는 촉매의 제조가 필요하다. Pd/CeO2 촉매의 Pd 종은 다양한 상태로 존재할 수 있으며, 일반적으로 금속 Pd나 PdO 나노입자 상태이지만, PdxCe1-xO2-δ 고용체를 형성하여 세리아 결함 부위에 안정화된 고분산된 Pd 이온으로도 존재할 수 있다. 각 Pd 종은 서로 다른 반응에 대해 서로 다른 촉매 활성을 나타낸다. 따라서, 우수한 활성을 보이는 촉매를 제조하려면 목표 반응에 적합한 Pd 상태에 대한 이해가 필요하다. 제 4장에서는 Pd/CeO2 촉매에서 Pd 전구체가 Pd 상태에 어떠한 영향을 끼치는지 연구하였다. 서로 다른 Pd 전구체를 사용하여 Pd/CeO2 촉매를 제조했으며 CH4 및 CO 산화 반응에 적용하였다. 팔라듐 나이트레이트 (Pd(2)/CeO2(N))를 사용하여 제조된 촉매는 CH4 산화 반응에 대하여 높은 활성을 보였으며, 반면에 팔라듐 아세테이트(Pd(2)/CeO2(A))를 사용하여 제조된 촉매는 높은 CO 산화 활성을 나타내었다. 촉매 활성 및 특성에 대한 전구체의 영향은 다양한 분석 기법을 사용하여 연구되었다. Pd(2)/CeO2(N) 촉매는 CH4 산화를 위한 활성상인 PdO 나노입자를 표면에 보유하였다. 반면, Pd(2)/CeO2(A) 촉매는 CeO2 결함 부위에 안정화된 Pd 이온을 가지고 있었으며, 이는 반응성이 매우 낮았다. 그러나, 높은 Pd 분산도는 CO 산화 반응에 유리하였다. 결론적으로, Pd 전구체는 촉매 활성에 중요한 Pd/CeO2 내의 Pd의 분산도 및 화학 상태를 명확하게 변화시켰다. Natural gas is one of the most abundant energy sources on earth and can be used for electric power generation, heating, and transportation. Methane, a major component of natural gas, produces less pollution during combustion than conventional petroleum-based fuels. Thus, a large number of gasoline and diesel engines are being replaced by natural gas engines to reduce the emission of nitrogen oxides, sulfur oxides, and hydrocarbons. However, unburned methane is emitted from natural gas vehicles. Because the global warming potential of methane is 25 times greater than that of CO2, methane emission regulation is being implemented for environmental protection. The catalytic combustion of methane (CH4 + 2O2 → CO2 + 2H2O) is one of the most effective methods for reducing methane emissions from natural gas engines. However, methane is very stable because of its strong C–H bonds; methane oxidation requires high temperatures and consumes a large amount of energy. Therefore, catalysts for low-temperature methane oxidation are urgently required to reduce the energy consumption of this process. Currently, Pd-based noble metal catalysts are the most widely used and most active catalysts for the methane combustion reaction. However, the volatile price of noble metals limits their industrial applications. Fortunately, transition metal oxide catalysts, such as perovskite oxides or spinel oxides, are attractive alternatives. Among these transition metal oxides, spinel-type cobalt oxide is widely used as an active catalyst for methane combustion reactions. However, the poor intrinsic activity of metal oxides relative to Pd catalysts limits their application in low-temperature methane oxidation reactions. In chapter 2, the poor activity of bulk cobalt oxide catalyst was enhanced by two different methods. First, the textural property of the bulk catalyst was enhanced by the introduction of mesoporosity. Second, the synthesized mesoporous cobalt oxide catalysts were treated with nitric acid to further modify the surface structure and chemistry. The prepared catalysts were applied to the methane combustion reaction and a significant enhancement in catalytic activity was observed for the acid-treated catalysts compared to the pristine catalyst. The properties of the acid-treated catalyst were investigated in detail using various characterization techniques. In particular, transmission and scanning electron microscopy images revealed a roughening in the surface morphology of the acid-treated catalysts. Meanwhile, the H2 temperature-programmed reduction results showed that the reducibility of the cobalt oxides was enhanced by the acid treatment. In addition, O2 temperature-programmed desorption and X-ray photoelectron spectroscopy analyses revealed that the acid-treated catalysts contained larger amounts of surface chemisorbed oxygen than the pristine catalyst, which may explain the enhanced reducibility and oxidation activity of the acid-treated catalysts. In conclusion, simple acid treatment is a useful post-treatment method for the synthesis of highly active bulk oxide catalysts for the methane combustion reaction. Although it was possible to significantly increase the methane oxidation activity of the cobalt oxide catalyst by the acid treatment, the catalytic activity was still inferior than the noble metal catalysts. Noble metal catalysts, especially those containing Pd, have attracted significant attention due to the excellent catalytic activities for various oxidation reactions. Because of their high cost, noble metal catalysts are generally prepared by dispersing a small amount of the precious metal on a support material, such as alumina, ceria, or zeolite. However, Pd catalysts are vulnerable to H2O in the exhaust gas, which is known to severely inhibit the initial activity and cause deactivation during the long-term operation. One of the possible solutions for synthesizing the catalyst that is resistant to H2O is using hydrophobic support. Among the various support materials, zeolite can be effective support for the methane oxidation catalyst because the hydrophobicity can be enhanced by increasing Si/Al2 ratio. Zeolite can be easily dealuminated by steam treatment at high temperature, which is often called as hydrothermal treatment. In chapter 3, the effects of dealumination on Pd/SSZ-13 catalysts for methane combustion were investigated. Pd(1)/SSZ-13 (HTA), which was prepared by treating the catalyst hydrothermally at 750 °C, displayed extremely low catalytic activity. On the contrary, Pd(1)/SSZ-13 (DeAl) catalyst, which was prepared by treating the SSZ-13 support hydrothermally and subsequent Pd impregnation, exhibited superior catalytic activity for methane oxidation. Catalytic properties, including zeolite hydrophobicity, Pd location and states were investigated by using various characterization techniques to clarify the differences between the catalysts. Most Pd species in the Pd/SSZ-13 (HTA) catalyst existed as Pd ions, which were not very active for the reaction. On the contrary, most Pd species in the Pd(1)/SSZ-13 (DeAl) catalyst existed as PdO nanoparticles at the external surface, which were highly active for the methane oxidation. In summary, the amount of external PdO nanoparticles should be maximized in order to synthesize highly active Pd/SSZ-13 catalyst for the methane combustion, which was achieved by dealuminating the SSZ-13 support before the Pd impregnation. Because it was revealed that the hydrophobicity of the zeolite support was not critical factor for the catalytic activity, we decided to use other support material. Ceria is generally utilized for oxidation reactions owing to the redox properties arising from the Ce4+/Ce3+ cycle, strong interaction with supported metals and large oxygen storage capacity. Because of the limited availability of noble metals, the synthesis of highly active catalysts with low metal loadings is essential for industrial applications. Pd species in Pd/CeO2 catalysts can exist in various states, most commonly as metallic Pd, PdO nanoparticles, and highly dispersed Pd ions stabilized in ceria defect sites by forming a PdxCe1-xO2-δ solid solution. Each Pd phases exhibits different catalytic activities for different reactions. Therefore, understanding the active phase of Pd for a specific reaction is important to synthesize a highly active catalyst with an appropriate Pd state. In chapter 4, the effects of Pd precursor on the state of Pd species in the Pd/CeO2 catalysts were investigated. The Pd/CeO2 catalysts were prepared using different Pd precursors and applied to CH4 and CO oxidation. Significantly, the Pd/CeO2 catalyst prepared using palladium nitrate (Pd(2)/CeO2 (N)) was highly active toward CH4 oxidation, whereas the catalyst prepared using palladium acetate (Pd(2)/CeO2 (A)) exhibited high CO oxidation activity. The effects of the precursor on the catalytic activity and other properties were studied using various characterization techniques. The Pd(2)/CeO2 (N) catalyst contained surface PdO nanoparticles, which were active sites for CH4 oxidation. On the contrary, the Pd(2)/CeO2 (A) catalyst possessed Pd ions stabilized at CeO2 defect sites, which were not very reactive. However, the high Pd dispersion was beneficial for the CO oxidation reaction. Therefore, the Pd precursor clearly altered the Pd dispersion and state in the Pd/CeO2, which are critical for catalytic activity.

NiO 산소공여입자를 사용하는 기포유동층 매체 순환식 메탄연소 공정의 모사 및 설계

자밀라 건국대학교 대학원 2013 국내박사

The capability to capture a pure carbon dioxide with high potential and without any extra energy makes the chemical looping combustion (CLC) one of the leading technologies for CO2 capture compared to other capture techniques; the compressed carbon dioxide can be injected into oil and natural gas reservoirs for the purpose of simultaneously enhancing oil recovery and reducing the CO2 emissions since it is considered as the main greenhouse gas causing global warming. The chemical looping combustion is an unmixed combustion concept [1-4]. It consists of two interconnected fluidized bed reactors, an air reactor (AR) and a fuel reactor (FR), using circulating metal oxide particles to transfer oxygen from AR to FR. Metal oxide oxidizes in AR with air and it is reduced in FR by methane, thus eliminating NOx formation [1,5-7] and producing almost pure carbon dioxide. The purpose of this study was to develop the CLC process for methane using NiO-based as an oxygen carrier to determine the proper operating conditions for complete combustion of methane to carbon dioxide and water in the FR, therefore enhancing the performance of the CLC system. The hydrodynamic characteristics of a gas-solid fluidized bed were carried out at operating pressures from atmosphere to 5.013 bars in a pressurized fluidized bed. The effects of the fluidizing velocity, solid inventory and the operating pressure on the solid-holdup in the dense bed were studied. The experimental results showed that the solid-holdup decreases with increase of fluidizing velocity or the operating pressure, but there was no significant effect of pressure on the minimum fluidization velocity in this operating pressure range. The performance of the CLC system was investigated under various operating conditions such as temperature, bed weight and solid circulation rate, in two interconnected bubbling fluidized beds, a high velocity fluidized bed for the AR and a low velocity fluidized bed for the FR .The proper operating conditions in both AR and FR for complete combustion of methane were discussed, with consideration of the particle attrition and required makeup of fresh oxygen carrier (OC) particles. The simulation results showed that the efficiency of combustion of methane was strongly affected by the distribution of OC between the air reactor (AR) and fuel reactor (FR) at a constant temperature, circulation rate of OC, and total bed mass. The range of OC distribution possible to achieve complete combustion became wider with increasing either the temperature or the circulation rate of OC at a constant total bed mass. In this range, the amount of elutriated OC particles decreased a little as the FR mass increased because of the higher rates of particle elutriation and attrition in AR than in FR. More particularly interesting results were presented in this work, illustrating the effect of each reactor on the behavior of the whole process. Ultimately lead to a better design procedure of the CLC process of pure methane in a continuous bubbling fluidized bed process with NiO-based oxygen carrier, by estimating the minimum requirements of solids inventory in each reactor and the minimum solids circulation rate to maintain a complete conversion of methane to CO2 and H2O over a wide range of operating parameters. It was found that the minimum requirements of solids inventory in each reactor, operating temperature and solid circulation rate were necessary and could not be compensated by any other parameter. Moreover, it was found that for gaining more flexibility, the increase of the temperature is preferred rather than increasing of the total solids inventory, regarding to the economic considerations.

