Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
OXYGEN ENHANCED COMBUSTION OF BIOMASS
Field of the Invention
The present invention relates to combustion of biomass, especially in
power plants that generate steam.
Background of the Invention
Growing demand for electrical power obtained from fuel-fired power
plants, combined with growing interest in using biomass as fuel for such
plants,
has increased interest in finding efficient methods for combusting biomass in
power plants. The moisture content of biomass is typically very high. For
example green wood typically contains 40 to 60 % moisture. This increased
moisture content, and its low energy density, are among the primary issues
with
firing biomass in boilers and especially boilers that were designed for other
fuels
such as coal. For example, converting a coal-fired boiler to fire biomass
typically
cause the boiler to be derated by 30-50%.
Many boilers are 'flue gas limited' and can only handle up to a specific
amount of flue gas. This flue gas limitation may be due to the capacity of
fans if
present for impelling flow of flue gas, or may be based on design limits. For
example, boiler design considerations, such as the maximum allowable velocity
in
the convective section, can limit flue gas volume. Since the flue gas volume
per
unit heat input, or "specific flue gas volume", increases dramatically when a
fuel
such as coal is replaced with biomass, it causes a large impact on the
distribution
of heat absorption in the furnace. A boiler is typically designed for a
relatively
narrow range of specific flue gas volume. Within this range the boiler is
designed
for a specific amount of heat absorption in the furnace, or radiant section,
and the
convective section. When the specific flue gas volume is increased more heat
is
'pushed' from the radiant section into the convective section. This increase
in
heat transfer in the convective section often requires the use of water sprays
into
the steam flow to maintain the desired steam temperature, which may decrease
overall efficiency. This shifting of heat transfer from the radiative furnace
section
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to the convective section of the boiler further reduces or derates the boiler
capacity.
Conversion of an existing boiler to biomass firing can also significantly
degrade the combustion performance of the unit. The reduction in combustion
performance is due to both changes in the fuel characteristics and the firing
system. The high moisture content the fuel makes it more difficult to ignite
and
bum. This problem is compounded by the fact that grate firing systems often
suffer from uneven fuel distribution over the grate and non-uniform mixing
between the air and the fuel ¨ leading to incomplete combustion on parts of
the
grate and high CO emissions in flue gas. To overcome both of these problems
boiler operators typically operate the boiler at increased stoichiometric
ratios
(defined as the ratio of air supplied to that required to bum the fuel). The
stoichiometric ratio is often measured as the amount of oxygen left in the
flue gas
at the end of the combustion process. For example, a typical coal-fired boiler
will
operate with 3% "excess oxygen". This means the flue gas contains 3% oxygen
(by volume, wet basis). In contrast the flue gas from a biomass-fired boiler
typically contains at least 4.5% 02 (vol, wet basis) to control CO emissions
within
regulatory limits.
The extra air further increases the flue gas volume and impacts both the
thermal efficiency of the boiler, and the auxiliary power required for the
boiler. In
the first case the extra air volume carries heat out the stack, increasing the
sensible
heat loss. The extra air also increases the power required by both the blower
that
pushes combustion air into the boiler (typically called the forced draft, or
FD,
fan), and the blower used to draw the flue gas from the boiler (typically
called the
induced draft, or ID, fan). Therefore the overall effect of the excess air is
to
increase the specific flue gas volume, which is the gas volume per units of
energy
output (further limiting the amount of fuel that can be fired), reduce the
thermal
efficiency (allowing less of the fuel that is fired to be used to raise
steam), and
increase the auxiliary power (reducing the net power available
The present invention provides an improved method for combustion of
biomass in boilers.
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Brief Summary of the Invention
One aspect of the present invention is a method of combustion, comprising
(A) providing apparatus that includes a combustion chamber in which fuel
having a given moisture content and a given specific energy content fed into
the
combustion chamber at a given mass feed rate can be combusted in air to
produce
heat energy at a given rate,
(B) feeding into said combustion chamber fuel that contains biomass and
that has a specific energy content lower than said given specific energy
content, so
that combustion in air of said fuel fed at said given mass fed rate in said
combustion chamber in air produces heat energy at a rate lower than said given
rate, while feeding oxygen into said combustion chamber so that said fuel is
in
contact with gaseous oxidant whose oxygen content exceeds that of air by up to
5
vol.% above that of air, and
(C) combusting the fuel with said gaseous oxidant in said combustion
chamber.
