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lignite blends
Powder Technology 286 (2015) 39–47
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Evaluation on ash fusion behavior of eucalyptus bark/lignite blends Meiqian Chen ⁎, Dong Yu, Yuanhang Wei Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 3 March 2015 Received in revised form 6 June 2015 Accepted 28 July 2015 Available online 8 August 2015 Keywords: Ash fusion Kinetics Eucalyptus bark Lignite Blending ratio
a b s t r a c t One of the most crucial problems in using biomass fuel is the amount of solid wastes produced, which will cause serious deposition and corrosion. This work is mainly focused on an evaluation of ash fusion characteristics of eucalyptus bark, lignite and their blends by using thermal analysis technique. The ash deformation temperatures of eucalyptus bark and lignite were 1181 °C and 1320 °C. The differences between ash softening temperature and hemispherical temperature were 31 °C and 13 °C for biomass/lignite blends with biomass blending ratios of 20% and 80%. Also, the influences of blending ratio and heating rate on the co-fusion kinetics of eucalyptus bark and lignite were revealed based on two-stage scheme, which referred to pre-peak and post-peak around maximum reaction rate point in the main ash fusion region. The activation energy of ash fusion for eucalyptus bark/lignite blends in the pre-peak stage and the post-peak stage was in the range of 419–828 kJ/mol, and 322–702 kJ/mol, respectively. The biomass blending ratio should be controlled within 40% in order to reduce the possibility of sintering for eucalyptus bark/lignite blends. The ash fusion kinetic characteristics of mixture samples had no linear relation with blend ratios due to interaction between biomass and coal. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The co-firing of biomass and coal would reduce CO2 emissions, NOx and SOx levels [1,2], and reduce the dependency on fossil fuels [3]. Biomass fuels also contain mineral matter, generally in proportions strongly different from those found in coal [4]. However, inorganic species in biomass fuels such as alkali oxides and salts can aggravate agglomeration, deposition and corrosion problems on heat transfer surfaces in boilers [5]. The ash fusion characteristics of biomass are mainly dependent on the high-temperature molten material built up of quartz, potassium iron oxide and silicates [6]. Initial deformation temperatures of Capsicum stalks ashes, cotton stalks ashes and wheat stalks ashes increase with decreasing K2O and go up with increasing MgO, CaO, Fe2O3 and Al2O3 [7]. SiO2 and Al2O3 of biomass fuels are all favorable to increase the ash fusion temperature, and the Al2O3 is more effective than SiO2 in reducing the slagging tendency [8]. The initial deformation temperature decreases with the base to acid ratio increasing if the base to acid ratio is less than 1.4. When co-fired with coal, biomass fuels may significantly lower the ash melting temperature [9]. The formation of low temperature eutectics is considered as the initiator of agglomerates, and the ash fusibility test has been the most accepted
⁎ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (M. Chen).
http://dx.doi.org/10.1016/j.powtec.2015.07.043 0032-5910/© 2015 Elsevier B.V. All rights reserved.
method of assessing whether an ash will foul or slag on the heat transfer surfaces of boilers [10]. Due to the diversity and variable amount of biomass combusted with coal in industrial processes, produced ash has different characteristics and composition compared to coal ash and has not been characterized to date [11]. Blends of straw with coal contents between 5 wt.% and 15 wt.% are able to inhibit ash and reduced ash quantity [12]. When lignite is blended with some agro residues in proportions 95:5, 90:10, 80:20, Ca and Fe levels in fly ash are reduced, whereas Al and Si levels are increased, suggesting lower deposition and corrosion problems, in comparison to the combustion of lignite alone [13]. Ash melting temperatures for co-combustion of corn straw with coal firstly decrease and then increase with the content of the corn straw increase [14]. Combustion of each fuel alone for lignite or agro residue can provoke medium or high deposition problems. Biomass causes a significant decrease in the content of some metals in fly ash from co-combustion of coal and biomass including Al2O3, Fe2O3, SiO2, K2O, TiO2, and CaO [11]. By blending potassium-rich hazelnut shell with lignite in the ratios of 5 or 10 wt.%, the sintering temperatures are reduced to 919 and 730 °C, respectively [10]. The antagonistic influence of hazelnut shell on the thermal behavior of ash is attributed to the interaction of potassium from biomass with silicon compounds found in mineral matter of lignite. The softening temperature of the biomass fuels examined is substantially lower than that of a brown coal [15]. Switching from pure coal combustion to co-firing herbaceous biomass fuels with coal will require lowering the operation temperature of the boiler, otherwise
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M. Chen et al. / Powder Technology 286 (2015) 39–47
softening deformation of the composite ash can form deposits and slagging, with potential damage to the heat exchange surfaces. Therefore, extensive research is needed to reduce the operational costs and improve the reliability of the existing and newly built co-fired and/or biomass-fired power plants [16]. Rice straw, pine sawdust and leaf show serious slagging/fouling due to their high alkali content, and the ash content decreases with the increase in ashing temperature, and the alkali metals are relatively more volatile with the increase in ashing temperature [17]. Addition of straw into coal lowers the viscosity of the produced ash fractions, and the stickiness of the produced ash particles increases at lower temperatures with increasing the percentage of straw in the blends [16]. There is a great deal of eucalyptus bark for woody waste biomass species in south China. Eucalyptus is a short period of growth of industrial timber tree species. All countries tend to increase the share of biomass in the process of industrial combustion, e.g. the EU has introduced regulations to increase its amount in the process of the combustion of coal (hard bituminous coal and lignite) from 7.5% (2010) to 14% (2020) [11]. Lignite has some advantages over higher rank coals such as lower mining cost, high volatile matter content, high reactivity and low amount of pollution-forming impurities such as sulfur, nitrogen, and heavy metals [18]. Lignite resources in China are more than 1.3 × 1014 tons, which is about 13% of the total world coal reserves [19]. The properties of ash material formed during combustion of a blend cannot be predicted from the known characteristics of the ash formed from each fuel, and interaction between ashes from different fuels is poorly understood [13,20]. Therefore, knowledge of the influences of the mixtures on the ash fusion properties can make it possible to avoid fuel combinations with unwanted properties. Despite the extensive research on melting developments of biomass and coal ash, little attention is placed on their kinetic aspect of melting transition. An assessment about the effect of blending ratio in the fuel blends on the ash fusion kinetics is also scarce. In this paper, the ash melting kinetics of a typical biomass (eucalyptus bark), lignite and eucalyptus bark/lignite blends based on the two-period scheme are investigated using thermal analysis experiments at heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1. Furthermore, the ash analyses and ash fusing temperatures of eucalyptus bark, lignite and eucalyptus bark/lignite blends are determined. 2. Methods 2.1. Experimental facility and test samples A kind of Chinese pulverized lignite and a typical biomass sample, eucalyptus bark were selected. Table 1 illustrated proximate analyses and ultimate analyses of coal and biomass. Proximate analyses of the samples were performed in a thermogravimetric analyzer (TGA/ SDTA851, Mettler Toledo) with a precision of 0.001 mg, the detailed measuring methods were reported in Refs. [21,22]. The heating values in Table 1 were estimated by using the method reported in Ref. [23]. The ultimate analyses of eucalyptus bark and lignite were conducted in an Element Analyzer (EA3000, Leeman). There were no specific standards for biomass ash analysis in China, so the standards for coal ash analysis were usually used to determine the property of biomass ash [17]. The samples were dried at 50 °C and crushed into powder with diameters less than 0.2 mm. Then, these samples were ashed in a muffle furnace. 1 g of sample in a corundum crucible was placed into
a muffle furnace below 100 °C and then heated up to the temperature of 500 °C at 10 °C min−1 heating rate. The sample was kept on being heating up to 815 °C after 500 °C was maintained for 30 min, and the samples burnt for 2 h under 815 °C to ensure complete ashing. The ash sample was removed from the muffle furnace, and then it was placed in a dryer to be cooled down to room temperature after being cooled in the air. The sample was again burnt by above procedure, until the final ash sample was obtained up to two consecutive mass difference of sample less than 0.001 g. The residual ash was respectively milled to smaller particles (b 0.1 mm) in an agate mortar. The eucalyptus bark/lignite ash blends were prepared and homogenized with biomass ash mass percentages of 20%, 40%, 60% and 80%, respectively. Ash fusion temperatures of samples were measured according to GB/T 219-2008 (Chinese norms). A triangular ash cone with a bottom of 7 mm equilateral triangle and a height of 20 mm was made. The triangular ash cone was heated in weak reducing atmosphere. The sample was heated at the heating rate of 15 °C min−1 before 900 °C and 5 °C min−1 after it. The shape change of the ash cone was detected in the furnace by using a thermal microscope (BYTHR-9F, Boyuntian), and the deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT) were measured. TG/DTG/DSC experiments were carried out by a synchronous thermal analyzer (SDT-Q600, TA) with a precision of ±2%. The samples were heated in nitrogen (80 ml/min) in a Pt crucible (using Al2O3 as a reference material), in the temperature range between 30 and 1450 °C, with heating rates of 10, 15 and 20 °C min−1. To ensure reproducibility, the experiments were repeated three times. The results indicate that a good reproducibility is maintained for each run because the relative deviation was generally within ±1.5%. The uncertainties of the measurement in the experiment were dependent on the experimental conditions and the measurement instruments. The uncertainties of the measured parameters were estimated by using the method in Refs. [24–26]. The uncertainty of temperature was determined by
pffiffiffi δT 0:01=2 3 ¼ 0:01% ¼ 20 T
where T is the minimum value of the temperature during the experimental process, δT the standard resolution uncertainty of the temperature. Similarly, the uncertainty of the mass is given by
pffiffiffi δm 0:001=2 3 ¼ ¼ 0:006% m 5
where m is the minimum value of the sample mass during the experimental process, δm the standard resolution uncertainty of the mass. Powder X-ray diffraction was utilized to determine the inorganic compounds in the ashes, using a S4 Explorer diffractometer (Bruker AXS). The ash analyses of samples were shown in Table 2. Compounds in ashes can be divided into two groups by ionic potential difference [27]. One is acidic group compounds with high ionic potential like SiO2, Al2O3, and TiO2 which function as network formation agents. The other is basic group compounds with low ionic potential like Fe2O3, CaO, MgO, and so on. They can damage polymers in ash and help melting.
