Temperature and residence time of the combustion products in a waste-to-energy plant

The reduction of the dioxin levels in the exhausts of today waste-to-energy plants relies on the control of the thermo-fluid-dynamic processes occurring within the combustion chamber, rather than on policies aimed at restricting the amount of chlorine in the waste material to be treated. This is a consequence of the fact that waste-to-energy plants currently receive the bulk of discarded PVC and other chlorine sources that are deliberately burned in order to increase the waste heating value. Indeed, severe law regulations are into force in many industrialized countries, posing constraints on the value of some relevant in-chamber thermo-fluid-dynamic variables, such as temperature and residence time of the gases resulting from the combustion process, whose accurate experimental monitoring is extremely expensive and difficult to achieve. The present work analyses the shortcomings of the methods generally employed in full scale plants for the verification of the temperature and residence time of gases produced during the combustion process, and presents the advantages of using a new procedure developed by authors, based on the numerical simulation of the waste combustion process to optimize monitoring of the quantities of interest. The verification of the developed model, which accounts for both the solid and the gaseous phases, and for the various modes of heat and mass transfer between these phases, is obtained through a comparison with the results of an experimental campaign carried out on a full scale plant in Italy. The temperature distribution in the combustion chamber is calculated considering various waste compositions, and both forced and mixed convection. In fact, it is also shown that neglecting buoyancy effects may lead to appreciable errors.