PT PLN (Persero) Pembangkitan Tanjung Jati B 1

EXECUTIVE SUMMARY

Today’s strict environmental regulation requires power plant operators to reduce their

harmful emissions. Sulfur Dioxide (SO2) is one of emission produced from boiler combustion

that can create acid rain. This acid rain is very harmful both to the human health and

environment. It contributes to environment damage, increased illness and premature death

from heart and lung disorder.

Flue Gas Desulfurization is a system used to remove the emission of Sulfur Dioxide (SO2) in

combustion flue gas emitted by Coal fired power plant.Typically FGD system is capable to

remove approximately 80% of SO2emission from the flue gas stream. The by-product of FGD

system is gypsum (CaSO4) which can be used for various industrial applications.

PLTU Tanjung Jati B, a relatively new power plant with net capacity of 4 x 660 MW,

operates all of its boilers with FGD. In normal operating conditions, the exhaust gas emitted

to the environment consists of CO2, H2O,NOx, and a small part of SOx. However, recently

Tanjung Jati B Unit 1&2 FGD system started toexperience malfunction,

causingenvironmental and operational problems. The problem causes carryover of salt and

gypsum in the flue gas resulting in local “salt rain”. This situation is harmful tothe

agricultural fieldsaroundthe plant, creates a corrosion problem and corona problem to the

high voltage equipment. Due to the aforementioned problems, the FGD system must be

periodically maintained and as consequences, the corresponding power plant unit also has to

be shut down.

In this paper, schematic of FGD system in Tanjung Jati B power plant Unit 1&2, the working

principle of FGD, chemical processes inside FGD system, and the phenomena that can cause

the malfunction of the FGD are explained in detail. The focus of this paper is the operation

within the FGD absorber system. The solution to the FGD problem and proposed

improvement for FGD operation are described in order to maintain proper operation of FGD

and availability of the power plant.

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I INTRODUCTION

I.1 Background

Tanjung Jati B Power Plant is one of the biggest Coal Fired Power Plant (CFPP) in Java Bali

system. It has 4x660MW net generating capacity and contributes to approximately 11%

energy in Java Bali Madura Electricity system. The availability and reliability of Tanjung

Jati B greatly impacts the system and the soundness of operation condition of the

aforementioned system.

Tanjung Jati B Power Plant Consumes mostly Medium Calorific Value (MCV) Coal

approximately 6750 ton/day per unit, and produce large amount of fly/bottom ah, CO2, SO2,

NOx, etc. These by products contribute significantly to acid rain formation. As widely

known, acid rain is very harmful forthe environment and human health.

Figure1 :Tanjungjati B Power Plant Units 1-4

One of the equipment that can be used to reduce SO2 emission is the Flue Gas

Desulfurization (FGD) system. FGD system can reduce the amount of SO2 emitted to the

environment by method of wet scrubbing of the flue gas using limestone slurry. In Coal Fired

power Plant, FGD system is installed between Electrostatic Precipitator and exhaust Stack. A

single FGD in Tanjung Jati B power plant is capable of removing 516,24 kg SO2 per hour

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contained in the flue gas. Figure 2 shows the schematic of FGD in Tanjung Jati B Power

Plant.

Figure2 : Location of FGD in Coal Fired Power Plant System

Figure 3 :TanjungJati B Unit 2 FGD Absorber

FGD System

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Figure 4 : Typical FGD Schematic. Oxidation air system does not reflect configuration in

FGD unit 1&2

Figure 4 shows the layout of FGD absorber. This system is the main equipment in FGD

system, that functions as SO2 absorber, and gypsum producer. The absorber is divided into

several areas. SO2scrubbing area absorbsSO2from flue gas stream and converts it into

Calcium Sulfite (CaSO3). Mist eliminator area filters slurry carryover in the flue gas stream.

The forced oxidation area oxidizes Calcium Sulfite into Calcium Sulfate/Gypsum (CaSO4) by

supplying air oxidation air.

The operating principle of FGD absorber is as follows:

1. Flue Gas from ESP (Electrosatic Precipitator) enters the FGD and is scrubbed by

Limestone Slurry to remove SO2content, creating Calcium Sulfite (CaSO3).

2. The Flue gas flows through mist eliminator for removal of slurry carry over. The

treated flue gas exiting the eliminator has low level of SO2 and solid particles, and is

ready to be safely exhausted to the environment.

3. The limestone and calcium sulfite slurry that is used to scrub the flue gas drops into

the Forced Oxidation area, which is then converted into Gypsum.

Mist Eliminator

Area

Inlet Flue Gas

(SO2)

SO2 Scrubing

Area

Forced Oxidation

Area

Treated Flue

Gas

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4. Gypsum from the Forced oxidation area is continually removed to separator area to

be dried and shipped.

