Combustion of Synthetic Fuels - ACS Publications - American

8 Synthetic F u e l Character Effects on a

Rich-Lean

Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 1, 2018 | https://pubs.acs.org Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch008

Gas Turbine Combustor LEONARD C. ANGELLO and WILLIAM C. ROVESTI Electric Power Research Institute, Palo Alto, CA 94303 THOMAS J. ROSFJORD United Technologies Research Center, East Hartford, CT 06108 RICHARD A. SEDERQUIST United Technologies Corp., South Windsor, CT 06074 Five fuels including No. 2 fuel oil, SRC II, Η-Coal, and EDS middle distillates, and hydrotreated Paraho shale oil residual were tested in a subscale 5-inch diameter, staged rich-lean com bustor at conditions representative of baseload and part power settings of 30-MW utility combustion turbine. A minimumNO emission level corrected to 15% oxygen of approximately 35 ppmv was attained for all the fuels despite fuel bound nitrogen levels of up to 0.8 percent by weight. Smoke emissions did depend on fuel properties and ranged between a SAE Smoke Number of 20 to 45 at baseload operation. Indication of increased smoke and liner heating with reduced fuel hydrogen content was observed, although the indicated trends were not as consistent as those for lean combustors. x

Rich-lean combustion systems are a recent generic c l a s s of s t a t i o n a r y gas turbine combustors capable of low Ν 0 emission performance w i t h f u e l s c o n t a i n i n g high concentrations of n i t r o gen. Several r i c h - l e a n combustor designs are c u r r e n t l y under development by u t i l i t y gas turbine manufacturers as part of the ongoing DOE/NASA Low Ν 0 Heavy Fuel Combustor Concepts Program (1)· As i l l u s t r a t e d i n Figure 1 the r i c h - l e a n combustor concept i s s i m i l a r to the f u e l staging technique used i n b o i l e r com b u s t i o n systems f o r c o n t r o l l i n g Ν 0 emissions from f u e l s con t a i n i n g high f u e l - n i t r o g e n . In b r i e f , a small amount of primary a i r i s mixed w i t h the f u e l i n the head-end of a r i c h lean combustor. This creates a f u e l r i c h combustion zone t o r e l e a s e n i t r o g e n from f u e l s c o n t a i n i n g n i t r o g e n compounds and maximizes the e a r l y formation of molecular n i t r o g e n . This r i c h burn step i s followed by the r a p i d i n t r o d u c t i o n of secondary a i r χ

χ

χ

0097-6156/83/0217-0151$06.50/0 © 1983 American Chemical Society

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

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COMBUSTION OF SYNTHETIC FUELS

to achieve complete combustion of unburned hydrocarbons and carbon monoxide under f u e l l e a n c o n d i t i o n s to minimize the formation of thermal Ν 0 · The combustion process i s optimized i n the r i c h stage to minimize the formation of Ν 0 and molecules such as NHg and HCN which would convert r e a d i l y to Ν 0 i n the lean stage. S u f f i c i e n t residence time i n the lean stage assures complete combustion of even poor q u a l i t y f u e l s . The purpose of t h i s paper i s to present the r e s u l t s of an experimental program sponsored by the E l e c t r i c Power Research I n s t i t u t e (EPRI). The purpose of the e f f o r t was to determine, by subscale combustor r i g t e s t and data a n a l y s i s , the e f f e c t s of s y n t h e t i c f u e l property v a r i a t i o n s on the emissions, performance, and d u r a b i l i t y c h a r a c t e r i s t i c s of r i c h - l e a n combustion systems. Fuel property v a r i a t i o n s were i n v e s t i g a t e d by t e s t i n g f i v e f u e l s i n c l u d i n g No. 2 petroleum d i s t i l l a t e f u e l , c o a l - d e r i v e d middle d i s t i l l a t e f u e l s produced by the SCR-II, HCoal and EDS processes, and a shale o i l r e s i d u a l f u e l . Tests with these f u e l s provided data f o r r e a l i s t i c ranges of f u e l v i s c o s i t y , and hydrogen and n i t r o g e n content. Tests were performed at conditions r e p r e s e n t a t i v e of f u l l - and p a r t i a l power s e t t i n g s of a 30-MW u t i l i t y combustion t u r b i n e . χ

