By Don Newburry, Miratech Corp.
For owners and operators of stationary diesel engines, exhaust-stream diesel particulate matter (DPM) has become a growing concern. DPM causes a variety of respiratory ailments and has been linked to lung cancer, heart disease and global warming. Stationary engine DPM has been targeted for increasing regulation at local, state and national levels.
DPM regulation and reduction technologies are familiar to operators of mobile diesel engines. Now, some of the same DPM emission limits are being applied to stationary diesels. Vehicular DPM reduction technologies are being adapted for larger stationary engines. New Jersey, Wisconsin, California and their local air quality districts have imposed strict limits on industrial DPM (0.1 gram per brake horsepower per hour). Last year the U.S. Environmental Protection Agency (EPA) proposed New Source Performance Standards for stationary compression-ignition engines starting with model year 2007. The proposed standards essentially impose on stationary engines the same multi-tier schedule of tightening limits for DPM and other pollutants as they do for mobile diesels.
Diesel particulate consists mainly of soot – carbon and ash particles. As surfaces of the particles adsorb a sticky layer of liquid sulfates and heavy hydrocarbons from unburned fuel and lubricants, they clump together. The adsorbed hydrocarbons, known as the soluble organic fraction (SOF) of DPM, include carcinogenic toxins and can make up as much as 40 percent of DPM by weight. Along with SOF-plastered carbon particles, DPM also includes free hydrocarbon and sulfate particles (or droplets) and sulfuric acid.
DPM particles are small and generally are sorted by regulations into two size-ranges. PM10 particles are those with diameters up to 10 microns – less than half the width of a human hair. PM2.5 particles have diameters of less than 2.5 microns. Because DPM particles are so small, they can penetrate deep into the lungs, affecting tissues and air passages. Size also allows PM10 particles to stay in the atmosphere for hours; PM2.5 particles can stay aloft for days or weeks. Airborne PM10 particles can travel as far as 30 miles, and PM2.5 particles can travel hundreds of miles.
The EPA has estimated that nearly 100 million Americans live in areas with unhealthy levels of particulate matter in the ambient air. The EPA designates these locations as non-attainment areas for PM10 or PM2.5.
Over the last decade, diesel manufacturers have made great progress in reducing engine-out emissions. In its proposed new source performance standards, EPA places the burden of DPM reduction on engine manufacturers. But currently available engine-based solutions are not able to achieve the low DPM levels called for in these proposed federal regulations. At state and local levels regulations are now in effect and compliance often requires retrofit exhaust treatment devices.
The most common way to achieve high particulate reduction is to use filters that trap exhaust particulates. Particulate reduction devices are generally referred to as “diesel particulate filters” (DPFs), “particulate traps” or (less accurately) “soot filters” or “traps.” As an added benefit, DPFs also reduce visible diesel smoke and can reduce odor. In standby generation for healthcare facilities, for example, this added benefit is particularly important.
Trapped particulates would quickly build up and clog a filter, blocking exhaust flow and shutting down the engine if the particulates were not removed. The process of removing trapped particulates from a filter (allowing engine operation without intolerable backpressure buildup) is called filter regeneration.
All standard industrial DPFs take one of two basic approaches to regeneration: combustion or non-combustive chemical oxidation on the filter. In turn, two basic approaches exist to DPM combustion on a filter: active and passive regeneration. Active regeneration uses a heat source independent from the engine and exhaust stream to burn off trapped DPM. Passive regeneration relies solely on exhaust heat.
Whatever method of regeneration is used, DPM oxidation leaves only a fine ash on the filter. Periodically this can be washed off, vacuumed off or blown off with high-pressure air. Removing this ash is usually called filter cleaning and can be done much less often than filter regeneration.
