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Cleaning and Dewatering of Low-Rank Coal by OilAgglomerationR. C. TIMPE a , R. A. DEWALL a & T. A. POTAS aa Energy and Environmental Research Center , University of North Dakota , Box 8213,University Station, Grand Forks, North Dakota 58202, USAPublished online: 08 Feb 2007.

To cite this article: R. C. TIMPE , R. A. DEWALL & T. A. POTAS (1992) Cleaning and Dewatering of Low-Rank Coal by OilAgglomeration, Coal Preparation, 11:1-2, 1-10, DOI: 10.1080/07349349208905203

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Cool Prepora~~on, 1992 Vol. II, pp. 1-10 Photocopying permitted by license only IQ 1992 Gordon and Breach Science Publishers S.A. Printed in the United Kingdom

Cleaning and Dewatering of Low-Rank Coal by Oil Agglomeration R. C. TIMPE, R . A. DEWALL and T. A. POTAS

Energy and Environmental Research Center. University of North Dakota, Box 8213. University Station. Grand Forks, North Dakota 58202. USA

The preparation of managcable low-ash. low-moisture coal combustion and/or conversion (i.e.. lique- faction. gasification) feedstocks is a priority for future energy development. Physical cleaning by conven- tional washability techniques, followed by dilute acid leaching. has produced lignite, subbituminous, and brown coal products with less than I wt. % ash on a dry basis on a laboratory scale. Agglomeration yields have ranged from 85 to 99wt. %. Ash contents have been less than 2 wt. % moisture- and oil-free for n North Dakota lignite. After air-drying the agglomerates overnight. Karl-Fischer moisture analysis indi- cates the nioisture levels are less than 5 wt. % in the agglomerates. Aner thermal drying of the coal, the moisture levels are less than 0.5 wt. %.

Kc,? , ~ . o r d ~ : Oil agglomcrstion. Coal dewatering. Coal cleaning. Low-rank coal clwning. Lignite agglomeration. Low-rank coal beneficiation

INTRODUCTION

In the quest for ensuring ample supplies of energy for the next several hundred years. one keeps stumbling over the large reserves of low-rank coals (LRCs). Lignites and subbituminous coals are plentiful between the Mississippi River and the Rocky Mountains.' LRCs are also relatively accessible. with an abundance of large strip- minable deposits along the southern coast of thiscountry and in Wyoming, Montana, and North Dakota.* These coals are quite reactive and, therefore, make good conver- sion feedstock as well as fuel for combustion. They are generally low in sulfur and low in ash.' These factors make LRCs quite attractive to industries. particularly utilities and chemical companies that are able to build their plants near the fuel supply. There would be a great deal more interest in these LRCs, however, if the energy content were higher, and the mineral content, particularly sodium, were lower, making i t more cost-effective ro move the fuel to plants located at sites far from the mines. Industry is not interested in paying transportation costs on fuel that is 25-40 wt. % moisture (though the cost/ton at the mine is still quite modest) and contains sodium, the major cause of ash fouling in boilers.

Dewatering, besides improving calorific value and thus lowering transportalion costs kcallkg, also increases coal reactivity. Enhanced reactivity may or may not yield a positive effect. I t is positive in the case where the coal is reacted for its intended use.

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2/[204] R. C. TIMPE et al.

However, it can be negative in the case where the dewatered coal is allowed to standin air and become oxidized.

Demineralization reduces combustion ash disposal problems, as well as operationalconcerns such as fly ash removal and ash fouling. Some of the sulfur and nitrogen inthe coal is present as mineral matter and can be reduced on demineralizing the coal.

One means of upgrading lignites and subbituminous coals is the oil agglomerationprocess, variations of which have been studied and patented.':" Recently, a successfuloil agglomeration process was developed at the University of North Dakota Energyand Environmental Research Center (UNDEERC). It is a successful bench-scaleprocess that satisfies the qualitative needs of the LRCs as to ash, moisture, Btucontent, and handleability. Ash reductions of up to 80%, moisture reductions of> 85% accompanied by Btu content comparable to that of bituminous coal, and theproduction of uniform., hard, transportable agglomerates make this process a viablemeans of producing a high-quality fuel and/or conversion feedstock.

Physical cleaning by conventional washability techniques, followed by dilute acidleaching, has produced low-rank coal products with less than I wt. % ash on a drybasis on a laboratory scale." The EERC oil agglomeration process incorporates ionexchange and surface conditioning of fine coal followed by oil agglomerationto accomplish similar effects using common chemicals, equipment, and brief,uncomplicated operating conditions in a single processing scheme.

