Particulate dynamics and flue gas thermodynamics: what governs wet scrubbing

Artur Cokaj · 16 October 2026 · Updated 21 October 2026

The physics of water-based scrubbers: flue gas cooling, droplet size, particle capture, liquid separation and material selection.

The effectiveness of a water-based scrubber depends on how flue gas meets the liquid, particle characteristics and installation management. Cooling, capture and droplet separation are connected stages, each with operating conditions that must be respected.

Wood smoke is a mixture

Wood combustion produces a mixture of gases and microscopic particles. Visible soot alone is therefore insufficient to describe it: specify which components are to be measured and treated. The EPA information sheet on wood combustion specifically notes this mixed nature of the effluent.

Treating gaseous substances also requires a specific assessment. Absorption into the liquid depends on compound solubility, temperature and gas–water contact. Good dust removal does not, by itself, demonstrate equivalent results for gaseous organic compounds or odours. EPA, spray system information sheet.

Cooling is part of the design

Scrubbing changes flue gas temperature and humidity: some water may evaporate, removing heat from the stream. Outlet temperature must be determined from the system's heat and water balance. A value close to 50 °C may describe a particular operating condition but is not a universal target. EPA, scrubber manual, section 2.5.3.

The useful sizing figure is therefore the flow rate under declared conditions. Actual and normalised cubic metres per hour describe different references: temperature, pressure and vapour content must be explicit when comparing measurements or performance.

Droplets and particles: establishing contact conditions

Capture may occur through inertial impaction, interception and diffusion. The contribution of each mechanism changes with particle size and the relative motion of gas and liquid.

Droplet size distribution must also suit the working conditions. Smaller droplets offer greater surface area per unit volume, but they may be more easily entrained by the stream and lose velocity relative to particles. Nozzle geometry, water flow rate and liquid distribution also matter. Atomisation pressure alone does not describe efficiency. EPA, scrubber manual, sections 2.2.1 and 2.4.1.6.

In spray chambers, liquid pressure and flow contribute to contact energy. These systems may have limitations in capturing fine particulate matter: selection must start with emission characteristics and the required performance, rather than treating every scrubbing architecture as equivalent. EPA, spray system information sheet.

Separate droplets and maintain the circuit

Droplets that have collected contaminants must be separated from the stream before discharge. Liquid carry-over, blocked nozzles or insufficient water flow may impair operation. Droplet separation is therefore part of the cleaning process.

During operation, checks must match the design: pressure drop, liquid flow rate, circuit condition and, where relevant, pH and solids concentration. In recirculating systems, make-up and blowdown help maintain operating conditions. EPA, wet scrubber monitoring.

Materials: selection depends on the actual environment

Corrosion resistance depends on liquid composition, pH, temperature and the presence of aggressive substances such as chlorides. Welds, surface finish, stagnant areas and deposits also matter. The steel family designation alone is insufficient to predict machine service life. Outokumpu, stainless steel corrosion resistance.

AISI 316L must therefore be assessed against actual conditions, rather than presented as a universal requirement for every scrubber. Laboratory material tests must be interpreted in light of differences from actual service; where uncertain, testing in the process fluid may be needed. Outokumpu, material selection criteria.

From physical principles to measured performance

These principles explain the variables governing scrubbing. Reading experimental results instead requires examining test conditions and distinguishing concentration from mass flow rate: this is the subject of Dust and organic compound removal efficiency: evidence from stack tests. Overall dust removal efficiency does not automatically describe efficiency for every particle size.

Standards and safety: references serve different purposes

UNI 11859-1:2022 concerns checking the safety of combustion product evacuation systems in existing civil-use installations fired by liquid or solid fuel. The UNI catalogue expressly excludes its use as a standard for design, installation or upgrading. It is therefore not the reference for assigning scrubber efficiency classes.

Assessment of impacts on neighbours remains separate: Article 844 of the Civil Code refers to normal tolerability, taking local conditions into account. A removal percentage does not resolve this assessment on its own.

A design's quality is assessed by relating effluent characteristics, sizing and documented performance under the conditions of use. In the next article, we will examine how water recirculation, make-up and blowdown affect consumption and treatment continuity.

Sources and interpretation

References consulted on 16 October 2026. The 2002 EPA manual is used for physical principles and is still cited by the EPA monitoring page. For UNI 11859-1:2022, the reference is the official catalogue summary. Links in the text provide access to the sources; the Outokumpu documentation concerns material selection and resistance.