Sampling for CEMS

As with any analytical system, continuous emissions monitoring systems (CEMS) are only as good as the quality of the sample presented to them. Robert E Sherman discusses the development of modular substrate sample conditioning systems and their benefits.

Continuous emissions monitoring systems are a necessary ‘evil’ required to keep many chemical and hydrocarbon processing industry plants operating. Known as CEMS, these systems monitor and report the emissions from vents or stacks to atmosphere for sulphur dioxide, oxides of nitrogen, carbon dioxide and other possible pollutants. In the US they are legislated by the US EPA, while in the UK they come under the Environmental Agency’s MCERTS approval scheme (see panel overleaf).

Typical CEMS have had a life-cycle cost upwards of $500 000 and typically require a $100 000 climate-controlled building to house just one or two of them. But these amounts can be reduced to 60% or 65% of those figures by utilising ‘Modular Substrate Sample Conditioning’ technology, as described in ANSI/ISA 76.00.02-2002. Here we will show how design considerations of ANSI/ISA 76 systems can reduce both the physical package size of the CEMS and the maintenance time required to keep the system operating to legislated specifications.

Development of this important new technology was initiated in 1998 by Don Mayeaux of A+ Corporation. He proposed replacing the classic double-block-and-bleed design with a dual seal and captive vent design. This detail did not survive the transition to ANSI/ISA 76 but the concept was championed by the team of Peter Van Vuuren and Dan Podkulski of ExxonMobil Chemicals, Robert Dubois of Dow Chemical and Mel Koch, director of CPAC, the Centre for Process Analytical Chemistry.

These people met and the result was a technical tour-de-force called NeSSI (New Sampling Sensor Initiative) that was placed within the University of Washington-Seattle CPAC. This placement solved the legal problem of monopoly and restraint-of-trade between the two giant chemical companies and allowed a venue to post all NeSSI information for free exchange among process industry professionals.

Essentially, all papers relating to modular sample conditioning systems now reside here and everything from early history to latest presentations ends up there — an innovative and outstanding solution to allow an industry-wide, trend-setting initiative to grow quickly.

The next benchmark in development was ExxonMobil’s John Sablatura’s paper ‘Analyser Key Performance Indicators’, which established the maintenance requirements for conventional (also legacy or classic) sample conditioning systems and forced industry to realise the true life-cycle cost of process analyser systems. This is often twice the initial installed cost over a service life usually estimated at 10-15 years.

Then in January 2000, ExxonMobil’s Gunnell and Van Vuuren addressed both ‘Today’s Cost of Build’ (initial outlay) and ‘Today’s Cost of Ownership’ (on-going expenditure) to bring more attention to the ‘hidden life-cycle costs’. They analysed project data and determined that 38% of initial outlay is spent on the process analyser, 30% on the sample system and 27% on providing a controlled environment for these. They forecast that a 40% savings on ‘cost of build’ could be achieved by reducing the costs of the sample transport and conditioning system and eliminating the need for a climate-controlled housing.

They also projected a 35% savings in the ‘cost of ownership’ by increasing the number of analysers a technician can support, eliminating the need for a dedicated site analyser engineer and reducing the cost of spares holding.

It will be seen that NeSSI accomplishes the above reductions and eliminations. The next step was ANSI (the American National Standards Institute) approval in 2002 of the ISA/NeSSI-proposed ‘modular component interfaces for surface-mount fluid distribution components’ design. This then resulted in the international standard ANSI/ISA 76 and a common basis for modular component and system design.

Continuous emissions monitoring systems typically have to be housed in climate-controlled enclosures, but new sampling technology can help cut lifecycle costs

Looking at the present, we now have three major valve and fittings vendors on second generation ANSI/ISA 76 modular substrate system designs, and over 25 third-party component manufacturers producing ANSI/ISA 76-compliant components to populate the substrate systems provided by the valve and fittings vendors.

To see the benefits of this approach, we first need to consider classic CEMS designs. The four basic designs (with approximate shares of the market) are: in-situ (1%); wet process/wet sample (10%); wet process/dry sample (64%); and dilution probe (25%).

The analytical elements of in-situ units are enclosed within the stack or duct and require exclusion of the process gas with a ‘shutter’ device for calibration. Since the sample gases are usually hot and wet, these units often have problems with dirty optics or malfunctioning shutter assemblies, which prevent proper operation and calibration.

Dilution probe systems utilise a probe in the stack or duct without optics, using tubing to direct dilution air through a precision orifice to bring a calibrated amount of stack gases with it. They rely on dilution air being free of measured pollutants and their popularity has declined. Their analysers must be very sensitive, usually of research laboratory quality and needing a very stable thermal environment for accuracy, and therefore not tolerating process plant environments. This is where the $1000/ft2 controlled environment process analyser shelter enters the equation. One benefit of the installed cost of these systems is that the sample transfer lines typically only need to be freeze-protected rather than kept at an elevated temperature and therefore can be simpler and significantly less costly.

Wet process/wet sample CEMS require temperature-controlled heat-traced insulated sample transfer lines costing up to $45/ft (plus installation), a heated and insulated sample conditioning system enclosure and heated measurement cell analysers to keep the sample above its dew point at all times. Process analysers having a heated cell are the exception rather than the rule and are usually more expensive. These systems also require a controlled environment process analyser shelter.

Wet process/dry sample systems also require heat-traced insulated sample transfer lines, but only a freeze-protected insulated sample conditioning system enclosure. The process analysers for these systems are the simplest of all CEMS types and can be rugged units that are stable in their operation with no heated analysis cells.

Looking to the future, to realise the perceived benefits of the modular substrate sample conditioning system technology we have to analyse the current state of ANSI/ISA 76 analyser systems. Mechanical modular substrate components are readily available from Circor and others. Integrated analysers of varying types are available from Equitech, Panametrics and Teledyne.

Smart diagnostics/control on modular substrate analyser systems can be accomplished by substituting electronic sensors/controllers for mechanical/visual components. While this presently represents an increase in initial purchase price, given a near-future NeSSI standard data bus structure the answer is to replace a technician making daily rounds of the systems with a technician checking an ‘Exceptions Log’ printout for out-of-specification flows or pressures.

This can give dramatic life-cycle/cost-of-ownership savings and the advantage will increase as electronics become more cost-effective and the volume of these devices in ANSI/ISA 76 format increases.

Robert E Sherman is with Circor Instrumentation Technologies in the US.