344
Energy & Fuels 2001, 15, 344-349
Equilibrium Chemistry of Biomass Combustion: A Round-Robin Set of Calculations Using Available Computer Programs and Databases M. Blander,† T. A. Milne,‡ D. C. Dayton,*,‡ R. Backman,§ D. Blake,‡ V. Ku¨hnel,| W. Linak,⊥ A. Nordin,¶ and A. Ljung¶ QUEST Research, 1004 East 167th Place, South Holland, Illinois 60473, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, Åbo Akademi University, Data City, FIN 20520, Turku, Finland, Camp, Dresser & McKee International (CDM), AM UMWELTPARK-5, Bochum, Germany D-44793, NRMRL, MD65, EPA Research Triangle Park, North Carolina 27711, and Department of Inorganic Chemistry, Umeå University S-901 87 Umeå Sweden Received June 5, 2000
Equilibrium calculations of three problems regarding biomass combustion, at various levels of sophistication, were performed at six laboratories using seven combinations of computer programs and databases. The objective was to test the adequacy of the programs and databases for calculating both condensed and gas-phase behavior. The first problem was a simplified calculation for the combustion of a woodlike material with added sulfur to possibly form an ideal molten salt solution of potassium and calcium sulfate. The second and third problems were to simulate aspen wood and wheat straw combustion, respectively, and required a relatively sophisticated database on high-temperature solutions to describe condensed phases. All the participants performed calculations of the gas phases, which were reasonably accurate when their databases were adequate. For problem I, most of the participants were also able to calculate a reasonable set of condensed phases. However, for problems II and III, only four of the participants, using the two most sophisticated computer programs and databases, had the ability to produce rational results for the condensed phases. This round robin identified two computer programs and their associated databases that could prove useful for calculating the condensed-phase equilibrium chemistry of biomass combustion when coupled with experimental programs and the capability to expand databases as new experimental data become available. Such calculations can greatly enhance our understanding of the total equilibrium chemistry of biomass combustion.
Introduction Modern computer programs used to calculate the equilibrium chemistry of complex systems can be important tools for increasing our understanding of, and optimizing conditions for, biomass combustion. When coupled with experimental research, such calculations can provide information on the conditions for forming fouling and corrosive liquids as well as on the total equilibrium chemistry of combustion processes. Realistic and reliable equilibrium models could help minimize the impact of such liquids and control the effluents of combustion. Two aspects of the behavior of ash-forming elements are of special interest: (1) the behavior of condensed material, in particular whether molten phases will form under given conditions; and (2) the gas-phase speciation of these components, as they are involved in transport through the gas phase to points of condensation or aerosol formation. †
QUEST Research. National Renewable Energy Laboratory. Åbo Akademi University. | CDM. ⊥ NRMRL, EPA. ¶ Umeå University. ‡ §
To determine how well the many associated computer programs and databases would perform these tasks, a round-robin set of equilibrium calculations was performed at six laboratories using seven combinations of computer programs and databases. Each program and database combination has a central free energy minimization program that uses a database containing the standard free energies of formation of a large number (often thousands) of gas, liquid, and solid species, as well as data, concepts, and theories for describing the properties of liquid or solid solutions. The elemental composition of a biomass fuel and the mixture of nitrogen and oxygen to simulate combustion air were input into the thermochemical equilibrium codes. An excess of air was used in all of the calculations such that the product gas contained 4-5% of O2. The standard sources of data in the databases include the JANAF tables.1-3 Incorporating unique data on (1) Stull, D., Prophet, A., Eds. JANAF Thermochemical Tables, NSRDS NBS No. 37, National Standards Reference Data System, National Bureau of Standards, U.S. Government Printing Office: Washington, DC, 1969; 2nd ed., 1986. (2) Barin, I.; Knacke, O.; Kubachewski, O. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1977. (3) Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, Germany, 1989.
