Energy & Fuels 1993, 7, 743-745
743
Fuel Minerals, Fouling, and Slagging S . A. Benson’ Energy and Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202-9018
J. N. Harb Department of Chemical Engineering, Brigham Young University, 350 CB, l’rovo, Utah 84602 Received April 8, 1993. Revised Manuscript Received June 2, 1 9 9 P
Thrust Area 2, Fuel Minerals, Fouling, and Slagging, is focused on obtaining a clear understanding of the role of fuel inorganic components in combustion processes. During the combustion process, the inorganic components are transformed to ash. The principal problems associated with ash are deposition on heat-transfer surfaces, erosion, corrosion, and formation of fine particulate that is difficult to collect. In addition, catalytic inorganic components impacts are noted in a number of coal combustion processes, e.g., burnout and devolatilization. The specific objectives of Thrust Area 2 are the following: (1) to characterize the chemical and physical transformations of inorganic components, (2) to characterize the catalytic role of inorganic components on oxidation and devolatilization, (3) to establish submodels for mineral behavior, (4)to perform key experiments to define model parameters, (5) to incorporate submodels into comprehensive code, and (6) to evaluate models.
Introduction Coal is a very heterogeneous material consisting of both organic and inorganic material. The portion of coal that is inorganic is usually small, but can and often does have a profound impact on combustion and gasificationsystems. This material greatly affects boiler design and has an impact on the flexibility the operator has to alter fuel sources and system operating procedures. The principal problem noted in combustion systems due to coal minerals is ash deposition, leading to reduced heat transfer and often reduced unit availability. In addition to ash deposition, catalytic mineral impacts are noted in a number of coal combustion processes, e.g., burnout and devolatilization. These impacts are greatly affected by the form of the inorganic material found in the coal. The efforts of Brigham Young University’s Advanced Combustion Engineering Research Center (ACERC) in minerals behavior build on the many activities currently underway at numerous university, government, and industrial laboratories around the world. ACERC researchers have established collaborative relationships with several of the recognized expert groups and continue to expand these relationships. The ACERC focus is on fireside modeling. These cooperative relations will enable ACERC researchers to utilize the information and, in some cases, submodels developed by other researchers to accomplish the goal of describing minerals behavior during combustion. Project Overview This program in minerals behavior attempts to provide necessary information related to model development. Work is underway on mineral matter catalysis, transformation, and subsequent deposition processes. Key issues identified include the following:
* Abstract published in Adocmce ACS Abstracts, October 15, 1993.
1. Characterization of Minerals in Coal and Chars. Detailed information on concentrations, mineralogy, and local chemistry is needed for representative (ACERC)coals and chars. 2. Role or Catalytic Minerals in Char Oxidation. Knowledge of the kinetics and mechanisms of catalytic reaction during pyrolysis and char oxidation during combustion is needed. 3. TransformationlParticle Formation. I t is important to know the various phases present as ash moves through the system. In addition, the size and other physical characteristics, such as the viscosity of the liquid phases and partitioning between solid, liquid, and vapor phases, must be known. 4. Transport Processes. Ash only sticks to a surface if it arrives at the surface; thus, the movement of all classes of material must be determined. 5. Sticking Behavior. Once the material reaches the surface, the combination of the target (deposit surface) and incoming material must be such that the impinging material sticks. A basic understanding of the chemical1 physical processes governing sticking behavior is greatly needed. 6. Deposit Growth. The deposition of material on a heat-transfer surface is a dynamic system. Reactions involving incoming material-including ash particles, gas diffusion (SOz), and migration of mobile species-may all affect the changing chemistry within the deposit. In addition, deposit shedding and erosion influence the buildup of deposits. A stationary deposit occurs when the buildup, shedding, and erosion mechanisms come into balance. The time required to balance may vary between 1day and years. These mechanisms of growth and removal need to be understood and modeled. Accomplishments Significant progress has been made in all projects within Thrust Area 2, Fuel Minerals, Fouling, and Slagging. The
