Renewable Energy 126 (2018) 954e959

Contents lists available at ScienceDirect

Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Development of biomass fast proximate analysis by

thermogravimetric scale

zquez- Martí a, *, J. Gaibor-Cha
B. Vela
vez b, Z. Nin
~ o-Ruiz b, E. Corte

s-Rojas b
a
Departamento de Ingeniería Rural y Agroalimentaria, Universitat Polit
ecnica de Valencia, Camino de Vera s/n, 46022 Valencia Spain
b
n de Ambiente, Departamento de Investigacio
Grupo de Biomasa, Centro de Investigacio
n, Universidad Estatal de Bolívar, Guaranda Ecuador

a r t i c l e i n f o a b s t r a c t

Article history: EN norms set the methods for determining the ash and volatile content in biomass. These establish the
Received 17 August 2017 use of a muffle to heat the samples at temperatures of 550
C and 900
C respectively, with a minimum
Received in revised form analysis time of 4 h as standard method. The objective of this work was to reduce significantly the
10 February 2018
analysis times, making very short heating periods using a thermogravimetric scale (TGA), and to apply an
Accepted 7 April 2018
Available online 9 April 2018
equation to the residual weight to obtain the weight of ash, volatiles and fixed carbon in biomass
samples. We analyzed the factors: the temperature ramp, atmosphere and airflow in the determination.
In this work new validated methods were developed with an analysis time of 10e20 min.
Keywords:
Biomass
© 2018 Elsevier Ltd. All rights reserved.
Biofuel
Pruning residues
Energy wood

1. Introduction Committee for Standardization. The sample requires a minimum

mass of 1 g which must be pre-dried in an oven at 105
C for 1 h.
The use of thermogravimetric scales in the characterization of Once dried, the sample is introduced into the muffle, where tem-
biomass is a technique that is widely used in laboratories [1,2]. The perature is uniformly raised to 250
C for a period of between
thermogravimetric analysis evaluates the loss of weight of a sample 30 min and 50 min (i.e. with a rise between 4.5
C/min and 7.5
C/
when the temperature is increased in a controlled atmosphere. This min). Then, this temperature is maintained for 60 min to allow
can be an oxidizing atmosphere (air) or an inert atmosphere (Ni- volatiles being evaporated before ignition. Subsequently the tem-
trogen or Helium). perature in the oven is continuously raised to 550 ± 10
C for a
Current equipment allows setting the method of each experi- period of 30 min, or an elevation of 10
C/min, and this temperature
ment. A method of analysis fixes the ramp of temperature increase is maintained for at least 120 min. By counting the time required in
(R K/min), which can be performed in one or several stages or the different segments, the minimum test duration for ash deter-
segments, intercalating constant temperature periods [3]. On the mination is 40 þ 60 þ 30 þ 120 ¼ 250 min.
other hand, each method allows to fix an injection of a gas flow on EN-ISO 18123:2015 [6] establishes the method for the deter-
the sample in each segment. This programming capability of the mination of volatile matter in solid biofuels. This was also devel-
different stages of the process allows to devise more versatile and oped by the Technical Committee CEN/TC 335 of the European
fast methods than those established in the norms for the proximate Committee for Standardization. According to the established pro-
analysis of biomass, whose objective is to determine the amount of cedure, the sample of at least 1 g in a ceramic crucible with a lid,
ash, volatile and fixed carbon [4]. without contact with ambient air, is heated to 900
C ± 10
C for
EN-ISO 18122: 2015 [5] establishes the method for the deter- 7 min. The percentage of volatile matter is calculated from the mass
mination of ash in solid biofuels. This standard method was loss of the test portion after deduction of the mass loss due to
developed by the Technical Committee CEN/TC 335 of the European moisture. The disadvantage of the application of this method in
muffle is the control of the increase of temperature and the with-
drawal of the sample at 900
C. To remove the sample it is neces-
sary to wait for muffle cooling, which can distort the measurement.
* Corresponding author. Departamento de Ingeniería Rural y Agroalimentaria, The objective of this work was to analyze the factors: the ve-
cnica de Valencia, Camino de Vera s/n, 46022, Valencia, Spain.
Universitat Polite
E-mail address: borvemar@dmta.upv.es (B. Vela
zquez- Martí). locity of temperature increase, atmosphere and airflow in the

https://doi.org/10.1016/j.renene.2018.04.021
0960-1481/© 2018 Elsevier Ltd. All rights reserved.

