Basics of Steam Boiler
Basics of Steam Boiler
Steam is the gas formed when water (H2O) passes from the liquid to the gaseous state. At the molecular level, this is when H2O molecules manage to break free from the bonds (i.e., hydrogen bonds) keeping them together. In liquid water, H2O molecules are constantly being joined together and separated. As the water molecules are heated, however, the bonds connecting the molecules start breaking more rapidly than they can form. Eventually, when enough heat is supplied, some molecules break free. These ‘free’ molecules form the transparent gas. This gas is known as steam, or more specifically dry steam. High-pressure steam is a working fluid of steam prime movers such as reciprocating steam engines and steam turbines.
Traditionally, a boiler is an enclosed container which provides a means for heat from combustion to be transferred into the working media (normally water) until it becomes heated or a gas (steam). It can be simply stated that a boiler is as a heat exchanger between combustion heat and water. The boiler is the part of a thermal power plant process which uses high pressure steam for power generation. Boilers are used for the generation of steam or for heating water. The steam or hot water under pressure can then be used for transferring the heat to a process which consumes the heat in the steam and turns it into work. Steam boiler is also referred to as steam generator. Steam produced in a boiler can be low pressure steam, medium pressure steam, or high-pressure steam.
Shell and tube-saturated steam boilers of the current packaged form are being manufactured since the time which is before the second world war, and their lineage can be traced directly back to the Cornish boilers of the early nineteenth century, invented by the British inventor and mining engineer Richard Trevithick.
A steam boiler fulfils the statements (i) it is part of a type of heat engine or process, (ii) heat is generated through combustion (burning) or waste heat from an industrial process can be used, (iii) it has a working fluid, also known as heat carrier which transfers the generated heat away from the boiler, (iv) the heating media and working fluid are separated by walls.
In an industrial / technical context, the steam produced is used as process steam in various industrial processes or for driving turbines for the production of electric power. The concept of steam boiler includes the whole complex system for producing steam. It includes all the different phases of heat transfer from flames to water / steam mixture (economizer, boiler, superheater, reheater and air preheater). It also includes different auxiliary systems (e. g., fuel feeding, water treatment, and flue gas channels including stack).
The heat is generated in the furnace part of the boiler, where fuel is combusted. The fuel used in a boiler contains either chemically bonded energy (like solid, liquid, or gaseous fuel, or waste and biofuels) or nuclear energy. Sometimes solar energy is also used for the production of steam. A boiler is required to be designed to absorb the maximum quantity of heat released in the process of combustion. This heat is transferred to the boiler water. The relative percentage of each is dependent upon the type of boiler, the designed heat transfer surface, and the fuels used for combustion.
Principles of a steam boiler
Boilers use a combination of radiation, convection, and conduction to convert heat energy into steam energy. Proper boiler operation depends on controlling several variables, including boiler feedwater quality, water flow and level in the boiler, furnace temperatures and pressures, burner efficiency, and air flow etc.
In order to describe the principles of a steam boiler, a person is to consider a very simple case, where the boiler is simply a container, partially filled with water (Fig 1). Combustion of fuel produce heat, which is transferred to the container and makes the water evaporate. The vapour or steam can escape through a pipe which is connected to the container and be transported to the place of its use. Another pipe brings water (known as feedwater) to the container to replace the water which has evaporated and escaped.
Since the pressure level in the boiler is to be kept constant (in order to have stable process values), the mass of the steam which escapes has to be equal to the mass of the feedwater which is added. If steam leaves the boiler faster than water is added, the pressure in the boiler falls. If water is added faster than it is evaporated, the pressure rises. If more fuel is combusted, more heat is generated and transferred to the water. Hence, more steam is generated and pressure rises inside the boiler. If less fuel is combusted, less steam is generated and the pressure sinks.
In a simple power plant cycle, the steam boiler provides steam to a heat consumer, normally to power an engine (Fig 1a). In a steam power plant, a steam turbine is used for extracting the heat from the steam and turning it into work. The turbine normally drives a generator which turns the work from the turbine into electricity. The steam, used by the turbine, can be recycled by cooling it until the steam condenses into water and then the condensate is returned as feedwater to the boiler.
Fig 1 Simplified steam boiler and Rankine cycle
The condenser, where the steam is condensed, is a heat exchanger which typically uses water from a nearby sea or a river to cool the steam. In a typical power plant, the pressure, at which the steam is produced, is high. But when the steam has been used to drive the turbine, the pressure has dropped drastically. A pump is hence needed to get the pressure back up. Since the work needed to compress a fluid is around a hundred times less than the work needed to compress a gas, the pump is located after the condenser. The cycle which this process forms, is called a Rankine cycle and is the basis of the majority of modern steam power plant processes (Fig 1b).
Design and operation
Combustion boilers are designed to use the chemical energy in fuel to raise the energy content of water so that it can be used for heating and power applications. Several types of fossil and non-fossil fuels are fired in boilers. During the combustion process, oxygen reacts with carbon, hydrogen and other elements in the fuel to produce a flame and hot combustion gases. As these gases are drawn through the boiler, they cool as heat is transferred to water. Eventually the gases flow through a stack and into the atmosphere. As long as fuel and air are both available to continue the combustion process, heat gets generated.
