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Cite this article: Olugbemide AD, Lajide L, Adebayo A, Owolabi BJ (2016) Production of Second-Generation Biofuel from Five Tropical Lignocellulosic Materi-als: Effects of Particle Size and Dilution Ratio. Chem Eng Process Tech 2(2): 1030.

*Corresponding author

Akinola David Olugbemide, Auchi Polytechnic, Auchi, Edo State, Nigeria, Tel: 2348067340581; Email:

Submitted: 26 August 2016

Accepted: 28 September 2016

Published: 30 September 2016

ISSN: 2333-6633

Copyright© 2016 Olugbemide et al.

OPEN ACCESS

Keywords•Biogas•Second-Generation Biofuel•Dilution ratio•Particle size•Anaerobic digestion

Research Article

Production of Second-Generation Biofuel from Five Tropical Lignocellulosic Materials: Effects of Particle Size and Dilution RatioAkinola David Olugbemide1*, Labunmi Lajide2, Albert Adebayo2, and Bodunde Joseph Owolabi2

1Department of Basic Sciences, Auchi Polytechnic, Nigeria2Department of Chemistry, Federal University of Technology, Nigeria

Abstract

Effects of particle size and dilution ratio on biogas production and efficiency of anaerobic digestion (AD) of five lignocellulosic materials (LMs) were investigated. The LMs studiedwere oil fibre (OF), rice husk (RH), corn cob (CC), Elephant grass (EG) and Huracrepitan (HC) leaves. Two particle sizes and three dilution ratios were used in the batch AD studies at mesophilic temperature of 37+2oC for a retention time of twenty-one days. All the substrates had the capability to be used for biogas production, but rice husk (RH) produced the best result (2500ml) which was obtained at the 250µm particle size and 1:8 dilution ratio. RH had a production rate of 885ml/day and technical digestion time DT90 of five days. Kinetic studies of the best five scenarios based on optimum conditions were studied using modified Gompertz to compare experimental data with predicted results. A good correlation between the predicted and experimental results was confirmed. Biodegradability constants ranged between 0.0276-0.8794 d-1. Based on biogas production, biodegradability constant and DT90, RH had the best performance. Results from this research are expected to serve as a reliable guide in the operation of AD of these LMs on a large-scale.

ABBREVIATIONSLMs: Lignocellulosic Materials; AD: Anaerobic Digestion;

C/N: Carbon-Nitrogen ratio; RH: Rice Husk; OF: Oil Fibre; CC: Corn Cob; EG: Elephant Grass; HC: Huracrepitan; RHD: Rice Husk Digestate; G(ml):Volume of Biogas Accumulated after a Period of Time; Gm: Maximum Accumulated Gas at an Infinite Time, Ko: Biodegradability Rate Constant; G(t):Cumulative Biogas Yield at a Digestion Time; Go: Biogas Potential of Substrate; Rmax: Maximum Biogas Production Rate; λ: Lag Phase Period prior to Biogas Formation; t: time

INTRODUCTIONBiogas production from lignocellulosic materials (LMs) has

become a remarkable phenomenon today [1]. This is due largely to their renewable nature, relative abundance and also because of the problems associated with the use of fossil fuels which include their nonrenewable nature, the degradation of the environment and negative impact on human health [2].

LMs consist primarily of lignin, cellulose and hemicellulose. The carbohydrate fraction (cellulose and hemicellulose) of LMs is the biodegradable portion that anaerobic microorganisms can use to produce biogas [3]. Lignin on the other hand is not easily biodegraded by microorganisms [4].

Anaerobic digestion is an environmentally friendly process that does not only produce methane-rich biogas but also produces digestates which can be used as biofertilizers for food production. Besides, AD helps in waste management by converting organic wastes into valuable products and reducing the volume of wastes that ends up in landfills. AD can also help in mitigating deforestation since most rural dwellers use firewood as a source of fuel. Also, the health hazards women in the rural areas are exposed to by using fire woods to cook can be curtailed by the use of AD processes to produce biogas [5]. AD can be categorized into four stages, namely; hydrolysis, acidogenesis, acetogenesis and methanogenesis. Hydrolysis which is the first stage of the process is considered the rate determining step [6]

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plays a major role in determining the optimum retention time for AD process.

There are various factors that affect the efficiency of the AD process. These include pH, substrate type, temperature, loading rate, carbon-nitrogen ratio (C/N), particle size, dilution ratio (substrate-to-water) [7]. All these factors are inter-related one way or the other to enhance maximum biogas production.

