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2021年1月21日发(作者：fridge怎么读)
外文文献：

Biomass co-firing options on the emission reduction and electricity
generation costs in coal-fired power plants
Abstract
Co-firing
offers
a
near-term
solution
for
reducing
CO2
emissions
from
conventional fossil fuel power plants. Viable alternatives to long- term CO2 reduction
technologies such as CO2 sequestration, oxy-firing and carbon loop combustion are
being
discussed,
but
all
of
them
remain
in
the
early
to
mid
stages
of
development.
Co-firing, on the other hand, is a well-proven technology and is in regular use though
does not eliminate CO2 emissions entirely. An incremental gain in CO2 reduction can
be achieved by immediate implementation of biomass co-firing in nearly all coal-fired
power plants with minimum modifications and moderate investment, making co-firing
a
near-term
solution
for
the
greenhouse
gas
emission
problem.
If
a
majority
of
coal-fired
boilers
operating
around
the
world
adopt
co-firing
systems,
the
total
reduction
in
CO2
emissions
would
be
substantial.
It
is
the
most
efficient
means
of
power generation from biomass, and it thus offers CO2 avoidance cost lower than that
for
CO2
sequestration
from
existing
power
plants.
The
present
analysis
examines
several
co-firing
options
including
a
novel
option
external
(indirect)
firing
using
combustion or gasification in an existing coal or oil fired plant. Capital and operating
costs of such external units are calculated to determine the return on investment. Two
of
these
indirect
co-firing
options
are
analyzed
along
with
the
option
of
direct
co-firing of biomass in pulverizing mills to compare their operational merits and cost
advantages with the gasification option.
1. Introduction




The
evidence
of
the
effects
of
anthropogenic
emission
on
global
climate
is
overwhelming
[1]
. The threat of increasing global temperatures has subjected the use
of fossil fuels to increasing scrutiny in terms of greenhouse gas (GHG) and pollutant
emissions. The issue of global warming needs to be addressed on an urgent basis to
avoid catastrophic consequences for humanity as a whole.

Socolow
and
Pacala
[2]
introduced
the
wedge
concept
of
reducing
CO2
emissions
through
several
initiatives
involving
existing
technologies,
instead
of
a
single
future
technology
or
action
that
may
take
longer
to
develop
and
stronger
willpower
to
implement.
A
wedge
represents
a
carbon- cutting
strategy
that
has
the
potential to grow from zero today to avoiding 1 billion tons of carbon emissions per
year
by
2055.
It
has
been
estimated
[3]
that
at
least
15
strategies
are
currently
available that, with scaling up, could represent a wedge of emissions reduction.






Although a number of emission reduction options are available to the industry,
many
of
them
still
face
financial
penalties
for
immediate
implementation.
Some
measures
are
very
site/location
specific
while
others
are
still
in
an
early
stage
of
development.
Carbon dioxide sequestration or zero emission power plants
represent
the future of a CO2 emissions-free power sector, but they will take years to come to
the mainstream market. The cost of CO2 capture and sequestration is in the range of
40e60 US$$/ton of CO2, depending on the type of plant and where the CO2 is stored
[4,5]
.
This
is
a
significant
economic
burden
on
the
industry,
and
could
potentially
escalate the cost of electricity produced by as much as 60%.

Canada
has
vast
amounts
of
biomass
in
its
millions
of
hectares
of
managed
forests,
most
of
which
remain
untapped
for
energy
purposes.
Currently,
large
quantities of the residues from the wood products industry are sent to landfill or are
incinerated
[6]
. In the agricultural sector, grain crops produce an estimated 32 million
tons of straw residue per year. Allowing for a straw residue of 85% remaining in the
fields to maintain soil fertility, 5 million tons would still be available for energy use.
Due
to
an
increase
in
land
productivity,
significant
areas
of
land
in
Canada,
which
were
earlier
farmed,
are
no
longer
farmed.
These
lands
could
be
planted
withfast-growing energy crops, like switch-grass offering potentially large quantities
of biomass for energy production
[6]
.




Living
biomass
plants
absorb
CO2
from
the
atmosphere.
So,
its
combustion/gasification for energy production is considered carbon neutral. Thus if a
certain amount of biomass is fired in an existing fossil (coal, coke or oil) fuel fired
plant generating some energy, the plant could reduce firing the corresponding amount
of fossil fuel in it. Thus, a power plant with integrated biomass co-firing has a lower
net CO2 contribution over conventional coal-fired plants.
Biomass
co-firing
is
one
technology
that
can
be
implemented
immediately
in
nearly all coal-fired power plants in a relatively short period of time and without the
need
for
huge
investments.
It
has
thus
evolved
to
be
a
near-term
alternative
to
reducing
the
environmental
impact
of
electricity
generation
from
coal.
Biomass
co-firing
offers
the
least
cost
among
the
several
technologies/
options
available
for
greenhouse gas reduction
[7]
. Principally, co-firing operations are not implemented to
save energy but to reduce cost, and greenhouse gas emissions (in some cases).
In a
typical co-firing plant, the boiler energy usage will be the same as it is operated at the
same
steam
load
conditions
(for
heating
or
power
generation),
with
the
same
heat
input as that in the existing coal- fired plant. The primary savings from co-firing result
from reduced fuel costs when the cost of biomass fuel is lower than that of fossil fuel,
and avoiding landfill tipping fees or other costs that would otherwise be required to
dispose
of
unwanted
biomass.
Biomass
fuel
at
prices
20%
or
more
below
the
coal
prices would usually provide the cost savings needed
[8]
.
2. Co-firing options




