Boiler Design Steam Beoiler
The meaning of Bolier.
Boilers are closed vessels where combustion heat is flowed into the water until hot water or steam is formed. Hot water or steam at a certain pressure is then used to flow heat to a process. Water is a useful and inexpensive medium for delivering heat to a process. If water is boiled to steam, the volume will increase by about 1,600 times, producing energy that resembles explosive gunpowder, so that the boiler is equipment that must be managed and maintained very well.
Types of Boilers Based on Sytems They Work
Boiler feed water system (feed water system)
Feed water system provides water for boilers automatically according to steam requirements. Various faucets are provided for maintenance and repair purposes.
Boiler (steam system)
Steam System is collecting and controlling steam production in a boiler. Steam is flowed through the pipeline to the user’s point. In the whole system, the steam pressure is regulated using a valve and monitored by a pressure monitor.
Fuel system (fuel system).
The fuel system (fuel system) is all the equipment used to provide fuel to produce the heat needed. Equipment needed in the fuel system depends on the type of fuel used in the system Water supplied to the boiler to be converted into steam is called feed water. Two feed water sources are:
Condensate or condensing steam returned from the process.
Makeup water (treated raw water) which must be fed from outside the boiler room and process plant.
Another system is the use of an economizer to preheat feed water using waste heat in the exhaust gas, to obtain a higher boiler efficiency.
Heat energy generated in the boiler system has a value of pressure, temperature, and flow rate that determines the utilization of steam to be used. Based on these three boiler systems recognize the conditions of low pressure (LP), and high pressures (high pressure / HP), with that difference the utilization of steam coming out of the boiler system is utilized in a process to heat up the liquid and run a machine (commercial and industrial boilers), or generate electrical energy by converting heat energy to mechanical energy then turning the generator to produce electrical energy (power boilers). However, there are also those who combine the two boiler systems, which utilize high temperatures to generate electricity, then the remaining steam from the turbine under low pressure conditions can be utilized into industrial processes with the help of heat recovery boilers.
The boiler system consists of a feed water system, a steam system, and a fuel system. The feed water system provides water for the boiler automatically according to steam requirements. Various faucets are provided for maintenance and repair of the feed water system, handling feed water is needed as a form of maintenance to prevent damage from the steam system. The steam system collects and controls steam production in the boiler. Steam is run through a piping system to the user’s point. In the whole system, steam pressure is regulated using a tap and monitored with a pressure monitor. The fuel system is all equipment used to provide fuel to produce the heat needed. Equipment needed in the fuel system depends on the type of fuel used in the system.
Before explaining the diversity of boilers, it is necessary to know the components of boilers that support steam generation, along with boiler components:
This component is a place to burn fuel. Some parts of the furnace include: refractory, fireplace room, burner, exhaust for flue gas, charge and discharge door.
Ø Steam Drum
This component is a hot water reservoir and steam generation. Steam is still saturated.
This component is a place for drying steam and is ready to be sent by playing the steam pipe and is ready to drive a steam turbine or run an industrial process.
Ø Air Heater
This component is a heating room that is used to heat the absorbed outside air to minimize the humid air that will enter the furnace.
This component is a heating room that is used to heat water from condensed water from the previous system and new feed water.
Ø Safety valve
This component is a steam exhaust line in the event of a situation where steam pressure exceeds the boiler’s ability to withstand steam pressure.
Ø Blowdown valve
This component is a channel that functions to remove deposits that are in the steam pipe.
1.4 WATER CONDITIONS BOILER FEED
Water used in the treatment process and boiler feed water is obtained from river water, reservoir water, bore wells and other spring sources. The water quality is not the same even though using similar water sources, this is influenced by the environment from which the water originates. Generally, river water sources have been polluted by population activities and industrial activities, therefore purification is necessary.
Boiler feed water must meet specified specifications so as not to cause problems with boiler operation. The water must be free of unwanted minerals and other impurities that can reduce the working efficiency of the boiler.
Feed water must meet certain prerequisites as described in the table below:
1.5 BOILER PROBLEMS
A boiler or steam generator that is operated without good water conditions, sooner or later will cause problems related to the performance and quality of the steam generator system. Many of the problems caused by the lack of handling and special attention to the use of boiler feed water.
