Syngas production from downdraft gasification of oil palm fronds

Energy 61 (2013) 491e501

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Syngas production from downdraft gasification of oil palm fronds Samson Mekbib Atnaw a, *, Shaharin Anwar Sulaiman a, Suzana Yusup b a b

Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2013 Received in revised form 30 July 2013 Accepted 14 September 2013 Available online 8 October 2013

Study on gasification of OPF (oil palm fronds) is scarce although the biomass constitutes more than 24% of the total oil palm waste. The lack of research related to gasification of oil palm fronds calls for a study on gasification behaviour of the fuel. In this paper the effects of reactor temperature and ER (equivalence ratio) on gas composition, calorific value and gasification efficiency of downdraft gasification of OPF were investigated. The heating value of syngas and the values of cold gas and carbon conversion efficiencies of gasification obtained were found to be comparable with woody biomass. The study showed that oxidation zone temperature above 850
C is favourable for high concentration of the fuel components of syngas CO, H2 and CH4. Average syngas lower heating value of 5.2 MJ/Nm3 was obtained for operation with oxidation zone temperatures above 1000
C, while no significant change in heating value was observed for temperature higher than 1100
C. The average and peak heating values of 4.8 MJ/Nm3 and 5.5 MJ/Nm3, and cold gas efficiency of 70.2% at optimum equivalence ratio of 0.37 showed that OPF have a high potential as a fuel for gasification. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Biomass Gasification Oil palm fronds

1. Introduction There is a huge amount of sustainable and renewable energy resource in Malaysia in the form of biomass waste. Oil palm waste is reported to be the second largest biomass energy potential in the country, next to forest residue [1]. Currently Malaysia accounts for nearly 50% of the world palm oil production [2]. The oil-palm plantation coverage in the country reached about 4.69 million hectares in 2009 [3], with harvestable biomass of 50e70 tons per hectare per year [4]. Considering the total life time of oil palm tree plantation, only 10% by weight is converted to the final product (namely palm oil and kernel oil); while the remainder 90% becomes biomass waste [5] in the form of EFB (empty fruit bunches), kernel shells, POME (palm oil mill effluent), trunks and OPF (oil palm fronds). While EFB and shells are partially utilized in direct burning applications as boiler fuels other wastes from the estates like OPF are simply left in the plantation, for erosion control and soil nutrition. Though OPF accounted for more than 24% of the total oil palm biomass, they were not utilized for energy use [6]. It was estimated that 2.7 million tons of felled fronds were available every year [4] and they are available all year round when the palms are pruned to collect fresh fruit brunches, and also during replanting.

* Corresponding author. Tel.: þ60 192535248. E-mail addresses: [email protected], [email protected] (S.M. Atnaw). 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.09.039

There are few numbers of studies on thermochemical conversion of oil palm biomass most of them focused on pyrolysis and liquefaction of EFB’s, shells and fibres [7e12]. Only very recently since 2011, studies were reported on gasification of EFB [13e15]. The studies investigated the performance of fluidized bed gasification of EFB in terms of syngas heating value, carbon conversion efficiency and cold gas efficiency of gasification and the results showed that EFB give gasification results comparable with sawdust biomass. Whereas studies on gasification and pyrolysis of OPF biomass are very rare [16,17]. Studies on the characterization of OPF and preliminary investigation on their feasibility as a gasifier fuel showed that OPF have a high potential to be used for gasification [18,19]. Ultimate and proximate analysis results showed that the volatile matter content of OPF (85.1%) is comparable with that of beach wood and sugarcane bagasse (82.50 and 85.61% respectively) while it is higher compared to rice husk and coconut husk biomass [4,20]. This high volatile matter content of OPF implies its high reactivity and significant potential to be used as a fuel in thermochemical energy conversion like pyrolysis and gasification. The chemical composition of OPF consists of 49.8% cellulose, 23.5% hemicellulose, 20.5% lignin and 2.4% ash [6,21]. Compared to other oil palm biomass types like EFB, shells and trunks OPF have the highest cellulose and lowest lignin and ash contents. An experimental investigation carried out by Hanaoka et al. [22] suggested that gasification conversion of cellulose, hemicellulose and lignin is 97.9%, 92.2%, and 52.8% on carbon basis, respectively. Lignin is the most difficult component in thermal decomposition and accounts

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for most of the unconverted matter in ash and char due to its high fixed carbon content [10]. Therefore, high cellulose composition and low lignin and ash compositions of OPF imply its high potential as a gasification fuel. Studies showed that the use of syngas from gasification, in a combined cycle with internal combustion engines and turbines, for power generation has the advantage of high efficiency (up to 45%) as compared to the lower efficiency (25e35%) obtained from direct combustion of solid biomass fuels [23e25]. However, the relative composition of the component gases produced via gasification (CO, H2, CH4 and CO2) and energy value of the syngas are highly dependent on the type of biomass material [26,27]. This is due to the wide variations in the key characteristics of different biomass materials in terms of chemical and physical properties, lignocellulosic composition, morphological characteristics, energy density, ash content, mass/bulk density, ash melting behaviour, charring behaviour etc [28,29]. Moreover, the complex heterogeneous solidegas and homogenous gasegas chemical reactions taking place during gasification are highly dependent on operating conditions like ER (equivalence ratio) of gasification, reactor temperature and the gasifying medium used (atmospheric air, pure oxygen, steam etc.) [13,24,30,31]. The objectives of this work were to study the performance of OPF as a fuel for downdraft gasification and investigate the influences of the main operating parameters; reactor temperature and equivalence ratio on produced syngas composition and calorific value. The study was conducted by measuring temperature profiles, syngas composition, and its calorific value. In addition, investigation of energy efficiency of gasification and carbon conversion efficiency were carried out. The heating value and composition of syngas produced from downdraft gasification of OPF were compared with results for woody biomass materials from literature. In addition, the results from an equilibrium model of gasification were presented in comparison with experimental data. The equilibrium model was developed with ASPEN PLUS process simulator software based on the method of minimization of the Gibbs free energy. The minimization of Gibbs free energy approach was previously used by different authors to study gasification of different biomass materials [32,33]. The detailed account of the equilibrium model development and the simulation results was given in previous publications [34,35]. 2. Methodology 2.1. Material Newly pruned and green OPF feedstock used for the study was collected from oil palm plantation site located at Bota Kanan, Seri Iskandar, Malaysia. The scope of feedstock treatment in this study includes cutting/chipping to proper size that is suitable for downdraft gasification, and drying under the sun. The feedstock used for the gasification was of average dimensions 20 mm square with 10 mm thickness. The proximate and ultimate analysis results of OPF are given in Table 1 [4]. 2.2. Experimental setup and procedures The experimental rig used was a batch feed downdraft gasifier. The operation was carried out at low pressure with atmospheric air as gasification medium. The schematic of the experimental setup is shown in Fig. 1. The design capacity of the lab scale downdraft gasifier (4) used was of 50 kW of thermal output, with cylindrical reactor of height 1000 mm and diameter 400 mm. A necking or throat of slope angle of 70
was provided near the grate of the gasifier in order to ensure smooth down flow of the biomass by