Another aspect of the invention is a method of increasing fuel combustion
rate in a combustion chamber with a convective heat transfer zone in which
fuel
that contains biomass is combusted with combustion air in said combustion
chamber to produce flue gas containing a specific oxygen concentration between
3 vol. % and 8 vol.% at a given maximum fuel feed rate limited by the capacity
of
an FD fan if present for feeding said combustion air, the capacity of an ID
fan if
present to evacuate flue gas from said combustion chamber, the flue gas
velocity
in said convective heat transfer zone, or the carbon monoxide concentration in
said flue gas, feeding into said combustion chamber additional fuel containing
biomass and additional oxidant containing at least 50 vol. % 02 , reducing
said
combustion air flow rate by the amount that reduces said oxygen concentration
in
said flue gas by 0.1 to 5.0 vol.% and combusting said additional fuel without
exceeding said FD fan capacity, said ID fan capacity, said flue gas velocity,
nor
said carbon monoxide concentration.
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Yet another aspect of the invention is a method of increasing fuel
combustion rate in a combustion chamber with a grate for combustion of fuel
with
a convective heat transfer zone in which fuel that contains biomass is
combusted
with combustion air in said combustion chamber to produce flue gas containing
a
specific oxygen concentration between 3 vol. % and 8 vol.% at a given maximum
fuel feed rate limited by the carbon monoxide concentration in said flue gas,
feeding into said combustion chamber additional fuel containing biomass and
additional oxidant containing at least 50 vol. % 02 to one or more oxygen
deficient areas on said grate to maintain or reduce said carbon monoxide.
Brief Description of the Drawings
Figure 1 is a cross-sectional view of one embodiment of combustion
apparatus in which the present invention can be practiced.
Detailed Description of the Invention
The present invention is an improvement in the combustion of fuel
comprising biomass in a combustion chamber. "Biomass," for the purposes of the
present invention, means any material not derived from fossil resources and
comprising at least carbon, hydrogen, and oxygen. Biomass includes, for
example,
plant and plant-derived material, vegetation, agricultural waste, forestry
waste,
wood, wood waste, paper waste, animal-derived waste, poultry-derived waste,
and
municipal solid waste. Other exemplary feedstocks include cellulose,
hydrocarbons, carbohydrates or derivates thereof, and charcoal. Typically
biomass
can include one or more materials selected from: timber harvesting residues,
softwood chips, hardwood chips, tree branches, tree stumps, leaves, bark,
sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice straw,
sugarcane
bagasse, switchgrass, miscanthus, animal manure, municipal garbage, municipal
sewage, commercial waste, grape pumice, almond shells, pecan shells, coconut
shells, coffee grounds, grass pellets, hay pellets, wood pellets, cardboard,
paper,
plastic, and cloth. The present invention can also be used for fuels that also
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comprise carbon-containing feedstocks other than biomass, such as a fossil
fuel
(e.g., coal or petroleum coke), i.e. mixtures of biomass and fossil fuels.
The present invention is especially applicable to combustion of biomass in
a combustion chamber that is part of a system that includes, in addition to a
combustion chamber, heat exchangers that absorb heat of combustion into, for
instance, water. Preferred systems include power generation boilers,
especially in
which heat exchange to boiler feed water is achieved by radiant heat transfer
and
by convective heat transfer. The heat exchange produces steam, superheated
steam, and/or supercritical steam, which can be used to generate electric
power.
The present invention is especially applicable to combustion of biomass in
a combustion chamber including a grate on which fuel rests as it is being
combusted. However, the present invention can be practiced in systems wherein
the fuel is combusted in the combustion chamber by grate firing, suspension
firing, or a combination of grate firing and suspension firing, or by firing
in a
bubbling fluidized bed or in a circulating fluidized bed.
The following description refers to Figure 1, and illustrates practice of the
invention in one embodiment in which grate firing is employed.
Combustion chamber 1 includes grate 2 on which fuel can rest after the
fuel is fed into combustion chamber 1, for instance as fuel stream 3. Grate 2
is
solid and includes a plurality of openings through which gas can flow,
including
primary air which is fed as primary air stream 4. Optionally, overfire air
stream 5
can also be fed into the combustion chamber 1.
Combustion of fuel in combustion chamber 1 produces heat of
combustion, and flue gas which exits combustion chamber 1 as stream 7. The
heat
of combustion can be transferred to feed water flowing through boiler tubes in
the
walls of combustion chamber 1, to heat the feed water. Heat of combustion can
also be transferred from flue gas by indirect heat exchange to feed water, or
to
steam, in heat section 6 which generally includes a region (the "radiant
section")
in which heat transfer occurs predominantly by radiative heat transfer, and a
region (the "convective section") in which heat transfer occurs predominantly
by
convective heat transfer.
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In the present invention, oxygen is fed in small amounts into the region
below grate 2, into the region of the fuel on grate 2, or into both regions.
Sufficient oxygen is fed so that the gaseous atmosphere in contact with the
fuel
has an oxygen content higher than that of air, i.e. at least 21 vol.%, up to 5
vol.%
higher than that of air and preferably not more than 1 vol.% higher than that
of air.
The oxygen can be fed into the region below grate 2 in any of numerous
ways, such as by mixing it with primary air that is fed as stream 4, or
inserting a
lance 8 into the region below grate 2 and feeding the oxygen through the lance
into the region below grate 2 where it then can mix with primary air.
The oxygen can be fed into the region above grate 2 in any of numerous
ways, such as by inserting a lance 9 into the region above grate 2 so that
oxygen
emerging from the lance 9 can contact fuel present on the grate, and feeding
oxygen through the lance 9.
The oxygen that is fed below or above the grate 2 is preferably fed as a
stream comprising at least 50 vol.% oxygen preferably 90 vol.% oxygen .
Streams having such oxygen content are readily available from commercial
sources. Alternatively, streams having such oxygen content can be formed in
apparatus located near the combustion chamber such as VPSA units that separate
oxygen from air.
The practice of the present invention provides numerous advantages in its
own right, and especially compared to prior practice relating to combustion of
biomass.
The moisture content of fuel comprising biomass is typically very high.
This increased moisture content, and its low energy density, are among the
primary issues with firing biomass in boilers and especially boilers that were
designed for other fuels. For example, converting a 50MWfiet coal-fired boiler
(heat rate of 11,500 Btu/kWhnet) to fire biomass would be expected to cause
the
boiler to be derated by 20-45% just to account for the moisture in the fuel.
The
shift in boiler heat transfer balance and the increased excess air requirement
increase the required derate to 30-50% for many boilers. The present invention
permits efficient combustion of biomass fuels, even in boilers that were
designed
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for combustion of fuels having lower water contents, ancUor higher energy
density, than biomass. The invention is useful when the fuel containing the
biomass has a water content of at least 25 wt.%, or when the fuel containing
the
biomass has an energy content less than 7500 BTU/lb or even less than 5000
BTU/lb.
In the present invention, the addition of only a small amount of oxygen
enhances and controls combustion both on and above the grate as a means to
recover lost generating capacity. The enhanced combustion, in turn, enhances
flame stability and ensures more complete burnout. Oxygen injection over the
grate can also stabilize and improve the combustion process. In general, by
using
oxygen in the combustion environment according to the present invention, it is
possible to reduce the excess air flow, and thereby reduce the specific flue
gas
volume. The lower specific flue gas volume allows the boiler operator to
increase
the firing rate to regain some of the generating capacity lost when the boiler
was
converted to biomass firing. Even small reductions in excess air can allow
boiler
capacity lost during the conversion to biomass to be recovered (reducing the
required boiler derate).
Another operational benefit of oxygen injection according to the present
invention is that less heat will be 'pushed' into the convective section due
to both
the reduced specific flue gas volume and the increased temperature near the
fuel
bed on the grate. Both of these effects lead to increased heat absorption in
the
radiative part of the boiler ¨ reducing the need to spray in cooling water to
control
superheat and reheat temperatures in the convective section.
In the present invention oxygen could be added by combination of being
directly injected or mixed with combustion air (enrichment). For example, one
might enrich the undergrate air to ensure there are no "hot spots" nor "cold
spots"
on the grate, while using high momentum lances to inject oxygen above the
grate
to promote good mixing and volatiles/CO burnout. The over-bed oxygen lances
can also be used to move heat (by influencing mixing) into different parts of
the
grate. For example, some of the heat from the volatile combustion zone of the
grate can be moved into the drying portion of the grate to facilitate drying.
