Production of methane from anaerobic digestion of

Applied Energy 93 (2012) 148–159

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Production of methane from anaerobic digestion of jatropha and pongamia oil cakes R. Chandra a,⇑, V.K. Vijay b, P.M.V. Subbarao c, T.K. Khura a a

Department of Farm Power and Machinery, College of Agricultural Engineering and Post Harvest Technology (Central Agricultural University), Ranipool, Gangtok, Sikkim 737 135, India b Centre for Rural Development and Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India c Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

a r t i c l e

i n f o

Article history: Received 14 November 2009 Received in revised form 30 July 2010 Accepted 12 October 2010 Available online 7 January 2011 Keywords: Jatropha Pongamia Oil seed cake Anaerobic digestion Biogas Methane

a b s t r a c t The experimental study was carried out on anaerobic digestion of jatropha (Jatropha curcas) and pongamia (Pongamia pinnata) oil seed cakes in a 20 m3/d capacity floating drum biogas plant under mesophilic temperature condition. The average specific methane production potential of jatropha oil seed cake was observed as 0.394 m3/kg TS and 0.422 m3/kg VS. The average content of methane and carbon dioxide in the produced biogas over 30 days of retention time period was found as 66.6% and 31.3%, respectively. Cumulative methane yield over 30 days of retention time period was found as 131.258 m3 with a 259.2 kg of input volatile solids, with an average total volatile solids mass removal efficiency of 59.6%. However, in case of pongamia oil seed cake average specific methane production was observed as 0.427 m3/kg TS and 0.448 m3/kg VS. The average value of methane and carbon dioxide content in the produced biogas over 30 days of retention was found as 62.5% and 33.5%, respectively. Cumulative methane yield over 30 days of retention time period was found as 147.605 m3 with a 255.9 kg of input volatile solids, with an average total volatile solids mass removal efficiency of 74.9%. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction One of the major thrust towards sustainable socio-economic development of the world in 21st century is cultivation of energy resources and technologies as the depletion of petroleum fuel is at alarming level. The associated energy experts over the world are searching for supplementing the fossil fuel energy resources with cultivated bio-fuel energy resources. Development of sustainable and commercially viable technologies for production of alcohols, biogas, producer gas and bio-diesel are good examples on this scenario. The commercial viability of any bio-resource energy technology strongly depends on level of utilization of the cultivated resources and the amount of energy consumption for production of useful fuel. Bio-diesel has high potential as a new and renewable energy source in forthcoming future as a substitution fuel for petroleum derived diesel. Presently, more than 95% of bio-diesel of the world is produced from edible oil, which is available at large scale from the agricultural industry. However, continuous and large-scale production of bio-diesel from edible oil without proper planning may cause negative impact such as depletion of food supply lead to economic imbalance. A possible solution to overcome this problem is to use non-edible oil for production of bio-diesel [1].

⇑ Corresponding author. Tel./fax: +91 3592 251390. E-mail address: [email protected] (R. Chandra). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.10.049

India is endowed with more than 100 species of tree born nonedible oil seeds occurring in wild or cultivated sporadically to yield oil in considerable quantities [2]. The country has a huge potential of tree born non-edible oil seeds. Therefore, the attempts are being made for utilization of non-edible and under-exploited oils for biodiesel production. A National Mission on bio-diesel in India has been launched in the year 2003 under demonstration phase with the objective to produce enough bio-diesel to meet 20% blending of total diesel requirement using various non-edible oils by the year 2011–2012 [3]. In this context, cultivation of jatropha and pongamia (non-edible oil seed bearing plants) on 40 million hectare waste land has been started to meet the oil seed requirement. However, there are critical issues, which need to be addressed to make the production of bio-diesel as a techno-economically viable and ecologically acceptable renewable substitute or additive to diesel. Present method of utilization of only extracted vegetable oil from the bio-diesel resource results in generation of huge unutilized biomass. In general, 50% (dry weight basis) of the collected fruits of bio-diesel resource are seeds (kernels). Out of these seeds, at the most 35% is converted into vegetable oil and remaining 65% material is rejected as toxic oil seed cake. In short, more than 85% of cultivated bio-resource (seed’s pericoat and oil seed cake) is remaining unutilized in bio-diesel production. This toxic oil seed cake can neither be used as cattle feed nor as a bio-fertilizer for growing plants, due to presence of phorbol ester (a toxic compound). The current annual production of toxic jatropha oil seed cake alone has been estimated to be about 60,000 tonnes [4]. The

