ACERC
Abstracts - 1994 |
ACERC
Abstracts - 1994 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
Bartholomew, C.H.
Catalytic Hydroprocessing of Petroleum and Distillates, Marcel Dekker,
Inc., New York, NY, 1994. Funded by Brigham Young University.
Hydrotreating, the catalytic conversion and removal of organic sulfur, nitrogen, oxygen and metals from petroleum crudes at high hydrogen pressures and accompanied by hydrogenation of unsaturates and cracking of petroleum feedstocks to lower molecular hydrocarbons plays an ever increasing key role in the refinery. Indeed, hydrotreating capacity has been growing steadily (at about 6% per year since 1976) and represents today nearly 50% of the total refining capacity. The increased application of hydrotreating can be ascribed to (i) the ever decreasing availability of light, sweet crudes and thus the increasing fraction of heavy, sour crudes that must be processed and (ii) the trend to increase upgrading of feedstocks for improvement of downstream processing such as catalytic reforming and catalytic cracking.
Hydrotreating of petroleum residua feedstocks involves three important reactions: hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodemetallization (HDN) for removal of organically-bound sulfur, nitrogen, and metals respectively. Sulfided Mo, CoMo, and NiMo catalysts used in these reactions are prepared by impregnating catlyst extrudates with solutions of Co, Mo and Ni followed by drying, calcinations at 400-500°C, and sulfiding with H2S/H2 at 350-400°C. The active sites for HDS and HDN are thought to be sulfur vacancies at the surface of a sulfide phase, e.g. CoxMoyS.
Hydrotreating involves a number of catalytic steps. For example, reaction steps in HDS include: (i) adsorptions of H2 and the organic sulfide, (ii) hydrogenolysis of the carbon-sulfu bond, (iii) hydrogenation of unsaturates, (iv) hydrocracking, and (v) desorptions of hydrocarbons and H2S. An important objective in hydrotreating is to maximize the rates of S, N, and metal removal, while minimizing the rates of hydrogenation and hydrocracking and therewith hydrogen consumption.
Sulfided resid hydrotreating catalysts are deactivated over a period of months by coke, metals and nitrogen compounds. The deactivation process involves a combination of uniform poisoning, pore mouth poisoning and pore blockage by (i) decomposition of organometallic compounds and (ii) buildup of soft coke and its transformation over a period of time to hard, crystalline coke. These problems are minimized by careful selection of uard beds, reactor design, and catalyst design; moreover, it is possible to regenerate coked catalysts with an oxygen burn.
Deactivation of hydrotreating catalysts has been fairly extensively studied. Several previous reviews of the literature and an international symposium have covered in some depth most aspects of this subject. Nevertheless, an updated overview of the key aspects of residua hydrotreating deactivation, including coke formation chemistry, metals deposition chemistry, catalyst and reactor design, and the use of mathematical models to simulate the deactivation process may be timely.
This review focuses on the deactivation of sulfide Mo, CoMo, and NiMo catalysts in hydrotreating of heavy residuum feedstocks. Coke formation, metals deposition, the roles of catalyst and reactor design in minimizing catalyst decline, and the application of modeling to design and prediction of deactivation rates are discussed in this review in the sections which follow.
Harb, J.N.; Slater, P.N.
and Richards, G.H.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis,
Washington, DC, 1994. Funded by ACERC.
A theoretical study was performed to investigate the effect of ash chemistry and non-constant thermal properties on the calculated heat flux through a coal ash deposit. A mathematical model, previously developed to describe the build-up of furnace wall deposits, was used to predict the rate of deposit growth, thermal conductivity, and porosity of the deposit, as well as the heat transfer through the deposit. In this study, a method to predict deposit emittance and absorbance as a function of composition and particle size was added to the deposition model. Simulations showed that the ash chemistry had a significant effect on the thermal and physical properties of the deposit and, consequently, the heat flux through the deposit. Significant differences in the predicted heat flux through the deposit were observed when constant values (not varying with time and/or position) for the deposit emittance and thermal conductivity were assumed. The observed differences are largely due to the inability of the constant properties to adequately predict the resistance of the inner layer of the deposit that significantly affects the heat transfer through the deposit.
Yu, H.; Marchek, J.E.; Adair,
N.L. and Harb, J.N.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis,
Washington DC, 1994. Funded by ACERC.
