Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal

Accepted Manuscript Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal J.P. Makwana, A.K. Joshi, G. Athawale, Dharminder Singh, Pravakar Mohanty PII: DOI: Reference:

S0960-8524(14)01370-4 http://dx.doi.org/10.1016/j.biortech.2014.09.111 BITE 14001

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

21 July 2014 20 September 2014 22 September 2014

Please cite this article as: Makwana, J.P., Joshi, A.K., Athawale, G., Singh, D., Mohanty, P., Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.09.111

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Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal J.P.Makwanaa, A.K.Joshia, G. Athawaleb, Dharminder Singhb, Pravakar Mohantya* a: Sardar Patel Renewable Energy Research Institute (SPRERI), V.V.Nagar-388120 (Gujarat), India b: M.Tech (Thermal) student, Sardar Patel Renewable Energy Research Institute, V.V.Nagar-388120 (Gujarat), India

Abstract An experimental study of air gasification of rice husk was conducted in a bench-scale fluidized bed gasifier (FBG) having 210 mm diameter and 1600 mm height. Heating of sand bed material was performed using conventional charcoal fuel. Different operating conditions like bed temperature, feeding rate and equivalence ratio (ER) varied in the range of 750-850˚C, 25-31.3 kg/h, and 0.3-0.38, respectively. Flow rate of air was kept constant (37 m3/h) during FBG experiments. The carbon conversion efficiencies (CCE), cold gas efficiency, and thermal efficiency were evaluated, where maximum CCE was found as 91%. By increasing ER, the carbon conversion efficiency was decreased. Drastic reduction in electric consumption for initial heating of gasifier bed with charcoal compared to ceramic heater was ~ 45%. Hence rice husk is found as a potential candidate to use directly (without any processing) in FBG as an alternative renewable energy source from agricultural field. Keywords: Air Gasification, Fluidized bed gasifier, Rice husk, dolomite, charcoal heating

*

Corresponding authors, Tele: + 912692-231332, 235011, Fax: +912692-237982 E-mail: [email protected], [email protected], (P. Mohanty)

1.

Introduction Currently, the utilization of agriculture residues like rice husk, rice straw and sugar cane

bagasse for thermal energy and/or electricity generation has received tremendous attention as it do not interfere in the matter of “food/fuel” controversy. (Lim et al, 2012). According to Timmer et al., the world wide rice consumption in 2020 will hit to 450 million tons, with a growth of 6.6% in comparison to 422 million tons in 2007. China and India are the top most rice producing countries in the world, while both of the countries contributing around 50% of total world production. The Asia region alone produces more than 90% of the total global rice output. Rice is the second largest cereal crop, producing the largest amount of crop residues in the world (Soest et al., 2006). Rice, rice husk and rice straw are the main products of paddy cultivation and processing (Binod et al., 2009). The average ratio of (rice grain: rice husk: rice straw) is coming to (1:0.25:1.25) by wt.% (Haefele et al., 2011). The majority (83- 88%) of rice husk particles were varies in the range of 0.21- 0.85 mm. As reported by Subramaniam et al., (2011), These values for Indian rice husk varies in the range of 0.075-3 mm. Rice residues are renewable and available in surplus which can be used as an energy source in thermochemical conversion processes such as gasification and combustion or in bioconversion processes for the production of bioethanol and biogas etc (Yoon et al., 2012, Delivand et al., 2011, Lv et al., 2004). The ash produced from gasification and combustion processes can be used further as supplementary material in cement and ceramic manufacturing process (Zain et al., 2011, Yoon et al., 2012, Delivand et al., 2011, Lv et al., 2004) and the spent material from bioconversion can be used as an animal feed if necessary (Bisaria et al., 1997, Boateng et al., 1992). While focusing fluidized bed gasification and its different operating conditions like equivalence ratio (ER) can be chosen as primary factors which influence the operation quite