10.1021/ef0001181 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/24/2001
Equilibrium Chemistry of Biomass Combustion
individual substances has also customized many of the databases. In addition, each program treats liquid and solid solutions differently. The treatment of solutions is the major difference between the most sophisticated and the less sophisticated programs. A significant problem arising from biomass combustion is the production of fouling and corrosive liquid solutions. Because of the large deviations from ideal solution behavior of many liquid solutions formed in biomass combustion, condensed-phase behavior must be calculated using solution concepts and theories that have been validated to realistically predict the molten biomass ash properties. The gas phase is generally considered to be ideal except for substances that tend to associate strongly (e.g., alkali halide vapor molecules), for which the deviations from ideal behavior are represented by the formation of different polymers or associated species of the primary monomeric molecules. The participants were asked to use readily available thermodynamic data routinely used with their programs, regardless of the treatment of nonideality. In several cases, the programs are routinely used mainly to determine the gas-phase chemistry of inorganics, including heavy metals, or to carry out engineering assessments of energy and mass balances in chemical processes. We were interested in learning whether the assumption of ideal behavior of condensed phases significantly affected the vapor chemistry. The calculations presented were performed mostly during 1997, and the results were presented at the Engineering Foundation Conference on “The Impact of Mineral Matter Impurities in Solid Fuel Combustion” held Nov 2-7, 1997 in Kona, Hawaii, but not published in the proceedings.4 Even the most sophisticated programs lacked some of the tools and data needed to realistically describe the solid and liquid solutions expected to form on the basis of the problems posed. For each type of biomass investigated, one should perform preliminary calculations to determine the activities of a wide variety of combustion products and judge which species are likely to form solid and liquid solutions. On the basis of this information, one can create a database for such possible solutions. For complex multicomponent solutions of, for example, molten salts and silicates, in which deviations from ideality are often very large, one must use solution theories that can predict the multicomponent properties from the properties of lower order (unary and binary) systems. The database then requires incorporation of optimized models of experimental data (e.g., phase diagrams, activities, vapor pressures, thermal analysis, etc.) of these lower order systems. When the parameters used in the models are optimized to represent key data of a known phase diagram, for example, the lowest measured ternary eutectic temperature of 540 °C in the Na2O-K2O-SiO2 ternary system,5 the melting behaviors and tendencies for fouling and corrosion in biomass combustion can be reliably calculated. Thus, one may need to customize the computer program and the (4) Gupta, R. P.; Wall, T. F.; Baxter, L. Impact of Mineral Impurities in Solid Fuel Combustion; Kluwer Academic/Plenum: New York, 1999. (5) Levin, E. M.; Robins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists; American Ceramic Society: Columbus, OH, 1964; p 146, Figure 381.
