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J. E. Radway Basic Chemicals Division Basic Incorporated Cleveland, Ohio 441 14
How more ash Adding more ash to that already in fuel oil can lead to reduced particulate emission. Dispersed magnesium oxide is the key
The fuel burned by utilities on the East Coast, from Maine to Florida, is predominantly oil. Until recently, most West Coast and Gulf Coast power plants burned natural gas, but now they, too, are switching to oil. In doing so, they are discovering what East Coast utilities learned a long time ago-namely that when burning oil, they must combat “acid smut” fallout and particulate emission. The means used to combat these problems, that is, adding more ash in the form of dispersed magnesium oxide (MgO), leads to an unexpected result-reduced emissions. A careful review of the chemistry involved shows that more solids input to the boiler can mean less output to the stack. Actually, there are three forms of particulate emission from boilers fired with residual fuel oils. These are fuel ash, combustibles (carbon or coke) and sulfates. Of the three, sulfatescreated by the combustion processprovide the largest portion of particulate solids, and are the key factor that must be addressed, in order to comply with emission standards. However, neither the fuel ash nor the combustibles can be ignored. For example, if all the ash in a residual oil containing 0.1% ash were emitted, approximately 50% of the allowable new source performance standards (NSPS) of 0.1 lb of particulates per million Btu would escape. Also, combustible emissions, though highly dependent on burner/boiler design, operation, and maintenance, typically approach the levels reported for ash. Needless to say, therefore, efforts to control these emissions are also important. Ash content of residual fuel oil seldom exceeds 0.2%. Nevertheless, even this small amount of ash is capable of causing severe problems of external deposits and corrosion in boilers. Of the many elements that may appear in oil-ash deposits, the most important are vanadium, sodium, and sulfur. Compounds of these elements are found in almost every deposit in boilers fired by residual fuel oil.
Needed: more ash A proven way to prevent the problems of ash corrosion and deposition, and to help keep fuel ash from going up the stack, is to add more ash to the fuel oil burned under the boiler. This ad-
ditional ash is added in the form of dispersed magnesium oxide (MgO) metered into the residual fuel oil. An excellent example of how this is being done effectively and profitably can be found at the Long Island Lighting Company (LILCO). LILC O S boilers burn relatively high-sulfur, high-ash fuel (2.5% S, 0.05% ash). Their information on ash retention is probably the most complete available, because they are in the business of recovering and selling vanadium from ash in their boilers. The vanadium produced by LILCO in 1976 was equivalent to 9% of U S . production. Indeed, published reports show that they sold 362 tons for $1.2 million. These reports also show that 33% of the ash is continuously collected and removed from the furnace as bottom ash. Another 57% is accumulated as powdery deposits between the furnace outlet and the stack tip. This latter ash is removed by washing the furnace at 4-6 month intervals. The high ash retention is in part attributed to the use of LiquiMagO, a magnesia additive manufactured by Basic Chemicals, and to the high sodium and vanadium content of the fuel. Personnel at LILCO define the term “ash” as including the magnesia additive. Agglomeration effects Apparently, the magnesia additive contributes to the retention of fuel ash in the boiler by helping to agglomerate a portion of the particulates. Some of these agglomerations become too large to be carried out of the unit at normal gas velocities. Evidence for this agglomeration effect is provided by published data showing a direct relationship between the amount of magnesia fed to the burners and the percent of ash collected in the boiler ash pit. A reduction in ash burden in the flue gases at the furnace outlet was also shown, despite increasing addition of magnesia solids; those solids themselves become an ash component. The agglomerating effect of magnesia additive is supported by data collected by another utility burning low-sulfur, low-ash fuel (0.8% S and 0.01% ash) on a new Combustion Engineering corner-fired boiler equipped with mechanical collectors. Without additive, these collectors were ineffective in capturing ash (0% efficiency). When the MgO additive was fired with the fuel, an unofficial efficiency test showed 75% collection. Supporting evidence for the improved collection is provided by the dramatic increase in captured vanadium pentoxide (VZO~), as bottom ash with MgO: 9.3% with, compared to 2.9% without.
Emlsslom, lb1106 BIU
LIqulMafl k e d ‘ab Ib/lOB Blu
Exoess 0 2 YO
Total psrtlculale
Carbon
Ire”
-
0 0.0250 0.0375
0.50 0.27 0.27
0.0887 0.0742 0.0557
0.0222 0.0111 0.0084
0.0133 0.0059 0.0045
Source: Data horn Ponlbrule Statlon of Inlerbrabant, S A , Belgium, burning3 4 % S,0.01 % ash residual 011.
