Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02727638 2011-01-05
DESCRIPTION
Field of the invention:
This invention relates to a multi-system process that recycles waste system
heat sources,
specifically hot flue gases, from a combustion unit to dry wet biomass, which
is used as feedstock
for said combustion unit process. This invention improves the thermal content
of said biomass used
in thermal conversion processes by lowering the moisture content. The drying
unit consists of a
fluidized bed dryer wherein the bed consists of wet biomass, which is
fluidized by hot flue gases and
dried by the heat of condensation provided by the hot flue gases. The
fluidized bed dryer can be
incorporated to different power production processes. A storage tank unit
which stores the dried
biomass to be used for the thermal conversion process is also claimed and is
placed after the drying
unit.
BACKGROUND OF THE INVENTION
Research and development has been devoted to finding new sources of renewable
energy due
to the depletion of fossil fuels and environmental issues associated with
their use. Combustion of
fossil fuels produces high levels of carbon dioxide as well as NOX and SO,,
gas emissions which are
harmful to the environment (Alauddin, Z. 2010).
Renewable resources are part of our natural environment. Renewable energy is
beneficial for
the overall global energy consumption. Among renewable energy sources is
biomass, which is a
broad term that encompasses all solids derived from plant matter and wastes
(US2007220805-Al).
Carbonaceous biomass is derived from agricultural residues and industrial
wastes (McKendry, P.
2002). Energy can be extracted from biomass through thermal conversion
processes such as
gasification, pyrolysis and combustion. These processes either convert biomass
to another fuel, such
as liquid or gas, with a higher calorific value than raw biomass or directly
extract heat from biomass
for the production of work (Bridgwater, A. V. 1994). The relative energy value
of biomass is
determined by the chemical and physical properties of the large molecules from
which it is made
(McKendry, P. 2002.).
One such process to extract energy from carbanaceous biomass includes
combustion to
produce heat that can be exploited for the production of steam. In order to
efficiently extract useful
energy from biomass while maintaining low greenhouse gas emissions, thermal
conversion
processes can be designed to avoid high emissions and result in a high energy
yield. (Lundgren, J.
2009).
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Carbonaceous biomass can be combusted in a circulating fluidized bed
combustor. It
comprises two chambers, a combustion chamber and a cyclone (CA 1261123). The
combustion
chamber contains a bed of material, sand or limestone. The use of limestone is
preferred because it
35 lowers sulfur dioxide emissions of biomass combustion. Biomass is fed into
the combustion
chamber into which primary and secondary air is injected to fluidize the bed
material and combust
biomass according to a method described in CA-2148597 to transport the solid
particles between
chambers. Biomass is combusted in the combustion chamber during this process.
Combustion
products are carried to the cyclone separator which expels the cooled flue
gases through the top of
40 the cyclone while combustion products and hot bed material are recirculated
to the combustion
chamber.
A few caveats in the production of energy using biomass as the fuel are:
acquiring a high
grade biomass, the time needed for growth and cultivation of biomass, and the
limited availability of
biomass. The cost for collection and processing of biomass can be very
expensive and time
45 consuming. Therefore, when using biomass for the production of energy, a
route for maximum
efficiency and the ability to capture as much energy as possible, needs to be
achieved. To obtain
the most efficient route of energy production, the grade and moisture content
of the biomass must be
accounted for. (Gomez-Barea, A. 2010)
The use of high quality biomass will lead to an efficient route for energy
production
50 (Pimentel, D. 1981). The moisture content, particle size, and ash melting
behaviour play a factor in
the quality of fuel. High grade fuel consists of biomass with low moisture
content, small particle
size and high corrosive ash content (Loo, S. 2008). In combustion systems, the
moisture content in
the fuel must first be driven off, before combustion can occur efficiently.
The presence of water
reduces the combustion temperature below optimal operating conditions;
therefore, the combustion
55 of the fuel may be incomplete and cause unhealthy emissions (Hatfield, J.
2008). The water may
also recondense into the combustion system, which may lead to the corrosion of
the equipment,
causing potential hazards within the system such as fire (Stringer, J. 1996).
To avoid such issues
with biomass containing high moisture content, a method to dry biomass prior
to its thermal
conversion is required.
60 Several methods to dry biomass to an acceptable moisture level have been
developed,
however there are drawbacks to these methods. Although, in most cases, these
methods utilize waste
heat sources to render the process more economical, they usually require
energy to power the
mechanical parts incorporated into these biomass dryers. The general design of
these dryers uses
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CA 02727638 2011-01-05
concurrent or counter flow of a hot waste source, such as flue gases or steam,
with the biomass fed
65 into the dryer to drive off excess moisture.
