A high-sodium lignite from the Freedom mine in North Dakota was tested in the transport gasifier at the Power Systems Development Facility (PSDF). During the first use of the high-sodium lignite in October 2003, agglomerated deposits formed at various locations in the transport gasifier system. An extensive laboratory testing program was carried out to characterize the deposits, understand the mechanism of the deposit formation, and test various methods of preventing or minimizing the agglomeration. The results of the deposit analysis and initial lab studies suggested that sodium released from the lignite was deposited on the surface of the sand bed material, resulting in the formation of sticky sodium silicates. Additional laboratory tests indicated that the agglomeration could be avoided or minimized by replacing the sand with a nonreactive bed material (e.g., coarse coal ash), operating at slightly reduced temperatures and using certain types of additives. By using these procedures, we completely eliminated the deposition problems in a subsequent gasification run in August 2004.

Various additives were evaluated to assess their ability to prevent ash agglomeration during the gasification of high-sodium lignite. Additives that showed promise in simple muffle furnace tests included meta-kaolin, vermiculite, two types of silica fume, and one type of bauxite. Additives that were tested and rejected included dolomite, calcite, sand flour, kaolinite, fine kaolin, and calcined bauxite. Based on the muffle furnace test results, the meta-kaolin was selected for a follow-on demonstration in a pilot-scale coal gasifier. Pilot-scale testing showed that the addition of coarse (minus 14-mesh, 920-μm mean size) meta-kaolin at a feed rate roughly equivalent to the ash content of the lignite (∼10 wt %) successfully prevented agglomeration and deposition problems during gasification of high-sodium lignite at a maximum operating temperature of 927 °C (1700 °F).

No abstract