Air emissions legislation review for internal combustion engines

Transcription

1 Air emissions legislation review for internal combustion engines

2 Fig. 1 Wärtsilä 20V34SG and 6R32DF prime movers. Introduction General The reciprocating engine is the natural choice for the shipping industry, while in the power plant sector stationary enginedriven power plants are popular today (Fig. 1). Larger baseload engine-driven power plants with outputs up to 300 MW electricity and also smaller decentralized combined heat and power (CHP) production plants are both common. Reciprocating engines offer many advantages: High thermal efficiency (low fuel consumption) Optimal matching at different loads (fast load response and good load-following characteristics) Flexible fuel choice Easy maintenance and robust design. Engine-driven power plants also have a short construction time, they are compact and water conserving (Fig. 2), and they can be located close to the end user. Different types of reciprocating engines exist on the market and are operated according to various principles. The most common engine types and fuel alternatives are: Diesel engines operating on diesel oil, heavy fuel oil, crude m3/h Assumption: hardness of raw water is max 5 dh Diesel and gas engine based powerplant,radiatorcooled 130 MW power plants e Diesel and gas engine based power plant, cooling tower Steam boiler power plant, cooling tower 0 Fig. 2 Raw water consumption in different power plants with primary flue gas cleaning methods, for example appropriate fuel choice and dry nitrogen oxides (NO X ) reduction such as Low-NO X burners for the boiler and a Low-NO X combustion diesel engine. 2

3 CO 2 emissions in g/kwh (electricity) Coal-fired, steam boiler 800 Gas turbine, fuel oil, single cycle 750 Gas turbine, natural gas, single cycle 600 Diesel engine, fuel oil, single cycle Gas engine, natural gas, single cycle oil, natural gas (high-pressure gas bar), biofuels (gas and oils), and Orimulsion Spark-ignited (SG) Otto-type engines operating on low-pressure gas fuels Dual-fuel engines (DF) that run on low-pressure natural gas (with liquid fuel as pilot fuel) or on liquid fuels such as diesel oil (as a back-up fuel) and heavy fuel oil. These engines can operate at full load in both fuel modes. In liquid mode they work according to the diesel process, and in gas mode according to the Otto process. Energy efficiency (low specific CO 2 emissions) Reciprocating engines have the highest rate of energy conversion to mechanical output among simple cycle prime movers, which means the lowest fuel consumption and therefore the lowest specific CO 2 and SO 2 (sulphur dioxide) emissions for a given specific fuel quality. CO 2 ( the most important greenhouse gas ) emissions are in focus today due to the Kyoto Protocol. These emissions can be reduced by increasing the efficiency of the prime mover or by switching the plant s fuel, or both. Other measures of reducing CO 2 emissions are increased combined heat and power (CHP) production in efficient decentralized power plants close to the end user and replacing old inefficient power stations with efficient new ones. Typical energy efficiencies (mechanical output) for simple cycle applications are % (calculated at the lower heat value of the fuel), where smaller units have lower and large 2-stroke engines have the highest efficiency. Figure 3 gives typical specific CO 2 emissions for different prime movers. Reciprocating internal combustion engines run at high 600 Fig 3 Typical CO 2 emissions for different prime movers. 450 efficiency over a broad load range, which is a significant advantage in ships or other applications where engine loading varies considerably. The high efficiency at part load together with the consecutive use of engines in a multi-engine installation enables power plant turndown ratios to as low as 10%. Multi-engine installations also increase operating safety and availability by providing a redundant solution and giving the possibility to perform corrective or preventive maintenance on part of the plant while the rest continues to produce power and heat. The energy efficiencies of Wärtsilä engines have increased substantially in recent decades. This trend reflects both better engine performances and bigger engine sizes. Engine efficiencies have generally risen as a result of increased firing pressures, higher compression ratios, shorter fuel injection duration, optimized valve timings and improved combustion processes. The improvement of efficiency of Wärtsilä engines has a significant impact on the environment from the lifecycle point of view, since the operative life of a reciprocating internal combustion engine is normally between 25 and 50 years and in some cases even longer. The CHP plant can be situated in urban or industrial areas close to the consumers of the heat and electricity it produces, so the need for land and transmission lines, with their associated energy losses, can be minimized. Reciprocating engines are well suited for cogeneration, e.g. for hot water production, steam generation (sometimes with an additional steam turbine for enhanced electrical efficiency), sea water desalination, district cooling systems and for heating air for industrial processes. The heat-to-power ratios for the engine in CHP applications typically range from 0.5 to 1.3. As an example, the specific CO 2 emissions of a cogeneration (CHP) plant producing electricity and useful heat are about 370 g/kwh (heat + electricity) when operating on heavy fuel oil at a total plant efficiency of about 80%. The total efficiency will vary from case to case depending on the plant configuration. It is not only the efficiency of the prime mover that affects the resulting CO 2 gas emissions; the choice of fuel also has an impact, for example if oil is used instead of coal, or if natural gas or gases from renewable sources are used instead of oil. The modern reciprocating engine is fuel versatile; it can be run on both a conventional liquid or bioliquid fuel and also gaseous (natural or bio) fuels (depending on the engine type). 3

