Air-blown gasification of woody biomass in a bubbling fluidized bed gasifier

Applied Energy 112 (2013) 414–420

Contents lists available at SciVerse ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Air-blown gasification of woody biomass in a bubbling fluidized bed gasifier Young Doo Kim a, Chang Won Yang a, Beom Jong Kim b, Kwang Su Kim b, Jeung Woo Lee a, Ji Hong Moon c, Won Yang a,b, Tae U Yu a,b, Uen Do Lee a,b,⇑ a b c

Department of Green Process and System Engineering, University of Science and Technology, Republic of Korea Energy System R&D Group, Korea Institute of Industrial Technology, Republic of Korea Yonsei University, Seoul, Republic of Korea

h i g h l i g h t s  Air-blown gasification of woody biomass was investigated in a pilot-scale BFB gasifier.  The performance of the gasifier was investigated as a function of equivalence ratio and fluidization conditions.. 3

 The average heating value of the product gas was above 4.7 MJ/Nm .  Stable operation of the integrated gasification-power generation system was achieved.

a r t i c l e

i n f o

Article history: Received 25 September 2012 Received in revised form 13 March 2013 Accepted 25 March 2013 Available online 24 April 2013 Keywords: Biomass gasification Bubbling fluidized bed Air-blown gasifier Syngas Power generation

a b s t r a c t Air-blown gasification of woody biomass was investigated in a pilot-scale bubbling fluidized bed gasifier. Air was used as the gasifying agent as well as a fluidizing gas. Fuel was fed into the top of the gasifier and air was introduced from the bottom through a distributor. In order to control the composition of the product gas, the amounts of feedstock and gasifying agent being fed into the gasifier were varied, and the temperature distribution in the gasifier and the composition of the syngas were monitored. It was shown that the distribution of the reaction zones in the gasifier could be controlled by the air injection rate, and the composition of the syngas by the equivalence ratio of the reactants. Although the obtained syngas had a low caloric value, its heating value is adequate for power generation using a syngas engine. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Increased industrialization and energy consumption has led to stiff international competition to secure energy resources. According to recent research (Jacques et al., 2010), the price of oil and gas will be twice that at present, and therefore, the widespread exploitation of renewable energy is essential to avoid energy shortages, as well for minimization of the greenhouse effect [1]. Currently, there is much interest in renewable resources in combustible including woody biomass, sewage sludge and wastes, and various method have been tried to convert these renewable resources to

⇑ Corresponding author at: Energy System R&D Group, Korea Institute of Industrial Technology, Republic of Korea. Tel.: +82 41 578 8574; fax: +82 41 589 8323. E-mail addresses: [email protected] (Y.D. Kim), [email protected] (C.W. Yang), [email protected] (B.J. Kim), [email protected] (K.S. Kim), jwlee93@kitech. re.kr (J.W. Lee), [email protected] (J.H. Moon), [email protected] (W. Yang), [email protected] (T.U Yu), [email protected] (U.D. Lee). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.03.072

easy-to-use fuels such as gas or liquid fuel more effectively [2–6]. Biomass gasification, in which biomass is converted to quality syngas containing hydrogen, carbon monoxide, and methane without any impurities, is of particular interest [7,8]. Gasification can be classified by the type of gasifier, which can be Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB), moving/fixed bed, or entrained flow gasifier and also by whether the reaction occurs under high- or low-pressure conditions [9]. It is also classified by the oxidant, namely, air [10] steam [11], air–steam [12–14], oxygen–steam [15–17], or excess oxygen. One application of syngas with negligible tar and dust contents and with a high heating value is as the fuel for syngas engines for the direct generation of electrical power [18]. Air-blown gasification is a simple method in comparison to steam or oxygen-blown gasification, as the composition of the syngas can easily be controlled through the heating load, and fuel with various physical and chemical characteristics can be used by layering [19]. There have been various approaches for investigating the effects of a gasifier operation on product gas

415

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420

Nomenclature BFB CFB LHV LNG LPG

Bubbling Fluidized Bed Circulating Fluidized Bed lower heating value liquefied natural gas liquefied petroleum gas

