Apparatus and method for flaring waste gas

Abstract

 The invention describes a combustion system for the flaring of waste gas. The system is comprising a combustion device (105), a heat exchanger (110) and a stack (115). The combustion device (105) is comprising a waste gas feed pipe (120), a support gas feed pipe (125), an air feed system (130), a mixing chamber (135) for mixing air with waste gas and/ or with support gas, and a gas permeable combustion surface (140) onto which the waste gas will be burnt after the premix has flown through it, thereby producing flue gas. The stack (115) connects the combustion device (105) to the heat exchanger (110), thereby creating flue gas flow from the combustion device (105) into the heat exchanger (110). The heat exchanger (110) comprises channels for the flue gas and for at least one fluid to be heated.
The invention also describes a method for use of such a combustion system.

Description

Technical Field

[0001] The invention relates to the technical field of waste gas combustion (also known as flaring), where it is essential that low emissions are achieved. An apparatus is provided for the combustion of waste gas with minimum emissions and recuperation of heat from the flue gas; as well as a method for using such systems.

Background Art

[0002] Flare systems are widely used for combustion of waste gas, e.g. in the biogas, tank park, waste water treatment, chemical or petrochemical industries. Such flaring systems exist that mix the waste gas with air, and where the mixture of waste gas and air flows through a porous or perforated burner membrane and the waste gas is burnt at the outside of the porous or perforated burner membrane. Examples of such flaring systems can be found in NL1011009, in WO2006/010693 and in WO2008/055829. It is a benefit of such flaring systems that low emissions of volatile organic components (VOC) can be reached. Legislation is imposing ever more stringent emission limits.

[0003] It is a disadvantage of such flaring systems that flue gas is disposed of in the environment at high temperature, e.g. at 1100 - 1200 °C as a lot of energy is blown and lost into the environment. It is desirable to recuperate the energy of the flue gas. However, it is a problem that heat recuperation systems have to withstand difficult and demanding conditions because of the high temperature of the flue gas and because of the likely corrosive composition (e.g. because of the presence of aggressive acids, e.g. hydrogen sulphide) of the flue gas due to the composition of the waste gas, which can even be inconsistent in composition over time, reducing the lifetime of the heat recuperation systems.

Disclosure of Invention

[0004] A first objective of the invention is to provide a long lifetime flaring system for combustion of waste gas with low emissions and which is incorporating a heat exchanger for the recuperation of heat from the flue. The flaring system is comprising a porous or perforated burner membrane (for minimizing emissions of VOC) for combustion of a premix of waste gas and air.

[0005] A second objective of the invention is to provide a method of operation of a flaring system provided with a heat exchanger for recuperation of heat of the flue gas, in a way that ensures long lifetime of the complete system.

[0006] According to a first aspect of the invention a combustion system for the flaring of waste gas is provided. The combustion system is comprising a combustion device, a heat exchanger and a stack. The combustion device is comprising

• a waste gas feed pipe,
• a support gas feed pipe,
• an air feed system,
• a mixing chamber for mixing air with waste gas and/ or with support gas, thereby creating a premix,
• and a gas permeable combustion surface onto which the waste gas will be burnt after the premix of combustible gas and air has flown through it, thereby producing flue gas.

The stack is connecting the combustion device to the heat exchanger, as a result, when the combustion system is in operation, the flue gas will flow from the combustion device into the heat exchanger. Preferably, the stack is insulated in order to prevent loss of heat (e.g. via radiation) to the environment. In the heat exchanger, channels are provided for the flue gas and for at least one fluid to be heated by heat provided by the flue gas. For example, the stack can have a length between 2 and 3 meter.

[0007] Preferably the heat exchanger is a plate heat exchanger. A preferred steel alloy for the construction of the heat exchanger is 316Ti (UNS S31635 or DIN/EN 1.4571) which is a high temperature resistant stainless steel grade.

