Review of Phosphorus Chemistry in the Thermal Conversion of Biomass: Progress and Perspectives

Phosphorus is vital for all life, which means that phosphorus is present in all biomass to some extent. Some types of biomass, such as certain agricultural residues, animal biomass, and sewage sludge, contain high levels of phosphorus. In thermal conversion of biomass, high levels of phosphorus can cause various operational problems. Extensive work has been carried out to understand the phosphorus chemistry taking place at higher temperatures. However, until now, knowledge about transformation chemistry and fate of phosphorus during thermal conversion of biomass was not easily accessible in one place. In this work, the outcome of an extensive literature review has been summarized. First, an introduction to the role of phosphorus in biomass, and in what forms it may be present, is given. Different analytical techniques relevant for characterization of phosphorus in biomass, biomass char, and biomass ash are described. A classification of phosphorus-rich biomass is proposed, and the potential of different phosphorus-rich biomass types is presented. To provide a common basis for the field of high-temperature phosphorus chemistry, fundamental chemistry relevant for thermal conversion of biomass is first discussed. This includes the gas phase chemistry and reaction pathways of light phosphorus species, pyrolysis, and combustion of organophosphorus compounds, and solid, liquid, and gaseous interactions with other ash-forming elements. Thereafter, an in-depth review of phosphorus transformations in biomass, including decomposition of organic phosphorus, ash transformations in the condensed phase, release to the gas phase, and formation of particulate matter in the flue gas follows. Finally, the review covers research focusing on phosphorus in different application aspects, such as the use of phosphorus as an additive to mitigate operational problems related to slag and deposit formation, the behavior of phosphorus in fluidized bed technologies, corrosion, and the deactivation of SCR catalysts.