Why every Urea Plant needs a Continuous N/C ratio measurement ?

Why every Urea Plant needs a Continuous N/C ratio measurement ? continuous N/C ratio measurement ?



Spie Nederland B.V.
Mr. Jan Huijben

Contents

̶  Why every Urea Plant needs a Continuous N/C ratio measurement ?
̶  In general
̶  Introduction
̶  General set-up of the N/C metering instrument
̶  Technical description of the system
̶  The N/C metering system has the following features
̶  Example of cost reduction
̶  Plant advantages
̶  Spie introduction
1. To achieve the most efficient production of good quality urea. 

The figure on the right side shows the urea concentration in the outlet of the urea reactor versus the N/C ratio and it clearly shows that there is an optimum N/C ratio at which the urea concentration is maximum. Operating continuously at this optimum N/C ratio leads to the lowest energy consumption figures, the lowest ammonia emission figures and you will be able to increase the plant capacity by means of creep.
You will obtain the most efficient urea production with the lowest  urea cost price.  Several factors can easily cause that one drifts away from the optimum N/C ratio; think for example about different ambient and cooling water temperatures during day and night. The more efficient production and increased capacity leads to a payback time of less than one year!

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Nations urged to combine environmental and development goals

World leaders should set six new sustainable development goals to achieve global prosperity, scientists argue

'Humans are transforming the planet in ways that could undermine any development gains,' the paper warns. Photograph: AFP/Getty Images

Degradation of the natural world is undermining efforts to reduce poverty, warn scientists, who say the only chance of achieving global prosperity is for all countries to combine poverty and environmental targets.
World leaders should set six goals around universal clean energy, an end to water and food shortages, thriving lives and livelihoods, and healthy and productive ecosystems, they say.
Prof David Griggs, director of the Monash Sustainability Institute in Australia, argues in an article in the journal Nature that it is no longer enough for countries to solely pursue the poverty alleviation targets enshrined in the millennium development goals (MDG) that were agreed in 2000 but run out in 2015.

"Humans are transforming the planet in ways that could undermine any development gains. Mounting research shows that the stable functioning of Earth systems – including the atmosphere, oceans, forests, waterways, biodiversity and biogeochemical cycles – is a prerequisite for a thriving global society," he writes, with colleagues.

Instead, the authors say that the old goals should be combined with global environmental targets drawn from science and from existing international agreements to create new "sustainable development goals" (SDGs).

"Pursuing a post-2015 agenda [which is] focused only on poverty alleviation could undermine the agenda's purpose. Growing evidence and real-world changes convincingly show that humanity is driving global environmental change and has pushed us into a new geological epoch. Further human pressure risks causing widespread, abrupt and possibly irreversible changes to basic Earth-system processes. Water shortages, extreme weather, deteriorating conditions for food production, ecosystem loss, ocean acidification and sea-level rise are real dangers that could threaten development and trigger humanitarian crises across the globe," say the authors.

Countries began the political process of adopting new post-2015 targets earlier this month at the inaugural meeting of the open working group on sustainable development goals at the UN headquarters in New York. Most developing countries argued, as they have done throughout the long-running UN climate negotiations, that rich countries should do more than developing countries to alleviate environmental pressures on the basis that they have been largely responsible for the problems and have greater resources to tackle them. However, developed countries want to see ecological improvements included as an overarching priority in the future goals of developing nations.
The scientists' hopes rests on countries combining existing, agreed UN targets and adopting a new definition of sustainable development. It is presently defined as: "Development that meets the needs of the present without compromising the ability of future generations to meet their own needs." They propose: "Development that meets the needs of the present while safeguarding Earth's life-support system, on which the welfare of current and future generations depends."
"None of this is possible without changes to the economic playing field. National policies should, like carbon pricing, place a value on natural capital and a cost on unsustainable actions. International governance of the global commons should be strengthened, for example through binding agreements on climate change, by halting the loss of biodiversity and ecosystem services and by addressing other sustainability concerns," says the article in Nature.
"A small number of goals is essential for focus; others could be added but should build on the core six. But the first step is for policymakers to embrace a unified environmental and social framework for the SDGs, so that today's advances in development are not lost as our planet ceases to function for the benefit of a global population."

Global warming predictions prove accurate

Analysis of climate change modelling for past 15 years reveal accurate forecasts of rising global temperatures

Predictions of rising temperatures due to human-induced climate change have proved accurate. Photograph: Graham Turner for the Guardian

Forecasts of global temperature rises over the past 15 years have proved remarkably accurate, new analysis of scientists' modelling of climate change shows.
The debate around the accuracy of climate modelling and forecasting has been especially intense recently, due to suggestions that forecasts have exaggerated the warming observed so far – and therefore also the level warming that can be expected in the future. But the new research casts serious doubts on these claims, and should give a boost to confidence in scientific predictions of climate change.
The paper, published on Wednesday in the journal Nature Geoscience, explores the performance of a climate forecast based on data up to 1996 by comparing it with the actual temperatures observed since. The results show that scientists accurately predicted the warming experienced in the past decade, relative to the decade to 1996, to within a few hundredths of a degree.
The forecast, published in 1999 by Myles Allen and colleagues at Oxford University, was one of the first to combine complex computer simulations of the climate system with adjustments based on historical observations to produce both a most likely global mean warming and a range of uncertainty. It predicted that the decade ending in December 2012 would be a quarter of degree warmer than the decade ending in August 1996 – and this proved almost precisely correct.
The study is the first of its kind because reviewing a climate forecast meaningfully requires at least 15 years of observations to compare against. Assessments based on shorter periods are prone to being misleading due to natural short-term variability in the climate.
The climate forecast published in 1999 is showed by the dashed black line. Actual temperatures are shown by the red line (as a 10-year mean) and yellow diamonds (for individual years). The graph shows that temperatures rose somewhat faster than predicted in the early 2000s before returning to the forecasted trend in the last few years. Photograph: Nature Geoscience The new research also found that, compared to the forecast, the early years of the new millennium were somewhat warmer than expected. More recently the temperature has matched the level forecasted very closely, but the relative slow-down in warming since the early years of the early 2000s has caused many commentators to assume that warming is now less severe than predicted. The paper shows this is not true.
Allen said: "I think it's interesting because so many people think that recent years have been unexpectedly cool. In fact, what we found was that a few years around the turn of the millennium were slightly warmer than forecast, and that temperatures have now reverted to what we were predicting back in the 1990s."
He added: "Of course, we should expect fluctuations around the overall warming trend in global mean temperatures (and even more so in British weather!), but the success of these early forecasts suggests the basic understanding of human-induced climate change on which they were based is supported by subsequent observations."

