On Friday the 19th of July 2019 a massive explosion and fire at the coal gasification plant of the Henan Gas Group caused more than 30 fatalities and major injuries. Damage to property within 3km of the facility resulted from the blast. The root cause lay within the Air Separation Unit complex where oxygen was produced to feed the gasification reaction.
This incident which took place in Yima city, Henan province, is not common but is not isolated. There have been eight explosions on Chinese ASUs since 1973. And the problem is not restricted to China. On the 6th of August 2007 an explosion at an Air Separation Unit, or ASU, operated by NIGC feeding the SABIC Ethylene Oxide production facility in Al-Jubail, Saudi Arabia suffered an explosion that ceased ASU operations on the site for many months and delayed the ramp up of Ethylene Oxide and MEG production significantly.
Why do ASUs explode? The most feared reason is accumulation of hydrocarbons in liquid oxygen which can take place in the main condenser unit. The intense concentration of combustible hydrocarbons with the densely packed oxidising potential of liquid oxygen is the principle that fired some of the first rockets more than 79 years ago.
Intake of hydrocarbons with the air that is sucked into the ASU is inevitable. Extremely low concentrations of methane, acetylene and other light hydrocarbons are present in the air from natural causes or due to emissions from neighbouring petrochemical processing operations. At low concentrations in gaseous air, these hydrocarbons present no risk because the gas mixture is not within a flammable range.
However, when the air is separated to nitrogen and oxygen the hydrocarbons accumulate with the liquid oxygen. Over time, they can build up to levels that form an explosive mixture and detonation would be catastrophic for the ASU and surrounding people, property and process equipment.