Steam Boiler: Flue Gas Treatment

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Combustion Air Preheater

The combustion air preheater is definitely one of heat exchanger applications. Based on Figure 1 below, flue gas simply leaves steam boiler and passes via air preheater. The combustion air is passed through this equipment too to increase its temperature before being combined with boiler fuel.

Because the temperature of combustion air is lower than the temperature of flue gas, combustion air receive heat transfer from flue gas through combustion air preheater in the process of convection heat transfer. The heat transfer make temperature of flue gas lower and consequently minimizes its heat loss and also decreases the air temperature to stack. The additional heat throughout the combustion air going into boiler furnace improves the process of combustion. This decreases the fuel need in quantity equivalent in the value of heat to quantity of heat which have been taken in the application of heat exchanger such as combustion air preheater, therefore increasing efficiency.

Through the utilization of air preheater application, about 1 % of fuel is unspent for every single 40 F go up in the temperature of combustion air.

Figure 1: Combustion Air Preheater
(Source: Book-Boiler Control System Engineering-G.F Gilman)

Orsat Analysis

Orsat analysis is one of techniques to analyze flue gases. Orsat analysis is even now regarded as the most beneficial technique to analyze flue gases correctly. Orsat device as shown in Figure 1 contains some components (labeled) such as:

- Label A is straight burette which flow from bottom to top and contains 100 sections

- Label B is aspirator container

- Label D1, D2, and D3 are pipettes that are numbered 3 pieces comprising absorbents stuffed with beads or glass tubes intended for improving absorption areas. Each pipettes contains solution as follow:

· CuCl2 (cuprous chloride) in HCl intended for CO

· Alkaline pyrogallol intended for O2

· KOH (Potassium hydroxide) intended for SO2 dan CO2)

Each one pipette is linked to an unoccupied pipette lurking behind it so absorbent could subside directly into this when the gas is accepted. Label C is the primary separating cock pertaining to gas maintenance in the process in conjunction with Label F1, F2, and F3 which is 3 separating cocks intended for 3 pipettes.

Figure 1: Orsat Device for Orsat Analysis
(Source: Book-Boiler for Power and Process-Kumar Rayaprolu)

The process of orsat analysis is as follow:

Aspirator container aspirates gas approximately 100 cc.
3 absorbents absorbs gas in certain series
Record absorption through variation in the burette. Caution, tight adherence to the right planning of absorbents and the process to specific focus are required to get mistake-free outcomes. To get correct samples on a continual base, a suction pump is utilized to suck up flue gas, because the aspirator only is normally not sufficient.

Characteristic Output of ESP (Electrostatic Precipitator)

Characteristic output of ESP (Electrostatic Precipitator) is a comparison between voltage and current density. Characteristic voltage and current generated in the control system is a combination of high voltage rectifier transformer and precipitator as a supplier of high voltage DC. Dust that passes through the precipitator plate can be likened to the particle charge carriers from the negative plate to positive plate.

Figure 1: Characteristic of ESP (Electrostatic Precipitator)

From the Figure 1 above show that the output voltage and current density as a characteristic, dust deposition efficiency depends on the magnitude of the voltage difference on each plate precipitator. On the other hand, through the addition of the operating voltage potential output will cause a magnetic field that would cause a greater potential as well.

Fly Ash Bin in Electrostatic Precipitator

Fly ash bin is one of electrostatic precipitator parts. Steam boiler use electrostatic precipitator for filter flue gas, so pollutant can be eliminated. Dust particles screening result from electrostatic precipitator (ESP) unit will be accommodated on the fly ash bin. The air from compressor pipe goes into fly ash bin will be thrown back into atmosphere with the help of fan. So the air inside fly ash remains vacuum. Installation of fly ash bin is equipped with double screw ash conveyor as shown in Figure 1 to throw dust particles into transport vehicle.

Figure 1: Double Screw Ash Conveyor

Dust Hopper in Electrostatic Precipitator

Electrostatic precipitator parts consist of some equipment, one of them is dust hopper. Electrostatic precipitator has similar function like bag house filter and muti cyclone. Electrostatic precipitator as filter flue gas boiler has many advantages than other equipment. But this equipment is more expensive than baghouse and muti cyclone.

Hopper in the electrostatic precipitator is the shelter of dust particles or disposal of dirt particles. In one unit electrostatic precipitator is usually installed three dust hoppers. Sensor electrode phone is used to determine amount of dust particle in each hopper. In the tank there is air lock system which is driven by motor which is equipped with gears. Placement of air lock system as shown in Figure 2, there is reservoir of dust (dust hopper and fly ash bin.

