Impact of organically bonded potassium on torrefaction: Part 1. Experimental

Abstract

Torrefaction is a promising heat-treatment method being developed for biomass to increase the use of biomass in its thermochemical conversion processes. This type of pre-treatment can improve the properties of biomass for thermal conversion by improving grindability, heating value, reducing the hydrophilic nature, and increasing its resistance to biodegradation. In this work, we studied the impact of organically bound K, Na, Ca and Mn on mass loss of biomass during torrefaction. These elements were of interest because they have been shown to be catalytically active in solid fuels during pyrolysis and/or gasification.In this work, we studied spruce and pine as coniferous woods, aspen as a deciduous wood and miscanthus as an herbaceous biomass. The biomasses were first acid washed to remove the ash-forming elements and then organic sites were doped with K, Na, Ca or Mn. The doping was performed in a nitrate solution of each metal. The resultant fuels were then torrefied at fixed temperatures between 240 and 280°C in a thermogravimetric analyzer.The results show that K and Na bound to organic sites can significantly increase the mass loss during torrefaction at temperatures between 240 and 280°C for a fixed time and temperature. It is also seen that Mn bound to organic sites increases the mass loss while Ca addition does not influence the mass loss rate during torrefaction. This increase in mass loss during torrefaction with alkali addition is unlike what has been found in the case of pyrolysis where alkali addition resulted in a reduced mass loss. These results are important for the future operation of torrefaction plants which will likely be designed to handle various biomasses with significantly different contents of K. The results imply that shorter retention times are possible for high K-containing biomasses.