4 Reduction technologies It is generally preferable to apply primary methods to reduce air emissions at their source rather than attempting to remove them from the exhaust gases. Wärtsilä is putting heavy emphasis on developing new primary methods for its engines while closely following the development of the secondary methods. Primary methods 1. Nitrogen oxides (NO X ) Nitrogen oxides (NO X ) are formed in the combustion process by oxidation of nitrogen (from the atmosphere and fuel) to NO and NO 2. The NO X formation rate in a engine is largely temperature driven and consequently a function of the local high-temperature areas and their duration during combustion. To be able to reduce NO X emissions it is necessary either to prevent their formation in the cylinder (primary method) or to remove the NO X from the exhaust gases in an after-treatment system (secondary method). There are two basic primary methods of reducing NO X emissions in diesel engines, the first dry and the second wet : In-cylinder combustion control measures without water introduction: Low-NO X combustion for diesel engines The lean-burn approach for gas-fired spark-ignited (SG) or dual-fuel engines (DF) Introducing water into the combustion process by: Injecting water directly into the combustion chamber (applicable only on liquid-fuel-fired diesel engines), or Humidifying the combustion air, or Water emulsion (e.g. a water/fuel oil emulsion) Dry methods Low-NO X combustion research is focusing on optimizing the closing timing of the inlet valve (technology called Miller valve timing ); early inlet valve closing suppresses the in-cylinder combustion temperatures, which reduces NO X formation. So far this method has achieved a NO X reduction of about 35% (reduction from the beginning of 1990) with unaffected or slightly improved engine-specific fuel consumption. Further efforts are being made to achieve higher reductions. In the lean-burn approach natural gas and air are premixed before introduction into the cylinders, which results in a lower combustion temperature. This low fuel/air ratio, called lean-burn, reduces NO X efficiently. Water/steam introduction It has long been known that water has a positive influence on reducing NO X formation by cutting temperature peaks in the combustion process. Various methods of introducing water have been evaluated and tested such as water-in-fuel emulsions, humidification of the combustion air by various methods, and Fig. 4 In the engine laboratory in Vaasa (Finland) new innovative engine designs and primary emission reduction methods are developed and tested. 4

