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Recent developments in particulate control

World Coal,

There is increasing concern about the effects of airborne particulate matter on public health. Particulate matter less than 10 µm (PM10) can be inhaled into the respiratory tract and PM2.5 can get into the alveoli and pass though the mucus membrane into the blood. In order to prevent adverse health effects, the World Health Organisation (WHO) has set guidelines for maximum mean annual ambient concentrations of PM10 at 20 µg/m3 and PM2.5 at 10 µg/m3.

Methods of particulate control

Emissions of fly ash from the stacks of pulverised coal combustion (PCC) plants contribute to ambient concentrations of particulate matter. In order to lower these concentrations, fly ash emissions from PCC plant must be reduced by a form of particulate control. The majority of countries now have some form of emission limit values (ELV) for particulate matter from PCC plant. Historically, electrostatic precipitators (ESP) have dominated the market for particulate control in a variety of combustion and industrial processes, including PCC plant, incineration plant, cement kilns, steel manufacture, oil refineries and paper industries. ESP negatively charge particulates with a discharge electrode in order to collect them on a positively charged plate electrode. ESP have a high collection efficiency, low pressure drop, good reliability and relatively low capital, operational and maintenance costs.

Fabric filters (FF) have been used for particulate control in smaller processes. FF capture fly ash by passing the flue gas through a filter, through which the particulates are too large to pass – the same principle is found in a vacuum cleaner. FF are becoming more popular with large PCC plant because of higher collection efficiencies of PM2.5 and their ability to capture other pollutants by the use of sorbent injection. Unfortunately, FF have high pressure drops and higher capital, operating and maintenance costs.

Usually, collected fly ash is sold to the construction industry, providing it meets required quality parameters. Collected fly ash excessively contaminated by sorbents and pollutants potentially cannot be sold and disposal costs will follow.

Challenges to particulate matter control

Due to concerns about human health and air opacity, ELV for particulate matter have become more stringent with time. ELV in some countries are now extending to specific toxic metals and PM2.5 as they are hazardous to health and have low collection efficiencies in existing particulate control. Lower ELV are generally met by increasing the efficiency of the existing particulate control via numerous enhancements.

However, as PCC plants are progressively operating outside the design conditions, enhancements are made difficult due to two reasons. Firstly, fly ash properties are altered because of fuel variation (different coals or coal blends to design specifications and cofiring with biomass) and installation of emission reduction technologies in the boiler (such as low NOx burners) and post combustion (such as SCR). Secondly, PCC plants undergo cyclic operation, as opposed to the designed baseload operation. These two reasons adversely affect performance of older ESP. Other challenges are space restrictions onsite for the expansion of equipment or inadequate working area. Fortunately, however, the addition of wet FGD can more than halve the particulate control outlet emissions.

Boosting collection efficiency

Despite this, increases in collection efficiency of ESP and FF have been achieved with additional benefits such as lower parasitic load, increased fly ash sales and longer equipment life. ESP maintenance procedures can be used to upgrade certain parts. Important upgrades include power supply for superior charging and microprocessor control for sophisticated operation. Uniform flow distribution can be ensured using CFD analysis and subsequent addition, modification or removal of flow control devices. When the resistivity of the fly ash becomes too high, collection efficiency decreases: a common example of this is when there is a switch to firing lower rank coals.

However, fly ash resistivity can be decreased – and collection efficiency restored – by adding chemicals to the flue gas in a process known as flue gas conditioning. For conventional ESP, the collection efficiency for fine particulates is low; this problem can be solved by retrofitting an agglomerator, which further agglomerates particulates. For an improvement in collection efficiency along with other advantages, the following variations of ESP technology could prove suitable: colder side ESP, wet ESP, moving electrode ESP, electromechanical and double-zone ESP. These variations can be retrofitted to the existing ESP or built as an additional polishing device. Cross flow ESP illustrates that the conceptual development of ESP technology continues. Modern ESP are designed to have high collection efficiencies of all types of fly ash, some are marketed as applicable to worldwide coal firing; collection efficiencies are now up to 99.81%.

FF are benefiting from advances in synthetic fabrics, coatings and treatments for increased resistance to chemical degradation, thermal deterioration and mechanical wear. For fabric filters with the pulse jet cleaning system, the control can be upgraded to microprocessor control for sophisticated cleaning routines. Uniform flow distribution through the FF can be ensured using CFD analysis. Flue gas conditioning and agglomeration can be used for creating optimum fly ash cohesivity for more effective filter cake development. A humidification system can be integrated in FF to provide flue gas temperature control with other advantages.

Sorbents can be injected upstream of the fabric filter for capture of other pollutants in the filter cake. This is known as multi-pollutant control – a popular example is the injection of activated carbon to capture mercury. A modern pulse jet FF will achieve collection efficiencies greater than 99.95%. Furthermore, new hybrid ESP/FF systems aim to become more viable than the individual technology by utilising the advantages of both technologies. Generally R&D is ongoing in all areas and many technologies are moving towards multi-pollutant control devices.

Kyle Nicol is a consultant at the IEA Clean Coal Centre. His full report is available from the IEA Clean Coal Centre Bookshop.

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