GASIFICATION OF CARBONACEOUS MATERIALS USING PULSED OXYGEN

A process for the gasification of carbonaceous materials for the product of syngas. Pulsed oxygen is used to maintain the temperature of the gasification zones and to avoid hot spots in the gasification reactor.

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Description
CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Provisional Applications 61/214,482 filed Apr. 24, 2009; 61/270,645 filed Jul. 10, 2009; and 61/295,355 filed Jan. 15, 2010.

FIELD OF THE INVENTION

The present invention relates to the gasification of carbonaceous materials for the product of syngas. Pulsed oxygen is used to maintain the temperature of the gasification zones and to avoid hot spots in the gasification reactor.

BACKGROUND OF THE INVENTION

Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass into predominantly carbon monoxide and hydrogen (syngas) by reacting the carbonaceous material at high temperatures with a controlled amount of oxygen. Syngas may be burned directly in internal combustion engines, used to produce methanol, dimethyl ether, or hydrogen, or converted via the Fischer-Tropsch process into synthetic fuels. Syngas can also be used to produce other products.

Gasification of fossil fuels is currently widely used to generate electricity. However, almost any type of organic material can be used as the raw material for gasification, including biomass and plastic waste. Thus, gasification has the potential to be an important technology for renewable energy, and is typically carbon neutral.

Since gasification is an endothermic reaction, heat must be supplied to the carbonaceous material either indirectly (through exchange with a hot transfer area or through the simultaneous heat release associated with partial oxidation due the introduction of oxygen or air into the reactor. Most carbaneous material derived from biomass contain significant amounts of inorganic material (i.e. silica, potassium and other elements) which do not undergo gasification and can agglomerate and fuse into a phase commonly called slag when exposed to elevated temperatures (typically >1800 F). Gasifiers which are designed to minimize slag formation and use partial oxidation to generate the required thermal energy for gasification (aka directly heated gasifier) must control the amount of oxygen in order to avoid excessive temperatures within the partial oxidation zone.

The direct injection of oxygen or air into a gasifier chamber typically leads to high temperatures within the gas jet associated with the nozzle or injection device used to introduce the oxidant. Several gasifier designs or systems such as U.S. Pat. No. 6,613,111B2 and U.S. Pat. No. 6,680,137B2 utilize two fluid bed reactors consisting of inert and carbaneous solids. Gasification occurs within one bed (first stage) and the effluent solids from this bed consisting of inorganic and carbaneous materials are collected and routed to a second fluid bed (second stage) where they undergo oxidation to raise the temperature of the inorganic solids. The heated inert solids are then sent back to the gasifier section (first stage) in order to supply heat for further gasification. In this type of design, the amount of heat generated in the oxidation bed is critical since it must be sufficient to maintain the desired gasification temperature. If the amount of oxidation is excessive (too much carbaneous material with excessive air), the solids may undergo an unacceptably high temperature rise, resulting in either slag formation (loss of fluidization) or the volatilization and redeposition of undesired inorganic material in the colder sections of the gas conduits. Operating the oxidation zone at lower than desired temperatures can lead to the accumulation of carbaneous material in both the gasification and oxidation stages of the gasifier system.

It is important to point out that either full or partial oxidation of the carbaneous material within the second stage can occur. The most important objectives in the oxidation stage include removal of carbonaceous material to prevent accumulation and to generate the appropriate amount of thermal energy to drive the gasification reactions.

Gasifier designs based on indirect heating through hot transfer areas are best represented by U.S. Pat. No. 5,059,404, U.S. Pat. No. 5,306,481 and related patents. In these gasifier systems the heat required for driving the gasification reactions occurs through heat transfer tubes located within the fluidized bed.

This invention teaches a method in which air or oxygen can be introduced into any gasifier system in such a way as to better control the temperature within either the gasifier or oxidation stage. The invention is applicable to any gasifier system such as the direct indirectly heated systems.

While there is much activity in the field of gasification, especially for converting biomass to fuel products, there is still a need in the art for improved and more efficient processes and equipment.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a process for converting carbonaceous materials to a syngas in a gasification process unit, which process comprising:

a) introducing an effective amount of fluidizing gas such as steam into a gasification reactor containing a fluidized bed of solids;

b) introducing a fluidizing gas through a plurality of nozzles located at the bottom of said gasification reactor containing solids thereby resulting in and maintaining a fluidized bed of said solids;

c) operating said gasification reactor at a temperature of about 1000° F. to about 1600;

d) introducing a carbonaceous material, having an organic fraction and an inorganic fraction, into said gasification reactor containing said fluidized bed of solids wherein the residence time of said carbonaceous in said gasification reactor is from about 5 to 90 seconds, thereby resulting in a gaseous phase biomass product stream and a carbon-rich biomass particulate product;

e) pulsing oxygen, through said plurality of nozzles, into said gasification reactor in order to keep the temperature in the range from about 1000° F. to about 1600° F., and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said carbonaceous material, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times;

f) The air or O2 can be introduced into any sections of the gasifier which require thermal energy to either heat fluidizing solids or promote gasification

