Fuel Processing Technology 87 (2006) 383 – 408
www.elsevier.com/locate/fuproc
Compositional constraints on slag formation and potassium volatilization
from rice straw blended wood fuel
Peter Thy a,⁎, Bryan M. Jenkins b , Charles E. Lesher a , Sidsel Grundvig c
a
b
Department of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA
Department of Biological and Agricultural Engineering, University of California, One Shields Avenue, Davis, CA 95616, USA
c
Department Earth Sciences, Aarhus University, DK-8000 Århus C, Denmark
Accepted 1 August 2005
Abstract
Experimental melting of biomass ash blends demonstrates that the addition of rice straw to a dominantly wood-based fuel causes a marked
freezing point depression in the liquidus temperature of the inorganic slag from well above 2000 °C to a minimum of about 1260 °C. The
minimum temperature is achieved for ash blends with about 30% rice straw ash. The melting interval (liquidus to solidus) for the ash blends is
typically 100–200 °C. The solidus shows a systematic decrease from about 1350 °C to as low as 800 °C for pure rice straw ash. Potassium is
completely lost from slag for blends with less than 30% rice straw ash content. The addition of more than 30% rice straw ash results in an
enhanced retention of potassium in the solid slag. Potassium loss for fuel blends with above 30% rice straw ash is further positively correlated with
melting temperature. As the temperature approaches the solidus, potassium is increasingly bound in the melt as well as in potassium–aluminum
silicate minerals (leucite) and, therefore, partially retained in the slag. There are indications that melting temperatures above the ‘true’ liquidus for
rice straw-rich blends cause partial potassium loss and consequently a rise in the liquidus. This will result in an apparent extending of the melting
interval for blends with above 30% rice straw ash. The liquidus silicate mineralogy of the slag changes as a function of increasing rice straw ash
from larnite, to åkermanite, wollastonite, and diopside. This mineralogical sequence reflects an increase in the Si/Ca ratio and polymerization of
the melt. The experimental slag shows favorable similarities to the mineralogy and composition of slag formed in commercial biomass-fueled
boilers, suggesting that the simplified conditions of the experimental melting study can be used to predict combustion conditions in commercial
biomass-fueled boilers. Thus, small additions of straw to a predominantly wood fuel should have the effect of lowering slag melting temperature
and relatively reducing potassium loss to the flue gas. If combustion temperature can be controlled to within, or below, the melting interval of the
ash (b 1260 °C), the relatively loss of potassium can be minimized. Boiler operation below the minimum solidus temperature (∼1050 °C) will
further strongly restrict loss of potassium.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Biomass fuel; Combustion; Rice straw; Wood; Ash; Slag; High temperature; Phase relations; Liquidus; Solidus; Freezing point depression; Potassium
loss
1. Introduction
Development of biomass and other renewable power
generation has distinct economic and environmental advantages. Despite this, the situation today in California is such that
herbaceous fuels are virtually unusable by many existing
biomass power generators using direct-combustion technologies. The reason is that build-up of residual deposits on firesides
⁎ Corresponding author. Tel.: +1 530 752 0350; fax: +1 530 752 0951.
E-mail address: [email protected] (P. Thy).
0378-3820/$ - see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2005.08.015
and heat transfer surfaces in furnaces and boilers are so severe
that plants experience rapid decline in efficiency and increasing
metal corrosion leading to increasing maintenance costs and
reductions in energy revenues. For the practical concern of
achieving economically viable biomass power generation, an
understanding of the high temperature behavior of inorganic
components in biomass fuel systems requires detailed information on the condensed and volatile phases over wide ranges of
temperatures and fuel compositions [1–10].
Systematic studies under controlled combustion temperatures (500–1400 °C) exist for ash of wood and rice straw fuels
384
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
[11–15]. There are experimental suggestions that the addition of
straw to conventional biomass boiler fuels in some instances
may reduce fouling. The results of an evaluation for urban wood
fuel ash [13] show that potassium is strongly partitioned into the
vapor phase, while sodium is preferentially retained by solid
and liquid phases. Thy et al. [13] presented a parameterization
that predicted a reduction in potassium loss for commercial
wood fuels as a function of silica in the inorganic fuel
component. Other work [14] illustrates that fundamental
differences exist at superliquidus conditions between wood
and rice straw ash melts. Contrary to expectations, potassium is
retained in rice straw slag, but strongly volatilized from wood
slag. This may relate to differences in the extent of
polymerization of the melts. If the alkali metals occur in highly
depolymerized melts, such as wood ash melts, they will be
easily evaporated during prolonged heating and subsequently
deposited on heat exchangers. If the melt is highly polymerized,
such as rice and wheat straw ash melts, they are retained in the
polymerized network. The potential of our preliminary findings
[13,14] is that the addition of rice straw to conventional woodbased fuels may be beneficial and may reduce relative
potassium losses and thereby fouling (cf., [3]).
The addition of rice straw to wood fuels is expected to
decrease both solidus and liquidus temperatures (i.e., the
classic freezing point depression), but the magnitude of the
depression cannot be predicted based on the available
experimental data. In addition to the strong compositional
effects on melting temperatures, the severity of slag formation
and its ease of removal will depend on the amount of melt
present as well as its composition and polymerization. It is
plausible that typical boiler conditions during combustion are
within the melting temperature of slag from blended wood and
straw fuel and, therefore, that melt will be present in the slag.
A melting point depression, resulting from added rice straw,
may strongly increase melt fraction for the same combustion
temperature. An increase in the melt fraction and changes in
its composition will affect the physical properties (bulk
viscosity and surface tension) of the slag and, thereby, its
ease of removal after cooling. Likewise, potassium volatilization may be dependent on melt fraction as well as on melt
composition. Our previous results indicate that the addition of
highly polymerized rice straw melt will increase the retention
of potassium in the slag, thereby preventing its transport to
heat exchanger surfaces, but still yielding a slag with a lower
melting point.
We present here the results of a systematic study of the high
temperature melting relations of ashes produced by mixing rice
straw and wood. The blending interval we investigate is up to
50% rice straw ash. We use a high temperature vertical quench
furnace that lets us determine the phase relations at various
temperatures by allowing run products to be rapidly quenched
and recovered from the high-temperature environment. We
finally compare the results to slag formed in commercial
biomass-fueled boilers and conclude that the rapid vertical
quench furnace is an inexpensive testing tool that can be utilized
before more expensive laboratory, pilot or full scale, experiments are conducted [1,3,4,6,7].
2. Experimental techniques
2.1. Fuel selection
Two biomass fuels were used as starting materials for the
melting experiments. The first sample was mixed conifer (white
fir and ponderosa pine) whole-tree chips obtained during
January 2001 from Wheelabrator-Shasta Energy Company, Inc.,
Anderson, California. The trees were harvested from the
northeastern slopes of Mt. Shasta. This relatively clean and
high quality fuel is one of many types received at the plant.
However, reclaiming, conveying, and stacking operations at the
plant often produce a lower purity fuel that is stoked to the
Table 1
Compositions of fuels used as starting materials
Wood
Rice straw
Ultimate analysis
(% wet basis)
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Chlorine
Moisture (% wet basis)
Ash (%, dry basis) a
48.54
5.22
0.07
0.02
36.55
0.03
10.4
1.2
38.50
3.56
0.55
0.06
36.30
0.58
7.5
22.1
Ash analysis (% oxides)
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2 O
P2O5
LOI
Total
SO2
Cl
CO2
Volatile-free
9.35
14.01
0.13
0.19
3.12
4.68
1.14
1.71
1.76
2.64
4.93
7.39
32.06
48.05
0.39
0.58
10.72
16.06
3.13
4.69
27.59
94.32 100.00
0.69
b0.065
6.24
75.38
0.01
0.09
0.10
0.27
1.64
1.60
0.14
11.95
0.61
7.97
99.76
0.67
3.18
0.22
Ash fusion (°C)
Oxidizing atmosphere
Initial
Softening
Hemispherical
Fluid
Reducing atmosphere
Initial
Softening
Hemispherical
Fluid
1236
1244
1246
1249
1240
1378
1429
1470
1254
1261
1262
1263
1175
1367
1406
1420
Volatile-free
82.13
0.01
0.10
0.11
0.29
1.79
1.74
0.15
13.02
0.66
100.00
All other analyses are by Hazen. LOI is the loss-on-ignition determined from
heating the ash to 950 °C. Oxygen was determined by difference.
a
Determined at UCD. The ash analyses are done on ash produced at 525 °C.
Ash composition determined at the University of Aarhus, except SO2 and CO2
which were determined by Hazen Research, Inc., Golden, CO, USA, and Cl
which were determined using INAA at UCD.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
400
Intensity (Counts)
3.0482
200
2.5183
3.9014 3.2183
100
produced under controlled temperatures without ignition. Such
ignition could have resulted in uncontrolled high temperature
and possible elemental losses. The ash samples were stored in
airtight containers before and between uses.
Wood Ash 525 oC
300
300
2.3001
2.1037
1.9240
1.8822
1.6051
o
Rice Straw Ash 525 C
3.1798
385
2.3. Ash characterization
200
2.2376
100
0
1.8201
10
20
30
40
2-Theta
50
60
(o)
Fig. 1. XRD patterns (Cu Kα radiation) for the two test ashes produced at
525 °C. Measured d-values are in Å. Wood ash shows diffraction pattern for
calcite. The rice ash shows the diffraction pattern for sylvine (KCl) and
amorphous silica (hump in background between 20° and 30° 2θ). Intensity is
counts per second. 2-Theta is the angle 2θ (°).
boilers. The other sample was a medium grain Japonica (variety
M202) rice straw from Colusa Country, CA.
The fuels were dried in ambient air for a week and then
milled to a maximum 1/8 in. (3 mm) particle size. The final
moisture content for the air-dried wood fuel was determined as
10.4% after oven-drying at 105 °C. The ash content was 1.2%
(Table 1), calculated on a dry basis. The moisture content of
the rice straw fuel was 7.5% and the ash content was 22.1%
(Table 1).
2.2. Ashing
The two samples were ashed in air in a large-volume, electric
muffle furnace. Temperature was ramped at 20 °C/min to 100 °C
and then at 2 °C/min to a maximum of 525 °C. Temperature was
dwelled at 400 °C for 3 h and again at 525 °C for 4 h. The furnace
temperature was then dropped from the maximum 525 °C by
8 °C/min until 30 °C. The furnace and sample temperatures were
monitored during ashing by thermocouples inserted through the
roof of the furnace. The rice straw sample was ashed in open
ceramic containers. The wood sample was ashed in a semiclosed, steel container with airflow of 4.5 L/min admitted to the
container when temperature reached its maximum value. These
ashing procedures allowed relatively large ash volumes to be
The ultimate elemental compositions and ash fusing
temperatures for the two biomass fuels were determined
following the analytical recommendations of Miles et al. [7].
The ash compositions were determined by X-ray fluorescence
on Li-borate fused beads using the ash produced at 525 °C [15].
The analytical results are presented in Table 1. The two different
ashes contain relatively similar potassium contents (13 and
16 wt.% K2O, volatile free), and, therefore, blending rice straw
and wood ashes will have minor effect on the potassium
content. The main effects from blending are on the SiO2, CaO,
and Cl contents, which show much larger relative differences
for the two ashes. Rice straw blending results in significant
increases in ash and slag production (by weight), proportional to
the amount of added rice straw.
To further characterize the two ashes and to guide the
formulation of ash blending procedures, various light and
electron microscopic techniques were applied. Scanning
electron microscope images (back-scattered electron images
(BSE) and elemental X-ray dot-maps) show that the rice straw
ash particles have typical elongated shapes reflecting organic
growth forms. They typically measure up to 1 mm long and
50 μm wide and are highly porous with an inner intricate
network of walls. The elemental distributions of Si and K are
irregular with outer walls often containing higher concentrations
of Si. These particles are biogenic in origin. Irregularly shaped
high-Si particles, containing minor, but variable amounts of Ca,
Al, and Mg are also present and are of terrigenous origin (soil
particles). X-ray diffraction pattern (Cu Kα) reveals that in
addition sylvine (KCl) are present and that the main part of the
ash particles is composed of non-crystalline, amorphous
compounds (Fig. 1). Crystals of KCl are frequently observed
after ashing or burning of rice straw and other grass [16,17].
In contrast, the wood ash is very fine-grained and rarely
contains particles above 20 μm in length or diameter. Individual
Table 2
Summary of ash blends used in the experiments
Rice
straw
Rice
straw
SiO2 75.38 82.12
0.01
0.01
TiO2
Al2O3 0.09
0.10
Fe2O3 0.10
0.11
MnO
0.27
0.29
MgO
1.64
1.79
CaO
1.60
1.74
Na2O 0.14
0.15
K2O 11.95 13.02
0.61
0.66
P2O5
Total 91.79 100.00
R 50%;
W 50%
R 50%;
W 50%
R 40%;
W 60%
R 40%;
W 60%
R 30%;
W 70%
R 30%;
W 70%
R 20%;
W 80%
R 20%;
W 80%
R 15%;
W 85%
R 15%;
W 85%
R 10%;
W 90%
R 10%;
W 90%
Wood Wood
42.37
0.07
1.61
0.62
1.02
3.29
16.83
0.27
11.34
1.87
79.26
53.45
0.09
2.02
0.78
1.28
4.14
21.23
0.33
14.30
2.36
100.00
35.76
0.08
1.91
0.72
1.16
3.61
19.88
0.29
11.21
2.12
76.75
46.59
0.11
2.49
0.94
1.52
4.71
25.90
0.38
14.61
2.76
100.00
29.16
0.09
2.21
0.83
1.31
3.94
22.92
0.32
11.09
2.37
74.25
39.27
0.13
2.98
1.12
1.77
5.31
30.87
0.42
14.94
3.20
100.00
22.56
0.11
2.51
0.93
1.46
4.27
25.97
0.34
10.97
2.63
71.74
31.44
0.15
3.50
1.30
2.04
5.95
36.20
0.47
15.29
3.66
100.00
19.25
0.11
2.67
0.98
1.54
4.44
27.49
0.35
10.90
2.75
70.49
27.32
0.16
3.78
1.40
2.18
6.29
39.00
0.50
15.47
3.90
100.00
15.95
0.12
2.82
1.04
1.61
4.60
29.01
0.37
10.84
2.88
69.24
23.04
0.17
4.07
1.50
2.33
6.65
41.91
0.53
15.66
4.16
100.00
9.35 14.01
0.13
0.19
3.12
4.68
1.14
1.71
1.76
2.64
4.93
7.39
32.06 48.04
0.39
0.58
10.72 16.06
3.13
4.69
66.73 100.00
R, rice straw ash; W, wood ash. Each blended composition is calculated from Table 1 in the first column and normalized to 100% in the second column.
