Biomass Conversion and Biorefinery

https://doi.org/10.1007/s13399-023-04418-z

ORIGINAL ARTICLE

Investigation of the combustion and ash deposition characteristics

of oil palm waste biomasses
Fairuz Milkiy Kuswa1,2 · Hanafi Prida Putra1,2 · Prabowo1 · Arif Darmawan2 · Muhammad Aziz3 · Hariana Hariana1,2

Received: 30 March 2023 / Revised: 26 May 2023 / Accepted: 29 May 2023

© The Author(s) 2023

Abstract
Biomass serves as an alternative energy solution for decarbonizing coal-fired power plants, which have been reactivated in
several countries due to the global energy crisis. Oil palm waste, owing to its abundant availability, holds significant potential
as a biomass fuel. This study aimed to investigate the combustion performance of various oil palm wastes in comparison to
coal. Biomass combustion is associated with ash-related problems such as slagging, fouling, and corrosion, which may accel-
erate ash deposit acceleration, reduce heat transfer, and damage refractory equipment in boilers. Ash-related problems were
evaluated using the method commonly adopted for solid fuel, including experimental drop tube furnace combustion and ash
observation. The results indicate that each oil palm waste has different combustion characteristics. Palm leaves, empty fruit
bunch, and palm fronds with clean probe observation have a relatively low tendency of slagging and fouling and can be recom-
mended as biomass fuel for co-firing. However, their high alkali and iron contents need to be considered. Palm fiber has similar
combustion characteristics to coal, but it has a high slagging and fouling tendencies. The palm stems with high chlorine content
have a high corrosion tendency confirmed by probe observation, scanning electron microscopy, and X-ray diffraction analyses.

Keywords Oil palm waste · Biomass · Combustion · Slagging · Fouling

1 Introduction in higher levels of carbon dioxide (­ CO2), nitrogen oxides

(­ NOx), and sulfur oxides ­(SOx) [4]. These emissions lead to
Coal has long been the primary fuel for power generation, adverse environmental effects, such as acid rain production,
particularly in densely populated Asian countries like China, the ozone layer’s destruction, and the acceleration of global
India, and Indonesia. As a result of the energy crisis caused warming [5]. Therefore, it is challenging for nations with
by the disruption of gas lines due to the conflict between significant energy requirements to meet the urgent need to
Russia and Ukraine, several European countries are resorting improve environmental quality by reducing their coal con-
again to fossil fuels [1]. However, the emission of green- sumption. Moreover, following the Paris Agreement, the
house gases, as a consequence of coal burning, can contrib- global community has pledged to implement decarboniza-
ute significantly to the acceleration of climate change on a tion to lower the amount of ­CO2 in the atmosphere [6].
global scale [2, 3]. Research shows that burning coal results It is generally accepted that biomass may be used as a
clean and carbon-neutral renewable energy source, which
can replace fossil fuels, especially coal [7–9]. Biomass
* Muhammad Aziz
maziz@iis.u-tokyo.ac.jp grows by converting the solar energy into chemically
stored energy through photosynthesis. Energy from the
* Hariana Hariana
hariana@brin.go.id solar can later be converted into heat and power through
combustion. Conceptually, the amount of ­CO 2 released
1
Mechanical Engineering Department, Sepuluh Nopember into the atmosphere is the same as the ­C O 2 absorbed
Institute of Technology, Surabaya, Indonesia during the growth of plants. Therefore, this process can
2
Research Center for Energy Conversion and Conservation, be considered carbon neutral [10–12]. Biomass com-
National Research and Innovation Agency, South Tangerang, bustion is an intricate process that incorporates many
Indonesia
physical and chemical factors. The fuel composition and
3
Institute of Industrial Science, The University of Tokyo, the intended use of combustion products play significant
Tokyo 153‑8505, Japan

13
Vol.:(0123456789)
 Biomass Conversion and Biorefinery

roles in the combustion process [13]. However, biomass plants. A lab-scale combustion test was performed utilizing
combustion employing co-firing or single fuel still cre- a drop tube furnace (DTF) and thermogravimetric and dif-
ates issues and complicates the further investigation. ferential thermal analysis (TG-DTA) to examine the com-
As a tropical country, Indonesia has the advantage of hav- bustion behavior and ash deposit formation. In addition,
ing significantly high biomass potential from agricultural ash deposits from DTF combustion are further analyzed to
and forestry biomasses. In addition, according to the Index find ash-related problem tendencies using scanning electron
Mundi, Indonesia is the world’s largest producer of palm microscopy–energy dispersive spectroscopy (SEM-EDS)
oil commodities, making it a high opportunity to produce and X-ray diffraction (XRD).
green fuels [14–16]. Many parts of oil palm wastes can be
recycled and used as material or biomass fuel. Hamada et al.
[17, 18] have found that combustion residues from oil palm
2 Material and methods
wastes, such as mesocarp fiber and palm shells, can be used
as an aggregate material to obtain a more optimum concrete
Oil palm wastes and coal were analyzed for clarifying
mix. Several studies [19, 20] have examined the potential of
their proximate, ultimate, and ash analyses and ash fusion
oil palm trees, such as empty fruit bunch (EFB), palm fiber,
to obtain their characteristics as solid fuel by using coal test-
and palm kernel, as co-firing substitutes in power plants and
ing standards. Then, the empirical indices were employed
investigated in terms of power generation and efficiency.
to predict the slagging, fouling, abrasion, and corrosion ten-
Hariana et al. [15] have conducted an in-depth analysis of
dencies of biomasses and coal. TG-DTA was carried out to
the co-firing test between coal, EFB, and palm frond to eval-
compare the thermal behavior of biomasses with coal. For
uate slagging and fouling tendencies. Moreover, Idris et al.
the main experiment, a lab-scale DTF combustion test was
[21] have also discussed co-firing of oil palm waste biomass
performed to simulate the slagging and fouling with operat-
and coal in terms of emissions.
ing conditions adapted to the actual pulverized coal boiler.
Agricultural biomass contains larger amount of ash-
During the combustion test, the gas emission was analyzed.
forming chemicals than woody biomass since it has a rapid
The attached ash on the probe surface was observed visu-
metabolic rate and absorbs more nutrients during its growth
ally and weighted to find the ash deposit tendency. The ash
stage [22]. As shown in the previous study [23], rice husk
observation was strengthened with morphological analysis
and oil palm wastes, as agricultural biomass, have a higher
using SEM-EDS and mineral determination using XRD.
composition of silica or potassium, and have higher ash con-
From the series of tests, each biomass was evaluated and
tent than woodchip biomass which has a higher composition
compared to coal as a baseline sample to obtain and clarify
of calcium. Other studies show the average ash content in
the recommended oil palm wastes that have good combus-
wood and woody biomass is around 4.6 wt% compared to
tion characteristics with fewer ash-related problems.
agricultural biomass, which is around 8 wt% [24]. Slagging
and fouling on the boiler could result from the biomass’s high
ash content and occur in a two-step mechanism. In the first 2.1 Samples preparation
step, ash particles and vapors make it to the heat exchange
surface, where they will deposit. The second step involves the Seven oil palm wastes were prepared along with one bitu-
expansion of an ash deposit layer. These two steps can occur minous coal from East Kalimantan, Indonesia. The oil palm
sequentially according to the step or together simultaneously wastes consisted of palm fiber, palm leaves, upper stem, mid-
[25]. The high alkali metal content also causes slagging and dle stem, lower stem, EFB, and palm frond, as illustrated in
fouling, potentially lowering the sintering temperature of ash Fig. 1. The biomass and coal samples were dried (according
[26–28]. Slagging and fouling can reduce boiler efficiency to ASTM D3302) and pulverized to obtain a size of less than
due to the reduced ability for heat exchange. Furthermore, 250 μm for characteristics tests such as proximate, ultimate,
this will cause damage and expensive repair costs [29]. calorific value, and total chlorine. For the DTF combustion
A previous study [15] shows that the combustion of oil experiment and thermogravimetric analysis, the coal was
palm wastes shows a positive trend; however, at certain pulverized to a particle diameter smaller than 75 μm. On
proportions, it can cause ash problems. Moreover, several the other hand, the biomass was ground to an average par-
studies only focus on co-firing between coal and EFB, palm ticle diameter smaller than 250 μm after being dried at a
frond, and palm kernel shell [15, 19–21, 30], but the com- temperature of 60 °C for 1 h. The size and preparation were
bustion behavior of other oil palm wastes like fiber, stem, adjusted to pulverized coal boiler feed. The appearance of
and leaves is still rare. In order to fill this gap, this study the pulverized samples used in this study is shown in Fig. 2.
is aimed to investigate and evaluate the combustion aspect Each sample was combusted as a single fuel in the DTF
of seven different oil palm wastes compared to coal before to determine each biomass’s combustion behavior, which is
being implemented for co-firing in existing coal-fired power further compared to coal.

