Enhancement of Methane Concentration by Removing Contaminants from Biogas Mixtures Using Combined Method of Absorption and Adsorption

Abstract

We report a laboratory scale combined absorption and adsorption chemical process to remove contaminants from anaerobically produced biogas using cafeteria (food), vegetable, fruit, and cattle manure wastes. Iron oxide (Fe 2 O 3 ), zero valent iron (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">F</mml:mi><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>), and iron chloride (FeCl 2 ) react with hydrogen sulfide (H 2 S) to deposit colloidal sulfur. Silica gel, sodium sulfate (Na 2 SO 4 ), and calcium oxide (CaO) reduce the water vapour (H 2 O) and carbon dioxide (CO 2 ). It is possible to upgrade methane (CH 4 ) above 95% in biogas using chemical or physical absorption or adsorption process. The removal efficiency of CO 2 , H 2 S, and H 2 O depends on the mass of removing agent and system pH. The results showed that Ca(OH) 2 solutions are capable of reducing CO 2 below 6%. The H 2 S concentration was reduced to 89%, 90%, 86%, 85%, and 96% for treating with 10 g of FeCl 2 ,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">F</mml:mi><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>(with pH), Fe 2 O 3 ,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">F</mml:mi><mml:mi mathvariant="normal">e</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>, and activated carbon, respectively. The H 2 O concentration was reduced to 0.2%, 0.7%, 0.2%, 0.2%, and 0.3% for treating raw biogas with 10 g of silica gel and Na 2 SO 4 for runs R1, R2, R3, R4, and R5, respectively. Thus, given the successful contaminant elimination, the combined absorption and adsorption process is a feasible system for biogas purification.