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EVEG 4120 - Exam 2
Terms in this set (189)
Toxic Substances Control Act
Maritime Protection, Research and Sanctuaries
National Emission Standards for Hazardous Air Pollutants
Principal Organic Hazardous Constituents
Importance of thermal methods
destroy organic components of wastes--> combustion
converts waste to CO2, water, and inorganic products
treats solids, liquids, gases, etc
substance + oxygen --> oxygen + water
oxygen supplied w air, also contains nitrogen
chamber for burning waste, fitted with burner for controlled temperature
What keeps the incinerator itself from burning?
refractory material (silica brick) contains high temperature
*What is required for an incinerator permit under HSWA?
1. Trial Burn Plan is necessary
3. Facility standards (waste analysis, operating requirements)
*What are the three requirements for combustion in a reactor?
The Three T's
*How do incinerators supply oxygen?
air (~21% oxygen)
*What laws govern incineration?
3. Toxic Substance Control Act
4. Maritime Protection, Research and Sanctuaries Act
Principal Organic Hazardous Constituents
Specific organic compounds selected on the basis of their difficulty to incinerate
What are generally applicable incinerator standards?
1. Particulates - 0.08 grains per dry standard cubic foot (dscf) corrected time 7% O2
2. HCl emissions - 4 lb/hr or 99% control
3. CO emissions - 100 ppmv as 60 min rolling average corrected to 7% O2 on a dry basis
4. Metal emissions - emissions of 10 priority metals must meet risk based guidelines
5. DRE for dioxins, dibenzofurans, PCBs - 99.9999%
Why do incinerators use excess air?
Perfect combustion is not possible
Excess air is needed to ensure system is not oxygen limited
Why are conventional fuel used for combustion with hazardous waste?
HW has low heating values
Fuels must be used to bring temperature of the waste to a temperature at which rapid oxidation can occur
At what heating value can waste be burned without auxiliary fuel?
4,500 BTU/lb (2,500 kcal/kg)
What do compounds containing sulfur produce?
SO2 and SO3
What do compounds containing chlorine produce?
Cl2 and HCl
What do waste containing nitrogen produce?
NO and NO2
What do waste containing metals produce?
Metals oxidize and exit as ash
Some metals (Hg) are volatile and leave as flue gas
*What are the 3 types of incinerators?
1. Liquid injection
2. Solid waste
3. Gas (vapor)
*Liquid injection incinerator
For: pumpable liquid wastes
Waste burned directly in a burner or injected into the flame zone of incinerator chamber through atomizing nozzles
Supplemental fuel may be required
Goes to air pollution control device
*what are the parameters of liquid injection?
Temperature: 1000-1700 C
Residence time: milliseconds - 2.5 sec
Characterize: chemical composition, heat of combustion, viscosity, corrosivity, reactivity, potential for polymerization, ash content, ash fusion temperature
device for converting a liquid to a fine spray
Important factor in efficient operation of incinerators injecting liquids
Breaks liquid up into fine droplets, to develop the desired pattern for the liquid droplets in the combustion zone with sufficient penetration and kinetic energy, used to control the flow rate of the liquid discharged to combustion system
Fluid 1 (steam/ compressed air) mixed with fluid 2 (waste liquid)
Droplet size affects residence time (larger droplet, larger residence time)
For liquids, what is the burner designed to do?
Provide proper turbulence around atomized droplets
Why does a viscous waste require additional time, temperature, or turbulence (as compared to a low viscosity waste of same composition)?
As viscosity increases, droplet size increases
Need to decrease droplet size
What is needed for aqueous waste exhibiting ash content?
Waste is generally injected downstream from the burner to avoid clogging
*how is combustion carried out in solid waste incineration?
In suspension, on a grate, or on a solid hearth
*what is the most popular hazardous waste incineration?
Rotary kiln (solid waste, hearth type)
*what are different types of solid waste incineration systems?
1. rotary kiln
2. fixed/sloped hearth incinerator
3. multiple hearth incinerator
4. fluidized bed incinerator
waste combusted with energy recovery
What is typically used at waste-to-energy facilities?
High temperature combustion
*what type are most MSW incinerators?
Fixed hearth type
*how do rotary kilns work?
3 feeds: Air,solids & waste fuel & sludge
ash collects at bottom & empties out
rotary kiln is at an angle sloped down
secondary combustion chamber , then to air pollution control device
In a rotary kiln, where does complete combustion occur?
secondary combustion chamber
How fast does a rotary kiln rotate?