Also in accordance with the present invention there is provided a process for converting a biomass feedstock to a synthetic gas in a two-stage gasification process unit, which process comprises:

a) introducing an effective amount of steam into a first gasification stage containing a fluidized bed of solids;

b) introducing a fluidizing gas through a first plurality of nozzles located at the bottom of said first gasification stage containing solids thereby resulting in and maintaining a fluidized bed of said solids;

c) operating said first gasification stage at a temperature of about 1000° F. to about 1600° F.;

d) introducing a biomass feedstock in particulate form into a first gasification stage containing the fluidized bed of solids wherein the residence time of said biomass in said first gasification stage is from about 5 to 90 seconds, thereby resulting in a gaseous phase biomass product stream and a carbon-rich biomass particulate product;

e) pulsing oxygen, through said plurality of nozzles, into said first gasification stage in order to keep the temperature in the range from about 1000° F. but not greater than about 1600° F., and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said biomass, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times;

f) transporting at least a portion of said gaseous phase biomass product stream to a solid/gas separation unit wherein particles greater than a predetermined size are separated and returned to said first gasification zone and wherein the treated gaseous phase biomass product stream is transported to a second gasification stage;

g) transporting solids and particulates from said first gasification stage to a second gasification stage;

h) introducing, through a second plurality of nozzles, an effective amount of a fluidizing gas into said second gasification stage thereby resulting in a second fluidized bed of biomass particulates and fluidizing solids;

i) operating said second gasification zone in the temperature range from about 1600 F to about 2000 F, but at a temperature at least 50° F. greater than the first gasification stage and at a residence time from about 1 to 3 times that of said first gasification stage;

j) pulsing an effective amount of oxygen through said second plurality of nozzles of said second gasification stage in order to maintain said second fluidized bed in the temperature range of about 1600° F. to about 2000° F., 1700 to 1800, and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said biomass, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times, thereby resulting in said biomass being converted to a gaseous phase and a solid phase;

j) returning at least a portion of said solids to said first gasification stage;

k) passing said syngas stream to a solids/gas separation zone wherein substantially all of said remaining solids are removed, thereby resulting in a substantially solids-free syngas stream;

l) passing said syngas product stream to downstream processing; and

m) removing any excess solids from the gasification process unit to maintain a predetermined balance of solids.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a simplified drawing of the nozzle section needed for any gas injection into any section of the gasifier.

FIG. 2 hereof is a representation of a preferred embodiment of the gasification system of the present invention showing a generic two stage gasifier followed by a secondary gasifier and other main components.

FIG. 3 hereof is a simplified drawing showing what applicants believe to be the sequencing of pulsed oxygen into the gasification reactor of the present invention.

FIG. 4 hereof is a representation of the time sequencing of oxygen injection into the gasification reactor of the present invention.

 DETAILED DESCRIPTION OF THE INVENTION

Any suitable carbonaceous material (solid, liquid or gaseous) that is capable of being used as a fuel source can be used in the practice of the present invention. Non-limiting examples of such carbonaceous materials that can be used in the practice of the present invention include: i) petroleum derived carbonaceous materials such as methane, heavy hydrocarbonaceous oils, heavy and reduced petroleum crude oils, petroleum atmospheric bottoms, petroleum vacuum distillation bottoms, heavy hydrocarbon residues and asphalt; ii) bitumens, tar sand oil, pitch, and shale oil; iii) natural gas; iv) coals such as lignite, sub-bituminous, bituminous, and anthracite; v) coal derived materials including coal liquid products obtained from coal liquefaction as well as gaseous products obtained by coal gasification; and vi) biomass feeds.

Gasifiers designs can be broadly grouped into slagging/non-slagging and single or multistage devices. Non-slagging gasifiers operate at temperatures below the fusion point of the inorganic constituents contained within the feed stock. Some feed stocks contain inorganic constituents which readily vaporize or form fine particles which become part of the gas stream (i.e. silica). Typically, non-slagging gasifiers operate at <2000 F and in many biomass applications must operate below <1800 F in order to avoid slagging of the inorganic feed constituents. Many biomass gasifier designs incorporate a two stage design since the initial decomposition of the cellulose involves formation of quantities of carbaneous materials (commonly referred to as tar, carbon and soot) which react more slowly than the parent cellulose feed and require longer residence times and/or higher temperatures to completely gasify. This tar, carbon and soot material, is typically collected through cyclones or other solid-gas separation methods and routed to a second stage or volume in which it can undergo further gasification or in cases which utilize direct heat generation, this material is either partially or fully oxidized. Partial oxidation leads to the generation of additional syngas while supplying heat to the gasification stage. Full oxidation leads to only the generation of heat for further gasification.

The fluidized bed gasification process requires the attainment of appropriate fluidization with the maintenance of the appropriate gasification and in some gases the oxidation temperatures within each stage. Failure to maintain the appropriate fluidization conditions (gas velocities and solid particle properties) can easily lead to the generation of inappropriate syngas and loss of gas and/or solid throughput.

The gasifier system operating pressure strongly impacts the heat transfer and fluidization properties of the system. For example when operating at 300 psig, the gas throughput through the system is approximately 20 times higher than that of 15 psig. Consequently the amount of gas needed for fluidization and generation of heat for direct heated gasifiers is significantly higher resulting in a much higher heat generation rate in any stage requiring the addition of thermal energy. This higher heat generation rate can result in local high temperatures which exceed the desired maximum, resulting in slagging or other undesirable impacts on fluidization. Consequently most commercially available gasifier systems operate at low to modest pressures (<100 psig) in order appropriately balance the fluidization and temperature requirements. For indirectly fired systems, operations at higher pressures are difficult to increased heat transfer area and heat flux required to drive the appropriate extent of gasification.

Gasification systems able to operate at elevated pressures (>200 psig) offer significant economic advantages over the existing lower pressure systems, especially when the syngas is utilized in producing chemical or liquid transportation fuel products. With a low pressure gasifier, syngas compression is necessary to achieve the >400 psig necessary to produce most chemical or transportation fuels through commercially established catalytic processes. The cost of compression can easily be >10% of the total plant capital and the energy expenditure can amount to 10-15% of the incoming feed. Since steam (produced at >300 psig) is the primary fluidizing gas, gasification at elevated pressures is far more economically viable than that of low pressure.

There is presently limited or no commercial biomass high-pressure gasification processes, which is a preferred gasifier process of the present invention. A high-pressure gasification process producing syngas, a mixture of predominantly hydrogen, carbon monoxide, carbon dioxide, and methane would be beneficial since the subsequent conversion of syngas into other chemicals such as dimethyl ether, methanol, Fischer-Tropsch products, and ammonia occurs at high-pressure. Conventional low-pressure gasifiers thus require a very expensive and most often prohibitive (economically) gas compression step. As a result, the high pressure gasifier system of the present invention substantially decreases the size, and preferably eliminates, the compression step typically required for post-gasifier conversion processes.

The gasification process as applied to the conversion of carbonaceous materials actually involves many individual reactions associated with conversion of carbon, hydrogen, and oxygen into products involving steam, hydrogen, oxides or carbon, soot or tars and hydrocarbons. At the elevated temperatures (>1000° F.) associated with gasification, the major products are typically steam, hydrogen, CO2, CO and methane. Chars and soot represent compounds rich in carbon and may contain small amounts (<5%) of hydrogen.

The preferred gasification temperatures are in the range of 1600° to about 1800° F. when the desired product is a synthesis gas suitable for the production of chemicals such as dimethyl ether or transportation fuels such as diesel or gasoline. When generating synthesis gas for use in a gas or steam turbine, the preferred gasification route involves converting as much of the feed carbon into a gaseous fuel and minimizing the production of tars and soot.

Substantially all reactions occur simultaneously within the gasification process (when oxygen is present). Since the gasification process is endothermic in nature heat must be supplied in order to maintain the elevated temperatures. Gasifiers can be classified with respect to how they provide this heat. Indirect gasifiers utilize heat transfer tubes or other surfaces within the gasifier reactor. An external source of hot gas passes through the tubes to provide heat to the gasification reactor. The maximum operating temperature for these types of gasifiers is typically <1600° F. due to the material limitations associated with the heat transfer area. Expensive high temperature metal alloys or other heat transfer materials can be utilized however the mechanical complications associated with thermal stress prohibit operations in the desired range of 1800° F. High temperature gasifiers (>1800° F.) such as those utilized for coal, employ O2 in the feed and provide the necessary thermal energy for driving the endothermic reactions through partial oxidation. This use of internally generated heat is referred to as a “direct” or O2 blown gasifier which can achieve almost convert conversion of the feed carbon. Coal gasifiers (direct type) generally operate in what is referred as the slagging mode since the temperatures achieved within the partial oxidation zone is very high (>2000° F.) and the inorganic constituents (also referred to as ash) undergo “fusion” or are converted to liquids which collect at the bottom of the gasifier and are periodically or continuously drawn out of the system. However, when this technology is applied to biomass, issues arise due to the inorganic content within the feed matrix. Biomass typically contains higher concentrations of inorganic constituents which can vaporize at elevated temperatures and deposit on downstream equipment causing fouling of heat transfer surfaces and operational problems.

A major problem associated with the use of partial oxidation (O2 injection) as the heat source in biomass gasification involves the management of slag or vaporization of the inorganic constituents within the biomass. Due to the relatively low reactivity of coal towards gasification, commercial systems are designed to operate at very high temperatures (>2200° F.) thus potentially vaporizing an inappropriate amount of inorganic material of the biomass feed. Operating at lower temperatures reduces the efficiency of coal gasification process.

To date, all commercial gasifier systems that employ O2 to supply thermal energy through partial oxidation, generate localized hot spots at the injection point. The reaction of oxygen in the gasification environment is very fast and for all practical purposes occurs within the jet volume associated with the O2 injection nozzle. The O2 jet forms essentially a volume around the nozzle which is referred to as the partial oxidation or pox zone. Within this volume, localized temperatures (hot spots) can approach the adiabatic flame temperature determined by the combustion of the available oxygen and the local fuel which is typically synthesis gas. The endothermic reactions (gasification and pyrolysis) do not occur as fast as oxidation and consequently more chemical heat is generated than removed. One possible way to mitigate the high temperatures is to transfer cooler solids and gas through the pox region. A fluidized bed reactor using inert solids provides geometry to mitigate the higher temperatures. A solid with catalytic properties will provide additional heat mitigation through promotion of the steam reforming of gaseous hydrocarbons produced through pyrolysis.