Table 1 Proximate and ultimate analysis of OPF [4]. Proximate analysis (%)a Volatile matter (VM) Fixed carbon (FC) Ash Ultimate analysis (%)a C H N O (by diff.) LHV (MJ/kg) a

85.1 11.5 3.4 42.4 5.8 3.6 48.2 15.72

Dry weight basis.

gravity. The details on the reactor design are given in the work of Moni et al. [36]. The experimental setup consisted of a Japan made vortex blower (1) of 950 L per minute (lpm) nominal flow rate and working pressure of 16 kPa, for the supply of inlet air to the gasifier. Various syngas conditioning units were provided downstream of the gasifier for the cooling and cleaning of syngas, which include; a cyclone (7), cooling heat exchanger (8) and oil bath filter (9) which were provided before the cleaned gas sampling point (9). In addition, as shown in Fig. 1 a number of gas flare points (5) were provided on the outlet piping in order to check the combustibility of produced syngas and to burn poisonous gases like CO before being released to the atmosphere. The flow rate of inlet air was measured using a VFC (variable flow controller) Series Dwyer rotameter (2), which has an accuracy of 5%. Monitoring of the gasifier temperature profile was carried out using five Type N thermocouples (depicted by T1 to T5 in Fig. 1) which were mounted in the middle section along the gasifier wall, at 200 mm interval. The accuracy of thermocouples used was 2.5
C. The temperature readings were collected using a USB (universal serial bus) based temperature data logger (11) at every minute interval and the readings were stored in a computer (12). The amount of OPF fed into the gasifier was measured using a weigh scale with initial feed of 12 kg maintained for all gasification experiments. GC (Gas chromatography) unit was used to measure the composition of syngas produced by gasification in terms of volume concentration. The composition of the main component gases CO, CO2, CH4 and H2 was directly measured making use of a multipurpose EMERSON X2GP gas analyser. The measurement principles used by the GC are infrared measurement (IR), ultraviolet measurement (UV) and thermal conductivity measurement (TCM). The IR and UV measurement methods which used the principle of infrared/ultraviolet light absorbed by the sample gas were used to measure concentrations of CO, CO2 and CH4. The wave length of IR/ UV light characterized the gas components whereas the intensity of absorption was used for measuring concentration. The TCM method which operates based on the unique thermal conductivity characteristic of the gas was principally used to measure H2 concentration. The measuring accuracy of the GC unit used is 2%. Besides temperature and gas composition data, the ash, char and liquid tar produced were collected and measured at the end of each experimental run using a weighing scale, accounting for the total material balance of the system. The char and ash were collected from the grate and the ash box, respectively. The amount of liquid tar produced was quantified by draining the cyclone and gas outlet pipes immediately after each gasification run. In order to determine the carbon conversion efficiency of the system, ultimate analysis was conducted on the char and solid tar by using a Leco CHNS-932 unit. The effect of equivalence ratio on the gasifier performance was studied by varying the inlet air flow rate from 140 to 300 lpm, which resulted in a range of equivalence ratio between 0.27 and 0.59. The moisture content of the feedstock was kept at 16  2% on wet basis for all of the gasification experiments.

S.M. Atnaw et al. / Energy 61 (2013) 491e501

12

5

493

5

Feed

11

5

T5

6

T4

2

4

T3

7

8

10

T2

1

3

9

Grate

T1

Producer gas outlet

Thermocouples

Fig. 1. Schematic drawing of experimental setup (1) air blower, (2) rotameter, (3) air distribution line, (4) downdraft gasifier, (5) gas flare points, (6) raw gas sampling point, (7) cyclone, (8) cooling heat exchanger, (9) oil bath filter, (10) clean gas sampling point, (11) temperature data logger, (12) computer.

