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Technology Characterization: Steam Turbines
Prepared for:
Environmental Environmental Protection Agency Climate Protection Partnership Div Washington, DC
Prepared by: Energy Nexus Group 1401 Wilson Blvd, Suite 1101 Arlington, Virginia 22209 Sign up to vote on this title
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Disclaimer:
The information included in these technology overviews is for information purposes only a gathered from published industry sources. Information about costs, costs, maintenance, operatio any other performance criteria is by no means representative of agency policies, definition determinations for regulatory or compliance purposes.
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TABLE OF CONTENTS
I NTRODUCTION AND SUMMARY .................................................... ......................................... APPLICATIONS .................................................. .......................................................................................................... ........................................................ ............. ............ Industrial and CHP Applications ...................................................................................... Combined Cycle Power Plants .......................................................................................... District Heating Systems.................................................................................................... Systems .................................................................................................... TECHNOLOGY DESCRIPTION ................................................ .................................................................................................. .................................................. Basic Process and Components......................................................................................... Components ......................................................................................... Types of Steam Turbines .................................................................................................... Design Characteristics ...................................................................................................... PERFORMANCE CHARACTERISTICS ............................................... ........................................................................................ ......................................... Electrical Efficiency........................................................................................................... Efficiency ........................................................................................................... Process Steam and Performance Tradeoffs....................................................................... Tradeoffs ....................................................................... CHP System Efficiency ...................................................................................................... Performance and Efficiency Enhancements ...................................................................... Capital Cost ....................................................................................................................... Cost ....................................................................................................................... Maintenance....................................................................................................................... Maintenance ....................................................................................................................... Fuels................................................................................................................................... Fuels ................................................................................................................................... Availability......................................................................................................................... Availability ......................................................................................................................... EMISSIONS ............................................... ....................................................................................................... ........................................................ ...................... ..................... Nitrogen Oxides (NO x) ....................................................................................................... Sulfur Compounds (SO x ) .................................................................................................... Particulate Matter (PM) .................................................................................................... Carbon Monoxide (CO) ..................................................................................................... Carbon Dioxide (CO2 ) ....................................................................................................... Typical Emissions .............................................................................................................. Boiler Emissions Control Options - NO x ........................................................................... Sign up to vote on this title Boiler Emissions Control Options - SO x ............................................................................ Useful Not useful
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Technology Characterization – Steam Turbines Introduction and Summary
Steam turbines are one of the most versatile and oldest prime mover technologies still in ge production. Power generation using steam turbines has been in use for about 100 years, they replaced reciprocating steam engines due to their higher efficiencies and lower Conventional steam turbine power plants generate most of the electricity produced in the U States. The capacity of steam turbines turbines can range from 50 kW to several hundred MWs for utility power plants. Steam turbines are are widely used for combined heat and power ( applications.
Unlike gas turbine and reciprocating engine CHP systems where heat is a byproduct of p generation, steam turbines normally generate electricity as a byproduct of heat (st generation. A steam turbine turbine is captive to a separate heat source and does not directly co fuel to electric energy. The energy is transferred transferred from the boiler to the turbine through pressure steam that in turn turn powers the turbine and generator. This separation of fun enables steam turbines to operate with an enormous variety of fuels, from natural gas to waste, including all types of coal, wood, wood waste, and agricultural byproducts (sugar bagasse, fruit pits, pits, and rice hulls). In CHP applications, steam steam at lower pressure is ext from the steam turbine and used directly or is converted c onverted to other forms of thermal energy.
Steam turbines offer a wide array of designs and complexity to match the desired applic and/or performance specifications. Steam turbines for utility service may have several pre casings and elaborate design features, all designed to maximize the efficiency of the power For industrial applications, steam turbines are generally of simpler single casing design and complicated for reliability reliability and cost reasons. CHP can be adapted to both utility and indu steam turbine designs.
Applications
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While steam turbines themselves are competitively priced compared to other prime mover costs of complete boiler/steam turbine CHP systems are relatively high on a per kW of cap
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must be able to accommodate the increased flow rate of lower-Btu steam. In such situation decision involves only the added capital cost of the steam turbine, its generator, controls electrical interconnection, with the balance of plant already in place. Similarly, many fac faced with replacement or upgrades of existing boilers and steam systems often conside addition of steam turbines, especially if steam requirements are relatively large compar power needs within the facility.
