Electrostatic precipitators have been an integral part of flue gas cleaning systems for years. Stricter emission limits however require that electrostatic precipitators will step by step be replaced by fabric filters. This situation has induced manufacturers of electrostatic precipitators to introduce a new concept to the filtration market. The conversion of an electrostatic precipitator into a hybrid filter.

Function of hybrid filters:
The hybrid filter system represents a combination of the electrostatic precipitator and a fabric filter. The flue gas first flows through an electrostatic precipitator, in which a large percentage of the dust load is separated off by means of the electric filter principle. Then, the pre-cleaned flue gas passes through a fabric filter unit, in which the remaining particles are separated. Due to pre-cleaning by the electrostatic precipitator, the fabric filter will be loaded with considerably fewer dust particles. The advantage for the plant operator is, that the filter media are exposed to fewer mechanical impacts which can result in longer service lives.

Experiences with hybrid filters:
Together with ETS-ELEX India, a leading engineering company in India, BWF Envirotec has successfully equipped several hybrid filter plants with needlona® filter media. So, for example, 830 filter bags made of needlona® - PPS/PPS 551 have been in operation in a hybrid filter in a coal fired power plant for more than a year and a half. The expected service life for these filter bags is approximately 36 months. The guaranteed dust content of the cleaned gas is < 25 mg/Nm³. Other applications are, for example, in cement mills which have been equipped with needlona® - PE/PE 551 MPS CS17. These filter bags have already been operating for several years.

Here below, you may find some operating data of our reference plants:

Operating Data Reference 1 Reference 2

Application: Coal Fired Cement Mill
Power Plant
Temperature:
[°C] 140 - 150 75 - 90

Gas Volume:
[Nm³] approx. 93,000 approx. 68,000

Filter Area:
[m²] 1,900 1,130

Air to Cloth Ratio:
[m³/(m²*min)] 1.24 1.33

Emission
[mg/Nm³] 25 25

Needlona®- filter media have proved most successful in first hybrid filters. The emission values are guaranteed to be within the required limits.

Located in the German Land of North Rhine-Westphalia, the Leverkusen waste incinerator has been ecologically processing a grade of municipal and industrial waste similar to domestic waste since its start-up in 1970. The incinerator`s technology, developed to the environmental technology standards of the time, has been adapted to keep pace with increasingly stiffer standards. The modernisation work was finalised with the construction of a new flue gas cleaning system, which started operation in November 1996*. At that time, BWF Envirotec was awarded the contract to deliver more than 2,000 filter bags for the flue gas cleaning system.

The dust collector:
Operating temperature: approx. 106°C cont., 160°C peak
Gas volume: max. 237,465 Nm³/h
Air to cloth ratio: 0.82 m/min
Flue gas: O2 = 6 - 11 Vol.%, H2O = 30.5 Vol%

A fabric filter on the jet pulse system is used for dust removal. In normal operation, the flue gas reaches the system at a temperature of 106°C. During start-up brief peak temperatures of approx. 160°C are possible. The installed filter bags are made of a polyphenylene sulphide (PPS) filter material and coated with CS18®, a polytetrafluorethylene (PTFE) finish, to support the cleaning.

Recently, a routine examination of a used filter bag in our laboratory, confirmed the excellent cleaning performance of the filter bags made of needlona® - PPS/PPS 604 MPS CS18 throughout their whole service life. This excerpt relates to the most important findings of the latest analysis:

“The filter bag examined does not show any mentionable mechanical and chemical signs of wear. The needle felt cross section presents largely a surface-orientated dust separation across the whole filter bag. The tensile strength values are high and are in the range of average tensile strength values measured for the new quality sample. The clean gas side is almost free from dust.”

Relating to all relevant filtration properties, the installed filter bags made of needlona® PPS/PPS 604 MPS CS18 are in an excellent condition after almost ten years of operation. These results also agree with the positive experiences of the operator who reported that the measured annual emission value “dust” is well below legally prescribed limits. Hence, the filtration performance of these filter bags can be included in the list of successfully installed BWF Envirotec filter media in the flue gas cleaning systems of waste incinerators.