2.3. Theory and calculations The mass and species balances were calculated based on the mass of the by-products of the system (char, ash, and liquid tar) which were collected and measured at the end of each of the gasification run. The mass conversion efficiency of the gasifier was also determined based on the mass balance. The ultimate analysis results for char and solid tar produced for three selected gasification runs (with equivalence ratio of 0.27, 0.39 and 0.51) are shown in Table 2. The carbon conversion efficiency of the gasification process was calculated by Ref. [37]:

hC ¼ 1 

Cg Cf

(1)

where Cg is total rate of carbon in the outlet stream and Cf is the total rate of carbon in the inlet stream. The total amount of carbon in the outlet stream from the system was determined considering the rate of production of char and its carbon fraction from the ultimate analysis. The total amount of carbon in the input stream was determined considering the rate of consumption of feed and its carbon fraction from ultimate analysis of the biomass. The producer gas flow rate (Nm3/hr) was estimated using the Nitrogen balance method given by Refs. [38,39]:

Qg ¼

Qa  0:79 WC  N2 %

(2)

where Qa is inlet air flow rate (Nm3/hr), WC is flow rate of biomass (kg/hr) and N2% is the volume percentage of N2 in dry product gas. The cold gas efficiency of gasification was calculated using [37]:

Q H

g hth ¼ g  100 _ f  HS m

(3)

Table 2 Ultimate analysis of char and solid tar. Equivalence ratio (ER)

a

Dry weight basis.

s2xi ¼ B2xi þ Pxi2

(4)

For temperature measurements done using the Type N thermocouples of accuracy 1.5
C (0.5%), considering the random uncertainty arising from operation as 1.5% [37], the total uncertainty in temperature measurement, sT could be estimated from Eqn. (4) to be 1.6%. Similarly for the inlet air flow rate taking the 5% systematic uncertainty of the rotameter unit and taking the uncertainty generated from the reading as 1% [37], the total uncertainty was calculated to be 5.1%. The measurement of gas composition was done using a gas chromatograph with an accuracy of 2%. Though high value of random uncertainty of 5% is typically assumed for the case of manual sampling and injection, a value of 1% was taken for the current work considering the use of online sampling system. Hence, the total uncertainty in measuring gas composition was calculated using Eqn. (4) as 2.2%.

Ultimate analysis of chara C

0.27 0.39 0.51

where Qg is product gas flow rate (Nm3/hr), Hg is lower heating _ f is biomass feed rate (kg/hr) and HS is value of the gas (MJ/Nm3), m lower heating value of biomass (MJ/kg). Uncertainty analysis: As uncertainty analysis is a crucial part of reporting any experimental data, the uncertainties inherent in the various instrumentations used in the current study and those involved during the measurement process were investigated. The uncertainties in the measured parameters (temperature, air flow rate and gas composition) were determined considering both the systemic and random uncertainties involved in measurement taking. The cause of systemic errors could be due to imperfect calibration or changes in the environmental condition that affect the measurement. Such systemic errors are inherent to the device and the measurement method used. Hence they cannot be removed by repeating or averaging large number of results. The uncertainty values provided in the manufacturer manuals of the measuring equipment’s were used to estimate the systemic uncertainty. Random errors in experimental measurements arise from unknown and unpredictable changes in the experiment. Such random variations could be caused inside the measuring equipment, or the environment. The total measurement uncertainty was calculated based on Eqn. (4) considering the systematic uncertainty, Bxi and the random uncertainty Pxi of variable Xi [37,40]:

H

N

56.743 2.522 1.246 63.537 2.459 0.771 58.497 1.996 0.731 Ultimate analysis of solid tar 41.693 4.149 1.907

S 0.045 0.156 0.078 0.532

3. Results and discussions 3.1. TGA analysis The pyrolysis behaviour and thermal stability of OPF were characterized by using a thermogravimetric analyser (TGA) unit.

S.M. Atnaw et al. / Energy 61 (2013) 491e501

110 100 90 80 70 60 50 40 30 20 10 0

45 Weight loss

40

Weight loss rate

35 30 25 20 15 10

Weight loss rate (wt %/oC)

Weight (%)

494

5 0

100

200

300

400 500 600 700 Temperature (oC)

800

0 900 1000

Fig. 2. Weight loss and derivative weight loss curves for pyrolysis of OPF.

Three repetitive weight loss curves were obtained for a single sample in order to check the reproducibility of the results. Continuous flow of N2 at a rate of 100 ml/min, and heating rate of 50
C/min were used during the entire duration of the experiment. Shown in Fig. 2 is the variation of percent weight loss and derivative weight loss with temperature for pyrolysis of OPF samples with particle sizes of 250e500 mm. From the weight loss curve in Fig. 2 it can be seen that the weight loss was minimal for pyrolysis temperature below 270
C. This minimal weight loss of less than 15% at lower temperature range could mainly be attributed to moisture evaporation through drying. The most significant rate of weight loss of about 55% occurred in the temperature range between 270 and 400
C, as can be seen from the weight loss and the peaks of the weight loss rate curve in Fig. 2. Previous studies showed that the main reaction stage of biomass pyrolysis where up to 80% of weight loss occurs is at similar range of pyrolysis temperature as obtained in the current experiment [41,42]. Generally, the evolution of the volatile compounds during pyrolysis takes place in the sequence of hemicellulose, cellulose followed by lignin. Studies on pyrolysis behaviour of biomass materials indicated that decomposition of hemicellulose and cellulose take place around 220e300
C, and 300e340
C respectively, while decomposition of lignin took place almost the entire duration between temperature ranges of 150e 900
C usually without showing any significant peak [10,22]. This implies that the peak decomposition shown in Fig. 2 for pyrolysis temperature between 270 and 400
C could mainly be attributed to decomposition of the less stable components cellulose and hemicellulose. The ash and char residue of OPF remaining at the end of the pyrolysis process was minimal as expected from its lower ash and fixed carbon composition.

