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Effects of different operating parameters on the syngas composition in a two-stage gasification process
Renewable Energy 109 (2017) 135e143
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Effects of different operating parameters on the syngas composition in a two-stage gasification process Chiou-Liang Lin*, Wang-Chang Weng Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, 811, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2016 Received in revised form 2 February 2017 Accepted 8 March 2017 Available online 9 March 2017
This research used a two-stage fluidized-bed gasifier to investigate the effects of the temperature, equivalence ratio, and steam/biomass ratio on the syngas composition. When the operating temperature in the first stage increased from 700 C to 900 C, the proportion of H2 in the syngas increased significantly. After passing through the second stage (900 C), the syngas produced from the first stage underwent the thermal reaction again, and the proportion of H2 was further increased. When the ER value increased from 0.2 to 0.3, the proportion of H2 in the syngas increased; whereas, when the ER value increased to 0.4, the amount of H2 produced was reduced. For S/B ratio, an increase to 0.5 enhanced the steam content of the gasifier and accelerated the methaneesteam reforming reaction, thus producing more H2 (up to 52 mol%). Furthermore, when the operating temperature of the fluidized bed reactor at the second stage was set at 900 C, the proportion of H2 in the syngas could still be effectively improved to more than 42 mol% although the operating temperature at the first stage was only 700 C. The proportion of H2 was enhanced to more than 52 mol% with a combination of appropriate ER and S/B ratio values. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Two-stage gasifier Equivalence ratio Steam/biomass Syngas
1. Introduction Taiwan currently has 24 large incineration plants in operation, and nearly 600 million tons of waste can be processed each year. Since 2008, the disposal rate of municipal solid waste in Taiwan has been 99.9%; except for recycling, 97% of the remaining wastes are processed by incineration [1]. However, with increasing resource scarcity and the rise of the “zero waste” concept, waste has been increasingly considered as another resource. Since the majority of recycled waste is organic matter, the transition from incineration to gasification of wastes can not only solve the problem of waste disposal, but also produce reusable energy. With gasification technologies, organic materials are converted into reusable syngas (e.g. a mixed gas comprising CO, CH4, and H2) via partial oxidation for further purification and application. Generally, the common gasification operating parameters include the type of biomass, temperature, pressure, feed particle size, particle size of the bed material, type of gases, equivalence ratio (ER), and steam/biomass (S/B) ratio [2e5]; the operating temperature, ER, and S/B ratio all
* Corresponding author. E-mail address: [email protected] (C.-L. Lin). http://dx.doi.org/10.1016/j.renene.2017.03.019 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
exhibit significant impacts on the syngas composition from the gasification process. In the gasification process, the operating temperature directly affects the gasification reaction. A temperature increase changes the surface temperature of the bed material so that there is an improvement in the thermal conductivity between the bed material and biomass, thereby affecting the product gas composition and heating value. Kumar et al. [4] investigated the effect of operating temperatures in the range of 650e850 C on gasification and found that the highest carbon conversion efficiency (82%) and energy conversion efficiency (96%) were obtained at 850 C; the H2 concentration increased from 4% at 650 C to 15% at 850 C. A study by Luo et al. [6] suggested that when the vaporization temperature increased from 600 C to 900 C, the carbon conversion efficiency and the amount of gas generated increased from 61.96% to 92.59% and from 1.15 Nm3/kg to 2.53 Nm3/kg, respectively. Furthermore, mez-Barea et al. [7] also revealed that when the findings from Go operating temperature was in the range of 820e1200 C, an increase in operating temperature could increase the gas production rate from 67% to 81% and reduce the tar yield. Gao et al. [8] and Ma et al. [9] also addressed that an increase in operating temperature could increase the amount of H2 in the syngas. Luo et al. [6] suggested that an operating temperature increase could intensify the
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wateregas shift reaction, and Boudouard reaction, resulting in the production of more H2 and CO; the reaction mechanism is as follows: Wateregas shift reaction
CO þ H2 O4CO2 þ H2
(1)
Boudouard reaction
C þ CO2 42CO
(2)
Another important parameter in the gasification process is ER, which is the ratio of the actual airefuel ratio and the stoichiometric airefuel ratio. It is an essential parameter indicative of whether there is complete oxidation in the gasification process. Lv et al. [10] investigated the impact of ER adjustment upon the product gas composition in the gasification process. When ER was increased to 0.27 from 0.19, the proportion of CO2 gas in the produced gas showed an upward trend and those of CO and H2 were slightly decreased. Hence, when ER is increased, the amount of oxygen that enters the gasifier and undergoes the reaction also increases such that the carbon conversion efficiency is enhanced, the CO2 production is increased, and the CO and H2 productions decrease. A study by Chiang et al. [11] showed that after the ER value increased, the amount of oxygen in the gasifier increased, and the combustion efficiency improved. When the ER value increased from 0.2 to 0.3, the difference between the H2 production and the productions of CO2 and CO was small; however, when the ER value increased to 0.4, the CO2 production increased by 10%, and the productions of CO and H2 decreased by 8% and 3%, respectively. Gregorio and Zaccariello [12] found that when the ER value increased from 0.26 to 0.31, except for increased CO2 production, the productions of CO, H2, CH4, and tar all decreased. Based on the results of previous studies, an appropriate ER value for the gasification process should be controlled in the range of 0.2e0.4 [5]. The S/B ratio is the ratio of the amount of steam to the amount of biomass feed in the gasifier and is also an important parameter affecting gasification. A study by Ruiz et al. [13] showed that when the gasification temperature was maintained at 750 C whilst increasing the S/B ratio, the productions of CH4, H2, and CO2 gradually increased, while CO production decreased. A study by Dascomb et al. [14] suggested that when the S/B ratio increased from 0.7 to 4.5 and the temperature was maintained at 850 C, H2 production increased by 10%, but the energy conversion efficiency decreased from 68% to 42%. Furthermore, Wang et al. [15] also found that an increase in the S/B ratio (0.0e1.23) led to increases in the total gas yield and H2 