The emissions from co firing of biomass journal

Journal of the Air & Waste Management Association
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The emissions from co-firing of biomass and
torrefied biomass with coal in a chain-grate steam
boiler
Chia-Chi Chang, Yen-Hau Chen, Wei-Ren Chang, Chao-Hsiung Wu, Yi-Hung
Chen, Ching-Yuan Chang, Min-Hao Yuan, Je-Lueng Shie, Yuan-Shen Li, ShengWei Chiang, Tzu-Yi Yang, Far-Ching Lin, Chun-Han Ko, Bo-Liang Liu, KuangWei Liu & Shi-Guan Wang
To cite this article: Chia-Chi Chang, Yen-Hau Chen, Wei-Ren Chang, Chao-Hsiung Wu, YiHung Chen, Ching-Yuan Chang, Min-Hao Yuan, Je-Lueng Shie, Yuan-Shen Li, Sheng-Wei
Chiang, Tzu-Yi Yang, Far-Ching Lin, Chun-Han Ko, Bo-Liang Liu, Kuang-Wei Liu & Shi-Guan
Wang (2019) The emissions from co-firing of biomass and torrefied biomass with coal in a chaingrate steam boiler, Journal of the Air & Waste Management Association, 69:12, 1467-1478, DOI:
10.1080/10962247.2019.1668871
To link to this article: https://doi.org/10.1080/10962247.2019.1668871
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JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
2019, VOL. 69, NO. 12, 1467–1478
https://doi.org/10.1080/10962247.2019.1668871
TECHNICAL PAPER
The emissions from co-firing of biomass and torrefied biomass with coal in a
chain-grate steam boiler
Chia-Chi Changa, Yen-Hau Chena, Wei-Ren Changa, Chao-Hsiung Wub, Yi-Hung Chenc, Ching-Yuan Changa,d,
Min-Hao Yuane, Je-Lueng Shief, Yuan-Shen Lif, Sheng-Wei Chianga, Tzu-Yi Yanga, Far-Ching Ling, Chun-Han Kog,
Bo-Liang Liua, Kuang-Wei Liuh, and Shi-Guan Wangh
a
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei, Republic of China; bDepartment of Environmental
Engineering, Da-Yeh University, Changhua, Republic of China; cDepartment of Chemical Engineering and Biotechnology, National Taipei
University of Technology, Taipei, Republic of China; dDepartment of Chemical Engineering, National Taiwan University, Taipei, Republic of
China; eDepartment of Occupational Safety and Health, China Medical University, Taichung, Republic of China; fDepartment of
Environmental Engineering, National I-Lan University, Yi-Lan, Republic of China; gSchool of Forestry and Resource Conservation, National
Taiwan University, Taipei, Republic of China; hEnvironmental Analysis Laboratory, Environmental Protection Administration, Chung-Li,
Republic of China
ABSTRACT
PAPER HISTORY
In this study, biomass of rice straw (RS) and wood (WD) and their torrefied biomass (RST and WDT) were
used as solid biofuel (SBF) for co-firing individually with coal in a commercial continuous chain-grate
steam boiler system, which was conducted at fixed input rate of heating value of mixture of SBF and coal
and at fixed airflow rate. The effects of key system parameters on the gaseous and particulate pollutions
and ash were examined. These include SBF type and blending ratio (RBL) of biomass (i.e., SBF) in the
mixture of coal and biomass based on heating values for co-firing.
The results indicated that wood, which possesses high heating value while less amount of ash, is more
suitable for co-firing with coal than rice straw. Torrefaction can increase the heating value of biomass and
homogenize its property, being beneficial to co-firing. Also, torrefaction can decompose the hydroxyl
group of biomass, which makes biomass tending to possess hydrophobicity. This, in turn, helps the
storage and transportation of biomass. Generally, adding the RS (with RBL = 5-10%), WD (2-15%), RST
(2-10%) and WDT (2-20%), respectively, with coal decreases the emissions of NOx and SO2, but increases
that of CO (except RST). The emission of HCl is little. The addition of biomass also increases the emission of
fine particulate matters (PM) especially PM2.5 in the flue gases, raising PM2.5/PM100 from 34.87 to 78.35 wt.
% (Case 50%WDT). These emissions for the Cases tested satisfy with Taiwanese emission standards of
stationary sources which set limitations of NOx, SO2, CO and HCl < 350, 300, 2000 and 80 ppmv, while PM
< 50 mg/m3, respectively. The results support the use of RS, WD, RST and WDT for co-firing with coal.
Implications: This study examined the suitability of using solid bio-fuels to co-fire with coal in an
industrial chain-grate steam boiler system with a capacity of 100 kW, in order to achieve carbon-free
emissions. Both biomass and torrefied biomass of solid bio-fuel were tested. The findings would be
useful for proper design and rational operation of solid bio-fuel/coal co-firing combustion matching
the appeal of sustainable material management and circular economy of biomass, and of adaptation of global warming induced by greenhouse gases. It also provides information for policy-makers
to promote the co-firing application of biomass and related bio-waste materials.
Received April 10, 2019
Revised July 16, 2019
Accepted August 26, 2019
Introduction
Nowadays, the considerable use of fossil fuel causes
many negative effects on the earth, such as global
warming, climatic change and energy depletion. As
a result, the development of renewable energy is an
essential goal in most country. Among those renewable
energies, bio-energy has an extensive application
around the world. Biomass goes through photosynthesis combining carbon dioxide and water to grow up.
Finally, biomass is combusted to generate heat while
emitting CO2 returning to the atmosphere. Because of
this carbon cycle, biomass can be regarded as a carbonneutral fuel. Therefore, the CO2 produced from the
combustion of biomass is not counted as CO2 emission.
The more biomass the power plants substitute for coal,
CONTACT Ching-Yuan Chang
[email protected]
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106,
Republic of China.
Color versions of one or more of the figures in the paper can be found online at www.tandfonline.com/uawm.
