Producing Low Tar Gases in a Multi-Stage Gasifier
A system for gasifying solid matter uses multiple stages to produce low-tar combustible gas includes a first reactor having a fluidized bed to produce hydrogen containing gas, pyrolysis vapors, tars, and char particles at temperature less than the exit of the second reactor and a higher temperature partial oxidation combustor zones. A second reactor includes a higher temperature partial oxidation zone to activate hydrogen and cause cracking of aromatic ring compounds, a co-flow moving granular bed with a char gasification stage to catalyze tar reduction, and control char residence time, and a media screen comprising a parallel wire screen substantially vertically oriented supporting granular media.
This application claims the priority of provisional Application Ser. No. 61/397,765. The present invention relates, in general, to gasifying materials such as biomass and waste to produce high quality gas.
FIELD OF THE INVENTION Background“Tars are the Achilles heel of gasifiers, and many gasifier projects have failed because of insufficient attention to low tar production or efficient tar destruction”—Tom B. Reed (T. Milne 1998). The highly generic term “tar” was uniformly defined in 1998 (at the EU/IA/DOE conference in Brussels) as all organic contaminants of gasification that have a molecular weight larger than benzene. Several review articles have been published discussing the nature, formation and destruction of tar from biomass gasification. (Li 2009) (Han 2008) (T. Milne 1998).
A maturation process has been proposed for tar with temperature, progressing from mixed oxygenates (400° C.), to phenolic ethers (500° C.), then alkyl phenolics (600° C.), then heterocyclic ethers (700° C.), then polycyclic aromatics (800° C.), and then larger Polynuclear aromatic hydrocarbons (PAH), soot, and coke (900° C.). Elliot, D. C. “Relation of reaction time and temperature to chemical composition of pyrolysis oils.” Proceedings of the ACS Symposium Series 376, Pyrolysis Oils from Biomass. American Chemical Society, 1988. Polymerization and subsequent agglomeration of high molecular weight PAH is described as a homogeneous pathway to “soot” formation. Homann, K. H., Wagner, H. G., “Some new aspects of the mechanisms of carbon formation in premixed flames.” Eleventh International Symposium on Combustion. Pittsburgh: The Combustion Institute, 1967.
Several different classifications of tar have been established. These classifications are related to temperatures. Classification has been developed as follows: “primary tars” are vapors produced at lower temperatures and are the first evolved in thermal depolymerization of cellulose, hemicellulose, and lignin—these are mainly oxygenated compounds. Next, the secondary and tertiary reaction products of primary tars are termed “secondary tar” and “tertiary tar”. Tertiary tars were sub-classified as tertiary-alkyl and tertiary-polynuclear aromatic hydrocarbons (PAH). It is hypothesized that once tertiary tars are formed these may require even higher temperatures and additional residence time for thermal destruction.
There are several approaches to achieve adequate reduction of tar after an initial stage of gasification including thermal cracking, partial oxidation, and catalytic cracking using mineral catalysts or reforming with metal catalysts. One such method is indirect heat thermal cracking. This method has been discussed in the open literature to reduce tars in raw product gas. In the absence of char, a temperature of 900° C. is insufficient to achieve much tar destruction. Specifically, a slip stream was filtered at 450° C. to remove all char dust, but no measurable difference was found (after a fluid bed gasifier (8000 mg/Nm3 in feed gas from CFB operating at 825° C.) was filtered at 450° C. to remove all char dust). The application of a homogeneous phase reactor demonstrated only ˜25% reduction even with residence times as high as 12 seconds. Even at 1000° C. with 12 second residence time, only 75% reduction was achieved (˜2000 mg/Nm3 in product). (Houben, M. P. Analysis of tar removal in a partial oxidation burner. PhD Dissertation, Eindhoven: Technical University Eindhoven (Netherlands), 2004).
