RECOVERY BOILER

Figure 3.1 The position of recovery boilers in pulp mills

After a while the vacuum in to write about the recovery boiler because of the many activities, this time the author presents articles about the recovery boiler and the supporters.this time the author will present an article about the recovery boiler.

Recovery boiler boiler is a special unit used for obtaining soda or to purify inorganic chemical compounds (chemical recovery) contained in the black liquor from the digester and the rest of the cooking as well as generating high-pressure steam (high pressure steam).

Combustion occurs in the recovery boiler produced steam (high pressure steam) which is used as turbines and organic compounds melt like lava called smelt, smelt is then mixed with a weak white liquor (WWL) to green liquor (GL), to be transferred to recausticizing (RC). Recausticizing (RC) aims to process green liquor (GL) to white liquor (WL), which will be reused as an ingredient in cooking wood digester (Pulp making section).

Figure 3.2 Cycle of Chemical Process of HBL (Heavy Black Liquor)

The fuel used in the recovery boiler in the form of heavy black liquor (HBL). Levels of black liquor ranged between 15% -18% solid and should be concentrated prior to reaching solid levels above 62% to be burned in the recovery boiler furnace. Therefore the weak black liquor (WBL) was concentrated in a vacuum evaporator plant into heavy black liquor with a solid content of 70% -72%.

Heavy black liquor (HBL) contains 20-30% inorganic chemical compound with the main content of sodium carbonate (Na2CO3) and sodium sulphate (Na2SO4) and 40-50% of organic compounds derived from wood for cooking in the digester and the rest is water.

With heavy burning black liquor (HBL), heat energy is released around 3100-3500 kcal / kg dry solid. Furthermore, the thermal energy will partly be used to convert inorganic compounds and partly used to generate steam.

Heavy black liquor (HBL) with a solid content of between 60-70% is sent to the evaporator plant into the mixing tank. In the mixing tank, black liquor (BL) mixed with soda, ash from the hopper and ash coming from the electrostatic precipitator coupled with makeup salt cake (Na2SO4).

From the mixing tank mix of heavy black liquor (HBL), ash and salt cake was pumped through the spray gun to be sprayed into the furnace and dried by blowing hot air, and then clump together to form charbed bottom of the furnace and burned after reaching the point of combustion. The need for combustion air is blown through the primary, secondary, and tertiary windbox located around the bottom of the furnace wall.

Before arriving in the furnace, a process of drying, phyrolisis and gasivikasi by blowing hot air by the reaction:

CO + O2 CO2

H2 + O2 H2O

The division of air is set as follows:

- Primary air flow = 30-35% of the total air requirements

- Secondary water flow = 50-60% of the total air requirements

- Tertiary air flow = 10-15% of the total air requirements

Figure 3.3 Distribution of the combustion air in the furnace

To initiate combustion in the furnace and used to stabilize the combustion of diesel fuel is sprayed burner start-up and load into the furnace burner.

During combustion, the following process takes place in the furnace, namely:

1. Organic compounds burn off heat and partially turned into gas

2. Sodium sulphate (Na2SO4) is reduced to a compound of sodium sulphite (na2 ¬ S)

3. Organic compounds melt like lava called smelt

Na2SO4 reduction depends on the reaction between Na2SO4 direct contact with hot carbon compound (which results from the burning of organic compounds) at a pressure below atmospheric pressure, such as the following reaction:

Na2SO4 Na2S + 2CO2

Na2SO4 Na2S + 4CO2

Speed reduction can be calculated using the equation:

Speed Reducers = x 100%

If the conditions of combustion is complete, the speed reduction will reach above 95%. Inorganic compounds melt (smelt) will gather around the side charbed and flow out through the smelt dissolving tank into the spout. The result is called green liquor dissolving and sent to recausticizing section for further processing into white liquor which is then sent to the digester and reused as cooking ingredients (cooking liquor).

The rest of the air and combustion gases called flue gas still contains a high heat value. Flue gas is sucked / drawn by induce Draft Fan (IDF) in which the flue gas will pass through the boiler tubes that turned into a high-pressure steam (high pressure steam) which is then used to drive turbine generators for generating electrical energy. So the production side of the recovery boiler is a high-pressure steam (60 bar).

Reactions that occur during the furnace can be seen in the following figure.

Figure 3.4 Chemical Reactions Happens When Using Combustion Furnace Underway

While the physical events that occur can be seen in Figure 3.5.

