Abstract:
This application concerns methods and apparatus for use in industrial waste recovery operations such as recovery of non-consumed chemicals in industrial processes, with recovery of quick lime in a wood pulp process being an example. In some embodiments, methods comprise baking lime sludge in a kiln and controlling a temperature in a calcining zone of the kiln to be above about 2250° F. to vaporize sodium contained in the lime sludge. Interaction of the vaporized sodium with SO x  can deter accumulation of one or both of CaCO 3  and CaSO 4  on one or more inner surfaces of the kiln. In some embodiments, lime sludge can be rinsed to generate a filtrate comprising dissolved NaOH, and the filtrate can charge a scrubber for removing SO x  from an exhaust from the kiln. Embodiments of co-fired burners for heating such kilns by burning petroleum coke and natural gas are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of U.S. Provisional Patent Application 61/065,108, filed Feb. 8, 2008, which is hereby incorporated. 
    
    
     FIELD 
     This application concerns methods and apparatus for use in industrial waste recovery operations such as recovery of non-consumed chemicals in industrial processes, with recovery of quick lime in a wood pulp process being an example. 
     BACKGROUND 
     In making paper and other pulp products, cellulosic fiber, such as for example wood, is chemically digested in a continuous or a batch process. Usually, the fiber is charged together with a cooking liquid such as a white liquor having certain desirable chemicals for dissolving a majority of the lignin contents of the wood. Pulp so formed is typically washed or rinsed, and separated from the cooking liquid. The filtrate from the rinse forms a weak black liquor. 
     As used herein, “black liquor” means the waste product that results from separating the pulp from the cooking liquid subsequent to digesting the cellulosic fiber. Black liquor is usually rich in valuable chemicals, some of which can be recovered to produce additional and/or cooking liquid for use in the digester. For example, black liquor can be concentrated by evaporating a major portion of its water contents in an evaporation plant and some of the chemicals can be recovered in the form of Na 2 CO 3 . The concentrated black liquor is combusted in a recovering furnace to produce desirable process steam and a smelt having certain desirable chemicals that can be dissolved in water to form a green liquor. 
     The Na 2 CO 3  (sodium carbonate) is used to produce NaOH (sodium hydroxide), an ingredient used to produce the cooking liquid, by treating the concentrated green liquor with burnt lime, also known as quick lime, (CaO). The causticizing reaction just described and used to produce the sodium hydroxide is shown in Equations 1 and 2.
 
CaO+H 2 O→+Ca(OH) 2 +Heat  Equation 1
 
Na 2 CO 3 +Ca(OH) 2 →2NaOH+CaCO 3   Equation 2
 
     An additional recovery process is usually applied to “close” the cycle and recover quick lime from the lime sludge (also known as lime mud), which includes CaCO 3 . NaOH and solutions with dissolved NaOH, such as aqueous NaOH, can also be recovered from the lime sludge. After rinsing, the lime sludge is heated in a lime kiln to evaporate any remaining water, and then heated further in a reburning process to recover the quick lime according to the stoichiometric reaction shown in Equation 3.
 
CaCO 3 +energy→CaO+CO 2   Equation 3
 
     Many kilns used to recover quick lime from lime sludge are heated by a continuous heat source, such as by continuous combustion of natural gas. Fuel costs for kilns heated only by combustion of natural gas are high, and combustion of natural gas only usually leads to peak flame temperatures in excess of 2800° F., which undesirably forms oxides of nitrogen (NO N ). In addition, some chemicals in the cooking liquid and green liquor (and thus the lime sludge), as well as natural gas, contain sulfur. Consequently, the high combustion temperature of natural gas usually forms oxides of sulfur (SO x ) in addition to the NO N , which makes compliance with emissions requirements difficult. 
     SUMMARY 
     Methods for recovering lime from a manufacturing process are disclosed. Such methods include baking lime sludge in a kiln and controlling a flame temperature of a flame so that a temperature in a calcining zone of the kiln is above about 2250° F. to vaporize sodium contained in the lime sludge. Interaction of the vaporized sodium with SO x  deters accumulation of one or both of CaCO 3  and CaSO 4  on one or more inner surfaces of the kiln. 
