Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES
Article
Co-combustion of Tannery Sludge in a Bench-scale Fluidized Bed Combustor: Gaseous Emissions, Cr Distribution and Speciation Hao Dong, Xuguang Jiang, Guojun Lv, Fei Wang, Qunxing Huang, Yong Chi, Jianhua Yan, Weizhong Yuan, Xijiong Chen, and Weizhong Luo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01831 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
3
Co-combustion of Tannery Sludge in a Bench-scale Fluidized Bed Combustor: Gaseous Emissions, Cr Distribution and Speciation
4
Hao Dong,† Xuguang Jiang,*† Guojun Lv,† Fei Wang,† Qunxing Huang,†
5
Yong Chi,† Jianhua Yan,† Weizhong Yuan,‡ Xijiong Chen,‡ Weizhong Luo‡
1 2
6
†
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou
7
8 9
310027, China
‡
Zhejiang Zheneng Changxing Electric Power Generation Co., Ltd., Huzhou 313000, China
10
11
ABSTRACT: In this study, eleven coal mono-combustion tests and four co-combustion
12
tests of coal and tannery sludge were conducted on a 35kW fluidized bed combustor. The
13
combustion behavior and emission characteristics of the fuels were investigated on a
14
bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of an
15
excess air ratio, primary air rate, secondary air ratio and fuel type on flue gas emissions
16
were studied. The results showed that the fluidization status and temperature distribution
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
17
had direct influences on CO emission. Sufficient fluidization and high temperature
18
effectively reduced CO emission. NOx emission was relatively sensitive to the excess air
19
ratio and increased with increasing excess air ratio. By comparing BFB and CFB, we
20
found that CFBs have an advantage in optimizing combustion and controlling emissions
21
by enhancing mixing and increasing freeboard temperatures.
22
The Cr speciation and distribution among different ash types were extensively
23
investigated in four co-combustion tests. The results showed that the distribution modes
24
of Cr in BFBs and CFBs were different and determined by separate fluid dynamics
25
modes of sludge ash particles in the combustor. The extent of Cr oxidation in ash in CFB
26
tests was higher than that in BFB tests, particularly for bottom ash and heat exchanger
27
ash, due to longer residence times in high-temperature regions.
28
Introduction
29
The management of hazardous wastes that pose severe threats to the environment and
30
human health have drawn worldwide concern. In China, 39.76 million tons of industrial
31
hazardous waste were generated in 2015 according to the 2016 Environmental Statistics
32
Annual Report by the Chinese Environmental Protection Department. A lack of sufficient
33
disposal capabilities have made it challenging to compensate for ever-increasing amounts
34
of hazardous waste.1
ACS Paragon Plus Environment
Page 2 of 37
Page 3 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
35
Tannery sludge (TS), which is generated from tanning processes, contains a high
36
content of Cr2 and is classified under code HW21-193-001-21 in China’s Hazardous
37
Waste List. Only 60-70% of chromium salts used in the tanning process react with skins;
38
the remaining 30-40% are disposed in the tanning exhaust bath and end up as TS.3 Cr
39
toxicity is significantly influenced by its oxidation state. Hexavalent chromium (Cr(VI))
40
is known to have 100-fold higher toxicity than trivalent chromium (Cr(III))4. Therefore,
41
Cr speciation in waste streams is of special concern.
42
The co-combustion of hazardous wastes in fluidized bed combustion (FBC) systems is
43
a potential technology for effective disposal of hazardous wastes. Co-combustion in FBC
44
systems has been widely applied to many kinds of wastes, such as biomass, municipal
45
solid waste (MSW), refuse-derived fuel (RDF) and sewage sludge, and extensively
46
characterized based on combustion, emission, ash deposition and trace element
47
transformation.5-10 However, few studies have investigated the co-combustion of
48
hazardous wastes in FBC systems.11,
49
results.
12
Thus, the need remains for more meaningful
50
In this work, TS was co-combusted in a bench-scale FBC system for the first time.
