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Conventional diesel buses produce significant amounts of pollutant emissions - especially particulate matter (PM) and nitrogen oxides (NOx) - that cause a deterioration of air quality. In order to reduce these emissions, there are a variety of different emission control technologies available. These can be applied to new buses as well as to buses that have already been in use, which is called retrofitting.
The most important of these technologies are listed below:
Engine management techniques can be used to adjust the engine for low NOx emissions actions that are typically accompanied by increased CO, HC and particulate emissions. The oxidation catalyst can, however, be added to reduce the emissions of the latter pollutants.
Diesel oxidation catalysts (DOCs) reduce carbon monoxide, hydrocarbons and the soluble organic fraction (SOF) of particulate matter (1); see also (2). In addition, significant reductions of "unregulated emissions" (pollutants not regulated by laws and regulations) as well as reduced mutagenic and carcinogenic activity.
In most applications, a diesel oxidation catalyst consists of a stainless steel canister with an internal honeycomb structure called a substrate or catalyst support. There are no moving parts, just large amounts of interior surface area coated with catalytic metals such as platinum or palladium. In the case of diesel exhaust, the catalyst oxidizes carbon monoxide (CO), and hydrocarbons (HCs) associated with both gaseous and particulate phases.
Oxidation catalysts have proven effective at reducing particulate and smoke emissions on older vehicles. Under the U.S. EPA's urban bus rebuild/retrofit program, several manufacturers have certified diesel oxidation catalysts as providing at least a 25 percent reduction in PM emissions for in-use urban buses. Certification data also indicates that oxidation catalysts achieve substantial reductions in CO and HC emissions.
The sulfur contained in the fuel is oxidized in the catalyst to form sulfates thus contributing to the particulate emission. The emission of sulfates is directly related to the sulfur content of the fuel but the conversion to sulfate is also dependent on catalyst activity and temperature. Some catalyst formulations have been developed which selectively oxidize the organic compounds while minimizing oxidation of the sulfur dioxide and thus formation of sulfates; however, the lower the sulfur content in the fuel, the greater the opportunity to maximize the effectiveness of oxidation catalyst technology for both better total control of PM and greater control of toxic HCs. Lower sulfur fuel (500 ppm), which was introduced in 1993 throughout the U.S. and in the mid 1990's in the EU, has facilitated the application of catalyst technology to diesel-powered vehicles. Furthermore, the 50 ppm sulfur content available in the European market and more recently in the U.S. has further enhanced catalyst performance. In some European countries (e.g. Sweden), the standard diesel fuel is certified to <10 ppm sulfur content but is usually sold at sulfur contents <3 ppm. These low sulfur contents have reduced the particle emissions further and have open-up new possibilities for the use of Diesel Particulate Filters (DPF). The low sulfur concentration of the fuel has also reduced the potential for acidification of the environment. Sulfur in diesel fuel significantly affects the reliability, durability, and emissions performance of catalyst-based diesel particulate filters. Sulfur affects filter performance by inhibiting the performance of catalytic materials upstream of or on the filter. Sulfur also competes with chemical reactions intended to reduce pollutant emissions and creates particulate matter through catalytic sulfate formation. Catalyst-based diesel particulate filter technology works best when fuel sulfur levels are less than 15 ppm. In general, the less sulfur in the fuel, the better the technology performs.
Particulate filter technology can be successfully used in applications where the fuel sulfur level is >15 ppm, but only after a careful assessment has been made of the fuel sulfur level, the engine, the type of filter system, the operating conditions and the emission reductions desired. If the content of sulfur is more than 15 ppm, the intervals for maintaining the filter will be dramatically increased.
After introducing low sulfur diesel fuel with a maximum of 50 ppm sulfur, Hong Kong has initiated a diesel oxidation catalyst retrofit program in which manufacturers have certified systems achieving 35% initial PM reductions and 25% PM reductions after extended mileage accumulation.
Diesel particulate filters (DPFs), often combined with oxidation catalysts (2) (w1), reduce the particulate matter by filtering the exhaust stream (1) (2) (3). State of the art DPFs, showing PM reductions of >95%, also reduces the particle number emission in a uniform manner in the sense that large reductions are found over the entire particle size distribution.
