Solid Waste Management and Sustainability Technology (NOTE 4)

1. Solid Waste Management and Sustainability Technology (NOTE 4) Joonhong Park Yonsei CEE Department 2013. 9. 30.

2. Combustion and Energy Recovery • Heat value of refuse • Materials and thermal balances • Combustion hardware used for MSW • Undesirable effects of combustion

3. 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)

4. 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

5. Heat value: Calorimetry Calorimeter: to measure energy necessary to heat 1gram of water by 1 degree C Thermometer H2O To electrical contact O2 Bumb Cell

6. Heat value: Calorimetry Thermogram Linear Part Temp oC dT Time

7. Heat value: Calorimetry U = Cv * dT / M System characteristic 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

8. 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.

9. 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.)

10. Reaction - Thermodynamics Activation Energy (Barrier): activated by Catalyses/Enzymes Reactants (A and B) ΣΔGreactant Total RXT’n Chemical Free Energy, ΔGr = ΣΔGpro -ΣΔGrxt Products (C and D) ΣΔGproduct

11. 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)

12. Combustion Stoichiometry • Production 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)

13. 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

14. Combustion efficiency Emission Cold water Condenser Steam Combustion Turbine Fuel Generator Air Electricity

15. Combustion efficiency • 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

16. Thermal balance on a waste-to-energy combustor To vaporation To stack gases To steam From water To radiation From fuel To ash

17. 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)

18. Supercritical Fluid Soild Liquid Pressure CRITICALPOINT TRIPLEPOINT Super-cooled Liquid Saturated vapor Superheated vapor Super-cooled Vapor Temperature MELTINGPOINT BOILINGPOINT CRITICALTEMPERATURE

19. Waste-to-Energy Combustors Combination of combustion of waste with energy recovery. A typical MSW combustor Stack Overhead crane Feed hopper Steam generator Bag house Scrubber Solid Waste Storage pit Receiving area Stoker grate Ash conveyor Furnace

20. Combustion chamber Overfire air (oxygen and turbulence provider) Temperature (980-1090 oC) Grates • Reciprocating • Rocking • Traveling (functions: conveying refuse, producing turbulence, and underfire air) Q: If temp. is low? If temp. is high? Underfire air

21. Excess air and temperature relationship in MSW combustion Why not? (supercritical steam) 4000oF Why not? Operational air volume 3000oF 2000oF (1090oC) Remember Stoichiometric oxygen? 1000oF 0 50 100 150 % -50 Excess air, % above stoichiometric

22. Efficiency of energy recovery as related to quality of MSW as a fuel

23. 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.

24. 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. Gas 1200 Liquid Temp (oC) 800 Solid 400 100 101 102 103 104 105 106 1/heating rate (milliseconds per oC)

25. 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?

26. 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.)

27. ASTM RDF Designations Note: RDF-6 and -7 have been tried on a pilot scale but have not been found to be successful at full-scale plants.

28. Undesirable effects of combustion Waste heat Ash Air pollution Dioxin (of particular concern)

29. 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

30. 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).

31. 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.

32. 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.

33. 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)

34. 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)

35. 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

36. 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

37. Dioxin polychlorinated dibenzodioxins (PCDD) polychlorinated dibenzofurans (PCDF) Cl O Cl Cl O 2,3,7,8-TCDD is particularly toxic to animals.(molecular reason?) Cl

38. Air emissions of dioxins and difurans (US case) Discussion: Voluntary risk versus involuntary risk