GB2184226A - Gas turbines - Google Patents

Abstract

A gas turbine has a combustion chamber including a monolithic ceramic honeycomb, an active catalyst being supported on the surfaces of through passages in the honeycomb. The honeycomb has generally uniform cross-section primary through-passages, and auxiliary through-passages each having a lesser cross-section. Diagrammatic cross-sections of honeycomb are shown. <IMAGE>

Description

SPECIFICATION Gas turbines This invention relatesto gas turbines and in particularto gas turbines of the type wherein a gaseous mixture comprising a fuel and a gas containing free oxygen, e.g. air, is combusted in a catalytic combustion chamber and the resulting hot gas stream drives the turbine. In such gas turbines the combustion catalyst generally comprises an active metal such as platinum deposited on an inert supportthat is capable of withstanding the combustion temperatures. The support is normally a monolithic ceramic honeycomb.

As the gaseous mixture passes through the through passages of the catalyst-bearing honeycomb,the combustion reaction proceeds and, as a consequence, the temperature ofthe gas stream, and ofthe honey- comb, increases. For a gas mixture of given composition and inlettemperature, at some distancethroughthe honeycomb a rapid increase in temperature occurs until a peaktemperature is reached: thereafterthetemperature ofthe gas mixture remains approximately constant or drops slightly.The length ofthethrough passages of the monolithic honeycomb is generally designed such that, atthe maximum desired gas flow rate, this region of peaktemperature occurs well before the gas reaches the passage outlets, In this way, as the activity ofthe catalyst deteriorates with age, the peaktemperature region gradually moves towards the passage outlets. Of course, the higherthe gas inlet temperature, the closertothe passage inlet will bethe peaktemperature region. One or more components ofthe gas mixture is normally compressed priorto entry into the catalytic combustion chamber and such compression will effect heating to the inlettemperature.In some cases there may also be preheating of the gas mixture, or one or more components thereof, before or after compression, by heat exchange with the effluent gas leaving the turbine.

One problem that is encountered is that when the turbine is started up, the full gas flow rate cannot usually be employed since as insufficient reaction occurs within the honeycomb passages to achievethetemperature necessary to obtain the desired combustion and satisfactory running ofthe turbine. It is therefore usually necessary to start uptheturbine at a reduced gas flow rate and gradually increase the gas flow rateto the desired level as the honeycomb heats up. This is disadvantageous where rapid start-up is required.

Alternatively, or additionally, it is necessary to provide for additional preheating of the gas mixture so that its inlettemperature is increased.

We have devised aform of honeycomb construction that enables the turbine to be started-up at gas flow rates closer to the desired normal running rate.

Accordingly the present invention provides a gas turbine having a combustion chamberforthe combus- tion ofa preheated gaseous mixture comprising a fuel and a gas containing free oxygen, said combustion chamber including a monolithic ceramic honeycomb having an array of primary through passages ofgener- ally uniform cross-sectional configuration and bearing on their surfaces a combustion catalystforsaid gas- eous mixture, characterised in that the honeycomb has auxiliary th rough passages also bearing said combustion catalyst, each auxiliary passage having a smaller cross-sectional area than that of a primary passage.

As a result of a proportion ofthe gaseous mixture passing through the auxiliary passages and reacting therein, the gaseous mixture passing through adjacent primary passages is heated to cause more rapid combustion. As a consequence combustion can be started with an increased gaseous mixture initial flow rate.

Alternatively, oradditionally,the provision ofthe auxiliary passages can enablethe overall volume ofthe monolithic honeycomb to be reduced. Thus combustion ofthe gaseous mixture in the auxiliary passages causes the peak temperature to be achieved in those passages at a position closer to the inlet region ofthe honeycomb; in turn the gas mixture in the larger, primary, passages becomes preheated and thus achieves its peak temperature at a position closerto the honeycomb inlet. As a consequence the length of,and hence the volume occupied by, the honeycomb can be decreased.

The auxiliary passages required by the present invention are additional to any passages occurring at the periphery ofthe honeycomb monolith that have a smaller cross section than the primary passages. Such smali peripheral passages will generally result from the honeycomb being manufactured to an overall cross- sectional configuration with primary passages of such a shape and sizethat passages of smaller cross sec- tion are required atthe periphery to make up the desired overall cross-sectional configuration.Thuswhere, for example, the overall cross-sectional configuration ofthe honeycomb is circular and the primary passages therethrough are of,forexample square cross-section,then, atthe periphery ofthe honeycomb, smaller passages, for example those resulting from the intersection of such a square primary passage with an arcof the peripheral circle, may occur: the auxiliary passages required by the present invention are additional to those peripheral small cross-section passages. The auxiliary passages of the present invention are thus located inboard of any peripheral passages of cross section smaller than the primary passages.