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Test

Program

Test F a c i l i t y . The experimental program to document the consequences of burning f u e l s that possess chemical and/or p h y s i c a l p r o p e r t i e s which d i f f e r from f u e l s i n common use was conducted i n the t e s t f a c i l i t y shown s c h e m a t i c a l l y i n Figure 2. The t o t a l a i r f l o w was supplied to the t e s t c e l l by a p o s i t i v e displacement compressor, metered by a c a l i b r a t e d v e n t u r i and heated i n an e l e c t r i c a l r e s i s t a n c e - t y p e heater. The a i r f l o w which e x i t e d the heater was d i v i d e d i n t o a primary a i r f l o w , which f e d the r i c h - s t a g e combustor, and a secondary a i r f l o w which was i n j e c t e d through the combustor quench s e c t i o n . V a r i a t i o n s i n the primary-secondary a i r f l o w s p l i t were achieved by a c t u a t i n g a pneumatic c o n t r o l valve located i n the primary a i r l i n e ; a high temperature gate valve l o c a t e d i n the secondary a i r l i n e provided the supply system pressure drop necessary f o r c o n t r o l . A c a l i b r a t e d v e n t u r i was located i n the primary l i n e to meter the primary a i r f l o w and hence permit c a l c u l a t i o n of the r i c h combustor equivalence r a t i o . The secondary a i r f l o w rate was c a l c u l a t e d as the d i f f e r e n c e of the t o t a l and the primary a i r f l o w r a t e s . The model combustor used i n the present study was a copy o f one c o n f i g u r a t i o n ( C o n f i g u r a t i o n 2C) evaluated under the DOE/NASA Low Ν 0 Heavy Fuel Combustor Concepts Program, and c o n s i s t e d of four component s e c t i o n s : f u e l preparation, f u e l r i c h combustion, a i r quench, and f u e l - l e a n combustion s e c t i o n s ( 2 ) . A l l of the t e s t f u e l was i n j e c t e d i n t o a r i c h combustion χ


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ANGELLO E T A L .

Rich-Lean

153

Gas Turbine Combustor

Secondary airflow


Fuel — Primary—airflow ν

>~ 1.4-1.6

0-0.3

ν

y Fuel preparation

Figure

1.

Fuel-rich combustor

Elements

Rapid quench

Fuel-lean combustor

o f Rich-Lean Staged

Combustor

AIR S U P P L Y

VALVE TO CONTROL AIRFLOW

EXIT

SPLIT

COOLED COMBUSTOR SECTIONS

PRIMARY AIRFLOW VENTURI

Figure

2.

