Elsevier

Fuel

Volume 230, 15 October 2018, Pages 185-193
Fuel

Full Length Article
Benzene pyrolysis and PM formation study using a flow reactor

https://doi.org/10.1016/j.fuel.2018.04.009Get rights and content

Abstract

The main component of PM formed in the combustion field is soot, and its formation mechanism is hydrocarbon fuel partial oxidation. Then as the first step of this PM formation research work, benzene pyrolysis and formation of PM in a diluted benzene-oxygen mixture were studied using a flow reactor. Experimental studies were conducted under zero O2 condition and O2 exist condition, respectively, and the influence on benzene pyrolysis and formation of PM due to exist or absence of oxygen was discussed. The effects of temperature and oxygen on PM formation were also discussed by independent control of temperature and oxygen concentration. PM sampled from the flow reactor was dried up to separate volatile compositions from soot. PM composition change by oxygen concentration and temperature were analyzed in detail. As the result, it was found that PM formation behavior at low temperature condition was different from that of high temperature condition. The valley temperature between them was around 1273 K. It decreased with the addition of oxygen. Under low temperature condition, small amount of PM was formed from benzene-N2 mixture without oxygen. It increased with the addition of oxygen. Main composition of this PM was low boiling point SOF. Under high temperature condition, large amount of PM was formed from benzene-N2 mixture without oxygen. It decreased with the addition of oxygen.

Introduction

Reduction of PM formation from combustion field is required to keep the clean environment. Further, PM is troublesome problem for human health directly. Then PM exhausted from a combustion of hydrocarbon fuel should be reduced. PM is formed from incomplete combustion such as fuel rich combustion under a certain temperature condition. The main component of PM formed in the combustion field is soot. It is well known that the mechanism of soot formation is hydrocarbon fuel partial oxidation, but more detailed information of it is needed to explain the formation mechanism of soot in PM.

Lower hydrocarbon radicals with less number of carbon atoms than the fuel result from fuel pyrolysis. These lower hydrocarbon radicals grow up to Polycyclic Aromatic Hydrocarbon (PAH) through polymerization and aggregation reactions. PAH grows more and it becomes soot. Therefore, PAH is considered as a precursor of soot. A process of PAH formation was proposed by Frenklach and Wang [1]. In their proposed process, small hydrocarbon radicals with unsaturated chain structure are formed by fuel pyrolysis. These radicals grow up to first aromatic ring by polymerization. The first aromatic ring grows up to PAH. After then, Aggregated PAH changes to soot nucleus and it becomes soot by surface growth. In several other researches, behavior of hydrocarbon radicals in a flame has been investigated to find out the relationship with soot [2], [3], [4].

The hydrogen abstraction acetylene addition (HACA) reaction mechanism was proposed to be the main formation route from chain hydrocarbon to aromatic hydrocarbon [5], [6]. This mechanism is well known as a reaction route of soot precursor formation. The HACA mechanism is a repetitive reaction sequence of two principal steps. First step is abstraction of hydrogen atom from hydrocarbon by third molecule. It forms hydrocarbon radicals including acetylene. In the second step, acetylene is added to the molecular radical site formed by hydrogen abstraction. The HACA reaction progresses until aromatic hydrocarbon being formed. After then, formed aromatic hydrocarbon grows up to PAH by additional HACA mechanism [5], [6], [7]. It grows up to heavier PAH having more polycyclic structure. For the growth of aromatic hydrocarbon to PAH, other reaction mechanisms are also known. Shukla et al. investigated PAH formation in hydrocarbon pyrolysis and suggested that phenyl radical, benzyl radical, and cyclopentadienyl radical have an important role in PAH growth. They proposed phenyl addition cyclization (PAC) mechanism [8], [9], [10], [11], [12].

In order to clarify a growth process of soot, it is necessary to investigate the route and condition of PAH formation. The progress of HACA reaction requires hydrocarbon molecules in radical state and acetylene. However, formation processes of these species have not been clarified. Concerning PAH formation route by HACA mechanism, benzene formation is generally considered. Several research reports have suggested that the high temperature chemistry of benzene has significant impact on the formation of heavier PAH and soot [13], [14]. Kern et al. investigated pyrolysis of toluene, benzene, butadiene, and acetylene using a shock tube and indicated that large amount of acetylene and diacetylene were formed under the condition higher than 2000 K [15]. Knorre et al. investigated soot formation in benzene-acetylene mixture using a shock tube and indicated that the soot yield increased at temperature higher than 2000 K in benzene:acetylene mixture of 1:1 [16]. Hou et al. [17] and Bikau et al. [8] investigated pyrolysis of benzene using a flow reactor and suggested that biphenyl is a dominant product in the formation process of heavy PAH under temperature conditions lower than 1300 K. From the results of benzene pyrolysis using shock tube, Sivaramakrishnan et al. [18] and Bohm et al. [19] also suggested that the dominant products in the formation process of PAH are different between high and low temperature conditions. High temperature oxidation of benzene has been investigated in many literatures [20], [21], [22] but low temperature oxidation has been slightly investigated.

