WO1998058209A1

WO1998058209A1 - Landfill gas flare

Info

Publication number: WO1998058209A1
Application number: PCT/NZ1998/000085
Authority: WO
Inventors: John Payne Truscott
Original assignee: Envirowaste Services Limited
Priority date: 1997-06-16
Filing date: 1998-06-16
Publication date: 1998-12-23
Also published as: NZ502322A; AU7943598A

Abstract

The invention is directed to a waste gas burner apparatus particularly, but not solely, for use with waste gases emanating from landfills. The apparatus includes: a flare terminating in a waste gas outlet; a cowling about the outlet, the cowling having a substantially open top for the exhaust of waste gas and an open region below the outlet for the introduction of air; variable constriction means to vary the size of the outlet; and wherein the variable constriction means varies the velocity of the waste gas exiting the outlet thus varying the amount of air drawn into the cowling via the open region.

Description

LANDFILL GAS FLARE

FIELD OF THE INVENTION

The invention relates to the method and apparatus for the combustion of combustible gases, and in particular, although not necessarily solely, such combustible gases as would be produced by a landfill of waste products.

BACKGROUND OF THE INVENTION

Many landfills are provided with ducting throughout the landfill so that gases produced upon the composition of the waste material may be removed. The provision of such ducting facilitates both the reduction in the presence of such gases in the atmosphere as well as generally drawing a vacuum through the landfill to inhibit the build up of aromatic compounds.

The gases produced within a landfill can be drawn through the ducts and be provided for use in a variety of apparatus such as gas generators and the like. However, with the highly variable quantities and qualities of gases produced from landfills, there is regularly a need to burn off excess gas or, if no other use is made of the gas, the entire quantity of gas from the landfill will need to be burnt off. Current regulatory requirements for landfills require that the gas emissions from a landfill do not contain certain compounds above specified levels. Therefore, in order to establish a landfill and acquire the needed regulatory approval, the emitted gases need to be treated to remove such components to within acceptable levels.

Landfill gases are traditionally burnt off utilising flares. Such flares may be provided as a flare direct into the open air which utilises available oxygen in the surrounding atmosphere about the flame to bring the oxygen content of the gas mixture up to a sufficient level for combustion. This is the typical sort of flare seen in some petrochemical installations. One problem with such flares is that they are easily blown out by the wind and are less reliable in circumstances where the quality of the gas supplied to the flare is variable. To overcome some of these difficulties, enclosed flares which utilise a cowling about the region of combustion have been used. Although cowlings protect the region of combustion from the wind, they also restrict the access of oxygen into the combustion region. As a result, the temperature of combustion of the gas in the flare may not be adequate. Therefore, such known enclosed flares generally provide fans or other mechanical equipment to induce sufficient air into the chamber within the- cowling to ensure adequate combustion. The requirement for such mechanical equipment can greatly increase the cost of the overall apparatus. Furthermore, complex control equipment may be necessary to ensure sufficient oxygen is drawn into the cowling to achieve a suitable combustion level depending on the gas quantity and quality at the time.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a method and/or apparatus for the combustion of gas which overcomes some of these disadvantages or at least provides the public with a useful choice.

SUMMARY OF THE INVENTION

The invention may be seen to consist in a waste gas burner apparatus including:

a flare terminating in a waste gas outlet; a cowling about the outlet, the cowling having a substantially open top for the exhaust of waste gas and an open region below the outlet for the introduction of air; variable constriction means to vary the size of the outlet; and wherein the variable constriction means varies the velocity of the waste gas exiting the outlet thus varying the amount of air drawn into the cowling via the open region.

Preferably the variable constriction means includes a restrictive throat about the outlet from the flare, the throat including a movable member to vary the dimension of the outlet. Preferably the cowling provides a first chamber in which oxygen may be mixed with the waste gas and a second chamber above the first chamber in which further oxygen may be introduced to the waste gas stream.

Preferably the apparatus includes a heat sensor to measure the temperature of combustion and control means to control the velocity of the waste gas from the outlet such that the further oxygen drawn into the waste gas stream is controlled thereby maintaining the temperature of combustion of the waste gas.

Preferably the apparatus further includes a stack to contain the heat in the area of combustion.

Preferably the stack is formed from a combination of an inner layer of a ceramic material and an outer layer of aluminium.

Preferably the ceramic material is a ceramic fibre.

