Bioresource Technology 200 (2016) 789–794
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Effect of fuel origin on synergy during co-gasification of biomass and coal in CO2 Yan Zhang ⇑, Yan Zheng, Mingjun Yang, Yongchen Song School of Energy and Power Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian City 116024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synergy effects during the co-
gasification of coal and biomass were assessed via a congress-mass TGA mode. Potassium species in biomass ash is a direct consequence of synergy during co-gasification. Potassium transfer from biomass to coal surface occurs during copyrolysis/gasification. No inhibition effect was observed in this study. Low-ash coal and K-rich biomass was the best combination to achieve synergy.
a r t i c l e
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Article history: Received 17 September 2015 Received in revised form 21 October 2015 Accepted 24 October 2015 Available online 30 October 2015 Keywords: Synergy Co-gasification Coal Biomass TGA
a b s t r a c t The effect of fuel origin on synergy in coal/biomass blends during co-gasification has been assessed using a congruent-mass thermogravimetry analysis (TGA) method. Results revealed that synergy occurs when ash residuals are formed, followed by an almost complete gasification of biomass. Potassium species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The SEM–EDS spectra provided conclusive evidence that the transfer of potassium from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica eliminated synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. The best result in terms of synergy was concluded to be the combination of low-ash coal and K-rich biomass. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays coal is the main feedstock used for energy production because of its large reserves, and it is expected to be applied as the energy resource for over 110 years (BP Statistical review, 2014). However, the huge consumption of coal over the past few decades has caused serious environmental impact locally and globally. Biomass is a renewable and clean energy source because of its ⇑ Corresponding author. Tel./fax: +86 411 8470 9684. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.biortech.2015.10.076 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
replenishment, carbon neutrality, and low sulfur content, which can supply about 14% of the world’s energy consumption (Saxena et al., 2009). However, biomass fuels in their original forms have also widely dispersed and low energy-density character, and thus it is cost-prohibitive to run a stand-alone biomass conversion plants (Craig and Mann, 1996). Combining biomass and coal as feedstock for energy production can offer several advantages in terms of environmental friendliness, production economy, and improved thermal efficiency (Craig and Mann, 1996). The environmental benefit of this approach is that the utilization of biomass can contribute to a
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CO2 neutral cycle (Hernandez et al., 2010). The economic superiority of co-processing biomass and coal lies in the economies of plant scale that can reduce specific operating costs to allow better use of biomass than in the case of constructing new decentralized plants fed exclusively with biomass (Saw and Pang, 2013). Aside from the direct economic and environmental benefits listed above, a number of researchers have found synergy in coprocessing of coal and biomass, in particular co-gasification. This synergy implies an attractive possibility of improving overall efficiency of co-processing systems. Previous studies on this issue have been conducted through different types of reactors such as thermogravimetric analyses (TGA) (Brown et al., 2000; Krerkkaiwan et al., 2013; Habibi et al., 2013; Ding et al., 2014), fixed-bed reactor (Howaniec and Smolinski, 2013; Fermoso et al., 2010; Rizkiana et al., 2014; Jeong et al., 2014), fluidized-bed reactor (Saw and Pang, 2013; Sjöström et al., 1999; Seo et al., 2010), and even an entrained-bed reactor (Hernandez et al., 2010). However, lack of synergy effects in coal/biomass blends during cogasification was also reported. Collot et al. (1999) found some hints of synergy in the volatile yield in a fixed-bed reactor, but they were too small to constitute clear evidence of synergy. Kumabe et al. (2007) did not find any sign of synergies either in product distribution or in gas composition and process efficiency. Similar findings were reported by Aigner et al. (2011), who conducted cogasification of coal and wood in a dual fluidized bed gasifier. Moreover, in addition to synergy and additivity effects, Habibi et al. (2013) and Ding et al. (2014) reported that an inhibiting effect was observed during co-gasification of certain coal/biomass blends. The authors explained that this inhibiting effect was attributed to the formation of KAlSiO4 and comparable gasification rates of biomass char and coal char. In summary, synergy, additivity and inhibition behaviors were reported to occur for similar types of processes. Therefore, the occurrence of synergy during coprocessing coal and biomass is generally inconclusive. Contradictions on the existence of synergy between coal and biomass suggest that this issue requires further systematic research. In our previous paper (Zhang et al., 2015), we reported that the sample mass dependence of the char reactivity could be misdiagnosed as synergy or inhibition during co-gasification of coal and biomass, typically in the case of using conventional TGA as an experimental measure. A congruent-mass TGA method has been developed to overcome the limitations of the conventional TGA mode, which provides reasonable and reliable information on the synergy effect during co-gasification of coal and biomass (Zhang et al., 2015). This study reports further progress on this issue. Among all possible factors of governing synergy during coprocessing biomass and coal, we believe that fuel origin may be an important determinant that should be considered. The purpose of this study is to investigate any synergies that result from different characters of fuels: biomass samples with different contents of potassium and silica and coal samples with different ranks and ash contents.
100 lm in a hammer mill and dried at 105 °C for 2 h before use. The proximate and ultimate analyses of coal and biomass samples are shown in Table 1. The chemical compositions of the ashes derived from coal and biomass were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and were reported as the percentage of the total weight for each metal elemental oxide. The results are also given in Table 1.
2.2. Apparatus A simultaneous thermal analyzer (Mettler-Toledo TGA/DSC1100LF) was used for co-gasification of coal and biomass. It consists of a furnace chamber, a sample holder that connects to a horizontal arm, and a gas-intake tube. The furnace chamber is an alumina tube with an inner diameter of 1.3 cm and heated with an electric heater. The program temperature was controlled by a furnace thermocouple located on the alumina tube wall near the elliptical disc. The sample holder is a hollow alumina elliptical disc that is divided into reference and sample zones. The gas-intake tube is installed over the horizontal arm, which located in front of the outlet of the gas-intake where the reactant gas flows across the crucible mouth.