R. Chandra et al. / Applied Energy 93 (2012) 148–159

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Nomenclature C C/N CD d db DR g h H JC kg

carbon carbon–nitrogen ratio cattle dung day dry basis dilution ratio gram hour hydrogen jatropha oil seed cake kilo gram

estimated amount of jatropha oil seed cake could be a significant source of bio-energy production if it is utilized in a planned manner. Further, waste-to-energy provides a solution to waste management and energy generation. An integrated anaerobic waste valorization process is an interesting option for energy generation from non-edible oil seed cakes [5]. Anaerobic digestion is considered to be a sustainable bio-conversion technology as it produces biogas a renewable gaseous fuel and it also stabilizes and reduces the volume of waste. As a part of an integrated waste management system anaerobic digestion reduces the emission of green house gases into the atmosphere. The degradation process or digestion of solids in an anaerobic digester takes place in three stages. The first stage is the hydrolysis of particulate and colloidal wastes to solublise the waste in the form of organic acids and alcohols. The second stage is the conversion of the organic acids and alcohols to acetate, carbon dioxide, and hydrogen. The third stage is the production of gases mostly methane and new bacterial cells or sludge from acetate and hydrogen. In an anaerobic digester a great diversity of bacteria are required to perform phases of hydrolysis, acidogenesis and methanogenesis of the input substrate feed that contains diversified wastes in term of carbohydrates, fats and proteins [6]. The yield and constituents of biogas are greatly affected by carbohydrates, fats and proteins contents of the feed material. Anaerobic digestion of carbohydrates, fats and protein yields 886 l of biogas (with methane content of around 50%), 1535 l of biogas (with methane content of around 70%) and 587 l of biogas (with methane content of around 84%) per kg of VS destroyed, respectively [7]. The oil seed cakes of jatropha and pongamia are rich in fat and protein and therefore, are considered to be good feed material for biomethanation. The governing factors of anaerobic digestion process such as pH, retention time (RT), total solids (TS), volatile solids (VS) and organic loading rates (OLR) influence the sensitivity of bacteria, the response to toxicity and acclimatization characteristics [8]. Methanogens are sensitive to both high and low pH and perform well within pH of 6.5–8.0 [9]. Long retention time increases the potential for acclimatization and also minimizes the severity of response to toxicity. The heavy metals at higher concentration have toxic effect on bacterial activity. Further, at higher OLR non-toxic organic or inorganic substances become inhibitory to bacterial growth. The threshold toxic levels of inorganic substances depend on the conditions that whether these substances act alone or in combination. Certain combinations have a synergistic effect, whereas other display an antagonistic effect [10,11]. The carbon/ nitrogen (C/N) ratio of the feedstock has been found to be a useful parameter in providing optimal nitrogen level for bacterial growth. The optimal C/N ratio is 30 [12]. The actual available C/N ratio is a function of feedstock characteristics and digestion operational

l m3 N OLR PC STP TS TVSMRE VS

litre cubic metre nitrogen organic loading rate pongamia oil seed cake standard temperature and pressure total solids total volatile solids mass removal efficiency volatile solids

parameters and may vary from less than 10 to above 90. Since all of the carbon and nitrogen present in the feedstock are not available for digestion. Furthermore, it has been reported that at 37 °C temperature the amount of biogas production is reaches at maximum from each category of waste material under anaerobic digestion process [13]. Most of the experimental studies have been performed to find out the biogas generation potential of various feedstock mixture and its individual components of various categories of waste materials like animal dung, kitchen wastes, waste flowers, etc. In the anaerobic digestion, the pre-treated substrate produce higher amount of biogas as well as considerably reduce the total and volatile solids content in the digester. Furthermore, the chemical analysis of substrates indicates an improvement in nitrogen content after anaerobic digestion [14]. The potential biogas production from municipal garbage under batch anaerobic digestion at room temperature conditions (26 ± 4 °C) for 240 days of retention time was reported 0.661 m3 kg–1 of volatile solids. Total biogas yield from municipal garbage per kg dry matter was observed 0.50 m3 with an average methane content of the biogas of 70% by volume [15]. Anaerobic digestion of olive oil mill wastewater (OMW) mixed with diluted poultry manure (DPM) in pilot plant reactor of 100 l, containing 40% volatile solids produces biogas at a rate of 1.53 l/ d per unit volume of reactor with a methane content of 65% by volume. Co-digestion of wastewater together with local agricultural residues is a sustainable and environmentally attractive method to treat wastes and convert to useful resources. The biogas produced can be used for the generation of heat or electricity; apart from this energy co-digestion results in liquid and solid effluents that are also valuable as they retain all their nutrient constituents (nitrogen, phosphorus, trace elements, etc.). Thus, it can be used as bio-fertilizers and soil organic matter improvers [16]. The review of the literature showed that no study has been reported on anaerobic digestion of jatropha and pongamia oil seed cakes. Although, the production of these two oil seed cakes is expected to be very high in India. These feed materials could be a potential source of biogas production which would be used to supplement the petroleum demand in substantial amount.

2. Analysis of feed materials and experimental details of anaerobic digestion process 2.1. Proximate and ultimate analysis of feed material The proximate and ultimate analysis of jatropha and pongamia oil seed cakes were carried out as per standard procedure described below.

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2.1.1. Proximate analysis 2.1.1.1. Moisture content. The moisture content of the feed material was determined as follows: the initial weight of the samples of 50 g biomass with pre-weighed moisture boxes were taken by using an electronic balance with least count of 0.001 g. The samples were first heated at 60 °C for 24 h and then at 103 °C for 3 h using a hot air oven. The final weight or dried samples weight with pre-weighed moisture boxes were recorded. The percentage moisture content of the sample was then calculated by using:

MC ¼

Ww Wd 100 Ww

ð1Þ

where MC is the moisture content, % (wet basis); Ww is the weight of wet sample, g; and Wd is the weight of oven dried sample, g. 2.1.1.2. Oil content. The oil content of the mechanically expelled oil seed cakes of jatropha and pongamia were determined by Soxhlet extraction method. The samples of 200 g of jatropha and pongamia oil seed cakes were crushed using mechanical blender. The crushed samples of oil seed cake were packed in a thimble and the oil was extracted with the solvent n-hexane. The solvent n-hexane was finally removed by rotary evaporator (Laborta 4000-Heidolph Instruments, Germany) to recover the oil. 2.1.1.3. Total solids content. The total solids content of feed materials were determined as per the standard method [17]. The initial weight of the samples of 50 g biomass with pre-weighed porcelain boxes were taken by using an electronic balance with least count of 0.001 g. The samples were first heated at 60 °C for 24 h and then at 103 °C for 3 h using a hot air oven. The final weight or dried samples weight with pre-weighed porcelain boxes were recorded. The percentage total solids content of the sample was then calculated by using:

Wd 100 TS ¼ Ww

ð2Þ

where TS is the total solids, %; Wd is the weight of oven dried sample, g; and Ww is the weight of wet sample, g. 2.1.1.4. Volatile solids content and non-volatile solids content. The volatile solids content and non-volatile solids content of feed materials were determined as per the standard method [17]. The oven dried samples used for determination total solids content were further dried at 550 °C ± 50 °C temperature for 1 h in a muffle furnace and allowed to ignite completely. The dishes were then transferred to a desiccator for final cooling. The weight of the cooled porcelain dishes with ash were taken by the electronic balance. The volatile solids content and non-volatile solids content of the sample were calculated using:

ðW d W a Þ 100 Wd Wa 100 NVS ¼ Wd

VS ¼

ð3Þ ð4Þ

where VS is the volatile solids in dry sample, %; NVS is the non-volatile solids in dry sample, %; Wd is the weight of oven dried sample, g; Wa is the weight of dry ash left after igniting the sample in a muffle furnace, g. 2.1.2. Ultimate analysis Carbon, hydrogen and nitrogen contents in feed materials (cattle dung, jatropha oil seed cake and pongamia oil seed cake) were determined using fully automatic instrument ‘Vario EL’ elemental analyzer (Perkin Elmer, USA Made) which enables speedy and accurate quantitative analysis of CHN in the sample. The instru-