production as well as a decrease in the tar production. As an increase in the S/B ratio can increase the amount of H2 participating in the gasification process, an increase in H2 production via enhancement of the S/B ratio was observed. However, the S/B ratio should be controlled within a certain range, otherwise excessive water vapor may absorb heat inside the gasifier, thereby causing the gasification reaction to not proceed efficiently owing to the temperature decrease. Currently, numerous studies have focused on modifying the gasifier design to enhance syngas production, in order to increase the value of syngas for reuse. Tar, char, and other challenges can still simultaneously occur in the gasification process using fluidized-bed gasifiers; consequently, the two-stage gasification process is a relatively new technology. In a study by Soni et al. [16], waste from the slaughtering industry was employed as raw material to conduct oxidation, and the reactors of the first and second stages were fixedbed gasifiers. The difference between the first stage and the second stage gasification processes was compared, and the results suggested that the second-stage oxidation could effectively enhance the H2 yield (7.3%e22.3%) and total gas yield (30.8%e54.6%);
moreover, the tar yield decreased from 18.6% to 14.2%. Xiao et al. [17] used woodchips and other biomass as raw materials to conduct the second-stage gasification, and the syngas, tar, and additional products from the first-stage gasification reaction entered the second-stage reactor to undergo reactions. In contrast, tar and char were burned or cracked in the second-stage reactor, and the discharge of tar and char was reduced. In a study by Park et al. [18], two fixed-bed reactors were utilized for two-stage gasification, and there were complex hydrocarbons in the tar generated from the first stage. The tar entered the second-stage reactor to undergo a cracking reaction, and the possible reactions are as follows [18,19]:
Cn Hx /Cm Hy þ H2
(3)
Cn Hx þ H2 O/H2 þ CO
(4)
Cn Hx /Coke þ Other gases
(5)
As the fluidized bed reactor has advantages such as high heat transfer and high mass transfer efficiencies, its application in gasification will help enhance the overall gasification efficiency. Accordingly, if the two fluidized bed gasifiers are series-connected to conduct the two-stage gasification process, the gasification efficiency will improve, the generated tar will decrease, and the H2 production rate will increase. In this study, two-stage fluidized bed gasifiers were employed, and the operating temperature of the first-stage fluidized bed reactor, ER, and S/B ratio were changed to investigate their impact on the product gas composition from the second-stage fluidized bed gasification. 2. Experimental methods In this study, a laboratory-scale, two-stage fluidized bed gasifier was adopted, and the structural diagram is shown in Fig. 1. The gasifier was made of stainless steel (AISI-310), with a thickness of 0.49 cm and a height of 50 cm; the outer diameter of the gasifier body was 4.27 cm and the inner diameter was 3.29 cmdthe specifications were the same for the gasifier bodies of the first and second stages. A stainless steel distributor plate (Perforated mesh plate) was installed at the bottom of each fluidized bed reactor, with an open area of 15.2%. Although the distributor design is not entirely in compliance with the recommendations of Geldart and
Fig. 1. Two-stage bubbling 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.
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Table 1 Ultimate analysis, proximate analysis, and heating value analysis of artificial waste. Species Ultimate analysis (wt%) C H O N Proximate analysis (wt%) Moisture Volatile matter Fixed carbon Ash Lower Heating Value (MJ/kg)
Polypropylene (PP)
Wood chips
Polyethylene (PE)
86.3 12.77 0.35 0.57
45.98 7.32 46.51 0.18
85.71 13.04 0.39 0.86
0.01 99.99 <0.01 <0.01 44.27
7.27 76.25 16.48 <0.01 15.83
0.01 99.99 <0.01 <0.01 44.92
Baeyens [20], the fluidization appeared to be uniform, and the simple distributor design made the rig more economical. An electric heating system was utilized in the gasifier, and the outside of the gasifier body was insulated by fibers to prevent heat loss; three thermocouple sets were used to monitor and record the temperature changes in the gasifier. Two manually operated directional valves were designed for the feed inlet to prevent leaks of syngas generated from the gasifier or the outside air from entering the gasifier, which would affect the results. Silica sand was used as the two-stage bed material for this experiment, with a density of 2.6 g/ cm3; the composition of the silica sand was as follows: 97.8% SiO2, 2% Al2O3, and 0.07% Fe2O3. The mean particle size of the first-stage bed material was 0.775 mm, and the ratio of the bed height to the diameter was approximately two-fold (H/D ¼ 2); the mean particle size of the second-stage bed material was 0.545 mm, and the ratio of the bed height to the diameter was approximately one-fold (H/ D ¼ 1). The waste/biomass materials used in this study were prepared artificially by employing polyethylene (PE) plastic bags (0.222 g) to wrap wood chips (1.04 g) and polypropylene (PP) plastic particles (0.11 g). Each packet of artificial feed weighed 1.372 g and one packet was fed in per 20 s. The results from the ultimate analysis, proximate analysis, and heating value analysis of these materials are shown in Table 1. Before the experiment, a water manometer was used to measure the minimum fluidization velocity of the fluidized bed; the gas flow rate applied in the experiment was 1.3 times the minimum fluidization velocity, and the method was based on a study by Lin et al. [21]. Experimental parameters included the operating temperature of the first-stage fluidized bed reactor, S/B ratio, and ER values, and each test procedure is described in Table 2. In the experiment, the artificial waste/biomass was placed into the gasifier at a fixed rate for gasification. Additionally, water was directly added to the artificial wastes to simulate the different S/B ratios. During sampling, the syngas passed through a glass-fiber filter device, and most of the particles were trapped on a glass microfiber filter. Subsequently, the syngas passed through the impingers in a low temperature water bath, and the remaining fly ash and steam were filtered and absorbed by the glass microfiber filter and silica gel. Finally, an active sampling pump was utilized for sampling, and the samples were stored in air-sampling bags. The sampled syngas was then analyzed by a gas chromatograph coupled to a thermal conductivity detector (GCTCD). The injection port temperature was 200 C, and the detector temperature was 250 C. The initial oven temperature of 40 C was maintained for three minutes. Then the temperature was increased to 240 C at a rate of 30 C/min, and was held for one minute. The sample analysis duration was 10.67 min, and the target gases were H2, CH4, CO, CO2, and N2. The acquired results were analyzed, and the calculated results were expressed in terms of the product gas distribution ratio, total gas yield, and total gas heating value.