Some driers are composed of a rotary kiln in which moist biomass is dried
through passing
hot waste air through the rotating drum, such as that described by
JP2010163509-A issued to
Kataoka. The design of these driers results in a low residence time and high
drying efficiency due to
the well-mixing biomass with hot flue gases. However, these driers contain
mechanical parts which
70 require high maintenance and increased capital costs. They also lower
overall efficiency of the
process due to additional external work required for the rotation of the kiln
of the dryer.
Other driers contain perforated conveyor belts, which carries the biomass
across the dryer,
while passing hot air through the belt such as that described by R0109240-B1
issued to Tudose.
Such dryers require high residence times as well as uneven drying of biomass
due to poor mixing
75 which results in lower drying efficiency.
A biomass vacuum dryer described by JP200525721 1 -A issued to Doi J. requires
lower
residence times but requires the use of a vacuum pump which requires higher
maintenance costs to
operate the pump and a lower overall efficiency of the process.
Drying methods described by DE102007038105-A1 issued to Fudel and
DE202005016236-
80 U1 issued to Goebler utilize waste hot air injected into a drying vessel,
also contain mechanical
parts such as a circulating drag chain and stirrer to properly mix biomass and
increase the drying
surface area. Although these methods recycle waste heat sources, they also
require additional
maintenance costs and capital costs to operate these additional mechanical
parts.
The invention disclosed in this patent addresses these drawbacks by improving
the drying of
85 biomass and reducing maintenance and capital costs created by the use of
additional mechanical
parts in the dryer. The invention consists of a fluidized bed dryer through
which hot flue gases from
a circulating fluidized bed combustor boiler are injected into the dryer.
Moist biomass is fed into the
dryer and fluidized by the flow of hot flue gases that pass through a tube
protruding through the
distributor plate. Therefore, moist biomass circulates in the dryer which
improves the removal of
90 moisture from biomass by the hot flue gases. This drying method, thus,
strictly relies on hot flue
gases to fluidize and dry biomass without the use of additional mechanical
power. The fluidized bed
dryer can be incorporated in thermal conversion processes of biomass that
produce waste heat
sources which can be exploited for drying moist biomass. The efficiency of the
thermal conversion
process of biomass is increased by the removal of water and recycling of waste
heat sources.
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SUMMARY OF THE INVENTION
In the present specification, including the claims, the terms dry biomass' and
wet biomass'
signify biomass of approximately 30% and 60% moisture content on a wet basis,
respectively. The
terms `cold flue gas' and `hot flue gas' signify flue gas at a temperature of
approximately 120 C and
100 300 C respectively.
The following method of producing high quality biomass for combustion is
disclosed.
Specifically, the invention utilizes existing plant waste heat sources, such
as hot flue gases, to dry
moist biomass to improve their thermal content or processibility. Biomass
varies in moisture content
and is derived from a variety of sources. Depending on the climate that
biomass is extracted from,
105 biomass may contain a high level of moisture. This requires moist biomass
to be processed, through
pelletizing, briquetting or shredding, to form small and somewhat uniform
biomass fragments,
which improves handling. These processes not only make biomass more
manageable, but also lower
the moisture content and increase the thermal content of biomass fuel.
However, these processing
methods, while increasing the efficiency of the combustion process, require
energy input which
110 lowers the profitability of the process. Disclosed in this patent is a
novel dryer and drying process
which utilizes recycled waste heat sources from the combustion process, which
not only efficiently
dries biomass to increase its thermal content but also renders the multi-step
process more profitable.
The present invention includes a method for improving the process in drying
woody biomass
fuel and increasing the efficiency of combustion of said biomass, for power
generation, using a
115 multi-step system comprising of.
(a) A fluidized bed dryer to lower the moisture content of moist biomass;
(b) A storage tank used to keep fuel dry until needed for combustion in a
fluidized
bed boiler;
(c) A control element at the outlet of the storage tank to regulate amount of
dried fuel
120 fed into fluidized bed boiler; and
(d) A circulating fluidized bed combustion boiler, containing dry solids such
as
sand or limestone, which circulate with the dried fuel for combustion of said
fuel.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1: Overall process diagram.
125 Figure 2: Direction of flow in the circulating fluidized bed combustion
boiler.
CA 02727638 2011-01-05
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention relates to a method and apparatus for drying moist woody
biomass,
130 used as a fuel for firing a circulating fluidized-bed combustion (CFBC)
boiler.
Embodiments of the claimed process are hereafter described with general
reference to Figure
1 and Figure 2.