ment works on the principle of thermal conductivity detector (TCD). 2.2. Start up of anaerobic digester The major challenge in anaerobic digestion of jatropha and pongamia oil seed cakes is lack of inherent bacteria like in cattle dung. Apart from the existing bacteria in a digester, fresh cattle dung continuously adds more bacteria to the digestion system and stabilizes the anaerobic digestion process. However, lack of the inherent bacteria, demands a special attention for operation of digester with non-edible oil seed cakes. Another major drawback of oil seed cake is the presence of long chain free fatty acids, which can destroy the population of bacteria in the digester. Moreover, an appropriate amount of cattle dung with oil seed cake may stabilize the bacterial population. The time between initial digester feed sludge and stable operation of digester should be as short as possible for smooth start-up of the anaerobic digestion process. The steady-state condition for efficient operation of the digester is normally achieved approximately in one month. This condition is reflected by the production of burnable biogas and a stable volatile acid-to-alkalinity ratio [6]. The start-up is generally considered the most critical step in the operation of anaerobic digesters. Once an anaerobic digester has been started up successfully, it is expected to run without much attention as long as operating conditions are not significantly altered. The source of micro-organisms, the size of the inoculum and the initial mode of operation are important factors during start-up. Usually, the inoculum volume is at least 10% of the new digester volume and consists of an undefined mixed culture from an equivalent system that is actively digesting a similar feedstock [18]. 2.3. Preparation of efficient inoculum In the present study, a running 20 m3/d capacity floating drum type biogas plant with cattle dung substrate was selected as an environment. The feeding of cattle dung was stopped for three months to make sure that there is no unprocessed cattle dung present in the digester (no volatile matter) prior to feeding of non-edible oil seed cakes. Thereafter, feeding of pongamia oil seed cake with a dilution ratio of 3:1 (water:oil seed cake on weight basis) was carried as per following schedule. Schedule 1. 8 kg of oil seed cake substrate (2 kg pongamia oil seed cake with 6 kg water) with a dilution ratio of 3:1 for 5 days. The position of gas holder drum was remain unchanged for first 2 days of the experiment. However, addition of oil seed cake substrate was continued. On third day of the experiment a small rise (approximately 10 cm) of gas holder drum was recorded which is equivalent to 0.90 m3 volume of biogas. The same feeding pattern was continued for two more days and a rapid rise in gas holder drum was observed. This showed encouraging results in biogas production from pongamia oil seed cake. Schedule 2. 20 kg of oil seed cake substrate (5 kg pongamia oil seed cake with 15 kg water) with a dilution ratio of 3:1 for next 10 days: For the first 2 days of increased loading, a drop in gas production was observed as compared to fifth day of the experiment. The feeding was continued for few more days and positive results were observed on third day with rapid upward movement of gas holder drum. The gas holder drum reached at its highest position (30 cm) on fifth day and remained almost at the same level up to tenth day during the experiment.

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was calculated by adding daily biogas, methane and carbon dioxide production, respectively. Specific biogas production (m3/kg TS & m3/kg VS), specific methane production (m3/kg TS & m3/kg VS) and total volatile solids removal efficiency (%) were determined by using standard formulae.

This pattern of biogas production from pongamia oil seed cake showed the adaptation of bacteria to the changed environment offered by the new substrate possibly by developing a suitable strain. This acclimatization is due to the fact that, when the concentrations of inhibitory or toxic materials are slowly increased within the environment many micro-organisms rearrange their metabolic resources, and overcoming the metabolic block produced by the inhibitory or toxic materials. However, sufficient time must be available to the bacteria for the rearrangement of metabolic resources under sudden change in environment [19]. The slurry of the biogas plant fed with pongamia oil seed cake was used as inoculum for anaerobic digestion of jatropha and pongamia oil seed cakes substrates. The new micro-organisms present in the inoculum could act comfortably with jatropha and pongamia oil seed cake substrates. The above study lead to a set of important developments namely, an effective inoculum as a pool of new micro-organisms, an optimal size of the inoculum and a mode of operation of the anaerobic digester [18].

2.4.1. Theoretical calculation Observed daily biogas production was corrected at standard temperature and pressure (STP) condition using Eq. (5). STP refers to 0 °C (273 K) temperature and one atmospheric pressure.

