The diesel engine combustion process mainly produces NO (approximately 60–90%) and little NO2in the combustion chamber. They are considered a mixture called NOX but only NO2 is relevant for air hygiene as a pollutant input. Diesel engines systematically produce NO2 from NO in catalytic converters. deNOx systems use it to oxidize and efficiently reduce soot particulates.
In air, NO oxidizes to NO2, a gas that irritates mucous membranes and is caustic when combined with moisture (acid rain). It increases asthma sufferers’ physical stress, especially when they exert themselves physically. NO2 has a ‘‘fertilizing’’ effect on plants, i.e. it promotes growth.
According to authority, a limit concentration of NO2 of 40mg/m2 will be in force in the EU in 2010.
Achieving the aforementioned limits will necessitate considerable efforts in all sectors, i.e. not only in the transportation but also the major offshore industry and shipping businesses will be required to reduce NOX emission worldwide.
The IMO (International Maritime Organization) imposed limits on NOX emissions for marine diesel engines
with power outputs >130 kW as of January 1, 2000. It plans to later adapt the limits determined from the test cycles dependent on use (main propulsion or auxiliary engine) and the mode of operation (constant speed or propeller drive) specified.
In 1999, EPA adopted regulations requiring new marine diesel engines to comply with emission standards
beginning in 2004 (tier 1) and 2007 (tier 2).
In May 2008, EPA published new rules aimed at dramatically reducing air pollution from marine diesel engines.
↓ particulate matter (PM) emissions by 90%
↓ nitrogen oxide (NOx) emissions by 80 %
Tier 1 and Tier 2 emission standards currently applicable to new marine diesel engines
Tier 1 limits NOx emissions only, and applies to model years 2004 and later
Tier 2 limits NOx, CO, and PM, and applies to model years 2007 and later
Tier 3 “near term” emission standards for “existing” engines for most towboats, Tier 3 standards become effective in 2016
Tier 4 “long term” emission standards for “newly-built” engines mandates high-catalytic after-treatment application of high efficiency after technology for most of our towboats, Tier 4 standards may become effective in 2016
Growing opportunities for dual-fuel and gas-diesel engines in land and marine power markets have stimulated designs from leading medium speed and low speed enginebuilders. Development is driven by the increasing availability of gaseous fuels, the much lower level of noxious exhaust emissions associated with such fuels, reduced maintenance and longer intervals between overhauls for power plant.
A healthy market is targeted from floating oil production vessels and storage units, rigs, shuttle tankers, offshore support vessels and LNG carriers.
Valuable breakthroughs in mainstream markets have been made since 2000 with the specification of LNG-burning engines for propelling a small Norwegian double-ended ferry (Mitsubishi high speed engines), offshore supply vessels and a 75 000 cu.m. LNG carrier (Wärtsilä medium speed engines).
Natural gas is well established as a major contributor to the world’s energy needs. It is derived from the raw gas from onshore and offshore fields as the dry, light fraction and mainly comprises methane and some ethane. It is available directly at the gas field itself, in pipeline systems, condensed into liquid as LNG or compressed as CNG. Operation on natural gas results in very low emissions thanks to the clean-burning properties of the fuel and its low content of pollutants. Methane, the main constituent, is the most efficient hydrocarbon fuel in terms of energy content per amount of carbon. Wärtsilä’s dual-fuel (DF) four-stroke engines can be run in either gas mode or liquid-fuelled diesel mode. In gas mode the engines work according to the lean-burn Otto principle, with a lean premixed air-gas mixture in the combustion chamber. (Lean burn means the mixture of air and gas in the cylinder has more air than is needed for complete combustion, reducing peak temperatures). Less NOx is produced and efficiency increases during leaner combustion because of the higher compression ratio and optimized injection timing. A lean mixture is also necessary to avoid knocking (selfignition).
Marine engine designers in recent years have had to address the challenge of tightening controls on noxious exhaust gas emissions imposed by regional, national and international authorities responding to concern over atmospheric pollution.
Exhaust gas emissions from marine diesel engines largely comprise nitrogen, oxygen, carbon dioxide and water vapour, with smaller quantities of carbon monoxide, oxides of sulphur and nitrogen, partially reacted and non-combusted hydrocarbons and particulate material. Nitrogen oxides (NOx)—generated thermally from nitrogen and oxygen at high combustion temperatures in the cylinder—are of special concern since they are believed to be carcinogenic and contribute to photochemical smog formation over cities and acid rain (and hence excess acidification of the soil). Internal combustion engines primarily generate nitrogen oxide but less than 10 per cent of that oxidizes to nitrogen dioxide the moment it escapes as exhaust gas.
Sulphur oxides (SOx)—produced by oxidation of the sulphur in the fuel—have an unpleasant odour, irritate the mucus membrane and are a major source of acid rain (reacting with water to form sulphurous acid). Acid deposition is a trans-boundary pollution problem: once emitted, SOx can be carried over hundreds of miles in the atmosphere before being deposited in lakes and streams, reducing their alkalinity.
Sulphur deposition can also lead to increased sulphate levels in soils, fostering the formation of insoluble aluminium phosphates which can cause a phosphorous deficiency. Groundwater acidification has been observed in many areas of Europe; this can lead to corrosion of drinking water supply systems and health hazards due to dissolved metals in those systems. Forest soils can also become contaminated with higher than normal levels of toxic metals, and historic buildings and monuments damaged.
Particulate matter (PM) is a complex mixture of inorganic and organic compounds resulting from incomplete combustion, partly unburned lube oil, thermal splitting of HC from the fuel and lube oil, ash in the fuel and lube oil, sulphates and water. More than half of the total particulate mass is soot (inorganic carbonaceous particles), whose visible evidence is smoke. Soot particles (unburned elemental carbon) are not themselves toxic but they can cause the build-up of aqueous hydrocarbons, and some of them are believed to be carcinogens. Particulates constitute no more than around 0.003 per cent of the engine exhaust gases.
Noxious emissions amount to 0.25-0.4 per cent by volume of the exhaust gas, depending on the amount of sulphur in the fuel and its lower heat value, and the engine type, speed and efficiency.
De-NOx technology options are summarized as follows:
The primary NOx reduction measures can be categorised as follows:
-Water addition: either by direct injection into the cylinder or by emulsified fuel.
- Altered fuel injection: retarded injection; rate-modulated injection; and a NOx-optimized fuel spray pattern.
- Combustion air treatment: Miller supercharging; turbocooling; intake air humidification; exhaust gas recirculation; and selective non-catalytic reduction. (Miller supercharging and turbocooling are covered in the Pressure Charging chapter.)
- Change of engine process: compression ratio; and boost pressure.
The basic aim of most of these measures is to lower the maximum temperature in the cylinder since this result is inherently combined with a lower NOx emission.
Below courtesy of CATERPILLAR USA