In recent times there has been a major shift from the use of conventional feed stocks for AD, namely cow dung, poultry wastes and municipal solid wastes. The quest for alternative substrates to keep anaerobic digesters running all year round has been a formidable force in driving research along this frontier. First generation biofuels i.e. fuels from the edible parts of plants, have always generated controversy due to the fact that they could further worsen food crises already encountered in some part of the world. Second generation biofuels i.e. from non-edible parts of plants on the other hand enjoy much acceptance since they do not compete with food production [8]. It is therefore imperative that researchers in different part of the world study the potentials of lignocellulosic materials that fall under this category in their regions for biogas production. This is quite important because differences in climatic conditions make it rather difficult to compare accurately results from one part of the world with another [9]. Therefore, the objectives of this research were to i) evaluate biogas potentials of five tropical LMs through AD ii) determine the effects of particle size and dilution ratio on biogas production and efficiency iii) determine AD performances of samples based on lag phase and technical digestion time (DT90) which is the number of days 90% of biogas was produced iv) Study the kinetics of AD processes in order to determine biodegradability constants and compare measured values with predicted data using modified Gompertz model.

MATERIALS AND METHODS

Samples

Five samples were selected for this experiment. Four of the samples, corn cob (Zea mays), rice husk (Oryzasativa L.), Sandbox leaves (Huracrepitan L.) and Elephant grass

(Pennisetumpurpureum L.) were collected from Auchi, Etsako West Local Government Area, Edo State, Nigeria while oil palm fibre, a by-product of palm oil processing, was obtained from a local palm oil mill at Igarra, Akoko-Edo Local Government Area, Edo State, Nigeria.

The samples were rid of adhering dirt and other foreign matter and then air-dried, ground and screened to achieve 250 and 710 micron particle sizes. The samples were stored in air-tight containers prior to analysis and anaerobic digestion. Ground samples of the LMs are shown in (Figure 1).

Anaerobic digestion experiment

Batch laboratory AD of raw samples was carried out at two particle sizes of 250 and 710 microns. Also, three dilution ratios (substrate-to-water) of 1:4, 1:6 and 1:8 (100g each of the samples was used while the volume of water was varied) were employed for a retention time of 21 days at an average ambient temperature of 37+2oC to determine optimum conditions. Buchner flasks stoppered with rubber bungs were used as digesters. Cumulative biogas produced was measured by downward displacement method with saturated salt solution every 24 hours while the production rate was calculated [10]. Saturated salt solution was used to prevent the dissolution of biogas in the water. The biogas produced was tested using flame test as described by [11]. The digesters were designated in accordance with their names, mesh sizes and dilution ratios: RH214, RH714, CC214, CC714, EG214, EG714, HC214, HC714 and OF214 OF714 etc.

Analytical methods

Raw samples and digestates were subjected to the following proximate and chemical analyses according to standard procedures: moisture content, ash content, crude protein, crude fibre, ash, total solids, volatile solids, pH, carbon, nitrogen, lignin, cellulose and hemicellulose [12,13]. pH values were determined using a digital pH meter (HANNA, Instruments, Italy).

Kinetic studies

Biodegradability of samples was carried out according to the method proposed (equation 1) by [14] to determine

Figure 1 Images of the ground substrates used in the experiment a) corn cob b) rice husk c) Huracrepitan d) elephant grass e) oil fibre.

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biodegradation rate (ko) of the process.

( )1[ expexpmG G Kot= − −

1ln ln m

m

Gor KotequationG G

= −

ln [Gm/Gm-G] was plotted against digestion time t

G (ml) is the volume of biogas accumulated after a period of t (days). Gm (ml) is the maximum accumulated gas at an infinite time, ko (per day) is the biogas rate constant, t (days) is the digestion time.

Cumulative biogas yields of the samples were fitted to a modified Gompertz equation (equation 2) to investigate AD performance for biogas production. Experimental data were compared with predicted values.

( ) ( )00

1 2maxexp exp RG t G t equation

Gλ

= − − +

G(t) Cumulative biogas yield (mL) at a digestion time (t)

Go – B iogas potential of substrate (mL)

Rmax – Maximum biogas production rate (mL/day)

λ –lag phase period prior to biogas formation (day)

t – time (day)

Scanning Electron Microscopy (SEM)

Scanning electron micrographs of raw RH, and rice husk digestate (RHD) at different magnifications were obtained to verify the impact of anaerobic microorganisms on the structure of RH. Dried samples were mounted onto the stub and coated in gold using the sputter coaters and then viewed under the scanning electron microscope (PhenomProX Model).

Statistical analysis

The data collected were subjected to analysis of variance (ANOVA) and multiple comparison procedure using Duncan

Multiple Range Test (DMRT) at 5% level of significance. An optimized fit was determined with a non-linear regression using MATLAB software package (R2011a).

