Gasification and tar removal characteristics of rice husk in a bubbling fluidized bed reactor

Gasification and tar removal characteristics of rice husk in a bubbling fluidized bed reactor

Fuel xxx (2016) xxx–xxx Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Gasifica...

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Fuel xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Gasification and tar removal characteristics of rice husk in a bubbling fluidized bed reactor Jin Woo Kook a, Hee Mang Choi a, Bo Hwa Kim a,b, Ho Won Ra a, Sang Jun Yoon a,c, Tae Young Mun a, Jae Ho Kim a,c, Yong Ku Kim a, Jae Goo Lee a,c, Myung Won Seo a,b,c,⇑ a b c

Climate Change Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Graduate School of Energy Science and Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea Department of Advanced Energy and Technology, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

a r t i c l e

i n f o

Article history: Received 24 February 2016 Received in revised form 7 April 2016 Accepted 4 May 2016 Available online xxxx Keywords: Rice husk Gasification Temperature ER (equivalence ratio) Tar reduction

a b s t r a c t Technology for converting biomass such as rice husk into a useable energy sources are key to address energy consumption issues. The effects of temperature (600–900 °C), equivalence ratio (ER, 0.15–0.3), and addition of catalyst on the gasification characteristics of rice husk were investigated in a bubbling fluidized bed reactor with an inside diameter of 0.067 m and a height of 1.55 m. As the reaction temperature and ER were increased, the concentrations of CO and CO2 in the product gas decreased. Slight increases in CH4 and H2 concentrations were also observed with increasing temperature. Throughout the temperature range of interest, an increase in ER resulted in decrement of both the higher heating value of the product gas and the cold gas efficiency. Furthermore, the effect of operating condition and addition of bed material were determined in a bubbling fluidized bed reactor. An increase in reaction temperature and ER decreased the tar content. The addition of calcined dolomite and olivine in the bed material reduced the amount of tar during rice husk gasification in a bubbling fluidized bed reactor. These results have the potential to be applied to the conversion of biomass into a useable energy source. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Owing to the continuously increasing use of high-end technologies in industry, energy consumption is also growing worldwide. In response to the price fluctuation of fossile fuels and more stringent environmental laws and regulations, the major energy-consuming countries are investing extensively in research efforts to convert sustainable biomass sources into eco-friendly energy sources [1]. Among the various sources of biomass, agricultural byproducts such as rice husk and straws are produced worldwide in huge quantities, amounting to 470 million tons per year. Rice husk accounts for 20% of the weight of the rice plant, and 94 million tons of rice husk are produced globally every year [2]. While rice husk has been traditionally used in low value applications such as stall mats, cement filler, and compost, its potential as a high value added material is attracting increasing attention. For example, it has recently been used as a feedstock for gasifiers

⇑ Corresponding author at: Climate Change Research Division, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea. E-mail address: [email protected] (M.W. Seo).

and boilers to generate heat and electricity in co-generation systems, and can be used as a mineral source for the anode material of Li-ion batteries, solar cells, and nanostructured silicon [3]. Thus, technologies for converting biomass into a useable energy source can be categorized into three main categories: (i) biological methods, such as anaerobic digestion and alcoholic fermentation; (ii) thermochemical methods, such as gasification and pyrolysis; and (iii) physical methods, such as extraction and solidification [4]. For example, the gasification process is a technology that produces a product gas composed mainly of carbon monoxide (CO) and hydrogen (H2). This is accomplished by the partial oxidation of carbon-containing materials, such as biomass, coal, and petroleum coke. Examples of gasifying agents include oxygen, steam, and carbon dioxide. Using the product gas mixture, numerous products can be manufactured, including chemical substances ranging from ammonia and methanol to industrial gases [5,6]. In terms of the biomass gasification process, important operation parameters include the equivalence ratio (ER), gasifying agent, bed temperature, and catalyst. The ER can be defined as the oxidizer to biomass mass ratio divided by the stoichiometric oxidizer to biomass ratio. An ER equal to 1 represents theoretically complete combustion. In the

http://dx.doi.org/10.1016/j.fuel.2016.05.027 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

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0.20–0.45

Unknown

Air

Olivine, FCC catalyst Olivine, calcined dolomite Calcined dolomite Air/steam mixture Steam 0.3–0.9 Dual fluidized bed/wood pellet

