Page 246 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 231

Table 12.1— percentage weight by species and higher heating values for fuel types.
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Species Percentage Weight by Species for
Fuel Types
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Natural Gas No. 2 Oil Coal
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Carbon 74.7 87.0 75.1

Hydrogen 23.3 12.5 4.8

Higher Heating (Btu/lb) 22,904 19,520 13,380
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Table 12.2—flue gas losses due to dry gas and moisture.
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Fuel Min. Dry Min. Min.
Gas loss Moisture Stack Gas
Loss (%) Loss (%) Loss (%)

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Natural Gas 2.9 10.1 13.0

No.2 Oil 5.1 6.4 11.5

No.6 Oil 6.6 6.2 12.8

Coal 5.5 10.0 15.5
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an intermediate wall between the hot and cold 40°F the stack temperature is reduced there is a
streams. corresponding 1% efficiency increase. On the wa-
In boiler plants it is more common to hear ter side, an increase of approximately 1% in effi-
the heat recovery apparatus referred to by name, ciency is expected for each 11’F rise in feedwater
(i.e. air preheater, economizer, etc.) than regener- temperature.
ator or recuperator. Economizers have been in use for a long time
and it has been found that boilers operating at
pressures of 75 psig or greater are excellent appli-
CONVENTIONAL ECONOMIzER cations. One of the strongest points for installing
an economizer is its compact size compared to
An economizer performs two functions, it other options.
reduces stack temperature and also heats boiler Some general guidelines for economizer in-
feed water (Figure 12.5). stallations are:
A practical rule of thumb is that for every