Guide to Choosing a Continuous Emission Monitoring System (CEMS)

Processes involving industrial-scale combustion are consistently subject to some type of air-quality environmental regulation. These regulations vary based on the specific application and the pollutants in question. For example, the European Union’s Industrial Emissions Directive and regulations from the U.S. Environmental Protection Agency (EPA) require continuous monitoring of pertinent pollutants at their point of emission into the atmosphere. The EPA states: “A Continuous Emission Monitoring System (CEMS) is the total equipment necessary for the determination of a gas or Particulate Matter concentration or emission rate using pollutant analyser measurements and a conversion equation, graph, or computer program to produce results in units of the applicable emission limitation or standard. CEMS are required under some of the EPA regulations for either continual compliance determinations or determination of exceedances of the standards. The individual subparts of the EPA rules specify the reference methods that are used to substantiate the accuracy and precision of the CEMS.”

A CEMS is solely designed for monitoring emissions to ensure regulatory compliance and is not integrated with any process control systems. However, the data it collects can be utilized to assess and improve combustion processes.

Macrotec is an integration partner for SICK, providing process design, engineering, equipment selection, installation, maintenance, and support services.

Components typically regulated that might require measurement:

  • CO – Carbon Monoxide
  • CO₂ – Carbon Dioxide
  • NOx – Nitrogen Oxides
  • SO₂ – Sulphur Dioxide
  • HCl – Hydrogen Chloride
  • HF – Hydrogen Fluoride
  • NH₃ – Ammonia
  • Hg – Mercury
  • VOC’s – Volatile organic Compounds
  • Particulate Matter
  • O₂ – Oxygen (measured for reference)
  • Flow volume, pressure, temperature, humidity (measured for reference)
CEMS Cover

Selecting a Continuous Emission Monitoring System (CEMS) is often a challenging decision, particularly given its substantial initial investment and potentially even greater costs over its lifetime. This decision is frequently inescapable owing to regulatory mandates for these systems. Opting for the option with the lowest upfront cost can, paradoxically, turn out to be the costliest over the system’s operational life, which usually extends beyond a decade. Regrettably, there’s no one-size-fits-all guideline, since specific conditions at the plant can significantly impact the appropriateness and inherent expenses of the CEMS technology under consideration.

Currently, process industries subject to specific emission reduction mandates, such as the Cement and Incineration industries, have access to a diverse range of CEMS (Continuous Emission Monitoring Systems) options provided by various vendors. However, the CEMS market exhibits considerable variability, both in the appropriateness of the measurement technologies available for specific monitoring needs and in the suppliers’ ability to meet the regulatory and situational demands of each unique installation location.

INDEX

INTRODUCTION
Choosing a Continuous Emission Monitoring System (CEMS)

STEP 1
 Determine the Process Parameters, and Measurement Requirements.

STEP 2
Determine whether the CEMS needs to comply with certain national and/or international rules and standards.

STEP 3
Investigate if there are any upcoming regulations in the near future that could affect emission permits and reporting requirements.

STEP 4
Determine the current operational conditions of your CEMS, specifically focusing on the type of fuel being used in the kiln. Also, assess the likelihood of a change in the fuel type in the near future.

STEP 5
Determine if the raw materials used are sources of critical gas components, including organic compounds, ammonia, chlorine, sulphur, and similar substances.

STEP 6
Confirm if the operational conditions of the CEMS are expected to remain stable throughout its lifespan.

STEP 7
Consider the lifetime costs of your CEMS, rather than focusing solely on the initial investment.

STEP 8
Evaluate the prerequisites for operating and maintaining the CEMS. Ensure that you have a sufficient number of skilled personnel available for the specific technical solution you have chosen.

STEP 9
Consider the environmental and situational conditions specific to your site location.

STEP 10
Evaluate the capability of a potential supplier against all your major requirements to ensure they meet your specific needs.

STEP 1 

Determine the Process Parameters, and Measurement Requirements.

Identify the specific process parameters and gas components that your plant’s emission permit mandates for monitoring, the necessary measurement ranges, and the mounting location on the stack.

The selection of an appropriate Continuous Emission Monitoring System (CEMS) is greatly influenced by the specific components that environmental regulatory agencies mandate for continuous monitoring and reporting. Consequently, the initial step in this process involves compiling a comprehensive list of all the parameters that need monitoring, along with their associated emission limit values or the required measurement ranges.

For reference and scaling, it’s essential to monitor the temperature and pressure of the flue gas, as well as its oxygen content, in most stack settings. Additionally, measuring the moisture content in the flue gas is often necessary, for instance, to convert wet gas measurements to dry gas concentration values and vice versa.