*Source: Brochure from the Leverkusen waste incineration plant - technique for human and environment

In May 2001, the dust collector of Burgau waste pyrolysis plant was re-equipped with filter bags made of needlona®-PTFE/PTFE 704 MPS CS18 from BWF Envirotec within a maintenance inspection. After more than four years of operation, the plant operator and BWF Envirotec have now prepared a detailed review.

The Process:
In the waste pyrolysis plant, municipal and bulk waste, as well as industrial waste which is similar to domestic waste, are thermally treated at 600°C in two rotary kilns operating in parallel. The carbonization gas which develops during the process is burnt in a combustion chamber with an air supply at a temperature of approx. 1,250°C, and is then fed via a waste-heat boiler to the flue gas cleaning system. The latter consists of a fabric filter, a superposed dry sorption and an ancillary catalyst.

Technical operating data of the flue gas cleaning system:

Operating temperature: approx. 200°C - 230°C
Raw gas flow: max. 38,000 Nm³/h
Crude gas dust load: < 5 g/Nm³
Air to cloth ratio: 0.74 m/min
Emission value (dust): << 1 mg/Nm³ (beyond the detection limit)

A fabric filter on the jet pulse system is used for dust removal. In normal operation, the carbonization gas reaches the plant at a temperature of 200°C. During start up, brief peak temperatures of approx. 230°C are possible.

The installed filter bags are manufactured from polytetrafluorethylene (PTFE) needle felt and finished with a PTFE aiding de-dusting surface treatment. This filter material is the optimum filter medium for the application in the waste pyrolysis plant at Burgau, as it allows continuous operating temperatures of up to 250°C and short-term peaks of up to 280°C. Besides this, PTFE is the only fibre polymer which does not age, which means the technical characteristics throughout the whole operating period remain unchanged. This was one of the decisive reasons for the plant operator replacing the fibre glass previously used with a textile filter medium from BWF Envirotec.

The Results:
Our technical application services include the visual, mechanical and chemical investigation of a randomly selected filter bag in our R & D department at irregular intervals. Each time, the operator of Burgau waste pyrolysis plant receives the results obtained concerning the current status of the filter bags, as well as an estimation of the continuity of operation in a detailed laboratory investigation report. The following is an extract relating to the most important findings from the latest analysis.

The filter bags are in an excellent condition after 48 months of operation. There is no significant wear. The maximum tensile strengths in length and cross direction remain at a high level and lie in the range of the average tensile strength values of the sample first tested in 2001. The air permeability measured indicates a very good regeneration of the filter bag. The filter cake structure is surface-orientated over the whole filter bag. The clean gas side is free from dust. This corresponds to the positive findings of the operator, who reports an average emission of less than 1 mg dust/Nm³ below the detection limit throughout the whole life time. The emission limits, which are prescribed in a federal regulation to protect the environment from pollution, are well-below the defined values.

Compared with the fibre glass previously used, there are even more benefits with the installation of the BWF Envirotec filter bags. At maximum gas load the cleaning frequency of the filter bags from sequence to sequence was originally 5 - 7 sec. This meant, that with 52 sequences, the same filter bag was cleaned approx. every 5 min. Now, following the re-equipment with PTFE filter bags, cleaning is only short-term when there are peak gas loads, meaning that, the same filter bag is only cleaned 2 - 5 times per day. Result: due to its lower energy consumption, the fabric filter operation allows higher cost efficiency and significantly reduced emissions.

Summarizing, it can be said that the filter bags made of needlona® - PTFE/PTFE 704 MPS CS18 have been performing very well in the Burgau waste pyrolysis plant. Cleaning as well as emission performance indicate excellent results. The operator is most satisfied.

