Whats in the air for continuous emissions monitoring?

Working on the well-established principle that if you cant measure something you cant control it, regulatory authorities such as the U.S. Environmental Protection Agency (EPA) have for many years insisted on the continuous monitoring of emissions from major sources such as power plants in the utilities and chemical industries.

A continuous emissions monitoring system (CEMS) is the total equipment necessary for the determination of a gas or particulate matter concentration or emission rate using pollutant analyzer measurements and a conversion equation, graph or computer program to produce results in units of the applicable emission limitation or standard, according to the EPA.

Now that might be a fairly typical example of all-embracing officialese, but the EPA regulations which had their genesis with the Clean Air Act of 1963 and its subsequent amendments in 1977 and 1990 go on to specify in much more detail what is expected of the operators of plants emitting gases such as nitrogen oxides (NO_x), sulphur dioxide (SO₂), carbon monoxide and dioxide (CO, CO₂), as well as oxygen. Performance specifications for CEMS are laid down in EPA regulations 40 CFR Part 60 (new source performance standards) and 40 CFR Part 75 (covering utility boilers and turbines under the Acid Rain Program), and are constantly under review to reflect changing technologies and attitudes to possible pollutants.

In the power sector, for example, the Electric Power Research Institute (EPRI), Palo Alto, Calif., is running a CEMS program to help participating member companies keep abreast of the new pollutants that power plants will need to begin monitoring in the next few years the most likely ones being mercury, particulate matter mass concentrations, SO₃, NH₃, HCl and HF. Some of these measurements can provide more accurate, defensible Toxics Release Inventory (TRI) reporting and others can serve for process control. EPRI established the program to promote the commercial availability of robust, accurate, least-cost CEMS and gas species-based process monitors, says Chuck Dene, the programs technical lead.

Focus on mercury

Most CEMS manufacturers are currently concentrating on the upcoming mercury monitoring rules that take effect on January 1, 2009. By then all new and existing coal-fired power plants must have some form of continuous mercury-monitoring capability in operation. This follows the EPAs promulgation of the Clean Air Mercury Rule (CAMR) in May 2005, which creates a market-based, cap-and-trade program for mercury emissions.

Mercury is the hot topic right now, agrees Doug Kriebel, manager of systems and proposals with the Rosemount Analytical Division of Emerson Process Management, Solon, Ohio. Its mainly concerned with coal-fired power stations and most of these already have CEMS in place, so for them its just an added measurement to their existing system, rather than putting in a whole new system just to measure mercury. So far, though, the EPA has not laid down what it thinks is the best way.

Such uncertainty is certainly fuelling the market for mercury CEMSs at the moment. There are very, very few [mercury systems] installed right now, notes Frank Duckett, product manager for continuous gas analyzers at Thermo Fisher Scientific, Waltham, Mass., which is one of the largest suppliers of CEMSs. So between now and January 2009 there are 800 or so systems that need to be installed in the U.S. We have a very large program here, which we started two to three years ago, to develop an analyzer, calibrator, probes and sampling system specifically for mercury monitoring, and weve started to install these over the last six months or so. Thermos Mercury Freedom system has so far met or exceeded all of the EPAs performance specifications.

Field evaluations of mercury monitors are being carried out on the EPAs behalf by EPRI, which also has developed its own mercury sorbent trap called QuickSEM. An alternative to continuous emissions monitoring, the trap collects mercury for later laboratory measurement. If the EPA were to allow sorbent traps, EPRI says the savings could be as much as $80,000 per unit compared with CEMSs.
What is claimed to be the first mercury CEMS destined for continuous operation (as opposed to evaluation tests) in a U.S. coal-fired power plant has recently been ordered by PSEG Fossil for its Hudson Generating Station in Jersey City, N.J. This DM-6 mercury CEMS and data acquisition system is being supplied by Horiba Instruments, Irvine, Calif. It reduces all mercury compounds to elemental mercury at the stack prior to transporting the sample to a continuous cold vapor atomic absorption analyzer.

