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A comparison of thermal behaviors of raw biomass, pyrolytic biochar and their blends with lignite
Bioresource Technology 146 (2013) 371–378
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A comparison of thermal behaviors of raw biomass, pyrolytic biochar and their blends with lignite Zhengang Liu, Rajasekhar Balasubramanian ⇑ Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117576, Singapore
h i g h l i g h t s The biochars were prepared from woody and non-woody biomass by pyrolysis. The biochars had improved fuel qualities than their parent biomass. The addition of biomass (biochar) in lignite increased reactivities of the blends. Significant interactions were observed within all the fuel blends. Limited increase of slagging and fouling for biochar/lignite than biomass/lignite.
a r t i c l e
i n f o
Article history: Received 4 June 2013 Received in revised form 14 July 2013 Accepted 16 July 2013 Available online 25 July 2013 Keywords: Co-combustion Interactions Fuel quality Slagging and fouling Low rank coal
a b s t r a c t In this study, thermal characteristics of raw biomass, corresponding pyrolytic biochars and their blends with lignite were investigated. The results showed that pyrolytic biochars had better fuel qualities than their parent biomass. In comparison to raw biomass, the combustion of the biochars shifted towards higher temperature and occurred at continuous temperature zones. The biochar addition in lignite increased the reactivities of the blends. Obvious interactions were observed between biomass/biochar and lignite and resulted in increased total burnout, shortened combustion time and increased maximum weight loss rate, indicating increased combustion efficiencies than that of lignite combustion alone. Regarding ash-related problems, the tendency to form slagging and fouling increased, when pyrolytic biochars were co-combusted with coal. This present study demonstrated that the pyrolytic biochars were more suitable than raw biomass to be co-combusted with lignite for energy generation in existing coalfired power plants. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Concerns over depletion of fossil resources as well as increasing environmental pollution caused by large-scale utilization of fossil fuels have necessitated the research on renewable energy. Lignocellulosic biomass is considered to be a promising feedstock for renewable energy and its use as solid fuel for power or energy production is the most efficient and cost-effective option for biomass energy recovery (Berndes et al., 2003; Dai et al., 2008). Compared to direct combustion of biomass alone, biomass/coal co-firing in existing coal-fired boilers has received increasing attention because this approach can accommodate varying amounts of available biomass and does not require large investments in new, stand-alone biomass plants. Extensive research related to biomass/coal co-firing has been carried out over the years. The results from this research reveal that there are several technical constraints associated with this application (Dai et al., 2008; Sami ⇑ Corresponding author. Tel.: +65 65165135; fax: +65 67744202. E-mail address: [email protected] (R. Balasubramanian). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.07.072
et al., 2001). For example, the poor grindability caused by the fibrous structure of biomass feedstock leads to high energy consumption during the grinding. In addition, the low energy density resulting from the high oxygen content increases the cost of handing logistics. On the other hand, the big difference between biomass and coal in terms of their fuel quality makes it difficult to design the burner and control biomass/coal co-combustion process (Dai et al., 2008). All these issues are derived from the inherent properties of biomass materials, leading to low thermal efficiency and high emissions of air pollutants (Khan et al., 2009; Saidur et al., 2011). It is, therefore, necessary to upgrade raw biomass feedstocks to meet the requirements of existing coal-fired power plant prior to co-firing. Low temperature pyrolysis is one of the important thermochemical approaches for biomass feedstock upgrading, with the solid biochar as the target product (Park et al., 2012; Van der Stelt et al., 2011). Compared to raw biomass, the resultant pyrolytic biochar has improved fuel quality such as significantly enhanced grindability and increased energy density, which could overcome issues associated with grinding and logistics encountered during
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co-combustion of raw biomass and coal (Abdullah and Wu, 2009; Phanphanich and Mani, 2011). Many studies have been published on biomass upgrading by low temperature pyrolysis, especially within the temperature range of 200–300 °C (also called torrefaction), as a pre-treatment for further biomass-to-energy conversion (Libra et al., 2011; Park et al., 2012). For example, the gasification of pyrolytic biochars produced more H2 and CO than that of raw biomass (Chen et al., 2013). In most cases, biomass upgrading by low temperature pyrolysis focuses on the improved physicochemical properties including enhanced energy density and increased grindability. As a solid fuel, the improved combustion property of the biochar is more desirable for practical applications. However, only a few reports are currently available for the combustion behavior of the resultant pyrolytic biochar (Chen et al., 2011, 2012; Rousset et al., 2011). It has been reported that the pyrolysis performed at 200–300 °C cannot improve the combustion property of biomass feedstocks due to the high decomposition temperature of cellulose (generally higher than 300 °C), especially for woody biomass (Park et al., 2012). On the other hand, high temperatures significantly favor the liquid and gaseous products, and decrease energy recovery in the biochar. Therefore, pyrolysis temperature is one of the vital parameters for solid fuel production from biomass while maintaining high energy recovery and desired combustion property. In addition, most of the research reported in the literature has focused on woody biomass with less attention paid to abundant agricultural residues (Abdullah and Wu, 2009; Lee et al., 2012; Phanphanich and Mani, 2011). Due to the difference in chemical composition, the woody biomass and agricultural biomass show different degrees of decomposition under identical pyrolysis conditions. Therefore, the pyrolytic biochars produced from the agricultural biomass are needed to be systematically investigated in view of its abundant potential. One of the key concerns in utilizing biomass feedstocks as a solid fuel for energy generation is the occurrence of ash-related problems in boilers (Dai et al., 2008; Vamvuka and Kakaras, 2011). Biomass materials, especially agricultural residues, are known to be rich in alkali metals such as Na and K and these alkali metals can cause undesirable slagging and fouling in the high temperature furnaces and boilers due to the formation of low melting- or softening-point alkali silicates (Khan et al., 2009; Saidur et al., 2011). Moreover, the ash content and its chemical composition have an influence on the application of combustion technology. Therefore, the ash-related problems should be investigated during raw biomass/coal and pyrolytic biochar/lignite co-combustion as they control their combustion performances. To the best of our knowledge, there is a lack of information regarding the combustion characteristics of pyrolytic biochars and their blends with coal. In this study, the woody and non-woody biomasses were pyrolyzed with the aim to produce solid fuel biochars for co-combustion with low rank coal for energy generation. The pyrolytic biochars produced from woody biomass and agricultural biomass were compared to find out the similarity and difference between their fuel qualities. The combustion characteristics of raw biomass, corresponding pyrolytic biochars and their blends with lignite were investigated to evaluate their feasibilities for energy production in existing coal-fired power plants. In addition, the slagging and fouling issues were assessed when raw biomass was replaced by pyrolytic biochars for co-combustion with lignite.
