Improved Materials for High-Temperature Black Liquor Gasification (2006)

The laboratory immersion test system built and operated at ORNL was found to successfully screen samples from numerous refractory suppliers, including both commercially available and experimental materials. This system was found to provide an accurate prediction of how these materials would perform in the actual gasifier environment. Test materials included mullites, alumino-silicate bricks, fusion-cast aluminas, alumina-based and chrome-containing mortars, phosphate-bonded mortars, coated samples provided under an MPLUS-funded project, bonded spinels, different fusion-cast magnesia-alumina spinels with magnesia content ranging from 2.5% to about 60%, high-MgO castable and brick materials, spinel castables, and alkali-aluminate materials. This testing identified several candidate material systems that perform well in the New Bern gasifier. Fusion-cast aluminas were found to survive for nearly one year, and magnesia-alumina spinels have operated successfully for 18 months and are expected to survive for two years. Alkali-aluminates and high-MgO-content materials have also been identified for backup lining applications. No other material with a similar structure and chemical composition to that of the fusion-cast magnesium-aluminum spinel brick currently being used for the hot-face lining is commercially available. Other materials used for this application have been found to have inferior service lives, as previously discussed. Further, over 100 laboratory immersion tests have been performed on other materials (both commercial and experimental), but none to date has performed as well as the material currently being used for the hot-face lining. Operating experience accumulated with the high-temperature gasifier at New Bern, North Carolina, has confirmed that the molten alkali salts degrade many types of refractories. Fusion-cast alumina materials were shown to provide a great improvement in lifetime over materials used previously. Further improvement was realized with fusion-cast magnesia-alumina spinel refractory, which appears to be the most resistant to degradation found to date, exhibiting over a year of service life and expected to be capable of over two years of service life. Regarding the use of refractory mortar, it was found that expansion of the current chrome-alumina mortar when subjected to black liquor smelt is likely contributing to the strains seen on the vessel shell. Additionally, the candidate high-alumina mortar that was originally proposed as a replacement for the current chrome-alumina mortar also showed a large amount of expansion when subjected to molten smelt. A UMR experimental mortar, composed of a phosphate bonded system specifically designed for use with fusion-cast magnesium-aluminum spinel, was found to perform well in the molten smelt environment. Strain gauges installed on the gasifier vessel shell provided valuable information about the expansion of the refractory, and a new set of strain gauges and thermocouples has been installed in order to monitor the loading caused by the currently installed spinel refractory. These results provide information for a direct comparison of the expansion of the two refractories. Measurements to date suggest that the fusion-cast magnesia-alumina spinel is expanding less than the fusion-cast {alpha}/{beta}-alumina used previously. A modified liquor nozzle was designed and constructed to test a number of materials that should be more resistant to erosion and corrosion than the material currently used. Inserts made of three erosion-resistant metallic materials were fabricated, along with inserts made of three ceramic materials. The assembled system was sent to the New Bern mill for installation in the gasifer in 2005. Following operation of the gasifier using the modified nozzle, inserts should be removed and analyzed for wear by erosion/corrosion. Although no materials have been directly identified for sensor/thermocouple protection tubes, several of the refractory material systems identified for lining material applications may be applicable for use in this capacity. Results of the modeling studies suggest that the temperature distribution is higher at the bottom of the gasifier than previously thought. Therefore, it may be possible to reduce the refractory temperature in the gasifier by changing the liquor spray. Also, modeling showed that because of the strong swirl, a separation zone could be formed at the corner of the conical wall where it meets the vertical barrel wall, and some the liquor droplets could be suspended in this zone. The accumulation of droplets in this area could cause instabilities in the performance and also in corrosion of the refractory.