CEMS Thailand Guide to Factory Stack Emission Monitoring 2026

Among foreign-owned manufacturers operating plants in Thailand, CEMS stack emission monitoring has quietly become one of the most pressing compliance topics of the year. If your factory runs a boiler, an incinerator, or a metal melting furnace, it may already be required to install a Continuous Emission Monitoring System and transmit the readings online to Thailand’s Department of Industrial Works (DIW). The obligation has kept widening since it went nationwide in 2023, adding both new industry categories and new geographic areas, so plants that once concluded “this does not apply to us” are increasingly being pulled into scope by a revised size threshold or a location-based add-on rule. This article walks through how the regulation expanded, how to confirm whether your own site is in scope, and what the technical build and the cost structure look like from an OT/IoT perspective.

What stack emission monitoring (CEMS) actually is

CEMS stands for Continuous Emission Monitoring System. It measures air pollutants leaving the factory stack continuously, without shutting the process down, and keeps streaming those values to the Thai government’s online reporting platform. In practice it is a chain of OT equipment working together: an analyzer samples and measures the gas inside the flue, and a data logger conditions the numbers and forwards them over a communications link to DIW’s Industrial Environmental Monitoring Center (IEMC).

What gets measured: SO2, NOx, CO, O2 and opacity

The typical CEMS measurement set covers sulfur dioxide (SO2), nitrogen oxides (NOx, expressed as NO2 equivalent), carbon monoxide (CO), oxygen concentration (O2, used as the reference gas for correcting the other readings), and opacity, which stands in for particulate concentration in the exhaust. Depending on the industry, flue gas flow rate and temperature are added as well so that mass-based emissions (kilograms per hour, for example) can be calculated. Every one of these directly affects human health and ambient air quality, which makes them a fundamentally different measurement target from greenhouse gases.

Reporting units are defined per industry and per pollutant. Instantaneous readings are generally not sent as-is; they are averaged over a defined window (a rolling average or an hourly average) before transmission. Whether that averaging happens inside the analyzer or in the data logger is a question worth settling during model selection. If the system will be tied into an existing PLC, the recommended design is to keep the calculation logic in one place rather than implementing it twice.

Why this is not the same as GHG/CO2 Scope 1 and 2 accounting

The most common point of confusion is the difference between CEMS and CO2 emissions visualization or GHG Scope 1 and 2 accounting. As covered in our separate article on CO2 emissions visualization and Scope 1 and 2 accounting for Thai factories, Scope 1 and 2 accounting is an accounting-style exercise: greenhouse gas emissions are calculated from fuel and electricity consumption and then disclosed. CEMS, by contrast, physically measures air pollutants such as SOx, NOx and particulates at the stack itself and reports them to the regulator in near real time under a legal mandate.

The purpose, the substances measured, and the recipient of the report are all different, so implementing one does not remove the need for the other. Plenty of plants carry both obligations at once. Where that is the case, the practical way to control cost is to share the sensor network and the data collection platform across both, rather than building two isolated stacks. The same logic applies to electricity monitoring: a factory power monitoring system is another OT sensing layer that is often built entirely separately when it could reuse the same data backbone.

How Thailand’s regulation has expanded

The scope of the CEMS mandate has widened considerably over the past few years. Laying the changes out chronologically makes it much harder to miss the moment your own site became subject to them.

As the table shows, the rules have grown along two axes: wider geographic coverage and broader industry and size criteria. The driver is rising public concern about PM2.5 and ambient air quality, centered on the Bangkok metropolitan area. With dry-season spikes in fine particulate matter (PM2.5) making headlines every year, DIW has moved toward publishing stack emission data openly, and the data collected through the IEMC online system is now also made available to the public through a smartphone application. In other words, CEMS is shifting from a scheme that simply files data with the authorities into one where neighboring residents and business partners can see how a plant is performing.

Food, chemical, textile dyeing, paper, rubber and electronic component plants that operate boilers, autoclaves, drying ovens or incinerators should re-check their status periodically, even if they were clearly out of scope at the start. A very common pattern: a food processing plant upgrades or adds a boiler as part of a capacity expansion, the replacement unit crosses the 30 tons of steam per hour threshold, and the site only discovers it now owns regulated equipment once the installation work is finished. Factoring the CEMS question into the capital expenditure plan up front avoids a scramble for extra budget and schedule right before start-up.

How to check whether your factory is in scope

When the answer is not obvious, working through the following checks in order usually clarifies the picture.

• Does the site have a boiler, drying oven, incinerator, metal melting furnace, or any other combustion equipment that discharges exhaust gas through a stack
• For a boiler, does it generate 30 tons of steam per hour or more, or exceed 100 MMBTU per hour of heat input
• Does the business fall under thermal power generation, oil refining, metal smelting, waste incineration, glass, cement, pulp and paper, or any of the 13 industry categories added by the 2024 amendment
• Is the plant located within Bangkok, or does the industrial estate’s own management rules impose additional emission reporting
• Do the environmental impact assessment (EIA) report or the conditions attached to the factory operating licence (Ror Ngor 4) mention emission monitoring

If any single item applies, we recommend making a direct enquiry to the relevant DIW regional office or to the industrial estate administrator. Plants sitting just under a threshold need to re-check whenever production plans change, because a capacity increase alone can push equipment ratings over the line.

