The present study investigates the characteristics of Moderate or Intense Low-oxygen Dilution (MILD) oxy-combustion in a laboratory-scale furnace. Experiments using natural gas (NG), liquefied ...petroleum gas (LPG) and ethylene (C2H4) are carried out at a firing rate of 13kW. The furnace temperatures and exhaust emissions are measured for a range of equivalence ratios and external-CO2 dilution rates.
It is observed that MILD combustions occur for the three fuels even when using pure oxygen as oxidant. When diluting oxidant by CO2 at a fixed rate, the MILD combustion can be established as long as the equivalence ratio (Φ) is sufficiently high. The region of MILD combustion is found to be wider with dilution by CO2 than by N2. Notably, also, the operating range of MILD combustion is larger for NG than LPG or C2H4 as fuel.
Moreover, when Φ<1, as Φ is increased, the furnace temperature rises slightly but the NOx emission decreases. This cannot be explained when using the traditional thermal NOx mechanism. Indeed, using various NO mechanism models, our calculations show very low NO emissions resulting from the thermal, prompt and NNH routes but a much higher value from the N2O-intermediate route. Namely, only the latter mechanism plays a crucial role in forming NO. Also important is that the NO reburning appears to reduce NO emissions notably and so should not be ignored in the MILD combustion.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In this work a vertical furnace is used to investigate the MILD (Moderate or Intense Low-oxygen Dilution) combustion characteristics of pulverized Victorian brown coal. This paper reports the effect ...of turbulence on the volatiles’ release and reactions under vitiated co-flow conditions as well as the impact on the pollutants’ formation and emission. Loy-Yang brown coal from the Latrobe Valley, Victoria, Australia, with particle sizes in the range of 53–125µm, is injected into the furnace using CO2 as a carrier gas through an insulated and water-cooled central jet. The bulk jet Reynolds number was varied from Rejet = 5527 to Rejet = 20,000. The furnace walls as well as co-flow temperature and local oxygen concentrations are controlled by a secondary swirling burner using non-premixed natural gas combustion. The co-flow in the furnace was operated with an O2 concentration of 5.9% (db by volume). Detailed measurements of in-furnace temperatures and chemical species are presented and discussed, together with visual observations and CH chemiluminescence (CH*) imaging at the bottom, middle and top parts of the furnace. The CH* signal intensity is found to be significantly lower at the top part of the furnace which is an indication of slow rate of the heterogeneous combustion of char particles. The largest amount of CO concentrations are measured for the highest jet velocity (i.e., Rjet = 20,000) case which implies that with increasing turbulence there is a better mixing and a broad devolatilization zone is formed which produces more CO. The measured NO emission for any case was less than 125 ppmv (db at 3% O2) which provides evidence to the potential benefits of MILD combustion application to Victorian brown coal towards reducing NO emission. Complementary CFD model helped in shedding light on the flow field, turbulence intensity, volatiles’ release rate, combustion of volatile matters, and overall carbon consumption inside the furnace for the three cases. It was found that increasing the jet Reynolds number increases the volatiles release rates and decrease the rate of overall carbon consumption.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This paper reports a combined experimental and numerical investigation on the impact of particle size on the MILD combustion characteristics of pulverised brown coal. In particular, the work ...investigates the volatiles release and reactions from the coal particles with a vitiated co-flow and its impact on the formation and emission of CO, CO2, and NOx. The experimental vertical furnace is 1200mm long with an internal cross-section of 260×260mm2. High volatile brown coal from the Latrobe Valley, Victoria, with particle sizes in the range of 53–125μm and 250–355μm is injected into the furnace using CO2 as a carrier gas through an insulated water-cooled central jet. The complementary numerical results are in good agreement with the experimental measurements. It is found that, for both cases, a stable MILD combustion is established with a similar large recirculation vortex around the centre of the furnace. Devolatilisation starts earlier for the smaller particles' case and is completed at the end of the recirculation vortex, while for the larger particles' case the devolatilisation happened post the recirculation vortex. The difference is related to the particle dispersion within the jet and differences in Stokes number. The particle flow path and difference in residence time had influence on char burn-out and emission levels. The formation/destruction of NO is measured to be subtly varied by the combination of the physical and chemical nature of the MILD combustion characteristics related to both particle sizes. The measured NO emission of the larger particle case is 15% higher than that of smaller particle case. The model calculated about 95.5% of total NO is produced via the fuel-NO route. A strong NO-reburning mechanism is found for both cases, where ≈55% of total NO is reduced for the small particle case and ≈39% of total NO is reduced for the large particle case.
