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Simulation and Optimization of a Granular Limestone Flue Gas Desulfurization Process
Abstract
This chapter presents a process simulation model developed for the Limestone Emission Control process. The process involves contacting flue gas with a densely packed, wet bed of granular limestone. The process model includes the chemistry, mass transfer, and heat transfer associated with this system along with sorbent screening and recycle steps occurring outside the scrubber. Approximate cost correlations are applied to various system components and used to drive a design optimization for four different cases corresponding to two levels of sulfur capture and two levels of sulfur content in the coal. The results indicate that this technology is best suited for high-sulfur, small-scale applications. Future experimental work should evaluate the use of larger (1/4 inch) sorbent.
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Mathematical models for flue gas desulfurization processes using wet granular limestone beds are presented. The models include a first principles analysis of the chemical and physical processes in the reactor as well as overall material balance for the sorbent handling system.
Preface 1 Introduction 2 The First Law and Other Basic Concepts 3 Volumetric Properties of Pure Fluids 4 Heat Effects 5 The Second Law of Thermodynamics 6 Thermodynamic Properties of Fluids 7 Applications of Thermodynamics to Flow Processes 8 Production of Power from Heat 9 Refrigeration and Liquefaction 10 Vapor/Liquid Equilbrium: Introduction 11 Solution Thermodynamics: Theory 12 Solution Thermodynamics: Applications 13 Chemical-Reaction Equilibria 14 Topics in Phase Equilibria 15 Thermodynamic Analysis of Processes 16 Introduciton to Molecular Thermodynamics Appendixes A Conversion Factors and Values of the Gas Constant B Properties of Pure Species C Heat Capacities and Property Changes of Formation D Representative Computer Programs E The Lee/Kesler Generalized-Correlation Tables F Steam Tables G Thermodynamic Diagrams H UNIFAC Method I Newton's Method Author Index Subject Index
This paper discusses the fundamental processes in SOâ capture by calcium-based adsorbents for upper furnace, duct, and electrostatic precipitator (ESP) reaction sites. It examines the reactions in light of controlling mechanisms, effect of sorbent physical properties, and important process variables. Upper furnace reactivity is limited to 900-1200 C by rate and equilibrium constraints, respectively. Sulfation is a function of in-situ sorbent characteristics of porosity, particle size, and surface area. Conversion of the sorbent is ultimately limited by the formation of the calcium sulfate (CaSOâ) product layer. The in-duct reaction is accomplished through sorbent scavenging in the flue-gas stream by a water spray. The scavenging efficiency of the sorbent by the water droplets limits the process, while reaction is controlled by the dissolution rate of the sorbent. The E-SOX process in a modified ESP simulates a short time spray dryer through injection of a Ca(OH)â slurry. The Ca(OH)â undergoes aqueous phase reaction to remove SOâ. Evaporation of the droplets prior to the ESP field conditions the flue gas for more-efficient particulate-matter collection by lowering the gas resistivity.
Pressure drop and liquid holdup for two-phase concurrent downward flow in packed beds were correlated for various types of packings by taking into account two hydrodynamic regimes: a poor and a high gas-liquid interaction regime.Foaming and non-foaming systems have been considered.In the poor interaction regime, the pressure drop was calculated as due to the gas flowing in a bed restricted by the presence of the liquid. A correlation valid for a free liquid trickling, modified in order to take into account the effect of the pressure drop, is proposed and used to correlate liquid holdup in the presence of a concurrent gas flow.In the high interaction regime, empirical correlations were proposed for both foaming and non-foaming systems.All the employed correlations fit experimental results from several authors better than those proposed in the literature.
A 0.5 MW spray-dry scrubbing FGD pilot plant was used in the study of spray dryer performance over a wide range of operating conditions. Experimental findings were compared with a spray dryer model. During operation with large excesses of lime, the SO2 absorption was limited by gas phase diffusion. At operation with a shortage of lime, the rate limiting step was the dissolution rate of lime. In addition, the flow regime in a spray dryer can be best described as well mixed. The SO2 level in the flue gas was found to exert no direct effect on the efficiency of SO2 removal. The observed effects are attributed solely to the changes in the drying process, due to the inter-dependence of slurry composition and SO2 concentration.
Dry scrubbing that takes place after the air preheater (
ETS, Inc., a pollution consulting firm with headquarters in Roanoke, Virginia, has developed a dry, limestone-based flue gas desulfurization (FGD) system. This SOâ removal system, called Limestone Emission Control (LEC), can be designed for installation on either new or existing coal-fired boilers. In the LEC process, the SOâ in the flue gas reacts with wetted granular limestone that is contained in a moving bed. A surface layer of principally calcium sulfate (CaSOâ) is formed on the limestone. Periodic removal of this surface layer by mechanical agitation allows high utilization of the limestone granules. A nominal 5,000 acfm LEC pilot plant has been designed, fabricated and installed on the slipstream of a 70,000 pph stoker boiler providing steam to Ohio University`s Athens, Ohio campus. A total of over 90 experimental trials have been performed using the pilot-scale moving-bed LEC dry scrubber as a part of this research project with run times ranging up to a high of 125 hours. SOâ removal efficiencies as high as 99.9% were achievable for all experimental conditions studied during which sufficient humidification was added to the LEC bed. The LEC process and conventional limestone scrubbing have been compared on an equatable basis using flue gas conditions that would be expected at the outlet of the electrostatic precipitator (ESP) of a 500 MW coal-fired power plant. The LEC was found to have a definite economic advantage in both direct capital costs and operating costs. Based on the success and findings of the present project, the next step in LEC process development will be a full-scale commercial demonstration unit.