Table 1 Proximate analyses and ultimate analyses of samples. Samples
Lignite Eucalyptus bark
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Mar
Var
Aar
FCar
Car
Har
Oar
Nar
Sar
6.84 7.79
39.12 84.16
31.61 3.60
22.43 4.45
41.40 43.94
3.14 5.27
54.46 50.36
0.68 0.42
0.32 0.01
HHV (MJ/kg) 14.97 15.94
M. Chen et al. / Powder Technology 286 (2015) 39–47 Table 2 Ash analysis (wt.% dry basis) of samples. Samples
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
K2O
Na2O
Lignite Lignite (mean) [36] 20EB/80La 40EB/60L 60EB/40L 80EB/20L Eucalyptus bark Eucalyptus I [37] Eucalyptus II [37]
51.60 44.87 46.80 41.70 38.20 34.70 40.50 41 41
22.40 17.11 26.60 23.60 18.60 15.90 9.75
7.52 10.80 5.13 4.89 5.08 5.03 6.82
6.08 13.11 7.15 11.00 15.70 17.80 13.20 18 22
2.30 2.50 1.81 2.32 3.30 4.29 9.63 4.2 2.9
1.27 0.81 1.38 1.29 1.15 0.99 0.67
1.85 1.48 2.03 2.69 3.65 4.26 11.20 8.7 4.7
0.62 0.48 0.74 1.10 1.47 2.00 2.01 1.9 1.2
a
20EB/80L refers to 20% Eucalyptus bark/80% Lignite blend.
The base-to-acid ratio and the silica–alumina ratio for ash samples were usually applied for indicating slagging property of solid fuel [13,27]. Rb=a ¼ ð Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 OÞ=ðSiO2 þ Al2 O3 þ TiO2 Þ ð1Þ S=A ¼ SiO2 =Al2 O3
ð2Þ
For Rb/α b 0.5, deposition tendency is low, for 0.5 b Rb/α b 1 deposition tendency is medium and when Rb/α N 1 deposition tendency is high. For values b 0.31 or N3 of silica–alumina ratio, deposition tendency is low, while for values between 0.3 and 3 deposition tendency is high. The ratio of base to acid and the ratio of silica to alumina for ash samples were shown in Fig. 1. From Fig. 1, the ratio of base to acid and the ratio of silica to alumina for the eucalyptus bark ash were higher than the lignite ash. The two indexes increased with increasing the biomass blending ratio for eucalyptus bark/lignite blend ash. The ratios of silica to alumina for all eucalyptus bark/lignite blend ash, which were in the range of 1.76– 2.18, were much less than 4.15 for the pure eucalyptus bark, and near to that of the pure lignite. The ratio of silica to alumina for the 80EB/ 20L blend was almost 1/2 of the pure eucalyptus bark. The ratio of base to acid of the eucalyptus bark was above 3 times of the lignite. The ratios of base to acid of the 20EB80L, 40EB60L, 60EB40L, 80EB20L blends were 26.70%, 39.21%, 59.88% and 76.24% of the pure eucalyptus bark. In summary, ash fusion characteristics of the eucalyptus bark is improved by blending with lignite.
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order to calculate the kinetics of ash fusion much accurately, the peak of the main ash fusion process in the DSC curve may be taken as the demarcation point of two periods (pre-peak and post-peak). The reference value of activation energy of ash sample around peak is calculated by using the Ozawa–Flynn–Wall equation [29,30], which can be expressed as follows: AEa Ea −2:315−0:4567 lg β ¼ lg Rg ðα Þ RT
ð3Þ
where g(α) is integral representation of the mechanism function, β the heating rate (°C/min), T the absolute temperature (K), A the pre-exponential factor (s− 1), Eα the activation energy (J/mol), and R the universal gas constant [J/(mol·K)], α the degree of conversion at time t and it can be calculated by the following equation a¼
m0 −m m0 −m∞
ð4Þ
where m is the mass of samples at any time, m0 the initial mass of samples, and m∞ the mass of samples at the end of the ash fusion, respectively. By the way, the mass fraction of samples, longitudinal coordinate in TG curves can be expressed as M¼
m : m0
ð5Þ
By plotting lg (β) vs. 1/T, the reference value of Ea can be related to certain degree of conversion. To confirm the mechanism functions, the Coats–Redfern formula can be used [31–33], which is expressed as follows: h i ln g ðα Þ=T 2 ¼ ln ðAR=βEa Þ−Ea =RT:
ð6Þ
By substituting common solid reaction mechanism functions into Eq. (6) respectively, and plotting ln[g(α)/T2] vs. 1/T, Ea and A of the samples in each stage can be obtained. The mechanism function will be selected solely by comparing the Ea values with reference value obtained from the Ozawa–Flynn–Wall method. 3. Results and discussion 3.1. Ash fusing temperatures
2.2. Kinetic analysis Conversion degree of ash melting is directly proportional to the endothermic effect, which can be calculated from the area surrounded by the DSC curve and the base-line, melting kinetics can be established by DSC data [28]. The ash fusion process of biomass, coal and biomass/coal blends are all complex multiple step reaction, which makes it difficult to determine the overall reaction mechanism and formulate the reaction kinetics. In
Fig. 1. The ratio of base to acid and the ratio of silica to alumina of ash samples.
The ash fusing temperatures of eucalyptus bark/coal at different biomass blending ratio were shown in Fig. 2. The difference of ash fusion temperatures between eucalyptus bark and lignite was significant. The ash fusion temperatures of the
Fig. 2. Variation of ash fusing temperatures of eucalyptus bark/lignite with biomass blending ratios.
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eucalyptus bark were far less than those of the lignite. The deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT) for the eucalyptus bark ash were 1181, 1201, 1225 and 1236 °C, respectively, whereas they were 1320,
1373, 1423 and 1458 °C for the lignite ash, respectively. The ash fusion temperatures of biomass/lignite blends decreased with increasing biomass blending ratios. The DT values sharply decreased when biomass blending ratio was over 40%, and other three ash fusion temperatures
Fig. 3. TG–DTG–DSC curves of ash fusion for lignite, eucalyptus bark.
