Abstract: A study on gasification of Empty Fruit Bunch (EFB), a waste of the palm oil industry is investigated. The composition and particle size distribution of feedstock are determined and the thermal degradation behaviour is analysed by a thermogravimetric analysis (TGA). Then 300 g h-1 fluidized bed bench scale gasification unit is used to investigate the effect of the operating parameters on biomass gasification namely reactor temperature in the range of 700-1000°C and feedstock particle size in the range of 0.3-1.0 mm. The main gas species generated, as identified by a Gas Chromatography (GC), are H2, CO, CO2 and CH4. With temperature increasing from 700 to 1000°C, the total gas yield is enhanced greatly and has reached the maximum value (~ 92 wt. %, on the raw biomass sample basis) at 1000°C with big portions of H2 (38.02 vol.%) and CO (36.36 vol.%). Feedstock particle size shows some influence on the H2, CO and CH4 yields. The feedstock particle size of 0.3 to 0.5 mm, is found to generate a higher H2 yield (33.12 vol.%) and higher LHV of gas product (17.19 MJ m-3).
INTRODUCTION
Dependence on fossil fuels as the main energy sources has led to serious energy crisis and environmental problems. Therefore, due to the environmental considerations as well as the increasing demand for energy in the world, more attention has been paid to develop new energy sources (Ni et al., 2006). Owing to that, there has been interest in the utilization of biomass for production of environmental friendly biofuels. As known, biomass is a CO2 neutral resource in the life cycle while CO2 is a primary contributor to the global greenhouse effect. Hence, increasing attention is being paid to biomass as a substitute for fossil fuel to reduce the global greenhouse effect, particularly under the commitment of the Kyoto Protocol. Biomass used as an energy resource can be efficiently achieved by thermo-chemical conversion technology: pyrolysis, gasification or combustion. Gasification process is one of the most promising thermo-chemical conversion routes to recover energy from biomass. During gasification process, biomass is thermal decomposed to small quantities of char and ash, liquid oil and high production of gaseous products under presence of oxygen. The yields of end products of gasification and the composition of gases are dependent on several parameters including temperature, biomass species, particle size, heating rate, operating pressure and reactor configuration (Demirbas and Arin, 2002).
The concern of using biomass in gasification to produce a hydrogen rich product has been getting particular attention in recent years. The reasons may be attributed to: (1) hydrogen is a clean and efficient energy source and is expected to take an important role in a future energy demand; (2) hydrogen is a safe source and can be easily stored as a gas or a liquid; (3) hydrogen has good properties in fuelling engines in automobiles and (4) most important, current and future energy technologies are extensively increasing the possibility of utilizing hydrogen with economic acceptance. Apparently, how to force the biomass gasification process into shift towards the maximum hydrogen rich end product is becoming a priority topic (Chen et al., 2003b).
As shown in Table 1, for comparison among different fuels, hydrogen has the highest energy content. As a result, hydrogen energy is indeed an ideal energy carrier in the future.
Oil palm (Elaeis guianensis) originates from West Africa. It grows well in wet and humid places like Malaysia. In the present time Malaysia is the world’s largest producer and exporter of palm oil. It currently accounts for 51% of the world palm oil production and 62% of the world exports.
| Table 1: | Energy contents of different fuels |
In year 2006, the palm oil production in Malaysia increase to 16.5 million metric tons compared to 2.57 million metric tons in 1980 (Mongabay.com, 2007). Beside palm oil availability, Malaysian palm oil also generates huge quantity of oil palm biomass including oil palm trunks, oil palm fronds, Empty Fruit Bunches (EFB), shells and fibers in the production of palm oil. The annual generation of fiber, shell and empty fruit bunches are estimated to be 9.66, 5.20 and 17.08 million tons, respectively (Nasrin et al., 2008). Oil palm is a multipurpose plantation and also a prolific producer of biomass as raw materials for value-added industries (Basiron and Simeh, 2005). For example, fresh fruit bunch contains only 21% palm oil, while the rest 6-7% palm kernel, 14-15% fiber, 6-7% shell and 23% Empty Fruit Bunch (EFB) are left as biomass (Umikalsom et al., 1997).
A large number of research projects in the field of thermo-chemical of biomass, mainly on pyrolysis and gasification have been conducted. In the literature, large numbers of experimental studies concerning the gasification of biomass with air and oxygen have been presented and extensive research has been conducted on small and medium size air gasifiers to produce low BTU fuel gas and power (Demirbas, 2002; Chen et al., 2003a; Yang et al., 2006). In this study, the gasification of Empty Fruit Bunch (EFB), a waste of the palm oil industry has been investigated using fluidized bed reactor under different operating conditions (temperature and feedstock particle size), in order to achieve an improved performance of palm oil wastes conversion to energy with a higher yield of H2-rich gas.