ER FN PAHs RPM

compositions and quality. Low heating value (LHV) of syngas was studied in terms of equivalence ratio (ER) [14], and the effects of gasifier temperature, steam to biomass ratio and ER on gas composition was also investigated. More specifically, the effect of ER on the gas yield and properties of syngas were reported for different biomass fuels such as rubber wood chip and rice husk. In this study, air-blown gasification was investigated for the production of fuel for a syngas-engine power generator. Using air as the oxidant results in the syngas containing nitrogen and it thus has a low caloric value. Product gas compositions and temperature distribution in the gasifier were monitored as a function of

equivalence ratio fluidization number polycyclic aromatic hydrocarbons revolutions per minute

equivalence ratio and fuel feeding rate. And coupling effects of fluidization condition and gasification reaction were investigated. In addition, preliminary test of syngas power generation was conducted and it was found that the producer gas is adequate for power generation using a syngas engine. 2. Bfb systems Fig. 1 shows a schematic diagram of a BFB system and its reaction zones when feedstock is fed from the top and air is introduced from the bottom. Influenced by the heat and mass transfer

Biomass

Syngas

Biomass

Drying/ Pyrolysis/ Gasification

Biomass+Char

Product gas

Char

Gasification/ Combustion Char

Air+Productgas

Combustion/ Air preheating

Fig. 1. BFB gasification system with top feeding and reaction zones of the gasifier.

416

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420

between the fuel particles and environment of the gasifier, the biomass undergoes successive reaction process such as drying, pyrolysis, gasification, and combustion and the thermo-chemical conversion of biomass is strongly related to dimensions and shapes of feedstock [20,21]. The heat and product gases from the combustion of the biomass are used for gasification, pyrolysis, and drying under an appropriate atmosphere. Char, tar and non-condensable gas are representative by-products from gasification. Char is a residual solid material from devolatilization or pyrolysis of carbonaceous in biomass. Tars are variable mixture of phenols, polycyclic aromatic hydrocarbons (PAHs) and heterocyclic compounds. Because it is principally the char after the gasification process that is combusted. This reaction step is particularly important in the air-blown gasification since it supplies necessary reaction heat. An air-blown gasification system that uses air and biomass has the advantages of enhanced fuel efficiency, the simplicity of the system, and the ability to control the composition of the syngas. The vertical temperature profile in the system can be controlled by the biomass feeding rate and air-flow rate and it can lead different syngas composition as well as tar content in the syngas. 3. Experimental setup Fig. 2 shows schematic diagrams of the experimental setup used in this study. At the bottom of the gasifier, there is an additional

fuel-combustion chamber to preheat the whole system in the beginning. The screw feeder introduces biomass into the gasifier, and the cyclone located behind the gasifier separates off gas and entrained dust, sand, and unburned char. 3.1. Gasifier Fig. 3a shows the BFB gasifier. The inner diameter of the gasifier is 0.4 m and its height is 3.8 m. The gasifier is of the BFB type and is designed to process biomass at a rate of 10–80 kg/h. Thermocouples of K-type were installed at eleven points to measure the temperature profile inside the gasifier, as shown in Fig. 2. They are arranged at regular intervals, except that between the distributor and TC-1. 3.2. Feeder and feedstock Fig. 3b shows the screw-type feeder, which can continuously feed a uniform amount of biomass to the gasifier. The feed rate (in revolutions per minute, RPM) can be controlled, and the amount of biomass supplied by the feeder at a given RPM was determined before the gasification experiment was conducted. The proximate and ultimate analysis of the wood pellet is given in Table 1. The biomass consists of volatiles, fixed carbon, and

Biomass

Syngas

6 5

7

4

8

3

2 9 1

Fig. 2. Schematic diagram of gasification system and the temperature measurement points; 1: compressed air, 2: LPG, 3: preheat chamber, 4: gasifier, 5: screw feeder, 6: hopper, 7: cyclone, 8: tar trap, 9: gas analyzer.

417

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420

Fig. 3. Direct photographs of the experimental setup.

Table 1 Proximate and ultimate analysis of feedstock. Feedstock

Table 2 Experimental conditions. Wood pellet

Proximate analysis (as received wt.%) Moisture Volatiles Fixed carbon Ash

9.8 72.7 16.7 0.8

Ultimate analysis (d.a.f. wt.%) C H O N S LHV (MJ/kg) Single particle size Apparent density (kg/m3) Bulk density (kg/m3)

