Anaerobic processing of baby maize stover for bioenergy production under thermophilic and mesophilic conditions

Abstract

There is a growing consensus that purpose-grown crops should not be used for energy generation, especially if this involves competition for land and resources that could be used for food or animal feed (OECD, 2010). This highlights the need for the use of agricultural or industrial residues as sources of biomass. Tropical Power Ltd opened Africa’s first gird-connected anaerobic digestion (AD) plant in Kenya which uses baby maize stover as a substrate. The aim of this work was to assess the digestion of the novel substrate in AD, baby maize stover from Kenya.

This research conducted laboratory experiments and modelling, and investigated different approaches (e.g. operating temperatures, pre-treatments and trace elements (TE) supplementation) to improve the overall energy balance and digestion performance of this substrate, in comparison with traditional maize silage as a model for ligno-cellulosic agro-wastes. For laboratory experiments, this work was carried out over 3 hydraulic retention time (HRT) in ten 4 L continuously-stirred tank reactors (CSTR) under thermophilic (55℃) and mesophilic conditions (35℃). The pre-treatment was a thermophilic pre-hydrolysis step before mesophilic digestion. Modelling was conducted by hand calculation and modelling software 'AD assessment tool'. Laboratory experimental data and data from the biogas plant in Kenya were used for the modelling.

In laboratory experiments, the greatest methane yield of baby maize stover was found in mesophilic digesters with 5 TE (Fe, Co, Ni, Se, Mo) which was 0.333 L CH4 g-1 volatile solids (VS) at organic loading rate (OLR) 3 g VS L-1 day-1. In contrast, the methane yield in mesophilic digesters with 3 TE (Fe, Co, Ni) was significantly lower than that of thermophilic digesters. 5 TE supplementation helped to provide stable operation and higher methane yield. Even with 5 TE supplementation, however, mesophilic digesters showed signs of failure after 150 days. The volatile fatty acids (VFA) accumulation in thermophilic digesters with 5 TE only fell after W dosing, and it appears W may also help the stable operation. The two-stage system was tested three times under different conditions and the gas production from pre-hydrolysis improved every time; however, specific methane yield in the two-stage system remained less than that of single stage digestion.

In modelling, firstly, overall energy balance of the biogas plant in Kenya from 08.2015 to 07.2016 was assessed. The percentage of calculated fuel consumption per actual fuel consumption was 95.3 % so the modelling assumption was close to actual operation. Secondly, rationalisation of the biogas plant design based on actual feedstock availability was conducted because the biogas plant received only 21 % of target feeding and was too large for actual feed availability. This rationalisation was conducted by hand calculation and modelling software AD assessment tool. The required digester volume was 1360 m3 which was 24 % of actual digester volume and less than total volume of 1520 m3 for the hydrolysers in the main plant. While hand calculation considered a more limited range of parameters (energy requirement for heating and energy output as methane) than the AD assessment tool (transportation, heat, CHP, electricity, methane, process loss), both modelling results clearly indicated the net energy output in single-stage mesophilic digestion was greater than that of single-stage thermophilic digestion.

When mesophilic digesters fed on baby maize stover at OLR of 3-4 g VS L-1 day-1 received appropriate TE, the specific methane yield was close to that of thermophilic digesters. In terms of overall energy balance, single mesophilic digestion was thus better than single-stage thermophilic digestion and two-stage system for this substrate.

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ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

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