0SS7-0624/93/2507-0743$04.QQIQ 0 1993 American Chemical Society
744 Energy &Fuels, Vol. 7, No. 6,1993
most significant accomplishments obtained in the past year in the behavior of mineral matter involved model development and experimental work. Progress in the area of mechanisms of deposit formation involved determining the types of particles that initiate deposit formation. Deposit initiation studies were performed to examine selected major factors that influence fouling deposit initiation. The approach was to combust several ACERC coals of varying rank in a bench-scale, drop-tube furnace system. Deposits formed from these coals were examined by optical and scanning electron microscopy to determine the morphology and composition of strongly adhering particles. Mechanisms for ash deposit initiation that were observed will prove useful for developing and verifying ash transport and deposit initiation models. Progress in the area of char minerals and surface properties involved the investigation of the roles of calcium mineral catalysis and pore properties in the oxidation of Beulah Zap and Dietz coal chars. TPO studies of C, CO, and COZ on CAO provided new mechanistic evidence suggesting CO is formed on carbon and COz at the carbonCaO interface where it migrates to the CaO surface and eventually desorbs. Catalysis of the oxidation of a hightemperature Dietz coal char was investigated by TGA at low reaction temperatures and in an inverted drop-tube furance at high temperatures. The intrinsic reactivities from TGA of the original Dietz char and 4% CaO/Dietz char are the same, within experimental error, while that for demineralized Dietz char at 5-6 times lower. This catalytic effect is significant, but less than observed for Zap and synthetic chars. High-temperature reactivity data are presently being analyzed. The effects of air exposure on NZand COz surface areas of coals and chars were also investigated; apparently, exposures to air of 2-24 h have no measurable effect on surface area. An investigation comparing the physical properties of chars prepared in a flat flame burner (FFB) and in a high-pressure controlled profile (HPCP) drop-tube furnace was initiated. NZ surface areas of chars prepared in the FFB are 50-100 times larger than for the coal and for the chars prepared in the HPCP. True (solid) densities are larger, particle densities are smaller, and porosities are higher for the samples prepared in the FFB relative to those prepared in the HPCP. COZsurface areas are comparable for the chars prepared in the two different reactors. Research Projects Behavior of Mineral Matter i n Coal Combustion Systems (Project 2E). A detailed understanding of the inorganic components under combustion conditions is required in order to develop reliable predictions. This project incorporates both experimental and theoretical techniques to identify and quantify the processes that govern inorganic transformations in coal combustion systems. This project has two principal objectives: (1) fundamental investigation of inorganic component release, transformation, and deposition; and (2) incorporation of current knowledge into a submodel for use in the ACERC comprehensive computer codes. The objective of this project is to develop submodels to predict the behavior of inorganic components during pulverized coal combustion. The completed submodels will address heat recovery, catastrophic failure, and ash management strategy. The general approach involves
Benson and Harb identifying key processes, performing laboratory-scale simulations, simulating pilot-scale combustion testing, and developing the mineral matter submodel. The experimental work for the project involved several aspects. The first was to demonstrate the computercontrolled scanning electron microscopy (CCSEM) capabilities and continue development to include quantification of mineral/mineral associations. The second was work on application of FTIR techniques to examine ash deposits in situ. The third was to complete experimental work on the examination of mineral transformations under staged combustion. The most significant accomplishments obtained in the past year involved model development and experimental work. Key accomplishments in the area of model development included (1) the development and use of a model to describe deposit buildup, (2) acquisition and interfacing of a mineral transformation model developed at the Energy and Environmental Research Center (EERC) with Thrust Area 2 calculations, (3) qualitative comparison of predicted behavior with experimental results in the ABB-CE fireside performance test facility, and (4) the development of a method to close the overall energy balance in the comprehensive 2-Dcode. Key accomplishments occurred in three areas: (1) CCSEM, (2) in situ FTIR analysis of deposit, and (3) staged combustion experiments in a laboratory-scale reactor. Char