Fuel Processing Technology, 36 (1993) 117-122 117 Elsevier Science Publishers B.V., Amsterdam

Laboratory m e a s u r e m e n t of N release under combust ion condit ions and comparison wi th plant NOx formation

C.I~ Man, K.J. Pendiebury and J.R. Gibbins

Mechanical Engineering Department, Imperial College, London SW7 2BX, United Kingdom

Abstract The development of a new version of the wire-mesh apparatus, capable of

achieving both high heating rates and peak temperatures in excess of 1500°C, has allowed captive sample measurements of N release during devolatilisation under PF combustion conditions to be mode for the first time. Both volatile and N release yields differ significantly from previous wire-mesh data obtained for peak temperatures in the region of 1000°C. Significant differences are also found in N release patterns between different coal types. Comparison of the high-temperature wire-mesh apparatus N release data with NOx formation data from combustion plant, including full size boilers, shows that differences in NOx formation between coals with similar N contents can be related to their different patterns of volatile nitrogen release.

1. INTRODUCTION

Overall EC NOx emissions are set to fall from 1980 values by 10% in 1993 and 30% in 1998 under the EC Large Combustion Plant Directive [1]. Methods of reducing NOx to achieve these limits have concentrated on minimizing its formation during combustion using air-staged low-NOx burners.

The bulk of the NOx formed during coal combustion originates from fuel nitrogen, but it is observed that coals with similar overall nitrogen contents may give widely different levels of NOx during combustion. Figure 1 shows the simplified route for fuel NOx formation [2].

In an air-staged low-NOx burner, fuel NOx originates predominantly from the oxidation of nitrogen in the char [3]. Tests which can predict the partition between volatile and char nitrogen should therefore offer the potential for predicting the relative tendency of a coal for NOx formation. Existing standard devolatilisation tests are unsuitable since they do not reproduce the dispersed sample distribution, the heating rate or the temperatures encountered in a PF furnace [4]. The object of this paper is to investigate suitable coal test methods to predict NOx formation under air-staged low-NOx combustion conditions using a high-temperature wire-mesh apparatus. Compared to the entrained flow

0378-3820/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

118

reactors which can also be used for coal characterisation (e.g. [3]), this type of apparatus has the advantage of being relatively small and simple to operate, and also allows measurement of volatile yields using direct weighing instead of ash- tracer methods.

¢kl,,atl~Malmn

Vo~BtileN - - NOxlxllCU~O~ ~ NOx

N lkOeO~l : I'K:;N. ~ NH =. N =. 111oO0 etC, Nz

CO~N r e k m ~ t~o*

~ N Nz

el 8 ~4o z

-~2o

20 4o 60 Total ~ . (%d~

Figure 1. Simpi i fmd model f ~ convers ion of coal ni t rogen to NOx (after Pohl, 1976),

Figure 2. Volat i le N vs. total vokl t i les for UK bi tuminous coal [5]

2. EXPERIMENTAL

A suite of ten UK and world-traded bituminous coals were used in this study and the analyses are given in Table 1. VM contents varied from approximately 33 to 44% dafand ash contents from 6-18% (db). A particle size range of 106-150 1am (restricted by the aperture of the high-temperature mesh material) was used in all the experiments. After grinding and sieving, coal samples were dried overnight in a nitrogen-purged oven at 105°C and stored under flowing nitrogen until used.

