Chapter 5 described the comparison of RME oxidation results with diesel combustion. RME combustion results were simulated using a surrogate fuel model of n-hexadecane (C16H34) and of n-hexadecane + methyl acetate (C16H34 + CH3COOCH3). However, these models were not able to reproduce the experimental results in exact manner. So new kinetic mechanism needs to be developed for RME oxidation. This chapter will define chemical reaction mechanism and it will describe the modeling tools used in this study to identify the correct kinetics; CHEMKIN simulation package and Perfectly Stirred Reactor (PSR) code. It also covers the literature survey on chemical kinetic modeling study of Biodiesel combustion and structure of typical high temperature chemical kinetic mechanism. The last part of chapter explains the development of chemical kinetic mechanism for oxidation of RME.
A chemical reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. An elementary step is a basic building block of complex reaction. The elementary processes in a reaction mechanism describe the molecular reaction and changes it undergoes during reaction. It also describes the transition state or order that bonds form or break and rate of each elementary step. The reaction mechanism is also accounted for reaction intermediates, which are stable molecules and do not appear in experimentally determined rate low because they are formed in one step and consumed in a subsequent step.
The CHEMKIN package is software designed to facilitate simulations of elementary chemical reactions in flowing systems. CHEMKIN is a highly structured & modular package that requires the manipulation of a number of programs, subroutines & data files. This package developed at the National Sandia Laboratories of Livermore (Kee and al., 1993) allows modelling the whole gas phase. The advantage of CHEMKIN is its general-purpose and problem-independent structure which, allows the analyst to work with same chemical input regardless of the particular problem [6.1].
The Perfectly Stirred Reactor (PSR) code is a FORTRAN computer program that predicts the steady-state temperature and species composition in a perfectly stirred reactor. The model accounts for finite-rate elementary chemical reactions. The governing equations are a system of non-linear algebraic equations that are solved using a hybrid Newton/ time-integration method. The program runs in conjugation with the CHEMKIN package, which handles the chemical reaction mechanism [6.2].
6.1 CHEMKIN
The CHEMKIN package is composed of two blocks of FORTRAN code and two files:
An Interpreter (code)
The Gas-Phase Subroutine Library (code) The Thermodynamic Database (file) The Linking File (file)
As shown in figure 6.1, the typical input files to Chemkin Interpreter are Chem.inp and therm.dat. The Chem.inp file contains detailed reaction mechanism and species, elements appearing in the mechanism. The reaction mechanism section contains the number of chemical reactions, followed by values for a Pre-exponential factor, temperature exponent and activation energy representing a typical Arrhenius type rate expression [6.1]. The thermodynamic database file (therm.dat) contains thermodynamic data of species given as polynomial. The thermodynamic data of species is calculated using THERGAS code. THERGAS is based on group or bond additive approach. This thermodynamic data is a function of temperature as input file to THERGAS code contains a temperature range over which polynomial fits to thermodynamic data are valid.
The interpreter is a program that reads a symbolic description of a reaction mechanism and then extracts the needed thermodynamic data for each species involved from the thermodynamic database. The primary output from the Interpreter is a Linking file or binary file called Chem.bin. The other output is printed output file ‘Chem.out’, which contains list of elements, species and reaction mechanism. This file also shows diagnostic error messages [6.1].
Once the interpreter is executed and the Linking File is created, the user is ready to use Gas-Phase Subroutine Library. The Gas-Phase Subroutine Library has over 100 subroutines that give information on elements, species, reactions, equation of state, thermodynamic properties and chemical production rates. The selection of Chemkin subroutines for any given problem begins by finding the appropriate equations [6.1].
The different types of calculation/application codes like PSR and SENKIN are used. PSR allows modeling of the experiments conducted in Perfectly Stirred Reactor/ Jet Stirred Reactor, whereas SENKIN allows modeling of shock tube experiments [6.1]. The PSR code is described in detail later in this chapter. The input to the PSR is in Keyword format, which defines a particular reactor and parameters needed to solve it. The other input file is ‘Restart’ file, which contains the solution from a previously calculated computer solution in order to restart a simulation. There are three outputs from the PSR code; save file, Recover file and Print output. The ‘Save’ file is written after the successful completion of problem.
This file contains the first-order sensitivity coefficients and rate of production values. The ‘Recover’ file is written after every successful completion of converged Newton iterations. It is also used to restart a problem, which has terminated due to CPU time limits. The Print file contains solution in terms of exit mole fraction of all species. All input and output from code are in terms of mole fraction [6.2].