[0008] Preferably, the heat exchanger is a plate heat exchanger that is comprising plates, connection blocks and a housing. Preferably, the inlet and outlet for the fluid to the heat exchanger are connected via one or more connection blocks. The housing is connected to a frame structure (e.g. via welding). The plates of said plate heat exchanger are hanging in a housing. The plates are connected (e.g. via welding) to the connection blocks. Preferably, the plates are connected (e.g. via welding) to the connection blocks over a length of less than 25% of the length of the plate. The connection blocks are supported by the frame structure, in which preferably no connection is made between the connection blocks and the frame structure, but the connection blocks (and hence the plate heat exchanger) are via gravity forces supported by the frame structure.
It is a benefit of this embodiment that thermal expansion of the plates is possible without danger of damage to the heat exchanger which is a synergetic contribution to the lifetime of the combustion system. Even more preferable, the plates in between which flue gas is flowing, are not connected to each other. Preferably, the distance between the gas permeable combustion surface and the entrance of the flue gas into the heat exchanger is in the range of 1.5 to 2 meter. It is possible to position part of the heat exchanger inside the stack. These embodiments contribute to a longer lifetime of the combustion system.

[0009] In a preferred embodiment, the stack is comprising one or more hatches. The one or more hatches can be opened and closed by means of a control system, e.g. by means of an electrical or pneumatic driven hydraulic jack.

[0010] Preferably, the combined surface of the hatches is between 20% and 200% (and preferably between 50% and 150%; more preferably between 75% and 135%) of the surface area of the gas permeable combustion surface. Most preferred however is when the surface of the hatches is between 90% and 110% of the surface of the gas permeable combustion surface.

[0011] When the hatches are opened, flue gas will flow out of the hatches and will not (or only for a small part) enter the heat exchanger.

[0012] In a further embodiment of the invention, at the outside of the stack and above the location of the one or more hatches, one or more deflectors are provided at the outside of the stack. When the one or more hatches are opened the deflectors are deflecting the flue gas away from the combustion system. It is a benefit of the deflectors that they limit the contact of the flue gas flowing through the opened hatches, with outer parts of the combustion system (e.g. with outer parts of the heat exchanger). This way, the deflectors prevent damage of parts of the heat exchanger.

[0013] In a further preferred embodiment of the invention, the stack is provided with means for the stack to be able to slide vertically, thereby sliding in part over the heat exchanger. Preferably, the vertical movement is done without any horizontal movement. Preferably, the stack can slide over a distance of at least 200 mm, even more preferably of at least 250 mm. It is a benefit of this embodiment of the invention that free access to the combustion device can be created for maintenance of the combustion device, e.g. for replacing parts of it, e.g. the gas permeable combustion surface.

[0014] In a specific embodiment, insulation material is fixed to the inside of the stack. An example of a suitable insulation material is ceramic fibres. The insulation material is located between the inside of the stack and the housing in which the plate heat exchanger is fitted. The insulation material is not fitted (not connected) to the housing. The stack (with the insulation material fixed to it) can slide over a certain distance over the housing in which the plate heat exchanged is fitted.

[0015] The burner membrane can be a woven or knitted metal fibre membrane as known from WO97/04152 or WO2004/092647; or a sintered, possibly perforated, metal fibre membrane as e.g. known from WO93/18342 or a needled metal fibre membrane as e.g. known from EP982541, thereby creating low emissions. In a preferred embodiment, the gas permeable combustion surface is comprising stainless steel or ceramic fibres. Preferred stainless steel grades for stainless steel fibres of the gas permeably combustion surface are Fe Cr Alloys, as e.g. covered by DIN 1.4767.

[0016] According to a second aspect of the invention a method is provided for operating a combustion system for the flaring of waste gas. In the method, a combustion system is used as described in the first aspect of the invention. At start-up of combustion in the combustion device, support gas is introduced in the combustion device instead of waste gas. Examples of support gas that can be used in the invention are methane, natural gas, propane and LPG (liquefied petroleum gas). Such support gas is having known caloric properties and is sufficiently free from substances that would create corrosive flue gas.

[0017] Preferred is when prior to supplying support gas, air is supplied into the combustion system in order to purge the complete combustion system.

[0018] Preferred is when combustion is switched over from support gas to waste gas after reaching the steady-state operational temperature of the flue gas.

[0019] In a more preferred embodiment of the method, combustion is switched over from support gas to waste gas once the operating conditions of the combustion system are such that no condensation of flue gas will occur in the heat exchanger after starting to burn waste gas.

[0020] Even more preferably, combustion is switched over from support gas to waste gas, once the conditions of the inlet temperature ( and even more preferred also of the volume flow) of the fluid entering the heat exchanger are such that no condensation of flue gas will occur in the heat exchanger after starting to burn waste gas. For example switching over from support gas to waste gas is performed when the inlet temperature of the fluid in the heat exchanger is higher than a specific temperature.

[0021] When starting combustion with a premix comprising air and support gas, preferably the supply of support gas or the air volume is controlled (e.g. via a support gas modulating valve) in such a way to reach the operational temperature of the flue gas in the stack (e.g. a temperature between 1000°C and 1200 °C). The flue gas temperature in the stack can be measured by appropriate means, e.g. by means of one or more thermocouples. A control system is used to control the combustion parameters.

[0022] In a specific embodiment, the inlet temperature of the fluid (to which heat from the flue gas is to be transferred) in the heat exchanger is measured. Once the conditions are fulfilled that - taking the starting capacity (in Megawatt) of the combustion system into account, the volume flow and the inlet temperature of the fluid in the heat exchanger - no condensation of flue gas in the heat exchanger will occur after switching over to waste gas, (e.g. once the inlet temperature of the fluid in the heat exchanger is above a specific temperature dependent on the combustion system capacity and which can be calculated for each combustion system) waste gas is progressively supplied into the mixing chamber, and the supply of support gas is gradually reduced. The rates of gas flow (of support gas and of waste gas) are controlled by the control device, via measurement of the flue gas temperature. It is the objective to keep the flue gas temperature at its operational (steady state) level. This control is continued till no more support gas is entered in the mixing chamber (e.g. via closure of the support gas modulating valve) and the combustion device is fully operating on waste gas. Preferably, control is continued once 100% waste is combusted.

[0023] In a further preferred method, the combustion system is comprising a stack that is comprising one or more hatches. The hatches are open at the moment of starting combustion. When the hatches are open at the moment of starting combustion, the flue gas will flow out of the stack through the opened hatches, meaning that the flue gas will not (or only a small part of it) flow through the heat exchanger.

[0024] In a preferred method, the hatches are closed once the flue gas in the stack reaches a specified temperature, e.g. a temperature higher than 200°C, or temperature higher than 300°C or a temperature higher than 400°C.

[0025] It is a benefit of the hatches that they act as a safety device, protecting the combustion system (including the combustion device, the stack and the heat exchanger) against the negative effects of explosions that could occur at start-up of the system, e.g. when the ignition of the combustible gas is not instantaneous.

[0026] In a preferred method, the hatches are closed when the combustion system is not in operation. A preferred step when starting the combustion system, is starting the air supply (e.g. via the air supply fan of the combustion system) at high volume (and preferably at maximum volume) during a certain time. This operation purges the whole system (combustion device, stack as well as the heat exchanger). This can be done as a safety measure to remove combustible gas that could still be present in the system; such gas could lead to explosions in the combustion system if not removed properly. In a next step, the air supply can be set to a new value. The hatches are subsequently opened by means of the control system. Support gas is introduced in the combustion device and mixed with air in the mixing chamber. After the premix has flown through the gas permeable combustion surface it is burnt on the other side of the gas permeable combustion surface. To start the combustion. a spark generating electrode, a pilot burner or another suitable ignition device can be used. The flue gas is flowing through the stack. As long as the hatches are open, the preferential flow of the flue gas is through the opened hatches and not through the heat exchanger. The hatches are closed at a certain moment in time. In a preferred method, the hatches are closed once the flue gas in the stack reaches a certain temperature. One or more thermocouples (or other temperature measurement devices) in the stack can be used to measure the temperature of the flue gas. When reaching the pre-set temperature of the flue gas, the hatches are automatically closed via the control system of the combustion system. The pre-set temperature can e.g. be 200°C, or 300°C, or 400°C. Such a temperature has the benefit that flue gas is at a temperature below the temperature that could damage the outer parts of the combustion system when the hatches are closed.

