Post on 13-Jan-2016
Solid Waste Management and Sustainability Technology (NOTE 4)
Joonhong Park
Yonsei CEE Department
2013. 9. 30.
Combustion and Energy Recovery
Heat value of refuse
Materials and thermal balances
Combustion hardware used for MSW
Undesirable effects of combustion
Heat value: Unit
Amount of energy necessary to heat one unit mass of water one unit temperature degree.
British termal unit (BTU) energy amount to heat one pound of water by one degree Fo.
Calorie (Cal): energy to heat one gram of water by 1Co
Joule (J): kg m2/s2 (ML2T-2) 4.184 J = 1 Cal.
watt-hours (Wh): (kg m2/s3)* 3600(s/h)
See Table 7.1 (useful conversion factors)
Heat value: Determination methods
Ultimate analysis- the DuLong equation: Btu/lb = 145C + 620(H-O/8)
- alternatively: Btu/lb =144C+672H+6.2O+41.4S-10.8N
Compositional analysis- Btu/lb = 49R+22.5(G+P)-3.3W
- Btu/lb = 1238+15.6R+4.4P+2.7G-20.7W
Proximate analysis
-Btu/lb = 8000A +14,500B
Calorimetry
Heat value: Calorimetry
Calorimeter: to measure energy necessary to heat 1gram of water by 1 degree C
O2
H2O
Thermometer
To electrical contact
Bumb Cell
Heat value: Calorimetry
Thermogram
Temp oC
Time
dTLinear Part
Heat value: Calorimetry
U = Cv * dT / M
Here
U: heat value of unknown material, cal/g Cv: heat capacity of the calorimeter dT: rise in temperature from thermogram oC M: mass of the unknown material, gram
System characteristic
Heat value: Calorimetry
Higher heating value (HHV): the gross calorific energy
Lower heating value (LHV): the net calorific energy
HHV = LHV + latent heat of vaporization (occurring in the bomb calorimeter)
LHV is a more realistic value for design.
Heat value: Calorimetry
Calorimetry is the referee method of measuring heat value of a fuel
But it does not actually simulate the behavior of that fuel in a full-scale combustor.
Reason 1: Some metals oxidize at sufficiently high temperatures to yield heat (exothermic reaction) => It happens in calorimetry but not in a full-scale combustor.
Reason 2: All organic material will oxidize in a calorimeter but this will not occur in a full-scale combustor (time dependent efficiency.)
Reaction - Thermodynamics
Reactants (A and B)
Σ ΔGreactant
Products (C and D)Σ ΔGproduct
Total RXT’n Chemical Free Energy, ΔGr
= Σ ΔGpro -Σ ΔGrxt
Activation Energy (Barrier): activated by Catalyses/Enzymes
Reactions Stoichiometry and Kinetics
• Energetics : “thermodynamic fall”– When ΔGr is less than 0, thermodynamically favorable.– dCi = Ф (dGr) = Ф (masses of reacting constituents)
• Fundamental Governing Eq. (Stoichiometry)α1 A + α2 B < = > α3 C + α4 D αi: stoichiometric coefficient; Q: unit?forward rxn const. = [C] α3 [D] α4 /[A] α1 [B] α2
• Reaction Kinetics (the Mass Law)rate = dCi/dt = Ф (masses of reacting constituents) = function of (energetics, system characteristics)
Combustion StoichiometryProduction of hydrocarbons CO2 + sunlight + nutrients + H2O => (HC)x + O2
Combustion (rapid decomposition) (HC)x + O2 => CO2 + H2O + nutrients + heat energy
Two-step reaction•C+O => CO + 10,100 J/g•CO + O => CO2 + 22,700 J/g•Stoichiometric oxygen: one mole carbon + one mole of molecular oxygen (2.67 gO2/gC)
Example: Stoichiometric oxygen & combustion air
Problem 1: calculate stoichiometric oxygen required for the combustion of methane gas (CH4)
Problem 2:Calculate the stoichiometric oxygen required for the combustion of methane gas
Combustion efficiency
Combustion
Condenser
Turbine
Generator
Emission
Fuel
Air
Cold water
Steam
Electricity
Energy conservation 0 = Q0 – QU – QW
Q0: energy flow in
QU: useful energy out QW: wasted energy out
E(%) = QU/Q0 X 100
Carnot efficiency (Ec)
Ec(%) = 100 x (T1-T0) /T1 T1: absolute temp. of the boiler, oK T0: absolute temp. of the condenser, oK
Combustion efficiency
Thermal balance on a waste-to-energy combustor
From water
From fuel
To stack gases
To steam
To vaporation
To radiation
To ash
23年 4月 21日
Incinerators…Being too hot is not good.
“Incinerator” is a facility to burn refuse without recovering energy from MSW. “Incinerators”, a name no longer used by the industry because of the sorry record of these facilities (poor design, inadequate engineering, and inept operation combined to produce an ash still high in organics and smoke that even in the days of little industrial air pollution controlled caused many communities to shut down the incinerators.)Without energy recovery, the exhaust gas from these units was too hot => causes problems in dust control (electrostatic precipitators)
23年 4月 21日
Pressure
Temperature
Soild Liquid
Super-cooled Liquid
Super-cooled Vapor
Saturated vapor
Superheated vapor
MELTINGPOINT
BOILINGPOINT
CRITICALTEMPERATURE
CRITICALPOINT
TRIPLEPOINT
Supercritical Fluid
23年 4月 21日
Waste-to-Energy Combustors
Combination of combustion of waste with energy recovery.
A typical MSW combustor
Receivingarea
SolidWaste
Storagepit
Overheadcrane
Feedhopper
Stokergrate Ash
conveyor
Steamgenerator
Scrubber
Furnace
Baghouse
Stack
23年 4月 21日
Combustion chamber
Overfire air (oxygen and turbulence provider)
Underfire air
Temperature(980-1090 oC)
Q: If temp. is low?If temp. is high?
Grates-Reciprocating
-Rocking-Traveling
(functions:conveying refuse,
producing turbulence,and underfire air)
23年 4月 21日
Excess air and temperature relationship
in MSW combustion
1000oF
2000oF (1090oC)
3000oF
4000oF
-50 0 50 100 150 %
Excess air, % above stoichiometric
Operationalair volume
Why not?
Why not? (supercritical steam)
RememberStoichiometric
oxygen?
23年 4月 21日
Efficiency of energy recovery as related to quality of MSW as
a fuel
HeatValue
6500Btu/lb
6000Btu/lb
5000Btu/lb
4000Btu/lb
3000Btu/lb
Refuse % moisture % noncombustible % combustible
Steam generated (tons per ton refuse)
151471
4.3
181666
3.9
252055
3.2
322444
2.3
392833
1.5
23年 4月 21日
Another types of combustors
Rotary kiln: - furnace is rotating - provides excellent mixing, improving the efficiency of combustion.
Modular starved air combustors - two-stage combustion system (burned by starved air mode and then by fossil fuel) - typically, no recovery of energy - good for small scale (15-100 tons per day) - mainly used for destruction of some hazardous materials such as biohazards from hospitals.
23年 4月 21日
Pyrolysis (in principle)
Destructive distillation or combustion in the absence of O2. C6H10O5+heat energy => CH4 + H2 + CO2 + C2H4 + C + H2O
Produces a solid, a gas (methane), and a liquid (ethylene) Effect of temperature and heating rate in the formation of pyrolysis products.
100 101 102 103 104 105 106
400
800
1200
Temp(oC)
1/heating rate (milliseconds per oC)
Gas
Liquid
Solid
23年 4月 21日
Pyrolysis (theory vs. reality) Theoretically speaking, pyrolysis and gasification is - Environmentally excellent - Producing little pollution - Resulting in the production of various useful fuels. - Gasification appears to be able to meet the air emission requirements for solid waste combustion, including the strict dioxin standards.
Nevertheless, practically speaking…. - Success in pyrolysis of homogeneous and predictable fuels such as sugarcane bagasse. - Failure in pyrolysis of heterogeneous and unpredictable refuse. - Not a single unit has yet to be successfully field tested in full scale (could not convince PEOPLE).
=> Should we continue improving pyrolysis technology?
23年 4月 21日
Mass Burn vs RDF Mass burn unit: no preprocessing of the MSW prior to being fed into the combustion unit. RDF (refuse-derived fuel) unit: processed prior to combustion (i) to remove noncombustible items and (ii) to reduce the size of the combustible fraction.
Advantages of RDF - uniform heat value - reduction of the amount of excess air required for combustion (50% the excess air is sufficient). - less requirement for air-pollution-control devices. - some problem items (ex. Batteries) can be eliminated before combustion. - Possible to store them for a relatively long term.
Disadvantages of RDF - processing of MSW is not easy. - corrosion and erosion problems (due to high temp.)
