CATALYTIC COMBUSTION

Abstract: Combustion apparatus, e.g. for a gas turbine, has at least one catalytic combustion unit (14, 15, 16) having a plurality of through passages of which some are combustion passages (28) having a combustion catalyst on the passage walls and at least some others are bypass preheat passages (29) that are free of catalyst and are adjacent the combustion passages. A first gas stream containing air and fuel is supplied to the combustion and bypass preheat passages; catalytic combustion of the fuel occurs in the combustion passages (28) and heat is transferred through the passage walls to heat the gas in the bypass preheat passages (29). On emergence from the combustion unit (14, 15, 16) the preheated gas from the bypass passages (29) combusts in a downstream afterburn zone (20). A second gas stream containing air and a variable amount of fuel is supplied to the afterburn zone (20). Where there is more than one combustion unit, larger bypass passages (26) may be provided through the first combustion unit (14) so that part of the first gas stream passses through the first combustion unit (14) without preheating and mixes, in a zone (17) between the first and second combustion units (14, 15), with the gas from the combustion passages (28) and bypass preheat passages (29) of combustion unit (14) to form the gas stream fed to the second combustion unit (15).

Description: Catalytic combustion This invention relates to catalytic combustion and in particular to a catalyst structure for use in a catalytic combustion process, for example as encountered in gas turbines. Catalyst bodies for use in such processes may comprise a structure, such as a foam or honeycomb, having through passages supporting, or composed of, a catalyst active for the combustion process. In any combustion process there may be used an assembly of one or more such catalyst bodies. In a catalytic combustion process a fuel/air feed, at a temperature below that at which autoignition takes place is passed, normally at superatmospheric pressure, typically in the range 2 to 40 bar abs., through the catalyst body assembly wherein combustion takes place giving a hot gas stream. The fuel may be gaseous or liquid at ambient pressure and temperature, but most, if not all, of the fuel should be in the gaseous state at the temperature and pressure at which the fuel/air mixture is fed to the catalyst body. Examples of suitable fuels include natural gas, propane, naphtha, kerosene, and diesel distillate. At least part of the fuel may be the product of subjecting a hydrocarbon feedstock to catalytic autothermal steam reforming. A process describing the use of catalytic autothermal steam reforming of a hydrocarbon feedstock to produce a gas turbine feedstock is set out in EP-A-351094. Catalytic combustion processes such as those encountered in gas turbine applications are normally operated, at least once the catalyst has "lit-off", at very high gas velocities and this presents problems in maintaining combustion. The gas leaving the catalyst body passages during normal operation desirably has a linear velocity in the range 25-150, particularly 50-100, m/s. As the flow rate is increased, the rate at which heat is lost from the catalyst surface to the gas increases. The rate at which fuel is transferred to the catalyst surface also increases as the gas velocity increases. Provided the catalyst is of sufficient activity to burn the fuel, the rate at which heat is released at the catalyst surfaces thus increases as the gas velocity increases. Thus, provided the catalyst is of sufficient activity, the rate of heat release and the rate of heat loss both increase as the gas velocity increases and so the catalyst surface temperature changes little, if at all. As the gas velocity is increased further, eventually a flow rate is reached where the reaction rate cannot be increased and becomes kinetically limited. Further increase in the flow rate increases the heat loss and so the temperature of the catalyst surface falls. This reduces the rate of combustion on the catalyst surface, which results in a further fall in temperature, until a point is reached where combustion can no longer be sustained. The temperature at which combustion can no longer be sustained depends on a variety of factors such as the nature and concentration of the fuel in the combustible mixture, the gas velocity, and the nature and activity of the catalyst. [The switch from mass transfer to kinetic control is not as sharp as might be implied from the above: the net effect, however, is the sum of the limitations imposed by the mass transfer and the reaction rate]. Available catalysts that are able to tolerate the temperatures normally achieved unfortunately have insufficient activity to enable operation at the gas flow rates normally desired in gas turbine operations; ie the desired flow rates are greater than those at which combustion can be sustained. In some cases catalysts that can tolerate the temperatures normally achieved have insufficient activity to enable the catalyst to "light-off, or effect complete conversion, at acceptable preheat temperatures. While there are some catalysts with sufficiently high activity to perform the combustion at lower temperatures, these active catalysts tend to sinter and/or evaporate at the temperatures normally achieved and so the catalyst life is limited. These problems