Since 1994 Regenerative Thermal Oxidizers (RTOs) have been applied to direct wood-fired OSB dryers for the purpose of VOC abatement. In early direct wood-fired OSB dryer applications many OSB producers felt that upstream particulate control was not necessary. However, the ash from the combustion of wood waste created a host of new operational problems for the RTO systems. This paper provides assembled historical particulate data and the correlation of these data with heat exchange media life in a downstream RTO. Only RTO units with random ceramic saddles were evaluated. The resulting trends showed that solid particulate levels in the range of 0.005 gr/sdcf (11.5 mg/Nm3) were necessary for a ceramic bed life of five years or more. The most important recommendation is to insure that the upstream particulate control technology be properly maintained for maximum particulate collection in order to minimize operational problems with downstream RTO systems.
Prior to 1994, RTO technology had not been applied to direct wood-fired OSB dryers for VOC control. The direct-fired dryer was an entirely new application that posed new problems for RTO technology. The most costly problems were associated with the solid particulate emissions from the combustion and drying processes. In the early period of this application, there was no historical information available to help the OSB producer or the RTO supplier decide how much upstream particulate removal is necessary to allow trouble free operation and a long service life for the RTO. Today, approximately six years later, there is information available to aid in this decision.
This paper will discuss the nature of the problem, the mechanisms involved, and the potential solutions. In addition, historical data for RTOs using random packed ceramic saddles as heat exchange media will be presented to illustrate trends between inlet particulate levels and the longevity of ceramic heat exchange media.
Particulate Emissions From a Direct Wood-Fired OSB Dryer
Particulate emissions from a wood dryer are measured using US EPA Method 5/202. This method segregates the emissions into two categories: 1) filterable and 2) condensable particulate. The condensable, or back half, particulate is comprised of organic vapors that will condense at temperatures down to 68°F (20°C) using Method 202 (1). The analytical methods may vary somewhat from state to state. Regardless of any differences in analytical methods, these condensable particulate emissions are of little concern to the downstream RTO. This is because they are either organic compounds in the vapor phase or condensed organic vapors depending on the gas stream temperature entering the RTO. These organic vapors are oxidized within the RTO. They do, in fact, contribute as fuel to the process gas stream 'solvent load' entering the RTO thus reducing natural gas consumption.
The level of filterable or front half particulate load, on the other hand, is a very important factor in the operation of an RTO. Filterable particulate is that which is collected on a glass fiber filter held in a filter holder heated to a temperature of 248 +/- 25°F (120 +/- 14°C). The glass fiber filter is 99.95% efficient on 0.3 micron dioctyl phthalate smoke particlesl. Thus, for practical purposes the filter captures all of the solid particulate.
The filterable particulate from a direct-fired OSB dryer has a particle size distribution that is typically bi-modal. There is a component of large particles and a component of very small particles. The larger particles are either those produced in the rotary drum dryer by mechanical action or large fly ash particles from combustion that are not collected in the cyclone or multi-cyclone collectors. The efficiency of these devices determines the magnitude of the large particle component.
The small particle component is primarily composed of oxides of commonly occurring metallic elements such sodium, potassium, magnesium, calcium, and silica with smaller amounts of aluminum, iron, and zinc. These oxides are formed in the combustion process. At flame temperature they are vaporized into a gas then rapidly condense into a sub-micron particle or fume. Due to their small size, they easily penetrate the rotary drum dryer, the product recovery cyclone, and the multi-cyclone (if there is a multi-cyclone in the system). Unless they are removed by an additional particulate collection device, these oxides of alkaline earth metals will enter the RTO.