Since the filtered particles will otherwise accumulate and plug the filter, successful filter systems must provide a means of burning off the collected PM, so called regeneration, or otherwise removing the captured particles.
A variety of filter materials have been used in diesel particulate filters including: ceramic and silicon carbide materials, fiber wound cartridges, knitted silica fiber coils, ceramic foam, wire mesh, sintered metal substrates, and, in the case of disposable filters, temperature resistant paper. Filter efficiency has rarely been a problem with the filter materials listed above, but work has continued to: 1) optimize filter efficiency and minimize back pressure, 2) improve the radial flow of oxidation in the filter during regeneration, 3) improve the mechanical strength of filter designs, and 4) strategies to remove accumulated soot in the filter that otherwise will plug the filter (i.e. regeneration).
Because the regeneration of the filter is the most difficult function of the operation of the DPF, many different approaches have been developed. Some of these techniques are used together in the same filter system to achieve efficient regeneration. Both on- and off-board regeneration systems exist. The major regeneration techniques are listed below.
- Catalyst-based regeneration using an oxidation catalyst applied to the surfaces of the filter. A base or precious metal coating applied to the surface of the filter reduces the ignition temperature necessary to oxidize accumulated particulate matter.
- Catalyst-based regeneration using an upstream oxidation catalyst. In this technique, an oxidation catalyst is placed upstream of the filter to facilitate oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The nitrogen dioxide adsorbs on the collected particulate substantially reducing the temperature required to regenerate the filter.
- Fuel-borne catalysts. Fuel-borne catalysts reduce the temperature required for ignition of trapped particulate matter (a cerium-based system is used on production light duty vehicles in Europe.)
- Air-intake throttling. Throttling the air intake to one or more of the engine cylinders can increase the exhaust temperature and facilitate filter regeneration.
- Post top-dead-center (TDC) fuel injection. Injecting small amounts of fuel in the cylinders of a diesel engine after pistons have reached TDC introduces a small amount of unburned fuel in the engine's exhaust gases. The unburned fuel can then be oxidized in the particulate filter to increase filter temperature and to ignite accumulated soot to be combusted.
- On-board fuel burners or electrical heaters. Fuel burners or electrical heaters upstream of the filter can provide sufficient exhaust temperatures to ignite accumulated soot and thus initiates the regeneration of the filter.
- Off-board electrical heaters. Off-board regeneration stations combust trapped particulate matter by blowing hot air through the filter system.
In some nonroad applications, disposable filter systems have been used. A disposable filter is sized to collect particulate for a working shift or some other predetermined period of time. After a prescribed amount of time or when backpressure limits are approached, the filter is removed and cleaned or discarded.
To ensure proper operation, filter systems are designed for the particular vehicle and vehicle application taking special account of the overall duty cycle and exhaust temperature profile.
Installation of a filter system on a vehicle may cause a small fuel economy penalty due to the increased backpressure of the filter system. Also, those filter regeneration methods involve the use of fuel burners that will increase the fuel economy penalty.
Selective catalytic reduction (SCR) systems convert NOx to nitrogen and other gases through the addition of a reductant to the exhaust stream (3).
An SCR system uses a metallic catalyst and a chemical reagent (or reductant), usually an aqueous urea solution in mobile source applications, to convert nitrogen oxides to molecular nitrogen and oxygen in the exhaust stream. The reductant is added at a rate calculated from an algorithm that estimates the amount of NOx present in the exhaust stream based on engine parameters such as engine revolutions per minute (rpm) and load. As exhaust gases and the reductant pass over the SCR catalyst, chemical reactions occur that reduce NOx emissions by 75 to 90%, HC emissions up to 80%, and PM emissions 20 to 30%. SCR also reduces the characteristic odor produced by a diesel engine and diesel smoke. Under certain conditions, an SCR system can increase ammonia emissions so it is typical to add an oxidation catalyst downstream so as to minimize "ammonia slip". Like all catalyst-based emission control technologies, SCR performance is enhanced by the use of low sulfur fuel.
The SCR technique is still under development and is not yet commonly used for on-road applications in Europe.
