Effects of Steam on the Release of Potassium, Chlorine, and Sulfur during Char Conversion, Investigated under Dual-Fluidized-Bed Gasification Conditions

Abstract

The corrosion and fouling of heat-transfer surfaces and the agglomeration of bed materials in fluidized beds are some of the ash-related problems caused by the transformation and release to the gas phase of ash-forming elements from biomass during thermochemical conversion processes. The magnitudes of these problems are largely dependent upon the release of potassium (K), chlorine (Cl), and sulfur (S) from the biomass. We investigated the effects of steam on the release of K, Cl, and S during char conversion, under conditions relevant for dual-fluidized-bed gasification (DFBG). The study was carried out with wheat straw in a laboratory-scale bubbling fluidized-bed reactor in the temperature range of 800–900 °C. The release of K, Cl, and S from wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance that linked the masses of these elements in the wheat straw to the mass of the solid residue obtained at the end of each experiment. To facilitate analyses of the experimental results, leaching and the Brunauer–Emmett–Teller surface area measurement of the wheat straw and some of the solid residues were carried out. The results show that, during devolatilization, the release of volatile salts, e.g., KCl, is significantly limited by intraparticle diffusion resistance, owing to a compact char matrix (i.e., negligible porosity). However, during char gasification, steam renders the char less compact by expanding and/or creating new pores in the char. As a result, intraparticle diffusion resistance decreases, thereby facilitating the evaporation of volatile salts of K and S from the char matrix. The conversion of the char is also conducive to the release of char-bound K and S, especially at 900 °C. At temperatures of >800 °C, the relative proportions of the elements released and char gasified indicate that the release of K can somewhat be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The results also show that, during char combustion, the proportions of the char that can be combusted and the extent of the release of the elements are influenced by the extent to which the char is gasified in the gasification chamber.

In Situ Measurement of Potassium Release during Biomass Combustion Using Laser-Induced Breakdown Spectroscopy: Effect of Silicate on Potassium Release

“…As a result, the char-K was mainly transformed into K 2 CO 3 , and part of K 2 CO 3 was released to the gas phase during the char combustion stage. On the other hand, with the oxidation reaction of organic matter combined with K, K in char-K may also be directly released into the gas phase in the form of K or KOH . Nevertheless, nearly 68% of the total K remained in ash was in the form of K 2 CO 3 after char combustion.…”

“…On the other hand, with the oxidation reaction of organic matter combined with K, K in char-K may also be directly released into the gas phase in the form of K or KOH. 40 Nevertheless, nearly 68% of the total K remained in ash was in the form of K 2 CO 3 after char combustion. This indicated that the majority of char-K was transformed into K 2 CO 3 during the char combustion stage.…”

Steam Gasification of Biochar Derived from the Pyrolysis of Chrome‐Tanned Leather Shavings

“…At 800–900 °C, the vapor pressure of KCl is highly significant for evaporation. However, the compactness of the biochar is also one of the factors controlling the extent of evaporation of KCl from the biochar . The authors observed an increase in the release of both chlorine and potassium when the temperature of steam gasification of wheat straw biochar raised from 800 to 900 °C.…”

Impacts of Bed Material Activation and Fuel Moisture Content on the Gasification Rate of Biomass Char in a Fluidized Bed

“…Under gasification conditions (reducing atmosphere and T = 600−1000 °C), while the Ca in the ash layer is in solid form, the K is likely to form gaseous compounds, such as KOH or KCl (with the latter being formed if the fuel contains a substantial amount of Cl). 33,34 Under conditions relevant to steam gasification, the K in these compounds can originate from various sources, such as Ca/K/Si systems, 35 Ca/K/P systems, 35 Al/K/Si systems, 36 K 2 CO 3 , 37 and K 2 SO 4 . 38 In the present work, three theoretically plausible explanations are proposed for how the catalytic elements in the activated olivine come into contact with char to catalyze char gasification (Figure 1): (1) The outer surface (the Ca-rich ash layer) of the activated olivine particle comes into contact with the external surface of the char particle (i.e., solid−solid contact).…”

“…As mentioned in section , char gasification can also be catalyzed through the use of active bed materials. − Just as in the case of catalytic tar reduction, K or Ca (or a combination thereof) is likely to be responsible for the increase in char gasification rate when an active bed material is employed. Under gasification conditions (reducing atmosphere and T = 600–1000 °C), while the Ca in the ash layer is in solid form, the K is likely to form gaseous compounds, such as KOH or KCl (with the latter being formed if the fuel contains a substantial amount of Cl). , Under conditions relevant to steam gasification, the K in these compounds can originate from various sources, such as Ca/K/Si systems, Ca/K/P systems, Al/K/Si systems, K 2 CO 3 , and K 2 SO 4 …”