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Characteristics of rice husk char gasification with steam
Fuel 158 (2015) 42–49
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Characteristics of rice husk char gasification with steam Ming Zhai ⇑, Yu Zhang, Peng Dong, Pengbin Liu School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China
h i g h l i g h t s A drop-tube reactor was used for the measurements. The temperature is the primary factor that influences the steam gasification reaction of rice husk char. The conversion rate of rice husk char increases as the steam flow rate. The reactivity of rice husk char prepared at low temperature is relatively high. When the temperature is more than 850 °C, the diffusion through gas controls the overall reaction.
a r t i c l e
i n f o
Article history: Received 26 December 2014 Received in revised form 5 May 2015 Accepted 6 May 2015 Available online 16 May 2015 Keywords: Biomass char Rice husk char Gasification with steam Conversion rate Kinetics analysis
a b s t r a c t Biomass char gasification with steam refers to the reaction of the steam and biomass char under high temperature when the biomass char converts to combustible gas. Rice husk was selected as the raw material for char preparation. A gasification reactor was designed and built for the study of characteristics of rice husk char gasification with steam. Results show that the temperature is the primary factor that influences the steam gasification reaction of rice husk char. The conversion rate increases significantly from 27.7% to 90.73% with the temperature from 700 to 950 °C. H2 accounts for 46.9% of the product gas at 950 °C. The conversion rate of rice husk char increases with temperature. The conversion rate of rice husk char increases as the steam flow rate. H2 and CO gradually increase while CO2 and CH4 decrease as the steam flow rate. The conversion rate can be increased by decreasing particle size at low temperature, but the influence of the particle size becomes smaller above 900 °C. The reactivity of rice husk char prepared at low temperature is relatively high. Both surface reaction controlled shrinking core reaction model and homogeneous reaction model can describe the steam gasification reaction of rice husk char when the temperature is less than 850 °C. However, when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biomass char gasification with steam refers to the reaction of the steam and biomass char under high temperature when the biomass char converts to combustible gas. During the gasification process, the reduction reaction between the steam and carbon and the reforming reaction between the steam and gaseous products are involved [1,2]. Compared with biomass, volatile and oxygen content in biomass char are significantly reduced, in which fixed carbon composition is largely accumulated [3,4]. Therefore, biomass char has high reaction activity with less N and S levels, results in little environmental pollution relative to coal char. That is why it is considered to be a quality gasification material. The introduction of steam as the gasification medium into the gasification process ⇑ Corresponding author. Tel.: +86 18646218082. E-mail address: [email protected] (M. Zhai). http://dx.doi.org/10.1016/j.fuel.2015.05.019 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
can not only improve the gasification activity of biomass char, but also boost the hydrogen yield and gas production rate and optimize the quality of gas. Moreover, the composition of the product gas can be controlled by adjusting the flow rate of steam [5]. Chaudhari et al. [3] studied the steam gasification of two biomass chars at 700, 750, and 800 °C in a fixed bed microreactor at different steam flow rates. The results suggest a strong potential for producing H2 and syngas from biomass chars by a simple steam gasification process. Haykiri-Acma et al. [6] using the thermogravimetric analysis technique investigated gasification characteristics of some agricultural and waste biomass samples chars in a gas mixture of steam and nitrogen. It showed that gasification characteristics were fairly dependent on the ash and fixed carbon contents and the constituents present in the ash. Low ash content and high fixed carbon content biomass materials were recommended for the gasification processes. Ashish Bhat et al. [7] investigated the gasification of rice husk char in its original grain form in
43
M. Zhai et al. / Fuel 158 (2015) 42–49
a silica tube reactor with steam, and of rice husk char powder in a thermogravimetric balance in a carbon dioxide medium for determining the kinetic parameters. Experiments were conducted at temperatures of 750–900 °C. The data was analyzed based on the volume reaction and shrinking core models. However, there are still not enough experimental data to explain the typical characteristics of biomass char gasification with steam. In this paper, the rice husk is adopted as the raw material to produce biomass char in a muffle furnace under varying operating conditions. Then, the experimental setup is designed and established on which the experiment is performed to study the characteristics of the gasification reaction under varying operation conditions, and the kinetics analysis of the gasification reaction is performed. Thus, the experimental results can provide experimental and theoretical reference to engineering applications. 2. Methodology Rice husk was chosen as the raw materials for this study. Rice husk, an agricultural biomass resource, is composed of fiber, lignin, extractives, and ash. Unlike other agricultural biomass sources, rice husk is easy to collect and use since it accumulates primarily in rice processing plants. When making char samples, rice husks are not crushed, but are instead screened, and husks of similar particle size are chosen. A muffle furnace was used to produce rice husk chars. After that, a gasification reactor was designed and built for the study of characteristics of rice husk char gasification with steam.
Fig. 1. The experimental setup for rice husk char gasification with steam. 1. Nitrogen cylinder. 2. Valve. 3. Rotameter. 4. Thermocouple. 5. Steam generator. 6. Temperature controller. 7. Boat. 8. Drop-tube furnace. 9. Sampling tube. 10. Condenser. 11. Collection bottle. 12. Drying bottle. 13. Dust extraction pipe. 14. Aspirator pump. 15. Gas collecting bag.
2.1. Preparation of rice husk char The proximate and ultimate analysis of the rice husk is shown in Table 1. Instead of crushing, the rice husk is only screened to select the husk of similar particle size as the material. Then rice husk char is prepared by heating in a muffle furnace after being put into the sealed crucible. The muffle furnace is controlled by a temperature controller. The temperature program of the muffle furnace can be defined, and thus the temperature inside the furnace can be controlled. Therefore, the controller can be used to create various operating conditions (at the temperature of 600 °C, 700 °C, 800 °C, and 900 °C for 20 and 40 min) for the furnace while preparing the rice husk char. The main technical parameters of the muffle furnace are: the measurement range of temperature is from 0 to 1000 °C; the measurement resolution is 1 °C and the temperature control accuracy is ±5 °C. 2.2. Experimental setup for rice husk char gasification with steam The experimental setup for rice husk char gasification with steam is shown in Fig. 1, whose principal part is a drop-tube furnace composed of a heating furnace and a temperature controller. In the drop-tube furnace, the gasification reaction of rice husk char with the steam generated by a steam generator occurs. The product gas is collected by the gas collecting bag after passing the condenser, drying bottle, and dust extraction pipe. The collected gas is to be used for performing sampling analysis. The electrical heating is adopted for the drop-tube furnace, in which there is a corundum tube whose inner diameter is 40 mm, and the length is 1300 mm. Meanwhile, a heating resistance wire twines around
Table 1 Proximate and ultimate analysis of rice husk. Mad (%)
Vad (%)
Aad (%)
FCad (%)
Cdaf (%)
Hdaf (%)
Odaf (%)
Ndaf (%)
Sdaf (%)
5.08
63.05
14.98
16.89
46.18
6.08
45.02
2.62
0.10
the corundum tube and thus a high-temperature environment in the corundum tube is formed by heating the resistance wire. Meanwhile, an external temperature controller is attached to the drop-tube furnace, which can set the temperature program of the heating furnace, and thus the temperature in the furnace is controlled. Moreover, the steam generator which produces saturated steam at atmospheric pressure consists of a power adjustable heating mantle and a round-bottomed flask. Before the experiment, output steam amount of the steam generator needs to be calibrated. The four calibrated steam production amounts are shown in Table 2. Nitrogen is used as the protective gas of the experiment setup and an anoxic environment with high temperature is created in the heating furnace. After that, the water gas reaction occurs between the rice husk char and the steam in the furnace, and then the product gas goes into the condenser. In the condenser, some unreacted steam and macromolecule condensable gas flow into the collection bottle, and then the residual gas enters the gas collecting bag after drying and dust removal.
2.3. Measurement methods Measurement of temperature: in the experiment, the temperature rise procedure of the heating furnace can be controlled by the temperature controller and the K-type thermocouple is inserted into the boat to monitor the temperature variation during the reaction. Measurement of gas flow rate: install a rotameter at the outlet of the nitrogen gas cylinder to control the amount of nitrogen entering the furnace.
