Characteristics of rice husk gasification in an entrained flow reactor

Characteristics of rice husk gasification in an entrained flow reactor

Bioresource Technology 100 (2009) 6040–6044 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/loca...

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Bioresource Technology 100 (2009) 6040–6044

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Characteristics of rice husk gasification in an entrained flow reactor Yijun Zhao a, Shaozeng Sun a,*, Hongming Tian a, Juan Qian a, Fengming Su b, Feng Ling b a b

Combustion Engineering Research Institute, School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, PR China China Power Complete Equipment Co., Ltd., Beijing 100011, PR China

a r t i c l e

i n f o

Article history: Received 2 February 2009 Received in revised form 5 June 2009 Accepted 8 June 2009 Available online 8 July 2009 Keywords: Biomass Gasification Kinetic parameter Rice husk

a b s t r a c t Experiments were performed in an entrained flow reactor to better understand the characteristics of biomass gasification. Rice husk was used in this study. Effects of the gasification temperature (700, 800, 900 and 1000 °C) and the equivalence ratio in the range of 0.220.34 on the biomass gasification and the axial gas distribution in the reactor were studied. The results showed that reactions of CnHm were less important in the gasification process except cracking reactions which occurred at higher temperature. In the oxidization zone, reactions between char and oxygen had a more prevailing role. The optimal gasification temperature of the rice husk could be above 900 °C, and the optimal value of ER was 0.25. The gasification process was finished in 1.42 s when the gasification temperature was above 800 °C. A first order kinetic model was developed for describing rice husk air gasification characteristics and the relevant kinetic parameters were determined. Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.

1. Introduction The demand for renewable sources of energy is increasing, due to the increasing concern about global warming, climate changes and the declining of the fossil fuel reserves. Compared with other renewable energy resources, biomass is huger in annual production with a geographically widespread distribution in the world. Gasification of biomass fuel (such as wood-based materials, agricultural residues, forestry wastes, etc.) is a promising technology that provides a competitive means for producing chemicals and energy from renewable energy sources (Chen et al., 2004; Weerachanchai et al., 2008; Toonssen et al., 2008; Moghtaderi, 2007). The gasification process is the result of the combination of a series of complex and competing reactions which are similar in different gasifiers. It is necessary to understand the gas concentration distribution inside the gasifier in detail for performance evaluation and optimum design of gasifier. A lab-scale entrained flow reactor (EFR) is a good analytical tool which is often used to study different thermo-chemical conversion processes (Biaginia et al., 2005; Niu et al., 2008). In this study, experiments were performed in an EFR with air as fluid medium. This paper describes effects of the gasification temperature and the equivalence ratio (ER), which is defined as the ratio of the actual air supplied to the stoichiometric air required for complete combustion on the biomass gasification. The results provide vital experimental data for design and operation of biomass gasifier. Moreover, axial gas yield profiles in the

* Corresponding author. Tel.: +86 451 86412238; fax: +86 451 86412528. E-mail address: [email protected] (S. Sun).

EFR are also reported in this paper. The data can be used in the mathematical model of biomass gasification. 2. Experimental 2.1. Experimental material The biomass used in this study was rice husk which was obtained from Heilongjiang province, China. Rice husk sample was dried naturally in air and milled. The low heating value (LHV) and moisture content (air dry basis) of the sample are 13.67 MJ/kg and 8.30%. The elemental content (air dry basis) of C, H and O are 37.35%, 4.49% and 33.19%, respectively. 2.2. Experimental apparatus The EFR is shown in Fig. 1. The experimental system is made up of five parts: the main body of EFR, the fuel feeding system, the gas supplying system, the heating and temperature measuring system and the sampling system. The main body of EFR includes two parts: the horizontal chamber and the vertical chamber. The horizontal chamber is 100 mm in diameter and 850 mm in length. A gas burner is installed at the entrance of the horizontal chamber. Propane gas was supplied from the gas burner to pre-heating the EFR. Air was supplied from the air compressor, metered through a mass flow controller. The carrier gas was preheated in the horizontal chamber before it entered the vertical chamber in the experiments. The vertical chamber is 1900 mm in height and 100 mm in diameter. Five sampling probes named as Nos. 1–5 in turn and four

0960-8524/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.06.030

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Y. Zhao et al. / Bioresource Technology 100 (2009) 6040–6044

