# On the Issues of \(\mathbf{NO_{x}}\) as Greenhouse Gases: An Ongoing Discussion...
## 6 NOx in Waste Incineration and Co-Incineration Plants  
Thermal processing of waste, especially combustion, can generate NOx emissions. The combustion of MSW and RDF in incineration and co-incineration plants generates NOx inside a furnace. The underlying role of incineration and co-incineration plants in the issue of NOx emissions can be explained by the emerging concerns that the high nitrogen content (usually) in municipal waste can influence the enhanced NOx emissions. In fact, N-fuel directly impacts NOx emissions. Nevertheless, nowadays, these plants are equipped with an advanced flue gas cleaning system. The consideration of NOx emissions from the WHe plant is preliminarily considered as the issue of nitrogen content in burned fuels. It is known that the nitrogen content in solid fuel directly impacts NOx emissions. The low N content in biomass (especially for wood and woody biomass) is a big advantage due to decreased NOx and ammonia (HN3) emissions [80]. The nitrogen in woody biomass (mean value, dry-ash-free, daf state) is 0.4 wt%, whereas the nitrogen content in waste is usually higher and can reach a value of more than 2.3 wt% [81]. Considering the particular components of MSW, a higher nitrogen content was observed in plastics (polyurethane N = 6 wt%, db), biomass residues (garden trimmings N = 3.4 wt% db), and food wastes (N = 2.6 wt% db) [82]. The nitrogen content varies in the waste sources in terms of regions, countries, and season. For example, [PERSON] and colleagues [83] reported that nitrogen content in waste from China in the period of 2015-2021 varied between 0.37 and 2.11 wt% depending on the region. The NOx in MSW incinternators is mainly produced by fuel NOx and thermal NOx, accounting for about 80% and 20%, respectively [84]. Behind the fuel properties, the formation of NOx depends on the different processes and technological parameters, mainly the furnace type, the temperature in the combustion zone, air distribution, oxygen excess, turbulence, and residence time of gaseous compounds in the combustion and post-combustion zone [82]. It is accepted that the design of the furnaces can achieve gas temperatures that fit the conditions of selective non-catalytic reduction by ammonia or urea to remove about 50% of the NOx formed initially [85]. The nitrogen concentration in stack emissions of MSW incinternations was observed in the range of 90-150 mg/m\({}^{3}\) and 65-145 mg/m\({}^{3}\) (11% O\({}_{2}\), STP) for FBC and grate furnaces, respectively [82]. Nevertheless, the concentration of NOx formed in the combustor, measured before the flue gas cleaning system can be much higher (i.e., up to 950 mg/m\({}^{3}\) (11% O\({}_{2}\) STP)). The highest in-chamber NOx concentration was observed during coal and waste blends (i.e., 470-950 mg/m\({}^{3}\) for meat and bone meal + coal and 220-550 mg/m\({}^{3}\) for paper and plastic waste + coal) [82]. There are various ways to reduce NOx emissions from incineration plants. The most important techniques are divided in two ways, namely primary and secondary measures [37]. The primary measures include every technique realized inside the combustion chamber such  
as air and/or fuel staging and/or special additives such as iron-containing additives. The secondary measures included every method realized with additional and independent apparatus outside the combustion chamber. Within these secondary measures, the following methods are included: selective catalytic reduction, SCR, selective non-catalytic reduction SNCR, non-thermal plasma, O\({}_{3}\) application, adsorption, and absorption processes, and photocatalytic processes [37; 86; 87]. Note that SNCR can be included in primary or secondary measures depending on where the process is realized (i.e., where a NH\({}_{3}\)-containing additive is injected). The efficiency of NO\({}_{x}\) removal depends on the applied method.