Background

The US Environmental Protection Agency (EPA) issued a number of rules to control emissions of toxic air pollutants from existing stationary reciprocating internal combustion engines (RICE).

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The rules, entitled National Emission Standards for Hazardous Air Pollutants (NESHAP) for Reciprocating Internal Combustion Engines, are intended to reduce emissions of toxic air pollutants—such as formaldehyde (HCHO), acetaldehyde, acrolein, methanol and other air toxics—from several categories of previously unregulated stationary engines. The EPA has determined that carbon monoxide (CO) can be often used as an appropriate surrogate for formaldehyde. Since testing for CO emissions has many advantages over testing for emissions of hazardous air pollutants (HAP), most of the emission standards have been finalized in terms of CO as the only regulated pollutant.

The NESHAP regulations for stationary engines are published in Title 40, Part 63, Subpart ZZZZ (63.6580) of the Code of Federal Regulations (CFR). Regulatory documents as well as fact sheets and related information can be also found in the US EPA stationary engine pages [3115].

The NESHAP standards are applicable to existing engines. Separate regulations have been adopted to control emissions from new stationary engines.

Applicability

The applicability of the emission standards depends on the classification of the source of air toxics emissions. “Major sources” of air toxics are defined as those that emit 10 short tons per year of a single air toxic or 25 short tons per year of a mixture of air toxics. “Area sources” are those sources that are not “major sources”.

The NESHAP rules are applicable to “existing” diesel and SI engines, as determined by their date of construction or reconstruction:

  • “Area sources” of air toxics emissions: Engines constructed or reconstructed before June 12, 2006.
  • “Major sources” of air toxics emissions:
    • Engines ≤ 500 hp constructed or reconstructed before June 12, 2006.
    • Engines > 500 hp constructed or reconstructed before December 19, 2002.

Emergency Engines. The NESHAP requirements apply to engines used for non-emergency purposes. The following operational requirements must be met by emergency engines:

  • There is no time limit on the use in emergency situations (e.g., power outage, fire, flood).
  • The engine may be used for up to 100 hours per calendar year for maintenance checks, testing, and for emergency demand response (i.e., blackout and brownout prevention).
  • The engine may be used for up to 50 hours per year for certain non-emergency uses such as local reliability (the operation counts toward the above 100 hour limit).

Emission Standards

The NESHAP regulations include three types of emission standards:

  1. Emission Limits—Limits for lean-burn engines are expressed as volumetric, dry CO concentrations (ppm) at 15% O2. Limits for rich-burn SI engines are expressed as volumetric, dry concentrations of HCHO (ppm or ppb) at 15% O2. The standards must be met during any operating conditions, except during periods of start-up (of maximum 30 minutes). Emissions are tested at 100% load.
  2. Percentage CO/HCHO Reductions—Alternative compliance options are available in certain engine categories, expressed as percentage CO or HCHO/THC emission reductions. These reductions can be achieved by retrofitting engines with emission controls, such as oxidation catalysts (OC) on lean-burn engines or non-selective catalytic reduction (NSCR, three-way catalysts) on rich-burn engines.
  3. Equipment Standards—Engines must be retrofitted with emission controls: oxidation catalysts on lean-burn engines and NSCR catalysts on rich-burn engines.

The standards for stationary diesel engines are listed in the following table.

Table 1
NESHAP Emission Requirements for Stationary Diesel (CI) Engines
Engine CategoryEmission StandardAlternative CO Reduction
Area Sources
Non-Emergency 300 < hp ≤ 50049 ppm CO70%
Non-Emergency > 500 hp23 ppm CO70%
Major Sources
Non-Emergency 100 ≤ hp ≤ 300230 ppm CO-
Non-Emergency 300 < hp ≤ 50049 ppm CO70%
Non-Emergency > 500 hp23 ppm CO70%

Standards for spark ignition, gas-fired stationary engines are summarized in Table 2. The engine designations indicate two- or four-stroke (2S/4S) lean- or rich-burn (LB/RB) natural gas or landfill/digester gas (LFG/DG) engines.

Table 2
NESHAP Emission Requirements for Stationary Gas (SI) Engines
Engine CategoryEmission StandardAlternative CO/HCHO Reduction
Area Sources†
4SLB, Non-Emergency > 500 hpInstall OCa
4SRB, Non-Emergency > 500 hpInstall NSCRb
Major Sources
2SLB, Non-Emergency 100 ≤ hp ≤ 500225 ppm CO-
4SLB, Non-Emergency 100 ≤ hp ≤ 50047 ppm CO-
4SRB, Non-Emergency 100 ≤ hp ≤ 50010.3 ppm HCHO-
LFG/DG, Non-Emergency 100 ≤ hp ≤ 500177 ppm CO-
4SRB, Non-Emergency > 500 hp350 ppb HCHO76% HCHOc
† Standards applicable only to engines operated > 24 hrs/yr and installed in locations that are not “remote areas”. Remote areas include (1) offshore locations along that portion of the coast that is in direct contact with the open seas, (2) pipeline segments with 10 or fewer buildings intended for human occupancy and no buildings with four or more stories within 220 yards on either side of the centerline of any continuous 1‐mile length of pipeline, or (3) non gas-pipeline locations that have 5 or fewer buildings intended for human occupancy and no buildings with four or more stories within a 0.25 mile radius around the engine.
a The oxidation catalyst must provide a 93% CO emission reduction or a 47 ppm CO concentration.
b The NSCR catalyst must provide a 75% CO reduction or a 30% THC reduction or a CO concentration of 270 ppm.
c Alternative option: 30% THC reduction.

Other Provisions

Diesel Fuel. Certain categories of diesel engines are required to use ultra-low sulfur diesel (ULSD, max. 15 ppm S) fuel:

  • Stationary non-emergency engines greater than 300 hp with a displacement of less than 30 liters per cylinder, fully effective from 2013.
  • Stationary emergency engines ≥ 100 hp that operate for more than 15 hours per year for emergency demand response, effective from 2015.