U.S. patent application number 09/138020 was filed with the patent office on 2001-10-11 for gasifier system and method.
Invention is credited to ABRAMS, STAN E..
Application Number | 20010027737 09/138020 |
Document ID | / |
Family ID | 22480074 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010027737 |
Kind Code |
A1 |
ABRAMS, STAN E. |
October 11, 2001 |
GASIFIER SYSTEM AND METHOD
Abstract
An energy conversion system which comprises a solid fuel fed
combustor system having a first chamber portion with an inlet feed
for feeding a metered amount of a solid fuel thereto, a first
burner stage having a first traveling conveyor firebelt, a metered
amount of air introduced in progressively increasing proportions
along the length of the first traveling conveyor. A second burner
stage having a second traveling conveyor firebelt with air
introduced in a progressively decreasing amount along the length of
said second traveling conveyor firebelt and a controller for
controlling air introduced to the system. The chamber has a sloped
common roof section common to the burner stages and is made of
radiative energy reflective fire brick and angled to reflect
radiative energy generated from fuel traveling on the traveling
conveyor firebelts and directing the radiative energy on fuel
traveling on the first firebelt. The air introduced is controlled
in amounts so as to automatically minimize excess air to the
firebelt conveyors to minimize the quantity of carbon monoxide and
nitrogen oxides and other pollutants.
Inventors: |
ABRAMS, STAN E.; (CASTLE
ROCK, CO) |
Correspondence
Address: |
JIM ZEGEER
801 NORTH PITT STREET #108
ALEXANDRIA
VA
22314
|
Family ID: |
22480074 |
Appl. No.: |
09/138020 |
Filed: |
August 21, 1998 |
Current U.S.
Class: |
110/234 ;
110/186; 110/188; 110/191; 110/203; 110/229 |
Current CPC
Class: |
F23N 2223/02 20200101;
Y02E 20/12 20130101; F23N 5/003 20130101; F23J 2217/40 20130101;
F23G 2203/105 20130101; F23L 15/00 20130101; F23G 5/16 20130101;
F23G 2203/8016 20130101; Y02E 20/348 20130101; F23G 5/0276
20130101; F23N 1/02 20130101; F23N 2237/16 20200101; F23G 5/444
20130101; F23G 2201/303 20130101; F23J 2217/50 20130101; F23G
2206/203 20130101; F23G 2900/50006 20130101; F23N 2223/08 20200101;
F23G 2205/122 20130101; F23G 2205/18 20130101; F23G 5/006 20130101;
Y02E 20/34 20130101 |
Class at
Publication: |
110/234 ;
110/191; 110/229; 110/186; 110/188; 110/203 |
International
Class: |
F23B 007/00; F23G
005/027; F23N 005/00; F23J 015/00 |
Claims
What is claimed is:
1. A solid fuel fed combustor system comprising: a first chamber
portion having an inlet feed for feeding a metered amount of a
solid fuel thereto, a first burner stage having a first traveling
conveyor firebelt with a first air introducing means for
introducing a metered amount of air in progressively increasing
proportion along the length of said first traveling conveyor to
cause endothermic decomposition of said solid fuel, a second burner
stage having a second traveling conveyor firebelt with a second air
introducing means for introducing air in a progressively decreasing
amount along the length of said second traveling conveyor firebelt.
and to cause exothermic decomposition of solid fuel received from
said burner stage, and means for controlling air introduced to said
burners.
2. The solid fuel combustor system defined in claim 1 wherein said
first chamber portion has a sloped common roof section common to
said burner stages and made of radiative energy reflective fire
brick and angled to reflect radiative energy generated from fuel
traveling on said traveling conveyor firebelts and directing said
radiative energy on fuel traveling on said first firebelt.
3. The solid fuel combustor system defined in claim 1 wherein said
means for controlling is adapted to automatically minimize excess
air to said firebelts to minimize the quantity of carbon monoxide
and nitrogen oxides produced in combusting said solid fuel.
4. A solid fuel fed gasification system comprising: a first chamber
portion having an inlet feed for feeding a metered amount of a
solid fuel thereto, a first burner stage having a first traveling
conveyor firebelt with a first air introducing means for
introducing a metered amount of air progressively increasing
proportion along the length of said first traveling conveyor, a
further burner stage having a further traveling conveyor firebelt
with a second air introducing means for introducing air in a
progressively decreasing amount along the length of said further
traveling conveyor firebelt, said first chamber portion having a
sloped roof section common to said burner stage and made of
radiative energy reflective fire brick for reflecting radiative
energy generated from fuel traveling on said traveling conveyor
firebelts and directing said radiative energy on fuel traveling on
said first firebelt.
5. An energy conversion system comprising in combination: an inlet
solid fuel feed conveyor and solid fuel metering unit for
controlling the solid fuel flow through said system, a gasifier for
extracting combustible gaseous products from solid fuel, a boiler
for converting thermal energy to steam, a gasifier fire tube for
coupling various combustible gaseous products produced in said
gasifier to said boiler at said gasifier fire tube coupling to said
boiler, including means for feeding outside ambient air in
preheated condition and at a controlled rate and gaseous products
from said gasifier fire tube to said boiler, cyclone device for
removing particulate from the discharge unit of said boiler, a
baghouse for removing of particulate from said cyclone, a scrubber
for cleaning noxious gases from the air stream before discharge to
the atmosphere, and a control system for controlling the speed of
conveyors in said gasifier.
