Energy & Fuels 2001, 15, 95-105
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High-Temperature Combustion of Pyrolyzed Char Containing Calcium Sulfide A. C. Sheth* and S. A. Morton, III University of Tennessee Space Institute, 411 B. H. Goethert Parkway, MS-19, Tullahoma, Tennessee 37388 Received April 20, 2000. Revised Manuscript Received September 21, 2000
In response to the United States Department of Energy’s (U.S. DOE) national research program called “Combustion 2000,” the High Performance Power System (HIPPS) scheme was developed by the Foster Wheeler Development Corporation (FWDC). The HIPPS involved pyrolyzing a high sulfur-containing coal in the presence of limestone sorbent to capture sulfur as CaS and then combusting the resulting char containing spent and unspent sorbent and coal-ash with the fuel gas (mostly CO, H2, and CH4) in a high-temperature furnace called HITAF to drive the steam cycle. To maintain the overall energy efficiency of the HIPPS scheme and to meet the stringent limits on SO2-emissions established in the “Combustion 2000" program, it was necessary to minimize and avoid any SO2-evolution in the HITAF while combusting the CaS-containing pyrolyzed char. To determine the fate of sulfur during combustion of this pyrolyzed char, under a partial support from FWDC, the University of Tennessee Space Institute (UTSI), carried out a small bench scale experimental study. In this study, representative samples of chemical grade CaS powder, HIPPS char, and synthetic char mix were oxidized at desired temperature (7501315 °C) using a thermogravimetric analyzer (TGA) and a tube furnace setup under appropriate gaseous environment. On the basis of the literature survey, a thermodynamic analysis of the possible chemical reactions was also carried out using an equilibrium program developed by the NASA Lewis Research Center. From the thermodynamic study as well as bench scale experiments, it was found that under the proposed HITAF operating conditions, a significant level of SO2 will be produced from CaS in the char due to either thermal decomposition, oxidation, or reaction with CO2. As a result, it is recommended that the proposed HIPPS scheme be modified by either incorporating a two-stage HITAF combustion step or an appropriate flue gas desulfurization step.
Background In an era of ever decreasing fossil fuel resources and increasing environmental awareness, it is imperative to use highly efficient and environmentally acceptable methods to generate electric power. The United States Department of Energy (U. S. DOE) initiated a national research program called “Combustion 2000" with the goal of decreasing negative environmental impact while increasing the efficient use of valuable fossil fuel resources, namely coal. One of the projects under this initiative is a High Performance Power System (HIPPS) based combined cycle power generation scheme being developed by a team led by the Foster Wheeler Development Corporation (FWDC). In the Foster Wheeler concept of combined cycle power generation (see Figure 1), coal is first pyrolyzed, releasing a fuel gas which is used to operate a turbine cycle. The pyrolyzed char is then sent to a steam cycle combustor along with the turbine exhaust. The overall process efficiency is calculated to be more than 50% which satisfies the “Combustion 2000" goal. However, to meet the other goal of reduced environmental impact, what happens to the sulfur species present in the pyrolyzed char needs to be determined. Under partial * Corresponding author. E-mail:
[email protected], Fax: 931-3937201.
Figure 1. HIPPS commercial plant design schematic. Reprinted with permission from ref 1. Copyright 1997 National Technical Information Service, U.S. Department of Commerce.
support from FWDC, the University of Tennessee Space Institute (UTSI) carried out a small project involving theoretical as well as experimental studies to determine fate of the sulfur-containing species (especially calcium sulfide present in the pyrolyzed char) when subjected to oxidation in the High-Temperature Advanced Furnace (HITAF) of the steam cycle.
10.1021/ef000082c CCC: $20.00 © 2001 American Chemical Society Published on Web 11/14/2000
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Energy & Fuels, Vol. 15, No. 1, 2001
Sheth and Morton
Table 1. Comparison of HIPPS Design Coal to HIPPS Chara Pittsburgh No. 8 (as received) fixed carbon volatile matter ash moisture
Proximate Analysis (wt %) 48.15 35.91 9.94 6.00
HIPPS char (as received) 59.63 5.25 34.26 0.86
sulfur form
Pittsburgh No. 8 coal
HIPPS char
pyritic sulfur sulfate sulfur organic sulfur sulfide sulfur
1.37 0.01 0.81 0.0
0.0 0.0 2.04 2.61
overall sulfur
2.19
4.65
a
Ultimate Analysis (wt %) carbon 69.36 oxygen 11.41 hydrogen 5.18 sulfur 2.89 nitrogen 1.22 ash (ignited basis) 9.94
67.36 NDb 0.37 4.65 1.05 34.56
Ash Analysis (wt %) silicon dioxide 47.86 ferric oxide 24.08 aluminum oxide 22.19 potassium oxide 1.49 titanium dioxide 1.29 calcium oxide 1.29 sulfur trioxide 0.80 magnesium oxide 0.60 sodium oxide 0.30 phosphorus pentoxide 0.10 nickel oxide NIL
30.69 11.86 13.53 1.08 0.69 23.82 17.16 0.88