51
Fifteen tests were conducted under different operational conditions, including two modes
52
of fluidization, namely, bubbling fluidized bed (BFB) combustion and circulating
53
fluidized bed (CFB) combustion, two feedstock types, coal mono-combustion and
54
co-combustion of coal and TS. Other operational parameters, such as the primary and
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
55
secondary air flow rates and feeding rate, were also adjusted to determine the optimal
56
combustion conditions. The emission characteristics and Cr distribution and speciation
57
were investigated in this study.
58
Experimental procedures
59
Materials
60
TS was obtained from a tannery in Haining. The TS was predried at 105°C for more
61
than 24 h and subsequently milled in a ball miller. Proximate analysis, calorific value
62
analysis and ultimate analysis were performed for TS and coal (Table 1). The proximate
63
analysis was conducted using the Coal Industry Analysis Method (GB/T 212-2008). The
64
lower heating value was quantified by the Chinese Standard Method of Determination of
65
Calorific Value of Coal (GB/T 213-2008). The ultimate analysis was performed using the
66
elemental analyzer 1ECO-CMNS932. The elemental contents of TS were quantified
67
using X-ray fluorescence analysis (Table 2) with an ARL ADVADT’XIntelli Power
68
TM4200 instrument.
69
Procedures
70
Experiments were conducted on a 35 kWth FBC test rig;13 the system scheme is shown
71
in Figure 1. Tests were switched between BFB and CFB by controlling valves after the
ACS Paragon Plus Environment
Page 4 of 37
Page 5 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
72
furnace, as shown in Figure 1. The preparation steps performed before conducting
73
experiments, including cold fluidization and ignition tests, have been extensively
74
described elsewhere.13 Feeding coal was sieved under 1 cm. Dried TS was ground to a
75
mean diameter of less than 1 mm. For co-combustion tests, the percentage of TS in the
76
samples was 10% by weight which referring to co-combustion of sludges of which
77
caloric value are close to TS.8, 14 Because there was no heating surface in the bed area, the
78
feeding rate of coal must be limited during BFB tests to control bed temperature. For this
79
purpose, and due to limitation of capacity of feeder, we added 50%wt quartz sand in coal
80
during BFB tests. For each test, the emissions of gaseous pollutants were monitored at the
81
stack. The concentrations of gaseous products (CO and NOx) and moisture were
82
measured using the FTIR spectrometer of a gas analyzer GASMET FTIR Dx4000 (Temet
83
Instrument Oy, Finland). Ash samples were taken at four different places (marked in
84
Figure 1):
85
a. bottom ash (BA) from the combustor bed;
86
b. heat exchanger ash (HA) from the heat exchanger;
87
c. fabric filter ash (FA) from the baghouse; and
88
d. quartz glass fiber ash (QA) from the stack.
89
The concentrations of total Cr in the ash samples were measured using inductively
90
coupled plasma-atomic emission spectrometry (ICP-AES) after microwave digestion of
91
each sample in hydrochloric acid, nitric acid and hydrochloric acid. Cr(VI) in the ash
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
92
samples was extracted using U.S. Environmental Protection Agency Method 3060A and
93
quantified using the diphenylcarbazine method. Fifteen tests were conducted in this
94
study; the conditions of these tests are listed in Table 3. Eleven of these tests involved the
95
mono-combustion of coal and were labeled C1 through C11; the four other tests involved
96
the co-combustion of coal and TS and were labeled S1 through S4.
97 98
The actual excess air ratio was calculated using the measured levels of O2 and CO in the exhaust gas according to the following equation: Excess air ratio=
99
21 21 − ( − 0.5 )
This equation is based on a study by Basu et al.15 H2 and CH4 gases had sufficiently
100
low concentrations to be considered negligible.
