Dallas-based company Energy Transfer said an incident at its 20-inch pipeline occurred just off Spencer Highway in Deer Park.
The City of Deer Park said the pipeline is burning the chemical "Y Grade NGL," natural gas liquid, which a U.S. Energy Information Administration presentation generally identifies as a mixture of ethane, propane, butanes, and natural "gasoline," or pentanes.
Those words should be familiar in everyday life as flammables.
Oil Price Information Service, a Dow Jones-owned data digest for the fuel supply industry, simplifies NGLs as "removed from natural gas at natural gas processing plants as a combined stream, often called raw make or y-grade."
"This combined stream, like the strands in a rope, is then fractionated (separated) to produce the products ethane, liquefied petroleum gases (propane and butanes), and natural gasoline. Though they are derived from a vaporous stream, natural gas liquids are kept in a liquid state for storage, shipping, and consumption," OPIS wrote in a November 2022 explanation blog.
13 INVESTIGATES: Energy Transfer waits hours before providing statement on pipeline fire
What about the safety of NGLs? The evacuations and shelter-in-place orders should signal the severity of what's occurring.
Food & Water Watch, a nonprofit watchdog group, said NGLs are colorless and nearly odorless, and using a cell phone could spark a fire. The Associated Press additionally reported on a chemical smell in the air in the early stage.
Harris County's air monitoring dashboard, implemented in the wake of the 2019 ITC fire, has already identified the pipeline fire in multiple areas in Deer Park and La Porte.
SEE MORE: Deer Park pipeline fire will take hours to burn out, possibly stretching into Tuesday, officials say
Why fire crews are standing back
Letting the fire burn out is better, from an environmental perspective, than trying to attack the flames with some kind of suppressing foam or liquid, said Ramanan Krishnamoorti, a petroleum engineering professor at the University of Houston.
"Otherwise it's going to release a lot of volatile organics into the environment," he said.
Still, there will undoubtedly be negative environmental consequences, including a release of soot, carbons and organic material, he said.
Energy Transfer said air monitoring equipment was being set up near the plume of fire and smoke, which could be seen from at least 10 miles (16 kilometers) away.
A statement from Harris County Pollution Control on Monday afternoon said no volatile organic compounds had been detected. The statement said particulate matter from the smoke was moderate and not an immediate risk to healthy people, although "sensitive populations may want to take precautions."
Natural gas liquids are used primarily in the manufacturing of plastics and basic and intermediate chemicals, Krishnamoorti said.
The Associated Press contributed to this story.