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Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
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Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification Jia-Hong Kuo a, Chiou-Liang Lin b,*, Tsung-Jen Chang b, Wang-Chang Weng b, JingYong Liu a a
School of Environmental Science and Engineering and Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China b Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan
article info
abstract
Article history:
This paper describes the effect of equivalence ratio and steam/biomass ratio on the gas
Received 11 April 2016
production of a two-stage fluidized bed gasifier. The calcium oxide was employed as bed
Received in revised form
material in the second stage to evaluate the influence of gasification efficiency. The result
13 July 2016
shows the H2 molar percentage increased by approximately 2e3 mol% at the end of the
Accepted 15 July 2016
second stage fluidized bed reactor without adding CaO. The second stage fluidized bed
Available online xxx
gasifier plays an important role of cracking and reforming of tar which generated from the first stage one. On the other hand, when the calcium oxide was used as the bed material in
Keywords:
the second stage, it enhanced the generation of H2 under most parameters. Highest H2
Biomass
molar percentage in syngas was observed as 37 mol% at ER and S/B ratio is 0.3 and 2,
Gasification
respectively. Generally, more H2 generation through second stage gasifier was found than
Calcium oxide
that in first stage by 6.1e12.8%.
Syngas
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Fluidized bed
Introduction Gasification is a process where a partial oxidation reaction occurs in the degradation of biomass converted into reusable gases with medium and low heating values, such as carbon monoxide (CO), hydrogen (H2) and methane (CH4) [1], which can be used after purification. Recently, many studies regarding to gasification focused on the fluidized bed gasifier [2e4], because it has the advantages of good gas-fuel mixing and heat transfer efficiency, uniform temperature, ease of continuous operation, and many types of feed materials that
can be processed [5,6]. However, several operating parameters may impact the gasification efficiency of a fluidized bed gasifier such as bed material, operating temperature, different types of biomass, particle size of biomass, feed velocity, equivalence ratio (ER), and steam/biomass ratio (S/B). Those key factors mainly influence the gas yield, gas composition, carbon conversion efficiency and energy efficiency of a fluidized bed gasifier [7e11]. For several gasification researches, the ER and S/B ratios are important parameters. ER refers to the actual air fuel ratio divided by the stoichiometric air fuel ratio. Lv et al. [12] investigated the impact of adjusting ERs during the
* Corresponding author. Fax: þ886 7 5919376. E-mail address: [email protected] (C.-L. Lin). http://dx.doi.org/10.1016/j.ijhydene.2016.07.144 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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gasification process on the composition of generated gases. When ER was raised from 0.19 to 0.27, proportion of CO2 showed an increasing trend in the gas production, but slightly decreased in CO and H2. Ran and Li [13] also pointed that ER value affected the syngas composition and total gas yield. When he tested the ER from 0.2 to 0.35 during gasification, the ER was 0.2 that had highest proportion of H2 in the syngas. Research of fluidized bed gasification conducted by Li et al. [14] adjusted ER between 0.31 and 0.47, and they found that the contents of H2 and CO decreased when the ER was increased. This was because more oxygen entered the gasifier when the ER was raised, improving the carbon conversion efficiency and hence the yield of CO2, but the yields of CO and H2 decreased. Aznar et al. [15] illustrated that the gas heating value, total gas yield, amount of char and tar decreased with ER increased, when the ER value was controlled between 0.3 and 0.46. Additionally, Chiang et al. [16] also reported that an increase in ER would reduce the total gas yield, and the proportions of CO and H2 in the gas production decreased as CO2 increased. Accordingly, the suitable range of ER value was suggested in the range from 0.2 to 0.4 during gasification [7]. On the other hand, an important parameter to gasification is S/B ratio which defined as the ratio of steam to biomass in the gasifier. Kumar et al. [8] conducted the gasification experiment with S/B ¼ 0, 7.3 and 14.29 as the experimental conditions, and the result indicated that the yield of H2 increased when the S/B increased. He et al. [17] also noted that, after increasing S/B from 0.39 to 1.04, the proportion of H2 in the synthesis gas (syngas) increased from 27.5% to 53.22%. Loha et al. [18] reported that production of H2, CO2 and CH4 increased with S/B ratio increasing when the gasification temperature was constant at 750 C, but the amount of CO decreased with S/B ratio increasing. Song et al. [19] illustrated that the tar content in the gas reduced as increasing S/B ratio from 0.8 to 1.4 and Mayerhofer et al. [20] also had the same results when the S/B ratio was increased from 0.83 to 1.2. An increase in S/B could increase H2 yield because raising S/B could increase hydrogen in the gasification reactions, and more H2 was produced. However, the S/B must be controlled within a range to avoid too much moisture absorbs heat energy in the gasifier, that