Special Issue 13 14 NOVEL CARBON RESOURCE SCIENCES NEWSLETTER Development of Mechanisms on Primary Reactions during Coal Volatile Combustion: Numerical Investigation Agung Tri Wijayanta 1 , Alam Md. Saiful 2 , Koichi Nakaso 2 , Jun Fukai *2 1 Research and Education Center of Carbon Resources, Kyushu University, Fukuoka 816-8580, Japan 2 Department of Chemical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan * Corresponding author: jfukai@chem-eng.kyushu-u.ac.jp This research report resumes the results of research work for developing reaction mechanisms during coal volatile combustion based on our related publications. Developments of coal conversion become more familiar in the worldwide at the present. Computational Fluid Dynamics (CFD) plays a key role to explain these developments of different coal chemistry steps including devolatilisation, volatile combustion and reforming, char oxidation and char gasiication. In the research work, irstly, the authors develop the reduced mechanism that has a good agreement with the mechanism model of primary reaction of coal volatiles that consists of 255 chemical species and 1095 elementary reactions. Secondly, in the viewpoint of nowadays environmental issue for CO 2 reduction, carbon resource utilization especially how to increase syngas during coal gasiication, the authors investigate continuously the primary reactions of coal volatiles including the soot formation mechanism for O 2 /CO 2 environment to analyze the combustion components. The effects of CO 2 concentration, reaction temperature and pressure are also discussed in this research report for the soot model that provides a better result of the increase in product concentrations of CO, H 2 and the reduce in CO 2 . Keywords: reaction mechanism, reduced mechanism, soot formation, aromatics, coal volatiles, plug low reactor. Introduction As well known that coal has a potential energy value, the substantial worldwide attention is being focused on cleaner and more eficient use of coals. Coal gasiication process, in particular, allows an eficient use of this energy source with a low environmental impact. Continued utilization of coal is depending on the availability and development of the new process. Presently, about 40% of electric power generation in the world depends on coals. As coal heats up, the volatile components of the coal will diffuse into the gas stream. Volatiles generally are composed of H 2 , H 2 O, CO, CO 2 , hydrocarbon gases, hydrocarbon liquids, and polycyclic aromatic hydrocarbons (PAHs). Most of the compounds will continue to react to produce successively lighter gases as the more complex molecules decompose, eventually forming CO 2 and H 2 O provides if the suficient oxygen is available. PAHs are products of primary pyrolysis and PAHs are precursors of soot in secondary pyrolysis. The conversion of PAHs into soot during secondary pyrolysis is accompanied by release of CO and H 2 . New advance in instruments, computers and numerical methods have greatly increased in order to conduct the research efforts in coals. In this numerical research work, the authors divide the study into two parts 1-3) . One is to develop a numerical model that is validated with the experimental results and then to make a reduced model for CO 2 gasiication that shows similarity with primary mechanism consists of 255 chemical species and 1095 elementary reactions 4) . The reduced model contains 53 chemical species and 73 elementary chemical reactions and the comparison between the primary and reduced mechanisms is provided. The second part is to extend the previous work for developing mechanism including the soot formation. The reaction mechanism consists of 276 species and 3793 reactions 4) . The comparison between soot formation and without soot models is also provided. Moreover, the effects of CO 2 concentration, temperature and pressure for the appropriate soot model are also reported in this paper. Mathematical Model A plug low reaction model for reaction of coal volatiles is developed for O 2 /CO 2 gasiication. The model is numerically developed using the input data of coal volatile pyrolysis that has concentration of H 2 , CO, CH 4 , O 2 and CO 2 of 0.00049, 0.00283, 0.00211, 0.00729 and 0.02241 in mass fraction, respectively. Simulation is conducted inside the reactor of 28 mm in diameter and 1200 mm in length. The plug low reactor model can be used to model multiple reactions as well as reactions involving changing temperatures, pressures and densities of the low. In the ideal tubular reactor, which is called the plug low reactor, there is assumed to be complete mixing perpendicular to the direction of low (i.e. the radial direction) and no mixing in the direction of low. In the plug low, the residence time in the reactor is the same for all elements of luid. It means that the reaction proceeds as the reactants in a plug progress along the reactor tube. There is ideally no back-mixing in the reactor and there exists a uniform temperature, pressure and velocity proile across the radius. All the gas phase and surface phase reactions are considered in this mathematical model are as follows: (1) The lower case