Table 2 Steam generation with heating power. Heating power (w)
100
150
200
250
Steam generation (g/min)
1.13
1.85
2.64
3.23
M. Zhai et al. / Fuel 158 (2015) 42–49
Calculation of feed quantity: feed quantity is calculated using the method of subtraction. Firstly, the mass m is obtained by weighing the boat, and then the mass m1 is obtained by weighing the boat with rice husk char in it when the rice husk char does not leak out. Finally, the actual mass m2 of rice husk char that participated in the reaction process is calculated by:
m2 ¼ m1 m
ð1Þ
Calculation of conversion rate: collect and weigh the residue after gasification and the mass of the residue is defined as m3. Then, the mass of converted char can be obtained by subtracting the mass of the residue m3 from the mass m2 of rice husk char before the reaction. After that, divide the ash content Aad in industrial analysis by the yield of char, and the ash content A of rice husk char is got. Thus, the conversion rate is calculated as:
X¼
m2 m3 ð1 AÞm2
ð2Þ
Measurement of gas composition: the composition of gas resulted from gasification of rice husk char is identified by GC–MS produced by Agilent. The content of the gas composition of the sample is measured by the external standard method which obtains the standard curve of water gas first, and then compares the measurement curve of the sample with the standard curve to recognize the content of gas component in the sample. The rice husk char has complex pore structure with a high affinity. Before the experiment, the prepared rice husk char is dried in a drying closet at a temperature of 105 °C for 3 h to remove the moisture. Then 4 g char is weighed as samples for the study. After that, the drop-tube furnace is heated to the reaction temperature. Then the steam generator is switched on to produce steam that is carried by the nitrogen and react with the rice husk char in the furnace. 3. Results and discussion Cracking of biomass char occurring in the process of gasification generates some unsaturated hydrocarbon gas, but only little due to the reforming reaction of steam [8]. As expected, no peaks of unsaturated hydrocarbon were detected in the gas chromatograph, so this paper focuses on studying the characteristics of the gas that accounts for a high proportion of the product gas, such as CO, CO2, H2 and CH4. The reaction between rice husk char and steam is divided into two steps. The first step is the cracking reaction of rice husk char. It generates unsaturated hydrocarbon gas by analyzing the volatile content, and at the same time, carbon in the char and steam react to produce CO, CO2 and H2. The second step is the reforming reaction of steam, i.e., the reforming reaction between the generated hydrocarbon gas from the first step and steam and CO further converts to H2 and CO2. The composition of gas resulted from steam gasification of rice husk char can be seen as a combined consequence of the reactions (3)–(9) [9].
C þ H2 O ¼ CO þ H2
ð3Þ
Cx Hy Ox ! aH2 þ bCO2 þ cCO þ dCH4 þ eH2 O þ f C2
ð4Þ
Cn Hm þ 2nH2 O ¼ ð2n þ m=2ÞH2 þ nCO2
ð5Þ
CH4 þ H2 O ¼ CO þ 3H2
ð6Þ
CO þ H2 O ¼ CO2 þ H2
ð7Þ
C þ 2H2 ¼ CH4
ð8Þ
C þ CO2 ¼ 2CO
ð9Þ
In this paper, the reaction temperature, steam flow rate, reaction time, particle size and the influence of different char-producing temperature on conversion rate of rice husk char are studied. 3.1. The influence of reaction temperature on the gasification characteristics of rice husk char The primary reactions happened in the gasification of biomass char are endothermic reactions, and thus the reaction temperature is the main factor that affects the gasification characteristics and gas composition of biomass char [10,11]. The gasification characteristics of rice husk char under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, are studied. The rice husk char prepared at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacted 12 min under the condition when the steam flow rate is of 2.64 g/min. As can be seen from Fig. 2, the temperature has a significant influence on the reactivity of rice husk char. As the temperature increases, the conversion rate of rice husk char significantly increases. The conversion rate of rice husk char is 27.7% at the temperature of 700 °C, whereas when the temperature is raised to 950 °C, the conversion rate increases to 90.73%. Meanwhile, when the rice husk char reacts with the steam, the steam firstly spreads onto the surface of rice husk char and then reacts with the char on the surface. After that, it passes through the porous structure of the char into the interior to produce H2, CO, and other gasses. Then, the product gas diffuses further to open the internal pore structure of rice husk char. Moreover, as the temperature rises, the speed of diffusion of steam and product gas increases, and thus the reaction of rice husk char intensifies. The reaction (3) and (9) during the gasification process are both endothermic reactions, therefore as the temperature increases, the reaction can be enhanced [12]. Thus, more carbon involves in the reaction process, resulting in a considerable improvement in conversion rate of rice husk char. Some researchers have found that the reactivity of different biomass char is also greatly influenced by the ash content of the char [13]. Rice husk char contains much ash, and the ash accounts for 49.17% of the rice husk char prepared at the temperature of 800 °C. Therefore, a high ash content is also an important reason for the high conversion rate of rice husk char, because the ash contains metal oxides, such as MgO, K2O, Fe2O3, and so on. In the course of the gasification reaction, these metal oxides play a catalytic role in promoting the progress of the reaction. As shown in Fig. 3, the gas resulted from gasification of rice husk char is mainly composed of CO, CO2, H2, CH4, of which the content of methane is small, accounting for only about 5%. With
1.0 0.9
&RQYHUVLRQUDWH
44
0.8 0.7 0.6 0.5 0.4 0.3 0.2
750
800
850
900
950
7HPSHUDWXUH & Fig. 2. Rice husk char conversion rate with temperature.
45
M. Zhai et al. / Fuel 158 (2015) 42–49
9ROXPHIUDFWLRQ
50 40 30 20
+ &2 &2
10 0
&+
700
750
800
850
900
950
7HPSHUDWXUH R& Fig. 3. Rice husk char gasification gas composition with temperature.
increasing temperature, the content of H2 and CO gradually increases from 35.1% and 20.3% at the temperature of 700 °C to 46.9% and 26.8% at the temperature of 950 °C, respectively. The content of CO2 and methane decreases from 29.6% and 23.8% at the temperature of 700 °C to 6.5% and 2.2% at the temperature of 950 °C, respectively. It is because the reactions (3)–(6) all generate hydrogen and are endothermic reaction. Thus, the rising temperature facilitates these reactions, resulting in the content of hydrogen increases rapidly. Meanwhile, the high temperature can also facilitate the reaction (9) as well as the thermal cracking of CH4, resulting in a reduction in the content of carbon dioxide and methane. Therefore, it is indicated that the gas composition is mainly affected by the reaction (3) and (9) at high temperatures. 3.2. The influence of steam flow rate on the gasification characteristics of rice husk char Steam is the gasification medium of rice husk char, so the flow of it also affacts on the characteristics of gasification. Since the steam adopted is saturated steam under atmospheric pressure, the steam is heated to the ambient temperature when it enters the reaction environment. The rice husk char produced at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacts for 12 min at 950 °C and the steam flow rate is of 0 g/min, 1.13 g/min, 1.85 g/min, 2.64 g/min and 3.23 g/min, respectively. As shown in Fig. 4, when the steam flow rate is less than 2.64 g/min, the conversion rate of rice husk char increases rapidly by increasing steam flow rate to enhance the steam concentration in the furnace. When there is no steam, a further coking reaction of rice husk char occurs at 950 °C and it can be known that the
conversion rate is 12.95% by analyzing the volatiles. After the entering of steam, the gasification reaction between rice husk char and steam occurs. At this time, the conversion rate of rice husk char increases rapidly and reaches 90.73% when the steam flow rate is 2.64 g/min. However, the excess steam has little effect on the conversion rate of rice husk char. Fig. 5 shows the variation of gas composition with different amounts of steam flow rate. With the entering of steam, the carbon and steam in rice husk char react to produce large amounts of H2 so that H2 rapidly increases. Meanwhile, H2 accounts for 46.9% of the produced gas when the steam flow rate reaches 2.64 g/min, whereas the H2 content varies very little by further increasing the steam flow rate. At the same temperature, the increase in steam flow rate promotes the reforming reaction (6) and (7), and a portion of CO and CH4 is consumed to generate CO2, so that CO2 increases when CO and CH4 decrease. Experimental results show that the contents of CO, CO2 and H2 are mostly influenced by the water–gas shift reaction (5) [14]. 3.3. The influence of reaction time on the gasification characteristics of rice husk char Just like the reaction temperature, reaction time is also an important parameter affecting the characteristics of steam gasification of rice husk char. With the increase of reaction time, the heat and mass transfer effects of the particle are improved which is conducive to the steam gasification of rice husk char. In this paper, the rice husk char produced at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacts for 2 min, 4 min, 6 min, 8 min, 10 min and 12 min under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, respectively. The variation of conversion rate of rice husk char over reaction time at different temperatures is shown in Fig. 6. It can be seen that at different reaction temperatures, the conversion rate of rice husk char increases with the increase of reaction time, and the reaction rate of rice husk char is very slow when the temperature is less than 850 °C. At the temperature of 700 °C and 800 °C, the conversion rate of rice husk char increases from 10.2% and 11.7% of 2 min to 27.7% and 39.5% of 12 min, respectively. Meanwhile, the conversion rate rapidly increases to 90.73% from 29.6% when the temperature rises to 950 °C, and the reaction rate increases significantly. It indicates that the reaction rate of rice husk char accelerates with increasing temperature, which is consistent with Arrhenius law. Increased reaction time means that the rice husk char has more time to react with the steam. With the increase of reaction time, the carbon and steam in rice husk char can react
50
9ROXPHIUDFWLRQ
&RQYHUVLRQUDWH
1.00
0.75
0.50
0.25
0.00
0.0
0.7
1.4
2.1
2.8
3.5
6WHDPIORZUDWH JPLQ Fig. 4. Rice husk char conversion rate with steam flow rate.