At the operating pressure, temperature of the feedwater, prior to entering the boiler, is less than that at the saturated condition, i.e., the entering feedwater is under the sub-cooled condition. Within the boiler, the feedwater temperature is increased to the near-saturation temperature in the economizer. Thereafter, the sensible heat for bringing the feedwater to saturation and the latent heat for the evaporation of saturated feedwater is added in the water-wall for generating steam. This steam is separated from the steam-water mixture coming out from the water-wall and purified in the boiler drum to ensure the supply of saturated steam.
The saturated steam is further superheated to attain the desired energy level. The heat-transfer surfaces, e.g., economizer, water-wall, and superheaters / reheaters, etc., are located in the boiler in the flow path of products of combustion so that heat is absorbed efficiently in proper proportions in different heat transfer zones.
The evaporation zones are comprised of the boiler drum, downcomers, and riser tubes or simply risers. While the boiler drum and downcomers are located outside the heat transfer zone, the riser tubes are exposed to heat transfer. While flowing through the riser tubes the feedwater receives the heat of combustion for evaporation. The riser tubes in present-day boilers are arranged such a way so that they form the enclosure of the combustion chamber or furnace and receive the heat. Hence, riser tubes are normally called water-walls.
The feedwater from the economizer enters the boiler drum, flows down through the downcomers, passes through the pipes to the water-wall bottom header and rises through the riser tubes. In the riser tubes, the feedwater absorbs the heat, part of which is converted to steam and re-enters the boiler drum as a steam-water mixture. From this mixture, steam is separated in the boiler drum and purified. Dry saturated steam from the boiler drum is lead out through the saturated steam pipes to the superheaters. The complete flow path of working fluid is shown in Fig 2.
Fig 2 Flow of working fluids in a steam boiler
The process of separation and purification of steam in the boiler drum is accomplished by drum internals, e.g., cyclones, baffles, etc., chemical and feedwater admission piping, blow-down lines, etc. The process includes three steps namely separation, steam washing, and scrubbing. Separation is the process of removing the bulk mass of water from steam and is accomplished by any one of several means such as gravity, abrupt change in flow direction, centrifugal action, impact against a plate, and the use of baffles etc.
Boilers are manufactured in several different sizes and configurations depending on the characteristics of the fuel, the specified heating output, and the needed emission controls. Some boilers are only capable of producing hot water, while others are designed to produce steam. Boilers can burn coal, oil, gas (natural gas, or by-product gases), biomass as well as other fuels and fuel combinations.
Boilers can be classified as (i) fire tube and water tube boilers, (ii) high-pressure, medium-pressure, and low-pressure boilers, (iii) natural and forced circulation boilers, (iv) single tube and multi tube boilers, (v) stationary and portable boilers, (vi) coal-fired, oil-fired, and gas-fired boilers, and (vii) externally fired and internally fired boilers.
Majority of the boilers are classified as either water-tube or fire-tube boilers, but there are also other designs of boilers which are used in industry. Fig 3 shows typical fire tube and water tube boilers.
Fig 3 Types of boilers
Fire-tube boiler – Fire tube boiler consists of numbers of tubes through which hot gasses are passed. These hot gas tubes are immersed into water, in a closed vessel. In this boiler one closed vessel or shell contains water, through which hot gas tubes are passed. These hot gas tubes heat up the water and convert the water into steam and the steam remains in same vessel.
In fire tube boiler, as the name suggests, its normal construction is as a tank of water perforated by tubes which carry hot flue gases from the fire. The tank is normally cylindrical in shape to realize the maximum strength from simple structural geometry. The tank can be installed either horizontally or vertically. Typically, the furnace and the grate, on which fuel is burnt, are located underneath the front end of the shell. The gases pass horizontally to the rear, then either are released through the stack at the rear or reverse directions and pass through the horizontal tubes to stack at the front. The fire tubes can be placed horizontally or vertically or at an inclined plane in a furnace. In this type of boiler boiling takes place in the same compartment where water is stored. As a result, only saturated steam used to be produced in older designs of this type of boiler, but today, a fire-tube boiler can generate superheated steam as well.
Fire tube boilers are compact, of packaged construction and cheaper. These boilers are normally used for relatively small steam capacities and low to medium steam pressures. Today, these boilers are used extensively in the stationary engineering field. The steam generating capacity and outlet steam pressure of these boilers are limited, hence, they are unable to meet the needs of larger units. Another disadvantage of these boilers is that they are susceptible to explosions.
Water tube boiler – Water tube boiler is a kind of boiler where the water is heated inside tubes and the hot gasses surround them. This is just opposite of fire tube boiler. In this boiler, boiler feedwater flows through the tubes for evaporation and enters the boiler drum. Baffles are installed across the tubes to allow cross flow of flue gases to ensure maximum exposure of the tubes. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. On the basis of configuration of tubes inside the furnace, this boiler is further classified as ‘straight-tube boiler, or ‘bent tube boiler’.
In a water-tube boiler, water circulated in the tubes is heated externally by the hot flue gas. Fuel is burned inside the furnace, creating hot gas which heats up the water in the steam generating tubes. Cool water at the bottom of the steam drum returns to the feedwater drum of small boilers through large-bore ‘downcomer’ tubes, where it helps pre-heat the feedwater supply. In large utility boilers, feedwater is supplied to the steam drum and the downcomers supply water to the bottom of the water-walls. The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In large utility boilers water-filled tubes form the walls of the furnace to generate steam and saturated steam coming out of the boiler drum re-enter the furnace through a superheater to become superheated. The superheated steam is used for driving the turbines.
