Nitric oxide reduction by non-thermal plasma and catalysis

Combustion of fossil fuels produces millions of tons of air pollutants such as NOx and SO2. The high cost and operating difficulties of prevailing selective catalytic reduction (SCR) process are driving R&D efforts towards alternative technologies and modifications to the SCR technology. Non-thermal plasma technique has been found to be one of the most promising technologies for NOx removal. In the present study, a non-thermal plasma technique in the form of dielectric barrier discharge (DBD) has been extensively investigated for the removal of NOx. The main variables including electrical parameters, chemical compounds and flow conditions are identified and studied in terms of their effects on NO/NOx conversions. Significant increases in NO/NO x conversions of 90% and 40% are observed in the presence of both O 2 and H2O than in the presence of either O2 or H2O alone. Addition of 1000 ppm ethylene to the NO/O2/CO 2/N2 mixture almost promotes 100% NO oxidation to NO 2. The chemistry of plasma reactions is discussed. The DBD system is found to be effective for NO oxidization into NO2 and HNO 3. Kinetics studies have also been made for NO-O2 reaction under plasma conditions. A rate equation has been proposed, -d[NO]/dt = kp [NO]1/2[O2] 1/2 with kp = 0.0143exp(-1865/E d) showing that a reaction can be initiated at much lower activation energy under plasma conditions. A hybrid plasma-catalyst (P-C) system has been developed to achieve the synergy. gamma-Al2O3 and laboratory-prepared tungsten catalysts have been used in the hybrid P-C experiments. The improvement in NOx removal by the P-C system is about 15% compared to by the SCR alone when 1000--3000 ppm of methane or ethylene is added to the inlet gas stream. There is no formation of N2O (greenhouse gas) in the P-C system when inlet gas contains moisture or ethylene. DBD reactor design parameters have also been investigated in terms of SO2 oxidation to SO3 with respect to reactor geometry, dielectric material and thickness. The DBD technique has the great potential to replace the prevailing combined SCR and flue gas disulfurization (FGD) processes both technically and economically. The P-C system has the potential to be used for the removal of NOx from diesel engine exhausts