Energy efficiency, greenhouse gas reduction and cost price of a green gas supply chain were evaluated. This supply chain is based on co-digestion of dairy cattle manure and maize, biogas upgrading and injection into a distribution gas grid. A defined reference scenario reflects the current state of practice, assuming that input energy is from fossil origin. Possible improvements of this reference scenario were investigated. For this analysis two new definitions for energy input-output ratio were introduced; one based on input of primary energy from all origin, and one related to energy from fossil origin only. Switching from fossil to green electricity significantly improves the energy efficiency (both definitions) and greenhouse gas reduction. Preventing methane leakage during digestion and upgrading, and re-using heat within the supply chain show smaller improvements on these parameters as well as on cost price. A greenhouse gas reduction of more than 80 % is possible with current technology. To meet this high sustainability level, multiple improvement options will have to be implemented in the green gas supply chain. This will result in a modest decrease of the green gas cost price.
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One of the issues concerning the replacement of natural gas by green gas is the seasonal pattern of the gas demand. When constant production is assumed, this may limit the injected quantity of green gas into a gas grid to the level of the minimum gas demand in summer. A procedure was proposed to increase thegas demand coverage in a geographical region, i.e., the extent to which natural gas demand is replaced by green gas. This was done by modeling flexibility into farm-scale green gas supply chains. The procedure comprises two steps. In the first step, the types and number of green gas production units are determined,based on a desired gas demand coverage. The production types comprise time-varying biogas production, non-continuous biogas production (only in winter periods with each digester having a specified production time) and constant production including seasonal gas storage. In the second step locations of production units and injection stations are calculated, using mixed integer linear programming with cost price minimization being the objective. Five scenarios were defined with increasing gas demand coverage, representing a possible future development in natural gas replacement. The results show that production locations differ for each scenario, but are connected to a selection of injection stations, at least in the considered geographical region under the assumed preconditions. The cost price is mainly determined by the type of digesters needed. Increasing gas demand coverage does not necessarily mean a much higher cost price.
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Biogas production from codigestion of cattle manure and biomass can have a significant contribution to a sustainable gas supply when this gas is upgraded to specifications prescribed for injection into the national gas grid and injected into this grid. In this study, we analyzed such a gas supply chain in a Dutch situation. A model was developed with which the cost price per m n3 was presented as a function of scale level (m n3/hr). The hypothesis that transport costs increase with increasing scale level was confirmed although this is not the main factor influencing the cost price for the considered production scales. For farm-scale gas supply chains (approximately 150-250 m n3/h green gas), a significant improvement is expected from decreasing costs of digesters and upgrading installations, and efficiency improvement of digesters. In this study also practical sustainability criteria for such a supply chain were investigated. For this reason, the digestate from the digester should be used as a fertilizer. For larger scale levels, the number of transport movements and energy use in the supply chain seem to become a limiting factor with respect to sustainability. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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