M. Chen et al. / Powder Technology 286 (2015) 39–47
Fig. 4. TG–DTG–DSC curves of ash fusion for eucalyptus bark/lignite blends.
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M. Chen et al. / Powder Technology 286 (2015) 39–47
Fig. 4 (continued).
M. Chen et al. / Powder Technology 286 (2015) 39–47 Table 3 TG, DTG and DSC data of eucalyptus bark/lignite ash. TG Samples β °C min−1 T/°C
α/%
Tp1/°C Tp2/°C Ti/°C Tp/°C
Tf/°C
Q/ (kJ kg−1)
20EB/80L 10 15 20 40EB/60L 10 15 20 60EB/40L 10 15 20 80EB/20L 10 15 20
2.88 3.15 3.38 2.46 2.95 3.20 2.31 2.88 3.80 2.57 2.94 3.10
1061 1050 – 1045 1056 1064 1067 – – 1064 – –
1288 1292 1300 1296 1288 1300 1297 1290 1295 1360 1351 1360
2736 2684 2349 1861 1723 1513 1629 1617 1548 3676 2944 2425
826–1243 836–1237 850–1210 830–1188 857–1181 884–1190 844–1204 847–1200 850–1200 853–1204 874–1200 886–1203
DTG
DSC
1124 1107 1120 1117 1122 1128 1116 1126 1119 1130 1133 1148
1000 1248 1050 1008 1015 1017 1007 1034 1050 1014 1042 1080
1187 1224 1245 1187 1192 1196 1149 1168 1183 1190 1188 1197
Tp1 was the temperature corresponding to the first peak point in the DTG, Tp2 the temperature corresponding to the second peak point, Ti the initial temperature corresponding to the endothermic peak in the DSC, Tp the temperature corresponding to the endothermic peak, Tf the final temperature corresponding to the endothermic peak, Q the total endothermic heat flow.
also abruptly decreased with biomass blending ratio over 60%. The biomass blending ratio should be controlled in less than 40% in order to reduce the possibility of sintering. The difference between ST and HT decreased with increasing biomass blending ratios, which was 31 °C and 13 °C for biomass blending ratio of 20% and 80%.
3.2. Ash fusion profile for biomass and coal Fig. 3 illustrated TG–DTG–DSC curves of ash fusion for lignite and eucalyptus bark at heating rates of 10 °C min− 1, 15 °C min− 1 and 20 °C min−1. From Fig. 3(1), the ash fusion process of lignite in the DTG curves had a significant peak at about 1178 °C corresponding to mass loss rate of 2.33%, 2.45% and 3.23% in the TG curves at a heating rate of 10 °C min−1, 15 °C min−1 and 20 °C min−1. At different heating rates, the main reaction of the whole melting process in the DSC curves was an endothermic reaction and started between 1168 °C and 1215 °C, but ended at uncertain temperature due to the high fusion temperature of the lignite. The maximum heat flow occurred between 1195 °C and 1245 °C with a heating rate increase from 10 °C min−1 to 20 °C min−1. From Fig. 3(2), the ash fusion process of eucalyptus bark in the DTG curves had a significant peak at about 1140 °C corresponding to mass loss rate of 3.60%, 4.98% and 5.06% in the TG curves at a heating rate of 10 °C min−1, 15 °C min−1 and 20 °C min−1. At different heating rates, the main reaction of the whole melting process in the DSC curves started between 1084 °C and 1100 °C, and ended between 1246 °C
45
and 1268 °C. The maximum heat flow occurred between 1185 °C and 1222 °C with a heating rate increase from 10 °C min−1 to 20 °C min−1. Fig. 4 illustrated TG–DTG–DSC curves of ash fusion for eucalyptus bark/lignite blends at heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1. Based on Fig. 4, the main characteristic parameters of the ash fusion process for all eucalyptus bark/lignite blends were listed in Table 3. From Fig. 4 and Table 3, with a heating rate increase from 10 °C min−1 to 20 °C min−1, the total absorbed heat flow in the ash fusion process for all biomass/coal blends decreased, e.g., the heat flow of 40EB/60L decreased from 1861 kJ/kg to 1513 kJ/kg. The heat flow of 80EB/20L at a heating rate of 10 °C min−1 reached a maximum, 3676 kJ/kg, while the value of 40EB/60L at a heating rate of 20 °C min− 1 was minimum, 1513 kJ/kg. Furthermore, with the decrease of heating rate, the initial temperature and the temperature corresponding to the endothermic peak value slightly decreased, e.g., the temperature of 40EB/60L at maximum heat flow decreased from 1196 to 1187 °C. 3.3. Kinetic analysis The main ash fusion region may be divided into two stages of pre-peak and post-peak by the maximum value (endothermic peak) in DSC diagram. The linear fitting was shown in Fig. 5 by plotting lg (β) vs. 1/T (T indicates the temperature corresponding to α = 0.33 at each heating rate) for the 40EB/60L blend, the reference activation energy was obtained, 397.59 kJ/mol. Fig. 4 illustrated that there was an obvious endothermic peak around 1196 °C for the 40EB/60L blend at 10 °C min−1 heating rate, the temperature range for the pre-peak period varied from 1130 to 1180 °C. Based on Eq. (6), by plotting ln[g(a)/T2] versus 1/T as shown in Fig. 5(b), the dominant mechanism was fitted to the 3
Avrami–Erofeev equation, gðαÞ ¼ ½− ln ð1−αÞ4 , which was controlled by the random nucleation and then growth process. Therefore, the activation energy and pre-exponential factor of the sample were obtained, 424.26 kJ/mol and 2.20 × 1015 min− 1. Nucleation is the process by which the reaction interface is initially established [34], at perhaps a limited number of points in the reactant, usually at a surface and possibly at an imperfection. The initially generated germ nucleus, consisting of a few atoms only is generally regarded as unstable due to the high ratio of surface strain to volume. It may, however, develop into a growth nucleus, which increases in size through advance of the reactant– product interface into the bulk of the crystallite, so reducing the relative importance of the surface strain term. The same method was applied to the post-peak period of 40EB/60L, two periods of other samples. The results were presented in Table 4. From Table 4, ash fusion mechanisms of the lignite in the pre-peak stage and post-peak stage were identical under certain heating rate. Under heating rates of 10 °C min−1, 15 °C min− 1 and 20 °C min−1,
Fig. 5. Kinetic linear fitting in the pre-peak period for ash fusion of the 40EB/60L blend at 10 °C min−1 heating rate.