3.1.1. Kinetic analysis on pyrolysis of OPF One of the important parameters in chemical reactions that indicate the reactivity of a given fuel is the activation energy. The activation energy (E) which has the units of kilojoule per mole can be defined as the minimum amount of energy needed to start a chemical reaction. TGA data from pyrolysis decomposition at different heating rate with continuous N2 supply at a rate of 100 ml/ min was used to determine the activation energy, E of the OPF samples. Shown in Fig. 3 is the percent weight loss curves obtained for heating rates of 20
C/min, 50
C/min and 100
C/min. The model free method of iso-conversional method of estimating the activation energy was used. The value of activation energy (E) was estimated as a function of a, which is the extent of conversion given by the relation [8,43]:

a ¼

wo  w wo  wf

(5)

where, wo, w and wf stand for the initial, actual and final weight percentage of the sample respectively from percent weight loss data of TGA analysis. The simplified kinetic rate equation developed by Flynn-WalleOzawa was adopted in the kinetic analysis, which is given by the relation [44,45]:

lnðbÞ ¼ ln

 AE E  5:311  1:052 RgðaÞ RT



(6)

where, b is the heating rate, A is the Arrhenius parameter, R is the universal gas constant and g(a) is a mathematically defined integral reaction model, and T stands for temperature. Shown in Fig. 4 is the Arrhenius plot (ln(b) vs. 1/T) for pyrolysis of OPF corresponding to each a value considered (20%, 30%, 40%, 50% and 60%). The activation energy, E for the different values of a were computed by equating the slope of the plot ln(b) versus 1/T, with the constant values of the last terms of Eqn. (6), i.e. 1.052*E/R. After taking the average of E obtained corresponding to each a value, activation energy, E for pyrolysis of OPF was calculated to be 59.2 kJ/mol. This value of E obtained was found to be lower compared to other biomass types; corn stalk (66 kJ/mol), wheat straw (70 kJ/mol) and wood chips (85 kJ/mol) [41]. This lower value of E, for pyrolysis of OPF could most probably be due to its higher volatile matter composition of 85%, which makes it a highly reactive fuel. This low activation energy and high reactivity of OPF is a major advantage for its application as energy fuel in thermochemical conversion. 3.1.2. Kinetic analysis on combustion of OPF To study the combustion behaviour of OPF, TGA analysis of samples with size from 250 to 500 mm was carried out at four

90

20 K/min

80

50 K/min

70

0.6

100 K/min

60

ln[ß(K/sec)]

Weight loss (%)

100

50 40

0.4

20%

0.2

30%

0.0

40%

-0.2

50%

-0.4

30

-0.6

20

-0.8

10

-1.0

60%

-1.2

0 0

100

200

300

400 500 600 Temperature (oC)

700

800

900

1000

Fig. 3. Percentage weight loss with temperature for pyrolysis of OPF at different heating rate.

-1.4 0.00250

0.00275

0.00300 0.00325 1/T (K-1)

0.00350

0.00375

Fig. 4. Arrhenius plots for pyrolysis of OPF at various conversion rates.

S.M. Atnaw et al. / Energy 61 (2013) 491e501

T) for combustion of the OPF samples at different conversion extent (a) values. Following the same procedure discussed in Section 3.1.1, the average activation energy (E) for combustion of OPF was estimated to be 54.3 kJ/mol. As expected this value is lower than the activation energy for pyrolysis of the fuel due to introduction of oxygen which further facilitates the conversion process.

10 K/min

80

20 K/min

70

50 K/min

60

100 K/min

50

3.2. Gasification results

30 20 10 0 0

100

200

300

400 500 Temperature (oC)

600

700

800

900

Fig. 5. Percent weight loss curve with temperature for combustion of OPF.

different heating rates with the supply of oxygen at a rate of 10 ml/ min to the reactor. Shown in Fig. 5, is the percent weight loss with temperature for four different heating rates of 10, 20, 50 and 100
C/min. The TG curves for oxidation of OPF at different heating rates were found to be similar and in close range with each other. The weight loss at lower temperature was mainly due to pyrolysis, as the combustion reaction is expected to take place at higher temperature. As shown in Fig. 5 more than 80% weight loss occurred at temperatures below 600
C implying that OPF decomposition occurs mostly by pyrolysis rather than combustion. A study by Hanaoka et al. [22], showed that conversion of cellulose to gas occurs mainly due to pyrolysis, while hemicellulose and lignin are mainly decomposed via combustion. Hence, the higher conversion of OPF mainly by pyrolysis at lower temperature is expected considering the high cellulose and low lignin composition of the fuel. The decrease in the weight loss curve shown in Fig. 5 for higher heating rate of 100
C/min for temperature between 400 and 600
C could be most probably due to Fast pyrolysis where more than 75% of the solid fuel vaporises at high heating rate and short residence time. The conditions of fast pyrolysis include high heating rate, temperature of about 500
C and short residence time [46e48]. Hence the reason for the decrease in weight loss curve for temperature near 500
C at higher heating rate could be attributed to occurrence of fast pyrolysis and subsequent combustion of the volatiles released with air. The model free (iso-conversional method) was once again used to estimate the activation energy for combustion of OPF. Shown in Fig. 6 is the Arrhenius plot (ln(b) vs. 1/

3.2.1. Steady operation Experimental results for batch fed operation of a downdraft gasifier are presented in this paper. Unlike continuous feed operation the results for temperature, syngas composition and heating value show variation with operation duration for the case of batch fed operation due to sharp increase during startup and decrease near the end of the operation due to depletion of the fuel. Shown in Fig. 7(aec) are the variations of reactor temperature, concentration

1400

Reactor Temperature (oC)

40

T1

1200

T2

T3

T4

1000 800 600 400 200 0 0

10 20 30 40 50 60 70 80 90 100 Operation time (min)

30

Gas composition (% Vol.)

Weight loss (%)

100 90

495

CO 25

CH4 CH

H2 H

20 15 10 5 0 0

10

20

30 40 50 60 70 80 Operation time (min)

90 100

1.0 20%

7

30%

6

0.0

40%

-0.5

50% 60%

-1.0

5 4 3 2 1

-1.5

0

-2.0 -2.5 0.00230

LHV (MJ/Nm3)

ln[ß(K/sec]

0.5

0 0.00255

0.00280

0.00305

0.00330

10 20 30 40 50 60 70 80 90 100 Operation time (min)

0.00355

1/T (K-1) Fig. 6. Arrhenius plots for combustion of OPF at various conversion rates.