2.3. Combined co-pyrolysis/gasification tests Combined co-pyrolysis/gasification tests of coal and biomass samples were performed via a congruent-mass TGA mode. The detailed procedures of this method have been reported elsewhere (Zhang et al., 2015). Briefly, the following two experimental runs were conducted: in the first TGA run, 20 ± 0.1 mg of coal and 20 ± 0.1 mg of biomass were filled into two crucibles. The two crucibles were then placed in the sample and reference zones of the elliptical disc for subsequent TGA test. In the second TGA run, two crucibles were used to fill the coal/biomass blends with the mass blending ratio of 1:1. Each of them contains 10 ± 0.1 mg of coal and 10 ± 0.1 mg of biomass. Both the total sample mass (40 mg) and the individual masses of coal (20 mg) and biomass (20 mg) were exactly congruent with those used in the first TGA run. The two crucibles were then located on the sample and reference zones of the elliptical disc. For each experimental run, the sample-filled crucibles were heated under a nitrogen flow of 200 mL/min and heating rate of 50 °C/min to 900 °C, followed by an isothermal holding at this temperature for 10 min. The isothermal gasification of the sample was initiated by switching to a CO2 flow of 200 mL/min and completed until weight loss reached constant. In all cases, baseline correction was conducted by subtracting a ‘‘brank” signal that was recorded with an empty crucible from the TGA data of the sample determined under the same conditions. Char conversion (x) and isothermal reactivity (r, s1) of the char during the isothermal gasification step were calculated by Eqs. (1) and (2), respectively:
2. Experimental 2.1. Materials and analyses Three coals were used in this study. They were selected for variations in both coal rank and ash content: a bituminous coal (BC) from Shaanxi Province in China, a high ash lignite from the Inner Mongolia Autonomous Region in north of China (LC), and a low ash lignite from Indonesia (LI). Four kinds of biomass feedstocks were selected from either forestry wastes or agricultural residuals, including Chinese redwood (CR), soybean stalk (SS), orange peel (OP), and peanut shells (PS). All samples were pulverized to below
x¼
m0 mt m0 me
ð1Þ
r¼
dx dt
ð2Þ
where m0 is the initial sample mass at the start of the gasification; and mt is the instantaneous mass at gasification time t; and me is the sample mass at the end of gasification, which corresponds to the mass of the ash.
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2.4. Microscopic images and chemical composition of particle surface The surface state of coal and biomass particles before and after pyrolysis/gasification was observed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) (QUANTA450). Three sets of blended BC/SS samples were prepared for SEM–EDS analysis. The first one is the original BC/SS blend before co-pyrolysis/gasification; the second one is the BC/SS blended char obtained after pyrolysis at 900 °C with 10 min holding time; and the third one is the BC/SS blended char obtained when gasification conversion reached 0.5. The elemental composition of a sample is determined by examining the characteristic X-ray spectrum of the specimen. Elemental analysis was performed in ‘‘spot mode’’, in which the beam is localized on a given area manually chosen within the field of view. Each original sample and its derived chars were characterized by randomly selecting 3–4 particles of view.
3. Results and discussion 3.1. Characterization of original biomass and coal Table 1 presents the ultimate and proximate analyses of the investigated coal and biomass samples. LC and LI are apparently similar in coal rank and elemental composition, but significantly different in ash content (17.2 wt% in LC and 1.9 wt% in LI). BC is a higher-ranked coal than LC and LI, with the highest fixed carbon content among the three coal samples studied. Biomass samples commonly contain 71–78 wt% volatile matter, more than 2–3 times as much as the three coals contain. The ash content of biomass samples ranges from ca. 3 wt% to ca. 5 wt%. Table 1 also shows the chemical compositions of ashes derived from the three coals and four biomass samples. Significant differences in the mineral-phase chemical compositions of the three coal ashes were observed, particularly with respect to their CaO, SiO2, and Fe2O3 contents. The CaO content in BC ash is as high as 36.6 wt% but only 12 wt%–13 wt% in LC and LI ashes. The SiO2 content in LC ash is 53.5 wt%, which is more than twice that in LI ash (25.9 wt%). Table 1 indicates that CR ash has the highest CaO con-
tent of 58.3 wt% among all the studied samples, whereas SS and OP are more dominant in K2O with 35.4 wt% and 36.3 wt%, respectively, than in CaO with 17.1 wt% and 15.1 wt%, respectively. Among four biomass samples studied, only PS contains relatively higher SiO2 content (48.6 wt%), more than 2–3 times as much as the other three biomass origins contain (20–23 wt%).
3.2. Combined pyrolysis/gasification behavior of individual fuels Fig. 1 shows the weight loss (wt%) of individual coal and biomass samples during combined pyrolysis and gasification at 900 °C. These results were obtained by conventional TGA mode (a single crucible was used as a sample container); the initial sample mass of each sample used in this set of TGA tests was 20 ± 0.1 mg. Weight loss observed during non-isothermal pyrolysis and isothermal holding steps was principally attributed to devolatilization and decomposition processes that are responsible for the formation of water, gas, and tar. In Fig. 1, the weight losses of BC, LC, and LI coals at the pyrolysis step are 34.5 wt%, 35.1 wt%, and 51.7 wt%, respectively. These values are close to the results of volatile matter shown in Table 1. Likewise, the weight losses of four biomass samples that undergo pyrolysis are also close to that of volatile matter, ranging from 74 wt% to 78 wt%. The weight loss of all samples during isothermal gasification step is primarily a result of the gas–solid reaction process between CO2 and char produced in-situ.
3.3. Effect of biomass origin on synergy during co-gasification of coal and biomass Fig. 2 shows the TG curves of CR/BC and SS/BC sample sets obtained by congruent-mass TGA mode. The TG curve of each separated sample set at the non-isothermal pyrolysis and isothermal holding steps is almost entirely superposed with the TG curve of the corresponding blended sample set regardless of biomass origins. These results provided conclusive evidence that no synergy or interaction occurs between BC coal and biomass samples during the co-pyrolysis step.
