The Utah Division of Air Quality (UDAQ) recently issued guidance clarifying when emergency engines, such as backup generators, may avoid in-house 1-hour nitrogen dioxide (NO2) dispersion modeling during the permitting process. The guidance is particularly relevant for hospitals, universities, manufacturers, and other facilities with limited emergency generation capacity, as it establishes clear pathways that may eliminate the need for project-specific 1-hour NO2 modeling.
Historically, emergency engine projects have often required evaluation of potential 1-hour NO2 impacts during permitting, particularly where multiple generators are located at a single facility. Depending on the project, completing a modeling analysis can add cost, require additional coordination with regulators, and extend permitting timelines. The new guidance provides greater transparency regarding the circumstances under which UDAQ may accept operational restrictions in lieu of project-specific 1-hour NO2 modeling, allowing facilities to evaluate permitting strategies earlier in project development.
The guidance builds on Utah’s emergency engine permit-by-rule provisions in Rule R307-431. Facilities with emergency engines that qualify for the Approval Order exemption under R307-431 may also avoid 1-hour NO2 modeling by complying with operational restrictions already incorporated into the rule, including limitations on simultaneous testing of larger emergency engines. Qualifying engines must be located at minor sources, meet annual operating hour limits for non-emergency use, and satisfy other rule-specific requirements.
UDAQ also identified several situations where 1-hour NO2 modeling may not be necessary. Hospitals are exempt from modeling requirements due to life-safety considerations and National Fire Protection Association (NFPA) testing requirements. Universities may also avoid modeling if they accept permit conditions prohibiting the simultaneous testing of co-located emergency engines greater than 750 horsepower.
For facilities that do not qualify under R307-431, UDAQ established an additional screening pathway for certain minor sources. To qualify, emergency engines must meet criteria related to horsepower, EPA-tiered certification level, stack configuration, and distance from the facility boundary. Facilities meeting these criteria may avoid 1-hour NO2 modeling if they accept permit conditions that prohibit simultaneous engine testing and limit testing to daytime hours.
The guidance provides a clearer framework for evaluating emergency engine projects and determining when 1-hour NO2 modeling may be avoided through operational limitations. For facilities planning new emergency generators or permit modifications, evaluating eligibility early in project development may help streamline permitting, reduce regulatory uncertainty, and avoid potentially costly modeling efforts.
