| Description |
Refinery oil residue commonly considered the bottom of the barrel, can no longer be a waste. The increasing energy demand and the constant depletion of light oil supply make it crucial to find a suitable way to convert residual oils into valuable fuels. The gasification process represents a possible solution to this problem. Gasification is a thermo-process conducted in poor oxygen conditions, intending to obtain a hydrogen and carbon monoxide mixture, commonly named syngas. Gasification is widely implemented on an industrial scale to treat complex combinations such as biomasses, plastic waste, and coal. Most of the studies in the literature approach gasification modelling by studying the chemical equilibrium or with Computational fluid dynamics (CFD). However, the chemical equilibrium approach is well-performing in predicting the major gasification products, like hydrogen, carbon monoxide, carbon dioxide, and water. Thus, missing some crucial information like side-products formation or the evolution of conditions along the reactor, like temperature and species profile. The CFD approach overtakes the problems of the equilibrium approach, thus requiring a high level of complexity and being computationally expensive. The approach here proposed to model heavy oil gasification is based on the definition of a suitable kinetics model to target the evolution of all the essential variables along the reactor, thus with very low computational cost. The gasification process results from three different steps with different characteristic times. The first step is feed pyrolysis; liquid or solid feeds are exposed to very high temperatures. This triggers the thermal-decomposition reactions resulting in the volatilization of the feed in smaller gas molecules and the formation of a solid residue (CHAR). The second step is the partial combustion of gas compounds in homogeneous gas phase reactions. Finally, the last and slowest step is gasifying the solid products generated during the pyrolysis. The modelling approach is based on defining different reactive pathways for the three steps. The major challenge in modelling the first step is defining a proper framework for the feed characterization. A surrogate mixture is used to mimic feed chemical and physical properties. The surrogate is defined according to practical information on the feed; specifically, the SARA (Saturates, Aromatics, Resins, Asphaltenes) analysis and the elemental characterization are used to define the appropriate surrogate starting from a poll of nineteen key molecules. The pyrolysis of each surrogate molecule is described by a first-order irreversible reaction leading to the formation of gas and solid products (CHAR). The partial combustion is then described by coupling a gas phase mechanism. Depending on the required details, the gas phase mechanism can be either detailed or reduced. The gas phase mechanism accounts for the combustion of gas species released during the previous step. The last step describes the gasification of the CHAR generated in the first step, modelled with a series of global reactions defined empirically. The kinetics approach described above allows estimating major gasification products and eventual side products according to the level of details desired. It permits an assessment of much more information that cannot be extrapolated using a chemical-equilibrium approach like the reactor thermal profile and species evolution, and computational cost is much lower than CFD simulations. |