The invention may further be seen to consist in a waste gas burner apparatus including:

a flare terminating in an outlet for the waste gas; a cowling about the outlet, the cowling having a substantially open top for the exhaust of waste gas, an open region below the outlet for the introduction of air; a heat sensor to measure the temperature at which the waste gas is combusted; variable constriction means to vary the size of the outlet; and control means to control the velocity of waste gas from the outlet via the variable constriction means, such that the amount of air introduced into the waste gas may be altered to increase or decrease the temperature of combustion as measured by the heat sensor. In addition, the invention may also be seen to consist in a method of burning waste gas comprising: providing a flare terminating in an outlet for the waste gas to be combusted; providing a cowling about the outlet, the cowling having a substantially open top for the exhaust of combusted waste gas and an open region below the outlet for the introduction of air; and controlling the size of a constriction at the outlet to vary the velocity of waste gas exiting the flare thereby varying the air drawn into the cowling via the open region.

Preferably the method further includes the provision of a heat sensor and control means which operates to control the size of the constriction at the outlet in response to the heat of combustion of the waste gas as detected by the heat sensor.

Further aspects of this invention will become apparent to those skilled in the art upon reading the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the following drawings in which:

Figure 1 : Shows an end elevation of one embodiment of the invention;

Figure 2: shows a cross-sectional elevation through a portion of the apparatus of Figure 1 ;

Figure 3: shows a cross-sectional elevation through the portion of the apparatus of Figure 2 at a substantially orthogonal cross-section; Figure 4: shows plan view of a further embodiment of the invention;

Figure 5: shows an elevation of the apparatus of Figure 4; and

Figure 6: shows an elevation of a portion of the apparatus of

Figures 4 and 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, the invention can be seen to comprise a landfill gas flare 1 having a stack 2 extending upwards from a general gas burner assembly 3. The stack 2 is provided to ensure that the gases are contained and combusted at a suitable temperature for an extended period of time to ensure the destruction of sufficient combustible gas in the waste gas. Although this invention is generally described in reference to landfill waste gas, the invention may also be applied to other gas burners or gas flares for a variety of purposes such as use in the petrochemical industry or other industries where waste combustible gas is produced.

With reference to the combustion of landfill gas, the composition of this gas may typically have perhaps 60% methane together with carbon dioxide, nitrogen, oxygen and trace gases including H2S. The use of the flare seeks to convert the vast majority of the methane into carbon dioxide and reduce some of the other harmful products in the waste gas.

Various territories have regulations which govern on the temperature at which combustion must occur and the length of time for which this temperature must be held. In New Zealand this is 750°C for 0.5 seconds. This is intended to ensure that hazardous compounds contained in waste gases are suitably removed from the gas.

As the landfill gas does not generally carry sufficient oxygen for combustion itself, it is necessary to mix the gas with atmospheric oxygen to achieve combustion at a suitable level. The landfill gas is generally collected through ducting throughout the landfill and the use of vacuum pumps or similar to draw the gas from the landfill and supply it to the burner assembly 3 may be used as will be known to a skilled person. Although some gas flares operating without an outer ^~ stack may work under the passive pressure of the waste gas, enclosed flares such as the flare 1 generally operate from gas received under pressure. Some preprocessing of the gas may be necessary to remove excess condensation or similar material and the use of a condensation tank 4 prior to the burner assembly 3 may be common. 0

Referring to Figures 2 and 3, the burner assembly 3 can be seen in greater detail. The waste gas travels through the conduit 5 to the outlet 6 at which point the waste gas is outlet under pressure and at velocity. In this apparatus, the velocity may be increased through the use of a constriction 7 5 forming an angled throat about the outlet 6. The angle of this portion 7 can readily be determined through experiment as too great an angle at this point may create unwanted back pressure.

About the outlet 6 is a cowling 8 having an open top 9 for the _n exhausting of the flame and products of combustion. At the lower end of the cowling 8, an opening 10 is provided to allow air surrounding the flare 5 to be drawn up inside the cowling 8 and into a first mixing chamber region 1 1 .

The increased velocity attained by the gas as it exits the outlet 6 5 due to the constriction 7 creates a venturi effect, as will be known in the art, to draw greater quantities of atmospheric oxygen into the cowling 8, and thus into the gas stream, through the inlet 10.

