Search

The Xinjiang lignite mined from Shaerhu coalfield (SEHc) easily causes severe fouling and corrosion because of its high sodium and chlorine contents. Therefore, it is necessary to study the migration and transformation behavior of sodium and chlorine during combustion in order to reveal the mechanisms of fouling and corrosion, and propose the effective solutions of above problems. In this study, based on the 0.4 T/D circulating fluidized bed (CFB) test system, the migration and transformation behavior of sodium and chlorine in SEHc during combustion at 950 °C was explored. The migration and transformation paths of sodium and chlorine were proposed through the chemical characterization of ash samples along the flue gas flow direction, as well as the thermodynamic equilibrium calculation by the software of Factsage 6.1. The experimental studies show the sodium and chlorine mainly in the form of NaCl crystal in raw coal underwent a series of physical and chemical changes during combustion, and subsequently distributed in bottom ash/circulating ash, fly ash and gas phase in various forms including sodium aluminosilicates, chlorides and sodium oxides. Sodium was more inclined to be resided in ash in the form of aluminosilicates through the reactions with other minerals (SiO 2 and Al 2 O 3), while chlorine was easily released into the flue gas in forms of HCl, Cl 2 , NaCl, etc. The Cl-based species might result in the corrosion of metal heating surfaces because of the presence of corrosion products (metal chlorides) in fly ash. As temperature decreased, the sodium or chlorine vapors would successively deposit in fly ash via physical condensation or chemical reaction. At 840∼570 °C, the sodium-based species (Na 2 O and NaCl) would first deposit in fly ash, then gaseous chlorine species (NaCl, FeCl 3 and so on) primarily deposited at 570∼180 °C. • Na and Cl in SEHc mainly exist in the form of NaCl crystal. • The migration and transformation behaviors of Na and Cl greatly differed. • At 840∼570 °C, gaseous Na-based species migrated towards fly ash. • At 570∼180 °C, gaseous Cl-based species deposited on ash particles. [ABSTRACT FROM AUTHOR]

An effective bed material is crucial to remit slagging/agglomeration for high-sodium lignite during circulating fluidized bed (CFB) gasification. In this study, the exploration of effective bed material for use as slagging/agglomeration preventatives of high-sodium lignite mined from Zhundong district was conducted in a 0.4 T/D CFB test system. Three mineral materials, SiO 2 -based quartz sand, Al 2 O 3 -based corundum and CaO/Fe 2 O 3 -rich boiler ash collected from bottom ash of a CFB industrial boiler, were selected as the potential bed material. Their impacts on the de-fluidization tendency, transformation and migration of alkali metals, priority of reactions involving sodium-based species, formation of liquid slags and ash fusibility were investigated through the characterization of ash samples and thermodynamic equilibrium calculation by Factsage 6.1. The particle-size analysis results presented the improvement of three bed materials in the growth of ash particles followed the order of quartz sand > corundum > boiler ash. This order was ascribed to their reaction priority with sodium-based compounds in coal and the chemical property (mainly referring to ash fusibility) of corresponding products. Generally, reactions between sodium-based species and minerals (Al 2 O 3 and SiO 2 ) were the main way of sodium retention within the ash in gasifier. As boiler ash was used, however, Al 2 O 3 and SiO 2 preferentially reacted with the CaO enriched in boiler ash rather than sodium-based species in coal, resulting in very low sodium retention. Under such circumstances, computing results suggested the ash in gasifier could withstand higher gasification temperature, consistent with the measured results of ash fusibility. Additionally, the gasification temperature operating range could be largely widened by simply increasing CaO/Fe 2 O 3 content in gasified ash. Therefore, boiler ash is an effective and low-cost bed material for use as the slagging/agglomeration preventative of high-sodium lignite during CFB gasification. [ABSTRACT FROM AUTHOR]

A high-sodium, high-chlorine Xinjiang lignite was used as fuel in a 0.4 t/d circulating fluidized bed (CFB) test system to investigate ash deposition and corrosion characteristics using an air-cooled stainless steel probe. Different wall temperatures of the probe, exposed to the similar flue gas environment for 8 h, were achieved by adjusting cooling air flow separately at 1.0 m 3 /h, 3.0 m 3 /h and 5.0 m 3 /h. The probe wall temperature variation was analyzed, and deposited ash was characterized via a series of analytical techniques. Experimental results confirmed the disturbance of ash deposition to the heat flux through the probe, and this disturbance was enhanced due to the faster ash deposition rate at lower wall temperature. The selective deposition of mineral species including Na-, Cl-, K- and Mg-species occurred, resulting in a larger ash deposition amount on the lower-temperature probe. The corrosion caused by NaCl-rich deposited ash was accompanied with the oxidation of metallic matrix. It is found that the antioxidation of metals was in the order of Ni > Cr > Fe. During corrosion, metal chlorides with strong volatility were the main corrosion products, resulting in the presence of voids in deposited ash because of their release. This corrosion was accelerated at high temperatures and had repeatability. [ABSTRACT FROM AUTHOR]