Device for treatment of a gas flow having at least one body (3) that is adapted to cause a conversion in the composition of the gas. The device is characterized in that the body (3) has a modular construction comprising a plurality of sections (26, 26', 27, 27', 36) with different internal structures that allow gas to flow through the section, and that the sections (26, 26', 27, 27', 36) are arranged so that at least a portion of the gas flows through at least two sections with different internal structures during operation of the device.