Limited tests conducted by a major utility have also shown that ash retention in the furnace is affected by both the source and the particle size of the magnesia injected with the fuel. About twice as much is collected as bottom ash when magnesite-derived magnesia was used, as when brinederived MgO was used. Also, a much finer-sized magnesite-derived additive’s particle size (0.7 versus 2.0 fi, as measured by x-ray sedimentation)was about twice as effective as the standard-sized additive, in tests at two different utilities. Unburned carbon smoke The second form of particulate emission-combustibles-are really unburned fuel going up the stack as carbon. In the past, little attention was paid to the possibility that magnesium oxide could have an effect on these combustibles. Indeed, of the metal oxides known to have smoke suppressant effects, magnesia was considered one of the poorest.
Reduction in combustibles has always been considered a matter of excess air, proportioning of air to fuel, and burner maintenance. For example, a curve relating oxygen ( 0 2 ) in the flue gas to particulate emissions would go through a minimum at 2% 0 2 . Thus, increased emissions at lower oxygen levels were attributed to more unburned carbon, while those at higher values were explained in terms of sulfates. However, recent tests at a station in Belgium (later repeated at a second station) show a dramatic impact of dispersed magnesia injection on both total particulates and combustibles when a high sulfur/low ash residual oil is fired (3.4% S, 0.01% ash) (Table 1). Emissions, which were already below the EPA new source performance standard (NSPS), were reduced by a further 37% despite the injection of magnesia solids equivalent to 42% of what the particulate burden would have been without additive. Iron emissions, which are considered in-
Adding MgO. I f is w r y carefully metered
00 13-936X/78/0912-0388$01.OO/O @ 1978 American Chemical Society
Volume 12, Number 4, April 1978
389
dicative of boiler corrosion, were reduced by 64%. Carbon emissions were reduced by 50-62%, despite reduction of excess 0 2 from 0.50% to 0.27%. Normally, lowering of excess oxygen should have increased carbon emissions. Thus, it can only be concluded that the magnesia must be a factor in the reduction. Obviously, these findings are not directly translatable to other boilers because of the different nature of the fuels and combustion. Moreover, European oil burners are generally more efficient with low excess air than is the usual U.S. equipment. Nevertheless, they suggest that dispersed magnesia can help to reduce combustibles. Chemical leverage Dispersed magnesias can be thought of as chemical levers because, when injected into a boiler with the fuel, they can prevent 12-18 times their weight in particulate sulfate emissions. An explanation of how dispersed magnesias prevent particulate sulfates requires an explanation of the mechanism of sulfate formation. Conversion of fuel sulfur to sulfur trioxide (SO3) and sulfates in the boiler is governed by the laws of chemical equilibrium, and is influenced by sulfur concentration and excess air levels. It can be shown that the percentage of conversion of flue gas SO2 to SO3 (the sulfate precursor) increases as the fuel sulfur content decreases. It can also be shown that 0 2 levels in the flue gas below 0.5%-1% are necessary to limit SO3 formation. Since most U.S. boilers operate somewhat above this level, burning low-sulfur fuel does not yield the expected dramatic reduction in SO3and particulate sulfates. Indeed, a fivefold reduction in fuel sulfur lowers SO3by only 40%. According to the literature, a maximum of about 5% of the sulfur in a clean fuel is converted to SO3; i.e., equivalent to about 0.34 lb/ lo6 Btu. The SO3 does not remain a gas; it reacts with water vapor in the combustion products to form sulfuric acid, most of which condenses in the air heater, ducts, and stack. Stack temperatures on some boilers, particularly those designed for gas, can be below 240 OF where most, if not all of the acid condenses. If the acid did not attack the boiler, this would be a happy situation, since retention of the acid in the boiler avoids plume problems. Unfortunately, it does react, and thereby increases potential particulate emissions by as much as 3.5 lb for each Ib of SO3 formed. 390
