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