     A fluid fuel can provide a continuous ignition source for co-firing a pulverized solid fuel. The fluid fuel can be natural gas, and a flow rate of the natural gas can be between about 10 MCF and about 20 MCF. Petroleum coke can be co-fired with natural gas to produce the flame. In some embodiments, natural gas is continuously burned as a primary ignition source, and petroleum coke is injected into the primary ignition source from above. 
     The act of controlling the flame temperature can comprise one or more of selecting a volumetric flow rate of an oxidizer, selecting a volumetric flow rate of a fuel-supply inlet stream carrying entrained particles of petroleum coke, and selecting respective flow rates of petroleum coke and fluid fuel. A volumetric flow rate of a fuel-supply stream can be between about 550 CFM and about 850 CFM. A flow rate of petroleum coke can be between about 50 pounds per minute and about 60 pounds per minute. 
     Other methods of recovering lime are also disclosed. Such methods include rinsing a lime sludge with a rinse to generate a filtrate comprising dissolved NaOH and baking the rinsed lime sludge in a kiln exhausting at least some SO x . At least a portion of the SO x  can be scrubbed from the exhaust in a scrubber at least partially charged with the filtrate comprising dissolved NaOH. Quick lime can be removed from the kiln. 
     Kilns for recovering lime are also disclosed. Some such kilns have an entrance region for receiving lime sludge, and define a calcining region disposed opposite the entrance region. A co-fired burner for burning pulverized solid fuel can be located in or near the calcining region. The co-fired burner can include a fluid-fuel injector for providing a continuous ignition source and an injector body positioned above the fluid-fuel injector for injecting a pulverized solid fuel downwardly into the continuous ignition source. As noted above, the pulverized solid fuel can be petroleum coke. 
     In some kilns, the fluid-fuel injector comprises one or more turning vanes for mixing a fluid fuel with an oxidizer. The injector body can comprise a nozzle for turning a stream of the solid fuel between about 15 degrees and about 25 degrees. Some injector bodies comprise a tube having an inner-diameter of about 4 inches. Petroleum coke can be injected by such an injector body. In at least some kilns, the calcining region is positioned below the co-fired burner. 
     Systems for recovering lime are also disclosed. Such systems include a lime sludge washer for rinsing a lime sludge with a rinse and producing a filtrate. A scrubber in fluid connection with the washer can receive the filtrate from the washer. The filtrate can comprise a solution of NaOH. A kiln for baking lime sludge can have an exhaust in fluid connection with the scrubber for exhausting kiln exhaust products at least partially through the scrubber. The scrubber can be configured to scrub SO x  from the kiln exhaust products with the filtrate. 
     Kilns as disclosed herein can be used in such systems. For example, kilns having a co-fired burner can be used in such systems. Such co-fired burners can include a first injector for injecting natural gas for providing a continuous ignition source inside the kiln and a second injector positioned above the first injector comprising a nozzle for injecting a stream of petroleum coke into the continuous ignition source. 
     The foregoing and other features and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary liquor cycle in a wood pulp manufacturing process. 
         FIG. 2  illustrates a schematic of an exemplary apparatus for recovery of quick lime. 
         FIG. 3  illustrates a schematic of an exemplary co-fired burner that can be used in an apparatus for industrial waste recovery processes, such as the recovery of quicklime. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes embodiments of methods and apparatus for recovering materials from industrial waste, such as recovering lime from a pulp process. 
     The following makes reference to the accompanying drawings which form a part hereof, wherein like numerals designate like parts throughout. The drawings illustrate specific embodiments, but other embodiments may be formed and structural changes may be made without departing from the intended scope of this disclosure. Directions and references (e.g., up, down, top, bottom, left, right, rearward, forward, heelward, etc.) may be used to facilitate discussion of the drawings but are not intended to be limiting. For example, certain terms may be used such as “up,” “down,”, “upper,” “lower,” “horizontal,” “vertical,” “left,” “right,” and the like. These terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” surface can become a “lower” surface simply by turning the object over. Nevertheless, it is still the same surface, and the object remains the same. 
     Accordingly, the following detailed description shall not be construed in a limiting sense. 
     Exemplary Liquor Cycle 
       FIG. 1  illustrates an exemplary liquor cycle in an exemplary wood pulp manufacturing process. In the cycle shown in  FIG. 1 , wood chips  301  are combined with a cooking liquid that includes a mixture of white liquor  303  and black liquor  310  in a digester  302 . Gases from the digester  302  are transferred (e.g., by a pressure differential) to a blow tank  304  where they are condensed and discarded as a waste product  305  to a suitable waste site  315 . 
     The mixture of digested chips and cooking liquid is moved to a washer apparatus  306  where the pulp  307  is rinsed, such as for example by water, and separated from the mixture. The filtrate  308  is moved to a weak black liquor storage  309 . A portion  310  of the weak black liquor can be used to at least partially recharge the digester  302 . 
     The remaining weak black liquor  311  is concentrated by evaporating excess volatiles, including water, in one or more evaporators  312 . The gaseous products of evaporation  313  are blown off and condensed to be disposed of in the suitable waste site  315 . The black liquor so concentrated is stored in a strong black liquor storage apparatus  314  before entering a recovering furnace  316  where the black liquor is baked to form smelt  317 . 
     The smelt  317  is dissolved in a dissolving tank  318  and the resulting solution, green liquor, is transferred to a green liquor clarifier  319 . Dregs  320  are filtered out and transferred to a dregs washer  324  where the precipitated dregs are washed, for example, with water  323 . The resulting filtrate  327  can be stored in a weak liquor storage  328  and recombined with the smelt  317  in the dissolving tank  318 . The washed dregs  325  can be disposed of and transferred to a suitable waste site  326 . 
     The green liquor filtrate from the green liquor clarifier  319  can be stored in a green liquor storage  140  and/or transferred to a slaker  150 . In the slaker  150 , the green liquor is combined with lime  101 . By combining the green liquor and the lime  101 , the reaction of Equation 1 occurs. The reaction of Equation 2 can also be carried out in the slaker  150 , but as the reaction drives toward completion, it slows. The resulting mixture containing NaOH and CaCO 3  can then be transferred to the causticizer  160  after removing the grits for disposal. While in the slaker  150  and the causticizer  160 , the mixture can be kept well agitated to assist completion of the reaction and to prevent precipitation of the CaCO 3 . The causticizer  160  can be used to allow more time for the reaction of Equation 2 to complete. 
     The products of Equation 2, including precipitated CaCO 3  and NaOH in solution, can be transferred to the white liquor clarifier  321  where the white liquor can be filtered and transferred to the white liquor storage  322 . The clarifier  321  provides residence time and slow movement to allow the CaCO 3  to settle to the bottom of the clarifier. The corresponding lime mud (CaCO 3 )  327  can be collected from the bottom of the clarifier and transferred to the lime recovery cycle  200 , which can be referred to as the “Miller Process.” 
     In the lime recovery cycle  200 , the lime sludge  327  can be transferred to the lime mud washer  113  where the lime sludge  327  can be rinsed with a rinse  115  capable of dissolving NaOH in solution, e.g., water for an aqueous solution of NaOH. Filtrate  109  from the lime mud washer  113  containing a solution of NaOH, such as aqueous NaOH, can be transferred to a weak liquor storage  117 . A solution  119  containing sodium ion, such as for example stored weak liquor (e.g., the filtrate  109 ) can be fed to a scrubber  108  for scrubbing gas products  102  emitted from the kiln  100 . Such scrubbing is described in more detail below. 
     The washed lime sludge  116  can be transferred to a lime sludge thickener  114  (referred to as a “lime mud precoat filter” in  FIG. 2  and the corresponding description below) for thickening the lime mud  116  by, for example, vacuum removal of rinse solution through a filtration assembly. Additional filtrate  109  collected from the lime sludge thickener  114  can be transferred to the weak liquor storage  117 . As described in further detail below, a lime mud thickener  114  can be located at an entrance of a kiln  100  in some embodiments. Once sufficiently thickened, the lime sludge  84 , (e.g., primarily CaCO 3 ) can be transferred to a lime kiln  100  (or a baking region thereof, such as a calcining region) and baked at a sufficiently elevated temperature to cause the CaCO 3  to undergo the reaction of Equation 3. The resulting CaO (lime)  101  can then be removed from the lime kiln and readied for transfer to the slaker  150 . 
