A SIMPLE APPARATUS FOR BIOGAS QUALITY DETERMINATION

Misr J. Ag. Eng., 25(3): 1055- 1066

BIOLOGICAL ENGINEERING

A SIMPLE APPARATUS FOR BIOGAS QUALITY DETERMINATION Abdel-Hadi, M. A. ABSTRACT A prototype experimental setup was constructed and investigated to determine the quality of biogas produced during anaerobic fermentation of cow dung. The technique depends on measuring percentage of carbon dioxide volume using 40% potassium hydroxide. The biogas and carbon dioxide percentage were collected from a bench-scale batch anaerobic digester (vertical type) under room temperature every week for 16 weeks retention time without mixing to determine the biogas quality throughout the retention time. The volume of gases was re-calculated for standard temperature and pressure (STP). The result has been verified by determining the methane percentage of the same sample of biogas using a gas chromatography (Chrompack CP 9001) and the carbon dioxide percentage of the biogas was proportional to its quality. INTRODUCTION

B

iogas is mainly composed of methane (CH4), carbon dioxide (CO2) and low amount of other gases (Yadava and Hcssc, 1981). (GTZ, 1999) mentioned that, biogas is a mixture of gases that is composed chiefly of methane 40-70 vol.%, carbon dioxide 30-60 vol.% and other gases 1-5 vol.% including hydrogen (H2) 0-1 vol.% and hydrogen sulfide (H2S) 0-3 vol.%. (Tjalfe, 2003) reported the same percent of methane and carbon dioxide while the other gases 0 - 3 vol.%. The quality of biogas generated by organic waste materials does not remain constant but varies with the period of digestion (Khandewal and Mahdi, 1986). The ratio of CH4 to CO2 is normally stable in the reactor and a change of the ratio can be due to process imbalance. However, the methane ratio also depends on substrate composition, temperature, pH and pressure (Liu, 2003). Since the dissolution of CO2 is strongly dependent on pH, Assist. Professor, Agric. Eng. Dept., Fac. of Agric., Suez Canal Univ., 41522 Ismailia, Egypt. Misr J. Ag. Eng., July 2008

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fluctuation of pH can also change gas composition. The important processes in anaerobic digestion are hydrolysis, fermentation, acetogenesis, and methanogenesis. In the hydrolysis stage, complex organic materials are broken down into their constituent at parts such as amino acids, fatty acids, simple sugars and glucose (United Tech., 2003). In the Acidogenesis process, acidogenic bacteria turn the products of hydrolysis into simple organic compounds, mostly short chain (volatile) acids. The transition of the substrate from organic material to organic acids in the acid forming stages causes the pH of the system to drop. This is beneficial for the acidogenic and acetagenic bacteria that prefer a slightly acidic environment, with a pH of 4.5 - 5.5, and are less sensitive to changes in the incoming feed stream, but is problematic for the bacteria involved in the next stage of methanogenesis. Methanogens are very sensitive to changes and prefer a neutral to slightly alkaline environment (Gas Technology, 2003). If the pH is allowed to fall below 6, methanogenic bacteria cannot survive. A better indicator is therefore methane production (Hansson et al., 2002 and Liu, 2003). The change in pH can be both an indicator and the cause of process imbalance. The quality of biogas depends mainly on the presence of methane in it. A good quality of biogas has high percentage of methane. The percentage of methane in biogas is generally determined by the Orsat apparatus, gaschromatograph etc. (Holman, 1995). Savery and Cruzon (1972) suggested that, the three agents KOH, NaOH and Ca(OH)2 can be used in chemical scrubbing of biogas. The absorption of CO2 in alkaline solution is assisted by agitation. The turbulence in the liquid aids to diffusion of the molecule in the body of liquid and extends the contact time between the liquid and gas. Another factor governing the rate of absorption is concentration of the solution. A solution of potassium hydroxide (KOH), sodium hydroxide (NaOH) and water has enhanced scrubbing capabilities for CO2 removal because the physical absorption capacity of the water is increased by the chemical reaction of the KOH and NaOH. Konstandt (1976) mentioned that the percentage of methane CH4 can be estimated through recognition of CO2 percentage from this equation: CH4 = 100% - [CO2% + 0.2% H2S] vol.%