Water tube boiler is used when the steam demand as well as steam pressure requirements are high as in the case of boiler needed to meet the steam requirements for industrial processes as well as for power generation. The features of water tube boilers include (i) forced, induced and balanced draft provisions helping to improve combustion efficiency (ii) lesser tolerance for water quality hence necessity for water treatment plant, and (iii) higher thermal efficiency levels.
Natural circulation boiler – This is the boiler in which motion of the working fluid in the evaporator is caused by thermo-siphon effect on heating the tubes. In the natural circulation boilers, circulation of water depends on the difference between the density of a descending body of relatively cool and steam-free water and an ascending mixture of hot water and steam. The difference in density occurs since the water expands as it is heated, and hence, becomes less dense. All natural circulation boilers are drum-type boilers.
Forced / assisted circulation boiler – The density difference between the saturated liquid and saturated vapour starts diminishing at 18 MPa or higher fluid pressure, hence, it is difficult to maintain natural circulation of fluid flow in boiler tubes. In such cases fluid flow is ensured with the help of forced / assisted circulation using pumps. The forced / assisted circulation principle applies equally in both super-critical and sub-critical ranges.
Sub-critical boiler and super-critical boiler – The critical pressure is the vapour pressure of a fluid at the critical temperature above which distinct liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same, resulting in only one phase. The point at which the critical temperature and critical pressure is met is called the critical point. The critical pressure and critical temperature of water and steam are 22.12 MPa and 374 deg C respectively. Any boiler which operates below the critical point is called a sub-critical boiler, and one which operates above the critical point is known as a super-critical boiler.
Drum-type boiler – In a drum-type boiler, the drum acts as a reservoir for the working fluid. These boilers have one or more water drums, depending on the size and steam-generating capacity. The drum is connected to cold downcomers and hot riser tubes through which circulation of working media takes place. The lower portion of the drum with feedwater is called the water space and the upper portion of the drum occupied by steam is called the steam space. A drum-type boiler can be either the natural circulation type or forced / assisted circulation type. Drum-type boilers are essentially sub-critical boilers since they operate below the critical pressure of the working fluid. The economic design pressure limit of fluid in a drum-type boiler is around 18 MPa.
Once-through boiler – This type of boiler does not have a drum. Simply put, a once-through boiler is merely a length of tube through which water is pumped, heat is applied, and the water is converted into steam. In actual practice, the single tube is replaced by numerous small tubes arranged to provide effective heat transfer, similar to the arrangement in a drum type boiler. The fundamental difference lies in the heat-absorbing circuit.
Feedwater in this type of boiler enters the bottom of each tube and discharges as steam from the top of the tube. The working fluid passes through each tube only once and water is continuously converted to steam. As a result, there is no distinct boundary between the economizing, evaporating, and superheating zones. The circulation ratio of this type boiler is unity. These boilers can be operated either at sub-critical or at super-critical pressures.
Stoker-fired boiler – Prior to the commercial use of ‘fluidized-bed boilers’, stoker-fired boilers have been used. Stoker firing method have been the most economical method for burning coal in almost all industrial boilers rated for less than 100 ton per hour (tph) of steam. This type of boiler has been capable of burning a wide range of coals, from bituminous to lignite, as well as by-products of waste solid fuels. However, over the years, this type of boiler has become less popular because of the technological advancement.
In stoker-fired boiler, coal is pushed, dropped, or thrown on to a grate to form a fuel-bed. Stokers are divided into two general classes namely (i) overfeed, in which fuel is fed from the above, and (ii) underfeed, in which the fuel is fed from the bottom. Under the active fuel-bed, there is a layer of fuel ash, which along with air flow through the grate keeps metal parts at allowable operating temperatures. Stoker can be a chain-grate overfeed stoker, and a traveling grate overfeed stoker respectively.
Fossil fuel-fired boiler – In this type of boiler, coal, fuel oil, and natural gas are the main types of fossil fuel. These fuels can generate a substantial quantity of heat by reacting with oxygen. These fuels consist of a large number of complex compounds comprised of five principal elements: carbon, hydrogen, oxygen, sulphur, and nitrogen. Any of these three fuels, i.e., coal, fuel oil, and natural gas, can be used in steam power stations, but coal plays a large role in power generation because of the enormous reserves of coal in several countries around the world.
Pulverized coal fired (PCF) boilers – Coal-fired water tube boiler systems generate majority of the electric power generation worldwide. Pulverized coal fired boilers, which are the most popular utility boilers today, have a high efficiency but a costly SOx (oxides of sulphur) and NOx (oxide of nitrogen) control. Almost any kind of coal can be reduced to powder and burned like a gas in a PCF-boiler. The PCF technology has enabled the increase of boiler unit size from 100 MW in the 1950s to far over 1,000 MW.
New pulverized coal-fired systems routinely installed today generate power at net thermal cycle efficiencies ranging from 40 % to 47 % lower heating value, LHV, (corresponding to 34 % to 37% higher heating value, HHV) while removing up to 97 % of the combined, uncontrolled air pollution emissions (SOx and NOx).
Coal is a heterogeneous substance in terms of its organic and inorganic content. Since only organic particles can be combusted, the inorganic particles remain as ash and slag and increase the need for particle filters of the flue gas and the tear and wear of furnace tubes. Pulverizing coal before feeding it to the furnace has the benefit that the inorganic particles can be separated from the organic before the furnace. Still, coal contains a lot of ash, part of which can be collected in the furnace. In order to be able to remove ash the furnace easier, the bottom of the furnace is shaped like a ‘V’.
