Dry process

Dry processes typically inject a dry sorbent into the flue gas before a particulate separator with the exception of the packed bed absorber (PBA).  Usually, the particulate separator is either a bag filter or an electrostatic precipitator. They are designed to remove acidic gas components (mainly SO2, HCl, HF, SO3) by neutralizing them with alkaline sorbents. All dry processes can use calcium hydroxide Ca(OH)2 and have no residual liquid effluent to treat, in contrast to a wet system.

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Lhoist offers a full range of Sorbacal® sorbents for the different types of dry processes. Their benefits include:

  • superior gas removal performance
  • less sorbent consumption
  • reduced residue generation

Dry processes are also suitable for removing micro-pollutants through injection of Sorbacal® Micro sorbents. The main types of micro-pollutants to be removed are mercury (Hg), organic components such as dioxins/furans (PCDD/PCDF), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and odors. 

The micropollutant sorbent can be injected separately from the calcium hydroxide or in the form of specific blends to simultaneously remove acidic gas components and micro-pollutants. Volatile metals like selenium and arsenic are captured by chemical reaction with the calcium hydroxide sorbents and do not require additional sorbents. Dry processes in combination with tailor-made, high quality sorbents enable the best performance and minimize residue production and disposal cost.

Based on their extensive process experience, our experts can assist you to optimize your dry process.

Dry Sorbent Injection (DSI)

DSI is relatively simple and versatile flue gas treatment process. It is based on the dry injection of one or more sorbents into the flue gas before a filtering unit, usually a bag filter, or an electrostatic precipitator. The sorbent reacts with the pollutants in a single pass. DSI allows the simultaneous removal of acidic gas components with calcium hydroxide Ca(OH)2 for example, and micropollutants.
The reaction between the gas-phase pollutants and the sorbent occurs in the reactor or flue gas duct and continues in the filter, especially with a bag filter. The reaction products and fly ash from the process are captured by downstream filtering equipment.

Lhoist solution

DSI performance is influenced by process-related parameters:

  • flue gas temperature and humidity
  • pollutant concentration
  • sorbent dispersion
  • residence time
  • qualities of the calcium hydroxide, including available Ca(OH)2 ,specific surface area and pore volume
  • filtering unit

DSI with Sorbacal® products has been demonstrated to be capable of achieving very high removal efficiency (up to > 95% for SO2 and > 99% for SO3, HF and HCl).


  • Multi-pollutant approach
  • Low capital cost
  • Low footprint
  • Limited time from design phase to operation
  • Limited production loss during installation
  • No water consumption or release
  • No impact on the flue gas temperature
  • Very limited power consumption
  • Flexible add-on technology

The DSI process can be easily adapted to achieve more stringent emission limits or target removal of additional pollutants by determining the best sorbent to use and optimizing the DSI system.

Circulating Dry Scrubber (CDS)

CDS technology was first developed for SO2 removal in coal-fired power plants. Today it is also used in flue gas treatment for industrial furnaces and boilers that use biomass, industrial or municipal waste as fuels. The CDS process is able to remove both acidic gas components and micropollutants. It is based on the recirculation of filter residue which consists of sorbent reaction products and fly ash from the filter. CDS units operate exclusively with calcium-based sorbents.

The CDS process consists of a reactor followed by a particulate filter. A large part of the solids from the filter is recycled into the reactor, where the fresh sorbent is added. In most cases water is injected into the reactor or onto the solids. Addition of water plays a critical role for temperature control and to improve removal performance. Some installations use quicklime (CaO) that is hydrated prior to entering the CDS process.



Lhoist solution

CDS performance is primarily driven by the recirculation rate and the moisture content of the recirculating residue. Sorbacal® CDS can handle more than double the moisture content of standard hydrates which also improves flowability. This significantly enhances the efficiency and performance of CDS systems. If needed, micropollutants can also be removed, by adding PAC, lignite coke or Sorbacal® Micro.


Recirculation of the residue decreases sorbent consumption especially when combined with water addition on the residue. It can also enable systems to achieve high acid gas removal levels: more than 99% for SO2, SO3, HF and HCl.

Less capital intensive than wet processes and with a multi-pollutant approach, CDS technology is gaining momentum. Lhoist and equipment designers and suppliers are further developing this technology to improve acid gas treatment performance and decrease sorbent consumption.

Packed Bed Absorber (PBA)

This technology is mainly used in the ceramic brick and tile industry for flue gas treatment downstream of tunnel kilns, usually to capture HF, SO3 and HCl from a limited flue-gas flow rate.
In the PBA process flue gas passes in a cross- or counter-flow through a reaction chamber filled with a granular sorbent. The reagent is drawn by gravity through the reaction chamber and is removed at the bottom of the PBA. The removal performance of a conventional PBA using natural limestone is quite high for SO3 and HF (> 95-98%) but rather limited for HCl (20-30%) and SO2 (10-20%).

Lhoist solution

Sorbacal® C, a natural limestone CaCO3 sorbent in the form of “chippings”, is typically used for PBAs. For greater removal performance we recommend Sorbacal® G: spherical granulated particles consisting of CaCO3 and Ca(OH)2. The residual Ca(OH)2 content in combination with the higher porosity and surface area of the granulates enables significantly better removal rates for HCl (> 70%) and SO2 (30-35%). With multi-stage PBAs, even higher rates have been achieved for HCl and SO2 (>80%).


The PBA is particular suitable for industrial processes with a peak release of acidic pollutants. Sorbacal® G significantly increases the removal performance for HCl and SO2. This makes it an attractive alternative in PBA processes for new applications, like the upcoming desulphurization of marine diesel engines.

Furnace Sorbent Injection (FSI)

In this process dry calcium hydroxide (Ca(OH)2) is injected directly into the furnace at 850-1,050°C, i.e. before any additional FGT installation.

The sorbent instantly decomposes into a porous and extremely reactive form of quicklime that will selectively neutralize SO2. Under these conditions the acid gas capture is very efficient because the competitive reaction with CO2 cannot occur at such high temperatures. The capture of halogen acid gases is more limited. However, the injected sorbent will continue to react in the downstream process to remove these pollutants.

Lhoist solution

Sorbacal® SPS has been proven to be the most efficient sorbent for this demanding application.


Coupled with a downstream FGT installation, FSI is an extremely efficient and flexible solution for pretreating heavy SO2-loaded gases or treating SO2 peaks.

(Circulating) Fluidized bed boiler (CFB/FBB)

In a circulating fluidized bed (CFB) or fluidized bed boiler (FBB), the fuel particles are suspended in a hot (800-900°C) bed of fluidized solid materials: fuel, sand, ashes, etc. Air is blown through this bed to keep it fluidized and to provide the necessary oxygen for combustion. When limestone CaCO3 is added to the hot bed it decomposes, resulting in a selective SO2 removal similar to the FSI process.

Lhoist solution

Lhoist has developed a range of Sorbacal® C limestone products with grain size distributions adapted to the specific flow conditions of different CFB/FBB combustion technologies. Mixing this with the fuel ensures a longer residence time for the limestone particles in the fluidized bed, improving the performance of the in situ SO2 removal.


In situ removal of SO2 is an easy and effective solution for SO2 control in plants operating CFB/FBB technologies. When coupled with a downstream FGT installation, it enables the pretreatment of heavy SO2-loaded gases.

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Discover the optimum solution for your process and facility by contacting our experts.