decreases the temperature and suppresses the gasification reactions [21]. In addition to the above-mentioned operating parameters, the use of additives may also impact the gasification efficiency. For example, Chiang et al. [22] added 10% CaO in the biomass, and the result indicated that the proportion of CO2 in the gas yield was 7.5% less than that without adding CaO. The reason was that CaO could absorb CO2 in the gasification process. Kobayashi et al. [23] also noted that the use of CaO in the gasification process could absorb CO2 in the gasification process. According to the research results of Acharya et al. [24], with the addition of CaO (CaO/biomass ¼ 2) during gasification, the proportion of H2 in the gas yield increased by 54.43% and the proportion of CO2 decreased by 93%. The primary reason was the same as that proposed by Udomsirichakorn et al. [25] in their research on the circulating fluidized bed gasifier: CaO absorbed CO2, which decreased the proportion of CO2 in the gas yield and further stimulated the generation of more H2 from the water gas shift reaction. The reaction formulae were given as follows:
CaO þ CO2 /CaCO3
(1)
The gasification studies in recent years attached importance to the improvement of gasification efficiency. Soni et al. [26] compared two fixed-bed gasifiers with one-stage gasification to discuss the gasification efficiency after gasification. The results showed that two-stage gasification could more effectively raise the hydrogen yield (from 7.3% to 22.3%) and the total gas yield (from 30.8% to 54.6%), and it decreased tar production from 18.6% to 14.2%. Park et al. [27] also conducted the two-stage gasification experiment by using two fixed-bed reactors, and they revealed that the tars generated in the first stage gasifier could undergo the cracking reaction in the second stage gasifier. Tar cracking would produce H2, CO and other gases. The tar yield of the fixed bed gasifier was more, but the gas yield was less than that from the fluidized bed. In recent years, related work reported that, when a fluidized bed was used for the first stage and a gasifier operation was used for the second stage gasification, and after adding dolomite and activated carbon as additives in the second stage gasifier, the total gas yield was between 66.6 and 75.1% and the tar production was less than 1% [28]. It was found that the two-stage gasification could effectively enhance the gasification efficiency and decrease the generation of tar, but the current second-stage gasification studies mostly used the fixed bed gasifier as the second stage gasifier [26,29]. If the fluidized bed gasifier was utilized as the second stage gasifier for the second-stage gasification experiment, the gasification efficiency may be improved, the tar yield in the gasification process may be decreased, and the gas production may increase. However, few studies are available at present that have investigated this process. Thus, this research will discuss the impacts of different operating conditions (ER and S/B) on the gasification efficiency of the twostage fluidized bed gasifier, and it compares the impact on the gasification efficiency when the calcium oxide is used as an additive in the second stage gasifier.
Experimental This research utilized the laboratory-scale two stage fluidized bed gasifier, whose structure is detailed in Fig. 1. The gasifier is made of the stainless steel (AISI-310), with the thickness of 0.49 cm, the height of 50 cm, the outer diameter of 4.27 cm and the inner diameter of 3.29 cm. The first stage uses the same gasifier as the second stage. The connecting tube of two gasifiers is made of stainless steel with the thickness of 0.42 cm. A stainless steel distributor plate is installed at the bottom of each gasifier and its opening area is 15.2%. The electric heating system is used for heating the gasifier. The outer layer of the gasifier is enclosed by thermal insulation fiber to prevent heat loss. Three thermocouples are mounted to monitor and record the temperature changes in the gasifier. The feed inlet is made by the double valve design, so as to prevent gas escape or entering of external air from affecting the experimental results while the materials are feed into the gasifier. The artificial simulated wastes were used in the experiments, mainly composed of polypropylene (PP) plastic particles, wood chips and plant capsules. The basic element
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Fig. 1 e Two-stage fluidized bed gasifier. (1) PID controller, (2) mass flow meter, (3) distributor, (4) thermocouple, (5) electric heater, (6) gasifier, (7) manual feeder, (8) impingers and cooling system, (9) sampling pump, (10) glass filter, (11) induced fan.
analysis of various experimental materials was shown in Table 1. The feedstock included the plant capsules (0.098 g), PP plastic particles (0.022 g) and wood chips (0.163 g), each pellet of simulated wastes was 0.283 g, and 15 pellets were fed per minute (5 pellets/20 s). The experiments were conducted with the second stage fluidized bed gasifier. For the first and second stages, the operating temperature was controlled at 800 C, the equivalence ratio (ER) controlled at 0.2, 0.3 and 0.4, the steam/ biomass ratio (S/B) controlled at 0, 0.5, 1, 1.5 and 2, and the additive was calcium oxide (CaO). The resulting data were employed to evaluate the impacts of operating temperature and additives on the gasification efficiency of the two-stage fluidized bed gasifier. The experimental procedures were shown in Table 2.
