Method and system for catalytic gasification of liquid fuels

Abstract

The present invention is a method and a system for the gasification of a liquid fuel and includes providing a supply of a liquid fuel and an oxidant, atomizing the liquid fuel and mixing it with the oxidant, catalytically reacting the fuel oxidant mixture, and providing an ignition source for initiating the catalytic reaction. A hydrocarbon fuel can be mixed with oxygen, as a constituent of air, preferably forming a fuel rich fuel air mixture that passes through a catalytic reactor having an ultra-short channel length metal monolith substrate. The fuel air mixture is vaporized and partially oxidized.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method and system for the process of reactively converting a liquid fuel into a gasified stream. The method and system of the present invention also provide a means for quickly starting and operating the process at a variety of inlet conditions including ambient inlet conditions. More particularly, the method and system include the steps of atomizing a liquid fuel, mixing the atomized fuel with an oxidizer in fuel rich proportions, partially vaporizing the fuel and passing the resulting stream through a catalyst bed. The catalyst bed is suitable for supporting partial oxidation/reforming reactions at high space velocity and is integrated with an ignition source to initiate the reaction. This results in an integrated system comprised of a fuel injector/mixer/vaporizer/catalytic-reactor/igniter and represents a simplified fuel preparation/gasification and reformation approach eliminating the predisposition to coking, especially with liquid fuels.

The method of the present invention can be used in applications where gasification of long chain hydrocarbons is required or desired to improve performance. These pre-reforming reactors can be integrated directly with fuel cells, IC engines or burners For example, soot and coke are byproducts of long chain hydrocarbon combustion, such as in diesel engines. Soot is formed when liquid droplets of diesel fuel are combusted prior to vaporization. Application of this method will allow pre-vaporization and partial pre-reforming of diesel fuel prior to combustion. The diesel fuel is converted into a gaseous mixture of CO, H₂ and short chain hydrocarbons, the combustion of which does not produce coke and soot.

The integrated fuel injector/mixer/vaporizer/catalytic-reactor/igniter allows pre-reforming liquid hydrocarbon fuels without the need for external pre-heat or complete pre-vaporization. The reactor permits a simplified fuel injection, mixing and vaporization approach eliminating the predisposition to coking when using such fuels by taking advantage of a catalytic reaction and rapid mixing within the catalyst bed. One such catalytic reactor comprises catalytically coated, short-contact-time, ultra-short-channel-length substrates such as those disclosed in U.S. Pat. No. 5,051,241; the contents of which are incorporated herein by reference, particularly the teachings at Column 2, line 47 through Column 4, line 55. One such means is commercially available from Precision Combustion, Inc., as Microlith®. This technology provides the ability to impede soot formation kinetics, increase diffusive capabilities to boost mixing, and provides high surface area to augment vaporization.

2. Brief Description of the Related Art

Vaporization of liquid fuels (e.g., alcohols, hydrocarbons) typically is achieved by indirectly supplying heat into a stream of liquid fuel via heat exchange with a hot wall. One disadvantage of this method is that the rate of vaporization is limited by the rate of heat transfer such that relatively large surface area is required for fuel vaporization. Another disadvantage of this method, especially for vaporizing long chain hydrocarbons, is that heating the fuel stream to the vaporization temperature tends to cause fuel decomposition and formation of deposits. More specifically, coke formation is problematic. Moreover, preventing deposits from forming within the fuel passages in the nozzle during steady state operation due to heat-up of the nozzle from the downstream hot zone is challenging.

Another known method for gasification of a fuel stream comprises mixing atomized fuel with a hot gas such as superheated steam that supplies the heat required for fuel vaporization and prevents coke formation. However, the large amounts of superheated steam required in this method result in a large heat load for steam production.

Spray methods for atomization of liquid fuels known in the art include air-blast or pressure atomizers, ultrasonic and electrospray atomizers. These spray systems are capable of providing a uniform distribution of atomized fuel across the entrance of the catalyst bed. Such atomizers may include a co-flow of air that allows mixing of the fuel and oxidizer. However, very fine and uniform droplet size along with homogeneous fuel-air distribution, required to avoid coke formation and obtain temperature/mixture uniformity in the reactor, is difficult to achieve in practical systems.

Ignition devices, such as a spark or glow plugs, are widely used to ignite fuel-oxidizer mixtures at startup. These devices often are subject to failure due to the high operating temperatures by virtue of their location required for ignition.