Environmental Science & Technology
Sulfate formation can actually result in far greater particulate emissions than those stemming from so3 alone. This is because ferrous sulfate (FeS04-7H20) corrosion products, originating from iron in the boiler system, can further react with SO2 and 0 2 in the flue gas to yield highly corrosive ferric sulfate (Fe2(SO,)3). Below pH 3, the latter will attack the boiler metal, producing more ferrous sulfate, which will react with more S02, and so on. Obviously, the best emission control techniques involve: preventing SO3 formation maintaining the pH above 3 to break the ferrous-ferric-SO2 corrosion cycle. SO3 control with MgO It has been shown that most of the SO3in the flue gas of boilers is formed catalytically in the convection passes. It has also been shown that MgO interferes with that catalysis, reducing SO3 concentrations ahead of the air heater from 32 ppm to about 7 ppm. Recent field tests have demonstrated even lower SO3 concentrations at the air heater entrance, circa 2-3 ppm, with “LiquiMag” addition. At this level, even if all the SO3were reacted and emitted as hydrated corrosion products, about 0.02-0.03 lb/106 Btu, there would be little problem in meeting standards. It is important to note that the formation of SO3is catalyzed by the iron oxide surfaces in the boiler, but only at temperatures above about 900 O F . Thus, magnesia injected in the cold end of the boiler (downstream from the economizer) cannot provide the same leverage in controlling SO3 and particulate emissions as an equivalent quantity fed with the fuel, because it has no effect on the SOz-to-SO3 catalysis reaction. Magnesia injected at the cold end merely has a sacrificial function. It reacts with the condensed acid, preventing corrosion of the air heater, ducts, and other structures. However, the effect on total emissions is much less than when it is fed with the fuel or injected into the furnace.
It will be apparent from Table 2 that magnesia fed into the furnace is more efficient in reducing sulfate particulates than that injected downstream. The inhibition of SO3 formation by catalysis is the key. The benefit of dispersed magnesia on fuel ash retention and combustibles is also attainable only through furnace feed. The foregoing discussion assumes uniform formation and discharge of particulates. In the absence of magnesia treatment, these conditions rarely occur. Particulates are accumulated in the boiler at lower loads, because of the sticky nature of the condensed acid, and then discharged as “acid smut” when the load is increased. Thus, the boiler can be in compliance part of the time, and severely polluting at other times. The magnesia treatment minimizes such swings by controlling SO3. Published studies by Long Island Lighting Co. personnel have pointed out one other variable of significance in sulfate formation and plume visibility-boiler dirtiness. The dirtier the boiler, the greater the sulfate formation and plume visibility. Thus, emissions will change with time, and fuel treatment evaluations must allow for this factor. MgO and particulates Table 3 shows the cumulative impact of furnace feed of dispersed magnesia on the estimated emissions of ash, combustibles, and particulate sulfates for both “clean” and “dirty” residual oils. The maximum figures, though unlikely to occur on any operating boiler, were used to highlight the importance of sulfates in total particulates as well as in acid smuts. They also remind one of the old adage, “an ounce of prevention is worth a pound of cure.” Preventing sulfate formation by injection of magnesia in the furnace to slow the catalytic activity of the iron oxide surfaces is a key factor in compliance. The reduction in combustibles with dispersed magnesia can also affect total emissions favorably. It is of consequence from a fuel efficiency
Magnesia injection Untreated
After eConomiter
With fuel
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Maximum sulfate emissions, lb/106 Btu MgO fed rate, lb/1O8 Btu Note: Residual oil Is 2.5% S.
