Ravi K. Srivastava

Nitrogen Oxides Emission Control Options for Coal-Fired Electric Utility Boilers

Abstract Recent regulations have required reductions in emissions of nitrogen oxides (NOx) from electric utility boilers. To comply with these regulatory requirements, it is increasingly important to implement state-of-the-art NOx control technologies on coal-fired utility boilers. This paper reviews NOx control options for these boilers. It discusses the established commercial primary and secondary control technologies and examines what is being done to use them more effectively. Furthermore, the paper discusses recent developments in NOx controls. The popular primary control technologies in use in the United States are low-NOx burners and overfire air. Data reflect that average NOx reductions for specific primary controls have ranged from 35% to 63% from 1995 emissions levels. The secondary NOx control technologies applied on U.S. coal-fired utility boilers include reburning, selective noncatalytic reduction (SNCR), and selective catalytic reduction (SCR). Thirty-six U.S. coal-fired utility boilers have installed SNCR, and reported NOx reductions achieved at these applications ranged from 15% to 66%. Recently, SCR has been installed at >150 U.S. coal-fired utility boilers. Data on the performance of 20 SCR systems operating in the United States with low-NOx emissions reflect that in 2003, these units achieved NOx emission rates between 0.04 and 0.07 lb/106 Btu.

Investigation of Selective Catalytic Reduction Impact on Mercury Speciation under Simulated NOx Emission Control Conditions

Abstract Selective catalytic reduction (SCR) technology increasingly is being applied for controlling emissions of nitrogen oxides (NOx) from coal-fired boilers. Some recent field and pilot studies suggest that the operation of SCR could affect the chemical form of mercury (Hg) in coal combustion flue gases. The speciation of Hg is an important factor influencing the control and environmental fate of Hg emissions from coal combustion. The vanadium and titanium oxides, used commonly in the vanadia-titania SCR catalyst for catalytic NOx reduction, promote the formation of oxidized mercury (Hg2+). The work reported in this paper focuses on the impact of SCR on elemental mercury (Hg0) oxidation. Bench-scale experiments were conducted to investigate Hg0 oxidation in the presence of simulated coal combustion flue gases and under SCR reaction conditions. Flue gas mixtures with different concentrations of hydrogen chloride (HCl) and sulfur dioxide (SO2) for simulating the combustion of bituminous coals and subbituminous coals were tested in these experiments. The effects of HCl and SO2 in the flue gases on Hg0 oxidation under SCR reaction conditions were studied. It was observed that HCl is the most critical flue gas component that causes conversion of Hg0 to Hg2+ under SCR reaction conditions. The importance of HCl for Hg0 oxidation found in the present study provides the scientific basis for the apparent coal-type dependence observed for Hg0 oxidation occurring across the SCR reactors in the field.

Influence of droplet spacing on drag coefficient in nonevaporating, monodisperse streams

It is well established that droplet interactions profoundly influence the ignition and combustion behavior of droplet clouds and fuel sprays. Four-group combustion modes of a droplet cloud have been identified, with that of single-droplet combustion possibly being applicable in practice to only a very limited number of special situations. Such a special situation, however, can arise during the incineration of liquid hazardous wastes, where droplets with large diameters congregate at the outer edge of fuel-spray cones. One or more of these stray droplets then may individually pass through, or bypass, the main flame zone and lead to a failure mode in the incinerator. In this document, the influence of droplet spacing on the drag coefficient of individual drops injected into a quiescent environment has been determined through measurement of trajectories of single, monodisperse, nonevaporating droplet streams.

Pilot-Scale Study of the Effect of Selective Catalytic Reduction Catalyst on Mercury Speciation in Illinois and Powder River Basin Coal Combustion Flue Gases

Abstract A study was conducted to investigate the effect of selective catalytic reduction (SCR) catalyst on mercury (Hg) speciation in bituminous and subbituminous coal combustion flue gases. Three different Illinois Basin bituminous coals (from high to low sulfur [S] and chlorine [Cl]) and one Powder River Basin (PRB) subbituminous coal with very low S and very low Cl were tested in a pilot-scale combustor equipped with an SCR reactor for controlling nitrogen oxides (NOx) emissions. The SCR catalyst induced high oxidation of elemental Hg (Hg0), decreasing the percentage of Hg0 at the outlet of the SCR to values <12% for the three Illinois coal tests. The PRB coal test indicated a low oxidation of Hg0 by the SCR catalyst, with the percentage of Hg0 decreasing from ∼96% at the inlet of the reactor to ∼80% at the outlet. The low Cl content of the PRB coal and corresponding low level of available flue gas Cl species were believed to be responsible for low SCR Hg oxidation for this coal type. The test results indicated a strong effect of coal type on the extent of Hg oxidation.

