Low NOx Thermal Oxidizers

A low NOx thermal oxidiser is the right choice where the permit sets NOx limits below what a standard thermal oxidiser produces, typically below 50 to 100 ppm, as in urban areas with strict air quality rules, regions under the EU Industrial Emissions Directive and facilities near residential areas. Chemical, petrochemical, pharmaceutical, semiconductor, coating and printing plants are the usual buyers. NOx forms through thermal fixation of atmospheric nitrogen at high flame temperature and through oxidation of nitrogen compounds in the waste gas or fuel. A standard thermal oxidiser typically produces 100 to 300 ppm NOx or more, and CRA's low NOx design brings this below 50 to 100 ppm.

CRA's low NOx thermal oxidiser combines multi-stage burner technology, flue gas recirculation (FGR) and precise temperature control. Staged combustion runs a fuel-rich primary zone that lowers peak flame temperature and a lean secondary zone that completes combustion at lower temperature. FGR returns 10 to 30% of the cooled exhaust to the combustion zone, which dilutes the flame with CO₂ and water vapour and lowers the peak temperature that drives thermal NOx. Low NOx burners with internal recirculation, and lean-premix technology as an option able to reach below 25 ppm NOx, add further reduction. Extended residence time allows lower temperatures while VOC destruction stays above 99.9%, with no SCR catalyst needed.

A low NOx thermal oxidiser manufactured by CRA is designed for VOC and HAP destruction above 99.9% while limiting NOx to below 50 to 100 ppm, or lower depending on configuration, with CO typically below 100 ppm. Compared with a conventional design, NOx is reduced by 50 to 90% with no loss of destruction efficiency and comparable energy use. High temperature favours VOC destruction and raises NOx, while low temperature cuts NOx and risks incomplete oxidation, and staged combustion with precise control holds both. In operation the plant checks performance through continuous NOx and CO monitoring, periodic VOC stack testing and data logging.

A low NOx thermal oxidiser should be chosen when the permit limit of 50 to 100 ppm is reachable with primary measures, when capital cost matters, when operation is intermittent and when simplicity is preferred. It has no catalyst, SCR reactor or ammonia system, so there is no reagent consumption, catalyst replacement, catalyst warm-up or ammonia slip. A standard oxidiser with SCR is the better route when the limit is very low, below 25 to 50 ppm, when continuous operation justifies the catalyst investment, or when SCR is being retrofitted to an existing oxidiser. CRA evaluates the NOx limit, operating profile and lifecycle cost for each project before recommending.

The figures in these FAQs are general guidance for low NOx thermal oxidiser service, and the NOx and destruction performance of a specific unit depend on the waste gas composition, its nitrogen content, flow and concentration swings, the fuel, the permit limits and the site conditions. CRA reviews the process data, including turndown and upset cases, before sizing the burner, flue gas recirculation, chamber and controls. For a specific application, send the waste gas analysis, flow cases and the NOx, CO and VOC limits that apply, and CRA's engineering team reviews the design basis with you before any proposal is prepared.