BVo ¼

ð5Þ

where BVo is the volume of daily produced biogas at STP (at 0 °C), l or m3; BV is the volume of daily produced biogas at temperature T, l or m3; T is the observed biogas temperature, °C. The daily production of methane and carbon dioxide in produced biogas were determined by:

CH4Yield ¼

2.4. Experimental biogas plant and parameters of anaerobic digestion process

CH4 Conc: BVo 100

CO2Yield ¼

CO2 Conc: BVo 100

BVoSpecificTS ¼

BVoSpecificVS ¼

BVo DMF TS

BVo DMF VS

ð8Þ

(b)

10 Diamter ASB/CEM Pipe

Flange Plates

Central Guide Frame

15 Thick Partion Wall

Earth Filling

10 Diamter ASB/CEM Pipe

CC Foundation (1:3:6)

D

B1

B

C

D

30

30 15

7. 5 23

A

ð9Þ

where BVoSpecificTS is the specific biogas production, m3/kg TS; BVoSpecificVS is specific biogas production, m3/kg VS; DMF is the daily mass of feed, kg; TS is the total solids content, decimal; VS is the volatile solids content, decimal.

Ground Level

30

ð7Þ

where CO2Yield is the carbon dioxide yield at STP, l or m3; CO2 Conc. is the carbon dioxide concentration in biogas, %. The specific biogas production (per unit TS and VS) were calculated using:

Gas Holder Supporting Structure

F

ð6Þ

where CH4Yield is the daily methane yield at STP, l or m3 CH4 Conc. is the methane concentration in biogas, %

Anaerobic digestion of jatropha and pongamia oil seed cake substrates were carried out in a floating drum biogas plant of 20 m3/d capacity by continuous (daily) feeding of substrates for 30 days. Fig. 1a and b shows the schematic and pictorial view of experimental biogas plant. Table 1 shows daily feeding level of jatropha and pongamia oil seed cake substrates. Measurement of ambient temperature and substrate temperature (°C) was carried by using K-type thermocouple. Substrate temperature was measured by inserting a thermocouple into the digester of the biogas plant at a depth of 1.0 m. The daily biogas production (m3) at standard temperature and pressure (STP) was measured. The methane and carbon dioxide content in the produced biogas was measured by using a Biogas Analyzer (Model No. MG-609u) made by Chemtron Science Limited, Mumbai, India. This biogas analyzer was specially built for compositional analysis of biogas constituents (CH4 and CO2). The infrared sensors for methane and carbon dioxide have measurement range of 0–100%. The cumulative biogas, methane and carbon dioxide production over the study period

(a)

273 BV 273 þ T

23 7.5

All dimensions in centimetres

Fig. 1. (a and b) A view of biogas plant (20 m3/d) fed with jatropha and pongamia oil seed cakes.

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Table 1 Total solids and volatile solids concentration in the substrates. Treatment

Substrate concentration of the daily feed material Total solids

Jatropha oil seed cake substrate Pongamia oil seed cake substrate

Volatile solids

kg/d

%

kg/d

%

9.25 8.95

18.5 19.9

8.64 8.53

17.3 19.0

The specific methane production (per unit TS and VS) was determined using:

MVoSpecificTS ¼

CH4Yield DMF TS CH4Yield DMF VS

ð11Þ

where TVSMRE is the total volatile solids mass removal efficiency, %; TVSM is the total volatile solids input mass, kg.

ð15Þ

3. Results and discussion 3.1. Properties of feed materials Tables 2 and 3 show the proximate and ultimate analysis of feed materials. The proximate analysis of jatropha and pongamia oil seed cakes showed the volatile solids content of these oil seed cakes was more than six times higher than that of cattle dung. Table 2 shows that the non-volatile solids content of both oil seed cakes was very low in comparison to cattle dung. This is due to non presence of lingo-cellulosic materials in oil seed cake. Table 3 clearly shows that the carbon and hydrogen contents in the oil seed cakes were also higher than that of cattle dung.