6. The system defined in claim 5 including sensor means at each of
said inlet feed conveyor and metering unit, gasifier, gasifier fire
tube and connection to boiler, boiler, cyclone, bag house, and
scrubber, computer means connected to a said sensors for making
control adjustments for different types of fuel without changing
the equipment in the gasifier system.
7. A solid fuel gasification and energy system comprising in
combination: an inlet feed conveyor and fuel metering unit for
controlling the fuel flow through said system, a gasifier, a boiler
for converting thermal energy to steam, a gasifier fire tube for
coupling various gaseous products produced in said gasifier to said
boiler at said gasifier fire tube coupling to said boiler,
including means for feeding outside ambient air in preheated
condition and at a controlled rate, and feeding gaseous products
from said gasifier fire tube to said boiler, cyclone device for
removing particulate from the discharge unit of said boiler, a
baghouse for removing of particulate from said cyclone, a scrubber
for cleaning noxious gases from the air stream before discharge to
the atmosphere, a control system for controlling the speed of
conveyors in said gasifier, and sensor means at each of said inlet
feed conveyor and metering unit, gasifier, gasifier fire tube and
connection to boiler, boiler, cyclone, baghouse, and scrubber,
computer means connected to said sensors for making control
adjustments for different types of solid fuel without changing the
equipment in the gasifier system.
8. The invention defined in claim 7 wherein said inlet solid fuel
feed conveyor and solid fuel metering unit is controlled as a
function of thermal conditions at the output of said boiler.
9. The invention defined in claim 7 wherein said control system is
adapted to minimize excess air to said conveyors to minimize the
quantity of carbon monoxide produced.
10. The invention defined in claim 7 wherein said control system is
adapted to minimize the excess air and oxygen therein to said
conveyors so as to minimize the quantity of nitrogen oxides
produced.
11. The invention defined in claim 7 wherein said control system is
adapted to minimize the excess oxygen to said conveyors and said
firetube and the oxygen therein to minimize the quantities of
carbon monoxide and nitrogen oxides produced.
12. In a solid fuel fed combustor having an inlet feed for solid
fuel and a combustion chamber including a first burner stage having
a traveling conveyor firebelt with means to introduced air along
the length of aid conveyor from below solid fuel on said first
conveyor firebelt and a second burner stage having a second
traveling conveyor firebelt and means to introduce air along the
length of said second traveling conveyor firebelt from below solid
fuel on said second traveling conveyor firebelt, the method of
minimizing production of carbon monoxide and nitrogen oxide
pollutants comprising controlling the amount of air introduced in
said first and second burner stage and the speed of said conveyors
such that endothermic decomposition of fuel takes place in said
first burner stage and exothermic combustion and decomposition of
fuel takes place in said second burner stage.
13. The method defined in claim 11 wherein gaseous products from
said second burner stage are mixed and combusted with a controlled
amount of heated air from ambient in a firetube and the released
thermal energy converted to steam in a boiler coupled to said
firetube.
14. The method defined in claim 13 wherein gaseous products and
particulate from said boiler are filtered in a cyclone and baghouse
to remove particulates, and a scrubber for cleaning noxious gases
from the air stream before discharge to the atmosphere.
Description
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION
[0001] This invention relates to gasifier systems and methods for
the efficient conversion of solid fuels to usable heat energy.
DESCRIPTION OF THE PRIOR ART
[0002] The prior art is best exemplified by U.S. Pat. No. 5,284,103
entitled BIO-MASS BURNER CONSTRUCTION by Hand et al, which is a
division of U.S. Pat. No. 5,178,076 entitled BIO-MASS BURNER
CONSTRUCTION issued Jan. 12, 1993 to the same inventors. In these
patents, the burner utilizes a first burning chamber having a
falling fuel entrained bed zone positioned above a traveling grate
having a porous metallic woven belt. Primary air is directed
through the porous belt to establish an oxygen-starved first
burning chamber. A second burning chamber in fluid communication
with the first burning chamber has a restricted diameter and
effectively provides a hot-air gas nozzle. In larger sized units, a
plurality of conveyors constitutes the traveling grate with the
conveyors being arranged in head-to-head stepped relationship so
that unburned fuel received by gravity from the entrained bed zone
is agitated or jostled to enhance its burning.
SUMMARY OF THE INVENTION
[0003] According to the present invention, firebelts ensure that
the heat loss from heating unnecessary air is minimized. The
quantity of air at each point in the combustion process is
stringently controlled. This air control benefits the combustion
process in three ways:
[0004] First, by minimizing the heat loss of the combustion
process, this minimizes the amount of carbon monoxide which is
produced. Carbon monoxide, a priority pollutant, is produced
directly proportional to the combustion temperature. Therefore, by
minimizing excess air, the quantity of carbon monoxide produced is
minimized.