     The lime kiln gases  102 , for example from combustion of the fuel used to heat the lime kiln  100 , together with the CO 2  from the reaction of Equation 3, desirably are fed through the scrubber  108 . The scrubbed kiln gases  120  can then be emitted to the environment  121 , e.g., the atmosphere. 
     Exemplary Embodiment of an Apparatus for a Lime Recovery Process 
       FIG. 2  illustrates a schematic of one embodiment of an apparatus  1  for recovering lime in an industrial process, such as a wood pulp manufacturing process. The apparatus  1  shown in  FIG. 2  implements at least portions of the lime recovery cycle  200  described above and illustrated in  FIG. 1 . 
     In the apparatus shown in  FIG. 2 , the lime mud precoat filter  114  removes solids from the washed lime sludge  116  (see  FIG. 1 ) and deposits wet lime sludge  84  on a kiln feed conveyor  92  configured to deliver the wet lime sludge  84  to the entrance  61  to the kiln  100 . Preferably, the filter  114  is configured to strain precipitated CaCO 3  from a solution having dissolved NaOH, such as a solution resulting from rinsing the lime sludge with water. 
     Typically, the rinse solution added to the lime mud precoat filter  114  can be controlled to reduce the likelihood of any remaining sodium from being entrained in the thickened lime sludge  84 . With respect to the exemplary lime mud precoat filter  114 , the rinse solution can be controlled (e.g., flow rate, NaOH concentration) to maintain the sodium entrained in the thickened lime sludge  84  at a sodium-to-sulfur molar ratio of about 2:1 with respect to the sulfur in the gas products  102 . 
     In the exemplary kiln  100 , the co-fired kiln burner  2  (also see  FIG. 3 ) is disposed at an end opposite the entrance  61  to the kiln  100 . In one embodiment, after the conveyor  92  deposits a thickened lime sludge  84  into the entrance  61  of the kiln  100 , the thickened lime sludge moves countercurrent to a flow of gas products  102  (e.g., carbon dioxide, CO 2 , various oxides of nitrogen, NO and various oxides of sulfur, SO x , arising as, for example, products of combustion, evaporated wash and reburned lime sludge) toward the end having the co-fired burner  2  as the kiln  100  rotates about a longitudinal axis of the kiln. In this exemplary embodiment, the kiln is sloped downwardly from the entrance  61  to the end having the co-fired burner such that the lime “rolls” in a cascading fashion within the kiln. Although not a feature of all kilns (e.g., kilns having a lesser slope from the entrance to the exit), the exemplary kiln includes an internal dam for retaining the lime sludge  84  within the kiln for a longer period of time as compared to a kiln without the dam and having the same slope, increasing the time available for the reaction of equation 3 to complete. 
     The heated lime sludge  80  ( FIG. 3 ) undergoes the reaction of Equation 3, e.g., reburning. At the entrance  61  to the lime kiln  100 , the temperature can range, for example, from about 450° F. to about 650° F. At the end with the burner  2 , the temperature can range, for example, from about 1750° F. to about 1950° F. 
     After reburning, recovered lime  101  can be deposited in a lime crusher  83  in preparation for introduction into the slaker  150  (see  FIG. 1 ). 
     A primary air supply for the kiln  100  enters an air intake  79  in fluid connection with a primary blower  78 . The primary blower  78  provides sufficient head to deliver a primary air supply to the kiln  100 , such as between about 500 cubic feet per minute (CFM) and about 1000 CFM. A damper  76 , such as a throttle valve, can be used to control a flow rate of the primary air supply. The throttle valve can be, for example, a butterfly valve. A fluid conduit  74  conveys the air supply from the blower  78  to the kiln  100 , and can incorporate a flexible segment  75  for accommodating vibration and various tolerances in the assembly  1 . 
     Although not necessary for implementing the Miller Process, the co-fired burner  2  shown in  FIG. 2  can receive two fuels, for example a pulverized solid fuel (such as pulverized petroleum coke) and a fuel for providing a continuous ignition source (such as natural gas) for maintaining ignition of the pulverized solid fuel. A blower  28  can provide sufficient head to an airstream for entraining a pulverized solid fuel and injecting the entrained fuel in to the burner  2 . In some embodiments, the blower  28  delivers between about 550 CFM to about 850 CFM through a pipe with an approximately six-inch inner diameter. Pulverized solid fuel can be delivered from a pulverized solid fuel storage bin  26  by a pipe  27 , such as a duct, for conveying the solid fuel to the entrainment stream conveyed by the fluid connection  25 . A pipe  23  carries the entrained solid fuel stream  22  (see  FIG. 3 ) to the burner  2 . 
