Continuous monitoring of particles after high-temperature filtration

Electrodynamic instruments have been used to continuously monitor particle concentrations after ceramic filters used in coal gasification, co-generation and high-temperature metals and chemical processes.

Electrodynamic particle monitoring technology is suitable for installation in processes operating at over 1000°C, making it particularly suited for the long-term measurement of the condition of ceramic filters and to indicate the breakdown of filter elements.

Techniques for continuous particulate monitoring

The technique and instruments used for continuous monitoring of industrial emissions must be robust and rugged enough to tolerate the conditions in the stack. However, the need to monitor at very high temperatures (up to 1000°C or higher) after ceramic filters puts additional constraints on the selection of measurement instruments, and on the materials from which they are constructed. Electrodynamic instruments are capable of measuring at temperatures up to 1200°C, since the measurement technique is still valid at this temperature and it is practical to build in-situ instruments that can tolerate this temperature.

Even though there are several other techniques used in commercially available particulate emission monitors only a few may be used to operate at very high temperatures. The techniques have the following limitations:

• Opacity: A cross-stack optical method in which absorption of light is measured. The key limitation is the difficulty of sourcing windows and lenses that can tolerate high temperature.
• Dynamic opacity (scintillation): A cross-stack optical method in which the variation of absorbed light is measured. The key limitation at elevated temperature is interference from heat haze and the sourcing of suitable optical components.
• Back/side scatter: A method in which light reflected backwards from particles is measured. The key limitation is the sourcing of suitable optical components.
• Forward scatter: An in-situ or extractive method in which light is directed at the particles and the light that is scattered in a forward direction by the particles is measured. The key limitation at elevated temperature is the cooling of gas in extractive systems and sourcing of optical components for in-situ systems.
• Oscillating filter (vibrating tapered element): The change in weight of a filter collecting particles is measured by measuring the change in resonant frequency of the support element. The key limitation at elevated temperature is that a vibrating element cannot be sourced that will work above 400°C.
• Beta attenuation: An extractive method in which particles are collected on a filter and the absorption of beta radioactivity is measured. The key limitation at elevated temperature is difficulty in cooling gases sufficiently before the filter.
• Triboelectric: A method in which the DC current caused by particles colliding with a rod is measured. The key limitation at elevated temperature is cathodic interference current from the heated rod.

The electrodynamic technique

Principle of operation

In an electrodynamic system, a grounded metallic sensing probe is installed across part of the stack and is connected to signal processing electronics capable of amplifying and measuring an AC current in the order of 10 pA. Particles in the stack to be monitored carry charge as a result of upstream activity and these particles induce an AC signal as they pass the rod. The magnitude of the signal is a function of the average charge per particle and the variation in the spatial distribution of the particles. The signal is, therefore, proportional to total mass concentration in conditions where the charge per particle remains constant (a function of particle type, particle and size and the process conditions), and stack conditions where the particle number concentration is small (since the particle distribution follows a Gaussian distribution in steady flow conditions). The proportional relationship between particle concentration and instrument response has been validated in regulatory approvals in Germany by TÃœV and UK by MCERTS. Since the AC signal is primarily derived from charge induction from particles passing the rod (unlike triboelectric instruments which measure the direct current caused by particles colliding with the rod), the problems of rod contamination and velocity dependence are minimised.

Empirical tests have shown that the electrodynamic signal is inversely proportional to particle size. This is consistent with the theory that the signal is proportional to the total surface area of the particles, which affects the total charge per particle.

In many industrial processes, especially those controlled by ceramic and fabric filters where the filter element surface acts to precondition the particle charge, the charge per particle in the final emission stack is sufficiently constant to permit a reliable calibration in mg/m³ by comparison to the results of an isokinetic sample (gravimetric sample under matched velocity conditions). The technique is best used in processes with a constant particle type.

Practical considerations for high-temperature applications are:

• The sensor rod can be made of FeCrAl alloy, which can tolerate temperatures of up to 1200°C.
• Analysis of the frequency components of the AC signal can be used to improve accuracy.
• The sensor rod can tolerate contamination without reduction in performance since the measurement signal derives from induction rather than collision.

Figure 1: Charge induction in electrodynamic sensor caused by charged particles passing the rod.

