To reduce carbon monoxide and hydrocarbon emissions from a genset's exhaust, a diesel oxidation catalyst (DOC) can be installed. As the exhaust smoke travels through the catalytic material, much of it is changed into water and carbon dioxide, explains Plunkett. Carbon monoxide and hydrocarbons are reduced 70 to 99 percent. Even particulate matter is reduced by 20 to 30 percent. "If the genset is in good operating condition, a DOC can be added to an existing system," says Plunkett.
A more expensive, but also far more efficient exhaust smoke cleaner for gensets is a DPF, which uses a ceramic filter media. "Instead of 20 to 30 percent reductions of particulate matter as seen in DOCs, DPFs get 90 to 99 percent reductions," says Plunkett. "They basically eliminate the smoke and odor."
The ceramic filter media consists of many cells. Each cell is open on one end and blocked on the other in an alternating fashion. Exhaust enters the open end and then must pass through a porous wall into an adjoining chamber. Particulate matter or soot is trapped at the wall, allowing clean, filtered exhaust gas to exit.
"The coating on the catalytic material reduces the temperature needed to burn off hydrocarbons from 1,400 degrees F to 600 to 800 degrees F," explains Plunkett.
Trapped particulate matter is subject to high engine temperatures and, through the process of oxidation, is broken down by nitrogen dioxide into nitrogen and carbon dioxide, explains Pope. Nitrogen and carbon dioxide can pass through the filter walls into the outside air.
More localities are adding requirements for stationary engines. For instance, the CARB diesel emission reduction program has classifications for after-treatment devices that reduce particulates, ranging from greater than 25 percent (CARB Level 1) to greater than 85 percent (CARB Level 3+).
Diesel oxidation catalysts generally are considered in CARB Level 1, while diesel particulate filters are CARB Level 3+. The federal RICE NESHAP rule that goes into effect on May 13, 2013, exempts emergency generator sets. However, it does apply to all stationary engines in non-emergency duties, such as demand response, prime power, combined heat and power, and peak load shaving applications.
Air standards are also tightening across the country. As a result, facility managers need to explore the many ways that the black smoke and wet stacking experienced with older emergency power generator sets can be reduced or eliminated.
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