can be overcome to some extent by increasing the temperature at which the fuel and air are fed to the combustion apparatus. Thus if the feed temperature is sufficiently high it may be possible to sustain combustion even at high gas flow rates. However it is often not practical to supply the fuel/air mixture at a high enough temperature. The fuel/air mixture is usually produced by compressing air and then adding fuel under pressure into the compressed air. It is usually desired to supply the fuel/air mixture to the combustion apparatus at a temperature in the range 250-450°C, particularly 300-400°C corresponding to the delivery temperature of the compressor producing the compressed air. In our European Patent Application 91311042.5, now published as EP-A-491481, we describe a technique for overcoming this difficulty wherein part of the fuel/air feed is combusted catalytically in a preliminary catalyst body and the resultant heated gas stream is then mixed with the remainder of the feed, and that mixture is then combusted. The combustion of the mixture of the heated gas stream and the remainder of the fuel/air feed may be effected catalytically in a main catalyst body, or, provided that mixture is hot enough, the combustion may be homogeneous, ie in the gas phase. EP-A-491481 envisaged the use of one or more preliminary catalyst bodies having combustion passages wherein combustion was sustained to produce the heated gas stream and bypass passages, which may be free from catalyst, through which the remainder of the fuel/air feed passed. In that arrangement the fuel/air feed passing through the combustion passages of the preliminary catalyst body combusts so that the catalyst body reaches the adiabatic flame temperature relatively close to the entrance to the combustion passages while the average temperature of the combusting fuel/air feed increases more gradually, reaching the adiabatic flame temperature before leaving the combustion passages. This however necessitates that the catalyst has to be able to withstand the adiabatic flame temperature, and this imposes constraints on the catalysts that may be employed. It was envisaged in EP-A-491481 that the combustion passages could be distributed across the cross section of the preliminary catalyst body in clusters but such clusters should each contain a sufficient number of combustion passages that substantial heat loss to adjacent bypass passages is avoided. It has been proposed in US-A-4870824 to keep the temperature of the catalyst down by providing the catalyst unit in the form of a honeycomb wherein only some of the through passages have a catalyst on the passage walls while adjacent passages are catalyst-free. In operation heat is transferred from the catalyst-containing combustion passages to adjacent bypass passages: this heat transfer has the dual role of effecting preheating of the fuel/air feed passing through the bypass passages and decreasing the temperature to which the catalyst in the combustion passages is subject and so it is possible to employ catalysts and supports therefor that do not have high temperature stability. During passage through the bypass passages, the \ fuel/air feed passing through those bypass passages becomes sufficiently heated that gas phase combustion occurs downstream of the catalyst unit. The walls of the honeycomb passages extend the ignition delay time of the heated fuel/air mixture passing therethrough and so, by appropriate honeycomb design, it is possible to provide that the gas leaving the bypass passages has a temperature at which the ignition delay time in the passages is such that essentially no gas phase combustion occurs in the passages, whereas, on emergence from the passages, the ignition delay time is very short so that gas phase combustion occurs essentially immediately downstream of the catalyst body. The catalyst body design may be arranged such that combustion in the combustion passages proceeds to a high conversion, eg over 802. This leads to stability in operation, as the catalyst body may be designed so that, with fresh catalyst, such high conversions may e achieved well before the gas mixture leaves the combustion passages. As the catalyst deactivates with time, ie as the catalyst "ages", the position at which such high conversion is achieved moves towards, but remains before, the exit of the combustion passages. The achievement of high conversions, also means that fluctuations in fuel and/or air flow can be accommodated. In a conventional honeycomb wherein all the passages are combustion passages, it might be possible to control the exit temperature of the gas at a desired level below the adiabatic flame temperature when the catalyst is fresh, but ageing of the catalyst and fluctuations in the air and fuel flow cannot readily be accommodated. In additi . , with a conventional honeycomb, at pressure, the surface temperature of the catalyst rapidly rises to the adiabatic flame temperature with consequent rapid deactivation of the catalyst. A further advantage of the use of this type of combustion catalyst assembly where there are combustion passages and adjacent catalyst-free bypass passages to which heat is transferred from the combustion passages is that, since the , catalyst does not become heated to the adiabatic flame temperature, it is possible to employ fuel/air mixtures that have higher adiabatic flame temperatures than