Analyses of Biogas and Digestate from Cattle Dung Anaerobic Digestion

• S. S. Yaru

  Department of Mechanical Engineering, The Federal University of Technology, Akure, Nigeria

• I. K. Adegun

  Department of Mechanical Engineering, University of Ilorin, Ilorin, Nigeria

This paper analyzed biogas and the digestate from cattle dung after anaerobic digestion. Eight kilograms (8 kg) of sun- dried cattle dung mixed with equal volume of water was subjected to anaerobic digestion for 14 days in a digester made of mild steel. The digester had a diameter of 0.3 m, 0.6 m height and 2 mm thickness. The biogas resulting from this setup was daily tested and finally burnt with a blue flame on the 13th day of incubation. The biogas was refined through sodium hydroxide (NaOH) solution. In the chromatographic analysis, the raw biogas produced 56.20% CH4, 39.51% CO2, 1.92% CO, 1.84% H2S and 0.53% NH3. The refined one comprised 70.28% CH4, 27.82% CO2, 1.55% CO, 0.34% H2S and 0.00% NH3. The specific gas constant (Ri) of the biogas increased from 0.3027 kJ/kgK at the raw stage to 0.3459 kJ/kgK when refined. Similarly, the cv improved from 0.9082 kJ/kgK to 1.0376 kJ/kgK that of cp from 1.2080 kJ/kgK to 1.3803 kJ/kgK and the values for the ratio of the specific heats agreed with the standard values of perfect polyatomic and diatomic gases. The calorific values of the biogases in the same order were 19.15MJ/m3 and 22.62MJ/m3. The sun-dried cattle dung and the digestate analyses with atomic absorption spectrophotometer showed respectively 0.13 and 0.20 for Fe (%), 0.49 and 0.55 Ca (%), 0.14 and 0.32 K (%), 0.28 and 0.34 Na (%) and 0.59 and 1.46 P (%) while the values for Co and Mn remained at 0.0001% and 0.03% respectively. The Kjedahl test results for N2 (%) were 1.06 and 1.40 while the oil (%) Soxhlet test results were 2.48 and 0.16 respectively. It was therefore concluded that the digestate had higher values of nutrients than the undigested cattle dung.

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