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Overfire air 5 (air supplied through ports located at one or more elevation
from
the grate) can also be enriched to enhance volatile combustion. Alternatively
oxygen enrichment under the grate may be increased through the use of a lance
to
target areas where the grate is known to be 'cold', or combustion is poor. The
amount of oxygen required to recover capacity by enabling reduced excess
oxygen operation is much less than that estimated for a simple direct
replacement
of combustion air. For example, the stoichiometric oxygen requirement for a
typical dry ash-free wood is about 2,000 SCF (123 lb) per 1,000,000 Btu and
produces about 3,200 SCF of flue gas. Conversely, 1 lb of oxygen can combust
about 8130 Btu of fuel and produces 26 SCF of flue gas. In order to maintain
the
original flue gas volume and burn additional fuel a portion of the original
combustion air volume must be reduced and replaced with additional oxygen.
The oxygen requirement to increase the capacity (or fuel firing rate) by 10 %
under the condition of constant flue gas volume flow rate was calculated for
both
dry and wet wood with 45% moisture content at two different excess oxygen
levels ( 3 and 4.5 % by volume in wet flue gas) and summarized in Table 1. The
amount of oxygen required ranges from 2850 to 3410 SCF per MMBtu of
additional fuel input at the constant excess 02 in flue gas. By reducing the
excess
oxygen level by 1 vol. %, the amount of oxygen required is reduced to less
than
half, in a range from 1140 to 1510 SCF per MMBtu of additional fuel input.
TABLE 1
Biomass Oxygen required (SCF/MMBtu):
Constant excess 02 1% reduction in excess 02
Dry wood, 3% Excess 02 2850 1260
Dry wood, 4.5% Excess 02 2870 1140
Wet wood, 3% Excess 02 3410 1510
Wet wood, 4.5% Excess 02 3410 1330
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The current invention has several additional advantages. First, by using
only enough oxygen enrichment to achieve flame stability on and above the
grate,
gross changes to furnace operation can be avoided. For example, many furnaces
are designed for a specific heat absorption pattern. In a steam boiler for
power
generation the balance between heat transfer in the radiant (furnace) section
is
often carefully balanced with that in the convective section by the boiler
designer.
Variations in heat transfer pattern from the design point can cause
significant
upsets in boiler operation. When high oxygen enrichment levels, such as those
presented in the prior art (>25%) are used, the heat transfer to the radiant
section
is often dramatically increased. For a utility boiler this means the steaming
rate
(rate of steam production) is increased, but there is insufficient heat
available to
superheat the steam to the desired turbine inlet temperature. In the current
invention the transition to a high moisture fuel often leads to off-design
furnace
operation where heat transfer to the radiant section is reduced compared to
the
design case. By using a small amount of oxygen enrichment and thereby reducing
the excess air requirement the radiative/convective heat transfer balance can
be
restored, at least in part, without increasing the radiative heat transfer
past the
design limits.
The present invention also does not require exhaust gas recirculation for
over-grate mixing. This leads to a much lower capital requirement (EGR fans,
ducts, and the like) and reduced operating cost.
Additionally, by using the oxygen addition of the present invention only to
support combustion and thereby reduce the specific flue gas volume through
excess air reduction, the volume reduction compared to oxygen use is much
higher than in the prior art. This enhanced effectiveness of oxygen addition
for
flue gas reduction leads to much lower oxygen requirements.
A significant advantage of the current invention over the prior art is related
to the use of oxygen enrichment only to support combustion and thereby reduce
the specific flue gas volume through excess air reduction, the flue gas volume
reduction compared to the simple replacement of a portion of combustion air
with
oxygen is much higher than in the prior art. This enhanced effectiveness of
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oxygen addition for flue gas reduction leads to much lower oxygen
requirements.
