The last years saw a sharp increment in the interest laid on the renewable and the alternative energy sector, mainly due to the depletion of fossil fuel throughout the world for industrial and commercial use. Malaysia is the second largest producer of palm oil, currently holding up to 4.5 million hectares of palm oil plantation in its land. Currently produced at more than 40 million tons a year, only a small portion of Oil Palm Frond (OPF) is used as domestic animals forage and as raw material in small-scale furniture industry, while the rest is left at the plantation floor to naturally decompose. This study introduces OPF as a solid biomass fuel for gasification to produce synthesis gas that can be utilized for heat and energy generation in a cleaner and more efficient manner than direct combustion. OPF was gasified in the downdraft gasifier at 700-1000°C reactor temperature with a controlled air supply of 180 to 200 L min-1. The effects of reactor temperature and operation time to the quality of syngas produced from OPF downdraft gasification were investigated. At a calorific value at around 18 MJ kg-1, OPF was found to produce synthesis gas that sustainably burnt in air with a higher heating value of around 5 MJ Nm-3. OPF was found to be optimally producing syngas with desired energy content at a reactor temperature range of 700-900°C and within the first 45 min of gasifier operation.
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Until the petroleum industry was introduced in the late 1800 sec, mankind has exploited biomass as the main energy source for heat and power generation since the beginning of time. However, the recent years saw a quick depletion of the global petroleum resources while demands for heat and power increase steadily by the year. With the petroleum resources estimated to disappear completely in less than 50 years of so, scientists and researchers worldwide introduced several renewable and alternative energy sources; one of them is biomass. While biomass is most common as solid fuels to the large energy facilities, its exploitation in small scale application can be even more fascinating mainly for the higher prospect for heat recovery, less complexity in raw materials supply and much lower impacts to the environment. Moreover, small plants are applicable to a wider range of industry and consumer users than large plants, adding up to its generous versatility as a renewable source for heat and power generation. As in Malaysia, a tropical country located close to the equator, biomass supply is in abundance. Being currently the second largest palm oil producer responsible for 43% of the worlds supply, Malaysia utilized more than 4.5 million hectares of its land for the cultivation of oil palm trees (Malaysia Palm Oil Board, 2011). With increasing trend in the awareness of biomass potential as alternative energy resource, the palm oil industry has emerged to be an attractive platform for continuous and large biomass supply as depicted by Abdullah and Yusup (2010). Common examples of biomass from oil palm industry are Palm Oil Mill Effluent (POME), Empty Fruit Bunch (EFB), fiber, shells, kernels, trunks and Oil Palm Fronds (OPF) as widely discussed by Faizal et al. (2010), Wan Asma et al. (2010), Abdullah et al. (2011) and Razuan et al. (2010). OPF was not given much attention to unlike other biomasses produced by the oil palm tree. Other than being utilized as ruminant feedstock for cattle as reported by Atil (2004) and as a raw material in small-scale wood and furniture industry, a large amount of OPF would normally be left on the plantation floor as a natural fertilizer once pruned or used as nutrient sources for the cultivation of young palms according to Haron et al. (2007). Recent studies on OPF as a raw material for ethanol production as reported by Yutaka (2007) and biomass briquette by Nasrin et al. (2008) have been presented and discussed. This realization of OPF as a biomass source led to a few studies including this one to have been established to introduce alternative endings to OPF as a biomass with potential values. With biomass gasification re-emerging popularity among researchers and enthusiasts worldwide in the pursuit to create and promote the awareness in green technology, OPF is seen to be a potential candidate based on its abundant supply and considerable energy content to be processed as a solid fuel for gasification. Efforts in studying OPF gasification by simulation and experiment approaches was reported by Atnaw et al. (2011), bearing a potential result where OPF might be a prospective biomass fuel for heat and energy generation. Similarly, a torrefaction attempt on OPF was reported by Sulaiman and Anas (2012).
This study intended to utilize OPF for downdraft gasification process, in which the effects of the reactor temperatures and operation time to the quality of the produced syngas were studied. The outcome of this study would enable OPF to be utilized as a solid biomass fuel for gasification at a larger scale where its practicality can be further observed and studied for actual application. The public awareness about gasification and its benefit may be increased mainly due to the heightened interests in green technology and the worlds fuel crisis. The promising potential of OPF as gasification fuel would be one of the biggest solution to Malaysias yearly energy expenses on coal and other fossil fuels for heat and energy generation when applied. The outcomes of this present research would also generate a few more studies of OPF as a biomass fuel for other applications, if not for gasification, thus promoting more intellectual awareness of OPF as a new hope as a biomass fuel source.
Gasifier specification: The gasifier used for the experiment was a laboratory-scale stationary, batch-operated 50 kWth fixed-bed downdraft type. The arrangement of the gasifier system is shown in Fig. 1. Air was supplied into the gasifier by means of blowing using a 250 W vortex blower and the amount of supplied air was controlled using a ball valve and a bypass point and monitored using a pitot tube and a water manometer. The full capacity of the gasifier was 12 kg for 2.5-5.0 cm cubic OPF blocks with 70% compact factor.
Feedstock specification: Pre-processed OPF fuel in block form was prepared and utilized in the experiment. Every part of OPF was utilized except for the leaflets in order to maintain a uniform fuel particle size and morphology. Averagely, the dimension of each fuel block was 2.5-5.0 cm in cubic shape. The fuel was processed from green OPF and was pre-dried to achieve the desired moisture content of 12±2%. The calorific value of OPF fuel was found to be 17.65 MJ kg-1 by average on dry basis.
|Fig. 1:||Downdraft gasifier assembly, A: Vortex air blower, B: Air bypass outlet, C: Pitot tube point, D: Primary air route, E: Secondary air route, F: Downdraft glassifier, G: Gas exhaust pipe and H: Gas flare point|
Gasification setting: The downdraft gasification of OPF was conducted within a known operation range for OPF fuel. The supplied air into the gasifier was controlled in the range of 180-200 L min-1 to keep the reactor temperature in between 700-900°C. Conducted studies have shown that this setting was the most optimal for the gasification of OPF fuel as described previously. The reactor temperature was controlled by means of regulating the air supply into the gasifier. The intended gasifier operation time was 1 h before refueling was required.
Preheating procedure: Prior to each test, the reactor was first preheated to prepare for gasification. Preheating was done by burning a pilot fuel that comprised of shredded paper, garden refuse and rejected OPF fuel from the fuel processing stages in the gasifier to bring up the reactor temperature to more than 500°C. This process was important to form a layer of char bed above the reactor grate. With preheating, syngas was produced at a shorter time (5-10 min) than without (15-20 min) and the combustion of OPF fuel was found to be steadier and less problematic. The positive effects of gasifier preheating can be explained by considering the autoignition temperature of woody biomass including wood that is around 250-300°C as thoroughly discussed by Baker (1983), Boonmee and Quintiere (2002) and Cao et al. (2006). Following preheating, thermochemical reactions occurred almost as instantly on freshly-loaded OPF fuel blocks due to rapid heating of fuel, resulting into a quicker transition from instantaneous drying to pyrolysis. Such transition made syngas to produce faster than without preheating. Additionally, excess tar deposits on the internal reactor and pipe walls from previous operations were discovered to be consumed in the heat, showing another advantage of preheating in the caretaking of the gasifier system. This was due to the thermal cracking of tar at a temperature of 700 to above 1000°C according to Milne et al. (1998).
OPF DOWNDRAFT GASIFICATION
Influence of reactor temperature: The influence of reactor temperature on the quality of syngas from the downdraft gasification of OPF was investigated by comparing the components and the calorific values of syngas produced at various reactor temperatures. Figure 2 shows the production trends of CO, CO2, CH4 and H2 while Table 1 and 2 shows their average values at different reactor temperature ranges. CO and H2 were found to produce in an increasing manner with increasing reactor temperature. CO2 production showed a dropping pattern as the reactor temperature rose. H2 was increasingly produced as the reactor temperature reached 850°C but dropped following that point. CH4 production however was found to be slightly increasing with increasing temperature, although with very less significance.