386
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 3
Experimental conditions and principal results for wood ash
Run number
W-16
W-15
W-14
W-13
W-12
W-11
W-10
W-9
W-8
W-7
W-6
W-20
W-19
W-18
W-17
W-5
Mass before (g)
0.0511
0.0336
0.0593
0.0522
0.0553
0.0602
0.0533
0.0554
0.0493
0.0565
0.0464
0.0527
0.0501
0.0469
0.0555
0.0378
Mass after (g)
0.0305
0.0196
0.0353
0.0313
0.0328
0.0349
0.0312
0.0322
0.0300
0.0303
0.0275
0.0322
0.0293
0.0272
0.0330
0.0221
Loss (%)
40
42
40
40
41
42
41
42
39
46
41
39
42
42
41
42
Temperature (°C)
1541
1517
1510
1498
1494
1490
1484
1469
1468
1464
1445
1431
1420
1412
1402
1390
Run time (min)
270
200
360
435
375
1025
255
435
1785
1125
1350
390
1125
1440
1545
975
Phases present
Q melt
Larnite
Periclase
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
Q melt
–
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Periclase
Q melt, quenched melt. Experiments between 1132 and 1390 °C failed to detect melt/glass.
groups of particles range from fibrous to round in shape. The
general shapes of the grains can be related to their compositions.
Needle shaped particles are Si-rich, angular particles are Ca-rich
(likely carbonates), and irregular to rounded particles are Si- and
K-rich. The X-ray diffraction pattern (Cu Kα) shows that the
only crystalline material detectable is calcite (Fig. 1). The
needle-shaped grains are composed of amorphous silica.
These observations clearly demonstrate that ash interaction
and slag formation in boilers fired by fuel blends will be highly
heterogeneous. However, in order to understand the melting
relations of slag, and to obtain equilibrium conditions in the
melting experiments, it is essential that the two components are
thoroughly mixed and are milled to approximately similar grain
sizes.
2.4. Experimental procedures
About 2 g of ash for each sample were dry-pulverized in an
agate mortar to an estimated average grain size below 10 μm.
The ashes were then dried in a vacuum furnace at 105 °C for
24 h and subsequently stored in airtight containers. A total of six
ash blends with from 10% to 50% rice straw ash were prepared
by mixing during grinding a total of 2 g in the desired weight
proportions of wood and rice straw ash. About 50 mg powder
for each experiment were pressed into a pellet and mounted onto
a 0.004 in. diameter Pt wire. These mounted pellets were
suspended into the furnace at the desired temperature.
Temperature was monitored by a Pt/90Pt10Rh thermocouple
(S-type) that was positioned near the ash pellet. Duration of the
experiments varied from 70 to 4300 min. The experiments were
terminated in air by pulling the sample out of the furnace
(estimated quench rate of N 5000 °C/min).
The experimental procedures used in this study are in many
respects similar to those commonly used for studying silicate
systems using vertical quench furnaces (e.g., [18]) and wireloop techniques [19]. The principal difference is that the
biomass material contains elements that under certain condi-
tions are volatile, such as K and Cl. To avoid losing particularly
K, the powder is prepared dry without the normal grinding in
acetone. Also, the pellet is mounted without sintering it to the Pt
wire with an H2–O2 gas torch, as is commonly done in silicate
studies. This restricts unintended loss from heating or leaching
of the ash and is a further development of our previous
techniques [13,14].
2.5. Analytical procedures
The experimental products were mounted in plastic (Buehler
transoptic powder), sectioned, and surface polished to allow
microscopic examination. Since salt is not stable at the high
Table 4
Experimental conditions and principal results for rice straw ash
Run
number
Mass
before
(g)
Mass
after
(g)
Loss
(%)
Temperature
(°C)
Run
time
(min)
Phases present
Melt
Quartz
R-19
R-18
R-16
R-20
R-15
R-21
R-5
R-6
R-4
R-7
R-8
R-9
R-10
R-3
R-11
R-12
R-13
R-14
R-2
R-1
0.0304
0.0305
0.0349
0.0342
0.0386
0.0395
0.0341
0.0428
0.0485
0.0465
0.0433
0.0373
0.0419
0.0486
0.0452
0.0431
0.0432
0.0348
0.0485
0.0495
0.0243
0.0245
0.0278
0.0269
0.0319
0.0326
–
0.0358
0.0406
0.0392
0.0355
0.0312
0.0351
0.0405
0.0378
0.0365
0.0359
0.0301
0.0414
0.0418
18
16
17
18
16
15
–
14
16
14
15
13
13
17
13
13
14
11
15
16
1536
1490
1439
1417
1393
1372
1343
1270
1255
1230
1212
1193
1176
1167
1137
1117
1100
1080
1075
982
390
1270
1410
1285
1455
1430
1350
1485
1320
1575
1380
1410
1425
1425
1350
1290
1560
1392
1320
1530
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz
Quartz, SiO2 polymorph.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
experimental temperatures, polishing was done without water.
All experimental products were examined using optical and
scanning electron microscopes. The crystalline products were
analyzed using the electron beam microprobe. The microprobe
was operated at 15 kV, a beam current of normally 10 nA, and
counting times between 10 and 30 s on peaks and 5 s on
backgrounds. Natural minerals and synthetic oxide mixtures
387
were used as standards (Na, jadeite; Mg, forsterite; Al, anorthite;
Si, augite; P, apatite; K, orthoclase, Ca, wollastonite; Ti, TiO2;
Mn, rhodonite; Fe, fayalite). The glass phase was analyzed with
a 10 m beam size and a beam current of 5 nA in order to reduce
potassium (and sodium) losses during analyses. These volatile
elements were for the same reason also analyzed first. A fused
international rhyolite standard of a composition relatively
Table 5
Experimental conditions and principal results for rice straw and wood ash blends
Run number
Loss (%)
Temperature (°C)
Run time (min)
Phases present
50% rice straw ash and 50% wood ash
R50-9
0.0399
–
R50-8
0.0327
–
R50-4
0.0311
0.0225
R50-2
0.0337
0.0232
R50-5
0.0313
0.0224
R50-3
0.0355
0.0259
R50-6
0.0348
0.0250
R50-1
0.0444
0.0323
R50-7
0.0388
0.0280
Mass before (g)
Mass after (g)
–
–
27
31
28
27
28
27
28
1318
1296
1273
1252
1230
1205
1180
1163
1064
1410
1485
365
4275
1350
1410
1395
1415
1670
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
–
40% rice straw ash and 60% wood ash
R40-8
0.0424
0.0305
R40-7
0.0387
0.0258
R40-1
0.0391
0.0290
R40-6
0.0372
0.0259
R40-5
0.0376
0.0256
R40-4
0.0373
0.0253
R40-2
0.0363
0.0259
R40-3
0.0417
0.0288
28
33
26
30
32
32
29
31
1296
1268
1250
1223
1201
1182
1160
1134
350
1080
1330
1430
1470
1190
1345
2685
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
30% rice straw ash and 70% wood ash
R30-1
0.0278
–
R30-2
0.0432
0.0267
R30-9
0.0471
0.0289
R30-8
0.0370
0.0225
R30-3
0.0384
0.0252
R30-7
0.0450
0.0299
R30-6
0.0452
0.0295
R30-5
0.0496
0.0336
R30-4
0.0391
0.0270
–
38
39
39
34
34
35
32
31
1300
1275
1253
1228
1206
1182
1161
1115
1071
1440
1495
1500
1290
1290
1400
1315
1610
1430
20% rice straw ash and 80% wood ash
R20-9
0.0464
0.0269
R20-8
0.0347
0.0208
R20-7
0.0442
0.0251
R20-6
0.0380
0.0229
R20-2
0.0384
0.0236
R20-1
0.0303
0.0169
R20-5
0.0359
0.0228
R20-4
0.0454
0.0278
R20-3
0.0396
0.0207
42
40
43
40
39
44
36
39
48
1415
1390
1368
1344
1318
1299
1271
1249
1154
15% rice straw ash and 85% wood ash
R15-4
0.0412
0.0230
R15-3
0.0421
0.0262
R15-2
0.0370
0.0224
R15-1
0.0423
0.0224
44
38
39
47
10% rice straw ash and 90% wood ash
R10-3
0.0391
0.0223
R10-2
0.0515
0.0303
R10-1
0.0456
0.0271
43
41
41
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Diopside
Diopside
Diopside
Diopside
Diopside
Phosphate
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Leucite
Leucite
Leucite
Leucite
Diopside
–
–
Diopside
Phosphate
Melt
Melt
Melt
Melt
Melt
Melt
Melt
–
–
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Wollastonite
Åkermanite
Åkermanite
Åkermanite
Åkermanite
Åkermanite
Leucite
Leucite
Leucite
Phosphate
1275
1485
1275
1380
1455
1365
2700
1700
4245
Melt
Melt
Melt
Melt
Melt
Melt
Melt
Melt
–
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Larnite
Åkermanite
Åkermanite
Åkermanite
Phosphate
1543
1443
1369
1345
70
180
1180
1315
Melt
Melt
Melt
Melt
Larnite
Larnite
Larnite
Larnite
Q-unknown
Q-unknown
1530
1492
1345
135
170
3050
Q melt
Q melt
Q melt
Larnite
Larnite
Larnite
Periclase
Periclase
Periclase
Q melt, quenched melt. Q-unknown, unknown quenched phase.
388
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 6
Phase composition of wood ash experiments
Run ID
Temperature (°C)
Quenched melt
W-16
1541
W-15
1517
W-14
1510
W-13
1498
W-12
1494
W-11
1490
W-10
1484
W-9
1469
W-8
1468
W-7
1464
W-6
1445
W-20
1431
W-19
1420
W-18
1412
W-17
1402
Larnite
W-16
1541
W-15
1517
W-14
1510
W-13
1498
W-12
1494
W-11
1490
W-10
1484
W-9
1469
W-8
1468
W-7
1464
W-6
1464
W-20
1431
W-19
142
W-18
1412
W-17
1402
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
5
Std
4
Std
5
Std
4
Std
5
Std
4
Std
5
Std
4
Std
4
Std
4
Std
12
Std
5
Std
6
Std
5
Std
4
Std
9.40
2.61
10.00
0.68
4.94
0.55
7.17
0.63
8.09
0.19
7.53
0.66
7.55
1.27
8.28
0.43
9.36
1.00
8.48
0.38
8.19
0.84
5.61
0.57
5.80
0.19
5.58
0.10
2.56
1.53
0.39
0.05
0.41
0.06
0.39
0.04
0.35
0.03
0.35
0.02
0.36
0.01
0.35
0.02
0.38
0.03
0.32
0.02
0.33
0.02
0.36
0.04
0.34
0.04
0.33
0.04
0.35
0.02
0.42
0.03
15.63
3.94
13.45
0.54
17.48
0.50
15.16
0.77
14.83
0.56
15.02
0.83
15.40
1.89
15.30
0.49
15.86
0.93
14.48
0.26
16.34
1.23
18.54
1.64
17.62
0.44
17.79
0.52
19.55
1.17
5.95
1.11
5.65
0.24
8.01
0.52
6.67
0.41
6.17
0.21
6.54
0.48
6.86
0.36
6.95
0.27
6.67
0.54
6.14
0.10
7.39
0.39
7.66
0.57
7.75
0.21
8.25
0.02
9.11
1.00
5.35
1.00
5.09
0.21
7.21
0.47
6.00
0.37
5.55
0.19
5.89
0.44
6.17
0.32
6.26
0.24
6.00
0.48
5.52
0.09
6.65
0.35
6.90
0.52
6.97
0.19
7.43
0.02
8.20
0.90
7.36
1.18
7.19
0.34
10.02
0.77
8.09
0.33
7.93
0.78
8.29
0.25
9.76
3.28
8.75
0.81
8.26
1.21
7.20
0.35
7.31
0.86
8.89
0.62
8.20
0.59
7.88
0.55
13.14
2.05
4
Std
6
Std
5
Std
5
Std
5
Std
6
Std
5
Std
7
Std
3
Std
4
Std
3
Std
6
Std
6
Std
5
Std
5
Std
23.45
0.92
24.58
1.05
23.35
0.86
20.69
0.23
22.84
1.02
21.79
0.69
20.73
2.09
22.98
0.82
27.15
1.89
24.87
1.49
23.44
0.82
19.31
0.82
20.18
0.23
20.02
0.21
18.10
0.56
0.15
0.02
0.15
0.01
0.13
0.02
0.16
0.03
0.14
0.04
0.19
0.03
0.16
0.02
0.15
0.01
0.14
0.05
0.17
0.04
0.18
0.04
0.19
0.02
0.21
0.03
0.20
0.04
0.17
0.03
3.75
1.31
2.95
0.33
2.90
0.27
3.34
0.05
3.05
0.45
3.51
0.11
4.09
0.40
3.07
0.06
3.13
0.11
3.50
0.24
3.15
0.09
3.85
0.21
3.85
0.11
3.89
0.07
3.83
0.29
0.67
0.04
0.69
0.09
0.66
0.15
0.85
0.10
0.72
0.12
0.82
0.14
0.99
0.15
0.75
0.07
0.79
0.12
0.80
0.18
0.73
0.07
1.10
0.14
1.00
0.07
1.14
0.11
1.03
0.11
0.60
0.04
0.62
0.08
0.60
0.13
0.76
0.09
0.64
0.11
0.74
0.13
0.89
0.14
0.68
0.06
0.71
0.11
0.72
0.16
0.65
0.07
0.99
0.13
0.90
0.06
1.02
0.10
0.93
0.10
0.90
0.11
0.84
0.08
0.99
0.10
1.25
0.11
1.07
0.11
1.25
0.09
0.99
0.14
1.14
0.09
1.12
0.06
1.20
0.10
1.16
0.05
1.57
0.09
1.51
0.05
1.42
0.07
1.12
0.06
CaO
Na2O
K2O
P2O5
5.24
2.83
5.20
0.24
4.64
0.54
4.93
0.18
5.22
0.26
4.73
0.36
3.87
1.52
4.19
0.11
4.45
0.38
4.67
0.07
4.88
0.43
4.11
0.36
4.45
0.23
4.39
0.16
2.65
0.49
51.08
2.87
53.58
0.51
50.83
1.03
53.27
0.55
52.94
0.92
53.73
0.65
51.51
0.51
51.15
0.39
49.76
1.68
52.33
0.79
49.44
0.71
49.88
0.83
51.51
0.34
51.47
0.28
49.49
0.52
0.02
0.03
0.02
0.02
0.01
0.01
0.02
0.02
0.01
0.02
0.01
0.01
0.02
0.02
0.05
0.04
0.04
0.04
0.18
0.17
0.03
0.02
0.01
0.01
0.02
0.02
0.02
0.03
0.01
0.01
0.02
0.01
0.02
0.02
0.00
0.01
0.00
0.00
0.01
0.01
0.02
0.01
0.00
0.00
0.01
0.01
0.02
0.01
0.05
0.04
0.01
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
4.15
1.03
4.26
0.23
2.40
0.33
3.85
0.29
3.92
0.15
4.00
0.26
3.35
0.49
3.19
0.11
3.23
0.59
3.87
0.04
3.25
0.35
3.19
0.30
3.04
0.11
3.01
0.05
1.36
0.76
0.66
0.05
0.65
0.06
0.66
0.06
0.60
0.04
0.64
0.07
0.66
0.05
0.36
0.09
0.59
0.06
0.52
0.03
0.59
0.03
0.66
0.01
0.57
0.05
0.59
0.05
0.60
0.04
1.76
2.66
62.34
1.29
63.22
0.69
63.83
0.95
64.29
0.40
64.27
0.34
63.98
0.47
64.21
0.77
62.91
1.23
61.22
1.59
60.35
1.79
60.80
0.94
62.99
0.48
62.01
1.03
62.36
0.53
62.40
1.69
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
0.00
0.00
0.01
0.02
0.03
0.04
0.01
0.01
0.01
0.02
0.04
0.03
0.02
0.02
0.01
0.02
0.00
0.00
0.00
0.00
0.00
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
0.00
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
8.95
0.23
7.81
0.46
7.90
0.18
8.99
0.38
8.09
0.69
9.11
0.11
8.65
1.35
8.00
0.27
7.75
0.11
8.32
0.64
8.12
0.10
9.34
0.59
8.68
0.13
8.53
0.12
9.08
0.74
Total
99.23
99.78
98.73
99.52
99.48
100.23
98.68
98.25
97.96
97.73
97.20
98.23
98.72
98.75
98.30
100.89
100.92
100.44
100.19
100.82
101.35
100.22
99.61
101.85
99.85
98.25
98.92
98.03
98.16
97.50
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
389
Table 6 (continued)
Periclase
W-16
W-15
W-14
W-13
W-12
W-11
W-10
W-9
W-8
W-7
W-6
W-20
W-19
W-18
W-17
1541
1517
1510
1498
1494
1490
1484
1469
1468
1464
1445
1431
1420
1412
1402
2
2
3
2
2
2
2
2
3
1
2
3
3
3
1
0.44
0.71
0.56
0.40
0.69
0.37
2.69
1.86
2.83
1.41
1.34
0.02
0.57
0.05
0.02
0.00
0.02
0.01
0.01
0.01
0.01
0.00
0.03
0.01
0.00
0.01
0.01
0.01
0.00
0.01
0.22
0.20
0.44
0.21
0.86
0.18
0.19
0.16
0.19
0.59
0.20
0.17
0.49
0.42
0.34
0.32
0.33
0.37
0.37
0.52
0.30
0.34
0.20
0.34
0.45
0.45
0.39
0.43
0.40
0.44
0.29
0.30
0.33
0.34
0.47
0.27
0.30
0.18
0.31
0.40
0.41
0.35
0.38
0.36
0.40
2.22
2.23
2.35
2.38
2.38
2.26
1.97
2.18
2.16
1.83
2.27
2.08
2.14
2.00
1.49
94.94
95.15
96.79
97.60
94.83
98.37
94.07
94.06
93.04
93.60
94.88
94.79
95.98
94.59
96.20
1.19
1.03
0.97
1.10
2.32
0.92
0.84
0.87
0.94
1.16
0.68
0.90
2.60
0.81
0.66
0.00
0.00
0.01
0.00
0.02
0.02
0.00
0.01
0.00
0.03
0.03
0.00
0.01
0.03
0.00
0.01
0.00
0.01
0.00
0.01
0.02
0.00
0.00
0.00
0.00
0.00
0.01
0.01
0.00
0.01
0.00
0.00
0.00
0.00
0.08
0.00
0.02
0.01
0.00
0.01
0.02
0.02
0.21
0.01
0.00
99.35
99.68
101.53
102.08
101.73
102.45
100.11
99.38
99.52
99.08
99.89
98.39
102.46
98.32
99.19
Total
Table 7
Phase composition of rice straw ash experiments
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O2
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Melts
R-19
1536
R-18
1490
R-16
1439
R-20
1417
R-15
1393
R-21
1372
R-5
1343
R-6
1270
R-4
1255
R-7
1230
R-8
1212
R-9
1193
R-10
1176
R-3
1167
R-11
1137
R-12
1117
R-13
1100
R-14
1080
R-2
R-1
1075
982
5
Std
6
Std
5
Std
6
Std
6
Std
4
Std
4
Std
5
Std
4
Std
5
Std
4
Std
4
Std
7
Std
9
Std
6
Std
5
Std
5
Std
6
Std
2
4
Std
86.03
1.53
81.96
1.26
82.19
0.59
80.80
0.51
79.46
1.10
78.91
1.21
79.45
0.38
78.77
0.18
78.20
0.42
76.92
1.23
77.17
0.31
76.56
1.36
77.95
1.18
77.56
0.83
76.86
1.24
75.66
1.32
75.69
0.79
75.93
1.29
77.38
76.24
0.54
0.01
0.02
0.01
0.01
0.03
0.02
0.01
0.02
0.03
0.04
0.02
0.02
0.01
0.01
0.01
0.02
0.01
0.01
0.02
0.02
0.03
0.02
0.02
0.02
0.02
0.02
0.03
0.06
0.02
0.02
0.01
0.01
0.04
0.02
0.00
0.01
0.04
0.02
0.03
0.19
0.06
0.33
0.10
0.22
0.13
0.23
0.05
0.21
0.17
0.46
0.47
0.31
0.11
0.20
0.04
0.49
0.34
1.07
1.46
0.38
0.39
0.55
0.53
0.46
0.33
0.40
0.37
0.24
0.15
0.63
0.63
0.21
0.04
0.41
0.33
0.17
0.12
0.08
0.18
0.10
0.11
0.09
0.23
0.10
0.25
0.05
0.19
0.13
0.19
0.12
0.14
0.11
0.08
0.07
0.16
0.03
0.23
0.10
0.16
0.10
0.15
0.09
0.17
0.10
0.09
0.06
0.43
0.69
0.18
0.15
0.08
0.08
0.12
0.10
0.18
0.06
0.07
0.20
0.11
0.12
0.10
0.25
0.11
0.27
0.05
0.21
0.14
0.21
0.13
0.16
0.12
0.09
0.08
0.18
0.03
0.25
0.11
0.18
0.11
0.17
0.10
0.19
0.11
0.10
0.07
0.48
0.77
0.20
0.17
0.09
0.09
0.14
0.11
0.21
0.06
0.08
0.36
0.09
0.69
0.13
0.55
0.12
0.57
0.09
0.58
0.16
0.67
0.18
0.57
0.07
0.50
0.02
0.72
0.11
0.67
0.13
0.70
0.09
0.54
0.08
0.64
0.16
0.56
0.07
0.62
0.07
0.60
0.10
0.64
0.14
0.58
0.16
0.70
0.61
0.04
2.20
0.15
3.15
0.09
2.93
0.10
2.91
0.12
2.91
0.10
3.40
0.10
3.37
0.08
2.98
0.07
3.54
0.16
3.73
0.10
3.19
0.16
3.17
0.34
3.36
0.32
3.02
0.37
3.08
0.20
3.59
0.64
3.35
0.25
3.38
0.17
3.56
3.41
0.19
2.82
0.10
3.80
0.12
3.67
0.11
3.74
0.20
3.76
0.21
4.21
0.35
4.17
0.28
3.91
0.11
4.83
0.11
4.48
0.25
4.27
0.23
4.06
0.25
4.08
0.27
3.48
0.43
3.82
0.14
4.12
0.52
4.22
0.08
4.35
0.36
4.73
4.02
0.12
0.19
0.06
0.26
0.09
0.32
0.07
0.33
0.08
0.32
0.03
0.42
0.06
0.35
0.04
0.42
0.04
0.33
0.07
0.57
0.22
0.41
0.04
0.45
0.03
0.49
0.07
0.52
0.07
0.45
0.10
0.64
0.26
0.44
0.09
0.36
0.05
0.44
0.34
0.08
4.49
0.23
5.78
0.11
7.25
0.10
7.78
0.14
8.53
0.27
8.17
0.10
9.34
0.13
10.76
0.21
10.37
0.09
10.15
0.35
11.23
0.36
11.82
0.32
11.66
0.50
12.18
0.39
12.18
0.61
12.06
0.59
13.09
0.37
13.43
0.25
13.38
15.14
0.38
0.26
0.07
0.22
0.10
0.29
0.06
0.28
0.09
0.65
0.25
0.42
0.19
0.72
0.14
0.37
0.05
0.54
0.07
0.64
0.12
0.83
0.13
0.75
0.31
0.42
0.08
0.40
0.16
0.60
0.15
0.80
0.12
0.88
0.04
0.98
0.06
0.94
0.85
0.06
Quartz
R-5
1343
98.89
0.00
0.15
0.07
0.08
0.00
0.07
0.03
0.00
0.11
0.02
96.75
96.33
97.70
96.93
96.65
96.89
98.45
98.01
99.21
98.51
98.39
98.07
99.28
98.26
98.35
98.32
98.65
99.54
101.56
100.82
99.35
390
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 8
Phase composition of 10% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O2
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
Melt
R10-3
1530
R10-2
1492
5
Std
3
Std
21.11
0.42
22.00
1.24
0.43
0.03
0.45
0.04
12.79
0.83
12.06
1.88
4.49
0.34
4.42
0.63
4.08
0.31
4.02
0.58
4.44
0.37
4.11
0.31
11.87
0.87
12.17
0.19
40.0
0.88
39.78
0.70
0.02
0.02
0.01
0.00
0.00
0.00
0.02
0.02
3.18
0.19
3.10
0.45
Larnite
R10-3
1530
1492
28.19
0.62
28.35
0.23
0.04
0.02
0.03
0.03
0.52
0.15
0.24
0.03
0.10
0.07
0.13
0.06
0.09
0.07
0.12
0.05
0.56
0.06
0.65
0.05
2.74
0.13
3.01
0.08
60.47
0.31
59.88
0.46
0.03
0.02
0.02
0.03
0.02
0.02
0.02
0.03
7.99
0.57
7.57
0.37
100.65
R10-2
5
Std
5
Std
Periclase
R10-3
R10-2
1530
1492
0.02
0.01
0.01
0.02
0.80
0.79
1.47
1.43
1.34
1.30
3.62
3.46
93.67
93.18
0.53
0.37
0.00
0.00
0.01
0.00
0.00
0.03
100.12
99.28
2
1
Total
98.42
98.13
99.89
experiments form the foundation for testing the effects on melting temperature from blending the two ashes. These ash blends
have allowed the freezing point depression to be determined
within the blending range of interest. The principal results of the
experiments are summarized below for each ash or ash blend.
Estimates of the experimental phase proportions as well as the
elemental losses of the alkali earth metals are also included.
The compositions of the ashes and the ash blends are summarized in the Table 2, as they were mixed and as normalized to
100% volatile free. A summary of the experimental conditions
and the principal results for the pure ashes are given in Table 3
(wood ash) and Table 4 (rice straw ash). The experimental
conditions for the ash blends are given in Table 5. The detailed
analytical results are summarized for each experimental product
in Tables 6–13. For each experimental product and each mineral
similar to the rice straw glass was used to monitor losses during
analyzes (see Thy et al. [13]). The precision of the electron
microprobe analyses is generally within 1–3% for major
elements and 5% or above for minor elements (see [13] for
details). The oxides reported were SiO2, TiO2, Al2O3, Fe2O3,
MnO, MgO, CaO, Na2O, K2O, and P2O5. It is assumed that all
iron occurs as Fe3+. Other elements, including Cl, were not
present in sufficient amounts to be detected.