*Schematic process diagram = Source of MPA Burgau
*Pyrolysis = thermal decomposition of substances under exclusion of oxygen

New electrical conductive filter material for applications at a continuous temperature of 125°C (peaks: 140°C)

BWF Envirotec, manufacturer of electrical conductive filter materials, has extended its established ExCharge® concept. We now offer our ExCharge® design felts, which were especially developed for use in areas which are subject to explosion hazards, also for all Polyester-Polyacrylnitril (DT-PE) filter media. Areas of application are especially in the cement industry, in coal mills and in the chemical industry.

The measured resistance values of the ExCharge® product range are within the scope of < 104 Ω (according to DIN 54345, part 1 and part 5) which means they are well below the maximum permissible resistance value of < 108 Ω which is stipulated for conductive filter materials. Exam, an European testing institute for explosion protection-mining-test track, has verified and confirmed results of < 106 Ω for our filter material.

Advantages of ExCharge® are:
• permanent static dissipation by a continuous conductive matrix
• thermally stable at a continuous operating temperature of 125°C
• certified product (acc. to ATEX-directive 94/9/EG)

Please note the following when using antistatic material in your dust collector:

Electrostatic charges which might arise on the filter medium must be dissipated. Therefore, the electrical conductive connection between the filter medium and the supporting cage with the grounded housing of the dust collector must be assured.

The standard width of needlona®-DT-PE/DT-PE 601 ExCharge® is 220 cm. We would be pleased to personally discuss with you advantages and use of our ExCharge® design.

Contact us - we look forward to hearing from you!

It has been the target of our company to present our internationality also with respect to test norms.

We have been able to replace almost all German Norms (DIN) pre-viously used for quality control with international standards (ISO), without any changes to the test procedure. This means that, you as our customer, continue to receive the quality associated with BWF Envirotec. Without any "ifs" or "buts"!

Here below, you may find an overview of the old and new test norms as well as the associated changes.

Area Weight:
DIN 53 854 is replaced by ISO 9073-1. Test procedure and test equipment remain the same. There are no changes for you.

Thickness/Density/Pore Volume:
There is no applicable international test norm, which exists for this measurement. We have therefore created our own BWF Envirotec company norm, which is available on demand.

Air Permeability:
The previous DIN 53 887 is replaced by EN ISO 9237. This new norm indicates the air permeability in "mm/s@200Pa." For your information, we have additionally specified the earlier unit of measurement l/(dm_min)@200 Pa in the data sheet. Since it is a mathematical conversion, no change of the test procedure was necessary.

Tensile Strength:
The previous DIN 53 857 is replaced by ISO 9073-3. According to the new ISO-norm, it is required to specify the maximum value of the tensile strength and not, as before, the value of the elongation at first break. As a consequence, some qualities may have a higher value of the tensile strength in cross direction. The essential test parameters, like preloading of 2 N and deformation speed of 200 mm/min remain unchanged.

You will find attached an example of our revised technical specifications.

PyroGuard is a finish which has been specially created for the filtration of all kinds of combustible dust. As a follow-up to our tried and proven FireGuard finish, PyroGuard completely fulfills requirements in the area of pyrophorous dust separation. Our existing FireGuard finish, a heat-resistant top layer made of hi-tech fibre, keeps any red-hot sparks on the surface. Where non-combustible dust is involved, the filter medium is protected to a very large degree from flying sparks. However, when there is pyrophorous dust - needlona ® - PyroGuard is the solution!

PyroGuard is a heat-resistant, homogeneous coating of graphite on top of the felt. The particles swell up on contact with sparks. This property of PyroGuard coating significantly enhances the resistance capability of the filter medium to combustile dust.

This finish is suitable for all areas of application where flying sparks can occur. You can obtain our standard PyroGuard finish under its name needlona ® - PE/PE 524 PyroGuard.

We will be glad to send you further information such as, for example, samples, technical specifications – just get in contact with us!