zquez- Martí et al. / Renewable Energy 126 (2018) 954e959
B. Vela 955

determination of content of ash, volatile, and fixed carbon in determination. Before each test, the sample was dried in oven at
biomass samples by means of thermogravimetric balance (TGA) to 105
C following the indications of the norm ISO-EN 18134e3:2017
reduce the time of analysis in the laboratory. Some investigations [9].
have proposed methods of proximate analysis with TGA. Torcuato Evaluated materials were residual wood of the pruning of
et al. (2017) [7] found, using a series of thermogravimetric meth- Euphorbia lancifolia, cultivated in the province of Bolivar in Ecuador.
odologies, that heating rate and particle size are important factors This is a species of special relevance in terms of its use as biomass in
to be taken into account, whereas temperature and carrier gas (type Ecuador as well as its medicinal applications [10,11]. This plant
and flow rate) are critical to enable the proper quantification of provides a large amount of residue, and has a great propagation
volatiles and fixed carbon. In their experiments, the best condition capacity, with a very fast growth, both of the stem and of the
was achieved by applying 600
C and CO2 as carrier gas (instead of branches after pruning [12,13].
N2). It is the highlight of the proposal method regarding the con- The mass of each sample placed in the TGA ranged from 5 to
ditions often applied for this purpose. Furthermore, this method 7 mg. Samples with different percentages of wood and leaf were
has proved to be advantageous in three important aspects: A single evaluated: 100% wood, 90% wood-10% leaf, 80% wood-20% leaves,
measurement is enough for quantification of all properties, it can be 70% wood-30% leaves, 60% wood-40% leaves, 50% wood 50%
performed in a short time (1 h 27 min) in comparison with methods leaves, 100% leaves. This is very convenient because when pruning
performed in a muffle furnace, and it can be applied for different residues are used as biofuel, these residues present different per-
kinds of biomasses, from lignocellulosic to residue. However García centages of leaves [14e16]. For this reason this analysis was
et al. (2013) [8] had proposed a 25 min-last thermogravimetric performed.
method as a tool to determine biomass sample's proximate analysis In standard tests performed in muffle, according to EN-ISO
data (moisture, ash, volatile matter and fixed carbon contents) just 18122:2015 [5], the ash content was calculated according to
by direct measure of weight changes on each sample's TG chart. We equation (1), where m1 is the mass of the crucible, its lid and
in this work have taken up the idea of García et al. [8], but started sample before heating, in grams,; m2 is the mass of the crucible, its
from the hypothesis that if the sample is subjected to rapid similar lid and sample after heating, in grams,; mcrisol is the mass of the
temperature increases the residual weights are constant, and these empty crucible and its lid, in grams.
are related to the residual weights obtained when the sample is
m2  mcrisol
subjected to slow heating processes, obtaining from them the %Ash ¼ $100 (1)
percentage of ash, volatile and fixed carbon. This is demonstrated in m1  mcrisol
this paper, our purpose being to reduce the test to 10e15 min. In standard tests performed in muffle, according to EN-ISO
18123:2015 [6], the content of volatile matter in each experiment
2. Materials and methods was calculated according to equation (2), where m1 is the mass of
the crucible, its lid and the sample, in grams, before heating; m2 is
Initially ash and volatile tests were performed according to EN- the mass of the crucible, its lid and the sample, in grams, after
ISO 18122:2015 [5] and EN-ISO 18123:2015 [6] respectively, using a heating; mcrisol is the mass of the empty crucible and its lid, in
muffle. The values obtained served as reference for the validation of grams.
the methods developed with TGA.
The experimental design shown in Table 1 was followed. In the m1  m2
%volatile ¼ $100 (2)
experimental design, 4 factors were evaluated: On one hand, m1  mcrisol
temperature increasing speed (ramp factor) at two levels, one at
Fixed carbon content is obtained by difference, applying equa-
25
C/min and the other at 50
C/min; On the other hand, atmo-
tion (3).
sphere and flow factor were evaluated, also at two levels, 0 and
20 ml/s with airflow and nitrogen flow. Maximum temperature %fixed carbon ¼ 100  %volatile  %Ash (3)
reached 550
C for ash determination, and 900
C for volatile

Table 1
Statistical description of ash and volatile contents in wood and leaves mixtures analyzed with the standardized methods EN-ISO 18122: 2015 [5] and EN 15148: 2009 [4]
respectively.

% leaves Mean (% ash) Standard deviation Coefficient Coefficient Coefficient Minimum Maximum
(% ash) of variation of Skewness of Kurtosis (% ash) (% ash)

0 3.93 0.045 1.16% 0.66 0.44 3.89 3.98

10 5.23 0.075 1.43% 0.14 0.67 5.16 5.31
20 5.51 0.113 2.04% 1.21 0.97 5.44 5.64
30 6.49 0.139 2.14% 1.19 0.33 6.40 6.65
40 6.83 0.053 0.77% 1.03 0.96 6.79 6.89
50 7.63 0.047 0.62% 0.98 1.23 7.60 7.69
100 10.85 0.181 1.67% 1.07 0.26 10.65 10.99
Total 6.64 2.102 31.66% 1.73 0.44 3.89 10.99

% leaves Mean Standard deviation Coefficient Coefficient Coefficient Minimum Maximum

(% volatile) (% volatile) of variation of Skewness of Kurtosis (% volatile) (% volatile)

0 81.88 2.14 2.61% 0.68 0.99 79.54 83.74

10 83.16 0.79 0.95% 1.08 0.10 82.6 83.72
20 79.71 1.46 1.83% 0.51 0.52 78.68 80.75
30 77.77 0.52 0.66% 1.13 0.14 77.18 78.14
40 78.86 2.12 2.69% 1.09 0.36 77.36 80.37
50 76.87 1.43 1.86% 0.93 1.08 75.73 78.48
100 74.61 0.12 0.15% 0.09 0.56 74.5 74.73
Total 78.72 3.03 3.85% 1.60 0.83 74.5 83.74
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B. Vela

To evaluate if there were influences of the factors studied amount in unknown.