Synthetic

Fuel

Combustor R i g


INSTRUMENTATION


154

COMBUSTION OF SYNTHETIC FUELS

chamber through the f u e l p r e p a r a t i o n s e c t i o n . This s e c t i o n consisted of a s i n g l e , a i r - a s s i s t f u e l i n j e c t o r which was c e n t r a l l y mounted i n an annular, vane-type s w i r l e r ; the s w i r l e r nozzle assembly was recessed approximately 1.2 inches from the r i c h combustor i n l e t . Several i n j e c t o r models were evaluated during the combustor shakedown t e s t s ; excessive smoke l e v e l s or nozzle f a i l u r e ( e . g . , l o s s of nozzle resonator cap) were experienced with most. Acceptable o p e r a t i o n was a t t a i n e d with use of a D e l a v a n ® s w i r l a i r i n j e c t o r ; a l l reported data were acquired using t h i s i n j e c t o r . The nozzle a s s i s t a i r f l o w was metered with a c a l i b r a t e d v e n t u r i and was included i n c a l c u l a t i o n of the primary and t o t a l a i r f l o w r a t e s . Most of the data were acquired with an a i r - a s s i s t pressure of 300 p s i at the f u e l i n j e c t o r ; l i m i t e d t e s t s were performed to i n v e s t i g a t e the i n f l u e n c e of higher or lower a s s i s t - a i r pressure l e v e l s . The f u e l - r i c h combustion chamber was a 5 - i n c h diameter c y l i n d r i c a l s e c t i o n , 11.0 inches l o n g , with 1 . 9 - i n c h long c o n i c a l s e c t i o n s at both the i n l e t and e x i t (Figure 3 ) . The e n t i r e chamber was double-jacketed to allow a nominal 40 GPM water coolant flow r a t e . An H / 0 t o r c h was incorporated i n the design to i g n i t e the burner. The quench s e c t i o n was a 3 - i n c h diameter c y l i n d r i c a l s e c t i o n , 3 inches l o n g , containing 16 s l o t s to permit the a d d i t i o n of the secondary a i r f l o w to the r i c h combustor e f f l u e n t . The f u e l - l e a n combustor consisted of a 1 0 . 6 - i n c h long c o n i c a l d i f f u s e r followed by a 5 - i n c h diameter c y l i n d r i c a l s e c t i o n to give an o v e r a l l l e n g t h of 18 inches from the quench s e c t i o n e x i t to the exhaust measurement plane (Figure 3 ) . This combustor was a l s o d o u b l e - j a c k e t ; the water coolant used f o r the r i c h burner was a l s o used f o r the f u e l - l e a n device. The combustor e x i t conditions were documented by a f i v e - p o r t ganged sampling probe, a t h r e e - p o i n t thermocouple rake and a smoke probe. The water-cooled sampling probe spanned the combustor diameter, and contained f i v e 0.034-inch diameter i n l e t orfices. The probe was designed to achieve an aerodynamic quick-quench of the captured streams i n order to minimize chemical r e a c t i o n w i t h i n the probe. The captured sample was t r a n s f e r r e d i n an e l e c t r i c a l l y - h e a t e d sample l i n e to an emission a n a l y s i s system capable of continuously monitoring the emissions of carbon monoxide, oxygen, carbon d i o x i d e , unburned hydrocarbons and oxides of n i t r o g e n . A water-cooled smoke probe was designed i n accordance with SAE ARP1179. The probe, which had a sample i n l e t diameter of 0.07 i n c h e s , was s i z e d to i s o k i n e t i c a l l y sample the gas stream at the baseload condition. Three PT6RH/PT30RH thermocouples were mounted on a water-cooled s t r u t with a vented r a d i a t i o n s h i e l d around each sensor. 2

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155

Rich-Lean Gas Turbine Combustor


Test Fuels F i v e f u e l s were i n v e s t i g a t e d i n c l u d i n g : No. 2 petroleum d i s t i l l a t e f u e l , coal-derived middle d i s t i l l a t e f u e l s produced by the SRC-II, Η-Coal and EDS processes and shale o i l r e s i d u a l f u e l . D e t a i l e d analyses of each t e s t f u e l are presented i n Table 1. The No. 2 petroleum d i s t i l l a t e f u e l and the c o a l derived l i q u i d s had s i m i l a r d i s t i l l a t i o n and v i s c o s i t y ranges but d i s p l a y e d s i g n i f i c a n t l y d i f f e r e n t l e v e l s of hydrogen and fuel-bound n i t r o g e n content. The hydrocarbon-type also showed considerable v a r i a t i o n , with excess of 75% of the SRC-II f u e l having an aromatic character. In a d d i t i o n the i n t i a l b o i l i n g point of the Η-Coal d i s t i l l a t e f u e l was somewhat lower than the other d i s t i l l a t e f u e l s .

93-9 ALL

Figure

3.

D I M E N S I O N S IN C E N T I M E T E R S

Subscale

Rich-Lean

Combustor

Configuration



*At 49°C **Net Heat of Combustion

Distillation I n i t i a l b o i l i n g point 10% 50% 90% End point recovery Recovery Residue F 424 510 605

wt% wt% wt% wt% vol% vol% vol%

Chemical P r o p e r t i e s Carbon Hydrogen Surfur Nitrogen Saturates Olefins Aromatics

—

F F Btu/lb es es

Unit

Physical Properties S p e c i f i c Gravity 40°C Pour Point F l a s h Point Gross H t . of Combustion Kinetmatic V i s c o s i t y 40C 100C

