POROUS MEDIA BURNER FOR LOW CALORIFIC VALUE FUEL GASES

Abstract

A method and burner for combusting fuels having a heating value of <80 BTU/scf (<2.98 MJ/Nm3) in which the fuel premixed with air is injected into a porous inert media so that the fuel is combusted with the air. Oxygen may be optionally injected into the porous inert media separately from the premixed air/fuel.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

Background

1. Field of the Invention

The present invention relates to combustion of fuels having low calorific values of <50 BTU/scf (<1.863 MJ/Nm³) that are derived from reject streams from landfill gas or digester gas purification processes.

2. Related Art

Traditionally, fossil fuels such as natural gas, coal and petroleum have been used in industrial furnaces and boilers as energy sources. Several well-established combustion technologies (whether it is combusted with oxygen or air) are commercially available for burning such fuels. In some industrial processes, large amounts of by-product gases are generated that contain calorific values typically lower than the traditional fuels. While most of the by-product gases are utilized in the same process or plant, a significant portion is still flared without any energy recovery. Low calorific value (LCV) fuels having lower heating values (LHV) of less than 188 BTU/Nft³ (less than 7 MJ/Nm³) are often available from various industrial processes such as landfills or bio-gas plants. Reed, et al. reported typical LHVs for various LCV fuels which are listed below in Table I (Reed, R. J. “Combustion Handbook”, North American Mfg. Co., Third Edition, Vol. 1, 2001.).

TABLE I
Composition and lower heating values of LCV fuels
	Composition	LHV
Gas	CH4	C2H6	C3H8	CO	H2	CO2	O2	N2	H2O	BTU/scf	MJ/Nm3
BFG	—	—	—	22.7	2.3	19.3	0.7	—	—	80	2.98
COG	28.3	3.4	0.2	4.2	50.6	0.9	1.6	—	—	475	17.7
LG	45-65	—	—	0-1	0-1	34-55	0-5	0-1	sat.	240-550	9-20
BGP	1.84	—	—	2.1	21	12	—	43	—	190	7.07
BFG: blast furnace gas
COG: coke oven gas
LG: landfill gas
BGP: Batelle gasification process
sat.: saturated

One of ordinary skill in the art will recognize that the above LHV values may be converted to Wobbe Index values by the well known equation:

$\mathrm{Wobbe}\mathrm{Index}=\frac{\mathrm{LHV}}{\surd {\rho }_{\mathrm{re}}}$

Due to the presence of large amounts of diluents in the fuel composition, LCV fuels present certain combustion challenges. These fuels are prone to poor ignition and low flame stability. They are also susceptible to flame extinction or blow-off. In an effort to improve flammability, these fuels are sometimes blended with conventional fuels (called “sweetening”). These fuels may also be swirled, injected in a cyclone burner and/or preheated.

The flammability limits are characteristics of the fuel and depend on mixture composition and temperature. Determination of flammability limits depends on measurement of residence time and heat loss. Flammability limits of mixtures of fuels and diluents are estimated using le Chatlier rule as given below:

$x_{\mathrm{fm}}=\frac{100}{\frac{p1}{x_{f1}}+\frac{p2}{x_{f2}}+\frac{p3}{x_{f2}}}$

where:

xfm: the flammability of the mixture in vol %

xfi: the flammability of the submixture of component i with a diluents

pi: the vol % of the submixture of component i in the mixture

Thus, it is seen that a combination of lower flammability and higher vol % of one of the components can substantially lower the flammability of the mixture.

Fuels with LHV <50 BTU/Scf (<1.863 MJ/Nm³)—also called as ultra low BTU or ULBTU fuels—are extremely difficult to satisfactorily burn them with conventional burners. ULBTU fuel, which is often a reject stream from an industrial process, is therefore typically flared by blending it with a conventional fuel. One example of such a reject stream is disclosed by U.S. Pat. No. 7,025,803. After pre-treatment via filtration and adsorption, a landfill gas is separated into CO₂-rich and CO₂-deficient streams by one or more gas separation membrane stages. The CO₂-rich stream may or may not be used to regenerate a PSA adsorbent bed. Because of the difficulty of combusting the CO₂-rich stream (with or without being used as a regeneration gas for a PSA adsorbent bed), that stream is instead treated in a thermal oxidizer.

