Methods for Reducing Emission of Harmful Substances in the Combustion Chambers of GTE and GTP
Authors: Komarov E. M. 
Published: August 2018

A combustion chamber, as one of the crucial GTE components, plays a significant role in ensuring its environmental characteristics. Therefore, understanding the mechanisms of forming harmful substances (pollutants) and a possibility to predict their emission values, when changing the engine operation parameters and the external conditions, are some of the key issues to ensure ICAO (International Civil Aviation Organization) standards. The solution of these issues allows us to estimate the emission characteristics at the stage of engine design and to develop effective methods for preventing the formation of air pollutants, as well as to increase the efficiency of burning fuels. Since the first limitation introduced by the Committee on Aviation Environmental Protection (CAEP / 1) in 1986 there were several amendments. The (CAEP / 8) standard, which has come into force since January 1, 2014, is already being ready to be replaced by more stringent requirements, i.e. reducing emissions of nitrogen oxides (NOx) by 40% by 2020 (as compared to the (CAEP / 2). As to other pollutants (CO, HC, SN), the trend is similar.

Main difficulties in creating combustion chambers with low-emission pollutants arise from the fact that reducing CO and NOx requires mutually opposite measures. A rational combustion chamber design should represent some trade-off between the requirements arising from the task of reducing emissions of these two groups of polluting components. This can be achieved through improving operation of the primary, burnout, and mixing zones, rationally chosen volume of the flame tube (FT), and residence time in the combustion chamber.

To have a clearer idea of possible ways to reduce pollutant emission of the GTE combustion chamber, it is necessary to take into account the basic mechanisms of their formation.

The main methods of reducing CO emission are based on the physical-and-chemical patterns of its formation:

Supporting the mixture composition in the combustion zone to be closer to α = 1.1 ... 1.3;

Increasing the combustion zone volume and the residence time in it.

The above methods of reducing CO emissions are difficult to implement in low-emission combustion chambers because their using leads to the sharp increase of NOх formation. It is found that only in a very narrow temperature range (flame temperature Тпл = 1650 ... 1900 K) desirable levels of NOх and CO emissions can be simultaneously achieved.

To reduce the level of NOх emission, are used the following approaches:

-        liquid fuel combustion implemented at a small length of FT with a residence time in the high temperature zone (over 1920 K) 5 ... 6 milliseconds followed by intensive quench in the mixing zone, that is, the principle of "quick burn and quick quench» is used;

-        fuel combustion at the temperature of 1750 ± 50 K (i.e. below 1920 K), with an outlet temperature pattern formed through the air feed in the mixing zone or

-        from the zone of a combustion chamber flame tube head with no quench of product of combustion.

The analytical results of a total scope of developments in reducing pollutant emissions allow us to distinguish the following standard fuel combustion technologies in GTE combustion chambers, which meet the available environmental requirements:

1)      use of burning the lean pre-mixed fuel in "dry" combustion chambers (This technology process uses the following schemes: RQL (Rich-Quench-Lean) – rich mixture combustion, followed by rapid air blending and lean mixture afterburning; LPP (Lean Premixed Pre-vaporized) - combustion of a lean premixed and vaporized mixture; LDI (Lean Direct-Injection) - combustion with lean mixture injection directly into the combustion zone;

2)      catalytic combustion of a fuel-air mixture;

3)      use of "wet" combustion chambers with diffusion flame and water injection (steam);

4)      additional use of catalytic cleaning of GTP outlet gases.

At present, natural gas combustion chambers with emission of NOx and CO <10ppm are under design. This is almost the lowest achievable level for the operating conditions under consideration. In designing such combustion chambers a main task is to develop and improve methods that allow calculating the combustion kinetics of a gas mixture, improving the software systems for calculating and obtaining reliable data on emission of harmful substances, and also to develop experimental methods for creating and full-scale engineering of the low-emission combustion chambers for stationary units and advanced aircraft engines. The presented methods for reducing emission of harmful substances, namely improving techniques to feed fuel, zone arrangement of combustion, use of catalysts in the combustion chamber and at the outlet of the plant, when used, should result not only to reducing emissions, but also to improving the other important combustion chamber characteristics, especially extension of steady combustion limits. Studies to obtain ultra-low emission levels, based on the burning concept of the lean homogeneous mixture in the combustion chamber, are at an early stage. It is necessary to solve a number of important problems, such as a problem of «lean» flameout, of flash back, and also ensuring a sufficient evaporation of fuel and its mixing with air.