Table 1 e Characterization analysis of inlet materials. Species
Polypropylene (PP)
Ultimate analysis (wt%) C H O N Proximate analysis (wt%) Moisture Volatile matter Fixed carbon Ash Lower Heating Value (MJ/kg)
Wood chips
Vegetable capsule
86.3 12.77 0.35 0.57
45.98 7.32 46.51 0.18
47.76 8.15 43.81 0.27
0.01 99.99 0 0 44.27
7.27 76.25 16.48 0 15.83
5.0 86.8 7.56 0.64 17.91
Before experiment, the minimum fluidization velocity was measured by referring to the method of Lin et al. [30]. The minimum fluidization velocity was 0.10 m/s, and the gas flow rate for the experiments was 1.3 times of that. The incoming gas flow was adjusted by using the mass flow meter, and also the experimental ER value was controlled. The S/B ratio was adding water in the artificial simulated wastes before experiments according to the experimental conditions. The firststage bed material of the experiments was silica sand with the average particle diameter of 545 mm and the density of 2600 kg/m3. For composition of silica sand, SiO2 was 97.8%, Al2O3 was 2% and Fe2O3 was 0.07%. In the second stage, the bed material was calcium oxide (CaO), with the particle size of 545 mm, the weight of 25 g, and the purity of above 96%. When sampling, the gas first passed through the glass fiber filter and most of particles were intercepted on the filter paper. After filtering, the gas went through the impinger in the low temperature water bath, and then the filter paper and the silica gel were used to filter the remaining particles and absorb the
Table 2 e Operating conditions for the experiments. Run Stage 1/stage 2 temperature ( C) ER S/B Additive 1 2 3 4 5 6 7 8
800/800 800/800 800/800 800/800 800/800 800/800 800/800 800/800
0.3 0.2 0.3 0.4 0.3 0.3 0.3 0.3
0 0 0 0 0.5 1 1.5 2
None CaO CaO CaO CaO CaO CaO CaO
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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moisture. Finally the active air sampling pump was employed to sample, and the samples were stored in the air sampling bags. The sampled syngas was analyzed by using the gas chromatograph adopted the detector of TCD (GC-TCD). According to previous studies, generally, the distribution of syngas generated from biomass gasification can be considered that it is mainly consist of H2, CO, CO2, and CH4 as major gas components [31,32]. Indeed, there are still some trace compounds exist in syngas during gasification process, but the concentrations of those gaseous products are relatively low. Previous study has discussed the comparison of experimental and theoretical simulation for steam gasification of woody biomass. Both results presented the four main gases occupied more than around 93% in gas phase. Trace components such as C2H4 and C2H6 are relatively lower than those [33]. On the other hand, our previous works also have estimated the emission of organic compounds included polycyclic aromatic hydrocarbon (PAHs) and benzene, toluene, ethyl benzene, xylene (BTEX) during gasification. Experimental results illustrated the total concentration of both organics is only 0.26 g/ Nm3 at 800 C [34]. Gasification system is known to generate the syngas that is diluted by the inert nitrogen gas. Therefore, many literature referred to using relative concentration for simplicity [35e38]. Besides, a small amount of inlet biomass was fed for a laboratory scale test in this study. The absolute concentrations among those gaseous products appeared tinily due to N2 dilution. In order to evaluate the effect of calcium oxide on hydrogen production during gasification process, and then, the relative concentrations in dry basis for the four main gases were presented to have a better understanding (shown in Table S1).
Results and discussion
of the syngas, and it changed the gas composition. Therefore, even without any additive, the proportions of H2 and CH4 in the syngas generated from the second stage gasifier were increased by approximately 2e3%.
Impacts of steam/biomass ratio (S/B) and CaO on gas yield of the two-stage gasification process Fig. 3 showed the impacts of changing S/B on the gas composition distribution when the operating temperatures for the gasifiers at the first and second stages were controlled at 800 C at an ER of 0.3 with CaO as the additive in the second stage gasifier. The results indicated that, when the S/B increased from 0 to 2, the proportion of H2 in the first stage of gas production increased slightly from 21.54 mol% to 23.94 mol%, CH4 decreased from 9.61 mol% to 5.53 mol%, and CO2 tended to decrease. Syngas treated by the second stage gasifier revealed the yields of H2 and CO2 tended to increase, and those of CO and CH4 decreased. The primary reason was more moisture in the gasifier stimulated the methane-steam reforming reactions [39] (Eqs. (2)e(3)) and tar reforming and cracking reactions (Eqs. (4)e(6)) to generate more gas contents while S/B ratios increased. Methane-steam reforming reactions: CH4 þ H2 O4CO þ 3H2
(2)
CH4 þ 2H2 O4CO2 þ 4H2
(3)
Tar reforming and cracking reactions: Tar þ n1 H2 O4n2 CO2 þ n3 H2
(4)
Cn Hm ðTarÞ þ nCO2 4ðm=2ÞH2 þ 2nCO
(5)
Cn Hm ðTarÞ4ðm=2ÞH2 þ nC
(6)
Fig. 2 shows the gas composition distribution when the operating temperature for the gasifiers at both first and second stages was controlled at 800 C, without adding CaO in the second stage gasifier at an ER of 0.3. It was found that H2 and CH4 increased slightly after passing the second stage fluidized bed gasifier because the re-heating environment for reaction
According to Kumar et al. [8], the carbon conversion efficiency could be raised if the S/B increased, and the moisture as a result of increased S/B would facilitate the methane-steam reforming reactions. Thus, when the S/B increased, CH4 reforming was promoted to generate more H2 and CO2. When CO2 was absorbed by calcium oxide, this would stimulate the steam reactions to generate more H2, and hence H2 increased in the second stage compared with the first stage.
Fig. 2 e Syngas composition in two-stage fluidized bed gasifier without adding CaO.
Fig. 3 e The effect of different S/B ratios on syngas composition with CaO addition.