Table 1 Properties of coal and biomass samples.
a
Abbreviation
BC
LC
LI
CR
SS
Classification
Bituminous
Lignite
Lignite
Forestry wastes
Agricultural residuals
OP
Origin
China
PS
China
Indonesia
China
China
China
China
Proximate analysis (wt%, db) Ash 6.3 Volatiles 35.3 Fixed carbon 58.4
17.2 37.9 44.9
1.9 49.7 48.4
3.5 75.1 21.4
5.3 75.9 21.6
2.8 75.6 21.6
8.0 71.2 20.8
Ultimate analysis (wt%, db) C 75.8 H 4.5 N 0.9 (O + S)a 12.5
62.8 4.0 1.0 15.0
71.2 5.1 0.7 42.6
46.1 5.8 0.3 44.3
44.2 5.6 0.9 44.0
45.4 5.9 1.3 44.6
47.1 5.7 2.1 37.1
Ash composition (wt%, db) SiO2 28.5 Al2O3 11.6 Fe2O3 10.3 CaO 36.6 MgO 1.6 Na2O 1.6 K2O 0.6 TiO2 0.1 P2 O5 0.3 SO3 7.9
53.5 19.1 3.7 13.6 1.1 1.2 1.7 0.2 0.5 3.5
25.9 19.4 24.4 12.1 3.1 0.2 0.6 0.9 0.3 5.2
20.6 2.2 2.7 58.3 5.1 1.7 6.8 0.1 2.1 0.2
22.6 3.7 1.7 17.1 8.1 4.1 37.4 0.4 3.2 0.7
19.8 3.5 0.2 20.1 8.3 3.1 34.3 0.1 3.4 4.3
48.6 8.6 3.1 6.8 4.3 1.7 14.8 0.6 5.5 3.6
By difference.
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0
N2
CO2
TG [wt%]
-20
(a) BC LC LI CR OP PS SS
-40 -60 -80 -100 Time [min]
(b)
Fig. 1. Weight loss of individual coal and biomass during the combined pyrolysis/gasification.
To visually evaluate the synergy effect in coal/biomass blends during the CO2 gasification step, char conversions are plotted as a function of reaction time. Fig. 3 shows x–t curves of separated and blended BC chars along with four individual biomass chars. Because no interaction was observed between BC and biomass feedstocks during pyrolysis step (Fig. 2), the masses of the individual BC and biomass char in the two sets of TGA tests (separated and blended) should be nearly the same. Fig. 3 indicates that the overall gasification times required for complete gasification (taking 0.95 char conversion as a reference) are approximately 21 min for the four separated sample sets, regardless of biomass origins. By contrast, the times of the four blended sample sets are significantly different, showing strong biomass-origin dependence, i.e., the gasification times to reach 0.95 char conversion for the PS/BC, CR/BC, OP/BC, and SS/BC blending chars are approximately 18, 15, 9, and 7 min, respectively. Specifically, Fig. 3(a) indicates that the x–t curve of blended BC and PS chars nearly overlaps with that of their separated ones over the entire conversion range, which suggests that little synergy or interaction occurs between BC and PS. This scenario is the only case among the four biomass origins studied.
(a)
(b)
Fig. 2. TG curves of separated and blended (a) CR/BC and (b) SS/BC sample sets during the combined co-pyrolysis/gasification.
(c)
(d)
Fig. 3. Char conversion as a function of reaction time during co-gasification of separated and blended (a) PS/BC, (b) CR/BC, (c) OP/BC, and (d) SS/BC.
For the other three biomass origins, however, the x–t curve of each blended coal/biomass set always overlapped with that of the separated one until certain conversion, after which the gasification of the blended set is clearly faster than that of separated one (Fig. 3(b–d)). The results imply that the promoted reactivity or synergy occurs in the blends of BC char with CR, OP, and SS chars after the co-gasification reaction progress to certain conversions. In Fig. 3(b–d), the x–t curves of blended BC char with CR, OP, and SS chars branched at the gasification times to reach approximately 6.1, 3.7, and 1.6 min, respectively. These branching times are approximately equal to the overall time of individual biomass chars required for their complete gasification (to reach 0.95 conversion), i.e., 5.9, 3.6, and 1.7 min for CR, OP, and SS chars, respectively. These results strongly suggest that synergy started with ash formation followed by complete gasification of the biomass chars.
