The art of catalyst design involves choosing the appropriate active ingredient to create the desired chemistry in the HRSG application, the appropriate carrier to support the active ingredient under the unique conditions of the HRSG application, and finally the appropriate substrate to provide contact between the catalyst and the exhaust flow being treated in the HRSG application. The ability to take into account both the individual active material, carrier, and substrate's distinctive properties as well as the peculiarities of the HRSG application is what it takes to "bring it all together," though. For instance, HRSGs undergo heat cycles as part of their regular functioning by nature. The catalyst supplier must take these effects into account, just as the HRSG designer must take into account the effects of differential thermal expansion inside the HRSG components. Catalyst materials (such as substrates and module assemblies) frequently come into contact with substances with various coefficients of heat expansion. The catalyst provider must take this into account while designing.