Rotary kilns have been used for decades to generate energy from wastes. The opera­tion of this type of reactor has been demonstrated in a continuous mode at industrial scale and many problems have been reported. The very high temperatures during incineration promote NOx and SOx production as well as dioxins and furans, which are carcinogens. Moreover, leaching and slagging can affect rotary kiln incinerators operability. Notwithstanding these reports, the interest for using rotary drum reac­tors for pyrolysis applications is currently growing [15]. Operation of the reactor at lower temperature and without oxygen will likely decrease pollutant emissions as well as minimize risks of leaching and slagging. In most cement plants around the world, rotary kilns can have impressive dimensions (over 100 m long). For pyrolysis applications, some pilot-scale units were built and operated to process waste.

Typically, a rotary drum pyrolysis reactor processes raw materials such as MSW, RDF and waste tires. It consists of a horizontal cylindrical reactor rotating at a certain speed in order to mix the bed of materials and promote transport phenomena. For this application, as oxygen must be purged, heat supply is generally indirect, that is, gas burners are mounted underneath the rotary drum and the flue gases are circulated around the drum in a blanket-like chimney. Figure 11.3c shows a schematic of a rotary drum pyrolysis system with an electric heater.