G. Lu and Md M. HossainSchool of Engineering and Digital Arts, University of Kent
Q. Gu, T. Chen, F. Sher, and H. LiuFaculty of Engineering, University of Nottingham
School of Engineering and Digital Arts
Experimental investigation of oxy-combustion
behaviour of single biomass pellets using high-
speed imaging and colour processing techniques
12th ECCRIA, Cardiff, 4th-7th Sept 2018
Outline
• Background
• Experimental set-up
• Test conditions
• Measurement
• Results and discussions
• Concluding remarks
2
• Biomass fuels have widely been accepted as renewable energy in new and retrofitted power/thermal plants (pulverised, fluidised bed, or grate chain boilers).
• Biomass fuels, however, can vary widely in properties, composition and structure, leading to drastically different 'fuel performance', particularly under oxy-combustion conditions.
• Limited work has been undertaken for the fundamental understanding of the combustion behaviours of biomass fuel under oxy-conditions.
• A combination of high-speed and spectroscopic imaging, and image processing techniques is employed to investigate the combustion behaviours of single biomass pellets in a V-DTF (Visual Drop Tube Furnace) under both air and oxy conditions.
Background
3
Experimental set-up
4
EMCCD
camera
High-speed
camera
V-DTF
Quarts
work tube
(Fuel pass) Cooling water
Air/O2/CO2
inlet
Cooling water
Flue gas
outlet
SiC Heating
Elements Viewing
window
Supporting-
metal tube
Thermocouples
(N1, N2, N3)
N1
N3
N2
Biomass
pellet
• V-DTF- an electrically heated drop tube furnace equipped with a quartz tube@50 mm inner dia and 1400mm long, capable of maintaining gas temperature up to 1050 °C.
5
• Key features of imaging systems
• A high-speed camera (IDS UI-3130CP Rev. 2)- acquiring videos of
burning biomass pellets.
• An EMCCD camera (Andor iXonEM+ 897)- acquiring videos of
spectral intensities of alkali metals (Na and K) during the biomass
pellets combustion.
EMCCD
camera
High-speed
cameraEMCCD cameraSpectral range: UV to near IR
Multiplication factor: 1000
Cooling temp: −85˚C
Dark current: <0.001 e−/pix/s
Image resolution: 512 × 512 pixels
High-speed cameraSensor type: CMOS, RGB
Frame rate: 575 fps@800(H)x600(V),
up to 900fps with a reduced image resolution
Resolution: 0.48 MPix
Interface: USB 3
Experimental set-up
Fuel properties and test conditions
6
• Five typical biomass pellets
Proximate analysis (wt%, as received)
Ultimate analysis (averaged by wt%)Miscanthus Peanut Strew
Torrefied wood Wood
Biomass Moisture AshVolatile
matter
Fixed
carbon
Miscanthus 4.42 3.67 75.91 16.00
Peanut 7.73 2.78 68.50 20.99
Straw 4.00 7.57 73.96 14.47
Twood 6.40 1.97 72.90 18.73
Wood 7.32 0.35 76.96 15.37
Proximateanalysis(wt%,asreceived)
Biomass C H N
Miscanthus 45.55 6.13 0.66
Peanut 46.69 6.42 1.38
Straw 44.03 5.93 0.67
Twood 49.32 6.04 0.51
Wood 46.68 6.44 0.24
Elementanalysesofbiomasspellets(Averagebywt%)
Note: only results from Miscanthus are presented.
7
• The pellets of tested biomass burnt under the following conditions
for pre-set temperatures of 800˚C and 900˚C which replicate the
working temperatures of a typical biomass-fired fluidised bed boiler
(fixed bed).
• The size and mass of all the pellets were measured before the
tests. Their volume and density were then computed and used to
normalise the results for fair comparisons.
