2.1 Introduction to Boiler

Transcription

1 2 Boilers Syllabus Boiler Types, Combustion in boilers Performances evaluation of boilers, Analysis of losses Feed water treatment, Blow down Energy conservation opportunities.

2 2.1 Introduction to Boiler It is an enclosed Pressure Vessel Heat generated by Combustion of Fuel is transferred to water to become steam Process: Evaporation Steam volume increases to 1,600 times from water and produces tremendous force Care is must to avoid explosion. What is a boiler?

3 Boiler Specification Boiler Make & Year :XYZ & 2003 MCR(Maximum Continuous Rating) :10TPH (F & A 100 o C) Rated Working Pressure :10.54 kg/cm 2 (g) Type of Boiler : 3 Pass Fire tube Fuel Fired : Fuel Oil Heating surface : M 2

4 Indian Boiler Regulation IBR Steam Boilers means any closed vessel exceeding 25 liters in capacity and which is used expressively for generating steam under pressure As per section 28 & 29 of the Indian Boilers Act. 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

5 2.2 Boiler Systems Water treatment system Feed water system Steam System Blow down system Fuel supply system Air Supply system Flue gas system

6 Merits Low Capital Cost and fuel Efficient (82%) Accepts wide & load fluctuations Steam pressure variation is less (Large volume of water) Packaged Boiler 2.3 Boiler Types and Classifications Fire in tube or Hot gas through tubes and boiler feed water in shell side Fire Tubes submerged in water Application Used for small steam capacities ( upto 25T/hr and 17.5kg/cm 2 Fire Tube Boiler

7 Boiler Types and Classifications Water flow through tubes Water Tubes surrounded by hot gas Application Used for Power Plants Steam capacities range from t/hr Characteristics High Capital Cost Used for high pressure high capacity steam boiler Demands more controls Calls for very stringent water quality Water Tube Boiler

8 Package boilers are generally of shell type with fire tube design High heat release rate in small combustion space Packaged Boiler More number of passes-so more heat transfer Large number of small diameter tubes leading to good convective heat transfer. Higher thermal efficiency

9 Chain Grate or Traveling Grate Stoker Boiler Coal is fed on one end of a moving steel chain grate Coal burns and ash drops off at end Coal grate controls rate of coal feed into furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completely Bed thickness decreases from coal feed end to rear end and so more air at front and less air at rear end to be supplied Water tube boiler

10 Spreader Stoker Boiler Uses both suspension and grate burning Coal fed continuously over burning coal bed Coal fines burn in suspension and larger coal pieces burn on grate Good flexibility to meet changing load requirements Preferred over other type of stokers in industrial application

11 Pulverized Fuel Boiler Coal is pulverised to a fine powder, so that less than 2% is +300 microns, and 70-75% is below 75 microns. Coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Combustion takes place at temperatures from C Particle residence time in the boiler is typically 2-5 seconds One of the most popular system for firing pulverized coal is the tangential firing using four burners corner to corner to create a fire ball at the center of the furnace. See Figure Tangential firing

12 Pulverized Fuel Boiler (Contd..) Advantages Its ability to burn all ranks of coal from anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces. Disadvantages High power demand for pulverizing Requires more maintenance, flyash erosion and pollution complicate unit operation

13 Fluidized bed Combustion (FBC) boiler When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream Further, increase in velocity gives rise to bubble formation, vigorous turbulence and rapid mixing and the bed is said to be fluidized. Coal is fed continuously in to a hot air agitated refractory sand bed, the coal will burn rapidly and the bed attains a uniform temperature Fluidized Bed Combustion

14 Fluidised Bed Combustion

15 Fluidized-bed boiler (Contd..) Advantages : Higher rates of heat transfer between combustion process and boiler tubes (thus reduced furnace area and size required), combustion temperature 850 o C is lower than in a conventional furnace. The lower furnace temperatures means reduced NO x production. In addition, the limestone (CaCO 3 ) and dolomite (MgCO 3 ) react with SO 2 to form calcium and magnesium sulfides, respectively, solids which do not escape up the stack; This means the plant can easily use high sulfur coal. Fuel Flexibility: Multi fuel firing

16 2.4 Performance Evaluation of Boilers What are the factors for poor efficiency? Efficiency reduces with time, due to poor combustion, heat transfer fouling and poor operation and maintenance.deterioration of fuel and water quality also leads to poor performance of boiler. How Efficiency testing helps to improve performance? Helps us to find out how far the boiler efficiency drifts away from the best efficiency. Any observed abnormal deviations could therefore be investigated to pinpoint the problem area for necessary corrective action.

