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Frontiers in Energy

ISSN 2095-1701

ISSN 2095-1698(Online)

CN 11-6017/TK

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Front. Energy    2021, Vol. 15 Issue (2) : 431-448    https://doi.org/10.1007/s11708-021-0726-3
RESEARCH ARTICLE
Influence of nozzle height to width ratio on ignition and NOx emission characteristics of semicoke/bituminous coal blends in a 300 kW pulverized coal-fired furnace
Liutao SUN, Yonghong YAN, Rui SUN(), Zhengkang PENG, Chunli XING, Jiangquan WU
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
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Abstract

To improve the ignition behavior and to reduce the high NOx emissions of blended pulverized fuels (PF) of semicoke (SC), large-scale experiments were conducted in a 300 kW fired furnace at various nozzle settings, i.e., ratios (denoted by hf/b) of the height of the rectangular burner nozzle to its width of 1.65, 2.32, and 3.22. The combustion tests indicate that the flame stability, ignition performance, and fuel burnout ratio were significantly improved at a nozzle setting of hf/b = 2.32. The smaller hf/b delayed ignition and caused the flame to concentrate excessively on the axis of the furnace, while the larger hf/b easily caused the deflection of the pulverized coal flame, and a high-temperature flame zone emerged close to the furnace wall. NOx emissions at the outlet of the primary zone decreased from 447 to 354 mg/m3 (O2 = 6%), and the ignition distance decreased from 420 to 246 mm when the hf/b varied from 1.65 to 3.22. Furthermore, the ratio (denoted by SR/SC) of the strong reduction zone area to the combustion reaction zone area was defined experimentally by the CO concentration to evaluate the reduction zone. The SR/SC rose monotonously, but its restraining effects on NOx formation decreased as hf/b increased. The results suggested that in a test furnace, regulating the nozzle hf/b conditions sharply reduces NOx emissions and improves the combustion efficiency of SC blends possessing an appropriate jet rigidity.
Keywords rectangular jet burner      nozzle height to width ratio      ignition characteristics      pyrolyzed semicoke (SC) and bituminous blend      NOx formation     
Corresponding Author(s): Rui SUN   
Online First Date: 05 March 2021    Issue Date: 18 June 2021
 Cite this article:   
Liutao SUN,Yonghong YAN,Rui SUN, et al. Influence of nozzle height to width ratio on ignition and NOx emission characteristics of semicoke/bituminous coal blends in a 300 kW pulverized coal-fired furnace[J]. Front. Energy, 2021, 15(2): 431-448.
 URL:  
https://academic.hep.com.cn/fie/EN/10.1007/s11708-021-0726-3
https://academic.hep.com.cn/fie/EN/Y2021/V15/I2/431
Fig.1  Schematic diagram of PBCS system.
Fig.2  Schematic and photos of the three pulverized coal direct-flow burners and their arrangement.
Case
1 2 3
Primary-air nozzle height of burner/mm 43 51 60
Total primary-air nozzle width of burner without nozzle distance/mm 26.06 21.96 18.66
Height to width ratio= hf/b 1.65 2.32 3.22
Total area of primary-air nozzle of burner/mm2 1120 1120 1120
Rated total thermal power/kW 300 300 300
Rated coal-fired thermal power/kW 250 250 250
Rated gas-fired thermal power/kW 50 50 50
Gas feeding flow rate range/(N·m3·h−1) 2–10 2–10 2–10
Primary air/secondary air temperatures/°C 80/230 80/230 80/230
Maximum furnace operating temperature/°C 1500 1500 1500
Furnace inner diameter/mm 800 800 800
Single section furnace body length 640 640 640
Primary-air mass flow rate/(kg·h−1) 75.36 75.36 75.36
Secondary-air mass flow rate/(kg·h−1) 185.36 185.36 185.36
Primary-air velocity/(m·s−1) 18.37 18.37 18.37
Secondary-air velocity/(m·s−1) 18.88 18.88 18.88
Coal mass flow rate/(kg·h−1) 33 33 33
Excess air coefficient 0.9 0.9 0.9
Unburnt carbon content in fly ash measured at x = 5000 mm/% 9.73 0.74 3.62
Tab.1  Experiment arrangements
Coal type Bituminous coal SC Solid fuel used in experimental trials (at a mass blending ratio of 1:1)
Proximate analysis (as received, wt.%) Mar 6.29 2.13 4.295
Var 29.51 8.73 19.12
Aar 6.01 11.96 8.98
FCar 58.02 77.18 67.6
Ultimate analysis (as received, wt.%) Car 70.67 79.01 74.84
Har 4.35 1.09 2.72
Oar 11.08 4.55 7.82
Nar 1.12 0.93 1.02
Sar 0.31 0.33 0.32
Low heat value/(MJ·kg−1) 27.13 27.42 27.28
Tab.2  Characteristics of the coal used in the experimental trials
Fig.3  Geometry of furnace and burner.
Fig.4  Velocity contour and streamlines at different hf/b settings in y direction (Magnitude velocity unit: m/s).
Fig.5  Profiles of the temperature along the furnace centerline and the calculating method of ignition point position at hf/b = 1.65(□), 2.32(○), and 3.22(△)
Fig.6  flame images at x = 340 mm for different hf/b values.
Fig.7  Profiles of the O2, CO, and HCN concentration along the furnace centerline with different hf/b values.
Fig.8  Profiles of gas temperature and O2, CO, and NO concentration along radial direction at hf/b = 1.65(□), 2.32(○) and 3.22(△).
Fig.9  Profiles of NOx concentration along furnace centerline at different hf/b values.
Fig.10  2D color filled contours of gas temperature and O2, CO, and NOx concentration at hf/b = 2.32.
Fig.11  Average concentrations of gas components and temperature in high reduction zone at different hf/b values (High reduction zone: CO>0.0108).
Fig.12  Ratio of SR to SC for gas components and temperature at different hf/b values.
Fig.13  Gas average temperature and burnout ratios of residual solid sampled at 820 mm and different hf/b values.
PF Pulverized fuels
SC Semicoke
PA Primary air
SA Secondary air
NOx Nitrogen oxides
O2 Oxygen
CO Carbon monoxide
HCN Hydrogen cyanide
BCR Bias concentration ratio
CFD Computational fluid dynamics
DTFs Drop tube furnaces
PBCS Pilot-scale bias combustion simulator
FTIR Fourier transform infrared
NELRECC National Engineering Laboratory for Reducing Emissions from Coal Combustion
HIT Harbin Institute of Technology
x Distance between the measuring point and the burner outlet for the axial direction/mm
r Distance between the measuring point and the burner outlet for the radial direction/mm
hf Burner’s primary-air nozzle height/mm
b Total burner’s primary-air nozzle width without nozzle distance/mm
hf/b Height to width ratio
SR The high reduction zone area/mm
SC The combustion zone area/mm
SR/SC The high reduction zone area to combustion zone area ratio
  
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[1] Yonghong YAN, Liutao SUN, Zhengkang PENG, Hongliang QI, Li LIU, Rui SUN. Effects of pyrolyzed semi-char blend ratio on coal combustion and pollution emission in a 0.35 MW pulverized coal-fired furnace[J]. Front. Energy, 2021, 15(1): 78-90.
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