NOx adsorbers may be defined as materials that store NOx under lean conditions which is subsequently released and catalytically reduced under rich conditions (w2). This technology is only beginning to enter the marketplace in limited applications and is not considered appropriate for typical retrofit applications.
Exhaust gas recirculation (EGR) systems work by the recirculation of exhaust gases back into the intake stream in order to cool the combustion process and thereby reducing NOx (w3). The recirculated exhaust is introduced to the charger inlet, in the case of turbocharged engines, or intake manifold, in the case of naturally aspirated engines. In most systems, an intercooler lowers the temperature of the recirculated gases. The cooled recirculated gases, which have a higher heat capacity than air and contain less oxygen than air, lower combustion temperature in the engine and thereby reduce NOx formation. Diesel particulate filters are an integral part of any low-pressure EGR system, ensuring that large amounts of particulate matter are not recirculated to the engine. Both low-pressure and high-pressure EGR systems exist but low-pressure EGR is most suitable for retrofit applications because it does not require engine modifications. EGR retrofit systems have been introduced in pilot projects in Sweden since the beginning of 2002. When EGR systems are installed as a retrofit device there will always be a trade-off between NOx reduction and fuel consumption. To optimize an EGR system and to reach acceptable fuel penalty (up to 2-3 %), special calibration for each type of engine must be carried out.
DPFs, DOCs as well as SCR are described in some detail in (1). Source(4) provides a concise introduction. The above technologies can be used in various combinations with each other. All these technologies, except for EGR, require the use of low sulfur diesel (LSD) or preferably ultra low sulfur diesel (ULSD), for optimal performance.
Retrofit systems such as oxidation catalysts, particulate filters etc to be mounted on in-use trucks need a well developed national infrastructure to be a realistic alternative. Availability of stable low-sulfur fuels, well-organized I/M program, maintenance resources with well-educated staff in the workshops are examples of absolute conditions.
The potential of emission control devices to reduce pollutant emissions depends on a number of factors, including the type of technology used, whether or not low sulfur diesel is used, and whether adequate inspection and maintenance is carried out.
The US-EPA provides a very comprehensive overview of emission reduction potentials and other parameters such as prices, sulfur tolerance and fuel penalty for a number of emission control technologies (w4). The data are however not directly transferable to other countries with another composition of the vehicle fleet, emission performance of the vehicles, engine technology used, fuel quality and other local conditions.
Many different figures have been quoted in the literature with regard to emission reductions achieved through the individual technologies described. Some important examples are given below:
Diesel Particulate Filters: PM reductions range between 70% to over 90% (2)(5) (w1).
If the filters are combined with oxidation catalyst or coated with catalytic material, substantial reductions of HC and CO will also occur.
Diesel Oxidation Catalysts (DOCs): reduction rates are: PM 20-50%; HC 50-90%; CO 10-90% (2) (5) (w1).
As noted in the Hong Kong example above, an estimate of 25% PM reduction is reasonable if used with fuel containing no more than 50 ppm sulfur.
Selective Catalytic Reduction (SCR): reduction rates are: NOx 60-90%; PM 0-30%; HC 50-90%; CO 50-90% (3)(6) (w1).
Exhaust Gas Recirculation (EGR): reduction rates are up to 50% for NOx (w1) (2). In the Swedish retrofit example noted above, 35% NOx reductions are being achieved.
In New York City, CRT technology (particulate filter combined with a catalyst) used with ULSD (less than 50 ppm) achieved the following reductions in average emissions compared to baseline diesel engine: PM 88%; HC 92%; CO 94% (7).
To the extent that diesel bus retrofit systems increase fuel consumption they will tend to increase carbon dioxide emissions, a potent greenhouse gas.
Diesel buses in general can be considered to be very reliable, especially in comparison to newer technology employing alternative power trains. Diesel engines look back on a comparatively long evolution of continuous development. In reliability tests of alternative fuel buses, diesel buses serve as reference.
Experiences regarding the durability of individual emission control technologies have been reported by Manufacturer of Emission Controls Association (MECA) (2):
- "Field trials and emerging commercial experience in Europe with particulate filter technology do not disclose any major durability concerns."
