Ignition delay times of n-butane and i-butane under O2/CO2 atmospheres: Shock tube experiments and kinetic model

Abstract

Pressurized oxy-fuel combustion is deemed an advanced oxy-fuel combustion technique due to the lower cost and little decrease in the generating efficiency compared to conventional PC system without CO₂ capture. Butane is an important composition of petroleum and natural gas. In this work, the ignition delay times (IDTs) for n-butane and i-butane under O₂/CO₂ atmospheres were measured in a shock tube at different equivalence ratios (Φ) and pressures. Based on our previous C₁C₃ model Oxymech2.0 and the C₄ sub-model of Aramco3.0, a chemical kinetic model Oxymech2.0 Plus was updated. Oxymech2.0 Plus was validated by the newly measured IDTs of butane under O₂/CO₂ atmospheres. The model was also validated by the literature experimental data for IDTs of n-butane and i-butane at O₂/Ar and O₂/N₂ atmospheres, laminar flame speeds of n-butane and i-butane in air, and species profiles of n-butane and i-butane pyrolysis. The comparison between Oxymech2.0 Plus and Aramco 3.0 models was also conducted in detailed. The results show that the updated reactions in the study significantly improve the prediction of IDTs, laminar flame speeds and species profiles of butane in O₂/CO₂ atmospheres. The effects of equivalence ratios and CO₂ on the IDTs of n-butane and i-butane were analyzed.

Introduction

Pressurized oxy-fuel combustion is deemed an advanced oxy-fuel combustion technique due to the lower cost and little decrease in the generating efficiency compared to conventional PC system without CO₂ capture [1,2]. A 50-MWh natural gas-fired Allam Cycle plant has been built in La Porte, Texas, which generates energy by using oxy-fuel and direct-fired combustion in a high-pressure turbine [3]. Therefore, the study of ignition characteristics and chemical kinetic mechanisms for fuel combustion under pressurized oxy-fuel condition is essential for actual combustor design.

Butane is not only an important component of petroleum and natural gas, but also the smallest alkane with isomers. The ignition delay time (IDT), which can be measured in shock tubes, is a well-established feature for the validation of fuel combustion chemical kinetic models. The IDTs of n-butane and i-butane under O₂/Ar or O₂/N₂ atmospheres measured in shock tubes have been reported by many researchers [4], [5], [6], [7], [8], [9]. The shock tube experiments and conditions of butane IDTs were collected and listed in Table 1. Ogura et al. [4] proposed a model based on Gri3.0 [10] that well reproduced their measured IDTs. Healy et al. [5,6] proposed the NUIG model based on the kinetic models developed by Curran et al. [11,12] and Bourque et al. [13] that satisfactorily predicted their experimental results. In 2019, He et al. [7] found that the NUIG [5,6], USC2.0 [14] and Sandiego [15] models slightly underestimated the IDTs and updated the NUIG model to match their experiments. Zhou et al. [16] proposed the Aramco3.0 model, which is widely adopted due to the good performance for predicting the IDTs, laminar flame speeds and species profiles of C₁C₄ hydrocarbon and oxygenated hydrocarbon species. However, as far as we know, few studies on the IDTs of butane under O₂/CO₂ atmospheres exist at present.

Many studies found that widely adopted chemical kinetic models such as Aramco2.0 [17], Ranzi [18], USC 2.0 [14], FFCM-1 [19] and Gri 3.0 [10], cannot accurately predict the IDTs and laminar flame speeds of CH₄, C₂H₆ and C₃H₈ under O₂/CO₂ atmospheres [20], [21], [22]. In our previous work, Liu et al. [20,21] and Xia et al. [22] proposed a C₁C₃ kinetic model termed the Oxymech2.0 based on the Aramco2.0 model, which produced good prediction results of the IDTs, laminar flame speeds and species profiles of CH₄, C₂H₆ and C₃H₈ under O₂/N₂ and O₂/CO₂ atmospheres. It is necessary to expand the fuels tested by the Oxymech2.0 to include butane. Therefore, the butane sub-model needs to be updated and improved in O₂/CO₂ atmospheres in this work.

The updated reactions in the Oxymech2.0 can be divided into three parts. The first part includes reactions containing CO₂, such as CO + OH 〈=〉 CO₂ + H. The second part contains third-body collision reactions. The rate constants of many third-body collision reactions, such as H + O₂ (+M) 〈=〉 HO₂ (+M), CH₃ + CH₃ (+M) 〈=〉 C₂H₆ (+M), and C₂H₄ + H (+M) 〈=〉 C₂H₅ (+M), were updated. The third part represents fuel H abstraction reactions. The rate constants of fuel H abstraction reactions, such as CH₄ + OH 〈=〉 CH₃ + H₂O, C₂H₆ + HO₂ 〈=〉 C₂H₅ + H₂O₂, C₂H₆ + OH 〈=〉 C₂H₅ + H₂O, C₃H₈ + H 〈=〉 iC₃H₇ + H₂, C₃H₈ + OH 〈=〉 iC₃H₇ + H₂O and C₃H₈ + H 〈=〉 nC₃H₇ + H₂, were updated. According to the model development work in our previous work [22], the fuel decomposition and H abstraction reactions are crucial to fuel ignition. Therefore, the C₄ sub-model in Aramco3.0 needs to be updated and combine with Oxymech2.0 for the prediction of butane combustion in O₂/CO₂ atmospheres in this work.

Besides IDTs, species profiles and laminar flame speeds are two other commonly used features for validating fuel combustion kinetic models. In 2018, Li et al. [23] measured the species profiles of C₁–C₄ products in n-butane and i-butane pyrolysis experiments at air conditions. They proposed a model (Li-2018) based on their previous work [24,25]. The Li-2018 model well predicted the species profiles of both n-butane and i-butane. The laminar flame speeds of butane in O₂/N₂ atmospheres and in air conditions at 1 atm were measured by many research groups [26], [27], [28], [29].

In this work, shock tube experiments of IDTs for n-butane and i-butane diluted in carbon dioxide were conducted at pressures of 1.0 and 10.0 atm, at three different equivalence ratios (0.5, 1.0, 2.0), and at temperatures of 1200–1600 K. An updated oxy-fuel combustion kinetic model based on the Oxymech2.0 and the C₄ sub-model of Aramco3.0, was proposed and referred to as the Oxymech2.0 Plus. The Oxymech2.0 Plus was validated by the newly measured IDTs of butane under O₂/CO₂ atmospheres, and by the reported IDTs of butane under O₂/N₂ atmospheres [5,6] and O₂/Ar atmospheres [4]. The model was also validated by the selected experimental data of butane laminar flame speeds and species profiles [23,[26], [27], [28], [29]]. The prediction performance of the Oxymech2.0 Plus on IDTs, laminar flame speeds and species profiles were compared with that of the Aramco3.0. The effects of equivalence ratios and CO₂ on the ignition of butane were also discussed in detailed.