Groundnut shell gasification performance in a fluidized bed gasifier

0 downloads 0 Views 2MB Size Report
May 24, 2018 - cation in a pilot-scale fluidized bed gasifier with bubbling air is carried out .... air distribution plate was placed between plenum and reaction ...
Environmental Technology

ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20

Groundnut shell gasification performance in a fluidized bed gasifier with bubbling air as gasification medium Dharminder Singh, Sanjeev Yadav, V. M. Rajesh & Pravakar Mohanty To cite this article: Dharminder Singh, Sanjeev Yadav, V. M. Rajesh & Pravakar Mohanty (2018): Groundnut shell gasification performance in a fluidized bed gasifier with bubbling air as gasification medium, Environmental Technology, DOI: 10.1080/09593330.2018.1476592 To link to this article: https://doi.org/10.1080/09593330.2018.1476592

Accepted author version posted online: 14 May 2018. Published online: 24 May 2018. Submit your article to this journal

Article views: 3

View related articles

View Crossmark data

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tent20

ENVIRONMENTAL TECHNOLOGY https://doi.org/10.1080/09593330.2018.1476592

Groundnut shell gasification performance in a fluidized bed gasifier with bubbling air as gasification medium Dharminder Singh

a

, Sanjeev Yadava, V. M. Rajesha and Pravakar Mohantyb

a

Department of Chemical Engineering, Shiv Nadar University, Greater Noida, India; bSardar Patel Renewable Energy Research Centre, Vallabh Vidhyanagar, India ARTICLE HISTORY

ABSTRACT

This work was focused on finding the groundnut shell (GNS) gasification performance in a fluidized bed gasifier with bubbling air as gasification medium. GNS in powder form (a mixture of different particle size as given in table 8 in the article) was gasified using naturally available river sand as bed material, top of the bed feeding, conventional charcoal as bed heating medium, and two cyclones for proper cleaning and cooling the product gas. Experiments were performed using different operating conditions such as equivalence ratio (ER) between 0.29 and 0.33, bed temperature between 650°C and 800°C, and feedstock feeding rate between 36 and 31.7 kg/h. Different parameters were evaluated to study the gasifier performance such as gas yield, cold gas efficiency, carbon conversion efficiency (CCE), and high heating value. The most suitable ER value was found to be 0.31, giving the most stable bed temperature profile at 714.4°C with 5–10% fluctuation. Cold gas efficiency and CCE at optimal ER of 0.31 was found to be 71.8% and 91%, respectively.

1. Introduction To sustain the non-renewable energy sources and to complete the growing energy demand of the world, renewable energy sources (including biomass) are getting much more attention these days. Biomass is present in a huge quantity in different forms such as agricultural waste, forest waste, food waste, municipal solid waste, and paper sludge. Each type of biomass has its own specific properties which determine its use as a raw material in gasification. Wood (forest waste) is used to be the most preferred biomass for gasification, but owing to its high cost and environmental regulation, it is being replaced with nonwoody biomass. However, some non-woody biomass (especially agri-residues) exhibit complexities during gasification due to their different physio-chemical properties [1].

CONTACT Sanjeev Yadav Uttar Pradesh 201314, India

[email protected]

Received 7 August 2017 Accepted 6 April 2018 KEYWORDS

Bubbling fluidized bed gasifier; groundnut shell powder; equivalence ratio; conventional charcoal; carbon conversion efficiency

India holds the second largest production shares after China in total world production of groundnut shell. In 2014, the total world production of groundnut with shelled was 38.51 Million MT, Asia contributed with 22.94 Million MT, Africa was 11.54 Million MT, and America was 3.98%. The production in India was 6.56 Million MT and in China it was 15.78 Million MT in 2014 [2]. Groundnut shell (GNS) is also one such agri-residue, whose heating value is nearly equal to that of wood but physical properties like low density and poor material flow characteristics lead to problems in its processing, handling, and feeding into gasifier. These problems can be overcome by doing the pretreating of the GNS such as drying, sizing, screening, and bailing [3–5] or force-feeding it to the nearest possible point to the bed zone, as done in case of rice husk [6,7].

Department of Chemical Engineering, Shiv Nadar University, Village Chithera, G.B. Nagar, Greater Noida,

