<b>Exhaust Gas Systems / Treatments</b>
When internal combustion engines operate, substances such as sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO) and carbon dioxide (CO2), hydrocarbons, unburned carbon and fine dust are emitted. The fuel prices, which have risen steadily in recent years, have led to e.g. Container ships drive more slowly and thus consume considerably less fuel. Efforts are also being made to reduce the drive power through propulsion-improving measures. A low specific energy consumption also means lower CO2 emissions; in the case of drives with internal combustion engines, the CO2 emissions are roughly proportional to the fuel consumption by a factor of 3.
NOx reduction:
The NOx and sulfur dioxide emissions depend on the combustion process and the sulfur content in the fuel; they can be directly influenced by the type of fuel and by optimizing the combustion process. The NOx emission limit values are regulated in MARPOL Annex VI and designated as Tier I, Tier II and Tier III. The emission values for NOx are determined as a function of the maximum operating speed of the engine n (rpm). The Tier I and Tier II limit values are global, while the Tier III standards only apply in the ECA (Emission Control Area).
The strict Tier III emission values cannot be achieved by optimizing the combustion process alone; further procedures for reducing NOx are required here. The NOx fractions in the exhaust gas from diesel engines are approximately 95% NO and 5% NO2. The aforementioned NOx values in the ECA can be achieved using the selective catalytic reaction method with a DeNOx catalyst (SCR catalyst = selective catalytic reaction). The nitrogen oxides react stoichiometrically with ammonia or urea as reducing agents to form nitrogen (N2) and water H2O). These substances are natural components of the atmosphere. In the SCR process, urea is first blown into the exhaust gas flow of the internal combustion engine and mixed with the exhaust gas flow. The exhaust gas flow then passes through the DeNOx reactor. This contains extruded ceramic blocks with a honeycomb structure as a catalyst. These ceramic blocks are coated with the actual catalyst made of vanadium pentoxide, titanium dioxide, aluminum oxide or others. The reaction takes place at temperatures between 280 and 600 ° C.
SOx reduction:
Despite optimized combustion processes in internal combustion engines, the actual emissions of sulfur oxide (SOx) are still very high, especially when operating heavy oil machines. The heavy oil that is frequently used has a sulfur content of around 2.5%. The exhaust gases from ship engines operated with heavy oil therefore contain a high proportion of SO2.
According to MARPOL Annex VI, sulfur limits are therefore set in the fuel depending on the driving area. More and more ECA zones have set the maximum value, both in Europe and in many parts of Asia, to a limit of 0.1% sulfur in fuel. This can only be met by the so-called marine diesel (similar to car diesel). In order to comply with the requirements, ships can either bunker low-sulfur marine diesel or install systems to reduce smoldering emissions.
A distinction is made here between wet and dry desulphurisation. The wet desulphurisation systems, known as scrubbers, work with milk of lime as the reaction agent. The exhaust gas flow from the machine is directed in countercurrent to the scrubbing liquid, which is finely distributed and injected into the washer. The chemical reaction, the mass transfer, takes place at the phase boundary of the small drops. This can be improved by installing it in the washer. In order to eliminate droplets entrained with the exhaust gas at the outlet of the scrubber from the further exhaust gas flow, there is usually a droplet separator at the head of the washer. The flow velocity of the exhaust gas in the scrubber is between 1 and 4 m/s. The residence time of the gas in the scrubber is between 10 and 30 s. A pressure loss of 100 to 200 Pa can be assumed for the equipment design of a scrubber without internals.
Special scrubbers use seawater or fresh water with caustic soda to reduce sulfur oxides in the exhaust gas. The dry desulphurisation system binds the sulfur compound through chemical sorption on lime granulate (calcium carbonate (CaCO3) or calcium hydroxide (Ca (OH) 2)) with a large internal surface, which creates gypsum.
See also Marine Scrubber Systems (https://www.trusteddocks.com/products/category/515-marine-scrubber-systems)
Exhaust Gas Systems / Treatments
Advertisement: