Determining the Fate and Operational Implications of Alkalis in Chemical Looping Combustion of Biomass

Licentiate thesis, 2020

Chemical looping combustion (CLC) of biomass is a promising thermal conversion technology with integrated carbon capture. As with other fluidized bed fuel conversion processes, biomass CLC may be susceptible to issues of agglomeration, fouling and corrosion stemming from the release of alkalis during fuel conversion. How alkalis are released and what implications they have on CLC operation is currently largely unknown. The experimental studies presented in Papers I - III of this thesis aimed at addressing this knowledge gap.

Measurement of gas-phase alkali release in CLC was addressed through the design and construction of a modular and transportable surface ionization detector (SID) system, customized for alkali emissions measurement from the fuel reactor (FR) and the air reactor (AR) of CLC pilots. Gas-phase alkali emissions measurement showed that approximately 1-10% of fuel alkalis are released to the gas phase in CLC. The FR alkali emissions were found to rise with the fuel’s alkali content, while no definitive correlation was established for AR emissions. With respect to emissions distribution, AR emissions were found to be generally lower than that of the FR. Surprisingly, in several cases, AR emissions were equal or marginally higher than in the FR. Analysis presented in Paper III led to a preliminary conclusion that AR gas-phase alkali emissions likely occur due to char or ash carryover from the FR, whereby the transported alkali compounds release to the gas phase at the higher temperatures of the AR.

Papers I-III also established that >97% of fuel alkalis are retained in solid form. Although a major part of the alkali retention originates from fuel ash formation, significant retention likely occurs due to oxygen carrier interaction with the fuel’s inorganic content. In Paper III it was also found that in CLC, the steam-rich FR atmosphere enhances the gas-phase release of alkalis from decomposition of alkali carbonates and sulphates. This was established in tests comparing CLC operation with oxygen carrier aided combustion (OCAC).

Development of biomass CLC technology was also addressed in this thesis. Paper I demonstrated that gas conversion efficiencies of >96% can be achieved with mixed synthetic and natural oxygen carriers. Experiments in Paper II commissioned a new 10 kW CLC pilot and demonstrated that the implementation of a volatiles distributor in the FR can improve fuel gas conversion efficiency by up to 10 percentage points. Papers I and II also evaluated the interdependencies of the CLC system’s control parameters with gas-phase alkali release. It was concluded that CLC system control parameters, other than temperature, likely do not significantly influence and are not constrained by alkali release behavior.