Almond shell

Bubbling fluidized bed/Pine sawdust

Pfeifer et al. [23]

Rapagna et al. [24]

Narvaez et al. [18]

750–900/ ambient 700–820/ ambient 750–850/ ambient

N/A Air 0.18–0.37 Downdraft/corn stalk Guo et al. [22]

900/ambient

N/A Air 0–0.5 Fluidized bed/pine sawdust

Lv et al. [20]

Ghassemi and ShahsavanMarkadeh [21]

600–1400/ ambient

N/A

Increasing the ER decreased the hydrogen content but increased carbon conversion and energy efficiency The higher temperature contributed to increased hydrogen production, but too high a temperature lowered the gas heating value. The LHV of the fuel gas decreased with ER. The optimal value of ER was found to be 0.23 Increasing the ER increased the heating value of syngas, due to a decrease in of H2 as a valuable gas The cold gas efficiency exhibited a maximum at ER = 0.29. This is due to the increase in gas yield The LHV and cold gas efficiency were good with an ER of 0.25–0.27, and a low tar yield of 0.5 g/N m3 was achieved by increasing the ER The temperature range for the catalytic reactions should be from 800 to 900 °C to obtain high conversion rates The catalytic activity of the olivine and calcined dolomite mixture exhibits a maximum at 800 °C Increasing the ER from 0.20 to 0.45, decreased the heating value by 2 MJ/N m3 and the tar yield by 50 wt% N/A

Air/steam mixture Air/steam mixture 0.07, 0.15, 0.29 0.19–0.27 650–850/ ambient 700–900/ ambient

Catalyst Gasifying agent Equivalence ratio (ER) Temperature (°C/pressure)

Fluidized bed/Dried Distillers grains with solubles (DDGS) Fluidized bed/pine sawdust

Rice husk obtained from Gongju, South Korea, was used as the fuel material. As shown in Table 2 [25], rice husk has low density (qs = 505 kg/m3) and low sphericity (0.19), and so it has unfavorable

Kumar et al. [19]

2.1. Materials

Reactor/fuel

2. Materials and methods

Author

gasification environment, higher ER values tend to result in lower yields of H2 and CO, and higher yields of CO2. The appropriate ER value reported to date for gasification is approximately 0.2–0.3 [7]. Gasifying agents include air and steam, which are commonly used in the biomass gasification process. Despite the use of steam yielding a product gas with higher calorific value than when air is used, steam has one main disadvantage, in that heat needs to be continuously supplied to the gasification reactor to supply steam [8,9]. Bed temperature is also a key in determining the gasifier performance, given that reactions involving C–H2O and C–CO2 (i.e., the main gasification reactions) are endothermic. Theoretically, the higher the temperature, the higher the yield, however, the reaction temperature tends to be limited by constraining factors such as reactor material, ash fusion temperature, tar generation and the generation of contaminants, such as NOx [8,9]. The optimal operating temperature concerning aforementioned constraints appears to be 750–800 °C [10]. Tar produced during gasification can lower the reactor performance and trigger an abnormal pressure build-up by precipitating inside the reactor and at joints, or by entering the turbine pump [11]. Tar, which contains aromatic compounds comprising 1–5 benzene rings along with oxygen-containing and polycyclic aromatic hydrocarbons, is generated in large quantities during biomass gasification. It has a complicated structure and exists as an aerosol under certain conditions, tending to agglomerate with flying particles, or continuing to react further to form polymers [12–15]. However, tar can be removed by controlling reaction and operating condition or by decomposition with additives placed in the reactor bed or at the rearward end. For thermochemical techniques using the latter of these techniques, the use of catalysts such as Ni– Al- and Ni–Ca-based catalysts and fluid catalytic cracking (FCC) catalysts, or additives such as limestone, dolomite, and olivine, is being extensively researched [13,14]. The mechanism of the dolomite reaction in biomass gasification is yet to be clarified; moreover, dolomite is prone to fracture and wear and has a high CO2 concentration (48 wt%). Therefore, it should be employed in the form of calcined dolomite after CO2 removal at high temperatures [8,16,17]. To develop a reliable rice husk gasification process, it is necessary to use a bubbling fluidized bed (BFB) reactor, which is flexible in terms of raw materials and has a superior heat transfer rate. In addition, rice husk gasification behaviors should be thoroughly understood for optimal operation and control of the gasifier, for the design of new gasifiers, and for device scale-up [18]. Additionally, a high efficiency biomass gasification process should be developed by improving the operating conditions through the introduction of tar removal technology. Table 1 summarizes previous studies on gasification using a BFB [18–24]. Although gasifiers are already commercially used, only a small number of studies have been conducted on biomass gasification, and even fewer studies have derived ER values in the reactor, or have investigated the effects of direct catalyst addition. In the present study, the effects of temperature (600–900 °C), equivalence ratio (ER, 0.15–0.3), and addition of catalyst on the gasification characteristics of rice husk were determined in a bubbling fluidized bed reactor with an inside diameter of 0.067 m and a height of 1.55 m.

