In this study, 11 coal mono-combustion tests and four co-combustion tests of coal and tannery sludge were conducted on a 35 kW fluidized-bed combustor. The combustion behavior and emission characteristics of the fuels were investigated on a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of an excess air ratio, primary air rate, secondary air ratio, and fuel type on flue gas emissions were studied. The results showed that the fluidization status and temperature distribution had direct influences on CO emission. Sufficient fluidization and high temperature effectively reduced CO emission. NOx emission was relatively sensitive to the excess air ratio and increased with increasing excess air ratio. By comparing BFB and CFB, we found that CFBs have an advantage in optimizing combustion and controlling emissions by enhancing mixing and increasing freeboard temperatures. The Cr speciation and distribution among different ash types were extensively investigated in four co-combustion tests. The results showed that the distribution modes of Cr in BFBs and CFBs were different and determined by separate fluid dynamics modes of sludge ash particles in the combustor. The extent of Cr oxidation in ash in CFB tests was higher than that in BFB tests, particularly for bottom ash and heat exchanger ash, due to longer residence times in high-temperature regions.