Below 2.5 kg/hour: the periodic tiers in practice, and what moves your number

CEMS for palm oil mills — Part 2 of 4. A focused series for palm oil mill operators and EHS managers, drawn from the Clean Air Regulations 2014 and the DOE CEMS Guidelines (Version 8, 2025), which we helped develop.

Part 1 of this series covered the rule: under the Second Schedule of CAR 2014, a palm oil mill boiler’s dust load decides how often it’s tested, climbing from once a year to continuous monitoring as it approaches 2.5 kg/hour. Most mills live somewhere in that middle ground — tested periodically, comfortably below the CEMS line. This part is about that middle ground: what a periodic test actually involves, and the operational factors that determine which tier you sit in from one test to the next.

A quick recap of the tiers

The frequency table from Part 1, in short — testing frequency climbs as your dust load does:

• >0.44 – <1.0 kg/h — once a year
• ≥1.0 – <1.5 kg/h — twice a year
• ≥1.5 – <2.0 kg/h — three times a year
• ≥2.0 – <2.5 kg/h — four times a year
• ≥2.5 kg/h — continuous monitoring, a CEMS

If you haven’t read the full breakdown of where that table comes from and why palm oil mills are judged this way rather than by boiler capacity, start there.

What a periodic test actually involves

Periodic monitoring is its own regulation, not just an absence of continuous monitoring — Regulation 16 of CAR 2014 sets three specific requirements alongside the frequency tiers themselves:

• A competent person conducts it. Reg 16(2): unless DOE directs otherwise, periodic monitoring defaults to once a year and must be carried out by a competent person — the tiered schedule from Part 1 is exactly the kind of Schedule-specific frequency that overrides that default.
• An accredited laboratory analyses the samples. Reg 16(4): any samples taken must be analysed by an accredited laboratory, not just weighed by whoever is available on the day.
• New facilities have a clock too. Reg 16(3): first monitoring at a new facility must happen after three months, but no later than six, from the start of operations.

Beyond who does the test, how it’s run still affects whether the result means anything: the plant has to be running normally. A test taken during an unusually clean run — light fuel load, fresh out of a boiler service, running below typical throughput — tells you what your boiler can do under ideal conditions, not what it emits day to day. If you’re using the result to plan ahead (and you should be), it only means something if it reflects how the mill actually operates. We cover what a properly documented test report should contain — and the checks worth running your eye over before you file it — in reading a stack-test report, and why the manual test remains the anchor even once you’re on continuous monitoring in why a continuous monitor still needs manual stack tests.

One more Reg 16 requirement worth knowing before your next test: Reg 16(5) requires periodic-monitoring records to be kept for at least three years and made available to DOE on request — the same three-year retention duty that applies to CEMS records under a separate provision once a mill crosses into continuous monitoring. That record is exactly what makes the trend-reading in the next section possible — you’re already required to keep the history, so use it.

What actually moves your number

CAR 2014 sets the limit and the tiers; it doesn’t say what determines where your boiler lands between tests. That comes down to ordinary combustion and abatement variables — none of them exotic, all of them within a mill’s control to some degree.

Fuel mix. Palm oil mills typically burn a blend of empty fruit bunch, shell and mesocarp fibre, and these aren’t equivalent from a dust standpoint. Shell and fibre tend to burn more completely and consistently; empty fruit bunch carries more moisture and ash, and a boiler running a higher proportion of it can push more particulate into the flue for the same thermal output. This is also why biomass fuel changes are treated as significant for CEMS calibration elsewhere in our content — the same batch-to-batch variability that can trigger a fresh QAL2 on a CEMS is what nudges a periodically tested mill’s dust load up or down test to test. CAR 2014’s own fuel-quality rule — air-dried, natural composition, no coating or preservative — sets a floor, but within that floor, the mix still matters.

Dust-collector condition. Most mill boilers run a cyclone or multiclone ahead of the stack, and its condition does more to move your number than almost anything else. A well-maintained collector holds dust load down consistently; wear on the cyclone tubes, a leaking bypass damper, or simple neglect of routine cleaning is often the actual cause of a rising trend — not a change in fuel. If your dust load has crept up since your last test and your fuel hasn’t materially changed, the collector is the first place to look before assuming you’re approaching the 2.5 kg/hour line for good.

Combustion conditions. Excess air, firing rate relative to boiler capacity, and how consistently the boiler is run near its design point all affect how much particulate carries over. Recall from Part 1 that the Second Schedule requires boiler thermal efficiency of at least 90% — that figure isn’t a separate box to tick, it’s connected to the same combustion quality that keeps particulate emissions down. A boiler running inefficiently is more likely to be running dirty as well.

None of this is spelled out in CAR 2014 — the Second Schedule sets the destination, not the route. But these are the levers that, in our experience running reference-method tests across Malaysian mills, actually explain why one year’s number differs from the last.

Read your trend, not just your latest result

A single passing test tells you where you stood on the day it was taken. What matters more is the direction: is your dust load flat, or has it climbed across your last two or three tests? A mill sitting steadily at 1.2 kg/hour is in a very different position from one that’s moved from 1.2 to 1.8 to 2.3 kg/hour over three annual tests, even though both are currently compliant on periodic testing.

If your trend is climbing toward 2.5 kg/hour, you have a choice to make before the decision gets made for you: invest in your abatement and combustion practices to hold the line, or start planning for continuous monitoring on your own timeline rather than DOE’s. Part 1 of our Buying a CEMS series walks through what that process involves — registration, a QAL1-certified system through a DOE-registered consultant, and the QAL2 step that makes the data count. Planning that transition ahead of a mandatory crossing is considerably less stressful than reacting to it.

What this means for your mill

Two practical habits follow from all this: treat every periodic test as a data point in a series, not an isolated pass/fail, and keep an eye on your dust collector’s condition between tests rather than waiting for the next report to find out something changed. Both are cheaper than being surprised by a jump into the next tier — or past the threshold altogether.

Not sure whether your trend is heading somewhere you’d rather avoid? Talk to us — we run reference-method testing for palm oil mills across Malaysia and can help you read what your numbers are actually telling you.

This article is general guidance, not legal advice. The operational factors discussed here reflect general combustion and abatement practice, not requirements set out in CAR 2014 itself. For obligations specific to your mill, refer to the current Environmental Quality (Clean Air) Regulations 2014, the EQA 1974, and the DOE CEMS Guidelines, or speak with us directly.