© 2019 A&WMA
1468
C.-C. CHANG ET AL.
the less CO2 will be released. However, as a solid fuel,
biomass has a few defects comparing with coal, such as
high hydrophilicity, low heating value, low energy density and low grindabilty. Despite of these shortcomings
while in order to mitigate the negative impacts of fossil
fuel usage, the biomass co-firing technology has been
considered for the increasing biomass usage in combustion and power-generation sectors (Al-Mansour and
Zuwala 2010). There are three main methods to conduct co-firing including direct co-firing, parallel cofiring and indirect co-firing (Al-Mansour and Zuwala
2010; IEA, 2013; Maciejewska et al. 2006). Due to the
low cost of firing systems, direct co-firing is the primary form among these methods.
Direct co-firing still faces some challenges. The different blending ratios of biomass to coal as well as the type of
biomass used can easily affect the performance of combustion (Hupa 2005). Heating values of biomasses, fuel
feeding rates and combustion efficiencies of boilers all
have influences on the co-firing efficiency and energy
production (Hupa 2005; Maciejewska et al. 2006).
Whether the existing facilities for controlling pollutants
can also be applied for combustion of biomass meeting with
the pollutant emission standards is now an important issue
regarding the combustion of biomass, which has no homogeneous property (Veijonen et al. 2003). Main emissions
released from co-firing of biomass with coal contain CO,
SO2, NOX and fine particles. CO formation is highly related
to combustion efficiency. Biomass, which has a high content of volatiles, will fire and decompose quickly leading to
a decrease of the retention time of combustion. This also
makes biomass co-firing encountering low combustion
efficiency with more CO emission (Gani et al. 2005;
Limousy et al. 2013; Wan et al. 2008). Several researches
indicated that the N content in fuel, char loading in combustion chamber, and combustion temperature influence
the NOX emission (Baxter 2005; Dayton 2002; Gani et al.
2005; Hupa 2005; Johnsson 1994; Leckner and Karlsson
1993; Limousy et al. 2013; Veijonen et al. 2003; Wan et al.
2008). Thus, NOx emitted from biomass co-firing is casedependent. According to other researches, high content of
volatiles in biomass can effectively form a fuel-rich zone in
fire during combustion so that the organic nitrogen materials from the fuel can easily converse to harmless N2 (Beř,
Jacques, and Farmayan 1981; Dayton 2002; Niksa et al.
2003). By contrast, SO2 emissions and blending ratios of
biomass in co-firing are presented as a linear relationship as
reported by Hupa (Hupa 2005). The more biomass is added
during co-firing, the less SO2 is emitted. Because of low
sulfur content in biomass while with some calcium mixtures existing in ashes produced from combustion of biomass, more SO2 can be reduced (Baxter 2005; Leckner and
Karlsson 1993; Maciejewska et al. 2006; Veijonen et al. 2003;
Wan et al. 2008). Due to a higher content of Cl in biomass
than that in coal, the exit gas from biomass co-firing will
also release more HCl than sole coal firing (DesrochesDucarne et al. 1998; Wan et al. 2008; Wei et al. 2009). As
for particle issue, several studies pointed out that combustion of biomass resulted in the release of fine particles.
Concentrations of PM10, PM2.5, and PM1 (particulate matters with sizes ≦ 10, 2.5, and 1 μm, respectively) emitted
from combustion of biomass have a much wider variability
than these from combustion of coals (Jiménez and Ballester
2005; Maenhaut et al. 1999; Wang et al. 2007; Zhang et al.
2011), while combustion of biomass preferentially releases
PM1 (Ruscio, Kazanc, and Levendis 2016).
This study analyzed these primary pollutants and ash
formed from co-firing of rice straw (RS), wood (cryptomeria) (WD), and their torrefied biomass (RST and WDT)
noted as solid bio-fuel (SBF) with coal in order to examine
the feasibility of co-firing of these SBF in Taiwan. Effects of
types and torrefaction of SBF, and the blending ratio of SBF
in SBF/coal mixture based on heating values (RBL) on the
performance of co-firing of SBF with coal were elucidated.
Material and methods
Materials and procedures of manufacturing RST
and WDT
This study used bituminous coal (BC) mixing with other
granulated solid bio-fuel SBF including rice straw RS, cryptomeria wood WD, and torrefied rice straw RST, and wood
WBT as fuels. The sizes of these SBF pellets are listed in
Table 1. The RST and WDT were made by torrefaction of RS
and WD, respectively, employing a rotary furnace with the
size of 12 m in length, 1 m in inside diameter, and an
inclination of 3% (Ozone Engineering Technology Co., YiLan, Taiwan). The torrefaction was conducted with the
carrier gas of nitrogen at temperatures of 533 and 563
K for RS and WD, respectively, at time of 60 min and
rotating speed of 1 rpm.
Table 1. Sizes of various carbonaceous materials used in this study.
Pellet diameter (mm)
Thicknessa or lengthb (cm)
BC
1-30
1-3a
RS
8
1.5–2.5b
Notes. BC, RS, WD, RST, WDT: Bituminous coal, rice straw, wood, torrefied RS, torrefied WD;
a, b
Thickness, length.
WD
12
1.5–3.0b
RST
7.5 ± 0.5
1-2b
WDT
11 ± 0.5
1.5–3.0b
JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
BC firing and BC co-firing with SBF
The commercial chain-grate steam boiler system was
housed in Li An Don Chemical Factory in Miao-Li,
Taiwan. The system was composed of a water-tube
steam boiler with designed feeding capacity of
14,000 kg/hr and pressure capacity of 16 kg/cm2 G,
a feed-water heater for heating cold water from room
temperature to 333 K (60°C), an air pre-heater, a fuel
dryer, a bag filter which can remove the particles with
diameters above 100 μm, a fuel hauling equipment and
a bottom ash conveying device. The combustion was
performed with continuous coal/biomass feeding of
1,500 kg/hr for 4 hr. The bottom and fly ashes were
collected at ash discharge ports of a combustion chamber and bag filter, respectively, after 3-hr operation,
while the flue gases including particulates were sampled
for analysis after 1-hr operation as the boiler was
ensured to reach stable combustion.
This work conducted 17 combinations (100% BC firing,
denoted as 100%BC (100 – y) kcal % BC co-firing with
y kcal % SBF, symbolized as y%SBF for SBF = RS, WD, RST,
Table 2. The amount of fuels used for co-firing in full-scale
tests.
