defluidization on hydrogen generation during fluidized bed air gasification of modified biomass

defluidization on hydrogen generation during fluidized bed air gasification of modified biomass

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 0 9 e1 4 1 7

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Effect of agglomeration/defluidization on hydrogen generation during fluidized bed air gasification of modified biomass Jia-Hong Kuo a, Chiou-Liang Lin b, Ming-Yen Wey a,* a b

Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROC

article info

abstract

Article history:

This study presents the effect of particle agglomeration on syngas emission during the

Received 17 June 2011

biomass air gasification process. Various operating conditions such as operating temper-

Received in revised form

ature, equivalence ratio (ER), and amount of bed materials are employed. The concentra-

26 September 2011

tions of H2 and CO increase along with the operating time as agglomeration begins, while

Accepted 1 October 2011

CO2 decreases at the same time. However, there is no significant change in the emission

Available online 1 November 2011

concentration of CH4 during the defluidization process. The lower heating value increases while the system reaches the agglomeration/defluidization under various operating

Keywords:

parameters. When the system reaches the agglomeration/defluidization process, the LHV

Agglomeration

value sharply increases. The results are obtained when the system reaches agglomeration/

Defluidization

defluidization. The temperature increases while bed agglomeration occurs. A higher

Air gasification

temperature increases the production of H2 and CO, contributing to the LHV calculation.

Hydrogen production

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Fluidized bed

1.

Introduction

2 Boudouard reaction

Biomass gasification is a renewable and CO2-neutral energy resource, and fluidized bed is recognized as an important technique for gasification of biomass. Gasification involves a series of endothermic reactions supported by the heat produced from the combustion reaction. It yields combustible gases such as hydrogen, carbon monoxide, and methane through a series of reactions. The following are the four major gasification reactions [1e3] and their chemical reactions at different temperatures, as shown in Fig. 1 [4]:

DH ¼ þ13; 138 kJ=kg mol carbon

(1)

(2)

3 Wateregas shift conversion CO þ H2 O/CO2 þ H2

DH ¼ 4198 kJ=mol

(3)

DH ¼ 7490 kJ=mol carbon

(4)

4 Methanation C þ 2H2 /CH4

1 Wateregas reaction C þ H2 O/H2 þ CO

C þ CO2 /2CO DH ¼ þ17; 258 kJ=kg mol carbon

Accordingly, the operating temperature can affect product distribution during gasification according to the chemical

* Corresponding author. Tel.: þ886 4 22852455; fax: þ886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.001

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100

0

60

40

40

60

20

80

0 100

100 0

80

Water-gas-shift raction CO + H2O → CO2 + H2

20

60

40

40

60

20

80

0 100

CH4 concentration (vol%)

20

100 0

Methanation reaction C+2H2 → CH4

80

20

60

40

40

60

20

80

0 400

500

600

CO2 concentration (vol.%)

C+CO2 → 2CO

H2O concentration (vol%)

80

700

800

900

H2 concentration (vol%)

CO and H2 concentration (vol%)

CO concentration (vol.%)

Boundourd reaction

100 1000

Fig. 1 e Illustration of thermodynamic equilibrium of different gaseous products concentration (Pressure [ 1.0eatm) (Basu, 2006) [1].

reactions involved. Previous research has noted the chemical equilibrium trends of the carboneoxygenehydrogen system, which indicates that when the temperature increases, the product concentrations of H2 and CO increase but those of CO2 and CH4 decrease [4]. The equivalence ratio (ER) is commonly used in connection with the gasifier air supply. It is defined as the ratio between the actual air fuel and the stoichiometric air fuel. The quality of gas obtained from a gasifier strongly depends on the ER value, which must be significantly below 1.0 to ensure a condition far from complete combustion. An excessively low ER value (<0.2) results in several problems including incomplete gasification, excessive char formation, and low heating value of the gas product. In contrast, a too high ER value (>0.4) results in the excessive formation of products of complete combustion, such as CO2 and H2O, at the expense of desirable products such as CO and H2.

