U.S. patent application number 12/767546 was filed with the patent office on 2010-10-28 for gasification of carbonaceous materials using pulsed oxygen.
Invention is credited to DUANE A. GOETSCH, Jacqueline Hitchingham, Lloyd R. White.
Application Number | 20100269411 12/767546 |
Document ID | / |
Family ID | 42990833 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100269411 |
Kind Code |
A1 |
GOETSCH; DUANE A. ; et
al. |
October 28, 2010 |
GASIFICATION OF CARBONACEOUS MATERIALS USING PULSED OXYGEN
Abstract
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.
Inventors: |
GOETSCH; DUANE A.; (Andover,
MN) ; Hitchingham; Jacqueline; (Anoka, MN) ;
White; Lloyd R.; (Minneapolis, MN) |
Correspondence
Address: |
HENRY E. NAYLOR & ASSOCIATES
P.O. BOX 86060
BATON ROUGE
LA
70879-6060
US
|
Family ID: |
42990833 |
Appl. No.: |
12/767546 |
Filed: |
April 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61214482 |
Apr 24, 2009 |
|
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61270645 |
Jul 10, 2009 |
|
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61295355 |
Jan 15, 2010 |
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Current U.S.
Class: |
48/204 |
Current CPC
Class: |
Y02P 20/145 20151101;
C10J 3/721 20130101; C10J 2300/0959 20130101; C10J 3/723 20130101;
C10J 3/482 20130101; C10J 2300/0916 20130101; C10J 2300/0976
20130101 |
Class at
Publication: |
48/204 |
International
Class: |
C10J 3/00 20060101
C10J003/00 |
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.degree. 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.degree. F. to
about 1800.degree. 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 O.sub.2 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 O.sub.2 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.degree. F. to about 1600.degree. 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.degree. F. but not greater than about 1600.degree.
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.degree. 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.degree. F. to about 2000.degree. 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.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] 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
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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
[0011] 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:
[0012] a) introducing an effective amount of fluidizing gas such as
steam into a gasification reactor containing a fluidized bed of
solids;
[0013] 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;
[0014] c) operating said gasification reactor at a temperature of
about 1000.degree. F. to about 1600;
[0015] 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;
[0016] e) pulsing oxygen, through said plurality of nozzles, into
said gasification reactor in order to keep the temperature in the
range from about 1000.degree. F. to about 1600.degree. 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;
[0017] 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
[0018] 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:
[0019] a) introducing an effective amount of steam into a first
gasification stage containing a fluidized bed of solids;
[0020] 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;
[0021] c) operating said first gasification stage at a temperature
of about 1000.degree. F. to about 1600.degree. F.;
[0022] 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;
[0023] 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.degree. F. but not greater than about
1600.degree. 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;
[0024] 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;
[0025] g) transporting solids and particulates from said first
gasification stage to a second gasification stage;
[0026] 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;
[0027] 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.degree. F. greater than the first
gasification stage and at a residence time from about 1 to 3 times
that of said first gasification stage;
[0028] 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.degree. F. to about 2000.degree. 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;
[0029] j) returning at least a portion of said solids to said first
gasification stage;
[0030] 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;
[0031] l) passing said syngas product stream to downstream
processing; and
[0032] m) removing any excess solids from the gasification process
unit to maintain a predetermined balance of solids.
BRIEF DESCRIPTION OF THE FIGURE
[0033] FIG. 1 is a simplified drawing of the nozzle section needed
for any gas injection into any section of the gasifier.
[0034] 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.
[0035] 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.
[0036] 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
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.degree.
F.) associated with gasification, the major products are typically
steam, hydrogen, CO.sub.2, CO and methane. Chars and soot represent
compounds rich in carbon and may contain small amounts (<5%) of
hydrogen.
[0044] The preferred gasification temperatures are in the range of
1600.degree. to about 1800.degree. 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.
[0045] 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.degree. 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.degree. F. High temperature
gasifiers (>1800.degree. F.) such as those utilized for coal,
employ O.sub.2 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 O.sub.2 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.degree. 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.
[0046] A major problem associated with the use of partial oxidation
(O.sub.2 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.degree. 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.
[0047] To date, all commercial gasifier systems that employ O.sub.2
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 O.sub.2
injection nozzle. The O.sub.2 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.
[0048] 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.degree. F.
reduces the extent of volatility of these constituents, thereby
minimizing fouling on downstream equipment.
[0049] 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.degree.-1600.degree. F. vs. 1800.degree. F.) in
order to reduce the potential for high temperatures within the pox
zone.
[0050] 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 H.sub.2, CO, CO.sub.2,
H.sub.20 and CH.sub.4 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.
[0051] 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.
[0052] 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.
[0053] FIG. 2 hereof presents a simplified drawing of the pulsed
O.sub.2 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 O.sub.2 flow is cycled
from zero flow to some maximum and then decreased back to zero, the
jet including the O.sub.2 increases from zero to some maximum and
then back to zero. The case of zero O.sub.2 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 O.sub.2
introduced during the pulsed O.sub.2 time period. FIG. 4 presents
qualitative plot of the O.sub.2 injection rate. The amount of
O.sub.2 introduced during each pulse cycle will establish the
maximum temperature rise within the jet. The volume of O.sub.2
introduced in each pulse is established by integrating the flow
rate over the characteristic time period (t.sub.2-t.sub.1) and the
interval between pulses is designated by (t.sub.3-t.sub.2). 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
O.sub.2 which can lead to a high local temperature.
[0054] 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 O.sub.2
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/t.sub.detach proportional to(g/Q).sup.1/5
where t.sub.detach 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
(t.sub.3-t.sub.2) for O.sub.2 must be less then this characteristic
frequency which can be determined empirically or through direct
measurement.
[0055] The temperature rise within the jet is dependent upon the
flow rate of O.sub.2 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.degree. F. to
2000.degree. 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.
[0056] Once the local temperature rise for the appropriate amount
of O.sub.2 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 O.sub.2 can be
introduced without exceeding the maximum allowable temperature.
[0057] 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.
[0058] 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/m.sup.3 to about 4500
kgm.sup.3. 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%.
[0059] 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.
[0060] 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 CO.sub.2 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.
[0061] Because of the high temperatures required for both stages,
the system must be heated using direct methods, preferably by the
addition of O.sub.2 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.degree. F. to about 1800.degree. F. reduces the extent of
volatility of these constituents thereby minimizing fouling on
downstream equipment.
[0062] 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.degree.
F.-1700.degree. F. vs. 1700.degree. F. 2000.degree. F.) in order to
reduce the potential for high temperatures within the pox zone.
[0063] 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 H.sub.2, CO, CO.sub.2, H.sub.2O and CH.sub.4
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.
[0064] 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 H.sub.2: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 (H.sub.2/CO<2) and in other cases a higher
H.sub.2/CO ratio is preferred (>2.0).
[0065] 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.degree. to about 1800.degree.
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.
[0066] 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 CO.sub.2 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.
[0067] 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.
[0068] 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.degree. to about 1700.degree. 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.degree. to about 1900.degree. 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.
[0069] 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.degree. F. and
the more preferred is as high as about 2000.degree. 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.
[0070] 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.
[0071] 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.
[0072] 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 O.sub.2 (420), steam (430), CO.sub.2 (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).
[0073] Returning now to FIG. 2 hereof which presents a simplified
drawing of the use of pulsed O.sub.2. 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 O.sub.2 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.
* * * * *