Alternate combustion technology on track to reduce greenhouse gas emissions
Most of the world's energy needs are supplied through combustion in one way or another. Conventional combustion processes burn fossil fuels and emit greenhouse gases. Reducing greenhouse gas emissions is a high priority for industry and is driving investigations into alternate fuels and combustion technologies.
Dr Paul Medwell from the Institute for Mineral and Energy Resources is working on an emerging combustion technology with the potential to significantly reduce the pollutant emissions associated with combustion. Moderate or Intense Low oxygen Dilution (MILD) combustion technology uses recirculated heat and exhaust gases to achieve stable combustion at low temperatures. Combustion at lower temperatures can reduce the amount of pollutants produced, particularly nitrogen oxides, and increase thermal efficiency.
MILD combustion has been successfully used in furnaces in the steel industry that are powered by natural gas, but there remains a gap in fundamental research into MILD combustion techniques. Different fuels generally have different combustion characteristics and it is critical to see how these different fuels perform under MILD conditions. This will help determine the suitability of MILD combustion technology for use in other industries and with non-conventional fuels.
Dr Medwell and his colleague Associate Professor Bassam Dally have been investigating the performance of different fuels under MILD combustion conditions, exploring ignition reaction zone structures and flame stabilisation. Combustion of natural gas, ethylene and liquid petroleum gas under MILD conditions was assessed experimentally using a Jet in Hot Coflow burner and modelled using laminar flame calculations.
The team found that the reaction zone structure was similar for all three fuel types when they were mixed with hydrogen - both experimental and modelling data showed similar results. Hydrogen was necessary to stabilise the flame structure and may be added to the fuels to decrease sooting and help increase ignition of low calorific value fuels. Of the parameters measured - hydroxyl radical, formaldehyde and temperature - formaldehyde number density varied the most between the different fuel types, but its basic behaviour was consistent.
Different conventional fuels normally burn very differently. The different fuel types examined in the research showed similar combustion characteristics suggesting that MILD combustion could be readily adapted to different fuel types. Insensitivity to fuel type is a significant advantage for incorporating MILD combustion into industrial systems and thereby reducing GHG emissions from burning fossil fuels.
Dr Medwell's research has shown a high potential for successful use of MILD combustion with non-conventional fuels like biomass and bio-derived fuels (e.g. biogas and bioliquids). Many biofuels, and other waste fuels with the potential to reduce dependence on fossil fuels, are low calorific, making them more difficult to combust. MILD combustion technology has the potential to increase thermal efficiency of low calorific fuels.
Brown coal is another non-conventional fuel type that could be used with MILD combustion. While it is not often used because of its high water content and high pollutant rate compared to other fuel types, using brown coal could be more feasible if it is burnt under MILD conditions.
MILD combustion could be readily utilised in furnaces within the ceramic, glass and chemical industries as well as increasing the industrial use of non-conventional fuel types. Dr Medwell is continuing to work towards a better understanding of fuel performance under MILD conditions. His current research focuses on how flames are sustained under the unique low oxygen conditions and MILD combustion of solid fuels, including biomass.
For more information on the project, you can read about Dr Medwell's research or contact him at:
Dr Paul Medwell
Institute for Mineral and Energy Resources
School of Mechanical Engineering
The University of Adelaide, Adelaide SA 5005
Ph: +61 8 8303 4367
E-mail: paul.medwell@adelaide.edu.au
Photographs of natural gas (NG), ethylene (C2H4) and Liquid Petroleum Gas (LPG) flames, each diluted with hydrogen (1:1 vol/vol) at two coflow O2 levels. Jet Reynolds number of 10,000. Note the different exposure times (all other camera parameters held constant). Horizontal lines indicate measurement locations (35 mm and 125 mm downstream of jet exit plane). Photograph height: 300 mm. Source: Medwell & Dally (2012).