letters (a, b, c and d) represent stoichiometric coeficients, while the capital letters represent the reactants (A and B) and the products (C and D). k is the proportionality constant called the speciic rate coeficient for the above reaction and follows the Arrhenius equation. (2) where A f is a frequency factor (mole-m-sec-K), T is temperature (K), α is temperature exponent, E is activation energy (J/mole) and R is the gas constant (J K -1 mole -1 ). Reduced Mechanism The aim of this study is to reduce the number of species and reactions to get a reduced mechanism small enough to use in CFD calculations. There is no theoretical limitation of the number of species utilized in the mechanism calculations however the simulation time becomes unacceptably long. There is no species limit in Chemkin- CFD, but the practical 50 species limit was introduced in the code of Fluent. Species entering the combustion process and the resulting stable products of the reaction process are selected as important species. The authors irst consider only ive compound species (H 2 , H 2 O, CO, O 2 , and CO 2 ) that is obtained by iltering the species based on the mass fraction. Then the Rate of Production (ROP) analysis has been studied. ROP analysis is particularly useful for plug-low systems, where the computational expense for the added calculations is small and it is possible to consider data from a large reaction set. ROP analysis determines the contribution of each reaction to the net production or destruction rates of a species. The percentage of the contribution of the i-th reaction to the formation of or consumption, C ki of a species k is calculated as: (3) where k ω is the net molar production rate per unit volume of the k-th species. The authors successively consider the most dominant reactions for each species and make a new mechanism. Then, the size of the mechanism is gradually increased by adding the chemical species and number of dominant reactions, and the procedure is terminated when the deviation becomes smaller. The reactions are added or removed to modify the reduced mechanism by temperature sensitivity analysis. The resulting mechanism has 15 compound species and 73 elementary chemical reactions that include 37 intermediate species 1) . Reaction Mechanism including Soot Formation Nucleation is the process of forming new condensed- phase particles from a continuous phase, such as gas and vapor. Particle nucleation is the most important step in soot formation and results in generation of the solid particles. It is irreversible and all reactants must be gas phase species. The PAH molecules irst grow into planar molecules while simultaneously reacting with the gas phase species and colliding with other PAH molecules to form large molecules. The PAH compounds having large molecular mass about 2000 is considered soot and the reaction mechanism is described from benzene to coronene (C 24 H 12 ). Then, considering the concentration of H 2 , CO, CO 2 and soot in the outlet of the reactor the authors deine which model has signiicant regarding energy and environmental issue. This soot model consists of the following principal processes: initial aromatic ring formation during small hydrocarbon oxidation, formation of larger PAH, particle nucleation/inception through coalescence of PAHs, particle growth and particle oxidation. Particle oxidation produces the lower PAHs. The reaction mechanism developed in this model consists of 276 species, 2158 conventional gas phase reactions and 1635 surface phase reactions 4) . Based on the detection of compounds containing up to 160 carbon atoms is considered as a particle 5) , the remaining all aromatic compounds are considered as PAHs in this soot model. The main reactions considered in this model are: Reaction between PAH/PAH* and PAH/PAH*: PAH/PAH* + PAH/PAH* → PAH/PAH* + H/H 2 (4) Reaction between PAH/PAH* and C 2 H 2 : PAH/PAH* + C 2 H 2 → PAH/PAH* + H (5) Particle oxidation: PAH/PAH* + OH → PAH/PAH* + CO + H (6) where * indicates a radical entity. Results and Discussion Reduced Mechanism The reduced mechanism is tested against the primary mechanism. The comparison of the two mechanisms can be seen in Fig. 1 that shows the concentration proiles of the major combustion species (H 2 , CH 4 , H 2 O, CO and CO 2 ). The Fig. 1-a shows the result when the authors consider 10 compound species of H 2 , CH 4 , H 2 O, C 2 H 4 , C 3 H 6 , CO, CO 2 , O 2 , C 6 H 6 and chrysene. The concentration difference between primary mechanism and reduced mechanism for H 2 O and CO is large. But, when we consider 15 compound species of H 2 , CH 4 , H 2 O, C 2 H 4 , C 2 H 6 , C 5 H 6 , C 4 H 8 , C 3 H 6 , CO, CO 2 , O 2 , C 6 H 6 , C 7 H 8 , C 6 H 5 OH and chrysene, then the result provides the smaller difference for all major combustion species as shown in the Fig. 1-b. This good agreement mechanism has 15 compound species and 73 elementary chemical reactions that include 37 intermediate species. This reduced model shows good results when the tem- perature and pressure are maintained up to 1373K and 1.0 MPa, respectively 1) . Research Report aA+bB ↔cC+dD ᵏƒ ᵏb k = A ƒ T α exp −E ─ RT C ki = ─×100 ω ki ω k