Remarks

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Table 1 Summary of previous studies on biomass gasification.

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J.W. Kook et al. / Fuel xxx (2016) xxx–xxx Table 2 Characteristics of rice husk, rice husk char, silica sand, calcined dolomite and olivine. Properties Bulk (qb) Particle (qs)

Density (kg/ m3) Size (mm)

Sphericity, Us (–)

Bed voidage, e (–) Umf (m/s) Literature Theoretical calculation [26] Ut (m/s) Literature Theoretical calculation [27]

Rice husk

Rice husk ash

Silica sand

Calcined dolomite

Olivine

111 505 Length: 7–8 Width: 2–3 Thickness: 0.2 0.19 Theoretical calculation 0.78 0.46–0.60 0.35

262 1238 0.031

1249 2642 0.427

1201 3431 0.506

1584 2880 0.529

0.23–0.25 Theoretical calculation 0.79 0.16–0.30 0.008

0.76–1.1 1.0

0.7–0.8 0.17

0.92–0.96 Theoretical calculation 0.52 – 0.15 (R.T.) 0.08 (700 °C) – 1.93 (R.T.) 1.31 (700 °C)

0.34 Theoretical calculation 0.65 – 0.29 (R.T.) 0.14 (800 °C) – 4.03 (R.T.) 4.04 (800 °C)

0.79 Theoretical calculation 0.45 – 0.27 (R.T.) 0.13 (800 °C) – 3.77 (R.T.) 3.76 (800 °C)

Fig. 1. Furthermore, Table 3 shows the results of proximate and ultimate analyses of the rice husks employed in this study. The rice husk raw material contained: 66.39% volatile components, with volatility similar to that of common biomass; an ash proportion of 11.69%; and a predominance of SiO2. Moreover, with its lower heating value of 3844 kcal/kg, rice husk exhibits the essential properties required to serve as a fuel.

2.2. Apparatus

Fig. 1. Geldart’s classification (redrawn from [26]).

properties for fluidization. According to the Geldart’s classification (Fig. 1) [28] that categorizes the flow properties of particles according to particle size and density, rice husk (dp = 7–8 mm, qs = 505 kg/ m3) belongs to Geldart Group D. This means that it possesses a spoutable property, characterized by layer separation at a flow speed exceeding a given value, with air bubbles exploding in the upper layer. Rice husk should therefore be fluidized using Geldart Group B materials as bed materials; therefore silica sand particle with dp = 0.427 mm, qs = 2642 kg/m3, and sphericity of 0.92–0.96 was used in this study. Additionally, to investigate the tar removal characteristics in the BFB using an in-bed catalyst, calcined dolomite and olivine with 0, 25, 50, 75, and 100 wt% silica sand were mixed. As dolomite contains 48 wt% CO2, it was employed following sintering at 850 °C for 10 h. Due to the large particle size and density of the selected silica sand bed material, calcined dolomite particles were prescreened to discard particles of 6200 lm. Calcined dolomite and olivine were confirmed to belong to Geldart Group B with mean particle sizes of 0.506 mm and 0.529 mm, respectively as shown in