     In the illustrated embodiment, a plenum  90  collects gas products  102  from the lime kiln exhaust, including products of combustion from the co-fired burner  2  and any products from reburning the lime sludge  84 . A stream of lime kiln exhaust gases enters an induced-draft fan  104  used to draw exhaust from the lime kiln  100 . A fluid conduit  106  between the outlet side of the induced-draft fan  104  and the scrubber  108  carries the gas products  102  to the scrubber  108 . 
     Some representative scrubbers are gas atomized (e.g., high pressure drop) Venturi scrubbers. The scrubber  108  can be a caustic scrubber. Many Venturi scrubbers have a sudden expansion at the Venturi inlet (e.g., from the inlet duct to the scrubber) into a larger diameter convergent-divergent “cone,” or nozzle. Liquid for scrubbing gas products  102  (conventionally water, but in the exemplary embodiment, a solution  119 , such as the filtrate  109 ) can be introduced to the scrubber (e.g., at or near the throat of the Venturi) for mixing with the gas products  102  and washing the walls of any buildup that may occur. For example, the scrubber  108  can be supplied with the solution  119  containing sodium ion from the weak liquor storage  117 , as in the Miller Process. As with other Venturis, Venturi action, e.g., mixing, takes place near the throat. 
     For example, the liquid  102  (which can be recycled as indicated by  FIG. 2 ) can be delivered to the converging portion of the Venturi, where the kiln exhaust from the conduit  106  accelerates. The speed of the exhaust can approach, under some conditions, about 100,000 ft/min through the throat. At such high velocities, the stream of gas products  102  can atomize the injected scrubber liquid (which can later be separated from the gas stream in the separator  110 , as described below). A pressure drop across the throat of the Venturi can be used as a measure of scrubbing efficiency. 
     The small droplets can interact (e.g., by way of increased surface area) with the gas products  102 . Such interaction can remove particulate, and can also place chemicals, such as NaOH, that have been added to the liquid, e.g., the solution  119 , in close contact with components of the exhaust gas, such as SO x . 
     A slurry resulting from such scrubbing, particularly with a Venturi scrubber, can move at a relatively high-speed (“high-speed slurry”), and can be injected in an impinging stream into a flooded tank (e.g., an “elbow tank”). Such a flooded tank is shown near the base of the scrubber shown in  FIG. 2 . The high-speed slurry can subsequently be injected into a separator vessel, such as the separator  110 , where solid particulate in the slurry can be separated from liquid, for example, by way of a cyclonic separation process. 
     After passing through the scrubber  108  and a separator  110  for removing condensates, the scrubbed exhaust  120  can have a lower concentration of SO x  than the kiln exhaust  102 . In some instances, sufficient SO x  can be removed to allow the scrubbed exhaust  120  to be emitted to the environment, most typically the atmosphere, and still meet environmental regulations. 
     Supplying the scrubber with solution from the liquor storage  117  can cause the amount of filtrate  109  to be at least partially proportional to the amount of lime sludge being processed and substantially proportional to a rate at which SO x  is produced in the kiln  100 . Consequently, available solution  119  from the liquor storage  117  can be in part proportional to a rate of lime recovery and a rate of SO x  production, to the extent the filtrate  109  from the liquor storage  117  is used to provide the solution  119 , rather than using additional (e.g., make-up) water, as is common in the prior scrubbing art. Because the solution  119  is a product of the lime recovery process, using this solution for charging the scrubber  108  can reduce costs, water consumption and waste. 
     Passing the kiln gases  102  through the scrubber  108 , as shown in the recovery cycle  200  (see  FIG. 1 ), provides an efficient and cost effective method of removing excess SO x  from the kiln gases  102 . Particularly valuable is that the amount of solution  119  having sodium available for charging the scrubber  108  is at least partially proportionate to the amount of reburnt lime sludge and the rate of SO x  production. Thus, the scrubbing portion of the recovery cycle  200  can largely be performed without significant addition of material, thereby saving on material costs. Of course, the scrubber  108  can also be charged by an external source of sodium for scrubbing the excess SO x  from the kiln gases  102 , if desired. 