would otherwise be possible. Thus mixtures containing a greater concentration of fuel can be employed. This is advantageous as light-off of the catalyst becomes easier as the fuel concentration increases. Also the activity requirement for the catalyst is decreased as the adiabatic flame temperature of the air/fuel mixture increases. A further problem associated with the design and operation of catalytic combustion units for applications such as gas turbines is that it is often difficult to arrange for the combustion to be sustained at low loads, ie when the gas turbine is in a "turn-down" or "idling" state. We have now realised that the aforementioned technique of preheating gas in bypass passages by heat transfer across the passage walls from adjacent combustion passages to the temperature at which gas phase reaction would occur but is inhibited while still in the passages, enables a system to be designed wherein stable operation in respect to catalyst ageing, fluctuations in fuel and air flows, and "turn¬ down" can readily be achieved. Accordingly the present invention provides combustion apparatus comprising a) a catalytic combustion unit having a plurality of through passages of which some are combustion passages having a combustion catalyst on the passage walls and at least some others are bypass preheat passages that are free of catalyst and are adjacent said combustion passages; b) means to supply a first gas stream containing air and fuel to said combustion passages and bypass preheat passages; c) an afterburn zone downstream of said catalytic combustion unit; d) means to supply a second gas stream containing air to said afterburn zone; and e) means to introduce a variable amount of fuel into said second gas stream upstream of said afterburn zone. The invention also provides a combustion process comprising a) feeding a first gas stream containing fuel and air to the combustion and bypass preheat passages of a catalytic combustion unit having a plurality of through passages of which some are combustion passages having a combustion catalyst on the passage walls and at least some others are bypass preheat passages that are free of catalyst and are adjacent said combustion passages, whereby combustion of said fuel takes place in said combustion passages and heat is transferred through the passage walls to said adjacent bypass preheat passages thus preheating the air and fuel passing through said bypass preheat passages, and combustion of the fuel passing through said bypass preheat passages takes place downstream of said combustion and bypass preheat passages to give a hot combusted gas stream; b) adding a second gas stream containing air to said hot combusted gas stream; and c) varying the amount of fuel added to said second gas stream prior to addition thereof to said hot combusted gas stream. As described in the aforesaid TJS-A-4870824, fuel passing through the combustion passages is combusted catalytically but some of the heat evolved is transferred through the passage walls to the adjacent catalyst-free bypass passages and heats the gas passing through those bypass passages. As described below, the catalytic combustion can be effected in stages. The resulting mixture of the combusted gas and heated bypass gas will have a temperature determined by the temperature and composition of the gas entering the passages, the relative proportions of gas passing through the bypass and combustion passages, and the amount of combustion that has taken place in the combustion passages. The combustion passages are preferably disposed individually in the catalyst body; ie each wall of a combustion passage has a catalyst-free bypass passage on its other side. Thus preferably each catalyst-containing combustion passage is surrounded by catalyst-free bypass preheat passages. Alternatively, but less preferably, the combustion passages are in clusters but with each combustion passage having at least one adjacent bypass passage. Thus no combustion passage should be totally surrounded by adjacent combustion passages. It is preferred that each bypass passage has at least one adjacent combustion passage since this will ensure that the combustible mixture passing through each bypass passage is preheated. The combustion passages are preferably disposed regularly across the cross section of the catalyst body. The bypass preheat and combustion passages may be the same size or may differ in size. As indicated above, the catalytic combustion with part of the fuel/air mixture passing through combustion passages and part through adjacent catalyst-free bypass preheat passages may be conducted in stages. This may be desirable to ensure that the catalytic combustion can be readily started and sustained. Thus two or more catalyst bodies may be employed in series, each having catalyst-containing combustion passages and adjacent catalyst-free bypass preheat passages, with mixing zones between each catalyst body. The ratio of bypass preheat passages to catalyst-containing combustion passages may vary from one body to another. In addition to the bypass passages that are adjacent to combustion passages of a catalyst body, further bypass means may be provided to supply fuel/air mixture to the mixing zone between that catalyst body and the adjacent downstream catalyst body. For example such additional bypass means may be passages that are of larger hydraulic diameter than the