3. Experimental results
The initial melting tests involve the determination and characterization of the equilibrium phase relations of the two pure
ashes (rice straw and wood) as a function of temperature between
their respective liquidus and solidus conditions. These initial
Table 9
Phase composition of 15% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O2
FeO
MnO
MgO
CaO
Na2O
K2 O
P2O5
Q melt
R15-4
1543
1443
R15-2
1369
R15-1
1349
30.21
0.33
31.81
0.17
30.11
0.13
29.90
0.32
0.27
0.07
0.21
0.04
0.35
0.07
0.38
0.03
6.65
0.77
6.03
1.05
10.03
1.66
9.95
0.70
2.33
0.33
2.07
0.52
3.60
0.70
3.37
0.39
2.12
0.30
1.88
0.47
3.27
0.64
3.06
0.36
2.87
0.31
2.52
0.58
3.42
0.51
3.27
0.21
9.60
0.97
9.59
0.74
9.36
0.20
8.77
0.31
41.97
1.65
42.26
1.50
38.36
3.29
38.70
0.88
0.13
0.06
0.08
0.04
0.01
0.02
0.01
0.02
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
4.68
0.21
4.11
0.24
3.95
0.43
4.04
0.22
98.72
R15-3
4
Std
4
Std
4
Std
4
Std
Larnite
R15-4
1543
1443
R15-2
1369
R15-1
1349
28.33
0.32
28.65
0.24
27.36
0.20
26.94
0.37
0.02
0.02
0.04
0.03
0.01
0.02
0.01
0.03
0.19
0.05
0.38
0.16
0.24
0.04
0.19
0.04
0.09
0.06
0.14
0.13
0.08
0.10
0.07
0.07
0.08
0.05
0.13
0.12
0.08
0.09
0.06
0.06
1.00
0.06
1.20
0.19
1.12
0.10
1.26
0.09
3.64
0.20
3.67
0.12
3.66
0.09
3.44
0.12
56.72
0.60
57.10
0.28
56.50
0.47
55.97
0.31
0.11
0.04
0.08
0.03
0.01
0.02
0.00
0.00
0.00
0.00
0.01
0.02
0.00
0.00
0.00
0.00
7.75
0.25
7.95
0.21
9.20
0.23
9.92
0.63
97.85
R15-3
5
Std
5
Std
6
Std
5
Std
1
3
Std
33.99
33.30
0.18
0.04
0.04
0.03
0.46
0.15
0.01
0.25
0.07
0.02
0.23
0.06
0.02
0.65
0.74
0.05
11.59
11.48
0.17
50.77
50.31
0.61
0.00
0.02
0.02
0.00
0.01
0.02
2.44
3.22
0.12
100.20
99.35
Unknown (quench phase)
R15-2
1369
R15-1
1349
Total
98.69
99.18
98.38
99.21
98.18
97.80
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
and glass phase, the tables give the averages of several point
analyses and, when appropriate, standard deviations.
391
the reach of the experimental quench furnace. The actual
melting point can thus only be estimated by extrapolating the
present results to high temperatures.
The experimental products (Table 3) retain their original
cylindrical pellet shape until about 1400 °C, above which
irregular droplet shapes are seen. Below 1350 °C, the pellets are
largely unconsolidated and break easily when handled. Since
interstitial quenched melt is not present in experimental
products below 1390 °C, the solidus must be located between
1390 and 1347 °C. The detailed analytical results are given in
Table 6. The build-up of a typical experimental droplet is
illustrated in Fig. 2A by a reflected light photomicrograph. The
droplets are composed of large, air-filled vesicles with walls of
silicate and oxide materials (Fig. 2B). The oxide and silicate
phases present in the experimental products above 1400 °C are
illustrated in Fig. 3 by a back-scattered electron image (BSE,
mean-atomic density distribution). The low-density mineral
(dark) is periclase (MgO) that forms grains typically below
3.1. Wood ash
The wood ash was examined between temperatures of
1541 and 1132 °C (Table 3). The very high temperature that
was required to completely melt the ash was not anticipated
[13,14]. The furnace heating elements are rated to a
maximum set point of 1700 °C. This generally allows a
maximum sample temperature of 1630–1640 °C to be
reached. Such a temperature is still insufficient to completely
melt the wood ash and some of the ash blends. The present
results suggest a very steep liquidus slope characterized by
limited compositional variation with large variation in
temperature. This is supported by the synthetic system
CaO–SiO2–MgO that reaches liquidus temperatures of
1800 °C for comparable compositional intervals [20], beyond
Table 10
Phase composition of 20% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O2
FeO
MnO
Melt
R20-9
1415
R20-8
1390
R20-7
1368
R20-6
1344
R20-2
1318
R20-1
1299
R20-5
1271
R20-4
1249
4
Std
4
Std
4
Std
5
Std
6
Std
5
Std
6
Std
3
Std
33.86
0.20
34.04
0.27
34.21
0.30
34.58
0.25
34.76
0.34
32.89
0.23
32.08
0.37
31.38
0.33
0.22
0.03
0.19
0.03
0.21
0.05
0.26
0.03
0.23
0.01
0.52
0.05
0.85
0.05
1.54
0.08
4.39
0.14
4.49
0.12
4.67
0.13
4.91
0.16
5.52
0.45
5.53
0.07
5.10
0.13
4.89
0.25
1.55
0.15
1.64
0.15
1.91
0.07
1.73
0.12
1.96
0.22
3.67
0.12
4.84
0.22
5.01
0.24
1.41
0.13
1.49
0.13
1.73
0.07
1.57
0.11
1.78
0.20
3.34
0.11
4.40
0.20
4.56
0.22
2.04
0.16
1.92
0.15
1.87
0.09
2.13
0.16
2.43
0.16
3.41
0.28
3.81
0.17
4.12
0.20
Larnite
R20-8
1390
R20-7
1368
R20-6
1344
R20-2
1318
R20-1
1299
R20-5
1271
R20-4
1249
3
Std
4
Std
3
Std
6
Std
4
Std
4
Std
4
Std
28.25
0.26
28.13
0.32
27.79
0.14
27.01
0.32
25.89
0.65
23.87
0.30
21.86
0.88
0.03
0.04
0.01
0.01
0.01
0.02
0.02
0.02
0.03
0.02
0.02
0.04
0.06
0.05
0.15
0.06
0.16
0.02
0.17
0.02
0.37
0.04
0.20
0.05
0.17
0.03
0.23
0.19
0.18
0.04
0.06
0.07
0.13
0.04
0.18
0.12
0.26
0.10
0.20
0.21
0.36
0.15
0.17
0.04
0.05
0.06
0.12
0.04
0.17
0.11
0.24
0.09
0.19
0.19
0.33
0.13
4
Std
4
Std
4
Std
39.84
0.51
38.76
0.56
38.42
0.43
0.03
0.02
0.02
0.02
0.08
0.12
5.72
0.60
6.38
0.61
5.75
0.94
0.91
0.07
1.20
0.18
1.82
0.61
0.83
0.06
1.09
0.16
1.66
0.55
Åkermanite
R20-1
1299
R20-5
1271
R20-4
1249
MgO
CaO
Na2O
K2O
P2O5
Total
7.72
0.25
7.70
0.05
8.12
0.04
7.87
0.12
7.47
0.30
6.10
0.17
4.77
0.10
4.29
0.11
44.85
0.12
44.82
0.19
43.80
0.42
42.83
0.59
42.40
0.70
41.78
0.52
41.00
0.48
40.49
0.13
0.01
0.01
0.01
0.02
0.01
0.01
0.05
0.05
0.10
0.05
0.07
0.03
0.04
0.04
0.12
0.04
0.01
0.01
0.02
0.02
0.01
0.01
0.00
0.01
0.01
0.03
0.03
0.03
0.00
0.00
0.06
0.03
4.75
0.09
4.65
0.16
4.41
0.05
4.24
0.13
4.56
0.23
5.32
0.09
5.23
0.11
5.55
0.05
99.40
0.93
0.06
1.03
0.06
1.03
0.07
1.04
0.16
1.78
0.16
1.68
0.13
1.84
0.14
3.12
0.07
3.33
0.09
3.31
0.08
3.15
0.08
2.51
0.08
1.94
0.10
1.70
0.21
58.83
0.61
58.88
0.27
58.28
0.48
57.92
0.41
57.85
0.38
56.90
0.25
57.26
0.61
0.00
0.01
0.00
0.00
0.02
0.03
0.13
0.05
0.07
0.04
0.02
0.03
0.09
0.02
0.00
0.00
0.01
0.01
0.01
0.02
0.01
0.02
0.02
0.02
0.01
0.01
0.06
0.02
8.62
0.64
8.82
0.13
8.65
0.28
9.82
0.57
11.74
0.59
13.99
0.24
16.10
0.64
100.10
1.12
0.12
1.21
0.17
1.60
0.24
10.95
0.06
10.17
0.23
9.92
0.33
41.49
0.51
40.79
0.21
40.81
0.30
0.11
0.04
0.08
0.04
0.17
0.03
0.03
0.02
0.01
0.02
0.05
0.03
0.61
0.05
0.57
0.08
0.88
0.58
100.82
99.50
99.2
98.61
99.44
99.32
97.73
97.46
100.44
99.40
99.65
100.34
98.80
99.56
99.18
99.51
392
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 11
Phase composition of 30% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Melt
R30-1
1300
R30-2
1275
R30-9
1253
R30-8
1228
R30-3
1206
R30-7
1182
R30-6
1161
5
Std
4
Std
7
Std
3
Std
5
Std
4
Std
7
Std
43.88
0.36
43.36
0.29
40.00
0.26
40.92
1.40
38.26
0.76
36.07
1.83
36.76
0.94
0.16
0.06
0.16
0.02
0.26
0.05
0.21
0.05
0.30
0.05
0.70
0.20
0.74
0.15
3
Std
10
Std
5
Std
4
Std
5
Std
50.67
0.22
49.87
0.32
49.67
0.47
50.16
0.61
49.59
0.18
5
Std
4
Std
5
Std
3
Std
2
2
4
Std
5
Std
Wollastonite
R30-9
1253
R30-8
1228
R30-3
1206
R30-7
1182
R30-6
1161
Åkermanite
R30-9
1253
R30-8
1228
R30-3
1206
R30-7
1182
R30-6
1161
Leucite
R30-3
R30-7
1206
1182
R30-6
1161
Fe2O2
FeO
MnO
3.59
0.10
3.91
0.09
5.33
0.13
5.53
0.56
5.49
0.33
3.19
0.51
2.71
0.16
1.26
0.21
1.31
0.16
1.87
0.13
2.06
0.31
2.33
0.20
4.27
0.15
3.90
0.31
1.15
0.19
1.19
0.14
1.70
0.12
1.88
0.28
2.12
0.18
3.88
0.14
3.55
0.28
1.73
0.23
1.95
0.23
2.37
0.21
2.75
0.32
2.96
0.27
3.59
0.58
3.93
0.20
0.01
0.01
0.03
0.03
0.01
0.02
0.01
0.02
0.05
0.03
0.03
0.02
0.11
0.25
0.04
0.03
0.04
0.01
0.04
0.02
0.05
0.04
0.05
0.07
0.05
0.05
0.05
0.04
0.04
0.06
0.05
0.03
0.05
0.07
0.05
0.05
0.05
0.03
0.04
0.05
41.97
0.27
42.90
0.71
42.08
0.36
42.08
0.09
39.23
0.01
0.01
0.03
0.02
0.03
0.02
0.03
0.01
0.02
1.31
0.10
2.12
1.20
1.59
0.24
1.45
0.02
1.43
0.31
0.08
0.43
0.23
0.47
0.11
0.64
0.16
0.42
54.41
53.80
0.32
53.28
0.64
0.00
0.02
0.03
0.03
0.04
22.78
22.46
0.32
21.19
0.50
1.12
1.98
0.08
2.86
0.43
20 μm in size. Of the minor elements substituting into periclase,
Mn and Ca are the most important with a total of about 3% as
oxides. The variable Si and Al are probably due to the small
grain size and interference during analyses from the quenched
melt. The remaining elements calculate to an average periclase
composition of (Mg0.98Mn0.01Ca0.01)O, with iron in trace
amounts. The 100 μm, larger and rounded, intermediate density
mineral is a polymorph of dicalcium silicate approximating a
general larnitic formula (2CaO–SiO2 or Ca2SiO4). When
calculated to four oxygens, the average formula is ((Ca1.943+
Mn0.03Mg0.03)(Si0.64P0.21Al0.12Fe0.02
)O4). The substitution of
3−
2−
(PO4) for (SiO4) groups maintains the ionic balance from
the substitution of alumina. Phosphor-containing larnitic
polymorphs are commonly found in Portland cement and
other ceramic products and appear to be stabilized by high
MgO
CaO
Na2O
K2 O
P2O5
Total
6.42
0.12
6.21
0.13
6.29
0.17
5.99
0.82
5.47
0.06
4.42
0.45
4.92
0.31
38.67
0.58
38.42
0.23
34.88
0.26
34.50
1.36
32.16
0.67
29.70
1.11
26.14
0.51
0.17
0.08
0.30
0.05
0.39
0.05
0.35
0.01
0.40
0.06
0.62
0.15
0.76
0.09
0.09
0.03
0.37
0.06
1.39
0.11
1.74
0.23
3.47
0.14
4.86
0.31
8.12
0.37
3.40
0.14
3.89
0.08
5.39
0.11
5.35
0.37
7.28
0.68
7.80
1.08
8.32
0.30
99.39
0.10
0.09
0.11
0.13
0.03
0.04
0.10
0.04
1.14
0.59
0.06
0.02
0.13
0.23
0.04
0.02
0.02
0.02
0.99
0.58
49.32
0.70
49.30
0.74
48.97
0.55
46.78
0.47
46.90
1.69
0.00
0.01
0.03
0.03
0.00
0.01
0.01
0.02
0.02
0.03
0.01
0.01
0.05
0.10
0.01
0.01
0.05
0.03
0.07
0.08
0.84
0.19
0.78
0.21
0.75
0.08
0.62
0.07
0.74
0.11
101.10
0.28
0.07
0.39
0.21
0.43
0.10
0.58
0.15
0.38
1.01
0.08
1.25
0.08
1.28
0.11
1.83
0.09
1.59
12.22
0.27
11.97
1.17
12.02
0.36
11.28
0.30
10.95
40.27
0.14
39.30
2.38
39.68
0.28
37.85
0.43
40.12
0.39
0.03
0.43
0.03
0.40
0.02
0.48
0.05
0.46
0.31
0.03
0.97
0.94
0.63
0.12
0.71
0.05
1.12
0.58
0.08
0.98
0.37
0.66
0.16
0.48
0.08
3.52
98.38
1.02
1.80
0.07
2.60
0.39
0.00
0.07
0.05
0.20
0.17
0.07
0.08
0.02
0.31
0.12
0.05
0.10
0.03
0.67
0.77
0.10
0.04
0.03
0.03
0.03
20.45
19.42
0.44
20.20
0.60
0.00
0.00
0.00
0.15
0.15
99.88
98.16
99.40
98.11
95.22
96.31
100.46
99.59
97.85
99.58
100.39
98.84
96.82
98.85
98.97
97.96
98.93
concentrations of phosphor [21,22]. Since the crystal structure
has not been determined, the phase is referred to as larnitic or
larnite for simplicity. The often well-rounded shapes of the
larnitic grains may be related to very low viscosity of the melt
phase and large differences in surface tension.