RESULTS AND DISCUSSIONFigure 1 shows the images of the ground samples of the

LMs used in the research. Table 1 shows the physico-chemical properties of the samples. The initial pH values ranged from 4.73-6.17 while the final values ranged from 4.00-5.53. Three of the samples (RH, OF and CC), there was a decrease in the pH values at the end of the experiments which could be as a result of accumulation of volatile fatty acid leading to a decline in pH [15]. But for EG and HC there was an increase in the pH values at the expiration of AD. This shows that the two LMs had a better buffering capacity that is, the bicarbonate ions in the digesters were sufficient enough to resist rapid fluctuation in pH compared to other LMs [16]. Unlike some of the AD processes reported in literature, there was no pH adjustment at the beginning of the experiments in order to have a clear understanding of the buffering capacities of the substrates under experimental conditions.

The carbon/nitrogen ratios for the raw samples were within the optimum recommended for anaerobic digestion i.e. 10-30:1 [17] except for sample CC. The C/N ratio of CC was significantly high (p > 0.05) and could have been responsible for the low biogas yield [18]. C/N plays a crucial role in AD and imbalance in C/N ratio could impair biogas production. It has been reported that that a low C/N could lead to ammonia build up while a high C: N could lead to lack of nitrogen [19]; both situations are not healthy for smooth running of a biogas plant. However, CC could be used as a co-substrate for feed stocks that are deficient in carbon content in order to attain optimum ratio [20].

Volatile solid reduction (VSR) which is an indicator of substrate utilization [21] ranged from 14.84-52.60% with RH recording the highest value of 52.60% as depicted in table 1. VSR had a directly relationship with the amount of biogas produced i.e. the higher the VSR the higher the biogas production. The 52.60% VSR recorded for RH showed compared favorably with 50.80% reported for NaOH-treated corn stover by [15] while 45.04% for OF was similar to 49% reported for food waste digested under mesophilic conditions by [22].

Table 1: Physico-chemical Properties of Raw Lignocellulosic Materials.

Parameters RH OF CC EG HC

MC (%) 7.84 + 0.04 4.17 +0.03 3.83 +0.17 2.67 +0.04 8.17 +0.07

Ash Content (%) 14.67+ 0.00 10.33 +0.02 1.61 +0.05 11.15 + 0.05 9.50 +0.02

TOC (%) 47.41 +0.02 30.72 +0.05 52.31 +0.02 9.36 +0.02 50.28 +0.04

TN (%) 2.91+0.02 1.50+0.00 0.96+0.02 2.28+0.03 3.33+0.02

C/N 16.27+0.08 20.48+0.03 54.51+1.12 21.61+0.24 15.10+0.09

TS (%) 92.16+0.04 95.83+0.03 96.17+0.17 97.33+0.04 91.83+0.07

VS (%) 77.49+0.05 85.50+0.03 94.56+0.22 86.18+0.02 82.34+0.06

VSR (%) 52.60 45.04 14.84 36.54 34.79

pHi 5.30+0.10 5.83+0.06 6.17+0.15 4.73+ 0.06 4.87+0.06

pHf 4.13+ 0.06 4.00+ 0.00 5.53+ 0.06 5.23+ 0.06 5.07+ 0.06RH: Rice Husk; OF: Oil Fibre, CC; Corn Cob; EG:Elephant Grass; HC: Huracrepitan; MC: Moisture Content; TOC: Total Organic Content; TN: Total Nitrogen; C/N : Carbon Nitrogen Ratio; TS: Total Solid; VS: Volatile Solid; VSR: Volatile Solid Reduction; Phi:Initial Ph; Phf :Final Ph

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Table 2 shows changes in lignin, cellulose and hemicellulose content after AD. The percentage loss after AD was highest for cellulose, and negative values for lignin and hemicellulose showed that there was an increase in their contents. Increase in lignin and hemicellulose after AD has been reported in literature [23, 24]. In this study, OF had the highest percentage increase for lignin while CC had the least value. The increase in lignin content has been attributed to the reduction of cellulose in the substrates [25]. This was supported by the decrease in the content of cellulose in all the substrates after AD as shown in (Table 2). RH218 and OF218 had 70.53 and 61.50% percentage loss for cellulose. The results were lower than 76.93% reported for cow dung while 75.49% recorded for HC compared favourably. There was an increase in hemicellulose after AD in RH and HC but a decrease in other samples. These results showed that most of the biogas produced probably came from cellulose degradation; however, there was a poor relationship between the amount of biogas produced and the percentage of cellulose degraded. The highest biogas production corresponded to the second highest percentage loss in cellulose after AD. This may be due to the fact that there are other factors that could affect biogas production beside cellulose degradation. It is important to note that one major factor that affect the activities of microorganisms during AD process is the nature of the substrate. The results have shown that the anaerobic microorganisms were capable of degrading cellulose in the substrates, though at different rates.