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Experimental results illustrated the H2 molar percentage in syngas was highest (37 mol%) at ER ¼ 0.3 and S/B ¼ 2. Generally, the H2 generation level by passing second stage fluidized bed gasifier was more than that of first stage one by 6.1e12.8%. However, it was noteworthy that CO2 generated in the second stage did not appreciably decrease. It was estimated that the increase in S/B enhanced the carbon conversion efficiency and produced more CO2. As a result, the calcium oxide could not fully absorb CO2. Udomsirichakorn et al. [40] also noted that the calcium oxide absorbing CO2 could promote the water gas shift reaction and increase the proportion of H2 in the syngas. Fig. 4 shows the effects of changing S/B on the gas yield and the total heating value when the operating temperature for the gasifiers at the first and second stages was controlled at 800 C at an ER of 0.3, with CaO as an additive in the second stage gasifier. Results indicated that, when the S/B increased from 0 to 2, the gas yield from the first and second stages clearly increased owing to the increase of moisture enhanced the reactions of methane-steam reforming and tar cracking/ reforming. It was noted that the gas yield in the second stage was higher when the calcium oxide was used as an additive, and the primary reason was that CO2 in the generated gases was absorbed by the calcium oxide to simulate the water gas shift reaction, producing more H2. When the S/B increased from 0 to 0.5, the total heating value of the gas yielded from the first stage gasifier increased apparently, but when the S/B increased from 0.5 to 2, the total heating value of the gas showed a decreasing trend. This phenomenon could be observed in both first and second stage fluidized bed gasifiers. The reason was supposed that CO2 was absorbed by the calcium oxide to simulate the water gas shift reaction, consuming CO and producing more H2, but the heating value of H2 was lower than that of CO. Hence, the total heating value of the gas decreased. Additionally, when the S/B increased, more steam could be reformed with CH4 to produce H2, the heating value of CH4 was higher than that of H2, and the total heating value of the gas decreased [40].
Impacts of equivalence ratio (ER) and CaO on gas yield of the two-stage gasification process Fig. 5 shows the impacts of changing ER on the gas composition distribution when the operating temperature for the gasifiers at the first and second stages was controlled at 800 C
Fig. 4 e Effect of different S/B ratios on total gas yield and heating value with CaO addition.
5
Fig. 5 e Effect of different ERs on syngas composition with CaO addition.
with an S/B of 0, with CaO as an additive in the second stage gasifier. The results indicated that the combustion efficiency increased when the ER increased from 0.2 to 0.4. The proportion of CO in the first stage gasifier decreased slightly from 35.11 mol% to 28.04 mol%, while CH4 decreased from 9.87 mol % to 7.15 mol%, but CO2 increased from 38.06 mol% to 43.1 mol %. Experimental results of syngas proportion in the second stage gasifier also showed the same trend. According to Lv et al. [12], an increase in ER would increase the oxygen to stimulate a complete oxidation reaction and enhance the combustion efficiency. The generation of CO2 increased but CO and CH4 decreased. The similar conclusion was also reached by Chiang et al. [16]. It could be observed that the yield of CO2 generated from the second stage gasifier was less than that in the first stage gasifier when CaO was used as additives, but the yield of H2 was higher because more oxygen participated in the reactions and it enhanced the combustion efficiency when the ER increased from 0.2 to 0.4. Besides, the CO2 produced from second stage gasifier decreased due to absorption processes caused by calcium oxide through water gas shift reaction to generate more H2. However, when the ER was 0.4, the yield of CO2 in the second stage gasifier did not appreciably decrease. It can be assumed that the combustion efficiency was higher and generated more CO2 at an ER of 0.4, and the effortlessness of CO2 absorption by CaO was supposed.
Fig. 6 e Effect of different ERs on total gas yield and heating value with CaO addition.
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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The results of impacts of changing ER on the gas yields and the total heating values under an S/B of 0 with CaO additive in the second stage gasifier was shown in Fig. 6. When the ER increased from 0.2 to 0.3, the total heating value increased at the first stage gasifier, but it decreased at the second stage. Then, the total heating value decreased at both stages when the ER increased from 0.3 to 0.4. The reason was that raising ERs led to increase oxygen content in the reactions and that reduced both gas yield and total heating value. Additionally, according to Ran and Li [13], higher ERs could increase the oxidation reactions, and that consumed H2, CO and CH4 to generate CO2 and H2O. Thus, results of both total gas yield and heating value showed decreasing trends.
Conclusion This research discussed the impacts of various equivalency ratios (ERs) and steam/biomass (S/B) ratios on the gasification efficiency with application of calcium oxide additive in a twostage fluidized bed gasifier. The results indicated that the hydrogen yield increased by 2e3% in two-stage gasifier system without additives compared with that in a single gasifier environment. Application of two-stage stage fluidized bed gasifier provided a higher temperature environment for carbon reacting which is unburned from the first stage gasifier. The gas compositions are changed, and it also improved the gasification efficiency. Nevertheless, the proportion of H2 in syngas composition was the highest (37%) when the calcium oxide was used as the second stage additive at ER and S/B ratio of 0.3 and 0.2, separately. Consequently, it was found that more H2 content generated from the second stage gasifier than that in first stage by 6.1e12.8% in the tests.
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Acknowledgement
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Supported by Science and Technology Planning Project of Guangdong Province, China (2014A050503063, 2016A050502059), Guangzhou Special Fund for the IndustryUniversity-Research Institute Collaborative Innovation (2016201604030058).
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Appendix A. Supplementary data
[18]
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.07.144.
