Feed Quality
Historically fluid-bed gasifiers have tended to be operated on low-rank coals such as lignite, peat, or biomass, though not exclusively. This is logical, since low-rank coals have a higher reactivity, which compensates to some degree for the lower temperature. To the extent that the ash properties allow it, operation at a higher temperature improves the ability to process high-rank but less reactive bituminous coals.
Fluid-bed gasifiers need ground coal and therefore have few limitations when it comes to coal size. Particles with a size above 10 mm should be avoided, just as coals with too high a percentage of very small particles. However, because coal particles may differ in both size and shape, it is not an easy material to fluidize. Bubbling beds especially have to be monitored constantly, on the one hand for de-fluidization due to deposition of large coal particles in the bed, and on the other hand for lifting out too many fine particles out of the bed. The latter problem is aggravated as coal particles reduce in size through gasification and are entrained in the hot raw gas as it leaves the reactor. Most of these char and ash particles are recovered in a cyclone and recycled to the reactor, but with too many fines these will choke the system.