Simulation on SO2 and NOX Emission from Coal–Fired Power Plants in North-Eastern North America

MM5-SMOKE-CMAQ regional air quality modeling system was used to simulate pollutants emission from coal–fired power plants in North-Eastern North America. The effects of SO2 and NOX on air quality producing from coal-fired power plants in the summer of 2001 were analyzed. Simulations show the contributions of SO2 and NOX emission from coal-fired power plants using different scenarios, coal-fired power plants from US and Canada contribute 67.2% and 32.8% for total SO2 concentration, 17.6% and 6.0% for total NOX concentration in researched domain. Some control measures for coal-fired power plants were discussed. Further controls for the emissions of SO2 and NOX from coal-fired power plants are necessary to reduce the adverse environmental effects.


Introduction
There is a long history of public concern about the emission of SO 2 and NO X , these emissions contribute to acid deposition, ultimately leading to a wide range of environmental impacts, including damage to forests and soils, fish and other living things, materials, and human health.as well as the formation of fine particles and gases that can impair visibility.SO 2 and NO X all are the precursors of acid rain [1].Coal-fired power plants are regarded globally as the major contributor to local air quality degradation and to global environmental impacts such as acid rain and greenhouse phenomenon [2].In the United States, over 70 percent of this electricity was produced through the combustion of fossil fuels, primarily coal and natural gas, while 26 percent of this generation was produced from fossil fuels in Canada.
Legislation enacted for control the emission of these two pollutants during the past two decades.In the Clean Air Act Amendments (CAAA) of 1990, US Environmental Protection Agency (EPA) has promulgated several regulations and proposed rules to decrease these adverse effects by reducing the emissions of SO 2 and NO X from coal-fired power plants.Since Congress passed the Clean Air Act (CAA), power plants have cut emissions of SO 2 and NO X dramatically [3,4].Many of these reductions have been achieved by conversion to lowersulfur fossil fuels, primarily natural gas, with attendant increases in costs.An alternative to conversion is to use mitigation technologies to reduce the emissions.The most common such technologies are reducing fuel sulfur levels and filtering emissions using flue gas desulfurization (FGD) and denitrification systems [5,6].
Analyses of the environmental effects arising from power plants using a variety of models [7][8][9][10] suggest that air quality effects depend on a wide variety of local atmospheric parameters as well as on the combustion technology.In some cases [11], regional transport dominates local sources during pollution episodes, while in other cases [12], transport is strongly affects by local topology.In view of this variable sensitivity, the effects of thermal power generation on air quality should be assessed on a case by case basis for well defined geographical locations and time periods.Plumes from power plants in certain locations have been implicated in significantly reduced air quality [13,14].
In this study, we use regional atmospheric modeling to explore the air quality implications of coal-fired power plants in North-Eastern North America.We design different scenarios to study the contributions of coal-fired power plants from United States and Canada.The concerned pollutants are SO 2 and NO X , which all are the leading precursors of acid rain; they have large environmental influences on the acid sensitive regions of North-Eastern North America.Research results can be made references for pursuing further emission reductions in US and Canada.

Modeling Description
EPA's Models-3/Community Multi-scale Air Quality (C-MAQ) [15] modeling system is a numerical transport and chemical model which is developed by US EPA and is implemented in a full modular approach.It means that the user can select different chemical schemes (CBM-IV, V, SAPRAC-99, RADM, etc.) and different numerical schemes.It has different capabilities such as: Process analysis, IRR analysis, etc.).It can handle interactions at different dynamic scales and among multi-pollutants with one modeling system.In this research, CMAQ (v4.5) is employed to simulate SO 2 and NO X for the summer season of 2001 (May 1 through September 1).The model uses the Carbon Bond 4 (CB4) gas phase chemical mechanism with 2005 updates to extend the inorganic reactions [16].
2001 national emissions inventories was used and processed by the Sparse Matrix Operator Kernel Emissions Modeling System (SMOKE, v2.2) [17].SMOKE system provides an efficient tool for converting emissions inventory data into the formatted emissions files: gridded, temporalized and speciated, which are required by CM-AQ.Area sources, non-road sources, mobile emissions, biogenic emissions and point sources are treated separately and merged.
Meteorological input data for the modeling runs are processed using the National Center for Atmospheric Research (NCAR) 5th generation Mesoscale Model (MM5, v3.0) [18,19].Important MM5 parameterizations and physics options apply to each summer include mixed phase microphysics, planetary boundary layer (PBL), and the land surface module.Meteorology-Chemistry Interface Processor (MCIP, v2.0) is used to process the MM5 output fields and generate the meteorological parameter fields required by SMOKE and CMAQ as well as the dry deposition velocity fields of chemical species required by CMAQ.
The model, as applied here, uses a horizontal resoluteion of 36 × 36 km 2 , with 23 layers in the vertical.The model domain covers most Eastern of America in order to minimize the effects of boundary conditions on model results.Additional model simulation was performed over an embedded domain covering the North-Eastern North America including the Great Lakes, with a grid spacing of 12 km.Modeling coarse domain and 2-way nested 12km domain are shown in Figure 1.
We explore the effects of the three emission scenarios described below: 1) Scenario 1 (Base case): All emissions in the 2001 criteria inventories were used.
2) Scenario 2: Coal emissions from power plants of US and Mexico removed from point source inventory, but kept the emission inventory of Canada same as Scenario 1.
3) Scenario 3: Coal emissions from power plants in Canada were removed from point source inventory, but kept the emission inventories of US and Mexico same as Scenario 1.

