Various branches of industry use flue gas systems to improve their industrial processes, a perfect example is the fertilization of greenhouses with carbon dioxide, the concentration of CO2 in the exhaust gas is 5-6%. In addition to a significant amount of CO2, the exhaust gases also contain toxic NOx nitrogen oxides, which in excessive amounts destroy green bodies in plants and can cause leaf fall. An ideal solution is a exhaust gas system that controls the concentration of CO2 in the greenhouse, in the event of too high a concentration system will redirects the exhaust gas outlet to the atmosphere via a three-way valve. Such a system has an SCR catalytic converter reducing nitrogen oxides, in combination with a cogeneration heat exchanger which uses waste heat from exhaust gases or waste heat from the engine cooling system provide a constant temperature in greenhouses. That system in combination with carbon dioxide fertilization affects plant growth and the size of the harvest.