ð12Þ

The requirement of above approach is that the biogas volume and its contents should be accurately measured. The molecular weight of methane and carbon dioxide as well as dry biogas volume were then correlated to obtain total volatile solids mass removed. The following relationship (Eq. (13)) was used to obtain the total volatile solids mass removed in the anaerobic digestion process:

TVSMR ¼

2 Conc:Þ þ ½ð44CO100 BVo DBF 100 22:413 TVSM

TVSMRE ¼

where MVoSpecificTS is the specific methane production, m3/kg TS; MVoSpecificVS is the specific methane production, m3/kg VS. The loss of volatile solids during anaerobic digestion process occurs due to conversion of the volatile solids primarily into biogas. Thus, the total volatile solids mass removal efficiency was estimated based on biogas production rate. In this estimation method, it was assumed that the organic mass converted into biogas is equal to the mass of dry biogas produced. The methane and carbon dioxide content in the biogas was determined for every operating day during the entire period of operation. An assumption was made that biogas behaves as an ideal gas. The total volatile solids mass removed was then assumed to be equal to mass of methane and carbon dioxide produced as given in:

TVS mass removed ¼ Mass of CH4 þ Mass of CO2

½ð16CH4 Conc:Þ 100

ð10Þ

MVoSpecificVS ¼

Thus, for estimation of total volatile solids mass removal efficiency the Eq. (14) becomes:

½ð16CH4 Conc:Þ 100

2 Conc:Þ þ ½ð44CO100 BVo DBF 22:413

ð13Þ

where TVSMR is the total volatile solids mass removed, kg; BVo is the daily biogas volume at STP (at 0 °C), m3; DBF is the dry biogas factor. The constants 16 and 44 represent the molecular weight of methane and carbon dioxide, respectively. The volume of one mole of ideal gas at STP was taken as 22.413 l in the above equation.The total volatile solids mass removal efficiency of the anaerobic digestion process is expressed as the % removal of initial total volatile solids. The total volatile solids mass removal efficiency (TVSMRE) was calculated using the following relationship given in:

TVSMRE ¼ ðTotal VS mass removed=Initial total VS mass fedÞ 100 ð14Þ

3.2. Parameters of anaerobic digestion process 3.2.1. pH of input substrate The average pH of input substrate for the jatropha oil seed cake substrate, JC (4.0 DR, 0% CD) was found as 6.8. Similarly, the average pH of input substrate for pongamia oil seed cake substrate, PC (3.5 DR, 0% CD) was found as 6.1. It has been reported that, most anaerobic bacteria including methane-forming bacteria perform well within a pH ranges from 6.8 to 7.2. The pH in an anaerobic digester initially decreases due to the production of volatile acids. However, as methane-forming bacteria consume the volatile acids and alkalinity is produced, the pH of the digester increases and then stabilizes. At hydraulic retention time more than 5 days the methane-forming bacteria begin to rapidly consume the volatile acids. In a properly operating anaerobic digester the pH is maintained between 6.8 and 7.2 as volatile acids are converted into

Table 3 Carbon, hydrogen, nitrogen contents and carbon–nitrogen ratio of the feed materials. Sl. no.

Feed material

C (%)

H (%)

N (%)

C/N ratio

s1 2 3

Cattle dung Jatropha oil seed cake Pongamia oil seed cake

35.20 48.80 47.80

4.60 6.20 6.50

1.55 3.85 5.50

22.7 12.7 8.7

Table 2 Physiochemical properties of basic feed materials. Feed material

Cattle dung Jatropha oil seed cake Pongamia oil seed cake

Physiochemical properties Moisture content (%)

Oil content (%)

Total solids (%)

Volatile solids (%)

Non-volatile solids (%)

81.6 (442.5 db) 7.5 (8.1 db) 10.5 (11.7 db)

– 8.3 7.2

18.4 92.5 89.5

14.4 (78.8 db) 86.4 (93.0 db) 85.3 (94.8 db)

21.2 7.0 5.2


153

Fig. 2. Daily biogas production rate from jatropha oil seed cake substrate.

methane and carbon dioxide. The pH of an anaerobic system is significantly affected by the carbon dioxide content of the biogas [6]. 3.2.2. Daily biogas production Fig. 2 shows the relationship between the daily biogas production yield and substrate temperature with respect to retention time for jatropha oil seed cake substrate, JC (4.0 DR, 0% CD) at feeding rate of 9.25 kg TS/d. The average daily biogas production during

30 days retention time period was observed as 6.541 m3/d. Similarly, daily biogas production yield at feeding rate of 8.95 kg TS/d and substrate temperature with respect to retention time for pongamia oil seed cake substrate (PC (3.5 DR, 0% CD)) is depicted in Fig. 3. Average daily biogas production over a period of 30 days was found as 7.791 m3/d. It was observed that biogas production rate became stable after eighth day of digestion process. Figs. 2 and 3 depicted that the biogas production rate during initial days

Fig. 3. Daily biogas production rate from pongamia oil seed cake substrate.