bypass preheat passages and extend through or round the first, and optionally any intermediate, catalyst body, as described in the aforementioned EP-A-491481, so that some of the air/fuel mixture bypasses that catalyst body. In this case it is preferred that little or no preheating of the air/fuel mixture passing through those additional bypass passages occurs from the combustion passages or the bypass preheat passages. In order that the length of the catalyst assembly can be minimised, it is preferred, when using a series of catalyst -bodies, that the flow through the catalyst bodies, at least after the first, is turbulent. After passage through the passages of the catalyst body or bodies having both combustion passages and adjacent bypass preheat passages, the uncombusted fuel, ie that in the gas that has passed through the bypass preheat passages, combusts giving a hot, combusted, gas stream. If there is no dilution of this hot combusted gas stream by the addition of a cooler gas, the hot combusted gas stream will have a temperature corresponding to the adiabatic combustion temperature of the fuel/air mixture. This combustion may be effected catalytically, ie by passage through a catalyst body in which all of the passages have a coating of a catalyst which retains activity at high temperatures, but is preferably effected non-catalytically, ie homogeneously in the gas phase. Where there are two or more catalytic combustion stages, ie where there are two or more catalyst bodies having both combustion passages and bypass preheat passages, and/or where the the uncombusted fuel in the gas that has passed through bypass preheat passages is effected catalytically, the catalyst employed in the upstream stage or stages may differ from that in the downstream stage or stages. Thus the catalyst employed in the \ downstream stage or stages may need to withstand higher temperatures than the catalyst in the upstream stage or stages, but does not need to have as great a low temperature activity as the catalyst in the upstream stage or stages. For example where the uncombusted fuel in the gas that has passed through bypass preheat passages is combusted catalytically, the catalyst employed for this stage is preferably free from noble metals of Group VIII of the Periodic Table, or compounds of such metals. Suitable catalysts include a mixture of rare earth oxides, particularly mixtures of ceria, lanthana, and praseodymia as described in EP-A-472307. Other suitable catalysts include perovskites and manganese substituted barium aluminates. However the catalyst in the combustion passages of upstr' tages may contain Group VIII noble metals or noble metal compo~M..s. Indeed it is preferred that the catalyst in said combustion passages contains platinum, palladium, and/or rhodium or compounds thereof. Furthermore since the combustion catalyst in these combustion passages does not need to withstand the high temperatures corresponding to the adiabatic flame temperature of the fuel/air mixture, the catalyst boc- can be constructed of a less heat resistant material, for example cordierite, or a metal, than that employed for any subsequent catalytic combustion of the heated mixture. The use of such supports is advantageous as they are more readily able to withstand thermal shock, ie rapid temperature change as may be encountered when the ~ ~ em i r started-up or shut-down, than supports that have t. hε higher temperatures. The system is preferably designed such that the temperature of the mixture of the combusted gas from the combustion passages and the heated unc --busted gas from the bypass passages is above the temperature at w.„_ch gas phase, ie homogeneous, combustion of the fuel can be sustained, but below the temperature at which rapid deactivation of the catalyst, or melting or softening of the catalyst support, takes place. Also as indicated above, this temperature is preferably chosen so that the ignition delay time of the uncombusted fuel/air mixture in the bypass passages, relative to the delay time when unconfined by the passage walls, is such that gas phase combustion does not occur in the bypass passages but takes place essentially immediately upon exit from the bypass passages. Under normal running conditions, ie once start-up has been accomplished, it is preferred that the amount, and temperature, of air fed to the combustion catalyst system having the aforesaid combustion and bypass passages, is maintained essentially constant. It is preferred that there is provision for controlling the temperature of the gas leaving the catalyst bφdy: thus this temperature may be monitored by a suitable sensor and the amount of fuel in the fuel/air mixture fed to the catalyst body, ie to the combustion and bypass preheat passages, varied in response to a signal from the sensor to maintain the exit temperature at the desired level. Thus, after start-up, the catalyst system operates under essentially constant conditions. The second gas stream, containing air, is added to the hot combusted gas stream produced by the catalytic combustion and any subsequent gas phase combustion thereof. At least under non-idling conditions, fuel is added to this second gas stream. The amount and temperature of this second gas stream should be such that, under the desired idling conditions, the temperature of the mixture of this second gas stream and the hot, combusted, gas stream