Table 1 Properties of coals used

Coal TGA proximate analysis (wt% db) Ultimate analysis (wt% db)

VM FC Ash C H N

1 34.3 55.0 10.7 72.3 4.8 1.7 2 41.4 52.4 6.2 70.6 5.2 1.7 3 27.8 57.3 14.9 70.2 4.2 1.8 4 34.1 55.4 10.5 72.9 5.0 1.6 5 30.5 58.7 10.8 74.3 4.6 1.8 6 32.2 49.6 18.2 70.4 4.7 1.7 7 37.2 55.8 7.0 78.9 5.5 1.6 8 40.1 51.5 8.4 66.1 5.1 1.4 9 38.4 55.5 6.1 74.6 5.5 2.6

10 31.7 58.5 9.8 71.7 4.5 0.9

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A new high-temperature wire-mesh apparatus (WM) was used for devolatilisation experiments. Between 20 and 25 nag of coal is spread in a 20 m m

diameter circle between two layers of wire-mesh, which also act as an electrical resistance heater. Temperatures are measured using a two-colour IR pyrometer, connected to a computer-based temperature control system. An helium sweep gas flows across the wire-mesh sample holder to prevent volatiles redepositing. Stainless steel mesh (AISI 304, 36 pm wires x 63 ~n aperture) was used for experiments at 1000 K/s to 1000°C with 10 s hold, and molybdenum mesh (63 pm wires x 106 Inn aperture) for runs at 1000 K/s to 1450°C with 150 ms hold. Volatile yields can be measured directly by weighing the sample holder and coal, or char, before and after pyrolysis. This apparatus is described in more detail elsewhere [5, 6].

C H N analysis was performed on the coal and the resultant char samples using a Carlo Erba 1106 elemental analyzer. Duplicate runs were carried out on 1.5 mg samples. The distribution between volatile and char nitrogen was calculated from the char yields plus the coal and char nitrogen contents.

For comparison, the coals were also pyrolysed in a Stanton-Redcroft TG1000 thermogravimetric analyzer (TGA) to produce chars under conditions approximating to current standard tests. About 10rag of coal was heated in the TGA in nitrogen (flowrate 25 ml/min) at 1 K/s to 900°C and held at peak temperature for 5 minutes.

3. RESULTS AND DISCUSSION

Figure 2 shows the effect of temperature on total volatile and nitrogen release during devolatilisation for a typical UK bituminous coal [5]. At temperatures below about 1300°C volatile nitrogen is proportional to total volatiles for all conditions, but at higher temperatures and extended hold times further nitrogen can be evolved with little additional sample weight loss. To approximate to the conditions found in current low-NOx burners, high temperature (1450°C) runs with short residence times (150 ms) were carried out. Longer residence times might be required to match future overfire air-staged combustors.

I:= I

% d~ ssS. (TGA)

!.o

0 30

Figure 3. Correlation between VVM volatile yields Figure 4. Relationship between wire-mesh at 1000"C and 1450'~ and TGA volatile yields, and TGA volatile yield and volatile nitrogen,

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Wire-mesh volatile yields from the ten coals are plotted against TGA volatiles in Figure 3. Although there is a very general correlation between WM and TGA yields, the ratios (i.e. R factors [7]) vary considerably since the effects of both time-temperature history and volatile redeposition within the sample reduce the TGA yields [8].

Volatile nitrogen yields are plotted against the respective total volatiles yields in Figure 4. Predictably, fractional nitrogen release is highest at 1450°C, and approximately equal to the total volatile matter yields (see 1:1 line). The mtrogen release at 1000°C is lower, but still approximately a linear function of total volatile yield. TGA volatile nitrogen yields are both significantly lower and more scattered. This can probably be attributed to extensive redeposition of heavy nitrogen-bearing tars within the TGA coal sample.

Full size plant and burner test rig NOx levels have been published [9] for four of the coals (identified only as UI~ South African, Australian and Indonesian for commercial reasons) burned in air-staged low-NOx burners. As Figure 5 shows, NOx levels under these conditions appear to correlate quite well with NOx figures estimated [1] using char nitrogen contents measured in 1450°C WM runs. The overall char N to NOx conversion figure of 25% for the test rig data is in good agreement with values obtained under similar conditions elsewhere [3]. Apparent conversion values are slightly higher for the boiler, possibly reflecting higher thermal NOx contributions or the practical dii~culties of combustion control in a multi-burner installation.