•Stable MILD combustion of pulverised brown coal is achieved.•A semi-uniform temperature distribution is measured inside the furnace.•The CO and NO emissions in the exhaust are higher for the small particles case than the large particles case.•The numerical model predicts 95.5% of total NO is produced via the fuel-NO route.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
This paper reports the impact of surrounding oxygen (O2) concentrations on the burning characteristics of pulverised brown coal under MILD (Moderate or Intense Low-oxygen Dilution) combustion ...conditions. A combined experimental and computational study is conducted to probe the volatiles' release and reactions from the micro sized coal particles and its effect on the production of CO, CO2 and NOx in a vertical furnace. Pulverised high volatile Victorian brown coal was introduced into the furnace utilising CO2 as a carrier gas through a central jet with a constant bulk jet Reynolds number Rejet = 20,000, and two co-flow oxygen concentration of 5.9% and 8.9%. For all the cases investigated, stable MILD combustion was achieved, featuring a uniform temperature distribution within the furnace. It was found that the co-flow O2 concentration significantly influences the CO emission. The measured CO emission for the 5.9% co-flow O2 concentration case is approximately eight times higher than that of the 8.9% co-flow O2 concentration case. In addition, the CFD analysis showed that an increase in the local O2 concentration leads to an increase in the volatiles release and reaction rates. Also, up to 63% more NO is produced for the 5.9% co-flow O2 concentration case in comparison with the 8.9% O2 case through the fuel-NO route.
•Stable MILD combustion is achieved with no visible flame for all cases.•A uniform thermal field is measured inside the furnace.•The volatile release and reaction rates are increased with increasing the co-flow O2 concentration.•The lower O2 concentration case produces 63% more NO than the higher O2 concentration case through the fuel-NO routes.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Sinusoidal modifications to the leading edge of a foil, or tubercles, have been shown to improve aerodynamic performance under certain flow conditions. One of the mechanisms of performance ...enhancement is believed to be the generation of streamwise vortices, which improve the momentum exchange in the boundary layer. This experimental and numerical study investigates the formation and evolution of these streamwise vortices at a low Reynolds number of
$Re=2230$
, providing insight into both the averaged and time-dependent flow patterns. Furthermore, the strength of the vortices is quantified through calculation of the vorticity and circulation, and it is found that the circulation increases in the downstream direction. There is strong agreement between the experimental and numerical observations, and this allows close examination of the flow structure. The results demonstrate that the presence of strong pressure gradients near the leading edge gives rise to a significant surface flux of vorticity in this region. As soon as this vorticity is created, it is stretched, tilted and diffused in a highly three-dimensional manner. These processes lead to the generation of a pair of streamwise vortices between the tubercle peaks. A horseshoe-shaped separation zone is shown to initiate behind a tubercle trough, and this region of separation is bounded by a canopy of boundary-layer vorticity. Along the sides of this shear layer canopy, a continued influx of boundary-layer vorticity occurs, resulting in an increase in circulation of the primary streamwise vortices in the downstream direction. Flow visualisation and particle image velocimetry studies support these observations and demonstrate that the flow characteristics vary with time, particularly near the trailing edge and at a higher angle of attack. Numerical evaluation of the lift and drag coefficients reveals that, for this particular flow regime, the performance of a foil with tubercles is slightly better than that of an unmodified foil.