It is a common hypothesis that the minimal performance of the char-free thermal treatment at 1000° C. as compared to the downdraft gasifier at the same temperature suggests a catalytic role for char in tar reduction. It is possible that the nature of the fed tars (refractory tertiary tars present in fluid bed gas compared to primary or secondary tars in lower temperature pyrolysis gases) may also play a role in determining the thermal requirement for cracking. Even so, non-catalytic (homogenous phase) tar conversion to below 200 mg tar per Nm3 of gas is possible, starting with tar at 8000 mg/Nm3 by using ˜1150° C. for ˜4 seconds. (Houben 2004). It is also notable that indirect heating only below 1100° C. with short residence times (say 1075° for 2 seconds) initially increased the amount of 2+ ring polycyclic aromatics—quantifiable tars with two or more aromatic rings—but extended residence time mitigates this effect.
Partial Oxidation has been explored as an alternate method for achieving tar destruction. This method includes blast containing oxygen subsequently added to raw generated gas. The Energy Center of the Netherlands (ECN) performed experiments using an atmospheric circulating fluidized bed gasifier (operated at 850° C.) where air was added subsequently to increase the product gas temperature to 1100° or more. (Zwart, R. W. R. Gas Cleaning, downstream of biomass gasification status report. Public Report, Energy Center of the Netherlands (ECN), SenterNovem, 2009.) To achieve 100 mg tar/Nm3, a temperature of 1150° C. was required, resulting in a cold gas efficiency loss of 8%.
A custom low swirl number burner (swirl number less than 0.4) was employed by Houben to partially combust a relatively cool (20° and 200° C.) synthetic gas feed, and so the peak temperatures were also relatively low (less than 900° C.). This experiment isolated (somewhat) the partial oxidation effect from thermal effect for tar destruction, and also included no effect of char. The optimum amount of blast addition of approximately 0.2 equivalence ratio, λ, relative to the fed gas was reported to avoid growth in the PAH number (number of aromatic rings). Further, adding no oxygen with indirect heat promoted tertiary tar formation, but so did adding too much oxygen, for example λ>0.4, in Partial Oxidation.
The presence of hydrogen also seems to play a key role in tar destruction. A PAH “cracking” scheme described in Jess, A. “Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels.” Fuel 75, no. 12 (1996): 1441-1448 describes the alternate pathways of PAH growth or PAH cracking (fewer aromatic rings and lower carbon numbers in tar compounds) that may occur with varying hydrogen concentration. Similarly, Houben (2004) found that if hydrogen concentration of the inlet gas were more than about 20% vol., tar reduction was optimized. Decreasing hydrogen at the inlet below this level dramatically increased tar concentration in the products for the same equivalence ratio, λ. Naphthalene and tertiary PAH (3+ ring) were totally eliminated with an inlet hydrogen content greater than 30% vol, but single ring aromatics, e.g. toluene and benzene were retained. Therefore, gasifier operations that increase the fed hydrogen concentration should result in beneficial tar reduction for the same POX condition.
Catalytic tar reduction by contacting the gas with char in temperatures in the range of 900 to 1000° C.—notably lower than necessary for thermal destruction in the absence of char, but still elevated with respect to the typical biomass gasifier exit temperature (750 to 850° C.)—have also been disclosed. (Chen 2009). The natural minerals in biomass ash (MgO, CaO, K2O, etc.) are believed to contribute to the catalytic effect, but the state of prior preparation (temperature history, surface area or oxidative exposure) is also thought to impact performance. Using commercial biochar (active carbon) and laboratory produced biochars (using 500° C. pyrolysis) blended with sand, it was reported that naphthalene conversion was 99.6% and 94.4% at 900° C. with 0.3 seconds residence time (25 cm3 catalyst bed, 2 cm bed height), starting with 90,000 mg tars/Nm3 in fed gas, compared with the blank sand (2%), natural olivine and sand (55%), and dolomite and sand (61%).
Fixed carbon gasifies much more slowly (orders of magnitude more slowly) compared to the volatile matter fraction of a solid fuel at the same temperature when under non-oxidizing conditions. A portion of the initial fixed carbon feed is usually present as a residue of gasification. If an appropriate technology were available to capture this char and expose it to the flowing gas stream it could be employed as a catalyst.