Property

360 400 466 536 660 98.5 1.5

87 12.95 0.25 0.02 64.8 0.7 34.5

0.841 -5 152 18280** 2.53 1.04

No. 2

TABLE 1

312 326 408 500 560 98.0 2.0

86.04 8.97 0.2495 0.75 19.3 2.9 77

0.9610 -40 164 17161 3.49 1.114

SRC-II

240 425 468 568 590 98.0 2.0

88.21 11.28 0.08 0.32 39 0.04 60.09

0.872 -45 90 17725 1.58 0.69

H-Coal

TEST FUEL ANALYSES

390 616 747 848 596 98.5 1.5

88.12 11.31 0.01 0.02 40 0.1 60

0.894 -55 167 19266 2.40 0.95

EDS


855 94.0 6.0

480

— — —

86.7 12.69 0.02 0.46

0.863* 95 235 19350 12.9 3.77

Shale

8.

ANGELLO ET A L .

157


The shale o i l r e s i d u a l had been hydrotreated to a s u b s t a n t i a l degree, providing i t with a hydrogen content very s i m i l a r to the No. 2 petroleum d i s t i l l a t e f u e l . The shale o i l r e s i d u a l f u e l had v i s c o s i t y c h a r a c t e r i s t i c s s i m i l a r to a viscous No. 4 petroleum d i s t i l l a t e f u e l . The n i t r o g e n content of the hydrotreated shale o i l r e s i d u a l was 0.49 weight p e r c e n t .


Test Conditions Tests were performed over a matrix of conditions to e s t a b l i s h emissions and heat load c h a r a c t e r i s t i c s for each t e s t fuel. The t e s t s were s t r u c t u r e d to allow determination of both combustion tradeoffs ( e . g . , low Ν 0 emissions v s . low smoke emissions) and the i n f l u e n c e of varying s e l e c t e d f u e l p r o p e r t i e s ( e . g . , n i t r o g e n or hydrogen content) on the emission l e v e l s . Three categories of t e s t s were performed. The f i r s t category i n c l u d e d tests to determine the change i n combustor emissions as the primary combustor equivalence r a t i o ( φ ρ ) was v a r i e d between 1.0 and 1.8. These t e s t s were r e f e r r e d to as signature t e s t s , and were performed at both baseload and 50% power operating c o n d i t i o n s . For these t e s t s the t o t a l a i r f l o w and a i r f l o w s p l i t between the primary and secondary streams were held constant while the f u e l flow rate was v a r i e d . With t h i s technique, the primary combustor equivalence r a t i o was changed while h o l d i n g the residence time i n t h i s chamber n e a r l y constant. The second category of t e s t conditions included design point operation at peak, baseload and 70% load c o n d i t i o n s of an u t i l i t y gas t u r b i n e . These points were s e l e c t e d to match the t e s t points from the DOE/NASA Low Ν 0 Program allowing comparison with data from that program. The t h i r d category of t e s t c o n d i t i o n s focused on o f f - d e s i g n operation to i n v e s t i g a t e the i n f l u e n c e of r i c h combustor residence time and pressure on combustor emissions; o f f - d e s i g n t e s t s departed from baseload and 70% power design p o i n t s . χ

χ

Test Results Tests with No. 2 Petroleum D i s t i l l a t e F u e l . Tests were performed with No. 2 petroleum d i s t i l l a t e f u e l to e s t a b l i s h a b a s e l i n e for comparison with the other t e s t f u e l s . The Ν 0 emissions signature i s shown i n Figure 4 f o r operation at both the baseload and 70% power c o n d i t i o n s . The Ν 0 emissions s t a r t e d at very high l e v e l s for equivalence r a t i o s near u n i t y and decreased r a p i d l y to values l e s s than 50 ppmv for φρ>1.3. The minimum Ν 0 l e v e l of approximately 37 ppmv was equivalent f o r e i t h e r operating c o n d i t i o n and was achieved f o r both at φ ρ = 1 . 5 5 . E q u i l i b r i u m chemistry considerations would lead to the conclusion that the rapid d e c l i n e i n Ν 0 emissions was a r e s u l t of both the decreased flame temperature and reduced χ

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158

COMBUSTION O F SYNTHETIC F U E L S

oxygen concentration at the f u e l - r i c h operation c o n d i t i o n s . These e f f e c t s would stongly reduce thermal f i x a t i o n of molecular n i t r o g e n to Ν 0 . While these considerations o f f e r an explanation f o r the f i n a l l e v e l of Ν 0 achieved, i t must be r e a l i z e d that the f u e l was i n j e c t e d as a spray and not as a f u l l y - v a p o r i z e d a i r mixture. Hence for any equivalence r a t i o there were regions of near s t o i c h i o m e t r i c combustion with the attendant production of high l e v e l s of Ν 0 . Therefore, an a n n i h i l a t i o n mechanism must have been present to reduce these i n i t i a l l e v e l s to the very low Ν 0 emissions a t t a i n e d . The combustor smoke emissions v a r i e d with the primary zone equivalence r a t i o , as shown i n Figure 5. There was no apparent d i s t i n c t i o n between the smoke l e v e l s achieved at e i t h e r the baseload or the 70% operation c o n d i t i o n ; the data f e l l into a band with i n c r e a s i n g smoke production f o r higher primary combustor equivalence r a t i o . The t r a d e o f f of choosing e i t h e r to minimize Ν 0 emissions or smoke emissions i s shown i n Figure 6, which i s a c r o s s - p l o t of the previous two f i g u r e s . Operation at the threshold of v i s i b l e smoke (SAE Smoke Number=20) could be achieved with Ν 0 emissions l e s s than 50 ppmv; attempts to f u r t h e r reduce the smoke emissions to lower values would r e s u l t i n excessive Ν 0 emissions. One operating concern for a r i c h combustor i s the occurrence of high combustor w a l l temperatures. In a f u e l - r i c h combustor, a i r cannot be used to f i l m - c o o l the walls and other techniques ( e . g . , f i n cooling) must be employed. The temperature r i s e of the primary combustor coolant was measured and normalized to form a heat f l u x c o e f f i c i e n t which included both convective and r a d i a t i v e heat l o a d s . Figure 7 d i s p l a y s the dependence of t h i s heat f l u x c o e f f i c i e n t on primary combustor equivalence r a t i o . These data were acquired i n t e s t s i n which the combustor a i r f l o w was kept constant. I f convective heat t r a n s f e r were the dominant mechanism a constant heat f l u x c o e f f i c i e n t of approximately 25 B t u / f t -hr-deg F would be expected. The higher values of heat f l u x and i t s convex character i n d i c a t e that r a d i a t i v e heat t r a n s f e r was an important mechanism. Furthermore, there i s an apparent t r a d e o f f between the temperature of the r a d i â t i n g - m e d i u m and the e f f e c t i v e e m i s s i v i t y of the medium. That i s , at equivalence r a t i o s near u n i t y the gas temperatures would be maximum, but few carbon p a r t i c l e s have formed. As the equivalence r a t i o was i n c r e a s e d , the temperature decreased, but the number of r a d i a t i n g p a r t i c l e s began to increase r e s u l t i n g i n a net increase i n the r a d i a t i v e mechanism. At high equivalence r a t i o s , the temperature of the r a d i a t i n g p a r t i c l e s decreased s u f f i c i e n t l y to o v e r r i d e the abundance of r a d i a t i n g p a r t i c l e s and consequently the heat f l u x c o e f f i c i e n t diminished. Comparison with the previous figures reveals that the region of primary zone equivalence r a t i o d e s i r e d f o r low Ν 0 operation and acceptable smoke emissions χ

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ET A L .


ENGINE POWER L E V E L

1.8

1.2

1.6

10

PRIMARY RICH Z O N E E Q U I V A L A N C E RATIO -

Figure NO

ρ

4.

Dependence on P r i m a r y

Equivalence Distillate

βθι-

φ

Ratio

for

No.

