WO2004005797A1

WO2004005797A1 - Direct fired steam generator

Info

Publication number: WO2004005797A1
Application number: PCT/GB2003/002878
Authority: WO
Inventors: Nigel Edward Humphrey Feilden; Ross Gregor Hewitt
Original assignee: Aberdeen University
Priority date: 2002-07-05
Filing date: 2003-07-07
Publication date: 2004-01-15
Also published as: GB0215653D0; AU2003251158A1

Abstract

A direct fired steam generator for generating steam at atmospheric pressure comprises a burner (1) for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber (2) having an inlet (12) through which a flame (11) produced by the burner can enter the chamber, a wall wetting system (7) within the chamber comprising a distributor (10) circumjacent to the inlet for jetting water onto the inner wall of the chamber to provide a continuous film of water (9) around the circumference of the chamber, and an outlet (4) through which steam generated from the jetted water can leave the chamber.

Description

DIRECT FIRED STEAM GENERATOR

The present invention relates to a direct fired steam generator and a method of generating steam, in particular for use in soil disinfection.

Conventional methods of disinfecting soil use toxic chemicals, for example methyl bromide, with consequent environmental concerns. Alternatives to chemical disinfection are therefore sought.

Conventional steam disinfection of soil and other horticultural growing media typically employs a conventional steam boiler. Conventional steam boilers produce pure steam which, at atmospheric pressure, releases all its heat at 100°C. This is 20 to 30°C hotter than the optimum temperature for soil disinfection. Such boilers also waste a significant proportion of their fuel calorific value in flue gasses, which typically leave the boiler at a temperature in excess of 200°C. For mobile use, the maximum boiler capacity is limited by the weight of the boiler, and the large amount of water it contains .

One alternative identified by the present inventors is steam disinfection using a direct fired steam generator. Direct fired steam generators are smaller, are not pressure vessels, and contain a relatively small amount of water, with consequent savings in weight compared to conventional steam boilers. They are also more energy efficient because the heat in the flue gas is recovered. Furthermore, direct fired steam generators can deliver most of their heat within the desired temperature range of 70 to 80°C for soil disinfection, as opposed to 100°C at one atmosphere pressure from conventional boilers, because the steam is mixed with what would normally be flue gas. Static direct fired steam generators, fuelled by methane, have previously been used to cure concrete rapidly and pasteurise animal feed.

Direct fired steam generators generally comprise a vapour generator having a vertically oriented outer circular cross sectional cylinder, with a lower end having an axial outlet aperture for steam and an upper end having an axial burner nozzle through which a fuel air mixture is passed for combustion. An inner circular cross sectional cylinder is mounted coaxially within and spaced from the outer cylinder. The inner cylinder wall extends from the lower end of the outer cylinder substantially to the upper end of the outer cylinder. A water inlet is provided at the lower end of the outer cylinder. In operation, water is provided to fill the space between the two cylinders, the water reaching the uppermost edge of the inner cylinder and overflowing in the manner of a weir to run down the inwardly facing surface of the inner cylinder wall. The fuel/air mixture is ignited and the resultant flame extends downwards into the inner cylinder. The heat from the flame vaporises water from inner cylinder wall to form steam, which passes out of the outlet aperture whilst the wall is kept cool by the water in the sleeve. An example of such a construction can be found in US 4,288,978.

As a further example, US 3,980,137 discloses an arrangement in which the weir has a series of grooves so that water flows in a spiral path down the inwardly facing surface of the inner cylinder wall.

Thus, conventional direct fired steam generators have a double inner and outer wall arrangement to provide a water sleeve, the water being introduced into the space between the walls and overflowing at the upper edge of the inner wall.

