Hennepin Energy Recovery Center data

The county maintains the highest performance standards at the Hennepin Energy Recovery Center (HERC), including its state-of-the-art pollution control system. Controlled high temperature combustion prevents some air pollutants from forming. The exhaust system neutralizes others or scrubs and filters them out.

Air emissions are cleaned and treated so that emissions are consistently below the Minnesota Pollution Control Agency permitted levels.

The combustion process reduces the volume of waste by 90 percent. The material remaining after combustion is non-hazardous ash that is disposed of in a landfill.

Metals are removed and recycled before the ash is sent to a landfill. HERC recovers more than 16,000 tons of scrap metal each year, which is nearly triple the amount of metal collected in curbside recycling programs in the county.

HERC uses continuous monitoring to measure and record emissions for nitrogen oxide, sulfur dioxide, carbon monoxide, and carbon dioxide. Opacity is also tracked continuously.

In addition, HERC's air permit requires annual stack testing to measure additional pollutants: hydrogen chloride, dioxins/furans, hydrocarbons, lead, total particulate matter, cadmium, and mercury. See the annual stack testing from 2025 and 10-year average (PDF, 1MB).

Continuous monitoring

HERC's air emissions monitoring system continually extracts and tests exhaust samples. Plant operators monitor the results around the clock and make adjustments to control air emissions and reduce particulate matter from escaping.

The following data is an automated daily report from the continuous monitoring system of the facility. HERC has two boiler units and each is monitored separately. They are labeled Unit 1 and 2. If all readings for a boiler unit are zero, this means the boiler unit is off-line.

Data displayed here is delayed by one day to allow the continuous emissions monitoring system time to retrieve sensor data if an automatic report fails to send. Daily reports are posted automatically and displayed here without review or editing by county staff.

View full-size continuous monitoring report (Power BI)

Carbon monoxide monitoring

Carbon monoxide (CO) values are the maximum 4-hour block average for that day. Reported as parts per million.

Sulfur dioxide monitoring

Sulfur dioxide (SO2) values are the daily (24-hour) averages.

Exhaust gases pass through a dry scrubber, where a hydrated lime slurry (alkaline) is injected to control sulfur dioxide.

Nitrogen oxides monitoring

Nitrogen oxides (Nox) values are the daily (24-hour) averages.

Urea is injected into the furnace to control nitrogen oxides emissions. When the low-NOx system was installed in 2015, Nox emissions were reduced by 25%.

Opacity

The opacity value displayed is the maximum 6-minute average for the selected day.

Opacity is the degree to which a visible emission plume blocks the background from view. Sudden opacity spikes can indicate operational or equipment issues that warrant investigation.

Permit limits

Permit limits are derived from U.S. Environmental Protection Agency standards for Large Municipal Waste Combustors (Code of Federal Regulation citation: 40 CFR 60, Subparts A, Cb and Eb).

All emission values are reported in units consistent with the permit. Carbon monoxide, Sulfur dioxide and Nitrogen oxides are reported as parts per million, dry gas basis at 7% dioxygen (ppmdv7) and opacity as a percent. Sulfur dioxide compliance is based on concentration in parts per million. Regulators set a dry gas basis to standardize the water vapor content to make the measurements reliable and comparable. Continuous emissions monitoring systems are designed to remove moisture from the gas sample to match the dry gas standard before the sample is analyzed.