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-- DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Introduction

In order to examine the role of pore structure, studies will be conducted on coal

chars in the electrodynamic balance. Larger particles will also be examined using a

fluidized bed to examine diffusion control reactions, and soots will also be investigated

to examine the role of meso- and micro-pores without macro-pore interference. These

studies will allow a full range of particles sizes and temperatures to be investigated and

eventually modelled.

Progress Report

The project has examined the effect of the pore structure diffusivity changes on

the generation of NOx and N,O from the Fluidized Bed Combustor. Furthermore,

refinement of the techniques necessary to determine micropore characteristics from

TEM imaging have been further refined.

The main thrust of the work done in the last quarter has involved the

examination of the pore structure of two bituminous coals( Illinois #6 and Newlands)

during combustion in a fluidized bed. The two coals were combusted in the FBC at

1023 K at 4% oxygen. The chars were retrieved via a number 100 mesh cage like

apparatus from the system under Helium purge gas. The chars were then

subsequently analyzed using the BET method(Surface area vs. conversion is given in Figure 1).

It was found that the two coals exhibited different pore structure properties

during combustion. The Newlands coal maintained a relatively constant pore structure

throughout combustion, with an average pore diameter as measured by BET of

approximately 20 2.4 nm. The Illinois coal, under the same conditions, varied to a

much greater extent. From a start of 2nm, the Illinois #6 average pore diameter grew to

a size of approximately 4 nm, a two fold increase.

of NO/N20 development. Tulin et al[l] concluded that the NO and N,O trends they

observed coin single particle fluidized bed combustion could be explained by the

The question of the evolution of the pore structure is highly important to models

hypothesis that char nitrogen, in the presence of oxygen, reacts with oxygen to form

NO or with NO to form N20. Their mechanism depended on diffusion of species(N0,

O2 and N20) in and out of the char pores. NO and N20 are formed by the reaction of

char with O2 and NO, followed by diffusion out of the pores they are formed in, where

NO is partially reduced. Of critical importance to their model parameters was the

effective diff usivity. In order to model the diffusivity of our chars with the BET data we obtained, we

used the formulation of Johnson and Stewart to determine an effective diffusivity for

parallel pores. Their formulation can be expressed mathematically as

where AVg is the incremental pore volume and DB and D, are the bulk and Knudsen

diffusions respectively. For the calculations, an average tortuosity, zpl of 4[2] was taken,

and an average density of 1 g/cm3 was assumed. The experimentally determined

diffusivity values are plotted in Figure 2 versus the conversion for both coals.

The effective diffusivity change explains the differences in NO conversion

plotted in Figure 3 for both coals. The Newlands coal shows a constant rate of

conversion with time, where as the Illinois #6 coal shows a similar trend to the

Newlands coal at lower conversions. However, at higher conversions, where the

effective diffusivity is seen to increase, the conversion rate begins to change at the

same time that the diffusivity is seen to dramatically increase. The full details of this

analysis will be presented at the Pittsburgh coal Conference, and the attached paper

more fully describes the research to be presented. The study also aims to study the microporosity evolution during combustion

using a TEM. However, to fully exploit the trends we wish to observe, single particle

investigation was necessary as detailed in previous reports. The system that has been

designed consists of using a lacy carbon grid that is then subsequently coated with 7 nm of Aluminum using a thermal evaporator. The aluminum deposition is used to

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protect the grid in the reactive conditions that we are using. The grids have now been

manufactured and tested and can hold up to the conditions imposed on them with

minimum loss of sample. We project that this phase of the testing should be done by

the next progress report.

Future Work

Refinement of the paper to be submitted to Carbon continues in an effort to clarify the

need for density determination for small soot particles in our tests. Further analysis of

single particle work will continue, and evaluation of spherocarb combustion using the

TEM will be accomplished in the next quarter.

Publications

A presentation and paper entitled Char Nitrogen: Effect of Coal-type pore structure on

NO/N20 evolution has been accepted by the Pittsburgh Coal Conference. A copy of

the article to be presented is attached.

References: 1. Tulin, C.J. Goel, S., Morihara, A., Sarofim, A.F., and Beer, J.M., "NO and N,O Formation for Coal Combustion in a Fluidized Bed: Effect of Carbon Conversion and Bed Temperature," Energy and F u e l s , 7, 1993, pp. 796-802.

2. Satterfield, C.M., Heterogeneous Catalysis in Industrial Practice, McGraww-Hill, New York, 1991.

3

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c\i' E 250 -

a 150 - 8 4! 100 -

50 - 0 -

$- 200 -

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Illinois #6 BET Area Illinois #6 Micropore Area Newlands BET Area

e-- 4

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Figure 2. 80

20 40 100

70 4

20

60 80 Char Conversion, +

Evolution of Char N, Surface Area with Conversion.