is above the temperature at which gas phase combustion of the fuel is sustained. In some cases fuel may be present in the second gas stream when operating under idling conditions: thus in one form of the invention, fuel is mixed with compressed air, part of the resultant fuel/air mixture is fed, as aforesaid, as the first gas stream to the combustion and adjacent bypass passages of the catalyst unit, while the remainder is employed as the second gas stream. Under load conditions other than idling, fuel (or more fuel when some fuel is added to the second gas stream when under idling conditions) is introduced into the second gas stream, and on mixing with the hot combusted gas, combustion of this added fuel takes place giving a hot product gas which may be used for driving a turbine. It is preferred, however, that under idling conditions no fuel is present in the second gas stream, and this \ second gas stream is at the same inlet temperature as that fed to the catalytic combustion. While the amount of air in the second gas stream may be varied as well as the amount of fuel added thereto as the system is changed from idling to full load, it is preferred that the amount of air employed is essentially constant. Conveniently the air fed to the catalytic combustion and that used as the second gas stream is derive.d from the same source, eg compressor, and is fed to the system at the same temperature. The proportion of the total air that is used as the second gas stream is preferably determined by the geometry of the system and preferably no provision, such as variable baffles or vanes, is made for varying this proportion in any particular system. Thus the proportion of air that is used as the second gas stream is preferably determined by the fixed geometry of the system so that the hydraulic resistance of the flow paths determines the proportion of the second gas stream. Thus it is preferred that the control of the operation between idling and full load is by varying only the amount of fuel added to the second gas stream. As indicated above, the use of a series of two or more catalyst bodies may be desirable in order to assist start-up and/or to sustain combustion. The need for such a series of bodies will depend on the particular design conditions. Thus if higher air compressor exit temperatures can be employed, such problems can be diminished. Likewise in some cases, it may be possible to provide a pilot flame, preferably lean-burn, upstream of the catalyst assembly to preheat the air, or air/fuel mixture, fed to the catalyst assembly. Furthermore, the use of catalysts that exhibit hysteresis, ie the catalyst requires a particular inlet temperature to achieve light-off but will remain lit if the inlet temperature falls to below that light-off temperature (but remains above some lower temperature), may assist stability. Two embodiments of the invention will now be described by way of example and with reference to the accompanying drawings wherein Figure 1 is a diagrammatic representation (not to scale) of a first embodiment having an assembly of three catalyst beds; Figure 2 is a section along the line II-II of Figure 1; Figures 3 and 4 are cross sections of part of the first and third catalyst bodies of the assembly of Figure 1; and Figure 5 is a view similar to Figure 1 showing a second embodiment. In the first embodiment the apparatus comprises an outer cylindrical casing 10 having an inlet zone 11 at one end and an outlet zone 12 at the other end. A tubular sleeve 13 is located, by means not shown, within casing 10 between the casing inlet and outlet zones 11, 12. Within sleeve 13 there is provided a series of three cylindrical catalyst bodies 14, 15, 16 with zones 17, 18 between the bodies, a sleeve inlet zone 19 between the casing inlet zone 11 and the first catalyst body 14, and a sleeve combustion zone 20 between the third catalyst body 16 and the casing outlet zone 12. Between sleeve inlet zone 19 and body 14 is a perforate baffle 21. The space between outer casing 10 and sleeve 13 forms an annular bypass 22. The cross sectional area of the annular bypass is 6Z, and the cross sectional area of the sleeve 13 is 32, of the total cross sectional area within casing ιo. Means, not shown, are provided to supply a stream of compressed air to the casing inlet zone 11, and injection means 23, 24 are provided to inject fuel into the sleeve inlet zone 19 and into the annular bypass 22 respectively. Valve means 25 are provided to vary the amount of fuel supplied via injection means 24 into the annular bypass 22. The first catalyst body 14, having a cross sectional area of 912 of the total cross sectional area within casing 10, comprises a cordierite honeycomb of length 90 mm having a number of through holes 26 of 20 mm diameter spaced in a equilateral triangular pattern. The number of holes 26 and their spacing is such that the total cross section of the holes 26 represents 212 of the total cross sectional area within casing 10. In Figure 2 seven holes 26 are shown but the actual number employed will depend on the cross sectional area within casing 10. These holes 26 are surrounded by a region 27 of honeycomb configuration having a voidage of 752 provided by through passages of 1.1 mm hydraulic diameter. The internal surfaces of alternate honeycomb passages in region 27 are coated with an alumina wash coat and impregnated with palladium as a combustion catalyst. 