Perhaps the most important finding is that all potassium has
been lost from the experimental wood ash product with the
result that the K2O concentrations are systematically below
detection limits for all mineral phases and the quenched melt.
The mass loss during the experiments is relatively constant at
41% by weight (Table 3), attributed to the breakdown of
hydrous amorphous components and carbonate and the
complete loss of K2O during heating and melting.
Further insight into the elemental distributions between the
various phases can be obtained from examining Fig. 4
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
393
Table 12
Phase composition of 40% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Melt
R40-8
1296
R40-7
1268
R40-1
1250
R40-6
1223
R40-5
1201
R40-4
1182
R40-2
1160
R40-3
1134
5
Std
5
Std
5
Std
5
Std
5
Std
4
Std
5
Std
5
Std
47.39
0.11
48.79
0.58
46.50
0.25
47.63
0.31
47.09
0.52
45.58
0.32
48.81
0.54
54.23
0.93
0.11
0.06
0.18
0.05
0.11
0.03
0.15
0.03
0.17
0.04
0.17
0.03
0.23
0.05
0.29
0.05
5
Std
4
Std
5
Std
4
Std
3
Std
6
Std
4
Std
49.89
0.16
50.33
0.14
50.29
0.39
49.72
0.31
50.46
0.45
50.68
0.30
50.44
0.39
3
Std
4
Std
4
Std
3
Std
3
Std
Wollastonite
R40-7
1268
R40-1
1250
R40-6
1223
R40-5
1201
R40-4
1182
R40-2
1160
R40-3
1134
Leucite
R40-4
1182
R40-2
1160
R40-3
1134
Diopside
R40-5
1201
R40-3
1134
Fe2O2
FeO
MnO
2.87
0.09
3.47
0.04
3.46
0.15
4.35
0.24
4.72
0.14
4.55
0.02
4.30
0.11
3.76
0.16
0.96
0.08
1.17
0.22
1.35
0.14
1.36
0.17
1.77
0.18
1.53
0.15
1.83
0.10
2.91
0.29
0.87
0.07
1.07
0.20
1.23
0.13
1.24
0.16
1.61
0.16
1.39
0.14
1.67
0.09
2.64
0.27
1.54
0.10
1.70
0.08
1.83
0.11
1.67
0.10
1.76
0.12
1.95
0.04
2.01
0.11
2.01
0.29
0.01
0.02
0.00
0.00
0.03
0.03
0.03
0.02
0.00
0.01
0.02
0.02
0.00
0.01
0.03
0.03
0.02
0.02
0.05
0.04
0.01
0.01
0.11
0.09
0.06
0.04
0.03
0.04
0.04
0.06
0.02
0.03
0.06
0.05
0.04
0.05
0.05
0.04
0.03
0.03
0.06
0.05
0.04
0.05
0.01
0.03
0.06
0.04
0.04
0.05
0.04
0.03
0.02
0.03
0.05
0.04
54.82
0.10
55.92
0.41
55.01
0.16
0.00
0.00
0.03
0.03
0.01
0.01
20.70
0.36
19.08
0.41
19.00
0.37
1.70
0.18
1.80
0.30
2.43
0.15
53.51
0.51
53.70
0.64
0.05
0.05
0.06
0.02
0.41
0.05
0.21
0.03
0.74
0.22
0.77
0.19
showing X-ray dot-maps for the principal elements (Kα lines).
The elements Si, Ca, and P are principally partitioned into the
larnitic phase. Mg goes dominantly into periclase, while the
other elements, Fe, Mn, and Al, are mostly partitioned into
the melt. Better quantification of the elemental distributions,
calculated as oxides, can be obtained from inspecting the
analytical data (Table 6). The back-scattered electron image
shown in Fig. 4 illustrates that the melt quenches as a twocomponent, but unidentified, mixture. The first phase to
appear is a high-density, skeletal, fast growing mineral. The
final phase is a lower-density interstitial material. The Al Kα
dot-map shows that Al is preferentially partitioned into the
last forming interstitial phase (probably melt). Aluminum thus
MgO
CaO
Na2O
K2 O
P2O5
Total
5.51
0.23
6.65
0.21
6.83
0.14
6.92
0.17
6.41
0.07
7.00
0.05
7.12
0.12
4.84
0.15
29.84
0.54
26.94
0.37
26.81
0.34
23.30
0.53
21.30
0.52
20.59
0.27
16.11
0.29
12.57
0.88
0.47
0.03
0.49
0.08
0.48
0.07
0.53
0.08
0.64
0.10
0.64
0.10
0.69
0.04
1.04
0.12
6.32
0.11
5.18
0.13
6.36
0.17
7.44
0.19
8.02
0.08
9.95
0.07
12.35
0.16
13.58
0.27
2.99
0.09
2.62
0.15
3.87
0.13
3.71
0.22
4.11
0.24
5.09
0.11
3.86
0.21
2.65
0.45
0.05
0.05
0.04
0.04
0.77
0.08
0.75
0.28
0.73
0.20
0.69
0.19
0.88
0.25
0.09
0.03
0.06
0.03
2.21
0.18
1.69
0.52
1.58
0.24
1.22
0.41
1.23
0.34
49.03
0.26
48.56
0.17
45.49
0.49
45.64
0.88
46.23
0.98
46.71
1.22
45.62
0.61
0.01
0.01
0.02
0.02
0.04
0.03
0.03
0.03
0.02
0.01
0.01
0.02
0.05
0.02
0.01
0.01
0.05
0.04
0.01
0.02
0.08
0.05
0.26
0.16
0.17
0.16
0.18
0.09
0.57
0.13
0.74
0.08
0.61
0.11
0.60
0.08
0.71
0.07
0.74
0.08
0.69
0.10
1.55
0.16
1.63
0.27
2.21
0.13
0.03
0.02
0.13
0.06
0.15
0.01
0.73
0.05
1.07
0.03
1.04
0.10
0.20
0.10
0.83
1.24
0.25
0.30
0.07
0.01
0.04
0.03
0.10
0.06
19.58
0.05
19.25
0.77
19.15
0.30
0.00
0.00
0.01
0.02
0.01
0.02
97.83
0.67
0.20
0.70
0.17
0.76
0.02
1.19
0.12
17.50
0.13
17.33
0.06
25.74
0.33
24.99
0.61
0.07
0.03
0.04
0.03
0.02
0.02
0.03
0.04
0.38
0.08
0.39
0.07
99.18
98.00
97.19
97.61
97.04
95.99
97.07
97.31
97.87
99.73
99.84
99.58
98.58
100.15
100.33
99.18
98.16
97.16
98.71
is not incorporated to any significant extent into silicate and
oxide minerals until near the solidus.
3.2. Rice straw ash
The rice straw ash was examined between temperatures of
1536 and 982 °C (Table 4). The liquidus temperature has not
been bracketed. However, the amount of melt in the
experimental products indicates that the liquidus is located
only slightly above 1536 °C. This high liquidus was unexpected since a relatively similar rice ash investigated by
Thy et al. [14] melted completely at a temperature of
1074 °C. The experimental products retain their original pellet
394
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 13
Phase composition of 50% rice straw blend
Run ID
Temperature (°C)
No/Std Dev
SiO2
TiO2
Al2O3
Fe2O2
FeO
MnO
Melt
R50-9
1318
R50-8
1296
R50-4
1273
R50-2
1252
R50-5
1230
R50-3
1205
R50-6
1180
R50-1
1163
16
Std
5
Std
6
Std
6
Std
5
Std
3
Std
4
Std
8
Std
58.62
0.42
59.48
0.65
56.42
0.26
59.99
1.66
57.06
0.54
57.79
0.23
63.77
0.52
63.41
1.01
0.09
0.04
0.11
0.03
0.08
0.03
0.10
0.03
0.11
0.04
0.18
0.04
0.22
0.04
0.20
0.04
2.45
0.06
2.51
0.10
2.52
0.12
2.79
0.26
2.83
0.03
3.15
0.13
4.31
0.32
4.71
0.18
0.90
0.13
0.93
0.14
0.81
0.13
1.02
0.14
0.98
0.03
1.32
0.23
1.22
0.21
2.10
0.41
0.82
0.12
0.85
0.13
0.74
0.12
0.93
0.13
0.89
0.03
1.20
0.21
1.11
0.19
1.91
0.37
1.39
0.16
1.45
0.18
1.35
0.13
1.55
0.12
1.53
0.09
1.52
0.13
1.58
0.20
1.71
0.09
2
5
Std
5
Std
3
Std
7
Std
4
Std
4
Std
50.55
50.23
0.79
50.89
0.36
51.26
0.35
50.20
0.48
50.22
0.26
50.49
0.35
0.00
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.02
0.03
0.02
0.01
0.07
0.02
0.02
0.01
0.01
0.01
0.02
0.05
0.04
0.03
0.03
0.09
0.11
0.00
0.01
0.01
0.04
0.06
0.03
0.04
0.05
0.04
0.05
0.06
0.05
0.06
0.00
0.01
0.01
0.04
0.05
0.03
0.03
0.05
0.04
0.05
0.06
0.04
0.05
5
Std
6
Std
54.28
0.27
54.52
0.58
0.02
0.02
0.05
0.04
0.05
0.01
0.07
0.01
0.49
0.09
0.53
0.17
0.44
0.08
0.48
0.16
Wollastonite
R50-8
1296
R50-4
1273
R50-2
1252
R50-5
1230
R50-3
1205
R50-6
1180
R50-1
1163
Diopside
R50-6
1180
R50-1
1163
shape until about 1170 °C, above which the pellets are
characterized by expansion and bubble formation with thin
silicate walls.
MgO
CaO
Na2O
K2O
P2O5
Total
4.94
0.12
5.15
0.12
4.93
0.14
4.93
0.20
5.52
0.11
5.57
0.08
3.92
0.25
3.61
0.21
25.68
0.45
23.44
0.29
22.21
0.38
20.42
0.95
19.12
0.27
17.44
0.20
12.66
0.70
9.96
0.65
0.34
0.06
0.31
0.07
0.44
0.09
0.36
0.04
0.45
0.06
0.50
0.00
0.59
0.09
0.59
0.06
3.16
0.10
5.48
0.10
8.84
0.15
4.73
0.26
9.10
0.13
8.5
0.25
9.50
0.16
11.57
0.16
0.78
0.08
0.86
0.26
2.17
0.17
1.92
0.37
3.12
0.25
2.79
0.04
2.28
0.24
1.51
0.20
0.09
0.10
0.03
0.99
0.17
0.68
0.02
1.10
0.09
1.15
0.09
1.30
0.44
0.06
0.07
0.03
1.84
0.21
1.36
0.04
2.01
0.21
1.85
0.16
1.97
0.26
49.11
49.29
0.41
45.92
0.22
47.30
0.34
46.06
0.60
45.74
0.45
44.59
0.96
0.01
0.02
0.02
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.02
0.03
0.01
0.12
0.07
0.00
0.01
0.14
0.02
0.13
0.07
0.18
0.09
0.17
0.06
0.78
0.81
0.06
0.88
0.06
0.83
0.08
0.65
0.04
0.72
0.05
0.85
0.03
100.68
100.67
0.95
0.10
1.05
0.10
17.98
0.29
17.15
0.40
26.51
0.22
25.71
0.34
0.07
0.04
0.07
0.04
0.04
0.02
0.09
0.06
0.45
0.06
0.56
0.04
100.84
98.35
99.71
99.78
97.81
99.82
98.86
100.03
99.36
100.60
101.62
100.28
99.98
99.55
99.78
The silicate phases present in the experimental products are
illustrated in Fig. 5 as a reflected light image and a mean-atomic
density map (BSE). The low-density mineral (dark) is quartz (or
Fig. 2. Reflected light photomicrographs of polished sections of experimental wood ash products. (A) Experiment at 1510 °C (W-14, Table 3). The experimental
product is composed of irregular walls of solid material and an inner concentration of large vesicles (now in part filled by plastic or air, gray to dark rounded areas).
Scale bar is 1 mm. (B) Experiment product at 1541 °C (W-16, Table 3). Close-up shows that three phases are present, large grayish tinted and rounded grains (larnite),
small white grains with high contrast to the surroundings (periclase), and white interstitial material (quenched melt). Scale bar is 0.5 mm.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
395
The BSE image and X-ray dot-maps in Fig. 7 illustrate that
the melt quenches as an unidentified two-phase mixture. As for
the pure wood ash, Al is strongly partitioned into the quenched
melt together with Fe, Mn, and Mg. Potassium is completely
lost to the furnace gas as seen for the pure wood ash. The
elemental losses during the experiments amount to about 42%
by weight, very similar to that observed for pure wood ash.
3.4. 15% rice straw ash and 85% wood ash blend
Fig. 3. Back-scattered electron image of experimental wood ash product (W-15,
Table 3). The Ca–Si-rich larnite phase is gray with slight compositional zoning
toward the rims. Black grains are periclase (MgO). The interstitial melt has
quenched as a two-phase unidentified composite (white and gray). Scale bar is
20 cm.
a quartz polymorph, such as tridymite or cristobalite) that forms
tabular often radiating grains, mostly below 40 μm. The lighter
gray matrix is glass (or quenched melt). Additional silicates
were not detected, not even in the low melting temperature
experiments to 982 °C.
Some important observations distinguish the behavior of the
rice straw ash from that observed for the wood ash: (1) glass
forms easily in the rice straw material upon quenching; (2) the
vesiculation and bubble-formation are much more extensive in
the rice straw ash than the wood ash; and (3) the crystals
coexisting with the rice straw melt display euhedral growth
forms not seen for the wood ash. These differences are likely
related to large differences in the physical properties of the
melts, such as a much higher viscosity for the rice straw melt as
opposed to the wood melt.
The analytical results given in Table 7 clearly demonstrate
that potassium is partially retained in the glass and not
completely lost to the furnace atmosphere, as was the case for
the wood ash. The mass loss during the experiments is relatively
constant at 16% by weight (Table 4), attributed to the
breakdown of hydrous amorphous compounds and the loss of
residual carbon as well as some loss of potassium.
