Elsevier

Combustion and Flame

Volume 154, Issue 3, August 2008, Pages 529-545
Combustion and Flame

Sensitizing effects of NOx on CH4 oxidation at high pressure

https://doi.org/10.1016/j.combustflame.2008.01.012Get rights and content

Abstract

The CH4/O2/NOx system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NOx on CH4 oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600–900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NOx and C1–2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NOx. A similar effect is observed with increasing pressure. The sensitizing effect of NOx is related to the hydrocarbon chain-propagating NO/NO2 cycle operated by NO2+CH3NO+CH3O and NO+CH3OONO2+CH3O as well as the formation of chain-initiating OH radicals from interactions between NO/NO2 and the H/O radical pool. At low temperatures, reactions between NO/NO2 and CH3O/CH2O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH3NO2) as a characteristic high-pressure phenomenon. The formation of CH3NO2 represents an inactivation of NOx, which may result in a temporary reduction of the overall hydrocarbon conversion rate.

Introduction

High-pressure combustion of natural gas has a number of important applications including engines and gas turbines. Pressure is a driving force in the partial oxidation of natural gas to oxygenated hydrocarbons that play an important role as fuels as well as feedstock in a range of industrial processes. There is a requirement for detailed and fundamental knowledge of the complex mechanisms that govern the homogeneous oxidation of natural gas under a wide range of reaction conditions, including high pressure and presence of nitrogen oxides (NOx). Detailed chemical kinetic modeling (DCKM) is a powerful tool to disclose these issues on an elementary reaction level, but it is dependent on the availability of experimental data to verify the model performances and this is limited in the field of high pressure combustion.

The presence of even small amounts of nitrogen oxides can have a significant impact on fuel oxidation characteristics [1], [2]. This interaction has been investigated for a wide range of fuels, including H2 [3], [4], CO [4], [5], [6], [7], CH2O [3], [8], CH4/natural gas [3], [4], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], C2 hydrocarbons [4], [12], [22], [23], [24], higher hydrocarbons [4], [12], [25], [26], [27], [28], and oxygenated hydrocarbons [29], [30], [31], [32], [33], [34], [35]. The sensitizing behavior is a result of interactions with the radical pool as well as direct reactions between NOx and stable species, e.g., H2, O2, CH2O, CH4, and C2H6. A number of reactions take place that recycle NO and NO2 directly or through intermediate formation and subsequent decomposition of nitrous acid (HONO), within both the H/O/OH/HO2 and the hydrocarbon radical pool. These NO/NO2 recycling mechanisms promote the hydrocarbon oxidation scheme, either indirectly via formation of important chain-carrying radicals, such as OH and CH3—e.g., NO2 + H ⇌ NO + OH, NO + HO2 ⇌ NO2 + OH, NO2 + HO2 ⇌ HONO/HNO2 + O2, NO2 + CH4 ⇌ HONO/HNO2 + CH3, and HONO (+M) ⇌ NO + OH (+M)—or by direct acceleration of the hydrocarbon oxidation chain—most importantly, NO + CH3OO ⇌ NO2 + CH3O and NO2 + CH3 ⇌ NO + CH3O.

The objective of this paper is to analyze the interaction of methane and nitrogen oxides at high pressures. Only a few of the previous studies were conducted above atmospheric pressure. Dagaut and co-workers investigated the mutual oxidation of CH4/natural gas and NOx in jet-stirred reactors at pressures up to 10 atm [18], [19], while Sivaramakrishnan et al. [21] extended the pressure range to 50 bar through shock-tube experiments (1000–1500 K, lean conditions). In the present work experimental data are obtained for the CH4/O2/NOx system under well-defined conditions in a high-pressure laminar flow reactor at pressures from 20 to 100 bar, moderate temperatures in the range of 600–900 K, and high dilution in N2. The experimental results are interpreted in terms of a novel detailed chemical kinetic model, established with emphasis on the direct interactions between NOx and the hydrocarbon oxidation chemistry. The primary application of interest has been the direct partial oxidation of natural gas to methanol (GTL), but the present results may also be important for ignition in engines and gas turbines.

Section snippets

Experimental

The experimental work has been conducted in a laboratory-scale high-pressure laminar flow reactor designed to approximate plug flow. The setup is described in detail elsewhere [36] and only a brief description is provided here. The system enables well-defined investigations of homogeneous gas phase chemistry at pressures from 10 to 100 bars, temperatures up to 925 K, and flow rates of 1–5 SLPM. The reactions take place in a tubular quartz reactor, enclosed in a stainless steel tube that acts as

Detailed kinetic model

The proposed reaction mechanism is composed of the fundamental combustion mechanisms of H2/O2, CO/CO2, NO/NO2, and C1–2 hydrocarbons that are drawn directly from previous work of the authors [36], [43], including thermodynamic properties of the involved species. In the following, these reaction subsets are supplemented by elementary reactions for interactions between NOx and C1–2 hydrocarbons, drawing partly on recent work on CH3OH/NOx interactions [44].

