Liquid Minerals Group Inc. - Bunker Fuel

Bunker fuel is also known by other names: heavy oil, #6 oil, resid, Bunker C, blended fuel oil, furnace oil and other often locally used names.  No matter the origin of bunker fuel it has common properties where ever found: color, viscosity, contaminants, and operator problems.

Origin of Bunker Fuel

       The origin of the bunker fuel being considered is crude oil.  When crude oil is subjected to refining, the lighter fractions (gasoline, kerosene, diesel, etc.) are removed by distillation.  The heaviest materials in crude petroleum are not distilled - the boiling points are too high to be conveniently recovered.  These materials (asphaltenes, waxes, very large molecules, etc.) carry through refining and become residual oil (or resid). During various operations in the refinery (principally heating at high temperatures), rearrangement of molecules may take place forming even larger molecular materials that have still higher boiling points.  These materials also become part of the resid.  Finally, any contaminants in the crude will not be distilled from the crude and will also be in the resid.  This includes any salts (chemical elements that are typically soluble in water), sediment (oil-wetted solids), and the heavy organic molecules from various sources.  Just as salt water leaves a residue of salt behind when it evaporates, so too does the refining process leave solids behind when the lighter materials are removed.  Before selling resid as bunker fuel, a refiner will very often dilute it to meet various sales specifications for trace metals, sulfur and/or viscosity.

Color

       The color of resid/bunker fuel is always black, dark brown, or at least very dark. This color arises from the asphaltenes in the crude oil.  Asphaltenes are very large molecules containing carbon, hydrogen, oxygen, sulfur, and some nitrogen.  Their true structures have never been completely determined, but the manner in which they are stable in oil has.  Asphaltenes are completely insoluble in oil.  However, they are stabilized in the oil by molecules called maltenes.  The maltenes are "attached" to the asphaltenes through various bonding mechanisms.  When crude oil is pumped from the ground, there is a delicate balance between the asphaltenes and maltenes.  This balance causes the asphaltenes to appear stable and soluble in the oil.  As various processes are carried out on petroleum to transport it, refine it and store it, this balance is often changed causing asphaltenes to no longer be stabilized by the maltenes.  This change in stability can cause the asphaltenes to precipitate and/or coagulate from the fuels.

Because asphaltenes (and maltenes) are very large, complex molecules, they have high boiling points and carry into the resid.  Since most asphaltenes end up in the "heavy" oils these oils are very often intensely black. (And similarly, since the asphaltenes don't distill into other petroleum fractions, distillates are normally light in color.)  Asphaltenes when separated from fuels are shiny black solids and are very hard.  The common definition of asphaltene is it is insoluble in aliphatic solvents, but very soluble in aromatic solvents. The common test for asphaltenes, IP 148, takes advantage of this property to determine asphaltenes in petroleum fractions.

Viscosity

       Due to the nature of the components that make up resid, it is generally viscous, especially as first produced at the refinery.  Resids can actually be solids at ambient temperatures.  To facilitate transport, resids are often blended with other refinery fractions to lower their viscosity.  (Blending can also be required to meet various sales specification needs.)  This blending comes at a price - the blend "solvents" normally have a higher value than the resid.  This results in a blended price of the resid that is higher than the resid would be otherwise.  Also, if the blending process is not closely monitored or if inappropriate solvents are used it can further destabilize the maltene/asphaltene balance, which can lead to further problems.  (The very simple ASTM Spot test D2781, to determine fuel stability has been developed to help avoid this problem.)  To control viscosity in use, resid is almost always stored and transferred under heated conditions.  And when being fed to burners (of whatever description) the temperature of the resid is normally raised even further to lower its viscosity still more so the correct atomization can be achieved for complete combustion. Correct atomization is required to avoid incomplete combustion and smoke formation.

