Change in Catalytic Activity of Potassium during CO₂ Gasification of Char

Abstract

Potassium (K)-catalyzed CO 2 gasification of lignite char was studied with a particular focus on the change in catalyst activity with the char conversion (X) at 800−900 °C. Char samples were prepared from an Indonesian lignite by a sequence of complete removal of inherent metallic species and mineral matter, K-loading by ion-exchange, and pyrolysis. The catalytic activity of K (k cat ′ ) was defined as the rate of catalytic gasification (after elimination of the rate of non-catalytic gasification and that of K volatilization from total mass release rate from char) per amount of K retained by the gasifying char. k cat ′ increased by a factor of 5− 20 with X over its range up to 0.98−0.99, depending on the initial K concentration in the char (m cat,0 ), ranging 0.16−1.4 wt %-daf. Such significant increase in k cat ′ was due to the change in not the intrinsic reactivity of char but its porous nature, that is, the size and volume of pores that retained the K catalyst. At X < 0.4, the entire portion of the K catalyst was confined in micropores (width <2.0 nm) having relatively small k cat ′ , although it increased gradually. At X > 0.4, the gasification created greater mesopores (width >2.0 nm), providing spaces for growth in the size of the K catalyst and allowing promotion of its activity. However, for low m cat,0 , its major portion continued to stay in micropores with a limited increase in k cat ′ .

Potassium-Induced Phenomena and Their Effects on the Intrinsic Reactivity of Biomass-Derived Char during Steam Gasification

“…At 800 °C, the instantaneous rate is sharply increased at the initial stage of conversion, as shown in Figure b (left insert), also observed at 750 °C (see Supporting Information) but to a lower degree, whereas it was not found for 700 °C. In view of the experimental reactivity rate versus conversion, this results in a conversion progression exhibiting two peaks, as displayed in Figure , a pattern also observed in other studies. , …”

“…In the present study, an inflection point of K where R starts to decrease, as identified for the high-ash containing char and potassium-impregnated coal char gasification, ,,, was not observed. Additionally, the determined increase of the instantaneous reaction rate as shown in Figure is not proportional, as reported by Karlström et al, who observed a proportionality between the instantaneous conversion rate and the K/C ratio in the conversion range of 0 to 80% during CO 2 char gasification of K-rich agricultural biomass.…”

“…After the initial peak at X < 0.2, the reaction rates for the chars diverge up to the rapid increase relates to the catalytic effect of potassium. 63 The observed variation in reaction rates between 0.2 and 0.7 of conversion for the water-leached chars can be attributed to differences in the content of water-soluble calcium species, as indicated in Table 1 . These differences may lead to variations in the development of the porous structure, with higher Ca content promoting larger surface area.…”

“…In view of the experimental reactivity rate versus conversion, this results in a conversion progression exhibiting two peaks, as displayed in Figure 3 , a pattern also observed in other studies. 62 , 63 …”

“…In view of the experimental reactivity rate versus conversion, this results in a conversion progression exhibiting two peaks, as displayed in Figure 3, a pattern also observed in other studies. 62,63 The peak at a low conversion is plausibly attributed neither to catalytic activity by potassium 45 nor to the initial specific surface area. In the latter case, the observed reaction reactivity R for each char shows the opposite trend with the highest for OC and the lowest for 48LC compared to the changes in specific surface areas (Table 1).…”