Modeling of the Gasifier

It is well-documented that with an equilibrium model, it is possible to predict the thermodynamic limits of chemical reactions that take place in the gasification process (Sharma, 2008). There is a wide variety of literature reports that proposes very similar gasification reactor models, which are based on thermochemical equilibrium and are applied to different types of feedstocks. The main difference between all literature models lies in the reactions chosen to represent the equilibrium between the gaseous compounds produced, in how the authors deal with the equilibrium of the involved reactions, and in the considerations and assumptions made. Some authors set the conversion of carbon into the gas phase or even consider 100% conversion (Zainal et al., 2001 Jarungthammachote and Dutta, 2007 Melgar et al., 2007), while others calculate it (Altafini et al., 2003 Mountouris et al., 2006 Sharma, 2008  

Yoshida et al, 2008). Likewise, some authors study independently the oxidation and reduction zones of the gasifier (Melgar et al, 2007 Sharma, 2008 Yoshida et al, 2008), while others consider the gasifier as a global system (Zainal et al, 2001 Mountouris et al, 2006 Choi et al, 2007 Jarungthammachote and Dutta, 2007). 

To calculate the yield of synthesis gas from petroleum vacuum residue, a modification of the model published by Yoshida et al. (2008) is proposed here. This model was chosen because it calculates the conversion of carbon into the gas phase, which is done by modeling the oxidation zone and then obtaining the composition of the synthesis gas produced by global modeling of the gasifier. 

Typically, petroleum residue is characterized by proximate analysis, which only quantifies the fraction of fuel material (FM), ash and moisture, and elemental (ultimate) analysis (C, H, O, N, S). In the proximate analysis, the fuel fraction is usually divided into two parts fixed carbon (FC) and volatile material (VM). For calculation purposes, both fractions are totalized as ash-free dry fuel matter (expressed in weight%). With the elemental analysis (reported in weight% dry basis for each element), it is possible to obtain the condensed formula of the fuel fraction and thus its molecular weight. 

Equations 4.18 through 4.23 are obtained by an atomic balance in Equation 4.17, where nj-Wg are the moles obtained of gasification products per mole of vacuum residue  

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With the results of these measurements the gas composition of the product gas can be calculated for different steam-fuel ratios and temperatures. The next step will be to improve the model of the gasifier on basis of this measurements. 

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