Development of an equilibrium model for the simulation of thermochemical gasification and application to agricultural residues

The recent crisis in Italian agriculture has forced operators to find new ways of deriving income from their activities. One of the most promising of these is the use of agricultural products and residues as an energy source. In particular, the exploitation of agricultural, industrial and forestry residues and byproducts can be a definite advantage in the generation of electrical and thermal energy. This is due to the fact that these materials need to be disposed of and, therefore represent cost, and can be then transformed into an opportunity for revenue. Among the various technologies available for converting biomass into biofuels, thermochemical gasification appears to be the most suitable for the conversion of crop residues (corn and sunflower stalks, rapeseed straw, etc.) as these are characterized by low humidity and a high C/N ratio. On the other hand, the operational problems encountered with current plant architectures, such as the blocking of the biomass flow in the reactor due to low-melting ash content, have limited the use of crop residues in gasification so far. In this paper, a model for the simulation of the biomass thermochemical gasification process is presented. The model is based on a chemical equilibrium hypothesis, i.e. it is assumed that the chemical transformations inside the gasifier can be described by a series of equilibrium reactions independent of each other and, therefore, a precise relationship between products and reactants can be determined. The model is validated by a comparison with data from literature and a sensitivity analysis on process parameters is presented. Finally, the model is applied to the study of the reliability of the gasification of crop residues currently popular in Italy, both in terms of produced gas quality and quantity.