Plastics as a reburning fuel: pyrolysis at reburning temperatures and NO-reburning by simulated pyrolysis gas

Using waste plastics as a reburning fuel is promising for both reducing NOx emissions and managing plastic waste. To gain a better understanding of the chemistry involved in plastic reburning, pyrolysis products of different types of plastics at typical reburning temperatures (900–1200 ◦C) were investigated experimentally, and the reburning performance of simulated plastic pyrolysis gases was studied through experiments and chemical kinetic modeling. The plastics involved in the pyrolysis experiments included high-density polyethylene (HDPE), polypropylene (PP), polyamide 6 (PA6), and acrylonitrile–butadiene–styrene copolymer (ABS). HDPE exhibited the highest hydrocarbon gas yield (up to ~ 70 wt% at 900 ◦C) while ABS had the lowest gas yield (<20 wt%) among the four plastics at the investigated temperatures. The hydrocarbon gas was composed mainly of methane, ethylene, and acetylene, with ethylene decreasing and acetylene increasing with temperatures rising from 900 to 1200 ◦C. Hydrogen cyanide was found to be the dominant gaseous nitrogen-containing compound for PA6 and ABS. Solid byproducts such as soot were generated at pyrolysis temperatures exceeding 1100 ◦C, leading to a reduction of the gas yield for all plastics. This indicates a potentially important contribution of heterogeneous NO reduction mechanisms during plastic reburning at high temperatures. Concerning the homogeneous NO reduction, reburning experiments using simulated plastic pyrolysis gases composed of methane, ethylene, and/or acetylene were carried out at 700–1200 ◦C in a flow reactor with an initial NO concentration of 1000 ppm and an excess air ratio λ of 0.7. Results for the simulated plastic pyrolysis gases showed NO reduction efficiencies ranging from 30 % to 46 % within the temperature range 950–1200 ◦C. The presence of acetylene in the simulated pyrolysis gas mixtures appears to facilitate NO reduction. A detailed chemical kinetic model applied to simulate the experiments gave a reasonable prediction of the NO reduction and revealed the important role of the ketenyl radical in removing NO.