WHAT IS THE BLACK LIQUOR
03.22
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Concentrated black liquor contains organic dissolved wood residue in addition to sodium sulfate from the cooking chemicals added at the digester. Combustion of the organic portion of chemicals produces heat. In the recovery boiler heat is used to produce high pressure steam, which is used to generate electricity in a turbine. The turbine exhaust, low pressure steam is used for process heating.
Combustion of black liquor in the recovery boiler furnace
needs to be controlled carefully. High concentration of sulfur requires optimum
process conditions to avoid production of sulfur dioxide and reduced sulfur gas
emissions. In addition to environmentally clean combustion, reduction of
inorganic sulfur must be achieved in the char bed.
The recovery boiler process has several unit processes:
- Combustion of organic material in black liquor to generate steam
- Reduction of inorganic sulfur compounds to sodium sulfide, which exits at the bottom as smelt
- Production of molten inorganic flow of mainly sodium carbonate and sodium sulfide, which is later recycled to the digester after being re-dissolved
- Recovery of inorganic dust from flue gas to save chemicals
- Production of sodium fume to capture combustion residue of released sulfur compounds
First recovery boilers
Some features of the original recovery boiler have remained
unchanged to this day. It was the first recovery equipment type where all
processes occurred in a single vessel. The drying, combustion and subsequent
reactions of black liquor all occur inside a cooled furnace. This is the main
idea in Tomlinson’s work.
Secondly the combustion is aided by spraying the black
liquor into small droplets. Controlling process by directing spray proved easy.
Spraying was used in early rotary furnaces and with some success adapted to
stationary furnace by H. K. Moore. Thirdly one can control the char bed by having primary air
level at char bed surface and more levels above. Multiple level air system was
introduced by C. L. Wagner.
Recovery boilers also improved the smelt removal. It is
removed directly from the furnace through smelt spouts into a dissolving tank.
Some of the first recovery units employed the use of Cottrell’s electrostatic
precipitator for dust recovery.
Babcock & Wilcox was founded in 1867 and gained early
fame with its water tube boilers. The company built and put
into service the first black liquor recovery boiler in the world in 1929.[2] This
was soon followed by a unit with completely water cooled furnace at Windsor
Mills in 1934. After reverberatory and rotating furnaces the recovery boiler
was on its way.
The second early pioneer, Combustion Engineering based its
recovery boiler design on the pioneering work of William M. Cary, who in 1926
designed three furnaces to operate with direct liquor spraying and on work by
Adolph W. Waern and his recovery units.
Recovery boilers were soon licensed and produced in
Scandinavia and Japan .
These boilers were built by local manufacturers from drawings and with
instructions of licensors. One of the early Scandinavian Tomlinson units
employed a 8.0 m high furnace that had 2.8*4.1 m furnace bottom which expanded
to 4.0*4.1 m at superheater entrance.[3]
This unit stopped production for every weekend. In the
beginning economizers had to be water washed twice every day, but after
installation of shot sootblowing in the late 1940s the economizers could be
cleaned at the regular weekend stop.
The construction utilized was very successful. One of the
early Scandinavian boilers 160 t/day at Korsnäs, operated still almost 50 years
later.[4]
Development of recovery boiler technology
The use of Kraft recovery boilers spread fast as functioning
chemical recovery gave Kraft pulping an economic edge over sulfite pulping.[4]
The first recovery boilers had horizontal evaporator
surfaces, followed by superheaters and more evaporation surfaces. These boilers
resembled the state-of-the-art boilers of some 30 years earlier. This trend has
continued until today. Since a halt in the production line will cost a lot of
money the adopted technology in recovery boilers tends to be conservative.
Tube spacing wide enough for normal operation of a coal
fired boiler had to be wider for recovery boilers. This gave satisfactory
performance of about a week before a water wash. Mechanical sootblowers were
also quickly adopted. To control chemical losses and lower the cost of
purchased chemicals electrostatic precipitators were
added. Lowering dust losses in flue gaseshas
more than 60 years of practice.
