Enhanced waste to energy operability under feedstock uncertainty by synergistic flue gas recirculation and heat recuperation

Variations in quantities and composition of received wastes in waste to enegy (WTE) plants lead to throughput and power losses (lower profits). By disturbing the mean residence time of flue gases in the air-pollution-control-system they result in temperature and offgas flow variations affecting combustion efficiency and actual pollutant emissions. Besides energy savings, integration by flue gas heat recovery (FHR) in a heat exchanger (recuperator) enables maintaing high throughput under feedstock uncertainty (e.g. poor wastes). An effective method for reducing WTE atmospheric pollution, mainly NOx emissions, flue-gas-recirculation (FGR) - mass recirculation of a fraction of flue gases to the combustor - may be used for the same purpose. Both FHR and FGR are related to robustness issues, limiting the actual range and effect of manipulation. Recent results indicate that FHR and FGR have opposite effects on WTE performance - increasing FGR cools down the combustor, while FHR boosts up combustion. The present work demonstrates the possibility of improving operability of WTE facilities by combined use of FHR and FGR, utilizing multiple waste mixtures with uncertain feedrates, heating value, or composition. It brings forth a key dimensionless parameter, determining the direction and magnitude of the manipulation and leads to explicit expressions for the sensitivities of power production, throughput and capacity constraints with respect to FGR and FHR ratios. Synergistic use of FHR and FGR enables maximization of throughput and power production within the process capacity constraints, without detrimental effects on destruction efficiency or final emissions. A Case Study is analyzed for a facility under a public-private-partnership contract, with received waste ranging from a guaranteed minimum 150.000-200.000 TPY and composition range: biodegradables 52-70% ww, recyclables (paper, plastics, metals, glass) 25-45% ww.