Generator Emissions Monitoring


What Diesel Generator Engines Require A CEMS?

The Environmental Protection Agency (EPA) and some States continuously develop laws that regulate emissions from nonroad and stationary compression ignition internal combustion (diesel) engines. All of their laws stem from the Code of Federal Regulations (CFR). The EPA is actually established by the Code of Federal Regulation (CFR 40._.1) as is the EPA's Clean Air Act (CFR 40._.1.C.60.A._.2).

Code of Federal Regulations (CFR) The challenge is to know specifically what regulation language applies to you so you don't have to rely on hearsay. There are numerous stories throughout the internet of major fines being assessed on owners and operators for not meeting regulatory requirements. We tend to think that some of those fines were the result of owner operators being given bad information by so called experts who thought they correctly knew the code, but had it wrong. Since we have seen this happen from time to time we decided to find the actual language for ourselves.

In our research to determine what generators require a continuous emissions monitoring system (CEMS), we discovered that its actually easier to determine the generators that DO NOT require one. So the way we look at it is if a diesel generator engine does not meet one of the conditions described below, it probably requires a continuous emissions monitoring system.

We came up with 4 conditions that we believe exempt nonroad and stationary compression ignition internal combustion (diesel) engines from continuous emissions monitoring.

1.  Ultra Low Sulfur Diesel

As of 2014, units (diesel engines) less than 25 megawatts (33,525 hp) that use Ultra-Low Sulfur Diesel (ULSD) do not require continuous emissions monitoring. Here's our justification:

The California Energy Commission's Energy Almanac, under Definitions of Motor Fuels and Refined Products, states that EPA Ultra Low Sulfur No. 2 Diesel Fuel (EPA Highway ULS Diesel) "has a sulfur level no higher than 0.0015% by weight (15ppm)". That puts it well under the .05 maximum indicated in the EPA's Continuous Emissions Monitoring Fact Sheet under To Whom Do These Requirements Apply?, where it states that "All units over 25 megawatts and new units under 25 megawatts that use fuel with a sulfur content greater than .05 percent by weight are required to measure and report emissions under the Acid Rain Program" (The Acid Rain Program is where continuous emissions monitoring is called for). From that it is logical to infer that units (diesel engines) less than 25 megawatts (33,525 hp) that use Ultra-Low Sulfur Diesel (ULSD) do not require a continuous emissions monitoring system (CEMS).

The US Department of Energy's information on Ultra-Low Sulfur Diesel, says that "As of December 1, 2010, all diesel fuel sold in the U.S. must be ULSD." In the EPA's' Diesel Fuel page it is stated that "Ultra Low Sulfur Diesel (ULSD) fuel will be phased in for nonroad, locomotive, and marine (NRLM) engines from 2007-2014." Based on that you should now only be using ultra-low sulfur diesel (ULSD) and if your unit (diesel engine) is less than 25 megawatts you should not be required to provide continuous emissions monitoring.

It should be noted that the State of California's' specification for Ultra-Low Sulfur Diesel (ULSD) fuel is slightly more stringent than the EPA's. For details see Ultra-Low Sulfur Diesel (ULSD) - CARB and Ultra-Low Sulfur Diesel (ULSD) - EPA.

2.  Permit Does NOT Call For Emissions Monitoring

CFR 40._.1.C.70._._.3.(c-d) and CFR 40._.1.C.71.A._.6 requires that permits to construct and operate nonroad and stationary compression ignition internal combustion (diesel) engines indicate required monitoring. We interpret this to mean that if the permit does not call for continuous emissions monitoring it is not required.

Note-1: While CFR 40._.1.C.60.IIII.?.4211.(d) seems to indicate that continuous emissions monitoring is required, it also indicates some exceptions when it references CFR 40._.1.C.60.IIII.?.4204.(c) and CFR 40._.1.C.60.IIII.?.4205.(d). If emissions monitoring is not called for on a permit it may be because the permitting agency applied some of these exceptions.

Note-2: CFR 40._.1.C.60.IIII.?.4209 calls for a backpressure monitor that notifies the owner or operator when the high backpressure limit of the engine is approached. This applies to diesel engines with a diesel particulate filter (DPF). However, if the permit does not call for a backpressure monitor then it is logical to infer that CFR 40._.1.C.70._._.3.(c-d) and CFR 40._.1.C.71.A._.6 applies and a backpressure monitor is not required.

Note-3: Per CFR 40._.1.C.60.IIII.?.4203, generator set engine emissions certified by the manufacturer must not be exceeded for the life of the engine.

3.  Non-Utility Unit

CFR 40._.1.C.75.A._.2 states " Any unit not subject to the requirements of the Acid Rain Program due to operation of any paragraph of § 72.6(b) of this chapter". § 72.6(b) states that "The following types of units are not affected units subject to the requirements of the Acid Rain Program: . . . (8) A non-utility unit." For definition click utility unit. Therefore, if a generator engine is not a utility unit, we believe it's safe to say that continuous emissions monitoring is not required.

4.  Tier-4 Engine

The EPA's web page on Nonroad Diesel Engines references CFR 40._.1.U.1039 as the "Tier 4 emission standards and certification requirements". In reference to CFR 40._.1.U.1039, the EPA also states on this page that "To meet these Tier 4 emission standards, engine manufacturers will produce new engines with advanced emission control technologies similar to those already expected for highway trucks and buses {which do not have continuous emissions monitoring systems}. Exhaust emissions from these engines will decrease by more than 90 percent. Because the emission control devices can be damaged by sulfur, we have also adopted requirements for in-use diesel fuel to decrease sulfur levels by more than 99 percent. The resulting Ultra Low Sulfur Diesel fuel has a maximum sulfur concentration of 15 parts per million." These references imply that as of 2013 (date of CFR 40._.1.U.1039) all new engines will be manufactured with emissions control devices that eliminate the need for continuous emissions monitoring.


Keep in mind that all emissions regulations are based on engines designed, constructed, tested, and certified to applicable regulations in place at the time of construction. Any changes to the equipment after the engine is constructed and shipped could result in large fines and costly corrective measures when discovered by a governing authority. Also keep in mind that even though an engine may not require a continuous emissions monitoring system, governing authorities may inspect it at any time. They can also require emissions testing from time to time as indicated in CFR 40._.1.C.60.IIII.?.4212 and CFR 40._.1.C.60.IIII.?.4213.


Do not make any decisions based on the content of this article. Always consult your local state or federal agency before drawing any conclusions, making any decisions, and taking any actions.

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