Solid Fuel and Fly-Ash Control

Plants intending to swap their natural gas boilers for those capable of combusting solid fuels must consider which fly-ash abatement technology will best meet their needs at the right price.
By Ron Kotrba | May 09, 2008
Ethanol producers whose process energy comes from a natural gas line, and will remain so indefinitely, may want to skip over this article. For those investigating boiler retrofits or new installations involving biomass or coal, however, it's important to make the right decision on which fly-ash control technology to install for meeting state regulations and protecting downstream equipment from fouling. There is no need to reinvent the wheel. Ethanol makers in search of an alternative energy path may benefit from the wood-products industry and its many years of experience.

Before a veneer manufacturer applies heat, pressure and glue to make its final product, the moisture content of the incoming green wood must be dried down considerably. Green wood can contain up to 50 percent water by weight, which must be reduced to 5 percent moisture before becoming veneer, particle board or wood pellets. Many of these companies burn their wood waste as a means to dry the good wood.

In Florida, what's been billed as the largest wood pellet manufacturing facility in the world—the 500,000-ton per year Green Circle Bio Energy plant in the panhandle city of Cottondale producing wood pellets for the European power industry—generates heat for its biomass dryers by combusting pine bark. The options for fly-ash abatement technology available to the company included a dry electrostatic precipitator (ESP), a wet ESP, a baghouse ash collection system, a wet scrubber and a multiclone fly-ash collector.

Gerry Graham of PPC Industries wrote a paper in which he compared various fly-ash control technologies. In that paper, he explains what a wet scrubber is and details its shortfalls: "A scrubber saturates the gas stream in order to remove the dry fly ash. The wet ash has to separate from the water in settling ponds or through a de-sludging unit, which increases the annual labor and operating cost. It is not uncommon to see 150 to 300 horsepower fans on scrubber installations in the wood industry. The energy necessary to separate the particulate from the gas stream can require 15 to 20 [inches] wc (water columns) of pressure drop through a typical venturi. These are huge and wasteful power consumers, increasing the plant's overall operating cost."

With today's high energy costs and narrow production margins, plants want to minimize energy consumption. Amidst those and additional concerns of equipment corrosion, cold weather performance and the regulatory push to decrease water discharge from industrial facilities, wet scrubbers may not be the best choice for abating fly ash.

Baghouse Technology
A technology with which most ethanol plant operators are familiar is baghouses, which are used to control fugitive dust from distillers grains. Red Trail Energy LLC, a 50 MMgy coal-fired ethanol plant in Richardton, N.D., employs baghouse technology from the Industrial Accessories Co., to capture its fugitive coal ash. Plant Manager Edward Thomas tells EPM the baghouse technology was part of the plant's inclusive design. "Red Trail didn't have a whole lot of extra control in the picking of its abatement equipment on the back end of the plant," Thomas says. "It was primarily handled through ICM Inc." An ethanol plant combusting coal for process energy must not only mitigate fly ash but also sulfur emissions. "This particular facility on the back end has an SBC [sodium bicarbonate] system," Thomas says. SBC essentially is a sulfur scrubber. To make its 50 MMgy of ethanol, Red Trail consumes approximately 100,000 tons of coal a year and produces 11,000 tons of ash.

At Red Trail, fly ash created in the coal boiler is taken through an ash collection system, which works off a set of blowers. "The ash collection system is going to take ash from the boiler itself and from our fly-ash baghouse on this facility," Thomas says. "The ash is pneumatically conveyed then to a holding silo which has a baghouse on it to clean the air that we are essentially blowing in there. From there, there's an unloading system which shares the same baghouse as the pneumatic conveying system, so that when we load ash out we put a little bit of negative pressure on the dump spout and control dust through that."

Environmental regulations differ from state to state so some agencies might require fugitive dust testing on the baghouse for the ash silo itself—but not in the largely rural North Dakota area where Red Trail is located. "At this facility, all we are required to do is watch the pressure drop," he says. "We don't have a physical test that we have to do." Not only would this vary from state to state but also within a region where a major emitter may have to meet different regulations than a minor emitter.

Graham notes the most common problem attributed to using baghouses as ash control devices: "The high temperatures and periodic cinders from the plant boiler can cause fire problems with baghouses." Ron Renko, regional sales manager for the Geoenergy Division of AH Lundberg Associates Inc., expresses the same safety concerns to EPM that Graham writes about. "One big concern is fire," he says. Geoenergy is an emissions controls vendor for biomass-powered plants. Renko says baghouses reside somewhere at the bottom of his fly-ash tech list. Geoenergy Manager Steve Jaasund tells EPM that baghouses just won't work for fly-ash control in a plant like the Florida pellet mill. His company supplied the monster wood pellet mill with its wet ESP equipment. "A baghouse is not feasible," Jaasund says. "The reason is three-fold: First, the gas stream exits the dryer much too close to the water vapor dew point so water condensation would be a big worry. Second, the condensible hydrocarbons (from the drying of wood biomass) at high temperatures will tend to plug the filter media. And finally, the great majority of the particulate—large fiber and condensible hydrocarbons—are combustible."

