# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 5 Results and Discussion
### Regulated Emissions: Experimental Results and Literature Comparisons
NMVOCs are mainly aldehydes, PAHs, dioxins and other compounds arising during the various stages of the combustion process but which do not reach complete oxidation. On the contrary, methane VOCs result from the fuel methane quota that remains unburned. The values listed in Table 2 refer to total VOCs while the limit set by the standards refers to NMVOCs because methane VOCs are not considered harmful to human health. Due to NMVOCs high global warming potential, they are harmful only to the environment. This aspect was not clear in the first Italian Standard [55]. Thus, the authors measured total VOCs. In this way, comparisons can be done also with the data available in the literature because they do not distinguish between methane and non-methane VOCs. It is interesting to note that biogas and natural gas total VOCs are comparable. To the authors' knowledge, in the literature only a few works have analysed biogas emissions. For this reason, after an in-depth literature review, the authors selected the work by [PERSON] et al. [41], which disclosed the emission factors detected at biogas and natural gas plants located in Denmark in the years 2000. In [PERSON] et al. 's investigation, biogas is mainly derived from agricultural wastes (manure), but they also included emissions analysis where biogas is produced at waste deposit sites and waste-water treatment plants. The general composition of biogas is: 65% methane and 35% CO\({}_{2}\); a very different content compared with those measured in this experimental campaign [35]. However, it is still valuable to compare [PERSON] et al. 's data with the authors' average weighted values of the emissions, despite the different biomass composition in input, diverse stages of ICEs development and the unknown regulatory limits set in Denmark during [PERSON] et al. 's experimental measurements. Despite the descrepances showed in Table 3, a general reduction in both biogas and natural gas emission values can be observed. NO\({}_{\text{x}}\) in biogas and natural gas emissions is largely reduced. Thus, from the 2000s to today, NO\({}_{\text{x}}\) emissions have more than halved. An reduction observable also in the case of CO, as previously, for both fuels. PM emissions in the authors' biogas plants are an order of magnitude lower than those reported by [PERSON] et al. while, in the case of natural gas, the authors' value is reduced by half of [PERSON] et al.'s. The SO\({}_{2}\) emissions of [PERSON] et al. and of the authors are comparable only in the case of biogas, while nothing can be said for natural gas due to the lack of reference. The only exception is the VOCs content in biogas units. Under analysis biogas units are characterised by a higher VOCs value compared to that detected by [PERSON] et al., while in the case of natural gas, the authors' computations show a lower value. This is probably related to actions on the legal limits and/or a technical evolution of the gas engines. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{**Authors**} & \multicolumn{2}{c}{**[PERSON] et al. [41]**} \\
**Substance** & **Biogas** & **Natural Gas** & **Biogas** & **Natural Gas** \\ \hline NO\({}_{x}\) & 626.00 & 195.00 & 1500.00 & 439.00 \\ CO & 494.00 & 166.00 & 758.00 & 457.00 \\ PM 2.5 & 0.05 & 0.27 & 0.57 & 0.42 \\ SO\({}_{2}\) & 61.00 & 0.27 & 52.80 & / \\ COV & 1087.00 & 985.00 & 706.00 & 1266.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average regulated emissions in the present work and in [PERSON] et al. [41]. The emissions are referred to the electrical energy unit and given as \(g\)\(G\)\(J_{(output)}^{-1}\).