GB2468366A

GB2468366A - Gas incinerator with fragmentary insulation bed

Info

Publication number: GB2468366A
Application number: GB0904914A
Authority: GB
Inventors: Stephen Warren
Original assignee: Hamworthy Combustion Engineering Ltd
Current assignee: Hamworthy Combustion Engineering Ltd
Priority date: 2009-03-07
Filing date: 2009-03-23
Publication date: 2010-09-08
Anticipated expiration: 2029-03-23
Also published as: CN101825286A; KR20100101053A; GB0903990D0; GB0904914D0; GB2468366B

Abstract

An incinerator 20 includes a generally vertical combustion chamber 40 where gas G, admitted into a proximal end of chamber 40, is burned in the presence of air A, and combustion products C are discharged at a distal end (outlet 60), and a thermally insulating fragmentary bed 62 (e.g. pumice supported by metal tray 64, formed to drain water from bed 62) extending generally horizontally across chamber 40. Bed 62 may include fragments of about 50mm diameter, to a vertical depth of 150mm to 200mm, held in compartments. Chamber 40 may have an inner 48 and outer 50 wall, forming a first passage 52, and a flame shield 54, forming a second passage 56, through which dilution air D flows before mixing with combustion products C in chamber 40. Incinerator 20 may be installed on an LNG carrier (10, Figure 1) (i.e. ship) to burn boil-off gas.

Description

I

INCINERATOR

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.

Copending European Patent Application No. 05 802 431.6 (the contents whereof are hereby imported by reference) concerns such an incinerator including a combustion section into which the boil-off gas is admitted at a proximal end to be burned in the presence of combustion air. A feature of this io incinerator as described in European Patent Application No. 05 802 431.6 is the provision of a thermal barrier of thermally insulating material extending across the combustion section at or near its proximal end.

It is an object of the present invention to provide an incinerator such as that of European Patent Application No. 05 802 431.6 with an improved thermal barrier.

Thus according to the invention there is provided an incinerator ::::. comprising a combustion section wherein gas is burned in the presence of air, **. . . . the combustion section having a proximal end where the gas is admitted and s.

extending generally vertically therefrom to a distal end whereby combustion * 20 products are discharged, and a thermal barrier extending generally horizontally across the combustion section adjacent its proximal end, characterised in that the thermal barrier comprises a fragmentary bed of thermally insulating material.

In this description, fragmentary' means that the bed comprises separate pieces of the thermally insulating material put together but not joined together. There are two notable advantages in this arrangement. First, water entering the incinerator (which is almost inevitable in the marine environment of the LNG carrier) does not accumulate in the combustion section where it may cause corrosion and/or blockages, but rather passes out through the fragmentary bed. Second, thermally insulating material tends to be frangible and thus a solid plate of this material would be liable to break and become ineffective under the vibration, stresses induced by pitching and rolling and other movement of a vessel at sea, whereas any breakage in a fragmentary bed simply results in more pieces without any substantial diminution of its effectiveness as a thermal barrier. To be effective as a thermal barrier the pieces of thermally insulating material must be laid in the bed so that there is io no direct opening therethrough whereby heat can radiate from the combustion section.

The thermally insulating material preferably comprises pumice, which may be nodules of nominal 50 mm diameter.

The bed, which may be 150 mm to 200 mm thick, is preferably supported by a metal tray, which may be formed to provide a drain for water in the incinerator.

: Dilution air may be passed over the combustion section to cool the S...

same and then mixed with the combustion products. The combustion section may have an inner wall and an outer wall together defining a first passage S...

through which the dilution air is passed before admission to the combustion section. The combustion section may contain a flame shield within and spaced * apart from the inner wall of the combustion section to define therewith a second passage through which the dilution air is passed before admission to the combustion section. The flame shield may contain the bed of thermally insulating material.

The bed of thermally insulating material is preferably configured and arranged to allow water to pass therethrough and the incinerator preferably comprises a drain arranged below the bed of thermally insulating material to carry away water passed therethrough.

The invention extends to an LNG carrier including the incinerator.

The invention will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and in which-Figure 1 is a vertical cross section through an LNG carrier, viewed from the port beam; Figure 2 is a vertical cross section through an incinerator of the LNG to carrier of Figure 1, shown to a larger scale and incorporating a thermal barrier; Figure 3 shows the thermal barrier of Figure 2 in plan view from above; Figure 4 shows a view from below corresponding to Figure 4; and Figure 5 is a cross-section, further enlarged, at X-X of Figure 3.

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 (which may be one of a plurality). The carrier 10 is driven by a propulsion S...

: system 16 that in this case comprises diesel engines but which may otherwise *5. S be a diesel-electric system, a dual-fuel system or a steam-powered system. (It may be noted here that even where there is a steam boiler in which boil-off gas S...

20 may be burnt, some further means of disposing of boil-off gas is required in * .* case the boiler ceases to function for any reason).

:. 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. The liquefaction plant is powered from the propulsion system 16, so that if this breaks down it is no longer possible to reliquify the boil-off gas.

(Even if the liquefaction plant 18 is independently powered, a break down of the propulsion system 16 may be of such duration that the liquefaction plant 18 cannot cope with the demand). Accordingly the carrier 10 is provided with means for disposing of boil-off gas in the form of an incinerator 20. 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 either the propulsion system 16 or the liquefaction system 18, 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 repairs are made, or otherwise as long as necessary.

The construction and operation of the incinerator 20 will now be described in more detail with reference to Figure 2. The gas line 22 delivers boil-off gas G to a gas inlet 42 at the proximal (ie lower) end of a combustion I...

section 40, which is generally cylindrical and extends vertically. Combustion air A is blown into the combustion section 40 by way of an air inlet 44 coaxial with the gas inlet 42 and the air-gas mixture ignited (by means not detailed but * 20 which will be readily understood by those skilled in the science). Thus the boil- *:*::* off gas burns as indicated at 46 and the combustion products C flow upwards through the combustion section 40, as indicated by the black arrows in Figure 2.

It will be understood that the combustion process generates a great deal of heat, both radiant and convected, and the incinerator 20 includes live means whereby this is controlled, which five means will now be described.

First, the combustion section 40 has an inner wall 48 and an outer wall which together define a generally annular first passage 52. A fan, not shown, delivers dilution air D to this first passage 52, and as indicated by the white arrows in Figure 2 this dilution air D is passed therethrough so as to cool the walls 48 and 50 of the combustion section 40. It will be noted that the dilution air D flows generally downward through the first passage 52, in the opposite direction from the generally upward flow of the combustion products C, thus providing counterfiow heat exchange.

Second, the air in the first passage 52 provides a thermally insulative layer around the combustion section 40, limiting radially outward heat io transmission therefrom.

Third, a flame shield 54 is located within the combustion section 40 and spaced apart from its inner wall 48 so as to define a second passage 56. This second passage 56 is in communication with the first passage 52 at the proximal (lower) end of the combustion section 40 so as to receive the dilution air D passed therethrough. The dilution air D is thus turned at the lower end of the incinerator 20 and fed upwards into the combustion section 40 therein to : mix with the combustion products C. Accordingly the dilution air D dilutes and S...

cools the combustion products C, and the diluted mixture M exhausts to atmosphere (as indicated by grey arrows in Figure 2) by way of a curved flue . 20 58 having a generally horizontal outlet 60.

Fourth, a proportion D of the dilution air is diverted through ports 64 in the upper part of the combustion section 40 and then mixed directly with the combustion products C to cool them before they are discharged. This ensures that no flame or "plume" issues from the incinerator and that the exhaust gases are below 535°C, which is the auto-ignition temperature for the boil-off gas.

Finally, a thermal barrier extends substantially horizontally across the lower end of the flame shield 54 to prevent excessive heating in this area of the incinerator 20 (where the gas is delivered). The thermal barrier comprises a fragmentary bed 62 of thermally insulating material supported by a metal tray 64. The thermal barrier is shown more clearly in Figures 3 to 5.

Referring to Figures 3 and 4, the inner wall 54 is seen to be generally circular in plan view (as is the incinerator as a whole). It is circumjacent, and supports, the metal tray 64, which is similarly circular in plan. The tray 64 supports the fragmentary bed 62 of thermally insulating material, which is laterally contained by the inner wall 54. The mutually coaxial inlets 42 and 44 extend through the bed 62 to deliver boil-off gas and air, respectively, for combustion.

Referring to Figure 5, the bed 62 is shown to comprise a mass of pumice nodules, 150mm to 200 mm thick (that is, in the vertical dimension).

Pumice is a thermally insulating material and the nodules, which are of a nominal 50 mm diameter, fill the area of some 5000 mm diameter within the inner wall 54 of the incinerator. It will be understood that the material from S...

which the tray 64 is made must be such as to withstand the relatively low heat conducted through the bed 62 but does not need to accommodate the * : substantially greater heat within the combustion section of the incinerator. S...

The particular advantages of the fragmentary form of the bed 62 may *:*::* now be noted. Thermally insulating material tends to be frangible and therefore vulnerable to vibration, stresses induced by pitching and rolling and other mechanical shocks common in a ship at sea. This vulnerability naturally increases with size, and a thermally insulating sheet as large as 5000 mm diameter would be at considerable risk of breakage. Even if (as would be normal) such a sheet were carried on a metal substrate, impact on or strain of the sheet could result in fracture of the sheet, reducing its effectiveness as a thermal barrier and possibly leading to delamin.ation of the thermally insulating material from the substrate and/or corrosion of the substrate and surrounding structures. The fragmentary bed 62 is substantially invulnerable to vibration, shock and other movement, which merely causes the individual pieces of thermally insulating material to move around somewhat. In essence, a fragmentary bed cannot "break" in the sense of becoming unfit for its purpose.

Even if one piece should fracture, there are many more in situ to preserve the effectiveness of the bed 62 as a thermal barrier. This contrasts with a solid thermal barrier of thermally insulating material, which can crack, rendering it io not just less effective but possibly dangerous and in addition creating the risk of a runaway failure as water and/or heat will tend to seek out any such crack and cause more damage.

As indicated in Figure 5, albeit only schematically, the pumice nodules forming the bed 62 are laid one on top of another (three-or four-deep, or possibly more). This means that there is nowhere any straight-line route through the bed 62 whereby radiant heat can escape. But small gaps among .. : the nodules provide a great many indiscriminate passages through the bed 62.

This means that water can escape. In the marine environment of an LNG carrier, there is of course water, water, everywhere"; when this is trapped it S...

causes corrosion and/or blockages; and in incinerators for boil-off gas this is *:::* hazardous, because proper operation of these on time and every time is * crucial to the vessel's safety, and faults in gas and air inlets and igniters etc *..

are unacceptable. The multiplicity of passages through the bed 62, between the pumice nodules, allows any water in the incinerator to percolate through the bed 62 and drain away. A thermal barrier of fragmentary form as in the present invention is generally much easier to make, transport and install than a unitary shield and a shield made from elements bolted together, and it is easy to repair by simply topping up with more pumice nodules. Those skilled in the art will also appreciate that the fragmentary form means that the thermal barrier is not liable to damage by thermal changes resulting in expansion or contraction of water, ice or steam.

Pumice has particular benefits. It is non-friable and therefore the bed does not crumble. As nodules it has a generally rounded form, which makes it easy to handle. Yet, particularly in mass as in the present invention, it does not tend to roll around and become displaced.

Tests on an incinerator of the kind aforedescribed for disposing of boil-off gas have shown that the temperature of the hot face of the bed 62 (le the.

face thereof exposed to the combustion section) does not exceed 700 °C.

Pumice melts at about 1200 °C and can therefore safely be used in this application. It should be pointed out that in some other applications such as ground flares the hot face floor temperature can exceed 1200 °C, in which case pumice is not a satisfactory thermally insulating material as it would melt during combustion and then solidify, losing its insulating properties and its : permeability to water. In such other applications other thermally insulating materials may be used to form the fragmentary bed 62.

As noted in copending European Patent Application No. 05 802 431.6, thermally insulating material applied to the wall of an on-board incinerator cannot be expected to remain in place over long periods because of the amount of vibration -not least from the propulsion system -plus pitching and rolling and and other mechanical shocks. In the present invention, no thermally insulating material is applied to the incinerator wall or any other vertical surface, and as explained above this is substantially immune to vibration and other shocks. Accordingly an incinerator according to the present invention can have extended maintenance schedules and a longer working life.

Modifications of the arrangements set forth herein, and applications other than those described, and will be apparent to those skilled in the art. For instance, although not detailed in Figure 5 (for simplicity of illustration) the tray 64 may be formed with a plurality of compartments, which. helps to keep the pumice nodules in position, or the tray 64 may be replaced by a metal grating.

Finally, it should also be understood that, whilst the invention has been developed especially for use in the incineration of boil-off gas on LNG carriers, the fragmentary thermal barrier (especially if of pumice) may have benefits in land-based incinerators. * ..* * * S *S S **** * * *.S. S...

S S... * * *. * . . * *. S..

S

Claims

Claims 1. An incinerator comprising a combustion section (40) wherein gas (G) is burned in the presence of air (A), the combustion section (40) having a proximal end where the gas (G) is admitted and extending generally vertically therefrom to a distal end whereby combustion products (C) are discharged, and a thermal barrier (62) extending generally horizontally across the combustion section adjacent its proximal end, characterised in that the thermal barrier (62) comprises a fragmentary bed of thermally insulating material.
2. An incinerator as claimed in claim I characterised in that the thermally insulating material comprises pumice.
3. An incinerator as claimed in claim 2 characterised in that the pumice comprises nodules of nominal 50 mm diameter... :
4.' An incinerator as claimed in claim 3 characterised in that the bed * *** has a vertical thickness between 150 mm and 200 mm. **�. *

* 20
5. An incinerator as claimed in any preceding claim characterised in that the bed is supported by a metal tray (64). S.

S
6. An incinerator as claimed in claim 5 characterised in that the tray (64) is formed with compartments to hold the thermally insulating material.
7. An incinerator as claimed in claim 5 or claim 6 characterised in that the tray (64) is formed to drain water from the bed.
8. An incinerator as claimed in any preceding claim characterised in that dilution air (D) is passed over the combustion section (40) to cool the same and then mixed with the combustion products (C).
9. An incinerator as claimed in claim 8 characterised in that said combustion section (40) has an inner wall (48) and an outer wall (50) together defining a first passage (52) through which the dilution air (D) is passed before admission to the combustion section (40).
10. An incinerator as claimed in claim 9 characterised in that the combustion section (40) contains a flame shield (54) within and spaced apart from the inner wall (48) of the combustion section (40) to define therewith a second passage (56) through which the dilution air (D) is passed before admission to the combustion section (40).:
11. An incinerator as claimed in claim 10 characterised in that the *.flame shield (54) contains the bed of thermally insulating material.S S... 20
12. An incinerator substantially as hereinbefore described with :5: reference to and as shown in the accompanying drawings. 5SS13. An LNG carrier including an incinerator as claimed in any preceding claim, which incinerator is configured and arranged for the disposal of boil-off gas from the LNG.