# On the Issues of \(\mathbf{NO_{x}}\) as Greenhouse Gases: An Ongoing Discussion...
## 6 NOx in Waste Incineration and Co-Incineration Plants
For example, by applying air staging, NO\({}_{x}\) emissions can typically be reduced by 30-60%, whereas during the application of SCR and SNCR techniques, efficiency at a level of 50-85% can be achieved [82]. [PERSON] and colleagues [84] reported that during the optimal conditions of coupling technologies (i.e., flue gas recirculation and SNCR), the total NO\({}_{x}\) removal efficiency reached over 76% for the grate furnace incinerator. The regulations governing flue gas emissions were becoming more stringent and varied. Depending on the country, the emission limits for NO\({}_{x}\) are in the range of 131.7-522.6 mg/m\({}^{3}\) (11% O\({}_{2}\), STP) [88]. The regulations of the emissions can help to estimate the maximal possible emission of NO\({}_{x}\) to the atmosphere from incineration and WtE plants. It is assumed that the emission of NO\({}_{x}\) from particular plants can reach a level up to the regulation limit. In fact, the emission of gaseous pollutants and particulates from these plants is usually lower than the limits. Moreover, it should be explained that despite the EU regulations and standards (Directive 2010/75/EU of the European Parliament and of the Council of 24 November 2010 on industrial emissions), other documents introduce additional requirements for the emission. Namely, best available techniques are more strict (i.e., 120 mg/m\({}^{3}\) 11% O\({}_{2}\) STP) compared to the EU standards (i.e., 200 mg/m\({}^{3}\) 11% O\({}_{2}\) STP). The analysis presented below includes the different scenarios in terms of waste ratio during co-combustion as well as the capacity of incineration or co-incination plants. EU Directives as well as BAT regulations were taken into account. It was assumed that both the EU Directives and BAT regulations should be fulfilled, thus in every case, the most restrictive (EU or BAT) value was used in the analysis. The results of the calculations of standards are presented in Tables 4 and 5. All values are presented as mg referred to as 11% O\({}_{2}\) (STP). It should be mentioned that the presented values are the required concentrations that cannot be exceeded (as daily averaged values). These requirements should be fulfilled (executed) by the combustion/incineration/co-incination plant. In other words, the acts of law define the standard values and the values that cannot be exceeded. For example, in the case of coal combustion and installations in the range of 1 to 5 MW, the standard value is 267 mg/m\({}^{3}\) (11% O\({}_{2}\) STP), but the required value (it cannot be exceeded) for continuous measurement is 400 mg/m\({}^{3}\) (11% O\({}_{2}\) STP) as the 150% of standard value. The most restrictive standards are observed for the co-incineration of biomass and RDF for large plants. In the case of a 305 MW plant and 10% co-firing, the required value is 75 mg/m\({}^{3}\). Table 5 represents the calculations of the value for the co-firing of biomass and RDF in the share range of 0-100% (by energy). These results are also presented (required values only) in Figure 4. For installations with a fuel capacity of 1.5 MW (standard range 1 \(\div\) 5 MW), the NO\({}_{x}\) standard decreases with the increase in waste share in the fuel mixture. Thus, the addition of waste to the mixture reduces the permissible emission of NO\({}_{x}\) to the environment. The opposite tendency was observed for the installations of a capacity higher than 50 MW. Namely, the increase in waste share caused the increase in NO\({}_{x}\) standards. Thus, in the case of larger installations, the addition of waste caused less restrictive limits of NO\({}_{x}\) emissions. In the case of medium-capacity installations (i.e., 15 MW), the addition of waste to the fuel mixture does not significantly change the emission limit. Consequently, the decrease in NO\({}_{x}\) emissions (due to legislation impact) will be suspected when co-incineration is realized in smaller plants.