EGEE 451 Energy Conversion Processes

Lecture 2 – Stationary Combustion Processes 9/07/12

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

overview of technology   Will then “dissect” overall

plant into smaller “boxes”

  Try to see where inefficiencies in energy are

  Where can improvements be made

  First step   Pulverize coal

  For various coals   Lignite and subbituminous

coal – size spec 70-75% -200 mesh (≤ 74 μm)

  Bituminous coal – size spec usually 80-85% -200 mesh

  Anthracite can be used for PC combustion, but little market for anthracite currently (high carbon content)

Stationary Combustion   Pulverization means the coal

will undergo one or more size reduction operations   Will ignore these for now

  But need to recognize that crushing or grinding operations are energy intensive

  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

  Boiler usually a rectangular steel box.

  For now, can ignore:   1. how burners are designed

  2. the array of the total number of burners

Coal & Air Coal & Air

Stationary Combustion   As coal injected into boiler,

pulverized coal ignites and burns in a large, hot turbulent flame

  Combustion occurs in 2 stages   1. Volatiles are driven out

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 burnout

Coal & Air Coal & Air

  First major energy conversion   CHEMICAL TO THERMAL

  Chemical – enthalpy of combustion of the fuel

  ΔHcombs

  Generation of heat is to get water to boil   One major wall of boiler is made

of tubes/pipes through which water circulates – water wall

  At this point dominant heat transfer mechanism is radiation

  Sometimes called the radiant section of boiler

  Hot combustion gases proceed through a flue (chimney) as they exit boiler   Additional tubes/pipes are

mounted in the flue as well

  Here dominant mechanism is convection. Region in boiler is sometimes called convection section or convection pass

Hot Gasses

Electricity Generation   Follow the steam path and

consider environmental issues

  High-pressure, high-temperature steam fed to turbine   Second major energy

conversion

  THERMAL TO MECHANICAL

  Enthalpy in steam converted to rotary mechanical work in turbine

  Turbine is coupled directly to rotary generator.   Third major energy

conversion

  MECHANICAL TO ELECTRICAL

  Therefore, net conversion to plant is   CHEMICAL TO

ELECTRICAL

  Efficiency combined, roughly –

  eC = 33%

  Exact number varies with age of plant, how well it’s run, parasitic energy losses, etc.

Steam   Steam exits turbine and is

condensed back to water.

  Typically condenser is heat exchanger that uses natural water source as working fluid.

  Why many power plants are located along rivers or on lakes

  Condensate is returned to the boiler   Water must be extremely pure

  Avoid corrosion in boilers tubes and/or turbine blades

  Can be stricter than for drinking water

  Condenser heat is transferred from steam (including heat & condensation) to condenser water

  Therefore water leaving condenser will be hot or warm

  If dumped directly into water source and hot, will alter microclimate and local ecology   Called thermal pollution

  Cooling towers used to cool condenser effluent

Steam Flow   Steam flow and

condensing water flow complex

  Also have to consider environmental issues

Boiler

Pump

Condenser

Turbine

Cooling Tower

Air

Air

Water

Water

Reservoir

High P,T Steam

Low P, T Steam

Water

Water

Environmental Issues   Ash   Ash partitions between

material falling to the bottom of the boiler and fine particles entrained in the hot 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)   N2 + O2 2NO   N2 + 2O2 2NO2

Fly ash (PM) SOX NOX CO2

Bottom ash

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

technologies represent parasitic energy losses

  Anything done to cool inside of boiler (to combat thermal NOX formation) reduces steam temp, which will affect efficiencies in the turbine

  Also CO2 production   Problem with putting CCS

on power plant stem partly from CO2 concentration in flue gas being ~10-15%

  Makes effective carbon capture difficult to do

  Whole operation is complex plant

  Several factors impact eC

  Incomplete combustion

  Ineffective heat transfer

  Heat losses

  Inefficiencies in turbine

  Inefficiencies in generator

  Parasitic energy losses

  Next lecture will begin to examine these effects

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/steam by 2 mechanisms   1. Radiation – in furnace

section of boiler, this is dominant heat transfer mechanism

  2. Convection – in flue, hot combustion gases enter, and this is the dominant heat transfer mechanism

  Each accounts for about 50% of heat transfer

  Method of interference   Some ash can adhere to the

tubes in the convection section or on the water wall of the radiant section.

  Deposition results from partially or wholly molten components of ash impacting one of these heat transfer surfaces and sticking there

  Continued impacting builds up sticky layer on steel surface

  This will trap particles that are not molten

Interference with Ash   Ash adhering to heat

transfer surfaces is solid, problem called ash deposition or ash fouling

  If deposits are semi- or fully molten, they are called slag deposits

  Can also be referred to as slagging

  From perspective of boiler efficiency, ash or slag deposits act as insulators

  Reduce heat transfer to the water/steam

  To overcome and maintain same rate of steam production (and electricity production) is to increase temperature in boiler

  Produces vicious cycle of more fouling or slagging, which requires still higher temperatures, causing more fouling or slagging….

Interference with Ash   Remedial measures for

fouling/slagging   Soot blowing   Shotgunning   Dynamiting   Coming off line for

detailed maintenance

  Boiler structure itself is extremely hot   Peak temp of “fireball”

could be ~1500°C   Not all heat will be

captured internally – some heat lost through walls

  Hot combustion gases pass a succession of steam tubes in the convection section   To recover as much heat as

possible

  At very end – heat exchanger to preheat the combustion air

  At this point, gases entering stack will be above ambient temp

Energy Losses in Boiler   Energy losses include:

  Energy in “so-called” dry gas – sensible heat in the gas energy is the moisture in gas

  Stack gas will be at some temp above the dew point, have to consider sensible heat and latent heat of moisture

  Where does moisture in stack gas come from?

  Moisture present in fuel and vaporized during combustion

  Moisture that formed chemically as a result of combustion of hydrogen in fuel

  4CH0.5 + 4 ½O2 H2O + 4CO2   Moisture that came into the system

with combustion air

  All air contains some amount of moisture

  Other class of loss – Unaccounted Losses   This could be a highly variable

number

  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

energy inputs and energy losses, where * denotes the big contributions

  Energy In   * Enthalpy of combustion

fuel   Preheating combustion air   Air heating by fans, blowers,

etc.

  Losses   *Stack gas losses   * Inefficient heat transfer

and unaccounted loss   Incomplete combustion   Furnace heat loss

  Since EnergyOUT = EnergyIN – Losses

  Efficiency = (EnergyIN – Losses)/EnergyIN

  The following are quantities of losses estimated for a boiler running on 25% excess air   Stack heat loss = 9%   Loss in heat transfer & unaccounted loss

= 6%   Incomplete combustion = 0.5%   Furnace heat loss = 0.5%

  Therefore boiler efficiency is 84%

  Long way from combined efficiency of 33%

  Need to look at efficiency of turbine and generator