Chemical-looping combustion characteristics of Zhundong coal semicoke with hematite oxygen carrier and transformation of alkali and alkaline earth metals

The low-carbon, clean, and efficient utilization of the Zhundong coal is greatly limited by some severe problems such as ash accumulation, slagging, and CO₂ emissions in the conventional combustion and utilization processes. The adoption of low-rank coal pyrolysis at low and moderate temperatures to produce semicoke, tar, and pyrolysis gas is a leading technology for the utilization of classified coal. Based on the characteristic of high alkali gasification activity of pyrolysis semicoke, the chemical-looping combustion technology is adopted to achieve a low-carbon, clean, and efficient conversion of the Zhundong coal semicoke. The in-situ gasification chemical-looping combustion characteristics of the Zhundong coal semicoke are studied in a fixed bed, in which CO₂ is selected as the gasification agent and hematite is considered as the oxygen carrier. The study of migration and transformation laws of alkali and alkaline earth metals (AAEMs) in the Zhundong coal semicoke also is performed. The experimental result indicates that the addition of the hematite oxygen carrier can significantly improve the CO₂ concentration of the flue gas at the outlet of the fixed bed reactor. However, CO₂ concentration increases first and then remains constant with the increase in the mass ratio of the oxygen carrier and semicoke (m_OC/m_C). The optimal m_OC/m_C is determined as 50∶1. In this case, the CO₂ selectivity of the flue gas at the outlet of the fixed-bed reactor is decreased by 8.95 percentage points at the pyrolysis temperature of 700 ℃ after acid washing and deashing treatment, indicating that the chemical-looping combustion performance of the Zhundong coal semicoke can be significantly affected by the catalytic activity of AAEMs in the semicoke. The distribution of AAEMs on the surface of the Zhundong coal semicoke is affected by the hematite oxygen carrier. Compared with the absence of the hematite oxygen carrier, more obvious cluster enrichment on the surface of semicoke for Na and K elements is found. An obvious dependency relationship for the distribution of Ca and Mg elements is observed. When the in-situ gasification chemical-looping combustion is adopted, the active AAEMs elements in the semicoke are migrated and transformed into hematite. High melting point minerals such as nepheline (NaAlSiO₄), potassium feldspar (KAlSi₃O₈), gehlenite (Ca₂Al₂SiO₇), and magnesium olivine (Mg₂SiO₄) are generated, effectively suppressing the volatilization of AAEMs in the Zhundong coal semicoke.