Lesson 12: Waste to Energy (Incineration)

Objectives:

• To describe the basic concepts associated with waste incineration including combustion control, system components, and air pollution concerns
• To present the basic calculations associated with analyzing waste combustion

Goals:

• Explain the three Ts of combustion
• Describe the main components in a waste to energy (WTE) facility
• Describe the primary air pollution concerns
• Perform basic combustion calculations

• Definition (combustion or incineration) - a process of burning in the presence of oxygen resulting from the rapid oxidation of substances
• Used for municipal solid wastes, industrial (hazardous) waste, sludges, fossil fuel
• Typically operated with excess air (EA), the oxygen supplied is greater than the oxygen requirements for complete combustion
• Common acronyms
  • WTE - waste to energy
  • MWC - municipal waste combustion (no energy recovery)

Advantages

• Volume and weight reduced (approx. 90% vol. and 75% wt reduction)
• Waste reduction is immediate, no long term residency required
• Destruction in seconds where LF requires 100s of years
• Incineration can be done at generation site (ex. medical waste incinerators)
• Air discharges can be controlled (low health risk)
• Ash residue is usually non-putrescible, sterile, inert
• Small disposal area required
• Cost can be offset by heat recovery/ sale of energy

Disadvantages

• High capital cost
• Skilled operators are required (particularly for boiler operations)
• Not all material are incinerable (noncombustable solids)
• Some material require supplemental fuel
• Public disapproval
  • Risk imposed rather than voluntary
  • Incineration will decrease property value (perceived not necessarily true)
  • Distrust of government/industry ability to regulate

History

• Late 1960s - 300 plants in the US (capacity of 30 million tons/yr)
• 250 plants closed between 1965 and 1980 as a result of Clean Air Act regulations (non-compliance)
• 1970’s source separation and mechanical separation (trommel, screens, hammermills, conveyors, magnets, air classifiers ) were used to produce refuse derived fuel (RDF) to be fired in conventional coal fired boilers by 1980 all but one of these operations closed due to material handling problems.
• Other issues
  • RCRAs emphasis on reduction and  recycling,
  • better materials handling equipment,
  • more effective pollution control equipment available,
  • better understanding of combustion technology,
  • LF siting problems
• General Chronology
  • 1983 - 50 MWC @ 76. million tons/yr
  • 1986 - 81 MWC @12 million tons/yr
  • 1990 - 168 MWC @31 million tons/yr
  • 1991 - 176 MWC (137 with energy recovery) @31.4 million tons/yr, producing enough electricity to power 1.2 million homes
  • 1992 - 190 MWC, 142 WTE @33.6 million tons/yr capacity in 34 states
  • 1993 -
    • 16% of waste disposed in incinerators
    • 100,000 tpd capacity
    • Power generation = 1.3 million homes
    • 164 MWC units, 125 WTE plants
    • 7 under construction, 37 planned
  • 1994
    • 15.5% of waste disposed in incinerators
    • 89,000 tpd
  • 2000 - anticipated that 20% of MSW will be combusted
  • Future
    • Some forms of RCRA reauthorization have called for a moratorium on MWC, expensive, strong market for recyclables, CAA impacts
    • Expect recent rate increase to drop over next few years: limited revenue, lack of project support, regulationss unclear

• Three Ts
  • time
  • temperature
  • turbulence
• Refuse receipt and storage
  • Scales
  • Sufficient length of road to entrance to avoid backup
  • Tipping area enclosed to prevent nuisance conditions
  • Tipping area large enough to permit more the 1 truck to maneuver
  • Storage for 2-3 days (also seasonal variations) so that continuous incinerator operation is possible
• Refuse feeding
  • Batch feeding is not desirabel - variation in furnace temperture due to air leakage leads to incomplete combustion
  • Small plants use rams to push waste into furnace
  • Large plants use traveling bridge cranes to transfer waste from pit to charging hopper (1.5-8 yd³ bucket)
  • Charging hopper with steep slope feeds waste  into furnace by ram, grate, or screw
• Grate system - most crucial
  • Transport refuse through furnace, promote combustion by adequate agitation and mixing with air, excessive turbulence leads to excessive carryover of particulates
  • 75-100 tons/ft²/hr or 250-300,000 BTU/ft²/hr
  • Types = traveling grate ( no longer used), rocking grate, reciprocating grate, rotary kiln, other proprietary grates (see Fig 13-3 and 13-4, pp. 621)
• Air Supply
  • Underfire Air - combustion achieved by injection of combustion air below grates
    • provided by fans,
    • cools grates
    • 40 to 100% of total air
    • low supply inhibits combustion leads to high grate temps, slagging which blocks grate,  and clinkers
  • Overfire Air - injection above grate
    • supplied by forced air blower,  induced draft, or both.
    • above air injection line parallel to grate plane
    • ensures complete combustion of flue gases and particulates
    • promotes turbulence
    • particularly important for temperature control where energy recovery is not provided
• Furnace volume
  • primary - area above grates
  • secondary combustion chamber - few seconds sufficient to retain gases in high temp zone for max. fuel volatilization. to ensure complete combustion
• Supplementary Fuel - used to control temp if heat content of primary  fuel (waste) insufficient
• Refractory Lined furnace -
  • no heat recovery - greater excess air requirements to control temps
  • conductive heating - heat transfer by progressive heating of adjacent elements (ex..pot on a stove), 100-200% EA required
• Waterwall units,
  • in furnace - most common, primary combustion chamber fabricated from closely spaced steel tube with water recirculation, 50-100% EA required for cooling
  • radiation chamber - heat transfer between 2 bodies not in direct physical contact and at different temperaturs (water and burning fuel)
• Boilers - heat recovery, water converted to steam, water flows countercurrent to gas flow
  • Economizers - heat boiler feedwater by extracting gases as they leave convective section
  • Convection tube - heat transfer from hot gases moving past tubes - boiler tubes perpendicular to flow of gas as exits incinerator, saturated steam produced
  • Super heater - tubular section upstream of convective section hot incerator gases superheat steam generated at convective tube

Flue Gases

• Heat Content
• Composition, Temperature - Higher temp important for plume lift, waster condensing problem for public (visual)
  • Know BTU in waste
  • Know gas composition
  • With either flue gas Temp or Enthalpy known, the other can be calculated
• Pollutants of concern
  • Particulates
  • Acid Gases (SO_x, HCl, HFl)
  • NOx, primarily NO and NO₂
  • Carbon Monoxide, organics (PIC)
  • Heavy Metals
  • Carbon dioxide not significant, if all MSW burned, it would contribute <2% of that produced during E production in US
• Emission Control
  • Remove or alter certain waste stream components, mercury in batteries, HM, Yard waste
  • Regulate combustion efficiency (design and operate furnace to maximize conversion of organic matter to CO₂ and water) Good Combustion Practices, GCP
  • Use properly maintained and operated emission control devices, Best demonstrated technology
• Particulates (smoky fire)
  • Solid - noncombustable materials released into flue gas as fly ash, Dia <1 micron to 100s of microns, inorganic oxides, Heavy metals, unburned matter
  • Condensable - refuse vaporized after passing out of system, cool, condense, ex: mercury, organic compounds
  • Causes
    • Too low of a comb T (incomplete comb)
    • Insuff. oxygen or overabundant EA (too high T)
    • Insuff mixing or residence time
    • To much turbulence, entrainment of particulates
  • Control
    • Electrostatic precipitator - after heat recovery, ESP induces a charge particulates, gas stream passes between plates w/ opposite charge, particulates attracted to plat, 93% removal of dia <2u, 99.8% removal of larger, cannot always meet stds
    • Fabric Filters, FF (baghouses) - like vac cleaner, flue gas pulled thru densely woven fabric, superior effic >99.99% (>effic for smaller diam.)
      • Tech of choice for new MWC
• Acid Gases
  • From Cl, S, N, Fl in refuse (in plastics, textiles, rubber, yd waste, paper)
  • Uncontrolled incin - 18-20% HCl with pH 2
  • Acid gas scrubber (SO₂, HCl, HFl) usually ahead of ESP or baghouse
    • Wet scrubber - older type of system, good mercury removal (cools gas) but generates waste water
    • Spray dryer - inject reagent slurry into a vessel where the water I the slurry evaporates, cooling the flue gas and allowing the acid gases to react with the reagent, produces dry powder collected by ESP or FF
      • BDT for use with ESP and FF
    • Dry Scrubber Injectors - introduces a totally dry, highly pulverized lime sorbent into flue gas or in-furnace, easy to retrofit existing units but limited efficiency (98%)
    • 95% removal of HCl, HFl - 95% required
    • 86% removal of SO₂ - 80% required

Assignments:

• Carefully review the example problems in Chapter 7