Condensing Economizers

Condensing Economizers

As discussed earlier, one may permit condensation of acid vapor as well as water vapor on the heating surface and take advantage of the tremendous energy available in the flue gas in the form of latent heat of water vapor by condensing the flue gas to below water dew point. This is about 55°C for flue gases from combustion of natural gas, which has about 18% volume of water vapor. Boiler efficiency can approach 100% if exit gas temperature is reduced to values close to 55°C.

There would be significant benefits to cooling the flue gas to temperatures below water vapor and acid dew points, provided the acid corrosion problems can be overcome in a cost-effective way. With stack temperatures below the water vapor dew point, condensed water vapor would provide a source of water for use in power plant cooling; recovered latent and sensible heat from the flue gas improves the plant efficiency significantly. Steam for deaeration can be reduced if the condensate water is preheated in a condensing econo- mizer. Enormous environmental benefits will also be available as the amount of flue gas to be handled in the cleanup equipment is lesser along with a lower temperature (volume)

and the cost of removing CO 2 at the back end of the boiler will be lower due to the lower vol- ume of flue gases to be handled. Fan size and cost used in pollution control system would

be smaller. However, while estimating the cost of the system, one should also consider the cost of handling the dilute or concentrated liquid condensate from condensation of acid vapor and the treatment costs to meet local pollution control regulations. Sometimes, the cost of meeting the handling cost may outweigh the benefit of energy recovery. It has to be decided on a case-to-case basis.

Consider the case where the flue gas contains both the sulfuric acid vapor and water vapor. As flue gas is reduced in temperature below the sulfuric acid dew point, the acid first condenses as a highly concentrated liquid solution of sulfuric acid and water. If more heating surfaces are provided at lower than acid dew point temperature to, say, below the water dew point, more water is condensed, and the liquid mixture of water and sulfuric acid, which forms on low-temperature surfaces, is approximately a few orders of magnitude more dilute in sulfuric acid than the highly concentrated acid solu- tions, which form at temperatures above the water vapor dew point temperature but below the sulfuric acid dew point temperature. Economizer and air heater are prone to acid corrosion, and suppliers of these suggest minimum fluid temperature to avoid corrosion of back-end heating surfaces.

According to literature survey, 304 stainless steel is the best candidate for heat exchangers that operate at temperatures below the water vapor dew point temperature

310 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

FIGURE 6.1

Arrangement of a condensing economizer. (Courtesy of Condensing Heat Exchanger Corp., Timmins, Ontario, Canada.)

(handling dilute acid), while Teflon and Alloy 22 for heat exchangers that operate at tem- peratures above the water vapor dew point temperature but below the sulfuric acid dew point temperature (handling concentrated acid). The cost of Alloy 22 is about 12 times that of 304 stainless steel. Alloy 22 (containing 20%–22.5% chromium, 12.5%–14.5% Mo) is the preferred alloy for the high acid concentration due to its low corrosion rate, avail- ability, and ability to be readily fabricated. The major attribute of Inconel Alloy 22 is outstanding resistance to a broad range of corrosive media. It resists oxidizing acids as well as reducing acids such as sulfuric and hydrochloric. Figure 6.1 shows the arrange- ment of a condensing economizer in a plant.