A four-step reduced mechanism for syngas combustion
Introduction
The development of IGCC technologies, involving gas-turbine combustion of the syngas derived by air or O2 gasification of pulverized coal, has recently promoted interest in studies of CO/H2 combustion. Several detailed mechanisms are now available for accurate description of the combustion process under conditions of practical interest [1], [2], [3], although chemical-kinetic uncertainties still exist for low-temperature autoignition processes at elevated pressure [4] and also for laminar flame propagation at high-pressure fuel-rich conditions and in strongly preheated mixtures [5]. Because of the relatively large number of chemical species and elementary rates appearing in these detailed mechanisms, their use in computations of high-Reynolds-number flows in complex geometries is still prohibitively expensive for most purposes, given the present computational capabilities. Reduced mechanisms, systematically derived from detailed chemical schemes by introduction of steady-states for intermediate species, represent an attractive alternative to shorten computational times, while providing sufficient accuracy to yield reliable computational results.
Regardless of the coal type and gasification technology, the syngas mixture always contains significant amounts of CO and H2 as the main reactive species along with diluents such as N2, CO2 and H2O, while the hydrocarbon content, mainly CH4, is in general very limited, especially when O2-enriched gasification is employed. In deriving chemistry descriptions for syngas combustion, it therefore appears reasonable to focus on the chemistry of CO and H2, neglecting the contribution of the hydrocarbon chemistry to the overall combustion process. Since the H2/CO volume ratio in most syngas mixtures typically exceeds 0.25 and often takes on values that are of the order of 0.5 or above, it is found that the hydrogen chemistry plays a dominant role in syngas combustion. A result is that most syngas mixtures exhibit large burning rates and small autoignition times, comparable to those found in hydrogen combustion.
Section snippets
The reduced chemistry
The CO–H2 submechanism of the so-called San Diego mechanism [2], comprising 30 elementary reactions among 11 reactive chemical species (CO, CO2, HCO, O2, H2, H2O, H2O2, O, H, OH and HO2), will be used as a detailed-chemistry description for validation purposes.
Of the 21 steps in this mechanism that do not involve carbon atoms, a subset of 12 elementary steps, numbered 1–12 in Table 1, with the subscripts f and b employed to denote forward and backward reactions, recently has been found [6] to
Validation of the reduced mechanism
To test the degree of accuracy associated with the chemical simplifications, laminar flame velocities and induction times determined numerically with the reduced mechanism were compared with those obtained from both detailed-chemistry computations and computations with the skeletal mechanism of Table 1. All computations were performed with the COSILAB code [8] with radiative heat transfer neglected and with a multicomponent transport description including thermal diffusion. In addition, to test
Concluding remarks
The mechanism presented here can be used over a wide range of combustion conditions that include, in particular, most of those of gas-turbine operation. Calculations have indicated that its use decreases computation times by a factor, exceeding two, that may be as large as five depending on the specific test case. In view of the previously identified discrepancies between predictions of current chemical-kinetic mechanisms and experimental measurements [4], [5], further improvements of the
Acknowledgements
This work was supported by the UE Marie Curie ITN MYPLANET, by the Comunidad de Madrid through project #S2009/ENE-1597, and by the Spanish MCINN through projects #ENE2008-06515 and CSD2010-00011.
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