# Greenhouse Residues' Potential for Biogas Production
## 1 Introduction  
Currently, the energy crisis is dominant due to the growing population and the total dependence of human civilization on depleting mineral resources. Therefore, the interest of scientists has been focused on developing environmentally friendly technologies using renewable energy sources. The gross electricity production in the EU was driven by renewable energy sources in 2020 (1086 TWh), surpassing the electricity produced from all fossil fuels (1012 TWh) [1]. It is a fact that the electricity from renewables doubled from 2000 to 2020, reaching a 39% share of the total electricity produced in 2021 in the EU [1]. However, the electricity production from biogas has remained a small fraction of renewable electrical energy (5-6%) from 2011 to 2021, while the share in electricity coming from hydroelectric plants decreased from 87% to 34.5%, and the energy from wind and solar photovoltaic systems has increased from 4.9% to 36.6% and 0.0% to 12.8%, respectively, from 2000 to 2021 [1]. The relatively high costs of biogas power generation compared to wind and solar photovoltaic energy systems may limit the further development of the biogas sector. On the other hand, biogas plants are more flexible to operate. Wherever heat off-take is available, an additional energy source (waste heat) can be exploited [2]. The vast opportunities of exploiting biomethane from biogas upgrading are expected to boost the biogas sector. It is recognized that although the combined biogas and biomethane share in the global bioenergy demand is approximately 5% (in 2018), they are the rapidly growing bioenergy sectors, and they are expected to reach a 12% share according to the Stated Policies Scenario(STEPS), and a 20% share according to the Sustainable Development Scenario SDS (STEPS), by 2040 [2]. There is a variety of feedstocks (usually referred to as biomass) which can be used for biogas production. The municipal biomass consists of domestic organic waste, green waste, such as grass from parks and gardens, and sewage sludge. The agro-industrial sector produces agricultural residues (crop residues left in the field after harvesting, such as stems, stalks, leaves, or residues during crop processing) and industrial wastes from food/fruit manufacturing, mainly peels and oil cakes [3]. In addition, biomass also includes livestock waste (manure), energy crops, and wood processing waste (wood chips, wood logs). Recently, plant biomass has been used for energy production. Approximately 5 billion hectares are estimated to be used for agricultural production worldwide [4], generating large quantities of plant residues. In Greece, there were more than 63,000 hectares cultivated for vegetable production in 2019, according to the Hellenic Statistical Authority [5], most of which produce vast amounts of residues in the form of leaves and stems. Each year, agricultural residues are disposed of by incineration, land application (mulching, composting), and landfilling [6]. The common practice of burning the residual biomass in the fields releases particulate matter, carbon dioxide (CO\({}_{2}\)), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), ammonia (NH\({}_{3}\)), volatile organic compounds (VOC), and non-methane hydrocarbons (NMHC) [7, 8], causing acid rain and environmental pollution. In addition, it introduces toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs) and phenols, into the atmosphere [9, 10]. These pollutants create smog, which is dangerous for public health. Moreover, leaving the agricultural residues in the fields for a herd of grazing animals might be a good option, but it should be done in an organized way [11, 12].