502 of the honeycomb passages are thus catalyst-containing combustion passages 28 (designated "C" in Figure 3), and each combustion passage 28 is surrounded by catalyst-free bypass preheat passages 29 (designated "B" in Figure 3). Baffle 21, which is spaced from the surface of body 14, has tubular projections 30 corresponding to, and aligned with, holes 26 and perforations 31 to allow gas to pass from sleeve inlet zone 19 to the space between baffle 21 and the honeycomb region 27 of body 14. The second catalyst body 15 has a length of 190 mm and has a honeycomb configuration over all of its cross section. The honeycomb configuration is the same as that of the honeycomb region 27 of body 14, ie voidage 752, passage hydraulic diameter 1.1 mm, and alternate passages are catalyst-containing combustion passages and catalyst-free bypass preheat passages. The third catalyst body 16 has a length of 200 mm and, like catalyst body 15, has a honeycomb configuration over all its cross section. The honeycomb passages have a voidage of 752 and a hydraulic diameter of 1.1 mm as in bodies 14, 15. However in body 16 only 37.52 of the passages are catalyst-containing combustion passages, ie three passages in each group of eight passages. The catalyst-containing passages 28 are disposed so that no two catalyst-containing passages are adjacent to one another. A suitable configuration for this honeycomb is shown in Figure 4 where "C" signifies a catalyst-containing combustion passage and "B" signifies a catalyst-free bypass preheat passage. if, in normal running, air at 300°C is fed, at 10 bar abs. and at a rate of 100 kg/s (per m 2 of the total interior cross section of the casing 10), to the casing inlet zone 11, it is calculated that 272 of the air passes through the annular bypass 22 while the remaining 732 enters the sleeve inlet zone 19. The perforations 31 in baffle 21 are of such size that the hydraulic resistance of baffle 21 and of the honeycomb passages in region 27 is such that 892 of gas entering sleeve inlet zone 19, ie 652 of the total air fed to the casing inlet zone 11, passes through the holes 26 and only 112, ie 82 of the total air fed to casing inlet zone 11, passes through the passages in honeycomb region 27. If fuel, eg natural gas, is added through injector 23 at such a rate that the fuel/air mixture entering catalyst body 14 has a composition that the combustion of all the fuel would give a gas mixture at 1200°C, and it is assumed that 952 of the fuel entering the combustion passages 28 of body 14 combusts during passage through those combustion passages, it is calculated that the temperature of the hot gas stream emerging from the honeycomb region 27 of body 14, ie the combusted gas that has passed through the combustion passages 28 of honeycomb region 27 and the heated, but uncombusted, gas that has passed through the bypass passages 29 of honeycomb region 27, would have a temperature of 746°C. The gas that passes through holes 26 enters zone 17 at essentially the same temperature at which it was fed to the sleeve inlet zone 19. The hot gas stream from honeycomb region 27 mixes in zone 17 with the unreacted fuel/air mixture that has passed through the holes 26 to give a gas mixture at a temperature of 351°C. This gas mixture then passes through catalyst body 15. If it is assumed that 902 of the fuel/air mixture passing through the catalyst-containing combustion passages of body 15 combusts during passage through those combustion passages, it is calculated that the gas entering the zone 18 between bodies 15 and 16, ie the mixture of the combusted gas that has passed through the combustion passages of body 15 and the heated gas that has passed through the catalyst-free bypass passages of body 15, would have a temperature of 748°C. The gas mixture from zone 18 then enters the third catalyst body 16. If it is assumed that 892 of the gas entering the catalyst-containing combustion passages combusts during its passage through those combustion passages, it is calculated that the gas emerging into sleeve combustion zone 20, ie the mixture of the combusted gas that has passed through the combustion passages of body 16 and the gas that has passed through the bypass passages of body 16 but has been heated during its passage through those bypass passages, would have a temperature of 904°C which is hot enough to sustain homogeneous, ie gas phase, combustion of a methane-containing fuel such as natural gas. This temperature is however sufficiently low that the ignition delay time in areas of high surface to volume ratio, ie within the honeycomb passages, is sufficiently long, in relation to the time that the gas mixture is within the passages, that essentially no homogeneous combustion takes place in the passages. The ignition delay time at this temperature in regions of lower surface to volume ratio, ie in the sleeve combustion zone 20 downstream of the passages, is however sufficiently short that the gas mixture combusts virtually instantaneously in sleeve combustion zone 20 giving a hot gas\ stream at 1200°C, which enters the casing outlet zone 12. The air (amounting to 272 of the total air fed to the casing inlet zone 11) passes through the annular bypass 22 essentially without heating from its inlet temperature of 300°C. if no fuel is added through the injector 24 into this air passing through the annular bypass 22, the hot