(temperature, flow, atmosphere type and maximum temperature)
in the calculated ash and volatiles, an analysis of variance with a
3.2. Determination of ash with TGA
95% confidence level was performed.
For the validation of the methods with respect to the values
Table 2 shows the ANOVA analysis where the influence of the
given by the reference, tests of the paired samples, based on the
factors was studied on the residual final weight obtained in each of
Student distribution, were performed.
the analysis methods for determination of ash tested with TGA:
temperature increase (ramp), flow rate, atmosphere type and leaf
3. Results and discussion percentage. The P-values prove the statistical significance of each of
the factors. It can be seen that all factors except the type of atmo-
3.1. Ash and volatiles with standardized methods sphere had significant simple effects. Since their P-values are less
than 0.05, these factors have a statistically significant effect on re-
Table 1 shows the statistical description of the percentage of ash sidual weight with a 95.0% confidence level.
and volatiles obtained by the standardized methods EN-ISO 18122: It is also observed that the interactions of the factors have sig-
2015 [5] and EN-ISO 18123:2015 [6] respectively in the different nificant influence in the results. This fact obliges us to treat all
sample types. The values of the asymmetry and kurtosis co- tested methods independently. It is not possible to group the re-
efficients were within the range of 2 and þ2, which means that sults of the different methods since the results are statistically
they follow a normal distribution. It can be observed that the ash different.
percentage of the sample with 100% wood was the lowest, with a Mean results obtained are shown in Table 3. It can be observed
mean of 3.93% and a standard deviation of 0.045% ash. However, the that residual weight of the sample increases when the percentage
ash percentage of samples with 100% leaves was the highest, with a of leaves increases in all methods. An analysis of paired samples
mean of 10.85% and a standard deviation of 0.181% ash. It is based on T-Student was performed to verify that there are signifi-
detected that there is a linear increase in ash percentage when the cant differences between the residual weights derived from the
percentage of leaves increases in the sample. The linear relation- TGA methods and the ash values obtained by the standardized
ship is shown in Fig. 1, where the equation obtained has an r2 of method. According to this analysis, the values obtained with all
0.99. It is important to note that the overall average of all samples is methods with TGA were statistically different from the standard-
6.64% ash. This value is suggested when combusting pruning resi- ized method with a 95% confidence level. This means that no
dues where the leaves have not been removed, and therefore, their method is validated directly. However, it is possible to obtain pre-
content is unknown a priori. cise equations that relate these values.
It is also observed that the amount of volatiles decreases slightly Fig. 2 shows the decrease of the weight of the sample in relative
with the percentage of leaves in the sample. In Fig. 1 it can be seen
that the slope is negative but very close to zero. The linear rela-
Table 2
tionship is shown in Fig. 1, where the equation obtained has an r2 of
Analysis of Variance (ANOVA) for residual weight of ash analysis methods with TGA.
0.72.
The average volatile content in the wood is 81.88% with a Factor Sum of squares Gl Mean Square F P-value
standard deviation of 2.14%. The average volatile content in the MAIN EFFECTS
leaves is 74.61% with a standard deviation of 0.12%. It is detected A:Ramp 810.708 1 810.708 103.35 0.0000
that the standard deviations of the measurements of volatiles are B:Flow 357.626 1 357.626 45.59 0.0000
C:Atmosphere 16.4449 1 16.4449 2.10 0.1521
significantly higher than those obtained in the determination of D:Leaves 4018.66 6 669.776 85.39 0.0000
ash. In addition, when the relation between the percentage of INTERACTIONS
leaves and the percentage of volatiles is studied, the coefficient of AB 0.317138 1 0.317138 0.04 0.8412
determination is lower. This may be due to the method of mea- AC 0.525523 1 0.525523 0.07 0.7965
AD 170.345 6 28.3908 3.62 0.0035
surement of volatiles being more imprecise than that the ash
BC 658.754 1 658.754 83.98 0.0000
method because the decrease in temperature from 900
C to room BD 110.036 6 18.3393 2.34 0.0408
temperature influences more uncontrollably matter decomposi- CD 189.588 6 31.5981 4.03 0.0016
tion. It is important to note that the overall mean of all samples is RESIDUES 549.079 70 7.84399
78.72% volatile. This value is suggested when pruning residues are TOTAL 7427.04 100

combusted and the leaf has not been removed, therefore the leaf All F-ratios are based on the mean residual error square.

12 86
Percentage of ash

y = -0.0812x + 81.921
Porcentage of vola es

10 84
R² = 0.7201
82
8
80
6
78
4 y = 0.0666x + 4.2629 76
2 R² = 0.9887
74
0 72
0 20 40 60 80 100 0 20 40 60 80 100
Percentage of leaves Percentage of leaves
Fig. 1. Variation of the percentage of ash and volatiles with the proportion of leaves.

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B. Vela 957

Table 3
Mean and standard deviation (x±s ) of the different ash determination methods with TGA.

