CURRENT RESEARCH Influence of Heavy Fuel Oil Composition and Boiler Combustion Conditions on Particulate Emissions H. Lawrence Goldstein" and Charles W. Siegmund Exxon Research and Engineering Co., Linden, N.J. 07036
Studies determined the influence of fuel composition and combustion conditions on particulate mass loading and size distribution. The fuel composition aspect of this study was concerned with determining the effect of the change from conventional high-sulfur fuel oil (Bunker C) to desulfurized fuel oils that have lower ash and asphaltene (hexane insolubles) contents. Varying operating conditions, such as combustion chamber residence time and excess air, produced notable differences in particulate loading and size distribution. Particulate emissions produced by the combustion of heavy fuel oil may be divided into three basic types: smoke, cenosphere, and ash residues. Smoke, traditionally associated with atmospheric pollution, is composed of submicron carbon particles formed in the vapor phase around burning fuel droplets. The mechanism for its formation is generally considered to include cracking of fuel molecules in a zone where there is insufficient oxygen to burn, followed by polymerization of the cracked fragments to form hydrogen-deficient multiple-ring structures. While smoke formation is only influenced to a slight extent by fuel composition, it is very responsive to combustion conditions. When smoke is emitted from the flame, it is the result of failure to mix smoke particles with oxygen a t a temperature high enough for combustion. Even when the smoke emitted is dense and very objectionable, it represents a very small percentage of the fuel. For example, correlations developed with distillate fuel indicate that a Bacharach smoke number of 9 can represent less than 0.1 wt %particulate ( I ) . Cenospheres are the residues of spray droplets, generally in the 10-50-pm size range. As the droplet passes through the flame, volatile components vaporize, fragments crack from nonvolatile molecules, and both burn in the vapor phase around the droplet. The nonvolatile carbonaceous residue forms a solid skeletal particle, full of void spaces, and roughly the size of the original droplet. This is called a cenosphere. The rate at which cenospheres burn out is limited by diffusion of 0 2 to and COn from the particles. This process is slow compared with vapor phase combustion of volatiles. Normally cenosphere and ash residue account for a major part of the weight of particulates emitted from heavy fuel oil firing. As such they have a major effect on emission factors. Both of these types of particulate are highly dependent upon fuel composition. The single most important fuel factor is the concentration of asphaltenes (hexane insoluble material), generally considered to be precursors of cenospheres. The fuel ash content represents the minimum level of particulate that will be emitted if all the carbonaceous material is consumed. Most of the heavy fuel oils used on the East Coast of the United States are of Venezuelan origin and, prior to the sulfur regulations (19701, had ash contents in the range of 0.05-0.10 w t % and hexane insoluble contents of up to 12%. In general,
the low-sulfur fuel oils in use today have much lower ash and asphaltene contents. At the present time, particulate regulations primarily address the problem of how much pollutant (mass loading) rather than what type pollutant (size distribution) is emitted from stationary combustion sources. In the States, where particulate restrictions have been imposed on existing units, they have tended to be in the range of 0.2-0.3 wt % on fuel. In general, the larger the stationary combustion source (heat input), the more severe is the performance standard applied to the unit. At the federal level, the Environmental Protection Agency has issued new source performance standards for newly installed, large (>250 X lo6 Btu/h) boilers. These standards, which apply mainly to electric utility boilers, limit particulate emissions to 0.10 lb per million Btu heat input (0.18 wt % on fuel) for liquid fuel oil. Means of achieving compliance with these standards have concentrated on removing the largest particles-i.e., cenospheres ( > l o pm), since each particle removed has many times the mass of smaller particles. Yet the large particles remain in the ambient air for a short time and have little effect on visibility while they are suspended. Since it is the submicron particles that decrease visibility and are believed to cause adverse health effects, the trend in particulate regulations is expected to be toward restricting size and composition since these are more important than total weight. The study summarized in this paper was undertaken to assess the influence of fuel composition, as affected by the change from high-sulfur to low-sulfur fuel oil, and the effect of certain key combustion conditions on total particulate emissions and their size distribution. These studies were performed in a small package boiler primarily because it afforded the opportunity to closely control and monitor the operating conditions. Since the boiler burns real residual fuel oil and provides realistic cooling of the combustion product gas stream, the results provide an indication of the changes that can occur in large size boilers.