Did you know 10 Common Habits That Damage the Kidneys~

Did you know 10 Common Habits That Damage the Kidneys~

1. Not emptying your bladder early:
2. Not drinking enough water:
3. Taking too much salt:
4. Not treating common infections quickly and properly:
5. Eating too much meat:
6. Not eating enough:
7. Painkiller abuse:
8. Missing your drugs:
9. Drinking too much alcohol:
10. Not resting enough

 

Air pollutant concentrations

Air pollutant concentrations

 

Air pollutant concentrations, as measured or as calculated by air pollution dispersion modeling, must often be converted or corrected to be expressed as required by the regulations issued by various governmental agencies. Regulations that define and limit the concentration of pollutants in the ambient air or in gaseous emissions to the ambient air are issued by various national and state (or provincial) environmental protection and occupational health and safety agencies.

Such regulations involve a number of different expressions of concentration. Some express the concentrations as ppmv (parts per million by volume) and some express the concentrations as mg/m3 (milligrams per cubic meter), while others require adjusting or correcting the concentrations to reference conditions of moisture content, oxygen content or carbon dioxide content. This article presents methods for converting concentrations from ppmv to mg/m3 (and vice versa) and for correcting the concentrations to the required reference conditions.

All of the concentrations and concentration corrections in this article apply only to air and other gases. They are not applicable for liquids.

Converting air pollutant concentrations

The conversion equations depend on the temperature at which the conversion is wanted (usually about 20 to 25 °C). At an ambient sea level atmospheric pressure of 1 atm (101.325 kPa):



Notes:

• 1 atm = absolute pressure of 101.325 kPa
• mol = gram mole and kmol = 1000 gram moles
• Pollution regulations in the United States typically reference their pollutant limits to an ambient temperature of 20 to 25 °C as noted above. In most other nations, the reference ambient temperature for pollutant limits may be 0 °C or other values.
• Although ppmv and mg/m3 have been used for the examples in all of the following sections, concentrations such as ppbv (i.e., parts per billion by volume), volume percent, mole percent and many others may also be used for gaseous pollutants.
• Particulate matter (PM) in the atmospheric air or in any other gas cannot be expressed in terms of ppmv, ppbv, volume percent or mole percent. PM is most usually (but not always) expressed as mg/m3 of air or other gas at a specified temperature and pressure.
• For gases, volume percent = mole percent
• 1 volume percent = 10,000 ppmv (i.e., parts per million by volume) with a million being defined as 106.
• Care must be taken with the concentrations expressed as ppbv to differentiate between the British billion which is 1012 and the USA billion which is 109 (also referred to as the long scale and short scale billion, respectively).

Correcting concentrations for altitude

Air pollutant concentrations expressed as mass per unit volume of atmospheric air (e.g., mg/m3, µg/m3, etc.) at sea level will decrease with increasing altitude. The concentration decrease is directly proportional to the pressure decrease with increasing altitude. Some governmental regulatory jurisdictions require industrial sources of air pollution to comply with sea level standards corrected for altitude. In other words, industrial air pollution sources located at altitudes well above sea level must comply with significantly more stringent air quality standards than sources located at sea level (since it is more difficult to comply with lower standards). For example, New Mexico's Department of the Environment has a regulation with such a requirement.

The derivation of an equation for relating atmospheric pressure to altitude has been published online by the Portland State Aerospace Society and that equation can be rearranged and used as follows:



As an example, given an air pollutant concentration of 260 mg/m3 at sea level, the equivalent pollutant concentration at an altitude of 2800 meters (2.8 km) is:

• C h = 260 × [ { 288 - (6.5)(2.8) } / 288 ] 5.2558
• = 260 × 0.71 = 185 mg/m3

Note:

• The above equation for the decrease of air pollution concentrations with increasing altitude is applicable only for about the first 10 km of altitude in the troposphere (the lowest atmospheric layer) and is estimated to have a maximum error of about 3 percent. However, 10 km of altitude is sufficient for most purposes involving air pollutant concentrations.

Correcting concentrations for reference conditions

Many environmental protection agencies have issued regulations that limit the concentration of pollutants in air pollution emissions and define the reference conditions applicable to those concentration limits. For example, such a regulation might limit the concentration of nitrogen oxides (NOx) to 55 ppmv in a dry combustion flue gas (at a specified reference temperature and pressure) corrected to 3 volume percent of oxygen (O2) in the dry gas. As another example, a regulation might limit the concentration of total particulate matter to 200 mg/m3 of an emitted gas (at a specified reference temperature and pressure) corrected to a dry basis and further corrected to 12 volume percent carbon dioxide (CO2) in the dry gas.

Environmental agencies in the USA often use the terms "dscf" or "scfd" to denote a "standard" cubic foot of dry gas. Likewise, they often use the terms "dscm" or "scmd" to denote a "standard" cubic meter of gas. Since there is no universally accepted set of "standard" temperature and pressure, such usage can be and is very confusing. It is strongly recommended that the reference temperature and pressure always be clearly specified when stating gas volumes or gas flow rates.