13
Biomass Conversion and Biorefinery

Fig. 1  Part of an oil palm tree

Fig. 2  Powdered samples of

(a) coal, (b) palm fiber, (c)
palm leaves, (d) upper stem, (e)
middle stem, (f) lower stem, (g)
EFB, and (h) palm frond

2.2 Ash fusion temperature (AFT) temperature at which the initial signs of ash from solid fuel
begin to melt. ST is a measure of the clinkering tendency
The ash melting properties were identified using an ash of coal ash and is usually used as a reference where the
fusion analyzer equipped with a camera and image process- coal ash in the boiler becomes thicker and sticks to the sur-
ing software to record photos and identify changes from face of the boiler. HT is the temperature when the ash shape
the solid phase to the liquid phase as the temperature rises. resembles a hemispherical shape, where the height reaches
These tests followed the conventional procedure for measur- almost half its initial width. FT is the temperature at which
ing AFTs, and previous investigations have used a similar the ash sample is completely fused until the height of the ash
technique [31, 32]. The sample’s ash was shaped into dense approaches the flat surface approximately with a maximum
pyramids cone and dried. Then, the ash fragments were fixed height of 1.6 mm, and only a small portion of the ash does
on ceramic slabs and placed within the ash fusion analyzer’s not melt [27, 33].
furnace. Regulated gas, with a composition of 60 vol% CO
and 40 vol% C ­ O2, was flown with a flow rate of approxi- 2.3 Empirical indices of solid fuels
mately 1.5 times the furnace volume for the reducing test,
while for the oxidizing test, the used gas was regulated air Various studies have relied on the theoretical prediction of
stream. The furnace was heated to a maximum tempera- coal as a first basis for estimating the likelihood of slagging,
ture of 1500 °C, and the collected pictures were analyzed to fouling, corrosion, and abrasion [26, 34–40]. Theoretical
identify the ash’s initial deformation (DT), softening (ST), prediction calculation is shown in Table 1. This study used
hemispherical (HT), and fluid (FT) temperatures. DT is the prediction calculations commonly adopted in coal analysis

13
 Biomass Conversion and Biorefinery

Table 1  Empirical indices for Parameter Formula Low Medium High

solid fuels
Slagging indication
B/A ratio Fe2 O3 +CaO+MgO+Na2 O+K2 O < 0.5 0.5–1.0 > 1.0
SiO2 +AI 2 O3 +TiO2
Silica ratio SiO2
∙ 100 > 72 65–72 < 65
SiO2 +Fe2 O3 +CaO+MgO
Fusibility (4 x DTreducing)+HToxidizing > 1343 1232–1343 <1232
5
Composite index SiO
1.24 BA + 0.28 AI O2 − 0.0023ST < 1.5 1.5–2.5 > 2.5
−0.019 Sr + 5.4
2 3

Slagging index B
.S < 0.6 0.6–2.0 > 2.0
A
Iron in ash Fe2O3 <8.0 8.0–15.0 > 15.0
Fouling indication
Fouling index B ( < 0.2 0.2–0.5 > 0.5
)
A
. Na2 O
Sodium in ash Na2O < 2.0 2.0–6.0 > 6.0
Total alkali Na2O + K2O < 2.0 2.0–3.0 > 3.0
Abrasion indication
Abrasion index qc + 0.5 pc + 0.2 Ash < 4.0 4.0–8.0 > 8.0
qc = 0.01xAshx(SiO2 − 1.5Al2O3)
pc = 1.3x(Sulfur − 0.3)
Corrosion indication
Sulfur/chlorine S > 4.0 2.0–4.0 < 2.0
Cl

with justified parameters for the biomass samples in oil palm a portable gas analyzer Bacharach PCA-400. ­SO2 and NOx
waste. emissions were measured following the quality standards
of the Indonesian Ministry of Environment (KLHK). The
gas analyzer was corrected to 7% ­O2 concentration using
2.4 Combustion characteristics using TG‑DTA an equation from Li et al. [44] to meet the requirements of
the Indonesian government policy.
The thermogravimetric analyzer was used to obtain the Furthermore, the ash formed during the combustion
combustion characteristics of solid fuels. Shimadzu DTG- was collected from a probe having a diameter of 50 mm
60 was utilized in the atmospheric environment to carry out (shown in Fig. 3) located at the end of DTF. The probe was
the analysis. Samples weigh approximately 8–10 mg and inserted, and the height was adjusted so that the tempera-
were heated from ambient temperature to 800 °C at a rate ture sensor on the probe surface showed a temperature of
of 10 °C/min. The temperature was held for 10 min at the 550 and 600 °C. These temperature conditions simulate
final temperature. slagging and fouling areas in the superheater and econo-
mizer areas in the boiler.
After 1 h of residence time, the probe was removed
2.5 Combustion of solid fuels in DTF from DTF. The ash collected on the probe surface was
weighed to determine the ash deposit weight. The ash
DTF testing has been carried out in several recent studies was brushed, and the remained ash attached to the probe
[38, 41–43] to find the tendency of slagging and fouling surface was visually examined following the method of
in power plants, especially in pulverized coal boilers. The Ohman et al. [45]. Category 1 indicates that the ash adher-
DTF has been demonstrated in the previous study [41]. A ing to the probe does not coalesce and sinter to the probe’s
ceramic tube was made of alumina with a length of 1.5 m surface. The ash can break with only a light touch. Cat-
and an outer diameter of 76.2 mm. The tube was placed egory 2 means that some ashes adhere to the surface of
in a chamber heated by 1-kWth electric heaters with a the probe, but this ash can also break with a light touch.
temperature range of 1200 to 1250 °C using a convection Category 3 indicates that the ash is sintered and adhered
heating system. In addition, the solid fuel was fed at a rate with the probe’s surface to form small material slag, which
of 50 g/h via primary air heated at 100–150 °C. Secondary is still breakable using one bare hand. Category 4 is ash
air was added to the combustion chamber to ensure perfect that sticks and sinters with the surface of the probe to
combustion with 3–5% excess oxygen. Exhaust gas meas- become a more extensive slag material that is not easily
urements were carried out at the bottom of the DTF using broken with one bare hand.

13
Biomass Conversion and Biorefinery

Fig. 3  Testing probe with stain-

less steel material

2.6 Ash mineralogy characteristics > middle stem > EFB > lower stem > palm frond. This is
probably due to the large amounts of Si and Fe, which play a
Ash from DTF combustion was analyzed by SEM-EDS role in ash deposit formation. The high composition of Si in
using Quanta 650 and XRD using an Aeris-type Bragg- biomass tends to produce more ash deposit [39]. Moreover, a
Brentano Diffractometer to characterize ash’s microstruc- high alkali composition (Na and K) in palm leaves can form
tural and minerals. Backscattered electron imaging (BSE) low eutectic alkali-aluminosilicate [40]. In addition, a high
and EDS were utilized in this investigation to analyze the Fe composition on palm leaves can accelerate ash deposit
fly ash samples. BSE are electrons that have high energy formation because of its low melting point [50].
directly reflected from the surface of the object being tested The volatiles owned by biomass appear to be more domi-
with differences in grayscale intensity between the chemi- nant than coal, corresponding to the lower fixed carbon con-
cal phase of an object. On the other hand, the EDS detec- tent in biomass. Moreover, certain ash-forming elements in
tor could identify elements with an atomic number [46]. biomass can increase volatilization and hazardous compo-
XRD was performed with Cu as the X-ray source and angle nent [22]. The calorific value of biomass is relatively lower
measurement from 10 to 90° with a 0.02° measurement step. than coal [15, 51]. Only palm fiber has an as-received calo-
Then XRD pattern was analyzed using MAUD to obtain the rific value close to coal with a value above 4000 kcal/kg.
minerals composition in ash. Three parts of the stem (upper, middle, lower) have a very
low calorific value, ranging only from 1500 to 1800 kcal/kg
in ­Qgrar. This is due to the high moisture content possessed
3 Results by the palm stem. This is also evident in EFB and palm
frond, which also have calorific values under 3000 kcal/kg
3.1 Solid fuel analysis due to their high moisture content. However, when moisture
on biomass is removed, the heating value of the biomass has
Initial analysis for solid fuel characterization is very impor- a relatively similar value above 4000 kcal/kg in the follow-
tant for energy calculations and early prediction of prob- ing order: palm fiber > palm leaves > EFB > palm frond >
lems in a power plant [47]. All results of the proximate and lower stem > middle stem > upper stem.
ultimate analysis are on a dry basis (db) except for moisture The chlorine content in biomass shows a larger value
content and calorific value (Qgr) on an as-received basis than in coal. Moreover, palm stem has a very high chlorine
(ar), as shown in Table 2. The moisture content of the stem value compared to palm leaves, palm fiber, EFB, and palm
samples is relatively high compared to other biomass, espe- frond, which has chlorine value under 3000 ppm. Fly ash
cially the middle stem (65.22 wt%), since the stem stores tends to condense in superheaters because of chlorine, which
more food reserves in the form of water [48]. The moisture allows for developing low-melting eutectics and chlorides.
possessed by palm fiber and palm leaves is relatively low By reacting with the alkali in the biomass, the chlorine will
compared to coal. The lowest moisture content is owned by further precipitate on the heating surface and react with the
palm fiber (8.43 wt%), making it have a high as-received metal surface’s protective oxide layer, thereby triggering
calorific value ­(Qgrar). The combination of high moisture corrosion [26, 52, 53].
content and particle size of biomass injected in the pulver- Elements present in biomass ash, like potassium and chlo-
ized boiler may result in delayed combustion and lower peak rine, can cause low AFTs and produce low-temperature and
flame temperatures [49]. The ash content of palm leaves is low-viscosity melting ash, which leads to slagging, fouling, cor-
the highest compared to coal and other oil palm wastes. The rosion, and abrasion [22, 54, 55]. The elements in the ash of all
order of the ash content from the highest to the lowest is samples that are the subject of this study are shown in Table 2.
as follows: palm leaves > coal > upper stem > palm fiber The richest S­ iO2 content is found on the upper stem compared

13
 Biomass Conversion and Biorefinery

to coal and other biomass. Silica has a good effect because its

Chlorinedb (ppm)
high melting point can raise the AFT [52, 56]. On the other
hand, Zevenhoven et al. [57] explained that the interaction

1577.66
1523.27
4559.66
4808.24
4590.31
2597.76
1766.60
between high Si and K in biomass, especially agricultural-

191.31
based, can cause severe ash-related problems during combus-
tion due to the low melting point of K-silicate. This compound
Qgrdb (kcal/kg)

adheres with other particles to form slag in the boiler.