As a result of the lack of handling of boiler feed water will cause the following problems:
a) Crust formation
Crust formed on the boiler wall occurs due to the presence of crustal minerals, such as hardness ions such as Ca2 + and Mg2 + and due to the effect of evaporation gases. Diamping can also be caused by a concentration mechanism in the boiler due to heating. The types of crust that are common in boilers are calcium sulfate, silicate compounds and carbonates. Substances can form hard and dense crusts so that the length of handling will be very difficult to remove. Silica is deposited together with calcium and magnesium so that it makes the crust harder and harder to remove.
The crust that covers the boiler surface has an effect on surface heat transfer and shows two main consequences, namely the reduction of heat transferred from the kitchen to the water which results in increasing the temperature around the kitchen, and decreasing the efficiency of the boiler.
To reduce the occurrence of crust formation on boilers, prevention can be carried out as follows:
– Reducing the amount of minerals with the softener unit
– Regularly blow down the amount
– Providing anti-crust chemicals
Dissolved and suspended substances found in all natural water can be removed / reduced in the pre-treatment process (pre-treatment) which is proven to be economical. Prevention of existing crust can be done by:
* On-line cleaning, which is softening old crust with chemicals while the boiler is operating normally.
* Off-line cleaning (acid cleaning) that is dissolving old crust with special acids but the boiler must stop operating.
* Mechanical cleaning: with brushes, chisels, scrubs, etc.
b) Corrosion events
Corrosion can be caused by oxygen and carbon dioxide contained in condensed vapors. Corrosion is a metal event returning to its original form in for example iron to iron oxide, aluminum and others. Koros events can occur due to:
– Corrosive gases such as O2, CO2, H2S
– Crust and deposit
– Metal differences (galvanized corrosion)
– pH that is too low and so on
The type of corrosion found in boilers and steam systems is general corrosion, pitting (hole formation) and embrittlement (steel cracking). The presence of dissolved gas, oxygen and carbon dioxide in boiler feed water is the main cause of general corrosion and pitting corrosion (electro chemical and diffrential oxygen types). The solubility of these gases in boiler feed water decreases when the temperature rises. Most oxygen will separate in the steam room, but a small amount of residue will be left in the solution or trapped in the bags or under the deposit, this can cause corrosion of the boiler metals. Because it is important to do the deoxygenation process of boiler water.
The average amount of corrosion or electrochemical attack will increase if the pH value of the water decreases. In addition, boiler feed water will be chemically conditioned to reach a relatively high pH value. Uncommon but dangerous forms of corrosion are forms of embrittlement corrosion or inter crystalline cracks in steel that occur when they are at high pressure and an improper chemical environment. Caustic embrittlement or crystalline inter flattening in steel that occurs when it is at high pressure and an improper chemical environment. Caustic embrittlement occurs in the blockage joint and extends to the end of the tube where the gap allows the development of a concentrated caustic environment. Hydrogen embrittlement is another form of intercrystalline cracks that occurs in boiler water tubes caused by high pressure and conditions
) Establishment of a deposit
Deposits are events of clotting substances in boiler feed water caused by the presence of suspended solids such as iron oxide, copper oxide and others. This event can also be caused by vapor contamination from the products produced by the production process. Sources of deposits in water such as dissolved salts and substances suspended in boiler feed water. Heating and with the presence of suspended substances in water on the boiler causes a number of loads to settle which decreases the solubility, if the temperature is raised. This explains why crust and sludge are formed. Crust is a form of deposits that remain on the surface of the boiler while sludge is a form of deposits that do not settle or soft deposits.
In high-pressure kettle, young silica settles with steam and can form a deposit that makes it difficult for the turbine leaves.
Prevention – prevention that can be done to reduce the occurrence of deposit events can be done including:
* Minimizing the entry of minerals that can cause deposits such as iron oxide, copper oxide and others.
* Preventing corrosion in the condensate system with the neutralization process (adjusting pH 8.2–9.2) can also be done by preventing the occurrence of air leaks in the condensate system.
* Prevents vapor contamination then uses chemicals to disperse minerals that cause deposits.
Prevention of existing deposits can be done with acid cleaning, online cleaning, and mechanical cleaning.
d) Steam contamination (steam carryover)
When boiler water contains high concentrations of dissolved salt and suspended substances, there is a tendency for it to form excess foam so that it can cause steam carryover solids and impurities into the steam.