Monoliths are commonly used catalyst substrates for the gasification of liquid fuel. Fuel oxidizer mixture inhomogeneities are usually detrimental to these substrates as they lead to localized lean or rich zones respectively causing hot spots or carbon precipitation regions. Since there is no opportunity for these zones to re-mix within the long, separated channels of a monolith, these substrates are particularly vulnerable. In addition, carbon precipitation is favored in monoliths due to the boundary layers that develop in these substrates.

Combustion of liquid fuels in fuel cell or internal combustion engine systems poses significant problems, especially for fuels with high aromatic content and wide boiling point distribution. This can be attributed to the propensity of the heavier aromatic compounds in the fuel to form deposits or coke when vaporized at high temperatures. Accordingly, there is a need for a pre-reforming reactor capable of operating with a range of liquid fuels. It is therefore an object of the current invention to provide a pre-reforming reactor for partially oxidizing and cracking the heavy components of the fuel. The pre-reformed fuel subsequently can be further reformed or combusted to power fuel cell systems, internal combustion engines, burners, and other known devices.

U.S. Pat. No. 4,381,187 to Sederquist (the “'187 Patent”) discloses a method in which a partially pre-vaporized fuel stream mixed with air, at an overall equivalence ratio greater than 3, is passed through a monolith catalytic structure thereby achieving gasification of the fuel in the stream. The '187 Patent requires mixing the fuel stream with the heated air stream and partial vaporization of the fuel prior to its introduction into the catalyst bed. Air temperature specified for the method is between 580 and 660° C. Thus, coking may occur. The method of the '187 Patent therefore requires supplying external heat for pre-heating the air. The method of the '187 Patent also requires the catalyst to be in a shape having wall surfaces extending in a downstream direction defining a plurality of parallel cells, for example, a conventional monolith. This configuration results in a comparatively low conversion rate of the reactants to the desired products. Moreover, in the method of the '187 Patent, the catalyst is chosen such as to initiate and sustain complete combustion, namely oxidation of part of the fuel to CO₂ and H₂O releasing heat. The '187 Patent discloses at column 1, line 49, that “once in vaporous form, fuel may be catalytically partially oxidized and reformed in an autothermal catalytic reactor.” Therefore, a separate reactor is required if H₂-rich gas stream is desired.

It is therefore another object of the current invention to provide a catalyst substrate that facilitates mixing of the stream flowing therethrough, for example a substrate having plurality of voids in random order and short channels extending in the downstream direction the length of which is similar to the channel diameter. Such a configuration results in a comparatively high conversion rate of the reactants to the desired products and minimizes break through of unreacted fuel. It also is an object of the current invention to provide a catalytic reactor for the gasification of liquid fuels comprising a catalyst that yields partial oxidation products, such as CO and H₂. This results in a higher level of fuel conversion for the same amount of added air and produces hydrogen-rich gas directly from the gasifier reactor. It is a further object of the current invention to provide a method whereby steam or atomized water and/or CO₂ may be added to the fuel/air stream to adjust the amount of hydrogen in the product stream. It also is a further object of the current invention to provide a method whereby no external pre-heating of either air or fuel is required.

The dependence of fuel conversion on oxygen-carbon ratio (O:C) is known to one skilled in the relevant art. Tests of a conventional gasifier comprising a catalytic reactor and prevaporized, premixed fuel and air inlet indicated a linear increase in fuel conversion with increasing air. With increased air, or a higher O:C, the catalyst temperature increased and higher fuel conversion can be achieved, though at the expense of higher heat release and higher catalyst temperatures. It thus is a further object of the current invention to provide a method whereby gasification of liquid fuels is achieved employing a fuel-rich fuel air mixture with an O:C ratio suitable for efficient fuel conversion.

DESCRIPTION OF THE INVENTION

Soot and coke can form when atomized long-chain hydrocarbon fuels are combusted. The present invention avoids coke formation by combining the elements of the two consecutive processes of liquid vaporization and catalytic reforming into one gasification process. The gasification process of the present invention limits or prevents coke formation by conversion of liquid fuel into gaseous fuel and/or conversion of liquid fuel into a hydrogen-rich stream. An example of the first application is combustion of liquid fuels and an example of the second application is fuel reforming.

A system according to the present invention comprises a cooled fuel injector, a short-contact-time, ultra-short-channel-length substrate catalytic reactor, and an ignition device (e.g. glow plug). The liquid fuel and air (atomizer air and secondary air) are injected into a chamber and the partially premixed fuel-air mixture introduced into the catalytic reactor. Another embodiment of the present invention comprises the use of a coiled, radial flow, short-contact-time, ultra-short-channel-length substrate catalytic reactor. The advantages of such a configuration are disclosed in U.S. patent application Ser. No. 10/324,464 filed Dec. 19, 2002, the contents of which are incorporated herein by reference, particularly the teachings at Paragraphs 0014-0031 including FIGS. 1 and 2.

