Characterization of ashes from biomass incineration and investigation of usability
M. Kröppl - Characterization of ashes from biomass incineration and investigation of usability - Phd Thesis, TU Wien / Inst. f. Chem. Tech. u. Analytik, Austria, 2011, pp. 1-154
The use of biomass incineration for the generation of heat and electricity has
increased in recent years and this trend will continue also in the years and decades
to come. Ashes are the inorganic residues of incineration. Apart from nutrients they
also contain heavy metals and unburnt carbon. Bottom ashes have low levels of
heavy metals and are therefore usually used as fertilizers in forests and on fields. Fly
ashes are collected in a gas cleaning system as particles must not leave the
incinerator and contaminate the environment - they must be properly disposed of.
In view of the rising amount of biomass ashes, this doctoral thesis analyzes different
ways for the utilization of biomass ashes – especially heavy metal contaminated fly
ashes. Fly ashes could be used as valuable materials, costs of disposal be reduced.
After a validation of the analytical method for the elements Al, As, B, Ba, Cd, Co, Cr,
Cu, Fe, Mg, Mn, Na, Ni, Pb, Sb, Sr, Ti, V, and Zn, elemental concentrations of ashes
from biomass incinerators and input materials were determined. Extraction
experiments investigated the mobility of heavy metals and nutrients. Different
durations and extraction media were tested. In acidic leaching experiments the best
conditions for dissolving heavy metals from the ashes were analyzed. The aim was to
determine whether the cleaned ashes could be used as fertilizing agent. The
influence of fly ashes on plant growth was also researched. An air classification of
the fly ashes investigated the elemental concentrations in different grain sizes. The
possibility of separating the finest fly ash fraction with the highest amounts of heavy
metals was investigated, so that the remaining ashes can be used as fertilizer. The
possibility of using fly ashes as secondary material in the cement industry was also
Findings showed that elemental concentrations from different biomass incinerators
vary depending on input material, location of biomass growth and the type of
combustion system employed. Shorter extraction times with water or weak salt
solutions release only low amounts of heavy metals from fly ashes. Nevertheless,
longer leaching – especially under more acidic conditions – can set free higher
amounts. Also an influence of fly ashes on plant growth was observed in a laboratory
experiment. By leaching under acidic conditions, lactic acid released most of the
harmful heavy metals, with the exception of cadmium, already at a moderate pH of 5.
By separation of the finest parts of the fly ashes (< 30 μm), the cadmium
concentration in the remaining ash fractions falls below the limit. Unfortunately, also
high quantities of potassium are removed (about 75% by removing two thirds of the finest ashes). This result strongly depends on the original concentration of cadmium.
In lower contaminated ashes, less fine ashes would be necessary for separation and
more potassium would remain in the coarser part. A way to recapture potassium from
the finest fly ashes is by leaching them with water or weak salt solutions. So,
potassium can be dissolved whereas heavy metals remain in the ashes. Together
with the coarser ash fraction the leachate can be used as fertilizing agent. A
granulation of the ashes using the leachate for moistening the ash would be the most
efficient way for using biomass fly ashes without exceeding limits of heavy metals.
Recycling of fly ashes in the cement industry is possible, but high amounts of alkali
and earth alkali metals and chloride can cause operation problems – so also here
maximum limit values have to be recognized.