Computer Controlled Scanning Electron Microscopy was used to analyze the mineral composition of two eastern US bituminous coals. In addition, a procedure was developed to determine the association of the minerals with the coal matrix. The mineral content of each of the coals was also determined on a particle-by-particle basis and used to examine the association of various minerals in the coal. Examination of the mineral/mineral associations showed that the distribution of minerals in the coal was not random. In particular, results show lower levels of association for pyrite than for the other minerals examined.
Erickson, T.A.; O'Leary,
E.M.; Folkedahl, B.C.; Ramanathan, M.; Zygarlicke, C.J.; Steadman, E.N.; Hurley,
J.P. and Benson, S.A.
The Impact of Ash Deposition on Coal-Fired Plants, Taylor & Francis,
Inc., 1994. Funded by US Department of Energy, Electric Power Research Institute,
Tow, Texaco, Shell, Union Electric, Kansas City Power and Light, Minnesota Power
and Northern States Power.
Over the past five years, computer-based research tools have been increasingly applied in making important economic and operational decisions in the utility power industry. These tools-which include models, indices, databases, and data manipulation programs-are used by researchers, operators, and managers in the evaluation of coal utilization as an efficient and environmentally acceptable source of energy. Applicable tools that have been developed at, and are currently used by, the Energy & Environmental Research Center (EERC) include Partchar©, MINCLASS©, VISCAL©, MANAGER©, ATRAN, LEADER©, PHOEBE©, and PCQUEST©. These software applications range from databases for retrieving coal and coal product analysis, to computer codes to process coal and coal product analysis, to advanced models and indices to evaluate the operational impacts of specific systems.
Richards, G.H.
Investigation of Mechanisms for the Formation of Fly Ash and Ash Deposits for
Two Powder River Basin Coals, Ph.D./BYU, December 1994. Advisor: Harb
Galbreath, K.C.; Zygarlicke,
C.J.; Casuccio, G.S.; Moore, T.A.; Gottlieb, P.J.; Agron-Olshina, N.; Huffman,
G.P.; Shah, A.; Yang, N.Y.C.; Vleeskens, J.M. and Hamburg, G.
Fuel, 1994 (in press). Funded by US Department of Energy/Morgantown Energy
Technology Center and Brigham Young University.
Six laboratories collaborated in an international study of the computer-controlled scanning electron microscopy (CCSEM) method of quantitative coal mineral analysis. A total of five analyses were performed by most of the laboratories on three bituminous coal samples: Pittsburgh No. 8, Illinois No. 6, and Prince. Repeatability relative standard deviation was less than 20% for the four minerals analyzed: calcite, kaolinite, pyrite, and quartz. Reproducibility relative standard deviations (RSDR) ranged from 21% to 83%. Reproducibility of the kaolinite results was the poorest, with an average RSDR of 60%, and pyrite was the best with an average RSDR of 22%. The reproducibility of calcite and quartz analysis results was similar, with an average RSDR of 38% and 36%, respectively. Although pyrite content was determined the most precisely, normative mineral calculations indicate that the results are overbalanced. Improvement in the interlaboratory agreement of CCSEM results will require the development of a standardized calibration procedure.
Gopalakrishnan, R.; Fullwood,
M. and Bartholomew, C.H.
Energy & Fuels, 8:984-989, 1994. Funded by ACERC.
Catalysis by CaO, CaCo3, and CaSO4 of the oxidation of a well-defined, high purity synthetic char, Spherocarb, was investigated at low reaction temperatures using thermogravimetric analysis. The results indicate significant catalytic effects-up to 160-fold increase for CaCO3 catalysis, 290-fold increase for CaSO4, and up to 2700 times for CaO. Oxidation rates were likewise measured for fresh, demineralized, and Ca-loaded chars prepared from Beulah-Zap lignite coal in a flat flame burner at 1473 K. The oxidation rates of CaO-catalyzed Spherocarb and Zap are the same within experimental error, suggesting that the high reactivity of the Zap char is due in large part to catalysis by CaO. It was also found that chlorine added to Ca-loaded char had a negligible effect on its low-temperature reactivity.
Folkedahl, B.C.; Steadman,
E.N.; Brekke, D.W. and Zygarlicke, C.J.
The Impact of Ash Deposition on Coal-Fired Plants, Taylor & Francis,
Inc., 1994. Funded by US Department of Energy, Brigham Young University, Electric
Power Research Institute, Dow, Texaco and Shell.
