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The reduction of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">C</mi><msub><mrow><mi mathvariant="normal">O</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></semantics></math></inline-formula> emissions in hard-to-abate industries is described in several proposals on the European and National levels. In order to meet the defined goals, the utilization of sustainable, non-fossil fuels for process heat generation in industrial furnaces needs to be intensified. The focus mainly lies on hydrogen (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">H</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></semantics></math></inline-formula>) and its derivates. Furthermore, biofuels, e.g., dimethyl ether (DME), are considered. Besides possible changes in the process itself when substituting natural gas (NG) with alternative fuels, the emission of nitrogen oxides (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">N</mi><msub><mrow><mi mathvariant="normal">O</mi></mrow><mrow><mi mathvariant="normal">x</mi></mrow></msub></mrow></semantics></math></inline-formula>) is a major topic of interest. In current European standards and regulations, the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">N</mi><msub><mrow><mi mathvariant="normal">O</mi></mrow><mrow><mi mathvariant="normal">x</mi></mrow></msub></mrow></semantics></math></inline-formula> emissions are specified in mg per m³ of dry off-gas and refer to a reference oxygen concentration. Within this study, this limit specification is investigated for its suitability for the use of various fuel-oxidizer combinations in industrial combustion applications. Natural gas is used as a reference, while hydrogen and DME are considered sustainable alternatives. Air and pure oxygen (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi mathvariant="normal">O</mi></mrow><mrow><mn>2</mn></mrow></msub></mrow></semantics></math></inline-formula>) are considered oxidizers. It is shown that the current specification, which is built on the use of fossil fuels, leads to non-comparable values for alternative fuels. Therefore, alternative <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">N</mi><msub><mrow><mi mathvariant="normal">O</mi></mrow><mrow><mi mathvariant="normal">x</mi></mrow></msub></mrow></semantics></math></inline-formula> limit definitions are discussed in detail. The most suitable alternative was found to be mg per kWh. This limit specification is finally being investigated for its compliance with current regulations on various aspects of Continuous Emission Monitoring Systems.