Chemical oxidation – One type of DPF takes a two-stage approach to regeneration. An oxidation catalyst, installed upstream of the DPM filter, removes from engine exhaust carbon monoxide (CO) and hydrocarbons (HC), as well as some EPA-listed hazardous air pollutants (HAPs). As a by-product of this first-stage process, nitrogen dioxide (NO2) is created in the exhaust stream. In a ceramic soot filter installed downstream, this NO2 oxidizes trapped particulates, but oxidation efficiency is dependent on temperature and the ratio of NO2 to soot.
This two-stage scheme effectively reduces DPM as well as CO and HC. But the approach requires strict maintenance of a ratio of NOx to DPM as high as 20:1, as well as a narrow exhaust temperature band (greater than 482 F at least half of the time) and low-sulfur fuel (maximum 50 ppm). Also, NO2 is an extremely toxic substance and may increase the emissions of nitro-polycyclic aromatic hydrocarbons (NPAH), which can cause cancer. Thus, the release of NO2 with this approach can be a serious risk.
Electrical regeneration – This approach uses electrical resistance heating to regenerate the filter. A stand-alone heater may be used, or the filter can be made of an electrically conductive, sintered metal fiber material. Triggered by sensors monitoring backpressure to indicate DPM buildup, electricity applied to this filter causes DPM combustion and filter regeneration.
Electrical regeneration has the advantage of reducing DPM regardless of engine load or exhaust temperature. Electrical regeneration does not require low-sulfur diesel fuel. What’s more, it creates none of the by-products of catalytic processes. However, its relative complexity tends to add to its cost and threatens reliability compared to DPFs that use passive regeneration. The sintered metal fiber filter removes much less DPM from diesel exhaust than other types of filters – about 65 percent to 85 percent, compared with 90 percent or more with some other filters. The filter material is susceptible to melting in high-temperature regeneration in applications with heavy engine-out DPM levels. Also, there can be a significant fuel penalty due to the heater’s energy requirement. Finally, electrical regeneration does nothing to reduce regulated diesel exhaust pollutants such as CO and non-particulate HC.
Catalysts – Without some ignition temperature-reducing mechanism, DPM won’t burn until it reaches minimum temperatures in the range of 1112 – 1202 F. These DPM ignition temperatures exceed typical diesel exhaust temperatures. To lower DPM oxidation temperature for filter regeneration, most DPFs use catalysts.
Fuel-borne catalysts (FBCs) are organometallic or bi-metallic additives – including platinum, cerium or iron, for example – dissolved into diesel fuel. An FBC lowers the burn temperature of DPM by about 212 – 482 F. At the lower temperatures, DPM will ignite and oxidize in a particulate filter lightly washcoated with a precious metal group (PMG) catalyst, typically platinum-based.
This approach succeeds in removing 85 percent or more of exhaust-stream DPM. But it is critical to keep FBC-treated fuel separate from diesel used in other applications and to make sure fuel is dosed with the right amount of fuel-borne catalyst. Typically, a dosing system needs to be installed as part of the setup. Since the FBC approach uses PMG catalysts, which are vulnerable to sulfur poisoning, FBC systems require the operator to use low-sulfur diesel fuel. As fuel-borne catalysts combust, they also add to ash buildup on particulate filters.
Catalyzed filters – Catalyzed filters have catalysts applied to the filters themselves. Most use PMG catalysts, which leave them vulnerable to sulfur poisoning. Technically, sulfur does not poison PMG catalysts in the way lead does, for example, permanently preventing catalyst action through chemical bonding with the catalyst. But sulfurous coatings on PMG catalysts do keep these catalysts from coming into contact with exhaust gases, and impede catalyst action in a way that is difficult to correct. DPFs using PMG catalysts thus require costly low-sulfur fuel.
One brand of DPF uses filters coated not with PMG catalysts but with a base-metal catalyst. Catalyst action is similar to that of PMG catalysts: dramatic reduction in the ignition temperature of DPM trapped by the filter. But a DPF using a base-metal catalyst resists sulfur poisoning and does not require low-sulfur diesel; it will work with regular No. 2 diesel or any other grade. Another benefit is that a base-metal catalyst does not produce toxic NO2 as a by-product of catalytic action.
