Process Technology · Emission Control
Measuring raw gas and clean gas at the same moment: proving guaranteed removal efficiency at commissioning, and controlling it for the life of the plant.
The true performance of a wet gas scrubber can only be established by measuring the process gas at two locations simultaneously: at the scrubber inlet (raw gas) and at the outlet (clean gas). By operating the same analyser technology at both points at the same time, the actual removal efficiency is derived directly from the concentration difference between the two readings.
This single principle delivers two distinct benefits. During commissioning, it is the only reliable way to verify that a new scrubber meets its guaranteed performance. In routine operation, it becomes a permanent continuous emission monitoring system (CEMS) that feeds the scrubber's own control loop. This article explains how simultaneous inlet/outlet measurement works, which analysers are deployed, and why these two roles matter.
01 Regulatory context
Increasingly stringent environmental legislation places rising demands on industrial operators to demonstrate continuous compliance with emission limit values (ELVs). Across sectors from chemical manufacturing and power generation to food processing and waste treatment, the wet gas scrubber remains one of the most widely applied technologies for removing gaseous and particulate pollutants from a process gas stream. It brings the gas into intimate contact with an absorption liquid, or scrubbing medium, before the treated gas is discharged to the stack.
Pollutant removal is, however, only part of the operator's obligation. The operator must also demonstrate how effectively the scrubber is actually performing — continuously and to the satisfaction of the permitting authority. This requires knowledge not only of the emission leaving the stack, but of the pollutant load entering the scrubber.
Measuring the outlet alone confirms whether the ELV is met, but not how much pollutant the scrubber has actually removed. To quantify this, inlet and outlet must be measured at the same moment, with identical analyser technology and referenced to the same conditions.
Only then does the true removal efficiency become visible. That single differential measurement is the subject of this article, and it earns its keep in two distinct ways: as a commissioning tool and as a permanent control instrument.
02 Scrubbers measured at inlet and outlet
What sets Lechler apart is the ability to cover both sides of the challenge within one engineered package. Combining spray-technology expertise with the manufacture of custom wet gas scrubbers, Lechler designs and integrates the scrubber alongside the analyser system that quantifies its performance. Rather than being adapted from a standard catalogue, every unit is engineered to order around the customer's specific gas composition, contaminant load and regulatory requirements, with the spray stage optimised to maximise mass transfer between the absorption liquid and the process gas.
To verify that the scrubber performs as intended, Lechler Process Technology installs identical analysers at both the scrubber inlet and outlet and operates them concurrently. Using the same measurement technology at both points — inline particulate (dust) monitors and gas analysers — makes the two data sets directly comparable. These analysers are particularly suited to the task: they measure multiple components simultaneously and in real time, so the inlet and outlet concentration of each pollutant can be tracked together and the removal efficiency calculated for each component individually. Because the analysers are integrated with the scrubber control system, the measured inlet/outlet difference can directly govern the blowdown of the absorption liquid, closing the loop between the scrubber's actual performance and its operation.
The result is a single-source solution. The open spray scrubber delivers robust, low-pressure-drop cleaning that copes well with high dust loads and fouling contaminants, while the simultaneous inlet/outlet analysers provide continuous, high-accuracy verification of both the residual emission and the actual removal efficiency — plus direct feedback control the moment performance begins to drift. The customer receives one complete engineered system. It is this pairing of spray-technology expertise with modern analyser instrumentation, applied on both sides of the absorption stage, that enables industrial operators to meet ever-stricter environmental legislation reliably and cost-effectively.
03 Scrubber technology and measurement know-how
These two disciplines seldom reside in one company. Spray-technology specialists master droplet formation, nozzle behaviour and interfacial gas–liquid contact, yet treat the analyser as an assembled component; instrumentation manufacturers master ppm-level measurement and sample conditioning yet never design scrubber internals. Bridging the two is normally left to the operator, who must then own every interface between them. Lechler holds the rare middle ground, mastering both the open spray absorption technology that captures the contaminant and the measurement expertise — built on high-end analysers — that verifies its removal.
This combined competence brings practical benefits. Because Lechler designs and builds the scrubber, the open scrubber's behaviour is understood in detail; and because Lechler Process Technology also specifies and installs the analysers, matched instruments can be positioned at inlet and outlet where each reading is most representative, cross-referenced to one another, fed directly into blowdown control, and the entire loop calibrated as a single instrument. Absorption performance and measurement accuracy are engineered to one specification, from a single source. For the operator, what would otherwise be split between a scrubber supplier and an instrument supplier is consolidated into one point of accountability, uniting two separately optimised components into one demonstrably compliant system.
