CONVERSION OF FLARE GAS TO POWER USING SUPERCRITICAL CO2 POWER CYCLE – ENERGY, EXERGY, AND THERMOECONOMIC ANALYSES

Abstract

An increase in oil and gas production is associated with the increase in energy demand around the world. This increase in producing fuels is correlated with an increase in flaring process, which is a wasted energy unless utilized properly. Several utilization techniques were applied in open literature to make use of the flare gas including producing natural gas liquids (NGL), liquefied natural gas (LNG), natural gas Hydrates (NGH), and other products. Furthermore, flare gas was utilized in power generation in several applications, but it was only applied on conventional power cycles that have limited efficiency with impractical layouts. To utilize the flare gas efficiently, in this thesis two new cycles are proposed that integrate flare gas and supercritical CO2 power cycle. In the first proposed cycle, referred to as flare-to-power 1 (FTP 1), flare gas is mixed with natural gas and burned in flare heat recovery unit (FHRU) to heat the working fluid (the supercritical CO2) before entering the gas turbine. While in the second cycle (FTP 2), flare is mixed with natural gas and burned to perform a reheating process for primary heated working fluid in a heating unit (HU) using natural gas only after the process of partial expansion in a high-pressure turbine. Energy, exergy, and thermoeconomic analysis were performed comparing both configurations along with sensitivity analysis to study the effect of main parameters on the system. Moreover, a comparison between proposed cycles using indirect-combustion (IDC) with similar cycles analyzed previously using direct-oxy-combustion (DOC) flare power cycle 1 (FPC1) and flare power cycle 2 (FPC 2) is conducted. Six different flare compositions were investigated in the analysis, and it was found that the highest thermal efficiency is obtained with FPC 1 configuration using flare sample 1 with a value of 59.16% and an exergy efficiency of 77.69% at 𝑇𝑚𝑎𝑥=650 ℃, 𝑇𝑚𝑖𝑛=32℃, 𝑃𝑚𝑎𝑥=25 𝑀𝑃𝑎, and 𝑃𝑚𝑖𝑛=4 𝑀𝑃𝑎. However, the lowest levelized cost of electricity (LCOE) is obtained using FTP 1 with flare sample 4 with 3.91 ¢/kWh at 𝑃𝑚𝑎𝑥=30 𝑀𝑃𝑎, 𝑇𝑚𝑎𝑥=650 ℃, 𝑇𝑚𝑖𝑛=32℃, and 𝑃𝑚𝑖𝑛=4 𝑀𝑃𝑎. These proposed cycles integrating flare gas and supercritical CO2 power cycle are superior to existing conventional cycles in terms of efficiencies, reduction of CO2 emissions, and their compact size. This study also suggests several directions of research for further investigation including analyzing the effect of variable flare gas flowrate on energy and exergy performance, and the design of cycle components in terms of size and materials. However, an experimental set-up needs to be built for results verification and validation of different aspects of the supercritical CO2 power technology.