Fuel properties and test conditions
Condition O2 (l/min) CO2 (l/m)
Air (l/min) 10
O2@21% 2.1 7.9
O2@25% 2.5 7.5
O2@30% 3.0 7.0
Measurement
8
• Size and Shape of flame
• Ignition time
• Burning velocity/rate
• Temperature and its distribution
• Spectral intensity of radicals
(Alkali metals such as Na and K)
• Volatile combustion • Char combustion
• Burning velocity/rate
• Temperature and its distribution
Biomass
particleWater
evaporation
Pyrolysis
(devolatilisation)
Volatile
combustion
Char burning
Ash
Char
combustion
Volatiles burning
Pre-heating and devolatilisation
Phases of biomass combustion
9
• For each test condition
• Three samples of each biomass fuel were burnt.
• Videos of the complete combustion process of each sample were
recorded.
• Videos of alkali metals (Na and K) emissions were also recorded by
placing band-pass filters at the front of the EMCCD camera.
• Temperatures (flame, surface and inner) of the burning pellet were
also measured concurrently using the thermocouples.
• Videos are processed and the characteristic parameters of the
burning pellet were defined and computed for different
combustion phases.
• The combustion behaviours of the burning pellet in relation to
the test condition are then quantified.
Measurement
10
Results and discussions
Miscanthus,
O2@21%,
CO2@79%
800°C
11
Results and discussions
• Example flame/char images of Miscanthus pellets
800⁰C 900⁰C
Air O2@21% O2@25% O2@30%
Volatile
flame
Burning
char (enhanced
images)
Air O2@21% O2@25% O2@30%
12
0
100
200300
400500
600
700800
900
10001100
1200
38:52.8 39:36.0 40:19.2 41:02.4 41:45.6 42:28.8 43:12.0 43:55.2
Temp(°C)
Time
W25
N1 N2 N3
O2@30%
N3
N2
Thermocouples
(N1, N2, N3)
N1
0100200300400500600700800900
100011001200
16:48.0 17:31.2 18:14.4 18:57.6 19:40.8 20:24.0 21:07.2 21:50.4
Temp(°C)
Time
M24
N1 N2 N3
O2@25%
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
17:46 18:29 19:12 19:55 20:38 21:22 22:05 22:48 23:31
Temp(∘C)
Time(mm:ss)
M20
N1 N2 N3
O2@21%
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
00:57.6 02:24.0 03:50.4 05:16.8 06:43.2
Temp(°C)
Time
M1
N1 N2 N3
Air
Time (mm:ss) Time (mm:ss)
Time (mm:ss) Time (mm:ss)
Ignition point
Ignition point
Ignition point
Ignition point
Results and discussions
• Temperature of a burning Miscanthus pellet under air and oxy combustion for the pre-set furnace temperature of 800 °C
13
• Temperature of a burning Miscanthus pellet under air and oxy combustion for the pre-set furnace temperature of 900 °C
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
08:09.6 08:52.8 09:36.0 10:19.2 11:02.4 11:45.6 12:28.8
Temp(°C)
Time
M7
N1 N2 N3
0100200300400500600700800900
100011001200
09:50.4 10:33.6 11:16.8 12:00.0 12:43.2
Temp(°C)
Time
M10
N1 N2 N3
O2@21%
O2@25%
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
55:55.2 56:38.4 57:21.6 58:04.8 58:48.0 59:31.2 00:14.4 00:57.6
Temp(°C)
Time
M4
N1 N2 N3
Air
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
12:00.0 12:43.2 13:26.4 14:09.6 14:52.8 15:36.0
Temp(°C)
Time
M14
N1 N2 N3
O2@30%
Time (mm:ss) Time (mm:ss)
Time (mm:ss) Time (mm:ss)
Ignition point
Ignition point
Ignition pointIgnition point
Results and discussions
N3
N2
Thermocouples
(N1, N2, N3)
N1
• Samples and burning times
(Note: only Miscanthus is included)
Results and discussions
Condition SampleVolumn
(cm3)
Density
(g/cm3)
Total
Combusiton
Volentile
Combustion
Char
Combustion
M1 1.27 0.52 242.0 240.0 38.0 202.0
