Analytical Instrumentation & Environmental Monitoring


The principles of dust emission monitoring: Part 1

May 2005 Analytical Instrumentation & Environmental Monitoring

For environmental monitoring purposes it is vital to ensure that the data reported from each emission source is directly comparable and referenced to a known identifiable standard.

The two basic methods for reporting are to identify emissions as mass concentration in mg/m3 or as a mass flow in gm/sec or kgm/hr. Providing all the necessary information is available, it is possible to convert one standard into the other. Both methods are consistent.

Mass concentration

The most common form of emission reporting is to express the data as a mass concentration. It is important to recognise what is meant by mass concentration.

Dust released to atmosphere from an industrial process is in the form of small particles suspended in a flow of air or exhaust gas. Assuming a homogeneous mixture of dust particles and air, if we were to microscopically examine the emissions we would find that within each cubic metre of air emitted there would be a number of dust particles, and if we knew the particle size distribution and density of those particles we could quickly calculate the mass of dust within that cubic metre of air. Alternatively, we could theoretically filter the cubic metre of air, collect the particles and weigh them, which is precisely what happens in an isokinetic sample test, where a sample of gas is extracted from the duct and the dust particles collected on a filter paper.

The result would be the same: a mass of dust per cubic metre of air, otherwise known as mass concentration.

Suppose that the process producing the dust had constant characteristics and that the rate of mass emission of dust was constant. Suppose then that we were able to double the flow of air that carries the particles by, say, injecting clean air into the duct. While the total mass emission would be constant, we would have diluted the suspension of dust particles in the air by a factor of 2, and the mass concentration would be halved, since the same mass would now be distributed throughout 2 cubic metres instead of one.

Similarly, if instead of increasing the amount of air, we increased the air temperature from 300K to 600K, we would also double the volume occupied by the air, assuming that the pressure remained constant. This too would produce a reduction in the mass concentration of dust by a factor of 2. The same effect occurs if the air pressure reduces. A reduction of 5% in atmospheric pressure will produce a 5% increase in the volume occupied by the air and a 5% decrease in mass concentration.

The measurement of mass concentration is thus dependent on any factor that can change the volumetric characteristics of the carrier gas, be it air or combustion exhaust gases. These factors fall into two groups:

a) Gas law effects: the effects of temperature and pressure.

b) Dilution effects: the effects of excess air and water vapour levels.

Data normalisation

It is clear that in order to report dust emissions it is necessary to ensure that the data is presented under a set of standard conditions for describing the volume of the carrier gas. It is standard practice to report the data as a mass per normal cubic metre of dry gas, at a specified level of oxygen.

This means that the actual measured concentration is corrected back to a value consistent with the carrier gas being at 0°C and 101,3 kPa pressure. This takes care of any variation in results due to temperature and pressure.

For measurements of emissions from combustion sources the dilution factor due to ingress of excess air can be determined from a measurement of the oxygen level of the exhaust gas. Since different processes require different levels of excess combustion air for optimum operation, the level of oxygen quoted for the emission normalisation varies from process to process, but must always be stated for the report to be complete and meaningful.

Exhaust emissions from combustion sources always contain gaseous water, produced from oxidation of the hydrogen within the fuel. The level of water vapour present depends upon the type of fuel being burned. In order to have a unique identifiable standard, it is usual to define the normal gas as a dry gas, thus removing errors in data reporting due to variations in water vapour levels in the emitted gas.

The consequences of all this for dust monitoring from combustion processes can become quite serious. Instead of a relatively simple measurement of dust concentration, in order for the data to be rigorously reported, it becomes necessary to monitor levels of exhaust gas temperature, pressure, oxygen and water vapour. A simple measurement has suddenly become a complex measurement in which the cost of measuring the gas normalisation parameters is greater than the cost of the primary dust measurement. For Schedule A processes, where the normalisation data is already available as part of the gas analysis requirements, this is not a problem. For many Schedule B processes, which are only required to measure dust, the problem becomes severe.

For non-combustion sources, most of which are Schedule B processes, the problem is not merely one of cost, but one of actual feasibility. In a process where the exhaust carrier gas is air, the effects of dilution cannot be measured or detected, since the oxygen level will always be 20,9%. For example, a process that has an allowed mass emission limit of 100 mg/m3, but is emitting 150 mg/m3 of dust, can merely dilute the exhaust gas with air to bring the emitted concentration below 100 mg/m3 and be technically legal, although the total amount of dust being emitted is not reduced by the dilution.

In such situations, specifying an emission limit in mass concentration becomes a worthless and futile exercise. Reported values of dust emission become meaningless, if they cannot be referred to any standard.

