Acidic gases are neutralized in the cleaning process because in concentrated form they are corrosive and harmful to health and must not be released into the atmosphere untreated. Neutralization converts reactive acid compounds into stable, environmentally compatible salts and is therefore an indispensable step in any modern exhaust gas purification system. The following sections explain which acidic gases are produced during thermal desorption, which limit values apply, and how the entire cleaning process works technically.
Which acidic gases are produced during thermal soil treatment?
During thermal soil treatment, acidic gases are produced primarily through the decomposition of chlorinated, sulfur-containing, and fluorine-containing pollutants in the rotary kiln. The most important compounds are hydrogen chloride (HCl), sulfur dioxide (SO₂), and hydrogen fluoride (HF). In addition, nitrogen oxides (NOₓ) may also occur depending on the contamination.
In the rotary kiln, contaminated soil is treated at temperatures of up to approximately 600 °C under oxygen exclusion. Pollutants such as PCB, PCDD/PCDF, chlorinated hydrocarbons, or PFAS are thermally destroyed or converted into the gaseous state. Chlorine, sulfur, and fluorine, which are chemically bound in these compounds, are released during desorption and react with water vapor to form the corresponding acid compounds. The raw gas leaving the kiln contains these acids in gaseous form and must be reliably cleaned before being released into the ambient air.
What happens if acidic gases are not neutralized?
If acidic gases are not neutralized, corrosive compounds such as HCl and SO₂ enter the atmosphere untreated. They cause acidification of soils and bodies of water, damage vegetation, and can irritate the respiratory tract and mucous membranes upon direct contact. At the same time, the acids attack plant components and significantly shorten their service life.
From a legal perspective, the discharge of uncleaned acidic exhaust gases is not permitted in Germany. Operators of thermal treatment plants are obligated under the Federal Immission Control Act (BImSchG) to keep all emissions below specified limit values. A failure or inadequacy of the neutralization stage therefore immediately leads to exceedances of limit values and thus to regulatory consequences up to and including operational shutdown. Neutralization is therefore not only necessary for environmental protection reasons, but also for the safe and permit-compliant operation of the plant.
How does the neutralization of acidic gases work technically?
The neutralization of acidic gases in flue gas purification is typically carried out by adding alkaline reagents such as hydrated lime (Ca(OH)₂) or sodium hydroxide (NaOH), which react with the acids to form neutral salts. In modern plants, this step is carried out in a wet or semi-dry scrubber connected directly downstream of the thermal afterburner.
The multi-stage exhaust gas purification in thermal desorption generally includes the following stages:
- Dedusting: Separation of solid particles from the raw gas, for example by fabric filters or electrostatic precipitators
- Thermal afterburning: Destruction of organic pollutants at high temperatures, during which acidic gas compounds can simultaneously be produced
- Flue gas scrubbing (neutralization): Introduction of alkaline scrubbing liquid or dry sorbent for the neutralization of HCl, SO₂, and HF
- Activated carbon filter: Adsorption of remaining organic trace substances and mercury as the final purification stage
In the scrubber, for example, hydrogen chloride reacts with hydrated lime to form calcium chloride and water. Sulfur dioxide is converted to calcium sulfite or calcium sulfate. These reactions take place at controlled pH values and temperatures that are continuously monitored to ensure complete neutralization.
Which limit values apply to acidic exhaust gases in Germany?
In Germany, the emission limit values for acidic exhaust gases from thermal treatment plants are regulated by the 17th Federal Immission Control Ordinance (17. BImSchV). For hydrogen chloride, a daily mean value of 10 mg/m³ applies, for sulfur dioxide 50 mg/m³, and for hydrogen fluoride 1 mg/m³. These limit values are binding and are monitored by continuous emission measurements.
The 17. BImSchV applies to plants for the thermal treatment of waste and prescribes, in addition to half-hourly and daily mean values, annual values and measurement procedures as well. Authorities may impose additional site-specific requirements as part of the permit under BImSchG. Plant operators are obligated to document the measurement results and report them regularly to the competent authorities. Quality management in accordance with ISO 9001 and 14001 supports the systematic compliance with and documentation of all relevant requirements.
What are the by-products of gas neutralization?
The neutralization of acidic gases produces as by-products primarily inorganic salts such as calcium chloride, calcium sulfate, and calcium fluoride, as well as water of reaction. Depending on the type of scrubber, these substances occur as sludges, filter cakes, or dusts and must be disposed of properly.
In wet flue gas scrubbing, an aqueous solution containing the dissolved salts is produced. This scrubbing liquid is processed, and the remaining solid fraction is discharged as residue. In dry or semi-dry sorption, dusty reaction products are formed, which are separated in the fabric filter. Both types of residue are generally classified as hazardous waste, as they may contain residual concentrations of heavy metals or organic compounds. Material flow management of such process residues therefore requires careful characterization and permit-compliant disposal.
How does neutralization affect the quality of the treated soil?
The neutralization of acidic gases does not directly affect the treated soil, as it takes place exclusively in the exhaust gas line and has no contact with the solid material being treated. The quality of the soil after thermal desorption is determined by the treatment temperature, the residence time in the rotary kiln, and the pollutant characteristics.
Indirectly, however, gas neutralization is crucial for the overall quality of the process: only when the exhaust gas purification is fully functional can the plant be operated in a permit-compliant manner and the treated soil be released. A failure of the neutralization stage would jeopardize the entire operation. Furthermore, stable exhaust gas purification helps to avoid re-contamination or emissions that could burden the cleaned soil or the plant environment. Information on the classification of the treated soil in the material cycle can be found in the Soil and Sustainability section.
How ARE Deutzen GmbH helps with the safe treatment of acidic gases
ARE Deutzen GmbH has been operating a thermal soil treatment plant at the Deutzen site in western Saxony since 1994 and has more than 30 years of experience in the treatment of complexly contaminated mineral waste. The plant’s multi-stage exhaust gas purification ensures reliable neutralization of acidic gases and compliance with all statutory emission limit values under the 17. BImSchV. The range of services specifically includes:
- Thermal desorption in the rotary kiln at up to approximately 600 °C under oxygen exclusion
- Multi-stage exhaust gas purification with dedusting, thermal afterburning, flue gas scrubbing, and activated carbon filter
- Treatment of soils contaminated with HCl-forming pollutants such as PCB, chlorinated hydrocarbons, PCDD/PCDF, and PFAS
- Proper disposal of all process residues from gas neutralization
- Continuous emission monitoring and complete documentation for authorities and clients
If you are planning a remediation project involving contaminated soils or mineral waste and require reliable, permit-compliant treatment, contact ARE Deutzen GmbH directly. Get in touch and describe your project so that we can develop the right solution together.