GB2471909A

GB2471909A - Boil-off gas incinerator with dilution air blown across a combustion products stream

Info

Publication number: GB2471909A
Application number: GB0912525A
Authority: GB
Inventors: Stephen Arthur Bulbick; Simon O'connor
Original assignee: Hamworthy Combustion Engineering Ltd
Current assignee: Hamworthy Combustion Engineering Ltd
Priority date: 2009-07-18
Filing date: 2009-07-18
Publication date: 2011-01-19
Anticipated expiration: 2029-07-18
Also published as: GB0912525D0; CN101956988A; KR20110008140A; GB2471909B; GB2471909C; CN101956988B

Abstract

A boil-off gas incinerator for an LNG carrier or ship has at the upper end of its combustion section 82 a plenum chamber 98 or the like for pressurised dilution air, a proportion DPof the dilution air being blown across the combustion section 82 via nozzles 102 in the form of radial jets that penetrate the stream of combustion products C and break it up into a turbulent mixture M. The nozzles 102 can be offset from each other at different distances along the stream. A proportion DQof the dilution air is directed downwards in an outer annular passage 88 and then turned into an inner annular passage 90. The proportion Drmay be approximately 55%. The stream of combustion products C may contain hot spots, but the mixture M is below the boil-off gas auto-ignition temperature (TA, fig 7) and does not contain mixing devices.

Description

I

Incinerator For Boil-Off Gas This invention concerns incinerators especially but not necessarily exclusively for disposing of boil-off gas on ships carrying cargoes of liquefied natural gas (LNG) by sea, such ships being commonly known as LNG carriers.

Our copending European patent application No 05 802 431.6 concerns such an incinerator in which inter a/ia the boil-off gas is admitted at a proximal end of a combustion section therein to be burned in the presence of combustion air and dilution air is mixed with the combustion products to cool them. The function of the dilution air is principally to cool the combustion products to a temperature below the auto-ignition temperature of the gas.

Whilst the introduction of dilution air is effective in reducing the mean temperature of the combustion products to an acceptable level, some parts of the combustion products are commonly hotter than the target maximum temperature while others are colder. In particular there is a tendency for the combustion products to form a stream with a relatively hot core -that is, at a temperature higher than the mean temperature of the stream and potentially above the auto-ignition temperature of the gas. Unless this structure is broken up, there is a risk that some part(s) of the exhaust from the incinerator will be above the auto-ignition temperature of the gas, with possibly dangerous consequences.

Previous attempts to break up the combustion stream have entailed the provision of some kind of mixing device within the incinerator. In one such known arrangement the combustion section is capped with a perforated metal dome which spreads out the combustion products before being mixed with the dilution air. Another known arrangement has a metal disc which directs the combustion products towards the sides of the incinerator, there to mix with the dilution air. Being directly exposed to the hot combustion products, such mixing devices have a limited working life owing to high temperature oxidation.

It is an object of the present invention to reduce the temperature of the combustion products as a whole to an acceptable level without the need for a mixing device exposed to the combustion products.

Thus according to the invention there is provided an incinerator for disposing of boil-off gas on an LNG carrier, which incinerator comprises a combustion section wherein the boil-off gas is burned in the presence of air to form a stream of combustion products and wherein dilution air is admitted to the incinerator to cool the combustion products, characterised in that at least a proportion D of the dilution air is blown across the incinerator to break up the stream of combustion products.

It is preferred that said proportion D of the dilution air mixes turbulently with the combustion products and the mixture is brought to a substantially uniform temperature before being discharged from the incinerator.

Preferably no part of the mixture has a maximum temperature at or above the auto-ignition temperature of the boil-off gas.

The incinerator preferably comprises a plurality of nozzles arranged and directed to blow said proportion D of the dilution air across the incinerator.

The nozzles may each be directed substantially radially across the incinerator and they are preferably located out of the stream of combustion products.

Turbulence of the mixing process may be improved by locating the nozzles at respectively differing distances along the stream of combustion products. The incinerator may comprise a plenum chamber or the like (which may be a room) whereby the dilution air is delivered to the nozzles. Preferably the dilution air is pressurised and each nozzle formed to accelerate the dilution air passing therethrough.

A proportion D° of the dilution air may be passed over the combustion section to cool the same before being mixed with the combustion products.

The combustion section may have a wall formed with a first passage through which the proportion D° of the dilution air is passed before admission to the combustion section. The combustion section may have an inner wall and an outer wall together defining said first passage, and the first passage may comprise a plurality of channels. The proportion D° of the dilution air preferably flows through said first passage in counter-flow to the combustion products in the combustion section.

The combustion section may contain a flame shield within and spaced apart from the wall of the combustion section to define therewith a second passage communicating with the first passage at or near a proximal end and extending therefrom into the combustion section.

Other aspects of the invention will be apparent from the following description which is made by way of example only with reference to the accompanying schematic drawings, in which -Figure 1 is a vertical cross section through an LNG carrier, viewed from the port beam, including an incinerator for boil-off gas; Figure 2 is a vertical cross section through a known incinerator for boil-off gas on an LNG carrier such as that of Figure 1, shown to an increased scale; Figure 3 is a vertical cross section through another known incinerator for boil-off gas on an LNG carrier such as that of Figure 1, to substantially the same scale as Figure 2; Figure 4 is a vertical cross section through an incinerator according to the present invention, to a further increased scale; Figure 5 is a schematic plan view of the incinerator of Figure 4, illustrating the radial disposition of dilution air nozzles; Figure 6 illustrates a typical temperature profile across a diameter d at X-X in Figure 4; and Figure 7 illustrates a typical temperature profile across a diameter d at Y-Y in Figure 4.

Referring first to Figure 1, this shows an LNG carrier indicated generally at 10. The carrier 10 carries a cargo 12 of LNG contained in an insulated tank. The carrier 10 is driven by a propulsion system 16 that in this case comprises diesel engines but which may otherwise be a dual fuel (gas/oil) system, a duel fuel (gas/oil) diesel-electric system, or a duel fuel (gas/oil) steam-powered system.

Although the tank 14 is insulated, some of the cargo necessarily boils off during a voyage, and the carrier 10 is therefore equipped with a liquefaction plant 18, connected to the tank 14 and operative to reliquify the boil-off gas produced. In case the liquefaction plant 18 cannot cope with demand, or breaks down, the carrier 10 is provided with means for disposing of boil-off gas in the form of an incinerator 20. (For completeness and to avoid uncertainty it may be noted here that carriers not equipped with a liquefaction plant also need an incinerator to dispose of excess boil-off gas when the regular gas consumers such as dual fuel engines are consuming only part of the boil-off gas). For simplicity of illustration the incinerator 20 is shown adjacent the stern of the ship 10, but those skilled in the science will appreciate that it may well be incorporated in the ship's funnel assembly or otherwise disposed.

The incinerator 20 is connected to the tank 14 by way of a gas line 22.

A control system 24 is linked at 26 and 28 to each of the propulsion system 16 and the liquefaction system 18, and at 30 to the incinerator 20. In the event of failure of the liquefaction system 18 (or of the regular gas consumer) or if only part of the boil-off gas is consumed, the gas line 22 is opened and the incinerator 20 actuated to burn the boil-off gas. This means of disposing of the boil-off gas can be continued until operating conditions ease or repairs are made, or otherwise as long as necessary.

Various incinerators have been previously proposed for the disposal of boil-off gas on LNG carriers, and one example is shown in Figure 2, in which arrows indicate the flows of gas, air and combustion products. The known incinerator 40 illustrated by Figure 2 comprises a combustion section 42 within an outer shell 44, each generally cylindrical and extending vertically. Each of the combustion section 42 and the outer shell 44 is made of steel, and in addition the combustion section 42 is lined with refractory bricks 46. The gas line 22 (Figure 1) delivers boil-off gas G to a gas inlet 48 at the proximal (ie lower) end of the incinerator 40. Combustion air A is blown into the combustion section 42 by means of a combustion air fan 50, and the mixture of gas G and combustion air A is ignited (by means not detailed but which will be readily understood by those skilled in the science). Thus the boil-off gas burns as indicated at 52 and a stream of combustion products C flows upwards from the combustion section 42, through a perforated dome 42a thereof. It will be understood that the combustion process generates a great deal of heat, and to cool the combustion products C they are mixed with dilution air D. Dilution air fans 54 blow the dilution air D into the annular space between the combustion section 42 and the outer shell 44, from where the dilution air D rises to meet the combustion products C, the perforated dome 42a serving to mix the combustion products C with the dilution air D. The resulting mixture M of combustion products and dilution air is then delivered to an exhaust (not shown).

