The industrial feasibility of low temperature DeNOx in the presence of SOx: a project case in a medium coking plant

Catalyst poisoning by SOx has hindered the industrial application of selective catalytic reduction (SCR) technology for the DeNOx of low temperature flue gas for many decades. The current engineering process of placing the SCR unit after the desulphurization and dedusting units can lead to high project and operational costs owing to low DeNOx efficiency at low temperatures. Based on our previous pilot results, a DeNOx project case was built before the desulfurization unit in a medium coking plant to explore the industrial feasibility of low temperature DeNOx in the presence of SOx. A new engineering process was considered and designed to overcome the problem of SOx, including the elimination of SO3 with NH3 before the SCR reactor, the filtration of ash by a foam metal plate and in situ regeneration technology. The project case could run continuously for six months with above 70% DeNOx efficiency and less than 10 ppm NH3 slip at 250 °C and a space velocity of 4000 h−1 in the presence of 260–300 mg m−3 SOx. The activity loss for the catalyst itself was not obvious after it had been running for six months, but blocking of the honeycomb channels by the sedimentary ash on the honeycomb catalyst modules owing to the low linear velocity resulted in decreased DeNOx efficiency and an increased pressure drop. A improved DeNOx process with gravitational dust collectors was also proposed to upgrade the present DeNOx project case for further continuous and stable operation.