On safety and the potential for baghouse fires, Thomas says Red Trail has not encountered any problems with its baghouse system. Graham, Renko and Jaasund all mention the possibility of moisture, tars and plugging of the fabric filter media. Thomas says Red Trail has experienced minor interruptions in production due to media clogging, but nothing debilitating. "We've had a little bit of that, but what we've done is work with the baghouse manufacturer and through some of the controls we've been able to work what's in the PLC [programmable logic controller] and fine tune the cycles of when the bag is cleaned," he says. "Our baghouse in this facility has six cells and we can run with five of them while one is off-line. If one was off-line, obviously you're reducing your air capacity and you could see some plants go down because of that, but to date it hasn't been limiting for us. A lot of that comes into play with the type of fuel being used and its ash content, the different additives used and how those materials are combusted."

Thomas mentions, however, some unforeseen factors which could affect the performance and longevity of baghouses. "The temperature of that flue gas going into it can degrade your bags over time," he says. Graham agrees. "Periodic bag replacement is a definite operating cost consideration," he writes. Thus, while capital costs to install a baghouse are much lower than that of installing the more sophisticated ESP, all aspects—including state regulations and plant operational needs, performance, safety, longevity, and capital and operational costs—must be weighed heavily before project leaders make a decision.

Electrostatic Precipitators
The term "electrostatic precipitator" is not as sexy as "extrasensory perception," but a sixth sense will not appease state regulators or protect a regenerative thermal oxidizer from fouling. A wet ESP, however, can effectively do both. In the wood products industry, hot gas from wood combustion is usually routed to the dryer for heat. In most cases, the fly ash from wood combustion is high in alkaline earth metals such as sodium, potassium, magnesium and calcium, which Jaasund says are aggressive against the heat-exchange media in the regenerative thermal oxidizers. The point of an ESP system wet or dry is to effectively isolate and trap particles in a hassle-free system, which cleans itself as needed. "The gas goes into the electrostatic precipitator and it passes adjacent to a high-voltage discharge electrode, which charges all the particles," Jaasund tells EPM. There are three types of particles: fly ash, larger-than-fine particles introduced from the high-velocity air in the dryer and condensed organics from drying biomass. "Because of the high voltage on the discharge electrode, it gives off electrons and the electrons attach to the particles. Then, because of the electric field—the high voltage on the discharge electrode versus the ground potential of the collecting electrode—those particles are all pushed over to a collecting surface where they accumulate."

An ESP works like a giant particle magnet. When it's time to unload the material for disposal, the magnet reverses its charge to force the particles away instead of attracting them. Upon exiting the ESP, the gas stream is largely free of particles and ready for the RTO where the volatile organic compounds (VOCs) will be thermally oxidized and released.
In 2005. Central Ethanol Co-op in Little Falls, Minn., broke ground on its new energy system—a Primenergy gasifier and combined-heat-and-power system—designed to consume 280 tons of wood chips per day. The system came on line last year but has been plagued with operational setbacks. Also, the fly-ash collection system included in the engineering package failed to meet Minnesota's particulate matter, 10 micron regulations.

"We had problems with our combustion tube so our gasification technology is down right now, but the method of abatement we were employing simply didn't work," General Manager Kerry Nixon tells EPM. As the plant's energy island was being built the ash collection system chosen was a multiclone. That is essentially a very small cyclone inside of a steel structure, and the drop in pressure going through it was supposed to drop the fly ash out—collect it out—and the air would continue to the exhaust," he says. "But the fly ash was so light that it could not take it out. We even tried to spray moisture to add weight to the ash so it would come out, but it just wouldn't work." Shortcomings in the gasification technology are being addressed, and Nixon says he hopes it will be worked out sooner rather than later. In the meantime, engineers are weighing the pros and cons of baghouse systems and ESPs so an informed and effective decision can be made—the second time around.

"I think they're leaning toward an ESP, but we've talked with others using baghouses and they've not had many problems," Nixon says. "So it all depends on the temperature of the fly ash and how it's all handled up until that point—it all comes down to what's going to work best for the dollars."

In his comparative document, Graham writes that wet ESPs have found "renewed interest from oriented strand board (OSB), particle board, and plywood veneer manufacturers for controlling dryer exhaust." He also says that dry ESPs are still considered the best available control technology for wood-fired boilers. Jaasund, however, gives compelling reasons why wet is better.

