WO2004022918A1

WO2004022918A1 - Tunnel exhaust air treatment

Info

Publication number: WO2004022918A1
Application number: PCT/AU2003/001137
Authority: WO
Inventors: Stephen James Walker; David Hawkins
Original assignee: Hyder Consulting Pty Limited; Burns And Roe Worley Pty Ltd
Priority date: 2002-09-06
Filing date: 2003-09-05
Publication date: 2004-03-18
Also published as: AU2002951255A0

Abstract

The present invention relates to the use of a heating mechanism such as a flue burner, heater or gas turbine (210) to treat pre-filtered vehicle emissions which would otherwise accumulate or be discharged as tunnel exhaust. The heating mechanism is used to raise the temperature of the pre-filtered tunnel exhaust (202). The elevated temperature of the exhaust discharge promotes dispersion. The pre-filter has the added benefit or serving as an air filter, particularly a PM10 filter for all of the tunnel exhaust which passes through it.

Description

Tunnel Exhaust Air Treatment

Technical Field

The invention pertains to exhaust air treatment and more particularly to methods and apparatus for treating the exhaust of a tunnel.

Background Art

Vehicle exhaust emissions accumulate in tunnels. These emissions must be removed. Some tunnels use a tunnel exhaust stack in addition to conventional ventilation fans.

A typical 3 km long road tunnel requires an average air flow under normal operation of about 470 cubic metres per second and generates an exhaust plume which is discharged from the interior of the tunnel, at ambient temperature, through a single stack. The untreated exhaust plume from the tunnel emits CO (about 27g per second), NO_x (about 6g per second), PMlO (about 0.16g per second), and HC (about 5g per second). These discharges are a source of considerable community concern and objections are taken to both the content and the appearance of the runnel exhaust.

Summary Disclosure of Invention

It is an object of the invention to provide methods and apparatus for treating tunnel exhaust gasses so as to reduce the impact of any of the following undesirable components: visual pollution, NO_x, CO, particulates, hydrocarbons or overall CO₂ emissions. Accordingly there is provided a device comprising a pre-filter which receives a supply of untreated tunnel exhaust. The pre-filter has an output which enters a diverter. The diverter or plenum supplies a first portion of the output to the intake of a gas turbine and creates a second portion which bypasses the gas turbine. The gas turbine is supplied with fuel and produces a turbine exhaust at elevated temperature. The second portion and the turbine exhaust enter a gas mixer. The mixer combines the second portion and the gas turbine exhaust and produces a gas mixture which is lower in temperature than the turbine exhaust but higher in temperature than the second portion. The device further comprises a selective catalytic reactor which receives the gas mixture and generates a treated tunnel exhaust which is discharged through a stack.

In preferred embodiments, the turbine exhaust is about 495C.

In other preferred embodiments, the gas mixture is about 200C.

The invention also provides a method for treating tunnel exhaust comprising the steps of introducing tunnel exhaust into a pre-filter which produces an output stream; supplying the output to a device for raising the temperature of the stream, then introducing the elevated temperature stream of gas into, if required, an optional selective catalytic reactor generating a treated tunnel exhaust which is discharged through a stack.

The invention also provides a method for treating tunnel exhaust comprising the steps of introducing tunnel exhaust into a gas turbine pre-filter which produces an output, supplying a first portion of the output to the intake of a gas turbine and creating a second portion which bypasses the gas turbine. The gas turbine is supplied with fuel and produces a turbine exhaust at elevated temperature. The second portion and the turbine exhaust enter a gas mixer to produce a gas mixture which is lower in temperature than the turbine exhaust but higher in temperature than the second portion. The mixture enters, if required, an optional selective catalytic reactor, thus generating a treated tunnel exhaust which is discharged through a stack. The invention also provides a tunnel exhaust treatment device comprising a pre-filter for receiving a supply of untreated tunnel exhaust, the pre-filter having an output which supplies a filtered stream of gas to device for heating the gas; the device for heating the gas producing an output stream at elevated temperature which is supplied to a first (optional) selective catalytic reactor, the selective catalytic reactor generating a treated tunnel exhaust which is discharged through a stack.

Brief Description of the Drawings

Figure 1 is a schematic diagram of a device for treating tunnel exhaust illustrating the use of a duct burner.

Figure 2 is a schematic diagram of a device for treating tunnel exhaust showing the use of gas turbine and various optional bypass arrangements and optional SCRs.

Best Mode and Other Modes for Carrying Out the Invention

The present invention relates to the use of a gas turbine or alternately a duct burner (or both of these) to treat pre-filtered vehicle emissions which would otherwise accumulate or be discharged as tunnel exhaust. The turbine, or duct burner, or both of these are used to raise the temperature of the pre-filtered tunnel exhaust. This has the effect of breaking down some of the noxious components of the tunnel exhaust. The turbine, or duct burner, or both of these create a gas stream at an elevated temperature which may be introduced into a selective catalytic reactor (SCR) before being discharged. The discharged exhaust disperses more easily than untreated air because it is hotter and more buoyant. The use of a pre-filter has the added benefit of serving as an air filter, particularly a PMlO filter, for all of the tunnel exhaust which passes through it. In preferred embodiments, all of the tunnel exhaust fumes pass through the pre-filter and (depending on a number of factors) some or all of the pre-filtered air is introduced into the turbine.