- 4- Newlands BJHDiameter - - 4 Illinois #6 BJH Diameter

Illinois #6 Deff

4- Newlands D e e -

- -

- -

- -

I 1 I

5e-4

4e-4

3e-4

2e-4

1 e-4

Oe+O 100 20 40 60 80

Char Conversion, X, Figure 3. Effective Diffbsivity for Newlands and Illinois #6 chars. The Newlands Deff is

essentially constant, while the Illinois #6 char De, increases during reaction.

0.00 0.25 0.50 0.75 1 .oo Char Carbon Conversion, X,

Figure 3. Char Nitrogen cumulative conversion to NO and N20. The Illinois #6 char produces significantly more NO than the Newlands, while producing less N20.

t

CHAR NITROGEN: EFFECT OF COAL TYPE PORE STRUCTURE ON NO/N20 EVOLUTION

Adel F. Sarofim, Angel0 W. Kandas, and Shakti Goel Chemical Engineering Department 66-572

Massachusetts Institute of Technology Cambridge, MA 02139

ABSTRACT

The conversion of char nitrogen to nitrogen oxides depends upon a large number of surface reactions and also pore diffisivity. As a consequence, there can be a wide variation in the amount of nitrogen oxides produced by different coals. In this paper, we will present results on the char nitrogen conversion to both NO and N,O in a fluidized bed reactor as a knction of carbon conversion for two coals. The results show that the increased char nitrogen conversion to NO with char burnout is consistent with the changes in pore diffusion with conversion, with higher conversions corresponding to higher char nitrogen conversions. However, the differences in char nitrogen conversion between coals shows that the changes in surface kinetics as well as pore diffusivity must be taken into account. The results have important implications to pulverized coals flames in which the volatile nitrogen conversion to NO is suppressed by deep staging and the char nitrogen contributes significantly to the residual NO formation.

INTRODUCTION

The conversion of char nitrogen to NO and N,O during fluidized bed combustion is not well understood. The most thorough study of char nitrogen conversion to N,O by de Soete[ 13 and Goel et al.[2] identified many of the reactions governing conversion of organically bound nitrogen to NO and N,O. Studies by Kramer and Sarofim[3] and Horio and coworkers[4] have shown that the presence of oxygen is important to the formation of N,O fiom NO in char during combustion. The critical step in the over-all conversion of char nitrogen to nitrogen oxides is the opening up of heterocyclic rings containing bound nitrogen by oxidation, leading to the formation of NO, or in the presence of NO, to N20[2][3][4].

Tuliin et a1[5] concluded that the NO and N,O trends they observed could be explained by the hypothesis that char nitrogen, in the presence of oxygen, reacts with oxygen to form NO or with NO to form N,O. Their mechanism depended on diffusion of species(N0, 0, and N20) in and out of the char pores. NO and N,O are formed by the reaction of char with 0, and NO, followed by diffision out of the pores they are formed in, where NO is partially reduced. As the particle decreases in size with increasing conversion, the NO will diffise out faster,

.

lessening the reaction of NO with the char, increasing total NO generation.

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Subsequent papers[2] hlly developed the model. The model was fitted to the data and an optimal solution for the kinetic parameters for the heterogeneous reaction and effective dfisivity were calculated. One conclusion reached was that the diasivity, following the hypothesis of D ' h o r e et al[6], corresponed to mesopore diffusion. This paper aims to determine how NO and N20 formed during oxidation of char depends upon the differences in the physical and chemical parameters for different coals.

EXPERIMENTAL METHOD

A 57-mm-diameter quartz glass fluidized bed reactor was electrically heated to 1023 K and a batch of coal particles(5-6) of approximately 4 mm diameter introduced to the bed. The release and combustion of volatiles occurs first followed by char combustion. Helium was used as a carrier gas with different concentrations of oxygen to maintain fluidization of the sand bed(d= 210pm), with a gas flow rate of 2.5 Llmin at S.T.P. at the bed entrance. The flue gas was analyzed for CO, C02, NO, N20 and CH4 using a Nicolet 520 FTlR with a low volume gas sampling cell of 233 cm3. Full details can be found in [7]. The compositions of the two coals used, Newlands and Illinois #6, are given in Table 1.

The experimental concentration vs time data for NO and N20 were converted to cumulative conversion of char nitrogen to NO and N20 using

with a similar equation used for FNo. N/C is the atomic ratio of nitrogen to carbon present in the char, which is assumed to be constant throughout the experiment[S].

