Air Emissions: Backup vs. Primary Power

A data center can have dozens or even hundreds of combustion engines on site, but the air-quality implications depend heavily on how those engines are allowed and expected to operate.

Emergency or backup generators are normally idle. They operate during qualifying emergencies and may also run for testing and maintenance subject to applicable federal and state requirements. Because their operating hours are limited, their annual fuel use and emissions profile can be very different from engines that run routinely.

Primary, prime or other non-emergency generation is different. If engines or turbines routinely supply electricity to the computing load—including some behind-the-meter arrangements—they can operate for many more hours. That changes total fuel consumption, annual emissions and the regulatory requirements that may apply.

Combusting natural gas or diesel can produce pollutants including nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), volatile organic compounds (VOCs) and greenhouse gases. Actual emissions depend on fuel, engine or turbine technology, operating load, maintenance and pollution-control equipment.

EPA's stationary-engine rules explicitly distinguish emergency engines from non-emergency engines. Stationary combustion turbines are covered by separate federal standards where applicable. Ohio projects may also require permits through Ohio EPA depending on equipment, emissions and operating configuration.

So when reviewing a data-center project, do not stop at the number of generators. Ask: Are they emergency-only or routine power? What fuel do they burn? How many hours may they operate? What emissions limits and controls apply? And what are the expected annual emissions under the proposed operating scenario?