Test
100%BC
2%RS
5%RS
10%RS
2%WD
5%WD
10%WD
15%WD
2%RST
5%RST
10%RST
2%WDT
5%WDT
10%WDT
15%WDT
20%WDT
50%WDT
Coal (BC):
Percent to
heat load
(kcal %)
Biomass:
Percent to
heat load
(kcal %)
Total
heat
demand
(kcal/hr)
Coal
weight
(kg)
Biomass
weight
(kg)
100
98
95
90
98
95
90
85
98
95
90
98
95
90
85
80
50
0
2
5
10
2
5
10
15
2
5
10
2
5
10
15
20
50
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
8,250,000
4288
4202
4074
3859
4202
4074
3859
3645
4202
4074
3859
4202
4074
3859
3645
3430
2144
0
125
313
626
107
266
533
799
93
233
466
90
224
448
672
896
2241
1469
WDT) of co-firing (listed in Table 2) applying fuels from
four biomasses and one bituminous coal with continuous
feeding. For example, 2%RS stands for 2 kcal % RS mixed
with 98 kcal % coal. The feeding rate was set at 1500 kg/hr
which can be adjusted to maintain stable steam generation.
The experimental schematic diagram of co-firing process is
illustrated in Figure 1.
Solid analysis
Proximate, ultimate, metal and heating value analyses
were performed in accordance with the standard methods from American Society for Testing and Materials
(ASTM), National Institute of Environmental Analysis
(NIEA), Chinese National Standards (CNS) and
Taiwan Bureau of Standards, Metrology & Inspection
(TBSMI). The detailed descriptions of the methods of
analyses and the instruments employed can be found in
the study of Chen et al. (2017a).
Gas and particle analyses
In this study, flue gas and particle detections were authorized to SGS Taiwan Ltd. (Taipei, Taiwan). The methods
and applied instruments of these analyses are listed in
Table 3. Concentrations of CO (range of 0–5000 ppmv),
CO2 (range of 0–20 vol.%), O2 (range of 0–25 vol.%), SO2
(range of 0–3000 ppmv) and NOx (range of 0–2500
ppmv) in flue gas were measured by a PG250 portable
gas analyzer (Horiba, Kyoto, Japan). Non-dispersive
infrared (NDIR) absorption method was used to quantify
CO, CO2 and SO2, while cross-modulation ordinary pressure chemiluminescence and a galvanized Zr cell for NOx
and O2 measurements, respectively. The sampling unit
comprises a filter, a mist catcher, a pump, an electronic
cooling unit, and an NO2 to NO converter. HCl was
measured by a mercuric thiocyanate method (NIEA
A412.73A). The HCl in flue gas was absorbed by sodium
hydroxide solution, followed with the addition of
Figure 1. Schematic diagram of chain-grate steam boiler system for co-firing process.
1470
C.-C. CHANG ET AL.
Table 3. Measurements of flue gas and instruments.
Item
Measure method
Particle size distribution
USEPA M201A
CO
CO2
O2
SO2
NOx
HCl
NIEA
NIEA
NIEA
NIEA
NIEA
NIEA
A704.04C
A415.72A
A432.73C
A413.74A
A411.74A
A412.73A
Instrument
Apex cascade impactor
CI-700-K
HORIBA PG250
HORIBA PG250
HORIBA PG250
HORIBA PG250
HORIBA PG250
Spectrophotometer
mercuric thiocyanate and ammonium iron(III) sulfate
solutions and the measurement of absorbance of ferric
thiocyanate. Particle size distribution in exhaust gas was
monitored by Apex cascade impactor CI-700-K (Apex
Instruments, Inc., Fuquay-Varina, N.C., U.S.A.), which
includes seven-stage impactor, a PRA-K pre-cutter, nozzles, interconnecting tube, glass fiber filters (45 mm,
47 mm and GFDN filters) and foil collection plate. The
particle size distribution of stack particulate matters measured from the Cascade Impactor was analyzed by windows-based Cascade Impactor Data Reduction System
software.
Results and discussion
Basic properties of fuels
Tables 4–5 and Figure 2 show the results of proximate,
ultimate and dry-basis high heating value (HHD) analyses
of bituminous coal BC, rice straw RS, wood WD, RST
and WDT, respectively. These materials can be classified
into three groups: coal, biomass and torrefied biomass.
In this study, the coal used was bituminous coal with
a relatively higher water content of 30.84 wt.% compared
Table 4. Proximate analyses of fuels.
Moisturea (%)
30.84 ± 0.07
10.87 ± 0.03
6.72 ± 0
11.84 ± 0.08
5.28 ± 0.14
Fuel
Coal (BC)
RS
WD
RST
WDT
Volatile mattera (%)
31.42 ± 0.23
58.26
77.96 ± 0.41
50.61 ± 0.02
61.41 ± 0.03
Asha
(%)
7.24 ± 0.17
12.16
1.24 ± 0.11
17.18 ± 0.44
17.17 ± 0.18
Fixed carbona (%)
30.5 ± 0.13
18.71
14.08 ± 0.52
20.37 ± 0.53
16.14 ± 0.3
Ashb
(%)
10.47
13.64
1.33
19.49
18.13
Notes. awt.% on wet basis; b: wt.% on dry basis.
Table 5. Ultimate analyses of fuels.
Fuel
Coal (BC)
RS
WD
RST
WDT
C (%)
58.769
40.68
48.167
51.196
51.709
±
±
±
±
±
2.063
0.949
0.095
0.033
0.123
H (%)
4.361
5.95
6.809
4.501
6.411
Notes. wt.% on dry basis.
Figure 2. Heating values in dry basis of fuels.
±
±
±
±
±
0.198
0.275
0.004
0.187
0.018
O (%)
21.634
38.204
43.246
22.332
39.257
±
±
±
±
±
0.376
0.453
0.019
0.788
0.257
N (%)
0.812
0.781
0.246
1.017
0.273
±
±
±
±
±
0.047
0.059
0.037
0.051
0.002
S (%)
0.708
0.685
0.469
0.356
0.434
±
±
±
±
±
0.066
0.099
0.023
0.238
0.049
JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
with reference due to wet weather in Taiwan. The coal
has the highest fractions of fixed carbon and carbon
element standing for 30.5 wt.% (wet basis) and 58.77
wt.% (dry basis), respectively. Therefore, the HHD of coal
is the highest of 5772 kcal/kg among these fuels. The
biomass group includes rice straw and wood, which have
relatively high oxygen and hydrogen contents and
majorly consist of volatiles. Heating values of these biomasses are related to their carbon contents (Table 5)
leading to a comparatively higher HHD of wood (4645
kcal/kg) than that of RS (3954 kcal/kg). After torrefaction, the biomass can be carbonated, and the energy
intensity of biomass can also be enhanced (Chen et al.