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In other words, biomass ashes play an important role in catalyzing the gasification reaction of coal char. Further assessment of the effect of biomass origin on synergy has been performed by using a synergy index (SI), which is defined as follows:
SI ¼
tx¼0:95 se tx¼0:95 bl
ð3Þ
where tx¼0:95 and tx¼0:95 are the gasification times required when se bl conversion reached x = 0.95 of separated and blended chars. According to the definition of SI in Eq. (3), a larger the magnitude of SI corresponds to a greater degree of synergy of the blends. Based on Fig. 3, the SI values of PS/BC, CR/BC, OP/BC, and SS/BC blending chars are calculated as 1.18, 1.41, 2.44, and 3.13, respectively. The differences in the degree of synergy among the four blended chars should correlate with the chemical composition of biomass ash. Interparticle mobility of alkali metals in biomass enables them to transfer from a biomass feedstock to a second feedstock, which weakens the strength of C–C bond, thus promoting the reaction rate of the char with CO2. The fact that the higher K content of OP and SS ashes in Table 1 (34.3 wt% and 37.4 wt%, respectively) corresponds to larger SI magnitudes of OP/BC and SS/BC blends (2.44 and 3.13, respectively) supports this view. However, the comparison between CR/BC and PS/BC blends seemed to be contradictory. The content of alkali metals in PS ash (14.8 wt %) is higher than that in CR ash (8.5 wt%), while the SI magnitude of PS/BC blend (1.18) is lower than that of CR/BC blend (1.41). This exception may be connected with the higher silica content of PS ash (48.6 wt%). This component is known to react with alkali metals to form silicates and reduce catalytic activity of the alkali species (Risnes et al., 2003). However, in spite of higher silicate content of PS ash, no inhibition effect was observed during the co-gasification of PS and BC (Fig. 3(a)). This result was different from the observation reported by Ding et al. (2014), who observed obvious inhibiting effects during co-gasification of coal/biomass blends in steam via a conventional TGA mode, i.e., coal/biomass mixtures gasified more slowly than coal itself. Further efforts have been made to certify the role of alkali metals, particularly potassium, on the synergy during co-gasification of coal and biomass. For this purpose, SEM-EDX was used to analyze the surface elemental composition of coal and biomass particles before and after co-pyrolysis/gasification. The SEM-EDX spectra revealed that potassium concentration at the surface of the BC particles was different before and after co-pyrolysis/gasification. The spectrum of the original BC shows mainly mineral elements of Ca, Al and Si. No K is observed in the spectrum of the original BC. By contrast, the spectra of co-pyrolyzed and co-gasified BC chars with SS indicate strong K peaks, which provide conclusive evidence that the transformation of K from SS to the surface of BC char occurred during co-pyrolysis/gasification. The focus is to know how such transfer of K affects char reactivity. As shown in Fig. 3 (d), the x–t curves of blended and separated SS/BC chars overlapped until the reaction time reached ca. 1.6 min. This result implies that K species transferred from SS to BC in pyrolysis step was a non-catalytic activity for the initial gasification of BC char. This phenomenon may be explained by the chemical structure of K species formed during pyrolysis. For example, K species that adhered on the surface of BC char immediately after co-pyrolysis was mainly attributed to the volatile K released from the SS, most of which should be chemically bounded with cellulosic fragments. Moreover, the gasification of the blended SS/BC char enhanced after the gasification reaction progress to ca. 1.6 min compared with the separated one. This reaction time is approximately equal to the overall time of the individual SS char (1.7 min). Thus, K species detected on the surface of BC char at conversion of 0.5 are
mainly attributed to the free mineral K in ash produced from SS. This result suggests that K species that exist in ash phase are catalytic activity, which is a direct response of synergy in SS/BC blends.
3.4. Effect of coal type on synergy during co-gasification of coal and biomass Fig. 4 shows the x–t curves of OP char with the three coal chars under CO2 atmosphere at 900 °C. The results indicate evident coalorigin dependence for both separated and blended chars. First, the times required to reach 0.95 conversion of separated LC/OP, LI/OP, and BC/OP chars are 20, 83, and 21.3 min, respectively. The difference in the reaction times among the three separated char sets is primarily attributed to the difference in the gasification reactivity of individual LC, LI, and BC chars. Second, Fig. 4 shows that the gasification of each blended set is clearly faster than its corresponding separated one; the times required to reach 0.95 conversion for LC/ OP, LI/OP, and BC/OP char blends are 9.9, 8.4, and 9.1 min, respectively. The discrepancy in the reaction time between separate and blended chars should reflect the degree of synergy among the three blended char sets. Essentially, the SI values of LC/OP, LI/OP, and BC/ OP blends are 2.02, 10.34, and 2.34, respectively (signed in Fig. 4). Basically, the reasons behind the difference in magnitude of SI must be examined from the chemical and compositional properties
(a)
(b)
(c)
Fig. 4. Char conversion as a function of reaction time during co-gasification of separated and blended (a) LC/OP, (b) LI/OP, and (c) BC/OP chars.
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of coals. Table 1 indicates evident or remarkable differences in the properties of the three coal samples studied. First, LC and LI are apparently similar in coal rank and younger than that of BC. However, the fact that the SI value of BC (2.34) is between those of LC (2.02) and LI (10.34) implies that coal rank has no relation to SI. Second, LC coal has the highest ash content (17.2 wt%) among the three coal samples studied, being 9.1 and 2.7 times larger than those of LI (1.9 wt%) and BC (6.3 wt%) coals, respectively. Clearly, the ash contents of the three coals correlate with their SI values. Many researchers also emphasized the effect of ash on gasification behavior (Habibi et al., 2013; Ding et al., 2014). Third, Table 1 also indicates that LC coal ash contains high alumina and silica contents than LI and BC. These two components can deactivate K and Ca by forming catalytically inactive silicates, thereby reducing subsequent char gasification rates (Risnes et al., 2003). The above results demonstrate that ash content and its chemical composition of coal play an important role in governing the magnitude of SI. Although some authors have reported inhibition effects during cogasification of coal and biomass chars with higher silica or alumina content by using conventional TGA (Habibi et al., 2013; Ding et al., 2014), different insights are obtained by congruent-mass TGA mode in this study. That is, whether in coal and biomass, higher silica, or alumina content, inhibition effect was not observed during the co-gasification of coal and biomass. 4. Conclusions Congruent-mass TGA provided conclusive evidence that synergy occurs only when free mineral potassium species are formed after complete gasification of biomass in coal/biomass blends. The SEM–EDS spectra revealed that the transfer of potassium from biomass to the surface of coal chars occurs during co-pyrolysis/ gasification. High silica content in biomass could eliminate the catalytic activity of potassium, but not to the extent of inhibiting the reaction of coal/biomass blended char and causing it to become slower than the reaction of separated one. The combination of low ash-coal and K-rich biomass was the best choice with respect to synergy achievement. Acknowledgement The authors are grateful to the financial supports from the National Natural Science Foundation of China (Grant No. 51376031).
