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Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars
Accepted Manuscript Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars
Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin PII:
S0360-5442(18)30603-0
DOI:
10.1016/j.energy.2018.04.013
Reference:
EGY 12652
To appear in:
Energy
Received Date:
02 December 2017
Revised Date:
17 March 2018
Accepted Date:
03 April 2018
Please cite this article as: Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin, Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars, Energy (2018), doi: 10.1016/j.energy.2018.04.013
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ACCEPTED MANUSCRIPT 1 2
Catalytic effect of oil shale ash on CO2 gasification of
3
leached wheat straw and reed chars
4 5
Siim Link*
6
Department of Electrical Power Engineering and Mechatronics, School of Engineering,
7
Tallinn University of Technology
8
Ehitajate tee 5, 19086, Tallinn, Estonia
9
E-mail: [email protected]
10
*corresponding author
11 12
Khanh-Quang Tran
13
Department of Energy and Process Engineering, Norwegian University of Science and
14
Technology
15
NO-7491 Trondheim, Norway
16 17
Quang-Vu Bach
18
Department of Energy and Process Engineering, Norwegian University of Science and
19
Technology
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NO–7491 Trondheim, Norway
21 1
ACCEPTED MANUSCRIPT 22
Patrik Yrjas
23
Johan Gadolin Process Chemistry Centre, Åbo Akademi University
24
Piispankatu 8, Turku, FI-20500, Finland
25 26
Daniel Lindberg
27
Johan Gadolin Process Chemistry Centre, Åbo Akademi University
28
Piispankatu 8, Turku, FI-20500, Finland
29 30
Stelios Arvelakis
31
Bioresource Technology Unit, Laboratory of Organic and Environment Technologies,
32
Department of Chemical Engineering, National Technical University of Athens
33
Zografou Campus, GR-15700, Athens, Greece
34 35
Argo Rosin
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Department of Electrical Power Engineering and Mechatronics, School of Engineering,
37
Tallinn University of Technology
38
Ehitajate tee 5, 19086, Tallinn, Estonia
39 40 41
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1 Abstract
43
Oil shale ash is a material that could be used as a low cost catalyst for biomass gasification
44
processes. This article analyses the catalytic effect of oil shale ash on CO2 gasification of reed
45
and leached wheat straw chars. A thermogravimetric analyser operated at atmospheric pressure
46
and two different partial pressures of CO2 such as 0.09 atm and 1 atm were applied. The results
47
indicate that oil shale ash has positive effect on char reactivity at the CO2 partial pressure of
48
0.09 atm: the addition of oil shale ash by 30% to reed char lowered the reaction time by 1.3
49
times and to leached wheat straw char by 1.4 times. At the partial pressure of CO2 1 atm, the
50
addition of oil shale ash resulted in the negative effect on the char conversion due to the binding
51
of CO2. The average calculated CO2 binding capacity of oil shale ash was 13.7 mg CO2 per 100
52
mg of oil shale ash.
53 54
Keywords: biomass, oil shale ash, char, gasification, catalyst, reactivity
55 56
2 Introduction
57
Power generation in Estonia is based mainly on oil shale fuel [1], which is characterized by
58
very high mineral matter contents (60-75%) [2]. As a consequence, 5-7 million tonnes of oil
59
shale ash are produced and sent to the landfills in Estonia annually. Only a small percentage of
60
the ash produced finds its way to secondary uses, either as a stabilizing agent of roadbeds in
61
construction, or soil conditioner in agriculture [3]. New methods of reusing or recycling of the
62
ash would help reduce the burden to the landfills and contribute to the use of oil shale for
63
sustainable heat and power generation in Estonia.
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Oil shale ashes contain various Ca compounds [4], of which many exhibit their catalytic effect,
65
enhancing the char gasification reactivity [5]. Calcium and several calcium containing
66
compounds (Ca(OH)2, Ca(Ac)2, CaCO3, CaC2O4, Ca(NO3)2) have been studied as catalysts to
67
improve the gasification reactivity of carbon containing materials [6-10]. H. Risnes et al. [6]
68
have reported that the effect of calcium addition as calcium sugar/molasses solutions on straw
69
significantly affects the ash chemistry and the ash sintering tendency but much less the char
70
reactivity. Perander et al. [7] found that the gasification rate of the char increases linearly with
71
an increase in the concentration of Ca and the catalytic activity of Ca is higher than K at the
72
beginning of char gasification but the catalytic effect of Ca decreases earlier than the catalytic
73
effect of potassium. Suzuki et al. [8] studied the different loading levels of Ca and found that
74
the gasification rate increases with the loading level. However, the gasification rate decreased
75
rapidly after the char conversion degree of 50%. Struis et al. [9] showed that after char
76
conversion degree of 20%, the reaction rate course of Ca(NO3)2 impregnated charcoal with
77
progressing gasification levels down to that of pure char without additive.
78
A number of low cost materials such as limestone and/or dolomite have been tested as catalysts
79
for biomass gasification. However, most of them have focused on improving the gas
80
composition and tar reduction [11-13] rather than on the gasification reactivity of biomass chars
81
[5]. Moreover, oil shale ash has not been tested as catalyst for biomass gasification. Additives
82
used in the biomass gasification process require detailed knowledge about their impact on the
83
process and the catalytic activity is strongly influenced by the chemistry of mineral matters of
84
the fuel [6]. Therefore, the study reported in this work was carried out, looking at the ability of
85
OSA improving catalytically the gasification of biomass chars. For this purpose, it is reasonable
86
to choose biomass materials with low reactivity as feedstock; otherwise it would be difficult to
87
observe the catalytic effect of OSA. From our previous studies [14-16], both reed and leached
88
wheat straw chars exhibited lower gasification reactivity compared to chars derived from wood
4
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under the conditions investigated. Moreover, reed and wheat straw differ by the content and
90
composition of mineral matter (ash). The choice of biomass materials for the study helps better
91
estimate the use of OSA for feedstocks with diverged ash content and composition. The
92
objective of this study is to determine the effect of OSA on the CO2 gasification reactivity of
93
the selected biomass (common reed and leached wheat straw) chars.
94 95
3 Materials and Methods
96
3.1 Materials
97
The biomass samples selected for the investigation were the following: pelagian reed,
98
originating from the west coast shorelines of Estonia and the islands of Estonia and wheat straw
99
from the area of Thessaly in Greece. The wheat straw sample was pre-treated using the leaching
100
method developed by Arvelakis et al. [17]. The oil shale ash was collected from the heat
101
exchanger of circulating ash (called INTREXTM according to the Foster Wheeler, the supplier
102
of the boiler) of a circulating fluidized bed boiler in the Eesti power plant (one of the Narva
103
power plants of the national energy company Eesti Energia AS).
104
The reed and leached wheat straw (hereafter referred as R and LWS, respectively) were
105
analyzed and characterized regarding to proximate and ultimate analyses, and their gross
106
calorific value, as well as ash chemical analysis in accordance with the American Society for
107
Testing and Materials (ASTM) standard methods, including D 1102-84, D 3175-89a, D 5142-
108
90, D5373-93, D 4208-88, D 2015-95.
109
The proximate analysis of reed char and leached wheat straw char samples (hereafter referred
110
to as RC and LWSC, respectively) was performed by means of a TA Instruments SDT Q600
111
thermogravimetric analyser (TGA). First, the sample was heated at a rate of 20 °C min-1 from
5
ACCEPTED MANUSCRIPT 112
ambient temperature to 105 °C and kept for 10 min in nitrogen gas flow of 90 mL min-1 to
113
determine the moisture content. Then, the temperature was increased, with a heating rate of 50
114
°C min-1, up to 900 °C, at which it was kept for 7 min to determine the volatile content of the
115
sample. Thereafter the temperature was lowered to 600 °C in 20 min, followed by supplying air
116
at the flow rate of 100 mL min-1 to initiate burning for determining the ash content [18].
117
The oil shale ash (hereafter referred to as OSA) fraction with particles smaller than 125 µm in
118
size was selected and characterized regarding the ash chemistry and mineralogy analyses. The
119
ash chemistry analysis was performed in accordance with various standard methods including
120
EVS-EN 196-21, DIN 51729, STI-1-20015, STI-2-20015, and ISO 334. The mineralogy
121
analysis was carried out by means of a Rigaku Ultima IV diffractometer using the X-ray powder
122
diffraction (XRD) method with Cu Kα radiation (λ = 1.5406 Å, 40 kV at 40 mA) and a silicon
123
strip detector D/teX Ultra.
