An Experimental Data Based Correction Method of Biomass Gasification Equilibrium

performance prediction. The model, implemented in the MATLAB-SIMULINK ® environment, is able to calculate the reactor main operating parameters such as reaction temperature, gas composition, gas flow rate and solid product (typically charcoal). The comparison of model output with experimental data puts in evidence the insufficient precision of equilibrium models due to their incapability of taking into account the nonequilibrium effects always present in the gasification process. To obtain a better prediction of measured values, the pure equilibrium model has been modified on the basis of literature experimental data, introducing semi-empirical relations with the aim to consider the most meaningful effects of nonequilibrium. The results demonstrate that this modification leads to an increased precision of the model in reproducing experimental data. DOI: 10.1115/1.4001463 The implementation of a biomass gasifier model can be performed following two different approaches: equilibrium EQ modeling based on the hypothesis of equilibrium for chemical reactions and kinetic modeling based on the simulation of chemical reactions kinetics, taking into account transport and diffusion phenomena occurring in the reactor. The relationship between the two mentioned modeling approaches is similar to that occurring between the ideal cycle and the real cycle of an internal combustion engine: equilibrium modeling provides the maximum useful effect attainable by the process while kinetic modeling introduces efficiency-limiting constraints. In literature, many examples of the application of kinetic 1‐5 and equilibrium 6‐10 modeling of gasification are reported. According to several authors 7,10, it is possible to affirm that downdraft gasifiers operate close to equilibrium conditions, thanks to the high temperature and long gas residence time in the reaction zone. It is dutiful to mention that equilibrium condition attainment is also connected to the size of fuel particles: larger solid pieces will take longer time to be gasified 11. Two different methodologies exist to deal with the construction of an equilibrium model, substantially equivalent in the results 12,13: a “stoichiometric approach,” based on the calculation of equilibrium constants for the reactions involved in the process and a “nonstoichiometric approach,” based on the minimization of Gibbs free energy. The first approach is more strictly connected to the physics of phenomena since it imposes the equilibrium of well defined reactions occurring in the system; the second approach solves the problem by minimizing the Gibbs free energy function with the imposition of non-negativity and mass-closure constraints, not considering particular chemical reactions and species involved. The equilibrium approach to gasification modeling is simpler than kinetic modeling. However, its accuracy is limited especially in the low temperature ranges, where kinetics plays a nonnegligible role in the development of the reactions. So, although particular gasifier designs, such as the downdraft or the fluidized bed types, may operate close to equilibrium, a disagreement between model results and measurements is always present. In particular, comparing the output values of an equilibrium model with the experimental measurements can be noticed that model predictions underestimate reaction temperatures, methane yield and solid carbon charcoal production at the highest air/fuel ratios. It stands to reason that part of the carbon and of the hydrogen introduced in the reactor with the feeding do not reach equilibrium, appearing in the products as charcoal and methane in excess. In this paper, a system of correction based on experimental literature data has been applied to an equilibrium model in order to exploit the versatility and simplicity of equilibrium modeling while simultaneously achieving an acceptable degree of precision. The idea is to subtract part of the incoming reactants at the gasifier inlet in order to modify the equilibrium state in the core of the reactor, adding the subtracted mass as “nonequilibrium products,” methane and char at the gasifier exit. The results show an improved agreement of the corrected model with respect to experimental data despite the limited increase in computing power required and complication of the model.