40 30 20
+
10
&2 &2
0
&+
0.0
0.7
1.4
2.1
2.8
3.5
6WHDPIORZUDWH JPLQ Fig. 5. Rice husk char gasification gas composition with steam flow rate.
46
M. Zhai et al. / Fuel 158 (2015) 42–49
1.0
700 & 800 & 850 & 900 & 950 &
&RQYHUVLRQUDWH
0.8 0.6 0.4 0.2 0.0
2
4
6
8
10
12
5HDFWLRQWLPH PLQ Fig. 6. Rice husk char conversion rate with reaction time.
to generate gases, so that the pore structure of char is opened, enhancing the effects of heat and mass transfer. Moreover, it is conducive to the further reaction between rice husk char and steam, increasing the conversion rate. 3.4. The influence of particle size on the gasification characteristics of rice husk char During the process of steam gasification of biomass char, the particle size is an important parameter manifesting the characteristics of biomass char and it also has some effects on the gasification characteristics of biomass char. Rice husk char of different sizes has different specific surface area, which affects the contact area between rice husk char and steam to a certain extent, thus affecting the reaction properties of rice husk char. Meanwhile, small size particles have a feature of well heat and mass transfer, and thus there is a relatively low temperature gradient inside and outside of the particle, which is beneficial to improve the conversion rate of char [15]. Since temperature and particle size have effects on the gasification characteristics of rice husk char [16,17], this paper examines the particle size on the gasification characteristics of rice husk char under five different temperature conditions. The rice husk char produced at the temperature of 800 °C for 40 min whose particle size is between 0.63–0.7 mm, 0.7–1 mm, 1–1.2 mm and greater than 1.2 mm is selected as the object. The gasification characteristics of rice husk char under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, are studied respectively when the steam flow rate is of 2.64 g/min. The variation of conversion rate of rice husk char with the temperature is shown in Fig. 7. It can be seen that at the same
1.0
0.7
0.98
0.6 0.5 0.4 0.3 0.2
0.96 0.94 0.92 0.90 0.88
0.1 0.0
Rice husk char prepared at different temperatures possess different internal structures, and the preparation temperature also has some influence on steam gasification characteristics of rice husk char. Min and Li et al. studied the influences of different pyrolysis temperatures on char production characteristics by XRD method, and found that the lower the temperature, the better the char reactivity and steam gasification reaction. Meanwhile, as the temperature rises, non carbon atom is gradually detached from biomass and the structure of char tends to be in order, making further pyrolysis gasification become difficult [18–20]. In this paper, rice husk char prepared at the respective temperature of 600 °C, 700 °C, 800 °C and 900 °C for 40 min is selected. The rice husk char reacts for 12 min at the gasification temperature of 950 °C and the steam flow rate is of 2.64 g/min. The variation of conversion rate of rice husk char with the char preparation temperature when the gasification temperature is 950 °C is shown in Fig. 8. It can be seen that the lower the char preparation temperature, the higher the conversion rate of rice husk char. Meanwhile, the conversion rate of rice husk char reaches 98.7% that almost equals to complete conversion when the char preparation temperature is 600 °C, whereas it is 87.3% when the char preparation temperature is 900 °C. It means that the char prepared at lower temperatures is more liable to gasification than that prepared at higher temperatures. It is found that the
&RQYHUVLRQUDWH
&RQYHUVLRQUDWH
0.8
3.5. The influence of char preparation temperature on the gasification characteristics of rice husk char
1.00
0.63-0.7PP 0.7-1PP 1-1.2PP >1.2PP
0.9
temperature, the conversion rate of small particle size rice husk char is higher, which is because that rice husk char of small particle size has a larger specific surface area, and thus in the course of the reaction, it has a larger contact area with the steam. Moreover, the small particle size also helps to improve the heat and mass transfer rate and reduce the temperature gradient inside and outside of the particle, so that the gasification reaction is intensified to improve the conversion rate of rice husk char. It can be found that the influence made by temperature on the conversion rate of rice husk char is larger than that made by the particle size, and that the influence of particle size is more evident when the temperature is low. Moreover, the conversion rate of rice husk char whose particle size is greater than 1.2 mm is of 27.7% when the temperature is 700 °C, but it increases to 42.99% when particle size reduces to 0.63– 0.7 mm. However, there are few differences in the conversion rates of rice husk char with four particle sizes when the temperature rises to 900 °C, which is almost negligible. It is because that at high temperatures, the advantage of small particle sizes in improving the heat and mass transfer rate is not obvious so that the influence of the particle size becomes smaller.
700
800
850
900
950
7HPSHUDWXUH & Fig. 7. Rice husk char conversion rate with particle size.
0.86
600
650
700
750
800
850
900
7HPSHUDWXUH & Fig. 8. Rice husk char conversion rate with the temperature of making char.
47
M. Zhai et al. / Fuel 158 (2015) 42–49
3.6. Reaction kinetics analysis on rice husk char gasification with steam The steam gasification and coal gasification of biomass char have some similarities, but more researches are done on the reaction model of coal gasification currently. Therefore, we can refer to the coal gasification model to select the appropriate model for describing the steam gasification reaction of rice husk char. The literatures have summed up several common kinetics models of coal gasification: homogeneous reaction model, shrinking core reaction model, mixed reaction model and distribution activation energy model [19]. In the study of gasification reaction among coal, CO2 and steam, Molina and Fanor [20] found by comparing the above-mentioned models that in the study of gas–solid reactions, shrinking core reaction model and homogeneous reaction model can be adopted to describe the reaction process if only the description of the relationship between conversion rate and time is required. Because the two models have simple mathematical formulas and enough accuracy. Therefore, the two models are selected as the basis on which the reaction model suitable for describing the steam gasification of rice husk char is determined by conducting fitting analysis on the experimental results and the selected model and the relevant reaction kinetics parameters are calculated. In the shrinking core reaction model, the expression of steam gasification reaction rate can be expressed as:
2 dx Ea ð1 xÞ3 ¼ k0 PnH2 O exp dt RT
dx Ea ð1 xÞ ¼ k0 PnH2 O exp dt RT
ð15Þ
It is handled similarly as the shrinking core reaction model and the following equations are obtained:
Ea kV ¼ k0 exp RT
ð16Þ
lnð1 xÞ ¼ kV t
ð17Þ
where kv is the reaction rate constant of homogeneous reaction model, s1. According to the variation of conversion rate of rice husk char over time based on the experimental results, the relationship between 1 (1 x)1/3 t and ln (1 x) t can be obtained, respectively. The characteristics of rice husk char gasification with steam using shrinking core reaction model and homogeneous reaction model are shown in Figs. 9 and 10, respectively.As can be seen from Figs. 9 and 10, 1 (1 x)1/3 and ln (1 x) both have a linear relationship with reaction time when the temperature is below 850 °C. However, if the temperature is higher than 850 °C and the reaction time is longer than 8 min, the relationship is not linear. Therefore for higher temperatures, the rate-controlling step
0.6 700&
0.5
800& 850&
0.4
900& 950&
0.3 0.2 0.1 0.0
2
4
6
8
10
12
W PLQ
ð10Þ
The rate-controlling step is the surface reaction. Keeping the steam partial pressure constant, the equation can be simplified as: 2 dx ¼ kG ð1 xÞ3 dt
In homogeneous reaction model, the expression for gasification reaction rate can be expressed as:
[
rice husk char prepared at high temperatures has greater specific surface area and more apertures structure, which enlarges the contact area between rice husk char and steam and is conducive to the precipitation of the produced gas during the process of gasification reaction. However, the experimental results demonstrate that the rice husk char under a high temperature has a lower reactivity, due to two reasons. First, the content of volatiles is relatively high in rice husk char prepared at a low temperature, and the volatiles can be further dissolved out at the temperature of 950 °C, resulting in a higher conversion rate. Second, the carbon structure of rice husk char prepared at high temperature is more compact and orderly, which is detrimental to the occurring of the gasification reaction between rice husk char and steam.