Correcting to a dry basis

If a gaseous emission sample is analyzed and found to contain water vapor and a pollutant concentration of say 40 ppmv, then 40 ppmv should be designated as the "wet basis" pollutant concentration. The following equation can be used to correct the measured "wet basis" concentration to a "dry basis" concentration:



As an example, a wet basis concentration of 40 ppmv in a gas having 10 volume percent water vapor would have an equivalent dry basis concentration of:

• C dry basis = 40 ÷ ( 1 - 0.10 )
• = 44.4 ppmv

Correcting to a reference oxygen content

The following equation can be used to correct a measured pollutant concentration in a dry emitted gas with a measured O2 content to an equivalent pollutant concentration in a dry emitted gas with a specified reference amount of O2:^[5]



As an example, when corrected to a dry gas having a specified reference O2 content of 3 volume %, a measured NOx concentration of 45 ppmv in a dry gas that has a measured 5 volume % O2 is:

• C r = 45 × ( 20.9 - 3 ) ÷ ( 20.9 - 5 )
• = 50.7 ppmv of NOx

Note:

• The measured gas concentration C m must first be corrected to a dry basis before using the above equation.

Correcting to a reference carbon dioxide content

The following equation can be used to correct a measured pollutant concentration in an emitted gas (containing a measured CO2 content) to an equivalent pollutant concentration in an emitted gas containing a specified reference amount of CO2:



As an example, when corrected to a dry gas having a specified reference CO2 content of 12 volume %, a measured particulates concentration of 200 mg/m3 in a dry gas that has a measured 8 volume % CO2 is:

• C r = 200 × ( 12 ÷ 8 )
• = 300 mg/m3

Note:

• The measured gas concentration C m must first be corrected to a dry basis before using the above equation.

References

1. ^ Draft Programmatic Environmental Impact Statement (EIS) for Stockpile Stewardship and Management, Volume 4 See section 03.08 by going to pdf pages 135-136 for discussion of New Mexico's regulation.
2. ^ Air Quality Impact Analysis, Section 2.2 Developed for the United States Bureau of Land Management, Socorro Field Office, New Mexico. See pdf page 18 of 44 pdf pages.
3. ^ A Quick Derivation relating altitude to air pressure December 2004.
4. ^ Reference conditions of gas temperature and pressure
5. ^ David A. Lewandowski (1999), Design of Thermal Oxidation Systems for Volatile Organic Compounds, 1st Edition, CRC Press, ISBN 1-56670-410-3.
6. ^ Same as Reference 5

Air pollution emissions

Air pollution emissions

Air pollutant emissions ( commonly referred to as simply emissions) is the term used to describe any gases, liquid droplets and solid particulates which are emitted or discharged into the atmospheric air and adversely affect the health of humans, animals, ecosystems or the usefulness of a natural resource.

Air pollution emissions may be categorized as being either anthropogenic (that is, resulting from human activities) or natural and not resulting from any human activities.

The focus of this article is to define the term "air pollutant emissions" as well as identifying and discussing the anthropogenic and natural sources of air pollutant emissions.

This article does not include any discussion of the specific chemicals and other substances that are designated as air pollutants, the health effects of air pollution, the governmental regulations for controlling and limiting the emissions of air pollutants or the available technologies for reducing air pollution emissions.

Anthropogenic sources of air pollution

The anthropogenic sources of air pollution emissions include:

• Stationary point sources: A stationary point source is a single, identifiable source of air pollutant emissions. For example, the emissions from a combustion furnace flue gas stack.
• Mobile sources: Mobile sources include the exhaust emissions from vehicles driven by fuel-burning engines. For example: automobiles, trucks, buses, trains, marine vessels, airplanes, etc.
• Area sources: An area source is a two-dimensional source of diffuse air pollutant emissions. For example, the emissions of methane and ammonia from piggeries and other livestock operations.
• Evaporative sources: Evaporative sources are volatile liquids that, when not completely enclosed in a tank or other container, evaporate and release vapors over time. For example, liquids such as paints, solvents, pesticides, perfumes, hair sprays, aerosol sprays and gasoline.
• Controlled burns: Controlled burning is a useful technique practiced in forestry management and in agriculture. Such controlled burns result in the formation and release of smoke, ash, dust, carbon dioxide, nitrogen oxides and other air pollutants.
• Waste disposal landfills: Microbes and chemical reactions act upon the waste and generate landfill gas that contains methane and carbon dioxide as well as small amounts of ammonia, mercaptans and other sulfides. Eventually, that gas escapes from the landfill and is released into the atmosphere.

Natural Sources of air pollution

The natural sources of air pollution emissions include:^[3]

• Volcanoes: Volcanic activity produces smoke, ash, carbon dioxide, sulfur dioxide and other air pollutants.
• Geysers: The air pollutants emitted by geysers include hydrogen sulfide, arsenic and other heavy metals.
• Digestive gases: Methane and other gases generated by the digestion of food and emitted by animals such as cattle.
• Oceans, Rivers and Estuaries: These are sources of methane emissions thought to be caused by the digestive systems of marine life, methanogenesis in sediments and drainage areas along coastal regions, and possibly seepage from methane hydrates on the ocean floors.
• Dust: Windblown dust from areas with little or no vegetation such as desert areas.
• Sea salt: Wind-blown sea water which evaporates in the atmosphere and releases sodium chloride and other particulates into the atmosphere.
• Radioactive decay: Radon gas is released into the atmosphere by radioactive decay occurring in the Earth]]'s crust.
• Forest fires: Forest fires created by lightning, or other natural causes, result in the formation and release of smoke, ash, dust, carbon dioxide, nitrogen oxides and other air pollutants.
• Plants and trees: Biogenic sources such as pine trees and certain other plants and trees which release volatile organic compounds (VOC). About 80% of the overall emissions of VOC are from biogenic sources.
• Wetlands: Microbial action in wetlands result in significant amounts of methane being formed and released to the atmosphere. In fact, wetlands are the largest natural source of methane emissions.
• Termites: Termites are the second largest natural source of methane emissions. The methane is produced by their normal digestive process.
• Lightning: Lightning converts atmospheric nitrogen to nitrogen oxides.
• Soil outgassing: Another biogenic source wherein microbial action in soils result in the formation and release of significant amounts of nitrogen oxides.

Primary and secondary air pollutants

Primary air pollutants are those that are directly emitted from an emission source (such as listed above). Secondary air pollutants are those that are formed by reactions between the primary air pollutants and normal atmospheric constituents. In some cases, the reactions that produce secondary air pollutants utilize energy derived from sunlight.

Some examples of secondary air pollutants are: sulfuric acid, nitric acid, nitrogen dioxide, ozone, formaldehyde, peroxyacetyl nitrate (PAN), ammonium nitrate and ammonium sulfate.

