A HIGH TEMPERATURE DOUBLE KNUDSEN CELL MASS SPECTROMETRY STUDY OF GAS SPECIES EVOLVED FROM COAL-
PETCOKE MIXED FEEDSTOCK SLAGS
Jinichiro Nakano1,2, Takashi Nagai3, James Bennett1, Anna Nakano1, and Kazuki Morita4
1 U.S. Department of Energy National Energy Technology Laboratory, 1450 Queen Ave., Albany, OR 97321 USA
2 AECOM, P.O. Box 1959, Albany, OR 97321 USA
3 Department of Mechanical Science and Engineering, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba, 275-0016, Japan
4 Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656 Japan
Keywords: Gasification, Slag, Vanadium, Coal, Petcoke, Vapor pressure Abstract
In this work, ion currents of gaseous species from synthetic slag mixtures mimicking those from coal-petcoke feedstock were measured by double Knudsen cell mass spectrometry at a temperature range of 1000 °C to 1300 °C and an oxygen partial pressure of approximately 10-10 atm. The majority of gaseous vanadium was found to be present in the form of V2O3, whose vapor pressure rapidly increased with increasing petcoke slag addition to coal slag. Effects of temperature, vanadium content in slags, and coal/petcoke ratios, with an emphasis on vanadium and alkali vapor species, are discussed.
Introduction
An increased use of petcoke with/without coal as carbon feedstock in gasification has introduced vanadium oxide to the slag system, leading to unknown chemical and physical slag properties in the gasifier environment. Appreciable quantities of vapors from volatile components (including vanadium oxide) of the feedstock slag can interact with gasifier construction materials (such as the refractory liner), the radiant or convective syngas coolers, and/or thermocouple sensor; causing increased refractory wear, system fouling, or component failure.
Thermodynamic databases for vanadium-containing slags are not commercially available, which has been in part limiting optimization of gasification operation to a trial and error basis. In order to enable predictions of the thermodynamic nature of vanadium slags existing during gasification, a long-term study of experimental vanadium-bearing gasifier slags was initiated by NETL [1-5].
With an emphasis placed on phase equilibria of coal-petcoke mixed slags in the past [2, 5], a study involving thermochemical properties was needed to aid in the creation of thermodynamic databases for vanadium containing slag systems.
In this work, vapor pressures of gaseous species evolved from synthetic coal-petcoke slag mixtures were determined using ion currents measured by double Knudsen cell mass spectrometry. Effects
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
of temperature, vanadium oxide content, and coal slag/petcoke slag ratios on the evolution of gas species from synthetic coal/petcoke slag mixtures are discussed.
Experimental
Synthetic slag samples used in this study were prepared from reagent grade oxide powders (Alfa Aesar, 99.5%+). Coal slag and petcoke slag compositions were determined based on literature data [6-8]. The mixtures were heated at 1450 °C for 1 hour in a 64 at.% CO-36 at.% CO2 gas mixture (an oxygen partial pressure of 3×10-9 atm) at 30 ml/min, then furnace cooled to room temperature in the same atmosphere. After cooling, the samples were ground to powders using a WC shutter box and stored in Ar sealed glass vials prior to designated analysis. The sample chemistry of the premelted slags was conducted using an inductively coupled plasma with optical emission spectrometry (ICP-OES) technique, with results listed in Table I. The synthetic slag compositions were varied from 0 to 100 wt.% petcoke slag, the balance being coal slag.
Table I. Chemistry of synthetic coal-petcoke slag mixtures used in this study (normalized on an oxide basis)
A schematic image of the high temperature double Knudsen cell mass spectrometer is shown in Figure 1. Details of the equipment are described elsewhere [9]. A sample chamber was kept in an oxygen partial pressure of approximately 10-10 atm by evacuating the chamber during measurements. A sample and a reference substance (V2O3, 99.7%, Alfa Aesar) were placed in separate Knudsen cells (Mo) which were heated by Ta resistance heating elements. Temperatures of the samples and cells were measured by three thermocouples located in holes at the bottom of the cell. An atomic beam of evaporated species from the Knudsen cells was detected by a quadrupole mass spectrometer. For each sample, ion currents of the evolved gas species were measured three times at each temperature (1000 °C through 1300 °C at 50 °C increments. This procedure was repeated three times using different portions of the samples at each temperature for reproducibility.
