Thermodynamic Modeling of Trace Elements in South African Coals

Fernando Martinez-Colon, Research Scholar Faculty Mentor: Dr. Sharon Miller, Research Associate, The Energy Institute and Dr.

Harold Schobert, Professor and Director, The Energy Institute, Department of Energy and Geological and Environmental Engineering

The Pennsylvania State University

Abstract: A thermodynamic model called FactSage has been used to model the species of certain trace elements, having potential health hazards specifically mercury (Hg), lead (Pb), copper (Cu) and arsenic (As), that might form during coal combustion. A series of Zambian coals were used to determine the concentrations of trace elements that would be used as input for the program. The reason that these coals are being studied is that it is being used as a household energy source and the people who are using it are burning the coal in low budget stoves. In other words there is no control of the emissions which can cause, over an extended period of time, detrimental health effects. After completing the study and analyzing the data it can be said that FactSage does predict environmental hazard components that can be emitted as gases or particulate during the process of coal combustion. Introduction:

Energy is needed by everybody as a resource for everyday life. Energy needs can be as simplistic as heat needed for home cooking or as sophisticated as generating electricity for a whole city. In developing third world nations, there is a growing demand for energy from fossil fuels, specifically coal, to create a better standard of living. In Africa, the majority of people use coal in very rudimentary stoves for basic home cooking and heating. The stoves often consist of a five gallon can with holes punched in the bottom to facilitate the entrance of air and therefore facilitate the burning of the coal. The stoves are used in poorly ventilated shacks where the fumes produced from burning the coal are trapped. The occupants are exposed over a long period of time to these fumes which contain organic and inorganic compounds that are known to be toxic. It is the objective of this study to model the species of certain inorganic elements, having potential health hazards, specifically copper (Cu), lead (Pb), arsenic (As) and mercury (Hg) that might form during combustion of a series of Zambian coals using a thermodynamic model called FactSage.

Background:

Coal is made up of organic and inorganic elements. The organic portion is made

up of carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) and forms part of the combustible part of the coal which produces heat. The inorganic portion of coals is mostly non-combustible and forms the ash that remains after the coal is burned (West

Virginia Geological & Economic Survey, 2002). The majority of naturally occurring elements have been found in coals. Trace elements are elements that are found in coal at concentrations that are extremely low, for example under or less than 0.1 wt% of the coal (Rubinson & Rubinson, 2000). Trace elements come from different natural sources such as soil, seawater and volcanic eruptions although human activities also lead to substantial emissions of some elements (Clark and Sloss, 1992). Some of these trace elements are essential for healthy plant and animal life, while some are toxic if present in sufficient quantities (Clark and Sloss, 1992)(Swaine and Goodarzi, 1995).

The modes of occurrence of trace elements in coal are important factors used in

anticipating the behavior of the element during coal cleaning and combustion. There are different factors that determine the modes of occurrence of trace elements, the dominant ones are: environment of coal deposition, nature of country rock, tectonic setting and hydrologic conditions, and age and rank of coal (coalification). Depending on the occurrence of these trace elements is how they are going to react under combustion conditions. Some elements are primarily associated with organic matter as part of the matrix of the coal and other elements are just associated with the minerals in coal (Swaine and Goodarzi, 1995). The trace elements that are associated with the organic matter may be chemically bound or physically bound (e.g., -COOH). The elements that are associated with the inorganic matter, the mineral part of the coal, may be present as non-essential elements substituting for major elements in common mineral structures (Clark and Sloss, 1992). Examples of minerals in coal are quartz (SiO2), pyrite (FeS2), chalcopyrite (CuFeS2), arsenopyrite (FeAsS), millerite (NiS), ullmannite (NiSbS) and sphalerite (ZnS) (Swaine and Goodarzi, 1995).

