Energy Fuels 2010, 24, 4153–4160 Published on Web 07/07/2010
: DOI:10.1021/ef100398z
Influence of the Temperature on the Release of Inorganic Species during High-Temperature Gasification of Hard Coal Marc Bl€ asing,* Tobias Melchior, and Michael M€ uller Institute of Energy Research (IEF-2), Leo-Brandt-Strasse 1, 52425 J€ ulich, Germany Received March 31, 2010. Revised Manuscript Received June 15, 2010
Several volatile inorganic species are of concern in future integrated gasification combined cycle (IGCC) power systems because of their relation to erosion, corrosion, fouling, and slagging. The aim of this work was to obtain more information on the influence of the temperature on the release of Na, K, Cl, and S species during gasification. Therefore, six hard coals were gasified in lab-scale experiments in a helium/ oxygen atmosphere at 1100, 1400, and 1700 °C. The results represent conditions in an entrained-flow gasifier. Hot-gas analysis was performed by molecular beam mass spectrometry. Species of interest were HCl, H2S, COS, SO2, NaCl, Na2SO4, KCl, and KOH. In principle, the results show a strong influence of the temperature and the coal composition on the release of the species under investigation; e.g., the release of KOH is strongly increased with an increasing temperature.
studies showed that these species form an important factor during fouling and corrosion in gasification and combustion facilities.6-9 Scandrett10 summarized several studies, and in general, the amount of gas-phase sodium and potassium in the flue gas entering the gas turbine should be less than 20 ppbw. Coal contains a variety of inorganic constituents in major (e.g., Si, Al, and S), minor (e.g., Na, K, Cl, and Ca), and trace (e.g., Hg) amounts in variable quantities, mostly depending upon the coal rank and origin.11-17 High-temperature gasification, e.g., entrained-flow gasification with temperatures about 1250-1600 °C, melts nearly all of the origin mineral phases and destroys all metal-organic bonds within the coal matter, leading to exhaustive volatilization of weakly bound species.5 Even part of the refractory mineral matter can be released during high-temperature gasification of coal.18
1. Introduction The world demand for energy is rapidly increasing. Much of the energy is generated by use of fossil fuels. Because fossil fuel resources are limited, there is a growing need to use the remaining fuel as effectively as possible and to use low-grade fuels (e.g., high-ash lignite or hard coals with a higher ash content) extensively in the future.1 Because of a growing awareness for the environmental protection, the development of cleaner and more efficient technology for energy generation is an important topic of energy research.2 High-temperature gasification systems have been proven to be efficient in energy generation and at minimizing harmful emissions while handling low-grade, high-ash fuels.3-5 One auspicious high-temperature process is the integrated gasification combined cycle (IGCC) process with an entrained-flow gasifier. State of the art single-stage pressurized entrained-flow gasifiers have an efficiency of 38-50% lower heating value (LHV) by now.5 However, there are several problems connected with the use of coal in advanced gasification power plants. The product gas must be cleaned of Na, K, Cl, and S species, because various
(9) Srivastava, S. C.; Godiwalla, K. M.; Banerjee, M. K. Review— Fuel ash corrosion of boiler and superheater tubes. J. Mater. Sci. 1997, 32, 835–849. (10) Scandrett L. A. The removal of alkali compounds from gases at high temperature. Ph.D. Thesis, Darwin College, Cambridge, U.K., 1983; p 162. (11) Raask, E. Mineral Impurities in Coal Combustion: Behaviour, Problems, and Remedial Measures; Hemisphere Publishing Corporation: New York, 1985. (12) Yudovich, Y. E.; Ketris, M. P. Chlorine in coal: A review. Coal Geol. 2006, 67, 127–144. (13) Tillman, D. A.; Duong, D.; Miller, B. Chlorine in solid fuels fired pulverized boilers—Sources, forms, reactions, and consequences: A literature review. Energy Fuels 2009, 23 (7), 3379–3391. (14) Vassilev, S. V.; Kitano, K.; Vassileva, C. A. Some relationships between coal rank and chemical and mineral composition. Fuel 1996, 75, 1537–154. (15) Osborn, G. A. Review of sulfur and chlorine retention in coalfired boiler deposits. Fuel 1992, 71 (2), 131–142. (16) Maes, I. I.; Gryglewicz, G.; Machnikowska, H.; Yperman, J.; Franco, D. V.; Mullens, J.; Van Poucke, L. C. Rank dependence of organic sulfur functionalities in coal. Fuel 1997, 5, 391–395. (17) Groen, J. C.; Craig, J. R. The inorganic geochemistry of coal, petroleum, and their gasification/combustion products. Fuel Process. Technol. 1994, 40, 15–48. (18) Quann, R. J.; Sarofim, A. F. Vaporization of refractory oxides during pulverized coal combustion. Proceedings of the 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1429-1440.