In general, steam turbine applications are driven by balancing lower cost fuel or avoided dis costs for the waste fuel, with the high capital cost and (hopefully high) annual capacity fact the steam plant plant and the combined combined energy plant-process plant application. For these r steam turbines are not normally direct competitors of gas turbines and reciprocating engines Industrial and CHP Applications
The primary locations of steam turbine based CHP systems is industrial processes where so waste fuels are readily readily available for boiler use. In CHP applications, steam extracted extracted fro steam turbine directly feeds into a process or is converted to another form of thermal en The turbine may drive an electric generator or equipment such as boiler feedwater pu process pumps, air compressors, and refrigeration chillers. chillers. Turbines as industrial drive usually a single casing machine, either single stage or multistage, condensing or non-conde depending on steam conditions and the value of the steam. Steam turbines operate at speed when driving an electric generator and operate over a speed range when driv refrigeration compressor. For non-condensing applications, steam exhausted from the turb turb at a pressure and temperature sufficient for the CHP heating application.
There were an estimated 19,062 MW of boiler/steam turbine CHP capacity operating i United States in 2000 located at over 580 industrial and institutional facilities. Figure 1 the largest amount of capacity is in the chemicals, primary primary metals, and paper industries. industries. and paper mills are often often an ideal industrial/CHP application for steam turbines. Such fac operate continuously, have a high demand for steam, and have on-site fuel supply at low, or negative costs (waste that would have to be otherwise disposed of). Sign up to vote on this title
Not useful byUseful existing steam turbine CHP capacity boiler fuel fuel type. While coals Figure 2 illustrates existing much of the installed boiler/steam turbine system base, large amounts of capacity are fuele wood, waste, and a variety of other fuels.
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Figure 1. Existing Boiler/Steam Turbine CHP by Industry 19,062 MW at 582 Sites
Comm/Inst 7%
Other Industrial 19%
Food 7%
Chemicals 22%
Primary Metals 11%
Paper 34%
Figure 2. Existing Boiler/Steam Turbine CHP by Boiler Fuel Type 19,062 MW at 582 Sites
Wood 13%
Waste 19%
Other 12%
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Combined Cycle Power Plants
The trend in power plant design is the combined cycle, which incorporates a steam turbin bottoming cycle with a gas turbine. Steam generated in in the heat recovery steam steam gen (HRSG) of the gas turbine is used to drive a steam turbine to yield additional electricity improve cycle efficiency. efficiency. Combined cycle CHP applications use an extraction-condensing of steam turbine. District Heating Systems
There are many cities and college campuses that have steam district heating systems w adding a steam turbine between the boiler and the distribution system may be an attra application. Often the boiler is capable of producing moderate-pressure moderate-pressure steam distribution system needs only low-pressure steam. In these cases, the steam turbine gene electricity using the higher-pressure steam, and discharges low-pressure steam into distribution system.
Technology Description Basic Process and Components
The thermodynamic cycle for the the steam turbine is the Rankine cycle. The cycle is the bas conventional power generating stations and consists of a heat source (boiler) that converts to high-pressure steam. steam. In the steam cycle, water is first pumped to to medium to high pressu is then heated to the boiling temperature corresponding to the pressure, boiled (heated liquid to vapor), and then most frequently superheated (heated to a temperature above th boiling). A multistage turbine expands the pressurized pressurized steam to lower pressure and the ste then exhausted either to a condenser at vacuum conditions or into an intermediate tempe steam distribution system that delivers the steam to the industrial or commercial applica The condensate from the condenser or from the steam utilization system returns to the feed pump for continuation of the cycle. Figure 3 shows theSign primary components up to vote on this title of a boiler/s turbine system.
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Figure 3. Components of a Boiler/Steam Turbine System Steam Turbine
Fuel Power out Boiler
Pump
Process or Condenser
Heat out
The steam turbine itself consists of a stationary set of blades (called nozzles) and a moving s adjacent blades (called buckets or rotor rotor blades) installed within a casing. The two sets of b work together such that the the steam turns the shaft of the turbine and the connected load. stationary nozzles accelerate the steam to to high velocity by expanding it to lower pressur rotating bladed disc changes the direction of the steam flow, thereby creating a force o blades that, because of the wheeled geometry, manifests itself as torque on the shaft on whic bladed wheel is mounted. The combination of torque and speed is the output power turbine.