Experimental
Residual Fuel Boiler. All combustion experiments were carried out in a 50-hp Cleaver Brooks package boiler with a nominal firing rate of 56.8 dm"/h (15 gph) of residual fuel oil. The boiler is a horizontal fire-tube type; by means of appropriate baffles and heat exchanger tubes, the combustion gases are forced to pass the length of the boiler four times before being emitted into the stack. In most experiments, a conventional high-sulfur (2.2%) residual fuel oil produced from Venezuelan crude was burned, as this ensured a large inventory of stack particulate. T o establish trends, however, various low-sulfur oils were also burned. Typical analyses of these fuels are presented in Table I. Combustion conditions were closely monitored during these tests and held constant except for the variable under study. Volume 10, Number 12, November 1976
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For the majority of experiments, a fuel firing rate of 28.4 dm3/h (7.5 gph) was employed and accurately determined in each test by monitoring periodically the loss in weight of the oil supply drum. Atomization of fuel was accomplished by passing air a t 68.8 kPa (10 psi) through the burner gun. Oil viscosity a t the atomizing nozzle was maintained a t 30 CSby heating the fuel to a predetermined temperature with an electrical heater-for example, the high-sulfur fuel to about 96 "C (205 OF) prior to the burner gun. Secondary air for combustion was furnished by a centrifugal blower mounted in the boiler head. The air was forced through a diffuser plate to mix thoroughly with the atomized oil before combustion. The amount of secondary air was controlled by means of a damper regulated to maintain the desired oxygen concentration in the flue gas, normally 2% (-10% excess air). For monitoring boiler performance, the concentration of CO, COe, and 0 2 in the flue gas was continuously measured by Beckman instruments. Both carbon monoxide and dioxide were measured using nondispersive infrared analysis while oxygen was determined polarographically. To ensure operational stability, the package boiler was allowed to warm up for a minimum of l h before the start of a run that normally lasted from 1.5-3.0 h. During the test, ad-
Table I. Representative Residual Fuels Analysis
Sulfur, O/O Carbon, O/Q Hydrogen, O/O Nitrogen, O/O API gravity Vis. SSU @ 38 " C (100
Con carbon, O/Q Hexane insoluble, YO Ash,
Yo
O F )
Conventional high sulfur
Intermediate
2.2 86.25 11.03 0.41 17.3 3138 12.51 10.33 0.08
0.96 87.11 11.23 0.26 21.5 586 5.64 4.72 0.04
0.50 87.94 11.85 0.16 24.7
350 41 25 13
155 20 10 9
70 10 lo 1-10 10pm
1-10pm
10pml-10
24 2
Environmental Science & Technology
pm
110 3
< I pm
137 8
of the package boiler accounts for most of the remainder of the vanadium oxide. Combustion Chamber Residence Time Effects. Another study was conducted to determine the effect of residence time in the high-temperature combustion zone on total particulates and size distribution. Residence time in this zone is one of the major differences between large and small boilers. As boilers increase in size, firebox residence time normally increases. Experimentally, residence time in the boiler was varied by a combination of two procedures. The first was simply changing the firing rate-the reciprocal of the firing rate 1IFr is roughly proportional to residence time. In the second technique, the fire chamber length was increased by inserting 2-ft-long sections of refractory. These may be slid down the length of water-backed wall to butt up against the existing chamber. Each added section of refractory increased combustion zone volume by 0.044 m3 or about a 66% increase over the normal single chamber. As expected, increasing residence time gave a marked decrease in total particulate. I t also changed the size distribution of the particulate. Figure 5 shows results on high-S fuel oil and Figure 6 shows those for 0.5%-S fuel. With the high-S fuel oil, increasing residence time by a factor of 3.3 or 4.7 (cutting firing rate in half and adding 1 or 2 chambers) reduced total particulate emissions about in half, from about 0.2 wt % on fuel to about 0.1 wt % on fuel. However, submicron particles actually increased a t the longer residence time. Apparently, the