Elsevier

Combustion and Flame

Volume 159, Issue 6, June 2012, Pages 2028-2055
Combustion and Flame

A comprehensive chemical kinetic combustion model for the four butanol isomers

https://doi.org/10.1016/j.combustflame.2011.12.017Get rights and content

Abstract

Alcohols, such as butanol, are a class of molecules that have been proposed as a bio-derived alternative or blending agent for conventional petroleum derived fuels. The structural isomer in traditional “bio-butanol” fuel is 1-butanol, but newer conversion technologies produce iso-butanol and 2-butanol as fuels. Biological pathways to higher molecular weight alcohols have also been identified. In order to better understand the combustion chemistry of linear and branched alcohols, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high-temperature and low-temperature reaction pathways with reaction rates assigned to describe the unique oxidation features of linear and branched alcohols. Experimental validation targets for the model include low pressure premixed flat flame species profiles obtained using molecular beam mass spectrometry (MBMS), premixed laminar flame velocity, rapid compression machine and shock tube ignition delay, and jet-stirred reactor species profiles. The agreement with these various data sets spanning a wide range of temperatures and pressures is reasonably good. The validated chemical kinetic model is used to elucidate the dominant reaction pathways at the various pressures and temperatures studied. At low-temperature conditions, the reaction of 1-hydroxybutyl with O2 was important in controlling the reactivity of the system, and for correctly predicting C4 aldehyde profiles in low pressure premixed flames and jet-stirred reactors. Enol–keto isomerization reactions assisted by radicals and formic acid were also found to be important in converting enols to aldehydes and ketones under certain conditions. Structural features of the four different butanol isomers leading to differences in the combustion properties of each isomer are thoroughly discussed.

Introduction

The production of butanol from biomass feedstock (i.e., bio-butanol) is not novel. Jones and Woods [1] provide a detailed history of bio-butanol production via fermentation prior to 1986. Louis Pasteur first documented the fermentation of sugars to 1-butanol in 1861, although the yields he achieved were not sufficient for commercialization. In the early 1900s, a German born chemist, Chaim Weizmann, was attempting to produce butanol for synthetic rubber production, and discovered that the Clostridium acetobutylicum organism was capable of converting large amounts of sugars into a mixture of acetone–butanol–ethanol (ABE) in the molar ratio of 3:6:1. This biological fermentation process reached large commercial scales in the 1930s and 1940s. After World War 2, there was a rapid decline in ABE production from biomass for two reasons; the cost of biomass feedstock rose sharply and petroleum derived solvents became more cost effective.

More recently, there has been renewed interest in biological fermentation of starches and sugars to produce fuels. In an effort to produce biofuels with better fuel properties and environmental performance than conventional bio-ethanol, researcher scientists and entrepreneurs have invested significant resources in bio-butanol production. Butanol has a higher energy density than ethanol and is less polar, so it can be more easily blended with hydrocarbon fuels like gasoline both in on-board transportation fuelling systems and in fuel pipelines. Nigam and Singh [2] briefly reviewed the recent activities in bio-butanol research and indicated that biological pathways exists for the production of 1-butanol, iso-butanol, and 2-butanol. The fourth isomer of butanol, tert-butanol, is a petrochemical product used as an octane enhancer in gasoline fuels. Thus, combustion studies on all four butanol isomers are warranted, as all have either a current or future use in engines.

Many previous combustion studies on the various butanol isomers have been conducted, so a brief summary is presented herein to set the tone of the present study. 1-Butanol has been studied as a fuel or as a blending agent for use in spark-ignition engines [3], [4], [5], [6], [7], [8], [9], [10], [11], compression–ignition engines [5], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], and motored engines [23]. iso-Butanol has also received attention from the engine community with studies performed in spark-ignition engines [24], [25], [26] and in compression–ignition engines [27], [28]. Research articles studying 2-butanol and tert-butanol in engines have not been presented to the authors’ knowledge. Although engine related studies provide important information on the feasibility and performance of butanol isomers in practical systems, fundamental combustion science research is also needed to benefit the design and optimisation of engine technologies.

To investigate the combustion properties at a more fundamental level, experimental combustion research on the butanol isomers includes studies on pyrolysis [29], [30], [31], [32], [33], [34], premixed laminar flame propagation [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], premixed laminar flame structure [45], [46], [47], non-premixed laminar diffusion flame structure [37], [41], [48], [49], [50], [51], shock tube and rapid compression machine ignition delay [52], [53], [54], [55], [56], [57], [58], [59], [60], spray chamber combustion [61], and jet-stirred reactor oxidation chemistry [37], [62], [63], [64], [65].

