<div class="section abstract"><div class="htmlview paragraph">A new tool for SCR (selective catalytic reduction) testing is developed. This side stream test bench has an advantage to test smaller catalysts with a proper exhaust. The exhaust mass flow and temperature are adjustable. A control program is developed to adjust and save the needed parameters. NO_x sensors are utilized in the test bench to measure the levels upstream and downstream of SCR reactor. In this study one SCR with a volume of 40.5 dm³ is tested with exhaust flows ranging from 400 to 600 kg/h and exhaust temperatures from 280 to 360°C. The tests are conducted using exhaust gas from a medium-speed diesel engine running on heavy fuel oil. A decrease from 75% to 99% (depending on test conditions) was observed in NO_x over the catalyst. In addition HC and PM were found to decrease while CO was increased over the catalyst. The controlling of exhaust flow and temperature with the test bench succeeded with only minor errors. The measured value of exhaust flow deviated from the set value in the order of 1% and temperature in the order of 0.1%. The results of NO_x sensors correlated well with the standardized NO_x measurement results with an exception being the case when ammonia slip was formed. When ammonia was found in the exhaust the NO_x sensors were observed to misleadingly measure the ammonia as NO<sub/>. The results with different exhaust flow rates revealed that SCR was performing better when the flow was lower, i.e., the space velocity was lower. At higher temperatures higher NO_x conversions were observed. At 360°C the NO_x conversion was near 95% while at 280°C the conversion was 75%.</div></div>