[0005] Secondly, nitrogen oxides, another priority pollutant, is
produced by combining the nitrogen in the air with the oxygen in
the air. This combination of nitrogen and oxygen only occurs at
high temperatures. The higher the temperature, the greater the
quantity of nitrogen oxides that are produced. While the combustor
of the present invention utilizes high temperatures, the formation
of nitrogen and oxides is minimized since there is no excess oxygen
to combine with the nitrogen. All the oxygen is used in the
combustion process.
[0006] Thirdly, by minimizing the amount of air supplied to the
combustion process, the amount of energy required to move air to
the combustor is minimized. Electrical energy costs are typically
20% less than similar combustion systems where the air is not
stringently controlled.
[0007] A further feature of the present invention is in the use of
reflected infrared energy. Heat is a form of electromagnetic energy
similar to light where the rays can be refracted or reflected.
Radiation produced from heat is of a longer wavelength than visible
light and is called infrared rays. By reflecting a certain amount
of the heat produced from a combustion process, this invention is
able to supply heat to the gasification process. The reflected heat
will be of benefit in two ways:
[0008] Firstly, the heat is reflected to a point where the heat can
be used to assist the combustion process. This is generally where
the fuel first enters the combustion process. At this point, the
fuel must be heated and the water removed. These processes require
addition of energy which can be added for heat of the combustion of
a part of the fuel or from the reflected energy. Using a part of
the fuel to preheat the remaining fuel is inefficient, leaving less
total heat available for production of electricity. Using reflected
heat removes or minimizes this inefficiency. The second way this
benefits the overall combustion process is in that the energy is
transferred in a beneficial way--not wasted by irradiating and
heating in the combustion chamber. Heat that is absorbed by the
combustion chamber is generally wasted since there is no direct
benefit from this radiation. A small portion is used in the
maintenance of the necessary combustion temperature but the
majority of irradiating heat is wasted as low-level heat
irradiating from the combustor exterior. Reflective heat added to
the fuel will benefit the overall combustion efficiency, and this
is a feature of the present invention.
[0009] Another feature of the invention is that the speed of the
conveyor drive and the rate of inlet air and the control of inlet
air is much more closely controlled so as to achieve high
efficiency. Still another feature of the invention is that the fuel
feed ramping is based on thermal conditions at the boiler
output.
[0010] Still another feature of the invention is that the induced
draft fan control is based on the draft pressure and boiler airflow
rate.
[0011] Still another feature of the invention is in the method to
control catalyst feed levels based on pollutant levels in the stack
as measured at a continuous emission monitoring system point.
[0012] Finally, the invention features a control system which is
based on operational parameters sensed at different stages in the
process.
[0013] Accordingly, the object of the invention is to provide an
improved gasifier system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The above and other objects, advantages and features of the
invention will become more apparent when considered with the
following specification and accompanying drawings wherein:
[0015] FIG. 1 is a diagrammatic sectional illustration of a
gasifier system incorporating the invention,
[0016] FIG. 2 is a cross-sectional illustration of a walking floor
trailer and inclined conveyor incorporated in the invention for
feeding fuel to the combustor unit,
[0017] FIG. 3 is a diagrammatic illustration, of the bin feeder and
rotary air lock incorporated in the invention,
[0018] FIG. 4 is a diagrammatic illustration of the boiler for
converting thermal energy to steam,
[0019] FIG. 5 is a diagrammatic illustration of the cyclone and
baghouse particulate collection division,
[0020] FIG. 6 is a diagrammatic illustration of the scrubber for
removing acid gases from the exhaust gas stream,
[0021] FIGS. 7A, 7B and 7C are flow diagrams illustrating the
combustion process and location of various sensors and control
elements incorporated in the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Referring now to FIG. 1, the gasifier system comprises an
inlet feed conveyor and fuel metering unit 10, a gasifier 11, a
gasifier firetube with connection to boiler 12, the boiler itself
13, a cyclone 14, baghouse 15, scrubber 16, and control system 17
which control system is diagrammatically illustrated in FIGS. 7A,
7B and 7C.
[0023] Inlet Feed Conveyor With Fuel Metering Unit 10
[0024] This section delivers the prepared fuel to the gasifier. It
includes the bin conveyor and rotary airlock. The control system
determines the required fuel flow for proper gasification and the
quantity to sustain the output of steam from the boiler. It then
determines the required speed of the conveyor and the airlock.
[0025] Walking Floor Trailer and Inclined Conveyor (FIG. 2)
[0026] The walking floor trailer 2-10 is used to store and deliver
fuel to the combustor with the inclined conveyor. The bin conveyor
2-12 monitors the amount of fuel and calls for additional fuel from
the walking floor trailer when necessary.
[0027] The walking floor trailer 2-10 works in the following
manner: The floor 2-14 is comprised of a number of strips that
independently move. To move the fuel to the rear of the trailer
(left end in FIG. 2), the strips all move together rearward. At the
end of the cycle (approximately four inches), each strip
independently moves forward, leaving the fuel undisturbed. This
cycle is repeated as required to move the fuel rearward as much as
necessary. A monitor or sensor 2-15 is located at the input end of
inclined conveyor 2-11. The inclined conveyor 2-11 monitors the
quantity of fuel located at the end of the trailer (the beginning
of the inclined conveyor) and moves the fuel rearward in the
walking floor trailer as required to maintain sufficient fuel in
the inclined conveyor.