An example for converting a 20 MWõ,t coal-fired boiler to fire biomass is
shown
in Table 2. For these calculations the flue gas volume was held constant,
consistent with a flue gas limited boiler. The baseline generating capacity
was
defined as that after the boiler was converted to biomass firing (using a 32%
moisture fuel) and was 14.7 MWnet in this example. The increased generating
capacity was first estimated assuming the oxygen concentration in the flue gas
was held constant at 4.5% (vol, wet) and combustion air was replaced with
increasing levels of oxygen. This condition is the conventional 'volume
reduction' strategy where the nitrogen in the combustion air is simply removed
by
using oxygen in place of a portion of the air. As can be seen in Table 2, the
generating capacity can be increased significantly, but the oxygen
requirements
are high enough that oxygen use may not be economically justified. In the case
of
the current invention, kinetic data was used to estimate the increase in
firing rate
from oxygen enrichment. The air injection rate was reduced by the amount of
oxygen injected and the firing rate increased ¨ resulting in a lower oxygen
concentration in the flue. With injection targeted to particular locations in
the
combustion chamber, such as described below, the oxygen consumption may be
even lower. The data in Table 2 show that the oxygen use is dramatically lower
for a given increase in capacity for the current invention. Using oxygen in
this
way can be economically viable.
TABLE 2
Increase in generation Oxygen required (SCF/MW baseline):
(% of baseline) Volume reduction Present invention
2% 710 70
10% 3500 340
22% 7420 1020
"Volume reduction" means operating such that the reduction in specific flue
gas
volume is attained only by the replacement of air with an equal amount of
oxygen.
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"Present invention" means operating such that the reduction in specific flue
gas
volume is attained in part by reduction in the amount of excess air.
The optimal embodiment of the current invention uses small amounts of oxygen
to support the various stages of biomass combustion. These stages include:
= Preheating/drying,
= Volatile release,
= Volatile combustion,
= Char combustion.
In a grate fired-combustor, such as that shown in Figure 1, these steps can
occur in-flight or on the grate, depending on the fuel characteristics (size)
and fuel
spreader/boiler design. For example, fine particulate are likely suspended as
they
are 'thrown' into the furnace. Therefore for the fine materials the entire
combustion process occurs in flight. For the largest particles they may dry
slightly as they exit the fuel spreader but land on the grate before drying is
complete. Therefore, for these particles the combustion process occurs
primarily
on the grate. Combustion problems can occur when the fuel and air distribution
are not matched across the grate and overfire air. For example, if too much
fuel is
deposited on a specific portion of the grate the combustion air may be
insufficient
to burn the material. Although optimal overfire air designs promote good
mixing
above the grate, there may still be regions where the oxygen levels are too
low to
complete combustion (and other areas where the excess air is much higher than
required for combustion). Further, the heat release pattern from the volatile
combustion may not match that required to promote drying/devolatilization of
materials that have landed on the grate ¨ causing material on portions of the
grate
to 'smolder' instead of burn.
It is known that high levels of oxygen enrichment can enhance combustion
and overcome problems associated with air/fuel distribution and heat release
mismatches. However, the objective of the current invention is to use the
least
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amount of oxygen to enable the excess air to be reduced (and thereby enable an
increase in boiler firing rate). Therefore the optimal embodiment is to use a
lance,
or lances, above the grate to inject oxygen into oxygen-deficient areas above
the
grate. Often the oxygen deficient area looks darker than the rest of the grate
as
the local temperature is colder. Such area can be detected by in-furnace video
camera, by an optical pyrometer or by visual observation. Other methods of
detecting the oxygen deficient area include gas analysis using a gas sampling
probe and by an optical gas species measurement device. With careful lance
design mixing can be controlled between the injected oxygen and the oxygen
deficient (and likely high CO) flue gas. Further, by targeting the injected
oxygen
jet tragectory high oxygen containing flue gas 'pockets' in the furnace
atomosphere can be drawn into the oxygen deficient area. The combination of
aerodynamic effects from the lance design and the kinetic effect of high
oxygen
concentrations enhance volatiles and CO combustion. The over-grate lances can
also be used to 'move' volatile combustion to add heat to cooler portions of
the
grate to support the combustion process on the grate.
In addition to the over-grate lances the optimal embodiment can also use
directed oxygen enrichment under the grate to enhance combustion on specific
regions of the grate. For example, if the windbox under the grate has
partitions to
divide the airflow to different parts of the grate, different levels of oxygen
enrichment could be used in the different partitioned areas (through use of
oxygen
distributors in the air supply duct for each partition). Alternately a
carefully
designed oxygen injection lance could be installed either below the grate or
immediately above the grate to enrich the combustion air in the immediate
vicinity of a known 'cold spot', or oxygen deficient areas.
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