Fig. 7. Variation of gasification data with operation time (a) temperature profile, (b) syngas composition, (c) heating value.

S.M. Atnaw et al. / Energy 61 (2013) 491e501

35 30 25 20 15 10 5 0

CO2 composition ( %)

CO composition (%)

496

25 20 15 10 5 0

500 600 700 800 900 1000 1100 1200 Oxidation zone temperature (oC)

500 600 700 800 900 1000 1100 1200

Oxidation zone temperature (oC)

(b) CH4 composition ( %)

H2 composition (%)

(a) 20 15 10 5 0 500

600

700

800

900 1000 1100 1200

5 4 3 2 1 0 500 600 700 800 900 1000 1100 1200

Oxidation zone temperature (oC)

Oxidation zone temperature (oC)

(c)

(d)

Fig. 8. Variation of syngas composition with oxidation zone temperature on dry gas basis (a) CO composition in syngas, (b) CO2 composition in syngas, (c) H2 composition in syngas (d) CH4 composition in syngas.

of fuel components of syngas and heating value for a typical gasification experiment. The dynamic temperature profile was developed by measuring the temperature values of each zone of gasification at every minute interval over the entire operation duration. From Fig. 7(a) it can be seen that the oxidation zone temperature increased from room temperature to a value above 800
C within the first 5 min of operation. After the 10th minute of operation it had been observed that the temperature of the reactor was found to stabilize till the 70th minute. The decrease in temperature after the 70th minute of operation was caused due to the batch fed operation, whereby most of the fuel will be consumed near the end of the experimental run. Therefore, the time interval between the 10th and 70th minutes of operation could be taken as the stable duration of operation. Similarly the relative concentration the fuel component gases (CO, CH4 and H2) and the heating value of syngas (Fig. 7 (b) and (c) respectively) were found to be relatively stable for the duration between the 10th and 70th minutes of operation. The variations at startup and near the end of the operation which were caused due to batch operation were avoided and the average values for the steady operation between 10th and 70th minute were reported in this paper for temperature, syngas composition and heating value. Similar approach was followed in the work of Plis and Wilk [49]. Hence the results from the current study could be reasonably extended to approximate the case of continuous feed operation. 3.2.2. Effect of temperature on syngas composition and heating value The influence of oxidation zone temperature on composition of the main syngas components CO, CO2, CH4 and H2 was studied by measuring the syngas composition corresponding to a given zone temperature. Shown in Fig. 8(aed) are variations of measured gas composition with reactor temperature for downdraft gasification of OPF. The data points for the composition of gas components (CO, CO2, CH4 and H2) in Fig. 8 are taken corresponding to the temperature values recorded during the gasifier operation. The scattering

and variation of the experimental data for composition of various gas components in Fig. 8 could be attributed to the typical temperature variation generally observed in fixed bed gasifiers. One of the reasons for the small variations and temperature peaks is due to the difference in the temperature of the solid and gaseous volatiles released during the reactions. Whenever the thermocouple is exposed to the red hot solid particles in the fuel bed undergoing combustion it reads peak temperature values. In addition the complex interaction between the endothermic and exothermic, homogenous (gasegas) and heterogenic (gasesolid) reactions causes variation in temperature with time. These sudden variations in temperature could be the causes for the scattering of the readings in gas composition taken at various temperatures. Though there are variations and scattered data points, the results were found to estimate the trend of the various gas compositions and the optimum range of temperature that result in higher concentration of CO, H2 and CH4. As can be seen from Fig. 8(a) the CO concentration in syngas increased from a range of about 5%e28% (Vol. %) for increase in oxidation zone temperature from 500 to 1200
C. Generally higher temperature is known to favour the reactants in exothermic reactions and the products in endothermic reactions [24,50]. Hence, the significant increase in CO composition at higher temperature could be explained considering the endothermic reduction reactions in Eqns. (7)e(9), which will have a better chance of reaching equilibrium at higher temperature [50e52]:

C þ H2 O þ 131:4 kJ=mol4CO þ H2

(7)

C þ CO2 þ 176:6 kJ=mol42CO

(8)

CH4 þ H2 O þ 206:3 kJ=mol4CO þ 3H2

(9)

The concentration of H2 in syngas increased for oxidation zone temperature range of 500e850
C, and it slightly drops at higher temperature as shown in Fig. 8(c). Whereas, the concentration of

S.M. Atnaw et al. / Energy 61 (2013) 491e501

LHV of syngas (MJ/Nm3)

6 5 4 3 2 1 0 500

600 700 800 900 1000 1100 1200 Oxidation zone temperature (oC)

Fig. 9. Variation of heating value of syngas with oxidation zone temperature.

CO2 decreased from a value above 15% to a range of 7%, for increase in oxidation zone temperature from 500 to 1200
C as shown in Fig. 8(b). The decrease in the relative concentration of CO2 could be explained in terms of the increase in the production of other permanent gases mainly CO and H2 with temperature. In addition, part of the CO2 produced from initial oxidation reaction given in Eqn. (10) would be consumed in the endothermic reduction reaction (Eqn. (8)).