Modeling Evaluation
To validate the basic calculation, we compared the values of SO 2 and NO X produced by the model for the time period from May 1 to September 1, 2001 with the corresponding measurements from AQS air quality monitoring stations located in Domain 2.
The simulation for SO 2 and NO X aren't very good, the model has an under-prediction for SO 2 and NO X average concentration; Normalized Mean Bias (NMB) and Normalized Mean Error (NME) for SO 2 are -51.75% and 71.28%; for NO X are -9.77% and 55.38% respectively.The reason that modeled deviates from observations is that measurements are made near the sources where the model emission schemes will never be able to reproduce small-scale fluctuations observed.Sub-grid scale variability in emissions will have a major impact on the comparison between the model and observation.More efforts should be done to improve the simulation for SO 2 and NO X .Moreover, the study's conclusions are obtained mainly by subtracting the scenarios from base case, thereby reducing the effects of errors in research results.ing and other industrial processes.SO 2 emissions from power plants react with other chemicals in the atmosphere to form sulfate particles, an important contributor to the fine particle mix that circulates with the air we breathe.Figure 2 and Figure 3 show the average SO 2 emission flux and SO 2 concentration in North-Eastern North America in the summer season of 2001.The average SO 2 emission flux was 0.021mol/s in whole domain in the summer.Several regions such as Chicago, southern Ohio, northern West Virginia and western Pennsylvania etc. were responsible for a significant proportion of SO 2 emissions.Many large coal-fired power plants encompasses in Ohio River Valley, which is a well-known emission source of precursors of acid rain in the North America.Same as emission, these areas had higher SO 2 average concentration in this summer.

Modeling Results and Analysis
Figure 4 shows the average difference of SO 2 concentration from base case and Scenario 2, which produce by the average of SO 2 concentration simulating from base case minus those of Scenario 2. The regions of high SO 2 concentration are the areas affected by the SO 2 emission of coal-fired power plants apparently.The highest difference of SO 2 concentration exists at the region of west Pennsylvania about 0.012 ppmv, it is easy to see that the Ohio River Valley is biggest source of SO 2 from coalfired power plants in researched domain.
Figure 5 shows the average SO 2 concentration simulating from three scenarios in the summer of 2001.The average SO 2 concentration of base case in domain 2 is 0.001 ppm in this summer.Average SO 2 concentration getting from Scenarios 2 and 3 are 0.00033 and 0.000672 ppm, these mean coal-fired power plants from US and Canada contribute 67.2% and 32.8% for total SO 2 concentration in researched domain.
According to EPA report, power plants account for 69% of total SO 2 emission in the US in 2001 [20].So, this is consistent with our research results for the most part.

SO2 Emission Control from Coal-Fired Power Plants
To help reduce acid rain, EPA is implementing a program to reduce releases of SO Four main technology strategies for SO 2 emissions control have been used by the electricity industry [22]: 1) Tall gas stacks that disperse emissions away from immediate areas; 2) Intermittent controls which involve routine operational adjustments to reduce power plant SO 2 emissions in response to atmospheric conditions; 3) Pre-combustion reduction of sulfur from fuels; and 4) Removal of SO 2 from the post-combustion gas stream, the main method is FGD.There are a wide variety of FGD techniques, of which the most common are wet scrubbers.Wet scrubbers work by using a slurry or solution to absorb SO 2 , producing an initially wet byproduct.Frequently, limestone is used as the absorbent, generating gypsum as a by-product.Typically, FGD can achieve SO 2 removal efficiency more than 90%.