Fig. 9. Characteristics of rice husk char gasification with steam using shrinking core reaction model considering the surface reaction as the rate-controlling step (the trend lines are linear fit straight lines. When temperature is higher than 850 °C, the data for reaction time of 2, 4, 6, 8 and 10 min are linear fitted).
ð11Þ 2.5
where
Ea kG ¼ k0 exp RT
2.0
It can be obtained by taking the logarithm on both sides of the equation that:
1.5
ln kG ¼ ln k0
Ea RT
ð13Þ
where k0 is the pre-exponential factor, s1; Ea is the apparent activation energy of the gasification reaction, kJ/mol; kG is the reaction rate constant of shrinking core reaction model, s1; R is the gas constant value, which is 8.314 Jmol1 K1; x is the conversion rate. To assess the reaction model with the experimental results, it can be obtained by integrating the Eq. (11) on both sides that: 1 3
1 ð1 xÞ ¼ kG t
ð14Þ
OQ [
ð12Þ
700& 800& 850& 900& 950&
1.0 0.5 0.0
2
4
6
8
10
12
W PLQ Fig. 10. Characteristics of rice husk char gasification with steam using homogeneous reaction model (the trend lines are linear fit straight lines. When temperature is higher than 850 °C, the data for reaction time of 2, 4, 6, 8 and 10 min are linear fitted).
48
M. Zhai et al. / Fuel 158 (2015) 42–49
-3.2
Table 3 Kinetic parameters.
-3.6
OQN*
-4.0 -4.4 -4.8 -5.2 0.80
0.85
0.90
0.95
1.00
Correlation coefficient
Activation energy (kJ/mol)
Shrinking core reaction model (surface reaction) Shrinking core reaction model (diffusion through gas) Homogeneous reaction model
66.5
23.3
0.9893
52.7
11.5
0.9842
74.8
200.3
0.9799
1.05
7 .
1.0
Fig. 11. The relationship between ln kG and 1000/T for shrinking core reaction model considering the surface reaction as the rate-controlling step.
0.9
700 & 800 & 850 & 900 & 950 &
0.8 0.7 0.6
[
-2.0
0.5 0.4
-2.4
0.3 0.2
-2.8
OQN9
Preexponential factor (min1)
Model
0.1 0.0
-3.2
2
4
6
8
10
12
W PLQ -3.6 -4.0 0.80
0.85
0.90
0.95
1.00
1.05
Fig. 13. Characteristics of rice husk char gasification with steam using shrinking core reaction model considering the diffusion through gas as the rate-controlling step.
7 . -2.6
Fig. 12. The relationship between ln kV and 1000/T for homogeneous reaction model.
-3.0 -3.2
OQN*
is not only surface reaction but could also be influenced by diffusion. The phenomena is similar with Ref. [21]. When the temperature is lower than 850 °C, the reaction rate constants kG of shrinking core reaction model and kV of homogeneous reaction model can be obtained by conducting linear regression analysis on 1 (1 x)1/3 and t, and ln (1 x) and t, respectively. The relationship between ln kG and 1000/T, ln kV and 1000/T is shown in Figs. 11 and 12, respectively. Do a linear fitting of every point in the graph first. Then, according to the Eqs. (13) and (16), the average apparent activation energy and pre-exponential factor of steam gasification of rice husk char described by shrinking core reaction model and homogeneous reaction model can be calculated with the slope and intercept of the fitting straight-line, respectively. Moreover, their values are shown in Table 3. It can be seen by comparing Figs. 9 and 10 that shrinking core reaction model and homogeneous reaction model both can describe the gasification reaction of rice husk char well, but the shrinking core reaction model has a greater correlation coefficient. If the diffusion through gas is considered as the rate-controlling step for the shrinking core reaction model, Eq. (14) will be x = kGt. The characteristics of rice husk char gasification with steam using shrinking core reaction model considering the diffusion through gas as the rate-controlling step are shown in Fig. 13. They have better linear fitting with the experimental results at all temperatures. The relationship between ln kG and 1000/T for shrinking core reaction model considering the diffusion through gas as the rate-controlling step is shown in Fig. 14 and the average apparent activation energy and pre-exponential factor are shown in Table 3.
-2.8
-3.4 -3.6 -3.8 -4.0 -4.2 0.80
0.85
0.90
0.95
1.00
1.05
7 . Fig. 14. The relationship between ln kG and 1000/T for shrinking core reaction model considering the diffusion through gas as the rate-controlling step.
From Figs. 9 and 13, it can be concluded that both surface reaction and diffusion through gas influence the reaction, but when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction. The shrinking core reaction model considering the diffusion through gas as the rate-controlling step can be used for describing the reaction behavior of steam gasification. 4. Summary and conclusions The characteristics of rice husk char with steam are studied by experiments. The following conclusions can be made after analyzing the influences of reaction temperature, steam flow rate, particle size, reaction time and char preparation temperature on the conversion rate and the composition of produced gas of rice husk char.
M. Zhai et al. / Fuel 158 (2015) 42–49
(1) Temperature is the primary factor that influences the steam gasification reaction of rice husk char. When the temperature rises from 700 °C to 950 °C, the conversion rate of rice husk char increases significantly from 27.7% to 90.73% after reacting for 12 min. The high temperature contributes for the production of H2. When the temperature is 950 °C, H2 accounts for 46.9% of the product gas. (2) Steam flow rate also has great influence on the gasification reaction of rice husk char. The conversion rate of rice husk char increases as the steam flow rate. H2 and CO gradually increase while CO2 and CH4 decrease as the steam flow rate. (3) The particle size has some effects on the conversion rate of rice husk char. The conversion rate can be increased by decreasing particle size at low temperature, but the influence of the particle size becomes smaller above 900 °C. (4) The conversion rate of rice husk char can be improved by increasing the reaction time. When the temperature is less than 850 °C, more reaction time is needed due to low reaction rate. (5) The reactivity of rice husk char prepared at low temperature is relatively high. Meanwhile, when the gasification temperature is 950 °C, the conversion rate of rice husk char reaches 98.65% if the char preparation temperature is 600 °C, whereas the conversion rate reduces to 87.3% if the char preparation temperature is 900 °C. (6) Both surface reaction controlled shrinking core reaction model and homogeneous reaction model can describe the steam gasification reaction of rice husk char when the temperature is less than 850 °C, and the shrinking core reaction model behaves better fitting. However, when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction.
References [1] Minkova V, Marinov SP, Zanzi R, Björnbom E, Budinova T, Stefanova M, et al. Thermochemical treatment of biomass in a flow of steam or in a mixture of steam and carbon dioxide. Fuel Process Technol 2000;61(1):45–52.
