Simultaneously achieving flame stability enhancement and nitric oxide emission reduction in a small-scale burner using rare-earth composite oxide catalytic coatings

Achieving low nitrogen oxide emissions in high-efficiency fuel conversion is a crucial objective in combustion science. This study systematically investigates the effects of inert Al₂O₃ and catalytic Y₂O₃-BaO-ZrO₂ coatings on the combustion stability and pollutant emission characteristics of premixed methane/air flames confined within a narrow channel. Experimental measurements, including flame extinction limits, spatially resolved OH chemiluminescence intensities, axial flame temperature profiles, and CO/NO concentrations in exhaust gases, were conducted across a wide operating range. Results demonstrated that the quenching distance of bare stainless-steel wall exhibited a pronounced nonlinear dependence on wall temperature. In contrast, the quenching date for Y₂O₃-BaO-ZrO₂ catalytic coating were reduced linearly by up to 71.8% and 85.4% as wall temperature increasing to 1073 K at stoichiometric and fuel-lean conditions. Although Al₂O₃-coated wall yielded the higher peak OH chemiluminescence intensity in flame cores across all equivalence ratio than the case of Y₂O₃-BaO-ZrO₂, the latter promoted significantly higher peak flame temperature and markedly extended high-temperature reaction zones, strongly confirming superior combustion enhancement by wall modification with rare-earth composite oxide catalytic coatings. Crucially, the Y₂O₃-BaO-ZrO₂ coating enabled synergistic control of pollutant emissions: at a-4 mm channel width, catalytic CO conversion exceeded 95% on average, while NO emissions were suppressed to near-zero levels under fuel-rich conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy further revealed accelerated CH₄ oxidation and NO decomposition over the Y₂O₃-BaO-ZrO₂ surface, suggesting the possible involvement of a Mars-Van-Krevelen-type redox pathway driven by lattice oxygen mobility and surface oxygen vacancy regeneration.