04 Two roles: commissioning and permanent control
Simultaneous inlet/outlet measurement is valuable for two clearly different reasons, and it helps to keep them apart. The first is a one-off task; the second runs for the entire operating life of the plant.
Commissioning
When a new scrubber starts up, its guaranteed removal efficiency must be demonstrated. Measuring only the clean-gas side cannot achieve this, because it never captures the incoming load. By measuring inlet and outlet concurrently during commissioning, the guaranteed efficiency can be demonstrated directly and on site, under real operating conditions rather than in a laboratory. This is exactly the kind of evidence that acceptance tests, permit sign-off and type-approval increasingly require.
Permanent scrubber control
Once the plant is running, the same two analysers remain in place and continue to operate. Because the removal efficiency is known at every moment, the measurement becomes part of the scrubber's process control: the live difference between inlet and outlet steers the blowdown of the absorption liquid, so the scrubber is continuously adjusted to the actual gas duty. The instrument that proves performance at commissioning therefore goes on to protect it, day after day.
05 The principle of simultaneous inlet/outlet measurement
The removal efficiency of a scrubber is defined by the fraction of the incoming pollutant load it captures. It therefore cannot be determined from a single measurement point. Two analysers — one in the raw-gas duct upstream of the absorption stage, one in the clean-gas duct downstream — are operated simultaneously and on the same basis, so that the inlet and outlet concentrations refer to the same gas as it passes through the scrubber.
Removal efficiency η = (cin − cout) / cin × 100%
where cin is the raw-gas concentration and cout the clean-gas concentration, both measured at the same moment and referenced to the same conditions.
From these two concurrent readings the removal efficiency follows directly as the proportional reduction between inlet and outlet concentration, while the outlet reading alone confirms compliance with the emission limit value. Simultaneous — rather than sequential — measurement is essential: for a wet gas scrubber the reference literature identifies temperature, pH and the gas–liquid (L/G) ratio, together with the incoming pollutant load, as the parameters that govern absorption performance, and all of these fluctuate during operation. Only concurrent readings referenced to the same conditions therefore yield a valid efficiency figure. Reference guidance for gas scrubbers is explicit on this point, requiring that both the ingoing and the outgoing gas concentration be measured to establish the efficiency of the installation. Using identical analyser technology at both points eliminates any systematic bias between the two measurements.
The choice of analysers is dictated by the process chemistry: the contaminants present in the raw gas and the reagents used to neutralise them determine the required instrument set at each measurement point. A gas scrubber operates across a wide range and the contaminant spectrum it must handle is correspondingly broad.
| Scrubber chemistry | Operating condition | Components removed |
|---|---|---|
| Acid scrubber | Low pH | Alkaline components such as ammonia (NH₃) and amines |
| Alkaline scrubber | High pH | Acidic components such as HCl, HF, SO₂ and H₂S |
06 From differential measurement to closed-loop control
Because inlet and outlet are measured continuously and side by side, two things are visible at all times: how clean the treated gas is, and how hard the scrubber is actually working. The outlet analyser confirms that the gas leaving the stack stays below the limit; the inlet analyser shows the incoming load.
Together they answer a question a single sensor never can: if the outlet reading climbs, is it because more pollutant is entering, or because the scrubber itself is losing performance? With both points measured, the two causes can be distinguished immediately.
This benefits both operator and regulator. Since the readings reflect the real gas on each side of the scrubber, the blowdown can be driven directly from the measured performance, topping up or refreshing the liquid to match the incoming load. Nothing slips past unnoticed, and the operator, the permitting authority and the environmental inspectorate all reference the same clear set of inlet, outlet and efficiency values.
07 Analysers
Lechler Process Technology uses the following analysers for testing and controlling wet gas scrubbers.
Inline environmental dust monitoring
Identical tribo-electric probes in the raw- and clean-gas ducts measure particulate at inlet and outlet; the difference gives dust removal efficiency and confirms emissions stay below the limit. A sustained rise in dust level signals a fault or filter rupture.
Portable aerosol measurement
Portable aerosol spectrometers measure multiple mass fractions at once and are ideal for mobile field surveys and exposure assessments, offering particle counts across 31 size channels for efficiency testing. For fixed monitoring, a compact weatherproof monitor with a heated inlet provides a robust basis for continuous outdoor operation.
