The Challenge
Many industrial furnaces operate in oxygen depleted conditions, emitting high levels of ammonia, NOX, and other nitrogen-bearing compounds. This presents a significant challenge for manufacturers striving to meet air quality regulations and limit carbon output.
Emission streams that contain nitrogen bearing hazardous air pollutants (HAPs) cannot be thermally treated in a traditional oxidizer system without forming additional NOX, making adherence to an air permit difficult or impossible. In such cases, a secondary abatement device, downstream of the thermal oxidizer is required for additional NOX reduction.
Historically, the industry has relied on Selective Catalytic Reduction (SCR) systems to achieve this. The add-on technology injects ammonia or urea into the process stream before passing it through a specialized catalyst that converts the NOX into nitrogen gas (N2) and water vapor. However, this approach comes with significant operational burdens:
- The need for ammonia or urea storage, pumps, and customized control systems to maintain injection rates.
- High operating costs due to chemical consumption, catalyst maintenance, and periodic replacement.
- Fresh air must be added to dilute the inert process streams to safe levels, increasing the treatment volume and leading to condensation issues.
- Additional winterization equipment to prevent freezing in cold water climates.
It essentially requires a small chemical operation to remove the NOX and the staff to support it.
The Solution
Over the past four decades, Anguil has provided thousands of thermal and catalytic oxidizers around the world, many of which incorporated SCR technology. Recognizing the inefficiencies, engineers at Anguil came up with a better solution. The objective was to maintain very high levels of HAP removal, eliminate the catalyst and associated maintenance, prevent the need to dilute the stream making it safer to operate, and eliminate the use of add-in chemicals.
This solution was a Multi-Staged Direct Fired Thermal Oxidizer (MS-DFTO) – a groundbreaking approach that compartmentalizes the oxidation process across multiple temperature-controlled stages with varying oxygen levels, to dramatically reduce the formation of greenhouse gases.
In the MS-DFTO the nitrogen bearing HAPs are introduced into the first stage of the thermal oxidizer, the reducing zone, that operates at elevated temperatures, but without oxygen. This serves to disassociate the HAPs while not forming NOX from any nitrogen bearing compounds in the absence of oxygen. Special operational sequences ensure starved oxygen is maintained in the first stage. The high temperature, oxygen depleted gas leaving this first stage is then rapidly cooled in a second stage to near the oxidation temperature. At this cooler temperature, the gases are reintroduced to air in the third chamber where the complete combustion of compounds occurs. Design considerations are made to ensure that the temperature after oxidation does not escalate to where thermal NOX could be formed, generally above 1,800°F (982°C).
Specialized refractory insulation and highly automated controls for the various stages of the Anguil MS-DFTO minimize auxiliary natural gas consumption. Properly staging the operating conditions and temperature profile throughout the MS-DFTO will result in destruction rates of the hazardous compounds and volatile organic compounds with minimal NOX formation in a single system.
In select applications, a fourth stage of the MS-DFTO has been added to provide Selective Non-Catalytic Reduction (SNCR) as a safety factor to ensure even fewer NOX emissions. Experience has shown that these fourth SNCR stages are not necessary and rarely used even when supplied as the NOX emissions have always met permit requirements without the need for this polisher.
The Result
This staged combustor from Anguil is reshaping how industries approach greenhouse gas abatement from process furnaces. On this application, the Anguil MS-DFTO was treating 5,000 SCFM (8,025 Nm3/hr) of process air in a single abatement device with well over 99% HAP destruction and NOX emissions that were a small fraction of the allowable permit value. The equipment does not require chemical injections or active catalyst maintenance, meaning a heavily reduced operational cost relative to an SCR. Inert process streams can remain inert, meaning a safer system can be supplied that does not require upstream dilution. Treating a fraction of the air also means the upstream process ducting can be smaller, saving cost all around.
Industrial facilities continue implementing Environmental, Social, and Governance (ESG) strategies to comply with regulations, improve sustainability, and meet stakeholder expectations. This ESG initiative met those objectives and reduced equipment cost, footprint, and maintenance costs, while eliminating the carbon footprint from past chemical purchases.