Energy & Fuels, Vol. 15, No. 2, 2001 345
database by including unique nonideal solution models and species for each type of biomass. The goal of this study, however, was to evaluate available thermodynamic equilibrium programs and databases to determine the level of accuracy that could be obtained without customization. On the basis of studies that required added capabilities for calculations involving complex inorganic systems, Blander (A) had a customized version of F*A*C*T available for this study. Despite this, other participants (B-D), using the two computer programs F*A*C*T and ChemSage, produced credible solutions to most of the problems posed. Both programs have extensive databases and include the most recent, reliable solution theories for molten salt solutions, molten oxide solutions (including silicates), and solid solutions.6-9 These theories are essential for describing the thermodynamic properties of liquid silicate solutions and some molten salt solutions that may form during biomass combustion. The programs used by the remaining participants (E-G) did not contain these nonideal solution models and treated the condensed phases as mixtures of pure compounds or as ideal solutions. The programs used by the participants, and the codes used in Tables 1-4 are as follows: (A) Blander, F*A*C*T*, customized version;10 (B) Kuhnel, F*A*C*T*, version 2.1;10 (C) Backman, ChemSage, version 3.2;11 (D) Nordin and Ljung, ChemSage, version 3.1;11 (E) Dayton, STANJAN with SRI revision of database;12 (F) Blake, HSC Chemistry for Windows, version 1.1;13 (G) Linak, NASA-Lewis, CET89.14,15 Note: The F*A*C*T and ChemSage programs have recently been combined into a single package known as FACTSage.16 Problem I Problem I is a woodlike biomass with an artificial composition that combusts to form an artificially ideal liquid solution of K2SO4 (K(SO4)0.5) and CaSO4 as well as solids and a gas phase. The compound K2Ca(SO4)2 was ignored for simplicity. The input composition in moles is as follows: N, 44.18; O, 14.22; C, 4.29; H, 6.21; S, 0.006 23; Ca, 0.003 80; K, 0.002 07. The total number of moles of sulfur in this problem is greater than that (6) Pelton, A. D. Calphad J. 1988, 12, 127-142. (7) Pelton, A. D.; Bale, C. W. Met. Trans. 1986, 17A, 1211-1215. (8) Pelton, A. D.; Blander, M. Met. Trans. 1986, 17B, 805-815. (9) Blander, M.; Pelton, A. D. Geochim. Cosmochim. Acta 1987, 51, 85-95. (10) Bale, C. W.; Pelton, A. D.; Thompson, W. T. Facility for the Analysis of Chemical Thermodynamics (FACT); CRCT, Ecole Polytechnique de Montreal: Montreal. (11) ChemSage; SGTE Thermochemical Database, Scientific Group Thermodata Europe, 1994. Eriksson, G.; Hack, K. Met. Trans. 