Boiler operation
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Boiler Operation
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Boiler operation
- 1. Boilers & Thermal Fluid Heaters 1
- 2. Training Agenda: Boiler Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities 2
- 3. Introduction What is a Boiler? • Vessel that heats water to become hot water or steam • At atmospheric pressure water volume increases 1,600 times • Hot water or steam used to transfer heat to 3
- 4. Introduction STEAM TO EXHAUST GAS VENT PROCESS STACK DEAERATOR PUMPS ECO- NOMI- ZER VENT BOILER BURNER WATER SOURCE BLOW DOWN SEPARATOR FUEL BRINE CHEMICAL FEED SOFTENERS 4 Figure: Schematic overview of a boiler room
- 5. Training Agenda: Boiler Introduction Thermal Equipment/ Type of boilers Boilers Assessment of a boiler Energy efficiency opportunities 5
- 6. Types of Boilers What Type of Boilers Are There? Thermal Equipment/ 1. Fire Tube Boiler 2. Water Tube Boiler Boilers 3. Packaged Boiler 4. Fluidized Bed (FBC) Boiler 5. Stoker Fired Boiler 6. Pulverized Fuel Boiler 7. Waste Heat Boiler 8. Thermic Fluid Heater (not a boiler!) 6
- 7. Type of Boilers 1. Fire Tube Boiler Thermal Equipment/ • Relatively small steam Boilers capacities (12,000 kg/hour) • Low to medium steam pressures (18 kg/cm2) • Operates with oil, gas or solid fuels (Light Rail Transit Association) 7
- 8. Type of Boilers 2. Water Tube Boiler • Used for high steam demand and pressure requirements • Capacity range of 4,500 – 120,000 kg/hour • Combustion efficiency enhanced by induced draft provisions • Lower tolerance for water quality and needs (Your Dictionary.com) water treatment plant 8
- 9. Type of Boilers 3. Packaged Boiler • Comes in complete package To • Features Chimney • High heat transfer • Faster evaporation • Good convective heat transfer • Good combustion Oil efficiency Burner • High thermal efficiency (BIB Cochran, 2003) • Classified based on number of passes
- 10. Type of Boilers 4. Fluidized Bed Combustion (FBC) Boiler • Particles (e.g. sand) are suspended in high velocity air stream: bubbling fluidized bed • Combustion at 840° – 950° C • Capacity range 0,5 T/hr to 100 T/hr • Fuels: coal, washery rejects, rice husk, bagasse and agricultural wastes • Benefits: compactness, fuel flexibility, higher combustion efficiency, reduced SOx & NOx 10
- 11. Type of Boilers 4a. Atmospheric Fluidized Bed Combustion (AFBC) Boiler • Most common FBC boiler that uses preheated atmospheric air as fluidization and combustion air 4b. Pressurized Fluidized Bed Combustion (PFBC) Boiler • Compressor supplies the forced draft and combustor is a pressure vessel • Used for cogeneration or combined cycle power generation 11
- 12. Type of Boilers 4c. Atmospheric Circulating Fluidized Bed Combustion (CFBC) Boiler • Solids lifted from bed, rise, return to bed • Steam generation in convection section • Benefits: more economical, better space utilization and efficient combustion (Thermax Babcock & Wilcox Ltd, 2001) 12
- 13. Type of Boilers 5. Stoke Fired Boilers a) Spreader stokers • Coal is first burnt in suspension then in coal bed • Flexibility to meet load fluctuations • Favored in many industrial applications 13
- 14. Type of Boilers 5. Stoke Fired Boilers b) Chain-grate or traveling-grate stoker • Coal is burnt on moving steel grate • Coal gate controls coal feeding rate • Uniform coal size for complete combustion (University of Missouri, 2004) 14
- 15. Type of Boilers 6. Pulverized Fuel Boiler • Pulverized coal powder blown with combustion air into boiler through burner nozzles • Combustion temperature at 1300 -1700 °C • Benefits: varying coal quality coal, quick response to load changes and high pre- heat air temperatures Tangential firing 15
- 16. Type of Boilers 7. Waste Heat Boiler • Used when waste heat available at medium/high temp • Auxiliary fuel burners used if steam demand is more than the waste heat can generate • Used in heat recovery from exhaust gases from gas turbines and diesel Agriculture and Agri-Food engines 16 Canada, 2001
- 17. Type of Boilers 8. Thermic Fluid Heater • Wide application for indirect process heating • Thermic fluid (petroleum-based) is heat transfer medium • Benefits: • Closed cycle = minimal losses • Non-pressurized system operation at 250 °C • Automatic controls = operational flexibility • Good thermal efficiencies 17
- 18. Type of Boilers 3. Heat transfer through heat 8. Thermic Fluid Heater exchanged User equipment 2. Circulated to user 4. Fluid equipment returned to heater Control panel Insulated outer wall 1. Thermic fluid heated Blower Exhaust motor in the heater unit (Energy Fuel oil filter Machine India) 18
- 19. Training Agenda: Boiler Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities 19
- 20. Assessment of a boiler 1. Boiler 2. Boiler blow down 3. Boiler feed water treatment 20
