Effect of Operating Parameters on Oxidation Characteristics of Soot under the Synergistic Action of Soluble Organic Fractions and Ash.

Diesel particulate filter is used to reduce particulate matter (PM) emission due to the stringent emission standards. The accumulated PM has been oxidized by the periodical regeneration method to avoid pressure buildup. The innovation of this study is to explore the oxidation performance of Printex-U (PU), which is mixed with ash and soluble organic fractions, under different operating conditions. Different aspects of operating parameters, such as the oxygen ratio in an O2/N2 atmosphere, total flow rate, initial PU mass, and heating rate, on PU oxidation properties have been critically discussed using a thermogravimetric analyzer. The oxygen ratio in the O2/N2 atmosphere is positively correlated with the oxidation characteristics of PU. The comprehensive oxidation index (S ) of PU under the 20% O2/80% N2 atmosphere increases by 184% compared with the 10% O2/90% N2 atmosphere. When the initial PU mass is 3 mg, the combustion stability coefficient (R w) and S reach the best values, which are 55.53 × 105 and 2.03 × 107 %2min-2 ° C-3, respectively. With the increase in the heating rate, the oxidation properties of PU become sensible and deflagration occurs easily, so that 10 °C/min heating rate is the best option. This study provides a theoretical basis for the optimization design of diesel particulates during the regeneration process.

Introduction

In the last decades, diesel engines have attracted increasing interests from vehicle producers and the public due to their higher thermal efficiency, less carbon monoxides, and unburned hydrocarbon emissions compared with gasoline engines.[1−4] Seventy percent of commercial highway freight vehicles are diesel vehicles, which accounts for 20% of total greenhouse gas emissions.[5] Many studies have classified diesel emissions into four categories, including nitrogen oxides (NO, NO2, N2O, etc.), particulate matter (PM) (dry soot smoke, soluble organic fraction, ash, sulfate, etc.),[6,7] carbon monoxide, and unburned hydrocarbons.[8−11] Environmental issues pose huge threats to public health, particularly the damage caused by small-diameter PM.[12−16] Short-term or long-term exposure to PM instigates adverse health effects upon the cardiovascular (CV) system[17] and cancer.[14]

Diesel particulate filtering (DPF) are considered one of the most efficient technologies in removing diesel engine PM. DPF have been widely used in after-treatment systems.[2,3,18−21] However, DPF capturing PM for a long time will lead to lower capture efficiency and the blockage of the DPF and the increase in exhaust backpressure, thus affecting the normal operations of the engine.[22−24] There are two main types of regeneration methods of the DPF, the active regeneration and passive regeneration method.[19,25,26] Many research results show that the maximum exhaust temperature of a diesel engine is about 400 °C, which is lower than the ignition temperature of PU without a catalyst about 550–600 °C.[27,28] The ignition temperature of soot is closely related to the oxidation performance of soot.

Many operating factors affect the oxidation performance, such as the oxygen ratio in the O2/N2 atmosphere, heating rate,[28] contents of soluble organic fractions (SOF), species and concentrations of the catalyst,[29] type and content of ash,[24] and optimal initial sample mass.[30,31] Liang et al. found out the impact of lubricant-derived ash on the oxidation performance of soot and concluded that the ash can accelerate the oxidation of soot.[32] Meng et al. studied the oxidation behavior of soot at different heating rate, and the results have shown that the activation energy of the soot reaction decreases and the reaction rate increases with the increase in the heating rate.[28] Liang et al. revealed the effect of lube base oil on the oxidation activity of diesel soot. The disordered nanostructure of soot accelerates the oxidation of phosphorus soot.[33] Fang et al. applied a thermogravimetric analyzer (TGA) to study the influence of different ash types on the oxidation performance of soot and indicated that the PU/ZnO mixture has the maximum comprehensive oxidation index (S ) compared with PU/Al2O3 and PU/MgO mixtures.[34] Furthermore, Zhang et al. used a thermogravimetric analyzer to study the interaction effect of SOF and ash on diesel soot oxidation, and the result shows that the mass ratio of PU/ZnO/15 W at 1/1/0.1 has a best combustion performance under an O2/N2 atmosphere.[24] Soot oxidation characteristics are studied from the aspects of ash content, type of lubricating oil, the microstructure of soot, catalyst, and different operating parameters. The oxidation performance of soot under different synergistic conditions is not explored. Therefore, it is necessary to explore the oxidation performance of soot under the combined action of various factors, which can provide a theoretical basis for the diesel exhaust after-treatment process.