Fixed char beds with direct blast addition help to achieve low tar by gaining elevated temperatures in the char bed and also by presenting the char to the gas for possible catalytic benefit, but this approach is prone to upsets such as high temperature excursions. On the other hand, by separating the partial oxidation zone to a location above the char bed, the hot gases can be exposed to the active catalytic properties of char without blast input to the delicate char bed. However, a mechanical grate supporting low density char dust requires low superficial velocities and this is also prone to its own possible solids flow upsets (bridging, chanelling (“rat holing”), local hot spots, etc.) due to the chaotic flow behavior of low density solids.
A partial oxidation zone that achieves higher temperatures (1150° C.) can be used to help effectively convert tars with or without passing through a fixed bed of char. The presence of hydrogen in the fed gas is an important feature to achieve maximum POX performance where opening aromatic rings can be favored over PAH growth. It is thought that the type of tars produced may also impact their ability to be subsequently reformed on a bed of char, and may impact the cracking performance in the POX stage; however, this has remained an unproven possibility.
The combination of these theories and principles (hydrogen rich gas production, followed by partial oxidation, exposure to the catalytic properties of char, and supported char bed) in a robust and scalable industrial design is not presently known to the state of the art. The classic fixed bed downdraft gasifier (and other techniques that employ a fixed bed of char alone) is not scalable over about 10 MWth due to the anisotropic shape and chaotic flow potentials in low density char beds. Blast addition directly into the fixed bed of char would not be sufficiently robust for commercial deployment and would not be scalable to industrial capacities (>10 MWth). The fixed bed of char alone suffers from solids flow irregularities (“bridging”) and other process upsets (“rat holes”) that occur due its low bulk density and the anisotropic nature and non-uniform particle size of the produced char.
Generally known gasifiers are of several types. The downdraft gasifier having an integrated fixed bed is a classic technology that is well known to those skilled in the art for low-tar gas production (<300 mg/Nm3). Increasing superficial velocity through the downdraft gasifier, even when there is no secondary air injection into the char bed, also results in an increase of peak temperatures in the char bed. Lowest tar yields are observed with high peak temperatures >1000° C. (Reed 1999) Referring now to
An alternative downdraft gasifier having a separated POX zone is another possibility (See FIG. 4)._One of the challenges with the classical two-stage, fixed bed, downdraft gasifier is that injection of blast into a fixed bed of char can lead to difficult operational problems—slagging (temperatures well over the ash fusion point), clinkering (fused ash particles), chanelling (“rat holing”), fuel bridging and material degradation in the blast input tube.
A multi-stage gasifier system (Viking II) was developed by the Danish Technical University (DTU) between 1980 and 1990 based on the principles of the downdraft gasifier, but separated the blast addition from the fixed bed to improve operability. The DTU gasifier incorporates a separate low temperature pyrolysis stage (52) (500 to 600° C.) that is configured above a vortex flow partial oxidation section (55) operated to achieve peak temperatures ˜1150° C.—this stage is the only zone of direct blast addition. This partial oxidation zone (505) is situated above a downdraft, dense “fixed” bed of char (57) supported on a mechanical grate (58) comprised of pivoting angle iron.
The DTU design suffers from limited scale-up potential (due to the fixed bed of char (57) and indirectly heated feed auger (51)). On the other hand, the DTU gasifier system proved to yield very low tars (<25 mg/Nm3) and produced a rich gas with ˜25% hydrogen without steam addition, and ˜35% hydrogen with steam addition. The gas quality was greatly enhanced by the recuperative indirect heat stage that can include indirect drying (52). The main difficulty with the DTU system is that the gasifier system still relied on a fixed bed of char, which consists of low bulk density solids that are irregular in particle size and shape. A relatively low superficial velocity is believed to be required for achieving a char pile without disruption on the mechanical grate (58) (previously described)—which indicates a costly scale-up for this reaction stage. The low density bed of char exhibits chaotic solids flow properties that would be unmanageable in an industrial-scale system with commercial reliability requirements.
