Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
t~ 6~2~
A PROCESS FOR RECOVERING CHENICALS AND ENERGY FROM
CELL~IOSE ~AST~ LIQ~OR
Backaround of the invention
The kraft process is currently the dominant chemical
pulping process. During pulping large quantities of
recoverable energy in the form of black liquor are
generated. Worldwide some 2.8 billion GJ (780 TWh) of
black liquor was produced in 1990 at kraft pulp mills. ;;~
The kraft recovery system has two principal functions~
i) Recovery and recure of the inorganic pulping -~
chemicals.
ii) Recovery of the energy value of the organic
material, mainly as process steam and electrical
power.
The chemical recovery process contributes significantl~
to the capital intensity of the kraft process. About
35% of the capital cost of a modern pulp mill is attri~
butable to the recovery process. ~ ~ ;
The predominant method today for recovery of chemicals ~ -~
and energy from black liquor is the Tomlinson recovery
boiler, a technology which was introduced well over
fifty years ago. Although an established technology,
there are some wellknown disadvantages wi~h conventional
recovery technology.
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Most often the recovery boiler with its inherent in-
flexibility constitutes the main production bottleneck
in the pulp mill. Economics of scale dictate large
capacity units.
Other disadvantages include the low thermal efficiency
and risk of smelt water explosions which in turn consti-
tute a safety problem.
These and other areas of concern have been the driving
force for development of new methods and principles for
recovering chemicals and energy from black liquor. One
of the more promising routes is gasification of the
liquor in entrained or fluidized beds. In some cases -~
these alternative processes can be intalled as incremen- `
tal capacity boosters, providing an opportunity to
eliminate the recovery boiler bottleneck.
One of the major driving forces for development of new
recovery technology has been to improve thermal effi~
ciency accompanied with higher power to steam output
ratios. The present invention relates to a major
improvement in this area, using technology based on
gasification and energy recovery in an externally fired
gas turbine cycle (EFCC).
Gasification of black liquor can be performed at various
temperatures and pressures, resulting in different forms
of the recovered inorganic constituents and different
calorific values of the combustible process gas.
The inorganics, mainly alkali compounds, are withdrawn -~
from the gasification system and solubilized to form an
6~g
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aqueous alkaline liquid, which liquor is used for
cooking liquor preparation.
Kraft pulp mills are significant producers of biomass
energy, and today most mills are designed to use the
biomass fuel available at the kraft mill to meet on site
steam and electricity needs via back pressure steam
turbine cogeneration system. Electricity demand is
often higher than internally generated, in particular
for integrated mills, and often electricity is imported
from the grit.
Process steam requirements for a modern kraft pulp mill
is in the order of 10 GJ per ton of air dried pulp. The
internal electricity demand is around 600 kWh/ton of air
dried pulp.
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The biomass gasification gas turbine cogeneration system
of the present invention will meet mill steam demand and
has the potential to produce excess electricity for
export.
The present invention can be practised using various ~ -
types of gas generators and gasification principles,
exemplified in prior art documents. ~- ~
In US 4,917,763 and US 4,692,209, gasification of spent ~-
cellulose liquor, such as black liquor, is described.
The gasificatiOn temperature is in the range of 1000
1300C, resulting in the evolvement of molten inorganics -
and a combustible gas. The molten alkaline chemicals
are withdrawn from the gas stream in a cooling and -
quenching stage, where an aqueous solution is sprayed
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into the gas stream. The product alkaline sOlution is
cooled to below 200C.
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The combustible gas is used for generating steam or as a
synthesis gas.
Another gasification method is described in US --~
4,808,264, where recovery and energy from black liquor
is carried ou~ in three distinct and separate steps,
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whereas in the first step concentrated black liquor is
gasified in a pressurized gasification reactor by flash
pyrolysis at 700 to 1300C, in which the inorganic
chemicals of the black liquor are contained in the form
of molten suspended droplets.
Energy is recovered from the resulting process gas for
generation of steam and/or electric power in a gas
turbine/steam turbine cycle. The steam turbine is of ~ -
back pressure type, preferably selected to fit the needs
of process steam for the mill.
In Wo 91/15665 is described a method and apparatus for
generation of electricity and steam from a pressurized
black liquor gasification process. Energy is recovered
in a gas turbine/back pressure steam turbine system.
Excess steam generated in the mill is recirculated into -
the gas turbine or the combustor thereof for increasing -~
the generation of electricity. This procedure is known
to the industry as a steam injedted gas turbine, herein-
after referred to as STIG.
Common for US 4,808,264 and W0 91/15665 is that they
both are based on energy recovery using a direct fired
gas turbine combined cycle, including a back pressure
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steam turbine. The direct fired cycle presents certain
difficulties in connection with gasification of
cellulose waste liquors, which will be further explored
below.
All direct fired cycles require fuel enhancement to
reduce ash components in the process gas to enable safe
operation of the gas turbine. Combustion products
passing through the gas turbine present limitations not -~i
only on the maximum temperature at entry to the gas
turbine, but also on the peak temperature to which the
mineral matter has been exposed during combustion. A
proportionally large fraction of alkali, increasing with
increasing temperature, is vaporized during pyrolysis ;~
and gasification and forms a submicron aerosol upon
condensation as the gas cools down, which alkali
deposits on the turbine blades. Alkali compounds, i.. :-.. ~ i~.' .. ...
comprising sodium and potassium, are particularly
aggressive at high gas turbine inlet temperatures.
Yarious forms of hot gas clean up systems including
ceramic filters are currently under development.
.
An alternative to the hot gas clean up system of W0 -
91/15665 is the approach in US 4,808,264, where the
combustion products are cooled down to temperatures
below about 200C, which facilitates the removal of -~
harmful inorganic components in the gas. A drawback
with this system is however the lower overall efficiency
and electricity generation potential. -~
The objective of the present invention is to provide a
safe, efficient and less capital intensive process for
production of electric power and process steam from
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gasification of ~lack liquor, based on energy recovery
in an externally fired gas turbine cycle with indirect
heating of the gas turbine motive fluid by counter~
current heat `exchange with hot gas generated during
gasification. Before entering the main heat exchange
zone, a substantial portion of the alkali inorganics
formed during the gasification are removed from the gas,
which removed alkali is further processed to cooking
liquors.
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In a specific embodiment of the present invention, the -~
relatively clean process gas from the gasifier is trans-
ferred to an externally fired gas turbine system, in
which gas turbine motive fluid steam or water is in-
jected to increase motive fluid mass flow and provide
better conditions for heat transfer in the heat ex-
changer.
Although conventional gas turbine cycles have inherent
thermodynamic advantages, simple cycle gas turbine
systems suffer from some wellknown disadvantages as
well, such as the large parasitic load of cooling air on
the system to decrease the turbine inlet temperature.
This effect is minimized when practising the present
invention, as will be further explained below.
Furthermore, in light of the strong scale economics of ;
gas and steam turbine cycles and other factors described
herein, conventional direct fired gas turbine and steam
turbine cycles, based on heavy duty ndustrial turbines, ;
may not be the best candidates for applications in the
relatively modest scales in conjunction with black ;
liquor gasification -
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213682~
The exhaust from the gas turbine contains a large
quantity of sensible heat, and if discharged to
atmosphere, large quantities of potentially useful -~
energy are wasted. However, this exhaust heat can be
exploited in various ways, for example to produce steam
in a heat recovery steam generator (HRSG), which can be - ~- -
used for process needs direct or in a cogeneration
figuration, or to produce more power in a condensing
steam turbine.
The principal use of the gas turbine exhaust in the
present invention is as oxidant in the gasifier and/or
for final oxidation of the process gas before entering
into the heat exchanger. ~ -
Yet another method to exploit the heat content of ~ -
turbine exhaust in the present invention is to raise
superheated steam which is recirculated and injected in
the compressed gas stream, thereby increasing motive
fluid mass flow, see e.g US patent No 3,978,661. Steam
injection in biomass gasifier gas turbine cogeneration
systems for forest product industry applications is for
example described in PU/CEES Working Paper No 113 by Dr
Eric Larson, Princeton, February 1990.
It is to be understood that a major drawback of direct
fired gas turbine cycles as exemplified in prior art
documents is the high sensitivity to fuel gas quality. -
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Indirect fired or externally fired gas turbine cycles
are considerably less sensitive and can accept fuels of
approximately the same quality as steam generators.
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Indirect cycles, currently under development for coal
gasification applications, can accommodate a wide
variety of conventional equipment. Advanced combustors
and high temperature heat exchangers are commercially
available or under development.
Stack gas recirculation to use all the cycle air for --~
combustion can be attractive in indirect cycles,
minimizing N0x emissions and lowering capital cost.
As will be subsequently explained herein, use of an
indirect fired gas turbine cycle in combination with
steam injection or compressed air humidification by --
water injection is an attractive alternative embodiment -~
of the present invention.
The practice of the present invention will be described
by reference to the appended description, example and
figure as applied to the recovery from black liquor. It
should, however, be recognized that the invention is ~ -~
applicable to the recovery of othér cellulose waste
liquors, such as for example spent sulfite or soda
pulping liquors. `
General descri~tion of the invention
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In the subject process, a cellulose waste liquor
containing hydrocarbonaceous material and inorganic ~ - -
alkali compounds is reacted with an oxygen containing
gas in a free flow gas generator to produce a com~
bustible gas. The gas generator operates at a reaction
zone temperature of between 600-1500C and at a pressure
of 1-100 bar. -
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During gasification, molten inorganic particles compsing sodium and potassium compounds are formed, which
molten particles are entrained in the gasflow as a
combustion residue.
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A large portion of the molten inorganics are separated
from the hot gas stream in one ore more separate gas
diversion and residue separation zones. The first gas
diversion and residue separation zone may be directly -
connected to the gas generator.
The hot raw gas stream comprising the molten inorganics
undergoes a change in direction in the first separation
zone, and is transferred to a second separation zone
consisting of a staggered array of refractory tubes,
acting as an impact separator.
In the first separation zone the molten particles and
slag pass through an outlet in the first gas diversion
zone and drop by gravity into a pool, comprising an
alkaline liquid.
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Optionally a small stream of bleed gas of the hot gas
stream from the reactor is passed through the outlet
promoting the separation of slag from the diverted main
gas stream.
The discharge of molten particles and slag in the first
separation zone could preferably be arranged as a quench
system including a cooled dip tube for directing the
particle flow into the pool of alkaline liquid.
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The second separatiOn zone is located in the hot gas
stream, acting as an impact separator for particles and
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slag entrained in the gas leaving the first separation
zone. Particle inertia tends to counteract the gas
fluid drag forces which act to sweep the particles past
separator tubes arranged in the gas stream. -
Impact separators are quite effective for removing
particles above a certain diameter, ranging from 5-50
microns. The second separator thus acts as a slag
screen. At the slag screen exit, the hot gas is
directed to a slag collection zone before being
decelerated to and passed through a ceramic filter. -
Ceramic filters, utilized for hot gas filtration of gas
streams, will be exposed to a variety of conditions in
addition to heat. Ceramic barrier filter devices -~
currently under development and commercial use include
candles, cross flow, tubes, bags and granual beds. Any
of those alone or in combination may be used as final
hot gas clean-up before entering the main heat
exchanger, when practising the present invention.
-
The separated smelt particles and slag comprising alkalicomponents are withdrawn from the separation zones and
further processed to form cooking liquors.
The hot clean gas, leaving the last separation zone, is
directed to a combustion zone, where the gas is com-
busted in the presence of an oxygen containing gas, --~
preferably part of the gas turbine exhaust. ;
The heating value of the process gas leaving the gas
generator is dependent on the type and amount of oxidant ;~
used in the gas generator. The use of air as oxidant
results in that a large portion of the product gas
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consists of nitrogen, thus resulting in a gas with a
rather low calorific value.
An important feature of the present invention is that
sulfur compounds, introduced into the gasifier by the
cellulose waste liquor or other sulfur containing
streams entering the gasifier, is reduced to sulfides
and furthermore that a large portion of these sulfides
are bound into alkali. ~ ~
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Thus, formation of hydrogen sulfide and sulfur dioxide
is to be minimized during gasification. The conditions
in the gasifier have to be carefully selected to prevent
excessive hydrogen sulfide formation.
Binding of sulfur into the alkali smelt is promoted by
high temperature in the gasifier and low gasification -
pressure. Addition of various forms of metal oxides,
such as titandioxide, mangandioxide or calcium com-
ponents to the reaction zone, may be employed to
minimize formation of alkali carbonates and hydrogen
sulfide formation during gasification. ~-
The oxidant to fuel ratio in the gasifier has further-
more to be adjusted to minimize formation of sulfates
and sulfur dioxide.
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Taking these restrictions into consideration, a
preferred operating range of the gasifier is at
temperatures above about 800C and a reactor pressure
between 1 and 5 bar.
The oxidant air supply to the reactor should be kept
below 95% of the stochiometrical value for complete
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oxidation of the incoming streams to the reactor and is
normally in the range of 30-80 %. The oxidant is pre-
ferably parts of or all of the gas turbine exhaust.
Final oxidation of the gas is performed in a gas com-
bustor after the inorganics have been separated and
withdrawn from the gas, and in this step no restriction
of oxidant supply is encountered.
After final oxidation the hot gas is directed to an
indirect fired gas turbine cycle system.
A distinguishing feature of the present invention is the
use of a heat exchanger, which transfers a large portion
of the gas combustor exhaust energy to preheat gas
turbine motive fluid.
The efficiency of the indirect exhaust fired cycle is
directly related to the size and performance of the heat -
exchanger. The heat exchanger in the hot zone must
operate at very high temperatures beyond the practical
range for today's metal technology.
Practical ceramic heat exchangers are under development
and are expected to be commercial in the near future.
These ceramic heat exchangers are highly resistant to ;~
erosion and corrosive attacks. Ash deposits on the tube
surfaces are amenable to control by the use of conven-
tional boiler soot blowers.
After the temperature has fallen to below about 800C,
more conventional types of heat exchange equipment may
be used for heat transfer to gas turbine motive fluid.
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The indirect cycle of the present invention may be
combined with a steam turbine system in numerous ways
The range of operating conditions in the indirect gas
turbine cycle of the present invention is not as limited ~ -
as those for the typical steam generation systems of ~ -
conventional combined cycle power plants. For example,
energy available from the combustion system for evapora-
tion and superheating can be used to circumvent pinch
point restrictions normally encountered in the waste
heat steam recovery systems of combined cycle power
plants.
To improve the power generation potential of the present ;~
invention, the mass flow through the turbine can be -~
increased by injecting water or steam into the gaseous
stream entering the gas turbine.
In a preferred embodiment, the compressed air stream
used as motive fluid in the gas turbine is cooled after
compression by adding water to the air stream in a ` ~
humidification tower, in which all or part of the in- ~;
jected water evaporates. The dewpoint decides maximum
water addition. In the following heat exchanger, the
humid compressed air is heated by heat exchange with
process gas combustor exhaust.
Naximum heat is recovered from the exhaust gas when the
temperature of the air at the inlet of the heat
exchangers is equal to the dewpoint temperature. The
evaporative regeneration can be performed in one or more
steps with humidification towers before the heat
exchanger.
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~1368~9
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By injection of water in the compressed air stream in
this way, at least two objectives are reached. The
resulting increased mass flow through the gas turbine
increases power output and heat transfer conditions in
the heat exchanger are improved. -
Another specific advantage of the process of the present
invention is that it can utilize low level heat from the
discharged flue gases, a compressor intercooler or from
the gasification process, or utilize low level heat from
elsewhere in the mill to preheat water used for
evaporation cooling of the compressed air and/or fuel
gas, and hence improve overall efficiency.
Yet another method to increase power output when practi-
sing the present invention is to inject steam into the -
gaseous stream entering the gas turbine or injection of
steam into the gas turbine before expansion.
A disadvantage with water or steam injected cycles is
that water added to the system is lost, if no method to
recapture the vapor from the exhaust gas is used. For
gas turbine systems with evaporative regeneration, the
water consumption for humidifcation is in the order of
0.1-0.8 kg water per kWh power and about twice as much
for power efficient steam injection systems. In both
cases the water has to be processed to boiler feed water
guality.
The gas turbine cycle in the present invention can be
integrated with a facility for production of de-
mineralized water to be used for injection. Such a
demineralization plant could be based on various prin-
ciples known from the sea water desalination industry.
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2~36823
Demineralization plants based on distillation processes
are most preferred for use in the present invention,
since they can use heat from the exhaust stream direct
or use surplus steam or low level heat from elsewhere in
the mill.
The invention will be further explained by the following
example and appended figure disclosing a preferred
embodiment of the present invention, practising steam
injection.
Exam~le
A kraft market pulp mill produces 1070 ton/day bleached ~ ~ -
pulp, generating a black liquor flow of 1662 ton/day as ~
dry solids. The mill's internal steam requirement -
amounts to 112 ton 5 bar steam and 36 ton 13 bar steam.
This steam is supplied from bark an hog fuel firing
Electricity consumption in the mill is 600 kWh~ton of
pulp or 642 MWh/day.
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The black liquor is fed to a gasification system inte~
grated with an externally fired gas turbine plant. The ~;
black liquor has the following data at the gasifier
entrance~
Dry solids content 78%
Temperature 170C
Higher heating value 18.8 W /kg DS
Flow rate 19.24 kg DS/s
The gasifier is operated at a pressure of 1,3 bar and a
reaction zone temperature of 900C. Air is extracted - ; ~-
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3~829
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from the gas turbine exhaust stream and used as oxidant
in the gasifier.
The smelt formed in the gasifier is separated from the
hot process gas in a separation zone arranged in the
bottom of the gasifier. Additional slag and particles
are removed from the hot gas stream in a slag screen
arranged at the gasifier exit.
The smelt from the gasifier have the following composi~
tion (potassium calculated as sodium): ~ ;
Na2C3 52,5%
Na2 x 33,8
NaOH 12,8
Na2S4 0,017% -
NaCL 0,9
The smelt comprising the alkali sulfides are withdrawn
from the gasifier and used for preparation of cooking
liquors. -
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The hot process gas is oxidized in a gas combustor inthe presence of gas turbine exhaust gas. The oxidized
hot gas stream is directed to the shell side of a heat
exchanger, exchanging heat to a compressed air stream
used as motive fluid in the gas turbine.
: ~:
Clean filtered air enters the compressor, where it is
pressurized to approximately 1.2 MPa and a temperature
of 361C. This air stream is humidified by injection of
steam and directed to the tube side of the ceramic heat
exchanger, where temperature is raised to 882C. This
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2~ 3682~
17
high temperature, pressurized air stream enters the gas
turbine, where it expands and generates power.. -
The humidified air exits the turbine at a temperature of :: .
377C to become the combustion air supplied to the ~-
gasifier and gas combustor.
The cooled shell side gas stream and excess turbine ~ :
exhaust stream is directed to a waste heat steam genera-
tor generating steam powering a condensing steam turbine
plant.
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The gas turbine have the following main design criteria~
Efficiencies ~ ~-
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Compressor adiabatic efficiency 0.89 ::-~
Turbine adiabatic efficiency 0.91 -
Generator efficiency 0.99
Ambient alr conditions at com~ressor inlet
Temperature 15C
Pressure 1.033 atm
Relative humidity 60%
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Gas turbine__nl_t_conditions
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Pressure 12.8 atm : : .
Temperature 882C
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Miscellaneous
Combustion and mechanical efficiencies are assumed to
be 1Ø
Steam generator exit gas temperature 125C.
Auxuiliary power is assumed to be negligible.
No provision is made for additional gas turbine
cooling. -~
Power out~ut
_________ __
Net power output gas turbine 40 MW
Net power output steam turbine 51 MW
Additional embodiments
The modern kraft mill often has hog and/or bark fired
boilers or gasifiers integrated. Other mills have
natural gas available for various purposes, such as lime
kiln fuel.
The present invention can be practised in combination
with combustion or gasification of other gaseous and
liquid hydrocarbon fuels available at the mill. As an
example, additional natural gas or biogas can be fired
in a preburner in the compressed air stream increasing
gas turbine inlet temperature and power output.
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~1~682~
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The same objective can be reached by blending the
combustible gas from the gasifier with another
hydrocarbonaceous fuel in the combustor ahead of the
heat exchangers. -~
.
Yet another method to increase power output in the
present invention is to inject high pressure steam in -~
various locations in the gas turbine motive fluid and/or
into the gas turbine.
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Obviously, various modifications of the invention as
herein set forth may become apparent to those skilled in
the art without departing from the spirit and scope
thereof. Thus, for example, humidification may be -~
employed in one or more steps with subsequent preheat, - - -~
and water or steam may be injected at different loca~
tions in the cycle to increase motive fluid mass flow.
Therefore, only such limitations should be made as are
indicated in the appended claims.
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