The innovation of this study is to explore the oxidation performance of soot, which is mixed with ash and SOF, under different operating conditions. This research aims to provide detailed information under the synergistic effects of ash and SOF on the effect of the (1) oxygen ratio in an O2/N2 atmosphere, (2) total flow rate, (3) initial PU combustion mass, and (4) heating rate on soot oxidation performance in an O2/N2 atmosphere.

Experimental Apparatus and Material

The influence of operating parameters on soot oxidation performance of soot under the synergistic action of ash and 15 W lubricating oil has been studied by using a TGA (TG209F3 from NETZSCH, Germany), and the main parameters of the TGA are listed in Table . The properties of real diesel particle soot are affected by engine working conditions, so they are very difficult to obtain.[35] PU has a similar microstructure to soot and the reactivity of PU follows the same trend as diesel soot,[29,36] so it is widely used to replace diesel particulates.[10,29,37−39] In this study, diesel soot is replaced by PU (Degussa GmbH, Frankfurt, Germany), and the physical properties are given in Table . The characteristic parameters of ZnO ash (nanoparticles are purchased from Shanghai Aladdin Biochemical, China) are summarized in Table . The experimental bench is shown in Figure .

Table 1

Main Parameters of Types TG209F3

parameters	value
balance sensitivity (μg)	0.1
heating rate (°C/min)	0.001–100
range of temperature (°C)	45–1000
size of the crucible (mm)	Φ 6.8
measuring dynamic range (g)	0–2

Table 2

Physical Properties of PU

soot	diameter (nm)	BET (m2/g)	oil absorption (g/100 g)	ash content (%)
PU	25	92	460	0.02

Table 3

Physical Properties of Ashes

ash	particle diameter (nm)	metal’s basis
ZnO	50 ± 10	99.8%

Figure 1

Experimental bench diagram.

Experimental Method and Data Analysis

The influence of operating parameters, such as the oxygen ratio in the O2/N2 atmosphere, total flow rate, the initial PU mass of combustion, and heating ramp on soot oxidation performance under the synergistic effect of ZnO ash and SOF (15 W lubricating oil[32]) are investigated, and the mixing mass ratio for PU/ZnO/15 W is 1/1/0.1. A vacuum drying chamber is set at 110 °C to dry the samples for 1 h. PU particles are in a tight contact mode with the catalyst and SOF in many studies.[32,35,40] To make the sample in tight contact, the experiment uses a vortex oscillator to mix the sample in a certain proportion for 15 min. In each experiment, the mixture is transferred to an alumina crucible with an inner diameter of 6.8 mm and a height of 7.4 mm and heated from 45 to 800 °C.

The controlled variable method has been used in this experiment to study the effects of different operating parameters on the oxidation performance of soot. The experiment is divided into four groups, and the operating parameters of the four groups are as follows. (1) Four oxygen concentrations have been tested, which include 5, 10, 15, and 20% O2 in O2/N2 atmosphere experiments. The total flow rate, the initial PU mass, and the heating ramp is 100 mL/min, 3 mg, and 10 °C/min, respectively. (2) Five total flow rates have been tested, which include 80, 90, 100, 110, and 120 mL/min. The oxygen concentration, the initial PU mass, and the heating ramp is 20%, 3 mg, and 10 °C/min, respectively. (3) Five initial PU masses have been tested, which include 2, 3, 4, 5, 7, and 9 mg. The oxygen concentration, the total flow rate, and the heating ramp are 20%, 100 mL/min, and 10 °C/min, respectively. (4) Five heating rates have been tested, which include 10, 20, 30, 40, and 50 °C/min. Five initial PU masses have been tested, which include 2, 3, 4, 5, 7, and 9 mg. The oxygen concentration, the total flow rate, and the initial PU mass are 20%, 100 mL/min, and 3 mg, respectively. Each test is running in duplicate and the ± error indicates the standard deviation of the two results.

