The Chemistry of Low-Rank Coals - American Chemical Society

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Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch016

Cationic Effects During Lignite Pyrolysis and Combustion BRUCE A. MORGAN and ALAN W. SCARONI Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802 A Texas lignite was acid-washed to remove its ion exchangeable cations then alkaline-earth metals (Ca, Mg) were back exchanged in various concentrations. The raw and modified lignites were pyrolyzed and combusted in an entrained-flow reactor. The effect of the cations on pyrolysis and combustion behavior was investigated. Their presence markedly reduced the evolution of volatile matter during pyrolysis. This is attributed to an increase in secondary char-forming reactions involving the ion exchangeable cations and volatile matter molecules, particularly tars. The rate of combustion of the lignite was enhanced by the presence of Ca in ion exchangeable form. The comparative behavior of a lignite treated with Ca, a good catalyst for the C-O reaction, and Mg a poor catalyst, indicates that the heterogeneous C-O reaction may be important during a short intermediate stage of lignite combustion. 2

2

In the U.S. a major usage of fuels is for electric power generation (1). Fuel o i l is used to generate about 15% of the nation's electricity (1). However, i t is possible with present technology along with new developments in coal combustion to reduce reliance on petroleumbased fuels by replacing them with coal. This would make more o i l available to other areas such as the production of chemicals and pharmaceuticals. The purpose of the present work is twofold. First, it is aimed at gaining understanding of the fundamentals of lignite combustion and the factors which most influence combustion behavior. This is predicated on the fact that the U.S. contains vast quantities of low-rank coals, including lignites. Recent estimates put these deposits at approximately 38 billion tons (2). An advantage of using lignites in industrial combustors and boilers is their generally low sulfur content. The formation of SO2 and SO3 within the combustor will be minimal, thereby reducing environmental concerns associated with acid rain. 0097-6156/ 84/0264-0255$06.00/0 © 1984 American Chemical Society Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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THE CHEMISTRY OF LOW-RANK COALS

There can be problems associated with the use of lignites for power generation, however. In general, low-rank coal deposits are located in sparsely populated areas, for example, in the Northern Great Plains Province. Also, lignites have a propensity to slag in combustors and form deposits on heat transfer tubes in boilers. This has been attributed to the presence of alkali and alkalineearth metals which exist, in part, as ion-exchangeable cations locat ed on the carboxyl groups of low-rank coals ( 3 ) . Removal of these cations may produce a more desirable fuel which could conceivably be transported to large population centers. As examples, coal-water and coal-liquid CO2 slurry pipelines are currently being advocated as effective modes of transportation. The coal-liquid CO2 pipeline is favored by some because of the unavailability of large quantities of water in some western states. The second objective of this research is to modify, and thereby improve, the combustion behavior of lignites so that they can be used more extensively as an energy source. In particular, i t is considered necessary to know whether or not the presence of ionexchangeable cations significantly affects the combustion behavior of lignites. This would indicate whether there is any practical advantage of removing or adding additional cations prior to combus tion. American lignites have primarily alkali and alkaline-earthmetals exchanged on their carboxyl groups (4). Tanabe (5) suggests that these metal oxides act as polymerization catalysts for hydro carbons. Longwell et a l . (6) are currently investigating the effect of calcium oxide on the cracking of aromatics and other hydrocarbons. They find that calcium oxide cracks aromatics more efficiently than it does other hydrocarbons such as aliphatics. However, the finding that coke and tar were the principal products implies that calcium oxide was selectively polymerizing rather than cracking the aromatic compounds. Pyrolysis occurs to some extent in a l l coal conversion process es. Much work has been done on the effect of various parameters such as atmosphere, temperature and heat flux on the pyrolysis of coal (7,8). The present work, however, is concerned more with the effect of an indigenous property of coal, its inorganic constituents, rather than processing variables. Morgan (4) has discussed in detail how mineral matter in low-rank coal exists in three distinct forms; as discrete minerals such as clays, as mineral matter associated with the organic matter such as ion-exchangeable cations and as trace elements. During combustion, the mineral forms undergo physical and/or chemical transformations leading to slag and deposit formation on various heat transfer surfaces in combustors and boilers. This has the effect of reducing the heat transfer efficiency of the boiler, and, to some extent, combustion efficiency because of dras tically reduced free space in the combustor, thereby reducing par ticle residence times below those required for complete combustion of the fuels. Several researchers (9,10) have studied the effect of ion-ex changeable cations on weight loss rate during rapid pyrolysis, that is pyrolysis at heating rates of 1θ4-1θ5 K/s. There is general con sensus that ion exchangeable cations promote secondary char-forming reactions (cracking and/or polymerization), thereby reducing volatile

Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

16.

