Combustion and Combustors for MGT Applications

An introductory overview of the micro and small-scale devices for the combustion in micro-gas turbines is presented at the beginning of this paper in order to identify and select the main topics to be discussed in the following. The wide variety of combustion systems for this particular application, together with the different strategies and design concepts that are proposes for setting up efficient MGT combustors, suggests the opportunity of outlining the several levels of approach to the studies on this subject. Therefore, the paper proceeds with a general thermo-kinetic analysis of the combustion conditions which are expected for both different fuel supplies and variable MGT load levels. The successive step is the comparison of different combustor concepts for a small-case turbine of the 100 kW range, and the CFD analysis gives indications on the behaviour of the several combustion systems. The same computational tool is finally employed for an exhaustive analysis of a lean-premixed combustor also when operated with low BTU fuels or under part-load conditions. 1. FOREWORDS This lecture aims at a discussion of the main characteristics that the combustion chamber within a microgas turbine should present consistently with the requirements of a small-size energy conversion system. Many authors have been dealing in recent years with the problems related with both micro-combustion and micro-combustors and their studies, combined with the manufacturers’ experience, contributed to the practical applications which can be already be found within actually operating systems. Defining a commonly recognized “size scale” for the micro-combustors is not properly straightforward, since the examples which can be found in technical and scientific literature start from some demonstration devices on a real micro-scale, with power outputs of the order of magnitude of a few Watts, coming up to micro (or mini) gas turbines in the 10 – 300 kW range. The first cases are developed with the main purpose of demonstrating the possibility of rising up the power density of ultra-compact energy systems, while the second ones correspond to really employable MGT based plants addressed either to the single electric power generation or to the combined production of power and electricity. In these latter cases, the reliability of the combustion system is a fundamental requirement to be complied with, and different strategies can be followed by the manufacturers in order to set up efficient MGT combustors, say: • Proceeding with a combustor design expressly accounting for the particular operating conditions of the MGT; • Adapting, in some sense, the well consolidated experience in the field of combustion chambers for industrial gas turbines to this novel application. Tuccillo, R.; Cameretti, M.C. (2005) Combustion and Combustors for MGT Applications. In Micro Gas Turbines (pp. 5-1 – 5-56). Educational Notes RTO-EN-AVT-131, Paper 5. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp.