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Thermal Analysis of Charcoal from Fast-Growing Eucalypt Wood: Influence ofRaw Material Moisture ContentAndré Luiz Missio a , Bruno Dufau Mattos b , Darci Alberto Gatto a b &Edson Alves de Lima ca Forest Engineering (PPGEF), Forest Products Laboratory , FederalUniversity of Santa Maria , Santa Maria , RS , Brazilb College of Materials Engineering (PPGCEM) , Federal University ofPelotas , Pelotas , RS , Brazilc Embrapa Forestry , Colombo , PR , BrazilPublished online: 11 Dec 2013.

To cite this article: André Luiz Missio , Bruno Dufau Mattos , Darci Alberto Gatto & Edson Alvesde Lima (2014) Thermal Analysis of Charcoal from Fast-Growing Eucalypt Wood: Influence of RawMaterial Moisture Content, Journal of Wood Chemistry and Technology, 34:3, 191-201

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Journal of Wood Chemistry and Technology, 34:191–201, 2014Copyright © Taylor & Francis Group, LLCISSN: 0277-3813 print / 1532-2319 onlineDOI: 10.1080/02773813.2013.852588

Thermal Analysis of Charcoal from Fast-GrowingEucalypt Wood: Influence of Raw Material

Moisture Content

ANDRE LUIZ MISSIO,1 BRUNO DUFAU MATTOS,2 DARCIALBERTO GATTO,1,2 AND EDSON ALVES DE LIMA3

1Forest Engineering (PPGEF), Forest Products Laboratory, Federal University ofSanta Maria, Santa Maria, RS, Brazil2College of Materials Engineering (PPGCEM), Federal University of Pelotas,Pelotas, RS, Brazil3Embrapa Forestry, Colombo, PR, Brazil

Abstract: This study aimed to investigate the influence of raw material moisture contenton the properties of charcoal from fast-growing Eucalyptus benthamii wood. For that,three treatments were performed with 0, 30, and 50% in relation to the wet basis moistureof the wood samples. The carbonization process used an electric kiln with a heatingrate of 1.60◦C min−1, initial temperature of 25 to 600◦C at the end, kept constant atthat temperature for two hours. Variables were statistically analyzed for charcoal yield,non-condensable gases, liquor, and fixed carbon and properties: fixed carbon content,volatile, ash, bulk density, and moisture content of charcoal. According to the results,the charcoal produced from dry wood presented the highest properties, yet the charcoalproduced from wood with 30% moisture content showed similar properties.

Keywords: Eucalyptus benthamii, pyrolysis, yield, correlation, DSC, TGA

Introduction

The evolution of global energy production based on fossil fuels led humanity to an insecure,expensive, and negative energy matrix for the environment. This scenario has caused manycountries to search for other energy sources that are renewable and less expensive, includingwood. Currently, wood is part of global energy production and its utilization is affected byvariables such as level of country development, availability of forests, environmental issues,and economic competition with other energy sources (oil, natural gas, hydroelectricity, andnuclear power).[1] Brazil is the greatest global producer and consumer of charcoal, whichis used in industrial scale mainly in steelworks[2] for the production of pig iron, steel, alloysteel, and ferroalloys and silicon metal.[3]

Wood is carbonized during the charcoal production due to the action of elevated tem-peratures that cause chemical, physical, colorimetric, and dimensional changes in relationto its original state. According to Syred et al.,[4] charcoal production is separated into

Address correspondence to Andre Luiz Missio, Federal University of Santa Maria, P.O. Box221, Santa Maria RS, 97105-900, Brazil. E-mail: andreluizmissio@gmail.com

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192 A. L. Missio et al.

three steps: first, wood is dried to eliminate all the moisture; second, the kiln temperatureincreases to 270◦C and the wood begins to decompose in an endothermic reaction witha spontaneous pyrolysis. Finally, in the third stage, final heating between 500 and 600◦Coccurs in order to eliminate the tar and the fixed carbon to an acceptable level.

From quality charcoal, it is known that the higher the content of fixed carbon and thesmaller the contents of ash and volatile materials, the higher the heating value of charcoal.[5]

In addition, high levels of fixed carbon and low content of volatile matter tend to burn slowerand remain in the furnaces for the full burning for a long time.[6,7]

According to Protasio et al.,[8] the attainment of high gravimetric yield is desirabledue to the great use of wood in carbonization kilns and the low yield of liquids and non-condensable gases, as these are by-products of the pyrolysis process. Mok et al.[9] also assertthat the highest yields of charcoal are obtained from species with highest lignin contentand lowest hemicellulose content.

Forest species from the Eucalyptus genus are constantly studied for bioenergy pro-duction. The Eucalyptus benthamii is from Australia and recognized for its adaptability toSouthern Brazil due to its tolerance to cold weather. Fast-growing plantations in Brazil pro-duce wood with high growth stress, which generate defects during wood sawing that limittheir use, such as warping, twisting, and curling. Thus, the energy production sector is analternative to increase added value of the wood through its use for the production of charcoal.

Previous studies[10–12] showed that variables involved in the process of pyrolysis, suchas temperature, carbonization time, and heating rate, influence on the properties of charcoal.Moreover, ignition, combustibility, and maintenance of heat in kilns are important propertiesthat should be studied in order to increase the efficiency of biomass use for energy, accordingto Leroy et al.[13] The same authors indicated that these properties are easily studied bythermogravimetric analysis and differential scanning calorimetry.

Likewise, since the variation of moisture content influences the physical and mechani-cal properties of wood, it is believed that this property has direct influence on the propertiesof charcoal. Therefore, this study aimed to investigate the influence of raw material moisturecontent on properties and thermal behavior of charcoal from Eucalyptus benthamii wood.

Experimental

Charcoal Production

Four trees from a seven-year-old Eucalyptus benthamii plantation with tree spacing of3.0 by 2.0 m, productivity of 338 m3 ha−1 in seven years, and average annual incrementof 48.37 m3 ha−1 yr−1, without previous thinning, at Embrapa Forestry, Colombo-PR,Brazil (25◦17′30′′S and 49◦13′27′′W, at altitude of 1027 m), were randomly selected andharvested. Immediately after, 35-mm-thick disks were cut from the trees at diameter breastheight (DBH, 1.30 m) and samples of whiteout sapwood, sized 35 × 35 × 120 mm3, wereprepared, according to Figure 1.

The samples of Treatment 1 (T1) were dried in an electric kiln at 102 ± 3◦C untilconstant weight, while the samples of Treatment 2 (T2) and Treatment 3 (T3) were satu-rated in a water-assisted vacuum system until reaching moisture content of 30 and 50%,respectively.

An electric muffle furnace (internally measuring 300 × 150 × 150 mm3, length,thickness and width, respectively) was equipped with a gases output, condensation system,and liquids collection that were used in the carbonization process (Figure 2).

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Wood Moisture Content on Charcoal Properties 193

Figure 1. Preparation of wood samples.

The wood samples were carbonized separately in the tubes, in which four samples(n = 4) were used for each moisture content. This process was carried out in order tomaintain the accuracy of the yield calculations. The heating rate was 1.60◦C min−1 andvariation of temperature was between 25◦C and 600◦C, which remained constant at 600◦Cfor two hours[4,11,14–16].

Charcoal Characterization

Yields of charcoal, liquor, and non-condensable gases were measured after each carboniza-tion process (Equations (1), (2), and (3)) and yield of fixed carbon was calculated byEquation (4), as previously utilized in other research.[8,11,17] After this stage, the charcoalwas milled in order to analyze moisture content, ash, volatile matter, and fixed carbon(Equations (5) to (8)) according to Brazilian Association of Standards Techniques[18] andthe bulk density (Dc) according to Brazilian Association of Standards Techniques.[19] Theanalysis of the high heating value (HHV) was performed with an adiabatic calorimeter

on/off

600°C

Cool Water Exit of Water

Electric Muffle furnace

Muffle internalsystem

Carbonization tubes (200 mm length and 40 mm diameter)

Gas condensation system

Wood sample

Figure 2. Sketch of electric muffle furnace and condensation gases system used for wood samplescarbonization.

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194 A. L. Missio et al.

IKA R©, model C 5000.

CY = mc

mw·100 (1)

LY = m4

mw·100 (2)

GY = 100 − (CY + LY) (3)

FCY = CY · FC

100(4)

MC = m0 − m1

m0·100 (5)

AC = m2

m0·100 (6)

VM = m0 − m3

m0·100 (7)

FC = 100 − (MC + AC + VM) (8)

where m0 = sample weight (g) at 0, 30, or 50% MC; m1 = sample weight after 100◦C;m2 = sample weight after 700◦C; m3 = sample weight after 900◦C; m4 = liquor weightcollected in the flask; mw = wood weight; mc = charcoal weight; AC = ash content (%);VM= volatile matter (%); FC = fixed carbon (%); CY = charcoal yield (%); FCY = fixedcarbon yield (%); GY = gases yield (%); LY = liquor yield (%).

DSC and TG Analysis

The Differential Scanning Calorimetry (DSC) measurements were carried out using aShimadzu DSC-60 in nitrogen atmosphere (gas flow of 20 ml min−1) at 10◦C min−1 using1.0–1.5 mg of charcoal samples in aluminum pans. The samples were heated from roomtemperature to 100◦C and maintained in isothermal conditions for 10 min, then heated to600◦C.

Thermogravimetric analysis (TGA) and derivate (DTG) measurements were carriedout using a Shimadzu DTG-60H equipment in dynamic nitrogen atmosphere (gas flow of20 ml min−1) at a temperature range of 10 to 900◦C using 10◦C min−1 heating rate, withcharcoal sample masses between 5.0 and 6.0 mg in a platinum pan.

Statistical Analysis

The data were analyzed through the Kruskal-Wallis nonparametric tests at 5% of signif-icance. The Kruskal-Wallis test is an alternative to analysis of variance (ANOVA) andrecommended for small sampling.[20] Moreover, a simple correlation analysis (Pearson)between variables was performed in order to assist the interpretation of results.

Simple statistical models have been proposed based on linear correlation coefficients forthe prediction and interpretation of the variables. The models were selected by coefficient ofdetermination (R2) using significance tests of model parameters. The adjustment of qualityof models was evaluated by the coefficient of determination (R2) and the standard errorof mean in percentage (SEM %); i.e., SEM is corrected as a function of the dependentvariable mean in order to compare between models with dependent variables that have

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Wood Moisture Content on Charcoal Properties 195

Table 1Kruskal-Wallis test and descriptive statistics for the properties of charcoal

Raw material moisture content (%)

Properties 0 30 50 P-value

Dc 323.81 ± 12.67a 288.73 ± 18.59b 250.76 ± 33.36c 0.012MCc 2.72 ± 0.85a 2.61 ± 0.21a 1.76 ± 0.06b 0.013AC 0.88 ± 0.05a 0.53 ± 0.09c 0.60 ± 0.07b 0.000VM 17.83 ± 1.77a 18.46 ± 2.91a 18.95 ± 0.92a 0.169FC 78.57 ± 2.04a 78.41 ± 3.11a 77.64 ± 3.10a 0.785HHV 32.83 ± 0.28a 32.83 ± 0.65a 32.16 ± 0.53b 0.022

Dc = charcoal bulk density (Kg.m−3); MCc = moisture content of charcoal (%); AC = ash content(%); VM = volatile matter (%); FC = fixed carbon (%); HHV = High heating value (MJ. kg−1).Means followed by the same letter in the line do not have statistically significant difference, inaccordance with the Kruskal-Wallis test.

different units. All parameters were significant at usual significance levels and test resultsof Durbin-Watson, White, and Shapiro-Wilk corroborate the conditions of independence ofresidues, homogeneity of variance, and normality of the residuals determined in the finalmodels.

Results and Discussion

The influence of raw material moisture content on the properties of charcoal from Euca-lyptus benthamii is shown in Table 1 through the Kruskal-Wallis test. The bulk density,moisture content of charcoal, ash content, and high heating value were significant as afunction of raw material moisture content in the carbonization process. When the moisturecontent increased, the charcoal bulk density was lower, which is undesirable.