3. Results and discussion Fig. 2 presents the distribution of syngas composition over time when both gasification temperatures of the first stage and second stage were 900 C. Silica sand (SiO2) was used as the bed material for both stages; the particle sizes of bed materials for the first stage and second stage were 0.775 mm and 0.545 mm, respectively. With an ER value of 0.3 and an S/B ratio of 0.0, sampling at three-minute intervals started in the fifth minute of operation to observe the gas distribution. The results revealed that gas composition after 14 min of operation tended to be stable. Therefore, the data of the syngases sampled after the 14th minute of the experiment were used for the discussion in this study. 3.1. Effect of operating temperature on the product gas composition from the second-stage gasification Fig. 3(a) shows the distribution of syngas composition when the first-stage temperatures were set at 700, 800, and 900 C, the second stage temperature was set at 900 C, and the ER and S/B ratio values were set at 0.3 and 0.0, respectively. When the operating temperature of the first stage increased from 700 C to 900 C, the proportions of H2 and CH4 increased by 5.79 mol% and 0.32 mol %, respectively, and those of CO and CO2 decreased by 0.23 mol% and 5.88 mol%, respectively. The experimental results indicated that an increase in the operating temperature in the first stage could elevate the proportions of H2, CO, and CH4, and relatively reduce the CO2 proportion. According to a study by Luo et al. [6], when the temperature increased, tar cracking and reforming
Table 2 The operating conditions for each experiment. Run
Temperature ( C)
S/B
ER
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
800 800 800 800 800
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
900 900 900 900 900
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
Stage 1
Stage 2
1 2 3 4 5
700 700 700 700 700
6 7 8 9 10 11 12 13 14 15
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100
H2 CO CO2
Gas composition (mol%)
80
CH4 S-1 S-2
60
40
20
0 5
8
11
14
17
Time (min) Fig. 2. The syngas composition at different operation times. (Stage 1: 900 C, Stage 2: 900 C, ER: 0.3 and S/B: 0.0).
reactions were intensified; thereby increasing the H2 and CO yields. Additionally, CH4 underwent the reforming reaction again with increasing temperature, and the proportions of H2 and CO2 increased and decreased, respectively. Additionally, Niu et al. [22] showed that when the temperature increased, the amount of tar produced was reduced during gasification process and increased the proportions of H2, since the steam cracking and tars reforming. The results of Guo et al. [23], Kook et al. [24] and Robinson et al. [25] had the same results. Therefore, an increase in the operating temperature of the first stage is beneficial in increasing the proportion of H2 in the syngas. The product of the first stage can undergo the reactions again after entering the second-stage fluidized bed reactor. As can be seen in Fig. 3(a), after the gasification with an operating temperature of 900 C in the second stage, the proportion of H2 in the syngas increased significantly; the highest proportion of H2 gas was 44 mol%. When the operating temperature of the first stage was 700 C or 800 C, the H2 ratio increased by approximately 17e19 mol%; when operating temperatures of both the first stage and second stage were 900 C, the H2 proportion only increased by 10 mol%. Nevertheless, regardless of the operating temperature of the first stage, the proportion of H2 in the syngas from the second stage was more than 40 mol% because the second-stage gasifier provided a high-temperature environment and extended the product residence time such that the gas produced in the first stage was reformed in the second-stage gasifier, and tar underwent the reforming cracking reactions. As a result, the H2 production from the second stage was higher than that from the first stage. Fig. 3(b) displays the total gas yield and the syngas heating value when the operating temperatures of the first stage were 700, 800, and 900 C, respectively, and the operating temperature of the second stage was 900 C. The results showed that when the operating temperature of the first stage increased from 700 C to 800 C and 900 C, the total gas yield increased from 0.497 Nm3/kg to 0.614 Nm3/kg, but the total gas yield of the second stage decreased. According to a study by Alauddin et al. [5], temperature is the most important factor affecting the gas composition and gas heating value during the operation, and an increase in the temperature will result in a relative increase in the total gas yield. The decrease in the
total gas yield of the second stage compared with that of the first stage was a result of the temperature rise in the second stage intensifying the methaneesteam reaction, thus enhancing H2 production, reducing the productions of CO, CH4, and CO2, and reducing the total gas yield. With regard to the total gas heating value, when the operating temperature increased from 700 C to 900 C, the total gas heating value of the syngas exhibited an upward trend. When the temperature of the first stage was raised from 700 C to 800 C and 900 C, the heating value of the first stage increased from 0.7 MJ/kg to 1.46 MJ/kg. The heating value of the second stage decreased because the temperature increase intensified the methaneesteam reaction, thus enhancing the H2 production and reducing the productions of CO, CH4, and CO2. However, the heating value of H2 was lower than those of the other gases such that after the second-stage reactions, the total heating value of the syngas decreased. 3.2. Effect of ER on the product gas composition of the two-stage gasification process Fig. 4 presents the effect of increasing the ER from 0.2 to 0.4 on the product gas composition when the operating temperatures for the first stage were 700, 800, and 900 C, the temperature of the second stage was 900 C, and the S/B ratio was 0.0. When the ER was 0.4, the proportion of H2 in the syngas decreased with firststage operating temperatures of 700, 800, or 900 C, while the proportion of CO2 increased. Niu et al. [22], Kook et al. [24] and Gil et al. [26] also pointed that when ER increased, the proportion of H2 in the syngas decreased. However, ER increased helps to reduce tar generation to generate clean syngas [26e28]. Ran and Li [29] pointed that an ER increase provided more oxygen, improved the burning efficiency, and accelerated the oxidation reaction so that the CO2 production increased and the productions of H2, CH4, and CO relatively decreased. However, if the outlet syngas composition of the second stage was compared, the proportion of H2 significantly increased when the syngas passed through the second stage. The largest increment in H2 production (19.42 mol%) for the second stage compared with the first stage was observed when the ER and the operating temperatures in the first stage and the second stage were 0.3, 800 C, and 900 C, respectively. Fig. 4 also indicates that
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100 H2
(a)
CO CH4
Gas composition (mol%)
80
CO2 S-1 S-2
60
40
20
0 700/900
800/900
900/900
Temperture (oC)
2.5
0.8 H2
(b)
CO CH4
2.0
1.5 0.4 1.0
Gas yield (Nm3/kg)
LHV (MJ/kg)
0.6
S-1 S-2 S-1 S-2
LHV LHV Gas Gas
0.2 0.5
0.0
0.0 700/900
800/900
900/900
Temperature (oC) Fig. 3. The effect of different operating temperature in Stage 1 on gasification efficiency. (a) Syngas compositions; (b) Syngas heating value and syngas yield.
when the first stage was at a low temperature (700 C), the ER exhibited a relatively large impact on the H2 proportion in the outlet syngas of the second stage. If the temperature of the first stage increased to 800 C or 900 C, the impact of the ER on the H2 proportion of the overall syngas was not significant. Fig. 5 describes the effect of the variation in the ER on total gas yield and the total gas heating value when the temperatures of both stages were 900 C and the S/B ratio was 0.0. When the ER value increased from 0.2 to 0.4, the total gas yield from the first stage was gradually decreased, suggesting that an ER increase can increase the amount of oxygen in the system. Consequently, the organic gases can be oxidized to CO2, causing a decline in gas production. Meanwhile, the effect of ER on the total heating value is shown in Fig. 5. The total heating value of the outlet syngas of the first stage or the second stage decreased with increasing ER value, indicating an increase in the ER can also enhance the burning efficiency, reduce the productions of H2, CO, and CH4, and reduce the total heating value and gas yield. A study by Alauddin et al. [5] indicated that a high ER value in the gasifier system could intensify the combustion, produce a large amount of CO2, and reduce the productions of H2, CH4, CO, and other gases. Therefore, although the passing of the syngas through a second stage helps to improve the proportion of H2 in the syngas, an increased ER will gradually decrease the total heating value of the syngas.
