Utilization of a Method to Estimate NO_2 Concentrations from a NO_X simulation for Thermal Power Plants.

摘要

In Brazil,atmospheric dispersion studies for natural gas or diesel fired utility plants are based on mathematical modeling recommended by USEPA,such as ISCST3.These small to medium sized plants emit a number of pollutants,including nitrogen oxides,NO_X,a general group of compounds that includes nitrogen moNO_Xide(NO),nitrogen dioxide(NO_2)and small percentages of other nitrogen compounds.Over 95% of the NO_X emitted to the atmosphere is composed of nitrogen moNO_Xide(NO),while less than 5% is composed of nitrogen dioxide(NO_2).When in the air,nitrogen oxide is submitted to photochemical transformations,slowly and partially yielding NO_2,and vice versa.Despite the low concentrations,NO_2 emissions are a major concern in terms of environmental impacts.Calculations on the concentrations of NO_2 are more complex,since the NO_2/NO_X ratio varies with the quantity of NO_X in the air,the time of day,ozone concentrations,climatic seasonality,air temperature and UV radiation.Although chemical conversion rates can be used to estimate NO_2 concentrations,studies generally assume that either all NO_X is found in the atmosphere as NO_2 or some percentage of the NO_X in the air is assumed as NO_2,which imply excessively conservative calculations.Scientific methods can be found in the literature,addressed to estimating concentrations of NO_2 as a function of the total emission of NO_X.The method utilized in this paper allows obtaining concentrations of NO_2 from maximum concentrations of NO_X as calculated by ISCST3 modeling.This method was field tested for a number of emissions from utility plants and considers NO_X to NO_2 correction factors as a function of the distance to the source,according to the diurnal variability of meteorological parameters.The modeling outputs were related to data from the automatic air quality monitoring station in the site where the thermal power plant targeted by this study is located.