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[2] Brink DL, Thomas JF. The pyrolysis gasification–combustion process: energy effectiveness using oxygen vs. air with wood fuel systems. New York: Academic Press; 1977. [3] Chaudhari ST, Dalai AK, Bakhshi NN. Production of hydrogen and/or syngas (H2+CO)via steam gasification of biomass-derived chars. Energy Fuels 2003;17(4):1062–7. [4] Klein MT, Virk PS. Modeling of lignin thermolysis. Energy Fuels 2008;22(4):2175–82. [5] Xu Guiying, Zhang Guogang, Cheng Hui, Wang Yang. Steam gasification of biomass char. Trans CSAE 2011;27(11):273–7. [6] Haykiri-Acma H, Yaman S, Kucukbayrak S. Gasification of biomass chars in steam nitrogen mixture. Energy Convers Manage 2006;47:1004–13. [7] Ashish Bhat JV, Bheemarasett Ram, et al. Kinetics of rice husk char gasification. Energy Convers Manage 2001;42(18):2061–9. [8] Guoxin Hu, Hao Huang, et al. Hydrogen-rich gas production from pyrolysis of biomass in an autogenerated steam atmosphere. Energy Fuels 2009;23(3):1748–53. [9] Feng Yan. Investigation on biomass fast pyrolysis and steam gasification of biomass char. Huazhong University of Science and Technology; 2010. [10] Jin-Hyung K, Yu L, Kyoung-Min S, et al. Temperature effect on the pressure drop across the cake of coal gasification ash formed on a ceramic filter. Powder Technol 2008;181(1):67–73. [11] He MY, Hu ZQ, Xiao B, et al. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): influence of catalyst and temperature on yield and product composition. Int J Hydrogen Energy 2009;34(1):195–203. [12] He MY, Hu ZQ, Xiao B, et al. Syngas production from catalytic gasification of waste polyethylene: influence of temperature on gas yield and composition. Int J Hydrogen Energy 2009;34(3):1342–8. [13] Xiaoying Kong, Lianhua Li, Xiuli Yin, Yongming Sun, Longlong Ma, Zhenhong Yuan, et al. Experiment on pyrolysis product and pyrolysis characteristics of marine alga with high ash content. Acta Energlae Solaris Sinica 2010;31(5):536–9. [14] Wei LG, Xu SP, Zhang L, et al. Steam gasification of biomass for hydrogen-rich gas in a free-fall reactor. Int J Hydrogen Energy 2007;32(1):24–31. [15] Encinar JM, Bernalte A, Ramiro A, et al. Pyrolysis of two agricultural residues: olive and grape bagasse. Influence of particle size and temperature. Biomass Bioenergy 1996;11(5):397–409. [16] Luo SY, Xiao B, Guo XJ, et al. Hydrogen-rich gas from catalytic steam gasification of biomass in a fixed bed reactor: influence of particle size on gasification performance. Int J Hydrogen Energy 2009;34(3):1260–4. [17] Sergio R, Ajmal L. Steam gasification of almond shells in a fluidized bed reactor: the influence of temperature and particle size on product yield and distribution. Biomass Bioenergy 1997;12(4):281–8. [18] Klnse Wolfgang, Wokli Michael. On the intrinsic reaction rate of biomass char gasification with carbon dioxide and steam. Fuel 2005;84:8. [19] Yinhua Xiang, Yang Wang, Jianmin Zhang, Jiejie Huang, Jiantao Zhao. A study on kinetic models of char gasification. J Fuel Chem Technol 2002;30(1):21–6. [20] Molina Alejandro, Mondragon Fanor. Reactivity of coal gasification with steam and CO2. Fuel 1998;77. 123-12. [21] Bhat A, Bheemarasetti JVR, Rao TR. Kinetics of rice husk char gasification. Energy Convers Manage 2001;42:2061–9.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Characteristics of rice husk char gasification with steam Ming Zhai ⇑, Yu Zhang, Peng Dong, Pengbin Liu School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China
h i g h l i g h t s A drop-tube reactor was used for the measurements. The temperature is the primary factor that influences the steam gasification reaction of rice husk char. The conversion rate of rice husk char increases as the steam flow rate. The reactivity of rice husk char prepared at low temperature is relatively high. When the temperature is more than 850 °C, the diffusion through gas controls the overall reaction.
a r t i c l e
i n f o
Article history: Received 26 December 2014 Received in revised form 5 May 2015 Accepted 6 May 2015 Available online 16 May 2015 Keywords: Biomass char Rice husk char Gasification with steam Conversion rate Kinetics analysis
a b s t r a c t Biomass char gasification with steam refers to the reaction of the steam and biomass char under high temperature when the biomass char converts to combustible gas. Rice husk was selected as the raw material for char preparation. A gasification reactor was designed and built for the study of characteristics of rice husk char gasification with steam. Results show that the temperature is the primary factor that influences the steam gasification reaction of rice husk char. The conversion rate increases significantly from 27.7% to 90.73% with the temperature from 700 to 950 °C. H2 accounts for 46.9% of the product gas at 950 °C. The conversion rate of rice husk char increases with temperature. The conversion rate of rice husk char increases as the steam flow rate. H2 and CO gradually increase while CO2 and CH4 decrease as the steam flow rate. The conversion rate can be increased by decreasing particle size at low temperature, but the influence of the particle size becomes smaller above 900 °C. The reactivity of rice husk char prepared at low temperature is relatively high. Both surface reaction controlled shrinking core reaction model and homogeneous reaction model can describe the steam gasification reaction of rice husk char when the temperature is less than 850 °C. However, when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Biomass char gasification with steam refers to the reaction of the steam and biomass char under high temperature when the biomass char converts to combustible gas. During the gasification process, the reduction reaction between the steam and carbon and the reforming reaction between the steam and gaseous products are involved [1,2]. Compared with biomass, volatile and oxygen content in biomass char are significantly reduced, in which fixed carbon composition is largely accumulated [3,4]. Therefore, biomass char has high reaction activity with less N and S levels, results in little environmental pollution relative to coal char. That is why it is considered to be a quality gasification material. The introduction of steam as the gasification medium into the gasification process ⇑ Corresponding author. Tel.: +86 18646218082. E-mail address: [email protected] (M. Zhai). http://dx.doi.org/10.1016/j.fuel.2015.05.019 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
can not only improve the gasification activity of biomass char, but also boost the hydrogen yield and gas production rate and optimize the quality of gas. Moreover, the composition of the product gas can be controlled by adjusting the flow rate of steam [5]. Chaudhari et al. [3] studied the steam gasification of two biomass chars at 700, 750, and 800 °C in a fixed bed microreactor at different steam flow rates. The results suggest a strong potential for producing H2 and syngas from biomass chars by a simple steam gasification process. Haykiri-Acma et al. [6] using the thermogravimetric analysis technique investigated gasification characteristics of some agricultural and waste biomass samples chars in a gas mixture of steam and nitrogen. It showed that gasification characteristics were fairly dependent on the ash and fixed carbon contents and the constituents present in the ash. Low ash content and high fixed carbon content biomass materials were recommended for the gasification processes. Ashish Bhat et al. [7] investigated the gasification of rice husk char in its original grain form in
43
M. Zhai et al. / Fuel 158 (2015) 42–49
a silica tube reactor with steam, and of rice husk char powder in a thermogravimetric balance in a carbon dioxide medium for determining the kinetic parameters. Experiments were conducted at temperatures of 750–900 °C. The data was analyzed based on the volume reaction and shrinking core models. However, there are still not enough experimental data to explain the typical characteristics of biomass char gasification with steam. In this paper, the rice husk is adopted as the raw material to produce biomass char in a muffle furnace under varying operating conditions. Then, the experimental setup is designed and established on which the experiment is performed to study the characteristics of the gasification reaction under varying operation conditions, and the kinetics analysis of the gasification reaction is performed. Thus, the experimental results can provide experimental and theoretical reference to engineering applications. 2. Methodology Rice husk was chosen as the raw materials for this study. Rice husk, an agricultural biomass resource, is composed of fiber, lignin, extractives, and ash. Unlike other agricultural biomass sources, rice husk is easy to collect and use since it accumulates primarily in rice processing plants. When making char samples, rice husks are not crushed, but are instead screened, and husks of similar particle size are chosen. A muffle furnace was used to produce rice husk chars. After that, a gasification reactor was designed and built for the study of characteristics of rice husk char gasification with steam.
Fig. 1. The experimental setup for rice husk char gasification with steam. 1. Nitrogen cylinder. 2. Valve. 3. Rotameter. 4. Thermocouple. 5. Steam generator. 6. Temperature controller. 7. Boat. 8. Drop-tube furnace. 9. Sampling tube. 10. Condenser. 11. Collection bottle. 12. Drying bottle. 13. Dust extraction pipe. 14. Aspirator pump. 15. Gas collecting bag.
