The Intergovernmental Panel on Climate Change (IPCC) estimates that baseline global GHG emissions may increase 25-90% from 2000 to 2030, with carbon dioxide (CO 2 ) emissions growing 40-110% over the same period. On-road vehicles are a major source of CO 2 emissions in all the developed countries, and in many of the developing countries in the world. Similarly, several criteria air pollutants are associated with transportation, for example, carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). Therefore, the need to accurately quantify transportation-related emissions from vehicles is essential. The new U.S. Environmental Protection Agency (EPA) mobile source emissions model, MOVES2010a (MOVES), can estimate vehicle emissions on a second-by-second basis, creating the opportunity to combine a microscopic traffic simulation model (such as VISSIM) with MOVES to obtain accurate results. This paper presents an examination of four different approaches to capture the environmental impacts of vehicular operations on a 10-mile stretch of Interstate 4 (I-4), an urban limited-access highway in Orlando, FL. First (at the most basic level), emissions were estimated for the entire 10-mile section "by hand" using one average traffic volume and average speed. Then three advanced levels of detail were studied using VISSIM/MOVES to analyze smaller links: average speeds and volumes (AVG), second-by-second link drive schedules (LDS), and second-by-second operating mode distributions (OPMODE). This paper analyzes how the various approaches affect predicted emissions of CO, NOx, PM2.5, PM10, and CO 2 . The results demonstrate that obtaining precise and comprehensive operating mode distributions on a secondby-second basis provides more accurate emission estimates. Specifically, emission rates are highly sensitive to stop-and-go traffic and the associated driving cycles of acceleration, deceleration, and idling. Using the AVG or LDS approach may overestimate or underestimate emissions, respectively, compared to an operating mode distribution approach.Implications: Transportation agencies and researchers in the past have estimated emissions using one average speed and volume on a long stretch of roadway. With MOVES, there is an opportunity for higher precision and accuracy. Integrating a microscopic traffic simulation model (such as VISSIM) with MOVES allows one to obtain precise and accurate emissions estimates. The proposed emission rate estimation process also can be extended to gridded emissions for ozone modeling, or to localized air quality dispersion modeling, where temporal and spatial resolution of emissions is essential to predict the concentration of pollutants near roadways.
Nitrogen oxides (NOX) and sulfur oxides (SOX) are criteria air pollutants, emitted in large quantities from fossil-fueled electric power plants. Emissions of SOX are currently being reduced significantly in many places by wet scrubbing of the exhaust or flue gases, but most of the NOX in the flue gases is NO, which is so insoluble that it is virtually impossible to scrub. Consequently, NOX control is mostly achieved by using combustion modifications to limit the formation of NOX, or by using chemical reduction techniques to reduce NOX to N2. Low NOX burners are relatively inexpensive but can only achieve about 50% reduction in NOX emissions; selective catalytic reduction (SCR) can achieve high reductions but is very expensive. The removal of NOX in wet scrubbers could be greatly enhanced by gas-phase oxidation of the NO to NO2, HNO2, and HNO3 (the acid gases are much more soluble in water than NO). This oxidation is accomplished by injecting liquid hydrogen peroxide into the flue gas; the H2O2 vaporizes and dissociates into hydroxyl radicals. The active OH radicals then oxidize the NO and NO2. This NOX control technique might prove economically feasible at power plants with existing SO2 scrubbers. The higher chemical costs for H2O2 would be balanced by the investment cost savings, compared with an alternative such as SCR. The oxidation of NOX by using hydrogen peroxide has been demonstrated in a laboratory quartz tube reactor. NO conversions of 97% and 75% were achieved at hydrogen peroxide/NO mole ratios of 2.6 and 1.6, respectively. The reactor conditions (500 °C, a pressure of one atmosphere, and 0.7 seconds residence time) are representative of flue gas conditions for a variety of combustion sources. The oxidized NOX species were removed by caustic water scrubbing.
Research at the University of Central Florida has determined that the injection of hydrogen peroxide (HO) into a simulated flue gas stream effectively oxidizes NO to NO, and NO to HNO and HNO. These oxides of nitrogen are much more soluble in water than NO, and therefore may be more easily scrubbed from the flue gas in a typical wet scrubber. Oxidation and NO removal efficiencies of greater than 90% were demonstrated in the laboratory. An economic comparison between the HO injection-wet scrubbing method and the selective catalytic reduction (SCR) method of NO removal was conducted for a design base case and a variety of alternative cases. This study illustrates the trade-off between capital and operating costs for the two alternatives. The single largest factor in determining whether the total cost of the HO injection-wet scrubbing method compares favorably with the total cost of the SCR method is the HO:NO molar ratio. At the HO:NO molar ratio demonstrated in the laboratory (1.92:1.0), the HO injection-wet scrubbing method of NO removal was shown to be uneconomical. However, the molar ratio in a full-size coal-fired power plant could be lower than that found in the laboratory. Based on all the cost assumptions stated in this article, at a molar ratio of 1.37:1.0, the hydrogen peroxide injection method was calculated to be an economically feasible alternative to the SCR method for NO control.
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