Boiler recovery is part of Kraft pulp making process whereby chemicals for white liquids are recovered and reformed from black liquor, containing lignin from previously processed wood. Black liquors are burned, generating heat, which is usually used in process or in generating electricity, just like in conventional steam power plants. Recovery boiler recovery by G.H. Tomlinson in the early 1930s was an important milestone in the advancement of the kraft process.
Recovery boilers are also used in the wood sulphite process (less common); this article only discusses the use of recovery boilers in the Kraft process.
Video Recovery boiler
Fungsi boiler pemulihan
Dark black waste contains organic soluble wood residues other than sodium sulfate from chemicals added to the digester. Burning an organic part of a chemical produces heat. In heat recovery the boiler is used to produce high pressure steam, which is used to generate electricity in the turbine. Turbine exhaust, low pressure vapor is used for heating process.
The burning of black liquids in the boiler recovery furnace needs to be carefully controlled. High sulfur concentrations require optimal process conditions to avoid sulfur dioxide production and reduce sulfur gas emissions. In addition to clean burning of the environment, the reduction of inorganic sulfur should be achieved in the bed of charcoal.
Some processes occur in the recovery boiler:
- Burning organic matter in black liquor to produce heat.
- Reduction of inorganic sulphate compounds into sodium sulfide, which comes out at the bottom as a fusion
- Production of liquid inorganic streams especially sodium carbonate and sodium sulfide, which are then recycled to the digester after reconstitution
- Inorganic dust recovery from flue gas to save chemicals
- Production of sodium smoke to capture residual burning of detached sulfur compounds
First recovery boiler
Some of the original recovery boiler features remain unchanged to this day. This is the first type of recovery equipment in which all processes occur in one ship. Drying, burning and subsequent reactions of black liquor all occur in the refrigerated furnace. This is the main idea in Tomlinson's work.
Second, burning is aided by spraying black liquor into small droplets. The process of controlling by directing the spray proved easy. Spraying is used in an early-stage furnace and with some successes adapted to stationary furnaces by H. K. Moore. The three people can control the charcoal bed by having the main air level on the surface of the char bed and more levels above. The multi-level air system was introduced by C. L. Wagner.
Boiler recovery also increases sewerage. This is taken directly from the furnace through the spouts sucked into the dissolution tank. Some of the first recovery units use the use of Cottrell electrostatic precipitators for dust recovery.
Babcock & amp; Wilcox was founded in 1867 and gained initial fame with its water-tube boilers. The company built and started serving the world's first black liquor recovery boiler in 1929. This was soon followed by a unit with fully water cooled furnaces at Windsor Mills in 1934. After a reverberatory and rotating furnace, a recovery boiler was on its way.
The second early pioneer, Combustion Engineering bases its recovery boiler design on the work of William M. Cary, who in 1926 designed three stoves to operate by direct and workplace drinking by Adolph W. Waern and its recovery unit.
Boiler recovery is immediately licensed and produced in Scandinavia and Japan. This boiler is built by a local manufacturer of the image and with instructions from the licensor. One early Scandinavian Tomlinson unit employs a 8.0 m high furnace which has a 2.8 * 4.1 m lower furnace expanded to 4.0 * 4.1 m at the entrance of the superheater.
This unit stops production for every weekend. At first economizers had to be washed water twice daily, but after mounting a shot of sootblowing in the late 1940s the economizers could have been cleaned on a regular weekend stop.
The construction used is very successful. One of the early 160 t/day Scandinavian boilers in KorsnÃÆ'¤s, operated still almost 50 years later.
Development of recovery boiler technology
The use of the Kraft recovery boiler quickly spreads as a functional chemical recovery gives Kraft pulping the economic edge over the pulp sulphite.
The first recovery boiler has a horizontal evaporator surface, followed by a superheater and more evaporative surfaces. This boiler resembles a state-of-the-art boiler about 30 years earlier. This trend continues to this day. Due to a stop on the production line will spend a lot of tech money being adopted in a conservative tending recovery boiler.