5 direct water injection into the combustion space. Each alternative has its own merits and drawbacks. The water must be of good quality to prevent clogging of the system, and the fuel consumption also increases slightly with most water/steam introduction methods. Fuel consumption depends on the method used and the NO X reduction rates; at high NO X reduction rates the emissions of unburned CO, HC and particulates tend to increase. Direct Water Injection (DWI) Injecting water into the cylinder (applicable on some liquid-fuel-fired diesel engine types) reduces the temperature in the cylinder and in this way prevents the formation of NO X. Direct Water Injection can reduce the NO X level by up to 50 60% (depending on the engine type) without adversely affecting power output or engine components. The method requires the minimum of space, which makes it suitable for retrofitting at low investment cost. NO X reduction will be most efficient from loads of 40% and higher of nominal engine output. To reach the maximum NO X reduction, water consumption is slightly over half of the fuel oil consumption, and the water used can be evaporated or technical water. DWI is applicable for bigger engine types such as the W32, W38, W46 and W64 in marine applications. More than 50 marine engines with DWI are already installed or on order. Humidification of the combustion air Combustion air can be humidified in different ways including injecting steam before the inlet to the turbocharger or in the charge air cooler. A new technology under development is CASS (Combustion Air Saturation System), which is being pilot tested on a Wärtsilä 32 Low-NO X engine. CASS technology seems to be more efficient than the DWI system but with the drawback that the water consumption is higher. Water emulsion Tests with Orimulsion have given a NO X reduction typically up to 30% compared to normal heavy fuel oils. Water/fuel oil emulsion will normally reduce NO X by % compared to the fuel oil case. 2. Sulphur dioxides and particulate The primary method of reducing SO X and particulate emissions is to use a low sulphur/ash fuel oil or natural gas, whenever commercially available. 3. Unburned emissions (CO, NMHC, etc.) Due to its high combustion temperature, a diesel engine produces low levels of unburned gaseous components, and thus meets most existing emissions legislation governing stationary power plants. The primary methods of keeping emissions low are normal maintenance and the power plant s operating profile. Gas-fired spark-ignited and dual-fuel engine types have higher levels of unburned emissions compared to a diesel engine and, depending on the stationary power plant legislation in question, sometimes these installations must be equipped with a secondary method (oxidation catalyst). Fig. 5 The Samalpatti power plant (106 MW e ) in India was installed with Low-NO X Wärtsilä 46 engines in order to comply with the Indian requirements. 5

6 Fig. 6 The RoRo vessel Mistal powered with a Wärtsilä 16V46 main engine equipped with Direct Water Injection. 4. The smokeless engine The need for non-visible smoke operation in the marine market has been boosted in recent years especially by the cruise ship industry. Since most harbours visited and routes operated by cruise ships are close to densely populated or environmentally sensitive areas the demand for non-visible smoke operation is considered to be increasingly important. Wärtsilä has responded to these needs with the introduction of common rail fuel injection technology. The apparent darkness of a stack plume depends upon many parameters such as concentration, size distribution and the colour of the particulate matter in the effluent, the gas temperature at the stack exit, the depth of the plume (i.e. the duct diameter), natural lighting and background conditions. To avoid visible smoke it is necessary to prevent fuel droplets from coming into contact with metal surfaces around the combustion space. High fuel injection pressures generate small fuel droplet sizes. With conventional mechanical injection systems the fuel injection pressure drops at low loads, resulting in large fuel droplets. Some of these will survive as droplets until they hit the combustion space surfaces, generating smoke emissions. The common rail fuel injection system on the other hand keeps the injection pressure high and constant over the whole load range, thus enabling operation without visible smoke over the entire operation field. The smokeless engine concept is available for bigger 4-stroke engine types and for 2-stroke engines. The 2-stroke smokeless engine is called the RT-Flex engine (Fig. 7). The key feature of the RT-Flex system is that it gives complete freedom in the Fig. 7 The bulk carrier Gypsum Centennial is equipped with a Sulzer 6RT-Flex58T-B main engine. timing and operation of fuel injection and exhaust valve actuation. This flexibility is employed to reduce engine running costs and exhaust emissions, and to ensure steady operation at very low speeds. This is made possible by the precise control of injection, together with the higher injection pressures achieved at low speed, and the sequential shut-off of the injector. Consequently RT-Flex engines can run very steadily, and without smoking, at 10 12% of nominal speed. Secondary methods NO X : Selective Catalytic Reduction (SCR) Selective Catalytic Reduction is the only suitable secondary method today for reducing NO X typically by 85 90%. A reducing agent, such as an aqueous solution of ammonia or urea, is injected into the exhaust gas at a temperature of 6

