LPG 및 암모니아 BOG 처리를 위한 듀얼 모드 재액화 시스템 설계 및 실증 연구

국문 초록 (Abstract)

본 연구는 암모니아(Ammonia) 및 액화석유가스(LPG) 운반선에서 발생하는 BOG (Boil Off Gas)를 효과적으로 재액화하여 에너지 손실을 최소화하고, 국제 해사 환
경 규제 및 탄소중립 요구사항을 충족할 수 있는 듀얼모드 재액화 시스템의 설계 와 실증에 관한 것이다. 기존의 BOG 처리 시스템은 특정 화물의 물성에 맞춰 설계 되어 있어, 암모니아와 LPG와 같이 상변화 조건과 열역학적 특성이 상이한 화물에 대해서는 동일 시스템으로 안정적인 처리가 어려운 한계가 있었다. 이러한 제약은 선박의 화물 운송 유연성을 저해하고, 추가 설비 설치나 별도의 운전 기능 구성이 필요한 비효율성을 초래해왔다.

본 연구에서는 이러한 문제를 해결하기 위해, 단일 재액화 시스템에서 암모니아 와 LPG 두 종류의 BOG를 모두 안정적으로 처리할 수 있는 듀얼모드 운전 기술을 개발하였다. 제안된 시스템은 3단 피스톤 타입의 고압 BOG 압축기, 고효율 열교환 기와 응축기, 냉매 계통 제어 밸브, 안전 계통 및 보호 로직으로 구성되며, 각 구성 요소는 두 유체의 증발 압력, 응축 온도, 잠열 특성 등을 반영하여 최적화되었다. 특히, 실제 선박 운항 중에 발생하는 BOG 조건을 육상 시험 환경에서 재현하기 위해, 폐루프 기반의 실증 시험 설비를 구축하고, 여기에 별도의 BOG 발생 Skid를 연계하여 다양한 부하 조건에서 반복 실험을 수행하였다.

운전 실증에서는 암모니아와 LPG 각각에 대해 압축기 부하 조건(50%, 100%), 냉각수 온도, BOG 유량, 증발 압력 등을 조정하며, 다양한 운전 시나리오에서의 성능
데이터를 확보하였다. 또한 밸브 유량계수(Cv) 조정과 운전 로직 전환을 통해 동일한 하드웨어 구성에서도 유체별 최적 운전이 가능함을 입증하였으며, 냉각수 유량,
응축 온도 등의 파라미터 변화가 재액화 효율에 미치는 영향도 분석하였다. 본 연구에서 개발한 시스템은 4,800kg/h의 BOG 처리 용량을 목표로 설정하였으며, 실
증 결과 실제 처리 용량은 5,187.8kg/h로 나타났다. 이는 목표 용량을 초과 달성한 수치로, 91k 급 LPG 운반선에서 발생하는 평균 BOG 발생량을 충분히 처리할 수
있는 성능을 갖추었음을 의미한다. 따라서 본 시스템은 해당 선박 규모를 고려할 때, 안정적이고 효율적인 운전이 가능함을 입증하였다.

암모니아는 상온에서 기체 상태로 존재하며, 상압에서의 액화 온도는 약 –33℃ 이다. 실증 운전 결과 암모니아는 액화 후 압력 15.5barg와 상압 조건 액화 온도
-33.3℃를 달성하였으며, 이는 상압 기준 액화 온도와 거의 일치하여 시스템의 효율적 운전 가능성을 확인하였다.

LPG는 주로 프로판과 부탄의 혼합물로 구성되며, 상압에서의 액화 온도는 약 –42℃이다. 실증 운전에서는 상압 조건 액화 온도 –59.4℃를 달성하였으며, 액화
후 압력 27.2 barg는 LPG의 상압 액화 압력보다 높은 값으로, 고압 조건에서도 안정적 운전이 가능함을 보여준다.

이와 같이 실증 운전 결과는 유체별 특성에 따른 최적 운전 조건이 동일한 하드웨어 구성에서 구현 가능함을 뒷받침하며, 향후 다양한 운전 시나리오에서도 안정
적이고 효율적인 시스템 운전이 가능함을 입증하였다.

연구 결과, 제안된 듀얼모드 재액화 시스템은 선박 화물의 종류에 따라 운전 조건을 유연하게 전환할 수 있는 실용성과 안정성을 확보하였으며, 기존 단일 연료
기반 재액화 시스템 대비 공간 활용 효율, 운용 편의성, 에너지 절감 측면에서 우수한 경쟁력을 나타냈다. 이러한 기술은 향후 암모니아 및 LPG를 포함한 다양한
친환경·탄소중립 선박 연료의 활용 확대에 이바지할 수 있는 핵심 기반 기술로 평가되며, 해운 산업의 탈탄소화와 환경 규제 대응에 실질적으로 이바지할 것으로
기대된다.

다국어 초록 (Multilingual Abstract)

This study focuses on the design and demonstration of a dual-mode reliquefaction system capable of effectively reliquefying boil-off gas (BOG) generated from ammonia and liquefied petroleum gas (LPG) carriers. The objective is to minimize energy loss while meeting international maritime environmental regulations and carbon-neutrality requirements. Conventional BOG handling systems are typically designed for a specific cargo, and therefore have limitations when applied to multiple cargoes such as ammonia and LPG, which exhibit different phase-change and thermodynamic properties. Such constraints reduce operational flexibility and lead to inefficiencies due to the need for additional equipment or separate operation modes.