1990, 21B, 1013-1023. (12) Reynolds, W. J. The Element Potential Method for Chemical Equilibrium Analysis: Implementation in the Interactive Program STANJAN; Department of Mechanical Engineering: Stanford University: Stanford, CA, 1986. (13) HSC Chemistry for Windows, version 1.1;. Outokumpo Research Oy: Pori, Finland. (14) Gordon, S.; McBride, B. J. Finite Area Combustion Theoretical Rocket Performance; NASA TM-100785; National Aeronautics and Space Administration: Washington, DC, 1988. (15) Gordon, S.; McBride, B. J. Computer Program for Calculations of Complex Chemical Equilibria; NASA RP-1311; National Aeronautics and Space Administration: Washington, DC, 1994. (16) Pelton, A. D.; Ericksson, G.; Hack, K.; Petersen, S.; Koukkari, P. Demonstration of FACTSage and ChemApp at HTMC-X. In HighTemperature Materials Chemistry, Abstracts of the 10th International IUPAC Conference; Hilpert, K., Froben, F. W., Singheiser, L., Eds.; Forschungszentrum Ju¨lich GmbH: Julich, Germany, 2000; p 77.
346
Energy & Fuels, Vol. 15, No. 2, 2001
Blander et al.
Table 1. Gas-Phase Species Potentially Affected by Nonideal Behavior of Condensed Phase (Mole Fractions in Phase) temp, °C SO2 SO3 SO2 SO3 SO2 SO3 K2SO4 KOH SO2 SO3 K2SO4 KOH KOH KNO2 KNO3 KOH K2SO4 KNO2 KNO3 SO2 KOH K2SO4 KNO2 KNO3 SO2 SO3 HCl SO2 KOH KCl K2SO4 SO3 HCl NaCl NaOH K2Cl2 Cl
A
B
C
1.5 × 10-5 3.0 × 10-5 3.9 × 10-5 6.4 × 10-6 4.5 × 10-5 1.4 × 10-6 1.2 × 10-6 2.0 × 10-6 2.0 × 10-4 1.7 × 10-6 3.4 × 10-6 6.0 × 10-5
1.5 × 10-5 3.0 × 10-5 3.9 × 10-5 6.4 × 10-6 4.5 × 10-5 1.4 × 10-6 2.1 × 10-6 2.6 × 10-6 2.0 × 10-4 1.7 × 10-6 3.4 × 10-6 6.0 × 10-5
850
9.8 × 10-6
9.8 × 10-6
950
5.7 × 10-5 2.5 × 10-7
5.7 × 10-5 2.6 × 10-7
1050
9.2 × 10-8 5.8 × 10-5 2.2 × 10-6
1.0 × 10-7 5.7 × 10-5 2.3 × 10-6
9.4 × 1.9 × 7.7 × 1.5 × 7.3 × 6.3 × 1.0 × 2.7 × 1.4 × 6.5 × 3.7 ×
7.3 × 1.9 × 3.1 × 8.5 ×
600 800 1000
1200
Problem I 1.5 × 10-5 3.0 × 10-5 2.8 × 10-5 6.4 × 10-6 4.6 × 10-5 1.4 × 10-6 2.0 × 10-6 2.6 × 10-6 2.0 × 10-4 1.7 × 10-6 3.4 × 10-6 6.0 × 10-5 Problem II 1.7 × 10-5 1.0 × 10-6 2.9 × 10-6 5.4 × 10-5 2.7 × 10-7 1.8 × 10-6 1.6 × 10-6 1.0 × 10-7 6.0 × 10-5 2.3 × 10-6 1.0 × 10-6 3.6 × 10-7 Problem III
D
E
F
G
1.5 × 10-5 3.0 × 10-5 3.9 × 10-5 6.3 × 10-6 4.6 × 10-5 1.4 × 10-6 2.7 × 10-6 2.9 × 10-6 2.0 × 10-4 1.7 × 10-6 3.4 × 10-6 6.0 × 10-5
1.5 × 10-5 3.1 × 10-5 3.9 × 10-5 6.4 × 10-6 4.9 × 10-5 1.5 × 10-6 2.7 × 10-7 1.4 × 10-6 1.7 × 10-4 1.5 × 10-6 1.4 × 10-6 5.7 × 10-5
1.5 × 10-5 3.0 × 10-5 3.9 × 10-5 6.3 × 10-6 4.9 × 10-5 1.4 × 10-6
6.2 × 10-5
1.5 × 10-5 3.0 × 10-5 3.9 × 10-5 6.4 × 10-6 4.6 × 10-5 1.4 × 10-6 3.3 × 10-6 3.3 × 10-6 2.0 × 10-4 1.7 × 10-6 3.4 × 10-6 6.0 × 10-5
2.4 × 10-5
7.7 × 10-6 5.1 × 10-7 1.3 × 10-6 3.5 × 10-5 3.4 × 10-9 1.1 × 10-6 1.0 × 10-6 3.9 × 10-9 5.4 × 10-5 3.6 × 10-8 9.8 × 10-7 3.5 × 10-7
1.4 × 10-5
2.7 × 10-5
4.9 × 10-5
6.7 × 10-5 6.9 × 10-10
5.7 × 10-5
7.5 × 10-11 6.7 × 10-5 2.4 × 10-9
6.3 ×
1.5 × 10-5 1.8 × 10-4 1.5 × 10-5 2.0 × 10-4 1.1 × 10-30 5.6 × 10-28
5.6 × 10-5 2.6 × 10-7 1.0 × 10-7 5.7 × 10-5 2.4 × 10-6
500 1000
10-6 10-4 10-7 10-4 10-7 10-6 10-4 10-6 10-8 10-7 10-7
6.9 × 2.0 × 3.1 × 8.8 × 1.2 × 6.3 × 1.5 × 2.2 × 2.6 × 2.2 × 5.3 ×
10-5 10-4 10-7 10-5
1.5 × 10-4 2.5 × 10-5
7.0 × 1.9 × 5.9 × 1.2 × 4.4 × 6.4 × 1.7 × 2.5 × 1.2 × 4.2 × 3.9 ×
10-6 10-4 10-7 10-5 10-7 10-6 10-4 10-5 10-8 10-7 10-7
10-6 10-4 10-7 10-4 10-7 10-6 10-4 10-5 10-7 10-7 10-7
3.1 × 10-5 1.8 × 10-4 1.5 × 10-6
1.8 × 10-16 2.2 × 10-15 7.8 × 10-7 4.8 × 10-5 6.9 × 10-7 1.1 × 10-4 6.0 × 10-8 1.6 × 10-6 9.3 × 10-5 2.1 × 10-5 1.3 × 10-7 2.6 × 10-7 3.4 × 10-7
10-9
2.6 × 10-4 1.8 × 10-4
6.3 × 10-6 2.6 × 10-4 2.7 × 10-16 5.6 × 10-19
4.8 × 10-7 2.0 × 10-5 2.3 × 10-5 9.5 × 10-7
9.2 × 10-7
Table 2. Problem I Liquid-Phase Results (Mole Fractions in Phase) A
