Release of P from Pyrolysis, Combustion, and Gasification of Biomass—A Model Compound Study

Abstract

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.

Ash Transformation during Fixed-Bed Combustion of Agricultural Biomass with a Focus on Potassium and Phosphorus

“…In general, phosphorus is mainly present in biomass as phosphates associated with C, H, K, Ca, and Mg. Phosphates associated with C and H are not thermally stable during combustion and thus transformed into inorganic phosphates (K, Ca, Mg–P x O y ) through the release of CO 2 and H 2 O. , In the local environment of the fuel, this may lead to the formation of (K, Ca, Mg)-pyrophosphates and (Na, K)-metaphosphates. , Similarly, it is feasible that the initial decomposition of the fuel could lead to the release of P 2 O 5 or KPO 3 to the gas phase and subsequent formation of KH 2 PO 4 , identified in the PM 1 of the WGR experiment. The formation of P 2 (g) is unlikely during combustion due to the extremely low oxygen partial pressures and high temperatures required to reduce phosphate to phosphorus …”

Thermal Conversion of Sodium Phytate Using the Oxygen Carrier Ilmenite Interaction with Na-Phosphate and Its Effect on Reactivity

“…P-195). The elemental composition of the used Na-phytate was previously determined, and the composition is shown in Table …”

“… a Reproduced with permission from Lidman Olsson et al Copyright 2021 American Chemical Society. The atomic ratio of carbon/phosphorus/sodium is 1:0.96:0.93. …”

“…The experiments were carried out in a fixed bed using a horizontal tube reactor setup described in detail previously . A fixed bed was used to ensure contact between the ilmenite and model compound(s) in a controlled manner.…”

“…In this work, the interaction between the oxygen carrier ilmenite and a commercially available Na-phytate was investigated in a horizontal tube reactor up to a temperature of 1100 °C. Na-phytate is a model compound mimicking a fuel containing alkali, phosphorus, and carbon and is known to form NaPO 3 and char upon thermal decomposition . It is worth mentioning that phosphorus-rich biomass normally contains other ash elements that may interact with phosphorus and alkali, for example, magnesium and calcium.…”