Online Speciation of Alkali Compounds by Temperature-Modulated Surface Ionization: Method Development and Application to Thermal Conversion

A novel method for online speciation of potassium- and sodium-containing compounds has been described and demonstrated. The method is based on a temperature-modulated surface ionization (TMSI) technique and may be used to determine the concentrations of alkali chlorides, hydroxides, carbonates, and sulfates in high-temperature processes. The measurement device is a further development of a surface ionization detector (SID) commonly used for online alkali measurements in combustion, gasification, and pyrolysis research. Discrimination between sodium and potassium compounds is made possible by differences in their aerosol evaporation characteristics as a function of temperature combined with the desorption kinetics of alkali on a hot platinum filament. The method is evaluated in laboratory experiments with known alkali salt concentrations. An experimental procedure where the platinum filament in the SID is regularly shifted between three temperatures is concluded to provide sufficient selectivity and time resolution for common applications. The TMSI method is successfully applied to characterize the emission of alkali compounds during pyrolysis of pine wood. The emissions during low-temperature pyrolysis are dominated by KOH, while similar amounts of KOH and NaOH are subsequently emitted from the remaining char and ash. The ability of real-time characterization of individual sodium and potassium compounds opens up new means to understand and optimize solid fuel conversion of common fuels such as low-grade biomass, waste, and coal.