Percentage of leaves

Ramp of temperature Flowml/min Atm. 0 10 20 30 40 50 100

25 C/min þ 1min 550 C 0 Air 1.27 ± 0.08 3.77 ± 0.43 4.11 ± 0.43 10.31 ± 0.43 6.58 ± 0.43 13.49 ± 0.43 22.01 ± 0.43
50
C/min þ 1min 550
C 0 Air 2.08 ± 0.07 14.32 ± 2.91 9.05 ± 0.43 19.83 ± 0.43 15.50 ± 0.43 21.36 ± 0.43 36.85 ± 0.43
25
C/min þ 1min 550
C 20 Air 2.88 ± 0.10 16.91 ± 1.28 11.56 ± 0.43 21.41 ± 0.43 26.03 ± 0.43 19.81 ± 0.43 31.19 ± 0.43
50
C/min þ 1min 550
C 20 Air 2.40 ± 0.07 23.53 ± 0.08 23.68 ± 0.43 25.62 ± 0.43 27.18 ± 0.43 26.69 ± 0.43 32.71 ± 0.43
25
C/min þ 1min 550
C 0 N 2.39 ± 0.06 6.43 ± 3.29 13.69 ± 0.43 11.75 ± 0.43 19.32 ± 0.43 14.87 ± 0.43 26.58 ± 0.43
50
C/min þ 1min 550
C 0 N 2.65 ± 0.11 15.63 ± 0.74 16.73 ± 0.43 20.60 ± 0.43 26.20 ± 0.43 20.74 ± 0.43 28.77 ± 0.43
25
C/min þ 1min 550
C 20 N 2.35 ± 0.12 8.06 ± 1.85 4.92 ± 0.43 13.21 ± 0.43 19.81 ± 0.43 8.74 ± 0.43 26.12 ± 0.43
50
C/min þ 1min 550
C 20 N 2.61 ± 0.09 14.17 ± 0.32 18.42 ± 0.43 19.03 ± 0.43 25.16 ± 0.43 22.16 ± 0.43 28.68 ± 0.43

terms (% of weight) versus temperature and time in the air atmo- time to levels of 10e15 min, none of the direct methods tested with
sphere, ash determination test, without any flow. It can be observed the TGA is directly feasible. However, according to the results, the
that the profile obtained versus temperature with ramps of 25 and combination of a direct and an indirect method is proposed. This
50
C/min are very similar. However, when it is plotted against new method tries to relate by equation the residual weights in
time, the graphs are different. This means that the weight decrease experiments with ramps of 25
C/min and 50
C/min with the ash
is linked to the temperature, regardless of the speed at which it is content obtained by standardized methods.
reached. The relationship between the percentage of residual weight in
As can be seen, weight loss undergoes 4 stages. First it slowly each method with TGA and the ash content are shown in Table 4. It
decreases until 250
C are reached. That is at 10 min when using a can be seen that the methods with the highest coefficient of
25
C/min ramp, or 5 min when using a 50
C/min ramp. Subse- determination were when a ramp of 25
C/min without flow was
quently there is a rapid drop in weight much more pronounced used, with r2 of 0.95, and when a ramp of 50
C/min with a flow of
between 250 and 350
C. From this temperature the decrease in 20 ml/min was used with r2 of 0.90. This second method is more
weight is attenuated slightly until residual weight is reached, convenient because it uses less analysis time.
18 min with a ramp of 25
C/min. It is important to note that when a
ramp of 50
C/min is used, all volatiles have not been released when 3.3. Determination of volatiles with TGA
550
C (11 min test) are reached, but only a part of them. This leads
us to conclude that, if it is not desired to exceed the temperature of Same as for the methods evaluated for ash determination, all
550
C, it is necessary to increase the time in which this tempera- factors studied in the methods for determination of volatiles with
ture is maintained; However this increases the test time. TGA influence the final residual weight obtained. The analysis of
If the object of the method to be developed is to reduce the test variance, shown in Table 5, shows that both the rate of increase of

120 120

100 25ºC/min
Percentage of weight
Percentage of weight

100
25ºC/min
80 80 50ºC/min
50ºC/min
60 60

40 40

20 20
0 0
0 200 400 600 0 5 10 15 20 25
Temperature (ºC) Time (min)
Fig. 2. Variation of the weight percentage of the sample versus temperature and time in the ash determination test in air atmosphere and without any flow.

Table 4
Equations that relate the residual weight from the methods with TGA and standard ash content.