However, biomass gasification has a greater tendency toward bed material agglomeration. Previous research have investigated the mechanisms of bed material agglomeration and other control methods [5e13]. Olofsson et al. (2002) [14] found that a high content of alkali and alkaline earth metals exists in biomass materials. They also reported two possible mechanisms of particle agglomeration included (1) homogeneous agglomeration which is considered a slight agglomeration on the particle surface, and than it results a uniform particle size distribution of agglomerates. Moreover, (2) heterogeneous agglomeration is recognized a fast increasing or variation of particle size of agglomerates lead to defluidization, and K, Na, Si, and Ca are assumed to be the sources of particle agglomeration. Fryda et al. (2008) [15] studied three different biomasses, namely, giant reed, sweet sorghum bagasse, and olive, for the fluidized bed agglomeration process. The results presented that the agglomerations of the giant reed and sweet sorghum bagasse are due to their K-rich content. Other study has estimated the agglomeration behavior during gasification processes, indicating that the initial agglomeration temperature in gasification is lower than that in a combustion atmosphere [8]. A material balance of the reaction in biomass fluidized bed gasification is widely studied in previous studies [16e18]. He et al. [19] indicated the municipal solid waste as a biomass source in the gasification process, and the results showed that the average yields of gas, char and tar phases are 0.21 Nm3/kg, 25.86 wt%, and 38.54 wt%, respectively. Moreover, Gil et al. [20] provided the effect of different type of gasifying agent on biomass gasification, the yield of the gas and tar are 1.25e2.45 Nm3/kg, and 3.7e61.9 g/kg while air is taken as gasifying agent during the gasification process. The decomposition of tar or char plays an important role on the generation and formation of hydrogen in the gasification process. Nevertheless, to crack the tar component into the useful gases to increase the heat value is widely studied. Many studies reported several ways to control the tar formation in the fluidized bed gasification process included catalytic cracking, thermal cracking, self-modification, mechanism controlling, and plasma controlling [21e25]. However, recently researches only focused on the agglomeration behavior during the biomass gasification process [26,27]. Particle agglomeration in fluidized bed operation is also a complex process during the biomass combustion and gasification, the effect of agglomeration on the tar decomposition is not clear. Therefore, the aim of this study is to evaluate the effect of particle agglomeration on behavior of the generation and proportion of gaseous products. According to the evaluation of previous researches on the effect of the operating condition on fluidized bed gasification for gaseous product distribution, most biomass fuels contain alkali metals that could cause particle agglomeration during fluidized bed operation. The agglomeration behavior influences not only the fluidization itself but also product distribution. Bed agglomeration causes serious problems, and it may result in an unscheduled system shut down. Therefore, this study focuses on the effect of particle agglomeration on product distribution such as H2, CO, CO2, and CH4. Various operating conditions, which include the operating temperature, ER, and amount of bed materials, are considered.

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Fig. 2 e Bubble fluidized bed incinerator. (1) auto-feeder controller, (2) PID controller, (3) data collector, (4) auto-feeder, (5) blower, (6) flowmeter, (7) thermocouple, (8) pressure detector, (9) preheater chamber, (10) sand bed, (11) electric resistance, (12) sampling place, (13) U manometer, (14) secondary fluidized bed combustor, (15) secondary air supply, (16) induce fan.

2.

Experimental

2.1.

Apparatus

The programmable logic controller is employed to control the temperature of the fluidized bed. Two pressure detectors are used to detect the difference between the pressure of the sand bed and the pressure of the freeboard chamber. These

Fig. 2 demonstrates the fluidized bed incineration system used in the experiments. The reactor is a bubbling fluidized bed incinerator made up of a preheated chamber (50 cm long) and a main chamber (105 cm high) with an inner diameter of 10 cm. The reactor is made of stainless steel (3 mm thickness, AISI 310) and is enclosed by an electrically resistant material packed with ceramic fibers to thermally insulate the system. The stainless steel porous plate functions as a gas distributor with a 15% open area. Three thermocouples are used to measure the temperatures of the preheated chamber, sand bed, and freeboard chamber.