The requirement to measure the amount of dust particles in the exhaust gas is also common. The specific monitoring needs here can vary; in some cases, assessing the flue gas opacity is sufficient as an indicator of dust load. In other situations, it may be necessary to determine dust concentration in terms of mg/m^3 or to calculate the total mass flow in g/h. Furthermore, if mass flow data are needed for gaseous or particulate emissions, it becomes essential to continuously measure the volume flow in the stack.

The monitoring requirements for gaseous compounds at a given plant can vary significantly for several reasons. At some facilities, it might be sufficient to measure the primary flue gas components such as O2, CO₂, CO, and NO. However, other sites may also need to track SO₂ and unburned hydrocarbons, collectively referred to as Volatile Organic Compounds (VOCs). The adoption of alternative fuels can directly influence the minimum variety of gaseous compounds that need to be continuously monitored. Consequently, the total number of gaseous components monitored by a CEMS can range from fewer than five to over fifteen.

As indicated in the CEMS technology overview in Table 1, a CEMS solution will invariably comprise a combination of technologies to meet all monitoring requirements. Certain technologies can measure a specific set of components, and some components, like VOCs, are exclusively measured by a particular analyser technology – in this case, Flame Ionization Detection (FID). In addition to the technology mix, the measurement approach can also vary. Many parameters can be measured in-situ, directly in the stack. Generally, the in-situ approach is preferred, as sample extraction and conditioning are often sources of operational issues and maintenance demands. However, as the number of monitored components increases, the analyser hardware required for an all-in-situ CEMS solution can become quite large and necessitate multiple openings in the stack. When the number of gases to be measured exceeds a certain threshold, an extractive multi-component technology might be considered a more suitable option.

Table 1 HD V3

1. NOx via converter
2. Only via heated sampling line

Step 1 – Checklist

Process:

    • Stack gas conditions and composition.
    • What components must be measured?
    • What are the measurement ranges?
    • Must reference parameters such as temperature, pressure, moisture, or O₂ content be measured?

Measuring location:

    • Analyser mounting position and stack/duct info.
    • Calibration ports.
    • Platform availability.
    • Instrument air.
    • Connectivity for remote monitoring.

STEP 2 

Determine whether the CEMS needs to comply with certain national and/or international rules and standards

Typically, the reporting guidelines for CEMS are based on national regulations or adhere to internationally recognized rules and standards, such as those established in the European Community or by the US Environmental Protection Agency (EPA). It’s important to note that certain CEMS technologies may not be compliant with the local regulations applicable to a specific site. Furthermore, some suppliers might not meet the necessary criteria to satisfy the regional requirements in these regulated markets, like having a functional quality management system, which is a prerequisite in the European Community. The responsibility for ensuring the compliance of the chosen CEMS lies with the plant operator. Therefore, it’s crucial that the legal requirements for the CEMS are well understood and that both the chosen system and its supplier are capable of meeting these requirements.

The range of options available for a CEMS solution can be constrained by these regulatory demands. Regardless, a competent supplier should be able to propose a solution that is the most cost-effective while considering the operational and situational specifics of the plant.

Step 2- Checklist

    • What national regulations and standards apply?
    • Is it necessary to take international standards such as EU directives or US EPA standards into consideration?
    • Do additional specifications apply to certain measuring technology due to specific plant requirements?

STEP 3

Investigate if there are any upcoming regulations in the near future that could affect emission permits and reporting requirements.

Environmental regulations are dynamic, reflecting a global shift towards a more sustainable and eco-friendly economy. This evolution often leads to the adoption of higher technical standards when they become economically viable. As a result, emission monitoring requirements are likely to become more stringent over time. A notable example is the recent introduction of a new Maximum Achievable Control Technology (MACT) rule by the EPA for cement plants, which includes the additional monitoring of Hydrogen Chloride (HCl) and Mercury (Hg). When there’s a need to measure new components or if the emission limits for an existing component change, it’s essential to reassess and possibly update the existing CEMS to ensure compliance.

In such scenarios, multi-component systems that can measure a wide range of gaseous components are advantageous, as they offer the flexibility to add new components at a relatively low cost. Meanwhile, multi-technology solutions provide the option to replace a specific analyser if it becomes unsuitable for the new requirements. Ideally, a CEMS solution should be versatile enough to accommodate future regulatory changes.

In situations where continuous monitoring of mercury emissions is required, especially at very low levels (in the microgram range), a dedicated, state-of-the-art analyser technology is necessary. Recent advancements suggest that it’s now possible to achieve these low measuring ranges, even below 10 micrograms, with systems that are both cost-effective and operationally feasible.

