To address these complex emission profiles, modern treatment strategies combine advanced
reagent technologies with increasingly efficient process configurations. The objective is not only
to meet regulatory limits but also to optimize cost, reliability, and overall environmental
performance.
Dry Processes: Simplicity and Flexibility
Dry flue gas treatment processes are widely applied because of their operational simplicity,
minimal infrastructure requirements and absence of liquid effluents. In a typical dry
configuration, a dry sorbent such as calcium hydroxide is injected directly into the flue gas
stream upstream of a particulate control device such as a bag filter or an electrostatic
precipitator. The sorbent reacts with acidic components, neutralizing them and forming solid
reaction products which are then captured with the particulates.
Dry sorbent injection is a flexible solution suitable for a broad range of installations, including
existing plants where retrofitting needs to be fast and minimally disruptive. Advanced variations,
such as circulating dry scrubbers or packed bed absorbers, offer improved contact efficiency
and higher removal rates. Dry treatment is particularly advantageous when water management
is a concern or when space constraints make wet systems less practical.
Semi-Wet Processes: Balancing Performance and Water Use
Semi-wet processes, often referred to as spray dry absorbers, use lime slurry or milk of lime as
a reagent. The suspension is atomized into the hot flue gas, where water evaporation lowers the
temperature and disperses fine reactive particles. This promotes a highly efficient gas-solid
reaction while keeping overall water consumption relatively low. The result is a dry powder that
can be collected downstream without generating liquid effluents.
The semi-wet approach offers a balance between removal efficiency and operational cost,
making it attractive for many municipal waste incineration plants, coal-fired power stations and
steel sinter plants. In recent years, improvements in atomization technologies and process
control have further enhanced performance and reduced reagent consumption.
Wet Processes: High Removal Rates for Large Installations
Wet flue gas treatment processes, or wet scrubbers, are widely used where very high removal
efficiencies are required, particularly for large combustion units. In these systems, the flue gas is
cooled below its dew point, causing acid gases to condense. The gas is then washed with a
scrubbing liquor, typically lime slurry or a limestone suspension, which dissolves and neutralizes
the pollutants.
Wet scrubbers are known for their ability to achieve removal efficiencies exceeding 99% for acid
gases such as HCl, HF and SO₂. They also allow the use of low-cost limestone as a sorbent,
which can be advantageous in applications with stable, high-volume gas streams. The main
trade-offs involve higher capital investment, the need for wastewater treatment, and more
complex operational management.
Combined Processes: Expanding the Control Envelope
No single technology can address every pollutant profile optimally. This is why combined
process configurations are gaining attention across sectors. A common strategy is to integrate
dry sorbent injection either before or after a semi-wet or wet treatment stage. By doing so, one
can improve peak control capabilities, reduce reagent costs and target additional pollutants
such as SO₃, heavy metals or micropollutants.
The principle behind combined processes is straightforward but powerful: allow a lower-cost or
more robust technology to address the bulk of the emissions, and then apply a more selective or
efficient technology downstream to polish the final flue gas. This layered approach provides a
more adaptable response to changing regulatory requirements and varying operating
conditions.
Targeting Micropollutants
Beyond traditional acid gases, modern flue gas treatment strategies are increasingly addressing
micropollutants. Activated carbon or lignite coke injection can be integrated with dry or semi-wet
processes to capture dioxins, furans and mercury. The growing emphasis on these pollutants
reflects a shift from simple compliance to comprehensive emission control strategies, driven by
both regulatory frameworks and corporate sustainability objectives.
Efficiency, Flexibility and Compliance
Selecting the right flue gas treatment configuration is no longer just about meeting emission
limits. It is a strategic decision that involves balancing reagent costs, water use, maintenance,
energy consumption, plant footprint and long-term reliability.
Dry processes offer simplicity and ease of retrofit. Semi-wet technologies provide a middle
ground with solid performance and moderate water demand. Wet scrubbers remain the
benchmark for very high removal efficiencies, especially in large-scale installations. Combined
configurations offer a flexible path to handle complex and evolving emission profiles.
As environmental expectations and regulatory pressures increase, industries are moving toward
integrated emission control strategies that combine technical performance with economic
sustainability. The future of flue gas treatment lies not in a single technology, but in intelligent
combinations designed to optimize performance, cost, and compliance over the entire life cycle
of the plant.