Ramp of temperature Flow Atmosphere Equationa r2

25 C/min þ 1min 550 C 0 ml/min Air y ¼ 0:2631,x þ 4:3253 0.95
50
C/min þ 1min 550
C 0 ml/min Air y ¼ 0:1709,x þ 3:7347 0.79
25
C/min þ 1min 550
C 20 ml/min Air y ¼ 0:238,x þ 2:0547 0.67
50
C/min þ 1min 550
C 20 ml/min Air y ¼ 0; 558,x  7; 6593 0.90
25
C/min þ 1min 550
C 0 ml/min Nitrogen y ¼ 0:2255,x þ 3:5905 0.72
50
C/min þ 1min 550
C 0 ml/min Nitrogen y ¼ 0; 198,x þ 2; 6807 0.65
25
C/min þ 1min 550
C 20 ml/min Nitrogen y ¼ 0:2071,x þ 4:233 0.68
50
C/min þ 1min 550
C 20 ml/min Nitrogen y ¼ 0:2078,x þ 2:547 0.67
a
y ¼ percentage of ash; x ¼ %residual weight.
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B. Vela

Table 5 method with the highest coefficient of determination was obtained

Analysis of Variance (ANOVA) for residual weight from methods of analysis of vol- with the ramp of 50
C/minþ1min 900
C with a nitrogen atmo-
atiles with TGA.
sphere. This reduces the analysis time to 19 min.
Factor Sum of squares Gl Mean Square F P-value Fig. 3 describes the weight variation experienced by the sample
MAIN EFFECTS when ramps of 25 and 50
C/min, with air atmosphere, and without
A:Ramp 12.0234 1 12.0234 6.65 0.0119 flow are used. Like in the ash tests, it can be observed that the
B:Flow 21.9039 1 21.9039 12.12 0.0008 graphs of weight loss versus temperature reached are very similar
C:Atmosphere 27.3523 1 27.3523 15.13 0.0002
regardless of the ramp used. The same conclusion was obtained by
D:Leaves 373.98 6 62.33 34.49 0.0000
INTERACTIONS Xu et al. (2017) [17] working with rape straw with ramps of 20
C/
AB 8.26598 1 8.26598 4.57 0.0358 min, 30
C/min and 40
C/min. However they are different when
AC 12.6422 1 12.6422 7.00 0.0100 the weight loss is analyzed according to time. The weight loss is
AD 34.2242 6 5.70403 3.16 0.0083
faster as the rate of increase in temperature is higher.
BC 8.6885 1 8.6885 4.81 0.0315
BD 7.76562 6 1.29427 0.72 0.6378
It can be seen that the weight drops down slowly to about
CD 43.1326 6 7.18877 3.98 0.0017 250
C. That is up to 10 min when using a ramp of 25
C/min, or
RESIDUES 131.93 73 1.80727 5 min when using a ramp of 50
C/min. Subsequently there is a
TOTAL 694.375 103 rapid drop in weight much more pronounced between 250 and
350
C. From this temperature the decrease in weight is attenuated
slightly until reaching residual weight at approximately 550
C,
temperature (ramp), flow, type of atmosphere and percentage of from which weight descends very slowly. This is from 22 min when
leaf had P-value less than 0.05, so they have a statistically signifi- using a ramp of 25
C/min or 11 min when using a ramp of 50
C/
cant effect with a 95.0% confidence level. This forces to do the min. The profile of weight loss with temperature is a characteristic
analysis of comparison with the standardized value separately with of the species or material. Morin et al. (2017) [18] described
each method. The values of each method evaluated with TGA can different behaviors for thermogravimetric analysis of the coal ob-
not be grouped since they are statistically different. tained by pyrolysis. On the other hand, according to Ozsin € and
The standardized volatile content can be compared to the 100-% Pütün, (2017) [19], the composition of the gas released at each stage
residual weight in each TGA experiment. The mean and standard is assumed to be different. They worked with a TGA device coupled
deviation of the values obtained are shown in Table 6. It is shown to a mass spectrometer (MS) and a FTIR.
again that increasing the percentage of leaf content decreases the
volatile content. The paired sample tests showed that there are
4. Conclusions
significant differences between the values obtained with TGA and
those provided by the standardized method. The values with TGA
In this work we have developed methods for the determination
all differ by excess, which means that thermal decomposition of the
of ash and volatiles with thermogravimetric balance (TGA) for
mineral fraction may have occurred. However, due to the repro-
mixtures of wood and leaves of Euphorbia lancifolia. The results
ducibility of the experiments, the residual values can be related to
show that the residual weights after applying ramps of 25
C/min
the volatile content standardized through the use of regression
and 50
C/min up to 550
C and 900
C show significant differences
models.
with the values obtained with the standardized methods EN-ISO
The regression models that relate the results of each method to
18122:2015 [5] and EN-ISO 18123:2015 [6] respectively. However,
the volatile content are shown in Table 7. It can be observed that the
residual weights obtained with TGA present fixed relations

Table 6
Mean and standard deviation (x±sÞ of volatiles from the different methods of determination with TGA.

