The production of biogas through anaerobic digestion is one of the technological solutions to convert biomass into a readily usable fuel. Biogas can replace natural gas, if the biogas is upgraded to green gas. To contribute to the EU-target to reduce Green House Gases emissions, the installed biogas production capacity and the amount of farm-based biomass, as a feedstock, has to be increased. A model was developed to describe a green gas production chain that consists of several digesters connected by a biogas grid to anupgrading and injection facility. The model calculates costs and energy use for 1 m3 of green gas. The number of digesters in the chain can be varied to find results for different configurations. Results are presented for a chain with decentralized production of biogas, i.e. a configuration with several digesters, and a centralized green gas production chain using a single digester. The model showed that no energy advantage per produced m3 green gas can be created using a biogas grid and decentralized digesters instead of one large-scale digester. Production costs using a centralized digester are lower, in the range of5 Vct to 13 Vct per m3, than in a configuration of decentralized digesters. The model calculations also showed the financial benefit for an operator of a small-scale digester wishing to produce green gas in the cooperation with nearby other producers. E.g. subsidies and legislation based on environmental arguments could encourage the use of decentralized digesters in a biogas grid.
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A transparent and comparable understanding of the energy efficiency, carbon footprint, and environmental impacts of renewable resources are required in the decision making and planning process towards a more sustainable energy system. Therefore, a new approach is proposed for measuring the environmental sustainability of anaerobic digestion green gas production pathways. The approach is based on the industrial metabolism concept, and is expanded with three known methods. First, the Material Flow Analysis method is used to simulate the decentralized energy system. Second, the Material and Energy Flow Analysis method is used to determine the direct energy and material requirements. Finally, Life Cycle Analysis is used to calculate the indirect material and energy requirements, including the embodied energy of the components and required maintenance. Complexity will be handled through a modular approach, which allows for the simplification of the green gas production pathway while also allowing for easy modification in order to determine the environmental impacts for specific conditions and scenarios. Temporal dynamics will be introduced in the approach through the use of hourly intervals and yearly scenarios. The environmental sustainability of green gas production is expressed in (Process) Energy Returned on Energy Invested, Carbon Footprint, and EcoPoints. The proposed approach within this article can be used for generating and identifying sustainable solutions. By demanding a clear and structured Material and Energy Flow Analysis of the production pathway and clear expression for energy efficiency and environmental sustainability the analysis or model can become more transparent and therefore easier to interpret and compare. Hence, a clear ruler and measuring technique can aid in the decision making and planning process towards a more sustainable energy system.
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One potential renewable energy resource is green gas production throughanaerobic digestion (AD). However, only part of the biogas produced (up to50-60%) contains the combustible methane; the remainder are incombustiblegasses with the biggest being carbon dioxide. These gasses are often not usedand expelled in the atmosphere. Through the use of BIO-P2M where hydrogenis mixed with the remaining CO2 additional methane can be produced,increasing the yield and using the feedstocks more effectively. Within thisresearch the environmental sustainability and effectiveness of BIO-P2M isevaluated using the MEFA and aLCA method, expressed in; net green gasproduction, efficiency in (P)EROI, emissions in GWP100, and environmentalimpact in Ecopoints. The functional unit is set as a normal cubic meter ofGroningen quality natural gas. Results indicate a net improvement of allindicators when applying BIO-P2M in several configurations (in situ, ex situ).When allocating the production of renewable energy to the BIO-P2M systemenvironmental impacts for wind the results are still positive; however, whenusing solar PV as an energy source the environmental impact in Ecopointsexceeds that of the reference case of Groningen natural gas. An additionaloption for improving the indicators is optimization of the process. When usingBIO-P2M combined with heat and power unit for producing the internalelectricity and heat demands all indicators are improved substantially. On anational scale when utilizing al available waste materials for the BIO-P2Msystem around 1217 MNm3/a of green gas can be produced, which is 3% ofthe total yearly consumption in the Netherlands and around 60% more thanwhen using normal AD systems. Within the context BIO-P2M is an interestingoption for increasing green gas output and improving the overall sustainabilityof the AD process. However, the source of green electricity needs to be takeninto account and process optimization can ensure better environmentalperformance.
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