Particulate matter (PM) is present in the atmosphere as both a primary and a secondary air pollutant. Primary PM is released into the atmosphere directly from a source, such as ash in the flue gas emitted from a coal-fired furnace. Secondary PM is produced in the atmosphere in the form of ammonium nitrate and ammonium sulfate. Most of the secondary PM is the respirable fraction known as PM2.5 which is very small particulate matter having a size of 2.5 μm or less.

The distinction between "emissions" and "effluents"

In general, the term "emissions" is applied to air pollutants released or "emitted" into the atmospheric air. By contrast, the term "effluents" is generally applied to pollutant-containing wastewaters released from industrial or public facilities, agricultural operations and other sources.

References

1. ^ Air Pollution Emissions Overview From the website of the U.S. Environmental Protection Agency
2. ^ Terms of Environment: Glossary, Abbreviations and Acronyms From the website of the U.S. Environmental Protection Agency (scroll to P terms)
3. ^ Methane and Nitrous Oxide Emissions From Natural Sources From the website of the U.S. Environmental Protection Agency
4. ^ Natural Sources - Global Emissions From the website of the U.S. Environmental Protection Agency.
5. ^ Note: In the context of this article, a biogenic source is one that produces air pollutants by the action of living organisms.
6. ^ Emission Inventory, January 2009 From the website of the California Air Resources Board
7. ^ A Regional Simulation to Explore Impacts of Resource Use and Constraints Final Report, December 2006, Oak Ridge National Laboratory (ORNL)
8. ^ Same as Reference 4
9. ^ Stanley E. Manahan (2004), Environmental Chemistry, 8th Edition, CRC Press, SBN 1-56670-633-5.
10. ^ Note: PM2.5 is defined as being respirable because it is small enough to pass through the nose and penetrate deep into the gas exchange region of the lungs.

Air quality monitoring network design

Air quality monitoring network design

The design of the air quality monitoring network basically involves determining the number of stations and their location, and monitoring methods, with a view to the objectives, costs and available resources. The typical approach to the network design, appropriate over the city-wide or national scale, involves placing monitoring stations or sampling points at carefully selected representative locations, chosen on the basis of required data and known emission/dispersion patterns of the pollutants under study. This scientific approach will produce a cost effective air quality monitoring program. Sites must be carefully selected if measured data are to be useful. Moreover, modeling and other objective assessment techniques may need to be utilized to "fill in the gaps" in any such monitoring strategy.

An air monitoring network properly designed is a key component of any air quality control program. In practice, the operation and maintenance of air quality monitoring stations are expensive, so it is desirable to use as few stations as possible to meet monitoring goals. Another consideration in the basic approach to the network design is the scale of the air pollution problem:

• The air pollution is of predominantly local origin. The network is then concentrated within the urban area to monitor nitrogen oxides (NO2), sulfur dioxide (SO2), particulate matter smaller than 10 um (PM10), particulate matter smaller than 2.5 um (PM2.5), carbon monoxide (CO) and volatile organic compounds (VOC).
• There is a significant regional contribution to the problem and more emphasis will be on monitoring ozone (O3) and particulate matter..
• Large-scale phenomena, such as winter or summer smog episodes or dust cloud (local impacts should be avoided).

The number of sites will depend upon the size and topography of the urban area, the complexity of the source mix and the monitoring objectives.[1][6] The U.S. Environmental Protection Agency (U.S. EPA) and the European Union (EU) Directives specify a minimum number of stations to be established dependent upon the population, and it also indicates what types of areas should be monitored.

Monitoring Objectives

The air quality monitoring program design will be dependent upon the monitoring specific objectives specified for the air quality management in the selected area of interest. What are the expected outputs of the monitoring activity? Which problems need to be addressed?

Defining the output will influence the design of the network and optimize the resources used for monitoring. It will also ensure that the network is specially designed to optimize the information on the problems at hand. There might be different objectives for the development of the environmental monitoring and surveillance system. Normally, the system will have to provide online data and information transfer with a direct /automatically/ on-line quality control of the collected data. Several monitors, sensors and data collection systems may be applied to make on-line data transfer and control possible.[6]

The main objectives for the development of an air quality measurement and surveillance program might be related to:

• Population exposure and health impact assessment
• Identifying threats to natural ecosystems
• Determining compliance with national or international standards
• Informing the public about air quality and establishing alert systems
• Providing objective input to air quality management and to transport and land-use planning
• Identifying and apportioning sources
• Developing policies and setting priorities for management action
• Developing and validating management tools such as models and geographical information systems
• Quantifying trends to identify future problems or progress in achieving management or control targets.

Screening Studies and Operational Sequence

Before a final program design is presented it is also important to undertake a preliminary field investigation often referred to as a screening study. This may consist of some simple inexpensive measurements (e.g. using passive samplers) and simple dispersion models. The data will give some information on the expected air pollution levels, highly impacted areas and the general background air pollution in the area.

The number of monitoring stations and the indicators to be measured at each station in the final permanent network may then be decided upon as based on the results of the screening study as well as on the knowledge of the sources and prevailing winds. Once the objective of air sampling is well-defined and some preliminary results of the screening study are available, a certain operational sequence has to be followed. The best possible definition of the air pollution
problem, together with the analysis of the personnel, budget and equipment available, represent the basis for the decision on the following questions:

• What spatial density of sampling stations is required?
• How many sampling stations are needed?
• Where should the stations be located?
• What kind of equipment should be used?
• How many samples are needed and during what period?
• What should the sampling (averaging) time and frequency be?
• What additional background information is needed? (For example: meteorology, topography, population density, emission sources and rates)
  • Meteorology
  • Topography
  • Population density
  • Emission sources and rates
• What is the best way to obtain the data (configuration of sensors and stations)?
• How will the data be accessible, communicated, processed and used?

The answers to these questions will vary according to the particular need in each case. Most of the questions will have to be addressed in the site studies and in the selection of sites as discussed below.