Step 3 – Checklist of Common new requirements

    • Mercury (Hg)
    • Hydrogen fluoride (HF)
    • Greenhouse gasses (GHGs)

STEP 4

Determine the current operational conditions of your CEMS, specifically focusing on the type of fuel being used in the kiln. Also, assess the likelihood of a change in the fuel type in the near future.

When the kiln process is functioning correctly, alternative fuels typically have a minimal impact on emission values. However, certain process conditions can pose challenges for a CEMS, particularly due to the presence of substantial amounts of aggressive gas compounds. These compounds require special handling and treatment to prevent adverse effects on the measurement accuracy and lifespan of the CEMS.

In-situ devices and hot wet sampling systems have demonstrated effective lifetime cost performance, even with high concentrations of aggressive gas compounds like condensates or aerosols. These systems inherently avoid contact with such compounds at any wetted part of the analyser. Conversely, cold dry sampling systems can also be adapted for these conditions, but this often comes at the cost of additional filtration steps. These steps necessitate frequent maintenance and increase operational risks, as the sampling and conditioning system must continuously protect the analyser equipment from fouling.

The choice of fuel also influences regulatory requirements. Plants using fossil fuels are generally subject to the same moderate monitoring requirements as power utilities. However, the use of alternative fuels may subject a plant to the stricter emission regulations applicable to waste incineration plants, which demand more rigorous monitoring. If a plant intends to use alternative fuels, it is crucial for the CEMS to be capable of meeting these heightened operational and regulatory demands, both current and future.

Step 4 – Checklist 

    • What fuel is currently used or is to be deployed in the near future?
    • When using alternative fuels, does the system meet the stringent thermal requirements for monitoring waste treatment processes?

STEP 5

Determine if the raw materials used are sources of critical gas components, including organic compounds, ammonia, chlorine, sulphur, and similar substances.

These gas components are recognized as challenging for analyser systems, particularly when using cold extractive sampling methods. It’s crucial that the design of the CEMS sampling system takes into account the anticipated high concentrations of these gas compounds. In-situ and hot wet sampling solutions have proven to be significantly less susceptible to issues caused by these aggressive and difficult-to-manage components compared to cold dry sampling methods.

STEP 6

Confirm if the operational conditions of the CEMS are expected to remain stable throughout its lifespan.

The implementation of gas cleaning steps, such as DeNOx systems or wet scrubbers, can significantly reduce pollutant levels, potentially necessitating much lower measurement ranges for the monitored gases during their operation. It’s important that the analyser technology within the CEMS is capable of adjusting to these lower measurement ranges. Additionally, changes in temperature and moisture levels in the flue gas stack can affect the suitability and efficiency of the CEMS.

The introduction of process additives like ammonia or urea in a Selective Non-Catalytic Reduction (SNCR) DeNOx system can lead to substantially higher NH3 concentrations in the flue gas. This increase can, as previously mentioned, result in reduced availability and higher lifetime costs for the CEMS.

Step 6 – Checklist of common changes

    • Switch from electrostatic precipitator to bag or ceramic filtration.
    • Installation of wet scrubber.
    • NO reduction due to Selective non-catalytic reaction (SNCR).
    • SO₂ reduction due to flue gas desulphurisation.

STEP 7

Consider the lifetime costs of your CEMS, rather than focusing solely on the initial investment.

When considering the costs for your CEMS, it’s crucial to look beyond the initial investment and factor in the lifetime costs. The operational lifespan of a CEMS typically extends well beyond 10 years. The total lifetime cost can vary significantly based on the chosen measurement approach (such as in-situ, dilution, cold dry, or hot wet sampling) and the mix of analyser technologies. In some cases, the lifetime cost can exceed the initial investment by more than threefold. Therefore, evaluating the total lifetime cost scenario for a CEMS system is more insightful than merely comparing initial system prices.

Although they may have a higher initial price, all-in-situ CEMS systems often offer the best value in terms of lifetime cost. This is because they do not require expensive maintenance for the gas sampling and conditioning system. In-situ analyser technology is also designed to withstand harsh environmental and operational conditions, leading to more robust and stable product designs.

It’s generally observed that the main sources of operational failures and maintenance requirements in a CEMS are not the analysers themselves, but the sampling and conditioning systems. A malfunction in the sampling system can lead to severe damage in subsequent system components, such as the sensitive sensor technologies in the analysers. Therefore, if an extractive approach is chosen, a simpler setup is preferable for better availability, cost performance, and reduced operational risks. In this context, the hot wet sampling approach has inherent advantages over dilution and cold dry solutions.

