The invention relates to a method for burning off gas, according to which method gas is mixed with air, after which the air/gas mixture is burned in a substantially vertically extending combustion space, whereby a torch-like flame is formed.

The invention also relates to a device suitable for carrying out such a method, which device comprises a substantially vertically extending combustion chamber, which is near its bottom side provided with at least one air/gas mixture supply channel.

In the drilling of oil and/or gas wells and the production from said wells large amounts of gas are frequently released, which need to be burned off on site, which in practice often takes place by passing the gas through a chimney and subsequently burning it therein. During this process a torch-like flame is formed, which rises several metres from the chimney. In order to avoid any risks to the surroundings the chimney needs to be relatively long, therefore.

Another drawback is the fact that a plant comprising such a chimney is often dimensioned for burning a flow of air/gas mixture of a certain magnitude, whereby incomplete combustion may take place when the flow deviates from said magnitude.

The object of the invention is to provide a method for burning off gas, wherein the above drawbacks are avoided. With the method according to the invention this objective is accomplished in that steam is supplied at the lower side of the flame, which steam is used to regulate the size of the flame.

It has become apparent that the size of the flame can be reduced considerably by supplying steam near the lower side of the flame. This makes it possible to design the combustion chamber in such a manner that the entire flame is surrounded by the combustion chamber, whereby the total height of the combustion chamber is less than the height required with the known device. The steam being supplied furthermore effects an increased turbulence of the air/gas mixture, which results in an improved combustion. In addition to that the steam causes the air/gas mixture to expand, which likewise results in an improved combustion.

One embodiment of the method according to the invention is characterized in that a relatively small flow of air/gas mixture is first ignited when the burning of the air/gas mixture is started, whereby a torch-like flame is formed, after which water is sprayed into the flame, which water is evaporated by the flame and rises to the upper side of the flame, after which the supply of the air/gas mixture to the combustion chamber is increased considerably, whereby the steam being formed is drawn down along walls of the combustion chamber, whereby the flame is reduced to a desired size. In this manner the combustion process can be started.

The water being sprayed into the flame, which subsequently rises to the upper side of the flame, forms a block of steam above the flame, as it were. By increasing the flow of air/gas mixture into a first part of the combustion chamber relatively shortly thereafter, an overpressure is generated at that location. In the other, second part of the combustion chamber an underpressure is generated, which draws the block of steam downwards, thereby pushing the flame down, as it were. This flame of a reduced height can be maintained by subsequently supplying steam to the lower side of the flame. The object of the invention is furthermore to provide a device wherein the drawbacks of the known device are avoided.

This objective is accomplished with the device according to the invention, whilst the device is furthermore provided with a steam supply channel near the bottom side of the combustion chamber. Supplying steam through the steam supply channel makes it possible to regulate the size of the torch-like flame which is formed when gas is burned off.

It is noted that from German patent application DE-A1- 26.54.438 a device is known wherein steam is supplied to the combustion chamber. The steam helps to ensure that the gas is burned without producing smoke. It is not known from the said document, however, to supply steam in such a manner that the torch-like flame formed during the burning off of the gas can be controlled as regards its size.

The invention will be explained in more detail with reference to the drawings, wherein:

Figure 1 is a perspective view of a device according to the invention;

Figure 2 shows a longitudinal section of a part of the device shown in Figure 1;

Figure 3 shows a bottom side of the combustion chamber of the device shown in Figure 1; Figure 4 shows a detail of the view shown in Figure

3;

Figure 5 shows a cross-section of the view shown in Figure 3, seen in the direction indicated by arrows V-V;

Figure 6 is a bottom view of the device shown in Figure 2;

Figures 7 and 8 show details of the device shown in Figure 2;

Figures 9A-9C show a sound absorber of the device; and

Figure 10 shows a heat-resistant baffle of the sound absorber shown in Figures 9A-9C.

Like parts are numbered alike in the Figures.

Figure 1 shows a device 1 according to the invention, wherein the most essential parts of the device according to the invention are schematically illustrated. Device 1 comprises a combustion chamber 2, which is bounded by a double-walled cylinder 3. Double-walled cylinder

3 extends substantially vertically and is suspended in a metal frame 4.

Cylinder 3 is at its bottom side closed by a metal plate 5, through which tubes 6, 7 extend. Tubes 7 form steam supply channels and are connected to pipes present in double-walled cylinder 3 (see Figure 2). A cylindrical drum 8 is disposed parallel to plate 5, at some distance therefrom, to which drum a gas supply pipe 9 is connected.

