(362f) Rigorous Thermodynamic Analysis of a Baseload LNG Chain with Different Boil-Off Gas Minimization Strategies
The primary focus of the study is to identify exergy losses across a baseload LNG chain supplying more than 3 MTA of LNG. In this research, a rigorous exergy analysis was performed on an entire actual LNG chain that was simulated using ProMax® and Aspen Plus® simulation software. Exergy loss across each component of the entire chain was determined taking into consideration both physical and chemical contributions. The analysis was not limited to the main processing units, as it was extended to quantify exergy loss across the chain utility sections (i.e. power and steam generators, cooling water loop, etc.). Results revealed that the main contributor to the total exergy loss is the LNG plant utility section, accounting for 69% of the total exergy loss. Within the main LNG process, significant amounts of losses were found to occur in the liquefaction, sweetening and sulfur recovery units; corresponding for 33%, 30% and 30.4% of the plant total exergy loss, respectively, excluding the plant utility. Components responsible for the highest exergy consumption were also highlighted, with the main consumers being the compressors and their drivers, LNG storage, columns (absorbers, distillations) and heat exchangers. The contribution to the total exergy loss provided valuable insights on locations where improvements are needed to translate into environmental and energy benefits. Different optimization options were identified, rapidly evaluated, and found to be attractive for rigorous synthesis and optimization. One optimization opportunity was concerned with the LNG loss during storage, also known as boil-off gas (BOG) generation.
BOG generation takes place because of high pressure LNG flashing to storage condition, heat leaks, and other factors. The evaporation of the stored LNG means loosing portion of the compression energy provided for NG liquefaction. Nearly 12 MW of compression energy is needed to liquefy one MTA of LNG. Our simulation shows that a 1 MW loss in the liquefaction results in 3.3 MW loss in the compressors gas turbine drivers. Thus, LNG loss through BOG generation should be minimized, or effectively utilized in the chain. Different BOG minimization and utilization strategies were investigated using a combination of thermodynamic, synthesis, and simulation techniques taking into consideration loading and holding (i.e. no ship loading) operation modes. Valuable insights were obtained and utilized to recommend new practices for BOG minimization and utilization.