Fig. 2 shows the lab-scale 5 kg/h BFB (0.067 m I.D.  1.55 m height) used in the present experiment. As can be seen, the gasifier is composed of a rovo feeder, which can supply powder particles at a quantified regular rate; a bubble cap-type air distributor; a main reactor; a cyclone; a product gas cooling condenser; and a wet gas meter, for measuring the product gas flow rate. Rice husk is introduced into the bed section in a premeasured feeding quantity at a feeder rotation speed set by the calibration curve. An embedded 10 kW electric heater was installed as an air preheater to heat the gasifying agent to ca. 400 °C. Inside the main reactor, a bubble cap-type air distributor was installed to ensure that gasification could be smoothly implemented by the gasifying agent fed from the lower section. K-type thermocouple and pressure transducer (DPLH series, Sensys Korea) were installed in the axial direction to measure the pressure and temperature inside the reactor. An electric heater capable of heating the bed material up to 1000 °C was mounted on the outer part of the bed, and heat loss was minimized by wrapping the gasifier with a cowl. The product gas generated in the bed was passed through the cyclone to remove the solid particles contained in the product gas. Most of the product gas passed the condenser, MFM (Mass Flow Measurement), and vent system. Part of the product gas was then gone through the spray particle-collection filter followed by the gas cooler to remove residual steam or tar. After collection of the tar, the product gas was passed through a nondispersive infrared (NDIR) sensor type gas analyzer (AO2000 series, ABB) to carry out real time composition analysis. The temperature, pressure and flow rate data were controlled and collected using a human machine interface (HMI).

Table 3 Proximate and ultimate analysis of rice husk. Sample

Analysis Proximate analysis (air dry basis)

Rice husk

Ultimate analysis (dry basis)

LHV (kcal/kg)

M

VM

Ash

FC

C

H

O

N

S

9.96

66.39

11.69

11.96

55.13

6.43

38.43

0.01

0.00

3844

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Fig. 2. Schematic diagram of lab-scale 5 kg/h BFB.

The possible gasification reactions inside the bubbling fluidized bed reactor are as follows.

Table 4 Experimental conditions. Experimental variable

Operational range

Temperature (°C) Equivalence ratio, ER Air velocity (m/s)

600–900 0.15–0.3 0.06–0.17 (0.3 Umf–3 Umf) 6.6 Only sand 0, 25, 50, 75, 100 (Sand + calcined dolomite) 0, 25, 50, 75, 100 (Sand + olivine)

Feeding rate (g/min) Bed material (wt%)

2.3. Experimental method Table 4 outlines the operating conditions for the rice husk gasification experiment, where air was used as the gasifying agent, and was introduced into the gasifier after being preheated to 250 °C. The air flow rate was set at 0.02–0.17 m/s taking into account the temperature. The reactor temperature was maintained between 600 and 900 °C. A rovo feeder was used to feed a set amount of rice husks to the gasifier at a rate of 6.6 g/min. As bed materials, the three mixtures/materials used were sand alone, a mixture of sand and calcined dolomite, and a mixture of sand and olivine. The tar produced following the gasification reaction was dried at 110 °C and weighed. 3. Results and discussion 3.1. Effect of temperature Operating temperature is an important parameter in the gasification process, influencing not only the composition of product gas, but also the gas yield, carbon conversion, HHV of product gas, cold gas efficiency, and tar yield. The effect of operating temperature on gas composition, gas product yield, carbon conversion, HHV (Higher Heating Value) of product gas and cold gas efficiency are shown in Figs. 3–5. The carbon conversion was calculated from the yield of CO, CO2 and CH4.

