New fabric filter ensures low emissions and high efficiency at Duna-Dráva Cement Ltd.

6 July 2020

Background and objective

The Vác cement plant in Hungary is one of two owned by Duna-Dráva Cement, a joint venture between HeidelbergCement Group and SCHWENK Zement KG. In response to tightening European regulations over dust emissions, in November 2016, the plant decided to replace its existing kiln/raw mill ESP with a new fabric filter from FLSmidth.
We supplied the new filter on an EPC basis, involving the engineering, supply, dismantling of the old precipitator and mechanical installation of the new system. Scope of supply also included a new filter fan, with motor, variable speed drive and transformer, and a new dust transport system.

airtech filter duna drava — 3D model of the new fabric filter supplied to Vác cement plant. The colored elements are part of FLSmidth scope of supply.

A challenging proposition

The main project drive was the European IPPC Directive which limits dust emissions to <10 mg/Nm³ (10% O2 reference), while the existing Vác ESP for kiln/raw mill dedusting, after technical update in 2006, was able to reach <30 mg/Nm³.

Additionally, Vác cement plant wanted a system in which raw mill output was collected by the filter without prior separation. This means a very large dust load of about 800 g/Nm³ reaching the filter. In these conditions, a perfect distribution of gas and dust is fundamental to ensure the desired performance.

Finally, the fabric filter had to be installed on the existing ESP support structure, within a very narrow footprint, in order to reduce project CAPEX.

Our technical solution based on the FLSmidth’s long bag technology which combines the use of 10+ meter long bags with a unique flow distribution was able to meet all the above expectations.

CFD modelling rises to the challenge

Gas flow has a significant impact on filter performance. Flow uniformity, compartment-to-compartment flow balance, pressure loss and dust distribution must all be optimised to achieve high filter performance. And key to managing gas flow is the design of the internal structures for gas distribution.

At the Vác plant, gas and dust are exposed to the filter media from both the bottom and sides of the filter bags. We produced a two-phase CFD model representing both gas and dust flow through the filter and the ductwork before and after the filter, including all parts that would affect the flow field, such as guide vanes, screens, general flow internals, and the bags. We placed particular focus on modelling the bags – a critical element when using long bags (10 m) technology.

Gas distribution in the filter

An optimized gas distribution system is installed to split the gas across the bottom and side of the bag’s area to reach an even gas velocity though the complete area of the bag.

Dust distribution in the filter

The dust flow in the upwards direction is evenly distributed, leading to even dust load on all filter bags.

The modelling allowed us to verify various parameters:

Precise pressure loss
Optimal gas velocity when approaching the bags
Optimal dust distribution when approaching the bags
Optimal flow balance
Minimum pressure loss

This detailed analysis allowed us to design a new fabric filter that is able to meet current requirements but is future proofed against further tightening of dust emissions limits.

With the two-phase CFD study it was also possible to estimate precise pressure loss across the circuit. Which meant we could design the fan with an efficiency that is 8% higher than was guaranteed in the supply agreement.

A smart fabric filter

Performance of a filter does not simply depend of the design. How the system is operated also plays an important role. Particularly, how the differential pressure (dP) across the filter is controlled. If the dP is too high, power consumption will be excessive. Too low, and the filter may not meet requirements.

To fight this problem, we developed the Smart Pulse Controller EVO II as an add-on for existing cement plant control systems.It was installed and its impact on dP was tested at the Vác installation over a period of twenty days: ten days with Smart Pulse Controller EVO II deactivated and then 10 days with the controller activated.

Without the controller , the filter ran at between 10 mbar and 12 mbar in normal operations. This likely reflected the filter’s original fixed dP settings. But there are problems with a fixed dP setting that does not take into account the varying production load conditions that are a natural part of plant operations.

Setting the dP for high production loads, for example, will mean that when loads are lower, the dP is excessively high, resulting in higher consumption by the ID fans.

By varying dP settings to take into account varying load conditions, greater efficiencies are possible. Which is what Smart Pulse Controller EVO II does. The controller automatically adjusts dP settings based on the plant’s actual production load. This optimises filter operation and reduces power use.

Counting the benefits

Taken together, the benefits accrued by higher-than-guaranteed fan efficiency and EVO II control of dP setting are substantial. Due to lower pressure air consumption and fan electrical energy consumption the system has a significant operational cost saving.

We are impressed with the EVO II. It has optimised and reduced our dP by a minimum of 10%. It’s fully automatic and requires no interaction from our operators. As a main result, the fan efficiency is higher than it was expected.
Duna-Dráva Cement

A sustainability success story too

Not only has the new fabric filter enabled Duna-Dráva Cement to control its dust emissions at the Vác plant, the lower power consumption translates into CO₂ savings too.

The use of Smart Pulse Controller EVO II and the high-efficiency fan lowered the carbon dioxide emission footprint. Such improvements in the carbon intensity of cement operations are vital in ensuring the sustainability of the industry – something we are committed to under our MissionZero goals.