One should also note square headers in the 1940 recovery
boiler. The air levels in recovery boilers soon standardized to two: a primary
air level at the char bed level and a secondary
above the liquor guns.
In the first tens of years the furnace lining was of
refractory brick. The flow of smelt on the walls causes extensive replacement
and soon designs that eliminated the use of bricks were developed.
Improving air systems
To achieve solid operation and low emissions the recovery
boiler air system needs to be properly designed. Air system development
continues and has been continuing as long as recovery boilers have existed.[6] As
soon as the target set for the air system has been met new targets are given.
Currently the new air systems have achieved low NOx, but are still working on
lowering fouling. Table 1 visualizes the development of air systems.
Table 1: Development of air systems.[6]
|
Air system
|
Main target
|
But also should
|
|
1st generation
|
Stable burning of black liquor
|
|
|
2nd generation
|
high reduction
|
Burn liquor
|
|
3rd generation
|
decrease sulfur emissions
|
Burn black liquor, high reduction
|
|
4th generation
|
low NOx
|
Burn black liquor, high reduction and low sulfur emission
|
|
5th generation
|
decrease superheater and boiler bank fouling
|
Burn black liquor, high reduction, low emissions
|
The first generation air system in the 1940s and 1950s
consisted of a two level arrangement; primary air for maintaining the reduction
zone and secondary air below the liquor guns for final oxidation.[7]The
recovery boiler size was 100 – 300 TDS (tons of dry solids) per day. and black
liquor concentration 45 – 55 %. Frequently to sustain combustion auxiliary
fuel needed to be fired. Primary air was 60 – 70 % of total air with
secondary the rest. In all levels openings were small and design velocities
were 40 – 45 m/s. Both air levels were operated at 150oC.
Liquor gun or guns were oscillating. Main problems were high carryover, plugging and low reduction. But the
function, combustion of black liquor, could be filled.
The second generation air system targeted high reduction. In
1954 CE moved their secondary air from about 1 m below the liquor guns to about
2 m above them.[7] The
air ratios and temperatures remained the same, but to increase mixing
50 m/s secondary air velocities were used. CE changed their
frontwall/backwall secondary to tangential firing at that time. In tangential
air system the air nozzles are in the furnace corners. The preferred method is
to create a swirl of almost the total furnace width. In large units the swirl
caused left and right imbalances. This kind of air system with increased dry
solids managed to increase lower furnace temperatures and achieve reasonable
reduction. B&W had already adopted the three-level air feeding by then.
Third generation air system was the three level air. In Europe the use of three levels of air feeding with
primary and secondary below the liquor guns started about 1980. At the same
time stationary firing gained ground. Use of about 50 % secondary seemed
to give hot and stable lower furnace.[8] Higher
black liquor solids 65 – 70 % started to be in use. Hotter lower furnace
and improved reduction were reported. With three level air and higher dry
solids the sulfur emissions could be kept in place.
Fourth generation air systems are the multilevel air and the
vertical air. As the feed of black liquor dry solids to the recovery boiler
have increased, achieving low sulfur emissions is not anymore the target of the
air system. Instead low NOx and low carryover are the new targets.
[edit]Multilevel air
The three-level air system was a significant improvement,
but better results were required. Use of CFD models offered a new insight of
air system workings. The first to develop a new air system was Kvaerner
(Tampella) with their 1990 multilevel secondary air in Kemi , Finland
[edit]Vertical air
Vertical air mixing was invented by Erik Uppstu.[10] His
idea is to turn traditional vertical mixing to horizontal mixing. Closely
spaced jets will form a flat plane. In traditional boilers this plane has been
formed by secondary air. By placing the planes to 2/3 or 3/4 arrangement
improved mixing results. Vertical air has a potential to reduce NOx as staging
air helps in decreasing emissions.[11] In
vertical air mixing, primary air supply is arranged conventionally. Rest of the
air ports are placed on interlacing 2/3 or 3/4 arrangement.
