Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYSTEM AND METHOD FOR GASIFICATION-COMBUSTION
PROCESS USING POST COMBUSTOR
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority to US Provisional
Application 61/080,805, filed July 15, 2008 and US Patent Application
12/467,887, filed
May 18, 2009, the disclosure of which is incorporated by reference in its
entirety.
TECHNICAL FIELD
[0002] The present invention relates to a system and method for implementing a
gasification-combustion process that converts waste or solid fuel into energy,
while
producing a minimal amount of undesirable emissions.
BACKGROUND
[0003] Municipal solid waste ("MSW") is the gross product collected and
processed by municipalities and governments. MSW includes durable and non-
durable
goods, containers and packaging, food and yard wastes, as well as
miscellaneous inorganic
wastes from residential, commercial, and industrial sources. Examples include
newsprint,
appliances, clothing, scrap food, containers and packaging, disposable
diapers, plastics of
all sort including disposable tableware and foamed packaging materials, rubber
and wood
products, potting soil, yard trimmings and consumer electronics, as part of an
open-ended
list of disposable or throw-away products. A traditional method of waste
disposal is a
landfill, which is still a common practice in some areas. Many local
authorities, however,
have found it difficult to establish new landfills. In those areas, the solid
waste must be
transported for disposal, making it more expensive.
[0004] As an alternative to landfills, a substantial amount of MSW may be
disposed of by combustion at a municipal solid waste combustor ("MWC"), which
is also
known as a waste-to-energy plant ("WTE"). The typical MWC has a moving grate
that
enables the movement of waste through the combustion chamber and thus allows
complete
combustion of the waste. The MWC usually includes a primary air source and a
secondary air source. Primary air is supplied from under the grate and is
forced through
the grate to sequentially dry (evolve water), de-volatilize (evolve volatile
hydrocarbons),
and burn out (oxidize nonvolatile hydrocarbons) the waste bed. The quantity of
primary
air is typically adjusted to maximize burn out of the carbonaceous materials
in the waste
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bed, without having any excess air. Secondary air is supplied through nozzles
located
above the grate and is used to create turbulent mixing that destroys the
hydrocarbons that
evolved from the waste bed. The total amount of air (primary and secondary)
used in a
typical MWC is approximately 60% to 100% more than the amount of air required
to
achieve stoichiometric conditions (i.e., the theoretical conditions under
which a fuel is
completely burned).
[0005] One of the problems associated with the conventional combustion of MSW
and other solid fuels is that it creates undesirable and harmful byproducts,
such as NOx,
carbon monoxide, and dioxins. For example, NOx is formed during combustion
through
two primary mechanisms. First, fuel NOx is formed by the oxidation of
organically bound
nitrogen (N) found in MSW and other fuels. When the amount of 02 in the
combustion
chamber is low, N2 is the predominant reaction product. However, when a
substantial
amount of 02 is available, an increased portion of the fuel-bound N is
converted to NOx.
Second, thermal NOx is formed by the oxidation of atmospheric N2 at high
temperatures.
Because of the high activation energy required, thermal NOx formation does not
become
significant until flame temperatures reach 1,100 C (2,000 F).
[0006] There are several known technologies for reducing the harmful emissions
created by conventional MSW combustion systems. For example, there are two
groups of
technologies known to control NOx emissions: combustion controls and post-
combustion
controls. Combustion controls limit the formation of NOx during the combustion
process
by reducing the availability of 02 within the flame and by lowering combustion
zone
temperatures. Post-combustion controls involve the removal of the NOx
emissions
produced during the combustion process (e.g., selective non-catalytic
reduction (SNCR)
systems and selective catalytic reduction (SCR) systems).
[0007] Despite the improvements made in reducing the harmful emissions of
conventional combustion systems, there is still a need for alternative methods
and systems
that efficiently convert MSW or other solid fuels to energy while producing a
minimal
amount of undesirable emissions.
SUMMARY OF THE INVENTION
[0008] The present invention relates to a gasification-combustion system and
method that converts waste or other solid fuels to energy while producing
significantly
lower quantities of NOx, carbon monoxide, dioxins, and other undesirable
emissions than
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conventional mass combustion. Gasification is the partial combustion of a
solid fuel that
produces a gas mixture. The gasifier of the present invention operates at
lower
temperatures and introduces less air than conventional combustion systems, and
thus it
produces a lower amount of undesirable emissions. According to the present
invention, a
post combustor uses the gas mixture produced by the gasifier to generate
thermal energy.
The post combustor controls combustion of the gas mixture using adjustable
injection
nozzles. The nozzles can be adjusted based on the composition of the specific
gas mixture
entering the post combustor, so as to achieve optimal combustion conditions
with minimal
emissions. In sum, the gasification-combustion process using the post
combustor of the
present invention significantly reduces the amount of undesirable emissions
produced
when converting waste or solid fuel to energy. The above-described method and
system is
just one example of the present invention, which can vary in other
embodiments.
