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Spational modeling of a sustainable gas supply chain by matching supply and demand
Biogas production from co-digestion 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 several relevant aspects of such a supply chain were analyzed: the cost price (€/mn3, based on a net present value calculation), as a function of scale level (mn3/hr), the effect of scale level on energy use and transport movements, and the influence of seasonal variations in gas demand on the cost price. The model includes practical sustainability criteria such as using the digestate from the digester as a fertilizer and limits on land use for biomass production. To model the seasonal gas demand, gas demand data were supplied by a gas grid company. These data comprise the gas demand of five gas receiving stations (GRS) during a year on an hourly basis, which depict the gas demand of a geographical area. Of each GRS the dominant types of users of the gas are known, f.i. mainly households or large companies, which obviously have a different demand pattern. For each GRS, the influence of four scenarios on cost price were analyzed, namely the influence of variable biogas production by a digester, use of a gas storage facility in the supply chain, and a big and a smaller digester.
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 mn3/hr green gas) a significant improvement is expected from decreasing costs of digesters and upgrading installations, and efficiency improvement of digesters. At larger scale levels the number of transport movements and energy use in the supply chain seem to become a limiting factor in respect to sustainability. Results also show that flexible production is cheaper than gas storage for small gas demand following capacities. When gas demand has to be followed for a longer period, gas storage is cheaper. Moreover, the cost price decreasing effect seems to be more significant when the gas delivery area mainly consists of households than when it consists of large companies. This might influence the optimal locations for digesters and thus locations for biomass production. The consequences of this are currently investigated in further research.