Optical Diagnostics for Quantitative Potassium Chemistry in Biomass Thermochemical Conversion Processes

Abstract: Biomass is an essential sustainable and carbon-neutral energy source. Through thermochemical conversion processes, such as combustion and gasification, biomass can generally be used in boilers to provide heat and/or power. Biomass fuels contain varying amounts of potassium, chlorine and sulfur. The release of these elements, especially in the form of potassium chloride (KCl), in the thermal conversion processes causes crucial problems such as slagging and corrosion of heat transfer surface, and bed material agglomeration in operating furnaces. Potassium sulfate (K2SO4) is less corrosive and has a higher melting point than KCl. K2SO4 could be formed through sulfation of potassium using sulfur-containing additives, such as sulfur oxides (SO2). Between KCl/KOH and SO2, the sulfation can occur through gas-phase reactions. The understanding of the sulfation processes is based on gas-phase K-Cl-S chemistry, and the work in this thesis focuses on the development and application of in-situ optical diagnostics for quantitative measurements of key species in the K-Cl-S chemistry.As a preliminary investigation (Paper I) of the homogeneous sulfation between gas-phase KCl/KOH and SO2, a specially designed counter-flow reactor was adopted, where neither wall-based reactions nor other heterogeneous reactions could be involved. Homogenous sulfation was observed in the reactor, and the sulfation of KOH was observed to be more rapid than that of KCl. However, due to the counter-flow structure, quantitative measurement of potassium species was hard to be achieved. In order to achieve quantitative measurement and evaluate a detailed K-Cl-S mechanism, a laminar flame burner (Paper II) was designed to provide well-defined homogenous hot gas environments. The homogenous hot gas could be widely adjusted both in temperature and gas composition to mimic real operation conditions of furnaces. The temperature was measured using two-line atomic fluorescence (TLAF) thermometry. Different homogenous reactions could be investigated with seeding certain additives in gas or liquid phase, such as K2CO3, KCl and SO2.UV absorption spectroscopy was used to quantify concentrations of KOH, KCl, OH radicals and SO2. However, UV absorption cross sections of KOH, KCl and SO2 at the temperature over 1200 K were absent. Thus, they were accurately measured in this work (Paper III, IV). Tunable diode laser absorption spectroscopy (TDLAS) was adopted for the measurement of the concentration of K atoms. Two TDLAS systems, at 769.9 nm and 404.4 nm, were employed to extend the measurement dynamic range of potassium atoms, from below ppb-level to over ten ppm.The K-Cl-S chemistry (Paper V, VI) was investigated in oxidative and reducing environments having temperatures from 1120 K to 1950 K with additives of K2CO3, KCl and SO2. Based on the quantitative measurements of KOH, KCl, K atoms and OH radicals, a detailed K-Cl-S mechanism was evaluated and a reasonable agreement between the modelling and experimental results was obtained. SO2 sulfated KOH/KCl to K2SO4 in an oxidative environment at a temperature below 1550 K. The sulfation of KOH was more rapid compared to that of KCl. The chain-terminating reactions between KOH / K atoms and OH radicals promoted the consumption of OH radicals and inhibited the oxidation of CO and H2 as the environment temperature below 1550 K (Paper VII). Sulfation reactions consumed KOH and K atoms and eliminated the chain terminating reactions. In a reducing environment with SO2 seeding, potassium was only transformed into KOSO, and UV absorption spectrum of KOSO was obtained for the first time.