Model study on the formation of fuel NOx from cokes on combustion: Effect of metal oxides

Model study on the formation of fuel NO, from cokes on combustion: effect of metal oxides Chikao

Yokokawa,

Hirokazu

Department of Chemical Engineering, yama, Suita, Osaka, 564, Japan (Received 29 September 1980)

Oda, Akira

Fukawa”

Faculty of Engineering,

and Kengo Tanaka+ Kansai University,

Senri-

To elucidate the process of ‘fuel NO,’ formation from cokes and chars on combustion, carbons obtained from urea-resin and melamine-resin were subjected to combustion under a stream of pure oxygen at temperatures between 600°C and 1000°C. The carbons subjected to combustion without any additives produced a relatively small amount of NO,, but, it was observed that the NO,formation predominated at lower temperatures of combustion. When the combustion was carried out with the addition of metal oxides, such as iron oxides and cupric oxide, the amount of NO, formed increased markedly but silicon dioxide had no such effect.

Fossil fuels give NO, in various amounts when they are burnt in air and the NO, has been seen as a serious source of air-pollution. The formation of ‘thermal NO,’ has been studied and is considered to depend mainly on the flame temperature and on the excess of oxygen. However, the formation of ‘fuel NO,’ has also been studied and recently the ‘fuel NO,’ from coals has been examined’ -6. It is generally accepted that fuel NO, from coals on their combustion originates mainly from the nitrogen contained in volatile matter (gaseous and tarry products evolved in the process of combustion) and the contribution of the nitrogen in coke or char is very small, even though the distribution of coal nitrogen in volatile matter and in fixed carbon are considered comparable. Systematic and extensive studies are required in the field of fuel NO, formation from cokes, because the emission of NO, from the sintering process of iron-ore is considered to be a serious problem’. This paper examines the formation of fuel NO, from cokes using nitrogen-rich carbons as model substances. The carbons obtained from urea-resin and melamine-resin were subjected to combustion with pure oxygen and the amount of NO, evolved was determined. The effect of metal oxides on the formation of NO, was also investigated. EXPERIMENTAL Materials used Urea resin was prepared in our laboratory by the standard procedure from urea and aqueous solution of formaldehyde with an addition of aqueous ammonia. * Present address: Kohonoshima Chemical Industries Co. Ltd., 17-42, Dohjima, Kita-ku, Osaka, 530, Japan 7 Present address: Yoshida Kogyo Co. Ltd., 1300, Horikiri, Kurobe, Toyama, 938, Japan

001fw2361/81/060495-04$2.00 01981 IPC Business Press

Melamine resin was prepared by Nippon Carbide Industry Co. Ltd. on a commercial scale. Ferric oxide, ferrous-ferric oxide, cupric oxide and silicon dioxide were commercially obtained as G.R. grade. Oxygen and argon were supplied commercially in pressure bottles. Pyrolysis

of the resins

Approximately 60 g of resins, contained in a silica crucible of 300 ml capacity, were heated in an electric furnace under an argon atmosphere. The temperature was increased at a rate of 5°C min-’ to 600°C and was maintained at this temperature for 1 h. Approximately 4g of the 600”C-carbons, contained in an alumina boat, were then heated in an electric furnace, under a stream of argon, to between 600 and 15OO”C, the final temperature being maintained for 1 h. The yield and nitrogen content (determined by Kjeldahl procedure) of the carbons are listed in Table 1. Combustion and determination of NO, The equipment used for the combustion experiments is shown schematically in Figure 1. Approximately 20 mg of carbon were accurately weighed into an alumina boat which was placed at one end of the silica combustion tube (9 mm diameter, 450 mm length). To investigate the effect of metal oxides, finely pulverized metal oxides were mixed with the carbons in various ratios and the mixtures containing 20 mg of carbon were subjected to combustion. The combustion tube was heated to a desired temperature between 600 and lOOO”C, while a stream of pure oxygen was introduced at a rate of 10 ml min- ’ and directed out via tap (1). The flask of 1000 ml capacity (5), which was used as a gasometer, was evacuated to a pressure of < 650 Pa. The separation funnel (6) was filled with 50 ml of an aqueous solution of 0.3% hydrogen

FUEL,

1981,

Vol 60, June

495

Formation

of fuel NO, from

cokes

Table 1 Yield and nitrogen content melamine-resin and urea-resin

on combustion:

of carbons obtained

C. Yokokawa

from

et al.

