OPTIMIZATION OF BIOGAS PURIFICATION USING WATER-SCRUBBING TECHNOLOGY FOR ENHANCED GAS QUALITY

Abstract

Biogas has been found to be a green source of power. It consists majorly of methane (CH4), carbon dioxide (CO2), hydrogen sulphide (H2S) and water vapour. Use of biogas has never reached its potential with the problem of inherent impurities that compromises its quality and restricts its usage. Purification of biogas is important in ensuring the gas is apt in a number of applications within the renewable energy sector. Amines scrubbing, physical scrubbing, cryogenic separation, pressure swing adsorption, membrane separation and water scrubbing are some of the methods which have been employed to upgrade biogas. Water scrubbing has, however, been highly applied in the enrichment of CH4 due to its nature of availability and ease of use. Various studies have applied water scrubbing to remove impurities from biogas; however, little has been done from these studies to establish the optimal working parameters applicable to small household operators. This study, therefore, focused on optimizing the water scrubbing process for optimal biogas quality using water scrubbing technology. The study optimized the following variables: water flow rate (WFR), the gas flow rate (GFR), pressure (P), and gas retention time (RT). WFR was varied at the rates of 3.5, 4.0, 4.5, 5.0, and 5.5 litres per hour (l/hr), while the GFR was varied at 7.5, 8.0, 8.5, 9.0, and 9.5 litres per minute (l/min). P was varied at 0.04, 0.05, 0.06, 0.07, and 0.08 bar, while RT was varied at 30, 45, 60, 75, and 90 seconds. A set of 30 runs was experimented and the percentage gas composition in the upgraded biogas recorded using a SKY2000-M4-WH multi-gas detector. The results of the experiment were analyzed and optimized using ANOVA, Response Surface Methodology, and Design Expert software version 13. The raw biogas of an initial composition of 80.66% Methane, 15.76% Carbon dioxide, and 10 mg/m3 Hydrogen Sulphide was used as a control experiment. This study found that the optimal CH4 enrichment of 91.45% was observed at the following values of WFR, GFR, P, and RT of 5.0 l/hr, 8.0 l/min, 0.07 bar, and 75 seconds, respectively. A combination of these factors gave the optimal results: CH4 improved by 10.79% (from 80.66% to 91.45%), CO2 had the lowest value of 6.91%, and H2S was zero mg/m3. The predictive models developed revealed that methane enrichment increases with the WFR and RT, while it decreases with an increase in the GFR. It is recommended that further research be conducted to establish the rank of influence of the factors for further optimization.