Table 1 e Proximate analysis and ultimate analysis of strawdust. Proximate analysis (dry basis, wt%) Moisture Volatile matter Ash Ultimate analysis (dry, ash-free basis, wt%) C H N O LHV

6.3 93.2 0.6 43.1 5.8 5.0 46.1 3194 kcal/kg

Table 2 e Operating conditions in experiment. Amount of Run Temperature ER (–) bed materials ( C) (g) 1 2 3 4

600 700 800 900

0.5

1400

5 6 7 8

800

0.4 0.5 0.6 0.7

1400

9 10 11 12

600 700 800 900

0.5

1400

13 14 15 16

800

0.4 0.5 0.6 0.7

1400

17 18 19 20

800

0.5

1400 1600 1800 2000

Na concentration (wt.%) 0

1.2

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Table 3 e Heat of combustion gases. Gas

Higher heating value (MJ/kg-mol)

Lower heating value (MJ/kg-mol)

CO H2 CH4

282.99 285.84 890.36

282.99 241.83 802.34

probes are connected to the different pressure transmitters (Huba control 692). The pressure signals are digitized and recorded by a data acquisition system (ADVANTECH PCLabCard PCI-1711 and ADAMView software). The pressure drop can be used to measure agglomeration during fluidization. According to previous studies, when the agglomeration condition occurs, a drastic increase in the temperature ensues as well as a simultaneous and rapid decrease in the bed pressure drop [28,29]. Therefore, the pressure drop is used to measure agglomeration during fluidization, and the typical pressure profile has been reported in previous paper [30]. The secondary fluidized bed is set after the fluidized bed reactor to combust the exhaust organics behind the main gasifier, as shown in Fig. 2.

2.2.

Fig. 3 e Effect of operating temperature on gas composition. (Operating condition: ER [ 0.5, amount of bed materials: 1400 g; gas composition calculated by N2-free environment).

Preparing artificial feed wastes

The artificial solid waste included sawdust (2.25 g) and a polyethylene (PE) bag (0.3 g). The information of proximate and ultimate analysis of the sawdust is showed in Table 1. The total mass was 3.55 g. An agglomeration promoter (Na) was added as nitrates (NaNO3) with 1.2 wt% of one artificial waste pack. The metals to be investigated were dissolved as nitrates in distilled water, and the weight percentage of Na was calculated as atoms of the metal, not as nitrate. The metal solution (1 mL) was then added to the sawdust and was packed in a PE bag. Before the experiment, artificial wastes were stored for a day to

a

b

c

d

Fig. 4 e Effect of defluidization process on selectivity of gases at different operating temperature (a) 600  C, (b) 700  C, (c) 800  C, (d) 900  C. (A-H2, --CO2, :-CO, 3-CH4) (Operating condition: ER [ 0.5, amount of bed materials: 1400 g; gas composition calculated by N2-free environment) (red-straight line: time to reach the defluidization). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5 e Effect of equivalence ratio on gas composition. (Operating condition: Temperature: 800  C, amount of bed materials: 1400 g; gas composition calculated by N2-free environment).

ensure that the sawdust could absorb the metal solution completely. Silica sand, with an almost constant density (rp ¼ 2600 kg/m3), was used as the bed material in the experiment. The pressure-versus-time profile and visual observation were used to evaluate defluidization time.

2.3.

Experimental procedure

The input air (relative to the theoretical air) was then used under the different operating temperatures. Input air at

