Whiter gypsum, cleaner air
Effective filtration maximizes the environmental benefits of flue gas desulfurization

As world electricity demand soars, coal-fired power plants will continue to meet a large part of global power demand for many years to come. To protect the environment, coal-burning plants need flue gas desulfurization (FGD). Choosing the right filter fabrics for gypsum dewatering can maintain high FGD plant availability and reduce operating costs.
Ottilie Steffen
World energy consumption is increasing steadily, and by 2030 it will probably be 70 percent higher than it was in 2000. China’s energy requirement alone will more than double between 2008 and 2030. In 2000 more than half of the world’s energy was used by industrialized countries. By 2030, however, the developing countries—with 82 percent of the world’s population—are likely to have overtaken the industrialized countries in energy use. This shift in energy use is important because electricity generation in developing countries is likely to depend on coal for many years to come. Half of China’s new generating capacity, for instance, will come from coal-fired plants.

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

With coal comes the threat of pollution: not just carbon dioxide but also sulfur dioxide, nitrogen oxides and particulates. The International Energy Agency forecasts that by 2030, emissions from coal-fired plants will be more than 60 percent higher than they are today. Two-thirds of this increase will come from developing countries, especially China and India.

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

Gypsum dewatering

Wet-scrubbing FGD plants use limestone (calcium carbonate) to convert the sulfur dioxide in flue gas to calcium sulfate, or gypsum (CaSO42 H2O). Power-station operators use horizontal vacuum belt filters to dewater the gypsum, yielding up to 1 t/h gypsum per m2 of filter fabric.

Low-quality gypsum is sent to landfill; better-quality gypsum can be sold to make plasterboard and other building materials, helping to offset the running costs of
the FGD plant. Salable gypsum requires thorough washing, to improve its color, followed by dewatering to less than ten percent moisture by weight. This in turn requires a well-specified belt filter equipped with a suitable filter fabric. For best filtration, the filter fabric must combine high permeability, homogeneous pore size distribution, superior cake release, easy cleanability throughout its life, and good tracking behavior on the filter rollers. Filter fabrics are of two kinds (see left picture on page 47). In surface media, which use monofilament fabrics, most of the filtration occurs on the filter cake itself, as solid particles bridge between the filaments. Depth media, such as multifilament fabrics and fleeces, trap gypsum particles in the spaces between the fibers. Multifilament fabrics create
small pore sizes, aiding the removal of fine particles, but these small pores also clog easily.

Monofil for easy cleaning

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

Experience has shown that the best choice for vacuum belt filters is Sefar’s range of polyester double-layer weave (DLW) fabrics. These combine a fine, smooth filtration layer with the robust support provided by a medium-weight backing fabric. Polyester is used in preference to polypropylene because it has clear advantages in tracking behavior and dimensional stability. For indexing or reciprocating tray belt filters in gypsum dewatering, on the other hand, polypropylene DLW fabric is used successfully because in this type of filter the fabric does not have to withstand such high mechanical forces.

Achieving a uniform cake thickness requires homogeneous distribution of the pores to provide efficient and uniform drainage. In gypsum dewatering, the fabric is generally graded in terms of its nominal pore size or air permeability; special finishing processes allow these parameters to be reproduced routinely and precisely. Smoothing the surface gives the fabric a non-stick quality, which promotes even cake thickness and improves cake release. In the ideal case the gypsum should fall off the belt by itself, without the use of a scraper.For a horizontal vacuum belt filter in continuous operation, the service life of the filter medium is determined by how easily it can be cleaned. This cleanability can be considerably improved by choosing the most suitable properties for the thread and the fabric. Sefar double-layer fabrics have clear advantages in this area over conventional 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 generally better at providing the stable running behavior needed by the large belt filters that are increasingly used.

However, heavier fabrics cost more to buy, which is an economic disadvantage if the heavier material does not have an appropriately longer life-span. Sefar’s policy of continuous product development ensures that users are able to stay abreast of this and other trends. To sum up, coal-fired power plants are retaining and even increasing their importance worldwide, as suppliers not just of electricity but also of gypsum. New filter fabrics specifically designed for gypsum dewatering can help power producers meet the ever-increasing demands to cut emissions, while reducing operating costs. n