A reactor for reducing the concentration of NOx in a stream comprising: an inlet for the stream; an outlet for a stream containing a reduced concentration of NOx; one or more catalyst beds comprising a ceramic or metallic foam with a NOx reduction catalyst; one or more flow paths from the inlet to the outlet that passes through at least one catalyst bed wherein the catalyst beds are closed at the top and bottom so that the flow path through the catalyst bed passes through the sides of the catalyst bed in a lateral flow is described.