Table 3 shows the results of ash analysis of coal and oil
palm waste biomasses. Coal samples, followed by palm

4381
4385
4362
4241
4071
4437
4762
6566

leaves, own the highest A ­ l2O3 content. In contrast, palm

stems in all biomass parts had the lowest A ­ l2O3 content.
Qgrar (kcal/kg)

Generally, aluminum in biomass may be in the form of alu-

minosilicates when impurities are present. The high value
of aluminosilicates may increase the AFT and react to cap-
2272
1902
1834
1475
1505
3636
4313
4889

ture the alkaline vapor during combustion [52]. Palm frond

samples contain more F ­ e2O3 and CaO than other biomass;
Sdb (wt%)

this poses a risk to the boiler due to the low melting point,
and it will result in slagging when exposed to temperatures
0.11
0.08
0.05
0.14
0.15
0.20
0.14
0.87

above a certain threshold [58, 59]. Besides that, the largest

amount of MgO is owned by the palm frond and the lower
Odb (wt%)

stem. MgO may improve the combustion characteristics and

M moisture, Ash ash content, VM volatile matter, FC fixed carbon, Qgr gross calorific value, ar as-received, db dry basis
44.92
42.94
45.13
45.10
41.55
32.96
37.54
18.63

mitigate slagging and fouling in the boiler [60]. Overall,

oil palm waste biomass has a lower composition of ­Na2O
Ndb (wt%)

and ­SO3 than coal. However, it cannot be denied that the

potassium ­(K2O) content in this oil palm waste is normally
0.49
0.65
0.29
0.69
2.15
2.30
0.72
1.42

greater than coal, especially EFB, which can lower ash melt-
ing temperature.
Hdb (wt%)

The solid phases of biomass and coal ash may react

5.16
5.31
4.43
2.90
4.14
4.23
5.09
3.65

with one another to generate minerals or liquid phases

when heated [61, 62]. As shown in Fig. 4, coal has a fairly
Cdb (wt%)

good AFT (with a DT of 1190 °C) with medium slagging

and fouling tendency [37]. Deformation value means the
47.11
47.65
47.78
46.35
45.82
46.03
51.42
66.79

temperature at which ash begins to melt, then proceed with

the softening temperature, which indicates a change in the
FCdb (wt%)

ash becoming a liquid phase. Line graphs in Fig. 4 that

coincide with coal are palm frond and palm fiber samples,
15.17
18.30
17.41
19.44
20.00
18.26
18.95
48.36

which means these two samples have similar characteris-

tics close to coal in the AFT aspect. Palm frond and palm
VMdb (wt%)
Table 2  Coal and oil palm waste biomass characteristics

fiber also have a medium tendency of slagging and fouling

because the range of their DT is between 1100 and 1300
82.61
78.34
80.27
75.73
73.81
67.46
75.95
42.99

°C [63]. Palm frond, which has a medium tendency, has

the second highest DT value of 1240 °C influenced by the
Ashdb (wt%)

highest MgO content of 12.89 wt% compared with other

biomass [15]. High MgO leads to the production of high
14.28

2.22
3.36
2.32
4.83
6.19
5.09
8.65

melted particles, which result in high AFTs [2, 60]. Palm

fiber has high ­SiO2 and ­Al2O3 content that cause medium
Mar (wt%)

AFT with DT value at 1140 °C. Palm leaves are the sam-
ples with the highest DT, reaching 1305 °C, making them
48.13
56.63
57.94
65.22
63.04
18.05
25.55
8.43

have the lowest tendency of slagging and fouling [63]. It is

probably influenced by lower potassium and higher ­Al2O3
Middle stem
Palm leaves

Lower stem
Upper stem

Palm frond

contents in palm leaves [26].

Palm fiber
Samples

The lower stem has a higher AFT than the middle and
Coal

EFB

upper stems because of a higher MgO content, which is a

13
Biomass Conversion and Biorefinery

Table 3  Ash analysis of coal Samples Ash analysis (wt%)

and oil palm waste biomasses
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 Na2O K 2O Mn3O4 P2O5 SO3

Coal 55.70 17.12 8.92 4.50 3.36 0.89 2.09 2.12 0.06 0.43 4.52
Palm fiber 60.05 10.00 2.56 9.13 5.91 0.18 0.18 10.02 0.18 0.18 0.12
Palm leaves 51.93 12.90 11.05 10.22 2.79 0.14 2.83 2.95 0.83 1.78 2.27
Upper stem 72.80 5.02 1.08 4.46 3.82 0.05 0.18 11.07 0.16 0.10 0.18
Middle stem 57.26 8.78 4.61 7.37 7.69 0.12 0.19 13.15 0.27 0.10 0.18
Lower stem 50.06 7.63 4.85 10.00 11.16 0.09 0.19 14.85 0.35 0.10 0.21
EFB 33.85 1.61 5.44 7.28 5.01 0.08 0.40 32.58 0.09 4.23 2.03
Palm frond 22.03 1.80 28.59 19.72 12.89 0.23 0.38 5.58 0.18 2.27 2.62

Fig. 4  AFT comparison of coal and oil palm wastes

similar case to the high AFT of the palm frond. As can be 3.2 Theoretical prediction of samples
observed, the ash fusion temperatures (DT, ST, and HT)
can be ordered as follows: upper stem < middle stem < Table 4 shows the scores for determining the tendency
lower stem, while the FT of those three samples shows sim- range for several parameters. Green highlights indicate low
ilar values. The upper stem has a high slagging and fouling tendency, with a score point of 0.0; yellow highlights indi-
tendency due to low DT, below 1100 °C, while the others cate medium tendency, having a score point of 0.5; and red
show a medium tendency. The lowest AFT of all biomasses highlights indicate high tendency, with a score point of 1.0.
is shown by EFB, with a DT of below 1000 °C, due to The slagging parameters (a total of six parameters) are con-
higher contents of ­K2O and CaO, which can decrease the sidered to show a high slagging tendency if the total score
solid fuel ash melting temperature [26]. EFB has the high- of slagging is > 3.5, a medium slagging tendency if the
est potential of melting ash, which can react with hazard- total score is 2.5–3.5, and a low slagging tendency if the
ous components such as chloride and then accommodate total score is < 2.5. Likewise, the fouling parameters (a total
slagging and fouling in the boiler area [26, 34]. of three parameters) are considered to have a high fouling

13
 Biomass Conversion and Biorefinery

Table 4  Calculated and Palm Palm Upper Middle Lower Palm

predicted indexes of coal and oil Parameter Coal EFB
Fiber Leaves Stem Stem Stem Frond
palm waste biomass
B/A Ratio 0.28 0.40 0.46 0.26 0.50 0.71 0.86 1.10
Silica Ratio 76.85 77.33 68.34 88.61 74.43 65.81 68.86 44.11
Fusibility 1216 1166 1336 1032 1122 1143 1112 1271
Composite Index 2.44 3.52 2.74 5.62 3.81 4.20 3.89 3.86
Slagging index 0.24 0.06 0.09 0.04 0.07 0.03 0.16 0.12
Iron in ash 8.92 2.56 11.05 1.08 4.61 4.85 6.06 8.97
2.0 out 2.0 out 2.5 out 2.0 out 2.0 out 3.0 out 3.0 out 4.0 out of
Total Slagging
of 6.0 of 6.0 of 6.0 of 6.0 of 6.0 of 6.0 of 6.0 6.0
Fouling Index 0.6 0.07 1.30 0.05 0.09 0.13 0.40 3.26
Sodium in ash 2.09 0.18 2.83 0.18 0.19 0.19 0.47 2.97
Total Alkali 0.27 10.20 5.78 11.25 13.34 15.04 23.81 5.97
2.0 out 1.0 out 2.5 out 1.0 out 1.0 out 1.0 out 1.5 out 2.5 out of
Total Fouling
of 3.0 of 3.0 of 3.0 of 3.0 of 3.0 of 3.0 of 3.0 3.0
Abrasion index 4.21 2.98 6.70 4.52 2.36 1.03 2.71 1.03
Total Abrasion 0.5 out 0.0 out 0.5 out 0.5 out 0.0 out 0.0 out 0.0 out 0.0 out of
of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 1.0
Corrosion index 40.90 0.71 0.69 0.29 0.26 0.09 0.50 0.61
Total Corrosion 0.0 out 1.0 out 1.0 out 1.0 out 1.0 out 1.0 out 1.0 out 1.0 out of
of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 of 1.0 1.0

Tendencies : Low Medium Severe

tendency if the total score of fouling is > 2.5, a medium a low tendency. In the aspect of corrosion, only coal has a
fouling tendency if the total score is between 1.5–2.5, and a low tendency from the S/Cl ratio, while all biomass has a
low fouling tendency if the total score is < 1.5. high tendency due to the high chlorine value of the biomass.
As shown in Table 4, palm fiber, upper stem, and middle
stem show a low slagging tendency with a total score of 2.0 3.3 Combustion characteristics
out of 6.0, similar to coal with different high-tendency slag-
ging parameters. Coal, palm fiber, upper stem, and middle From the results of the TGA, the basic combustion param-
stem have the same high tendency in fusibility but different eters are shown in Table 5, followed by the thermogravi-
composite indexes. metric (TG) and thermogravimetric derivatives (dTG) of
Meanwhile, the other biomasses have a medium to high the coal and palm-based biomass samples in Fig. 5. The
slagging tendency. Furthermore, a striking parameter dif- ignition temperature (Tig), which means material ignition
ference is found in the B/A ratio, which is owned by coal, spontaneously triggered by external heat [65], shows that
palm fiber, palm leaves, upper stem, and middle stem, which palm biomass has a lower value than coal according to the
tend to have a lower tendency than other biomass, which steepness of the TG graph. Tig of coal is higher (341 °C)
means these five samples have less base content than the
other samples.
Palm fiber and stems show a low fouling tendency with Table 5  Combustion parameter obtained from TG-DTA
a tendency score of 1.0 out of 3.0 due to lower ­Na2O con- Samples Tig Tbo Tbo-Tig (Tq) Tmax Rmax
tent. ­Na2O content in ash is used in all fouling indices and °C °C °C °C mg/s
become an important fouling indicator [64]. Moreover, N ­ a2O
Coal 341.00 561.20 220.20 394.30 0.01
has a low melting point below 1200 °C, which is lower than
Palm fiber 302.28 487.23 184.95 353.48 0.08
the furnace temperature. In addition, ­K2O also needs to be
Palm leaves 247.16 482.55 235.39 369.95 0.08
considered because of its low melting point (700 °C) [24].
Upper stem 248.81 458.65 209.84 312.29 0.04
Coal, palm leaves, and palm frond have a high tendency of
Middle stem 292.67 387.00 94.33 321.59 0.10
fouling compared to other biomass. However, coal still has a
Lower stem 272.94 407.29 134.35 325.88 0.12
lower total alkali content than all palm biomass samples. For
EFB 233.44 389.19 155.75 335.98 0.13
the abrasion parameter, only coal, palm leaves, and upper
Palm frond 258.64 365.67 107.03 336.42 0.13
stem have a medium tendency, while other samples have