Fig. 1. Schematic diagram of the entrained flow reactor.

thermocouples named as T1–T4 in turn are located along the axial direction of the reactor. To compensate for heat losses of the vertical part during the experiment, external electrical heating of 6 kW was provided. PID temperature controlling system is used and the operation temperature could be adjusted according to the wall temperature of the EFR. The chamber is made up of corundum tube. The air tightness tests were performed before experiments. Rice husk was continuously fed into the reactor by a screw feeder from the hopper on the top of the reactor, where rice husk mixed with gasification agent. To improve the uniformity of the feeding process, a brush-like device has been placed above the screw conveyor. The carrier gas is nitrogen. The gas flow rates are measured by mass flow controllers. The experiments were performed in atmospheric pressure. The sampling system consists of a fiber filter, an ice water condenser, gas washing bottles (acetone), a vacuum pump and an on line gas analyzer, as shown in Fig. 1. The solids in the produced gas were removed with a hot quartz fiber filter. The sampling probes were heated to 200 °C to prevent tar condensation. The produced gas concentration of CO, CO2, H2, CH4 and C2H4 were monitored continuously. Gasboard-3020 (WuHan Cubic Optoelectronics Co., Ltd., China) was used to measure the concentration of CO, CO2 and H2. Concentrations of CO and CO2 were measured with the principle of non-dispersive infrared spectroscopy (NDIR). Hydrogen was measured using a thermal conductivity detector. CH4 and C2H4 were analyzed on a GC6890 Series (Agilent Technologies, USA). The flow rate of sampling gas ranged from 0.7 to 1.2 L/min. To ensure the reliability of tests data, each experiment was repeated two times and the results had a good agreement. 3. Results and discussion 3.1. Effect of the reactor temperature The temperature is crucial for the overall biomass gasification process. In the present work, the reactor temperature was varied from 700 to 1000 °C in 100 °C increment. The feeding rate of rice husk and the air flow rate were 10 g/min and 9.5 L/min, respectively. The flow rate of the carrier gas (nitrogen) was 41 L/min. Test results are presented in Table 1 and Fig. 2.

The gasification process consists of three basic processes, namely pyrolysis, oxidization reactions and reduction reactions. An instantaneous pyrolysis of the fuel is supposed to occur as soon as it is injected into the gasifier. Then a fraction of products of the pyrolysis and char are burnt with oxygen. In the reduction zone, char, tar and hydrocarbons are then gasified by reactions with carbon dioxide and water vapour to give a fuel gas composed mainly of CO, H2, CH4 and C2H4 (Mathieu and Dubuisson, 2002; Gabra et al., 2001). As shown in Fig. 2, the concentration of CH4 remained about 1.3%. It shows that the temperature has little effect on the rate of the reaction between CH4 and oxygen. The concentration of C2H4 decreased from 0.65% to 0.19% with the increase of the temperature from 700 to 1000 °C because of cracking. The results suggest that higher temperature is favorable of the reforming and cracking reactions of heavier hydrocarbons. The variations of the concentrations of CO, CO2 and H2 are shown in Fig. 2. When the temperature increased from 700 to 900 °C, the concentration of CO decreased from 20.25% to 16.67%, while the concentrations of CO2 and H2 increased from 11.34% to 14.42% and from 5.17% to 7.97%, respectively. It implies that water–gas shift reaction between CO and H2O is more dominant, which would explain the increase in the H2/CO ratio and the decrease in the CO/CO2 ratio, as presented in Table 1. Furthermore the Boudouard reaction between char and CO2 seems less impor-

Table 1 Effect of the reactor temperature on rice husk air gasification. Fuel

Rice husk

Fuel feed rate (g/min) Air flow (L/min) The carrier flow (L/min) Temperature (°C) H2/CO H2/CO2 CO/CO2 Low heating value (MJ/Nm3) Fuel gas production (Nm3/kg) Carbon conversion (%) Cold gas efficiency (%)