51.02 7.16 41.73 0.09 0.004 18 Ø 8 mm  20 mm 1050 600

Case

Startup

1

2

3

4

Biomass feeding rate (kg/h) Air (Nm3/hr) ER

10 52 1.3

34 37 0.27

55 54 0.24

55 43 0.19

25 33 0.32

into the atmosphere. The syngas was collected by a port in the stack, and a continuous gas analyzer was used to analyze the syngas for H2, CO, CO2, O2, and CH4. The data from the gas analyzer was monitored in real-time and recorded on a computer. For syngas analysis, the tar in the syngas must be removed, and therefore, a tar trap (Fig. 3c) device was installed to absorb the tar from the high-temperature syngas. 3.5. Syngas engine system

ash, and the volatile and fixed-carbon contents were 72.7% and 16.7%, respectively. 3.3. Preheat chamber Fig. 3d shows the preheat chamber. The preheat chamber was located at the bottom of the gasifier in place of a wind-box to supply air to the gasifier. The chamber was equipped with an LPG gas burner and the LPG combustion gas was supplied to the gasifier through the distributor during the startup. When the gasification condition is achieved, the LPG supply was stopped and the chamber acted as a simple wind-box. 3.4. Gas analyzer Fig. 3e shows the device to analyze the syngas produced by the gasifier. The syngas is ignited in the stack before being released

Table 3 shows the specification of syngas engine. Four cycle type syngas engine was ready to use syngas from gasifier to generate electric power. Fuel of syngas engine is available for selection either LNG or Syngas. Syngas from gasifier of atmosphere condition was supplied by ring blower and introduced into the engine while premixing with air. The maximum capacity of the syngas engine is about 30 kWe. Electricity from syngas engine was spent at an electronic load resistance. 4. Experiment 4.1. Experimental method In this study, silica sand was employed as the bed material. At the start of the experiment, to heat up the gasifier, the LPG burner at the bottom of the gasifier was used to preheat the air, which, in turn, heated the sand up to the temperature at which spontaneous

418

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420

Table 3 Syngas engine specification. 4 Cycle syngas engine Syngas 4 Turbo charger 20–30 kW 1800 RPM

Case 2

Case 3

ER : 0.24 LHV:5.3MJ/Nm 3

ER : 0.19 LHV:5.7MJ/Nm 3

900

TC-1 TC-2 TC-3 TC-4 TC-5 TC-6 TC-7 TC-8 TC-9 TC-10 TC-11

800

o

Temperature ( C)

Engine type Fuel Number of cylinder Intake system Power RPM

Case 1 ER : 0.27 LHV:4.7MJ/Nm 3

combustion occurs (400 °C). The temperature of the gasifier was monitored by a computer. After biomass was input into gasifier, its spontaneous combustion caused the temperature to rapidly increase, and the LPG burner was turned off [22,23]. Then, a uniform amount of biomass was fed by the screw feeder such that the equivalence ratio (ER) (see Section 4.2) was maintained at about 1.3.

ER ¼

Actual air fuel ratio Stoichiometric air fuel ratio

700

600

500 0

10

20

30

40

50

60

Time (m)

ð1Þ

Fig. 4. Temperature of the gasifier for each case.

The combustion conditions were maintained until the bed temperature of the gasifier had reached 800 °C at TC-3. The air inflow rate was reduced to change ER when the bed temperature was sufficiently high. The heat from biomass combustion was used to heat the sand and for the endothermic pyrolysis and gasification reactions.

The experimental conditions were controlled by the feed rates of biomass and air, as shown in Table 2. The biomass feed rate ranged from 25 to 55 kg/h, and air-flow rate from 33 to 54 Nm3/ h. The ratio of the biomass and air feed rates is defined as the equivalence ratio (ER), and the four representative experimental conditions of this study were ER = 0.32, 0.27, 0.24, and 0.19. CASE 2 and CASE 3 are the two cases in which the biomass feeding rate is the same (around 55 kg/h) but the air feeding rate is different, and CASE 3 has the lowest ER (0.19). In addition, long-term operation performance was tested over two days (CASE 4). For all cases, the heating value of the syngas satisfies the minimum requirement of the power generation engine while showing the effect of fluidization number and equivalence ratio on the gasification process in the bubbling fluidized bed gasifier. 5. Results 5.1. Temperature of the gasifier The temperature profiles in the gasifier are shown in Fig. 4. The temperature of the gasifier is mainly controlled by the ER and fuel feed rates of the biomass. For fixed feed rates of biomass, the average temperature increases as ER decreases and for fixed ER, it is proportional to the fuel feed rates. It is notable that the temperature distribution is significantly affected by ER or fuel feed rates. As shown in Fig. 4, the difference between the maximum and minimum temperature in the gasifier becomes large when ER decreases or fuel feed rates decreases. In a bubbling fluidized bed with top feeding, the gasifier temperature is also highly coupled with fluidization number (FN) which represents fluidization condition of the fluidized bed. With higher FN, fluidization occurs more vigorously and it enhances fuel mixing and heat transfer in the reaction zone. In order to investigate the details of the thermo-chemical conversion process of the gasifier, vertical temperature distribution