[0028] Bin Feeder and Rotary Air Lock
[0029] The bin feeder (FIG. 3) receives the fuel from the inclined
elevator 2-11 (FIG. 2). The bin feeder is used to meter the fuel to
the combustor. Depending upon the quantity of fuel required by the
combustor to produce sufficient stream, the bin feeder 3-11 speed
will be varied to introduce sufficient fuel to the combustor. A
sensor 3-10 is used on the bin feeder to ensure that sufficient
fuel is present on the bin feeder belt. If additional fuel is
needed, a signal is sent to the inclined conveyor/walking floor
trailer (FIG. 2) for additional fuel.
[0030] The rotary air lock 3-12 is used to provide a mechanical
seal to minimize the quantity of unwanted air introduced into the
combustor.
[0031] Gasifier 11
[0032] The fuel gasification process takes place in gasifier 11
shown in FIG. 1. device. This process is described hereafter in the
section entitled "COMBUSTION PROCESS DESCRIPTION". The area of this
gasification is referred to as stage 1. The gasification in this
stage is controlled totally by the control system by taking the
following parameters and the system computer determines the
variables that need to change or remain the same during the
gasification process:
[0033] Stage 1a
[0034] Solid fuel feed rate,
[0035] Airflow rate,
[0036] Oxygen concentration,
[0037] Firebelt 20 speed,
[0038] Gasification temperature,
[0039] Draft pressure.
[0040] Stage 1b
[0041] Air flow rate,
[0042] Oxygen concentration,
[0043] Firebelt 21 speed,
[0044] Oxidation temperature,
[0045] Draft pressure.
[0046] Firebelt (Conveyor) Air Flows
[0047] The variable control of air through the firebelts
(conveyors) 20, 21 is the primary reason for the extremely low air
emissions from the combustor disclosed herein. Controlling the
amount of air that passes through each unit area of the firebelts
governs the quantity and quality of the combustion process.
[0048] The combustion process requires two components; fuel and an
oxidizer. Normally, air is used as an oxidizer but air contains
many gases, most of which do not contribute to the oxidation
process. In fact, the other gases in air can have a deleterious
effect on the overall combustion efficiency.
[0049] The entire combustion process is a series of discrete steps
where heat is either created or it is used. In general, the
following steps occur in the solid fuel combustion process:
1 Firebelt 20 Heat solid fuel to operating temperature requires
energy Remove adsorbed water from solid fuel requires energy Heat
combustion air to operating temperature requires energy Gasify fuel
components in solid fuel requires energy Decompose fuel gases into
elemental fuels requires energy Reflected heat from firebelt 21
produces energy Combust small percentage of solid fuel for heat
produces energy Net energy: Produces very little energy Heat loss:
Negligible Firebelt 20 Heat combustion air to operating temperature
requires energy Additional heating of fuel and ash from requires
energy 1,500.degree. F. to 2,500.degree. F. Reflect heat to
firebelt 20 requires energy Combust carbon into carbon monoxide
produces energy Preheat air from hot bottom ash produces energy Net
energy: Produces little energy Heat loss: Small - from hot bottom
ash Firetube 12 Heat combustion air to operating temperature
requires energy Additional heating of fuel and ash from requires
energy 2,500.degree. F. to 4,000.degree. F. Combust fuels produces
energy Preheat combustion air from combustor refractory produces
enemy Net energy: Produces significant energy Heat loss: Negligible
Boiler Superheater Heat transferred to steam requires energy Net
energy: Requires significant energy Heat loss: Negligible Boiler 11
Heat transferred to boil water requires energy Net energy: Requires
significant energy Heat loss: Negligible Boiler Preheater Heat
transferred to water requires energy Net energy: Requires
significant energy Heat loss: Negligible Cyclone 14 Heat lost via
radiant heating produces energy Heat lost in fly-ash produces
energy Net energy: Produces unusable energy Heat loss: Significant
Baghouse 15 Heat lost via radiant heating produces energy Heat lost
in fly-ash produces energy Net energy: Produces unusable energy
Heat loss: Significant Stack Fan Heat lost via radiant heating
produces energy Heat lost in atmosphere produces energy Net energy:
Produces unusable energy Heat loss: Significant Heat Balance Heat
Produced 100% Heat used to produce steam 82% Heat loss through
radiant convection 5% Heat loss discharge to atmosphere 11% Heat
loss from ash 2% Heat loss through combustor, fire tube and boiler
<0.001%
[0050] To ensure that the heat loss from heating unnecessary air is
minimized, the quantity of air at each point of the combustion
process is stringently controlled. This air control benefits the
combustion process in three ways.
[0051] The first way is by minimizing the heat loss in the
combustion process. This minimizes the amount of carbon monoxide
produced. Carbon monoxide, a priority pollutant, is produced
directly proportional to the combustion temperature. Therefore, by
minimizing excess air we minimize the quantity of carbon
monoxide.