The internal flow passages of a steam turbine are similar to those of the expansion section around gas turbine (indeed, gas turbine engineering came directlySign from steam turbine design up to vote on this title years ago). The main differences are the different gas density, molecular molecular weight, isen Useful Not useful expansion coefficient, and to a lesser extent viscosity of the two fluids.
Compared to reciprocating steam engines of comparable size, steam turbines rotate at m
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lines used primarily for space heating and food preparation, to medium pressure us industrial processes and cogeneration, to high-pressure use in utility power genera Generally, as the system gets larger the economics favor higher pressures and temperatures their associated heavier walled boiler tubes and more expensive alloys.
In general, utility applications involve raising steam for the exclusive purpose of p generation. Such systems also use a water-cooled condenser to exhaust the steam from turbine at the lowest practical pressure. pressure. Some utility turbines turbines have dual use, power gene and steam delivery at higher pressure into district heating systems or to neighboring indu plants at pressure, pressure, and consequently do not have condensers. These plants are actually cogeneration/CHP plants. Boilers
Steam turbines differ from reciprocating engines and gas turbines in that the fuel is burned piece of equipment, the boiler, which is separate from the power generation equipmen steam turbogenerator. As mentioned mentioned previously, this separation separation of functions enables turbines to operate with an enormous variety of fuels.
For sizes up to (approximately) 40 MW, horizontal industrial boilers are built. This enable car shipping, with considerable cost savings and improved quality as the cost and quali factory labor is usually both lower in cost and greater in quality than field labor. Large assembled boilers are typically capable of firing only gas or distillate oil, as there is inade residence time for complete combustion of most solid and residual fuels in such designs. field-erected industrial boilers firing solid and residual fuels bear a resemblance to utility b except for the actual solid fuel injection. injection. Large boilers usually burn pulverized coal, how intermediate and small boilers burning coal or solid fuel employ various types of solids feed Types of Steam Turbines
The primary type of turbine used for central power generation is the condensing turbine. T Sign up to vote on this title power-only utility turbines exhaust directly to condensers that maintain vacuum conditions Not discharge of the turbine. An array of tubes, cooled by Useful river, lake, oruseful cooling tower w 1 condenses the steam into (liquid) water. The cooling water condenses the steam turbine ex steam in the condenser creating the condenser vacuum. As a small amount of air leaks
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CHP applications use two types of steam turbines: non-condensing and extraction. Non-Condensing (Back-pressure) Turbine
Figure 4 shows the non-condensing turbine (also referred to as a back-pressure turbine) exh its entire flow of steam to the industrial process or facility steam mains at conditions close t process heat requirements.
Figure 4. Non-Condensing (Back-Pressure) Steam Turbine High pressure steam
Power Out Turbine
Low pressure steam To process
3
Usually, the steam sent into the mains is not much above saturation temperature. The “back-pressure” refers to turbines that exhaust steam at atmospheric pressures and above. specific CHP application establishes establishes the discharge pressure. pressure. 50, 150, and 250 psig are the typical pressure levels for steam distribution systems. systems. District heating systems most ofte the lower pressures, and industrial processes use the higher pressures. Industrial processes Sign upsteam to vote on this titlefor driving h include further expansion for mechanical drives, using small turbines Useful generation Not usefulcapability equipment that runs continuously for long periods. Power significantly when steam is used at appreciable pressure rather than being expanded to vacu a condenser. Discharging steam into a steam distribution system system at 150 psig can sac
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extracted steam may be used for process purposes in a CHP facility or for feedwater heating the case in most utility power plants.
Figure 5. Extraction Steam Turbine High pressure steam
Power Out Turbine
Medium/low pressure steam To process Condenser
The steam extraction pressure pressure may or may not be automatically regulated. Regulated extra permits more steam to flow through the turbine to generate additional electricity during pe of low thermal demand by the CHP system. In utility type steam turbines, there may be se extraction points, each at a different different pressure corresponding corresponding to a different different temperature. facility’s specific needs for steam and power over time determine the extent to which steam extraction turbine is extracted for use in the process.
