Effects of n-Butanol Addition on the Combustion Characteristics of n-Heptane Counterflow Diffusion Flame at Elevated Pressure

Abstract

This study focused on the effects of n-butanol addition on the combustion characteristics of n-heptane counterflow diffusion flame under pressures of 1, 3, and 5 atm by a detailed kinetic simulation. The added n-butanol volume fraction ranged from 0 to 50%. The mass averaged velocity of fuel streams was selected to ensure momentum flux balance and was approximately constant for the investigated flames. Flame structures and mole fraction profiles impacted by n-butanol addition for major species, free radicals, and intermediate species were analyzed by concentrating on the formations of soot precursors and oxygenated air pollutants. The results showed that with the addition of n-butanol, the flame temperature decreased and the formation of the main soot precursors such as C₂H₂ and C₆H₆ was inhibited. This can be attributed to the reduced rate of production of these species. The flame temperature increased significantly, and the profile moved towards the fuel side with the increasing pressure. Moreover, the production of C₂H₂ and C₆H₆ was observably promoted as the pressure increased. The concentrations of free radical H, O, and OH decreased significantly as the pressure increased but slightly decreased with the increasing n-butanol addition, which might have been caused by the chemical effect of n-butanol.

1. Introduction

Energy plays an important role in the development and progress of human society. The development of science and technology promotes the improvement of the energy utilization level and scale, which continuously facilitates the growth of human society to a new stage. At present, the overall energy consumption of the world is still dominated by fossil fuels [1,2,3]. However, the utilization of fossil fuels also brings problems. The combustion of fossil fuels releases air pollutants and greenhouse gases, polluting the environment and contributing to global warming [4,5]. Moreover, the incomplete burning of fossil fuels generates soot, which is harmful to human health [6,7,8]. It is urgent and necessary to develop and use new renewable energy sources.

Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, as alternatives to petroleum-based transportation fuels, have been of increasing interest in recent years due to the long-term promise of fuel-source renewability and climate-impact reduction [9]. Alcohols as oxygenated compounds stand out as good alternative fuels or fuel additives for the reduction of soot emissions and have been widely investigated [10,11,12]. Butanol is a very promising fuel alternative or additive that has higher energy density [13], lower water absorption, and better miscibility with present fuels, compared to ethanol [9].

Investigations of the effects of n-butanol on combustion fundamental characteristics have been extensively performed experimentally and numerically. Uyumaz [14] investigated the combustion and performance characteristics of n-heptane, n-butanol, and isopropanol in an HCCI engine; the result showed that adding high octane number alcohols could extend the range of the HCCI operation and control autoignition. Liu et al. [15,16] also investigated to reveal the combustion characteristics and soot-reduction capability in HCCI combustion with different control strategies, and they found that the blends of gasoline and n-butanol in diesel significantly reduced soot emissions, and n-butanol addition presented a higher soot-reduction ability at a low diesel replacement ratio (30%). Wu et al. [17] simulated the combustion process in a diesel engine with n-butanol/diesel blends and found that the maximum combustion pressure and temperature increased and the accumulated heat of release reduced with n-butanol addition. Chen et al. [18] conducted an experimental study on the effects of the energy substitution ratio on the combustion characteristics and performance of a diesel engine fuelled with methanol, ethanol, and n-butanol addition, and the results showed that the alcohol additions decreased the particle concentration. The effect of n-butanol addition with isooctane and n-heptane on ignition delay times was obtained in a constant volume combustion chamber; it was presented that n-butanol addition inhibited the ignition delay time and reduced reactivity [19]. Zheng et al. [20] experimentally investigated the influence of two-stage injection on the combustion and emission characteristics in a single-cylinder diesel engine with pure diesel, and blended, fuels of gasoline and n-butanol, and the results showed that with the addition of gasoline and n-butanol, a slight increase in indicated thermal efficiency was induced and the smoke emissions were more sensitive to the change in post-injection strategy. An experimental study of combustion characteristics with n-heptane direct injection and n-butanol pipe injection in an HCCI engine was carried out by Zhang et al. [21], and they found that n-heptane direct injection could improve n-butanol combustion and reduce HC and CO emissions. Yu et al. [22] used n-butanol as a substituted fuel for spark ignition aviation piston engines to study the engine performance, and they found that the heat release of n-butanol was more concentrated and n-butanol could achieve higher brake thermal efficiency. The combustion and emission characteristics of n-heptane with three oxygenated fuels, including n-butanol, benzaldehyde, and anisole, were experimentally studied on a heavy-duty diesel engine, and the results showed that the n-butanol addition led to lower soot emissions and higher NOx emissions with increasing injection pressure [23].

To our knowledge, the study of the detailed effects of n-butanol addition on the formations and consumptions of important species, including some soot precursors and oxygenated air pollutants at elevated pressure, is limited. This is the first time a comprehensive numerical study is being performed on a counterflow diffusion flame of n-butanol/n-heptane blended fuel for a thorough understanding of the flame chemistry at elevated pressure. The present study aims to explore the effects of n-butanol addition on n-heptane flames, with an emphasis on the pollutant species by kinetic analysis.

2. Kinetic Modeling and Analysis Method

The detailed chemical reaction mechanism for n-heptane and n-butanol combustion used here is POLIMI_ BIOGASOLINE_171R developed by Ranzi et al. [24], which consists of 171 species and 3860 reactions. The reduced mechanism is derived by including n-butanol in the previous ternary mixture of n-heptane, iso-octane, and toluene. The accuracy of the mechanism has been proved by numerous works and studies [25,26,27]. The simulations performed in this work were conducted by OpenSMOKE/CounterFlow-Flame1D code [28].

The counterflow configuration considered in this numerical study mimics a counterflow burner designed by Advanced Combustion Laboratory in NJUST. Only a brief overview is provided here as a comprehensive description is available in Ref [29]. The upper nozzle and the lower nozzle are used for the fuel and the oxidizer stream, respectively. The separation distance between the nozzles is 8.0 mm. On the fuel side, n-heptane was replaced by n-butanol ranging from 10% to 50% in volume fraction. On the oxidizer side, 79% nitrogen and 21% oxygen were mixed as the oxidizer. The detailed flame conditions were listed in

Table 1. The detailed flame conditions.

No.	Pressure (atm)	Fuel Side				Oxidizer Side
		Volume Fraction (%)			Vf (cm/s)	VOX (cm/s)
		NC7H16	N1C4H9OH	N2
1	1	10	0	90	16.39	20
2		9	1		16.51
3		8	2		16.63
4		6	4		16.89
5		5	5		17.01
6	3	10	0		16.39
7		9	1		16.51
8		8	2		16.63
9		6	4		16.89
10		5	5		17.01
11	5	10	0		16.39
12		9	1		16.51
13		8	2		16.63
14		6	4		16.89
15		5	5		17.01

. The boundary temperatures of the fuel and oxidizer nozzle were maintained at 570 K and 300 K, respectively. The simulated pressure was 1, 3, and 5 atm.

3. Results

The mole fraction profiles and rate of production of vital components are calculated for all the simulated flames. Only the profiles that are thought to be most relevant to fuel consumption and important pollutant formations are discussed in this paper. The simulation results show that the variations of the flame temperature and mole fraction profiles of these species present a uniform trend with the n-butanol addition at the same pressure, respectively. There is no mutation with the increasing n-butanol addition. Thus, to make the figures clearer to read, typical conditions are selected to show the changing tendency with 0%, 20%, and 50% n-butanol addition at each pressure. Moreover, the rate of production analysis for reactant n-heptane and n-butanol, and some essential intermediate species, including acetylene (C₂H₂) and benzene (C₆H₆), are shown.

3.1. Flame Temperature Profiles

The intensity of flame burning can be measured by measuring multiple indicators, such as the light intensity of the flame, the flame temperature distribution, the fuel consumption rate, and the product generation rate. The flame temperature distribution is one of the most intuitive and effective indicators to identify the flame burning intensity.

The flame temperatures are shown in Figure 1a, with 0%, 20%, and 50% n-butanol additive at pressures of 1, 3, and 5 atm, and the locally enlarged view of flame temperature is presented in Figure 1b. It can be seen that at constant pressure, the flame temperature gradually decreased with the increasing n-butanol addition. The peak temperature decreased the most, and the temperature distribution was slightly narrowed when n-butanol was added. However, with the increase in pressure, the peak value of the flame temperature increased significantly, and the temperature peak position moved to the fuel side. The distribution of the flame temperature was also narrowed, which indicated that the reaction zone was narrowed at elevated pressure.

3.2. Major Species and Radicals

Figure 2 showed the mole fraction profiles of n-heptane with different levels of n-butanol addition at different pressures. The mole fraction of n-heptane decreased as the mole fraction of n-butanol increased since the total mole fraction of the mixed fuel was 10% on the fuel side (the other 90% was the diluted nitrogen) at the same pressure. However, the position where the n-heptane mole fraction decreased to zero was almost unchanged, which illustrated that the consumption of n-heptane was faster at higher n-heptane concentrations. The position of n-heptane started to react and moved to the oxidizer side when the pressure increased. At the same time, the slope of the mole fraction distribution became larger, indicating that the reaction rate of n-heptane increased with the increasing pressure. The position where the n-heptane was depleted was moving toward the oxidizer side slightly.

