Page 245 - Boiler plant and distribution system optimization manual

230 Boiler Plant and Distribution System Optimization Manual
WHAT IS FLUE GAS?
Before we go on, let’s explore some basics in
the field of waste heat recovery. The first applies
to the nature of flue gasses which contains both
dry gas and moisture. The amount of moisture in
flue gasses varies with the amount of hydrogen
in the fuel, each pound of hydrogen in the fuel
combines with oxygen to form approximately 9
pounds of water (Table 12.1). This water is in the
superheated state containing more than half the
energy in the flue gas stream. As each fuel has a
different carbon to hydrogen ratio the energy in
the moisture in the flue gas stream will vary from
fuel to fuel.
CARBON AS FUEL
The dry gas in the flue gas is formed from
the combustion of carbon to carbon dioxide plus
nitrogen which does not take part in the combus-
tion process and any excess air above and beyond
the amount needed for combustion.
DRY GAS AND MOISTURE LOSSES
The carbon and hydrogen composition of
various fuels determines the dry gas and mois-
ture losses. Table 12.2 shows how this affects flue
gas losses and the minimum stack losses that can
be expected.
SENSIBLE AND LATENT HEAT
Sensible heat is that heat which can be sensed
with a thermometer or other temperature sensing
instrument.
The formula for sensible heat transfer is:
Q = M C (T – T )
1
2
p
Q = Heat content Btu/hr
M = Flow rate LB/HR
C = Specific heat BTU/LB/F
p
T = Stream temperature
1
T = Reference temperature
2
Latent heat is the heat required for phase
change, i.e. to change a liquid to a vapor (water
to steam) or vice versa, without a change in tem-
perature.
The formula for latent heat transfer is:
Q=M H
Q = Heat content Btu/hr
1
M = Flow rate lb/hr
H = Heat of vaporization Btu/hr
FUEL
Combustion products from burning fuels
with higher hydrogen content contain more wa-
ter vapor and larger amounts of latent heat loss
potential. Gas fired boilers are inherently less effi-
cient than heavy oil fired units and represent bet-
ter candidates for heat recovery. The type of fuel
will also affect the maintainability and service life
of a heat recovery system.
Natural gas is a clean-burning fuel and caus-
es minimal corrosion problems in heat recovery
hardware.
Fuel oil contains varying amounts of sulfur,
which leads to acid corrosion problems.
REGENERATORS AND RECOUPERATORS
Different terminologies have developed over
the years in different industries referring to heat
recovery process. The term regenerators has come
to refer to the alternate heating and cooling of a
media, such as plates or brickwork or other heat
absorbing material with hot exhaust gasses, and
then recapturing the heat by warming combus-
tion or process air over the same media by ma-
nipulating gas and air streams.
Recuperators refers to the continuous op-
erating (static) type or heat recovery unit using