2C þ O2 ¼ 2CO DH ¼ 221:4 kJ=mol C þ O2 ¼ CO2

DH ¼ 393:5 kJ=mol

C þ CO2 ¼ 2CO DH ¼ þ172:4 kJ=mol C þ 2H2 ¼ CH4

DH ¼ 74:3 kJ=mol

CH4 þ H2 O ¼ CO þ 3H2 CH4 þ 2H2 O ¼ CO2 þ 4H2

DH ¼ þ206 kJ=mol DH ¼ þ165 kJ=mol

ð1Þ ð2Þ ð3Þ ð4Þ ð5Þ ð6Þ

CH4 þ CO2 ¼ 2CO þ 2H2

DH ¼ þ247 kJ=mol

ð7Þ

CO þ 2H2 O ¼ CO2 þ 2H2

DH ¼ 41 kJ=mol

ð8Þ

From Fig. 3(a), it can be seen that CO and CO2 concentration, which was mainly determined by reactions (1) and (2), decreased with increasing temperature according to Le Chatelier’s principle. Increasing temperature favors reverse reaction of (1) and (2), therefore CO and CO2 decreased with increasing temperature. Lv et al. [20] also reported that the exothermic reaction such as (1) and (2) favors with decreasing temperature. This is consistent with the results of several previous studies [20,29,30]. In terms of the relationship between temperature and gas yield, CO yields of 0.078–0.045 kg/kg-fuel were obtained, indicating inverse relationship with temperature as shown in Fig. 3(b), and also an inverse relationship was observed between temperature and yield for CO2, with CO2 yields of 0.3456–0.2336 kg/kg-fuel being obtained. The content of CH4 showed an opposite trend with increasing temperature. In the absence of water condition, CH4 was determined by reaction (4) and pyrolysis of rice husk. Some studies have reported that CH4 concentration was not significantly affected by an increase in operating temperature, while others have claimed that a temperature rise is favorable to CH4 generation, because high temperatures favor methanation and reactions (5) and (6) [20,29–33]. H2 yields of 0.000508–0.001413 kg/kg-fuel were

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Fig. 3. Effect of operating temperature on product gas composition and gas yield.

Fig. 4. Effect of operating temperature on carbon conversion, HHV of product gas, and cold gas efficiency.

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3.2. Effect of equivalence ratio

Fig. 5. Effect of operating temperature on tar content.

obtained at 600–900 °C, with no significant variation in yield observed with an increase in temperature. Both the carbon conversion and the HHV of product gas decreased slightly with decreasing CO and CO2 concentrations. The carbon conversion and HHV of product gas ranged from 0.68 to 0.95 and from 173 to 2528 kJ/m3, respectively with increasing temperature as shown in Fig 4(a) and (b). Cold gas efficiency was also affected by a change in temperature as shown in Fig. 4(c), with the highest cold gas efficiency (0.374) being obtained under the following conditions: ER = 0.15, temperature = 700 °C. As the ER increased from 0.20 to 0.30, the cold gas efficiency decreased from 0.2766 to 0.1843, respectively, with the highest value being obtained at 800 °C, and so this was considered the optimal operating temperature for cold gas efficiency. The tar contents were 0.58, 0.37, 0.34, and 0.22 g/N m3 at ER = 0.20 when the temperature was varied between 600 and 900 °C as shown in Fig 5. As reported from Wolfesberger et al. [34], the tar content tended to decrease as the reaction temperature was increased, thus demonstrating that high reaction temperatures effectively reduce tar production.

The ER, i.e., the stoichiometric air to fuel ratio, is also a determining factor for the gasification performance. The effects of ER on rice husk gasification were examined by measuring the gas concentration, gas product yield, carbon conversion, HHV of product gas, and cold gas efficiency at 800 °C, as presented in Figs. 6–8. As the ER was increased, the concentrations CO, H2 and CH4 decreased as shown in Fig. 6(a). The gas yield of CO2 increased with increasing ER as can be seen in Fig. 6(b). The Kuo and Chen [35] also reported that the H2 and CO concentrations decreased and CO2 composition increased as the ER value increased. Generally with increasing ER, reaction (2) is dominant than reaction (1) as reported from Andre et al. [29] and Pinto et al. [30]. H2, CO and CH4 gave yield ranges of 0.001793–0.000595 kg/kg-fuel, 0.0944– 0.0338 kg/kg-fuel and 0.0074–0.0038 kg/kg-fuel respectively, thus verifying that H2, CO and CH4 yields decrease with an increase in ER. CO2 yield increased from 0.231 to 0.278 kg/kg-fuel as the ER increased. In terms of the carbon conversion, no clear ER-dependent tendencies were observed as shown in Fig. 7(a), with carbon conversions of 0.82–0.91, 0.84–0.95, 0.82–0.89, and 0.68–0.83 at 600, 700, 800, and 900 °C, respectively. At all temperature ranges, an increase in ER resulted in a decrease in the HHV of product gas; e.g., the HHV of product gas were calculated as 608–2528 kJ/m3 at 600 °C, 355–2308 kJ/m3 at 700 °C, 599–2186 kJ/m3 at 800 °C, and 173–1602 kJ/m3 at 900 °C as shown in Fig. 7(b). It appears that the carbon conversion drop slightly at 900 °C. A similar tendency was observed for the cold gas efficiency as shown in Fig. 7(c), i.e., at all temperatures range, the cold gas efficiency decreased as the ER value increased. It can therefore be inferred that the ER should be maintained as low as possible, being sufficient to meet the gasification operating conditions. Similar to Arena et al. [36], the tar content tended to decrease with an increase in ER as shown in Fig. 8. At 600 °C, the tar content decreased from 1.82 to 0.58, 0.33, and 0.29 g/N m3 as the ER was increased from 0.15 to 0.20, 0.25, and 0.30, respectively, thus demonstrating that the higher the ER value, the higher the tar removal efficiency.

