STUDY ON NEAR-ZERO CO_2 EMISSION THERMAL CYCLES WITH LNG CRYOGENIC EXERGY UTILIZATION

摘要

Compared with other fossil fuels, liquefied natural gas (LNG) supplies relatively clean energy because its low contribution to environmental pollution. Furthermore, while 10% of the gas energy was required for its liquefaction, the valuable high cryogenic (at ～ 110 K) exergy of the LNG is wasted in many LNG receiving terminals worldwide, by uselessly cooling the seawater used for its re-vaporization. This cryogenic exergy is, however, as an excellent power source. In addition, due to the increasing concern about global climate change, the development of power system which minimizes CO_2 emission is of great interest. In this paper, new cycles are proposed which integrate LNG cryogenic exergy utilization and CO_2 recovery. The cycles employ both the fuel chemical exergy and LNG cryogenic exergy for power generation, they use no cooling water, and on the contrary, allow the easy removal and recovery of water and CO_2 generated from combustion to thereby offer both energy saving and greenhouse gas emission mitigation. The cycles employ CO_2 as the main working medium. Oxygen and fuel methane are introduced at stoichiometric ratio, and thus the turbine exhaust is merely a mixture of carbon dioxide and water steam. The LNG coldness is used to improve the power generation efficiency by (1) serving as a low temperature heat sink of the power cycle, and (2) cooling the working fluid prior to compression, to reduce compressor power consumption. Without consuming additional power, the water and extra carbon dioxide generated from combustion can be easily separated from the main stream during the exothermic process when integrated with the LNG evaporation process. Internal combustion and recuperation are adopted to increase the average heat absorption temperature. The proposed cycles are simulated using the commercial ASPEN PLUS code, and their performance is computed. The results indicate that the proposed cycles indeed reduce CO_2 emissions significantly with very attractive thermal performances, able to attain thermal and exergy efficiencies of 60-65% and 45-53% respectively.