2. Methods 2.1. Materials Pinewood (PW) and coconut fiber (CF) were selected as representative biomass samples for the woody and agricultural biomass,
respectively. The raw biomass was crushed to less than 125 lm for subsequent pyrolytic biochar production. The corresponding pyrolytic biochar was prepared in a horizontal fixed-bed tubular quartz reactor (Liu et al., 2013b). Argon gas was introduced to maintain an inert atmosphere and the biomass was heated to 350 °C with a heating rate 15 °C/min. The biomass was pyrolyzed at 350 °C for 20 min and then cooled down to room temperature maintaining the inert atmosphere. A temperature of 350 °C was chosen because previous experiments have shown that it is suitable for upgrading other biomass feedstocks to obtain improved combustion behavior while maintaining higher energy yield (Park et al., 2012). The pyrolytic biochars produced from CF and PW were labeled as PY-CF and PY-PW, respectively. The lignite sample was obtained from Pasar, Indonesia and the fuel blends were prepared using lignite weight fraction of 25% in the blends. 2.2. Characterization The carbon, hydrogen, nitrogen and sulfur contents were determined on a Vario MACRO cube Elemental Analyzer (Germany). Proximate analysis was carried out according to the Standard Practice for the Proximate Analysis of Coal and Coke (GB/T212–2001) using a 5E-MAG6600 Automatic Proximate Analyzer (China). The surface morphology of the fuel was analyzed using a scanning electron microscopy JSM-5600LVF (Japan). The higher heating value (HHV) of the fuel was measured using a Kaiyuan 5E-KC5410 Express Calorimeter (China). The fuel sample was combusted in an oxygen-rich environment inside a closed vessel that was contained within a water bath. The increase in temperature of the water bath was used to calculate the HHV of the fuel sample. 2.3. Combustion analysis A thermobalance TGA-Q500 (USA) was used for the combustion experiments. All combustion experiments were conducted at atmospheric pressure, using temperatures ranging from room temperature to 650 °C with a heating rate of 20 °C/min and an air flux of 50 ml/min. To minimize the effects of mass and heat transfer limitations, small amounts of biomass samples (around 8 mg) were loaded into an alumina crucible. Three replicates of each TGA experiment were performed to ensure the reproducibility. 2.4. Ash analysis The lignite and blends ashes were prepared at 750 °C according to the ASTM. As for the ash analysis, the ash was digested in mixed solutions (2 ml 65% HNO3, 2 ml 30% H2O2 and 0.5 ml 48% HF) at 180 °C for 30 min in Milestone microwave digester (USA). The digestion solution was then evaporated to dryness to remove the fluorides and the resultant residues were dissolved in 1:1 HNO3 solutions, followed by the dilution by de-ionized water to desired volume. Subsequently, metal concentrations (K, Na, Ca, Fe, Si, Al and Mg) in the digestion solution were quantified using ICP-OES (Perkin Elmer 3000DV, USA). 3. Results and discussion 3.1. Proximate and ultimate analysis of raw biomass, pyrolytic biochar and lignite Table 1 provides results of proximate and ultimate analysis, as well as the calorific values of raw biomass, pyrolytic biochars and lignite. As expected, the pyrolytic biochars have increased carbon contents and decreased oxygen contents than their parent
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CF
PY-CF
Ultimate analysis (dry ash free, wt%) C 47.75 67.51 H 5.61 3.95 N 0.90 1.01 S 0.23 0.37 Oa 45.51 27.16 Proximate analysis (dry base, wt%) Volatile matter 80.69 30.01 Fixed carbon 10.26 60.81 Ash 9.05 9.18 Fuel ratio 0.13 2.03 HHV(MJ/kg) 19.23 25.96 a
PW
PY-PW
Lignite
48.15 6.70 1.35 0.20 43.60
76.00 4.65 1.40 0.34 17.61
61.64 5.72 1.74 0.77 30.13
85.45 13.15 1.40 0.15 19.83
34.31 63.46 2.23 1.85 32.62
48.76 40.98 10.26 0.84 25.75
By difference.
biomass. Nitrogen and sulfur contents are a key concern for a solid fuel because they could be transformed into toxic NOx and SO2 in the subsequent combustion process. The contents of nitrogen and sulfur in pyrolytic biochars are higher than those of raw biomass, especially for the sulfur (0.37% in PY-CF and 0.23% in CF; 0.34% in PY-PW and 0.20% in PW), but their values are still much lower than those of the lignite. This implies that as with raw biomass combustion, considerable environmental benefits can also be achieved by the use of pyrolytic biochars (Khan et al., 2009; Williams et al., 2012). To examine the changes in atomic composition, the atomic H/C and O/C ratios of raw biomass and corresponding biochars were plotted in a van Krevelen diagram (lignite for comparison), as illustrated in Fig. 1. The biochars have decreased H/C and O/C ratios than raw biomass and well below the values of those of lignite. The low H/C and O/C ratios suggest reduced energy loss, smoke formation and water vapor during biochars combustion process (Khan et al., 2009; Williams et al., 2012). From Fig. 1, it is expected that the biochars have improved fuel qualities than either raw biomass or lignite. In addition, the woody PW and PY-PW exhibited higher fuel qualities than those of CF and PY-CF. Fuel ratio is indicated as a ratio of fixed carbon against volatiles (fixed carbon/volatiles) and is a characteristic value representing the property of a solid fuel, classifying coal rank in ASTM 388. The proximate analysis confirmed that the contents of the volatiles decreased and fuel ratios significantly increased due to pyrolysis upgrading (shown in Table 1). For example, the fuel ratio increased from 0.13 of CF to 2.03 of PY-CF, and from 0.15 of PW to 1.85 of
PY-PW. The values of pyrolytic biochars are well higher than that of lignite (0.84). The increased fuel ratios suggest improved combustion performance and reduced emissions of air pollutants when raw biomass is replaced by pyrolytic biochars (Khan et al., 2009; Williams et al., 2012). In general, the raw biomass feedstocks have low calorific values and high oxygen contents compared to coal. This creates technological challenges in co-combustion of biomass and coal. In this study, however, the calorific values of the biochars (25.96 and 32.62 MJ/kg for PY-CF and PY-PW, respectively) were higher than that of the lignite (25.75 MJ/kg). In addition, it is generally considered that the HHV should exceed 20 MJ/kg for a solid fuel to ensure auto-thermal combustion (Liu et al., 2012). Based upon the results obtained in this study, both biochars well exceeded this HHV threshold and were higher than that of the lignite, and thus are promising alternatives for solid fuel applications. The pyrolytic biochars showed higher ash contents than parent biomass. This is opposite to another biochar (hydrochar) produced from hydrothermal carbonization, which has decreased ash content than parent biomass (Liu et al., 2013a). In comparison to hydrochars, the relatively higher ash content is one of disadvantages for pyrolytic biochars, implying the presence of ash-related problems which are often encountered during raw biomass combustion. 3.2. Combustion behavior of individual fuel Thermal gravimetric (TG) and differential thermal gravimetric (DTG) profiles depicting the combustion process of lignite, raw biomass and corresponding pyrolytic biochars are presented in Figs. 2 and 3, respectively. The characteristic temperatures and peak points of the combustion process are summarized in Table 2. As shown in Fig. 2, the DTG curve of the lignite showed two peaks: the first peak (centered at 84 °C and a weight loss of 8.95% at temperatures below 200 °C) corresponds to the evaporation of moisture contained in the lignite and the second broad peak (centered at 404 °C) relates to the decomposition/oxidation of the lignite (Liu et al., 2012). The results in Fig. 3 indicate that the combustion behaviors of the two biomasses are very different from that of the lignite. It is reported that due to differences in reactivity between the volatiles and resulting chars, the combustion of raw biomass usually produces two peaks in the DTG curve (excluding the moisture loss peak) (Liu et al., 2012). In agreement with previous reports, the DTG curves of CF and PW in this study showed two separated
120
2.0
Lignite
0.15
1.8
100
PW
1.6 80
1.2
0.8
0.05
Lignite
1.0
60
40
PY-PW
20
PY-CF
0.00
0.6 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Atomic O/C ratio Fig. 1. Atomic H/C and O/C ratios of raw biomass, pyrolytic biochars and lignite.
0
0
100
200
300
400
500
o
Temperature ( C) Fig. 2. TG and DTG profiles of the lignite.
600
700
DTG (%/sec)
0.10 TG (%)
Atomic H/C ratio
CF
1.4
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120
0.90
A
CF PY-CF
100
DTG (%/sec)
0.60
80 TG (%)
0.75
0.45 60 0.30 40 0.15 20 0.00 0
0
100
200
300 400 o Temperature ( C)
500
600
700
120
B
PW PY-PW
100
0.45
60 0.30 40
DTG (%/sec)
TG (%)
80
0.60
0.15 20 0.00 0 0
100
200
300
400
500
600
700
o
Temperature ( C) Fig. 3. TG and DTG profiles of (A) CF and PY-CF and (B) PW and PY-PW.