The lower calorific value of syngas (LCVsyngas) and the H2:CO ratio are shown in Fig. 3. The experimental results showed that H2:CO ratio decreased while LCVsyngas increased with increasing reactor temperature.
The reason behind the increment of LCV can be explained using the following correlation:
where, LCVsyngas is the total lower calorific value of syngas while LCVn is the specific lower calorific value of a gas component species i.e., CO and CO2. The summation of the lower calorific values of all gas component species (CO, CO2, H2 and CH4) will bear LCVsyngas. The typical LCV for each gas component is as shown in Table 3.
The increment in LCVsyngas with increasing reactor temperature was speculated due to the increasing concentration of CO in syngas, where the LCV of CO is slightly higher than that of H2, hence also explained why LCVsyngas still increased even when with reducing H2 amount in syngas: the superiority of energy content of CO at increasing concentration overcame the loss in syngas energy due to the reduction of H2 in syngas as the reactor temperature rose to above 1000°C. While this may be beneficial to increase LCVsyngas, the combustibility of syngas may be compromised due to the high CO concentration for the fact that CO, although combustible, is also a non-supporter of combustion.
|Fig. 2:||Production trends of gas components in syngas produced from downdraft gasification of OPF chips as a function of reactor temperature at 180 to 200 L min-1of air|
|Fig. 3:||Lower calorific value and H2:CO ratio of syngas produced from downdraft gasification of OPF chips as a function of reactor temperature|
|Table 1:||Gas components in syngas produced from downdraft gasification of OPF chips as a function of reactor temperature range|
|Table 2:||Characteristic of syngas produced from downdraft gasification of OPF chips as a function of reactor temperature range|
|Table 3:||Typical LCV of syngas main gas components|
|Table 4:||Characteristic of syngas produced from the downdraft gasification of OPF chips at a reactor temperature range of 700 to 900°C|
Higher H2 concentration in syngas is therefore still favorable for this reason. This increasing amount CO compared to decreasing H2 with increasing reactor temperature also caused the H2:CO ratio to radically drop.
The cold gas efficiency of syngas (CGEsyngas) is shown in Fig. 4. The experimental CGEsyngas values were calculated using the following correlation:
where, Vsyngas is the flow rate of syngas leaving the reactor, mOPF is the mass feed rate of OPF fuel in the reactor and LCVsyngas and LCVOPF are the lower calorific values of syngas and OPF, respectively. Due to the inability to measure the actual Vsyngas owing to the incapability of the existing measuring instrument, it was estimated that every kilogram of OPF produced 2.5 m3 of gas by average amount of gas produced from 1 kg or biomass according to the GEK developers (AllPowerLabs, 2010) while mOPF was estimated to be 10 kg h-1. LCVsyngas were calculated from syngas compositions while LCVOPF was defined to be 18 MJ kg-1 by average.
It was observed that CGEsyngas increased with increasing reactor temperature, mainly due to the increment in syngas energy as attributed to the rising concentration of CO (Table 4). The highest CGEsyngas value was found to be 76.28% at a reactor temperature range of 1150±50°C.
|Fig. 4:||Cold gas efficiency (CGE) of syngas produced from downdraft gasification of OPF chips as a function of reactor temperature|
This observation was related to the increasing amount of CO in syngas as discussed previously and to note that CGEsyngas was mostly in between 50 to 70%, it showed a good indication that the pyrolysis process inside the reactor was good enough to extract volatiles from OPF fuel as concluded by Kennedy and Lukose (2006).
INFLUENCE OF OPERATION TIME
The influence of operation time to syngas characteristic has been an important interest to this study in order to determine the maximum operation time until the gasifier needs to halt the supply of syngas for refueling mainly due to decreasing syngas quality. The characteristic of syngas was monitored by the concentration of its gas components, lower calorific value (LCVsyngas), H2:CO ratio and CGEsyngas. The values were the average of five repeated operations and are shown in Table 5 and 6.