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M. Chen et al. / Powder Technology 286 (2015) 39–47
Table 4 Kinetic parameters of ash fusion of eucalyptus bark/lignite. Samples
β °C min−1
T °C
Ea kJ mol−1
A min−1
R
100L
10
1120–1170 1180–1230
502.49 687.54
8.14E + 16 2.04E+18
−0.99 −0.99
15
1160–1200 1210–1260
428.32 489.16
3.76E+15 6.26E+16
−0.98 −0.99
20
1200–1250 1260–1310
511.84 751.42
9.83E+16 7.64E+19
−0.99 −0.97
10
1130–1170 1180–1220 1150–1190 1200–1250 1180–1230 1240–1290 1130–1180 1200–1250
778.23 679.45 814.38 660.51 827.46 701.64 424.26 440.73
9.84E+19 1.33E+18 2.65E+20 8.19E+17 6.52E+20 9.07E+18 2.20E+15 3.14E+15
−0.99 −0.98 −0.99 −0.99 −0.97 −0.98 −0.99 −0.98
1120–1170 1180–1230 1140–1180 1190–1250 1100–1150
503.17 479.15 588.36 322.48 586.78
8.89E+16 4.98E+16 2.31E+17 7.46E+13 2.04E+17
−0.99 −0.99 −0.98 −0.99 −0.99
g(α)/mechanisms Random nucleation and then growth, Avrami–Erofeev model: 2
½− ; ln ð1−αÞ5 Three dimensional diffusion, G–B model: 2
20EB/80L
15 20 40EB/60L
10 15 20
60EB/40L
10
1− 23 α−ð1−αÞ3 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2 G–B model: 2
1− 23 α−ð1−αÞ3 Random nucleation and then growth, Mampel Power model: α2 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ4 Avrami–Erofeev model 3
½− ; ln ð1−αÞ2
Avrami–Erofeev model 2
1160–1210 1110–1160 1170–1220 1120–1170 1180–1230
511.13 428.11 430.54 811.39 698.85
7.93E+16 3.76E+15 6.82E+15 1.15E+20 8.54E+18
−0.99 −0.98 −0.99 −0.99 −0.98
10
1140–1180 1190–1240
419.33 403.69
2.02E+15 1.28E+15
−0.97 −0.98
15
1120–1170 1180–1230 1110–1160
425.54 397.54 677.18
2.89E+15 9.67E+14 9.24E+17
−0.99 −0.98 −0.98
1170–1220
604.87
8.67E+16
−0.99
1150–1200
394.45
6.99E+14
−0.99
15 20
80EB/20L
20
½− ; ln ð1−αÞ5 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2
Mampel Power model: α2 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2 Mampel Power model: α2 Avrami–Erofeev model: 3
100EB
10
½− ; ln ð1−αÞ2 Mampel Power model: α2 Avrami–Erofeev model: 2
15 20
1210–1260 1130–1180 1190–1240 1180–1220 1230–1270
377.33 405.78 418.91 664.12 622.84
2.61E+14 1.44E+15 1.97E+15 7.74E+17 3.66E+17
the dominant ash fusion mechanisms of lignite were described by the Avrami–Erofeev model, the G–B model and the Avrami–Erofeev model, respectively. The Avrami–Erofeev model suggests that ash fusion mechanism is mainly controlled by the random nucleation and then growth process, while in conjunction with the G–B model, ash fusion mechanism is mainly controlled by the three dimensional diffusion process. Diffusion-controlled solid-state reaction kinetics in a sphere, where diffusion in all three directions is all-important. In a diffusioncontrolled reaction, numerous chemical reactions or micro-structural changes in solids take place through solid-state diffusion, i.e., the movement and transport of gas molecules in the solid phase [35]. The dominant ash fusion mechanisms of the eucalyptus bark were described by the Avrami–Erofeev model, or the Mampel Power model. The Mampel Power model a means that ash fusion mechanism is also controlled by the random nucleation and then growth process. The dominant ash fusion mechanisms of eucalyptus bark/lignite blends were described by the Avrami–Erofeev model, the G–B model or the Mampel Power model. The activation energy of ash fusion for lignite in the pre-peak stage and the post-peak stage was in the range of 428–512 kJ/mol, and 489–752 kJ/mol, respectively. The activation energy of ash fusion for eucalyptus bark in the pre-peak stage and the post-peak stage was in the range of 394–664 kJ/mol, and 377–623 kJ/mol, respectively. The
−0.97 −0.97 −0.99 −0.98 −0.99
½− ; ln ð1−αÞ5 Mampel Power model: α2 Avrami–Erofeev model: 2
½− ; ln ð1−αÞ5
activation energy of ash fusion for eucalyptus bark/lignite blends in the pre-peak stage and the post-peak stage was in the range of 419– 828 kJ/mol, and 322–702 kJ/mol, respectively. The activation energy of the 20EB80L blend was towards a maximum among these blends, indicating its high ash fusion inertia, which was helpful for reducing the release of alkali metal. Furthermore, with the increase of heating rate, expect for the 60EB/40L blend and lignite, the activation energy of ash fusion for other samples in the pre-peak stage and the post-peak stage increased. The ash fusion kinetic characteristics of eucalyptus bark/lignite blends had no linear relation with blend ratios due to interaction between biomass and coal. 4. Conclusions Both heating rate and blending ratio have much appreciable influence on melting of eucalyptus bark/lignite blends. With a heating rate increase from 10 °C min−1 to 20 °C min− 1, the total absorbed heat flow in the ash fusion process for all biomass/coal blends decreased. The heat flow of 80EB/20L at a heating rate of 10 °C min−1 reached a maximum, 3676 kJ kg−1, while the value of 40EB/60L at a heating rate of 20 °C min−1 was towards to a minimum, 1513 kJ kg−1. The biomass blending ratio should be controlled within 40% in order to reduce the possibility of sintering for eucalyptus bark/lignite blends.
M. Chen et al. / Powder Technology 286 (2015) 39–47
The ratio of silica to alumina for all eucalyptus bark/lignite blend ash was much less than the pure eucalyptus bark, and very close to the pure lignite. The ratio of silica to alumina for the 80% eucalyptus bark/20% lignite blend was almost 1/2 of the pure eucalyptus bark. The ratio of base to acid of the eucalyptus bark was above 3 times of the lignite. Under heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1, the dominant ash fusion mechanisms of lignite were described by the Avrami–Erofeev model or the G–B model, while for the eucalyptus bark, those were described by the Avrami–Erofeev model or the Mampel Power model. The dominant ash fusion mechanisms of eucalyptus bark/lignite blends were described by the Avrami–Erofeev model, or the G–B model or the Mampel Power model. Nomenclature a degree of conversion A the pre-exponential or frequency factor (min−1) apparent activation energy (kJ mol−1) Ea R the universal gas constant (J mol−1 K−1) T temperature (°C, K) Greek letter β heating rate (°C min−1) Subscripts and superscripts 0 initial ∞ burnout Acknowledgments This work was supported by the National Natural Science Foundation of China under No. 51376017. References [1] D.-W. Lee, J.-S. Bae, Y.-J. Lee, S.-J. Park, J.-C. Hong, B.-H. Lee, C.-H. Jeon, Y.-C. Choi, Two-in-one fuel combining sugar cane with low rank coal and its CO2 reduction effects in pulverized-coal power plants, Environ. Sci. Technol. 47 (2013) 1704–1710. [2] L.I. Darvell, J.M. Jones, B. Gudka, X.C. Baxter, A. Saddawi, A. Williams, A. Malmgren, Combustion properties of some power station biomass fuels, Fuel 89 (2010) 2881–2890. [3] C.H. Pang, B. Hewakandamby, T. Wu, E. Lester, An automated ash fusion test for characterisation of the behaviour of ashes from biomass and coal at elevated temperatures, Fuel 103 (2013) 454–466. [4] M. Pronobis, The influence of biomass co-combustion on boiler fouling and efficiency, Fuel 85 (2006) 474–480. [5] J.M. Jones, L.I. Darvell, T.G. Bridgeman, M. Pourkashanian, A. Williams, An investigation of the thermal and catalytic behaviour of potassium in biomass combustion, Proc. Combust. Inst. 31 (2007) 1955–1963. [6] Y. Niu, W. Du, H. Tan, W. Xu, Y. Liu, Y. Xiong, S. Hui, Further study on biomass ash characteristics at elevated ashing temperatures: the evolution of K, Cl, S and the ash fusion characteristics, Bioresour. Technol. 129 (2013) 642–645. [7] Y. Niu, H. Tan, X. Wang, Z. Liu, H. Liu, Y. Liu, T. Xu, Study on fusion characteristics of biomass ash, Bioresour. Technol. 101 (2010) 9373–9381. [8] Q.H. Li, Y.G. Zhang, A.H. Meng, L. Li, G.X. Li, Study on ash fusion temperature using original and simulated biomass ashes, Fuel Process. Technol. 107 (2013) 107–112. [9] A.A. Tortosa Masiá, B.J.P. Buhre, R.P. Gupta, T.F. Wall, Characterising ash of biomass and waste, Fuel Process. Technol. 88 (2007) 1071–1081. [10] H. Haykiri-Acma, S. Yaman, S. Kucukbayrak, Effect of biomass on temperatures of sintering and initial deformation of lignite ash, Fuel 89 (2010) 3063–3068.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Evaluation on ash fusion behavior of eucalyptus bark/lignite blends Meiqian Chen ⁎, Dong Yu, Yuanhang Wei Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 3 March 2015 Received in revised form 6 June 2015 Accepted 28 July 2015 Available online 8 August 2015 Keywords: Ash fusion Kinetics Eucalyptus bark Lignite Blending ratio
a b s t r a c t One of the most crucial problems in using biomass fuel is the amount of solid wastes produced, which will cause serious deposition and corrosion. This work is mainly focused on an evaluation of ash fusion characteristics of eucalyptus bark, lignite and their blends by using thermal analysis technique. The ash deformation temperatures of eucalyptus bark and lignite were 1181 °C and 1320 °C. The differences between ash softening temperature and hemispherical temperature were 31 °C and 13 °C for biomass/lignite blends with biomass blending ratios of 20% and 80%. Also, the influences of blending ratio and heating rate on the co-fusion kinetics of eucalyptus bark and lignite were revealed based on two-stage scheme, which referred to pre-peak and post-peak around maximum reaction rate point in the main ash fusion region. The activation energy of ash fusion for eucalyptus bark/lignite blends in the pre-peak stage and the post-peak stage was in the range of 419–828 kJ/mol, and 322–702 kJ/mol, respectively. The biomass blending ratio should be controlled within 40% in order to reduce the possibility of sintering for eucalyptus bark/lignite blends. The ash fusion kinetic characteristics of mixture samples had no linear relation with blend ratios due to interaction between biomass and coal. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The co-firing of biomass and coal would reduce CO2 emissions, NOx and SOx levels [1,2], and reduce the dependency on fossil fuels [3]. Biomass fuels also contain mineral matter, generally in proportions strongly different from those found in coal [4]. However, inorganic species in biomass fuels such as alkali oxides and salts can aggravate agglomeration, deposition and corrosion problems on heat transfer surfaces in boilers [5]. The ash fusion characteristics of biomass are mainly dependent on the high-temperature molten material built up of quartz, potassium iron oxide and silicates [6]. Initial deformation temperatures of Capsicum stalks ashes, cotton stalks ashes and wheat stalks ashes increase with decreasing K2O and go up with increasing MgO, CaO, Fe2O3 and Al2O3 [7]. SiO2 and Al2O3 of biomass fuels are all favorable to increase the ash fusion temperature, and the Al2O3 is more effective than SiO2 in reducing the slagging tendency [8]. The initial deformation temperature decreases with the base to acid ratio increasing if the base to acid ratio is less than 1.4. When co-fired with coal, biomass fuels may significantly lower the ash melting temperature [9]. The formation of low temperature eutectics is considered as the initiator of agglomerates, and the ash fusibility test has been the most accepted
