# On the Issues of \(\mathbf{NO_{x}}\) as Greenhouse Gases: An Ongoing Discussion...
## 1 Introduction
### No\({}_{x}\): The Matter of Definition  
To find the right direction in the debate over the role of NO\({}_{x}\) in the GHE, the definition of NO\({}_{x}\) should be reiterated: NO\({}_{x}\) emissions are the sum of NO and NO\({}_{2}\)[7]. The classification of NO and NO\({}_{2}\) into one group (NO\({}_{x}\)) is related to the phenomenon of NO conversion in the atmosphere into NO\({}_{2}\) in the presence of ozone (this is a simplified representation of a complex process). The time taken for NO to turn into NO\({}_{2}\) in atmospheric air conditions is a few minutes. Subsequently, NO\({}_{2}\) can be transformed into other nitrogen compounds within a period of hours to days [8]. It should be noted that in addition to NO and NO\({}_{2}\), N\({}_{2}\)O can also be emitted from the combustion process. The highest emission of N\({}_{2}\)O (from combustion processes) is observed in fluidized-bed combustors. The concentration in flue gas from these combustors can be in the range of 40-100 ppm [9]. Although N\({}_{2}\)O belongs  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Case** & **Assumptions** \\ \hline Coal combustion (100\%) & LHV = 22 MJ/kg, new installation, continuous measurement (daily average values) \\ Biomass combustion (100\%) & LHV = 18 MJ/kg, new installation, continuous measurement (daily average values) \\ RDF combustion (100\%) & LHV = 15 MJ/kg, new installation, continuous measurement (daily average values) \\  & LHV = 22 MJ/kg (coal) and LHV = 15 MJ/kg (RDF), \\  & RDF share of 10\% (by energy), new installation, \\ Coal/RDF co-firing (10\% by energy) & continuous measurement (daily average values), \\  & installation capacity (thermal power in fuel) of 1.5 MW, \\  & 15 MW, 105 MW, and 305 MW \\  & LHV = 22 MJ/kg (coal) and LHV = 15 MJ/kg (RDF), \\  & RDF share of 10\%, 20\%, 30\%, 40\%, 50\%, 60\%, 70\%, \\ Biomass/RDF co-firing & 80\%, and 90\% (by energy), new installation, \\  & continuous measurement (daily average values), \\  & installation capacity (thermal power in fuel) of 1.5 MW, \\  & 15 MW, 105 MW, and 305 MW \\ \hline \hline \end{tabular}
\end{table}
Table 1: The assumptions during the calculation of NO\({}_{x}\) emission standards in different cases.  
to the group of direct GHGs, it cannot be classified as NO\({}_{\rm x}\). In addition, the term NO\({}_{\rm y}\) is also seen in the literature, which contains all reactive nitrogen compounds that exist in atmospheric air. Examples of NO\({}_{\rm y}\) are HNO\({}_{\rm 3}\), C\({}_{2}\)H\({}_{3}\)NO\({}_{\rm 5}\) (peroxyacetyl nitrate (PAN)), HONO, N\({}_{2}\)O\({}_{5}\), HO\({}_{2}\)NO\({}_{\rm 2}\), NO\({}_{\rm 3}\), and NO\({}_{\rm x}\)[8]. Because NO\({}_{\rm x}\) belong to NO\({}_{\rm y}\), the remainder of the listed compounds are referred to as NO\({}_{\rm z}\). Therefore, NO\({}_{\rm x}\) + NO\({}_{\rm z}\) = NO\({}_{\rm y}\). The main conversion processes of NO\({}_{\rm x}\) into NO\({}_{\rm z}\) (NO\({}_{\rm x}\) \(\rightarrow\) NO\({}_{\rm z}\)) are NO\({}_{\rm 2}\) + NO\({}_{\rm 3}\)\(\rightarrow\) N\({}_{\rm 2}\)O\({}_{\rm 5}\) and PAN formation, which account for 55% and 34% of the total NO\({}_{\rm x}\)\(\rightarrow\) NO\({}_{\rm z}\), respectively. The opposite process (NO\({}_{\rm z}\)\(\rightarrow\) NO\({}_{\rm x}\)) is also possible, and the main process components of this are the reverse reactions N\({}_{\rm 2}\)O\({}_{\rm 5}\)\(\rightarrow\) NO\({}_{\rm 2}\) + NO\({}_{\rm 3}\) and PAN decomposition [10].