대형기관 모사 정적연소실에서 메탄의 연소 특성에 관한 연구

권순태 서울과학기술대학교 2014 국내박사

This study is the combustion characteristics of the methane gas in a constant volume combustion chamber for a large displacement volume commercial engine, which discusses the fundamental characteristics of the fuel in the aspect of the thermo-chemistry and thermodynamics and compares these results with the experimental ones. In addition, The combustion processes are discussed on the basis of pressure measurement. Also torch device is applied to the constant volume combustion chamber and the effects of orifice diameter and volume of torch device are analyzed through the heat release patterns and the flame visualization. In the aspect of the thermo-chemistry and thermodynamics, the results show that the final pressures from theoretical analysis are varied under the same heating value due to the change of constant volume specific heat, caused by the difference of the burned gas composition according to the fuel gas compositions and the stoichiometric ratios and the trends of the analytic and experimental pressures coincide very well, however, some minor differences are observed between two. The root cause of the difference is the heat transfer, which changes the specific heat and lowers the temperature considerably, in the real combustion process. In addition, the large chamber volume and the ignition position promote the transfer to the wall. Also, the fuel conversion efficiency increases as the methane mol fraction decreases and is maximum when the stoichiometric ratio is from 0.8 to 0.9. For these increments due to the composition and the stoichiometric ratio could sufficiently compensate the decrement by the specific heat ratio drop, bio fuel like LFG might be more advantageous than pure methane in the real engine. In the analysis of the combustion process base on the pressure measurement, the bi-modal peak pressure phenomenon, which is caused by the interaction of the heat release and the heat transfer, are more apparent as the mixtures are more favorable to the combustion and the magnitudes of the pressures depend on the unburned fraction. In addition, there exist 4 main inflection points during the heat release due to the variation process of the heat transfer area related to flame propagation from the ignition point. Also, the inflection points increase as the mixture quality is worse because of the extended burn duration. Consequently, the sophisticated interactions between the heat transfer area changing pattern due to the flame propagation and the transfer duration might cause very peculiar heat release patterns. On the other hands, for the improvement of the combustion, torch devices are applied to the combustion chamber. The results show the there exists optimum orifice-diameter ratio regardless of the torch volume and little or adverse effects on the combustion are observed in case of the excessive small ratio. In addition, the torch ignition reduce the burn duration in the first place and then the decrease of heat transfer caused by shortening time contributes to raise of the peak combustion pressure. Finally, the torch mostly plays positive role in shortening main burn duration as the combustion condition is worse due to lower methane