A system for treating an exhaust stream from an engine includes a main exhaust passageway adapted to receive the exhaust stream from the engine. A side branch is in communication with the main exhaust passageway. A regeneration unit is positioned within the side branch for combusting a fuel and heating the exhaust flowing through the main exhaust passageway. A lean NOx catalyst is positioned within the main exhaust passageway downstream of the regeneration unit. A reductant injector is positioned downstream of the regeneration unit and upstream of the lean NOx catalyst to inject reductant particles into the exhaust stream. A controller operates the regeneration unit to increase the exhaust temperature as well as operates the reductant injector to reduce NOx within the lean NOx catalyst.