3.3. Effect of the S/B ratio on the product gas composition of the second-stage gasification process Fig. 6 depicts the effect of the change in the S/B ratio from 0 to 0.5 on the product gas composition when the operating temperatures for the first stage were 700, 800, and 900 C, the operating temperature of the second stage was 900 C, and the ER was 0.3. As can be seen in Fig. 6(a), when the operating temperature of the first stage was 700 C, an increased S/B ratio exhibited no significant impact on the proportion of the H2 in the syngas. When the operating temperature of the first stage was 800 C or 900 C, an increase in the S/B ratio from 0.0 to 0.5 could relatively enhance the proportions of H2, CO2, and CH4 in the syngas. The findings of Luo et al. [6] indicated that steam could intensify the methane reforming reaction, promote tar cracking and reforming reactions, produce more H2, and increase carbon conversion efficiency. However, if the system was operated at low temperatures, excessive steam entering the system reduced the reaction temperature of the system, and therefore, the impact of the S/B ratio at a low temperature was not significant. In Fig. 6(c), the operating temperatures of both stages were 900 C. When the S/B ratio increased to 0.5, the proportion of H2 in the syngas of the first stage pronouncedly increased, indicating a significant impact on the steam at a high temperature. Meanwhile, the proportion of H2 in the outlet syngas of the second stage also increased to a maximum of
140
100
100 H2
H2
CO2 S-1 S-2
60
40
20
80
Gas composition (mol%)
(a)
CO CH4
(b)
CO2 S-1 S-2
60
40
20
0
0
0.2
0.3
0.4
0.2
Equivalence ratio
0.3
0.4
Equivalence ratio
100
(c)
H2 CO CH4
Gas composition (mol%)
80
CO2 S-1 S-2
60
40
20
0 0.2
0.3
0.4
Equivalence ratio Fig. 4. The effect of different ER values on syngas compositions. (a) Stage 1: 700 C, Stage 2: 900 C; (b) Stage 1: 800 C, Stage 2: 900 C; (c) Stage 1: 900 C, Stage 2: 900 C.
C.-L. Lin, W.-C. Weng / Renewable Energy 109 (2017) 135e143
Gas composition (mol%)
80
CO CH4
C.-L. Lin, W.-C. Weng / Renewable Energy 109 (2017) 135e143
2.5
141
0.8 H2 CO CH4
2.0
Gas yield (Nm3/kg)
LHV (MJ/kg)
0.6
1.5 0.4 1.0
S-1 S-2 S-1 S-2
LHV LHV Gas Gas
0.2 0.5
0.0
0.0 0.2
0.3
0.4
Equivalence ratio Fig. 5. The effect of different ER values on syngas heating value and syngas yield. (Stage 1: 900 C, Stage 2: 900 C).
100
100
(a)
CO2 S-1 S-2
60
40
H2 CO CH4
80
Gas composition (mol%)
CO CH4
80
Gas composition (mol%)
(b)
H2
CO2 S-1 S-2
60
40
20
20
0
0 0
0.25
0
0.5
Steam/Biomass ratio
0.25
0.5
Steam/Biomass ratio
100
(c)
H2 CO CH4
Gas composition (mol%)
80
CO2 S-1 S-2
60
40
20
0 0
0.25
0.5
Steam/Biomass ratio
Fig. 6. The effect of different S/B ratios on syngas compositions. (a) Stage 1: 700 C, Stage 2: 900 C; (b) Stage 1: 800 C, Stage 2: 900 C; (c) Stage 1: 900 C, Stage 2: 900 C.
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2.5
0.8 H2 CO CH4
2.0
1.5 0.4 1.0
Gas yield (Nm3/kg)
LHV (MJ/kg)
0.6
S-1 S-2 S-1 S-2
LHV LHV Gas Gas
0.2 0.5
0.0
0.0 0
0.25
0.5
Steam/Biomass ratio Fig. 7. The effect of different S/B ratios on syngas heating value and syngas yield. (Stage 1: 900 C, Stage 2: 900 C).
52.1 mol%. In contrast, with increasing S/B ratio, the difference between the first stage and the second stage on the H2 proportion in the outlet syngas became smaller. This result suggested that if a high-temperature operation is performed and sufficient steam is provided in the first stage, a high proportion of H2 can be obtained, but with no substantial increase from the operation of the second stage. Fig. 7 shows the effect of the changes in the S/B ratio on the total gas yield and syngas heating value when the operating temperatures of both stages were 900 C and the ER was 0.3. When the S/B ratio increased, the proportion of H2 in the syngas increased, and the proportions of CO, CH4, and other gases relatively decreased such that the total heating value of the syngas exhibited a downward trend when increasing the S/B ratio. After passing through the second stage, the syngas underwent reforming reactions and the H2 production increased; thus, the total heating value of the outlet syngas of the second stage also decreased when increasing the S/B ratio. Findings by Palumbo et al. [30] suggested that a hightemperature operation and an increased amount of steam could accelerate the methaneesteam reforming reaction so that CH4, CO, and CO2 underwent the reactions, and H2 production improved. As H2 has a low heating-value compared with CO, CO2, and CH4, the total gas yield and heating value were reduced.
4. Conclusions In this study, the effects of different operating parameters (temperature, ER and S/B ratio) upon the product gas composition of a two-stage fluidized bed gasifier were explored by mainly changing the temperature of the fluidized bed gasifier in the first stage in addition to the ER and S/B ratio values. The results showed that when the operating temperature increased from 700 C to 900 C, the proportion of H2 in the syngas increased significantly. Moreover, when the operating temperature of the fluidized bed reactor in the second stage was controlled at 900 C, the proportion of H2 in the syngas was effectively enhanced to more than 42 mol%, although the operating temperature of the first stage was only 700 C. When appropriate ER and S/B ratio values were utilized, the proportion of H2 was still enhanced to more than 52 mol%.
