Upgrading ESPs to hybrid filters

CaSE: aalborg Portland cement plant, Denmark In just 7 days, existing ESPs were upgraded to efficient hybrid filter solutions delivering low emission level and reduced power consumption

The background Emissions from a cement mill ESP is dependent on the operation of the mill. Especially during the start-up phase with varying operational conditions installation of bags in the casing ensures low dust emission at all times.

An increasing environmental focus on emissions called for an upgrading of Aalborg Portland’s two ESPs originally designed for an emission of 500 mg/Nm3.

Decision-makingFulfilling the new requirements for low dust emission including today’s requirement for low energy consumption made the plant evaluate three solutions:

1. Modernisation and enlargement of the existing ESPs2. Installation of new fabric filters3. Replacing ESP fields with fabric filter technology

The space around the filters were too limited to allow for extensions and installing new fabric filters required investment in new fan (due to increase in pressure drop) and an increase in power consumption.

As the mechanical condition of the 40 years old installations was very good, it was decided to replace ESP fields with fabric filter technology creating a hybrid filter solution. This allowed the plant to improve the performance of the filters within the existing casing and footprint and at the same time achieve a constant low emission level in spite of varying operational conditions.

The solutionThe roof on the last part of the precipitator was removed and top-boxes with tube sheets for bag support and jet pulse cleaning nozzles were installed directly in the existing casing.

The total installation period from stop of the mill to start-up after the modification was only 7 days. The existing fan was reused to reduce the overall cost of the upgrading project.

Results The upgrading of the two 40 years old ESPs has resulted in a significant reduction of the emission level. The actual emission was well within the requirements of the environmental authorities.

The performance test verified a pressure drop of 40 mmWG (400 Pa) for one unit and 70 mmWG (700 Pa) for the second unit and dust emissions at 8 mg/Nm3 and 17 mg/Nm3. The first set of filter bags was not replaced until after 7 years of operation.

With the cost-effective upgrading of ESPs to hybrid filters Aalborg Portland achieved an emission level well below the requirements.

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Hybrid filter solution

A hybrid filter combines ESP and fabric filter technologies. First, the dust is collected by the precipitator fields at the inlet of the installation.

Here the majority of the dust particles are captured leading to a reduced dust load to the fabric filter part. This minimizes wear on the bags and reduces cleaning cycles by compressed air leading to an increased filter bag life time.

In addition, as the dust load on the bags is reduced, the pressure drop across the bags is also reduced leading to reduced power consumption for the fan after the filter.