Moving granular beds have also been used in prior art to present char as a catalyst to produced gas but in a cross flow moving granular bed filter, see
Another moving granular bed filter which is shown in
The present invention differs from the above referenced inventions and others similar in that these prior devices do not provide features that can be readily scaled up to industrial operational levels. What was needed was a gasifier system able to meet the low tar requirements while producing high quality gases, and which is feasible and operable in an industrial setting.
SUMMARYThis summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used in isolation as an aid in determining the scope of the claimed subject matter. At a high level, embodiments of the invention relate to a gasification system for converting feedstocks such as biomass and waste to combustible gases with low tar levels.
Embodiments of the invention include a gasifier wherein the char bed can be scaled up while managing the low bulk density solids and the irregularities in operation caused by variations in superficial velocity. Some embodiments of the invention include a gasifier that provides a partial oxidation zone(s) to allow maximum advantage of the high temperatures required for lowest tar production. Additionally, embodiments of the invention provide a gasifier that fosters high quality gas production. Further embodiments of the invention include a gasifier constructed to provide disengagement screen scrubbing.
The utility of the present invention is to convert biomass and waste feedstock (solids) into a combustible gas at elevated temperature and pressure with substantially reduced tar concentrations. There may be applicability of this invention to gasification of other higher volatile matter solid fuels, including for example, various low rank coals, brown coal, peat, and lignite. Achieving low tar gas is the key to unlocking quantitative gas conditioning needed for advanced high efficiency gas-to-power systems (engines, combustion turbines, solid oxide fuel cells, etc.) and advanced synthesis technology for biofuels (ethanol, mixed alcohols, and Fischer-Tropsch liquids) and chemicals such as hydrogen and ammonia.
Embodiments of the invention utilize an entrained flow reactor coupled downstream of a fluidized bed reactor. An entrained flow reactor is a reactor in which the reactant feedstock and oxidant are fed into the top of the reactor so that the oxidant stream surrounds (e.g., “entrains”) the feedstock and carries the feedstock through the reactor. A fluidized bed reactor is one in which a fluid is forced upward through a granular bed at velocities sufficient to cause the granular material to behave, in many respects, as a fluid. In certain embodiments, the entrained flow reactor incorporates a moving granular bed that captures and supports a catalytic char bed.
A first illustrative embodiment of the present invention relates to a multi-stage reaction system for producing low-tar combustible gas. In certain embodiments, the system includes a fluidized bed reactor that includes a partial oxidation zone, in which a portion of the solid feedstock is partially oxidized, thereby creating a gas and a plurality of char particles. The illustrative embodiment further includes an entrained flow partial oxidation reactor situated downstream from the fluidized bed reactor, and where the entrained flow partial oxidation reactor includes a moving granular bed.
A second illustrative embodiment of the present invention relates to a multi-stage reaction system for producing low-tar combustible gas. In certain embodiments, the system includes a fluidized bed reactor that includes a partial oxidation zone, in which a portion of the feedstock is partially oxidized thereby creating a gas and a plurality of char particles. The illustrative embodiment further includes an entrained flow partial oxidation reactor situated downstream from the fluidized bed reactor. The entrained flow partial oxidation reactor includes a moving granular bed. In certain embodiments, a media screening device screens media from the moving granular bed and a media recycle system returns the screened media to the entrained flow partial oxidation reactor.
A third illustrative embodiment of the present invention relates to a method for controlling an operating pressure of a two-stage gasification system. In certain embodiments, the method includes performing a partial oxidation of a portion of the feedstock in a fluidized bed reactor; elutriating the resulting plurality of char particles and the gas from the fluidized bed reactor as a mixture of gas and char; receiving the mixture into an entrained flow reactor that includes a moving granular bed of filtering media; and allowing the mixture to flow through the moving granular bed of char and media. As the mixture is pushed through the granular bed, embodiments of the method further include capturing a portion of the plurality of char particles in the filtering media; screening a portion of the filtering media to remove captured char particles; and returning the screened filtering media to the entrained flow reactor.
These and other aspects of the invention will become apparent to one of ordinary skill in the art upon a reading of the following description, drawings, and the claims.
