본 “미활용 에너지로써의 LNG 냉열을 이용한 발전공정”은 산업통상자원부
지원 과제로써 “CCUS기술 인력양성 고급트랙”의 세부과제로 공주대학교 화학
공학부와 지질환경과학과에서 공동으로 5년간 수행하는 세부과제의 일환으로 저
술되었다. 우리나라의 경우에 연간 수입하는 LNG는 약 3,500만 톤으로 일본과
중국에 이어서 세계 3위의 수입국이며, 특히 단일 인수기지로는 평택과 인천이 세
계 1위와 2위를 기록하고 있으나, LNG 냉열의 활용은 일본이 60%, 중국이 10%
인 반면에 우리는 거의 전무한 실정이다.
LNG는 천연가스를 상압 조건하에서 ?163도까지 냉각하면 액화되어 얻어지는
액화천연가스로써 천연가스에 비해서 수송 및 저장이 용이하다. 국내에서 수입하
는 천연가스는 대부분이 LNG의 형태로 수입한다. 천연가스를 액화하게 되면 냉동
사이클을 가동해야 하므로 과정에서 수많은 전기를 소모하게 된다. 그런데 LNG를
다시 사용하기 위해서는 증발시켜야 하는데 우리나라의 경우에는 대부분 바닷물
을 이용해서 재기화시키고 있는 실정이다. 더욱이 바닷물의 온도가 낮은 겨울철에
는 LNG를 기화시키기 위해서 천연가스를 연소시켜서 열을 공급하고 있다. 따라서
본 저에서는 천연가스를 액화시키는 과정에서 소요된 전기에너지의 일부를 다시
전기 에너지로 회수하거나 증기 재압축을 이용한 냉동 사이클에 소모되는 전기에
너지의 대부분을 회수하는 방법을 제안하고 있다.
제 1장에서는 천연가스와 액화천연가스에 대해서 개괄적인 내용을 다루고 있으
며, 2장에서 4장까지는 천연가스 구성물질과 냉매 성분들에 대한 물성 추산을 위
한 열역학 모델식에 대해서 설명하였으며, 5장과 7장에서는 증기 재압축을 활용
한 냉동사이클에서 소모되는 전력을 절감하기 위해서 LNG와 열교환을 통해서 절
감되는 전기에너지를 냉매의 공급온도에 따라서 상관 관계식을 도출하였으며, 여
러 가지 작동유체를 활용한 동력생산공정과 천연가스 개질반응을 통해서 수소를 생산할 적에 도출되는 이산화탄소의 양을 LNG의 조성에 따라서 정량적으로 산출
하였다.
본 저서를 완성하기까지 도움이 되는 많은 자료를 구해주신 김동선 교수님께 감
사를 드리며, CCUS 사업단의 이영미 선생님과 이 책이 출판될 수 있도록 도움을
주신 아진 출판사의 김근배 사장님께도 감사를 드리는 바이다.
2019년 12월 연구실에서 편저자 일동
제1장 천연가스와 액화천연가스 ?1
1.1 천연가스란 무엇인가? ·····························································1
1.2 천연가스 처리공정 ···································································4
1.2.1 전처리 공정 ··········································································5
1.2.2 천연가스 액화공정 ······························································6
1.2.3 NGL 회수공정 ·····································································8
제2장 상평형 일반 ?11
2.1 화학공정의 모사에 있어서 열역학 올바른 열역학 모델식
선정의 중요성 ·········································································11
2.2 상평형 추산에 있어서 One Model Approach와 Two Model
Approach ················································································23
2.3 기액 평형 관계식 ···································································25
2.4 순수성분의 퓨개시티 계수 표현식의 유도 ·······················27
2.5 혼합물 중의 ‘i' 성분의 퓨개시티 계수 표현식의 유도 30
2.6 액액 평형 관계식 ···································································33
2.7 과잉 Gibbs 자유에너지와 액체 활동도계수 사이의 관계
식의 유도 ·················································································38
2.8 엔탈피의 계산 ·········································································43
2.9 밀도의 계산 ·············································································48
2.9.1 Redlich-Kwong 식을 이용한 기체의 밀도계산 ······49
2.9.2 Redlich-Kwong 식을 이용한 액체의 밀도계산 ······51
2.10 SRK 상태방정식에 적용한 혼합물 중의 ‘i' 성분의 퓨개
시티 계수 표현식의 유도 ···················································54
2.11 PR 상태방정식에 적용한 혼합물 중의 ‘i' 성분의 퓨개
시티 계수 표현식의 유도 ···················································63
제3장 상태방정식 모델식의 소개 ?75
3.1 이상기체 거동으로부터 벗어나는 편차와 반데르발스 상태
방정식의 소개 ·······································································75
3.1.1 반데르발스 상태방정식의 매개변수 a와 b의 소개 ···77
3.1.2 반데르발스 상태방정식의 변형 ·····································79
3.2 Redlich-Kwong 상태방정식 (1949년) ··························85
3.2.1 Redlich-Kwong 상태방정식의 매개변수 a와 b값의
계산 ·····················································································86
3.2.2 Redlich-Kwong상태방정식의 변형 ····························90
3.3 Soave-Redlich-Kwong (SRK) 상태방정식 (1972년) ····91
3.3.1 Soave-Redlich-Kwong 상태방정식의 매개변수 a와
b값의 계산 ·········································································93
3.3.2 Soave-Redlich-Kwong 상태방정식의 변형 ···········97
3.4 Peng-Robinson (PR) 상태방정식 (1976년) ··············98
3.4.1 Peng-Robinson 상태방정식의 파라미터 a와 b의 계산 ·99
3.4.2 Peng-Robinson 상태방정식의 변형 ························106
3.5 일반적인 2 매개변수 삼차형 상태방정식 ······················106
3.6 순수성분의 증기압을 잘 추산하기 위한 Alpha Functions ·108
3.7 혼합물의 K-value를 잘 추산하기 위한 Mixing Rules ·113
3.8 일반적인 2 매개변수 삼차형 상태방정식에 적용한 Panagiotopoulos
혼합규칙의 유도 ··································································115
3.9 SRKM과 PRM ···································································127
제4장 전기식 냉동기에서 냉매의 공급온도에 따른 액화천연가스의
톤당 냉열 가격 ?129
4.1 서론 ························································································129
4.2 전산모사 ················································································134
4.2.1 노말 부탄을 냉매로 사용한 냉동사이클의 전산모사 ···134
4.2.2 LNG의 질량유량을 결정하기 위한 전산모사 ··········136
4.3 결과 ························································································137
제5장 액화천연가스를 활용한 개방형 랭킨 사이클에 적용한 냉열
발전의 최적화에 대한 연구 ?141
5.1 서 론 ······················································································141
5.2 개방형 랭킨 사이클에 대한 전산모사 ····························142
제6장 천연가스 조성에 따른 수소 생산 시에 발생하는 이산화탄소
배출량 산출 ?99
6.1 서론 ······················································································149
6.2 전산모사 ················································································151