RESULTS Formath

Melamine-resin Heat-treatment temperature (“C)

_ 600

700 800

Urea-resin

Yield (o/o)

N (%I*

Yield (%I

N (%I’

100 55 26 17 14 14 11 11 11 10 8

44.7 50.1 35.5 24.6 14.1 9.1 5.1 3.7 4.0 1.6 0.8

100 24 18 17 16 15 14 _ _ _ -

30.5 25.8 20.4 14.4 6.8 6.0 3.6 _ _ _

a Determined by the Kjeldahl procedure. The reliability of the procedure was confirmed by comparing the data with these obtained by CHN-coder for several carbons; the data agreed within experimental error for each pair

AND DISCUSSION qf’@el

NO,’

on combustion

of carbons

The changes of the conversion of fuel nitrogen to NO, with the combustion temperature are shown in Figure 2 and 3 for urea-600”C-carbon and for melamine-600”Ccarbon, respectively. It is clear from the Figures that the 600 ‘C-carbons show a conversion of less than 1% when they are burnt in the absence of ferric oxide. Figures 4 and 5 illustrate the results for lOOO”C-carbons. The lOOO”Ccarbons show relatively large values ofconversion, but the values scarcely exceed 10% when the carbons are burnt alone. The large difference observed between the conversions for 600’C-carbons and for lOOO”C-carbons can be attributed to the different mode of nitrogen atoms in the carbons. It is difficult to ascertain the structural nature of the nitrogen in the carbons and therefore also difficult to distinguish the difference between the nature of nitrogen in the 600”C-carbons and the lOOO”C-carbons, for instance, i.r. spectra for the carbons have no distinctive

Thermometer

Thermocbuple

-

Vacuu

Flask (7) Gasometer Figure

1

Experimental

apparatus

for the combustion

(5) of carbons

800

600

Combustion peroxide, acidified with sulphuric acid. After the temperature reached the desired and constant value, tap (2) was gradually opened and the flow rate of oxygen was adjusted to w 10 ml min-‘. The boat was then quickly inserted into the centre of the tubing so that the sample could be combusted. The stream of oxygen was maintained for w 90 min to collect all of the effluent gas in the gasometer; the inside pressure of the gasometer was still less than atmospheric pressure. 50 ml of the aqueous hydrogen peroxide was poured into the gasometer from the funnel (6) and after standing for one night to ensure that all the amount of NO, was dissolved in the solution, the solutions in the gasometer and flask (7) were collected and quantitatively made up with water to 250 ml. 50 ml of the solution was taken for the determination of NO, using the JIS method*s. The value of the conversion of nitrogen to NO, quoted in the text indicates the percentage of the weight of nitrogen in the carbon appearing as NO,. To determine the error involved in the measurements, a standard gas sample, which consisted of nitrogen and 910 ppm of nitrogen monoxide (99.9%), was introduced into the combustion apparatus and the amount of NO, recovered was determined. By this means the analytical error was found to be within 1%. *

A calorimetric

496

method using phenoldisulphonic

FUEL, 1981, Vol 60,

June

acid.

temperature

PC

1

Figure2 Change in conversion of fuel nitrogen into NO, with the combustion temperature for urea-600°C-carbon. 0, Without additives; 0, equal amount by weight of Fez03 added

600 Combustion

700

a00 tempemture

900 PC)

Change in conversion of fuel nitrogen into NO, Figure 3 the combustion temperature for melamine-600°C-carbon. 0, Without additives; 0, equal amount by weight of Fe203

with added

Formation

of fuel NO, from

6 a, 20s j, b lo-

/ -0

I 6cm

700 800 900 Combustion tempemture

1000

1100

(T)

Figure4 Change in conversion of fuel nitrogen into NO, with the combustion temperature for urea-1000°C~carbon. 0, Without additives;., equal amount by weight of Fez03 added;., equal amount by weight of Fe304 added

25 I

01

500

I

600

I

700 800 900 Combust Ion temperature (“C)

C. Yokokawa

Ejfect of metul oxides on the formation

L

? s 0 500

on combustion:

et al.

expected. Yoshida et al.“,” have found that >98’;/, of NO, in a flue gas were removed by coke at a certain temperature above 800°C and they considered that the NO, was reduced by carbon to molecular nitrogen. The rate of the reduction depends on the temperature so that this may provide the explanation for the conversion obtained in the present experiments.

l

?

cokes

I

I

1000

Figure 5 Change in conversion of fuel nitrogen into NO, the combustion temperature for melamine-1000”Gcarbon. 0, Without additives; 0, equal amount by weight of Fez03

1100 with

of“jiie1

NO,

The effect of ferric oxide and ferrous-ferric oxide on the formation of NO, is shown in Figures 2, 3 and 4, 5 for 6000C-carbons and for lOOO”C-carbons, respectively. In the studies, an equal amount of oxide by weight was added to the carbons. The iron oxides promote the NO, formation considerably, i.e., for 1000°C carbons the conversion observed in the presence of iron oxide is approximately 2 or 3 times larger than that observed in the absence of the iron oxides and the tendency is more distinct for 600”C-carbons. The values of the conversion vary with the combustion temperature, passing through a maximum at a certain temperature between 700°C and SOO’C, with the exception of the melamine-600”C-carbon (Figure 3)*. The maximum observed in Figures 2,4 and 5 indicates that there is a competition between catalytic oxidation of the fuel nitrogen to NO, by iron oxides+ and reduction of NO, to molecular nitrogen by carbon, and that the former predominates at temperatures below lOOO”C, especially between 700°C and 800°C. Figure 6 shows the effect of the amount of ferric oxide added to the 1000°C carbons on the formation of NO, and it is shown that the addition of a half amount of iron oxide by weight is sufficient to promote the NO, formation. Cupric oxide and silicon dioxide were added to the 1000°C carbons in an equal amount by weight and the mixtures were cornbusted in the same way. The results were almost the same for the two carbons, and therefore only the results for the urea-1000LC-carbon are shown in Figure 7. Cupric oxide has a weak effect in promoting

