Modelling and simulation of a bifunctional two-stage micro-methanizer for simultaneous H2 and CH4 production from African organic substrates

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A mechanistically coupled mathematical model of a bifunctional two-stage anaerobic digestion (TSAD) system for the simultaneous co-production of biohydrogen (H 2 ) and biomethane (CH 4 ) from African organic substrates was developed and implemented in MATLAB R2023a. Stage 1 integrates a modified Monod kinetic framework with competitive H 2 inhibition, a Newton–Raphson charge-balance pH solver, and an Arrhenius thermal correction. Stage 2 implements a simplified IWA Anaerobic Digestion Model No. 1 (ADM1) retaining acetoclastic and hydrogenotrophic methanogenesis as the rate-limiting processes. Dynamic inter-stage coupling is achieved via piecewise cubic Hermite interpolation (PCHIP) of the Stage 1 volatile fatty acid (VFA) effluent trajectory, replacing the fixed-feed simplification adopted in the majority of published two-stage models. The framework was applied to four African lignocellulosic substrates — olive pomace, wheat straw, millet stalk, and peanut shell — characterised by a biodegradability correction combining the Chandler lignin model with the Triolo sigmoidal estimate. Stage 1 achieves near-complete substrate conversion (99.1%), with dissolved H 2 reaching 0.3457 mol m −3 (compared against an operational reference value of 0.30 mol m −3 used here to delimit the hydrogen accumulation regime). Dynamic coupling reduces the acetate supply to Stage 2 by 45.1% relative to the fixed-feed reference (16.47 vs. 30.00 mol m −3 ), i.e. the fixed-feed assumption overestimates the dynamically coupled value by 82.1%; this constitutes the primary novel finding. The total recoverable energy at laboratory scale ( V 1 = 5 L, V 2 = 10 L) is 1.416 kWh (94.4 Wh L reactor − 1 , LHV basis), with H 2 and CH 4 contributing 21.5% and 78.5%, respectively. One-at-a-time sensitivity analysis identifies k L a , H 2 and Y H 2 as the dominant parameters governing cumulative H 2 yield, and the framework provides a predictive modelling tool for decentralised TSAD biorefinery design in Sub-Saharan and North Africa.