Scanning electron microscopy/electron microprobe analysis (SEM/EMPA) techniques can quantify the inorganic phases present in complex coal combustion materials, as well as provide information on the morphology of the sample. Recent advancements in SEM instrumentation, data manipulation, and automated image analysis techniques have made the routine analysis of a large number of samples and data points possible. The scanning electron microscopy point count (SEMPC) routine, developed at the Energy & Environmental Research Center (EERC), randomly selects a statistically valid number of points and analyzes them for chemistry. This routine also allows for the concurrent storage of digitized images and the measurement of sample porosity. A mineral classification of the chemical point data is determined using a new data manipulation program, MINCLASS©. This new program uses carbon and oxygen region of interest (ROI) x-ray counts to differentiate between oxides and metals as well as sulfates and sulfides. A user-friendly graphical user interface (GUI) to execute under Windows© has been implemented, which makes the application very easy to learn and use. The GUI allows the user to easily select data sets that may be used to define and classify the chemistries of raw data. Mineral definition data sets have been developed for several types of samples, including coal combustion and gasification. Users may also enter their own mineral definitions for the program to use in the classification of Materials into mineral phases. In conjunction with this new program. MINCLASS©, a viscosity calculation program, VISCALC©, can be used to determine viscosity of silicate materials. This viscosity model will produce visual displays of viscosity distributions. The viscosity calculation also associates a viscosity value with each analysis point in a file. The user can retrieve a stored image and inspect the morphology of the area surrounding each point and relate it to the calculated viscosity. The measurement of porosity, calculation of viscosity, and determination of mineral-phase distribution for a sample are great assets in the characterization and determination of strength development in coal combustion deposits and by-products. The association of chemistry, mineralogy, and morphology is realized, utilizing the SEMPC technique developed at the EERC.
Karner, F.R.; Zygarlicke,
C.J.; Brekke, D.W.; Steadman, E.N. and Benson, S.A.
Power Engineering, 98:35-38, 1994. Funded by US Department of Energy,
Brigham Young University and Electric Power Research Institute.
Severe boiler fouling can be controlled. But, it demands a thorough understanding of how minor changes in the chemical and physical properties of coal and ash and operating conditions can cause hardened deposits. Traditional analytical techniques have helped reveal some of these mechanisms in the past, but limitations inherent to the analysis left many questions unanswered.
Now, however, scanning electron microscopy and electron microprobe analysis (SEM/EMPA) provide researchers with the analytical tools necessary to truly understand deposit formation mechanisms. These new techniques are exposing the size, shape, and chemistry of the myriad individual ash particles formed when coal burns. This and other data can be used to predict combustion behavior and its impact on ash deposition, waterwall slagging, and other steam tube deposits.
Microanalysis is capable of helping operators to identify more efficient and cost effective corrective measures as well. Recognized control methods (i.e., soot-blowing or plant capacity reductions) are often expensive and ineffective. The same is true for trial and error operating changes. Logic dictates analyzing coal and ash deposits first and then modifying operating procedures accordingly; SEM/EMPA facilitates the process.
Zygarlicke, C.J.; McCollor,
D.P. and Benson, S.A.
Proceedings of the Pacific Rim International Conference on Environmental
Control of Combustion Processes, Maui, HI, August 1994. Funded by US Environmental
Protection Agency, US Department of Energy/Morgantown Energy Technology Center
and Brigham Young University.
Trace metals pose a potential problem to emerging coal gasification electric power-generating systems. Some of the trace metals in coal are considered air toxics when released into the atmosphere and can also cause the degradation of fuel cell efficiency due to contamination. The fate of trace metals during coal conversion in integrated gasification combined cycle and integrated gasification fuel cell systems is closely tied to how the trace metals are associated in the coal and gasification conditions. Bench-scale gasification experiments were performed using Illinois No. 6 coal to determine the partitioning of Hg, Se, As, Ni, Cd, Pb, and Cr into gas, liquid, and solid phases as a function of gasification conditions and coal composition. Entrained ash was generated in a pressurized drop-tube furnace and collected using a multicyclone and impinger sampling train. Coal analysis revealed arsenic, Hg, Ni, Pb, and Se to be primarily associated with pyrite. Cr was associated primarily with clay minerals, and Cd appeared to have either a sulfide or an organic association. Cr, Pb, and Ni are enriched in the ash particulate fraction (collected in multicyclones), while Cd, As, Se, and Hg are depleted in the particulate and are more enriched in the very fine particulate or vapor-phase fraction (collected in the final filter and impingers). Oxygen contents were varied to represent both combustion and gasification systems. Most of the work was conducted at lower oxygen/carbon (O/C) ratios. Lower O/C ratios resulted in more Hg being driven to the vapor phase. Under the constant O/C ratios, Hg, Se, and Cd showed increasing volatility with increasing temperature in the reaction zone.