M O R G A N A N D SCARONI

Lignite Pyrolysis and Combustion

257


matter yields and changing product compositions. Specific mechanisms for these reactions have not been determined. However, i t is suggested that ion-exchangeable cations either react chemically with the volatile matter or prevent the escape of volatile matter molecules from coal particles by physically blocking pores which act as exit routes. Upon investigation of the volatile matter composition, Tyler and Schafer (11) found no large difference in the quantity and quality of light hydrocarbon gases evolved from acid-washed coals. Thus i t appears, as was suggested by Tanabe (_5 ) , that cations selectively polymerize rather than crack aromatic tars. Heterogeneous Combustion. An attendant difficulty in studying the effects of ion-exchangeable cations on the heterogeneous char combustion stage of lignite combustion is the existence of a concurrent or at least overlapping pyrolysis process. To overcome this d i f f i culty, many researchers purposely separate the pyrolysis stage from the char combustion step. Generally, the coal is first pyrolyzed under controlled conditions then heterogeneous char combustion is investigated subsequently. For example, this approach has been used widely (12-14) to investigate and compare the effect of alkali and alkaline-earth metals on the gasification behavior of various carbon and coal chars. Walker et a l . (12) and McKee (13) have done extensive work in a thermogravimetric analyzer at 0.1 MPa of air at various temperatures on the reactivity of graphite impregnated with various metals. They found that alkali and alkaline-earth metals, spec i f i c a l l y Na and Ca, are excellent catalysts for the C-O2 reaction. This was attributed to the ability of the catalysts to undergo redox reactions. That i s , they believed Ca, for example, catalyzed the reaction by supplying oxygen for the C-O2 reaction according to the following mechanism: CaO + h 0 -> Ca0 2

2

Ca0 + C -*· CaO + CO 2

Radovic, Walker and Jenkins (14) studied the effect of various forms of mineral matter in chars on their reactivity in air. It was found that ion-exchangeable cations particularly Ca, greatly enhanced char reactivity whereas discrete mineral phases had no significant effect. A redox reaction was again postulated as the mechanism of catalysis by the ion-exchangeable cations. As discussed previously, several attempts have been made to interpret specific effects due to cations on the combustion behavior of coals. However, most concentrate on studying the effect of cations on a single stage of the overall combustion process. Even though this does improve understanding of the mechanisms involved in each stage, it may not be appropriate when both pyrolysis and combustion occur interactively. In coal combustion, these processes overlap thereby affecting the chemistry occurring within each. This project is an attempt to investigate, in situ, the effects of cations on the individual and overall combustion processes.


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THE CHEMISTRY OF LOW-RANK COALS


Experimental Lignite Preparation. The coal chosen is a Texas lignite, PSOC-623, from the Darco seam. The coal was obtained from the PSU-DOE coal sample bank and data base. This particular coal was chosen because it has been previously well characterized (4) and because i t contains a large quantity of ion-exchangeable cations. The as-received lignite had a large particle size distribution. To obtain a narrow particle size range, the coal was first crushed using a General Purpose Mill reducing a l l particles to below 1 mm in diameter. Next, the 1 mm particles were pulverized in a Hammer Screen Mill thereby reducing particle size to less than 150 ym. The pulverized coal was sieved to separate a 200x270 mesh (U.S. Standard sieves) fraction. The mean weight particle size was 62 ym and the dispersion parameter 8.4 according to the Rosin-Rammler dis tribution (15). The size graded particles were dried under vacuum at 338 Κ for 8 hours, then stored in a dessicator until used. Prior to combustion testing, 50 grams of the 200x270 mesh frac tion were acid-washed in 900 ml of 0.4 M HC1. The coal and acid solution were stirred continuously for 24 hours. After that time, the coal was filtered and acid-washed a second time for 4 hours. The coal was again filtered and washed with 250 ml of cool, deionized water to remove chloride ions. Further washing to remove any remain ing chloride ions was accomplished by stirring the coals with 900 ml of deionized water and heating to 238-258 Κ for 2 hours. This solu tion was filtered and checked for chloride ions by addition of a drop of silver nitrate to the filtrate. If reaction occurred, that is, i f a cloudy white precipitate formed, the coal was washed repeat edly with cool deionized water until no reaction occurred upon the addition of silver nitrate. The acid-washed coal was dried under vacuum at 338 Κ for 8 hours. The acid-washed sample was back exchanged with alkali (Na, K) and alkaline-earth (Ca, Mg) metals by placing 50 grams of the coal into 900 ml of 1 M metal acetate solution. The exchange time was varied according to the desired cation loading with a minimum time of 24 hours. The ion exchanged coal was washed with cool, deionized water (250 ml) and dried under vacuum at 338 Κ for 8 hours. Data for the lignite treated with Na and Κ will be reported elsewhere. Table I gives the ultimate analysis of the raw Texas lignite and the proximate analyses of the raw and modified coals. Acid-washing decreased the ash content of the coal by 33% (from 15.9 to 10.7 wt%). Subsequent addition of ion-exchangeable cations increased the ash content by between 18 and 30% depending on the loading. The volatile matter content (wt% daf) was lower for the acid-washed coal but was not significantly different for the cation-containing samples. The distribution of the cations on the raw coal and the quantity of each cation back exchanged on the acid-washed coal is shown in Table II. These determinations were made by atomic absorption spec troscopy. The acid-washing was fairly efficient, removing over 99% of the cations present on the raw coal. Less than 0.01 wt% cations remained on the acid-washed lignite. The predominant cations on the raw coal were Ca and Mg, these two accounting for over 90% of the total quantity. As shown in Table II, the quantity of cations back exchanged on the acid-washed coal was greater than that



Ca

1.4 0.7

Raw

73.0

wt% (dry) meq/g (dry)

% (daf)

C

1.3

Ν 1.3

S

TABLE II

19.0

0 (diff) Raw Acid Washed Ca (1.0) Ca (2.0) Mg (1.3) 10.8 7.8 5.3 9.2 6.8

2

H0

0.26 0.20

M£

0.03 0.01

Na 0.03 0.01

Κ 0.03 0.002

Ba 0.02 0.002

Sr 1.8 0.9

Total

The Chemistry of Low-Rank Coals - American Chemical Society

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