A chemical equilibrium model for biomass gasification. Application to Costa Rican coffee pulp transformation unit

Highlights: • A thermodynamic equilibrium model is established for coffee gasification. • The model accounts for both gasifier heterogeneous and homogeneous sections. • Biochar Formation Enthalpy is included in the thermodynamic model. • Biochar Formation Gibbs Free Energy is determined numerically via regression. • Experimental syngas and biochar yields are used for gasifier model validation. -- Abstract: The present study considers the establishment of a biomass gasification model using for its validation, experimental data obtained in a 15–20 kg/h coffee pulp downdraft gasifier unit, operated at ICAFE-Costa Rica. The gasification model proposed considers a heterogeneous "downflow" central section and a homogeneous "upflow" exit section, both operating at chemical equilibrium. Each of the sections includes a number of relevant independent chemical reactions. Biomass is incorporated into the model using its elemental composition as well as its water content. Different reaction paths are examined in order to analyze the applicability of the various assumptions considered. The established model accounts for biochar thermodynamic properties as well as gasifier operating temperatures in the homogenous "upflow" section, using a numerical fitting method. With this approach, the proposed model successfully predicts the molar fractions of various syngas components, most notably, the CH₄ and H₂ fractions via the CO₂ hydrogenation reaction, as well as the carbon converted. It is anticipated that the developed model will provide a valuable simulation and design tool for scaled-up downdraft biomass gasifiers.