Misr J. Ag. Eng., July 2008

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Methane content was measured by absorption of carbon dioxide with10%, 33% and 40% of KOH (APHA et al., 1992; Hamilton and Stephen, 1964; Okeke and Ezekoye, 2006) respectively. The assumption by using this method was that biogas mainly constituted of methane and carbon dioxide gas, where the other gases produced during anaerobic process were neglected. Ergüder et al. (2000) reported that, the gas production in batch reactors was determined by a water displacement device. The content of CH4 in biogas was determined as follows: A known volume of the headspace gas (V1) produced in a serum bottle used in biochemical methane production (BMP) experiments was syringed out and injected into another serum bottle which contained 20 g/l KOH solution. This serum bottle was shaken manually for 3-4 min so that all the CO2 was absorbed in the concentrated KOH solution. The volume of the remaining gas (V2), which was 99.9% CH4, in the serum bottle was determined by means of a syringe. The ratio of V2=V1 provided the content of CH4 in the headspace gas. This paper is an attempt to ease the biogas quality determination using a simple apparatus that use a chemical method such as potassium hydroxide solution to determine the CO2 percentage. The CO2 percentage in the biogas is proportional to its quality. MATERIALS AND METHODS Biogas Digester A bench-scale of cylindrical biogas digester (vertical type) as shown in fig. (1) was constructed at the Agric. Eng. Dep., Fac. of Agric., SuezCanal Univ. The digester was fabricated from galvanized steel sheet of 1.5 mm thickness, 45 cm length and 25 cm diameter with total capacity of 22 liters and digestion volume of 16 liters. The digester has inlet and outlet tube of 50.8 mm (2 in.) diameter for feeding by organic wastes and rejecting the digested materials. To follow up the digestion processes, orifice for releasing the produced gas was provided to the digester. Released gas volume was collected in gasholder and determined by using the wetted displacement with a previously calibrated scale in liter. The digestion system was batch anaerobic fermentation under room


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temperature (measophilic reaction) for 16 weeks retention time without mixing to obtain the biogas production.

Gas outlet Water Gasholder Scale (Liter)

Pressure balance chamber Biogas

Substance inlet

Biogas removal Substance outlet

Fig. (1): Schematic diagram of bench-scale biogas digester (vertical type) Biogas Quality Determination The biogas digester and KOH apparatus for determine the biogas quality were designed and constructed at the Agric. Eng. Dept., Fac. of Agric., Suez Canal Univ. The simple apparatus consists of glass U-tube shape with 12 mm internal diameter for filling by potassium hydroxide solution. The U-tube hitched with tap to adjust the level of solution with Misr J. Ag. Eng., July 2008

1058

atmospheric pressure after removal of CO2. The tube was fitted with injection of samples as a biogas inlet and with gas outlet to release gases after removal of CO2 as shown in fig. (2).

1 2 8 3

7 6

4 5 1 - Funnel 5 - Flask

2 - Holder 3 - Glass tube 4 –Tap 6 - Injection of samples 7 - Scale 8 – Gas outlet

Fig. (2): KOH apparatus to determine methane and carbon dioxide. The temperature and pH were measured regularly every week using Jenway temperature and pH meter, model 370pH/mv hand held meter (Jenway ® 2006) for re-calculated the biogas volume. Methane percentage was measured using the potassium hydroxide solution in the laboratory scale investigation. The released gas was fractioned in a percentage (i.e. methane and CO2 percentage) using the 40% potassium hydroxide. All measurements were carried out under room temperature and atmospheric pressure. The volume gases were recalculated for standard temperature and pressure STP (Hansen et al.,


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2004). The percentage of methane in each sample was determined using a gas chromatography GC (Chrompack CP 9001) at a flow rate of 18 ml/min with helium as the carrier gas. The flame-ionization-detector (FID) operated at a flow rate of H2 24 ml/ min and makeup N2 of 30ml/ min. The percentage of CO2 was calculated as follow: Percentage of CO2 

