The Release, Distribution, and Implications of Alkalis in Chemical Looping Combustion of Biomass

Doktorsavhandling, 2022

Chemical looping combustion (CLC) of biomass is a promising technology for power generation with a potential net negative CO2 footprint. Like other fluidized bed biomass conversion technologies, biomass CLC may be susceptible to alkali-induced  agglomeration, fouling, and corrosion. The mechanisms, distribution, and implications of alkali release in CLC systems presents a significant knowledge gap that is critical for upscaling and commercialization of biomass CLC.

To investigate gas-phase alkali release in CLC, a CLC-specific surface ionization detector (SID) measurement system was developed. This system was validated in Papers V and VI against two independent alkali measurement techniques. Alkali emissions were measured in four experimental campaigns using three different CLC pilots, four oxygen carriers (OC), and nine different biomass fuels. The SID-based emissions measurements showed that up to 17% of the fuel alkalis are released to the gas phase of the fuel reactor (FR), and up to 7% to the air reactor (AR). Thermodynamic modelling of alkali release, and chemical analysis of flue gas alkalis indicated that gaseous FR alkalis are dominated by KOH(g) and KCl(g). FR alkali emissions levels were found to rise with temperature and steam concentration. Two key alkali release processes were identified: 1) sublimation of KCl(s) to KCl(g), and 2) decomposition of alkali salts, such as K2CO3, to yield KOH(g). It was determined that higher temperature accelerates both of these processes, while steam enables and accelerates the second path. AR emissions were independent of temperature and other operating parameters within the performed tests. Experimental evidence suggests that AR emissions occur via carryover of char from the FR to AR. Ash-bound and OC-bound carryover mechanisms were also proposed, but could not be experimentally validated. Modelling of alkali release and AR fly ash chemical analyses suggest that AR emissions are essentially KCl-free. Alkali retention in condensed phases was found to be >77%, with OC alkali uptake accounting for approx. 30%. A balance on potassium in the 10 kW pilot indicated that up to 60% of fuel alkalis can be elutriated from the FR as solid ash particles, while AR fly ash accounted for up to 3% of fuel alkalis.

Biomass CLC experiments indicate that alkali release behavior does not constrain the fuel conversion and carbon capture performance of CLC systems. Furthermore, CLC technology offers inherent advantages for conversion of high alkali biomass. The key advantage is that the low corrosion potential of the AR flue gases should allow for higher steam temperatures, and thus more efficient operation of the main process heat exchangers in the AR. Further work is needed to evaluate long-term effects of alkali release on OC performance.