Data Analysis

The change line of the sample mass with temperature is defined as a thermogravimetric (TG) curve in Figure , which is obtained by a TGA. The DTG curve is the derivative of the TG curve. To better represent and evaluate the oxidation characteristics of particles, the combustion start temperature (Ts), peak temperature (Tp), and end temperature (Te) are defined in Figure , respectively.[34,39,41]Tp represents the corresponding temperature to the peak of the DTG outline, which represents Wmax. The average value of the mass loss rate from the beginning of the reaction to the end is Wmean. Ts is the point where the sample begins to burn, and Te is defined as the temperature when the elementary combustion process is realized. Ts, Tp, and Te are obtained as follows. Tp is the intersection of the abscissa and the vertical line across the peak point A of the DTG curves, which crosses the point B on the TG curves. The blue line is tangent to the TG curves passing through B. Ts is the temperature at point C, which is the blue line across the horizontal line whose ordinate is 100%. Te is the temperature at point D, which is the blue line across DTG curves.

Figure 2

Definition of the combustion characteristic temperature with the TG-DTG tangent method.

The S and the combustion stability index (Rw) include the ease of ignition, the firing velocity, and the end and peak temperature. S reflects the ignition, combustion, and burnout properties of a sample and it can be used to evaluate the oxidation property of the sample.[41−44]Rw represents the stability in the process of sample combustion.[24,45] With the increase in S and Rw, the combustion process improves. S and Rw are, respectively, defined as following:where Wmax and Wmean represent the maximum mass loss rate and average mass loss rate, respectively. Ts, Tp, and Te represent the combustion start temperature, the combustion peak temperature, and the combustion end temperature, respectively.

Results and Discussion

Influence of the Oxygen Ratio in an O2/N2 Atmosphere on Soot Oxidation

The oxygen present in a diesel engine exhaust is between 5 and 18%,[22,31,46] and it can reach 20% in a large load.[14] As a result, 5% O2, 10% O2, 15% O2, and 20% O2 are taken to investigate the effect of oxygen concentration on the oxidation characteristics of PU. The TG-DTG curves of the PU/ZnO/15 W mixture at different oxygen ratios in O2/N2 atmospheres are displayed in Figure . The oxidation properties of PU became better with the increase in the oxygen ratio, which is consistent with the result of Shi et al.[47] Their results show that increasing the oxygen volume fraction promotes the reaction between coal and oxygen. TG-DTG curves and the inflection point shift to the low-temperature region, and the oxidation reaction range of PU, Tp, and Te gradually decreases with the increase in the oxygen ratio in the O2/N2 atmosphere. The reason for this phenomenon is that the reaction has more diffusion of oxygen atoms with the increase in the oxygen ratio.[42] With the increase in the oxygen ratio in the O2/N2 atmosphere, PU can fully come in contact with oxygen molecules and quickly react.

Figure 3

TG-DTG curves at different oxygen ratios in O2/N2 atmospheres.

Figure shows that Rw and S increase with the oxygen ratio in the O2/N2 atmosphere. With the increase in the oxygen ratio in the O2/N2 atmosphere, Te and Tp decrease, while Wmax and Wmean increase. At a 20% oxygen ratio in the O2/N2 atmosphere, Rw and S reach the largest values at 56.43 × 105 and 2.04 × 107 %2min–2 ° C–3, respectively (Table ). The reason for this phenomenon can be explained as follows: (1) A higher oxygen ratio will allow more sufficient diffusion of oxygen;[48] (2) with the increase in the oxygen ratio, more oxygen can be absorbed by the active structure of PU. To sum up, the increase in the oxygen ratio in the O2/N2 atmosphere promotes the oxidation regeneration of PU and makes TG-DTG curves move to the direction of the low-temperature zone, and it reduces Te and Tp and increases Rw and S.

Figure 4

Comparison of S and Rw at different oxygen ratios in O2/N2 atmospheres.