Table 1 contains thermodynamic properties

Results and discussion

The effect of NOx on the CH4 oxidation chemistry has been investigated at three different pressures: 100, 50, and 20 bars. The experiments were conducted with high dilution in N2 under reducing (116<ϕ<99), stoichiometric (ϕ1), and oxidizing conditions (ϕ0.04) with a constant addition of NOx of about 200 ppm. The experimental conditions are summarized in Table 3, and the experimental results are presented in Fig. 2, Fig. 3, Fig. 4, representing reducing, stoichiometric, and oxidizing

Conclusions

The CH4/O2/NOx system diluted in N2 has been investigated in a laboratory-scale high-pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NOx on CH4 oxidation at high pressures and medium temperatures. Experiments have been conducted at 100, 50, and 20 bars, 600–900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results have been interpreted in terms of a detailed kinetic model drawn from previous work of

Acknowledgments

The work is part of the CHEC (Combustion and Harmful Emission Control) research program. It was financially supported by the Technical University of Denmark and the Danish Technical Research Council.

References (137)

  • T. Faravelli et al.

    Combust. Flame

    (2003)
  • P. Glarborg

    Proc. Combust. Inst.

    (2007)
  • M. Hori et al.

    Proc. Combust. Inst.

    (1992)
  • J. Roesler et al.

    Combust. Flame

    (1995)
  • P. Glarborg et al.

    Combust. Flame

    (2003)
  • M. Hori et al.

    Proc. Combust. Inst.

    (1998)
  • A.A. Konnov et al.

    Proc. Combust. Inst.

    (2005)
  • P. Dagaut et al.

    Combust. Flame

    (2005)
  • A. Doughty et al.

    Proc. Combust. Inst.

    (1996)
  • A.A. Konnov et al.

    Combust. Flame

    (2005)
  • J. Bromly et al.

    Proc. Combust. Inst.

    (1992)
  • P. Nelson et al.

    Proc. Combust. Inst.

    (1994)
  • M. Hori et al.

    Proc. Combust. Inst.

    (2002)
  • R. Lyon et al.

    Combust. Flame

    (1990)
  • V.I. Vedeneev et al.

    Catal. Today

    (1992)
  • M.W. Slack et al.

    Combust. Flame

    (1981)
  • F. Yamada et al.

    Chem. Phys. Lett.

    (1981)
  • R. Guirguis et al.

    Combust. Flame

    (1985)
  • J.T. Jodkowski et al.

    Chem. Phys. Lett.

    (1993)
  • J.A. McCaulley et al.

    Chem. Phys. Lett.

    (1985)
  • E. Martínez et al.

    Chem. Phys. Lett.

    (2000)
  • N. Murakami et al.

    Nenryo Kyokaishi

    (1982)
  • P. Glarborg et al.

    Combust. Sci. Technol.

    (1995)
  • P. Dagaut et al.

    Int. J. Chem. Kinet.

    (2003)
  • H.B. Dixon et al.

    Mem. Proc. Manchester Lit. Philos. Soc.

    (1930)
  • R.G.W. Norrish et al.

    Proc. R. Soc. London Ser. A

    (1934)
  • J.H. Bromly et al.

    Combust. Sci. Technol.

    (1996)
  • T. Amano et al.

    Proc. Combust. Inst.

    (1998)
  • Y. Yamaguchi et al.

    J. Phys. Chem. A

    (1999)
  • K. Tabata et al.

    J. Phys. Chem. A

    (2000)
  • A.B. Bendtsen et al.

    Combust. Sci. Technol.

    (2000)
  • P. Dagaut et al.

    J. Phys. Chem. A

    (2006)
  • J.M. Zalc et al.

    Ind. Eng. Chem. Res.

    (2006)
  • R. Sivaramakrishnan et al.

    Phys. Chem. Chem. Phys.

    (2007)
  • P. Dagaut et al.

    Combust. Sci. Technol.

    (2005)
  • P.A. Glaude et al.

    Energy Fuels

    (2005)
  • N. Murakami et al.

    Nenryo Kyokaishi

    (1982)
  • K. Hjuler et al.

    Ind. Eng. Chem. Res.

    (1995)
  • M. Alzueta et al.

    Israel J. Chem.

    (1999)
  • P. Dagaut et al.

    Combust. Sci. Technol.

    (2001)
  • M. Alzueta et al.

    Energy Fuels

    (2001)
  • M. Alzueta et al.

    Energy Fuels

    (2002)
  • C.L. Rasmussen, J. Hansen, P. Marshall, P. Glarborg, Int. J. Chem. Kinet., in...
  • O. Levenspiel

    Chemical Reaction Engineering

    (1972)
  • O. Levenspiel

    The Chemical Reactor Omnibook

    (1993)
  • D.W. Rytz et al.

    Ind. Eng. Chem. Res.

    (1991)
  • S. Ozturk et al.

    Ind. Eng. Chem. Res.

    (2000)
  • A.A. Konnov et al.

    Combust. Sci. Technol.

    (2004)
  • C.L. Rasmussen, P. Glarborg, submitted for...
  • C.L. Rasmussen, K.H. Andersen, K. Dam-Johansen, P. Glarborg, Int. J. Chem. Kinet., in...
  • Cited by (0)

    View full text