In large, often complex, industrial plants, additional steam may be admitted (flows int casing and increases the flow in the steam path) to the the steam turbine. Often this happens using multiple boilers at different pressure, because of their historical historical existence. These turbines are referred to as admission turbines. At steam extraction and admission admission locations Sign up to vote on this title are usually steam flow control valves that add to the steam and control system cost.
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Numerous mechanical design features increase efficiency, provide for operation over a ran conditions, simplify manufacture and repair, and achieve other practical purposes. The
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sometimes returned to the boiler and reheated to high temperature and then returned t turbine for further (safe) (safe) expansion. In a few large, high pressure, utility utility steam systems systems i double reheat systems.
With these choices the designer of the steam supply system and the steam turbine hav challenge of creating a system design which delivers the (seasonally varying) power and s which presents the most favorable business opportunity to the p lant owners.
Between the power (only) output of a condensing steam turbine and the power and combination of a back-pressure steam turbine essentially any ratio of power to heat output c supplied. Back-pressure steam turbines turbines can be obtained with a variety of back pressures, fu f increasing the variability of the power-to-heat ratio. Design Characteristics
Custom design:
Steam turbines are designed to match CHP design press temperature requirements and to maximize electric effic while providing the desired thermal output.
Thermal output:
Steam tu turbines ar are ca capable of of op operating ov over a broad r steam pressures. Utility steam turbines operate with inlet inlet s pressures up to 3,500 psig and exhaust vacuum conditions a as one inch of Hg (absolute). (absolute). Steam turbines are custom des to deliver the thermal requirements of the CHP applica through use of backpressure or extraction steam at approp pressures and temperatures.
Fuel flexibility:
Steam turbines offer a wide range of of fuel flexibility using a v of fuel sources in the associated boiler or other heat so including coal, oil, natural gas, wood and waste products.
Reli Reliab abil iliity and and life life::
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Stea Steam m turbi urbine ne life ife is extr extrem emel ely y long long.. When hen prop proper erlly oper operat at Useful Not useful maintained (including proper control of boiler water chemi steam turbines are extremely reliable, only requiring over every several years. They require controlled thermal transie
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Performance Characteristics Characteristics Electrical Efficiency
The electrical generating efficiency of standard steam turbine power plants varies from a hi 37% HHV4 for large, electric utility plants designed for the highest practical annual cap factor, to under 10% HHV for small, simple plants which make electricity as a byprodu delivering steam to processes or district heating systems.
Steam turbine thermodynamic efficiency (isentropic efficiency) refers to the ratio of p actually generated from the turbine to what would be generated by a perfect turbine wi internal losses using steam at the same inlet conditions and discharging to the same downs pressure (actual enthalpy drop divided by the isentropic enthalpy drop). Turbine thermodyn efficiency is not to be confused with electrical generating efficiency, which is the ratio o power generated to total fuel input to the cycle. Steam turbine thermodynamic effi measures how efficiently the turbine extracts power from the steam itself. Multistage (mod to high-pressure ratio) steam turbines have thermodynamic efficiencies that vary from 65% small (under 1,000 kW) kW) units to over 90% for large industrial and utility utility sized units. single stage steam turbines can have efficiencies efficiencies as low as 50%. When a steam turbine exh to a CHP application, the turbine efficiency is not as critical as in a power only conde mode. The majority of the energy not extracted by the steam turbine satisfies satisfies the thermal Power only applications waste the exhaust turbine steam energy in condensers.
Table 1 summarizes performance characteristics for typical commercially available s turbines and for typical boiler/steam CHP systems in the 500 k W to 15 MW size range.