Certified environmental fine dust monitor
For continuous fine-dust monitoring, various configurations are available with durable pumps for 24/7 operation. The core is typically a 19-inch rack-mounted monitor for an analyser shelter; mobile, climate-controlled cabinets also allow outdoor deployment.
Continuous emission monitoring system (CEMS)
A CEMS continuously records emission concentrations online, housing an industrial computer, a heated sampling system and an FTIR analyser, optionally with an FID for total hydrocarbons and an oxygen analyser. It quantifies many gaseous components at once.
Portable FTIR emission analysis
An FTIR analyser designed for periodic on-site work in harsh industrial conditions. The heated sample path prevents condensation and the loss of water-soluble components. In a single instrument, FTIR measures acidic gases, hazardous compounds and greenhouse gases simultaneously.
Portable flame ionisation detection (FID)
Portable FID instruments reliably measure gaseous organic emissions (total organic carbon, TOC) during periodic emission control.
Gas chromatography (GC)
Gas chromatography selectively separates and quantifies volatile organic compounds (VOCs), in ambient-air monitoring especially the BTEX group: benzene, toluene, ethylbenzene and the xylenes. A compact micro-GC with a photo-ionisation detector (PID) determines BTEX selectively without cross-sensitivities.
08 Why measure both points
One measuring point
- Outlet only: proves compliance, but gives no insight into scrubber performance
- Inlet only: describes the load, but says nothing about the emission
- A rising outlet reading cannot be attributed to a cause
Two measuring points, simultaneously
- Proves the guaranteed efficiency at commissioning
- Delivers live efficiency figures in operation
- Higher accuracy and immediate regulatory transparency
- Closed-loop blowdown control on real scrubber performance
Measuring both simultaneously gives the complete picture: at commissioning it proves the guaranteed efficiency, then becomes a permanent monitoring system in which the outlet emission combined with the inlet conditions delivers live efficiency figures. Combined with dust, FID, FTIR or GC analysers, the simultaneous inlet/outlet approach offers higher accuracy, immediate regulatory transparency and closed-loop control based on real scrubber performance. Compliance is shifting from one-off certification to proof of actual operating performance — and data from both points supplies exactly that proof, from commissioning through continuous operation.
Frequently asked questions
Why is measuring the outlet alone not enough?
The outlet reading confirms whether the emission limit value is met, but it never captures the incoming load, so it cannot show how much pollutant the scrubber has actually removed. Removal efficiency is a ratio between two concentrations and therefore cannot be derived from a single measurement point.
Why must the two measurements be simultaneous rather than sequential?
Temperature, pH, the gas–liquid (L/G) ratio and the incoming pollutant load all govern absorption performance, and all of them fluctuate during operation. Readings taken one after the other would refer to different operating conditions. Only concurrent readings referenced to the same conditions yield a valid efficiency figure.
Why use identical analyser technology at both points?
Using the same instrument class at inlet and outlet makes the two data sets directly comparable and eliminates any systematic bias between them. Any difference in the readings can then be attributed to the scrubber rather than to the instrumentation.
How does the differential measurement control the scrubber?
The analysers are integrated with the scrubber control system, so the live inlet/outlet difference governs the blowdown of the absorption liquid: the liquid is topped up or refreshed to match the incoming load, and the scrubber is continuously adjusted to the actual gas duty rather than to a fixed schedule.
If the outlet reading rises, how do you know what caused it?
With both points measured, the two possible causes can be distinguished immediately: the inlet analyser shows whether more pollutant is entering the scrubber, or whether the incoming load is unchanged and the scrubber itself is losing performance. A single sensor cannot answer that question.
Which analysers are selected for a given scrubber?
The process chemistry dictates the instrument set. Acid scrubbers running at low pH remove alkaline components such as ammonia and amines, while alkaline scrubbers neutralise acidic components such as HCl, HF, SO₂ and H₂S. The inlet and outlet instruments are matched to whichever chemistry applies, and the same instrument class is deployed at both points.
Do the commissioning analysers stay in place afterwards?
Yes. The same two analysers that demonstrate the guaranteed removal efficiency during commissioning remain installed and continue to operate as a permanent continuous emission monitoring system, feeding the scrubber's own control loop for the operating life of the plant.
Standards & certifications referenced: EN 14181 (QAL1) · EN 15267-4 · EN 12619 · TÜV QAL1 dust monitoring · QAL1 for PM10 / PM2.5 · ATEX
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Custom wet gas scrubbers with matched inlet and outlet analysers, engineered and calibrated as one system.