gas stream produced in the sleeve combustion zone 20 mixes, in casing outlet zone 12, with the air that has passed through the annular bypass 22 to give a gas mixture at a temperature of 950°C. If fuel is then added, via injector 24, to this air passing through annular bypass 22, on mixing with the hot gas mixture from the sleeve combustion zone 20, the mixture in the casing outlet zone 12 will be hot enough to combust homogeneouslv. Combustion will thus occur giving a gas mixture at a tern ,ture, above 950°C, dependent on the amount of fuel adc via injector 24. \ The second embodiment, shown in Figure 4, is similar to that of Figure 1, but the third catalyst body of the Figure 1 embodiment is omitted. In this second embodiment, the annular bypass 22 has a cross section corresponding to 172 of the total interior cross section of the casing 10 and the holes 26 in body 14 have a total cross sectional area amounting to 102 of the cross sectional area within casing 10. A constriction 32 is provided in annular bypass 22. As in the Figure 1 embodiment the honeycomb passages of each of bodies 14, 15 have a voidage of 752, a hydraulic diameter of 1.1 mm, and only alternate passages have a catalyst coating. In this embodiment the catalyst bodies 14 and 15 are 130 mm and 190 mm long respectively. Constriction 32 of annular bypass 22 is of such size that, if, in normal running, air at 300°C is fed, at 10 bar abs. and at a rate of 100 kg/s (per m 2 of the total interior cross section of the casing 10), to the casing inlet zone 11, 432 of the air passes through the annular bypass 22 while the remaining 572 enters the sleeve inlet zone 19. The perforations 31 in baffle 21 are of such size that the hydraulic resistance of baffle 21 and of the passages in honeycomb region 27 is such that 42.52 of the total air fed to the casing inlet zone 11, ie about 752 of the air entering sleeve inlet zone 19, passes through the holes 26 and only about 14.52 of the total air fed to casing inlet zone 11, ie about 252 of the air entering sleeve inlet zone 19, passes through the passages in honeycomb region 27. If a methane-containing fuel such as natural gas is added through injector 23 at such a rate that the fuel/air mixture entering catalyst body 14 has a composition that the combustion of all the fuel would give a gas mixture at 1400°C, and it is assumed that 90.52 of the fuel entering the combustion passages 28 of body 14 combusts during passage through those combustion passages, it is calculated that the temperature of the hot gas stream emerging from the honeycomb region 27 of body 14 would have a temperature of 827°C. This stream mixes in zone 17 with the unreacted fuel/air mixture that has passed through the holes 26 to give a gas mixture at a temperature of 440°C. , This gas mixture then passes through catalyst body 15. If it is assumed that 932 of the fuel/air mixture passing through the combustion passages of body 15 combusts during passage through those combustion passages, it is calculated that the gas entering the sleeve combustion zone 20 would have a temperature of 905°C which is hot enough to sustain virtually instantaneous homogeneous, ie gas phase, combustion of a methane-containing gas downstream of body 15 but is such that the ignition delay time while within the passages of body 15 in relation to the dwell time within those passages, is such that essentially no homogeneous combustion occurs within those passages. The gas mixture in sleeve combustion zone 20 thus combusts, giving a hot gas stream at 1400°C which enters the casing outlet zone 12. The air (amounting to 432 of the total air fed to the casing inlet zone 11) passes through the annular bypass 22 essentially without heating from its inlet temperature of 300°C. If no fuel is added through the injector 24 into this air passing through the annular bypass 22, the hot gas stream produced in the sleeve combustion zone 20 mixes, in casing outlet zone 12, with the air that has passed through the annular bypass 22 to give a gas mixture at a temperature of 950°C. If fuel is then added, via injector 24, to this air passing through annular bypass 22, on mixing with the hot gas mixture from the sleeve combustion zone 20, the mixture in the casing outlet zone 12 will be hot enough to combust homogeneously. Combustion will thus occur giving a gas mixture at a temperature, above 950°C, dependent on the amount of fuel added via injector 24. It will be appreciated that instead of providing constriction 32, annular bypass 22 could be made narrower to restrict the flow through the bypass to 432 of the total air fed to the casing inlet zone 11. It is thus seen that in the above examples, the combustion can be varied, from idling to full load, by increasing the amount of fuel, by means of valve 25, injected into annular bypass 22. However, during this variation, the catalytic combustion in the catalyst bodies is unchanged. Also it is seen that the catalyst temperature is maintained below about 950°C, and so catalysts having activity at low temperatures, but which lose their activity when subjected to temperatures above about 950°C can be employed, thereby facilitating start-up. It will be appreciated that in a gas turbine, ballast gas, eg steam or a further quantity of air, will normally be added to the hot gas from the casing outlet zone 12 to moderate the temperature before the mixture enters the turbine.

