Breathe easy

For many organisations today, one aspect of the manufacturing process that needs to be monitored and controlled, for both efficiency and environmental benefit, is gaseous emissions.

Plants are constantly looking for means to increase production and decrease costs. Increases in production are generally associated with an increase in fuel consumed and, in turn, a subsequent increase in emissions generated. In addition, regulatory entities are increasingly implementing standards and setting requirements for monitoring and reporting data on plant emissions. In order to improve efficiency throughout a manufacturing process and at the same time meet the needs of sustainable development goals, industry is finding an increased need for robust, reliable and accurate gas analysis methods.

To meet the needs of this demand, different technologies for gaseous emissions sampling and analysis have been developed over the years. One particular technique is to extract a gas sample and dilute that sample prior to analysis. This article explains some advantages of the dilution extractive method.

Methods of gas analysis

The means to generate an analysis from a process gas can be broken into three main categories — in-situ gas analysis, full extractive sampling gas analysis and dilution extractive gas analysis.

In-situ

In-situ analysis consists of measuring and analysing the gas right at the point desired. In other words, the analysis instrument is located at the process point where the measurement is needed. An analysis performed within a very short distance of the instrument is termed 'in-situ point' analysis. An example of in-situ point analysis would be an electro-optical or electro-mechanical device situated at the measurement point.

An analysis that is made over a distance, through a gas stream is termed 'in-situ path' analysis. An example of in-situ path analysis would be an instrument that passes some type of energetic wave (such as light or sound) through the medium to be measured.

The interaction of the energetic wave with the medium is then detected and measured. In either case, the measurement is made right at the point desired.

While in-situ analysis avoids sample transportation lines, some concerns can include the following:

• Electronics and components are located remotely at the stack making maintenance and repair somewhat difficult.
• Instruments are typically exposed to the elements and harsh stack conditions.
• The technologies employed may have difficulty measuring low concentrations of gas.
• The technologies employed may suffer from spectral interference from other gas components.

Full extractive

The full extractive method samples a representative portion of a gas stream and transports the sample to an analyser.

While a fairly straightforward concept, gas sample transportation can be tricky and sometimes problematic. Gas from a process generally contains moisture in suspension and this moisture can create difficulty from a transportation and gas analysis standpoint. Should the gas cool below its dew point during transportation, the suspended moisture will condense to form water droplets. This condensed water can cause plugging conditions by itself or with particulate matter in the system.

Other issues such as corrosion can also sometimes be encountered. Some gases are partially soluble in water (SO₂, NO₂) and others fully soluble in water (NH₃, HCl, HF). Should vapour condense, acidic compounds can be created and corrosion induced on or within system components.

Because of these issues, sample lines on fully extractive systems are heated along their entire path to prevent the gas from cooling below the dew point. Occasionally, however, heated lines can fail or 'cool spots' along the heat trace can develop, still allowing moisture to condense (see Figure 1).

Figure 1: Full extractive system.

Measurement made on a full extractive sample can be done either with the moisture still present in the sample or with a 'dried' sample. If the measurement is done with the moisture still present, this is referenced as a hot, wet or 'wet basis' analysis. The wet analysis methodology can have drawbacks associated with the moisture in the gas as noted above. Therefore, to avoid these issues, a majority of full extractive systems cool the sample under controlled conditions to condense the water out and dry the sample. Once the sample has been cooled and dried, it is then sent to an analyser. This method is referenced as the cool, dry or 'dry basis' analysis.

As noted above, some gases are either partially or fully water soluble. By cooling the sample and removing the water for a cool dry analysis, some of these gases that have dissolved in the moisture can also be removed from the gas as the water is condensed. As a result, an error can be induced on gases such as SO₂ and NO₂. Along these same lines, it is impossible to measure highly water soluble gases like NH₃, HCl and HF.

Dilution extractive

The dilution extractive method extracts a very small representative portion of a gas stream and very accurately dilutes the sample with air before transportation to an analyser. By significantly diluting the gas sample, the dew point of the gas is lowered far beyond the lowest possible ambient temperature. This method therefore makes it impossible for any moisture in the gas sample to condense out of suspension and form water droplets.

There are two basic ways to dilute an extracted sample. One technique for diluting the gas sample takes place within the sample probe itself. In other words, the dilution mechanism is part of the probe. This is termed 'in-stack dilution'. The other technique for dilution takes place immediately after the probe, right at its discharge. This is termed 'out-of-stack dilution'. For all intents and purposes, both techniques accomplish similar desired results. Once diluted, the sample is then transported to the analyser.

Dilution extractive sampling: a closer look

Dilution extractive sampling and analysis techniques have the considerable benefit of avoiding many of the extra sample transportation and handling requirements of a full extractive sample system. The lowering of the dew point obviates the need for heated sample lines, and equipment to cool and dry the sample gas prior to analysis is not required (see Figure 2).

Figure 2: Dilution extractive system.