The rate of consumption of n-heptane with 0%, 20%, and 50% butanol addition was presented in Figure 3. The dominant consumption reactions of n-heptane were R742: NC₇H₁₆ = 0.2CH₃ + 0.4C₂H₅ + 0.4NC₃H₇ + 0.4NC₄H₉P + 0.5NC₅H₁₁ + 0.1NC₇H₁₅ and R2584: NC₇H₁₆ + H = H₂ + NC₇H₁₅. Figure 3a,b shows the influence of R742 and R2584 on the consumption rate of n-heptane under different flame conditions. When the pressure was kept constant, the reaction rates of R742 and R2584 both decreased with the increasing addition of n-butanol. The position of the reaction rate curve hardly moved, which indicated that the consumption rate of n-heptane decreased due to the decrease in n-heptane concentration. The reaction rates of R742 and R2584 significantly increased with the increasing pressure.

N-butanol was the fuel additive for this study. Figure 4 describes the distribution of the n-butanol mole fraction in different flame conditions. It was found that the position of the depletion of n-butanol was almost unchanged with the increasing n-butanol addition ratio, which can be attributed to the faster reaction rate of butanol in the reactions with higher mole fractions. When the pressure increased, the depleted point of n-butanol moved slightly toward the oxidizer side, and the slope of the mole fraction distribution curve increased, which revealed that n-butanol consumption was improved with higher pressure.

Figure 5 gives the dominant consumption reactions of n-butanol at elevated pressures. The main reactions responsible for the consumption of n-butanol were R942: N1C₄H₉OH = H₂O + NC₄H₈, R936: N1C₄H₉OH = CH₂OH + NC₃H₇, and R3665: N1C₄H₉OH + H = H₂ + CH₃CH₂CH₂CHOH. The consumption rates of the three reactions all increased with the increased n-butanol addition at the same pressure. When the fuel composition remained unchanged, the consumption rate of n-butanol significantly increased as the pressure increased. Furthermore, the reaction rate curves moved to the oxidizer side at higher pressure conditions. The dramatic increase in the reaction rates of n-butanol consumption was a good explanation for the rapid decrease in n-butanol under high pressure, as shown in Figure 4.

From the view of the main reactions responsible for the consumption of n-heptane and n-butanol, the H radical played a vital role in the combustion. The mole fraction profile of H radical with different n-butanol fractions at elevated pressure is presented in Figure 6a. It can be seen that the peak mole fraction of the H radical decreased with the addition of n-butanol at constant pressure, and the peak position moved toward the fuel side. As the pressure increased, the peak value of H radical dropped sharply and the peak value position moved to the fuel side significantly. The peak value of the H radical at 5 atm reduced to about one-third of that at 1 atm. Moreover, the distribution curves were notably narrowed. The increased pressure inhibited the generation of H radicals.

The mole fraction profiles of O and OH radicals shown in Figure 6b,c exhibited similar variation trends to H radicals. The peak mole fractions of the O and OH radicals decreased with the increasing n-butanol addition under each pressure, which revealed that n-butanol addition inhibited the production of O and OH radicals. When the pressure increased, the peak concentrations of O and OH radicals reduced dramatically, and the narrowed peak position moved toward the fuel side. The pressure showed an inhibition effect on these three radicals, and it was more obvious in reducing the H and O radicals.

The mole fraction profiles of CO and CO₂ with different levels of n-butanol additives are presented in Figure 7a,b. CO and CO₂ concentrations decreased with the replacement of n-heptane by n-butanol at the same pressure. This illustrated that the n-butanol could slightly inhibit the formation of CO and CO₂, and the higher the ratio of n-butanol addition, the more obvious the inhibition effect, while with the same volume addition of n-butanol, the peak mole fraction values of CO decreased drastically when the pressure increased. On the contrary, the peak values of CO₂ concentrations increased with the increasing pressure, and the peak values positions moved toward the fuel side. This indicated that more carbon element was converted to CO₂ rather than CO. Furthermore, the distribution of the peak value of CO₂ kept the same variations as that of the fuels n-heptane and n-butanol corresponding to the former analysis.

3.3. Intermediate Species

As acetylene (C₂H₂) and benzene (C₆H₆) are important soot precursors, and formaldehyde and acetaldehyde are typical and harmful oxygenated air pollutants, they are discussed in detail.

