GB2306347A

GB2306347A - Flare assemblies

Info

Publication number: GB2306347A
Application number: GB9522338A
Authority: GB
Inventors: Brian Grady
Original assignee: Hi Lo Flare Systems & Services
Current assignee: Hi Lo Flare Systems & Services
Priority date: 1995-11-01
Filing date: 1995-11-01
Publication date: 1997-05-07
Anticipated expiration: 2015-11-01
Also published as: GB9522338D0; GB2306347B

Abstract

A flare assembly for the burning of gas produced by a land-fill site has a collection manifold 10 to which a collector for gas produced by the site is connected. A duct 12 leads from the manifold 10 to a flare tip 17 at which the collected gas is burnt, a fan 15 being arranged within the duct 12 to draw gas from the manifold 10 and an adjustable flow rate valve 16 being provided within the duct 12 to regulate the gas flow. A sensor 23 is provided in or adjacent the manifold 10 and controls the operation of the flow rate valve 16 dependent on the sensed pressure. An analysis unit 19 also controls the adjustment of the valve 16 by determining the proportions of at least CH 4 and O 2 in the collected gas but possibly also the proportion of CO 2 .

Description

FLARE ASSEMBLIES This invention relates to a flare assembly, in particular but not exclusively intended for use in connection with a flare system arranged to burn combustible gases produced from a land-fill site.

Once a land-fill site has been capped, usually there are produced various gases by the decomposing organic matter previously deposited in the site. The gases may be of a variable analysis, but in the main comprise methane (CH4), carbon dioxide (CO2) and nitrogen (N2). These gases are the result of the action of micro-organisms on the organic waste matter, leading to the biodegradation of that matter.

As the rate of gas generation within a capped land-fill site increases, there is a tendency for the gas to migrate towards the boundary areas of the cap and then escape into the atmosphere through the soil.

The escaping gas has at best a nuisance value, having regard to the usual smell thereof, but can be a significant fire or even explosion risk, in turn leading to the destruction of property or the loss of life. As a consequence, attempts are usually made at collecting the gas produced by capped land-fill sites and then burning the gas in a safe and controlled manner.

Known systems for burning land-fill gases are aimed primarily at eliminating the methane content of the gas, but this can produce undesirably large amounts of carbon monoxide (CO). Tests on existing sites have shown that as much as 3 or 4 tonnes of carbon monoxide can be produced in one day.

The present invention aims at providing a flare assembly for use for example to burn the gas produced by a land-fill site, which assembly is configured to ensure the optimum combustion of any methane in the gas being burned and also to maintain the production of carbon monoxide at the lowest practical level.

According to the present invention, there is provided a flare assembly for the burning of gas produced by a land-fill site, comprising a collection manifold to which means to collect gas produced by the land-fill site is connected, a flare tip for the burning of the collected gas, a duct leading from the manifold to the flare tip, the duct including fan means to draw gas from the manifold, adjustable flow control means to regulate the flow of gas along the duct to the flare tip, means to sense the gas pressure in or adjacent the manifold, and control means to effect adjustment of the flow control means dependent upon the sensed pressure.

In the past, gases collected from a land-fill site have simply been supplied to a flare assembly and burned there, without attempting to control the products of combustion so as to fall within acceptable limits. In the present invention, the combustion of the collected gases is optimised and this is achieved by sensing the pressure of the collected gases before those gases are supplied to the flare tip, and then controlling the pumping of the gases in order that optimum gas extraction is achieved. In turn, this assists the optimum combustion.

The flare tip itself usually is associated with an inlet orifice for combustion air, and it is preferred for at least one of said inlet orifice and an outlet from the flare tip for the combustible gases to be adjustable. In the case of the latter, the flare tip may have a circular opening through which the combustible gases may pass and an axially adjustable valve member in the form of a cone which cone may obstruct to a greater or lesser extent the circular opening. As the annular area of the opening increases, so the gas exit velocity will reduce, and vice versa, thus giving control over the gas turbulence for a given flow rate. Conversely, for a variable flow rate, the turbulence may be optimised.

The combustion of the gases may be further optimised by providing means to determine at least some of the constituent proportions of the collected gas, the control means being arranged to effect adjustment of the flow control means and at least one of the flare tip and the inlet orifice for air, dependent upon the sensed proportions. In this case, the determining means advantageously determines the proportions of at least any methane and oxygen in the collected gas.

However, the determining means may also determine the proportion of carbon dioxide in the collected gas, the proportion being taken into account as well, in adjusting the combustible gas flow rate. These measures ensure that the land-fill site is not overdrawn, thereby pulling atmospheric oxygen into the site, and that the site is not underdrawn, thereby allowing outward migration of the gas to occur. If the inlet manifold pressure is maintained at a pre-set level, variations in the gas producing characteristics of the field can be detected and allowed for, by adjusting the adjustable flow control means.

The adjustable flow control means may comprise an adjustable flow control valve arranged in said duct.

In this case, the flow control valve preferably is located downstream of the fan means.

Further to assist the optimum combustion of the gases drawn from the site, the temperature of combustion, or of the exhaust gases from the flare tip may be monitored, or in the alternative the composition of the exhaust gases may be determined, that composition in turn being indicative of whether the combustible gases have been burned at the correct temperature. The control means can make adjustments to the flare tip, in order either to raise or lower the temperature of combustion. This may be achieved by opening or closing the combustion air inlets, by changing the gas exit velocity at the flare tip, or by a combination of these measures.

There may be advantages in monitoring the operation of the system over an extended period of time. For example, the combustion conditions may be recorded in a suitable memory module, the contents of which can be downloaded into a computer so that trends can be recorded over extended periods of time. Such operations may be performed remotely, using conventional telemetry technology, so allowing the offsite control and operation of a flare system.

By incorporating in an overall system the various preferred aspects of this invention, the combustion of gases produced from a land-fill site may be optimised so that the noxious emissions are reduced to a minimum and the principal products of combustion are only carbon dioxide and water vapour.

By way of example only, one specific embodiment of the present invention will now be described in detail, reference being made to the accompanying drawings in which: Figure 1 is a flow diagram showing the component parts of a flare system arranged in accordance with the present invention; Figure 2 is a plan view on a land-fill flare assembly arranged to operate as shown in Figure 1; and Figure 3 is a side view of the land-fill flare assembly of Figure 2.

Referring initially to Figure 1, there is shown an inlet manifold 10 for combustible gases drawn from a capped land-fill site (not shown). Pipes from the site are connected to inlets 11 to the manifold 10, so that the gases may accumulate within the manifold. A pipe 12 connects the manifold 10 to a knock-out pot 13 (a gas de-watering and coarse filtration device) and the outlet from that pot 13 passes through a flame arrester 14 to the inlet of a motor driven centrifugal fan 15.

The outlet from the fan passes through an adjustable flow control valve 16 and is led to a flare tip 17 whereat the combustible gases are burned. A shroud 18 surrounds the flare tip.

An analysis unit 19 has one inlet connected to a duct leading from the valve 16 to the flare tip, to sense the combustible gas composition. A second inlet is connected to sensor 20 mounted in the vicinity of the flare tip 17, to sense the products of combustion produced by the burning of the gases. The analysis unit is arranged to detect the presence of methane, oxygen and carbon dioxide in the gases collecting with the inlet manifold 10, and carbon monoxide, carbon dioxide and oxides of nitrogen in the exhaust gases.

Outputs from the analysis unit 19 are supplied to a controller, typically in the form of a PLC 21.

Waste gas from the analysis unit 19 is fed back to the flare tip shroud 18, along pipe 22.

Other sensors associated with the system described above include a pressure sensor 23 for gas pressure in pipe 12, temperature, flow rate and pressure sensors 24, 25 and 26, for the gases in the pipe leading to the flare tip 17, and a temperature sensor 27 for the combustion temperature within the shroud 18. All of these sensors provide outputs connected to the PLC 21.

The fan 15 is driven by an electric motor 28, switch unit 29 controlling the supply of power to the motor. The switch unit 29 may in turn be controlled by the PLC 21. Also, the setting of the adjustable flow control valve 16 is under the control of the PLC 21, by adjustment unit 30.

The flare tip 17 is adjustable, to permit the velocity of the gases leaving the tip to be adjusted to suit the combustion conditions. This is performed by a tip controller 31, again operated by the PLC 21.

Initial combustion of the gases at the flare tip is performed by a pilot 32 supplied with combustible gas from a cylinder 33 through a solenoid valve 34, or alternatively by a slipstream arrangement of landfill gas through pipe 36.

When the combustion of gases is to be started, the valve 34 is opened by the PLC 21 if the landfill gas is not sufficient to ignite, and a pilot igniter 35 is operated to light the pilot 32.

Referring now to Figures 2 and 3, there is shown a practical embodiment of a flare assembly which operates on the principles illustrated in Figure 1. In the drawings of this flare assembly, the same reference numbers are used as have been used in connection with Figure 1, to denote like parts.

The apparatus is constructed as an integral unit, ready for installation at a land-fill site. Pipes are installed below the capped site to lead biogas generated by the degrading organic matter to the manifold 10, the pipes being connected appropriately to the inlets 11. The fan 15 draws the gas from the manifold 10 through pipe 12, the K/O pot 13 and through the flame arrester 14, and supplies the gas to the flare tip 17, through the control valve 16. As can be seen in Figures 2 and 3, the flare tip is disposed within the shroud 18 and the lower end of the shroud is fitted with volume control dampers 40, arranged so as to allow the supply of combustion air for the burning of the biogas. The exhaust gases are sampled by pipe 41 taking a small portion of the overall exhaust back to the analysis unit 19.

As can be seen in Figures 2 and 3, the flare tip includes an outer casing 42 defining a circular central opening 43. A conical element 44 is mounted point-down within the opening, such that raising and lowering of the conical element 44 alters the effective gas flow area, through the tip. In this way, the velocity and so also the turbulence of that gas can be controlled so as to fall within appropriate values, having regard to the volume flow. The turbulent gas then mixes with air drawn through the volume control dampers 40 for combustion within the shroud.

In operation, the incoming biogas is analysed, to determine the methane content along with any oxygen and carbon dioxide content and the rate of flow of that gas to the flare tip is controlled so as to minimise the oxygen content. The conditions within the shroud are also optimised by control of the flare tip, to ensure the most effective burning of the biogas, to minimise the production of gases other than water vapour and carbon dioxide. In particular, the carbon monoxide content of the exhaust gas is determined and the operating conditions adjusted to ensure the carbon monoxide content is minimised.

Claims

1. A flare assembly for the burning of gas produced by a land-fill site, comprising a collection manifold to which means to collect gas produced by the land-fill site is connected, a flare tip for the burning of the collected gas, a duct leading from the manifold to the flare tip, the duct including fan means to draw gas from the manifold, adjustable flow control means to regulate the flow of gas along the duct to the flare tip, means to sense the gas pressure in or adjacent the manifold, and control means to effect adjustment of the flow control means dependent upon the sensed pressure.

2. A flare assembly as claimed in claim 1, wherein the flare tip is associated with an inlet orifice for combustion air, and at least one of an outlet opening from the flare tip and said inlet orifice is adjustable.

3. A flare assembly as claimed in claim 2, wherein means are provided to determine at least some of the constituent proportions of the collected gas, and the control means effects adjustment of the flow control means and at least one of an outlet opening from the flare tip and said inlet orifice dependent upon the determined proportions.

4. A flare assembly as claimed in claim 3, wherein the determining means determines the proportions of at least CH4 and 2 in the collected gas.

5. A flare assembly as claimed in claim 4, wherein the determining means further determines the proportion of CO2 in the collected gas.

6. A flare assembly as claimed in any of claims 2 to 5, wherein means are provided to determine at least some of the constituent proportions of the products of combustion of the collected gas, and the control means effects adjustment of the flow control means and at least one of the outlet opening from the flare tip and said inlet orifice dependent upon the determined constituent proportions.

7. A flare assembly as claimed in claim 6, wherein the determining means determines the proportion of at least the carbon monoxide content of said products- of combustion.

8. A flare assembly as claimed in claim 7, wherein the determining means further determines the proportions of carbon dioxide and oxides of nitrogen in said products of combustion.

9. A flare assembly as claimed in any of the preceding claims, wherein the adjustable flow control means comprises an adjustable flow control valve arranged in the duct.

10. A flare assembly as claimed in claim 9, wherein the valve is located downstream of the fan means.

11. A flare assembly as claimed in any of the preceding claims, wherein the flare tip defines a circular outlet opening and there is mounted a conical valve member within said outlet opening which conical valve member is adjustable axially of the opening thereby to adjust the effective gas outlet opening from the flare tip.

12. A flare assembly as claimed in any of the preceding claims, wherein at least one of the temperature, flow rate and pressure of the collected gas flow to the flare tip is monitored and the control means adjusts the combustion conditions dependent thereon.

13. A flare assembly as claim in any of the preceding claims, wherein the temperature of the combustion of the collected gases is determined and the control means adjusts the combustion conditions dependent thereon.

14. A flare assembly as claimed in claim 1 and substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

15. A method of burning gas produced by a land-fill site whenever employing a flare assembly according to any of claims 1 to 14.