US20130340872A1

US20130340872A1 - System and apparatus for connecting a gas source to a thermal oxidiser

Info

Publication number: US20130340872A1
Application number: US13/989,623
Authority: US
Inventors: David John Cork
Original assignee: Corkys Management Services Pty Ltd
Current assignee: Corkys Management Services Pty Ltd
Priority date: 2010-11-24
Filing date: 2011-11-24
Publication date: 2013-12-26
Also published as: CN103328769B; CA2820643A1; AU2011334608A1; NZ612115A; EP2643551A4; EP2643551A1; WO2012068628A1; AU2011334608B2; CO6741146A2; CN103328769A

Abstract

A duct for connecting a source of gas including a volatile organic compound to a thermal oxidiser wherein the duct has a sufficiently sized cross-sectional diameter to reduce the velocity of the gas to below 7 m/s prior to entering the thermal oxidiser

Description

The present invention relates to a method and/or apparatus for the safe connection of thermal oxidisers, gas engine or boilers etc to a working coal mine whereby methane derived from the mine's ventilation air can be safely converted to carbon dioxide; thus significantly reducing greenhouse gas emissions without compromising the safety of the coal miners.

BACKGROUND

Methane may be released from underground coal mines as part of the ventilation air and is known as Ventilation Air Methane (VAM). The volume of mine ventilation air is typically very large. Ventilation air exhaust streams typically range from 80 to 500 m³/s.
VAM typically has a methane concentration level of less than 1% volume. However, within any month the daily average VAM can vary between 0.0% and 1.5% volume. Thus the combustion characteristics of VAM are highly variable. The flammability range of the methane in VAM is typically well below the lower flammability limit, which is 5.0% volume methane in air.
At very extraordinary times the concentration of methane may suddenly surge to between 8% to 15% methane when there are events underground. More frequently, but still at very low frequency, surges may occur between 2 to 8% methane. For example seismic activity could release methane previously caught in the goaf as a plug of methane. There may be no warning of the timing, and no ability to predict the concentration of the methane plug. Coalminers depend on there being no source of ignition at times when these incidents occur to ensure a working environment that is as safe as possible.
As methane is a powerful greenhouse gas, a number of thermal oxidiser processes have been proposed to burn out VAM as a greenhouse abatement technology (e.g. VAM RAB™, Voxidiser™, and Vamox™). VAM may also be used as combustion air in gas engines, gas turbines, boilers and other like devices. However, these thermal oxidisers, and other VAM combustion systems represent a possible ignition source. Therefore, if these greenhouse abatement technologies are to be used, then there needs to be a system that can safely, and consistently, isolate the thermal oxidisers from the coal mine within seconds. Such a system would be ideally scalable to handle air volumes between 80 to 600 m³/s.
The required speed of the safety system and the large volume flows implies very large duct isolation and diverter doors. To move freely, these doors should not be a tight fit so that they can reliably move to their correct position in the event of an emergency.
The safety system must reduce the unmitigated risk to below the tolerable risk. For large mines this is expected to be less than 1×10⁻⁶failures per year for all countries and less than 1×10⁻⁹failures per year in some countries. If unmitigated risk is greater than risk tolerance, independent protection layers are required.
The tolerable risk is achieved by introducing Independent Protection Layers (IPL). These layers must be:

- Independent in terms of operation
- Sufficiently independent so that the failure of one IPL does not adversely affect the probability of failure of another IPL
- Designed to prevent the hazardous event, or mitigate the consequences of the event
- Designed to perform its safety function during normal, abnormal, and design basis conditions
- Auditable for performance

Protection layers may be full or partial systems. A full system completely prevents a cause from developing into a consequence, unless it fails to operate. A partial system may not fully prevent a cause from generating a consequence, even if it operates as it should, an example being an alarm. Protection Layers can also be preventive or provide mitigation.
Both fire curtains and smoke vents are available as existing, commercial safety products. The fire curtain fail closed and the vents are spring loaded and fail open. These products can be used together to provide a safe system of connect to a mine fan. Using the products together, and in layers of protection, allows products developed for alternative markets in this specific application.

SUMMARY

According to one aspect the present invention provides a duct for connecting a source of gas including a volatile organic compound to a thermal oxidiser wherein the duct has a sufficiently sized cross-sectional diameter to reduce the velocity of the gas to below 7 m/s prior to entering the thermal oxidiser.
In one form the source of gas includes methane and is derived from a coal mine ventilation system which is introduced via a fan into an inlet of the duct remote from the thermal oxidiser, or which is drawn through the duct into the inlet of the thermal oxidiser by a induce draft fan located at the outlet of the thermal oxidiser.
In one form the duct includes a barrier at a point along the length of the duct between the inlet of the duct and the thermal oxidiser, the barrier being moveable between an open position where the gas may pass along the duct between the fan and the thermal oxidiser and a closed position where the gas is prevented from moving beyond the barrier along the length of the duct towards the thermal oxidiser.
In one form the duct further includes a vent located at a point along the length of the duct between the inlet of the dut and the barrier wherein the vent is moveable between an open state and a closed state.
In one form the duct further includes a dilution door to introduce air from outside the duct into the gas passing through the duct to dilute the concentration of the volatile organic compound in the gas. In one form the dilution door also acts as a vent in the event the gas pressure within the duct reaches a predetermined level, or other event trigger.
In one form the duct further includes one or more safety vents which are retained in a closed position until the pressure within the duct reaches a predetermined level, or in the event of another event trigger, at which time the safety vents move to an open position allowing the gas within the duct to escape to atmosphere. In one form the one or more safety vents are formed in a frangible manner wherein the vent opens when the pressure within the duct reaches a predetermined level thereby moving to an open position. In one form the predetermined level is 250 Pa gauge. In an alternative form the vent opens in the event of another event trigger such as for example the draft has fallen to −100 Pa gauge.
In one form the duct further includes an isolation valve located immediately before the thermal oxidiser which provides that the thermal oxidiser may be isolated from the duct and the source of the gas when the isolation valve is closed.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The present invention will become better understood from the following detailed description of preferred but non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

FIG. 1 is an elevation cross section of an underground mine and duct transporting VAM to thermal oxidiser in accordance with one embodiment;

FIG. 2 is an infra red methane monitor response curve;

FIG. 3 is a layer of protection analysis diagram;

FIG. 4 is an elevation cross section of an underground mine and duct transporting. VAM to thermal oxidiser in accordance with another embodiment; and,

FIGS. 5 & 6 are isometric diagrams of two 6 m by 6 m duct 36 m long leading to a VAM distribution system.

DETAILED DESCRIPTION OF EMBODIMENTS AND THE ACCOMPANYING FIGURES

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.
In the context of this specification, the word “comprising” means “including principally but not necessarily solely” or “having” or “including”, and not “consisting only of”. Variations of the word “comprising”, such as “comprise” and “comprises” have correspondingly varied meanings.
As used herein, the terms ‘door’, ‘vent’, ‘barrier’, ‘valve’, ‘curtain’ or ‘louver’ are used interchangeably to mean a device that allows gas flow when open and restricts gas flow when closed. These words are not meant to limit interpretation or imply exclusion of other devices that perform the same function.

- Door is use to imply large valve that opens and closes regularly
- Vent is used to imply a large valve where the safest condition is open
- Barrier is used to imply a large valve where the safest condition is closed

As used herein, the term “event trigger” is intended to include any one of the following:

- high methane concentration below ground, such as for example above 1.5%;
- low oxygen concentration below ground, such as for example below 20.6%;
- sudden pressure wave below ground such as for example of the magnitude of 100 Pa;
- a high temperature within the thermal oxidiser such as for example above 1050° C.; and/or,
- reduce draft (high pressure) at inlet of thermal oxidiser such as for example above −100 Pa.
  These various examples of event triggers are not meant to limit interpretation or imply exclusion of other events or devices that perform the same function. It is expected that a number of independent event triggers will be used to activate the independent layers of protection in accordance with various embodiments of the present invention. Substitution of one event trigger with another does not limit the intent or interpretation or imply exclusion, as one or more of the IPLs can be triggered by one or more “event triggers”.

The invention uses a large duct to transport the VAM between the mine fan and the thermal oxidiser. The large duct is used rather than a smaller duct because:

- 1. The larger duct includes a large cross sectional area and provides slower gas velocity and therefore ensures more time before a slug of methane reaches the thermal oxidiser. This extra time allows vents, barriers and doors to move to their required position and for gas monitor processing time.
- 2. The larger duct allows for larger vent areas to be installed into the duct and provides for slow gas velocity through those vents. A smaller duct would save capital cost, but has the consequence of increasing the pressure drop through any installed vents which increases the static electricity risk and also may provide that entrained particles could damage the opening and closing mechanism of the vents. More importantly small vents do not allow rapid exhausting and may lead to dangerous pressure build up.
- 3. The larger duct allows a greater ratio of seal area per leak area around the edge of the vent and barrier. For example 15 mm clearance may be significant in a 1 m duct but insignificant in a 5 m duct.

In certain embodiments, it is desirable that the duct has a large cross-sectional area prior to the thermal oxidiser where the VAM velocity is reduced below 7 m/s, preferably below 3 m/s but above 2 m/s to act as mud drop out zone. The VAM velocity is above 2 m/s so that it is much faster than the methane flame speed at atmospheric pressure.
In certain embodiments the duct includes a spring loaded vent before a barrier to the thermal oxidiser to encourage methane to be expelled to atmosphere. The vent would typically be in the form of a louver type construction and the barrier in the form of a roller door or roller curtain system. Other fast acting doors may also be suitable such as large louver systems. These can restrict or open flow in a large duct very quickly. Independent of the hardware chosen, the combination of vents and barriers bypasses methane rich VAM to atmosphere and stops it flowing to the thermal oxidiser and/or other VAM combustion systems through the duct.
Even in a duct 8 m by 8 m in cross section, a louver arrangement may only need move 300 mm to open or close the barrier. Roller curtains have the advantage for vertical barriers because they are fully out of the flow stream of the VAM and do not foul with mud.
In the event of a power failure or high methane event it is desirable that the vents located in the duct open in less than half a second and the barriers closes in about 5 to 7 seconds. A vent is typically in the roof of the duct as methane is buoyant and will self evacuate through a vent in the roof It is typical to have a plurality of vents and a plurality of barrier systems in the one duct to provide increased independent protection layers (IPLs). The lower the tolerable risk the more likely it is to have redundant vents and barriers. In a preferred form, the total vent area located in the duct is equal to, or larger than, the total barrier area of the duct and the total barrier area is equal to, or larger than, the largest section of the ventilation fan outlet. The vent also acts as a vent area to relieve pressure in the very unlikely event of a deflagration in the duct.
It is very common to have multiple fans to draw VAM out of a mine. Therefore, the vent barrier system will need to be applied to each fan duct or after the ducts from each fan join. For ease of maintenance it may be desirable to have a vent barrier system per fan. Alternatively, and to reduce capital cost, one ventilation fan may act as the bypass fan and induce draft fans located after the outlet of the thermal oxidiser can, replace the duty of one or more of the mine ventilation fans, In this arrangement only the duct need have the vent and barrier system.
To help aid the shedding of water on the duct assembly, the vents may be located on an inclined roof of the duct. The vent may be placed on both sides of the roof hip or one side of hip or on a skillion roof. The vents may also be placed in the walls of the duct in an alternative embodiment.
The vent and barriers need to be fast acting so fouling is avoided. As such, it is desirable that the vents and barriers are not completely gas tight. The induced draft fan needs to be sized so that a small amount of air is drawn in through the vents. In such a design even a failure to open will still vent methane in an emergency. The barrier may leak slightly, but the residual methane will be vented in the next vent/barrier arrangement or be diluted to safe concentration by dilution doors further along the duct. The dilution doors situated along the length of the duct could be also of a louver construction or any other fast acting door. The area of the dilution doors is preferably larger than the duct cross-sectional area. In a preferred form, multiple dilution doors are located along the length of the duct so that there is 100% redundancy for this step. It is also preferred that the dilution doors fail open in a power failure or high methane event.
The dilution doors may also be used as a process control mechanism to lower the thermal oxidiser temperature by reducing the fuel concentration of the YAM being directed to the thermal oxidiser. In addition, the dilution doors may also act as vent area to relieve pressure in the very unlikely event of a deflagration in the duct.
The mine ventilation fan pressure (velocity and static head) must be a smaller portion of the total system pressure drop compared to the pressure drop created by the induce draft fans. This allows the dilution doors to suck in more fresh air than the flow of air from the mine and thus means that a 10% methane event can be diluted down below 5% methane.
The duct may also have multiple fixed vents that will fail open once a predetermined pressure is reached should all the other vents, barriers and dilutions doors fail to operate as designed. It is desirable that some of these fixed vents, or safety vents, are constructed in a “frangible” form and are spaced along the duct evenly so that their total area exceeds the total dilution door area. These frangible vents may be triggered at a point inside the safety duct at say a 1.5% methane, 250 Pa overpressure. It is desirable that the frangible fixed vents can open in tens of milliseconds. It is also desirable in certain embodiments to use a combination of frangible panels which are triggered by overpressure working against a mechanical bias, such as a spring, or air struts, in combination with panels that are triggered by a pressure measurement to reduce the risk of common cause failure.
In addition the duct also includes reversal valves which isolate the thermal oxidiser when a methane spike is detected. The reversal valves fail close to isolate the thermal oxidiser along the length of the duct nearest the thermal oxidiser. This isolation valve may be slower to operate than the other barriers, vents and doors to allow the duct and thermal oxidiser to be filled with air before the plug of methane reaches the duct.
Referring now to FIG. 1 the mine fan 1 sucks the VAM from the coal face 3 to the surface via the shaft 2. A series of gas monitors are operating in the shaft 2 to yield early warning that a plug of methane is approaching the fan 1. It is envisaged that many types of gauges and monitors may be used to detect various event triggers which in turn trigger one or more of the IPLs.
A vent 4 is set in the roof of the duct 5. The vent 4 remains in the closed position during normal operation and when opened allows the VAM out from the duct 5 and away from the thermal oxidiser 10. The opening of vent 4 could be triggered by a signal or combination of signals including either high methane signal from the gas monitors located in the shaft 2 of the mine, a high pressure signal from the mine, high temperature from mid thermal oxidiser, high pressure from thermal oxidiser, physical high pressure within the duct or warmth within the duct.
A barrier 6 is located after the vent 4 along the length of the duct 5 and is open under normal operation and closes in a power failure, a high methane event, or other event. trigger, to increase flow out of the duct 5 through the vent 4. Typically if any of the triggers to open vent 4 occur then barrier 6 is automatically closed. A typical trigger point would be between about 1.5% and about 2.0% methane content which is well below the VAM lower explosive limit.
The duct 5 also includes dilution doors 7 which are located further along from the bather 6. The dilution doors 7 are normally closed during typical operation of the thermal oxidiser. They are also designed to progressively open when the thermal oxidiser is too hot which has the effect of diluting down the methane concentration of the VAM entering the thermal oxidiser. For example the first dilution door 7 may open at 1075° C., the second dilution door 7 at 1125° C. or similar combinations of temperature. Likewise, the first door 7 may open at 1.0% methane, the second door 7 at 1.5% methane or similar combination of methane concentration below the VAM lower explosive limit.
The dilution doors 7 can also be opened by a signal or combination of signals including either high methane signal recorded by the sensors located in the mine shaft, a high pressure signal from the mine, high temperature from mid thermal oxidiser, high pressure from thermal oxidiser, physical high pressure within the duct or warmth within the duct. They also act as vents. When the dilution doors 7 are in the open position the induced draft fan 11 sucks harder to pull the extra air volume provided by the open dilution doors 7 through to the thermal oxidiser effectively diluting and purging the duct from VAM.
Along the top of the duct 5 is a series of frangible vents 8 which are designed to fail open at a low pressure of about 0.2 kPa gauge via a pressure signal trigger and others at about 0.25 kPa via the pressure in the duct working against an opening mechanism. This is known as a frangible design. It is likely that a combination of blast panels and spring loaded louvers could be used in this application to provide multiple opening points with different opening mechanisms to improve the systems safety integrity level.
The reversal valve 9 located in the duct 5 opens and closes with normal operation. In the event of a power failure or high methane event the reversal valve 9 fails closed isolating the thermal oxidiser 10 from the fan 1. This event should occur ideally sometime after the dilution doors 7 open which has the effect of purging the duct 5 with air. The induced draft side of the thermal oxidiser remains open and in communication with the induced draft fan 11 so any stray methane that does enter the thermal oxidiser is vented in the normal manner. In one form the reversal valve has a labyrinth seal to ensure that any leakage always flows towards the induced draft fan not the mine ventilation fan 1.
In the event the induce draft fan 11 fails at just the wrong point in time then blast panels 12 will be activated. These blast panels will open via the pressure at the base of the thermal oxidiser at about 250 Pa. If vents 4 and frangible vents 8 are triggered by different opening signals then this vents can be the same type if desired. Similarly, dilution doors 7, frangible vent 8 and blast panel 12 can be the same type of opening if desired.
In the event that an event trigger has been recorded, a signal is generated to open vents, close barriers and open dilution doors and close reversal valves. The preset methane concentration could for example be a fraction of the Lower. Explosive Limit (LEL), say between 1 to 3%.
FIG. 2 illustrates the response time for an infrared methane monitor with a T90 of 6.0 seconds. Step change 14 at time zero seconds results in monitor response 15. This is not a particularly fast IR monitor but is use to illustrate the timing. Assuming we are managing a step change 14 then all dilution doors 7 are open by 0.2 seconds. The vent 4 would start opening by 0.4 seconds and be open by 0.9 seconds. To improve venting and dilution the curtains will start to descend at 0.4 seconds and be closed by 6.5 seconds. The duct between the barrier 6 and the thermal oxidiser will be full of air not VAM. The reversal valve will start to close at 0.4 seconds and be closed full by 15.4 seconds thus ensuring the duct is purged with air and the thermal oxidiser is isolated as the methane step change emerges from the ventilation fan 1. A methane monitor with a T90 of 24 seconds will trip approximately one second slower than the example above, but still fast enough to allow the system to work.
If vent 4 and barrier 6 only partial deploy this still improves the efficiency of dilution doors 7. If vent 4 and barrier 6 completely fail to deploy then dilution doors dilute the VAM to one third the original concentration. In this case 0.17% being a third of the original 0.5%. If isolation valve 9, vent 4 and barrier 6 fail then the VAM entering the thermal oxidise is approximately 3.3% assuming that the dilution doors worked.
Frangible vents 8 and blast panel 12 are only needed should all the above safety features fail. Should all other safety features fail then the vents 8 and 12 would need to be sized to keep the overpressure within the thermal oxidiser and the duct below, say, 2 kPa. Reversal valve 9 isolation coupled with blast panels 12 ensure that should all the other vents, barriers and doors fail, the deflagration still moves towards the induced draft fans 11 not the mine ventilation fan 1.
Other safety features may exist in the design of the thermal oxidiser such as the one described in PCT application PCT/AU2010/001217 such as height of chequer pack effectively acting as a flame arrestor and pressure relief vents on top of the oxidiser tower. Such features would need to be considered in any layer of protection analysis.
FIG. 3 illustrates a version of the apparatus with higher levels of redundancy and therefore a lower residual risk. In this example, an extra vent, barrier and dilution doors have been added. To illustrate how the individual parts fall into Independent Layers of Protection (ILP) systems; the systems have been labelled:

- By-pass system 16 labelled ILP 1
- Dilution doors 17 labelled ILP 2
- Safety system of each different thermal oxidiser ILP 3
- Reversal isolation valve 9 ILP 4
- Frangible panels 18 labelled ILP 5
  Otherwise the labels are the same as those that have been included with reference to FIG. 1.

FIG. 4 shows this process of adding extra layers of protection. The thickness of the black arrows between ILPs is proportional to risk frequency starting from completely unmitigated risk at the thickest end falling to a fully mitigated risk at the point of the arrow. As more ILPs are added the risk reduces. ILP 3 refers to the thermal oxidiser systems (different internal system, or even no systems depending on the design of the thermal oxidiser. ILP 6 refers to further auxiliary systems such as flame arrestors, fire fighting foams, sprinklers and the like.
Adding other safety devices into the duct is possible, as required; to manage tolerable risk, for example flame arrestor or more chequer bricks and is listed as ILP 6. These additional layers of protection do not change the base concept. This duct design is focused on avoiding the unsafe events not on reaction after the event. Many of the commercially available system are reactionary and are therefore less desirable in a layer of protection analysis (LOPA). Likewise, but in the opposite direction in terms of safety, reducing layers of protection does not impact on the concept.
FIGS. 5 and 6 are an isometric representations to illustrate the size and complexity of a real application.
In certain embodiments the conditioning duct has a large cross-sectional area duct prior to the thermal oxidiser where the VAM velocity is reduced below 7 m/s, preferably below 5 m/s but above 2 m/s, to act as mud drop out zone.
In certain embodiments, a vent before a barrier in the duct between the fan and thermal oxidiser to encourage methane being expelled to atmosphere.
In certain embodiments the vent is a fast acting louver system that leaks methane out even in the louver closed position. In certain embodiments the barrier is a fast acting fire curtain.
In certain embodiments, the conditioning duct includes one or more vent and barrier systems. In certain embodiments, the conditioning duct includes one or more vents after the barrier systems. (these have been referred to as dilution doors in this document)
In certain embodiments one or more safety doors open or close when a fraction of the lower explosive limit value for at least one of the volatile organic chemicals is detected in the gas stream passing through the conditioning duct.
In certain embodiments the high methane event is detected well down stream of the duct, say more than 10 second before high methane VAM reaches vent and barriers.
In certain embodiments fast measure methane monitors where the monitor T90 time is less than 5 seconds.
In certain embodiments the fixed vents open at low pressure should there be a deflagration in the duct.
Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

1. A duct for connecting a source of gas including a volatile organic compound to a thermal oxidiser wherein the duct has a sufficiently sized cross-sectional diameter to reduce the velocity of the gas to below 7 m/s prior to entering the thermal oxidiser.

2. A duct according to claim 1 wherein the source of gas includes methane and is derived from a coal mine ventilation system which is introduced via a fan into an inlet of the duct remote from the thermal oxidiser, or which is drawn through the duct into the inlet of the thermal oxidiser by an induce draft fan located at the outlet of the thermal oxidiser.

3. A duct according to claim 1 wherein the duct includes a barrier at a point along the length of the duct between the inlet of the duct and the thermal oxidiser, the barrier being moveable between an open position where the gas may pass along the duct between the inlet of the duct and the thermal oxidiser and a closed position where the gas is prevented from moving beyond the barrier along the length of the duct towards the thermal oxidiser.

4. A duct according to claim 3 wherein the barrier moves from the open position to the closed position upon the occurrence of an event.

5. A duct according to claim 3 wherein the duct further includes a vent located at a point along the length of the duct between the inlet of the duct and the barrier wherein the vent is moveable between an open state and a closed state.

6. A duct according to claim 5 wherein the vent moves from the closed state to the open state upon the occurrence of an event.

7. A duct according to claim 1 wherein the duct further includes a dilution door to introduce air from outside the duct into the gas passing through the duct to dilute the concentration of the volatile organic compound in the gas.

8. A duct according to claim 7 wherein the dilution door also acts as a vent in the event the gas pressure within the duct reaches a predetermined level.

9. A duct according to claim 1 wherein the duct further includes one or more safety vents which are retained in a closed position until the pressure within the duct reaches a predetermined level at which time the safety vents move to an open position allowing the gas within the duct to escape to atmosphere.

10. A duct according to claim 9 wherein the one or more safety vents are formed in a frangible manner wherein the safety vent opens when the pressure within the duct reaches a predetermined level.

11. A duct according to claim 9 wherein the predetermined level is 250 Pa. gauge.

12. A duct according to claim 1 wherein the duct further includes an isolation valve located immediately before the thermal oxidiser which provides that the thermal oxidiser may be isolated from the duct and the source of the gas when the isolation valve is closed.

13. A duct according to claim 12 wherein the isolation valve is closed upon the occurrence of an event.

14. A duct according to claim 4 wherein the event is selected from one or more of the following: a power failure, a methane concentration in the coal mine ventilation system above 1.5%. high pressure in the coal mine ventilation system.

15. A system for safely connecting a source of gas including a volatile organic compound to a thermal oxidiser, the system including:

a. a duct according to claim 1; and

b. a monitor located at some point associated with the source of the gas: wherein the monitor is able to detect an event