FuelProcessing Technology,36 (1993) 129-135
Elsevier Science Publishers B.V., Amsterdam
Effect of coal ash properties and burning temperature on behavior of minerals with v i t r i f i c a t i o n and sintering of ash T. Kojimaa, T. Ohtanib, T. Shimizuc and T. Furusawad aDepartment of Industrial Chemistry, Faculty of Engineering, Seikei University, Kichijojikitamachi, Musashino-shi, Tokyo 180, Japan bCF Project, Tonen Corporation, Chiyoda-ku, Tokyo lO0, Japan
Palace Side Bldg. l - l - l ,
CDepartment of Chemical System Engineering, Faculty of Engineering, Niigata University, Ikarashi Ninocho, Niigata 950-21, Japan dLate Abstract Temperature at the center of burning char particles was measured. Effects of the temperature and coal properties on behavior of minerals in the coal ash with v i t r i f i c a t i o n and sintering of the ash were discussed. The maximum temperature inside the burning char particle was 20-400 K higher than the surrounding temperature. For the coals with r e l a t i v e l y low melting points of ash, the maximum temperature almost linearly decreased with the increased surrounding temperature, while for the coals with r e l a t i v e l y high melting points of ash, the temperature increment was almost kept at 200-300 K. For all coals, the temperature increment reached to zero above 1623 K. Quartz peak intensity measured by X-ray diffraction decreased with the maximum temperature and the peak disappeared by v i t r i f i c a t i o n of ash, at about 1673 K irrespective of i t s melting properties. Effects of the v i t r i f i c a t i o n on leachability of various elements in ash were also discussed. I . INTRODUCTION
In coal processing, coal ash may cause serious problems; clinker trouble in fluidized bed gasifiers, fouling of heating surface in combustors, vaporization of harmful elements from the ash, and so on. For example, the temperature of fluidized bed gasifiers must be controlled below the softening temperature of the ash, however i t is well known that the temperature of the burning particle in the oxidizing zone is much higher than the surrounding temperature. I t is generally believed that application of ash agglomerating gasifiers, such as U-gas[l] and Westinghouse processes, may be limited by the ash melting property of coal employed. The properties of coal ash are remarkably different among coals and change with coal conversion in coal processing. They are affected by process temperature, atmosphere, pressure and their history as well as the ash composition. In the U-gas and Westinghouse processes, coal ash is
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withdrawn as ash agglomerates from the bottom of the g a s i f i e r . On the o t h e r hand, ash i s withdrawn as melted slag from entrained type g a s i f i e r s . The produced ash from these processes can be used f o r various a p p l i c a t i o n s , such as b u i l d i n g , f e r t i l i z a t i o n and reclamation [ 3 ] , because i t has s o l i d s t r u c t u r e and the s o l u b i l i t y of harmful elements from the ash i s expected to be reduced through the v i t r i f i c a t i o n process of the ash in these reactors, I t should also be noted t h a t some elements may vaporize in the coal conversion processes. They effuse from stacks and are d i s t r i b u t e d i n t o the a i r , or are condensed on s o l i d surface. The condensed elements o f t e n show t h e i r high l e a c h a b i l i t y . I t is suggested t h a t high process temperature decreases l e a c h a b i l i t y of elements in the ash body by i t s vitrification w h i l e i t increases t h e i r v o l a t i l i t y . The coal ash i s thought to come from three o r i g i n s ; p l a n t organism i t s e l f , simultaneous d e p o s i t i o n w i t h p l a n t and i n f l o w a f t e r coal f o r m a t i o n . Ash from the f i r s t o r i g i n is thought to be w e l l dispersed in organic m a t e r i a l s in coal. Thus, the organic a f f i n i t y of element gives informat i o n s on the o r i g i n of the element. The behavior of the elements in coal conversion processes i s thought to be a f f e c t e d by t h e i r o r i g i n . The organic a f f i n i t y of elements, A, was defined by the f o l l o w i n g e q u a t i o n [ 4 ] where Y i s w e i g h t f r a c t i o n of ash in a s i n k - f l o a t separate and ~ s t h a t in the o r i g i n a l coal. Z and Z0 are values of weight f r a c t i o n an element f o r s i n k - f l o a t separate and o r i g i n a l coal, r e s p e c t i v e l y . Z/Z 0 = A(Yo/Y) + B