In most applications, oxygen as a constituent of air is a preferred oxidizer. The ratio of the fuel stream to the oxidizer stream should be such that there is insufficient amount of oxidizer to completely oxidize all the fuel, namely, the ratio should be fuel rich. According to the method of the present invention, an air stream, into which the liquid fuel is atomized, may be at any temperature, either hotter or colder than ambient temperature. It was found that heat generated in the catalyst bed is sufficient to support fuel vaporization at the level required for stable oxidation reactions to proceed throughout the catalyst bed.

The catalyst bed comprises a catalyst suitable for supporting partial oxidation reactions. Preferably, the catalyst should be one of the metals of group VIII of the periodic system of elements. The substrate on which the catalyst is supported preferably provides good mixing for the fuel/oxidizer mixture passing therethrough. To provide good mixing capabilities, the substrate preferably comprises a multiplicity of void volumes in random order. This may be best achieved by using porous metal or ceramic substrates, or by using multiple ceramic or metal screens or foams.

It is preferred that the rate of flow of fuel and oxidizer into the catalyst bed is sufficiently high such that significant amount of partial oxidation products, namely CO and H₂, are formed. When partial oxidation products are formed, less heat is released resulting in lower catalyst bed temperatures. In another embodiment of the present invention, water in the form of liquid or steam may be added to the fuel/oxidizer stream entering the catalyst bed to help control the catalyst bed temperature, the degree of fuel conversion in the catalyst, and the exit mixture composition.

The method and system of the present invention provide gasification of liquid fuel without a requirement for supplying external heat or steam to the gasifier. Fuel and air may be supplied to the gasifier at ambient temperatures. This allows a smaller mixing volume, since the catalytic bed tolerates partial unmixedness, and a simpler fuel and air delivery system design. This also allows a means for start up and operation in the absence of heat at the reactor inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic representation of an embodiment of a gasification system according to the present invention.

FIG. 2 depicts a schematic representation of another embodiment of a gasification system according to the present invention.

FIG. 3 depicts a diagrammatic representation of a detailed design of a gasification system according to the present invention.

FIG. 4 provides a graphical representation of lightoff temperature versus time in the gasification system depicted in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

As depicted schematically in FIG. 1, a gasification system (10) according to the present invention comprises a path (12) defining a flow of air (14). Fuel stream (16) is introduced into injector (18), which atomizes fuel stream (16). Atomized fuel (20) and air (14) enter catalyst bed (22) where fuel (20) is additionally mixed, vaporized and partially reformed. Gasified fuel stream (24) leaves the catalyst bed (22). Ignition source (26), in close proximity with catalyst bed (22), is used to initiate the process.

FIG. 2 schematically depicts gasification system (110) according to the present invention. Functional elements corresponding to those depicted in FIG. 1 are referenced by corresponding 100-series reference numbers. In this embodiment, catalyst bed (122) defines a cylindrical shape and comprises a wound catalytically coated, short-contact-time, ultra-short-channel-length substrate. Atomized fuel (120) and airflow (114) enter into the inner diameter (128) of catalyst bed (122) and flow out radially (130) through catalyst bed (122). The igniter (126) in this embodiment comprises an electric glow plug (132) placed inside inner diameter (128) of catalyst bed (122). Glow plug (132) may be coated with catalyst (134) to further assist the start up process. Electric current initially is supplied to glow plug (132) to preheat catalyst bed (122) to the start up temperature. Fuel stream (116) is introduced into injector (118), and air (114) is then mixed with atomized fuel (120) causing catalyst bed (122) to heat up to the operating temperature at which point the electric current to the glow plug (132) is stopped. Gasified fuel stream (124) exits the system (110).

A gasification method according to the present invention provides at least partial reforming of the liquid fuel yielding hydrogen, carbon monoxide and short chain hydrocarbons, without a requirement for complete fuel pre-vaporization. Alternatively, such a gasification method also could be used for partially or completely reforming gaseous fuels. This improves fuel properties for many downstream applications, such as minimizing soot formation in fuel combustion or producing reformed fuels for use with fuel cells, such as solid oxide fuel cells. Water or steam and/or other components may be added to the fuel and air stream in other embodiments of the present invention to optimize the performance or the reforming reaction and to adjust the composition of the product mixture.