     Exemplary Co-Fired Burner 
       FIG. 3  illustrates an exemplary co-fired kiln burner  2  that can be used to heat a kiln, such as the kiln  100 . The exemplary burner  2  is configured for co-fired combustion, such as combustion of a fuel capable of continuous combustion (e.g., natural gas) and combustion of a second fuel, such as a fuel having a high combustion temperature (e.g., a pulverized solid fuel, such as petroleum coke). As shown in  FIG. 3 , the burner  2  comprises a first fuel injector  34  for providing a continuous ignition source for igniting a high-combustion-temperature fuel from a second injector (such as, for example, the nozzle  24 ). The burner illustrated in  FIG. 3 , a frame  10  supports injector body  20  and the main burner  30 , which extend through the firewall  70  isolating the firing end of the kiln  100  from the environment. The exemplary firewall partially forms a firing end hood disposed about the first and second injectors. An exemplary injector body  20  is a pipe with an approximately four-inch inner diameter in fluid connection with the pipe  23  ( FIG. 2 ) carrying entrained pulverized solid fuel. 
     A typical ignition temperature of a pulverized solid fuel can be about 1800° F. As noted, some embodiments of co-fired burners are natural gas co-fired burners that continuously burn natural gas for igniting the solid fuel. 
     An inlet stream  22  of solid fuel, such as a stream of air with entrained particles of petroleum coke, can enter the body  20  at a first end and be discharged at a second end having an injector nozzle  24 . In some embodiments, the inlet stream  22  delivers between about 550 CFM and about 850 CFM of air and entrained fuel, carrying between about 50 pounds per minute and about 60 pounds per minute (lbs/min) of entrained fuel, such as pulverized petroleum coke. 
     The nozzle  24  desirably can be configured as a pulverized solid-fuel injector nozzle, such as a nozzle for injecting pulverized petroleum coke into a continuous ignition source from above. In the illustrated embodiment, the nozzle  24  injects pulverized petroleum coke at an angle  8  between about 15 degrees and about 25 degrees below a horizontal line  59 . In other words, the illustrated nozzle  24  turns the inlet stream  22  by about 15 degrees and about 25 degrees in the direction of gravity. In at least one embodiment, the nozzle is formed by approximately cutting in half a 45-degree bend configured for a four inch inner-diameter pipe to form a pipe fitting having about a 22.5-degree bend. 
     As noted above, a main burner  30  can provide a continuous ignition source for igniting a high-combustion-temperature fuel. An inlet stream of fluid fuel  32  (e.g., gaseous natural gas) enters the burner  30 . In the illustrated embodiment, the main burner  30  is configured as a natural gas burner for continuously burning between about 10,000 cubic feet per hour (10 MCF) and about 75,000 cubic feet per hour (75 MCF). The illustrated fuel injector  34  is a natural gas injector having a plurality of turning vanes (not shown) to enhance mixing of the fuel stream  32  with an oxidizer, such as, for example, air. 
     In the embodiment shown in  FIG. 3 , the injection stream  40  of a pulverized solid fuel mixes with the continuous ignition source  50 . In the case of a natural gas burner, the ignition source  50  is a continuous flame produced by burning the injection stream of the natural gas. Individual particles  42  of a pulverized fuel burn when mixed with the ignition source  50 . A resulting flame  52  can be generally characterized as less intense and at a lower temperature than a flame resulting from burning the fluid fuel  32  alone. A desirable flame reduces erosion of refractory materials within the kiln  100  and also reduces the tendency of the kiln exhaust  102  to entrain CaCO 3  dust from the lime sludge  80 . Mixing of the solid fuel particles  42  with the continuous ignition source  50  can be enhanced by the presence of turning vanes in the vicinity of the nozzle  24  inside the kiln. 
