Field of the Invention

The present invention relates to systems and methods for mitigating at least one component, such as methane, from a fluid stream, such as mine ventilation air. The present invention also relates to combustion modules for use in such systems and methods.

Background to the Invention

It is often desirable to mitigate a component or components from a fluid stream, particularly where the fluid stream is a gaseous emission from a process which contains compounds which are harmful to humans and/or the environment. Examples of such compounds include volatile low molecular weight hydrocarbons such as methane. Fugitive methane emissions occur from a variety of sources including coal, oil and gas production, transport, mining, agriculture, waste disposal, livestock, and land use (forestry). In fact, depending on coal mine site specifications, approximately 50-85% of all coal mining related methane is emitted to the atmosphere in mine ventilation air.

Existing technologies used to mitigate methane from mine ventilation air have employed a range of techniques based on combustion, membrane separation and adsorption. In conventional combustion-based systems, the methane-containing fluid stream is generally passed over a hot medium to oxidize the methane to carbon dioxide and water which are then vented to the atmosphere. Ideally, the heat discharged during oxidation is retained by the medium thereby sustaining its temperature and avoiding the need for further heating from external sources. However, due to design inefficiencies, this type of system often requires additional cleaners and/or filters to treat the incoming stream or purify fluid streams. Moreover, this type of system requries more energy for its operation due to high pressure drop through the medium. In general, none of these technologies excel in treating gas emissions produced in high dust environments such as exhibited in ventilation air from coal mines.

The present invention aims to provide alternative systems and methods for mitigating one or more components from a fluid stream, in particular methane from mine ventilation air. Furthermore, the invention aims to provide combustion modules for use in such systems and methods.

Summary of the Invention

According to a first aspect of the invention there is provided a system for mitigating a volatile component from a gaseous feed, the system including: an inlet in fluid communication with the gaseous feed; a combustion module for mitigating the volatile component from the gaseous feed; a control device for controlling flow of the gaseous feed through the combustion module between a forward flow cycle and a reverse flow cycle; and an outlet through which a gaseous emission from which the volatile component has been mitigated may be emitted from the system; wherein said combustion module includes a body portion formed from a refractory material and having a plurality of bores extending from a first end thereof to an opposing second end thereof, the bores facilitating the flow of the gaseous feed through the body portion and transfer of heat to the gaseous feed, thereby mitigating the volatile component in use.

As used herein the term "bore" includes within its scope individual discrete channels extending through the body portion and includes connected channels that form a network extending within and through the body portion. The term includes channels formed subsequent to formation of the body portion, for example by drilling or the like, and also includes channels formed during formation of the body portion, for example during moulding of the body portion.

Advantageously, the provision of bores extending through the body portion facilitates passage of the gaseous feed through the combustion module with minimal pressure drop. This in turn improves operational costs of the system relative to prior art models. Also, for high dust environments, such as seen in ventilation air from coal mines, clogging of the bores is less of an issue compared with prior art models.

As will be appreciated from the above discussion of the field of the invention, in one application of the system of the invention the gaseous feed is ventilation air derived from a coal mine and the volatile component is methane. It will also be appreciated, however, that the system of the invention is not so limited. For example, the system may be equally applicable to other applications involving the mitigation of a volatile component from a gaseous feed, such as mitigation of methane from other sources including oil and gas production, waste disposal, livestock and land use (forestry) as discussed above. Additional applications of the system will be appreciated by those in the art and are considered to fall within the scope of the system of the invention.

In order to minimise any resistance to flow of the gaseous feed through the combustion module, it is generally preferred that the combustion module be orientated such that the bores are disposed parallel with the flow of the gaseous feed. It may be, though, that in certain applications the combustion module may be orientated such that the bores are disposed on an angle to the flow of the gaseous feed. This may be determined on a case by case basis. In one embodiment, the body portion of the combustion module defines a multi-passage structure. That is, the bores are disposed in the body portion in an array. The bores will generally be disposed across the entire cross section of the body portion.

The width, which as used herein includes reference to diameter, of each bore is preferably from about 1mm to about 100mm. More preferably, the width of the bores is from about 5mm to about 20mm and even more preferably about 10mm. The width of the bores may or may not be constant across the length of the bores. Preferably, however, each of the bores has a width that is constant across their length. Furthermore, the width of bores within the body portion may or may not be uniform. Generally, it will be desirable that all of the bores in the body portion be of identical or similar width. However, it is envisaged that in certain applications it may be suitable for some bores to have a larger width than others. This may be somewhat dependent on the particular field of use, or volume of gas to be passed through the article.

The bores may be of regular or irregular cross sectional shape. Preferably, the bores are regular in cross sectional shape and extend parallel to one another from the first end of the body portion to the opposing second end of the body portion. The cross sectional shape of the bores is not intended to be limited in any way. For example, the bores may have a square, triangular, circular, hexagonal, octagonal or pentagonal cross sectional shape. Preferably, the bores have a circular cross sectional shape.

The body portion includes internal walls that define the bores and that separate them from one another. It will be appreciated that the width of the internal walls may or may not be constant along their length. Preferably, the width of the internal walls is constant along the length of the internal walls. Moreover, the width of individual internal walls may differ compared with other internal walls. In that regard, it will be appreciated that the width and cross sectional shape of the bores will affect the width and shape of the internal walls.

The spacing of the bores may be dependent on the particular field of application. Generally, it will be preferable for the bores to be spaced apart at a distance of from about 2mm to about 200mm, more preferably about 15mm to about 35mm. In that regard, the distance between the bores as used herein is intended to mean the distance between a centre point of a bore to a centre point of an adjacent bore or bores.

The height of the body portion is not intended to be particularly limited.

Preferably, the height of the body portion may be up to about 5 metres. More preferably, the height of the body portion is from about 1 metre to about 4 metres, even more preferably from about 1.5 metres to about 3 metres. Again, it will be appreciated that the height of the body portion will be somewhat dependent on the field of application of the system of the invention.

The shape and configuration of the body portion is not particularly limited. For example, the body portion may be cubic, cylindrical or prismatic in shape. In certain embodiment, it is envisaged that the body portion may be a prism of triangular, rectangular, square, pentagonal, hexagonal, or octagonal shape. Generally, however, the body portion is square or rectangular prismatic such that the body portion complements a housing in which it is fitted.

The body portion of the combustion module may be manufactured from any suitable refractory material. Preferably, the refractory material is a ceramic material, alumina, silica, magnesia, lime, fireclays, zirconia, dolomite, mullite, castable refractory cement or mixtures thereof.

The body portion need not be a unitary article. For example, the body portion may consist of a number of segments or blocks having the desired physical characteristics of the body portion.

The body portion may contain at least one catalyst to assist with mitigation of the component(s) from the fluid stream. In particular, the body portion may include at least one catalyst coated on internal walls of the bores, or disposed in the refractory material. The type of catalyst may depend on the component(s) to be mitigated and the environmental conditions in which the article is used. For example, for methane oxidation, Pd and Pt would be the best candidates. Table 1 summarises the kinetic data for selected catalysts.

Table 1

In such cases, a washcoat slurry of base metals such as alumina and ceria, on which the noble metal catalysts are placed, can be coated on the internal walls of the bores. Preferably, the catalyst is located near the central section of the body portion along the flow direction. One or more catalyst sections may be intermittently coated along the flow direction.

The control device advantageously facilitates control of the flow of the gaseous feed through the system. More particularly, in a particular embodiment during the forward cycle the gaseous feed is fed to the first end of the body portion at a relatively low inlet temperature and is heated within the body portion until combustion of the volatile component occurs resulting in emission of a relatively high temperature gaseous emission from the opposing second end and heating more towards the opposing second end of the body portion, and during the reverse cycle the gaseous feed having a relatively low inlet temperature is fed to the opposing second end of the body portion resulting in emission of the relatively high temperature gaseous emission from the first end and heating more towards the first end of the body portion, and the control device automatically switches flow between the forward cycle and reverse cycle, and vice versa, dependent on the temperature of the opposing second end and the first end of the body portion respectively and/or dependent on the temperature at a predetermined inner distance from the opposing second end and the first end of the body portion respectively.

The control device for effecting a change in directional flow of the fluid stream across the at least one article is not intended to be particularly limited. For instance, the control device may be a valve, regulator or tap, which will generally be associated with and operated by a computer control. The control device may be operated independently or used in conjunction with one or more temperature measuring devices (such as a thermocouple) that are positioned at suitable locations within the combustion module. The temperature measuring devices may send a signal to the computer control when a particular section of the body portion within the combustion module reaches a predetermined temperature, thereby activating the control device.

The way in which the article achieves mitigation of the volatile component(s) is not intended to be particularly limited. Generally, however, during mitigation the article is heated in order to oxidize the component(s). The degree of heating will depend on the particular component(s) to be mitigated as well as whether or not the body portion contains a catalyst. It should be appreciated that depending on the application in question, the system may be provided with any number of combustion modules. These may be arranged in series or in parallel as desired. Likewise, if more than one combustion module is included, the combustion modules need not be identical. These may include body portions formed from different refractory materials having the same or different catalysts applied or included. Likewise, the bores extending through different body portions may be of different dimensions and the body portions may be of different dimension relative to one another. In summary, the features of the combustion modules may be taken individually or collectively from those described above.

The orientation of the combustion module(s) is not particularly limited. For example, the combustion module(s) may be horizontally or vertically mounted to facilitate either substantially horizontal or vertical introduction of the gaseous feed into and through the bores of the body portion(s).

The body portion of the combustion chamber may be heated by any suitable source, for example, an external source such as an electrical heating element. Heating may be applied continually if desired. If heating is applied, heating elements are preferably located in the centre of the body portion. The body portion of the combustion module may also be heated to an initial desired temperature by an external heating source immediately prior to passing the gaseous stream through the body portion of the combustion module, for example by a duct burner. Heat generated during mitigation of the volatile component may be retained by the body portion thereby maintaining the desired temperature for the body portion and foregoing the need for continued operation of the external heating source.

In order to provide further environmental advantages to the system, the system preferably includes a carbon dioxide removal system for capturing and removing carbon dioxide from the gaseous emission prior to release to the atmosphere. The carbon dioxide removal system may equally be positioned such that carbon dioxide is removed from the gaseous feed prior to introduction to the combustion module if desired. The volume of carbon dioxide in the gaseous emission/feed is expected to be between about 0.2 to 2%. Suitable devices or systems for removing such amounts of carbon dioxide would be readily appreciated by those of skill in the art, but may include, for example, carbon dioxide scrubbers and sinks, and/or other suitable absorbent or adsorbent components. Geo-sequestration or other forms of carbon dioxide sequestration may also be employed if appropriate.

Additional undesirable components may also be removed from the gaseous emission if present using suitable methods and systems as known in the art.

In order to improve the operating efficiency of the system as a whole, waste heat from the system is preferably captured and utilised within the system or in an associated side process. For example, waste heat from the system may be utilised in a coal drying process, for heating of water, for steam generation for cooling or water desalination.

According to a second aspect of the invention there is provided a method of mitigating methane from ventilation air derived from a coal mine, the method including: passing the ventilation air through a combustion module including a body portion formed from a refractory material, at least part of the body portion being at or above the auto-ignition temperature of methane; wherein the body portion includes a plurality of bores extending from a first end thereof to an opposing second end thereof, the bores facilitating the flow of the ventilation air through the body portion and transfer of heat to the ventilation air, thereby mitigating the methane. In this specific method, a section of the body portion is heated to a temperature suitable for mitigation of the methane. Generally, this will be a section that is centrally disposed along the axis of the body portion. For example, the section of the body portion where mitigation of methane is to occur, in the absence of a catalyst, will preferably be heated to a temperature of from about 900 ⁰ C to about 1200 ⁰ C.

The dimensions of the body portion, dimensions of the bores and materials of construction may be gleaned from the above description of the first aspect of the invention. These features equally apply to the second aspect. Likewise, the body portion of the combustion module may preferably include at least one catalyst coated on internal walls of the bores, or disposed in the refractory material.

When a catalyst in provided to assist in the mitigation of the methane, the temperature of the section of the body portion where mitigation occurs may be less than if a catalyst is not employed. As such, in this instance, the section of the body portion is heated to a temperature of from about 200 ⁰ C to about 700 ⁰ C, preferably from about 350 ⁰ C to about 500 ⁰ C. Such low temperatures are advantageous from at least a costs perspective.

The heat generated during the combustion of methane is preferably retained by the body portion and the method preferably includes switching flow of the ventilation air between a forward cycle and a reverse cycle, and vice versa, dependent on the temperature of the second opposing end and the first end of the body portion respectively and/or dependent on the temperature at a predetermined inner distance from the opposing second end and the first end of the body portion respectively. This advantageously improves efficiency of the method and facilitates continuous operation. Preferably, switching between the forward cycle and the reverse cycle is conducted automatically by means of a control device. In order to reduce environmental impact, carbon dioxide is preferably removed from the ventilation air prior to combustion of the methane in the combustion module, or is removed from a gaseous emission exiting the combustion module. Carbon dioxide removal may be effected by any suitable means as noted above in respect of the previous aspect of the invention.

Advantageously, waste heat from the combustion of the methane may be captured and utilised. For example, waste heat may be utilised in a coal drying process, for heating of water, for steam generation for cooling, or for water desalination as mentioned above.

In arriving at the invention, the inventor has also arrived at a combustion module suitable for use in the system of the first aspect of the invention and/or the method of the second aspect of the invention.

As such, in a third aspect of the invention there is provided a combustion module for use in the above described system and/or method including a body portion formed from a refractory material and having a plurality of bores extending from a first end thereof to an opposing second end thereof, the bores facilitating the flow of a gaseous feed through the body portion and transfer of heat to the gaseous feed.

Calculations conducted by the inventor suggest that it will be preferable that the bores be substantially parallel to one another and have a width of from about 5mm to 20mm, more preferably about 10mm, and be spaced apart a distance of from about 15mm to 35mm, and the body portion have a height of from about 1.5 metre to about 3 metres. It will, however, be appreciated that any of the dimensions and features previously described may also apply to this aspect of the invention.

Again, the body portion of the combustion module may include at least one catalyst coated on internal walls of the bores, or disposed in the refractory material.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a system for mitigating a volatile component from a gaseous stream according to the invention; and

Figure 2 is a perspective view of a body portion for use in a combustion module according to the invention.

Detailed Description of the Invention

Referring to Figures 1 and 2, a system 10 for mitigating a volatile component from a gaseous feed 11 is provided. The gaseous feed 11 has a methane concentration between 0.01% to 1.5%, such as seen in ventilation air from a coal mine. The gaseous feed enters the system 10 via an inlet 12 that may be provided with a fan 13.

According to a forward cycle, the gaseous feed 11 flows through a forward flow 14 that includes passing through a combustion module 15. The combustion module, which will be described in more detail below, effects combustion of the volatile component, in this case methane.

During start up of the system 10, a duct burner 16 brings the body portion 20 (illustrated in Figure 2) of the combustion module 15 to a desired temperature, generally about 1200 ⁰ C. Inclusion of a catalyst in the body portion, for example within the material of the body portion itself or applied to the walls of the bores extending therethrough, may dictate a relatively low start up temperature of from about 20O ⁰ C to about 700 ⁰ C. This will be most advantageous in terms of minimising operational costs and so on. Once the body portion 20 is at the desired temperature, the duct burner 16 may be turned off. Alternatively, electrical heating elements (not shown) located in the centre of the body portion 20 may be used to start up the system 10 instead of the duct burner 16.

A draft fan (not shown) forces the gaseous feed 11 through the combustion module 15, including passing through bores of the body portion 20. The gaseous feed is initially at a temperature of about 25 ⁰ C (i.e. ambient temperature) and increases temperature as it passes through the combustion module 15. The temperature of the gaseous feed 11 rises in the combustion module 15, as a function of the flow rate of the gaseous feed 11 and the axial length of the combustion module 15, by absorbing heat from inner walls of the bores of the body portion 20.

At a certain distance from an inlet 17 of the combustion module 15, the gaseous feed 11 reaches the auto-ignition temperature of the volatile component (in this case methane). At that point, oxidation takes place resulting in relatively high temperature gaseous emission that provides heat to the body portion 20 as it exits the combustion module 15 through outlet 18 and is emitted from the system 10 through an exhaust 19.

When the outlet 18 of the combustion module 15, according to the forward cycle 14, reaches a predetermined temperature, or when other positions along the axial length of the body portion 20 reach predetermined temperature(s), a control (not shown) switches flow to a reverse cycle 14' which effectively reverses flow through the combustion module 15.

Referring particularly to Figure 2, the body portion 20 includes an array of bores 21 that extend through the body portion 20 from a first end 22 to an opposing second end 23. The bores 21 have a circular cross section with a diameter of 10mm and are spaced apart a distance of 20mm, taken from the centre points of the bores 21. The outermost bores 21 defining the edge of the array are located 25mm from the edge of the body portion 20, again taken from the centre points of the bores 21 defining the edge of the array. The body portion 20 has a height of from 1 metre to 2 metres, a length of from about 2 metres to 4 metres and a width of about 2 metres. It will be appreciated that these dimensions may vary as described above.

As mentioned above, the provision of bores 21 advantageously avoids, or at least alleviates issues relating to dust clogging the passage through the body portion 20 of the combustion module 15. The bores 21 also advantageously ensure minimal pressure drop across the combustion module 15 which provides for relative good economy for the system 10.

As previously noted, the system 10 has a forward cycle 14 flow and a reverse cycle 14' flow. This is also illustrated in Figure 2. According to the forward cycle 14, the gaseous feed 11 enters bores 21 of the body portion 20 of the combustion module 15 from a first end 22. As the gaseous feed 11 passes through the body portion 20, its temperature increases due to heat exchange with the walls of the bores 21 until it reaches a section of the body portion 20 that is at or above the auto-ignition temperature of the volatile component in the gaseous feed 11. This will generally be in a mid- region of the body portion 20.

Once the volatile component is ignited, the resultant high temperature emission raises the temperature of the opposing second end 23 of the body portion 20. Once the opposing second end 23 of the body portion 20, which generally constitutes the outlet to the combustion module 15, reaches a predetermined temperature, the control device switches flow to the reverse cycle 14' thereby maintaining the thermal environment necessary to continue with the mitigation, for example the auto-oxidation of methane. Examples

Energy

Energy required for start-up

Similar to the MEGTEC TFRR, CANMET CFRR and VAMOX systems, the centre of the combustion module (i.e. the body portion), which is effectively a refractory cast regenerative bed (heat transfer bed), may be preheated to the methane auto-ignition temperature. For the MEGTEC unit, the bed is initially heated to 1000 ⁰ C by electrical heating coils. Practically, this electrical heating method can be used for the body portion of the combustion module, in the form of a honeycomb-shape refractory cement cast block VAM mitigator, by heating the centre of the body portion. Electrical heating may only be needed at first start-up, and the mitigator will be self-sustaining when methane concentration in ventilation air is above 0.3%. For a VAMOX demonstration unit at Jim Walter Resources' No.4 mine, a propane fed burner is used for its start-up.

In the body portion, in the form of a refractory cement cast block, heat exchange, flow reversal and methane oxidation occur in the individual passages, allowing use of a duct burner for start-up. It is expected that the duct burner can be installed in the top of the cast block body portion, and hot combustion gases flow through the individual passages to heat the bed for start-up. Mine drainage gas with a methane concentration of not less than 30% can be used by the burner. Once the mitigation process is started, the mitigator will be self-sustaining. The cost of the duct burner would be cheaper than that of electrical coils, however the electrical heating process should be simpler than using the duct burner. When the mitigator is in self-sustaining operational status, the start-up heating equipment is not needed unless the temperature of bed centre decreases due to lower methane concentration than the minimum requirement over a time period. The start-up method may be determined during the design.

Based on calculations, heat transfer and fluid dynamics in the honeycomb- shape refractory cement cast block, the energy required for first start-up is estimated as:

It is preliminarily calculated that 1.2 m ³ of the cast bed could process 0.362 m ³ /s of ventilation air, very close to that of existing thermal flow reverse reactors;

Volume of the passages (each 15mm diameter, approximately 2050 passages, passage length of 1.2m) is 0.4345m ³ (this is estimated based on the preliminary calculation as a case here);

The volume of the refractory cement cast solid is 0.7655m ³ ;

Assume 40% of the cast solid is heated to 1000 ⁰ C, and this should be a reasonable case. Heat capacity of the cast solid is 1.3kJ/kg-°C at 100O ⁰ C, and its density is 2230kg/m ³ . Hence, the heat required to achieve this is calculated as 40%χ0.7655χ2230χl.3χ(1000-25)=865.5 MJ, which is equal to the heating value of 24 m ³ methane;

Hence, for a VAM mitigation plant processing 200m ³ /s ventilation air, the energy required for start-up is estimated to be 478,167MJ, equivalent to the heat value of 13,282 m ³ of methane. This methane is

~10% of the average amount of methane (at 100% concentration) drained in one day at Appin. It is expected that actual energy requirement for start-up should be less than the calculated amount, and should be close to that of the existing thermal flow reverse reactor. Energy consumption during operation

At present, there are several MEGTEC and VAMOX demonstration units in the world, which are summarised in the following: ■ MEGTEC demonstration units:

WestCliff Colliery, Australia: 17m ³ /s per unit, four units installed; Consol Energy, USA: 14m ³ /s per unit, one installed; Zhengzhou mine, China: 17m ³ /s per unit, one installed. VAMOX demonstration unit: - Jim Walter Resources, USA: 14m ³ /s per unit, one installed.

For the VAMOX unit installed at Jim Walter Resources, a 75kW fan is required for its operation. So far there is no report on fan size for the abovementioned MEGTEC demonstration units, but it is estimated that an at least 15OkW fan is required to run two MEGTEC units of 34m ³ /s VA in total.

As discussed above, one of the advantages for the system of the invention is its low pressure drop through the honeycomb monolithic structure bed. Based on the preliminary design calculations, the pressure drop is about 25Pa through the honeycomb block with a height of 1.2m and passage size of 15mm diameter. Hence, for processing 14m ³ /s ventilation air, the power consumed is about 0.35kW to overcome the pressure drop inside the bed. In a comparison, for a same height of packed bed, the pressure drop could be 13,500Pa depending on the packed bed materials, which corresponds to a power consumption of 189kW.

In general, during its normal operation, the power consumption for operating the VAM mitigation unit is a sum of power for overcoming the pressure drop through the unit system and power for instrumentation. Plant Configuration and Size

As discussed above, depending on mine site specifications, a combined

VAM mitigation and utilisation plant may consist of the VAM capture units, lean-burn turbine units and VAM mitigation units. If a mine just employs

VAM mitigation units, under the site condition the mitigation plant can have a number of mitigation units, which can be configured in parallel, similar to the configuration of 4 MEGTEC units at WestCliff. Advantageously, each unit would have a capacity of processing 14-17m ³ /s ventilation air, which should lead to a good portability.

The VAM mitigation and/or utilisation plant size should be reasonably large as it processes air flow rate of 200-400m ³ /s. For example, for a 220-250MW pulverised coal fire power station, its flue gas flow rate is about 300m ³ /s (slightly various depending on type of power plant and coal properties). This could be used as an indication of any potential VAM plant size.

In the specification the term "comprising" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the term "comprising" such as "comprise" and "comprises".

The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to those of skill in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.