Influência do tratamento do carvão da pirólise do lodo de esgoto na purificação do biogás, simulando os gases obtidos do processo de fermentação de dejetos suínos

Abstract

Biogas is a valuable source of renewable energy because it is composed mainly of methane (60-80%), carbon dioxide (20-40%), and traces of ammonia and hydrogen sulfide. The main application of biogas is the use of methane as fuel for electric motors and generators. In order to increase the calorific value of biogas to the same level as that of methane, the other gases have to be removed. One of the most common processes used to purify biogas is adsorption using activated carbons. The objective of this work was to the study the adsorption of carbon dioxide, ammonia, and hydrogen from a mixture with the same composition as that of the biogas from the fermentation of swine manure. The adsorbent used was the char from the pyrolysis of sewage sludge at 500 °C. The chars underwent physical ativation (at 700 °C, for 2 hours, under N2), chemical activation using HCl (7.5 M) or KOH (3.0 M), and were characterized for composition and textural properties via XRD, XRF, BET, and SEM. The experiments were carried out in a fixed-bed column (adsorber) at 25 °C and 1 atm. The gas flow rate was 50 mL/min, and the mass of char were 18.4 or 6.0 g. The results showed that the chemical activation significantly enhanced the adsorption. Regarding the adsorption of H2S and CO2, the best result was for the char treated with HCl. The char had high specific area (248 m2 /g), high breakthrough time (400 min for H2S, and 175.5 min for CO2), high saturation time (1020 min for H2S, and 484.5 for CO2), and high adsorption capacities (670,33 mg H2S/gads e 218.385 mg H2S/gads). The chars were selective for NH3 and had no affinity for CH4. The percentages of adsorption were 61,4% for H2S and 75% for CO2 using CAHCl. For the gas mixture, the percentages of adsorption were 71,8% for H2S and 55% for CO2. The dynamic adsorption was modeled by a system of differential equations describing mass balances for gas and particle phases. The equations were solved numerically against experimental data (molar fraction) in order to obtain estimates for parameters such as axial diffusivity, effective intraparticle diffusivity, and external mass transfer coefficient for H2S and CO2 as well as others present in the mixture involving NH3 and CH4.