Carryover steam occurs when minerals from the boiler come out along with steam to tools such as superheaters, turbines, and others. These contaminations can be deposited again on a steam system or substances that will contaminate the process or materials needed by steam.
Carryover steam can be avoided by holding dissolved solids in boiler water below a certain level through a systematic analysis and control of the administration of chemicals and blowdown. Carbon dioxide carryover can restore vapor and condensed acids.
TYPE – BOILER TYPES
2.1 BOILER TYPES
This section explains about sharing types of boilers: Fire tube boilers, Water tube boilers, Fluidized bed combustion boilers, Atmospheric fluidized bed combustion boilers, Pressurized fluidized bed combustion boilers, Circulating fluidized bed combustion boilers, Stoker fired boilers, Pulverized fuel boilers, Waste heat boilers and thermic fluid heaters.
A. Fire Tube Boiler
In a fire tube boiler, hot gas passes through the pipes and boiler feed water is inside the shell to be converted into steam. Fire tube boilers are usually used for relatively small steam capacities with low to moderate steam pressures. As a guideline, fire tube boilers are competitive for steam speeds up to 12,000 kg / hour with pressures up to 18 kg / cm2. Fire tube boilers can use fuel oil, gas or solid fuel in their operations. For economic reasons, most fire tube boilers are constructed as “package” boilers (assembled by factories) for all fuels.
B. Water Tube Boiler
In the water tube boiler, boiler feed water flows through the pipes into the drum. Circulated water is heated by the combustion gas to form steam in the steam area in the drum.
This boiler is chosen if the steam demand and steam pressure are very high as in the case of a boiler for power plants. A very modern water tube boiler designed with a steam capacity between 4,500 – 12,000 kg / hour, with very high pressure. Many water tube boilers are constructed in packages if fuel and gas are used.
For water tubes that use solid fuel, it is not commonly designed in packages.
Characteristics of water tube boilers as follows:
* Forced, induced and balanced drafts help to improve combustion efficiency.
* Less tolerant of the quality of water produced from a water treatment plant.
* Allows for a higher level of heat efficiency.
C. Boiler Package
Called a boiler package because it is available as a complete package. When sent to the factory, it only requires steam pipes, water pipes, fuel supplies and electrical connections to operate. The boiler package is usually a shell and tube type with a fire tube design with both heat transfer and high convection.
Characteristics of packaged boilers are:
* The small amount of combustion space and the high heat released results in faster evaporation.
* The large number of small diameter pipes makes it have a good convective heat transfer.
* Forced or induced draft systems produce good combustion efficiency.
* A number of passes / passes produce better overall heat transfer.
* Higher thermal efficiency compared to other boilers.
The boilers are grouped according to the number of passes – that is, how many times the combustion gas crosses the boiler. The combustion chamber is placed as the first track after that, then one, two, or three sets of fire pipes. The most common boilers in this class are three-pass units with two sets of fire-tubes / fire pipes and exhaust gases coming out from behind the boiler.
D. Combustion Boiler with Fluidized Bed (FBC)
Fluidized bed (FBC) combustion appears as a possible alternative and has significant advantages over conventional combustion systems and provides many benefits – compact boiler design, flexible to fuel, high combustion efficiency and reduced harmful pollutant emissions such as SOx and NOx. The fuel that can be burned in this boiler is coal, repellent goods from the washing place for clothes, rice husks, bagasse & other agricultural wastes. Fluidized bed boilers have a wide range of capacities which are between 0.5 T / hr to more than 100 T / hr.
When evenly distributed air or gas is passed up through a bed of solid particles such as sand supported by a fine filter, the particles will not be disturbed at low speeds. Once the air velocity gradually rises, a state is formed where the particles suspended in the air stream – the bed is called “fluidized”.
With the subsequent increase in air velocity, bubble formation, strong turbulence, rapid mixing and the formation of a tight bed surface occur. Bed solid particles display the properties of boiling liquid and look like fluid – “bubbling fluidized bed”.
If the sand particles in the fluid state are heated to the flame temperature of the coal, and coal is injected continuously into the bed, the coal will burn quickly and the bed reaches a uniform temperature. Fluidized bed combustion (FBC) takes place at temperatures around 840OC to 950OC. Because this temperature is far below the fusion temperature of ash, the melting of ash and the problems associated with it can be avoided.