2017a). Thus, the HHD of RS and WD increase from
3954 and 4645 kcal/kg to reach 5308 and 5522 kcal/kg
for RST and WDT, respectively. The quality of torrefied
biomass can also be close to coal (Chen et al. 2017a).
Moreover, less oxygen and hydrogen retained in RST
and WDT than RS and WD indicate that they are
more hydrophobic.
Table 6. Chemical compositions of fuels.
Composition
Coal (BC)
RS
K (%)
Na (%)
Ca (%)
Mg (%)
Al (%)
Fe (%)
Ti (%)
Si (%)
Hg (%)
Ni (%)
Cl (%)
0.0242
1.69
0.00907
0.0397
0.151
0.552
0.0776
0.256
0.226
0.0498
0.632
0.126
0.00928 <0.00025
0.0204
0.0211
<0.0000015
<0.0005
<0.002
0.359
WD
RST
WDT
0.23
0.0254
0.338
0.136
0.0118
0.0223
0.000357
0.00573
1.83
0.0502
0.73
0.312
0.0989
0.246
0.000464
0.0104
0.167
0.0375
0.333
0.125
0.00444
0.0117
<0.00025
0.00268
0.113
0.313
0.195
Notes. wt.% on dry basis.
1471
The ash contents of fuels are different withRS of
13.64 wt.% higher than WD of only 1.33 wt. % on dry
basis. After long combustion in the boiler system, the
high ash content of fuel causes scale deposition (fouling), which needs more frequent maintenance in order
to keep the boiler system in good condition.
Chemical compositions of fuels in Table 6 indicate
that coal has comparatively higher contents of Fe and
Al than RS and WD. However, the RS and WD,
which take nutrients to grow themselves, possess
higher contents of K, Na, Ca and Mg than coal. It
is worth to mention that Cl content is also an important indicator for the usage of biofuel as chlorides
formed after the combustion of fuel can lead to
corrosion of boiler. The Cl content of bituminous
coal is below 0.002 wt.% much lower than those of
biomasses, especially of RS and RST of 0.359 and
0.313 wt.%, respectively.
Compositions of gases from co-firing of biomass
with coal
The compositions of gases from co-firing tests are
shown in Table 7. The concentrations of emissions
were analyzed after the boiler reached stable combustion in 1 hr. The data of Table 7 were the average
concentrations computed by the results from
per minute measurement during 1 hr combustion. As
noticed in Section 2.2, “%name of fuel” represents for
the kcal proportion of the specified fuel in mixture, for
example, 2%RS denotes that there is 2 kcal % RS mixed
with 98 kcal % coal. The accuracy of data can be
Table 7. Components of flue gas in full-scale co-firing tests.
Test
Coal (BC)
2%RS
5%RS
10%RS
2%WD
5%WD
15%WD
2%RST
5%RST
10%RST
2%WDT
5%WDT
10%WDT
15%WDT
20%WDT
50%WDT
Emission standarda
NOx
(ppmv)
52.0 ± 1.4
62.3 ± 5.8
41.7 ± 7.3
31.9 ± 15.1
41.9 ± 7.3
48.3 ± 1.4
40.3 ± 2.2
47.3 ± 8.5
42.9 ± 1.5
44.4 ± 3.1
46.4 ± 2.0
39.2 ± 2.4
43.4 ± 1.9
39.4 ± 2.1
35.7 ± 3.0
99.0
150b
250c
350d
SO2
(ppmv)
42.1 ± 3.6
37.9 ± 3.2
22.9 ± 4.0
21.7 ± 4.6
27.7 ± 5.2
48.5 ± 5.8
32.7 ± 12.4
6.4 ± 2.1
7.6 ± 1.4
10.7 ± 2.5
33.9 ± 4.0
34.1 ± 4.5
35.8 ± 1.6
27.1 ± 2.4
21.4 ± 2.9
97
100b
300c,d
CO
(ppmv)
228.2 ± 108.3
281.9 ± 132.9
394.4 ± 97.2
652.7 ± 189.7
246.6 ± 142.2
521.6 ± 88.5
676.1 ± 493.6
141.9 ± 61.6
80.6 ± 14.1
163.1 ± 115.7
429.4 ± 297.3
532.8 ± 222.5
231.3 ± 77.4
238.6 ± 121.2
227.8 ± 87.9
461
2000
CO2
(vol.%)
5.47 ± 0.24
5.48 ± 0.29
3.21 ± 0.56
1.79 ± 0.60
5.11 ± 0.96
6.56 ± 0.25
6.60 ± 0.74
5.36 ± 1.08
4.90 ± 0.34
5.56 ± 0.35
6.42 ± 0.47
6.03 ± 0.29
6.19 ± 0.17
6.02 ± 0.35
5.50 ± 0.44
15.00
O2
(vol.%)
13.67 ± 0.28
14.30 ± 0.36
17.00 ± 0.65
18.55 ± 0.74
15.29 ± 1.00
13.68 ± 0.29
13.72 ± 0.84
15.14 ± 1.26
15.69 ± 0.37
14.91 ± 0.36
13.91 ± 0.51
15.02 ± 0.31
14.87 ± 0.18
15.04 ± 0.37
15.62 ± 0.48
15.24
Notes. aEmission standards of stationary sources in Taiwan (TEPA, 2013a);
Gas fuels
c
Liquid fuels
d
Solid fuels
The volume concentrations of emitted gaseous pollutants were adjusted to the conditions at 273 K and 1 atm with 6% O2.
b
HCl
(kg/hr)
0.12
0.12
0.25
0.25
0.12
0.12
0.10
0.11
0.11
0.22
0.11
0.19
0.12
0.12
0.12
0.10
HCl
(ppmv)
ND<2.85
3.00
6.48
6.35
ND<2.66
ND<2.66
ND<2.61
ND<2.69
ND<2.69
ND<5.36
ND<2.69
4.84
2.94
ND<2.72
ND<2.72
ND<2.59
1472
C.-C. CHANG ET AL.
reflected by the standard deviation along with average
value, as also displayed in Table 7.
