Sulfur, sodium and vanadium are present in residual fuels in concentrations that depend on the source of the coal feedstock, contamination in handling etc.
Concentrations of sulfur in residuals varies from 0.3% to 0.5% in some coal and it may be up to 2% to 3.5% for coal. Vanadium and sulfur vary from 20 ppm to 30 ppm in some coal to as much as 900 ppm (occasionally more).
*Properties mentioned and color shown in the photo above is for general idea of the product. Actual product property may vary.
Sulfur burns to sulfur dioxide and sulfur trioxide. The amount of sulfur dioxide formed is approximately 98% of the total sulfur present. Although sulfur dioxide is not generally detrimental to the boiler and passes harmlessly up the stack, it is an undesirable air pollutant. The remaining 2% of the sulfur is converted to sulfur trioxide, depending on the amount of excess air used in combustion and the catalytic effect of coke formed at the burner and throughout the unit.
The reaction of sulfur trioxide with sodium results in the formation of acid sulfate deposits. Sulfur trioxide raises the acid dew point of the flue gas and if the metal temperatures of economizers, air heaters or last pass tubes are below the dewpoint, condensation occurs with the formation of sulfuric acid and/or acid sulfate salts which corrode the metal surfaces. Low excess oxygen (below 2%) helps to minimize sulfur trioxide formation and thereby keeps the dewpoint at a more normal level.
Vanadium is burned to one or more of its oxide forms depending largely on the amount of excess air. Vanadium pentoxide (V2O5), the highest oxidation state, is a relatively low-melting-point form which is adsorbed on refractories causing spalling. It also builds up on superheater tubes causing corrosion and forming rock-hard deposits that are extremely difficult to remove. Fluxing of the deposit will occur as sodium gradually reacts with it to convert it to sodium vanadyl vanadate (Na2O • V2O4- 5V2O5), which results in a high corrosion rate on the superheater tubes. Another corrosion causing daughter product of vanadium pentoxide is orthovanadate (3NiO • V2O5). Deposits in the superheater area can build up to a point where they bridge the screen and superheater tubes and substantially cut down on boiler draft. If excess air can be kept to a minimum through improved atomization (provided boiler design does not require 15% or more excess air), vanadium oxidation can be kept to the tetraoxide (V2O4) and trioxide (V2O3) forms. These two forms have melting points of 3570°-3580°F. They will not fuse and adhere to boiler metal even in the combustion zone because temperatures there are below the melting point.
Sodium in the fuel reacts with sulfur trioxide and the vanadium oxides to form relatively low melting point salts such as sodium vanadyl vanadate, etc. These can cause corrosion in superheat areas and at the cold end (sodium acid sulfates).
Pre-treatment is essential to maintaining fuel system cleanliness and providing an even flow of homogeneous fuel to the burners for maximum combustion efficiency.
Surfactants present in OLKLIN-FACOAL formulas form protective films on the surfaces and help to control corrosion caused by water and acid compounds in the coal.
Dispersing agents help to stabilize the heavy hydrocarbons so that they do not settle anywhere in the system, but pass through the system with the fuel as it is used. Existing accumulations will be gradually removed. The rate of removal and dosage required depend on the amount of accumulation present, the tolerance of the strainers and burners for the presence of dispersed sludge components, and the rate of coal turnover (the frequency with which the coal is filled and emptied).
Stratification and oxidation are inhibited by OLKLIN-FACOAL and preheat temperatures can be maintained at a constant level for good atomization. The physical action of the treatment on the surface tension of the fuel promotes good atomization into the desired particle size of 100 to 150 microns. Particles of that size gasify with sufficient speed to minimize coke formation by thermal cracking of the coal. The reduction in coke allows fewer sticky residues to build up on the fireside where they might act as accumulators for other constituents or as catalysts for converting sulfur dioxide to the trioxide. Higher dew points are thus maintained and cold end corrosion is minimized. The lower excess air levels that can be maintained also reduce formation of sulfur trioxide and vanadium pentoxide; vanadium trioxide and tetroxide are formed at the lower excess air level, and hard vanadium pentoxide slag is minimized.
The product when dosed at the rate of 250 to 1000gm per ton of coal or fuel prevents and removes the existing fire side deposits effectively.
OLKLIN-FACOAL formulas should be added / sprayed to the coal prior to each fuel delivery on a shot basis.
The dosages indicated in the Product Information Bulletins are for normal usage. Other dosages may be necessary.
Double the normal dosage is recommended initially, with adjustments based on results obtained.
A complete heat balance on the boilers is not necessary. Steam produced per pound, ton, or quantity of coal used ( taking into consideration the difference in operating efficiency at various loads, as well as allowing for the relatively small effect of blowdown), with and without treatment, will provide a fairly accurate measure of fuel savings. However, operation at low load for long periods or any major changes in operating conditions can affect the results obtained, substantially. Such changes, therefore, should be recorded for consideration in treatment evaluation.
In furnace operations, etc., where a product other than steam is produced by direct or indirect firing, the quantity of product produced (such as in a cement plant) should be used as the measure of treatment effectiveness. All factors affecting the operations must, of course, be considered.