The present invention is described in detail below with reference to the attached drawing figures, wherein:
The subject matter of embodiments of the invention disclosed herein is described with the specificity required to meet statutory requirements. However, the description itself is not intended to limit the scope of claims in this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different features or steps, or combinations of features or steps, similar to the ones described in this document, in conjunction with other technologies. Moreover, although the term “step” is used herein to connote different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.
Referring to the drawings, and particularly to
For example, in some embodiments, the gasification system 100 can include any number of additional components such as, for example, those illustrated in
With continued reference to
In operation, the first reactor 101 creates a hydrogen-rich partial oxidation zone 112 in its upper section 106 and preferably includes direct blast addition and/or indirect heat addition through the port 116. Embodiments of the invention allow for influence and control of the hydrogen concentration in the raw gas, thereby facilitating the subsequent cracking of tars, which occurs in the partial oxidation zone 112 of the first reactor 101 and/or in a partial oxidation zone 126 of the second reactor 102.
With continued reference to
Embodiments of the invention can include a number of different options for configuring blast nozzles 142 around the periphery the first reactor 101 and/or the second reactor 102. The configuration of the blast nozzles 142 facilitates forming localized regions of oxidative thermal intensity (by virtue of the mixing pattern), rather than achieving more uniform mixing patterns achieved by typical approaches to designing gas burners for lean fuel conditions. Turning briefly to
For example, as shown in
Turning to
In the embodiment illustrated in
According to certain embodiments of the invention, one or more auxiliary blast zones will include multiple nozzles configured around the perimeter of the vessel so as to create at least two tangent target circles. In some embodiments, as depicted in
Moreover, according to embodiments of the invention, mixing performance associated with the blast zones 146 and 162 can be optimized through computational fluid dynamics (CFD) calculations. For example, CFD software can be used to create 3-D patterns with thermally intense zones having various peak temperatures. In certain embodiments, mixing performance can be optimized by varying relative diameters of the target circles, adjusting swirl number (e.g., utilizing a swirl number less than 0.4), and by optimizing the equivalence ratio of the total auxiliary blast addition. The equivalence ratio, λ, is the blast to fuel ratio, relative to the stoichiometric blast to fuel required to just burn the fed gas and char. According to embodiments of the invention, the total auxiliary blast input is less than about 25% of the stoichiometric blast-to-fuel ratio calculated relative to the fed feedstock analysis. Additionally, in some embodiments, the total auxiliary blast can be about 50%, or more (and even up to 100%, particularly when indirect heat is supplied during the first stage), of the entire blast input to the reaction system.
For example, in one embodiment, the blast nozzle configuration is developed using CFD software to model at least one thermally intense zone having a peak temperature of ˜1150° C. The total auxiliary blast addition is controlled such that it has an equivalence ratio, λ, of approximately 0.2 (or less) in oxygen limited partial oxidation and incorporates a majority (>50%) of the total blast addition through auxiliary ports, configured to achieve localized zones of peak temperature of approximately 1150° C. Configuring the blast nozzles accordingly can facilitate achieving desired performance objectives during operation.
A peak temperature of between about 1000° C. and about 1200° C. generally is sufficient for activating hydrogen molecules in the manner necessary for cracking aromatic ring compounds and providing the necessary termination to avoid ring polymerization. Too high a temperature in the bulk gas may cause melting of ash and slag formation that can interfere with operation. Accordingly, the partial oxidation zones are configured to include local thermally intense zones rather than high bulk gas temperatures. These localized thermally intense zones facilitate activation of hydrogen radicals that can subsequently initiate chemical reactions in the adjacent bulk gas. For example, hydrogen facilitates terminating the activated carbon atom in an aromatic ring that has been thermally cracked open, thereby providing for tar reduction rather than tar polymerization.
Returning to
By controlling the operating pressure of the gasification system 100 for a given gas production rate, it is possible to control (to a degree) the maximum particle size and the rate of release of char 199, and the maximum particle size of the char 199, through the elutriation mechanism 200 associated with the first reactor 101. The particle size of the char 199 affects the catalytic performance of the char 199 for gas temperatures of less than 1000° C. In other words, a smaller particle size tends to produce more tar reduction for the same temperature, particularly if the temperature is less than 1000° C. For example, at 900° tar reduction in one study was 88% for one particle size range (1 to 2 mm) and 96% for another (0.1 to 0.15 mm). Accordingly, certain embodiments of the invention incorporate a method of operating the gasification system to control the char 119 particle size and elutriation rate.
