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HIGH-SODIUM LIGNITE GASIFICATION WITH THE PSDF TRANSPORT GASIFIER

High-sodium lignite from the Freedom mine in North Dakota was tested in a Transport Gasifier at the Power Systems Development Facility (PSDF). The tests occurred in October 2003 and August 2004 for a total of 445 hours of operation. Due to its high sodium content, the Freedom mine lignite presented unusual challenges to gasifier operations. This paper discusses high-sodium lignite gasification operation and quantifies the potential for deposit formation. The paper also includes the results of extensive lab tests on deposit characterization, briefly covers the mechanics of deposition, and explores the use of additives to prevent deposits from agglomerating in the unit. During the first use of the high-sodium Freedom lignite in October 2003, agglomerated deposits were formed at various locations in the Transport Gasifier system. An extensive laboratory testing program was carried out to characterize the deposits, to understand the mechanism of the deposit formation, and to 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 replacement of the sand with a non-reactive bed material (e.g., coarse coal ash), by operating at slightly reduced temperatures, and by the use of certain types of additives. In the second use of the high-sodium Freedom lignite in August 2004, several of the remedial procedures that were tested in the laboratory were applied during the gasifier operation. Prior to the introduction of the high-sodium Freedom lignite, much of the sand bed material was replaced with coarse ash from PRB coal, and gasifier operating temperatures were slightly reduced (about 100°F below the temperatures used in the October 2003 run). Dolomite was also fed into the gasifier to help minimize the contact between any sticky particles. By the use of these procedures, the deposition problems that were seen in the October 2003 run were completely eliminated in the August 2004 run.