7 C. The urea reagent in the exhaust gas decays into ammonia, which is then put through a catalyzing process that converts the NO X into harmless nitrogen and water. It is important to note that at high NO X reduction rates the control system of the SCR is critical due to its operation within a narrow window. At high reduction rates the size of the SCR reactor increases and more complicated premixing and reagent injection systems are needed, which raises the investment cost. A high NH 3 /NO X ratio is needed at high NO X reduction rates, i.e. more reagent is needed, which results in higher operating costs. A high NH 3 /NO X ratio may also lead to increased emission of ammonia (ammonia slip). A typical SCR plant consists of a reactor containing several catalyst layers, a dosing and storage system for the reagent, and a control system. In marine vessels, where available space is limited, the reactor is designed to incorporate the exhaust gas silencer a solution called Compact SCR. The size of the reagent tank depends on the size of the engines, the load profile and how often the tank can be refilled. The lifetime of the catalyst elements is typically 3 5 years for liquid fuels and longer if the engine is operating on gas. The high running costs of the catalyst result from the consumption and price of the reagent and from replacement of the catalyst layers. The reagent consumption depends on the stipulated NO X limit. SCR technology can be applied on all Wärtsilä engines, 2-stroke as well as 4-stroke. Experience in the application of SCR in diesel engine plants has highlighted the following points: SCR is a sensitive method: a certain minimum temperature is needed to avoid salt formation ( SO X sensitivity) on the SCR elements. Some trace metals which might be present in the fuel oil act as catalyst poisons and deactivate the catalyst A soot blowing system should be installed in the reactor containing the catalyst elements (especially when operating on liquid fuels). SCR technology is used on many ferries in the Baltic Sea and currently about 60 marine engines are fitted or have been ordered with SCR. Around the world about 1000 MW e of stationary power plants equipped with Wärtsilä engines are equipped with SCR. SO 2 and particulates The emissions of sulphur dioxide and particulates are mainly fuel related. If a low-sulphur/ash fuel or natural gas is not commercially available and the stipulated emission limit is strict, a secondary exhaust gas cleaning method should be used. A wet flue gas desulphurization (FGD) unit is used mainly for SO 2 removal and an electrostatic precipitator (ESP) for particulate removal. A semi-dry FGD removes SO 2 and particulates simultaneously. Several types of FGD are available in the power plant market and the choice of method depends on many factors such as plant size, the availability and quality of water resources and reagent, and legislation (concerning SO 2, particulates, the minimum outstack exhaust gas temperature, and end product disposal requirements, etc.). At the moment FGD is installed in about 1000 MW e of stationary power plants equipped with Wärtsilä engines around the world. The most used FGD methods are NaOH in smaller plants and CaCO 3 scrubbers in bigger plants. Due to the different temperature and composition of the diesel engine flue gas, the electrical properties of the diesel particles are different compared to particles from a boiler s flue gas. Wärtsilä therefore extensively tested the ESP (Electrostatic Precipitator) performance in a diesel engine power plant during Based on this experience Wärtsilä is currently building the first commercial diesel engine power plant (capacity about 150 MW e ) to be equipped with ESP. Secondary exhaust gas cleaning equipment is bulky and its investment cost is high. Operational costs will vary a lot depending on the electricity need, the byproduct disposal cost (ESP and FGD) and, for FGD, the additional reagent and water costs. Unburned emissions (CO, NMHC (NonMethaneHydroCarbon), etc.) Bigger diesel engines fulfil most existing stationary power plant legislation on unburned gaseous emissions such as CO through good engine maintenance. The use of oxidation catalysts is not recommended in the case of fuels containing sulphur as the oxidation catalyst might oxidize a large amount of the SO 2 to SO 3, which will form sulphate (a submicron particulate), and the catalyst might get deactivated by the flue gas. Diesel engines (mainly high-speed) operating on good quality brands of light fuel oil are occasionally equipped with oxidation catalysts. Gas-fired spark-ignited and dual-fuel engines are sometimes equipped with oxidation catalysts depending on the stationary power plant legislation in force. The performance of the oxidation catalyst depends considerably on the flue gas temperature. Wärtsilä engine power plants with outputs of about 800 MW e are equipped with oxidation catalysts. 7

8 20 SPECIFIC NO EMISSIONS (g/kwh) x x RTA Direct water injection Direct water injection IMO limit Low-NOx combustion rpm SCR W64 W46/ZA40 W38 W32 W26/W20 W200 Fig. 8 The NO X limit in the Annex VI of MARPOL, as adopted by the MARPOL 1997 Conference. Emission standards: Marine Wärtsilä s minimum development standard for Wärtsilä and Sulzer engines for marine use is that these engines comply with the requirements of the International Maritime Organization (IMO). Wärtsilä has developed, and is developing, NO X reducing technologies that comply with even more stringent national or regional legislation expected in the future. 1. MARPOL Annex VI After a ratification process lasting several years, the IMO MARPOL 73/78 Annex VI legislation seems to have gained the necessary support of the member states in 2003 to become ratified and enter into force internationally one year later. The IMO MARPOL 73/78 Annex VI sets limits on NO X and SO X emissions from ships and also on other air emissions like VOC and ozone-depleting substances. These other air emission limits do not, however, concern ship machinery (Fig. 8) NO X All Wärtsilä standard engines can meet the NO X limits set by Annex VI. To show compliance, Wärtsilä has tested selected parent engines on the test bed since 2000 and subsequent, approved engines are delivered with an EIAPP (Engine International Air Pollution Prevention) Statement of Compliance SO X The Baltic Sea and the North Sea have been declared emission control areas; the sulphur content in fuel used on board ships in a SO X emission control area is not permitted to exceed 1.5%. IMO/MEPC is further studying the application of a voluntary Greenhouse Gas Emission index for ships. A working group is preparing an IMO greenhouse gas strategy resolution for adoption by the IMO Assembly in EPA The US Environmental Protection Agency (EPA) issued new legislation concerning air emission legislation for US coastal shipping in early 2003 (Table 1). Existing legislation already covers engines from 2.5 litres/cylinder upwards. This new legislation covers C3 category engines, i.e. new marine compression-ignition engines at or above 30 litres/cylinder, and the limit on NO X emissions is the same as the IMO s limit. However, the EPA has announced that they will review and tighten the legislation in

9 Fig. 9 Carnival Spirit was the first vessel to be equipped with a Wärtsilä 46 common rail engine. New EPA regulations from January Cylinder displacement Litres/cylinder HC + NO x g/kwh PM g/kwh CO g/kwh Implemen - tation date Engines displ. < < displ. < < displ. < < displ. < < displ. < W20 15 < displ. < W26 power < 3300 kw 15 < displ. < W26 power > 3300 kw 20.0 < displ. < < displ. < WV32LN displ. > 30.0 IMO (NO X) 2004 W32, W38, W46, W64, RTA, ZA40 EPA has not finalized Tier 2 standards for engines with a displacement exceeding 30 litres/cylinder. EPA will announce final Tier 2 standards for these engines by April 2007 Table 1. Environmental Protection Agency (EPA) Tier 1 Emission Standards for Marine Engines, 40 CFR Parts 9 and EU The European Union is also active in imposing legislation related to NO X and SO 2 emissions in certain sensitive sea areas and inland waterways. However, this is still under development. 4. Local regulations River Rhine Limits on air emissions from ships on the River Rhine have been in force since These limits apply to NO X, CO, THC and particles. Alaska Alaska operates limits on the smoke emitted by ships. Other Economic instruments for reducing emissions have been adopted in some countries. A system of environmentally differentiated fairway dues was introduced in Sweden in 1998 and an environmental differentiation of tonnage tax in Norway Complementary reductions in port dues are offered by many Swedish ports, and also by the port of Mariehamn in the Åland Islands and by the port of Hamburg. Vessels with the Green Award certificate are entitled to a rebate on port fees in 50 ports around the world. 9