Black liquor dry solids
Net heating values of industrial black liquors at various
concentrations
As fired black liquor is a mixture of organics, inorganics
and water. Typically the amount of water is expressed as mass ratio of dried
black liquor to unit of black liquor before drying. This ratio is called the
black liquor dry solids.
If the black liquor dry solids is below 20 % or water
content in black liquor is above 80 % the net heating value of black
liquor is negative. This means that all heat from combustion of organics in
black liquor is spent evaporating the water it contains. The higher the dry
solids, the less water the black liquor contains and the hotter the adiabatic
combustion temperature.
Black liquor dry solids have always been limited by the
ability of available evaporation.[12] Virgin
black liquor dry solids of recovery boilers is shown as a function of purchase
year of that boiler.
Virgin black liquor dry solids as a function of purchase
year of the recovery boiler
When looking at the virgin black liquor dry solids we note
that on average dry solids has increased. This is especially true for latest
very large recovery boilers. Design dry solids for green field mills have been
either 80 or 85 % dry solids. 80 % (or before that 75 %) dry
solids has been in use in Asia and South America. 85 % (or before that
80 %) has been in use in Scandinavia and Europe .
[edit]High
temperature and pressure recovery boiler
Development of recovery boiler main steam pressure and
temperature was rapid at the beginning. By 1955, not even 20 years from birth
of recovery boiler highest steam pressures were 10.0 MPa and 480oC.
The pressures and temperatures used then backed downward somewhat due
to safety.[13] By
1980 there were about 700 recovery boilers in the world.[8]
Development of recovery boiler pressure, temperature and
capacity.
[edit]Safety
One of the main hazards in operation of recovery boilers is
the smelt-water explosion. This can happen if even a small amount of water is
mixed with the solids in high temperature. Smelt-water explosion is purely a
physical phenomenon. The smelt water explosion phenomena have been studied by
Grace.[14] By
1980 there were about 700 recovery boilers in the world.[8] The
liquid - liquid type explosion mechanism has been established as one of the
main causes of recovery boiler explosions.
In the smelt water explosion even a few liters of water,
when mixed with molten smelt can violently turn to steam in few tenths of a
second. Char bed and water can coexist
as steam blanketing reduces heat transfer. Some trigger event destroys the
balance and water is evaporated quickly through direct contact with smelt. This
sudden evaporation causes increase of volume and a pressure wave of some 10 000
– 100 000 Pa.
The force is usually sufficient to cause all furnace walls to bend out of
shape. Safety of equipment and personnel requires an immediate shutdown of the
recovery boiler if there is a possibility that water has entered the furnace.
All recovery boilers have to be equipped with special automatic shutdown
sequence.
The other type of explosions is the combustible gases
explosion. For this to happen the fuel and the air have to be mixed before the
ignition. Typical conditions are either a blackout (loss of flame) without
purge of furnace or continuous operation in a substoichiometric state. To
detect blackout flame monitoring devices are installed, with subsequent
interlocked purge and startup. Combustible gas explosions are connected with
oil/gas firing in the boiler. As also continuous O2 monitoring
is practiced in virtually every boiler the noncombustible gas explosions have
become very rare.