3.3. 10% rice straw ash and 90% wood ash blend
The melting relations of this ash blend were determined
between 1530 and 1345 °C (Table 5) without reaching the
liquidus for the same reasons as for the pure wood ash.
Typical experimental products are illustrated in Fig. 6. The
detailed analytical results are summarized in Table 8. The
experimental products are essentially similar to those obtained
for the pure wood ash containing larnite and periclase
together with a quenched melt phase. The products are compact with few, if any, vesicles and are composed of closely
packed larnitic and periclase grains with quenched interstitial
melt.
Only four experiments were conducted on this ash blend.
These were done between 1543 and 1345 °C (Table 5) without
reaching the liquidus for similar reasons as for the pure wood
ash and the 10% rice straw ash blend. The detailed analytical
results are summarized in Table 9. Typical experimental
products are as for the 10% rice straw blend. As seen for the
pure wood ash and the 10% ash blend, the melt quenches to a
mixture composed of two phases. One of these phases are
elongated grains of larnitic composition ((Ca1.48Mn0.02Mg0.47)
(Si0.92P0.07Al0.01)O4) with much higher Mg and lower Ca and P
content than the larnitic phase that appears in the quench melt
for the pure wood. As for the previous ash blend, the elemental
losses amount to an average 42% by weight. All potassium is
lost to the furnace gas during the experiments.
3.5. 20% rice straw ash and 80% wood ash blend
A series of nine experiments were conducted to constrain the
melting relations of this ash blend. These were done between
1415 and 1154 °C (Table 5). The highest temperature
experiment constrains the liquidus (1403 ± 12 °C), while the
lowest temperature experiments were below the solidus since no
melt was detected. The total melting interval for this blend was
determined to be 150–200 °C. From about 1150 °C, the
experimental pellets are well sintered, partial melting occurs at
about 1250 °C, and near complete melting occurs at about
1300 °C. The detailed analytical results are summarized in
Table 10. Most experimental products are highly heterogeneous
with zones of melt and minerals (Fig. 8A). The mineral phases
observed in the experimental products are larnite at the liquidus
and nearly pure åkermanite (Ca2.02(Mg0.71Si1.79Al0.32)O7, with
minor amounts of Mn, Fe3+, P, and Na) appearing at 1309 ±
10 °C (Fig. 8B). A phosphate phase occurs in the lowest
temperature experiment as grains too small to be analyze
individually. As seen for the pure wood ash and the 10% and
15% blends, the melt quenches to a mineral–glass mixture. A
dendritic unidentified quenched mineral appears in the
dominating glass. These dendrites are rich in Si, Mg, and
Ca and poor in Al. Aluminum is thus retained only in the
melt. As for the previous ash blend, the elemental losses
amount to an average 41% by weight. All potassium is lost to
the furnace gas during the experiments.
3.6. 30% rice straw ash and 70% wood ash blend
A series of nine experiments was conducted between 1300
and 1071 °C (Table 5) to constrain the melting relations of this
396
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Fig. 4. X-ray density dot-maps for the main elements in the experimental wood ash product (W-18, 1412 °C; Table 3). Kα lines for Si, Al, Fe, Mn, Mg, Ca, and P. BSE,
back-scattered electron image.
ash blend. The highest temperature experiment constrains
the liquidus at 1288 ± 13 °C. The lowest temperature experiment was below the solidus that thus was loosely constrained
at 1138 ± 23 °C. The total melting interval for this blend was
determined to be about 150 °C. The detailed analytical results
are summarized in Table 11. The experimental products
Fig. 5. Rice ash product. (A) Reflected microscope image of R-21 (1372 °C, Table 4) showing that the experimental product is composed of a large bubble with a thin
outer wall. Scale bar is 3 mm. (B) Back-scattered electron image of the silicate wall in panel A. The dark tabular grains are quartz. Light gray is glass. Scale bar is 50 cm.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
397
Fig. 6. Product from heating 10% rice straw ash blend. (A) Reflected microscope image (R10-2, 1472 °C, Table 5) showing that the product is a compacted droplet of
rounded larnitic grains and an interstitial quenched melt. Scale bar is 1 mm. (B) Back-scattered electron image of the center of the same experimental product as in
panel A. The lighter rounded grains are larnite. The quenched interstitial melt is composed of two phases. Scale bar is 50 cm.
indicate a complex mineralogy composed of wollastonite,
åkermanite, and leucite. Phosphate was only encountered in the
lowest temperature experiment. Wollastonite ((Ca1.01Si0.97)O3,
with minor amounts of Mn, Mg, and P) and åkermanite
((Ca1.96Mg0.81Mn0.06)(Si1.93Al0.09)O7, with minor amounts of
Fe 3+ , K, and P), are the liquidus phases. Leucite
(K0.94Al0.96Fe3+0.06Si1.99O6, with minor amounts of Mg and
Ca) appears at 1217 ± 11 °C.
Most experimental products are highly heterogeneous with
zones of melt and minerals (Fig. 9). For example, the experiment
at 1206 °C contains a marginal zone of leucite, followed by
åkermanite, and toward the center contains abundant quenched
melt (or glass). The leucite may display well-developed crystal
surfaces (Fig. 9). Fig. 10 shows X-ray dot-maps for 1228 °C
intergrowth of wollastonite, åkermanite, and glass. Wollastonite
appears as large grains (∼200 μm) particularly seen on the Mg
and Ca Kα maps, but difficult to distinguish from the melt on the
mean-atomic density BSE image. Åkermanite appears as small
irregular grains (b50 μm). Fig. 11 shows X-ray dot maps of 1161
°C complex mineral intergrowth including leucite and with
relatively small amounts of glass present. On the BSE image,
wollastonite and åkermanite form a complex intergrowth that
cannot easily be discriminated. In contrast, leucite stands out as
dark grains and the glass as light patches. The K and Al Kα maps
clearly identify the leucite and the glass, while Mg and Ca Kα
maps as before identify wollastonite and åkermanite. The melt
readily quenched to a glass in contrast to the wood ash and low
rice straw blends.
The elemental losses vary from 39% to 31% by weight
positively correlated with temperature (Table 5). This variation
is consistent with the observation that potassium is partially lost
to the furnace gas and that the loss decreases with decreasing
temperature and the appearance of leucite in the products.
3.7. 40% rice straw ash and 60% wood ash blend
A series of eight experiments was conducted between 1296
and 1134 °C (Table 5) to constrain the melting relations of this
ash blend. The highest temperature experiment constrains the
liquidus at 1282 ± 14 °C. The lowest temperature experiment
was well above the solidus. The detailed analytical results are
summarized in Table 12. The experimental products indicate a
complex mineralogy composed of wollastonite, leucite, and
diopside. Wollastonite appears on the liquidus at 1282 ± 14 °C.
Leucite and diopside ((Ca1.00Mg0.95Mn0.03)Si1.97O6, with
minor amounts of Al, Fe3+, and P) appear nearly simultaneously at 1212 ± 11 °C. The appearance of wollastonite is
conspicuous as this phase forms large euhedral grains with
well-developed crystal surfaces. Two products at 1182 and
1160 °C did not appear to contain diopside; however, the
failure to detect this phase may be related to the large grain
and the small section sizes. Phosphate grains appear at the
lowest temperature investigated, but are too small to be
analyzed. Typical BSE images of the experimental product are
shown in Fig. 12. The wollastonite forms large euhedral to
skeletal grains that may exceed 1 mm in size (Fig. 12A).
Large elongated and euhedral diopside appears at lower
temperature together with rounded to irregular shaped
wollastonite. The rectangular shape of the diopside in crosssection is characteristic for this phase (Fig. 12B). Leucite
appears intergrown with wollastonite (Fig. 12C).
The elemental losses are relatively constant at about 30% by
weight (Table 5). This amount is similar to the low temperature
loss observed for the 30% rice straw blend and reflects that most
of the potassium is being retained in the experimental products.
3.8. 50% rice straw ash and 50% wood ash blend
A series nine experiments was conducted between 1318 and
1064 °C (Table 5) to constrain the melting relations this ash
blend. The highest temperature experiment constrains the
liquidus at 1307 ± 11 °C. The lowest temperature experiment
constrains the solidus loosely at 1114 ± 50 °C. The detailed
analytical results are summarized in Table 13. Wollastonite is
the liquidus phase. Diopside appears at 1263 ± 11 °C.
Phosphate grains appear at the lowest temperature investigated,
but are too small to analyze individually. Typical experimental
products are illustrated in Fig. 13. Fig. 13A shows a large
398
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
4. Elemental losses and phase appearances
4.1. Mass balance calculations
The proportions of the solid phases and the coexisting liquid
in the experimental products can be estimated by least-squares,
linear approximations of the compositions of the phases in the
experimental products (Tables 6–13) to the volatile-free
composition of the starting ash mixtures (Table 2). The results
of these calculations are summarized in Table 14, giving the
estimated phase proportions on a wt.% basis. The labels indicate
the individual melting experiments. In addition to the phase
proportions, this type of calculation also allows estimates of the
elemental losses, either directly from the deviations from the
actual composition of ash mixtures or alternatively, as done
here, by including the element in question as an oxide in the
calculations. All analyses used in the calculations are
recalculated to 100% on a volatile free basis (H2O, SO2, CO2,
and Cl free as given in Table 2).
4.2. Elemental losses
Fig. 7. BSE image and X-ray density dot-maps for the main elements in the
experimental product of an ash blend with 10% rice straw ash (R10-2, 1492 °C,
Table 5). Kα lines for Si, Al, Fe, Mn, Mg, Ca, and P. BSE, back-scattered
electron image. Scale bar is 20 cm.
wollastonite liquidus crystal (N 500 m). Fig. 13B illustrates
smaller wollastonite grains that have nucleated on the inner
surface of the melt droplet. Fig. 13C shows large diopside
crystals in a matrix of smaller wollastonite grains and
interstitial melt.
The elemental losses are relatively constant at about 28% by
weight (Table 5). This amount is similar to the low temperature
loss for the 30% and 40% rice straw blends and reflects that
most of potassium is being retained in experimental products.
Only the oxides K2O and Na2O show detectable losses. The
calculated losses of K2O are illustrated in Fig. 14 as percentages
of original ash composition. It is seen that K2O is strongly lost
from the pure wood ash as well as from the wood ash blended
with small amounts of rice straw ash (10% and 15%). On the
other hand, K2O is partly retained in the blends with higher rice
straw ash as well as in the pure rice straw ash. The loss
correlates positively with the experimental temperature for most
ashes and ash blends (Table 14; Fig. 14). Thy et al. [14] melted
a slightly different rice straw ash just above its liquidus
temperature at about 1070 °C over a range in experimental
duration to 11,000 min without observing detectable loss of
K2O. In contrast, Jenkins et al. [6] better in accord with the
present result obtained an estimated liquidus of about 1400 °C
for a not very different rice straw ash. The present finding
stresses that super-liquidus temperatures, well exceeding the
liquidus temperature, for some ash compositions can result in
significant losses of K2O (see Section 4.3 for further
discussion). The rate control on the losses was not investigated
in the present work since the majority of the experiments were
of duration well above 20 h. However, Thy et al. [14] found a
strong time dependence of losses from an urban wood fuel with
the result that the potassium concentration was reduced to
below 20% of the original after only 30 h. The very low silica
content of the wood fuel in this study suggests a much stronger
rate control on potassium loss than found by Thy et al. [14].
Despite that Na2O is low in both starting compositions (0.14–
0.60 wt.% Na2O), Na2O is still observed to be strongly lost
from the wood ash-dominated blends and is completely retained
in the rice ash-dominated blends with above 40% rice ash
(Table 14).
Potassium loss is dependent on temperature (Fig. 15). The
pure rice straw ash shows a relatively modest loss of 8% K2O
per 100 °C increase in temperature. With decreasing rice straw
component in the blends, the K2O loss becomes more marked.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
399
Fig. 8. Experimental products of heating 20% rice straw blend. (A) Back-scattered electron image of R20-1 (1299 °C, Table 5). The experimental product is highly
heterogeneous. The dark grains are åkermanite and the lighter rounded grains are larnite. The intermediate gray shaded areas are quenched melt. Scale bar is 50 μm. (B)
Back-scattered electron image of R20-3 (1218 °C, Table 5). Rounded larnite grains and a two-phase quenched melt. Scale bar is 50 cm.
For 30% rice straw blend, the loss in K2O increases to 27% per
100 °C. The results for the 40 and 50% blends suggest an
intermediate loss of about 18% K2O per 100 °C despite large
variation. Blends with below 30% rice straw are not shown in
Fig. 15 because potassium is completely lost.
There is some independent support for these calculated
losses. The weight losses during the experiments were
measured for most of the melting products (Tables 3–5) and
can be compared to the calculated losses. The experiments on
the pure rice straw ash show measured 13–17 wt.% losses
(Table 5) that can be completely accounted for by the calculated
losses from the mixing experiments (Table 14) together with the
determined loss-on-ignition of the ash (∼8%, Table 1).
Similarly, the measured weight losses during the experiments
on the wood ash vary from 39% to 46% (Table 3) and can be
reasonably accounted for by the calculated mass balance losses
(Table 14) together with the determined loss of ignition of the
ash (∼27%, Table 1). The loss on ignition is largely associated
with residual carbon not driven off during ashing. This suggests
that other elements were not to a significant extent lost during
the experiments. Nevertheless, it is probable that some low-
Fig. 9. Back-scattered electron image of R30-3 (1206 °C, Table 5) showing
irregular to euhedral leucite (dark), åkermanite (intermediate gray), and melt
(light gray). Scale bar is 20 cm.
concentration elements are not well known for the wood ash
(FeO, MgO, and P2O5). These oxides show poor fits in the
mixing calculations and add to the total calculated losses. This
can account for a small overestimated loss based on the mass
balance calculations for the pure wood ash (1–2% absolute).
4.3. Phase proportions and phase appearances
The results of the calculation of the phase proportions are
summarized in Table 14, giving the estimated phase proportions
on a wt.% basis. The calculated results for the two end-member
ashes are illustrated in Fig. 16 and for the ash blends in Fig. 17,
both as functions of melting temperature.