Site selection

The urban air quality monitoring program will normally provide the information to support and facilitate the assessments of the air quality in a selected area and to meet the objectives as stated by the users. Some of the objectives have been presented above. This normally means that for designing a monitoring program in an urban area, several monitoring stations are needed for characterizing the air quality in the region. The areas are generally divided into urban, suburban and rural areas. Measurements should be undertaken in different microenvironments within these areas, where people are present. In a typical urban air pollution measurement program, the microenvironments selected are often classified as:

• Urban including traffic, residential, commercial, and background
• Suburban (traffic and industrial)
• Rural sites (background areas)

When measuring air quality or analyzing the results from measurements it is important to bear in mind that the data you are looking at are a sum of impacts or contributions originating from different sources on different scales.
In any measurement point in the urban area the total ambient concentration is a sum of:

• Natural background concentration
• Regional background
• City average background concentration
• Local impact from traffic along streets and roads
• Local impacts from small area sources like open air burning (waste and cooking)
• Impact from large point sources such as Industrial emissions and power plants

To obtain the information about the importance of these different contributions it is therefore necessary to locate monitoring stations, so that they are representative for different impacts. In addition to the air pollution data, we will often need meteorological data to identify and quantify the sources contributing to the measurements. It is also important to carefully characterize the representativeness of the monitoring sites, and to specify what kind of stations we are reporting
data from. More than one monitoring site is often needed in order to characterize the air quality in the urban area.[4][5]

Additional factors points to consider in this case when selecting specific site locations include:

• Past and current monitoring results
• Site accessibility
• Power accessibility
• Topographical effects
• Local interferences
• Security

When considering the location of individual samplers, it is essential that the data collected are representative for the location and type of area without undue influence from the immediate surroundings.

Air quality indicators

Air quality indicators have been selected for different environmental issues and challenges. Not all indicators are specific enough to address only one issue. The nature of the air pollution involves some indicators addressing several issues. Some of the issues that have to be addressed are:

• Climate change
• Ozone layer depletion
• Acidification
• Toxic contamination
• Urban air quality
• Traffic air pollution.

As it can be seen from the list, the indicators have to cover all scales of the air pollution problems (in space and time) to address different type of impacts and effects.[6]

The most commonly selected air quality indicators for urban and industrial air pollution are:

• Nitrogen dioxide (NO2)
• Sulfur dioxide (SO2)
• Carbon monoxide (CO)
• Particles with aerodynamic diameter less than 10 μm and 2.5 μm (PM10 and PM2.5)
• Ozone (O3).
• Volatile organic compounds (VOC)

The US EPA refers to the first five compounds listed above as the priority pollutants.[1] They are also given in the Air Quality Daughter Directives of the European Union with specific limit values for the protection of health and the environment.[6] The World Health Organization guideline values also include the above indicators.[2][3]

Other elements in the design

In the design of the air quality monitoring program we will also have to include the measurements of meteorology. Weather stations should be located in order to assess the general wind flow over the study area. Weather stations do not need to be placed at all air monitoring sites, but some co-locations will decrease the total cost of these measurements. Before the air quality data can be used to assess the situation in the area, it is important to assure that the data collected are real concentration values, which may be compared to similar information from other areas and countries. For each pollutant, which is measured as the input to the air quality assessment and evaluation, the following main questions may be asked:

• Have the suitable quality assurance procedures been set up for all stages and activities?
• Is technical advice available?
• Is monitoring being carried out at suitable locations?
• Have suitable arrangements for data handling and storage been made and implemented?

The documentation to support the credibility of data collection and the initial data quality assurance are the responsibility of the data provider. This includes the process of data collection, application of calibration factors, Quality Assurance procedures (QA/QC), data analysis, data “flagging”, averaging and reporting. A combination of data record notes, data quality flags and process documentation are all part of this first phase of processing. During the data collection phase, one role of the data provider is to assist in maintaining the process credibility and validity of the data. Good data quality is essential for adequate reporting of the
air quality.

References

1. Monitoring ambient air quality for World health impact assessment, World Health Organization (WHO) Regional Office for Europe. WHO Regional Publications, European Series, No. 85 (1999), ISBN 92-890-1351-6.
2. National Ambient Air Quality Standards (NAAQS), 1990, From the website of the U.S. EPA.
3. "Methodology for assessment of exposure to environmental factors in application to epidemiological studies", World Health Organization (WHO) Regional Office for Europe, The Science of the Total Environment, Volume 168, Number 2, 16 June 1995, pp. 93-100(8).
4. Air Quality Guidelines for Europe, World Health Organization (WHO) Regional Office for Europe, 2nd Edition. WHO Regional Publications, European Series, No. 91 (2000), ISBN 92-890- 1358-3.
5. Air Quality Guidelines Global Update 2005, World Health Organization (WHO) Regional Office for Europe, 2006, ISBN 92-890-2192-6.
6. Criteria for EUROAIRNET - The EEA Air Quality Monitoring and Information Network, Technical Report No. 12, (1999).

Publishing Note:

This article was written by Massoud Estiri, a member of this wiki, and uploaded by him in a pdf format. It was then transformed into this wiki's format using the Wikitext markup language and the Wikitext Editor by Milton Beychok, the organizer of this wiki..

Air quality measurement instruments and methods

Air quality measurement instruments and methods

 

Instruments for measurements of air quality may vary strongly in complexity and price from the simplest passive sampler to the most advanced and most often expensive automatic remote sampling system based upon light absorption spectroscopy of various kinds. Air monitoring methodologies can be divided into four main generic types, covering a wide range of costs and performance levels: continuous analyzers, active manual samplers, passive samplers and remote sensing devices. Each of these methodology types have advantages and disadvantages. Each type can be particularly useful in achieving certain monitoring objectives. Therefore it is important to consider each type of monitoring method in optimizing a monitoring network design.

The table below lists four typical types of instruments, their abilities and prices:


Relatively simple equipment is usually adequate to determine background levels (for some pollutants), to check Air Quality Guideline values or to observe trends. Also for undertaking simple screening studies, passive samplers may be adequate. However, for complete determination of regional air pollution distributions, relative source impacts, hot spot identification and operation of warning systems more complex and advanced monitoring systems are needed.

