For decades the electrostatic precipitators (ESPs) have been identified as the traditional solution for particle filtration of substantial gas volumes, especially within the fields of power plants and the cement industry (Fig. 1). In spite of continuously optimizing and expanding the ESP concept, the ESP technology now seems to have been caught up by the restricted emission requirements, resulting from the focus of the authorities on a targeted reduction of the industrial air pollution over the past few years. Until now the ESP technology has been thought of as the optimum particle separation method fulfilling the legal emission requirements of the time. Today a large number of ESPs are globally subject to the requirement for retrofitting or replacement by up-to-date technology, such as modern pulse-jet bag filters.
ESPs are in operation in a wide range of industrial segments, with a dominant presence in:
– Power plants (flue gas cleaning in connection with the combustion of fossil fuel)
– The cement industry
– Smelting furnaces (ferrous and non-ferrous products)
Many plants are heavyweights, operating with gas volumes of more than 1 million m3/h.
The ongoing discussions of how to combat climatic changes often bring about the improvement to be achieved by replacing fossil fuel by more CO2-neutral ones, such as biomass fuel. Seen from a fundamental basis it is a difficult job for an ESP to ensure an efficient cleaning of the flue gas from
biomass combustion compared to the flue gas from a coal-fired boiler.
If there is an explicit demand for a marked reduction of the emission from an existing ESP plant, there are basically three possible solutions:
– Demolition and replacement of the existing ESP by a new pulse-jet bag filter
– Retrofitting of a pulse-jet bag filter (existing housing and steel structures are reused)
– Retrofitting of a combined ESP and pulse-jet bag filter (a hybrid filter in existing housing, Fig. 2)
Self-cleaning and temperature-resistant pulse-jet bag filters have for several years been in operation in flue gas cleaning applications. Technical improvements of the filtering fabric as well as the cleaning systems (typically pulse-jet cleaning of the filter bags by compressed air) have put the pulse-jet filter technology in a position at the forefront of the tightened, legal emission requirements. Today’s filtering fabrics are fully applicable in the high-temperature flue gas environments and economically ensure a long lifetime of the filter bags. The applied membrane and microfibre technology ensures an efficient separation of even submicron particles. Furthermore the pulse-jet bag filter technology is relatively easily combined with processes for the sorption of acid gases, heavy metals, and dioxins.
The space available will often form the basis when deciding on the size of filter. Depending on the flue-gas flow, the choice will be between square or cylindrical modular filter designs. The features of the pulse-jet system range from traditional high-pressure systems to advanced low-pressure systems with a high pulse energy. The latter system consumes only very little energy, which may have a decisive influence on large filter plants.
In recent years a large number of ESPs have been retrofitted into pulse-jet bag filters. With typical retrofitting the internal parts (discharge electrodes and collecting plates) of the ESP are removed for replacement by fabric filter bags. Finally there is a custom-designed layout including a traditional pulse-jet cleaning system with conventional pulse-jet valves. The task of creating sufficient filter area within the frames of the existing filter housing may pose a challenge, as an ESP housing usually offers plenty of height, but only a limited cross-sectional area. In such cases the capability of providing efficient pulse-jet cleaning of long filter bags is crucial.
When retrofitting existing ESPs, a logical consideration is to evaluate the possibility of combining the ESP facilities with the pulse-jet bag filter facilities in one unit. As it is not very costly to maintain part of the ESP – and the pulse-jet bag filter could benefit by a pre-separator - the transformation into a so-called hybrid filter is obvious.
The hybrid filter has obvious advantages over the common pulse-jet bag filter:
– A reduced dust load on the filter bags ➝ A longer lifetime of filter bags
– A lower pressure drop across the filter bags
– A reduced filter area
– Protection against sparks
– Better gas distribution
Most ESPs are built in modules of serial fields. Whenever possible, the first serial field at the inlet section is maintained and the other fields are replaced by pulse-jet bag filter modules. The transformation into a hybrid filter including part of the existing ESP, place high demands on the design of the pulse-jet bag filter as well as its ability to pulse-jet clean extra long filter bags.
Simatek A/S recently introduced the next generation of pulse-jet bag filter technology, distinguished as the most advanced technology on the market for replacement and retrofitting of ESPs. This state-of-the-art filter technology has a number of features outpacing known technology, which are essential for the retrofitting of ESPs.
– The conventional pulse-jet valves have been exchanged for new technology ensuring efficient cleaning of very long filter bags – typically of 16 metres in length.
– Servo-controlled indexing system facilitating cleaning of more than 10 000 m² filter area per pulse valve.
– Massive reduction of energy consumption for cleaning of the filter area.
– Intelligent cleaning function, automatically adapting the jet-pulse profile and energy to the actual requirements.
– Modular system for easy retrofitting of almost any kind of ESPs.
Unlike traditional filter technology, the SimPulse 3CS filter modules provide for the required filter area without any need to expand the existing ESP housing. The SimPulse 3CS filter modules are vacuum resistant, – no need for extraordinary pressure-reinforcement of a traditional Walk In Plenum (Fig. 3). The concept is based on standard modules which are easily implemented into existing ESPs, involving only a small amount of engineering work.
The servo-controlled pulse-jet system has a modular design for flexible and easy integration into electrostatic precipitators (ESPs). The concept ensures a uniform cleaning of all filter bags at exactly the same high pulse-jet energy at a low cleaning pressure (0.3 – 0.5 bar). The PADs are in 6 different standard sizes to match different ESP layouts and most capacities. Typically 2 to 6 PAD modules will meet the required capacity within the framework of the ESP.