FIG. 3 provides a detailed design (200) of an embodiment of a gasification system (210) according to the present invention. In the embodiment shown in FIG. 3, a catalytic reactor (212) is provided comprising a short-contact-time, ultra-short-channel-length substrate wound into a radial flow coiled catalyst bed (214) with an inner diameter of 0.3″ and an outer diameter of 2″. The edges (216) of coiled catalyst bed (214) are sealed such as to prevent bypassing of the catalyst bed (214) by the reactants. The flow of reactants enters the inner plenum (218) of catalyst bed (214) and then flows through the bed in radial direction from the inside outward. The inlet fuel/air (and optionally steam) mixture enters inner plenum (218) of catalyst bed (214) from a first side (224). Electrical heating element (222), typically a commercially available glow plug, provides initial pre-heat during the start up of the reactor (212). Glow plug (222) is positioned inside the inner plenum (218) of catalyst bed (214) from a second or opposite side (226) of catalyst bed (214).

Liquid fuel (228) is atomized by fuel nozzle (230), for example an ultrasonic nozzle. Fuel nozzle (230) is fastened to mounting block (232) by any conventional means, for example mounting pins (234). Mounting block (232) can be cooled by any conventional means, for example cooling water passages (236) as shown in FIG. 3. Prior to entering the inner plenum (218) of catalyst bed (214), liquid fuel (228) is injected into air stream (238) provided by a supply source, for example air inlet conduit (240). Steam may be added to air stream (238) if operating in Auto-Thermal Reformer (ATR) mode. Accordingly, the gasification system described herein does not require pre-heat, complete pre-vaporization, or complete premixing of the fuel. Moreover, additional water/air-cooling of fuel nozzle (230) further reduces the coking tendency of the heavy hydrocarbon fuels that tends to occur over hot surfaces. The preferred amount of cooling is system-design dependent and provides sufficient cooling to keep the fuel nozzle temperature below the point where fuel may start to coke in the presence of radiative heating from the hot catalyst bed. During steady-state operation, a cooling mechanism for the glow plug may also be employed.

Continuing with FIG. 3, atomized fuel is introduced into a reduced mixing and fuel evaporation region. Liquid fuel (228) is metered into fuel nozzle (230) and atomized at the fuel nozzle horn (242). Air inlet plenum (244) is in fluid communication with air inlet conduit (240), and air stream (238) enters air inlet plenum (244) and is directed into mixing region (246). A secondary air source, ambient temperature air, is supplied around the fuel nozzle horn (242) for cooling. Glow plug (222) further comprises electrical power leads (254) and is positioned in close proximity to catalytic reactor (212) to enable rapid startup of gasification system (210).

Vaporized and partially oxidized fuel exits catalytic reactor (212) and passes through post reaction chamber (248), exhaust gas plenum (250) and passage (252). The embodiment shown in FIG. 3 also includes instrumentation egress ports (256).

Results from a startup test of gasification system (210) using liquid fuel JP-8 at fuel feed rate of 5 ml/min is shown in FIG. 4. The reactor components were at room temperature and no heat was provided to the reactor prior to start up. According to the present invention, the reactor was started by energizing the glow plug with a 24V DC source and less than 50 W_e. The fuel nozzle was powered and the fuel pump was started with a 1-2 second delay. The glow plug was shut down after the catalyst temperature started to rise. The tests indicated lightoff in about 5 to 10 seconds and the reactor reached steady-state operation in less than 30 seconds.

The preferred O:C ratio at the reactor inlet of a gasification system according to the present invention is fuel dependent. Using dodecane for fuel, gasification system (210) was varied by changing the amount of air provided at the reactor inlet while maintaining a fuel feed rate of 5 ml/min. Room temperature inlet air was used. In this embodiment, the gasified fuel is combusted in a downstream flame and achieving high fuel conversion is not required. Operating the gasifier at O:C between 0.5 and 0.7 is sufficient for obtaining a stable, soot free flame while providing efficient fuel conversion.

Although the invention has been described in considerable detail, it will be apparent that the invention is capable of numerous modifications and variations, apparent to those skilled in the art, without departing from the spirit and scope of the invention.

Claims

1. A method for gasification of a liquid fuel comprising: a. providing a supply of a liquid fuel; b. providing a supply of an oxidant; c. atomizing the liquid fuel; d. mixing the atomized fuel with the oxidant; e. catalytically reacting the fuel oxidant mixture; and f. providing an ignition source for initiating the catalytic reaction.

