Controlling NO_x emission from boilers using waste polyethylene as reburning fuel

Literature lacks data on the performance of waste plastics in controlling NO_x emissions in practical combustors operating typically in the temperature window of 1000–1200 °C, under a range of fuel-oxygen equivalence ratios, preventing recycling of plastics in this application. In this contribution, we demonstrate that waste plastics, in particular polyethylene, can serve as a reburning fuel for converting nitrogen oxides (NO_x) into environmentally-benign combustion products, in practical large-scale combustors such as pulverised coal power plant, circulation fluidised bed combustion plant, entrained flow boilers, and incinerators. The experiments involved a high-temperature vertically-entrained reactor operating in the range of 600–1200 °C, in conjunction with online infrared spectroscopy, chemiluminescence and gas chromatography. Chemical kinetic modelling, supported by DFT calculations with DMol³ package revealed the underlying chemistry of the NO_x mitigation reactions, especially the importance of C₂H₅ and C₂H₃ radicals. We modelled the process kinetically and performed a techno-economic assessment of the new technology, to prove its financial feasibility. The reaction of pyrolytic fragments with NO_x yields excellent removal efficiency of nitrogen oxides of up to 82% and selectivity to N₂ up to 85%, within the temperature range of 1000 °C–1200 °C, and fuel-oxygen equivalent ratios of Φ = 0.8–1.2. While we observed the formation of HCN, the overall nitrogen selectivity shifts towards the formation of N₂. Both the conversion of NO_x and the selectivity to N₂ can be improved further by increasing the residence time. The economic assessment indicates that, the use of waste plastics is comparable to other mainstream solid fuels and becomes less expensive when considering renewable benefits.