The first recovery boilers had severe problems with fouling.
Large enough tube distance for normal operation of coal-fired boilers should be wider for boiler recovery. This provides a satisfactory performance about a week before water washing. Sootblowers mechanics are also quickly adopted. To control chemical losses and lower the cost of chemicals purchased electrostatic precipitators are added. Lowering the dust loss in the flue gas has more than 60 years of practice.
One should also note the square header in the recovery boiler 1940. The air level in the recovery boiler is immediately standardized to two: the primary air level at the charcoal and secondary bed level above the liquor weapon.
In the first decades the furnace layers are refractory bricks. The smelly flow on the wall led to extensive and immediate replacement of designs that eliminated the use of developed bricks.
Fix the air system
To achieve solid operation and low emissions, air recovery boiler system needs to be designed properly. The development of the air system continues and continues as long as there is a recovery boiler. Once the target set for the air system has been met, a new target is given. Currently the new air system has achieved low NOx, but still works to lower fouling. Table 1 visualizes the development of the air system.
Table 1: Development of air systems
The first-generation air systems of the 1940s and 1950s consisted of two-tier arrangements; the main air to keep the reduction zone and secondary air under the liquor for final oxidation. The recovery boiler size is 100 - 300 TDS (ton of dry solids) per day. and the concentration of 45-55% black liquor. Often to maintain burning auxiliary fuel is required to be fired. The primary air is 60 - 70% of the total air with the remaining secondary. At all small opening levels and the design speed is 40 - 45 m/s. Both air levels are operated at 150 o C. Weapons or oscillating liquor guns. The main problems are high accumulation, blockage and low reduction. But function, burning black liquor, can be filled.
Second-generation air systems target high reductions. In 1954, CE moved its secondary air from about 1 m under a liquor to about 2 m above them. The ratio and temperature of the air remain the same, but to increase the mixing, 50% secondary air velocity is used. CE changed their frontwall/backwall into secondary due to tangential firing at that time. In a tangential air nozzle system is at the corner of the furnace. The preferred method is to make the vortex almost a total width of the furnace. In large units, the vortex causes left and right imbalances. Such air systems with increased dry solids increase the temperature of the lower furnaces and achieve a reasonable reduction. B & amp; W had adopted a three-level airflow at the time.
The third-generation air system is a three-level air. In Europe, the use of three primary and secondary airfeeding levels under a liquor gun began around 1980. At the same time, stationary shootings occurred. The use of about 50% of the secondary seems to provide a low heat and stable stove. Higher black liquid solids 65 - 70% are used. Lower heated stoves and better reductions are reported. With three levels of air and higher dry solids, sulfur emissions can be maintained in place.
The fourth-generation air system is air and vertical air. Since the black liquor dry solid feed to the recovery boilers has increased, achieving low sulfur emissions is no longer the target air system. In contrast low NOx and low residual are new targets.
Multilevel air
The three-level air system is a significant improvement, but better results are needed. The use of the CFD model offers new insights into the workings of the air system. The first to develop a new air system was Kvaerner (Tampella) with their 1990-level secondary air in Kemi, Finland, which was later adapted to a large number of recovery boilers. Kvaerner also patented a four-level air system, where additional air levels are added above the tertiary air level. This enables significant NOx reduction.
Vertical air
The vertical air mixing was created by Erik Uppstu. The idea is to convert traditional vertical mixing to horizontal mixing. Jet close range will form a flat field. In traditional boilers, the aircraft is formed by secondary air. By placing the plane into 2/3 or 3/4 the mixing settings are improved. Vertical air has the potential to reduce NOx as a staging air aid in reducing emissions. In vertical air mixing, the primary air supply is regulated conventionally. The rest of the air port is placed in a 2/3 or 3/4 interlace arrangement.