B
C
D
E
F
G
1.0 6.9 × 10-7
8.4 × 10-7
0
3.5 × 10-3
1.0
0
1.0 2.6 × 10-7
7.1 × 10-6
0.35 0.65 5.1 × 10-3
0.49
0.78
1.9 × 10-4 1.4 × 10-3
0.49 0.01 2.3 × 10-7
0.22 2.6 × 10-6
T ) 600 °C CaSO4 K2SO4 KOH CO2 N2 H2O total moles
1.0
0
0
0
0
T ) 800 °C CaSO4 K2SO4 KOH CO2 N2 H2O total moles CaSO4 K2SO4 KOH Ca(OH)2 N2 H2O K2CO3 total moles
0
0
0.61 0.39
0.56 0.44
2.5 × 10-3
0 T ) 1000 °C 0.49 0.51
1.7 × 10-3
1.8 × 10-3
0
0
T ) 1200 °C CaSO4 K2SO4 KOH Ca(OH)2 N2 H2O K2CO3 total moles
0.99 5.2 × 10-3
0
0
0
needed to fully react with Ca, K, and O to form sulfates. This leads to the formation of relatively high melting sulfates.17 Problem I was designed to be simple enough to be solved using computer programs incorporating less than optimal condensed-phase solution models. For the gas phase, the various sets of calculations generally match well for the species presented. One (17) Blander, M. Biomass Bioenergy 1997, 12, 289-293.
0
3.7 × 10-4 8.0 × 10-5 1.1 × 10-4
0.99
8.0× 10-5
0
exception is with the STANJAN program, where the assumption of an ideal solution of condensed phases appears to lead to perhaps an order of magnitude lower partial pressure for K2SO4 over the solution. The fundamental abilities of all the programs seem to enable them to perform acceptable calculations for the gas phase when the thermodynamic database is adequate. An abbreviated listing of mole fractions of gas-phase
Equilibrium Chemistry of Biomass Combustion
Energy & Fuels, Vol. 15, No. 2, 2001 347
Table 3. Problem II Liquid-Phase Results (Mole Fractions in Phase) A CaSO4 K2SO4 K2CO3 KOH Ca(OH)2 CaCO3 FeO MgO K 2O Al2O3 CaO H2O total moles
B
C
2.1 × 10-2 1.5 × 10-2 0.41
2.0 × 10-2 1.4 × 10-2 0.41
0.53
0.55
0.56
0.47
1.9 × 10-3
D
E
F
G
2.4 × 10-5
0
T ) 850 ˚C
3.4 × 10-3
7.1 × 10-4 6.8 × 10-2 0.28 0.37 0.27 1.0 × 10-4
0
0.61 0.32 0.56
1.0
9.5 × 10-10 3.0 × 10-2 2.9 × 10-5
T ) 950 °C CaSO4 K2SO4 K2CO3 KOH Ca(OH)2 FeO MgO K 2O Al2O3 CaO total moles CaSO4 K2SO4 K2CO3 KOH Ca(OH)2 CaCO3 total moles
0
0
0
0.85 0.15
1.4 × 10-4
1.4 × 10-4
0.40 0.41 0.16
0.43 0.57
2.1 × 10-5
1.5 × 10-5
0
T ) 1050 °C
6.3 × 10-2 0.93 3.9 × 10-3 2.6 × 10-4 8.7 × 10-5
1.8 × 10-3 7.5 × 10-2 0.25 0.38 0.29 1.0 × 10-4
0.82 0.13 4.0 × 10-2
0
0
0
0
Table 4. Problem III Liquid-Phase Results (Mole Fractions in Phase) A
B
C
D
E
F
G
T ) 500 °C SiO2 K2O Na2O Al2O3 MgO CaO K2SO4 Na2SO4 KCl CaSO4 NaCl KOH H2O K2CO3 CaCl2 NaOH MgSO4 KNO3 total moles
0.77 0.12 4.7 × 10-2 4.9 × 10-2 1.5 × 10-4 9.8 × 10-3 2.2 × 10-3 8.3 × 10-4 2.8 × 10-3 1.8 × 10-4 1.1 × 10-3
0.78 0.15 2.7 × 10-2 3.6 × 10-2 1.9 × 10-3 4.6 × 10-3 8.6 × 10-4
7.7 × 10-6 1.2 × 10-4 2.6 × 10-6 0.23
0.32
0
0
7.6 × 10-4 0.98
0.31
6.0 × 10-3
0.69
1.4 × 2.0 ×
7.5 × 10-5
10-2 10-4
0
T ) 1000 °C SiO2 K2O Na2O Al2O3 MgO CaO K2SO4 Na2SO4 KCl CaSO4 NaCl KOH H2O K2CO3 CaCl2 K2Si4O9 Na2Si2O5 NaOH MgSO4 MgCl2 Na2CO3 MgCO3 CaCO3 total moles
0.83 0.11 2.8 × 10-2 1.6 × 10-2 1.1 × 10-2 4.9 × 10-3 6.5 × 10-4 1.6 × 10-4 3.2 × 10-5 2.8 × 10-5 8.0 × 10-6
0.83 0.12 2.5 × 10-2 1.6 × 10-2 1.7 × 10-4 5.3 × 10-4 4.4 × 10-4
0.83 0.12 2.6 × 10-2 1.6 × 10-2 3.0 × 10-5 2.0 × 10-4
0.71 0.23 5.7 × 10-2 7.8 × 10-6 4.6 × 10-4 6.8 × 10-4
2.4 × 10-8 7.0 × 10-7
6.4 × 10-5 1.6 × 10-6 6.0 × 10-9 2.4 × 10-9 1.1 × 10-9 0.71
0.16 0.12
0.16 0.34
3.7 × 10-2 1.7 × 10-4 4.1 × 10-4 1.3 × 10-6
