Catering for King Coal

The 1979 Three Mile Island accident and 1986 Chernobyl disaster, along with increasing costs, effectively ended the rapid growth of nuclear power.

The role of nonwovens as high-performance filter media in the production of coal is perhaps little recognised in the wider world.

Pulverized coal fired boilers, however, dominate the electric power industry, providing steam to drive the large turbines. The basic concept is to make use of the entire volume of the furnace for the combustion of solid fuels and as a consequence the coal, often mixed with added biomass and other materials, is ground into a fine grain, mixed with air and burned in the flue gas flow.

The coal contains mineral matter which is converted to ash during combustion and has to be removed at the furnace bottom – in the so-called ‘baghouse’.

Baghouses are not confined to the energy industry, but also installed in the plants of steel mills, pharmaceutical and chemical producers, food manufacturers and other industrial companies. They came into widespread use in the late 1970’s following the development of high-temperature fibres capable of withstanding temperatures of over 175°C.

These fibres include polyphenylene sulphide (PPS) meta-aramids and even higher performance polyimide (PI) and polytetrafluoroethylene (PTFE). Recently, an Australian company, Metis Technologies, has introduced polyacrylate fibres (PCAs) as a much cheaper alternative to PI and PTFE fibres. PCAs were also initially developed back in the 1970’s, but in the past few years their fire resistant properties have been significantly enhanced.

Eight metres long

Baghouses typically have a particulate collection efficiency of 99% or better, even when particle size is very small. Most employ long, cylindrical filter bags that can be up to eight metres long and are usually needlepunched nonwovens or woven fabrics, reinforced with scrims. For applications where there is relatively low dust loading and gas temperatures do not exceed 170°C, pleated nonwoven cartridges can be used instead of bags.

The dust-laden coal gas enters the baghouse through hoppers and is directed into the baghouse, where it is drawn through the bags – either on the inside or the outside depending on the cleaning method – and accumulates on the filter media surface until air can no longer move through it.

When sufficient pressure drop occurs, the cleaning process begins. Baghouses are very efficient particulate collectors because the dust cake is formed on the surface of the bags. The nonwoven fabric provides a surface on which dust collects through four basic mechanisms:

• Inertial collection – the dust particles strike the fibres placed perpendicular to the gas-flow direction instead of changing direction with the gas stream.
• Interception – the particles that do not cross the fluid streamlines come in contact with fibres due to their size.
• Brownian movement – sub-micrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces.
• Electrostatic forces – the presence of an electrostatic charge on the particles and the filter can significantly enhance dust capture.

It is a combination of these four mechanisms which results in the formation of the dust cake on the filter, eventually increasing the resistance to gas flow and requiring periodic cleaning or even bag replacement. The effective lifetime of nonwoven filter bags, however, is generally between two and three years, and high abrasion resistance is a necessary property of the fabrics.

In addition to efficient dust removal, the filter media also effectively neutralise harmful gases.

Growth

Anywhere between 10,000 and 25,000 filter bags are employed in each baghouse of a coal-fired energy plant, and – with a 2- 3 year’ lifespan per bag - future growth is inevitable.

In 2012 the New York Times reported that the lion’s share of the new coal plants planned worldwide will be in the People’s Republic of China, with 363 further plants under construction to add new capacity of around 560,000 MW, and in India, with 520,000 MW to be supplied by 455 new plants.

New and tougher emissions standards in China are only intensifying the demand for nonwoven filter bags.

Stricter emissions standards for sulphur dioxide (SO₂), nitrogen oxide (NOx) and particulate matter (PM) came into force for newly-constructed Chinese power plants in January 2012, replacing those that had been in effect since 2003. They were also applied to the country’s vast iron, steel and cement industries. As a consequence, many factories have had to install new bag filters, as well as being obliged to replace their used ones at an earlier stage than had previously been the case, to meet the new requirements.

This year, all new and existing Chinese power plants will also be subject to mercury emissions standards and, in addition, nine key regions in China with the most severe air pollution problems will face even stricter emissions standards. China is also reported to be planning “very ambitious” goals to cut its carbon-dioxide output after 2020 and may announce an overall cap on emissions during the first half of 2016.

It is not just in the rapidly developing nations of Asia, however, that new coal-fired energy plants are being constructed, with 36 currently under construction in the USA to add a further 20,236 MW of capacity.

In Europe, Poland is currently building 13 new plants and even in Germany, ten new coal-fired plants are being added to the energy generation mix.

Oil shale

Estonia, meanwhile, is rich in high quality oil shale deposits which are mainly to be found in the north east of the country. 

Eesti Energia has its largest power stations running on oil shale near the city of Narva, close to the Russian border, covering the energy requirements of more than 500,000 people. The production of liquid fuels, power and heat from oil shale is a major economic factor in this region and the Eesti Narva power plants generate over 90% of the electricity produced in Estonia, although the utmost priority is given to compliance with environmental constraints.

The main Eesti power plant – as the biggest oil shale fired power plant in the world – has recently placed a major order with German nonwovens manufacturer BWF Envirotec for some 30,000 filter bags.

 BWF says its filter media are individually tailored to each application, with important factors including the characteristics of the dust to be filtered, compliance with the emission limit values and the efficient neutralisation of noxious gases.

EDANA has calculated that the global market for filter media has grown at an annual 7% over the past five years, and will reach a value of $3.5 billion this year, ensuring the sector is well represented at the next INDEX show which takes place at Palexpo from 4th-7th April 2017.

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