Effects of Buffering and Urea on the Quantity and Quality of Biogas From Banana Leaves as Alternative Renewable Energy Resource.

Abstract

Buffer solutions of different molar concentrations (O.1.M, 0.2M, 0.3M, .4M and 0.5M) were prepared using buffer salts (Na₂HPO₄ and NaH₂PO_4.H₂O). The pKa of the buffer salts was determined from the ionization constant, Ka (7 x 10^-8) of the phosphoric ions (HPO₄^2- and H₂PO₄^-) of the buffer salts using the relation, pKa = - log Ka. The pKa of the buffer salts obtained was further used in Headerson-Hasselbaltch’s equation, pH = pKa + log [base]/[acid], where the volumes (125cm³ each) of the prepared buffer solution of required pH (6.8, 7.0, 7.2, 7.4 and 7.6) were prepared. Slurries were prepared using 20g banana leaves in 125cm³ of the prepared buffer solutions. For the investigation of the effect of buffering on biogas yield and its composition, 0.1M, 0.2M, 0.3M, 0.4M and 0.5M buffer solutions (pH 7.0 each) were used in making the slurry; for the investigation of the effect of pH of buffer solution on the quantity and composition of biogas, buffer solutions of pH (6.8, 7.0, 7.2, 7.4 and 7.6) were separately used in making the slurry; for the investigation of the effect of addition of urea (as nutritive additive), 0.1g, 0.15g, 0.20g, 0.25g and 0.30g of urea, were respectively added into the five (5) replica slurries prepared from 20g banana leaves and 125cm³ distilled water, which were labeled A,B,C, D and E and; for the investigation of the combined effect of buffering and addition of urea, 0.1g, 0.15g, 0.20g , 0.25g and 0.30g of urea (as additive), were  respectively added into another five (5) replica slurries prepared from 20g banana leaves and 125cm³ of 0.2M buffer solutions of pH 7.4, which were also labeled A,B,C,D and E. Compositional analysis of the biogas produced was also carried out by passing the gas collected (biogas) through concentrated NaOH solution (10M) and a  solution of (CH₃COO)₂Pb in a 3 molar CH₃COOH, where CO₂ and H₂S were, respectively absorbed from the biogas. The work revealed that, the use of buffer solution at certain molar concentrations (0.1M – 0.3M) and pH (7.0 – 7.4); the use of urea (0.1g/125cm³ – 0.25g/125cm³)  equivalent to 0.8g/dm³ – 2.0g/dm³ and; the use of the combination of buffer solution and urea, each enhanced biogas yield. The compositional analysis carried out, revealed that the biogas (secondary biofuel) produced was of good fuel value (60 – 68.78%  CH₄) and low environmental hazards (CO₂ below 40% and H₂S below 0.5%).

Keywords: Biogas; buffer solution; urea; molar concentration;  ionization constant (pKa) of phosphoric ions; pH of slurry.