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Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification Jia-Hong Kuo a, Chiou-Liang Lin b,*, Tsung-Jen Chang b, Wang-Chang Weng b, JingYong Liu a a
School of Environmental Science and Engineering and Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China b Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan
article info
abstract
Article history:
This paper describes the effect of equivalence ratio and steam/biomass ratio on the gas
Received 11 April 2016
production of a two-stage fluidized bed gasifier. The calcium oxide was employed as bed
Received in revised form
material in the second stage to evaluate the influence of gasification efficiency. The result
13 July 2016
shows the H2 molar percentage increased by approximately 2e3 mol% at the end of the
Accepted 15 July 2016
second stage fluidized bed reactor without adding CaO. The second stage fluidized bed
Available online xxx
gasifier plays an important role of cracking and reforming of tar which generated from the first stage one. On the other hand, when the calcium oxide was used as the bed material in
Keywords:
the second stage, it enhanced the generation of H2 under most parameters. Highest H2
Biomass
molar percentage in syngas was observed as 37 mol% at ER and S/B ratio is 0.3 and 2,
Gasification
respectively. Generally, more H2 generation through second stage gasifier was found than
Calcium oxide
that in first stage by 6.1e12.8%.
Syngas
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Fluidized bed
Introduction Gasification is a process where a partial oxidation reaction occurs in the degradation of biomass converted into reusable gases with medium and low heating values, such as carbon monoxide (CO), hydrogen (H2) and methane (CH4) [1], which can be used after purification. Recently, many studies regarding to gasification focused on the fluidized bed gasifier [2e4], because it has the advantages of good gas-fuel mixing and heat transfer efficiency, uniform temperature, ease of continuous operation, and many types of feed materials that
can be processed [5,6]. However, several operating parameters may impact the gasification efficiency of a fluidized bed gasifier such as bed material, operating temperature, different types of biomass, particle size of biomass, feed velocity, equivalence ratio (ER), and steam/biomass ratio (S/B). Those key factors mainly influence the gas yield, gas composition, carbon conversion efficiency and energy efficiency of a fluidized bed gasifier [7e11]. For several gasification researches, the ER and S/B ratios are important parameters. ER refers to the actual air fuel ratio divided by the stoichiometric air fuel ratio. Lv et al. [12] investigated the impact of adjusting ERs during the
* Corresponding author. Fax: þ886 7 5919376. E-mail address: [email protected] (C.-L. Lin). http://dx.doi.org/10.1016/j.ijhydene.2016.07.144 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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gasification process on the composition of generated gases. When ER was raised from 0.19 to 0.27, proportion of CO2 showed an increasing trend in the gas production, but slightly decreased in CO and H2. Ran and Li [13] also pointed that ER value affected the syngas composition and total gas yield. When he tested the ER from 0.2 to 0.35 during gasification, the ER was 0.2 that had highest proportion of H2 in the syngas. Research of fluidized bed gasification conducted by Li et al. [14] adjusted ER between 0.31 and 0.47, and they found that the contents of H2 and CO decreased when the ER was increased. This was because more oxygen entered the gasifier when the ER was raised, improving the carbon conversion efficiency and hence the yield of CO2, but the yields of CO and H2 decreased. Aznar et al. [15] illustrated that the gas heating value, total gas yield, amount of char and tar decreased with ER increased, when the ER value was controlled between 0.3 and 0.46. Additionally, Chiang et al. [16] also reported that an increase in ER would reduce the total gas yield, and the proportions of CO and H2 in the gas production decreased as CO2 increased. Accordingly, the suitable range of ER value was suggested in the range from 0.2 to 0.4 during gasification [7]. On the other hand, an important parameter to gasification is S/B ratio which defined as the ratio of steam to biomass in the gasifier. Kumar et al. [8] conducted the gasification experiment with S/B ¼ 0, 7.3 and 14.29 as the experimental conditions, and the result indicated that the yield of H2 increased when the S/B increased. He et al. [17] also noted that, after increasing S/B from 0.39 to 1.04, the proportion of H2 in the synthesis gas (syngas) increased from 27.5% to 53.22%. Loha et al. [18] reported that production of H2, CO2 and CH4 increased with S/B ratio increasing when the gasification temperature was constant at 750 C, but the amount of CO decreased with S/B ratio increasing. Song et al. [19] illustrated that the tar content in the gas reduced as increasing S/B ratio from 0.8 to 1.4 and Mayerhofer et al. [20] also had the same results when the S/B ratio was increased from 0.83 to 1.2. An increase in S/B could increase H2 yield because raising S/B could increase hydrogen in the gasification reactions, and more H2 was produced. However, the S/B must be controlled within a range to avoid too much moisture absorbs heat energy in the gasifier, that decreases the temperature and suppresses the