MATERIALS AND METHODS
Feedstock preparation and properties: The EFB sample investigated in this study was collected from palm oil mill located in Dengkil, Selangor. EFB used in this work is the biomass remaining as a by-product of industrial process after removal of the nuts. Samples received were relatively dry having less than 10 mf wt. % moisture and were in the form of whole bunches. Particle size reduction was required to allow gasification of the EFB on the available 300 g h-1 reactor.
| Table 2: | Particle size distribution of EFB |
| Table 3: | Properties of EFB (mf wt. %) |
The bunches were first manually chopped into small pieces that could be fed in to a shredder. After that, a Fritsch grinder with a screen size of 1.0 mm was used in order to reduce the feedstock size to less than 1.0 mm. The distribution of feed particle size after grinding is given in Table 2.
After extensive feeding trials, it is found that only particles between 0.3-1.0 mm are easily fed. Both the size fraction below and above this range frequently led to blockage of the available feeder.
The proximate and elemental analyses were carried out in a TGA (Mettler-Toledo 851) and CHNS/O analyzer (Perkin-Elmer 2400II), respectively. The results are listed in Table 3. EFB have a very high volatile content, >80 wt% and contain low amounts of fixed carbon, <10 wt%. The calorific value of EFB studied (~18 MJ kg-1) was measured in a bomb calorimeter (Parr 1341); this is lower than that of coal, possibly due to the low fixed-carbon and high oxygen contents in the EFB (McKendry, 2002). The ultimate analysis indicates that EFB is environmental friendly, with trace amounts of nitrogen, sulfur and mineral matter.
Experimental procedure: A fluidized bed bench scale gasification unit operating at atmospheric pressure was employed for all runs. Figure 1 shows a schematic diagram of this unit, which consists of three main parts: reactor part (gasification reactor and heating furnace), condenser and purification part (condenser, glass wool filter and dryer) and gas storage part (gasbags). The reactor is a cylindrical configuration made of stainless steel with a length 600 mm and a diameter of 20 mm. Three thermocouples were inserted in the middle of the heating furnace, middle of the reactor tube and bottom of the reactor tube, respectively.
| Fig. 1: | Flow diagram of the 300 g h-1 fluidized bed gasification system |
Biomass was fed into the reactor by feeder on the top of the reactor, which were continuously carried out at a constant flow rate. The feeding capacity of biomass was 5 g min-1. The heating medium in the reactor was inert sand of size between 0.3-0.5 mm. Air, entering from the base of the reactor was used as a fluidizing gas.
The condensable vapors were collected in the liquid products collection system which consists of cooled condenser, glass wool filter and silica gel filter. The incondensable gases leaved the system and then collected by the gasbags needed to be analyzed by Gas Chromatography (GC) to assess the quantity and type of gas produced. The GC utilized was an Agilent Technologies with nitrogen and argon as the carrier gases and one column (molar sieve).
RESULTS AND DISCUSSION
Thermogravimetric analysis of EFB: The thermogravimetric analysis (TGA) was performed under 10 mL min-1 air with a heating rate of 10°C min-1. The thermal degradation characteristics of different particle size dried feedstock are displayed in Fig. 2 and 3 by thermogravimetry (TG) and differential thermogravimetry curves (DTG), respectively. The EFB samples showed a small DTG peaks around 100°C which are indicative to the moisture content of 3.3%, followed by big peaks around 300°C which are indicative to the decomposition of cellulose and hemicellulose, while the humps apparent around 450°C are indicative to the decomposition of lignin. The total weight loss between 100 and 550°C is 83.94%, residue of 7.08% indicates to the ash content.
| Fig. 2: | Thermogravimetric analysis of EFB |
Effect of reactor bed temperature on product yields: The yields of final products from EFB gasification under different temperatures are listed in Table 4 and 5. As shown in Table 4, with temperature increasing from 700 to 1000°C, the total gas yields increased sharply from 62.68 to 91.70 wt. %, while liquid, char and tar yields reduced steadily. Meanwhile, varying temperature showed a great influence on gas product components. The main gas products were H2, CO, CO2, CH4 and some C2 hydrocarbons (C2H4 and C2H6).
As shown in Table 5, H2 content increased steadily from 10.27 to 27.42 vol. % as temperature increased from 700 to 900°C and with a further increase of temperature to 1000°C, the H2 content increased significantly to 38.02 vol. %. Yield of CH4 also increased from 5.84 to 14.72 vol. %., while CO2 content decreased in general with temperature increasing, particularly at 1000°C.
| Fig. 3: | Differential thermogravimetric analysis of EFB |
| Table 4: | Products yields from EFB gasification at different temperature |
| Table 5: | Yields of gas products from EFB gasification at different temperature |
The yield of CO increased first from 21.87 to 33.35 vol.% as temperature increased to 800°C, then decreased to 33.08 vol.% at temperature 900°C, however, it increased again to 36.36 vol.% as temperature continuously increased to 1000°C. The yields of C2H4 and C2H6 are relatively small and the influence of temperature is insignificant. At high temperature (1000°C), H2 (38.02 vol.%.) and CO (36.36 vol.%) have the highest yields. The thermal cracking of gas-phase hydrocarbons at high temperature might explain the variation of gas product distribution observed (Papadikis et al., 2009). At high furnace temperature, prior to being quenched in the condenser, the gas species generated from biomass gasification could undergo further reactions (secondary reactions) such as tar cracking and shifting reaction, leading to much more incondensable gases (including H2) generated. Therefore, the total yield of gases products increased significantly as temperature increased from 700 to 1000°C. From the above analysis, it can be concluded that higher temperature (1000°C) is favorable for thermal cracking of tar and shift reaction.
