# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 5 Results and Discussion
### Regulated Emissions: Experimental Results and Literature Comparisons  
Table 2 lists the content of regulated emissions in both biogas and natural gas ICEs exhausts. Measurements reveal that nitrogen oxides exceed the prescribed limits in three cases. This is a consequence of a slow deviation of the engines from the best operating point. The cause is the variability of the biogas composition along the year. The natural gas engine \(\rm NO_{x}\) emission is 59% lower than the regulatory limit. Due to the constant fuel composition, it is easier to calibrate the NG ICE and run it at the best operating point. For this reason, since \(\rm NO_{x}\) are compounds easily measurable, it may be useful to equip both type of engines with a continuous measurement system. In this way, limit exceedance is avoided also in the case of fuel composition variations. Hydrogen chloride and carbon monoxide, regulated substances only for biogas, are always under the fixed limits. In particular, CO emissions are at least 40% lower than the prescribed limit, while HCl is higher than the instrument threshold only in the case of BGP 4. Nonetheless, the detected value is approximately 86% lower than the prescribed limit (10 mg \(\rm Nm^{-3}\)). \(\rm SO_{x}\) and particulates are regulated emissions only for natural gas ICEs. For them a maximum value of 35 mg \(\rm Nm^{-3}\) and 5 mg \(\rm Nm^{-3}\) is set, respectively. In the analysed NG plant, only \(\rm SO_{2}\) was detectable. As clearly shown in Table 2, the measured values for both \(\rm SO_{x}\) and PM are two orders of magnitude lower than the regulatory limits. Comparing the natural gas engine's CO, HCl, \(\rm SO_{2}\) and PM emissions with the biogas ones reveals that the BG plants' \(\rm SO_{2}\) emissions are not comparable with the NG ICE emissions. Only three plants do not exceed 35 mg \(\rm Nm^{-3}\) while the BGP 4 \(\rm SO_{2}\) emissions are 3.8 times higher than the regulatory limit set for natural gas ICEs. This outcome underlines the need, on the one hand, to monitor such substances but, on the other hand, to introduce an appropriate legal limit for \(\rm SO_{x}\) also in the case of biogas ICEs. VOCs are another set of compounds which need to be tracked only in the case of biogas. Analysing the VOCs values, it is clear that they exceeded the limit of 100 mg \(\rm Nm^{-3}\). However, in this case, a thorough discussion is needed. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Substance** & **Unit** & **BGP 1** & **BGP 2** & **BGP 3** & **BGP 4** & **BGP 5** & **BGP 6** & **NG** \\ \hline _Biogas_ & & & & & & & & \\ CH\({}_{4}\) & \% vol. & 53 & 53 & 51 & 53 & 55 & 47 & - \\ CO\({}_{2}\) & \% vol. & 44 & 44 & 48 & 45 & 44 & 41 & - \\ N\({}_{2}\) & \% vol. & 1.9 & 1.9 & 0.7 & 1.7 & 1.1 & 9 & - \\ O\({}_{2}\) & \% vol. & 0.4 & 0.4 & 0.1 & 0.4 & 0.1 & 2.4 & - \\ \hline _Exhaust gases_ & & & & & & & & \\ Flow rate (wet) & \(\rm Nm^{3}\) h\({}^{-1}\) & 5100 & 4850 & 4000 & 4130 & 4300 & 4900 & 4900 \\ Flow rate (dry) & \(\rm Nm^{3}\) h\({}^{-1}\) & 4200 & 4030 & 3500 & 3350 & 3700 & 4300 & 4250 \\ Temperature & \({}^{\circ}\)C & 503 & 505 & 470 & 171 & 267 & 227 & 149 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Exhaust gases physical characteristics. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Substance** & **BGP 1** & **BGP 2** & **BGP 3** & **BGP 4** & **BGP 5** & **BGP 6** & **NG** \\ \hline \(\rm NO_{x}\) & 420 & 455 & 599 & 547 & 486 & 363 & 143 \\ CO & 474 & 238 & 356 & 406 & 465 & 163 & 122 \\ HCl & \(<\)0.1 & \(<\)0.1 & \(<\)0.2 & 1.4 & \(<\)0.2 & \(<\)0.1 & 0.2 \\ VOCs & 809 & 1060 & 658 & 862 & 546 & 789 & 724 \\ SO\({}_{2}\) & 74 & 24 & 18 & 132 & 48 & 26 & 0.2 \\ PM & - & - & - & 1.1 & \(<\)0.1 & 0.1 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Content of regulated emissions in both biogas and natural gas engines exhausts. Values are given in mg \(\rm Nm^{-3}\). Total VOCs are composed of methane and non-methane VOCs.