In the soot formation studies of near-sooting premixed flame using benzene-oxygen mixture, large mole fractions of C6H6O and C5H6 early in the flame suggest that O-atom attack might be a main route of benzene consumption [23]. Frenklach et al. investigated soot formation in benzene-oxygen mixture at temperature higher than 1400 K using a shock tube and indicated that soot formation is suppressed by the existence of oxygen [24]. In the soot formation studies of laminar diffusion flame using propane, hexane, and benzene, it was suggested that temperature has influence on growth process from PAH to soot. It was also suggested that oxygen has influence on the formation amount of soot [25], [26].

Wang et al. [27], Frenklach et al. [24] and Leusden et al. [28] reported that oxygen suppresses soot formation from toluene and acetylene. The results obtained in these works indicated that, depending on the experimental conditions, oxygen also promotes the formation of soot. Promotion of soot formation by oxygen occurs under lower pressure and lower temperature conditions [24]. It is considered that the dominant product in the formation process of soot might be changed by oxygen between high and low temperature conditions. In order to clarify the formation of soot, it is necessary to investigate the critical temperature at which soot suppression process changes to soot promotion process. However, there are no research under the low temperature condition lower than 1300 K, and the oxygen effect on PM formation under low temperature conditions is not clear. The soot formation process from benzene-air mixture is complicated by temperature and oxygen conditions, and detail of benzene pyrolysis has not been clarified.

As for soot formation process from benzene-air mixture, it is considered that benzene has two functions for soot formation. The first function is the initiation molecule as a source of radicals. Hydrocarbon radicals and acetylene are formed by benzene pyrolysis, and PAH is formed by the progress of HACA mechanism. The second is that benzene becomes a PAH nucleus by itself. It is considered that these functions of benzene have diffluent ways of influence on PAH and soot formation. However, detail of benzene pyrolysis has not been clarified.

In order to investigate the effects of temperature and oxygen on PAH formation in a flame, it is necessary to separate these two effects. For separation of these effects, temperature and oxygen have to be controlled separately in a flame. However, independent control of temperature and oxygen concentration in the flame is usually difficult. On the other hand, when a flow reactor is used for soot formation study, it can control the temperature independently from the oxygen concentration [29]. Further, it can control the fuel-oxygen ratio and can control the dilution ratio of the fuel-oxygen mixture. Endothermic and exothermic reactions in a fuel-oxygen mixture result a change of the mixture temperature. The temperature change by these reactions can be suppressed by the dilution of fuel-oxygen mixture. Further, the temperature of reaction field can be controlled by heating and cooling of the flow reactor. When the reactions of diluted fuel-oxygen mixture is investigated using a flow reactor, it is possible to clarify the relationship between the temperature and these reactions by suppressing the temperature influence caused endothermic and exothermic reactions.

Then as the first step of this PM formation research work, benzene pyrolysis and formation of PM in a diluted benzene-oxygen mixture were studied using a flow reactor. The effects of temperature and oxygen on PM formation were investigated by independent control of temperature and oxygen concentration. PM sampled from the flow reactor was dried up to separate volatile compositions from soot. PM composition change by oxygen concentration and temperature were analyzed in detail. Obtained result was a fundamental relationship between benzene pyrolysis and PM formation.

Section snippets

Experimental setup

Experimental setup for benzene pyrolysis and oxidation is shown in Figs. 1 and 2 is the overview of it. This setup consists of a fuel supply system and a flow reactor with heating furnace. Liquid benzene was regulated by a micro tube pump, and was supplied to a heated evaporation tank. N2 and air mixed with vaporized benzene in the tank. Here benzene flow rate was 0.36 g/min and total flow rate of the mixture was 5.4 L/min as the main experimental condition. It corresponded 1.9 vol% of benzene

Overall reaction behavior in the flow reactor

Overall reaction behavior along the reactor is shown in Fig. 7. The test conditions were a benzene concentration of Cb-IN = 1.9 vol%, an oxygen concentration of CO2-IN = 2.3 vol%, and a furnace temperature of Tf = 1273 K. The residence time in the heating zone was 0.41 s. Upper part of the figure shows concentrations of benzene, oxygen, and carbon dioxide. The middle part shows the formation mass of “Dry 120 °C PM”, and the lower part shows the temperature along the reactor.

The benzene

Conclusions

Benzene pyrolysis and formation behavior of PM were investigated using a flow reactor. Experimental studies were conducted under zero O2 condition and O2 exist condition, and the relationship between them was investigated. As a result of the experiment, the PM formation behavior under O2 exist condition can be explained by oxidation reaction overlapping on the PM behavior under zero O2 condition. The main results are summarized as follows:

  • 1.

    Both of temperature and oxygen dominated the PM

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