The effects of fuel washing techniques on alkali release from biomass

Abstract

The influence of different washing techniques on the alkali release during pyrolysis of biomass is studied. After washing and drying, samples of wheat straw, wood waste and cellulose are subjected to a constant heating rate in a N₂ atmosphere, and the release rate of alkali compounds from the sample is measured continuously by a surface ionization technique. Alkali is released from the untreated biomass in two temperature intervals: (1) in connection with the pyrolysis process taking place at 200–500°C, and (2) from the material remaining after pyrolysis at temperatures above 600°C. Separate vacuum pyrolysis experiments show that the alkali release is dominated by potassium-containing compounds, with minor contributions from sodium-containing compounds. The effect of water washing of the biomass is compared with a more thorough acid leaching technique. In the temperature range 200–500°C, washing with water reduces the alkali emission from wood waste and wheat straw by 5–30%, while acid leaching is more effective and reduces the emission by around 70%. Above 600°C where the vaporization of alkali compounds from untreated wheat straws increases sharply, the washing procedures are sufficient for a reduction in the measured alkali release by more than 90%. Experiments with pure cellulose (ash content 0.07%) indicate that the washing methods are ineffective in removing alkali bound to the organic structure of the biomass. The results support the conclusion from earlier studies that relatively simple washing techniques can improve the combustion properties of biomass fuels with a high ash content. For fuels with a lower ash content like woody biomass, the concentration of alkali bound to the organic structure limits the effect of the washing techniques.