severely. The ER is defined as the ratio of the amount of oxygen (air) supplied and the amount of oxygen (air) needed for stoichiometric combustion of the fuel or biomass feedstock (Narvaez et al., 1996, Ghani et al., 2012). Narvaez et al., has also carried out the FBG operation and emphasized on the ER value of 0.2 to 0.04, as it affects temperatures, as well as the gas composition, heating value, tar and final SPM contents. Values for ER ≤ 0.18 are not practically feasible due to higher tar formation (process close to pyrolysis). Values of ER ≥ 0.45 produce undesired syngas composition with low heating value. In this study experiments were conducted by considering the ER in between 0.3 to 0.38. For optimized gasifier design and operation, several “measures” has been considered both in terms of design and operation, like adequate feeding device and better feeding insertion to the bed, air distribution plate, control of gas residence times both in the bed and in freeboard section, maintaining high temperature in freeboard reaction, high H2O/C and/or H/C ratios (Narvaez et al., 1996, 1997) and the use of an appropriate bed material in combination (dolomite + sand) etc. Lignocellulosic biomass differs much in their physical, chemical, morphological properties and LHV, which affect the characteristics of the gasification process (Moilanen et al., 2009, Nemtsov et al., 2008, Riehl et al., 2013). Ash as produced from the rice husk contains more than 95% (wt.%) silica and minor quantities of other minerals (Preto et al., 1987; Natarajan et al., 1998a, b). Narva´ez et al., (1996, 1997) has reported some improvements in gasification with controlled air, similar to the study conducted by Olivares et al., (1997) by selecting some catalyst with it. They have depicted how different percentages of dolomite in the bed improve the gasifier performance. During their analysis they have conducted with 20 wt. % of calcined dolomite in the bed and found it as self sacrificed low cost material for the tar reduction. When the flue gas

needs an exhaustive cleanup or polishing (below 200 mg of tars/Nm3), another catalytic bed in dual is being tried (Narvaez, et al., 1996, 1997), whereas some groups suggested for three step tar cracking. Orio et al., (1997) and Perez et al., (1997) has pointed about the difficulty and several challenges involved in the substantial tar reduction steps from higher concentration to lower of 200 mg of tars/ Nm3 which is necessary to be addressed. Further considering the high temperature in freeboard zone, high H2O/C and/or H/C ratios, (Narvaez et al., 1997) and the use of an in-bed dolomite is being focused through this work utilizing the high ash content paddy husk for thermal application. Rice husk has poor flow characteristics, rendering it difficult to handle and fed into the fluidized bed. It has low bulk density (100±4 kg/m3), abrasive and has interlocking nature. Feeding of such low-density biomass materials into the FBG is difficult, therefore, it is usually force-fed as near as possible into the hot bubbling bed zone to achieve a higher burnout rate in the fluidized bed, as the minimum fluidization velocity of rice-husk ash was found to be 22 cm/s (Preto et al., 1987; Natarajan et al., 1998a, b). This is done either mechanically by a screw feeder or pneumatically by air (Natarajan et al., 1998a, b). Various feeding arrangement reported include feeding at the bottom of the bed (Armesto et al), feeding via a dual distributor type feeding mechanism which feeds rice husk to the bottom of the distributor, (Mansaray et al., 1999) feeding just above the bed (Bhattacharya et al., 1984) and feeding at the freeboard region (Zhao et al., 2004, Zhao et al., 2009) etc. All those need attention in terms of its handling, pattern of charging and power consumption. Feeding of feedstock directly into the bed might affect the hydrodynamics of the bed (mixing and fluidizing characteristics) and possibility of agglomeration due to longer retention time. Above addressed issues are being tried to solve through this research work in pilot scale fluidized bed gasification system.

2.