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different ERs was determined. The ER is defined the ratio of the actual supply air divided by the stoichiometric air required for complete combustion. Once the temperature of the sand became steady, the artificial waste was fed into the chamber at a rate of 1 bag per 10 s. The total amount of bed materials used was 1400 g. After each experiment, the combustion chamber was cooled down to room temperature, and the bed materials were collected. The different operating parameters employed, such as operating temperature, ER, and the amounts of bed materials, are presented in Table 2. This experiment was estimated under three operating parameters such as operating temperature (600, 700, 800, and 900  C), ER (0.4, 0.5, 0.6, and 0.7), and amount of bed materials (1400, 1600, 1800, and 2000 g) under a Na ratio of 1.2 wt% of the simulated biomass. Besides, the gaseous products, such as H2, CO, CO2, and CH4, were sampled during the experimental processes. After defluidization, the synthetic waste was continuously fed into the gasification chamber, and the products were then sampled. In addition, The LHV calculation is calculated from the gaseous products such as H2, CO, and CH4 which are showed in Table 3, and the definition of the lower heating value is difined as below: LHVðMJ=kg  moleÞ ¼ 0:0224ð241:83  H2 þ 282:99  CO þ 802:34  CH4 Þ

a

b

c

d

Fig. 6 e Effect of defluidization process on selectivity of gases at different equivalence ratio (a) 0.4, (b) 0.5, (c) 0.6, (d) 0.7. (A-H2, -eCO2, :-CO, 3 -CH4) (Operating condition: Temperature: 800  C, amount of bed materials: 1400 g; gas composition calculated by N2-free environment) (red-straight line: time to reach the defluidization). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Sampling and analysis

To capture the gaseous products, the tar and char should be removed. In this study, char was removed by a cyclone, and the tar was collected by the sampling procedure modified from the method of the European Committee for Standardization (CEN)/technical Specification [31]. The modified sampling chain was used in this study to collect the gaseous products, which were analyzed by gas chromatographer combined with thermal conductivity detector (GC/TCD).

3.

Result and discussion

3.1.

Effect of operating temperature

Fig. 3 shows the effect of the operating temperature on gas composition during air gasification. The proportion of H2 in syngas increases, whereas the proportion of CO plots a minor increase as the operating temperature increases. There is a decreasing trend in the result of the proportions of CO2 and CH4 as the temperature increases. Many studies indicate that the production of H2 is directly proportional to the temperature [5]. Han et al. [32] indicated the increasing temperature presented opposite effect on the water gas shift reaction because this reaction was controlled by chemical reaction kinetics and higher temperature was favorable for the reaction. In addition, Acharya et al. [33] presented high

temperature was not beneficial for the exothermic water gas shift reaction. Enhancing H2 production was that the reforming reactions were all promoted with the increase of temperature. Moreover, Skoulou et al. [34] illustrated the high temperature steam reforming of CH4 or even tar thermal cracking might contribute to the slight further increase of CO. Consequently, temperature controlling is a key factor for the hydrogen production during biomass gasification application. In the air gasification system, wateregas shift reaction is an exothermic procedure; thus, the effect of temperature is minimal. However, other reactions, such as wateregas, boudouard, and methanation, and tar decomposition are endothermic such that they are reinforced by an increase in temperature. Higher temperature causes a change in the syngas composition. Wateregas reaction, boudouard reaction, and tar decomposition contribute to the increase in H2 and CO content. Methanation reaction also favors a higher temperature, but the changes are difficult to find due to the low emission concentration. Heat energy is given from both biomass combustion and electric heater to provide the air gasification reactions in the fluidized bed operation. At high temperature, the wateregas shift reaction still occurs that cannot neglectable. However, according to the experimental results which show a decreasing trend of CO2, the wateregas shift reaction is probably weak in the comparison of other reactions described in the introduction. Base on the Le Chatelier’s principle, higher temperature favors the reactants to react in exothermic reactions and the products in

