Whiter gypsum, cleaner air

Tag : polypropylene monofilament
Process Worldwide-02-2008
Whiter gypsum, cleaner air
Effective filtration maximizes the environmental benefits of fluegas desulfurization

As world electricity demand soars, coal-fired power plants willcontinue to meet a large part of global power demand for many yearsto come. To protect the environment, coal-burning plants need fluegas desulfurization (FGD). Choosing the right filter fabrics forgypsum dewatering can maintain high FGD plant availability andreduce operating costs.
Ottilie Steffen
World energy consumption is increasing steadily, and by 2030 itwill probably be 70 percent higher than it was in 2000.China’s energy requirement alone will more than doublebetween 2008 and 2030. In 2000 more than half of the world’senergy was used by industrialized countries. By 2030, however, thedeveloping countries—with 82 percent of the world’spopulation—are likely to have overtaken the industrializedcountries in energy use. This shift in energy use is importantbecause electricity generation in developing countries is likely todepend on coal for many years to come. Half of China’s newgenerating capacity, for instance, will come from coal-firedplants.

The world’s coal reserves are widely distributed—theUSA has around 27 percent of the total, Russia 17 percent, China 13percent and India 10 percent—and large enough to keep pricesstable for an estimated 60 years, even with increasing demand.

With coal comes the threat of pollution: not just carbon dioxidebut also sulfur dioxide, nitrogen oxides and particulates. TheInternational Energy Agency forecasts that by 2030, emissions fromcoal-fired plants will be more than 60 percent higher than they aretoday. Two-thirds of this increase will come from developingcountries, especially China and India.

But with modern emissions control technology, coal-fired plants canmeet the targets set by governments and environmentalists. Asenergy demand rises, moreover, good cleanup processes are likely tobecome cheaper. Flue gas desulfurization (FGD) is a key technologyin reducing one source of pollution from coal-fired power plants:sulfur dioxide, the source of “acid rain”. Modernwet-scrubbing FGD plants remove more than 98 percent of the sulfurfrom the flue gas. FGD can be fitted to existing as well as newplants.

Gypsum dewatering

Wet-scrubbing FGD plants use limestone (calcium carbonate) toconvert the sulfur dioxide in flue gas to calcium sulfate, orgypsum (CaSO42 H2O). Power-station operators use horizontal vacuumbelt filters to dewater the gypsum, yielding up to 1 t/h gypsum perm2 of filter fabric.

Low-quality gypsum is sent to landfill; better-quality gypsum canbe sold to make plasterboard and other building materials, helpingto offset the running costs of
the FGD plant. Salable gypsum requires thorough washing, to improveits color, followed by dewatering to less than ten percent moistureby weight. This in turn requires a well-specified belt filterequipped with a suitable filter fabric. For best filtration, thefilter fabric must combine high permeability, homogeneous pore sizedistribution, superior cake release, easy cleanability throughoutits life, and good tracking behavior on the filter rollers. Filterfabrics are of two kinds (see left picture on page 47). In surfacemedia, which use monofilament fabrics, most of the filtrationoccurs on the filter cake itself, as solid particles bridge betweenthe filaments. Depth media, such as multifilament fabrics andfleeces, trap gypsum particles in the spaces between the fibers.Multifilament fabrics create
small pore sizes, aiding the removal of fine particles, but thesesmall pores also clog easily.

Monofil for easy cleaning

Filter fabric supplier Sefar prefers the philosophy of usingmonofilament fabrics made only from polyester or polypropylene.These have better cleaning properties and less tendency to becomeblocked, so they provide greater throughput and longer life.

Experience has shown that the best choice for vacuum belt filtersis Sefar’s range of polyester double-layer weave (DLW)fabrics. These combine a fine, smooth filtration layer with therobust support provided by a medium-weight backing fabric.Polyester is used in preference to polypropylene because it hasclear advantages in tracking behavior and dimensional stability.For indexing or reciprocating tray belt filters in gypsumdewatering, on the other hand, polypropylene DLW fabric is usedsuccessfully because in this type of filter the fabric does nothave to withstand such high mechanical forces.

Achieving a uniform cake thickness requires homogeneousdistribution of the pores to provide efficient and uniformdrainage. In gypsum dewatering, the fabric is generally graded interms of its nominal pore size or air permeability; specialfinishing processes allow these parameters to be reproducedroutinely and precisely. Smoothing the surface gives the fabric anon-stick quality, which promotes even cake thickness and improvescake release. In the ideal case the gypsum should fall off the beltby itself, without the use of a scraper.For a horizontal vacuumbelt filter in continuous operation, the service life of the filtermedium is determined by how easily it can be cleaned. Thiscleanability can be considerably improved by choosing the mostsuitable properties for the thread and the fabric. Sefardouble-layer fabrics have clear advantages in this area overconventional single-layer mono- and multifilament fabrics.

Continuous development

The current tendency in gypsum dewatering is to use heavy fabrics(1000 g/m2 and higher). This is because heavy fabrics are generallybetter at providing the stable running behavior needed by the largebelt filters that are increasingly used.

However, heavier fabrics cost more to buy, which is an economicdisadvantage if the heavier material does not have an appropriatelylonger life-span. Sefar’s policy of continuous productdevelopment ensures that users are able to stay abreast of this andother trends. To sum up, coal-fired power plants are retaining andeven increasing their importance worldwide, as suppliers not justof electricity but also of gypsum. New filter fabrics specificallydesigned for gypsum dewatering can help power producers meet theever-increasing demands to cut emissions, while reducing operatingcosts. n