Figure 2 shows the cumulative biogas production of the samples at various dilution ratios and particle sizes. The choice of the best substrates in each of the category of the samples was based on cumulative biogas production, biogas yields, biogas production rate, lag phase, dilution ratio, particle size and DT90

The optimum dilution ratio for RH, CC and OF was 1:8 while for EG and HC leaves were 1:6 and 1:4 respectively. It is important to determine the appropriate dilution ratio for each substrate since the appropriate dilution ratio facilitates movement and growth of bacteria, dissolution and transport of nutrients and also prevents scum formation [26].

The optimum particle size for RH, OF and EG was 250 microns while that of CC and HC was 710 microns. For CC and HC further decrease in size reduction did not improve biogas yield, but for RH, OF and EG an opposite trend was observed. It is noteworthy

that substrates with lower particle size had better volatile solid reduction compared to larger particle size as depicted in (Table 1). Increased specific surface area has been reported to increase biogas production by making more of the substrate accessible to anaerobic microorganisms [27]. Be that as it may, it can be seen from the results that appropriate particle size for AD may depend largely on the substrate type. There was no biogas production in digesters OF216, OF716, EG716, CC216 and HC216. The dilution ratio, which incidentally was the same in all the digesters, might be one of the reasons for the failure encountered. It has been mentioned earlier that sub-optimal dilution ratio could be detrimental to AD process. Another plausible explanation could be the rapid accumulation of volatile fatty acids (VFAs) which could lead to lowering of the pH and eventual failure of the digesters. VFAs have been reported to inhibit hydrolysis which is the rate limiting step in AD [28].

Flammability tests were conducted. Blue luminous flame was produced when a fire came in contact with biogas which was a confirmation that flammable biogas was produced.

Biogas production rate

Figure 3 and Figure 4 show production rates and peak values of OF218, RH218, HC716, CC718 and EG214. RH218 reached its peak value of 885 on day 1 which was earlier than other digesters, OF218 and CC718 reached their peaks on day 2 producing 555 and 100ml/day respectively. HC716 and EG214 produced 165 and 120ml/day on day 7. This finding has economic significance since a shorter time of attaining the peak value would reduce the cost of operation on a large-scale. Figure 5 shows the biogas yields of the LMs. Biogas yield has been defined as the biogas production per gram of VS added to a digester. The values ranged from 5.18-32.26ml/g VS with RH218 having the highest value. It can therefore be inferred that RH218had a better utilization of the feedstock than other LMs. Overall, there was a decline in biogas production as the experiments progressed, which might be due to a reduction in the amount of substrates available for anaerobic microorganisms or decrease in pH [29].

The biodegradability constants of the samples ranged between 0.0276-0.8794 d-1 (Table 3). At optimum conditions for each of the samples, that is, 250µm and dilution ratio 1:8, 710µm and dilution ratio 1:8, 710µm and dilution ratio 1:6 and 250µm and dilution ratio 1:4, designated RH218, OF218, CC718, HC716

Table 2: Changes in Lgnin, Cellulose and HemicelluloseContents after Anaerobic Digestion.

Parameters (%) RH OF CC EG HC

Lignin BD 35.80+ 0.09 23.35+ 0.01 34.57+ 0.03 28.01+0.02 25.70+ 0.02

Lignin AD 48.05+ 0.05 49.70+0.00 37.83+0.01 33.82+0.04 37.18+0.03

% loss AD -34.22 -112.85 -9.43 -20.74 -44.70

Cellulose BD 40.52+ 0.41 37.94+0.43 12.32+0.02 39.01+0.01 53.65+0.02

Cellulose AD 11.94+ 0.06 15.60+0.00 11.41+0.01 36.10+ 0.00 13.15+0.10

% loss AD 70.53 61.50 7.98 10.91 75.49

Hemicellulose BD 23.69+0.50 38.72+0.43 53.11+0.01 33.00+0.00 20.65+0.01

Hemicellulose AD 40.00 + 0.00 34.70+0.00 50.77+0.02 30.08+0.03 49.67+ 0.08

% loss AD -68.85 10.38 4.41 8.85 -140.53

*Data are means of three samples (n = 3) with standard deviation, *Negative sign indicates an increase in content after anaerobic digestion

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Figure 2 Cumulative Biogas Production of LMs at Various Dilution Ratios.