When data are needed for model verification and performance, expensive monitoring systems are usually required.

Passive Samplers

Simple passive samplers have been developed for surveillance of time integrated gas concentrations. These types of samplers are usually inexpensive in use, simple to handle and have an adequate overall precision and accuracy dependent upon the air pollution concentration level in question. This method has been used in industrial areas, in urban areas and for studies of indoor/outdoor exposures for variety of pollutant like as ammonia (NH3), benzene-toluene-xylenes (BTX), sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3), hydrogen fluoride (HF), hydrogen chloride (HCl), aldehydes, and volatile organic compounds (VOC).

Passive samplers include items such as diffusion tubes and badges. They tend to be simple and low cost, and can be deployed in large numbers with no reliance on access to electrical connections. This type of sampler is useful for screening studies, for mapping, and for baseline studies. While the samplers are often used for monitoring O3, NOx and SO2, the technology is unproven for some pollutants. Passive samplers are labor-intensive for their deployment and analysis. Passive samplers generally provide only monthly or weekly averages.

Passive samplers are an excellent tool for saturation sampling. This involves the collection of many samples in a small, well-defined area over a short duration, to
provide an in-depth characterization. Saturation sampling is typically conducted to gather data necessary to properly site long-term monitoring devices. The passive sampler incorporates an adsorbing surface, pre-treated depending upon the target gas. The adsorbing surface is placed within a cylindrical enclosure that has a diffusive surface, allowing the target gas to reach the adsorbing surface. The sample is exposed for between 1 and 7 days or more depending upon the target gas and the expected ambient concentration. The sample is then extracted and analyzed using a variety of standard laboratory methods.

Active samplers

Active samplers draw ambient air through a collecting medium for some specified time, typically 24 hours, with the volume of air being metered. The collecting medium is subsequently analyzed and the concentration of pollutant in the sampled air is determined. Active sampling methods are usually low cost and easy to operate. The active sampling methodologies offer reliable performance, with an extensive historical database because most of these methods have been in
operation for many years. Active sampling methods require labor-intensive sample collection and analysis, and require laboratory analysis after the ambient air sample is collected.

Manual sampling is event-specific, that is, the sampler usually operates over a fixed period of time accumulating and integrating sample. Integrating measurement methods, although fundamentally limited in their time resolution, are useful for the assessment of long-term exposure, as well as being invaluable for a variety of area-screening, mapping and network design functions. Manual sampling is still widely used world-wide because manual methods offer a wider variety of pollutant monitoring and can be relatively straightforward.

In the past, active sampling of gaseous pollutants was typically carried out. This can be done by using wet absorption techniques, where sample air is introduced into a liquid reagent through impingers. The pollutant is absorbed in the reagent and the reagent is then analyzed using various methods (usually some sort of chromatography) to determine the concentration of pollutant in the batch sample. Another method of batch sampling is where sample air is drawn through a porous bed of solid adsorbent over a period of time. The pollutant is then extracted from the adsorbent and analyzed. Collection efficiencies from this type of sampling apparatus can often exceed 90%. Sampling for most ambient air toxics involves the collection of grab samples and subsequent analysis using
gas chromatography–mass spectrometry (GC-MS) or high performance liquid chromatography. The grab sample is usually collected on an absorbent material or in a specially treated, chemically inert cylinder or bag. The sampling method is dependent upon the target compound, analysis method and sampling environment. Detailed information on the standard sampling methods used in the U.S. Environmental Protection Agency (U.S. EPA) air toxics sampling program is available online.[1]

Canister sampling

Canister sampling can be used for volatile hydrocarbons up to C9. Air samples are collected in stainless steel canisters by the aid of a pump or just by opening the valve of an evacuated canister. The canisters are sent to the laboratory for analysis and then cleaned by evacuating it.

Adsorbent tubes

Adsorbent tubes can be used for sampling of a wide number of volatile organic compounds. The tubes can be filled with different kinds of adsorbents, depending of which components of interest. When used as a passive sampler, there is no need for any extra equipment. To decrease the minimum sampling period or to improve the detection limit, the tube can be connected to a pump. Adsorbent tubes are not suitable for some of the most volatile hydrocarbons.

Absorption bottles

The most commonly used active device for gaseous sampling has been the bubbler with an absorption solution, often together with a filtration system. A chemical solution is used to stabilize the pollutant for subsequent analysis with minimum interference by other pollutants. Samplers have also been used with impregnated filters based on the iodide absorption method. The flow is set with a restrictor and measured with a mass flow meter. In the sequential version of these
samplers the desired start time can be set to start sampling at the same start time every day at 24 hour intervals.

Impregnated filter sampling

A relatively simple alternative to the use of solutions for absorption and chemical reaction is to use chemically impregnated filters. These filters are prepared by dipping filters into a solution of the selected chemical and drying them before sampling commences. This sampler consists of a glass bulb with an impregnated filter inside. The impregnated filter bulb is connected to a calibrated pump that draws a steady airflow through the filters. After exposure, the filter and the pollutant of interest react with the chemical on the filter. The filter is sent to the laboratory for analysis. The detection limit is better than for the other methods but the method is more labor intensive and depends of extra sampling equipment such as a high precision electric pump.

High and low volume sampler

For measurements of ambient suspended particles the most accurate way to determine aerosol mass concentration is to pass a known volume of air through a filter. Each filter has to be weighed unexposed, before being installed in the sampler. The weighing should be performed in weighing, the filter is placed in the plastic bag with zip tightening and marked with station identification and/or number.
Size selective samplers
A variety of sampling devices are available that segregate collected suspended particulate matter into discrete size ranges based on their aerodynamic diameters. These particle samplers may employ one or more fractionating stages. The physical principle by which particle segregation or fractionation takes place is inertial impaction. Therefore, most such devices are called impactors. Other impactors have been developed to fractionate suspended particles into two size fractions, i.e., coarse (from 2.5 to 10 μm) and fine (less than 2.5 μm). Although these virtual or dichotomous impactors operate like a typical inertial unit, large particles are impacted into a void rather than an impervious surface.

