NOx calculation: comparing the old and new TNO method

New TNO method for Dutch nitrogen calculation

The nitrogen issue has kept construction and infrastructure in the Netherlands on edge for years. A new calculation method from TNO enables a more precise calculation of NOx emissions from mobile machinery, thanks to a large-scale measurement programme. In the Dutch permitting process, that precision can be exactly what makes the difference between getting a permit or not. We explain what's changed between the old and new method, and what that can mean for projects.

Measurement programme by Topsector Logistiek

The measurement programme is an initiative of Topsector Logistiek, in collaboration with TNO, and falls under the broader Clean and Emission-free Construction programme (Schoon en Emissieloos Bouwen). It was launched because of a lack of practical data: the existing calculation methods for NOx emissions from mobile machinery were largely based on assumptions, and those assumptions tended to be conservative. That led to a structural overestimation of actual nitrogen emissions. The programme has three goals: collecting practical data at scale on real construction projects, using that data to refine the existing calculation methods, and recording the results in a manual and in AERIUS, the Dutch tool for calculating nitrogen deposition, so the whole sector can use them.

What were the starting points of the old method?

Until recently, the calculation methods for NOx emissions from mobile machinery relied on a limited amount of practical measurements. Without hard figures per machine, calculations fell back on fixed assumptions: 35% engine load to estimate fuel consumption, and 6% AdBlue consumption relative to that. Operating hours were generally assumed to be a fixed 8 hours per day. In practice, this often deviates significantly from what a machine actually does, and therefore from the actual emissions your project reports.

What has the measurement programme delivered?

To collect real practical data, Topsector Logistiek measures fuel consumption, operating hours and engine load on more than 500 construction machines at 40+ companies. On 150 of those machines, NOx emissions are also measured directly at the exhaust, using a sensor.

When Topsector Logistiek started measuring, the difference with practice turned out to be substantial: across the whole measurement programme, the actually measured NOx emissions were on average about 40% lower than what you would calculate using measured fuel and hours, but with the standard 6% AdBlue added. Based on all these measurements, the calculation methods have since been adjusted so the outcome comes closer to practice.

A few clear patterns emerge from the measurements:

Engine load determines emissions per litre. More so than engine type or year of construction. The higher the load, the lower the NOx emissions per litre of fuel.
That load can be estimated well. Using fuel consumption, or even based on operating hours alone, adjusted coefficients already come surprisingly close to measured practice.
Idling is costly. At low load, aftertreatment works less effectively, which means more NOx is released per litre of fuel.
The assumption of 8 hours per day is too high. In practice, almost all machine types fall well below that, averaging around 6 hours. Calculating with actual hours directly lowers the calculated NOx by around 25%.

How does the AUB method work now, and what has changed?

The AUB method calculates using AdBlue, hours and fuel. The problem was always the first component: measuring AdBlue consumption proves difficult in practice, so it was usually unknown. Without that data, a standard 6% AdBlue was assumed, as a percentage of fuel consumption.

TNO has now found a way to estimate that AdBlue percentage without measuring it, based purely on fuel. The logic: if you know how much fuel a machine consumes, and you combine that with the engine power of that specific machine (in kW, from the engine plate), you can calculate how heavily the engine is loaded on average. And if you know how heavily an engine is loaded, you also know how well the catalytic converter has been able to work, and therefore how much nitrogen has actually been captured.

Concretely, the calculation chain works like this:

Fuel consumption (litres per hour)
Engine load (derived from that consumption, plus year of construction and engine power)
AdBlue percentage (from a table, matching that load)

The exact formulas can be found in the new TNO manual.

Engine loadAdBlue %

0–10%	4.3%
10–15%	5.3%
15–20%	6.2%
20–25%	6.4%
25–35%	6.8%
35–50%	6.9%
50–100%	7.0%

An important nuance: the new calculation rule doesn't automatically produce a lower emission figure. On a machine that runs at low load, such as a lot of idling, the exhaust gas stays relatively cool. A catalytic converter needs heat to convert NOx effectively into harmless substances: at too low a temperature, little AdBlue is dosed, and conversion works poorly. The result is that relatively more NOx leaves the exhaust unconverted. The old 6% assumption didn't account for that, and so turned out to be too optimistic in those situations. Only on machines that are working hard and running warm enough does the new calculation come out noticeably lower than before.

How does the U method work, and what has changed?

The U method is the simplest route: only operating hours and engine power, no fuel or AdBlue needed. Useful, for example, early in a project when only the machine type and an estimate of the hours are known.

You need three things:

Maximum engine power (kW) of the machine
Stage category of the engine (for the correct coefficient)
Number of operating hours over the project period

This was already an existing method, but the coefficients have been refined based on the measurement programme, making the estimates sharper and closer to actually measured values.

How accurate is this now, compared to an actual measurement?

TNO has tested all methods against machines where NOx emissions were also actually measured with a sensor at the exhaust. This shows that both new methods, the AUB method with the new AdBlue calculation rule and the adjusted U method, come noticeably closer to that measured reality than the old approach. The AUB method comes up to 64% closer to reality. The U method up to 33%.

What does this mean for your project?

Recalculating 48 permit applications from the participant group showed that 23 of them originally calculated a nitrogen deposition contribution. With the updated methods and coefficients, 10 of those 23 no longer produced a deposition contribution in the calculation.

These are specific applications, not a guarantee for every project: the emission estimate in AERIUS remains dependent on the fuel and AdBlue figures you enter yourself, and the total deposition calculation depends on more sources than mobile machinery alone. But it does show that calculating with more realistic assumptions can, in specific cases, make a difference in a permitting process.

The adjusted coefficients and the AdBlue calculation rule will be incorporated into the AERIUS update in October. Until then, you can already apply the new calculation rules via the Emissievoorspeller (Topsector Logistiek's calculation tool) or manually, following the formulas in the manual.

Automatically calculating emissions for your entire fleet, based on the latest calculation rules

For a single machine, this is still manageable with the new formulas: read off fuel consumption, look up engine power, calculate the load factor, look up the corresponding AdBlue percentage in the table, and do the sum. For an entire fleet of dozens or hundreds of machines, every month again and broken down per project, that's no longer an occasional calculation but a structural task.

Flowter's emissions reporting is based on this new TNO manual. For machines where CAN bus data is available, Flowter automatically calculates fuel consumption, operating hours and engine load, and converts that into CO2, NOx and NH3 emissions according to the current calculation methods. That means you don't have to determine the load factor for each machine yourself and look it up in the right table: you simply see, per project and period, what the emissions of your entire fleet are.

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