Written by M.M.Noor, Feb 2013, USQ, Australia.

Title: The importance of MILD combustion, impact on the industry and global warming

The demand for energy is dramatically increasing due to the growth of the world’s population and substantial economic development in countries such as China and India. Some of the major challenges are to provide efficient energy and limit greenhouse gas (GHG) emissions. History and prediction of world energy unwanted increase of earth temperature [1,2]. Combustion of fossil fuel is projected to fulfill about 80% of this energy needs. Combustion pollution is related to an increase in the earth temperature, unstable weather, increase in ocean levels and ice melting in the North and South Poles.  A more efficient use of fuel with low GHGs emission as well as carbon capture and storage (CCS) might be effective ways to gradually reduce the GHG emissions [3,4]. Several researchers [5,6] reported that CO₂ contributed 77% of the greenhouse gas emissions with combustion accounting for 27%, making it a major contributor to global climate change. To counter this issue, the improvement of combustion efficiency with lower emissions has led researchers to have more interest in new combustion technology and combustion modeling [7,8]. Preheating of the reactants by the hot flue gas is one method to improve the combustion efficiency. By implementing this concept, a new combustion technology called MILD combustion was invented. Currently, MILD combustion has been applied in closed furnaces where the oxygen is diluted and the air-fuel mixture is preheated by internal flue gas circulation.

MILD combustion produces high combustion efficiencies with very low emissions. The re-use or recycling of the waste heat from flue gases will increase the thermal efficiency of MILD combustion by 30%, while also reducing NO_x emissions by 50% [9]. This research will also consider the use of biogas as a combustion fuel. The performance of MILD combustion with biogas compared to natural gas is about the same [10], but because biogas is renewable and CO₂ produced from biogas combustion will be reused by feedstock that produces biogas (closed loop), it is more beneficial to use biogas. Biogas is a type of renewable energy. The production of biofuels is normally based upon locally-available feedstocks including soybean, rapeseed, jatropha seed, palm oil, sunflower, cottonseed, tallow (animal fat) or even waste cooking oil. In Australia and the US, most biofuels are derived from soybeans while in Europe rapeseed is the largest source. Carbon dioxide (CO₂) is currently the primary concern for greenhouse gases, global warming and its consequences [11]. When fossil fuels are combusted, CO₂ will be released as a waste by-product. Biogas produced from biomass uses CO₂ in the photosynthesis stage, hence will reduce the CO₂ in the atmosphere.

This research will concentrate on open furnace MILD combustion using biogas as the fuel because open furnaces are more common in the industry. Generally, the setup for an open furnace is simpler and cheaper than a closed furnace because the latter needs a thick and solid wall. However, open furnaces have additional complexity because of their requirement for preheating [9,12,13]. It is believed that currently there is no reported data about MILD combustion in open furnace applications [13]. In order to achieve MILD combustion, the supplied air must be diluted to reduce the oxygen content and preheat the air-fuel mixture to reach the auto-ignition temperature. The exhaust gas recirculation (EGR) concept is utilized to achieve the dilution of oxygen and the preheating of the mixture by collecting the exhaust gas and mixing it with a reactant. To collect the exhaust gases, the furnace needs to be enclosed to capture the required fraction of exhaust gas. The design and draft for the open furnace were done with the assistance of AutoCAD® and ANSYS Fluent® (CFD). The CFD was used to study and modified the furnace and nozzle design and these processes repeated until the simulations reach the MILD combustion state [14].

 References

1. IEA (International Energy Agency) Organisation for Economic Co-Operation and Development, 2009 World Energy Outlook (WEO), Int. Energy Agency, IEA, Paris
2. Maczulak A. 2010 ‘Renewable Energy, Sources and Methods’, Facts on File Inc., New York, USA
3. IEA (International Energy Agency) 2006 World Energy Outlook (WEO), Int. Energy Agency, IEA, Paris
4. Orr F. 2005 ‘Energy and climate: challenges and solutions’, GCEP. Stanford University
5. IEA/OECD, 2002 CO2 Emissions from Fuel Combustion: 1971–2000, Organisation for Economic Cooperation and Development and Int. Energy Agency, Paris
6. Jonathan, P 2006 Responses to questions on the design elements of a mandatory market-based greenhouse gas regulatory system, World Resources Institute, Washington
7. Merci, B., Naud, B. and Roekaerts, D., 2007 Impact of Turbulent Flow and Mean Mixture Fraction Results on Mixing Model Behaviour in Transported Scalar PDF Simulations of Turbulent Non-premixed Bluff Body Flames Flow, Turbulence and Combustion, 79, p. 41-53.
8. Smith ST and Fox RO 2007 A term-by-terms direct numerical simulation validation study of the multi-environment conditional PDF model for turbulent reacting flows, Phys Fluids, 19, p. 085102.
9. Tsuji H, Gupta A, Hasegawa T, Katsuki M, Kishimoto K, and Morita M 2003 ‘ High-Temperature Air Combustion, From Energy Conservation to Pollution Reduction’, CRC Press, Boca Raton, Florida
10. Colorado, A. F., Herrera, B. A. and Amell, A. A. (2010), Performance of a Flameless Combustion Furnace using Biogas and Natural Gas’, Bioresource Technology 101(7), 2443-2449.
11. Volk, T 2008 CO2 Rising; the World’s Greatest Environmental Challenge, MIT Press, Cambridge, Massachusetts, London, England.
12. Li PF, Mi J, Dally, BB, Craig RA Wang PF 2011a, Premixed Moderate or Intense Low-Oxygen Dilution (MILD) Combustion from a Single Jet Burner in a Lab-Scale Furnace, Energy Fuels, 25, pp. 2782-2793
13. Noor, MM, Wandel AP, and Yusaf T, 2012 MILD Combustion: A Technical Review towards Open Furnace Combustion, 2nd Malaysian PG Conference (MPC), 7-9 Jul, Bond University, Gold Coast, Australia, Paper No.: MPC2012-28, pp. 79-100.
14. Noor, MM, Wandel AP, and Yusaf T, 2012, A Preliminary Study of Control Parameters for Open Furnace MILD Combustion using CFD, 2nd Malaysian Postgraduate Conference (MPC), 7-9 Jul, Bond University, Gold Coast, Australia, Paper No. MPC2012-16, pp. 46-60.

Thanks.
M.M.Noor, UMP, Malaysia.