Maps Recovery boiler
Black liquor dry powder
The fired black liquor is a mixture of organic, inorganic and water. Usually the amount of water expressed as the mass ratio of the black dry liquid to the black liquor unit before it is dried. This ratio is called dry solid black liquor
If dry solids of black liquor is below 20% or moisture content in black liquor above 80%, the net warming value of black liquor is negative. This means that all the heat from organic combustion in the black liquor is spent to evaporate the water it contains. The higher the dry solids, the less water containing the black liquid and the hotter the temperature of the adiabatic combustion.
Black liquor dry solids are always limited by the evaporation capabilities available. Virgin black liquor dry solids from the recovery boilers are shown as a function of the year of purchase of the boiler.
When looking at the dry solids of black liquor, we note that the average dry solids have increased. This is especially true for very large recovery boilers. The dry solid design for the green field plant is 80 or 85% dry solids. 80% (or before that 75%) dry solids have been used in Asia and South America. 85% (or before that 80%) has been used in Scandinavia and Europe.
High temperature and pressure recovery boiler
Development of steam pressure and primary temperature of the early recovery boilers early. In 1955, even 20 years from birth, the highest boiler vapor pressure was 10.0 MPa and 480 o C. The pressure and temperature used then retreated downward due to safety. In 1980 there were about 700 recovery boilers in the world.
Development of boiler pressure, temperature, and boiler capacity.
Security
One of the major hazards in boiler recovery operations is the smelly water explosion. This can occur even if a small amount of water is mixed with solids in high temperatures. Smelt-water explosion is a purely physical phenomenon. The phenomenon of smelly water explosion has been studied by Grace. In 1980 there were about 700 recovery boilers in the world. The liquid-liquid fluid explosion mechanism has been established as one of the main causes of recovery boiler explosions.
In a water explosion smelling even a few liters of water, when mixed with a molten melt it can vigorously turn into steam in a few tenths of a second. Sleep Char and water can coexist because the vapors envelop reduce heat transfer. Some trigger events damage the balance and water evaporates quickly through direct contact with the odor. This sudden evaporation causes an increase in volume and pressure waves of about 10 000 - 100 000 Pa. This style is usually enough to cause all the walls of the furnace to warp out of shape. Security of equipment and personnel requires immediate termination of the recovery boiler if there is a possibility that water has entered the furnace. All recovery boilers must be equipped with special automatic shutdown sequence.
Another type of explosion is a combustible gas explosion. In order for this to happen, fuel and air must be mixed before ignition. A common condition is blackout (flame loss) without cleaning furnace or continuous operation in a substoichiometric state. To detect a flame monitoring device installed, with subsequent cleaning and activation of the interlock. A combustible gas explosion is connected to the oil/gas firing in the boiler. As well as continuous monitoring of O 2 in almost every boiler, non-combustible gas explosions have become very rare.
Modern recovery boiler
The modern recovery boiler is a single drum design, with a vertical steam-generating bank and a wide-spaced superheater. The design was first proposed by Colin MacCallum in 1973 in a proposal by GÃÆ'¶taverken (now Metso Power inc.) For a large recovery boiler that has a capacity of 4,000,000 Â £ black solids per day for boilers in SkutskÃÆ'¤r, Sweden , but this design was rejected because it was too advanced at that time by the prospective owner. MacCallum presented the design on BLRBAC and on "The Radiant Recovery Boiler" paper printed in Tappi magazine in December 1980. The first boiler of this single drum design was sold by GÃÆ'¶taverken at Leaf River in Mississippi in 1984. The construction of a steam-producing bank vertical similar to vertical saver. Vertical bank boilers are easy to clean. The distance between superheater panels increases and flattens over 300 but below 400 mm. The wide spacing in the superheater helps minimize fouling. This arrangement, in combination with freshwater efforts, ensures maximum protection against corrosion. There are many improvements in boiler recovery materials to limit corrosion.