High volume PUF-sampler

The high volume PUF-sampler can be used for sampling of a wide range of organic pollutants like poly-aromatic hydrocarbons (PAH), dioxins, pesticides (like DDT), etc. The sampler consists of a glass cylinder and a filter holder. The glass cylinder holds two polyurethane foam (PUF) plugs for trapping the gas phase of the pollutants. The filter holder in front contains a glass fiber for collecting pollutants condensed on particles. The air is drawn through the sampler by a pump and 500 m3 of air would be a typical sample volume for a 24-hour sample.

Continuous monitors

The analyzers are connected to a data acquisition system and an automatic gas calibration unit to provide regular quality control checks for the data. Continuous analyzers provide high resolution measurements (typically hourly averages or better) at a single point for most of the "Criteria Pollutants" (SO2, NO2, CO, O3 and PM) as well as for other important species such as VOC.

The sample is analyzed on-line and in real time, usually by electro-optic methods: UV or IR absorption, fluorescence, or chemiluminescence are common detection principles. To ensure that data from continuous emission monitoring systems are accurate and reliable, a high standard of maintenance, operational and quality assurance and quality control procedures is invariably required.

The advantages of continuous analyzer systems are that they offer a proven technology, high performance, hourly data, and/or on-line information. Disadvantages of continuous emission monitoring systems include the complexity and cost of the instrumentation, the requirement for a high level of skill in the operation of the instrumentation, and high recurrent costs.

Some monitoring networks incorporate mobile monitoring stations to improve spatial scales. It is possible continuous analyzers are fitted into a special enclosure on the back of a truck or on a trailer. Mobile stations can be good for special studies including complaint investigation. However, there can be durability and stability issues with the instruments, particularly when driven over rough roads. It can also take time to ensure the instruments are stable and therefore, it may not be possible to move the station on a daily basis. A good source of power supply is necessary and it may be difficult to ensure stable and continuous power at all locations.

Methods and instruments for measuring continuous air pollutants must be carefully selected,
evaluated and standardized. Several factors must be considered:

• Specificity: respond to the pollutant of interest in the presence of other substances
• Sensitivity: range from the lowest to the highest concentration expected
• Stability: remain unaltered during the sampling interval between sampling and analysis
• Precision: accurate and representative for the true pollutant concentration in the atmosphere where the sample is obtained
• Response time: short enough to record accurately rapid changes in pollution concentration
• Ambient temperature and humidity: no influence on the concentration measurements

Remote sensors

Remote sensors have recently been developed. They use long-path spectroscopic techniques to make real-time concentration measurements of a range of pollutants. The data are obtained by integrating along a path between a light source and a detector. Long-path monitoring systems can have an important role in a number of monitoring situations, particularly in proximity to sources. Remote sensing systems provide path or range-resolved data with multi-parameter
measurements and are useful near emission sources.

However, the remote sensing systems are very complex, expensive and difficult to support, operate, calibrate, and validate. Data from remote sensing systems are not readily comparable with point data, and the operation of remote sensing systems is susceptible to problems due to atmospheric visibility and and other
interferences.

Typical air pollutant concentrations and methods of measurement

The table below lists the typical air pollutant concentration of interest involved in monitoring air quality:



The most commonly used methods for automatically monitoring air pollutants such as those above are:

• Sulfur dioxide (SO2): Measured by the fluorescent signal generated by exciting SO2 with UV light.
• Nitrogen oxides (NOx): Measured by the chemilumiscent reactions between NOx and O3.
• Ozone (O3): Measured by an ultraviolet absorption analyzer which determines the ozone concentration by the attenuation of 254 nm UV light along a single fixed path cell.
• Particulate matter (PM-10, PM-2.5 and TSP): Measured by gravimetric methods including true micro weighing technology.For automatic monitoring an instrument named "Tapered Element Oscillating Microbalance (TEOM)" has been most frequently used. Measurement on filter tape using the principles of beta attenuation for estimating 30 minute or one hour average concentrations of PM-10 or PM-2.5 has also been used.
• Carbon monoxide (CO): In urban air pollution studies, a non-dispersive infrared photometer utilizing gas filter correlation technology and state-of-the-art optical and electronic technology is used to measure low concentrations of CO accurately and reliably.
• Hydrocarbons (Methane and NMHC): Measured using a flame ionization detector (FID). However, problems in power supplies may interrupt these continuous measurements.
• Volatile Organic compounds (VOC): Measured by gas chromatography and photo-ionization detector (PID).

Meteorological data

Meteorological data are important input data to a system used for information, forecasting and planning purposes. Meteorological data are needed from the surface, normally collected from that are 10 m high, as well as up to the top of the atmospheric boundary layer.

Meteorological surface data such as winds, temperatures, stability, radiation, turbulence and precipitation are automatically measured and transferred to a central computer via radio communication, telephone or satellite. Measuring such data requires sensors for at least the most important parameters such as:

• Wind speeds
• Wind directions
• Relative humidity
• Temperatures or vertical temperature gradients
• Net radiation
• Wind fluctuations or turbulence
• Atmospheric pressure
• Precipitation

by Dr. Amar Nath Giri

Temperature conversions

Temperature conversions

Temperature conversion refers to the methods for converting temperature values expressed in one unit to values expressed in another unit. The four most widely used temperature measurement units are those of the Kelvin, Celsius, Fahrenheit and Rankine temperature scales. This article defines and compares the four scales and also provides the methods for converting temperatures from any one of those four scales to the other three.

Definition of the temperature scales and their reference points

Table 1 below defines and compares the Kelvin, Celsius, Fahrenheit and Rankine temperature scales in terms of their three reference points, namely their values at absolute zero, the melting point of water and the boiling point of water:

Conversion methods

Table 2 presents the the simple equations used to convert temperatures from any one of the four scales to the other three:

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Acid dewpoint

Acid dew point

The acid dewpoint (also acid dew point) of a flue gas (i.e., a combustion product gas) is the temperature, at a given pressure, at which any gaseous acid in the flue gas will start to condense into liquid acid^]

The acid dew point of a flue gas, at a given pressure, is often referred to as the point at which the flue gas is "saturated" with gaseous acid, meaning that the flue gas cannot hold any more gaseous acid.