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of fuel origin on synergy during co-gasification of biomass and coal in CO2 Yan Zhang ⇑, Yan Zheng, Mingjun Yang, Yongchen Song School of Energy and Power Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian City 116024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synergy effects during the co-
gasification of coal and biomass were assessed via a congress-mass TGA mode. Potassium species in biomass ash is a direct consequence of synergy during co-gasification. Potassium transfer from biomass to coal surface occurs during copyrolysis/gasification. No inhibition effect was observed in this study. Low-ash coal and K-rich biomass was the best combination to achieve synergy.
a r t i c l e
i n f o
Article history: Received 17 September 2015 Received in revised form 21 October 2015 Accepted 24 October 2015 Available online 30 October 2015 Keywords: Synergy Co-gasification Coal Biomass TGA
a b s t r a c t The effect of fuel origin on synergy in coal/biomass blends during co-gasification has been assessed using a congruent-mass thermogravimetry analysis (TGA) method. Results revealed that synergy occurs when ash residuals are formed, followed by an almost complete gasification of biomass. Potassium species in biomass ash play a catalytic role in promoting gasification reactivity of coal char, which is a direct consequence of synergy during co-gasification. The SEM–EDS spectra provided conclusive evidence that the transfer of potassium from biomass to the surface of coal char occurs during co-pyrolysis/gasification. Biomass ash rich in silica eliminated synergy in coal/biomass blends but not to the extent of inhibiting the reaction rate of the blended chars to make it slower than that of separated ones. The best result in terms of synergy was concluded to be the combination of low-ash coal and K-rich biomass. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays coal is the main feedstock used for energy production because of its large reserves, and it is expected to be applied as the energy resource for over 110 years (BP Statistical review, 2014). However, the huge consumption of coal over the past few decades has caused serious environmental impact locally and globally. Biomass is a renewable and clean energy source because of its ⇑ Corresponding author. Tel./fax: +86 411 8470 9684. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.biortech.2015.10.076 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
replenishment, carbon neutrality, and low sulfur content, which can supply about 14% of the world’s energy consumption (Saxena et al., 2009). However, biomass fuels in their original forms have also widely dispersed and low energy-density character, and thus it is cost-prohibitive to run a stand-alone biomass conversion plants (Craig and Mann, 1996). Combining biomass and coal as feedstock for energy production can offer several advantages in terms of environmental friendliness, production economy, and improved thermal efficiency (Craig and Mann, 1996). The environmental benefit of this approach is that the utilization of biomass can contribute to a
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CO2 neutral cycle (Hernandez et al., 2010). The economic superiority of co-processing biomass and coal lies in the economies of plant scale that can reduce specific operating costs to allow better use of biomass than in the case of constructing new decentralized plants fed exclusively with biomass (Saw and Pang, 2013). Aside from the direct economic and environmental benefits listed above, a number of researchers have found synergy in coprocessing of coal and biomass, in particular co-gasification. This synergy implies an attractive possibility of improving overall efficiency of co-processing systems. Previous studies on this issue have been conducted through different types of reactors such as thermogravimetric analyses (TGA) (Brown et al., 2000; Krerkkaiwan et al., 2013; Habibi et al., 2013; Ding et al., 2014), fixed-bed reactor (Howaniec and Smolinski, 2013; Fermoso et al., 2010; Rizkiana et al., 2014; Jeong et al., 2014), fluidized-bed reactor (Saw and Pang, 2013; Sjöström et al., 1999; Seo et al., 2010), and even an entrained-bed reactor (Hernandez et al., 2010). However, lack of synergy effects in coal/biomass blends during cogasification was also reported. Collot et al. (1999) found some hints of synergy in the volatile yield in a fixed-bed reactor, but they were too small to constitute clear evidence of synergy. Kumabe et al. (2007) did not find any sign of synergies either in product distribution or in gas composition and process efficiency. Similar findings were reported by Aigner et al. (2011), who conducted cogasification of coal and wood in a dual fluidized bed gasifier. Moreover, in addition to synergy and additivity effects, Habibi et al. (2013) and Ding et al. (2014) reported that an inhibiting effect was observed during co-gasification of certain coal/biomass blends. The authors explained that this inhibiting effect was attributed to the formation of KAlSiO4 and comparable gasification rates of biomass char and coal char. In summary, synergy, additivity and inhibition behaviors were reported to occur for similar types of processes. Therefore, the occurrence of synergy during coprocessing coal and biomass is generally inconclusive. Contradictions on the existence of synergy between coal and biomass suggest that this issue requires further systematic research. In our previous paper (Zhang et al., 2015), we reported that the sample mass dependence of the char reactivity could be misdiagnosed as synergy or inhibition during co-gasification of coal and biomass, typically in the case of using conventional TGA as an experimental measure. A congruent-mass TGA method has been developed to overcome the limitations of the conventional TGA mode, which provides reasonable and reliable information on the synergy effect during co-gasification of coal and biomass (Zhang et al., 2015). This study reports further progress on this issue. Among all possible factors of governing synergy during coprocessing biomass and coal, we believe that fuel origin may be an important determinant that should be considered. The purpose of this study is to investigate any synergies that result from different characters of fuels: biomass samples with different contents of potassium and silica and coal samples with different ranks and ash contents.
100 lm in a hammer mill and dried at 105 °C for 2 h before use. The proximate and ultimate analyses of coal and biomass samples are shown in Table 1. The chemical compositions of the ashes derived from coal and biomass were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and were reported as the percentage of the total weight for each metal elemental oxide. The results are also given in Table 1.
2.2. Apparatus A simultaneous thermal analyzer (Mettler-Toledo TGA/DSC1100LF) was used for co-gasification of coal and biomass. It consists of a furnace chamber, a sample holder that connects to a horizontal arm, and a gas-intake tube. The furnace chamber is an alumina tube with an inner diameter of 1.3 cm and heated with an electric heater. The program temperature was controlled by a furnace thermocouple located on the alumina tube wall near the elliptical disc. The sample holder is a hollow alumina elliptical disc that is divided into reference and sample zones. The gas-intake tube is installed over the horizontal arm, which located in front of the outlet of the gas-intake where the reactant gas flows across the crucible mouth.