Figure 1. A schematic of the high temperature double Knudsen cell mass spectrometer used in this work
A vapor pressure of species i, Pi, was converted from ion current, Ii, measured from the mass spectrometer, using the following relationship [10]:
𝑃𝑃𝑖𝑖=𝐼𝐼𝑆𝑆𝑖𝑖
𝑖𝑖𝑇𝑇 (1)
where Si is the device dependent constant including ionization cross-section and efficiency of the ion detector and T the absolute temperature of a sample.
Results and Discussion
During the ion current measurements from 1000 to 1300 °C, only Na, K, and V2O3 were detected as vapor species evolving from the synthetic slag samples. Wang et al. reported the presence of vanadium based gas species V, VO, and VO2 when testing vanadium-containing slags (Al2O3- CaO-MgO-SiO2-V2O3) at elevated temperatures [11]. The difference may be attributed to a combination of experimental parameters; including higher temperatures (1550 – 1650 °C), lower vanadium concentrations (0.01 – 0.10 at.% V2O3), and higher CaO/SiO2 basicity (0.8 – 2.0).
Vapor pressures of the species found in this study are presented in Figure 2. Absent points represent no vapor pressure, i.e., essentially zero pressure detected. In general, the vapor pressures tend to increase with increasing petcoke slag/coal slag ratios for the slag compositions studied.
Note that the vanadium concentration increases as the quantity of petcoke added as carbon feedstock increases. Vanadium oxide seems to promote increases in the vapor pressure of Na and K evolved from the slags. No Na vapor was detected for 0 wt.% vanadium oxide in the slag at 1000 °C and 1050 °C. A slag composition based on at least 80 wt.% petcoke slag was required for a K vapor pressure to become detectable at 1000 °C, while it was detected in a 60 wt.% petcoke slag at 1050 °C and a 40 wt.% petcoke slag at 1100 °C.
Depressions of the Na and K vapor pressures occurred toward the mid petcoke slag concentrations, as seen in Figure 2 (b), which becomes more apparent at lower temperatures. Nakano et al. reported the presence of calcium aluminosilicate and spinel in vanadium-bearing gasifier slags [1]. In the report, the formation of calcium aluminosilicate was only noted at 1200 °C (not at or above 1350
°C). With more vanadium in the slag, the stability of calcium aluminosilicate shifted toward calcium-richer, possibly by vanadium atoms replacing aluminum atoms in the crystal structure.
Calcium deficiency in slag also influences slag basicity (C/S ratio), which would affect the immiscibility of slag at lower temperatures. The decrease in the Na and K vapors with increasing vanadium oxide at a low range of the petcoke slag concentrations could be attributed to the spinel phase (Fe(Al, V)2O4) accepting more vanadium over aluminum [1], releasing aluminum into the molten slag forming an amorphous aluminosilicate which incorporate alkali metals.
Vapor pressures of V2O3 were not detected (or detectable) below 1250 °C in any of the slag samples. A petcoke slag concentration of at least 60 wt.% was needed to release a sufficient quantity of V2O3 vapor at 1300 °C, and at least 80 wt.% petcoke was needed to evolve V2O3 vapor at 1250 °C. The vapor pressure of V2O3 is likely related to the stability of the crystalline V2O3
phase and the saturation of V2O3 in slags.