6.3 × 10-2 0.10 0.20
0.47 0.21 3.5 × 10-4
0.70
0.70
species for all problems is given in Table 1. The species in Table 1 were reported because their gas-phase concentrations could be affected by how the condensed phases were treated.
0.62
3.8 × 10-2
0.92 0.08
3.7 × 10-2
In Table 2 the results for calculations of the liquid phases for problem I, at 1 atm and 600, 800, 1000, and 1200 °C are presented. For the condensed phases, most of the participants (A-E and G) deduced similar rea-
348
Energy & Fuels, Vol. 15, No. 2, 2001
sonable results for the solid phases at 600 and 800 °C. For the liquid phase at these two temperatures, participants A-D and G had consistent results indicating that no liquid phase was present. For 1000 and 1200 °C, only four participants (A-D) obtained generally plausible, consistent results for the liquid and solid phases. Participant E’s calculations led to the appearance of liquid water at 600 and 800 °C (with a minor amount of KOH in solution at 800 °C). This was a consequence of the assumption of only a single, ideal condensed phase (e.g., low activities of minor components) and the extrapolation of low-temperature data above the critical temperature of water. Self-consistent results were found by participants A-D for all the condensed phases, and participant G reported pure solid K2SO4 and CaSO4 rather than the ideal liquid solution at 1000 °C. Problem II This problem involves the combustion of a measured composition of aspen wood in an excess of air at 850, 950, and 1000 °C and 4 atm. The input composition in moles is as follows: N, 44.18; O, 14.22; C, 4.29; H, 6.21; Ca, 0.003 80; K, 0.002 07; S, 0.001 57; Mg, 0.001 05; P, 0.000 274; Fe, 3.35 × 10-5; Al, 3.74 × 10-5. For this composition, there is not enough sulfur to completely react with the Ca, K, and O to form sulfates. For aspen wood, this leads to a relatively low melting carbonate in addition to some high-melting sulfate. The low sulfur content in aspen wood is the root cause of potentially fouling and corrosive carbonate liquids formed in aspen wood combustion.18 This composition is likely to produce liquid solutions of sulfates and carbonates of Ca and K as well as solid solutions and many stoichiometric solid compounds. The calculations of the condensed-phase equilibrium chemistry of molten salt solutions in this problem require a reasonably extensive database and the inclusion of sophisticated nonideal solutions models. Because of the basic nature of the combusted wood products, very little SO2 and SO3 are present in the gas, and except for minor gaseous products such as OH, NO2, K2SO4, KNO2, KNO3, K, Ca(OH)2, SO2, and CO, which are either missing from some of the databases or were below 10-7 mole fraction in most of the calculations (e.g., SO2, CO, Ca(OH)2, and K), all the calculations produced similar results for the gas phase (see Table 1). Again, the STANJAN program (E) appears to predict lower gasphase inorganic sulfate partial pressures because of the condensed-phase assumption. The differences, though real, are not likely to have a significant impact on the results for the condensed phases or on estimates of the main species responsible for alkali transport through the vapor phase. The results for the calculated liquid solutions of problem II are presented in Table 3. Participants A-C produce a molten potassium, calcium carbonate at 850 °C that contained minor amounts of sulfates in A and B. Participant D found a liquid oxide at 850 °C, high in K2O, CaO, and Al2O3. It is likely (from a knowledge of the K2O-Al2O3 and CaO-Al2O3 phase diagrams) that such liquids do not exist at these temperatures. These (18) Blander, M.; Ragland, K. W.; Cole, R. L.; Libera, J. A.; Pelton, A. D. Biomass Bioenergy 1995, 8, 29-38.
