Getting particular with particulates

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Despite the growing popularity of natural gas, diesel remains the fuel of choice for many engine generating-set specifiers and operators. But pollutants resulting from diesel-fuel combustion are attracting increasing regulatory attention. In fact, between 2012 and 2014, allowable emissions of one of these pollutants—particulate matter—will be reduced by 90%.

Diesel particulate filters (DPFs) offer one means for meeting this aggressive target, but successful operation requires attention to both initial sizing and ongoing maintenance.

Combustion basics

It is a chemical certainty that when atomized diesel fuel is ignited (at 15 “events” per second at 1,800 rpm), the subsequent combustion and energy release will have several consequences:

1. The energy contained in the fuel will convert rapidly to heat energy and increase in volume, forcing the piston down to turn the crankshaft. This mechanical energy then will be converted into electrical energy by the generator.

2. The high temperatures at the injector flame tip will create nitrogen oxides (NOx).

3. Incomplete combustion in some areas of the combustion chamber will create carbon monoxide (CO).

4. The relatively cool areas in the combustion chamber—cylinder walls, inlet valve(s)—will contribute to incomplete combustion and create the hydrocarbons (HC) and particulate matter (PM, often referred to as soot).

The PM created during combustion now is attracting greater scrutiny. Particulates comprise a carbon core, to which other combustion products become attached, including microscopic metal particles and unburned fuel and oil particles. Individual particulates are virtually invisible, but collectively they appear as the smoke you see from older diesel engines on start up, and when the genset takes the electrical load.

PM is a serious health hazard now being targeted by the U.S. Environmental Protection Agency, the California Air Resources Board (CARB), and other regulators around the world. PM is an airborne pollutant and, because it is so small, it travels deep into the lungs, attaching to our lungs’ walls. According to medical experts, PM can:

• Cause lung damage, potentially leading to premature death

• Cause eye irritation

• Aggravate respiratory conditions such as asthma and bronchitis

• Cause cancer in humans.

• Also, PM reacts with NOx and sunlight creating ozone at ground level, leading to:

• Haze or smog restricting visibility

• Acid rain

• Global climate change.

What can be done?

There are three potential contributors to reducing the amount of PM emitted from an engine’s exhaust system: improving fuel quality, optimizing the combustion cycle, and treating post-combustion exhaust.

Dirty, impure fuel has an adverse effect on exhaust emissions. The introduction of low and ultra-low sulfur diesel fuel for on-road use in October 2006 increases the effectiveness of exhaust after-treatment devices. We will see use of this fuel spill over to the stationary engine/power-generation industry during the next few years.

In addition, engine manufacturers have made enormous progress in reducing PM creation during the combustion process. For example, raising combustion temperatures reduces PM; however, this action also increases NOx, so this approach requires a fine balance and some compromise. Success has been achieved primarily through fuel management using electronic engine-control modules and electronically activated fuel injectors. Most engines now have a small pre-injection, followed by the main injection of fuel, followed by a post injection—three separate injection events for a more complete and cleaner combustion, a round that repeats itself 15 times per second.

EPA regulations have aggressively targeted PM emissions, particularly in its Tier 4 regulations, which become effective for most horsepower nodes from 2012 through 2014. Under these new rules, PM has to be cut from the Tier 3 level of 0.15 g/bhp/hr to 0.015 g/bhp/hr, a 90% reduction. It is unlikely that engine manufacturers will be able to meet the new targets through cleaner-fuel and combustion improvements. So, exhaust after-treatment devices offer promise for providing the added particulate removal these regulations will require.

CARB also has pursued PM reduction in the regulations it has issued for diesel engines operating in California. As part of this effort, the agency has established three performance levels for diesel engine emission-control devices, as follows:

• 0% — 24% reduction of particulates, no category

• 25% +, Level 1. Many diesel oxidation catalysts meet this level

• 50% +, Level 2. Several hybrid systems fall into this level

• 85% +, Level 3. Most DPFs and an active DPF meet level 3

How DPFs work

DPFs are a valuable tool in the effort to clean up diesel fuel exhaust. DPFs work just like the air filter at the front end of the combustion chamber. Exhaust is forced to enter the device, and particulate is trapped by the filter media, generally manufactured from cordierite (a ceramic material) and silicon carbide. These media are typically 45% to 60% porous.

Exhaust gas enters the DPF from the engine exhaust manifold or turbocharger. The face of the DPF consists of many cells (usually about 200/sq in.). Every open cell at the entrance is blocked at the other end. Every alternate cell is blocked at the entrance but open at the other end.

The exhaust therefore, only has one option: it enters an open chamber but has to pass through the porous wall of the filter into an adjoining chamber that is open at the other end. The PM and soot are trapped at the wall, and only clean, filtered exhaust gas is able to exit (see Figure 1).

Regeneration

What happens to the filter-trapped PM? At high engine temperature there is a chemical reaction known as oxidation, in which nitrogen dioxide (NO2) breaks down the PM and coverts it to nitrogen (N) and CO2, which are able to flow through the filter walls and harmlessly into the air. This oxidation process is called “regeneration.” The chemical reaction creates heat energy, referred to as an exothermic reaction, and this further aids the oxidation process by raising the temperature.

