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
Introduction
Definition (combustion or incineration)
- a process of burning in the presence of oxygen resulting from the rapid
oxidation of substances
Pyrolysis - combustion in the absence
of oxygen to produce solid (carbon), liquid (ethylene), and gaseous (methane)
high energy products, low pollutant emissions
Gasification - combustion with limited
oxygen producing only gasious products with high energy content
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)
Mass burn - combustion with little to
no waste processing
Refuse derived fuel (RDF) - removal
of noncombustables, reduction in waste size, increase homogeneity
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
Design
- typically a turn key approach; design, cconstruct, turn over the key
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
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 Combustors
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)
Starved air combustors
smaller capacity (15-100 tpd)
two stage, first stage starved air,
second stage complete combustion
no heat recovery
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 (SOx, 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
Too much turbulence, entrainment of
particulates
Control
Cyclones - not effective for removal
of small particulates
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 ddensely woven fabric, superior
effic >99.99% (>effic for smaller diam.) Technology 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
Nitrogen removal
Source removal to avoid fuel NOx prodcution
T < 1500 F to avoid thermal NOx
Denox sytems - selective catalytic reaction
via injection of ammonia
