Three-step chemical kinetic reaction
As we are well known about hydrogen is a clean energy medium that can be found from most of the renewable energy source and growing as a key clean energy sources. Although there are lots of factors which have to address before considering power generation form hydrogen like, hydrogen power economy and safety associated with accidental explosion. Lots of analytic studies of combustion process (ignition) above crossover temperature are studied [1, 2], however in this report we have investigated hydrogen-oxygen reaction phenomena that comprises deflagration, flame balls, diffusion flames and ignition (spontaneous and forced). Hydrogen-oxygen combustion at steady state is easier than the oxidation of other fuel due to it requires fewer species and fewer elementary steps for ignition [3]. 1.1 Brief IntroductionItalic text Chemical kinetics is the study of the speed (rate) where chemical reaction takes place and the speed which is affected by the factors. Three Step Reaction is step by step of elementary reaction process till the third steps that occurs on a molecular level in chemical reactions. In the elementary reaction process, more than one or more molecules are changing their geometry and/or disturbing molecules by eliminating or adding another reactant molecule that consist of multiple elementary processes. However, there are different factors which has to keep in mind while determining the rate of reaction such as concentration, surface area, temperature, and catalysts [4]. Three-Step Mechanism 2 A + 2 B C + D Step 1: A + A X (fast) Step 2: X + B C + Y (slow) Step 3: Y + B D (fast)
1.2 Objective of three step chemical kinetics studiesBold text - Analysis of the overall reaction sequence of elementary steps. For example: The Steady-State Approximation - Determination of absolute rate of reaction and its individual elementary steps. According to discussion given in [3, 5] suggested that at steady state assumption is a reasonable approximation for three step than other steps (four) chemistry when the homogeneous ignition of hydrogen-oxygen mixture under the crossover temperature and pressure at initial condition. In the reduction mechanism, includes , , and , design by using skeletal mechanism of eight elementary step under the steady state condition [5]. HO2 consumption through occurs fast to take place this intermediate at required low temperature, thus further reducing into two global steps and . This processes compiled with effective temperature sensitivity of corresponding overall rates that assists the activation energy asymptotics is used to explain the resulted thermal runaway, and ignition time for different initial temperature, pressure, and composition.
2 Reduced three step mechanism for autoignitioBold textn From many discussion given in [5, 6], ignition processes explained by the overall reaction with ignition time increase linearly. The chain termination via becomes significant at low temperature (M is third body as an energy carriers [7]), increases induction time [8], the reaction rate parameter is included in table called San-Diego mechanism [9]. Most interesting things is to be noticed that chain termination via. includes in chain branching at temperatures at below crossover [5] and other hand alternative branched chain path enables to ignite via , therefore eight elementary reactions is needed for low temperature description which is shown in Table 1 [5], for the numerical validation H atom will be removed rapidly through , at steady state ignition below crossover condition and final result can be adduced as three steps [10].
In the numerical integrations of adiabatic, isobaric homogeneous reactors, the result found using three step mechanism which are plotted and compared in Fig. 1.
Figure 1: solid line indicates ignition time with 21-step detailed chemistry and dashed curve indicates three step reduced mechanism [5].
From the discussion given in [5], activation energies and Arrhenius rate constant (k8) are very large and the results shown in Figure 2 corresponding to hydrogen and air combustion at different pressure. And resulted equivalence ratios within flammability condition range changes form very lean to very rich mixtures, where we have noticed that temperature and error at very lean mixtures has directly proportional relationship.
Figure 2: Below crossover temperature of hydrogen-oxygen ignition. Left-hand-side indicates variation of ignition time with equivalence ratio and right-hand-side indicates by dashed line experimentally determined explosion limits of hydrogen-oxygen and solid line representing analytic prediction developed by third explosion [8].
3 Conclusion
From the many studies, the chemical kinetics of hydrogen-air combustion below crossover, ignition delay varies inversely with pressure and directly with temperature. The studies found that the tested ignition delay has been used to foresee accurate ignition time that are 20 % better than wide range of pressures, temperature and compositions.
REFERENCES
[1] R. R. Craig, "A shock tube study of the ignition delay of hydrogen-air mixtures near the second explosion limit," DTIC Document1966. [2] R. Kushida, "The reaction of hydrogen and oxygen at high temperature," National Engineering Science Company, Air Force Contract No. AF3396160–8606, 1962. [3] P. Boivin, "Reduced-kinetic mechanisms for hydrogen and syngas combustion including autoignition," 2011. [4] T. A. M. U. Department of Chemistry. (2015). Kinetics : Factors Affecting Reaction Rates. Tilgjengelig: https://www.chem.tamu.edu/class/majors/tutorialnotefiles/factors.htm [5] P. Boivin, A. L. Sánchez, og F. A. Williams, "Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover," Combustion and Flame, vol. 159, ss. 748-752, 2012. [6] P. Boivin, A. L. Sánchez, og F. A. Williams, "Four-step and three-step systematically reduced chemistry for wide-range H2–air combustion problems," Combustion and Flame, vol. 160, ss. 76-82, 1// 2013. [7] S. McAllister, J.-Y. Chen, og A. C. Fernandez-Pello, Fundamentals of combustion processes: Springer, 2011. [8] Antonio L. Sánchez, César Huete, a. Daniel Martínez-Ruiz, og D. M. Boza. Reactive Flows. Tilgjengelig: http://asanchez.ucsd.edu/ [9] P. Saxena og F. A. Williams, "Testing a small detailed chemical-kinetic mechanism for the combustion of hydrogen and carbon monoxide," Combustion and Flame, vol. 145, ss. 316-323, 2006. [10] U. Maas og J. Warnatz, "Ignition processes in hydrogen oxygen mixtures," Combustion and flame, vol. 74, ss. 53-69, 1988.