"When we're fully operational with both projects, we'll be able to collect about 95% of the CO2 produced during ethanol production," says McDonald, summarizing the project's prime environmental benefit, since the gas otherwise would be vented to the atmosphere.
A Lot Like Oil fields
McDonald says his team is utilizing off-the-shelf components to collect CO2 in an approach similar to that employed in oil fields, where natural gas must be dehydrated and compressed.
"The concept calls for initial capture of CO2 from corn-to-ethanol fermenters, then routing it through a 36-inch pipeline to a gas scrubber tower, where circulating water removes any hydrocarbons while the CO2 is at atmospheric pressures," says McDonald. Centrifugal blowers then boost the pressure to about 20 psig, after which the CO2 is transported to a six-cylinder reciprocating compressor, where pressure is further increased in four stages.
"The first two stages employ two cylinders apiece," McDonald says. "The third boosts pressure to about 685 psig." Discharge from stage-three compression is routed to a dehydration plant, a necessary step since CO2 is saturated after fermentation.
After drying, the CO2 returns for a fourth stage of compression, where it reaches approximately 1,435 psig and displays properties common to both gas and a liquid. Thereafter, a centrifugal pump further compresses the CO2 to 1,950 psig or more. Next, a 6-in. pipeline transports the CO2 to the injection well, though an 8-in. line will supply the IL-ICCS project.
The injection wells are designed to deliver the pressurized, dense (liquid phase) CO2 to close to the bottom of the 1,600-ft-thick Mount Simon Sandstone, a porous, brine-filled layer identified as a potentially sound reservoir for long-term CO2 storage, in part due to relatively impermeable layers below and above it. The layer also has a storage capacity of more than 29 billion tons.
"Although the wells share a lot in common with standard oil field wells, there also are notable differences," says Eric Berlin, project manager at Schlumberger.