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C.-L. Lin, W.-C. Weng / Renewable Energy 109 (2017) 135e143 [18] Y. Park, T. Namioka, S. Sakamoto, T.J. Min, S.A. Roh, K. Yoshikawa, Optimum operating conditions for a two-stage gasification process fueled by polypropylene by means of continuous reactor over ruthenium catalyst, Fuel Process. Technol 91 (2010) 951e957. [19] T.Y. Mun, P.G. Seon, J.S. Kim, Production of a producer gas from woody waste via air gasification using activated carbon and a two-stage gasifier and characterization of tar, Fuel 89 (2010) 3226e3234. [20] D. Geldart, J. Baeyens, The design of distributors for gas-fluidized beds, Powder Technol. 42 (1985) 67e78. [21] C.L. Lin, M.Y. Wey, S.D. You, The effect of particle size distribution on minimum fluidization velocity at high temperature, Powder Technol. 126 (2002) 297e301. [22] M. Niu, Y. Huang, B. Jin, X. Wang, Oxygen gasification of municipal solid waste in a fixed-bed gasifier, Chin. J. Chem. Eng. 22 (2014) 1021e1026. [23] F. Guo, Y. Dong, L. Dong, C. Guo, Effect of design and operating parameters on the gasification process of biomass in a downdraft fixed bed: an experimental study, Int. J. Hydrogen Energ 39 (2014) 5625e5633. [24] J.W. Kook, H.M. Choi, B.H. Kim, H.W. Ra, S.J. Yoon, T.Y. Mun, J.H. Kim, Y.K. Kim, J.G. Lee, M.W. Seo, Gasification and tar removal characteristics of rice husk in a
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Effects of different operating parameters on the syngas composition in a two-stage gasification process Chiou-Liang Lin*, Wang-Chang Weng Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung, 811, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2016 Received in revised form 2 February 2017 Accepted 8 March 2017 Available online 9 March 2017
This research used a two-stage fluidized-bed gasifier to investigate the effects of the temperature, equivalence ratio, and steam/biomass ratio on the syngas composition. When the operating temperature in the first stage increased from 700 C to 900 C, the proportion of H2 in the syngas increased significantly. After passing through the second stage (900 C), the syngas produced from the first stage underwent the thermal reaction again, and the proportion of H2 was further increased. When the ER value increased from 0.2 to 0.3, the proportion of H2 in the syngas increased; whereas, when the ER value increased to 0.4, the amount of H2 produced was reduced. For S/B ratio, an increase to 0.5 enhanced the steam content of the gasifier and accelerated the methaneesteam reforming reaction, thus producing more H2 (up to 52 mol%). Furthermore, when the operating temperature of the fluidized bed reactor at the second stage was set at 900 C, the proportion of H2 in the syngas could still be effectively improved to more than 42 mol% although the operating temperature at the first stage was only 700 C. The proportion of H2 was enhanced to more than 52 mol% with a combination of appropriate ER and S/B ratio values. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Two-stage gasifier Equivalence ratio Steam/biomass Syngas
1. Introduction Taiwan currently has 24 large incineration plants in operation, and nearly 600 million tons of waste can be processed each year. Since 2008, the disposal rate of municipal solid waste in Taiwan has been 99.9%; except for recycling, 97% of the remaining wastes are processed by incineration [1]. However, with increasing resource scarcity and the rise of the “zero waste” concept, waste has been increasingly considered as another resource. Since the majority of recycled waste is organic matter, the transition from incineration to gasification of wastes can not only solve the problem of waste disposal, but also produce reusable energy. With gasification technologies, organic materials are converted into reusable syngas (e.g. a mixed gas comprising CO, CH4, and H2) via partial oxidation for further purification and application. Generally, the common gasification operating parameters include the type of biomass, temperature, pressure, feed particle size, particle size of the bed material, type of gases, equivalence ratio (ER), and steam/biomass (S/B) ratio [2e5]; the operating temperature, ER, and S/B ratio all
* Corresponding author. E-mail address: [email protected] (C.-L. Lin). http://dx.doi.org/10.1016/j.renene.2017.03.019 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
exhibit significant impacts on the syngas composition from the gasification process. In the gasification process, the operating temperature directly affects the gasification reaction. A temperature increase changes the surface temperature of the bed material so that there is an improvement in the thermal conductivity between the bed material and biomass, thereby affecting the product gas composition and heating value. Kumar et al. [4] investigated the effect of operating temperatures in the range of 650e850 C on gasification and found that the highest carbon conversion efficiency (82%) and energy conversion efficiency (96%) were obtained at 850 C; the H2 concentration increased from 4% at 650 C to 15% at 850 C. A study by Luo et al. [6] suggested that when the vaporization temperature increased from 600 C to 900 C, the carbon conversion efficiency and the amount of gas generated increased from 61.96% to 92.59% and from 1.15 Nm3/kg to 2.53 Nm3/kg, respectively. Furthermore, mez-Barea et al. [7] also revealed that when the findings from Go operating temperature was in the range of 820e1200 C, an increase in operating temperature could increase the gas production rate from 67% to 81% and reduce the tar yield. Gao et al. [8] and Ma et al. [9] also addressed that an increase in operating temperature could increase the amount of H2 in the syngas. Luo et al. [6] suggested that an operating temperature increase could intensify the
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wateregas shift reaction, and Boudouard reaction, resulting in the production of more H2 and CO; the reaction mechanism is as follows: Wateregas shift reaction
CO þ H2 O4CO2 þ H2
(1)
Boudouard reaction
C þ CO2 42CO
(2)
Another important parameter in the gasification process is ER, which is the ratio of the actual airefuel ratio and the stoichiometric airefuel ratio. It is an essential parameter indicative of whether there is complete oxidation in the gasification process. Lv et al. [10] investigated the impact of ER adjustment upon the product gas composition in the gasification process. When ER was increased to 0.27 from 0.19, the proportion of CO2 gas in the produced gas showed an upward trend and those of CO and H2 were slightly decreased. Hence, when ER is increased, the amount of oxygen that enters the gasifier and undergoes the reaction also increases such that the carbon conversion efficiency is enhanced, the CO2 production is increased, and the CO and H2 productions decrease. A study by Chiang et al. [11] showed that after the ER value increased, the amount of oxygen in the gasifier increased, and the combustion efficiency improved. When the ER value increased from 0.2 to 0.3, the difference between the H2 production and the productions of CO2 and CO was small; however, when the ER value increased to 0.4, the CO2 production increased by 10%, and the productions of CO and H2 decreased by 8% and 3%, respectively. Gregorio and Zaccariello [12] found that when the ER value increased from 0.26 to 0.31, except for increased CO2 production, the productions of CO, H2, CH4, and tar all decreased. Based on the results of previous studies, an appropriate ER value for the gasification process should be controlled in the range of 0.2e0.4 [5]. The S/B ratio is the ratio of the amount of steam to the amount of biomass feed in the gasifier and is also an important parameter affecting