Strong black liquor is introduced into a mixing tank by pumps from the evaporator
. Usually there are two parallel [lines] pumps. Also the ash from
recovery boiler hoppers and the electrostatic precipitator, which consists
mostly of sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3), is introduced
into the mixing tank. Alternatively, the ash can be added to the black liquor
in the evaporation plant before the last concentration. In this case the liquor’s
tendency to solidify on the heat transfer surfaces of the concentrator is diminished.
If sodium sulfate (Na2SO4) is added as a make-up chemical to the chemical circulation,
it is usually added into the mixing tank. While the original kraft pulping
process was named after this procedure, modern mills do not usually require
added sodium sulfate.
The hot ash comes through rotary feeders into the mixing tank and is then mixed with the black liquor by rotating agitators. The level in the mixing tank is controlled by allowing only a certain amount of liquor to enter the tank. Vent gas produced in the mixing tank needs to be removed. Due to tighter emissions limits, these gases can no longer be released to the environment through scrubbers.
There should be great enough underpressure in the mixing tank to prevent the vent gas from going into the boiler and ESP hoppers, which are underpressurized by an induced draft fan. The traditional way to process the gases from the mixing and dissolving tank was to direct them to a separate scrubber. The problem with this kind of processing is the TRS level of the incoming gases, which requires a high pH level from the chemicals so that the sulfur compounds can effectively be removed. This is why vent gases are today being burned in recovery boilers.
The liquor is introduced into a suction box through a mechanically cleanable strainer. It is then pumped forward in the system by spray gun pumps. There can also be two mixing tanks, either parallel or in series. In the latter case, the second tank is usually bigger and functions as a level tank. There are drain and overflow piping from the mixing tank to the liquor dump tank. Also, the mixing tank is usually equipped with piping straight from the evaporator to the spray gun pumps.
There are usually two parallel spray gun pumps, one being used as a back-up. The pumps are specially designed centrifugal pumps made of acid-resistant material.
Virgin black liquor concentrated in the evaporator (dry solids content 60 – 85%) is called strong liquor. Strong liquor with added ash from the recovery boiler is called black liquor as fired.
After the spray gun pumps are black liquor preheaters. To conserve energy, black liquor is stored at a temperature that keeps its viscosity just high enough so that it can be pumped safely. Preheaters normally use direct 10–14 bar steam blown through nozzles directly into the black liquor. Or, they can be similar to heat exchangers where 3–5 bar steam is used and the steam and the liquor are on separate sides of the piping inside the heat exchanger. Normal spraying temperature of the black liquor is 115–130 ºC. The temperature, and therefore the viscosity, of the liquor have a great impact on the spraying droplet size and efficient combustion. Therefore, black liquor temperature is an important control variable.
After heating, the black liquor enters the furnace from the mixing tank (or
straight from the evaporator) through liquor guns 
. The liquor guns 
are situated approx. 6-8 meters from the furnace bottom,
usually symmetrically on each wall . There are 20 liquor guns in a large recovery boiler.
The black liquor is broken down into droplets in the liquor gun's splashplate.
The average droplet diameter is approx. 2 mm. The aim is that the droplets produced
will fall onto the bottom of the furnace and dry on the way down. The actual
droplet combustion occurs in the vicinity of the bottom, on the char bed.
It is important to minimize the carry-over, i.e. droplets reaching to the top of the furnace. This can be achieved by avoiding too small droplets from liquor guns. It is also important to create sufficient amount of non-combustible alkali coke on the furnace floor, which is needed for the char bed reduction reactions. The droplet size is determined on the basis of the height of the char bed.
There are several factors that affect the droplet size, e.g. the shape and size of the splashplate on the liquor gun, the size of the liquor gun nozzle, and the black liquor viscosity and pressure. The size and number of liquor guns need to be determined for each individual situation so that the best possible droplet size can be obtained by adjusting the temperature of the black liquor lines. The droplet size can be adjusted by keeping the pressure of the black liquor lines constant and adjusting the temperature (and thus the viscosity) of the liquor.
There are usually only one or two parallel black liquor pipes to the liquor guns. For recycling purposes there is a backflow line, which usually leads to mixing tank. The piping is equipped with nozzles for cleaning and emptying. In addition, heat tracing is applied to keep the liquor warm during start-up and short shutdowns. The black liquor piping together with all related equipment is usually made of acid-resistant steel.