Catalytic combustors can be used to burn ULBTU fuels, however, catalyst degeneration, poisoning, and regeneration is a major concern. Thermal oxidation is a technology that can be used to oxidize ULBTU fuels, but thermal oxidizer units are complex and expensive.

Several burners have been developed that can be used with LCV fuels. Niska, et al. of MEFOS and Linde developed a blast furnace gas oxy-fuel burner for steel reheating furnaces. This burner (the “S3 burner”) is based on the REBOX® flameless technology and uses an optional booster fuel. While the burner yielded lower NOx and CO emissions, the authors concluded that, for high productivity, propane boosting should be employed.

U.S. Pat. No. 7,448,218, U.S. Pat. No. 5,433,600, U.S. Pat. No. 5,447,427, and US 2004/0175663 A1 disclose other examples of burners developed for LCV fuels.

Catalytic combustors are typically employed for combustion of ULBTU fuels. For example, U.S. Pat. No. 6,393,821 discloses a process in which gaseous fuel from natural evolution associated with rotting of materials (e.g. landfills, gas digesters, livestock refuse) is mixed with air, compressed, pre-heated, and catalytically combusted. The combustion products are directed to a turbine to produce electricity. Methane concentrations as low as 1% by volume can be combusted in a catalytic combustor. As discussed above, catalyst degeneration/poisoning and regeneration are major concerns.

Similarly, US 2007/0098604 discloses a catalytic reactor for reacting a fuel rich mixture of a low-BTU fuel and air, without an excessive pressure drop. The reactor includes an assembly of catalyst-coated tubes having an exit area that is at least 50% larger than a close-packed assembly. A fuel-rich mixture of fuel and air is in contact with the catalyst coating on the outside surface of the tubes. Cooling air flows inside the tubes and gets pre-heated. The catalytically reacted fuel-air mixture meets with the preheated air at the exit of the tubes and achieves complete combustion. This combustor aids the combustion of low-BTU fuels having lower flame temperatures by catalytically reacting a portion of the fuel and completing the rest of the combustion non-catalytically. As discussed above, catalyst degeneration/poisoning and regeneration are major concerns.

Oxygen enhancement of LCV fuel combustion is less common. US 20110195366 discloses a method for combustion of a low-grade fuel (a mixture of low-grade and high-grade fuels with a LHV≦7.5 MJ/Nm³) using an existing air burner. The burner supplies the fuel from an opening that normally supplies air and oxidant from an opening that normally supplies fuel. Both openings open out into a combustion zone downstream. The oxidant is preferably 95% oxygen by weight. While combustion of LCV fuels having an LHV of less than or equal to 7.5 MJ/Nm³ are disclosed, there is no disclosure of combustion of LCV fuels having an LHV of <80 BTU/scf (1.863 MJ/Nm³).

Thus, there is a need for methods and burners for combustion of fuels having low calorific values of <80 BTU/scf (<2.98 MJ/Nm³) without requiring additional high calorific value fuels and without requiring catalysts requiring replacement or regeneration.

SUMMARY

There is provided a burner for combusting fuels having a heating value of <80 BTU/scf (<2.98 MJ/Nm³), comprising: an inner tube having open first and second ends; an outer tube concentrically disposed around the inner tube and having a closed first end and a second end, the inner and outer tubes defining annular space between them for allowing a flow of premixed air/fuel therethrough; and a porous inert media comprising metal or ceramic and having first and second ends that is disposed within the inner tube adjacent the inner tube first end. The annular space has a second end that is closed by a face that extends between the second ends of the tubes. The face includes one or more openings for allowing a flow or flows of air and fuel. The outer tube first end extends past the inner tube first end.

