COAL REFUSE TO ENERGY 

by: Ronald D. Neufeld 

Professor of Civil Engineering 

University of Pittsburgh 

Pittsburgh, PA 15261 

Abstract 

The objective of this research is to develop basic energy production and 

environmental information for the in-situ combustion of coal refuse piles. 

Laboratory pilot system results show exhaust gas temperatures from in-situ 

combustion of over 1 ,OOO°C, with 65% to 96% utilization of the original calorific 

content of the pile. The relative quantity of fines has great influence on gas 

permeability of refuse, and operational efficiency of combustion, EPA-TCL2 

leachates of "post bum" coal refuse are nonhazardous under current RCRA 

regulations. Combustion volatilized significant quantities of Al, Pb, Zn, Hg, 

and Cd, and removed 40% to 99% of the original pyritic sulfur, thereby 

attenuating acid mine drainage potentials. 

KEYWORDS : Coal refuse ; waste-to-energy; combustion; leachates; 

coal preparation plants; trace metals; 

COAL REFUSE PILES: NATURE, LOCATION, SAMPLING 

Coal refuse piles originate fromthe coal preparation plant waste residuals 

of yesterday. -It is estimated that about 40% of the volume of all coal mined 

in the U.S. since the 18th century has been discarded as waste, covering about 

177,400 acres of land throughout the coal producing regions of the U.S. Today, 

coal preparation plants are somewhat more efficient and produce waste at the rate 

of about 25% by volume of coal feed (1,2,3). 

Coal refuse piles contain coal, carbonaceous shales, and pyrites. Some 

piles are on fire, and pose inherent safety hazards; acid mine drainage 

conditions are also common resulting in environmental blights on the landscape. 

Little data is available characterizing the physical and chemical properties of 

coal refuse piles. 

Pennsylvania Department of Environmental Resources (DER) provided 

topographic maps with locations of all known coal refuse piles within the State. 

Using this data, a sampling protocol was developed, and 9 different sites were 

evaluated within the Western Pennsylvania region. 

445 
COAL REFUSE ANALYSIS 

Samples from each site were crushed and riffled to assure samples were 

Coal refuse and ashed refuse samples were digested 

Details 

representative for analysis. 

with HC1 and HF in teflon lined vessels for subsequent metal analysis. 

of analytical techniques may be found in Schmidt(4). 

Table 1 is a summary of analysis conducted on 9 different coal refuse pile 

sites in Western Pennsylvania. Results of the 9 sites show a sample mean value 

of 4,923 BTU/lb dry weight (with a range of 2,328 BTU/lb to 7,187 BTU/lb). 

TABLE 1 

ANALYSIS WESTERN PENNSYLVANIA COAL REFUSE PILES 

- Site 

Banning 

Browns town 

Cramer 

Export 

Homer City 

Mather 

Nemacol in 

Revloc 

Ebenezer 

BTU/LB . 4,380 

7,178 

6,467 

3,665 

5, 844 

3,117 

2,328 

6,518 

4,8710 

% ASH 

65.6 

48.2 

53.6 

68.4 

57.4 

71.9 

76.5 

51.1 

63.0 

% VOLATILES 

18.7 

23.4 

19.9 

17.4 

18.9 

17.6 

17.1 

18.6 

21.5 

%FIXED CARBON 

15.8 

28.3 

26.6 

14.2 

23.6 

10.5 

6.4 

30.3 

15.5 

&"e 2.328 - 7.178 48.2-76.5 17.1-23.4 6.4-30.3 

Table 2 is a summary of sulfur forms in coal refuse. Note that the 

average total sulfur content is 1.08% (with a range of 0.58% to 3.04%). Further 

elemental characterization data is provided by Neufeld (5). 

TABLE 2 

SULFUR FORMS IN WESTERN PENNSYLVANIAN COAL REFUSE PILES 

(8 BY WEIGHT) 

Site 

Banning 

Browns town 

Cramer 

Export 

Homer City 

Mather 

Nemacol in 

Revloc 

Ebenezer 

.60 

3.04 

1.11 

.61 

.79 

.97 

1.59 

.91 

* 58 

Or panic 

.32 

1.32 

.64 

.36 

.39 

.24 

.17 

.37 

.30 

.06 

1.20 

.13 

.06 

.09 

.07 

.12 

.05 

.09 

+ Sulfate 

.22 

.52 

.34 

.19 

.31 

.66 

1.30 

.49 

.19 

Range .58-3.04 .17-3.02 .os-1.20 .19-1.30 

446 
COAL REFUSE RECYCLING 

Coal refuse is attractive for recycling due to its BTU value. Currently, 

modular coal preparation type plants using "float/sink" technology are reclaiming 

some coal refuse sites. Using such technology, a high BTU value coal product may 

be "mined" for blending with the coal feed to local utility plants. Wastes from 

such systems are simply returned to the refuse pile. Alternatively, coalrefuse 

may be combusted inbulk for energy production. Two techniques for direct firing 

include fluidized bed combustion, and in-situ combustion (4,5,6). 

Insitu Combustion: EXDerimental Procedure Figure 1 is a diagram of the 

University of Pittsburgh in-situcombustion system capable of handling a 25 pound 

charge of coal refuse. The combustor exhaust pipe was connected through a gas 

cooling system, to an exhaust fan capable of pulling 75 cfm at a water head of 

50 inches. The purpose of the coolers were NOT to generate steam, but rather to 

cool gases sufficiently to assure the integrity of down stream air pollution 

sampling equipment, and the air moving fan. 

RESULTS 

Figure 2 is an example of data from the in-situ combustion of coal refuse 

from the "Cramer" site. Note exhaust gas temperatures rapidly reached a maximum 

of 946OC before it declined due to completion of combustion. The temperature 

profile of the exhaust gas and the lower elevation of the reactor follows similar 

trends. Further data of a series of runs comparing refuse of high and low ash 

contents shows that high ash content material could also achieve substantial 

exhaust gas temperatures and energy production, and therefore shouldnot be ruled 

out for commercial considerations (4). 

GAS PERMEABILITY/PARTICLE SIZE 

The particle size distribution of the refuse influenced permeability, which 

in turn influenced time for startup, and fraction of calorific value recovered 

during a run. Summary data of runs from all locations shows that start-up 

periods appears to fairly constant at permeability values greater than about 300 

Darcys, with drastically increased values at smaller permeabilities. Further 

analysis shows that permeability and start-up period increase drastically as the 

"D-90" particle size (ie.. the effective diameter at which 90% of all particles 

are larger than) falls below 0.6 mm. Higher permeability allows more air to 

flow into the fire zone, which, in turn, feeds more oxygen to the fire and avoids 

pyrolysis conditions. 

Gaseous Pollutant Concerns 

An air pollution sampling train was installed to monitor on-line SO*, NO,, 

combustion gases, and total hydrocarbons. In general, the nature of air 

emissions is a function of the type of combustor used. In this case, in-situ 

combustion is by nature a fuel rich system, with operator control only with the 

exhaust air fan. Summary data shows a sharp increase in Total Hydrocarbons in 

the exhaust gas as its temperature decreases below 60OoC. Similar data for CO 

illustrates the same critical temperature. Operation at lower temperatures 

resulted in pyrolysis conditions, and emission of tars and oil that coated the 

fan impeller blade. Maintenance of exhaust gas temperatures over 6OO0C allowed 

for relatively clean burning of coal refuse with no noticeable hydrocarbons. 

447 
Similarly, noticeable levels of H2S were liberated under lower 

temperature/pyrolysis conditions with SO2 being liberated at elevated 

temperatures. Analysis shows some sulfur oxide controls would be necessary for 

commercial scale in-situ coal refuse combustors. 

Trace Metals: 

Material balances conducted for trace metals shows that significant 

fractions of Cd, Pb, Hg, Al, S, and Zn appear to be volatilized during coal 

combustion: the data for the significant fractions volatilized is presented in 

Table 3. 

TABLE 3 

ELEMENTS VOLATILIZING DURING COAL, REFUSE COMBUSTION 

"Percent Volatilized" 

Site 

Banning 

Browns town 

Cramer 

Export 

Homer City 

Mather 

Nemacolin 

Revloc 

Eb ene z er 

Cd 

75.0 

28.6 

66.7 

0 

50.0 

53.8 

12.5 

50.0 

78.1 

Pb 

79.3 

81.1 

72.0 

73.2 

59.6 

91.7 

91.7 

87.2 

85.7 

HP 

71.7 

83.8 

92.4 

76.6 

72.7 

68.8 

81.3 

81.8 

40.7 

A1 

0 

4.8 

0 

33.3 

25.3 

21.8 

23.3 

21.2 

22.2 

S Zn 

83.3 69.9 

71.4 39.4 

76.6 20.0 

82.0 0 

77.2 41.7 

76.3 10.6 

57.9 0 

65.9 0 

75.9 33.9 

Ranee 0-78 60-92 69-96 0-33 58-83 0-70 

Normalizing auantities of metals released to energy production, Table 4 

shows calculated air emission potentials for key volatilized trace metals as 

follows : 

TABLE 4 

AIR EMISSION POTENTIALS FOR 

Al. Cd. HB. Pb, and Zn 

Element Range 

(lbs. of meta1/100 MW day) ......................................... 

A1 0-407,500 

Cd 0-2.3 

Pb 150-395 

Zn 0-516 

Hg 1.0-10.1 

Similarly, elements Ba, Fe, Mn, Ca, and Mg appears to be retained strongly 

Results of a mass balance showing "percent retained in 

This data 

in the combustion ash. 

ash" of these elements is given on a site-by-site basis on Table 5. 

448 
was developed by acid solubilization of aliquots of pre and post burn coal refuse 

samples, followed by AA analysis for metals of concern. 

TABLE 5 

ELEMENTS RETAINED IN COAL REFUSE COMBUSTION ASH 

"Percent Retained in Ash" 

Site Ba 

Brownstown 100 

Cramer 100 

Export 96.1 

Homer City 99.5 

Mather 97.5 

Nemacolin 100 

Revloc 100 

Ebenezer 92.1 

Banning 100 

Fe 

100 

100 

100 

91.5 

76.8 

95.0 

83.7 

94.7 

84.2 

Mn 

100 

100 

100 

100 

100 

100 

100 

100 

100 

Ca 

100 

78.3 

100 

80.0 

100 

89.4 

100 

60 

55 

ME - 

93.3 

92.3 

100 

96.3 

86.0 

91.4 

94.1 

100 

86.7 

Range - 92-10 84-100 100 53-100 87-100 

LEACHATES and ACID MINE DRAINAGE POTENTIALS: 

Pre-burn and post burn coal refuse samples were leached in accord with both 

the ASTM distilled water, and EPA-EP acetic acid based techniques. In addition, 

measures of pH, Hot Acidity, and major anion and cation species were made on the 

distilled water leachates. Heavy metal based "EP-toxicity" appears not be an 

issue. Other data, however show that on the average the pH of leachate solutions 

changed from clustering about pH = 3.4 to clustering about pH = 7.2. This is 

significant, since it indicates that one beneficial environmental effect of coal 

refuse combustion is to cause drainages to go from the acid range, to the 

neutral/slightly alkaline range. This is a particular advantage for in-situ 

combustion where the refuse is not moved. 

A better measure of attenuation abilities of acid mine drainage potentials 

is the "net alkalinity", (Net Alkalinity = Alkalinity - Hot Acidity), which 

accounts for the ultimate oxidation of inorganic species. Distilled water 

leachates from combusted coal refuse ash confirm that relatively acidic leachates 

are converted into a neutral leachates, with concurrent small increases in 

solubilization of K, Mg, and Na. 

A summary compositional analysis of all pre and post burn coal refuse ash 

is developed for all sites, and is available from Neufeld (5). 

449 
CONCLUSIONS: 

1. Coal refuse originated as reject material produced during the mining or 

beneficiation of coal. These wastes pose potentially major environmental 

problems in the form of fire potentials, and producers of acid mine 

drainage. Structural weaknesses in coal refuse banks have led to 

landslides leading to loss of human life. In addition, burning refuse 

piles contribute significantly to potentially serious air pollution 

problems, and may cause additional underground coal fires within the coal 

mining regions of the U.S. 

2. One approach for the simultaneous generation of energy, remediation of 

serious environmental problems, and recycling of resources is combustion 

of coal refuse via fluidized bed reactors, or via an in-situ technique. 

3. Using a bench scaled in-situ combustion technique, Exhaust temperatures in 

excess of 1,300 OC were easily achievable, however, overall energy 

utilization efficiencies were achievable only when piles did not contain 

excessive quantities of "fines. Particle size and the permeability of the 

refuse are significant variables that influence "extractability", or 

energy utilization (% of the original BTUs utilized) of a pile. Below a 

D90 particle size of 0.50 mm (ie. .90% by weight of coal refuse has a 

particle size > the D90 size), the energy utilization of the burn 

decreases from 90+ % to less than 65%. Particle size is related to gas 

flow permeability. In a similar manner, at gas flow permeabilities of 200 

Darcys or less, the energy utilization of the burn follows a similar 

trend. 

4. Leachates produced by the residual material remaining after combustion do 

not containmetal concentrations in excess of 100 times the drinkingwater 

standard and therefore the residual ash material from Burnout Control 

would be non-hazardous under RCRA regulations. Pre-burn coal refuse 

material exhibited tendencies to acid mine drainage formation as measured 

by "net alkalinities" (net alkalinity - alkalinity - hot acidity). These 

tendencies were usually confirmed by visual observation at the time of 

sampling by the Principal Investigator and research team. Net 

alkalinities of pre-burn materials ranged from -524 mg/L as CaC03 to -14 

mg/L as CaC03. Leachates from post combustion residuals were 

characterized by a net alkalinity range of -12 mg/L as CaC03 to 36 mg/L as 

CaC03 and a pH range of 4.8 to 10.1. 

5. Based on the results of mass balances, fractions of Al, Cd, Hg, Pb, and Zn 

are volatilized during coal refuse combustion. Up to 33% of the solid 

phase aluminumvolatilized, aluminumbeing present in the solid phase from 

6.3% to 9.1% by weight. Cd, Hg, Pb, and Zn were found to volatilize to as 

high as 70% by weight, however, in the solid phase Cd, Hg, Pb and Zn are 

present at <.01% by weight. 

450 
REFERENCES 

1. Heaton, R.C., Thode, E.F., and Wagner, P., Trace Element Recoverv from 

Coal PreDaration Wastes, Los Alamos National Laboratory, Los Alamos, NM, 

LA-9175-MS, 1982, 

2. Cavalet, J.R. "Coal Cleaning Plant Refuse Characterization" EPRI report 

# CS-4095 June, 1985 

3. McGown, Frank., and Gillespie, Barry, L., Nanaeement - of Coal Waste by 

Enerev - Recoverv: Mild Gasification/Flash Pvrolvsis of Coal Preparation 

Wastes, DOE/MC/21108, (1985) 

4. Schmidt, K. "Leachate ProDerties and Mass Balances Resultine from 'Burnout 

Control' of Coal Refuse Pi1 est' MS Thesis, Department of Civil 

Engineering, School of Engineering, University of Pittsburgh, 1989 

5. Neufeld, R. D. et a1 "Burnout Control and Enerev Production from Coal 

Refuse Piles" final report to the Pennsylvania Energy Development 

Authority under contract ME 486-004, December 31, 1989 (University of 

Pittsburgh Research in Civil Engineering series report CE/EE-02) 

6. Chaiken, Robert F., Neth od for Controlled Burnout of Abandoned Coal Mines 

and Waste Banks, U.S. Patent No. 4,387,655 (1983). 

This project, entitled BURNOUT CONTROL AND ENERGY PRODUCTION FROM 

COAL RE FUSE PILES, was supported by the Pennsylvania Energy 

PeveloDment Authority award number ME 486-004, to the University of 

Pittsburgh, Dr. Ronald D. Neufeld, Principal Investigator. 

451 
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454