Figure 1. The modes of occurrence of trace elements in coal (Clark and Sloss, 1992)

Trace elements

Organic association Inorganic association

Physically bound Chemically bound Discrete minerals

These modes of occurrence of a trace element can be inferred from indirect

evidence such as chemical analysis, from statistical correlations with other elements or with other coal characteristics such as ash yield, from the element’s geochemical characteristics, or from behavior during heating or leaching of the coal (Swaine and Goodarzi, 1995). Some modes of occurrence for several individual trace elements could be for example, Antimony (Sb) may be present in solid solution in pyrite and as minute

Organic functional groups

Fine-grained mineral matter in organic matrix

Essential elements in

mineral structureComplexes and chelates Substitution of non-

essential trace elements in mineral structure

accessory sulfides dispersed throughout the organic matrix. Arsenic (As) may appear associated with pyrite (FeS2) as arsenopyrite (FeAsS). In the majority of coals arsenic is primarily associated with massive or late-stage pyrite. Sometimes it may be found associated with fine-grained pyrite and other sulfides or even organically associated to the matrix. Analytical data indicate that most of the Copper (Cu) in coal occurs as chalcopyrite (CuFeS2). The Zambian coals used for this study came from an area mined for its copper (Cu) ore in the form of chalcopyrite. Lead (Pb) occurs predominantly as sulfides (e.g. PbS) or in association with sulfides minerals (e.g. ZnS). Much of the mercury (Hg) in coal is present in association with pyrite (Fe1-xHgxS2). Trace elements can be emitted or liberated from the coal during its combustion. The definition of combustion is the rapid chemical combination of oxygen (O2) with the combustible elements of the coal, which are the organic components or organic associated components (Babcock &Wilcox, 1978). During combustion or gasification the particles undergo complex changes including the formation of ash particles, and vaporization of volatile elements (Clark and Sloss, 1992). During the combustion these trace elements may be liberated into the air as vapor or as solid in particulate. The degree of vaporization determines how each element is divided between the various solid residues and the gas. It is possible to predict the group to which element belongs based on the boiling points of the elements and their compounds and the temperatures at which phase changes occur (Clark and Sloss, 1992).

Substances, all of them can be hazardous if taken in high amounts or

concentration. Many of the trace elements can have serious health effects if they are taken in excess. Maybe a little of anyone of the elements would not have serious effects but taking them every day for a lifetime can cause some harmful health problems. The gases and particles inhaled by people over an extended period of time can result in severe respiratory diseases. Particles irritate lung tissue and many trace metals can be absorbed into the body. Some trace elements with known toxic responses in test systems and in humans appear in Table 1.

Table 1. Selected trace elements emitted by coal fired power stations with known

toxic responses in test systems and in humans (US DOE, 1989) Element Health effects

Arsenic (As) Anemia, gastric disturbances, ulceration, skin and lung carcinogen in humans

Copper (Cu) Biologically essential but if taken in excess it can be toxic

Mercury (Hg) Neural and renal damage, cardiovascular disease; methylmercury is teratogenic in humans

Lead (Pb) Anemia, cardiovascular, neurological, growth retarding, and gastrointestinal effects, some compounds are possible human

carcinogens, and probably teratogenic to humans

There is no evidence that the responses listed in the table above are occurring as a

result of the low concentrations of trace elements in the ambient air around power stations, but there is concern that pollutants may be carried long distances and be deposited on the soil or in water bodies, and become accumulated through the food chain (Neme C, 1991). Trace elements present in the form of dust or particles are not so readily available for uptake by plants, but may still be ingested or inhaled by animals and humans (Schroeder, Dobson, Kane and Johnson, 1987). There is a need to understand the hazardous effects that trace elements have and more important promote investigations on the different coals utilized around the world so to prevent harmful health consequences.

The coals being studied come from the South of Africa specifically from Zambia.

The reason that it is being studied is that it is being used as a household energy source and there is need to know if the concentrations of the trace elements it possesses are possible hazards to the people who are utilizing these coals. Most of the people who are using these coals as a source of energy are poor people who probably are burning it in low controlled systems, which mean that they are inhaling all those vapors produced by the reactions of combustion by the burning coal. The most affected by these trace elements are women and children because they are the ones who are most exposed to the emissions of the burning coal (West Virginia Geological & Economic Survey, 2002).