combustion zones, centering around 296 and 444 °C for CF, and 342 and 455 °C for PW. In addition, the main weight losses of raw biomass occurred at the low combustion temperature zone, with 50.59% within the temperature range of 274–323 °C for CF and 49.52% within the temperature range of 309–361 °C for PW. The weight losses in higher temperature zones only account for 13.71 and 28.22% for CF and PW, respectively. The low combustion temperature corresponding to the main weight loss and the separated combustion zone indicate the higher reactivity of the raw
biomass, and low combustion efficiency and high emissions when raw biomass is combusted (Khan et al., 2009). The pyrolytic biochars exhibited different DTG curve patterns compared to their parent biomass and the combustions shifted towards higher temperature zone. Compared to CF, two combustion zones of PY-CF were closer and the weight loss decreased significantly at the low temperature zone (39.53%) and increased (31.38%) at the high temperature zone. As for the minor peaks around 570 °C in both CF and PY-CF, they are ascribed to the decomposition of the inorganic components in the ash (Liu et al., 2012). In the case of PY-PW, it is totally different from PW and its combustion took place within one temperature zone of 338– 499 °C and with only one peak centering around 426 °C (with a shoulder peak) in the DTG curve. It can be seen that the combustion behavior of PY-PW resembles that of the lignite and this similarity suggests the promising potential of PY-PW and lignite cofiring on existing lignite-fired power plants. Compared to raw biomass, the higher combustion temperature favors the higher combustion efficiency and reduced emissions when pyrolytic biochars are combusted (Liu et al., 2012; Williams et al., 2012). In addition, the moisture content also plays an important role in fuel combustion performance (Khan et al., 2009; Zhong et al., 2006). From the first weight loss stage related to moisture content, it can be seen that the moisture contents remarkably decreased for the biochars compared to those of the parent biomass (from 5.84% to 3.37% for CF and PY-CF, and from 8.56% to 3.36% for PW and PYPW, respectively). Compared to raw biomass, the decreased moisture contents not only elevate combustion temperature but also reduce CO emission when the biochar is combusted (Zhong et al., 2006). Taking into account that the values of the Rmax and temperature at which Rmax is achieved (Tmax) are inversely proportional to the reactivity of the fuel, the pyrolytic biochars show decreased reactivities due to the losses of volatile matter and decreased H/C and O/C ratios (Liu et al., 2012). It should be noted that the reactivities of pyrolytic biochars are comparable to or even higher than that of the lignite in spite of lower H/C and O/C ratio of pyrolytic biochars than those of the lignite. This result can be ascribed to the porous structure produced from the volatiles release during pyrolysis (Fig. S1, see Supplementary information). The pyrolytic biochars had coarser surface than the lignite and some porous structures were observed in pyrolytic biochars, which caused more accessibility of the oxygen gas to active site than the lignite. From the above results, it can be concluded that the pyrolytic biochars exhibited improved combustion characteristics; the combustion shifted towards higher temperatures and occur in continuous temperature zones. The improved combustion characteristics coupled with the decreased moisture contents suggest that the biochars are more suitable to be used as solid fuel for energy
Table 2 Combustion parameters of raw biomass, corresponding pyrolytic biochar and the lignite. Fuel
CF PY-CF PW PY-PW Lignite CF/lignite PY-CF/lignite PW/lignite PY-PW/lignite
Tia (°C)
First combustion zone Temp. range (°C)
Tpeak (°C)
Rmax (%/ sec)
Weight loss(%)
Temp. range (°C)
Tpeak (°C)
Rmax (%/ sec)
Weight loss (%)
274 274 309 338 337 262 293 292 333
274–323 274–313 309–361 338–499 337–540 262–324 293–483 292–345 333–513
296 286 342 426 404 288 386 325 380
0.81 0.64 0.55 0.31 0.14 0.18 0.33 0.16 0.19
50.59 39.53 49.52 90.04 70.63 16.35 74.58 16.46 76.16
323–497 313–422 361–465 NA NA 324–446 NA 345–487 NA
444 400 455 NA NA 360 NA 375 NA
0.06 0.42 0.34 NA NA 0.32 NA 0.18 NA
13.71 31.38 28.22 NA NA 51.58 NA 59.25 NA
NA = Not available. a Ti = Ignition temperature; Tb = Burnout temperature.
Second combustion zone
Tb (°C)
Total burnout (%)
497 422 465 499 540 446 483 487 513
90.64 90.05 98.09 97.48 91.30 91.85 91.14 93.26 93.17
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generation than raw biomass. In addition, woody biomass PW and the corresponding pyrolytic biochar PY-PW showed higher fuel qualities than CF and PY-CF, respectively.
3.3. Combustion behaviors of fuel blends Fig. 4 presents the TG and DTG profiles for the raw biomass/lignite blends and pyrolytic biochar/lignite blends. Raw biomass/lignite blends, namely CF/lignite and PW/lignite blends, showed similar combustion patterns. Generally, in DTG curves, there are two weight loss peaks during the combustion due to the high reactivity of the raw biomass. Two relatively separated temperature zones were observed for CF/lignite blend combustion, 262–324 °C (centered at 288 °C with Rmax 0.18%/s) and 324–446 °C (centered at 360 °C with Rmax 0.32%/s). In the case of PW/lignite blend, two DTG peaks also were present, while the combustion occurred at relatively higher and continuous temperature range: 292–345 °C with weight loss of 16.46% and 345–487 °C with weight loss of 59.25%, respectively. Compared to raw biomass/lignite blends, more combustion zones overlap between pyrolytic biochars and lignite due to the elevated combustion temperature of the biochars. As a consequence, the biochar/lignite blends combusted at a continuous temperature zones (293–483 °C for PY-CF/lignite blend and 333–513 °C for PY-PW/lignite blend, respectively) and only
A
0.35
CF/lignite blend
one main DTG peak was observed, centered at 386 and 380 °C for PY-CF/lignite and PY-PW/lignite, respectively. In order to find out whether interactions between the components of the blends occur, the theoretical DTG curves of the blends were calculated as the sum of the curves of each individual component according to the below equation (Eq. (1)):
DTGBlend ¼ 25% DTGBiomassðBiocharÞ þ 75% DTGLignite
ð1Þ
where DTGBiomass (Biochar) and DTGLignite are DTG values for the raw biomass (biochar) and lignite in the blends, respectively. As shown in Fig. 4, no obvious difference between experimental and theoretical values was observed during moisture evaporation and initial devolatilization process for all the blends. The experimental values deviated significantly from the theoretical ones at high temperature combustion zones, indicating the presence of synergistic interactions between biomass/lignite and biochar/lignite during combustion. As a result of the interactions, the overall duration for the combustion of the blends was shortened and the blends had increased reactivities than expected values: increased Rmax and decreased Tmax. The least deviations between theoretical and experimental values were observed within PY-PW/lignite blend among all the blends. The theoretical total burnouts of the blends were also calculated and compared with experimental values. The total burnouts increase by 0.17–0.78% for the all the blends (increase from
B
0.42
Theoretical Experimental
Theoretical Experimental
PY-CF/lignite blend
0.35 0.28 DTG (%/sec)
DTG (%/sec)
0.28 0.21
0.14
0.07
0.21 0.14 0.07
0.00
0.00 0
100
200
300
400
500
600
0
700
100
200
o
C
0.24
400
500
600
700
Temperature ( C)
Theoretical Experimental
PW/lignite blend
D
0.25
0.20
Theoretical Experimental
PY-PW/lignite blend
0.20
DTG (%/sec)
0.16 DTG (%/sec)
300
o
Temperature ( C)
0.12
0.15
0.10
0.08 0.05 0.04 0.00 0.00 0
100
200
300 400 o Temperature ( C)
500
600
700
0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. Experimental and theoretical DTG profiles of the blends, (A) CF/lignite, (B) PY-CF/lignite, (C) PW/lignite and (D) PY-PW/lignite.