Other companies offering EPA-field-tested systems include Tekran Instruments, Knoxville, Tenn., with its Series 3300 based on the companys patented cold vapor atomic fluorescence technology, and Cemtek Systems, Linden, N.J., which offers complete mercury solutions using Tekran and Thermo Instruments analyzers for which it is a preferred integrator.

The MIP Division of Ducon Technologies, Farmingdale, N.Y., is one of many other companies looking to that 2009 deadline. Its SM4 mercury monitor is said to be the first instrument to use a thermocatalytic principle to avoid wet chemical sample treatment. Underscoring that interest in mercury monitoring is by no means a U.S. phenomenon. For instance, process analyzer and instrumentation company Sick Maihak, Reute, Germany, and Houston, Texas, offers its TÜV- and EPA-approved (for measurements of 0-45μg/m³) MERCEM stack gas analyzer.

While perhaps not immediately relevant to the many chemical plants that burn oil or gas, the CAMR does indicate how regulation can influence product development. It wouldnt surprise me to see other nations bringing EPA-like regulations on board by around 2012 to 2014. In reality though, in the U.S., once you hit January 2, 2009, this market will drop dramatically, says Alan Matta, manager for Thermos industrial hygiene and safety products.

Established role

The general market for conventional CEMSs, however, continues to be one of either steady replacement of aging systems, or installation of new systems driven by plant expansion both in the U.S. and, increasingly, in the rapidly growing areas of the Middle East and Asia. Overall, CEMSs have been around for decades, but they are still probably the most accurate way of measuring emissions. Typically, they are viewed as environmental compliance instruments, but they can also serve as process monitoring instruments, even though the stack is the last place you can measure in the process, says Kriebel.

Apart from the mercury developments, Kriebel notes the EPA is always pressing for new ways of measuring [pollutants]. From a CEMS perspective, he says the laser tuned diode which measures in situ on the stack rather than taking samples for analysis seems to be making its presence felt for moisture, ammonia and other measurements that you cant get very easily with CEMS.

But whether your measurements are taken in situ or via samples sent down heated lines to the typical CEMS shelters and cabinets on the plant floor, the regulatory demands for constant on-going calibration and verification of the analytical instrumentation are always there. And with them, often quite literally hand in hand, is the demand for constant maintenance and monitoring of the systems themselves.

Maintenance is a factor, agrees Kriebel, and someone might be prepared to pay a little more for something thats easier to maintain, rather than have someone climb a stack to maintain something a little cheaper. The drawback of CEMSs is that you have to maintain them.

Rosemounts field-mountable MicroCEM system offers the choice of locating its analysis enclosure either right on the stack or in a ground-level enclosure. The goal of the two-box unit was to make it more compact to fit into expanding plants that may have limited ground space for shelters and cabinets, says Kreibel.

The CEMplicity monitoring system from Forney Corp., Carrolton, Texas, follows a similar approach. Dubbed a low-cost solution for CEMS and process monitoring, it provides the same cost savings as in situ type systems without the drawbacks associated with in process monitors, claims the company. Using EPA referenced method analyzers (chemiluminescent for NOx and paramagnetic for oxygen), CEMplicity is a close-coupled system packaged in a compact climate-controlled NEMA 4 enclosure.

Predictable interest

Meanwhile, an emissions monitoring technique that holds out the promise of little, if any, maintenance is attracting increasing interest. Evolving from work in the early 1990s on using neural networks to accurately predict NO_x emissions from a natural-gas-fired boiler, Predictive Emissions Monitoring Systems (PEMSs) are proving themselves on a wide range of applications in the chemical and petrochemical sectors.

Figure 1. Some systems based on models now include real-time sensor validation. Courtesy: Pavilion Technologies

As the name suggests, PEMSs are effectively model-based alternatives to the physical measurements required of CEMSs. They predict emission levels from a knowledge of process and combustion plant parameters. More than 250 such systems have been installed worldwide and have been proven on many types of process units, combustion set-ups and fuel types, says Paul Reinermann, director of environmental solutions with PEMS pioneer Pavilion Technologies, Austin, Texas. All of our customers will tell you that PEMS will cost less than a hardware-based CEMS typically around 50% less both for total cost of ownership and initial capital cost, he adds, while acknowledging that it can be almost site dependent as to whether a company opts for CEMS or PEMS.

