BioCH4 from the Anaerobic co-Digestion of the Organic Fraction of Municipal Solid Waste and Maize Cob Wastes

Abstract

Maize Cob Waste (MCW) is available in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the fields despite its negligible impact in soil fertility. It can be used as substrate in Anaerobic co-Digestion (AcoD) and as precursor to produce Activated Carbons (ACs). In this context, a biorefinery concept was developed based on two purposes: 1) the pretreated MCW can be valorised as co-substrate in the AcoD with Organic Fraction of Municipal Solid Wastes (OFMSW), and 2) MCW can be used as a precursor of ACs for biomethane (bioCH4) conditioning. The AcoD of OFMSW with chemically pre-treated MCW in presence of H2O2 at 23 ºC increased the biogas and CH4 yields by 65% and 48%, respectively, when compared to AD of standalone OFMSW., providing higher biogas quality and a more stable AcoD process than with non-pre-treated MCW. Among the ACs produced, the physically MCW(PA)3h AC performed better in H2S removal than commercial and impregnated by liquid digestate ACs. Textural properties seemed to be more important than the mineral content for H2S removal and the presence of O2 on MCW(PA)3h surface may have favoured H2S catalytic oxidation. The MCW(PA)3h AC was also the most suitable candidate for CO2 separation due to its more favourable textural properties, sufficient selectivity and higher working capacity than the others ACs produced. The adsorption equilibrium measurements of CO2 and CH4 showed that the Sips isotherm model and the Adsorption Potential Theory (APT) can be confidently employed to correlate the experimental data, as well as the axial dispersed plug-flow and Linear Driving Force (LDF) model is able to correlate the fixed bed experimental data. The environmental Life Cycle Assessment of a biorefinery case study was performed on the hypothesis of implementing, at an existing Portuguese Anaerobic Digestion plant processing OFMSW, (i) an AcoD unit using MCW as co-substrate; (ii) an H2S unit using MCW(PA)3h as adsorbent, and (iii) a Pressure Swing Adsorption (PSA) upgrading unit. The cogeneration of the biogas produced during AcoD with non-pre-treated MCW is more sustainable than with pre-treated MCW. The environmental impacts associated with biogas upgrading to bioCH4 at optimized H2S adsorption capacity of MCW(PA)3h, if fossil natural gas used for the OFMSW transport is substituted by the produced bioCH4, decreased significantly, giving lower impacts than cogeneration in five categories.