Phosphorus-rich biomass can cause
operational problems in combustion
units. Na-phytate, a model compound used to simulate phosphorus in
biomass, was studied in a laboratory-scale reactor under temperature
and gas atmosphere conditions relevant for pyrolysis, combustion,
and gasification in fixed bed or fluidized bed reactors to understand
the P and Na release behavior. Solid residues from Na-phytate thermal
conversion were analyzed using ICP with optical emission spectrometry
in order to quantify the P and Na release. The release mechanism was
evaluated based on FTIR spectroscopy analysis of the residues, measurement
of the flue gas CO/CO2 concentration, characterization
of flue gas particles using SEM with EDS, and thermodynamic equilibrium
calculations. Na-phytate decomposed in several steps under a nitrogen
atmosphere, starting with condensation of the phosphate OH groups,
followed by carbonization in the temperature range 300–420
°C. In the carbonization process, the phosphate units detached
from the carbon structure and formed cyclic NaPO3. Above
800 °C, the C in the char reacted with the melted NaPO3 to form CO and gaseous elemental P. When the char produced from
flash pyrolysis of Na-phytate at 800 °C for 10 min was exposed
to 1% O2, 10% CO2, or 10% H2O (in
N2), the release of Na and P to the gas phase in the temperature
range 800–1000 °C was around 0–7%. However, the
release of P in an inert atmosphere, with a holding time of 2 h or
until full char conversion had been achieved, increased from around
4% at 800 °C to almost 30% at 1000 °C. The results indicated
that carbothermic reduction reaction is responsible for the release
of P and that NaPO3 vaporization is not the dominating
mechanism for P and Na release at temperatures below 1000 °C.
A small amount of P was released in the O2, CO2, and H2O containing gases because these gas species consumed
the char and thereby inhibited the release of P.
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