Another previously proposed incinerator for disposing of boil-off gas on LNG carriers is shown in Figure 3, in which arrows indicate the flows of gas, air and combustion products. The incinerator 60 illustrated by Figure 3 comprises a combustion section 62 within an outer shell 64, each generally cylindrical and extending vertically, and each made of steel. The gas line 22 (Figure 1) delivers boil-off gas G to a gas inlet 66 at the proximal (ie lower) end of the incinerator 60. Air A is blown into the combustion section 62 by means of fans 68, and the mixture of gas G and combustion air A is ignited (by means not detailed but which will be readily understood by those skilled in the science). Thus the boil-off gas burns as indicated at 70 and a stream of combustion products C flows upwards through the combustion section 62.

To cool the combustion products C some of the air A is channelled through the annular space between the combustion section 62 and the outer shell 64 and then through ports in the combustion section 62 to serve as dilution air.

This air does not mix very thoroughly with the combustion products C, so to effect the necessary mixing a metal plate 72 extends transversely across the path of the combustion products C to direct them outwards to mix with the air A. The resulting mixture M of combustion products and dilution air is then delivered to an exhaust (not shown).

The known arrangements of Figure 2 and Figure 3 each suffer from a serious problem in that the mixing devices (the perforated dome 42a of Figure 2 and the transverse plate 72 of Figure 3) are exposed to the stream of combustion products C. Being each made of steel, these mixing devices inevitably have a limited working life under the temperatures to which they are subjected.

The present invention overcomes this problem by avoiding the need for a (physical) mixing device in the stream of combustion products, as will now be described with reference to Figure 4 in which, as before, arrows indicate the flows of gas, air and combustion products. The incinerator 80 illustrated by Figure 4 comprises a combustion section 82 within an outer shell 84, each generally cylindrical and extending vertically, and each made of steel. Upstanding from the proximal (ie lower) end of the combustion section 82 is a generally cylindrical heat shield 86. The combustion section 82, the outer shell 84 and the heat shield 86 are thus so arranged as to form an outer annular passage 88 between the combustion section 82 and the outer shell 84 and an inner annular passage 90 between the combustion section 82 and the heat shield 86.

The gas line 22 (Figure 1) delivers boil-off gas G to a gas inlet 92 at the proximal end of the combustion section 82. Combustion air A is blown into the combustion section 82 by means of a combustion air fan 94, and the mixture of gas G and combustion air A is ignited (by means not detailed but which will be readily understood by those skilled in the science). Thus the boil-off gas burns as indicated at 96 and a stream of combustion products C flows upwards through the combustion section 82.

At the distal (ie upper) end of the combustion section 82 is a plenum chamber or the like 98 for dilution air delivered by dilution air fans 100 which pressurise the dilution air. The dilution air is divided into two parts: a proportion D is blown across the combustion section 82 by way of nozzles 102 and a proportion D° is directed downwards through the outer annular passage 88.

Each nozzle 102 is formed to accelerate the dilution air D passing therethrough and thus create a radial jet of dilution air D that penetrates the stream of combustion products C and breaks it up. This results in a turbulent mixture, indicated at M, of combustion products and dilution air. The turbulence is enhanced by locating the nozzles 102 at somewhat different distances along the stream of combustion products C, as can be seen in Figure 4 from the relative offset of the two broken arrows representing the jets emanating respectively from the two nozzles 102 shown therein.

As shown in Figure 5 the incinerator has four nozzles 102 arranged at right angles and each radially directed, but it will be understood that there may be more, or fewer.

Returning now to Figure 4, the proportion D° of dilution air directed downwards through the outer annular passage 88 serves first to cool the wall of the combustion section 82, moving in counter-flow to the generally upward flow of combustion products C. At the bottom of the combustion section 82 this dilution air D° is turned upwards into the inner annular passage 90, from where it is channelled into the combustion section 82 to dilute and cool the combustion products C. For completeness it may be noted that the air in the inner and outer passages 90 and 88 provides a thermally insulative layer around the combustion section 82, limiting radially outward heat transmission therefrom, and the incinerator may also be equipped near its bottom end with a horizontal heat shield such as a fragmentary bed of thermally insulative material.

The dilution air D° channelled into the combustion section 82 mixes only incompletely with the combustion products C, as can be ascertained by determining temperature profiles across the combustion section 82. Figure 6 illustrates a typical temperature profile at Section X-X. The mean temperature of the stream 110 (that is, averaged across the Section X-X) is below the auto-ignition temperature TA of the boil-off gas. However, parts 112 (notably the outer parts) of the stream 110 are at a temperature substantially below TA whilst other parts 114 are at a temperature above TA.

Those skilled in the art will appreciate the hazard if a stream 110 containing such hot spots is discharged in the vicinity of airborne boil-off gas vented from the cargo tanks of an LNG carrier.

In the present invention this problem is overcome by the action of the transverse jets of dilution air D upon the steam of combustion products C. This can be seen by comparing the temperature profile upstream of the jets, illustrated by Figure 6, with the downstream temperature profile, at section Y-Y, illustrated by Figure 7. It is clear from Figure 7 that after the jets of dilution air D have penetrated the stream of combustion products C, the resulting stream 120 has a substantially uniform temperature profile across the diameter d and its maximum local temperature is wholly below TA. There is therefore no hazard in discharging this stream 120 in the vicinity of possible airborne boil-off gas vented from the cargo tanks of an LNG carrier.

Although other arrangements may be possible we have found that pressurising the dilution air up to about 20 mbar (typically 12-14 mbar) is effective. Also, the proportions of dilution air are approximately D 55% and D° 42% (the remainder being bleed air for flanges.

Modifications of the arrangements set forth herein, and applications other than those described, and will be apparent to those skilled in the art.

Claims

Claims 1. An incinerator for disposing of boil-off gas on an LNG carrier, which incinerator comprises a combustion section wherein the boil-off gas is burned in the presence of air to form a stream of combustion products and wherein dilution air is admitted to the incinerator to cool the combustion products, characterised in that at least a proportion D of the dilution air is blown across the incinerator to break up the stream of combustion products.
2. An incinerator as claimed in claim I characterised in that said proportion D of the dilution air mixes turbulently with the combustion products and the mixture is brought to a substantially uniform temperature before being exhausted from the incinerator.
3. An incinerator as claimed in claim I or claim 2 characterised in that no part of the mixture has a maximum temperature at or above the auto-ignition temperature of the boil-off gas.
4. An incinerator as claimed in any preceding claim characterised in that the incinerator comprises a plurality of nozzles arranged and directed to blow said proportion D of the dilution air across the incinerator.
5. An incinerator as claimed in claim 4 characterised in that the nozzles are each directed substantially radially across the incinerator.
6. An incinerator as claimed in claim 4 or claim 5 characterised in the nozzles are each located out of the stream of combustion products.
7. An incinerator as claimed in any of claims 4 to 6 characterised in that the nozzles are respectively located at differing distances along the stream of combustion products.
8. An incinerator as claimed in any of claims 4 to 7 characterised in that the incinerator comprises a plenum chamber or the like whereby the dilution air is delivered to the nozzles.
9. An incinerator as claimed in claim 8 or claim 9 characterised in that the dilution air is pressurised and each nozzle is formed to accelerate the dilution air passing therethrough.
10. An incinerator as claimed in any preceding claim characterised in that a proportion D of the dilution air is passed over the combustion section to cool the same before being mixed with the combustion products.
11. An incinerator as claimed in claim 10 characterised in that the combustion section has a wall formed with a first passage through which the proportion D° of the dilution air is passed before admission to the combustion section.
12. An incinerator as claimed in claim 11 characterised in that the combustion section has an inner wall and an outer wall together defining said first passage.
13. An incinerator as claimed in claim 11 or claim 12 characterised in that said first passage comprises a plurality of channels.
14. An incinerator as claimed in any of claims 10 to 13 characterised in that the proportion D° of the dilution air flows through said first passage in counter-flow to the combustion products in the combustion section.
15. An incinerator as claimed in any of claims 10 to 14 characterised in that the combustion section contains a flame shield within and spaced apart from the wall of the combustion section to define therewith a second passage communicating with the first passage at or near a proximal end and extending therefrom into the combustion section.
16. An incinerator for disposing of boil-off gas on an LNG carrier substantially as hereinbefore described with reference to and as shown in the accompanying drawings.19. An LNG carrier including an incinerator as claimed in any preceding claim.