For A>O, the element i s concentrated in ash in the organic p a r t r a t h e r than i n the ash in the i n o r g a n i c p a r t , i . e . w i t h organic a f f i n i t y . A=O i n d i c a t e s t h a t the element i s u n i f o r m l y d i s t r i b u t e d in the "ash" w h i l e A=I i n d i c a t e s i t s uniform d i s t r i b u t i o n in the " c o a l " . We determined the a f f i n i t y of each element f o r a Japanese coal w i t h high ash contents of 37.8 %. For the main elements of Si, AI, Fe, Ca, K and Mg, the values of "A" were almost zero except t h a t A(Na) was around -0.2, In the o t h e r minor and t r a c e elements showed the A values between 0 and I , except Mn w i t h small negative A value, The organic a f f i n i t y was found to be high f o r elements of Sr, V, and B (A>O.8), decreased w i t h the order o f Sc (A=O.6), Be, Ba, Sn, Ni, Y, Cr, Ti (A=O,5), Zr, As, Co, Cu, Zn (A=0,35), and small f o r Pb (A=O.2), La, Mo, Cd. These r e s u l t s w e l l agreed w i t h the previous r e s u l t s [ 5 , 6 ] f o r low ash coals. In the present paper, d e t a i l e d d i s c u s s i o n s are conducted on the e f f e c t of the temperature of burning char p a r t i c l e s on behavior of minerals in coal ash w i t h v i t r i f i c a t i o n and s i n t e r i n g of ash. A summary i s presented on the e f f e c t s of c o n d i t i o n s of coal conversion process and the organic a f f i n i t y of elements on t h e i r v o l a t i l i t y and s o l u b i l i t y from the ash.
EFFECT OF BURNING CHAR TEMPERATURE ON BEHAVIOR OF MINERAL
2.1, Experimental Coal samples employed in the present study are shown in Table I . The samples were p u l v e r i z e d , sieved to the diameter smaller than 0,297mm, g r a n u l a t e d to the diameter of 13-18 mm and then carbonized at 1173 K i n a n i t r o g e n f l o w f o r more than 15 min. The carbonized sample was placed i n an experimental apparatus  w i t h an i n n e r diameter of 45mm as shown in Figure I , The sample was heated up to a desired temperature in n i t r o g e n ,
then a i r was introduced at the rate of 12 cm/s (room temperature base). The temperature inside the granulated particle was measured with a naked CA thermocouple of diameter 0.32 mm or a PR thermocouple covered with a ceramic tube of a diameter O.5mm. At higher temperatures than 1673 K, combustion was conducted on a ceramic boat. After pulverization of the produced ash, mineral matters in coal ash were measured by X-ray d i f f r a c tion (Cu/Ni, 30 kV). Table l Analyses of Coals Taka- Erme- M i l l e r - Lithshima lo blend gow Proximate moist. I%] 1.9 4.2 3.6 2.8 ash [dry%] 2 1 . 9 1 3 . 5 1 7 . 5 18.2 F.C.[dry%] 4 1 . 0 5 6 . 3 5 0 . 8 52.0 V.M.[dry%] 37.1 3 0 . 2 3 1 . 6 29.8 ASTM ash melt., reducing atm. [K] i n i t . def. 1628 1743 1693 softening hemisphere .fluid
1633 1643 1648
>1773 >1773 >1773
>1773 >1773 >1773
Taka- Erme- M i l l e r - Lithshima lo blend gow Ultimate [d.a.f.%] C 80.5 8 1 . 8 8 3 . 3 83.4 H 6.5 4.8 5.6 5.4 N 1.4 1.9 1.7 1.8 com. S 1.0 l.l 0.8 0.5 O(dif.) I0.5 10.4 8.6 8.7 JIS ash melt., oxidizing atm. [ K ] 1733 s o f t e n . 1593 1568 >1773 melt. 1623 1603 >1773 fluid 1673 1713 -
+ "!u2+ i 1
2 COMPRESSOR 3 VALVE 40RFICE METER 5 4-WAY VALVE 6 FURNACE 7 QUARTZ TUBE 8 GAS PREHEATER A SAMPLE
~ C A -TC Te=1353K
t 5 TIME i'nin]
Figure I . Experimental apparatus.
Figure 2. Typical example of time variation curve of temperature inside burning char (Takashima).
' " ~
1273 1473 Te IKI
Figure 3. Maximum temperature increment against surrounding temperature for coals with low ash melting temperature.