2.3. Combined co-pyrolysis/gasification tests Combined co-pyrolysis/gasification tests of coal and biomass samples were performed via a congruent-mass TGA mode. The detailed procedures of this method have been reported elsewhere (Zhang et al., 2015). Briefly, the following two experimental runs were conducted: in the first TGA run, 20 ± 0.1 mg of coal and 20 ± 0.1 mg of biomass were filled into two crucibles. The two crucibles were then placed in the sample and reference zones of the elliptical disc for subsequent TGA test. In the second TGA run, two crucibles were used to fill the coal/biomass blends with the mass blending ratio of 1:1. Each of them contains 10 ± 0.1 mg of coal and 10 ± 0.1 mg of biomass. Both the total sample mass (40 mg) and the individual masses of coal (20 mg) and biomass (20 mg) were exactly congruent with those used in the first TGA run. The two crucibles were then located on the sample and reference zones of the elliptical disc. For each experimental run, the sample-filled crucibles were heated under a nitrogen flow of 200 mL/min and heating rate of 50 °C/min to 900 °C, followed by an isothermal holding at this temperature for 10 min. The isothermal gasification of the sample was initiated by switching to a CO2 flow of 200 mL/min and completed until weight loss reached constant. In all cases, baseline correction was conducted by subtracting a ‘‘brank” signal that was recorded with an empty crucible from the TGA data of the sample determined under the same conditions. Char conversion (x) and isothermal reactivity (r, s1) of the char during the isothermal gasification step were calculated by Eqs. (1) and (2), respectively:
2. Experimental 2.1. Materials and analyses Three coals were used in this study. They were selected for variations in both coal rank and ash content: a bituminous coal (BC) from Shaanxi Province in China, a high ash lignite from the Inner Mongolia Autonomous Region in north of China (LC), and a low ash lignite from Indonesia (LI). Four kinds of biomass feedstocks were selected from either forestry wastes or agricultural residuals, including Chinese redwood (CR), soybean stalk (SS), orange peel (OP), and peanut shells (PS). All samples were pulverized to below
x¼
m0 mt m0 me
ð1Þ
r¼
dx dt
ð2Þ
where m0 is the initial sample mass at the start of the gasification; and mt is the instantaneous mass at gasification time t; and me is the sample mass at the end of gasification, which corresponds to the mass of the ash.
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2.4. Microscopic images and chemical composition of particle surface The surface state of coal and biomass particles before and after pyrolysis/gasification was observed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) (QUANTA450). Three sets of blended BC/SS samples were prepared for SEM–EDS analysis. The first one is the original BC/SS blend before co-pyrolysis/gasification; the second one is the BC/SS blended char obtained after pyrolysis at 900 °C with 10 min holding time; and the third one is the BC/SS blended char obtained when gasification conversion reached 0.5. The elemental composition of a sample is determined by examining the characteristic X-ray spectrum of the specimen. Elemental analysis was performed in ‘‘spot mode’’, in which the beam is localized on a given area manually chosen within the field of view. Each original sample and its derived chars were characterized by randomly selecting 3–4 particles of view.
3. Results and discussion 3.1. Characterization of original biomass and coal Table 1 presents the ultimate and proximate analyses of the investigated coal and biomass samples. LC and LI are apparently similar in coal rank and elemental composition, but significantly different in ash content (17.2 wt% in LC and 1.9 wt% in LI). BC is a higher-ranked coal than LC and LI, with the highest fixed carbon content among the three coal samples studied. Biomass samples commonly contain 71–78 wt% volatile matter, more than 2–3 times as much as the three coals contain. The ash content of biomass samples ranges from ca. 3 wt% to ca. 5 wt%. Table 1 also shows the chemical compositions of ashes derived from the three coals and four biomass samples. Significant differences in the mineral-phase chemical compositions of the three coal ashes were observed, particularly with respect to their CaO, SiO2, and Fe2O3 contents. The CaO content in BC ash is as high as 36.6 wt% but only 12 wt%–13 wt% in LC and LI ashes. The SiO2 content in LC ash is 53.5 wt%, which is more than twice that in LI ash (25.9 wt%). Table 1 indicates that CR ash has the highest CaO con-
tent of 58.3 wt% among all the studied samples, whereas SS and OP are more dominant in K2O with 35.4 wt% and 36.3 wt%, respectively, than in CaO with 17.1 wt% and 15.1 wt%, respectively. Among four biomass samples studied, only PS contains relatively higher SiO2 content (48.6 wt%), more than 2–3 times as much as the other three biomass origins contain (20–23 wt%).
3.2. Combined pyrolysis/gasification behavior of individual fuels Fig. 1 shows the weight loss (wt%) of individual coal and biomass samples during combined pyrolysis and gasification at 900 °C. These results were obtained by conventional TGA mode (a single crucible was used as a sample container); the initial sample mass of each sample used in this set of TGA tests was 20 ± 0.1 mg. Weight loss observed during non-isothermal pyrolysis and isothermal holding steps was principally attributed to devolatilization and decomposition processes that are responsible for the formation of water, gas, and tar. In Fig. 1, the weight losses of BC, LC, and LI coals at the pyrolysis step are 34.5 wt%, 35.1 wt%, and 51.7 wt%, respectively. These values are close to the results of volatile matter shown in Table 1. Likewise, the weight losses of four biomass samples that undergo pyrolysis are also close to that of volatile matter, ranging from 74 wt% to 78 wt%. The weight loss of all samples during isothermal gasification step is primarily a result of the gas–solid reaction process between CO2 and char produced in-situ.