700
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91.14 to 91.85% for CF/lignite; from 90.99 to 91.14% for PY-CF/lignite; from 93.00 to 93.26% for PW/lignite, and from 92.85 to 93.17% for PY-PW/lignite). The increase of the total burnout also confirms the existence of the synergistic interactions within the blends and suggests the increased combustion efficiency achieved during the blends combustion. The interactions between raw biomass, biochar and lignite blends during co-combustion can be seen by comparing the experimental and theoretical results shown in Fig. 4. To further compare the interactions within different blends, deviations between experimental and theoretical weight loss at different temperatures were calculated according to the following equation (Eq. (2)):
Deviationð%Þ ¼
Experimental Theoretical 100 Theoretical
ð2Þ
In general, as shown in Fig. 5, the same trend was observed for all the blends: the deviations increased sharply to reach a maximum value at certain temperature and then decreased. The addition of the raw biomass and pyrolytic biochar increased the weight loss of the blend at most temperatures except for PY-CF/lignite blend at a temperature range of 270–340 °C and CF/lignite at a temperature range of 310–343 °C. This result is consistent with previous reports that the biomass addition enhanced the devolatil-
A
15
Deviation (%)
0
-15
-30
-45 CF/lignite blend PY-CF/lignite blend
-60
ization of the coal during biomass/coal co-processing (Farrow et al., 2012; Liu et al., 2013b). The CF/lignite and PY-CF/lignite blends showed similar deviations between the experimental weight loss and expected theoretical values at most of the combustion temperatures. In contrast, when comparing PW/lignite and PY-PW/lignite blends, less deviation was observed for PY-PW/lignite blend than that of PW/lignite blend. It is worth noting that contradictory results have been reported regarding the interactions between coal and raw biomass during co-combustion in the literature (Gil et al., 2010; Idris et al., 2012; Vamvuka and Sfakiotakis, 2011). For example, no significant interactions were detected between pine sawdust and coal within all the blends with coal in the range of 5–80% during co-combustion (Gil et al., 2010). In contrast, obvious interactions were observed during Turkish Elbistan lignite and woody shell of hazelnut co-combustion, resulting in higher burnout than expected values (Haykiri-Acma and Yaman, 2008). As for the biomass/lignite and biochar/lignite blends in this study, significant deviations of the experimental weight loss from the expected values confirm that the interactions do occur between biomass/biochar and lignite during combustion. In addition, it was reported that compared to high rank coals, obvious synergistic interactions were observed between biomass and low rank coals during co-combustion, and the absent or weak synergistic interactions between the high rank coals and biomass were attributed to no or little overlap in the combustion regions (Liu et al., 2012; Moon et al., 2013; Sahu et al., 2010). Therefore, the interactions between biomass and coal during co-combustion are expected to be enhanced with the elevated fuel quality of biomass. However, the results in this study are contradictory to previous reports. The minor interactions were observed within PY-PW/lignite blends among all the blends in spite of PY-PW exhibiting the highest fuel quality. The difference is attributed to complex, but poorly understood interactions between lignite and biomass/biochar during the combustion process. The above results confirm the presence of the interactions between biomass/biochar and lignite during co-combustion. Furthermore, the interactions are complex and vary with the biomass feedstock, indicating that the specific mechanisms involved in the co-combustion process require further investigation. 3.4. Slagging and fouling formation with the use of the fuel blends
-75
0
100
200
300
400
500
600
700
o
Temperature ( C) 15
B 0
Due to the diverse mineral constituents and relatively high contents of alkali and alkaline earth metals (AAEMs) in biomass feedstocks, the fouling and slagging problems are key concerns during biomass/coal co-combustion (Dai et al., 2008). In this study, the fouling and slagging formation with the use of fuel blends and lignite were investigated and the corresponding indices were calculated by the follows equations (Eq. (3) and (4)) (Pronobis, 2005):
Slagging indexðSIÞ ¼ ðB=AÞ S%
ð3Þ
Fouling indexðFIÞ ¼ ðB=AÞ ðNa2 O þ K2 OÞ
ð4Þ
Deviation (%)
-15
-30
-45 PW/lignite blend PY-PW/lignite blend
-60
0
100
200
300
400
500
600
700
o
Temperature ( C) Fig. 5. Deviations between experimental and theoretical weight loss of the blends at different temperatures.
where B/A = (Fe2O3 + CaO + MgO + Na2O + K2O)/(SiO2 + Al2O3 + TiO2); S is the percent of the sulfur in the dry fuel sample. Table 3 presents major ash problems related metal oxides contents in ash obtained from the blends and lignite. It can be seen that the addition of raw biomass and pyrolytic biochar to lignite increased the AAEM oxides contents in blend ashes in comparison to the ash of lignite alone, especially for CF and PY-CF. The SI value, 0.20, indicates that the lignite had low slagging formation potential when it was combusted alone (the acceptable range for low slagging inclination SI < 0.60). No or less increases were observed for SI values of the blends for woody PW addition (0.20) and PY-PW addition (0.22) in comparison to those of agricultural CF and PY-CF addition (0.24 and 0.29 for CF/lignite and PY-CF/lignite,
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Z. Liu, R. Balasubramanian / Bioresource Technology 146 (2013) 371–378 Table 3 Major ash problem-related metal oxide contents in lignite ash and blend ashes. Fuel ash
PW/lignite
PY-PW/lignite
CF/lignite
PY-CF/lignite
Lignite
MgO (%) Na2O (%) K2O (%) CaO (%) Al2O3 (%) SiO2 (%) TiO2 (%) Slagging index Fouling index
1.91 1.79 1.21 10.93 20.59 53.25 1.31 0.20 1.06
2.18 2.15 1.83 12.33 23.57 49.28 1.21 0.22 1.35
1.75 2.50 6.07 10.79 18.49 51.24 1.26 0.24 3.20
1.77 2.81 8.63 11.93 18.54 49.25 1.21 0.29 4.92
1.63 1.79 0.94 10.03 21.37 55.26 1.36 0.20 0.70
respectively). Regarding the fouling issue, all blends were in relatively high fouling formation ranges (FI ranged from 1.60 to 4.92 for blends) (Pronobis, 2005). Significant increases for FI values were observed when CF and PY-CF were added to the lignite than those of PW and PY-PW addition. This is ascribed to higher AAEM contents in agricultural biomass than woody biomass. When comparing the slagging and fouling problems of the blends, limited increase was observed by the use of pyrolytic biochars than raw biomass. The above results confirm that the slagging and fouling problems encountered during raw biomass and coal co-combustion were still present when pyrolytic biochars were co-combusted with coal, especially for agricultural pyrolytic biochars. To overcome these problems, some pre-treatment should be taken to reduce the AAEM contents in pyrolytic biochars prior to cocombustion with coal. It has been reported that washing treatment efficiently removed AAEMs originally contained in the raw biomass and that commodity fuels were prepared by combining washing and torrefaction (Saddawi et al., 2012). Therefore, washing treatment is a potential option to reduce the AAEM contents in the biochars and detailed slagging and fouling inclinations of biochar combustion by washing technique need further investigation. 4. Conclusions The low temperature pyrolysis significantly increased fuel quality of the biomass such as increased energy density and fuel ratio. The pyrolytic biochars exhibited improved combustion characteristics than their parent biomass. The addition of both raw biomass and pyrolytic biochar in lignite increased the reactivities of the blends. The synergistic interactions were obviously observed within all the blends and consequently improved thermal efficiencies of the blends were expected, especially for pyrolytic biochar/lignite blends. Compared to raw biomass/lignite blends, limited increases for slagging and fouling inclinations were observed for pyrolytic biochars/lignite blends. Acknowledgement This work was financially supported by M3TC (Minerals, Materials and Metals Technology Centre), NUS. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 07.072. References Abdullah, H., Wu, H., 2009. Biochars as a fuel: 1. Properties and grindability of biochars produced from the pyrolysis of mallee wood under slow-heating conditions. Energy Fuels 23, 4174–4181.