10 9.5 41 700 0.26 0.46 1.79 4.00 1.22 60.66 35.88

10 9.5 41 800 0.28 0.41 1.46 3.72 1.22 59.69 33.25

10 9.5 41 900 0.48 0.55 1.16 3.63 1.27 60.41 33.59

10 9.5 41 1000 0.65 0.84 1.30 4.36 1.45 74.88 46.17

Y. Zhao et al. / Bioresource Technology 100 (2009) 6040–6044

CH4

CO

H2

C2H4

18 15 12 9 6 3 0 700

750

800

850

900

Reactor temperature (

950

1000

)

Fig. 2. Effect of the reactor temperature on the concentration of the produced gas.

tant at higher temperature. CO and H2 are the main products of biomass pyrolysis. Higher yields of CO and H2 are obtained by rapid pyrolysis at higher temperature (Dupont et al., 2008). When the temperature was higher than 900 °C, with the increase of temperature, the concentrations of CO and H2 increased rapidly, while the concentration of CO2 increased slowly. The evolutions of LHV, fuel gas production, carbon conversion and cold gas efficiency versus the temperature are presented in Table 1. The LHV is defined as the amount of the heating value of the combustible component and the fuel gas production is defined as the amount of gas produced per unit weight of fuel. The LHV of the produced gas decreased from 4.00 MJ/Nm3 at 700 °C to 3.63 MJ/Nm3 at 900 °C and increased to 4.36 MJ/Nm3 at 1000 °C as indicated in Table 1. The fuel gas production increased slowly from 1.22 to 1.45 Nm3/kg with the increase of the temperature. The carbon conversion is defined as the degree to which the carbon in the fuel has been converted into gaseous products. It is an important parameter in biomass conversion process which is determined by the produced gas composition and gas yield. As presented in Table 1, the carbon conversion remained about 60% when the temperature was below 900 °C, and increased rapidly from 60.41% to 74.88% with the temperature rising up to 1000 °C. That is due to the increase of the yields of CO and CO2 as mentioned before. The cold gas efficiency is used to evaluate gasification performance which is defined as the ratio of the heat content of the fuel gas generated by the gasification of the rice husk to the heat content of that rice husk when it is completely burnt. When the temperature was below 900 °C, the cold gas efficiency decreased with the increasing of temperature, because the increase of the fuel gas production can not compensate the decrease in the LHV of the produced gas. When the temperature rose up to 1000 °C, the cold gas efficiency increased to 46.17%. The results suggest that the optimal gasification temperature may be above 900 °C, because higher temperature is favorable to energy and carbon conversion, as well as gas yield. The main disadvantage of using higher temperature is the higher heat loss and costs. 3.2. Effect of ER ER is a crucial factor affecting gas quality. In this study, ER varied from 0.22 to 0.34 through changing the air flow rate while holding the fuel feeding rate at 10 g/min. The gasification temperatures were 800 and 1000 °C, respectively. The Effect of ER on the produced gas composition when the temperatures are 800 and 1000 °C are shown in Figs. 3 and 4,

respectively. Fig. 3 shows that concentrations of CH4 and C2H4 vary little in the whole range of ER. It indicates that the oxidization reactions between CnHm and oxygen are less important. Effect of ER on the rice husk air gasification is summarized in Table 2. As can be seen in Table 2, Figs. 3 and 4, when ER increased from 0.22 to 0.25, the yields of CO and CO2 increased because of the reactions between char and oxygen. When the ER was higher than 0.25, the concentration of CO decreased rapidly from 24.49% to 14.84% at 800 °C and from 20.87% to 14.52% at 1000 °C, respectively. While the concentration of CO2 increased from 10.55% to 13.39% at 800 °C and from 14.83% to 15.50% at 1000 °C, respectively. The yield (amount of CO and CO2) is presented in Table 2, which remained constant with the increase of ER. The results indicate that the reaction between CO and O2 plays a more prevailing role with higher ER. It can be concluded that the increase of ER led to the combustion of H2 and consequently the decrease of H2 concentration. As presented in Table 2, when ER increased from 0.22 to 0.34, the LHV of the produced gas decreased from 4.74 to 3.08 MJ/Nm3 at 800 °C and from 5.20 to 3.59 MJ/Nm3 at 1000 °C, respectively. It is due to the decrease of the combustible gas and dilution by the addition of nitrogen. Furthermore, the fuel gas production increased from 1.04 to 1.39 Nm3/kg at 800 °C and from 1.28 to 1.57 Nm3/kg at 1000 °C, respectively with the increase of ER. Due

Concentrations of the producer gas ( )

CO2

21

27

CO2

CH4

CO

H2

C2H4

24 21 18 15 12 9 6 3 0 0.22

0.24

0.26

0.28

0.30

0.32

0.34

Equivalence ratio Fig. 3. Effect of ER on the concentration of the produced gas at 800 °C.

