Hot Gas Filtration is the Hot Topic Hour on July 2, 2015 at 10am CST
This session will build on Hot Gas Filter Route Map and Summary. Panelists will summarize the options available and reference documents in the Route Map. Additions to this website are encouraged in advance. Participants are encouraged to review this site prior to the meeting.
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Coverage of April 29, Hot Gas discussion in Charlotte
The Hot Gas Decisions session at AFS yesterday zeroed in on key issues and options relative to selection and operation of filters and precipitators to capture a range of pollutants. As Tom Hart of AEP pointed out to the attendees, power plants must evaluate mercury, water and solid waste impacts along with any decision to reduce particulate. Tom contributed additional power points on the regulatory maze. These have been incorporated into the PPAQ decision orchard. This service is free of charge to power plant personnel as explained in Power Plant Decisions
The first decision is whether to keep the existing precipitator or to replace it with a baghouse. The key factor is whether MATS particulate limits can be met where particulate is the surrogate for metal toxics. Mick Chambers of SEI presented a number of examples where precipitators are achieving efficiencies suitable to meet MATS.
A hot discussion was generated by the question of whether to use a precipitator or a baghouse for a new plant. Tom Hart unhesitatingly chose the baghouse. When challenged by Mick, Tom explained some of the operating problems he has seen at AEP plants. Maintaining efficiency in a baghouse is easier, in his opinion, due to the ability to easily replace bags. The same cannot be said for precipitator internals.
Steve Feeney of B&W said that the particulate decision is greatly influenced by the scrubber type selected. He believes that dry scrubbers are the better choice. Dry scrubber efficiency has improved over the years. This makes it a candidate for a range of coals. Steve also talked about successfully capturing and preventing re-emission of mercury in the wet scrubber. This also makes the wet scrubber option more attractive.
The scrubber now functions to capture a number of pollutants which results in what Tom Hart described as “scrubber soup”.
There was agreement regarding the difference between a 200 and a 500 SCA precipitator. The good news and bad news is as follows: the good news is that you can increase precipitator efficiency just by increasing the size. The bad news is that the cost rises proportionately. Also there is difficulty expanding the size of an existing precipitator.
This SCA sizing also impacts the capture of activated carbon laden with mercury. Some operators are finding higher mercury readings with sorbent traps than CEMS. The conclusion is that particulate mercury is captured in the sorbent trap but not measured by the CEMS. The further conclusion is that this could be a serious problem once EPA concludes that there is a big hole in the theory of just measuring gaseous mercury.
The opposite experience was communicated by Tom who says that their measurements may even show higher readings with CEMS than sorbent traps but acknowledged that they have large well operated precipitators with high total efficiency. But not all the power plants are operating in this highly efficient mode. As Tom, himself pointed out earlier, it is difficult to maintain precipitators except by shutting them down for repairs.
If a baghouse is chosen, a decision must be made on cleaning type. Reverse air and pulse jets are the options. Glass and synthetics are the common fiber options but ceramics and metals have suddenly joined the list.
Rich Miller of Solaft championed the use of extended surface area bags such as their star bag. He also recommended software to track bag life, predict change outs and manage operations. This was part of a theme addressed by many of the panel members. It is the maintenance and operation which determine performance. With the new continuous mass monitoring requirements the operator cannot operate for any length of time with broken bags without exceeding the limits.
The panel had proponents for membranes on glass and synthetics and proponents for non-woven felts including those with fiber blends by type and size e.g. nanofibers. Clint Scoble of Testori maintained that non-woven blends can meet MATS requirements. P84 fiber has a different shape than PPS. By blending the two the efficiency/pressure drop ratio can be improved.
Eddie Ricketts of Donaldson cited the efficiency and ease of cleaning associated with membranes. John McKenna of ETS agreed that membranes do provide the highest efficiency. Several of the panelists then discussed initial vs. long-term performance. If the membrane breaks, there is a problem. On the other hand, if the felt pressure drop builds up despite increased pulsing, there is also a problem.
John Eleftherakis of Filtration Group covered the extensive experience on glass furnaces, biomass boilers and incinerators with their ceramic hot gas element. When catalyst is embedded and dry sorbent injection is utilized results of less than 5 mg/Nm3, 90% SO2 and even 90% NOx removal can be achieved.
Martin Schroter of Dürr provided insight into a unique combination of processes. Very finely powdered limestone is injected in the boiler with the fuel. A ceramic catalytic element follows the economizer. High SO2 removal efficiency is obtained due to the contact time of the calcium particles on the filter element. The clean gas then enters a heat exchanger which can extract lots of valuable heat. Boiler efficiency is also enhanced by the lower volume of gas moved by the ID fan. This results in lower fan horsepower. Elimination of the rotary air heater also eliminates the extra flue gas volume created by air heater leakage.
John McKenna of ETS believes that combining nanofiber and catalyst technology could be a big step forward.
Pavlos Papadopoulos of Purolator displayed results showing that sintered metal fibers perform better than sintered powdered metal. Very high removal efficiencies are achieved at low pressure drop. Purolator has deep and varied experience on a range of applications including smelters. This technology may have some new uses in coal-fired boiler operation according to some new concepts offered by McIlvaine.
Coal flyash has been found to contain high percentages of rare earths in contrast to mined coal. China has concluded that flyash is, therefore, a top source for rare earth extraction. The process of capturing flyash is a beneficiation process. But what about refining this even further? What if there were two more particulate separation stages? If the rare earth percentage in one stage was much higher than in others, it could make the rare earth production from flyash even more attractive.
This hot gas session was a positive step toward improving the hot gas decisions program. We will keep updating the decision guide and the decision orchard. We welcome your input for both.