Berndes, G., Hoogwijk, M., Broek, R.V.D., 2003. The contribution of biomass in the future global energy supply: a review of 17 studies. Biomass Bioenergy 25, 1– 28. Chen, W.H., Chen, C.J., Huang, C., Shen, C.H., Hsu, H.W., 2013. A comparison of gasification phenomena among raw biomass, torrefied biomass and coal in an entrained-flow reactor. Appl. Energy. (http://dx.doi.org/10.1016). Chen, W.H., Cheng, W.Y., Lu, K.M., Huang, Y.P., 2011. An evaluation on improvement of pulverized biomass property for solid fuel through torrefaction. Appl. Energy 88, 3636–3644. Chen, W.H., Lu, K.M., Tsai, C.M., 2012. An experimental analysis on property and structure variations of agricultural wastes undergoing torrefaction. Appl. Energy 100, 318–325. Dai, J., Sokhansnji, S., Grace, J.R., Bi, X., Kim, C.J., Melin, S., 2008. Overview and some issues related to co-firing biomass and coal. Can. J. Chem. Eng. 86, 367–386. Farrow, T.S., Sun, C., Snape, C.E., 2012. Impact of biomass char on coal char burn-out under air and oxy-fuel conditions. Fuel. (http://dx.doi.org/10.1016/j.fuel). Gil, M.V., Casal, D., Pevida, C., Pis, J.J., Rubiera, F., 2010. Thermal behaviour and kinetics of coal/biomass blends during co-combustion. Bioresour. Technol. 101, 5601–5608. Haykiri-Acma, H., Yaman, S., 2008. Effect of co-combustion on the burnout of lignite/biomass blends: a Turkish case study. Waste Manage. 28, 2077–2084. Idris, S.S., Rahman, N.A., Ismail, K., 2012. Combustion characteristics of Malaysian oil palm biomass, sub-bituminous coal and their respective blends via thermogravimetric analysis (TGA). Bioresour. Technol. 123, 581–591. Khan, A.A., Jong, W., Jansens, P.J., Spliethoff, H., 2009. Biomass combustion in fluidized bed boilers: potential problems and remedies. Fuel Process. Technol. 90, 21–50. Lee, J.W., Kim, Y.H., Lee, S.M., Lee, H.W., 2012. Optimizing the torrefaction of mixed softwood by response surface methodology for biomass upgrading to high energy density. Bioresour. Technol. 116, 471–476. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.D., Neubauer, Y., Titirici, M.M., Fuhner, C., Bens, O., Kern, J., Emmerlich, K.H., 2011. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 2, 89–124. Liu, Z., Quek, A., Kent Hoekman, S., Balasubramanian, R., 2013a. Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 103, 943–949. Liu, Z., Quek, A., Kent Hoekman, S., Srinivasan, M.P., Balasubramanian, R., 2012. Thermogravimetric investigation of hydrochar-lignite co-combustion. Bioresour. Technol. 123, 646–652. Liu, Z., Quek, A., Parshetti, G., Jain, A., Srinivasan, M.P., Kent Hoekman, S., Balasubramanian, R., 2013b. A study of nitrogen conversion and polycyclic aromatic hydrocarbon (PAH) emissions during hydrochar-lignite co-pyrolysis. Appl. Energy 108, 74–81. Moon, C., Sung, Y., Ahn, S., Kim, T., Choi, G., Kim, D., 2013. Effect of blending ratio on combustion performance in blends of biomass and coals of different ranks. Exp. Thermal Fluid. Sci. 47, 232–240. Park, S.W., Jang, C.H., Baek, K.R., Yang, J.K., 2012. Torrefaction and low-temperature carbonization of woody biomass: evaluation of fuel characteristics of the products. Energy 45, 676–685. Phanphanich, M., Mani, S., 2011. Impact of torrefaction on the grindability and fuel characteristics of forest biomass. Bioresour. Technol. 102, 1246–1253. Pronobis, M., 2005. Evaluation of the influence of biomass co-combustion on boiler furnace slagging by means of fusibility correlations. Biomass Bioenergy 28, 375–383. Rousset, P., Aguiar, C., Labbe, N., Commandre, J.M., 2011. Enhancing the combustible properties of bamboo by torrefaction. Bioresour. Technol. 102, 8225–8231. Saddawi, A., Jones, J.M., Williams, A., Coeur, C.L., 2012. Commodity fuels from biomass through pretreatment and torrefaction: effect of mineral content on torrefied fuel characteristics and quality. Energy Fuels 26, 6466–6474. Sahu, S.G., Sarkar, P., Chakraborty, N., Adak, A.K., 2010. Thermogravimetric assessment of combustion characteristics of blends of a coal with different biomass chars. Fuel Process. Technol. 91, 369–378. Saidur, R., Abdelaziz, E.A., Demirbas, A., Hossain, M.S., Mekhilef, S., 2011. A review on biomass as a fuel for boilers. Renew. Sust. Energy Rev. 15, 2262–2289. Sami, M., Annamalai, K., Wooldridge, M., 2001. Co-firing of coal and biomass fuel blends. Prog. Energ. Combust. Sci. 27, 171–214. Vamvuka, D., Kakaras, E., 2011. Ash properties and environmental impact of various biomass and coal fuels and their blends. Fuel Process. Technol. 92, 570–581.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
A comparison of thermal behaviors of raw biomass, pyrolytic biochar and their blends with lignite Zhengang Liu, Rajasekhar Balasubramanian ⇑ Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117576, Singapore
h i g h l i g h t s The biochars were prepared from woody and non-woody biomass by pyrolysis. The biochars had improved fuel qualities than their parent biomass. The addition of biomass (biochar) in lignite increased reactivities of the blends. Significant interactions were observed within all the fuel blends. Limited increase of slagging and fouling for biochar/lignite than biomass/lignite.
a r t i c l e
i n f o
Article history: Received 4 June 2013 Received in revised form 14 July 2013 Accepted 16 July 2013 Available online 25 July 2013 Keywords: Co-combustion Interactions Fuel quality Slagging and fouling Low rank coal
a b s t r a c t In this study, thermal characteristics of raw biomass, corresponding pyrolytic biochars and their blends with lignite were investigated. The results showed that pyrolytic biochars had better fuel qualities than their parent biomass. In comparison to raw biomass, the combustion of the biochars shifted towards higher temperature and occurred at continuous temperature zones. The biochar addition in lignite increased the reactivities of the blends. Obvious interactions were observed between biomass/biochar and lignite and resulted in increased total burnout, shortened combustion time and increased maximum weight loss rate, indicating increased combustion efficiencies than that of lignite combustion alone. Regarding ash-related problems, the tendency to form slagging and fouling increased, when pyrolytic biochars were co-combusted with coal. This present study demonstrated that the pyrolytic biochars were more suitable than raw biomass to be co-combusted with lignite for energy generation in existing coalfired power plants. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Concerns over depletion of fossil resources as well as increasing environmental pollution caused by large-scale utilization of fossil fuels have necessitated the research on renewable energy. Lignocellulosic biomass is considered to be a promising feedstock for renewable energy and its use as solid fuel for power or energy production is the most efficient and cost-effective option for biomass energy recovery (Berndes et al., 2003; Dai et al., 2008). Compared to direct combustion of biomass alone, biomass/coal co-firing in existing coal-fired boilers has received increasing attention because this approach can accommodate varying amounts of available biomass and does not require large investments in new, stand-alone biomass plants. Extensive research related to biomass/coal co-firing has been carried out over the years. The results from this research reveal that there are several technical constraints associated with this application (Dai et al., 2008; Sami ⇑ Corresponding author. Tel.: +65 65165135; fax: +65 67744202. E-mail address: [email protected] (R. Balasubramanian). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.07.072
et al., 2001). For example, the poor grindability caused by the fibrous structure of biomass feedstock leads to high energy consumption during the grinding. In addition, the low energy density resulting from the high oxygen content increases the cost of handing logistics. On the other hand, the big difference between biomass and coal in terms of their fuel quality makes it difficult to design the burner and control biomass/coal co-combustion process (Dai et al., 2008). All these issues are derived from the inherent properties of biomass materials, leading to low thermal efficiency and high emissions of air pollutants (Khan et al., 2009; Saidur et al., 2011). It is, therefore, necessary to upgrade raw biomass feedstocks to meet the requirements of existing coal-fired power plant prior to co-firing. Low temperature pyrolysis is one of the important thermochemical approaches for biomass feedstock upgrading, with the solid biochar as the target product (Park et al., 2012; Van der Stelt et al., 2011). Compared to raw biomass, the resultant pyrolytic biochar has improved fuel quality such as significantly enhanced grindability and increased energy density, which could overcome issues associated with grinding and logistics encountered during
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co-combustion of raw biomass and coal (Abdullah and Wu, 2009; Phanphanich and Mani, 2011). Many studies have been published on biomass upgrading by low temperature pyrolysis, especially within the temperature range of 200–300 °C (also called torrefaction), as a pre-treatment for further biomass-to-energy conversion (Libra et al., 2011; Park et al., 2012). For example, the gasification of pyrolytic biochars produced more H2 and CO than that of raw biomass (Chen et al., 2013). In most cases, biomass upgrading by low temperature pyrolysis focuses on the improved physicochemical properties including enhanced energy density and increased grindability. As a solid fuel, the improved combustion property of the biochar is more desirable for practical applications. However, only a few reports are currently available for the combustion behavior of the resultant pyrolytic biochar (Chen et al., 2011, 2012; Rousset et al., 2011). It has been reported that the pyrolysis performed at 200–300 °C cannot improve the combustion property of biomass feedstocks due to the high decomposition temperature of cellulose (generally higher than 300 °C), especially for woody biomass (Park et al., 2012). On the other hand, high temperatures significantly favor the liquid and gaseous products, and decrease energy recovery in the biochar. Therefore, pyrolysis temperature is one of the vital parameters for solid fuel production from biomass while maintaining high energy recovery and desired combustion property. In addition, most of the research reported in the literature has focused on woody biomass with less attention paid to abundant agricultural residues (Abdullah and Wu, 2009; Lee et al., 2012; Phanphanich and Mani, 2011). Due to the difference in chemical composition, the woody biomass and agricultural biomass show different degrees of decomposition under identical pyrolysis conditions. Therefore, the pyrolytic biochars produced from the agricultural biomass are needed to be systematically investigated in view of its abundant potential. One of the key concerns in utilizing biomass feedstocks as a solid fuel for energy generation is the occurrence of ash-related problems in boilers (Dai et al., 2008; Vamvuka and Kakaras, 2011). Biomass materials, especially agricultural residues, are known to be rich in alkali metals such as Na and K and these alkali metals can cause undesirable slagging and fouling in the high temperature furnaces and boilers due to the formation of low melting- or softening-point alkali silicates (Khan et al., 2009; Saidur et al., 2011). Moreover, the ash content and its chemical composition have an influence on the application of combustion technology. Therefore, the ash-related problems should be investigated during raw biomass/coal and pyrolytic biochar/lignite co-combustion as they control their combustion performances. To the best of our knowledge, there is a lack of information regarding the combustion characteristics of pyrolytic biochars and their blends with coal. In this study, the woody and non-woody biomasses were pyrolyzed with the aim to produce solid fuel biochars for co-combustion with low rank coal for energy generation. The pyrolytic biochars produced from woody biomass and agricultural biomass were compared to find out the similarity and difference between their fuel qualities. The combustion characteristics of raw biomass, corresponding pyrolytic biochars and their blends with lignite were investigated to evaluate their feasibilities for energy production in existing coal-fired power plants. In addition, the slagging and fouling issues were assessed when raw biomass was replaced by pyrolytic biochars for co-combustion with lignite.