V1  V2 x 100 vol.% V1

(1)

Where V1 : volume of biogas before removal of CO2, ml V2 : volume of methane and the other gases after removal of CO2, ml The volume of biogas, methane and the other gases were re-calculated for standard temperature and pressure (STP: 0 oC and 1 bar) by using ideal gas law. PV = NRT (2) The atmospheric pressure (P) equal the gas pressure (Pm) during adjustment the solution level in the apparatus.

V V Where

Vm .Pm .T VmP.T.Tm Tm

(3) (4)

V : gas volume at STP, ml, Vm : gas volume at room temperature, ml, T : standard temperature, 0 oC (273 oK) and Tm : room temperature in oK. Percentage of CH4 = 100% - [CO2% + 3% (H2S and the other gases)] vol.,% (5) Where


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3% is the average of H2S and the other gases percentage 1-5 vol.% (GTZ, 1999) The SPSS statistical package, version 10.0 (SPSS Inc., Michigan, USA), was used for the statistical analysis. From the data obtained in the experimental, Bivariate correlations analysis was done to establish the presence or absence of significant differences in the methane percentage determination by potassium hydroxide solution 40% and gas chromatography. RESULTS AND DISCUSSIONS The anaerobic process (acetogenesis stage) in digester will normally leaded to accumulation of volatile fatty acids (VFA) resulting in a decrease in pH. At the same time, the CO2 gradually increased after the second week and then reached its to maximum value at the fifth week. After the fifth week, the pH value slowly gradually increased (fig. 3) this refers to the digested material has a high buffer capacity. At that time, the CO2 gradually decreased and methane production gradually increased to balance the process in the digester. The results obtained from the experiments can be used to analyze the quality of biogas as a function of its CO2 percentage. On the other hand, when CO2 percentage decreased in biogas produced the percentage of methane will be increased. Fig. 4 shows the variation of methane percentage and biogas production at different retention times. It can be seen that the methane percentage of biogas sample is gradually increases after the second week and then reaches a maximum value at the middle of the retention time. It can also be observed that, due to no production of biogas a very negligible percentage of methane in biogas samples before the second week and after sixteen weeks from retention time. Fig. 5 shows the result of methane percentage by GC and KOH solution method. The methane percentage by GC was less than from KOH method in the beginning of the retention time anaerobic process due to the other gases such as H2S and ammonia were high percentage. However, it was by both methods approximately congruent and the same direction from fifth week to the end of the retention time.


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100

8.00 Methan

Carbon dioxide

7.80

pH value

80

7.60

70

7.40

60

7.20

50

7.00

40

6.80

30

6.60

20

6.40

10

6.20

0

6.00 0

1

2

3

4 5

6

7

8

9 10 11 12 13 14 15 16 17 18

Hydraulic retention time, week

100 90 80 70 60 50 40 30 20 10 0

3.5

Methane

3.0

Biogas production

2.5 2.0 1.5 1.0 0.5

Biogas production, l u

Methane, %

Fig. (3): Relationship among the methane, carbon dioxide percentage and the pH value during the retention time.

0.0 0 1 2

3 4 5 6

7 8 9 10 11 12 13 14 15 16 17 18


Fig. (4): Variation of methane percentage and the biogas production in different weeks of the retention time. Misr J. Ag. Eng., July 2008

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pH value

Methane and Carbon dioxide, %

90

Methane, %

100 90 80 70 60 50 40 30 20 10 0

Methane, (KOH) method Methane, (GC) analysis 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18


Fig. (5): Comparison between methane percentage by KOH method and GC analysis. Table (1) shows a comparison between the Std. deviation, the mean and correlation between the derived KOH solution and GC method. Table (1): Significance levels of the correlation between the KOH and GC methods. Std. N Mean correlations Deviation KOH

