Watering Down the Problem-Literally

Burning solid fuels to heat biomass dryers produces two broad types of emissions: fly ash and volatile organic compounds (VOC). Before the dryer exhaust stream is introduced to the regenerative thermal oxidizer for VOC destruction, fly ash must be abated to avoid irreparable damage downstream. Biomass Magazine details an increasingly popular and effective way for biomass processors to accomplish this.
By Ron Kotrba
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 contains up to 50 percent water by weight and needs to 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. The combustion of wood produces fly ash subject to control by environmental regulations or the plant itself, especially if costly emissions controls are jeopardized by particles downstream. But the amount of wood a plant handles will determine in large part the extent to which its emissions are regulated, and the throughput of most wood-pellet mills has been relatively low so far. These plants produce 30,000 to 50,000 tons of wood pellets a year, helping fuel a wood-stove renaissance in residential heating.

Companies are now taking this to another level. Utility-sized pellet plants have begun to emerge. Case in point: The 500,000-ton per year Green Circle Bio Energy Inc. facility in Cottondale, Fla. (see November 2007 Biomass Magazine). "In addition to the Green Circle Bio Energy plant, there are four or five more U-sized pellet mills under development," says Ron Renko, regional sales manager with Geoenergy, a division of A.H. Lundberg Associates. Geoenergy is the emissions-controls provider for the Cottondale pellet plant. "We're seeing a lot of activity in this direction and, frankly, it's all offshore money," he says. "Green Circle is from Sweden, and we're tracking other jobs with financing from Germany. These newer facilities are geared for utility boiler fuel augmentation in Europe." With European directives in place, U-scale pellet mills are being developed to serve these renewable energy demands; and project development activity is expected to rise sharply when U.S. power companies are eventually persuaded by carrot or stick to follow suit. For those investigating such a U-sized biomass endeavor, it is important to learn what state-of-the-art in emissions abatement is today-and tomorrow.

The electrostatic precipitator or ESP is not as sexy as extrasensory perception, but a sixth sense will not protect a regenerative thermal oxidizer (RTO) from fouling. A wet ESP, however, can capture fly ash to protect important downstream equipment. In the wood products industry, hot gas from wood combustion is usually routed to the dryer for heat. "Burning wood or biomass gives rise to inorganic particulate matter or fly ash, so right off the bat your dryer is going to have fly ash," says Steve Jaasund, professional engineer and manager of Geoenergy. The wood is dried aggressively with heat-an environment conducive to the formation ofvolatile organic compounds (VOCs). Therefore carry-over fly ash from combustion and VOCs born in the dryer constitute the two major categories of emissions from dryers heated by wood. "The best and most efficient way to destroy VOCs is with a regenerative thermal oxidizer," Jaasund says. But exhaust from the dryer cannot go straight into the RTO for VOC destruction because of the fly ash particles present in the stream. In most cases, the fly ash from wood combustion is high in alkaline earth metals like sodium, potassium, magnesium and calcium, which Jaasund says are aggressive against the heat-exchange media in the RTO. Gerry Graham from PPC Industries agrees. In a technical paper explaining various controls for stack emissions, he states, "The wet precipitator may be a necessary pretreatment item for … [downstream] systems which do not handle particulate emissions very well."

The point of an ESP system-wet or dry-is to effectively isolate and trap particles in a hassle-free system that "cleans itself" as needed, and protects the RTO from those abated particles. "The gas goes into the electrostatic precipitator and it passes adjacent to a high-voltage discharge electrode, which charges all the particles," Jaasund tells Biomass Magazine. 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 (wood). "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." Essentially it 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 VOCs will be thermally oxidized and released.

Make it Wet
While Graham notes wet ESPs have found "renewed interest from OSB (oriented strand board), particle board, and plywood veneer manufacturers for controlling dryer exhaust," he also says dry ESPs are still considered the best available control technology for wood-fired boilers. Jaasund gives three compelling reasons why a wet ESP is recommended for U-scale pellet mills. "Biomass dryers typically operate near the dew point of the gas stream," he says. "In other words, 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." Jaasund 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 build-up on the equipment. Also, a dry ESP must still contend with the larger combustible particles coming out of the dryer. Given the high oxygen content of the dryer off-gases and the sparking characteristics of any ESP "it's a big invitation for a fire," Jaasund adds.

Condensing tar-like 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 build up on the front of what's called the cold face of the heat-exchange media," he explains. Facing those three problems-fire, condensation and condensible organics-"that's 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."

Here's how the wet ESP system Geoenergy installed at GCBE 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, thus 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 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 little as once every four hours or as often as every 90 minutes. The removal process lasts about a minute-and-a-half. From above the system hot water flushes the tubes free of particles. Geoenergy also mixes caustic soda in the ESP flush water 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 a gallon or two per minute to keep a dissolved-solids equilibrium in the tank. Where that bleed-off stream goes is plant specific, but in GCBE'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.

A wet ESP is not the only method to control particulates. PPC Industries' Graham covers the competition. First there is the dry ESP, the challenges to using a dry ESP in applications such as U-sized pellet mills have already been discussed. What's called wet scrubbers work, but Graham says they increase labor and operational costs. "The energy necessary to separate the particulate from the gas stream can require 15 to 20 inches of WC (water column) pressure drop through a typical venturi. These are huge and wasteful power consumers, increasing the plant's overall operating cost." Baghouses are another common particle containment device. However, "The high temperatures and periodic cinders from the plant boiler can cause fire problems with baghouses," Graham writes. "Periodic bag replacement is a definite operating cost consideration." Geoenergy's Renko says there are many reasons baghouses reside at the bottom of the list for particle collection. "One big one is fire," he says. Other concerns with baghouses are condensation and tars plugging the filter media.

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 TO 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 as energy gets more precious and margins thin as more U-sized pellet mills come on line, people will begin to look for ways to reduce costs. "The next logical thing is to make an RTO a catalytic system," he says. "The way an RTO becomes a catalytic RTO is you put 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 using 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 degrees to 800 degrees-the catalytic RTO consumes way less energy." However, the capital costs are higher for a catalyzing oxidizer.

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 future energy prices get too high, operators will be able to consider solid-fuel-fired dryers with catalytic RTOs." BIO

Ron Kotrba is a Biomass Magazine senior writer. Reach him at rkotrba@bbibiofuels.com or (701) 738-4962.