Use and limitations of the technology

Electrodynamic instruments are used to satisfy qualitative and measurement requirements on bag filters and ceramic filters in the metals, mineral and chemical industries. Their adoption in the UK is extensive and their use in Europe, Japan and Australia is widespread. The technical limitations of electrodynamic technology are:

• The use of electrodynamic technology for particulate measurement requires applications with a predictable particle type and pre-charge, non-condensing conditions, and a minimum particle velocity of 5 m/s. There are only minor effects of changing velocity if the velocity is greater than 8 m/s.
• The instrument cannot be used for measurement in the presence of water droplets, however this is rarely an issue in high-temperature applications. The instrument can, however, discriminate between solid particles and water vapour.
• The technology has limitations in applications in which the pre-charge on the particle is likely to change. In practice, this covers electrostatic precipitators (where charge on the particle may be changed by precipitator condition), processes where the particle type may change and pre-arrestment locations in combustion applications in which flame conditions vary significantly (since this affects ionisation, changing the charge on the particles). However, in practice most industrial applications are more constant and, therefore, this limitation is in practice relatively small.
• The process limits for the technique are that it measures all particles from 0.1 micron and larger (response is inversely proportional to particle size); it measures particle concentration from below 0.1 to over 1000 mg/m³ and should be used in applications where there is a minimum velocity of 5 m/s.

Construction of electrodynamic instruments

An electrodynamic instrument consists of a sensor which is mounted in the stack via a coupling or flange connection. The version of the sensor used at temperatures to 1200°C includes ceramic insulation (to isolate the rod from the stack wall), a FeCrAl sensor rod and a heat shield to protect the sensor electronics from the stack temperature. Other variants of the sensor are available for operation up to 250, 400 and 800°C.

The sensor is connected to a control unit via a single cable that provides all sensor power and communication with the sensor. In versions using Modbus for communication the cable length can be up to 2000 m in length. In practical systems, the control unit is used for the user interface and instrument set-up, and provides 4-20 mA, RS232/485 and ethernet outputs for connection to external plant control systems, PLCs and plant LANs.

Applications

Filtration systems are used in a growing number of high-temperature applications in coal gasification, co-generation, incineration and high-temperature metals and chemical processes. Ceramic filters are able to operate at temperatures in excess of 600°C and provide high levels of particulate removal in these applications. The continuous monitoring of the efficiency of the filtration system can be important for a number of reasons.

Plant protection

High-temperature filters are often fitted before further plant used to remove energy from the high-temperature gas, and therefore protecting this plant from damage is of real relevance.

One way electrodynamic sensors are used is to provide information that particle levels have increased so that the plant can be shut down before damage to expensive downstream equipment can occur. For example, in coal gasification processes, high-pressure and high-temperature syngas is cleaned by a ceramic filter before passing into the turbine for electricity generation. The condition of the filter is critical to ensure particles do not pass into the turbine causing damage to the turbine blades and casing.

Environmental legislation

The chemical, metals and mineral industries are increasingly fitting continuous monitors to satisfy environmental legislation and meet ISO 9001 commitments.

For example, the carbon black industry uses electrodynamic sensors to monitor emissions for two reasons - the high temperature of the flue gas from the carbon black manufacturing process (furnace process) and the resilience of the sensor to any contamination caused by carbon.

Reduced filter running costs

Plant operators can use the graphics screens on electrodynamic instruments to view emissions trends and hence diagnose the condition of filters. This helps to minimise the damage that can occur when a ceramic filter fails, by stopping the plant before failure, or before consequential damage to other filter elements in the vicinity of the failed filter. This helps reduce down steam particle carryover and reduces the often considerable cost of replacing failed filter elements.

The technology can also be used to assist maintenance personnel to locate the leaking element in a large filter system. This is done by synchronising the output from the particulate monitor to the cleaning sequence of the filter and using the dynamics of the dust signal to pinpoint which row of elements, when cleaned, is causing high dust levels and, hence, is beginning to fail.

Summary

Electrodynamic particle emission monitors can be constructed to work in industrial applications with temperatures of up to 1200°C and have been used in high-temperature applications in incineration, coal gasification and the metals industry using high-temperature filtration, including ceramic filters. The technique requires no optics or active sensor components that can be a limiting factor in opacity and light scatter particle measurement techniques in high-temperature applications.

The technique measures the AC current induced in a sensor rod inserted across the emission stack. In spite of cross-sensitivities to particle size, particle material and particle charge, the system can be calibrated in mg/m³ by reference to an isokinetic sample (in industrial processes with emission control equipment where these conditions remain sufficiently constant).

Practical instruments using this technique are being used in high-temperature industrial applications in UK, Germany and Japan. These instruments are sufficiently accurate to meet regulatory approvals in the UK (MCERTS) and Germany (TÃœV) for continuous emission measurement.

Conclusion

High-temperature electrodynamic particle emission instruments (designed to operate at 800°C) are suitable for the long-term, continuous monitoring of emissions from industrial processes and for monitoring the condition of ceramic filters. Since the early detection of filter failure is critical to reducing damage and minimising downtime in a ceramic filter application, these instruments can assist in reducing filter running costs. These instruments have sufficient accuracy and reliability to meet the emerging need for high-temperature particle emission measurement, both practically and cost effectively. In the future, the proportion of electrodynamic instruments used at even higher temperatures (up to 1200°C) is forecast to increase in line with the growth of high-temperature filters in industrial processes.

By William Averdieck, PCME Ltd