Combustion of Synthetic Fuels - American Chemical Society

11 E f f e c t of L i q u e f a c t i o n P r o c e s s i n g C o n d i t i o n s

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch011

on C o m b u s t i o n C h a r a c t e r i s t i c s of S o l v e n t - R e f i n e d C o a l R. W. BORIO, G. J. GOETZ, T. C. LAO, Α. Κ. MEHTA, and N. Y. NSAKALA Combustion Engineering, Inc., Kreisinger Development Laboratory, Windsor, CT 06095 W. C. ROVESTI Electric Power Research Institute, Palo Alto, CA 94303 SRC-I processing has been performed using three variations in the manner in which mineral matter and unconverted coal are separated from the hot coal liquid. These processes are the Pressure Filtration Deashing (PFD), Anti-Solvent Deashing (ASD), and Critical Solvent Deashing (CSD). Since these processing conditions may influence the combustion of SRC-I solids produced, an experimental program was carried out at both the bench and pilot plant scale to determine the influence of processing (i.e., solids separation method) and com bustion conditions on carbon burnout of these three SRC's. Included in this study was an examination of NO emissions (particularly for the CSD and PFD SRC'S) with the objective of attaining low ΝO emissions without adversely affecting combustion efficiency. Reactivity andNO emissions results from the SRC testing were compared with those obtained from two coals that were previously tested and used as reference coals. One of these coals was a high reactivity Wyoming sub -bituminous coal and the other was a low reactivity Kentucky high volatile bituminous coal. X

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The Solvent Refined Coal-I (SRC-I) process (1) provides a way in which coal, by way of direct hydrogenative liquefaction, can be transformed into an environmentally clean fuel for the electric u t i l i t i e s . Earlier tests (_2, _3> A» 1 ) pulverized SRC-I solid fuel (SRC), while considered successful, indicated the need for concern in two areas: carbon in the fly ash and nitrogen oxides (Ν0 ) emissions. Although good combustion efficiencies (generally greater than 98%) were attained there was a substantial amount of carbon in the particulates (generally greater than 60%). This w i l l pose a collection problem i f an electrostatic precipitator (ESP) is envisioned for particulate collection because of the very low resistivity imparted by carbon. In addition, the high nitrogen contents (1.8-1.9%) of SRC indicate that there is a potential for high Ν0 emissions. 0097-6156/83/0217-0201 $06.00/0 © 1983 American Chemical Society w i t h

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Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COMBUSTION O F S Y N T H E T I C F U E L S

202

SRC has been produced using three d i f f e r e n t schemes for sep a r a t i n g the m i n e r a l matter and unconverted c o a l from the hot c o a l liquid. These schemes are designated as Pressure F i l t r a t i o n De ashing (PFD, 2) A n t i - S o l v e n t Deashing (ASD, 6) and C r i t i c a l Solvent Deashing (CSD, ^7). As these processing conditions may i n f l u e n c e the combustion of SRC s o l i d s produced, Combustion E n g i n e e r i n g , under a contract with EPRI, conducted an experimental program to determine the i n f l u e n c e of processing ( i . e . , s o l i d s s e p a r a t i o n method) and combustion operating conditions on carbon burnout of PFD, ASD, and CSD SRC. Included i n t h i s study was an examination of Ν 0 emissions ( p a r t i c u l a r l y f o r the CSD and PFD SRC) with the o b j e c t i v e of a t t a i n i n g low N 0 emissions without adversely a f f e c t i n g combustion e f f i c i e n c y . R e a c t i v i t y and Ν 0 emissions r e s u l t s from the SRC t e s t i n g were compared with those obtained from two p r e v i o u s l y tested reference c o a l s , a low r e a c t i v i t y Kentucky high v o l a t i l e bituminous c o a l (KHB) and a high r e a c t i v i t y Wyoming subbituminous c o a l (WSB). The primary o b j e c t i v e of t h i s study was to determine the i n fluence of SRC-I processing ( i . e . , s o l i d s separation) and combus t i o n operating conditions on carbon burnout under combustion conditions s i m u l a t i n g those achievable i n b o i l e r s o r i g i n a l l y de signed f o r c o a l f i r i n g . The secondary o b j e c t i v e was to examine combustion operating conditions that r e s u l t e d i n low Ν 0 emissions while simultaneously achieving high carbon burnout. The primary research t o o l s used i n t h i s program were C - E s Drop Tube Furnace System (DTFS), a bench s c a l e entrained laminar flow furnace and the C o n t r o l l e d Mixing H i s t o r y Furnace (CMHF), a p i l o t s c a l e entrained plug flow furnace. Both the DTFS and CMHF by v i r t u e of t h e i r a b i l i t y to r e s o l v e combustion time i n t o distance along t h e i r r e s p e c t i v e furnace lengths were used to examine carbon burnout phenomena associated with the SRC and reference c o a l s . In a d d i t i o n , the CMHF by v i r t u e of i t s staged combustion c a p a b i l i t i e s was used e x t e n s i v e l y to evaluate Ν 0 emissions and to e s t a b l i s h conditions conducive to low Ν 0 . χ