For comparison purposes, the carbon conversion was computed fiom the following equation

The total carbon calculated fiom the denominator of Equation 2 was compared to the amount of the carbon content of the coal fed to the bed, and the largest discrepancies were found to be approximately 10%.

The chars were retrieved fiom the FBC in order to study their properties with the use of a basket made of number 100 mesh steel wire. The retrieved chars were then analyzed using an ASAP 2000 automated BET system for both their surface area and their pore size distribution. The evolution of the surface area and pore structure were then compared to the NO and N20 profiles.

RESULTS

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Representative CO,, COY CH,, NO and N20 concentration profiles are shown in Figure 1 for the lllinois #6 and the Newlands coals. The initial rapid buildup and decay are due to the devolatilization of the coal particles. The subsequent evolution of the concentration profiles are due to char bum out.

The surface area of the char particles is shown in Figure 2 as a function of char conversion. The figure shows an initially high BET surface area for both chars after devolatilization. Approximately two-thirds of the surface area resides in the micropore region throughout conversion for both chars. At higher conversions, both chars exhibit a slight increase in their surface area.

DISCUSSION

The cumulative conversions of NO and N20 were calculated using equation 1 and are plotted in Figure 3. The slope of the fiaction of char nitrogen converted to N,O is essentially constant throughout combustion for both coals. The cumulative fiaction of NO generated, however, show a different trend for the two coals. The Newlands char essentially exhibits a constant slope until very high conversions. The Illinois char, on the other hand, exhibited an increasing slope starting at a fairly low conversion(-40%). This increasing trend is not fully explained by the change in radius due to the shrinking core assumption used in[2], as at 40% conversion the change in char radius is minor.

Of the parameters used in the NOx models, the Thiele Modulus and the Biot number both depend on the effective diffisivity. One can use the BJH desorption data of pore size and volume in the mesopore range to determine a value for the effective dfisivity, De, Following the approach suggested by Johnston and Stewart[B], and assuming the pores are parallel cylindrical pores, the flux may be integrated. The experimentally determined pore size distribution may be used to obtain the integrated flux, which can then be used to calculate the effective difisivity. This may be expressed mathematically as

where AV, is the incremental pore volume and DB and Dk are the bulk and Knudsen dffisions respectively. For the calculations, an average tortuosity, zpy of 4[10] was taken, and an average density of 1 g/cm3 was assumed.

The average BJH diameter and the dffisivity calculated for the two coals is shown in Figure 4. The two coals exhibit two distinct profiles. The Newlands char parameters remain essentially constant. However, the Illinois #6 char exhibits a constantly increasing pore diameter that doubles in size during the course of the experiment. This change in diameter is responsible for the large increase in difhsivity for the Illinois char, while the Newlands char's dffisivity increases only slightly. This change in difhsivity would explain the constantly increasing slope in conversion for the Illinois char. When DeEgoes up, the concentration of NO in the pores will go down, increasing overall NO conversion(Fig. 4), although, due to the different chemical parameters for the coals, overall conversion between coals may differ.

Practical Implications

The mathematical models have shown that the conversion of char nitrogen to NO is a function of the char oxidation kinetics, the fraction of char nitrogen forming NO versus N,O, and the decomposition reaction of NO and N,O, and the transport of the reactants and products both within the pores ad in the external boundary layer. These relationships apply to both 5uidized bed combustion and pulverized coal flames. The present paper shows that the increase in the char nitrogen conversion with increasing burnout is consistent with the increased diffusion of NO out of the pores. The increased diaision at high burnout is a measure of both the increased diffisivity and the shorter diffusion path as the char particle burns out. The differences in NO formation between coals, however, also shows the importance of differences in the surface kinetics, as the coal with the higher diffusivity does not yield the higher NO conversion.

In a fluidized bed coal combustion, the net NO and N,O emissions are found to be governed more by the rates of their reduction by the char in the bed than by the rates of their formation[l 13. In this case one of the more important properties of char affecting the net NO emission is the kinetics of char oxidation which determines the char loading.

By contrast, in pulverized coal flames the secondary reactions with char are unimportant. The N,O, however, is very rapidly decomposed by homogeneous reaction at high temperatures, but NO is essentially unaffected so that the rate of emission of NO is very close to that of its formation. The results of this paper show that the NO formation is controlled by differences in both the heterogeneous chemical kinetics and the pore diffhsivity. Some control of the NO formation from char may be achievable by controuing the rate of heating in the burner zone which has an impact on the plastic behavior of the bituminous coals and therefore of the diffisivities of the chars that are produces. Such strategies may become important for low- NOx burners for which the char nitrogen conversion represents a major fraction of the residual NO formation.