A spray of tiny droplets containing water and calcium hydroxide is used to absorb and react with sulfur dioxide. Simultaneously, water evaporates from the droplets, and solid particles are formed. These particles contain calcium sulfite (the reaction product), unreacted calcium hydroxide, and a small amount of equilibrium moisture. The process is modeled as a spray of isolated droplets that undergo no internal circulation. Sulfur dioxide mass transfer is treated as a series of resistance in the gas and liquid. In the liquid, dissolved sulfur dioxide diffuses through a stagnant film to react with calcium hydroxide. A monodisperse spray is considered first, followed by a polydisperse spray. The effects of important process variables on the conversion of sulfur dioxide are explored in both systems. For instance, as more water is sprayed into the gas, the conversion of sulfur dioxide increases, because the liquid surface area is greater and the droplets take longer to dry. The conversion also increases as more calcium hydroxide is added to the liquid, because the liquid-phase resistance decreases. The conversion decreases substantially at high sulfur dioxide concentration, because less sulfurous acid ionizes. Backmixing the gas often increases the conversion of sulfur dioxide, because the droplets dry more slowly. Reducing the size of the monodispersion, or the mean size of the polydispersion, also increases the conversion, because the equilibrium moisture has greater surface area. The conversion of sulfur dioxide in a monodispersion is compared with the conversion of sulfur dioxide in a polydispersion having the same volume to surface ratio.
The dry reaction between SO2 and limestone has been investigated at low temperatures. The study was focused on the wet-dry scrubbing application. Parameters investigated included: temperature: 313–353 K, SO2 concentration: 50–4000 ppm, oxygen concentration: 0–9 percent, carbon dioxide concentration: 0–10 percent, relative humidity 0–92 percent, limestone panicle diameter: 4–100 microns, and limestone conversion: 0–95 percent. The study has revealed that the relative humidity, the particle diameter and the limestone conversion have the most dramatic impacts on the reaction rate. A suggested reaction mechanism is outlined in great detail.
Equilibrium concentrations of liquid and vapor were measured over the liquid concentration range 0.59 to 4.48 grams per 100 grams and the temperature range 30° to 80° C. A unique static cell that employs mercury to sample the gas phase is described. The data are correlated by taking into account the dissociation of the molecular species in solution and the variation of the activity coefficient of the ionic species. The resulting equations are used to construct a table presenting data over the range 0.01 to 20.0 grams per 100 grams and 0° to 130°C. These values are compared with data reported in the literature.
Previous experimental data on mass transfer between particles and fluid in fixed and fluidized beds are reanalyzed and correlating equations are developed for the various situations.
Bench scale studies of the reaction between sulfur dioxide and alkaline sorbents were performed in an integrated fixed bed reactor at low temperature, simulating the fabric filter dust layer of dry FGD systems with SO2 laden gas passing through it. Three major groups of sorbent were tested: calcitic and dolomitic hydrated lime, sodium bicarbonate, and limestone (calcite aragonite) with salt additives such as CaCl2, Ca(NO3)2, and NaNO3. Relative humidity significantly affected SO2 absorption by lime and limestone plus additives, but had less impact on NaHCO3 reactivity. Different reaction patterns for various sorbents were observed: the SO2/hydrated lime reaction rate was reduced with conversion of the sorbent, resulting in limited lime utilization, whereas the reactions between SO2 and NaHCO3 and SO2 and CaCO3 plus additives seem to proceed a constant rate, potentially leading to high sorbent utilizations. Results are discussed relating to sorbent screening for dry FGD systems.
A model of pore structure evolution has been developed for gas-solid reactions exhibiting pore closure behavior. A cylindrical pore network of constant coordination number is used to represent the porous medium, and both discrete and continuous distributions of pore size are considered. The mathematical model takes into consideration the effects of pore overlapping and nonuniform pore growth, and follows the formation of inaccessible volume in the interior of the reacting particles by use of percolation theory concepts. The model is applied, along with an appropriate diffusion and reaction model, to the investigation of the transient behavior of calcined limestone particles in an environment of sulfur dioxide.
Incluye bibliografía e índice
Thesis (Ph. D.)--Ohio University, March, 1991.
Thesis (M.S.)--Ohio University, June, 1993.
Thesis (M.S.)--Ohio University, November, 1989.
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