Figure 3.5 Physical Events Occurring On When Burning Furnace Progress in

3.1 Feed Water, deaerator, and Steam Drum

Feed water (boiler feed water) is a mixture of steam condensate and makeup water demineralizer mixed in the reservoir tank. Feed water is pumped to the deaerator where the feed water is sprayed over the top of the deaerator were greeted with a blast of steam from the bottom of the feed water to heat up to a temperature of about 1300C and simultaneously releasing free oxygen seyawa of feed water.

Furthermore, feed water flows through three layers of perforated plate into steam scrubbing vessel and over flow into the deaerator tank. Some steam together with oxygen-free exit through the venting deaerator. Warming events and the release of oxygen in the deaerator steam depending on usage and settings deaerator pressure and level. To enhance the removal of free oxygen that is still attached to the feed water, chemical compounds injected into the deaerator Hydrazine (N2H4) is that the current or the Deha (heptyl diethyl adipate).

From the deaerator, feed water is pumped into the boiler by using a feed water pump. Feed water pump consists of 3 units (50 Hz) and 1 unit (60 Hz) are smaller capacity. In normal operation the unit used feed water 1 (50 Hz), and other water feed stand by.

Feed water from the deaerator is pumped into the bottom header economizer I through pipes with outside diameter 159 mm. From the economizer header I water moves upward toward the economizer header I through 67 line pipe economizer. From the upper economizer header I move down to the water below the header economizer II, from there the water upward through pipe line 67 II to economizer header above. Feed water is heated to temperatures up to 2300C and 1300C will flow to the steam drum through 6 Conductor pipe plumbing expenses.

There are 6 pipe down comer overall, two down comer pipe connected to the boiling tube header panel or generation bank as a liaison water from the steam drum to the generation bank. Mixture of water and steam generation for banks will flow to the steam drum and water in the steam drum boiler liquid phase and vapor phase are separated by a cyclone separator, where the vapor that forms collected in the steam drum level is called saturated steam and boiler water level below steam drum re-circulated.

Due to release of steam is formed, the levels of minerals in the steam drum more and more viscous. So to stabilize the mineral content of a small amount of water in the boiler steam drum disposed towards continual continuous blow down expantion tank. In the continual blow down expantion tank boiler water turns into steam and partly flowed into deaerator, while the rest are discarded to blow down tank.

To maintain the quality of boiler water, injected a chemical compound sodium hydroxide (NaOH) and sodium Posphate (Na3PO4) diperpipaan feed water into economizer I.

3.2 superheater and Main Steam

Saturated vapor collected in steam drum is called saturated steam, passed through the screen towards the superheater tube I, II, III. This area is heated vapor from saturated steam conditions up steam superheater.

Penginjeksian feed water area located between the superheater header superheater I II and III superheater. In normal operating conditions, the temperature steam superheater region can be controlled as follows:

- Temperature steam after superheater I = 3430C

- Temperature steam after superheater II = 3870C

- Temperature steam after superheater III = 4550C

3.3 Burning Black Liquor (black liquor firing)

The main fuel recovery boiler is heavy black liquor concentration results in the evaporator plant WBL. Heavy black liquor is pumped to the mixing tank and mixed with ash (dust) and saltcake makeup (Na2SO4 powder) and stirred with an agitator. Heated with steam to the low temperature of about 105-1150C to obtain a perfect mix and does not clot. HBL then pumped to the pipeline system into spray gun and heated again with the steam press medium in direct contact (direct steam heater) to reach the appropriate temperature, around 115-1200C.

HBL then sprayed into the furnace through 6 sets of spray gun are placed symmetrically on the wall of the furnace between the level of secondary and tertiary water ring water ring with 3 sets are 3 sets on the left and on the right is a wall furnace.

The factors that affect the combustion HBL is:

-% Of total solid

- Viscosity

- Temperature

- Angle spraying

- The type and size of nozzle spray gun spray gun

HBL viscosity depends on% total solids and temperature HBL. When high viscosity will cause the spray HBL too rough so difficult to burn in the furnace, and vice versa if low viscosity will result in HBL too fine spray and increase the occurrence and chemical loss carry over (TRS)

HBL Flow spraying spraying depends on pressure, nozzle size and number of spray gun is used. Charbed formation at the base of the furnace depends on the viscosity, flow HBL spraying, spraying angle, the type of spray gun and the amount of air delivery and pressure.

