SOURCE AND NATURE OF FIRESIDE DEPOSITION

Fireside fouling of combustion equipment is caused by the deposition of fuel ash components. Table 21-1 shows analyses of typical high, medium, and low ash liquid fuels. Oils containing more than 0.05% ash are considered high ash oils; those containing less than 0.02% ash are considered low ash oils.

Table 21-1. Typical residual oil analyses.

Sulfur emission regulations have severely restricted the use of high sulfur oils. Generally, high sulfur oils (greater than 1.0% sulfur) have high ash contents. These oils are usually imported from the Caribbean area. Prior to 1972, most East Coast boilers were burning high sulfur, high ash oils.

The combustion of high ash oils produced troublesome deposits on boiler convection surfaces such as steam generating, superheat, reheat, and economizer sections. The firing of high ash oils (even in those units which were originally designed to burn coal) produced convection surface deposition that was difficult to remove by soot blowing.

The most troublesome oil ashes are those which contain a vanadium/sodium ratio of less than 10:1. In Table 21-1, the fuel with the medium ash content has a relatively low vanadium/sodium ratio and produced an extremely tenacious deposit on superheater surfaces (as shown in Figure 21-1).

Vanadium reacts with oxygen to form various oxides in the furnace. When formed, vanadium pentoxide condenses within the furnace when gas temperatures approach its solidification point.

Sodium also reacts with oxygen and sulfur trioxide to form fouling compounds. Thermo-dynamics favor the formation of sodium sulfate rather than the relatively unstable sodium oxide.

Nickel can also contribute to deposition by forming oxides. Aluminum can be present in the oil in the form of cracking catalyst fines (aluminum that is introduced during the refining of oil). Although it is generally not a troublesome component, iron is occasionally present in fuels at relatively low levels. When waste fuels are burned, some contaminants (such as lead) can cause extremely dense and tenacious deposits.

Black liquor has been used as a boiler fuel for many years in the Kraft paper industry. It contains a significant amount of combustible material, along with sodium salts. Black liquor burning can produce relatively adherent, low bulk density sodium sulfate deposits on recovery boiler convective surfaces. In some instances, it is advisable and economically beneficial to control or limit deposition with magnesium-based additives. These materials are blended compounds which contain magnesium oxide and/or aluminum oxide.

The combustion of coal tar in boilers often produces objectionable amounts of sodium salts and/or iron compounds on boiler convective surfaces. Often, ash deposits are similar in composition to coal ash.

Combustion of solid fuels such as coal and bark (also referred to as "hog fuel") can also lead to fireside slagging. Sodium, calcium, silica, iron, and sulfur content are of primary concern in the burning of solid fuels. Other metal oxides, such as alumina, titania, and potassium oxide, can also aggravate slagging. Figure 21-2 shows a heavy accumulation of slag in a coal-fired boiler.

In addition to proper boiler furnace design, operational considerations should be explored to prevent fireside slagging. Ash fusion testing has been used by boiler manufacturers to assess slagging potential (see Figure 21-3). Where design and operational improvements are impractical or insufficient, chemical treatments (such as combustion catalysts and antifoulants) should be considered.