17 Boiler Efficiency There are two methods of assessing boiler efficiency. 1) The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel. 2) The Indirect Method: Where the efficiency is the difference between the losses and the energy input. Boiler Efficiency Evaluation Method 1. Direct Method 2. Indirect Method

18 Efficiency Calculation by Direct Method Example: Type of boiler: Coal fired Boiler Heat input data Qty of coal consumed :1.8 TPH GCV of coal :3200K.Cal/kg Heat output data Qty of steam gen : 8 TPH Steam pr/temp:10 kg/cm 2 (g)/180 0 C Enthalpy of steam(sat) at 10 kg/cm 2 (g) pressure :665 K.Cal/kg Feed water temperature : 85 0 C Enthalpy of feed water : 85 K.Cal/kg Find out the Find efficiency? Find out the Evaporation Ratio?

19 Boiler efficiency ( ): = Q x (H h) x 100 (q x GCV) Where Q = Quantity of steam generated per hour (kg/hr) H = Enthalpy of saturated steam (kcal/kg) h = Enthalpy of feed water (kcal/kg) q = Quantity of fuel used per hour (kg/hr) GCV = Gross calorific value of the fuel (kcal/kg) Boiler efficiency ( )= 8 TPH x1000kg/tx (665 85) x TPH x 1000Kg/T x 3200 = 80.0% Evaporation Ratio = 8 Tonne of steam/1.8 Ton of coal = 4.4

20 What are the losses that occur in a boiler? Steam Output 6. Surface loss 1. Dry Flue gas loss 2. H2 loss 3. Moisture in fuel 4. Moisture in air 5. CO loss Fuel Input, 100% Boiler Flue gas 7. Fly ash loss Air 8. Bottom ash loss Efficiency = 100 ( ) (by In Direct Method)

21 Boiler Heat Balance:

22 Example: The following are the data collected for a typical oil fired boiler. Find out the efficiency of the boiler by indirect method and Boiler Evaporation ratio. Ultimate analysis of Oil C : 84.0 % H 2 : 12.0 % S: 3.0 % O 2 : 1.0 % GCV of Oil Steam Generation Pressure Enthalpy of steam : kcal/kg : 7kg/cm 2 (g)-saturated : 660 kcal/kg Feed water temperature : 60 o C Percentage of Oxygen in flue gas: 7 Percentage of CO 2 in flue gas: 11 Flue gas temperature (T f ) : C Ambient temperature (T a ) : 27 0 C Humidity of air : kg/kg of dry air

23 Solution Step-1: Find the theoretical air requirement [( 11.6 xc) {34.8 x( H 2 O2 /8)} (4.35 x S)]/100 = kg/kg of oil =[(11.6 x 84) + [{34.8 x (12 1/8)} + (4.35 x 3)]/100 kg/kg of oil =14 kg of air/kg of oil Step-2: Find the %Excess air supplied O % 7% 2 x100 x O2% Excess air supplied (EA) = = 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

24 Step-4: Estimation of all losses i. Dry flue gas loss Percentage heat loss due to dry flue gas = m xc p x( T T GCV of fuel f a ) x 100 m= mass of CO 2 + mass of SO 2 + mass of N 2 + mass of O x x 64 21x 77 m m = 21 kg / kg of oil (21 14) x x0.23x( ) x 100 = 9.14 %

25 Alternatively a simple method can be used for determining the dry flue gas loss as given below. a) Percentage heat loss due to dry flue gas = m xc x( T GCV of T fuel x 100 Total mass of flue gas (m) = mass of actual air supplied + mass of fuel supplied = =22 p f a ) %Dry flue gas loss = 22 x0.23 x( ) x %

26 ii. Heat loss due to evaporation of water formed due to H 2 in fuel = Where, H 2 percentage of H 2 in fuel = = 7.10% 9 x H2 x {584 Cp (Tf - Ta )} GCV of fuel 9 x 12 x { ( )} x x iii. Heat loss due to moisture present in air AAS x humidity xc p x( Tf Ta ) = x 100 GCV of fuel 21x0.018 x0.45 x(220 27) = x 100 =

27 iv Heat loss due to radiation and other unaccounted losses For a small boiler it is estimated to be 2% Boiler Efficiency i. Heat loss due to dry flue gas : 9.14% ii. Heat loss due to evaporation of water formed due to H 2 in fuel : 7.10 % iii. Heat loss due to moisture present in air : % iv. Evaporation Heat loss due Ratio to = radiation Heat utilised and other for steam unaccounted generation/heat loss addition : 2% to the steam = x 0.83/ (660-60) Boiler Efficiency = = 100- [ ]

28 2.5 Why Boiler Blow Down? When water evaporates Dissolved solids gets concentrated Solids precipitates Coating of tubes Reduces the heat transfer rate

29 TABLE 2.3 RECOMMENDED BOILER WATER LIMITS (IS 10392, Year 1982) Factor Upto 20 kg/cm kg/cm kg/cm 2 TDS, ppm Total iron dissolved solids ppm Specific electrical conductivity at 25 0 C (mho) Phosphate residual ppm ph at 25 0 C Silica (max) ppm

30 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 etc) within prescribed limits so that steam quality is not likely to be affected TDS level keeps varying fluctuations of the water level in the boiler. substantial amount of heat energy is lost with intermittent blow down.

31 Continuous Blowdown 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 purity. This type of blow down is common in highpressure boilers.

32 The quantity of blowdown required to control boiler water solids concentration is calculated by using the following formula: (Continuous Blow down) Steam 10 T/hr TDS(T) =0 TDS(S) in feed water 100 ppm TDS (C) =3500 ppm Allowable) Blow down(b) B =(Sx100)/C Blowdown %= TDS in FWx100 TDSin Boiler Blow down flow rate=3%x 10,000kg/hr=300kg/hr =(100/3500)x100 =3%

33 2.6 Boiler Water Treatment Method : It is carried out by adding chemicals to boiler to prevent the formation of scale by converting the scaleforming compounds to free-flowing sludges, which can be removed by blowdown. Limitation: Applicable 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 consideration. Internal treatment alone is not recommended.

34 External Water Treatment Propose: 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). Different treatment Process : ion exchange; demineralization; reverse osmosis and de-aeration.

35 Ion-exchange process (Softener Plant) In ion-exchange process, 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. 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. Softening reaction: Na2R + Ca(HCO3)2 «CaR + 2 Na(HCO3) Regeneration reaction CaR + 2 NaCl «Na2R + CaCl2

36 Demineralization 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.

37 De-aeration All natural waters contain dissolved gases in solution. Certain gases, such as carbon dioxide and oxygen, greatly increase corrosion. When heated in boiler systems, carbon dioxide (CO 2 ) and oxygen (O 2 ) are released as gases and combine with water (H 2 O) to form carbonic acid, (H 2 CO 3 ). In de-aeration, dissolved gases, such as oxygen and carbon dioxide, are expelled by preheating the feed water before it enters the boiler. Figure 2.9 Deaerator

38 Reverse Osmosis

39 2.7 Energy Conservation Opportunities in Boilers

40 1. Reduce Stack Temperature Stack temperatures greater than 200 C indicates potential for recovery of waste heat. It also indicate the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning. 22 o C reduction in flue gas temperature increases boiler efficiency by 1%

41 2. Feed Water Preheating using Economiser For an older shell boiler, with a flue gas exit temperature of 260 o C, an economizer could be used to reduce it to 200 o C, Increase in overall thermal efficiency would be in the order of 3%. Condensing economizer(n.gas) Flue gas reduction up to 65 o C 6 o C raise in feed water temperature, by economiser/condensate recovery, corresponds to a 1% saving in fuel consumption

42 3. Combustion Air Preheating Combustion air preheating is an alternative to feedwater heating. In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 o C.

43 4. Incomplete Combustion (c c c c c + co co co co) Incomplete combustion can arise from a shortage of air or surplus of fuel or poor distribution of fuel. In the case of oil and gas fired systems, CO or smoke with normal or high excess air indicates burner system problems. Example: Poor mixing of fuel and air at the burner. Poor oil fires can result from improper viscosity, worn tips, carbonization on tips and deterioration of diffusers. With coal firing: Loss occurs as grit carry-over or carbon-in-ash (2% loss). Example :In chain grate stokers, large lumps will not burn out completely, while small pieces and fines may block the air passage, thus causing poor air distribution. Increase in the fines in pulverized coal also increases carbon loss.

44 5. Control excess air for every 1% reduction in excess air,0.6% rise in efficiency. The optimum excess air level varies with furnace design, type of burner, fuel and process variables.. Install oxygen trim system TABLE 2.5 EXCESS AIR LEVELS FOR DIFFERENT FUELS Fuel Type of Furnace or Burners Excess Air (% by wt) Pulverised coal Completely water-cooled furnace for slagtap or dry-ash removal Partially water-cooled furnace for dry-ash removal Coal Spreader stoker Water-cooler vibrating-grate stokers Chain-grate and traveling-grate stokers Underfeed stoker Fuel oil Oil burners, register type Multi-fuel burners and flat-flame Natural gas High pressure burner 5-7 Wood Dutch over (10-23% through grates) and Hofft type Bagasse All furnaces Black liquor Recovery furnaces for draft and sodapulping processes 30-40

45 Blowdown Heat Recovery Efficiency Improvement - Up to 2 percentage points. Blowdown of boilers to reduce the sludge and solid content allows heat to go down the drain. The amount of blowdown should be minimized by following a good water treatment program, but installing a heat exchanger in the blowdown line allows this waste heat to be used in preheating makeup and feedwater. Heat recovery is most suitable for continuous blowdown operations which in turn provides the best water treatment program.

46 8. Reduction of Scaling and Soot Losses In oil and coal-fired boilers, soot buildup on tubes acts as an insulator against heat transfer. Any such deposits should be removed on a regular basis. Elevated stack temperatures may indicate excessive soot buildup. Also same result will occur due to scaling on the water side. High exit gas temperatures at normal excess air indicate poor heat transfer performance. This condition can result from a gradual build-up of gas-side or waterside deposits. Waterside deposits require a review of water treatment procedures and tube cleaning to remove deposits. Stack temperature should be checked and recorded regularly as an indicator of soot deposits. When the flue gas temperature rises about 20 o C above the temperature for a newly cleaned boiler, it is time to remove the soot deposits

47 9. Reduction of Boiler Steam Pressure Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery, a similar reduction in the temperature of the flue gas temperature results. Potential 1 to 2% improvement. Steam is generated at pressures normally dictated by the highest pressure / temperature requirements for a particular process. In some cases, the process does not operate all the time, and there are periods when the boiler pressure could be reduced. Adverse effects, such as an increase in water carryover from the boiler owing to pressure reduction, may negate any potential saving. Pressure should be reduced in stages, and no more than a 20 percent reduction should be considered.

48 10. Variable Speed Control for Fans, Blowers and Pumps Generally, combustion air control is effected by throttling dampers fitted at forced and induced draft fans. Though dampers are simple means of control, they lack accuracy, giving poor control characteristics at the top and bottom of the operating range. If the load characteristic of the boiler is variable, the possibility of replacing the dampers by a VSD should be evaluated.

49 11. Effect of Boiler Loading on Efficiency As the load falls, so does the value of the mass flow rate of the flue gases through the tubes. This reduction in flow rate for the same heat transfer area, reduced the exit flue gas temperatures by a small extent, reducing the sensible heat loss. Below half load, most combustion appliances need more excess air to burn the fuel completely and increases the sensible heat loss. Operation of boiler below 25% should be avoided Optimum efficiency occurs at 65-85% of full loads

50 12. Boiler Replacement if the existing boiler is : Old and inefficient, not capable of firing cheaper substitution fuel, over or under-sized for present requirements, not designed for ideal loading conditions replacement option should be explored. Since boiler plants traditionally have a useful life of well over 25 years, replacement must be carefully studied.