Multihearth furnaces

Mu/tihearth Furnace. Multihearth furnaces are most often used for incineration of municipal and industrial sludges, and for generation and reactivation of char. The main components of the multihearth are a refractory-lined shell, a central rotating shaft, a series of soHd flat hearths, a series of rabble arms having teeth for each hearth, an afterburner (possibly above the top hearth), an exhaust blower, fuel burners, an ash removal system, and a feed system. 

The devolatilized coal particles are transported to a direct-fired multihearth furnace where they are activated by holding the temperature of the furnace at about 1000°C. Product quaUty is maintained by controlling coal feed rate and bed temperature. As before, dust particles in the furnace off-gas are combusted in an afterburner before discharge of the gas to the atmosphere. Finally, the granular product is screened to provide the desired particle size. A typical yield of activated carbon is about 30—35% by weight based on the raw coal. 

Because powdered activated carbon is generally used in relatively small quantities, the spent carbon has often been disposed of in landfills. However, landfill disposal is becoming more restrictive environmentally and more costiy. Thus large consumers of powdered carbon find that regeneration is an attractive alternative. Examples of regeneration systems for powdered activated carbon include the Zimpro/Passavant wet air oxidation process (46), the multihearth furnace as used in the DuPont PACT process (47,48), and the Shirco infrared furnace (49,50). 

Numerous industrial solid wastes are disposed of in incinerators that have energy recovery capability. Most of these systems are smaller than MSW incinerators. The compositions of specific industrial wastes are more uniform than those of MSW, but the range of waste categories is so broad that special hardware and furnaces must sometimes be used. Rotary kilns, multihearth furnaces, and fluidized-bed incinerators have been employed for industrial waste incineration systems. 

Multihearth furnaces (fig. 1.9) are a variation of the rotary hearth furnace with many levels of round stationary hearths with rotating rabble arms that gradually plow granular or small lump materials radially across the hearths, causing them to eventually drop through ports to the next level. 

Scrap oxidation is performed either in push-type or in rotary furnaces. Also, multihearth roasting furnaces are employed. 

Continuous dryers, ovens, incinerators, and furnaces take any of a variety of forms such as rotary drum, tower, shaft, tunnel oven, multihearth (Herreshoff) kiln, and fluidized bed. As with all continuous furnaces, their design is very dependent on how the load(s) can be moved through the furnace (or occasionally, how the furnace can be moved over the loads). 

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