Besides the obvious economic and environmental benefits associated with cleaningand dewatering of coal, oil agglomeration is effective in reclaiming excessively smallfines produced during coal preparation. In addition, formation of the agglomerateprotects the coal from the rapid weathering that occurs following moisture removalfrom LRCs, thus reducing Btu loss due to surface oxidation and decreasing thepotential for hazards involving spontaneous combustion and dust explosions.

Successful agglomeration of Northern Plains lignites and subbituminous coals aswell as Alaskan and European lignites has been carried out at EERC, with resultscomparable to those reported in this paper. To date, every LRC tested in this processhas been successfully agglomerated. To illustrate the effect of the dewatering-deashingprocess by oil agglomeration, this paper reports the results of agglomerating twolignites and two subbituminous coals using coal-derived oil.

EXPERIMENTAL

Two North Dakota lignites and two Wyoming subbituminous coals were successfullyagglomerated with a phenol-rich coal-derived oil from the Dakota GasificationCompany at Beulah, North Dakota. The agglomerated coals included Beulah lignite,Center lignite, Kemmerer subbiturninous, and Eagle Butte subbituminous. Theproximate analyses are shown in Table 1.

All coals were ground to pass 30 mesh (595 pm) and were agglomerated withoutfurther sizing. Additional identical tests were carried out on - 30 x + 200-mesh(595 pm > particle size> 74 f.Lm) Beulah lignite. Tests involved 50 g of coal from thewhole coal or the screened fraction containing the - 30 x + 200-mesh particles.Operating temperature and pressure were those of the laboratory in which the testswere performed, i.e., ca. 730 Torr and 25°C.

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OIL AGGLOMERATION OF LOW-RANK COAL

TABLE I

[205]/3

Proximate analysis of center and Beulah lignites and Eagle Butte and Kemmerer subbituminous coals

Wt%

Beulah Center Eagle Butte Kemmerer

Moisture 24.4 33.9 24.3 18.2Volatiles, mf 42.7 43.3 44.2 41.8Fixed carbon. mf 49.0 48.0 48.0 53.7Ash, mf 8.4 8.7 7.8 4.5

Four different nitric acid concentrations: 0.67, 0.34, 0.17, and 0.08 M (moles/liter),all of which had pH < 2., were used in the acid-cleaning step of the lignites. 50 g of coalwere cleaned with 100ml of acid by stirring at ca. 5500 rpm for 30 min. Required acidstrength appeared to be coal specific, although 0.34 M was as good as 0.67 M for thecoals tested. When the pH became> 2 on contact with the coal, the cleaning was lesseffective. Thus 0.08 M acid appeared to be too weak for cleaning these coals, and0.17 M was marginal.

The oil used in the agglomeration was a coal-derived oil consisting primarily ofphenol and phenol derivatives. 50 ml of oil were added to agglomerate the cleanedcoal. The mixture was stirred in a beaker with a mechanical stirrer at 800 to 1000rpmfor 10min. to form agglomerates. The agglomerates were collected on a 30-meshscreen and allowed to air dry overnight. Thermal deoiling was used to recover excessoil. Thermogravimetric analysis (TGA) indicated that as much as 90% of the oil couldbe removed from the agglomerates by the thermal deoiling, and an apparatus toconfirm these results using oil mass balance is being designed. Operating parametersare shown in Table II, and a schematic of the process is shown in Fig. 1. Cells I and2 in Fig. 1 are the same cell shown twice to illustrate the change from high-shear tolow-shear mixing. A schematic of the oil recovery process is shown in Fig. 2.

Analysis of agglomerates was by thermogravimetric analysis (TGA). Moisture(assumed to be water), oil, volatiles, fixed carbon, and ash were determined using amodified TGA proximate analysis method. The method involves heating the sampleunder argon flowing at ~ 160crrr'jrnin from ambient to 1looe at 20°Cjmin andholding for 5 min, then heating to 2500e at 100°Cjmin and holding for 10min, then

TABLE II

Operating parameters for EERC low-rank coal agglomeration process

Experimental parameters

Coal Particle Size

Acid StrengthCoal-Acid Mixing SpeedAcid Contact TimeOil-Mixing SpeedOil-Mixing TimeAgglomerate deoilingtemperature

- < 30 by 200 mesh< 30 mesh by 0

-6.0 to 0.75wt. %- 5500 rpm-30min- 800 to 1000 rpm-IOmin-200°C

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-30 mesh coal (25-35%)

I High Shear

(1 5-25%) Acid (1 -6%) Low Shear

Mixing I

30 mesh Screen ~&$omerates

4 Fines, Water &Excess Oil

FIGURE I Schematic of EERC low-rank coal agglomeration process.

heating to 900°C and holding for 20min. At the 32-minute mark of the analysis, the argon flow is stopped and replaced by air flow at - 240cm'/min. An example of the results obtained by the TGA method is shown in Fig. 3.

