Gaseous Emissions

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Abstract: A brief characterization of the chemical and physical properties of several regulated and unregulated emissions from internal combustion engines, including nitrogen oxides, hydrocarbons, carbon monoxide, sulfur dioxide, nitrous oxide and hydrogen.

Nitrogen Oxides—NOx

NameNitric oxideNitrogen dioxide
Formula Weight30.0146.01
AppearanceColorless gasRed-brown gas
Density1.0367 (rel. to air)...
Melting Point-161°C-9.3°C
Boiling Point-151°C21.3°C

Nitrogen oxides—as commonly defined by emission regulations and regulatory measurement protocols—include two gases: nitric oxide NO and nitrogen dioxide NO2. Physical properties of both gases are listed in the table on the right [175]. From the chemical point of view, there are several other nitrogen oxides. One of them, nitrous oxide (N2O), is discussed below.

NOx is considered one of the critical pollutants found in emissions from all types of internal combustion engines. Nitrogen oxides are highly active ozone precursors playing an important role in the smog chemistry. They can also form secondary nitrate particulates in the atmosphere.

Concentrations of NOx in untreated diesel exhaust are typically between 50 and 1000 ppm. If concentrations are given in mass units, NOx is usually expressed as NO2 equivalent.

Nitric oxide (NO) is a colorless and odorless gas. In the laboratory, it may be synthesized directly from nitrogen and oxygen under high temperature and pressure:

N2 + O2 ↔ 2NO - 182.4 kJ/mole(1)

The negative heat effect represents an endothermic reaction. Equation (1) can also represent the overall reaction of NO formation from the elements in the engine cylinder, where temperature and pressure are high. The exact NOx formation mechanisms in the diesel engine, however, are much more complex; they were discussed under Emission Formation in Diesel Engines.

At low temperature and pressure, the chemical equilibrium moves to the left side of Equation (1). Thermodynamically, nitric oxide has a tendency to decompose to nitrogen and oxygen under conditions in diesel exhaust. The rate of decomposition, however, equals practically zero and NOx control from diesel engines requires sophisticated in-cylinder and aftertreatment techniques.

In older, naturally aspirated diesel engines, approximately 95% of nitrogen oxides were composed of NO and only 5% of NO2. The proportion of NO2 in total NOx in turbocharged diesel engines (without aftertreatment) is typically higher, reaching up to about 15%. According to British data, the fraction of NO2 in vehicle NOx emissions (all fuels) increased from around 5-7% in 1996 to 15-16% in 2009 [2338]. NO can be oxidized by oxygen into nitrogen dioxide:

2NO + O2 ↔ 2NO2 + 113.8 kJ/mole(2)

NO2 is a red-brown gas of an unpleasant irritating odor. NO2 is extremely reactive, exhibits strong oxidation properties and is more toxic than nitric oxide. NO2 chemistry also plays a role in several types of emission control catalysts, where NO2 reactions may include oxidation of hydrocarbons, carbon monoxide as well as diesel particulates.

At ambient conditions, the equilibrium of the reaction is shifted to the right, but the reaction rate is slow, Figure 1 [3727]. At low NO concentrations on the order of a few ppm—which can be representative of exposures to engine emissions—a number of days may be required to achieve a substantial level of conversion to NO2.

Figure 1. Oxidation of NO to NO2 as a function of time and initial concentration

It should be noted that the kinetics of NO → NO2 oxidation shown in Figure 1 is applicable in environments without ozone or UV radiation (sunlight), for instance in underground mining. In the presence of ozone, NO can be oxidized very fast according to Equation (3), while a number of further reactions involving ozone and various nitrogen oxides are catalyzed by UV radiation. Hence, the rate of NO oxidation in polluted urban air will also depend on O3 concentration and other variables.

NO + O3 → NO2 + O2(3)

With increasing temperatures, the equilibrium of Equation (2) is shifting to the left, but NO2/NOx ratios of 70-80% are still possible at temperatures typical for diesel engine exhaust. The reaction rate of NO oxidation can be accelerated by catalysts. Therefore, the NO2/NOx ratios are typically much higher in diesel engines equipped with catalysts or catalytic particulate filters than in engines without aftertreatment. The average NO2/NOx ratio in US 2007 heavy-duty truck engines (with catalytic aftertreatment) tested by the ACES study was 68% [2245]. Further production of NO2 according to Equation (2) occurs spontaneously—albeit at a slower rate—in the NO-air mixture after exhaust gases are discharged into the atmosphere.