gasification reactions [21]. In addition to the above-mentioned operating parameters, the use of additives may also impact the gasification efficiency. For example, Chiang et al. [22] added 10% CaO in the biomass, and the result indicated that the proportion of CO2 in the gas yield was 7.5% less than that without adding CaO. The reason was that CaO could absorb CO2 in the gasification process. Kobayashi et al. [23] also noted that the use of CaO in the gasification process could absorb CO2 in the gasification process. According to the research results of Acharya et al. [24], with the addition of CaO (CaO/biomass ¼ 2) during gasification, the proportion of H2 in the gas yield increased by 54.43% and the proportion of CO2 decreased by 93%. The primary reason was the same as that proposed by Udomsirichakorn et al. [25] in their research on the circulating fluidized bed gasifier: CaO absorbed CO2, which decreased the proportion of CO2 in the gas yield and further stimulated the generation of more H2 from the water gas shift reaction. The reaction formulae were given as follows:
CaO þ CO2 /CaCO3
(1)
The gasification studies in recent years attached importance to the improvement of gasification efficiency. Soni et al. [26] compared two fixed-bed gasifiers with one-stage gasification to discuss the gasification efficiency after gasification. The results showed that two-stage gasification could more effectively raise the hydrogen yield (from 7.3% to 22.3%) and the total gas yield (from 30.8% to 54.6%), and it decreased tar production from 18.6% to 14.2%. Park et al. [27] also conducted the two-stage gasification experiment by using two fixed-bed reactors, and they revealed that the tars generated in the first stage gasifier could undergo the cracking reaction in the second stage gasifier. Tar cracking would produce H2, CO and other gases. The tar yield of the fixed bed gasifier was more, but the gas yield was less than that from the fluidized bed. In recent years, related work reported that, when a fluidized bed was used for the first stage and a gasifier operation was used for the second stage gasification, and after adding dolomite and activated carbon as additives in the second stage gasifier, the total gas yield was between 66.6 and 75.1% and the tar production was less than 1% [28]. It was found that the two-stage gasification could effectively enhance the gasification efficiency and decrease the generation of tar, but the current second-stage gasification studies mostly used the fixed bed gasifier as the second stage gasifier [26,29]. If the fluidized bed gasifier was utilized as the second stage gasifier for the second-stage gasification experiment, the gasification efficiency may be improved, the tar yield in the gasification process may be decreased, and the gas production may increase. However, few studies are available at present that have investigated this process. Thus, this research will discuss the impacts of different operating conditions (ER and S/B) on the gasification efficiency of the twostage fluidized bed gasifier, and it compares the impact on the gasification efficiency when the calcium oxide is used as an additive in the second stage gasifier.
Experimental This research utilized the laboratory-scale two stage fluidized bed gasifier, whose structure is detailed in Fig. 1. The gasifier is made of the stainless steel (AISI-310), with the thickness of 0.49 cm, the height of 50 cm, the outer diameter of 4.27 cm and the inner diameter of 3.29 cm. The first stage uses the same gasifier as the second stage. The connecting tube of two gasifiers is made of stainless steel with the thickness of 0.42 cm. A stainless steel distributor plate is installed at the bottom of each gasifier and its opening area is 15.2%. The electric heating system is used for heating the gasifier. The outer layer of the gasifier is enclosed by thermal insulation fiber to prevent heat loss. Three thermocouples are mounted to monitor and record the temperature changes in the gasifier. The feed inlet is made by the double valve design, so as to prevent gas escape or entering of external air from affecting the experimental results while the materials are feed into the gasifier. The artificial simulated wastes were used in the experiments, mainly composed of polypropylene (PP) plastic particles, wood chips and plant capsules. The basic element
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Fig. 1 e Two-stage fluidized bed gasifier. (1) PID controller, (2) mass flow meter, (3) distributor, (4) thermocouple, (5) electric heater, (6) gasifier, (7) manual feeder, (8) impingers and cooling system, (9) sampling pump, (10) glass filter, (11) induced fan.
analysis of various experimental materials was shown in Table 1. The feedstock included the plant capsules (0.098 g), PP plastic particles (0.022 g) and wood chips (0.163 g), each pellet of simulated wastes was 0.283 g, and 15 pellets were fed per minute (5 pellets/20 s). The experiments were conducted with the second stage fluidized bed gasifier. For the first and second stages, the operating temperature was controlled at 800 C, the equivalence ratio (ER) controlled at 0.2, 0.3 and 0.4, the steam/ biomass ratio (S/B) controlled at 0, 0.5, 1, 1.5 and 2, and the additive was calcium oxide (CaO). The resulting data were employed to evaluate the impacts of operating temperature and additives on the gasification efficiency of the two-stage fluidized bed gasifier. The experimental procedures were shown in Table 2.