Contaminants

       Anything that doesn't distill during refining carries into the residual oil.  This includes not only water soluble metal salts sodium (Na), potassium (K), calcium (Ca), sulfates (=SO4), and several others, but also the oil soluble metals vanadium (V), lead (Pb), nickel (Ni) and others.  Oil wetted materials such as rust and metal particles will also be present.  The water-soluble materials enter the refinery contained in very small droplets of water dispersed through out the crude oil.  As refining proceeds, the water is boiled away leaving the contaminants behind.  Sodium, the most prevalent water-soluble contaminant, comes from salt water normally produced with the crude oil. ("Salt" in this sense is sodium chloride.)  Additional amounts of sodium can be introduced at the refinery if low cost caustic (sodium hydroxide) is used as a neutralizer for chlorides in the oil.  Sodium can also be added during ocean transportation, as salt water is normally used as ballast for ships when they sail (especially when empty).  There are methods to remove vanadium from oil (solvent dilution, etc.).  However, these methods are not economically attractive.  Therefore nearly all refiners simply concentrate the vanadium in the inexpensive resid fractions.

       It is important to note the oil soluble metals vanadium and nickel are present as chemical molecules known as porphyrins.  These come from the primordial materials that became petroleum.  Porphyrins are very large molecules. As such they have very high boiling points.  Therefore, they do not distill during refining.  Because of the concentration effects when roughly 90% of each liter of crude is removed, these oil soluble contaminants are concentrated approximately 10 times in the resulting resid. Thus for a crude oil containing 15 ppm vanadium, the resulting resid would contain about 150 ppm vanadium.  The contaminant lead is rarely encountered today. Lead does not exist in nature as a crude oil contaminant.  When found in oils or resids it is almost always due to fuel contamination with leaded gasolines.  As the use of lead in gasolines has diminished, the amount of lead seen in fuels has correspondingly decreased.  However, due to the very corrosive attack by lead in fired equipment a check for lead should always be made.

       Another contaminant encountered in nearly all fuels is sulfur.  Sulfur exists as both a water-soluble contaminant (as contaminant-metal sulfates, sulfites, and sulfides) and as an oil soluble contaminant (polysulfides, thiols, mercaptans, pyrroles, etc.).  Except for adding to deposits in fired equipment, sulfur problems normally occur after going through the combustion process.  The level of sulfur found in a resid is normally controlled by the specifications from the fuel purchaser.  Environmental laws in nearly all countries have required a reduction in the amounts of sulfur that can be combusted. Sulfur can also be removed from oil, but the cost has never met with widespread acceptance.  The normal practice is to reduce the amount of sulfur in fuels that are sold by blending with low sulfur solvents.  In the

, the current level of sulfur that can be burned is about 0.75% without stack scrubbers.  The cost of scrubbers to remove the sulfur combustion products has forced many smaller fuel users to burn much cleaner - albeit more expensive - fuels.

       Contaminants occurring in resids should also include various suspended solids (rust, catalyst fines, etc.) and other materials that are introduced in the refinery (corrosion inhibitors, soaps, water wetted solids, etc.).  These contaminants typically cause problems as filter plugging materials and in some cases as particulate emissions from stacks.

Deposits

       Various metal contaminants are present in all fuels (of greatest concern are vanadium, sodium, potassium, nickel, and lead).  Sodium and potassium are water-soluble.  They are contained in the water that is in the fuel and could be "washed out." Vanadium, nickel, and lead are oil soluble and cannot be removed economically from fuel.  When all of these metal contaminants are combusted, they form various oxides, sulfates, and eutectic mixtures (two or more materials that melt at lower temperatures than any of the components).  The metals that cause the most problems (V, Na, K, and Pb) form oxides with melting points in the typical operating temperature range of boilers, gas turbines, and diesel engine exhaust systems.  These molten metal oxides deposit on the cooler surfaces forming sticky deposits.  When eutectic mixtures of these metal oxides form they can remain liquid to very low temperatures.  If the metal oxides remain molten they have the possibility of causing corrosion.

