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Technology-Based Methodology for Calculating Digester-Gas-Related Greenhouse-Gas Emissions
Author: John Willis, Patrick Griffith, Don Trueblood, Zhiguo Yuan, and David Checkel
Date: 10/13
Preprint, WEFTEC 2013, Chicago, IL, Oct. 5-9, 2013

Current greenhouse gas (GHG) reporting protocols use uniform emission factors to estimate the emissions of methane (CH4) attributable to the combustion of anaerobic digester biogas. In general, these protocols have gravitated to an assumption that approximately 1% of all combusted digester-gas CH4 is released to the atmosphere as a fugitive, anthropogenic emission. This simplification is still commonly used despite considerable, already-available data collected through stack testing required for air compliance permitting that suggest combustion effectiveness is better correlated to the type of device used for combustion. Combustion of digester gas in turbines, microturbines, boilers, and/or low-NOx flares approaches completion with CH4 emissions of less than 0.1%. At the other end of the spectrum, conventional flares may operate between 94 and 97 percent efficiency, but this performance can degrade significantly on windy days or with poorer quality fuel. EPA has estimated that conventional candle-stick-type flares typically operate with between 1% and 2% CH4 emissions. EPAs values have been shown to be very optimistic through wind tunnel experiments conducted as part of The Flare Research Project (U. of Alberta, 2004). This paper suggests a new methodology that uses stack testing data to develop technology-based emissions factors for digester gas GHG-emissions reporting (i.e. that a specific amount of gas combusted in a device of a certain kind releases a certain percentage of that fuel input as CH4).