By Tushar Patel and Jacob Duffney

To meet global emissions targets, traditional power generation cycles have aimed to reduce and capture greenhouse gas emissions, such as CO2 and other pollutants. However, the equipment needed to do so often requires significant expense.

The system’s effectiveness comes at the cost of the cycle’s efficiency. Electrical costs, for example, have increased markedly in cycles using external removal systems to capture, at most, 90% of the CO2 released.

An oxy-fired, trans-critical CO2 Allam Cycle with a low-pressure-ratio turbine has been found to be efficient and effective. It captures the CO2 produced by the combustion of hydrocarbon fuels and uses a combination of heating, cooling and compression to transform it into a supercritical state whereby it can be recuperated and recirculated. This has the potential to realize low cost and clean generation.

A key design choice is the use of a combined turbine-compressor train. However, implementing a supercritical CO2 compressor-turbine train poses certain challenges. An initial design consideration is to find the type of compressor solution best suited for this application.

Operating in the environment of sudden higher CO2 density and increased force levels on rotating equipment, the Atlas Copco compressor provides reliability and high-efficiency values. This allows it to use around 30% less energy than a standard single-shaft compressor.

Supercritcal pressure

This compressor also delivers high-pressure CO2 in supercritical CO2 power cycle applications. Today, applications require more than gaseous CO2. CO2 must be delivered under high, sometimes supercritical, pressure and in large quantities.

The emerging power cycle through oxyfuel combustion uses supercritical CO2 (sCO2) as a working fluid and operates above the supercritical point of CO2, where distinct liquid and gas phases do not exist.

Instead of conventional phase changes to recover the energy, sCO2 undergoes drastic density changes over the small temperatures and pressure gradients at high temperatures. This allows energy recovery in relatively smaller equipment.

The entire cycle depends on efficiency of the CO2 compressor and its design. For instance, in the high-pressure stages, dry face seals enhance the machine’s reliability and minimize seal leakage losses while providing savings on operating expense.

By segregating the stages of the inline compressor, intercooling can be implemented between stages. This increases efficiency by creating an even isothermal compression process.

The machine is designed to handle high forces that result from a dense, supercritical gas. The high-pressure ratio and efficiency of an integrally geared (IG) compressor can adapt rotor speeds to the impeller behavior.

One application of this technology was a CO2 compressor with an inlet pressure of approximately 30 bar, and inlet temperature of near ambient (achieved via cooling by conventional cooling towers). It also had a discharge pressure sufficiently high to achieve a specific gravity near liquid water when cooled to near ambient temperature again. The outlet pressure was about 90 bar.

A large range in pressure means the compressor must accommodate variations in volumetric flow. Furthermore, to facilitate startup and other modes of operation, it requires inlet guide vanes (IGVs) on the first stage that have the potential for a flow turn-down of up to 35%. Given the large amount of flow sent through the CO2 compressor, inter-cooling became necessary to reduce power consumption.

Note: the demonstration nature of the coupled turbine resulted in a compressor driver speed that was higher than typical synchronous speeds. Nonetheless, eliminating the use of an intermediary gearbox was preferred.

Delicate balance

Due to its nearly closed-loop process, the sCO2 Allam Cycle relies on a delicate balance of heating, cooling, compression and venting. Every aspect of the compressor-turbine train, from aerodynamics and process control to lube oil and seal support systems, must be carefully considered to prevent damaging or hazardous conditions.

The CO2 centrifugal compressors, including the referenced gas turbine-driven centrifugal compressor used in the sCO2 Allam Cycle, have typically been offered and supplied as in-line (between bearing) API 617 Ch.2 technology. Although this approach has had a long and proven history, it also presents limitations and drawbacks when it comes to CAPEX, maintenance and flexibility.

An IG compressor addresses these limitations. However, a typical gearing arrangement limits the size and number of stages that can be mounted on the machine.

An external intermediate gearbox can be used to vary the IG compressor input speed. But the industry typically rejects this solution due to the perceived complexity of two separate gearboxes.

Technological challenge

Along with the technological challenges of allocating the necessary combination of machine components in a compressor-turbine-generator train, there are additional considerations. These range from aerodynamics and rotordynamics to the design of the process sealing system, lube oil, and control systems.

With in-line compressor technology, the aerodynamic components are mounted on the same shaft, requiring them to be sized-based on a given driver speed. IG compressors have stage design speeds that can be selected independent of the optimum driver speed.

This makes it possible for components to be tailored to thermodynamic requirements without being bound by speed and enables higher loading per compression stage.

As each impeller is a stage with its own casing and inlet and discharge connections, IG compressors accommodate intercooling between stages to reduce adiabatic losses.

This allows for greater flexibility and more efficient compression. IG compressors are considered high-head compressors due to their ability to match stage speed to aerodynamic requirements. As a result, fewer impellers are needed to reach the same compression-ratio-to-head requirement, providing a more compact footprint.

IG compressors offer ready access to major components due to their horizontally split gearbox design. Any compression stage can be accessed separately and repaired onsite, without removing the process connections to the other stages, thus reducing downtime for maintenance.

IG compressor casings can also be rotated to orient the nozzles in any direction, allowing for significant layout variability and therefore compact machine footprints.

The typical gearing arrangement of the IG compressor limits the size and the number of stages that can be mounted on the machine. When an IG compressor is employed in a gas- or steam-driven train, it is often offered with an external intermediate gearbox to vary the input speed, or it is driven through a pinion.

Authors: Jacob Duffney and Tushar Patel work for Atlas Copco Gas and Process Division.