Concentrations of the producer gas ( )

Concentrations of the producer gas ( )

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24

CO2

CH4

CO

H2

C2H4

21 18 15 12 9 6 3 0

0.22

0.24

0.26

0.28

0.30

0.32

0.34

Equivalence ratio Fig. 4. Effect of ER on the concentration of the produced gas at 1000 °C.

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Y. Zhao et al. / Bioresource Technology 100 (2009) 6040–6044 Table 2 Effect of the ER on rice husk air gasification. 800

Fuel feed rate (g/min) Air flowrate (L/min) ER CO2 yield (Nm3/kg) CO yield (Nm3/kg) CO2 + CO (Nm3/kg) Low heating value (MJ/Nm3) Fuel gas production (Nm3/kg) Carbon conversion (%) Cold gas efficiency (%)

10 7.4 0.22 0.107 0.254 0.361 4.74 1.04 56.40 35.99

1000 10 8.4 0.25 0.124 0.289 0.413 4.73 1.18 64.51 40.77

10 9.5 0.28 0.156 0.228 0.384 3.72 1.22 59.70 33.25

10 10.5 0.31 0.169 0.213 0.382 3.27 1.29 59.04 30.90

to the effect of the LHV and production of the produced gas, the cold gas efficiency reached the maximum value of 40.77% at 800 °C and 50.64% at 1000 °C, respectively when ER was 0.25. Similar tendency was observed for the carbon conversion, which reached the maximum value of about 65% at 800 °C and about 75% at 1000 °C, respectively. Therefore, the optimal value of ER was 0.25 in the present study. 3.3. Axial gas profiles in EFR The behavior of the axial gas yield profiles versus EFR height for the gasification of the rice husk at 800 and 900 °C are shown in Figs. 5 and 6, respectively. In the experiments, ER was 0.25 when the temperature was 800 °C and ER was 0.28 when the temperature was 900 °C, respectively. As shown in Fig. 5, with the distance from the fuel injector increasing from 410 mm (No. 1 sampling probe) to 1530 mm (No. 5 sampling probe), the yield of CO increased from 0.21 to 0.29 Nm3/kg and the yield of H2 increased from 0.065 to 0.075 Nm3/kg, while the yield of CO2 decreased from 0.146 to 0.124 Nm3/kg, respectively. It is expected that the residence time is an influential parameter on Boudouard reaction between char and CO2. There were no significant variations in the yields of CH4 and C2H4. It can be concluded that reactions of CnHm are less important in the gasification process except cracking reactions which occur at higher temperature. No significant changes were detected in the yields of main gases after 1250 mm (No. 4 sampling probe) from the fuel injector. The trend shows that the gasification process is finished in a residence time about 1.42 s when the gasification temperature is 800 °C. Trends are similar when

0.32

CO2

CH4

CO

H2

C2H4

Gas yield (Nm3/kg fuel)

0.28 0.24 0.20 0.16

10 11.5 0.34 0.186 0.206 0.392 3.08 1.39 60.81 31.23

0.24

10 7.4 0.22 0.179 0.267 0.446 5.20 1.28 68.45 48.58

10 8.4 0.25 0.207 0.281 0.488 4.96 1.40 74.75 50.64

CO2

CH4

0.08 0.04 0.00 400

600

800

1000

1200

1400

1600

Distance from the fuel injector (mm) Fig. 5. Axial gas yield profiles versus EFR height for the gasification of rice husk at 800 °C (ER = 0.25).

