Supercritical CO2 Heat Recovery System Finds Application in Oil and Gas Operations
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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.
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 the supercritical region and experiences no change in phase. Consequently, its temperature gradient can more-closely track with the heat source, allowing for a minimized temperature differential area between the heat source and working fluid. This translates to an effective heat-transfer profile, the ultimate goal of every waste-heat-recovery cycle (Fig. 1).
Additionally, using CO2 allows for a compact and flexible system that is a more-manageable retrofit option and comes at a far lower cost than many other approaches to waste heat recovery. For example, the physical properties of supercritical CO2 mean that it can interact more directly with the heat source, eliminating the need for a secondary thermal loop and consequently reducing the complexity of the solution and its total installed cost.
Power output can be optimized to the specific application, allowing the economic production of emission-free power. This makes it possible to reduce primary energy consumption, lower energy cost, and reduce greenhouse gas emissions from the fuel-intensive operations typically found in oil and gas operations, improving performance and meeting higher environmental standards.
Using patented technologies, the supercritical CO2 heat-recovery system presented in this paper is based on a closed-loop Rankine engine with a directly heated cycle. The CO2 is pumped to high pressure and conveyed through a recuperator and a waste heat exchanger, where heat is exchanged with the waste heat source. The heated CO2 is then expanded in the CO2 power turbine, where it is converted ultimately to electrical power, then cooled in a condenser and returned to the start of the cycle. Because no phase change occurs in the waste heat exchanger, the temperature of the working fluid can more-closely track with the heat source, allowing for thermal load following, more-effective heat transfer, and higher cycle efficiency.
The cycle is suited ideally to constant temperature heat sources such as those found in gas turbine exhaust streams, with combined cycle overall efficiencies that can be greater than 50%. The system is modeled around reference conditions on waste heat supply temperatures of 990°F with a flow rate of 540,644 lb per hour and a waste heat input of 114 MMBTU per hour, but can operate over a wide range of conditions around this reference.
By using supercritical CO2 as the working fluid heat recovery, the solution does not merely represent emission-free energy. Environmentally benign, nontoxic, nonflammable, low-cost, and readily available, use of CO2 as a fluid means that this approach does not have to use any water; and, unlike ammonia or other common closed-loop organic Rankine cycle fluids, it is completely safe in the unlikely event of a loss of working fluid.
Furthermore, the use of CO2 as a noncorrosive and nonabrasive fluid extends the integrity and lifespan of system components while minimizing any maintenance requirements and, in turn, supporting extended service intervals. Designed as a low-maintenance system, the heat recovery unit is fully automated with connectivity for remote operation and monitoring, and does not require on-site personnel. The complete paper presents a detailed description of the system configuration, installation, and operation, and compares steam and organic Rankine cycle technologies.
Applying CO2 Cycles in the Oil and Gas Industry
Following an extensive factory-testing program that included hundreds of hours of turbopump and power-turbine operations together with control systems, stability, and partial-load-endurance tests, TC Energy (formerly TransCanada) will be among the first companies to adopt the supercritical CO2-based technology in oil and gas operations. The company is developing a commercial waste heat power generation facility in Alberta using the technology that is, according to the authors, the first of its kind. Petrobras also is exploring the technology.
The supercritical CO2 cycle is simpler, more compact, and easier to operate than traditional power technologies such as ordinary steam or organic Rankine cycles. Such a system is estimated to have a 25 to 40% smaller footprint than an equivalent steam unit and no need for operational expenditure on boiler operators, for example. Consequently, the system is suitable for minimally invasive retrofitting programs at both onshore installations such as compression stations and liquefied natural gas facilities, and, in the future, offshore platforms, where space is at a premium and restrictions on weight often exist. The approach can also be a totally dry closed-loop process that offers considerable flexibility, including a water-free option that is advantageous in arid regions.
Following its own work on the concept of supercritical CO2, the US National Energy Technology Laboratory recently concluded that power cycles based on this technology offer benefits for stationary power production with a broad range of potential sources of energy. The supercritical CO2 heat-recovery system described in the complete paper derives its value through its unique combination of a lower cost per unit of electricity produced, a compact footprint, higher energy recovery from waste-heat streams, ability to generate power from a wide range of sources, and production of environmentally friendly low-cost electricity from a wasted resource without the need for water.
By recovering heat loss and increasing efficiency, the operators of production facilities and transmission systems can make dramatic improvements and yield new commercial opportunities. For instance, energy recovery may potentially allow profits to be realized from fields that might otherwise be marginal prospects, or may represent an additional revenue stream for gas-pipeline transmission and distribution-network operators.
Supercritical CO2 Heat Recovery System Finds Application in Oil and Gas Operations
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