The liquor dump tank is equipped with heating and mixing devices. Liquor that is too weak, that is having a low dry solids content, for the black liquor tank (e.g. liquor that has been used in spooling the black liquor lines) is collected in the dump tank. When the level of weak liquor in the dump tank reaches a certain point, the weak liquor is automatically pumped back to the evaporator.
Green liquor equipment includes the dissolving tank with related equipment, green liquor pumps, and piping.
The dissolving tank is a large cylindrical, oval, or figure-eight shaped tank that has a horizontal floor and roof. The tank is usually made of black plate and lined inside by waterproof concrete or acid-resistant plate. The concrete lining functions as sound isolation as well because the smelt running down to the dissolving tank causes small explosions as it hits the liquid. The smelt is dissolved with weak white liquor conducted to the tank. Salts tend to solidify in the tank at the bottom, so powerful agitators are used in addition to possible pump rotation. The pump rotation may be arranged only for the purpose of breaking down the smelt. The smelt is usually broken down by channeling a steam jet to the smelt flow coming down the spout.
Dissolving the smelt creates vent gases. As with the vent gases in the mixing tank, these vent gases are also conducted to a separate washer where dust and sulfur compounds are washed away. Today, the vent gases are burned in recovery boilers.
There are usually two green liquor lines: one that takes green liquor to causticizing and the other that brings white liquor to the dissolving tank. The lines are switched from time to time due to green liquor’s tendency to block up. The amount of green liquor to be removed is determined either by the level of the liquor or the dissolvent concentration. The weak liquor input is controlled by the alternative left over. Or, the weak liquor input can be controlled based upon its concentration and the green liquor intake can be based upon overflow. There is usually only one green liquor circulation pump. Green liquor pumps are specially designed centrifugal pumps made of acid-resistant steel, as is all the related equipment.
Smelt spouts move the smelt from the furnace bottom to the dissolving tank
. The smelt is very aggressive especially in oxygenous
environment and will corrode the spout quickly unless the cooling is efficient.
There are usually 3 to 9 smelt spouts in a recovery boiler and they have a
double casing . Cooling water
flows between the casings. The cooling water is demineralized water.
A recovery boiler requires oxygen in the air in order to create combustion.
The air system should be flexible and feed oxygen where it is needed based
on the furnace reactions .
Flue gas is no longer used in recovery boilers for preheating the air. The air is heated usually by steam or by hot water and steam. Hot water comes from the boiler feedwater. Extra water is circulated through the feedwater preheater (economizer) and it is then conducted to air preheater batteries and pumped back to the feedwater line before the water preheater. The air preheater is usually located on the pressure side of the fans.
Primary, secondary, and tertiary air have their individual combustion air fans
. Then, controlling one circuit will not have a significant
effect on the others. Fans are often equipped with rotation speed controls,
which can be adjusted. The air can be measured separately in suction ducts so
that temperature changes will not have a significant effect on the result. In
addition to the fan control, the air can be controlled by dampers of different
zones. The primary air zone can also have wall or group-specific dampers. The
air enters the recovery boiler furnace through air nozzles. The air nozzles
usually have also dampers for fine control. They can be either automatic or
manually operated.
The recovery boiler water and steam circulation system recovers the heat energy
produced during black liquor combustion and cools down the boiler’s hottest
parts. The heat energy released helps to vaporize the water and adds to the
temperature and pressure of the steam produced. The superheated steam can be
used to produce electricity with the help of a turbine and a generator . Low pressure steam , which is cool after the turbine, is used in many
places, e.g. in the evaporator and the bleach plant.
Recovery boiler water and steam circulation is based on natural circulation so that the flow is automatic due to the differences in temperature, pressure, and height.
Most of the recovery boiler's feedwater is recovered back from the heat exchangers as condensate. This way, only a small amount of demineralized water is required to make up for condensate loss. Condensate loss occurs most often due to direct steam heating and sootblowing.
The recovery boiler water system requires high quality water. Normal water used at the mill is first cleaned in the raw water pre-treatment system, which can include filters, precipitation, and aeration. This removes the coarse impurities and colloid compounds. In addition to this, salts are removed from the water by ion exchangers. There are usually two of them: one removes the anionic (negative) salts and the other the cationic (positive) salts. Any corrosion chipped off from the water piping is filtered from the condensate going back to the feedwater tank.
