Development Of Low Emission Combustor Research Articles

International Experience in Developing Low-Emission Combustors for Land-Based, Large Gas-Turbine Units: Mitsubishi Heavy Industries’ Equipment

This is the second paper in a series of publications summarizing the international experience in the development of low-emission combustors (LEC) for land-based, large (above 250 MW) gas-turbine units (GTU). The purpose of this series is to generalize and analyze the approaches used by various manufacturers in designing flowpaths for fuel and air in LECs, managing fuel combustion, and controlling the fuel flow. The efficiency of advanced GTUs can be as high as 43% (with an output of 350–500 MW) while the efficiency of 600–800 MW combined-cycle units with these GTUs can attain 63.5%. These high efficiencies require a compression ratio of 20–24 and a temperature as high as 1600°С at the combustor outlet. Accordingly, the temperature in the combustion zone also rises. All the requirements for the control of harmful emissions from these GTUs are met. All the manufacturers and designers of LECs for modern GTUs encounter similar problems, such as emissions control, combustion instability, and reliable cooling of hot path parts. Methods of their elimination are different and interesting from the standpoint of science and practice. One more essential requirement is that the efficiency and environmental performance indices must be maintained irrespective of the fuel composition or heating value and also in operation at part loads below 40% of rated. This paper deals with Mitsubishi Series M701 GTUs, F, G, or J class, which have gained a good reputation in the power equipment market. A design of a burner for LECs and a control method providing stable low-emission fuel combustion are presented. The advantages and disadvantages of the use of air bypass valves installed in each liner to maintain a nearly constant air to fuel ratio within a wide range of GTU loads are described. Methods for controlling low- and high-frequency combustion instabilities are outlined. Upgrading of the cooling system for the wall of a liner and a transition piece is of great interest. Change over from effusion (or film) cooling to convective steam cooling and convective air cooling has considerably increased the GTU efficiency.

Low-Emission combustion of fuel in aeroderivative gas turbines

The paper is the first of a planned set of papers devoted to the world experience in development of Low Emission combustors (LEC) for industrial Gas Turbines (GT). The purpose of the article is to summarize and analyze the most successful experience of introducing the principles of low-emission combustion of the so-called “poor” (low fuel concentration in air when the excess air ratio is about 1.9–2.1) well mixed fuelair mixtures in the LEC for GTs and ways to reduce the instability of combustion. The consideration examples are the most successful and widely used aero-derivative GT. The GT development meets problems related to the difference in requirements and operation conditions between the aero, industrial, and power production GT. One of the main problems to be solved is the LEC development to mitigate emissions of the harmful products first of all the Nitrogen oxides NOx. The ways to modify or convert the initial combustors to the LEC are shown. This development may follow location of multiburner mixers within the initial axial envelope dimensions or conversion of circular combustor to the can type one. The most interesting are Natural Gas firing GT without water injection into the operating process or Dry Low emission (DLE) combustors. The current GT efficiency requirement may be satisfied at compressor exit pressure above 3 MPa and Turbine Entry temperature (TET) above 1500°C. The paper describes LEC examples based on the concept of preliminary prepared air–fuel mixtures' combustion. Each combustor employs its own fuel supply control concept based on the fuel flow–power output relation. In the case of multiburner combustors, the burners are started subsequently under a specific scheme. The can type combustors have combustion zones gradually ignited following the GT power change. The combustion noise problem experienced in lean mixtures' combustion is also considered, and the problem solutions are described. The GT test results show wide ranges of stable operation at needed levels of NOx and CO emissions. The world experience analysis and generalization and investigation of the further development directions for the high performance GT will assist development of domestic LEC for prospective GTs.

Development of Low Emission Combustor for 110 MWe Gas Turbine

Background: This is a review article. The stages of development and finishing of Low-Emission Combustor (LEC) for GTE- 110 gas turbine with capacity of 110 MW are presented. The main problem referred to in the article is pressure pulsations arising in certain modes of LEC operation. Methods: A detailed account of numerical and experimental research is given for detection of contributing factors on arising of unstable combustion, leading to pressure pulsations with high magnitudes (more than 2% of air pressure at the inlet of LEC). A computational and experimental approach to research of processes in LEC is described. Method of influence upon the border of steady combustion area is shown. Findings: Relationship between position and shape of flame in the volume of flame tube and stability of combustion process is described. Design criterion of stable combustion, allowing comparison of modifications of LEC structures and operation modes is presented. Technical solution developed on the basis of calculation and experimental studies, allowing implementation of stable and low-emission operation of LEC in all design operation modes of Gas Turbine (GT), is presented. The results of tests at pressures up to 450 kPa in the form of diagrams of combustion efficiency and NOx emissions over the entire load range of GT are presented. Improvements: Control method of combustor to achieve these characteristics was developed.

Experimental Investigation of a New Concept of Fuel Prevaporization

The established emission standards for aircraft engines require the development of low emission combustors which incorporate new concepts of fuel prevaporization and pre-mixing systems. At MTU-Muenchen a fuel injection system was designed which greatly suppresses droplet combustion and avoids burning at stoichiometric air fuel ratios. Thereby the large quantities of NO produced at adiabatic peak temperatures are omitted and soot formation as a result of low velocity droplet combustion is avoided. Tests with a combustor incorporating the fuel injection system yielded high combustion efficiencies, improved combustor outlet temperature distributions and low pollutant emissions. Comparing the measured emission indices for CO, NO, and unburned HC with values required for civil aircraft engines showed a promising development potential of achieving the standards.

Pollution control in continuous combustion engines

Brief consideration is given to the formation of smoke, carbon monoxide, unburned hydrocarbons and nitric oxides in continuous combustion systems, and to the manner and extent to which their exhaust concentrations are influenced by changes in engine operating conditions. The main objective, however, is to review the various techniques that have been used or advocated for pollution control. It is shown that the development of low emission combustors is proceeding along two main lines. The simplest and most direct approach is through various minor modifications to established hardware, e.g., by changes in liner geometry and airflow distribution and by the adoption of more sophisticated methods of fuel injection. These modifications may be supplemented, where feasible, by compressor air bleed at low power operation and water injection at high power conditions. The other approach is towards radically new concepts which involve major combustor redesign. Of these the most, promising appear to be variable geometry and staged combustors, and also “prevap/premix” systems in which the fuel is vaporized and thoroughly mixed with all the air required for combustion upstream of the combustion zone. How far the potential of these advanced concepts will be developed must clearly depend on the severity of future legislation on emissions control.