Such conventional direct fired steam generators have a number of drawbacks. For example, they use gaseous fuel, require level ground during usage, and are typically unsuitable for mobile applications. Gaseous fuels, for example propane, tend to be expensive, with consequently expensive operating costs. Furthermore, during disinfection of soil it is not always possible to find level ground for operating the direct fired steam generator, and hence possible sites for usage may be limited. It is also desirable to have a mobile steam generator which may be operated whilst in motion, i.e. when the ground is not always level or stationary. Direct fired steam generators also tend to be heavy, due to the inner and outer cylinders containing the water sleeve, which can impair mobility of the steam generator.

An object of the present invention is to provide a direct fired steam generator which seeks to alleviate the above identified problems.

According to the present invention there is provided a direct fired steam generator for generating steam at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber comprising a distributor circumjacent to the inlet for jetting water onto the inner wall of the chamber to provide a continuous film of water around the circumference of the chamber, and an outlet through which steam generated from the jetted water can leave the chamber. The direct fired steam generator of the present invention is inexpensive to run using liquid fuel, such as diesel or kerosene, as opposed to more expensive gaseous fuels such as propane. The single wall chamber allows for the generator of the present invention to be 50% to 60% lighter than conventional direct fired steam generators, and is robust enough to withstand movement typically encountered during mobile use, for example in fields or greenhouses. The output kW (of heat) to weight ratio can therefore be higher than conventional steam boilers. The generator of the present invention can also be used other than when vertically upright, and can be used at, for example, up to 10° to vertical.

In use, the inner wall of the chamber should always be wet and coated with water, otherwise so-called "hot spots" can develop on the chamber surface which may cause overheating. Hot spots may be avoided by jetting water around the circumference of the inner wall to ensure a continuous film of water around the circumference of the inner wall, by roughening the surface of the inner wall to increase wetting of the inner wall by the water, or by wetting the inner wall to excess.

Preferably, the distributor comprises an array of jets, equally spaced and separated by from 10 to 100mm. More preferably, the jets are spaced by from 20 to 35mm.

The distributor jets are preferably selected so as to provide a desired irrigation rate per unit perimeter for a particular application. The jets preferably have sufficient pressure drop so that changes in hydrostatic head, for example due to tilting, are negligible, and an opening width so as to be resistant to clogging and easy to manufacture. For example, in a chamber having a diameter of 500mm, a distributer having 60 jets can jet 60 to 100 litres of water per minute onto the inner wall of the chamber, i.e. 38 to 64 litres per minute per metre of chamber circumference.

In preferred embodiments of the present invention, the inner wall of the chamber has a roughness height of from 1.6 to 6 micrometers. As mentioned above, inner wall surface roughness can help ensure the presence of a continuous film of water around the circumference of the inner wall of the chamber, and help prevent formation of rivulets of water, which may lead to "hot spots" and exposure of dry areas of the inner wall to the flame.

The presence of a continuous film of water around the circumference of the inner wall of the chamber reduces "hot spots", which can in turn reduce the sensitivity of the generator of the present invention to tilting during movement or use.

The generator may further comprise a baffle plate positioned within the chamber towards the outlet for preventing a flame produced by the burner from entering the outlet and to prevent water mist being recycled into the flame. Preferably, the baffle plate is positioned such that the chamber volume between the baffle plate and the inlet is greater than the chamber volume between the baffle plate and the outlet. More preferably, the ratio of the chamber volume between the baffle plate and the inlet to the chamber volume between the baffle plate and the outlet is from 1:1 to 3:1, for example substantially 2:1.

In preferred embodiments of the present invention, a water mist system is provided, positioned towards the outlet for controlling the temperature of steam exiting the generator through the outlet. Conveniently, the water mist system is positioned within the chamber towards the outlet, and is preferably positioned between the baffle plate and the outlet. The water mist system is conveniently directed towards the outlet, to help prevent quenching of the flame. Preferably, the water mist system can produce droplets of water which are smaller than droplets of water which can be produced by the wall wetting system.

Preferably, the generator further comprises recycling means for recycling unvaporised water for use in the wall wetting system and/or the water mist system if present.

According to the present invention there is also provided a direct fired steam generator for generating steam • at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber to provide a continuous film of water around the circumference of the inner wall of the chamber, the inner wall having a roughened surface to maintain the continuous film of water, and an outlet through which steam generated from the water can leave the chamber.

According to the present invention there is further provided a direct fired steam generator for generating steam at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber to provide a continuous film of water around the circumference of the chamber whereby the inner wall is wetted to excess, and an outlet through which steam generated from the water can leave the chamber.

According to the present invention there is still further provided a method of generating steam at atmospheric pressure using a direct fired steam generator as herein described, which method comprises providing a continuous film of water from the wall wetting system on the inner wall around the circumference of the chamber, burning an air/fuel mixture in the burner to produce a flame, allowing the flame produced by the burner to enter the chamber through the inlet, allowing the flame to at least partially vaporise the water to generate steam, and allowing the steam to leave the chamber through the outlet.

Preferably, in the method of the present invention a water mist is provided from a water mist system towards the outlet for controlling the temperature of steam exiting the generator through the outlet. The droplets of water in the water mist ^! are preferably smaller than droplets of water provided by the wall wetting system.

The water mist can allow the outlet steam temperature to be controlled. For example, for disinfection of soil the temperature of steam is preferably 70°C to 80°C, which is optimal for heat transfer to soil.

Water which is not vaporised may be recycled, and water to the wall wetting system and/or water mist if present preferably comprise (s) recycled water. In this way, dissolved solids present in the chamber can pass into the water mist system to then pass out through the outlet, and hence build-up of particulate matter can be reduced or even substantially removed.

Preferably, the present invention also provides a method of disinfecting soil, which method comprises generating steam by a method as herein described, and applying the steam to soil to be disinfected.

Alternatively, the present invention may provide a method of curing concrete, the method comprising generating steam by a method as herein described, and applying the method to the concrete to be cured.

Further alternatively, the present invention may provide a method of pasteurising biological materials which may include animal feed, the method comprising generating steam by a method as herein described, and applying the method to the material to be pasteurised.

A preferred embodiment of the present invention will now be' described in detail by way of example with reference to accompanying Figure 1, which shows a schematic view of a generator of the present invention.

Referring to Figure 1, a generator of the present invention comprises a burner 1 for producing a flame 11 by burning an air/fuel mixture, a single walled circular cross sectioned chamber 2 having an inlet 12 through which the flame 11 enters the chamber 2, a wall wetting system 7 within the chamber 2, and an outlet 4 through which steam can leave the chamber 2. The wall wetting system 7 comprises a distributor 10 circumjacent to the inlet 12 for jetting water onto the inner wall of the chamber 2 to provide a continuous film of water 9 around the circumference of the inner wall of the chamber

2.

The wall wetting system 7 comprises 60 circumferentially spaced jets (not shown) to jet water onto the inner wall of the chamber 2. The jet holes are drilled to 4 mm diameter and are lined with silicone tubing using epoxy adhesive to avoid rusting. Each jet has an internal diameter of 2.4 mm (with tubes installed) . In use, the total flow to the jets is 60 to 100 litres per minute, i.e. 38 to 64 per metre width of water film if the internal diameter of the chamber is 500 mm. In this embodiment, the spacing between the jets is 26.2 mm, but this can be varied according to the particular generator in question, although an upper limit of 40 mm spacing between jets is preferred. A larger spacing between jets results in a larger jet of water, and larger vertical drop before the water films from adjacent jets join together. Larger jets are also easier to drill and are less likely to be blocked.

In order to help prevent the development of "hot spots", the surface of the inner wall of the chamber 2 is roughened to from 1.6 to 6 micrometers. This level of roughness is estimated by comparison with a GAR S-22 microfinish comparator. The surface may be roughened due to parallel corrugations, although randomly orientated grooves, or grooves within 45 degrees of the horizontal plane are preferred. The roughening is intended to promote surface wetting by aiding capillary movement of water.