Minerals/Surface Properties (Project 2H). This project involves investigation of (1) the catalysis (rates and mechanisms) of the oxidation of synthetic chars and coal chars by calcium minerals and (2) surface/pore properties of coal chars prepared from rank representative coals. The studies on char minerals catalysis and characterization involve (1) measurements over a representative range of temperatures of oxidation rates of mineralfree and CaO-catalyzed chars (synthetic and coal-derived); (2) measurement of active site concentrations on carbon and mineral phases of the same chars; (3) studies of adsorption and reaction of oxygen, CO, and COZon CaO; and (4) study of the effects of burnout on iron mineral chemistry. The studies on char pore/surface properties involve (1) measurements of surface areas and pore-size distribution of selected coals and chars, (2) measurements of densities and porosities of selected coals and chars, (3) determination of active surface site concentrations in organic and inorganic phases in chars, and (4) determination of functional groups on the surfaces of char as a function of burnout. Work on the catalysis of Dietz char oxidation by CaO was performed on char produced a t high temperatures from the raw coal and demineralized coal. The oxidation rates were obtained using a TGA 10% 02.Based on the calculated rates at 600 K, a 6-fold decrease in rate was observed after the removal of the active minerals by HC1 acid washing of the char. Studies of the surface properties were divided into four areas: (1) measurement of surface areas and densities were conducted on untreated, demineralized, and remineralized chars prepared from Beulah Zap coal, the measured solid densities for the Zap coal, char, and demineralized char being 1.45, 1.95, and 1.81 g/cm3,respectively; (2) standard operating procedures for surface area measurements were developed using a new surface area analyzer; (3) an investigation, conducted to determine the effects of air exposure on surface areas of coal, indicated that for a 24-h exposure to a stream of air, no measurable effects on the surface area of coals or chars
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Fuel Minerals, Fouling, and Slagging were noted; and (4)collaboration with other investigators on the surface and pore properties of chars was conducted. Minerals Behavior Measurements for Model Evaluation (Project 21). Studies were performed to examine selected major factors that influence fouling deposit initiation. The approach was to combust several ACERC coals of varying rank in a bench-scale, drop-tube furnace (DTF) system. Deposits formed from these coals were examined by optical and scanning electron microscopy to determine the morphology and composition of strongly adhering particles. Mechanisms for ash deposit initiation observed will prove useful for developing and verifying ash transport and deposit initiation models. Several ACERC coals, prepared as 70-80% -200-mesh samples, were analyzed using standard coal characterization techniques and CCSEM: Beulah lignite, Dietz subbituminous, Utah Blind Canyon bituminous, Pittsburgh No. 8 bituminous, and Illinois No. 6 bituminous coals. In addition, a non-ACERC Illinois No. 6 coal and a synthetic coal containing pyrite and kaolinite were included for comparison. A laminar flow DTF was used to combust the coals and collect deposits on a cooled cylindrical steel substrate. To provide a cleaner surface for potentially adhering particles, several deposit-generating tests incorporated a simulated “soot blower” to remove loosely adhering particulate material at intervals, as the deposit was grown. Fly ash and deposits were collected for each coal. CCSEM was used to analyze the fly ash and the initial deposits. The
deposits, however, required a more customized version of the CCSEM analysis. Approximately 2000 particles were analyzed for each fly ash sample, and about 200 adhering ash particles were analyzed for the deposits. The primary outputs of CCSEMare particle average diameters, particle aspect ratios or shape factors, particle composition expressed as normalized X-ray energy counts for 12 major elements, and a close-to-stoichiometrically-correctmineral identity.
Plans Efforts will continue towards the stated Thrust Area objectives and the development of anticipated products. Specifically, we plan to: 1. Continue development of analytical tools to examine coal minerals, fly ash, and deposits (at the EERC and
BYU). 2. Perform additional experiments to further elicit our understanding of mineral transformations. 3. Develop a quantitative description of observed mineral matter behavior. 4. Continue investigation of mineral catalysis of char oxidation in order to identify and quantify the key mechanisms. 5. Continue development of a mineral matter submodel(s) for use with the comprehensive ACERC computer codes.