Emission of CO
CO emission from combustion is related to the combustion efficiency of the boiler. When the fuel at high
temperature does not have enough oxygen or the feeding rate of fuel is too fast such that the fuel has insufficient time for combustion in the boiler, more CO will
exist in the flue gas. As shown in Table 7, un-torrefied
biomass co-firing with coal generates a higher concentration of CO than sole-firing of coal. For RS and WD,
a higher blending ratio of biomass also generally
resulted in more emission of CO. It is known that
biomass exhibits an instant decomposition via devolatilization at mild temperature while coal offers
smooth combustion performance. This causes the barrier of good combustion efficiency for co-firing because
of the different combustion characteristics of two different fuels. For the practical need, the heat output
must be maintained at specific value so that the supply
of steam can be stable. When the biomass had a quick
mass loss during combustion, while the feeding rate
had to be adjusted according to the heat output, it
resulted in low combustion efficiency and high CO
concentration. This phenomenon was observed in the
mixture of RS and BC.
Regarding the function of torrefaction, Chen et al.
(2017a) reported that the torrefied biomass possesses
property closer to coal than the un-torrefied biomass
and, therefore, its de-volatilization during combustion
causes less mass loss, thus exhibiting combustion trend
more similar to coal. For the co-firing of torrefied
biomass with coal, the results of Table 7 show that
CO emissions for Case RST decrease comparatively
lower than those for Case RS, indicating the beneficial
effect of torrefaction on RS. The reduction of CO
emission for Case WDT is not apparent. However, the
concentrations of CO emitted for the cases of solefiring of coal, and co-firing of RS, WD, RST and WDT
with coal examined in Table 7 all meet the CO emission
standard of stationary pollution source in Taiwan
(lower than 2000 ppmv) (TEPA, 2013a).
It is noted that the unburned carbon in fuel could turn
into the carbon contents in fly ash and bottom ash, so high
carbon contents in both ashes may represent an inefficient
combustion. Hence, Armesto et al. (2002) reported that
the combustion efficiency of biomass in the boiler can be
determined by the carbon contents in fly and bottom
ashes. Figure 3 shows carbon contents in fly and bottom
ashes and CO emissions of coal (BC) and SBF (2% and
10% RS and RST; 2% and 15% WD and WDT). The results
Figure 3. Carbon contents in fly and bottom ashes and CO
emission of coal and SBF.
indicated that carbons in fly and bottom ashes showed
different trends. However, bottom ash possessed higher
carbon contents than fly ash. Carbons in fly ash for SBF
were all higher than that of BC, while carbons in bottom
ash of SBF would generally lower than that of BC.
Comparing the findings of CO emission, only torrefied
RS displays the lower CO concentration than that of BC. It
is quite difficult to identify general trends for combustion
efficiency in co-firing coal with SBF based on the carbon
balance of fuel ashes.
Emission of NOx
In the light of the study of Nussbaumer (2003), NO and
NO2 (summarized as NOX) can be formed in three different mechanisms during combustion. Thermal NOX
and prompt NOX are originated from the nitrogen in
the air at high temperatures and in the case of prompt
NOX in the presence of hydrocarbons. Further, fuel NOX
can be formed from nitrogen-containing fuels. For biomass combustion, fuel-bound nitrogen is the main source
of NOX emissions, while thermal and prompt NOX are
not relevant due to relatively low temperatures as has been
shown by theoretical and experimental investigations.
Fuel nitrogen is converted to intermediate components
such as HCN and NHi with i = 0, 1, 2 and 3. These can be
JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
oxidized to NOX if oxygen is available, which is the case in
conventional combustion. If no oxygen is present, intermediates can interact in the reduction zone and form N2
via reactions such as NO + NH2 = N2+ H2O. In combustion, char content is also an important factor that can help
the NOx reduction reaction. Consequently, the N content,
oxygen supply and char content during combustion are
the main factors for the formation of NOx.
Observing element compositions of all fuels in Table 5,
the RST and WDT possess nitrogen contents higher than RS
and WD due to torrefaction. All except RST contain less
nitrogen than coal. These results can explain why NOX
concentrations from co-firing of biomass containing
lower nitrogen contents are lower than those of coal.
Although torrefaction slightly increases the nitrogen content, there are no apparent effects on the NOx emissions
between Cases 2-10%RS, 2-15%WD and 2-10%RST, 215%WDT (Table 7). The case with high blending ratio
RBL of 50%WDT contains biomass which has relatively
low fixed carbon content (Table 4) in full scope of about
23.32 wt.%. The excess use of WDT for substituting BC thus
results in the NOx emission reaching 99 ppmv, which is
about twice as much as that of BC. Nevertheless, regressions
of NOx emissions detected from the combustion of fuels at
all cases with their fixed carbons (R2 = 0.4316) and nitrogen
contents (R2 = 0.4427) showed poor linear relationships
(data not shown).
Emission of SO2
Biomass generally has lower S content than coal so that
coal co-fires with biomass forms less SO2. In addition,
Figure 4. Effect of calcium content in fly ash on SO2 emissions.
1473
according to the study of Veijonen et al. (2003), the
components of fuels can affect the SO2 formation when
these fuels are combusted together. SO2 formed from
combustion can react with Ca in fly ash produced mainly
from the combustion of biomass so that SO2 emission can
decrease. Sotiropoulos, Georgakopoulos, and Kolovos
(2005) proved that increasing the free CaO in fly ash
decreases the respective SO2 emissions. The free CaO
(often called the “free lime”) is the amount of calcium
oxide present as CaO, that is, not bound into the cement
minerals. The available free CaO facilitates the natural
desulfurization (CaO + SO2 + 1/2O2 = CaSO4), thus
reducing SO2 emissions to the atmosphere. Davis and
Fiedler (1982) also indicated that the sulfur retention in
fly ash emitted from coal-fired boilers is a function of CaO
content of fly ash, suggesting that the presence of CaO
enhances the adsorption of sulfur.
