Method for controlling temperatures in the afterburner and combustion hearths of a multiple hearth furnace

Claim 1

In a method of incinerating combustible waste in a multiple hearth furnace containing a series of superimposed hearths which comprises feeding the combustible waste at the upper end of the furnace and passing the waste downward through a series of combustion hearths, supplying air to the combustion hearths to combust the waste material, and discharging the inert solid products of combustion at the lower end of the furnace, while the gaseous products of combustion flow upward countercurrent to the flow of waste material through the hearths and into an afterburner to remove the malodorous gases and/or pollutants, said afterburner being located after the uppermost waste handling hearth, the improvement wherein the temperatures of the afterburner and individual combustion hearths of the multiple hearth furnace are simultaneously controlled by: (A) splitting the waste feed between (1) the uppermost waste handling hearth and (2) the hearth directly below the uppermost waste handling hearth in such proportions as to control the temperature of the afterburner to a temperature within preselected limits; and (B) controlling the supply of combustion air to the individual combustion hearths in sufficient quantities so as to operate the combustion hearths at a temperature at or below a preselected maximum temperature; wherein said steps (A) and (B) are synchronized in response to the temperature of the afterburner and also in response to the temperatures in the individual combustion hearths by (I) controlling the temperature of the uppermost waste handling hearth below temperatures which would result in thermal stress of the furnace parts beyond safe operating limits and yet high enough to maintain the temperature of the afterburner within preselected limits to remove malodorous exhaust gases, and (II) controlling the temperatures of the combustion hearths by supplying air to the individual hearths in amounts sufficient to control the temperatures of said individual hearths at temperatures at or below preselected maximum temperatures, which temperatures are below that which would cause thermal stress in the furnace parts.

Claim 5

The method as claimed in claims 1 or 2 in which the step of controlling the supply of combustion air to the individual hearths comprises directing high velocity jets of small amounts of air into the respective individual combustion hearths at constant air flow rates in amounts sufficient to create turbulence to ensure uniform mixing of the air and combustion gases so that the temperature measured in an individual hearth accurately represents the combustion conditions therein, and directing large cross-section low velocity streams of air from main air combustion jets in the respective individual combustion hearths for supplying the bulk of the combustion air to control the combustion in the hearths, said air flow rates from the main combustion jets being varied in accordance with the amount of air needed to control the temperatures in the individual hearths in response to the respective temperatures of the individual hearths.

Claim 8

The method as claimed in claims 1 or 2 which comprises incinerating autogenous sludge as the combustible material.

Claim 22

The method as claimed in claims 18 or 19 which comprises directing at least the high velocity jets are directed into the hearths near the top of the hearths, whereby the return flow sweeps over the bed of materials on the hearth with reduced turbulence for entraining emitted gases without kicking up dust from the bed of material.