Figure 1: Dust Hopper and Air Lock in Electrostatic Precipitator

Hammer Beater Plate in Electrostatic Precipitator

Hammer beater plate is one of electrostatic precipitator parts. Electrostatic precipitator is equipment in steam boiler which is used to filter flue gas before pass through stack and discharged to atmosphere. Hammer beater plate in electrostatic precipitator is used to clean collector electrode plates from dust particle that settles due to electrostatic precipitator process. Furthermore hammer beater plate is driven by low speed motor (see Figure 1).

Hammer beater plate is designed with different position, so the beating on the collector electrode plate alternately ongoing. The position of hammer beater is along electrode hammer shaft and coupling shaft through transmission gear to motor shaft.

Figure 1: Construction of Hammer Beater Plate in Electrostatic Precipitator

Discharge Electrode in Electrostatic Precipitator

Discharge electrode is one of electrostatic precipitator parts. Function discharge electrode as shown in Figure 1 in electrostatic precipitator unit is giving negative charge on dust particles. Discharge electrode plate is located between two collector plates and it is suspended on a wire mounted on an insulator at the top, and defended his position by providing weight on the bottom. The use of discharge electrode plates are made with rods and both ends are tapered to facilitate the generation of plasma corona. Because volume levels of dust particle which enter always change, then the current flow into electrostatic precipitator will also vary.

Dust particles and other material through plate electrostatic precipitator is medium conductor of electricity flow from positive plate to negative plate, while the insulation material is air around the plate.

Figure 1: Construction of Discharge Electrode in Electrostatic Precipitator

Collection of Parallel Plates in Electrostatic Precipitator

Collection of parallel plate is one of electrostatic precipitator parts. The parallel plates serve as dust collector. These plates will be given high contrary voltage. Supporting plates of electrostatic precipitator are as follow:

Support Insulator

Support insulator serves as a holder of positive and negative plates in electrostatic precipitator unit as shown in Figure 1 below.

Figure 1: Support Insulator Discharge Electrode

Another function of support insulator is as medium barrier (insulator) between frame (body) of precipitator and negative plate. Support insulator is equipped with heater and thermostat. The purpose of heater is to make temperature of insulator stable. If the temperature change over the insulator will be broken or damaged, so temperature must be maintained constant.

Collector Plates

Collector plates as shown in Figure 2 is place of dust particle attach on it when it is given positive voltage source. Size of dust particles attached on collector plates ranged from 0.3 to 40 microns. Dust on collator plates are periodically removed through vibration (rapping) into dust hopper.

Figure 2: Construction of Collector Plates

High Voltage Power Supply

High voltage power supply is one of electrostatic precipitator parts. With this equipment the AC input voltage is increased through step up transformer, and then converted to be high voltage DC. Static electricity is needed in the process of dust deposition obtained through converter AC/DC through step up transformer. Control voltage is performed on each cable conductor. The control is done feed back to changes in variable peak current, average current and over voltage. On the secondary side of transformer is connected to the bridge rectifier with four diodes with equal rating.

Figure 1: Connection Diagram Rectifier

Gas Distribution Wall (GDW)

The function of gas distribution wall (GDW) is one of electrostatic precipitator parts. Gas distribution wall as shown in Figure 1 is a place for filtering dust from flue gas (product of combustion process in furnace steam boiler). GDW in principle operate without the use of electrical energy which consists of composition of iron plate made with holes.

Dust particle which is sucked by induced draft (ID) fan before entering electrostatic precipitator unit will pass through gas distribution wall (GDW), the level of dust particles that have bigger size than the size of holes will be stuck on the wall plate and fall down.

Figure 1: Construction of Gas Distribution Wall (GDW)

Electrostatic Precipitator Parts

The main parts of electrostatic precipitator are a room or passageway containing electrodes in the form of parallel plates, collection of tubes or just inner wall of collector. Distance between one plate to other plate about 0.5 m, air will flow through plate spacing. The plates are acting as electrodes which serve to collect dust particles.

Figure 1: Construction Electrostatic Precipitator Parts
(source: Encyclopedia Britannica, Inc.)

Electrostatic precipitator parts ash shown in Figure 1 above can be mentioned as follow:

Gas Distribution Wall (GDW)
High Voltage Power Supply
Collection of Parallel Plates, consist of:

1. Support Insulator

2. Collector Plates

Electrostatic Filter Precipitator Work

In a simple Electrostatic filter Precipitator (ESP) working principle, opposite the high voltage (positive and negative) is imposed on the two plates or grids. Negative grid charge particles and positive grid attract (collect) particles. Working principle of electrostatic filter precipitator in Figure 1 can be written sequentially as follow:

Electrodes attached to the exhaust filter provide a negative charge by the plasma generation of corona glow discharge of dust particulates.
Dust particles are negatively charged are then captured on plate slab with a positive charge.
By mechanical vibrations or shocks generated hammer beater plate and dust particles fall and are collected in the hopper (collector) and then disposed to further process.