However, regeneration only occurs when the engine exhaust is hot enough for this reaction to occur. In cooler temperatures, the PM will not get oxidized and will build up in the filter, just like the dust in an air filter. Gradually exhaust backpressure will increase as the DPF captures more PM. All engine manufacturers publish their Maximum Allowable Backpressure figure, and it should not be exceeded or:

• The turbocharger will not be able to provide a fast enough boost for the engine to take the block load

• Crankcase pressure will increase

• Fuel consumption will increase.

The tipping point at which the volume of particulates entering the DPF is equal to the particulates being oxidized is called the balance point temperature (BPT); at this point there is no gain or reduction in the PM stored on the filter. If the temperature is below BPT, PM will build up in the filter, but as the temperature rises above BPT, PM will be reduced. Adding a catalytic coating to the DPF lowers the unit’s BPT significantly. Platinum is a common catalytic coating for DPFs, and can oxidize carbon monoxide (CO) and HC from the exhaust. And, since platinum also reduces the BPT, the DPF oxidizes the PM at a lower exhaust temperature.

To ensure PM buildup doesn’t create back-pressure conditions that exceed manufacture recommendations, DPFs should be provided with a pressure differential switch. This sensor will constantly monitor the pressure in and out of the DPF and provide an alarm and/or pre-alarm for the system, which can be connected to the genset’s control and annunciator panels.

DPF suppliers size their products based on specific application requirements, using exhaust-flow and temperature data developed under both normal and maximum operating conditions. An engine may require one or more DPFs—in parallel, not series—to maintain operation at an acceptable backpressure for the engine, and to provide a level of PM storage capacity. DPFs usually are combined with an exhaust silencer, in order to make the best use of available space. But care must be taken on a retrofit application to ensure that the genset structure is capable of supporting the added weight of the DPFs. If not, an independent means of support for the DPF housing may be needed.

Prior to installing a DPF (or any catalyst) in a new exhaust system for the first time, the engine should be run under load for about an hour without the catalyst to ensure the exhaust system is clean and clear and prevent any loose pieces from impinging on the front face of the device. However, local environmental officials should be consulted first, to ensure this procedure does not violate any regulations.

Application-specific concerns

Plans also must consider whether the engine will be used in a prime-power or standby-power setting. Prime-power applications, including cogeneration, combined heat and power, rental, demand response, and distributed generation typically will be constantly regenerating a DPF, because such generator loads create exhaust temperatures well above the BPT.

Standby generators, however, typically are run automatically on a weekly 1-hour exercise cycle, and then shut down, with no load and at a very low exhaust temperature. Under these conditions, the DPF will not self-regenerate, so plant personnel need to carry out a planned regeneration before the backpressure reaches a critical point for the engine. There are several options for working with standby generators, including:

1. Increase the time between auto start exercises beyond the traditional one week and decrease the running time in order to extend the time required before regeneration.

2. Apply the facility load to the genset, thus creating higher exhaust temperature. This will also test the entire generating system, including the transfer switch—providing greater assurance to the facility management that their emergency power source is truly ready to take over in the event of utility power failure.

3. Arrange for the set to be run with a load bank, which also will benefit the engine. Load bank service is available from most generator set distributors. Many specifying engineers now recognize the benefits of routine genset tests under (partial) load conditions. This has resulted in an increase in the number of new standby generator sets being supplied with a radiator mounted load bank.

4. Remove the filter from its housing and send it to be externally regenerated by an approved service provider. There are procedures for external regeneration in order to prevent thermal stressing of the filter.

Additionally, small amounts of lubricating oil in the exhaust stream will burn, leaving ash deposits on the walls of the filter that eventually will need removal. This ash is inorganic and non-toxic, so it may be removed simply by blowing it out with compressed air at the filter’s outlet end and applying a vacuum cleaner at the inlet end to collect the soot.

A tested technology

Diesel particulate filters are a mature technology in the United States and many other parts of the world. Most U.S. diesel trucks manufactured since 2007 have DPFs installed, and DPFs have become standard equipment in European diesel automobiles, which make up 50% of that region’s new auto purchases. An increasing number of engines in the power generation-industry will have a DPF in the exhaust system, even before being required by the Federal government.

No more exhaust smoke on start up, and none of the traditional smell of diesel. What is the world coming to?

Test	HC	CO	NOx	NO 2	CO 2	PM2.5
0 hour control	-81%	-98.1%	-1.3%	1.1%	-0.2%	-98.5%
167 hours control	-61%	>-99%	0.2%	0.9%	1.7%	-98.6%
500 hours	>-99%	-93%	5.0%	-0.9%	-1.7%	-99.4%

Author Information

Pope is senior sales engineer and marketing manager for Süd-Chemie Inc. He worked for two engines manufacturers and an engine/generator set distributor before joining the company. He has been a member of EGSA for many years, has served on the board of directors and is a past chairman of the EGSA Education Committee.

Documented results

Dramatic reduction of PM, CO, and HC is common with the installation of a catalyzed DPF. The data in Table 1 are from an independent test laboratory (University of Riverside, Calif.) taken during the CARB verification testing of a Süd-Chemie Inc. EnviCat cDPF. The engine was a Caterpillar D3406TA.

Variations in the BPT and conversion efficiency of the HC and CO are common. Ambient conditions, engine load, engine condition, and fuel are the variables that can cause different conversion ratios.

The DPF’s filtration efficiency likely will improve above the baseline level. As the filter collects more particulates, that layer acts as a pre-filter.

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