It is preferred that the honeycomb is of such size that substantially complete combustion of the fuel in the gas mixture occurs.

But the term "substantially complete combustion" we mean that at least 90%, preferably at least 95%, of the fuel is combusted. In many cases combustion of at least 98% ofthefuel is desirable. Although it isto be preferred, it is not necessarythatthe combustion takes place only within the honeycomb: some ofthecombustion, e.g. up to about 10% of the fuel that is combusted, may occur downstream of the honeycomb if the temperature ofthe gas mixture is raised sufficiently within the honeycomb.

Honeycombs having some passages of reduced cross-sectional area have been proposed in US-A-4521532 for a totally different application, viz automobile exhaust catalytic converters. There the different passage sizes were employed to spread the position of maximum temperature through the honeycomb to avoid hot spots which could lead to melt-down ofthe honeycomb. The present invention is based on the realisation thatthis principle of providing auxiliary passages of smallercross-section passages can lead to fasterstart- up of a gas turbine.

In the honeycomb used in the invention there are through passages of different cross-sectional dimensions; the larger, or largest, passages herein termed the primary passages, are dimensioned such that, afterstart-up and when the turbine is operating at the design gas flow rate, combustion of the gas mixture flowing th rough the primary passages occurs before the gas mixture leaves the primary passages: on the other hand the dimensions are chosen to minimise the pressure drop through the honeycomb atthe design gas flow rate.While the provision of auxiliary passages of smaller cross-sectional area than the primary passages has the effect of increasing the pressure drop, reducing the length of the honeycomb, as is often made possible as a result of the present invention, has the opposite, and thus a compensatory effectwhich may, in some cases, result in an overall decrease in the pressure drop.

In additiontothe primary passages there are also auxiliary passages, of cross-sectional area smallerthan that ofthe primary passages. As will be described hereinafter there may be auxiliary passages of different sizes butfor sim pl icity it wil I be assumed in the following description of the working ofthe invention thatthe auxiliary passages are all of the same size.

Compared to the primary passages, the auxiliary passages have a greater surface area, and hence more active catalyst, per unit passage volume. Also, for a given pressure difference between the inlet and outlet of the honeycomb passages, the gas linearvelocitywill be somewhat less in the auxiliary passages than in the primary passages, so that the gas mixture has a greater residence time in the auxiliary passages. Thus, even at high start-up gas flow rates, the greater residencetime ofthe gas mixture in the auxiliary passages and the greateramount of active catalyst per unit passage volume in the auxiliary passages enablesthecatalytic combustion to occurtherein. The heat produced in this reaction heats gas in adjacent passages.The auxiliary passages are dimensioned such that tile peaktemperature is reached in the auxiliary passages at the design operating gas flow rate sufficiently nearto the auxiliary passage inlets to effect preheating ofthe gas mixture flowing through adjacent primary passagesto such an extentthatthe peaktemperature is achieved within those adjacent main passages.

As the honeycomb warms up the peaktemperature position in those adjacent primary passages will then move towards the passage inlets and this will enable heatto be transferred from those primary passagesto any adjacent primary passages that have not been heated by adjacent auxiliary passages. It is seen thatthe combustion can thus spread through the honeycomb array even through initiated at only one auxiliary passage location.

It is preferred, however, to have auxiliary passages positioned at intervals, preferably regular, through the honeycomb array. The auxiliary passages may be isolated or in groups of adjacent auxiliary passages so that each group of adjacent auxiliary passages acts as a larger heat source: in this way more rapid heating ofthe primary passages adjacent the group of auxiliary passages may be achieved than if each auxiliary passage had only adjacent primary passages.

Where the auxiliary passages are isolated or in groups each containing less than ten adjacent auxiliary passages, the dimensions and number of auxiliary passages relative to the primary passages are preferably such that between 2 and 50%, particularly between 5 and 33%, of the gas mixture flows through auxiliary passages.

Where the auxiliary passages are in groups of more than ten adjacent auxiliary passages, their dimensions and number can be such that a smaller proportion ofthetotal gas flows through the auxiliary passages. In some cases the proportion flowing through the auxiliary passages may be as small as 0.1%, or even less, of the total gas flow and is preferably between 0.1 and 30% of the total gasflow.

The passages may have a circular cross-section but are preferably of polygonal cross-section. In thefollow ing description reference is made to the effective passage diameter: by this term we mean the effective hydraulicdiameterwhich is defined as 4 x cross-sectional area ofthe passage perimeter of the passage cross-section For a circularcross-section passage this of course gives an effective diameter equal to the actual crosssectional diameterwhilefora regular polygon the effective diameter is the diameter of the inscribed circle.