0.5 - 2 rpm on its longitudinal axis
How much is rotary kiln sloped?
1 - 2 degrees
Why is a rotary kiln sloped?
so waste moves horizontally as well as vertically
How can ash be removed in a rotary kiln?
1. removed when it begins to interfere
2. use ram to continuously remove
*What type of waste can be incinerated in a rotary kiln?
combination of solid and liquid waste
*What type of incinerator is typical of MSW waste-to-energy facilities?
fixed/sloped hearth incinerator
How many combustion chambers are there in a fixed/sloped hearth incinerator?
Where is the ashpit door located in fixed/sloped hearth incinerator?
primary combustion chamber
In fixed/sloped hearth incinerators, where are auxiliary burners located?
primary & secondary combustion chambers
*how do fixed/sloped hearth incinerator work?
Enter through charging door
step grate goes down to primary combustion chamber with auxiliary burner, ashpit door at bottom
secondary combustion chamber with auxiliary burner
tertiary combustion chamber to air pollution control device
Air Pollution Control Device
*What is a multiple hearth incinerator?
similar to two fixed/sloped hearth incinerators connected
ash out bottom & rest out top to APCD
*What is fluidized bed incinerators commonly used for?
sludges (with high water content)
Why would incineration be used for gases and vapors?
destory volatile organic compounds (VOCs)
What are the types of gas and vapor incinerators?
1. fume/gas incinerator
2. catalytic oxidation (CatOx)
Why would a gas incinerator be used over an open flare?
*How does a fume/gas incinerator work?
Feed: combustion air & supplemental fuel
to combustion chamber and exit
*How does a waste to energy facility work?
solid waste storage pit
overhead crane moves waste to feed hopper
stoker grate moves waste down
ash conveyor and steam generator
gas to scrubber then baghouse then stack
What are MSW combustion units?
Mass burn units or refuse-derived fuel (RDF) units
Mass burn unit
-no solid waste preprocessing before combustor
-overhead crane loads waste to feed chute that enters the combustor
-solid waste is preprocessed, removes noncombustible items and reduces size of combustible fraction
Why does an RDF system have more appeal than a mass burn unit?
1. RDF produces more uniform fuel with a higher heating value
2. RDF requires less excess air, around 50% (bc fuel is more uniform); a mass burn requires around 100% excess air
3. Mass burn requires more air and larger more expensive APCD
4. The pre-processing of RDF removes problematic materials (batteries)
What is a downside of RDF?
pre-processing waste is difficult and expensive
after solid waste is combusted, most organic content is transformed but some remains as ash
25% of MSW recovered as ash
what can ash be recovered as?
1. bottom ash
2. fly ash
-recovered from combustion chamber
-consists of inorganic material and some uncombusted organics
particulates removed from gaseous emissions using air pollution control equipment (bag house or ESP)
fly ash + bottom ash
basically a hazardous waste
How is ash diposed of?
Non-hazardous: conventional landfill
hazardous: HW landfill
*What are the 4 characteristics of hazardous waste?
*What is the test for toxicity?
(toxicity characteristic leaching procedure)
process by which a soluble contaminant (adsorbate) is removed from water or gas by contact with a solid surface (adsorbent)
adsorbate vs adorbent
Adsorbate: contaminant, gets absorbed
adsorbent: does absorbing
What is the most common adsorbent?
carbon that has been processed to significantly increase the surface area
What can activated carbon be made from?
coal, wood chips, coconut hulls
*How is activated carbon made?
1. material is pyrolyzed (heated in an atmosphere with insufficient oxygen for complete combustion)
2. Activated using steam, phosphoric acid, strong base, or other agent
Granular Activated Carbon
activated carbon commercially available in different particle sizes and shapes
*How can activated carbon differ?
1. different starting materials
2. different manufacturing processes
3. different sorption affinities for compounds
How is activated carbon used?
contactor or column
How is an activated carbon column used?
1. water is fed from top of column
2. water exits through underdrain system at the bottom
Why might backwashing be used in an activated carbon column?
remove particulates that may build up during processing
What is used activated carbon called?
spent activated carbon
*How is an activated carbon column set up?
Top --> Bottom
1. water enters
2. carbon bed
3. plenum plate (support)
4. underdrain system
When would upflow beds be used with activated carbon?
high solids content
What is the most common set up of activated carbon beds?
downflow mode - water flow down through beds
If downflow is used and solids are not filtered, what needs to be done?