Figure 4. Maximum temperature increment against surrounding temperature for coals with high ash melting temperature. 2.2. Temperature of burning char p a r t i c l e s Typical examples of the time variation curve of temperature inside the burning char p a r t i c l e s are shown in Figure 2 for Takashima coal. The maximum temperature, Tm~, was 20-400 K higher than the surrounding tempera~:~;eT~romF~et~p~t~S~
at Te : 1273 K ( f o r Takashima coal) and at Te = 1323 K ( f o r Ermelo). On the other hand, the lower pattern in Figure -3 was not found at Te lower than 1373 K f o r the coals with r e l a t i v e l y high melting points of ash, i . e . , Millerblend and Lithgow coals. The maximum temperature increment, Tm x - T_ are plotted against the surrounding temperature, Tp in Figure 3 ~or Ta~ashima and Ermelo with low ash melting points and in Figure 4 for Millerblend and Lithgow with high ash melting points. In Figure 3, the maximum temperature increment, Tmax
- Te, almost l i n e a r l y decreased w i t h the increased surrounding temperature and the increment almost reached zero at about 1673 K, w h i l e i n Figure 4, the temperature increment was almost kept at 200-300 K in the range o f Te < 1373 K, the increment d r a s t i c a l l y decreased w i t h the increased surrounding temperature in the range of Te > 1373K, and the value o f Tmax was a l most same as T_ at above 1623 K. I t was foun~ t h a t the time v a r i a t i o n curve o f temperature and the maximum temperature increment through the combustion were a f f e c t e d by the ash m e l t i n g temperature as w e l l as by the ash content and surrounding temperat u r e as suggested in our previous p a p e r [ 7 ] . E f f e c t of oxygen c o n c e n t r a t i o n on the burning char temperature was also measured by d i l u t i n g a i r by n i t r o g e n at surrounding temperature of 1273 K. I t was found t h a t the temperature increment l i n e a r l y increased w i t h oxygen c o n c e n t r a t i o n , t h e r e f o r e i t was suggested t h a t the rate c o n s t a n t was a l most kept unchanged, nevertheless the burning char temperature increased, This suggests t h a t the r e a c t i o n proceed under the c o n d i t i o n o f mass t r a n s fer controlling. 2.3, Behavior o f mineral matter in ash T y p i c a l examples o f X-ray d i f f r a c t i o n are shown in Figure 5. The peak i n t e n s i t i e s f o r v a r i o u s m i n e r a l s were p l o t t e d a g a i n s t Tmax, For most of the minerals, t h e i r peak i n t e n s i t y decreased w i t h Tmax or T~, which suggests the progress o f ash v i t r i f i c a t i o n w i t h temperature increment, Espec i a l l y , the quartz peak i n t e n s i t y was w e l l c o r r e l a t e d w i t h Tmax f o r the ashes from the f o u r kinds of coal w i t h d i f f e r e n t ash m e l t i n g p r o p e r t i e s , The r e s u l t s are shown in Figure 6. The peak i n t e n s i t y decreased w i t h Tmax and the peak disappeared at about 1673 K i r r e s p e c t i v e o f the m e l t i n g p r o p e r t i e s of ash.
_ 600 xlo21--~
• Lithgow_~J - 2000
8 0 0 - ,
12?3 1473 Tmox (Ki
Figure 6. E f f e c t of maximum temperature on quartz peak intensity.
Figure 5. X-ray d i f f r a c t i o n
( A . H : a n h y d r i t e , H:hematite) (Q:quartz, A : a n o r t h i t e )
I t is suggested that the reduction in the temperature increment and the change in the time v a r i a t i o n curve of char temperature are explained by the progress of the v i t r i f i c a t i o n of coal ash. Thus the present r e s u l t s indicate that the v i t r i f i c a t i o n of coal ash mainly proceeds around T_ax = 1600 K f o r a l l coals employed f o r the present study. I t is conclude~ that the temperature range where the v i t r i f i c a t i o n proceeds is not so d i f f e r e n t as expected from the difference in the melting temperatures among the four coal samples. 3. VOLATILITY AND SOLUBILITY OF VARIOUS ELEMENTS FROMASH Behavior of various elements with combustion below 1773K, and t h e i r s o l u b i l i t y into acidic water from the ash were investigated. The o r i g i n a l coal sample and the ash sample obtained in the previous section were employed f o r the analyses of 28 elements, AI, Fe, Ca, Na, Mg, K, Ti, Ba, Sr, Mn, Sr, As, Sn, W, V, Pb, Zn, Cr, Cu, Ni, La, Y, Co, Sc, Mo, B, Be and Cd by ICP (Inductive coupled plasma) emission spectroscopy. Most of the elements remained in the ash under the present experimental condition, while the vaporization of B and Zn was found in the range T > 1523 K, and s l i g h t vaporization of Na and Cd at about T : 1773 K. Dried and powdered ash sample was weighed, mixed with acidic solution and shaken f o r 6 hrs. The supernatant was analyzed. 24 elements except Sn, Cr, Cd, Ni were q u a n t i t a t i v e l y detected. The dissolved f r a c t i o n s for a l l elements increased with the increase in proton concentration. The s o l u b i l i t i e s of a l k a l i n e earth metals were remarkably high, while they d r a s t i c a l l y decreased with the increased maximum temperature at about 1600 K, where the v i t r i f i c a t i o n of coal ash mainly proceeded. Almost same tendency was found f o r Mn, La, AI, Fe, V, Sc, Y and Pb. The dissolved f r a c tion f o r the other elements showed a smaller decrease in the temperature range around 160OK. Close r e l a t i o n with the value of the organic a f f i n i t y was hardly found. Detailed discussion w i l l be necessary. 4. Conclusion The progress of the v i t r i f i c a t i o n and the reduction of the s o l u b i l i t y of elements from the ash were mainly found at around 1600 K f o r a l l coals employed in the present study, i r r e s p e c t i v e of the ash melting properties measured according to JIS or ASTM. 5. REFERENCES 1 2 3 4 5 6 7
D.M. Mason and J.G. Patel, Fuel Process. technol., 3 (1980) 181. C.W. Schwartz, L.K. Rath and M.D. Freier, Chem. Eng. Prog., A p r i l (1982) 55, L.B. Clarke, Applications for coal-use residues, IEA Coal Research, London, 1992. T. Kojima and T. Furusawa, J. Fuel Soc. Jpn. (Japanese), 65 (1986) 143. L. Horton and K.V. Aubrey, J. Soc. Chem. Ind. Sup., I-S (1950) 41. P. Zubovic, Coal Science, Ad. in Chem. Ser. 55 (1966) 221. T. Kojima, T. Shimizu, D. Kunii and T. Furusawa, J. Fuel Soc. Jpn. (Japanese), 65 (1986) 194.
135 Discussion Effects of coal ash properties and burning temperature on behavior of minerals with vitrification and sintering of ash T. Kojima, T. Ohtani, T. Shimizu and T. Furusawa Question: J.A. Moulijn According to which criteria did you select the elements you have studied? I would expect that a few elements you did not select are very important from an environmental point of view, viz., Hg, Se, Cr, F. Why did you not select these elements? Amw~ One reason comes from the experimental limitation. The content of Hg is extremely low and it easily vaporized even at 200°C, which is the temperature to dissolve coal in acid. F cannot be analyzed by ICP. As to the harmful elements, I analyzed as many as I could, e.g., Cd, As, Sn, AI. Na and K are included because they cause corrosion problems. Second reason why we did so many elements is that our objective is to correlate leachability with other elemental properties. But it is not yet fully successful at this stage. The third reason why we include the main elements, e.g., Fe, Ca, Mg, Ba, are that they may affect the leachability of other elements. Especially, under the conditions close to neutral pH, dissolution of Ca, Mg, etc. affects the pH value, giving many problems, whereas they are not harmful themselves. Question: K.A. Nater 1. Could the so-called affinity for the organic phase not better be called 'affinity to the 'float"? 2. Leachability has been studied with diluted nitric acid for a fixed period of time. Are time series considered? Remark: reference given to standard leachability test. Answer I. 'Float' has low ash content, and 'sink' has high content. 'Float' has high 'organic' content. In our analysis, element content is expressed as a function of ash content. Main ash constituents are inorganic 'SiO2, A1203...' and ash free coal is organic. Furthermore, we obtained a good linear relation for most of the elements as shown in Fig. 1. So, we call A 'organic affinity'. 2. We confirmed that 6 h is enough to obtain saturation in leached amount of elements, thus we selected 6 h. This is the reason why we use pulverized ash to test the leachability.