The liquid–mineral relations for wood ash were only
determined in a narrow interval between 1400 and 1550 °C.
Within this temperature interval, the melt proportion increases
and the larnite proportion decreases with increasing temperature. The proportion of periclase is relatively constant except for
a slight drop at the highest melting temperatures. We can
extrapolate the melt proportion to rather uncertain liquidus and
solidus temperatures of 1950–2050 °C and 1200–1100 °C,
respectively. The liquidus phase was also not determined, but
can only be larnite and/or periclase. Lime (CaO) was not
detected in any of the experiments, despite that this phase
occupies a large area in the CaO–SiO2–MgO system [20]. The
solidus phase assemblage is unknown, but must involve an
Al2O3 bearing phase as this oxide is strongly enriched in the
melt. It is thus possible that, near the solidus, a SiO2–Al2O3–
K2O phase will appear (such as leucite) and, as a consequence,
stabilize potassium in the slag.
The addition of rice straw ash to the wood ash causes a
strong drop in liquidus temperature (Fig. 17). We here relate
phase crystallization to the liquidus; however, the discussion is
equally applicable to melting above the solidus that may better
apply to many features of slag formation and temperature
cycling in furnaces and boilers. For 10% added rice straw ash,
the liquidus temperature is still too high to be directly or
indirectly determined. The mineral phases detected well below
the liquidus are, as for the pure wood ash, larnite and periclase.
400
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Fig. 10. X-ray density dot-maps for the main elements in the experimental ash blend with 30% rice straw ash (R30-8, 1228 °C, Table 5). Kα lines for Ca and Mg. BSE,
the back-scattered electron image. Scale bar is 50 cm.
For 15% added rice straw ash, the liquidus has dropped
sufficiently to allow an indication of the liquidus phase as
larnite. Periclase is no longer detected at any melting
temperature. For 20% added rice straw ash, the liquidus mineral
is larnite at 1403 ± 11 °C (Fig. 18). Åkermanite appears (or
melts) at 1299 ± 10 °C. An uncertain solidus temperature of
1220 °C can be estimated. For 30% added rice straw ash,
wollastonite and åkermanite appear relatively simultaneously
on the liquidus at 1264 ± 11 °C and are followed by leucite at
1217 ± 11 °C. The solidus can be estimated by extrapolation to
about 1090 °C. For 40% added rice straw ash, the liquidus
phase is wollastonite at 1282 ± 14 °C and with leucite and
diopside appearing relatively simultaneously at a temperature
of 1212 ± 11 °C. The solidus temperature can be estimated by
extrapolation to about 1020 °C. For 50% added rice straw ash,
the liquidus phase is wollastonite at 1307 ± 11 °C and diopside
appears at a temperature of 1263 ± 11 °C. The solidus
temperature can be estimated by extrapolation to a very low
temperature of about 1000 °C. A phosphate phase, too small
to be analyzed, appears in many of the low temperature
experiments.
The sequence of liquidus minerals from larnite, to åkermanite, wollastonite, diopside, and silica reflects the bulk
compositions of the blends and the coexisting melt structure.
The mineral structure becomes increasingly dominated by
network bonding silica at the same time as modifying cations
(Ca, Mg) decrease. This is a reflection of the melt structure that
similarly becomes increasingly polymerized with increasing
rice straw ash.
All the ash blends show systematic increases in the
proportions of both minerals and melt with decreasing melting
temperatures. This allows an estimate of the solidus temperature
to be estimated, despite uncertainty often due to large
extrapolated temperature intervals. Because temperature-dependent losses of K2O are seen for most blends (Fig. 15), it is
possible that the estimated liquidus temperatures in part reflect
the changing bulk composition of the slag. This means that
estimates of melting relations and liquidus temperature cannot
be based on bulk ash compositions, as is often done, but must
take into consideration temperature-dependent changes in bulk
composition. The latter effect of changing bulk composition has
strongly influenced the results for the pure rice straw ash, as
discussed below.
The retention of K2O in the slag can be correlated with the
appearance of leucite (Table 14). Leucite is a K2O and Al2O3
silicate that is stabilized at relatively low temperature and low
melt fraction in melts relatively enriched in the same oxides.
The appearance of leucite, thus, strongly signifies retention of K
in the slag. This is seen for the 30% and 40% rice straw blends.
A similar appearance of leucite in the 50% rice straw blend
would be predicted from the significant drop in the calculated
loss of K2O for the melting experiments below 1200 °C.
Because of the heterogeneous nature of the low temperature
experiments, and the small volume examined, leucite (or other
potassium silicates) may have been undetected in these latter
experiments. For all experiments for which a potassiumcontaining mineral phase was detected, this phase is leucite
(KAlSi2O6). However, it is possible [13] that other potassium
silicates (e.g., kalsilite or potassium tetrasilicate) with much
higher potassium contents relative to silica may appear near the
solidus for ash blends with high potassium and low aluminum
contents.
The melting relations and phase proportions as a function of
melting temperature for the pure rice straw ash poses some
difficulties that cannot easily be explained in terms of
equilibrium melting relations. The diagram in Fig. 18 shows
nearly constant proportions of the melt and mineral proportions
independent of temperature, except perhaps at temperatures
above 1500 °C. Thus, these phase proportions cannot be
extrapolated to an estimated solidus temperature and only
allows the liquidus temperature to be extrapolated based on a
single melting experiment at 1536 °C. Melting experiments by
Jenkins et al. [6] on a nearly identical rice straw suggest a
solidus around 800 °C and a liquidus above 1400 °C. All the
current melting experiments contain a quartz polymorph as the
only mineral phase. The lack of another K2O-rich phase is
conspicuous and is difficult to understand based on expectations
from the simple SiO2–K2O systems [20].
We base a tentative interpretation in part on the previous
determination of an ∼1050 °C liquidus temperature for a
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
401
Fig. 11. X-ray density dot-maps for the main elements in the experimental ash blend with 30% rice straw ash (R30-6, 1161 °C, Table 5). Kα lines for Mg, Al, K, Ca, and
Mn. BSE, the back-scattered electron image. Scale bar is 20 cm.
relatively similar rice straw ash [13] and the observation that the
K2O content of the melt in the present experiments decreases
with increasing melting temperature (Fig. 16). We suggest that
depletion in K with increasing melting temperature results in a
progressive increase in melting temperature with approximately
identical proportions of melt and quartz. This would result in an
extended subliquidus range that reflects a range of ash
compositions from the ‘equilibrium’ K2O content of 12 wt.%
to an uncertain amount of perhaps 2–3 wt.% remaining in the
melt. This implies volatile loss of K with increasing
temperature. The same may have affected the liquidus
determinations for rice straw-rich blends.
In summary, however, the observed variation in the behavior
of potassium for the range of slag compositions investigated in
this study can be explained by a marked decrease in potassium
loss with increasing rice straw in the slag for the same
temperature.
4.4. Liquidus and solidus temperatures and freezing point
depression
The liquidus temperature (or complete melting point) was
bracketed in four series of experiments. Liquidus temperature
for pure rice straw was extrapolated to approximately 1575 °C.
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P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Fig. 12. Back-scattered images of typical experimental products from blending 40% rice straw ash with 60% wood ash (Table 5). (A) Large skeletal and euhedral
grains are wollastonite (R40-7, 1268 °C). Scale bar is 200 cm. (B) Rectangular shaped diopside and irregular wollastonite grains (R40-5, 1201 °C). Scale bar is 50 cm.
(C) Intergrowth of wollastonite (light) and leucite (dark gray) (R40-5, 1201 °C). Small amount of interstitial glass appears with an intermediate gray shade. Scale bar is
20 cm.
This value is substantiated by a significant increase in melt
proportion for the 1536 °C experiment, but may only apply to a
bulk composition with relatively low K2O. Liquidus temperatures were determined as 1307 ± 11 °C for the 50% rice
straw ash blend; 1282 ± 14 °C for the 40% rice straw ash blend;
1264 ± 11 °C for the 30% rice straw ash blend, and 1403 ± 13 °C
for the 20% rice straw ash blend. For the 10% and 15% rice
straw ash blends, the experiments only gave minimum values of
1530 °C and 1543 °C, respectively. For the 15% blend, this
value is relatively close to the liquidus temperature, judging
from the amount of melt in the experimental products. For pure
wood ash, a liquidus temperature was estimated by extrapolation to an uncertain value of 1950–2050 °C.
The resulting freezing point depression as a function of the
percentage of rice straw ash in the blends is shown in Fig. 18. It
is seen that the addition of small amounts of rice straw ash to the
blend will strongly affect the melting points until an amount of
about 20% rice straw ash (∼25 °C/wt.% rice ash). From about
20% rice straw, the liquidus slope levels out and reaches a
minimum at about 30% rice (1264 °C). With increasing rice
straw ash, the liquidus temperature rises steadily to 50% straw
ash, and probably also beyond, to an apparent 1536 °C for pure
rice straw ash (4 °C/wt.% rice ash). The extrapolated solidus
shows a systematic decrease with increasing rice straw and does
not suggest a minimum, as for the liquidus.
The estimated solidus shows a systematic fall (Fig. 18) with
increasing rice straw in the blends from about 1220 °C for 20%
blend to around 1000 °C for 50% blends without a clear
minimum as seen in the liquidus. The true solidus for pure rice
straw may reach as low as 800 °C as found by Jenkins et al. [6].
The liquidus and to a lesser extent the solidus determinations
have been affected by variable potassium losses and thus do not
record the melting conditions for the starting compositions as
given in Table 2. This is particularly substantiated for pure rice
straw ash for which the liquidus may only apply to a bulk
composition with relatively low K2O. The ‘true’ liquidus for a
starting composition with 12 wt.% K2O may very well be below
1100 °C as found by Thy et al. [14], although this composition
would suggest a much higher liquidus based on the binary
K2O–SiO2 system [23] or the ternary K2O–SiO2–Al2O3 [20]
and K2O–SiO2–MgO [24] systems. The pure wood and blends
with rice straw of 30%, and below, record the liquidus for
Fig. 13. Back-scattered images of typical products of blending 50% rice straw ash with 50% wood ash (Table 5). (A) Large skeletal and euhedral grains of wollastonite
(R50-8, 1296 °C). Scale bar is 100 cm. (B) Grains of wollastonite nucleated and grown on the inner surface of the melt pellet (R50-2, 1252 °C). Scale bar is 200 cm.
(C) Intergrowth of wollastonite (light) and diopside (dark gray) in an intermediate gray glass (R50-6, 1180 °C). Scale bar is 100 cm.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
403
Table 14
Calculated phase proportions and elemental losses using least-squares mixing calculations
Run ID
Temperature (°C)
Melt (%)
Lar (%)
Per (%)
W-16
W-15
W-14
W-13
W-12
W-11
W-10
W-9
W-8
W-7
W-6
W-20
W-19
W-18
W-17
R-10-3
R-10-2
R-15-4
R-15-3
R-15-2
R-15-1
R-20-9
R-20-8
R-20-7
R-20-6
R-20-2
R-20-1
R-20-5
R-20-4
R-30-1
R-30-2
R-30-9
R-30-8
R-30-3
R-30-7
R-30-6
R-40-8
R-40-7
R-40-1
R-40-6
R-40-5
R-40-4
R-40-2
R-40-3
R-50-9
R-50-8
R-50-4
R-50-2
R-50-5
R-50-3
R-50-6
R-50-1
R-19
R-18
R-16
R-20
R-15
R-21
R-5
R-6
R-4
R-7
R-8
1541
1517
1510
1498
1494
1490
1484
1469
1468
1464
1445
1431
1420
1412
1402
1530
1492
1543
1443
1369
1345
1415
1390
1368
1344
1318
1299
1271
1249
1300
1275
1253
1228
1206
1182
1161
1296
1268
1250
1223
1201
1182
1160
1134
1318
1296
1273
1252
1230
1205
1180
1163
1536
1490
1439
1417
1393
1372
1343
1270
1255
1230
1212
23
29
18
20
23
21
17
21
25
26
22
15
15
15
12
38
40
72
71
61
65
100
100
100
98
98
49
34
17
100
100
85
70
52
42
29
100
94
93
80
75
64
64
44
100
99
99
89
88
82
64
57
91
65
69
67
68
59
61
65
55
56
61
70
66
75
74
71
73
76
73
70
69
72
78
78
79
81
60
59
28
29
39
35
7
7
8
8
8
8
8
8
7
7
7
8
8
8
7
2
1
b1
b1
2
2
13
16
18
Ake (%)
Woll (%)
Leu (%)
Dio (%)
Qtz (%)
K2O loss
Na2O loss
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.5
0.5
0.4
0.4
0.5
0.5
0.5
0.5
0.5
0.4
0.4
0.4
0.6
0.3
0.3
0.2
0.1
0.1
0.1
9
35
31
33
32
41
39
35
45
44
39
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.9
15.5
15.5
15.4
15.4
15.4
15.4
15.2
15.2
15.2
15.2
15.2
15.2
15.6
15.2
14.8
14.6
13.8
13.6
11.6
11.3
10.2
8.9
10.1
9.0
8.9
8.5
7.2
6.9
9.0
11.5
9.4
6.0
10.4
6.7
7.6
8.7
8.1
8.7
9.0
7.9
7.6
7.0
7.9
7.1
5.9
7.2
7.2
6.0
38
50
64
6
18
24
32
37
9
12
16
18
23
8
8
11
6
7
20
24
30
36
43
1
6
b1
b1
1
1
11
12
18
28
33
b1
b1
b1
13
b1
b1
b1
8
10
(continued on next page)
404
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Table 14 (continued)
Run ID
Temperature (°C)
R-9
R-10
R-3
R-11
R-12
R-13
R-14
R-2
R-1
1193
1176
1167
1137
1117
1100
1080
1075
982
Melt (%)
Lar (%)
Per (%)
Ake (%)
Woll (%)
Leu (%)
Dio (%)
63
61
68
65
59
60
59
56
62
K2O loss
Qtz (%)
37
39
32
35
41
40
41
44
38
Na2O loss
5.4
5.8
4.5
4.9
5.7
5.0
4.9
5.5
3.7
Lar, larnite; Per, periclase; Ake, åkermanite; Woll, wollastonite; Leu, leucite; Dio, diopside; Qtz, SiO2 polymorph.
potassium-free starting compositions. Variable loss of potassium may thus have contributed to the observed increase in the
liquidus for 40–50% rice straw ash. This is suggested by the
lack of a corresponding variation for the solidus estimates. For
practical purposes, however, the liquidus temperature shown on
Fig. 18 may be more relevant than the ‘true’ liquidus
temperature for the starting compositions.