Methodology: The procedure of this research involves the use of a thermochemical modeling software package called FactSage. This computer program has been established mainly in the field of complex chemical equilibria and process simulation where the software has unique capabilities. For this research the equilibrium module was used which calculates the concentrations and phases of chemical species when specified elements or compounds react or partially react to reach a state of chemical equilibrium. It employs the Gibbs energy minimization algorithm, which is the difference in energy between starting materials and products in their standard states, most stable form at 25 oC and 1 atmosphere of pressure (Maitland Jones, 2000). The program offers great flexibility in the way the calculations may be performed (Chalpad, 2002). For example there are a variety of units that can be used, in temperature it can be chose to work with Kelvin, Centigrade or Fahrenheit, for pressure, energy and quantities of species it can do the same thing. The input to run the program may be elements, compounds, solutions and solid solutions. The only conditions that need to be specified are the temperatures of reaction and pressure at which the reaction is going to occur. FactSage is limited to predicting products formed at equilibrium, and the reactions of combustion of the burning coal are not necessarily at equilibrium. For the purpose of this research the conditions under which the program is run are: temperature starting from 550 oC to 1150 oC at intervals of 200 oC, at 1 atmosphere of pressure. For the input several samples of the same coal at different depths and diverse sizes as well as different coals were used. A total of fourteen samples of Zambian coal were provided. A total of nine samples were run via FactSage. Five of the

coals had incomplete analysis. Figures 2, 3 and 4 show the concentration of mercury (Hg), arsenic(As), lead (Pb) and (CO) in the original coal sample. Each sample was provided by the USGS (United States Geological Survey) with its field number. Some of the names for example were Zm-99-2, ZM-2000-1 and Zambia2-1. Examples of the input are in Table 2.

Table 2. List of input in grams for selected species in the coal sample

*ND - nondetected

Species ( x 10-2 g) Samples CuFeS2 FeAsS PbS HgS FeS2 Fe2O3Zm-99-2 43.61 0.4847 2.933 9.28 x 10-4 46.57 30.99

ZM2000-1 3.495 0.7346 1.443 0.002900 140.4 93.43 Zambia2-1 0.8751 0.005651 0.4550 ND 48.03 31.96 Zambia2-9 2.279 0.3391 1.135 0.06610 573.1 381.4

Results: The equilibrium products predicted by FactSage are shown in Table 3. All the mercury (Hg) present in the coal was present as gas phase at temperatures ranging from 550 oC to 1150 oC at intervals of 200 oC. The mercury was present as a variety of compounds as follows: Hg2 (g) molecular mercury, HgO (g) mercury oxide, HgS (g) mercury sulfide, HgCl (g) mercury chloride, HgCl2 (g) mercury dichloride. The Hg that has gone into the gas phase is going to be part of the vapor that is being released by the burning coal. This may not represent a direct hazard, but the problem is that all of this Hg in the gas phase can react with components in the atmosphere and form methylated mercury, CH3Hg, which can be incorporated or consumed by the fish in the water and by this way enter the food chain. Once it enters the food chain it has to be considered as a health hazard because people can be infected by eating the fish. Gas phase mercury species can also react with unburned carbon particles which can be inhaled by humans. Arsenic (As) was not completely volatilized forming both gas and solid compounds. Some of the compounds that were predicted by the model are: As2S3 (liq), Ca3(AsO4)2 (s) calcium di(arsenicoxide), Cu3As (s) copper arsenate, it also had a variety of molecular forms for arsenic in the gas phase, As4O6 (g) arsenic oxide, AsS (g) arsenic sulfide and AsCl3(g) tricloro arsenate. Although there were many gas phase As compounds they formed the lowest percentage of total As products. In two of the Zambian coal products the total gas phase accounted for 2.1% of the total As and the solid phases accounted for the remaining 97.9%. For the rest of the samples the percentages rounded closer to a 100% for the gas phase and for the other phases, that are liquid and solid were closer to if not zero. Chronic inhalation of both gas and solid particulate containing As is a health hazard. The South African coal came from near a Zambian copper ore and it had high concentrations of copper (Cu). The copper is a biologically essential trace element, but it

