Investigative Study on the Use of De-Oiled Palm Kernel Cake for Biogas Production

: Availability of Palm Kernel Cake (PKC) has increased due to the increase in the number of cottage oil palm processing industries in developing countries. A quest for clean energy from bio-waste is also on the increase. This study aims at investigating the biogas generating capacity of de-oiled PKC and its corresponding methane content. De-oiled PKC and a mixture of de-oiled PKC and fresh sugar cane chips were used as the two bio-feed samples in a laboratory anaerobic digestion set up. A theoretical approach was also used to determine the expected methane content in the biogas. Laboratory results for de-oiled PKC gave the volume by weight of bio-feed for biogas and methane to be 12.7 ml/g and 4.2 ml/g respectively and that of the combination of de-oiled PKC and fresh sugar cane chips to be 3.15 ml/g and 1.25 ml/g respectively. The measured methane composition for de-oiled PKC and that of the combination of de-oiled PKC and fresh sugar cane chips to be 33% and 40% respectively while the theoretical estimates were 33.5% and 41.1% respectively. The study shows that de-oiled palm kernel cake has biogas/methane generation potential whose quality can be improved by the addition of other biogas producing wastes.


INTRODUCTION
Biogas is a mixture of gases mainly carbon dioxide and methane that results from anaerobic fermentation of organic matter by bacteria (GreenLearning Canada Foundation, 2017). Fig. 1 shows the schematic of the process. Biogas is mainly composed of 50 to 70 percent methane, 30 to 40 percent carbon dioxide (CO 2 ) and low amount of other gases (Hydrogen 5-10%, Nitrogen 1-2%, Water vapour 0.3%, and traces of hydrogen sulphide). Its calorific value is 20 MJ/m 3 and burns with 60% efficiency in a conventional biogas stove (Regattieri et al, 2018). Biogas and electricity are generated from effluent management, and several biochemicals such as ethanol, fatty acids, waxes and others which could be obtained through application of biotechnology. Conversion to energy is a good means of obtaining carbon credit facility for sustainable management (Sridhar and AdeOluwa 2009). Closed tank digester system with biogas capture and utilisation can contribute to the sustainable development rather than open air disposal of palm oil industry wastes. This method has been developed for treating palm oil mill effluents (Understanding Energy, 2017). The biogas generated is captured and directed to flaring or used as boiler fuel or for power generation. For the controlled anaerobic tank digester method with mixing, the gross treatment efficiency has been estimated to be in the range of 90-95% in terms of BOD removal. COD treatment efficiency is experienced in the range of 80-90%. Methane content in the biogas generated has been reported in the range of 54-70% with an average of 64%. The major part of the balance of the biogas is CO 2 (36%) with traces of hydrogen sulphide (Understanding Energy, 2017). Oil palm industry generates a large quantity of residues and wastes in the form of empty fruit bunch, palm kernel shells, trunk of the plant, fibre, leaves and others. Empty fruit bunches and palm kernel shells have been successfully converted into compost and were useful in developing oil palm nurseries and other food crops. The various uses of the by-products and waste from palm oil mills are shown in Figure 2.
Wastewater generated through typical palm oil processing averages 0.5 m³/ton of fresh fruit bunches (FFB). Some palm oil mill processes about 450,000 tons FFB and thus 210,000 m³ of wastewater per year. For such a company, a digester volume of 6,000 m³, will allow handling the daily load of about 700 m³ of waste water (Understanding Energy, 2017). Palm kernel cake (PKC) is a byproduct of oil extraction from palm kernel nut and it is abundant in the tropical areas of the world (Rhule, 1996). In recent times however, there has been an increase in the number of cottage oil palm processing industries in developing countries like Nigeria due to ban on the importation of vegetable oil which has resulted in abundant availability of PKC. The PKC so obtained varies considerably in chemical composition (protein, fibre or lipids), depending on source, the extent and methodology of oil removal and the proportion of endocarp remaining (Rhule, 1996;Adesehinwa, 2007). Presently PKC is being used as a filler in animal feeds, used for land filling or burnt directly as fuel. From the chemical composition of PKC, Adesehinwa (2007) showed that PKC is a potential energy source. This study therefore aims at investigating biogas production potential as an alternative use for PKC. Chaikitkaewa et al (2015), evaluated three biomass residues from palm oil mill plant including empty fruit bunches (EFB), palm press fiber (PPF) and decanter cake (DC) for methane production by solid state anaerobic digestion at 25% total solids content. The highest cumulative methane production of 2180 mLCH4 was obtained from EFB followed by PPF (1964 mL CH4) and DC (1827 mL CH4). Methane production from EFB, PPF and DC by solid state anaerobic digestion was 55, 47 and 41 m 3 CH4/ton, respectively, which suggested that decanter cake could be a promising substrate for methane production by solid state anaerobic digestion.
This paper seeks to investigate the biogas potential after oil palm kernel oil has been extracted from Palm kernel cake with a focus on its quality in terms of the percentage methane content.

A. Theoretical Calculation of Biogas Composition
Based on chemical formula, Buswell devised the equation to predict theoretical yield of component products from bio-digestion. The Buswell's equation is given as Where a, b, c, d are the moles of Carbon, Hydrogen, Oxygen and nitrogen respectively present in the waste sample. The simplification of the Buswell equation and Boyle's law to give the contribution of some chemical components in waste samples is given in the Table 1 (Renewable Energy Concepts, 2018; Czepuck et al). These estimates were used to determine the theoretical gas composition of the waste samples.  3 and 5) were set up simultaneously for the anaerobic digestion process and this was repeated twice (Experiment 1& 2). In each set-up, 50 g of de-oiled palm-kernel cake was measured into conical flasks (which served as mini-digesters). The wastes in the conical flasks were diluted with 500 ml of water to make a total solid value (TS) of 10% and were thoroughly stirred. The conical flasks were sealed at the top to prevent the escape of gas while a hose was fixed at the side opening. The hose serves as a passage for the gas to the point of collection.

