Wet lime scrubbing

Synonyms, abbreviations and/or process names

—  Limestone-gypsum process
—  IFO (in-situ forced oxidation process)

Removed components

—  SO2
—  HCl
—  HF

Diagram

Process description

The wet lime injection process combines the advantages of scrubbers and (semi) dry lime injection. This flue gas cleaning technique is mainly used for desulphurisation of flue gases, while other acid-forming components are also bonded.

During wet lime injection, a slurry of limestone (CaCO3) is sprayed into the spray column. The presence of SO2 causes gypsum (CaSO4.2H2O) to be produced.

Originally, the process was made up of two absorbers and an oxidation unit. SO2 and limestone are absorbed in the absorber, whereby calcium sulphite is created. In the oxidation barrel under the absorber, this is then converted into dihydrate gypsum, CaSO4.2H2O, at a relatively low pH (5.5 - 6).  The gypsum is then dewatered.

This original concept has evolved into a 1-reactor process, the so-called in-situ forced oxidation process (IFO), whereby air is injected into the reaction tank, which converts the existing sulphite into sulphate. This makes it possible to realise considerable investment savings.

Variants 

Instead of limestone, Ca(OH)2 can also be used. The residue will be the same.

Efficiency

—  SOx:    90 – 97 %

Boundary conditions

—  Flow rate:                                   50 – 500 000 Nm3/h
—  Temperature:                            5 - 80 °C
—  SOx:                                            Broad range
—  HCl:                                            Broad range
—  HF:                                              Broad range

Auxiliary materials

—  Lime, limestone
—  Water

Environmental aspects

Waste water must be treated or discharged. Gypsum must be deposited or disposed of. Gypsum depositing can only take place if particular quality criteria, which have been implemented by the gypsum industry, have been satisfied.

Transport costs for the dewatered residue is determined by the type of residue.

Energy use

No data

Cost aspects

  • Investment
    —  Investment costs for an IFO installation with limestone at an power plant with a capacity between 100 and 1000
          MW, amounts to 540 – 200 EUR/kW [7].
  • Operating costs
    —  Personnel costs:   ca. 20 000 EUR per year (1 day per week)
    —  Auxiliary and residual materials: The released waste water must be purified prior to being discharged. The
          created gypsum must be dewatered and disposed of. Transport costs are determined by the type of residue.

                        Inert: ca. 75 EUR/ton
                        Chemical: 150 – 250 EUR/ton
                        Cost aspects CaCO3: ca. 60 EUR/ton

For flue gases from a household-waste incineration plant with a volume of ca. 100 000 Nm3/h and with an average gas composition for acidic components:

Acidic component

Concentration 

(mg/Nm3)1

Boundary limit 

(mg/Nm3)2

SO2

70 - 300

50

HCl

500 – 1 000

10

HF

0,4 – 5

1

1: measurements performed by VITO at a household-waste combustion plant

2: day averages VLAREM II

This means lime usage amounting to 133 kg/h or 3 192 kg/day. On an annual basis, this means ca. 12 ton per year per 1 000 Nm3/h or 706 EUR per year per 1 000 Nm3/h.

Advantages and disadvantages

  • Advantages
    — 
    The used sorbent and limestone are cheap;
    —  Compared to semi-dry systems, there is no need to place a dust filter after the reactor;
    —  The sorbent is use at a stoichiometric ratio of 1;
    —  High yield are possible;
    —  A lot of experience;
    —  Reliable;
    —  Both SO2 and SO3 are removed;
    —  Re-usable product;
    —   Can be used at relatively high temperatures (50 – 80 °C)
  • Disadvantages
    —   In order to form high-quality gypsum, interfering components (flue gas, pollutants) must be removed in an
           earlier phase;
    —   Waste water needs to be treated;
    —   High water use (8 - 20 l/Nm3).

Applications

As a flue gas purification technique for combustion processes in:

—  Waste incineration installations;
—   Electricity production

References

  1. Factsheets on Air-emission reduction techniques, www.infomil.nl, Infomil
  2. Common waste water and waste gas treatment and management systems in the chemical sector. BREF document, European IPPC Bureau, http://eippcb.jrc.es
  3. Elslander H., De Fré R., Geuzens P., Wevers M. (1993). Comparative evaluation of possible gas purification systems for the combustion of household waste. In: Energie & Milieu, 9
  4. Vanderreydt I. (2001). Inventory of the waste incineration sector in Flanders. Vito, 2001/MIM/R/030
  5. Work-book on environmental measures: “Metal and electro-technical industry” (1998 ). VNG publishers
  6. Supplier information
  7. Srivastava R. K., Jozewicz (2000). Controlling SO2 emissions: An analysis of technologies. EPA/600/SR-00/093
  8. VDI 3679, Nassabscheider, Abgasreinigung durch Absorption
  9. VDI 3927, Abgasreinigung, Abscheidung von Schwefeloxiden, Stickstoffoxiden und Halogeniden aus Abgasen (Rauchgasen) von Verbrennungsprozessen