C þ O2 4CO2 þ 393:8 KJ=mol

(10)

30

slight decrease for higher temperature above 1000
C, which could be attributed to its consumption by the methane reforming reaction (Eqn. (9)). From the results in Fig. 8(aed) it can be concluded that oxidation zone temperature above 850
C was found to be favourable for higher concentration of the fuel components of syngas CO, H2 and CH4. The heating value of the producer gas is highly important for its application as a fuel. The LHV (lower heating value) of syngas produced from downdraft gasification of OPF was calculated from measured gas composition values recorded at different oxidation zone temperatures. Shown in Fig. 9 is variation of heating value of syngas with oxidation zone temperature. The heating value showed a sharp increase with reactor temperature for the range between 500 and 900
C. Average syngas lower heating value of 5.2 MJ/Nm3 was obtained for operation with oxidation zone temperatures above 1000
C while no significant change in heating value was observed for temperature higher than 1100
C as shown in Fig. 9. 3.2.3. Effect of equivalence ratio on syngas composition and calorific value Equivalence ratio of gasification is a parameter that quantifies the amount of air/oxygen per unit mass of fuel, as compared to the theoretical amount of air/oxygen needed for complete combustion. Hence the optimum equivalence ratio that favours gasification (incomplete combustion) resulting in combustible gases like CO, rather than the case of complete combustion with excess air supply that mainly produces CO2 need to be determined [24]. In this study a number of gasification experiments were done by varying the equivalence ratio from 0.27 to 0.59, while keeping other parameters like amount of feed and moisture content of fuel constant. Fig. 10(aed) shows variations in concentration of the four major product gases from gasification (CO, CO2, H2 and CH4) with equivalence ratio. The standard deviations of the compositions of the

CO2 composition (% Vol.)

CO composition (% Vol.)

Similar results were reported in literature with increasing trends of CO and H2 in syngas and decreasing trend in the concentration of CO2 [27,50]. The concentration of CH4 in syngas was found to be in trace amount at lower temperature and it stayed at an average of about 1% for temperature range between 700 and 900
C, as shown in Fig. 8(d). The CH4 concentration showed a

25 20 15 10 5 0 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Equivalence ratio (ER)

497

18 16 14 12 10 8 6 4 2 0 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Equivalence ratio (ER)

(a)

(b)

2.5

H2 composition (% Vol.)

CH4 composition (% Vol.)

12

2.0 1.5 1.0

10 8 6 4

0.5

2

0.0 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

0 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

Equivalence ratio (ER)

Equivalence ratio (ER)

(c)

(d)

Fig. 10. Variation of syngas composition with equivalence ratio on dry gas basis (a) CO composition (b) CO2 composition (c) H2 composition (d) CH4 composition.

498

S.M. Atnaw et al. / Energy 61 (2013) 491e501 Table 4 Mean errors between predicted and experimental values.

80 70

Predicted parameters

CO

CO2

H2

N2

60

Mean error

0.14

0.32

0.27

0.07

50 40 30 20 10 0 0.25

0.30

0.35 0.40 0.45 0.50 Equivalence ratio (ER)

0.55

0.60

Fig. 11. Variation of N2 composition in product gas with equivalence ratio of gasification.

gases recorded over the total operation time for each experiment carried out at the different equivalence ratio values are shown graphically by the error bars in Fig. 10. Considering the maximum deviation value in CO composition (2.9% vol.) for gasification at equivalence ratio of 0.35, for 95% confidence interval the composition of CO varied from 16.2 to 27.8%. This shows that the variation in composition over operation time of the run was acceptable as the expected range of CO composition for biomass gasification commonly vary between 15 and 30% [53]. Shown in Fig. 10(a) is variation of CO composition in syngas with equivalence ratio. At lower values of equivalence ratio CO composition was found to be low and it stars to rise until the optimum equivalence ratio value of 0.37 and later drops for higher equivalence ratio values as shown in Fig. 10(a). The maximum CO and H2 average composition values of 22.2 and 9.6% respectively were obtained for gasification at ER values of 0.37, where the relative concentration of CO2 was found to be minimum at a value of 8.2%. CO2 also showed an inverse relation with CO as the reactions that produce those gases are competing for the same reactant namely carbon. The concentration of CO2 is generally expected to be minimum at the optimum equivalence ratio range between 0.35 and 0.4 and increased at higher equivalence ratio. However, the experimental results showed a slight decrease in CO2 concentration at higher equivalence ratio of 0.59. This decrease in CO2 concentration could be attributed to the higher dilution of the product gas with N2 because of the higher air flow rate for operation at higher equivalence ratio. Generally, CO2 composition was found to increase with increase in equivalence ratio of gasification as shown in Fig. 10(b). As shown in Fig. 10(c), the CH4 content increased with equivalence ratio up to equivalence ratio of about 0.45 and it starts to decrease for higher values. Similar trend with that of CH4 was observed for H2 composition as shown in Fig. 10(d). Higher H2 and CH4 composition values of 9.6% and 1.4% were obtained corresponding to equivalence ratio of 0.37 and 0.39 respectively. In addition to the composition of the major four gases in the product gas, the variation of N2 with equivalence ratio of gasification is Table 3 Comparison of predicted and experimental gas composition at different equivalence ratio of gasification. Equivalence ratio

0.35 0.37 0.39 0.41 0.51 0.59

CO % vol.

H2 % vol.

CO2 % vol.

N2 % vol.