2. The method of claim 1 wherein the liquid fuel comprises hydrocarbons.

3. The method of claim 1 wherein the oxidant is a constituent of air.

4. The method of claim 1 comprising the additional step of introducing water to the fuel oxidant mixture.

5. The method of claim 4 wherein the water is introduced in the form of steam.

6. The method of claim 1 wherein the step of mixing the atomized fuel with the oxidant further comprises forming a fuel rich fuel oxidant mixture.

7. The method of claim 1 wherein the step of catalytically reacting the fuel oxidant mixture includes providing a catalytic reactor comprising an ultra-short channel length reactor substrate.

8. The method of claim 7 wherein the step of providing a catalytic reactor further comprises providing a plurality of the substrates thereby providing void volumes in random order.

9. The method of claim 7 wherein the step of providing a catalytic reactor further comprises providing a catalyst on at least a portion of the substrate, the catalyst comprising one of the metals of group VIII of the periodic system of elements.

10. The method of claim 7 wherein the substrate is provided in a coiled configuration.

11. The method of claim 10 wherein the substrate is mounted to avoid bypass of the substrate by the fuel oxidant mixture.

12. The method of claim 1 wherein the step of providing an ignition source for initiating the catalytic reaction further comprises providing a glow plug.

13. The method of claim 1 wherein the step of catalytically reacting the fuel oxidant mixture includes vaporizing and partially oxidizing the fuel oxidant mixture.

14. A system for gasification of a liquid fuel comprising: a. a supply of a liquid fuel; b. a supply of an oxidant; c. a fuel nozzle for atomizing the liquid fuel; d. a mixing region for mixing the atomized fuel with the oxidant; e. a catalytic reactor for catalytically reacting the fuel oxidant mixture; and f. an ignition source for initiating the catalytic reaction.

15. The system of claim 14 wherein the liquid fuel comprises hydrocarbons.

16. The system of claim 14 wherein the oxidant is a constituent of air.

17. The system of claim 14 further comprising a supply of water and introducing the water to the mixing region.

18. The system of claim 17 wherein the water is in the form of steam.

19. The system of claim 14 wherein the mixing region further comprises forming a fuel rich fuel oxidant mixture.

20. The system of claim 14 wherein the catalytic reactor comprises an ultra-short channel length reactor substrate.

21. The system of claim 20 wherein the catalytic reactor further comprises a plurality of the substrates thereby providing void volumes in random order.

22. The system of claim 20 wherein the catalytic reactor further comprises a catalyst on at least a portion of the substrate, the catalyst comprising one of the metals of group VIII of the periodic system of elements.

23. The system of claim 20 wherein the substrate defines a coiled configuration.

24. The system of claim 23 wherein the substrate is mounted to avoid bypass of the substrate by the fuel oxidant mixture.

25. The system of claim 14 wherein the ignition source comprises a glow plug.

26. The system of claim 14 further comprising a supply of coolant for cooling the fuel nozzle.

27. The system of claim 14 wherein the catalytic reactor for catalytically reacting the fuel oxidant mixture further comprises vaporizing and partially oxidizing the fuel oxidant mixture.

28. A system for gasification of a liquid fuel comprising: a. means for providing a supply of a liquid fuel; b. means for providing a supply of an oxidant; c. means for atomizing the liquid fuel; d. means for mixing the atomized fuel with the oxidant; e. means for catalytically reacting the fuel oxidant mixture; and f. means for an ignition source for initiating the catalytic reaction.

29. The system of claim 28 wherein the means for atomizing the liquid fuel comprises a fuel nozzle.

30. The system of claim 28 wherein the means for mixing the atomized fuel with the oxidant further comprises a means for forming a fuel rich fuel oxidant mixture.

31. The system of claim 28 wherein the means for catalytically reacting the fuel oxidant mixture comprises a catalytic reactor that comprises an ultra-short channel length reactor substrate.

32. The system of claim 31 wherein the catalytic reactor further comprises a plurality of the substrates thereby providing void volumes in random order.

33. The system of claim 31 wherein the catalytic reactor further comprises a catalyst on at least a portion of the substrate, the catalyst comprising one of the metals of group VIII of the periodic system of elements.

34. The system of claim 31 wherein the substrate defines a coiled configuration.

35. The system of claim 34 wherein the substrate is mounted to avoid bypass of the substrate by the fuel oxidant mixture.

36. The system of claim 28 wherein the means for an ignition source for initiating the catalytic reaction comprises a glow plug.

37. The system of claim 28 further comprising a means for cooling the means for atomizing the liquid fuel.