Figure 1: Electrostatic Precipitator (ESP) Working Principle

In the screening process of dust particles by using electrostatic filter precipitator (ESP), dust that passes though plate will experience multi stages as shown in Figure 2. Therefore, the dust particles which are not attached in the first stage will pass through the next stage so can increase efficiency of electrostatic filter precipitator (ESP).

Figure 2: Electrostatic Precipitator Multi Stage (top view)

Electrostatic Precipitator Efficiency

Electrostatic precipitator design should consider about efficiency because it determine overall efficiency of power plant. There are some factors affecting the efficiency of electrostatic precipitator as follow:

The amount of dust collector plates

Amount of plates and surface area of collector plate can affect the magnitude of desired efficiency. The more number of plates and larger area of collector plates will make higher efficiency which can be obtained.

Velocity of flue gas

Increased flow velocity of flue gas which is sucked by Induced Draft (ID) fan may lead dust are not ionized by the rod electrodes perfectly. Furthermore dust particles are not attached on collector plates. If velocity of flue gas is too high will make electrostatic precipitator difficult to catch dust particle and finally decrease electrostatic efficiency.

Temperature

Changes of temperature in electrostatic precipitator can affect changes in efficiency. If temperature in electrostatic precipitator decrease can lead dust will be moisture and dust particle will be attached permanently on collector plates and difficult to clean it. Thermocouple is installed in electrostatic precipitator to measure temperature in this equipment.

Fuel

The selection process of fuel for steam boiler combustion is very important, besides the economic aspect also concern about resulted efficiency. The fuel is commonly used is coal or oil palm shell. The amount of fuel which is used depends on the load generated and calorie content of fuel.

Electroventuri System

Flue gas is product of combustion process. Before exhausted to atmosphere flue gas boiler should be treated to filter pollutant particle which cause air pollution. There are some flue gas cleaning methods. One of them is electroventuri system.

The working principle of electroventuri system as shown in Figure 1 below is based on attraction between molecules that are not similar or adhesion force in the opposite type of molecules. Dust particles are given negative charge and collected in water which has positive charge.

Basically electroventuri method is almost same as electrostatic precipitator system, except that this method operates in wet environment. Using electroventuri system will gain efficiency less than 95% while smallest size of dust particle which can be captured is 0.5 µC. Application of electroventuri system is applied in cement industries and pulp and paper industries. The disadvantageous of using electroventuri system is this system need large area for its placement.

Figure 1: Electroventuri System

Artificial Rain System (Scrubber)

Method of artificial rain system in Figure 1 below is one of flue gas boiler cleaning methods. This method using medium water as filter for waste dust, water used containing certain active chemicals. Flue gas is passed through artificial rain curtain, so the grain of dust is fallen by grains of artificial rain then some dust particle will separate from flue gas flow. Furthermore waste dust is filtered by artificial rain system.

The method of artificial rain system (scrubber) is less effective because only grains of dust which is contacted directly by artificial rain can be separated from flue gas while grains of dust which is not contacted by artificial rain will pass through stack to atmosphere.

The efficiency gained from the method of artificial rain (scrubber) is less than 90% while the size of maximum dust particle obtained is 5 µC. The application of this method is used for industrial process that uses fuel with high NOx content. The disadvantageous of this system is this method produces liquid waste which is generated from artificial rain mixed with dust particle.

Figure 1: Artificial Rain System (Scrubber)

Flue Gas Boiler Cleaning Methods

Flue gas boiler before pass stack and exhausted to atmosphere should be cleaned and filtered first to reduce and eliminate pollution air. Pollution air is caused by pollutants which can destroy and damage environment. The following below are the cleaning methods to clean and filter flue gas boiler and this equipment are usually located before stack or chimney:

Each system has advantageous and disadvantageous in term of filtered size of particle, amount of filtered flue gas, price and efficiency. The best cleaned method is chosen based on these parameters.

Electrostatic Precipitator as Flue Gas Boiler Filter

Electrostatic precipitator is one of flue gas filter equipment in boiler. There are some choices to determine flue gas filter equipment. There are multi cyclone, bag house and electrostatic precipitator. Basic working principle of electrostatic precipitator is passing the exhaust gas or flue gas (product of combustion from furnace boiler) to the room which containing the electrode plates, made of copper or brass.

Attached electrodes in the construction of electrostatic precipitator is energized by direct current electric (DC) with positive and negative charges. Between the electrode trunks which has negative charge and plate dust collector which has positive charge is energized by voltage 70-90 KV. The grains of dust that pass through the electrode trunk will be induced by negative charge and so now the grains of dust have negative charge. Because their charge is negative, the electrode plate which has positive charge attracts the grains of dust so the grains of dust stick to the plate.

Presence of vibration (rapping) that touch the plate dust collector will cause dust fall into dust hopper, then the dust will be separated from flue gas in the electric precipitator. By passing static electric current will precipitate dust effectively so the impact of air pollution can be eliminated. This method can reach efficiency above 90% and can capture small particle dust until less than 2µC. Electrostatic precipitator can be applied to cement plant, pulp and paper plant and power plant.

The factors that determine the selection of flue gas filter equipment in boiler are as follow:

- The efficiency to filter dust particle

- The size of captured dust particle

- The fuel used in the combustion furnace

- Capacity of fuel in the combustion furnace

- Development and maintenance cost

Air Pollution from Steam Boiler

Parameters of air pollutants resulting from the combustion furnace of the steam boiler are:

1. Sulfur Dioxide

Air pollution by sulfur oxides is mainly caused by two components which in the form gas and colorless, namely sulfur dioxide (SO2) and sulfur trioxide (SO3). Both are called as sulfur oxides (SOx). Sulfur dioxide has a characteristic sharp odor and not flammable in the air while triode is not reactive component.

The problem posed by the pollutants created by humans are in terms of its distribution is uneven, so concentrated in certain regions. Pollution from natural sources is usually more spread out evenly. Source of SOx pollution can be found in the burning of fuels in industry such as fuel coal. SOx pollution will give bad impacts on humans and animals and also damage plants if occurs at levels of 0.5 ppm. The primary effect of SOx pollutants on humans is respiratory system irritation. To suppress the emission of SOx gases, a unit of FGD (Flue Gas Desulfurization) is always used.

2. Carbon Monoxide

Carbon monoxide is a compound that has characteristic such as odorless, tasteless, and at normal air temperature will form colorless gas. CO compounds have the potential to be dangerous because the toxins are capable to forming strong bonds with blood pigment.

Artificial sources of CO include motor vehicles, especially those that use gasoline, while the source does not move like the burning of coal, oil from industrial and domestic waste burning.

The impact of CO varies depending on a person's health status; the influence of high CO levels is against central nervous system. To suppress the emission of CO, the unit of scrubber is used in the chimney or stack.

3. Nitrogen Dioxide

Nitrogen oxides (NOx) are a group of nitrogen contained in the atmosphere consists of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen monoxide is colorless and odorless gas while nitrogen dioxide is reddish brown and pungent.

The largest source of NOx pollution is from human activities in urban and industrial activities. The impact of NOx pollution is as toxic material so will be danger for human’s lung. To suppress the emission of NOx is steam boiler should use fuel with low content of NOx.

4. Dust Particle

The combustion product especially for dust can be divided into two types namely bottom ash and fly ash. Effect of liquid or solid form of dust particulate in the air depends on its size. The size of dust particulates are harmful to health has range generally between 0.1 microns to 10 microns. To suppress this emission, steam boiler can be equipped with multi cyclone, bag house or Electrostatic Precipitator (ESP).

Multi Cyclone in Steam Boiler

There are three type of dust collector; multi cyclone, baghouse, and electric static precipitator. The steam boiler which has stoker type, usually use multi cyclone to collect dust. Dusts from flue gas are separated by centrifugal and gravitation force.

Figure 1: Multi Cyclone in Steam Boiler

Multiple cyclone has some cyclones to separate dust from flue gas. Multi cyclone usually receives flue gas from air heater ducting, collect dust from flue gas flow then distribute flue gas into stack. Top section of cyclone is used for impact effect to flue gas flow, so debris or dust can be filtered and fallen down into hopper. The multi cyclone is connected by two flanges of ducting, outlet and inlet. Flue gas flow hit cyclone and make its flow to be centfrifugal flow. Because of gravitation, dust will fall down into hopper and separation of dust from flue gas happen.

Flue Gas Treatment in Steam Boiler

Some of treatments of flue gas in steam boiler should be performed to reduce bad effect of emission (NOx and Sox) before distributed to atmosphere. The followings are treatment to reduce content of NOx and SOx

1. Treatment to reduce content of NOx

a) Select low content NOx of fuel such as for coal must less than 1.5%

b) Increase sizing of furnace to reduce temperature and NOx production.

c) Restrict the use of oxygen by low excess air when react with nitrogen to form NOx.

d) Use over fire air to add combustion perfectly.

e) Install flue gas filter such as multi cyclone, baghouse or electric precipitator to reduce NOx content in flue gas.

f) Use mechanical ash collection like hopper to collect unburned fuel.

2. Treatment to reduce content of SOx

a) Select low content of SOx fuel.

b) Use scrubber wet non regenerative, use additional chemical like limestone to react with SOx to be combined compound. The compound can be disposed or be sold for gypsum.

c) Use scrubber wet generative to make faster the process of collection by use sodium carbonate.

d) Use scrubber dry, collecting solid product result from heating process of flue gas, so dries up the moisture. The moisture can be taken from adding chemical in water based spray to flue gas.