P*

E*

P*

E*

P*

E*

P*

E*

20.2 18.8 17.8 16.7 11.41 7.7

17.9 20.7 15.8 16.2 13.7 10.0

13.0 12.0 11.2 10.4 6.3 3.4

10.7 11.9 9.8 9.2 9.0 6.9

15.8 16.4 16.8 17.2 19.2 20.7

12.3 11.4 14.0 13.9 15.5 14.5

50.6 52.5 54.0 55.4 62.8 68.1

58.0 56.7 58.9 59.6 60.1 68.0

shown in Fig. 11. As shown in Fig. 11, the composition of N2 in the product gas was higher at lower equivalence ratio values, because of lower generation of other gas components at lower values of equivalence ratio. The N2 content is shown to decrease up to equivalence ratio value of about 0.45 as relative concentration of other component gases (CO, CO2, CH4 and H2) increase in the product gas mixture. The N2 content increases with equivalence ratio for values above 0.45, probably due to higher air flow resulting in greater dilution of product gas with N2 and the reduction of other component gases generation rate at higher values of equivalence ratio. Comparison between model predictions and experimental date was carried out for the range of equivalence ratio between 0.35 and 0.59. This range was selected as the theoretical minimum air fuel ratio and corresponding equivalence ratio of gasification for OPF biomass was calculated to be 1.71 and 0.36 respectively [17]. Shown in Table 3 is comparison of equilibrium model prediction and typical experimental data for CO, CO2, H2 and N2 composition in product gas for equivalence ratio between 0.35 and 0.59. The mean error values between prediction and experimental data calculated based on the sum square deviation method for composition of each gas component is given in Table 4. The equilibrium model reasonably predicted the composition of CO, CO2 and N2 as compared to the experimental results as shown in Table 3. H2 concentration predicted by the model was found to be slightly higher than experimental data for equivalence ration between 0.35 and 0.41 as shown in Table 3. Such higher prediction of H2 composition in syngas from equilibrium model of gasification was reported in other studies as well [32,54,55]. In addition, the equilibrium model of gasification predicted the composition of CH4 in syngas to be in trace amount while experimental results in the range of 0.66e1.83% were obtained for the range of equivalence ratio considered. Similarly prediction of only trace amount of CH4 concentration in syngas [54,55] was reported by other authors using the equilibrium model of gasification. Equilibrium model of downdraft gasification developed by Zainal et al. [54] predicted only trace amount of CH4 (0.64) as compared to experimental values in the range of 1.58% reported in the study. Shown in Fig. 12 is comparison of predicted and experimental values of LHV of syngas for the range of equivalence ratio between 0.35 and 0.59. For the practically optimum range of equivalence

6

LHV of Syngas (MJ/Nm3)

N2 composition (% Vol.)

90

5 4 3 2 1 0 0.32

LHV Predicted LHV Experimental 0.37

0.42 0.47 0.52 Equivalence ratio (ER)

0.57

Fig. 12. Comparison of predicted and experimental heating value of syngas at different equivalence ratio.

S.M. Atnaw et al. / Energy 61 (2013) 491e501

Biomass type

Dry gas composition (% vol.) CO

CO2

H2

CH4

LHV (MJ/Nm3)

References

Current study [55] [56] [31] [37]

OPF

25.3

8.2

9.6

1.2

4.8

Coconut shells Hazelnut shells Furniture wood Woody biomass

21.3 19.6 24.0 20.3

11.8 10.0 14.7 8.3

13.5 12.7 14.2 17.8

1.5 2.0 2.0 1.7

4.9 4.7 5.5 5.3

ratio from 0.36 to 0.45, the heating value estimated based on predicted syngas composition decreased from 5.5 MJ/Nm3 to 3.6 MJ/ Nm3. It can be seen from the figure that the predicted and experimental results are in good agreement, and the equilibrium model predicted the syngas heating value with a satisfactory level of accuracy with a mean error of only 0.13. Table 5 shows comparison of gas composition and heating value for gasification of OPF obtained in the current study (for equivalence ratio of 0.37), with optimum values reported in the literature for coconut shells [56], hazelnut shells [57], furniture wood [31] and woody biomass [37], of which all were the result of downdraft gasification process. The gas compositions obtained from gasification of OPF are shown in Table 3 to be comparable with those of other biomass materials reported for optimum operating conditions. Downdraft gasification of OPF resulted in the highest CO content and the lowest CO2 content in comparison to other biomass materials. On the other hand slightly lower H2 and CH4 contents are shown for OPF. The heating value of syngas from gasification of OPF is shown in Table 5 to be comparable with coconut shells but higher than that of hazelnut shells. The highest average lower heating value of syngas obtained from gasification of OPF was 4.83 MJ/Nm3 for gasification experiment with equivalence ratio of 0.37. Peak lower heating value of 5.48 MJ/Nm3 was recorded for the gasification run with the optimum equivalence ratio of 0.37. A comparable average heating value of 5.62 MJ/Nm3 was reported in the work of Zainal et al. [31] for operation at optimal equivalence ratio value of 0.38 for gasification of woody biomass in a downdraft gasifier. 3.2.4. Effect of equivalence ratio on gasification and carbon conversion efficiencies The cold gas efficiency accounts for percentage of the energy output of the gasifier (in terms of heating value of syngas) to the 6 Gasification efficiency

80

LHV

5

70 60

4

50 3 40 30

2

20

LHV of syngas (MJ/Nm3)

Gasification efficiency (%)

90

1 10 0

0 0.27

0.31

0.35 0.37 0.39 0.41 Equivalence Ratio, ER

0.51

0.59

Fig. 13. Effect of equivalence ratio on gasification efficiency and calorific value of syngas.

total energy input as heating value of biomass. Hence, the cold gas efficiency determines the performance of energy conversion during the gasification process. Shown in Fig. 13 is variation of cold gas efficiency and heating value of syngas with equivalence ratio. Both the calorific value and the resulting gasification efficiency shown in Fig. 13 is the lowest at the lowest equivalence ratio value of 0.27, but increases sharply up to equivalence ratio of 0.37 at which heating value is at peak. The cold gas efficiency is also shown to reach the maximum value at equivalence ratio of 0.37. Both the cold gas efficiency and heating value of syngas showed a decreasing trend for equivalence ratio range of 0.37e0.51. The result showed that both the heating value and resulting gasification efficiency decreased sharply for ER values lower than 0.35 and higher than 0.51. From the results it can be deduced that gasification of OPF under such conditions would result in acceptable cold gas efficiency and heating value of syngas at equivalence ratio range of 0.35 and 0.51. The optimum equivalence ratio value is shown to be 0.37, with corresponding maximum cold gas efficiency of 70.2% and average heating value of 4.8 MJ/Nm3. At optimum equivalence ratio (0.37) the peak heating value is shown to be 5.5 MJ/Nm3. The results obtained for gasification of OPF are found to be within the range of those reported in literature for downdraft gasification of woody biomass; i.e. cold gas efficiency of 47e88.6% and heating value of 4.2e5.6 MJ/Nm3 [31,37]. Lower values of 50e60% cold gas efficiency and heating values of 4.2e5.8 MJ/Nm3 were reported for downdraft gasification of rice husk [58,59]. Hence the results showed that OPF have a good potential to be used as a gasification fuel and could produce syngas of acceptable heating value with cold gas efficiency comparable to that of woody biomass. The mass conversion efficiency is an important parameter that determines the percentage of solid feed converted to permanent gases. High mass conversion efficiency signifies lower generation of by products like tar, char and ash. The carbon conversion efficiency indicates how much of the initial carbon content of fuel is converted to gaseous products, and thus affecting the amount of unconverted carbon in char and tar by products from gasification. Shown in Fig. 14 is variation of mass conversion and carbon conversion efficiencies with equivalence ratio. Both mass and carbon conversion efficiencies are shown in Fig. 14 to be minimal at lower equivalence ratio of 0.27, and increase sharply up to equivalence ratio of 0.35. The minimum values of mass and carbon conversion efficiencies observed for equivalence ratio around 0.4 could be attributed to variation in flow characteristics of the fuel bed, and or as a result of ash and particulates carry over from the grate. For equivalence ratio values between 0.35 and 0.59, average mass and carbon conversion efficiencies of 92% and 93% respectively were obtained. These values of mass and carbon conversion efficiencies obtained for OPF were found to be comparable with results for gasification of woody biomass which were reported to be 75e98% 100

Carbon conversion Mass convertion

95

Conversion (%)

Table 5 Comparison of syngas composition and heating value for gasification of OPF with other biomass.

499

90 85 80 75 70 0.27

0.31

0.35 0.37 0.39 0.41 Equivalence ratio (ER)

0.51

0.59

Fig. 14. Effect of equivalence ratio on mass conversion and carbon conversion efficiencies.

500

S.M. Atnaw et al. / Energy 61 (2013) 491e501

and 81.4% of mass and carbon conversion efficiencies respectively [31,60]. Studies showed that the carbon conversion efficiency of high ash content fuels would be lower than that of OPF; for example 75%, for rice husk [58]. The high mass and carbon conversion efficiency of OPF which could be mainly due to its low ash and fixed carbon composition showed its high potential as a fuel for thermal conversion processes. 4. Conclusions Gasification experiments were conducted using a lab scale downdraft gasifier with OPF fuel to investigate the effects of equivalence ratio. The study showed that oxidation zone temperature above 850
C is favourable for high concentration of the fuel components of syngas CO, H2 and CH4. Average syngas lower heating value of 5.2 MJ/Nm3 was obtained for operation with oxidation zone temperatures above 1000
C, while no significant change in heating value was observed for temperature higher than 1100
C. The optimum range of operation in terms of equivalence ratio was between 0.35 and 0.51. At the optimum equivalence ratio of 0.37 the gasification efficiency and carbon conversion were 70.2% and 93% respectively. The optimum equivalence ratio also resulted in average syngas lower heating value of 4.8 MJ/Nm3 and mass conversion efficiency of 92%. The results obtained in the current study for gasification of OPF were found to be comparable with results of other studies for downdraft gasification of woody biomass. The heating value of syngas produced from gasification of OPF was found to be comparable with those of coconut shells biomass and better than hazelnut shells. The cold gas efficiency, carbon and mass conversion percentage of OPF were found to be comparable with those of woody biomass and were higher compared to that of rice husk. The results showed that OPF has a high potential as a fuel for gasification. Acknowledgements This work is funded by Fundamental Research Grant Scheme (FRGS) of the Malaysian Ministry of higher Education, and Universiti Teknologi PETRONAS. References [1] Jaafar MZ, Kheng WH, Kamaruddin N. Greener energy solutions for a sustainable future: issues and challenges for Malaysia. Energy Policy 2003;31(11):1061e72. [2] Shuit SH, Tan KT, Lee K, Kamaruddin A. Oil palm biomass as a sustainable energy source: a Malaysian case study. Energy 2009;34(9):1225e35. [3] Wahid MB. Overview of the Malaysian Oil Palm Industry 2009. Malaysian Palm Oil Board; 2010. Retrived on May 25, 2010, from: http://econ.mpob.gov. my/economy/EID. [4] Wahid C, Weng CK. Availability and potential of biomass resources from the Malaysian Palm Oil Industry for generating renewable energy. Oil Palm Bull 2008;56. Retrived May 25, 2010, from: http://pamoilis.mpob.gov.my/ publication/opb56.html. [5] Basiron Y. Palm oil production through sustainable plantations. Eur J Lipid Sci Technol 2007;109(4):289e95. [6] Mohammed MAA, Salmiaton A, Wan Azlina W, Mohammad Amran MS, Fakhru’l-Razi A, Taufiq-Yap YH. Hydrogen rich gas from oil palm biomass as a potential source of renewable energy in Malaysia. Renew Sust Energy Rev 2010;15(2):1258e570. [7] Akhtar J, Kuang SK, Amin NAS. Liquefaction of empty palm fruit bunch (EPFB) in alkaline hot compressed water. Renew Energy 2010;35(6):1220e7. [8] Guo J, Lua AC. Kinetic study on pyrolytic process of oil-palm solid waste using two-step consecutive reaction model. Biomass Bioenergy 2001;20(3):223e33. [9] Lee DH, Yang H, Yan R, Liang DT. Prediction of gaseous products from biomass pyrolysis through combined kinetic and thermodynamic simulations. Fuel 2007;86(3):410e7. [10] Yang H, Yan R, Chin T, Liang DT, Chen H, Zheng C. Thermogravimetric analysis-Fourier Transform infrared analysis of palm oil waste pyrolysis. Energy Fuel 2004;18(6):1814e21.

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