Percentage of leaves

Ramp of temperature Flow ml/min Atm. 0 10 20 30 40 50 100

50
C/min þ 1min 900
C 0 Air 99.46 ± 0.40 97.29 ± 0.43 96.51 ± 0.02 94.21 ± 2.65 95.44 ± 0.07 95.50 ± 0.01 93.10 ± 0.40
50
C/min þ 1min 900
C 0 Air 99.79 ± 0.09 97.68 ± 0.11 96.89 ± 0.18 96.11 ± 0.04 90.91 ± 3.69 95.11 ± 0.04 92.69 ± 0.13
50
C/min þ 1min 900
C 20 Air 97.92 ± 0.36 96.72 ± 0.23 96.84 ± 0.17 95.88 ± 0.10 94.58 ± 1.84 92.90 ± 0.67 93.02 ± 0.06
50
C/min þ 1min 900
C 20 Air 98.16 ± 0.25 96.70 ± 0.39 95.55 ± 2.39 94.89 ± 1.63 86.20 ± 3.31 89.97 ± 1.17 90.01 ± 3.65
50
C/min þ 1min 900
C 0 N 98.50 ± 0.08 97.70 ± 0.08 97.25 ± 0.31 96.25 ± 0.22 96.22 ± 0.58 95.00 ± 0.33 92.74 ± 0.56
50
C/min þ 1min 900
C 0 N 98.73 ± 0.14 97.87 ± 0.33 97.34 ± 0.15 96.68 ± 0.18 95.80 ± 0.10 94.36 ± 1.83 92.91 ± 0.22
50
C/min þ 1min 900
C 20 N 98.46 ± 0.07 97.33 ± 0.06 96.43 ± 0.85 96.29 ± 0.02 95.33 ± 0.43 94.98 ± 0.87 92.78 ± 0.28
50
C/min þ 1min 900
C 20 N 98.07 ± 0.12 97.05 ± 0.45 96.51 ± 0.09 95.70 ± 0.22 95.64 ± 0.11 95.25 ± 0.40 92.44 ± 0.01

Table 7
Equations relating the residual weight of the methods with TGA and standard volatile content.

Ramp of temperature Flow Atmosphere Equationa r2

25 C/min þ 1min 900 C 0 ml/min Air y ¼ 1:4471,x  59:933 0.78
50
C/min þ 1min 900
C 0 ml/min Air y ¼ 0:4199,x þ 39:699 0.58
25
C/min þ 1min 900
C 20 ml/min Air y ¼ 1:0747,x  22:913 0.47
50
C/min þ 1min 900
C 20 ml/min Air y ¼ 0:2015,x þ 60:534 0.75
25
C/min þ 1min 900
C 0 ml/min Nitrogen y ¼ 1:5771,x  72:627 0.71
50
C/min þ 1min 900
C 0 ml/min Nitrogen y ¼ 1:5409,x  69:422 0.84
25
C/min þ 1min 900
C 20 ml/min Nitrogen y ¼ 1:7682,x  90:408 0.59
50
C/min þ 1min 900
C 20 ml/min Nitrogen y ¼ 2:324,x  143:86 0.61
a
y ¼ percentage of volatiles; x ¼ 100-%residual weight.

zquez- Martí et al. / Renewable Energy 126 (2018) 954e959
B. Vela 959

120 120

100 100

Percentage of weight
50ºC/min 50ºC/min
Percentage of weight 25ºC/min 25ºC/min
80 80

60 60

40
40
20
20
0
0 0 10 20 30 40
0 100 200 300 400 500 600 700 800 900
Temperature (ºC) Time (min)

Fig. 3. Variation of the weight percentage of the sample versus temperature and time in the test of determination of volatiles in air atmosphere and without flow.

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