Previous studies[21–24] of the combustion of wet biomass stated that a delay in thepyrolysis of these materials occurs due to the presence of moisture. Likewise, Diego et al.[25]

studied the combustion of dried and wet pine wood and observed that the devolatizationrate of dried raw material could be three times higher than those found in wet raw material.

This delay in the devolatilization due to the presence of moisture implies a longerand drastic time of water extraction from the wood during the carbonization process. Aphenomenon very similar to a hot-water extraction occurs when the biomass is subjectedto this situation, in which the hemicelluloses, that is the polysaccharides more fragile in thewood, are hydrolyzed, leached, and extracted.[26–28]

These associated facts generate hypotheses and interesting conclusions regarding theresults found in this study. The lower bulk density of charcoal produced from the rawmaterials with higher moisture content can be explained by these phenomena. Moreover,these phenomena also contribute to higher porosity of the charcoal, which makes it inter-esting when this material is used as filters, for replacement of soil, and other value-addedapplications.[29–31]

All charcoals, regardless of raw material moisture, have a fixed carbon greater than75% and volatile matter content less than 20%. Charcoal moisture content is generallybelow 3% and typically ash content is below 1%. In most cases, according Syred et al.,[4]

charcoal moisture content reduces the calorific or heating value, since energy is required

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196 A. L. Missio et al.

to heat and evaporate the moisture; however, this was not observed here. The values ofcharcoal moisture content decreased with increasing moisture content of the raw materialand showed the expected inverse behavior.

Results from properties of charcoal of Eucalyptus benthamii wood, e.g., are in agree-ment with the characteristics demanded by a steel blast furnace plant in Brazil using charcoalas a fuel. For this world industrial destination, charcoal is considered good to excellent bycriteria previously established by FAO[12] (FC = 75–80%, AC = 3–4% or less, VM =20–25%, Dc = 250–300 kg.m−3, among other properties).

For the ash content, the best results were obtained of charcoal T2 and T3. However,decrease in ash content was not linear; i.e., the lowest ash content was observed in T2. Twodistinct phenomena occurred simultaneously, in which the presence of high moisture in thesamples caused leaching of inorganic materials during the carbonization process, resultingin a lower ash content in charcoal produced with the wet samples (T2 = 0.53% and T3 =0.60%) than that found in charcoal from the dried samples (T1 = 0.88%).

On the other hand, a slight increase (T2<T3<T1) was observed when the ash contentof T2 is compared to ash content of T3. T2 <T3 occuring in the ash content of charcoal isassociated with the high degradation of raw material during the carbonization due to thedelay in the devolatization caused by the high moisture content. This loss of components(hemicelluloses, cellulose, and lignin) of the original structure of wood resulted in a charcoalwith lower bulk density (higher porosity) and an ash content higher when the moisture variedfrom T2 to T3. The quantification of inorganic materials (%AC) is always performed bythe proportion of organic fraction in the charcoal; i.e., if the organic fraction or moisturedecreased, the ash content theoretically should increase, which was observed in the presentstudy.

According to Bustamante-Garcıa et al.,[32] the ash is an industrial pollutant productin which low ash content is a great result. Moreover, moisture content of wood did notinfluence the values of fixed carbon and volatile matter.

For volatile matter, neither moisture content of the raw material met the standards ofthe European market, which allows a maximum rate of 12% of volatile materials.[33] Thisis not suitable for metallurgical charcoal, which often contains 10–15%. Concentration ofvolatile materials between 12% and 16% reduced smoke release.[34] However, charcoal fordomestic use typically contains 20–30% of volatile matter.

The yields of charcoals prepared under the experimental conditions are presented inFigure 3. The charcoal yield and the yield of by-products were sensitive to the raw materialmoisture content at carbonization, as can be seen; the charcoal yield gradually decreaseswith increasing raw material moisture content. The charcoal yield of T1 and T2 is accordingto Brazilian standard,[35] which is about 35%; however, this level was not obtained with thecarbonization of wood at high moisture content (T3).