Blander et al.
results are incorrect because they were calculated using data that were extrapolated from binary data on molten silicates. The complex nature of the interaction between Al2O3 and K2O in silicate solutions renders such an extrapolation unreliable. However, because few appropriate experimental data are available, the formation of potassium-aluminum silicates cannot be ruled out. Participant A made several calculations from low to high alumina compositions that suggested the stability of an unrealistic alumina-rich liquid. Understanding the fundamental properties of such materials and how the computer programs handle these situations is necessary to judge the reliability of such calculations. A correction of this deficiency is under development for the F*A*C*T system.10 Participant A found a K2SO4-rich liquid at 1050 °C, whereas participants C and D found a K2SO4rich solid solution and participant B found pure K2SO4. These differences are small and are undoubtedly embedded in the uncertainty of the melting point of K2SO4, which was probably slightly lower for A than for B-D. The other solids formed are very different for each participant, largely because of the differences in the databases. With a large collection of possible solids in the database, including the most stable solids found in nature, such as apatite Ca5(PO4)3OH, leaving some of the solids out of the database might have a slight influence on the other phases. A detailed account of these differences, which provide notable but not crucial information, is beyond the scope of this paper. Problem III This problem is concerned with the combustion of a measured composition of wheat straw in an excess of air at 500 and 1000 °C and 1 atm. The input composition in moles is as follows: N, 297.2; O, 101.0; H, 49.02; C, 27.27; Si, 0.6528; K, 0.1923; Cl, 0.0542; S, 0.0425; Na, 0.0406; Mg, 0.0387; Ca, 0.0298; Al, 0.0228. This composition suggests the probable formation of high silicacontaining molten silicates.5,19 The presence of a very low melting eutectic temperature (540 °C) at a high silica composition in the K2O-Na2O-SiO2 ternary system5 indicates that low-melting silicate condensates are possible. Calculating the thermochemical properties of silicates requires a high degree of sophistication. The gas phases of the participants are largely the same down to the level of 10-7 mole fraction; the differences are usually consistent with uncertainties in the databases. Differences between the participants for the solids and solid solutions can be ascribed to (a) differences in the thermochemical data (some have compounds with greater stabilities) and (b) choices of solids. For example, A chose stoichiometric diopside, CaMgSi2O6, whereas B-D chose two solid solutions of CaSiO3-MgSiO3, one with a high (>1) Ca/Mg ratio and one with a low (