Fluidized-bed boiler – In modern large-capacity coal-fired boilers, coal is burnt in suspension. Fluidized-bed combustion ensures burning of solid fuel in suspension, in a hot inert solid-bed material of sand, limestone, refractory, or ash, with high heat transfer to the furnace and low combustion temperatures (800 deg C to 950 deg C). The combustor-bed material consists of only 35 % coal. Fluidized-bed combustion is comprised of a mixture of particles suspended in an upwardly flowing gas stream, the combination of which shows fluid-like properties.
Fluidized-bed burners are capable of firing a wide range of solid fuels with varying heating value, ash content, and moisture content. In this type of boiler, pollutants in products of combustion are reduced concurrently with combustion much of the ash and hence the particulate matter as well as sulphur is removed during the combustion process. Further, lower temperature combustion in the fluidized bed results in lower production of NOx (nitrogen oxides) and removes any slagging problem.
There are two main types of fluidized bed combustion boilers namely (i) bubbling fluidized bed (BFB), and (ii) circulating fluidized bed (CFB) boilers. In the bubbling type, since the velocity of the air is low, the medium particles are not carried above the bed. The combustion in this type of boiler is generated in the bed. In the circulating type, the velocity of air is high, so the medium sized particles are carried out of the combustor. The carried particles are captured by a cyclone installed in the outlet of combustor. Combustion is generated in the whole combustor with intensive movement of particles. Particles are continuously captured by the cyclone and sent back to the bottom part of the combustor to combust unburned particles. This contributes to full combustion.
Waste-heat recovery boiler or heat-recovery steam generator – As the name implies, waste-heat recovery boiler (WHRB) or a heat-recovery steam generator (HRSG) is a boiler where heat, generated in different processes, is recovered and used to generate steam or boil water. The main purpose of this boiler is to cool down flue gases produced by metallurgical or chemical processes, so that the flue gases can be either further processed or released without causing harm. The steam generated is only a useful by-product.
A WHRB or a HRSG is a heat exchanger which recovers heat from a gas stream and in turn produces steam which can be used in a process or to drive steam turbines. WHRB or HRSG uses excess or waste heat from a process to produce steam. This boiler has two functions namely (i) to produce steam and to provide cooling for a process in order for it to proceed or to recover heat which otherwise is released to the atmosphere, losing a tremendous quantity of usable energy. Fig 4 shows a waste heat recovery boiler which can be used to recover waste heat energy and cool the flue gas stream from a turbine exhaust.
Fig 4 Typical diagram of a waste heat boiler
Water circulation
The circulation of boiler water is based on the principle of convection. A fluid which is heated expands and becomes less dense, moving upward through heavier, denser fluid. Convection and conduction transfer heat through pipe walls and water currents, resulting in unequal densities. Cold water flows through the downcomer to the bottom of the mud drum and then flows upward through the riser (water wall tubes) as it is heated.
In a water tube boiler, circulation occurs because the temperature of the fluid in the downcomer is always lower than the temperature in the boiler and steam generating (riser) tubes. Steam bubbles are formed as the liquid temperature continues to increase. These bubbles increase the circulation as they move up the riser tubes. The pressure builds as the water vapour collects in the upper drum. Each time the water passes through the tubes, it picks up more heat energy. As the pressure increases, the boiling point of the water increases. When the target pressure is achieved, steam is delivered to the steam header. To maintain this pressure, makeup water is to be added, heat is be continually applied, and circulation is to be controlled. In a fire tube boiler, the water level in the boiler shell is to be maintained above the tubes to prevent overheating of the tubes.
Steam– Saturated steam is steam in equilibrium with water (e.g., steam which holds all of the moisture it can hold and still remain a vapour). Saturated steam can be used to purge process equipment or perform other functions, or it can be superheated. As long as the steam and water are in contact with each other, the steam is in a saturated condition. Saturated steam cannot absorb additional water vapour, but the boiler can continue to add heat energy to it.
Steam which continues to take on heat energy or get hotter is known as super-heated steam. Super-heated steam, which is produced downstream of the steam drum (typically in the firebox), is steam which has been heated to a temperature above its saturation temperature. Super-heated steam is typically 100 deg C to 150 deg C hotter than the saturated steam. Typical uses of superheated steam include (i) driving turbines, (ii) catalytic cracking, (iii) product stripping, (iv) maintenance of steam pressures and temperatures over long distances, and (v) producing steam for systems which need dry, moisture-free steam.
Superheated steam is not the best choice for heat transfer in some heat exchangers since the quantity of energy given up by superheated steam is relatively small compared to the energy given up by saturated steam. Also, some facility processes cannot tolerate the high temperatures of superheated steam. The process of cooling the superheated steam is called desuperheating. Desuperheated steam is superheated steam from which some heat has been removed by the reintroduction of boiler feedwater. Typically, desuperheating does not occur at the boiler but at specific points in the process where boiler feedwater is injected into superheated steam.
Water treatment methods
Raw water can come from a variety of sources, such as lakes, rivers, or wells. Each water source has its own components and treatment requirements. However, in general, the water chemistry needed for steam production is required to meet standards. The water needs to be filtered and have minerals and oxygen removed. Raw water goes through the several steps of treatment as given below to become boiler feedwater.
Cleaning of the water – This step removes suspended solids. Depending on water source this can include (i) coagulation / sedimentation, and (ii) filtration.
Removal of minerals – This step is done to the clean water (from previous step) to remove minerals which can build-up on steam turbines or other process equipment. Depending on the water source, this step can be done by one or more of these processes (i) softening, (ii) demineralization (ion exchange), and (iii) reverse osmosis (membrane).