124 125
3.2 Leaching
126
The leaching pre-treatment of the wheat straw was performed using tap water according to the
127
method developed by Arvelakis et al. [17]. During the leaching process, the sample was put in
128
a 200-mesh plastic grid, tied up and submerged into tap water in a plastic 75 l volume barrel.
129
The used mass/water ratio was 66.6 g/L and the retention time was 12 h. At the end of the
130
process, the sample was allowed to dry naturally in air until reaching a constant weight under
131
the sun of the Mediterranean summer within a period of 5 days. During the drying process, the
132
bed thickness of the drying sample was kept below 15 cm to guarantee the fast-drying process
133
without material loss due to possible mould bacterial activity.
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3.3 Char preparation by pyrolysis
135
A cylindrical atmospheric fixed-bed reactor with an electrical heater was used to prepare char
136
from the selected biomass samples. A quartz pipe inside the reactor protects the heating wire
137
against corrosive compounds possibly present in the evolved gas mixture during the pyrolysis.
138
The sample is loaded into the sample holder made of a stainless-steel SS316 bound net with 80
139
μm openings and placed in the quartz pipe. The applied heating procedure was the following:
140
(i) hold the sample at 50 °C for 45 min, (ii) heat the sample to the end temperature of 800 °C at
141
a heating rate of 10 °C min-1, (iii) hold the sample at the end temperature for 15 min, and (iv)
142
cool down. Nitrogen was used as a carrier gas with a flow rate of 1 L min-1 in all pyrolysis
143
experiments. The methodology together with a detailed description of the system is reported in
144
[19].
145
3.4 Test method for CO2 gasification reactivity of char
146
A thermogravimetric analyser (TGA) of TA Instruments SDT Q600 was used for this test. Two
147
partial pressures of CO2, 0.09 and 1.0 atm, were employed for the test at atmospheric pressure,
148
considering possible interferences of CO2 chemistry during the process on the catalytic effect
149
of OSA. The char reactivity was tested at a CO2 partial pressure of 0.09 atm with the following
150
procedure. The sample was heated up from room temperature to 850 °C at a heating rate of 50
151
°C/min in a nitrogen flow rate of 100 mL min-1. When the temperature (850 °C) had been
152
established and kept constant, the gas composition was changed to include CO2 to ensure the
153
flow rate of N2 and CO2 at 91 and 9 mL min-1, respectively. After that, the TGA reactor was
154
held under these conditions until no mass change was observed. Similarly, the char reactivity
155
was tested at a CO2 partial pressure of 1 atm with the following procedure. The sample was
156
heated up from room temperature to 850 °C, at a heating rate of 50 °C/min in a nitrogen flow
157
rate of 100 mL min-1. When the temperature (850 °C) had been established and kept constant,
7
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the gas was changed to include only CO2 with a flow rate of 100 mL min-1. Then, the TGA
159
reactor was held under these conditions until no mass change was observed.
160
The gasification reactivity is expressed as the char gasification rate over a degree of char
161
conversion. The char gasification rate in units of min-1 at any particular conversion value is
162
defined as shown in Eq. (1).
163
r=dX/dt
164
where r is the reaction rate, and X is expressed as (Eq. (2)):
165
X=(Mo-M(t))/(Mo-Mf).
166
Mo represents the initial mass of the char, M(t) is the mass of the char at time t, and Mf is the
167
mass of the gasification residue.
168
3.5 Thermodynamic calculations
169
In our study, the thermodynamic equilibrium calculations were performed to determine the
170
composition of mineral matter under gasification conditions. For calculations, FactSage 7.1
171
software package was used [20]. The following thermodynamic databases were used for the
172
calculations: FToxid, which includes data for solid oxides and silicates and liquid slag; FTSalt
173
for liquid alkali salts (NaCl-Na2CO3-Na2SO4-KCl-K2CO3-K2SO4) and corresponding alkali salt
174
solid solutions; FactPS for the gas phase and other stoichiometric solid phases. The input for
175
the calculations was the chemical composition of the fuel ashes in Table 2, the temperature was
176
set to 850 °C and the partial pressure of CO2 was set to be 0.09 atm or 1.0 atm.
(1)
(2)
177
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4 Results and discussion
179
4.1 Material characterization
180
Reed (R) and leached wheat straw (LWS) differ by their composition (as seen in Tables 1 and
181
2). The ash content of R and LWS is 3.2% and 5.8% respectively, as dry basis. The main
182
component of reed ash is SiO2. Besides SiO2, the ash of LWS contains also 15% of CaO.
183
Table 1
184
Fuel characterization of the selected biomass samples.
Proximate analysis, dry
Ultimate analysis, dry basis (wt%)
Gross
basis (wt%)
Moisture
calorific
Material (wt%)
value,
Volatile
Fixed
Ash
matter
carbon
N
C
H
S
Cl
O
(MJ/kg)
R
5.4
3.2
80.3
16.5
0.4
47.4
5.7
0.2
n.d.
43.1
20.41
LWS
5.8
5.8
80.7
13.5
0.6
46.3
5.3
0.2
0.1
41.7
20.03
185 186
Table 2
187
Chemical analysis of the feedstock ashes (wt%).
Material
K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 TiO2 SO3
P2O5 Cl
Reed
5.9
8.4
2.9
1.4
73.7
n.d.
1.1
0.1
5.2
1.0
0.6
LWS
4.0
1.9
15.3 2.5
49.9
1.9
0.6
0.1
3.8
n.d.
1.8
OSA
1.0
0.2
43.9 9.6
17.6
5.3
1.5
n.d.
15.3 n.d.
0.4
188 189
On the other hand, the char yield (dry basis) after pyrolysis was 19.4% for reed sample and
190
27.3% for leached wheat straw sample. After pyrolysis, the content of fixed carbon and ash
9
ACCEPTED MANUSCRIPT 191
increases, but the content of volatile matter decreases compared to parent fuel samples (see also
192
Tables 1 and 3).
193
Table 3
194
Char characterization of the selected biomass samples.
Moisture
Proximate analysis, dry basis (wt%)
Material (wt%)
Ash
Volatile matter
Fixed carbon
RC
1.2
17.7
15.0
67.3
LWSC
2.1
34.6
14.7
50.7
195 196
Results from the XRD analysis of oil shale ash (OSA) are presented in Table 4, which shows
197
that the content of CaO is 2.9% and that of Ca(OH)2 is 18.0%. Kuusik et al. [21] reported 19.9%
198
CaO and 2.1% Ca(OH)2 for the ash collected from the heat exchanger of the circulating ash
199
(INTREX ash) of the fluidized bed boiler. Bityukova et al. [4] have studied also the composition
200
of oil shale ashes. They found that the INTREX ash contains 9.9% CaO and 12.3% Ca(OH)2.
201
The differences between the content of CaO and Ca(OH)2 can be attributed to the chemistry of
202
CaO in the presence of moisture, according to Eq. (3), forming thermally more stable product
203
of calcium hydroxide (portlandite) [4].
204
CaO + H2O = Ca(OH)2 + heat
205
Table 4
206
Mineral composition of the oil shale ash (wt%).
(3)
SiO2 CaO Ca(OH)2 CaSO4 MgO Ca2SiO4 Ca2Mg(Si2O7) CaSiO3 KAlSi3O8 CaCO3 12.0
2.9
18.0
41.0
4.4
5.0
207
10
5.0
5.1
2.6
4.0
ACCEPTED MANUSCRIPT 208
4.2 Thermogravimetric analysis of pure oil shale ash
209
First of all, the behavior of the oil shale ash was thermogravimetrically analyzed under different
210
CO2 partial pressures, as described above. The partial pressure of 0.09 atm characterizes the
211
partial pressure of CO2 in product gas under atmospheric air blown gasification condition [22],
212
and the partial pressure of 1 atm could be considered as the partial pressure of CO2 under
213
pressurized gasification conditions depending on the total pressure of product gas [23-25]. The
214
results of these analyses are shown in Fig. 1.
98
700
96
600
94
500 Phase I: 100% N2
92
400
Phase II: 100% CO2
90
300
Phase II: 9% CO2 + 91% N2
200
Temperature
3000
2800
2600
2400
2200
2000
1600
1400
1200
1000
800
0
600
84
400
100
200
86
0
Temperature, oC
800
88
215
900
100
1800
Mass loss, M/Mo (%)
Phase II
Phase I
102
Time, s
216
Fig. 1. Thermogravimetric analysis of the oil shale ash used for the test.