A baffle plate 3 is positioned within the chamber 2 towards the outlet 4 for preventing the flame 11 from entering the outlet 4. The baffle plate 3 is positioned such that the ratio of the chamber 2 volume between the baffle plate 3 and the inlet 12 to the chamber 2 volume between the baffle plate 3 and the outlet 4 is substantially 2:1.

A water mist system 6 is positioned towards the outlet 4 within the chamber 2, between the outlet 4 and baffle plate 3 for controlling the temperature of steam exiting the generator through the outlet 4. The water mist system 6 is fed by the high pressure pump 5 via the pressure regulator 8, and directs a water mist towards the outlet 4. The droplet size of the water mist is preferably smaller than the water droplet size produced by the wall wetting system 7. The water mist system preferably produces water droplets in the 10 to 50 micrometre diameter range, using a feed pressure of 60 to 110 bar.

The water mist can produce a significant reduction in the temperature of steam leaving the generator, and facilitates control of that temperature. The water mist is directed towards the outlet 4 to reduce the likelihood of the flame 11 being quenched. Transfer of heat from the flame 11 to the water and steam present in the upper volume of the chamber 2 above the baffle 3 is primarily by means of radiative transfer, whilst in the lower volume of the chamber 2 heat transfer is primarily by means of convective transfer.

In this preferred embodiment, the wall wetting system 7 is operated to ensure an excess of water 9 on the inner wall of the chamber 2, i.e. not all of the jetted water is vaporised into steam. The water is jetted onto the inner wall of the chamber 2 as a continuous stream so as to spread out to form a continuous film around the circumference of the inner wall of the chamber 2. Water which is not vaporised flows as a film 9 down the inner wall of the chamber 2, and collects in a sump formed by the internal projection of the exhaust 4 above the floor of the chamber 2. The level of water 20 is maintained by a float switch (not shown) which opens valve 19 to let fresh water in from the supply 17, the volume of which may optionally be measured by a meter 18. Water in the sump is thus a mixture of unvaporised water from the water film 9 and fresh water. The sump water is pumped by circulation pumps 15 and 16 via the recirculation loop 14 to the wall wetting system 7. Sump water is also pumped via pipe 13 to the water mist system 6 via high pressure pump 5. Some of the water mist leaves the generator via the outlet 4, carrying with it particulate matter and dissolved solids from the chamber 2. This has the benefit that particulate matter and dissolved solids present in the chamber can pass into the water mist system and out of the generator through the outlet 4. In this way, significant build-up of dissolved solids and their deposition as scale can be avoided.

Thus, in use , water is supplied to the wall wetting system 7 to provide a continuous film of water 9 around the circumference of the inner wall of the chamber 2, and the water mist system 6 via high pressure pump 5 and pressure regulator 8. An air/fuel mixture is burned in the burner 1 to produce a flame 11, which enters the chamber 2 via the inlet 12 to partially vaporise the film of water 9. Steam generated from the film of water 9 leaves the chamber 2 through the outlet 4, the temperature of the steam being controlled by the water mist system 6. Water which is not vaporised is recycled via circuit 14 for use in the wall wetting system 7 and via circuit 13 to water mist system 6.

It will be understood that the embodiment illustrated describes the invention in one form only for the purposes of illustration. In practice, the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

Claims

1. A direct fired steam generator for generating steam at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber comprising a distributor circumjacent to the inlet for jetting water onto the inner wall of the chamber to provide a continuous film of water around the circumference of the chamber, and an outlet through which steam generated from the jetted water can leave the chamber.

2. A generator according to claim 1 wherein the distributor comprises an array of jets, equally spaced and separated by from 10 to 100mm.

3. A generator according to claim 2 wherein the jets are spaced by from 20 to 35mm.

4. A generator according to claim 1, 2 or 3 wherein the inner wall of the chamber has a roughness height of from 1.6 to 6 micrometers.

