Abstract: The present disclosure deals with a gas cleaning system (1) for cleaning process flue gas. The gas cleaning system (1) comprises a reactor inlet duct (13) having a longitudinal axis and a reactor duct (14) fluidly connected perpendicularly to the reactor inlet duct and positioned downstream from the reactor inlet duct. The reactor duct likewise has a longitudinal axis. Within the reactor duct is a gas cleaning device (20), such as a catalytic reactor, and a gas flow rectifier (30) for rectifying flue gas flow from the reactor inlet duct (13) into the reactor duct (14). The gas flow rectifier is arranged in the reactor duct upstream of the gas cleaning device (20), wherein the gas flow rectifier (30) comprises at least one expanded screen (30a).