Benson, S.A.
Proceedings of the Second International Conference on Energy and Environment:
Transitions in East Central Europe, Prague, Czech Republic, November 1994.
Funded by US Department of Energy and Energy & Environmental Research Center.
The inorganic components (ash-forming species) associated with coals significantly affect boiler design, efficiency of operation, and lifetimes of boiler parts. During combustion in conventional pulverized fuel boilers, the inorganic components are transformed into inorganic gases, liquids, and solids. This partitioning depends upon the association of the inorganic components in the coal and combustion conditions. The inorganic components are associated as mineral grains and as organically associated elements, and these associations of inorganic components in the fuel directly influence their fate upon combustion. Combustion conditions, such as temperature and atmosphere, influence the volatility and the interaction of inorganic components during combustion and gas cooling, which influences the state and size composition distribution of the particulate and condensed ash species. The intermediate species are transported with the bulk gas flow through the combustion systems, during which time the gases and entrained ash are cooled. Deposition, corrosion, and erosion occur with the ash intermediate species are transported to the heat-transfer surface, react with the surface, accumulate, sinter, and develop strength. Research over the past decade has significantly advanced understanding of ash formation, deposition, corrosion, and erosion mechanisms. Many of the advances in understanding and predicting ash-related issues can be attributed to advanced analytical methods to determine the inorganic composition of fuels and the resulting ash materials. These new analytical techniques have been the key to elucidation of the mechanisms of ash formation and deposition. This information has been used to develop algorithms and computer models to predict the effects of ash on combustion system performance.
Zygarlicke, C.J.; Benson,
S.A. and Borio, R.W.
Coal Blending and Switching of Low-Sulfur Western Fuels, 281-300, American
Society of Mechanical Engineers, New York, NY, 1994. Funded by US Department
of Energy and Electric Power Research Institute.
Combustion ash formation and deposition behavior of Wyoming subbituminous/Oklahoma bituminous coal blends were examined using small-scale testing equipment. Inorganic constituents in the parent coals affected ash behavior. Wyoming entrained ash was finer-sized and enriched in calcium aluminosilicates, while the Oklahoma fly ash was larger-sized and enriched in silica. The 70/30 Wyoming/Oklahoma blend produced worse slag deposits while the 90/10 Wyoming/Oklahoma blend produced worse fouling deposits.
McCollor, D.P.; Zygarlicke,
C.J.; Toman, D.L. and Evenstad, D.N.
The Impact of Ash Deposition on Coal/Fired Plants, Taylor & Francis,
Inc., 1994. Funded by Brigham Young University and US Department of Energy.
A bench-scale method has been developed to measure in situ the force required to remove coal ash deposits grown under simulated fouling conditions. A suite of eight coals, representing a range of rank and known fouling behavior, were examined using this method. Statistical analysis of the data resulted in a good correlation of adhesion strength with an independently derived index of high-temperature fouling propensity and with 1(coal feed time), number of soot blowings, and number of noneffective soot blowings. Using this method, a coal or coal blend can be rapidly screened for fouling behavior under conditions simulating those attained in a particular utility boiler.
Richards, G.H.; Harb, J.N.;
Baxter, L.L.; Bhattacharya, S.; Gupta, R.P. and Wall, T.F.
25th Symposium (International on Combustion, 1994 (in press). (Proceedings
of the 25th Symposium (International) on Combustion, Irvine, CA, August
1994). Funded by Australian Research Council, US Department of Energy (Pittsburgh
Energy Technology Center) and ACERC.
Emission Fourier transform infrared (FTIR) spectroscopy data provide in situ,. Time-resolved, spectral emissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3 h of deposit growth on a tube in a cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelengths (>7 µm), emissions features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emissivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with experimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typical of deposits formed from western U.S. coals (0.45-0.55). Calculated total absorptivities are found to be lower than the corresponding emissivities.
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