Abstract:
A grate-kiln pelletizing furnace includes a grate that conveys pelletized material to a rotary kiln, a cooler that cools pelletized material from the rotary kiln, and a gas flow apparatus that directs a stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln. The gas flow apparatus also directs a stream of gas from the grate to the rotary kiln to vitiate the preheated process air.

Description:
TECHNICAL FIELD 
     This technology includes grates, rotary kilns, coolers, and other components of grate-kiln pelletizing furnaces. 
     BACKGROUND 
     The grate-kiln pelletizing process is a means of indurating iron ore into pellets suitable for transportation and subsequent use in blast-furnaces and steel-making. The iron ore fines are mixed with other materials such as dolomite and bentonite, and formed into round balls, which are then loaded onto a moving grate, where they are dried, preheated, and partially hardened. Final hardening takes place when the pellets are discharged from the grate into a large rotary kiln, where they are heated to 2400-2500 F by means of a large burner firing into a process air stream, with an excess of oxygen in the products of combustion. (In some cases oxidation of iron in the ore also provides heat input to the process.) The pellets are then cooled in a cooler by forcing a stream of ambient air through the pellets. The process air stream for the kiln is the hot air generated from cooling the pellets in the cooler combined with products of combustion from the kiln burner. 
     Since the heat transferred to the pellets in the grate and kiln sections is regenerated into the process air in the cooler, the process is very energy efficient, but the process requirement for large excess of oxygen in the kiln combined with the high air temperature in the air entering the kiln from the cooler also results in very high NOx. It would be valuable to be able to reduce the NOx generated by the kiln burner while still maintaining the high process efficiency by using the process air. 
     There are other similar processes that incorporate rotary kilns fired by a burner and supplied with a process air stream that has been pre-heated by cooling the product in a cooler. The invention is applicable to those processes as well. 
     Typical prior art grate-kiln pelletizing furnaces incorporate a large rotary kiln fired by one or two very high capacity (100 to 500 MMBtu/h) kiln burners which combust hydro-carbon fuels, usually natural gas, fuel oil, coal, or biomass, in an excess of high-temperature preheated air, to provide a high temperature (2400-2500 F) oxidizing environment which is needed to indurate iron ore pellets. The typical kiln is fired by a single large burner, with a very long high temperature flame. The large flame envelope results in a very large interface area between the flame and the high-temperature oxidant, and in long residence times. Thus, the large flame envelope, high preheat temperature, high flame temperature, and large excess of oxygen in the combustion zone all combine to generate very high NOx emissions. 
     The prior art design is very fuel efficient, in that the heat stored in the pellets is transferred to preheat air to temperatures as high as 2000 F; this air is then subsequently used for drying and heating the pellets, and as oxidant for the fuel needed to heat the process gases to the required temperature. The problem is that the factors that make the process fuel-efficient contribute strongly to the formation of NOx. Most of the strategies used by prior art low-NOx burners either do not work very well in the high temperature highly oxidizing environment, or have significant negative impacts on fuel efficiency. 
     The only other means available for NOx reduction have been after-treatment methods such as SCR, SNCR, and LO-TOX. These methods are either very expensive to implement, require significant additional energy input to the process, or are impractical to incorporate into the process. 
     In the context of maximizing fuel efficiency with unregulated emissions, the prior art arrangements make intuitive sense, as the highest temperature streams of recuperated cooling air are used in the highest temperature part of the process. 
       FIGS. 1 ,  2 , and  3  show configurations typical of the prior art. In  FIG. 1 , hot indurated pellets are discharged from a kiln  10  into a cooler  12 . A cooling air blower  14  blows a cooling air stream over the pellets in the cooler  12 , cooling the pellets and heating the cooling air. The blower  14  is part of a gas flow apparatus that includes blowers, burners, ducts, flow control devices, controllers, and other known devices as needed, in a configuration that provides the heated process air and other reactants for the indurating process. The cooler  12  is typically segmented into stages or sections  20 , with the cooling air leaving the sections  20  closest to the discharge end  22  of the kiln  10  hotter than the cooling air leaving the sections  20  farther from the discharge end  22  of the kiln  10 . In the case of coolers in which the travel is rotational, such as annual coolers, the terms “closest” and “farther,” as used above, refer to the distance that the pellets have travelled along the path of rotation of the cooler  12 , as opposed to the linear distance from the discharge end of the kiln  10 . 
