Investigation of additives for preventing ash fouling and sintering during barley straw combustion

• Kaolin and zeolite 24 have high capacities to capture KCl.
• KCl capturing efficiencies of kaolin and zeolite 24 decreased at a high temperature and with long reaction time.
• Barley straw ash has a high sintering and melting tendency.
• Addition of kaolin and zeolite 24A increased melting temperature of the barley straw ash.
• High melting points K–Al-silicates formed in the barley straw ash upon additive addition.

Formation of potassium chloride reduces ash sintering temperature and causes fouling deposits in biomass combustion applications. In the present work, the capacity of two mineral additives zeolite 24A and kaolin to capture KCl were investigated. A series of thermogravimetric experiments were carried out to measure fractions of KCl retained in the two additives as function of reaction temperature and heating time. The residues from additive-KCl mixtures after heating treatment were analyzed by X-ray diffractometry (XRD). When heated at 900 °C for 1 h, the overall KCl capturing efficiencies of the two additives were 60% and 45% for zeolite 24A and kaolin respectively, which slightly decreased to 50% and 43% as the heating time increased to 12 h. At 1000 °C, the fractions of KCl captured by zeolite 24A and kaolin significantly decreased from 50% and 40% to 26% and 17%, as the KCl-additive mixtures were heated for 1 and 12 h, respectively. The decrease in of the overall KCl capturing efficiencies is mainly attributed to reduction of surface areas and chemically active compounds of the two additives with increasing temperature and heating time. The XRD analysis results showed that both zeolite 24A and kaolin can react with KCl to form different potassium aluminium silicates. It indicates that chemical reactions play an important role in the overall capturing process. The effects of zeolite 24A and kaolin on sintering behaviors of the barley straw ash were also investigated. The residues from sintering tests were analyzed by a combination of X-Ray diffractometry (XRD) and scanning electron microscopy equipped with energy dispersive X-Ray analysis (SEM-EDX). The barley straw ash melted intensively at elevated temperatures. Together with XRD analysis, the SEM-EDX analysis results revealed that severe melting of the barley straw ash was due to formation and fusion of low temperature melting potassium silicates. Addition of kaolin and zeolite 24A significantly reduced the sintering tendency of the barley straw ash. Upon additive addition, high temperature melting potassium aluminium silicates formed in the barley straw ash as revealed by XRD and SEM-EDX analyses. Formation and presence of the refractory potassium aluminium silicates partly explain the improved sintering behaviors of the ash-additive mixtures.