Figure 3 Biogas production rates of LMs.

Figure 4 Peak values of five substrates at optimum conditions.

Figure 5 Cumulative Biogas Yields of Lignocellulosic Materials.

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and EG214 respectively. The biodegradability constants (ko) were at their peaks, denoting high digestion efficiency i.e. higher hydrolysis rates, compared to other substrates.

Kinetic studies

Table 3 and Table 4 show the parameters used in determining the performances of the digesters. The lower lag phase (λ) (0-3 days) recorded showed a good start-up or a high hydrolysis rate in all the digesters. This shows that the microorganisms adapted quickly to the substrates. The biodegradability constants of the best samples in each of the categories namely RH218, OF218, CC718, HC716 and EG214 ranged between 0.1651-0.8794 d-1. The relatively high biodegradability constants (ko) for RH218, OF218, CC718, HC716 and EG214 indicated a higher digestion efficiency i.e. higher hydrolysis rates, compared to other scenarios studied. These values were higher than the range reported by [30], that is, 0.054 – 0.097 d-1 for organosolv pretreated biomass. The DT90 for the five samples was between 3-10 days. They are relatively

low compared to 19 days recorded for RH716. DT90 is a useful tool in recommending a minimum retention time under continuous operations [31]. This information is vital when considering the economic viability of AD process on a large scale.

Table 4 shows the percentage difference between experimental and predicted results. There was a good correlation between the measured cumulative biogas production and the predicted data using the modified Gompertz model. The nearness of the experimental and predicted value further confirms proper utilization of the substrates. The percentage differences were lower than 19.5% reported for brewery grain waste while 10.51% recorded for HC716 was close to 9.2% reported for bread waste by [32]. The predicted lag phase and the experimental were close too but there were remarkable differences in the Rmax predicted and the experimental.

Scanning electron microscopy

Figure 6 represents images of RH and RH digestate (RHD)

Table 3: Parameters for anaerobic digesters’ efficiencies.Sample DT90 Ko (d-1) lag phaseRH214 16 0.1658 0RH216 17 0.1245 0RH218 5 0.4980 0RH714 13 0.2640 0RH716 19 0.0276 2RH718 17 0.0943 0OF214 5 0.3734 1OF218 3 0.7675 0OF714 12 0.1910 2OF718 6 0.4298 1CC214 7 0.1916 0CC218 18 0.0697 0CC714 15 0.0540 2CC716 17 0.2967 0CC718 6 0.3561 0HC214 17 0.1325 1HC218 9 0.1563 2HC714 14 0.1443 1HC716 8 0.2979 1HC718 8 0.2123 1EG214 10 0.3708 1EG216 7 0.2199 1EG218 8 0.2834 1EG714 14 0.1004 1EG718 14 0.0757 3

Table 4: Comparison of Measured and Predicted Data Using Gompertz Model.

Sample Go measured (ml)

Go predicted (ml) % difference Rmaxmeasured

(ml)Rmax predicted

(ml)λ measured

(day)λ predicted

(day)RH218 2500 2569.39 2.78 885 688 0 -0.78*

OF218 1050 1046.00 0.38 555 317 0 1.27

CC718 415 427.00 2.89 100 83.75 0 -0.41

HC716 655 721.50 10.51 165 74.29 1 -0.38

EG214 670 704.00 1.57 120 56.82 1 1.46*Negative values for λ means no lag phase

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obtained by SEM to verify structural changes caused by the activities of anaerobic microorganisms during the twenty-one days of the AD process. Raw RH had a regular and compact surface structure compared to RHD. The rough, disrupted and disintegrated surface of RHD revealed the impact of anaerobic microorganisms on the feedstock.

CONCLUSIONExperimental studies on AD of some selected LMs namely

OF, HC, RH, EG and CC showed that they all had the potential to be used as substrates for biogas production. The criteria used to choose the best samples were cumulative biogas production, biogas yields, biogas production rate, lag phase, dilution ratio, particle size, biodegradability rate constant (Ko) and DT90. Based on these criteria, rice husk with particle size of 250 microns digested at ratio 1:8 (RH218) proved to be the most promising biomass for second-generation biofuel production of all the substrates.

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a b

c d

Figure 6 Scanning electron microscope images of RH and RHD a) RH, magnified 1500x, b) RHD, magnified 500x, c) RHD, magnified 1000x d) RHD, magnified 2500x.

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Olugbemide AD, Lajide L, Adebayo A, Owolabi BJ (2016) Production of Second-Generation Biofuel from Five Tropical Lignocellulosic Materials: Effects of Particle Size and Dilution Ratio. Chem Eng Process Tech 2(2): 1030.

Cite this article

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