Another major factor contributing to the lifetime costs of a CEMS is the requirement for consumables such as instrument air and calibration gases. Analyzer technologies that necessitate daily span and zero checks using test gases will incur considerably higher lifetime costs compared to those employing internal validation standards. For instance, TDLS-based analysers with internal reference gas cells are renowned for their ability to operate for years without the need for calibration using test gases. This is particularly advantageous for measuring sensitive gas components like HF and NH3, where the benefit of not requiring calibration gas bottles is a significant consideration.

Step 7 – Checklist 

    • In-Situ vs Hot and Wet vs Cold and Dry.
    • Will location effect availability of calibration gas?
    • Will location effect availability of spares or support?
    • Cost of consumables, spare parts, and maintenance.
    • Service and maintenance intervals.

STEP 8

Evaluate the prerequisites for operating and maintaining the CEMS. Ensure that you have a sufficient number of skilled personnel available for the specific technical solution you have chosen.

Under the European quality standard, CEMS are required to demonstrate a field-proven availability (measurement uptime) of over 95%. This figure includes all maintenance periods and validation cycles, which contribute to the system’s total downtime. While extractive systems can meet these availability requirements, they often necessitate significantly more maintenance effort compared to in-situ systems.

Among extractive systems, those that incorporate a gas cooler to remove moisture from the flue gas are particularly demanding in terms of maintenance. This is especially true when dealing with water-soluble and/or acid-forming gas compounds in the sample, as gas conditioning in these cases becomes a delicate process. A failure in any upstream filter step can lead to severe damage to subsequent system components.

If maintaining a constantly available and well-trained CEMS maintenance team is not feasible, it may be more practical to consider an in-situ or hot wet extractive approach. These systems typically require less intensive maintenance and can be more forgiving in environments where dedicated maintenance resources are limited.

Step 8 – Checklist 

    • Target system availability of > 95%.
    • Downtime caused by filter exchange or gas extraction.
    • Availability of maintenance staff on-site.
    • Availability of supplier technicians.
    • Availability of consumables on-site.
    • Availability of critical spares.
    • Training of maintenance staff.
    • Remote monitoring and assistance.

STEP 9

Consider the environmental and situational conditions specific to your site location.

The environmental conditions at your site are a crucial factor in determining the appropriate system approach. A CEMS designed for use north of the Arctic Circle, for instance, will differ significantly from one intended for the Arabian Desert.

Cement plants are often situated near raw material sources or large construction projects, such as dams that require substantial amounts of concrete. These locations can be quite remote, which makes the availability of spare parts and the logistics of consumables an important consideration. Certain CEMS require regular, sometimes even daily, validation using bottled test gases. Some of these gases, like NH3 and HF, are not only expensive but also challenging to handle due to their limited shelf lives, corrosiveness, toxicity, and other hazardous properties.

The ability of a supplier to perform system diagnostics remotely through an internet connection, whether it’s via fibre, satellite, or mobile connection, can be highly beneficial for sites located in remote areas. This capability allows for straightforward identification of simple system failures. Moreover, if a visit from a field service technician is necessary, they can be informed in advance about the specific issue, enabling them to bring the appropriate spare parts. Remote diagnostics can prevent unnecessary field service trips, thereby saving both time and money.

Step 9 – Checklist 

Site conditions:

      • Indoors or outdoors?
      • Shelter?
      • Ambient temperature for location and plantroom.
      • Accessibility to analyser installation point.
    • How will site location effect spares, service, and availability of test gasses?
    • Internet connection availability.

STEP 10

Evaluate the capability of a potential supplier against all your major requirements to ensure they meet your specific needs.

As outlined in the previous nine steps, selecting a CEMS is a multifaceted process, requiring detailed consideration of various preconditions specific to each installation site. Table 2 encapsulates the key decision-making steps and provides an overview of the general suitability of different CEMS approaches.

In light of this complexity, it’s crucial that the CEMS supplier is capable of understanding and supporting this decision-making process, enabling them to propose a solution tailored to the customer’s specific needs. Key indicators of a suitable supplier include a broad range of CEMS expertise and analyser technology options, comprehensive regulatory knowledge, and a portfolio of solutions tailored to the unique demands of the cement industry.

The design of the CEMS offered should be driven by the specific requirements of the site, rather than being limited to the preferred technology approach of a particular supplier. Additionally, the availability of field service and spare parts is an important factor; hence, the supplier should have a robust support structure in place to ensure efficient and effective service delivery.

Table 2 V3

* Depending on supplier; FC: filter correlation; FT-IR: fourier transformation infraed spectrocopy