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Table 1. Boiler/Steam Turbine CHP System Cost and Performance Characteristics* Cost & Performance Characteristics5
System 1
System 2
Steam Turbine Parameters Nominal Electricity Capacity (kW) Turbine Type Equipment Cost ($/kW) 6 Total Installed Cost ($/kW) 7 Turbine Isentropic Efficiency (%) 8 Generator/Gearbox Efficiency (%) Steam Flow (lbs/hr) Inlet Pressure (psig) Inlet Temperature (° Fahrenheit) Outlet Pressure (psig) Outlet Temperature (° Fahrenheit) CHP System Parameters
500 Back Pressure 540 918 50% 94% 21,500 500 55 0 50 29 8
3,000 Back Pressure 225 385 70% 94% 126,000 600 575 150 366
80% 6.4% 26.7 19.6 5,740 79.6% 0.09 4,515 75.6%
80% 6.9% 147.4 107.0 31,352 79.5% 0.10 4,568 75.1%
Boiler Efficiency (%), HHV CHP Electric Efficiency (%), HHV9 Fuel Input (MMBtu/hr)10 Steam to Process (MMBtu/hr) Steam to Process (kW) Total CHP Efficiency (%), HHV11 Power/Heat Ratio 12 Net Heat Rate (Btu/kWh) 13 Effective Electrical Efficiency (%), HHV14
System 3 15,000 Back Press 205 349 80% 97% 450,000 700 650 150 366 80% 9.3% 549.0 386.6 113,291 79.7% 0.13 4,388 77.8%
* For typical systems commercially available in 2002
Sign up to vote on this title Characteristics for “typical” commercially commercially available steam turbine generator generator systems. Steam turbine data bas Useful Notturbine. useful MW information from: TurboSteam, Inc for 500 kW and 3 MW; General Electric for 15 6 Equipment cost includes turbine, gearbox, generator, controls, and switchgear; boiler and steam system cos not included. 5
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Operating Characteristics
Steam turbines, especially smaller units, leak steam around blade rows and out the end s When an end is at a low pressure, as is the case with condensing steam turbines, air can also into the system. system. The leakages cause less power to be produced than expected, and the m water has to be treated to avoid boiler and turbine material material problems. Air that has leak needs to be removed, which is usually done by a compressor removing non-condensable from the condenser.
Because of the high pressures used in steam turbines, the casing is quite thick, and consequ steam turbines exhibit large thermal thermal inertia. Steam turbines must must be warmed up and c down slowly to minimize the differential expansion between the rotating blades and stationary parts. parts. Large steam turbines can take over ten hours to warm up. While smaller have more rapid startup times, steam turbines differ appreciably from reciprocating eng which start up rapidly, and from gas turbines, which can start up in a moderate amount of and load follow with reasonable rapidity. rapidity. Steam turbine turbine applications usually continuously for extended periods, although the steam fed to the unit and the power deli may vary (slowly) during such periods of continuous operation. Process Steam and Performance Tradeoffs
The amount and quality of recovered heat is a function of the entering steam conditions an design of the steam turbine. Exhaust steam from the the turbine is used directly in a process converted to other forms of thermal thermal energy, including hot or chilled chilled water. Steam discharg extracted from a steam turbine can be used used in a single- or double effect absorption absorption chiller. steam turbine can also be used as a mechanical drive for a centrifugal chiller. CHP System Efficiency
Steam turbine CHP systems generally have low power to heat ratios, typically in the 0.05 t range. This is because electricity electricity is a byproduct of heat generation, with the system 15 optim Sign up to vote on this title for steam production. Hence, while steam turbine turbine CHP system electrical efficiency may Useful Not low, it is because the primary objective is to produce large amounts ofuseful steam. The effe 16 electrical efficiency of steam turbine systems, however, is generally high, because almo the energy difference between the high-pressure boiler output and the lower pressure tu
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enhancing options to incorporate in terms of their incremental effects on performance and cost, and select appropriate steam turbine inlet inlet and exhaust conditions. Often the steam is going into a system that already exists and is being modified, so that a number of steam sy design parameters are already determined by previous decisions, which exist as system hard characteristics.
As the stack temperature of the boiler exhaust combustion products still contain some tradeoffs occur regarding the extent of investment in heat reclamation equipment for the sa efficiency improvement. Often the stack exhaust temperature is is set at a level where furthe recovery would result in condensation of corrosive chemical species in the stack, consequential deleterious effects on stack life and safety.
Steam Reheat Higher pressures and steam reheat increase power generation efficiency in large industrial utility) systems. systems. The higher the pressure ratio (the ratio of the the steam inlet pressure to the s exit pressure) across the steam turbine, and the higher the steam inlet temperature, the power it will produce per unit of mass flow. To avoid condensation the inlet steam tempe tempe is increased to the economic practical limit of materials. materials. This limit is now generally in in the of 800 to 900°F for small industrial steam turbines.
When the economically practical limit of temperature is reached, the expanding steam can a condition of temperature and pressure where condensation to (liquid) (liquid) water begins. amounts of water droplets can be tolerated in the last stage of a steam turbine provided tha droplets are not too large or numerous. At pressures higher than that that point the steam is ret to the boiler and reheated in temperature and then returned to the expansion steam turbin further expansion. When returned to the next stage of the turbine, the steam steam expands w condensation.
