Like all combustion processes and nearly all waste management processes, WTE facilities produce air emissions. To minimize them, WTE facilities employ a carefully controlled combustion process and sophisticated air pollution control equipment. Continuous emission monitoring is complemented with periodic testing performed by regulator-approved third parties. Of the air coming out of a WTE stack, 99.9% is water vapor, nitrogen, oxygen and CO2.
What Comes Out of Our Stacks?
Since the implementation of stringent air pollution standards, such as the Clean Air Act Amendments of 1990, emissions from the industry have dropped dramatically in response to the closure of outdated facilities and installation of new air pollution control equipment. Since beginning our comprehensive approach to sustainability in 2007, we have reduced our emissions of conventional air pollutants by up to 69%. Today, we are finding ways to maintain our levels of emissions performance more efficiently by optimizing our operations.
View Detailed 2020 Performance by Facility.
WTE facilities are strictly regulated by the federal and state/provincial environmental agencies. A WTE facility operates under various environmental permits, including both solid waste and air permits. Our facilities’ air permits contain emissions limitations and requirements for carbon monoxide (CO), filterable particulate matter (PM), nitrogen oxides (NOx), sulfur dioxide (SO2), dioxins and furans, mercury, cadmium and lead. Most of our permits contain additional limits.
By contrast, landfills, which receive most of the waste remaining after recycling in the U.S., do not employ air pollution control measures beyond a combustion device and do not engage in continuous emissions monitoring.
Controlling Air Emissions
Our facilities use state-of-the-art control technologies to remove air pollutants associated with combusting MSW as part of the WTE process:
Our boilers are designed specifically to ensure complete combustion, thereby recovering as much energy as possible from the waste resource, while reducing volatile organic compounds and other organic compounds.
Nitrogen Oxides (NOx) Control:
Most boilers are equipped with selective noncatalytic reduction (SNCR) systems, which inject ammonia or urea into the furnace to chemically convert NOx into gaseous nitrogen, a harmless gas that makes up most of our atmosphere. Covanta has installed its proprietary low nitrous oxide system (Low NOx™) in 28 units, which helps control NOx emissions and reduce reagent consumption. Covanta also operates one facility located in Palm Beach County, which uses a catalyst installed after the baghouse for even greater NOx reduction. This process is termed selective catalytic reduction or SCR.
After leaving the boiler, combustion gases travel through an extensive air pollution control system. At many of our plants, activated carbon is added to the flue gas stream as it exits the boiler. Gaseous phase contaminants such as mercury and dioxins adsorb to the surface of the carbon so it can be removed downstream in the baghouse (see below).
A scrubber neutralizes acid gases, including sulfur dioxide and hydrochloric acid, by spraying a lime slurry into the exhaust stream.
Operating like a very efficient vacuum cleaner, the baghouse removes over 99.5% of the particulate matter from the combustion gases. As air is drawn through the baghouse, particulate matter and fly ash are caught on the surface of the bags. Periodically, the bags are cleaned by temporarily reversing the airflow or, in other designs, pulsing the bags with a strong jet of air. The particulate and fly ash are removed from the bottom of the baghouse.
Emissions from WTE facilities are determined both through routine stack tests (performed at least once a year) and through continuous emissions monitoring systems (CEMS). As required by our facilities’ operating air permits, CEMS monitor flue gases continuously for carbon monoxide, nitrogen oxides, sulfur dioxide, opacity and carbon dioxide and/or oxygen. Opacity is a measure of particulate matter that helps us assess the performance of our baghouses. Facilities operators monitor these parameters and adjust as needed to ensure both proper operation and regulatory compliance. For example, monitoring carbon monoxide (CO) levels continuously provides a surrogate for the combustion process, and allows operators to make adjustments as needed to keep the process operating properly.
Other regulated pollutants are checked through a rigorous stack testing program performed by a regulator-approved third party. The operating parameters under which the stack test is conducted set the criteria for the facility’s operation until the next stack test is completed. Examples of operating parameters so measured include activated carbon addition rate and steam flow rates. Operating the combustion process and air pollution control equipment in accordance with these standards ensures compliance. These tests are scheduled well in advance of their performance, and facility operators are prohibited from altering operations in any way to improve emissions performance during the test.
For more information, read Covanta’s white paper, Waste-to-Energy Emissions.
Our Approach to Air Emissions Management
Our operations teams manage our performance on air emissions compliance through our Environmental Management Information System (EMIS), technical standards, training, and robust environmental procedures. Our EMIS allows us to track timely completion of compliance requirements and manage associated compliance data, which is reviewed monthly by the company’s senior management team alongside other environmental performance metrics.
Facilities that have the greatest opportunity to improve are automatically enrolled in our Environmental Improvement Plan (EIP), which requires the facility to identify the root causes of its air exceedances and create a customized plan to improve performance. Quarterly conference calls and face-to-face meetings are held to maintain progress and encourage communication between subject matter experts, facility personnel and corporate personnel. Facilities remain enrolled in an EIP until their performance improves.
In recent years, we have increased the number of facilities enrolled in the EIP to further leverage its proven track record of improving performance.
Covanta Fleet-Wide 2018–2020 WTE Emissions Compared to 2007
Covanta Americas 2018–2020 WTE Emissions Compared to Federal Standards
Select an individual U.S. facility to see detailed 2020 environmental performance on life-cycle GHG emissions, electricity generation, metals recycling, emissions performance relative to federal guidelines and emissions compared to total emissions in the local county.