© 2018 Informa UK Limited, trading as Taylor & Francis Group

2

D. SINGH ET AL.

Though India is the second largest producer of groundnut in the world and GNS contains considerable amount of energy [8], but not much work has been done to study its utility as biomass feedstock for producing bioenergy through gasification. In this work, the study of GNS gasification in a pilot-scale fluidized bed gasifier with bubbling air is carried out and performance of this gasifier is evaluated using equivalence ratio (ER) as a primary parameter. ER refers to the ratio of the AFR (actual air to fuel ratio) to stoichiometric air of biomass [9]. ER less than 0.18 generally results in higher tar formation, and ER greater than 0.45 produce syngas with undesirable composition and low heating value [9]. Therefore, ER values are neither kept very low or nor very high. High heating rates produced a large amount of light gases and a less amount of char and Tar, which suggest that gas composition and gas yield are directly related to heating rate of biomass particles [10]. Very fine particles have faster heating rate due to larger surface area in comparison to large particles size. Small particles need low air presser during combustion process for better endothermic reaction than large particles. Fine particles generate more CH4, CO, and CO2 because of the pyrolysis process controlled by kinetics reactions, while in large particles, the process is controlled by gas diffusion. So, for biomass gasification, small particle size is recommended in literature. However, small particle size increases SPM in literature. However segregation of SPMs must be important factor in gasification process. To achieve proper segregation of SPM, double cyclone has been recommended by various authors in literature. Second cyclone also performed as coolant device for syngas [11,24]. In the literature, the study on gasification of different biomass (including GNS) in a large-scale fluidized bed gasifier was done and the effects of different process parameters were investigated [12]. This work showed that carbon conversion in a large-scale gasifier depends upon the activity of solid char residue, and the activity of solid char residue is a function of temperature, pressure, partial pressure of reactants (H2O, CO2), and product (H2, CO). Influence of catalyst addition on GNS gasification in fluidized bed reactor was also reported in the literature and it was shown that tar cracking took place and helped increasing the tar removal efficiency up to 50% by the addition of catalyst [13]. For lab-scale gasification of GNS, a study was reported in which a lab-scale fluidized bed gasifier was used where they showed the effect of GNS particle size and reactor temperature on producer gas quality, gas yield, and heating value [8]. This work did not try the powder form of GNS. In another work on GNS gasification [14], thermal integration unit was added to fixed bed gasifier in which they found improved gasification reaction by

lowering tar content and increasing gas production efficiency. Various method has been used for heating the gasifier bed to start up gasifier operation such as electric furnace [15,16], programmable temperature controller [17,18], burner and pre-heated air [19], electrically heated oven [20], liquid petroleum gas [21,22] and wall heater [23]. All of these methods were used to supply external heating to achieve the desired operating bed temperature. All these methods consume time around 45–60 min to achieve desired bed temperature for gasification, same as in the case of charcoal heating; however, charcoal heating is better on the aspects of cost effectiveness in comparison to these methods [24]. In this work, gasification of GNS was carried out in air bubbling fluidized bed gasifier in powder form with feeding at the top of the bed for proper mixing of biomass and bed material to achieve high rate of heat and mass transfer between bed material and biomass. Internal heating of bed was done by the use of conventional charcoal to make it economically efficient. For proper segregation of solid particles from syngas, double cyclone separators were used, which also cools the product gas.

2. Materials and methods 2.1. Experiment apparatus A bubbling-type fluidized bed (BFB) gasifier was used for experiments, which has also been used for air gasification of rice husk [24]. The schematic representation of BFB is given in Figure 1. The reaction chamber of gasifier was cylindrical type and it was divided into two sections, bed section and free board section. Bed section contained sand as bed material and it was used for fluidization and combustion process. Free board section involves gasification and pyrolysis processes. Gasifier rector design parameters and bed material properties have been described in Table 1. Table 2 describes materials and their properties used in manufacturing of gasifier. A reaction chamber operating at a high temperature (600–900°C) was constructed with the use of high heat resistance stainless steel 304. Total height of chamber was 1600 mm in which bed section was 600 mm and freeboard section was 1000 mm. The inner diameter of reactor was 200 mm. It was externally covered by heat insulation. A plenum was connected to bottom of reaction chamber, i.e. bottom of bed section. A multi-perforated circular type air distribution plate was placed between plenum and reaction chamber for proper distribution of air throughout the bed and chamber. It also reduced the backflow

ENVIRONMENTAL TECHNOLOGY

3

Figure 1. Schematic representation of air bubbling fluidized bed gasifier [24].

Table 1. Design parameters of reactor and bed properties. Parameter Gasifier type Total reactor height (mm) Thickness of rector sheet (mm) Bed section height (mm) Freeboard section height (mm) Inner diameter of reactor (mm) Biomass Gasification agent Bed material Sand density (kg/m3) Sand particle size (mm) Bed height (mm) Heating medium Thermocouples Distributer plate diameter (mm) Thickness of distributer plate (mm) No. of orifices in distributer plate Inner diameter of orifice (mm) Time required, initial to start-up operation (min)

Description Bubbling fluidized bed gasifier 1600 5 600 1000 200 GNS powder Atmospheric air River Sand 1470 0.4–0.6 300 Charcoal K-type 220 5 102 2 45–60 (Approx.)

of bed material towards the plenum. Diameter of distributer plate was 220 mm with 5 mm thickness. Diameter plate also contained 102 orifices of 2 mm inner diameter (Table 3). Two cyclones were used for separating char from gas. First cyclone was installed in gas venting line after the exit of freeboard section to discern the char, ash, and dust particles from the product gas. Second cyclone was placed between the first cyclone and burner, to Table 2. Different gasifier parts and their material proprieties. Gasifier part

Material

Bed section and freeboard section Distributor plate Cyclone and gas outlet Insulation Hooper Auger

S.S 304 S.S 310 M.S Cerawool MS S.S 304

4

D. SINGH ET AL.

Table 3. Particle separator and collector dimensions. Parameter No. of cyclone and ash pit 1st and 2nd Cyclone diameter (mm) 1st and 2nd Cyclone body cylindrical height (mm) 1st and 2nd Cyclone total height (mm) 1st and 2nd Cyclone solid outlet diameter (mm) 1st and 2nd ash pit diameter (mm) 1st and 2nd ash pit height (mm) 1st and 2nd ash pit solid exit diameter (mm) Gas pipe diameter (mm)