154


(1–7 days) was very low as compared to than later days of retention time. It has been reported that when the hydraulic retention time is more than 5 days, the methane-forming bacteria begin to rapidly consume the volatile acids, and thereby more biogas production occurs [6]. The substrate temperature during anaerobic digestion of jatropha oil seed cake and pongamia oil seed cake were observed in the range of 26.3–34.5 °C and 33.7–35.2 °C, respectively. The observed substrate temperature indicates that the digester was operating in mesophilic temperature range. It has been reported that

mesophilic methanogens come into play in the temperature range of 20–45 °C and the biogas production reaches the maximum when the process temperature is maintained around 35 °C [20]. Furthermore, it has also been reported that most of the methane-forming bacteria are active in mesophilic range from 30 to 35 °C [21]. 3.2.3. Methane and carbon dioxide content of produced biogas Fig. 4 shows the variation of methane and carbon dioxide content in the produced biogas from jatropha oil seed cake substrate. The maximum and minimum values of methane and carbon

Fig. 4. Variation of methane and carbon dioxide content in produced biogas from jatropha oil seed cake substrate.

Fig. 5. Variation of methane and carbon dioxide content in produced biogas from pongamia oil seed cake substrate.


dioxide were found to vary from 68.0% to 60.7% and 32.7% to 29.0%, respectively starting from first day to thirtieth day of retention time. The average methane and carbon dioxide content over 30 days were found as 66.6% and 31.3%, respectively. Similarly, the variation of methane and carbon dioxide content of biogas produced from pongamia oil seed cake substrate is shown in Fig. 5. The maximum and minimum values of methane and carbon dioxide were found to vary from 65.3% to 56.0% and 38.3% to 31.7%,

155

respectively. The average value of methane and carbon dioxide content over 30 days of retention time period were found as 62.5% and 33.5%, respectively. The observed values of methane concentration in the produced biogas from jatropha and pongamia oil seed cake substrates have showed a significantly higher methane percentage than the produced biogas from cattle dung. This fact is due to degradation of fats and proteins give more methane content (70–84%) in the

Fig. 6. Cumulative biogas, methane and carbon dioxide yields from jatropha oil seed cake substrate.

Fig. 7. Cumulative biogas, methane and carbon dioxide yields from pongamia oil seed cake substrate.

156


biogas than 50% from carbohydrates [6]. It was also observed that the variation of the average methane content in the biogas produced from jatropha oil seed cake substrate was marginally higher in comparison to pongamia oil seed cake substrate. 3.2.4. Cumulative biogas, methane and carbon dioxide production Fig. 6 shows cumulative biogas, methane and carbon dioxide production over 30 days retention period for jatropha oil seed cake

substrate. The cumulative biogas, methane and carbon dioxide production were found as 196.224, 131.258 and 61.271 m3, respectively with total 259.2 kg of input volatile solids. Fig. 7 shows cumulative biogas, methane and carbon dioxide production over 30 days retention period for pongamia oil seed cake substrate. The cumulative biogas, methane and carbon dioxide production were found as 233.725, 147.605 and 77.625 m3, respectively with total 255.9 kg of input volatile solids.

Fig. 8. Variation of specific biogas yield on jatropha oil seed cake substrate.

Fig. 9. Variation of specific biogas yield on pongamia oil seed cake substrate.


3.2.5. Specific biogas production rate The variation in specific biogas production yield per unit TS and per unit VS in case of jatropha and pongamia oil seed cake substrates are depicted in Figs. 8 and 9, respectively. The observed range of specific biogas production yield with jatropha oil seed cake substrate with in 30 days retention time period was observed in the range of 0.16–0.71 m3/kg TS and 0.17–0.76 m3/kg VS. Simi-

157

larly, the average specific biogas production yield over 30 days retention time period on pongamia oil seed cake substrate was found in the range of 0.17–0.87 m3/kg TS and 0.18–0.91 m3/kg VS. The average specific biogas yield with jatropha oil seed cake substrate over the 30 days retention time period was recorded as 0.598 m3/kg TS and 0.640 m3/kg VS. Similarly, the average value of specific biogas production yield with pongamia oil seed cake

Fig. 10. Variation of specific methane yield on jatropha oil seed cake substrate.

