GB2502298A

GB2502298A - Burner and Combustor for Gaseous Hydrogen

Info

Publication number: GB2502298A
Application number: GB201209004A
Authority: GB
Inventors: Bhupendra Khandelwal; Vishal Sethi; Riti Singh; David G Macmanus; Priyadarshini Murthy
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2012-05-22
Filing date: 2012-05-22
Publication date: 2013-11-27
Also published as: GB201209004D0

Abstract

A burner 10 for a micromix combustor has a flow of oxidant along an axial passage 20, and at least one injector 50 arranged to feed at least one of a gaseous oxidant or a gaseous hydrogen fuel into the oxidant flow. The injector feed is in a direction 60 transverse to, and radially offset 70 with respect to the burner passage axis 30. The burner arrangement may include several injectors, spaced around a circumferential wall 25 of the burner, and where four injectors are use, two may feed oxidant and two may feed hydrogen. Injectors may also be arranged diametrically opposite relative to the burner axis, and the axial passage may include a swirler (100 Fig 2). The burners may be used in a micromix combustor (Fig 5A, 5B), and the burners may be arranged in at least one, but preferably several concentric rings (120, 121, 122, Fig 5B) with injectors being serviced by a manifold arrangement (130, 140, Fig 5B).

Description

TITLE: MICROMIX COMBUSTOR FOR GASEOUS HYDROGEN

DESCRIPTION

TECHNICAL FIELD

The present invention relates to micromix combustors for gaseous hydrogen.

BACKGROUND ART

Due to its high reactivity, gaseous hydrogen is not amenable to premixed combustion of the kind conventionally used to achieve low NOx emissions with gaseous fuels such as methane. This is because premixed hydrogen and air are prone to ignite in the conduits leading to the combustor -a phenomenon known as "flashback". Instead, it is known to obtain low NOx emissions by buming gaseous hydrogen in so-called "micromix" combustors. Rather than pre-mixing the hydrogen and air, gaseous hydrogen alone is injected into the combustion chamber through many nozzles, resulting in many local mixing zones bctwccn the hydrogen and air. Thc air and hydrogen mix by cross flow interaction and the mixture then bums in or directly adjacent each mixing zone in a diffusive type flame.

Compared to hydrogen burners using few, large-scale diffusion flames where layers of stoichiometric mixtures give rise to high local temperatures and high NOx production, the many difftision flames of a micromix combustor result in lower temperatures and thus lower NOx. Figure 24 of US 6,267,585 (Suttrop) illustrates how, by increasing the density of difftision flames from the 0.1/inch2 (I50/m2) found in a conventional burner to 10/inch2

I

(15000/rn2) the relative concentration of NOx in the exhaust gas can be reduced from 30 parts per million (ppm) to the level of 10 ppm, described in US'585 as a "practical level". Further increasing the density to a "preferable" 20/inch2 (30000/rn2) results in a thither reduction in NOx to 6 ppm. US'S 85 suggests that the upper limit of several thousand dififisive microcombustion flames distributed over the entire available burner surface facing into the combustion chamber may be reached on the one hand when the miniaturization is no longer economically feasible, or technically when the diffusive microcombustion flames are no longer stable due to the high number of flames per square inch.

Figures 8-17 of US 585 disclose a particular construction of micromix combustor for combusting hydrogen comprising two perforated plates spaced from each other by tubes mounted in the perforations. Each tube has an air illlet port and an air outlet port directly into the combustion chamber. Hydrogen fuel enters through holes passing through the walls of each tube as close as possible to the exit port of each tube next to the combustion chamber.

Figure 9 ofUS'SSS shows the flow direction of distinct hydrogen jets at a right angle to the air flow direction to the tubes, while figure 14 shows a hydrogen jet impinging on an air jet at a right angic and circulating both downsfrcam and upstream. US'585 notes that this rcsults in excellent mixing but that hydrogen jets may be directed to impinge on the air streams at a angle other than a right angle, for example by directing the hydrogen injection holes at a respective angle through the wall of the respective tube. The example is given of each tube comprising six holes and a combustor comprising 500 tubes, resulting in a total of 6x5003000 diffusive microcombustion flames being formed when operating the cornbustion chamber. It is suggested that this large number of difñsive microcombustion flames reduces the NOx production to less than 20% of the NOx production in a conventional burner of comparable size but with few large combustion flames.

Marek C.J., Smith T.D. and Kundu K., AIAA-2005-3776, discloscs alternative low emission micromix hydrogen combustors for gas turbines using lean direct injection (LDI).

