# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 3 Biogas and Natural Gas Facilities Design Features  
The experimental campaign is preceded by an in-depth analysis of both the Italian biogas sector and the plant structure. In this manner, a significant number of biogas units can be selected and, then, experimentally analysed. Two basic concepts are followed during the selection of the facilities:  
* Units need to have the same nameplate thermal power of the ICE. * The mix of feedstock used as input needs to be different in all the investigated units. In this manner, plants are comparable because the ICEs have the same thermal and electrical power. However, being the mix of feedstock different, similarities and discrepancies in both the biogas composition and the engines' regulated and unregulated emissions can be highlighted. The sector analysis firstly highlights that the most widespread ICE design power is 999 kWel. Historical reasons dating back to 2011-2012 lie behind this size. In particular, the development of the Italian biogas sector was driven by the most generous feed-in tariff ever set in Europe: 280 EUR/MWh guaranteed for 15 years to units with design power up to 999 kWel. So, in 2011-2012, Italy registered a rocketing growth in biogas installations and the vast majority of the installed units were built with the maximum admissible electric power: 999 kWel. Based on this findings, the six selected units are made up of the same devices and powered by an internal combustion engine characterised by thermal and electrical nameplate power equal to 2459 and 999 kW, respectively. The ICE electrical efficiency is equal to 40.58% while the engine compression ratio and speed are respectively 12.5:1 and 1500 rpm. Additional details about the engine construction characteristics can be found in the manufacturer's datasheet [53]. As for the biogas units, the NG ICE is selected after a detailed analysis of the Italian cogeneration sector [54]. The selected engine has the same nameplate power, manufacturer and specifications. It also started operating in 2012 as the biogas ones. Therefore, biogas and natural gas engines are characterized by the same technological advances. In this way, their emissions are comparable. In terms of fuel consumption, a biogas ICE requires 547 Nm3 h-1 of raw biogas while an NG ICE takes 281 Nm3 h-1 of natural gas from the grid. Biogas ICEs release 5312 kg h-1 of waste gases at a temperature of 457 degC into the atmosphere. Note that the exhaust gases temperature is extremely high because the heat content of the waste gases is not recovered with a waste heat recovery unit (WHRU). Nonetheless, biogas engines are CHP units because the heat content of both the ICE cooling water and the lube oil is used to heat up a water flow rate which flows into the digester walls and guarantees control of their operating temperature. Among the analysed plants, BGP 1, 2 and 3 are not equipped with WHRU. Hence, the stack temperature varies from 470 to 500 degC. In the case of BGP 4, the engine is equipped with an organic Rankine cycle, while BGP 5, BGP 6 and NG ICE are equipped with a heat exchanger which produces hot water or steam. The installation of the WHRUs guarantees exhaust gases temperatures lower than 200 \({}^{\circ}\)C.  
Figure 1 presents the adopted diet and emissions limits for each biogas unit. Although the considered biogas plants are subject to emissions limits set in [55] and characterised by the same layout and power unit, the local authority sets different limits. In particular the BGP 2 NO\({}_{\rm x}\) and CO maximum values are respectively 10% and 50% lower than those set for BGP 1. No reasons can be found to explain such values, especially because both plants are located in the same Italian region and just a few kilometres away from each other. However, to fulfil the legal limits, the ICE calibration and control strategy are different compared to the other biogas units. Note also that, the VOCs limit set for BGP 1 and BGP 2 is 50% higher than in other facilities. Fortunately, in this case, the reason is easy to explain.