Controlling Emissions in a Growing Wood Pellet Marketplace

A large wood pellet plant looked to experience in the panelboard industry for a proven approach to controlling its emissions. A reliable technology provided the answer for its low emissions goal.
By Steve Jaasund
The drive for carbon-neutral energy sources has given rise to increasing focus on biomass for energy. A large component of the world's biomass energy resource is wood, and pellets are a logical form to provide heat and power.

The U.S. has a great potential to meet the growing demand for wood pellets. Because of its favorable climate and topography, growing enough trees is not a problem. Also, both the pulp and paper and panelboard industries are mature with well-established markets and technology for the harvesting, transportation and processing of wood. This situation makes pellet production in the U.S. an attractive option.

That's how Green Circle Bioenergy LLC saw it. Beginning in 2006, Green Circle began the process of planning a major pellet facility in order to meet the market demand for green fuel in Europe. Because of the value of carbon dioxide emission offset credits within the European Economic Community, green fuels such as wood pellets command a market premium over traditional fuels such as coal.

In early 2007, Green Circle began construction on its facility in Cottondale, Fla., approximately 25 miles north of Panama City. Start-up occurred in the spring of 2008. At full production, Green Circle produces 550,000 tons of pellets per year, making it one of the largest pellet manufacturing plants in the world. Most of the pellets are shipped to Europe for use in power-generating boilers.

Pellet Manufacturing Emission Challenges
The green chip drying and handling area of a wood pellet production facility includes a complex array of machinery and choices that can be integrated to work together. For emissions control, gases from the dryer and the heat energy system must be cleaned in order to meet local, state and federal requirements. Basically, this comes down to meeting the standards for emissions of volatile organic compounds and particulate matter. Included in these two categories are special categories of emissions known as hazardous air pollutants that generally have even more restrictive requirements for abatement. For example, formaldehyde in the gas stream is part of the general category of volatile organic compounds and is considered a hazardous air pollutant by the U.S. EPA. Similarly, manganese will be present as a particulate and is also considered a hazardous air pollutant.

The drying process described above creates significant quantities of all these pollutants. More specifically, the combustion of wood and the subsequent intimate contact of the hot flue gases with green wood chips for drying results in an emission profile that has three main categories of particles: inorganic fly ash from combustion, organic condensibles from the green wood chips, and coarse wood particles from the tumbling action of the dryer. Each of these particles must be abated in a single piece of equipment before the gas stream is treated for the volatile organic compounds. This contaminated gas stream profile presents a complex emission control challenge.

The cornerstones of the emission control system that answers this challenge are two technologies designed for different, yet synergistic, duties. The first is the wet electrostatic precipitator, whose main function is to reduce the concentration of particulate matter in the gas stream to levels that are both acceptable for discharge to the atmosphere and suitable for treatment in downstream volatile organic compound control equipment.

The second technology is the regenerative thermal oxidizer, whose main function is to destroy the volatile organic compounds with high temperature combustion.

Panelboard Industry Experience
Since the early 1990s, regenerative thermal oxidizers have been employed for emission control from wood dryers used in the manufacture of panelboard products such as plywood, particleboard and oriented strand board. Regenerative thermal oxidizers effectively incinerate volatile organic compounds using a minimum of energy.

Volatile organic compound-laden gas is routed into a heat recovery chamber that is filled with ceramic media. By passing through the inlet heat recovery chamber, the emission stream is preheated to a temperature near that of the combustion chamber, in which a natural gas burner maintains the temperature to approximately 1,500 degrees Fahrenheit (the temperature required for complete thermal oxidation).

Upon exiting the combustion chamber, the emission stream enters the outlet heat recovery chamber. The gas stream passes through the outlet heat transfer media bed where the heat energy gained from the inlet heat recovery chambers and combustion chamber is transferred to the ceramic heat exchange media (heat sink). This is the final step in the regenerative process. Typical discharge temperatures from regenerative thermal oxidizer systems are approximately 75 degrees Fahrenheit above the inlet temperature. Finally, the emission stream exits the regenerative thermal oxidizer system through the outlet diverter valves and is transferred to the stack via the induced draft fan.

After a prescribed period of time (typically two to six minutes) the gas stream is reversed. This back-and-forth, regenerative operation allows the regenerative thermal oxidizer to recover up to 95 percent of the heat generated in the combustion chamber to greatly minimize fuel costs.

Unfortunately, much of the early regenerative thermal oxidizer experience in the panelboard industry was not good. Most wood dryers in this industry are directly heated with flue gas from the combustion of wood. The inorganic fly ash particles in the flue gas are comprised principally of oxides of sodium and potassium, compounds that proved to cause great harm to the internal components of regenerative thermal oxidizer. In addition, other particulate contaminants such as condensed tar from the drying process were also found to be troublesome because they tend to build up and foul the regenerative thermal oxidizer.

To counter these problems the panelboard industry employed wet electrostatic precipitators as a solution for the collection of particulates. Since the 1980s, dozens of these units have been used at plywood, particleboard, medium density fiberboard and oriented strand board plants and are presently considered the technology of choice for abating particulates in the dryer off gas streams.

Wet electrostatic precipitators work by first cooling the hot gas stream with water sprays. This step serves two functions. First, it cools the gas to the lowest practical temperature to help condense high molecular weight organic compounds, turning organic vapors into liquid particles. It also serves to preclean the gas stream by scrubbing out coarse dust particles. The next step is treatment in an electrostatic precipitator for the collection of the condensed organic particles and the remaining fine, inorganic, fly ash particulate.

The final step is the removal of the collected particulate matter from the system.

Once the gas stream has been cleaned of most of the organic and inorganic particulate matter it can be treated in the regenerative thermal oxidizer for the destruction of volatile organic compounds and organic hazardous air pollutants.

The adaptation of the wet electrostatic precipitator/regenerative thermal oxidizer technology combination has proven to be successful in the panelboard industry. Many facilities now operate in complete compliance with restrictive volatile organic compound emission limitations without the requirement for costly down time to clean or replace regenerative thermal oxidizer media.

Green Circle's Green Decision
For emission controls Green Circle selected E-Tube wet electrostatic precipitators and GeoTherm regenerative thermal oxidizers supplied by the Geoenergy Division of A.H. Lundberg Associates. The combination emission control system at the Green Circle plant is designed to reduce particulate emissions to less than 0.01 grains/scfd (approximately 20 milligrams per Nm3) and destroy the volatile organic compounds by 95 percent. Included in these design emissions levels is compliance with regulations for hazardous air pollutants emissions.

In summary, treatment of these gases with the combined wet electrostatic precipitator/regenerative thermal oxidizer system results in a gas stream that exceeds all modern standards for the emission of particulate matter volatile organic compounds and hazardous air pollutants. In addition, energy consumption is minimized and operational reliability is assured through the use of technology that has been demonstrated in similar wood drying panelboard installations.

Steve Jaasund is manager of the Geoenergy Division of A.H. Lundberg Associates Inc. Reach him at [email protected] or (425) 283-5070.