a

b

c

d

Fig. 7 e Effect of defluidization process on selectivity of gases at different amount of bed materials (a) 1400 g, (b) 1600 g, (c) 1800 g, (d) 2000 g. (A-H2, --CO2, :-CO, 3 -CH4) (Temperature: 800  C, ER [ 0.5, amount of bed materials: 1400 g; gas composition calculated by N2-free environment) (red-straight line: time to reach the defluidization). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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temperature is maintained at 800  C. The results show that the proportion of H2, CO, and CH4 decreases as the ER increases, whereas the proportion of CO2 increases when the ER is set from 0.4 to 0.6. As the ER is set to 0.7, the trend of gas composition is difficult to find because the high flow rate affects the sampling procedure during air gasification. The increase in ER causes the increase in partial combustion of different gaseous compounds. There was an increase in the proportion of CO2 in the experiment. Mansaray et al. [38,39] noted that at a higher ER/or fluidization velocity, more air is supplied, and hence more char can be combusted to form CO2, at the expense of other combustible gases such as H2 and CH4. High fluidization gas velocity results in a shorter residence time for the char in the gasification system, causing an increase in the gas yield and a decrease in the tar and char yields [40]. The selectivity of the gaseous products during bed agglomeration/defluidization is shown in Fig. 6. The emission concentration of H2 and CO increases when the system shifts to defluidization, but that of CO2 and CH4 decreases. The selectivity of each gaseous product also presents the same trend. Consequently, the same observation applies to the experimental results of the temperature effect. Thus, the emission behavior of gaseous products after defluidization has less relevance to the various operating conditions. The fluidization bed shifts to a fix-bed type during bed agglomeration, and the high temperature accumulation on the surface of the sand bed promotes the increase in H2 and CO [41,42].

Fig. 8 e Effect of various operating conditions on the lower heating value during defluidization process. (a) Temperature, (b) Equivalence ratio, (c) Amount of bed materials. (red asterisk: time to reach the defluidization). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

endothermic reactions. The results are in accordance with those in the literatures [35e37]. Fig. 4 shows the effect of the operating conditions on the emission concentration of gaseous products and the selectivity of each gas during the air gasification process. As shown in the figure, the concentrations emitted during the operating time are similar under the various operating conditions. The concentrations and selectivities of H2 and CO increase along with the operating time as the agglomeration began, whereas those of CO2 decrease. However, there is no significant change in the emission concentration of CH4 during the defluidization process. The reason is that when the system reaches defluidization, the fluidized bed system shifts to a fix-bed state, and the temperature increases on the surface of the sand bed.

3.2.

Effect of equivalence ratio

Fig. 5 indicates the effect of ER on gas composition during air gasification. The ER is set from 0.4 to 0.7, and the operating

3.3.

Effect of the amount of bed materials

Fig. 7 presents the effect of the defluidization process on the gas composition at different amounts of bed materials. Based on the Fig. 7, the emission concentration and proportion of each gaseous product show the same trend in comparison with the other operating conditions, such as operating temperature and ER. The increasing amount of bed materials does not influence the chemical reaction but instead affects the particle agglomeration tendency during the defluidization process.

3.4. Estimation of the changes in the lower heating value (LHV) Fig. 8 indicates the effect of the different operating conditions on the LHV during the defluidization process. The LHV value increases along with an increase in the operating temperature. However, when the system reaches agglomeration/ defluidization, the LHV sharply increases. The increase in temperature during bed agglomeration, which results in an increase in the production of H2 and CO, contributes to LHV calculation. The LHV also gradually increases under the other operating parameters, such as ER and the amount of bed materials, during the bed agglomeration/defluidization process. The results are in accordance with those in previous works [28,35,36].

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Conclusion

This study presents the effect of particle agglomeration during the biomass air gasification process. Different Na ratios with various operating conditions, such as operating temperature, ER, and amount of bed materials, are considered. The selectivity of H2 and CO increases along with the operating time as agglomeration begins, but the selectivity of CO2 decreases simultaneously. When the system reaches defluidization, the fluidized bed system is shifted to a fix-bed state and the temperature increases on the surface of the sand bed. There is no significant change in the emission concentration of CH4 during the defluidization process. The LHV value increases along with the operating temperature. When the system reaches agglomeration/defluidization, the LHV sharply increases. Thus, the temperature increases during bed agglomeration, leading to an increase in the production of H2 and CO and contributing to the LHV calculation. The LHV gradually increases after the system reaches the agglomeration/ defluidization under the various operating parameters such as ER and the amount of bed materials those chosen in this study. Therefore, the increasing of LHV is controlled by the agglomeration behavior according to the experimental results.

Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 98-2221-E-005-014-MY3.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 0 9 e1 4 1 7

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