Table 4

Summary of Characteristic Parameters in All the Cases of This Study

case	operating parameters	Ts (°C)	Te (°C)	Tp (°C)	Wmax (%/min)	Wmean (%/min)	Rw (105)	S × 107 %2min–2 ° C–3
1–2	5%O2	557 ± 2	678 ± 1	611 ± 4	9.35 ± 0.25	1.56 ± 0.02	23.55 ± 0.65	0.69 ± 0.02
3–4	10% O2	545 ± 2	641 ± 1	599 ± 6	12.8 ± 0.11	1.57 ± 0.03	33.70 ± 0.11	1.05 ± 0.01
5–6	15% O2	538 ± 5	611 ± 9	580 ± 1	16.4 ± 1.31	1.38 ± 0.20	45.31 ± 3.07	1.27 ± 0.10
7–8	20% O2	542 ± 2	590 ± 5	568 ± 6	20.2 ± 0.51	1.74 ± 0.03	56.43 ± 1.52	2.04 ± 0.07
9–10	80 mL/min	540 ± 1	593 ± 3	569 ± 3	20.2 ± 0.87	1.78 ± 0.50	56.53 ± 3.35	2.08 ± 0.15
11–12	90 mL/min	540 ± 4	594 ± 3	568 ± 1	19.5 ± 0.15	1.74 ± 0.02	54.86 ± 0.03	1.97 ± 0.04
13–14	100 mL/min	541 ± 2	591 ± 1	567 ± 1	19.6 ± 0.94	1.76 ± 0.02	55.53 ± 2.35	2.04 ± 0.08
15–16	110 mL/min	536 ± 3	592 ± 3	569 ± 1	21.1 ± 0.58	1.79 ± 0.01	59.09 ± 1.33	2.18 ± 0.05
17–18	120 mL/min	540 ± 1	593 ± 3	568 ± 2	19.6 ± 0.02	1.72 ± 0.01	55.11 ± 0.29	1.95 ± 0.02
19–20	PU 2 mg	540 ± 3	594 ± 2	567 ± 3	19.4 ± 0.38	1.72 ± 0.03	53.08 ± 0.41	1.92 ± 0.05
21–22	PU 3 mg	536 ± 4	591 ± 1	567 ± 1	19.6 ± 0.94	1.76 ± 0.02	55.53 ± 2.35	2.03 ± 0.09
23–24	PU 4 mg	537 ± 1	593 ± 1	567 ± 1	17.8 ± 0.30	1.77 ± 0.01	50.35 ± 1.76	1.85 ± 0.07
25–26	PU 5 mg	529 ± 1	591 ± 1	564 ± 1	15.4 ± 1.22	1.78 ± 0.03	44.43 ± 2.35	1.66 ± 0.12
27–28	PU 7 mg	505 ± 7	595 ± 0	564 ± 2	12.7 ± 0.47	1.79 ± 0.20	38.31 ± 0.77	1.50 ± 0.02
29–30	PU 9 mg	501 ± 12	604 ± 3	570 ± 5	10.3 ± 0.71	1.72 ± 0.01	31.02 ± 1.15	1.17 ± 0.01
31–32	10 °C/min	540 ± 1	589 ± 3	566 ± 1	20.6 ± 0.04	1.78 ± 0.00	58.16 ± 0.28	2.14 ± 0.02
33–34	20 °C/min	550 ± 4	633 ± 0	605 ± 1	26.5 ± 0.35	3.30 ± 0.00	67.86 ± 0.29	4.49 ± 0.02
35–36	30 °C/min	549 ± 1	659 ± 3	621 ± 1	27.8 ± 1.05	4.74 ± 0.03	71.22 ± 3.07	6.88 ± 0.38
37–38	40 °C/min	554 ± 4	689 ± 3	646 ± 1	31.5 ± 0.36	6.01 ± 0.01	78.87 ± 0.18	8.99 ± 0.01
39–40	50 °C/min	571 ± 7	706 ± 2	660 ± 3	37.7 ± 0.34	7.42 ± 0.09	86.03 ± 0.73	10.21 ± 0.04

Influence of the Total Flow Rate on Soot Oxidation

Figure shows the change of soot oxidation characteristics at different total flow rates (80, 90, 100, 110, and 120 mL/min). It can be seen from Figure that the oxidation process of soot can be divided into three stages: 45–200 °C, 200–500 °C, and 500–650 °C. The first-order TG curves decrease by about 5%, which is due to the evaporation of water in the sample.[34] The second stage shows a slow decline at about 10%, because volatile components such as SOF are broken down. The oxidation reaction of soot mainly occurs in the third stage, and the sample reacts rapidly with the increase in temperature.

Figure 5

TG-DTG curves with different total flow rates.