Case 3

Case 2

FN=3.7 ER=0.19

FN=3.2 ER=0.27

FN=4.6 ER=0.24

4

Gasifier height (m)

4.2. Experimental conditions

Case 1

Drying/ Pyrolysis/ Gasification

2

Gasification/ Combustion

Combustion/ Air preheating

0 500

600

700

(a)

800500

600

700

800500

(b)

600

700

800

(c) o

Temperature ( C) Fig. 5. Vertical temperature distribution of the gasifier as a function of FN.

of each case was plotted in Fig. 5. It is notable that varying the feed rates of biomass or air induced changes the temperature profile which leads to the reaction conditions inside the gasifier significantly. As discussed in Fig. 1, a bubbling fluidized bed with top fuel feeding system has sequential reaction pathway from the top. In the view point of fuel, it experiences drying, pyrolysis, gasification and combustion. In the view point of oxidizer, it experiences preheating and combustion and it turns into combustion gases and then take part in the gasification process. Again, the product gas from gasification, acts as a heat transfer medium during the pyrolysis and drying process. It is interesting that changes of reaction zones can be estimated from the vertical temperature profile of the gasifier. As depicted in Fig. 5, each reaction zone is overlapped to each other but we can estimate a boundary or layer of a specific reaction zone between the overlaps. For convenience, we can tell that main gasification zone is coincided with the constant temperature region though some partial oxidation or pyrolysis occurs simultaneously in the same region. Fig. 5a shows the temperature profile of Case 1. Comparing to Cases 2 and 3, main gasification zone is relatively narrow and difference between the maximum and minimum temperature is

419

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420 Table 4 Concentration of syngas composition.

Table 5 Comparison of gasifier performance with previous studies.

Case

1

2

3

4

Gas composition (dry vol.%)

14.5 16.0 0 13.8 4.0 48.2 4.7

16.1 16.0 0 15.0 4.6 51.6 5.3

16.5 16.4 0 16.1 5.3 54.2 5.7

15.2 17.0 0 11.8 4.3 48.3 4.7

H2 CO2 O2 CO CH4 Total Lower heating value (LHV) (MJ/Nm3)

Case 1

Case 2

ER : 0.27

ER : 0.24

Type Fuel Method Temperature (°C) CO H2 CO2 CH4 H2O H2/CO

SEI

ISU

EPI

This study

BFB Wood Air 650–815 15.5 12.7 15.9 5.72 Dry 0.8

BFB Corn Air 730 23.9 4.1 12.8 3.1 Dry 0.2

BFB Wood Air 650 17.5 5.8 15.8 4.65 Dry 0.3

BFB Wood pellet Air 750–800 16.1 16.5 16.4 5.3 Dry 1

SEI – Southern Electric International Inc. / ISU – Iowa State University / EPI – Energy Product of Idaho.

Case 3 ER : 0.19

CO2

3

O2 CO CH4

20

4 15

10 2

o

5

0

0 0

10

20

30

40

50

Time (m)

25 20

CO CO2

15

H2

10

CH4 O2

5 0

800

Temperature ( C)

Gas composition (dry vol.%)

6

Lower heating value (MJ/Nm )

LHV H2

25

Gas composition (dry vol.%)

30

TC-1 TC-2 TC-3 TC-6 TC-8 TC-10

700

600

500

Fig. 6. Syngas composition and lower heating value as a function of ER.

0

5.2. Syngas composition Table 4 shows the syngas composition as a function of ER, and Fig. 6 shows how its composition varies with time as well as ER.

1

2

3

Time (day) Fig. 7. Continuous operation results of BFB systems.