Oxygen and carbon dioxide in the water can corrode the boiler and the condensate system. They are removed in a deaerator, where the gases are separated from the demineralized water and the purified condensate. The oxygen is removed by heating the water. At the boiling point, the solubility of the gas to water is zero. There is, however, some gas in the boiling water so that the partial pressures of the gas in the liquid and vapor state above it attempt to reach the same level. The steam used in the deaerator is conducted into the water in the feedwater tank, so that when coming through the deaerator, the steam effectively removes oxygen from the water flow. To ensure oxygen removal, the rest of the oxygen is removed from the water by chemicals.
The condensate and the demineralized water are conducted from the deaerator to the feedwater tank.
The pure condensates coming from the heat exchangers as well as the purified
demineralized water are conducted into the feedwater tank . The water is stored in the feedwater tank at its
boiling point (110-140°C depending upon tank pressure). However, due to
the pressure in the tank being higher than the air pressure, the water does
not boil in the feedwater tank.
The water leaving the feedwater tank is taken to the feedwater preheater (economizer),
where it is heated close to the boiling point with the help of the flue gas
exiting the recovery boiler. The temperature of the flue gas has already dropped
by the time it reaches the economizer, as it has transferred heat in the superheater
and the generating bank . The feedwater can also be
preheated by steam before it enters the economizer. Depending on the cleanliness
of the flue gas, the economizer tubes can be either in a horizontal or vertical
position. When using horizontal flow tubes, the flue gas needs to be cleaner,
i.e. the ash content cannot be as high as is the case when using vertical flow
tubes. Today, a vertical tube structure is more common.
The feedwater is conducted from the economizer to the spray water condenser.
There the feedwater condenses the steam from the steam drum into water. The
condensed water is very clean so it is fed into the desuperheaters located in
between the superheaters. In the desuperheater, the feedwater temperature rises.
From there the feedwater is conducted into the steam drum .
The portion of feedwater not vaporized in the steam drum is conducted into the downcomers. There are 4-6 downcomers
next to the recovery boiler. From the downcomers, the water moves into the dense
piping and horizontal tubes under the boiler. There the water/steam mixture
continues its way up inside the boiler vertical tubes. Most of the water vaporization
occurs due to the heat transferred from the furnace. Partially vaporized water
rises up through the sidewall risers to the generating bank where the vaporization continues due to the flue gas
heat. From the generating bank the water and steam are conducted back into the
steam drum.
Water is separated from the steam in the steam drum . The steam
is conducted to the superheaters and the water into the downcomers and back
to the boiler bottom to be circulated once again.
The superheaters raise the steam temperature and pressure to the level
required by the turbine. Due to the desuperheaters located between the superheaters
and the oversized heat transfer surface area of the superheaters, it is possible
to maintain the steam temperature constant over a large load range and prevent
overheating of superheater parts. The desuperheaters spray a certain amount
of pure water from the spray water condenser into the steam flow going from
one superheater to the other. The amount sprayed is controlled by steam temperature
measurement and very rapid control. The steam temperature in the superheater
needs to be controlled because the sodium sulfate of the flue gases starts to
smelt and solidify in the superheater if the temperature is too high. The practical
temperature limit is 480°C, however, this may vary based on the boiler structure
and the presence of harmful chemicals (e.g. potassium and chlorine) in the chemical
circulation. To prevent the superheaters from excess heat from the furnace,
screen tubes can be used or the superheaters can be located behind
the nose.
The superheated, high pressure steam flows from the superheaters to the turbine through the main steam line. Some of the high pressure steam is taken for boiler sootblowing.
Heavy fuel oil is normally used as an auxiliary fuel. Some boilers also use natural gas or light fuel oil. The oil burning equipment consists of a pumping unit with preheaters, piping, and burners. The recovery boiler has two kinds of oil burners:
which are used in heating the boiler, assisting the
black liquor combustion, forming the char bed, and in final combustion. The burners are usually steam or air pressure atomizing. According to the regulations, all new and modernized recovery boiler oil burners must have flame control instrumentation, which automatically stops the oil flow should the flame in the burner go out for any reason. There are one or two emergency shutdown valves with actuators for shutting down specific oil lines.
Ash, which consists mostly of sodium sulfate and sodium carbonate, is collected from the recovery boiler ash hoppers and electrostatic precipitators (ESP) and is then mixed with the black liquor in the mixing tank in order to recover the chemicals. The ash falls through the hoppers and the rotary feeders onto dustproof scraper conveyors, which are straight under the hoppers. The rotary feeders prevent the air and mixing tank vent gas from moving into the flue gas ducts. In ash conveying it is important to ensure that if there is any possible leakage in the heat surface, the leaking water cannot erupt from the lower starting point of the scraper conveyor into the black liquor in the mixing tank.
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