added

profiles. However, it is considered feasible that the 6OO”Ccarbons still possess the organic character which allows the existence of nitrogen atoms in such structures as amine, imine etc. However, lOOO”C-carbons would be recognizably inorganic in nature and the nitrogen in them would be cemented in the domain of the carbons as given by C,N,. It is suggested that in the course of heattreatment at temperature between 600°C and lOOO”C, the nitrogen present in the 600°C-carbons escapes as molecular nitrogen* and therefore it is considered that the nitrogen evolved during the combustion is not oxidized at these temperatures+. The difference in the conversion of nitrogen between urea-carbons and melamine-carbons is barely noticeable. It is noteworthy that the conversion of fuel nitrogen into NO, decreases with an increase of the combustion temperature, in contrast to what might generally be * The gaseous product evolved from the 6OO”C-carbons during the pyrolysis at the temperature between 600°C and 1OOWC was analysed and no appreciable quantities of nitrogen containing materials such as NH, and HCN were detected, also, condensable product (tar) was not formed in an a’ppreciable quantity t Air was passed through the combustion tube, filled with and without ferric oxide, at temperature between 600°C and 1000°C. The outlet gas was analysed and no NO, was detected

* The reason for the exceptional behaviour of the melamine-600 Ccarbon may be attributed to its high nitrogen content, but a definite explanation can not be given yet t Mixtures of the carbons and ferric oxide were heated at 1000 C under a stream of argon. The outlet gases contained no appreciable amount of NO,. It seems, therefore, that the reaction between ferric oxide and nitrogen in the carbons to form NO, need not be considered

Q

I

I

05 Ratlo

”

10 of Fe03

15 added to carbons

2 5

20 (g/g)

Figure 6 Effect of the amount of ferric oxide added on the conversion of fuel nitrogen into NO, at 900°C. 0, Urea-lOOO”Ccarbon; A, melamine-lOOO°C-carbon

FUEL, 1981, Vol 60, June

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Formation

of fuel NO, from cokes on combustion:

C. Yokokawa

CONCLUSJON

7”

a

z * 830s Z

5 .z 20 6

5

t

\

I

I

‘\

“:; j 500

1

600

700 Comb&Ion

800 900 tempemture PC)

1000

1100

Figure 7

Effect of cupric oxide and silicon dioxide on the conversion of fuel nitrogen of the urea-lOOO”C-carbon into NO, at various temperatures of combustion. 0, Without additives; 0, equal amount by weight of CuO added;*, equal amount by weight of SiOz added

NO, formation but silicon dioxide has no or a slight retarding effect on the NO, formation. The fact that silicon dioxide has no promoting effect seems to eliminate the possible explanation that the acceleration of NO, formation could be attributed to the dilution effect of the oxides, which might disturb the contact of NO, formed with carbon surface and therefore disturb the reduction reaction of NO, to molecular nitrogen by the carbons.

498

et al.

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1981,

Vol 60, June

Nitrogen in carbons is converted into NO, on their combustion to a relatively small extent but the conversion is promoted markedly by the presence of iron oxides. The NO, formed seems to be reduced by excess of carbon giving molecular nitrogen at temperature above 800°C however, the promotion by the iron oxides also predominates at temperature. It is suggested that a layer of coke without mixing with iron-ore would be effective in reducing the NO, emission from the sintering process of iron-ore, if the layer was placed in such a way as to be in contact with the outlet gas while it is at a sufficiently high temperature. REFERENCES 1 2

Fine, D. H., Slater, S. M., Sarolim, A. F. and Williams, G. C. Fuel 1974,53, 120 Pereira, F J., Beer, J. M., Gibbs, B. and Hedley, A. B. 15th Symposium (In:) on Combustion, The Combustion Institl:ie, Pittsburgh, 1975, p. 1149 Pershing, D. W. and Wendt, J. 0. L. 16th Symposium (Int) on Combustion, The Combustion Institute, Pittsburgh, 1977, p. 389 Vogt, R. A. and Laurendeau, N. M. Fuel 1978, 57, 232 Solomon, P. R. and Colket, M. B. Fuel 1978, 57, 749 Ogura, T., Watanabe, M. and Hiraki, A. J. Fuel Sot. Japan 1978, 57, 758 ‘The Annual Review of Fuel Sot. of Japan. 1. Fuel Sot. Japan 1978, 57, 654 Japanese Industrial

Standard, ‘Method for Determination of Nitrogen Oxides in Exhaust Gases’, JIS-K0104, 1974 Yoshida, T., Tazawa, R., Ueno, Y. and Koshitani, Z. J. Fuel Sot. Jupun 1975,54, 766 Yoshida, T., Kimura, N., Ueno, Y. and Koshitani, Z. J. Fuel Sot. Japan 1979, 58, 868