Stationary Combustion First system to evaluate Pulverized coal combustion for electricity generation Reasons for doing this: 1. Dominant technology domestic use of coal. 85-90% of coal used goes to electric power generation 2. A majority of electricity generated in US comes from PC fired power plants A couple of years ago, >50% of electricity was generated from coal Changed recently – only about 40% currently due to increased use of natural gas 3. Sort of “state of the art” large scale electricity generation Base case to compare to other technologies
Stationary Combustion Begin by looking at For various coals overview of technology Lignite and subbituminous Will then “dissect” overall coal – size spec 70-75% plant into smaller “boxes” -200 mesh (≤ 74 μm) Try to see where Bituminous coal – size inefficiencies in energy are spec usually 80-85% -200 Where can improvements mesh be made Anthracite can be used for PC combustion, but little First step market for anthracite Pulverize coal currently (high carbon content)
Stationary Combustion Pulverization means the coal Boiler usually a rectangular will undergo one or more size steel box. reduction operations Will ignore these for now For now, can ignore: But need to recognize that 1. how burners are designed crushing or grinding operations 2. the array of the total number are energy intensive of burners Done onsite represents parasitic energy losses and reduces electricity out of plant Often, last stage of grinding is done in mill just ahead of feed to burners Mills can be swept with hot gases to remove moisture Pulverized coal is blown with air through burners into the boiler Coal & Air Coal & Air
Stationary Combustion As coal injected into boiler, Combustion occurs in 2 pulverized coal ignites and stages burns in a large, hot 1. Volatiles are driven out turbulent flame of the coal (thermally), ignite and burn in gas phase 2. the residual solid char (i.e., fixed carbon) – ignites and burns as a process of heterogeneous combustion called char burnoutCoal & Air Coal & Air
First major energy conversion Hot combustion gases CHEMICAL TO THERMAL proceed through a flue Chemical – enthalpy of combustion of the fuel (chimney) as they exit ΔHcombs boiler Additional tubes/pipes are Generation of heat is to get mounted in the flue as well water to boil One major wall of boiler is made Here dominant mechanism of tubes/pipes through which is convection. Region in water circulates – water wall boiler is sometimes called At this point dominant heat convection section or transfer mechanism is radiation convection pass Sometimes called the radiant section of boiler Hot Gasses
Electricity Generation Follow the steam path and Turbine is coupled directly consider environmental to rotary generator. issues Third major energy conversion High-pressure, high- MECHANICAL TO temperature steam fed to ELECTRICAL turbine Second major energy Therefore, net conversion conversion to plant is THERMAL TO CHEMICAL TO MECHANICAL ELECTRICAL Enthalpy in steam Efficiency combined, converted to rotary roughly – mechanical work in turbine eC = 33% Exact number varies with age of plant, how well it’s run, parasitic energy losses, etc.
Steam Steam exits turbine and is Condenser heat is transferred condensed back to water. from steam (including heat & condensation) to condenser Typically condenser is heat water exchanger that uses natural water source as working fluid. Therefore water leaving condenser will be hot or warm Why many power plants are located along rivers or on If dumped directly into water lakes source and hot, will alter microclimate and local Condensate is returned to the ecology boiler Called thermal pollution Water must be extremely pure Cooling towers used to cool Avoid corrosion in boilers condenser effluent tubes and/or turbine blades Can be stricter than for drinking water
Steam Flow Steam flow and High P Steam ,T condensing water flow Turbine complexBoiler Low P T Steam , Also have to consider environmental issues Water Condenser Water Pump Water Air Reservoir Water Cooling Tower Air
Environmental Issues Ash Ash partitions between Fly ash (PM) material falling to the bottom SOX of the boiler and fine NOX particles entrained in the hot CO2 combustion gases Sulfur undergoes conversion to SO2 and SO3, or SOX Small amount of NOX comes from nitrogen in coal (fuel NOX) Most comes from nitrogen in air at high temperatures of combustion system (thermal NOX)Bottom ash N2 + O2 2NO N2 + 2O2 2NO2
Pollutant Clean Up Fly ash Typically dealt with in one of two technologies Electrostatic precipitator Baghouse filtration SOX is commonly treate in scrubbers where it reacts with aqueous slurry of lime Ca(OH)2 + SO2 + ½ O2 CaSO4 + H2O Ca(OH)2 + SO3 CaSO4 + H2O Precipitated CaSO4 called scrubber sludge Need to dispose of ~25% is used in sheetrock (wallboard)
Pollutant Clean Up NOX can be treated by reduction with ammonia 6NO + 4NH3 5N2 + 6H2O 6NO + 8NH3 7N2 + 12H2O Or urea 6NO + 2 CO(NH2)2 5N2 + 4H2O + 2CO2 6NO + 4 CO(NH2)2 7N2 + 5H2O + 4CO2 Alternative technologies involve fuel gas recirculation or staged combustion (e.g., overfire air or low-NOX burners)
Pollutant Clean Up Environmental Whole operation is technologies represent complex plant parasitic energy losses Several factors impact eC Anything done to cool Incomplete combustion inside of boiler (to combat Ineffective heat transfer thermal NOX formation) Heat losses reduces steam temp, which Inefficiencies in turbine will affect efficiencies in the turbine Inefficiencies in generator Parasitic energy losses Also CO2 production Problem with putting CCS Next lecture will begin to on power plant stem partly examine these effects from CO2 concentration in flue gas being ~10-15% Makes effective carbon capture difficult to do
Stationary Combustion Electricity production in PC-fired power plant involved 3 major energy conversion processes 1. Chemical to thermal – enthalpy of comb of coal enthalpy in steam 2. Thermal to mechanical – enthalpy in steam rotation of turbine/generator 3. Mechanical to electrical – rotation of generator electrical energy And with these energy conversions, if draw “box” around whole process (eC or “big box” conversion), value of eC = 33% Not particularly good. If viewed another way, two out of three tons of coal is wasted. Want to determine 1. where the inefficiencies are and 2. what, if anything, can be done about them. Therefore, useful to divide “big box” into three smaller “boxes”, corresponding to one of three energy conversion processes
Stationary Combustion Chemical Energy Will concentrate on boiler “box” today Effective energy output going to be energy input minus the losses. So we can look at these different items as “little” boxes. Major energy input will be enthalpy of combustion of the fuel As noted previously, Fuel from coal is pulverized to 75-85% that is ≤ 74 μm Typically, last stage of pulverization is effected by pulverizers directly upstream of the burners Often pulverizer output is swept directly into the burners
Stationary Combustion Chemical Energy Combustion occurs in two steps Volatiles from coal ignite and burn in homogenous gas- phase combustion Char ignites and burns out in heterogeneous gas-solid combustion Time for combustion of a coal particle is 0.25-1 sec Important to assure that abundant oxygen is available for complete combustion If reaction 2C + O2 2CO occurs to any extent Less heat is evolved than for C + O2 CO2 Incomplete combustion (or non-combustion) leaving unburned carbon can lead to smoke and soot emission in addition to being wasteful of energy. Boilers are then run with 20-30% excess air
Stationary Combustion Chemical Energy Two other energy inputs, though neither is as important as the fuel combustion Previously discussed convection section of boiler 1. At the end of the convection section, before gaseous products of combustion go to the stack, is a heat exchanger to preheat combustion air Typically combustion air is used at 55-80°C Can count the “extra” heat as a contribution to the total energy input And, 2. Small but measureable contribution comes from the fact that air will be passing through devices like fans, pumps, pulverizers, etc. These devices will add slight amount of heat to the air Where does this heat go? Want it to go to energy in steam generated
Stationary Combustion Chemical Energy Heat transferred to water/ Method of interference steam by 2 mechanisms Some ash can adhere to the 1. Radiation – in furnace tubes in the convection section of boiler, this is section or on the water wall dominant heat transfer of the radiant section. mechanism Deposition results from 2. Convection – in flue, hot partially or wholly molten combustion gases enter, and components of ash this is the dominant heat impacting one of these heat transfer mechanism transfer surfaces and Each accounts for about sticking there 50% of heat transfer Continued impacting builds up sticky layer on steel surface This will trap particles that are not molten
Interference with Ash Ash adhering to heat Reduce heat transfer to the transfer surfaces is solid, water/steam problem called ash deposition or ash fouling To overcome and maintain same rate of steam If deposits are semi- or fully production (and electricity molten, they are called slag production) is to increase deposits temperature in boiler Can also be referred to as Produces vicious cycle of slagging more fouling or slagging, which requires still higher From perspective of boiler temperatures, causing efficiency, ash or slag more fouling or slagging…. deposits act as insulators
Interference with Ash Remedial measures for Hot combustion gases fouling/slagging pass a succession of Soot blowing steam tubes in the Shotgunning convection section Dynamiting To recover as much heat as possible Coming off line for detailed maintenance At very end – heat exchanger to preheat the Boiler structure itself is combustion air extremely hot Peak temp of “fireball” At this point, gases could be ~1500°C entering stack will be Not all heat will be above ambient temp captured internally – some heat lost through walls
Energy Losses in Boiler Energy losses include: Moisture that formed chemically as a Energy in “so-called” dry gas result of combustion of hydrogen in – sensible heat in the gas fuel energy is the moisture in gas 4CH0.5 + 4 ½O2 H2O + 4CO2 Stack gas will be at some Moisture that came into the system temp above the dew point, with combustion air have to consider sensible All air contains some amount of heat and latent heat of moisture moisture Other class of loss – Unaccounted Where does moisture in Losses stack gas come from? This could be a highly variable Moisture present in fuel number and vaporized during combustion However, in practice when boiler efficiency tests are done, results are not accepted if losses “unaccounted for” are > 2%
Energy Losses in Boiler So in summary, here are Since EnergyOUT = EnergyIN – Losses energy inputs and energy losses, where * denotes the Efficiency = (EnergyIN – Losses)/ big contributions EnergyIN Energy In The following are quantities of losses * Enthalpy of combustion estimated for a boiler running on 25% fuel excess air Preheating combustion air Stack heat loss = 9% Air heating by fans, blowers, Loss in heat transfer & unaccounted loss etc. = 6% Incomplete combustion = 0.5% Losses Furnace heat loss = 0.5% *Stack gas losses * Inefficient heat transfer Therefore boiler efficiency is 84% and unaccounted loss Incomplete combustion Long way from combined efficiency of 33% Furnace heat loss Need to look at efficiency of turbine and generator
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