2.1. Preparation of rice husk char The proximate and ultimate analysis of the rice husk is shown in Table 1. Instead of crushing, the rice husk is only screened to select the husk of similar particle size as the material. Then rice husk char is prepared by heating in a muffle furnace after being put into the sealed crucible. The muffle furnace is controlled by a temperature controller. The temperature program of the muffle furnace can be defined, and thus the temperature inside the furnace can be controlled. Therefore, the controller can be used to create various operating conditions (at the temperature of 600 °C, 700 °C, 800 °C, and 900 °C for 20 and 40 min) for the furnace while preparing the rice husk char. The main technical parameters of the muffle furnace are: the measurement range of temperature is from 0 to 1000 °C; the measurement resolution is 1 °C and the temperature control accuracy is ±5 °C. 2.2. Experimental setup for rice husk char gasification with steam The experimental setup for rice husk char gasification with steam is shown in Fig. 1, whose principal part is a drop-tube furnace composed of a heating furnace and a temperature controller. In the drop-tube furnace, the gasification reaction of rice husk char with the steam generated by a steam generator occurs. The product gas is collected by the gas collecting bag after passing the condenser, drying bottle, and dust extraction pipe. The collected gas is to be used for performing sampling analysis. The electrical heating is adopted for the drop-tube furnace, in which there is a corundum tube whose inner diameter is 40 mm, and the length is 1300 mm. Meanwhile, a heating resistance wire twines around
Table 1 Proximate and ultimate analysis of rice husk. Mad (%)
Vad (%)
Aad (%)
FCad (%)
Cdaf (%)
Hdaf (%)
Odaf (%)
Ndaf (%)
Sdaf (%)
5.08
63.05
14.98
16.89
46.18
6.08
45.02
2.62
0.10
the corundum tube and thus a high-temperature environment in the corundum tube is formed by heating the resistance wire. Meanwhile, an external temperature controller is attached to the drop-tube furnace, which can set the temperature program of the heating furnace, and thus the temperature in the furnace is controlled. Moreover, the steam generator which produces saturated steam at atmospheric pressure consists of a power adjustable heating mantle and a round-bottomed flask. Before the experiment, output steam amount of the steam generator needs to be calibrated. The four calibrated steam production amounts are shown in Table 2. Nitrogen is used as the protective gas of the experiment setup and an anoxic environment with high temperature is created in the heating furnace. After that, the water gas reaction occurs between the rice husk char and the steam in the furnace, and then the product gas goes into the condenser. In the condenser, some unreacted steam and macromolecule condensable gas flow into the collection bottle, and then the residual gas enters the gas collecting bag after drying and dust removal.
2.3. Measurement methods Measurement of temperature: in the experiment, the temperature rise procedure of the heating furnace can be controlled by the temperature controller and the K-type thermocouple is inserted into the boat to monitor the temperature variation during the reaction. Measurement of gas flow rate: install a rotameter at the outlet of the nitrogen gas cylinder to control the amount of nitrogen entering the furnace.
Table 2 Steam generation with heating power. Heating power (w)
100
150
200
250
Steam generation (g/min)
1.13
1.85
2.64
3.23
M. Zhai et al. / Fuel 158 (2015) 42–49
Calculation of feed quantity: feed quantity is calculated using the method of subtraction. Firstly, the mass m is obtained by weighing the boat, and then the mass m1 is obtained by weighing the boat with rice husk char in it when the rice husk char does not leak out. Finally, the actual mass m2 of rice husk char that participated in the reaction process is calculated by:
m2 ¼ m1 m
ð1Þ
Calculation of conversion rate: collect and weigh the residue after gasification and the mass of the residue is defined as m3. Then, the mass of converted char can be obtained by subtracting the mass of the residue m3 from the mass m2 of rice husk char before the reaction. After that, divide the ash content Aad in industrial analysis by the yield of char, and the ash content A of rice husk char is got. Thus, the conversion rate is calculated as:
X¼
m2 m3 ð1 AÞm2
ð2Þ
Measurement of gas composition: the composition of gas resulted from gasification of rice husk char is identified by GC–MS produced by Agilent. The content of the gas composition of the sample is measured by the external standard method which obtains the standard curve of water gas first, and then compares the measurement curve of the sample with the standard curve to recognize the content of gas component in the sample. The rice husk char has complex pore structure with a high affinity. Before the experiment, the prepared rice husk char is dried in a drying closet at a temperature of 105 °C for 3 h to remove the moisture. Then 4 g char is weighed as samples for the study. After that, the drop-tube furnace is heated to the reaction temperature. Then the steam generator is switched on to produce steam that is carried by the nitrogen and react with the rice husk char in the furnace. 3. Results and discussion Cracking of biomass char occurring in the process of gasification generates some unsaturated hydrocarbon gas, but only little due to the reforming reaction of steam [8]. As expected, no peaks of unsaturated hydrocarbon were detected in the gas chromatograph, so this paper focuses on studying the characteristics of the gas that accounts for a high proportion of the product gas, such as CO, CO2, H2 and CH4. The reaction between rice husk char and steam is divided into two steps. The first step is the cracking reaction of rice husk char. It generates unsaturated hydrocarbon gas by analyzing the volatile content, and at the same time, carbon in the char and steam react to produce CO, CO2 and H2. The second step is the reforming reaction of steam, i.e., the reforming reaction between the generated hydrocarbon gas from the first step and steam and CO further converts to H2 and CO2. The composition of gas resulted from steam gasification of rice husk char can be seen as a combined consequence of the reactions (3)–(9) [9].
C þ H2 O ¼ CO þ H2
ð3Þ
Cx Hy Ox ! aH2 þ bCO2 þ cCO þ dCH4 þ eH2 O þ f C2
ð4Þ
Cn Hm þ 2nH2 O ¼ ð2n þ m=2ÞH2 þ nCO2
ð5Þ
CH4 þ H2 O ¼ CO þ 3H2
ð6Þ
CO þ H2 O ¼ CO2 þ H2
ð7Þ
C þ 2H2 ¼ CH4
ð8Þ
C þ CO2 ¼ 2CO
ð9Þ
In this paper, the reaction temperature, steam flow rate, reaction time, particle size and the influence of different char-producing temperature on conversion rate of rice husk char are studied. 3.1. The influence of reaction temperature on the gasification characteristics of rice husk char The primary reactions happened in the gasification of biomass char are endothermic reactions, and thus the reaction temperature is the main factor that affects the gasification characteristics and gas composition of biomass char [10,11]. The gasification characteristics of rice husk char under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, are studied. The rice husk char prepared at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacted 12 min under the condition when the steam flow rate is of 2.64 g/min. As can be seen from Fig. 2, the temperature has a significant influence on the reactivity of rice husk char. As the temperature increases, the conversion rate of rice husk char significantly increases. The conversion rate of rice husk char is 27.7% at the temperature of 700 °C, whereas when the temperature is raised to 950 °C, the conversion rate increases to 90.73%. Meanwhile, when the rice husk char reacts with the steam, the steam firstly spreads onto the surface of rice husk char and then reacts with the char on the surface. After that, it passes through the porous structure of the char into the interior to produce H2, CO, and other gasses. Then, the product gas diffuses further to open the internal pore structure of rice husk char. Moreover, as the temperature rises, the speed of diffusion of steam and product gas increases, and thus the reaction of rice husk char intensifies. The reaction (3) and (9) during the gasification process are both endothermic reactions, therefore as the temperature increases, the reaction can be enhanced [12]. Thus, more carbon involves in the reaction process, resulting in a considerable improvement in conversion rate of rice husk char. Some researchers have found that the reactivity of different biomass char is also greatly influenced by the ash content of the char [13]. Rice husk char contains much ash, and the ash accounts for 49.17% of the rice husk char prepared at the temperature of 800 °C. Therefore, a high ash content is also an important reason for the high conversion rate of rice husk char, because the ash contains metal oxides, such as MgO, K2O, Fe2O3, and so on. In the course of the gasification reaction, these metal oxides play a catalytic role in promoting the progress of the reaction. As shown in Fig. 3, the gas resulted from gasification of rice husk char is mainly composed of CO, CO2, H2, CH4, of which the content of methane is small, accounting for only about 5%. With
1.0 0.9
&RQYHUVLRQUDWH
44
0.8 0.7 0.6 0.5 0.4 0.3 0.2
750
800
850
900
950
7HPSHUDWXUH & Fig. 2. Rice husk char conversion rate with temperature.
45
M. Zhai et al. / Fuel 158 (2015) 42–49
9ROXPHIUDFWLRQ
50 40 30 20
+ &2 &2
10 0
&+
700
750
800
850
900
950
7HPSHUDWXUH R& Fig. 3. Rice husk char gasification gas composition with temperature.