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RESEARCH FACILITIES AND PROCEDURES A number of standard and s p e c i a l bench s c a l e t e s t s along w i t h the Drop Tube Furnace System (DTFS) and p i l o t s c a l e C o n t r o l l e d Mixing H i s t o r y Furnace (CMHF) were employed i n t h i s program. Standard t e s t s consisted of proximate, u l t i m a t e , higher heating v a l u e , ash composition, ash f u s i b i l i t y temperatures, Hardgrove g r i n d a b i l i t y , and screen analyses. S p e c i a l bench s c a l e c h a r a c t e r i z a t i o n t e s t s consisted of micro-proximate a n a l y s i s and m i c r o ultimate a n a l y s i s (C, Η, N ) ; micro-proximate and m i c r o - u l t i m a t e analyses were performed on p a r t i c u l a t e samples c o l l e c t e d from varying stages of combustion i n the DTFS and CMHF. In a d d i t i o n , s e l e c t e d samples of SRC and chars from p a r t i a l combustion or p y r o l y s i s of the SRC were submitted f o r Thermo-Gravimetric analyses. Thermo-Gravimetric Analyses were performed on ASTM v o l a t i l e matter char residues ground to -200 mesh. Thes^ residues £ 4 - 5 mg) were heated i n n i t r o g e n and then burned i s o t h e r m a l l y (700 C) i n a i r .


11.

BORIO E T A L .

203

Liquefaction Processing Conditions

The Drop Tube Furnace System (DTFS) c o n s i s t s e s s e n t i a l l y of an e l e c t r i c a l l y heated 2 inch I . D . χ 18 inch long furnace where f u e l (1 gm/min) and preheated secondary gas ( a i r or i n e r t s ) are introduced. The h i s t o r y of combustion i s monitored by s o l i d s / g a s sampling at various points along the length of the furnace. The p i l o t s c a l e C o n t r o l l e d Mixing H i s t o r y Furnace (0.5 χ 10 Btu/hr) i s based on the p r i n c i p l e of plug flow which r e s o l v e s time i n t o distance along the length of the furnace. By sampling at different ports along the length of the furnace, i t i s p o s s i b l e to examine the burnout and Ν 0 formation h i s t o r y of a f u e l . The CMHF also has f l e x i b i l i t y f o r c o n t r o l l i n g the primary and secondary a i r / f u e l r a t i o s and f o r delaying and/or staging secondary a i r i n t r o d u c t i o n (at any of seven l e v e l s along the length of the furnace). 6


