a principal feature of the invention re - directs some of the process gas and recuperated air from the cooler to maintain efficiency or at least minimize efficiency losses , while firing the kiln with a lower average oxidant temperature , and to provide the kiln with oxidant that has been somewhat vitiated . this can lower nox emissions while maintaining high process efficiency . the invention may replace some or all of the ambient cooling air with exhaust gases leaving the drying or preheating stage as a first stage of cooling media in the cooler . this then becomes a source of vitiated high temperature oxidant for the kiln , which can further reduce the oxygen level in the kiln and reducing nox . another principal feature of the invention re - routes some or all of the highest temperature air leaving the cooler to the grate preheating and / or drying sections , instead of directing it to the kiln . lower temperature air can be provided to the kiln to replace the higher temperature air that was re - routed , for example by increasing the capacity of the combustion air blower providing air to the kiln burner . the reduced air temperature resulting from re - routing high temperature air from the cooler and replacing it with lower temperature air ( for example , increased combustion air to the kiln burner ) can reduce nox . this reduced temperature air to the kiln additionally provides the benefit of allowing the kiln to be fired with a lean pre - mix or other low nox burner which further reduces nox . although redirecting part of the higher temperature air stream a longer distance ( to the grate section instead of directly into the kiln ), may in some cases result in a more expensive installation , it can allow the high process efficiency typical of the prior art configuration to be maintained . if the high temperature air were not re - directed , a choice might have to be made between high efficiency and low nox , but the invention is expected to eliminate the need to choose — both high efficiency and low nox can be realized in a grate - kiln indurating furnace environment . one embodiment of the invention is shown in fig4 . in this embodiment , the cooling media supplied to the first stage 20 of the cooler 12 has been changed . in the prior art of fig1 - 3 , only ambient air is provided to the cooler 12 . in this embodiment of the invention , some or all of the process gas from either the drying or preheat stages 42 , 50 , 34 of the grate 36 is transported by ducts 70 and a process gas blower 72 to the first section 20 of the cooler 12 . the hot process air may be mixed with the cooler ambient air at or inside the cooler 12 . ideally , the process gas supplied to the cooler 12 should be the lowest temperature and lowest oxygen process gas available . the typical cooler area will probably have to be increased to compensate for the fact that the process gas will be hotter than ambient air , so will provide less cooling , but using the process gas in the hottest section 20 of the cooler 12 will help to mitigate this effect by maintaining the highest possible temperature difference between the product being cooled and the cooling media . the reduced oxygen stream leaving the first section 20 of the cooler 12 may be then routed directly to the kiln 10 . the reduced oxygen content in the process gas stream will reduce the nox in the process , even if none of the other steps or embodiments of the invention are incorporated , but this step may be most effective if combined with one or more of the further steps and embodiments described below . in the embodiment shown in fig5 , the duct structure 24 is configured to divert some of the high temperature oxidant from the first stage 20 of the cooler 12 to mix with the lower temperature oxidant going to the grate 36 , thus raising the temperature of the oxidant from ( in the example shown ) 1300 f to 1500 f — about the same temperature that is achieved by incorporating the air heater in the prior art of fig2 . since less high temperature air is going to the kiln 10 , the combustion air blower 76 supplying the kiln burner 26 may now be configured to supply additional ambient air , replacing the high temperature air that is being diverted to the grate . this will result in lower nox in the kiln 10 in potentially two ways . first , just maintaining the same total air mass flow into the kiln 10 at a lower air temperature will reduce the kiln nox , and second , using increased air supply to the kiln burner 26 will allow replacement of the typical sub - stoichiometric kiln burner with any one of a number of types of low - nox burners . if enough air is diverted , a lean - premix type low nox burner can be used on the kiln 10 , which will result in much lower nox emissions . diverting the high temperature air to the grate 36 allows several other options which will maintain the efficiency benefits from using the high temperature air in the process . one option ( not shown ) is that the air heater 52 , and thus the fuel input 78 to the burner 54 at the air heater 52 , can be eliminated while still maintaining the same air and heat input to the grate 36 . this will compensate for the extra fuel that will have to be used by the kiln burner 26 because of using lower temperature air in the kiln 10 . another option is to keep the air heater 52 as shown in fig5 , but not to fire the burner 54 on it during normal operation . this option keeps the air heater 52 and its burner 54 