14

10.1151 61.7286

GC

14

12.1758 61.4786

0.968**

** Correlation is significant at the 0.01 level


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CONCLUSION - When pH value gradually increased the methane production gradually increased and at that time the CO2 gradually decreased. - The CO2 percentage of biogas mixture should be proportional indicator of its quality. - The result indicated that, the methane percentage by GC different than by KOH solution method in the beginning of the retention time anaerobic process. However, it was nearly in line direction in middle and end the retention time. - The statistical analyses result showed that, the correlation was found significant between the derived KOH solution and GC method at the 0.01 level - The KOH apparatus is simple and the using easier than other conventional methods especially in development country and cheaper than others methods. REFERENCES APHA, AWWA, and WEF. (1992). Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington. Ergüder, T. H., E. Güven, And G. N. Demirer (2000). Anaerobic treatment of olive mill waste in batch reactors. Process Biochem. 36: 243–248. Gas Technology, I. (2003). HIMET-A Two-Stage Anaerobic Digestion Process for Converting Waste to Energy. Institute of Gas technology, Chicago IL. GTZ-GATE (1999). Biogas Digest (Volume I. Biogas Basics) GTZGATE. Eschborn, Germany. http://www2.gtz.de/dokumente/bib/045364.pdf Hamilton, L. F. and G. S. Stephen (1964). Quantitative chemical analysis. The machmillan company, New York, pp. 454 – 459. Hansen, T. L., J. E. Schmidt, I. Angelidaki, E. Marca, J. Jansen, H. Mosbaek and T. H. Christensen, (2004) Method for determination of methane potentials of solid organic waste. Waste Management 24: 393–400.


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Hansson, M., A. Nordberg, I. Sundh and B. Mathisen (2002). Early warning of disturbances in a laboratory-scale MSW biogas process. Water Science and Technology, 5 (10): 255-260. Holman, J. P. (1995). Experimental methods for engineers, 6th ed. New Delhi: Tata McGraw-Hill, p. 539-43. Jenway ® (2006). Operating Manual of the 370 pH/mv meter. Jenway, Gransmore Green Felsted, Dunmow Essex CM6 3 LB, England. Khandewal, K. C. and S. S. Mahdi, (1986). Biogas technology: practical handbook. New Delhi: Tata McGraw-Hill, p. 51-2. Konstandt, H. G. (1976). Engineering’s operation and economics of methane gas production. Seminar on Microbial Energy Conversion, Gottingen, Erich Goetze Verlag, Germany Liu, J. (2003). Instrumentation, Control and Automation in Anaerobic Digestion. Ph.D. dissertation, Department of Biotechnology, Lund University, Sweden. Okeke, C.E. and V.A. Ezekoye, (2006). Design, construction, and performance evaluation of plastic biodigester. The Pacific Jo. Sc. Tec. 7(2), Nsukka, Nigeria. Savery, W. C. and D. C. Cruzon (1972). Methane recovery from chicken manure. J Water Pollut Control Fed 44: 2349-54. Tjalfe G. Poulsen (2003). Solid waste management. Chapter 5 Anaerobic digestion, Aalborg University. Denmark. United Tech, I. (2003). Anaerobic Digestion, UTI Web Design. Yadava, L. S. and P. R. Hcssc (1981). The Development and Use of Biogas Technology in Rural Areas of Asia (A Status Report 1981). Improving Soil Fertility through Organic Recycling. FAO/UNDP Regional Project RAS/75/004, Project Field Document No. 10.