The reaction taking place in wet scrubbing using a CaCO3(Limestone) slurry produces CaSO3

(calcium sulfite) and can be expressed as:

CaCO3 (s) + SO2 (g) → CaSO3 (s) + CO2 (g)

The CaSO3 (calcium sulfite) is further oxidized to produce CaSO4 · 2H2O (Gypsum) via

forced oxidation method:

CaSO3 (s) + H2O (l) + ½O2 (g) → CaSO4 (s) + H2O

Figure 5 : FGD Absorber Reaction Schematic

In Tanjung Jati B, The average SO2 concentration in flue gas entering the FGD absorber is

1500ppm. The typical FGD exhaust SO2 level is approximately 300ppm. Thus, the condition

meets the environment regulation forSO2emission which must be less than 750 ppm.The

limestone consumption needed depends on the SO2 production which affected by coal quality

and quantity. In common operation, Tanjung Jati B consume approximately 2500-3000 ton of

limestone per month per Unit operation.

I.2 Problems Experienced In TanjungJati B FGD Unit 1&2

One of the problems experienced in FGD system is due to salt and gypsum carry over caused

by mist eliminator malfunction. When the mist eliminator performs under malfunction

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operation, the solid material in flue gas could not be filtered and released to the environment

along with the flue gas. The material carried over from FGD causes environmental problem

and become a big issue. The corrosive and pollutant characteristic of the carry over material

contributes to equipment corrosion and damage. Carryover also cause corona problem to the

high voltage equipment forcing shutdown of the unit to conduct insulator cleaning. Due to

the current condition, proper operation of FGD is strongly needed, and improvement must be

established to perform the excellence unit operation.

I.3 Benefit

The improvements proposed aims to improve FGD system in order to increase reliability,

availability and long life operation capability of the unit. The aim of improvement is to

provide sustainable and economic electric energy supply with consideration of environmental

aspect. It is important for Tanjung Jati B Power Plant to perform safely, reliable and

environmentally friendly.

I.4 Purpose

The purpose of the improvement modification is to provide solution and improvement in

order to maintain proper FGD operation.

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II. CONTENT

The chemical reaction taking place in FGD absorber system is a delicate process that requires

control of limestone slurry supply, PH level, oxidation air supply, and sulfite build up.All

aspects must be properly controlled to maintain adequate SO2 absorption, and acceptable

parameters within the FGD system. Despite efforts to control these parameters, there are

several operational problems experienced in unit 1&2 FGD due to incorrect operating

parameters. The said problems are as follow;

- Increase of Sulfite level in Absorber

- “Sulfite Blinding” causing Occasional Temporary Inability of SO2 Removal and

loss control of PH

- Hardening of Slurry matter in FGD Absorber.

- Frequent Inability of FGD to filter carry over into exhausted flue gas.

Details of the aforementioned problems are explained in subchapters III.1 to III.4.

II.1 Increase of Calcium Sulfite level in Absorber

The absorber is designed to remove sulfur dioxide emission of 516,24 kg/hr by converting

SO2 into Calcium Sulfite (CaSO3) The Calcium sulfite is then turned into Calcium

Sulfate/Gypsum (CaSO4) by forced oxidation method. The Limestone forced oxidation

system typically has a very low concentration in dissolved Calcium Sulfite, with typical

concentration less than 30 ppm. The sulfite level can be controlled by supplying oxidation air

to the forced oxidation area, by means of Air blower and diffusers in the bottom of the

Absorber.

Despite the typically acceptable Calcium Sulfite conditions, increase in Calcium Sulfite

concentration can occasionally occur in FGD absorber. This is caused by lack of oxidation

air amount in relation with the Calcium Sulfite added into the forced oxidation system. The

cause oflack of supply is insufficient air supply from the oxidation blower, blocking of

oxidation air holes in the absorber, and elevated SO2 level in flue gas beyond the capability of

the absorber.

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Figure 6 :Hibbon air blowers supplying oxidation air to FGD absorber

The condition mentioned above causes theCalcium Sulfite to Gypsum reaction to be

incomplete, and leaving a fraction of sulfite not converted, thus gradually increasing sulfite

concentration. Increase in sulfite concentration can be detrimental towards FGD operation

because it leads to other problems, such as “Sulfite Blinding”and Hardening of Slurry Matter

that will be explained in the next subchapters.

Shown in figure 6 and 7 are photos of one of oxidation air pipe blockage, reducing absorber

capability to oxidize Calcium Sulfite into Gypsum.

Figure 7: Blocked oxidation air nozzle. In operating condition, this pipe is submerged in

gypsum slurry Shutting down of oxidation air will result in slurry entering the pipes and

increase chance of blockage.

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Figure 8 : Removed Oxidation Air pipe containing solid gypsum blockage

II.2 Sulfite Blinding

Sulfite Blinding is a phenomenon characterized by loss control of Absorber PH, regardless of

increased supply of limestone slurry. This situation is caused by increase of sulfite

concentration in the absorber. At some concentration, there is enough sulfite to start blinding

the limestone. In the area around a calcium carbonate particle (the active ingredient in

limestone) there is a relatively high PH in the range of 6-8. The bulk of the slurry is typically

in a PH range of 5.2 to 5.6. The solubility of calcium sulfite decreases with increasing PH.

This causes sulfite to precipitate into surface of limestone and reduce the active surface area

of limestone, thus resulting in loss control of PH.

Figure 9: SEM photograph of Calcium sulfate (flat plates) blindunreacted limestone. The

elevated concentration of calcium sulfate will cause the inability of limestone to convert

SO2into Calcium Sulfite.

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Sulfite Blinding will reduce the ability of Limestone slurry to convert SO2into Calcium

Sulfite, causing addition of SO2instead of Calcium Sulfite in the forced oxidation area.

Introduction of SO2into the forced oxidation area will continually decrease the PH level due

to the inherent acidity of SO2. Adding limestone will not increase the PH, it will make the

problem worse. Also, the flue gas exhausted into the environment will not be properly

treated, and contains high level of SO2.

II.3 Hardening of Slurry matter in FGD Absorber

Due to its flat molecular shape, Calcium Sulfite is prone to form solidmaterial and scale in

the FGDabsorber. This condition will createundesirable operating condition and can threat

the FGD operation. The hardened material needs to be removed, and shutting down of FGD

is necessary toperform the action.

Figure 10: Large amount of scales and hard material removed from Unit 1 Absorber.

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Figure 11: The scale collected from FGD absorber was large and had may layers

II.4 Frequent Inability of FGD to filter carryover material

The most known problem of Unit 1&2 FGD is carryover of Gypsum and Salt in the

exhausted flue gas. The FGD design to filter the carry over by series of tight chevrons called

the Mist Eliminator. The chevron filters the carry over material, and is periodically sprayed

every 15 minutes to drop the carry over into the forced oxidation pool.

Figure 12: Mist eliminator profile

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The carry over problem is caused by blocking of mist eliminator by carry over material,

rendering it ineffective to properly function. One of the causes is blockage of spray nozzle

due to barnacle Shells carried bysea water supply. This leaves parts of eliminator being

unwashed, leaving residual material carried by flue gas. Another cause of blockage is the

occasional increase in Calcium Sulfite that increases the chance of solid formation in the mist

eliminator. Both of these factors contribute in the ineffectiveness of mist eliminator causing

unavoidable carry over.

Figure 13: Properly working Mist Eliminator Spray Nozzle

Figure 14: Mist eliminator Nozzle block by seashell

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Figure 15: Major Blockage on mist eliminator by slurry

II.5 Secondary Problems caused by improper condition of FGD

The improper FGD condition also cause environmental problem within and outside of the

Power Plant area, mainly due to Carry Over in flue gas. The environmental problems are as

follow;

- Gypsum Carry over creating corrosion problem in units 1to4

- Gypsum carry Over creating corona problem in high voltage isolators

- “Gypsum Rain” causing crop failures in neighboring crop fields, resulting in an

environmental and Public Relations problem.

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III CONCLUSION AND RECOMMENDATIONS

Analysis of the situation results in these factors as cause of problem;

- Elevated SO2 concentration in flue gas beyond the capability of FGD.

- Lack of oxidation air supply

- Blockage of Mist Eliminator

- Blockage in mist eliminator spray

- Blockage in oxidation air nozzle

Based on analysis on the situation, the following actions are proposed to relieve the situation.

- Utilization of fresh water instead of sea water for supply of Mist eliminator

spray.

- Replacement of oxidation air system with new design to eliminate blockage.

- Increase of oxidation air supply.

Details of the aforementioned improvement solutions are explained in subchapters IV.1 to

IV.3.

III.1 Utilization of fresh water to supply of Mist eliminator spray

Fresh water can be utilized to supply mist eliminator spray, replacing sea water as current

supply. The main purpose is to eliminate blocking of nozzle by seashell. Several

modifications need to be done to achieve this such as rerouting of supply water, and new

piping.

III.2 Improvement of oxidation air system design

The purpose of the existing oxidation system is to create small bubbles via small pipe holes,

creating a large contact area to optimize forced oxidation reaction. However, the existing

system is prone to blockage, rendering the forced oxidation to be less effective. This can be

relieved by employing a new design, which utilizes agitators instead of small holes to diffuse

the oxidation air. Such system is installed in Tanjung Jati B FGD for Unit 3&4. Figure 15

shows illustration of the proposed improvement.

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Figure 16: Improved oxidation air system as employed in Tanjung Jati B FGD absorber

unit 3&4

III.3 Increase in oxidation Air Supply

It is understood that coal supplied to Tanjung Jati B Power Plant has high sulfur content.

This results in high SO2 emitted into the FGD to sometimes rise above 2000ppm. The

oxidation air of the FGD is not sufficient to convert the elevated Sulfite concentration due to

high SO2, and will increase sulfite level resulting in Sulfite Blinding. This situation can be

relieved by increasing the supply of oxidation air, by adding bigger air blowers.

III.4 Conclusion

The appropriate application of the proposed improvements will reduce the intensity of the

aforementioned malfunction, and improve the condition of Tanjung Jati B FGD System. This

is made possible because the improvements eliminate the root cause of the problem. Such

improvements are most needed to ensure excellent operation of the Power Plants.

Oxidation air

supply with

agitators as

diffuser