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Ash Formation and Behavior in Utility Boilersby: Steven A. Benson, Ph.D.

as printed in

Microbeam

Part 6: Ash Deposit Strength Development and Slag Flow: Low Temperature Process

(2000

oF). The discussion was focused primarily on

flow of a viscous liquid. In this newsletter, I will discuss ash deposit strength

development that results when the vaporized ash components condense and/or react upon

gas cooling. The elements that vaporize consist mainly of sodium, potassium, sulfur,phosphorus, chlorine, and minor amounts of other elements. In the vapor state the

elements can exist in the elemental, oxide, hydroxide, chloride, sulfate, sulfide,

phosphate, and other forms. The form of the vapor phase species depends upontemperature and gas composition. As the temperature of the gases decrease due to the

transfer of heat in the boiler more of the inorganic vapor phases species condense. These

species can condense heterogeneously on the surfaces of ash particles, deposits, and heattransfer surfaces or homogeneously to form submicron aerosols. In addition to

condensation, direct reaction of vapor phase species with solid ash particles can occur

(ie., reaction of SO2/SO3 with CaO). Low melting point and low viscosity liquid phases

are usually produced as a result of the condensation/reaction of these components.

Figure 1 illustrates the type of liquid and other bonding phases present in deposits as a

function of temperature. At lower temperatures, condensed phases dominate, while at

higher temperatures the bulk of the ash particles produce molten phases such as silicates.As discussed in the last newsletter, in low-temperature fouling, the bonding is a result of

the formation of condensed phases including sulfates, chlorides, and phosphates.

Condensed sulfur species, principally in the form of sulfate, are the dominant phases thatform the matrix or bonding material in the low-temperature deposits when firing coals or

biomass containing high levels of alkali and alkaline earth elements.

High temperature fouling occurs in regions of the utility boiler where temperaturesexceed the stability of the sulfate-bearing phases ( >2000

oF). The higher temperature

causes melting and interaction of the particles to form a liquid phase depicted by region

(v) in Figure 1. Once a liquid phase has formed on the outside of the deposit, it becomes

an efficient collector of ash particles, regardless of the individual melting characteristics

of the particles.

Low-temperature ash deposition due to condensed phases occurs at temperatures in the

range of 1000 - 1650oF. In systems which exhibit low temperature fouling, the sulfate

phases dominate the matrix or bonding mechanism between particles. Detailed

examination of low-temperature deposits shows high levels of sodium, potassium and/or

calcium in the form of sulfates, chlorides, and phosphates in the deposits. These phases

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may have a low melting point due to the formation of Na-Ca-sulfate eutectic (very low

melting points) phases. In addition, formation of low-temperature deposits is dependentupon the availability of small calcium oxide particles and the process of sulfation.

Calcium oxide-rich particles in a deposit undergo sulfation through reaction with sulfur

dioxide in the gas stream. This reaction produces calcium sulfate which causes particle-

to-particle bonding and fills in the available pore space in the deposits. This pore fillingproduces very strong, brick-like deposits which are difficult to remove. The low

temperature deposition processes depend upon the abundance of alkali and alkaline earth

elements in the fuel and their ability to react with volatilized sulfur, phosphorus, andchloride species to form liquid phases or solid phases that cause particle to particle

bonding in deposits. The stability of these phases is temperature dependent and most are

not stable in a liquid or solid phase at temperatures above 2000oF in the presence of

silicates.

Figure 1. Type of liquid and other bonding phases present in deposits as a function of

temperature.