t; -.’ . P

NEWSFRONT

filters are breaking temperature records and removing more than dust

as cleaning temperatures have been moving up ever since G heat-resistant fabrics boosted

the limit for baghouse filtration to 250°C (CE, April 1989, p. 30). Now, commercial hot-gas cleaning products, based on ceramics, absorbent beds and sintered metal, have trebled this tem- perature limit, allowing even the hottest fluegases to be filtered without cooling. Breakthroughs promise to raise temperature limits even hrther, as new processes that run at well over 1,OOO”C leave the lab for the test site.

In addition to withstanding such high temperatures, these new gas cleaning systems can operate at pres- sures that would destroy any fabric bag. Even more importantly, many of them can remove chemical pollutants as well as particulates. Compared with traditional gas cleaning systems based on cyclones and electrostatic precipita- tors (ESPs), the new systems reduce emissions by one order of magnitude.

So far, demand for hot gas cleaning is strongest in advanced coal-fired power plants, using pressurized flu- idized-bed combustion (PFCB) or inte- grated gasification combined cycle (IGCC) processes (CE, August 1991, p. 47). In such units, filtering the gas while hot allows abrasive dust to be re- moved before it can damage expensive turbines - some new systems can even remove NOx and SOX.

In conventional power plants, hot gas cleaning eliminates the need to cool fluegas to keep it from destroying filter bags. Cooling is typically achieved by diluting fluegas with air, a step that raises both equipment and treatment costs, requiring extra fans and increas-

28 CHEMICAL ENGINEERINGNULY 1994

GAS

Ktracted JSt

it

Ceramic candle filters permit gas cleaning at extremely high temperatures and -

pressures, as found in coal- fired power plants, incinera- tors, and refineries. Here, a unit is outlined and shown being installed at a coal gasification plant in Germany. The ceramic candles that make up each filter are shown at far left

CLEANING COMES OUT OF THE BAG

ing the amount of gas to be filtered. In contrast, hot filtration eliminates

the need for extra equipment, reduces gas volume for treatment, and even al- lows waste heat to be recovered. For most plants, this means a 2-3% in- crease in operating efficiency. ‘When integrated into a process, hot gas filtra- tion gives something back as it removes pollutants,’’ says Wolfgang Peukert, R&D manager for Hosokawa Micropul (Cologne). It can also prevent dioxins from forming, he adds.

Reflecting these benefits, the world bought $70-75 million worth of hot gas

cleaning systems in 1992, and Lutz Bergmann, an analyst at Filter Media Consulting, Inc. (La Grange, Ga.), ex- pects that figure to reach $170 to $180 million by 1996-1997. While its leading market to date is in coal-fired power plants, hot gas filtering is also being used in incinerators and other combus- tion processes, and in refineries, to re- cover catalyst from fluid catalytic cracking (FCC) units.

The technique is not without its drawbacks. For many industries, capi- tal costs remain a major obstacle. Ce- ramic candles for high temperature fil-

ters are said to cost about five times more than high-temperature ceramic bags, which, in turn, cost ten times more than conventional fabric bags. In addition, salts can be difficult to re- move from ceramic filters, and they tend to clog heat exchangers, prevent- ing heat recovery. Nevertheless, a growing number of process plants find that the benefits of hot gas cleanup still outweigh its liabilities.

Roots in clean coal processes The development of hot gas filtration has closely tracked that of new coal- fired power plant technology. Deutsche Babcock Energie und Umwelttechnik AG (Oberhausen, Germany), for exam- ple, began working on high-tempera- ture filtration back in 1987, when it started developing its PFCB process. The company's hot filtration process was eventually incorporated into a 15- MW pilot plant in Friedrichsfeld, and an improved version is now available through Lurgi Lentjes Babcock Ener- gietechnik GmbH (LLB; Oberhausen). Designed to run from 600°C to 850°C, the system can reduce dust levels from 200 to below 5 mg/Nm3, says LLBs se- nior manager George von Wedel. The unit's filter is made up of several layers

3M

of candles - as many as 2,000 per unit. In addition to its test facility in

Friedrichsfeld, which has operated for about 3,000 hours, LLB installed a 600- candle unit last year for Rheinbraun AG's (Berrenrath, Germany) high-tem- perature Winkler gasification test facil- ity, which operates at about 300°C and 10 bar. The unit is using the DIA Schu- malith, a ceramic filter element devel-

oped by Schumacher Umwelt und Trenntechnik GmbH (Crailsheim, Ger- many). In the Netherlands, Schu- macher recently completed an 864-can- dle unit for a local consortium. The unit is designed to clean 400,000 m3h of syngas for a 250-MW coal gasifica- tion plant in Buggenum. The plant, which is just starting up now, is using Shell BV's gasification process (CE,

Schumacher CHEMICAL ENGINEERINGNULY 1994 29

NEWSFRONT Babcock and Wilcox

Several new hot gas cleaning

systems remove air pollutants as well

as particulates. This system, for

example, removes both NOx and SOX

January, 1992, p. 39). In another large demonstration project, LLB's technol- ogy has been selected for an IGCC pro- ject in Puertollano, Spain, to start up in 1996. Using Krupp Kopper's Prenflo gasification (CE, March, 1992, p. 21) process, the facility will consist of two 900-candle vessels running at 270°C and 25 bars.