⁎ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (M. Chen).
http://dx.doi.org/10.1016/j.powtec.2015.07.043 0032-5910/© 2015 Elsevier B.V. All rights reserved.
method of assessing whether an ash will foul or slag on the heat transfer surfaces of boilers [10]. Due to the diversity and variable amount of biomass combusted with coal in industrial processes, produced ash has different characteristics and composition compared to coal ash and has not been characterized to date [11]. Blends of straw with coal contents between 5 wt.% and 15 wt.% are able to inhibit ash and reduced ash quantity [12]. When lignite is blended with some agro residues in proportions 95:5, 90:10, 80:20, Ca and Fe levels in fly ash are reduced, whereas Al and Si levels are increased, suggesting lower deposition and corrosion problems, in comparison to the combustion of lignite alone [13]. Ash melting temperatures for co-combustion of corn straw with coal firstly decrease and then increase with the content of the corn straw increase [14]. Combustion of each fuel alone for lignite or agro residue can provoke medium or high deposition problems. Biomass causes a significant decrease in the content of some metals in fly ash from co-combustion of coal and biomass including Al2O3, Fe2O3, SiO2, K2O, TiO2, and CaO [11]. By blending potassium-rich hazelnut shell with lignite in the ratios of 5 or 10 wt.%, the sintering temperatures are reduced to 919 and 730 °C, respectively [10]. The antagonistic influence of hazelnut shell on the thermal behavior of ash is attributed to the interaction of potassium from biomass with silicon compounds found in mineral matter of lignite. The softening temperature of the biomass fuels examined is substantially lower than that of a brown coal [15]. Switching from pure coal combustion to co-firing herbaceous biomass fuels with coal will require lowering the operation temperature of the boiler, otherwise
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M. Chen et al. / Powder Technology 286 (2015) 39–47
softening deformation of the composite ash can form deposits and slagging, with potential damage to the heat exchange surfaces. Therefore, extensive research is needed to reduce the operational costs and improve the reliability of the existing and newly built co-fired and/or biomass-fired power plants [16]. Rice straw, pine sawdust and leaf show serious slagging/fouling due to their high alkali content, and the ash content decreases with the increase in ashing temperature, and the alkali metals are relatively more volatile with the increase in ashing temperature [17]. Addition of straw into coal lowers the viscosity of the produced ash fractions, and the stickiness of the produced ash particles increases at lower temperatures with increasing the percentage of straw in the blends [16]. There is a great deal of eucalyptus bark for woody waste biomass species in south China. Eucalyptus is a short period of growth of industrial timber tree species. All countries tend to increase the share of biomass in the process of industrial combustion, e.g. the EU has introduced regulations to increase its amount in the process of the combustion of coal (hard bituminous coal and lignite) from 7.5% (2010) to 14% (2020) [11]. Lignite has some advantages over higher rank coals such as lower mining cost, high volatile matter content, high reactivity and low amount of pollution-forming impurities such as sulfur, nitrogen, and heavy metals [18]. Lignite resources in China are more than 1.3 × 1014 tons, which is about 13% of the total world coal reserves [19]. The properties of ash material formed during combustion of a blend cannot be predicted from the known characteristics of the ash formed from each fuel, and interaction between ashes from different fuels is poorly understood [13,20]. Therefore, knowledge of the influences of the mixtures on the ash fusion properties can make it possible to avoid fuel combinations with unwanted properties. Despite the extensive research on melting developments of biomass and coal ash, little attention is placed on their kinetic aspect of melting transition. An assessment about the effect of blending ratio in the fuel blends on the ash fusion kinetics is also scarce. In this paper, the ash melting kinetics of a typical biomass (eucalyptus bark), lignite and eucalyptus bark/lignite blends based on the two-period scheme are investigated using thermal analysis experiments at heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1. Furthermore, the ash analyses and ash fusing temperatures of eucalyptus bark, lignite and eucalyptus bark/lignite blends are determined. 2. Methods 2.1. Experimental facility and test samples A kind of Chinese pulverized lignite and a typical biomass sample, eucalyptus bark were selected. Table 1 illustrated proximate analyses and ultimate analyses of coal and biomass. Proximate analyses of the samples were performed in a thermogravimetric analyzer (TGA/ SDTA851, Mettler Toledo) with a precision of 0.001 mg, the detailed measuring methods were reported in Refs. [21,22]. The heating values in Table 1 were estimated by using the method reported in Ref. [23]. The ultimate analyses of eucalyptus bark and lignite were conducted in an Element Analyzer (EA3000, Leeman). There were no specific standards for biomass ash analysis in China, so the standards for coal ash analysis were usually used to determine the property of biomass ash [17]. The samples were dried at 50 °C and crushed into powder with diameters less than 0.2 mm. Then, these samples were ashed in a muffle furnace. 1 g of sample in a corundum crucible was placed into
a muffle furnace below 100 °C and then heated up to the temperature of 500 °C at 10 °C min−1 heating rate. The sample was kept on being heating up to 815 °C after 500 °C was maintained for 30 min, and the samples burnt for 2 h under 815 °C to ensure complete ashing. The ash sample was removed from the muffle furnace, and then it was placed in a dryer to be cooled down to room temperature after being cooled in the air. The sample was again burnt by above procedure, until the final ash sample was obtained up to two consecutive mass difference of sample less than 0.001 g. The residual ash was respectively milled to smaller particles (b 0.1 mm) in an agate mortar. The eucalyptus bark/lignite ash blends were prepared and homogenized with biomass ash mass percentages of 20%, 40%, 60% and 80%, respectively. Ash fusion temperatures of samples were measured according to GB/T 219-2008 (Chinese norms). A triangular ash cone with a bottom of 7 mm equilateral triangle and a height of 20 mm was made. The triangular ash cone was heated in weak reducing atmosphere. The sample was heated at the heating rate of 15 °C min−1 before 900 °C and 5 °C min−1 after it. The shape change of the ash cone was detected in the furnace by using a thermal microscope (BYTHR-9F, Boyuntian), and the deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT) were measured. TG/DTG/DSC experiments were carried out by a synchronous thermal analyzer (SDT-Q600, TA) with a precision of ±2%. The samples were heated in nitrogen (80 ml/min) in a Pt crucible (using Al2O3 as a reference material), in the temperature range between 30 and 1450 °C, with heating rates of 10, 15 and 20 °C min−1. To ensure reproducibility, the experiments were repeated three times. The results indicate that a good reproducibility is maintained for each run because the relative deviation was generally within ±1.5%. The uncertainties of the measurement in the experiment were dependent on the experimental conditions and the measurement instruments. The uncertainties of the measured parameters were estimated by using the method in Refs. [24–26]. The uncertainty of temperature was determined by
pffiffiffi δT 0:01=2 3 ¼ 0:01% ¼ 20 T
where T is the minimum value of the temperature during the experimental process, δT the standard resolution uncertainty of the temperature. Similarly, the uncertainty of the mass is given by
pffiffiffi δm 0:001=2 3 ¼ ¼ 0:006% m 5
where m is the minimum value of the sample mass during the experimental process, δm the standard resolution uncertainty of the mass. Powder X-ray diffraction was utilized to determine the inorganic compounds in the ashes, using a S4 Explorer diffractometer (Bruker AXS). The ash analyses of samples were shown in Table 2. Compounds in ashes can be divided into two groups by ionic potential difference [27]. One is acidic group compounds with high ionic potential like SiO2, Al2O3, and TiO2 which function as network formation agents. The other is basic group compounds with low ionic potential like Fe2O3, CaO, MgO, and so on. They can damage polymers in ash and help melting.