Because of the way sample dilution is accomplished, the amount of gas collected at the probe for measurement in a dilution extractive system is considerably lower than a full extractive system. Much of the volume of gas going to the analyser is from the dilution air so the actual volume of gas from the stack or process is quite low. Flow rates into the sample probe can be two orders of magnitude lower than a full extractive system (eg, 0.22 L/min versus 22 L/min). This means that there will be a significantly lower amount of particulate matter that needs to be filtered. This in turn means filter life is greatly extended compared to a full extractive system. Also, with such a low volume of gas entering the sample probe, particulate matter travelling in the main gas stream is more likely to remain in the main stream and not enter the probe. The dilution extractive sampling method is especially appropriate for hot, dusty and moist environments.

How dilution extraction works

Dilution of the gas sample must be extremely precise and robust to function properly. Fortunately, this is relatively simple to do and is accomplished by the use of various static components. These non-moving, static components reside either within the probe itself, for in-stack dilution, or within an assembly located just downstream of the probe, for out-of-stack dilution. The main components of the probe and dilution assembly are:

• coarse filter,
• fine filter,
• critical orifice,
• ejector (venturi) with a sample gas inlet and a dilution air inlet,
• diluted sample outlet.

The coarse filter prevents large particles from entering the sample probe and the fine filter removes small particulate matter from the gas sample. After the filters, sample dilution takes place. The sample gas passes through what is called a 'critical orifice' — this is one of the main components in a dilution assembly. The critical orifice allows only a very precise and fixed amount of sample gas into the assembly. The metering, and hence dilution ratio, is done by creating a 'sonic flow' through this critical orifice. The conditions for sonic flow are reached and maintained by a fixed pressure drop across the orifice. The pressure drop is a difference in pressure between the stack pressure and the internal pressure of the probe. The internal pressure of the probe is controlled by the other main component in the assembly, the 'ejector'. The ejector is a venturi driven by dilution air. The dilution air is directed down the throat of the venturi at a very precise pressure, creating a vacuum. The inlet line of sample gas coming from the critical orifice is attached to the venturi tube where a vacuum is being generated by the dilution air. The vacuum created by the venturi provides the appropriate pressure drop across the critical orifice to create sonic flow conditions and pulls sample gas into the dilution assembly. The venturi itself is also a critical orifice and operates with sonic flow conditions. The combination of the ejector and critical orifice precisely controls the dilution rate (see Figure 3).

Figure 3: Dilution probe (courtesy of James A Jahnke).

Hence, there are two precisely controlled flows through the ejector pump: the undiluted sample gas from the critical orifice, Q₂, and the dilution air going into the ejector, Q₁. The outflow of the ejector is the diluted sample gas consisting of Q₁+Q₂ and the dilution ratio is calculated as:

The critical orifice, ejector and dilution air pressure are all designed and set to deliver a pre-determined, fixed dilution ratio.

Additional considerations of dilution extractive sampling

Dilution ratios of 100:1 or higher are not uncommon and care should be taken in considering the measurement range and subsequent performance requirement which results from that dilution. For example, if a 100:1 dilution ratio is selected and the gas to be sampled has a nominal value of 60 ppm, then the diluted sample will have a nominal value of 0.6 ppm. The analyser selected should be able to operate accurately and precisely at this low gas concentration.

The critical orifice is a function of the pressure difference between the stack and the internal assembly of the probe, and pressure changes at the stack therefore have an effect on the dilution. Fortunately, these changes are linear, predictable and easily addressed. A pressure gauge becomes part of the system and is used to easily correct for changes in stack pressure.

One of the main benefits of a dilution extraction system is to lower the dew point of the gas sample so low that moisture will remain in suspension and will not form water droplets. Because there is generally no need to dry the gas, it can go directly to an analyser and gas concentrations reported by the analysers are already on a wet basis, eliminating the need for conversions.

Calibration of the system is simple and is accomplished by sending span or zero gas directly to the centre of the probe inlet. The calibration gas then goes through the same dilution as the sample gas, leading to a true calibration of the full system under actual operational conditions.

Advantages of dilution extractive systems

There are genuine advantages to a dilution extractive system over a full extractive system which offer the user significant economic benefits both in operation and in maintenance that can be summarised as:

• No heated sample lines are required.
• Chiller equipment for condensing moisture is not required.
• Maintenance is reduced.
• Direct wet basis measurement is used.
• Water-soluble gases are not lost.

In addition, less calibration gas is needed. Because such low volumes of sample gas are required, calibration gases last much longer. An extractive dilution system typically only requires 0.2 to 0.4 L/min of calibration gas as opposed to the 20–50 litres required for a full extractive system. Even with daily calibrations, a cylinder can last between one and two years when using the dilution extraction method.

Summary

Process plants today are finding an increasing need to accurately monitor and control plant emissions and in turn are seeking robust and reliable means to do so. Of the three main methods used for gas sampling and analysis, the dilution extractive method seems to offer a very attractive means to meet this need for many of the hot, dirty environments found at many plants.