The classic stumbling block looks like this. A food processing plant had been running a boiler rated at roughly 20 tons of steam per hour. To keep up with demand it replaced the unit with a 35 tons per hour class boiler, and the new equipment crossed the size threshold, converting the site into regulated equipment. At the approval stage for a boiler replacement, the comparison tends to focus on energy efficiency and fuel cost, and the question of whether CEMS compliance is triggered often never makes it onto the evaluation list. Checking threshold applicability while the investment is still being scoped, and securing installation budget and schedule alongside the boiler work if needed, avoids an unplanned follow-up investment after the line is already running.

The OT architecture of a CEMS build-out

CEMS is not a single instrument. It is a small system in which several pieces of OT equipment work together. Breaking it into four layers makes the whole picture much easier to evaluate.

Layer 1: the analyzer

Installed either directly in the flue or downstream of a sampling line, the analyzer measures gas concentrations such as SO2, NOx, CO and O2 along with opacity. Two installation methods dominate: the in-situ method, which inserts a probe directly into the flue, and the extractive method, which draws gas out and measures it in a separate analysis cabinet. Which one suits a given stack depends on exhaust temperature, moisture content and dust loading.

Whichever method is chosen, the single most important shared decision is where to place the sampling point relative to the existing flue geometry. If the sampling point sits where the gas flow is skewed, the system will faithfully report unrepresentative numbers forever, and no amount of calibration will fix it. For retrofits onto existing equipment, we recommend a simple check of the gas flow profile inside the flue before installation begins.

Layer 2: the data logger and calculation

This layer receives the analyzer signal (4-20 mA, or a digital protocol such as Modbus), applies oxygen correction and unit conversion, and computes the time-averaged values. In plants that already run a PLC or SCADA platform, the design fork is how the data logger is positioned within the OT network. The core question is whether to ride on the existing supervisory platform or to split the regulatory reporting path off as an independent system. That decision also determines where responsibility sits when data goes missing, which we return to below.

Layer 3: communications and network security

Data is transmitted to DIW’s IEMC online and near-continuously. What often gets overlooked is that the moment a regulatory reporting path connects to an external network, it becomes an OT security concern. Following the zoning approach described in our separate article on OT security for factories in Thailand, the CEMS communications path should be logically separated from the production control network, with firewall rules that permit only the traffic actually required. If a legally mandated reporting path shares a segment with the production line control network, a security incident on the plant floor can propagate straight into the compliance reporting system.

Layer 4: handling data gaps and abnormal conditions

Communications outages, power failures and calibration work all create periods when data cannot be transmitted. You need an operating rule that records the reason for each gap and the time to recovery, and reports it to DIW where required. A gap is not automatically a violation, but gaps that stretch on without explanation, or that recur frequently, make a site a likely candidate for an on-site inspection. Backup power, spare parts sourcing and calibration scheduling all belong in the design. The point is to build something that keeps running continuously, not merely to install an instrument.

What CEMS costs

CEMS costs vary widely depending on how many gas species are in scope, what equipment already exists, and how developed the communications infrastructure is. Splitting the estimate into the five layers below makes quotations far easier to compare against each other.

Installation is the line item most sensitive to local conditions, particularly the existing flue structure and whether the work can proceed without stopping production. Tie-in work that cannot be done on a live plant is normally scheduled to coincide with a planned shutdown maintenance window. As a practical rule of thumb, factories that need to modify existing equipment typically plan for somewhere between a few months and about half a year from the start of evaluation to the beginning of live operation.

Maintenance cost is the other commonly underestimated item. Periodic calibration with zero and span gas consumes gases and consumables, and it also requires taking the analyzer temporarily out of the measurement path, so it has to be built into the annual schedule alongside the production plan and the data gap recording rules. One consideration specific to overseas sites: lead times for calibration standard gases and spare parts tend to be longer than they would be in Japan, which is worth allowing for when the maintenance plan is drawn up.

Rollout steps

Deploying CEMS is not a matter of ordering an instrument and bolting it on. It needs to be planned as a continuous sequence running from confirming your regulatory position through to steady-state operation. A standard sequence looks like this.

• Confirm applicability: compare boiler ratings and industry classification against the criteria, and make a direct enquiry to the relevant DIW regional office or industrial estate administrator where needed
• Survey the flue and select the sampling point: check the geometry, temperature, moisture and dust loading of the existing flue, then decide between in-situ and extractive
• Design the integration with existing OT equipment: decide whether to ride on the existing SCADA or energy monitoring platform or split the compliance reporting path off as an independent system, and design the communications security at the same time
• Plan the installation work: identify what can be built without stopping production, and align any tie-in work with a scheduled maintenance shutdown
• Commission and perform initial calibration: calibrate with zero and span gas, and verify the communications path with a test transmission to DIW’s IEMC
• Establish operating rules: set the calibration schedule, the procedure for recording and reporting data gaps, and the handover documentation for staff changes

Sharing information across maintenance, IT/OT and environmental management from the planning stage onward prevents the gaps in ownership that otherwise surface after installation, such as who performs calibration and who responds when the data link drops. At Thai sites in particular, the head office environmental function and the local plant frequently hold different assumptions about reporting formats and units. Bringing head office into the loop early, while the reporting mechanism is still being defined, makes later enquiries far easier to handle.