The correspondence between the proportion of coal nitrogen volatilised and WM total volatiles in Figure 3 also suggests that NOx levels might be correlated without actual char nitrogen analyses, an obvious advantage under commercial conditions. The apparent feasibility of this approach, for the limited data available, is shown in Figure 6.

40O

2.500

Io NOx ¢onwrsion

A 6OO

JE Conv~iorl. assuming char N content = oosl N content

Figure 5. Relationship between NOx emission Figure 6. Relationship between NOx emission levels and 1450"C char N content, levels and 1450"C char yields,

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4. CONCLUSIONS

Devolatilisation under the unrealistic conditions that can be applied in TGA apparatus (and by inference conventional proximate analysis) is of limited value for predicting NOx production under PF combustion conditions.

As an alternative, a captive-sample WM test has been demonstrated which, at peak temperatures of 1450°C and 150 ms hold time, can apparently achieve reasonably realistic nitrogen release during devolatilisation. Results obtained have been found to correlate fairly closely with a limited amount of available test rig and plant data. As a further simplification, the assumption that coal nitrogen is distributed proportionally between WM char and volatiles appears reasonable for the conditions used. This could minimise the need for char micro elemental analysis when making a pre]imlnary assessment.

However, although the present results are encouraging, the validity of the observed correlations must be exRmlued for a wider range of coals before firm conclusions can be drawn.

5. ACKNOWLEDGEMENTS

The authors would like to thank Babcock Energy Limited and in particular J.L. King and G. Hesselm_~nn for their help and advice in preparing this paper. The work was funded by the Science and Engineering Research Council under Grant GR/F89817.

6. R E F E R E N C E S

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2.

3.

4.

5.

6.

7. 8.

9.

J.L. Vernon, ~mission standards for coal-fired plants', IEACR/11, IEA Coal Research, London, 1988. J.H. Pohl and A.F. Sarofim, Sixteenth Symp. (Int.) on Combustion, 1976, 491-501. T. Nakamura, J.P. Smart, W.L. van de Kamp, M.E. Morgan, Proc. 1991 Int. Conf. on Coal Sci., Newcastle, 331-334. N.M. Skorupska, 'Coal specifications - impact on power station performance', IEACR/52, IEA Coal Research, London, 1993. C.K. Man, K.J. Pendiebury and J.R. Gibbins, ACS DFC Prepr. 37 (1), 1992, 102-107. K.J. Pendlebury, C.K. Man and J.R. Gibbins, Rev. Sci. Instrum. (in preparation). G.M Kimber and M.D. Gray, Combust. Flame 11, 1967, 360-362. J.R. Gibbins, K. Khogali and R. Kandiyoti, Fuel Proc. Tech. 24(1), 1990, 3-8. K.B. Lam, P.J. Wilkinson and J.L. King, 9th Conference of the Electric Power Supply Industry (CEPSI), Hong Kong, 22-27 Nov., 1992.

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Discussion

Laboratory measurement of N release under combustion conditions and comparison with plant NOx formation

C.K. Man, K.J. Pendlebury and J.R. Gibbins

Question: K.A. Nater

Which of the stages of a swirl (staged) burner do you simulate?

Answer

These data apply to an air staged low-NO x burner system. The experiment attempts to simulate the initial stage of pyrolysis in the burner. This region is reducing and the quantity of char-N is set for the possible oxidation to NOx.

Question: J.A. Moulijn

I have a comment rather than a question. We also compared TGA with high-heating rate methods. We used an entrained flow reactor and came to the same conclusion: TGA cannot be used in predicting an R factor. We did not blame it directly to macerals but to plastic/nonplastic behavior. But anyway, you have to investigate each coal in an instrument working at high heating rates (1000-2000 K/s).

Answer

Proximate V.M. is unrepresentative for volatile determination. The R factor of most coals are constant at a particular condition but there are a few which behave differently and therefore we need to test each coal on an individual basis. There are also differences in the levd of volatile nitrogen released especially for overseas coals, e.g., South African and Australian coals.