217
During Phase I, three steps of mass loss could be observed. The first step up to 180 °C is related
218
to the removal of moisture. The second step of mass loss between the temperatures of 375 –
219
450 °C is related to the pyrolysis of unburnt hydrocarbon entrapped in the ash. The third mass
220
loss between 600 – 850 °C is attributed to the decomposition of carbonates [26].
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It is seen that at the CO2 partial pressure of 0.09 atm, there is no increase in mass in Phase II.
222
This indicates that CaO does not react with CO2 under the conditions of our investigation. At
223
the CO2 partial pressure of 1 atm, CaO reacts with CO2 and the mass of ash sample was
224
increased. The average calculated CO2 binding capacity of oil shale ash is 13.7 mg CO2 per 100
225
mg of oil shale ash sample, which is in agreement with previous studies [27].
226
Based on the composition of oil shale ash in Table 2, the equilibrium calculations at 850 °C
227
under different partial pressures of CO2 were performed as well, as shown in Table 5.
228
Table 5
229
Equilibrium calculation of oil shale ash (wt%).
PCO2 (atm) 0.09 1.00
MgO 2.9 2.8
CaO 3.9 0.0
Mineral composition Ca3MgAl4O10 Ca3MgSi2O8 Ca2Fe2O5 11.6 52.0 2.8 11.2 50.5 2.7
CaCO3 0.0 6.7
CaSO4 26.9 26.1
230
The thermodynamic calculations support the experimental data that CO2 is consumed by CaO
231
at the elevated partial pressure of CO2.
232
The observed behavior of oil shale ash under different partial pressures of CO2 is in agreement
233
with the chemical reaction between CaO and CO2 described by Eq. (4), considering the effect
234
of CO2 partial pressure on the equilibrium [28,29]. Indeed, the forward reaction, which reduces
235
the partial pressure of CO2, is favored when the applied partial pressure of the CO2 is increased,
236
considering Le Chatelier’s principle.
237
CaO + CO2 ↔ CaCO3
(4)
238 239
4.3 Char gasification reactivity and the catalytic effect of oil shale ash
240
Figs. 2 and 3 show the data of reaction rates plotted versus the conversion degree for the selected
241
char samples without and with OSA addition, under the CO2 partial pressure condition of 1 atm.
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ACCEPTED MANUSCRIPT 242
Fig. 2 shows the reed sample and Fig. 3 the leached wheat straw. The char conversion degree
243
plotted versus the time of the gasification tests is illustrated in Fig. 4 (reed) and Fig. 5 5 (LWS).
244
As can be seen from Figs. 2 and 3, the LWS char is more reactive than the reed char under
245
identical gasification conditions. The reaction rates increase from the start to the points where
246
the char conversion degree is about 20-30%, and then decline. With the addition of OSA, the
247
gasification reactivity of both char samples decreases. The more OSA is added, the lower the
248
reactivity is. In addition, the rate curves for the reed char (Fig. 2) exhibit double peaks, but it is
249
not the case with the LWS char (Fig. 3). 4.5 Reed char
Reaction rate, %/min
4.0
R/10%_OSA
3.5
R/20%_OSA
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
250 251
10
20
30
40
50
60
70
Char conversion degree, % Fig. 2. Reaction rate of reed char, pCO2=1 atm.
252
13
80
90
100
ACCEPTED MANUSCRIPT 4.5
Reaction rate, %/min
4.0 3.5 3.0 2.5 2.0 1.5 LWS char
1.0
LWS/10%_OSA
0.5
LWS/20%_OSA
0.0 0
10
20
40
50
60
70
80
90
100
Char conversion degree, %
253 254
30
Fig. 3. Reaction rate of leached wheat straw char, pCO2=1 atm.
100
Char conversion degree, %
90 80 70 60 50 40 Reed char
30
R/10%_OSA
20
R/20%_OSA
10 0 0
255 256
1500
3000
4500
6000
Time, s Fig. 4. Conversion degree versus time of reed char, pCO2=1 atm.
257
14
7500
9000
ACCEPTED MANUSCRIPT
Char conversion degree, %
100 90 80 70 60 50 40 30
LWS char
20
LWS/10%_OSA
10
LWS/20%_OSA
0 0
258
1000
2000
3000
4000
5000
6000
Time, s
259
Fig. 5. Conversion degree versus time of leached wheat straw char, pCO2=1 atm.
260
The reaction rates versus the char conversion degree of the char samples under the CO2 partial
261
pressure condition of 0.09 atm are shown in Figs. 6 and 7 for the reed and LWS char sample,
262
respectively. The char conversion degrees plotted versus the time of the gasification tests are
263
shown in Figs. 8 and 9. As can see from Figs. 6 and 7, the rate curves for the CO2 gasification
264
of reed and LWS without OSA additions are similar, having one peak only. However, when
265
OSA is added, the peak disappears in all cases. In the case of reed char, OSA additions give
266
rise to increased reaction rate until the conversion degree reaches 15-20% approximately.
267
Within this section of the curves, the more OSA is added, the higher the reaction rate is. In the
268
second section, within 15-35% as char conversion degree, OSA additions seem to have negative
269
effect on the reaction rate. However, the differences between the rate curves are very small. The
270
effects turn to be positive again for the last section from 35% to 100% with pronounced
271
differences and the order of the curves is the same as in the first section.
272
15
ACCEPTED MANUSCRIPT 1.0
Reaction rate, %/min
R/30%_OSA 0.8
R/20%_OSA R/10%_OSA
0.6
Reed char
0.4
0.2
0.0 0
10
30
40
50
60
70
80
90
100
Char conversion degree, %
273 274
20
Fig. 6. Reaction rate of reed char, pCO2=0,09 atm.
275
Reaction rate, %/min
1.4 LWS/30%_OSA
1.2
LWS/20%_OSA LWS/10%_OSA
1.0
LWS char
0.8 0.6 0.4 0.2 0.0 0
276 277
10
20
30
40
50
60
70
Char conversion degree, % Fig. 7. Reaction rate of leached wheat straw char, pCO2=0,09 atm.
278
16
80
90
100
ACCEPTED MANUSCRIPT 100
Char conversion degree, %
90 80 70 60 50
R/30%_OSA
40
R/20%_OSA
30
R/10%_OSA
20
Reed char
10 0 0
10000
30000
Time, s
279 280
20000
Fig. 8. Conversion degree versus time of reed char, pCO2=0,09 atm.
100
Char conversion degree, %
90 80 70 60 50 LWS/30%_OSA
40 30
LWS/20%_OSA
20
LWS/10%_OSA
10
LWS char
0 0
281
5000
10000
15000
20000
25000
Time, s
282
Fig. 9. Conversion degree versus time of leached wheat straw char, pCO2=0,09 atm.
283
In the case of LWS char gasification with OSA additions, the rate curves (Fig. 7) are similar in
284
shape with those of reed char gasification under identical conditions. The curves also exhibit
285
three sections. The main differences between the two cases are that the second section of LWS
17
ACCEPTED MANUSCRIPT 286
curves shows pronounced negative effects of OSA additions on the rate. However, the order of
287
the curves with OSA additions is not consistent.
288
4.4 Discussion
289
Our results show that under the gasification conditions with high CO2 partial pressure, e.g. 1
290
atm, the catalytic effect of oil shale ash is not favored. In fact, as described above, the partial
291
pressure of CO2 is above the equilibrium value of CaCO3 composition. Therefore, CaO is
292
deactivated by reacting with CO2, producing more CaCO3 [30]. This is revealed from Fig. 1
293
under brief discussion in Section 4.2. Consequently, the more OSA is added, the lower the
294
reaction rate. Together with the deactivation of CaO as catalyst, the whole OSA material
295
became non-active. In this case, the non-active OSA blocks also the active sites of char and the
296
contact between CO2 and char is inhibited. Therefore, the lower reactivity (see also Fig. 2 and
297
Fig. 3) under CO2 partial pressure of 1 atm with increasing share of OSA is believed to occur
298
due to the blockage of active sites of char by OSA.
299
Interestingly, in the case of the reed char samples, some fluctuations in the reaction rate were
300
observed after the maximum peaks. It is, however, believed to be caused by the data acquisition,
301
co-considering that previous studies of reed gasification showed no similar behavior [14].
