Civil Engineering Reference
In-Depth Information
2. Moisture loss due to the water contained in the fuel is equal to the kilograms of
water per kilogram of fuel multiplied by the enthalpy difference between the
water vapor mixture in the stack exit gas and water at ambient temperature;
3. Moisture loss due to the combustion of the hydrogen in the fuel equals the
weight fraction of hydrogen in the fuel multiplied by 9 (as stated in Sect.
6.1
where it is pointed out that the combustion of 1 kg of hydrogen produces 9 kg of
liquid water) multiplied by the enthalpy difference between the water vapor
mixture in the stack and liquid water at ambient temperature. This loss is
considered only if the Higher Heating Value is assumed as heat input in the fuel.
The losses due to incomplete combustion are totally attributed to CO in stack
gases and to the combustible in the refuse. In the latter case they are evaluated as the
kilograms of dry refuse per kilogram of fuel multiplied by the heating value of the
refuse determined by laboratory tests.
The radiation losses are estimated by reference to the above-mentioned ABMA
charts or equivalent charts.
Additional losses, generally ranging between 0.5 and 1.5 %, are introduced to
take into account losses neglected in the indirect method computation.
Graphic solutions were developed by using the above-mentioned or similar
procedures in order to estimate stack gas losses (dry flue gas losses; all the moisture
losses together) and losses due to incomplete combustion.
The heat-loss method requires the measurement of stack flue gas parameters
(temperature; O
2
or CO
2
and CO concentrations as % by volume or as ppm) and of
combustion air temperature.
The measurement of O
2
is generally preferred to that of CO
2
because simpler
instrumentation achieves the same accuracy. Portable analyzers are available for
both O
2
and CO
2
measurements.
Carbon monoxide is generally measured by means of handheld chemical absorb-
ing analyzers (Orsat or similar analyzers) and length of stain detectors.
Stack opacity or smoke density is assumed as an index of the combustion
conditions. It can be measured by means of hand pump filter paper or a similar
tester where the color assumed by the paper is compared to a standard scale.
Generally, index 0 means no opacity. Optimum values range between 0 and
3 (Bacharach index).
The relationships between excess air and stack gas concentration of O
2
and CO
2
for different fuels are shown in Fig.
6.7
.
Figure
6.8
shows a typical relationship between total stack gas losses and stack
temperature for different values of stack excess O
2
and for a specific fuel (natural
gas in this case). Notice that stack temperature increases with the excess of O
2
because complete combustion is achieved. Similar curves are available for different
fuels. Figure
6.9
shows the relationship between losses due to unburned
combustibles and stack excess O
2
for different values of CO emissions. Sets of
values referring to both Lower and Higher Heating Values are reported.
Other methods, based on the evaluation of total stack losses and unburned losses
through curves and standard coefficients are widely used:
