Exhaust Gases
The polluting emissions by the diesel engines exhaust usually consist of a mixture of gas, vapour, liquid aerosols and solid substances in the particulate state. Analysing in detail the composition of the exhaust gases of engines using HFO and MDO, it is made up of 90% nitrogen and oxygen (non-polluting substances), and the remaining 10% of: carbon dioxide (CO2), water vapour and, in minor quantity, carbon monoxide (CO), sulphur oxides (SOx), nitrogen oxides (NOx), unburnt hydrocarbons (HC) and particulate matter (PM), originating from the combustion reactions.
The amount with which these substances are presented in the gases depends on several factors:
- Quality of used diesel fuel
- Engine type
- Engine tuning
- Regulation of the fuel feed pump
- Motor load
- Motor temperature
- Engine maintenance status
Table 2.1 shows the values relating to the typical composition of the exhaust gases of a medium-fast 4-stroke diesel engine.
Table 2.1. Typical composition of the exhaust gases of medium-fast 4-stroke diesel engines
Main components of the exhaust gases | Concentration of the component [% in vol] | Emission index[g/kWh] | ||||||||
Nitrogen | N2 | 74.0-76.0 | 5020-5160 | |||||||
Oxygen | O2 | 11.6-12.6 | 900-980 | |||||||
Carbon dioxide | CO2 | 5.2-5.8 | 560-620 | |||||||
Vapour | H2O | 5.9-8.6 | 260-370 | |||||||
Nitrogen oxides | NOx | 0.08-0.15 (Relatively high, if not controlled) | 9.6-16 | |||||||
Sulphur oxides | SOx | 0.08 (Linked to the type of fuel used) | 12 | |||||||
Carbon monoxide | CO | 0.006-0.011 (Relatively low, thanks to the good quality of combustion) | 0.4-0.8 | |||||||
Hydrocarbons and volatile organic compounds | HC, VOC | 0.04-0.13 | 0.4-1.2 | |||||||
Particulate Matter | MGO* [mg/Nm3] | HFO** [mg/Nm3] | MGO4 [g/kWh] | HFO5 [g/kWh]] | ||||||
Soot (elemental carbon) | 50 | 50 | 0.3 | 0.3 | ||||||
Fuel ash*** | 4 | 40 | 0.03 | 0.25 | ||||||
Lubricant ash | 3 | 8 | 0.02 | 0.04 |
*Marine Gas Oil DM-A grade with 0.01% fuel ash
content and 1.5% lubricant ash content
**Heavy Fuel Oil RM-B grade with 0.1% fuel ash
content and 4% lubricant ash content
***Due to low load, start-up phase and rapid load
increase
Table 2.2 shows the values relating to the typical composition of the exhaust gases of a 2-stroke diesel engine.
Table 2.2. Typical composition of the exhaust gases of 2-stroke diesel engines
Main components of the exhaust gases | Concentration of the component [% in vol] | Emission index [g/kWh] | |
Nitrogen | N2 | 74.0-76.0 | 5020-5160 |
Oxygen | O2 | 13.0 | 1070 |
Carbon dioxide | CO2 | 5.2 | 590 |
Vapour | H2O | 5.4 | 250 |
Nitrogen oxides | NOx | 0.15 | 18.63 |
Sulphur oxides | SOx | 0.05 | 8.52 |
Carbon monoxide | CO | 0.006 | 0.41 |
Hydrocarbons and volatile organic compounds | HC, VOC | 0.02 | 1.17 |
Particulate Matter | 120 mg/Nm3 | 0.66 |