Environmental Engineering Reference
In-Depth Information
Solution
The elemental composition data in the table are on a daf basis; thus, first convert
the main elemental composition to a db:
Y i , daf
Y i , db =1
Y ash , db
The ash content from the proximate analysis is given on an
ar
basis, but we
have to calculate it on a db:
Y ash , ar =1
Y ash , db =1
=
1
Y moisture , ar
½
=
ð
1
0
:
065
Þ
0
:
135 = 0
:
144
Then,
Y C , db =1
ð
0
:
144
Þ
0
:
423 = 0
:
362; Y H , db =1
ð
0
:
144
Þ
0
:
05 = 0
:
043;
Y S , db =1
ð
0
:
144
Þ
0
:
006 = 0
:
005; Y N , db =1
ð
0
:
144
Þ
0
:
031 = 0
:
027;
Y O , db =1
ð
0
:
144
Þ
0
:
489 = 0
:
419
kg −1
Substituting the calculated values in Equation (2.8) gives HHV = 13.1 MJ
(db), a relative difference of approximately
23%.
Exercise : repeat this calculation for other wood types.
Note : This outcome indicates that care should be taken for using empirical correla-
tions for
biomass types. In this case, the biomass contained a high
amount of silica-rich (soil derived, adherent) ash.
deviating
2.7 ASH CHARACTERIZATION TECHNIQUES
Despite the comparatively low content of mineral matter in biomass, it is often the
cause of diverse ash-related challenges that operators face in the (thermal) conversion
of the fuel. Although in the case of firing a single fuel it is possible to predict ash
behavior with relatively low effort, it becomes more difficult to predict ash behavior
in the case of firing multiple fuels in proportions that vary in time due to, e.g., seasonal
changes or extreme heterogeneity (Miller and Miller, 2007). Blending of fuels might
lead to nonadditive behavior.
In biomass thermochemical conversion systems, ash melting (slagging) causes
severe problems, such as agglomeration of the reactor contents and slagging of heat
exchangers with negative implications for safe and reliable reactor operation and heat
transfer performance. Therefore, the ash content and ash melting behavior are impor-
tant properties of solid biomass fuels.
Ash melting behavior is determined by preparing ash samples in specific shapes,
such as pyramids or cubes, and then heating these samples in a reducing or oxidizing
atmosphere in an oven. The oven temperature is raised to a point not much below the
expected deformation temperature. Subsequently, the oven temperature is slowly
increased at a uniform heating rate of 3
7 C
min −1 . The shape of the samples can
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