# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 3 Results and Discussion
### Estimation of Gas Emissions during Firing Process
This is due to the fact that clay contains one of the most important sources of fluorine such as phyllosilicates, therefore, the less clay in the mixture, the greater the reductions in fluorine emissions. Incorporating up to 90% of FAA and FAB reduced the emissions of HF by 42% and 60% compared to the 100% clay brick. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Raw Materials**} & \multicolumn{6}{c}{**Initial Content (\%)**} & \multicolumn{6}{c}{**Emissions (mg/kg) \({}^{*}\)**} \\ \cline{2-10}  & **Cl** & **F** & **S** & **C** & **N** & **HCl** & **HF** & **SO\({}_{2}\)** & **CO\({}_{2}\)** & **N** \\ \hline Clay B & 0.02 & 0.02 & 0.02 & 0.3 & - & 119.28 & 122.14 & 31.93 & 454.27 & - \\ FAA (low LOI) & 0.02 & 0.01 & 0.17 & 4.7 & - & 11.81 & 6.05 & 2992 & 17.844 & - \\ FAB (high LOI) & 0.02 & 0.01 & 0.31 & 24.4 & - & 76.19 & 39.01 & 6090 & 122,160 & - \\ \hline \hline \end{tabular} (-) <Detection Limit; (\({}^{*}\)) Gas emissions calculated according to the mass balance of Equation (1).  
\end{table} 
Table 4: Content and pollutants emissions of clay B and fly ashes A and B. The introduction of fly ash in the mixtures produced an increase in SO\({}_{2}\) emissions due to the high content of S present in the fly ash. The FAB/clay mixtures showed extremely high levels due to the presence of the combination of different sulphur associated with inorganic, pyritic, and organic carbon. The introduction of fly ash in the mixtures FAA/clay and FAB/clay increases the emissions of CO\({}_{2}\) due to the high contents of unburnt carbon that fly ash present. The increase in CO\({}_{2}\) emissions was higher for FAB due to the high contents of unburnt carbon (27.03%) before the firing process compared to FAA (5.43%) and clay. In this case, the CO\({}_{2}\) emissions from inorganic carbon were not important in an overall assessment. Considering the technological properties to market this type of brick according to AENOR Fired Clay Piece HD Clinker R-55 and the leaching behaviour, it could be possible to incorporate up to 20% of FAA and 10% of FAB. The reduction in emissions of HCl with these additions was 13% and 5%, respectively, according to reference emission values. As the HCl emission limit value was \(<\)160 ppm, the addition of 20% FAA and 10% FAB did not exceed the acceptable emission value proposed. In the case of HF, the introduction of FA supposes a reduction of 10-11%, respectively. The HF emission limit value was \(<\)180 ppm, therefore, the addition of 20% FAA and 10% FAB did not exceed the acceptable emission value. In the case of SO\({}_{2}\), the introduction of FA produced an increase of SO\({}_{2}\) emissions was 61% and 65%, respectively. The SO\({}_{2}\) emission limit value was \(<\)450 ppm, therefore, it exceeds the acceptable emission value, and an intervention to review the value would be advisable. For 20% of FAA and 10% of FAB in the mixtures, the increase in CO\({}_{2}\) emissions was 62% and 87%, respectively. It is important to note that the level of CO\({}_{2}\) emissions must be estimated based on the total annual production of the ceramic factory. Figure 12: Estimation of HCl, HF, SO\({}_{2}\), and CO\({}_{2}\) emissions for mixtures performed with FAA/clay B and FAB/clay B during the firing process including reference emission values proposed by [PERSON] et al. (2018).