The lower combustion temperature is achieved due to the high heat transfer coefficient as a result of rapid mixing in the fluidized bed and effective heat extraction of the bed through heat transfer on the pipe and bed walls. The gas speed is reached between the minimum fluidization speed and the particle entry speed. This ensures stable bed operation and avoids carrying particles in the gas path.
E. Atmospheric Fluidized Bed Combustion (AFBC) Boilers
Most boilers that operate for this type are Atmospheric Fluidized Bed Combustion (AFBC) Boilers. This tool is only a conventional conventional boiler shell coupled with a fluidized bed combustor. Such a system has been installed combined with a conventional water tube boiler / water pipe boiler.
Coal is crushed into sizes 1-10 mm depending on the level of coal and the type of air feeder to the combustion chamber. Atmospheric air, which acts as fluidizing and combustion air, is put under pressure, after being preheated by the fuel exhaust. Pipes in beds that carry water generally act as evaporators. The combustion gas product passes through the super heater section of the boiler then flows to the economizer, to the dust collector and air pre-heater before being discharged to the atmosphere.
F. Pressurized Fluidized Bed Combustion (PFBC) Boilers
In the Pressurized Fluidized bed Combustion (PFBC) type, a compressor supplies air Forced Draft (FD), and the burner is a pressurized tank. The heat released in the bed is proportional to the pressure of the bed so that the deep bed is used to extract large amounts of heat. This will increase the combustion efficiency and absorption of sulfur dioxide in the bed. Steam is produced in two pipe bonds, one in the bed and the other in the top. Hot gas from the chimney drives a power-generating gas turbine. The PFBC system can be used to generate cogeneration (steam and electricity) or power plants with combined cycles. Combined cycle operations (gas turbines & steam turbines) increase overall conversion efficiency by 5 to 8 percent.
G. Atmospheric Circulating Fluidized Bed Combustion Boilers (CFBC)
In the circulation system, the parameters of the bed are maintained to form a floating solid from the bed. The solid is lifted in a phase that is relatively dissolved in the solids lift, and a down-comer with a cyclone is a solid circulation flow. There is no steam generator pipe located in the bed. Generating and overheating steam takes place in the convection section, the water wall, at the output of the riser.
CFBC boilers are generally more economical than AFBC boilers, for applications in the industry it requires more than 75 – 100 T / hour steam. For large units, the higher the characteristics of the CFBC boiler furnace will provide better use of space, larger fuel particles, the residence time of absorbent materials for efficient combustion and increasing SO2 capture, and the easier the application of combustion techniques for NOx control than the AFBC steam generator.
H. Stoker Fired Boilers
Stokers are classified according to the method of feeding fuel into the furnace and by the type of the gas. The main classification is the spreader stoker and chain-gate or traveling-gate stoker.
1) Spreader stokers
Spreader stockers utilize a combination of combustion suspension and grate combustion. Coal is fed continuously to the furnace above the coal combustion bed. Fine coal is burned in suspension; larger particles will fall into the grate, where they will be burned in thin, fast burning coal beds. This combustion method provides good flexibility to load fluctuations, because ignition almost occurs quickly when the combustion rate increases. Because of this, spreader stoker is preferred over other stoker types in various applications in the industry.
2) Chain-grate or traveling-grate stoker
Coal is fed to the end of a moving steel grate. When it moves along the furnace, coal burns before falling on the test as ash. Certain skill levels are needed, especially when adjusting grate, damper and baffles, to ensure clean burning and produce as little as possible the amount of carbon that is not burned in ash.
The coal feed hopper extends along the entire end of the coal feed on the furnace. A coal grate is used to control the speed of coal fed to the furnace by controlling the thickness of the fuel bed. The size of coal must be uniform because large chunks will not burn perfectly when they reach the end of the grate.
I. Pulverized Fuel Boiler
Most coal-fired power station boilers use fine coal, and many water pipe boilers in larger industries also use fine coal. This technology is developing well and throughout the world there are thousands of units and more than 90 percent of coal combustion capacity is this type.
For bituminous coal, coal is ground to a fine powder, measuring +300 micrometers (µm) less than 2 percent and measuring below 75 microns at 70-75 percent. It should be noted that too fine powder will waste grinding energy.
Conversely, a powder that is too coarse will not burn perfectly in the combustion chamber and cause greater losses because the material is not burned. The powdered coal is blown out with a portion of the combustion air going into the boiler plant through a series of burner nozzles. Secondary and tertiary air can also be added.
Combustion takes place at temperatures from 1300 – 1700 ° C, depending on the quality of the coal. The residence time of particles in a boiler is usually 2 to 5 seconds, and particles must be small enough for perfect combustion.
This system has many advantages such as the ability to burn various coal qualities, a fast response to changes in cargo load, the use of high preheated air temperatures etc.
One of the most popular systems for fine coal combustion is tangential combustion using four burners from all four corners
J. Hot Waste Boilers
Wherever waste heat is available at medium or high temperatures, waste heat boilers can be installed economically. If the steam needs more than the steam produced using hot exhaust gas, additional burners that use fuel can be used.
If indirect steam can be used, steam can be used to produce electrical power using a steam turbine generator. This is widely used in the reuse of heat from exhaust gases from gas turbines and diesel engines.
K. Thermic Fluid Heaters
At present, thermic fluid heaters have been used extensively in various applications for indirect heating processes. By using petroleum fluid as a heat transfer medium, the heater provides a constant temperature. The combustion system consists of a fixed grate with a mechanical draft arrangement.
Modern thermic fluid oil heaters consist of a double coil, three pass construction and are installed with a pressure jet system. The thermic fluid, which acts as a heat carrier, is heated in a heater and circulated through the user’s equipment. Here the fluid transfers heat to the process through a heat exchanger, then the fluid is returned to the heater. The thermic fluid flow at the end of the user is controlled by a pneumatically operated control valve, based on the operating temperature. The heater operates on a high or low fire depending on the temperature of the returning oil which varies depending on the system load.
The advantages of these heaters are:
* Closed system operation with minimum loss compared to steam boiler.
* Non-pressurized system operation even for temperatures around 2500C compared to 40kg / cm2 steam pressure requirements in a similar steam system.
* Automatic control settings, which provide operating flexibility.
* Good thermal efficiency due to the absence of heat loss caused by blowdown, condensate removal and flash steam.
The overall economic factor of a thermal fluid heater depends on the specific application and basis of reference. Coal-fired thermic fluid heaters with a heat efficiency range of 55-65 percent are the most convenient to use compared to most boilers. Combining the heat recovery equipment in the bar will enhance the next thermal efficiency level.
2.2 TYPE OF WATER TUBE BOILER
In the water tube boiler, boiler feed water flows through the pipes into the drum. Circulated water is heated by the combustion gas to form steam in the steam area in the drum. This boiler is chosen if the steam demand and steam pressure are very high as in the case of a boiler for power plants. A very modern water tube boiler designed with a steam capacity between 4,500 – 12,000 kg / hour, with very high pressure. Many water tube boilers are constructed in packages if fuel oil and gas are used. For water tubes that use solid fuel, it is not commonly designed in packages.
This type of fire tube water pipe boiler has the characteristics: it produces high capacity and steam pressure.
2.3 HOW TO WORK TYPE OF WATER TUBE BOILER
The working method of the boiler water tube type is: the ignition process occurs outside the pipe, then the heat produced heats the pipe containing water and previously the water is conditioned first through an economizer, then the steam produced first is collected inside a drum-drum. Until the pressure and temperature are appropriate, through the secondary superheater stage and the new superheater primary steam is released into the main distribution pipe. In a water pipe, flowing water must be conditioned on minerals or other substances which dissolve in the water. This is a major factor that must be considered for this type.
Ø Other characteristics of water tube boilers are as follows:
* Forced, induced and balanced drafts help to improve combustion efficiency.
* Less tolerant of the quality of water produced from a water treatment plant.
* Allows for a higher level of heat efficiency.
2.4 BENEFITS AND LOSS OF BOILER WATER TUBE
* Large steam capacity of up to 450 TPH
* Operating pressure reaches 100 bar
* The value of the efficiency is relatively higher than the fire tube boiler
* Furnaces are easy to reach for inspection, cleaning and repairs.
* The construction process is more detailed.
* Initial investment is relatively more expensive.
* The handling of water entering the boiler needs to be maintained, because it is more sensitive for this system, it needs supporting components for this.
* Because it can produce greater steam capacity and pressure, the construction requires a large area.
From various sources…