Cleaning brown coal – the green approach

A long with black coal, natural gas and nuclear power, one of the most important sources of energy is brown coal. But one of its by-products, gypsum, has to be dealt with in an environmentally friendly way before it can be re-used. That's where GKD's filter belts and meshes come in.

The dewatered FGDP discharge.
The dewatered FGDP discharge.
Visible for miles: the white vapours from the cooling towers.
Visible for miles: the white vapours from the cooling towers.

Experts estimate that the world's greatest brown coal supplies are to be found in Russia, the USA, China and Australia. Every year, about 180 million tons of brown coal are open-cast mined in Germany and used in coal-fired power stations to produce electricity. But even the available German reserves will last for at least another three or four generations. German coal is open-cast mined at three major brown coal fields – in the Lower Rhine Bight, in Central Germany and in Lausitz.

Conveyor belts or rail vehicles continuously transport the brown coal from the open-cast mines to the bunkers of the power stations. Here, it is first finely ground and dried before being fired in the steam generator. The combustion heat released is transferred to feed water piped in under pressure, which then turns to steam. The steam drives turbine generators, producing electricity which is fed to households via the grid.

One method used to clean the flue gas produced during coal firing in an environmentally compatible way is the flue gas desulphurisation plant (FGDP). A by-product of this process is the FGDP gypsum resulting from gypsum slurries. Filter belts made by GKD– Gebr. Kufferath AG facilitate the efficient dewatering of FGDP gypsum on vacuum belt filter plants and thus play a key role in the further processing of FGDP gypsum in the building materials industry.

GKDis an owner-managed technical weaver and a market leader in the field of woven meshes made of metal, plastics and fibres, for architecture and design, for filter media and for process belts. It has seven production facilities, two of them in Germany, the others in the USA, England, Spain, South Africa and China and another branch in Dubai.

Every year, about 180 million tons of brown coal are open-cast mined in Germany and used in coal-fired power stations to produce electricity.

The flue gases produced during combustion are cleaned in several stages to comply with the legal emission limits. First, an electro-filter ensures the separation of ash and dust components in the gas. Due to the 0.2 to 0.6% sulphur content of raw brown coal, the flue gas also contains about 600 to 3.500 mg/m3 of sulphur dioxide. To extract this, the flue gas is fed into the flue gas desulphurisation plant – a process prescribed by law since 1988 under the Ordinance on Large-Scale Firing Installations for all power stations with a performance of 300 MWth or more. In the scrubber, the sulphur dioxide in the flue gas reacts with a lime solution to form a watery gypsum suspension (Ca SO4). After desulphurisation through this lime scrubbing process, the flue gas is then fed via cooling towers or chimneys into the atmosphere. The gypsum slurry is pumped out of the scrubbers into the gypsum dewatering section, where it is first thickened in hydrocyclones and then dewatered by means of vacuum on the continuous filter belt of the vacuum belt filter plant.

GKD belts for belt filter plants

GKD's Weave In Motion Process belts are also used in the wood-processing industry along with numerous industrial and environmental fields ranging from the processing of minerals and sewage sludge dewatering to the production of nonwovens or laminates. They are also used for baking and for deep-freezing meat, fruit or dough as well as for the production of fruit juices.

The range of products encompasses four types of belt: plastic woven belts, metal woven belts, process belts made from a combination of metal and plastic, and process meshes for the wood-processing industry. All are manufactured on heavy-duty looms for metallic meshes and finished using processes such as thermal fixing, edge welding, individual edge treatments and a wide range of seams.

The mesh apertures of the GKDfilter belts vary from 20 μm up to 400 μm making them suitable for most vacuum belt filters. Belts made by GKDare also used for a variety of purposes in the filtration of abrasives and coolants and in the refinement of phosphoric acid or in rock salt processing. GKDsays that they have a service life, absolute chemical and thermal resistance and very good cleaning capabilities. The various stages of the continuously running vacuum belt filter plant, from feed of the gypsum slurry via washing to dewatering, put great demands on the continuous filter belt. GKDdouble-weave twill belts made from polyester monofilaments with additional multifilament weft wires (Type 2035) or staple fibres (Type 2025) are suitable products for the dewatering of FGDP gypsum. These mesh types have optimal material properties for the filtration process. Another precondition for use on belt filter plants is the stability of the belts. Because they are produced on special, ultra-modern looms, GKDbelts have great cross-stability, even in woven widths of up to seven meters and lengths of 120 m.

Due to the usefulness of GKDbelts GKDfiltration meshes are used by brown coal-fired power stations worldwide on their vacuum belt filter plants. In China, for example, GKDbelts over 80m long and up to 4m wide are in use.

The RWE Power AG, which operates five brown coal-fired power stations in Germany, is also a customer of the Düren-based technical weaver. Like the RWE locations Frimmersdorf and Weisweiler, the location Niederaussem near Bergheim relies on GKDcompetence. About 90,000 tons of brown coal a day arrive at the Niederaussem power station, which, with its 3,800 MW capacity, is the highest-output power station in the Rhine coalfield.

RWE has been applying the flue gas desulphurisation process since 1987. Today, a total of 11 scrubbers are in operation in Niederaussem. The two belt filters have been running since the end of 2004 with filter belts of Type 2035 and process around 8,000 tons of gypsum slurry into about 800 tons of FGDP gypsum per day.

With belts 62.5 m long and 3.35 m wide, belt stability in particular is one of the decisive selection criteria for RWE Power AG. The plants operate around the clock and must produce optimal results in spite of fluctuations in the amounts and properties of the gypsum. RWE Power AG was looking for a belt with stability and a long service life. After initial laboratory tests, a filter belt was delivered in September 2004. Together with RWE, GKDclosed the endless belt on site with a flexible clipper seam and put it into operation.

In addition to stability, the GKDbelts used at RWE ensure a dewatering of the gypsum slurry to FGDP gypsum with the required residual moisture. A guaranteed residual moisture of the gypsum is crucial in the choice of filter belts, since the FGDP gypsum produced is later used in the building materials industry, where the material is further processed into countless gypsum products like drywalls or self-leveling screed. In Niederaussem, this is done over by a neighboring company.

“[…] Our competence is now also in demand at other German and international brown coal-fired power stations,” says Helmut Jakob, sales engineer of GKD's Weave In Motion business unit.