[0052] Second, nitrogen oxides, another priority pollutant, is
produced by combining the nitrogen in air with the oxygen in the
air. This combination of nitrogen and oxygen only occurs at high
temperatures. The higher the temperature, the greater the quantity
of nitrogen oxides that is produced. While the combustor disclosed
herein utilizes very high temperatures, the formation of nitrogen
oxides is minimized since there is no excess oxygen to combine with
the nitrogen. All of the oxygen is used in the combustion
process.
[0053] Third, by minimizing the amount of air supplied to the
combustion process, this also minimizes the amount of energy
required to move the air to the combustor. Electrical energy costs
are typically 20% less than similar combustion systems where the
air is not stringently controlled.
[0054] Reflection of Infrared Energy
[0055] Heat reflection is another innovative feature of the
combustor of this invention. Heat is a form of electromagnetic
energy, similar to light where the rays can be refracted or
reflected. Radiation produced from heat is of a longer wavelength
than visible light and is called infrared rays.
[0056] By reflecting a certain amount of the heat produced from the
combustion process, we can supply additional heat to the
gasification process. The reflected heat will be of benefit in two
ways:
[0057] The first way is that by the heat reflected to a point where
the heat can be used to assist the combustion process. This is
generally where the fuel first enters the combustion process. At
this point, the fuel must be heated and the water removed. These
processes require the addition of energy which can be added from
either the combustion of a part of the fuel or from the reflected
energy. Using a part of the fuel to preheat the remaining fuel is
inefficient, leaving less total heat available for the production
of electricity. Using the reflected heat removes or minimizes this
inefficiency.
[0058] The second way reflected heat benefits the overall
combustion process is that the energy is transferred in a
beneficial way and is not wasted by irradiating and heating the
combustion chamber. Heat that is absorbed by the combustion chamber
is generally wasted since there is no direct benefit from this
radiation. A small portion is used in the maintenance of the
necessary combustion temperature, but the majority of the radiative
heat is wasted as low level heat radiated from the combustor
exterior. The reflection of the heat back onto the fuel will
benefit the overall combustion efficiency.
[0059] Gasifier Firetube with Connection to Boiler 12
[0060] This is actually a part of the gasifier and is referred to
as stage 2. It is the connecting tube to the boiler 13. Outside
ambient air is preheated between outer skins of the gasifier and
injected at a rate controlled by the control system. When this
oxygen rich air meets the gas from the gasifier, ignition takes
place in the firetube and thus enters the boiler. The following
parameters are taken in the firetube for the control system's
use:
[0061] Stage 2
[0062] Air flow rate,
[0063] Oxygen concentration,
[0064] Carbon dioxide concentration,
[0065] Carbon monoxide concentration,
[0066] Firetube draft pressure,
[0067] Firetube temperature,
[0068] Boiler draft pressure,
[0069] Boiler temperature.
[0070] Boiler 13 (See FIG. 7B)
[0071] The boiler 13 (FIG. 4) converts the thermal energy to steam.
The following parameters (see FIG. 7B) are taken in the boiler for
the control system use for control of the feed rate and the steam
output:
[0072] Air flow rate,
[0073] Steam pressure,
[0074] Steam temperature.
[0075] The boiler 4-10 (13) used in this embodiment of the
invention is of the Scotch Marine Type of boiler. Other types of
boilers such as A-frame, H-frame may be used in other
installations.
[0076] The boiler 4-10 absorbs the heat produced from the
combustion of the fuel and transfers it to water, which is
converted into steam. The steam is used to produce mechanical work
such as electrical generation, heating, etc.
[0077] The Scotch Marine Boiler is a three-pass type of boiler. The
first pass is through a large diameter central tube 4-11.
Approximately 40% of the usable heat is absorbed during the first
pass. This heat is used to heat the water and to convert part of
the water into steam. The second pass is in the reverse direction
through a series of small tubes 4-12. An additional 40% of the
usable heat is absorbed during the second pass. The remaining water
is converted into steam by the heat absorbed during the second
pass. The third pass is again through a large diameter tube 4-14
where the remaining 20% of the heat is absorbed. This heat is used
to, in essence, superheat the steam and ensure that no liquid water
remains.
[0078] An economizer may be attached after the boiler to preheat
the water and improve efficiency. This is not shown in this
embodiment.
[0079] Cyclone 14
[0080] The cyclone 14 (FIG. 5) is the first mechanical device that
removes particulate. The design of the cyclone is such that when
the air flows through it from the boiler the largest particulate
will drop from the airflow through the bottom of the cyclone to a
storage container. The following parameters are measured (see FIG.
7B) for the control system 17 in the cyclone:
[0081] Inlet temperature,
[0082] Inlet pressure,
[0083] Outlet temperature,
[0084] Outlet pressure.
[0085] The cyclone 5-10 and baghouse 5-11 operate as particulate
collection devices. In the combustion process, as the fuel is
combusted a small percentage of ash remains. Some of this ash is
entrained within the combustion air stream and is carried along
with the exhaust gases, called fly-ash. The cyclone 5-10 and
baghouse 5-11 remove the fly-ash so that it is not emitted into the
atmosphere.
[0086] The cycle acts through centripetal action. The gas spins
around in the cyclone and separates the heavy particles from the
gas based upon weight. The ash particles are collected at the
bottom of the cyclone and removed through a rotary air lock and a
vacuum removal system (eductor).