CO

10 10.5 0.31 0.229 0.253 0.482 4.07 1.52 73.76 45.28

H2

10 11.5 0.34 0.243 0.228 0.471 3.59 1.57 72.21 41.28

C2H4

0.16 0.12 0.08 0.04 0.00 400

600

800

1000

1200

1400

1600

Distance from the fuel injector (mm) Fig. 6. Axial gas yield profiles versus EFR height for the gasification of rice husk at 900 °C (ER = 0.28).

the temperature is 900 °C, as shown in Fig. 6, and the gasification process is finished in a residence time about 1.29 s. Therefore, the residence time for the gasification process decreases with the increase of the gasification temperature. 3.4. Model section 3.4.1. Kinetic evaluation method Biomass pyrolysis and gasification are complex processes due to the existence of different chemical components in the biomass material. Hence, no single kinetic model can be used to explain universally the mechanism of thermal decomposition in all types of biomass. The approach based on the Arrhenius equation were frequently used for the calculation of kinetic parameters (Munir et al., 2009; Xiu et al., 2006; Williams et al., 2000). A simple kinetic model for thermal decomposition was adopted to describe the conversion process, which can be expressed by the following equation (Guo and Lua, 2001):

  dw E ¼ Aðw1  wÞexp  dt RT

0.12

10 9.5 0.28 0.215 0.279 0.494 4.36 1.45 74.88 46.17

0.20

Gas yield (Nm3/kg fuel)

Temperature (°C)

ð1Þ

where A and E are the frequency factor (s1) and the activation energy (kJ/mol), respectively. R is the universal gas constant, 8.3145 (J/ mol); T is the absolute temperature (K) of the gasification process; W is the weight loss fraction at time t, and W1 is the maximum weight loss fraction of the sample. The weight loss of biomass during thermal decomposition should be determined directly by the difference between the weight of the feed biomass powder and chars. The weight loss has been calculated indirectly using the socalled ash tracer method (Xiu et al., 2006). The ash content in the

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Y. Zhao et al. / Bioresource Technology 100 (2009) 6040–6044

  dw 4183 ¼ 171:9ð73  wÞ exp  dt T

Table 3 Weight loss fraction of the solid particles versus the residence time. Temperature (k)

Sampling point

The residence time (s)

Weight loss fraction (%)

973

1 2 3 4 5

0.447 0.826 1.205 1.584 1.963

54.42 64.45 67.17 69.45 70.61

1073

1 2 3 4 5

0.399 0.738 1.077 1.416 1.754

65.94 68.15 70.35 71.84 72.43

1173

1 2 3 4 5

0.363 0.671 0.978 1.286 1.594

69.20 69.77 71.35 72.11 72.74

ð7Þ

4. Conclusions Experimental investigation of characteristics of rice husk air gasification in an entrained flow reactor was carried out. Reactions of CnHm are less important in the gasification process except cracking reactions which occur at higher temperature. In the oxidization zone, reactions between char and oxygen have a more prevailing role. The optimal gasification temperature of the rice husk could be above 900 °C, and the optimal value of ER is 0.25. The gasification process is finished in 1.42 s when the gasification temperature is above 800 °C. A first order kinetic model is developed for describing biomass air gasification. Acknowledgement

char can be used to estimate the weight loss because all the ash retains in the char. Since the gasification temperature is a fixed value, A exp (E/RT) is constant. Then,

Financial support from the Heilongjiang Provincial Natural Science Foundation (Contract No.:1307396) is gratefully acknowledged.

K ¼ A expðE=RTÞ

References

ð2Þ

Substituting Eq. (2) into Eq. (1), the expression can be written as

dw ¼ Kðw1  wÞ dt

ð3Þ

where w = 0 as t = 0. Integrating Eq. (3), the following expression can be obtained:

  w1 K ¼ ln t w1  w

ð4Þ

According to the experimental data, K can be calculated from Eq. (4). Expressing Eq. (2) in logarithmic form:

lnðKÞ ¼ lnðAÞ 

E 1000 1000R T

ð5Þ

3.4.2. Model results Table 3 shows the weight loss fraction of the solid particles versus the residence time when the temperature were 700, 800 and 900 °C. Unfortunately, it was difficult to collect adequate solid particles at each sampling point when the temperature was 1000 °C in the experiment. As shown in Table 3, the maximum weight loss fraction of the solid particles, w1 is no more than 73%. The value of K can be calculated, K700 = 2.282, K800 = 3.658 and K900 = 4.728. Using linear regression, the relation between ln(k) and 1000/T is

LnðkÞ ¼ 5:147  4:183ð1000=TÞ

ð6Þ

According to Eq. (5) and (6), A and E/R could be obtained, A = 171.9 and E/R = 4183. Therefore, the rice husk air gasification characteristics can be described as

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