Where the auxiliary passages are all of the same size they preferably have an effective diameter between 20 and 75%, particularly between 30 and 60%, ofthe effective diameter of the primary passages. Where there are auxiliary passages of differing sizes, it is preferred that 30 to 70% ofthe auxiliary passages have effective diameters between 50 to 80% ofthe effective diameter of the primary passages and the remainder of the auxiliary passages have effective diameters between 20 and 50% of the effective diameter of the primary passages.

The primary passages preferably have an effective diameter in the range 1 to 5 mm. The passage lengths, which is preferably the same for both the primary and auxiliary passages, is typically in the range 50to 600 mm.

The honeycomb can be made by extrusion of suitable ceramic material through an appropriatelyconfigured die,forexample as described in GB-A-1385907. The honeycomb is preferably formed from an alumina composition: particularly suitable extrudable alumina compositions are described in EP-A-134138.

The catalytic material is preferably a platinum group metal, particularly platinum, rhodium or palladium, or atransition metal oxide such as cobalt or chromium oxides and may be incorporated directly on to the ceramicsupportor may be present in a wash-coat applied to the ceramicsupport.

The present invention is of particular merit where the gas turbine is powered by a fuel such as natural gas which has a relatively high ignition temperature. Thus with such fuels, unless the catalyst is particularly active, the rapid rise in temperature only occurs after the gas mixture has reached a temperature of the order of 450-500 C and so rapid start-up with such fuels is not normally possible when using a honeycomb having only primary passages since the heating given by the precombustion compression is generally no more than about 300-4000C. With such fuels the peaktemperature will typically be above 1 200 C and is generally in the range 1250-1550"C.

In some applications there is a risk that during or after start-up, when the monolith reaches a sufficiently high temperature the gas upstream ofthe monolith may reach its auto-ignition temperature with the result that flash-back of the combustion to a location upstream of the combustion catalyst occurs. Where such premature combustion is undesirable, a flametrap, e.g. a wire mesh, may be positioned upstream ofthe combustion catalyst. Such a wire mesh will also act as a radiation shield to reduce heating ofthe gas upstream of the mesh by radiation from the monolith.

The invention is illustrated by reference to the accompanying drawings wherein Figures 1 and2 are graphs showing howthetemperature ofthe gas mixture varies as it passes through the honeycomb: Figure 1 is illustrative of conventional arrangements wherein the passages are all of the same size while Figure 2 illustrates the present invention.

Figures 3 to 7 are cross-sections of different honeycomb configurations: for simplicity the thickness ofthe honeycomb webs has been shown as being negligible.

In Figures 1 and 2the gas mixturetemperatureTis plotted against distance, d. d1 and do indicatethe positions of the passage inlets and outlets respectively. Tj indicates the ignition temperature for the gas mixture, ie the temperature at which rapid reaction occurs.

In Figure 1 curve A illustrates how the temperature increases if the gas flow rate, rate I, is too high at start-up: it is seen that the gas leaves the honeycomb before the rapid reaction has taken place - in this case substantially complete combustion may not occur. By starting up at a significantly reduced flow rate, rate II, the rapid increase in temperature as illustrated by curve B, can be achieved within the honeycomb. Afterthe combustion has been achieved atthe gas flow rate corresponding to curve B,the gasfiow rate is gradually increased, and, since the honeycomb has been heated up by the reaction atthe low gas rate, substantially complete combustion at the full gas rate I can be achieved: this is shown by curve C.

In Figure 2, illustrative of the temperature profile atconstanttotal gas flow rate in a honeycomb with primary and auxiliary passages in accordance with the invention, curves D and E referto thetemperature profile in the primary passages and curves F and G refer to the temperature profile in the auxiliary passages.

Curves D and F show the profiles as if the auxiliary passages had no effect on the primary passages and vice versa - thus in this case, since the linear flow rate through the auxiliary passages is sig nificantly lessthan that through the primary passages, curves D and Fcorrespondto the situation of curves A and B respectively of Figure 1. Howeverthe auxiliary and primary passages do have an effect on one another as shown by curves E ang G. Thus as shown by curve G the auxiliary passages do not heat up quite as quickly since the gas flowing through adjacent primary passages will extract heat from the auxiliary passages.However sufficient heat is transferred from the auxiliary passages to the primary passages that, as shown by curve E, the gas in the adjacent primary passages is more rapidly heated to its ignition temperature and so the rapid rise in temperature will occur in those primary passages well before the gas reaches the honeycomb outlet.

In Figure 3 one form of array of primary and auxiliary passages is shown: here the primary passages are in the form ofregularoctagons and the auxiliary passages are the squares at the centre of each group offour primary passages. In this case the effective diameter of each ofthe auxiliary passages is about 40% of that of the primary passages and atthe gas flow rates normally prevailing in gas turbine combustion chambers, about 10% of the gas will flow through the auxiliary passages. The precise figures values ofthe above quoted percentages will of course depend on the wall thickness ofthe honeycomb for the effective diameter percentage and on the wall thickness and the actual gas flow rateforthe gas flow percentage.

In the embodiment of Figure 1 each primary passage is of square configuration while the auxiliarypassages comprise primary passages subdivided into four by diagonal partitions across the square. In this case again the effective diameter of the auxiliary passages is about 40% ofthat of the primary passages. However as there arefourtimes as many auxiliary passages as there are primary passages, about3l% ofthetotal gas mixturefiowthrough the auxiliary passages.

In Figure 5 an arraysimilarto that of Figure 4is shown bywhereinthe auxiliary passages are oftwo different sizes. Thus the primary passages are of square configuration, intermediate auxiliary passages correspond to a primary passage divided into two by a diagonal partition while, as in the Figure 4embodiment, the small auxiliary passages correspond to primary passages divided into four by two diagonal partitions.It is seen thatthere are four small auxiliary passages, and four intermediate auxiliary passages, for everyfive primary passages. ltshould be noted that in each group offive primary passages, the central primarypas- sage has no adjacent auxiliary passage and each of the otherfour primary passages has an adjacent small auxiliary passage and two adjacent intermeidate auxiliary passages (each ofthe latter being adjacent to two ofthesefour primary passages). The effective diameter of the small auxiliary passages is, as in Figure 4; about40% ofthat of the primary passages and that of the intermediate auxiliary passages is about 60% ofthat ofthe primary passages.About 16% ofthe gas mixture flows through the intermediate auxiliary passages and about7%through the small auxiliary passages.

The arrays shown in Figures 3 to 5 are regular: it is of course not necessary, but is preferred, that the arrays are regular.

In the embodiment shown in Figure 6the primary passages are ofrightangled isoscelestrianglecross section while the auxiliary passages are one hundred adjacent auxiliary passages grouped in a square.

in the embodiment of Figure7the primary passages are of square configuration: diagonal bands of primary passages are divided into bands of adjacent intermediate auxiliary passages by diagonal webs.

Where the two diagonal bands intersectthe intermediate auxiliary passages are each divided into two to form a cluster of adjacent small passages.

In atypical examplethe overall diameterofthe combustion catalyst honeycomb is about45 cm and the length about 15cm with the primary passages in the form of right angled isosceles triangle cross section having an effective diameter of about 1.3 mm and auxiliary passages in one or more clusters of about 100 auxiliary passages grouped to form a square as depicted in Figure 6.

Typical operating conditions are Airlnaturalgas 40:1 (by volume) airflow rate 23kg/sec in let temperature 300-400, eg 360"C inlet pressure 10 barbs.

Claims (10)

1. A gas turbine having a combustion chamberforthe combustion of a preheated gaseous mixture comprising a fuei and a gas containing free oxygen, said combustion chamber including a monolithic ceramic honeycomb having an array of primary through passages of generally uniform cross-sectional configuration and bearing on their surfaces a combustion catalystforsaid gaseous mixture, characterised in thatthe honeycomb has auxiliary th rough passages also bearing said combustion catalyst, each auxiliary passage having a smallercross-sectional area than that of a primary passage.

2. A gas turbine according to claim 1 wherein the auxiliary passages are disposed at regular intervals throughout the honeycomb array.

3. A gas turbine according to claim 1 or claim 2 wherein the auxiliary passages are disposed in groups of adjacent auxiliary passages.

4. A gas turbine according to claim 3wherein the auxiliary passages are disposed in groups of morethan ten auxiliary passages and are of such dimensions and number in relation to the primary passagesthatthe proportion ofthe gas mixture flowing through the auxiliary passages iffrom 0.1 to 30% ofthetotal gasflow.

5. Agasturbine according to anyone of claims 1 to 3 wherein the auxiliary passages are disposed in groups of less than ten adjacent auxiliary passages and are of such dimensions and number in relation to the primary passagesthatthe proportion ofthe gas mixture flowing through the auxiliary passages is between 2 and 50% ofthe total gas flow.

6. A gas turbine according to any one of claims 1 to 5 wherein the auxiliary passages are all of the same size and have an effective diameter between 20 and 75% of the effective diameter of the primary passages.

7. A gas turbine according to any one of claims 1 to 5 wherein there are auxiliary passages of morethan one size.

8. A gas turbine according to claim 7wherein 30 to 70% of the auxiliary passages have an effective diameter between 50 and 80% of the diameter of the primary passages and the remainderoftheauxiliary passages have an effective diameter between 20 and 50% of the diameter ofthe primary passages.

9. A gas turbine according to any one of claims 1 to 8 wherein the auxiliary passages have a cross-section configuration corresponding to subdivision ofthe primary passages by one or more partitions.

10. Agasturbine according to anyone of claims 1 to 9wherein the primary passages have an effective diameter in the range 1 to 5 mm.