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EXPERIMENTAL PROGRAM The t e s t program was set up i n three phases: bench s c a l e , DTFS, and CMHF. Bench s c a l e and DTFS t e s t s were performed on a l l three f u e l s , while the CMHF t e s t s were performed only on the CSD and PFD SRC f u e l s . The low melting temperatures of the SRC r e s u l t e d i n pluggage of both DTFS and CMHF f u e l i n j e c t i o n systems. Special water cooled f u e l i n j e c t o r s were f a b r i c a t e d to a l l e v i a t e t h i s problem. Testing i n the DTFS involved examining the e f f e c t s of furnace w a l l temperature, p a r t i c l e s i z e , and combustion medium on burnout. More extensive t e s t i n g was conducted on the CSD SRC sample i n both the DTFS and CMHF as recommended by EPRI. In the CMHF the e f f e c t of two stage combustion was examined. S p e c i f i c a l l y , f i r s t stage s t o i c h i o m e t r y , f i r s t stage residence time, and o v e r a l l excess a i r upon burnout and NO formation of the CSD SRC sample were examined. Based on the CSD SRC r e s u l t s , a l i m i t e d t e s t matrix was e s t a b l i s h e d for the PFD SRC sample to examine the e f f e c t s of f i r s t stage s t o i c h iometry and o v e r a l l excess a i r on burnout and Ν 0 . A plug flow char combustion model was used to p r e d i c t the com b u s t i o n e f f i c i e n c i e s of SRC under simulated commercial b o i l e r operating c o n d i t i o n s . Inputs were based on the v o l a t i l e y i e l d s and char c h a r a c t e r i s t i c s measured i n the CMHF. χ

RESULTS Fuel Analyses A n a l y t i c a l r e s u l t s (Table 1) show that SRC have very high v o l a t i l e matter and n i t r o g e n contents (52-60% and 1.8-1.9%, respec t i v e l y , on a d r y - a s h - f r e e b a s i s ) and very low moisture and ash contents (0.1-0.3%, a s - r e c e i v e d b a s i s i n each c a s e ) . The Higher Heating Values for the SRC (15,920-16,115 B t u / l b , d r y - a s h - f r e e b a s i s ) are much higher than those of reference c o a l s (13,290 and 14,110 B t u / l b f o r the WSB and KHB c o a l s , r e s p e c t i v e l y ) .


Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 39.4

39.4

15860

1210

Higher Heating Value, (HHV), Btu/lb

F1aamab111ty Index °F

15920 1270

16050

16115

1270

15880

15940

1040

10900

13290

100.0

1170

12730

14110

100.0 100.0 100.0 100.0 100.0

100.0

100.0

100.0

100.0

Total

8.6 14.9

0.3

-

0.1

-

0.3

Ash

-

12.5 11.3 21.1 17.3 3.0

3.1

3.4

3.4

2.9

2.9

Oxygen (D1ff.)

1.4 1.3 1.3 1.1 1.9

1.9

1.8

1.8

1.9

Nitrogen

0.8 0.7 0.4 0.3 0.7

0.7

1.0

1.0

1.9

59.4

88.5

88.1

87.7

87.3

1.0

1.0

Sulfur

88.4

88.0

Carbon

4.9 80.4 72.5

4.8 72.4

3.9

5.9

5.9

6.1

6.1

5.8

5.8

Hydrogen

w

1.2 4.4

3.1

-

0.1

Moisture (Total)

-

100.0

0.1

100.0

-

100.0

0.3

Ultimate, Ut. Percent

100.0

100.0

100.0

100.0

100.0

100.0

100.0

Total

8.6

37.3 62.7 56.6

59.8

49.0

39.6 14.9

40.0

33.6

40.2

1.2

as-rec. daf

33.0

daf

Kentucky High Vol. Bit. Coal

60.4

3.1

as-rec.

0.3

48.2

0.1 60.2

59.8

0.3

as-rec. daf

59.6

daf

Wyoming Subbltumlnous Coal

0.1

Fixed Carbon

51.8

as-rec.

Ant1 Solvent Deashed SRC

0.3

48.0

Volatile Matter

daf

Critical Solvent Deashed SRC

Ash

0.1

51.6

Moisture (Total)

Proximate, Ut. Percent

Analysis

Pressure Filtered Deashed SRC

Table I ANALYSES OF SRC AND REFERENCE COALS


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BORIO E T A L .