fraction, on the other hand the torch reduces initial burn duration rather than main burn as methane fraction increases. The torch volume can be changed via adjusting the height. As the torch height increases, the peak combustion pressure is higher and the combustion duration is reduced. Especially, combustion pressure and time is improved regardless of kind of torch in the range of < 0.02 and case of = 0.01 shows the highest improvement. Finally, the heat release patterns and visualization images show that the jet and/or spout from torch promote combustion by accelerating the flame front in the main combustion chamber. In addition, there exists hot gas jet when the orifice diameter is 4 ㎜, while flame passes through orifice directly if the diameter is 6 ㎜ and over. Also the effect of torch ignition is different according to the combinations of the methane fraction, the torch volume and orifice size because various combustion processes occur due to the interaction of these parameters. Finally, the suitable torch might satisfy not less than 6 ㎜ orifice diameter and not more than 0.15 of area ratio concurrently for securing the consistency of combustion process in the real engine. 본 논문은 대형 상용기관을 모사한 정적연소실에서 가스연료의 연소 특성에 대한 논문으로 연소특성을 연소화학양론 및 열역학적 측면에서 분석하고 이를 실험적 결과와 비교하였다. 또한 정적연소실의 연소압력 측정을 기반으로 연소과정을 해석하였으며, 토치를 적용하여 토치의 오리피스직경과 체적에 따른 효과를 분석하였다. 열 발생 특성과 가시화 결과로 연료 조성에 따른 연소 특징 및 토치 연소의 특징을 논하였다. 연소화학양론 및 열역학적 분석에서 연료 중 메탄 분율과 당량비에 따라 기연가스의 조성이 변화하고 이에 따라 정적비열이 변화하여 동일 발열량에 최종압력은 변화하며, 이것은 실제 연소압력 측정결과와 대체로 일치한다. 메탄 분율과 당량비에 따른 상대적 압력변화 및 연소기간도 분석결과와 실험결과가 경향 상 일치한다. 분석과 실험 결과 사이의 세부적 차이는 온도와 연소 기간에 의한 열전달량의 차이에 기인한 온도저하 및 이에 따른 정적비열의 변화가 주요 원인이며, 연소실 체적과 점화위치도 연소기간과 온도에 큰 영향을 준다. 최종적으로 정적연소에서 연료변환효율은 메탄 분율이 작을수록 그리고 당량비 0.8 ~ 0.9 사이에서 최대가 되며, 이러한 연료변환효율의 증가는 실물기관에서 비열비 감소의 효과를 상쇄하고 남으므로 순수 메탄보다 불순물이 포함된 매립지가스와 같은 바이오연료의 연소가 효율 면에서 유리하다. 연소압력 측정을 기반으로 연소과정을 해석한 결과 연료에 포함된 메탄의 분율이 높아 연소에 유리한 조건일수록 두 개의 압력 정점이 존재하며, 이는 연소에 의한 열발생과 열전달에 의한 냉각효과의 상호 작용이며 두 정점의 크기는 미연가스 분율에 따라 달라진다. 또한 연소과정 중 열발생에는 4개의 주요 변곡점이 발생하고, 이는 점화위치로부터 화염전파에 따른 전열 면적 변화과정이 주원인이며 연소에 불리한 조건일수록 변곡점은 증가하고 열발생은 복잡한 형태를 지니는데, 이는 연소기간 연장이 주원인이다. 결론적으로 점화위치와 관련된 화염전파 과정 및 전열 면적의 변화과정 그리고 대형 연소실에 의한 연소기간 연장의 효과가 상호 복잡하게 작용하면서 매우 특이한 형태의 열발생 곡선이 생성된다. 화염 생성 및 연소 효율 향상을 위하여 정적연소실에서 토치 장치를 적용하였다. 실험 결과 연소를 개선시키는 최적의 오리피스 비율이 존재하며 오리피스가 매우 작은 경우 압력 상승 비율은 직경 변화에 무관하게 일정 수준을 유지하거나 소폭 감소한다. 아울러 토치의 효과는 일차적으로 연소 시간단축에 영향을 주고 시간단축에 따른 전열량의 감소가 이후 압력상승에 기여한다. 마지막으로 메탄 분율이 적어 연소 조건이 나쁠수록 토치는 주로 주 연소 기간을 단축시키고, 연소 조건이 좋은 경우에는 주로 초기 연소를 단축시키는 효과가 있다. 또한 토치의 높이가 증가함에 따라 연소 압력은 상승되었고, 연소 시간은 개선되었다. 특히 < 0.02에서는 모든 토치에서 압력 향상과 시간단축이 있었으며 토치의 높이에 관계없이 = 0.01에서 최고 압력 상승과 최고 시간 단축율을 보였다. 하지만 토치의 높이(h)가 토치 직경(D)보다 큰 경우(under square)의 형태에서는 토치 내부에서 냉각이 발생하여 압력상승과 시간단축의 폭이 크지 않았으며 토치 내부에서의 과도한 냉각을 피하는 토치의 형태인 정행정(square)의 형태를 유지하는 경우에 효과가 뚜렷하게 났다. 열 발생 특성과 가시화한 결과로 토치의 특성을 살펴본 결과 토치 연소는 주 연소실에 형성된 화염 면을 토치에서 분출되는 가스가 가속시킴으로 연소를 촉진한다. 이때 오리피스 직경이 4 ㎜일 때는 고온의 가스가 분출되며 6 ㎜이상에서는 토치에서 형성된 화염이 직접 오리피스를 통과한다. 또한 토치의 체적이 증가하면 분출되는 화염은 더욱 가속화 되었으며, 연소실 하면에서부터 연소가 진행되었다. 메탄 분율, 토치 체적 그리고 오리피스 크기가 상호 연관적으로 작용하여 다양한 연소 형태를 야기하고 이에 따라 토치 연소의 효과가 매우 다르게 나타난다. 마지막으로 실물기관에서 연소 과정의 일관성을 보장하는 적절한 토치의 조건은 6㎜ 이상의 오리피스 직경과 0.15 이하의 면적비을 동시에 만족시키는 것이다. 토치의 형태는 직경(D)과 높이(h)가 비슷한 형태이어야 연소의 개선이 크다.