major effect of residence time is in burnout of cenospheres, which permits formation of smaller particles, probably through an ash vaporization condensation mechanism. The metals such as vanadium and sodium contained in the ash will form volatile oxides as the cenospheres burn out. Under the conditions existing in the combustion chamber, these oxides will vaporize and, as the combustion gases move through the cooler parts of the boiler, will recondense as fine particles. The opposite effect was seen with the l o w 3 fuel oil (Figure 6). For this fuel, there was relatively little large particulate emitted in the base case. The largest size fraction was the submicrons which accounted for nearly two-thirds of the total mass (but note that it was significantly lower than the submicron fraction for a high-S fuel a t the same conditions). Increasing residence time by a factor of 3.3 cut total particulates by over 80% and submicrons by 75%. Thus stack solids were essentially a t the fuel ash level and were mostly (>80%) submicron. The reduced solids burden in the combustion zone, coupled with the additional residence time in this case, permitted both large and small carbon particles to burn out almost completely. Since large utility boilers in general provide long residence times a t higher temperatures, stack particulate will tend to be very low. However, as inferred from this study the emissions will tend to be predominantly submicron. Effect of Excess Combustion Air. The effect of varying excess combustion air is illustrated in Figure 7. The top curve shows the response of a conventional high-S oil to various levels of excess air, while the bottom curve compares the response of a low-S (0.3%) oil. The major difference between fuels is the asphaltene content; the level of carbonaceous particulate emissions is roughly proportional to asphaltene content. The results of this study clearly show there is an optimum level of excess air that corresponds to minimum particulate emissions. Above and below this level particulate emissions increase. In these tests, burnout of particulate was maximized a t an excess combination air level of 60%, equivalent to 8%oxygen in the flue gas. With a h i g h 3 fuel oil, this gave a particulate loading of 0.12 wt % (on fuel) representing a decrease of close to 40% compared to the base case a t 10% excess air. For the low-S fuel, the equivalent mass loading
5
*0°1
200
0.22 wt. 7"
7
Ln 0
? '4
169 P
1
123
r i w
p
0 . 0 6 3 w l . 'la
I
I \\
0.11
wt.
".
80
4 U I3
F n
40
2 n
BASE F . R 2 8 . 3 dm3/h 1 Chamber
1 4 . 2 dm3/h 2 Chambers
BAS E F . R 2 8 . 3 r,13/l 1 Chanber
1 4 . 2 d71~/11 3 Chambers
Figure 5. Effect of residence time on particulate size distribution (high4 fuel oil)
amounted to 0.01 wt % or roughly one-third of the base case amount. Although 60%excess air was found t o be optimum for particulate burnout, it is far from optimum for boiler efficiency. I t is normal practice to operate a boiler with the minimum excess air that gives tolerable smoke-usually about 10-25% excess air. Higher excess air levels decrease thermal efficiency by increasing the volume of hot flue gases carrying heat out the stack-far outweighing the small amount of heat obtained by burning carbon in the particulates. Thus, adjusting excess air level as a means of controlling particulates almost always entails some loss in absorption efficiency. The generally accepted mechanism for burnout of carbonaceous cenosphere particulate involves diffusion of oxygen to the surface of the particles followed by gas-solid combustion. This is a relatively slow process, especially since it occurs in a part of the combustion zone where oxygen may be depleted. A high level of excess air and a long residence time a t high temperature favor oxygen diffusion to the surface and promote cenosphere burnout. These complementary effects are illustrated for a h i g h 3 fuel in Figure 8. By adding an additional refractory chamber, combustion zone residence time was increased by roughly 66% over the normal single chamber case. The moderate increase in submicron particulate accompanying the decrease in large ( > l o pm) and intermediate (1-10 pm) solids is primarily due to concentration of ash material into the small size range.