2. Methods 2.1. Materials Pinewood (PW) and coconut fiber (CF) were selected as representative biomass samples for the woody and agricultural biomass,
respectively. The raw biomass was crushed to less than 125 lm for subsequent pyrolytic biochar production. The corresponding pyrolytic biochar was prepared in a horizontal fixed-bed tubular quartz reactor (Liu et al., 2013b). Argon gas was introduced to maintain an inert atmosphere and the biomass was heated to 350 °C with a heating rate 15 °C/min. The biomass was pyrolyzed at 350 °C for 20 min and then cooled down to room temperature maintaining the inert atmosphere. A temperature of 350 °C was chosen because previous experiments have shown that it is suitable for upgrading other biomass feedstocks to obtain improved combustion behavior while maintaining higher energy yield (Park et al., 2012). The pyrolytic biochars produced from CF and PW were labeled as PY-CF and PY-PW, respectively. The lignite sample was obtained from Pasar, Indonesia and the fuel blends were prepared using lignite weight fraction of 25% in the blends. 2.2. Characterization The carbon, hydrogen, nitrogen and sulfur contents were determined on a Vario MACRO cube Elemental Analyzer (Germany). Proximate analysis was carried out according to the Standard Practice for the Proximate Analysis of Coal and Coke (GB/T212–2001) using a 5E-MAG6600 Automatic Proximate Analyzer (China). The surface morphology of the fuel was analyzed using a scanning electron microscopy JSM-5600LVF (Japan). The higher heating value (HHV) of the fuel was measured using a Kaiyuan 5E-KC5410 Express Calorimeter (China). The fuel sample was combusted in an oxygen-rich environment inside a closed vessel that was contained within a water bath. The increase in temperature of the water bath was used to calculate the HHV of the fuel sample. 2.3. Combustion analysis A thermobalance TGA-Q500 (USA) was used for the combustion experiments. All combustion experiments were conducted at atmospheric pressure, using temperatures ranging from room temperature to 650 °C with a heating rate of 20 °C/min and an air flux of 50 ml/min. To minimize the effects of mass and heat transfer limitations, small amounts of biomass samples (around 8 mg) were loaded into an alumina crucible. Three replicates of each TGA experiment were performed to ensure the reproducibility. 2.4. Ash analysis The lignite and blends ashes were prepared at 750 °C according to the ASTM. As for the ash analysis, the ash was digested in mixed solutions (2 ml 65% HNO3, 2 ml 30% H2O2 and 0.5 ml 48% HF) at 180 °C for 30 min in Milestone microwave digester (USA). The digestion solution was then evaporated to dryness to remove the fluorides and the resultant residues were dissolved in 1:1 HNO3 solutions, followed by the dilution by de-ionized water to desired volume. Subsequently, metal concentrations (K, Na, Ca, Fe, Si, Al and Mg) in the digestion solution were quantified using ICP-OES (Perkin Elmer 3000DV, USA). 3. Results and discussion 3.1. Proximate and ultimate analysis of raw biomass, pyrolytic biochar and lignite Table 1 provides results of proximate and ultimate analysis, as well as the calorific values of raw biomass, pyrolytic biochars and lignite. As expected, the pyrolytic biochars have increased carbon contents and decreased oxygen contents than their parent
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CF
PY-CF
Ultimate analysis (dry ash free, wt%) C 47.75 67.51 H 5.61 3.95 N 0.90 1.01 S 0.23 0.37 Oa 45.51 27.16 Proximate analysis (dry base, wt%) Volatile matter 80.69 30.01 Fixed carbon 10.26 60.81 Ash 9.05 9.18 Fuel ratio 0.13 2.03 HHV(MJ/kg) 19.23 25.96 a
PW
PY-PW
Lignite
48.15 6.70 1.35 0.20 43.60
76.00 4.65 1.40 0.34 17.61
61.64 5.72 1.74 0.77 30.13
85.45 13.15 1.40 0.15 19.83
34.31 63.46 2.23 1.85 32.62
48.76 40.98 10.26 0.84 25.75
By difference.
biomass. Nitrogen and sulfur contents are a key concern for a solid fuel because they could be transformed into toxic NOx and SO2 in the subsequent combustion process. The contents of nitrogen and sulfur in pyrolytic biochars are higher than those of raw biomass, especially for the sulfur (0.37% in PY-CF and 0.23% in CF; 0.34% in PY-PW and 0.20% in PW), but their values are still much lower than those of the lignite. This implies that as with raw biomass combustion, considerable environmental benefits can also be achieved by the use of pyrolytic biochars (Khan et al., 2009; Williams et al., 2012). To examine the changes in atomic composition, the atomic H/C and O/C ratios of raw biomass and corresponding biochars were plotted in a van Krevelen diagram (lignite for comparison), as illustrated in Fig. 1. The biochars have decreased H/C and O/C ratios than raw biomass and well below the values of those of lignite. The low H/C and O/C ratios suggest reduced energy loss, smoke formation and water vapor during biochars combustion process (Khan et al., 2009; Williams et al., 2012). From Fig. 1, it is expected that the biochars have improved fuel qualities than either raw biomass or lignite. In addition, the woody PW and PY-PW exhibited higher fuel qualities than those of CF and PY-CF. Fuel ratio is indicated as a ratio of fixed carbon against volatiles (fixed carbon/volatiles) and is a characteristic value representing the property of a solid fuel, classifying coal rank in ASTM 388. The proximate analysis confirmed that the contents of the volatiles decreased and fuel ratios significantly increased due to pyrolysis upgrading (shown in Table 1). For example, the fuel ratio increased from 0.13 of CF to 2.03 of PY-CF, and from 0.15 of PW to 1.85 of
PY-PW. The values of pyrolytic biochars are well higher than that of lignite (0.84). The increased fuel ratios suggest improved combustion performance and reduced emissions of air pollutants when raw biomass is replaced by pyrolytic biochars (Khan et al., 2009; Williams et al., 2012). In general, the raw biomass feedstocks have low calorific values and high oxygen contents compared to coal. This creates technological challenges in co-combustion of biomass and coal. In this study, however, the calorific values of the biochars (25.96 and 32.62 MJ/kg for PY-CF and PY-PW, respectively) were higher than that of the lignite (25.75 MJ/kg). In addition, it is generally considered that the HHV should exceed 20 MJ/kg for a solid fuel to ensure auto-thermal combustion (Liu et al., 2012). Based upon the results obtained in this study, both biochars well exceeded this HHV threshold and were higher than that of the lignite, and thus are promising alternatives for solid fuel applications. The pyrolytic biochars showed higher ash contents than parent biomass. This is opposite to another biochar (hydrochar) produced from hydrothermal carbonization, which has decreased ash content than parent biomass (Liu et al., 2013a). In comparison to hydrochars, the relatively higher ash content is one of disadvantages for pyrolytic biochars, implying the presence of ash-related problems which are often encountered during raw biomass combustion. 3.2. Combustion behavior of individual fuel Thermal gravimetric (TG) and differential thermal gravimetric (DTG) profiles depicting the combustion process of lignite, raw biomass and corresponding pyrolytic biochars are presented in Figs. 2 and 3, respectively. The characteristic temperatures and peak points of the combustion process are summarized in Table 2. As shown in Fig. 2, the DTG curve of the lignite showed two peaks: the first peak (centered at 84 °C and a weight loss of 8.95% at temperatures below 200 °C) corresponds to the evaporation of moisture contained in the lignite and the second broad peak (centered at 404 °C) relates to the decomposition/oxidation of the lignite (Liu et al., 2012). The results in Fig. 3 indicate that the combustion behaviors of the two biomasses are very different from that of the lignite. It is reported that due to differences in reactivity between the volatiles and resulting chars, the combustion of raw biomass usually produces two peaks in the DTG curve (excluding the moisture loss peak) (Liu et al., 2012). In agreement with previous reports, the DTG curves of CF and PW in this study showed two separated
120
2.0
Lignite
0.15
1.8
100
PW
1.6 80
1.2
0.8
0.05
Lignite
1.0
60
40
PY-PW
20
PY-CF
0.00
0.6 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Atomic O/C ratio Fig. 1. Atomic H/C and O/C ratios of raw biomass, pyrolytic biochars and lignite.
0
0
100
200
300
400
500
o
Temperature ( C) Fig. 2. TG and DTG profiles of the lignite.
600
700
DTG (%/sec)
0.10 TG (%)
Atomic H/C ratio
CF
1.4
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120
0.90
A
CF PY-CF
100
DTG (%/sec)
0.60
80 TG (%)
0.75
0.45 60 0.30 40 0.15 20 0.00 0
0
100
200
300 400 o Temperature ( C)
500
600
700
120
B
PW PY-PW
100
0.45
60 0.30 40
DTG (%/sec)
TG (%)
80
0.60
0.15 20 0.00 0 0
100
200
300
400
500
600
700
o
Temperature ( C) Fig. 3. TG and DTG profiles of (A) CF and PY-CF and (B) PW and PY-PW.
combustion zones, centering around 296 and 444 °C for CF, and 342 and 455 °C for PW. In addition, the main weight losses of raw biomass occurred at the low combustion temperature zone, with 50.59% within the temperature range of 274–323 °C for CF and 49.52% within the temperature range of 309–361 °C for PW. The weight losses in higher temperature zones only account for 13.71 and 28.22% for CF and PW, respectively. The low combustion temperature corresponding to the main weight loss and the separated combustion zone indicate the higher reactivity of the raw
biomass, and low combustion efficiency and high emissions when raw biomass is combusted (Khan et al., 2009). The pyrolytic biochars exhibited different DTG curve patterns compared to their parent biomass and the combustions shifted towards higher temperature zone. Compared to CF, two combustion zones of PY-CF were closer and the weight loss decreased significantly at the low temperature zone (39.53%) and increased (31.38%) at the high temperature zone. As for the minor peaks around 570 °C in both CF and PY-CF, they are ascribed to the decomposition of the inorganic components in the ash (Liu et al., 2012). In the case of PY-PW, it is totally different from PW and its combustion took place within one temperature zone of 338– 499 °C and with only one peak centering around 426 °C (with a shoulder peak) in the DTG curve. It can be seen that the combustion behavior of PY-PW resembles that of the lignite and this similarity suggests the promising potential of PY-PW and lignite cofiring on existing lignite-fired power plants. Compared to raw biomass, the higher combustion temperature favors the higher combustion efficiency and reduced emissions when pyrolytic biochars are combusted (Liu et al., 2012; Williams et al., 2012). In addition, the moisture content also plays an important role in fuel combustion performance (Khan et al., 2009; Zhong et al., 2006). From the first weight loss stage related to moisture content, it can be seen that the moisture contents remarkably decreased for the biochars compared to those of the parent biomass (from 5.84% to 3.37% for CF and PY-CF, and from 8.56% to 3.36% for PW and PYPW, respectively). Compared to raw biomass, the decreased moisture contents not only elevate combustion temperature but also reduce CO emission when the biochar is combusted (Zhong et al., 2006). Taking into account that the values of the Rmax and temperature at which Rmax is achieved (Tmax) are inversely proportional to the reactivity of the fuel, the pyrolytic biochars show decreased reactivities due to the losses of volatile matter and decreased H/C and O/C ratios (Liu et al., 2012). It should be noted that the reactivities of pyrolytic biochars are comparable to or even higher than that of the lignite in spite of lower H/C and O/C ratio of pyrolytic biochars than those of the lignite. This result can be ascribed to the porous structure produced from the volatiles release during pyrolysis (Fig. S1, see Supplementary information). The pyrolytic biochars had coarser surface than the lignite and some porous structures were observed in pyrolytic biochars, which caused more accessibility of the oxygen gas to active site than the lignite. From the above results, it can be concluded that the pyrolytic biochars exhibited improved combustion characteristics; the combustion shifted towards higher temperatures and occur in continuous temperature zones. The improved combustion characteristics coupled with the decreased moisture contents suggest that the biochars are more suitable to be used as solid fuel for energy
Table 2 Combustion parameters of raw biomass, corresponding pyrolytic biochar and the lignite. Fuel
CF PY-CF PW PY-PW Lignite CF/lignite PY-CF/lignite PW/lignite PY-PW/lignite
Tia (°C)
First combustion zone Temp. range (°C)
Tpeak (°C)
Rmax (%/ sec)
Weight loss(%)
Temp. range (°C)
Tpeak (°C)
Rmax (%/ sec)
Weight loss (%)
274 274 309 338 337 262 293 292 333
274–323 274–313 309–361 338–499 337–540 262–324 293–483 292–345 333–513
296 286 342 426 404 288 386 325 380
0.81 0.64 0.55 0.31 0.14 0.18 0.33 0.16 0.19
50.59 39.53 49.52 90.04 70.63 16.35 74.58 16.46 76.16
323–497 313–422 361–465 NA NA 324–446 NA 345–487 NA
444 400 455 NA NA 360 NA 375 NA
0.06 0.42 0.34 NA NA 0.32 NA 0.18 NA
13.71 31.38 28.22 NA NA 51.58 NA 59.25 NA
NA = Not available. a Ti = Ignition temperature; Tb = Burnout temperature.
Second combustion zone
Tb (°C)
Total burnout (%)
497 422 465 499 540 446 483 487 513
90.64 90.05 98.09 97.48 91.30 91.85 91.14 93.26 93.17
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generation than raw biomass. In addition, woody biomass PW and the corresponding pyrolytic biochar PY-PW showed higher fuel qualities than CF and PY-CF, respectively.
3.3. Combustion behaviors of fuel blends Fig. 4 presents the TG and DTG profiles for the raw biomass/lignite blends and pyrolytic biochar/lignite blends. Raw biomass/lignite blends, namely CF/lignite and PW/lignite blends, showed similar combustion patterns. Generally, in DTG curves, there are two weight loss peaks during the combustion due to the high reactivity of the raw biomass. Two relatively separated temperature zones were observed for CF/lignite blend combustion, 262–324 °C (centered at 288 °C with Rmax 0.18%/s) and 324–446 °C (centered at 360 °C with Rmax 0.32%/s). In the case of PW/lignite blend, two DTG peaks also were present, while the combustion occurred at relatively higher and continuous temperature range: 292–345 °C with weight loss of 16.46% and 345–487 °C with weight loss of 59.25%, respectively. Compared to raw biomass/lignite blends, more combustion zones overlap between pyrolytic biochars and lignite due to the elevated combustion temperature of the biochars. As a consequence, the biochar/lignite blends combusted at a continuous temperature zones (293–483 °C for PY-CF/lignite blend and 333–513 °C for PY-PW/lignite blend, respectively) and only
A
0.35
CF/lignite blend
one main DTG peak was observed, centered at 386 and 380 °C for PY-CF/lignite and PY-PW/lignite, respectively. In order to find out whether interactions between the components of the blends occur, the theoretical DTG curves of the blends were calculated as the sum of the curves of each individual component according to the below equation (Eq. (1)):
DTGBlend ¼ 25% DTGBiomassðBiocharÞ þ 75% DTGLignite
ð1Þ
where DTGBiomass (Biochar) and DTGLignite are DTG values for the raw biomass (biochar) and lignite in the blends, respectively. As shown in Fig. 4, no obvious difference between experimental and theoretical values was observed during moisture evaporation and initial devolatilization process for all the blends. The experimental values deviated significantly from the theoretical ones at high temperature combustion zones, indicating the presence of synergistic interactions between biomass/lignite and biochar/lignite during combustion. As a result of the interactions, the overall duration for the combustion of the blends was shortened and the blends had increased reactivities than expected values: increased Rmax and decreased Tmax. The least deviations between theoretical and experimental values were observed within PY-PW/lignite blend among all the blends. The theoretical total burnouts of the blends were also calculated and compared with experimental values. The total burnouts increase by 0.17–0.78% for the all the blends (increase from
B
0.42
Theoretical Experimental
Theoretical Experimental
PY-CF/lignite blend
0.35 0.28 DTG (%/sec)
DTG (%/sec)
0.28 0.21
0.14
0.07
0.21 0.14 0.07
0.00
0.00 0
100
200
300
400
500
600
0
700
100
200
o
C
0.24
400
500
600
700
Temperature ( C)
Theoretical Experimental
PW/lignite blend
D
0.25
0.20
Theoretical Experimental
PY-PW/lignite blend
0.20
DTG (%/sec)
0.16 DTG (%/sec)
300
o
Temperature ( C)
0.12
0.15
0.10
0.08 0.05 0.04 0.00 0.00 0
100
200
300 400 o Temperature ( C)
500
600
700
0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 4. Experimental and theoretical DTG profiles of the blends, (A) CF/lignite, (B) PY-CF/lignite, (C) PW/lignite and (D) PY-PW/lignite.
