# Flue Gas Composition and Treatment Potential of a Waste Incineration Plant
## 2 Methods and Materials
### Nitrogen Oxide (Nox) Emissions and Reduction  
Nitrogen oxides (NOxs) are the main cause of acid rain and smog and are formed by a burning process known as oxidation. NOx is formed in two ways, either by oxidation of fuel-bound nitrogen compounds called fuel NOx, or by oxidation of the nitrogen present in the air at high temperatures, known as thermal NOx. Consequently, the NOx produced by combustion remains in the range of 250-450 mg/Nm\({}^{3}\) at 11% dry O\({}_{2}\)[13; 24]. See Table 8 for the new directives regarding NOx emissions, where the average NOx for new and old plants is around 135 mg/Nm\({}^{3}\). Normally, NOx emissions are reduced by reaction with ammonia by using the selective non-catalytic reduction (SNCR) DeNOx type. The ammonia comes from an aqueous ammonia solution or from the thermal decomposition of a solid or aqueous urea solution. The following reduction reactions take place in the temperature range of 850-1050 \({}^{\circ}\)C [28]:  
\[4\text{NO}+4\text{NH}_{3}+\text{O}_{2}\to 4\text{N}_{2}+6\text{H}_{2} \text{O}\]  
\[2\text{NO}_{2}+4\text{NH}_{3}+\text{O}_{2}\to 3\text{N}_{2}+6\text{H}_{2} \text{O}\] \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Facility**} & \multirow{2}{*}{**Waste Type**} & **Flow Rate** & **CO\({}_{2}\)** & **O\({}_{2}\)** & **Auxiliary Fuel** & **Amount of Daily** & **Gross Calorific** \\  & & **(m\({}^{3}\)/h)** & **Volume \%** & **Volume \%** & **(m\({}^{3}\)/day)** & **Waste Input** & **Value (KJ/kg)** \\ \hline A & MSW & 50,888 & 8.56 & 10.6 & 2779 & 176 & 14,690 \\ B & MSW & 57,250 & 9.75 & 9 & 1390 & 213 & 13,840 \\ C & MSW & 57,423 & 10.2 & 8.79 & & 236 & 14,650 \\ D & MSW & 36,120 & 9.72 & 10 & & 114 & 13,540 \\ E & IW & 62,371 & 6.95 & 12.7 & & 173 & 32,260 \\ F & IW & 32,712 & 8.68 & 10.8 & & 90 & 25,470 \\ G & IW & 15,082 & 8.29 & 11.4 & & 66 & 31,460 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Characteristics of real incineration facilities (adapted from [23]). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Reactants**} & \multicolumn{3}{c}{**Resulting Material**} \\ \cline{2-7}  & \(\text{C}_{6}\text{H}_{10}\text{O}_{5}\) & \(\text{O}_{2}\) & \(\text{N}_{2}\) & \(\text{CO}_{2}\) & \(\text{H}_{2}\text{O}\) & \(\text{N}_{2}\) \\ \hline No. of moles & 1 & 6 & (6 \(\times\) 3.76) & 6 & 5 & 22.56 \\ Atomic weights & 162.16 & 32 & 28.02 & 44.01 & 18.02 & 28.02 \\ Molecular weights & 162.16 & 192 & 632.13 & 264.06 & 90.1 & 632.13 \\ Material weight to & 1 & 1.19 & 3.9 & 1.63 & 0.56 & 3.9 \\ carbon weight & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The chemical properties of cellulose interacting with air. It should be noted that the maximum reduction in NOx emissions is limited to between 60 and 70%. This is because of the NH\({}_{3}\) slip at the stack prevention. Ammonia is usually dosed at a maximum molecular ratio (\(\beta\)) of between 1 and 1.25 [28]. Sometimes, the SNCR technique is not adequate to raise the concentration of NOx emissions to the required level of the BAT; therefore, an advance reduction phase is required by adding a selective catalytic reduction (SCR). In the SCR, the DeNOx reactions take place with a high efficiency on the surface of the catalyst, at a lower temperature range of around 350 \({}^{\circ}\)C. To increase the required oxygen atoms, vanadium pentoxide (V\({}_{2}\)O\({}_{5}\)) is used as a catalyst because it can recover oxygen from the flue gas content. As mentioned, SCR working with a low temperature field can be erected either at the end-tail of the flue gas treatment and filtering unit or in an in-between position (between a filter unit and electrostatic precipitator [ESP]) (as shown in Figure 3). For the SCR tail-end arrangement, there are two possible options; hot, 220-260 \({}^{\circ}\)C, or cold, 190 \({}^{\circ}\)C. These two options provide some flexibility for working in different temperature profiles of the flue gas treatment stream.