Elsevier

Combustion and Flame

Volume 158, Issue 6, June 2011, Pages 1059-1063
Combustion and Flame

A four-step reduced mechanism for syngas combustion

https://doi.org/10.1016/j.combustflame.2010.10.023Get rights and content

Abstract

A four-step reduced chemical-kinetic mechanism for syngas combustion is proposed for use under conditions of interest for gas-turbine operation. The mechanism builds upon our recently published three-step mechanism for H2–air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010), which was derived from a 12-step skeletal mechanism by assuming O, OH, and H2O2 to be in chemical-kinetic steady-state and includes a correction to account for the failure of the O and OH steady-states during autoignition. The analysis begins by appropriately extending the number of elementary steps in the skeletal description to enable computation of the CO chemistry for mixtures with appreciable H2 content, giving a total of 16 elementary steps. It is seen that the formyl radical HCO, which appears as the only additional relevant intermediate in the extended chemical description, follows accurately a steady-state approximation, which can be used along with the steady-state approximations for O, OH, and H2O2 to derive the reduced description, involving the three global steps of our previous H2–air mechanism, 3H2+O2 ⇌ 2H2O+2H, 2H+M ⇌ H2+M, and H2+O2 ⇌ HO2+H, along with the additional step CO+ H2O ⇌ CO2+H2. Expressions are given for the rates of the four global reactions in terms of those of the elementary steps of the skeletal mechanism, with concentrations of the different steady-state species also given in explicit form. Comparisons of results of computations of laminar burning velocities and induction times with published experimental data for H2/CO/O2 mixtures with different diluents at atmospheric and elevated pressures and for varying equivalence ratios and initial temperatures indicate that the reduced description can be applied with reasonable accuracy in numerical studies of gas-turbine 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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