Mass flow

The second method of data reporting is to express the emissions in the form of a mass flow in grams per second or kilograms per hour, representing the total mass of dust emitted per unit time, by that process. The measurement is related to mass concentration.

Mass flow = mass concentration x volumetric flow

= mg/m3 x m3/sec

= mg/sec

Note that for this measurement, no normalisation data is required. The value for mass flow is absolute and does not depend in any way on gas temperature, pressure, oxygen or water vapour values, or on any form of dilution of the exhaust gases.

This is illustrated by the earlier example of dust emissions being diluted by a factor of 2 by doubling the airflow. While the mass concentration is halved the volumetric flow is doubled, leaving the value of mass flow unchanged.

Total mass emissions

While a measurement of normalised mass concentration allows a direct comparison of the dust abatement efficiency of different emission sources, it does not help assess the impact on the environment of those sources.

A small stack of 300 mm diameter emitting 500 mg/m3 of dust at 10 m/sec velocity would be ejecting 1,5 gm/sec of dust into the environment. A large power station stack of 6 m diameter emitting only 100 mg/m3 of dust at 10 m/sec would be producing 10 gm/sec of dust.

It is only mass flow that can be related to environmental impact. From measurements of mass flow it is simple to calculate total mass emissions over any period of time, for individual stacks, industrial sites and geographical areas. Emissions reported in mass concentration must be converted to mass flow in order for this information to be obtained, thus requiring the measurement of both mass concentration and gas flow. (To be continued.)

For more information contact Stuart Truebody, Environmental Process Analytics, 012 661 6656, [email protected]





Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Biofilm monitoring system
Instek Control Analytical Instrumentation & Environmental Monitoring
Alvim, through Instek Controls, provides innovative, high-tech solutions for biofilm and biofouling monitoring in industrial plants.

Read more...
WearCheck Water earns AdBlue/DEF analysis accreditation
Wearcheck Analytical Instrumentation & Environmental Monitoring
WearCheck Water recently became the first laboratory in Africa to be officially ISO17025 accredited to test AdBlue diesel exhaust fluid by the South African National Accreditation System.

Read more...
Streamlining strain gauge load cell integration
Vepac Electronics Analytical Instrumentation & Environmental Monitoring
Vepac’s data acquisition hardware provides an efficient and effective all-in-one solution for customers looking to simplify, enhance and optimise their strain gauge load cell systems.

Read more...
Oxygen measurement in beverages
Anton Paar Analytical Instrumentation & Environmental Monitoring
[Sponsored] Anton Paar offers a complete range of oxygen measurement instruments for total package oxygen (TPO) measurements, at-line quality control (QC), and in-process monitoring. These instruments help beverage manufacturers achieve accurate, reliable oxygen control at every stage of production.

Read more...
Sensors and controls for food, beverage and pharmaceutical
Instek Control Analytical Instrumentation & Environmental Monitoring
Included in Instek Control’s range is Anderson-Negele, which has adopted ‘Hygienic By Design’ as its guiding principle, with a particular focus on meeting the stringent regulatory requirements found in industries such as dairy, brewery or pharmaceutical.

Read more...
Vertical labelling of test tubes in clinical laboratories
Omron Electronics Analytical Instrumentation & Environmental Monitoring
Werfen has implemented a new automated machine for the supply of reagents to drug toxicology laboratories, built by MACCO in collaboration with OMRON and Marini Pandolfi. It uses OMRON SCARA robots and advanced vision systems to ensure reagent quality through vertical handling and labelling process of test tubes.

Read more...
Keeping a close eye on product quality and purity
Endress+Hauser South Africa Analytical Instrumentation & Environmental Monitoring
Colour measurements are necessary in many processes to avoid product losses and ensure safe production and batching.

Read more...
The importance of environmental monitoring and visibility at data centres
Legrand Analytical Instrumentation & Environmental Monitoring
Data centres are one of the most energy-intensive building types, consuming up to 50 times the energy per floor space compared to a typical commercial office building. With organisations embracing advanced technologies, data centres powering these technologies are under increasing pressure around the globe to increase capacities and improve efficiencies.

Read more...
Analysers for use in high ambient temperature environments
Analytical Instrumentation & Environmental Monitoring
The 993X series of analysers from Ametek Process Industries are now IECEx Zone 2 certified for use in locations with up to 60°C ambient temperature. Built with IP66-rated enclosures and using an integrated cooling system, they can be installed outdoors or in minimally temperature-controlled enclosures, reducing complexity while lowering capital and operating costs.

Read more...
ATEQ is adapting to evolving markets
ATEQ South Africa Analytical Instrumentation & Environmental Monitoring
ATEQ is a global company that has established itself as a world leader in leak test technologies and industrial quality control equipment. The company’s mission is to help its customers remain compliant with regulations and maintain product quality through its range of support services.

Read more...