Wet electro filter

Synonyms, abbreviations and/or process names

—  Wet E-filter
—  Wet ESP
—  Wet electrostatic precipitator

Removed components

—  Dust, particles, aerosols

Diagram

Process description

The wet electrostatic precipitator mostly works on the same principle as the dry electrostatic precipitator, but has been specifically developed to realise higher yields for water-soluble aerosols, which are more difficult to separate with a dry electrostatic precipitator.

When using a wet electrostatic precipitator the added air must be pre-saturated (in an earlier phase) with water vapour.

The collected dust is removed using rinsing fluid. This takes place by periodically spraying the filter or by collecting droplets from the added gas stream. In both cases, the liquid droplets move, under the influence of the electrical field, to the collector electrode so that a downward-flowing liquid film is created on the electrode.

This problem is resolved in dust-removal by spraying water, which means it is used more regularly. The disadvantage of this approach is that relatively large droplets exist when water is sprayed - even when a water nozzle is used, they may be 30 - 200 µm in size. Therefore, compared to the dry electrostatic precipitator, the used voltage must be reduced by 20 to 25 % in order to prevent the electric field from disappearing completely due to strong electrical discharges. As a result, in wet electrostatic precipitators, spraying almost always takes place intermittently, or additional filters are placed in series. By spraying intermittently, there will be a periodic fall in separation yield. 

Variants 

The condensation electrostatic precipitator is regarded as an extended version of the wet electrostatic precipitator. The condensation electrostatic precipitator has been developed to realise a higher separation yield for aerosols. By cooling the collector electrode, a homogenous liquid film is created on it. The filter is often set up as a set of pipe filters in a cooled sheath.

At high field voltages, a consistently high separation yield is achieved.

Efficiency

The removal yield is 97 - >99% depending on particle size.

Low residual emissions are possible, depending on the application.

In dust-removal using collection via droplets, it is sometimes difficult to form an homogenous film on the collector electrode. This may lead to dry spots, which negatively influences the yield.

Boundary conditions

—  Flow rate:           1 800 - 900 000 Nm3/h
—  Temperature:     < 80 °C
—  In-coming concentrations: 1 -110 g/Nm3

Auxiliary materials

Rinsing water: Use can be minimised by cleaning and recirculation.

Environmental aspects

Collected dust and rinsing liquid as waster water stream.

Energy use

Energy use varies between 0.17 – 0.35 kWh per 1 000 Nm3 depending on implementation. Further, usage of 0.17 – 0.5 kWh must be accounted for the ventilator [5].

Cost aspects

  • Investment
    — 
    The investment costs are ca. three times higher than for dry electrostatic precipitation and amount to 60 too
          300 EUR per m3 for systems of 30 000 – 200 000 m3/h. [1,2]
  • Operating costs
    — 
    Personnel costs:    ca. 0.25 mh/day
    —  Auxiliary and residual materials: The separated dust must be dewatered and disposed of. The water must be
          treated prior to being discharged. Transport costs for the separated dust are determined by the type of residue.
                                                Inert: ca. 75 EUR/ton
                                                Chemical: 150 – 250 EUR/ton

             —  Operational costs: 0.05 – 0.1 EUR per 1 000 Nm³/h for systems greater than 50 000 m3/h.

Advantages and disadvantages

  • Advantages
    — 
    The wet electrostatic precipitator has been developed to separate water-soluble aerosols. It allows to realise
          high separation yields.
    —  Low energy use due to almost no electrical discharges;
    —  The system can be installed in modules;
    —  By simultaneously reducing temperature, it is also possible to separate volatile metals such as mercury;
    —  Partial separation of acid gases still takes place;
    — There is a very high yield for sub-micron particles;
    —  When there is a high field voltages (>50 kV), separation is almost independent of residence time, so a
          compact construction possible.
  • Disadvantages
    — 
    A waste water stream is produced which needs treating;
    —  The filter is very heavy;
    —  In the condensation electrostatic precipitator, the dew point of the to-be-treated gas stream must be high
          enough to realise effective condensation. It is assumed that cooling by a few degrees is enough to create a
          film thickness of 50 to 150 µm;
    —  It is necessary to use cooling water;
    —  For aggressive flue gas components, the filter must be made of a synthetic material;
    —  It is necessary to treat rinsing water;
    —  There is less practical experience compared to the dry electrostatic precipitator;
    —  Less suited to high dust loads;
    —  Uncertainty about residual emissions;
    —  High investment costs;
    —  Little practical experience.

Applications

Is used as aerosol filter and mercury separator at waste incineration plants.

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. Work-book on environmental measures: “Metal and electro-technical industry” (1998 ). VNG publishers
  5. Supplier information