There is also disclosed a method for combusting fuels having a heating value of <80 BTU/scf (<2.98 MJ/Nm³) that includes the following steps. Air is mixed with a gaseous fuel having a heating value of <80 BTU/scf (<2.98 MJ/Nm³) to provide a flow of premixed air/fuel. The flow of premixed air/fuel is directed through an annular space defined by inner and outer tubes of a burner. The flow of premixed air/fuel is injected into a porous inert media disposed within the inner tube. The fuel is combusted with the air to produce a flow of combustion products within a downstream portion of the inner tube in a direction opposite that of the flow of premixed air/fuel. Heat is exchanged between the flows of combustion products and premixed air/fuel across the inner tube. The burner and/or method may include one or more of the following aspects:

• • an oxygen lance is included having first and second ends and extending through a centrally disposed aperture formed in the closed first end of the outer tube so that the lance second end is disposed adjacent the first end of the media, wherein a space is defined by the outer tube first end, an outer surface of the lance, and an inner surface of the outer tube adjacent its first end.
  • A screening material is disposed directly between the lance second end and the media first end.
  • The lance second end abuts directly against the media first end.
  • A flow of oxygen is injected into the porous inert media separate from the flow of premixed air/fuel for assisting the combustion of the fuel with the air.
  • The fuel is a reject stream from a landfill gas or digester gas purification process.
  • The oxygen is at least 95% pure oxygen.
  • A flame resulting from combustion of the fuel with the air and oxygen is stabilized within the media.
  • A flame resulting from combustion of the fuel with the air and oxygen is stabilized at a downstream face of the media.
  • The fuel has a heating value of <50 BTU/Nft³ (<1.86 MJ/Nm³).
  • The fuel is a reject stream from a landfill gas or digester gas purification process.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a cross-sectional elevation view of an exemplary burner for use in the invention.

FIG. 2 is a top plan view of the burner of FIG. 1.

FIG. 3 is a cross-sectional elevation view of the burner of FIG. 1 taken along line 3-3.

FIG. 4 is a cross-sectional elevation view of the burner of FIG. 1 taken along line 4-4.

FIG. 5 is a cross-sectional elevation view of the burner of FIG. 1 taken along line 5-5.

FIG. 6 is a bottom plan view of the burner of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solution proposed here is based on stabilizing a flame in or on an inert porous matrix created by combustion of LCV fuels having a LHV of <80 BTU/scf (<2.98 MJ/Nm³) or even as low as <50 BTU/Nft³ (<1.86 MJ/Nm³). Unlike conventional burners, there is no free flame. The presence of the inert porous media establishes heat feedback from the hot combustion products so that the reactants may be preheated. This increases the flame velocity, improves flame stability, and therefore, allows combustion of LCV gases that are harder to burn with traditional burners. Oxygen may be optionally injected into the inert porous media in order to further enhance the ignitability of the fuel and provide a higher flame temperature in comparison to one produced from combustion of only the premixed fuel and air. While the air and fuel are premixed, for safety reasons the oxygen and the premixed air/fuel mixture are separately supplied to the inert porous media.

The inert porous media is disposed within an upstream end of an inner tube of the burner. In order to inject the optional oxygen separately from the air/fuel mixture, it is injected from a lance directly into an upstream face of the porous inert media. The open downstream end of the lance typically directly abuts against the upstream face. However, one of ordinary skill in the art will recognize that the downstream end of the lance may be separated from the upstream face by a screen that is designed to support the media within the burner.

Different types of media are known in the field of porous media burners. Typically, the media is a metallic or ceramic foam. One example for such media is open-cell silicon carbide coated carbon foam manufactured by Ultramet (Pacoima, Calif.). Engineered foams made from yttrium-stabilized zirconia can also be used for this purpose. There can be one or more layers of porous media each having a same or different porosity and number of pores per inch. The porous media layers can be stacked on top of one another or separated with a gap.

A combustion chamber is formed by the downstream end of the burner inner tube. In other words, the combustion chamber is defined by the downstream face of the inert porous media at one end, by the open downstream end of the inner tube at the other end, and on all sides by the downstream portion of the inner tube.

The combustion chamber of the burner is integrated with a flow path of the premixed air/fuel so that heat from combustion may be used to preheat the premixed air/fuel before it is introduced into the inert porous media. This is done in counter-flow fashion so that as the premixed air/fuel flows in a direction opposite that of the combustion products. This is accomplished with the provision of an outer tube concentrically disposed around the inner tube. The downstream end of an annular space between the inner and outer tubes receives a flow of the premixed air/fuel. The flow continues upstream through this annular space, which is of course counter to the downstream direction of the flow of combustion products on the inside of the inner tube. The outer tube extends farther upstream of the inner tube and has a closed upstream end. On the other hand, the upstream end of the inner tube is open. Thus, the flow of premixed air/fuel exits the annular space, continues radially inward and then is introduced into the upstream face of the inert porous media.