A primary contribution of the aforementioned experimental work is to aid in the development and validation of detailed chemical kinetic models for the isomers of butanol, such as those presented previously by [33], [37], [40], [51], [54], [59], [60]. These fundamental kinetic models aim to be predictive in nature with the ultimate goal of advancing design and optimization of practical engine systems through coupling with computational fluid dynamic (CFD) models. Despite the recent leaps in butanol kinetic modeling, a comprehensive model valid at the temperature and pressure ranges and oxidation conditions found in practical engines is yet to be developed.

The goal of this study is to develop a novel chemical kinetic model for the four isomers of butanol: 1-butanol (butan-1-ol), 2-butanol (butan-2-ol), iso-butanol (2-methylpropan-1-ol), and tert-butanol (2-methylpropan-2-ol). This study differentiates itself from the previous butanol chemical kinetic models by including detailed high- and low-temperature pathways with reaction rate constants derived using rate rules. The developed reaction pathways and rate rules draw extensively from work performed by theoretical and experimental chemists in the atmospheric and combustion research communities, as cited in the model development section of this paper. The model is widely validated against recently published experimental speciation data from low-pressure laminar premixed flames [45], [46], atmospheric pressure premixed laminar flame velocities [39], [41], elevated pressure rapid compression machine (RCM) ignition delay times [57], [58], atmospheric and high pressure shock tube (ST) [52], [53], [60] ignition delay times, and elevated pressure jet-stirred reactor (JSR) species profiles [62], [64]. By presenting a validated rule-based chemical kinetic model for the four isomers of butanol, this study aims to provide a platform and methodology for developing similar models for higher molecular weight alcohols. Furthermore, this study attempts to guide future butanol related combustion chemistry research by elucidating those reactions that are the most significant and uncertain at the present time.

Section snippets

Chemical kinetic model development

Alcohols contain a hydroxyl group that brings about different behavior of its associated rate constants and product channels compared to hydrocarbons. These effects include pre-reaction complexes when reacting with OH that cause negative temperature dependence in its rate constants at low-temperature [66]. Also its molecular structure allows a faster reaction of its 1-hydroxy (i.e., alpha-hydroxy) radical with O2 compared to an analogous hydrocarbon radical with O2 [67]. This unique behavior

Model validation studies

The proposed model for the four isomers of butanol is validated against a wide range of experimental data covering low-temperature and high-temperature oxidation conditions. There has been a wide range of experimental validation made available for the isomers of butanol including jet-stirred reactor speciation [37], [62], [64], counter-flow non-premixed diffusion flame speciation [37], [51], co-flow laminar non-premixed flame speciation [49], counter-flow flame non-premixed ignition [41], rapid

Premixed laminar flame velocity simulations

Atmospheric pressure Premixed laminar flame velocities (Su0) have been reported for 1-butanol [39], [40], [41], iso-butanol [39], [41], 2-butanol [39], and tert-butanol [39]. Su0 was modeled using the PREMIX flame code in CHEMKIN PRO [121]. A high-temperature version of the detailed mechanism was used for these simulations, wherein reaction classes 11–30 were removed. The simulations accounted for thermal diffusion (i.e., Soret effect), assumed mixture-averaged transport, and the solutions were

Summary

This paper presented a new model for the four isomers of butanol and compared it to experimental data from a wide variety of fundamental combustion devices. The kinetic model includes comprehensive low-temperature and high-temperature reaction pathways specific to alcohol fuel chemistry. Rate constant rules were developed by surveying available experimental and theoretical literature on alcohols, as well as by using estimation procedures discussed in this paper.

In the low-pressure premixed

Research outlook

Butanol combustion chemistry is currently a topic of intense research in the combustion community. Although the present model well predicts a wide variety of combustion data, additional fundamental and applied research can aid substantial advances in the predictive accuracy and mechanistic realism of future models. The present study addresses many issues regarding the current state-of-the-art in comprehensive chemical kinetic modeling of alcohol combustion, so it is worthwhile to summarize

Acknowledgments

The US Department of Energy, Office of Vehicle Technologies and Office of Basic Energy Sciences supported the portion of this work performed at LLNL, and the authors thank program managers Kevin Stork and Wade Sisk. Research at LLNL was performed under the auspices of the US Department of Energy under Contract DE-AC52-07NA27344. The work at RWTH Aachen is part of the Cluster of Excellence “Tailor Made Fuels from Biomass”, which is funded by the Excellence Initiative by the German federal and

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