In this study, the combustion and emission characteristics of slurry from an Indonesian lignite washery tailing (LWT) and a bituminous coal were studied and compared in both a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of excess air ratio, feeding rate, secondary air ratio, and secondary air location on the flue gas emissions were analyzed to investigate the optimal operating conditions. The results show that it is feasible to directly fire LWT slurry in a CFB. The temperature of the dense-phase bed firing the LWT slurry can reach 800 degrees C but is lower than that of a normal BFB/CFB firing coal. Both SO2 and NO emissions of bituminous coal are higher than those of LWT slurry, but their emission characteristics are still below the limits for both EU and Chinese regulations. Improved emission characteristics are observed when firing LWT slurry and coal in CFB compared to BFB. Two main differences were found between LWT slurry and coal: (1) Low primary air (PA) flow rates facilitate the defluidization of the FB combustor when firing LWT slurry. (2) High feeding rates of LWT slurry may reduce the temperature along FB instead. These factors should be paid special attention when designing and operating an FB system. Optimal secondary air (SA) ratios were determined for both LWT slurry and coal. The SA should not be located near the distributor. All of the analyses in this study provide useful information for the design and operation of a commercial FB system.