Fluidized Bed Disposal of Secondary Sludge High in Inorganic Salts

The authors have presented an excellent example of the total systems approach to overcoming a dif­ ficult waste disposal problem. They have presented useful guideline data for fluid bed incinerators which are increasingly being adopted for thermal destruc­ tion of difficult organic sludges. Although not of concern in the design described, the fluid bed incinerator has the special capability of being shutdown for relatively long periods but still being able to effect a rapid startup due to the thermal fly wheel effect of the bed. This consideration is especially important for the incineration of sludges such as those described by the authors. Implied, but not stated, is that the incinerator will operate with low excess air ratios. While I agree with the authors that the thermal fly wheel effect in a fluidized bed is considerably greater than the other processes evaluated, I am sure that automatic control provisions have been made to prevent the temperatures in the freeboard and the bed from dropping below a predetermined minimum or going above a predetermined maximum and a brief description would have strengthened the presentation. There was no mention of the provisions made for bringing the incinerator to start-up temperatures; presumably this important function is performed by an auxiliary burner, since direct injection of fuel gas into the bed can only be accomplished near the normal operating temperature range. From the process design viewpoint, the authors should have stated the conditions at which the super­ ficial fluidizing velocity is reported, that is, is it at operating temperature and atmospheric pressure or is another basis chosen? Also, useful guidelines which could have been provided are the design heat release rate on some basis such as superficial cross-