Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of Biomass─Experimental Investigation in a 10 kW_th Pilot

Abstract

Alkali release was studied in a 10 kW th chemical looping pilot operated with a Linz–Donawitz (LD) slag oxygen carrier (OC) and three biomass fuels. Experiments were performed at three temperatures and in three operation modes: chemical looping combustion (CLC), chemical looping gasification (CLG), and oxygen-carrier-aided combustion (OCAC). Gas-phase alkali release was measured with a surface ionization detector (SID). Fuel reactor (FR) gas-phase alkali emissions increased with the temperature. This occurred as a result of increased evaporation of KCl and enhanced decomposition of alkali salts during char conversion. Air reactor (AR) alkali emissions were lower than in the FR and independent of the operating temperature. In comparison of operating modes, CLC and CLG modes resulted in similar gas-phase alkali emissions due to the similar extent of char conversion. In contrast, operation of the reactor system in OCAC mode resulted in significantly lower levels of gas-phase alkalis. The difference in alkali emission was attributed to the steam-rich atmosphere of CLC. The effect of steam was further investigated in CLC and OCAC tests. Lowering steam concentrations in CLC operation resulted in lower gas-phase alkali emissions, while introducing steam to the FR during OCAC operation resulted in higher alkali emissions. It was concluded that steam likely enhances gas-phase K release through a reaction of K 2 CO 3 within the fuel char with steam to produce KOH(g). Solid sampling and analysis for K content was used along with SID measurements to develop a K mass balance for the reactor system. Mass balance results for the straw pellet fuel tests showed that LD slag OC absorbs approximately 15–51% of fuel K, 2.2% of fuel K is released to the gas phase, and up to 3.4% of fuel K is captured in the AR fly ash. The residual 40–80% of fuel K was determined to leave the FR as K-rich fly ash.

Chemical Looping Combustion of Waste─Opportunities and Challenges

“…Hence, most of the impurities in the fuel, such as alkali, chlorine, and sulfur, also end up in the fuel reactor off-gas. This trend has been confirmed by alkali measurements during CLC of biomass in a 10 kW th unit . Only a small fraction of the impurities will be carried over to the air reactor in the form of char or ash and will be released there as a result of char oxidation or the further temperature increase from around 900 to 1000 °C.…”

“…In CLC, these impurities could lead to interactions with the oxygen carrier, agglomeration/plugging in the fluidized bed reactor system, and fouling/slagging/corrosion on heating surfaces, leading to significant cleanup requirements. CLC tests with biomass in a 10 kW th unit show that up to 50% of alkali are absorbed by the oxygen carrier, which was Linz–Donawitz (LD) slag in this case . Experiments with organic solid waste show that chlorine can be absorbed to a high degree by the oxygen carrier when red mud is used .…”

Production of aviation fuel with negative emissions via chemical looping gasification of biogenic residues: Full chain process modelling and techno-economic analysis

“…CLG eliminates the requirement of ASU by achieving inherent air separation through the utilization of oxygen carriers [12]. It also results in less cleaning requirement and less corrosion of the heat transfer surfaces downstream the AR compared to an Indirect Gasifier as there is no or very limited fuel conversion in the AR [13,14]. The CLG system is made up of two fluidized bed reactors that are linked together: an Air Reactor (AR) and a Fuel Reactor (FR).…”

2022 Pioneers in Energy Research: Anders Lyngfelt

“…Nemati reports fundamental modeling and batch experiments with random metal packings for chemical looping combustion in a packed fluidized bed reactor. Gogolev investigates the effects of the temperature, operation mode, and steam concentration on alkali release in a 10 kW th pilot chemical looping conversion of biomass. Li tests the performance of volatile distributors with different configurations under different fluidization regimes.…”