Combustor 2

Petroleum

Fuel

ENGINE POWER L E V E L Δ

PEAK

Ο

BASE

Ο

70%

SMOKE THRESHOLD

Δ

. _ O-

0.8

j 1.0

1.2

JL 1.4

1.6

PRIMARY RICH Z O N E E Q U I V A L A N C E RATIO -

Figure 5. Smoke D e p e n d e n c e o n P r i m a r y Equivalence Ratio for No. 2 Distillate Fuel

1.8 φ.

Conbustor Petroleum


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

2501-


SMOKETHRESHOLD

0

20

40

10

M

SAE SMOKE NUMBER -

Figure 6. T r a d e o f f o n NO

and

Smoke

100

SN

93-15

Emissions

for

χ No.

2 Petroleum

Distillate

Fuel

Μι-

Μ

ENGINE POWER L E V E L

0.8

1.0 PRIMARY

1.2

1.4

Δ

PEAK

Ο

BASE

Figure 7. Average Heat Transfer t o Wall for No. 2 Petroleum

1.8

1.6

RICH Z O N E E Q U I V A L A N C E RATIO -

φ„

P r i m a r y Combustor D i s t i l l a t e Fuel


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a l s o produced the maximum heat t r a n s f e r to the primary combustor walls. Test Results with Middle D i s t i l l a t e C o a l - D e r i v e d F u e l s . Tests were performed with middle d i s t i l l a t e f u e l s produced by the SCR-II, Η - C o a l , and EDS processes. The composition of these f u e l s represented s i g n i f i c a n t v a r i a t i o n s i n the fuel-hydrogen content, the f u e l - n i t r o g e n content, and the mix of c h a r a c t e r i s t i c hydrocarbon compounds. Ν 0 emissions signatures were obtained for the SCR-II, Η-Coal and EDS f u e l s at baseload and part-power t e s t conditions. For each f u e l the Ν 0 s t a r t e d at a very high l e v e l (greater than 200 ppmv) f o r equivalence r a t i o s near u n i t y and decreased r a p i d l y with i n c r e a s i n g equivalence r a t i o . For a l l three fuels the minimum Ν 0 l e v e l was independent of the simulated power c o n d i t i o n , with a l e v e l of 35 to 40 ppmv achieved f o r φ ρ = 1 . 5 5 . This minimum value i s most s i g n i f i c a n t for the SCR-II f u e l which contained 0.75 percent n i t r o g e n i n the fuel. If t h i s f u e l n i t r o g e n content were f u l l y o x i d i z e d , the Ν 0 l e v e l would approach 350 ppmv at baseload o p e r a t i o n . Again i t i s b e l i e v e d that high concentrations of Ν 0 are formed i n the r i c h combustor but that a n n i h i l a t i o n r e a c t i o n s are s u f f i c i e n t vigorous to reduce the emissions to the observed v a l u e s . The combustor smoke emission increased with primary combustor equivalence r a t i o . The data i n d i c a t e d a s i g n i f i c a n t d i f f e r e n c e between o p e r a t i o n at baseload and 50% power c o n d i t i o n s f o r the SCR-II and Η-Coal f u e l . For these two f u e l s , smoke emissions were s i g n i f i c a n t at high power o p e r a t i o n but n e a r l y non-existent for operation at pressures below 100 p s i . There was no systematic d i s t i n c t i o n between these operating c o n d i t i o n s f o r the data acquired using EDS f u e l ; the smoke l e v e l s were more c h a r a c t e r i s t i c of high power pressure o p e r a t i o n . The three f u e l s are compared i n Figure 8 to show the tradeoff i n choosing e i t h e r to minimize Ν 0 emissions or smoke emissions. It would be d e s i r a b l e to operate i n the lower l e f t - h a n d region of t h i s p l o t and hence a t t a i n low emissions of both s p e c i e s . The data acquired for Η-Coal and EDS i n d i c a t e the a b i l i t y to a t t a i n Ν 0 emissions l e s s than 50 ppmv f o r SAE SN=10. Higher emissions of e i t h e r species would have to be expected i f operating on SCR-II fuel.


