Taking Your Digester Gas on the Road: The Case to Upgrade Digester Gas for Vehicle Fuel

摘要

Often digester gas generated by anaerobic digestion at WWTPs is used to produce heat andelectricity through cogeneration, but under the right market conditions digester gas may providebetter economic and environmental value as a vehicle fuel. Electricity is relatively inexpensive insome parts of the country and the payback for the installation of cogeneration is noteconomically attractive. While the use of digester gas for anaerobic digester process and spaceheating in a boiler is still often inexpensive, not all of the digester gas is used throughout theyear—especially during the spring and summer months. This leaves some or all of the digestergas available for an alternative end use.Two case studies—the City of Tacoma, Washington, and Pima County (Tucson), Arizona—arepresented where digester gas end-use alternatives were evaluated and resulted in vehicle fuelproduction being identified as the most cost-effective alternative, over other more traditional enduses. Both of these locations have relatively low electricity costs (less than about $0.06 perkilowatt-hour). A quantity of digester gas sufficient to make 100 kilowatt-hours of electricity candisplace roughly 23 to 26 liters of diesel fuel or 26 to 30 liters of gasoline for vehicle fuel. Thispaper discusses the technologies, life-cycle costs, and carbon emissions reductions thatdemonstrate that for these utilities, converting their digester gas to vehicle fuel was the besteconomical and a good environmental use of their resource.As the role of wastewater utilities expands from treatment to resource recovery, understandingthe best use of those recovered resources will serve only to improve the long-term sustainabilityof operations, fiscal and environmental. This paper discusses the technical aspects of vehicle fuelproduction and the likely impacts to long-term operating costs and capital budgets, along withthe benefits to plant operators, design engineers, and utility managers.