WOOD PELLETS- A GROWING MARKETPLACE

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1 WOOD PELLETS- A GROWING MARKETPLACE The present drive for carbon neutral energy sources has given rise to increasing focus on biomass for energy. A big component of the world s biomass energy resource is wood, and pellets are a logical form. The United States has the potential to meet the growing demand for wood pellets. Because of our favorable climate and topography, growing enough trees is not a problem. Also, the pulp and paper and panelboard industries are mature industries with well-established markets and technology for the harvesting, transporting, and processing of wood. This situation makes the pellet production in the USA an attractive option. At least Green Circle Bioenergy, Inc. saw it that way. Starting in 2006, Green Circle began the process of planning to build a major pellet facility in order to meet the market demand for the green fuel in Europe. Because of the value of CO2 emission offset credits within the EU, green fuels like wood pellets command a market premium over traditional fuels such as coal. In early 2007, Green Circle began construction at their facility in Cottondale, Florida, fifty-five miles north of Panama City. As of this writing, construction is largely complete with startup scheduled for April, When the plant is up to full production, Green Circle anticipates an annual production of 550,000 tons, making it the largest pellet manufacturing plant in the world. All production will be shipped by sea 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 which can be integrated to work together. With regard to emission control, gases from the dryer and the heat energy system must be cleaned in order to meet local, state, and federal requirements for air emissions. Basically, this comes down to meeting the standards for emissions of volatile organic compounds (VOCs) and particulate matter. Included in these two categories are special categories of emissions known as hazardous air pollutants (HAPs) that generally have even more restrictive requirements for abatement. For example, formaldehyde in the gas stream is part of the general category of VOC and is considered a HAP by the US EPA. Similarly, manganese will be present as a particle and is also considered a HAP. 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 VOCs. This contaminated gas stream profile presents a complex emission control challenge. The cornerstones of the emission control systems that answers this challenge are two technologies designed for very different, yet synergistic duties. The first is the wet electrostatic precipitator (wet ESP) whose main function is to reduce the particulate matter in the gas stream to levels that are both acceptable for discharge to the atmosphere and suitable for treatment in downstream VOC control equipment. The second is the regenerative thermal oxidizer (RTO) whose main function is to destroy the VOCs with high temperature combustion.

2 PANELBOARD EXPERIENCE Since the early 1990s, RTOs have been employed for emission control from wood dryers employed in the manufacture of panelboard products such as plywood, particleboard, and OSB. RTOs are an effective, low-energy way of incinerating VOCs. As shown in the diagram to the right, VOC-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 very near the combustion chamber temperature. In the combustion chamber, a natural gas burner maintains the temperature to approximately 1,500 F, 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 RTO systems are approximately 75 F above the inlet temperature. Finally, the emission stream exits the RTO system through the outlet diverter valves and is transferred to the stack via the induced draft fan. After a prescribed period of time (2 to 6 minutes) the gas stream is reversed. This back-and-forth regenerative operation allows the RTO to recover up to 95%of the heat generated in the combustion chamber which greatly minimizes fuel costs. 2

3 RTO PROCESS Unfortunately, much of the early RTO experience in the panelboard industry has not been positive. Most wood dryers in this industry are directly heated with flue gas from the combustion of wood. The inorganic fly ash particles in this flue gas are comprised principally of oxides of sodium and potassium, compounds that proved to cause great harm to RTO internals. 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 RTO. To counter these problems, the panelboard industry employed wet ESPs as a solution for the collection of particulates. Since the 1980s, dozens of these units have been used at plywood, particleboard, MDF, and OSB plants and are presently considered the technology of choice for abating particulates in the dryer off gas streams. Fouled RTO Media support structure Fouled RTO Media (left) and new RTO media (right) 3

4 WET ESPS Wet ESPs 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 pre-clean the gas stream by scrubbing out coarse dust particles. After the spray cooling, 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 RTO for the destruction of VOCs and organic HAPs. The adaptation of the wet ESP/RTO technology combination has proven to be very successful in the panelboard industry. Many facilities now operate in complete compliance with restrictive VOC emission limitations without the requirement of costly down time to clean or replace RTO media. GREEN CIRCLE BIOENERGY For emission controls, Green Circle selected E-Tube Wet ESPs and GeoTherm RTOs 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 (~20 mg/nm3) and destroy the VOCs by 95%. Included in these design emissions levels is compliance with regulations for HAP emissions. 4

5 CONCLUSION In summary, treatment of these gases with the combined wet ESP/RTO system results in a gas stream that exceeds all modern standards for the emission of particulate matter, VOCs, and HAPs. 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. Geoenergy s Wet ESPs and RTOs during installation at Green Circle 5