ARTICLE pubs.acs.org/IECR
Influence of Diesel Fuel Characteristics on Soot Oxidation Properties Harrie Jansma,† Debora Fino,‡ Renate Uitz,§ and Michiel Makkee*,† †
Department of Chemical Engineering, Section Catalysis Engineering, Delft University of Technology, Julianalaan 136, NL 2628 BL Delft, The Netherlands ‡ Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy § Shell Global Solutions (Germany) GmbH, Hohe-Schaar-Strasse 36, D-21107 Hamburg, Germany ABSTRACT: This study provides an overview on the impact of fuels on certain soot characteristics, such as oxidation behavior and morphology. The aim is to understand how fuel parameters affect the ease at which soot can be oxidized. The oxidation behavior of soot samples obtained from different types of diesel fuel was determined by means of Temperature Programmed Oxidation in a Thermo-Gravimetric Analyzer and a six-flow reactor; the structural morphology of soot was examined via Transmission Electron Microscopy. The soot oxidation temperature was found to be dependent to a great extent on the type of the diesel fuel, when oxygen is the only oxidant with or without a Pt-catalyst upstream. Soot oxidation in the presence of NO and a Pt-catalyst upstream resulted in a reduction in the oxidation temperature; this environment also led to smaller differentiation between the soot samples compared with the pure oxygen conditions. The amount of sulfur in the fuel had only a minor impact on the soot oxidation temperature, whereas the aromatic compound content affected this temperature significantly. A low-aromatic fuel results in a soot with a significantly higher oxidation temperature than a fuel with a large amount of aromatics (particularly diaromatics). This may have an impact on induced diesel soot abatement and regeneration strategies in automotive Diesel Particulate Filters as well as on the design of a new class of diesel fuels.
’ INTRODUCTION During operation of a diesel powered engine, diesel fuel is injected into the cylinder. The liquid atomizes into small droplets, which vaporize and mix with air under pressure and burning. The fuel distribution is nonuniform, and the generation of unwanted emission depends to a great extent on the degree of nonuniformity. Carbonaceous soot is formed in the center of the fuel spray, where the air/fuel ratio is low. A nonideal mixing of fuel and air creates small pockets of excess fuel, in which solid carbonaceous soot particles (a solid and a soluble organic fraction, SOF) are formed. The soot structure, observed in a transmission electron microscope (TEM) image, revealed a disordered turbo-stratic particle with a graphite-like structure.1 Diesel particulate matter (DPM), as defined in the EPA regulations and sampling procedure, is a complex aggregate of solid and liquid material.1DPM is generally divided into three basic fractions: dry carbon particles, commonly known as soot, heavy hydrocarbons that are adsorbed and condensed on the carbon particles, called Soluble Organic Fraction (SOF), and sulfate fraction, hydrated sulfuric acid.1 The actual composition of DPM depends on the diesel composition, engine type, and its load/speed conditions. “Wet” particulates may contain up to 60% of the hydrocarbon fraction (SOF), while “dry” particulates are mainly composed of dry carbon. The amount of sulfates is directly related to the sulfur contents of the diesel fuel. Diesel particulates are very small. The primary (nuclei) carbon particles have a diameter of 0.01 0.08 μm, while the agglomerated particle diameter is in the 0.08 to 1 μm range.2 4 In Euro 5 emission standards for cars and light duty trucks, which was implemented in 2009 for the new car models, the level of soot in the exhaust gases was decreased almost five times in comparison r 2011 American Chemical Society
with Euro 4 regulations.5 In order to meet these soot emission requirements a diesel particulate filter (DPF) has to be installed in the exhaust system in most vehicles. Diesel soot is normally oxidized at temperatures of around 600 °C, whereas the main exhaust gas temperature of a modern light duty diesel engine is about 250 °C. Soot is collected on a filter and further oxidized at high temperatures. Several types of filters have been described in the literature,6 9 but the wall-flow monolith is by far the most studied and commercially applied particulate trap.10 It consists of a ceramic structure with parallel channels of which half are closed in an alternate checker board manner at the upstream side and the others are closed at the downstream end.11 As a consequence, the exhaust gases are forced to flow through the porous channel walls that act as filters; in this way high particulate trapping efficiency (>95%) can be achieved. However, in order to prevent clogging of the filter, it has to be regenerated from time to time by post injection of fuel that promotes catalytic combustion of some fuel within the exhaust line,12,13 and this increases the overall fuel consumption.8 If the fuel itself produces lower engine-out PM emissions, less filter regenerations will in principle be required. Conversely, nitrogen dioxide (NO2) is known to be a more powerful oxidant than oxygen;14 it can in fact convert soot into CO and CO2 at temperatures as low as 275 300 °C. In a Pt-Catalyzed Soot Special Issue: Russo Issue Received: August 16, 2011 Accepted: November 22, 2011 Revised: November 14, 2011 Published: November 22, 2011 7559
dx.doi.org/10.1021/ie201823u | Ind. Eng. Chem. Res. 2012, 51, 7559–7564
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. List of Selected Fuel Characteristics density @ 15 °C
fuel 1
fuel 2
fuel 3
fuel 4
fuel 5
fuel 6
fuel 7
fuel 8
fuel 9
fuel 10
kg/m3
822.7
823.5
860.2
861.2
831.6
840.3
862.4
845.9
836.2
854.7
distillation °C
179.8
178.7
209.0
204.0
171.0
171.6
183.5
155.2
176.4
161.0
5% vol
°C
192.6
192.7
224.3
221.8
202.2
193.1
207.9
214.4
205.5
176.6
10% vol
°C
198.5
197.8
229.4
227.7
215.4
200.6
218.2
230.1
220.2
181.0
20% vol
°C
205.3
205.7
236.2
234.7
232.9
212.2
231.1
250.0
238.6
191.4
30% vol
°C
212.2
212.4
242.8
241.8
246.6
224.6
241.9
267.8
254.3
205.8
40% vol 50% vol
°C °C
220.0 228.1
220.3 228.4
251.8 263.1
250.9 261.5
259.2 270.5
240.3 258.2
252.0 261.5
283.7 299.5
267.5 280.3
228.1 254.8
60% vol
°C
237.4
237.9
276.9
274.5
281.5
273.9
272.5
313.6
293.6
272.3
70% vol
°C
247.5
247.7
292.2
289.7
294.2
288.6
286.5
325.2
308.1
286.5
80% vol
°C
257.8
258.5
309.1
306.5
310.5
306.5
304.1
334.0
323.7
301.8
90% vol
°C
270.9
271.5
330.7
327.5
332.8
331.3
329.3
342.4
340.6
326.3
95% vol
°C
281.3
281.5
346.1
340.9
349.8
351.6
349.8
352.0
355.6
349.9
ending boiling point
°C
291.7
293.6
356.8
350.6
363.5
363.3
363.4
361.9
363.3
361.5
yield loss + residue
% vol % vol
97.8 2.2
98.2 1.8
98.4 1.6
99.0 1.0
98.6 1.4
98.0 2.0
79.9 2.1
98.3 1.7
97.5 2.5
97.5 2.5
E250
% vol
∼ 73
∼ 72.5
39.1
39.2
32.9
45.4
37.7
20.0
27.2
48.0
E300
% vol
--
--
75.5
76.5
74.1
76.6
77.8
50.3
64.4
78.9
E350
% vol
--
--
96.6
97.9
95.4
94.7
95.0
94.4
93.6
95.0
cetane number
-
47.0
46.7
44.4
43.6
55.6
45.7
43.1
57.2
55.0
39.3
sulfur amount
Ppm