Removal of oxygen – Dissolved oxygen and other gases primarily consisting of carbon di-oxide (CO2) in boiler feedwater are major cause of boiler system corrosion. While oxygen results in localized corrosion (pitting), CO2 forms carbonic acid and damages condensate piping. Removal of oxygen can be done by (i) deaeration, or (ii) oxygen scavenging.
Coagulation – It is done by adding chemicals to reduce coarse suspended solids, silt, turbidity, and colloids through the use of a clarifier. The impurities gather together into larger particles and settle out of the chemical / water solution (sedimentation).
Filtration – It removes coarse suspended matter and sludge from coagulation or from water softening systems. Gravel beds and anthracite coal are common materials used for filter beds. Softening is the treatment of water to remove dissolved mineral salts such as calcium and magnesium, known as hardness, in boiler feedwater. Softening methods include the addition of calcium carbonate (lime soda), phosphate, and / or zeolites (crystalline mineral compounds).
Demineralization – It is the removal of ionized mineral salts by ion exchange. The process is also called deionization, and the water produced is called deionized water.
Reverse osmosis – It uses pressure to remove dissolved solids from boiler feedwater by forcing the water from a more concentrated solution through a semi-permeable membrane to a less concentrated solution.
Deaeration – It removes oxygen or other gases from boiler feedwater by increasing the temperature, using steam, to strip out the dissolved gases.
Specific terms describe the water as it moves through these steps. Water starts as raw water and becomes demineralized water, then deaerated water, and finally boiler feedwater. Steam which has been condensed (condensate) is already clean and can be fed back into the system at the deaerator. This saves the cost of treating more raw water, making it practical to recycle any used steam condensate.
Components of a boiler system
The main components in a boiler system are boiler feedwater heaters, deaerators, feed pump, economizer, superheater, attemperator, steam system, condenser, and condensate pump. In addition, there are sets of controls to monitor water and steam flow, fuel flow, air flow, and chemical treatment additions. Broadly speaking, the boiler system comprises a feedwater system, steam system, and fuel system.
The feedwater system supplies water to the boiler and regulates it automatically to meet the steam demand. Different valves are there to provide access for maintenance and repair. The steam system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout this system, steam pressure is regulated using control valves and checked with steam pressure gauges. The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment needed in the fuel system depends on the type of fuel used in the system. Fig 5 shows flow diagram of a steam boiler system showing different components.
Fig 5 Flow diagram of a steam boiler system showing different components
Feedwater system – In this system, the water, which is supplied to the boiler and which is converted into steam, is called feedwater. The two sources of feedwater are condensate or condensed steam returned from the process and makeup water (treated raw water) which comes from the plant processes outside the boiler.
Boiler feedwater levels and flows are critical to proper boiler operation. If feedwater flow is reduced and the water level decreases to the point where the boiler runs dry, the tubes get overheat and fail. If the boiler water level becomes too high, excess water is carried over into the steam distribution system. This negatively affects process facility steam consumers and can damage turbines and other equipment.
Feedwater heater – The purpose of feedwater heater is to preheat the feedwater with the heat energy of the spent steam. This improves the boiler efficiency. Heaters are shell and tube heat exchangers with the feedwater on the tube side (inside) and steam on the shell side (outside). The heater closest to the boiler receives the hottest steam. The condensed steam is recovered in the heater drains and pumped forward to the heater immediately upstream, where its heat value is combined with that of the steam for that heater. Ultimately the condensate is returned to the condensate storage tank or condenser hotwell.
Deaerators – Feedwater frequently has oxygen dissolved in it at objectionable levels, which comes from air in-leakage from the condenser, pump seals, or from the condensate itself. The oxygen is mechanically removed in a deaerator. Deaerators function on the principle that oxygen is decreasingly soluble in water as the temperature is increased. This is done by passing a stream of steam through the feedwater. Deaerators are normally a combination of spray and tray type. One issue with the control of deaerators is ensuring adequate temperature difference between the incoming water temperature and the stripping steam. If the temperature is too close, not enough steam is available to strip the oxygen from the make-up water.
Economizers – Economizers are the last stage of the feedwater system. They are designed to extract heat value from exhaust gases to heat the steam still further and improve the efficiency of the boiler. They are simple finned tube heat exchangers. Not all boilers have economizers. Normally, economizers are found only on water tube boilers using fossil fuel as an energy conservation measure. A feedwater economizer reduces steam boiler fuel requirement by transferring heat from the flue gas to incoming feedwater. By recovering waste heat, an economizer can frequently reduce fuel requirement by 5 % to 10 % and pay for itself in less than 2 years.
A feedwater economizer is appropriate when insufficient heat transfer surface exists within the boiler to remove combustion heat. Boilers which exceed 75 kilowatts boiler rating, operating at pressures higher than 0.5 MPa, and those which are considerably loaded all the year need economizer.
Steam system – Steam moves from a high-pressure location to a lower pressure location. In doing so it uses some of its own energy which is seen as pressure drop. Additionally, heat losses during distribution causes a fraction of the steam to condense to form condensate. This is fundamental to understanding the operation of steam systems and how they are designed. There are a number of items of basic line equipment which are common to all steam systems such as different types of valves, pressure and temperature gauges, strainer and steam traps etc.