In this work, the structure of laminar premixed ethylene/air flat flames at low pressure are studied experimentally. The aim of the work is to exploit the spatial expansion of laminar 1D flames and ...advanced laser techniques to better understand the conditions and precursors affecting soot particles inception. Soot volume fraction (
f
v
) profiles were measured using laser-induced incandescence (LII), while spatial distribution of the CH
*
and C
2
*
radicals was measured using spatially resolved emission spectroscopy. Spatially resolved laser-induced fluorescence has been used to record emission from 2–3 rings, 3–4 rings and > 5 rings. The temperature of soot particles (
T
s
) was evaluated through fitting the spectrally resolved soot luminosity, while flame gaseous temperature (
T
g
) was measured using a fine thermocouple. The laminar flow velocity was modeled and used to evaluate the reaction time at each HAB. Taking advantage of the expanded flame structure at low pressure, the profiles of CH
*
, C
2
*
, soot and temperatures, as a function of the height above the burner, were well resolved. It was found that CH
*
and C
2
*
chemiluminescence overlap in space. The thickness of CH
*
layer is larger than that of C
2
*
, and it peaks at slightly different location, CH
*
appears approximately 1 mm before C
2
*
. The distance between the two peaks decreases linearly with the increase in pressure. The lowest value of the initial soot volume fraction (
f
v
) was 0.19 ppb, measured at pressure 27 kPa. It was found that
f
v
scales with the pressure following a power function of the form
f
v
α Pr
n
, where n is 2.15 ± 0.7. It was observed that, in all the flames investigated, the initial soot particles first appear at a common critical inception temperature,
T
inception
, of 1465 ± 66 K. It was found that the
T
inception
is lower than the maximum flame temperature,
T
max
,
by ~ 45° and appeared ~ 1 mm further than the location of
T
max
. Using the Lagrangian quantity
df
v
/dt
and
f
v
, it was possible to reveal the soot growth rate,
k
SG
.
At pressure of 27 kPa, the value of
k
SG
was evaluated as 20 s
−1
.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
•MILD combustion of prevaporised liquid fuels under elevated pressure is examined.•MILD combustion is achieved for all fuels under atmospheric pressure.•Combustion stability is highly dependent on ...fuel type.•Pressure, jet velocity and carrier gas show great impacts on NOx emissions.
This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverse-flow MILD combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and n-heptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with n-heptane being the most unstable due to its fast ignition under all high-pressure conditions studied. Measured CO emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low CO and NOx emission becomes narrower under elevated pressure as NOx emissions emitted from all fuels increased with pressure. The enhanced NOx formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher NOx emissions as pressure increases. The NOx emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The NOx emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing NOx emissions under these conditions, especially at elevated pressures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•MILD combustion characteristics of grape marc are experimentally investigated.•No visible flame has observed for any of the experimental cases.•A semi-uniform thermal field was measured inside the ...furnace.•The highest NO is produced for the 250–355 µm sized particles’ case.•No remarkable NO-reburning process is found for any cases.
In this experimental study, the burning characteristics of grape marc, as a biomass fuel, under MILD (Moderate or Intense Low-oxygen Dilution) combustion conditions are explored. Specifically, the impact of grape marc particle size on flame structure, stability and pollutant emissions, under MILD combustion, is investigated. Three different particle sizes of the grape marc are considered, in the range of 150–250 μm, 250–355 μm and 355–500 μm. The particles are injected through a central insulated jet into a hot and vitiated coflow inside a vertical MILD combustion furnace. All other operating conditions remain the same. CO2 is employed as a carrier gas and the central jet bulk Reynolds number is set to the value of Rejet = 13,040. The vitiated coflow had ~6% oxygen molar fraction and a flow bulk temperature of about Tco-flow = 1330 K. The heat input from the grape marc fuel is kept constant at 10 kW, corresponding to a mass flow rate of 1.63 kg/h. Stable MILD combustion conditions have been successfully achieved for all cases. The temperature gradient along the vertical and the radial direction of the furnace is limited to below 100 K for all the cases. The variation of particle size does not significantly impact on the furnace temperature and exhaust emissions. Albeit, the case of largest particle size exhibits a higher temperature by ~40 K as compared to the other two cases, and the highest NO production is identified for the middle particle size's case showing peak value of approximately 478 ppm at the exhaust at 3% excess O2. No remarkable NO-reburning process is observed for any cases, which points towards the intense volatile combustion of grape marc under MILD combustion conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The role of hydrogen addition on the structure of the Moderate or Intense Low oxygen Dilution (MILD) combustion regime is examined using a combination of experimental techniques and laminar flame ...calculations. Laser diagnostic imaging is used to simultaneously reveal the in situ distribution of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature using the Jet in Hot Coflow (JHC) burner. The fuels considered are natural gas, ethylene, and LPG (each diluted with hydrogen 1:1 by volume). Hydrogen addition to the primary fuel was found necessary to stabilise the flames. Further to the role of hydrogen in the stabilisation of the flames, hydrogen addition also leads to the reaction zone exhibiting similar structure for different primary fuel types. The independence of the reaction zone structure with hydrogen addition suggests that a wide variety of fuels may be usable for achieving MILD combustion.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