[0027] Potential damage to the outside of the stack due to the temperature of the flue gas is further prevented in case deflectors are mounted on the outside of the stack, above the position of the hatch or hatches.

[0028] Preferably, when the hatches are closed, the combustion device is still burning support gas and not yet waste gas.

[0029] Preferably in steady-state operation of the combustion system, flue gas is entering the heat exchanger at a temperature of more than 1000°C, more preferably at more than 1100°C. Preferably, a control system is used to create stable combustion conditions, e.g. via controlling the air volume (e.g. via the speed of the air supply ventilator) that is supplied in the mixing chamber in order to keep the flue gas temperature constant at a preset level (e.g. a temperature between 1000°C and 1200 °C). Controlling the temperature of the flue gas is helpful to minimize emissions of the combustion system.

[0030] Preferably, in steady-state operation of the combustion system, flue gas is exiting said heat exchanger at a temperature below 500°C, preferably below 400 °C, more preferably below 250°C, even more preferably below 200°C, in order to have a high efficiency of recuperation of heat from the flue gas.

Brief Description of Figures in the Drawings

[0031] 

Figure 1 shows a combustion system according to the invention.

Figure 2 shows an alternative example of a combustion system according to the invention.

Figure 3 shows an example of a plate heat exchanger that can be used in a combustion system according to the invention.

Mode(s) for Carrying Out the Invention

[0032] Figure 1 shows a combustion system 100 according to the invention. The combustion system is comprising a combustion device 105, a heat exchanger 110 and a stack 115. The combustion device is comprising

• a waste gas feed pipe 120,
• a support gas feed pipe 125,
• an air feed pipe 130, whereby the air is supplied by means of a fan 132,
• a mixing chamber 135 for mixing air with waste gas and/ or with support gas, thereby creating a premix,
• and a gas permeable combustion surface 140 onto which the waste gas will be burnt after the premix of combustible gas and air has flown through it, thereby producing flue gas. The gas permeable combustion surface 140 can advantageously comprise a knitted or woven fabric made out of metal fibres, e.g. of steel grades according to DIN 1.4767.

The stack 115 is connecting the combustion device 105 to the heat exchanger 110, as a result, when the combustion system is in operation, the flue gas will flow from the combustion device 105 into the heat exchanger 110. The stack can e.g. have a length in the range of 2 to 3 meter. It is possible that the heat exchanger is partly inside the stack, the distance between the gas permeable combustion surface 140 and the entrance of the flue gas in the heat exchanger 110 can e.g. be in the range of 1.5 to 2 meter. In the heat exchanger 110, channels are provided for the flue gas and for at least one fluid 150 to be heated by heat provided by the flue gas.

[0033] It is possible that the stack 115 includes a diverting part 153, e.g. for connecting a cylindrical combustion device 105 to a beam shaped stack 115, when a beam shaped heat exchanger 110 is used.

[0034] One or more thermocouples 155 are used to monitor the flue gas temperature in the stack. The temperature measurement of the thermocouples 155 is used in the control of the combustion system. An ignition device 158 is used to ignite the premix gas when starting the combustion system. The ignition device 158 can e.g. be a spark generating electrode or a pilot burner.

[0035] Figure 2 shows an alternative example of a combustion system according to the invention. The combustion system is comprising a combustion device 105, a heat exchanger 110 and a stack 115. As in the example of figure 2 the combustion device 105 is cylindrical and the stack 115 and the heat exchanger 110 (which is supported by a frame structure 160, 162) are beam shaped, the stack 115 comprises a diverter part 153 to connect the cylindrical combustion device 105 to the beam shaped part of stack 115. The stack 115 is provided with a hatch 170 and a deflector 180 positioned above the hatch 170. In the example, the surface of the hatch is equal to the surface of the gas permeable combustion surface (not shown in figure 2). It is possible that the heat exchanger 110 starts already inside the stack 115, e.g. just above hatch 170. The stack 115 can slide vertically over a distance (and hence over part of the heat exchanger 110), till against the horizontal bars 162 of the frame structure 160, 162 which is supporting the heat exchanger 110. This way, access is provided to the combustion device 105, e.g. for replacing its gas permeable combustion surface.

[0036] In an example of the method of the invention, a combustion system is used as in figure 1. Thermal oil of a closed circuit is the fluid to which the flue gas will transfer its heat in the heat exchanger 110. In use, the flow of thermal oil is e.g. 17000 kg/h. When starting the combustion system, it is verified that thermal oil is flowing through the heat exchanger 110. After a purge of the combustion system with air via blowing of the air fan, the combustion device is started on propane gas which is used as support gas, wherein ignition device 158 is igniting the premixed gas and air flow. The supply of propane gas and air is controlled in order for the flue gas temperature (as detected by thermocouple 155) to reach the target temperature of e.g. 1100°C for clean combustion to occur and for the steady state capacity of e.g. 1.5 MW of the combustion device. Once the process conditions are such that the temperature of the thermal oil at the inlet of the heat exchanger 110 reaches 110°C, the waste gas valve is opened and waste gas is introduced in the combustion device. The control system will gradually decrease the supply of support gas and increase the supply of waste gas, till the supply valve of support gas is fully closed and the valve of the waste gas supply is fully open. After that moment is reached, the speed of the air fan 132 is modulated in order to maintain the flue gas temperature at the required temperature (e.g. 1100°C) as measured by thermocouple 155. If use of heat of the thermal oil is insufficient to cool the thermal oil sufficiently prior to its entrance in the heat exchanger, it is possible to use an air fan to create air flow over channels of the thermal oil, thereby ensuring that the temperature of the thermal oil is not higher than a specified temperature, e.g. 200°C.

[0037] Figure 3 shows a cross section of an example of a plate heat exchanger 300 that can be used in a combustion system according to the invention. The plate heat exchanger 300 is comprising a housing 311, which is connected via welding to a frame structure 365. The frame structure 365 is supporting the housing 311. The plate heat exchanger 300 is further comprising a stack of plates 313, an inlet 341 and an outlet 343 for fluid to be heated. The inlet 341 and outlet 343 are connected to the connection blocks 372, 373. The plates 313 are arranged in pairs of plates. A pair of plates is bonded to each other (e.g. by welding or by brazing) to close off the sides of the plates.

[0038] Fluid to be heated will flow between such two plates, e.g. in U-shape via a rib connection 355 in the middle of the pair of plates. A multiple of such pairs of plates is provided in the plate heat exchanger, through each pair, fluid to be heated is flowing. In between each two of said pairs of plates, flue gas is flowing (from the stack 315 into the plate heat exchanger in the direction of arrow 361, out of the plate heat direction in the direction of arrow 363). There is no bond between plates of neighboring pairs of plates. The plates 313 are free hanging in the housing 311. The plates 313 are connected (e.g. welded) to the connection blocks 372, 373 over a length L1 (e.g. 162 mm) of the total length L (e.g. 1441 mm) of the plates 313. The connection blocks 372, 373 are hanging on the frame structure 365, in this example without any connection, but supported by gravity.

