5 Nonequilibrium Distillation Effects in Vaporizing Droplet Streams
Downloaded by RUTGERS UNIV on January 1, 2018 | http://pubs.acs.org Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch005
S. HANSON, J. M. BEER, and A. F. SAROFIM Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139
The combustion of fossil fuels in the United States will be complicated by an increased reliance on fuels with high nitrogen and low hydrogen content. The problem will be aggravated by the displacement of petroleum-based fuels by coal-derived synthetic fuels and shale oil. Previous studies on the contribution of fuel nitrogen to ΝO emissions (1,2) used specific combustion systems and fuels doped with fuel nitrogen. These experiments provided useful information for the systems studied and indicated that the most effective met hod for limiting the formation of NO from fuel nitrogen is staged combustion. Staged combustion takes advantage of the fact that fuel nitrogen in the gas phase is strongly dependent on the oxygen available. This method consists of a fuel-rich first stage, in which the fuel nitrogen is released and converted mostly to N , followed by a second stage in which the remaining oxygen is added to complete the combustion. The process requires that the fuel nitrogen be evolved into the gas phase prior to exiting the first stage, and with sufficient residence time to participate in the gas phase reactions that form N . Therefore, the objective of this study has been to determine the rate of nitrogen evolution from vaporizing fuel droplets. x
x
2
2
In order to obtain detailed information on the evolution of fuel nitrogen, a laminar flow drop tube furnace, able to simulate conditions found in actual combustion systems, was adopted for this study. The nitrogen in fuels consists of complex, mostly heterocy clic compounds. In petroleum crudes, these include pyrroles, in doles, isoquinolines, acridines, and porphyrins. During refining most of these concentrate in the heavy resin and asphaltene frac tions, which might suggest their relatively late release in the 0097-6156/83/0217-0095$06.00/0 © 1983 American Chemical Society
Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
96
COMBUSTION OF SYNTHETIC FUELS
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combustion of r e s i d u a l f u e l o i l s . However, the e v o l u t i o n of n i t r o gen from a heavy f u e l o i l d r o p l e t during i t s v a p o r i z a t i o n i n a flame d i f f e r s s i g n i f i c a n t l y from the r e s u l t s of e q u i l i b r i u m d i s t i l l a t i o n obtained under slow h e a t i n g c o n d i t i o n s at atmospheric pressure. The e v o l u t i o n of a p a r t i c u l a r compound i s determined by the r a t e of d r o p l e t v a p o r i z a t i o n , the r e l a t i v e v o l a t i l i t y and d i f f u s i v i t y i n the l i q u i d phase of the compound i n question. Because of the complex composition of f u e l o i l s , i t i s d i f f i c u l t to separate the r e l a t i v e importance of these e f f e c t s s i n c e the measured n i t r o gen e v o l u t i o n i s a summation of many f r a c t i o n s , each of which i s i n f l u e n c e d to a d i f f e r e n t extent. In order to r e v e a l the r o l e of d i f f u s i o n and r e l a t i v e v o l a t i l i t y i n the e v o l u t i o n of f u e l n i t r o gen, a complementary set of experiments were performed using ndodecane doped w i t h p y r i d i n e , q u i n o l i n e or a c r i d i n e . In the f o l l o w i n g , experimental r e s u l t s are presented of the e v o l u t i o n of f u e l n i t r o g e n during e q u i l i b r i u m d i s t i l l a t i o n and i n e r t p y r o l y s i s of d r o p l e t arrays i n the laminar flow furnace f o r three f u e l o i l s and a doped model f u e l .
EXPERIMENTAL The experimental furnace i s a v e r t i c a l l y o r i e n t e d laminar f l o w drop tube furnace having a 30 cm long uniformly hot t e s t sect i o n w i t h o p t i c a l access. The f u e l d r o p l e t array i s introduced on the l o g i t u d i n a l a x i s c o n c u r r e n t l y w i t h the ambient gas. The dropl e t stream i s i n t e r r u p t e d at s e v e r a l p o i n t s i n i t s t r a j e c t o r y by a sampling probe i n s e r t e d a x i a l l y from the base of the furnace. The probe quenches and t r a n s p o r t s the e n t i r e f l o w to a sampling t r a i n which recovers the f u e l d r o p l e t residue f o r a n a l y s i s . The above process i s repeated at s e v e r a l furnace temperatures f o r each f u e l . A d e t a i l e d d e s c r i p t i o n of the system i s to be found i n references (3) and (4).
RESULTS Table 1 presents the f u e l p r o p e r t i e s and composition of the f u e l s s t u d i e d , namely, an Indo-Malaysian r e s i d u a l petroleum f u e l , a Gulf #6 petroleum f u e l , and a raw Paraho shale o i l . These f u e l s a l l have a C/H atomic r a t i o of about 0.6 and have s i m i l a r b o i l i n g curves. However, when one considers the n i t r o g e n content, d i f f e r ences i n composition and behavior emerge. The Indo-Malaysian r e s i d u a l petroleum f u e l contains the l e a s t n i t r o g e n , 0.25% by weight; the raw Paraho shale o i l contains the most, 2.15% by weight. The Gulf #6 petroleum f u e l resembles the Indo-Malaysian petroleum f u e l more c l o s e l y i n source and composition, having 0.44% by weight n i -
Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
5.
HANSON E T AL.
TABLE 1: Downloaded by RUTGERS UNIV on January 1, 2018 | http://pubs.acs.org Publication Date: April 29, 1983 | doi: 10.1021/bk-1983-0217.ch005
97
Nonequilibrium Distillation Effects
API G r a v i t y at 1 5 ° C
FUEL OIL ANALYSIS
Raw Paraho Shale O i l
Indo-Malaysian Petroleum
Gulf #6 Petroleum
37
21.8
13.2
100 27 12
300 50 18
49
99
68
27
16
4.5
Heat of Combustion Gross B t u / l b Net Btu/lb
19,400 18,240
19,070 17,980
18,400 17,260
Elemental A n a l y s i s Carbon % Hydrogen % Nitrogen % Sulfur % Ash % Oxygen %
83.55 11.69 2.15 0.74 0.09 1.65
V i s c o s i t y ssu at 40°C ssu at 70°C ssu at 100°C F l a s h Point
e
C
Pour Point °C
87.08 12.05 0.25 0.23 0.036 0.48
Bartok; Combustion of Synthetic Fuels ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
88.29 12.31 0.44 · χ
V 1300 Κ Ο 1400Κ
COAST
NO. 6 P E T R O L E U M
1.3 1.2 1.1 1.0
IΝ D O - M A L A Y SI A Ν NO. 6
PETROLEUM
1.3
oc