23年 4月 21日
ASTM RDF Designations
Name Description
RDF-1RDF-2RDF-3
RDF-4
RDF-5
RDF-6
RDF-7
Unprocessed MSW (the mass burn option)MSW shredded but no separation of materialsOrganic fraction of shredded MSW. This is usually produced in a materials recovery facility (MRF) or from source-separated organics such as newsprint.Organic waste produced by a MRF that has been further shredded into a fine, almost powder, form sometimes called “fluff.”Organic waste produced by a MRF that has been densified by a pelletizer or a similar device. These pellets can often be fired with coal in existing furnaces.Organic fraction of the waste that has been further processed into a liquid fuel such as oil.Organic waste processed into a gaseous fuel.
Note: RDF-6 and -7 have been tried on a pilot scale but have not been found to be successful at full-scale plants.
Undesirable effects of combustion
Waste heat
Ash
Air pollution
Dioxin (of particular concern)
2005-11-18
Ash 25% of the original mass is ash, with a high density of about 1200-1800 lb/yd3
Bottom ash - recovered from the combustion chamber - consists of the inorganic material as well as some unburned organics
Fly ash - the particulates removed from the gaseous emissions
Waste heat
The second law of thermodynamics: when energy is changed from one form to another, it may not be possible subsequently to change it all back to its original form (i.e., part of change is irreversible) - Enthalpy (H) = Internal energy (U) + Work (PV) - Free energy (G) = H – entropy (TS)
The steam generated by a combustion plant is useful for driving turbines but the remaining steam has little industrial use, unless it is located sufficiently close to buildings to use it for heating.
The residual steam is condensed into (hot) water => 90% is treated (cooled) and reused. Because of uncertain effect in ecosystem, discharging hot used water into environment is prohibited (cooling pond and cooling tower are needed).
Ash
Ash from MSW combustion comes perilously close to being classified as a hazardous waste by EPA.
The major problem with ash from MSW is the presence of heavy metals (Lead 3,100 mg/kg of ash; Aluminum 17,800 mg/kg of ash; see Table 7-13)
Leachability test: when leached with a solvent, if the concentration of a leached compound exceeds 100 times the drinking water standard, the waste is classified as hazardous.
Leachability of heavy metal is a function of pH. Fly ash is often classified as hazardous. Combined with the bottom ash most often meets the requirement.
Ash Ash disposal is either in special landfills or in regular municipal solid waste landfills.
If the ash is compacted, the ash is highly impermeable, with a permeability as low as 10-9 cm/sec.
Alternatively, the ash can be used for- Road base material- Structural fill- Gravel drainage ditches- Capping strip mines- Mixing with cement to make building blocks(Q: Reuse, recycle, or recovery?)
Metals can be recovered from ash.
Air Pollutants
Gases and particulates
Primary pollutants: products of the combustion process that can be shown to be harmful in the form they are emitted.
Secondary pollutants: those that are formed in the atmosphere as a direct result of the emission primary pollutants.
Example for secondary pollutant: - S + O2 => SO2 (sulfur dioxide) - O2 + 2SO2 => 2SO3 (sulfur trioxide) - SO3 + H2O => H2SO4 (acid rain; pH < 4.5)
Air Pollutants as secondary pollutants
Photochemical smog(NO2 + light => ) - see Table 7-14 - 75% of fuel NOx + 25% Thermal Nox
Escape of heavy metal with the emission gases (lead, cadimium, and mercury*: don’t dump batteries in MSW bin; reduction of mercury use)
Global warming gases (CO2 and CH4; CH4 is 17 times more potent as a greenhouse gas then CO2 => waste combustion is better than methane in landfill)
Control of particulates
Settling chambers (> 100 micro-m) Cyclone Bag filters (fine particulates) Wet scrubber (large particulates) Q: good for heavy metal recovery? Why not? Dry scrubbers Electrostatic precipitators
Control of gaseous pollutants Removal of the pollutant from the gaseous emissions, a chemical change in the pollutant, or a change in the process producing the pollutants.
Wet scrubbers (chemical addition and reaction; dissolution of some pollutants) after ESP or baghouse.
Dry scrubber (efficient to control sulfur oxides; injection of lime slurry).-Ca(OH)2 + heat => CaO + H2O -SO2 + CaO => CaSO3
* denox system: ammonia + NOx => N2 + H2O
2005-11-18
Dioxin polychlorinated dibenzodioxins (PCDD) polychlorinated dibenzofurans (PCDF)
O
OCl
Cl
Cl
Cl
2,3,7,8-TCDD is particularly toxicto animals.(molecular reason?)
2005-11-18
Air emissions of dioxins and difurans (US case)
Emission source % of total
Hospital waste combustionMunicipal waste combustionWood burningCement kilnsNonferrous metal refiningDiesel fuel Hazardous waste incinerationAll other sources
55.432.63.93.82.70.90.40.3
Discussion: Voluntary risk versus involuntary risk