- 21. Assessment of a Boiler 1. Boiler performance • Causes of poor boiler performance -Poor combustion -Heat transfer surface fouling -Poor operation and maintenance -Deteriorating fuel and water quality • Heat balance: identify heat losses • Boiler efficiency: determine deviation from best efficiency 21
- 22. Assessment of a Boiler Heat Balance An energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and losses Stochiometric Excess Air Un burnt Stack Gas FUEL INPUT STEAM OUTPUT Convection & Blow Ash and Un-burnt parts Radiation Down of Fuel in Ash 22
- 23. Assessment of a Boiler Heat Balance Balancing total energy entering a boiler against the energy that leaves the boiler in different forms 12.7 % Heat loss due to dry flue gas 8.1 % Heat loss due to steam in fuel gas 1.7 % 100.0 % Heat loss due to moisture in fuel BOILER 0.3 % Fuel Heat loss due to moisture in air 2.4 % Heat loss due to unburnts in residue 1.0 % Heat loss due to radiation & other unaccounted loss 73.8 % Heat in Steam 23
- 24. Assessment of a Boiler Heat Balance Goal: improve energy efficiency by reducing avoidable losses Avoidable losses include: - Stack gas losses (excess air, stack gas temperature) - Losses by unburnt fuel - Blow down losses - Condensate losses 24 - Convection and radiation
- 25. Assessment of a Boiler Boiler Efficiency Thermal efficiency: % of (heat) energy input that is effectively useful in the generated steam BOILER EFFICENCY CALCULATION 1)DIRECT METHOD: 2) INDIRECT METHOD: The energy gain of the The efficiency is the working fluid (water and steam) different between losses is compared with the energy and energy input 25 content of the boiler fuel.
- 26. Assessment of a Boiler Boiler Efficiency: Direct Method Heat Input x 100 Q x (hg – hf) x 100 Boiler efficiency (η) = = Heat Output Q x GCV hg -the enthalpy of saturated steam in kcal/kg of steam hf -the enthalpy of feed water in kcal/kg of water Parameters to be monitored: - 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/cm2(g)) and superheat temperature (oC), if 26 any
- 27. Assessment of a Boiler Boiler Efficiency: Direct Method Advantages • Quick evaluation • Few parameters for computation • Few monitoring instruments • Easy to compare evaporation ratios with benchmark figures Disadvantages • No explanation of low efficiency • Various losses not calculated 27
- 28. Assessment of a Boiler Boiler Efficiency: Indirect Method Efficiency of boiler (η) = 100 – (i+ii+iii+iv+v+vi+vii) Principle losses: i) Dry flue gas ii) Evaporation of water formed due to H2 in fuel iii) Evaporation of moisture in fuel iv) Moisture present in combustion air v) Unburnt fuel in fly ash vi) Unburnt fuel in bottom ash 28 vii) Radiation and other unaccounted losses
- 29. Assessment of a Boiler Boiler Efficiency: Indirect Method Required calculation data • Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content) • % oxygen or CO2 in the flue gas • Fuel gas temperature in ◦C (Tf) • Ambient temperature in ◦C (Ta) and humidity of air in kg/kg of dry air • GCV of fuel in kcal/kg • % combustible in ash (in case of solid fuels) • GCV of ash in kcal/kg (in case of solid fuels) 29
- 30. Assessment of a Boiler Boiler Efficiency: Indirect Method Advantages • Complete mass and energy balance for each individual stream • Makes it easier to identify options to improve boiler efficiency Disadvantages • Time consuming • Requires lab facilities for analysis 30
- 31. Assessment of a Boiler 2. Boiler Blow Down • Controls ‘total dissolved solids’ (TDS) in the water that is boiled • Blows off water and replaces it with feed water • Conductivity measured as indication of TDS levels • Calculation of quantity blow down required: Feed water TDS x % Make up water Blow down (%) = Maximum Permissible TDS in Boiler water 31
- 32. Assessment of a Boiler Boiler Blow Down Two types of blow down • Intermittent • Manually operated valve reduces TDS • Large short-term increases in feed water • Substantial heat loss • Continuous • Ensures constant TDS and steam purity • Heat lost can be recovered • Common in high-pressure boilers 32
- 33. Assessment of a Boiler Boiler Blow Down Benefits • Lower pretreatment costs • Less make-up water consumption • Reduced maintenance downtime • Increased boiler life • Lower consumption of treatment chemicals 33
- 34. Assessment of a Boiler 3. Boiler Feed Water Treatment • Quality of steam depend on water treatment to control • Steam purity • Deposits • Corrosion • Efficient heat transfer only if boiler water is free from deposit-forming solids 34
- 35. Assessment of a Boiler Boiler Feed Water Treatment Deposit control • To avoid efficiency losses and reduced heat transfer • Hardness salts of calcium and magnesium • Alkaline hardness: removed by boiling • Non-alkaline: difficult to remove • Silica forms hard silica scales 35
- 36. Assessment of a Boiler Boiler Feed Water Treatment Internal water treatment • Chemicals added to boiler to prevent scale • Different chemicals for different water types • Conditions: • Feed water is low in hardness salts • Low pressure, high TDS content is tolerated • Small water quantities treated • Internal treatment alone not recommended 36
