Emissions Eliminator

Companies that embrace biomass-to-energy applications face stiff emissions and pollution control requirements. Biomass Magazine examines Eisenmann Corp.'s dual flow wet electrostatic precipitation technology and boiler emissions compliance system, which will be installed in a major bourbon distillery in Kentucky.
By Anna Austin
The emissions from a biomass boiler vary depending on the fuel source, whether it's wood waste, manure or coal. The composition of these emissions can be a deciding factor when a company purchases a system to mitigate various pollutants.

The electrostatic precipitator (ESP), first patented by chemistry professor Frederick Cottrell in 1907, is a particulate collection pollution control device that removes particles from a flowing gas, such as air, using the force of an induced electrostatic charge.

Typically electrostatic precipitation is a dry process, but spraying moisture such as water into the air flow containing the particles reduces the electrical resistance of the incoming dry material, making the process more effective. A wet electrostatic precipitator (WESP) combines the operational methods of a traditional wet scrubber with a dry ESP.

Eisenmann Corp.'s WESP-2F is designed exclusively for stringent multipollutant applications such as biomass boiler systems. The WESP-F2 will soon make its first commercial debut in a major bourbon distillery in Kentucky, which will be a showcase for demonstrating the technology, says Joseph Shulfer, P.E. engineering product manager for Eisenmann.. "The primary reason they chose our technology is it will allow them to change biomass sources without having to change the type of pollution control equipment they are planning to use," he says.

Inside a WESP
As one might imagine, the type of pollutants produced by a biomass plant differ depending on the technology and the biomass. Many plants use a variety of feedstock sources. "There is a very limited selection of multipollutant control equipment available-there are technologies that focus on specific pollutants, such as a flue gas desulfurization scrubber for sulfur dioxide control," Shulfer says. For example, Eisenmann, which is well-known in the biofuels industry, provides regenerative thermal oxidizers to treat emissions from drying distillers grains in ethanol plants. "These are designed exclusively for the abatement of carbon monoxide and VOCs (volatile organic compounds), much like our WESP-2F is designed specifically for the treatment of biomass boiler emissions," he says

Eisenmann's WESP-2F not only addresses particulate matter emissions, but also an array of other pollutants. "One thing that we've seen in the past year, is that with the increased costs of oil and natural gas, a lot of U.S. plants are looking for alternative ways to produce steam," Shulfer says. "One operational difference between a natural-gas-fired boiler and a biomass system is the increase in untreated emissions. This will result in the need for a more robust pollution-control system."

When operating, biomass boilers require combustion air, which is ultimately turned into flue gas and sent through the smokestack before being released into the atmosphere, but not before being treated by pollution control equipment.

In a wet pollution control system, the gas first travels through a quench duct which forces the temperature of the gas to drop from about 450 degrees Fahrenheit down to the saturation temperature, which may be as low as 160 degrees F, Shulfer explains. "This is done because in order for a WESP system or a scrubber to work, the gas must be saturated with water-a point at which a drop in temperature will result in condensation," he says.

As the quenched gas enters the WESP-2F system, it travels through an open spray tower scrubber where nitric oxide, which isn't soluble in water, soluble nitrogen dioxide, sulfur dioxide and hydrogen chloride are removed. "The spray tower removes these acid gases through the liquid-gas absorption as the flue gas travels in the vertical direction counter flow to the scrubber spray," Shulfer says. "It's like a rain shower inside. It models the same ‘scrubbing' the earth does after a hard rain, absorbing the pollutants and particulates."

After passing through some duct work, the gas reaches the first WESP field. Because particulate matter is a difficult pollutant to remove due to its size, the system must rely on forces such as an electric field to pull it out of the air stream. "Inside of the wet electrostatic precipitator there are multiple tubes which particulate laden air enters," Shulfer says. "The tubes are constructed with a rigid electrode placed in the center of the tube. These two components form the high-voltage electric field required for the removal of particulate matter."

The collector tube walls inside the WESP have opposite polarity of the electrodes, which when combined function as a large capacitator. "The electrical potential between them is kept as high as possible for maximum efficiency while controlling the spark rate for adequate ozone production-an oxidizing agent used in the reduction of NOx (nitrogen oxide) and mercury," Shulfer says.

Voltage is applied across the space between the electrode and the wall. As particles enter the system they become charged and are attracted to the wall. All of the particles and dust that accumulate on the wall have to be disposed of at some point, Shulfer says. "Dry electrostatic precipitators have rappers that pound on the wall to shake the collected dust into a hopper-a technique that can result in particulate re-entrainment," he says. "In a WESP, water is injected in the form of a super-fine mist that continuously cleans the collected particulate from the collector tube walls." Ash resistivity issues associated with dry ESPs are also eliminated because of the wet film slurry formed from the collected water and particulate.

In the last phase, the gas travels through an up-flow WESP field. Shulfer says this is done is to ensure the removal of mercury. Elemental mercury is one of the most difficult types of mercury to remove, he says. "In the first field, ozone is produced and the mercury is converted to mercuric oxide, which can be removed with the final WESP field because it takes on the form of a particulate," Shulfer says. Basically, the mercury is converted to a gas because of the specific temperature and then combines with the ozone and condenses to form a solid piece of mercuric oxide dust, which is attracted to the second WESP field, he says.

The final field also serves as a demisting device and removes most of the remaining droplets of water from the gas as it goes up and out through the smokestack.

Why Choose a WESP?
In selecting a WESP over a dry ESP, water consumption is something to consider. That can be determined by measuring the amount of water needed to quench the incoming flue gas, according to Shulfer. "We have new technology that can be easily implemented with this system that reduces the amount of water consumed by over 60 percent," he says. "It will also reduce the combined boilers and WESP blowdown to zero-liquid discharge."

The WESP-2F allows for the ability to switch to any fuel without worrying about additional emissions, Shulfer says.

To make the system easier to purchase, Eisenmann has partnered with a key biomass boiler manufacturer in the United States to offer package deals. The cost of a biomass-to-energy system can range from $5 million to $50 million, according to Shulfer. "Typically our system represents about 10 percent of the total capital investment," he says, adding that customers can expect a three- to five-year payback on the entire system based on fuel savings.

Overall, the WESP-2F is the result of much research and dedication. In support of developing the WESP-2F for the biomass boiler industry, Eisenmann desired to entirely understand the technology and its implications. "We invested a lot of time, including the development of a small-scale pilot unit," Shulfer says. "This was an ideal way to develop this technology. We have employees on our staff who have dedicated their entire careers to air pollution control technology and, specifically, electrostatic precipitation." BIO

Anna Austin is a Biomass Magazine staff writer. Reach her at aaustin@bbiinternational.com or (701) 738-4968.