In many industrial combustion processes, the flue gas is cooled by the recovery of heat from the hot flue gases before they are emitted to the atmosphere from the final flue gas stack (commonly referred to as a chimney). It is very important not to cool the flue gas below its acid dew point because the resulting liquid acid condensed from the flue gas can cause serious corrosion problems for the equipment used in transporting, cooling and emitting the flue gas.

Chemistry and mechanism

Sulfuric acid dew point

As a broad generality, flue gases from the combustion of coal, fuel oil, natural gas, or biomass are primarily composed of gaseous carbon dioxide (CO2) and water vapor (H2O) as well as gaseous nitrogen (N2) and excess oxygen (O2) remaining from the intake combustion air. Typically, more than two-thirds of the flue gas is nitrogen. The combustion flue gases may also contain small amounts of particulate matter, carbon monoxide (CO), nitrogen oxides (NOx), and sulfur oxides in the form of gaseous sulfur dioxide (SO2) and gaseous sulfur trioxide (SO3). The SO3 is present because a portion of the SO2 formed in the combustion of the sulfur (S) compounds in the combustion fuel is further oxidized to SO3. The gas phase SO3 then combines the vapor phase H2O to form gas phase sulfuric acid (H2SO4):

Calculated sulfuric acid dew points of typical combustion flue gases, as a function of sulfur trioxide content, and water vapor.

• H2O + SO3 → H2SO4
• water + sulfur trioxide → sulfuric acid

Because of the presence of gaseous sulfuric acid, the sulfuric acid dew point of most flue gases is much higher than the water dew point of the flue gases. For example, a flue gas with 12 volume % water vapor and containing no acid gases has a water dew point of about 49.4 °C (121 °F). The same flue gas with the addition of only 4 ppmv (0.0004 volume %) of SO3 will have a sulfuric acid dew point of about 130.5 °C (267 °F).

The acid dew point of a combustion flue gas depends upon the composition of the specific fuel being burned and the resultant composition of the flue gas. The adjacent graph depicts how the amounts of water vapor and gaseous SO3 present in a flue gas affect the sulfuric acid dew point of the flue gas.

Given a flue gas composition, its acid dew point can be predicted fairly closely. As an approximation, the sulfuric acid dew points of flue gases from the combustion of fuels in thermal power plants range from about 120 °C to about 150 °C (250 to 300 °F).

Other acid dew points

Sulfurous acid
Some of the sulfur dioxide in flue gases will also combine with water vapor in the flue gases and form gas phase sulfurous acid (H2SO3):

• H2O + SO2 → H2SO3
• water + sulfur dioxide → sulfurous acid

Nitric acid
The nitrogen in flues gases is derived from the combustion air as well as from nitrogen compounds contained in the combustion fuel. Some small amount of the nitrogen is oxidized into gaseous nitrogen dioxide (NO2) and some of that gas phase nitrogen oxide then combines with water vapor to form gas phase nitric acid (HNO3):

• H2O + NO2 → H2NO3
• water + nitrogen dioxide → nitric acid

Hydrochloric acid

Some flue gases may also contain gaseous hydrochloric acid (HCl) derived from chloride compounds in the combustion fuel. For example, municipal solid wastes contain chloride compounds and therefore the flue gases from municipal solid waste incinerators may contain gaseous hydrochloric acid which will condense into liquid hydrochloric acid if those flue gases are cooled to a temperature below the acid dew point of hydrochloric acid.

Prediction of acid dew points

These equations can be used to predict the acid dew points of the four acids that most commonly occur in typical combustion product flue gases:

Sulfuric acid (H2SO4) dew point:
(1)1000/T=1.7842−0.0269log10(PH2O)−0.1029log10(PSO3)+0.0329log10(PH2O)log10(PSO3)
or this equivalent form:
(2)1000/T=2.276−0.02943loge(PH2O)−0.0858loge(PSO3)+0.0062loge(PH2O)loge(PSO3)

Sulfurous acid (H2SO3) dew point:
(3)1000/T=3.9526−0.1863loge(PH2O)+0.000867loge(PSO2)+0.000913loge(PH2O)loge(PSO2)

Hydrochloric acid (HCl) dew point:
(4)1000/T=3.7368−0.1591loge(PH2O)−0.0326loge(PHCl)+0.00269loge(PH2O)loge(PHCl)

Nitric acid (HNO3) dew point:
(5)1000/T=3.6614−0.1446loge(PH2O)−0.0827loge(PHNO3)+0.00756loge(PH2O)loge(PHNO3)
where:
T = The acid dew point temperature for the indicated acid, in kelvins
P = Partial pressure, in atmospheres for equation 1 and in mmHg for equations 2, 3, 4 and 5

Compared with published measured data, the acid dew points predicted with equations 3, 4 and 5 are said to be within 6 kelvins, and within 9 kelvins for equations 1 and 2

Predicting the sulfur trioxide content of flue gases

As can be seen in the above equation for the sulfuric acid dew point of a flue gas, the partial pressure of sulfur trioxide in the flue gas is required. That partial pressure can be readily determined given the total pressure of the flue gas and the volume percent of sulfur trioxide in the flue gas, since the partial pressure of any component of a gaseous mixture may be obtained by simply multiplying the total gas pressure by the component's volume fraction of the gaseous mixture.

Determining the volume percent of sulfur trioxide in a flue gas by theoretical calculations is quite difficult and unreliable. However, the volume fraction of the sulfur dioxide in the flue gas can be determined by assuming that 90 percent or more of the sulfur in the combustion fuel will be oxidized into gaseous sulfur dioxide when the fuel is combusted. Then it is commonly assumed that about 1 to 5 percent of the sulfur dioxide will be further oxidized into sulfur trioxide. In other words, if the sulfur dioxide in the flue gas is determined to be 0.3 volume percent and it is assumed that 3 percent of that will be further oxidized to sulfur trioxide, the volume fraction of sulfur trioxide in the flue gas will be (0.003)(0.03) = 0.00009 and, if the flue gas pressure is essentially 1 atm (760 mmHg), the partial pressure of the sulfur trioxide will be (0.00009)(760) = 0.0684 mmHg.