Moisture and ash in slected samples were also determined by ASTM Method D27I for comparison with TGA data. Although TGA and ASTM values compared favor- ably, TGA ash values were typically higher and, therefore, the more conservative dcnshing value. A basis of comparative success in deashing the coal was the clean coal ash eficiency index, El. The El is calculated according to the following equation:

whcrc Y is clcan coal weight recovery in %, A , is refuse ash weight in %, and A,, is clean coal ash in %.

A more accurate measure of small amounts of water than that of the TGA or ASTM dctcrmination was obtained by the Karl-Fischer method." This titrimetric mclhod, which measured water as a reactant rather than on the weight loss on vaporization, enabled the analyst to measure water in the presence of organic com- pounds. Moisture, as measured by weight loss as in the TGA and ASTM methods,

Ice lsopropyl Water Alcohol- Bath Dry Ice

Bath

FIGURE 2 Schematic of agglomerate deoiling process.

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OIL AGGLOMERATION OF LOW-RANK COAL

0 10 20 30 40 50 Time (rnin)

FIGURE 3 TGA ;ln;~lysis of lignite oil agglomerate before deoiling.

was shown by Karl-Fischer data to be as misleading as measurements for water in the agglomerates.

RESULTS AND DISCUSSION

Physical C l e a n i n g Reducing the particle size of Beulah, North Dakota lignite to < 595pm resulted in ca. 20wt. % of the coal having particle sizes i 74pm. The fines fraction was removed from the sample, and an ash content was determined on the remainder. The cleaned fraction of Beulah lignite was 5.9 wt. % compared with the whole coal ash content of 8.4%, as shown in Table I , a reduction of 29%. The elemental content of the coal and the retained fraction are shown in Fig. 4. The silicon, iron, titanium, and sodium were reduced more than 50% by screening alone. The only element wt.% found to be greater in the retained fraction than in the whole sample was magnesium.

FIGURE 4 Elemental analysis of ash from - 30 x 0-mesh raw Beulah lignile. - 30 Beulah lignite. and -30 x 200-mesh Beulah lignite agglomerate.

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6/[2081 R . C. TlMPE cr ul.

TABLE Ill

pH of aqueous phase during steps in the agglomeration of Beulah lignite

Acid cleaning Agglomeration Run # Volume Bind. BZOA % Acid Acid Before After After Oil %

In the agglomeration process, the fines produced during the physical coal cleaning were not separated from the cleaned fraction. The acid cleaning which followed removed some of the mineral matter from the fines, making them suitable for agglomeration.

C h e m i c a l C l e a n i n g a n d D e w a t e r i n g Much of the mineral content of low-rank coals was in the form of exchangeable cations as shown in Fig. 4, the data for Beulah lignite. Calcium, magnesium, potassium, and sodium ions were relatively easily removed from LRCs by classical ion exchange reactions with H + . Figure 4 shows that acid leaching of the screened Beulah lignite resulted in > 90% reduction of the calcium, magnesium, sodium, and sulfur when calculated on the basis of fixed carbon in parent coal, screened fraction, and agglomerates. Potassium and phosphorus were reduced by two-thirds. Iron and duminum were reduced by one-third. Silicon and t~tanium were not appreciably affected by the acid treatment.

E f f e c t o f t h e a g g l o m e r a t i o n p r o c e s s o n pH. The [Hf ] of the aqueous phase during acid cleaning and agglomeration changes significantly during partial demineralization and coal-surface conditioning, as shown by the data for Beulah lignite in Table 111. The increase in pH, indicating a decrease in [H'], is particularly obvious when cleaning with the weaker nitric acid solution, i.e., 1.5% and 0.75%. I t is apparent from the change in pH before and after acid cleaning that reactions of the acid did occur with the coal, normally resulting in a decrease in the acid concen- tration. In three tests (BZOA-3, 4, 5), the increases in acid strength (decrease in pH) during acid cleaning were attributed to inadequate temperature compensation by the meter. The compensation is necessary because of temperature increases resulting from high-shear rates. The increased temperature causes increased ionization of the acid leading to lower pH readings.

Figure 5 shows ash reduction as a function of pH. In the pH range 1.5-2.0, ash reductions were over 70% Tor both the Beulah and Center, North Dakota lignites.

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OIL AGGLOMERATION OF LOW-RANK COAL

FIGURE 5 Percent ash reduction versus pH in the oil agglomeration of Beulah and Center Lignites.

However, at pH > 5, ash reduction was decreased to less than 50% for the Center lignite. Ash reduction in the Beulah lignite still exceeded 60% at pH 5, but then decreased to only 40% at pH 6.

