Dev. Chem. Eng. Mineral Process., 7(3/4), pp.387-395, 1999.

Effect of Ash Components on the Ignition

and Burnout of High Ash Coals

B. Feng, R. Yan, and C.G. Zheng' National Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China

The ef/ect of the ash components on the ignition and burnout of four Chinese

high ash coals were studied by thermogravimetric analysis. To investigate the

influence of the ash components, comparative experiments were carried out with

original, deashed and impregnated coals. Eleven types of ash components, such

as 2302, CaCO3, MgO, Na2CO3, K2CO3, Al2O3, Ti02, Fe2O3, FeS2,

NH4Fe(SOa)z.l2H20 and FeS04(lVHa).6H20 were used in the present study. It

was found that most of the ash components have negative effects. The strong

influence of some ash components suggests that the combustion characteristics

of high ash coal may be determined by the ash composition.

Introduction China has abundant deposits of low-grade coals, the coals with high content of ash.

The low-grade coals are burnt for power generation in China although they associate

with many operating problems such as flame extinguish and slagging. The efficiency

of utilization is also not satisfactory. The study on the combustion characteristics of

high ash coals is important for efficient and safe use. The ash components in the coal

ash, mainly emanating from the mineral matter are expected to influence the

combustion characteristics.

* Author for correspondence

387

B. Feng, R. Yan and C.G. Zheng

The effect of mineral matter on combustion, gasification and pollutant emission

has been studied by some researchers [ 1-41. Kbpsel and Zabawski [ 1,2] studied the

catalytic effects of ash components in low rank coal gasification. The conversion rate

for ash-containing lignite was 30-50 times higher than that for deashed lignite.

Another interesting finding is that the rate of conversion when sodium and potassium

were used in combination was considerably higher than the sum of the conversion

rates when these elements were used alone.

Morgan and Jenkins [3] studied the effects of Ca, Mg, Na, K, Sr and Ba on the

rapid pyrolysis of lignite in a dilute-phase entrained flow reactor. Cations were

added to the acid-washed lignite by ion exchange. The presence of the cations

was found to decrease the rate of volatile release, the total release of volatiles and

the apparent activation energy for devolatilization.

Aho et al. [4] studied the effects of Fe and A1 on peat combustion and the

formation of nitrogen oxides under conditions of rapid combustion and slow

combustion. Iron has a strong catalytic effect on both the slow and rapid

combustion, and it decreased the ratio of N20 to NO in the combustion gases.

Aluminium did not have a detectable effect on the combustion rate but it

depressed the formation of nitrogen oxides slightly.

Kopsel and Halang [5 ] studied the effect of ash elements on NOx formation in

char combustion. However, there are few studies on the effect of the ash components on the oxidation of char. The present paper studies the effect of Si,

Ca, Al, Na, K, Mg, Fe on the combustion of four Chinese high ash coals. The

indices for ignition and burnout are used.

Experimental Four Chinese high ash coals were deashed and impregnated with different ash

components. The original, deashed and impregnated coal samples, were burnt in a

thermobalance under the following conditions: sample weight 10 mg, heating rate 20

Wmin, oxygen flow rate 5OmVmin. The TG, DTG and DTA curves were used to

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Ignition and burnout of high ash coals

calculate the indices for ignition (T1 and M) and burnout ("2 and N). The values

were compared.

Table I analyses of coal samples

Proximate analvsis (%) Coal Type A B C D

a , I

Moisture 0.77 3.22 3.69 2.03 Volatile Matter 7.62 7.49 10.67 17.49 Ash 34.88 29.03 49.48 3 1.36 Fixed Carbon 56.73 60.29 36.16 49.13 Ultimate analysis (%, as received) Carbon 58.60 62.78 60.34 56.52 Hydrogen 2.0 1 1.94 2.04 2.82 Nitrogen 1.70 1.21 1.53 1.61 Sulfur 1.10 0.80 0.93 0.82 Oxygen 0.94 1.02 0.96 0.95 Mineral Matter (%)

389

B. Feng, R. Yan and C.G. Zheng

Deashing

Interference in the coal structure caused by deashing was limited as much as possible.

The porosity and the surface area showed no significant change after deashing. For

deashing, 10 g of 0.1-0.5 mm coal fraction was boiled with 100 ml of 20% HF acid

for 1 h at temperature of 333 K, washed with distilled water, and dried under vacuum

at 363 K. 95 wt?? of the ash was removed. The proximate and ultimate analyses and

trace elements in the original coals are shown in Table 1. The ash consists of mainly

Si02, Fez03 and Al2O3.