Figure 2. Partial pressures of (a) Na, (b) K, and (c) V2O3 evolved from synthetic coal-petcoke mixed slags with respect to petcoke slag concentration
The effect of temperature on the partial pressures of gas species evolved from the slag samples is presented in Figure 3. The vapor pressures of all gas species observed (K, Na, and V2O3) tend to increase with increasing temperature. The Na and K curve depressions found in Figures 2 (a) and (b) are clearly reflected in Figure 3. The presence of vanadium oxide in slags at certain
concentrations (estimated between 23 – 34 wt.%) lowered the vapor pressures of alkali metals (Na and K) while increased that of V2O3. The vanadium effect on lowering the Na and K vapor evolution from molten slags is, however, minimized at temperatures higher than 1200 °C (as previously noted, 1250 °C is the temperature where V2O3 starts to have a measurable vapor pressure). In order to reduce the vapor pressure of V2O3, crystal phases that incorporate vanadium into them may be considered. If a linear regression is assumed, an approximation of vapor pressures at higher temperature would generate useful information for the gasification industry.
Figure 3. Partial pressures of (a) Na, (b) K, and (c) V2O3 evolved from synthetic coal-petcoke mixed slags with respect to temperature
Conclusions
Evolution of vapor species from synthetic slags simulating mixtures of coal slag and petcoke slag in gasifiers was investigated using a high temperature double Knudsen cell mass spectrometry technique. Na, K, and V2O3 species were detected at temperatures between1000 and 1300 °C in an oxygen partial pressure of approximately 10-10 atm. Vapor pressures of the vapor species generally increased with increasing petcoke slag concentrations and temperature. Vapor pressures of Na and K were lowered at approximately 40 – 60 wt.% petcoke slag especially below 1250 °C, which was likely caused by a combination of thermodynamic parameters. Vapor pressure information, as thermochemical data, will contribute to the thermodynamic database development.
Acknowledgement
This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. This research was supported in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education.
Disclaimer: ‘‘This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof’’.
References
1. J. Nakano, et al., "Crystallization of Synthetic Coal-Petcoke Slag Mixtures Simulating Those Encountered in Entrained Bed Slagging Gasifiers," Energ Fuel, 23 (2009), 4723-4733.
2. J. Nakano, et al., "Phase Equilibria in Synthetic Coal-Petcoke Slags (SiO2-Al2O3-FeO-CaO- V2O3) under Simulated Gasification Conditions," Energ Fuel, 25(7) (2011), 3298-3306.
3. J. Nakano, et al., "Interactions of refractory materials with molten gasifier slags," Int J Hydrogen Energ, 36(7) (2011), 4595-4604.
4. J.P. Bennett, et al. Causes of Phosphate Migration in High Cr2O3 Gasifier Refractories and the Impact on Slag Wear and Spalling. in UNITECR 2015. 2015. Vienna, Austria.
5. J. Nakano, et al., "Thermodynamic effects of calcium and iron oxides on crystal phase formation in synthetic gasifier slags containing from 0 to 27 wt.% V2O3," Fuel, 161 (2015), 364-375.
6. W.A. Selvig and F.H. Gibson, Analyses of Ash from United States Coals. Bureau of Mines Bulletin. Vol. 567. 1956, Washington, USA: United States Government Printing Office.
7. R.W. Bryers, "Utilization of Petroleum Coke and Petroleum Coke/Coal Blends as a Means of Steam Raising," Fuel Process Technol, 44(1-3) (1995), 121-141.
8. M.L. Swanson and D.R. Hajicek. Advanced High-Temperature, High-Pressure Transport Reactor Gasification. in International Conference on Fluidized Bed Combustions. May 31, 2001. Reno, NV.
9. T. Nagai, et al., "Mass spectrometric study on phosphorus in molten carbonsaturated iron," Isij Int, 47(2) (2007), 207-210.
10. M. Heyrman, et al., "Improvements and new capabilities for the multiple Knudsen cell device used in high-temperature mass spectrometry," Rapid Commun Mass Sp, 18(2) (2004), 163- 174.
11. H. Wang, et al., "High-temperature mass spectrometric study of the vaporization processes of V2O3 and vanadium-containing slags," Rapid Commun Mass Sp, 24(16) (2010), 2420-2430.