Common pitfalls

Most CEMS projects that run into trouble do so because of operational design rather than measurement accuracy.

• Deploying it as an isolated compliance-only system. A standalone terminal installed purely for reporting, with no link to production management or energy monitoring systems, tends to lose its operational know-how the moment the responsible person transfers or leaves
• Leaving the calibration schedule out of the maintenance plan. Missed periodic calibration casts doubt on the reliability of the readings and raises the risk of findings during an audit
• Deferring the network security design. If the network is not designed from the outset around the fact that this path needs permanent external connectivity, retrofitting security later is expensive
• Failing to re-check after a capacity increase when sitting near a threshold. Expanding production capacity can push a boiler past the size criteria, quietly turning it into regulated equipment
• Leaving data gap recording and reporting to the shop floor to figure out. Unless who records what, and when, is agreed in advance, the response to an actual gap will always be reactive

Frequently asked questions

How is CEMS different from GHG/CO2 emissions reporting?

CEMS is a legal obligation to measure air pollutants (SOx, NOx, particulates and so on) directly at the stack and report them to DIW. GHG emissions accounting is an accounting-style calculation of greenhouse gas emissions derived from fuel and electricity consumption. The substances covered, the calculation method and the recipient of the report are all different. Plants that need both can improve investment efficiency by sharing sensors and data infrastructure wherever the two overlap.

How much does CEMS cost?

It ranges considerably depending on the number of measured parameters and what equipment is already installed. We recommend breaking the quotation into the five cost layers set out above (analyzer hardware, sampling system, data logger and communications, installation work, and maintenance) and confirming each against your own flue structure and operating conditions. Note that two systems with an identical parameter list can still differ substantially in total cost because of installation difficulty alone.

Does a factory with a small boiler still fall in scope?

If steam generation is under 30 tons per hour and heat input is under 100 MMBTU per hour, current nationwide criteria do not immediately bring the site into scope. That said, coverage has been widening in stages, as the 2024 industry expansion and the 2026 Bangkok add-on rule both show, so it is important to re-check periodically for equipment rating changes driven by capacity increases and for any additional rules imposed by your industrial estate.

What happens if we fail to report to DIW?

Persistently neglecting a reporting obligation attached to the conditions of your factory operating licence can expose the site to on-site inspection and corrective orders from the regulator. When data gaps do occur, being able to document and explain the cause and the expected recovery time is the practical way to reduce the burden of an audit.

Can multiple plants be monitored on one dashboard?

Where CEMS is installed at several sites, standardizing the data logger output format makes it technically feasible to build a dashboard that lets head office or the environmental management function compare emissions across locations, separately from the individual reports filed with DIW. If you connect this to an existing energy monitoring platform or production management system, keeping the regulated reporting path logically separate from the internal visualization path means that changes to one are far less likely to disturb the other. Demand for side-by-side comparison of environmental data across multiple countries and sites is growing, and when each site uses a different model and data format, the conversion work needed to consolidate later can easily outweigh the benefit.

Summary

Thailand’s CEMS stack emission monitoring regime has widened its industry and geographic coverage in stages since it went nationwide in 2023. Given the trajectory through the 2024 expansion to 13 industry categories and the 2026 Bangkok add-on rule, it is reasonable to assume that mid-sized foreign-owned plants previously outside the scope will continue to be drawn in by capacity increases and further regulatory revisions. CEMS is a four-layer OT system covering the analyzer, the data logger, communications and data gap handling. Treating it not as an instrument installation but as a system designed to keep running continuously, aligned with your existing SCADA and OT security design, pays off in both compliance and operational workload. The value of confirming threshold applicability early is greatest for exactly those factories currently planning a boiler or incinerator replacement or expansion.

We are happy to help at any stage, from a first-pass check of whether your plant falls within CEMS scope through to designing the integration with your existing OT equipment and energy monitoring platform, including while you are still weighing up the options. If your factory operates a boiler or incinerator and you want to confirm whether the size thresholds or industry categories apply, or you are still working out how CEMS should coexist with an existing SCADA or energy monitoring system, tell us about your current equipment configuration and we will work through the requirements and the path forward with you. Questions alone are always welcome, so please feel free to contact us.

References

• Thailand: Revised and expanded requirements on air pollution monitoring and reporting for factories, Global Compliance News
• Thailand partially revises regulations on mandatory installation of air pollution measurement equipment for certain industries, Enviliance ASIA
• Thailand tightens air quality rules with mandatory factory emissions monitoring, Envirotech Online
• What is CEMS: five things industrial factories need to know, sci-master.com
• Ministry of Industry notification applying the requirements to the Bangkok metropolitan area, Department of Industrial Works (DIW)
• Continuous Emission Monitoring System (CEMS), ALS Thailand
• CEMS (Continuous Emission Monitoring System), automation.co.th