Page 261 - Wastewater Solids Incineration Systems

Incinerator Operations 223

An example follows:
• Feed rate: 4536 wet kg/h (10 000 lb/hr) or 108 864 wet kg/d (240 000 lb/d)
• Dry solids: 33%
(0.33) (108 864)
35 925 dry kg/d (79 200 lb/d)
• Volatile solids: 70%
(0.70) (35 925 dry kg/d)
25 147 kg/d (55 440 lb/d) volatile solids
• Inerts (ash) 35 925 25 147 kg/d 10 778 kg/d (23 760 lb/d)
• Heating value 25 575 kJ/kg (11 000 Btu/lb) combustibles (volatile solids)
• Q (Btu volatile solids/d)/(dry solids/d)
(25 575 kJ/kg 25 147 kg volatile solids/d)/35 925 dry solids/d
17 902 kJ/kg (7700 Btu/lb) of dry solids
• P (8141)/(17 902 8141) ) 100 31% dry solids
An incinerator that is operated far below the point of autogenous combustion
will result in excessive use of auxiliary fuel. Operating far above the point of autoge-
nous combustion will result in the use of additional cooling air to prevent high incin-
erator temperatures and clinker formation. Optimum combustion and incinerator
process control occur when the percentage of solids in the feed cake are at or slightly
greater than the autogenous combustion point. The theoretical point of autogenous
combustion should be compared to the actual operational point of autogenous com-
bustion. To establish autogenous conditions, the operator must frequently observe
conditions inside the incinerator through a burner view port to watch for signs of
combustion when auxiliary burners are shut down.
Optimum conditions at startup occur when the auxiliary burners are fired to
match the heat energy required for excess moisture evaporation. When the oper-
ator observes that feed cake solid ignition has occurred, the auxiliary burners
should be reduced. The extent of the auxiliary burner reduction depends on the
heat energy produced from the burning feed. To ensure that the combustion reac-
tion continues, total heat available from the auxiliary burners and the feed solids
combustion must be equal to or greater than the heat required for evaporation.
Therefore, as more of the feed solids mass begins to burn, the auxiliary burners
should be decreased accordingly.