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‫الملخص العربي‬

‫جھاز بسيط لتقدير جودة الغاز الحيوي‬ ‫د‪ .‬محمد على عبد الھادي‬ ‫يھدف ھذا البحث إلي تصميم جھاز بسيط لقياس جودة الغاز الحيوي بطريقة كيميائية يعتمد علي‬ ‫تقدير نسبة غاز ثاني أوكسيد الكربون في الغاز الحيوي نتيجة امتصاصه بمحلول كيماوي‬ ‫ھيدروكسيد البوتاسيم ‪.%٤٠‬‬ ‫صمم وصنع الجھاز بالوحدة التجريبية للغاز الحيوي بقسم الھندسة الزراعية ‪ -‬كلية الزراعة ‪-‬‬ ‫جامعة قناة السويس‪ .‬والجھاز عبارة عن أنبوب زجاجي علي شكل حرف ‪ U‬بقطر داخلي ‪ ١٢‬مم‬ ‫مفتوح من أحد جانبيه إلضافة المحلول الكيماوي ومرتبط باألنبوب صنبور لضبط منسوب‬ ‫المحلول في كال الجانبين أثناء القياس لمعادلة الضغط الجوي‪ .‬أجريت تجربة معملية علي روث‬ ‫األبقار بالتخمر الالھوائي نظام تغذية مرة واحدة تحت درجة حرارة الغرفة )‪ ٢٢‬م‪ (o‬وبدون تقليب‬ ‫لوقت استبقاء ‪ ١٦‬أسبوع في مخمر رأسي مصنع من الحديد المجلفن بسمك ‪ ١.٥‬مم وبقطر‬ ‫وارتفاع ‪ ٢٥‬و ‪ ٤٥‬سم علي التوالي وحجم صافي لتخمر المادة ‪ ١٦‬لتر‪.‬‬ ‫استخدمت التجربة المعملية في قياس كمية الغاز الحيوي الناتج باللتر باإلزاحة الحجمية يوميا ً مرة‬ ‫كل أسبوع‪ .‬كذلك تم تقدير نسبة غاز ثاني أوكسيد الكربون بطريقة ھيدروكسيد البوتاسيم ‪%٤٠‬‬ ‫وحساب نسبة الميثان في الغاز الحيوي الناتج مع األخذ في االعتبار درجة الحرارة والضغط‬ ‫الجوي أثناء القياس )المعادلة العامة للغازات( تم مقارنة النتائج المتحصل عليھا بھذه الطريقة مع‬ ‫التقدير بطريقة الغاز كروماتوجراف )‪gas chromatography (Chrompack CP 9001‬‬ ‫كطريقة قياسية لنفس العينات‪.‬‬ ‫وقد توصلت النتائج الي‪:‬‬ ‫‪ ‬بتقدير نسبة غاز ثاني أوكسيد الكربون في الغاز الحيوي يمكن حساب نسبة غاز الميثان مع‬ ‫األخذ في االعتبار نسبة ‪ %٣‬غازات أخري‬ ‫‪ ‬وجود عالقة عكسية بين نسبة الميثان وثاني أوكسيد الكربون في الغاز الحيوي‬ ‫‪ ‬وجود عالقة طردية بين رقم األس الھيدروجيني داخل المخمر ونسبة الميثان المتحصل عليھا‬ ‫وفي نفس الوقت عالقة عكسية مع نسبة ثاني أوكسد الكربون ‪ ،‬وذلك في مدى األس الھيدروجيني‬ ‫من ‪ ٦.٤‬إلى ‪٧.٢‬‬ ‫‪ ‬يوجد فروق بسيطة بين نسبة غاز الميثان المقدرة بطريقة ھيدروكسيد البوتاسيم ‪%٤٠‬‬ ‫والمقدرة بتحليل الغاز كروماتوجراف ‪ gas chromatography‬خاصةً في بداية وقت االستبقاء‪.‬‬ ‫‪ ‬وجد ارتباط معنوي مقداره **‪ .968‬عند مستوي ‪ 0.01‬بين التقدير بطريقة ھيدروكسيد‬ ‫البوتاسيم ‪ %٤٠‬وتحليل الغاز كروماتوجراف‪.‬‬ ‫‪ ‬تصميم وتشغيل الجھاز يعتمد علي المعادلة العامة للغازات )الحرارة والضغط أثناء القياس(‪.‬‬ ‫‪ ‬سھولة االستخدام ورخص الثمن بالمقارنة مع الطرق األخرى خاصةً في المناطق النامية‪.‬‬

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‫‪Misr J. Ag. Eng., July 2008‬‬