Simulation of dispersion of a power plant plume using an adaptive grid algorithm

A new dynamic adaptive grid algorithm has been developed for use in air quality modeling. This algorithm uses a higher order numerical schemeFthe piecewise parabolic method (PPM)Ffor computingadvective solution fields; a weight function capable of promoting grid node clustering by moving grid nodes; and a conservative interpolation equation using PPM for redistributing the solution field after movement of grid nodes. Applications of the algorithm to a model problem, in which emissions from a point source disperse through the atmosphere in time, reflect that the algorithm is able to capture not only the regional ozone plume distribution, but also the small-scale plume structure near the source. In contrast, the small-scale plume structure was not captured in the correspondingstatic grid solution. Performance achieved in model problem simulations indicates that the algorithm has the potential to provide accurate air quality modelingsolutions at costs that may be sig nificantly less than those incurred in obtainingequivalent static grid solutions. r 2001 Elsevier Science Ltd. All rights reserved.

Development of low-diffusion flux-splitting methods for dense gas-solid flows

The development of a class of low-diffusion upwinding methods for computing dense gas-solid flows is presented in this work. An artificial compressibility/low-Mach preconditioning strategy is developed for a hyperbolic two-phase flow equation system consisting of separate solids and gas momentum and continuity equations. The eigenvalues of this system are used to devise extensions of the AUSM+ [1] and LDFSS [2] flux-splitting methods that provide high resolution capturing of bubble growth and collapse in gas-solid fluidized beds. Applications to several problems in fluidization are presented.

Simulation of a reacting pollutant puff using an adaptive grid algorithm

A new dynamic solution adaptive grid algorithm, DSAGA-PPM, has been developed for use in air quality modeling. In this paper, this algorithm is described, and is evaluated with a test problem. Cone-shaped distributions of various chemical species undergoing chemical reactions are rotated to simulate the transport and chemistry processes that occur in the atmosphere. The results obtained by using DSAGA-PPM are more accurate than those obtained from a static grid with the same number of nodes. The computational cost associated with a static grid solution with the same level of accuracy is prohibitive. Because of its efficient use of computational resources, DSAGA-PPM has the potential to improve the accuracy, or efficiency, or a combination of both in air quality models.

Reduction of Multi-pollutant Emissions from Industrial Sectors: The U.S. Cement Industry – A Case Study

Carbon dioxide (CO2) accounts for more than 90% of worldwide CO2-eq greenhouse gas (GHG) emissions from industrial sectors other than power generation. Amongst these sectors, the cement industry is one of the larger industrial sources of CO2emissions. In 2005, this industry accounted for about 6% of the global anthropogenic CO2emissions. Further, global production of cement has been growing steadily, with the main growth being in Asia. Considering these trends, the worldwide cement industry is a key industrial sector relative to CO2emissions.

Initial Application of the Adaptive Grid Air Quality Model

Grid size (or resolution), when inadequate, can be an important source of uncertainty for air quality model (AQM) simulations. Coarse grids used because of computational limitations may artificially diffuse the emissions, leading to significant errors in the concentrations of pollutant species, especially those that are formed via non-linear chemical reactions. Further, coarse grids may result in large numerical errors. To address this issue, multi-scale modeling and grid nesting techniques have been developed (Odman and Russell, 1991; Odman et al., 1997). These techniques use finer grids in areas that are presumed to be of interest (e.g., cities) and coarser grids elsewhere (e.g., rural locations). Limitations include loss in accuracy due to grid interface problems and inability to adjust to dynamic changes in resolution requirements. Adaptive grids are not subject to such limitations and do not require a priori knowledge of where to place finer grids. Using grid clustering or grid enrichment techniques, they automatically allocate fine resolution to areas of interest. They are thus able to capture the physical and chemical processes that occur in the atmosphere much more efficiently than their fixed grid counterparts. The adaptive grid methodology used here is based on the Dynamic Solution Adaptive Grid Algorithm (DSAGA) of Benson and McRae (1991), which was later extended for use in air quality modeling by Srivastava et al. (2000). It employs a structured grid with a constant number of grid nodes. The modeling domain is partitioned into M N × quadrilateral grid cells. The grid nodes are re-positioned in a twodimensional space throughout the simulation according to a weight function which represents the resolution requirements. The areas of the grid cells change due to grid node movements but the connectivity of the grid nodes remains the same. Further, since the number of grid nodes is fixed, refinement of grid scales in some regions is accompanied by coarsening in other regions where the weight function has smaller values. This results

Particle flow, mixing, and chemical reaction in circulating fluidized bed absorbers

Abstract A mixing model has been developed to simulate the particle residence time distribution (RTD) in a circulating fluidized bed absorber (CFBA). Also, a gas/solid reaction model for sulfur dioxide (SO 2 ) removal by lime has been developed. For the reaction model that considers RTD distribution inside the core and annulus regions of a CFBA, a macrochemical reaction can be simulated based on microchemical reaction dynamics. The presented model can predict SO 2 and lime concentration distributions inside the CFBA, and give the amount of lime needed to remove a given percentage of SO 2 . It is found that SO 2 concentration decreases with the increase of CFBA distance from the bottom in the core region. However, lime concentration exhibits a very slight variation in the core region. This means that lime is efficiently utilized to remove SO 2 . The model also predicts that SO 2 partial pressure at the exit of the CFBA decreases with the increase in the percentage of fresh lime injected in the CFBA.