Table 1 Proximate analyses and ultimate analyses of samples. Samples
Lignite Eucalyptus bark
Proximate analysis (wt.%)
Ultimate analysis (wt.%)
Mar
Var
Aar
FCar
Car
Har
Oar
Nar
Sar
6.84 7.79
39.12 84.16
31.61 3.60
22.43 4.45
41.40 43.94
3.14 5.27
54.46 50.36
0.68 0.42
0.32 0.01
HHV (MJ/kg) 14.97 15.94
M. Chen et al. / Powder Technology 286 (2015) 39–47 Table 2 Ash analysis (wt.% dry basis) of samples. Samples
SiO2
Al2O3
Fe2O3
CaO
MgO
TiO2
K2O
Na2O
Lignite Lignite (mean) [36] 20EB/80La 40EB/60L 60EB/40L 80EB/20L Eucalyptus bark Eucalyptus I [37] Eucalyptus II [37]
51.60 44.87 46.80 41.70 38.20 34.70 40.50 41 41
22.40 17.11 26.60 23.60 18.60 15.90 9.75
7.52 10.80 5.13 4.89 5.08 5.03 6.82
6.08 13.11 7.15 11.00 15.70 17.80 13.20 18 22
2.30 2.50 1.81 2.32 3.30 4.29 9.63 4.2 2.9
1.27 0.81 1.38 1.29 1.15 0.99 0.67
1.85 1.48 2.03 2.69 3.65 4.26 11.20 8.7 4.7
0.62 0.48 0.74 1.10 1.47 2.00 2.01 1.9 1.2
a
20EB/80L refers to 20% Eucalyptus bark/80% Lignite blend.
The base-to-acid ratio and the silica–alumina ratio for ash samples were usually applied for indicating slagging property of solid fuel [13,27]. Rb=a ¼ ð Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 OÞ=ðSiO2 þ Al2 O3 þ TiO2 Þ ð1Þ S=A ¼ SiO2 =Al2 O3
ð2Þ
For Rb/α b 0.5, deposition tendency is low, for 0.5 b Rb/α b 1 deposition tendency is medium and when Rb/α N 1 deposition tendency is high. For values b 0.31 or N3 of silica–alumina ratio, deposition tendency is low, while for values between 0.3 and 3 deposition tendency is high. The ratio of base to acid and the ratio of silica to alumina for ash samples were shown in Fig. 1. From Fig. 1, the ratio of base to acid and the ratio of silica to alumina for the eucalyptus bark ash were higher than the lignite ash. The two indexes increased with increasing the biomass blending ratio for eucalyptus bark/lignite blend ash. The ratios of silica to alumina for all eucalyptus bark/lignite blend ash, which were in the range of 1.76– 2.18, were much less than 4.15 for the pure eucalyptus bark, and near to that of the pure lignite. The ratio of silica to alumina for the 80EB/ 20L blend was almost 1/2 of the pure eucalyptus bark. The ratio of base to acid of the eucalyptus bark was above 3 times of the lignite. The ratios of base to acid of the 20EB80L, 40EB60L, 60EB40L, 80EB20L blends were 26.70%, 39.21%, 59.88% and 76.24% of the pure eucalyptus bark. In summary, ash fusion characteristics of the eucalyptus bark is improved by blending with lignite.
41
order to calculate the kinetics of ash fusion much accurately, the peak of the main ash fusion process in the DSC curve may be taken as the demarcation point of two periods (pre-peak and post-peak). The reference value of activation energy of ash sample around peak is calculated by using the Ozawa–Flynn–Wall equation [29,30], which can be expressed as follows: AEa Ea −2:315−0:4567 lg β ¼ lg Rg ðα Þ RT
ð3Þ
where g(α) is integral representation of the mechanism function, β the heating rate (°C/min), T the absolute temperature (K), A the pre-exponential factor (s− 1), Eα the activation energy (J/mol), and R the universal gas constant [J/(mol·K)], α the degree of conversion at time t and it can be calculated by the following equation a¼
m0 −m m0 −m∞
ð4Þ
where m is the mass of samples at any time, m0 the initial mass of samples, and m∞ the mass of samples at the end of the ash fusion, respectively. By the way, the mass fraction of samples, longitudinal coordinate in TG curves can be expressed as M¼
m : m0
ð5Þ
By plotting lg (β) vs. 1/T, the reference value of Ea can be related to certain degree of conversion. To confirm the mechanism functions, the Coats–Redfern formula can be used [31–33], which is expressed as follows: h i ln g ðα Þ=T 2 ¼ ln ðAR=βEa Þ−Ea =RT:
ð6Þ
By substituting common solid reaction mechanism functions into Eq. (6) respectively, and plotting ln[g(α)/T2] vs. 1/T, Ea and A of the samples in each stage can be obtained. The mechanism function will be selected solely by comparing the Ea values with reference value obtained from the Ozawa–Flynn–Wall method. 3. Results and discussion 3.1. Ash fusing temperatures
2.2. Kinetic analysis Conversion degree of ash melting is directly proportional to the endothermic effect, which can be calculated from the area surrounded by the DSC curve and the base-line, melting kinetics can be established by DSC data [28]. The ash fusion process of biomass, coal and biomass/coal blends are all complex multiple step reaction, which makes it difficult to determine the overall reaction mechanism and formulate the reaction kinetics. In
Fig. 1. The ratio of base to acid and the ratio of silica to alumina of ash samples.
The ash fusing temperatures of eucalyptus bark/coal at different biomass blending ratio were shown in Fig. 2. The difference of ash fusion temperatures between eucalyptus bark and lignite was significant. The ash fusion temperatures of the
Fig. 2. Variation of ash fusing temperatures of eucalyptus bark/lignite with biomass blending ratios.
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M. Chen et al. / Powder Technology 286 (2015) 39–47
eucalyptus bark were far less than those of the lignite. The deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT) for the eucalyptus bark ash were 1181, 1201, 1225 and 1236 °C, respectively, whereas they were 1320,
1373, 1423 and 1458 °C for the lignite ash, respectively. The ash fusion temperatures of biomass/lignite blends decreased with increasing biomass blending ratios. The DT values sharply decreased when biomass blending ratio was over 40%, and other three ash fusion temperatures
Fig. 3. TG–DTG–DSC curves of ash fusion for lignite, eucalyptus bark.