"Biomass dryers typically operate near the dew point of the gas stream," Jaasund says. "You want your dryer to be as efficient as possible, so you don't want to be spitting red-hot gases out of the dryer—that's just money down the drain." He says if a dry ESP is employed to treat the gas stream at a near-dew-point condition, condensation is coating the machinery all the time. This leads to excessive corrosion and buildup. Also, a dry ESP must still contend with the larger combustible particles coming out of the dryer in some industrial arrangements. Given the high oxygen content of the dryer off-gases and the sparking characteristics of any ESP, Jaasund says, "It's a big invitation for a fire."

Condensing tarry materials and heavier solids from the drying process can also present problems downstream when a dry ESP is in play. The heavier materials will condense at relatively high temperatures and cause trouble in the RTO. "They buildup on the front of what's called the cold face of the heat-exchange media," he tells EPM.

Facing those three problems—increased potential for fire, condensation and condensible organics is "when you throw in the towel and make the whole system wet," Jaasund says. "You spray water in there and quench it down to the lowest possible temperature and you just deal with the goo."

This is how the wet ESP system Geoenergy installed at the Green Circle Bio Energy wood pellet plant works. Hot gas from the dryer enters the system. It's not saturated or at the dew point yet, so large quantities of recycled water are sprayed to quench the gas to its dew point between 140 and 170 degrees Fahrenheit, cooling it down but not losing energy. "This is an adiabatic process so we're just exchanging sensible heat—temperature—for latent heat, which is the evaporation of the energy tied up in evaporating water," Jaasund says. Below the ESP unit sits a pool of recycled water. A pump carries the water to spray nozzles for quenching and it drains back down into the pool. The heavier solids mixed in the spray water descend to the pool while other condensed solids make their way in through the fan-driven exhaust stream to the ESP system above. The particles moving upward accumulate inside the surface of the ESP's array of tubes.

Eventually, the material amassed on the ESP's charged surfaces tubes must be discharged, which occurs as infrequently as once every four hours or as often as every 90 minutes. The removal process lasts about a minute-and-a-half. The system uses hot water to flush the tubes free of particles. Geoenergy also mixes caustic soda in the ESP flush-water from above to help dissolve the material from the collecting surfaces. "All that stuff runs down off the tubes and then feeds into that recycle tank," Jaasund says. "So all solids from the quench step and the precipitation step end up in that recycled water," which must also be cleared to avoid clogging in the tank, pump or spray nozzles.

A decanter centrifuge is constantly treating a side stream of the recycled water, isolating and removing the "organic goo" while returning the centrates—the nonsolids—back to the tank. While the centrifuge removes the suspended solids, the plant needs to bleed off 1 or 2 gallons every minute to keep the dissolved solids equilibrium within the tank. Where that bleed stream goes is plant specific, but in the pellet mill's case it goes back into the dryers. "One might say, ‘Now you're just going to get it back,' but you don't because that stream is water with dissolved solids and the dryer will dry off the water but leave the solids in the dryer with the biomass—it actually goes out with the product," Jaasund says.

Downstream: RTOs and Beyond
The whole point of an RTO is to oxidize VOCs with heat as a nonregenerative thermal oxidizer does, but using less energy to do so. Through the utilization of heat-exchanging media the plant consumes less energy thereby dropping operational costs, but the RTO costs more than a thermal oxidizer does.

"The RTO is what I call a box of rocks," Renko says. "It's a heavy-metal structure filled with ceramic media and you have burners and fans and your typical electrical motor controls." Jaasund says that, as energy gets more precious and margins thin out, people will begin to look for ways to reduce costs. "The next logical thing is to make the RTO a catalytic system," he says. "The way an RTO becomes a catalytic RTO is by putting a layer of catalyst on top of media beds so now you've got 8 feet of stoneware media and 1 foot or 6 inches of catalytic media." A catalytic RTO can be made with with base metals, typically manganese dioxide, or noble (precious) metals such as platinum or palladium. "Either approach would allow the oxidization to occur at much lower temperatures," he continues. "So the combustion chamber is no longer really a combustion chamber because you can set your burner operation down from 1,600 to 800 degrees—the catalytic RTO consumes way less energy." Of course, the capital costs are higher for a catalyzing oxidizer than a noncatalytic one, but nevertheless Renko says his company is providing three catalytic RTOs to Pacific Ethanol Inc., for three ethanol plant projects.

The question is, with a wood-fired dryer, is catalytic oxidation feasible in the presence of even small amounts of sub-micron inorganic particulate? "We believe that the answer lies in the performance of the upstream wet ESP," Jaasund says. "While today's wet ESPs provide good RTO media protection they are not sized to clean the incoming gas to a level that will also protect the catalyst. However, they can be. The important technical hurdle is not whether we have the tools to operate catalytically but rather how to adapt them. This information will come as we go down the road so that when energy prices get too high, operators will be able to consider solid-fuel-fired-dryers with catalytic RTOs."

Ron Kotrba is an Ethanol Producer Magazine senior writer. Reach him at or (701) 738-4962.