Turning to figure 1, moist biomass is fed into the bottom portion of the
fluidized bed dryer
via fuel inlet 20. The fluidized bed dryer 10 has a conical bottom and a roof
that is at a 45 angle
135 directed to the storage tank 30. The angled roof facilitates transporting
biomass from the fluidized
bed dryer 10 to the storage tank 30. Hot flue gas from the CFBC boiler 40 is
fed via the flue gas
connection pipe 50, either through the conical bottom of the fluidized bed
dryer 10 passing through
the distributor plate 70, or through a separate pipe 60 extended from the flue
gas connection pipe
that protrudes through the dryer distributor plate 70. Flow of flue gas is
redirected and controlled
140 through either pipe by a controlling valve 80. This setup controls whether
the biomass is fluidized in
the fluidized bed dryer 10, when the flue gas enters through the flue gas
connection pipe 50 which
protrudes through the distributor plate 70. Biomass, thus, recirculates in the
fluidized bed dryer 10
and is dried through heat transfer between hot flue gases and biomass. The
flow of flue gases in
fluidized bed dryer 10 is shown in Figure 2. The flue gases provide the
required heat to remove the
145 excess moisture from moist biomass, which remains fluidized in the
fluidized bed dryer 10 until the
moisture content of the biomass reaches approximately 30%. When desired
moisture content is
attained, the flue gas is redirected by the controlling valve 80 to pass
through the conical bottom of
the fluidized bed dryer 10 through the dryer distributor plate 70 and the
biomass is transported to the
storage tank 90 via the top of the fluidized bed dryer 10. This will provide
sufficient velocity to
150 transport the biomass via the top of the fluidized bed dryer 10 to the
storage tank 30. The flue gas
remains directed through the dryer distributor plate 70 until all fluidized
dried biomass particles
have been transported to the storage tank 30. Particles are transferred to the
storage tank 30 via the
dryer-storage transfer pipe 90.
Dry fluidized biomass is transferred from the fluidized bed dryer 10 to a
cylindrical steel
155 storage tank 30. The storage tank 30 has a conical bottom and a roof. The
biomass is transferred to
the top of the storage tank 30 from the fluidized bed dryer 10 via dryer-
storage transfer pipe 90. The
dried biomass is stored in the storage tank 30 which is fixed with a control
element 100 which
regulates the feed rate into the CFBC boiler 40. Flue gases, cooled from 300 C
to approximately
120 C, exit via exhaust pipe 110 located on the roof of the storage tank 30.
The dried biomass settles
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160 to the bottom of the storage tank 30 and accumulates. The bottom of the
storage tank 30 is conical so
that the accumulated biomass can fall due to gravity directly into the storage-
CFBC boiler screw
feeder 120. As dried biomass continues to accumulate at the bottom of the
storage tank 30, it forms
aggregates. The aggregation of biomass at the bottom of the storage tank 30
makes it difficult to
extract the biomass fuel to be transported to the CFBC boiler 40. Biomass is
transferred, as needed
165 by the CFBC boiler 40, using the storage-CFBC boiler screw feeder which
has a sharp blade. As
rotation of the storage-CFBC boiler connection screw 120 occurs, biomass
aggregates are sheared
and then moved through the storage-CFBC boiler screw feeder 120 to the CFBC
boiler 40. This
ensures controllable feed into the CFBC boiler 40 via the CFBC boiler inlet
130.
Dried fuel from storage tank 40 is fed to the CFBC boiler 50 via the CFBC
boiler inlet 140,
170 connected to the storage tank 40 by the storage-CFBC connection screw 130.
Through combustion
of the fuel, flue gases are produced and expelled though the gas outlet pipe
140 at the top of the
cyclone. The flue gases heat steam through a heat exchanger 150. The waste
flue gases are then fed
to the fluidized bed dryer 10 via the flue gas connection pipe 50.
REFERENCES
175 Alauddin, Z., P. Lahijani, M. Mohammadi. 2010. Gasification of
lignocellulosi biomass in fluidized
beds for renewable energy development: A review. Renewable & Sustainable
energy
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Bridgwater, A. V. 1994. Catalysis in thermal biomass conversion. Applied
Catalysis A: General
180 116.-5-47.
Gomez-Barea, A., B. Leckner. 2010. Modeling of biomass gasification in
fluidized bed. Progress in
Energy and Combustion Science, 36 (4): 444-509.
185 Hatfield, J. 2008. Effect of moisture content in biomass material. Crown
Royal Corp. Fluid
Journal. ]-11.
Loo, S., J. Koppejan. 2008. Biomass Combustion and Co-firing. ISBN 978-1-84407-
249-1.
190 Lundgren, J., E. Pettersson. 2009. Combustion of horse manure for heat
production Bioresource
Technology, 12:3121-3126.
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McKendry, P. 2002. Energy production from biomass (part 1): Overview of
biomass. Bioresource
Technology 83: 37-46.
195 McKendry, P. 2002. Energy production from biomass (part 2): conversion
technologies.
Bioresource Technology 83: 47-54.
Pimentel, D., et al. 1981. Biomass Energy from Crop and Forest Residues.
Science, 221:110.
200 Stringer, J., A. J. Minchener. 1996. High temperature corrosion in
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