M. Chen et al. / Powder Technology 286 (2015) 39–47
Fig. 4. TG–DTG–DSC curves of ash fusion for eucalyptus bark/lignite blends.
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M. Chen et al. / Powder Technology 286 (2015) 39–47
Fig. 4 (continued).
M. Chen et al. / Powder Technology 286 (2015) 39–47 Table 3 TG, DTG and DSC data of eucalyptus bark/lignite ash. TG Samples β °C min−1 T/°C
α/%
Tp1/°C Tp2/°C Ti/°C Tp/°C
Tf/°C
Q/ (kJ kg−1)
20EB/80L 10 15 20 40EB/60L 10 15 20 60EB/40L 10 15 20 80EB/20L 10 15 20
2.88 3.15 3.38 2.46 2.95 3.20 2.31 2.88 3.80 2.57 2.94 3.10
1061 1050 – 1045 1056 1064 1067 – – 1064 – –
1288 1292 1300 1296 1288 1300 1297 1290 1295 1360 1351 1360
2736 2684 2349 1861 1723 1513 1629 1617 1548 3676 2944 2425
826–1243 836–1237 850–1210 830–1188 857–1181 884–1190 844–1204 847–1200 850–1200 853–1204 874–1200 886–1203
DTG
DSC
1124 1107 1120 1117 1122 1128 1116 1126 1119 1130 1133 1148
1000 1248 1050 1008 1015 1017 1007 1034 1050 1014 1042 1080
1187 1224 1245 1187 1192 1196 1149 1168 1183 1190 1188 1197
Tp1 was the temperature corresponding to the first peak point in the DTG, Tp2 the temperature corresponding to the second peak point, Ti the initial temperature corresponding to the endothermic peak in the DSC, Tp the temperature corresponding to the endothermic peak, Tf the final temperature corresponding to the endothermic peak, Q the total endothermic heat flow.
also abruptly decreased with biomass blending ratio over 60%. The biomass blending ratio should be controlled in less than 40% in order to reduce the possibility of sintering. The difference between ST and HT decreased with increasing biomass blending ratios, which was 31 °C and 13 °C for biomass blending ratio of 20% and 80%.
3.2. Ash fusion profile for biomass and coal Fig. 3 illustrated TG–DTG–DSC curves of ash fusion for lignite and eucalyptus bark at heating rates of 10 °C min− 1, 15 °C min− 1 and 20 °C min−1. From Fig. 3(1), the ash fusion process of lignite in the DTG curves had a significant peak at about 1178 °C corresponding to mass loss rate of 2.33%, 2.45% and 3.23% in the TG curves at a heating rate of 10 °C min−1, 15 °C min−1 and 20 °C min−1. At different heating rates, the main reaction of the whole melting process in the DSC curves was an endothermic reaction and started between 1168 °C and 1215 °C, but ended at uncertain temperature due to the high fusion temperature of the lignite. The maximum heat flow occurred between 1195 °C and 1245 °C with a heating rate increase from 10 °C min−1 to 20 °C min−1. From Fig. 3(2), the ash fusion process of eucalyptus bark in the DTG curves had a significant peak at about 1140 °C corresponding to mass loss rate of 3.60%, 4.98% and 5.06% in the TG curves at a heating rate of 10 °C min−1, 15 °C min−1 and 20 °C min−1. At different heating rates, the main reaction of the whole melting process in the DSC curves started between 1084 °C and 1100 °C, and ended between 1246 °C
45
and 1268 °C. The maximum heat flow occurred between 1185 °C and 1222 °C with a heating rate increase from 10 °C min−1 to 20 °C min−1. Fig. 4 illustrated TG–DTG–DSC curves of ash fusion for eucalyptus bark/lignite blends at heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1. Based on Fig. 4, the main characteristic parameters of the ash fusion process for all eucalyptus bark/lignite blends were listed in Table 3. From Fig. 4 and Table 3, with a heating rate increase from 10 °C min−1 to 20 °C min−1, the total absorbed heat flow in the ash fusion process for all biomass/coal blends decreased, e.g., the heat flow of 40EB/60L decreased from 1861 kJ/kg to 1513 kJ/kg. The heat flow of 80EB/20L at a heating rate of 10 °C min−1 reached a maximum, 3676 kJ/kg, while the value of 40EB/60L at a heating rate of 20 °C min− 1 was minimum, 1513 kJ/kg. Furthermore, with the decrease of heating rate, the initial temperature and the temperature corresponding to the endothermic peak value slightly decreased, e.g., the temperature of 40EB/60L at maximum heat flow decreased from 1196 to 1187 °C. 3.3. Kinetic analysis The main ash fusion region may be divided into two stages of pre-peak and post-peak by the maximum value (endothermic peak) in DSC diagram. The linear fitting was shown in Fig. 5 by plotting lg (β) vs. 1/T (T indicates the temperature corresponding to α = 0.33 at each heating rate) for the 40EB/60L blend, the reference activation energy was obtained, 397.59 kJ/mol. Fig. 4 illustrated that there was an obvious endothermic peak around 1196 °C for the 40EB/60L blend at 10 °C min−1 heating rate, the temperature range for the pre-peak period varied from 1130 to 1180 °C. Based on Eq. (6), by plotting ln[g(a)/T2] versus 1/T as shown in Fig. 5(b), the dominant mechanism was fitted to the 3
Avrami–Erofeev equation, gðαÞ ¼ ½− ln ð1−αÞ4 , which was controlled by the random nucleation and then growth process. Therefore, the activation energy and pre-exponential factor of the sample were obtained, 424.26 kJ/mol and 2.20 × 1015 min− 1. Nucleation is the process by which the reaction interface is initially established [34], at perhaps a limited number of points in the reactant, usually at a surface and possibly at an imperfection. The initially generated germ nucleus, consisting of a few atoms only is generally regarded as unstable due to the high ratio of surface strain to volume. It may, however, develop into a growth nucleus, which increases in size through advance of the reactant– product interface into the bulk of the crystallite, so reducing the relative importance of the surface strain term. The same method was applied to the post-peak period of 40EB/60L, two periods of other samples. The results were presented in Table 4. From Table 4, ash fusion mechanisms of the lignite in the pre-peak stage and post-peak stage were identical under certain heating rate. Under heating rates of 10 °C min−1, 15 °C min− 1 and 20 °C min−1,
Fig. 5. Kinetic linear fitting in the pre-peak period for ash fusion of the 40EB/60L blend at 10 °C min−1 heating rate.