3.3. Effect of biomass origin on synergy during co-gasification of coal and biomass Fig. 2 shows the TG curves of CR/BC and SS/BC sample sets obtained by congruent-mass TGA mode. The TG curve of each separated sample set at the non-isothermal pyrolysis and isothermal holding steps is almost entirely superposed with the TG curve of the corresponding blended sample set regardless of biomass origins. These results provided conclusive evidence that no synergy or interaction occurs between BC coal and biomass samples during the co-pyrolysis step.
Table 1 Properties of coal and biomass samples.
a
Abbreviation
BC
LC
LI
CR
SS
Classification
Bituminous
Lignite
Lignite
Forestry wastes
Agricultural residuals
OP
Origin
China
PS
China
Indonesia
China
China
China
China
Proximate analysis (wt%, db) Ash 6.3 Volatiles 35.3 Fixed carbon 58.4
17.2 37.9 44.9
1.9 49.7 48.4
3.5 75.1 21.4
5.3 75.9 21.6
2.8 75.6 21.6
8.0 71.2 20.8
Ultimate analysis (wt%, db) C 75.8 H 4.5 N 0.9 (O + S)a 12.5
62.8 4.0 1.0 15.0
71.2 5.1 0.7 42.6
46.1 5.8 0.3 44.3
44.2 5.6 0.9 44.0
45.4 5.9 1.3 44.6
47.1 5.7 2.1 37.1
Ash composition (wt%, db) SiO2 28.5 Al2O3 11.6 Fe2O3 10.3 CaO 36.6 MgO 1.6 Na2O 1.6 K2O 0.6 TiO2 0.1 P2 O5 0.3 SO3 7.9
53.5 19.1 3.7 13.6 1.1 1.2 1.7 0.2 0.5 3.5
25.9 19.4 24.4 12.1 3.1 0.2 0.6 0.9 0.3 5.2
20.6 2.2 2.7 58.3 5.1 1.7 6.8 0.1 2.1 0.2
22.6 3.7 1.7 17.1 8.1 4.1 37.4 0.4 3.2 0.7
19.8 3.5 0.2 20.1 8.3 3.1 34.3 0.1 3.4 4.3
48.6 8.6 3.1 6.8 4.3 1.7 14.8 0.6 5.5 3.6
By difference.
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0
N2
CO2
TG [wt%]
-20
(a) BC LC LI CR OP PS SS
-40 -60 -80 -100 Time [min]
(b)
Fig. 1. Weight loss of individual coal and biomass during the combined pyrolysis/gasification.
To visually evaluate the synergy effect in coal/biomass blends during the CO2 gasification step, char conversions are plotted as a function of reaction time. Fig. 3 shows x–t curves of separated and blended BC chars along with four individual biomass chars. Because no interaction was observed between BC and biomass feedstocks during pyrolysis step (Fig. 2), the masses of the individual BC and biomass char in the two sets of TGA tests (separated and blended) should be nearly the same. Fig. 3 indicates that the overall gasification times required for complete gasification (taking 0.95 char conversion as a reference) are approximately 21 min for the four separated sample sets, regardless of biomass origins. By contrast, the times of the four blended sample sets are significantly different, showing strong biomass-origin dependence, i.e., the gasification times to reach 0.95 char conversion for the PS/BC, CR/BC, OP/BC, and SS/BC blending chars are approximately 18, 15, 9, and 7 min, respectively. Specifically, Fig. 3(a) indicates that the x–t curve of blended BC and PS chars nearly overlaps with that of their separated ones over the entire conversion range, which suggests that little synergy or interaction occurs between BC and PS. This scenario is the only case among the four biomass origins studied.
(a)
(b)
Fig. 2. TG curves of separated and blended (a) CR/BC and (b) SS/BC sample sets during the combined co-pyrolysis/gasification.
(c)
(d)
Fig. 3. Char conversion as a function of reaction time during co-gasification of separated and blended (a) PS/BC, (b) CR/BC, (c) OP/BC, and (d) SS/BC.
For the other three biomass origins, however, the x–t curve of each blended coal/biomass set always overlapped with that of the separated one until certain conversion, after which the gasification of the blended set is clearly faster than that of separated one (Fig. 3(b–d)). The results imply that the promoted reactivity or synergy occurs in the blends of BC char with CR, OP, and SS chars after the co-gasification reaction progress to certain conversions. In Fig. 3(b–d), the x–t curves of blended BC char with CR, OP, and SS chars branched at the gasification times to reach approximately 6.1, 3.7, and 1.6 min, respectively. These branching times are approximately equal to the overall time of individual biomass chars required for their complete gasification (to reach 0.95 conversion), i.e., 5.9, 3.6, and 1.7 min for CR, OP, and SS chars, respectively. These results strongly suggest that synergy started with ash formation followed by complete gasification of the biomass chars.