Figure 8 and Figure 9 show the mole fraction distributions of C₂H₂ and C₆H₆ with different levels of n-butanol additives at elevated pressures. It can be seen that the variation trends of C₂H₂ and C₆H₆ were consistent. When the pressure was constant, the concentrations of C₂H₂ and C₆H₆ significantly decreased with the increasing n-butanol additives, which illustrated that n-butanol addition could inhibit the formation of soot precursors. Westbrook et al. [30] concluded that nearly all of the oxygen atoms initially present in the oxygenate react directly to generate CO through a detailed chemical kinetic modeling investigation. As a result, the reduction in the two soot precursors can be attributed to the existence of the O element in n-butanol and oxidized more carbon elements to CO rather than C₂H₂ and C₆H₆.

On the contrary, with the increase in pressure, the concentrations of C₂H₂ and C₆H₆ increased, and the distributions moved toward the oxidizer side. The peaks of the C₂H₂ distribution curve increased to a small degree and were not sensitive to pressure changes. The peak value of benzene at 5 atm was about twice that at 1 atm, and the increase was very obvious. The production areas of C₂H₂ and C₆H₆ were narrowed with increasing pressures. The total mole fractions of C₂H₂ and C₆H₆ were enhanced by the increased pressures.

The rates of production of C₂H₂ with 0%, 20%, and 50% n-butanol additions were given in Figure 10. The main formation reaction of acetylene was R82: C₂H₃ (+M) = C₂H₂ + H (+M), and the main consumption reaction was R323: C₂H₂ + O = H + HCCO. It showed that with the increasing n-butanol additive, the reaction rates of the production and consumption reactions all decreased. The overall effects of n-butanol addition resulted in a decrease in C₂H_2, as shown in Figure 8. When the fuel composition was kept constant, the reaction rates of R82 and R323 both increased with the increased pressure, and the reaction areas were narrowed. The combined effect promoted the generation of C₂H₂. As pointed out by the well-known hydrogen abstraction carbon addition (HACA) mechanism [31], C₂H₂ is a key species that contributes to the growth of poly-aromatic hydrocarbon (PAH). Subsequently, it might result in higher soot formation with increasing pressure.

Figure 11 presents the rate of production of C₆H₆ with different levels of n-butanol at different pressures. The main reactions responsible for C₆H₆ formations were R415: C₆H₅ + H = C₆H_6, and R419: 2C₃H₃ (+M) = C₆H₆ (+M). The dominant reaction for C₆H₆ consumption was R444: C₆H₆ + H = H₂ + C₆H₅. The formation and consumption reactions of C₆H₆ all decreased with increased n-butanol additives at constant pressure. The total C₆H₆ concentration was reduced as n-heptane was replaced by n-butanol. When the pressure increased, the formation and consumption reactions of C₆H₆ all increased and the final C₆H₆ concentration increased, as shown in Figure 9. By blending n-butanol into n-heptane, the oxygenated fuel increased oxygen atoms, which could effectively reduce the formation of soot precursors [32].

Figure 12 and Figure 13 present the concentrations of CH₂O and CH₃CHO in different flame conditions. The mole fractions of CH₂O and CH₃CHO increased significantly with the addition of n-butanol. The simulation results indicated an enhancement effect for increasing n-butanol in the reaction mixture. This was because the addition of n-butanol increased the oxygen content on the fuel side, which promoted the formation of oxygen-containing pollutants [33]. The mole fraction of CH₂O decreased as the pressure increased, and the position of the peak value of CH₂O moved toward the oxidizer side. While the peak concentration of CH₃CHO nearly kept the same at different pressures, the position of the peak value of CH₃CHO moved to the oxidizer side as well.

4. Conclusions

This study investigated the effects of n-butanol addition on the combustion characteristics of n-heptane counterflow diffusion flame under pressures of 1, 3, and 5 atm. The flame temperature and major species of reactants, soot precursors, and oxygenated air pollutants variations were analyzed. The results can be summarized as follows:

• The flame temperature decreased with the addition of n-butanol, while the temperature increased with increasing pressures.
• The addition of n-butanol inhibited the formation of CO, C₂H₂, and C₆H₆ but promoted the formation of CO₂, CH₂O, and CH₃CHO. This can be attributed to the increased oxygen content on the fuel side; thus, the oxygen-containing species increased.
• The H, O, and OH radicals slightly decreased with n-butanol addition, while they dramatically reduced under higher pressures, especially the H and O radicals.
• The increased pressure significantly enhanced the rates of the main production and consumption reactions and promoted total combustion.