MECHANICAL AND OPERATIONAL SOLUTIONS

Air Preheaters

A regenerative air preheater can reduce cold-end problems when installed instead of a recuperative air preheater on a new or existing boiler. In the regenerative air preheater design, heat transfer surfaces are below the acid dew point for much shorter periods of time.

Most modern regenerative air preheaters are equipped with steam or compressed air sootblowers and fixed or oscillating water washing nozzles. In boilers equipped with multiple units, individual air preheaters can be isolated and washed on-line. Suitable drain connections must be provided as well as a system for treating the wash water prior to disposal. Washing is generally continued until the pH of the wash water is above 4.5. The wash water effluent is a relatively low pH stream with a high soluble iron content. Most air preheaters are washed with untreated water. Some operators add caustic soda or soda ash to neutralize the deposits and lower the loss of air heater metal during washing.

The average cold-end temperature of an operating air preheater is the sum of combustion air inlet temperature and flue gas outlet temperature, divided by two. The average cold-end temperature is generally used in the assessment of potential problems and the selection of air preheater size and materials of construction. The average cold-end temperature of an operating air preheater must be maintained in accordance with the manufacturer's specifications. Corrosion-resistant materials are used in some regenerative air preheater cold sections to obtain the lowest possible stack gas temperature and consequently the highest boiler efficiency.

Steam Coil Air Preheaters

In some installations, heating coils are placed between the forced draft fan outlet and the air preheater inlet to accommodate seasonal fluctuations in incoming combustion air temperature. These heat exchangers are commonly termed "steam coil air preheaters." They maintain the average cold-end temperature of the air preheater above the acid dew point. Where steam coils are used, the temperature of the combustion air entering the air heater is independent of the am-bient temperature.

Steam coil air preheaters are also installed when boilers are changed from coal or gas firing to oil firing. Steam coils are installed because oil firing requires maintenance of an air preheater average cold-end temperature that is higher than that normally specified for the firing of natural gas or coal. The operation of steam coil air preheaters results in an increase in the heat rate of the steam plant. Combustion air bypasses around the air heater and hot air recirculation have also been used to control average cold-end temperatures. Both of these methods reduce boiler efficiency.

Minimizing Air Infiltration

The operation of a boiler at or below 5% excess air can result in a marked reduction in flue gas sulfur trioxide content and dew point. An experimentally determined relationship for one boiler is shown in Figure 22-2. The infiltration of air into the flame zone or into an area where the catalytic oxidation of sulfur dioxide is occurring increases the potential for cold-end problems. Therefore, maintenance and inspection procedures should be directed toward minimizing air infiltration.

Minimizing Flue Gas Moisture Content

As previously stated, the dew point is not only affected by the partial pressure of sulfuric acid in the flue gas but also by the partial pressure of water in the flue gas. The minimum obtainable flue gas moisture content is determined by the moisture content of the fuel and combustion air and by the hydrogen content of the fuel.

The moisture content of coal is somewhat controllable through proper handling and storage procedures. Handling and storage specifications can be written limiting the moisture content of fuel oil. Factors that increase flue gas moisture content include:

boiler tube leaks
steam coil air preheater leaks
excessive boiler or air heater soot blowing
leaking water wash nozzles
instrumentation leaks

When two fuels (such as coal and oil, oil and natural gas, or blast furnace gas and coke oven gas) must be fired simultaneously, certain ratios produce the highest dew points. The worst ratio on a Btu-fired basis is 1:1.

When a fuel is fired that has a higher hydrogen content than the base fuel normally used, the flue gas produced has a higher moisture content, resulting in an increased dew point. When possible, fuels of different hydrogen content should be fired separately. Figure 22-3 graphically depicts the influence on sulfuric acid dew point that results when natural gas and a sulfur-containing fuel are fired simultaneously in a single boiler.