is toxic if it is present in excess. Most of the copper stays in the solid form except for CuCl (liq) which is a liquid. The predominant species for the solid form of Cu predicted by FactSage are: CuO(s) copper oxide, Cu5FeS4 (s) iron sulfide cuprate, Cu2S (s) copper sulfide, (Cu2O)(Fe2O3) (s) complex between cupper oxide with iron oxide, Cu3As (s) cupper arsenate. This means that the copper is going to form part of the fly ash particulate emitted by the burning coal. In other words it can be inhaled and cause irritation on lungs tissue. Lead (Pb) was present in the coal at low concentration, however; with chronic exposure Pb is accumulated in the body and can reach toxic levels. None of the Pb products predicted by the model were found in the gas phase, one of them was found to be liquid, PbCl2 (liq), and the other ones were solids. Examples of the solid phase lead are: Pb2Fe2O5 (s) lead iron oxide and (PbO)(Al2O3)6 (s) which is a lead complex hexaaluminate lead oxide. These findings indicate that lead is going to form part of the particulate which can be inhaled or deposited on the soil and water. Because of this, it is children who are at greatest risk from lead poisoning given that they persist in eating soils and their consumption of dust particles by hand to mouth activities. Figure 5 presents the results in a graphic form. On the vertical axis is the vapor percent and on the horizontal axis is increasing temperature in Centigrade. What it represents is the vapor percent of selected trace elements as a function of temperature. After completing the study and analyzing the data it can be said that FactSage does predict environmental hazard components that can be emitted as gases or particulate under the simplified conditions used to mimic coal combustion. In research it is always important to find a way of validating the findings of the study. In this case one way of validating the results is taking some of the coal samples and burn them with the same conditions that were in the input for the program. This way it can be said that the findings of the program are the same as if the coal being studied was burned, if this is the result of burning the coal. Future Works: The study conducted with the thermochemical model FactSage is the beginning of further studies. For example, to prove if FactSage really does model the species of trace elements that might form during the process of coal combustion, tests by burning the coal under the same conditions that were established in the program could be conducted and analysis carried out of the gas and solid products. Another interesting follow up research project would be to investigate to what extent the reactions of combustion are completed, if they are completed at all, to see if the health effects of the trace elements in coal is due to incomplete combustion.

References:

1. Babcock &Wilcox, Steam its generation and its use, 39th ed., New York: Babcock & Wilcox company, 1978.

2. Chalpad, FactSage Thermochemical Software and Databases, Vol 26, No 2, Published by Elsevier Science Ltd, 2002.

3. Clark Lee B., Sloss Lesley L., Trace elements - emissions from coal combustion and gasification, IEA Coal Research, IEACR / 49, p. 13, 21-24, 26, 91-93, 1992.

4. Maitland Jones Jr., Organic Chemistry 2nd edition, W.W. Norton & Company Inc., p. 110, 275, 2000.

5. Neme C (1991) Electric Utilities and Long-Range Transport of Mercury and Other Toxic Air Pollutants, Center for Clean Air Policy, Washington, DC, USA, p.125 (Nov 1991).

6. Rubinson K., Rubinson J., Contemporary instrumental analysis, Prentice-Hall, Inc. Chapter 1, p. 10-11, 2000.

7. Schroeder W H, Dobson M, Kane D M, Johnson N D, Toxic trace elements associated with airborne particulate matter: a review. JAPCA, 37(11), p. 1267-1285, 1987.

8. Swaine D. J., Goodarzi F., Environmental Aspects of Trace elements in Coal, Kluwer Academic Publishers, 1995.

9. US DOE – United States Department of Energy (1989) Clean coal technology demonstration program, final programmatic environmental impact statement. DOE/EIS-0146, US Department of Energy, Washington, DC, USA, vp (NOV 1989).

10. West Virginia Geological & Economical Survey, Trace Elements in West Virginia coals, www.wvgs.wvnet.edu/www/datastat/te/, March 1, 2002.

Acknowledgements: Financial support was provided by the (SROP) Summer Research Opportunities Program and The Energy Institute. The co-directors of the SROP program are acknowledged for always being ready to help about anything and everything. Thanks to Dr. Harold Schobert for taking me into his research team and giving me the opportunities to make networks with people I need to know for graduate studies. Special thanks to Dr. Sharon Falcone Miller for taking me by the step and helping with the research and everything. Special thanks to Cyndi Freeman Fail for contacting me and giving me the opportunity to for part of Pennsylvania State University during this summer.

Table 3. Output of Potential Environmental Hazardous Compounds Predicted by FactSage at 750° and 1150°C (grams) Output 750 C

Species Zm 99-2 Z Z Z a Zam bi bia bia m 99-4 M 2000-1 ambia 2-1 Z mbia 2-2 bia 2-5 Zam a 2-6 Zam 2-8 Zam 2-9 Pb2Fe2O5(s) 0 .010 PbCl2 (liq) 0.017 (PbO)(Al2O3)6(s) 0 0.0 0.019 .050 39 CuO(s) 0 0. .003 004 Cu5FeS4 (s2) 0.239 CuCl(liq) 0.02 1 CuFeS2(s) 0.066 Cu2S(s3) 0 .005 (Cu2O)(Fe2O3)(s) 0.0 0.017 31 Cu3As (s) 0 .012 As2S3 (liq) 0.004 0.000 Ca3(AsO4)2 (s) 0 0 0. .019 .001 001

*Hg(g) 8.00E-06 8.00E-05 2 3.90E- 2.60E-

0E5.70E-.50E-

05 0.00E+0

0 0.00E+0

0 05 05 3.6 -04 05

**As(g) 4.57E-05 2.94E-19 1 0.00107

5.04E-

04 4.61E-

13 6.13E-

13 0.00143

0 0 0.00 761 0.00127

5 * Total gas phase Hg species: Hg, Hg2, HgO, HgS, HgCL, HgCl2 **Total gas phase As species: As, As2, As3, As4, As4O6, AsS, AsCl3

Output 1150 C Species Zm 99-2 Z Z Z Zam m bi bia bia m 99-4 M 2000-1 ambia 2-1 bia 2-2 Za bia 2-5 Zam a 2-6 Zam 2-8 Zam 2-9 PbFe10O16 (s) 0 .034 (PbO)(TiO2)(s2) 0 0.0 .018 09 PbCl2 (liq) CuO 0 .005 Cu(liq) 0 .004 (CuO)(Fe2O3)(liq) 0.0 0.027 49 (CuO)(Fe2O3)(s2) 0.0 0.000 00 Cu5FeS4 (s2) 0.238 0.036 Cu3As (s) 0 .012 As2S3 (liq) *Hg(g) 8.00E- 8 2 . 3. 6 0E 0.00E-05 .50E- 0.00E+0 0 00E+0 90E- 2. 0E- 3.6 -04 5.7 E-

06 05 0 0 05 05 05

**As(g) 0.00223

71 1.53E- 1.09E- 0.00143 0.00107

50.00156

0 .53E-16 .05E-

05 07 06 0 0 0.00 761 0 * Total gas phase Hg species: Hg, Hg2, HgO, HgS, HgCL, HgCl2 **Total gas phase As species: As, As2, As3, As4, As4O6, AsS, AsCl3

Figure 2. Quantity of copper (Cu) and lead (Pb) in original coal

0.00E+00

2.00E-02

4.00E-02

6.00E-02

8.00E-02

1.00E-01

1.20E-01

1.40E-01

1.60E-01

Zm-99-1

Zm-99-2

Zm-99-3

Zm-99-4

ZM-2000

-1

Zambia

2-1

Zambia

2-2

Zambia

2-3

Zambia

2-4

Zambia

2-5

Zambia

2-6

Zambia

2-7

Zambia

2-8

Zambia

2-9

Samples

gram

s (g

)

CuPb

Figure 3. Quantity of Mercury (Hg) in original coal

0.00E+00

5.00E-05

1.00E-04

1.50E-04

2.00E-04

2.50E-04

3.00E-04

3.50E-04

4.00E-04

Zm-99-1

Zm-99-2

Zm-99-3

Zm-99-4

ZM-2000

-1

Zambia

2-1

Zambia

2-2

Zambia

2-3

Zambia

2-4

Zambia

2-5

Zambia

2-6

Zambia

2-7

Zambia

2-8

Zambia

2-9

Samples

gram

s (g

)

Hg

Figure 4. Quantity of Arsenic (As) in original coal

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

7.00E-03

8.00E-03

Zm-99-1

Zm-99-2

Zm-99-3

Zm-99-4

ZM-2000

-1

Zambia

2-1

Zambia

2-2

Zambia

2-3

Zambia

2-4

Zambia

2-5

Zambia

2-6

Zambia

2-7

Zambia

2-8

Zambia

2-9

Samples

gram

s (g

)

As

Figure 5. Percent Vapor Phase as a Function of Temperature for Selected Trace Elements

0

10

20

30

40

50

60

70

80

90

100

550 750 950 1150

Temperature (C)

Vapo

r %

HgPbCuAs