TREATMENT FOR FOULING

Additives are used to control fouling by elevating the melting point of the deposits, by physically diluting deposits, or by providing a shear plane to assist in removal by soot blowing. Additives used to control fouling contain magnesium, silica, manganese, and/or aluminum. Figure 21-4 illustrates the effect of treatment.

The melting point of untreated ash constituents can be as low as 1000°F. The introduction of appropriate metal oxides elevates the melting point of ash components by several hundred degrees. The additive components most commonly used to raise the deposit melting point are magnesium and/or aluminum. Dosages depend on ash levels in the fuel and the ratio of various ash components.

Fuel additives are intended to control fouling by forming a friable, nonadherent deposition that can be removed by soot blowing.
When melting temperatures are raised, the physical characteristics of the deposits are altered. Often, the heaviest deposition occurs in areas where the gas temperature is lower than the melting temperature of a deposit. Therefore, a treatment program designed solely to elevate the melting point of the deposit will not solve the problem, and it is necessary to introduce additive components that change the physical characteristics of the deposit, making it more friable. Additives used for this purpose contain compounds of magnesium or aluminum. Aluminum is usually the most effective material for increasing friability of deposits.
Newer technology has been developed that offers certain advantages over magnesium or magnesium/aluminum combination treatments. Silicon-based materials have been used to control fouling and form friable nonadherent oxides. The silicon component acts like a sponge to adsorb low melting point oxides, preventing their agglomeration and subsequent deposition. The silicon-based materials contribute far fewer solids than traditional treatment. This reduces fouling and minimizes loading on the solids collection system.

Additive dosage should be fixed at the minimum level necessary for proper conditioning of depo-sits. Overfeeding of additives, particularly magnesium oxide, can cause troublesome deposition. Magnesium oxide can react with sulfur trioxide in the flue gas to form magnesium sulfate deposits. These deposits generally form in convection areas, such as primary superheaters, steam generating sections, and economizers.

Boiler design has proven to be a very important consideration in the determination of magnesium additive levels. The addition of aluminum to magnesium-based additives reduces magnesium sulfate deposition.

The greatest need for fuel additives to control deposition is in boilers previously used to fire other fuels, such as natural gas or coal. Generally, units used to fire coal have sufficient soot blower placement for adequate removal of properly conditioned deposits.

Natural gas-burning units generally do not have the desired soot blowing equipment. It is advisable to contact the manufacturer so that the proper number of soot blowers and their location can be specified. It is virtually impossible to burn a fuel containing greater than 0.2% ash without the use of soot blowers, even with the aid of a conditioning additive.

Boilers designed for oil firing incorporate design parameters that allow for relatively trouble-free operation from a fireside standpoint. Combustion gas velocity, tube spacing, furnace gas exit temperature, and economizer configuration are adjusted to account for oil ash characteristics. The most common problems associated with oil-design boilers are air preheater fouling and corrosion. Deposition and corrosion can occur when the oil used is of lesser quality than specified for the boiler, or if multiple oils are used.

HIGH-TEMPERATURE FIRESIDE CORROSION

Fuel ash corrosion in high-temperature areas can cause extensive boiler damage. The corrosion is caused principally by complex oxide-slags with low melting points. Corrosion by slag components, such as sodium vanadyl vanadate, progresses rapidly between 1100 and 1650°F.

Sodium sulfate is also a primary corroding medium that can be present with sodium vanadyl vanadate. It also is the primary corroding material of gas turbine blades. High-temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct contact with the metal. The liquid-phase deposits corrode by fluxing of the protective oxide layer on the metal surface as shown in Figure 21-1. Deposits also promote the transport of oxygen to the metal surface. Corrosion is caused by the combination of oxide layer fluxing and continuous oxidation by transported oxygen.

Tube spacers, used in some boilers, rapidly deteriorate when oil of greater than 0.02% ash content is fired. Because these spacers are not cooled and are near flue gas temperatures, they are in a liquid ash environment. Fuel oil additives can greatly prolong the life of these components.

High-temperature corrosion caused by sulfidation is a common problem in gas turbines. The corroding medium is sodium sulfate. Sodium enters the gas turbine either with the fuel or combustion air. The sodium content of the fuel can be lowered by means of water washing. It is relatively difficult to remove all sodium-containing mists or particulates at the intake of the gas turbine. Due to salt water mist ingestion, blade sulfidation problems are common in gas turbines used for marine propulsion.

Reheating furnaces in the iron and steel industry have suffered wastage of metallic recuperators. The problem is more prominent where steel mills have converted reheat furnaces from natural gas to residual oil firing. The gas inlet temperature at the recuperator is high enough to provide an environment for high-temperature corrosion.

Firing of residual oil in refinery process heaters has caused numerous corrosion failures of convection tube support members. The tube supports are generally not cooled by water or air and, therefore, are at a temperature close to that of the gas. In some cases, tube supports fabricated from nickel and chromium alloys have been installed to alleviate this problem.

Additives containing magnesium and aluminum oxides have been successful in controlling these and other high-temperature corrosion problems. The additives function by elevating the melting point of the deposits. Enough magnesium oxide must be added to enable the metallic constituents to remain in the solid phase as they contact the metal surface.

In conjunction with an additive program to control high-temperature corrosion, it is advisable to operate the unit at minimum excess air levels. The corrosion rate is influenced by the oxygen content of the combustion gas. The second phase of the corrosion mechanism is oxidation; therefore, the partial pressure of oxygen in the combustion gas can be lowered to reduce corrosion rates.
In residual oil-fired equipment, it is often necessary to feed a 1:1 ratio of magnesium and vanadium to achieve low corrosion rates. This level of feed is generally higher than that which is necessary to control fouling.