The project will design, build, and test a high-pressure linear-motor-driven leak recovery compressor for the cost-effective recovery of methane leaks within the transmission, storage, gathering, and processing sectors of the natural gas value chain.

Gas Technology Institute (GTI); Des Plaines, Illinois
University of Texas Center for Electromechanics; Austin, Texas
 

Large reciprocating and centrifugal compressors are central to the natural gas value chain. Across the U.S., around 8,000 compressors move gas through over 250,000 miles of large transmission lines and store it underground in high-pressure geological formations for use during high demand periods. These compressors are concentrated at about 2,000 compressor stations where they are maintained and operated year-round. These compressor stations also contain pneumatic controllers, storage tanks, and purge systems that are required to operate the compressors or direct flow in and out of the station.

These compressor stations represent potential methane emission sources, and though relatively small in number, they contribute a disproportionately large percentage (about 20%) of the total methane emissions from the entire value chain.  The primary challenge preventing the capture and mitigation of these leaks is not a lack of will or desire, but rather the absence of a suitably engineered and priced solution.
 

The project team will design, build, and test a fully functional linear-motor-driven leak recovery compressor and package it onto a full leak recovery system designed specifically to gather and recompress methane emissions from midstream transmission and storage compressor stations. The linear motor leak recovery compressor is uniquely suited to provide variable flow capabilities to match the variable leak rates inherent to this application. It is being designed for a high-pressure discharge to compress the gas back to midstream operating pressures, often over 1,000 psig. The team’s goal is to develop a unit that is less expensive than current compressors while still capable of reaching a discharge pressure of 1500 psig or more.

• Simulated compressor performance computationally predicts inlet and outlet flows and pressures in response to changes in stroke, frequency, piston diameters, valve sizes, etc.  •    Includes losses associated with friction, pressure drop, internal leaks, and motor inefficiencies to ensure the compressor operates within the motor specifications.
• Sized components for 4 stages of pistons and valves; later modified design to 3 stages with double acting stage 1 to better control flow rate.
• Modeled motor force requirements and identified a discrepancy.
• Built a motor test rig and used it effectively to measure motor efficiency and operation.
• Selected proper motors and started necessary modifications.
• Completed design of compressor sections that balances forces and provides target flows and pressures.
• Sized all ancillary components (valves, heat exchanges, etc.).
• Completed horizontal motor frame prototype and initiated procurement of components.
• Completed economic modeling that shows market potential.
• Completed compressor P&ID using information from gas utility partners.
• Identified control and data acquisition hardware.  Purchased hardware, tested, and verified operation of motor controller and data acquisition hardware on an existing linear motor test frame.
• Evaluated several approaches to system control logic and currently refining strategy.
• Simulating aggressive leak cases that jump from 10% capacity to 100% capacity in half a second.
• Controls are maintaining simulated vent gas system within 2 inches water column of target pressure.

Figure 2. Simulated results using control strategy based on inlet pressure. The mass flow graph is showing the leak rate into the leak recovery buffer tank which varies both sinusoidally and using a step change. Input tank pressure shows the variation in the suction tank pressure as the leak recovery compressor control system adapts to the changing leak rate. The suction pressure is always maintained within 500 pascals (2 inches water column) of the target pressure, ensuring vents at compressor stations are not over-pressurized.

The project team is running additional simulations with different control strategies to identify the most effective approach.  
GTI is working to finalize the compressor design using 3 stages of compression.  The design will include necessary force balance, flow, heat transfer, and system integration/maintenance considerations.  GTI is starting sourcing of all the components required for preliminary testing, including motor, compressor, and balance of plant components.

$1,499,918

$375,002

NETL – Kyle Clark (kyle.clark@netl.doe.gov or 304-244-9178)
Gas Technology Institute – Jason Stair (jstair@gti.energy or 224-235-3579)