Proceedings World Bioenergy 2010

It is seen that the total data set, existing of 164
products, have a broad range of differences in ash content
and composition. The lowest ash values are found for soft
wood consumer pellets (0,20-0,64%). Hard, mixed and
waste wood pellets have ash contents between 0,8-2,5%.
Miscellaneous non-pellets woods (such as chips, logs,
saw dust, flour, bark) have been studied with ash contents
in the range of 0,23-5,9%. Non-wood plant based
products were studied with ash contents varying between
0,8-14,7%. Waste stream or sludge products had ash
contents of 3,5-46%. The main elements of wood based
products are calcium, silicon, potassium and magnesium.
Calcium contents range from 386 mg/kg for a soft wood
pellet to 12990 mg/kg for a product consisting of mixed
bark. The waste stream and sludge products are much
more divers in composition then the wood or plant based
products, having also significant differences in Al, Fe, Na
and P contents. Typically sewage sludge products have
high iron contents in the range of 10-20 g/kg.
The sintering melting temperatures (SST) are found
in the range of 578°C for rye straw to 1585°C for kenaf.
One has to take into account, while interpreting the
results, that the determination of ash melting
temperatures can have a standard deviation of up to 50°C.
As expected, the melting T of ash of soft wood consumer
pellets are higher than of the less clean industry pellets.
Typically low melting ash is obtained from grasses and
cereal based products (non wood based plants).
Figure 2: %Ca of total ash versus ash melting
temperature of all products. Logarithmic fit is shown
without ash consisting of mixed ash from waste and
sewage sludge ash [10].
In figure 2 the ash melting T is shown as a function
of %Ca of the total ash for the complete data set. It is
known from literature that calcium plays an important
role in the ash melting behavior. Authors [9,11,18,19]
describe the increase of ash melting T by increasing the
Ca content of incineration ash. However, it is clear that
calcium content itself is not enough to predict ash melting
temperatures of the products. In particular waste, sludge
and non-wood plant based products show less correlation
between T and %Ca of the ash.
The ash melting temperature is correlated to all of
wt% per element in the incineration ash (Ca, Mg, Fe, Si,
Al, P, Na) for the complete data set studied (n=164).
Clearly seen is that calcium is the most important element
for affecting ash melting leading to an increase in ash
melting T. Silicon and potassium are the 2nd and 3rd
most important elements affecting ash melting by
lowering the melting T. Fe and Al seem to have a limited
effect while, within the data set studied. Na, P and Mg
90 world bioenergy 2010
show hardly any significant effect. The results were
confirmed with principle component analyses (PCA).
Remarkable is that magnesium shows on average for this
data set almost no negative influence on ash melting. In
literature some authors have stressed the importance of
magnesium in lowering ash melting T but others also
have seen increased melting T. Of course, one has to take
into account that probably also the interactions between
the various elements play an important role. Mixed
oxides will all have their different melting characteristics
depending also on thermal-physical history of the ash.
It is very likely that the total composition of the ash is
of great importance for the ash melting behavior.
Therefore, it is thought that the ratios of various elements
are important. In figure 3 the most important example is
given of ash melting T as function of a typical ratio of
elements (Ca/[K+Si]).
Figure 3: Ratio combined elements (mg/kg product)
versus ash melting T of all products.
It is seen that again, as expected, in particular
calcium is having a positive influence on the effect of
Mg, Si en K on the ash melting. The Ln-fit of the ratios
Ca/K, Ca/Si en Ca/Mg show a reasonable correlation.
The effect of calcium on sodium is less dominant. It is
thought that calcium containing silicates have higher
melting points than mixed Mg-K-Na silicates. The
presence of Fe en Al probably gives a slight increase in
the melting of mixed silicates compared to silicates,
which have high Na and K content. Mg and P show no
correlation with Si. Therefore, it is seen that the Ln of the
ratio of the most dominant elements Ca/(K+Si) gives a
good correlation with a R 2 of 0,65. The correlation with
ratios without Si, K or Ca are worse or even bad. A slight
improvement is found for ratios were Al or Fe are
included as a positive effect and Na as a negative effect
on the Tmelt. However, the improvements are small.
Including Mg gives no significant improvement in the fit
compared to the ratio Ca/(K+Si). The prediction using
the Ln fit of the ratio Ca/(K+Si) is already good as well.
Research was extended by using statistics (linear
regression analyses) to gain more insight in the
interactions between the elements. The main results are
shown in figure 4 and 5.