Wednesday, September 05, 2007

Thermal Oxidation for VOC Control

Volatile organic compounds generally are fuels that are easily combustible.Through combustion, which is synomonous with thermal oxidation and incineration , the organic compounds are oxidised to CO2 and water, while trace elements such as sulfur and chlorine are oxidised to species such as SO2 and HCL.
Three combustion processes that control vapor emisions by destroying collected vapors to prevent release to the environment are
1. thermal oxidation - flares
2. thermal oxidation and incineration
3. catalytic oxidation.
Each of these procecses has unique advantages and disadvantages tha require deep considiration for proper method.As an illustration, flares are designed for infrequent large volumes of concentrated hydocarbon emissions.On the other hand thermal oxidizers are designed for high - efficiency treatment of contınuous.mixed-hydrocarbon gas streams.The design of the basics processes can be modified for specific applications,resulting in the overlap of the distinctions between processes.

Saturday, August 04, 2007

Nitrogen Dioxide

Measurement of Nitrogen Dioxide

For measuring nitrogen dioxide concentration in ambient air the chemiluminesence method is used primarly.Nitrongen dioxide concentrationas are determined indirectly from the difference between the NO and NOX concentrations in the atmosphere.These concentrations are determined by measuring tje light emitted from the chemiluminesece reaction of NO with O3 and the light output is proportional to NO present in the ambient air stream.

Reaction in the Atmosphere

NO is a colorless gas that has some harmful effects on human health , but these effects are substantially less than those of an equivalent of Nitrogen Dioxide.In the atmosphere NO reacts with O2 to form nitrogen dioxide , a brown- coloured gas that is a serious repspiartory irritant.Its brown color is strong enoughtaht is often possible to see a distinct brown colr emerging from a power plant stack or from the vent of any process using nitric acid,which realese NO2.

Methods for measuring Ozone

The principal method used for measuring ozone is based on chemiluminescence.When ozone and ethylen react chemically,products are formed which are in exited electronic state.These products fluoresence,relasing light.The principal components are a cocstant source of ethylen,an inlet sample line for ambient air,a reaction chamber, a photomultuplier tube , and signal processesing circuitry.The rate at which light is recieved by the photomultipler tube is dependent on the ozone conentration and the ethylene.If the concentration of ethylene is made much higher than the ozone concentration to be measured, the light emitted is proportional only to concentration of ozone.

Carbon Monoxide Measurement Technique


The primary method used for measuring carbon monoxide in US is based on nondispersive infrared photometry (NDIR).The analyser has a hot filament sorce of infrared radiation, a chopper , a sample cell, reference cell and a detector.The reference cell is filled with a non- infrared- absorbing gas,and the sample cellis continuously flushed with ambient air containnig unknown amount of CO.The detector cell is divided into two compartments by a flexible membrane, with each filled with CO.Movement of the membrane causes a change in elektrical capacidance in control circuit whose signal is processed and fed to recorder.

Analysis and Measurement of Gaseous Pollutants

The two major goals for air pollutants are identification and quantification of a sample of ambient air.
Hundreds of chemicals are present in the urban atmospheres.The gaseous air pollutantsmost commonly monitored are CO,OZONE,NO2,SO2 and nonvolatile organic ompounds(NMVOCs).

Monitoring Systems

Ambient air quality data may be obtained through the use of mobile or fixed sampling networks and the use of integrated samplers or continuous monitors.Decisions regarding these monitoring techniques constitude the first important steps in design a monitoring network.

Fixed VS Mobile Sampling

In fixed sampling systems , staions are permanent or at least long term instalations.In a mobile sampling network, the monitoring instuments are rotated on schedule among selected locations.
An advantage of fixed sampling is that measurements are made concurrently at all sites, providing directly comparable information, which is particularly important in determining relationship of polluting source to local air quality and in tracing dispersion of pollutants troughout area.The chif advantage of mobile sampling is that air quality measurements can be made at many sites - far more than wopuld be feasible in a fixed sampling program.


Continuous vs İntegrated Sampling

Continuous monitoring is conducted with devices that operate as both sampler and analyser.Pollutant concntrations are instantaneously displayed on a meter, continuously recorded on a chart, magnetic tape or a disk.Integrated sampling is done with devices that collect a sample over some specified time interval after which sample is sent to a laboratory for analysis.This is an olderly techniquce and limited use currently.

Friday, August 03, 2007

Ambient Air Quality Sampling Program

Air pollution monitoring is conducted to determine either emission concentration or ambient air quality.The range of emission concentrations,temperature and pressures encountered is sometimes different than found in tha ambient air sampling situations.For that reason sampling and analysis technuques and equipment are different for each case.

OBJECTİVES OF SAMPLİNG PROGRAM

The object would be to measure the amount of gaseous and particulate matter at enough locations to make the data statistically significant.It is not uncommon to find each tation in a network equipped with simple, unsophisticated grab sampling devises.However quite afew complicated devises have been developed which contains continuous monitors with telemetry and computer control.Meterological variables are also monitored and corelated with the concentration data.The information is then used;

1.To establish and evaluate control measure
2.To evaluate atmosphric-diffusion model paremeters
3.To determine areas and time periods when hazardous levels of pollution occurs in the atmosphere
4.for emergecncy warning systems

Best Available Control Technology of Air Pollution

The Federal New Source Review program requires that Best Avaible Control Technology(BACT) be appiled to major new sources and mjor modifications in attainment areas.Many local agencies also require that BACT be applied to smaller new sources of air pollution.
BACT is a technology-based standard that can be met by available technology.Typically it is specified in an air permit as a maximum emission rate or concentration that can be monitored and verified.
The EPA has adopted a " top down" methodology for determining BACT.This means that the control technology with the highest degree of pollutant reduction must be evaluated first.İf that technology meets all the criteria for feasibility,cost effectiveness ,air quality impacts and other impacts, it is determined to be BACT for the proposed new source and analysis ends.İf not, the next most effective control technology is evaluated for BACT.
As a minimum level, BACT must at least meet the emission limitations of New Source Performance Standards(NSPS).Established NSPS have already been based on a thorough technology and economic evaluation.But as control technology improves , BACT may become more stringent than NSPS.

Effects of Air Pollution on Human Health

In Bhopal, India, in December 1984, a release of methly isocyanate from a pessticide plant killed about 2500 people. Similar leakages of hydrogen sulfide from natural gas processing plants have killed houndreds of people. These tragic events attract wide attention.

Their effects dont result from a single exposure these gases but from repeated exposure to low concentration for long periods.

If we wish to establish the dose-response curve for a pollutant gas, we have 3 possible approaches; animal experiments, loboratory experiments with humans and epidomiological studies of human populations.