302
On the other hand, both of the pure reed and leached wheat straw char samples (Fig. 2 and 3,
303
Fig. 6 and 7) exhibited lower reactivity when the pCO2 = 0.09 atm, compared to pCO2 = 1 atm.
304
The maximum reaction rates decreased by an order of magnitude. The reaction time for reed
305
char was prolonged 4.4 and for leached wheat straw char 5.1 times (see also Fig. 4 and 5, Fig.
306
8 and 9). The decrease in the reaction rate with the partial pressure of CO2 is in agreement with
307
previous studies [31].
308
Under the gasification conditions of 100% CO2 as a gasification agent, the reed char and leached
309
wheat straw char samples (see also Fig. 2 and Fig. 3) exhibited similar behavior compared to
18
ACCEPTED MANUSCRIPT 310
previous studies [18]. The reactivity of leached wheat straw char is moderately higher than that
311
of reed, i.e. 1.7 times, considering the total reaction times (see also Fig. 4 and Fig. 5).
312
The gasification tests without the addition of OSA (Fig. 2 and 3, Fig. 6 and 7) show that the
313
reaction rate increased at the beginning of the gasification process, achieving a maximum, after
314
which the reaction rate declined. Previous studies suggest that several factors affect the
315
reactivity, which include porous structure, active sites, and mineral matter. According to
316
Radović et al. [32], the reactivity is rather more dependent upon carbon active sites than the
317
total surface area. Van Heek et al. [33] concluded that the extension of total surface area could
318
not be the only dominating factor. It is more likely that the properties of the surface, such as
319
activity and accessibility, give the possibility of blockage areas by minerals. Bar-Ziv et al. [34]
320
suggested that the reactivity evolution during gasification is influenced by changes in the porous
321
structure, and coalescence of microcrystals can be used to represent the change in the
322
concentration of the active sites. Livneh et al. [35] confirmed that the most significant change
323
in reactivity occurs in the range of 0-30% conversion, and the slowdown of reactivity at the
324
conversion rate above 55% can be explained by the consumption of small microcrystals, which
325
are generally more reactive than large ones. The initial increase in the reaction rate observed in
326
our study could be associated with the increase of the surface area as well as active sites in the
327
early phase of gasification. The maximum in the reaction rate is thought to arise from two
328
opposing effects, namely, increase in the reaction surface area as micropores grow and their
329
decline as pores collapse progressively at their intersection (coalescence) [36]. The decrease in
330
the reaction rate is probably due to catalyst deactivation along the process.
331
On the other hand, the gasification tests with OSA additions under the partial pressure of CO2
332
0.09 atm, exhibited high reaction rates in the beginning, up to the conversion degree of 10-20%.
333
After that the rate started to decline. The earlier part of the reaction rate curve is similar to that
334
presented by Struis et al. [9], using Ca(NO3)2 impregnated biomass samples. Many authors have
19
ACCEPTED MANUSCRIPT 335
attributed the loss of activity of CaO to the sintering of CaO [8,9]. Unfortunately, the
336
explanations based on the studies reported and X-ray diffraction (XRD) data are not convincing
337
enough to prove the sintering of CaO. For instance, (i) Struis et al. [9] explain the sintering of
338
CaO based on the XRD of lignite chars performed by Radović et al. [37]. The signal of CaO is
339
not visible in the case of a coal sample, but can be seen in the pyrolysis chars of coal. However,
340
this is not a proof of sintering of CaO; (ii) Suzuki et al. [8] related this phenomenon to gradual
341
sintering of CaO, evidenced by XRD measurement after gasification (gasification temperature
342
was 900 °C). Unfortunately, the results of XRD measurement were not given. Hence, the
343
deactivation of CaO is believed to be unrelated to the sintering of pure CaO because of the
344
interaction with other mineral matter such as Si and K compounds [6]. Additionally, we
345
performed thermodynamic equilibrium modelling with FactSage (seen also in Table 6). As can
346
be seen from Table 6, the Ca is bound in different compounds, mainly with Si, K, S and Mg
347
when OSA is added to biomass chars.
348
Table 6
349
Equilibrium calculation of char mineral matter and oil shale ash (wt%), pCO2=0.09 atm and 850 °C.
R SiO2 Na2Mg2Si6O15 Na2Ca3Si6O16 NaAlSiO4 NaAlSi3O8 KAlSi2O6 KAlSi3O8 CaSiO3 CaMgSi2O6 Ca2MgSi2O7 CaAl2Si2O8 Ca3P2O8 Ca3Fe2Si3O12 CaSO4
79.8 6.4 12.2 1.7 -
R R R 10%OSA 20% OSA 30% OSA 27.2 3.6 9.0 5.3 8.7 2.2 7.4 8.7 27.9 24.2 26.8 39.6 29.2 36.1 0.9 4.8 4.9 4.8 15.5 1.7 11.1
350
20
LWS 32.9 14.1 28.7 18.3 2.6 3.4
LWS LWS LWS 10%OSA 20%OSA 30%OSA 9.9 2.6 2.5 5.5 10.6 4.6 15.6 1.2 29.7 30.3 26.2 27.7 32.9 27.1 13.1 6.0 3.2 3.6 3.9 11.4 15.9 16.5
ACCEPTED MANUSCRIPT 351
Among the studied shares of OSA to the char samples, OSA addition of 30% exhibited the
352
fastest reactions times, i.e. the addition of 30% to the reed char lowered the reaction time by
353
1.3 times and to the leached wheat straw char by 1.4 times.
354
Tuomi et al. [25] observed that the dolomite was more active on tar decomposition in the
355
fluidized-bed gasification tests than in the laboratory scale tests under identical atmospheric
356
pressure conditions and at the same temperature. Similarly, oil shale ash could exhibit enhanced
357
catalytic effects on char conversion in fluidized bed compared to the laboratory scale
358
experiments performed in our study. The importance of even distribution and dispersion, and
359
good contact between catalysts and active sites has been pointed out in other studies as well
360
[7,9,37,38]. Therefore, the impact of OSA as a bed additive in the fluidized bed gasification
361
process needs further investigation.
362 363
5 Conclusions
364
In this study, the catalytic effect of oil shale ash on CO2 gasification of reed and leached wheat
365
straw chars were studied by means of a thermogravimetric analyser, TA Instruments SDT
366
Q600, operated at atmospheric pressure with two different partial pressure levels of CO2: 1.0
367
atm and 0.09 atm. It was observed that the effect was positive when the CO2 partial pressure of
368
0.09 atm was applied. The addition of OSA by 30 wt% to reed char lowered the reaction time
369
by 1.3 times and to leached wheat straw char by 1.4 times, presumably due to the presence of
370
CaO in the OSA. In contrast, when the CO2 partial pressure of 1.0 atm was applied, the effect
371
was negative. This is presumably due to the binding of CaO with CO2 to form CaCO3,
372
considering that CaO but not CaCO3 is directly responsible for the catalytic effect of OSA.
373
Despite of this, the result from this study suggests that OSA can be used as additive to catalyse
374
CO2 gasification of biomass. However, further investigations are needed to optimize the
21
ACCEPTED MANUSCRIPT 375
gasification process with respect to the CO2 partial pressure to be applied. On the other hand, it
376
also suggests that OSA could be used as additive for removing CO2 from hot gas steams such
377
as process or flue gases containing high content of CO2.
378 379
Acknowledgements. Arvo Mere, Peter Backman, Maaris Nuutre, Raaja Aluvee, and Birgit
380
Maaten are acknowledged for their comprehensive assistance in conducting our tests. Rain
381
Veinjärv and Rustam Hasjanov from Enefit Energiatootmine AS are acknowledged for their
382
help in collecting oil shale ash. The work is financially supported by Estonian Ministry of
383
Education and Research (IUT19-4), and by the European Regional Development Fund through
384
the project TK141 “Advanced materials and high-technology devices for energy recuperation
385
systems“.
386 387
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Dolomite at Elevated Pressure. 1. Sorbent Performance. Ind Eng Chem Res 1996;35:176-83.
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ACCEPTED MANUSCRIPT
The impact of oil shale ash on gasification is dependent on partial pressure of CO2. Oil shale ash could act as catalyst under atmospheric gasification conditions. Oil shale ash could bind CO2 under pressurized gasification conditions.
Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin PII:
S0360-5442(18)30603-0
DOI:
10.1016/j.energy.2018.04.013
Reference:
EGY 12652
To appear in:
Energy
Received Date:
02 December 2017
Revised Date:
17 March 2018
Accepted Date:
03 April 2018
Please cite this article as: Siim Link, Khanh-Quang Tran, Quang-Vu Bach, Patrik Yrjas, Daniel Lindberg, Stelios Arvelakise, Argo Rosin, Catalytic effect of oil shale ash on CO2 gasification of leached wheat straw and reed chars, Energy (2018), doi: 10.1016/j.energy.2018.04.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1 2
Catalytic effect of oil shale ash on CO2 gasification of
3
leached wheat straw and reed chars
4 5
Siim Link*
6
Department of Electrical Power Engineering and Mechatronics, School of Engineering,
7
Tallinn University of Technology
8
Ehitajate tee 5, 19086, Tallinn, Estonia
9
E-mail: [email protected]
10
*corresponding author
11 12
Khanh-Quang Tran
13
Department of Energy and Process Engineering, Norwegian University of Science and
14
Technology
15
NO-7491 Trondheim, Norway
16 17
Quang-Vu Bach
18
Department of Energy and Process Engineering, Norwegian University of Science and
19
Technology
20
NO–7491 Trondheim, Norway
21 1
ACCEPTED MANUSCRIPT 22
Patrik Yrjas
23
Johan Gadolin Process Chemistry Centre, Åbo Akademi University
24
Piispankatu 8, Turku, FI-20500, Finland
25 26
Daniel Lindberg
27
Johan Gadolin Process Chemistry Centre, Åbo Akademi University
28
Piispankatu 8, Turku, FI-20500, Finland
29 30
Stelios Arvelakis
31
Bioresource Technology Unit, Laboratory of Organic and Environment Technologies,
32
Department of Chemical Engineering, National Technical University of Athens
33
Zografou Campus, GR-15700, Athens, Greece
34 35
Argo Rosin
36
Department of Electrical Power Engineering and Mechatronics, School of Engineering,
37
Tallinn University of Technology
38
Ehitajate tee 5, 19086, Tallinn, Estonia
39 40 41
2
ACCEPTED MANUSCRIPT 42
1 Abstract
43
Oil shale ash is a material that could be used as a low cost catalyst for biomass gasification
44
processes. This article analyses the catalytic effect of oil shale ash on CO2 gasification of reed
45
and leached wheat straw chars. A thermogravimetric analyser operated at atmospheric pressure
46
and two different partial pressures of CO2 such as 0.09 atm and 1 atm were applied. The results
47
indicate that oil shale ash has positive effect on char reactivity at the CO2 partial pressure of
48
0.09 atm: the addition of oil shale ash by 30% to reed char lowered the reaction time by 1.3
49
times and to leached wheat straw char by 1.4 times. At the partial pressure of CO2 1 atm, the
50
addition of oil shale ash resulted in the negative effect on the char conversion due to the binding
51
of CO2. The average calculated CO2 binding capacity of oil shale ash was 13.7 mg CO2 per 100
52
mg of oil shale ash.
53 54
Keywords: biomass, oil shale ash, char, gasification, catalyst, reactivity
55 56
2 Introduction
57
Power generation in Estonia is based mainly on oil shale fuel [1], which is characterized by
58
very high mineral matter contents (60-75%) [2]. As a consequence, 5-7 million tonnes of oil
59
shale ash are produced and sent to the landfills in Estonia annually. Only a small percentage of
60
the ash produced finds its way to secondary uses, either as a stabilizing agent of roadbeds in
61
construction, or soil conditioner in agriculture [3]. New methods of reusing or recycling of the
62
ash would help reduce the burden to the landfills and contribute to the use of oil shale for
63
sustainable heat and power generation in Estonia.
3
ACCEPTED MANUSCRIPT 64
Oil shale ashes contain various Ca compounds [4], of which many exhibit their catalytic effect,
65
enhancing the char gasification reactivity [5]. Calcium and several calcium containing
66
compounds (Ca(OH)2, Ca(Ac)2, CaCO3, CaC2O4, Ca(NO3)2) have been studied as catalysts to
67
improve the gasification reactivity of carbon containing materials [6-10]. H. Risnes et al. [6]
68
have reported that the effect of calcium addition as calcium sugar/molasses solutions on straw
69
significantly affects the ash chemistry and the ash sintering tendency but much less the char
70
reactivity. Perander et al. [7] found that the gasification rate of the char increases linearly with
71
an increase in the concentration of Ca and the catalytic activity of Ca is higher than K at the
72
beginning of char gasification but the catalytic effect of Ca decreases earlier than the catalytic
73
effect of potassium. Suzuki et al. [8] studied the different loading levels of Ca and found that
74
the gasification rate increases with the loading level. However, the gasification rate decreased
75
rapidly after the char conversion degree of 50%. Struis et al. [9] showed that after char
76
conversion degree of 20%, the reaction rate course of Ca(NO3)2 impregnated charcoal with
77
progressing gasification levels down to that of pure char without additive.
78
A number of low cost materials such as limestone and/or dolomite have been tested as catalysts
79
for biomass gasification. However, most of them have focused on improving the gas
80
composition and tar reduction [11-13] rather than on the gasification reactivity of biomass chars
81
[5]. Moreover, oil shale ash has not been tested as catalyst for biomass gasification. Additives
82
used in the biomass gasification process require detailed knowledge about their impact on the
83
process and the catalytic activity is strongly influenced by the chemistry of mineral matters of
84
the fuel [6]. Therefore, the study reported in this work was carried out, looking at the ability of
85
OSA improving catalytically the gasification of biomass chars. For this purpose, it is reasonable
86
to choose biomass materials with low reactivity as feedstock; otherwise it would be difficult to
87
observe the catalytic effect of OSA. From our previous studies [14-16], both reed and leached
88
wheat straw chars exhibited lower gasification reactivity compared to chars derived from wood
4
ACCEPTED MANUSCRIPT 89
under the conditions investigated. Moreover, reed and wheat straw differ by the content and
90
composition of mineral matter (ash). The choice of biomass materials for the study helps better
91
estimate the use of OSA for feedstocks with diverged ash content and composition. The
92
objective of this study is to determine the effect of OSA on the CO2 gasification reactivity of
93
the selected biomass (common reed and leached wheat straw) chars.
94 95
3 Materials and Methods
96
3.1 Materials
97
The biomass samples selected for the investigation were the following: pelagian reed,
98
originating from the west coast shorelines of Estonia and the islands of Estonia and wheat straw
99
from the area of Thessaly in Greece. The wheat straw sample was pre-treated using the leaching
100
method developed by Arvelakis et al. [17]. The oil shale ash was collected from the heat
101
exchanger of circulating ash (called INTREXTM according to the Foster Wheeler, the supplier
102
of the boiler) of a circulating fluidized bed boiler in the Eesti power plant (one of the Narva
103
power plants of the national energy company Eesti Energia AS).
104
The reed and leached wheat straw (hereafter referred as R and LWS, respectively) were
105
analyzed and characterized regarding to proximate and ultimate analyses, and their gross
106
calorific value, as well as ash chemical analysis in accordance with the American Society for
107
Testing and Materials (ASTM) standard methods, including D 1102-84, D 3175-89a, D 5142-
108
90, D5373-93, D 4208-88, D 2015-95.
109
The proximate analysis of reed char and leached wheat straw char samples (hereafter referred
110
to as RC and LWSC, respectively) was performed by means of a TA Instruments SDT Q600
111
thermogravimetric analyser (TGA). First, the sample was heated at a rate of 20 °C min-1 from
5
ACCEPTED MANUSCRIPT 112
ambient temperature to 105 °C and kept for 10 min in nitrogen gas flow of 90 mL min-1 to
113
determine the moisture content. Then, the temperature was increased, with a heating rate of 50
114
°C min-1, up to 900 °C, at which it was kept for 7 min to determine the volatile content of the
115
sample. Thereafter the temperature was lowered to 600 °C in 20 min, followed by supplying air
116
at the flow rate of 100 mL min-1 to initiate burning for determining the ash content [18].
117
The oil shale ash (hereafter referred to as OSA) fraction with particles smaller than 125 µm in
118
size was selected and characterized regarding the ash chemistry and mineralogy analyses. The
119
ash chemistry analysis was performed in accordance with various standard methods including
120
EVS-EN 196-21, DIN 51729, STI-1-20015, STI-2-20015, and ISO 334. The mineralogy
121
analysis was carried out by means of a Rigaku Ultima IV diffractometer using the X-ray powder
122
diffraction (XRD) method with Cu Kα radiation (λ = 1.5406 Å, 40 kV at 40 mA) and a silicon
123
strip detector D/teX Ultra.