Another was to mitigate the high temperatures is to use pulse oxygen injection so as to keep the maximum temperature within the oxygen injection region (referred to as the flame zone) below the fusion temperature of the biomass. This method for controlling this temperature involves the periodic injection of oxygen at a flow rate and frequency that prevents the attainment of temperatures approaching or exceeding the fusion temperature of the inorganic constituents within the biomass feed. Additionally, operating at temperatures in the range of about 1800° F. reduces the extent of volatility of these constituents, thereby minimizing fouling on downstream equipment.

Temperature control using pulsed oxygen is practiced in both the primary and secondary gasifier sections. The biomass is feed introduced at or near the bottom of the fluid bed primary gasifier in which both pyrolysis and gasification occur simultaneously. The feed system is orientated to provide maximum contact of the biomass with the oxygen, steam and other fluidizing gases within the fluid bed. The use of both steam and oxygen minimizes the extent of pyrolysis however this reaction will still proceed to some extent resulting in the production of tars, soot and other carbon rich solids which inherently gasify at a much slower rate than the parent biomass feed. The heat required in the primary gasifier is significant since most of the biomass gasification and all of the pyrolysis occurs in this section (endothermic reactions). The primary gasifier operates at a lower temperature than the secondary gasifier (1500°-1600° F. vs. 1800° F.) in order to reduce the potential for high temperatures within the pox zone.

The products from the primary gasifier section include tars and other carbon rich intermediates arising from pyrolysis as well unreached biomass. The gas phase contains H2, CO, CO2, H20 and CH4 as well as other hydrocarbons arising from the pyrolysis reaction. Both gas and solid products from the primary gasifier are sent to the secondary gasifier which will operate at a higher temperature in order to facilitate the gasification of the tars and other carbon rich solids.

The instant invention will be better understood with reference to the figures here. FIG. 1 hereof represents the basic form of the present invention as applied to any stage of the gasifier system in which fluidizing gas containing air or oxygen is injected. The nozzles introducing the fluidizing gas (preferably steam) and the oxygen are spaced in accordance to that required to secure the appropriate fluidization within the gasifier stage. The nozzles are referred to the conduits in which gas is transferred into the gasifier stage in such a manner so as to adequately fluidize the carbonaceous and inert particles. The conduits in which fluidizing gas is conveyed to each gasifier stage is referred to as nozzles. However, the gas injection geometry can also include any device which adequately conveys the fluidizing gas into the gasifier stage in such a manner which provides acceptable fluidization. For one skilled in the art there are several geometries which can be utilized such as bubble caps.

Referring to FIG. 1, for any stage within the gasifier system, this represents the section in which fluidizing gas is introduced showing a pressure containing boundary 600 which originates at the plane in which gas is introduced 610 to the upper portions of the fluidized bed 620. In this drawing, the nozzles 630, 640, and 650 which introduce a fluidization gas represent a subset of the plurality of nozzles required for the system. For simplicity, they are shown to be on a single plane but variations in height above the bottom 610 of the gasifier stage can also be utilized. The conduit required for transferring the fluidization gas from the source to the gasifier stage 600 are denoted as 660, 670, and 680. There can be a single conduit for each nozzle or multiple nozzles can be connected in one or more fluidizing gas conduits. The conduit for the introducing solids into the gasifier stage is shown as 690. This can be one or more conduits and is not significant with respect to this invention. The conduit conveys solids into the gasifier which can encompass feed for gasification or partially reacted feed containing char, carbon and/or soot that will undergo either additional gasification, partial oxidation or complete oxidation, depending upon the nature of the gasifier stage. In the majority of applications, inert solids used to promote fluidization and heat transfer will also be conveyed through conduit represented by 690.

FIG. 2 hereof presents a simplified drawing of the pulsed O2 sequence. In this example the nozzles conveying the fluidizing gas are shown on a single plane 200. Each nozzle 210 consists of the appropriate diameter or geometry to convey the appropriate amount of fluidizing gas over the cross section of the gasifier stage. A shroud 220 can be part of the nozzle geometry om prder tp facilitate the entrainment of the bulk fluidized gas and solids into the volume of the jet or bubble associated with the fluidization gas 230 and 240. When periodically introducing oxygen into the fluidization gas, there will be a local increase in temperature within the gas volume associated with the jet. This jet can also be considered a bubble forming at the exit of the nozzle and extending into the fluidized bed. As the O2 flow is cycled from zero flow to some maximum and then decreased back to zero, the jet including the O2 increases from zero to some maximum and then back to zero. The case of zero O2 flow is not shown in FIG. 3. Within this jet volume a local temperature rise will occur due the relatively high oxidation rate compared to gasification. The temperature rise will dependent upon the volume of the O2 introduced during the pulsed O2 time period. FIG. 4 presents qualitative plot of the O2 injection rate. The amount of O2 introduced during each pulse cycle will establish the maximum temperature rise within the jet. The volume of O2 introduced in each pulse is established by integrating the flow rate over the characteristic time period (t2−t1) and the interval between pulses is designated by (t3−t2). FIG. 4 refers to two classes of nozzles with “A” and “B” designations. This is a simple example in which adjacent nozzles (A and B) alternate pulsing in order to avoid a local high concentration of O2 which can lead to a high local temperature.