At one side facing plate 5 drum 8 is provided with a plate 10 comprising a plurality of air supply channels 12 opening into drum 8 and extending therethrough. The tubes 6 in plate 5 are disposed co-axially above gas supply channels 11. The space between plates 5, 10 constitutes a air/gas mixing chamber 13. Double-walled cylinder 3 comprises two concentrically disposed tubes 14, 15, which are interconnected at their bottom sides and at their upper sides by an annular plate 16. Tube 14 forms the inner wall of combustion chamber 2. Combustion chamber 2 is open at an upward side, and opens into an expansion space 17. A sound absorber as shown in Figures

9A-9C and Figure 10 is provided above expansion space 17.

Figure 2 shows a longitudinal section of the device 1 shown in Figure 1, from which certain parts have been left of for the sake of clarity. A plurality of substantially vertically extending pipes 18 are present in the space between tubes 14, 15 of double-walled cylinder 3, said pipes being open at one upper side and being connected to a U- shaped pipe 19 at a bottom side facing away from said upper side, which pipe 19 is connected to a tube 7A. An annular partition plate 20 divides the space within the double-walled cylinder into a first space 17 and a second space 21 present above said first space. Second space 21 is at an upper side connected to a pipe 22 extending along the outside of double- walled cylinder 3, said pipe at the bottom side of double-walled cylinder being connected to substantially vertically extending pipes 23, which open into tubes 7b. Device 1 is furthermore provided with a water supply pipe 24, which comprises a pipe 25 extending along the outside of double-walled cylinder 3 and two pipes 26 disposed substantially horizontally within cylindrical spaces 17, 21. Device 1 is furthermore provided with a water spray supply pipe 27, which comprises a vertical pipe 28 extending into combustion chamber 2 and a substantially horizontally extending pipe 29. Horizontally extending pipes 26, 29 are provided with a plurality of water passages 30. Device 1 is furthermore provided with a water pipe 31, by means of which water can be sprayed under pressure into combustion chamber 2.

Tube 14 is provided with a bellows-shaped part 32, which functions to absorb expansion differences between tubes 14, 15. Figure 3 is a view of a part of the bottom side of the device shown in Figure 1, showing drum 8 and plate 5 positioned above said drum. For the sake of clarity only a single tube 11 and a single tube 6 are shown. As can be seen in Figure 3, drum 8 comprises four gas supply pipes 9a-9d, whereby gas supply pipe 9d is positioned behind gas supply pipe 9b. Each gas supply pipe 9a-9d opens into a gas supply chamber 40a-40d associated therewith. Air supply channels 12 extend through gas supply chambers 40a-40d. Gas supply chambers 40a-40d are provided with a plurality of vertically extending gas supply tubes 11, which taper off near plate 5. Plate 5 is provided with air/gas mixture supply tubes 6, which taper off at a side facing the drum and extend coaxially to the gas supply tubes 11. The opposite conical parts 41, 42 of tubes 11, 6 constitute a venturi, whereby gas flowing through tubes 11 at a relatively high velocity and

pressure draws along air from air/gas mixture chamber 13 and takes it into combustion chamber 2 through tubes 6. Air/gas mixture chamber 13 is supplied with fresh air through air supply channels 12.

Figure 4 clearly shows the venturi made up of the conical parts 41, 42 of tubes 11 and 6 respectively.

Figure 5 shows a cross-section of the drum 8 shown in Figure 3. A plurality of radially extending partition plates 43 divide drum 8 into a plurality of gas supply chambers or sections 40a-40d, with empty sections 44 present therebetween. Figure 6 shows a bottom side of the device 1 shown in

Figure 2. Pipes 22 branch into a plurality of vertically extending pipes 23 associated with various gas supply chambers or sections 40a-40d. In the device shown in Figure 6 each section 40a-40d comprises four U-shaped pipes 19 and steam supply tubes 7a connected thereto. Figure 7 shows a detail of the device 1 shown in Figure

2, which illustrates the connection of pipe 22 to the space 21 between tubes 14, 15.

Figure 8 shows a detail of a bottom side of a device according to the invention, which illustrates U-shaped pipe 19 and steam supply tube 7a connecting thereto.

Figures 9A-9C show various views of a sound absorber 5 according to the invention. Sound absorber 50a comprises two parallel side walls 51 and baffles 52 extending therebetween. Side walls 51 are provided with parabolic openings 53. Baffles 52 include an angle oι with a completely open bottom face 54 of sound absorber 50. Combustion gases flow through bottom face 54 into sound absorber 50 in the direction indicated by arrow PI, after which the combustion gases are deflected by baffles 52, in the direction indicated by arrows P2, after which the combustion gases exit sound absorber 50 along side faces 55 and upper face 56. Sound absorber 50 comprises side walls 57 near side faces 55, said side walls being spaced from baffles 52. The combustion gases can also exit from sound absorber 50 through parallel opening 53. Baffles 52 comprise a frame 60 built up of U-like sections, stiffening plates 61 and a stainless steel, heat-resistant mesh (seen Figure 10). Baffles 52 muffle the sound from the combustion gases.

The surface area of the surface 54 forming the inlet opening is considerably smaller than the surface area of the surfaces 55,

56, 51 forming the outlet opening, as a result of which the velocity and the temperature of the combustion gases flowing through the sound absorber are considerably reduced.

The operation of the device 1 according to the invention is as follows. Water is pumped into spaces 17, 21 of double-walled cylinder 3 via water pipes 24, 25, 26, up to a predetermined level therein. The device is provided with sensors and a regulator, by means of which the water levels within spaces 17, 21 can be sensed and regulated. Then gas is supplied to drum 8 via gas supply channels 9a-9d, and supplied to the combustion chamber via gas supply chambers 40a-40d, tubes 11 and tubes 6 positioned thereabove. When the gas flows through tubes 6, the gas flow draws the air present in air/gas mixing chamber 13 along into the combustion chamber. The air/gas mixture is ignited by means of an automatic igniter (not shown), whereby a torch-like flame is formed. The heat of the flame will heat the water present inside the cylinder wall and form steam near the upper side of spaces 17, 21. When a predetermined period has elapsed, a predetermined cylinder wall temperature or a predetermined steam pressure is reached, water is sprayed into the flame via pipes 31, 27, 28, 29, which water will be evaporated by the heat of the flame and rise to the upper side of the combustion chamber in the form of steam. Immediately thereafter the gas flow is increased considerably, whereby an overpressure is generated near the central axis of the combustion chamber. An underpressure is generated near the wall of the combustion chamber, which underpressure draws the steam present at the upper side of the combustion chamber downwards. This block of steam formed at the upper side of the combustion chamber forces the flame in downward direction, as it were, thereby reducing the size of the flame. The supply of water via pipes 31, 27, 28, 29 is stopped right after that. At the same time a sufficient amount of steam has been generated in spaces 17, 21, which steam is forced to the steam supply tubes 7a, 7b via pipes 18, 19, 22, 23, and which is blown against the lower side of the flame. As a result of the presence of the steam the flame will retain the size imposed by the block of steam. The steam supplied to combustion chamber 2 through steam supply tubes 7 will limit the size of the flame to maximally the height of combustion chamber 2. Furthermore it has become apparent that the steam being supplied leads to an increased turbulence and expansion of the air/gas mixture being supplied, thus improving combustion. The

combustion gases exit combustion chamber 2 at the upper side and expand in expansion space 17. Then the combustion gases exit the device 1 via sound absorber 50, whereby a further reduction of the velocity of the combustion gas is obtained as a result of the difference in surface area between the inlet opening and the outlet opening of sound absorber 50. It has become apparent that when drum 8 is divided into a plurality of separated gas supply chambers 40a-40d, the gas pressure in the combustion chamber is smaller, given an unchanged gas flow, than when a single gas supply chamber is used. Furthermore it has become apparent that it is advantageous when the steam supply tubes 7 located near the edge of combustion chamber 2 are smaller than the air/gas supply tubes 6 and the steam supply tubes 7 located near the centre are longer than said air/gas supply tubes 6. In one embodiment of a device 1 according to the invention the total height of the device amounted to about 15 m, with 75,000 m ³ of gas being burned per hour. The gas is forced into combustion chamber 2 through tubes 6 at a velocity of about 800 km/h. The temperature at the bottom side of the sound absorber was about 500 - 800 °C. By causing the combustion gas flow to expand within the sound absorber the exit velocity had been reduced considerably, and also the exit temperature had been reduced to 300 - 400 °C.

It is possible to vary the distance between the plate 5 comprising the air/gas mixture tubes 6 and the plate 10 comprising the gas supply tubes 11. Varying this distance makes it possible to change the amount of air drawn along by the gas flow and adjust it to a desired air/gas ratio.

Heat-conducting baffles may be disposed between tubes 14, 15, by means of which baffles the temperature of the inner tube 14 can be transferred more quickly to the water present between tubes 14, 15.