124 125
3.2 Leaching
126
The leaching pre-treatment of the wheat straw was performed using tap water according to the
127
method developed by Arvelakis et al. [17]. During the leaching process, the sample was put in
128
a 200-mesh plastic grid, tied up and submerged into tap water in a plastic 75 l volume barrel.
129
The used mass/water ratio was 66.6 g/L and the retention time was 12 h. At the end of the
130
process, the sample was allowed to dry naturally in air until reaching a constant weight under
131
the sun of the Mediterranean summer within a period of 5 days. During the drying process, the
132
bed thickness of the drying sample was kept below 15 cm to guarantee the fast-drying process
133
without material loss due to possible mould bacterial activity.
6
ACCEPTED MANUSCRIPT 134
3.3 Char preparation by pyrolysis
135
A cylindrical atmospheric fixed-bed reactor with an electrical heater was used to prepare char
136
from the selected biomass samples. A quartz pipe inside the reactor protects the heating wire
137
against corrosive compounds possibly present in the evolved gas mixture during the pyrolysis.
138
The sample is loaded into the sample holder made of a stainless-steel SS316 bound net with 80
139
μm openings and placed in the quartz pipe. The applied heating procedure was the following:
140
(i) hold the sample at 50 °C for 45 min, (ii) heat the sample to the end temperature of 800 °C at
141
a heating rate of 10 °C min-1, (iii) hold the sample at the end temperature for 15 min, and (iv)
142
cool down. Nitrogen was used as a carrier gas with a flow rate of 1 L min-1 in all pyrolysis
143
experiments. The methodology together with a detailed description of the system is reported in
144
[19].
145
3.4 Test method for CO2 gasification reactivity of char
146
A thermogravimetric analyser (TGA) of TA Instruments SDT Q600 was used for this test. Two
147
partial pressures of CO2, 0.09 and 1.0 atm, were employed for the test at atmospheric pressure,
148
considering possible interferences of CO2 chemistry during the process on the catalytic effect
149
of OSA. The char reactivity was tested at a CO2 partial pressure of 0.09 atm with the following
150
procedure. The sample was heated up from room temperature to 850 °C at a heating rate of 50
151
°C/min in a nitrogen flow rate of 100 mL min-1. When the temperature (850 °C) had been
152
established and kept constant, the gas composition was changed to include CO2 to ensure the
153
flow rate of N2 and CO2 at 91 and 9 mL min-1, respectively. After that, the TGA reactor was
154
held under these conditions until no mass change was observed. Similarly, the char reactivity
155
was tested at a CO2 partial pressure of 1 atm with the following procedure. The sample was
156
heated up from room temperature to 850 °C, at a heating rate of 50 °C/min in a nitrogen flow
157
rate of 100 mL min-1. When the temperature (850 °C) had been established and kept constant,
7
ACCEPTED MANUSCRIPT 158
the gas was changed to include only CO2 with a flow rate of 100 mL min-1. Then, the TGA
159
reactor was held under these conditions until no mass change was observed.
160
The gasification reactivity is expressed as the char gasification rate over a degree of char
161
conversion. The char gasification rate in units of min-1 at any particular conversion value is
162
defined as shown in Eq. (1).
163
r=dX/dt
164
where r is the reaction rate, and X is expressed as (Eq. (2)):
165
X=(Mo-M(t))/(Mo-Mf).
166
Mo represents the initial mass of the char, M(t) is the mass of the char at time t, and Mf is the
167
mass of the gasification residue.
168
3.5 Thermodynamic calculations
169
In our study, the thermodynamic equilibrium calculations were performed to determine the
170
composition of mineral matter under gasification conditions. For calculations, FactSage 7.1
171
software package was used [20]. The following thermodynamic databases were used for the
172
calculations: FToxid, which includes data for solid oxides and silicates and liquid slag; FTSalt
173
for liquid alkali salts (NaCl-Na2CO3-Na2SO4-KCl-K2CO3-K2SO4) and corresponding alkali salt
174
solid solutions; FactPS for the gas phase and other stoichiometric solid phases. The input for
175
the calculations was the chemical composition of the fuel ashes in Table 2, the temperature was
176
set to 850 °C and the partial pressure of CO2 was set to be 0.09 atm or 1.0 atm.
(1)
(2)
177
8
ACCEPTED MANUSCRIPT 178
4 Results and discussion
179
4.1 Material characterization
180
Reed (R) and leached wheat straw (LWS) differ by their composition (as seen in Tables 1 and
181
2). The ash content of R and LWS is 3.2% and 5.8% respectively, as dry basis. The main
182
component of reed ash is SiO2. Besides SiO2, the ash of LWS contains also 15% of CaO.
183
Table 1
184
Fuel characterization of the selected biomass samples.
Proximate analysis, dry
Ultimate analysis, dry basis (wt%)
Gross
basis (wt%)
Moisture
calorific
Material (wt%)
value,
Volatile
Fixed
Ash
matter
carbon
N
C
H
S
Cl
O
(MJ/kg)
R
5.4
3.2
80.3
16.5
0.4
47.4
5.7
0.2
n.d.
43.1
20.41
LWS
5.8
5.8
80.7
13.5
0.6
46.3
5.3
0.2
0.1
41.7
20.03
185 186
Table 2
187
Chemical analysis of the feedstock ashes (wt%).
Material
K2O Na2O CaO MgO SiO2 Al2O3 Fe2O3 TiO2 SO3
P2O5 Cl
Reed
5.9
8.4
2.9
1.4
73.7
n.d.
1.1
0.1
5.2
1.0
0.6
LWS
4.0
1.9
15.3 2.5
49.9
1.9
0.6
0.1
3.8
n.d.
1.8
OSA
1.0
0.2
43.9 9.6
17.6
5.3
1.5
n.d.
15.3 n.d.
0.4
188 189
On the other hand, the char yield (dry basis) after pyrolysis was 19.4% for reed sample and
190
27.3% for leached wheat straw sample. After pyrolysis, the content of fixed carbon and ash
9
ACCEPTED MANUSCRIPT 191
increases, but the content of volatile matter decreases compared to parent fuel samples (see also
192
Tables 1 and 3).
193
Table 3
194
Char characterization of the selected biomass samples.
Moisture
Proximate analysis, dry basis (wt%)
Material (wt%)
Ash
Volatile matter
Fixed carbon
RC
1.2
17.7
15.0
67.3
LWSC
2.1
34.6
14.7
50.7
195 196
Results from the XRD analysis of oil shale ash (OSA) are presented in Table 4, which shows
197
that the content of CaO is 2.9% and that of Ca(OH)2 is 18.0%. Kuusik et al. [21] reported 19.9%
198
CaO and 2.1% Ca(OH)2 for the ash collected from the heat exchanger of the circulating ash
199
(INTREX ash) of the fluidized bed boiler. Bityukova et al. [4] have studied also the composition
200
of oil shale ashes. They found that the INTREX ash contains 9.9% CaO and 12.3% Ca(OH)2.
201
The differences between the content of CaO and Ca(OH)2 can be attributed to the chemistry of
202
CaO in the presence of moisture, according to Eq. (3), forming thermally more stable product
203
of calcium hydroxide (portlandite) [4].
204
CaO + H2O = Ca(OH)2 + heat
205
Table 4
206
Mineral composition of the oil shale ash (wt%).
(3)
SiO2 CaO Ca(OH)2 CaSO4 MgO Ca2SiO4 Ca2Mg(Si2O7) CaSiO3 KAlSi3O8 CaCO3 12.0
2.9
18.0
41.0
4.4
5.0
207
10
5.0
5.1
2.6
4.0
ACCEPTED MANUSCRIPT 208
4.2 Thermogravimetric analysis of pure oil shale ash
209
First of all, the behavior of the oil shale ash was thermogravimetrically analyzed under different
210
CO2 partial pressures, as described above. The partial pressure of 0.09 atm characterizes the
211
partial pressure of CO2 in product gas under atmospheric air blown gasification condition [22],
212
and the partial pressure of 1 atm could be considered as the partial pressure of CO2 under
213
pressurized gasification conditions depending on the total pressure of product gas [23-25]. The
214
results of these analyses are shown in Fig. 1.