In general, the fixed carbon yield was high and showed great results even in highermoisture conditions of raw material. However, the gas yield and the fixed carbon yieldsignificantly decreased with increasing raw material moisture content, mainly to the gases.

On the other hand, the liquor yield increased as a function of the moisture content ofthe raw material, which was significant only for T3. This fact is associated with the dilutionin water of pyrolignous liquor due to the large presence of moisture in the raw materialduring carbonization.

The influence of raw material moisture content for charcoal production was previouslystudied in Eucalyptus grandis trees,[2] in which the highest charcoal yield (30.4%) was foundin dry raw material, while in the other two moisture contents studied (15.5 and 110%) thevalues of charcoal yield were slightly lower (29.8 and 29.3%). This considerable change

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Wood Moisture Content on Charcoal Properties 197

36.34a 34.79

a 30.30b

0

5

10

15

20

25

30

35

40

T1 T2 T3

Charcoalyield(%)

Kruskal-Wallis (P = 0.01831)

28.52a

27.27b 23.78

c

0

5

10

15

20

25

30

35

T1 T2 T3

Fixed-carbonyield(%)

Kruskal-Wallis (P = 0.007276)

24.42a 20.94

a

3.28 b0

5

10

15

20

25

30

T1 T2 T3

Gasesyield(%)

Kruskal-Wallis (P = 0.01831)

39.23b

44.25b

67.32a

0

10

20

30

40

50

60

70

80

T1 T2 T3

Liquoryield(%)

Kruskal-Wallis (P = 0.01831)

Gas

es y

ield

(%)

Liqu

or y

ield

(%)

Fixe

d ca

rbon

yie

ld (%

)

Cha

rcoa

l yie

ld (%

)

Figure 3. Yields of charcoal and by-products of pyrolysis at three moisture contents in woodcarbonization.

in yield as a function of raw material moisture content was also observed by Darmstadtet al.[36] for charcoal from maple bark, spruce, fir, and pine.

In previous studies, Kumar et al.,[37] with final temperature carbonization of 600◦C,observed a charcoal yield of 25.95% for Eucalyptus globolus wood, lower than that obtainedin the present study, even with 50% of moisture content in the raw material. For Eucalyptuscalmadulensis and Eucalyptus saligna wood,[12] the charcoal yields obtained were 36.2 and35.4%, respectively, similar to those obtained in T1 and T2.

Table 2 shows the matrix of Pearson linear correlation between the properties of char-coal from Eucalyptus benthamii. A positive and strong correlation between the high heatingvalue, charcoal yield, fixed carbon yield, and the bulk density of charcoal is observed.

The volatile matter presented a high and negative correlation to the fixed carbon. Thiscorrelation is high because the increase of temperature in the carbonization process causesthe break of structural wood components (cellulose, hemicellulose, and lignin), while thewater, CO, CO2, and other gases (VM) are removed from the wood. Thus, according toNishimiya et al.,[38] the evaporation of these gases results in the condensation of carbonand in aromatization, which forms graphite chains (FC).

The statistical models proposed for the charcoal characteristics of Eucalyptus ben-thamii, the probability value, standard error of the mean in percentage, and coefficient ofdetermination are shown in Table 3.

All models are significant at 95% confidence level and we obtained a very significantcoefficient of determination with the FCY, AC, HHV, and Dc (dependent variable) and MCw

(independent variable). Moreover, some charcoal properties produced from Eucalyptusbenthamii wood were determined just by variation of the raw material moisture content,eliminating the carbonization process for verification purposes.

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198 A. L. Missio et al.