available for operation under special conditions , such as during start - up , when it is useful to have to help bring the process on - line . a third option is that the air heater 52 and its burner 54 can be used in conjunction with the higher temperature input stream to increase the total energy input to the grate 36 . this will allow less energy to be input by the kiln burner 26 , which will make the process more efficient and also reduce nox . this option reduces nox further because the kiln burner 26 is the source of most of the nox generated by the process , and generates nox at a level much higher than the air heater burner 54 , because of the higher operating temperature in the kiln 10 . a slightly different embodiment is illustrated by fig6 , which shows the invention applied to the prior art of fig3 . in this case , the additional heat input to the grate 36 achieved by diverting the high temperature cooler air as described above is used to replace or augment the heat provided from preheat burners 60 . the same options and benefits apply to the preheat burners 60 as described with respect to the air heater 52 in fig5 . this feature of the invention can also be applied to configurations that combine both preheat burners and an air heater ( not shown ). fig6 also shows an additional feature of the invention which may be implemented independently or in combination with others . the kiln burner 26 and kiln burner combustion air blower 76 are supplied with air at approximately 800 f from an intermediate section 20 of the cooler . this embodiment preferably includes a filter 79 or other means for cleaning the gas before it enters the blower 76 . up to a temperature of about 900 f , this air can also be used as combustion air for a lean premix type burner . since the lean premix burner produces very low nox emissions , the heat in this air can be used in the process , helping to keep the efficiency high while still providing very low nox emissions . if used as shown in fig6 , depending on the range of possible air temperatures from the cooler 12 , the supply to the combustion air blower 76 may require a dilution air source and temperature control loop ( not shown ), such as are known in the art , to protect the combustion air blower 76 from damage which might be caused by excessive temperature , and to prevent flashback from occurring if a lean premix burner is used for the kiln burner 26 . fig7 shows an alternative feature that may be incorporated if it is desired to use the 800 f air from the intermediate section 20 of the cooler 12 in another part of the plant . in this step , the air from the final , coolest section 20 of the cooler 12 , which is at perhaps 300 f , is used as combustion air to supply the combustion air blower 76 for the kiln burner 26 . fig8 shows part of the process in greater detail than the previous figures , in order to illustrate another feature of the invention . as shown in fig8 , part of the high temperature air that went to the kiln 10 in the prior art configurations is diverted to combine with the air from the intermediate stage 20 of the cooler 12 in order to increase the temperature of the air supplied to the grate 36 , as in the embodiments of fig5 , 6 and 7 . fig8 also illustrates the additional step of diverting part of the high temperature air from the high temperature stage 20 of the cooler 12 to mix with ambient air to create a combined 800 f degree stream of combustion air supplied to the combustion air blower 76 and kiln burner 26 . if there are practical limits due to retrofit or other constraints that prevent diverting all of the available high temperature air to the grate 36 , this feature of the invention allows using more of this air in the kiln 10 while still using a lean premix burner for the kiln burner 26 , which will provide very low nox emissions and increased efficiency compared to being required to reject this high temperature air to atmosphere without using the energy contained in it . a controller 80 operates flow control devices 82 in response to one or more temperature sensors 84 to limit the air temperature to the combustion air blower 76 to a safe level ; such as 800 f for example , but the actual temperature will depend on the specific process and equipment selected for a particular installation . fig9 shows an embodiment in which the feature of the invention described above with reference to fig4 , i . e . supplying the hottest section 20 of the cooler 12 with process exhaust gas via a process exhaust blower 72 in lieu of ambient air from the cooling air blower 14 , is combined with the diverting feature of fig5 . fig1 shows an embodiment in which the high temperature ( perhaps 2000 f ) air stream from the hottest cooler section 20 is divided into two or three process streams ; one stream going directly to the kiln 10 ; one stream going to mix with the intermediate stage cooler air to provide a higher temperature ( 1500 f ) stream going to the grate 36 , and one stream going to the kiln combustion air blower 76 . as in fig8 , a flow control device 82 , such as a damper and actuator , is installed in the high temperature stream and also in an ambient air stream . the controller 80 modulates the opening of the two control devices 82 to maintain a desired value read by a thermocouple or other temperature device 84 . similarly , it may be desirable to control the amount of flow , or the mixed fluid temperature , or both , of the combined