An optimal algorithm to assess the compliance with the T2s requirement of Waste-to-Energy facilities

The gas resulting from the incineration of waste must be raised, after the last injection of combustion air, in a controlled and homogeneous fashion and even under the most unfavorable conditions, to a temperature of at least 850 °C for at least two seconds (Art. 50.2 Directive 2010/75/EU). This norm and its variations (i.e. 1,100 °C for 2 s if the chlorine content of the incinerated waste exceeds 1% by mass), called "T2s requirement", oblige all Waste-to-Energy (WtE) plant operators to monitor the post-combustion conditions and to turn on auxiliary burners in the occurrence of noncompliance with such a requirement. In a WtE boiler, the determination of the mean temperature reached by combustion gas in the post-combustion zone, after an ideal residence time of 2 s, is carried out by an algorithm implemented in the Distributed Control System (DCS) of the plant. Currently, since many different algorithms are used, it appears that further investigation on this subject is required. This work considers, as a case study, an existing WtE boiler and, by means of a calibrated long-furnace model of the post-combustion zone, investigates all the possible operating conditions as well as their connections with the monitored variables. The most relevant influences on the T2s temperature are highlighted and some control algorithms are proposed. The results so far obtained show that the T2s is affected both by boiler load and gas-side fouling in the same way and for the same extent. Therefore, since gas-side fouling in the post-combustion zone is an uncontrollable variable, boiler load is not usable in as input variable of a reliable algorithm. Moreover, the results highlight the significant role that can be played in the algorithm for the estimation of the T2s by the oxygen content in secondary flue gas.