- 37. Assessment of a Boiler Boiler Feed Water Treatment External water treatment: • Removal of suspended/dissolved solids and dissolved gases • Pre-treatment: sedimentation and settling • First treatment stage: removal of salts • Processes a) Ion exchange b) Demineralization c) De-aeration 37 d) Reverse osmoses
- 38. Assessment of a Boiler External Water Treatment a) Ion-exchange process (softener plant) • Water passes through bed of natural zeolite of synthetic resin to remove hardness • Base exchange: calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ions • Does not reduce TDS, blow down quantity and alkalinity b) Demineralization • Complete removal of salts • Cations in raw water replaced with hydrogen ions 38
- 39. Assessment of a Boiler External Water Treatment c) De-aeration • Dissolved corrosive gases (O2, CO2) expelled by preheating the feed water • Two types: • Mechanical de-aeration: used prior to addition of chemical oxygen scavangers • Chemical de-aeration: removes trace oxygen 39
- 40. Assessment of a Boiler External Water Treatment Mechanical Vent de-aeration Spray Boiler Feed Nozzles • O2 and CO2 removed by Water Stea heating feed water Scrubber m Section • Economical treatment (Trays) process Storage • Vacuum type can reduce Section O2 to 0.02 mg/l De-aerated • Pressure type can Boiler Feed Water reduce O2 to 0.005 mg/l ( National Productivity Council) 40
- 41. Assessment of a Boiler External Water Treatment Chemical de-aeration • Removal of trace oxygen with scavenger • Sodium sulphite: • Reacts with oxygen: sodium sulphate • Increases TDS: increased blow down • Hydrazine • Reacts with oxygen: nitrogen + water • Does not increase TDS: used in high pressure boilers 41
- 42. Assessment of a Boiler External Water Treatment d) Reverse osmosis • Osmosis • Solutions of differing concentrations • Separated by a semi-permeable membrane • Water moves to the higher concentration • Reversed osmosis • Higher concentrated liquid pressurized • Water moves in reversed direction 42
- 43. Assessment of a Boiler External water treatment d) Reverse osmosis Pressure Feed Fresh Water Water More Concentrated Solution Concentrate Water Flow Flow Semi Permeable 43 Membrane
- 44. Training Agenda: Boiler Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities 44
- 45. Energy Efficiency Opportunities 1. Stack temperature control 2. Feed water preheating using economizers 3. Combustion air pre-heating 4. Incomplete combustion minimization 5. Excess air control 6. Avoid radiation and convection heat loss 7. Automatic blow down control 8. Reduction of scaling and soot losses 9. Reduction of boiler steam pressure 10. Variable speed control 11. Controlling boiler loading 12. Proper boiler scheduling 45 13. Boiler replacement
- 46. Energy Efficiency Opportunities 1. Stack Temperature Control • Keep as low as possible • If >200°C then recover waste heat 2. Feed Water Preheating Economizers • Potential to recover heat from 200 – 300 oC flue gases leaving a modern 3-pass shell boiler 3. Combustion Air Preheating • If combustion air raised by 20°C = 1% improve thermal efficiency 46
- 47. Energy Efficiency Opportunities 4. Minimize Incomplete Combustion • 47 • Smoke, high CO levels in exit flue gas
- 48. Energy Efficiency Opportunities 5. Excess Air Control • Excess air required for complete combustion • Optimum excess air levels varies • 1% excess air reduction = 0.6% efficiency rise • Portable or continuous oxygen analyzers Fuel Kg air req./kg fuel %CO2 in flue gas in practice Solid Fuels Bagasse 3.3 10-12 Coal (bituminous) 10.7 10-13 Lignite 8.5 9 -13 Paddy Husk 4.5 14-15 Wood 5.7 11.13 Liquid Fuels Furnace Oil 13.8 9-14 LSHS 14.1 9-14 48
- 49. Energy Efficiency Opportunities 6. Radiation and Convection Heat Loss Minimization • Fixed heat loss from boiler shell, regardless of boiler output • Repairing insulation can reduce loss 7. Automatic Blow Down Control • Sense and respond to boiler water conductivity and pH 49
- 50. Energy Efficiency Opportunities 8. Scaling and Soot Loss Reduction • Every 22oC increase in stack temperature = 1% efficiency loss • 3 mm of soot = 2.5% fuel increase 9. Reduced Boiler Steam Pressure • Lower steam pressure = lower saturated steam temperature = lower flue gas temperature • Steam generation pressure dictated by process 50
- 51. Energy Efficiency Opportunities 10. Variable Speed Control for Fans, Blowers and Pumps • Suited for fans, blowers, pumps • Should be considered if boiler loads are variable 11. Control Boiler Loading • Maximum boiler efficiency: 65-85% of rated load • Significant efficiency loss: < 25% of rated load 51
- 52. Energy Efficiency Opportunities 12. Proper Boiler Scheduling • Optimum efficiency: 65-85% of full load • Few boilers at high loads is more efficient than large number at low loads 13. Boiler Replacement Financially attractive if 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 52