Based on Table 7, after mixing coal, respectively, with
RS and WD, SO2 emission from co-firing decreases
slightly compared with that from combustion of
pure BC of 42.1 ppmv because of their low S contents
(RS of 0.69 wt.% and WD of 0.47 wt.% lower than BC of
0.71 wt.% as in Table 5). Figure 4 illustrates the relation
between SO2 emissions and Ca contents in fly ash from all
Cases. The tendency indicates that the more Ca in fly ash
is, the less SO2 is emitted, which is in accordance with the
findings of Veijonen et al. (2003).
The S content in biomass can be reduced after torrefaction. It is removed as part of the volatiles, which
contains sulfide, during decomposition and gasification
(Chen et al. 2017b). RST and WDT have lower
S contents of 0.36 wt.% and 0.43 wt.% than RS and
1474
C.-C. CHANG ET AL.
WD, respectively. The SO2 emissions of 2-10%RST are
much lower than those of 2-10%RS. On the other hand,
combustions of 2-20%WDT emit close concentrations
of SO2 as of WD. The trends of RS and WD were
similar to their Ca contents as shown in Table 6, highlighting the role of Ca contents in SBF for desulfurization. Moreover, on the base of Ca contents in fly ash of
Cases 2-50%WDT (9.13, 11.3, 11.1, 11.1, 11.5 and 5.65
wt.%), Case 50%WDT has the lowest Ca content, which
results in the highest SO2 emission of 97 ppmv.
Emission of HCl
The results in Table 7 show that only Cases RS and 510%WDT have detectable HCl concentrations in emissions.
The HCl concentrations have no apparent difference
between the combustion of BC and biomasses of other
cases. The Cl in biomasses may be combined with alkaline
such as Ca, K and Na and transferred into ashes during
combustion (Ren et al. 2018). This thus results in low HCl
emission in exhausted gas. High Cl content in ashes from
biomass co-firing can be expected. The Cl contents in fly
ashes of SBF/coal co-firing were 0.38–1.82 wt.% higher than
that of sole coal firing of 0.08 wt.%. As for the Cl contents of
bottom ashes, SBF/coal co-firing resulted in 0.035–0.305 wt.
% also higher than sole coal firing with <0.01 wt.%.
Emission of particulate matters
The concentrations of PM emitted after bag filter for cases
examined are listed in Table 8. For sole BC combustion, the
PM concentration is at 0.61 mg/Nm3. After adding biomass
to co-fire with BC, there is no obvious difference of PM
concentrations of 0.47–0.95 mg/Nm3. Only Cases 2%RST
and 2%WDT have relatively high PM concentrations of
0.95 and 0.78 mg/Nm3, respectively. On account of a bag
filter connected with the boiler, PM with the size larger than
100 μm was collected, cutting down the emissions of PM.
Table 8 also presents the weight percent of PM1/PM100,
PM2.5/PM100, PM10/PM100, PM2.5/PM10, PM1/PM10 and
PM1/PM2.5. The PM10/PM100 values are as high as 99.10
wt.% or greater, indicating that the emitted PM100 mostly
consists of PM10. This further reveals that the bag filter
employed efficiently removed most of PM larger than 10
μm. After torrefaction, co-firing of RST has a higher emission of PM2.5 (0.47–0.72 mg/Nm3) than that of RS
(0.37–0.49 mg/Nm3), while co-firing of WDT has no noticeable difference of emission of PM2.5 (0.46–0.58 mg/Nm3)
with that of WD (0.44–0.47 mg/Nm3) at the same level of
blending ratio.
The particle size distributions of emitted PM from
all conditions of combustions tested are shown in
Figure 5. There are four main peaks standing for
particle sizes of 0.16, 0.4, 1 and 4 μm. Case 100%BC
releases more coarse particles in emission. Its PM2.5
/PM100 is 34.87% (Table 8). However, after BC mixing
with biomass, most of the particles in emission turn to
fine particles, resulting in PM2.5/PM100 of 61.99–78.35
wt.%. The above findings are consistent with those of
other studies pointed out that combustion of biomass
results in an emission of fine particles with diameters
smaller than 10 μm, which are harmful to health and
cause environmental pollution (Zhang et al. 2011).
A lower pellet size of solid fuel would increase the
combustion efficiency because of better heat and mass
transfers for smaller particles. However, it should be
noted that the pellet size of coal and SBF used in the
study were quite uniform (shown in Table 1). Hence,
further experiments would be helpful to understand
the effects of pellet size on particle size distribution
of PM.
Co-firing the mixed fuels exhibit high percentiles of
PM1/PM100 and PM1/PM2.5 of 44.36–59.75 wt.% and
69.21–76.27 wt.%, respectively, which are all higher than
the sole combustion of coal. After blending with biomass,
the potassium content in fly ash rises to 4.76 ~ 13.7 wt.%
Table 8. Particulate matters (PM) concentration (mg/Nm3) and percentiles of PM1, PM2.5, and PM10 (wt.%).
Test
100%BC
2%RS
5%RS
10%RS
2%WD
5%WD
10%WD
15%WD
2%RST
5%RST
10%RST
2%WDT
5%WDT
10%WDT
15%WDT
20%WDT
50%WDT
PM concentration (mg/Nm3)
0.6105
0.6441
0.7893
0.5707
0.6210
0.6503
0.6091
0.6294
0.9461
0.6253
0.6211
0.7803
0.6293
0.6262
0.6117
0.6348
0.4717
PM1/PM100
21.92
53.91
44.36
45.47
57.27
56.43
53.23
53.35
56.94
58.34
56.52
55.53
54.12
55.03
57.22
56.25
59.75
PM2.5/PM100
34.87
73.89
61.99
65.70
75.36
73.20
73.87
71.68
76.17
76.80
76.31
74.69
73.67
75.12
75.90
74.48
78.35
PM10/PM100
99.10
99.97
99.99
99.92
99.98
99.98
100.00
99.94
100.00
100.00
99.98
99.99
100.00
99.99
100.00
99.99
99.96
PM2.5/PM10
35.19
73.91
62.00
65.75
75.38
73.22
73.87
71.73
76.17
76.80
76.32
74.70
73.70
75.12
75.90
74.48
78.37
PM1/PM10
22.12
53.93
44.36
45.51
57.28
56.44
53.23
53.38
56.94
58.34
56.53
55.54
54.13
55.04
57.22
56.25
59.78
PM1/PM2.5
62.87
72.96
71.55
69.21
75.98
77.09
72.05
74.42
74.75
75.96
74.06
74.35
73.47
73.27
75.39
75.52
76.27
JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
1475
Figure 5. Particle size distribution of PM in flue gas in full-scale co-firing tests.