C. Experimental Procedure using de-oiled palm kernel cake and sugar cane
Two units labelled B and D (Figures 4 and 6) were set up simultaneously for the anaerobic digestion of the mixture and this was repeated twice (Experiment 1& 2). A mix of 50 g of de-oiled palm-kernel cake and 50 g of fresh sugar cane chips were measured into conical flasks (which served as minidigesters). The wastes in the conical flasks were diluted with 500 ml of water and were thoroughly stirred. The conical flasks were sealed at the top to prevent the escape of gas while a hose was fixed at the side opening. The hose serves as a passage for the gas to the point of collection.

D. Volume Measurement
In all the experiments, the volume of gas produced was measured through the displacement method. In set-ups A and B, the gas was collected directly while in set-up C and D the gas collected was passed through Calcium Hydroxide, Ca(OH) 2 to remove the CO 2 present. This leaves only the methane gas as the gas collected (Mel et al, 2014). The ambient temperature was measured and recorded daily using a thermometer. The data obtained from the volume measurement was recorded and compared to determine the amount of biogas that can be recovered from the amount of waste used for the experiment and the total energy potential of the waste samples.

A. Theoretical Gas Estimates
Applying the composition percentage to de-oiled palm kernel cake gives the results in Table 2 while its result from the application to the mixture of de-oiled palm kernel cake and fresh sugar cane chips is shown in Table 3.
De-oiled palm kernel cake has a volatile solids content of 76.6% weight by weight of solids. Therefore, the expected theoretical methane content of de-oiled palm kernel cake is given by 0.766 x 43.75 which is equal to 33.51%. Fresh sugar cane chips have a volatile solids content of 97.7% weight by weight of solids. Therefore, the expected theoretical methane content of fresh sugar cane chips is given by 0.977 x 49.868 which is equal to 48.72%.
Expected theoretical methane content from mixture of de-oiled palm kernel cake and fresh sugar cane chips is a 50% contribution by both materials since an equal weight of 50g each was used. The value is given by 0.5(33.51 + 48.72) which is equal to 41.12%.

B. The Anaerobic Digestion Process
The cumulative biogas volume measured with time from the anaerobic digestion of de-oiled palm kernel cake and fresh sugar cane chips is illustrated in Fig. 7. It was observed that there were traces of biogas production within 24 h. The average hourly production was 2.5 ml with peak production indicated at about 140 h for de-oiled palm kernel cake and 90 h for de-oiled palm kernel cake with fresh sugar cane chips. The production then began to decline with some negative values of gas volumes recorded. The negative values indicate that some other gases present within the biogas had dissolved in the water used to take measurement (Mel et al, 2014). For the weight of material used, a detention time of about 260 h

C. Methane Measured in the Laboratory
Methane content within the biogas was noticed within 24 h. The cumulative volume measured with time is shown in Figure  8. From the results, it was observed that the volume of methane was considerable low when compared with the volume of biogas produced by the same sample. CO 2 removal from biogas is mandatory to meet the specifications of a natural gas grid since CO 2 reduces the heating values of natural gas. The percentage of methane present in biogas as against that of CO 2 determines the quality of the biogas.
The comparison of the total volume and percentage of biogas present in the waste samples is shown in Table 4. It was observed that the waste sample containing only de-oiled palm kernel cake waste produced an average biogas and methane of 12.7 ml/g and 4.2 ml/g of waste respectively while the sample containing fresh sugar cane chips produced an average of 3.15 ml/g and 1.25 ml/g respectively. The methane yield from the sample containing de-oiled palm kernel cake and fresh sugar cane chips was higher than that of de-oiled palm kernel cake only. In terms of methane content in the measured biogas, the methane content from the deoiled palm kernel cake is 33% while that of de-oiled palm kernel cake and sugarcane is 40%.
DPKC: De-oiled Palm Kernel Cake, FSCC: Fresh Sugar Cane Chips Good biogas quality is expected to have a methane content of at least 50%. The sugar cane content added resulted in an increase in the methane content indicating that de-oiled palm kernel cake has a biogas generation potential which can be boosted by the addition of other products such as sugar cane.

D. Comparison of Laboratory and Theoretical Values
The laboratory measured methane content in de-oiled palm kernel cake was 33% which is 1.5% lower than the theoretical value of 33.5%, while the methane content of the sample with de-oiled palm kernel cake and fresh sugar cane chips is 40%, which is 2.68% lower than the theoretical value of 41.1%.
Variations between experimental and theoretical values have been noticed by other researchers, stating that the theoretical value is an indication of the maximum amount of methane that can be produced by the sample under ideal conditions. Factors responsible for the variations include Temperature, Heat, Mixing, Carbon/Nitrogen ratio, Volatile     solids content and Lipid content in waste sample (Czepuck et al, 2006;Chaikitkaewa et al, 2015).
IV CONCLUSION The results of the test digesters used for the laboratory research showed that de-oiled Palm kernel cake after going through anaerobic digestion produced biogas. Addition of other types of waste such as fresh sugar cane chips increased the amount of methane present in the biogas and therefore its quality. A comparison of the percentage of methane in the biogas from theoretical calculation and the laboratory showed a variation of less than 3%. It can therefore be concluded that de-oiled palm kernel cake has biogas/methane generation potential whose quality can be improved by the addition of other biogas producing wastes.