To overcome these limitations, a dual-mode operation technology was developed to enable stable processing of both ammonia and LPG BOG using a single relique-faction system. The proposed system consists of a three-stage piston-type high-pressure BOG compressor, high-efficiency heat exchangers and condensers, a refrigerant circuit with control valves, and a safety and protection logic. Each component was
optimized considering the vapor pressure, condensation temperature, and latent heat characteristics of both fluids. To reproduce the actual BOG generation conditions occurring during ship operation, a closed-loop test facility was constructed, integrated with a dedicated BOG feeding skid, enabling repeated experiments under various load conditions.

During the experimental operation, performance data were obtained for both ammonia and LPG by adjusting compressor load (50% and 100%), cooling water temperature, BOG flow rate, and evaporation pressure. The results demonstrated that optimal operation for each fluid can be achieved within the same hardware configuration through adjustment of the valve flow coefficient (Cv) and operation logic. Furthermore, the effects of varying parameters such as cooling-water flow and condensation temperature on reliquefaction efficiency were analyzed.
The developed system was designed with a target BOG handling capacity of 4,800kg/h, and the experimental results showed an actual capacity of 5,187.8kg/h, exceeding the design target. This indicates that the system possesses sufficient performance to handle the average BOG generation rate of a 91 k-class LPG carrier, confirming its stable and efficient operation capability.

Ammonia exists as a gas at ambient temperature and has a liquefaction temperature of approximately -33℃ at atmospheric pressure. Experimental results showed that ammonia was successfully liquefied at a pressure of 15.5 barg and a liquefaction temperature of –33.3 ℃, nearly identical to its saturation condition at atmospheric pressure, demonstrating efficient operation.
LPG, composed mainly of propane and butane, has a liquefaction temperature of approximately –42 ℃ at atmospheric pressure. During the test, a liquefaction temperature of –59.4℃ and a condensation pressure of 27.2barg were achieved, confirming stable operation even under high-pressure conditions.

These results confirm that optimal operating conditions for each fluid can be realized within a single hardware configuration, ensuring both stable and efficient operation under various load scenarios. The proposed dual-mode reliquefaction system demonstrated practicality and reliability by enabling flexible mode transition according to cargo type.
Compared to conventional single-fuel reliquefaction systems, it exhibits superior performance in terms of spatial efficiency, operational convenience, and energy savings. This technology is expected to serve as a key foundation for expanding the use of eco-friendly and carbon-neutral marine fuels such as ammonia and LPG, thereby contributing to the decarbonization of the maritime industry and compliance with global environmental regulations.

목차 (Table of Contents)

• List of Tables ····································································································· iii
• List of figures ···································································································· iv
• List of Acronyms & Nomenclature ································································· vi
• Abstract ············································································································· vii
• 1. 서 론 ··················································································································1
• 1.1 연구 배경 및 필요성························································································· 1
• 1.2 연구 목적········································································································2
• 1.3 연구 범위 및 한계···························································································· 2
• 1.4 논문의 구성·····································································································3
• 2. 이론적 배경 및 관련 연구 ····················································································4
• 2.1 BOG (Boil-Off Gas)의 개요 및 물성 비교 ······················································ 4
• 2.1.1 BOG의 정의 및 발생 원리············································································4
• 2.1.2 암모니아와 LPG의 물성 비교······································································4
• 2.2 기존 BOG 재액화 시스템 구조 및 기술 동향 ····················································· 5
• 2.2.1 LNG BOG 재액화 시스템············································································5
• 2.2.2 암모니아 BOG 재액화 기술 동향·································································6
• 2.2.3 LPG BOG 재액화 시스템··········································································· 7
• 2.2.4 시스템 비교 분석························································································8
• 2.3 듀얼모드 재액화 시스템 관련 선행연구 검토······················································ 9
• 2.4 성능 평가 지표·································································································10
• 3. 시스템 설계 및 구성 ····························································································· 12
• 3.1 듀얼모드 재액화 시스템 개요·············································································· 12
• 3.2 주요 구성 요소 및 기능························································································16
• 3.2.1 BOG feeding skid ···················································································· 16
• 3.2.2 재액화 시스템·······························································································23
• 3.3 배관 및 계장 설계 (P&ID 구성 원리)····································································34
• 3.4 운전 모드 전환 로직 (LPG ↔ 암모니아)·····························································37
• 3.4.1 운전 전환의 필요성과 설정 항목 ····································································38
• 3.4.2 퍼징 절차를 포함한 전환 시퀀스 ···································································39
• 3.4.3 전환 후 운전 설정 및 안정화··········································································41
• 4. 실증 운전 조건 및 절차 ··························································································43
• 4.1 시험 장비 및 계측 설정························································································43
• 4.2 운전 조건 설정···································································································44
• 4.3 LPG 운전 조건 및 절차 ······················································································44
• 4.4 암모니아 운전 조건 및 절차················································································ 46
• 4.5 데이터 수집 방법································································································ 46
• 5. 실증 결과 및 분석···································································································48
• 5.1 암모니아 운전 결과····························································································· 48
• 5.1.1 구간별 열역학적 변화 해석··············································································49
• 5.1.2 압력 및 온도 변화 특성····················································································51
• 5.1.3 응축 성능 분석 및 안정성 평가 ········································································52
• 5.2 LPG 운전 결과 ···································································································54
• 5.2.1 구간별 열역학적 변화 해석··············································································55
• 5.2.2 압력 및 온도 변화 특성···················································································56
• 5.2.3 응축 성능 분석 및 안정성 평가 ·······································································59
• 5.3 듀얼모드 운전 비교 및 고찰················································································· 62
• 5.4 실증 결과 및 기술적 시사점················································································· 63
• 5.5 시스템 통합 운전 시 개선점 및 고찰 ···································································· 64
• 6. 결론 ······················································································································66
• 참고문헌 ····················································································································68
• 초록 ····················································································································70
• 감사의 글 ···················································································································72