10 Emission standards: Power plants Wärtsilä s product development strategy for diesel power plants is to fulfil the World Bank s stack emission guidelines Thermal Power - Guidelines for New Plants 1998" for installations located in a non-degraded airshed by using primary methods. This includes a suitable choice of fuel, and the use of the Low-NO X combustion method on the engine. Secondary flue gas treatment methods such as FGD, SCR and ESP are available for installations located in a degraded airshed or subject to more strict national limits or when poor, low-cost fuel qualities are the only economical fuel choice, see Figure 10 (NO X limit), Figure 11 (particulate limit) and Figure 12 (SO 2 limit). German TA-LUFT regulations have been widely applied to gas engines in the European market. Wärtsilä s strategy for lean-burn engines, including the spark-ignited engine and dual-fuel engine in gas mode, is to comply with the German TA-LUFT regulation using primary techniques as far as practicable. Compliance with the German TA-LUFT regulation today normally requires a CO oxidation catalyst, see Figure 13 (NO X limit). In modern environmental legislation, emission norms are technology-specific, i.e. each prime mover type (boilers, gas turbines and reciprocating engines) has its own limits. National legislation or guidelines on specific emission limits for reciprocating engines can be found in Japan, South Korea, Taiwan, India, UK, France, Germany, Italy, Portugal, Ecuador, and Finland. Internationally, the World Bank s guideline Thermal Power Guidelines for New Plants 1998 is widely used as the minimum norm if the host country does not have its own specific legislation for engine-driven power plants (see examples in Figures 14 and 15). Technology-specific flue gas emission concentration limits must closely correspond to actual conditions as these best describe the performance of secondary cleaning equipment, if needed. For bigger reciprocating engines this means 15 %-vol O 2. In the World Bank Guidelines, India, Ecuador and the UK, for example, a reference oxygen concentration of 15 %-vol O 2 for emissions is used for reciprocating engines. The most cost-effective emissions norm is one based on the environmental quality need approach (taking into account both environmental aspects and cost). Examples include the World Bank s Thermal Power Guidelines for New Plants 1998 and the Japanese diesel engine norm. The most important stack emissions are NO X, particulates and SO 2. In some countries national legislation also regulates Fig. 10 NO X limit (WB Thermal Power- Guidelines for New Plants 1998) Fig. 11 Particulate limit (WB Thermal Power- Guidelines for New Plants 1998) Fig. 12 SO 2 limit (WB Thermal Power- Guidelines for New Plants 1998) CO and NMHC emissions. Some legislation takes into account the existing infrastructure when determining SO 2 and particulate limits and thus expensive secondary cleaning equipment, such as FGD that produces a byproduct and consumes valuable water resources, can be avoided. Plant size and location (urban/rural) also sometimes affect the limits. 10

11 Fig. 13 NO X limit for gas engines (TA-LUFT 2002) Fig. 14 Some HFO diesel engine particulate norms. References: UK: The Environmental Protection Act 1990, part 1 (1995 Revision), (PG1/5(95): Secretary of State s Guidance-Compression Ignition Engines, MW Net rated Thermal Input" Achievable Releases to Air; HM Inspectorate of Pollution; Processes Subject to Integrated Pollution Control, Chief Inspector s Guidance Note, Series 2 (S2), S Combustion Processes: Compression Ignition Engines 50 MW th and Over (September 1995)" Germany: Technische Anleitung zur Reinhaltung der Luft - TA-Luft October India: Environment (Protection) Third Amendment Rules, 2002" Japan: Nationwide general limits Ecuador: Standard for Emissions to the Air from Stationary Combustion Sources", December 2002 Portugal: Resolutions 286/93 and 1058/94 World Bank: World Bank Guidelines Thermal Power - Guidelines For New Plants" 1998; ame/thermalpowerguidelinesfornewplants/$file/hand bookthermalpowerguidelinesfornewplants.pdf Annex VI of MARPOL 73/78, Regulations for the Prevention of Air Pollution from Ships International Maritime Organisation ttp:// Environmental Protection Agency Fig. 15 Some HFO diesel engine NO X norms. 11

12 W-P / Bock s Office Wärtsilä Corporation is the leading global ship power supplier and a major provider of solutions for decentralized power generation and of supporting services. In addition Wärtsilä operates a Nordic engineering steel company Imatra Steel and manages a substantial shareholding to support the development of its core business. For more information visit WÄRTSILÄ is a registered trademark. Copyright 2003 Wärtsilä Corporation. Wärtsilä Finland Oy P.O.Box 252, FIN Vaasa, Finland Tel: Fax: