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 combustionof over 

### 1,OOO°C, with 65% to96% utilizationof the original calorific content of the pile. The relative quantity of fines has great influence ongas 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 quantitiesofAl,Pb,Zn,Hg, and Cd, and removed 

### 40% to 99% of the original pyritic sulfur, thereby attenuating acid mine drainagepotentials.

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. -Itis estimated that about 

### 40% of the volume of all coal mined in the U.S. since the 18th centuryhas been discarded as waste, covering about 

### 177,400acres of land throughout the coalproducing regions of the 

### U.S. Today, coalpreparation plants are somewhat more efficient and producewaste 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 environmentalblights on the landscape. Little data is available characterizing the physical and chemicalproperties 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.



### 445COAL 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 subsequentmetal analysis. of analytical techniques may be found in Schmidt(4).

Table 

### 1 is a summary of analysis conducted on 9 different coal refuse pile Results of the 9 sites show a sample mean value sites in Western Pennsylvania. of 4,923BTU/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 Brownstown 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 Further average total sulfur content is 1.08% (witha range of 0.58% to 3.04%). elemental characterization data is provided by Neufeld (5). 

# TABLE 

# 2

SULFUR FORMS IN WESTERN PENNSYLVANIAN

### COAL REFUSE PILES (8 

### BY WEIGHT)

Site Banning Brownstown Cramer Export Homer City Mather Nemacol in Revloc Ebenezer .60 3.04 1.11

.61 .79 .97 1.59

.91

*58 

### Orpanic .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



### 446COAL REFUSE RECYCLING

Coal refuse is attractive for recycling due to its BTU value. Currently, modular coal preparationtypeplantsusing "float/sink" technology are reclaiming

some coal refuse sites. Using such technology,ahigh BTU value coalproductmay be "mined" forblending 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 fordirect firing include fluidized bed combustion, and in-situcombustion (4,5,6).

Insitu Combustion: EXDerimental Procedure Figure 

### 1 is a diagram of the University of Pittsburgh in-situcombustionsystem capable ofhandling a 25pound 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-situcombustion 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 energyproduction,and therefore shouldnotbe ruled
out for commercial considerations 

### (4).

GAS PERMEABILITY/PARTICLE SIZE

Theparticle sizedistributionof the refuse influencedpermeability, 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 atpermeabilityvalues greater thanabout 

### 300 Darcys,with drastically increased values at smaller permeabilities. Further analysis shows that permeability and start-upperiod increase drasticallyas 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-lineSO*, 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 onlywith 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 impellerblade. Maintenance 

### of exhaust gas temperatures over 6OO0C allowed for relatively clean burning of coal refuse with no noticeable hydrocarbons.



### 447Similarly, 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-situcoal 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 Brownstown Cramer Export Homer City Mather Nemacolin Revloc Ebenez 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-sitebasis on Table 

### 5.

448was developedby acid solubilization of aliquots ofpre andpost burn coal refuse samples, followed by 

### AA analysis for metals of concern. 

# TABLE 

# 5

ELEMENTS RETAINED IN COAL REFUSE COMBUSTIONASH "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-burnandpost burn coal refuse samples were leached in accord withboth the ASTM distilled water, andEPA-EP acetic acid based techniques. Inaddition,

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 theaverage 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 acidmine drainage potentials is the "net alkalinity", (Net Alkalinity = Alkalinity 

### - Hot Acidity), which accounts for the ultimate oxidation of inorganic species. Distilled water leachates fromcombusted coal refuse ash confirm that relativelyacidic 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).



### 449CONCLUSIONS:



### 1. Coal refuse originated as reject material produced during the mining or beneficiationof 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 fluidizedbed reactors,or via an in-situtechnique.

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

excess of 1,300 OC were easily achievable, however, overall energy utilization efficiencieswere achievable only when piles did not contain excessive quantities of "fines. Particle size and thepermeability 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 flowpermeability. Ina similarmanner,at gas flowpermeabilities of 200 Darcys or less, the energy utilization of the burn follows a similar trend.

4. Leachatesproduced by the residual material remaining after combustiondo not containmetal concentrations inexcess 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 formationas measured by "netalkalinities" (netalkalinity

# - 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-burnmaterials ranged from -524mg/L as CaC03 to -14 mg/L as CaC03. Leachates from post combustion residuals were characterizedby a net alkalinity range of -12mg/L as CaC03 to 36mg/L as CaC03 and a pH range of 4.8 to 

### 10.1.

5. Based on the results ofmass balances, fractions ofAl, Cd,Hg,Pb,and 

### Zn are volatilized during coal refuse combustion. Up to 33% of the solid phase aluminumvolatilized, aluminumbeingpresent in the solidphase from 6.3% to 9.1%by weight. Cd,Hg,Pb,and 

### Znwere 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.

450REFERENCES

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., Nethod for Controlled Burnout ofAbandoned 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 Universityof Pittsburgh,Dr. Ronald D. Neufeld, Principal Investigator.



### 451

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