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206

COMBUSTION OF SYNTHETIC FUELS

Table II depicts the major compositional d i f f e r e n c e s between the CSD, PFD, and ASD SRC are i n the s o l u b l e f r a c t i o n s . The CSD SRC has s i g n i f i c a n t l y l e s s benzene i n s o l u b l e s ( i . e . , p r e - a s p h a l tenes) than do the other two SRC. This compositional d i f f e r e n c e may be r e s p o n s i b l e f o r the CSD SRC greater v o l a t i l i t y as discussed later i n this report. Both the CSD and ASD SRC have melting temperatures c a . 100 F lower than the PFD SRC, which could s i g n i f i c a n t l y a f f e c t the design of f u e l handling and i n j e c t i o n systems. TABLE II


ADDITIONAL ANALYSES* OF SRC

Analysis

Pressure Filtered Deashed SRC

Cricital Solvent Deashed SRC

Anti-Solvent Deashed SRC

Solvent E x t r a c t i o n s Oils Asphaltenes Benzene I n s o l . Calc. Total

WT% WT% WT% WT%

18.9 56.8 24.3 100.0

24.2 66.2 11.6 102.0

26.5 52.1 21.4 100.0

Vacuum D i s t i l l a t i o n - 1 0 0 0 ° F Fraction

WT%

2.7

5.8

11.0

Softening Point

°F

356

248

257

Fusion Point

°F

383

284

289

*Provided by EPRI

Physical Characteristics Pasting occurred on the r a c e , bowl, and b a l l s of the Hardgrove machine during the g r i n d a b i l i t y index determination on a l l three SRC.In each case an a d d i t i o n of as l i t t l e as 1% moisture (by weight) eliminated the pasting problem. The e f f e c t of moisture a d d i t i o n i s c l e a r l y shown i n F i g u r e 1. The HGI of the SRC are i n the 136-156 range, i n d i c a t i n g that these m a t e r i a l s are more e a s i l y ground than coals ( c o a l s HGI are t y p i c a l l y l e s s than 100). 1

Thermo-Gravimetric Char R e a c t i v i t i e s Thermo-gravimetric a n a l y s i s r e s u l t s are presented i n F i g u r e 2. They i n d i c a t e that: (1) the PFD SRC char i s r e l a t i v e l y more r e a c t i v e than the CSD and ASD chars; (2) PFD char r e a c t i v i t y i s between those of WSB and KHB c o a l chars; (3) CSD and ASD char r e a c t i v i t i e s are both comparable to that of the KHB c o a l char, but


BORio ET AL.



11.

Figure 1. Photographs of SRC-1 a f t e r g r i n d i n g i n the Hardgrove machine.


207

COMBUSTION OF SYNTHETIC

FUELS


208

TIME, MINUTES

Figure 2. Thermogravimetric burn-off curves i n a i r at 700°C f o r SRC and reference coal v o l a t i l e matter chars.


11.

BORio E T AL.


209

s u b s t a n t i a l l y higher than that of the a n t h r a c i t e char; and (4) the l i g n i t e char i s the most r e a c t i v e of a l l the chars under t h i s study. The TGA char r e a c t i v i t y data i n d i c a t e that the CSD SRC has a r e l a t i v e l y low r e a c t i v i t y ( s i m i l a r to KHB c o a l ) . However, the extremely high v o l a t i l e y i e l d s of the CSD SRC ( i l l u s t r a t e d i n DTFS and CMHF r e s u l t s below) leaves very l i t t l e char f o r subsequent burnout and r e s u l t s i n a high o v e r a l l combustion e f f i c i e n c y . P y r o l y s i s , Combustion and Ν 0 C h a r a c t e r i s t i c s


