M. E. Prudich’s research while affiliated with Ohio University and other places

Dry scrubbing that takes place after the air preheater (<350°F) is the mode of operation that is the primary focus of this chapter. At the relatively low temperatures that occur in this region of operation, the rate of the gas-solid reaction that drives SO2 capture in the convective (800–1200°F) and the combustion (1600–2400°F) zones is too slow to be significant. At lower temperatures, the presence of either liquid-phase or vapor-phase water is required in order to mediate SO2 capture and to produce reasonable capture rates. More specifically, this chapter focuses on work performed in support of the Limestone Emission Control (LEC) process. The LEC process is a unique system employing standard quarry-sized limestone to remove SO2 from coal-fired boiler flue gases. In the LEC process, hot flue gases (<350°F) are contacted with a bed of 1/32 to 1/4 inch limestone granules covered with a thin film of water. Sulfur dioxide is absorbed from the flue gas into the water film where it subsequently reacts with dissolved limestone. A layer of reaction products, primarily calcium sulfate and calcium sulfite, forms on the surface of the limestone as the reaction proceeds. The LEC process has demonstrated the ability to remove in excess of 90% (and under some conditions in excess of 99%) of the SO2 found in coal-combustion flue gases. This chapter includes a description of a mechanistic model for SO2 capture by a wetted limestone particle, process models for both fixed-bed and moving-bed LEC reactors, and support studies dealing with limestone solubilities and dissolution rates.

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.

This chapter is a reprint of a literature review prepared for the Ohio Coal Development Office by the Ohio Coal Research Consortium in 1990. It represents an assessment of the state of the art in flue gas desulfurization technologies at that time. It also served to motivate the work of the consortium starting in 1991, the results of which are presented in the following chapters. The discussion begins with a basic review of different technologies which are distinguished first by the amount of moisture in the sorbent feed and product, and second by the temperature range for the sorbent injection. The high temperature range corresponds to furnace injection; the medium temperature range corresponds to economizer injection; and the low temperature range corresponds to any system in place after the air heater. In each case, both process chemistry and contacting technologies are discussed. Low temperature technologies are further divided into duct injection, spray drying, capture in particulate control devices, and other special devices. The chapter concludes with a discussion of the fundamentals of sorbent and sorbent additive chemistry and the fluid mechanics, mass, and heat transfer.

Sorbent preparation techniques used today have generally been adapted from techniques traditionally used by the lime industry. Traditional “dry” hydration and slaking processes have been optimized to produce materials intended for use in the building industry. These preparation techniques should be examined with an eye to optimization of properties important to the SO2 capture process. The study of calcium-based sorbents for sulfur dioxide capture is complicated by two factors: (1) little is known about the chemical mechanisms by which the “standard” sorbent preparation and enhancement techniques work, and (2) a sorbent preparation technique that produces a calcium-based sorbent that enjoys enhanced calcium utilization in one regime of operation [flame zone (>2400°F), in-furnace (1600–2400°F), economizer (800–1100°F), after air preheater (<350°F)] may not produce a sorbent that enjoys enhanced calcium utilization in the other reaction zones. Again, an in-depth understanding of the mechanism of sorbent enhancement is necessary if a systematic approach to sorbent development is to be used. The long-term goals of the experimental program that resulted in this report were (1) defining the effects of slaking conditions on the properties of calcium-based sorbents, (2) determining how the parent limestone properties and preparation techniques interact to define the SO2 capture properties of calcium-based sorbents, and (3) elucidating the mechanism(s) relating to the activity of various dry sorbent additives. This chapter documents (1) the collection, production, and characterization of a series of limestone/hydrated lime/quicklime samples representing a range of Ohio limestone products, (2) an investigation of the importance of lime solubility enhancement in the evolution of surface area during the slaking process, and (3) a model/paper study of the effects of chemical additives on the performance in spray drying and in-duct injection processes.

The objective of this research is to develop and optimize preliminary process designs and cost estimates for a moving bed granular limestone desulfurization process typified by the LEC (Limestone Emission Control) process. The basic simulation programs have been written, debugged, and used to simulate the operation of a desulfurization plant with a cross flow moving bed configuration. The programs are based on first principles type mathematical models which reflect the underlying transport and chemical reactions which are involved in the desulfurization process. Spatial and temporal variations in component concentrations and temperature are considered. The simulation program centers around a resistance in series model which is used to calculate the sulfur dioxide transport rate. Resistances include mass transfer from the bulk gas to the condensed liquid surface, diffusion of sulfate and calcium ions in the liquid layer, and dissolution of calcium ions from the limestone surface. Sorbent blinding due to the formation of a precipitative layer and dry capture have also been incorporated into the model.

A novel wet/dry desulfurization post-Furnace process (ETS' Limestone Emission control (LEC) process) for SO[sub 2] removal has been described by Prudich et al.(1988). In this process hot flue gases are contacted with a bed of quarry-sized (1/4in. to 1/32in.) wet limestone granules. This thesis represents the development of a second generation model of ETS' LEC process. The first generation model developed by Prudich et al.(1988) the use of a fixed limestone bed. The work done on this has been well documented by Appell (1989) and Visneski (1991) process under consideration for this thesis involves a continuously moving limestone bed. The moving bed simplifies the process flowsheet and facilitates limestone reactivation. The moving-bed LEC process involves the use of a cross-flow pattern with the flue gas flowing horizontally across the limestone bed at speeds of around 1 to 2 feet per second and the limestone moving vertically downward through the LEC reactor at speeds of around 1 to 15 feet per hour. The primary parameters considered in the mathematical modeling of the moving-bed LEC process are inlet sulfur dioxide concentration, inlet gas-phase water concentration, inlet flue gas and limestone temperatures and water spray addition rate over the bed. For solution of the process, mass and energy balance equations derived as a function of the positions of the flue gas limestone are solved using a predictor-corrector method. The Adams-Bashforth (modified Euler's) method is used with a second order corrector.

The objective of this research is to develop preliminary process designs and cost estimates for four different configurations of scrubbers which contact sulfur dioxide laden flue gas with a wet, densely packed bed of 1/4 inch limestone gravel. The four configurations include a fixed limestone bed (ii), cocurrent flow of limestone and flue gas ({number_sign}2), counter-current flow (13), and a cross-flow moving bed configuration ({number_sign}4). First principles models are used to predict the performance of the scrubbers. A standardized nomenclature has been developed and maintained during the year. A standardized set of physical property and transport correlations has been developed. A general set of material and energy balances based on a first principles analysis of the underlying chemical and physical processes has been generated. Detailed equations for process configurations 1 through 4 have been developed. The numerical solution algorithms for configurations 1 through 4 have been coded and debugged. The simulation programs for configurations 1 through 4 have been tested for the effects of varying numerical step size. Design study cases have been developed using two representative coals with 1.5 and 3.5 wt% sulfur and assuming two levels of sulfur dioxide removal, 90 and 98%. Cost equations for capital and operating expenses have been developed and coded for the reactors and associated sorbent handling equipment. Case studies which examine the effect of bed depth on the overall economics have been generated. The results to date have been largely as expected. The major goal of developing a systematic and computationally efficient simulation package which can model a variety of process configurations has been achieved.

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.

A resistance-in-series kinetic model for the low temperature reaction of sulfur dioxide with limestone is presented. The resistances considered are the gas-phase transport of sulfur dioxide, the liquid-phase diffusion of both the sulfur species and the calcium species and the solid-phase dissolution of limestone. The model uses film theory to predict the liquid concentrations of the dissolved species and assumes an instantaneous reaction between the sulfur species and calcium species. The kinetic model incorporates three rate equations for the removal of sulfur dioxide. When the rate of removal is limited by the diffusion of sulfur dioxide across the gas film surrounding the limestone particle, a gas-phase controlled rate equation is used. When the diffusion of the reacting species through the liquid film covering the limestone particle is the predominant resistance, a liquid-phase controlled rate equation is used. When the rate is limited by the dissolution of limestone, a solid-phase controlled rate equation is used. The kinetic model is incorporated into a flow model for the fixed-bed Limestone Emission Control (LEC) system. The LEC system employs a fixed-bed of standard quarry-sized limestone to remove sulfur dioxide from coal-fired boiler flue gases. The flow modeling equations for the fixed-bed LEC system, which include simultaneous heat and mass transfer as applied to water-phase evaporation and condensation are also presented. The combined kinetic and flow model is subjected to a parametric study and the modeling predictions are compared with experimental results.

An experimental program is being carried out for the purpose of (1) defining the effects of slaking conditions on the properties of calcium-based sorbents, (2) determining how the parent limestone properties and preparation techniques interact to define the SOâ capture properties of calcium-based sorbents, and (3) elucidating the mechanism(s) relating to the activity of various dry sorbent additives. Accomplishments during the first year include: (1) the collection of a series of limestone/hydrated lime/quicklime samples representing a range of Ohio limestone products, (2) initial analyses of the suite of Ohio limestones, (3) the set-up/shakedown of a mercury porosimeter, (4) the design and construction of a bench-scale slaking reactor, and (5) the design and construction of a bench-scale calcination reactor. Project Year No. 2 research work included: (1) a continuation of the collection and characterization of a set of Ohio limestones, (2) the production of a complete set of calcined and hydrated products from the set of Ohio limestones, and (3) an investigation of the importance of lime solubility enhancement in the evolution of surface area during the slaking process.