1. separate sand filtering system
2. moving beds with continuously added carbon
process by which a component moves from one phase to another across some boundary
sorption takes places at a surface
Under the Sorption theory, what controlled the rate of sorption?
film and intraparticle transport (slowest)
Under the Sorption theory, what are the driving forces of sorption?
1. electrical attraction
2. chemical affinity of contaminant for adsorbent
3. van der waals forces (weak attrative forces)
4. hydrophobic nature of the organic
5. pH (for some compounds)
*What factors affect carbon adsorption?
1. Solubility - less soluble adorbed more easily
2. Molecular structure - branch chain organics adsorb better than straight chain organics
3. Molecular weight - larger molecules adsorb better
4. Polarity - less polar organics sorb more easily
5. Hydrocarbon saturation - double/triple bond organics sorb better
plot of the amount of a contaminant adsorbed per unit mass of carbon (X/M) versus the concentration of the contaminant in bulk solution (C) at equilibrium
What is a sorption isotherm?
plot of X/M vs C
from measurements used to determine the affinity of the contaminant for the adsorbent
How do is a isotherm experiment conducted?
1. Place contaminated water in containers with fixed amounts of contaminant and varying amounts of carbon
2. Put in oscillating shaker until equilibrium (1 day)
3. Separate carbon from solution (centrifugation or filtration)
4. Measure contaminant concentration in remaining solution
5. Mass sorbed (X) = Volume x (Cinitial - Cfinal)
one type of sorption isotherm
What are constants in the freundlich isotherm?
How can the freundlich isotherm be graphed?
1. plot x/m vs Ce on log-log scale
2. plot x/m vs Ce, use power curve fit
assumes: fixed number of adsorption sites
x/m = abCe/(1 + bCe)
What are constants in the Langmuir isotherm?
How is the Langmuir isotherm plotted?
Plot Ce/(X/M) vs Ce
Use: Ce/(X/M) = a/ab + 1/aCe
Why do isotherms give limited information?
Equilibrium is assumed, may not be reached in full scale
Why do batch systems not maximize the use of activated carbon?
Potential sorption capacity is not maximized
adsorption zone (mass transfer zone)
volume within the bed where adsorption takes place
What happens in the adsorption zone as the carbon becomes exhausted?
all sites are filled to the level at equilibrium with influent concentration & adsorption zone moves through the column in the direction of water flow
time where some specified amount of influent is detected in the effluent (5-10%)
Bed Depth Service Time
Cout/Cin vs time (days)
service time (days) vs bed depth (meters)
for indicators of breakthrough ( 10%, 90% of feed concentration)
*What does BDST graph tell you?
1. critical bed depth: x intercept
2. time required for AZ to pass through the critical depth: y intercept
3. velocity of adsorption zone: 1/slope
horizontal distance between feed concentration breakthroughs
CU = Cross-sectional area x (1/a) x unit weight of carbon
*minimum number of columns = (AZ/d) + 1
= (AZ/d) + 1
d= height of single column
t = aX + b
*When might activated carbon be used?
1. groundwater remediation: removing contaminants from groundwater
2. Removing residual contaminants from industrial WW effluents following other treatment
3. Removing VOCs and odor compounds from contaminated air streams
4. drinking water treatment
*what is done with spent activated carbon
1. regenerate activated carbon
2. dispose of it (landfill, incineration)
Where do large systems typically regenerate activated carbon?
Where do small systems typically regenerate activated carbon?
How is activated carbon regenerated?
steam heats carbon
Why does some carbon need to be added after regeneration?
some carbon is lost in regeneration process
a mass transfer process that enhances the volatilization of compounds from water to improve the transfer between air and water phases
*What is air stripping widely used for?
removing low concentrations of VOCs from water
*What is one application of air stripping
remove BTEX contamination from groundwater in a pump and treat system
*What is air stripping primarily useful for?
compounds with dimensionless Henry's Law constant, H', greater than 0.01
*How can air stripping be carried out?
diffused or mechanical aeration
*How is a packing tower air stripper set up?
Top --> Bottom
1. Air outlet at top
2. Water inlet below air outlet
3. stripping material
4. Air inlet (blower)
5. Water Outlet
Mass balance equation of air stripper
Qw(Cin - Cout) = Qa(Aout - Ain)
Simplify: uncontaminated air in and 100% removal
Cin = Qa
How is concentration in air outlet calculated?