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Two reference coals and the CSD SRC were pyrolyzed i n an argon atmosphere i n the DTFS at 2 8 0 0 ° F furnace w a l l temperature (Figure 3) In a residence time of about 330 m i l l i s e c o n d s , Wyoming subbituminous (WSB) c o a l evolved about 50% of i t s mass as v o l a t i l e matter the Kentucky high v o l a t i l e bituminous (KHB) c o a l evolved about 53% v o l a t i l e matter and the CSD SRC evolved about 99% v o l a t i l e matter. These are much higher than the ASTM v o l a t i l e matter values of 40.2, 37.3, and 59.8% for the WSB, KHB, and CSD f u e l s , r e s p e c t i v e l y . The d i f f e r e n c e between these values i s f a r more pronounced f o r the SRC than for the c o a l s . Figures 4, 5, and 6 show the s o l i d conversion e f f i c i e n c i e s of the three SRC and the reference coals i n a i r i n the DTFS at three temperatures (furnace w a l l temperatures of 2500, 2700, and 2800 F ) . The CSD and PFD SRC and WSB reference c o a l achieved a high s o l i d conversion e f f i c i e n c y (>75%) i n l e s s than 50 m i l l i s e c o n d s , while the ASD SRC and the KHB reference c o a l r e s u l t e d i n lower i n i t i a l conversion e f f i c i e n c i e s , l e s s than 60%. The i n i t i a l high degree of conversion of the CSD and PFD SRC r e s u l t s i n r e l a t i v e l y low amounts of r e s i d u a l char to be burned i n the l a t t e r stages of combustion. The CSD SRC showed the highest i n i t i a l and o v e r a l l s o l i d conver s i o n e f f i c i e n c y of a l l the fuels s t u d i e d . The r e s u l t s f u r t h e r show that the s o l i d conversion e f f i c i e n c i e s increase more d r a m a t i c a l l y with temperature than with residence time i n c r e a s e s . For the CSD SRC s o l i d conversion e f f i c i e n c i e s increased from 91 to 95% at a furnace temperature of 2500 F , as the residence time increased from 0.05 second to 0.3 second. The corresponding e f f i c i e n c i e s at 2800 F were 97% and 99% r e s p e c t i v e l y i n d i c a t i n g the pronounced e f f e c t of higher temperatures. A r e l a t i v e comparison of the f i v e fuels (three SRC and two reference coals) i n d i c a t e s that the CSD SRC has a high s o l i d con v e r s i o n e f f i c i e n c y s i m i l a r to the Wyoming subbituminous c o a l (WSB). This WSB c o a l has been found to y i e l d high combustion e f f i c i e n c i e s in u t i l i t y boilers. The PFD SRC i s seen to be l e s s r e a c t i v e com pared to the CSD SRC and the WSB c o a l i n the e a r l y stages of com b u s t i o n , however, o v e r a l l s o l i d conversion e f f i c i e n c y d i f f e r e n c e s are reduced with i n c r e a s i n g residence time, e s p e c i a l l y at the higher temperatures s t u d i e d . The ASD SRC, however, i s seen to give lower s o l i d conversion e f f i c i e n c i e s than the marginal KHB reference c o a l at a l l three temperatures s t u d i e d . At a furnace


COMBUSTION

210

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OF SYNTHETIC

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FUELS

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T E S T CONDITIONS

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TW = 2 8 0 0 ° F 30

R E G U L A R GRIND FUEL

20

T E S T No.

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0.3

0.5

0.4

RESIDENCE TIME, SEC

Figure at

TW

3.

DTFS p y r o l y s i s

e f f i c i e n c i e s of f u e l s

i n argon

= 2800OF.


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BORIO E T A L .

211



11.

RESIDENCE TIME, SEC

Figure 4- DTFS s o l i d conversion e f f i c i e n c i e s a i r at TW = 2500°F.

of fuels i n


212

COMBUSTION


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R E G U L A R GRIND

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2

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WSB C O A L

WSB-2

•

KHB COAL

KHB-2

Δ

CSD S R C

CSD-2

•

PFD SRC

PFD-2

ASD SRC

ASD-2

•

ι

-L

L 0.1

0.2 0.3 RESIDENCE TIME, SEC

ι

0.4

I 0.5

Figure 5. DTFS s o l i d conversion e f f i c i e n c i e s of fuels i n a i r at TW = 2700°F.


FUELS

BORio ET AL.

213



11.