Table 1 e Characterization analysis of inlet materials. Species
Polypropylene (PP)
Ultimate analysis (wt%) C H O N Proximate analysis (wt%) Moisture Volatile matter Fixed carbon Ash Lower Heating Value (MJ/kg)
Wood chips
Vegetable capsule
86.3 12.77 0.35 0.57
45.98 7.32 46.51 0.18
47.76 8.15 43.81 0.27
0.01 99.99 0 0 44.27
7.27 76.25 16.48 0 15.83
5.0 86.8 7.56 0.64 17.91
Before experiment, the minimum fluidization velocity was measured by referring to the method of Lin et al. [30]. The minimum fluidization velocity was 0.10 m/s, and the gas flow rate for the experiments was 1.3 times of that. The incoming gas flow was adjusted by using the mass flow meter, and also the experimental ER value was controlled. The S/B ratio was adding water in the artificial simulated wastes before experiments according to the experimental conditions. The firststage bed material of the experiments was silica sand with the average particle diameter of 545 mm and the density of 2600 kg/m3. For composition of silica sand, SiO2 was 97.8%, Al2O3 was 2% and Fe2O3 was 0.07%. In the second stage, the bed material was calcium oxide (CaO), with the particle size of 545 mm, the weight of 25 g, and the purity of above 96%. When sampling, the gas first passed through the glass fiber filter and most of particles were intercepted on the filter paper. After filtering, the gas went through the impinger in the low temperature water bath, and then the filter paper and the silica gel were used to filter the remaining particles and absorb the
Table 2 e Operating conditions for the experiments. Run Stage 1/stage 2 temperature ( C) ER S/B Additive 1 2 3 4 5 6 7 8
800/800 800/800 800/800 800/800 800/800 800/800 800/800 800/800
0.3 0.2 0.3 0.4 0.3 0.3 0.3 0.3
0 0 0 0 0.5 1 1.5 2
None CaO CaO CaO CaO CaO CaO CaO
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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moisture. Finally the active air sampling pump was employed to sample, and the samples were stored in the air sampling bags. The sampled syngas was analyzed by using the gas chromatograph adopted the detector of TCD (GC-TCD). According to previous studies, generally, the distribution of syngas generated from biomass gasification can be considered that it is mainly consist of H2, CO, CO2, and CH4 as major gas components [31,32]. Indeed, there are still some trace compounds exist in syngas during gasification process, but the concentrations of those gaseous products are relatively low. Previous study has discussed the comparison of experimental and theoretical simulation for steam gasification of woody biomass. Both results presented the four main gases occupied more than around 93% in gas phase. Trace components such as C2H4 and C2H6 are relatively lower than those [33]. On the other hand, our previous works also have estimated the emission of organic compounds included polycyclic aromatic hydrocarbon (PAHs) and benzene, toluene, ethyl benzene, xylene (BTEX) during gasification. Experimental results illustrated the total concentration of both organics is only 0.26 g/ Nm3 at 800 C [34]. Gasification system is known to generate the syngas that is diluted by the inert nitrogen gas. Therefore, many literature referred to using relative concentration for simplicity [35e38]. Besides, a small amount of inlet biomass was fed for a laboratory scale test in this study. The absolute concentrations among those gaseous products appeared tinily due to N2 dilution. In order to evaluate the effect of calcium oxide on hydrogen production during gasification process, and then, the relative concentrations in dry basis for the four main gases were presented to have a better understanding (shown in Table S1).
Results and discussion
of the syngas, and it changed the gas composition. Therefore, even without any additive, the proportions of H2 and CH4 in the syngas generated from the second stage gasifier were increased by approximately 2e3%.
Impacts of steam/biomass ratio (S/B) and CaO on gas yield of the two-stage gasification process Fig. 3 showed the impacts of changing S/B on the gas composition distribution when the operating temperatures for the gasifiers at the first and second stages were controlled at 800 C at an ER of 0.3 with CaO as the additive in the second stage gasifier. The results indicated that, when the S/B increased from 0 to 2, the proportion of H2 in the first stage of gas production increased slightly from 21.54 mol% to 23.94 mol%, CH4 decreased from 9.61 mol% to 5.53 mol%, and CO2 tended to decrease. Syngas treated by the second stage gasifier revealed the yields of H2 and CO2 tended to increase, and those of CO and CH4 decreased. The primary reason was more moisture in the gasifier stimulated the methane-steam reforming reactions [39] (Eqs. (2)e(3)) and tar reforming and cracking reactions (Eqs. (4)e(6)) to generate more gas contents while S/B ratios increased. Methane-steam reforming reactions: CH4 þ H2 O4CO þ 3H2
(2)
CH4 þ 2H2 O4CO2 þ 4H2
(3)
Tar reforming and cracking reactions: Tar þ n1 H2 O4n2 CO2 þ n3 H2
(4)
Cn Hm ðTarÞ þ nCO2 4ðm=2ÞH2 þ 2nCO
(5)
Cn Hm ðTarÞ4ðm=2ÞH2 þ nC
(6)
Fig. 2 shows the gas composition distribution when the operating temperature for the gasifiers at both first and second stages was controlled at 800 C, without adding CaO in the second stage gasifier at an ER of 0.3. It was found that H2 and CH4 increased slightly after passing the second stage fluidized bed gasifier because the re-heating environment for reaction
According to Kumar et al. [8], the carbon conversion efficiency could be raised if the S/B increased, and the moisture as a result of increased S/B would facilitate the methane-steam reforming reactions. Thus, when the S/B increased, CH4 reforming was promoted to generate more H2 and CO2. When CO2 was absorbed by calcium oxide, this would stimulate the steam reactions to generate more H2, and hence H2 increased in the second stage compared with the first stage.
Fig. 2 e Syngas composition in two-stage fluidized bed gasifier without adding CaO.
Fig. 3 e The effect of different S/B ratios on syngas composition with CaO addition.