The fuel is then combusted with the air (of the air/fuel mixture) and, if selected, the optional oxygen (injected directly into the inert porous media from the open end of the oxygen lance). While the flame can be stabilized on the surface of the porous inert media, greater heat exchange between the products of combustion and the premixed air/fuel may be realized when the flame is stabilized within the media. Thus, combustion is initiated either within the media itself or within the combustion chamber just downstream of the downstream face of the media. In order to stabilize the flame in the desired location, the pressure of the premixed air/fuel and/or the pressure of the optional oxygen may be increased so as to increase the velocity of those gases through the media.

The stability of the flame may also be enhanced by varying the allocation of oxidant between the air (of the premixed air/fuel) and the oxygen. Typically, as much as 5% of the oxidant bill may be supplied by the oxygen with the balance supplied by the air in the premixed air/fuel. One of ordinary skill in the art will recognize that the amount of air within the premixed air/fuel may be varied by increasing or decreasing the pressure of the air to be mixed with the fuel, and optionally conversely, by decreasing or increasing the pressure of the fuel to be mixed with the air. Such a one will further recognize that devices for mixing air and gaseous fuel are well known in the art and their details need not be repeated herein. In any case, no particular device for mixing the air and fuel is essential to the invention. Indeed, it is within the scope of the invention to separately feed air and fuel to the annular space and allow them to be mixed therein and/or inside the porous inert media. Typically, the air and fuel are mixed by injecting or aspirating one of the two into a flow of the other of the two. The oxygen is industrially pure oxygen that may be sourced from any type of oxygen production technology used in the industrial gas business, including but not limited to:

• • an air separation unit that cryogenically separates air gases into predominantly oxygen and nitrogen streams in which case the gaseous oxygen has a concentration exceeding 99% vol/vol,
  • vaporization of liquid oxygen which was liquefied from oxygen from an air separation unit, in which case it, too, has a purity exceeding 99% vol/vol, or
  • produced by a vacuum swing adsorption (VSA) unit in which case it typically has a purity of about 92-93% vol/vol.

The gaseous fuel has a LHV of <50 BTU/Nft³ (<1.863 MJ/Nm³). While the invention is not limited to any particular source of fuel, examples of such gaseous fuels include certain low calorific value blast furnace gases and off-gases from carbon black plants. One additional type of gaseous fuel is a low-methane reject stream from a landfill gas or digester gas purification process. These processes use one or more gas purification techniques, such as gas separation membranes or pressure swing adsorption (PSA) that produce a low-methane content stream typically having a high CO2 and/or N2 content . A particular example is the reject stream disclosed in U.S. Pat. No. 7,025,803 that is otherwise ordinarily sent to a thermal oxidizer (i.e., stream 22). These fuels share a common attribute in that their calorific content is too low for separating the high calorific components from the low calorific components. It is important to note that the gaseous fuel does not include any supplementary or boosting fuel and that in practice of the invention, no other fuel is combusted with the gaseous fuel in order to supplement or boost its calorific value.

Raw low calorific value fuels typically contain significant amount of impurities (such as sulfur, H₂S, NH₃, particulates, etc.) which need to be cleaned to some level before being utilized in energy recovery. For instance, blast furnace gas exits the furnace with a dust loading of 18 to 34 g/m³ due to its passage through coke, iron ore and limestone. As another example, biogas typically contains ammonia and hydrogen sulfide. Well known techniques for gas cleaning include: i) low temperature cleaning and ii) high temperature cleaning. In low temperature cleaning method, the raw LCV gas is typically cooled in a water-scrubber and desulfurized to remove particulates and alkali metals and also to reduce sulfur and ammonia concentration.

A typical embodiment of a burner for use in the invention (including injection of oxygen) will now be described.

As best illustrated in FIG. 1, the burner includes an inner tube 11 concentrically surrounded by an outer tube 13 that define between them an annular space 15. The downstream end of the annular space 15 is closed by a downstream face 17 of the burner. A premixed flow of air/fuel is received into the annular space 15 via air/fuel inlet conduit 19.

The outer tube 13 extends further upstream than does the inner tube 11 so that an inner wall of the furthest upstream portion of the outer tube defines a space 21. The upstream face 23 of the burner is also closed so as to direct the flow of air/fuel from the annular space 15 radially inward through the space 21. An oxygen lance 25 is disposed co-axial with the inner and outer tubes 11, 13 and includes a downstream end 27 that abuts against an upstream face 29 of a porous inert media 31. A flame resulting from combustion of the fuel with the air and oxygen oxidants may be stabilized within the porous inert media itself 31 or stabilized on a downstream face 33 of the media 31. The products of combustion (N₂, CO₂, H₂O) flow in the upstream to downstream direction through the combustion chamber 35 and out of the burner.

In operation, the premixed air/fuel is introduced into the annular space 15 via the conduit 19. Alternatively, separate conduits may separately feed the fuel and the air into the annular space. The flow of premixed air/fuel within the annular space 15 flows in a direction opposite that of the combustion products within the combustion chamber 35. The flow of premixed air/fuel is directed radially inwardly at the upstream face 23 of the burner, through a space 21, and is introduced into the porous inert media 31 via its upstream face 29. A flow of oxygen is injected into a central portion of the inert porous media 31 directly from the downstream end 27 of the lance 25. As discussed above, directly injected does not exclude the possibility of including a screen or similar feature in between the downstream end 27 and the upstream face 29. The fuel is combusted with the air and oxygen oxidants with the flame stabilized either within the media 31 itself, or on the downstream face 33 of the media 31. The products of combustion flow in an upstream to downstream direction within the combustion chamber 35 and out the burner.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims.

Claims

1. A burner for combusting fuels having a heating value of <80 BTU/scf (<2.98 MJ/Nm3), comprising:

an inner tube having open first and second ends;

an outer tube concentrically disposed around the inner tube and having a closed first end and a second end, the inner and outer tubes defining annular space between them for allowing a flow of premixed air/fuel therethrough, wherein: the annular space has a second end that is closed by a face that extends between the second ends of the tubes, the face includes one or more openings for allowing a flow or flows of air and fuel, the outer tube first end extends past the inner tube first end; and

a porous inert media comprising metal or ceramic and having first and second ends that is disposed within the inner tube adjacent the inner tube first end.

2. The burner of claim 1, further comprising an oxygen lance having first and second ends and extending through a centrally disposed aperture formed in the closed first end of the outer tube so that the lance second end is disposed adjacent the first end of the media, wherein a space is defined by the outer tube first end, an outer surface of the lance, and an inner surface of the outer tube adjacent its first end.

3. The burner of claim 1, further comprising a screening material disposed directly between the lance second end and the media first end.

4. The burner of claim 1, wherein the lance second end abuts directly against the media first end.

5. A method for combusting fuels having a heating value of <80 BTU/scf (<2.98 MJ/Nm3), comprising the steps of:

mixing air with a gaseous fuel having a heating value of <80 BTU/scf (<2.98 MJ/Nm3) to provide a flow of premixed air/fuel;

directing the flow of premixed air/fuel through an annular space defined by inner and outer tubes of a burner;

injecting the flow of premixed air/fuel into a porous inert media disposed within the inner tube;

combusting the fuel with the air to produce a flow of combustion products within a downstream portion of the inner tube in a direction opposite that of the flow of premixed air/fuel; and

exchanging heat between the flows of combustion products and premixed air/fuel across the inner tube.

6. The method of claim 5, further comprising the step of injecting a flow of oxygen into the porous inert media separate from the flow of premixed air/fuel for assisting the combustion of the fuel with the air.

7. The method of claim 6, wherein the fuel is a reject stream from a landfill gas or digester gas purification process.

8. The method of claim 5, wherein the fuel is a reject stream from a landfill gas or digester gas purification process.

9. The method of claim 5, wherein the oxygen is at least 95% pure oxygen.

10. The method of claim 5, wherein a flame resulting from combustion of the fuel with the air and oxygen is stabilized within the media.

11. The method of claim 5, wherein a flame resulting from combustion of the fuel with the air and oxygen is stabilized at a downstream face of the media.

12. The method of claim 5, wherein the fuel has a heating value of <50 BTU/Nft3 (<1.86 MJ/Nm3).

13. The method of claim 12, wherein the fuel is a reject stream from a landfill gas or digester gas purification process.