98
700
96
600
94
500 Phase I: 100% N2
92
400
Phase II: 100% CO2
90
300
Phase II: 9% CO2 + 91% N2
200
Temperature
3000
2800
2600
2400
2200
2000
1600
1400
1200
1000
800
0
600
84
400
100
200
86
0
Temperature, oC
800
88
215
900
100
1800
Mass loss, M/Mo (%)
Phase II
Phase I
102
Time, s
216
Fig. 1. Thermogravimetric analysis of the oil shale ash used for the test.
217
During Phase I, three steps of mass loss could be observed. The first step up to 180 °C is related
218
to the removal of moisture. The second step of mass loss between the temperatures of 375 –
219
450 °C is related to the pyrolysis of unburnt hydrocarbon entrapped in the ash. The third mass
220
loss between 600 – 850 °C is attributed to the decomposition of carbonates [26].
11
ACCEPTED MANUSCRIPT 221
It is seen that at the CO2 partial pressure of 0.09 atm, there is no increase in mass in Phase II.
222
This indicates that CaO does not react with CO2 under the conditions of our investigation. At
223
the CO2 partial pressure of 1 atm, CaO reacts with CO2 and the mass of ash sample was
224
increased. The average calculated CO2 binding capacity of oil shale ash is 13.7 mg CO2 per 100
225
mg of oil shale ash sample, which is in agreement with previous studies [27].
226
Based on the composition of oil shale ash in Table 2, the equilibrium calculations at 850 °C
227
under different partial pressures of CO2 were performed as well, as shown in Table 5.
228
Table 5
229
Equilibrium calculation of oil shale ash (wt%).
PCO2 (atm) 0.09 1.00
MgO 2.9 2.8
CaO 3.9 0.0
Mineral composition Ca3MgAl4O10 Ca3MgSi2O8 Ca2Fe2O5 11.6 52.0 2.8 11.2 50.5 2.7
CaCO3 0.0 6.7
CaSO4 26.9 26.1
230
The thermodynamic calculations support the experimental data that CO2 is consumed by CaO
231
at the elevated partial pressure of CO2.
232
The observed behavior of oil shale ash under different partial pressures of CO2 is in agreement
233
with the chemical reaction between CaO and CO2 described by Eq. (4), considering the effect
234
of CO2 partial pressure on the equilibrium [28,29]. Indeed, the forward reaction, which reduces
235
the partial pressure of CO2, is favored when the applied partial pressure of the CO2 is increased,
236
considering Le Chatelier’s principle.
237
CaO + CO2 ↔ CaCO3
(4)
238 239
4.3 Char gasification reactivity and the catalytic effect of oil shale ash
240
Figs. 2 and 3 show the data of reaction rates plotted versus the conversion degree for the selected
241
char samples without and with OSA addition, under the CO2 partial pressure condition of 1 atm.
12
ACCEPTED MANUSCRIPT 242
Fig. 2 shows the reed sample and Fig. 3 the leached wheat straw. The char conversion degree
243
plotted versus the time of the gasification tests is illustrated in Fig. 4 (reed) and Fig. 5 5 (LWS).
244
As can be seen from Figs. 2 and 3, the LWS char is more reactive than the reed char under
245
identical gasification conditions. The reaction rates increase from the start to the points where
246
the char conversion degree is about 20-30%, and then decline. With the addition of OSA, the
247
gasification reactivity of both char samples decreases. The more OSA is added, the lower the
248
reactivity is. In addition, the rate curves for the reed char (Fig. 2) exhibit double peaks, but it is
249
not the case with the LWS char (Fig. 3). 4.5 Reed char
Reaction rate, %/min
4.0
R/10%_OSA
3.5
R/20%_OSA
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
250 251
10
20
30
40
50
60
70
Char conversion degree, % Fig. 2. Reaction rate of reed char, pCO2=1 atm.
252
13
80
90
100
ACCEPTED MANUSCRIPT 4.5
Reaction rate, %/min
4.0 3.5 3.0 2.5 2.0 1.5 LWS char
1.0
LWS/10%_OSA
0.5
LWS/20%_OSA
0.0 0
10
20
40
50
60
70
80
90
100
Char conversion degree, %
253 254
30
Fig. 3. Reaction rate of leached wheat straw char, pCO2=1 atm.
100
Char conversion degree, %
90 80 70 60 50 40 Reed char
30
R/10%_OSA
20
R/20%_OSA
10 0 0
255 256
1500
3000
4500
6000
Time, s Fig. 4. Conversion degree versus time of reed char, pCO2=1 atm.
257
14
7500
9000
ACCEPTED MANUSCRIPT
Char conversion degree, %
100 90 80 70 60 50 40 30
LWS char
20
LWS/10%_OSA
10
LWS/20%_OSA
0 0
258
1000
2000
3000
4000
5000
6000
Time, s
259
Fig. 5. Conversion degree versus time of leached wheat straw char, pCO2=1 atm.
260
The reaction rates versus the char conversion degree of the char samples under the CO2 partial
261
pressure condition of 0.09 atm are shown in Figs. 6 and 7 for the reed and LWS char sample,
262
respectively. The char conversion degrees plotted versus the time of the gasification tests are
263
shown in Figs. 8 and 9. As can see from Figs. 6 and 7, the rate curves for the CO2 gasification
264
of reed and LWS without OSA additions are similar, having one peak only. However, when
265
OSA is added, the peak disappears in all cases. In the case of reed char, OSA additions give
266
rise to increased reaction rate until the conversion degree reaches 15-20% approximately.
267
Within this section of the curves, the more OSA is added, the higher the reaction rate is. In the
268
second section, within 15-35% as char conversion degree, OSA additions seem to have negative
269
effect on the reaction rate. However, the differences between the rate curves are very small. The
270
effects turn to be positive again for the last section from 35% to 100% with pronounced
271
differences and the order of the curves is the same as in the first section.
272
15
ACCEPTED MANUSCRIPT 1.0
Reaction rate, %/min
R/30%_OSA 0.8
R/20%_OSA R/10%_OSA
0.6
Reed char
0.4
0.2
0.0 0
10
30
40
50
60
70
80
90
100
Char conversion degree, %
273 274
20
Fig. 6. Reaction rate of reed char, pCO2=0,09 atm.
275
Reaction rate, %/min
1.4 LWS/30%_OSA
1.2
LWS/20%_OSA LWS/10%_OSA
1.0
LWS char
0.8 0.6 0.4 0.2 0.0 0
276 277
10
20
30
40
50
60
70
Char conversion degree, % Fig. 7. Reaction rate of leached wheat straw char, pCO2=0,09 atm.
278
16
80
90
100
ACCEPTED MANUSCRIPT 100
Char conversion degree, %
90 80 70 60 50
R/30%_OSA
40
R/20%_OSA
30
R/10%_OSA
20
Reed char
10 0 0
10000
30000
Time, s
279 280
20000
Fig. 8. Conversion degree versus time of reed char, pCO2=0,09 atm.
100
Char conversion degree, %
90 80 70 60 50 LWS/30%_OSA
40 30
LWS/20%_OSA
20
LWS/10%_OSA
10
LWS char
0 0
281
5000
10000
15000
20000
25000
Time, s
282
Fig. 9. Conversion degree versus time of leached wheat straw char, pCO2=0,09 atm.
283
In the case of LWS char gasification with OSA additions, the rate curves (Fig. 7) are similar in
284
shape with those of reed char gasification under identical conditions. The curves also exhibit
285
three sections. The main differences between the two cases are that the second section of LWS
17
ACCEPTED MANUSCRIPT 286
curves shows pronounced negative effects of OSA additions on the rate. However, the order of
287
the curves with OSA additions is not consistent.
288
4.4 Discussion
289
Our results show that under the gasification conditions with high CO2 partial pressure, e.g. 1
290
atm, the catalytic effect of oil shale ash is not favored. In fact, as described above, the partial
291
pressure of CO2 is above the equilibrium value of CaCO3 composition. Therefore, CaO is
292
deactivated by reacting with CO2, producing more CaCO3 [30]. This is revealed from Fig. 1
293
under brief discussion in Section 4.2. Consequently, the more OSA is added, the lower the
294
reaction rate. Together with the deactivation of CaO as catalyst, the whole OSA material
295
became non-active. In this case, the non-active OSA blocks also the active sites of char and the
296
contact between CO2 and char is inhibited. Therefore, the lower reactivity (see also Fig. 2 and
297
Fig. 3) under CO2 partial pressure of 1 atm with increasing share of OSA is believed to occur
298
due to the blockage of active sites of char by OSA.