Fig. 6. Effect of equivalence ratio on product gas composition and gas yield.

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Fig. 7. Effect of equivalence ratio on carbon conversion, HHV of product gas, and cold gas efficiency.

3.3. Tar removal characteristics using an in-bed catalyst The gasification and tar removal characteristics of calcined dolomite and olivine were examined by mixing with silica sand at various ratios at 600 °C with ER = 0.15. Fig. 9 shows the gasification characteristics obtained using different mixing ratios of calcined dolomite and olivine. As the calcined dolomite and olivine ratio in the bed material was increased, the carbon conversion increased slightly, whereas the HHV of product gas and the cold gas efficiency decreased slightly. The reason of decreasing HHV of product gas and cold gas efficiency is due to increasing CO2 concentration with increasing calcined dolomite and olivine. In particular, at 50 wt% olivine content, both the HHV of product gas and cold gas efficiency showed their lowest values (1254 kJ/m3 and 0.1929, respectively). Considering that the HHV of product gas and cold gas efficiency decrease substantially when the mixing ratio of calcined dolomite and olivine was >50 wt%, a mixing ratio of 650 wt% should be employed for good gasification performance. Jo et al. [37] also reported that the optimal product gas was obtained using 50 wt% calcined dolomite mixed with silica sand. Fig. 10 shows the effect of calcined dolomite and olivine mixing ratio on tar content. This figure shows similar tendencies in the tar content for both this study and Jo et al. [37]. More specifically, the tar content began to decrease sharply at calcined dolomite to olivine mixing ratios of >25 wt%, and decreased further upon increasing the mixing ratio. This clearly indicates that the addition of

Fig. 8. Effect of equivalence ratio on tar content.

calcined dolomite and olivine to the bed material is efficient for tar removal. Indeed, when 100 wt% calcined dolomite was used, the tar content was 0.28 g/N m3, virtually preventing tar production, while olivine showed the highest tar removal efficiency (0.48 g/N m3) at a mixing ratio of 75 wt%.

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Fig. 9. Effect of calcined dolomite and olivine mixing ratio on carbon conversion HHV of product gas, and cold gas efficiency.

Fig. 10. Effect of calcined dolomite and olivine mixing ratio on tar content.

temperature reduced the product gas yields of CO and CO2, whereas the yield of CO2 increased as the ER increased. Both the HHV of product gas and the cold gas efficiency decreased with an increase in ER at all temperature ranges examined, while cold gas efficiency also tended to decrease with an increase in temperature. The operating temperature required to achieve optimal cold gas efficiency was found to be 800 °C. To improve the gasification performance such as HHV of product gas and cold gas efficiency, ER should be maintained at the minimum level. Moreover, an increase in reaction temperature and ER decreased the tar content, thus confirming that higher reaction temperatures gave higher tar removal efficiency. Upon increasing the mixing ratio of olivine, the heating value and cold gas efficiency of the product gas decreased slightly, while the tar content decreased significantly. In the tradeoff between tar removal efficiency, heating value, and cold gas efficacy, the optimal mixing ratio of calcined dolomite to olivine was 650%. Acknowledgments

4. Conclusions The gasification characteristics of rice husk were investigated using a bubbling fluidized-bed (BFB) at reactor temperatures of 600–900 °C and equivalence ratios (ER) of 0.15–0.30. In addition, after adding a mixture of calcined dolomite and olivine as an inbed catalyst to the bed material, the effect of mixing ratio on husk gasification characteristics was examined. An increase in

This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B6-2414). This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20143030090960). This work was

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J.W. Kook et al. / Fuel xxx (2016) xxx–xxx

also supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-1507-KIER).

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Please cite this article in press as: Kook JW et al. Gasification and tar removal characteristics of rice husk in a bubbling fluidized bed reactor. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.05.027