Mechanism of Ca Additive Acting as a Deterrent to Na₂CO₃ Deactivation during Catalytic Coal Gasification

Abstract

The deactivation of Na 2 CO 3 , caused by the reaction between Na and Si and Al minerals in coal, is one of the key technical bottlenecks for the large-scale application of catalytic gasification. Ca can inhibit the deactivation of Na, and the binary Na−Ca catalyst shows strong synergistic effects during catalytic gasification. In this study, the deactivation behaviors of Na 2 CO 3 and the inhibition mechanism of Ca were systematically investigated by thermogravimetric, X-ray diffraction, Fourier transform infrared (FT-IR), model compounds, and FactSage thermodynamic equilibrium. The results show that the Na−Ca catalyst shows synergistic effects and more Na remains active as water-dissolved Na after gasification with Na−Ca addition. The mechanism of the inhibiting effect of Ca on Na 2 CO 3 deactivation at low temperatures (<1000 °C) is different from that at high temperatures (>1000 °C). At temperatures lower than 1000 °C, Ca inhibits the reaction between Na and Si and Al minerals; as a result, more Na remains active during catalytic gasification, which is validated by FT-IR. Meanwhile, the FactSage thermodynamic equilibrium shows that more Na 2 CO 3 exists with increasing Ca addition. With Ca/Si = 0.1, the disappearing temperature of Na 2 CO 3 is 940 °C. When Ca/Si increases to 1, the disappearing temperature of Na 2 CO 3 further increases to 1166 °C, indicating that more Na survives with Ca addition. At high temperatures (>1000 °C), Ca captures Si and a part of Na in priority to form sodium calcium silicate (Na 2 Ca 3 SiO 8 ) and calcium silicate (Ca 2 SiO 4 ). As a result, the remaining Na and Al form NaAlO 2 , which has excellent catalytic performance and contributes to the synergistically promoted catalytic performance. At low and high temperatures, Ca can act as a good deterrent to the Na deactivation.

Calcite or Kaolinite Addition for Mitigating Bed Agglomeration during Combustion and Steam Gasification of Potassium-Rich Agricultural Residues with Olivine as the Bed Material

“…53,55−57 The priority of Ca over alkalis was also shown for Na when reacting with Si-or Al-minerals. 52 Second, the addition of Ca into the bed can have a dilution effect, which acts to improve the resistance of ash fusibility 24 or to form a less sticky ash coating 42 due to the increased Ca concentration in ash or layers. Bozaghian et al 58 compared the effectiveness of CaCO 3 in reducing agglomeration arising from straw using quartz sand and olivine as bed materials.…”

“…These observations strongly suggest that, under steam gasification conditions, the addition of calcite has suppressed the reactions that would otherwise occur between olivine and K substances from grape marc. Previous studies have suggested that the two main anti-agglomeration mechanisms occur for Ca-based additives in a bed of silica sand. ,,,,,− First, Ca behaves as a competitor of K during the interactions with silica to form Ca silicates and thus hinders the formation of low-melting-point K-silicates. − ,, This is because Ca 2+ has a stronger affinity to negatively charged oxygen in the silica structure than K + . As a result, more K contents are released to the gas phase instead of being retained in the melts. ,− The priority of Ca over alkalis was also shown for Na when reacting with Si- or Al-minerals .…”

“…Previous studies have suggested that the two main anti-agglomeration mechanisms occur for Ca-based additives in a bed of silica sand. ,,,,,− First, Ca behaves as a competitor of K during the interactions with silica to form Ca silicates and thus hinders the formation of low-melting-point K-silicates. − ,, This is because Ca 2+ has a stronger affinity to negatively charged oxygen in the silica structure than K + . As a result, more K contents are released to the gas phase instead of being retained in the melts. ,− The priority of Ca over alkalis was also shown for Na when reacting with Si- or Al-minerals . Second, the addition of Ca into the bed can have a dilution effect, which acts to improve the resistance of ash fusibility or to form a less sticky ash coating due to the increased Ca concentration in ash or layers.…”

Novel Assumption about the Mechanism of Catalytic Gasification: On the Basis of the Same Catalytic Effect of Alkali between C–CO2 and Fe–CO2 Reactions

“…Iron powder (Fe), sodium carbonate, sodium aluminate, and potassium carbonate were ordered from Sinopharm Chemical Reagent Co. Shenmu (SM) coal, collected from the Shannxi province, was ground and sieved to a particle diameter of less than 125 μm, and corresponding char (SM) was prepared by the fast pyrolysis of SM coal. 17 The proximate and ultimate analyses of SM coal and SM char are shown in Table 1. SM was used as one basic carbon material to measure the catalytic performance of alkali.…”

Novel Process for Production of H2-Rich Nitrogen-Free Syngas from Coal Using a Cu/Fe-Alumina Catalyst and Steam

“…11 A sodium−calcium binary catalyst has shown a better catalytic effect than the respective single component catalysts at 900 °C, and Ca acts as a good deterrent to Na deactivation. 12,13 Coals impregnated with alkali carbonates have shown promising results for coal gasification. 14 Composite catalyst consisting of sodium and iron has also been used as impregnation catalysts for coal gasification.…”