According to certain embodiments of the invention, the method includes, at least in part, maintaining a target velocity in the freeboard 110 of the first reactor 101 by modulating the pressure set point. It will be appreciated by individuals having skill in the relevant arts that pressure modulation can be accomplished in a number of ways such as, for example, modulating fuel and air inputs, modulating a downstream valve position (e.g., downstream from a particulate removal), and the like. For instance, as illustrated in
Additionally, pressure control can be achieved by controlling the flow of char 199 through the second reactor 102. The gas 215 engaging the moving granular bed 134 in the second reactor 102 moves in co-flow direction with granular material 135. In certain embodiments, the granular material is input via the main gas inlet 204 of the second reactor 102. In operation, the moving granular bed 134 captures and dilutes char 199 in a matrix of granular solids 135 that has a higher specific gravity, thereby improving the solids' 135 flow properties. In this manner, a zone of gas-char 215 is created such that the gas-char 215 contacts, with sufficient residence time, the char 199 solids for catalytic tar-reduction-by-char. The moving granular bed 134 captures and mixes the low density char 199 (usually <190 kg/m3) with other media (usually >1900 kg/m3), thereby improving the char 199 flow properties. In this manner, the flow of char 199 can be positively managed by its association with the co-flowing media matrix 134.
With continued reference to
Turning briefly to
As illustrated at step 316, the mixture is allowed to flow through the moving granular bed and, as the mixture moves through the moving granular bed, char particles are captured in the media of the moving granular bed, as indicated at step 318. To control the concentration of char particles in the moving granular bed (and thereby, to facilitate control over the char flow rate and particle size), a portion of the media of the moving granular bed is screened to remove char particles, as shown at step 320. At a final illustrative step, step 322, the screened media is returned to the moving granular bed. According to certain embodiments, the illustrative method 300 can be used alone, or in conjunction with other methods, to affect control over the operating pressure of the gasification system by controlling the char flow rate in the entrained flow reactor.
Returning now to
The moving granular bed 134 is operated to capture char 199 as a physical barrier. The media residence time is correlated with the char residence time (the period of time that the average char particle spends in the reactor), and this char residence time can be modulated in a controlled manner with the media screening and recycle subsystem (103/104). According to certain embodiments, the moving granular bed 134 also can be configured to provide a zone of narrow gas residence time distribution through the char bed 134 in a conceptually plug flow reactor that allows for maximum tar cracking. In certain embodiments, the gas residence time and char residence time can differ by several orders of magnitude; therefore, the differential velocity between the gas 215 and char 199 is very close to the local gas interstitial velocity through the bed 134. In certain embodiments, the char in the char bed 134 is continuously refreshed by the char 199 supply from the first reactor 101 thereby reducing or eliminating the need for high performance filtration, even though some small particles of char 199 may slip through the bed with the gas 215.
With continued reference to
Embodiments of the moving granular bed 134 of the present invention include features that are not known to the art and that have been described above. These features include, for example, a co-flow design that is preferred for its gas-char contacting zone for enhancing catalytic tar-reduction-by-char performance rather than for its filter performance; the lack of a need for a media retention screen for media retention at the gas engagement interface; and a substantially vertically oriented gas disengagement screen (e.g., which is steeper than the angle of repose of the blended char and media matrix to scrub the disengagement screen keeping it free of dust clogging).