The SCR process or Selective Catalytic Reduction method, takes advantage of the ability of ammonia (NH3) to reduce NOx to nitrogen and water, at a lower temperature and in the presence of a catalytic surface. There are various common formulations of catalyst material and specific catalyst structures or bed designs. In general, gaseous ammonia is injected with a carrier gas, typically steam or compressed air, into the flue gas upstream of the catalyst. The ammonia/flue gas mixture enters the catalyst, where it is distributed evenly through or across the catalyst bed. The flue gas then leaves the catalytic reactor. SCRs operate most efficiently at elevated temperatures and when the flue gas is relatively free of particulate matter, which tends to contaminate or "poison" the catalytic surfaces. SCR's may be used to advantage in conjunction with the present technology, but is not strictly speaking required to obtain a useful outcome in the area of tunnel exhaust treatment.

The turbine, when employed, is available to generate electricity and useful heat.

The electricity may be used for the operation of the tunnel or diverted into a local power grid. The potential for the turbine to generate electricity is so large that in many applications, the turbine must essentially be downsized from a theoretical maximum capacity device and therefore only a portion of the total untreated tunnel exhaust may be used. In any event, even if only a fraction of tunnel exhaust is treated in this way, there are significant benefits to be obtained by employing the methods and devices which are the subject of the invention.

As shown in Figure 1, one arrangement 100 for practicing the invention comprises a pre-filter 102, particularly a PM 10 filter, which receives all or part of a tunnel' s exhaust flow 104. The pre-filter 102 discharges a filtered stream 106 which is introduced into a heating mechanism such as a duct burner or gas turbine 108. The duct burner (or gas turbine) 108 creates a heated, filtered stream 110 for introduction into the (optional) selective catalytic reactor or SCR 112. The output of the heating mechanism 108 or of the SCR 114 is discharged from the tunnel, for example, through an exhaust stack. A more sophisticated approach 200 is depicted in Figure 2. In this example, untreated tunnel exhaust 202 containing air and vehicle emissions enters a pre-filter 204 of the type which is generally used in combination with a gas turbine. The pre-filter 204 improves the quality of the tunnel exhaust and removes particles which could potentially damage the turbine. In preferred embodiments, the pre-filter 204 has an output which enters a diverter or plenum 206. The diverter or plenum 206 supplies a first portion of the output 208 to the intake of the gas turbine 210 and creates a second portion 212 which bypasses the gas turbine. The second portion flow 212 may be regulated with a damper 214. In some situations, passing the full supply of pre-filtered air through the turbine would require a turbine which is too large or economically unfeasible for the particular tunnel. Thus, the bypass or second portion 212 has the benefit of providing a way to tailor the turbine size and provide a supply of cooler air which can be used to lower the temperature of the turbine output. The turbine 210 receives a supply of pre-filtered but otherwise untreated tunnel exhaust 208.

The gas turbine 210 is supplied with fuel, preferably natural gas 216 and produces a turbine exhaust output 218 at elevated temperature, for example and generally about 495C. The turbine also produces electricity 220 for tunnel use or supply to a grid. In some embodiments, the second portion 212 and the turbine exhaust 218 enter a gas mixer 222. The mixer 222 combines the second portion 212 and the gas turbine exhaust 218 and produces a gas mixture 224 which is lower in temperature than the turbine exhaust but higher in temperature than the second portion, for example and preferably about 200C.

The device may optionally incorporate one or more selective catalytic reactors 226, 228 which receive heated tunnel exhaust. In one embodiment, a single SCR 226 may be located downstream of the mixer 222. In this location, the SCR is well suited for NO_x removal. In another embodiment a single SCR is located between the turbine 210 and the mixer 222. In this warmer location, the SCR is well suited for CO (Carbon monoxide) removal. In a third embodiment, both types of SCR location 226, 228 are employed. In a fourth embodiment, no SCR is used.

One or more heat exchangers or heat recovery devices 230 may be located, for example, downstream from the turbme and the SCR 226 to recover waste heat and thereby utilise otherwise wasted heat for other processes or purposes. Heat exchangers or heat recovery devices may also be located more immediately adjacent to the turbine output stream 218 so as to capture more heat, so long as the operation of the one or more SCRs is not impacted adversely. A heat recovery module 230 can be fed from the output of the SCR 226 which is about 200C (or potentially from the turbine output 218) for the purpose of extracting heat (cogeneration) or power (combined cycle) from the hot exhaust. A steam turbine generator may be used to convert heat into electrical power. The heat or power so extracted may be used in the tunnel itself or provided to one or more users outside of the tunnel in the form of steam, hot water or electrical power.

Turbine treatment may be applied to the entirety of a tunnel's exhaust or only a portion, even where all of the tunnel's exhaust is pre-filtered. In one example, 25% of a tunnel's exhaust 208 is introduced into the turbine 210 and the remainder of the exhaust or some portion of the remainder 212 is mixed 222 with the turbine exhaust and introduced into the selective catalytic reactor 226.

In another embodiment of the invention and as suggested by Figure 2, a flow of untreated tunnel exhaust air 232 is regulated by a damper 234 and exhausted from the tunnel's stack 236 in conjunction with treated air 238 (if any). This arrangement is useful, for example, during periods of unusually high exhaust air flow, or when a full bypass is required for system service or maintenance purposes.

In an alternate embodiment, a duct burner 240 is located in or adjacent to the mixer 222 for raising the temperatures of the combined turbine exhaust and filtered second portion to about 200C. The duct burner thus improves the performance of any downstream SCR where the SCR's intake would otherwise not be hot enough. In preferred embodiments the stack 236 is considerably shorter than a conventional tunnel exhaust stack. This is because the treated exhaust is at an elevated temperature and has a higher exit or efflux velocity, less density and better dispersion characteristics than untreated tunnel exhaust. In some embodiments the treated exhaust exits the stack at about 150C and the (single) stack height is reduced by 30-70%. More than one stack may be employed.

While the invention has been disclosed with reference to particular details of construction and operation, these should be seen as examples and not as limitations to the scope or spirit of the invention as expressed in the claims.

Claims

What is claimed is:

1. A tunnel exhaust treatment device comprising a pre-filter for receiving a supply of untreated tunnel exhaust, the pre-filter having an output which supplies a diverter, the diverter supplying a first portion of the output to the intake of a gas turbine and creating a second portion which bypasses the gas turbine; the gas turbine producing a turbine exhaust at elevated temperature; the second portion and the turbine exhaust entering a gas mixer, the mixer combining the second portion and the gas turbine exhaust and producing a gas mixture.

2. The device of claim 1 wherein:

the second portion represents about 75 percent of the pre-filter output.

3. The device of either of claim 1 wherein:

the stack is shorter than an expected height by 30-70 percent.

4. The device of claim 1, further comprising: a regulated bypass which extends from a source of tunnel exhaust before the pre- filter to a point past the mixer.

5. The device of claim 1, further comprising: a duct burner which heats the gas mixture before it is discharged from the tunnel.

6. The device of claim 1, further comprising: a heat transfer module which extracts heat or power from an output which has been heated by the turbine.

7. A method for treating tunnel exhaust comprising the steps of: introducing tunnel exhaust into a pre-filter which produces an output; supplying a first portion of the output to the intake of a gas turbine and creating a second portion which bypasses the gas turbine, the gas turbine being supplied with fuel and producing a turbine exhaust at elevated temperature, the second portion and the turbine exhaust entering a gas mixer; combining in the mixer, the second portion and the gas turbine exhaust and producing a gas mixture which is lower in temperature than the turbine exhaust but higher in temperature than the second portion.

8. The method of claim 7, wherein: the second portion represents about 75 percent of the pre-filter output.

9. The method of claim 7, wherein: a duct burner is used to elevate the temperature of the gas mixture prior to introduction of the mixture into a selective catalytic reactor.

10. The method of claim 7, wherein: a heat transfer module is used to extract heat or power from the treated tunnel exhaust prior to discharge.

11. A tunnel exhaust treatment device comprising: a pre-filter for receiving a supply of untreated tunnel exhaust, the pre-filter having an output which supplies a filtered stream of gas to device for heating the gas; the device for heating the gas producing an output stream at elevated temperature which is supplied to a first selective catalytic reactor, the selective catalytic reactor generating a treated tunnel exhaust which is discharged through a stack.

12. The tunnel exhaust treatment device of claim 11, wherein: the device for heating the gas is a gas turbine.

13. The tunnel exhaust treatment device of claim 11, wherein: the pre filter supplies a diverter which creates a first stream for supply to the turbine and a second stream which bypasses the turbine.

14. The tunnel exhaust treatment device of claim 13, wherein: an output stream of gas from the turbine and the second stream are combined in a gas mixer prior to introduction into the selective catalytic reactor.

15. The tunnel exhaust treatment device of claim 11 , wherein: one or more heat recovery devices are located downstream of the turbine but before the stack.

16. The tunnel exhaust treatment device of claim 14, wherein: a duct burner is located in or adj acent to the mixer.

17. The tunnel exhaust treatment device of claim 11, wherein: the stack receives a stream of untreated tunnel exhaust from a second bypass.

18. The tunnel exhaust treatment device of claim 11 , further comprising: a second selective catalytic reactor for carbon monoxide, located closer to the turbine than the first selective catalytic reactor.

19. The device of claim 1, further comprising: a selective catalytic reactor which receives the gas mixture and generates a treated tunnel exhaust which is discharged through a stack.

20. The method of claim 7, wherein: the output of the mixer is fed into a selective catalytic reactor, the selective catalytic reactor generating a treated tunnel exhaust which is discharged through a stack.