[0039] Between the stack 315 and the housing 311 insulation material 392 is provided. Example of suitable insulation material 392 are ceramic fibres. The insulation material 392 is fixed to the stack 315, but not to the housing 311. Therefore, the stack 315 can slide over a certain distance upwards over the housing 311 (and hence over the plate heat exchanger 300).

[0040] The flue gas temperature at the inlet 361 of the heat exchanger is e.g. 1100°C; the flue gas temperature at the outlet 363 of the plate heat exchanger 110, 300 is e.g. 250°C.

[0041] The examples given are in no way to be understood as limiting the scope of the invention. Elements of different embodiments of the invention can be combined within the content of the invention.

Claims

1. Combustion system for the flaring of waste gas, comprising a combustion device, a heat exchanger and a stack;
wherein said combustion device is comprising a waste gas feed pipe, a support gas feed pipe, an air feed system, a mixing chamber for mixing air with waste gas and/ or with support gas, thereby creating a premix, and a gas permeable combustion surface onto which said waste gas will be burnt after the premix of waste gas and air has flown through it, thereby producing flue gas;
wherein said stack is connecting said combustion device to said heat exchanger, thereby creating that said flue gas will flow from said combustion device into said heat exchanger;
and wherein in said heat exchanger, channels are provided for said flue gas and for at least one fluid to be heated by heat provided by said flue gas.

2. Combustion system as in claim 1, wherein said heat exchanger is a plate heat exchanger that is comprising plates, connection blocks and a housing, wherein said housing is connected to a frame structure, wherein the plates of said plate heat exchanger are hanging in a housing, and wherein the plates are connected to said connection blocks and wherein said connection blocks are supported by said frame structure.

3. Combustion system as in any of the preceding claims, wherein said gas permeable combustion surface is comprising stainless steel fibres or ceramic fibres.

4. Combustion system as in any of the preceding claims, wherein said stack is provided with means for said stack to be able to slide vertically, thereby sliding in part over said heat exchanger.

5. Combustion system as in any of the preceding claims, wherein said stack is comprising one or more hatches and wherein said one or more hatches can be opened and closed by means of a control system.

6. Combustion system as in claim 5, wherein the combined surface of said hatches is between 20% and 200% of the surface of said gas permeable combustion surface.

7. Combustion system as in claim 5 or 6, wherein at the outside of said stack and above the location of said one or more hatches, one or more deflectors are provided.

8. Method for flaring waste gas,

- wherein a combustion system is used as in any of the preceding claims;

- wherein at start-up of combustion in said combustion device, support gas is introduced in said combustion device instead of waste gas.

9. Method as in claim 8, wherein combustion is switched over from support gas to waste gas after reaching the steady-state operational temperature of the flue gas.

10. Method as in claims 8-9, wherein combustion is switched over from support gas to waste gas once the operating conditions of the combustion system are such that no condensation of flue gas will occur in the heat exchanger after starting to burn waste gas.

11. Method as in claim 10, wherein combustion is switched over from support gas to waste gas, once the conditions of the inlet temperature of the fluid entering the heat exchanger are such that no condensation of flue gas will occur in the heat exchanger after starting to burn waste gas.

12. Method for flaring waste gas as in claim 8 - 11, wherein a combustion system is used as in claims 5 - 7; and wherein said hatches are open at the moment of starting combustion.

13. Method for flaring waste gas as in claim 12, wherein the hatches are closed once the flue gas in the stack reaches a specified temperature.

14. Method for flaring waste gas as in claims 8 - 13 wherein in steady-state operation of said combustion system, flue gas is entering said heat exchanger at a temperature of more than 1000°C.

15. Method for flaring waste gas as in claims 8 - 14, wherein in steady-state operation of said combustion system, flue gas is exiting said heat exchanger at a temperature below 500 °C, preferably below 400 °C, more preferably below 250 °C.

Drawing