Coal impregnation

Eleven types of ash components were added to the deashed coals by two ways:

adding in aqueous solution or mixing mechanically. For aqueous solution addition,

the deashed coal is emerged into the solution of various ash components for 2 days.

The concentration of the solutions was selected according to the absorption ability of

the coal. For mechanical mixing, the ash components were mixed with the deashed

coals in a dry state, and the mixtures were then moistened. In all cases, the coals were

subsequently dried under vacuum for 3 h at 363 K.

To examine the influence of the anion, the coal was impregnated with Fe2O3,

FeS2, NHdFe(S04)z 12H20 and FeSOd(NHd.6H20.

Thermogravimetric analysis

The original, deashed and impregnated coals were burnt in a thermobalance. A

typical TGA curve for coal A is shown in Figure 1. The ignition temperature TI,

burnout temperature T2, the temperature at which the coal is reacting at the

maximum rate T,,, and the indices for ignition and burnout, M and N can be

calculated from the curve. The indices M and N are defined as follows [7],

+ 0.0093e0.36W1mx + 0.01 le0.44G1 (1)

(2)

M = 47e-0.00526 + 4,6e4.0044qmw

N = 0.55G2 + 0.0043T2,, + 0 . 1 4 ~ ~ ~ + 0.72t6, + 3.7

where TI,,,,, Wlmax and T2m, are the temperature of the first peak, reaction rate

of the first and second peak respectively, as shown in Fig. 1. G1 and G2 are the

390

Ignition and burnout of high ash coals

area below the first and the second peak (the weight losses in different

combustion stages). t 9 8 and t'98 are the burnout times for char and coal

respectively. The indices, M and N, were found to reflect the ignition and burnout

characteristics of the coals in practical furnaces and is used in design of boiler

furnace in China [7].

0 200 400 600 800 loo0

Temperature (K)

Figure 1. Typical TGA curve for coal A

Results and discussion Generally, the deashed coal can be ignited more rapidly but burnt more slowly

according to the experiments. Most of the ash components have negative effects on

the ignition and burnout of deashed coal.

Comparation of original and deashed coal

The ignition temperature (T,), the burnout temperature (T2), the temperature the coal

reacting at its maximum rate (Tma) and the indices for ignition and burnout (M and N)

are listed in Table 2 for the four original coals and the deashed coals. The ignitability

391

B. Feng, R. Yan and C.G. Zheng

of the deashed coals is better than the original coal samples. It may be due to the

removal of the main components, Si02 and Al2O3, leading to the decrease in the

thermal stability of the coal [6]. However, deashing causes the burnout to be a little

more difficult for Coals A, B and D except for Coal C which burns much slowly.

Effect of the ash components on combustion of Coal C was studied as shown:

Table 2 Thermogravimetric analysis of original and deashed coals.

Coal type Tl(OC) T,ax(OC) T2(OC) M N Coal A 466 494 657 1.265 5.830 Coal A (deashed) 43 1 465 660 1.499 6.300 Coal B 46 1 484 692 1.409 5.308 Coal B (deashed) 437 475 700 1.579 6.55 1 Coal C 449 460 592 1.340 6.560 Coal C (deashed) 4 15 446 499 1.659 4.859 Coal D 385 409 607 1.756 6.047 Coal D (deashed) 268 317 589 2.569 6.365

Efect of addition method The solution addition method and the mechanically mixing addition method were

compared. No significant difference was found although the mechanically mixed coal

has a slight increase in the combustion rate than the solution-emerged coal.

Efect of ash components The results for Coal C are summerized in Figure 2. The figure can be divided into

four regions:

Region I: the ash elements in this region make difficult the ignition and burnout of

the deashed coal. Fe3, Fe2, Nay All, Cal and Ca3 are in this region. The original coal

is also in this region, meaning that the combined effect of all the elements leads to

more difficult ignition and burnout.

Region 11: the ash elements in this region allow easy ignition, but the burnout

is difficult. K2 and K3 are in this region.