ACKNOWLEDEMENTS

The Authors wish to thank the Department of Energy for funding this project through their University Coal Research Grant program(Grants DE-FG 22-91PC91294 and DE-FG and Kitachi Ltd. for providing some of our coal samples.

REFERENCES

1. de Soete, G.G., Twenty-nird Symposum (TnternationaI) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1257-1264. 2. Goel, S.K, Morihara, A., Tulling, C.J., and Sarofim, A.F., "Effect0 fo NO and 0, Concentration on N20 Formation During Coal Combustion in a Fluidized-Bed Combustor: Modeling Results, I' Twenty-F@h Symposum(lntemationaI) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 1257-1264 3. Krammer, G. F., and Sarofim, A.F. "Reaction of Char Nitrogen During Fluidized Bed Combustion - Influence of Nitric Oxide and Oxygen on Nitrous Oxide," Combustion and Flame, 97, 1984, pp 118-124. 4. Mochizuki, M., Koike, J., and Horio, M., 1992, Fifrh International Workship on Nitrous Oxide Emissions, Tsukuba, Japan.

i Property Newlands(%)

Ash 17.44

5. Tulin, C.J. Goel, S., Morihara, A., Sarofim, A.F., and Beer, J.M., "NO and N,O Formation for Coal Combustion in a Fluidized Bed: Effect of Carbon Conversion and Bed Temperature," Energy andFuels, 7, 1993, pp. 796-802. 6. D'Amore, M., Tognotti, L., Sarofim, A.F., "Oxidation Rates of a Single Char Particle in an Electrodynamic Balance," Presented at the Internation Workshop on Heterogeneous Combustion, Dead Sea, Israel, January 5-9, 1992. 7. Tulin, C., Sarofim, A.F., and Beer, J.M., "Formation of NO and N,O Coal Combustion: The relative importance of volatile and char nitrogen" Proc. I993 Inter. Con$ Fhidized Bed Combstion, pp 599-609. 8. Smoot, L., D.., "Pulverized coal df is ion flames: A perspective through modeling," Eighteenth Sympommflntemationao on Combustion, The Combustion Institute, Pittsburgh,

9. Johnson, M.F., and Stewart, W.E, Pore Strucutre and Gaseous Diffusion in Solid Catalysts," Joumal of Catalysis, 4, 1965, pp 248-252. 10. Satterfield, C.M., Heterogeneous Catalysis in Industrial Practice, McGraww-Hill, New York, 1991. 11. Goel, S., Beer, J., and Sarofim, A.F. "Sigfllficance of Destruction Reactions in Determining the Net Emmisions of Nitrogen Oxides," 13th Intl. Con$ on FBC, Orlando, F1, 1995, pp 887- 898. Table 1 : Proximate and Ultimate analysis of Newlands coal.

1981, pp 1185-1202.

Illinois #I6 (%)

12.84

Property Newlands(%)

Ash 17.44

Volatile Matter 26.49

Fixed Carbon 56.07

Total Carbon 68.83

Nitroaen 1.2

Illinois #I6 (%)

12.84

36.19

50.97

65.1

1.05

~~ ~ ~

Volatile Matter 26.49

Fixed Carbon 56.07

Total Carbon 68.83

Nitroaen 1.2

E, R.

36.19

50.97

65.1

1.05

73 0 SL

0 5 10 . 15 Time, min

0.0 2.5 5.0 7.5 . 10.0 12.5 Time. min

Figure 1. Experimental concentration profiles of lllinois(a) and Newlands@) coals.

c u i E 250

-

-

-

-

-

9

- -0- Illinois #6 BJH Diameter 4- Newlands BJHDiameter .+ Illinois #6 Deff

4- Newlands Deff

1

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Illinois #6 BET Area Illinois #6 Micropore Area Newlands BET Area

$- 200 - a

e I 0 0 - 50 -

Q) 150- 0

3

0 I I I I

Newlands Micropore Area

20 40 60 80 Char Conversion, +

Figure 2. Evolution of Char N2 Surface Area with Conversion.

100

u 0.00 0.25 0.50 0.75 1 .oo Char Carbon Conversion, X,

Figure 3. Char Nitrogen cumulative conversion to NO and N20. The Illinois #6 char produces significantly more NO than the Newlands, while producing less N20.

80

70 Qa Lf 60 B

50 n 2 40

30 E

20

.-

5e-4

4e-4

3e-4

2e-4

I e-4

Oe+O

20 I 00 40 60 80 Char Conversion, X,

Figure 4. Effective Diffusivity for Newlands and Illinois #6 chars calculated using Equation 3. The Newlands Deff is essentially constant, while the Illinois #6 char

De, increases during reaction.