Environmental Engineering Reference
In-Depth Information
or lingocellulosic waste in bio-based industrial processes. The contaminants are usually a com-
plex mixture of substances that include geogenic materials (soil particles, etc.) and atmospheric
pollutants.
The elemental composition of ash from biofuels tends to differ significantly from that of coal
ash (Vassilev et al ., 2010) and to be much more variable. It therefore exhibits different and more
complex chemical behavior at high temperatures during combustion and gasification. Together
with the considerable potential for contamination, this will necessitate the development of systems
for classifying and characterizing ashes in biomass feedstocks.
In a review of the elemental composition of biofuel ash, Vasssilev et al . (2010) defined seven
different biomass groups using a ternary diagram with the following elements (clusters) in each
corner of the triangular panel: (i) Si
+
Al
+
Fe
+
Na
+
Ti; (ii) Ca
+
Mg
+
Mn; and finally, (iii)
K
Cl. Biomass samples with high levels of elements from the first cluster are described
as “high acid” (S-type) samples, while those with high levels of elements from the second two
clusters are regarded as “low acid.” Within the “low acid” group, those with high levels of
Ca
+
P
+
S
+
Cl
are referred to as K-type biomasses. There is also a group of CK-type biomasses whose ash
elemental composition is intermediate between that of the C and K types. The seven groups
are defined by abundance boundaries set at 30% for either of the low acid oxide clusters, and
at 40% and 60% for the high acid cluster. This scheme defines three of the seven groups as
having intermediate acidity. The authors also stated that there are strong associations between the
abundances of the individual elements of the N-S-Cl, Si-Al-Fe-Na-Ti, Ca-Mg-Mn and K-P-S-Cl
groups.
Such classifications are very important in fully understanding the complex mixtures of sub-
stance in biofuel ashes. However, the number of elements involved and their various interactions
during ash formation make it difficult to provide a detailed overview of the processes involved.
This problem is often addressed by using ternary diagrams, but simple combinatorics state that
if each ternary diagram features 3 ash elements ( k = 3) and there are (for example) a total of 10
elements to consider ( n = 10) then a full overview would require the use of 120 ternary diagrams.
The expression for the number of possible permutations when making k choices of n possible
objects is:
+
Mg
+
Mn are defined as C-type biomasses, while those with high levels of K
+
P
+
S
+
n
k
n !
k !( n k )!
=
An illustrative four-dimensional ternary diagram is shown in Figure 2.5.
It would be necessary to use a very large number of diagrams to cover all of the inorganic
elements in ash. In order to reduce the number that have to be considered, Tao et al . (2012b)
used principal component analysis to gain an overview of the ash elements in biomass and the
relative abundance of SiO 2 ,K 2 O, CaO, MgO, Na 2 O, P 2 O 5 ,Al 2 O 3 ,Fe 2 O 3 and SO 2 in biofuel
ash. Principal component analysis was used to identify a small set of orthogonal components that
account for the greatest possible amount of the variation in the observed data and could thus be
used to briefly describe the properties of a given ash. The studied dataset comprised 367 different
ash composition analyses, and a number of interesting conclusions were drawn. The first was
that the abundance of Si increases and that of Ca decreases gradually in the following order:
woody groups, herbaceous dicots, C4 graminoids and C3 graminoids. C4 and C3 plants differ in
the way they handle the first steps of photosynthesis; C4 plants are mostly found in tropical and
subtropical regions. Examples of C4 graminoids are sugarcane, maize, millet, sweet sorghum,
switchgrass and miscanthus, while wheat, oat, barley, rye, rice, alfalfa, sunflower and reed canary
grass are C3 plants as are most woody species.
Second, it was found that levels of K, P, Mg and S were higher in herbaceous plants than in
graminoids and dicots (grasses). Woody species were generally associated with high levels of
Ca-Mg-P-K-S and herbaceous ones with Si; of all the elements involved, Ca and Si made the
 
Search WWH ::




Custom Search