392

Ignition and burnout of high ash coals

+ Fe3 I

original coal +

II

+ K2

*K3

K1 )" + Ca2 ..... .I .......... "..SIK @13.." ... "....... ..... I. ...... ". ............... " ........ burnout

FeP *Mg*A12 deashed coal

+Ti ignition I V 1 1 1 d i f f l c u l t h easy

easy U 1 1.5 2 2.5

Index for ignition

Figure 2 Effect of mineral matters on the ignition and burnout of Coal C. The

symbols indicate the indices after addition of the mineral matters: Si - 17.1%

Si02; All - 4.9% Al2O3; A12 - 10.6% A1203; A13 - 15.6% A1203; Cal - 1.6%

CaC03; Ca2 - 2.4% CaCO3; Ca3 - 3.6% CaCO3; Mg - 0.4% MgO; Na - 10.1%

Na2C03; Ti - 0.6% TiOz; K1 - 1.1% K2C03; K2 - 1.3% K2CO3; K3 - 7.2%

K2CO3; Fel - 3.3% Fe2O3; Fe2 - 13.7% FeSO4(NH4)S04,6H20; Fe3 - 24.5%

NH4Fe(SO&. 12H20.

Region 111: the ash elements in this region allow easy burnout, but the ignition

is difficult. Si, A13, A12, Fel, Mg and Ti are in this region.

393

B. Feng, R. Yan and C.G. Zheng

Region IV: the ash elements in this region allow easy ignition and burnout.

Only K1 and Ca2 are in this region.

EHect of ash composition The effect of the element concentration was studied by adding different weight

percentage of the additives. In Fig. 2, All, A12, A13 indicates that 4.9%, 10.6%

and 15.6% of A1203 were added respectively. Cal, Ca2, Ca3, K1, K2, and K3

stand for the addition of 1.6%, 2.4%, 3.6% CaCO3, 1.1 %, 1.3% and 7.2% K2CO3

respectively. It seems that there exists “optimum” concentration for the ash

components studied. For instance, Cal hinders both the ignition and burnout.

However, Ca2 promotes both while Ca3 has the same trend as Cal.

EHect of Fe additives Fe was added in four types: in Fe2O3, FeS2, NH4Fe(S04)2*12H20 and

FeS04(NH4)S04*6H20. The effects of various iron additives were quite different

as shown in Fig. 2. The effect of FeS2 is not shown in the figure. It was found

that it improves the ignition of Coals A and B. This is in agreement with Li’s

study [8]. It may be due to the reactions of Fe(II)+02+Fe(III), S(-I)+02+S02

favoring the earlier ignition. However, Fe 1, Fe3 and Fe4 hinder the ignition and

burnout of Coal C significantly. Especially for Fe3, the coal is very difficult to

bum. However, the effect of Fe2 and Fe3 may be not caused by the catalytic

effect of Fe, but be caused by the decomposition of them.

EfSect of coal type The effects of some ash components on the different coals are not always the same.

For instance, SiO2 improves the burnout of Coal C slightly, but reduces the burning

rates of Coals A and B. However, some ash components do have the same influence

on the different coals. K intends to improve both ignition and burnout. Si, Al, Fe, Mg,

Na and Ti intend to prolong the ignition time. But Ti increases the burning rate of the

coal.

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Ignition and burnout ofhigh ash coals

Conclusions The effect of the ash constituents on the ignition and burnout of four Chinese high ash

coals was studied in a thennobalance. The following conclusions can be drawn:

1. The deashed coals ignite earlier than the original ones, but the burning rates are

lower than those of the original ones;

2. Ash constituents influence the ignition and burnout non-linearly and non-

monotunel y ;

3. Ash constituents have different effects on the different coals.

Acknowledgement The financial support of the National Natural Science Foundation is gratefully

acknowledged.

References I . Kspsel, R. andikbawski. H 1990.Fue1, 69(3). 275-281. 2. Kspsel, R. and Zobawski, H. 1990. Fuel, 69(3), 282-288. 3. Morgan, M. E. and Jenkins, R.G. 1986, 65. 757- 762. 4. Aho, M.J.. T u m w o r i , J.L. and Hdmdkiinen, J.P. 1990. Fuel, 69(5), 639-642. 5. Kspsel, R and Halang, S. 1997. Fuel, 76(4), 345-350. 6. Wang, TH., Cui, Z.D. andaeng, M.D. 1992. Journal of Fuel Chemishy(Chinese) ,20(3), 56-63. 7. Llu. W.2 1991. Thermal Power Generation (Chinese), 6, 15-19. 8. Li. XJ. 1992, Thermal Power Generation (Chinese), 3, 35-42.

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