Colorado Regulation 26 already includes requirements for natural gas-fired engines greater than or equal to 1,000 hp operating in the state of Colorado. The changes proposed to Reg 26 serve to further ratchet down NO
X emission limitations for engines operating across the state by adding standards for large diesel engines and for smaller natural gas engines, and lowering standards for natural gas engines larger or equal to 1,000 hp that were placed in service/modified/relocated after January 30, 2024. The applicability of these new limitations is determined differently for natural-gas fired engines compared to diesel or dual-fuel fired units, and the specific limits these different types of engines would be subject to also vary. A key point of these new rules is that they apply
statewide, rather than solely in the Denver Metro/North Front Range (DMNFR) ozone non-attainment area. If you operate, or plan to construct any engines that meet the following applicability criteria
anywhere in Colorado, then this proposed rule would apply to you! See the exact changes currently proposed to AQCC Regulation 26
here.
Proposed Engines Applicability Criteria
Applicability for natural gas-fired engines differs for rich-burn and lean-burn units. Any rich-burn natural gas-fired engine rated between 100-1000 hp would be subject to these new rules, as would any lean-burn natural gas-fired engine rated between 250-1000 hp. However, note that under the proposed rule, owners and operators would be required to provide a list of any natural gas engines less than 100 hp that were placed in service, modified, relocated, or replaced before January 30, 2024, and would need to report the actual NOX emission rate (g/hp-hr) for the previous calendar year as well as the current permitted emission rate (g/hp-hr).
Applicability for diesel or dual-fuel fired engines is simpler. Any engine firing these fuels rated at or above 500 hp would be subject to the newly proposed rules discussed below.
Note that some engines meeting the above size criteria would remain exempt from these new rules. First, these rules specifically cover stationary sources. Non-road engines would not be subject to the proposed emission limitations, nor the associated performance testing, monitoring, recordkeeping, or reporting requirements. Additionally, emergency power generation units that meet existing APEN or permit exemption criteria defined under AQCC Regulation 3 are exempt from the proposed Regulation 26 requirements. Finally, natural-gas fired engines that combust less than 100 MMBtu/yr (rolling 12-month basis) would be exempt from these proposed rule updates.
Proposed Engine NOX Emission Limitations
The current rule contains requirements for large (≥ 1,000 hp) natural gas-fired engines. Under the proposed rule, the NOX standard for lean burn engines placed in service/modified/relocated after January 30, 2024 would be lowered from 0.7 g/hp-hr to 0.5 g/hp-hr, and the standard for rich burn engines placed in service/modified/relocated after January 30, 2024 would be lowered from 0.5 g/hp-hr to 0.3 g/hp-hr.
Under the proposed rule, natural gas-fired engines not previously subject to the rule (i.e., rich burn engines 100-1,000 hp and lean burn engines 250-1,000 hp), in service on or before January 30, 2024, would be subject to a NOX emission standard of 0.5 g/hp-hr. This would apply to both lean-burn and rich-burn units. However, the NOX standards for new engines (placed into service, modified, or relocated after January 30, 2024) would vary based on their lean-burn vs. rich-burn design. New lean-burn units would be subject to the same NOX standard as existing units, 0.5 g/hp-hr. However, new rich-burn units would instead be subject to a NOX standard of 0.3 g/hp-hr.
Diesel and duel-fuel fired engines, both new and existing, would be subject to EPA Tier IV emissions standards for NOX. These standards are well established under existing federal rules, and vary based on engine sizing and operation (specific standards for genset engines). However, NOX standards under Tier IV typically range from 0.3 g/hp-hr to 0.5 g/hp-hr. Under the proposed rule, existing engines may need to be retrofitted to meet the Tier IV NOX limit.
Proposed Compliance Deadlines
Compliance with the above NOX emission standards would be required upon startup for any new engines meeting the described applicability criteria. This means that any engine that is placed into service, modified, or relocated after January 30, 2024, would need to be designed and permitted to meet these emission standards. However, the proposed rules provide a phase-in schedule for existing engines, allowing for more flexibility in retrofitting and/or re-permitting of these existing engines to meet the new standards. Please see the following for the proposed phase-in schedule for existing units:
| Location of Subject Engines by Owner/Operator | Compliance Deadlines |
| May 1, 2025 | May 1, 2026 | May 1, 2027 | May 1, 2028 | May 1, 2029 |
| Percent (%) of engines that must comply with new NOx Standards |
| Inside, or inside and Outside 8-Hr Ozone Control Area | ≥ 34% of engines in 8-Hr Ozone Control Area | ≥ 67% of engines in 8-Hr Ozone Control Area, and ≥ 25% of engines outside 8-Hr Ozone Control Area | 100% of engines in 8-Hr Ozone Control Area, and ≥ 50% of engines outside 8-Hr Ozone Control Area | ≥ 75% of engines outside 8-Hr Ozone Control Area | 100% of all engines |
| Outsite 8-Hr Ozone Control Area only | ≥ 20% | ≥ 40% | ≥ 60% | ≥ 80% | 100% |
Note that engines that only require a permit modification to meet these new standards (meaning they do not require physical modifications or other changes in operating conditions) would need to submit the required modification application(s) no later than May 1, 2024, to meet the initial May 1, 2025, compliance deadline for the new standards. Engines requiring additional modifications, or introduction of new operating limits or requirements would be subject to the phase-in schedule above. Permit modification applications must be submitted at least 365 days in advance of the specified compliance deadlines!
Proposed Engine Performance Testing and On-Going Compliance Requirements
The proposed NOₓ standards described above are only one piece of the overall puzzle. Additionally, the proposed rule includes the following testing, monitoring, recordkeeping, and reporting requirements (proposed effective dates included in parentheses):
- Conduct initial performance tests to demonstrate compliance with reduced standards (May 1, 2025);
- Conduct annual portable analyzer monitoring (May 1, 2025);
- Monitor catalyst inlet temperature (daily) and pressure drop (monthly), if a catalyst is used to meet NOX limit (May 1, 2024);
- Install hour meter (May 1, 2024);
- Conduct annual inspections covering oil and filter changes, air cleaners, fuel filters, hoses & belts, and spark plugs (2025);
- Conduct annual combustion process adjustment (2025);
- Maintain records of performance tests, portable analyzer monitoring data, catalyst monitoring data, tuning/combustion process adjustments, fuel combustion (if claiming exemption based on < 100 MMBtu/yr), and hours of operation; and
- Submit an annual report on May 1 of each year (May 1, 2024).
Rulemaking Process Next Steps
The rulemaking hearing covering these proposed additions to AQCC Regulation 26 is scheduled for December 12-15, 2023, and the anticipated effective date for these requirements begins in January 2024. While this rule has not been finalized Trinity recommends the following steps to ensure you are not caught off-guard!
- Evaluate your current engine fleet to identify engines potentially impacted by the proposed rule;
- Explore options for retrofitting or replacing engines not meeting the proposed standards;
- Closely follow the rulemaking process, and consider providing comments on the proposed rule. Note that industry associations such as the Colorado Oil and Gas Association and Colorado Stone Sand & Gravel Association have filed for party status as part of the rulemaking process, and may be appropriate avenues to ensure your questions and concerns are heard.
Stay tuned for more information from Trinity following the rulemaking hearing (
Rulemaking Hearing Details), and in the meantime, please don’t hesitate to reach out for support in interpreting these proposed rules, planning on how to ensure compliance, or in preparing needed permit applications to capture these new requirements at your various facilities.
