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Syllabus Boilers: Types, Combustion in boilers, Performances evaluation, Analysis of losses, Feed water treatment, Blow down, Energy conservation opportunities.
2.1 Introduction
A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process. Water is a useful and cheap medium for transferring heat to a process. When water is boiled into steam its volume increases about 1, times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be extremely dangerous equipment that must be treated with utmost care.
The process of heating a liquid until it reaches its gaseous state is called evaporation. Heat is transferred from one body to another by means of (1) radiation, which is the transfer of heat from a hot body to a cold body without a conveying medium, (2) convection, the transfer of heat by a conveying medium, such as air or water and (3) conduction, transfer of heat by actual physical contact, molecule to molecule.
Boiler Specification
The heating surface is any part of the boiler metal that has hot gases of com- bustion on one side and water on the other. Any part of the boiler metal that actually contributes to making steam is heat- ing surface. The amount of heating surface of a boiler is expressed in square meters. The larger the heating surface a boiler has, the more efficient it becomes. The quantity of the steam produced is indicated in tons of water evap- orated to steam per hour. Maximum continuous rating is the hourly evaporation that can be maintained for 24 hours. F & A means the amount of steam generated from water at 100 °C to saturated steam at 100 °C.
Indian Boiler Regulation
The Indian Boilers Act was enacted to consolidate and amend the law relating to steam boilers. Indian Boilers Regulation (IBR) was created in exercise of the powers conferred by section 28 & 29 of the Indian Boilers Act.
Typical Boiler Specification
Boiler Make & Year : XYZ & 2003 MCR (Maximum Continuous Rating) : 10TPH (F & A 100°C) Rated Working Pressure : 10.54 kg/cm^2 (g) Type of Boiler : 3 Pass Fire tube Fuel Fired : Fuel Oil
IBR Steam Boilers means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is wholly, or partly under pressure when the steam is shut off.
IBR Steam Pipe means any pipe through which steam passes from a boiler to a prime mover or other user or both, if pressure at which steam passes through such pipes exceeds 3.5 kg/cm^2 above atmospheric pressure or such pipe exceeds 254 mm in internal diameter and includes in either case any connected fitting of a steam pipe.
2.2 Boiler Systems
The boiler system comprises of: feed water system, steam system and fuel system. The feed water system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves 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 the system, steam pressure is regulated using valves and checked with steam pressure gauges. The fuel system includes all equipment used to provide fuel to gener- ate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system. A typical boiler room schematic is shown in Figure 2.1.
Figure 2.1 Boiler Room Schematic
The water supplied to the boiler that is converted into steam is called feed water. The two sources of feed water are: (1) Condensate or condensed steam returned from the processes and (2) Makeup water (treated raw water) which must come from outside the boiler room and plant processes. For higher boiler efficiencies, the feed water is preheated by economizer, using the waste heat in the flue gas.
The features of package boilers are: Small combustion space and high heat release rate resulting in faster evaporation. Large number of small diameter tubes leading to good convective heat transfer. Forced or induced draft systems resulting in good combustion efficiency. Number of passes resulting in better overall heat transfer. Higher thermal efficiency levels compared with other boilers. These boilers are classified based on the number of passes – the number of times the hot combustion gases pass through the boiler. The combustion chamber is taken, as the first pass after which there may be one, two or three sets of fire-tubes. The most common boiler of this class is a three-pass unit with two sets of fire-tubes and with the exhaust gases exiting through the rear of the boiler.
Stoker Fired Boiler:
Stokers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications are:
- Chain-grate or traveling-grate stoker
- Spreader stoker
Chain-Grate or Traveling-Grate Stoker Boiler
Coal is fed onto one end of a moving steel chain grate. As grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. Some degree of skill is required, particularly when setting up the grate, air dampers and baffles, to ensure clean combustion leaving minimum of unburnt carbon in the ash.
Figure 2.5 Chain Grate Stoker
The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal grate is used to control the rate at which coal is fed into the furnace, and to control the thickness of the coal bed and speed of the grate. Coal must be uniform in size, as large lumps will not burn out com- pletely by the time they reach the end of the grate. As the bed thickness decreases from coal- feed end to rear end, different amounts of air are required- more quantity at coal-feed end and less at rear end (see Figure 2.5).
Spreader Stoker Boiler
Spreader stokers (see figure 2.6) utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fast- burning coal bed. This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favored over other types of stokers in many industrial applications.
Figure 2.6 Spreader Stoker
Pulverized Fuel Boiler
Most coal-fired power station boilers use pulverized coal, and many of the larger industrial water-tube boilers also use this pulverized fuel. This technology is well developed, and there are thousands of units around the world, accounting for well over 90% of coal-fired capacity. The coal is ground (pulverised) to a fine powder, so that less than 2% is +300 micro metre (μm) and 70-75% is below 75 microns, for a bituminous coal. It should be noted that too fine a powder is wasteful of grinding mill power. On the other hand, too coarse a powder does not burn completely in the combustion chamber and results in higher unburnt losses. The pulverised coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place
Fluidised bed combustion has significant advantages over conventional firing systems and offers multiple benefits namely fuel flexibility, reduced emission of noxious pollutants such as SOx and NOx, compact boiler design and higher combustion efficiency. More details about FBC boilers are given in Chapter 6 on Fluidized Bed Boiler.
2.4 Performance Evaluation of Boilers
The performance parameters of boiler, like efficiency and evaporation ratio reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance. Even for a new boiler, reasons such as deteriorating fuel quality, water quality etc. can result in poor boiler performance. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.
Boiler Efficiency
Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilised to generate steam. There are two methods of assessing boiler efficiency.
- The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
- The Indirect Method: Where the efficiency is the difference between the losses and the energy input.
a. Direct Method
This is also known as ‘input-output method’ due to 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 evalu- ated using the formula
Parameters to be monitored for the calculation of boiler efficiency by direct method are :
• Quantity of steam generated per hour (Q) in kg/hr.
• Quantity of fuel used per hour (q) in kg/hr.
• The working pressure (in kg/cm^2 (g)) and superheat temperature (°C), if any
• The temperature of feed water (°C)
• Type of fuel and gross calorific value of the fuel (GCV) in kCal/kg of fuel
Boiler Efficiency Heat Output 100 Heat Input
= ×
Boiler Efficiency Evaluation
Direct Method (^) Indirect Method
Where, h (^) g – Enthalpy of saturated steam in kCal/kg of steam h (^) f – Enthalpy of feed water in kCal/kg of water
Example
Find out the efficiency of the boiler by direct method with the data given below:
- Type of boiler : Coal fired
- Quantity of steam (dry) generated : 8 TPH
- Steam pressure (gauge) / temp : 10 kg/cm 2 (g)/ 180°C
- Quantity of coal consumed : 1.8 TPH
- Feed water temperature : 85°C
- GCV of coal : 3200 kCal/kg
- Enthalpy of steam at 10 kg/cm 2 pressure : 665 kCal/kg (saturated)
- Enthalpy of feed water : 85 kCal/kg
It should be noted that boiler may not generate 100% saturated dry steam, and there may be some amount of wetness in the steam.
Advantages of direct method:
Plant people can evaluate quickly the efficiency of boilers Requires few parameters for computation Needs few instruments for monitoring
Disadvantages of direct method:
Does not give clues to the operator as to why efficiency of system is lower Does not calculate various losses accountable for various efficiency levels
b. Indirect Method
There are reference standards for Boiler Testing at Site using indirect method namely British Standard, BS 845: 1987 and USA Standard is ASME PTC-4-1 Power Test Code Steam Generating Units’. Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. A detailed procedure for calculating boiler efficiency by indi-
( ) 8 (665^ 85)^1000 100 =80% 1.8 3200 1000
Boiler Efficiency η = ×^ −^ × × × ×
( ) ( )^ g^ f 100 Q x h h Boiler Efficiency q GCV
− η = × ×
Where, H 2 - kg of H 2 in 1 kg of fuel Cp - Specific heat of superheated steam (0.45 kCal/kg °C)
iii. Percentage heat loss due to evaporation of moisture present in fuel
Where, M – kg of moisture in 1kg of fuel C (^) p – Specific heat of superheated steam (0.45 kCal/kg)°C 584 is the latent heat corresponding to the partial pressure of water vapour.
iv. Percentage heat loss due to moisture present in air
C (^) p – Specific heat of superheated steam (0.45 kCal/kg °C)
v. Percentage heat loss due to unburnt in fly ash
vi. Percentage heat loss due to unburnt in bottom ash
vii. Percentage heat loss due to radiation and other unaccounted loss
The actual radiation and convection losses are difficult to assess because of particular emissivity of various surfaces, its inclination, air flow pattern etc. In a relatively small boiler, with a capacity of 10 MW, the radiation and unaccounted losses could amount to between 1% and 2% of the gross calorific value of the fuel, while in a 500 MW boiler, values between 0.2% to 1% are typical. The loss may be assumed appropriately depending on the surface condition.
Example: The following are the data collected for a typical oil fired boiler. Find out the effi- ciency of the boiler by indirect method and Boiler Evaporation ratio.
• Type of boiler^ : Oil fired
Efficiency of boiler ( η ) = 100 - (i + ii + iii + iv + v + vi + vii)
Total ash collected / kg of fuel burnt G.C.V of bottom ash 100 GCV of fuel
× = ×
Total ash collected / kg of fuel burnt G.C.V of fly ash 100 GCV of fuel
× = ×
C p x (T -Tf a) (^100) GCV of fuel
=^ AAS^ ×^ humidity factor × ×
M x {584 C p (T -Tf a)} (^100) GCV of fuel
=^ + x
• Ultimate analysis of Oil
C : 84.0 % H 2 : 12.0 %
S : 3.0 % O 2 : 1.0 %
• GCV of Oil^ : 10200 kCal/kg
• Steam Generation Pressure^ : 7kg/cm^2 (g)-saturated
• Enthalpy of steam^ : 660 kCal/kg
• Feed water temperature^ : 60 °C
• Percentage of Oxygen in flue gas^ : 7
• Percentage of CO 2 in flue gas^ : 11
• Flue gas temperature (T^ f)^ : 220 °C
• Ambient temperature (T^ a)^ : 27 °C
• Humidity of air^ : 0.018 kg/kg of dry air
Solution
Step-1: Find the theoretical air requirement
kg/kg of oil
kg/kg of oil
=14 kg of air/kg of oil
Step-2: Find the %Excess air supplied Excess air supplied (EA) = (O 2 × 100)/(21-O 2 ) = (7 × 100)/(21-7) = 50%
Step-3: Find the Actual mass of air supplied Actual mass of air supplied /kg of fuel = [ 1 + EA/100] x Theoritical Air (AAS) = [1 + 50/100] x 14 = 1.5 x 14 = 21 kg of air/kg of oil
Step-4: Estimation of all losses i. Dry flue gas loss m × Cp × (Tf – Ta ) × 100 Percentage heat loss due to dry flue gas = GCV of fuel
m= mass of CO 2 + mass of SO 2 + mass of N 2 + mass of O (^2)
=[(11.6 × 84) + [{34.8 × (12 – 1/8)} + (4.35 × 3)]/
= [(11.6 × C ) + {34.8 × ( H (^) 2 − O 2 / 8)} +(4.35 × S )]/
iv. Heat loss due to radiation and other unaccounted loss : 2% Boiler Efficiency = 100- [9.14 + 7.10 + 0.322 + 2] = 100 – 18.56 = 81 %(app) Evaporation Ratio = Heat utilised for steam generation/Heat addition to the steam = 10200 × 0.83/ (660-60) = 14.
Boiler Evaporation Ratio
Evaporation ratio means kilogram of steam generated per kilogram of fuel consumed. Typical Examples: Coal fired boiler: 6 Oil fired boiler: 13 i.e 1 kg of coal can generate 6 kg of steam 1 kg of oil can generate 13 kg of steam However, this figure will depend upon type of boiler, calorific value of the fuel and associated efficiencies.
2.5 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 will concentrate and may even- tually 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 car- ryover of water into the steam. The deposits also lead to scale formation inside the boiler, result- ing in localized overheating and finally causing boiler tube failure. It is, therefore, 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 – thus 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 significant source of heat loss, if improperly carried out. The maximum amount of total dissolved solids (TDS) concentration permissible in various types of boilers is given in Table 2.1.
TABLE 2.1 RECOMMENDED TDS LEVELS FOR VARIOUS BOILERS Boiler Type Maximum TDS (ppm)*
- Lancashire 10,000 ppm
- Smoke and water tube boilers (12 kg/cm^2 ) 5,000 ppm
- Low pressure Water tube boiler 2000–
- High Pressure Water tube boiler with superheater etc. 3,000–3,500 ppm
- Package and economic boilers 3,000 ppm
- Coil boilers and steam generators 2000 (in the feed water Note: Refer guidelines specified by manufacturer for more details *parts per million
purity at given steam load. Once blow down valve is set for a given conditions, there is no need for regular operator intervention.
Conductivity as Indicator of Boiler Water Quality
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 – intermittent and 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 requires large short-term increases in the amount of feed water put into the boiler, and hence may necessitate larger feed water pumps than if continuous blow down is used. Also, TDS level will be varying, thereby causing fluctuations of the water level in the boiler due to changes in steam bubble size and distribution which accompany changes in concentration of solids. Also substantial amount 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
Figure 2.9 Blowdown Heat Recovery System
solution as particulate solids, sometimes in crystallized forms and other times as amorphous particles. When solubility of a specific component in water is exceeded, scale or deposits develop. The boiler water must be sufficiently free of deposit forming solids to allow rapid and efficient heat transfer and it must not be corrosive to the boiler metal.
Deposit Control
Deposits in boilers may result from hardness contamination of feed water and corrosion products from the condensate and feed water system. Hardness contamination of the feed water may arise due to deficient softener system. Deposits and corrosion result in efficiency losses and may result in boiler tube failures and inability to produce steam. Deposits act as insulators and slows heat transfer. Large amounts of deposits throughout the boiler could reduce the heat transfer enough to reduce the boiler effi- ciency significantly. Different type of deposits affects the boiler efficiency differently. Thus it may be useful to analyse the deposits for its characteristics. The insulating effect of deposits causes the boiler metal temperature to rise and may lead to tube-failure by overheating.
Impurities Causing Deposits
The most important chemicals contained in water that influences the formation of deposits in the boilers are the salts of calcium and magnesium, which are known as hardness salts. Calcium and magnesium bicarbonate dissolve in water to form an alkaline solution and these salts are known as alkaline hardness. They decompose upon heating, releasing carbon dioxide and forming a soft sludge, which settles out. These are called temporary hardness-hard- ness that can be removed by boiling. Calcium and magnesium sulphates, chlorides and nitrates, etc. when dissolved in water are chemically neutral and are known as non-alkaline hardness. These are called permanent hard- ness and form hard scales on boiler surfaces, which are difficult to remove. Non-alkalinity hard- ness chemicals fall out the solution due to reduction in solubility as the temperature rises, by concentration due to evaporation which takes place within the boiler, or by chemical change to a less soluble compound.
Silica
The presence of silica in boiler water can rise to formation of hard silicate scales. It can also associate with calcium and magnesium salts, forming calcium and magnesium silicates of very low thermal conductivity. Silica can give rise to deposits on steam turbine blades, after been carried over either in droplets of water in steam, or in volatile form in steam at higher pressures. Two major types of boiler water treatment are: Internal water treatment and External water treatment.
Internal Water Treatment
Internal treatment is carried out by adding chemicals to boiler to prevent the formation of scale by converting the scale-forming compounds to free-flowing sludges, which can be removed by blowdown. This method is limited to boilers, where feed water is low in hardness salts, to low pressures- high TDS content in boiler water is tolerated, and when only small quantity of water is required to be treated. If these conditions are not applied, then high rates of blowdown are
required to dispose off the sludge. They become uneconomical from heat and water loss con- sideration. Different waters require different chemicals. Sodium carbonate, sodium aluminate, sodium phosphate, sodium sulphite and compounds of vegetable or inorganic origin are all used for this purpose. Proprietary chemicals are available to suit various water conditions. The specialist must be consulted to determine the most suitable chemicals to use in each case. Internal treat- ment alone is not recommended.
External Water Treatment
External treatment is used to remove suspended solids, dissolved solids (particularly the calcium and magnesium ions which are a major cause of scale formation) and dissolved gases (oxygen and carbon dioxide). The external treatment processes available are: ion exchange; demineralization; reverse osmosis and de-aeration. Before any of these are used, it is necessary to remove suspended solids and colour from the raw water, because these may foul the resins used in the subsequent treatment sections. Methods of pre-treatment include simple sedimentation in settling tanks or settling in clarifiers with aid of coagulants and flocculants. Pressure sand filters, with spray aeration to remove carbon dioxide and iron, may be used to remove metal salts from bore well water. The first stage of treatment is to remove hardness salt and possibly non-hardness salts. Removal of only hardness salts is called softening, while total removal of salts from solution is called demineralization. The processes are:
Ion-exchange process (Softener Plant)
In ion-exchange process, the hardness is removed as the water passes through bed of natural zeolite or synthetic resin and without the formation of any precipitate. The simplest type is ‘base exchange’ in which calcium and magnesium ions are exchanged for sodium ions. After saturation regeneration is done with sodium chloride. The sodium salts being soluble, do not form scales in boilers. Since base exchanger only replaces the calcium and magnesium with sodium, it does not reduce the TDS content, and blowdown quantity. It also does not reduce the alkalinity. Demineralization is the complete removal of all salts. This is achieved by using a “cation” resin, which exchanges the cations in the raw water with hydrogen ions, producing hydrochloric, sulphuric and carbonic acid. Carbonic acid is removed in degassing tower in which air is blown through the acid water. Following this, the water passes through an “anion” resin which exchanges anions with the mineral acid (e.g. sulphuric acid) and forms water. Regeneration of cations and anions is necessary at intervals using, typically, mineral acid and caustic soda respectively. The complete removal of silica can be achieved by correct choice of anion resin.
Softening reaction: Na2R + Ca(HCO 3 ) 2 « CaR + 2 Na(HCO 3 )
Regeneration reaction CaR + 2 NaCl « Na2R + CaCl
for de-aeration because:
- Steam is essentially free from O 2 and CO 2 ,
- Steam is readily available
- Steam adds the heat required to complete the reaction.
Chemical de-Aeration
While the most efficient mechanical deaerators reduce oxygen to very low levels (0.005 mg/litre), even trace amounts of oxygen may cause corrosion damage to a system. Consequently, good operating practice requires removal of that trace oxygen with a chemical oxygen scavenger such as sodium sulfite or hydrazine. Sodium sulphite reacts with oxygen to form sodium sulphate, which increases the TDS in the boiler water and hence increases the blowdown requirements and make-up water quality. Hydrazine reacts with oxygen to form nitrogen and water. It is invariably used in high pressures boilers when low boiler water solids are necessary, as it does not increase the TDS of the boiler water.
Reverse Osmosis
Reverse osmosis uses the fact that when solutions of differing concentrations are separated by a semi-permeable membrane, water from less concentrated solution passes through the membrane to dilute the liquid of high concentration. If the solution of high concentration is pressurized, the process is reversed and the water from the solution of high concentration flows to the weaker solu- tion. This is known as reverse osmosis. The quality of water produced depends upon the concen- tration of the solution on the high-pressure side and pressure differential ascross the membrane. This process is suitable for waters with very high TDS, such as sea water.
The semipermeable nature of the membrane allows the water to pass much more readi- ly than the dissolved minerals. Since the water in the less con- centrated solution seeks to dilute the more concentrated solution, the water passage through the membrane generates a noticeable head difference between the two solutions. This head difference is a measure of the concentra- tion difference of the two solu- tions and is referred to as the osmotic pressure difference.
When a pressure is applied to the concentrated solution which is great that the osmotic pressure difference, the direction of water passage through the membrane is reversed and the process that we refer to as reverse osmosis is established. That is, the membrane's ability to selectively pass water is unchanged, only the direction of the water flow is changed.
The feed water and concentrate (reject stream) ports illustrates a continuously operating RO system.
Figure 2.11 Reverse Osmosis
Recommended boiler and feed water quality
The impurities found in boiler water depend on the untreated feed water quality, the treatment process used and the boiler operating procedures. As a general rule, the higher the boiler oper- ating pressure, the greater will be the sensitivity to impurities. Recommended feed water and boiler water limits are shown in Table 2.2 and Table 2.3.