*To whom correspondence should be addressed. Telephone: þ492461-61-1574. Fax: þ49-2461-61-3699. E-mail:
[email protected]. (1) Wolk, R. H.; McDaniel, J. High efficiency coal fuelled power generation. Energy Convers. Manage. 1992, 33 (5-8), 705–712. (2) Beer, J. High efficiency electric power generation: The environmental role. Prog. Energy Combust. Sci. 2007, 33, 107–134. (3) M€ uller, M.; Pavone, D.; Rieger, M.; Abraham, R. Hot fuel gas cleaning in IGCC at gasification temperature. Proceedings of the 4th International Conference on Clean Coal Technologies; Dresden, Germany, May, 2009. (4) Valenti, M. Coal gasification: An alternative energy source is coming of age. Mech. Eng. 1993, 114 (1), 39–43. (5) Higman, C.; Van der Burgt, M. Gasification; Elsevier: Amsterdam, The Netherlands, 2009. (6) Bryers, R. W. Fireside slagging, fouling and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22 (1), 29–120. (7) Bakker, W. High temperature corrosion in gasifiers. Mater. Res. 2004, 7 (1), 53–59. (8) Moses, C. A.; Bernstein, H. Impact study on the use of biomassderived fuels in gas turbines for power generation. NREL Technical Report NREL/TP-430-6085; National Renewable Energy Laboratory (NREL): Golden, CO, Jan, 1994. r 2010 American Chemical Society
4153
pubs.acs.org/EF
Energy Fuels 2010, 24, 4153–4160
: DOI:10.1021/ef100398z
Bl€asing et al.
Much of the Na in hard coals can be found within mineral grains, occurring as either halide or to a smaller extent bound in Na silicates.12,13 In contrast, K is mostly bound in aluminosilicate and, therefore, more likely released during secondary reactions occurring within the mineral matter of hard coal.11 Yudovich et al.12 and Tillman et al.13 have recently given comprehensive reviews of Cl in coal. In general, two broad modes of occurrence of Cl in coal have been highlighted: inorganically bound Cl in the form of discrete minerals, e.g., salt-like chlorine (NaCl) or to a much smaller extend Clcontaining silicates, and organic bound Cl with “semi-organically” Cl sorbed to the coal matrix, which is the main form of organically bound Cl.12,13 The amount of Cl is also an indicator of the fraction of NaCl in the coal.12,13 S can be found in the form of inherent mineral matter, usually mainly in the form of FeS2 or as organically bound S fixed in the coal matrix (e.g., thiophene, thiole, and sulfide).14-16 The release of Na, K, Cl, and S species during thermal conversion of coal and coal ashes has been investigated by multiple groups using several different experimental approaches and thermodynamic calculations. Experimental investigations on the influence of the temperature on the release of inorganic species during high-temperature gasification of coal are rarely found. The fate of Cl during water steam gasification (up to 1100 °C) of coal has been investigated by Gyo et al.19 They highlighted the increasing amount of released Cl with increasing temperature. Wang et al. predicted the formation of Na species for temperatures up to 1927 °C and atmospheric pressure under reducing conditions by thermodynamic equilibrium analysis.20 They found that the speciation of Na in the gas phase depends upon the oxygen content and temperature. Main vapor species for temperatures above 1427 °C were Na, NaOH, NaO, and NaCl. They further reported the increase of the relative concentration of gaseous Na species with an increasing temperature. In general, no stable solid S species was predicted by the thermodynamic modeling with FactSage. Despite growing knowledge of the release of Na, K, S, and Cl species during coal gasification, there are still a lot of open questions, e.g., release at gasification conditions relevant for the IGCC. Therefore, high-temperature, lab-scale gasification experiments were performed with a broad range of different hard coals. The results complement current analytical capabilities and data sets and can help to develop hot-gas cleanup methods.
Figure 1. Setup used for the gasification experiments in the temperature range of 1100-1700 °C. Table 1. Chemical Composition of the Hard Coal Samples (Mass %, as Received) C H N O S Cl Al Ca Fe K Mg Na Si
STD-1
STD-2
STD-3
STD-4
STD-5
STN-2
83.6 4.22 1.72 3.33 0.75 0.116 0.96 0.15 0.52 0.16 0.094 0.053 1.4
80.3 4.72 1.65 4.95 0.77 0.156 0.91 0.43 0.55 0.16 0.2 0.088 1.6
78.4 4.98 1.68 8.09 0.89 0.185 0.9 0.26 0.43 0.17 0.11 0.072 1.3
65.2 3.67 1.36 5.17 0.77 0.136 3.3 0.78 1.3 0.75 0.49 0.16 5.7
59.8 4.14 1.41 10.6 0.94 0.237 3.1 0.77 1.3 0.87 0.44 0.19 5.9
74.8 4.52 1.25 15.3 1.25 0.009 1.35 1.09 0.99 0.17 0.25 0.50 2.46
2.1. Fuel Preparation. Samples were taken from six different hard coals (five German hard coals and one Spitsbergen hard coal). The coals were dried and crushed in a mill to a particle size