In one embodiment, the so-called "NASA GRC (Ni)" shown in fIgure 4 of the document, the combustor injector is circular and made up of coaxial outer, centre and inner rings, the outer and centre rings being formed with circumferentially-spaced bores for throughflow of air. Each bore has two orifices for injection of gaseous hydrogen into the air flowing through the bore, the orifices being supplied by annular manifolds defined between adjacent rings. The orifices direct the ifiel into the bore in a radial direction relative to the axis of the bore and direction of air flow. Another embodiment, shown in figure 3(d) of the document, uses a single centre hydrogen nozzle at the centre of each hole with a large amount of counter swirl (apparently generated by a swirler structure around each nozzle) to produce mixing.

DISCLOSURE OF INVENTION

According to a first aspect of the present invention, there is provided a burner for a meromix combustor for gaseous hydrogen, the burner comprising a passage for flow of gaseous oxidant alollg the axis thereof and at least one injector configured to feed gaseous hydrogen or gaseous oxidant into the flow in a direction transverse to and radially offset from said axis.

Feeding gaseous hydrogen or oxidant into the flow of gaseous oxidant (typically air) ill a direction transverse to and radially offset from the axis of flow results in swirl, which has numerous benefits including increased flame stability and shorter flame length.

The passage may be circular in cross-section and have a circumferential wall. The at least one injector may be formed by a hole in the circumferential wall. An injector may be configured to feed hydrogen or gaseous oxidant in a direction tangential to the circumferential wall.

A plurality of injectors may be regularly spaced around the circumferential wall. The burner may consist of only four regularly-spaced injectors. Two injectors may be configured to feed gaseous hydrogen or oxidant into the passage in opposite senses about the passage axis. An air swirler may be fitted in the inlet of the passage.

The burner may comprise a plurality of injectors, at least one injector being configured to feed gaseous oxidant and another being configured to feed gaseous hydrogen.

The gaseous oxidant and hydrogen may be fed into the passage in opposite directions about the axis.

All of the injectors may be configured to feed gaseous oxidant into thc flow in a directioll transverse to and radially offset from said axis. Spaced along the axis from these injectors may be at least one further injector configured to feed gaseous hydrogen into the flow. The thither injector maybe configured to feed gaseous hydrogen radially relative to the axis of the passage. The burner may comprise two injectors located diametrically opposite relative to the axis.

According to a second aspect of the invention, there is also provided a micromix combustor for gaseous hydrogen comprising a plurality of burners as set out above and configured to provide a density of diffusion flames of at least 15000/rn2. As explained above, such a density of diffusion flames is necessary to achieve a commercially practical level of NOx. The burners may be arranged in a ring. The burners may be arranged in a plurality of concentric rings. At least one manifold for supplying gaseous hydrogen or oxidant to the burners may lie concentric with a ring. A manifold may lie radially between adjacent rings.

Where a burner in a ring has a plurality of injectors, at least one of the injectors may be fed from a manifold radially outward of the ring and at least one of the injectors may be fed from a manifold radially inward of a ring.

BRIEF DESCRIPTION OF DRAWINGS

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figures 1A and B are transverse and longitudinal cross-sectional views of a first embodiment of a burner 10 for a micromix combustor according to the present invention; Figures 2A and B are transverse and longitudinal cross-sectional views of a second cmbodimcnt of a burncr 10 for a micromix combustor according to thc prcscnt invcntion; Figures 3A and B are transverse and longitudinal cross-sectional views of a third embodiment of a burner 10 for a micromix combustor according to the present invention; Figures 4A and B are transverse and longitudinal cross-sectional views of a fourth embodiment of a burner 10 for a micromix combustor according to the present invention; Figures SA is a plan view of a cornbustor incorporating a plurality of burners according to the present invention; Figure SB is a detailed view on X in figure SA.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Figurcs 1A and B arc transvcrsc and longitudinal cross-scctional vicws of a first cmbodimcnt of a burncr 10 for a micromix combustor for gascous hydrogcn according to thc present invention. Burner 10 comprises a passage 20 of circular cross-section and having a longitudinal axis 30. Gaseous oxidant, typically air, flows along the axis as indicated by arrows 40. The burner also comprises four injectors 50, regularly spaced about the circumference 25 of the passage. As indicated by arrows 60 in figure IB, these injectors feed hydrogen into the passage in a direction transverse to the axis. As illustrated in figure 1A, the injectors 50 also feed the gaseous hydrogen in a direction 60 radially off'et (by a distance 70) from the axis 30.

In the embodiment shown, direction 60 lies tangential to the circular wall 25 of the passage 20; however, other angles are possible. Moreover, as indicated at 60', gaseous hydrogen is fed into the passage in opposite senses about the axis 30.

As illustrated in figures 2A and B, an air swirler 100 can be fitted in the inlet 21 of the passage 20. By pre-swirling the incoming air, the mixing effect can be considerably increased, resulting in improved flame stability limits.

Referring to figures 3A and B, air 40' may introduced through some tangential (or otherwise angular) injectors 50' in a manner similar to the hydrogen injection 60 and in addition to the air 40 entering in the axial direction 30. This method will increase the amount of recirculation produced. As shown, the air and hydrogen are fed into the passage in opposite directions about the axis 30; however, they could alternatively be fed in the same sense. The number of injectors could also vary.

In the embodiment of figures 4A and B, air is injected tangentially into the passage 20 from four regularly-spaced injectors 50', whilst hydrogen is injected radially into the passage from two diametrically-opposed injectors 50 spaced further along the axis of the passage. The tangential air injection builds up strong swirl flows in the combustion zone, leading to the appearance of an axially symmetric flow. Here, NOx emissions can be considerably reduced as the combustion zone happens in limited area.

A micromix combustor utilising the burner of the invention is illustrated in figure 5A. As explained above, the number of burners in the combustor is chosen so as to provide a density of diffusion flames of at least 15000!m2. Referring to the detailed view of SB, burners are arranged in a plurality of concentric rings 120,121 and 122, manifolds 130,140 for supplying gaseous hydrogen or oxidant to the burners lying concentric with and radially bctwccn adjaccnt rings 121,122 and 121,120 respectively. Referring to a burner in thc middle ring 121, at least one of the injectors 50 is fed from the manifold 140 radially outward of the ring and at least one other of the injectors 50 is be fed from the manifold 130 radially inward of a ring.

It should be understood that this invention has been described by way of examples only and that a wide variety of modifications can be made without departing from the scope of the invention.

Claims

Claims I. A burner for a micromix combustor for gaseous hydrogen, the burner comprising a passage for flow of gaseous oxidant along the axis thereof and at least one injector configured to feed the gaseous hydrogen or S gaseous oxidant into the flow in a direction transverse to and radially offset from said axis.
2. Burner according to claim 1, wherein the passage is circular in cross-section and has a circumferential wall.
3. Burner according to claim 2, wherein the at least one injector is formed byahole in the circumferential wall.
4. Burner according to claim 2 or claim 3, wherein the injector is configured to feed 15 hydrogen or gaseous oxidants in a direction tangential to the circumferential wall.
5. Burner according to any one of the claims 2 to 4, wherein a plurality of injectors is regularly spaced around the circumferential wall.
6. Burner according to claim 5, wherein the burner consists of only four regularly -spaced injectors.
7. Burner according to any preceding claim, wherein two injectors are configured to feed gaseous hydrogen or oxidant into the passage in opposite senses about the passage axis.
8. Burner according to any preceding claim, wherein an swirler is fitted in the inlet of the passage.
9. Burner according to any preceding claim, wherein the burner comprises a plurality of injectors, at least one injector being configured to feed gaseous oxidant and another being configured to feed gaseous hydrogen.
lO.Burner according to claim 9, wherein the gaseous oxidants and hydroOgen arte fed into the passage in the opposite direction about the axis.
11.Burner according to any preceding claim, wherein all of the injectors are configured to feed gaseous oxidants into the flow in a direction transverse to and radially offset from said axis.
12. Burner accordiig to claim I I, wherein, spaced along the axis from S injectors, is at least one further injector configured to feed gaseous hydrogen into the flow.
13.Burner according to claim 12, wherein the ftirther injector is configured to feed gaseous hydrogen radially relative to the axis of the passage.
14.Burner according to any preceding claim, wherein the burner comprises two injectors located diametrically opposite relative to axis.
15.A rnicroniix combustor for gaseous hydrogen comprising a plurality of burners according to any preceding claim and configured to provide a density of diffusion flame of at least 15000/rn2.
16. Micromix combustor according to claim 15, wherein the burners are ananged in a ring.
17.Micrornix combustor according to claim 16, wherein the burners are ananged in a plurality of concentric rings.
1 8.Micromix cornbustor according to claim 17, wherein at least one manifold for supplying gaseous hydrogen or oxidant to the burners lies concentric with a ring.
l9.Micromix combustor according to claim 18, wherein a manifold lies radially between adjacent rings.
20.Micromix combustor according to any one of the claims 16 to 19, wherein a burner in a ring has a plurality of injectors, with at least one of the injectors being fed from a manifold radially outward of the ring and at least one of the injectors being fed from a manifold radially inward of the ring.