Fuel composition and combustors

The early promise of CFD-based design approaches was that the predictable nature of the calculations would enable studies of multiple gas turbine combustor designs prior to prototyping. As part of the development flow, simulation could predict important system attributes such as stability and emissions performance and reduce the need for costly experimental testing. Results vary widely, however, among organizations.

Further, engine manufacturers are spending more on computer hardware and software licenses to feed larger, more complex combustion CFD simulations with little tangible evidence of useful, predictive results for key design criteria such as CO and unburned hydrocarbons (UHC) at low load with non-traditional fuels and blends. This has given rise to greater use of experimental testing and higher demand for overbooked test facilities.

These software tools worked well enough when the objective was to simply drive the combustor’s flame temperatures down in an effort to reduce NOx. Now that ultra-low NOx combustion equipment dominates the market, the industry is facing a completely new set of challenges such as Lean Blow Off (LBO), ignition, flashback, fuel flexibility and emissions. That necessitates a better understanding of how fuel models are used in combustion simulation.

Combustion is typically envisioned as a single step where fuel and oxygen burn to produce carbon dioxide and water with some pollutants. The reality is that when fuel burns, it undergoes a transformation consisting of thousands of chemical reactions with shortly lived radical species. These dictate combustion performance such as ignition, extinction and pollutant formation. In addition, advanced combustion devices have different conditions than legacy systems due to increased flue gas recirculation (FGR), lean premixing and other combustion staging techniques.

As an example, the GRI-Mech model was developed and validated using 1990’s era boiler technology and works quite well for that application. However, it has proved deficient in predicting NOx emission from newer gas turbines and burner systems. Some have chosen to use combustion models in conjunction with look-up tables or progress variables that simplify the complex chemistry. This allows the use of slightly more detailed fuel models, but the simplifications that occur result in poor prediction of emissions and combustion stability. These problems are typically addressed by using a simple kinetic post-processor that  predicts well when the majority of the NOx is thermally generated.

However, most ultra low NOx combustors have already had thermal NOx substantially reduced leaving other pathways such as prompt and third-body as substantial contributors to the total NOx emissions. Ignition delay is a critical parameter that is required in order to determine the location of the flame and combustion stability.

The ignition delay predicted shows its greatest error in conditions that are present in ultra low NOx combustion. GRI-Mech proves successful only at 8 atm for higher temperatures that are typical only of older combustion systems. The MFC chemistry model predicts NOx over this pressure range showing how recent advances in combustion kinetics can substantially improve emissions predictions compared to older models. Flame speed is another key measure of the quality of a fuel model.

Accurate fuel chemistry can be applied with Equivalent Reactor Networks (ERN) to combustion problems. Emissions predictions have been achieved for over 30 years through the use of idealized chemical reactor modeling using chemistry simulation software packages such as Chemkin. But these packages do not take into account effects of the complex 3-D flow field and geometry.

Building ERNs to represent the local chemical reactions in appropriate regions of the geometry is a proven method of incorporating the effects of both the flow and the kinetics in a single simulation. It is critical that the ERN be a true representation of the actual combustor flow field in order for the simulation to be useful. Once the ERN is created through a careful devolution of the combustor flow field, a detailed chemistry model can be used to provide an understanding of chemical behavior and performance. Using a CFD case as the basis for ERN creation dramatically improves the quality of the results. 

(More in Turbomachinery Handbook 2011)