The lower heating value (LHV, kJ m-3) of the gas products can be calculated using the following equation (Papadikis et al., 2009).
| (1) |
where, CO, H2, CH4 and CnHm are in terms of molar ratio of each component in the gas product, respectively. The calculated results are shown in Table 5. The heating value of total gas products increases steadily as the temperature increased. At 1000°C, LHV of gas products reached 15.55 MJ m-3 which belongs to be medium level of heat values for gas fuels that can be directly used for gas engine, gas turbine or boiler for power generation. Also it can be used for the chemical formation of methanol and methane, etc. (McKendry, 2002).
Effect of feedstock particle size on product yields: The second series of experiment was performed to establish the effect of feedstock particle size on the gasification product yields, In this study, the experiments were conducted by using three different feedstock particle size ranges, namely size <0.3, 0.3-0.5 and 0.5-1.0 mm with temperature 850°C. The effect of particle size on the product yields from EFB gasification is listed in Table 6 and 7. The smallest particle size of < 0.3 mm produced a gas yield of 81.47%, about 10% higher than larger particle size of 0.5-1.0 mm which produced a gas yield of 70.43%, while particles size of 0.3-0.5 produced a gas yield 72.12%. As shown in Table 6, the total gas yield decreased with feedstock particle size increased, while bio-oil, char and tar yields increased with increasing of feedstock particle size.
An increase in feedstock particle size causes greater temperature gradient inside the particle so that at a given time the core temperature is lower than of the surface, which possibly gives rise to an increase in the char and liquids yields and decrease in gases (Encinar et al., 2000).
As shown in Table 7, the hydrogen yields were 28.00, 33.12 and 29.51 vol. % for the particle size of <0.3 mm, 0.3-0.5 mm and 0.5-1.0 mm, respectively. The highest hydrogen yield was 33.12 vol. % obtained at particle size of 0.3-0.5 mm and lowest hydrogen yield was 28.00 vol. % obtained at particle size of <0.3 mm. Yield of CO increased from 36.13 to 40.68 vol.% for particle size of <0.3 and 0.3-0.5 mm, respectively, then decreased to lowest value of 33.80 vol. % for particle size of 0.5-1.0 mm. The same thing happened to CH4 yield where it was first increased from 16.09 to 19.26 vol. % for particle size of <0.3 mm and 0.3-0.5 mm respectively, then decreased to reach lowest value 12.43 vol. % for particle size of 0.5-1.0 mm.
| Table 6: | Products yields from EFB gasification at different feedstock particle size |
| Table 7: | Yields of gas products from EFB gasification at different feedstock particle size |
The opposite situation happened to CO2 yield, where it was first decreased from 19.78 vol.% to lowest value 6.94 vol. % for particle size of <0.3 mm and 0.3-0.5 mm, respectively and then increased to highest value 24.26 vol. % for particle size of 0.5-1.0 mm. However, in this study it was observed that the smallest feedstock particle size of <0.3 mm obtained maximum yield of gas product, while the feedstock particle size in range of 0.3-0.5 mm obtained the optimum gas composition and highest LHV of gas product.
CONCLUSION
The main products of EFB gasification are solid charcoal, bio-oil liquids and hydrogen rich gas product. Gas yield increases greatly whilst solid, liquid and tar yields decrease straightly as temperature increases from 700 to 1000°C. The gas products mainly consist of H2, CO, CO2 and CH4 with trace amounts of C2H4 and C2H6. High temperature is favorable for the enhancement of flammable gas product including H2, CO and CH4. Consequently, the LHV of gas products increased greatly with temperature and obtain 15.55 MJ m-3 at 1000°C. The effect of feedstock particle size on EFB gasification products is also investigated. The total gas yield decreases with increasing of feedstock particle size whilst char, liquid and tar yields increase as feedstock particle size increases. The EFB particle size of 0.3-0.5 mm is favorable for the enhancement of gas product quality containing H2, CO and CH4 and the LHV of gas products. From an energy viewpoint, the heating values of the gases generated in the process are about 14-17 MJ m-3 which can be used directly for gas turbine, engine and boiler as gas fuel.
ACKNOWLEDGMENTS
The author’s would like to thank Department of Chemical and Environmental Engineering Technology, Faculty of Engineering, University Putra Malaysia for financial support on this project.