5. A generator according to any preceding claim further comprising a baffle plate positioned within the chamber towards the outlet for preventing a flame produced by the burner from entering the outlet and to prevent water mist being recycled into the flame.

6. A generator according to claim 5 wherein the baffle plate is positioned such that the chamber volume between the baffle plate and the inlet is greater than the chamber volume between the baffle plate and the outlet.

7. A generator according to claim 6 wherein the ratio of the 5 chamber volume between the baffle plate and the inlet to the chamber volume between the baffle plate and the outlet is from 1:1 to 3:1.

8. A generator according to claim 7 wherein the ratio is 10 substantially 2:1.

9. A generator according to any preceding claim further comprising a water mist system positioned towards the outlet for controlling the temperature of steam exiting the generator

15 through the outlet.

10. A generator according to claim 9 wherein the water mist system is positioned within the chamber towards the outlet.

20 11. A generator according to claim 9 or 10 wherein the water mist system is directed towards to the outlet.

12. A generator according to claim 11 which comprises a baffle as defined in any one of claims 5 to 7 wherein the

25 water mist system is positioned between the baffle plate and the outlet.

13. A generator according to any one of claims 9 to 12 wherein the water mist system can produce droplets of water

30 which are smaller than droplets of water which can be produced by the wall wetting system.

14. A generator according to claim 13 wherein the water mist system produces water droplets in the 10 to 50 micrometre diameter range, using a feed pressure of 60 to 110 bar.

15. A generator according to any preceding claim further comprising recycling means for recycling unvaporised water for use in the wall wetting system and/or the water mist system if present.

16. A direct fired steam generator for generating steam at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber to provide a continuous film of water around the circumference of the inner wall of the chamber, the inner wall having a roughened surface to maintain the continuous film of water, and an outlet through which steam generated from the water can leave the chamber.

17. A direct fired steam generator for generating steam at atmospheric pressure, comprising a burner for producing a flame by burning an air/fuel mixture, a single walled circular cross sectioned chamber having an inlet through which a flame produced by the burner can enter the chamber, a wall wetting system within the chamber to provide a continuous film of water around the circumference of the chamber whereby the inner wall is wetted to excess, and an outlet through which steam generated from the water can leave the chamber.

18. A method of generating steam at atmospheric pressure using a direct fired steam generator according to any preceding claim, which method comprises providing a continuous film of water from the wall wetting system on the inner wall around the circumference of the chamber, burning an air/fuel mixture in the burner to produce a flame, allowing the flame produced by the burner to enter the chamber through the inlet, allowing the flame to at least partially vaporise the water to generate steam, and allowing the steam to leave the chamber through the outlet.

19. A method according to claim 18 which further comprises providing a water mist from a water mist system towards the outlet for controlling the temperature of steam exiting the generator through the outlet.

20. A method according to claim 19 wherein the droplets of water in the water mist are smaller than droplets of water provided by the wall wetting system.

21. A method according to claim 19 or 20 wherein the steam is provided at a temperature of from 70 to 90°C.

22. A method according to any one of claims 18 to 21 wherein water which is not vaporised is recycled.

23. A method according to claim 22 wherein the water to the wall wetting system and/or water mist if present comprises recycled water.

24. A method of disinfecting soil, which method comprises generating steam by a method according to any one of claims 18 to 23, and applying the steam to soil to be disinfected.

25. A method of curing concrete, which method comprises generating steam by a method as herein described, according to any one of claims 18 to 23, and applying the steam to the concrete to be cured.

26. A method of pasteurising biological materials, which method comprises generating steam by a method as herein

5 described, according to any one of claims 18 to 23, and applying the steam to the materials to be pasteurised.

27. A method according to claim 26, wherein said biological materials include animal feed.

10

28. A direct fired steam generator substantially as hereinbefore described with reference to the accompanying drawings .

15 29. A method of generating steam substantially as hereinbefore described.