Biomass co-firing has been successfully demonstrated in over 150 installations
worldwide for a combination of fuels and boiler types
[9]
. The co-firing technologies
employed in these units may be broadly classified under three types:


i. Direct co-firing,


ii. indirect co-firing, and


iii. gasification co-firing.




In all three options, the use of biomass displaces an equivalent amount of coal
(on
an
energy
basis),
and
hence
results
in
the
direct
reduction
of
CO2
and
NOx
emissions to the atmosphere. The selection of the appropriate co-firing option depends
on
a
number
of
fuel
and
site
specific
factors.
The
objective
of
this
analysis
is
to
determine
and
compare
the
economics
of
the
different
co- firing
options.
Brief
descriptions of the three co-firing options are presented here.
2.1. Direct co-firing




Direct
co-firing
involves
feeding
biomass
into
coal
going
into
the
mills,
that
pulverize the biomass along with coal in the same mill. Sometime separate mills may
be used or biomass is injected directly into the boiler furnace through the coal burners,
or
in
a
separate
system.
The
level
of
integration
into
the
existing
plant
depends
principally on the biomass fuel characteristics.




Four different options are available to incorporate biomass cofiring in pulverized
coal power plants
[10]
. In the first option, the pre-processed biomass is
mixed with
coal
upstream
of the existing coal
feeders. The fuel
mixture is
fed into the existing
coal
mills
that
pulverize
coal
and
biomass
together,
and
distribute
it
across
the
existing coal burners, based on the required co-firing rate. This is the simplest option,
involving the lowest least capital costs, but has a highest risk of interference with the
coal
firing
capability
of
the
boiler
unit.
Alkali
or
other
agglomeration/corrosion-causing
agents
in
the
biomass
can
build-up
on
heating
surfaces
of
the
boiler
reducing
output
and
operational
time
[11]
.
Furthermore,
different combustion characteristics of coal and biomass may affect the stability and
heat
transfer
characteristics
of
the
flame
[12]
.
Thus,
this
direct
co-firing
option
is
applicable
to
a
limited
range
of
biomass
types
and
at
very
low
biomass-to-coal
co-firing ratios.





The
second
option
involves
separate
handling,
metering,
and
pulverization
of
the biomass, but injection of the pulverized biomass into the existing pulverized fuel
pipe-work
upstream
of
the
burners
or
at
the
burners.
This
option
requires
only
modifications
external
to
the boiler. One disadvantage would be the
requirement of
additional equipment around the boiler, which may already be congested. It may also
be
difficult
to
control
and
to
maintain
the
burner
operating
characteristics
over
the
normal boiler load curve.





The third option involves the separate handling and pulverization
of the biomass fuel with combustion through a number of burners located in the lower
furnace, dedicated to the burning of the biomass alone. This demands a highest capital
cost,
but
involves
the
least
risk
to
normal
boiler
operation
as
the
burners
are
specifically
designed
for
biomass
burning
and
would
not
interfere
with
the
coal
burners.





The final option involves the use of biomass as a reburn fuel for NOx emission
control.
This
option
involves
separate
biomass
handling
and
pulverization,
with
installation of separate biomass fired burners at the exit of the furnace. As with the
previous option, the capital cost is high, but risk to boiler operation is minimal.
2.2. Indirect or external co-firing
Indirect
co-firing
involves
the
installation
of
a
completely
separate
biomass
boiler to produce low-grade steam for utilization in the coal-fired power plant prior to
being upgraded, resulting in higher conversion efficiencies. An example of this option
is
the
Avedore
Unit
2
project
in
Copenhagen,
Denmark.
In
Canada,
Greenfield
Research
Inc.
has
developed
a
similar
CFB
boiler
design
that
utilizes
a
number
of
units
of
the
existing
power
plant
systems
like
ID
fan
etc.
to
reduce
the
capital
cost.
In
this
system,
a
subcompact
circulating
fluidized
bed
boiler
is
designed
specifically
to
have
a
piggy-back
ride
on
an
existing
power
plant
boiler.
Since
it
is
not a stand-alone boiler
it
does
not
need
many
of
the
equipment
or
component
of
a
separate
boiler.
This
unit
releases
flue
gas
at
relatively high temperature and joins the existing flow stream of the parent coal-fired
boiler
after
air
heater.
Thus,
the
flue
gas
from
the
co-firing
unit
does
not
come
in