Stoichiometry Control in Reduction Furnaces

Abstract

Abstract

Pyrolysis furnaces run in a reducing mode to destroy waste materials while minimizing NO_x formation. A subsequent reoxidation (re-ox) section, sometimes referred to as an afterburner, combusts any remaining combustibles exiting the reducing section. The oxygen level in the re-ox section can be easily controlled with a conventional O₂ analyzer and an air control loop. However, controlling the stoichiometry in the reducing section is more complicated because of the waste stream composition and flow rate variability. Various techniques have been used such as measuring the waste gas composition and flow rate, measuring the combustibles level, or measuring the differential temperature before and after the re-ox air injection. All of these have some challenges depending on the application. A new technique has been developed for estimating the stoichiometric ratio using the combination of the measured oxygen concentration (which is at trace levels in the reducing section) and temperature in the reducing section. This method can be applied to many waste compounds such as NH₃, HCN, NO_x, H₂, and saturated and unsaturated organic fuels. The error in the correlation predictions is typically less than 3% compared to equilibrium calculations. The accuracy of the correlation is not affected by the addition of inerts (such as nitrogen, argon, helium), oxygen, air or NO_x, to any of the fuel, waste, or air streams. This new technique for measuring and controlling the stoichiometry in pyrolysis furnaces is discussed here.