Supercritical CO2Heat Recovery System Finds Application in Oil and Gas Operations

Abstract

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 197965, “Transforming Waste Heat to Electric Power in Oil and Gas Compression Systems Using Supercritical Carbon Dioxide,” by Thomas Soulas, Siemens, prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. The complete paper describes an advanced Rankine cycle process-based system that converts waste heat into usable electrical power to improve the efficiency of gas-compression stations on gas-production platforms and pipelines. Instead of steam, this system uses industrial-grade carbon dioxide (CO2) in the supercritical state as the working fluid. Introduction Globally, rejected heat is estimated to correspond to approximately 65% of net energy input across the industrial infrastructure, with numbers varying from 60 to 70% depending on the region. Considerable waste heat is ejected from equipment such as the gas turbines commonly used in mechanical drive applications found in the compression processes of gas-production platforms and transmission pipelines. Recently, a technology that supports energy recovery from heat rejected from a broad range of industrial processes has become available to the oil and gas industry. The system recovers usable, but often wasted, heat and converts it into higher-value, usable electrical power. While most gas turbine heat-recovery systems use a bottoming steam cycle to improve thermal efficiency, the system described in the complete paper is based on an advanced Rankine cycle process. With revenue and cost predictability, the technology generates power at a competitive installed cost and delivers an estimated 10% increase in baseline efficiency for a gas-compression station to reduce effectively the overall cost of electricity. The main innovation of the technology lies in the selection of CO2 as the working fluid. Advantages of CO2 in Heat-Recovery Cycles CO2 has relatively moderate conditions for the supercritical state, with a critical pressure of 1,071 psi and a critical temperature of just above 88°F. With an approximately 50% increase in specific heat capacity at approximately the critical point and likely cycle conditions, and a reduced compressibility factor near the critical point, CO2 is ideally suited to recover heat energy across a broad range of temperatures and sources such as those found in exhaust gas streams.