Ecosustainable biomethane and fertilizerproduction through anaerobic co-digestionof animal manure and energy crops

In Italy and many European countries energy production from biomass is encouraged by strong economic subsidies so that renewable energy plants, anaerobic digestion plant producing biogas in particular, are getting large diffusion. Nevertheless, it is necessary to define the environmental compatibility as well as technological and economic issues dealing with the emerging renewable energy scenario. This evaluation should take into account global parameters as well as environmental impacts at regional and local scale coming from new polluting emissions. The environmental balances regarding new energy plants are of primary importance within very polluted areas such as Northern Italy where air quality limits are systematically exceeded, in particular for PM10, NO2 and ozone. The most important environmental shortcomings that should be solved or at least minimized as far as biogas production and utilisation are concerned are: 1. macro-pollutants emissions from biogas engine at the local scale and low fuel utilization index (biogas plants generally don’t recover all thermal energy at disposal); 2. indirect GHG emissions, mainly involving post-methanation emissions from the digestate storage; 3. ammonia emissions from the storage and land spreading of digested materials, low fertilising efficiency of manure and digestate, nitrate contamination of groundwater. The described emissions and energy inefficiency could involve negative environmental balances at the local scale, conflicting with the possible benefits arising from biomass energy production. An alternative technological choice for biogas valorisation could be biomethane production (also called green gas) through biogas purification and upgrading processes in order to remove CO2 and trace components. Biomethane production and its injection into natural gas grid (or its use as a transport vehicle fuel) could bring about strong energy and environmental benefits such as higher energy efficiencies and lower specific emissions (district heating CHP units, combined cycle gas turbines, methane powered vehicles). The present study mainly aims at analysing biogas upgrading techniques under the aspects of energy consumptions and environmental sustainability, with a specific focus on minimizing methane losses from the process by means of suitable design and operative choices (temperature, pressures, sorbents, recirculation strategies, etc.) that are fully described and simulated. The considered upgrading techniques are based on the principles of physical and chemical absorption and pressure/vacuum swing adsorption (PSA). The analysis highlights that there are strong differences among the examined upgrading techniques, as far as specific sorbent flows, absorbing tower dimensions, methane losses, power required, recoverable heat and environmental impacts (use of resources, gaseous releases of odorous and polluting molecules, GHG balances) are concerned. In particular, all the analysed upgrading techniques could be designed in order to achieve very low methane slip, below 0.1%, except PSA for which methane losses are hardly reducible below 2%, even at very high energy consumptions. The actual range of methane slip for the considered technologies is 0.1÷5% whereas the energy consumption to upgrade biogas lies in the range 0.05÷0.54 kWhe/m3 of raw biogas. The following analysis reports also some economic evaluations including electric energy costs, thermal energy requirements, biomethane sale incomes and external costs due to environmental impacts of biogas production+upgrading techniques. Within the described cost-benefit approach, the best overall balances seems to be assured by absorption with DEPG and chemical absorption with MEA. Finally, the last part of the present work shows a technical analysis of a specific digestate treatment process that could help reaching both the reduction of GHG and ammonia emissions and, at the same time, the production of fertilizers. The present analysis therefore confirms that biogas/biomethane technology is absolutely ready and suitable to reach very high levels of productivity, efficiency and environmental performances at sustainable costs and the right technological approach could solve many environmental problems regarding nitrate contamination of groundwater, ammonia emissions and global warming issues.