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The average heat f l u x c o e f f i c i e n t to the primary combustor w a l l i s p l o t t e d f o r the f u e l s i n Figure 9. The r e s u l t s i n general d i s p l a y e d a convex character as was observed with NO. 2 fuel. The l e v e l of the heat t r a n s f e r c o e f f i c i e n t and i t s convex trend i n d i c a t e s the importance of r a d i a t i v e heat t r a n s f e r f o r these f u e l s . The maximum value of the c o e f f i c i e n t f o r SCR-II f u e l exceeded the maximum f o r other f u e l s by 30%. The hydrogen content of SCR-II was l e s s than that f o r the other f u e l s t e s t e d which apparently r e s u l t e d i n a more intense r a d i a t i n g medium.



COMBUSTION OF SYNTHETIC

Figure 8. C o m p a r i s o n o f T r a d e - o f f o f NO a n d S m o k e Emissions at Baseload Conditions for Coal-Derived

Fuel

Figure 9. Comparison o f Primary Combustor W a l l Heat Loading for Coal-Derived Fuels at Baseload


FUELS

8.


ANGELLO ET A L .

163

Again the maximum i s observed at primary combustor equivalence r a t i o s c l o s e to that desired for minimum Ν 0 emission o p e r a t i o n .


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Test Results with Shale O i l Residual F u e l . Tests were performed with a shale o i l r e s i d u a l f u e l which had the v i s c o s i t y c h a r a c t e r i s t i c s of a heavy No. 4 petroleum d i s t i l l a t e f u e l . The t e s t s were performed a f t e r heating the f u e l to 160°F to reduce i t s v i s c o s i t y to 7 cs i n an attempt to enhance the f u e l atomization and v a p o r i z a t i o n process. Even with t h i s degree of h e a t i n g , t h i s f u e l has a v i s c o s i t y twice the l e v e l of other fuels t e s t e d . The Ν 0 signature at both baseload and 50% power conditions was s i m i l a r to the other fuels t e s t e d , reached a minimum of 40 ppmv at φ ρ = 1 . 5 . The corresponding smoke emissions were higher than f o r other fuels tested (Figure 10). These l e v e l s are a t t r i b u t e d to r e l a t i v e l y poor atomization because of the higher f u e l v i s c o s i t y . A s i n g l e data point i s shown for which the a i r - a s s i s t pressure was increased from 300 p s i to 500 p s i to improve the f u e l a t o m i z a t i o n . A s u b s t a n t i a l reduction i n smoke emissions was observed with the smoke number decreasing from SAE SN=40 to 3. The Ν 0 emissions increased from 32 to 50 ppmv with the enhanced a i r a s s i s t i n d i c a t i n g that the f u e l p r e p a r a t i o n process o f f e r s an emissions t r a d e o f f . In a l i m i t e d number of t e s t s , i t was determined that heating the a s s i s t a i r d i d not improve the smoke emissions. The Ν 0 l e v e l s obtained were independent of the operation of the primary combustor. Neither reducing the residence time nor changing the pressure l e v e l s i g n i f i c a n t l y a f f e c t e d the Ν 0 l e v e l s . The heat f l u x to the combustor w a l l was comparable to that f o r NO. 2 petroleum distillate fuel. χ

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D i s c u s s i o n of Test Results The t e s t data acquired were analyzed to determine the f u e l property e f f e c t s on the staged combustor performance. Influences on the emissions and the heat f l u x to the primary combustor w a l l are presented i n t h i s s e c t i o n . Comparisons with data acquired by Westinghouse E l e c t r i c Corporation i n another EPRI-sponsored c o n t r a c t u a l program (RP989-1) i n which s i m i l a r t e s t f u e l s were combusted i n a c o n v e n t i o n a l , lean combustor are also presented ( 3 ) · Ν 0 E m i s s i o n s . The n i t r o g e n content i n the d i s t i l l a t e f u e l s ranged from 0.0 to 0.75 wt%. The i n f l u e n c e of t h i s range on Ν 0 emissions i s d i s p l a y e d i n Figure 11. The values p l o t t e d correspond to the minimal Ν 0 l e v e l f o r each f u e l . Since the minima occurred over a small range of primary combustor equivalence r a t i o (1.5

Combustion of Synthetic Fuels - ACS Publications - American

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