gasification. A study by Ruiz et al. [13] showed that when the gasification temperature was maintained at 750 C whilst increasing the S/B ratio, the productions of CH4, H2, and CO2 gradually increased, while CO production decreased. A study by Dascomb et al. [14] suggested that when the S/B ratio increased from 0.7 to 4.5 and the temperature was maintained at 850 C, H2 production increased by 10%, but the energy conversion efficiency decreased from 68% to 42%. Furthermore, Wang et al. [15] also found that an increase in the S/B ratio (0.0e1.23) led to increases in the total gas yield and H2 production as well as a decrease in the tar production. As an increase in the S/B ratio can increase the amount of H2 participating in the gasification process, an increase in H2 production via enhancement of the S/B ratio was observed. However, the S/B ratio should be controlled within a certain range, otherwise excessive water vapor may absorb heat inside the gasifier, thereby causing the gasification reaction to not proceed efficiently owing to the temperature decrease. Currently, numerous studies have focused on modifying the gasifier design to enhance syngas production, in order to increase the value of syngas for reuse. Tar, char, and other challenges can still simultaneously occur in the gasification process using fluidized-bed gasifiers; consequently, the two-stage gasification process is a relatively new technology. In a study by Soni et al. [16], waste from the slaughtering industry was employed as raw material to conduct oxidation, and the reactors of the first and second stages were fixedbed gasifiers. The difference between the first stage and the second stage gasification processes was compared, and the results suggested that the second-stage oxidation could effectively enhance the H2 yield (7.3%e22.3%) and total gas yield (30.8%e54.6%);
moreover, the tar yield decreased from 18.6% to 14.2%. Xiao et al. [17] used woodchips and other biomass as raw materials to conduct the second-stage gasification, and the syngas, tar, and additional products from the first-stage gasification reaction entered the second-stage reactor to undergo reactions. In contrast, tar and char were burned or cracked in the second-stage reactor, and the discharge of tar and char was reduced. In a study by Park et al. [18], two fixed-bed reactors were utilized for two-stage gasification, and there were complex hydrocarbons in the tar generated from the first stage. The tar entered the second-stage reactor to undergo a cracking reaction, and the possible reactions are as follows [18,19]:
Cn Hx /Cm Hy þ H2
(3)
Cn Hx þ H2 O/H2 þ CO
(4)
Cn Hx /Coke þ Other gases
(5)
As the fluidized bed reactor has advantages such as high heat transfer and high mass transfer efficiencies, its application in gasification will help enhance the overall gasification efficiency. Accordingly, if the two fluidized bed gasifiers are series-connected to conduct the two-stage gasification process, the gasification efficiency will improve, the generated tar will decrease, and the H2 production rate will increase. In this study, two-stage fluidized bed gasifiers were employed, and the operating temperature of the first-stage fluidized bed reactor, ER, and S/B ratio were changed to investigate their impact on the product gas composition from the second-stage fluidized bed gasification. 2. Experimental methods In this study, a laboratory-scale, two-stage fluidized bed gasifier was adopted, and the structural diagram is shown in Fig. 1. The gasifier was made of stainless steel (AISI-310), with a thickness of 0.49 cm and a height of 50 cm; the outer diameter of the gasifier body was 4.27 cm and the inner diameter was 3.29 cmdthe specifications were the same for the gasifier bodies of the first and second stages. A stainless steel distributor plate (Perforated mesh plate) was installed at the bottom of each fluidized bed reactor, with an open area of 15.2%. Although the distributor design is not entirely in compliance with the recommendations of Geldart and
Fig. 1. Two-stage bubbling 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.
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Table 1 Ultimate analysis, proximate analysis, and heating value analysis of artificial waste. Species Ultimate analysis (wt%) C H O N Proximate analysis (wt%) Moisture Volatile matter Fixed carbon Ash Lower Heating Value (MJ/kg)
Polypropylene (PP)
Wood chips
Polyethylene (PE)
86.3 12.77 0.35 0.57
45.98 7.32 46.51 0.18
85.71 13.04 0.39 0.86
0.01 99.99 <0.01 <0.01 44.27
7.27 76.25 16.48 <0.01 15.83
0.01 99.99 <0.01 <0.01 44.92
Baeyens [20], the fluidization appeared to be uniform, and the simple distributor design made the rig more economical. An electric heating system was utilized in the gasifier, and the outside of the gasifier body was insulated by fibers to prevent heat loss; three thermocouple sets were used to monitor and record the temperature changes in the gasifier. Two manually operated directional valves were designed for the feed inlet to prevent leaks of syngas generated from the gasifier or the outside air from entering the gasifier, which would affect the results. Silica sand was used as the two-stage bed material for this experiment, with a density of 2.6 g/ cm3; the composition of the silica sand was as follows: 97.8% SiO2, 2% Al2O3, and 0.07% Fe2O3. The mean particle size of the first-stage bed material was 0.775 mm, and the ratio of the bed height to the diameter was approximately two-fold (H/D ¼ 2); the mean particle size of the second-stage bed material was 0.545 mm, and the ratio of the bed height to the diameter was approximately one-fold (H/ D ¼ 1). The waste/biomass materials used in this study were prepared artificially by employing polyethylene (PE) plastic bags (0.222 g) to wrap wood chips (1.04 g) and polypropylene (PP) plastic particles (0.11 g). Each packet of artificial feed weighed 1.372 g and one packet was fed in per 20 s. The results from the ultimate analysis, proximate analysis, and heating value analysis of these materials are shown in Table 1. Before the experiment, a water manometer was used to measure the minimum fluidization velocity of the fluidized bed; the gas flow rate applied in the experiment was 1.3 times the minimum fluidization velocity, and the method was based on a study by Lin et al. [21]. Experimental parameters included the operating temperature of the first-stage fluidized bed reactor, S/B ratio, and ER values, and each test procedure is described in Table 2. In the experiment, the artificial waste/biomass was placed into the gasifier at a fixed rate for gasification. Additionally, water was directly added to the artificial wastes to simulate the different S/B ratios. During sampling, the syngas passed through a glass-fiber filter device, and most of the particles were trapped on a glass microfiber filter. Subsequently, the syngas passed through the impingers in a low temperature water bath, and the remaining fly ash and steam were filtered and absorbed by the glass microfiber filter and silica gel. Finally, an active sampling pump was utilized for sampling, and the samples were stored in air-sampling bags. The sampled syngas was then analyzed by a gas chromatograph coupled to a thermal conductivity detector (GCTCD). The injection port temperature was 200 C, and the detector temperature was 250 C. The initial oven temperature of 40 C was maintained for three minutes. Then the temperature was increased to 240 C at a rate of 30 C/min, and was held for one minute. The sample analysis duration was 10.67 min, and the target gases were H2, CH4, CO, CO2, and N2. The acquired results were analyzed, and the calculated results were expressed in terms of the product gas distribution ratio, total gas yield, and total gas heating value.
3. Results and discussion Fig. 2 presents the distribution of syngas composition over time when both gasification temperatures of the first stage and second stage were 900 C. Silica sand (SiO2) was used as the bed material for both stages; the particle sizes of bed materials for the first stage and second stage were 0.775 mm and 0.545 mm, respectively. With an ER value of 0.3 and an S/B ratio of 0.0, sampling at three-minute intervals started in the fifth minute of operation to observe the gas distribution. The results revealed that gas composition after 14 min of operation tended to be stable. Therefore, the data of the syngases sampled after the 14th minute of the experiment were used for the discussion in this study. 3.1. Effect of operating temperature on the product gas composition from the second-stage gasification Fig. 3(a) shows the distribution of syngas composition when the first-stage temperatures were set at 700, 800, and 900 C, the second stage temperature was set at 900 C, and the ER and S/B ratio values were set at 0.3 and 0.0, respectively. When the operating temperature of the first stage increased from 700 C to 900 C, the proportions of H2 and CH4 increased by 5.79 mol% and 0.32 mol %, respectively, and those of CO and CO2 decreased by 0.23 mol% and 5.88 mol%, respectively. The experimental results indicated that an increase in the operating temperature in the first stage could elevate the proportions of H2, CO, and CH4, and relatively reduce the CO2 proportion. According to a study by Luo et al. [6], when the temperature increased, tar cracking and reforming
Table 2 The operating conditions for each experiment. Run
Temperature ( C)
S/B
ER
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
800 800 800 800 800
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
900 900 900 900 900
900 900 900 900 900
0.0 0.0 0.0 0.25 0.5
0.2 0.3 0.4 0.3 0.3
Stage 1
Stage 2
1 2 3 4 5
700 700 700 700 700
6 7 8 9 10 11 12 13 14 15
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100
H2 CO CO2
Gas composition (mol%)
80
CH4 S-1 S-2
60
40
20
0 5
8
11
14
17
Time (min) Fig. 2. The syngas composition at different operation times. (Stage 1: 900 C, Stage 2: 900 C, ER: 0.3 and S/B: 0.0).
reactions were intensified; thereby increasing the H2 and CO yields. Additionally, CH4 underwent the reforming reaction again with increasing temperature, and the proportions of H2 and CO2 increased and decreased, respectively. Additionally, Niu et al. [22] showed that when the temperature increased, the amount of tar produced was reduced during gasification process and increased the proportions of H2, since the steam cracking and tars reforming. The results of Guo et al. [23], Kook et al. [24] and Robinson et al. [25] had the same results. Therefore, an increase in the operating temperature of the first stage is beneficial in increasing the proportion of H2 in the syngas. The product of the first stage can undergo the reactions again after entering the second-stage fluidized bed reactor. As can be seen in Fig. 3(a), after the gasification with an operating temperature of 900 C in the second stage, the proportion of H2 in the syngas increased significantly; the highest proportion of H2 gas was 44 mol%. When the operating temperature of the first stage was 700 C or 800 C, the H2 ratio increased by approximately 17e19 mol%; when operating temperatures of both the first stage and second stage were 900 C, the H2 proportion only increased by 10 mol%. Nevertheless, regardless of the operating temperature of the first stage, the proportion of H2 in the syngas from the second stage was more than 40 mol% because the second-stage gasifier provided a high-temperature environment and extended the product residence time such that the gas produced in the first stage was reformed in the second-stage gasifier, and tar underwent the reforming cracking reactions. As a result, the H2 production from the second stage was higher than that from the first stage. Fig. 3(b) displays the total gas yield and the syngas heating value when the operating temperatures of the first stage were 700, 800, and 900 C, respectively, and the operating temperature of the second stage was 900 C. The results showed that when the operating temperature of the first stage increased from 700 C to 800 C and 900 C, the total gas yield increased from 0.497 Nm3/kg to 0.614 Nm3/kg, but the total gas yield of the second stage decreased. According to a study by Alauddin et al. [5], temperature is the most important factor affecting the gas composition and gas heating value during the operation, and an increase in the temperature will result in a relative increase in the total gas yield. The decrease in the
total gas yield of the second stage compared with that of the first stage was a result of the temperature rise in the second stage intensifying the methaneesteam reaction, thus enhancing H2 production, reducing the productions of CO, CH4, and CO2, and reducing the total gas yield. With regard to the total gas heating value, when the operating temperature increased from 700 C to 900 C, the total gas heating value of the syngas exhibited an upward trend. When the temperature of the first stage was raised from 700 C to 800 C and 900 C, the heating value of the first stage increased from 0.7 MJ/kg to 1.46 MJ/kg. The heating value of the second stage decreased because the temperature increase intensified the methaneesteam reaction, thus enhancing the H2 production and reducing the productions of CO, CH4, and CO2. However, the heating value of H2 was lower than those of the other gases such that after the second-stage reactions, the total heating value of the syngas decreased. 3.2. Effect of ER on the product gas composition of the two-stage gasification process Fig. 4 presents the effect of increasing the ER from 0.2 to 0.4 on the product gas composition when the operating temperatures for the first stage were 700, 800, and 900 C, the temperature of the second stage was 900 C, and the S/B ratio was 0.0. When the ER was 0.4, the proportion of H2 in the syngas decreased with firststage operating temperatures of 700, 800, or 900 C, while the proportion of CO2 increased. Niu et al. [22], Kook et al. [24] and Gil et al. [26] also pointed that when ER increased, the proportion of H2 in the syngas decreased. However, ER increased helps to reduce tar generation to generate clean syngas [26e28]. Ran and Li [29] pointed that an ER increase provided more oxygen, improved the burning efficiency, and accelerated the oxidation reaction so that the CO2 production increased and the productions of H2, CH4, and CO relatively decreased. However, if the outlet syngas composition of the second stage was compared, the proportion of H2 significantly increased when the syngas passed through the second stage. The largest increment in H2 production (19.42 mol%) for the second stage compared with the first stage was observed when the ER and the operating temperatures in the first stage and the second stage were 0.3, 800 C, and 900 C, respectively. Fig. 4 also indicates that
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139
100 H2
(a)
CO CH4
Gas composition (mol%)
80
CO2 S-1 S-2
60
40
20
0 700/900
800/900
900/900
Temperture (oC)
2.5
0.8 H2
(b)
CO CH4
2.0
1.5 0.4 1.0
Gas yield (Nm3/kg)
LHV (MJ/kg)
0.6
S-1 S-2 S-1 S-2
LHV LHV Gas Gas
0.2 0.5
0.0
0.0 700/900
800/900
900/900
Temperature (oC) Fig. 3. The effect of different operating temperature in Stage 1 on gasification efficiency. (a) Syngas compositions; (b) Syngas heating value and syngas yield.
when the first stage was at a low temperature (700 C), the ER exhibited a relatively large impact on the H2 proportion in the outlet syngas of the second stage. If the temperature of the first stage increased to 800 C or 900 C, the impact of the ER on the H2 proportion of the overall syngas was not significant. Fig. 5 describes the effect of the variation in the ER on total gas yield and the total gas heating value when the temperatures of both stages were 900 C and the S/B ratio was 0.0. When the ER value increased from 0.2 to 0.4, the total gas yield from the first stage was gradually decreased, suggesting that an ER increase can increase the amount of oxygen in the system. Consequently, the organic gases can be oxidized to CO2, causing a decline in gas production. Meanwhile, the effect of ER on the total heating value is shown in Fig. 5. The total heating value of the outlet syngas of the first stage or the second stage decreased with increasing ER value, indicating an increase in the ER can also enhance the burning efficiency, reduce the productions of H2, CO, and CH4, and reduce the total heating value and gas yield. A study by Alauddin et al. [5] indicated that a high ER value in the gasifier system could intensify the combustion, produce a large amount of CO2, and reduce the productions of H2, CH4, CO, and other gases. Therefore, although the passing of the syngas through a second stage helps to improve the proportion of H2 in the syngas, an increased ER will gradually decrease the total heating value of the syngas.
3.3. Effect of the S/B ratio on the product gas composition of the second-stage gasification process Fig. 6 depicts the effect of the change in the S/B ratio from 0 to 0.5 on the product gas composition when the operating temperatures for the first stage were 700, 800, and 900 C, the operating temperature of the second stage was 900 C, and the ER was 0.3. As can be seen in Fig. 6(a), when the operating temperature of the first stage was 700 C, an increased S/B ratio exhibited no significant impact on the proportion of the H2 in the syngas. When the operating temperature of the first stage was 800 C or 900 C, an increase in the S/B ratio from 0.0 to 0.5 could relatively enhance the proportions of H2, CO2, and CH4 in the syngas. The findings of Luo et al. [6] indicated that steam could intensify the methane reforming reaction, promote tar cracking and reforming reactions, produce more H2, and increase carbon conversion efficiency. However, if the system was operated at low temperatures, excessive steam entering the system reduced the reaction temperature of the system, and therefore, the impact of the S/B ratio at a low temperature was not significant. In Fig. 6(c), the operating temperatures of both stages were 900 C. When the S/B ratio increased to 0.5, the proportion of H2 in the syngas of the first stage pronouncedly increased, indicating a significant impact on the steam at a high temperature. Meanwhile, the proportion of H2 in the outlet syngas of the second stage also increased to a maximum of
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Equivalence ratio Fig. 4. The effect of different ER values on syngas compositions. (a) Stage 1: 700 C, Stage 2: 900 C; (b) Stage 1: 800 C, Stage 2: 900 C; (c) Stage 1: 900 C, Stage 2: 900 C.
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Fig. 6. The effect of different S/B ratios on syngas compositions. (a) Stage 1: 700 C, Stage 2: 900 C; (b) Stage 1: 800 C, Stage 2: 900 C; (c) Stage 1: 900 C, Stage 2: 900 C.
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52.1 mol%. In contrast, with increasing S/B ratio, the difference between the first stage and the second stage on the H2 proportion in the outlet syngas became smaller. This result suggested that if a high-temperature operation is performed and sufficient steam is provided in the first stage, a high proportion of H2 can be obtained, but with no substantial increase from the operation of the second stage. Fig. 7 shows the effect of the changes in the S/B ratio on the total gas yield and syngas heating value when the operating temperatures of both stages were 900 C and the ER was 0.3. When the S/B ratio increased, the proportion of H2 in the syngas increased, and the proportions of CO, CH4, and other gases relatively decreased such that the total heating value of the syngas exhibited a downward trend when increasing the S/B ratio. After passing through the second stage, the syngas underwent reforming reactions and the H2 production increased; thus, the total heating value of the outlet syngas of the second stage also decreased when increasing the S/B ratio. Findings by Palumbo et al. [30] suggested that a hightemperature operation and an increased amount of steam could accelerate the methaneesteam reforming reaction so that CH4, CO, and CO2 underwent the reactions, and H2 production improved. As H2 has a low heating-value compared with CO, CO2, and CH4, the total gas yield and heating value were reduced.
4. Conclusions In this study, the effects of different operating parameters (temperature, ER and S/B ratio) upon the product gas composition of a two-stage fluidized bed gasifier were explored by mainly changing the temperature of the fluidized bed gasifier in the first stage in addition to the ER and S/B ratio values. The results showed that when the operating temperature increased from 700 C to 900 C, the proportion of H2 in the syngas increased significantly. Moreover, when the operating temperature of the fluidized bed reactor in the second stage was controlled at 900 C, the proportion of H2 in the syngas was effectively enhanced to more than 42 mol%, although the operating temperature of the first stage was only 700 C. When appropriate ER and S/B ratio values were utilized, the proportion of H2 was still enhanced to more than 52 mol%.
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