[0087] The baghouse operates on a different principle. The gas
passes through a series of fiberglass bags 5-13 that have very
small openings within them. The gas can pass through but the
particles cannot and remain adsorbed to the exterior of the bag. At
appropriate intervals, high-pressure air is introduced inside the
bags. This air literally forces the particles off of the bags and
they fall to the bottom of the baghouse. The particles then are
collected through a rotary air lock and an eduction system similar
to the cyclone.
[0088] Baghouse 15
[0089] The baghouse 15 (FIG. 5) is the final removal equipment for
particulate. The mesh size of the bags will be determined by the
particulate discharge requirements. The following parameters are
taken in the baghouse for the control system use:
[0090] Inlet temperature,
[0091] Inlet pressure,
[0092] Outlet temperature,
[0093] Outlet pressure.
[0094] Scrubber (FIG. 6)
[0095] The scrubber 6-10 is used to remove acid gases from the
exhaust gas stream. A mixture of lime (calcium oxide, a strong
caustic) and water is sprayed 6-11 through the exhaust gas. This
liquid chemically reacts with the acid gases such as sulfur
dioxide, hydrogen chloride, etc. to remove the acid gases. Plastic
or ceramic open-frame balls are often used as packing to increase
the surface area of the contact surface to improve the efficiency
of the chemical reaction. After the liquid has reacted with the
gas, the gas stream passes through a series of impediments, called
demisters 6-13 to remove all excess liquid. The gas then proceeds
to the stack exhaust fan where the clean exhaust gas is vented to
the atmosphere.
[0096] After the liquid has reacted with the acid gases, it is
collected in a spent slurry collector and returned for treatment to
a source by pump 6-14 in the bottom of the scrubber where it is
pumped to a separation chamber and the lime solution recycled.
[0097] The scrubber 6-10 (FIG. 6) cleans the bad gases from the air
stream before discharge to the atmosphere. It is usually a wet or
dry lime injection system depending on the discharge requirements.
The following parameters are taken for the control system that then
determines the feed rate for the catalytic agent:
[0098] Particulate (opacity),
[0099] Sulfur dioxide,
[0100] Nitrogen oxides,
[0101] Carbon monoxide,
[0102] Volatile organic carbon,
[0103] Certain hydrocarbon species,
[0104] Hydrogen chloride,
[0105] Hydrogen fluoride,
[0106] Hydrogen sulfide.
[0107] Cleaned exhaust gases are fed by fan 6-17 to the exhaust
stack.
[0108] Control System 17
[0109] The control system (see flow charts in FIGS. 7A, 7B and 7C)
is the determining factor for the gasification system to operate
properly and to be in compliance with the regulatory requirements
for air discharge. It includes Programmable Logic Controllers
(PLC's) and variable speed drives, diagrammatically illustrated in
FIGS. 7A, 7B and 7C, that operate the various motors, fans and
drives that operate the gasifier system. The PLC'S are in turn
controlled by signals from a computer that is programmed to
recognize all the variables listed plus other minor items and to
react properly from the data base. The program is designed to make
adjustments for different types of fuels (with different BTU
content) without changing equipment in the gasifier system.
COMBUSTOR PROCESS DESCRIPTION
[0110] Thermodynamic Extraction of Chemical Potential Energy
[0111] The release of chemical potential energy is a two-stage
process: Stage 1 gasifies the carbon-based solid fuels and Stage 2
oxidizes the gasified fuels to produce heat.
[0112] Stage 1 is subdivided into two separate processes involved
in the gasification of solid fuels. Stage 1a uses thermal
decomposition of the solid fuel introduced into the combustor to
break the fuel into gaseous fractions of lower molecular weight or
elemental composition. All absorbed compounds in the fuel such as
water and other solvents are released in this stage.
[0113] A polymerized hydrocarbon based fuel (both plastic and
lignin/cellulose base fuel) is decomposed into short chain
aliphatic hydrocarbons, elemental carbon, carbon monoxide and
hydrogen through the addition of energy as heat. Other elemental
based polymers including sulfur and silicon based compounds are
similarly broken into appropriate monomers or elements using the
same process. The ash produced from Stage 1a is largely carbon with
small amounts of metal oxides.
[0114] The heat required for the endothermic decomposition of the
fuel is produced from heat supplied from stage 1b and from limited
oxidation of fuel in stage 1a. The oxidation of the solid fuel is
limited in this stage by careful control of air added to the
combustion process in Stage 1a. The amount of air injected into
Stage 1a is controlled by the amount of oxidation required to
maintain the minimum necessary decomposition temperature in this
stage.
[0115] Stage 1b utilizes an exothermic partial oxidation of the
carbon in the ash to produce carbon monoxide and heat. The
remaining solid ash consists entirely of metallic oxides. The
reaction in Stage 1b is limited to partial oxidation of the carbon
by controlling the air injected into Stage 1b.
[0116] The heat of reaction of the carbon oxidation is used in
Stage 1a to decompose the fuel as previously described.
Approximately 80% of the heat of reaction is utilized in this
process with the remaining heat passing to Stage 2.
[0117] Physical Process
[0118] Solid fuel is introduced to the combustor section 1a
(firebelt 20) where gasification and moisture removal occurs. A
minimal amount of air is introduced to Section 1a to maintain the
minimum gasification temperature necessary for the specific type of
fuel used. Radiative energy from Section 1b, firebelt 21 also added
to the energy required for gasification.
[0119] The gases exiting this section consist of primarily carbon
monoxide, hydrocarbons (short-chain and long-chain) and water vapor
with minimal quantities of carbon dioxide and the balance of
nitrogen. The ash produced through the gasification process
consists of carbon, long-chain, high-boiling-point hydrocarbons and
metallic oxides.
[0120] Control of the gasification process is accomplished by
modulation of the fuel feed rate, the quantity of air introduced
through firebelts as measured by the oxygen concentration, the
gasification temperature and the speed of the firebelts. Air
injected into the solid fuel is minimized to prevent quenching of
the air/fuel reaction and to prevent complete oxidation of carbon
to carbon dioxide. The firebelt speed is controlled so that the
solid fuel has been completely gasified at the end of the belt.
[0121] The carbon ash from firebelt 20 falls onto section 1b
(firebelt 21) where additional air, in decreasing quantities, is
supplied to combust the carbon to carbon monoxide as well as
decomposition of the long-chain hydrocarbons to carbon monoxide and
hydrogen. Section 1b gases consist of carbon monoxide (10-15%),
hydrogen (5-20%) and carbon dioxide (1-10%) with the balance of the
gas being nitrogen. The ash remaining from this process consists of
metallic oxides with trace quantities of carbon-based compounds.
Oxygen content is minimized in stage 1b to prevent oxidation of the
carbon to carbon dioxide. The firebelt speed is controlled to
ensure complete oxidation of the ash just before the end of the
belt.
[0122] Radiative energy produced from stage 1b is reflected off of
the refractory walls onto section 1a where it is used to gasify the
solid fuel. Control of the section 1b process is performed by the
firebelt 21 speed, stage 1b temperature, control of the air to fuel
ratio through firebelt 21 as measured by the oxygen concentration
and by the overall draft (negative pressure) of the combustor
system.
[0123] Stage 2 combustion occurs within the firetube and within the
boiler where the carbon monoxide and hydrogen gases are oxidized to
carbon dioxide and water by the addition of additional air. The
firetube is used for mixing of the air and fuel gas with final
combustion occurring within the boiler cavity.
[0124] The Stage 2 combustion process is controlled by the air/fuel
gas ratio, boiler temperature, firetube temperature, carbon
monoxide concentration, oxygen concentration, carbon dioxide
concentration and by the overall draft of the combustor system.
[0125] Pollution Control
[0126] The combustor of the invention is a remarkably simple
combustion system, and it is very easy to minimize the air
pollutants that are produced from the combustion of solid fuels.
This is not the case with other combustor systems currently on the
market. With two exceptions, the combustor disclosed herein is the
only combustion system that does not require a gaseous fuel
afterburner to remove excess pollutants. The remaining two
combustion systems are on the order of ten times as expensive to
accomplish the same degree of pollution control as the combustor of
this invention.
[0127] There are five different categories of air pollutants that
the Environmental Protection Agency regulates in solid fuel
combustion systems. The combustor disclosed herein has been
specifically designed to minimize the quantity of each five
categories of pollutants. Each of these five categories will be
discussed individually.
[0128] Particulates
[0129] Particulates are released into the atmosphere from materials
in the fuel which cannot be burned. Usually these particles are a
chemical part of the fuel and when burned, recombine as small
particles. Part of these particles agglomerate together in chunks
which then collect in the bottom of the combustor and are removed
as bottom ash. The remaining particles are carried in the flue
gas.
[0130] These particles could be released into the atmosphere unless
they are removed. In the combustor of this invention, the particles
are removed by a device called a baghouse. The baghouse is a large
chamber filled with cloth bags that collect the dust as the gas
passes through them. The dust is then removed and the cleaned gas
can be released into the atmosphere.
[0131] The technology of baghouse design and construction is well
advanced. There have been very few refinements in the baghouse
particulate removal system since the mid-1970's.
[0132] Sulfur Dioxide
[0133] Sulfur oxides (NO.sub.x) are produced by the nitrogen
combining with oxygen in the presence of high temperatures.
Generally, the higher the temperature, the higher the quantity of
nitrogen oxides produced. Because nitrogen oxides have been found
to be a contributing factor in the destruction of ozone in the
atmosphere, the emission of these compounds are regulated and must
be minimized.
[0134] For most solid fuel combustion sources, a system of reducing
nitrogen oxides must be added to the combustor to lower the
nitrogen oxide emissions to an acceptable level. This reduction
system uses the injection of ammonia gas into the combustion system
and a catalytic converter (similar to today's automobiles) to
remove the nitrogen oxides and the ammonia. This is an expensive
process, both in capital costs for the previous metal catalyst and
for operating costs of ammonia injection. Additionally, another
pollutant, ammonia, a highly toxic compound, has been introduced
into the atmosphere which must be monitored.
[0135] The present combustor uses a different method to reduce
nitrogen oxides. By closely controlling the amount of air that is
introduced into the combustion process, the formation of nitrogen
oxides is minimized. Nitrogen oxides cannot form if there is not
oxygen to combine with the nitrogen. In the combustion process
disclosed herein, only enough air is added to the fuel to perform
the necessary combustion of the fuel. Because there is no
additional oxygen, there are very low quantities of nitrogen oxides
produced. Excess air is added only at the very end of the
combustion process to ensure complete combustion of the fuel. Using
this process, very low concentrations of nitrogen oxides are
emitted into the atmosphere.
[0136] Carbon Monoxide
[0137] Carbon Monoxide (CO) is the result of incomplete combustion.
This is due to either low combustion temperatures or insufficient
combustion air. In the combustor of this invention, the combustion
temperature exceeds 3,000.degree. F. for all solid fuels and
4,000.degree. F. using tires. To ensure that the combustion process
is complete and no carbon monoxide remains, a small amount of
excess air is added to the final stage of combustion. This results
in low concentrations of carbon monoxide and low concentrations of
nitrogen oxides, a feat unable to be accomplished by any other
combustor without an afterburner or a catalytic converter.
[0138] Volatile Hydrocarbons
[0139] Volatile hydrocarbons or volatile organic carbons (VOC) are
a class of compounds that are regulated by the Environmental
Protection Agency. These compounds include a wide range of
chemicals that can be emitted into the atmosphere. Included in this
list are compounds like dioxins, polychlorinated biphenyls (PCBs),
polynuclear aromatics (PNAs) and other hazardous air pollutants
(HAPs).
[0140] These compounds are created as the result of incomplete
combustion. In the combustor of this invention, the formation of
these compounds are kept to an extremely low level, in many cases
unmeasurable due to the extreme temperatures present in the
combustion process. It is highly unlikely that the Environmental
Protection Agency or any other environmental regulatory agency will
pass in the foreseeable future, regulations governing the release
of volatile hydrocarbons that the present combustor will be unable
to meet.
[0141] Monitoring of Pollutants
[0142] Regulations in all states and in most countries require a
facility to demonstrate that they are complying with the applicable
air emission standards. To demonstrate compliance, a facility must
usually install a system that continuously monitors the quality of
the gas being released into the atmosphere. The system is called a
Continuous Emission Monitoring System (CEMS).
[0143] Some of the parameters that are monitored include:
[0144] Particulates (opacity),
[0145] Sulfur dioxide,
[0146] Nitrogen oxides,
[0147] Carbon monoxide,
[0148] Volatile organic carbon,
[0149] Certain hydrocarbon species,
[0150] Hydrogen chloride,
[0151] Hydrogen fluoride,
[0152] Hydrogen sulfide.
[0153] The monitoring system continuously records the rate at which
the pollutants are released into the atmosphere. These records must
be submitted to the regulators to prove that the facility did
indeed meet the required standards.
COMBUSTOR CONTROL DESCRIPTION (FIGS. 7A, 7B AND 7C)
[0154] The combustion control process is a series of nested control
loops that provide the necessary regulation of heat production.
[0155] The primary loop that controls the heat production is
regulated by the quantity of fuel that is admitted to the
combustor. The fuel feed must have a wide range of quantities due
to the variety of fuels used in the combustor.
[0156] Within the primary loop are combustion control loops that
regulate the combustion process in Stages 1a, 1b and Stage 2. This
is controlled by the speed of the combustion belts and the quantity
of air added to the fuel as it is combusted. The goal of these
control loops is to have the fuel completely consumed while
maintaining the required pollution control.
[0157] All of the components controlled in the combustion system
contain feedback to inform the control system if a component
malfunctions. Different component types use different types of
feedback; for example, the air control dampers include a position
sensor so that the damper position set by the controller is
returned to the controller. If the position of the damper differs
from the setpoint, the operator is informed and if the error is
beyond a certain limit, the combustor is shut down.
[0158] Where possible, the control system is designed so that minor
component malfunctions are either self-corrected or the programming
compensates for the error. If minor errors are noted by the control
system, the system operator and system maintenance personnel are
notified for repair or replacement. This gives the control system a
great deal of intelligence including, where possible, predictive
failures.
[0159] In summary, the following parameters are used to control the
combustion process:
[0160] Stage 1a
[0161] Solid fuel feed rate,
[0162] Air flow rate,
[0163] Oxygen concentration,
[0164] Firebelt 20 speed,
[0165] Gasification temperature,
[0166] Draft pressure.
[0167] Stage 1b
[0168] Air flow rate,
[0169] Oxygen concentration,
[0170] Firebelt 21 speed,
[0171] Oxidation temperature,
[0172] Draft pressure.
[0173] Stage 2
[0174] Air flow rate,
[0175] Oxygen concentration,
[0176] Carbon dioxide concentration,
[0177] Carbon monoxide concentration,
[0178] Firetube draft pressure,
[0179] Firetube temperature,
[0180] Boiler draft pressure,
[0181] Boiler temperature.
[0182] While the invention has been described in relation to
preferred embodiments of the invention, it will be appreciated that
various other embodiments, adaptations and modifications of the
invention will be readily apparent to those skilled in the art.
* * * * *