13
Biomass Conversion and Biorefinery

Fig. 5  (a) TG and (b) dTG curves of coal and biomass samples

than all of the biomass with a range of 200–300 °C due 3.4 Qualitative probe observation
to very low volatile matter compared to biomass [23, 66].
Biomasses had lower Tmax values than coal, meaning that Figure 6 shows that the ash deposits on the probe have been
biomass has higher reactivity than coal. It also relates to brushed, and the probe surface has been observed qualita-
Rmax as the maximum combustion rate. Biomass has a rela- tively [45, 69]. As can be observed, coal combustion shows
tively high combustion rate compared to coal, which only the cleanest probe compared with all biomass combustion.
has a maximum combustion rate of 0.01 mg/s. It means This ash probe sample can be categorized as category 1.
that biomass is easier to be ignited and combusted than The partly and sintered ash categories 2 and 3 can be found
coal. For burnout temperature (Tbo), which means the final in all biomass samples. EFB is a biomass that produces a
combustion of material to be burned out where in the TG cleaner probe than other biomass and is categorized as cat-
curve (Fig. 5), the heat flow rate is 0, the palm frond has egory 2, both at probe temperatures of 550 and 600 °C. Ash,
a very low Tbo value compared to other biomass, and the which is sintered on the probe, can be removed easily with a
Tbo value in coal is the highest. The difference in value light touch. Palm leaves and palm fronds are categorized as
between Tbo and Tig means the necessary time for the mate- category 3. Visualizing this probe looks like some sintered
rial to be burned out. The middle stem is biomass with the ash is attached to the probe’s surface. The remaining mate-
lowest Tbo – Tig compared to other biomass, meaning the rial was very difficult to remove but could still be removed
middle stem takes a shorter time to be burned out [67]. with some firm pressure by hand. Sintered ash, categorized
Coal has a volatile decomposition phase and a char in as category 4, is visible in the case of a palm fiber sample
one peak decomposition phase, which starts at a tempera- at a probe temperature of 550 °C; the remaining material
ture of 250–530 °C, so it can be said that the combustion on the surface of the probe is very difficult to remove and
of volatile matter and char in coal occurs sequentially. In completely fused with stainless steel material on the probe.
the coal sample, there are small peaks after the main peak, However, this fused slagging material was not found in the
indicating char residue combustion until the material mass case of the palm fiber sample at a probe temperature of 600
is completely used up [65]. Biomass usually consists of °C. The appearance of the probe, which looks very corrosive
two stages, where the first stage is the decay of hemicel- with ash adhered to the probe surface, is found in the palm
lulose and cellulose, and the second stage is the decay of stem sample in all parts, both upper, middle, and lower.
lignin, residual volatiles, and char [65, 68]. Palm fiber, Figure 7 shows ash deposits weight from 1-h combustion
middle stem, lower stem, palm frond, and EFB have simi- in DTF. However, it is possible that the ash component in
lar curves from oil palm biomass. The first stage occurs samples such as ­K2O, ­Na2O, ­SiO2, and ­Fe2O3 also causes
at a temperature of 250–300 °C and the second at a tem- a lot of ash deposits in combustion. A large amount of ash
perature of 300–375 °C. The two stages in palm leaves owned by the upper and middle stem results from high silica,
become one with a temperature range of 250 to 375 °C. potassium, and chlorine content. A high alkali metal and
Meanwhile, the first upper stem stage occurs at a tempera- silica content with low deformation temperature can be a
ture of 250–330 °C and the second stage at 430–460 °C. prefix for ash deposition on hot surfaces at low temperatures

13
 Biomass Conversion and Biorefinery

Fig. 6  Probe observation

Probe temperature of 550 Probe temperature of 600
Samples
Probe before Probe after Probe before Probe after
brushed Brushed brushed Brushed

Coal

Palm
Fiber

Palm
Leaves

Upper
Stem

Middle
Stem

Lower
Stem

EFB

Palm
frond

and then accommodate slag material which inhibits boiler deposit due to the high content of F
­ e2O3 and CaO, where
heat transfer [70, 71]. In contrast, the lower stem, which is these two elements play an important role in the formation
rich in potassium, has a low ash deposit due to low ash, high of ash deposits [50, 73].
SiO2, and high MgO compared to the other two parts of the
stem. Coal with high ­SiO2 and ­Al2O3 values results in low 3.5 Emission from DTF combustion
ash deposits below 0.1 g. Likewise, low ash was found in
palm fiber, lower stem, and EFB. Palm fiber with 60.05 wt% Based on gas emission analysis on coal and oil palm wastes,
­SiO2 and 10 wt% ­Al2O3 had a positive impact on the forma- the ­O2 value in the exhaust gas ranged from 3 to 5%, and
tion of ash deposits [72]. Meanwhile, EFB with low ash the ­CO2 value obtained was not significantly different. Coal
content produces low ash deposits even though it has high has a ­CO2 value of 16.1%, and biomass has a C ­ O2 value of
­K2O. Palm leaves and palm fronds have a relatively high ash around 15%, as shown in Fig. 8a, except for the EFB and

13
Biomass Conversion and Biorefinery

Fig. 7  Ash deposition

Fig. 8  (a) Mean value of excess oxygen and C

­ O2 from DTF; (b) ­SO2 and ­NOX emissions @ O
­ 2 7%

palm frond samples, which have a higher ­CO2 value of 17%. higher ­NOX value than the coal sample (544.88 mg/Nm3).
Generally, the carbon content of the fuel affects the value of However, the high ­NOx value in the EFB sample is prob-
­CO2 in the combustion products; additionally, the value of ably due to the volatile matter’s influence. Kim et al. and Y.
volatile matter can affect the exhaust gas ­CO2, which can Lin et al. [74, 75] have also confirmed that ­NOx is released
influence the rate of volatilization of the results of fuel com- at volatile combustion temperatures. That is similar to the
bustion, which is higher in the gas phase than the formation increased ­CO2 emission caused by the volatile matter in
of char. [74, 75]. biomass combustion.
As shown in Fig. 8b, the coal sample showed ­SO2 val-
ues of 112.4 mg/Nm3, higher than oil palm wastes except 3.6 SEM‑EDS analyses
for palm fiber and EFB, which have values of 148.68 and
160.04 mg/Nm3, respectively. Sulfur in coal and biomass The SEM results of the ashes from the combustion of sam-
is composed of organic and inorganic components formed ples with two different probe temperatures using DTF are
during the devolatilization phase. The lack of excess air shown in Figs. 9, 10, 11, 12, 13, and 14. The color spectrum
in the combustion process affects the high sulfur dioxide in morphological SEM is also presented to ease the obser-
value formed [76, 77]. The sulfur content of palm fiber is vation of elements contained in morphological images and
relatively high, consistent with the extra air present during is supported by the percentage of elemental composition
combustion. The ­NOx emission of coal is relatively high, using EDS.
with a value of 382.35 mg/Nm3, consistent with the high Figure 9a and b are the morphology of the coal sample as
nitrogen content of the coal sample compared to the bio- a reference. As can be observed, the particles contained in
mass sample. In contrast, almost all of the oil palm waste this sample are dominated with a size of 50–100 μm. Particle
samples had lower ­NOX levels except EFB, which had a C (Fig. 9b) is a spherical shape with a bright color observed

13
 Biomass Conversion and Biorefinery

Fig. 9  Morphology and elemental analysis results of coal samples: (a) probe temperature of 550 °C and (b) probe temperature of 600 °C

Fig. 10  Morphology and elemental analysis results of palm fiber samples: (a) probe temperature of 550 °C and (b) probe temperature of 600 °C

13
Biomass Conversion and Biorefinery

Fig. 11  Morphology and elemental analysis results of palm leaves samples: (a) probe temperature of 550 °C and (b) probe temperature of 600 °C

at the probe temperature of 600 °C. It looks more numerous such as particle A. Moreover, based on the morphological
compared to the probe temperature of 550 °C. This particle analysis, tubular particles, such as particle D, which is alu-
represents heavy metal in the form of iron oxides which melt minum oxide, seem more dominant at a probe temperature
during combustion [78]. According to EDS results, the per- of 600 °C (Fig. 10b). The color code spectrum reinforces
centage of Fe at a probe temperature of 600 °C has a portion it with the highlight color on the Al element. In contrast to
of 5.2 wt%, while at a probe temperature of 550 °C, it only coal, palm fiber has smaller portions of Si, Al, and Fe but
has a value of 3.9 wt%. Powdered dry shape with fine particles with a greater number of aggregated particles E, which are
below 10 μm, which are harmless and usually fall off easily sticky and sintered during the deposition process, indicating
into bottom ash in boilers [79], looks more dominant at a that this sample is rich in Ca and K components [79].
probe temperature of 550 °C. Particles with shapes of smooth For the palm leaves sample shown in Fig. 11, the ash
surfaces and sizes above 100 μm (particle A) are observed. sample at a probe temperature of 600 °C is dominated by
It means the particle has a high melting point and retains its particles A, supported by the EDS results, which show that
original shape when fired, representing the elements Si and the Si element has a portion of 30.1 wt%. However, similar
Al. Particle B is an amorphous particle that does not melt dur- to the palm fiber biomass sample, the palm leaves sample
ing combustion. It is similar to particle A in ash chemistry but is also rich in Ca and K (particle E). Particle F in Fig. 11a
with alkaline minerals condensed on its surface [79]. represents unburned carbon with dark shapes that exist in
Figures 10, 11, 12, 13, and 14 are the morphological the gaps of among other particles [80, 81]. These particles
result for the sample biomass. As shown in Fig. 10, many appear to be denser at a probe temperature of 550 °C than
shapes measure up to 100 μm with smooth surface shapes, 600 °C. Carbon is not harmful to the formation of slagging

13
 Biomass Conversion and Biorefinery

Fig. 12  Morphology and

elemental analysis results of
palm stem samples: (a) upper
stem, probe temperature of 550
°C; (b) upper stem, probe tem-
perature of 600 °C; (c) middle
stem, probe temperature of 550
°C; (d) middle stem, probe tem-
perature of 600 °C; (e) lower
stem, probe temperature of 550
°C; and (f) lower stem, probe
temperature of 600 °C