- 53. Boilers & Thermic Fluid Heaters THANK YOU FOR YOUR ATTENTION 53
Notas do Editor
- TO THE TRAINER This PowerPoint presentation can be used to train people about the basics of energy equipment. The information on the slides is the minimum information that should be explained. The trainer notes for each slide provide more detailed information, but it is up to the trainer to decide if and how much of this information is presented also. Additional materials that can be used for the training session are available on www.energyefficiencyasia.org under “Energy Equipment” and include: Textbook chapter on this energy equipment that forms the basis of this PowerPoint presentation but has more detailed information Quiz – ten multiple choice questions that trainees can answer after the training session Workshop exercise – a practical calculation related to this equipment Option checklist – a list of the most important options to improve energy efficiency of this equipment Company case studies – participants of past courses have given the feedback that they would like to hear about options implemented at companies for each energy equipment. More than 200 examples are available from 44 companies in the cement, steel, chemicals, ceramics and pulp & paper sectors
- The objectives with this training session is to learn how a boiler works and what different kinds of boilers that there are. In addition, we will also learn how to assess the performance and efficiency of a boiler and how to identify energy efficiency opportunities. Those of you who already have experience from evaluating the performance of a boiler, and from identifying energy efficiency opportunities are of course more than welcome to share your experiences.
- A boiler is an enclosed vessel that provides a means for combustion heat to be transferred to water until it becomes heated water or steam. When water at atmospheric pressure is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be an equipment that must be treated with utmost care The hot water or steam under pressure is then usable for transferring the heat to a process.
- This is a schematic overview of a boiler room: As you can see, the boiler system comprises of a feed water system (click and circle will appear); a steam system ( click and 2 circles will appear ); as well as a fuel system (click and circle will appear). 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 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, an economizer preheats the feed water using the waste heat in the flue gas. 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 generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system.
- There are different types of boilers based on different fuels and with various capacities. (Questions to audience) What type of boilers do you know of? What kind of boilers do you use in the industry where you work? (Discussion) (Click once and boiler types will appear) We will look closer at the following types of boilers: Fire Tube Boiler, Water Tube Boiler, Packaged Boiler, Fluidized Bed Boiler, Stoker Fired Boiler, Pulverized Fuel Boiler and Waste Heat Boiler.
- To begin with, we will look at the fire tube boiler: This is generally used for relatively small steam capacities and at low to medium steam pressures. The steam rates for fire tube boilers are up to 12,000 kg/hour with pressures of 18 kg/cm2. Fire tube boilers can operate on oil, gas or solid fuels. The figure illustrates how a fire tube boiler works. The fuel is burned and heats up the water to steam which is turn channeled to the process. Today, most fire tube boiler are in a packaged construction for all fuels.
- In a water tube boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers. Most modern water boiler tube designs are within the capacity range 4,500 – 120,000 kg/hour of steam, at very high pressures. Many water tube boilers are of “packaged” construction if oil and /or gas are to be used as fuel. Solid fuel fired water tube designs are available but packaged designs are less common. The features of water tube boilers are: Forced, induced and balanced draft provisions help to improve combustion efficiency. Less tolerance for water quality calls for water treatment plant. Higher thermal efficiency levels are possible
- Does anyone recognize what type of boiler this is? (Click once and name will appear) This is a packaged boiler. More specifically, it is a typical 3 pass, oil fired packaged boiler. The packaged boiler is so called because it comes as a complete package. Once delivered to a site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made to become operational. Package boilers are generally of a shell type with a fire tube design so as to achieve high heat transfer rates by both radiation and convection. The features of packaged 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.