The increase in pH accompanying deashing was the result of [ H t ] displacement of !he cations on organic functional groups, reducing the polarity of the groups and making them more oleophilic. This surface conditioning may also be occurring in the pore structure. The release of moisture (water), caused by the hydrophobicity and decreased hydration of the ion-exchanged organic matrix and encouraged by the high ionic strength of the aqueous medium, results in dewatering the coal. Hypothetically, as illustrated in Fig. 6, replacement of the water in the pores by oil could enhance particle-particle binding and promote agglomerate formation.

Effect of acid strength. Several different acids can be used to achieve acid cleaning of the LRCs tested. The two acids giving the best acid-cleaning results with the coals tested were hydrochloric acid (HCI) and nitric acid (HNO,). The former was found to leave a substantial chloride residue with the coal after cleaning with 6% acid and. therefore, was not the acid of choice for these tests. The results of tests to determine the effect of acid strength on ash reduction for the two lignites are shown in Table IV. Ash reduction was not significantly different for the 3% versus the 6% acid concentration. However, the 1.5% and 0.75% acid concentrations gave much less ash reduction than the higher two acid concentrations, which is evident from the EI values in Table IV.

Table IV shows that acid strength may be coal specific in moisture reduction, although the reduction was > 70% for all four acid strengths used to clean the Center and Beulah lignites. With the Center lignite, the acid strength of > 3% reduced the moisture by 15-20% more than with an acid strength of 0.75% or 1.5%. However, acid strength appeared to make little difference in moisture reduction of the Beulah lignite. Table V summarizes the moisture and actual water measurements as deter- mined on an arbitrarily selected set of agglomerates from the Beulah lignite test matrix.

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Before Dewater ing

After Dewater ing

F I G U R E 6 Coal-oil inlcrface bcfore and a k r dewatering.

Agglomeration Agglomeration of deashed and surface-conditioned coal particles was accomplished with low-shear mixing for only five to ten minutes. The oil water-coal combination was mixed for a short time and was followed by easily seen separation of the solid phase agglomerates containing > 90% of the oil and the coal. Fines of higher ash content separated and were removed on the 30-mesh screen. Yields of agglomerated

TABLE IV

Summ:wy of ash and moisture reduction, c o d yield, and ash efficiency index oroi l agglomeration of Beulah :md Center ligniles on a moisture- and oil-free basis

Agglomcralion Results Summary (Moisture- and Oil-Free Basis)

C O : ~ Acid Ash Moisture Coal Ash errtciency s:~rnple strength. % rcduction. % reduction, % yield, % index

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TABLE V

Comparison of TGA with Karl-Fischer moisture content of Beulah lignite agglomerales

Moisture, wt. % % K-F H,O

Sample K-F T G A Oil, wt. % in TGA Moisture

Raw 3 1.39 2 4.27 21.95

deoil 2 0.25 0.75 4 3.76 20.22 16.46

deoil 4 0.19 1.64 1.45 5 3.79 19.44 15.65 6 4.22 18.68 14.46

coal were typically > 90% on a fixed carbon basis. Table VI shows performance data obtained from agglomeration tests with the Beulah and Center lignites and the Eagle Butte and Kemmerer subbituminous coals carried out at conditions optimized for the Beulah lignite.

The air-dried agglomerates, hard enough to withstand handling but for which no quantitative standard was currently available, showed increased hardness following deoiling. The oil and acid were saved for use in subsequent tests.

CONCLUSIONS

Low-rank coals can be agglomerated by a nonrigorous process using no extraordinary temperatures or pressures. The agglomerates are low in ash and water, reducing environmental concerns regarding solid waste, transportation costs/Btu, and sulfur emissions. Weathering and dustiness affect the agglomerates less than the raw coal, due to the minimal amount of oil-coating remaining as a binder.

TABLE VI

Summary of oil agglomeration performance data with respect to deashing

Coal sample Center Beulah Eagle Butte Kemmerer

Raw coal ash, wt. % 12.0 9.3 7.8 4.5 Agglomerate yield, wt. % 98.6 96.8 93.9 98.0 ASTM ash, wt. %" N/A 1.89 1.79 1.87 TGA ash, wt. % 2.30 2.52 2.63 2.01 Rcfusc (leachate + fines)d ash. 75.5 43.8 47.6 36.8

wt. % mf Efficiency index ash" 3240 2240 2500 1800 % Ash reduction' 80.8 72.2 66.3 54.7

*ASTM ash data for agglomerates is moisture-free. hTGA ash data for agglomerates is moisture-binding oil-free. 'Wascd on TGA ash determination.

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Acknowledgements

The :~uthors are grateful to the U S Department of Energy through the Pittsburgh Energy Technology Center and Minncsot;~ Power for financial support to carry out this projecl.

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