       Normally it will be metallic elements that cause deposits.  These deposits result from differences in melting points.  ALL chemical compounds have a melting point; solid materials just tend to have higher melting points.  The exact value of each compound's melting point results from many physico-chemical interactions and will not be explained in this discussion.  It is sufficient to say they exist. Since all the equipment we are interested in operates at high temperatures (much greater than ambient), melting points become important.  As fuel passes through burners and is burned as flames (flame temperatures are often above 2000° C) the metallic elements are converted primarily to oxides.  Due to equipment operating temperatures, most of the oxides of interest will be in a molten state in the hot flame.  After leaving the flame, the operating temperature of the equipment of interest will very likely be above the melting point of the compounds that have been formed during combustion.  When temperatures are higher than a compound's melting point, the metallic oxide will remain molten.  To maximize the effective life of the equipment in the path of the hot flames, the surfaces are normally cooled.  In a boiler this cooling is effected by the water inside the boiler tubes, in a gas turbine by extra air passing through the blade cooling holes, and in a diesel engine by the mass of the engine itself and cooling water.

       When these sticky, molten metal oxides come into contact with the cooler surfaces, they can deposit on these cooler surfaces.  As these sticky materials stay on the surface they are cooled by the surface and will eventually reach their melting point (only from the other side: from hotter to cooler).  When this occurs, they become solids.  This process repeats: hot, sticky molten compounds impact cooler surfaces, themselves cooling, etc.  The end result is deposits grow.  As the deposits grow they can interfere with the proper operation of the equipment.

Hot Corrosions

       The metals discussed above cause hot Corrosion: sodium, vanadium, lead and potassium.  (Other metals may contribute to deposit formation, but only these four metals are considered to be corrosive.)  It is important and critical to be aware that the corrosion mechanism discussed only occurs while these metals are in a liquid or molten state.  The corrosion mechanism does not take place while these metal oxides are solids.

       Alloys used in high temperature applications are carefully selected to be able to withstand the mechanical stresses they are subjected to and also because they develop tightly adherent oxide coatings when exposed to the hot combustion gases under use conditions.  This oxide layer develops to protect the metallurgy from additional oxidation and from attack from some of the corrosive elements in the gases.  For example chromium is alloyed in a metal because it forms very thin, very tight layers of Cr2O3 on the exterior of a metal part.  (This is why many automobile trim parts have been made with a "chrome" coating, they resist corrosion.)  These oxide coatings will also repair themselves if local damage occurs.  The coatings will also reform if stripped off.  These properties are due to natural metal oxide surface protection.

       The problem of high temperature corrosion of metallic parts arises when the hot combustion gases contain materials that can deposit as a liquid on these parts.  The corrosive materials cause destruction of the previously described protective oxide layer. These corrosive materials may destroy the protective oxide layer, disrupt its continuity and adherence, or possibly prevent the formation of a new, tightly adherent layer to replace the old.  Among the elements known to form these corrosive agents in combustion systems are vanadium (V), sodium (Na), potassium (K), and lead (Pb). These metals can react with each other and with oxygen and sulfur in combustion gases to form volatile compounds such as oxides, alkali sulfates, and vanadates when the fuel is burned.  In passing through the system, the hot gases cool, high pressures may fall, and the partial oxygen pressure may rise.  As a result of all of these actions, liquid or solid residues may form on the surfaces of high temperature parts of the system.

       High temperature corrosion is believed to be due primarily to vanadium and sodium contained in the fuel. When fuel containing vanadium is combusted in fired equipment, vanadium combines with oxygen to typically form vanadium pentoxide (V2O5).  Vanadium pentoxide normally melts about 675° C. When sodium in the fuel is combusted, sodium sulfate (Na2SO4, M.P. 880° C) can be formed from the sodium and sulfur also from the fuel.  A lower melting eutectic can be formed when amounts of sodium sulfate are present in the vanadium pentoxide (as low as 300° C for 65% sodium sulfate, 35% vanadium pentoxide).  While in the liquid state vanadium oxides and sodium sulfate can use an "oxygen transport" mechanism to "dissolve" the natural metal oxides that form on the alloy surfaces.  When the metal oxide coating is dissolved by the vanadium oxides, the metal forms a new oxide coating which again dissolves, the metal forms a new oxide coating, etc., etc., in an endless cycle. Each cycle removes a thin layer of the alloy metal.  The overall effect is corrosion. It is also possible for vanadium oxide to penetrate into the grain boundaries of an alloy causing small particles of metal to be removed.  This leads to more rapid metal losses.