In addition, a member 1 2 can be used within the throat 7 to provide 0 further control over the size of the outlet 6. As can be seen in Figures 2 and 3, the member 1 2 may comprise a substantially diamond or conical shaped portion so that axial movement of the member 1 2 with relation to the throat 7 causes a greater or lesser obstruction within the outlet 6. This member 12 may be used to restrict the outlet 6 and increase the velocity of the waste gas coming from the outlet 6 according to the restriction imposed. Although a variety of devices known to the art may be suitable for providing a variable constriction on the outlet 6, this preferred form utilises a member 1 2 moving within the outlet 6 to provide a substantially central obstruction about which the gas flow must pass. This facilitates axial movement of the member 1 2 and as can be seen in Figures 2 and 3, a handle 13 with an appropriate threaded portion 14 may allow the member

1 2 to be screwed into or out of the outlet 6 to vary the constriction in a manual manner. Of course, if desired, this may be driven by a suitable motor such as an electric or hydraulic drive to move the member 1 2 into and out of its position relative to the outlet 6.

A secondary means of varying the oxygen availability is provided with a further constriction 1 5. This constriction varies the air that can pass into the chamber 1 1 and be mixed with the gas from the outlet 6. As shown in Figures 2 and 3, although this may comprise a variety of means, this preferred form provides the constriction 1 5 as a disc shaped portion having an aperture at 1 6 for the passage of the waste gas from the outlet 6. The region 1 7 between the cowling 8 and the outer circumference of the disc 1 5 can be varied by relative axial movement of the disc 1 5 in relation to the angled portion of the cowling 8 adjacent the disc 1 5. It can be seen that raising this disc 1 5 will create a greater restriction to the air flow from the inlet 10 to the chamber 1 1 .

Referring particularly to Figure 3, it can be seen that an initial mixing step occurs in the gas and air mix may travel through the chute 1 8 to a second mixing chamber 1 9. Apertures 20 are provided, in this preferred form, in the base of the chamber 1 9 allow a final introduction of air to bring the overall mix to that necessary for combustion. An igniter 21 may then be provided to ignite the gas air mix in this region.

The use of the two stage mixing process allows the quantity of oxygen in the gas air mix to be held just below that necessary for combustion in the first mixing chamber and near the outlet 6. The final portion of air necessary to bring the mix up to that ready for combustion can be provided in the second mixing chamber 1 9 and, with the separation by the conduit 1 8, there is a reduced risk of the flame travelling back down towards the outlet 6. This is particularly important should the pressure of the gas at the outlet 6 drop which may encourage the flame to progress downwards.

Referring particularly to Figure 2, a UV sensor or similar sensor 22 may be provided. This sensor is to ensure detection of the flame emanating from the top 9 of the cowling 8 and is provided as a safety measure. Should no flame be detected, the unit may be controlled to automatically shutdown and stop the gas supply before going through a restart sequence after sufficient time has elapsed to ensure that unburnt gas has been flushed from the stack.

In a preferred form, and with reference to Figs 4 and 5, automatic control may also be provided on this apparatus. A heat sensor 23 may be provided in the stack 2 to ensure that any minimum temperature requirements set by regulation can be adhered to. Normally the heat sensor would be placed generally in the region of the hottest point of the flame such as at the expected tip of the flame. This heat sensor may be connected to suitable control means (not shown) in the overall apparatus to control the gas/air mix so that the temperature may be increased should it drop below a required level or decreased should it increase above a set level.

As shown in Fig 2 a second heat sensor 23a may be provided for safety reasons. This determines if the flame has travelled back into the system (i.e. "flashback") and will cause an automatic shutdown of the system if this occurs.

In operation, the heat sensor 23 senses the temperature of combustion which may vary in response to gas quantity or quality from the outlet 6. In response to a drop in temperature the control means may adjust the member 1 2 restricting the outlet 6 to increase the velocity of the gas flow from the outlet 6. An increase in velocity will increase the air drawn in through the inlet 10 and, consequently, increase the oxygen content of the gas air mix to increase the temperature of the combustion. By ensuring that such a minimum temperature is met and by knowing the height of the overall stack, sufficient temperature can be maintained for a sufficient period of time to substantially remove the methane or other components requiring destruction.

In addition, the second variable control over oxygen content being the disc 1 5 may also be either automatically controlled or manually controlled. It may be desirable to manually set the secondary control 1 5 for a particular site and then provide the fine control with the automatic control of the member 1 2. Alternatively, they may be used in conjunction with each other. Such variations will be well known to a person skilled in the art.