0.28
0.25
.03
-
.07
.02
Dirty fuel (2.5% S, 0.1% ash)
Maxlmum untreated
Ash Combustibles Sulfates Total Additive feed rate
0.054a 0.046b
0.28 0.380
-
Clean fuel (0.5% S, . O l % ash)
Magnesia treated
Maximum untreated
Magnesia treated
0.037d
0.0054a 0.023b 0.1188 0.1464
0.0057d 0.0153” Q.0300‘ 0.0510 0.0066
0.023= 0.030‘
0.090 0.02
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100% ash emission. Based on unburned carbon heat loss of 0.1 % for dirty fuel and 0.05% for clean fuel. Expressed as hydrated corrosion products. Used about 30% retention. Bollers wlth fly ash reinjection have consistently retained 90%. e 50% reduction, based on European data. Based on about 3 ppm SO3 reactingto hydrated corrosion products. 0 Based on conversion of 5% of the 0.5% S In the fuel to corrosion products. Assumed lower reduction In combustibles than actually observed by high S. low ash European residual fuel. a
standpoint, primarily because it provides an opportunity to reduce excess oxygen. Increased fuel ash retention in the boiler with dispersed magnesia is also helpful in controlling total emissions. But, as with the “combustibles,” the effect is minor, compared to the impact on sulfates. The table also provides a perspective on the potential contribution of the magnesia fuel additive to emissions. They are very minor relative to sulfates they prevent, and to NSPS. The indicated feed rates for clean fuel are based on the published experience with a gas-designed boiler converted to residual oil at the Ritchie Station of Arkansas Power & Light Company. For the untreated cases, the “particulate sulfates” are primarily “corrosion products” or “iron compounds.” When dispersed magnesia is used, the less noxious magnesium sulfate will make up a significant proportion of the small quantities of sulfate emissions. Consequently, the particulates will be less acid, finer, lighter in color and less visible to the eye. The iron compounds can be oxides or “rust” as well as sulfates, particularly on boilers designed for gas and converted to oil. These units generally have more heat exchange surface, and operate with lower exit gas temperatures. It is not uncommon for such units to be below the acid dew point at full load, and below the water dew point at low loads. The result is a more dilute, and therefore more corrosive acid. Air heater “basket” life of only two years is not uncommon under such conditions, even for expensive enameled surfaces. Short basket life can be roughly equated to increased particulate emissions. Indeed, each pound of metal
lost could yield nearly five pounds of hydrated iron sulfates. Obviously, not all of the metal lost is converted to hydrated sulfates, nor are all of the corrosion products emitted from the stack. Nevertheless, the impact on emissions can be significant. Reducing the amount of acid by burning lower sulfur fuels does reduce acid corrosion, but the iron sulfates themselves can accelerate the “rusting” reaction. Thus, condensation of even small quantities of acid can lead to significant corrosion and emissions. Magnesia feeding options The magnesia necessary to control sulfates can be provided in various physical forms, and injected at various points in the boiler. The physical forms are listed in order of increasing cost and efficiency. Obviously, the preferred form for a given boiler will depend on an economic analysis, but the dispersed forms are generally more “cost effective.” Magnesium oxide powder was first air-aspirated into boiler furnaces many years ago. It required a major capital investment; presented serious handling, housekeeping, and maintenance problems; and necessitated at least four times the dosage of today’s oil dispersions. The relatively poor efficiency was caused by powder distribution problems, and by a relatively large “effective” particle size resulting from formation of tough agglomerates during oxide manufacture. More recently, powdered MgO has been used to control SO3 problems by aspiration into the ducts between the economizer and air heater. This technique can be effective in neutralizing acid once it is formed, but the leverage of preventing SO3 formation in the
convection passes is lost. Time for the acid-base reaction is also shortened, and chances for particle agglomeration on the air heater surfaces are increased. MgO oil dispersion, such as “LiquiMag,” is far more efficient than powdered MgO, because of its much finer particle size, and unilorm feed via addition to the fuel. It is much easier to handle because it is a stable, meterable fluid. The feed equipment required is also mukh simpler, and less expensive to purchase and maintain. A more recent product development by Basic Chemicals-UltraMagTM-is effective a t roughly one-half of the “LiquiMag” dosage because of its much finer particle size. With “UltraMag,” more of the ash is collected in the furnace hopper. Coating of the waterwalls, which raises furnace outlet gas temperatures, is virtually eliminated. Also, the availability of MgOoil dispersions with different characteristics, and of several additive feeding options (together with and separate from the fuel) allows tailoring of the chemical treatment to any particular boiler design. The efficacy of dispersed MgO in helping to control such particulate emissions is influenced by many factois, including particle size, chemical form, point of addition, and fuel chemistry. It is also affected by such boiler operating and design parameters as excess air, leakage rates, flue gas temperatures, furnace volumes and heat exchange area. Dispersed MgO is a significant control tool when it is integrated into the total program and tailored to the particular boiler. The cost of MgO-oil dispersions is in the range of 2$/bbl of fuel oil, and up. Actual costs with any given boiler will depend on quality of the fuel oil and on the quality of quantity of the additive used.
Jerrold E. Radway is assistant cice president, technology, at Basic Chemicals Dicision, Basic, Inc. He was responsible for the incention of LiquiMaga and has been instrumental in the decelopment of f l u e gas desulfurization processes. Radway is a member of ACS, AIChE, A S M E , and APCA. Coordinated by JJ Volume 12, Number 4, April 1978
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