In contrast, for example, the prior art moving granular bed filter disclosed in U.S. Pat. No. 7,309,384 to Brown et al., and illustrated herein in
According to various embodiments of the invention the residence time (increased internal age distribution) of the trapped char particles is controlled by modulating the media flow that captures the char through an external recycle loop. With reference to
Turning to
Turning now to
Turning to
To recapitulate, embodiments of the invention include a gasification system having a fluidized bed reactor situated upstream from an entrained flow reactor. The entrained flow reactor includes a moving granular bed that holds up char and so presents catalytic properties of char to the gas for the purpose of tar cracking. In some embodiments, char concentration in the granular solids matrix is controlled with the media screening and recycle system (such as the media screening device 103 and the recycle system 104 illustrated in
For example, tests were performed under various operating conditions in a laboratory-scale entrained flow reactor, configured according to embodiments of the invention, using yellow seed cord as a model feedstock. Air or oxygen was used as blast as indicated in
The entrained flow reactor consisted of a small partial oxidation burner that injected blast laterally through 6 small holes (having an internal diameter of 1.4 mm) with slight swirling action, in the configuration illustrated in
As another example, further tests were performed in which the same test conditions discussed above were analyzed for lower heating value—indicate that the heating value stabilized at ˜5 MJ/Nm3 (dry) for air blown tests, and 10 MJ/Nm3 (dry) for oxygen blown tests, so long as the total equivalence ratio was less than 0.35, as reflected in
A reactor, configured in accordance with embodiments of the invention, that combines a partial oxidation burner for tar reduction above a “pebble grate” (which is previously described as a bed of granular media supported by a vertical wire screen supporting the media at the gas disengagement) to support a bed of char has not heretofore been known to the art. The integration of a first reactor that operates at lower temperatures in sequence with a higher temperature partial oxidation stage (sub-stoichiometric combustion) with a subsequent heat recuperative device that transports thermal energy back to the first reactor to drive pyrolysis reactions is also not previously known. Because most moving granular beds are designed for filtration, the co-flow design of the present invention is also not known because it does not impart ideal filtration conditions but rather it imparts ideal gas-char contact conditions for tar cracking. Low density char—that otherwise has chaotic solids flow properties—is dispersed into a granular bed material that has approximately ten (10) times the bulk density, which imparts improved solids flow properties. This is an improvement over prior art and contrasts with any reactor that includes a fixed bed of char in downdraft type gasifiers (integrated or separated from partial oxidation zones) that have been known to experience upsets associated with poor solids flow, “solids bridging”, and “rat hole” formations due to the chaotic movement and anisotropic nature of low density char beds.
The present invention has been described in relation to particular embodiments, which are intended in all respects to be illustrative rather than restrictive. Alternative embodiments will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope. For example, different contact times and variations in temperatures for certain zones may be employed. Connections between reactor vessels and return lines can vary in general position. It will be appreciated by individuals having skill in the relevant arts that certain optimizations will be necessary depending on the source of feedstock.
From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects set forth above, together with other advantages, which are obvious and inherent to the system and method. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.
Claims
1. A multi-stage reaction system for producing low-tar combustible gas, the system comprising:
- a fluidized bed reactor that includes a partial oxidation zone, in which a gas and a plurality of char particles are created in said partial oxidation zone; and
- an entrained flow partial oxidation reactor positioned downstream from the fluidized bed reactor.
2. The system disclosed in claim 1 wherein said entrained flow partial oxidation reactor includes a moving granular bed.
3. The system of claim 1, wherein the fluidized bed reactor further comprises a freeboard, said freeboard operated at a velocity controlled to create a generally consistent char particle size feed.
4. The system of claim 2 wherein said plurality of char particles is of generally consistent char particle size and at least a portion of said plurality of char particles is provided to the entrained flow partial oxidation reactor.
5. The system of claim 3 wherein provision of said generally consistent char particle size comprises modulating a pressure of the fluidized bed reactor.
6. The system of claim 1, further comprising a sand recovery cyclone.
7. The system of claim 1, wherein the entrained-flow partial oxidation reactor further includes a partial oxidation zone and means for injecting a stream of combined gas and char into said partial oxidation zone.
8. The system of claim 7, further comprising a cyclone for concentrating the plurality of char particles out of a fed gas stream prior to injection into said partial oxidation zone.
9. The system of claim 8, wherein said cyclone concentrates the plurality of char particles out of the fed gas stream prior to injection into said partial oxidation zone.