4.5. Phase diagram
The CaO–SiO2–MgO (CSM) system [20] (Fig. 19) is a
reasonable model for the ash blends as this system includes the
majority of the oxides (71–86% of the total variance). Judging
from the mineralogy observed in the wood-rich ash blends, it
appears reasonable to combine Al2O3 and P2O5 with SiO2 (S)
and MnO with MgO (M). Using these components, the
‘multicomponent’ CSM now accounts for 81–87% of the
total variance of the ashes, with the only oxides excluded being
TiO2, Fe2O3, Na2O, and K2O. Considering that K2O is
completely lost to the furnace atmosphere for blends with
below 30% rice straw, the correspondence for these blends
becomes even better (97–99%).
The pure wood ash and the 10–15% rice straw blends plot
close to the larnite field in the CSM system supporting larnite as
the liquidus phase followed by periclase (Fig. 19). The presence
of lime (CaO) and Ca3SiO5 are not detected despite that pure
Wood
14
10% 15%
20%
30%
12
K O
2 C
onte
nt o
f As
h
1600
50%
40%
1500
10
Rice Straw
8
6
4
2
0
20
40
60
80
Wt. % SiO2 in Melt
Fig. 14. Losses of K2O (wt.% ash basis) as a function of SiO2 content of melt
phase. Line is the composition of the starting ashes from Table 2. See text for
detailed explanation of calculations. For 30–50% rice straw blends, there is a
weak positive correlation between melt composition and loss of K2O and SiO2.
Temperature (oC)
K2O wt. % Loss Ash Basis
16
wood ash falls within, or near, these fields. The 20% ash blend
crystallized larnite as the liquidus and was followed by
åkermanite, but did not indicate the presence of merwinite as
predicted by the CSM system. The remaining 30% to 50%
blends plots close to the wollastonite field and thus supports the
observed liquidus phases. It is further interesting that, consistent
with the CSM system, it is pure wollastonite (CaSiO3) which
appears in the experiments and not magnesium wollastonite
([CaMg]SiO3). Furthermore, the 30% blend predicts åkermanite
as the second phase and the 40% and 50% blends predict
diopside as the second or third phase, all in general agreement
with the predictions from the CSM system (Fig. 19). Because of
the partial retention of K2O for the 30% to 50% rice straw
blends, the deviation between the multicomponent slag and the
simplified CSM system will become significant for these
blends.
The tentative liquidus temperatures predicted from the CSM
diagram (Fig. 18) suggests temperatures exceeding 1800 °C for
pure wood ash and a marked fall to about 1400 °C for the 20%
blend, again consistent with the experimental determinations.
The 30–50% blends plot on or near a 1400 °C isotherm in
general supporting the plateau seen in the experimental
determined liquidus. The predictions from the CSM phase
diagram suggest a marked rise in liquidus for further increasing
silica. The CSM liquidus closely follows the experimental
determined liquidus for the compositionally more complex ash
blends. For high rice straw blends, the liquidus is in addition
controlled by K2O and the direct comparison with CSM breaks
down. However, it is interesting that the high liquidus that we
1400
Rice
50%
40%
30%
(0.08)
Rice
(0.18)
1300
40-50%
(0.27)
1200
30%
1100
1000
900
20
30
40
50
60
70
80
90
100
Percentage Loss of K2O
Fig. 15. Melting temperature (°C) as function of % loss of K2O. Blends with
below 30% rice straw ash are not shown since these have lost virtually all K2O.
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Wood Ash
1600
Rice Straw Ash
2000
1600
?
1500
1800
Larnite
1300
Quartz
1500
Melt
1200
1100
1450
Liquidus Temperature (oC)
1400
Melt
Temperature (oC)
Temperature (oC)
Periclase
1550
1000
1400
0
20
40
60
80 100 0
20
Wt. %
40
60
405
900
80 100
Wt. %
1600
CSM
1400
loss
?
Liquidus
1200
1100
Thy et al. 2000
Solidus
1000
Fig. 16. Phase proportions (wt.%) as a function of melting temperature (°C) for
pure wood and rice straw ashes calculated by least-squares approximations to
the starting material (Table 2). See text for details of the calculations. For pure
rice straw ash, the liquidus temperature can be extrapolated to ∼1575 °C. The
nearly constant melt content for rice straw ash is suggested to be a result of
progressive K2O loss during the experiment and does not reflect equilibrium
conditions (see text for discussion).
sium
as
Pot
900
Jenkins et al. 1996
800
20
0
60
40
80
100
% Rice Straw Ash in Blend
estimated for pure rice straw ash compares best with potassiumfree, silica-rich melts in the CSM system (Figs. 18 and 19). This
supports the proposal that the high liquidus estimated for pure
rice straw ash is controlled by loss of potassium. Another
observation that can be made from comparisons between the
experimental biomass ash results and the CSM system is that
melting temperature estimates based on the simple CSM system
appear to be 100–200 °C above the true liquidus for natural fuel
blends.
Fig. 18. Experimentally determined and estimated liquidus and solidus
temperatures as a function of wt.% rice straw ash in the ash blend. CSM refers
to CaO–SiO2–MgO system; see Fig. 19 and text for discussion.
The low liquidus temperature observed by Thy et al. [14] for
pure rice straw ash is difficult to explain from the present
results. The ash composition investigated by Thy et al. [14]
would suggest a liquidus temperature of 1300–1400 °C and a
solidus temperature of about 800 according to the K2O–SiO2–
1450
50% Rice Straw
Temperature (oC)
1400
40% Rice Straw
1350
Liquidus
1300
Liquidus
1250
woll
1200
1150
melt
melt
woll
dio
leu dio
1100
1450
Temperature (oC)
Liquidus
30% Rice Straw
1400
1350
melt
lar
1300
Liquidus
1250
1150
1100
0
aker
melt
aker
1200
leu woll
20
20% Rice Straw
40
60
Wt. %
80
100
0
20
40
60
80
100
Wt. %
Fig. 17. Melting relations for the intermediate fuel ash blends with 50%, 40%, 30%, and 20% rice straw ash, respectively. The phase proportions are determined by
least-squares approximations to the starting material (Table 2). See text and Table 14 for details of the calculations. Woll – wollastonite; Leu – leucite; Ake –
åkermanite; Lar – larnite; Dio – diopside.
406
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
Fig. 19. The ashes and ash blends plotted in the CaO–SiO2–MgO system (CSM) after Osborn and Muan [20]. To plot the ashes, S is calculated as SiO2 + Al2O3 + P2O5
(wt.%) and M as MgO + MnO. Other elements have been excluded (TiO2, Fe2O3, Na2O, and K2O). To the left are shown the ashes used in the experiment as well as the
simplified primary phase fields discussed in the text. To the right is shown the CSM diagram as drawn by Osborn and Muan [20]. Note that on the original phase
diagram, pure wollastonite (CaSiO3) is referred to as pseudowollastonite, while magnesium wollastonite is referred to as wollastonite.
MgO system [24] as well as found by Jenkins et al. [6]. This
composition, however, contains significant amounts of CaO and
Na2O that may contribute to lowering the liquidus below what
would be estimated by the ternary K2O–SiO2–MgO [25].
Further work is required to better understand the high
temperature behavior of rice straw-rich blends.
5. Discussion
Blending of rice straw with a dominating wood fuel
markedly affects the composition and properties of the resultant
inorganic slag (and ash). The melting experiments show that
potassium is completely lost from the ash blends with below
30% rice straw ash. The experimental products of these ashes
contain periclase (MgO), larnite (Ca2SiO4 with appreciable
amounts of Al and P substituting for Si), and quenched melt
without detectable concentrations of K2O. A nearly pure
åkermanite appears in the 20% rice straw blend (Ca2Mg
(SiAl)2O7). Wollastonite (CaSiO3) first appears in the 30% rice
straw blends and diopside (CaMgSi2O6) first appears in the 40%
rice straw blend. None of these phases contains detectable
potassium. It is first with the appearance of leucite as a
subliquidus phase (KAlSi2O6) at temperatures in general below
1200 °C that potassium becomes increasingly retained in the
slag (melt and leucite). There is thus a strong correlation
between the retention of potassium in the slag and the
appearance of potassium bearing silicates (leucite). This is
undoubtedly related to an increased polymerization of the melt
and a better accommodation of the large potassium ions. The
melt phase in the experiments with 30% rice straw ash, and
above, also readily quenched to a glass, while for the other
experiments the melt quenched to a two silicate intergrowth.
The experiments on the pure rice straw ash show that a
quartz polymorph (tridymite) was the only detectable mineral
present. The slag readily quenched to a glass that retains large
proportions of potassium of the starting ash. The retention of
potassium in the quenched melt is negatively correlated with
temperature. This partial retention of potassium in the slag and
its dependency on temperature result in a progressive rise of
liquidus temperature with increasing melting temperature and
loss of potassium (phase proportions independent on temperature). The melting behavior and the compositional and
physical properties of rice straw slag are thus dependent on
the thermal history of the slag.
Mass balance calculations of the experimental products
quantify the extent of potassium loss during melting and slag
formation. The loss strongly decreases for rice straw slag from
60% to 25% of the original K2O content with decreasing
melting temperature. Likewise, there is a significant reduction
in the loss of potassium from the ash blends with increasing rice
straw ash content.
The literature contains relatively few studies of related
biomass fuels. Olander and Steenari [12] reported the presence
of potassium calcium silicate, potassium feldspar, and larnite, in
addition to potassium sulfate and sylvite in a barley–wheat
mixture heated to 1000 °C. For bark–wood mixtures, they
P. Thy et al. / Fuel Processing Technology 87 (2006) 383–408
detected lime, periclase, rankinite (2CaO·2SiO2), merwinite
(3CaO·MgO·2SiO2), and larnite. Olander and Steenari [12]
concluded that potassium appeared in low melting components
and is an important element in slag sintering. Misra et al. [11]
studied the ash composition from wood fuels to 1300 °C and
detected various oxides (lime, pericline, and manganese oxides)
and larnite. Our own study from 1999 [13] examined in detail an
urban wood fuel with considerably high soil component. The
ash remaining after heating the ash to 1300 °C contained
åkermanite, phosphate, and andradite garnet. These results were
used to model potassium loss from urban wood fuels by a series
of normative minerals including garnet, åkermanite and sodium
melilite, rankinite, orthoclase, and potassium silicate [13].
Based on the present work, it appears that this list needs to be
adjusted to account for the mineralogy found in more pure fuel
blends, specifically to include larnite, wollastonite, diopside,
leucite, and silica.
The mineralogy of the melting experiments can be
compared to that found in slag that was retrieved from
commercially operating biomass fueled power plants. Petrographic examinations of glassy deposits [26,27] show that
these contain in general the same silicate minerals (monticellite, garnet, augite, wollastonite, melilite, and leucite). It is also
conspicuous that the glass compositions are largely similar to
those experimentally produced in this study. This suggests that
the simplified melting experiments conducted in this study
apply to a certain extent to the more complex conditions in
full-scale power plants although they do not model the
continuous flow of gas phase combustion products past the
slag mass.
The results of the vertical quench furnace experiments show
that two factors control the retention of potassium in the fuel
slag and ash. Increasing rice straw ash stabilizes potassium in
the slag and consequently reduces the relative loss of potassium
to the furnace gas. The effect of lowering the temperature is
likewise to stabilize potassium in the slag. For the wood ash
blends, the retention in the slag at low temperature can be
related to the stabilization of potassium–aluminum–silicate
minerals (e.g., leucite). However, this is not the case for the pure
rice straw ash for which the retention appears to be dependent
on changes in the melt structure with lowered temperature
(cf., [13]).
The wide variation in slag composition (SiO2, CaO) results
in a marked freezing point depression. The extrapolated melting
point for pure wood ash is estimated at 2000 °C. The addition of
rice straw ash lowers the liquidus markedly (∼25 °C/wt.% rice
straw ash) until a minimum of 1264 °C at about 30% rice straw
ash. There is an apparent increase in the melting point with
increasing rice straw ash in part due to the effect of progressive
depletion of K2O from the slag that causes an increase in the
liquidus temperature.
6. Conclusions
The blending of rice straw with wood-based biomass fuels
results in marked changes in composition and physical
properties of the inorganic slag. Important observations include:
407
• The silica content of the slag increases markedly from 14 to
82 wt.% SiO2, while calcium decreases from 49 to 2 wt.%
CaO as straw ash addition increases to 50%. The melting
point drops markedly from an estimated 2000 °C to a
minimum of 1264 °C for 30% added rice straw.
• Potassium is completely lost from blends with less than 30%
rice straw.
• Potassium is partly lost from intermediate rice straw blends
due to structural changes in the melt that also results in the
stabilization of leucite (or other potassium silicates) at
temperatures below 1200 °C.
• Potassium is partially retained in the slag of pure rice straw
but increasing temperature promotes loss of potassium.
• The melting point for pure rice straw slag increases as a
function of the progressive loss of potassium. This is in
contrast for the wood-based fuels for which potassium is
completely lost even near the solidus conditions.
• The addition of rice straw fuel to a dominantly wood-based
fuel reduces the relative loss of potassium, lowers the
melting point, and increases the total volume of ash and
slag.
• Due to the much higher ash content of rice straw, the total
volatile loss of potassium increases when straw is blended,
becoming nearly constant above about 20% straw. Retention
in the slag reduces the total potassium in the gas phase that
further can reduce downstream fouling.
Acknowledgements
Wheelabrator-Shasta Energy Company, Anderson, California, kindly provided the raw fuels used in this study. Access to
the Electron Microprobe Facility of the Department of Geology,
University of California, Davis, is gratefully acknowledged.
The study was supported by the California Energy Commission's Energy Innovation Small Grant Program.
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