Media selection for Coal-fired B

Media selection for Coal-fired Boiler Baghouses was the Hot Topic Hour January 8, 2015

Power plants from Russia to China are removing precipitator internals and substituting bags in order to meet new emission limits. Eskom estimates they will need 350,000 bags per year for their South Africa power stations. For many plants fabric filters are being encountered for the first time. They need guidance to help them make the best media choices.

This webinar was a step in a comprehensive initiative to help purchasers make media selections using a Global Decisions Decision System (GDPS™). There are many potential unique determinations. For example, 6 fiber types x 4 media types x 6 plant specific conditions x 6 gas conditions x 3 different element shapes x 4 major type cleaning designs = 10,368 different determinations. When other parameters are included there are nearly 1 million combinations.

Clint Scoble of Testori indicated that with their customization an infinite number of combinations of fibers, laminates, densities, and surface treatments are possible. Here are just some of the options.

fiber	pps	p84	ptfe	glass	ceramic	acrylic
Media	Non-woven	Membrane laminate	Woven	Sintered	Scrim?	Mix of fiber types and density
Plant conditions	Emission limit	Area available	Fan limits	Mercury removal	SCR	FGD
Gas conditions	Flyash load	Sulfuric acid	Sulfates	Activated carbon	Temperature	Other acid gases
Bag shape	Tubular	Pleated	Cartridge
Surface treatment	Variety of coatings and treatments
Cleaning	High volume, medium pressure air	High pressure air	Reverse air	Shaker

There are controversies over the maintenance of membranes, the efficiency of non-wovens, the new 5 mg/Nm³ limits in China, installing elements in an existing ESP, sulfuric acid corrosion, activated carbon bleed through with absorbed mercury, the ability to handle heavy sulfate loads, the cost of pleated elements and the influence of cleaning type on media selection.

The options as refined in the webinar are being posted in Power Plant Air Quality and will be continually available to power plants around the world free of charge. An article with highlights will appear in Filtration News. Further review will take place at the AFS Spring meeting.

A description of the the GDPS™ was followed by three presentations and very good discussion of the issues.

GDPS™ is the grouping of decision trees and a route to travel from tree to tree and then reverse course as needed. For example a thorough investigation of particulate removal options may cause a travel back to the retirement decision tree due to the magnitude of the investment. Reversing course many times will be necessary to make the best media selections. Here is the GDPS™ for the range of air pollution control systems.

Emission Limits: Will PM be used as a surrogate for toxic metals? If so media selection could be critical. Bob McIlvaine displayed recent regulations requiring emission limits as low as 5 mg/Nm³. These tough limits are now being imposed in many countries. China has just imposed this limit in certain cities and provinces. The speakers all addressed efficiency issues and how the media can be tailored to meet the 5 mg/Nm³ and lower. Clint Scoble stated that Testori does not make claims on micron cut points. This avoidance is supported by the fact that particles are not spheres. Furthermore the separation ability is a function of specific gravity. The cascade impactors which are used to determine micron cut points are influenced by particle shape and gravity. They attempt to relate this to a specific spherical diameter and a selected gravity.

An even more important consideration for the purposes of emission limits is the leakage through seams, bag tears, and leakage at the connection to the tube sheet. Eddie Ricketts of Donaldson Membranes said they commonly find installations where leak checks were not conducted and all sorts of problems result. John McKenna of ETS explained that when dust is on the clean side of the bag it can wedge between the cage and bag surface and cause significant wear problems. The wear problems quickly result in lower emissions. Several speakers pointed to the fact that only one or two broken bags can cause the unit to exceed the emission limit.

Emission limits during start up and shut down are another consideration. The non woven fabrics which rely on cake for initial efficiency do not function as efficiently on start up as do membrane bags. Another development is the switch to mass particulate emission measurement rather than opacity. A few broken bags allowed to operate for a short time can bring the mass emissions average for a lengthy period to above the limits.

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Area available: Retrofitting the existing precipitator housing or installing the fabric filter in a tight area can restrict space. Pleated bags can be an answer even if more expensive. Bob McIlvaine presented details of Enel’s and Eskom’s extensive retrofitting of bags in precipitators in Italy, Spain, Chile, Russia, and South Africa.

Fan limits: If the existing fan is going to be used and it is desirable to minimize energy consumption, glass bags with reverse air may be an option. Eddie Ricketts of Donaldson touted the lower pressure drop with membrane bags. Clint Scoble had some interesting slides showing performance at different air to cloth ratios. So both media construction and amount can be varied to try to accommodate an existing fan.

Mercury Removal: If bromine is used as fuel additive there can be corrosion issues. If activated carbon is used then the dust loading will be higher and this leads to maintenance, energy, and emissions issues.

SCR: If the particulate control device is preceded by an SCR, there is a potential sulfuric acid corrosion problem. This is also a function of the sulfur in the coal and whether a low conversion catalyst is used.