M2 1.37 0.48 226.0 224.0 41.0 183.0
M3 1.45 0.52 219.0 220.0 38.0 182.0
ave 1.36 0.51 229.0 228.0 39.0 189.0
M20 1.61 0.52 289.0 265.0 38.0 227.0
M21 1.55 0.48 338.0 275.0 37.0 238.0
M22 1.46 0.53 308.0 285.0 40.0 245.0
ave 1.54 0.51 311.7 275.0 38.3 236.7
M23 1.25 0.52 199.0 200.0 38.0 162.0
M24 1.35 0.54 214.0 209.0 37.0 172.0
M25 1.28 0.52 211.0 199.0 36.0 163.0
ave 1.29 0.53 208.0 202.7 37.0 165.7
M26 1.44 0.49 173.0 173.0 37.0 136.0
M27 1.18 0.51 168.0 160.0 37.0 123.0
M28 1.15 0.53 168.0 161.0 36.0 125.0
ave 1.26 0.51 169.7 164.7 36.7 128.0
Time(sec)Total
burningtime
takenintest
(sec)
800degC
Air
O2@21%
O2@25%
O2@30%
Char SampleVolumn
(cm3)
Density
(g/cm3)
Total
Combusiton
Volentile
Combustion
Char
Combustion
M4 1.37 0.50 207.0 210.0 39.0 171.0
M5 1.45 0.55 222.0 218.0 41.0 177.0
M6 1.62 0.43 205.0 207.0 40.0 167.0
7.22 ave 1.48 0.49 211.3 211.7 40.0 171.7
M7 1.43 0.49 204.0 199.0 42.0 157.0
M8 1.50 0.52 215.0 214.0 42.0 172.0
M9 1.48 0.57 228.0 225.0 42.0 183.0
6.50 ave 1.47 0.53 215.7 212.7 42.0 170.7
M10 1.21 0.56 177.0 173.0 38.0 135.0
M11 1.47 0.51 177.0 173.0 37.0 136.0
M13 1.54 0.53 189.0 184.0 41.0 143.0
7.82 ave 1.41 0.53 181.0 176.7 38.7 138.0
M14 1.46 0.50 147.0 145.0 36.0 109.0
M17 1.27 0.45 124.0 127.0 35.0 92.0
M19 1.26 0.52 150.0 149.0 39.0 110.0
9.82 ave 1.33 0.49 140.3 140.3 36.7 103.7
Burningrate Time(sec)Total
burningtime
takenintest
(sec)
900degC
14
• Burning rates of Miscanthus pellets
Remarks:• An increased O2 flow would increased the burning rate of char and thus the total combustion rate of
the biomass pellet.
• The pre-set furnace temperature has a little impact on the burning rate of volatile matters.
• Combustion behaviours of the biomass pellet under the O2@25% oxy-condition show to be similar to that under the air combustion.
Results and discussions
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
Air O2@21% O2@25% O2@30%To
tal b
urn
ing
ra
te (
x1
0-3
cm
3/s
ec)
Bu
rnin
g r
ate
of
vo
latile
&
Ch
ar
(x1
0-3
cm
3/s
ec)
Conditions@800 deg C
Total Char Volatile
0
5
10
15
20
25
30
35
40
45
50
0
5
10
15
20
25
Air O2@21% O2@25% O2@30%
To
tal b
urn
ing
ra
te (
x1
0-3
cm
3/s
ec)
Bu
rnin
g r
ate
of
vo
latile
&
Ch
ar
(x1
0-3
cm
3/s
ec)
Conditions@900 deg C
Total Char Volatile
15
Concluding remarks
• Experiments have been carried out on a V-DTF to study the combustion behaviours of individual biomass pellets through digital imaging and image processing techniques.
• The complete combustion process of five different biomass pellets were recorded for both air and oxy conditions under the pre-set furnace temperatures of 800 °C and 900 °C.
• The phases of combustion (total, volatile and char) of each biomass pellet have been separated and the associated periods of time are determined.
• The temperatures of the burning pellet (e.g., ignition and surface) have been measured and their relationship with the combustion phases are quantified.
• Results have shown a strong correlation between the burning rate of biomass pellets and oxy flows. In particular, the burning rate of char and thus the total combustion rate increases with the O2 flow.
• The data processing is continuing to quantify and compare the combustion behaviours of different biomass materials (e.g., colour, and spectral intensities of free radicals) for both air and oxy conditions .
16
UKCCSRC is acknowledged for providing
a grant in aid of this research.
Acknowledgement
17