Combustion Air Preheating In large industrial systems, air preheaters recover heat from the boiler exhaust gas stream use it to preheat the combustion air, air, thereby reducing fuel consumption. Boiler combusti Sign up to vote on this title preheaters are large versions of the heat wheels used for the same purpose on industrial furn
Capital Cost
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electrostatic precipitator cost can reach 80% of the steam turbine cost. A typical coal/wood boiler costs more than the steam turbine.18 The cost of complete solid fuel cogeneration p varies with many factors, with fuels handling, pollution control equipment and boiler co being major cost items. items. Because of both the size of such plants and the diverse sources components, solid fuel cogeneration plants invariably involve extensive system engineerin field labor during construction. Typical complete plant costs run well over $1,000/kW, little generalization except that for the same fuel and configuration, costs per kW of cap generally increase as size decreases.
Steam turbine costs exhibit a modest extent of irregularity, as steam turbines are made in with finite steps between the the sizes. The cost of the turbine is generally the same for the and lower limit of the steam flowing through it, so step-like behavior is sometimes seen in s turbine prices. Since they come in specific size size increments, a steam turbine that is used upper end of its range of power capability costs lest per kW generated than one that is used lower end of its capability.
Often steam turbines are sold to fit into an existing plant. In some of these application specifications, mass flow, pressure, temperature and backpressure or extraction condition not conditions for which large competition competition exists. These somewhat unique machines machines are expensive per kilowatt than are machines for which greater competition exists, for three rea 1) a greater amount of custom engineering and manufacturing setup may be required; 2) th less potential for sales of duplicate or similar units; and 3) there are fewer competitive bid The truly competitive products are the “off-the-rack” type machines, while “custom” mac are naturally more expensive.
Steam turbine prices vary greatly with the extent of competition and related manufact volumes for units of desired size, inlet and exit steam conditions, rotational speed standardization of construction. Quoted prices are are usually for an assembled assembled steam tu electrical generator package. The electrical generator can account for 20% to 40% assembly. As the steam turbine/electrical generator package is heavy, due in large part heavy walled construction of the high-pressure turbine casing, it requires careful mounting Sign up to vote on this title appropriate pedestal. The installation and connection to the boiler through high pressure Usefulexpertise. Not useful temperature steam pipes require engineering and installation installation As the high-pre steam pipes typically vary in temperature by 750°F between cold standby/repair status an power status, care must be taken in installing a means to accommodate the differential expa
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such small sizes, and the fraction of total cost due to system engineering and field constru are high.19
Boiler combustion produces the steam for a steam turbine and the temperature of the ste limited by furnace heat transfer design, manufacturing consideration, and boiler tube b design. Higher heat fluxes in the boiler enable more compact boilers, with less boiler material to be built; however, higher heat fluxes also result in higher boiler tube temperatur the need for the use of a higher grade (adequate strength at higher temperature) boiler material. Such engineering economic tradeoffs between temperature temperature (with consequ increases in efficiency) and cost appear throughout the steam plant.
Because of the temperature limitation on boiler tubes, which are exposed to the high temper and heat flux in the furnace, steam turbine material material selection is easier. An often-overlo component in the steam power system is the steam (safety) stop valve, which is immed ahead of the steam turbine and is designed to be able to experience the full temperature pressure of the steam supply. This safety valve is necessary because if the generator electric were lost (an occasional occurrence), the turbine would rapidly overspeed and destroy i Other accidents are possible, supporting the need for the turbine stop valve, which significant cost to the system Maintenance
Steam turbines are rugged units, with operational life often exceeding 50 years. Maintena simple, comprised mainly of making sure that all fluids (steam flowing through the turbine the oil for the bearing) are always clean and at the proper temperature. The oil lubric system must be clean and at the correct operating temperature and level to maintain p performance. Other items include inspecting auxiliaries such as lubricating-oil lubricating-oil pumps, c and oil strainers and checking safety devices such as the operation of overspeed ov erspeed trips.
In order to obtain reliable service, steam turbines require long warmup periods so that ther minimal thermal expansion stresses and wear concerns. Steam turbine maintenance cos Sign up to vote on this title quite low, typically less than $0.004 per kWh. Boilers and any associated solid fuel fuel proce Useful Not useful and handling equipment that is part of the boiler/steam turbine plant require their own typ maintenance.
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Fuels
Industrial boilers operate on a variety of fuels, including wood, coal, natural gas, oils (inclu residual oil), municipal solid waste, waste, and sludges. The fuel handling, storage, storage, and prepa equipment needed for solid fuels add considerably to the cost of an installation. installation. Thus, such are used only when a high annual capacity factor is expected of the facility, or when the material has to be disposed of to avoid an environmental or space occupancy problem. Availability
Steam turbines generally have 99% plus availability with longer than one year bet shutdowns for maintenance and inspections. This high level of availability applies only steam turbine, not the boiler or HRSG that is supplying the steam.
Emissions
Emissions associated with a steam turbine are dependent on the source of the steam. turbines can be used with a boiler firing any one or a combination of a large variety of sources, or they they can be used used with a gas turbine in a combined cycle configuration. configuration. emissions vary depending on fuel type and environmental conditions. Boilers emissions nitrogen oxide (NOx), sulfur oxides (SOx), particulate matter (PM), carbon monoxide (CO) carbon dioxide (CO2). Nitrogen Oxides (NO x)
The pollutant referred to as NOx is a mixture of (mostly) nitric oxide (NO) and nitrogen di (NO2) in variable composition. composition. NOx forms by three mechanisms: thermal NOx, prompt NO fuel-bound NOx. In industrial industrial boilers, boilers, thermal thermal and fuel-bound are the predominant formation mechanisms. Thermal NOx, formed when nitrogen and oxygen in the combustio combine in the flame, comprises the majority of NOx formed during the combustion of gase light oils. Fuel-bound NOx is associated with oil fuels and forms when nitrogen in the fue oxygen in the combustion air react. Sign up to vote on this title
The most significant factors influencing the level of NO x emissions from a boiler are the f Useful Not useful temperature and the amount of nitrogen in the fuel. Other factors include excess air leve combustion air temperature.
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lower than residual oils. For industrial and commercial boilers, the most effective effective method o control is use of higher-grade fuel, and ensuring proper burner setup, adjustment maintenance. Carbon Monoxide (CO)
CO forms during combustion when carbon in the fuel oxidizes incompletely, ending up a instead of CO2. Older boilers generally have higher levels of CO than new equipment older burner designs do not have CO controls. Poor burner design or firing condition responsible for high levels of CO boiler emissions. emissions. Proper burner maintenance or equip upgrades, or using an oxygen control package, can control CO emissions successfully. Carbon Dioxide (CO 2 )
While not considered a regulated pollutant in the ordinary sense of directly affecting p health, emissions of carbon dioxide are of concern due to its contribution to global war Atmospheric warming occurs because solar radiation readily penetrates to the surface o planet but infrared (thermal) radiation from the surface is absorbed by the CO2 (and polyatomic gases such as water vapor, methane, unburned hydrocarbons, refrigerants and vo chemicals) in the atmosphere, with resultant resultant increase in temperature of the atmosphere. amount of CO2 emitted is a function of both fuel carbon content and system system efficiency. Th carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and free) coal is 66 lbs carbon/MMBtu.
Typical Emissions
Table 2 below illustrates typical emissions of NOx, PM and CO for boilers by size of s turbine system and by fuel type. Table 2. Typical Boiler Emissions Ranges
Boiler Fuel Coal
Sign up to vote on this title Systems 2 and 3
System 1 500 kW NOx N/A
CO N/A
PM N/A
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NOx 0 20-1 24
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CO 0 02-0 7
P
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Boiler Emissions Control Options - NO x
NOx control has been the primary focus of emission control research and development in bo The following provides a description of the most prominent emission control approaches. Combustion Process Emissions Control
Combustion control techniques are less costly than post-combustion control methods an often used on industrial boilers for NOx control. Control of combustion combustion temperature has bee principal focus of combustion process control in boilers. Combustion control requires requires trad – high temperatures favor complete burn up of the fuel and low residual hydrocarbons and but promote NOx formation. Lean combustion dilutes the combustion process and combustion temperatures and NOx formation, and allows a higher compression ratio or firing pressures resulting resulting in higher efficiency. However, if the mixture is too lean, misfirin incomplete combustion occurs, increasing CO and VOC emissions.
Flue Gas Recirculation (FGR) FGR is the most effective technique for reducing NOx emissions from industrial boilers inputs below 100 MMBtu/hr. With FGR, a portion of the relatively cool boiler exhaust gas enter the combustion process, reducing the flame temperature and associated thermal formation. It is the the most popular and effective NOx reduction method for firetube and wate boilers, and many applications can rely solely on FGR to meet environmental standards.
External FGR employs a fan to recirculate the flue gases into the flame, with external p carrying the gases from the stack to the burner. A valve responding to boiler input contro recirculation rate. Induced FGR relies on the combustion air fan for flue gas recirculatio portion of the gases travel via ductwork or internally to the air fan, where they are premixed combustion air and introduced into the flame flame through the burner. Induced FGR in newer de uses an integral design that is relatively uncomplicated and reliable. The physical limit to reduction via FGR is 80% in natural gas-fired boilers andSign 25%upfor standard to vote on thisfuel title oils.
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Low Excess Air Firing (LAE) Excess air ensures complete combustion. However, excess air levels greater than 45% can in increased NO formation, because the excess nitrogen and oxygen in the combustio
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standard No. 2 oil. NOx reductions of up to 70% over NOx emissions from standard No. 2 have been achieved achieve d in firetube boilers utilizing flue gas recirculation.
Burner Modifications Modifying the design of standard burners to create a larger flame achieves lower f temperatures and results in lower thermal NOx formation. While most boiler types and size accommodate burner modifications, it is most effective for boilers firing natural gas and dis fuel oils, with little little effectiveness in heavy oil-fired oil-fired boilers. Also, burner modifications mu complemented with other NOx reduction methods, such as flue gas recirculation, to comply the more more stringent stringent environmental regulations. regulations. Achieving low NOx levels (30 ppm) thr burner modification alone can adversely impact boiler operating parameters such as turnd capacity, CO levels, and efficiency.
Water/Steam Injection Injecting water or steam into the flame reduces flame temperature, lowering thermal formation and overall NOx emissions. However, under normal normal operating conditions, water/ injection can lower boiler efficiency by 3 to 10%. Also, there is a practical limit limit to the am that can be injected without causing condensation-related problems. This method employed in conjunction with other NOx control techniques such as burner modifications o gas recirculation. When used with natural gas-fired boilers, boilers, water/steam injection can res NOx reduction of up to 80%, with lower reductions achievable in oil-fired boilers. Post-Combustion Emissions Control
There are several types of exhaust gas treatment processes that are applicable to indu boilers.
Selective Non-Catalytic Reduction (SNCR) In boiler SNCR, a NOx reducing agent such as ammonia or urea is injected into the b exhaust gases at a temperature in the 1,400 to 1,600°F range. The agent breaks down the N the exhaust gases into water and atmospheric nitrogen (N2). While SNCR can reduce boiler Sign up to vote on this title emissions by up to 70%, it is difficult to apply to industrial boilers that modulate or Useful be Not useful frequently because to perform properly, the agent must introduced at a specific flu temperature. The location of the exhaust gases at the necessary temperature is cons changing in a cycling boiler.
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SCR is however costly to use and can only occasionally be justified on boilers with inputs o than 100 MMBtu/hr. SCR requires on-site storage of ammonia, a hazardous chemica addition, ammonia can “slip” through the process unreacted, contributing to environm health concerns. Boiler Emissions Control Options - SO x
The traditional method for controlling SOx emissions is dispersion via a tall stack to limit gr level emissions. emissions. The more stringent SOx emissions requirements in force today demand th of reduction methods as well. These include use of low sulfur fuel, fuel, desulfurizing fuel, an gas desulfurization (FGD). Desulfurization of fuel primarily primarily applies to coal, and, like FG principally used for utility boiler emissions control. Use of low sulfur fuels is the mos effective SOx control method for industrial boilers, as it does not require installation maintenance of special equipment.
FGD systems are of two types: non-regenerable and regenerable. The most common, common, regenerable, results in a waste product that requires proper disposal. Regenerable FGD con the waste product into a product that is saleable, such as sulfur sulfur or sulfuric acid. FGD re SOx emissions by up to 95%.
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