Fuel and operational flexibility in micro Gas Turbine combustors for sustainable energy production

Published on 16/11/2023

 

Finding means to fight climate change is a major target of today's scientific research. To this aim, it is essential to limit greenhouse gas emissions, for instance by replacing fossil fuels with alternatives such as biogas or hydrogen, or with alternative  advanced gas turbine cycles for heat and power generation. Maintaining complete and stable combustion in such unconventional conditions is not straightforward. The research at the Thermal Engineering and Combustion Unit of the University of Mons focuses on sustainable energy production for buildings and industry. In the words of Alessio Pappa (researcher at the University of Mons):

 

Alessio Pappa: "The pollution and environmental problems induced by combustion have to be considered one of our society's biggest challenges if we do not want it to become the legacy of future generations. Therefore, the accurate control of turbulent flames in combustors is mandatory to answer the needs and challenges imposed by the advanced gas turbine cycles, and new fuels, such as the so-called green-hydrogen"

 

Alessio Pappa, is a freshly graduated PhD at the Thermal Engineering and Combustion Unit. In his thesis, he has theoretically investigated turbulent flows in combustors of gas turbines using HPC.

 

Modelling combustion with CFD

 

In a combustor, different fluids are present and their turbulent behavior needs to be accounted for in numerical simulations.

 

Alessio Pappa: "Up to recent times, Computational Fluid Dynamics (CFD) has been efficiently used in aerodynamic design improvement and in turbulent combustion modelling. CFD then appears as evidence to study and improve actual combustors."

 

In particular, to accurately model turbulences in combustion processes, Large Eddy Simulations (LES) are generally used. In LES, the smallest length scales are ignored by performing spatial and temporal averaging, which ensures a limited computational cost while maintaining a reasonably good accuracy. However, the involved systems are so complex that Tier-1 supercomputers are needed to perform the LES.

 

Alessio Pappa: "The objective of my research is to assess and analyze, using LES, the impact of various fuel and fuel blends, and unconventional diluted conditions such as water and exhausted gas, on the combustion stability, performance, and emissions, especially for micro Gas Turbine (mGT) combustors."

 

More than a fluid dynamic problem

 

Besides the general geometric complexity of real combustors, a wide range of coupled problems are involved in turbulent flames.

 

Alessio Pappa: "There are fluid mechanical properties describing the mixing and all transfer mechanisms occurring in turbulent flames, such as heat transfer, molecular diffusion, convection, and turbulent transport. A precise knowledge of the chemistry, using detailed chemical reaction schemes, is also required to predict ignition, stabilization, fuel consumption rate, or emissions."

 

If it already seems complicated enough, that's not all. Among the other modelling and numerical difficulties, two- or three-phase combustion may also be encountered whether it is the combustion of liquid or solid fuels; or real gas effects due to critical conditions limiting the use of the perfect gas law and usual thermodynamical properties.

 

Alessio Pappa: "All the aspects of turbulent combustion modelling are very broad. In this framework, LES offer a balanced solution to better assess the combustion behaviour under the studied specific conditions."

 

 

Micro gas turbines, the future of power generation?

 

Decentralized systems in combination with small-scale Combined Heat and Power (CHP) production units, like micro gas turbines (mGTs), and performing Power-to-Fuel to store the excess of electricity coming from renewable energy are two important studied paths for the future energy production sector. Both trends require combustion flexibility in terms of fuel utilization and operation. Considering these statements, mGTs offer advantages related to their easy and high adaptability and flexibility and can fulfil the requirements of the future power generation market.

 

Alessio Pappa: "The adaptability of the mGT system allows to perform cycle humidification, which enables a separated heat and power production control, increasing the efficiency. It also allows to perform exhausted gas recirculation (EGR) to reduce the operating cost of carbon capture systems."

 

The path to more flexible combustors

 

Nevertheless, due to a limited operating range, the mGTs combustor is the main limiting component to advanced cycle implementation. In addition to the numerical development, Alessio’s research focuses on two other axes: fuel flexibility and operational flexibility. Fuel flexibility means that the mGT combustor must be able to work with different fuels such as hydrogen produced from renewable energies using Power-to-Fuel. Operational flexibility means that the combustor must work under diluted conditions (water or CO2) coming from different advanced cycle modifications such as humidification or EGR. Therefore, accurate data assessing the impact of such diluted conditions and/or different containing fuels on the combustion performance, stability, and emissions for actual mGT combustor geometries and operating conditions are mandatory to address this need for flexibility.

 

These simulations performed using LES are really cumbersome, especially in unconventional diluted conditions such as humidified combustion or performing EGR. HPC infrastructures are thus needed to perform such computationally high-cost simulations.

 

Alessio Pappa: "A typical simulation of the reacting flow in a burner of 300kWth requires usually 25 days using 800 cores on a Tier-1 supercomputer."