Conclusions On the basis of these package boiler studies, the following conclusions can be reached: Fuel Quality Effects 1. The weight of particulate emitted from combustion of heavy fuel oil is roughly proportional to its asphaltene plus ash content if combustion conditions are held constant. 2. Most low-S fuel oils are low in ash and asphaltene content, especially those made by processing Venezuelan residua. Consequently the use of low-S fuel oil reduces particulate emissions in all size ranges. However, a disproportionate amount of the reduction occurs in the large and intermediate size range. A significant, but somewhat smaller, reduction occurs in the submicron size range. 3. Asphaltene content (hexane insolubles) plays a major part in determining the level of large (>lo pm) and intermediate (1-10 pm) size particulate emissions; fuel ash content plays an important part in determining the level of submicron particulates, especially under combustion conditions which minimize larger particles. 4. Vanadium and other volatile ash constituents tend to concentrate in the submicron size range.
21 4Chamber . 2 dm3/h
Figure 6. Effect of residence time on particulate size distribution (low-S fuel oil)
150-
120-
-
-
90
60
-
30
20
0
40
80
60
EXCESS COMBUSTION AIR,
100
'/a
Figure 7. Variation of particulate emissions with combustion air
200/
c t
>
0 . 2 2 wt. '
""
f -I
Fuel Flrlng Rate .. .
3
-
160
16 w t .
3
2
0.14.d.
40.
0-
60
30 EXCESS C O M B U S T I O Y AIR,
?e
Figure 8. Influence of excess air and combustion chamber residence on size distribution (weight)
Effect of Combustion Conditions 1. Increased combustion chamber residence time promotes burnout of the carbonaceous particle residue resulting in an overall reduction in total amount of particulate. Size distribution is shifted toward the submicron range. 2. There is a level of excess air that produces minimum particulate emissions; above and below this value, particulates increase rapidly. This level, which in the 50-hp package boiler occurs a t 60% excess air may be related to the combustion Volume 10, Number 12, November 1976
1113
chamber residence time and flame temperature. Normally boilers are operated a t excess air levels far lower than that which gives minimum particulates in order to improve heat absorption efficiency. Increasing excess air to reduce particulate emissions will reduce absorption efficiency of the boiler. 3. Any increase in particulate burnout will reduce total particulate loading but shift the size distribution toward the submicron range. Acknowledgment
The assistance of J. P. Singleton, who carried out the actual experimental work, is gratefully acknowledged. Literature Cited (1) Hunt, R. A,, “A Geometric Correlation of Smoke Measurement”,
Paper No. 61-6, API Combustion Conference, Chicago, Ill., 1961.
(2) Goldstein, H. L., Siegmund, C. W., “Collection Efficiencies of Stack Sampling Systems for Vanadium Emissions in Flue Gases”, EPA-600/2-76-096, 37-46 pp, Environmental Protection Technology Series, April 1976. (3) Environmental Protection Agency, “Standards of Performance for New Stationary Sources”, Fed. Regist., 36 (247), 24876-95 (December 23,1971). (4) Ranz, W. E., Wong, J. M., “Impaction of Dust and Smoke Particles”, Ind. Eng. Chem., 44 (61, 1371-80 (1952). (5) Barrett, R. E., Miller, S.E., Locklin, D. W., “Field Investigation of Emissions from Combustion Equipment for Space Heating”, EPA-R2-73-084a; NTIS No. P B 233-148; API Publication 4180, June 1973.
Received for review December 22, 1975. Accepted April 26, 1976. Presented at the 1st Chemical Congress of North America, ACS, Mexico City, December 4 , 1975. Work supported by the Products Research Diuision, Fuels Laboratory of Exxon Research and Engineering Co.
