|Overview of Victorian brown coal under MILD combustion conditions|
|By Manabendra Saha, (PhD Candidate), Professor Bassam Dally, Centre for Energy Technology, School of Mechanical Engineering, The University of Adelaide|
|Moderate or Intense Low oxygen Dilution (MILD) combustion has been identified as an innovative approach that offers ultra-low pollutant emissions, high thermal efficiency, enhanced combustion stability, thermal field uniformity, and broad fuel flexibility. MILD combustion differs from conventional combustion because of the absence of any visible or audible flame at optimised conditions. As a result, MILD combustion is often called ’flameless combustion’ or ’flameless oxidation‘.|
|Figure 1 illustrates the operating principles of MILD combustion: low oxygen concentration in the reaction region while the local temperature of the combustible mixture is greater than that of the self-ignition temperature (Tsi) of the reactants. The high temperature of reactants and low oxygen concentration alter the reaction zone to be classified as “volumetric”, leading to lower and more homogeneous temperatures. These characteristics reduce the generation of pollutant emissions, in particular controlling NOx, unburned hydrocarbons, particulate matter and trace element emissions.|
|Figure 1: Operation principle of MILD combustion (J. Wünning, U.S. Patent 5154599, 1992)|
|A knowledge base on MILD combustion has been developed progressively for gaseous and liquid fuels. The International Flame Research Foundation (IFRF) conducted a set of experiments using a semi-industrial 580kW furnace operating on natural gas and reported the potential of MILD combustion. Moreover, MILD combustion technology has been successfully implemented in various industrial sectors for gaseous fuels in various industrial sectors (e.g. beam furnace at Degerfors in Sweden; annealing furnace for steel industry at Terni in Italy; MILD combustion reformer for the generation of hydrogen fuel at Munich airport in Germany; Les Dunes plant by Ascometal in France; rotary hearth furnace in US etc.). |
Despite a successful application of MILD combustion technology for gaseous fuel, this technology is still in its infancy for solid fuels, especially for pulverised brown coal. Thus far, a few studies have been conducted on MILD combustion of pulverised fuels, finding that this technology has potential for increased combustion efficiency with a substantial NOx reduction. BCIA has provided financial support for my PhD project, to deepen understanding on the applicability of MILD combustion for the more efficient use of Victorian brown coal.
The main focus of my research was to investigate the burning characteristics of pulverised Victorian brown coal under MILD combustion conditions. In particular, to investigate the influence of jet inlet velocities of a stream of CO₂ and brown coal (at room
|temperature) on both the flame stability and the formation and destruction of pollutants. Moreover, the project investigated the impact of turbulence on the devolatilisation of brown coal and the reaction of volatile species under MILD combustion conditions.|
A new vertical co-flow furnace was designed and built for the experimental part of this project, as shown in Figure 2 (left). The furnace contained an insulated and water-cooled central jet surrounded by a hot and diluted co-flow. Loy Yang brown coal with two particle size distributions (i.e. 53-125 μm and 250-355 μm) was injected into the furnace
|using CO₂ as a carrier gas. Flameless combustion was successfully achieved with brown coal, as shown in Figure 2 (below right).|
|Figure 2: Schematic (left) of the Adelaide MILD combustion furnace and photographs of the inside of the furnace; photographs (right) of flameless combustion in the (A) top, (B) middle, and (C) bottom part of the furnace|
|In-furnace temperatures and chemical species were measured (as shown in Figure 3), together CH chemiluminescence (CH*) imaging at the bottom, middle and top parts of the furnace. The CH* signal intensity was found to be significantly lower at the top part of the furnace, which is an indication of the slow rate of heterogeneous combustion of char particles. The highest CO concentrations were measured for the highest jet velocity, suggesting that with increasing turbulence there is a better mixing and formation of a broad devolatilisation zone which produces more CO. Under all conditions, the measured NO emission was less than 125ppmv, which is about half the level reported for power stations in the Latrobe Valley. |
|Figure 3: Measured in-furnace temperature distribution (left) and temporal alterations of measured O2, CO, NO and CO₂ in the exhaust when operating under MILD combustion condition with Loy Yang brown coal carried by CO₂|
|In addition to the comprehensive experimental investigations, a computational fluid dynamics (CFD) model was developed to better understand the flow field, turbulence intensity, volatiles release rate, combustion of volatile matters, and overall carbon consumption inside the furnace. The model indicated that increasing the turbulence of the jet increases the volatiles release rate. It was found that, for all cases, stable MILD combustion is established with a similar large recirculation vortex around the centre of the furnace. Devolatilisation starts earlier with smaller particles and was completed by the end of the recirculation vortex, while the devolatilisation of larger particles occurred after the recirculation vortex. The difference was found to be related to the particle dispersion within the jet and differences in Stokes number. Importantly, no evidence for soot formation was found under any set of operating conditions.|
This study has provided valuable systematic data that has helped to create a fundamental understanding of the MILD combustion of Victorian brown coal. My project has established that MILD combustion of Victorian brown coal is feasible, and has resulted in a CFD modelling tool that can help design the next phase of research. This represents a significant step toward the development of a cleaner, more efficient way to utilise Victorian brown coal for power generation.