Steam and mud drums – A boiler system consists of a steam drum and a mud drum. The steam drum is the upper drum of a water tube boiler where the separation of water and steam occurs. Feedwater enters the boiler steam drum from the economizers or from the feedwater heater train if there is no economizer. The colder feedwater helps create the circulation in the boiler. The steam outlet line normally takes off from this drum to a lower drum by a set of riser and downcomer tubes.
The lower drum, called the mud drum, is a tank at the bottom of the boiler which equalizes distribution of water to the generating tubes and collects solids such as salts formed from hardness and silica or corrosion products carried into the boiler. In the circulation process, the colder water, which is outside the heat transfer area, sinks and enters the mud drum. The water is heated in the heat transfer tubes to form steam. The steam-water mixture is less dense than water and rises in the riser tubes to the steam drum. The steam drum contains internal elements for feedwater entry, chemical injection, blowdown removal, level control, and steam-water separation. The steam bubbles disengage from the boiler water in the riser tubes and steam flows out from the top of the drum through steam separators.
Boiler tubes – Boiler tubes are normally fabricated from high-strength carbon steel. The tubes are welded to form a continuous sheet or wall of tubes. Frequently more than one bank of tubes is used, with the bank closest to the heat sources providing the highest share of heat transfer. The boiler tubes also tend to be the most susceptible to failure because of flow problems or corrosion / deposition issues.
Superheaters – The purpose of the superheater is to remove all moisture content from the steam by raising the temperature of the steam above its saturation point. The steam leaving the boiler is saturated, that is, it is in equilibrium with liquid water at the boiler pressure (temperature). The superheater adds energy to the exit steam of the boiler. It can be a single bank or multiple banks or tubes either in a horizontal or vertical arrangement which is suspended in the convective or radiation zone of the boiler. The added energy raises the temperature and heat content of the steam above saturation point. In the case of turbines, excessive moisture in the steam can adversely affect the efficiency and integrity of the turbine.
Superheated steam has a larger specific volume as the quantity of superheat increases. This necessitates larger diameter pipelines to carry the same quantity of steam. Because of temperatures, higher alloy steels are used. It is important that the steam is of high purity and low moisture content so that non-volatile substances do not build up in the superheater.
Attemperator – In order to achieve the proper control of superheat temperature an attemperator is used. Attemperation is the primary means for controlling the degree of superheat in a superheated boiler. Attemperation is the process of partially de-superheating steam by the controlled injection of water into the superheated steam flow. The degree of superheat depends on the steam load and the heat available, given the design of the superheater. The degree of superheat of the final exiting steam is normally not subject to wide variation because of the design of the downstream processes.
A direct contact attemperator injects a stream of high purity water into the superheated steam. It is normally located at the exit of the superheater, but can be placed in an intermediate position. Normally, boiler feedwater is used for attemperation. The water is to be free of non-volatile solids to prevent objectionable buildup of solids in the main steam tubes and on turbine blades.
Since attemperator water comes from the boiler feedwater, provision for it has to be made in calculating flows. The calculation is based on heat balance. The total enthalpy (heat content) of the final superheat steam is to be the mass weighted sum of the enthalpies of the initial superheat steam and the attemperation water.
Condensate systems – Although not a part of the boiler itself, condensate is normally returned to the boiler as part of the feedwater. Accordingly, one is to take into account the quantity and quality of the condensate when calculating boiler treatment parameters. In a complex steam distribution system, there are several components. These include heat exchangers, process equipment, flash tanks, and storage tanks.
Heat exchangers are the places in the system where steam is used to heat a process or air by indirect contact. Shell and tube exchangers are the normal design, with steam normally on the shell side. The steam enters as superheated or saturated and can leave as superheated, saturated, or as liquid water, depending on the initial steam conditions and the design load of the exchanger.
Process equipment includes turbines whether used for HVAC (heating, ventilation, and air conditioning) equipment, air compressors, or turbine pumps. Condensate tanks and pumps are major points for oxygen to enter the condensate system and cause corrosion. These points need to be monitored closely for pH and oxygen ingress and proper condensate treatment applied accordingly.
Fuel system – Fuel feed systems play a critical role in the performance of boilers. The primary functions of fuel system include transferring the fuel into the boiler and distributing the fuel within the boiler to promote uniform and complete combustion. The type of fuel influences the operational features of a fuel system. The fuel feed system forms the most significant component of the boiler system.
Feed system for gaseous fuels – Gaseous fuels are relatively easy to transport and handle. Any pressure difference causes gas to flow, and most gaseous fuels mix easily with air. Since on-site storage of gaseous fuel is typically not feasible, boilers are to be connected to a fuel source through a gas pipeline. Flow of gaseous fuels to a boiler can be precisely controlled using a variety of control systems. These systems normally include automatic valves which meter through gas flow through a burner and into the boiler based on steam or hot water demand.
The purpose of the burner is to increase the stability of the flame over a wide range of flow rates by creating a favourable condition for fuel ignition and establishing aerodynamic conditions which ensure good mixing between the primary combustion air and the fuel. Burners are the central elements of an effective combustion system. Other elements of their design and application include equipment for fuel preparation and air-fuel distribution as well as a comprehensive system of combustion controls.
Feed system for liquid fuels – Like gaseous fuels, liquid fuels are also relatively easy to transport and handle by using pumps and piping networks which link the boiler to a fuel supply such as a fuel oil storage tank. For promoting complete combustion, liquid fuels are to be atomized to allow thorough mixing with combustion air. Atomization by air, steam, or pressure produces tiny droplets which burn more like gas than liquid. Control of boilers which burns liquid fuels can also be accomplished using a variety of control systems which meter fuel flow.
Feed system for solid fuels – Solid fuels are much more difficult to handle than gaseous and liquid fuels. Preparing the fuel for combustion is normally necessary and can involve techniques such as crushing and / or pulverizing. Before combustion can occur, the individual fuels particles are to be transported from a storage area to the boiler. Mechanical devices such as conveyors, augers, hoppers, slide gates, vibrators, and blowers are frequently used for this purpose. The method selected depends primarily on the size of the individual fuel particles and the properties and characteristics of the fuel.
Stokers are commonly used to feed solid fuel particles such as crushed coal, wood chips, and various forms of biomass into boilers. Mechanical stokers evolved from the hand-fired boiler era and now include sophisticated electro-mechanical components which respond rapidly to changes in steam demand. The design of these components provides good turn-down and fuel-handling capability. In this context, turn-down is defined as the ratio of maximum fuel flow to minimum fuel flow.
In the case of pulverized coal boilers, which burn very fine particles of coal, the stoker is not used. Coal in this form can be transported along with the primary combustion air through pipes which are connected to specially designed burners.
Burner – A burner is defined as a device or group of devices for the introduction of fuel and air into a furnace at the needed velocities, turbulence, and concentration to maintain ignition and combustion of fuel with-in the furnace. Burners for gaseous fuels are less complex than those for liquid or solid fuels since mixing of gas and combustion air is relatively simple compared to atomizing liquid fuels or dispersing solid fuel particles. The ability of a burner to mix combustion air with fuel is a measure of its performance. A good burner mixes well and liberates a maximum quantity of heat from the fuel. A good burner is engineered to liberate the maximum quantity of heat from the fuel and limit the quantity of pollutants such as CO (carbon mono-oxide), NOx (oxides of nitrogen), and PM (particulate matter) which are released. Burners with these capabilities are presently used routinely for the boilers to comply with mandated emission limitations.
Steam quality and steam purity – There are salts dissolved in feedwater which need to be prevented from entering the superheater and thereby into the turbine. Depending on the quantity of dissolved salt, some impurity deposition can occur on the inner surfaces of the turbine or on the inner surface of superheater tubes as well. Steam cannot contain solids (because of its gaseous form), and hence the water content of steam defines the possible level of impurities. The water content after the evaporator (before superheaters) is to be less than 0.01 % to avoid impurity deposition on the inner tube surfaces. If the boiler in question is a high sub-critical pressure or super-critical boiler, the requirements of the steam purity are higher (measured in parts per billion, ppb).
In case of steam purity, the solid contents are a measure of solid particles (impurities) of the steam. The boiler water impurity concentration, solid contents after the steam drum, and moisture content after the steam drum are directly connected. For example, if the boiler water impurity concentration is 500 ppm (parts per million) and the moisture level in the steam (after the boiler) is 0.1 %, the solids content in the steam (after the boiler) is 500 ppm * 0.1 % = 0.5 ppm.
Boiler mountings – These are the fittings, which are mounted on the boiler for its proper and safe functioning. There are several types of boiler mountings, which are important for proper and the safe functioning of the boiler. These include (i) water level indicator, (ii) pressure gauge, (iii) safety valve, (iv) steam stop valve, (v) blow off cock, (vi) feed check valve, and (vii) fusible plug. Fusible plug is fitted to the crown plate of the furnace or the fire. Its objective is to put off the fire in the furnace of the boiler when the level of water in the boiler fails to an unsafe limit and hence avoids the explosion which can take place because of the overheating of the furnace plate.
Basic overview of boiler controls – Steam is exported from the boiler solely by virtue of a pressure difference between the boiler and point of requirement. The rate of steam mass flow is hence determined by the process need for the steam. The quality of the steam, e.g., pressure, wetness, and cleanliness are affected by the rate of demand from the process, rate of change of demand, design and operating characteristics of the boiler supplying the steam and the condition of the distribution and condensate return systems. For the control of different boiler parameters, several instruments are used. Fig 6 shows boiler control instruments.
Fig 6 Boiler control instruments
Pressure and level instruments are used for the primary control of a boiler and indeed the boiler can be operated with this at minimum, though sub-optimally. For optimal control of the boiler, flow, temperature, and analytical instruments are also needed and in addition, further pressure and level control instruments.
Boilers are fitted with a pressure control which regulates the burner during normal operation. A separate high-pressure limiter is also fitted which, when deployed, forces the burner to shut down and lock out needing manual reset. For new boilers, the limiter is to be of a type which fails safe. As a further control measure, steam boilers are also fitted with an independent safety valve which acts in the event of failure of both the pressure control and limit devices.
Level controls is crucial to maintain a sufficient quantity of water within a boiler in order to prevent the heated surfaces becoming exposed to steam when the burner is firing (a dangerous low-water condition). A dangerous low-water condition places the boiler pressure vessel at risk of catastrophic failure. The level controls comprise of a control unit for the pump and two independent level limiters. The level controller regulates the feedwater pump. In the event of some failure of the controller, the two limiters are independently connected to the burner to force its shutdown and lock out needing manual reset. These controls are needed to be fail safe, of high integrity and self-monitoring. An additional high-water alarm and cut out can also be provided.
Flame failure devices (also known as photo-cells or magic eyes) are used to prevent dangerous occurrences whereby fuel continues to enter the furnace in the event when the flame unexpectedly goes out during normal operation. Modern devices detect a specific frequency within the electro-magnetic spectrum which is emitted by a flame. When activated, they force a shut down and lock out of the burner requiring manual reset. As with level limiters, they are to be fail safe, of high integrity, and self-monitoring.
Automatic controls are a big asset since they reduce manual control of the furnace, boilers, and auxiliary equipment. The types of controls in the boiler include float, pressure, combustion, flame failure, and operation controls.
Boiler blowdown
When water is boiled and steam is generated, any dissolved solids contained in the water remain in the boiler. If more solids are put in with the feed water, they get concentrated and can eventually reach a level where their solubility in the water is exceeded and they deposit from the solution. Above a certain level of concentration, these solids encourage foaming and cause carryover of water into the steam. The deposits also lead to scale formation inside the boiler, resulting in localized overheating and finally causing boiler tube failure. Hence, it is necessary to control the level of concentration of the solids and this is achieved by the process of ‘blowing down’, where a certain volume of water is blown off and is automatically replaced by feed water, hence maintaining the optimum level of total dissolved solids (TDS) in the boiler water.
Blow down is necessary to protect the surfaces of the heat exchanger in the boiler. However, blow down can be a considerable source of heat loss, if improperly carried out. Once blow down valve is set for a given conditions, there is no need for regular operator intervention.
Conductivity is an indicator of quality of the boiler water. Since it is tedious and time consuming to measure total dissolved solids (TDS) in boiler water system, conductivity measurement is used for monitoring the overall TDS present in the boiler. A rise in conductivity indicates a rise in the ‘contamination’ of the boiler water.
Conventional methods for blowing down the boiler depend on two kinds of blowdown namely (i) intermittent, and (ii) continuous.
Intermittent blowdown – The intermittent blown down is given by manually operating a valve fitted to discharge pipe at the lowest point of boiler shell to reduce parameters (TDS or conductivity, pH, silica and phosphates concentration) within prescribed limits so that steam quality is not likely to be affected. In intermittent blowdown, a large diameter line is opened for a short period of time, the time being based on a thumb rule such as once in a shift for 2 minutes.
Intermittent blowdown needs large short-term increases in the quantity of feed water put into the boiler, and hence can necessitate larger feed water pumps than if continuous blow down is used. Also, TDS level are varying, thereby causing fluctuations of the water level in the boiler because of changes in steam bubble size and distribution which accompany changes in concentration of solids. Also, substantial quantity of heat energy is lost with intermittent blowdown.
Continuous blowdown – There is a steady and constant dispatch of small stream of concentrated boiler water, and replacement by steady and constant inflow of feed water. This ensures constant TDS and steam. Even though large quantities of heat are wasted, opportunity exists for recovering this heat by blowing into a flash tank and generating flash steam. This flash steam can be used for preheating boiler feed water or for any other purpose. This type of blow down is common in high-pressure boilers.
Steam boiler efficiency
Thermal efficiency of boiler is defined as the percentage of heat input which is effectively utilized to generate steam. There are two methods of assessing boiler efficiency. These are (i) direct method where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel, and (ii) indirect method where the efficiency is the difference between the losses and the energy input.
Direct method – This is also known as ‘input-output method’ because of the fact that it needs only the useful output (steam) and the heat input (i.e., fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula – Boiler efficiency = (heat output/heat input) x 100.
Parameters to be monitored for the calculation of boiler efficiency by direct method are (i) quantity of steam generated per hour (Q) in kilograms per hour (kg/hr), (ii) quantity of fuel used per hour (q) in kg/hr, (iii) the working pressure [in kg/sq-cm (g)] and superheat temperature ( deg C), if any , (v) the temperature of feed water ( deg C), and (vi) type of fuel and gross calorific value of the fuel (GCV) in kilocalories per kilogram (kCal/kg) of fuel.
Boiler efficiency is given by Boiler efficiency = [Q x (hg-hf)/(q x GCV)] x 100 where, hg is the enthalpy of the saturated steam in kCal/kg of steam and hf is the enthalpy of feed water in kCal/kg of water.
It is to be noted that boiler does not generate 100 % saturated dry steam, and there can be some quantity of wetness in the steam. Advantages of direct method are (i) plant personnel can evaluate quickly the efficiency of boilers, (ii) it needs few parameters for computation, and (iii) it needs few instruments for monitoring. Disadvantages of direct method are (i) it does not give clues to the operator as to why efficiency of system is lower, (ii) it does not calculate different losses accountable for various efficiency levels
Indirect method – There are reference standards for boiler testing at site using indirect method for example BS 845-1, 1987 and ASME PTC-4, 2013. Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The practicing energy mangers in industries prefer simpler calculation procedures. The principal losses which occur in a boiler are (i) loss of heat because of dry flue gas, (ii) loss of heat because of moisture in fuel and combustion air, (iii) loss of heat because of combustion of hydrogen, (iv) loss of heat because of radiation, and (v) loss of heat because of unburnt fuel.
In the above, loss because of moisture in fuel and the loss because of combustion of hydrogen are dependent on the fuel, and cannot be controlled by design. The data needed for the calculation of boiler efficiency using indirect method are (i) ultimate analysis of fuel (hydrogen, oxygen, sulphur, carbon, moisture content, and ash content), (ii) percentage of oxygen or CO2 in the flue gas, (iii) flue gas temperature in deg C, (iv) ambient temperature in deg C and humidity of air in kg/kg of dry air, (v) GCV of fuel in kCal/kg, (vi) percentage combustible in ash (in case of solid fuels), and (vii) GCV of ash in kCal/kg (in case of solid fuels).
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