The tradeoff is that the base-metal catalyst removes less than half of exhaust-stream CO and only 75 percent to 88 percent of exhaust-stream HC. In areas with tighter limits on CO and HC, this DPF can be installed in combination with a sulfur-resistant diesel oxidation catalyst, which will reduce CO and HC to the strictest compliance levels at a total cost competitive with other DPF systems.
While DPFs using catalyzed filters typically rely on passive regeneration, some models offer the option of active regeneration via supplemental electrical, diesel or natural gas-fueled burners. This option ensures DPM compliance regardless of exhaust temperature or engine load. Another option to achieve the required temperature is to spray fuel into an oxidation catalyst upstream of the DPF.
With all DPFs, backpressure monitoring is essential and most DPF models include integrated backpressure monitoring devices.
Another factor affecting DPF reliability, service requirements and service-life is the filter material. Most catalyst filters consist either of layered panels of sintered metal on a wire mesh support structure or extruded ceramics. Sintered metal filters offer the advantages of high soot-loading capacity within acceptable backpressure limits and the ability to be shaped to fit a wide range of space constraints. However, sintered metal filters offer less thermal durability and shorter service-life than some extruded ceramic filters. They can also cost more to make.
The most common extruded ceramic filters are made from cordierite or silicon carbide (SiC). In 2005, Corning introduced a new line of filters, initially for light-load vehicular diesels, made of stabilized aluminum titanate. The properties and advantages of this material seem similar to those of SiC. In the process of ceramic extrusion, channels with thin, porous walls are formed in the filter material. In the wall-flow monolith design, alternating channels are left open and plugged at opposite ends. Engine-out exhaust flows through the open end to the blockage and is forced to flow through the porous channel wall to an adjacent channel. As exhaust flows through the filter, particulates are trapped on the surface of channel walls and even within the walls.
While cordierite and SiC filters have many properties in common, SiC filters are thermally more durable: they are more resistant to thermal shock and have a much higher thermal conductivity, which helps prevent localized overheating. This gives SiC filters a higher soot-loading and particulate storage capacity. One DPF model features stackable SiC filters allowing filtering capacity to be tailored to the engine size and application. Research reported by a leading European manufacturer of sintered metal filters found SiC filters to have soot-loading capacity equal to or greater than that of sintered metal. According to this same research, cordierite has lower soot-loading capacity than either SiC or sintered metal.
A catalyzed silicon carbide filter. Photo courtesy of MIRATECH.
These characteristics make a significant difference in performance. One DPF using SiC filters, the MIRATECH CombiKat, was tested by the California Air Resources Board (CARB) and maintained backpressure below OEM specifications through 24 successive cold starts to 30-minute idle times, without regeneration.
Questions You Should Ask
Before investing in a retrofit DPM solution, find out what type of DPF you’re considering and consider the advantages and disadvantages of the various approaches outlined here. It also makes sense to ask some particular questions:
CARB verification? – With California’s aggressive DPM-control measures and rigorous testing procedures, CARB verification gives you good assurance of performance-as-promised as well as compliance with the strictest regulations in the United States Level 3 verification is the highest CARB verification level, requiring at least 85 percent DPM reduction.
Service intervals? – Less frequent filter cleaning and other maintenance means lower cost of ownership.
How is maintenance performed? – With many DPFs, disassembly is required for cleaning and other maintenance. Look for doors or filter access ports in the housing for easier, less time-consuming maintenance.
Low or ultra-low sulfur fuel? – In areas where regulations do not require low sulfur fuel, as California air quality rules do, the option to use regular diesel fuel can save about 9 percent or more on engine operating cost.
Don Newburry is Research & Development Manager at MIRATECH Corp., a provider of stationary industrial engine emissions control. He was formerly Senior Engineer at Siemens Westinghouse where he worked on low-emissions combustor designs for gas turbines. He has an master’s degree in mechanical engineering from Vanderbilt University and a bachelor’s degree in mechanical engineering from the University of Missouri-Rolla.