increasing temperature, the content of H2 and CO gradually increases from 35.1% and 20.3% at the temperature of 700 °C to 46.9% and 26.8% at the temperature of 950 °C, respectively. The content of CO2 and methane decreases from 29.6% and 23.8% at the temperature of 700 °C to 6.5% and 2.2% at the temperature of 950 °C, respectively. It is because the reactions (3)–(6) all generate hydrogen and are endothermic reaction. Thus, the rising temperature facilitates these reactions, resulting in the content of hydrogen increases rapidly. Meanwhile, the high temperature can also facilitate the reaction (9) as well as the thermal cracking of CH4, resulting in a reduction in the content of carbon dioxide and methane. Therefore, it is indicated that the gas composition is mainly affected by the reaction (3) and (9) at high temperatures. 3.2. The influence of steam flow rate on the gasification characteristics of rice husk char Steam is the gasification medium of rice husk char, so the flow of it also affacts on the characteristics of gasification. Since the steam adopted is saturated steam under atmospheric pressure, the steam is heated to the ambient temperature when it enters the reaction environment. The rice husk char produced at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacts for 12 min at 950 °C and the steam flow rate is of 0 g/min, 1.13 g/min, 1.85 g/min, 2.64 g/min and 3.23 g/min, respectively. As shown in Fig. 4, when the steam flow rate is less than 2.64 g/min, the conversion rate of rice husk char increases rapidly by increasing steam flow rate to enhance the steam concentration in the furnace. When there is no steam, a further coking reaction of rice husk char occurs at 950 °C and it can be known that the
conversion rate is 12.95% by analyzing the volatiles. After the entering of steam, the gasification reaction between rice husk char and steam occurs. At this time, the conversion rate of rice husk char increases rapidly and reaches 90.73% when the steam flow rate is 2.64 g/min. However, the excess steam has little effect on the conversion rate of rice husk char. Fig. 5 shows the variation of gas composition with different amounts of steam flow rate. With the entering of steam, the carbon and steam in rice husk char react to produce large amounts of H2 so that H2 rapidly increases. Meanwhile, H2 accounts for 46.9% of the produced gas when the steam flow rate reaches 2.64 g/min, whereas the H2 content varies very little by further increasing the steam flow rate. At the same temperature, the increase in steam flow rate promotes the reforming reaction (6) and (7), and a portion of CO and CH4 is consumed to generate CO2, so that CO2 increases when CO and CH4 decrease. Experimental results show that the contents of CO, CO2 and H2 are mostly influenced by the water–gas shift reaction (5) [14]. 3.3. The influence of reaction time on the gasification characteristics of rice husk char Just like the reaction temperature, reaction time is also an important parameter affecting the characteristics of steam gasification of rice husk char. With the increase of reaction time, the heat and mass transfer effects of the particle are improved which is conducive to the steam gasification of rice husk char. In this paper, the rice husk char produced at the temperature of 800 °C for 40 min whose particle size is greater than 1.2 mm is selected as the object of study. The rice husk char reacts for 2 min, 4 min, 6 min, 8 min, 10 min and 12 min under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, respectively. The variation of conversion rate of rice husk char over reaction time at different temperatures is shown in Fig. 6. It can be seen that at different reaction temperatures, the conversion rate of rice husk char increases with the increase of reaction time, and the reaction rate of rice husk char is very slow when the temperature is less than 850 °C. At the temperature of 700 °C and 800 °C, the conversion rate of rice husk char increases from 10.2% and 11.7% of 2 min to 27.7% and 39.5% of 12 min, respectively. Meanwhile, the conversion rate rapidly increases to 90.73% from 29.6% when the temperature rises to 950 °C, and the reaction rate increases significantly. It indicates that the reaction rate of rice husk char accelerates with increasing temperature, which is consistent with Arrhenius law. Increased reaction time means that the rice husk char has more time to react with the steam. With the increase of reaction time, the carbon and steam in rice husk char can react
50
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1.00
0.75
0.50
0.25
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2.8
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6WHDPIORZUDWH JPLQ Fig. 4. Rice husk char conversion rate with steam flow rate.
40 30 20
+
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46
M. Zhai et al. / Fuel 158 (2015) 42–49
1.0
700 & 800 & 850 & 900 & 950 &
&RQYHUVLRQUDWH
0.8 0.6 0.4 0.2 0.0
2
4
6
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to generate gases, so that the pore structure of char is opened, enhancing the effects of heat and mass transfer. Moreover, it is conducive to the further reaction between rice husk char and steam, increasing the conversion rate. 3.4. The influence of particle size on the gasification characteristics of rice husk char During the process of steam gasification of biomass char, the particle size is an important parameter manifesting the characteristics of biomass char and it also has some effects on the gasification characteristics of biomass char. Rice husk char of different sizes has different specific surface area, which affects the contact area between rice husk char and steam to a certain extent, thus affecting the reaction properties of rice husk char. Meanwhile, small size particles have a feature of well heat and mass transfer, and thus there is a relatively low temperature gradient inside and outside of the particle, which is beneficial to improve the conversion rate of char [15]. Since temperature and particle size have effects on the gasification characteristics of rice husk char [16,17], this paper examines the particle size on the gasification characteristics of rice husk char under five different temperature conditions. The rice husk char produced at the temperature of 800 °C for 40 min whose particle size is between 0.63–0.7 mm, 0.7–1 mm, 1–1.2 mm and greater than 1.2 mm is selected as the object. The gasification characteristics of rice husk char under five temperature conditions, namely 700 °C, 800 °C, 850 °C, 900 °C and 950 °C, are studied respectively when the steam flow rate is of 2.64 g/min. The variation of conversion rate of rice husk char with the temperature is shown in Fig. 7. It can be seen that at the same
1.0
0.7
0.98
0.6 0.5 0.4 0.3 0.2
0.96 0.94 0.92 0.90 0.88
0.1 0.0
Rice husk char prepared at different temperatures possess different internal structures, and the preparation temperature also has some influence on steam gasification characteristics of rice husk char. Min and Li et al. studied the influences of different pyrolysis temperatures on char production characteristics by XRD method, and found that the lower the temperature, the better the char reactivity and steam gasification reaction. Meanwhile, as the temperature rises, non carbon atom is gradually detached from biomass and the structure of char tends to be in order, making further pyrolysis gasification become difficult [18–20]. In this paper, rice husk char prepared at the respective temperature of 600 °C, 700 °C, 800 °C and 900 °C for 40 min is selected. The rice husk char reacts for 12 min at the gasification temperature of 950 °C and the steam flow rate is of 2.64 g/min. The variation of conversion rate of rice husk char with the char preparation temperature when the gasification temperature is 950 °C is shown in Fig. 8. It can be seen that the lower the char preparation temperature, the higher the conversion rate of rice husk char. Meanwhile, the conversion rate of rice husk char reaches 98.7% that almost equals to complete conversion when the char preparation temperature is 600 °C, whereas it is 87.3% when the char preparation temperature is 900 °C. It means that the char prepared at lower temperatures is more liable to gasification than that prepared at higher temperatures. It is found that the
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3.5. The influence of char preparation temperature on the gasification characteristics of rice husk char
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0.63-0.7PP 0.7-1PP 1-1.2PP >1.2PP
0.9
temperature, the conversion rate of small particle size rice husk char is higher, which is because that rice husk char of small particle size has a larger specific surface area, and thus in the course of the reaction, it has a larger contact area with the steam. Moreover, the small particle size also helps to improve the heat and mass transfer rate and reduce the temperature gradient inside and outside of the particle, so that the gasification reaction is intensified to improve the conversion rate of rice husk char. It can be found that the influence made by temperature on the conversion rate of rice husk char is larger than that made by the particle size, and that the influence of particle size is more evident when the temperature is low. Moreover, the conversion rate of rice husk char whose particle size is greater than 1.2 mm is of 27.7% when the temperature is 700 °C, but it increases to 42.99% when particle size reduces to 0.63– 0.7 mm. However, there are few differences in the conversion rates of rice husk char with four particle sizes when the temperature rises to 900 °C, which is almost negligible. It is because that at high temperatures, the advantage of small particle sizes in improving the heat and mass transfer rate is not obvious so that the influence of the particle size becomes smaller.
700
800
850
900
950
7HPSHUDWXUH & Fig. 7. Rice husk char conversion rate with particle size.
0.86
600
650
700
750
800
850
900
7HPSHUDWXUH & Fig. 8. Rice husk char conversion rate with the temperature of making char.