13
Biomass Conversion and Biorefinery

Fig. 13  Morphology and elemental analysis results of EFB samples: (a) probe temperature of 550 °C and (b) probe temperature of 600 °C

in the boiler, but it leads to higher emissions due to incom- in the palm frond are smaller in the 50–100-μm range.
plete combustion [82, 83]. Smooth spherical ash particles (particle C) in Fig. 14 appear
Figure 12 represents the SEM results of three different to have been molten during combustion. According to EDS
parts of the palm stem with probe temperatures of 550 and results, these samples contained more Fe and O, indicating
600 °C, respectively. The morphology of all palm stems was much iron oxide in the ash.
dominated by particles E, rich in K, Ca, Na, S, and Mg. Ca,
S, and Mg, which may adhere to other materials, leading to 3.7 X‑ray diffraction
increased ash deposition in boilers [79]. In palm stem com-
bustion, it was revealed that corrosion and erosion occurred, XRD patterns are shown in Fig. 15a for the probe tempera-
as evidenced by the presence of particles G in Fig. 11d ture of 550 °C and Fig. 15b for the probe temperature of 600
with a very bright irregular shape indicating it is a heavy °C. At a probe temperature of 550 °C, coal contains 48.54
metal, which represents Cr in its color-coded spectrum. The wt% albite ­(NaAlSi3O8), 47.25 wt% quartz ­(SiO2), 2.18 wt%
presence of Cr in this sample is caused by the interaction anhydrite ­(CaSO4), and 2.03 wt% hematite ­(Fe2O3). A high
between S, K, and Fe to form alkali sulfates [84] shown on percentage of albite must be considered because this mineral
particle H, with a similar morphology shape as particle G, contains Na and has a melting point of 1118 °C. Albite here
making this component molten and reacting with stainless is formed because the high sulfur in coal reacts with alkaline
steel probe leading to corrosion and erosion, as shown from Na to form ­Na2SO4. Furthermore, ­Na2SO4 reacts with alumi-
the probe observations in Fig. 6. nosilicate to form albite at temperatures above 575 °C [24].
Figures 13 and 14 are EFB and palm frond samples whose Palm fiber has a high Na content resulting in high percentage
morphological ash looks denser than other biomass samples. of albite. Palm fiber at a probe temperature of 550 °C has
Particles contained in the EFB (see Fig. 13) appear to be 55.95 wt% albite, 14.20 wt% quartz, 14.03 wt% cristobalite
dominated by particles with smooth surfaces (particle A) ­(SiO2), 8.88 wt% kumdykolite (­ NaAlSi3O8), 3.65 wt% cal-
and aggregates of fine particles which sintered and adhered cite ­(CaCO3), 3.07 wt% anhydrite, and 0.23 wt% hematite.
onto the surface of the coarse ash particles, which are domi- Harmless minerals dominate palm leaves. Palm leaves at a
nated by element K and have a larger size estimated to be probe temperature of 550 °C contain 93.88 wt% quartz, 2.12
over 100 μm in size. In comparison, the particles contained wt% ­K2CO3, 2.04 wt% Ca, and 1.958 wt% hematite. Quartz

13
 Biomass Conversion and Biorefinery

Fig. 14  Morphology and elemental analysis results of palm frond samples: (a) probe temperature of 550 °C and (b) probe temperature of 600 °C

dominates palm leaves with a melting point of 1730 °C [24, 91]. Although more than 20 wt% of minerals in the lower
61, 68], which may contribute to the increased AFT of palm stem have a low melting point, this lower stem is still domi-
leaves. Interestingly, each part of the palm stem at a probe nated by harmless minerals with a high melting point higher
temperature of 550 °C has different mineral constituents. than 1350 °C such as hematite, magnesioferrite, quartz, and
The upper stem has 88.40 wt% diopside (­ MgCaSi2O6), 4.82 anhydrite [24, 61, 68, 92]. Then, EFB at a probe temperature
wt% quartz, 5.43 wt% moganite (­ SiO2), 0.41 wt% sylvite of 550 °C contains 53.78 wt% albite, 40.82 wt% quartz, 2.73
(KCl), and 0.95 wt% hematite. This upper stem is domi- wt% calcite, 1.59 wt% hematite, and 1.08 wt% cristobalite.
nated by diopside with a melting point of 1391 °C [85, 86]. As mentioned before, the domination of albite in EFB may
Harmful minerals dominate the middle stem where the min- have a negative effect because of sodium content and its
eral consists of 28.77 wt% sylvite, 26.47 wt% aphthitalite low melting point. Meanwhile, palm frond is dominated
­(K3Na(SO4)2), 23.00 wt% quartz, 5.18 wt% F ­ ePO4, 4.78 wt% by harmless minerals such as hematite and clinoenstatite
anhydrite, 4.66 wt% M­ g2P2O7, 3.75 wt% hematite, 2.63 wt% ­(MgSiO3) that possibly melt at a temperature of higher than
MgO, and 0.78 wt% magnesioferrite ­(MgFe2O4). Sylvite and 1350 °C [24, 93, 94]. This palm frond comprises 61.75 wt%
aphthitalite are harmful because of potassium and sodium hematite, 29.21 wt% clinoenstatite, 5.13 wt% magnesiofer-
content that may contribute to slagging and fouling [29]. In rite, 1.78 wt% ­S8, 1.29 wt% quartz, and 0.85 wt% calcite.
addition, sylvite melts at a temperature of 770–790 °C [24, As shown in Fig. 10b, there is no significant differ-
87, 88], and chlorine in sylvite also has a negative effect ence between the XRD results of probe temperature of
on corrosion [89]. The lower stem has 40.73 wt% hematite, 550 °C. Coal and palm fiber also contain a high percentage
19.31 wt% arcanite (­ K2SO4), 19.26 wt% magnesioferrite, of albite. Coal has 48.66 wt% albite, 46.93 wt% quartz,
5.56 wt% quartz, 5.55 wt% F ­ ePO4, 4.08 wt% anhydrite, 3.28 2.24 wt% anhydrite, and 2.17 wt% hematite. Palm fiber
wt% Ca, and 2.24 wt% ­Ca2Fe2O5. Arcanite, ­FePO4, Ca, and contains 49.24 wt% albite, 23.63 wt% quartz, 21.16 wt%
­Ca2Fe2O5 have a low melting point below 1250 °C [68, 90, cristobalite, 4.14 wt% calcite, and 1.84 wt% hematite.

13
Biomass Conversion and Biorefinery

(a) Probe Temperature of 550°C (b) Probe Temperature of 600°C

Fig. 15  XRD plotting diagram of coal and biomass samples: (A) (L) ­FePO4; (M) sylvite; (N) aphthitalite; (O) MgO; (P) ­Mg2P2O7; (Q)
quartz; (B) anhydrite; (C) hematite; (D) albite; (E) cristobalite; (F) magnesioferrite; (R) arcanite; (S) ­Ca2Fe2O5; (T) clinoenstatite; (U)
calcite; (G) kumdykolite; (H) berlinite; (I) S
­ O3; (J) K
­ 2CO3; (K) Ca; ­S8; (V) diopside; (W) moganite

At a probe temperature of 600 °C, palm leaves are also chlorine [29, 89]. Moreover, ­FePO4 melts at below 1000
dominated by harmless minerals containing 86.81 wt% °C [91], which may trigger the melting ash. Lower stem at
quartz, 4.395 wt% anhydrite, 3.76 wt% K ­ 2CO3, 3.67 wt% a probe temperature of 600 °C is still dominated by harm-
Ca, and 1.36 wt% hematite. Similar to the probe tempera- less minerals even though it contains more than 20 wt%
ture of 550 °C, each side of the palm stem has different harmful minerals such as arcanite, ­Ca2Fe2O5, ­FePO4, and
mineral constituents at a probe temperature of 600 °C. The Ca. The lower stem contains 42.70 wt% hematite, 18.08
upper stem contains 29.96 wt% quartz, 26.70 wt% diop- wt% magnesioferrite, 15.69 wt% quartz, 13.48 wt% arcan-
side, 18.34 wt% cristobalite, 9.66 wt% sylvite, 9.47 wt% ite, 3.32 wt% C ­ a 2Fe 2O 5, 2.49 wt% anhydrite, 2.13 wt%
hematite, 3.85 wt% aphthitalite, and 2.03 wt% anhydrite. ­FePO 4, and 2.09 wt% Ca. EFB at probe temperature of
This upper stem is still dominated by harmless minerals 600 °C consists of 51.10 wt% quartz, 21.09 wt% kumdyko-
such as quartz, diopside, and cristobalite. As mentioned, lite, 14.58 wt% ­SO3, 5.87 wt% albite, 3.69 wt% berlinite,
quartz and diopside have melting points at 1730 and 1391 1.89 wt% calcite, and 1.79 wt% hematite. Kumdykolite is
°C, respectively, while cristobalite melts at 1730 °C [61]. a polymorph of albite [95–98, 98]. As mentioned before,
Harmful minerals also dominate the middle stem with albite may trigger the problem of slagging and fouling
24.48 wt% sylvite, 18.87 wt% F­ ePO4, 14.93 wt% M­ g2P 2O 7, because of its low melting point and sodium content. S ­ O3
12.34 wt% aphthitalite, 5.65 wt% magnesioferrite, 5.35 also has a negative effect related to corrosion because it
wt% hematite, 3.93 wt% MgO, and 2.90 wt% anhydrite. As can react with other elements forming HCl and ­H2S in the
mentioned before, sylvite and aphthitalite have a negative gas phase [89, 99]. Similar to a probe temperature of 550
effect due to the content of potassium, sodium, and total °C, the palm frond at a probe temperature of 600 °C is

