An exploratory study of phosphorus release from biomass by carbothermic reduction reactions

Phosphorus (P) from biomass can cause operational problems in thermal conversion processes. In order to explore the release mechanism of P to the gas phase, carbothermic reduction of meta-, pyro-, and orthophos-phates of ash elements commonly found in biomass; sodium, potassium, magnesium, and calcium was in-vestigated. Mixtures of each phosphate and activated carbon were heated to 1135 & DEG;C in a laboratory-scale reactor, with the CO and CO2 evolving from the sample monitored, and the chemical composition of selected residues analyzed to quantify the release of P. Thermal gravimetric analysis was also performed on selected samples. The alkaline earth phosphates were reduced in steps, following the sequence meta & RARR; pyro & RARR; or-tho & RARR; alkaline earth oxide. However, the alkali metaphosphates appear to be reduced in one step, in which both alkali and P are released. Alkali pyro-and orthophosphate appear to undergo a two-step process. In the first step, mainly alkali is released and in the second step both alkali and P. An intermediate is produced in the first step, which has a K:P:O atomic ratio of about 2:1:2.7, indicating it might be a phosphite with the overall stoichiometry; K4P2O5. The reduction of alkaline earth phosphates could be interpreted using available thermodynamic data, whereas thermodynamic equilibrium calculations for the alkali phosphates did not correspond well to the experimental observations. Kinetics were derived for the different reduction reactions, and can be used to compare the reactivity of the phosphates. The work suggests that carbothermic reduction reactions are important for the release of P in the temperature range 850-1135 & DEG;C and relevant for biomass combustion, pyrolysis and gasification.& COPY; 2022 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

This study investigated the release mechanism of phosphorus (P) to the gas phase during the carbothermic reduction of meta-, pyro-, and ortho-phosphates commonly found in biomass. It was found that alkaline earth phosphates were reduced in steps, while alkali metaphosphates were reduced in one step, producing an intermediate phosphite. These findings are important for understanding the release of P in biomass combustion, pyrolysis, and gasification processes.