700
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91.14 to 91.85% for CF/lignite; from 90.99 to 91.14% for PY-CF/lignite; from 93.00 to 93.26% for PW/lignite, and from 92.85 to 93.17% for PY-PW/lignite). The increase of the total burnout also confirms the existence of the synergistic interactions within the blends and suggests the increased combustion efficiency achieved during the blends combustion. The interactions between raw biomass, biochar and lignite blends during co-combustion can be seen by comparing the experimental and theoretical results shown in Fig. 4. To further compare the interactions within different blends, deviations between experimental and theoretical weight loss at different temperatures were calculated according to the following equation (Eq. (2)):
Deviationð%Þ ¼
Experimental Theoretical 100 Theoretical
ð2Þ
In general, as shown in Fig. 5, the same trend was observed for all the blends: the deviations increased sharply to reach a maximum value at certain temperature and then decreased. The addition of the raw biomass and pyrolytic biochar increased the weight loss of the blend at most temperatures except for PY-CF/lignite blend at a temperature range of 270–340 °C and CF/lignite at a temperature range of 310–343 °C. This result is consistent with previous reports that the biomass addition enhanced the devolatil-
A
15
Deviation (%)
0
-15
-30
-45 CF/lignite blend PY-CF/lignite blend
-60
ization of the coal during biomass/coal co-processing (Farrow et al., 2012; Liu et al., 2013b). The CF/lignite and PY-CF/lignite blends showed similar deviations between the experimental weight loss and expected theoretical values at most of the combustion temperatures. In contrast, when comparing PW/lignite and PY-PW/lignite blends, less deviation was observed for PY-PW/lignite blend than that of PW/lignite blend. It is worth noting that contradictory results have been reported regarding the interactions between coal and raw biomass during co-combustion in the literature (Gil et al., 2010; Idris et al., 2012; Vamvuka and Sfakiotakis, 2011). For example, no significant interactions were detected between pine sawdust and coal within all the blends with coal in the range of 5–80% during co-combustion (Gil et al., 2010). In contrast, obvious interactions were observed during Turkish Elbistan lignite and woody shell of hazelnut co-combustion, resulting in higher burnout than expected values (Haykiri-Acma and Yaman, 2008). As for the biomass/lignite and biochar/lignite blends in this study, significant deviations of the experimental weight loss from the expected values confirm that the interactions do occur between biomass/biochar and lignite during combustion. In addition, it was reported that compared to high rank coals, obvious synergistic interactions were observed between biomass and low rank coals during co-combustion, and the absent or weak synergistic interactions between the high rank coals and biomass were attributed to no or little overlap in the combustion regions (Liu et al., 2012; Moon et al., 2013; Sahu et al., 2010). Therefore, the interactions between biomass and coal during co-combustion are expected to be enhanced with the elevated fuel quality of biomass. However, the results in this study are contradictory to previous reports. The minor interactions were observed within PY-PW/lignite blends among all the blends in spite of PY-PW exhibiting the highest fuel quality. The difference is attributed to complex, but poorly understood interactions between lignite and biomass/biochar during the combustion process. The above results confirm the presence of the interactions between biomass/biochar and lignite during co-combustion. Furthermore, the interactions are complex and vary with the biomass feedstock, indicating that the specific mechanisms involved in the co-combustion process require further investigation. 3.4. Slagging and fouling formation with the use of the fuel blends
-75
0
100
200
300
400
500
600
700
o
Temperature ( C) 15
B 0
Due to the diverse mineral constituents and relatively high contents of alkali and alkaline earth metals (AAEMs) in biomass feedstocks, the fouling and slagging problems are key concerns during biomass/coal co-combustion (Dai et al., 2008). In this study, the fouling and slagging formation with the use of fuel blends and lignite were investigated and the corresponding indices were calculated by the follows equations (Eq. (3) and (4)) (Pronobis, 2005):
Slagging indexðSIÞ ¼ ðB=AÞ S%
ð3Þ
Fouling indexðFIÞ ¼ ðB=AÞ ðNa2 O þ K2 OÞ
ð4Þ
Deviation (%)
-15
-30
-45 PW/lignite blend PY-PW/lignite blend
-60
0
100
200
300
400
500
600
700
o
Temperature ( C) Fig. 5. Deviations between experimental and theoretical weight loss of the blends at different temperatures.
where B/A = (Fe2O3 + CaO + MgO + Na2O + K2O)/(SiO2 + Al2O3 + TiO2); S is the percent of the sulfur in the dry fuel sample. Table 3 presents major ash problems related metal oxides contents in ash obtained from the blends and lignite. It can be seen that the addition of raw biomass and pyrolytic biochar to lignite increased the AAEM oxides contents in blend ashes in comparison to the ash of lignite alone, especially for CF and PY-CF. The SI value, 0.20, indicates that the lignite had low slagging formation potential when it was combusted alone (the acceptable range for low slagging inclination SI < 0.60). No or less increases were observed for SI values of the blends for woody PW addition (0.20) and PY-PW addition (0.22) in comparison to those of agricultural CF and PY-CF addition (0.24 and 0.29 for CF/lignite and PY-CF/lignite,
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Z. Liu, R. Balasubramanian / Bioresource Technology 146 (2013) 371–378 Table 3 Major ash problem-related metal oxide contents in lignite ash and blend ashes. Fuel ash
PW/lignite
PY-PW/lignite
CF/lignite
PY-CF/lignite
Lignite
MgO (%) Na2O (%) K2O (%) CaO (%) Al2O3 (%) SiO2 (%) TiO2 (%) Slagging index Fouling index
1.91 1.79 1.21 10.93 20.59 53.25 1.31 0.20 1.06
2.18 2.15 1.83 12.33 23.57 49.28 1.21 0.22 1.35
1.75 2.50 6.07 10.79 18.49 51.24 1.26 0.24 3.20
1.77 2.81 8.63 11.93 18.54 49.25 1.21 0.29 4.92
1.63 1.79 0.94 10.03 21.37 55.26 1.36 0.20 0.70
respectively). Regarding the fouling issue, all blends were in relatively high fouling formation ranges (FI ranged from 1.60 to 4.92 for blends) (Pronobis, 2005). Significant increases for FI values were observed when CF and PY-CF were added to the lignite than those of PW and PY-PW addition. This is ascribed to higher AAEM contents in agricultural biomass than woody biomass. When comparing the slagging and fouling problems of the blends, limited increase was observed by the use of pyrolytic biochars than raw biomass. The above results confirm that the slagging and fouling problems encountered during raw biomass and coal co-combustion were still present when pyrolytic biochars were co-combusted with coal, especially for agricultural pyrolytic biochars. To overcome these problems, some pre-treatment should be taken to reduce the AAEM contents in pyrolytic biochars prior to cocombustion with coal. It has been reported that washing treatment efficiently removed AAEMs originally contained in the raw biomass and that commodity fuels were prepared by combining washing and torrefaction (Saddawi et al., 2012). Therefore, washing treatment is a potential option to reduce the AAEM contents in the biochars and detailed slagging and fouling inclinations of biochar combustion by washing technique need further investigation. 4. Conclusions The low temperature pyrolysis significantly increased fuel quality of the biomass such as increased energy density and fuel ratio. The pyrolytic biochars exhibited improved combustion characteristics than their parent biomass. The addition of both raw biomass and pyrolytic biochar in lignite increased the reactivities of the blends. The synergistic interactions were obviously observed within all the blends and consequently improved thermal efficiencies of the blends were expected, especially for pyrolytic biochar/lignite blends. Compared to raw biomass/lignite blends, limited increases for slagging and fouling inclinations were observed for pyrolytic biochars/lignite blends. Acknowledgement This work was financially supported by M3TC (Minerals, Materials and Metals Technology Centre), NUS. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 07.072. References Abdullah, H., Wu, H., 2009. Biochars as a fuel: 1. Properties and grindability of biochars produced from the pyrolysis of mallee wood under slow-heating conditions. Energy Fuels 23, 4174–4181.
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