     In  FIG. 1 , air at approx. 2000 F air leaving the hottest section  20  of the cooler  12  passes through a combination of hood and duct structures  24  before entering the kiln  10 . A kiln burner  26  typically fires one or more fuels such as natural gas, fuel-oil, coal, biomass, etc. into the discharge end  22  of the kiln  10 . The kiln burner  26  is typically provided with a stream of combustion air which is much less than the amount required to completely combust the air. The process air from the cooler  12  includes a large excess of air compared to what is required to burn the fuel, so there are typically oxygen levels from 10% to 16% in the process air leaving the kiln  10  after fully combusting all of the fuel. 
     The process gases leaving the kiln  10  pass through one or more ducts  30  to the final preheat section  34  of a traveling grate  36 . Dried and partially hardened pellets discharge from the grate  36  into the kiln  10  where the indurating process is completed. The process gases at perhaps approximately 2400 F are induced by a process gas blower  38  to flow through the pellet bed on the grate  36 , preheating the pellets; in doing so, the process gases are cooled to perhaps 600 F before entering the process gas blower  38 . The process gas blower  38  then discharges the process gases through ducts  40  into the drying section  42  of the grate  36 . The pellets at approximately ambient temperature enter the drying section  42  at the feed end  44  of the grate  36 . In drying the pellets, the process gases are further cooled to a temperature typically between 200 and 400 F, before being discharged to atmosphere through an induced draft fan  46  and stack  48 . It is typical for the exhaust to also be processed by means of equipment such as cyclone separators, electro-static precipitators, or baghouses (none of which are shown) to remove particulates before being discharged into the stack. 
     Typically, there are also one or more intermediate stages  50  of drying and/or preheat sections between the first drying section  42  and the final preheat section  34 . In one typical configuration ( FIG. 1 ), hot air at perhaps 1300 F from an intermediate section  20  of the cooler  12  is directed through a duct  51  and further heated by an air heater  52  to about 1500 F before being ducted to one of the intermediate sections  50  of the grate  36 . The air heater  52  is fired by a burner  54  using a fuel (typically natural gas, propane, or fuel oil) to provide the heat necessary for raising the temperature of the hot air to the required level. The process gases from the air heater  52  are then drawn through one or more intermediate drying/preheat sections  50 , sometimes in a combination of updraft and downdraft configurations (not shown), before being processed through gas clean-up equipment (not shown) as described earlier and then being exhausted to atmosphere. 
     A slightly different known configuration is shown in  FIG. 2 . The difference between  FIG. 2  and  FIG. 1  is that in  FIG. 2 , the process gas from the intermediate section  20  of the cooler  12  does not pass through the air heater  52 . Instead, the air heater  52  incorporates a stream of dilution air from a dilution air blower  56  (or alternatively from a combined air heater combustion air and dilution air blower—this alternative not shown) to create a hot gas stream of perhaps 2000 F that mixes with the incoming air stream from the intermediate stage  20  of the cooler  12  to produce a combined process gas stream at 1500 F, with a higher total mass flow rate, which is then directed to the grate as in  FIG. 1 . 
     Another known configuration in the prior art is shown in  FIG. 3 . In  FIG. 3 , preheat burners  60  are installed in the roof or sides of the preheat and/or drying sections of the grate  36 . The preheat burners  60  may be used instead of the air heater  52  shown in  FIGS. 1 and 2  or in addition to the air heater  52 . 
     In each prior art arrangement of  FIGS. 1 ,  2 , and  3 , process air of approximately 800 F from the final (coolest) stages  20  of the cooler  12  may be directed through ducts  62  to other parts of the plant (such as for grinding), or may be exhausted to the atmosphere through a stack  64  at approximately 300 F. The elements that differ between the three prior art configurations are sometimes used in combination with each other, e.g. some configurations have both preheat burners  60  and an air heater  52 . 
     SUMMARY OF THE INVENTION 
     The invention applies to a grate-kiln pelletizing furnace including a grate that conveys pelletized material to a rotary kiln, a cooler that cools pelletized material from the rotary kiln, and a gas flow apparatus that directs a stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln. In a preferred embodiment of the invention, the gas flow apparatus also directs a stream of gas from the grate to the rotary kiln to vitiate the preheated process air. 
     Another embodiment of the invention includes a gas flow apparatus that directs a first stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln, directs a second stream of gas from the cooler to the grate to transfer heat from pelletized material in the cooler to pelletized material on the grate. In accordance with the invention, the gas flow apparatus diverts a portion of the first stream to mix into the second stream. 