47
M. Zhai et al. / Fuel 158 (2015) 42–49
3.6. Reaction kinetics analysis on rice husk char gasification with steam The steam gasification and coal gasification of biomass char have some similarities, but more researches are done on the reaction model of coal gasification currently. Therefore, we can refer to the coal gasification model to select the appropriate model for describing the steam gasification reaction of rice husk char. The literatures have summed up several common kinetics models of coal gasification: homogeneous reaction model, shrinking core reaction model, mixed reaction model and distribution activation energy model [19]. In the study of gasification reaction among coal, CO2 and steam, Molina and Fanor [20] found by comparing the above-mentioned models that in the study of gas–solid reactions, shrinking core reaction model and homogeneous reaction model can be adopted to describe the reaction process if only the description of the relationship between conversion rate and time is required. Because the two models have simple mathematical formulas and enough accuracy. Therefore, the two models are selected as the basis on which the reaction model suitable for describing the steam gasification of rice husk char is determined by conducting fitting analysis on the experimental results and the selected model and the relevant reaction kinetics parameters are calculated. In the shrinking core reaction model, the expression of steam gasification reaction rate can be expressed as:
2 dx Ea ð1 xÞ3 ¼ k0 PnH2 O exp dt RT
dx Ea ð1 xÞ ¼ k0 PnH2 O exp dt RT
ð15Þ
It is handled similarly as the shrinking core reaction model and the following equations are obtained:
Ea kV ¼ k0 exp RT
ð16Þ
lnð1 xÞ ¼ kV t
ð17Þ
where kv is the reaction rate constant of homogeneous reaction model, s1. According to the variation of conversion rate of rice husk char over time based on the experimental results, the relationship between 1 (1 x)1/3 t and ln (1 x) t can be obtained, respectively. The characteristics of rice husk char gasification with steam using shrinking core reaction model and homogeneous reaction model are shown in Figs. 9 and 10, respectively.As can be seen from Figs. 9 and 10, 1 (1 x)1/3 and ln (1 x) both have a linear relationship with reaction time when the temperature is below 850 °C. However, if the temperature is higher than 850 °C and the reaction time is longer than 8 min, the relationship is not linear. Therefore for higher temperatures, the rate-controlling step
0.6 700&
0.5
800& 850&
0.4
900& 950&
0.3 0.2 0.1 0.0
2
4
6
8
10
12
W PLQ
ð10Þ
The rate-controlling step is the surface reaction. Keeping the steam partial pressure constant, the equation can be simplified as: 2 dx ¼ kG ð1 xÞ3 dt
In homogeneous reaction model, the expression for gasification reaction rate can be expressed as:
[
rice husk char prepared at high temperatures has greater specific surface area and more apertures structure, which enlarges the contact area between rice husk char and steam and is conducive to the precipitation of the produced gas during the process of gasification reaction. However, the experimental results demonstrate that the rice husk char under a high temperature has a lower reactivity, due to two reasons. First, the content of volatiles is relatively high in rice husk char prepared at a low temperature, and the volatiles can be further dissolved out at the temperature of 950 °C, resulting in a higher conversion rate. Second, the carbon structure of rice husk char prepared at high temperature is more compact and orderly, which is detrimental to the occurring of the gasification reaction between rice husk char and steam.
Fig. 9. Characteristics of rice husk char gasification with steam using shrinking core reaction model considering the surface reaction as the rate-controlling step (the trend lines are linear fit straight lines. When temperature is higher than 850 °C, the data for reaction time of 2, 4, 6, 8 and 10 min are linear fitted).
ð11Þ 2.5
where
Ea kG ¼ k0 exp RT
2.0
It can be obtained by taking the logarithm on both sides of the equation that:
1.5
ln kG ¼ ln k0
Ea RT
ð13Þ
where k0 is the pre-exponential factor, s1; Ea is the apparent activation energy of the gasification reaction, kJ/mol; kG is the reaction rate constant of shrinking core reaction model, s1; R is the gas constant value, which is 8.314 Jmol1 K1; x is the conversion rate. To assess the reaction model with the experimental results, it can be obtained by integrating the Eq. (11) on both sides that: 1 3
1 ð1 xÞ ¼ kG t
ð14Þ
OQ [
ð12Þ
700& 800& 850& 900& 950&
1.0 0.5 0.0
2
4
6
8
10
12
W PLQ Fig. 10. Characteristics of rice husk char gasification with steam using homogeneous reaction model (the trend lines are linear fit straight lines. When temperature is higher than 850 °C, the data for reaction time of 2, 4, 6, 8 and 10 min are linear fitted).
48
M. Zhai et al. / Fuel 158 (2015) 42–49
-3.2
Table 3 Kinetic parameters.
-3.6
OQN*
-4.0 -4.4 -4.8 -5.2 0.80
0.85
0.90
0.95
1.00
Correlation coefficient
Activation energy (kJ/mol)
Shrinking core reaction model (surface reaction) Shrinking core reaction model (diffusion through gas) Homogeneous reaction model
66.5
23.3
0.9893
52.7
11.5
0.9842
74.8
200.3
0.9799
1.05
7 .
1.0
Fig. 11. The relationship between ln kG and 1000/T for shrinking core reaction model considering the surface reaction as the rate-controlling step.
0.9
700 & 800 & 850 & 900 & 950 &
0.8 0.7 0.6
[
-2.0
0.5 0.4
-2.4
0.3 0.2
-2.8
OQN9
Preexponential factor (min1)
Model
0.1 0.0
-3.2
2
4
6
8
10
12
W PLQ -3.6 -4.0 0.80
0.85
0.90
0.95
1.00
1.05
Fig. 13. Characteristics of rice husk char gasification with steam using shrinking core reaction model considering the diffusion through gas as the rate-controlling step.
7 . -2.6
Fig. 12. The relationship between ln kV and 1000/T for homogeneous reaction model.
-3.0 -3.2
OQN*
is not only surface reaction but could also be influenced by diffusion. The phenomena is similar with Ref. [21]. When the temperature is lower than 850 °C, the reaction rate constants kG of shrinking core reaction model and kV of homogeneous reaction model can be obtained by conducting linear regression analysis on 1 (1 x)1/3 and t, and ln (1 x) and t, respectively. The relationship between ln kG and 1000/T, ln kV and 1000/T is shown in Figs. 11 and 12, respectively. Do a linear fitting of every point in the graph first. Then, according to the Eqs. (13) and (16), the average apparent activation energy and pre-exponential factor of steam gasification of rice husk char described by shrinking core reaction model and homogeneous reaction model can be calculated with the slope and intercept of the fitting straight-line, respectively. Moreover, their values are shown in Table 3. It can be seen by comparing Figs. 9 and 10 that shrinking core reaction model and homogeneous reaction model both can describe the gasification reaction of rice husk char well, but the shrinking core reaction model has a greater correlation coefficient. If the diffusion through gas is considered as the rate-controlling step for the shrinking core reaction model, Eq. (14) will be x = kGt. The characteristics of rice husk char gasification with steam using shrinking core reaction model considering the diffusion through gas as the rate-controlling step are shown in Fig. 13. They have better linear fitting with the experimental results at all temperatures. The relationship between ln kG and 1000/T for shrinking core reaction model considering the diffusion through gas as the rate-controlling step is shown in Fig. 14 and the average apparent activation energy and pre-exponential factor are shown in Table 3.
-2.8
-3.4 -3.6 -3.8 -4.0 -4.2 0.80
0.85
0.90
0.95
1.00
1.05
7 . Fig. 14. The relationship between ln kG and 1000/T for shrinking core reaction model considering the diffusion through gas as the rate-controlling step.
From Figs. 9 and 13, it can be concluded that both surface reaction and diffusion through gas influence the reaction, but when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction. The shrinking core reaction model considering the diffusion through gas as the rate-controlling step can be used for describing the reaction behavior of steam gasification. 4. Summary and conclusions The characteristics of rice husk char with steam are studied by experiments. The following conclusions can be made after analyzing the influences of reaction temperature, steam flow rate, particle size, reaction time and char preparation temperature on the conversion rate and the composition of produced gas of rice husk char.
M. Zhai et al. / Fuel 158 (2015) 42–49
(1) Temperature is the primary factor that influences the steam gasification reaction of rice husk char. When the temperature rises from 700 °C to 950 °C, the conversion rate of rice husk char increases significantly from 27.7% to 90.73% after reacting for 12 min. The high temperature contributes for the production of H2. When the temperature is 950 °C, H2 accounts for 46.9% of the product gas. (2) Steam flow rate also has great influence on the gasification reaction of rice husk char. The conversion rate of rice husk char increases as the steam flow rate. H2 and CO gradually increase while CO2 and CH4 decrease as the steam flow rate. (3) The particle size has some effects on the conversion rate of rice husk char. The conversion rate can be increased by decreasing particle size at low temperature, but the influence of the particle size becomes smaller above 900 °C. (4) The conversion rate of rice husk char can be improved by increasing the reaction time. When the temperature is less than 850 °C, more reaction time is needed due to low reaction rate. (5) The reactivity of rice husk char prepared at low temperature is relatively high. Meanwhile, when the gasification temperature is 950 °C, the conversion rate of rice husk char reaches 98.65% if the char preparation temperature is 600 °C, whereas the conversion rate reduces to 87.3% if the char preparation temperature is 900 °C. (6) Both surface reaction controlled shrinking core reaction model and homogeneous reaction model can describe the steam gasification reaction of rice husk char when the temperature is less than 850 °C, and the shrinking core reaction model behaves better fitting. However, when the temperature is more than 850 °C, the diffusion through gas controls the overall reaction.
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