Ash behavior in the combustion of phosphorus rich biofuels - literature survey and experimental studies

Utilization of biofuels such as grain, bran and rapeseed meal in energy production is considered to have ecological and economical benefits. These seed-originated biofuels usually contain significantly higher phosphorus contents than other biofuels, which may induce some ash related operation problems.

In this report, the behaviors of inorganic species during the combustion of phosphorus rich biofuels are studied through literature review and experiments. It is found that the majority of P, Mg and K in these biofuels would be present as phytic acid/phytate or other inositol phosphate. During the combustion of phosphorus rich biofuels, the K, P, and S are the main inorganic species released to the gas phase.

The release of K and P occurs in a temperature range of 900-1100 oC, and the primarily reason is attributed to the vaporization of phosphate compounds. The addition of Kaolin and Ca-based additives shows some retention effects on the K or P release. By performing thermodynamic calculations, the interactions among the released K, P, and S in the flue gas are investigated, showing that the ash chemistry would be significantly affected by the molar ratio of the released K/P.

The results from the thermodynamic calculations are supported by some experimental findings, and are used to explain some practical problems observed in the combustion of bran in a grate-fired power plant. Other operational problems related with the combustion of phosphorus rich biofuels are also discussed and the possible countermeasures are suggested.