H2

Electronic Power 16

CO2

18

CO CH4

16 14 12

12

10 8

8

6

Electronic Power (kWe)

20

20

Concentration (vol.%)

larger than other cases. This is because the feed rates of biomass is less than the other cases and this means that combustion heat and hot product gas yield are less than the other cases while the heat loss to the wall or sensible heat loss of the product gas to the fresh feedstock is relatively similar to the other cases. It is also notable that the temperature of the very first measuring point (TC-1) of Case 1 is less than the second point (TC-2) which implies that combustion zone is also narrow for Case 1. The reason why is less fuel feed rates and lower FN. Since the amount of fuel is not enough to reach the very bottom of the gasifier, the main combustion reaction occurs within narrower regions comparing to other cases. In addition, leveling-off of temperature in the fluidized bed is less since FN is less than the other cases. For Cases 2 and 3 increase of fuel feed rates as well as air flow rate increases gasification zone. The area of gasification zone (i.e. constant temperature region) increases as FN increases. This is because of increase of bed expansion, increase of solid entrainment, and total mass flow of the product gas. Note that the temperature maximum occurs in the very first measuring point which implies that the main combustion zone is lower than Case 1 and fuel can reach to the very bottom of the gasifier. The enhanced mixing by increased FN can also contribute to the fuel transportation. The decrease of drying and pyrolysis zone is also another interesting result. For the same fuel feed rates (Cases 2 and 3), the drying and pyrolysis zone is highly affected by ER which results in the heat and mass flow of the product gas which becomes heating medium for the drying and pyrolysis process.

4

4

2 0 0

10

20

30

0 40

Time (min) Fig. 8. Preliminary test result of syngas power generation (electric power output and simultaneous syngas composition).

The concentration of syngas tended to increase as ER went from 0.27 to 0.19. The hydrogen concentration, which is important for combustion of CO in syngas engines, increased from 14.5% to 16.5%, carbon monoxide increased from 13.8% to 16.1%, and CH4 increased from 4.0% to 5.3%. The total volume of the product gas decreased as ER was reduced. The caloric value for each ER is

420

Y.D. Kim et al. / Applied Energy 112 (2013) 414–420

shown in Table 4, and it can be seen that in all cases it exceeded 4.7 MJ/Nm3, which means the syngas produced here is suitable to be used as fuel for syngas-engines. Table 5 shows a comparison of our result and the results of previous research, and demonstrates that the performance of our gasifier is good in comparison to other systems. It is notable that hydrogen content of our study is higher than the previous results, which is good for syngas engine operation. It can be explained by the long residence time of the reactant and product gas due to our gasifier configuration and top fuel feeding system. Water–gas shift reaction (i.e. CO + H2O  >H2 + CO2) also contributes to the increase of the hydrogen content because CO of this study is less than the other results and CO2 is more than the other results (Table 5). Fig. 7 shows long-term operation results of BFB system. The experimental condition and average syngas compositions are presented in Table 2 (Case 4) and Table 4 respectively. As shown in the Fig. 7, stable and continuous operation of BFB gasification system has been proved and syngas quality is appropriate for power generation with a syngas engine. 5.3. Syngas engine test Fig. 8 shows the preliminary test result of integration of biomass gasification-gas cleaning-syngas engine. A gas cleaning system composed of a gas cooler, bag filter, scrubber and I.D fan was adopted for removing tar and dust in the product gas. The original product gas composition of the gasifier was similar to Case 2, but the final product gas composition can be altered by some air leakage from the bag filter caused by the suction operation of I.D fan. However, this can be compensated in the premixing stage of the syngas engine operation. As a result, stable and continuous operation was conducted with the integrated system and electricity of 12–14 kWe was generated in the test run [24]. 6. Conclusions Air-blown gasification was investigated for the production of syngas in a (BFB) gasifier. The feed rates of biomass and air were controlled to change the ER and vary the internal conditions. Changes in the biomass and air feed rates affected the product gas composition and temperature profiles in the gasifier. Based on the temperature profiles, the dynamics of reaction zones were investigated in terms of ER, fuel feed rates, and FN. The composition of the syngas was significantly affected by ER. The concentration of hydrogen is relatively higher than previous researches and it results from the configuration of the gasifier: longer free board and top fuel feeding. The caloric value of the product gas was above 4.7 MJ/Nm3, and thus satisfactory for use in syngas engines. Preliminary test of integrated gasification-power generation was conducted. Acknowledgement The authors would like to gratefully acknowledge to institution support program by the Ministry of strategy and Finance of Korea.