Table 2Pearson linear correlation for properties of charcoal

Variables AC VM MCc FC Dc HHV FCY CY MCw

AC 1 −0.20 0.19 0.07 0.42 0.20 0.40 0.39 −0.77∗∗

VM 1 0.03 −0.89∗∗ −0.11 −0.08 −0.25 0.09 0.27MCc 1 −0.17 0.48 0.50 0.49 0.59 −0.58∗

FC 1 0.07 0.22 0.25 −0.09 −0.15Dc 1 0.65∗ 0.90∗∗ 0.88∗∗ −0.82∗∗

HHV 1 0.72∗∗ 0.70∗ −0.56FCY 1 0.94∗∗ −0.82∗∗

CY 1 −0.78∗∗

MCw 1

Dc = charcoal bulk density (kg.m−3); MCc = charcoal moisture content (%); MCw = raw materialmoisture content (%); AC = ash content (%); VM = volatile matter (%); FC = fixed carbon (%);HHV = High heating value (MJ. kg−1); CY = charcoal yield (%); FCY = fixed carbon yield (%); ∗

significant at 5% probability of error, ∗∗ significant at 1% probability of error.

Figure 4 shows the similarity of thermograms for the charcoal produced in the threeconditions of moisture of the raw material. The TGA curves at nitrogen atmosphere show ahigh thermal stability of charcoal, in which the weight loss occurs only in two stages. In thefirst stage, the initial loss of moisture of charcoal is characterized,[39–41] which is confirmedin the DSC curve as an endothermic process at initial temperature around 25◦C until totalelimination of charcoal moisture at 100◦C.

The second stage shows that the elimination of elemental carbon[39] was quick for allcharcoal samples, where the most intense exothermic peak was at 565◦C for the charcoalfrom dry wood and around 530◦C for the charcoal produced from wet wood (30 and 50%for moisture). Figure 4 shows anticipation on the second peak at different temperatureswith increasing moisture content of carbonized wood. The decrease in temperature of thesecond peak (exothermic intense from second stage of thermograms; see Figure 4) for thecharcoal from wet raw material in relation to the dry has occurred due to the vapor pressurein the carbonization process.

A slow liberation of energy in charcoal with dry raw material was observed through acurve (crescent and gradual) until 400◦C. In the other step, liberation of energy exponentially

Table 3Statistical models for the properties of charcoal as a function of the MCw

N◦ Statistical models P-value SEM % R2

1 FCY = 28.7375 − 0.00194228·MCw2 < 0.01 0.045 0.772

2 AC = 0.861924 – 0.0460052·MCw < 0.01 0.139 0.7293 HHV = 32.9091 – 0.00000588895·MCw

3 0.012 0.011 0.4814 Dc = 326.238 – 1.43679·MCw < 0.01 0.078 0.672

Dc = charcoal bulk density (kg.m−3); MCw = raw material moisture content (%); AC = ash content(%); HHV = High heating value (MJ. kg−1); FCY = fixed carbon yield (%); R2 = coefficient ofdetermination; SEM % = standard error of the mean in percentage.

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Wood Moisture Content on Charcoal Properties 199

Figure 4. Thermogravimetric analysis and differential scanning calorimetry study.

increases until final temperature (600◦C). The peak corresponding to temperature differenceof sample (DTA) is wide and not well-defined with tonset at 270◦C, maximum temperatureat 540◦C, and ends at 596◦C.

On the other hand, an almost null energy liberation until 350◦C (heating temperature)was observed in DSC curves of charcoal with wet raw material (T2 and T3). After thistemperature, energy liberation is intense until the final burn. This behavior occurs in DTAcurves with an intense and well-defined peak at 530◦C in T2 and at 520◦C in T3. The finaltemperature of DTA peak for T2 and T3 is approximately 560◦C, which is slightly belowfinal temperature of DTA in T1.

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200 A. L. Missio et al.

Conclusions

The process of heat liberation during heating of charcoal in this study was significantly in-fluenced by moisture content. The charcoal with dry raw material showed a slow and gradualliberation, while the charcoal with wet raw material presented a brusque liberation of energy.

The carbonization of Eucalyptus benthamii wood at 30% of moisture content producedcharcoal with similar high heating value and lower ash content than dry wood. This resultis interesting and advantageous for industrial uses tin which ash is an undesirable residue.

Finally, Pearson correlation and mathematical regressions proved that moisture contentof the raw material (MCw) influences the properties Dc, MCc, AC e HHV, CY, and FCY.

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