[0009] For example, in one configuration of the present invention, a system
for
gasifying and combusting waste is provided. The system may contain a gasifier
for
mixing syngas with air or recirculated flue gas; said gasifier may contain an
entrance duct
and a premixing nozzle designed to inject the air or recirculated flue gas
into the gasifier.
The system may also contain a post combustor. The post combustor may contain
an
entrance duct for receiving syngas from the gasifier; a cyclone-shaped chamber
positioned
near the end of the entrance duct designed to collect fly ash or heavy weight
particles; a
top injection nozzle for directing air to flow through the post combustor into
the cyclone
shaped chamber; tangential nozzles for directing air or recirculated flue gas
into the post
combustor; sensors for measuring temperature, moisture, and carbon dioxide; a
controller
for positioning and controlling the nozzles to make air flow and temperature
more uniform
in the post combustor; and an exit duct for allowing gas to leave the post
combustor. In
some embodiments the top injection nozzle may be positioned so that the air
flowing
through the nozzle forces fly ash or heavy weight particles into the cyclone-
shaped
chamber; the tangential nozzles may have a direction and a position; and/or
the controller
may rely upon information from the sensors to determine the direction and
position of the
tangential nozzles.
[0010] Another configuration of the present invention sets forth a method for
gasifying and combusting waste. The method may comprise one or more of the
following
steps: mixing syngas with air or recirculated flue gas in a gasifier;
receiving the syngas
from the gasifier at a post combustor; collecting fly ash or heavy particles
with a cyclone-
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shaped chamber; directing air to flow through the post combustor into the
cyclone shaped
chamber; directing air or recirculated flue gas into the post combustor; using
a sensor to
gather measurements relating to temperature, moisture, and carbon dioxide of
gas inside
the post combustor; analyzing these measurements for determining which
direction and
position tangential nozzles connected to the post combustor should face;
adjusting the
tangential nozzles so that they face the determined direction and position;
and/or allowing
gas to leave the post combustor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are included to provide further
understanding of the invention and are incorporated in and constitute part of
this
specification, illustrate embodiments of the invention and together with the
description
serve to explain the principles of the invention. In the drawings:
[0012] FIG. 1(a) is a side schematic view of an embodiment of the post
combustor
used in the gasification-combustion process of the present invention.
[0013] FIG. 1(b) is a top schematic view of an embodiment of the post
combustor
used in the gasification-combustion process of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Reference will now be made in detail to the preferred embodiments of
the
present invention, examples of which are illustrated in the accompanying
drawings.
Throughout the following drawings, like numerals indicate like elements.
[0015] The present invention relates to a system and method that converts MSW
or
other solid fuels into energy while producing a reduced amount of undesirable
emissions.
The first step of the present invention, gasification, involves the partial
combustion of a
solid fuel. The second step, combustion, involves using the gas mixture
produced during
gasification to generate thermal energy. Both steps of the gasification-
combustion process,
and the apparatuses used to perform them, will be described in detail below.
[0016] Gasification is the partial combustion of MSW or other solid fuels. It
results in the production of a gas mixture of hydrogen, carbon monoxide,
carbon dioxide,
and water vapor, known as syngas. Gasification of solid fuel has several
advantages over
the conventional process of complete combustion. First, complete combustion
generally
requires mixing the fuel with air in excess of the amount needed to achieve
stoichiometric
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conditions (i.e., the ideal conditions in which fuel is completely burned).
The high amount
of oxygen present during complete combustion facilitates the production of
harmful gases,
such as NOx and dioxins. In contrast, gasification involves only partial
combustion, and,
as a result, it requires significantly less air than complete combustion. More
specifically,
the gasifier of the present invention can perform gasification of a solid fuel
using a sub-
stoichiometric amount of air. There are several benefits to the reduction in
air that is
achieved by using gasification. Introducing less oxygen means that a lower
amount of
NOx and dioxins is produced by the solid fuel. In addition, the fuel bound
nitrogen, which
would normally bond with excess oxygen to form NOx, is more likely to form
ammonia or
hydrogen cyanide. This is significant because, as described in detail below,
the syngas
formed during gasification is subsequently combusted using a post combustor.
During this
subsequent combustion, the ammonia and hydrogen cyanide react with and
decompose
some of the NOx that is generated by combustion of syngas. And, lastly, using
less air
reduces the costs associated with operating a combustion system.
[0017] Furthermore, the gasifier of the present invention is designed to
operate at
significantly lower temperatures than a conventional combustion system. In a
preferred
embodiment, the gasifier operates at temperatures below the melting
temperature of ash.
This is significant because the combustion of solid fuel produces both bottom
ash and fly
ash. When a combustion system operates at high temperatures, the ash can melt
and cause
slag formation on the moving grate components, which may require substantial
maintenance. Thus, by sustaining an operating temperature below the melting
point of ash,
the gasifier of the present invention limits the potential for slagging. This
reduces the
overall maintenance costs associated with converting waste or solid fuel to
energy and
makes its more practical to use conventional moving grate technology. The low
temperature gasification of solid fuel is also advantageous because it
produces less
particulate emissions and noxious gases, such as NOx, than conventional high
temperature
combustion.
