duct 501 leads to screen 506 screen 506 has holes having a diameter such that flue gas is allowed to pass through screen 506 however at least some ash particles are too big to pass through the holes of screen 506 because these ash particles are too big to pass through the holes of screen 506 they accumulate in hopper 502 flue gas passes through screen 506 and enters duct 503 duct 503 channels flue gas 104 to selective catalytic reduction scr catalyst 504 which removes nitrous oxide from flue gas 104 the flue gas 104 leaving scr catalyst 504 may then be discharged into the atmosphere or cleaned further before discharge into the atmosphere screen 506 is a pleated screen as known in the art and described for example in u s pat no 7 531 143 after screen 506 has been in operation for some time ash particulate matter lodges in section 507 a of pleat 507 and blocks that section this blockage may cause the flue gas to flow through triangular pleat 507 at a non uniform velocity this non uniform velocity can cause deterioration of screen material and rupturing of the screen when this happens particulate matter that should be screened passes through screen 506 screens that are essentially flat panels such as flat screen 508 shown in fig5 b do not have the problem described.