101
Results and discussion
102
Temperature distribution
103
Figure 2 shows that for the mono-combustion of coal in BFB, i.e., experiments C1-7,
104
the temperatures above the distributor generally decreased as the excess air ratio
105
increased. This effect occurred because the excess air had a cooling effect on the
106
combustor. Tests C4-7 with excess air ratios ranging from 2.15-2.77 achieved more
107
uniform temperature profiles than the other three tests, C1-3, because the fluidization
108
velocity increased, and the combustion zone moved toward the top of the freeboard.16 For
ACS Paragon Plus Environment
Page 6 of 37
Page 7 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
109
the co-combustion tests, the temperatures decreased within the same excess air ratio
110
range because of the relatively low caloric value of TS. In addition, because the
111
combustibles of TS consisted predominantly of volatiles and the amount of fixed carbon
112
was negligible, the decreased temperatures were observed primarily within denser bed
113
regions. For the CFB tests (Figure 3), the temperature distributions were relatively
114
uniform because the particles were transported predominantly along the combustor.
115
Gaseous emissions
116
As an incomplete combustion product, the level of CO emissions reflects the
117
combustion efficiency. Generally, a decreased CO emission indicates better combustion.
118
The CO emissions of all BFB tests are shown in Figure 4a and Figure 5a, including data
119
for tests C1-3 conducted at low fuel-feeding rates and low primary flow rates; tests C4-7
120
conducted at high fuel-feeding rates and high primary flow rates; test 7 with secondary
121
flow; and tests S1 and S2 with TS mixed in the feedstock. Enhanced mixing, sufficient
122
residence time and high temperature were observed to be the dominant factors affecting
123
complete combustion. As shown in Figure 4a for tests C1-3, the amount of CO emission
124
decreased from 1852 mg/m3 to 1096 mg/m3 as the primary air flow rate increased from
125
70 L/h to 78 L/h. However, when the primary air flow increased to 89 L/h, CO emissions
126
increased to 1247 mg/m3. Because the excess air ratio positively correlated with the
127
primary air supply when the feeding rate was constant, the change in CO emission with
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
128
PA in Figure 5a was consistent with that for the excess air ratio for tests C1-3. The
129
highest emission occurred at the lowest fluidizing air flow (70 L/h). This effect was due
130
to poor fluidization at low fluidizing air velocity, resulting in poor mixing conditions.
131
Defluidization has been reported to occur when the PA flow rate decreases to 67 L/h in
132
present FB combustor.13 The decreased CO emission levels when the PA reached 78 L/h
133
suggested that better fluidization and mixing conditions were achieved and that
134
combustion was optimized. Subsequently, as the PA flow rate increased to 89 L/h, the
135
amount of CO produced moderately increased. As mentioned in the discussion on
136
temperature distribution, the temperature decreased with increasing excess air ratio
137
cooling the combustor. Thus, as a result of the cooling effect of the excess air flow, the
138
combustion reaction rate decreased. Moreover, with a higher PA flow rate, the residence
139
time of combustibles in the combustor was shortened because the air superficial velocity
140
was higher. Due to these low temperatures and short residence times, fewer CO
141
molecules were converted to CO2, and more CO was emitted under higher PA flow rates
142
in test C3. Similar phenomena have also been reported by Chen13, Madhiyanon et al.17
143
and Varol et al.18
144
For tests C4-6, as previously discussed, the temperature increased along the combustor
145
due to the high fuel-feeding rate and high PA flow rate. A previous analysis indicated that
146
high temperatures accelerate the oxidation of CO, whereas a high PA flow rate decreases
147
the residence time of CO and other incomplete combustion products. As shown in Figure
ACS Paragon Plus Environment
Page 8 of 37
Page 9 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
148
4a and Figure 5a, the levels of CO emissions for tests C4-6 were generally lower than
149
those for tests C1-3, which indicated that temperature, rather than residence time, has a
150
dominant impact on the conversion of CO to CO2. Unlike tests C1-3, the feeding rates of
151
tests C4-6 were not kept constant at 14, 14 and 11 kg/h. Thus, the excess air ratios of tests
152
C4-6 were not proportional to their PA flow rates, and the trends of CO emissions under
153
different excess air ratios were different from that under the corresponding PA flow rates.