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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Experimental results illustrated the H2 molar percentage in syngas was highest (37 mol%) at ER ¼ 0.3 and S/B ¼ 2. Generally, the H2 generation level by passing second stage fluidized bed gasifier was more than that of first stage one by 6.1e12.8%. However, it was noteworthy that CO2 generated in the second stage did not appreciably decrease. It was estimated that the increase in S/B enhanced the carbon conversion efficiency and produced more CO2. As a result, the calcium oxide could not fully absorb CO2. Udomsirichakorn et al. [40] also noted that the calcium oxide absorbing CO2 could promote the water gas shift reaction and increase the proportion of H2 in the syngas. Fig. 4 shows the effects of changing S/B on the gas yield and the total heating value when the operating temperature for the gasifiers at the first and second stages was controlled at 800 C at an ER of 0.3, with CaO as an additive in the second stage gasifier. Results indicated that, when the S/B increased from 0 to 2, the gas yield from the first and second stages clearly increased owing to the increase of moisture enhanced the reactions of methane-steam reforming and tar cracking/ reforming. It was noted that the gas yield in the second stage was higher when the calcium oxide was used as an additive, and the primary reason was that CO2 in the generated gases was absorbed by the calcium oxide to simulate the water gas shift reaction, producing more H2. When the S/B increased from 0 to 0.5, the total heating value of the gas yielded from the first stage gasifier increased apparently, but when the S/B increased from 0.5 to 2, the total heating value of the gas showed a decreasing trend. This phenomenon could be observed in both first and second stage fluidized bed gasifiers. The reason was supposed that CO2 was absorbed by the calcium oxide to simulate the water gas shift reaction, consuming CO and producing more H2, but the heating value of H2 was lower than that of CO. Hence, the total heating value of the gas decreased. Additionally, when the S/B increased, more steam could be reformed with CH4 to produce H2, the heating value of CH4 was higher than that of H2, and the total heating value of the gas decreased [40].
Impacts of equivalence ratio (ER) and CaO on gas yield of the two-stage gasification process Fig. 5 shows the impacts of changing ER on the gas composition distribution when the operating temperature for the gasifiers at the first and second stages was controlled at 800 C
Fig. 4 e Effect of different S/B ratios on total gas yield and heating value with CaO addition.
5
Fig. 5 e Effect of different ERs on syngas composition with CaO addition.
with an S/B of 0, with CaO as an additive in the second stage gasifier. The results indicated that the combustion efficiency increased when the ER increased from 0.2 to 0.4. The proportion of CO in the first stage gasifier decreased slightly from 35.11 mol% to 28.04 mol%, while CH4 decreased from 9.87 mol % to 7.15 mol%, but CO2 increased from 38.06 mol% to 43.1 mol %. Experimental results of syngas proportion in the second stage gasifier also showed the same trend. According to Lv et al. [12], an increase in ER would increase the oxygen to stimulate a complete oxidation reaction and enhance the combustion efficiency. The generation of CO2 increased but CO and CH4 decreased. The similar conclusion was also reached by Chiang et al. [16]. It could be observed that the yield of CO2 generated from the second stage gasifier was less than that in the first stage gasifier when CaO was used as additives, but the yield of H2 was higher because more oxygen participated in the reactions and it enhanced the combustion efficiency when the ER increased from 0.2 to 0.4. Besides, the CO2 produced from second stage gasifier decreased due to absorption processes caused by calcium oxide through water gas shift reaction to generate more H2. However, when the ER was 0.4, the yield of CO2 in the second stage gasifier did not appreciably decrease. It can be assumed that the combustion efficiency was higher and generated more CO2 at an ER of 0.4, and the effortlessness of CO2 absorption by CaO was supposed.
Fig. 6 e Effect of different ERs on total gas yield and heating value with CaO addition.
Please cite this article in press as: Kuo J-H, et al., Impact of using calcium oxide as a bed material on hydrogen production in two-stage fluidized bed gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.144
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The results of impacts of changing ER on the gas yields and the total heating values under an S/B of 0 with CaO additive in the second stage gasifier was shown in Fig. 6. When the ER increased from 0.2 to 0.3, the total heating value increased at the first stage gasifier, but it decreased at the second stage. Then, the total heating value decreased at both stages when the ER increased from 0.3 to 0.4. The reason was that raising ERs led to increase oxygen content in the reactions and that reduced both gas yield and total heating value. Additionally, according to Ran and Li [13], higher ERs could increase the oxidation reactions, and that consumed H2, CO and CH4 to generate CO2 and H2O. Thus, results of both total gas yield and heating value showed decreasing trends.
Conclusion This research discussed the impacts of various equivalency ratios (ERs) and steam/biomass (S/B) ratios on the gasification efficiency with application of calcium oxide additive in a twostage fluidized bed gasifier. The results indicated that the hydrogen yield increased by 2e3% in two-stage gasifier system without additives compared with that in a single gasifier environment. Application of two-stage stage fluidized bed gasifier provided a higher temperature environment for carbon reacting which is unburned from the first stage gasifier. The gas compositions are changed, and it also improved the gasification efficiency. Nevertheless, the proportion of H2 in syngas composition was the highest (37%) when the calcium oxide was used as the second stage additive at ER and S/B ratio of 0.3 and 0.2, separately. Consequently, it was found that more H2 content generated from the second stage gasifier than that in first stage by 6.1e12.8% in the tests.
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Acknowledgement
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Supported by Science and Technology Planning Project of Guangdong Province, China (2014A050503063, 2016A050502059), Guangzhou Special Fund for the IndustryUniversity-Research Institute Collaborative Innovation (2016201604030058).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.07.144.
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