     In the illustrated embodiment, heated lime sludge  80  passes beneath the flame  52  within the kiln  100 . The flame  52  can be controlled (e.g., temperature) by adjusting the damper  76  (see  FIG. 2 ) to control the primary air supply, by adjusting the blower  28  to control the air volume of the fluid supply inlet stream  22 , and/or by adjusting fuel flow rates. Desirably, the resulting flame  52  is a short bushy flame that “licks” the bed of the lime sludge  80  (the flame contacts the surface of the lime sludge). Co-fired burners as described herein typically provide better control of a flame than a burner configured to burn only natural gas. 
     By placing the nozzle  24  above the fuel injector  34  as shown by  FIG. 3 , the flame can be better controlled to achieve a particular intensity, e.g., temperature, flowrate, and degree of interaction with or licking of the bed of lime sludge  80 . In addition, co-fired burners typically provide better control of the flame  52  in a calcining zone than a single fuel burner, e.g., a natural gas burner. For example, co-fired flames are typically shorter and bushier compared to a single-fuel (e.g., natural gas) flame, which is typically also more intense. Consequently, a co-fired burner can provide better control of temperature and heat intensity throughout a larger portion of a calcining zone than a single fuel, natural gas burner. In certain embodiments, the flame from a co-fired burner is controlled to have a temperature ranging between about 1750° F. to about 1950° F. for the lime  101  as it exits the kiln  100 . 
     Combustion in a co-fired burner  2 , together with reburning lime sludge  80 , produces gaseous products  102 . As noted above, these typically include carbon dioxide, various oxides of sulfur (SO x ) and various oxides of nitrogen (NO x ). However, by maintaining the flame temperature below about 2800° F., the temperature at the firing end of the kiln (e.g., at the end with the burner  2 ) can be maintained within a range to sufficiently reduce emissions of NO to meet many statutory emissions requirements. In certain embodiments, a temperature at a firing end of the kiln  100  can be maintained in the range between about 1750° F. and about 2500° F., and between about 1750° F. and about 1950° F. in certain embodiments. In addition, lower flame temperatures as delivered by co-fired burners can further reduce SO x  concentrations in the gas products  102 . 
     In reburning lime sludge  80 , however, CaCO 3  and calcium sulfate (CaSO 4 ) tend to accumulate on interior walls of the kiln  100 , degrading kiln performance. In addition, CaCO 3  and CaSO 4  tend to accumulate, on the blower of the induced draft fan  104 , causing the blower to drift out of balance and degrade in performance. In addition, high concentrations of SO x  generally increase the accumulation of CaCO 3  and CaSO 4  on the kiln walls and blower. 
     In a co-fired burner fueled in part by petroleum coke, the resulting flame can be maintained to provide a peak temperature in the calcining zone of the kiln  100  sufficient to intentionally vaporize the sodium contained in lime sludge  80 . In particular embodiments, the flame temperature is maintained to provide a peak temperature in the calcining zone of the kiln in the range of above about 2250° F. to about 2500° F. Such vaporized sodium can in turn chemically react with high concentrations of SO x  in the gas products  102 . Interaction of the sodium with the SO x  can reduce, and in some cases eliminate, accumulation of CaCO 3  and CaSO 4  inside the kiln and maintain performance of kiln refractory and the induced draft fan  104 . Before emitting the kiln gases  102  to the environment, the gases can be passed through a scrubber, such as the scrubber  108  previously described, to remove at least some of the excess SO x  and comply with emissions requirements. 
     In addition to reducing excess emissions and accumulation of CaCO 3  and CaSO 4 , a co-fired burner can significantly reduce operating costs of recovering useful chemicals from industrial waste. Typically, petroleum coke is less expensive than natural gas when the cost of each is normalized according to its respective available energy from combustion. In a working embodiment of the lime recovery process, natural gas consumption dropped from about 75 MCF when using a natural gas only burner to between about 10 and about 20 MCF using a co-fired burner configured to burn petroleum coke using a natural gas flame as the continuous ignition source. This large drop in natural gas usage and corresponding costs can more than offset incremental additional costs of petroleum coke. 
     In view of the many possible embodiments to which the principles of the disclosed innovations may be applied, it should be recognized that the illustrated embodiments are only examples and should not be taken as limiting the scope of the following claims. I therefore claim as my invention all possible embodiments and their equivalents that come within the scope of these claims.