Sulphur oxide (SOx) and nitrogen oxide (NOx) reduction

Marine SOx reduction
The International Maritime Organization (IMO) revised its standards on the sulphur content of marine fuels in 2008. These were included in Annex VI to the International Convention for the prevention of Pollution from ships (MARPOL Convention). Since 2020, the current global limit for the sulphur content of the fuel oil used by ships has been 0.5% m/m (by weight). For vessels sailing in Sulphur Emission Control Areas (SECAs), the sulphur limit has been 0.1% m/m since 1 January 2015. 

Marine sulphur emissions can be reduced by:
• reducing the sulphur content of the fuel used 
• removing sulphur from the exhaust gas 
• operating on cleaner fuel such as liquefied natural gas (LNG)

Wärtsilä provides several solutions to help customers reduce emissions of SOx and comply with local and global regulations. Wärtsilä's technology development supports solutions that enable the use of fuels with different sulphur contents, as well as systems that clean sulphur from exhaust gas and enable the use of alternative fuels with close to zero sulphur content, e.g. natural gas. Wärtsilä exhaust gas cleaning systems (EGCS) not only reduce SOx emissions but also remove large levels of particulate matter (PM) and black carbon. The Wärtsilä EGCS can be customised for both the 0.1% limit in Emission Control Areas (ECA) and the global 0.5% cap agreed within the IMO.

Emissions reduction for gas power plants
Sulphur dioxide (SO2) and particulate matter (PM) emissions are insignificant for power plants running on natural gas. Nitrogen oxide (NOx) emissions are also low. 

Wärtsilä gas engines use a lean-burn combustion process. In this process, natural gas and air are premixed in a lean air/fuel ratio (lambda 2-2.5) before being fed into the cylinders. The lean-burn process efficiently reduces NOx emissions due to a lower combustion temperature. Another advantage with the lean-burn process is the increased output and efficiency of the engine. Wärtsilä gas engines have sufficiently low NOx emissions to comply with most national/local regulations using lean-burn primary method only.

In areas with more stringent control of NOx emissions, the engines can be equipped with SCR units. In the SCR, NOx is reduced by a catalyst, combined with a reagent that is either an aqueous solution of urea or ammonia. 

Emissions reduction for liquid fuel power plants
NOx, SO2 and particulate matter are the main emissions of interest regarding stationary liquid fuel engines. SO2 and PM emissions are mainly related to the quality of the liquid fuel. Wärtsilä liquid fuel engines have low carbon monoxide (CO) and hydrocarbon (HC) emissions thanks to their high thermal efficiency. Wärtsilä’s liquid fuel power plants are designed to meet the stack emission limits set by the World Bank/IFC Guidelines for liquid fuel power plants up to 300 MWth (120-140 MWe) in non-degraded airsheds by using dry primary methods. Secondary flue gas treatment methods are available for more strict regulations, or when only low grade liquid fuels are commercially available.

Wärtsilä’s liquid fuel power plants can be equipped with Selective Catalytic Reduction (SCR) units to reduce NOX emissions if required. NOX emissions are typically reduced by up to 80-90% by using a reagent that is either an aqueous solution of urea or ammonia. 

A dry Electrostatic Precipitator (ESP) unit can be used to reduce PM emissions. The ESP technique provides a stable, low pressure-loss option to reduce PM emissions. ESP’s dry end product, fly ash, needs to be disposed of in an environmentally acceptable way.