MBE of ideal air stripper
R = H'(Qa/Qw)
air to water ratio
What is the mass transfer/two film theory of an air stripper?
bulk liquid to liquid film to air film to bulk air
overall mass transfer (KLa)
rate at which the contaminant is transferred from water to air [sec^-1]
product of KL*a
What is KL?
liquid mass transfer coefficient (m/sec)
What is a?
surface area/volume ratio of the packing (m2/m3)
How is KLa determined?
1. determined experimentally
2. Sherwood-Holloway equation
3. Onda correlations
height of the transfer unit
HTU = L/(Mw*Kla)
total column height
What is air-water ratio controlled by?
What happens at a large pressure drop?
air flow is increased
air holds water back
What should the air-water ratio be to avoid flooding pressure drop?
200 and 400 N/m2
What controls the operating costs of the air stripping tower?
What controls the capital costs?
What are some other physicochemical processes?
1. steam stripping
2. chemical oxidation
3. Supercritical fluid extration & Supercritical water oxidation
4. Reverse osmosis & electrodialysis
What is steam stripping used for?
removal of volatile compounds from groundwater or wastewater
How does steam stripping work?
transfers compounds from liquid phase to gas phase
like an air stripper but stripping gas is steam
How does chemical oxidation work?
oxidizing agent used to chemically transform wastes
What does chemical oxidation do?
What are commonly used as oxidizing agents?
What are the two types of membrane processes?
What are the types of processes with supercritical fluids?
1. supercritical fluid extraction
2. supercritical water oxidation
*what is stabilization?
process employing additives (reagents) to reduce the hazardous nature of a waste by converting the waste and its hazardous constituents into a form to decrease the rate of contaminant migration and/or reduce toxicity
*what is solidification?
process by which sufficient quantities of solidifying material are added to the hazardous materials to result in a solidified mass of material
What does solidification do?
1. increase strength
2. Decrease compressibility
3. decrease permeability of waste
What are applications of stabilization?
1. waste stabilization prior to secure landfill disposal
2. site remediation (soil)
What are applications of solidification?
solidify industrial wastes (non-hazardous, unstable wastes like sludges)
What are the advantages of stabilization?
1. improve handling and physical characteristics of waste
2. decrease surface area across which transfer or loss of contaminants can occur
3. limit pollutant leachability
4. reduce contaminant toxicity
process by which contaminants are transferred from a solid matrix to water
*What are mechanisms of stabilization/solidification?
hazardous waste constituents are physically entrapped in larger structural matrix
Where is the waste in macroencapsulation?
in discontinuous pore spaces
In macroencapsulation, how much the waste be released?
stabilized mass breaks down over time
hazardous waste continents are entrapped in crystalline structure of solidified matrix at the microscopic level
most waste stays entrapped
process by which contaminants are taken up into the sorbent just as a sponge is taken up into water
What is often absorbed in absorption?
What are absorbents?
hay and straw
interaction by which contaminants are elctrochemically bonded to stabilizing agents within the matrix (surface phenomena)
What are used in adsorbents?
organically modified clays
What is adsorption used in conjunction with for more permanent treatment?
Micro or macroencapsulation
certain waste constituents precipitate in stabilization processes
What can precipitate?
Hydroxides, sulfides, silicates, carbonates, phosphate
mechanism that changes a chemical constituent to a form that less or non toxic
How can detoxification occur?
chemical reactions like oxidation or reduction that reduce toxicity
What is an example of detoxification?
Cr 4 to Cr 3 in concrete
*What types of technology are used for stabilization/solidification?
4. organic polymer
What are examples of cement-based technology?
1. Portland cement: mix of limestone & clay fired in kiln at high temp
2. Concrete: composite of hydrated cement and aggregate
What are the cement reactions?
-intial rapid reaction
What are cement based stabilization primarily used for?
inorganic wastes (metals, radinuclides)
What are pros of cement based stabilization?
2. high pH (neutralize acids and maintain metals in hydroxide and carbonate precipitate forms)
What is a drawback of cement based stabilization?
organic constituents interfere with cement hydration process
What is a Pozzolan?
material that can react with lime in the presence of water to produce a cement like material
What is Pozzolan made of?
aluminosilicate material, lime and water (pozzolanic concrete)
What are examples of Pozzolans?
ground blast furnace slag
cement kiln dust
What are Pozzolan based processes primarily used for?
inorganic substances (heavy metals)
What does lime do in Pozzolan based processes?
raise pH - metal precipitation reactions
melting and fusion of materials at high temperatures followed by rapid cooling in a non-crystalline form
What is one way of vitrification?
use electrodes to heat and melt soil
What is a con of vitrification?
very expensive (large energy requirements)
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