Figure 5 shows the concentration of the gas components in syngas as a function of gasifier operation time. The highest-in-concentration gas component, CO, was observed to peak at minute 75 at 21.69% after a steady climb before experiencing a sharp drop towards the end of operation. H2 showed almost the same pattern where its concentration peaked at minute 85 before experiencing a drop following that period. CO2 however, experienced very less change in concentration except at minute 55 where it suddenly peaked before stabilizing again and then gently increased after minute 75 towards the end of operation. CH4 experienced relatively almost no change at all in concentration along the gasification period and was observed to reduce in concentration after 95 min of operation.
|Fig. 5:||Production trends of gas components in syngas produced from downdraft gasification of OPF chips as a function of gasifier operation time|
|Table 5:||Gas components in syngas produced from downdraft gasification of OPF chips as a function of gasifier operation time|
|Table 6:||Characteristic of syngas produced from downdraft gasification of OPF chips as a function of gasifier operation time|
Figure 6 shows the trends of the lower calorific value of syngas, LCVsyngas and the H2:CO ratio as functions of gasifier operation time. Both LCVsyngas and H2:CO ratio showed a climbing trend before dropping towards the end of the gasifier operation. LCVsyngas experienced the most reduction after around 90 min of operation while the H2:CO ratio did not give a very conclusive pattern of change along the gasification period, although the polynomial trend line suggested a smooth reduction following the 60th min of operation.
|Fig. 6:||Lower calorific value and H2:CO ratio of syngas produced from downdraft gasification of OPF chips as a function of gasifier operation time|
|Fig. 7:||Cold gas efficiency of syngas produced from downdraft gasification of OPF chips as a function of gasifier operation time|
The polynomial trend line for LCVsyngas showed a steep drop at nearly the same time frame. The reduction in LCVsyngas was mainly due to the decreasing amounts of CO and H2 towards the end of the operation where as the OPF fuel has been consumed to a minimum level, the air-fuel ratio increased to nearly or more than 1.0, transitioning the otherwise substoichiometric gasification to complete combustion. This caused CO2 to be produced instead of H2 and CO, leading to the drops in LCVsyngas and H2:CO ratio. Cao et al. (2006) discussed the similar observation, where LCV of syngas dropped mainly due to the decreasing amount of combustible components in syngas.
The cold gas efficiency of syngas (CGEsyngas) as a function of gasifier operation time is shown in Fig. 7. The plotted chart did not give a clear trending pattern albeit the polynomial trend line showed that CGEsyngas rose from the start of the operation and peaked at minute 55 (73.77%) before experiencing a steady reduction towards the end of the operation. By average, CGEsyngas was found to be 52.82% with a standard deviation of 12.82%.
The observations on the change in syngas characteristic due to the influence of reactor temperature critically contributed to the gasifier operation demands where, in order to produce good quality syngas from OPF gasification, the temperature of the reactor has to be kept in between the range of 700 to 900°C, which, in order to keep the H2:CO ratio at above 0.5. At this temperature range, the characteristic of syngas is shown in Table 4. Although at a higher temperature LCVsyngas and CGEsyngas improved significantly, mainly attributable to the high concentration of CO, the ignitability of syngas was compromised due to the nature of CO as not a non-supporter of combustion. This was observed in the flare tests during operation where it was found that the flare, although still combustible, was harder to ignite and sustain combustion in air.
The influence of operation time was found to be a crucial remark in operating the gasifier to produce good quality syngas from downdraft gasification of OPF. The designed operating duration for the gasifier was intended to be 60 min and it was discovered experimentally that the gasifier met the intended specification. Following the operation period of above 60 min, the quality of syngas in terms of composition and energy content reduced to which it became less effective to futile to be utilized to generate heat and power. For this reason, the operation of the gasifier has to be limited to only 60 min for each full capacity run until refueling is required.
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