|MILD Combustion of Pulverised Brown Coal|
|by Manabendra Saha, PhD Candidate, The University of Adelaide; Supervised by Prof. Bassam Dally, Centre for Energy Technology, School of Mechanical Engineering, The University of Adelaide|
|Coal is the cheapest and most abundant fuel resource in many parts of the world. Whilst combustion of pulverised coal is linked to global warming and air pollution (where additional environmental controls are not used); it supplies 30% of the total global energy demands and is forecast to continue to do so until at least 2035.|
Pulverised coal combustion can contaminate the environment through emissions of nitrogen oxides (NOx), sulfur dioxide (SO2), unburned hydrocarbons (UHC), particulate matter (PM) and mercury to the atmosphere. These emissions are associated with a variety of environmental concerns such as the formation of acid rain and photochemical smog in urban air.
In many jurisdictions across the world, environmental agencies mandate the use of additional control measures such as particulate filters, Flue Gas Desulphurisation (FGD), and NOx controls, however implementing such measures adds cost. Thus, there is great interest in the development of advanced coal combustion technologies that alter the combustion conditions and prevent (at source) the generation of pollutant emissions, in particular controlling NOx, UHC, PM and trace element emissions.
In view of this, 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. This novel concept is based on the recirculation of exhaust gas and heat to create a volumetric reaction zone at reduced temperatures. Consequently thermal stress and pollutant emissions are strongly reduced in MILD combustion. MILD combustion differs from conventional combustion because of the absence of any visible or audible flame at optimized conditions. As a result, MILD combustion is often called ’flameless combustion‘ or ’flameless oxidation‘.
MILD combustion technology has been studied extensively for gaseous and liquid fuels, and has been implemented 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.). The use of solid pulverised coal under MILD combustion conditions has received much less attention than that of gaseous fuels, so its burning characteristics are poorly understood.
A preliminary investigation was conducted to investigate the MILD combustion characteristics of pulverised brown coal in a laboratory-scale self-recuperative furnace. Low rank and high volatile Kingston brown coal with particle size in the range of 38–180 μm were injected into the furnace using either CO2 or N2 as a carrier gas.
|Measurements of in-furnace gas concentration of O2, CO and NO, in-furnace temperatures and exhaust gas emissions were measured and analysed.|
MILD conditions were achieved in all cases, and no visible flame was observed. The temperature was almost uniformly distributed across the furnace, with a variation of less than 100°C. It was found that there was a strong NO reburning reaction inside the furnace, because of the strong recirculation of the combustion products. Ash content analysis showed that the extent of carbon burnout was incomplete, which is thought to be due to the relatively short residence times inside a small furnace.
To augment the experimental measurements, computational fluid dynamic (CFD) modelling was used to investigate the effects of coal particle size and inlet air momentum on furnace flow dynamics and global CO emissions. It was found that increasing the air jet momentum broadened the reaction zone and facilitated MILD combustion.
The results of this work was recently published in ‘Energy & Fuels’ (M. Saha, B.B. Dally, P.R. Medwell and E.M. Cleary. Moderate or Intense Low oxygen Dilution (MILD) combustion characteristics of pulverized coal in a self-recuperative furnace (2014). Energy & Fuels 28: 6046-6057.
Following the successful experimental and numerical investigation, further work is under way to better understand the formation and destruction of pollutants and the burning characteristics of pulverised brown coal under MILD combustion mode. In particular the interaction of the volatiles with the vitiated co-flow and its impact on the formation and emission of PM, NOx and other pollutants will be investigated.
To probe these parameters under controlled conditions a vertical furnace (Figure 1 below) with a cross section of 260 mm2 × 260 mm2 has been designed and built. The furnace wall, co-flow temperatures and local oxygen concentrations are controlled by the secondary swirling burner using non-premixed natural gas combustion. Loy Yang brown coal with particle size in the range of 53–125 μm will be injected into the furnace using either CO2 or N2 as a carrier gas through the insulated central jet.
Following the completion of this work, a better understanding of pulverised brown coal combustion in MILD mode will be achieved, and will provide better data for ongoing MILD combustion research. The outcomes of this research will be a step forward to allow industry to have better confidence in utilising the MILD combustion technology.
|Figure 1: Schematic (left) and photograph (right) of the Adelaide MILD Combustion Furnace.|
|Figure 2: Photograph of the bottom, middle, and exit of the furnace when operating under MILD combustion condition with|
Loy Yang brown coal carried by CO2.