EVENDALE, OH - A GE Marine Engines expert recently reported that its aeroderivative gas turbines yield lower emissions than other marine propulsion alternatives used by a majority of current commercial vessels. These gas turbines can help reduce marine exhaust emissions throughout the world.
Dr. Dave Luck, Marine Applications Manager for GE Marine Engines, was part of the industry panel "Gas Turbine Environmental Implications For Transportation in the Next Millennium" held during the recent ASME TURBO EXPO in Indianapolis, June 7-10.
Luck noted that there are three major contributing factors that affect marine exhaust emissions: propulsion system technology, fuel quality and operational profiles.
Propulsion System Technology
Luck cited recent studies which indicate that a gas turbine-based propulsion system is the most environmentally-friendly choice and can at the same time increase revenues for commercial ship owners and operators. Luck highlighted the following benefits of GE's compact aeroderivative gas turbines:
- lower NOx and SOx emissions compared to diesel engines;
- low levels of noise and vibration;
- high power density;
- high reliability and availability due to rapid repair of modular components;
- low maintenance and manning requirements;
- competitive first cost including acquisition and installation; and
- high thermal efficiency reduces overall ship fuel consumption.
Many ship owners and operators are taking advantage of these benefits: GE has 66 LM aeroderivative gas turbines slated for use or operating in commercial marine service.
Luck indicated that a majority of commercial ships today are powered by low or medium speed diesels. These engines produce higher NOx emissions since combustion takes place at high peak pressures and temperatures.
In contrast, combustion in gas turbines is a continuous process, with average temperatures and pressures that are lower than the peak levels of diesel engines. Hence gas turbines tend to have lower NOX emission levels.
Fuel/Operational Effect
The quality of fuel is becoming an increasingly important issue in worldwide marine environments. In fact, Luck noted that proposed U.S. and the International Maritime Organization (IMO) emissions limits will soon require the use of lower sulfur fuels in the marine environment.
SOx emissions are a function of sulfur content. According to Luck, diesels operate with heavy fuel oil that has higher concentrates of impurities and sulfur which produce higher levels of SOx emissions. This heavy fuel has up to 5% sulfur by specification, or about 3% to 3.5% average. In contrast, gas turbines fired by marine distillate fuels have a maximum 1% to 2% sulfur specification; however the average sulfur level used in gas turbine fuels is typically less than 0.5%. Consequently, SOx emissions are reduced with gas turbines that burn distillate fuel.
Luck noted that the world's first gas turbine-powered cruise ships, currently being built for Royal Caribbean International and Celebrity Cruises, will see an 80% to 98% reduction in exhaust emissions. These ships will use GE LM2500+ aeroderivative gas turbines in a COmbined Gas turbine and steam turbine integrated Electric drive System (COGES) arrangement. The gas turbines will be fueled by clean-burning distillate fuel versus the heavy fuel customarily used for most diesel-powered cruise ships.
Operational factors also play a role in reducing emissions. Ship owners and operators - especially those in restricted coastal areas -- can reduce emissions with current diesel technology by reducing speed and power and by improving fuel quality which can limit total volume of emissions.
GE's Emissions Technology
Today, GE's gas turbine technology already surpasses current and proposed EPA and IMO emissions limits. In addition to the low emissions characteristics of today's engines, GE has the following two proven gas turbine technologies which could be applied for marine applications should even stricter regulations be implemented:
- Wet technology can reduce NOx to less than 1 gram per kilowatt hour (gm/kW-hr). This control technology uses a concept proven in land-based industrial applications. The availability of water can limit operations at sea, but this technology may be suitable for limited operation in coastal areas.
- Dry Low Emissions (DLE) produce emissions much lower than current marine requirements, well below 1 gm/kW-hr. To date, there are more than 100 DLE-equipped LM gas turbines ordered for a variety of industrial applications, and 51 LM6000, LM2500+, LM2500 and LM1600 DLE-equipped units in operation throughout the world. This DLE-equipped fleet recently exceeded the 300,000-fired hour mark.
GE's DLE program commenced in 1990, and all along has been based on these objectives:
- Applicable to the entire LM product line.
- Easily retrofitted to existing LM gas turbines.
- Low guaranteed levels of NOx and CO over a wide range of power levels and engine inlet temperatures.
- The same hardware life as non-DLE combustors.
GE's lean premix DLE combustion technology accomplishes low emissions by maintaining a near-optimum fuel-air distribution throughout the combustion zone and maintaining the flame temperature in a narrow band that is favorable both to low NOx and low CO production. GE's DLE combustion systems offer significant advantages where clean water for gas turbine injection is prohibitively expensive or simply not available.
According to Luck, there continue to be opportunities where wet NOx control, either with water or steam injection, are economically attractive. The choice of a dry or a wet approach to emissions control continues to be a very project-specific choice that needs to be evaluated carefully.
Exhaust Emissions Reduction Strategy
Luck concluded his presentation by outlining a possible strategy to reduce ship exhaust emissions. He said it will be necessary to leverage technology by making emissions limits independent of engine type, and to restrict mass pollutants emitted per unit time per unit power. He also said that clean fuels such as marine gas oil need to mandated globally to reduce SOx and particulate emissions. Finally he said that emissions may be further reduced in the future as more sophisticated technologies and cleaner fuels become available.
GE Marine Engines is part of GE Aircraft Engines and is headquartered in Evendale, OH. GE Marine Engines is the world's largest designer, developer and manufacturer of aeroderivative gas turbines for a variety of commercial and military marine propulsion applications.