154
As shown in Figure 5a, the changes in CO emissions with excess air ratios of tests C4-6
155
followed similar changes as those of tests C1-3. At a minimal excess air ratio of 2.20, the
156
lowest PA was observed, and the resultant fluidizing velocity was the lowest, which is
157
not favorable for efficient fluidization and mixing. Poor mixing can be taken as the
158
reason why the CO emission of test C4 was the highest among tests C4-6. Furthermore,
159
because the mean diameter of the fuel particles was larger than that of the bed material, a
160
higher critical fluidization velocity would be required with more fuel fed per unit time.
161
Thus, the mixing condition was not readily improved, but the PA was maintained at a
162
relatively high level, i.e., 91-99 L/h, during tests C4-6. This result can be observed by
163
comparing test C4 and test C3. The PA flow rates of these two tests were nearly the same,
164
at 91 and 89 L/h. The temperature of test C4 was higher than that of test C3, whereas the
165
CO emission of test C4, which was 1242 mg/m3, was not significantly less than that of
166
test C3, which was 1247 mg/m3, implying that the fluidization of test C4 was worse than
167
that of test C3. At an excess air ratio of 2.45 in C5, a higher PA, 99 L/h, was introduced
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
168
in the combustor, and fluidization and mixing were consequently improved. As a result,
169
the CO emission was notably reduced to 621 mg/m3. At a higher excess air ratio of 2.77
170
in test C6, the combustor was cooled by the excess air, and the CO emission increased to
171
805 mg/m3. The calculated excess air ratio of test C5 with the highest air supply and
172
lowest fuel fed was not the largest among tests C4-6 as expected. This result may have
173
been caused by enhanced mixing, which promotes complete combustion. More directly,
174
the CO emission decreased as the PA increased for tests C4-6, as shown in Figure 4a,
175
which verified that the mixing conditions are dependent on the PA flow and play a major
176
role in combustion during tests C4-6.
177
Test C7 was the only BFB experiment in which the secondary air was added. Relative
178
to test C4, which had the same PA flow rate and feeding rate, we observed reductions in
179
the CO emissions and excess air ratio. Secondary air is known to enhance turbulence and
180
mixing in the freeboard and to promote the combustion of incomplete combustion
181
products.19 The results also correlate with the relatively high freeboard temperature of test
182
C7 shown in Figure 2. Tests S1 and S2 involved the co-combustion experiments of TS.
183
The various excess air ratios used in tests S1 and S2 were comparable with those of tests
184
C4-6. However, more CO was emitted in the co-combustion experiments, as shown in
185
Figure 5a. The air flow rates of tests S1 and S2 were less than those of tests C4-6,
186
whereas the feeding rates were nearly the same. Thus, the mixing conditions of tests S1
187
and S2 were not as optimal as those of tests C4-6, resulting in more CO being emitted.
ACS Paragon Plus Environment
Page 10 of 37
Page 11 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
188
Because of the relatively low temperature of FBC (below 1000°C), NOx was mostly
189
generated from organically bound nitrogen in the fuel and is termed fuel-N.20 The
190
conversion scheme of fuel-N to NOx is shown in Figure 6.21 The first step is fuel-N
191
devolatilization. The original nitrogen bound in the fuel thermally decomposes into
192
volatiles (termed ash volatile-N), including tar and light gaseous species, such as HCN
193
and NH3. The remaining nitrogen is retained in the char (termed char-N). Additionally, as
194
the temperature increases, the further cracking of tar and char results in more light species
195
released, including soot from tar. The subsequent step after devolatilization is the
196
oxidation of volatile-N and char-N. For volatile-N, oxidation eventually occurs through
197
the reactions of two light gaseous species, as shown in reactions 1 and 2.22
198
CN (1)
199
N (2)
200
,
,
,
,
,
As for char-N, the overall oxidation mechanism is similar to reaction 3, although the
201
details remain to be clarified. 21
202
( ) + → NO + ()(3)
203
where C(N) and C(O) denote species bound on the char surface containing nitrogen and
204
oxygen, respectively. Note that the NO formed in char surface may also be reabsorbed
205
and then reduced to N2. The reactions are shown in reactions 4 and 5.21 Cf denotes a free
206
carbon site.
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
207
2 + → C(N) + ()(4)
208
( ) + → + ()(5)
209
As a result, the conversion of char-N to NOx is relatively limited by the reduction of
210
NO on char. Overall, NOx formation may occur in bulk flow in the particle boundary
211
layer or on the internal or external particle surface.
212
Figure 4b and Figure 5b demonstrate NOx emissions under different PA rates and
213
excess air ratios, respectively. As shown in Figures 5a and b, NOx increases with the PA
214
rate and excess air ratio from 264 mg/m3 to 341 mg/m3 for tests C1-3. As the excess air
215
ratio increases, the oxygen concentration in the flue gas increases, facilitating the
216
generation of NOx from fuel-N.17, 18 Moreover, the higher PA rate enhances the mixing of
217
the high-temperature dense bed, which was the main area for NO formation. Better
218
mixing conditions of these dense beds ensures that oxygen can contact fuel-N species
219
more sufficiently and provide more favorable kinetic conditions for NO formation.
220
Similar changes with the excess air ratio are also observed in tests C4-6 and tests S1 and
221
2. However, the NOx release in tests C4-6 does not increase with the PA rates as observed
222
in tests C1-3, indicating that the air stoichiometry in the excess air ratio is a major factor
223
rather than the mixing condition and temperature factors in NO formation.
224
A comparison of tests C1-3 and tests C4-6 shows that the emission levels of NOx in the
225
former three tests are all below those of the latter. Because the temperatures of tests C4-6
226
show pronounced increases along the entire combustor relative to tests C1-3, the results
ACS Paragon Plus Environment
Page 12 of 37
Page 13 of 37
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
227
can be interpreted in terms of volatile-N evolution and oxidation. Conceivably, the
228
cracking of tar and char are facilitated under high temperatures. Experiments have
229
verified that the thermal cracking of tar at temperatures below 1373°C yields only small
230
amounts of light gaseous nitrogen.23 The concentrations of free oxygen, hydroxyl and
231
hydrogen radicals increase at high temperatures, leading to increased gas-N species
232
oxidation via reactions 1 and 2.22 However, the remarkable increase of NOx from tests
233
C1-3 to tests C4-6 can be attributed to competition between NOx and N2O formation.
234
Glarborg et al.21 found that increases in NO yields are correlated with similar-magnitude
235
decreases in N2O yields. There are two major routes for N2O formation. In a
236
homogeneous mechanism, fuel-N cracks to form the gas-phase product HCN.
237
Subsequently, HCN is oxidized by oxygen and hydroxyl radicals to NCO, forming N2O
238
with NO via reaction 224. In fact, HCN is the main precursor of N2O.
239
+ → + (6)
240
In a heterogeneous mechanism, reactions 7 and 8 occur on the char surface. C(NCO) is
241
an oxidized char nitrogen species.25
242
C(N) + → ( )(7)
243
C(NCO) + NO → + + (8)
244
The formation of N2O is significantly influenced by oxygen content and temperature.
245
First, a conversion of HCN to NH3 occurs at low oxygen concentrations,26 i.e., higher
ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
246
oxygen concentrations promote the formation of HCN to act as a precursor to form N2O.
247
Second, N2O is sensitive to high temperatures. When the reaction temperature is over
248
900°C, N2O decomposes to N2.20 Thus, N2O formation can be inhibited by high
249
temperatures.21 As a result, N2O formation decreases as the temperature increases, as has
250
been reported in several studies.27, 28 FBC has a lower temperature than grate furnaces
251
and pulverized coal fired boilers; therefore, N2O is typically formed at higher rates under
252
fluidized bed conditions. Typical N2O concentrations from CFBs and pulverized coal
253
fired boilers are 74 mg/Nm3 and
