Circulating fluidized bed (CFB) with low operating temperatures may become an appropriate solution to the utilization of high-Na Zhundong coal. In this paper, one kind of Zhundong coal with 3.92% Na2O in coal ash was used as fuel, and its slagging and fouling characteristics at different air equivalence ratios (ERs) were systemically investigated in 0.4 t/d CFB test system to determine a better utilization scheme of Zhundong coal. The results show that ash-related problems still occurred when this high-Na coal was used in CFB, which was greatly influenced by ER. At the reducing atmosphere (ER < 1.0), slagging obviously occurred in the riser at 940°C. Na-based species reacted with the SiO2-rich bed material to form low-melting-temperature sodium silicates (Na2O？nSiO2). The formed sodium silicates covered bed material particles and increased their stickiness. These sticky particles agglomerated and adhered onto the high-temperature metal surfaces by adhesion force or inertial force, resulting in slagging. During slagging process, Ca, Mg and Fe acted as fluxing agents. This type slagging related to the SiO2-rich bed material might be reduced or even avoided via the replacement of the bed material. At the oxidizing atmosphere (ER > 1.0), fouling on tail heating surfaces at medium-low temperatures (607–735°C) was the main ash related problem. The released AAEM-based species were transported towards low-temperature surfaces by the mechanisms of diffusion, condensation and aggregation, or thermophoresis, resulting in a layered deposition. The inner layer of deposits with micron grain sizes, which was mainly composed of Na2SO4 and K2SO4, played a key role at the initial deposition stage. Besides, two kinds of aggregated particles separately rich in alkali silicates and Ca-based species were also important for fouling.
In this paper, the influence of the air equivalence ratio on slagging and fouling characteristics of ZDc was studied in a 0.4 t/d circulating fluidized bed test device. Experimental studies show ashrelated issues still occurred during Zhundong coal utilization in CFB, compared with the high-operating-temperature reactor such as pulverized coal furnace. However, the situations were different, and the main conclusions are listed below.
(1) The ash-related problems involving slagging and fouling were greatly influenced by the reaction atmosphere. The element concentration in deposits varied with ER, which gave rise to different ash deposition problems at different ERs. For example, at the reducing atmosphere (ER < 1.0), slagging obviously occurred in the riser at 940°C, while at the oxidizing atmosphere (ER > 1.0), fouling on tail heating surfaces at medium-low temperatures (607–735°C) was the main ash-related problem.
(2) During ZDc gasification, Na-based species reacted with the SiO2-rich bed material forming low-melting-point sodium silicates (Na2O？nSiO2). The formed sodium silicates coated the bed material particles increasing their stickiness. These sticky particles agglomerated and adhered to the high temperature metal surfaces by adhesion force or inertial force, resulting in slagging. Ca, Mg and Fe in the slags acted as fluxing agents accelerating ash melting during slagging. This kind of slagging related to the SiO2-rich bed material might be reduced or even avoided via a lower operating temperature or replacement of the SiO2-rich bed material.
(3) During ZDc combustion, large amounts of AAEMs were released. When the gas temperature decreased, AAEM based species transported towards low-temperature surfaces by mechanisms of diffusion, condensation and aggregation, or thermophoresis, resulting in a layered deposition. The inner layer of deposits with micron grain sizes, which was mainly composed of alkali metal sulfates including Na2SO4 and K2SO4, played an important role during the initial deposition stage. In addition, two kinds of aggregated particles, which were rich in alkali silicates and Ca-based species, respectively, were found important for fouling.
Fig.1. Schematic diagram of 0.4 t/d circulating fluidized bed system.