0.2

0.3

RESIDENCE TIME, SEC

Figure 6. DTFS s o l i d conversion e f f i c i e n c i e s a i r at TW = 2800°F.

of fuels i n


COMBUSTION OF S Y N T H E T I C F U E L S

214

temperature of 2 8 0 0 ° F s o l i d conversion e f f i c i e n c i e s at 0.3 second residence time are 99% f o r the CSD and WSB f u e l s , 97% f o r the PFD SRC, 90% f o r KHB, and 77% f o r the ASD SRC. Both the CSD and PFD SRC were studied i n the CMHF as w e l l as two reference c o a l s . The e f f e c t of primary stage stoichiometry on Ν 0 emissions i s shown i n F i g u r e 7. For CSD SRC the maximum Ν 0 r e d u c t i o n was obtained at an opt imum primary stage stoichiometry of about 40%. At an o v e r a l l excess a i r value of 20%, b a s e l i n e , unstaged emissions were 570 ppm compared to the optimum, staged Ν 0 emissions of 240 ppm ( a l l values corrected to 3% 0 ) . For PFD SRC the minimum Ν 0 (230 ppm) was a t t a i n e d at a primary stage stoichiometry of about 55% at the same o v e r a l l excess a i r . The SRC, due to t h e i r r e l a t i v e l y high f u e l n i t r o g e n contents, have a high Ν 0 formation p o t e n t i a l under conventional f i r i n g c o n d i t i o n s . However, staging the combustion a i r can r e s u l t i n acceptably low Ν 0 emissions without j e o p a r d i z i n g t h e i r combustion e f f i c i e n c i e s . Varying the primary stage residence time (Figure 8) showed the importance of p r o v i d i n g a s u f f i c i e n t time i n the s u b s t o i c h i o metric primary stage f o r achieving low Ν 0 . The o v e r a l l s o l i d conversion e f f i c i e n c i e s were unaffected by changes i n t h i s p a r a meter. Ν 0 was found to increase s l i g h t l y with i n c r e a s i n g excess a i r for a l l f u e l s t e s t e d i n the CMHF (Figure 9 ) . The r a t e of increase i n Ν 0 was small and decreased with i n c r e a s i n g excess a i r . Ν 0 increased from 245 ppm to only 288 ppm as excess a i r increased from 0% to 35% f o r CSD SRC. This was not unexpected since a l l t e s t s were run under optimum low Ν 0 primary stage s t o i c h i o m e t r i e s . V a r i a t i o n i n excess a i r from 0 to 35% under optimized conditions was found to have l i t t l e i n f l u e n c e on the o v e r a l l s o l i d conversion e f f i c i e n c y i n the CMHF. This i s probably because SRC e x h i b i t s high p y r o l y s i s e f f i c i e n c i e s with low amounts of r e s i d u a l chars being produced f o r subsequent burnout and because primary stage conditions had been optimized. χ

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χ

γ

Mathematical M o d e l l i n g In order to e x t r a p o l a t e the l a b o r a t o r y r e s u l t s to the f i e l d and to make semiquantitative p r e d i c t i o n s , an in-house computer model was used. Chemical r e a c t i o n rate constants were derived by matching the data from the C o n t r o l l e d Mixing H i s t o r y Furnace to the model p r e d i c t i o n s . The d e v o l a t i l i z a t i o n phase was not modeled since v o l a t i l e matter r e l e a s e and subsequent combustion occurs very r a p i d l y and would not s i g n i f i c a n t l y impact the accuracy of the mathematical model p r e d i c t i o n s . The " o v e r a l l " s o l i d conver s i o n e f f i c i e n c y at a given residence time was obtained by adding both the simulated char combustion e f f i c i e n c y and the average p y r o l y s i s e f f i c i e n c y (found i n the primary stage of the CMHF).



10

,1

1 30

1

Figure 7.

1 20

τ

60

ι

1

70

ι

1

KHB COAL Δ 80

ι

stoichiometry on Ν 0 χ emissions.

90

100

WSBCOAL Ο

ι

PFD-SRC

φ

ι

1

1

PRIMARY STAGE STOICHIOMETRY, %

50

ι

1

E f f e c t of primary stage

1 40

1


110

ι

r

I 120

216


COMBUSTION OF SYNTHETIC

0.5

1.0

1.5

2.0

PRIMARY STAGE RESIDENCE TIME, SEC

Figure 8. E f f e c t of primary stage residence time on ΝΟχ emissions.


FUELS



ι—*

Combustion of Synthetic Fuels - American Chemical Society

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