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M. Chen et al. / Powder Technology 286 (2015) 39–47
Table 4 Kinetic parameters of ash fusion of eucalyptus bark/lignite. Samples
β °C min−1
T °C
Ea kJ mol−1
A min−1
R
100L
10
1120–1170 1180–1230
502.49 687.54
8.14E + 16 2.04E+18
−0.99 −0.99
15
1160–1200 1210–1260
428.32 489.16
3.76E+15 6.26E+16
−0.98 −0.99
20
1200–1250 1260–1310
511.84 751.42
9.83E+16 7.64E+19
−0.99 −0.97
10
1130–1170 1180–1220 1150–1190 1200–1250 1180–1230 1240–1290 1130–1180 1200–1250
778.23 679.45 814.38 660.51 827.46 701.64 424.26 440.73
9.84E+19 1.33E+18 2.65E+20 8.19E+17 6.52E+20 9.07E+18 2.20E+15 3.14E+15
−0.99 −0.98 −0.99 −0.99 −0.97 −0.98 −0.99 −0.98
1120–1170 1180–1230 1140–1180 1190–1250 1100–1150
503.17 479.15 588.36 322.48 586.78
8.89E+16 4.98E+16 2.31E+17 7.46E+13 2.04E+17
−0.99 −0.99 −0.98 −0.99 −0.99
g(α)/mechanisms Random nucleation and then growth, Avrami–Erofeev model: 2
½− ; ln ð1−αÞ5 Three dimensional diffusion, G–B model: 2
20EB/80L
15 20 40EB/60L
10 15 20
60EB/40L
10
1− 23 α−ð1−αÞ3 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2 G–B model: 2
1− 23 α−ð1−αÞ3 Random nucleation and then growth, Mampel Power model: α2 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ4 Avrami–Erofeev model 3
½− ; ln ð1−αÞ2
Avrami–Erofeev model 2
1160–1210 1110–1160 1170–1220 1120–1170 1180–1230
511.13 428.11 430.54 811.39 698.85
7.93E+16 3.76E+15 6.82E+15 1.15E+20 8.54E+18
−0.99 −0.98 −0.99 −0.99 −0.98
10
1140–1180 1190–1240
419.33 403.69
2.02E+15 1.28E+15
−0.97 −0.98
15
1120–1170 1180–1230 1110–1160
425.54 397.54 677.18
2.89E+15 9.67E+14 9.24E+17
−0.99 −0.98 −0.98
1170–1220
604.87
8.67E+16
−0.99
1150–1200
394.45
6.99E+14
−0.99
15 20
80EB/20L
20
½− ; ln ð1−αÞ5 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2
Mampel Power model: α2 Avrami–Erofeev model: 3
½− ; ln ð1−αÞ2 Mampel Power model: α2 Avrami–Erofeev model: 3
100EB
10
½− ; ln ð1−αÞ2 Mampel Power model: α2 Avrami–Erofeev model: 2
15 20
1210–1260 1130–1180 1190–1240 1180–1220 1230–1270
377.33 405.78 418.91 664.12 622.84
2.61E+14 1.44E+15 1.97E+15 7.74E+17 3.66E+17
the dominant ash fusion mechanisms of lignite were described by the Avrami–Erofeev model, the G–B model and the Avrami–Erofeev model, respectively. The Avrami–Erofeev model suggests that ash fusion mechanism is mainly controlled by the random nucleation and then growth process, while in conjunction with the G–B model, ash fusion mechanism is mainly controlled by the three dimensional diffusion process. Diffusion-controlled solid-state reaction kinetics in a sphere, where diffusion in all three directions is all-important. In a diffusioncontrolled reaction, numerous chemical reactions or micro-structural changes in solids take place through solid-state diffusion, i.e., the movement and transport of gas molecules in the solid phase [35]. The dominant ash fusion mechanisms of the eucalyptus bark were described by the Avrami–Erofeev model, or the Mampel Power model. The Mampel Power model a means that ash fusion mechanism is also controlled by the random nucleation and then growth process. The dominant ash fusion mechanisms of eucalyptus bark/lignite blends were described by the Avrami–Erofeev model, the G–B model or the Mampel Power model. The activation energy of ash fusion for lignite in the pre-peak stage and the post-peak stage was in the range of 428–512 kJ/mol, and 489–752 kJ/mol, respectively. The activation energy of ash fusion for eucalyptus bark in the pre-peak stage and the post-peak stage was in the range of 394–664 kJ/mol, and 377–623 kJ/mol, respectively. The
−0.97 −0.97 −0.99 −0.98 −0.99
½− ; ln ð1−αÞ5 Mampel Power model: α2 Avrami–Erofeev model: 2
½− ; ln ð1−αÞ5
activation energy of ash fusion for eucalyptus bark/lignite blends in the pre-peak stage and the post-peak stage was in the range of 419– 828 kJ/mol, and 322–702 kJ/mol, respectively. The activation energy of the 20EB80L blend was towards a maximum among these blends, indicating its high ash fusion inertia, which was helpful for reducing the release of alkali metal. Furthermore, with the increase of heating rate, expect for the 60EB/40L blend and lignite, the activation energy of ash fusion for other samples in the pre-peak stage and the post-peak stage increased. The ash fusion kinetic characteristics of eucalyptus bark/lignite blends had no linear relation with blend ratios due to interaction between biomass and coal. 4. Conclusions Both heating rate and blending ratio have much appreciable influence on melting of eucalyptus bark/lignite blends. With a heating rate increase from 10 °C min−1 to 20 °C min− 1, the total absorbed heat flow in the ash fusion process for all biomass/coal blends decreased. The heat flow of 80EB/20L at a heating rate of 10 °C min−1 reached a maximum, 3676 kJ kg−1, while the value of 40EB/60L at a heating rate of 20 °C min−1 was towards to a minimum, 1513 kJ kg−1. The biomass blending ratio should be controlled within 40% in order to reduce the possibility of sintering for eucalyptus bark/lignite blends.
M. Chen et al. / Powder Technology 286 (2015) 39–47
The ratio of silica to alumina for all eucalyptus bark/lignite blend ash was much less than the pure eucalyptus bark, and very close to the pure lignite. The ratio of silica to alumina for the 80% eucalyptus bark/20% lignite blend was almost 1/2 of the pure eucalyptus bark. The ratio of base to acid of the eucalyptus bark was above 3 times of the lignite. Under heating rates of 10 °C min−1, 15 °C min−1 and 20 °C min−1, the dominant ash fusion mechanisms of lignite were described by the Avrami–Erofeev model or the G–B model, while for the eucalyptus bark, those were described by the Avrami–Erofeev model or the Mampel Power model. The dominant ash fusion mechanisms of eucalyptus bark/lignite blends were described by the Avrami–Erofeev model, or the G–B model or the Mampel Power model. Nomenclature a degree of conversion A the pre-exponential or frequency factor (min−1) apparent activation energy (kJ mol−1) Ea R the universal gas constant (J mol−1 K−1) T temperature (°C, K) Greek letter β heating rate (°C min−1) Subscripts and superscripts 0 initial ∞ burnout Acknowledgments This work was supported by the National Natural Science Foundation of China under No. 51376017. References [1] D.-W. Lee, J.-S. Bae, Y.-J. Lee, S.-J. Park, J.-C. Hong, B.-H. Lee, C.-H. Jeon, Y.-C. Choi, Two-in-one fuel combining sugar cane with low rank coal and its CO2 reduction effects in pulverized-coal power plants, Environ. Sci. Technol. 47 (2013) 1704–1710. [2] L.I. Darvell, J.M. Jones, B. Gudka, X.C. Baxter, A. Saddawi, A. Williams, A. Malmgren, Combustion properties of some power station biomass fuels, Fuel 89 (2010) 2881–2890. [3] C.H. Pang, B. Hewakandamby, T. Wu, E. Lester, An automated ash fusion test for characterisation of the behaviour of ashes from biomass and coal at elevated temperatures, Fuel 103 (2013) 454–466. [4] M. Pronobis, The influence of biomass co-combustion on boiler fouling and efficiency, Fuel 85 (2006) 474–480. [5] J.M. Jones, L.I. Darvell, T.G. Bridgeman, M. Pourkashanian, A. Williams, An investigation of the thermal and catalytic behaviour of potassium in biomass combustion, Proc. Combust. Inst. 31 (2007) 1955–1963. [6] Y. Niu, W. Du, H. Tan, W. Xu, Y. Liu, Y. Xiong, S. Hui, Further study on biomass ash characteristics at elevated ashing temperatures: the evolution of K, Cl, S and the ash fusion characteristics, Bioresour. Technol. 129 (2013) 642–645. [7] Y. Niu, H. Tan, X. Wang, Z. Liu, H. Liu, Y. Liu, T. Xu, Study on fusion characteristics of biomass ash, Bioresour. Technol. 101 (2010) 9373–9381. [8] Q.H. Li, Y.G. Zhang, A.H. Meng, L. Li, G.X. Li, Study on ash fusion temperature using original and simulated biomass ashes, Fuel Process. Technol. 107 (2013) 107–112. [9] A.A. Tortosa Masiá, B.J.P. Buhre, R.P. Gupta, T.F. Wall, Characterising ash of biomass and waste, Fuel Process. Technol. 88 (2007) 1071–1081. [10] H. Haykiri-Acma, S. Yaman, S. Kucukbayrak, Effect of biomass on temperatures of sintering and initial deformation of lignite ash, Fuel 89 (2010) 3063–3068.
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