- "Field experience with DOCs indicated that they have durability that meets heavy-duty diesel engine manufacturers' requirements."
- "EGR technology is not different from other automotive systems developed with heavy OEM (original equipment manufacturer) participation. It is believed that once developed it will meet the durability requirements of heavy-duty diesel engine manufacturers."
In its "verified technology list" (w4), the EPA lists "all the diesel retrofit products that have been approved for use in engine retrofit programs" along with "the percent reduction (of certified or tested levels) that EPA will recognize for State Implementation Plan (SIP) credits."
It is important to emphasize that the experiences above emanate from vehicle fleets in Europe and US and the conditions of these countries.
The Manufacturers of Emission Controls Association (MECA) states that these "indicate that the technologies evaluated as a part of the test program do indeed offer cost-effective reductions in both NOx and PM emissions. It also demonstrates that the technologies can be used in combination for the simultaneous reduction of both pollutants. It should also be noted that the analysis does not include the benefits of the substantial reductions in CO, HC, and toxic pollutants that can be achieved."
Some cost estimations are available for most of the technologies described above, e.g. (1) (2) (9)(10) (w1). They differ with regard to their assumptions, leading to a range of values. For instance, increased sales volumes will lead to lower prices (9), and the incremental cost of an emission control device on a new bus will be lower than that of the retrofit cost (w4). The following price ranges refer to the investment costs of the respective technical equipment.
Diesel Particulate Filters (DPFs): US$ 6000 - 10,000
Diesel Oxidation Catalysts (DOCs): US$ 1,000 - 3,000
Exhaust Gas Recirculation (EGR): US$ 800 – 1,500
Selective Catalytic Reduction (SCR):US$ 10,000 - 35,000(w2).
Further costs that have to be taken into account include those for inspection and maintenance, fuel economy penalty and low sulfur fuel. The actual emerging costs will be a function of the individual applications and situations (w1). The US EPA estimated the additional cost for low sulfur fuel (15 ppm maximum compared to 500 ppm maximum) at 4 - 5 cents per gallon (w1). (For further cost estimates of low sulfur fuels, see Cleaner Fuels.) See also(11) for an assessment of economic implications of tighter emission regulations.
Regarding DPFs, "in Sweden, over 6,500 buses have been equipped with a passive filter system. DPFs have also been retrofitted on heavy-duty vehicles in Great Britain, Germany, Finland, Denmark, and France. In off-road applications, over 10,000 filter systems have been retrofitted on diesel engines over the past 10 years. In the US, diesel filter retrofit programs are underway in California and in New York City where the city plans to retrofit 3,500 buses with diesel particulate filters" (3). In June 2001, a figure of 50,000 retrofitted diesel particulate filters worldwide has been reported (12).
According to (3), "retrofit of DOCs has been taking place for well over 20 years in the off-road vehicle sector, particularly in the underground mining industry, with over 250,000 off-road engines retrofitted. Since 1995, over 20,000 systems have been retrofitted on buses and highway trucks in the U.S. and Europe. Over 3,000 trucks and buses have been retrofitted in Mexico. Hong Kong has begun to retrofit thousands of urban buses with DOCs."
In Europe, catalyzed exhaust filters such as CRT have been used for more than 10 years (13).
"A program conducted in Germany where 22 line-haul trucks were fitted with SCR systems achieved the performance targets of approximately 70 percent NOx, 80 percent HC, and 30 percent PM reduction. The fleet accumulated a combined 3,600,000 miles of operation, with several vehicles operating over 250,000 miles with excellent results" (5).
As (w3) states, "EGR systems have been used to reduce emissions of nitrogen oxides (NOx) from gasoline engines for almost 20 years. Because of tightening NOx standards, EGR systems are being developed for use in diesel engines, as well".
Within the Info Pool, the following projects are described which feature retrofitted buses: Cleaner Bus Fleets in New York City and Retrofit Projects in the United States.
Apart from the extra costs involved, the main obstacle to the effective use of retrofit systems is likely to be the availability of low sulfur diesel required for the proper operation of many emission control devices. Also where PM filters are used they must be carefully matched to the vehicles to be retrofitted giving careful consideration to the vehicle operating cycle and exhaust gas temperature.
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