[0018] According to the present invention, the syngas produced during
gasification
flows out of the gasifier and into a post combuster, where the syngas
undergoes
combustion. The post combustor subjects the syngas to turbulent air flow that
is only
slightly in excess of stoichiometric conditions (and thus still less than the
amount of air
used in conventional combustion systems). The post combustor operates at
higher
temperatures than the gasifier, which has the effect of reducing carbon
monoxide
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emissions and destroying most of the dioxins formed during gasification. In
addition, the
amount of excess air present in the post combustor is minimal, which, along
with the
ammonia and hydrogen cyanide formed during gasification, reduces the amount of
NOx
generated by combustion of the syngas. In a preferred embodiment of the
present
invention, the syngas is resident in the combustion chamber of the post
combustor for
longer than two seconds and the operating temperature is greater than 800 C.
The thermal
energy created by combustion of the syngas can be used in a variety of ways,
such as to
produce steam and generate electricity. In sum, the gasification-combustion
process of the
present invention can convert MSW or other solid fuel into energy while
generating
significantly lower emissions of carbon monoxide, NOx, and other organics such
as
dioxins than the conventional process of complete combustion.
[0019] FIGS. 1(a) and 1(b) show preferred embodiments of the post combustor 10
used in the gasification-combustion process of the present invention. As can
be seen in
FIG. 1(a), the syngas generated by the gasifier flows into the post combustor
10 through
an entrance duct 20. Prior to entering the combustion chamber 30 of the post
combustor
10, the syngas is premixed with air, flue gas recirculation (FGR), or another
oxidant such
as plasma that is injected into the entrance duct 20 via premixing nozzle 44.
Premixing
the syngas with an oxidant allows the combustion of the syngas to occur at a
lower
temperature than it would without such premixing. This is significant because
maintaining
a lower combustion temperature reduces the production of NOx.
[0020] The post combustor 10 is designed so that there is a cyclone shaped
chamber 50 at the end of the entrance duct 20, where the syngas enters the
combustion
chamber 30. The cyclone shaped chamber 50 is used to collect fly ash or heavy
weight
particles that are created during gasification or combustion. The cyclone
shaped chamber
50 is aided by the downward flow of air from the top injection nozzle 41. The
downward
air flow forces fly ash and other heavy weight particles downward into the
cyclone shaped
chamber 50, while allowing the syngas to enter the combustion chamber 30. The
fly ash
and other particles can either concentrate in the center of cyclone shaped
member 50 and
flow downward, or form slag on the walls of the cyclone shaped member 50 and
flow
downward.
[0021] The combustion chamber 30 of post combustor 10 includes multiple
nozzles for injecting air or another oxidant into the combustion chamber 30.
As explained
above, the top injection nozzle 41 is designed to inject air or another
oxidant into the
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combustion chamber 30 in a generally downward direction. Tangential injection
nozzles
42 and 43 are configured to inject air or another oxidant tangentially into
the combustion
chamber 30 from desired angles. The present invention contemplates that
additional
nozzles can be provided so as to achieve the desired injection of air into the
combustion
chamber 30. The nozzles 41, 42, and 43 can be positioned and controlled by a
controller
51 so that a uniform flow of air, as well as a uniform temperature, is
maintained
throughout the combustion chamber 30 during combustion of the syngas. This is
important because temperature variations, and specifically pockets of higher
temperatures,
promote the creation of NOx. Thus, by maintaining uniform air flow and
temperature, the
post combustor 10 of the present invention reduces the amount of NOx generated
during
combustion.
[0022] In a preferred embodiment of the present invention, the post combustor
10
measures certain characteristics, such as the temperature, moisture, and
carbon dioxide
content, of the syngas as it enters the post combustor 10 from the gasifier.
This
information is then used to adjust the nozzles 41-44, so as to obtain optimal
air flow and
conditions for combustion of the specific type of syngas entering the
combustion chamber
30. To obtain optimal conditions, the direction and amount of air flow from
each nozzle
41-44 is adjusted individually and independently of one another. Computational
fluid
dynamics ("CFD") is used to determine exactly how the nozzles 41-44 should be
adjusted
in response to the measurements taken as the syngas enters the combustion
chamber 30.
[0023] The post combustor 10 also includes an exit duct 60 that permits flue
gas to
leave the combustion chamber 30. As explained above, the flue gas can then be
injected
back into the combustion chamber 30 via the nozzles 41-44. This is known as
flue gas
recirculation ("FGR"). FGR lowers the amount of 02 in the combustion chamber
30 and
suppresses the temperature in the combustion chamber 30. As a result, FGR has
the effect
of reducing the amount of NOx generated by combustion of the syngas.
[0024] While exemplary embodiments of the invention have been shown and
described herein, it will be obvious to those skilled in the art that such
embodiments are
provided by way of example only. Numerous insubstantial variations, changes,
and
substitutions will now be apparent to those skilled in the art without
departing from the
scope of the invention disclosed herein by the Applicants. Accordingly, it is
intended that
the invention be limited only by the spirit and scope of the claims, as they
will be allowed.
[0025] It is hereby claimed.
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