Claims: Claims 1. Combustion apparatus comprising a) a catalytic combustion unit having a plurality of through passages of which some are combustion passages having a combustion catalyst on the passage walls and at least some others are bypass preheat passages that are free of catalyst and are adjacent said combustion passages; b) means to supply a first gas stream containing air and fuel to said combustion passages and bypass preheat passages; c) an afterburn zone downstream of said catalytic combustion unit; d) means to supply a second gas stream containing air to said afterburn zone; and e) means to introduce a variable amount of fuel into said second gas stream upstream of said afterburn zone. 2. Combustion apparatus according to claim 1 wherein the \ combustion unit comprises at least two catalyst bodies in series, each having catalyst-containing combustion passages and catalyst-free bypass preheat passages adjacent the combustion passages, and a mixing zone between each catalyst body. 3. Combustion apparatus according to claim 2 wherein the ratio of bypass preheat passages to catalyst-containing combustion passages of one catalyst body differs from that of another of said catalyst bodies. 4. Combustion apparatus according to claim 2 or claim 3 wherein, in addition to the bypass preheat passages of a catalyst body, further bypass means are provided to supply additional fuel/air mixture to the mixing zone between that catalyst body and the adjacent downstream catalyst body. Combustion apparatus according to any one of claims 1 to 4 wherein each catalyst-containing combustion passage is surrounded by catalyst-free bypass preheat passages. 6. Combustion apparatus according to any one of claims 1 to, 5 including two or more catalyst bodies in series having through catalyst-containing combustion passages wherein the catalyst of an upstream catalyst body differs from that of a downstream catalyst body. 7. A combustion process comprising a) feeding a first gas stream containing fuel and air to the combustion and bypass preheat passages of a catalytic combustion unit having a plurality of through passages of which some are combustion passages having a combustion catalyst on the passage walls and at least some others are bypass preheat passages that are free of catalyst and are adjacent said combustion passages, whereby combustion of said fuel takes place in said combustion passages and heat is transferred through the passage walls to said adjacent bypass preheat passages thus preheating the air and passing through said bypass preheat passages, and combustion of the fuel passing through said bypass preheat passages takes place downstream of said combustion and bypass preheat passages to give a hot combusted gas stream; b) adding a second gas stream containing air to said hot combusted gas stream; and c) varying the amount of fuel added to said second gas stream prior to addition thereof to said hot combusted gas stream. 8. A process according to claim 7 wherein the amount of air fed to said combustion and bypass preheat passages is constant, and the amount of fuel fed to said combustion and bypass preheat passages is controlled to maintain constant the temperature of the mixture of combusted gas from said combustion passages and the uncombusted gas from said bypass preheat passages. 9. A process according to claim 7 or claim 8 wherein air is compressed and part of that air is mixed with fuel and fed as the gas first stream to said combustion and bypass preheat passages and the remainder forms said second gas stream to which a variable amount of fuel is added. 10. A process according to any one of claims 7 to 9 wherein the amount of air in said second gas stream is constant.

Inventors: FOWLES MARTIN, LYWOOD WARWICK JOHN, SHIPLEY DAVID GRAHAM, FOWLES, MARTIN, LYWOOD, WARWICK, JOHN, SHIPLEY, DAVID, GRAHAM
Applicants: FOWLES MARTIN, ICI PLC, LYWOOD WARWICK JOHN, SHIPLEY DAVID GRAHAM, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1993
Main classification: F23R3
IPC codes: F23C13, F23R3