Moving towards carbon-free fuels

The transport sector is almost exclusively powered by combustion of oil derived fuels leading to the transport sector being responsible for 24% of direct CO2 emissions globally (IEA, 2020). The 2021 IPCC sixth assessment report found that stabilising the climate will require strong, rapid, and sustained reductions in greenhouse gas emissions, and reaching net zero CO2 emissions. In order to achieve this, alternative carbon-free fuels are needed. At the University of Leeds, researchers are studying the properties of ammonia aerosol combustion to provide critical insights to underpin the development of ammonia as a carbon free fuel.

The challenge of ammonia combustion

In the search for renewable and carbon-free fuels, the use of ammonia is considered an attractive solution for engine and gas turbine applications particularly for the shipping industry. When compared to other carbon-free fuels, such as hydrogen, ammonia has significant advantages. Not only is it easy to produce from renewable sources of nitrogen and hydrogen, it is safer to store and transport and has a higher energy content. Ammonia can also be produced, transported and distributed without changing the infrastructure already deployed by industries.

However, for the successful application of ammonia as a fuel, one main challenge related to its combustion needs to be overcome: its low reactivity requires a high ignition energy, and leads to a narrow flammability range and low burning velocity. This complicates the stabilisation of the combustion flame and thus inevitably causes unreliable ignition and unstable combustion.

Understanding ammonia aerosol combustion

The combustion of clouds of fuel aerosol is of practical importance in gas turbines, diesel and spark ignition engines, furnaces and hazardous environments with potential to improve combustion. Experiments conducted at the Bradley Combustion Laboratory at University of Leeds have shown that, contrary to expectations, flame propagation in aerosol clouds, under certain circumstances, is higher (possibly by up to a factor of 3) than that in a fully vaporised homogeneous mixture as is traditionally used in combustion processes. The enhancement of the combustion properties provided by clouds of fuel aerosol have the potential to provide more rapid burning of ammonia in a gas turbine required to make it a viable alternative fuel.

Ammonia aerosol combustion has not yet been extensively studied. Understanding the properties of ammonia aerosol combustion will provide insights to support the development of novel methods to stabilise ammonia flames required to advance ammonia as a carbon-free fuel. A team led by Drs Sven Van Loo and Junfeng Yang, at the University of Leeds, will use numerical techniques and hydrodynamics codes developed at Leeds for astrophysical research to improve understanding of ammonia aerosol combustion processes.

What next?

  • The team are developing a numerical model to describe the multi-phase (liquid droplets, hot gaseous flame), multi-scale (from droplet sizes of 10 μm to flame size of 10 cm), and multiphysics (droplet evaporation and flame propagation) processes of ammonia aerosol combustion.
  • The models will be used to perform and validate a suite of numerical simulations exploring the parameter space for the ammonia aerosol combustion process.
  • Further developments of the model will be defined with project partner Shell Global Solutions (UK) to ensure they provide a realistic representation of combustion in gas turbines.

This project is funded by the STFC Horizons programme: Investigation Solutions for Net Zero.

Sven Van Loo is Lecturer of Astrophysics in the School of Physics and Astronomy at the University of Leeds. He specialises in computational fluid dynamics and has in recent years applied his expertise to industrial applications including carbon-sequestration pipeline transport and combustion processes.
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