Materials and method

2.1

Materials and facility developed

Rice husk was obtained from the M/s Tulasi rice mill of the Limbasi village of Kheda district state of Gujarat, India. The proximate and ultimate analysis of the samples is being reported in Table 1. Naturally available river bed sand after sieving (particle size 0.4-0.6 mm, 1470 kg/m3 density) was selected as bed material as well as heating media with a bed height of 300 mm. Performing several experimentation only with sand bed, the incorporation of naturally available dolomite is also being tried with different ratio. Targeting the reduction pattern of tar and SPM in the producer gas locally available dolomite was selected as self sacrificed catalyst where its particle size ranges from 0.4- 0.6 mm along with same size range of sand bed for performance comparison. 2.2

Fluidization of the bed and feed material Initially to study the hydrodynamics and to measure the fluidization properties of

different bed and feed materials acrylic columns (for easy visualization) with same diameter and height as that of the SS-reactor FBG has also been developed. For measurement of velocity of the air in the aspirator hot wire anemometer was used at different points as well as at the exit of FBG reactor (Table 2). 2.3

Experimental setup The schematic diagram of the experimental facility which was developed is shown in

Figure 1. The reactor was made of SS-316 seamless pipe and the total height of reactor is 1600 mm with an internal diameter of 210 mm. K-type thermocouples installed at 6 different sections

of the FBG set-up to measure the temperatures of air inlet to the reactor (T1), static sand bed temperature (T2), fluidized bed temperature (T3), free board temperatures (T4 and T5) and gas temperature (T6). Sand bed was heated by approximately 1 kg of charcoal to eliminate the use of ceramic heater and hence reduce the electricity consumption (during initial start-up). Earlier the practice was to use a temperature controlled ceramic heater to heat up the sand bed to achieve 800oC. Ceramic heater consumes > 60% of the total consumption in initial 1 h. 10-15 mm diameter charcoal of 1 kg (330±10 gram at an interval of 10 minutes) was feeded from the screw feeder to increase uniform bed temperature up to 600oC before starting of the operation. Before starting the feeding of the charcoal in the reactor, it was ignited in the improved biomass cookstove and air was blown for making them red hot charcoal. Purpose of this kind of heating arrangement is to make the FBG operation self-sufficient, sustainable and fit for rural agricultural applications. Screw feeder was modified for the feeding of the rice husk and its speed can be reduced by a gear box (speed ratio 60:1) arrangement followed by 18 T X 48 T sprockets. Further speed is also controlled by variable frequency drive (VFD). For consistent operation of the gasifier and separation of the char and tar, two cyclone separators are placed in the gas venting line. Air was blown by the regenerative blower of 100 m3 capacity and equally distributed by the distributor plate having 2 mm orifice diameter and 101 no. of holes. It can be defined that a multi-perforated stainless steel plate, with about 2% open area, is used as distributor. River sand with bulk density of about 1470 kg/m3 has used as bed material. The minimum fluidization velocity of the sand is found to be 0.1 m/s by analyzing the pressure drop curve. Then the surge of air is periodically permitted to stir up the bed to homogenize its temperature. When the bed material reaches about 600°C, then the fuel feeding was started. The feedstock feeding is done, with the help of screw feeder, above the bed level so as to avoid poor

mixing, back pressure development, as void space is more in the rice husk bed material, due to low density of rice husk which is being used for this study. The biomass feedstock is filled in the hopper and it is continuously metered out into the fluidized sand bed by the calibrated screw. Fuel feed rate was controlled so as to achieve the equivalence ratio (ratio of actual air supplied to the stoichiometric air required per kg of rice husk) for the chosen air flow rate. The process of gasification is continued for long run operation, once after the steady state has achieved, which initially takes about 30 to 50 minutes. All six point temperature profile (T1 to T6), gas flow rate, tar and SPM content of the gas and unloaded char weight was analyzed with run time during each experimentation. 2.4

Experimental procedures Experiments were conducted in the FBG with four different equivalence ratios of 0.3,

0.33, 0.35 and 0.38. Rice husk was feeded at 31.35, 28.16, 27.1 and 25 kg/h to match different ER of 0.3, 0.33, 0.35 and 0.38. A series of gasification experiments were conducted to determine the effect of the bed temperature and equivalence ratio (ER), that governing the producer gas quality (composition, production) and the gasification performance (carbon conversion efficiencies). Bed was initially heated by conventional charcoal feeded from the screw feeder and then temperature of the bed further increased by controlled air flow and partial combustion of the feeded biomass. Once the bed temperature attained to 600oC, continuous feedstock feeding was maintained by the horizontal screw feeder at a feeding rate corresponding to its required ER. Method was adopted from Vyas et al., (2007) to determine the producer gas composition at a fixed time interval. After steady operation was achieved, the sample of producer gas was collected by water displacement method and analyzed using a Gas Chromatograph (Sigma Instrument make, Makarpura GIDC, Baroda, Gujarat (India). The Chromatograph consists of 2

columns-Molecular Sieve and Porapack N as stationary media and Argon as carrier gas. Chromatograph uses TCD as detector. Tar and SPM sampling was taken as per the method given in operating manual for field type tar and particulate sampling unit by department of mechanical engineering, Indian Institute of Technology, Bombay (As per MNRE report, 2000) . Sampling method was same as per manual but flow was measured by wet type gas flow meter and sampling pot was placed in pipe line followed by pump and gas flow meter. The flow of the gas was kept constant 50 liters for all samples. The Carbone conversion efficiency has been calculated by Eq. 1 as reported by Lahijani et al., (2013). It represents the ratio of volumetric percentage of carbonaceous gas species in the producer gas in comparison to solid carbon present in the biomass feedstock (Eq. 1):

ηc =

Y (CO% + CH 4 % + CO2 %) × 12 × 100% 22.4 × C %

(Eq-1)

Where Y = dry gas yield (Nm3 /kg) and C % = mass percentage of carbon in the feedstock. Generally, the effectiveness of the gasification process is assessed in terms of the high heating value (HHV) of the producer gas, which is defined as (Eq. 2):

HHV=(H 2 %×30.52+CO%×30.18+CH 4 %×95)×4.19

(Eq-2)

The nitrogen content of the biomass was ignored while calculating the dry gas yield, but the N2 content of air and producer gas, were considered during mass balance to obtain the dry gas yield (Eq. 3) (Xio et al., 2006);

Y=(

Qa × 0.79 ) Wb (1 − X ash ) × N 2 %

(Eq-3)

Where, Qa = flow rate of air (Nm3 /h), Wb = mass flow rate of biomass (kg/h), Xash = ash content in the feed, and N2% = volumetric percentage of N2 in the dry producer gas.

A comparison between the chemical energy of the producer gas and that of biomass fuel is indicated by cold gas efficiency, which is defined as (Eq. 4) (Xiao et al., 2006); (Eq-4)

η =(

H g ×Y Hb

) × 100%

Where, the heating value of the producer gas and biomass are represented by Hg (MJ/Nm3) and Hb (MJ/ Nm3) respectively. Ratio of the output rate [in terms of the energy content of the hot gas] to the Energy input through biomass consumption rate (Eq. 5) (As per MNRE report, 2000).

ηth =[

H g (Qg + Cv (Tg − Ts )) Hb

]

(Eq-5)

Where, Qg = flow rate of producer gas (Nm3/h), Cv = Volumetric Specific heat of producer gas (kcal/Nm3), Tg =Temperature of Producer gas at the gasifier outlet (oC), Ts =Standard temperature (oC). 3.

Result and discussion

3.1

Fluidization properties of the materials Fluidization properties of the bed material (sand), feed material (rice husk) and catalyst

(dolomite) has been analyzed by infrared based hot wire anemometer in transparent acrylic column. The fluidization properties of the bed material, feed material and catalyst which have been used in the experiments are shown in Table 2. 3.2

Chemical properties of the rice husk and its char The paddy was cultivated in the Limbasi region of Kheda district, Gujarat, India. The rice

husk was procured at a rate of 4 - 5 rupees per kg. After receiving the feedstock it has been

passed through a screen of 10 mesh to segregate the stones, gravels, bricks and some oversized mud impurities before charging into the feeding hopper. Referring to van Krevelen diagram of atomic ratios for biomass it has been understood that O/C and H/C ratio of biomass is always ≥ 0.56 and ≥ 1.2, respectively, whereas lower O/C and H/C ratio is mostly observed in case of fossil fuels such as coal and petroleum-based fuels. Lower the O/C ratio, higher is the calorific value, which is reversal in case of biomass feedstock. Rice husk was having certain peculiarities of higher ash content of 15- 20 wt. % on dry wt. basis with respect to other types of agro-wastes. Gasification of the rice husk was done in fluidized bed gasifier with two cyclone separators to separate the char from the producer gas. The rice husk and the separated char from all the four ER has been observed for considering the carbon conversion efficiency of the gasifier. The char which has been collected from both the cyclone is having the heating value of 1.66±0.1 (MJ/kg). Table 1 shows the proximate and ultimate analysis of the rice husk and char collected from the two consecutive cyclones. 3.3

Electric power consumption Referring to several literatures it has been observed that, in regular practice the heating of

the bed material was accomplished by ceramic heater. Natarajan et al., (1998a, b) has also reported that the sand particles closer to the reactor walls are quickly heated up. Then the surge of air is periodically permitted to stir up the bed to homogenize its temperature. When the bed material reaches about 600°C, then the fuel feeding was to be started. In this study initially the heating of the bed material has been performed by 3 kW ceramic heaters and the total electric power consumption for operating gasifier was 4.0 kWh. To reduce the power consumption and to heat-up the bed material 3 kW ceramic heater was substituted by conventional charcoal. So, the

average power consumption was reduced to 2.2 kWh by reducing the initial burden of power consumption by 45% in the first hour of operation. 3.4

Temperature profile During each ER operation, maintaining the bed temperature profile was found to be

stable except with ER of 0.38. Temperature of the sand bed was found increased for ER 0.38, primarily because of higher ratio of air to feedstock availability, which initiates the combustion process rapidly inside the FBG. Temperatures of the bed (T2) were found to be highest followed by temperature above bed (T3) and free board temperatures (T4 and T5). Figure 2, shows the temperature profile of the FBG reactor. The highest bed temperature achieved was 816oC at ER of 0.38, whereas the lowest bed temperature of the reactor was 677oC at ER of 0.33. In case of rice husk gasification lower temperature profile was desirable in the sand bed, to reduce the clinker formation during FBG operation. 3.5

Effect of ER on producer gas quality The concentration of CO, H2 and CH4 decreased with increase in E.R (decrease in rice

husk feed rate) and hence concentration of gases like CO2 and H2O vapors produced from the combustion process became higher. The better HHV was found to be 3.98±0.1 (MJ/Nm3) at an ER of 0.3 and lowest HHV was found to be 3.6±0.1 (MJ/Nm3) for the product gas at an ER of 0.38 (Fig. 3). At higher temperatures, the heating value of the gas was found to be decreased at faster rate as both dilution rate, due to higher ER and the reduced carbon conversion, due to hard surface coating happened simultaneously (Natarajan et al. 1998a, b) 3.6

Effect of ER on gasifier performance

Both Subramanian et al., (2011) and Xu et al., (1985) has reported that the percent of carbon conversion efficiency (% CCE) increases with an increase in temperature up to certain extend, then starts to decrease after 900oC and the maximum carbon conversion efficiency in rice husk is about 82 %. During the whole period of experimentation (ranging 5 -6 h for each ER) the tar and SPM content of the producer gas was measured and calorific value of the producer gas was measured simultaneously to evaluate the performance of FBG. Char formation was found to be reduced and stands at 36.2%, 31.2%, 28.3% and 24.4% for the corresponding ER of 0.3, 0.33, 0.35 and 0.38 respectively. Fig. 4, shows the tar and SPM content of the producer gas in grams per cubic meter of the gas corresponding to different ER and it was following a decreasing trend with the increasing ER, whereas %CCE increases with increasing the ER. When the temperature is increased, the tar content is found to decrease consistently due to improved secondary thermal cracking of heavier tar. The tar content is decreased from 13.4g/Nm3 to 2.73g/Nm3 as the temperature increased from 700 to 900°C (Fig. 4) (Brage et al., 1997). Highest %CCE was found to be 91.2% at ER of 0.38, which might be due to excess propagation of combustion in presence of higher flow rate of air with lower rate of biomass feeding. 3.7

Effect of catalyst on gasifier performance Long duration experiments with different ER has revealed that ER of 0.3 and 0.33

delivered better (% CCE) as well as both hot and cold gas efficiency comparing to ER of 0.35 and 0.38 (Fig. 5). Long duration trial (12 h continuous operation) has been performed to observe the stability of the operation and adequate variation in the bed temperature of the FBG. Fig. 2, depicted about the bed temperature variation, where it was higher at ER value ≥ 0.35 comparing to the ER of 0.3 and 0.33. So, experiments for the catalytic cracking were conducted only with ER of 0.3 and 0.33. Fig. 6 describes about the effect of catalyst on tar and SPM reduction in the

producer gas. The addition of 20% dolomite with the bed material has resulted an average reduction of 41 - 46 % of tar and SPM content in the producer gas. Addition of catalyst increases the (%) CCE by 1% compared to non-catalytic gasification might be a reason of subsequent secondary cracking of heavier tar during catalytic application in the FBG. Considering the catalytic effect and the char formation, with the corresponding ER of 0.3 and 0.33, it was reduced from 36.3% to 35.0% and 31.2% to 30.5% respectively. 3.8

Effect of catalyst on Flame temperature gas yield and HHV Javier et al., (1998) has reported that the apparent thermal efficiency (gas yield X

LHVgas/LHVbiomass) is passed through the maxima with the variation of ER. It is due to the fact that the gas yields increases and LHV decreases on increasing the ER. Table 3 shows the change in flame temperature, gas yield and HHV of the producer gases on addition of catalyst at two different ER (0.3 and 0.33) on FBG. With increasing the ER the gas yield increases and HHV of the gas decreases hence simultaneously the flame temperature also decreases. Figure 6 shows the change in thermal efficiency of gasifier with catalyst addition along with sand for the ER of 0.3 and 0.33. The addition of catalyst has been resulted an increase of 4% in the apparent thermal efficiency of gasifier. Natarajan et al., (1998a, b) has reported that the cold gas efficiency follows the trend similar to that of the carbon conversion efficiency (CCE %) and LHV of the producer gas and is found to be maximum at 66%, corresponding to feeding of 16 kg/h and 780°C. Ultimately in this case the thermal efficiency was found maximum of 68.1%, at ER of 0.3 (Fig. 6). 4.

Conclusions

Air gasification of rice husk was successfully performed in bench-scale bubbling FBG developed at SPRERI. There was no clinker formation, electric power consumption was low and

HHV of producer gas was ≥ 3.76±0.1 (MJ/Nm3) . During catalytic study, reduction in tar and SPM content of producer gas was found significantly lower for ER of 0.33 than 0.3, and the CCE (%) was also higher for ER of 0.33. Among all four ER, considering the most suitable ER of 0.33 and taking mixture of 20% dolomite with sand has resulted 41 - 46 % of tar and SPM content reduction in desired gases.

Acknowledgements

The authors wish to thank the All India Coordinated Research Project (AICRP) on Renewable Sources of Energy for Agriculture & Agro-based Industries (ICAR), Government of India, for providing financial support for this project. The authors also thankful to Dr. M. Shyam, Director, and Prof. B.S. Pathak (Ex-Director) of Sardar Patel Renewable Energy Research Institute, V V Nagar, Anand, Gujarat, India, for their valuable advice, support for this work and preparation of manuscript.

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Flame

Burner

T6 Cyclone seperator 2 Free board section T5 Cyclone seperator 1 T4

Feeding hopper T3

Screw feeder

Bed Section Temp. Indicator

Ash bin

T2

Controller Gear box

Distributor plate T1

VFD Pannel

Regenerative blower

Air inlet

Control valve

T1-T6 Point for therm o couple

Figure 1. Schematic of the fluidized bed gasifier (FBG) designed and developed at SPRERI, facilitated with cyclone separator and temperature indicators

Figure 2. Variation of average temperature profile with respect to equivalence ratio (ER), at selected points in the reactor (from T1 to T6)

Figure 3. Variation of the gas composition and temperature, Tar and HHV of the producer gases with equivalence ratio (ER)

Figure 4. Variation of the bed SPM and HHV of the producer gases with equivalence ratio (ER)

Figure 5. Effect of E.R on carbon conversion efficiency (ηcc, (%)) ,hot gas efficiency (thermal =ηth) and Tar and SPM content of the gases produced from FBG

Figure 6. Effect of catalyst on cold gas, hot gas (thermal) efficiency and tar & SPM content of the gases produced from the FBG.

Captions of Tables

Table 1. Proximate and ultimate analysis of the biomass (rice husk) and its generated char from FBG operation

Table 2. Fluidization properties of different materials used in the FBG operation

Table 3. Effect of with and without catalyst on gas yield and HHV of the producer gas during FBG operation at two most selective ER of 0.3 and 0.33

Captions of Figures

Figure 1. Schematic of the fluidized bed gasifier (FBG) designed and developed at SPRERI, facilitated with cyclone separator and temperature indicators

Figure 2. Variation of average temperature profile with respect to equivalence ratio (ER), at selected points in the reactor (from T1 to T6) Figure 3. Variation of the gas composition and temperature, Tar and HHV of the producer gases with equivalence ratio (ER)

Figure 4. Variation of the bed SPM and HHV of the producer gases with equivalence ratio (ER)

Figure 5. Effect of E.R on carbon conversion efficiency (ηcc, (%)) ,hot gas efficiency (thermal =ηth) and Tar and SPM content of the gases produced from FBG

Figure 6. Effect of catalyst on cold gas, hot gas (thermal) efficiency and tar & SPM content of the gases produced from the FBG.

Table 1: Proximate and ultimate analysis of the biomass (rice husk) and its generated char from FBG operation Rice husk

Rice husk char

Proximate analysis Moisture content (%)

8.3

2.18

Volatile matter (% db)

71.49

16.29

Ash content (% db)

19.36

79.72

Fixed carbon (% db)

9.138

3.97

Carbon (%)

36.1

12.8

Nitrogen (%)

4.8

0.55

Hydrogen (%)

1.94

2.3

Oxygen (%)

37.8

5.35

C.V.

3167±90 kcal/kg

397±25 kcal/kg

Ultimate analysis

Table 2: Fluidization properties of different materials used in the FBG operation Material

Sand

Dolomite

Rice husk

Minimum fluidization velocity(m/s)

0.11

0.1

0.09

Bubbling velocity(m/s)

0.31

0.28

0.28

Fast fluidization(m/s)

1.9

1.65

0.61

Escape velocity(m/s)

>3.2

2.82

2.06

Table 3: Effect of with and without catalyst on gas yield and HHV of the producer gas during FBG operation at two most selective ER of 0.3 and 0.33 Without catalyst

With catalyst

% Increase

Equivalence ratio

Unit

0.3

0.33

0.3

0.33

0.3

0.33

Flame temperature

o

704

676

735

703

4.40

3.99

Gas yield

m3/kg

2.13

2.15

2.18

2.22

2.35

3.26

HHV of the producer gas

kcal/m3

952.8

909

990.3

925

3.94

1.76

C

Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal

HIGHLIGHTS

1. Study of air gasification of rice husk in bench-scale SS-fluidized bed gasifier 2. Instead of electrical heating, initial heating of gasifier bed material done by charcoal 3. Carbon conver. effici., cold & hot gas efficiency are better with dolomite plus sand 4. Composition of 20% dolomite and 80% sand resulted 41-46 % of tar & SPM reduction

Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal

GRAPHICAL ABSTRACT Study of air gasification of rice husk (25-31.3 kg/h) has conducted in a bench-scale SS-fluidized bed gasifier (FBG), developed at SPRERI, where initial heating of FBG bed material was performed by burned charcoal to reduce the additional electrical consumption. The carbon conversion efficiency (91%), cold and hot gas efficiency, and HHV of the producer gases are found better with dolomite plus sand usage in comparison to sand bed alone. At ER of 0.33 with 20% dolomite plus rest of sand has resulted a substantial tar and SPM reduction of 41-46 % in the producer gas, found suitable for thermal application.