Figures and 6 represent that the total flow rate has little influence on the oxidation characteristics of soot. The change in the total flow rate is too small to cause an obvious increase in the number of oxygen molecules in the O2/N2 atmosphere, which cannot make an obvious difference of S and Rw at different total flow rates. Comparatively speaking, the values of S and Rw reach the highest values at a 110 mL/min total flow rate. The oxidation reaction of soot is the most stable at the 110 mL/min total gas flow, so 110 mL/min has been selected as the operating parameter for the next set of experiments.

Figure 6

Comparison of S and Rw at different total flow rates.

Influence of the Initial PU Combustion Mass on Soot Oxidation

Figures and 8, respectively, display the TG-DTG curves, the change of S and Rw under different initial combustion masses (2, 3, 4, 5, 7, and 9 mg) of PU. The peak value of the curves of the mass loss rate moderately increases first and then decreases, reaching a maximum value of 19.69%/min at 3 mg, as shown in Figure . DTG curves show a trend of decreaking gradually with the increase in the added PU mass, which is a sign of deterioration of the oxidation reaction. It can be seen from the figure that with the increase in the initial combustion mass of PU, the TG-DTG curves migrate to the low-temperature zone. Meanwhile, Rw and S slowly decrease first and then increase. When the initial mass of PU is 3 mg, the largest value of Rw and S reached are 55.53 × 105 and 2.03 × 107 %2min–2 ° C–3, respectively. The possible reason is that oxygen diffusion and total oxygen has been limited. The heat transfer of the sample in the crucible is mainly affected by three kinds of mass transfer resistance.[48] The heat transfer and the oxidation reaction of soot become worse when the mass of the initial PU increases. The initial combustion of PU mass in 3 mg has the most stable thermogravimetric laboratory selection.

Figure 7

TG-DTG curves at different initial PU combustion masses.

Figure 8

Comparison of S and Rw at different initial PU combustion masses.

Influence of the Heating Rate on Soot Oxidation

Figures and 10 display the oxidation characteristics of PU at different heating rates. TG-DTG curves shift to the high-temperature region with the increase in the heating rate, as shown in Figure . The fluctuation range, which is the decomposition of volatile components at the low-temperature zone, increases when the heating rate increases. The reasons for this phenomenon are heat-transfer lag and react lag. The increase of the heating rate makes the temperature difference larger between the actual sample temperature and the temperature displayed by the TGA, and the heat-transfer time becomes shorter. The residence time of the sample at the set temperature has been reduced when the heating rate increases. However, the oxidized reaction of the sample needs fixed times. Before the sample is fully reacted, the temperature displayed by the TGA rises to a higher level, which makes the reaction curves shift to the high-temperature region as a whole.

Figure 9

TG-DTG curves at different heating rates.

Figure 10

Comparison of S and Rw at different heating rates.

When the heating rate increases, more energy is injected into this reaction, which causes rapid oxidation and deflagration. The increase in the characteristic temperature is not as much as that of Wmax and Wmean, which makes S and Rw increase. As seen from Figure , the reaction lag and heat-transfer lag have a great impact on the oxidation of soot. Kinetic data can be extracted from thermal experiments only when the reaction is avoided to the diffusive-controlled state.[49] From this perspective, a low sample mass is preferred through assuring the accuracy and repeatability of the results, which is consistent with Rodríguez-Fernández and Vázquez’s conclusions.[50]

Conclusions

This research aims to provide detailed information under the influence of operating parameters including the oxygen ratio in an O2/N2 atmosphere, total flow rate, initial PU combustion mass, and heating rate on soot oxidation performance in an O2/N2 atmosphere. The following conclusions are obtained from the above experimental results.

(1) With the increase in the oxygen ratio in the O2/N2 atmosphere, TG-DTG curves move toward the low-temperature region.

(2) The total flow rate has little effect on the oxidation performance of soot. When the total flow rate is 110 mL/min, the maximum Rw and S are 59.09 × 105 and 2.18 × 107 %2min–2 ° C–3, respectively.

(3) The reaction shifts to the low-temperature zone with the increase in the initial PU combustion mass. Rw and S increase slowly at first and then decrease and reach the maximum values of 55.53 × 105 and 2.03 × 107 %2min–2 ° C–3 at 3 mg, respectively.

(4) TG-DTG curves shift to the high-temperature region and Wmax, Wmean, Rw, and S increase when the heating rate increases. A 10 °C/min heating rate is the best option in this study.