In addition, for memualai combustion in the furnace, to shut down and to stabilize the operating conditions, use start up and load burner. Start up the burner and burner load should always be treated and prepared for any time will be used. Diesel oil or diesel used as fuel penyanggga recovery boiler. The raw materials sector in the tank without solar accommodated isolated and without heated by the temperature of the surrounding air is hot enough.

The capacity of each oil burner is as follows:

- Start-up burner = 500 kg / h solar

- Load burner = 2000 kg / h solar

3.4 Combustion Air and Flue Gas

Air exhaled by the primary combustion air into the primary air ring fan, secondary air fan into secondary and tertiary water ring water ring fan towards tertiary water. Primary and secondary air is heated first steam water heater using low and medium pressure steam to a temperature of about 1500C before being distributed kesetiap water ring equipped with windbox damper-damper to regulate air pressure and equitable distribution.

The tertiary air tertiary didistribusikian to ring without heated water, this is to prevent the gas temperature is not too high superheater region so as to avoid the occurrence of excessive heat (over heating) in the superheater tubes. In normal operation with full load of fuel usage of the air in total around 160,000 Nm3 / h.

3.5 Electrostatic Precipitator and Induced Draught Fans

Flue gas from the furnace is inhaled using Induced Draught Fan where the flue gas will pass through the screen first and then pass superheater tube, boiling tube panel (generation banks), economizer II, economizer I go into Electrostatic Precipitator and drawn by Induced Draught Fan then exhaled out through the chimney (stack).

As we know, a large number of chemical dust (dust) will carry over from the boiler and when discharged directly into the atmosphere means a chemical (chemical dust) and lead to harmful air pollution. To solve this problem, the flue gas is passed through a function captures Electrostatic Precipitator ash (dust) from the flue gas and recycled into the mixing tank where dust is mixed with HBL and burned back in the furnace. Electrostatic Precipitator consists of two units, each consisting of three electric fields (fields) with their efficiency masi8ng 99%.

3.6 Soot Blower

Soot blower has a double function, namely to soot blowing in normal operation and for soot blowing water in washing water when shut down. The goal is the same, namely to remove the dust attached / collected on superheater heating surface area, generation bank, economizer economizer I and II.

3.7 Green Liquor

Green liquor is a major product of the recovery boiler. In the process, black liquor (BL) contains two types of compounds are organic compounds (lignin and wood fiber) and inorganic compounds (Na2CO3 and Na2S). So the organic compounds in the BL will burn to the gas, while inorganic compounds burn to melt compound called smelt. Melt chemicals (smelt) will gather around the charbed and flows out through a gap called the smelt spout toward the dissolving tank. In the dissolving tank. In the dissolving tank, the smelt will be dissolved with weak white liquor (WWL) of the RC section (Section Recaustizing) becomes green liquor (GL). After that, smelt GCC's late (green liquor) will be sent back to the RC for processing into white liquor through recaustizing reaction. White liquor is used as a cooking ingredient in the digester timber (pulp making section).

Figure 3.6 Storage Tank Green Liquor

3.8 Drain and venting

Drain operated to reduce or to empty the boiler water if needed. While venting operated to remove air bubbles or steam from within the system at start up. At shut down venting opened to release the remaining pressure from the boiler. Some of the drain is connected to the inlet header blow down before being discharged into the ditch.

3.9 Security System

Recovery boiler is also equipped with safety systems and auxiliary units, such as:

- Interlock System

This system serves to prevent damage in case of deviation boiler operating conditions.

- Safety valve

This tool serves to keep the boiler pressure does not exceed the limits specified security.

- System rappid drain

This system serves to empty the boiler water to a minimum, if there is a severe leak that caused water boiler piping into the furnace.

The system is operated at the time of emergency and went so fast that boiler avoid further damage.

3.10 Blow Down Tank Continious

Water quality is maintained by opening continious boiler blow down tank. The goal of getting rid of the water from the steam drum to keep the concentration of water and to help control the level of steam drum. Continious blow down open continuously so that impurities in the steam drum is lost.

3.11 Sample Boiler Water

For quality control of boiler water used five sampling lines through attemperatur. Boiler water samples attemperatur cooled using cooling water is indirect contact.

The source sampling are:

a. Demineralized water

b. Feed water

c. Boiler water

d. Saturated steam

e. Superheated steam

f. Deaerator water in

see also previous article"Operation of vacuum evaporator"

Maybe useful.

3.12 Personal Protective Equipment (PPE)

Personal protective equipment used in the recovery boiler is as follows: