System for controlling the operation of a multiple hearth furnace

In a method for combusting a combustible material in a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said method comprising the steps of:

• introducing a mixture containing the combustible material and water into the upper end of the furnace,
• passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustible material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture,
• supplying air to the combustion zone to provide oxygen for combusting the combustible material,
• passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• removing the cooled residue from the lower end of the furnace;
• the improvement comprising the steps of:
• recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• sensing the temperature in the combustion zone, and
• controlling the flow of said recirculated gases in response to said sensed temperature to provide the quantity of said recirculated gases in the combustion zone required to achieve a balance between the heat produced and the heat absorbed in the combustion zone.

In a method for combusting a combustible material in a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said method comprising the steps of:

• introducing a mixture containing the combustible material and water into the upper end of the furnace,
• passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustible material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture,
• supplying air to the combustion zone to provide oxygen for combusting the combustible material.
• passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• removing the cooled residue from the lower end of the furnace;
• the improvement comprising the steps of:
• recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• measuring the oxygen content of said recirculated gases,
• comparing the oxygen content of said recirculated gases with a predetermined minimum oxygen content, and
• controllably supplying additional air to the combustion zone should the oxygen content of said recirculated gases become less than said minimum content to provide the quantity of said additional air in the combustion zone required to achieve a balance between the heat produced and the heat absorbed in the combustion zone and to increase the oxygen content of said recirculated gases above said minimum content.

In a method for combusting a combustible material in a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said method comprising the steps of:

• introducing a mixture containing the combustile material and water into the upper end of the furnace,
• passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustion material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture,
• supplying air to the combustion zone to provide oxygen for combusting the combustible material,
• passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• removing the cooled residue from the lower end of the furnace;
• the improvement comprising the steps of:
• recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• measuring the oxygen content of said recirculated gases,
• reducing the flow of said recirculated gases should the oxygen content thereof become less than a predetermined minimum content,
• sensing the temperature in said combustion zone, and
• supplying additional air to said combustion zone in response to said sensed temperature to provide the quantity of said additional air in the combustion zone required to maintain said combustion zone temperature at a substantially constant predetermined temperature independently of the heat produced in the combustion zone and to increase the oxygen content of said recirculated gases above said minimum content.

In a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said furnace comprising:

• an opening in the upper end of the furnace through which a mixture containing a combustible material and water is introduced into the furnace,
• means for passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• means for passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustible material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture.
• means for supplying air to the combustible zone to provide oxygen for combusting the combustible material,
• means for passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• an opening in the lower end of the furnace through which the cooled residue is removed from the furnace;
• the improvement comprising:
• means for recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• means for sensing the temperature in the combustion zone, and
• means for controlling the flow of said recirculated gases in response to said sensed temperature to provide the quantity of said recirculated gases in the combustion zone required to maintain said combustion zone temperature at a substantially constant predetermined temperature independently of the heat produced in the combustion zone.

In a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said furnace comprising:

• an opening in the upper end of the furnace through which a mixture containing a combustible material and water is introduced into the furnace.
• means for passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• means for passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustible material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture,
• means for supplying air to the combustion zone to provide oxygen for combusting the combustible material,
• means for passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• an opening in the lower end of the furnace through which the cooled residue is removed from the furnace;
• the improvement comprising:
• means for recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• means for measuring the oxygen content of said recirculated gases,
• means for comparing the oxygen content of said recirculated gases with a predetermined minimum oxygen content, and
• means for controllably supplying additional air to the combustion zone should the oxygen content of said recirculated gases become less than said minimum content to provide the quantity of said additional air in the combustion zone required to achieve a balance between the heat produced and the heat absorbed in the combustion zone and to increase the oxygen content of said recirculated gases above said minimum content.

In a multiple hearth furnace having a plurality of superposed hearths defining an upper drying zone, an intermediate combustion zone and a lower cooling zone, said furnace comprising:

• an opening in the upper end of the furnace through which a mixture containing a combustible material and water is introduced into the furnace,
• means for passing the mixture downwardly through the drying zone wherein the mixture is dried by exposure to gases heated in the combustion zone,
• means for passing the dried mixture downwardly from the drying zone through the combustion zone wherein the combustible material is combusted using air as the oxidant, the gases produced by the combustion of said material flowing upwardly into the drying zone to provide heat for drying said mixture.
• means for supplying air to the combustion zone to provide oxygen for combusting the combustible material,
• means for passing the solid residue uncombusted by said combustion downwardly from the combustion zone through the cooling zone wherein the residue is cooled by exposure to gases cooler than the gases produced by said combustion, and
• an opening in the lower end of the furnace through which the cooled residue is removed from the furnace;
• the improvement comprising:
• means for recirculating gases from the drying zone back into the combustion zone to provide relatively low temperature gases in the combustion zone for absorbing excess heat generated by said combustion,
• means for measuring the oxygen content of said recirculated gases,
• means for reducing the flow of said recirculated gases should the oxygen content thereof become less than a predetermined minimum content,
• means for sensing the temperature in said combustion zone, and
• means for supplying additional air to said combustion zone in response to said sensed temperature to provide the quantity of said additional air in the combustion zone required to maintain said combustion zone temperature at a substantially constant predetermined temperature independently of the heat produced in the combustion zone and to increase the oxygen content of said recirculated gases above said minimum content.