[edit]Modern recovery boiler
The modern recovery boiler is of a single drum design, with
vertical steam generating bank and wide spaced superheaters. This design was
first proposed by Colin MacCallum in 1973 in a proposal by Götaverken (now
Metso Power inc.) for a large recovery boiler having a capacity of
4,000,000 lb of black liquor solids per day for a boiler in Skutskär , Sweden Mississippi
in 1984. The construction of the vertical steam generating bank is similar to
the vertical economizer. Vertical boiler bank is easy to keep clean. The
spacing between superheater panels increased and leveled off at over 300 but
under 400 mm. Wide spacing in superheaters helps to minimize fouling. This
arrangement, in combination with sweetwater attemperators, ensures maximum
protection against corrosion. There have been numerous improvements in recovery
boiler materials to limit corrosion.[15][16][17][18]
The effect of increasing dry solids concentration has had a
significant effect on the main operating variables. The steam flow increases
with increasing black liquor dry solids content. Increasing closure of the pulp
mill means that less heat per unit of black liquor dry solids will be available
in the furnace. The flue gas heat loss will decrease as the flue gas flow
diminishes. Increasing black liquor dry solids is especially helpful since the
recovery boiler capacity is often limited by the flue gas flow.
A modern recovery boiler consists of heat transfer surfaces
made of steel tube; furnace-1, superheaters-2, boiler generating bank-3 and
economizers-4. The steam drum-5 design is of single-drum type. The air and
black liquor are introduced through primary and secondary air ports-6, liquor
guns-7 and tertiary air ports-8. The combustion residue, smelt exits through
smelt spouts-9 to the dissolving tank-10.
The nominal furnace loading has increased during the last
ten years and will continue to increase.[19] Changes
in air design have increased furnace temperatures.[20][21][22][23] This
has enabled a significant increase in hearth solids loading (HSL) with only a
modest design increase in hearth heat release rate (HHRR). The average flue gas
flow decreases as less water vapor is present. So the vertical flue gas
velocities can be reduced even with increasing temperatures in lower furnace.
The most marked change has been the adoption of single drum
construction. This change has been partly affected by the more reliable water
quality control. The advantages of a single drum boiler compared to a bi drum
are the improved safety and availability. Single drum boilers can be built to
higher pressures and bigger capacities. Savings can be achieved with decreased
erection time. There is less tube joints in the single drum construction so
drums with improved startup curves can be built.
The construction of the vertical steam generating bank is
similar to the vertical economizer, which based on experience is very easy to
keep clean.[24] Vertical
flue gas flow path improves the cleanability with high dust loading.[25] To
minimize the risk for plugging and maximize the efficiency of cleaning both the
generating bank and the economizers are arranged on generous side spacing.
Plugging of a two drum boiler bank is often caused by the tight spacing between
the tubes.
The spacing between superheater panels has increased. All
superheaters are now wide spaced to minimize fouling. This arrangement, in
combination with sweetwater attemperators, ensures maximum protection against
corrosion. With wide spacing plugging of the superheaters becomes less likely,
the deposit cleaning is easier and the sootblowing steam consumption is lower.
Increased number of superheaters facilitates the control of superheater outlet
steam temperature especially during start ups.
The lower loops of hottest superheaters can be made of
austenitic material, with better corrosion resistance. The steam velocity in
the hottest superheater tubes is high, decreasing the tube surface temperature.
Low tube surface temperatures are essential to prevent superheater corrosion. A
high steam side pressure loss over the hot superheaters ensures uniform steam
flow in tube elements.
[edit]Future prospects
Recovery boilers have been the preferred mode of Kraft
mill chemical recovery since the 1930s and the process has been
improved considerably since the first generation. There have been attempts to
replace the Tomlinson recovery boiler with recovery systems yielding higher
efficiency. The most promising candidate appears to be gasification,[26][27] where Chemrec's technology
for entrained flow gasification of black liquor could
prove to be a strong contender.[28]
Even if new technology is able to compete with traditional
recovery boiler technology the transition will most likely be gradual. First,
manufacturers of recovery boilers such as Metso, Andritz andMitsubishi, can
be expected to continue development of their products. Second, Tomlinson
recovery boilers have a long life span, often around 40 years, and will
probably not be replaced until the end of their economic lifetime, and may in
the meantime be upgraded at intervals of 10 – 15 years.
see also previous article "Ncg
(non condensible gases)"
May be useful.
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