Cold - end corrosion

       Sulfur trioxide formed during the combustion of sulfur contained in the fuel condenses with water vapor to form sulfuric acid.  (Typically only about 2 - 5% of the sulfur dioxide that results from combustion of fuel-sulfur is further oxidized to the trioxide.)  This acid then condenses on the cooler equipment surfaces.  Not only is the acid corrosive, it also acts as a trap for ash particles especially in boiler applications.  (In gas turbine applications, system temperatures are typically well above the acid dew point, HOWEVER, when heat recovery equipment is on the back end of a gas turbine, a similar acid condensation can occur.)  The combination of acid and ash particles forms deposits, which further restrict the effectiveness of any heat transfer equipment.  Typical indicators for this problem are black, sticky deposits on heat transfer surfaces, blocked airflow passages, corrosion and/or deposits in stacks, and often a blue-white plume.

Magnesium Use to Solve Chemical Problems in Bunker Fuels

       The problems described above ONLY occur when the big four metals (vanadium, sodium, potassium and lead) are molten.  Magnesium solves these chemical problems by combining with these metals (to varying degrees) to form higher melting chemical compounds.

       The corrosive effects of vanadium have been known for many years. And just as quickly it was found that one metal, magnesium, stood out as the most economical and effective element to combat this corrosiveness.  Magnesium works by combining with vanadium in the flame to produce magnesium orthovanadate (and other compounds). This compound has a melting point well above 1200° C. Since magnesium orthovanadate is not a liquid, it will not be corrosive and it will not form deposits.

       The minimum treating ratio of 3 parts of magnesium to 1 part of vanadium for gas turbines was determined to be correct in the late 1960's to early 1970's.  Initially the treat rate was set at 3.5 to 1 to insure adequate magnesium would be added.  The more appropriate 3:1 was agreed upon as an industry standard since the early 1980's. The actual stoichiometric amount of magnesium required to just react with vanadium to make safe compounds is only about 0.7:1.  However, additional magnesium is added because not only is the desired magnesium orthovanadate formed, but other less desirable magnesium vanadium compounds can also be formed.  To force the reaction to the desired product, more magnesium is required.  Other magnesium products are also formed (magnesium oxide and magnesium sulfate).  More magnesium needs to be added to offset these less desirable compounds.  And finally, since the time allowed for the reaction is very short (high gas velocity in the region of the flame), the greater the amount of magnesium added, the greater are the opportunity for a vanadium atom to find a magnesium atom.

       In diesels and boilers, less magnesium is required to fully control the effects of vanadium.  Most research and empirical use of true oil soluble magnesium proves that suggested levels as low as 1 part of magnesium to 5 parts of vanadium, sodium, potassium, and lead for boilers and closer to 1 part to 1 part for diesel engines is sufficient.  The lower dosage amounts are believed to be possible because of greater amounts of reactive surface area provided by the nano particles in the oil soluble magnesium and temperatures are lower in equipment other than gas turbines and the residence time (time for the reaction to take place) is greater in boilers and diesel engines.  LMGI agrees with these suggestions and treatment rates, however; LMGI additives have provided some customers the opportunity to go to even lower treatment rates.  LMGI will suggest a starting dosage rate and after close examination, the customer may in fact be able to lower the dosage rate to maximize the return on investment without sacrificing results.

       Sulfur trioxide formation and subsequent formation of sulfuric acid can also be controlled with the use of magnesium.  This mechanism is not as well defined as the corrosion mechanism.  It appears that magnesium additives form very finely divided particles of magnesium oxide when they are combusted.  These particles both coat the internal surfaces of high temperature equipment and also are available to neutralize any acid that forms. By coating the internal surfaces, the catalytic effect of hot iron is removed from the sulfur trioxide formation reaction.