FGD: If wet scrubbers follow the filter then there will be some additional particulate removal. If dry scrubbers are selected and the fabric filter must capture both flyash and sulfates then there are cleaning, life, and emission issues. Speakers during the session did briefly distinguish between three types of dry scrubbers. One is dry sorbent injection. Another is spray driers and a third is dry fluid bed and entrained flow dry scrubbers. One significance for media selection is that spray driers inject reagent as a slurry. As a result the amount of sulfur removal is limited to the amount of H₂O which can be injected prior to condensation and plugging. Another significance is the higher solids loadings of the all dry versions. An exception is Korea Cottrell who uses a cyclone prior to the fabric filter.

Coal Selection: If the power plant wants to blend coals and select the most economical then it will want to consider the higher dust loadings with low sulfur coals and the higher SO₂ levels with high sulfur coals. The mercury content of the coal is important. One overlooked aspect is the failure of the fabric filter to capture all the particulate mercury. EPA made a very big mistake by specifying that only gaseous mercury has to be measured to determine compliance. The basis of the regulation was the lack of particulate mercury in uncontrolled boilers. EPA failed to anticipate that activated carbon converts the gaseous mercury to particulate mercury. So a power plant does not have to reduce mercury emissions as long as they convert to the particulate form. Unfortunately in the particulate form it will fall closer to the source and be much more hazardous. This is too big a problem to be ignored. In fact the sorbent trap mercury capture methods are demonstrating that large amounts of particulate mercury are going to be emitted. The question is to what extent will media selection limit the coals which can be used?

Heat Recovery: Speakers talked about sensitivity to temperature with PAM having a limit of around 240-250°F. There was not time to explore fabric selection under conditions when acid gas is removed ahead of the heat exchanger. URS and MHPS are two companies advocating that power plants inject lime or sodium compounds ahead of the heat exchanger. The acid dewpoint can be reduced to less than 160°F. The heat exchanger can then be designed to reduce outlet gas temperature to less than 195°F. The main advantage is the additional heat recovery. This can add 1% to plant efficiency. But an ancillary advantage is the ability to use lower temperature media in the fabric filter.

This webinar was a step in a comprehensive initiative to help purchasers make media selections using a Global Decisions Decision System (GDPS™).

John McKenna of ETS was the opening speaker. John reviewed fabric selection considerations. Gas stream temperature, moisture, chemistry and dust loading must be considered. Dust characteristics – abrasiveness, stickiness, explosiveness and flammability must also be considered. The fabric’s filtration performance, temperature maximum, release properties, pressure drop, durability and cost must be taken into account. Other considerations are ePTFE membrane, coatings/treatment, blends, scrim and hardware.

John stressed that membrane bags must be properly installed and maintained. The bags must be correctly fitted. Leak testing should be done after the bags are installed.

Tighter emission regulations mean that leaks in a single bag must be quickly repaired. One or two broken bags could mean that the plant is out of compliance. Preventing dust from entering the “clean side” of the baghouse and bags is a must.

Clint Scoble of Testori USA was the second speaker. Clint showed pictures of the cross section shapes of typical fibers for coal-fired boilers explaining that more surface area increased filtration efficiency in the same basis weight felt. He described major fiber blends for coal-fired boilers. Blends are designed to be a combination of different fibers in the batt or a combination of batt and scrim made with different polymers.

Virtually all coal-fired boiler baghouse felts are scrim supported. This means they are more dimensionally stable, able to resist pulse pressures better/minimize dust penetration and able to support heavy filter cake on long bags (up to 10 meter) without stretching (40-60 lbs of dust on 10 m bags).

Clint described different needle felt constructions along with finishing and treatments. He explained that European felts tend to be denser providing longer life. He provided charts to aid in media selection but cautioned that they should only be used as a guideline.

Clint showed pictures of the cross section shapes of typical fibers for coal-fired boilers explaining that more surface area increased filtration efficiency in the same basis weight felt.

Tags: 221112 - Fossil Fuel 化石燃料, Testori Group, Media Filter, Bags

Eddie Ricketts of Donaldson was the last speaker. He discussed the advantages of membrane filter media for coal-fired boilers. Many utilities will be forced to add and or increase their sorbent injection levels to comply with lower Hg/HCl/acid gas emission limits. Several of these sorbents including Powdered Activated Carbon (PAC), Trona and lime products can have portions of their particle size distributions less than 2.5 microns. A higher efficiency membrane filter will allow these very fine/submicron sorbent fractions to be filtered more reliably and efficiently.

Membrane surface filtration technology allows filter fabric’s to clean better (lower Δps), which leads to longer cleaning cycle times, more gross filter cloth area on line, less compressed air usage (savings), and less cleaning stress on the filters (longer life).

High moisture levels can cause PM to be extremely sticky and challenging to remove from conventional filter bags. Moisture excursions can occur periodically via tube leaks, dew point excursions, high humidity, startup/shut down conditions. The primary cake in conventional filters can cement within the media. This condition will increase Δp which will trigger the boiler to be derated. Membranes via surface filtration eliminate this risk and allow for improved recovery from moisture excursions.

Tags: 221112 - Fossil Fuel 化石燃料, Donaldson Membranes, Media Filter, Bags