# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 1 Introduction  
Biogas fuel is produced from the natural degradation of organic materials by micro-organisms under anaerobic conditions. The process is called anaerobic digestion (AD), and commonly used feedstocks are energy crops, animal manure and slurries, municipal organic waste, industrial and agricultural waste and residues, sewage sludge, etc. Considering that these feedstocks are available worldwide, biogas can be produced all around the world with micro to large facilities. Typically, the biogas fuel is composed of methane, carbon dioxide, and other impurities like water, hydrogen sulphide, nitrogen, oxygen, ammonia, siloxanes and particles. After the cleaning process, the produced biogas can be combusted in boilers for heating purposes or in internal combustion engines (ICEs) and gas turbines for electricity production only (see, e.g., [1; 2; 3]). Properly arranged ICEs, gas turbines or boilers equipped with steam or organic Rankine cycle turbogenerators allowbiogas to be burned and simultaneously generate heat and electricity, this arrangement being the so called combined heat and power (CHP) unit (see, e.g., [4; 5; 6]). Alternatively, if the application requires a high energy content fuel, the biogas needs to be upgraded. By cleaning the biogas from impurities and separating the carbon dioxide, the energy content of upgraded biogas (typically called biomethane) becomes comparable to that of natural gas (NG). Biomethane can be directly injected into the natural gas network and/or stocked in tanks and then used as a substitute for fossil fuels in automotive applications (see, e.g., [7; 8; 9; 10; 11]). Therefore, due to its availability and versatility of use, biogas can be, on the one hand, an opportunity for developed and developing countries to  
* Reduce their fossil fuels dependency. * Develop wastes and residues supply chains. * Extend the use of biofuels in both the energy and transport sectors. * Mitigate climate change effects thanks to the reduction of greenhouse gases emissions. On the other hand, biogas production is stable and programmable. Hence, the electricity generation from biogas does not suffer from variability and unpredictability like wind and solar power. There is no risk of a large and unforeseeable mismatch between electricity demand and production, a gap that is hard to manage and which can result in grid faults or blackout. Note that techniques and devices able to manage the issues induced by the variability of wind and solar production are being widely investigated in the last few decade. As shown by the large number of published works, researchers and, among them, also the authors of the present work, focused, firstly on fossil fuels-based units flexibilization [12; 13; 14; 15; 16] and, then on: (i) innovative energy storage technologies like pumped thermal energy systems [17; 18; 19; 20], liquid air energy storage [21], hydrogen [22], etc. and (ii) dynamic energy management of electric grids with high renewable penetration [23; 24]. Therefore, biogas is an attractive renewable energy pathway especially for countries strongly dependent on fossil fuels imports. However, it can also be an issue for the environment as well as a cause of the reduction or even disruption of food supplies [25; 26; 27; 28; 29; 30; 31]. In fact, massive government supports can force farmers to redirect large agricultural areas from food to energy crops cultivation and force the heavy use of chemical substances like pesticides, fertilisers, etc. Despite these justified concerns, biogas is playing a key role in the energy sector decarbonization. In 10 years (2008-2017), the biogas installed electricity capacity (IEC) grew from 6699 MW to 16,915 MW. However, despite the IEC double-up, it constituted only the 0.77% of the total renewable energy IEC [32].