Value 2 220 and 200 280 and 240 840 and 670 80 and 75 380 and 340 940 and 350 85 and 70 150

cool the gas and capture the remaining fine particles of char, ash, and dust. Dimensions of particle separators and ash pits are shown in Table 3. Ash pits were attached at the bottoms of the cyclones to collect char, ash, and dust particles separated by cyclones from the syngas (Figure 1). Feeding of feedstock is done through feeding system having three parts; a hopper, an auger, and a drive system. Their design parameters are shown in Table 4. Drive system was made up of variable frequency drive and a gear box. A funnel type hopper with face length and face width of 455 and 410 mm, respectively, base length and base width of 310 and 120 mm, respectively, and height of 430 mm was used to hold the material for feeding. For the continuous flow of feedstock, an auger was used for carrying the feedstock from hopper to reaction chamber. To adjust and control the speed of auger, a gear box was setup with speed ratio 60:1 followed by 18 T × 48 T sprockets. A variable frequency drive model no. IGBT LS-600 of range 0–50 Hz was used to maintain the gear box speed. For supplying air, a three-phase regenerative blower model 8.3 of 2.2 kW, 5 amps, 220 V with air flow capacity of 140 m3/h was used which also acted as a gasification agent in the chamber. A thermocouple (T1) was installed in the plenum to measure the inlet air temperature of gasifier. For measurement of gasifier temperature, six K-type tested thermocouples (T1–T6) of range 0–1200°C were used. These six thermocouples were installed at different points of gasifier, as shown in Figure 1. To measure the inlet air temperature, a thermocouple (T1) was installed in plenum, just below the distributor plate. The distance between T1 Table 4. Design parameters of biomass feeding system. Parameter Hooper type Top inlet face dimensions of Hooper (l × b) (mm) Base dimensions of Hooper (l × b) (mm) Hooper height (mm) Auger length (mm) Auger bore (Shaft) thickness (mm) Auger fins thickness (mm) Auger & Geared motor speed ratio VFD frequency range (Hz)

Description Rectangular funnel type 455 × 410 310 × 120 430 920 40.5 3.5 60:1 0–50

Table 5. Instruments with their manufacturer and measuring range. Instrument

Manufacturer/model no.

Hot wire anemometer Rotameter Gas flow meter Muffle furnace Regenerative blower Variable frequency drive K-type thermocouple Filter paper

Kusam meco 273/04-05 Nova instruments Nova instruments Sabar Engg. IGBT LS-600 Altop industries Whattman

Measuring range 0–40 m/s 15–150 m3/h 10 litre in 1 revolution 0–1000°C 0–140 m3/h 0–50 Hz 0–1200°C 1.2 microns

thermocouple and distributor plate was kept at 170 mm. T2 thermocouple was placed in bed section above the distributor plate at height of 170 mm to measure the bed temperature. T3 thermocouple was installed just above the biomass feeding point with 530 mm height from distributor plate. The distance of T4 and T5 thermocouples from distributor plate was 1100 and 1350 mm, respectively. T6 thermocouple was installed in gas outlet pipe between the first and second cyclone to measure the producer gas temperature. All the thermocouples were connected with 12point digital indicator. Gas sampling system contained gas cleaning and cooling kit, vacuum pump, and gas sampling bladders. Gas sampling bladders were rubber bladders of high abrasion resistance, chemical resistance, and leak proof. They were used for storage or preserving syngas for short period and then it was sent for further analysis in a gas chromatography machine. Gas cleaning and cooling kit consisted of two cold water-filled impinger bottles of 500 ml to clean and cool the syngas, one air-filled stainless steel impinger bottle to collect particles of condensed tar and water, and a silica gel tube was used to remove moisture from the syngas. All the impinger bottles and silica gel tube was connected in series and attached with gasifier with the help of silicon pipe. When the syngas passed through first water-filled impinger bottle, very fine particles of char and dust got dispersed in water. Impinger bottle also cool down the syngas which helped to condense the tar. Tar was adsorbed by water during scrubbing and settling down at the bottom of impinger bottle and cleaned the syngas form dust, chars, and tar. Hence, both water-filled impinger bottles worked as scrubbers. Furthermore, air-filled impinger bottle and silica gel tube made it moisture-free and dry gas. Vacuum pump followed the gas cleaning and cooling system kit to suck the gas from the gas outlet point placed at the exit pipe of the first cyclone and filled the samples of gas in the rubber balloons. Gas flow rate was controlled by the vacuum pump and found to be stable at 10 l/min (Table 5). Tar and suspended particulate matter (SPM) collection system consisted of a stainless steel gas filter probe, filter

ENVIRONMENTAL TECHNOLOGY

papers, vacuum pump, and a water-filled gas flow meter. For collection of Tar and SPM particles, a dried filter paper was placed in the filter probe. A suction pump was used to suck the gas from the gas outlet point near the exit of first cyclone. For measuring the flow rate of the gas, a water-filled gas flow meter was used. Filter paper was placed in the probe for three minutes and then replaced by another filter paper. Tar and SPM particles tend to get stuck on it during gas passing through it. Table 5 gives information about various instruments and their manufacturer and measuring range used in gasification of GNS powder.

2.2. Characterization of biomass and product gas GNS powder was used as biomass material in this work. GNS was obtained locally and converted into powder through grinding. The physical and chemical characterization of GNS powder was done as per ASTM 1131 – D standard method. Particle size distribution of GNS powder was calculated from Equations (1) and (2) from the Tyler standard screen method. According to Tyler, standard screen particles’ size (dp) of biomass is given by the below equation. dp =

S1 + S2 , 2

(1)

where S1 shows aperture size of screen through which biomass particles are passed out and S2 shows aperture size of screen at which biomass particles are rest. The quantity of particles having a size greater than or equal to 1.68 mm was 5.5% of 1 kg GNS powder. The quantity of particles having a size equal to or smaller than 0.42 mm particles was 26.5%. River sand was used as inert bed material in a BFB gasifier. It also acted as the heating medium for combustion process in a gasifier.

2.3. Fluidization of the feed and bed As reported in the literature [21], for the material having low bulk density, the fluidization velocity should neither be very high (4.5–8 Umf) and nor be very low (less than 3 Umf). Since, GNS powder is a low-bulk density material, analysis of its hydrodynamics properties and fluidization behaviour was needed along with the analysis of bed material, i.e. river sand. An easily visible acrylic column had been developed for this purpose with same height (1600 mm) and internal diameter (210 mm) as of BFB gasifier reactor chamber. Hot wire anemometers were installed at different points in aspirator as well as outlet to study the inner air velocity of aspirator. A rotameter was installed at the inlet of aspirator to measure the air flow.

5

2.4. Experimental procedure Three different ERs (0.29, 0.31, and 0.33) were used to conduct the experiments. ER was calculated with the help of Equations (2) and (3) [25]. ER =

Actual air to fuel ratio , Stoichiometric air

Stoichiometri air = (C% × 11.53)   1 + 34.34 H2 % − O% kg, 8

(2)

(3)

where C%, H2%, and O% are mass percentages in biomass. The range of ER used here is narrow because the size of GNS particles is not uniform. If ER was increased too high, then the smaller particles would blow away by the air and would not combust very well and that would result in poor gasifier performance. To maintain ER ratio of 0.29, 0.31, and 0.33, feeding rate of GNS powder was varied to 36, 34, and 31.7 kg/h, respectively with constant air flow rate of 37 m3/h. To make the bed height of 300 mm, the reactor was filled with river sand for each experiment. Four sets of 300 g charcoal were prepared and the first set was ignited with the help of cook stove. These red-hot charcoals were poured in the hopper and then they were discharged into bed by an auger. Air was supplied with the velocity of 0.11 m/s by regenerative blower in order to achieve good heat transfer and good mixing. As the bed temperature reached around 80°C, it became stable and then the next batch of ignited charcoals was sent to increase bed temperature around 200°C. To further increase the bed temperature, third batch of red hot charcoal was sent to increase the bed temperature around 320°C. Fourth and final set of red hot charcoals was poured into bed along with small quantity of biomass (GNS powder), which increased the temperature in the range of biomass pyrolysis, i.e. above 400°C and started producing diluted producer gas. After that, GNS powder was filled in the hopper and auger was started for the continuous feeding of biomass in the bed section of the reactor. Air was supplied in the reactor continuously. The reactor temperature increased gradually due to pyrolysis process occurring in the reactor. It took 15 min for bed temperature to become stable at different temperatures for different ER and bed temperature readings were taken at every 20 min interval from different points of gasifier. Finally, gas, tar, and SPM samples were taken for analysis. The gas samples collected were analysed with the help of A2 columns. Thermal conductivity detector (Sigma instrument make, Makarpura GIDC, Borada, Gujarat, India) was used to analyse the gas composition. Argon was used as a carrier gas in the chromatograph; Porapack N and

6

D. SINGH ET AL.

Molecular Sieves were used as stationary media. Table 6 represents air blowing fluidized bed gasifier performance for GNS powder gasification for different parameters along with different trails.

3. Results and discussion The physio-chemical properties of GNS powder are listed in Table 7. Gross calorific value of GNS powder was found to be 3802.6 kcal/kg and bulk density was found to be 233.6 kg/m3. The GNS powder was directly used for gasification after the grinding process, without going through sieving process; therefore, it contains different size particles in different quantities, as shown in Table 8. The mass percentage of very fine particles (≤0.42 mm) was 26%, which was nearly equal to mass percentage of particles size in the range of 1.68–0.85 mm (27%) and 0.85–0.50 mm (28%). The huge quantity of these fine particles influenced the various parameters of gasifier. By Tyler standard sieve method, selected particle size of sand was in the range of 0.4–0.6 mm. The bulk density of sand was 1470 kg/m3. The fluidization properties of sand (bed material) and GNS (feed material) are given in Table 9. The bubbling fluidization velocity of the sand and GNS was found to be 0.31 and 0.24 m/s, respectively.

3.1. Effects of ER on gasifier temperature Figure 2 shows the temperature of various parts of gasifier at different ERs. Ideally, bed temperature should

Table 6. Gasifier performance with different parameters along with different trails. Values Parameters

Trial 1

Trial 2

Trial 3

Air feeding rate (m3/h) GNSP feeding rate (kg/h) ER Gasifier operational time (min) Inlet air temperature, T1 (°C) Bed temperature, T2 (°C) Above bed temperature, T3 (°C) Middle of freeboard temperature, T4 (°C) Top of freeboard temperature, T5 (°C) Gas temperature, T6 (°C) HHV (MJ/Nm3) Tar and SPM (g/m3) Gas yield (Nm3 gas/kg biomass) Cold gas efficiency (%) Carbon conversion efficiency (%) Gas composition (vol. %) CO H2 CO2 CH4

37 36 0.29 35 77.6 746.5 600.3 565.33

37 34 0.31 55 74.8 714.4 607 566.8

37 31.7 0.33 120 71.5 742.6 574 529.6

443

435.8

416

241.5 4.95 9.5 1.84 57.23 85 13.89 13.41 17.92 3.71

213.6 5.68 11.8 2.01 71.8 91 12.94 13.77 13.5 5.74

190.8 5.09 12.1 2.15 68.85 89.5 11.99 13.97 13 4.5

Table 7. Characteristics of GNS powder and its char. Material

GNS powder

Proximate analysis Moisture (%) Volatile matter (% dry basis) Ash content (% dry basis) Fixed carbon (% dry basis) Ultimate analysis Carbon (%) Nitrogen (%) Hydrogen (%) Oxygen (%) Bulk density (kg/m3) Calorific value (kcal/kg)

6.59 68.14 9.62 22.24

GNS powder char 1.57 17.63 77.12 4.08

38.05 3.44 5.14 43.78 233.6 3802.6

13.01 0.44 2.8 6.31 – –

Table 8. Particle size distribution of groundnut shell powder. Sieve size/sieve opening +10 −10 + 20 −20 + 30 −20 + 40 −40

Particle size (mm)

Mass percentage (%)

>1.68 ≤1.68 > 0.85 ≤0.85 > 0.50 ≤0.50 > 0.42 ≤0.42

5.5 28 27 13 26.5

increase with an increase in ER, but in this case bed temperature first decreased and then increased on changing the ER from 0.29 to 0.33. This happened due to nonuniform particle size of GNS as fine particles combust faster than the large particles due to higher surface area. At small ER of 0.29 where feeding rate was highest (36 kg/h), at first, all particles start depositing on the bed surface, then the fine particles (38% by weight) started moving into bed due to sufficient void between the bed particles where they were rapidly burnt and released heat as soon as they came in contact with hot bed, raising the bed temperature (T2) continuously up to 800°C in 35 min. Large- and medium-size particles deposited on the bed surface underwent pyrolysis releasing moisture and other volatile products in the form of gases and vapours. There were large fluctuations in top of the bed temperature (T3) at this ER due to improper mixing and fluidisation. Top of the bed temperature (T3) rise was less than the bed temperature (T2) rise. At ER 0.31, there was less pressure on bed due to which air easily moved up from bottom of the bed and burnt GNS powder particles around middle and top of the bed which increased T3 temperature. Temperature T2 went down at this ER as there was more air but less biomass inside the bed, hence less combustion took place in this region. On further increasing the ER to Table 9. Fluidization characteristics of sand and GNS powder. Material Minimum fluidization velocity (m/s) Bubbling fluidization velocity (m/s) Fast fluidization velocity (m/s) Terminal fluidization velocity (m/s)

Sand

GNS powder

0.11 0.31 1.9 > 3.2

0.08 0.24 0.59 > 2.01

ENVIRONMENTAL TECHNOLOGY

7

Figure 2. Temperature profiles in the air bubbling fluidized bed gasifier at different ER.

0.33, bed temperature (T2) was found increasing because a large amount of air improved fluidized behaviour of bed by which large and heavy particles of GNS powder easily penetrated and combusted in bed and raised its temperature. Medium-size particles floating above bed underwent pyrolysis and gasification process, which consumed heat released by combustion process and decreased the temperatures T3, T4, and T5. Small particles are forced out unburnt with the producer gas due to a high air pressure at ER 0.33. For each ER, the temperature of the bed was increasing with time, but the rate of increase was less for ER 0.31. The reactor was stopped within 35 min for ER 0.29, within 55 min for 0.33, and within 120 min for 0.31 due to choking in ventilation pipe and cyclone. This choking happened because of the condensation of the hydrocarbons into

tar at low exit gas temperature (T6) along with deposition of SPM and ash particles there.

3.2. Effect of ER and bed temperature on Tar and SPM It was reported in the literature that higher tar conversion into gases could be achieved by increasing the temperature [26]. But here in GNS gasification, the amount of Tar and SPM content was high at lowest ER 0.29, where bed temperature was highest. Because of high generation of tar contents in gas at this point, gas outlet pipe was completely chocked. However, as shown in Figure 3, Tar and SPM content was lower at ER 0.29 than other ERs because gas samples were collected after 20 min from start of the operation, but by

8

D. SINGH ET AL.

Figure 3. Effects of ER and temperature on Tar and SPM.

that time, the pipe had chocked and due to high bed temperature, tar had started burning. The gasifier had to be stopped soon (Table 6), which resulted in less collection of Tar and SPM particles at ER 0.29. Though the temperature decreased at ER 0.31 but the amount of Tar and SPM collected increased because there was less choking. Choking did take place at ER 0.31 and 0.33 but it happened very late (Table 6) and reactor operated smoothly for longer time. At ER 0.33, there was less tar generation and more SPM generation, but their total amount was little less than the total amount of tar and SPM at ER 0.31. Less tar generation was due to higher temperature at ER 0.33 as more tar conversion took place at higher temperature. At higher ER of 0.33, air pressure was highest in the reactor due to which, very fine particles of GNS powder exist with the producer gas from the reactor without getting burnt, hence increasing the presence of large quantity of SPM in exit gas.

3.3. Effects of different ER and bed temperatures on gas yield For determining the dry gas yield, Equation (4) is used [27], where Qa is air flow rate in Nm3/h, Wb is feeding

rate of biomass in kg/h, Xash is ash content in feedstock, and N2% is volumetric percentage of nitrogen in dry producer gas.  Y=

 Qa × 0.79 Nm3 /kg. Wb (1 – Xash )× N2 %

(4)

The nitrogen content of biomass was ignored while calculating the gas yield. Only the nitrogen content of air and producer gas was considered during mass balance (Figure 4). As shown in Figure 4, the gas yield increased linearly with increase in ER from 0.29 to 0.33. However, gas yield does not change with change in bed temperature. At ER 0.29, the bed temperature is highest but the gas yield is lowest. As discussed earlier, the temperature at ER 0.29 went up due to non-uniform particles size of GNS as fine particles getting burnt faster than medium- and large-size particles. Less combustion of medium- and large-size particles resulted in less gas yield at this ER. However, on increasing the ER from 0.31 to 0.33, gas yield increased with an increase in bed temperature. The highest gas yield of 2.15 Nm3/kg was achieved at ER of 0.33 and bed temperature of 742.6°C.

ENVIRONMENTAL TECHNOLOGY

9

Figure 4. Effects of ER and bed temperature on gas yield.

3.4. Effect of ER on gas compositions and HHV HHV is a very good indicator of gasification performance. HHV of the producer gas has been calculated by Equation (5) in MJ/Nm3 (28): HHV = (H2 % × 12.75) + (CO% × 12.63) + (CH4 % × 39.72),

(5)

where H2, CO%, and CH4% are the volumetric percentage of producer gas (Figure 5). As shown in Figure 5, HHV of producer gas was lowest at the lowest ER of 0.29 and increased with increasing the ER to 0.31. HHV falls down again on further increase of ER from 0.31 to 0.33. This variation in HHV values was found due to lowest concentration of CH4 and H2 in producer gas at lowest ER of 0.29. This variation in CH4 and H2 happened due to dilution and burning of tar as the tar chocked gas pipe at ER 0.29, which increased pressure inside the reactor, causing the gas to burn along with tar, thus decreasing HHV. Figure 5 also showed that concentration of CO2 was highest at lowest ER of 0.29 and because of this reason that HHV was lowest at this ER as non-combustible gas does not contribute to HHV.

Concentration of CO2 was highest at 0.29 because of the burning of the tar in exit gas pipe. Highest concentration of CH4 was found at ER 0.31 while there was very slight increase in the concentration of H2 on changing the ER from 0.31 to 0.33. Increase in ER from 0.31 to 0.33 decreased the feed rate of GNS powder, requiring comparatively less air for combustion, hence, reducing the concentration of CH4 and CO in producer gas. At highest ER, the quality of producer gas also got deteriorated because of lowest feeding rate increased air pressure in reactor that would force very fine particles of GNS powder in char out with product gas.

3.5. Effect of ER on gasifier performance Cold gas efficiency (η) and carbon conversion efficiency (ηcc) are good indicator of gasification performance. Cold gas efficiency can be calculated according to the relation given in Equation (6). It is the ratio of chemical energy of producer gas to biomass fuel [27].   Hg × Y ×100%, (6) h= Hb

10

D. SINGH ET AL.

Figure 5. Effect of ER on gas composition and HHV.

Figure 6. Effect of ER on cold gas efficiency, carbon conversion efficiency.

– 800 – Ni-Dolomite

0.03





78.4

2.99 1.2 1.5–5 86 56.9 – – 750 780 850 – – 0.36 – 0.75 –

Wood Saw Dust Spruce wood pellet Pine Sawdust + coal + plastic waste Coconut shell

Groundnut Shell Saw Dust Rice Husk Rice Husk

Silica Sand Ni-alumina Dolomite

0.32 0.3–0.38

4.97 – 4–8

85–95

94.9

48–60 60.5 38.79 50–65 5.92 4.6 3.13 – – 800 812 750–850 0.24

2.15 41 – 25–31.3

5.03

38.7

13.28 14.3 14– 23 29.9 9.27 30.3 7–15

4.5

62.5 – –

42.71 55.4 – – 10.27 16.3 – – – 2.35 1.61 2.13–2.15

22.11 7.3 4.56 4–6

3.38 3.2 3.45 2– 3.5 4.21 1.49 2–10

– 56.7

20.73 17.9 11.1 14– 18 9.25 17.6 10– 20 35.7

CH4

5.74 2.9

CO2

13.5 14.45

H2

13.77 4.0 12.94 19.9 0.31 0.25

Sand Alumina sand – Ni-Alumina Sand Sand GNS Powder Rice husk

34 –

714.4 600–850

63.7 –

88 55–85

2.01 1.3–1.98

CO LHV (MJ/ Nm3) Bed temp. (° C) ER Feed rate (kg/ h) Bed material Biomass

Table 10. Comparison of different biomass and their gasification properties [24, 29–35].

Cold gas efficiency (%)

CCE (%)

Gas yield (m3 gas/kg biomass)

Gas composition (vol.%)

N2

ENVIRONMENTAL TECHNOLOGY

11

where Hg and Hb are the heating values of producer gas and biomass in terms of MJ/Nm3. CCE can be calculated as given in Equation (7) [22]. It gives the ratio of vol.% elements of carbonaceous species in producer gas in comparison to solid carbon of biomass material.

hcc =

Y (CO% + CH4% + CO2 %) × 12 × 100 %, 22.4 × C% (7)

where CO%, CH4%, CO2% are the volumetric percentage of producer gas, C% is the mass percentage of carbon in biomass feedstock, and Y defines the dry gas yield in Nm3/kg (Figure 6). As shown in Figure 6, cold gas efficiency (η) also known as gasifier efficiency of the BFB gasifier with GNS powder followed increasing trend with increase in ER, though the change were very small between ER 0.31 and 0.33. Carbon conversion efficiency (ηcc), first increased slightly on changing the ER from 0.29 to 0.31 and then decreased slightly on changing the ER from 0.31 to 0.33. Therefore, the optimal CCE value was found to be 91% at ER 0.31 (Table 10). Table 10 gives the comparison of different biomass gasification parameters under different conditions. Gas yield is higher in gasification with alumina sand than the natural sand. Same is the case with GNS powder gasification. Gas yield in GNS powder was in range of 1.84– 2.15 Nm3 gas/kg biomass as shown in Table 6 with natural sand as bed material. Cold gas efficiency is quite low with natural sand in comparison with other bed materials for gasification process. For different biomass with different bed material at different ER, cold gas efficiency is in range of 50–65% and CCE was over 80%. Efficiency of GNS powder gasification also falls in the same range.

4. Conclusions GNS gasification in powder form (mixture of different particles) was successfully carried out at different ER (0.29, 0.31, and 0.33) with atmospheric air as gasification agent. Different particle sizes in GNS powder had a significant effect on gasifier performance such as high temperature rise at smallest ER, high CO2 generation at smallest ER, choking of gas exit pipeline and burning of tar. Very high ER values showed reduction in the HHV of the producer gas because of decreased concentration of CH4 and other light hydrocarbons having large heating values. At higher ER of 0.33, tar and SPM content was high and temperature fluctuation was also high. Very small ER (0.29) was also found

12

D. SINGH ET AL.

unfavorable for GNS gasification as choking and burning of tar occurred very early during the operation. Therefore, the most suitable ER value for GNS powder gasification was found to be 0.31. At this optimal ER, bed temperature was found to be most stable at 714.4°C for long time with 5–10% fluctuations. Bed heating with conventional charcoal proved to be a good way of heating as it eliminated the use of external sources of heating that could result in less electric power consumption.

Acknowledgement The authors thank Sardar Patel Renewable Energy Research Institute, V V Nagar, Anand, Gujarat, India, for making us avail their gasifier facility.

Disclosure statement No potential conflict of interest was reported by the authors.

ORCID Dharminder Singh

http://orcid.org/0000-0003-2451-3081

References [1] Sethumadhavan R. Experimental investigation on gasification parameters of a co-current flow gasifier with a woody and non-woody loose biomass [dissertation]. Chennai: Anna University; 2014. [2] FAOSTAT. Food and Agricultural Organization of the United Nation. Available from: http://www.fao.org/ faostat/en/#home [cited 2016 Nov 25 at 21:00]. [3] Uslu A, Faaij APC, Bergman PCA. Pre-treatment technologies and their effect on international bioenergy supply chain logistics: techno-economic evaluation of torrefaction, fast pyrolysis and palletisation. Energy. 2008;33:1206–1223. doi:10.1016/j.energy.2008.03.007 [4] Gold S, Seuring S. Supply chain and logistics issues of bioenergy production. J Clean Prod. 2011;19:32–42. doi:10. 1016/j.jclepro.2010.08.009 [5] Knoef HAM, Roracher H, Gredinger A, et al. Handbook biomass gasification. Enschede: Biomass technology group; 2001. [6] Natarajan E, Nordin A, Rao AN. Overview of combustion and gasification of rice husk in fluidized bed reactors. Biomass Bioenergy. 1998;14:533–546. [7] Natarajan E, Oman M, Gabra M, et al. Experimental investigation on fluidized bed gasification of rice husk. Biomass Bioenergy. 1998;12:163–169. [8] Natarajan E, Sethputhy SB Gasification of groundnut shell. Energ Sourc Part A. 2015;37:980–986. doi:10.1080/ 15567036.2011.601791 [9] Narvez I, Orio A, Aznar MP, et al. Biomass gasification with air in an atmospheric bubbling fluidized bed: effects of six operational variables on the quality of produced raw gas. Ind Eng Chem Res. 1996;35:2100–2120.

[10] Blasi CD. Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Ind Eng Chem Res. 1996;35:37–46. [11] Singh D, Yadav S, Mohanty P. Study of temperature profile in an air bubbling fluidized bed gasifier for rice husk gasification with conventional charcoal as heating medium. International Conference on Advances in Petroleum, Chemical and Energy Challenges (APCEC); 2017. p. 101–108. [12] Moilanen A, Nasrullah M, Kurukela E. The effect of biomass feedstock type and process parameters on achieving the total carbon conversion in the large scale fluidized bed gasification of biomass. Environ Prog Sust Energy. 2009;28:355–359. doi:10.1002/ep.10396 [13] Hanping C, Bin L, Haiping Y, Guolai Y, et al. Experimental investigation of biomass gasification in a fluidized bed reactor. Energy Fuels. 2008;22:3493–3498. doi:10.1021/ ef800180e [14] Nisamaneenatea J, Atongb D, Sornkadeb P, et al. Fuel gas production from peanut shell waste using a modular downdraft gasifier with the thermal integrated unit. Renew Energ. 2015;79:45–50. doi:10.1016/j.renene.2014.09.046 [15] Miccio F, Moersch O, Spliethoff H, et al. Generation and conversion of carbonaceous fine particles during bubbling fluidised bed gasification of a biomass fuel. Fuel. 1999;78:1473–1481. [16] Wang T, Chang J, Zhu LVJ. Novel catalyst for cracking of biomass Tar. Energy Fuels. 2005;19 (1):22–27. doi:10. 1021/ef030116r [17] Dalai AK, Sasaoka E, Hikita H, et al. Catalytic gasification of sawdust derived from various biomass. Energy Fuels. 2003;7:1456–1463. doi:10.1021/ef030037f [18] Gusta E, Dalai, AK, Uddin MA, et al. Catalytic decomposition of biomass tars with dolomites. Energy Fuels. 2009;23:2264–2272. doi:10.1021/ef8009958 [19] Armesto L, Bahillo A, Veijonen K, et al. Combustion behavior of rice husk in a bubbling fluidized bed. Biomass Bioenergy. 2002;23:171–179. doi:10.1016/S0961-9534 (02)00046-6 [20] Fryda LE, Panopoulos KD, Kakaras E. Agglomeration in fluidised bed gasification of biomass. Powder Technol. 2008;181 (3):307–320. doi:10.1016/j.powtec.2007.05. 022 [21] Rozainee M, Ngo SP, Salema AA, et al. Effect of fluidizing velocity on the combustion of rice husk in a bench-scale fluidized bed combustor for the production of amorphous rice husk ash. Bioresour Technol. 2008;99 (4):703–713. doi:10.1016/j.biortech.2007.01.049 [22] Lahijani P, Zainal ZA. Gasification of palm empty fruit bunch in a bubbling fluidized bed: a performance and agglomeration study. Bioresour Technol. 2011;102 (2):2068–2076. doi:10.1016/j.biortech.2010.09.101 [23] Öhman M, Pommer L, Nordin A. Bed agglomeration characteristics and mechanisms during gasification and combustion of biomass fuels. Energy Fuels. 2005;19 (4):1742–1748. [24] Makwana JP, Joshi AK, Athawale G, et al. Air gasification of rice husk in bubbling fluidized bed reactor with bed heating by conventional charcoal. Bioresour Technol. 2014;178:45–52. doi:10.1016/j.biortech.2014.09.111 [25] Turns SR. An introduction to combustion: concepts and applications. Columbus (OH): McGraw Hill Edu;2002.

ENVIRONMENTAL TECHNOLOGY

[26] Gómez-Barea A, Ollero P, Leckner B. Optimization of char and tar conversion in fluidized bed biomass gasifiers. Fuel. 2013;103:42–52. doi:10.1016/j.fuel.2011.04.042 [27] Xiao R, Zhang M, Jin B, et al. High-temperature air/steamblown gasification of coal in a pressurized spout-fluid bed. Energy Fuels. 2006;20:715–720. [28] Waldheim L, Nilsson T. Heating value of gases from biomass gasification. Report prepared for: IEA bioenergy agreement, Task-20; 2001. [29] Li X, Grace J, Lim C, et al. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy. 2004;26:171–193. doi:10.1016/S0961-9534(03)00084-9 [30] Cao Y, Wang Y, Riley R, et al. A novel biomass air gasification process for producing tar-free higher heating value fuel gas. Fuel Process Technol. 2006;87:343–353. doi:10. 1016/j.fuproc.2005.10.003

13

[31] Kuhe A, Aliyu SJ. Gasification of ‘loose’ groundnut shells in a throatless downdraft gasifier. IJRED. 2015;4 (2):125–130. [32] Mansaray K, Ghaly A, Al-Taweel A, et al. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy. 1999;17:315–332. doi:10.1016/S09619534(99)00046-X [33] Aznar M, Caballero M, Sancho J, et al. Plastic waste elimination by cogasification with coal and biomass in fluidized bed with air in pilot plant. Fuel Process Technol. 2006;87:409–420. doi:10.1016/j.fuproc.2005.09.006 [34] Chaiprasert P, Vitidsant T. Promotion of coconut shell gasification by steam reforming on nickel–dolomite. Appl Sci. 2009;6:332–336. [35] Micci F, Piriou B, Ruoppolo G, et al. Biomass gasification in a catalytic fluidized reactor with beds of different materials. Chem Eng J. 2009;154:369–374.