ON THE REDUCTION OF NOX-EMISSION LEVELS BY PERFORMING LOW NOX FLIGHTS

摘要

For aero-engines using hydrocarbon fuels, the major exhaust products that are discharged into the atmosphere are carbon dioxide (CO_2) and water vapor (H_2O). Minor products of combustion may include soot (smoke), carbon monoxide (CO), a variety of unburned hydrocarbons (UHC), nitrogen oxides NO_x (that is, nitric oxide (NO) and nitrogen dioxide (NO_2)) , and sulfur dioxide (SO_2). A favorable feature of the kerosene fuel used for jet aircraft propulsion is that it contains almost no sulfur. Therefore, the emission of SO_2 by aircraft is very low and usually negligible. The modern high bypass ratio turbofan engines employed for the propulsion of transport airplanes have high compressor pressure ratios and therefore high turbine entry temperatures. Due to the resulting high flame temperatures in the combustion chambers, the NO_x - formation rates at takeoff and cruise conditions have been substantially increased during the past decades. When transmitted to or emitted directly into the stratosphere, nitrogen oxides act as catalysts in chemical reactions that contribute to the depletion of the ozone layer [1]. When emitted into the upper troposphere, NO_x may participate in the formation of tropospheric or "bad" ozone. Emissions of NO_x thus affect global warming indirectly through tropospheric ozone formation.At lower altitudes, aircraft emission products may contribute to the occurrence of acid rain and ground level smog. These effects may cause serious environmental problems in the foreseeable future if the world air fleet increases further and new advanced turbofan engines come into operation. Such enginescould have even larger compression ratios and, therefore, may emit increased NO_x emission rates than the present engines. It seems generally accepted that the bulk of the reduction of the harmful impact of aircraft emissions on the atmosphere is the responsibility of the gas turbine combustor designer [2]. Accordingly, at present, considerable effort is being made to lower the NO_x production in the combustion chamber well below the current levels.However, in addition, performing a so-called Low NO_x Flight may deliver a noticeable contribution to the reduction of the NO_x-emissions. Regrettably, when executed with the current aircraft types, this certainly will result in a considerable increase in fuel consumption and direct operating cost. Surely, growth of aviation would have to be severely restricted unless one is able to avoid the risk of any perturbation in climate and/or in health.