Its filter system is designed so that, if one of the candles breaks, valves shut off that section, allowing the rest of the plant to continue running, says LLB manager Von Wedel. In conventional plants, he says, if a single DM700 ($438) candle breaks - even if it is only one of 2,000 - the plant must be shut down completely, at a cost of DM500,OOO ($313,00O)/d.

Ceramics and absorbent beds Durability is also making ceramic fil- ters a more-attractive alternative to baghouses for incineration units, and units are now running in medical waste incinerators (CE, April 1994, p. 41) and at least one unit that burns low-level radioactive waste. Among the compa- nies exploring this expandug market is BWF Textil GmbH (Offingen, Ger- many), which has developed Pyrotex candles, which, it says, retain their structural integrity at up to 900'C and differential presures of up to 20 kPa.

The 1.81-m candles offer a filtration efficiency of 99.99% with an emission rate below 1 mg/m3, and have been tested extensively by the University of Essen and in a number of industrial processes; several small commercial units have already been installed.

Not only ceramic but nonceramic systems are taking high-temperature filtration beyond particulate removal, aiming for such troublesome pollutants as SOX, NOx and hydrogen sulfide. Here, absorbent bed systems are play- ing a critical role, since ceramics can be

GAS CLEANUP: THE HEAT'S ON!

The following companies or institu- tions offer high-temperature, high- pressure gas cleaning systems that minimize emissions. For more infor- mation, please circle the appropriate Reader Service Card number.

ABB Corporate Research Center 400 Babcock & Wilcox Go. 401 BWFTextil GmbH 402

Filter Media Consulting, Inc. 404 Hosokawa Micropul 405 Karlsruhe University's Institute for Mechanical Engineering 406

LTG GmbH 408

Mitsubishi Heavy Industries 41 0 Pall Europe Ltd. 41 1 Schumacher 41 2

Ceramic Gas Cleaning PIC 403

Kawasaki Heavy Industries 407

Lurgi Lentjes Babcock 409

difficult to clean. In some units, various beds of metal oxide absorbants are combined - in a few cases, they are even allied with ceramic filters.

In Japan, for example, Kawasaki Heavy Industries (KHI; Kobe) is devel- oping a process that removes dust, hy- drogen sulfide and carbonyl sulfide. Tests began last year ai a 4-m.t.id coai gasifier in Nakoso to evaluate the tech- nique, which uses a crossflow reactor with iron oxide absorbent in a moving- bed granular filter. For a l,000-m3k gas stream at 698 K and 25.7 atm, the system reduced sulfur content from 1,000-3,000 ppm to below 100 ppm. The absorbent traps sulfur, which is then oxidized at 673 K to produce SO,.

Parallel to KHI's unit, Mitsubishi Heavy Industries (Tokyo) is also devel- oping a fixed-bed filter unit combining a silicon carbide porous filter for dust removal and an iron- and titanium

oxide-based honeycomb fured bed for desulfurization. A 20-m.t./d unit is being tested on a 4,530-m3/h stream at 26 atm. Tests at a 2-m.t./d unit indicate a 99.9% dedusting efficiency to less than 100 ppm. The system works best at 300 to 500 'C.

Also combining dust filtration with desulhrization at high temperatures, Haldor Tops~e (CE, February, p. 18) has developed a unit that operates at 400°C. It uses a ceramic filter for dust removal with a zinc oxide k e d bed for H,S absorption. Meanwhile, to remove reactive organics as well as SOX and NOx, Babcock & Wilcox Co. (B&W Al- liance, Ohio) is commercializing its SOX-NOx-ROx box (CE, April, 1989, p. 301, aiming for new power plants. The process is expected to cost $260/kW for a 250-MW boiler fired with 3.5% sulfbr coal generating 1.2 Ibs of NOx per mil- lion Btu.

While at Karlsruhe University, Mikropul's R&D manager Peukert de- veloped a unit combining a fixed bed of quartz particles with a countercurrent bed filter using CaO or NaHCO, to clean secondary aluminum-plant flue- gas. Work with absorbent beds contin- ues at Karlsruhe, and, in the Nether- lands, at Delft University.

Tighter emissions limits are pushing more refmers to use high-temperature filtration to improve cyclone recovery of precious-metal-bearing FCC catalysts. Pall Europe Ltd. (Portsmouth, U.K.) of- fers a blowback filter incorporating porous sintered metal cylinders, which can remove 99.97% of the entrained catalyst particles down to 1 pm dia. from 600°C offgas. It has installed units at Mobil Oil refineries in Germany, the U.S. and the U.K., where its C o r y h unit has been recovering 3.5 m.t./d of catalyst since 1992. Pall is now also working with ceramics for FCC, a field that Schumacher entered in 1992, when it installed a unit in the Lindsey Oil Refinery (Grimsby, U.K.). A second installation is planned this year at Scanoff Refinery (Lysekill, Sweden).

Whatever medium is used, hot-gas filtration systems fill a real need. They may never completely replace old-fash- ioned fabric bags, but they are already helping engineers in their struggle to meet tighter emissions standards.

Gerald Ondrey with Stephen Moore

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CHEMICAL ENGINEERING/JULY 1994 31