An assessment of waste heat powered absorption cooling in the oil and gas industry

Abstract

The global rise in energy demand and prices, and environmental concerns, are prompting the oil and gas industry to improve its energy efficiency. In this study, waste heat utilization strategies for energy efficiency enhancement in oil and natural gas (NG) plants are investigated in terms of thermodynamic performance and economic feasibility. Emphasis is placed on operating conditions representative of Middle East facilities, which are constrained by high ambient temperatures and relative humidity over a major part of the year. An extensive review of the literature highlights that studies on waste heat utilization in the oil industry have generally been limited to isolated sections of the plants, with the result that the entire spectrum of available energy recovery technologies for a given application has not been considered. In the NG sector, waste heat utilization has received considerably less attention than in the oil industry. In this study, waste energy sources within oil refineries, NG processing plants and liquefied natural gas (LNG) regasification terminals, are identified and quantified for typical plant layouts. Generic waste heat utilization schemes are developed and implemented to a major natural gas liquid (NGL) plant in the United Arab Emirates, namely GASCO ASAB. Scheme design is based on the use of gas turbine (GT) exhaust gas waste heat-powered absorption refrigeration for power and steam generation, to provide enhanced cooling and heating capacities to the plant. The schemes involve energy efficiency improvement of thermodynamic cycles using waste heat-powered absorption chillers, utilization of LNG as a heat sink for Rankine and Brayton power cycle configurations, and LNG pressure energy recovery through direct expansion turbines. Thermodynamic models for gas turbines, combined gas and steam turbines, and absorption refrigeration systems (ARSs) are developed using Engineering Equation Solver (EES), and validated by comparison with published experimental data. In all cases, prediction discrepancies for key parameters are found to be less than 5%. The validated models are used to analyze the thermodynamic performance of both the generic and ASAB plant-specific energy recovery schemes developed. The modeling results indicate that 151.5 MW and 127.4 MW of gas turbine exhaust waste heat could be utilized to generate 71.3 kg/s and 60.3 kg/s of 894 kPa steam, at ASAB-0 and ASAB-1 NGL plant, respectively, using a waste heat recovery steam generator. At ASAB-0 plant, the steam produced is utilized to power thirteen double-effect H2O-LiBr absorption chillers, which generate an additional cooling capacity of 195 MW. This enhanced capacity provides i) gas turbine inlet air cooling, which generates approximately 21 MW of additional electric power, ii) eliminates air coolers, and iii) pre-cools the process gas fed to the propane chiller at a reduced temperature of 10°C, thereby reducing propane compressor work. In addition, a small portion of the waste heat is utilized for lean gas regeneration, which saves approximately 3 MW of process heat. The capital investment, operating cost and annual savings of the proposed energy recovery scheme at ASAB-0 plant are estimated to be 64.1 million US$, 3 million US$ and 27.4 million US$ respectively, with a payback period of approximately three years. At ASAB-1 plant, ten 15 MW double-effect H2O-LiBr ARSs, each utilizing 5.4 kg/s of steam, would provide 145 MW of process cooling capacity to the plant, which could either be utilized for process gas cooling, or gas turbine inlet air cooling. The capital and operating costs of the waste heat powered ARS are approximately 49.3 million US$, and 2.2 million US$, respectively. Annual savings and payback period for the gas turbine cooling option are estimated to be 8.9 million US$ and approximately 5 years, respectively, and for the process gas cooling option, 21.8 million US$ and three years, respectively. Therefore, it is recommended to utilize the additional absorption cooling capacity for process gas cooling. This study demonstrates both the thermodynamic and economic feasibility of waste heat powered absorption refrigeration at a major NGL plant. The waste heat recovery methods and thermo-economic analysis methodology employed in this work could be extended to other oil and gas plants to quantify their waste heat utilization potential, and develop comprehensive heat recovery schemes tailored to existing facilities. This offers tremendous opportunity for reducing energy losses and fuel consumption through recovery of waste heat. Future work programs are proposed to further develop this research area.

Date of Award	2011
Original language	American English
Supervisor	Peter Rodgers (Supervisor)