13
 Biomass Conversion and Biorefinery

also dominated by harmless minerals such as hematite and 5 Conclusions

clinoenstatite. Palm frond contains 64.30 wt% hematite,
30.04 wt% clinoenstatite, 2.21 wt% magnesioferrite, 1.52 Based on a series of analyses that have been carried out
wt% ­S8, 1.35 wt% quartz, and 0.57 wt% calcite. to investigate the oil palm waste biomass in the combus-
tion aspect as single fuel, it can be concluded that each
type of oil palm waste biomass has different character-
istics. Palm fiber has the highest calorific value among
4 Discussion oil palm wastes and similar combustion characteristics
to coal, but it has sintered ash particles attached to the
Coal is used as a baseline for comparison of combustion for surface of the probe. Palm leaves, EFB, and palm fronds
other oil palm waste biomass because it has good combustion have relatively clean probe observation; however, it needs
characteristics supported with a calorific that is suitable as the to consider K-based and Fe-based minerals that can be
main fuel for the pulverized fuel boiler. Probe observations formed. On the other hand, palm stems with high chlo-
produced by coal are also relatively clean without adhered rine content increase the tendency of corrosion during
ash on the probe’s surface. SEM-EDS and XRD analyses also combustion. According to this study, palm leaves, EFB,
show that coal was dominated by high Si and Al. and palm fronds can be recommended to be utilized as
The high volatility of oil palm wastes causes various biomass fuel for co-firing. However, further investiga-
ash components and may increase the slagging and fouling tion on co-firing between oil palm wastes, coal, or other
potential during combustion. It can be observed that palm solid fuel is recommended to find the compatibility and
fiber has combustion characteristics similar and close to blend composition for optimal combustion and mitigate
coal, indicated by the consistency of slagging and fouling ash-related problems.
prediction calculations, calorific values, AFTs, and several
combustion characteristics from TGA analysis, such as
Tig, Tbo, and Tmax. However, palm fiber has sintered ash Author contribution Fairuz Milkiy Kuswa: data curation, writing –
original draft, investigation. Hanafi Prida Putra: data curation, writing,
particles in probe observation and further confirmed by editing. Prabowo: conceptualization, supervision, methodology. Arif
K-based minerals such as albite are found in relatively high Darmawan: data curation, writing, editing. Muhammad Aziz: supervi-
amounts. Palm leaves with a less prominent ash compo- sion, writing, editing. Hariana: conceptualization, writing, methodol-
nent, but still high in S
­ iO2 and ­Al2O3, produce the highest ogy, supervision.
AFT among other palm biomass. The SEM-EDS and XRD Funding Open access funding is provided by The University of Tokyo.
analysis results show that Si dominates the palm leaves
ash in the form of quartz with high melting temperatures. Data availability Data will be made available on request
EFB, which is rich in potassium, has a high slagging
tendency. This is supported by the results of ash mineral- Declarations
ogy analysis using SEM and XRD, where mineral albite Ethical approval This material is the authors’ own original work, which
with low melting point predominates. However, it has has not been previously published elsewhere. The paper is not currently
cleaner probe observations and lower ash deposits, which being considered for publication elsewhere.
can be an added value for EFB. Palm frond is rich in iron This paper does not contain any data taken directly from any published
content, producing 28.59 wt% F ­ e2O3 in ash analysis, cor- article.
roborated by the results of combustion ash with higher This research did not contain any studies involving animal or human
participants, nor did it take place in any private or protected areas. No
Fe and mineral hematite contents in the SEM and XRD specific permissions were required for corresponding locations.
results. Iron has a low melting point and easily reacts to
other components, such as S and Cl, producing FeS2 and Competing interests The authors declare no competing interests.
FeCl2, which leads to slagging and corrosion [24].
From probe observation, palm stems with a high chlo- Open Access This article is licensed under a Creative Commons Attri-
rine value show a higher corrosion tendency. As proven by bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
SEM-EDS analysis, Cr elements were carried away when as you give appropriate credit to the original author(s) and the source,
the ashes were brushed from the probe. This shows that provide a link to the Creative Commons licence, and indicate if changes
the palm stems corrosiveness can peel off the outer layer were made. The images or other third party material in this article are
of steel. Moreover, Fe-based and alkali-based minerals are included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
observed in XRD analysis. In addition, palm stems also the article's Creative Commons licence and your intended use is not
have higher ash deposits due to the domination of low- permitted by statutory regulation or exceeds the permitted use, you will
melting alkali-based minerals and bright particles that indi- need to obtain permission directly from the copyright holder. To view a
cate heavy metals in the ash mineralogy results [79, 100]. copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

13
Biomass Conversion and Biorefinery

References 18. Hamada HM, Shi J, Abed F et al (2023) Recycling solid waste to
produce eco-friendly ultra-high performance concrete: a review
of durability, microstructure and environment characteristics.
1. Borowski PF (2022) Mitigating climate change and the develop-
Sci Total Environ 876:162804. https://​doi.​org/​10.​1016/j.​scito​
ment of green energy versus a return to fossil fuels due to the
tenv.​2023.​162804
energy crisis in 2022. Energies 15:9289
19. Salleh SF, Abd Rahman A, Abdullah TART (2018) Potential
2. Xu Y, Yang K, Zhou J, Zhao G (2020) Coal-biomass co-firing
of deploying empty fruit bunch (EFB) for biomass co-firing in
power generation technology: current status, challenges and
Malaysia’s largest coal power plant. 2018 IEEE 7th Int Conf
policy implications. Sustainability 12:3692. https://​doi.​org/​10.​
Power Energy. PECon 2018:429–433. https://​doi.​org/​10.​1109/​
3390/​su120​93692
PECON.​2018.​86841​24
3. Wu X, Wu K, Zhang Y et al (2017) Comparative life cycle
20. Up O, Ko O, Owamah HI, Ci S, Adingwupu AC (2022) Assess-
assessment and economic analysis of typical flue-gas clean-
ment of power generation potential from the co-firing of Elaeis
ing processes of coal-fired power plants in China. J Clean Prod
Guineensis Residues with coal pellets in a popular Nigerian
142:3236–3242. https://​doi.​org/​10.​1016/j.​jclep​ro.​2016.​10.​146
palm oil research institute. Int J Coal Prep Util 42:2804–2819.
4. Paraschiv S, Paraschiv LS (2020) Trends of carbon dioxide
https://​doi.​org/​10.​1080/​19392​699.​2021.​19082​72
(CO2) emissions from fossil fuels combustion (coal, gas and oil)
21. Mohd Idris MN, Hashim H (2021) Integrating palm oil biomass
in the EU member states from 1960 to 2018. Energy Reports
waste utilization in coal-fired power plants for meeting near-
6:237–242. https://​doi.​org/​10.​1016/j.​egyr.​2020.​11.​116
term emission targets. J Environ Manage 296:113118. https://​
5. Zapata-mina J, Restrepo A, Esteban J (2023) Case studies in
doi.​org/​10.​1016/j.​jenvm​an.​2021.​113118
thermal engineering assessment of the exergy , emissions , and
22. Vassilev SV, Vassileva CG, Song Y-C et al (2017) Ash contents
combustion characteristics of a diesel engine operating on low-
and ash-forming elements of biomass and their significance for
glyceride biodiesel blended with diesel fuel. Case Stud Therm
solid biofuel combustion. Fuel 208:377–409. https://​doi.​org/​
Eng 41. https://​doi.​org/​10.​1016/j.​csite.​2022.​102636
10.​1016/j.​fuel.​2017.​07.​036
6. Edenhofer O, Steckel JC, Jakob M, Bertram C (2018) Reports
23. Putra HP, Hilmawan E, Darmawan A et al (2023) Theoretical
of coal’s terminal decline may be exaggerated. Environ Res Lett
and experimental investigation of ash-related problems during
13. https://​doi.​org/​10.​1088/​1748-​9326/​aaa3a2
coal co-firing with different types of biomass in a pulverized
7. Yang Q, Zhou H, Bartocci P et al (2021) Prospective contribu-
coal-fired boiler. Energy 269:126784. https://​doi.​org/​10.​1016/j.​
tions of biomass pyrolysis to China’s 2050 carbon reduction and
energy.​2023.​126784
renewable energy goals. Nat Commun 12:1–12. https://​doi.​org/​
24. Vassilev SV, Baxter D, Vassileva CG (2013) An overview of
10.​1038/​s41467-​021-​21868-z
the behaviour of biomass during combustion: Part I. Phase-
8. Rehfeldt M, Worrell E, Eichhammer W, Fleiter T (2020) A
mineral transformations of organic and inorganic matter. Fuel
review of the emission reduction potential of fuel switch towards
112:391–449. https://​doi.​org/​10.​1016/j.​fuel.​2013.​05.​043
biomass and electricity in European basic materials industry until
25. Madejski P, Janda T, Taler J et al (2018) Analysis of fouling
2030. Renew Sust Energ Rev 120:109672. https://​doi.​org/​10.​
degree of individual heating surfaces in a pulverized coal fired
1016/j.​rser.​2019.​109672
boiler. J Energy Resour Technol Trans ASME 140:1–8. https://​
9. Deng L, Jin X, Long J, Che D (2019) Ash deposition behaviors dur-
doi.​org/​10.​1115/1.​40379​36
ing combustion of raw and water washed biomass fuels. J Energy
26. Chen C, Bi Y, Huang Y, Huang H (2021) Review on slagging
Inst 92:959–970. https://​doi.​org/​10.​1016/j.​joei.​2018.​07.​009
evaluation methods of biomass fuel combustion. J Anal Appl
10. Rosendahl L (2013) Biomass combustion science, technology
Pyrolysis 155:105082. https://​d oi.​o rg/​1 0.​1 016/j.​j aap.​2 021.​
and engineering. Woodhead Publishing, London
105082
11. Alami AH, Tawalbeh M, Alasad S et al (2021) Cultivation of
27. Oladejo JM, Adegbite S, Pang C et al (2020) In-situ monitor-
Nannochloropsis algae for simultaneous biomass applications
ing of the transformation of ash upon heating and the predic-
and carbon dioxide capture. Energ Source Part A:1–12. https://​
tion of ash fusion behaviour of coal/biomass blends. Energy
doi.​org/​10.​1080/​15567​036.​2021.​19332​67
199:117330. https://​doi.​org/​10.​1016/j.​energy.​2020.​117330
12. Verma M, Loha C, Sinha AN, Chatterjee PK (2017) Drying of
28. Qian X, Xue J, Yang Y, Lee SW (2021) Thermal properties and
biomass for utilising in co-firing with coal and its impact on
combustion-related problems prediction of agricultural crop
environment – a review. Renew Sust Energ Rev 71:732–741.
residues. Energies 14. https://​doi.​org/​10.​3390/​en141​54619
https://​doi.​org/​10.​1016/j.​rser.​2016.​12.​101
29. Li N, Vainio E, Hupa L et al (2018) Interaction of high
13. Loo S van, Koppejan J (2008) The handbook of biomass combus-
Al2O3 refractories with alkaline salts containing potassium
tion and co-firing. Antony Rowe, Chippenham, London
and sodium in biomass and waste combustion. Energ Fuel
14. Index Mundi (2022) Palm oil production by country in 1000
32:12971–12980. https://​d oi.​o rg/​1 0.​1 021/​a cs.​e nerg​y fuels.​
MT. https://​www.​index​mundi.​com/​agric​ultur​e/?​commo​dity=​
8b031​36
palm-​oil. Accessed 10 Oct 2022
30. Yacob NS, Mohamed H, Shamsuddin AH (2021) Investigation
15. Hariana P, Hilmawan E et al (2023) A comprehensive evaluation
of palm oil wastes characteristics for co-firing with coal. J Adv
of co-firing biomass with coal and slagging-fouling tendency in
Res Appl Sci Eng Technol 23:34–42. https://​doi.​org/​10.​37934/​
pulverized coal-fired boilers. Ain Shams Eng J 14(7):102001.
araset.​23.1.​3442
https://​doi.​org/​10.​1016/j.​asej.​2022.​102001
31. Niu Y, Tan H, Wang X et al (2010) Study on fusion characteris-
16. Darmawan A, Asyhari T, Dunggio I et al (2023) Energy harvest-
tics of biomass ash. Bioresour Technol 101:9373–9381. https://​
ing from tropical biomasses in Wallacea region: scenarios, tech-
doi.​org/​10.​1016/j.​biort​ech.​2010.​06.​144
nologies, and perspectives. Biomass Convers Biorefin. https://​
32. Putra HP, Karuana F, Ruhiyat AS et al (2022) Effect of MgO-
doi.​org/​10.​1007/​s13399-​023-​04223-8
based additive on the sintering behavior for slagging-fouling
17. Hamada HM, Al-Attar A, Shi J et al (2023) Optimization of sus-
mitigation in Indonesian coal combustion. Int J Coal Prep Util
tainable concrete characteristics incorporating palm oil clinker
1–11. https://​doi.​org/​10.​1080/​19392​699.​2022.​21182​57
and nano-palm oil fuel ash using response surface methodology.
33. Liang W, Wang G, Ning X et al (2020) Application of BP neu-
Powder Technol 413:118054. https://​doi.​org/​10.​1016/j.​powtec.​
ral network to the prediction of coal ash melting characteristic
2022.​118054