The application of the present invention involves estimating the local temperature rise of the jet during the time period in which oxygen is introduced. Before determining the O2 pulsation frequency and flow rate one must first establish the nozzle design required to achieve acceptable fluidization. This is relatively straight forward to one skilled in the art and involves establishing the fluidization properties for both the feed, reaction intermediates, and inert solids in the bed. Once established, a heat balance over the various stages of the gasifier is required to determine how much oxygen needs to be introduced in the gasifier stages. This is again straight forward to one skilled in the art of fluidized beds. The amount of oxygen to be introduced into each stage can then be distributed over the nozzle geometry established for fluidization. One then determines if this oxygen requirement can be introduced over one or more subsets of nozzles for each stage, recognizing that the jet, or bubble, detachment from fundamental principals follows the relationship;


1/tdetach proportional to(g/Q)1/5

where tdetach is the time frame in which gas from the gas entering the nozzle detaches and enters the fluidized bed, g is the gravitational constant, and Q is the flow rate. The detachment frequency is relatively insensitive to the total flow Q and in the application of this invention the total flow rate through each nozzle is not a significant consideration. The pulsing frequency (t3−t2) for O2 must be less then this characteristic frequency which can be determined empirically or through direct measurement.

The temperature rise within the jet is dependent upon the flow rate of O2 and the rate of local entrainment within each nozzle. Entrainment rates for specific nozzles must be empirically established since it is highly dependent upon the local geometry and local solids concentration. Empirical correlations exist allow one to estimate solids flux into a jet and from these estimates a local temperature rise within the jet can be established from the amount of oxygen which must be introduced into each nozzle. The invention requires that the local temperature rise based on the estimated entrainment of the bulk fluidization material (element 230 in FIG. 2 hereof) should not exceed the desired maximum operating temperature (in the range of about 1800° F. to 2000° F.). If this is the case, then the nozzle geometry for the fluidizing gas must be modified to allow less oxygen per nozzle. This modification can involve the use of smaller nozzle diameters, solids distribution system in the feed conduit(s) (690 in FIG. 1 hereof) or the use of entrainment devices (such as shrouds) to facilitate entrainment.

Once the local temperature rise for the appropriate amount of O2 to be added to each gasifier section is found to be acceptable, the required pulse frequency can be established for a specific gasifier section. In the case where local temperature are excessive in a specific gasifier section, it may be possible to find other portions of the gasifier system where O2 can be introduced without exceeding the maximum allowable temperature.

FIG. 4 hereof presents the major components of a preferred two-stage gasification system of the present invention. The gasification system is comprised of two fluid gasification stages depicted as the primary gasification/pyrolysis reactor (GPR) designated as vessel 10 and the secondary gasification reactor (SGR), designated as vessel 20. The two reactors shown in this figure are connected through three conduits designated as the intermediate syngas transfer line (IST) designated as 140, the main downcomer or standpipe, designated as 40, and the riser line designated as line 30. It is within the scope of this invention that additional conduits may also be used. The feed will preferably be a biomass having a particle size less than about 0.5 inches. In some instances the biomass will have to be comminuted to a particle size less than about 0.5 inches. Any suitable particle size reduction technique can be used for reducing the biomass feedstock to the desired particle size range. Non-limiting examples of such techniques include jet milling, cryogenic milling, ambient milling as well as the use of mills such as knife mills, hammer mills and disc mills.

The particulate biomass material is fed to primary, or first, gasification/pyrolysis reactor 10 via line 100. The feed system is preferably orientated to provide maximum contact of the biomass with the oxygen, steam and other fluidizing gases within the fluid bed. The size range for the fluidization solids will be those based on Group A an Group B of the Geldart Groupings. That is having a particle size range from about 20 microns to about 500 microns with densities between about 1400 kg/m3 to about 4500 kg·m3. The feed is preferably fed into the lower section of GPR 10 at an axial location appropriate for the fluidization properties of the material. Typically, multiple feed conduits will exist located at multiple axial and radial positions within the dense phase of GPR 10. For purposes of this invention it will be understood that all fluidized beds have a dilute phase zone and a dense phase zone and each are typically expressed as solid volume in that particular zone. For example, the dilute phase zone typically has a solid volume of from about 0.01% to about 15%, preferably from about 0.02% to about 1%, and more preferably from about 0.03% to about 0.1%. The dilute phase zone typically has about 1% or less of the solid volume contained in the dense phase zone, preferably about 0.1% or less, and more preferably about 0.01% or less. In one embodiment of the present invention the dense phase zone has a solid volume content of from about 20% to about 40%, preferably from about 15% to about 35%.

The specific axial position of the feed conduits will be dependent upon the density, particle size and the gas rate within the vessel. In addition to the chosen biomass feed particulates, inert or catalytic fluidization solids can be introduced into the fluid beds in order to facilitate heat transfer, to promote gasification, or both. These fluidization solids can be introduced with the primary feed within vessel 10 via line 100 or they can be fed separately through a dedicated nozzle represented by inlet 460 to secondary gasification reactor 20. They can also be fed at any other suitable location of the process unit by use of any suitable device that is used to incorporate a material into a pressurized vessel, which devices are well know in the art.

The fluidization gas for GPR 10 can be any suitable gas. Non-limiting examples of such gases include steam, carbon dioxide, nitrogen, natural gas, liquid hydrocarbons and biomass type materials that can be gasified to produce a synthesis gas. Steam is a preferred fluidization gas as well as CO2 generated from the biomass feedstock or a mixture of both. More preferred is steam. The fluidization gas is introduced into said the first gasification reactor via a first plurality of nozzles N1 and into said second gasification reactor via second plurality of nozzles N2. Oxygen, or an oxygen-containing gas, is also introduced at specified locations within the reactor configuration in order to generate the thermal energy required to drive the endothermic reactions associated with gasification and reforming. The feed rates of the biomass, oxygen, steam as well as other gases will be established by the criteria for establishing an acceptable gas fluidization rate and providing the appropriate carbon, hydrogen and oxygen ratios for achieving the desired syngas composition.

Because of the high temperatures required for both stages, the system must be heated using direct methods, preferably by the addition of O2 to both stages. The maximum temperature within the oxygen injection region (which is also sometimes referred to as the flame or pox zone) must be below the fusion temperature of the biomass. The preferred method for controlling this temperature involves the periodic injection of oxygen at a flow rate and frequency that prevents the attainment of the fusion temperature of the inorganic constituents of the biomass feed. Additionally, operating at temperatures in the preferred range of about 1700° F. to about 1800° F. reduces the extent of volatility of these constituents thereby minimizing fouling on downstream equipment.

Temperature control using pulsed oxygen is practiced in both the first gasification reactor as well as in the second gasification reactor. The use of both steam and oxygen minimizes the extent of pyrolysis, however this reaction will still proceed to some extent resulting in the production of tars, soot and other carbon-rich solids that will gasify at a slower rate than the parent biomass material. The heat required in the primary gasifier is significant since most of the biomass gasification and substantially all of the pyrolysis occurs in this reactor (endothermic reactions). The primary gasifier operates at a lower temperature than the secondary gasifier (1000° F.-1700° F. vs. 1700° F. 2000° F.) in order to reduce the potential for high temperatures within the pox zone.

The products from the first gasification reactor include a solid phase comprised primarily of tars and other carbon-rich intermediates arising from pyrolysis, as well unreacted biomass. By “carbon-rich” we mean greater than about 50 wt. % carbon, preferably greater than 60 wt. % carbon. A gas phase also results comprised primarily of H2, CO, CO2, H2O and CH4 as well as a small amount other hydrocarbons arising from the pyrolysis reaction. Both gas and solid products from the first gasification reactor are sent to the second gasification reactor which is operated at a higher temperature in order to facilitate the gasification of the tars and other carbon-rich solids.

The gasifier can be operated to adjust the desired composition of the resulting syngas. For example, although it is preferred that the resulting syngas be approximately 2:1 H2:CO ratio while maximizing the amount of CO within the product syngas, designated as line 180, the preferred ration for converting the syngas to DME is about 1.25:1. In some cases, a lower synthesis gas ratio is preferred (H2/CO<2) and in other cases a higher H2/CO ratio is preferred (>2.0).

Upon entry into GPR 10 the biomass feed immediately reacts with the stream containing the fluidization gas and undergoes both pyrolysis and gasification. The pyrolysis reactions lead to the formation of tars and soot-like solids comprised predominately of carbon. The temperature within GRP 10 should be as high as possible but below the fusion or slagging, or fusion, temperature of the inorganic components of the biomass. This temperature range is typically in the range of about 1600° to about 1800° F. In order to maintain this temperature, oxygen or an oxygen-containing gas is introduced into GPR 10 as previously described. Conduit 105 represents the inlet for the fluidizing gas which is preferably steam or recycle gas. The location of the inlet conduits for the fluidizing gases will be located at or near the bottom of the fluidized bed 110. Normal commercial practice is employed in this design based on achieving sufficient gas velocities to suspend the biomass and other solids present within the reactor.

As previously mentioned, the biomass within GPR 10 will undergo both gasification and pyrolysis which will lead to the formation of synthesis gas as well as carbon-rich solids. Pyrolysis can also lead to tar-like solids if allowed to exit the reactor in an insufficient time frame which does not allow further gasification and pyrolysis to occur. The solids generated in GPR 10 exits via conduit 190 and travels up through conduit 30 (riser) into SGR 20. The geometry of conduit 190 can be a simple L valve or it can be a curve shaped continuous riser. The fluidization characteristics of the solids generated in GPR 10 and the amount of gas to be moved define the preferred geometry of the riser. A transport gas is introduced into riser 30 through conduit 300 at a rate that is sufficient to transport the solids upwards into GPR 20. The composition of the riser transport gas in conduit 300 is preferably consistent with the overall carbon, oxygen and hydrogen content necessary to result in the desired product synthesis gas. Other non-reactive gases can be used to provide the necessary flow rates if the addition of steam and/or CO2 excessively perturbs the elemental balance needed to secure the preferred synthesis gas ratio. A plurality of riser conduits 30 and exit conduits 190 can be employed, especially when higher throughputs are desired.

The gases produced in GPR 10 exits the reactor through the cyclone 120. Solids transported with the gases into cyclone 120 are returned to GPR 10 through solids return leg 130. Some gases will pass through inter-vessel downcomer 40, but this does not correspond to a significant volume since the flow area of downcomer 40 corresponds to less than about 5% of the total cross sectional area of GPR 10. A plurality of exit cyclones 120 and inter-reactor downcomers 40 can be employed, especially when the desired throughput rate exceeds the practical limit of a single unit.

The total reactor volume available for gasification and pyrolysis preferably corresponds to a minimum solids residence time of 5 second based on the biomass feed volume at a temperature in the range of about 1000° to about 1700° F. Longer residence times are preferred. Consequently, riser 30 is sized appropriately to assist in maintaining the desired temperature of the gasifier. Operations at higher temperatures of about 1800° to about 1900° F. in the second gasification zone will allow shorter residence times while the converse is true at lower temperatures. The preferred operating temperature and residence time for GPR 10 is based on maximizing the amount of conversion of the biomass to synthesis gas or conversely minimizing the amount of carbon-rich solids (non-syngas products) produced. The depth of the fluid bed 110 within GPR 10 will be dependent upon the minimum depth required for stable fluidization and the required residence time as well as the gas velocity. Conventional fluid bed parameters can be used.

The gases produced in GPR 10 are transferred into SPR 20 through one of more conduits represented by 140. The secondary gasification vessel 20 consists of a fluidized bed 150 which gasifies the carbon-containing solids transferred to this vessel through conduits 30 and 140. The fluidization conditions for GPR 20 involve a much higher fraction of inert solids and the desired temperature range is higher in order to facilitate the gasification of the rich carbon containing solids generated through pyrolysis. The preferred temperature is greater than about 1700° F. and the more preferred is as high as about 2000° F. Conduits 320 and 330 represent nozzles in which fluidizing gas is introduced. Only two nozzles are shown but it will be understood that the number can vary from a minimum of one to a plurality of ten of more depending upon the amount of carbon and the operating temperature. The total amount of oxygen contained within the fluidizing gas is preferably sufficient to maintain the preferred temperature and to be introduced in the appropriate manner to avoid any excessive temperature zones which lead to liquid formation through slagging or fusion of the inorganic constituents within the solids. The depth and diameter of the fluid bed 150 in GPR 20 is determined by several criteria involving the following:

a) Minimum fluidization velocity to achieve sufficient mixing while maintaining as high a temperature as possible without slagging or otherwise forming a liquid phase from the inorganic constituents.
b) Achieving sufficient residence time for gasifying a high fraction (>90%) of the carbon containing solids transferred into vessel 20.
c) Introducing the oxygen over a sufficient area and volume to minimize the high temperature region associated with partial oxidation and combustion.

The cross sectional area and residence time for SPR 20 is larger and longer compared to GPR 10. These vessel conditions combined with a higher operating temperature ensure more complete gasification of the carbon containing solids formed during paralysis within GPR 10. Oxygen can be introduced through the one or more conduits (two are shown in the FIGS. 320 and 330) either continuously or in a pulse.

The effluent gas from SPR 120 will contain some solids which can be removed through one or more cyclones denoted with 160. The solids are returned to the fluid bed through the return line 170. There will be a small amount of solids in the effluent gas (conduit 180); however through the proper balancing of flow conditions and cyclones the amount of solids will not impact downstream operations. Solids produced in SPR 20 are removed via conduit 500 and an inert transport gas can be injected into this conduit via line 108 to facilitate transport of solids out of SPR 20.

The effluent line (180) can pass directly into heat exchangers to cool the gas prior to subsequent processing. Alternatively as shown in the figure, the hot gas can pass through a secondary or autothermal reformer denoted as 440 for further processing of the methane and light hydrocarbon gases into synthesis gas. This configuration is used when maximizing the amount of synthesis gas through conversion of the residual light hydrocarbons formed during paralysis. Before the effluent from SPR 20 is passed into the secondary or autothermal reformer (440), a variety of streams may be introduced into conduit 180. These can include O2 (420), steam (430), CO2 (400), and hydrocarbons (410). The introduction of one or more of these streams as well as their specific flow rates will depend on the requirements of the secondary or autothermal reformer (440) and the desired product gas (510).

Returning now to FIG. 2 hereof which presents a simplified drawing of the use of pulsed O2. At the onset of the pulse, the pox zone is relatively small with only a modest increase in temperature. As time elapses, the incoming oxygen allows the pox zone to fully develop leading to a larger volume and higher temperatures within the zone. During this period of development, the temperature within the pox zone is increasing due to a combination of increasing oxygen flow and a decrease in the surface area to volume ratio. The duration of the pulse must be less than the time required to fully develop the pox zone. This time is approximated by the velocity of the incoming oxygen jet over the length of the penetration of the jet. The velocity is determined by the flow rate and the O2 nozzle diameter while the jet penetration is established using existing correlations available in the literature and/or detailed momentum modeling (using computational fluid dynamics). The temperature within the pox zone during the pulsing period is determined through use of a heat balance relating the energy being released through pox and the cooling occurring due to the flux of cooler solids and gases passing through the pox zone. The heat balance can be solved within the boundaries defined by the extent of mass flux and the amount of endothermic reactions occurring within the pox zone. Using these boundaries, one can establish a temperature rise which is below the fusion and/or vapor pressure limit of the inorganic constituents within the biomass feed.

Claims

1. A method for controlling the temperature in a gasifier system designed for converting carbonaceous materials to a syngas:

a) Introducing an effective amount of fluidizing gas (preferably steam) into a gasification reactor containing a fluidized bed of solids;
b) Introducing a fluidizing gas through a plurality of nozzles located within each stage of the gasifier system where thermal energy can be added to support the gasification reaction;
c) Operating said gasification stage or stages at a temperature of about 1200° F. to about 1800;
d) Operating other gasification stages where the partial or complete oxidation occurs at temperatures of 1200 to 1800 F.
e) pulsing oxygen, through said plurality of nozzles, into said gasification reactor in order to keep the temperature in the range from about 1200° F. to about 1800° F., and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said carbonaceous material, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times;
f) the flow rate at which O2 is to be pulsed should be established such that the local temperature rise within the injection nozzle jet remains below 2000 F or less than the fusion temperature of the feed solids (whichever is lower)
g) The pulsing frequency of the O2 injection should be less than the characteristic bubble detachment frequency for the nozzle system designed to maintain the appropriate fluidization.

2. A process for converting a biomass feedstock to a synthetic gas in a two-stage gasification process unit, which process comprises:

a) Introducing an effective amount of steam into a first gasification stage containing a fluidized bed of solids;
b) introducing a fluidizing gas through a first plurality of nozzles located at the bottom of said first gasification stage containing solids thereby resulting in and maintaining a fluidized bed of said solids;
c) Operating said first gasification stage at a temperature of about 1000° F. to about 1600° F.;
d) introducing a biomass feedstock in particulate form into a first gasification stage containing the fluidized bed of solids wherein the residence time of said biomass in said first gasification stage is from about 5 to 90 seconds, thereby resulting in a gaseous phase biomass product stream and a carbon-rich biomass particulate product;
e) pulsing oxygen, through said plurality of nozzles, into said first gasification stage in order to keep the temperature in the range from about 1000° F. but not greater than about 1600° F., and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said biomass, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times;
f) transporting at least a portion of said gaseous phase biomass product stream to a solid/gas separation unit wherein particles greater than a predetermined size are separated and returned to said first gasification zone and wherein the treated gaseous phase biomass product stream is transported to a second gasification stage;
g) Transporting solids and particulates from said first gasification stage to a second gasification stage;
h) introducing, through a second plurality of nozzles, an effective amount of a fluidizing gas into said second gasification stage thereby resulting in a second fluidized bed of biomass particulates and fluidizing solids;
i) operating said second gasification zone in the temperature range from about 1600 F to about 2000 F, but at a temperature at least 50° F. greater than the first gasification stage and at a residence time from about 1 to 3 times that of said first gasification stage;
j) pulsing an effective amount of oxygen through said second plurality of nozzles of said second gasification stage in order to maintain said second fluidized bed in the temperature range of about 1600° F. to about 2000° F., 1700 to 1800, and to keep the partial oxidation zone of said nozzles below the fusion temperature of the inorganic fraction of said biomass, wherein said nozzles are divided into one or more sets of nozzles wherein each set is pulsing an effective amount of oxygen at the same or at different times, thereby resulting in said biomass being converted to a gaseous phase and a solid phase;
j) returning at least a portion of said solids to said first gasification stage;
k) passing said syngas stream to a solids/gas separation zone wherein substantially all of said remaining solids are removed, thereby resulting in a substantially solids-free syngas stream;
l) passing said syngas product stream to downstream processing; and
m) removing any excess solids from the gasification process unit to maintain a predetermined balance of solids.
Patent History
Publication number: 20100269411
Type: Application
Filed: Apr 26, 2010
Publication Date: Oct 28, 2010
Inventors: DUANE A. GOETSCH (Andover, MN), Jacqueline Hitchingham (Anoka, MN), Lloyd R. White (Minneapolis, MN)
Application Number: 12/767,546
Classifications
Current U.S. Class: Water (48/204)
International Classification: C10J 3/00 (20060101);