299
Interestingly, in the case of the reed char samples, some fluctuations in the reaction rate were
300
observed after the maximum peaks. It is, however, believed to be caused by the data acquisition,
301
co-considering that previous studies of reed gasification showed no similar behavior [14].
302
On the other hand, both of the pure reed and leached wheat straw char samples (Fig. 2 and 3,
303
Fig. 6 and 7) exhibited lower reactivity when the pCO2 = 0.09 atm, compared to pCO2 = 1 atm.
304
The maximum reaction rates decreased by an order of magnitude. The reaction time for reed
305
char was prolonged 4.4 and for leached wheat straw char 5.1 times (see also Fig. 4 and 5, Fig.
306
8 and 9). The decrease in the reaction rate with the partial pressure of CO2 is in agreement with
307
previous studies [31].
308
Under the gasification conditions of 100% CO2 as a gasification agent, the reed char and leached
309
wheat straw char samples (see also Fig. 2 and Fig. 3) exhibited similar behavior compared to
18
ACCEPTED MANUSCRIPT 310
previous studies [18]. The reactivity of leached wheat straw char is moderately higher than that
311
of reed, i.e. 1.7 times, considering the total reaction times (see also Fig. 4 and Fig. 5).
312
The gasification tests without the addition of OSA (Fig. 2 and 3, Fig. 6 and 7) show that the
313
reaction rate increased at the beginning of the gasification process, achieving a maximum, after
314
which the reaction rate declined. Previous studies suggest that several factors affect the
315
reactivity, which include porous structure, active sites, and mineral matter. According to
316
Radović et al. [32], the reactivity is rather more dependent upon carbon active sites than the
317
total surface area. Van Heek et al. [33] concluded that the extension of total surface area could
318
not be the only dominating factor. It is more likely that the properties of the surface, such as
319
activity and accessibility, give the possibility of blockage areas by minerals. Bar-Ziv et al. [34]
320
suggested that the reactivity evolution during gasification is influenced by changes in the porous
321
structure, and coalescence of microcrystals can be used to represent the change in the
322
concentration of the active sites. Livneh et al. [35] confirmed that the most significant change
323
in reactivity occurs in the range of 0-30% conversion, and the slowdown of reactivity at the
324
conversion rate above 55% can be explained by the consumption of small microcrystals, which
325
are generally more reactive than large ones. The initial increase in the reaction rate observed in
326
our study could be associated with the increase of the surface area as well as active sites in the
327
early phase of gasification. The maximum in the reaction rate is thought to arise from two
328
opposing effects, namely, increase in the reaction surface area as micropores grow and their
329
decline as pores collapse progressively at their intersection (coalescence) [36]. The decrease in
330
the reaction rate is probably due to catalyst deactivation along the process.
331
On the other hand, the gasification tests with OSA additions under the partial pressure of CO2
332
0.09 atm, exhibited high reaction rates in the beginning, up to the conversion degree of 10-20%.
333
After that the rate started to decline. The earlier part of the reaction rate curve is similar to that
334
presented by Struis et al. [9], using Ca(NO3)2 impregnated biomass samples. Many authors have
19
ACCEPTED MANUSCRIPT 335
attributed the loss of activity of CaO to the sintering of CaO [8,9]. Unfortunately, the
336
explanations based on the studies reported and X-ray diffraction (XRD) data are not convincing
337
enough to prove the sintering of CaO. For instance, (i) Struis et al. [9] explain the sintering of
338
CaO based on the XRD of lignite chars performed by Radović et al. [37]. The signal of CaO is
339
not visible in the case of a coal sample, but can be seen in the pyrolysis chars of coal. However,
340
this is not a proof of sintering of CaO; (ii) Suzuki et al. [8] related this phenomenon to gradual
341
sintering of CaO, evidenced by XRD measurement after gasification (gasification temperature
342
was 900 °C). Unfortunately, the results of XRD measurement were not given. Hence, the
343
deactivation of CaO is believed to be unrelated to the sintering of pure CaO because of the
344
interaction with other mineral matter such as Si and K compounds [6]. Additionally, we
345
performed thermodynamic equilibrium modelling with FactSage (seen also in Table 6). As can
346
be seen from Table 6, the Ca is bound in different compounds, mainly with Si, K, S and Mg
347
when OSA is added to biomass chars.
348
Table 6
349
Equilibrium calculation of char mineral matter and oil shale ash (wt%), pCO2=0.09 atm and 850 °C.
R SiO2 Na2Mg2Si6O15 Na2Ca3Si6O16 NaAlSiO4 NaAlSi3O8 KAlSi2O6 KAlSi3O8 CaSiO3 CaMgSi2O6 Ca2MgSi2O7 CaAl2Si2O8 Ca3P2O8 Ca3Fe2Si3O12 CaSO4
79.8 6.4 12.2 1.7 -
R R R 10%OSA 20% OSA 30% OSA 27.2 3.6 9.0 5.3 8.7 2.2 7.4 8.7 27.9 24.2 26.8 39.6 29.2 36.1 0.9 4.8 4.9 4.8 15.5 1.7 11.1
350
20
LWS 32.9 14.1 28.7 18.3 2.6 3.4
LWS LWS LWS 10%OSA 20%OSA 30%OSA 9.9 2.6 2.5 5.5 10.6 4.6 15.6 1.2 29.7 30.3 26.2 27.7 32.9 27.1 13.1 6.0 3.2 3.6 3.9 11.4 15.9 16.5
ACCEPTED MANUSCRIPT 351
Among the studied shares of OSA to the char samples, OSA addition of 30% exhibited the
352
fastest reactions times, i.e. the addition of 30% to the reed char lowered the reaction time by
353
1.3 times and to the leached wheat straw char by 1.4 times.
354
Tuomi et al. [25] observed that the dolomite was more active on tar decomposition in the
355
fluidized-bed gasification tests than in the laboratory scale tests under identical atmospheric
356
pressure conditions and at the same temperature. Similarly, oil shale ash could exhibit enhanced
357
catalytic effects on char conversion in fluidized bed compared to the laboratory scale
358
experiments performed in our study. The importance of even distribution and dispersion, and
359
good contact between catalysts and active sites has been pointed out in other studies as well
360
[7,9,37,38]. Therefore, the impact of OSA as a bed additive in the fluidized bed gasification
361
process needs further investigation.
362 363
5 Conclusions
364
In this study, the catalytic effect of oil shale ash on CO2 gasification of reed and leached wheat
365
straw chars were studied by means of a thermogravimetric analyser, TA Instruments SDT
366
Q600, operated at atmospheric pressure with two different partial pressure levels of CO2: 1.0
367
atm and 0.09 atm. It was observed that the effect was positive when the CO2 partial pressure of
368
0.09 atm was applied. The addition of OSA by 30 wt% to reed char lowered the reaction time
369
by 1.3 times and to leached wheat straw char by 1.4 times, presumably due to the presence of
370
CaO in the OSA. In contrast, when the CO2 partial pressure of 1.0 atm was applied, the effect
371
was negative. This is presumably due to the binding of CaO with CO2 to form CaCO3,
372
considering that CaO but not CaCO3 is directly responsible for the catalytic effect of OSA.
373
Despite of this, the result from this study suggests that OSA can be used as additive to catalyse
374
CO2 gasification of biomass. However, further investigations are needed to optimize the
21
ACCEPTED MANUSCRIPT 375
gasification process with respect to the CO2 partial pressure to be applied. On the other hand, it
376
also suggests that OSA could be used as additive for removing CO2 from hot gas steams such
377
as process or flue gases containing high content of CO2.
378 379
Acknowledgements. Arvo Mere, Peter Backman, Maaris Nuutre, Raaja Aluvee, and Birgit
380
Maaten are acknowledged for their comprehensive assistance in conducting our tests. Rain
381
Veinjärv and Rustam Hasjanov from Enefit Energiatootmine AS are acknowledged for their
382
help in collecting oil shale ash. The work is financially supported by Estonian Ministry of
383
Education and Research (IUT19-4), and by the European Regional Development Fund through
384
the project TK141 “Advanced materials and high-technology devices for energy recuperation
385
systems“.
386 387
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ACCEPTED MANUSCRIPT
The impact of oil shale ash on gasification is dependent on partial pressure of CO2. Oil shale ash could act as catalyst under atmospheric gasification conditions. Oil shale ash could bind CO2 under pressurized gasification conditions.