|Rotz, Clarence - Al
|MITLOCHNER, FRANK - University Of California
Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2010
Publication Date: 4/14/2010
Citation: Montes, F., Hafner, S.D., Rotz, C.A., Mitlochner, F.M. 2010. Temperature and air velocity effects on ethanol emission from corn silage with the characteristics of an exposed silo face. Atmospheric Environment. 44(16):1987-1995.
Interpretive Summary: Ozone air pollution can cause cardiovascular problems and premature death, which makes it a major health concern. Ozone is produced in the lower atmosphere during hot sunny days, as a by product of reactions of nitrogen oxides with volatile organic compounds (VOCs). Although most of these compounds are produced in internal combustion engines and industrial processes, agriculture is believed to be an important contributor to ozone formation in urban areas surrounded by large animal feeding operations. Recent research suggests that silage is the major source of VOCs emitted from dairy farms with ethanol being the most abundant compound emitted from corn silage. Ethanol was used as a representative VOC to document the pattern of emission through time and to quantify the effect of air velocity and temperature on VOC emissions. Ethanol emission rate was highest immediately after the sample was exposed to flowing air with a rapid decrease over the first hour. The emission rate then continued to decline at a slower rate through the remainder of the 12 hour measurement period. Emission rate and the accumulated emission increased with silage temperature and the air velocity over the silage surface. These results indicate that temperature, air velocity and porosity should be considered in the measurement and analysis of ethanol emissions from silage. This suggests that the use of techniques designed for diffusion-limited systems are not appropriate for measuring VOC emissions from silage as they occur on farms. This new process-level data provides information for calibrating and evaluating models that will ultimately be used to develop strategies to control and mitigate silage VOC emissions, thus improving the sustainability of our dairy farms.
Technical Abstract: Volatile organic compounds (VOCs) from agricultural sources are believed to be an important contributor to tropospheric ozone in some areas. Recent research suggests that silage is a major source of these VOCs, but only limited data exist on VOC emission from silage. Ethanol is normally the most abundant VOC emitted from corn silage, and therefore was used as a representative compound to characterize the pattern of emission over time and to quantify the effect of air velocity and temperature on VOC emissions. Ethanol emission was measured from corn silage samples obtained from a bunker silo where the compacted silage density was maintained. Measurements were made over 12 h for each of a range of air velocities (0.05, 0.5 and 5 m/s) and temperatures (5, 20 and 35 deg C) using a wind tunnel system. Ethanol flux ranged from 2.3 to 220 g/h-m**2, and 12 h cumulative emissions ranged from 7.4 to 270 g/m**2. Ethanol flux was highly dependent on exposure time. Flux declined rapidly over the first hour and then continued to decline more slowly for the remainder of the 12 h period. The cumulative emission was very sensitive to temperature and air velocity; the 12 h cumulative emission increased by a factor of 5 with a 30 deg C increase in temperature, and by a factor of 10 with a 100-fold increase in air velocity. Cumulative emission was also correlated with gas-filled porosity. Effects of air velocity, temperature, and porosity were generally consistent with a conceptual model of VOC emission from silage. Results suggest that these parameters should be considered when measuring VOC emission from silage, and that the use of techniques designed for diffusion-limited systems are not appropriate for measuring emissions as they occur on farms.