Boilers economics

Boiler Economizers. Heat exchangers that use boiler flue gases to preheat the boiler feedwater are termed boiler economisers. 

With shell boilers, economizers will generally only be fitted to boilers using natural gas as the main fuel and then only on larger units. It would be unlikely that a reasonable economic case could be made for boilers of less than 4000kg/hF and A100°C evaporative capacity. The economizer will incorporate a flue gas bypass with isolating dampers to cover for periods when oil is used and for maintenance. The dampers require electric interlocks to the selected fuel. 

With watertube boilers economizers are normally used, therefore the situation of cold feed to the boiler would not apply. However, the feed to the economizer should be treated similarly to that to a boiler to reduce thermal shock. 

There are various WT and FT boiler economizer designs, classified as either steaming economizer and nonsteaming economizer types according to thermal performance. These economizers are constructed in either bare tube or finned tube (extended surface) patterns. They may be positioned horizontally or vertically within the boiler system, in either cross-flow or counterflow arrangements. 

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During MSW combustion in a modern mass burn combustor with reciprocating grates, 11 of MSW is converted to heat gases such as C02, NOx, SOx, and H20 and about 350 kg of ash residuals partitioned into the various ash streams. For every tonne of MSW combusted, about 5 kg of grate siftings, 295 kg of grate ash, 5 kg of boiler/economizer ash, 20 kg of ESP ash, and 12 kg of dry scrubber solids are produced (Chandler et al. 1997). 

Catalyst bed input gas temperatures are readily adjusted by heating and cooling the gas in boilers, economizers, heat exchangers etc. Industrial input temperatures vary from -660 K to 720 K, Table 7.2. 

Note The static pressure is normally used in most fan-selection procedures because this is the pressure value used in computing pressure and draft losses in boilers, economizers, air heaters, and ducts. In any fan system, the total air pressure equals static pressure + velocity pressure. However, the velocity pressure at the fan discharge is not considered in draft calculations unless there are factors requiring its evaluation. These requirements are generally related to pressure losses in the fan-control devices. 

Ash that collects at the bottom of the boiler is called bottom ash and/or slag. Fly ash is a finer ash material that is borne by the flue gas from the furnace to the end of the boiler. Bottom ashes are collected and discharged from the boiler, economizer, air heaters, electrostatic precipitator, and fabric filters. 

Economizer sorbent injection is identical to furnace sorbent injection except for the injection point location. With economizer sorbent injection, hydrated lime is injected into the boiler economizer inlet. An optimum temperature range (900° to 1,0(X)°F) for SO2 reaction occurs near this location and 50% SO2 removal efficiency has been achieved. The process is currently undergoing pilot plant testing (Keeth et al., 1991B). 

The boiler feed water is condensed steam after electricity generation that has been conditioned in a deaerator. The boiler feed water is preheated in economizers prior to entering the heat recovery boiler. Economizers are heat exchangers that transfer heat to boiler feed water while cooling gases entering intermediate and final absorption towers (Chapter 9). 

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