The effect of increasing dry solids concentration has a significant influence on the main operating variables. Steam flow increases with increasing dry liquor solids content. An increase in pulp plant closure means that less heat per unit of black liquor dry solids will be available in the furnace. The heat loss of the exhaust gases will decrease as the flow of exhaust gas decreases. The increase in dry liquor of black liquor is helpful because the recovery boiler capacity is often limited by the flow of exhaust gases.
A modern recovery boiler consists of heat transfer surfaces made of steel tubes; furnace-1, superheaters-2, boilers produce bank-3 and economizers-4. The drum-5 steam design is a single drum type. Air and black liquors are introduced through primary and secondary air ports-6, 7-liquor and tertiary-8 air ducts. Burning residue, the odor out through the smelly spouts-9 to the 10-dissolution tank.
The loading of the nominal furnace has increased over the last ten years and will continue to increase. Changes in air design have increased the temperature of the furnace. This has enabled a significant increase in solids loading (HSL) with only a small increase in design at heart rate of heart release (HHRR). The average flue gas flow is decreasing because less water vapor is present. Thus the velocity of the vertical flue gas can be reduced even with the increase in temperature in the lower furnace.
The most obvious change was the adoption of a single drum construction. This change is partially influenced by more reliable water quality control. The advantages of a boiler drum as compared to bi drum are security and availability is improved. Single drum boilers can be built for higher pressure and larger capacity. Savings can be achieved by decreasing erection time. There are fewer tube connections in a single drum construction so that drums with improved startup curves can be built.
The construction of a vertical steam generator bank is similar to a vertical economizer, which is based on very easy experience to maintain cleanliness. Vertical exhaust flow path improves hygiene with high dust loading. To minimize the risk of clogging and maximize cleaning efficiency, both the producing bank and the regulator are arranged at a generous distance. Incorporating two drums of bank boilers is often caused by the tight spacing between the tubes.
The distance between superheater panels increases. All superheaters are now wide spaced to minimize fouling. This arrangement, in combination with freshwater efforts, ensures maximum protection against corrosion. With superheaters less width possible, easier deposit cleaning and lower sooty steam consumption. Increasing the amount of superheater facilitates the temperature control of superheater steam outlets especially during start up.
The bottom loop of the hottest superheater can be made of austenitic material, with better corrosion resistance. The velocity of the steam in the hottest superheater tube is high, lowering the temperature of the tube surface. The low surface temperature of the tube is very important to prevent superheater corrosion. High pressure steam side losses above the heat superheater ensure uniform vapor flow within the tube element.
Future prospects
Recovery boilers have become the preferred mode of recovery of Kraft chemical plants since the 1930s and the process has improved since the first generation. There is an effort to replace the Tomlinson recovery boiler with a recovery system that results in higher efficiency. The most promising candidate seems to be gasification, where Chemrec technology for the flow of black overturn gasification can prove to be a strong competitor.
Even if new technologies are able to compete with traditional recovery boiler technology, the transition will most likely occur gradually. First, recovery boiler manufacturers such as Metso, Andritz and Mitsubishi, can be expected to continue their product development. Secondly, the Tomlinson recovery boilers have long lifetimes, often around 40 years, and may not be replaced until the end of their economic life, and may be temporarily increased at 10-15 years intervals.
References
Bacaan lebih lanjut
- Adams, Terry N. Dan Frederick, William J., (1988). Proses boiler Pemulihan fizik dan kimia . American Paper Institute, Inc., New York. 256 p.
- Adams, Terry N., Frederick, Wm. James, Grace, Thomas M., Hupa, Mikko, Iis, Kristiina, Jones, Andrew K., Tran, Honghi, (1997). Boiler Pemulihan Kraft , AF & PA, TAPPI PRESS, Atlanta, 381 pp, ISBN, 0-9625985-9-3.
- Vakkilainen, Esa K., (2005). Boiler Pemulihan Kraft - Prinsip Day Practice . The Finnish Soda Boiler Association r.y., Valopaino Oy, Helsinki, Finland, 246 p., ISBN, 952-91-8603-7
Source of the article : Wikipedia
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