NO X Emission and Changes Resulting from Different Scenarios
NO X is the term used to describe the sum of nitric oxide (NO), nitrogen dioxide (NO 2 ), and other oxides of nitrogen.NO X plays a major role in the formation of acid rain, secondary PM, ground level ozone, and smog in the atmosphere through a complex series of reactions with volatile organic compounds (VOCs), sunlight and water vapors.The Relationship between VOC and NO X and O 3 has extensive studies [23][24][25][26].NO X also contributes to visibility impairment.The major sources of human-produced NO X emissions are high-temperature combustion processes such as those that occur in electrical power plants and automobiles.Anthropogenic NOx emissions in the US are estimated to total 22.2 Tg (as NO 2 ) per year, with 53% from transportation, coal-fired plants are another major sources of NO X because of the fuel they burn [27].Figure 6 and Figure 7 show the average NO X emission flux and concentration in North-Eastern North America in summer season of 2001.The average NO X emission flux was 0.20 mol/s in whole domain in the summer.Big cities such as Chicago, Detroit, and Toronto etc. have much more emission of NO X than other areas, which arising from local motor vehicle emissions primarily.The biggest average NOx concentration is 0.056 ppmv in Toronto city.atmosphere to have obvious effect on Toronto.Average NO X concentration getting from Scenario 2 and Scenario 3 is 0.00164 ppmv and 0.00187 ppmv.So, we can get coal-fired power plants from US and Canada contributing 17.6% and 6.0% for NO X concentration in researched domain.This is lower with same EPA research results [20], coal-fired power plants contributed about 22% of all U.S. NO X emissions in 2002, and this may be the cause of under-prediction of model for NO X and the different object domain.

NO X Emission Control from Coal-Fired Power Plants
In the past few years, EPA has promulgated several rules to reduce NO X emissions, new rules for stationary sources (the Clean Air Interstate Rule, the Clean Air Mercury Rule, and the Clean Air Visibility Rule), as well as State plans to attain the National Ambient Air Quality Standard (NAAQS) of fine particle and ozone, will also significantly reduce future NO X emissions.Beginning with the CAAA of 1990, existing generators also have faced increasingly stringent regulation of their nitrogen oxide (NO X ) emissions.Restrictions on summer emissions of NO X from electricity generators in a majority of eastern states are expected to become even tighter during the next decade with the implementation of the call for amendments to state implementation plans (SIPs) from the US EPA, known as the NO X SIP Call.This new regulation is designed to address the long-range transport of NO X as a contributing factor to summer air pollution in cities on the East Coast.Recent lawsuits filed by EPA and New York State also have raised the possibility that many existing generating sources were negligent in not bringing their facilities into compliance with New Source Performance Standard (NSPS) when they made substantial investments enabling greater electricity generation at these facilities [28].Reduction of NO X emissions from industrial combus-tion sources, especially coal-fired power plants is an important consideration in efforts undertaken to address the NO X environmental concerns.New regulations announced by the US EPA facilitate utilities to develop new, efficient, and robust post-combustion NO X control technologies [29].The popular primary control technologies in use in the United States are low-NO X burners (LNB) and over-fire air.The average NO X reductions for specific primary controls have ranged from 35% to 63% from 1995 emissions levels.The secondary NO X control technologies applied on U.S. coal-fired utility boilers include re-burning, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) [30].

Conclusions
In this study, we use a regional atmospheric modeling system-CMAQ to compute the air quality effects of coalfired power plants in North-Eastern North America.For the time period from May 1 to September 1, 2001, we consider three different scenarios to study the effects of SO 2 and NO X emission from coal-fired power plants.the contributions of the coal plants for SO 2 and NO X are found to be big, coal-fired power plants from US and Canada contribute 67.2% and 32.8% for SO 2 concentration, 17.6% and 6.0% for NO X concentration in researched domain.The use of existing emission reduction rules and technologies can diminish the contributions of coal-fired power plants for total emission of SO 2 and NO X .Further control measures for the emissions of SO 2 and NO X from coal-fired power plants are necessary to reduce the negative environmental effects.

Figure 2 .Figure 3 .
Figure 2. Average SO 2 emission flux in the summer season of 2001 in North-Eastern North America.

Figure 4 .Figure 5 .
Figure 4. Spatially resolved hourly average difference of SO 2 concentration from base case and Scenario 2 in the summer season of 2001 in North-Eastern North America.
2 and other pollutants from coal-fired power plants.The first phase began in 1995 for SO 2 and targets the largest and highest emitting power plants.The second phase (started in 2000) sets tighter restrictions on smaller coal-, gas-, and oil-fired plants.This program will reduce annual SO 2 emissions by 10 million tons (almost half the 1980 level) between 1980 and 2010.The control of SO 2 emissions is currently achieved using an innovative strategy called cap and trade, established by the CAAA of 1990 and administered by EPA, which is the world's first large-scale capand-trade program for air pollution.The program is designed to reduce electric power sector emissions of SO 2 through a national, market-based cap-and-trade system for SO 2 emissions with the goal of reducing the adverse effects of acid rain.The total SO 2 release allowed is set at a maximum of 8.95 million tons by the year 2010-approximately half of 1980 emissions [21].

Figure 8 Figure 6 .Figure 7 .Figure 8 .
Figure 6.Average NO X emission flux in summer season of 2001 in North-Eastern North America.