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In other words, biomass ashes play an important role in catalyzing the gasification reaction of coal char. Further assessment of the effect of biomass origin on synergy has been performed by using a synergy index (SI), which is defined as follows:
SI ¼
tx¼0:95 se tx¼0:95 bl
ð3Þ
where tx¼0:95 and tx¼0:95 are the gasification times required when se bl conversion reached x = 0.95 of separated and blended chars. According to the definition of SI in Eq. (3), a larger the magnitude of SI corresponds to a greater degree of synergy of the blends. Based on Fig. 3, the SI values of PS/BC, CR/BC, OP/BC, and SS/BC blending chars are calculated as 1.18, 1.41, 2.44, and 3.13, respectively. The differences in the degree of synergy among the four blended chars should correlate with the chemical composition of biomass ash. Interparticle mobility of alkali metals in biomass enables them to transfer from a biomass feedstock to a second feedstock, which weakens the strength of C–C bond, thus promoting the reaction rate of the char with CO2. The fact that the higher K content of OP and SS ashes in Table 1 (34.3 wt% and 37.4 wt%, respectively) corresponds to larger SI magnitudes of OP/BC and SS/BC blends (2.44 and 3.13, respectively) supports this view. However, the comparison between CR/BC and PS/BC blends seemed to be contradictory. The content of alkali metals in PS ash (14.8 wt %) is higher than that in CR ash (8.5 wt%), while the SI magnitude of PS/BC blend (1.18) is lower than that of CR/BC blend (1.41). This exception may be connected with the higher silica content of PS ash (48.6 wt%). This component is known to react with alkali metals to form silicates and reduce catalytic activity of the alkali species (Risnes et al., 2003). However, in spite of higher silicate content of PS ash, no inhibition effect was observed during the co-gasification of PS and BC (Fig. 3(a)). This result was different from the observation reported by Ding et al. (2014), who observed obvious inhibiting effects during co-gasification of coal/biomass blends in steam via a conventional TGA mode, i.e., coal/biomass mixtures gasified more slowly than coal itself. Further efforts have been made to certify the role of alkali metals, particularly potassium, on the synergy during co-gasification of coal and biomass. For this purpose, SEM-EDX was used to analyze the surface elemental composition of coal and biomass particles before and after co-pyrolysis/gasification. The SEM-EDX spectra revealed that potassium concentration at the surface of the BC particles was different before and after co-pyrolysis/gasification. The spectrum of the original BC shows mainly mineral elements of Ca, Al and Si. No K is observed in the spectrum of the original BC. By contrast, the spectra of co-pyrolyzed and co-gasified BC chars with SS indicate strong K peaks, which provide conclusive evidence that the transformation of K from SS to the surface of BC char occurred during co-pyrolysis/gasification. The focus is to know how such transfer of K affects char reactivity. As shown in Fig. 3 (d), the x–t curves of blended and separated SS/BC chars overlapped until the reaction time reached ca. 1.6 min. This result implies that K species transferred from SS to BC in pyrolysis step was a non-catalytic activity for the initial gasification of BC char. This phenomenon may be explained by the chemical structure of K species formed during pyrolysis. For example, K species that adhered on the surface of BC char immediately after co-pyrolysis was mainly attributed to the volatile K released from the SS, most of which should be chemically bounded with cellulosic fragments. Moreover, the gasification of the blended SS/BC char enhanced after the gasification reaction progress to ca. 1.6 min compared with the separated one. This reaction time is approximately equal to the overall time of the individual SS char (1.7 min). Thus, K species detected on the surface of BC char at conversion of 0.5 are
mainly attributed to the free mineral K in ash produced from SS. This result suggests that K species that exist in ash phase are catalytic activity, which is a direct response of synergy in SS/BC blends.
3.4. Effect of coal type on synergy during co-gasification of coal and biomass Fig. 4 shows the x–t curves of OP char with the three coal chars under CO2 atmosphere at 900 °C. The results indicate evident coalorigin dependence for both separated and blended chars. First, the times required to reach 0.95 conversion of separated LC/OP, LI/OP, and BC/OP chars are 20, 83, and 21.3 min, respectively. The difference in the reaction times among the three separated char sets is primarily attributed to the difference in the gasification reactivity of individual LC, LI, and BC chars. Second, Fig. 4 shows that the gasification of each blended set is clearly faster than its corresponding separated one; the times required to reach 0.95 conversion for LC/ OP, LI/OP, and BC/OP char blends are 9.9, 8.4, and 9.1 min, respectively. The discrepancy in the reaction time between separate and blended chars should reflect the degree of synergy among the three blended char sets. Essentially, the SI values of LC/OP, LI/OP, and BC/ OP blends are 2.02, 10.34, and 2.34, respectively (signed in Fig. 4). Basically, the reasons behind the difference in magnitude of SI must be examined from the chemical and compositional properties
(a)
(b)
(c)
Fig. 4. Char conversion as a function of reaction time during co-gasification of separated and blended (a) LC/OP, (b) LI/OP, and (c) BC/OP chars.
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of coals. Table 1 indicates evident or remarkable differences in the properties of the three coal samples studied. First, LC and LI are apparently similar in coal rank and younger than that of BC. However, the fact that the SI value of BC (2.34) is between those of LC (2.02) and LI (10.34) implies that coal rank has no relation to SI. Second, LC coal has the highest ash content (17.2 wt%) among the three coal samples studied, being 9.1 and 2.7 times larger than those of LI (1.9 wt%) and BC (6.3 wt%) coals, respectively. Clearly, the ash contents of the three coals correlate with their SI values. Many researchers also emphasized the effect of ash on gasification behavior (Habibi et al., 2013; Ding et al., 2014). Third, Table 1 also indicates that LC coal ash contains high alumina and silica contents than LI and BC. These two components can deactivate K and Ca by forming catalytically inactive silicates, thereby reducing subsequent char gasification rates (Risnes et al., 2003). The above results demonstrate that ash content and its chemical composition of coal play an important role in governing the magnitude of SI. Although some authors have reported inhibition effects during cogasification of coal and biomass chars with higher silica or alumina content by using conventional TGA (Habibi et al., 2013; Ding et al., 2014), different insights are obtained by congruent-mass TGA mode in this study. That is, whether in coal and biomass, higher silica, or alumina content, inhibition effect was not observed during the co-gasification of coal and biomass. 4. Conclusions Congruent-mass TGA provided conclusive evidence that synergy occurs only when free mineral potassium species are formed after complete gasification of biomass in coal/biomass blends. The SEM–EDS spectra revealed that the transfer of potassium from biomass to the surface of coal chars occurs during co-pyrolysis/ gasification. High silica content in biomass could eliminate the catalytic activity of potassium, but not to the extent of inhibiting the reaction of coal/biomass blended char and causing it to become slower than the reaction of separated one. The combination of low ash-coal and K-rich biomass was the best choice with respect to synergy achievement. Acknowledgement The authors are grateful to the financial supports from the National Natural Science Foundation of China (Grant No. 51376031).
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