Fig. 11. Variation of specific methane yield on pongamia oil seed cake substrate.

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over the 30 days retention time period was observed as 0.703 m3/ kg TS and 0.738 m3/kg VS. 3.2.6. Specific methane production rate The variation in specific methane production yield per unit TS and per unit VS in case of jatropha and pongamia oil seed cake substrates are shown in Figs. 10 and 11, respectively. The range of specific methane production yield with jatropha oil seed cake

substrate over the 30 days retention time period was found 0.097–0.473 m3/kg TS and 0.104–0.506 m3/kg VS. Similarly, the specific methane production yield over the 30 days retention time period with pongamia oil seed cake substrate was found in the range of 0.096–0.55 m3/kg TS and 0.100–0.577 m3/kg VS. The average specific methane production yield with jatropha oil seed cake substrate over the 30 days retention time period was recorded as 0.394 m3/kg TS and 0.422 m3/kg VS. The average specific

Fig. 12. Variation of total volatile solids mass removal efficiency of the anaerobic digestion process on jatropha oil seed cake substrate.

Fig. 13. Variation of total volatile solids mass removal efficiency of the anaerobic digestion process on pongamia oil seed cake substrate.


methane production yield over the 30 days retention time period with pongamia oil seed cake substrate was observed as 0.427 m3/ kg TS and 0.448 m3/kg VS.

3.2.7. Total volatile solids mass removal efficiency of the anaerobic digestion process Fig. 12 shows the variation in total volatile solids mass removal efficiency of the anaerobic digestion process of jatropha oil seed cake, JC (4.0 DR, 0% CD). The total volatile solids mass removal efficiency of jatropha oil seed cake substrate over the 30 days retention time period was found in the range of 15.7–71.8%. Similarly, Fig. 13 shows the variation in total volatile solids mass removal efficiency of the anaerobic digestion process of pongamia oil seed cake, PC (3.5 DR, 0% CD). The total volatile solid mass removal efficiency on pongamia oil seed cake substrate over the 30 days of retention time period was found in the range of 16.0–97.2%. The average total volatile solids mass removal efficiency of jatropha oil seed cake substrate over the 30 days retention time period was recorded as 59.6%. Further, the average total volatile solids mass removal efficiency over the 30 day retention time of pongamia oil seed cake substrate was observed as 74.9%. The study revealed that the biogas yield per unit TS and VS was found higher in case of pongamia oil seed cake substrate than that of jatropha oil seed cake substrate. This is due to lower content of non-volatile solids in pongamia oil seed cake (5.2%) than in jatropha oil seed cake (7.0%). Furthermore, it was observed that the pongamia oil seed cake has higher biodegradability than jatropha oil seed cake, may be due to higher concentrations of long-chain fatty acid oleates and stearates in jatropha oil seed cake.

4. Conclusions The proximate and ultimate analysis of jatropha and pongamia oil seed cakes confirmed that they have rich proportionate of volatile solids content. These oil seed cakes have low non-volatile solids content, higher content of hydrogen and carbon as compared to the cattle dung. Results show that the jatropha and pongamia oil seed cakes contain more than six times higher volatile solids content in comparison to that of cattle dung. Further, the anaerobic digestion of jatropha oil seed cake, JC (4.0 DR, 0% CD) is resulted into an average specific biogas and specific methane production potential of 0.640 m3/kg VS and 0.422 m3/kg VS, respectively with an average total volatile solids mass removal efficiency of 59.6%. Whereas, the anaerobic digestion of pongamia oil seed cake, PC (3.5 DR, 0% CD) yields an average specific biogas and specific methane production of 0.738 m3/kg VS and 0.448 m3/kg VS, respectively with an average total volatile solids mass removal efficiency of 74.9% over a retention time period of 30 days. The biogas produced from jatropha and pongamia oil seed cakes contains 15–20% more methane than the biogas produced from the cattle dung.

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