COMBUSTION CATALYSTS

Combustion catalysts have been used for all types of fuels. The combustion catalyst functions by increasing the rate of oxidation of the fuel. Some fuels are difficult to burn within a given fixed furnace volume. Combustion catalysts are applied to these fuels to comply with particulate and opacity regulations. Combustion catalysts are also used to improve boiler efficiency by reducing carbon loss in the flue gas.

FUEL ADDITIVES

Most soluble fuel additives contain metallo-organic complexes such as sulfonates, carbonyls, and naphthenates. These additives are in a very convenient form for feeding. Most dry fuel additive preparations are used to treat fuels with high ash content, such as coal, bark, or black liquor. Metal oxides are used for this purpose.

Cold-end corrosion can occur on surfaces that are lower in temperature than the dew point of the flue gas to which they are exposed. Air heaters and economizers are particularly susceptible to corrosive attack. Other cold-end components, such as the induced draft fan, breeching, and stack, are less frequently problem areas. The accumulation of corrosion products often results in a loss of boiler efficiency and, occasionally, reduced capacity due to flow restriction caused by excessive deposits on heat transfer equipment.

Acidic particle emission, commonly termed "acid smut" or "acid fallout," is another cold-end problem. It is caused by the production of large particulates (generally greater than 100 mesh) that issue from the stack and, due to their relatively large size, settle close to the stack. Usually, these particulates have a high concentration of condensed acid; therefore, they cause corrosion if they settle on metal surfaces.

The most common cause of cold-end problems is the condensation of sulfuric acid. This chapter addresses problems incurred in the firing of sulfur-containing fuels. Sulfur in the fuel is oxidized to sulfur dioxide:

A fraction of the sulfur dioxide, sometimes as high as 10%, is oxidized to sulfur trioxide. Sulfur trioxide combines with water to form sulfuric acid at temperatures at or below the dew point of the flue gas. In a boiler, most of the sulfur trioxide reaching the cold end is formed according to the following equation:

The amount of sulfur trioxide produced in any given situation is influenced by many variables, including excess air level, concentration of sulfur dioxide, temperature, gas residence time, and the presence of catalysts. Vanadium pentoxide (V2O5) and ferric oxide (Fe2O3), which are commonly found on the surfaces of oil-fired boilers, are ef-fective catalysts for the heterogeneous oxidation of sulfur dioxide. Catalytic effects are influenced by the amount of surface area of catalyst exposed to the flue gas. Therefore, boiler cleanliness, a reflection of the amount of catalyst present, affects the amount of sulfur trioxide formed.

The quantity of sulfur trioxide in combustion gas can be determined fairly easily. The most commonly used measuring techniques involve either condensation of sulfur trioxide or adsorption in isopropyl alcohol. Figure 22-1 is a curve showing the relationship of sulfur trioxide concentration to dew point at a flue gas moisture content of 10%. Higher flue gas moisture increases the dew point temperature for a given sulfur trioxide-sulfuric acid concentration. Cold-end metal temperatures and flue gas sulfur trioxide content can be used to predict the potential for corrosion problems.

At the same sulfur content, gaseous fuels such as sour natural gas, sour refinery gas, and coke oven gas produce more severe problems than fuel oil. These gases contain more hydrogen than fuel oil, and their combustion results in higher flue gas moisture. Consequently, dew points are raised. With any type of fuel, corrosion and fouling potentials rapidly increase below gas temperatures of 140 degrees F (60 degrees C), which is the typical water dew point for flue gases.

Cold-end corrosion and deposition are usually much less severe in coal-fired boilers than in oil-fired units. Usually, coal ash is alkaline, so it increases the pH of the deposits formed in cold-end sections. Thus, the extent of the corrosive attack by sulfuric acid is diminished. Also, the high level of ash present when coal is fired results in a lower concentration of acid in the ash particle. At the same sulfur content, coal firing dew points are generally 20-40 degrees F lower than oil firing dew points.

The most common cause of deposition within air preheaters is the accumulation of corrosion products. Most air preheater deposits contain at least 60% iron sulfates formed by the corrosion of air heater tube metal. Therefore, a reduced corrosion rate frequently reduces the fouling of air preheaters.