Combustion Characteristics of MCV/LCV Fuels - A Numerical Chemical Kinetics Study at Gas Turbine Conditions

Sammanfattning: The purpose of this Master’s Thesis is to develop a model for numerical simulations in order to kinetically characterize different Medium Calorific- Low Calorific Value (MCV-LCV) fuel gas mixtures. Adding fuel components with a very low heating value or inert species affect the stability of the standard gas turbine combustor design. Siemens Industrial Turbomachinery (SIT) believes that increasing the fuel flexibility of their current gas turbine combustors is important in maintaining a leading position on the industrial power generation market. The numerical model development has included a test of chemical reaction mechanisms suitable for modelling syngas co-fired with natural gas. Predictions of laminar flame speed and ignition delay time for various syngas fuel mixtures at different pressures and initial temperatures have been compared to experimental data. Two different reactor models suitable for ignition delay time calculations have also been evaluated. ChemKin is the primary software used in this work but a performance test of the open-source chemical kinetics code package Cantera has also been conducted for comparisons. An investigation of the global chemical time scale, which is a fuel mixture characteristic, was performed to find a possible correlation with experimental flameout/flameback data from rig tests with a SIT burner. A correlation between the global chemical time and the limit for flameout was found for nitrogen content up to 35% by volume in the natural gas fuel for the AEV LB000 reference burner. For very high inert content in the natural gas fuel additional fluid dynamic effects becomes increasingly important, hence further confirms that the ultimate flameout limit is governed by the flow and the specific burner. For an arbitrary burner, auto ignition in the burner mixing tube might be relevant for inducing a possible flameback. A general investigation of the ignition delay time scale has therefore been included. The risk of flameback/flashback is known to increase when adding highly diffusive species to the fuel. It is clear that auto ignition may also be a relevant mechanism in inducing/supporting a flameback, given a sufficiently high temperature for ignition. For the LB000 burner standard operating conditions, ignition delay is irrelevant due to low air preheat temperature. Ignition delay time simulations are preferably performed using a homogeneous constant-pressure reactor, determining ignition by the maximum temperature gradient. Further development of chemical mechanisms suitable for modelling laminar flame speed and ignition delay of syngas-natural gas fuel mixtures at gas turbine operating conditions is required. The laminar flame speed predictions of ChemKin however proved quite accurate for lean mixtures at low preheat temperatures. The open-source software Cantera has proven to be a flexible code consistently over-predicting the laminar flame speed compared to ChemKin. It is also less user-friendly due to the lack of code documentation and technical support.