A further embodiment of the invention as is shown in Figures 4, 5 and 6 includes the use of multiple burner assemblies 3 inside a single stack 2. As shown in Figures 4, 5 and 6, seven burners are provided although this number may be altered as desired. In utilising multiple burners 3, the control means may also have control over the operation of the individual burners as described previously. For example, should the temperature measured by the sensor 23 reduce and operation of the member 1 2 (shown in Figs 2 and 3), which may operated simultaneously in all the burners or individually if desired, be insufficient to increase the temperature, the control means may shut one or more of the burners down to increase the gas pressure and volume through the remaining burners. This may assist in achieving the temperatures required for the operating conditions imposed by regulations.

The stack 2 operates to contain the heat in the area of combustion thus ensuring that combustion temperature is maintained for as long as possible. The stack can be formed from any materials as will be known in the art. However, the stack is preferably formed of aluminium and a ceramic material, preferably a ceramic fibre, to reduce cost and weight of the apparatus.

The ceramic fibre is fitted to the inside of the stack in such a way as to prevent heat from combustion reaching the aluminium in the outer part of the stack.

The ceramic fibre is installed in layers so that there is no direct path for the heat to reach the aluminium in the outer part of the stack. The number of layers depends on the temperatures required or expected to be reached.

The ceramic fibre is preferably held in place by ceramic cups bolted to the aluminium in the outer part of the stack and heat resistant wire looped from cup to cup.

Ceramic fibre of use can be any sort as used in furnaces and boiler stacks as will be known in the art.

Thus, it can be seen that the invention provides a landfill gas flare or other form of gas flare which provides for some variation on the oxygen flow through the apparatus in response to the gas flow. This response varies according to the venturi effect caused by the gas stream from the outlet 6 so that there may be a reduction in the cost of fans or other mechanical equipment to pump oxygen into the gas flare. Furthermore, some fine control may be provided through the use of the tongue or member 12 operating within the outlet 6 to provide a variable restriction to increase the gas velocity from the outlet 6. A second variation through the use of the disc 1 5 may also assist.

Example

A typical landfill gas composition is shown in Table 1 .

Table 1 - TYPICAL LANDFILL GAS COMPOSITION

Typical amounts of the components at the Greenmount landfill indicated in Table I but not shown, are shown in Tables 2 and 3.

Table 2 Volatile Hydrocarbon compounds: Levels in Landfill Gas (mg/rr_3)

Benzene 25.8 toluene 423.6 ethylbenzene 214.6 m + p xylene 374 o xylene 103.4

2 methyl nonane 51 .7 limonene 134.2

1 -ethyl-4-methyl benzene 41 .8 n-propyl benzene 58.7

1 ,2,3 trimethyl benzene 50.7

2-methyl dodecane 61 .7 isopropyl toluene 62.7

2,9-dimethyl decane 13.2

Table 3 Volatile Organochlorine compounds: Levels in Landfill Gas (mg/rτ.3)

as Cl- chloroform 4.0 3.6

1 ,2 dichloroethane 7.9 5.6 carbon tetrachloride 1 .6 1 .5 trichloroethylene 21 .1 17 tetrachloroethylene 35.3 30

Total Cl- 57.7

The following test shows the results of use of a waste gas flare, as shown in Figs 4-6, in connection with waste gases at the Greenmount Landfill having a typical component makeup as given in Tables 1 -3 above. The results are shown in Table 4. This shows two analysis runs in comparison with a system calibration run ("Mass in Field Blank Extract"). Test Methods

Sampling was conducted in accordance with Unilabs Method ^~ SM047.1 , based on USEPA SW-846 Method 0031 (Draft). Gas samples were withdrawn through a stainless steel probe. Volatile organic compounds (VOCs) were collected downstream on a set of sorbent traps, containing Anasorb-747 and Tenax-GC. The traps had been pre-spiked with a range of 1 c 2 - labelled standards prior to use.

Analysis of the traps was carried out by thermal desorption purge- and-trap gas chromatography/mass spectrometry (GC/MS) in accordance with SW-846 Method 5041 .

Tenax-GC and Anasorb-747 field blanks which accompanied the samples were also analysed, and each component reported as the total mass found in the combined resin extracts.

Discussion

In general, the above sampling and analytical procedures can be used to quantify VOCs that have a boiling point between -1 5 and 1 21 °C. The efficiency of sampling and analysis for the VOCs of interest has been supported by laboratory gaseous dynamic spiking studies with GC/MS analysis according to SW-846 Method 5041 .

These procedures are generally not applicable to polar, water soluble VOCs such as methyl ethyl ketone, and compounds with boiling points greater than 1 21 °C, including octane (b.p. 1 26°C), ethylbenzene (b.p. 1 36 °C), the xylenes (b.p. approx 140°C) and cyclohexanone (b.p. 1 56°C). Accordingly, results for these compounds are only semi-quantitative. Table 4 - RESULTS OF ANALYSIS

Notes:

< means less than. Value stated is the limit of detection. ^* Results are semi-quantitative (refer previously). As is readily apparent, the waste gas flare apparatus of the present invention provides an efficient alternative to known apparatus for disposing of waste gases. The results in Table 4 show levels of emission readily 5 acceptable to regulatory authorities in most developed nations, and achieve the intention of removing the hazardous components of the gas to acceptable levels prior to emission.

Where in the foregoing description reference has been made to 10 specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth.

Although this invention has been described by way of example and with reference to possible embodiments thereof it is to be understood that 1 5 modifications or improvements may be made thereto without departing from the scope or spirit of the invention as defined in the appended claims.

_ _ TJ:MW:TSPEC63264

20

File Ref: 501685/142

25

30

Claims

WE CLAIM:

1 . A waste gas burner apparatus including:

a flare terminating in a waste gas outlet; a cowling about the outlet, the cowling having a substantially open top for the exhaust of waste gas and an open region below the outlet for the introduction of air; variable constriction means to vary the size of the outlet; and - wherein the variable constriction means varies the velocity of the waste gas exiting the outlet thus varying the amount of air drawn into the cowling via the open region.

2. The apparatus of Claim 1 wherein the variable constriction means includes a restrictive throat about the outlet from the flare, the throat including a movable member to vary the dimension of the outlet.

The apparatus of Claim 1 or Claim 2 wherein the cowling provides a first chamber in which oxygen may be mixed with the waste gas and a second chamber above the first chamber in which further oxygen may be introduced to the waste gas stream.

4. The apparatus of any one of the previous claims wherein the apparatus includes a heat sensor to measure the temperature of combustion and control means to control the velocity of the waste gas from the outlet such that the further oxygen drawn into the waste gas stream is controlled thereby maintaining the temperature of combustion of the waste gas.

5. The apparatus in any one of Claims 1 to 4 further including a stack to contain the heat in the area of combustion.

6. The apparatus of Claim 5 wherein the stack is formed from a combination of an inner layer of a ceramic material and an outer layer of aluminium.

The apparatus of Claim 6 wherein the ceramic material is a ceramic fibre.

8. A waste gas burner apparatus including:

a flare terminating in an outlet for the waste gas; a cowling about the outlet, the cowling having a substantially open top for the exhaust of waste gas, an open region below the outlet for the introduction of air; a heat sensor to measure the temperature at which the waste gas is combusted; variable constriction means to vary the size of the outlet; and control means to control the velocity of waste gas from the outlet via the variable constriction means, such that the amount of air introduced into the waste gas may be altered to increase or decrease the temperature of combustion as measured by the heat sensor.

9. The apparatus of Claim 8 wherein the variable constriction means includes a restrictive throat about the outlet from the flare, the throat including a movable member to vary the dimension of the outlet.

10. The apparatus of Claim 8 or Claim 9 further including a stack to contain the heat in the area of combustion.

1 1 . The apparatus of Claim 10 wherein the stack is formed from a combination of an inner layer of a ceramic material and an outer layer of aluminium.

1 2. The apparatus of Claim 10 wherein the ceramic material is a ceramic fibre.

1 3. A method of burning waste gas comprising:

providing a flare terminating in an outlet for the waste gas to be combusted; providing a cowling about the outlet, the cowling having a substantially open top for the exhaust of combusted waste gas and an open region below the outlet for the introduction of air; and - controlling the size of a constriction at the outlet to vary the velocity of waste gas exiting the flare thereby varying the air drawn into the cowling via the open region.

14. The method of Claim 1 1 further including the provision of a heat sensor and control means which operates to control the size of the constriction at the outlet in response to the heat of combustion of the waste gas as detected by the heat sensor.

1 5. A waste gas burner apparatus substantially as herein described with particular reference to any one of the attached figures.