Environmental Engineering Reference
In-Depth Information
2.5.2 Ultimate Analysis
The ultimate (or elemental) analysis gives the chemical composition and the HHV of
the fuel. This chemical analysis usually lists the carbon, hydrogen, oxygen (by differ-
ence), nitrogen, sulfur, and ash content of the dry fuel on a weight percentage basis.
The biomass sample is combusted in a combustion chamber using an O
2
atmosphere
with helium (He) as carrier gas. The product gases of combustion are CO
2
,H
2
O, NO,
NO
2
,SO
2
,SO
3
, and N
2
.SO
3
, NO, and NO
2
are reduced in a copper-containing
section downstream the combustion compartment of the device, which ensures con-
version to SO
2
and N
2
, respectively. H
2
O, SO
2
, and CO
2
are captured in different
adsorption columns. N
2
is not captured by the adsorption columns and is detected
by a thermal conductivity detector (TCD). Subsequently, H
2
O, SO
2
, and CO
2
are
released and introduced into the TCD. The weight percentage is determined integrally.
The C, H, N, and S contents can then be determined from the known sample weight;
O is determined from the difference between daf weights and the weights of the main
other elements that have already been determined. The method has been standardized
(see ASTM D3177-84 (
tinyurl.com/mjqmw5a
)).
The ultimate analyses of a number of biomass fuels and other solid fuels are given in
Table2.4.Note that biomass typicallyhas very lownitrogenandsulfur contents relative to
fossil fuels (especially coal). Also, the ash content of biomass is typicallymuch less than
that of coals, although some forms (e.g., sewage sludge) have a high ashcontent. Depend-
ing on the exact composition and quantity, this can lead to melting of the ash (known as
“
slagging
”
), which can cause severe problems in high-temperature process equipment.
Example 2.1 Calculation of the amount of air needed for the complete
combustion of wood
Wood pellets with a composition given in Table 2.3 (first entry) are to be com-
busted with dry air with 79 vol.% of N
2
and 21 vol.% O
2
(idealized composition).
Assume that the fuel is completely daf:
kg
−1
fuel) is needed to just completely, thus stoichio-
metrically, combust this fuel?
Given: MW
C
= 12.011 kg
a. How much air (in kg
kmol
−1
,MW
H
= 1.00797 kg
kmol
−1
,
kmol
−1
.
b. What is the volume percentage of CO
2
in the flue gas produced?
c. A CO
2
analyzer needs completely dry flue gas, so the gas is cooled before
entering the analyzer. What is the expected volume percentage the analyzer
quantifies in the cooled flue gas?
kmol
−1
, and MW
O
= 15.9994 kg
MW
N
= 14.0067 kg
Solution
a. The complete combustion reaction can be written as (the factor 3.76 is the
molar ratio between N
2
and O
2
; air can be considered an ideal gas)
xCO
2
+
1
C
x
H
y
O
z
+a O
2
+3
ð
:
76N
2
Þ!
2
yH
2
O+3
:
76a N
2
ð
RX
:
2
:
1
Þ
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