     In another embodiment of the invention, the gas flow apparatus a) draws successively cooler streams of gas from respective sections of the cooler, including a first stream from a first section and a second stream from a second section cooler than the first section, b) directs the first stream from the cooler to the grate to transfer heat from pelletized material in the cooler to pelletized material on the grate, and c) directs the second stream from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln. 
     In yet another embodiment of the invention, the rotary kiln has a burner, and the gas flow apparatus directs a stream of gas from the cooler to the burner to provide preheated combustion air to the burner. This embodiment preferably includes means for cleaning the stream of gas. 
     The invention also provides a method of operating an apparatus including a rotary kiln, a grate configured to convey pelletized material to the rotary kiln, and a cooler configured to cool pelletized material from the rotary kiln. The method may comprise the steps of directing a first stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln, and directing a second stream of gas from the grate to the rotary kiln to vitiate the preheated process air. 
     The method may alternatively comprise the steps of directing a first stream of gas from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln; directing a second stream of gas from the cooler to the grate to transfer heat from pelletized material in the cooler to pelletized material on the grate; and diverting a portion of the first stream to mix into the second stream. 
     In another alternative, the method may comprise the steps of drawing successively cooler streams of gas from respective sections of the cooler, including a first stream from a first section and a second stream from a second section that is cooler than the first section; directing the first stream from the cooler to the grate to transfer heat from pelletized material in the cooler to pelletized material on the grate; and directing the second stream from the cooler to the rotary kiln to provide preheated process air for pelletized material in the rotary kiln. 
     Another method is provided for operating an apparatus including a rotary kiln, a burner configured to fire into the rotary kiln, and a cooler configured to cool pelletized material from the rotary kiln. This method comprises the step of directing a stream of gas from the cooler to the burner to provide preheated combustion air to the burner. 
     The invention further provides a method of retrofitting an apparatus that has a capacity to provide heat input to a grate as a fraction of a total heat input provided to the grate and a rotary kiln. The retrofitting method configures the apparatus to have an increased capacity to provide heat input to the grate as a fraction of the total heat input provided to the grate and the rotary kiln, whereby the retrofitted apparatus can provide an equally decreased fractional heat input at the rotary kiln to yield less NOx from the rotary kiln. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Each of  FIGS. 1-3  is a schematic view of parts of a respective prior art grate-kiln pelletizing furnace. 
       Each of  FIGS. 4-11  is a schematic view of parts of a respective grate-kiln pelletizing furnace configured according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     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  FIG. 4 . In this embodiment, the cooling media supplied to the first stage  20  of the cooler  12  has been changed. In the prior art of  FIGS. 1-3 , 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  through the duct  24 . 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  FIG. 5 , 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  through the duct  51 , 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  FIG. 2 . 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  FIG. 5 , 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  FIG. 6 , which shows the invention applied to the prior art of  FIG. 3 . 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  FIG. 5 . This feature of the invention can also be applied to configurations that combine both preheat burners and an air heater (not shown). 
       FIG. 6  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 a duct  77  that draws 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  FIG. 6 , 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 . 
       FIG. 7  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 . 
       FIG. 8  shows part of the process in greater detail than the previous figures, in order to illustrate another feature of the invention. As shown in  FIG. 8 , 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  FIGS. 5 ,  6  and  7 .  FIG. 8  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. 
       FIG. 9  shows an embodiment in which the feature of the invention described above with reference to  FIG. 4 , 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  FIG. 5 . 
       FIG. 10  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  FIG. 8 , 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  FIG. 10 , 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. 
       FIG. 11  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  through a duct  90  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  FIG. 2  can be retrofitted to provide the embodiment of  FIG. 5 . This can be accomplished by configuring the duct structure  24  of  FIG. 2  to divert preheated gas to the grate  36  as shown in  FIG. 5 . Importantly, for a given set of operating conditions, the prior art apparatus of  FIG. 2  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  FIG. 5  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  FIG. 6 , the grate  36  is equipped with four preheat burners  60 , whereas the prior art apparatus of  FIG. 3  is shown to have only three preheat burners  60  at the grate  36 , and the prior art apparatus of  FIG. 2  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  FIG. 5  duct structure  24 , as shown in  FIG. 6 , or without that feature. Each of the embodiments shown in  FIGS. 7-11 , 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.