OCR Text |
Show Because of worldwide environmental concerns about air pollution, numerous nations are switching to natural gas, especially as a fuel for stationary combustion devices. Compared to coal and oil, natural gas burns cleaner and produces less soot, less NOx, and more energy per unit CO2 emitted. However, after mobile sources, gas-fired systems are the largest emitters of NOx and major emitters of CO. As a result, designers are faced with the challenge of developing burners with high combustion efficiency and ultra-low NOx emissions. The next generation of ultra-low NOx combustion systems are projected to operate extremely lean coupled with rapid mixing, using active control to maintain stability. However, such operating conditions may potentially exacerbate VOC emissions, even while maintaining high combustion efficiency. Phenomena responsible for the formation of VOCs include poor fuel-air mixing, insufficient reaction time, quenching, and excessively lean or rich burn [1-4]. From a chemical kinetics point of view, a methane molecule, the main component of natural gas, interacts with radicals present in the reaction zone and can follow, as shown in Fig. one of two kinetic pathways to VOC formation: oxidation or pyrolysis. Oxidation, favoured in the presence of large amounts of excess air, results in the production of aldehydes, such as formaldehyde. Pyrolysis yields alkenes and alkynes (e.g., ethene and acetylene) [1], followed by the possible formation of aromatic compounds (e.g., benzene, toluene) through the recombination and cyclization of acetylene with other intermediate hydrocarbons radicals. Under very rich conditions, aromatics can further recombine and agglomerate to form polycyclic aromatic hydrocarbons (PAHs), soot, and even dioxins in the presence of chlorine and chlorinated compounds [5]. The fate of each fuel molecule entering the reaction zone depends on local flow field dynamics, temperature, pressure, fuel-air equivalence ratio, and the concentration of radicals. 3 |