- 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 – the bed is called “fluidized”. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse & other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr. The fluidized bed combustion (FBC) takes place at about 840oC to 950oC. Fluidized bed combustion (FBC) has emerged as a viable alternative and has significant advantages over a conventional firing system and offers multiple benefits – compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. Three types of FBC boilers are explained on the next slides.
- Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC). In this boiler, atmospheric air, which acts as both the fluidization and combustion air, is delivered at a pressure, after being preheated by the exhaust fuel gases. In Pressurized Fluidized Bed Combustion (PFBC) type, a compressor supplies the Forced Draft (FD) air and the combustor is a pressure vessel. A deep bed is used to extract large amounts of heat. This will improve the combustion efficiency and sulphur dioxide absorption in the bed. The steam is generated in the two tube bundles, one in the bed and one above it. Hot flue gases drive a power generating gas turbine. The PFBC system can be used for cogeneration (steam and electricity) or combined cycle power generation
- This figure illustrates another type of fluidized bed combustion, the atmospheric circulating fluidized bed combustion boiler. (Click once) In a circulating system the bed parameters are maintained to promote solids elutriation from the bed. They are lifted in a relatively dilute phase in a solids riser, and a down-comer with a cyclone provides a return path for the solids. (Click once) There are no steam generation tubes immersed in the bed. Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser. (Click once) Benefits: CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam. For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators.
- Stoker fired boilers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications of stokers are “spreader stoker” and “chain-grate or traveling-grate stoker”. To begin with we will look at spreader stokers. These stokers utilize a combination of suspension burning and grate burning. Spreader stokers 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 the firing rate is increased. Due to this, the spreader stoker is favored over other types of stokers in many industrial applications.
- This picture illustrates a chain grate or traveling grate stoker. Coal is fed onto one end of a moving steel grate. As the grate moves along the length of the furnace, the coal burns before dropping off at the end as ash. The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal gate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completely by the time they reach the end of the grate.
- The coal is pulverized to a fine powder until less than 2% of the coal is +300 micro meter and 70-75% is below 75 microns for bituminous coal. The pulverized coal is then blown with part of the combustion air into the boiler plant through a series of burner nozzles. The combustion takes place at temperatures ranging between 1300-1700 degrees Celsius depending mainly on the coal grade. The particle residence time in the boiler is typically 2 to 5 seconds an dthe particles has to be small enough to be completely combusted during this time period. This system has many advantages such as ability to fire varying quality of coal, quick responses to changes in load, use of high pre-heat air temperatures etc. One of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace. This is shown in the figure.
- A waste heat boiler can be economically installed wherever waste heat can be available at medium or high temperatures. Wherever the steam demand is more than the steam generated during waste heat, auxiliary fuel burners are also used. If there is no direct use of steam, the steam may be let down in a steam turbine-generator set and power produced from it. It is widely used in the heat recovery from exhaust gases from gas turbines and diesel engines.
- In recent times, thermic fluid heaters have found wide application for indirect process heating. Instead of water, petroleum based fluids are the heat transfer medium, and provide constantly maintainable temperatures for the user equipment. The combustion system comprises of a fixed grate with mechanical draft arrangements. The advantages of these heaters are: Closed cycle operation with minimum losses as compared to steam boilers. Non-Pressurized system operation even for temperatures around 250 0C as against 40 kg/cm2 steam pressure requirement in a similar steam system. Automatic control settings, which offer operational flexibility. Good thermal efficiencies as losses due to blow down, condensate drain and flash steam do not exist in a thermic fluid heater system.
- The combustion system comprises of a fixed grate with mechanical draft arrangements. This is how it works: The thermic fluid, which acts as a heat carrier, is heated up in the heater Circulated through the user equipment. There it transfers heat for the process through a heat exchanger Fluid is then returned to the heater.
- Assessment of a boiler.
- We will go through three topics under the assessment of a boiler: Assessment of the boiler itself Boiler blow down Boiler feed water treatment
- The performance parameters of a 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 and water quality can result in poor boiler performance. We will now discuss heat balance and boiler efficiency, which are important in assessing the boiler performance: A heat balance helps us to identify avoidable and unavoidable heat losses. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.
- We will start with looking at heat balance. The combustion process in a boiler can be described in the form of an energy flow diagram. This shows graphically how the input energy from the fuel is transformed into the various useful energy flows and into heat and energy loss flows. The thickness of the arrows indicates the amount of energy contained in the respective flows
- Heat balance is an attempt to balance the total energy that enters a boiler against the energy that leaves it. This figure illustrates the different typical losses that occurs while generating steam. (Question ) Does anyone have any suggestions of what the two major heat losses are? ( Discussion) ( Click once and answer reveals ) They are dry fly gas that represents a heat loss of 12.7% and heat loss as a result of steam in the flue gas of 8.1%. ( Click once for other heat losses to appear ) Other heat losses are due to moisture in the fuel and in the air, as well as unburnts in residue and radiation. ( Click once ) This leaves 73.8% of heat that goes to steam.
- The goal of a Cleaner Production and/or energy assessment must be to reduce the avoidable losses, i.e. to improve energy efficiency. The following losses can be avoided or reduced: Stack gas losses: Excess air (reduce to the necessary minimum which depends from burner technology, operation, operation (i.e. control) and maintenance) and Stack gas temperature (reduce by optimizing maintenance (cleaning), load; better burner and boiler technology). Losses by unburnt fuel in stack and ash (optimize operation and maintenance; better technology of burner). Blow down losses (treat fresh feed water, recycle condensate) Condensate losses (recover the largest possible amount of condensate) Convection and radiation losses (reduced by better insulation of the boiler).
- We will now look at boiler efficiency. Thermal efficiency of a boiler is defined as the percentage of heat energy input that is effectively useful in the generated steam. There are two different methods to assess boiler efficiency. (Click once ) They are direct and indirect method. (Click once ) In the direct method, the energy gain of the working fluid, that is the water and steam, is compared to the energy content of the boiler fuel. (Click once ) In the indirect method, the efficiency is calculated as the difference between the losses and energy input. We will start with looking at the methodology of the direct method of calculating boiler efficiency.
- The direct method of determining boiler efficiency is also known as the “input-output” method. This is because it only needs the useful output, which is steam, and the heat input, which is fuel, in order to evaluate the efficiency. The efficiency is evaluated by using this formula where hg is the enthalpy of saturated steam and hf is the enthalpy of feed water. (Click once ) The parameters to be monitored for the calculation of boiler efficiency through the direct method are: Quantity of steam generated per hour; the quantity of fuel used per hour; the working pressure and superheat temperature if any; the temperature of feed water; the type of fuel and gross calorific value, GVC, of the fuel.
- Advantages include that staff can quickly evaluate boiler efficiencies; it only requires a few parameters for computation and needs few instruments for monitoring. It is also easy to compare evaporation ratios with benchmark figures. The disadvantages are that it does not give clues to the operator as to why efficiency of system is lower. It does not calculate various losses accountable for various efficiency levels either.
- Now, we will look at the indirect method. This is also referred to as the heat loss method. The boiler efficiency can be calculated by subtracting the heat loss fractions from 100 as shown here. The principle losses that occur in a boiler are due to: Dry flue gas; evaporation of water and evaporation of moisture in fuel; moisture present in combustion air; unburnt fuel in fly ash; unburnt fuel in bottom ash, radiation and other unaccounted losses.
- The data that is required to calculate boiler efficiency according to the indirect method include: Ultimate analysis of fuel in terms of H2, O2, S, C, moisture content and ash content. Percentage of oxygen or CO2 in the flue gas; flue gas temperature; ambient temperature and humidity of air; as well as GCV of fuel. In case of solid fuels, the percentage combustible in ash and GCV of ash.
- Advantages with the indirect method include a complete mass and energy balance can be obtained for each individual stream, which makes it easier to identify options to improve boiler efficiency. Disadvantages are that it is time consuming and requires lab facilities for analysis.
- When water is boiled and steam is generated, any dissolved solids contained in the water remain in the boiler. 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. The control of total dissolved solids (TDS) is achieved by 'blowing down‘: a certain volume of water is blown off and is automatically replaced by feed water Since it is tedious and time consuming to measure TDS in a 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. (Click once) The quantity of blow down required to control boiler water solids concentration is calculated by using the following formula: Blow down in percentage = feed water TDS x Make up water / maximum permissible TDS in boiler water.
- There are two methods for blowing down the boiler, they are intermittent and continuous ( click once ). Intermittent blow down is given manually by operating a valve that is fitted to discharge pipe at the lowest point of boiler shell. This is to reduce the total dissolved solids as well as conductivity, pH, silica and phosphates without affecting the steam quality. Intermittent blow down requires large short-term increases in the amount of feed water put into the boiler and might therefore require large feed water pumps. It should also be noted that substantial amounts of heat energy are lost during an intermittent blow down. (Click once) A continuous blow down ensures constant TDS and steam purity at given steam load through a steady and constant dispatch of small stream of concentrated boiler water that is replaced by a steady and constant inflow of water. Once blow down valve has been set for certain conditions it does not require further operation interventions. Large quantities of heat is wasted but this can be recovered by blowing into a flash tank and generating flash steam. This type of blow down is common in high-pressure boilers.
- Good boiler blow down control can significantly reduce treatment and operational costs that include: Lower pretreatment costs Less make-up water consumption Reduced maintenance downtime Increased boiler life Lower consumption of treatment chemicals
- Producing quality steam on demand depends on properly managed water treatment to control steam purity, deposits and corrosion. A boiler is the sump of the boiler system. It ultimately receives all of the pre-boiler contaminants. Boiler performance, efficiency, and service life are direct products of selecting and controlling feed water used in the boiler. 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.
- Deposits and corrosion result in efficiency losses and may result in boiler tube failures and inability to produce steam. Deposits also act as insulators and therefore slow heat transfer. Different types of deposits affect the boiler efficiency differently why it may be useful to analyze the deposits for their characteristics. The most important chemicals in water that influence the formation of deposits in the boilers are the salts of calcium and magnesium. These are known as hardness salts. Calcium and magnesium bicarbonate dissolve in water to form an alkaline solution and are therefore known as alkaline hardness 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 hardness chemicals and form hard scales on boiler surfaces, which are difficult to remove. Silica in boiler water can rise to the formation of hard silicate scales. Silica can also associate with calcium and magnesium salts and form calcium and magnesium silicates of very low thermal conductivity. Silica can also give rise to deposits on steam turbine blades.
- There are two major types of boiler water treatment, namely internal and external water treatment: internal and external. We will first explain internal water treatment. Internal treatment involves adding chemicals to a boiler to prevent the formation of scale. Scale-forming compounds are converted to free-flowing sludge, which can be removed by blow down. This method is limited to boilers, where feed water is low in hardness salts where low pressure, high TDS content in boiler water is tolerated when only a small quantity of water is required to be treated. If these conditions are not met, then high rates of blow down are required to dispose off the sludge. They become uneconomical considering heat and water loss. Different types of water sources require different chemicals. Internal treatment alone is not recommended.
- External treatment is used to remove suspended solids, dissolved solids (particularly the calcium and magnesium ions which are major a cause of scale formation) and dissolved gases (oxygen and carbon dioxide). 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 external treatment processes we will explain next are: Ion exchange De-aeration (mechanical and chemical) Reverse osmosis
- In the ion-exchange process, the hardness is removed when the water passes through a bed of natural zeolite or synthetic resin and without the formation of any precipitate. The simplest way to remove hardness through ion exchanges is ‘base exchange’ in which calcium and magnesium ions are exchanged for sodium ions. Since the base exchanger only replaces the calcium and magnesium with sodium, it does not reduce the TDS content, and blow down quantity. It also does not reduce the alkalinity. Demineralization is the complete removal of all salts. This is achieved by using a “cation” resin that 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 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. Ion exchange processes can be used for almost total demineralization if required, as is the case in large electric power plant boilers.
- In de-aeration the dissolved gases such as oxygen and carbon dioxide are expelled by preheating the feed water before in enters the boiler. As all natural waters contain dissolved gases in solution such as carbon dioxide and oxygen, these are released as gases when heated and combine with water to form carbonic acid. This way, two very corrosive gases are removed. The removal of non-condensable gases from the boiler feed water is essential to the longevity to the boiler equipment and also to the safety of the operation. De-aeration can be done through either mechanical or chemical de-aeration processes or both together.
- (Note: this slide can be skipped if limited time available) Mechanical de-aeration is typically utilized prior to the addition of chemical oxygen scavengers. Mechanical de-aeration is based on that the removal of oxygen and carbon dioxide can be accomplished by heating the boiler feed water as it reduces the concentration of oxygen and carbon dioxide in the atmosphere surrounding the feed water. Mechanical de-aeration can be the most economical external water treatment. Mechanical de-aeration can be of two types: vacuum or pressure type. The vacuum type of de-aerator operates below atmospheric pressure can reduce the oxygen content in water to less than 0.02 mg/liter The pressure-type de-aerators operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure. This type can reduce the oxygen content to 0.005 mg/litre.
- (Note: this slide can be skipped if limited time available) While the most efficient mechanical deaerators reduce oxygen to very low levels (0.005 mg/liter), 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 blow down requirements and make-up water quality. Hydrazine reacts with oxygen to form nitrogen and water. It is invariably used in high pressure boilers when low boiler water solids are necessary, as it does not increase the TDS of the boiler water.
- When solutions of differing concentrations are separated by a semi-permeable membrane, water from a less concentrated solution passes through the membrane to dilute the liquid of high concentration. This is called osmosis. 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 solution. This is known as reverse osmosis.
- The figure illustrates a continuously operating reversed osmosis system with feed water and concentrated reject stream The quality of water produced depends upon the concentration of the solution on the high-pressure side and pressure differential across the membrane. This process is suitable for waters with very high TDS, such as sea water.
- We will go through different energy efficiency opportunities for boilers.
- There are numerous areas for energy efficiency improvements for boilers. We will briefly explain these 14 areas.
- The stack temperature should be as low as possible. However, it should not be so low that water vapor in the exhaust condenses on the stack walls. This is important in fuels containing significant sulphur as low temperature can lead to sulphur dew point corrosion. Stack temperatures greater than 200°C indicates potential for recovery of waste heat. It also indicates the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning. (click once) Typically, the flue gases leaving a modern 3-pass shell boiler are at temperatures of 200 to 300 oC. Thus, there is a potential to recover heat from these gases. The potential for energy savings depends on the type of boiler installed and the fuel used. (click once) Combustion air preheating is an alternative to feed water heating. In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 oC. Most gas and oil burners used in a boiler plant are not designed for high air-preheat temperatures.
- Incomplete combustion can arise from a shortage of air, surplus of fuel or poor distribution of fuel. You can identify it from the color of the smoke. A quite frequent cause of incomplete combustion is the poor mixing of fuel and air at the burner. The root cause can be For oil fires it can be the result from improper viscosity, worn tips, carbonization on tips and deterioration of diffusers or spinner plates. For coal firing, non uniform fuel size could be one of the reasons for incomplete combustion. 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. In sprinkler stokers, stoker grate condition, fuel distributors, wind box air regulation and over-fire systems can affect carbon loss. Increase in the fines in pulverized coal also increases carbon loss.
- Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion and to ensure satisfactory stack conditions for some fuels. The optimum excess air level varies with furnace design, type of burner, fuel and process variables. It can be determined by conducting tests with different air fuel ratios. Controlling excess air to an optimum level always results in reduction in flue gas losses; for every 1 percent reduction in excess air there is approximately 0.6 percent rise in efficiency. Various methods are available to control the excess air: Portable oxygen analyzers and draft gauges can be used to make periodic readings to guide the operator to manually adjust the flow of air for optimum operation. Excess air reduction up to 20 percent is feasible. The most common method is the continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 10-15 percent can be achieved over the previous system. The same continuous oxygen analyzer can have a remote controlled pneumatic damper positioner, by which the readouts are available in a control room. This enables an operator to remotely control a number of firing systems simultaneously
- The external surfaces of a shell boiler are hotter than the surroundings. Therefore, the surfaces lose heat to the surroundings depending on the surface area and the difference in temperature between the surface and the surroundings. The heat loss from the boiler shell is normally a fixed energy loss, irrespective of the boiler output. Repairing or augmenting insulation can reduce heat loss through boiler walls and piping. (click once) Uncontrolled continuous blow down is very wasteful. Automatic blowdown controls can be installed that sense and respond to boiler water conductivity and pH.
- 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. An estimated 1per cent efficiency loss occurs with every 22oC increase in stack temperature. Therefore, stack temperature should be checked and recorded regularly as an indicator of soot deposits. It is also estimated that 3 mm of soot can cause an increase in fuel consumption by 2.5per cent due to increased flue gas temperatures. Periodic off-line cleaning of radiant furnace surfaces, boiler tube banks, economizers and air heaters may be necessary to remove stubborn deposits. (click once) Reduction of boiler steam pressure is an effective means of reducing fuel consumption by as much as 1 to 2 per cent. Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery, a similar reduction in the temperature of the flue gas is obtained. Steam is generated at pressures normally dictated by the highest pressure and temperature requirements for a particular process.
- Variable speed control is an important means of achieving energy savings. Generally, combustion air control is affected 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. In general, if the load characteristic of the boiler is variable, the possibility of replacing the dampers by a VSD should be evaluated. (click once) The maximum efficiency of the boiler does not occur at full load, but at about two-thirds of the full load. In general, efficiency of the boiler reduces significantly below 25per cent of the rated load and operation of boilers below this level should be avoided as far as possible.
- Since, the optimum efficiency of boilers occurs at 65-85 percent of full load, it is usually more efficient, on the whole, to operate a fewer number of boilers at higher loads, than to operate a large number at low loads. (click once) The potential savings from replacing a boiler depend on the anticipated change in overall efficiency. A change in a boiler can be financially attractive if the existing boiler is old and inefficient; not capable of firing cheaper substitution fuel; over or under-sized for present requirements; and not designed for ideal loading conditions.