13
 Biomass Conversion and Biorefinery

temperature. Fuel 260:116324. https://​doi.​org/​10.​1016/j.​fuel.​ emission and ash deposition improvements. J Clean Prod
2019.​116324 270:122418. https://​doi.​org/​10.​1016/j.​jclep​ro.​2020.​122418
34. Lachman J, Baláš M, Lisý M et al (2021) An overview of slag- 52. Niu Y, Tan H, Hui S (2016) Ash-related issues during biomass
ging and fouling indicators and their applicability to biomass combustion: alkali-induced slagging, silicate melt-induced slag-
fuels. Fuel Process Technol 217:106804. https://​doi.​org/​10.​ ging (ash fusion), agglomeration, corrosion, ash utilization, and
1016/J.​FUPROC.​2021.​106804 related countermeasures. Prog Energy Combust Sci 52:1–61.
35. Kitto JB, Stultz SC (2005) Steam: its generation and use, 41st https://​doi.​org/​10.​1016/j.​pecs.​2015.​09.​003
edn. Barberton, OH (United States) 53. Zi J, Ma D, Wang X et al (2020) Slagging behavior and mecha-
36. Raask E (1985) Mineral impurities in coal combustion: behavior, nism of high-sodium–chlorine coal combustion in a full-scale
problems, and remedial measures. Hemisphere Publishing Cor- circulating fluidized bed boiler. J Energy Inst 93:2264–2270.
poration, Washington, DC, United States https://​doi.​org/​10.​1016/j.​joei.​2020.​06.​009
37. Zhu C, Tu H, Bai Y et al (2019) Evaluation of slagging and foul- 54. Nunes LJR, Matias JCO, Catalão JPS (2016) Biomass combus-
ing characteristics during Zhundong coal co-firing with a Si/Al tion systems: a review on the physical and chemical properties
dominated low rank coal. Fuel 254. https://d​ oi.o​ rg/1​ 0.1​ 016/j.f​ uel.​ of the ashes. Renew Sust Energ Rev 53:235–242. https://​doi.​
2019.​115730 org/​10.​1016/j.​rser.​2015.​08.​053
38. Jeong T-Y, Sh L, Kim J-H et al (2019) Experimental investigation 55. Wang C, Tang G, Sun R et al (2020) The correlations of chemi-
of ash deposit behavior during co-combustion of bituminous coal cal property, alkali metal distribution, and fouling evaluation
with wood pellets and empty fruit bunches. Energies 12:2087 of Zhundong coal. J Energy Inst 93:2204–2214. https://​doi.​org/​
39. Garcia-Maraver A, Mata-Sanchez J, Carpio M, Perez-Jimenez 10.​1016/j.​joei.​2020.​06.​002
JA (2017) Critical review of predictive coefficients for biomass 56. Prismantoko A, Hilmawan E, Darmawan A, Aziz M (2023)
ash deposition tendency. J Energy Inst 90:214–228. https://​doi.​ Effectiveness of different additives on slagging and fouling
org/​10.​1016/j.​joei.​2016.​02.​002 tendencies of blended coal. J Energy Inst 107:101192. https://​
40. Ghazidin H, Prayoga MZE, Putra HP et al (2023) A comprehen- doi.​org/​10.​1016/j.​joei.​2023.​101192
sive evaluation of slagging and fouling indicators for solid fuel 57. Zevenhoven M, Yrjas P, Skrifvars B-J, Hupa M (2012) Charac-
combustion. Therm Sci Eng Prog 40:101769. https://​doi.​org/​10.​ terization of ash-forming matter in various solid fuels by selec-
1016/j.​tsep.​2023.​101769 tive leaching and its implications for fluidized-bed combustion.
41. Hariana PA, Ahmadi GA, Darmawan A (2021) Ash evaluation Energ Fuel 26:6366–6386
of indonesian coal blending for pulverized coal-fired boilers. J 58. Wang Y, Xiang Y, Wang D et al (2016) Effect of sodium oxides
Combust 2021. https://​doi.​org/​10.​1155/​2021/​84787​39 in ash composition on ash fusibility. Energ Fuel 30:1437–1444.
42. Wang C, Sun R, Zhao L et al (2020) Experimental study on https://​doi.​org/​10.​1021/​acs.​energ​yfuels.​5b027​22
fouling and slagging behaviors during oxy-fuel combustion of 59. Wei B, Wu W, Liu K et al (2020) Investigation of slagging
high-sodium coal using a high-temperature drop-tube furnace. characteristics on middle and low temperature heat transfers
Int J Greenh Gas Control 97:103054. https://​doi.​org/​10.​1016/j.​ by burning high sodium and iron coal. Combust Sci Technol
ijggc.​2020.​103054 194:1768–1787. https://​doi.​org/​10.​1080/​00102​202.​2020.​18307​
43. Sahu SG, Sarkar P, Chattopadhyay US et al (2019) Investigation 68
on the combustion behavior of coal at various level of washing 60. Akiyama K, Pak H, Ueki Y et al (2011) Effect of MgO addi-
in TGA and drop tube furnace. Trans Indian Ceram Soc 78:181– tion to upgraded brown coal on ash-deposition behavior during
186. https://​doi.​org/​10.​1080/​03717​50X.​2019.​16688​53 combustion. Fuel 90:3230–3236. https://​doi.​org/​10.​1016/j.​fuel.​
44. Li S, Xu T, Sun P et al (2008) NOx and SOx emissions of a 2011.​06.​041
high sulfur self-retention coal during air-staged combustion. Fuel 61. He C, Bai J, Ilyushechkin A et al (2019) Effect of chemical
87:723–731. https://​doi.​org/​10.​1016/j.​fuel.​2007.​05.​043 composition on the fusion behaviour of synthetic high-iron
45. Öhman M, Boman C, Hedman H et al (2004) Slagging tendencies coal ash. Fuel 253:1465–1472. https://​doi.​org/​10.​1016/j.​fuel.​
of wood pellet ash during combustion in residential pellet burn- 2019.​05.​135
ers. Biomass Bioenergy 27:585–596. https://​doi.​org/​10.​1016/j.​ 62. Li F, Fan H, Fang Y (2017) Investigation on the regulation mech-
biomb​ioe.​2003.​08.​016 anism of ash fusion characteristics in coal blending. Energ Fuel
46. Kutchko BG, Kim AG (2006) Fly ash characterization by SEM- 31:379–386. https://​doi.​org/​10.​1021/​acs.​energ​yfuels.​6b025​39
EDS. Fuel 85:2537–2544. https://d​ oi.o​ rg/1​ 0.1​ 016/j.f​ uel.2​ 006.0​ 5.​ 63. Vassilev SV, Baxter D, Vassileva CG (2014) An overview of the
016 behaviour of biomass during combustion: Part II. Ash fusion and
47. Kozlov A, Svishchev D, Donskoy I et al (2015) A technique ash formation mechanisms of biomass types. Fuel 117:152–183.
proximate and ultimate analysis of solid fuels and coal tar. J https://​doi.​org/​10.​1016/j.​fuel.​2013.​09.​024
Therm Anal Calorim 122:1213–1220. https://​doi.​org/​10.​1007/​ 64. Hare N, Rasul M, Moazzem S (2010) A review on boiler deposi-
s10973-​015-​5134-7 tion/foulage prevention and removal techniques for power plant.
48. Kosugi A, Tanaka R, Magara K et al (2010) Ethanol and lactic In: Recent Advances in Energy and Environment. Proceedings
acid production using sap squeezed from old oil palm trunks of the 5th IASME/WSEAS International Conference on Energy
felled for replanting. J Biosci Bioeng 110:322–325. https://​doi.​ & Enviroment (EE'10. vol 23. World Scientific and Engineer-
org/​10.​1016/j.​jbiosc.​2010.​03.​001 ing Academy and Society (WSEAS) Stevens Point, Wiscon-
49. Heinzel T, Siegle V, Spliethoff H, Hein KRG (1998) Investigation sin, United States, p 25
of slagging in pulverized fuel co-combustion of biomass and coal 65. Yan Y, Meng Y, Tang L et al (2019) Ignition and kinetic stud-
at a pilot-scale test facility. Fuel Process Technol 54:109–125. ies: the influence of lignin on biomass combustion. Energ Fuel
https://​doi.​org/​10.​1016/​S0378-​3820(97)​00063-5 33:6463–6472. https://d​ oi.​org/​10.​1021/​acs.​energy​ fuels.​9b0108​ 9
50. Rushdi A, Sharma A, Gupta R (2004) An experimental study of 66. Umar DF, Suganal S, Monika I et al (2020) The influence of
the effect of coal blending on ash deposition. Fuel 83:495–506. steam drying process on combustion behavior of Indonesian low-
https://​doi.​org/​10.​1016/j.​fuel.​2003.​08.​013 rank coals. Indones Min J 23:105–115
51. Hu J, Yan Y, Song Y et al (2020) Catalytic combustions of two 67. Marinov SP, Gonsalvesh L, Stefanova M et al (2010) Combus-
bamboo residues with sludge ash, CaO, and Fe2O3: bioenergy, tion behaviour of some biodesulphurized coals assessed by TGA/

13
Biomass Conversion and Biorefinery

DTA. Thermochim Acta 497:46–51. https://​doi.​org/​10.​1016/j.​ gaseous pollutant emission and ash properties. Sustain Energy
tca.​2009.​08.​012 Technol Assess 54:102836. https://​doi.​org/​10.​1016/j.​seta.​2022.​
68. Vamvuka D, Panagopoulos G, Sfakiotakis S (2020) Investigating 102836
potential co-firing of corn cobs with lignite for energy produc- 86. Choudhary R, Venkatraman SK, Bulygina I et al (2021) Biomin-
tion. Thermal analysis and behavior of ashes. Int J Coal Prep Util eralization, dissolution and cellular studies of silicate biocer-
00:1–12. https://​doi.​org/​10.​1080/​19392​699.​2020.​18560​99 amics prepared from eggshell and rice husk. Mater Sci Eng C
69. Lindström E, Sandström M, Boström D, Öhman M (2007) Slagging 118:111456. https://​doi.​org/​10.​1016/j.​msec.​2020.​111456
characteristics during combustion of cereal grains rich in phospho- 87. Fan H, Li F, Guo Q, Guo M (2021) Effect of biomass ash on
rus. Energ Fuel 21:710–717. https://​doi.​org/​10.​1021/​ef060​429x initial sintering and fusion characteristics of high melting coal
70. Liu Y, Cheng L, Ji J, Zhang W (2019) Ash deposition behavior in ash. J Energy Inst 94:129–138. https://​doi.​org/​10.​1016/j.​joei.​
co-combusting high-alkali coal and bituminous coal in a circulat- 2020.​11.​008
ing fluidized bed. Appl Therm Eng 149:520–527. https://d​ oi.o​ rg/​ 88. Yao X, Zhou H, Xu K et al (2019) Evaluation of the fusion and
10.​1016/j.​applt​herma​leng.​2018.​12.​080 agglomeration properties of ashes from combustion of biomass,
71. Yan T, Bai J, Kong L et al (2017) Effect of SiO2/Al2O3 on fusion coal and their mixtures and the effects of K2CO3 additives. Fuel
behavior of coal ash at high temperature. Fuel 193:275–283 255. https://​doi.​org/​10.​1016/j.​fuel.​2019.​115829
72. Hariana GH, Putra HP et al (2023) The effects of additives on 89. Míguez JL, Porteiro J, Behrendt F et al (2021) Review of the use
deposit formation during co-firing of high-sodium coal with of additives to mitigate operational problems associated with the
high-potassium and -chlorine biomass. Energy 271:127096. combustion of biomass with high content in ash-forming species.
https://​doi.​org/​10.​1016/j.​energy.​2023.​127096 Renew Sust Energ Rev 141:110502
73. Lin X, Yang Y, Yang S et al (2016) Initial deposition feature 90. Barbosa MM, Puschmann R, Thiele S et al (2014) Development
during high-temperature pressurized pyrolysis of a typical Zhun- of thermally sprayed Ca2Fe2O5 coatings for thermoelectrical
dong coal. Energ Fuel 30:6330–6341. https://​doi.​org/​10.​1021/​ applications. Proc ITSC 2014:485–490
acs.​energ​yfuels.​6b010​74 91. Lajmi B, Hidouri M, Rzeigui M, Ben Amara M (2002) Synthesis
74. Kim JH, Lee YJ, Yu J, Jeon CH (2019) Improvement in reactiv- and crystal structure of a new magnesium phosphate Na3RbMg7
ity and pollutant emission by co-firing of coal and pretreated (PO4)6. Mater Res Bull 37:2407–2416. https://​doi.​org/​10.​1107/​
biomass. Energ Fuel 33:4331–4339. https://​doi.​org/​10.​1021/​acs.​ S2056​98901​70063​63
energ​yfuels.​9b003​96 92. Xue P, He D, Xu A et al (2017) Modification of industrial BOF
75. Lin Y, Ma X, Ning X, Yu Z (2015) TGA-FTIR analysis of co- slag: formation of MgFe2O4 and recycling of iron. J Alloys
combustion characteristics of paper sludge and oil-palm solid Compd 712:640–648. https://​doi.​org/​10.​1016/j.​jallc​om.​2017.​
wastes. Energy Convers Manag 89:727–734. https://​doi.​org/​10.​ 04.​142
1016/j.​encon​man.​2014.​10.​042 93. Qu Y, Yang Y, Zou Z et al (2015) Melting and reduction behav-
76. Ren X, Sun R, Meng X et al (2017) Carbon, sulfur and nitrogen oxide iour of individual fine hematite ore particles. ISIJ Int 55:149–
emissions from combustion of pulverized raw and torrefied biomass. 157. https://​doi.​org/​10.​2355/​isiji​ntern​ation​al.​55.​149
Fuel 188:310–323. https://​doi.​org/​10.​1016/j.​fuel.​2016.​10.​017 94. Gu F, Peng Z, Tang H et al (2018) Preparation of refractory mate-
77. Yanik J, Duman G, Karlström O, Brink A (2018) NO and SO2 rials from ferronickel slag. Miner Met Mater Ser Part F8:633–
emissions from combustion of raw and torrefied biomasses and 642. https://​doi.​org/​10.​1007/​978-3-​319-​72484-3_​67
their blends with lignite. J Environ Manage 227:155–161. https://​ 95. Borghini A, Ferrero S, O’Brien PJ et al (2020) Cryptic meta-
doi.​org/​10.​1016/j.​jenvm​an.​2018.​08.​068 somatic agent measured in situ in Variscan mantle rocks: melt
78. Fisher GL, Prentice BA, Sllberman D et al (1978) Physical and inclusions in garnet of eclogite, Granulitgebirge, Germany. J
morphological studies of size-classified coal fly ash. Environ Sci Metamorph Geol 38:207–234. https://d​ oi.o​ rg/1​ 0.1​ 111/j​ mg.1​ 2519
Technol 12:447–451. https://​doi.​org/​10.​1021/​es601​40a008 96. Ferrero S, Ziemann MA, Angel RJ et al (2016) Kumdykolite,
79. Li J, Zhu M, Zhang Z et al (2016) Characterisation of ash depos- kokchetavite, and cristobalite crystallized in nanogranites from
its on a probe at different temperatures during combustion of a felsic granulites, Orlica-Snieznik Dome (Bohemian Massif): not
Zhundong lignite in a drop tube furnace. Fuel Process Technol an evidence for ultrahigh-pressure conditions. Contrib to Mineral
144:155–163. https://​doi.​org/​10.​1016/j.​fuproc.​2015.​12.​024 Petrol 171:1–12. https://​doi.​org/​10.​1007/​s00410-​015-​1220-x
80. Bailey JG, Tate A, Diessel CFK, Wall TF (1990) A char morphol- 97. Hwang S-LS, Chu H-TY, Sobolev NV (2010) Kumdykolite, an
ogy system with applications to coal combustion. Fuel 69:225– orthorhombic polymorph of albite, from the Kokchetav ultra-
239. https://​doi.​org/​10.​1016/​0016-​2361(90)​90179-T high-pressure massif, Kazakhstan. Eur J Mineral 21:1325–1334.
81. Mishra SB, Langwenya SP, Mamba BB, Balakrishnan M (2010) https://​doi.​org/​10.​1127/​0935-​1221/​2009/​0021-​1970
Study on surface morphology and physicochemical properties of 98. Schönig J, von Eynatten H, Meinhold G et al (2020) Deep subduc-
raw and activated South African coal and coal fly ash. Phys Chem tion of felsic rocks hosting UHP lenses in the central Saxonian
Earth 35:811–814. https://​doi.​org/​10.​1016/j.​pce.​2010.​07.​001 Erzgebirge: implications for UHP terrane exhumation. Gondwana
82. Bartoňová L (2015) Unburned carbon from coal combustion ash: Res 87:320–329. https://​doi.​org/​10.​1016/j.​gr.​2020.​06.​020
an overview. Fuel Process Technol 134:136–158. https://​doi.​org/​ 99. Bryers RW (1996) Fireside slagging, fouling, and high-tempera-
10.​1016/j.​fuproc.​2015.​01.​028 ture corrosion of heat-transfer surface due to impurities in steam-
83. Rybak W, Moroń W, Czajka KM et al (2017) Co-combustion of raising fuels. Prog Energy Combust Sci 22:29–120. https://​doi.​
unburned carbon separated from lignite fly ash. Energy Procedia org/​10.​1016/​0360-​1285(95)​00012-7
120:197–205. https://​doi.​org/​10.​1016/j.​egypro.​2017.​07.​165 100. Luan C, You C, Zhang D (2014) Composition and sintering
84. Natesan K, Purohit A, Rink DL (2003) Coal-ash corrosion of characteristics of ashes from co- fi ring of coal and biomass in a
alloys for combustion power plants. In: Proc 17th Annu Conf laboratory-scale drop tube furnace. Energy 1–9. https://​doi.​org/​
Foss Energy Mater April 22-24, 2003. Argonne National Lab., 10.​1016/j.​energy.​2014.​03.​050
Argonne, IL (US), pp 1–15
85. Guo S, Gao J, Zhao D et al (2022) Co-combustion of sewage Publisher’s Note Springer Nature remains neutral with regard to
sludge and Zhundong coal: Effects of combustion conditions on jurisdictional claims in published maps and institutional affiliations.

13