References [1] Robert Jacques, Lennert Moritz. Two scenarios for Europe: ‘‘Europe confronted with high energy prices’’ or ‘‘Europe after oil peaking’’. Futures 2010;42:817–24. [2] Xu ZR, Zhu W, Li M, Zhang HW, Gong M. Quantitative analysis of polycyclic aromatic hydrocarbons in solid residues from supercritical water gasification of wet sewage sludge. Appl Energy 2013;102:476–83. [3] Syred Nicholas, Abdulsada Mohammed, Griffiths Anthony, O’Doherty Tim, Bowen Phil. The effect of hydrogen containing fuel blends upon flashback in swirl burners. Appl Energy 2012;89:106–10. [4] Zhang Qinglin, Dor Liran, Zhang Lan, Weihong Yang, Blasiak Wlodzimierz. Performance analysis of municipal solid waste gasification with steam in a plasma gasification melting reactor. Appl Energy 2012;98:219–29. [5] Munajat Nur Farizan, Catharina Erlich, Reza Fakhrai, Fransson Torsten H. Influence of water vapour and tar compound on laminar flame speed of gasified biomass gas. Appl Energy 2012;98:114–21. [6] Jae Ik Na, So Jin Park, Yong Koo Kim, Jae Goo Lee, Jae Ho Kim. Characteristics of oxygen-blown gasification for combustible waste in a fixed-bed gasifier. Appl Energy 2003;75:275–85. [7] Cordiner S, De Simone G, Mulone V. Experimental-numerical design of a biomass bubbling fluidized bed gasifier for paper sludge energy recovery. Appl Energy 2012;97:532–42. [8] Boerrigter H, Rauch R. Review of applications of gases from biomass gasfication. ECN, Biomass, Coal Environ Res 2006 [85.25.64.83]. [9] Grobl T, Walter H, Haider M. Biomass steam gasification for production of SNG – process design and sensitivity analysis. Appl Energy 2012;97:451–61. [10] Kunii D, Levenspiel O. Fluidization Engineering 2nd ed., ButterworthHeineman, Boston; 1991, p. 1–10. [11] Narvaez I, Orio A, Corella J, Aznar MP. Biomass gasification with air in a bubbling Fluidized bed. Effect of six operational variables on the quality of the produced raw gas. Ind Eng Chem Res 1996;35(7):2110–20. [12] Herguido J, Corella J, Gonzalez-Saiz J. Steam gasification of lignocellulosic residues in a fluidized bed at small pilot scale. Effect of the type of feedstock. Ind Eng Chem Res 1992;31(5):1274–82. [13] Sadaka SS, Ghaly AE, Sabbah MA. Two phase biomass air–steam gasification model for fluidized bed reactors: part III-model validation. Biomass Bioenergy 2002;22:479–87. [14] Lv P, Chang J, Xiong Z, Huang H, Wu C, Chen Y. Biomass air–steam gasification in a fluidized bed to produce hydrogen-rich gas. Energy Fuel 2003;17:677–82. [15] Lv PM, Xiong ZH, Chang J, Wu CZ, Chen Y, Xhu JX. An experimental study on biomass air–steam gasification in a fluidized bed. Bioresour Technol 2004;95:95–101. [16] Corella J, Toledo JM, Molina G. A review on dual fluidized-bed biomass gasifiers. Ind Eng Chem Res 2007;46:6831–9. [17] Gil J, Aznar MP, Caballero MA, Francés E, Corella J. Biomass gasification in fluidized bed at pilot scale with steam–oxygen mixtures. Product distribution for very different operating conditions. Energy Fuel 1997;11:1109–18. [18] Wang Y, Kinoshita CM. Experimental analysis of biomass gasification with steam and oxygen. Sol Energy 1992;49:153–8. [19] Wang L, Weller CL, Jones DD, Hanna MA. Contemporary issues in thermal gasification of biomass and its application to electricity and fuel production. Biomass Bioenergy 2008;32:573–81. [20] Onay Ozlem, Mete Kockar O. Slow, fast and flash pyrolysis of rapeseed. Renew Energy 2003;28:2417–33. [21] Li Shiguang, Shaoping Xu, Liu Shuqin, Yang Chen, Qinghua Lu. Fast pyrolysis of biomass in free-fall reactor for hydrogen-rich gas. Fuel Process Technol 2004;85:1201–11. [22] Nataraja E, Nordin A, Rao AN. Overview of combustion and gasification of rice husk in fluidized bed reactors. Biomass Bioenergy 1998;14:533–46. [23] Jain V, Groulx D, Basu P. Study of heat transfer between an over-bed oil burner flame and a fluidized bed during start-up: determination of the flame to bed convection coefficient. Appl Energy 2010;87:2605–14. [24] Manya JJ, Sanchez JL, Abrego J, Gonzalo A, Arauz J. Influence of gas residence time and air ratio on the air gasification of dried sewage sludge in a bubbling fluidized bed. Fuel 2006;85:2027–33.