The reasons behind that decision often relate directly to the work involved in creating the PEMS model. Like most systems on the market, Pavilions PEMS is statistically based and so requires running a series of experiments on the specific unit to generate data to correlate process operating conditions with emissions produced. Although this may only take 24 hours, some plants may not have the flexibility to run through the experimental operating ranges.

Reinermann points to refineries as a case in point: When you ask them to vary the firing rates on a furnace or heater, they quickly add up how much crude theyre not going to be processing [while we are doing that]. Petrochemical plants on the other hand tend to have large banks of olefin furnaces, one or two of which will always be in flux, so its easier there. In fact, he says Pavilion has recently had considerable success in U.S. ethanol plants. I would estimate that a half of all new ethanol plants have our PEMS installed. The primary reason is that they dont want to hire additional staff to maintain hardware-based CEM systems.

Acquiring the data to create a PEMS model obviously also would be easier if the plant itself were equipped with a CEMS. While the plants data historian can provide the process data, emissions data in the U.S. has to come from an EPA-referenced source such as a CEMS. In practice though, as Reinermann points out, most PEMSs are destined for units without a CEMS, so an outside contractor offering stack testing services might have to be called in for the experimental stage.

Creating the model though is definitely not for outsiders. Even a simple boiler really requires a chemical engineer to build the right model, he stresses. Its to do with the basics of what we learn about combustion as chemical engineers, all about whats going inside that firebox or turbine. We have very good engineers that are expert at interviewing plant operators to acquire just that information and capture it in the model.

Gaining regulatory acceptance

Although not yet universally recognized by regulatory authorities, PEMSs are accepted for compliance with the EPAs 40 CFR Parts 60/75 subject to certification requirements that call for either a three- or 30-day test period depending upon application. To further assuage EPA concerns, Pavilion has incorporated a real-time sensor validation system and daily electronic data assessment tests of the PEMS to verify its accuracy.

Meanwhile, CMC Solutions, Wixom, Mich., has gained EPA certification for its SmartCEM PEMS, following trials at two sites operating large-frame gas turbines rated at 160 MW and 80 MW. CMC says these 2005 certifications were the first of their kind in the U.S., each allowing the source site to operate the turbine and PEMS without a CEMS as a non-peaking unit under the regulation.

Another company that started out like Pavilion by developing neural-network-based solutions is Manchester, U.K.-based Cogsys. It has now evolved into a rotating machinery emissions monitoring company, says business development manager Chris Dagnall. Part of its portfolio of software solutions is the alert performance and emissions monitoring system, a PEMS in all but name that is aimed mainly at gas turbine applications both on- and offshore in the oil-and- gas and petrochemical industries.

Our system sits on top of the DCS (distributed control system), explains Dagnall, which is very good at bringing in information from all parts of the plant but not so good at generating reports and doing the calculations to meet regulatory requirements. The alert system has been available through Cogsys sister companies Advantica and Stoner Software, Harrisburg, Pa., but Dagnall says Cogsys is now working with rotating machinery partners in the U.S. to provide solutions.

Some turbine manufacturers now treat PEMSs almost as standard equipment. GE Energy, Atlanta, Ga., for example, added PEMS to its Smart Services portfolio of turbine tools for remote monitoring and diagnostics and remote NOx tuning. The GE PEMS offering relies on a first principles rather than a statistical approach, employing equations that represent various mass and energy balances and thermokinetic reactions in the combustion process. This provides a generalized model that then is tuned to the specific turbine/fuel combination being measured.

Weel & Sandvig, Lyngby, Denmark, has been installing PEMSs on gas turbines since 1998, with its first installation at an industrial combined-heat-and-power plant. As with most other turbine PEMSs, the WS.GT-PEMS is easily extended to also provide performance monitoring of the turbine as an aid to optimizing its operation. And, of course, operating plants and equipment at their optimum levels goes a long way towards ensuring that sites stay on the right side of the regulators leaving the CEMS and PEMS to provide the evidence of that good operation.