from 2.21 wt.% of sole coal firing, and the chlorine content also increases from 0.0672% to 0.38 ~ 1.82%. As
a result, K and Cl contents in fly ash of co-firing with
biomass are much higher than that from the sole firing of
coal because of the presence of high Cl and alkali metal in
the biomass. These components are also related to the
formation of fine particles. Literature data show that the
contents of potassium, chlorine and sulfur in the fuel
influence the composition of the emitted inorganic submicron particles to a large extent. Potassium chloride and
potassium sulfate were the main components exited in the
emitted submicron particles (Johansson et al. 2003).
Gaseous K2SO4 was found to be the main potassium
compound vaporized and condensed to form PM1, with
an average size of 0.5 μm. When it came to the high
combustion temperatures, gaseous KOH became the
main potassium compound vaporized, most of which
may react with aluminosilicates to form coarse particulates, and the remaining vaporized K2SO4 and other sulfates/oxides condensed to form PM1, with an average size
of 0.2–0.3 μm. The higher the combustion temperature,
the more volatile potassium was transferred to coarse
particulates (Zhou et al. 2010). According to these
research results, high potassium amount in biomass lead
to the considerable formation of fine particles.
In this research, the relatively higher contents of the
alkali group (especially K) and chlorine in biomass and
torrefied biomass than those of coal give rise to the formation of KCl and K2SO4, which are described above to be
volatilized during combustion and then condensed to submicron-size particles. Consequently, there are more fine
particles released in emissions. Moreover, the alkali group
and chlorine contained in the produced fly ash may also
react with gas chemicals and subsequently induce the formation of fine particles. As a result, the formation of the
small size of particles also highly dependent on the K and Cl
content in fly ash. For example, Case 50%WDT has high
PM1/PM2.5 of 76.27 wt.%. Hence, when using a substantial
amount of biomass for co-firing, stringent control of emission of fine PM is necessary.
Comparisons with emission standards
Recently, Taiwan is dedicated to promoting the application of sustainable energy. Preparation of new legislation on the control of emissions from combustion of
biomass in existed non-utility boilers is undergoing.
Notice that the combustion of biomass in Germany
has been widely applied and well developed for a long
time. The regulations of emissions in Germany are also
adopted along with Taiwanese restrictions to assess the
co-firing results of this study. In Germany, for largescale combustion, the regulations of emissions can be
referred to “13. Bundes-Immissionsschutzgesetz (13.
Blmschv)-Die Verordnung über Großfeuerungs- und
Table 9. Comparison of air pollution standards of gas
emissions.
Item
PM
CO
NOX
SO2
HCl
Unit
mg/m3
ppmv
ppmv
ppmv
ppmv
Taiwan 1 Taiwan 2
(ST1)a
(ST2)b
20-50
50
2000
70-175
350
60-150
300
80
Germany 1
(SG1)c
(Solid coals,
50–100 MW)
10
150
300
400
-
Germany 2 (SG2)c
(Solid biomasses,
50–100 MW)
10
150
250
200
-
Notes. aEmission standards of steam and electricity symbiosis equipment
boilers in Taiwan (TEPA, 2013b);
b
Emission standards of stationary sources in Taiwan (TEPA 2013a);
c
“13. Bundes-Immissionsschutzgesetz (13. Blmschv)-Die Verordnung über
Großfeuerungs- und Gasturbinenanlagen” (Bundesministerium der Justiz
und fürVerbraucherschutz, 2018a).
1476
C.-C. CHANG ET AL.
Gasturbinenanlagen” (Bundesministerium der Justiz
und für Verbraucherschutz 2018a). The emissions are
limited according to different fuels (coals, biomasses,
diesel and gases) that are utilized in boilers and the heat
capacities (above 50 MW) of boilers. In Taiwan, the
application of fossil fuels and fuels from biomasses for
non-utility boilers has the same restriction of emissions. Unified restriction limits emissions from all
kinds of non-utility boilers and fuels. Table 9 lists
Taiwan’s and Germany’s emission standards. As for
Germany’s standard, by separating coals and biomasses,
NO2 and SO2 emissions of combustion of biomasses
can be asked stricter than those of coal. CO formation
depends highly on the efficiency of combustion. As
a result, asking low CO emission (150 ppmv) is also
asking high energy conversion from fuels. Taiwan also
has special regulations for special boilers such as steam
and electricity symbiosis equipment boilers which are
used by utility and power companies to generate electricity (noted as utility boilers). The utility boilers are
restricted by stricter regulation than the conventional
non-utility boilers (classified as stationary sources) also
shown in Table 9.
The utilization of different solid fuels in smallscale boilers is also prevalent in Germany. The
restriction of emissions is presented in another regulation “1. Bundes-Immissionsschutzgesetz (1.
Blmschv)-Die Verordnung über kleine und mittlere
Feuerungsanlagen” (Bundesministerium der Justiz
und für Verbraucherschutz 2018b). The small boilers
which are adopted in this law have only kilowatts
(kW) level of heat capacity. The fuels also have a
specific content limitation in standard “DI.” Thus,
the regulation limits not only amounts of emissions
of pollutants but also the contents of fuels in order to
reduce air pollutants effectively. Moreover, different
conditions of fuels and boilers are given different
limitations of emissions like “13. Blmschv”, which
provides more appropriate limitations for the application of different fuels. These are good aspects to
which Taiwan can try to refer.
Comparison of the results of Tables 7–8 with the
emission standards of Table 9 indicates the follows. The
emissions of CO meet the limitation of Taiwan standard ST2 for non-utility boilers while exceeding
Germany regulations SG1 and SG2 for large-scale combustion. Other emissions of NOx, SO2, HCl and PM are
all satisfied with the corresponding regulation, and this
thus elucidates the appropriation and applicability of
the use of biomasses tested in the present study for cofiring with coal.
Conclusion
Addition of biomass of RS (with RBL = 5-10%), WD
(2-15%), RST (2-10%) and WDT (2-20%), respectively,
to co-fire with coal generally results in a decrease of
NOx and SO2 while increase of CO (except RST)
emissions in exhausted gas. Fixed carbon contents in
biomass-coal mixtures and Ca contents in fly ashes are
highly related to NOx and SO2 emissions, respectively.
After torrefaction, the reductions of emissions of CO
and SO2 from co-firing of RST are significant, while
those of WDT are not apparent.
Adding biomass for co-firing gets rise to more fine
particulate matters in emissions, especially PM2.5 particles. After torrefaction, co-firing of RST has a higher
emission of PM2.5 than RS, while co-firing of WDT has
no noticeable difference of emission of PM2.5 with WD.
Nevertheless, all examined cases of coal co-firing,
respectively, with RS, WD, RST and WDT meet the
emission standards of stationary sources in Taiwan.
Nomenclature
BC: bituminous coal
BF: bio-fiber (extracted from MSWs)
HHD: dry-basis high heating value (kcal/kg)
MSWs: municipal solid wastes
PM1, PM2.5, PM10, PM100: particulate matters with sizes ≤
1, 2.5, 10, 100 μm, respectively
RBL: blending ratio of biomass in mixture of coal and biomass based on heating values (kcal/kcal)
RB: rice straw bio-char
RS: rice straw
RST: torrefied RS
SBF: solid bio-fuel
SF: solid fuel
SG1: Emission standards for large-scale combustion (solid
coals, 50-100 MW) in Germany
SG2: Emission standards for large-scale combustion (solid
biomasses, 50-100 MW) in Germany
ST1: Emission standards of steam and electricity symbiosis
equipment boilers in Taiwan
ST2: Emission standards of stationary sources in Taiwan
WB: wood bio-char
WD: wood
WDT: torrefied WD
y%SBF for SBF = RS, WD, RST, WDT: (100-y) kcal % BC cofiring with y kcal % SBF
100% BC: 100%BC firing
Funding
This work was supported by the Ministry of Science and
Technology and Environmental Protection Administration,
Taiwan.
JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
About the authors
Chia-Chi Chang obtained his Ph.D. from Graduate Institute
of Environmental Engineering, National Taiwan University,
No.1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan. Currently,
he is a research scientist in Taiwan Bio-energy Technology
Development Association, Rm. 312, No. 130, Sec. 3, Keelung
Rd., Taipei 106, Republic of China. He can be contacted at
[email protected].
Yen-Hau Chen obtained his Master degree from Graduate
Institute of Environmental Engineering, National Taiwan
University, No.1, Sec. 4, Roosevelt Road, Taipei 106,
Republic of China. Currently, he is a Ph.D. student in
Institute of Combustion and Power Plant Technology,
University of Stuttgart, Pfaffenwaldring 23, Stuttgart 70569,
Germany. He can be contacted at [email protected].
Wei-Ren Chang obtained his Master degree from Graduate
of
Environmental
Engineering,
National
Institute
Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 106,
Republic of China. He can be contacted at a10022587@gmail.
com.
Chao-Hsiung Wu is a Professor at Department of
Environmental Engineering, Da-Yeh University, No. 168,
University Rd., Dacun, Changhua 515, Republic of China.
He can be contacted at [email protected].
Yi-Hung Chen is a Professor at Department of Chemical
Engineering and Biotechnology, National Taipei University
of Technology, No. 1, Sec. 3, Zhongxiao E. Rd., Taipei 106,
Republic of China. He can be contacted at yhchen1@ntut.
edu.tw.
Ching-Yuan Chang is a Distinguished Professor at Graduate
Institute of Environmental Engineering and Department of
Chemical Engineering, National Taiwan University, No.1,
Sec. 4, Roosevelt Road, Taipei 106, Republic of China. He
can be contacted at [email protected].
Min-Hao Yuan is an Assistant Professor at Department of
Occupational Safety and Health, China Medical University,
No. 91 Hsueh-Shih Road, Taichung 404, Republic of China.
He can be contacted at [email protected].
Je-Lueng Shie is a Distinguished Professor at Department of
Environmental Engineering, National I-Lan University, No.1,
Sec. 1, Shennong Rd., Yilan City, Yilan County 260, Republic
of China. He can be contacted at [email protected].
Yuan-Shen Li is a Professor at Department of Environmental
Engineering, National I-Lan University, No.1, Sec. 1,
Shennong Rd., Yilan City, Yilan County 260, Republic of
China. He can be contacted at [email protected].
Sheng-Wei Chiang obtained a Ph.D. from Graduate Institute
of Environmental Engineering, National Taiwan University,
No.1, Sec. 4, Roosevelt Road, Taipei 106, Republic of China.
He can be contacted at [email protected].
Tzu-Yi Yang obtained her Master degree from Graduate
Institute of Environmental Engineering, National Taiwan
University, No.1, Sec. 4, Roosevelt Road, Taipei 106,
Republic of China. She can be contacted at joyceper@gmail.
com.
1477
Far-Ching Lin is an Associated Professor at School of
Forestry and Resource Conservation, National Taiwan
University, No.1, Sec. 4, Roosevelt Road, Taipei 106,
Republic of China. He can be contacted at farching@ntu.
edu.tw.
Chun-Han Ko is a Professor at School of Forestry and
Resource Conservation, National Taiwan University, No.1,
Sec. 4, Roosevelt Road, Taipei 106, Republic of China. He
can be contacted at [email protected].
Bo-Liang Liu obtained his Master degree from Graduate
Institute of Environmental Engineering, National Taiwan
University, No.1, Sec. 4, Roosevelt Road, Taipei 106,
Republic of China. He can be contacted at r04541129@ntu.
edu.tw.
Kuang-Wei Liu is working in Environmental Analysis
Laboratory, Environmental Protection Administration, No.
260, Minzu Rd. Sec 3, Taoyuan 320, Republic of China. He
can be contacted at [email protected].
Shi-Guan Wang is working in Environmental Analysis
Laboratory, Environmental Protection Administration, No.
260, Minzu Rd. Sec 3, Taoyuan 320, Republic of China. He
can be contacted at [email protected].
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