Chemical Kinetics of Homogeneous Atmospheric Oxidation of Sulfur Dioxide Stanley P. Sander and John H. Seinfeld” Department of Chemical Engineering, California Institute of Technology, Pasadena, Calif. 91 125
A systematic evaluation of known homogeneous SO2 reactions which might be important in air pollution chemistry is carried out. A mechanism is developed to represent the chemistry of NO, /hydrocarbon/SOz systems, and the mechanism is used .to analyze available experimental data appropriate for quantitative analysis of SO2 oxidation kinetics. Detailed comparisons of observed and predicted concentration behavior are presented. In all cases, observed SO2 oxidation rates cannot be explained solely on the basis of those SO2 reactions for which rate constants have been measured. The role of ozone-olefin reactions in SO2 oxidation is elucidated.
This work consists of a systematic evaluation of all known atmospheric SO2 reactions which might be important in air pollution chemistry. We begin by considering the photochemistry of SO2 in air, in mixtures of air and oxides of nitrogen, and in mixtures of air, oxides of nitrogen, and hydrocarbons. We then summarize available experimental data appropriate for quantitative analysis of SO2 oxidation kinetics, and present comparisons of predicted and measured concentrations of the species involved. Finally, we draw conclusions pertaining to additional information needed to understand more completely the atmospheric chemistry of SO2. A t m o s p h e r i c C h e m i s t r y o f Sulfur D i o x i d e
The oxidation of sulfur dioxide in the atmosphere occurs by both heterogeneous and homogeneous paths. Heterogeneous routes, involving the catalytic oxidation of SO2 in water droplets, are reasonably well understood (1-6). Homogeneous gas-phase reactions of SO2 in the atmosphere have been reviewed (7-12), although the detailed mechanism by which SO2 is oxidized in the presence of other air pollutants has not been established. The object of this work is to study the rates and mechanisms of the homogeneous atmospheric oxidation of SO2 through detailed analysis of available laboratory data on S02-containing systems. For a number of years there has been considerable interest in elucidating the mechanism of photochemical smog reactions (13-17). Although it has long been recognized that SO2 is generally present with hydrocarbons and oxides of nitrogen in the atmosphere, comparatively little experimental work has been conducted on systems containing SOz. The sulfate aerosol formation which often takes place in such systems has been viewed as an undesirable feature in laboratory smog chambers, particularly when the primary objective of the experiment is the study of hydrocarbon/NO, chemistry. It has been suggested that heterogeneous paths may account for most of the photochemical sulfate aerosol observed in the atmosphere (18).However, since the hydration of SO3 leads readily to aerosol, the homogeneous oxidation of SO2 to SO3 is a potential key step in sulfate aerosol formation. 1114
Environmental Science & Technology
In this section we summarize atmospheric reactions involving SO*, liydrocarbons, and oxides of nitrogen. Photochemistry of SO2. The photooxidation of SO2 alone or in mixtures of SO2 and 0 2 has been studied by several investigators (9). Rao et al. (19) and Okuda et al. (20) have suggested that the most active species involved in the photochemistry of SO2 is the excited triplet state, 3S02. The principal reactions in the photochemistry are believed to be those given in Table I. Where rate constant determinations are available, they are indicated in the table. Reactions 1-7 are apparently the major reactions which occur subsequent to light absorption by an SO2 molecule. Reaction 8a has a relatively low activation energy but because it is spin-forbidden also has a low preexponential factor (21).Reaction 8b is spin-allowed but endothermic. The oxygen atoms produced in Reactions 8a and 8b will combine readily with 0 2 to form 0 3 . Because there is no direct evidence to date for the existence of O3 or SO4 in irradiated SOz/air mixtures, Reactions 8a-8c are probably unimportant in this system. Although reported overall quantum yields for SO2 photooxidation vary over several orders of magnitude, even the most optimistic estimates place these reactions in a position of minor importance (56). Photochemistry of SO2 and NO,. The primary effect of adding NO2 to an SOn/air mixture is the oxidation of SO2 by Reaction 9 by the oxygen atoms formed from NO2 photolysis. A large number of studies of this reaction have resulted in a
