Methane enrichment of biogas using carbon capture materials

Abstract

An important percentage of biogas is made of CO2, which decreases its heating value. If CO2 is adsorbed two advantages can be achieved: CO2 capture and the increase of biogas heating value. Biomethane is a renewable fuel, which can provide energy autonomy and a reduction of greenhouse gases emissions. CO2 capture from power plants by using solid adsorbents is an effective method for the reduction of CO2 emission and an excellent solution for methane enrichment of biogas. This work evaluates the CO2 removal and methane enrichment of biogas by adsorption of gas molecules to solid surfaces of sorbents in a pilot-scale biogas upgrading system. The materials selected to remove CO2 from gases were three: calcium hydroxide, commercial activated carbon and solid amine adsorbent, loaded on commercial activated carbon. The amine adsorbent used in this work was polyethylenimine (PEI). The adsorbents were characterized by thermal stability through thermogravimetric analyzer (TGA), X-ray diffraction analysis (XRD), specific area, pore size distribution and particle size distribution. The CO2 adsorption capacities of the sorbents were measured using a thermogravimetric analyzer with pure CO2 at atmospheric pressure. The CO2 adsorption capacity test was 0.00653 mol/g for calcium hydroxide, 0.00219 mol/g for commercial activated carbon with 0,1 wt% of amine and 0.00168 mol/g for commercial activated carbon. The effect of adsorbent dosage as a function of time was also investigated. The result showed that the CO2 adsorption of the sorbents increases with adsorbent dosage. The results obtained from the upgrading tests conducted in the lab-scale system showed that a purity of 99.9 % methane was obtained using 15 g of calcium hydroxide, a purity of methane of 87 % was obtained using 30 g of commercial activated carbon with 0.1 wt% amine and a purity around 86 % methane was obtained using 30 g of commercial activated carbon.