Supercritical CO2 Heat Recovery System Finds Application in Oil and Gas

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.

The thermophysical properties of CO₂ as a working fluid offer high latent heat and density and fluid stability, thus maximizing heat absorption from the heat source at high temperatures (i.e., greater than 350°C) and ­consequently improving cycle conversion efficiency, which results in higher power output. This property is mainly beneficial at the waste heat exchanger, because CO₂ is a stable fluid even at the high temperatures and pressures observed in industrial processes. At these conditions in the waste heat exchanger, the CO₂ moves into…