10. The system of claim 2, wherein the entrained-flow partial oxidation reactor includes a plurality of blast inlet ports configured around a periphery of a vessel enclosing the reactor.
11. The system of claim 10, wherein said plurality of blast inlet ports is arranged in an alternating pattern such that each of the plurality of blast inlet ports targets a tangent curve of a tangent circle.
12. The system of claim 10, wherein said plurality of blast inlet ports is arranged in an alternating pattern such that a first one of the plurality of blast inlet ports targets a first tangent curve of a first tangent circle and a second one of the plurality of blast inlet ports targets a second tangent curve of a second tangent circle, wherein the first and second tangent circles are located at the same elevation.
13. The system of claim 10, wherein said plurality of blast inlet ports is arranged in an alternating pattern such that a first one of the plurality of blast inlet ports targets a first tangent curve of a first tangent circle and a second one of the plurality of blast inlet ports targets a second tangent curve of a second tangent circle, wherein the first and second tangent circles are located at different elevations.
14. The system of claim 7, wherein the moving granular bed operates in co-flow with respect to the stream of combined gas and char to create a gas-char contacting zone.
15. The system of claim 14, wherein a gas-media disengagement screen is oriented at an angle that is steeper than an angle of repose of the combined media and char mixture.
16. The system of claim 15, wherein the gas-media disengagement screen is oriented substantially vertically.
17. The system of claim 15, wherein the gas-media disengagement screen includes a plurality of parallel wires extending between an upper frame edge and a lower frame edge.
18. The system of claim 17, wherein each of the plurality of wires includes a cross section partially defined by a first side that converges with a second side at a vertex in the direction of the disengaging gas flow.
19. The system of claim 18, wherein the cross section is wedge shaped.
20. The system of claim 2 wherein the entrained-flow partial oxidation reactor further includes a partial oxidation zone, means for injecting a stream of combined gas and char into said partial oxidation zone, and a plurality of blast inlet ports configured around a periphery of a vessel enclosing the reactor.
21. The system of claim 20 wherein the moving granular bed operates in co-flow with respect to the stream of combined gas and char to create a gas-char contacting zone.
22. The system of claim 2 wherein the entrained-flow partial oxidation reactor further includes a partial oxidation zone and means for injecting a stream of combined gas and char into said partial oxidation zone and said moving granular bed includes a gas-media disengagement screen oriented at an angle steeper than the angle of repose of the combined media and plurality of char particles.
23. A method for controlling an operating pressure of a two-stage gasification system, the method comprising:
- performing a partial oxidation of a portion of biomass in a fluidized bed reactor, wherein the partial oxidation creates a gas and a plurality of char particles;
- elutriating at least a portion of said plurality of char particles and gas from the fluidized bed reactor, wherein said elutriating includes removing a mixture of gas and char particles from the fluidized bed reactor;
- receiving the mixture into an entrained flow reactor, wherein the entrained flow reactor includes a moving granular bed of filtering media;
- allowing the mixture to flow through the moving granular bed; and
- capturing a portion of the plurality of char particles in the filtering media.
24. The method of claim 23 further comprising screening a portion of the filtering media to remove captured char particles; and returning the screened filtering media to the entrained flow reactor.
25. A multi-stage reaction system for producing low-tar combustible gas, the system comprising:
- a fluidized bed reactor that includes a partial oxidation zone in which a portion of the feedstock is partially oxidized, wherein said partial oxidation creates a gas and a plurality of char particles;
- an entrained flow partial oxidation reactor situated downstream from the fluidized bed reactor, the entrained flow partial oxidation reactor including a moving granular bed;
- a media screening device that screens media from the moving granular bed; and
- a media recycle system that returns the screened media to the entrained flow partial oxidation reactor.
Type: Application
Filed: Jun 16, 2011
Publication Date: Dec 22, 2011
Inventors: Thomas J. Paskach ( Ames, IA), John P. Reardon (St. Louis, MO), Paul Evans (Miami, OK), Jerod Smeenk (Ames, IA)
Application Number: 13/162,241
International Classification: C10J 3/48 (20060101); C10L 3/00 (20060101);