stream going to the grate section 36 . as shown in fig1 , a flow control device 82 may be placed in each of the high temperature flow streams , and these devices 82 can be controlled by a controller 80 to maintain a desired temperature level . since the fluid temperature is very high , flow control devices such as dampers can be expensive . the temperature or flow target can be maintained by other means known in the art as well , including : size or operating speed of process gas blowers , aspirators or educators , relative sizing of ducts or flow restrictions , appropriate baffle placement within the cooler 12 or cooler cover . fig1 shows an embodiment that re - routes some or all of the highest temperature air leaving the cooler 12 to the preheating and / or drying sections 34 , 42 , 50 of the grate 36 , instead of directing it to the kiln 10 . lower temperature air can be provided to the kiln 10 from an intermediate section 20 of the cooler 12 as shown , or by replacing the higher temperature air that was re - routed , for example , by increasing the capacity of the combustion air blower 76 providing air to the kiln burner 26 . accordingly , the problem of high nox emissions can be solved by one or more of the following : a . vitiation of the high temperature air from the cooler by means of substituting process gas from the grate for ambient air as the source of cooling for the high temperature stage of the cooler . b . vitiation of the kiln burner combustion air by substituting vitiated process gas from the cooler as described above for part of the ambient combustion air provided to the kiln burner . c . reduction of the amount of high temperature air from the cooler that is provided to the kiln . d . increasing the fraction of heating done by the grate section and decreasing the heating done by the kiln . e . replacing hot air from the cooler with ambient or warm air provided to a low nox burner . f . replacing the sub - stoichiometric burner on the kiln with a low nox burner using stoichiometric or excess air . the problem of decreasing efficiency from implementing low nox measures can be solved by a combination of one or more of : a . diverting the air from the high - temperature end of the cooler to the grate section instead of rejecting it . b . using air from the high , intermediate , or low temperature parts of the cooler as some or all of the kiln burner combustion air . c . increasing the fraction of heating done by the grate section and decreasing the heating done by the kiln . the invention can thus reduce nox emissions from kilns that operate at high temperatures while using high temperature air recuperated from coolers as combustion air and process air . the invention accomplishes the reduction of nox emissions from high - temperature , high - excess air kiln furnaces with no fuel efficiency penalty , or with a smaller fuel - efficiency penalty , compared to the prior art . additionally , any of the various embodiments of the invention may be of retrofitted construction . for example , the prior art apparatus of fig2 can be retrofitted to provide the embodiment of fig5 . this can be accomplished by configuring the duct structure 24 of fig2 to divert preheated gas to the grate 36 as shown in fig5 . importantly , for a given set of operating conditions , the prior art apparatus of fig2 has a limited capacity to provide heat input to the grate 36 as a fraction of a total heat input provided to the grate 36 and the rotary kiln 10 . retrofitting the prior art apparatus by configuring it to divert preheated gas to the grate 36 would increase the capacity to provide heat input to the grate 10 as a fraction of the total heat input provided to the grate 36 and the rotary kiln 10 . for a given total heat input under given operating conditions , the embodiment of fig5 can thus provide an equally decreased fractional heat input at the rotary kiln 10 to yield less nox from the rotary kiln 10 . as shown in fig6 , the grate 36 is equipped with four preheat burners 60 , whereas the prior art apparatus of fig3 is shown to have only three preheat burners 60 at the grate 36 , and the prior art apparatus of fig2 has no preheat burners at the grate 36 . an increased fractional heat input capacity at the grate 36 can thus be obtained by installing one or more preheat burners 60 , or by replacing an existing preheat burner 60 with a preheat burner 60 having a greater heat input capacity . this increase could be provided either with the gas diverting feature of the fig5 duct structure 24 , as shown in fig6 , or without that feature . each of the embodiments shown in fig7 - 11 , as well as any other embodiment of the invention , can also be provided by retrofitting a prior art apparatus as needed to provide the elements of the invention as shown , described and claimed . this written description sets forth the best mode of carrying out the invention , and describes the invention so as to enable a person skilled in the art to make and use the invention , by presenting examples of elements recited in the claims . the patentable scope of the invention is defined by the claims , and may include other examples that occur to those skilled in the art . such other examples , which may be available either before or after the application filing date , are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims , or if they have equivalent elements with insubstantial differences from the literal language of the claims .