Webinar focuses on air emissions from pelletizing wood

By Sue Retka Schill | May 21, 2014

Tackling emissions at pellet production facilities was the topic of the latest offering in the Biomass Magazine’s webinar series, held Tuesday, May 20.

That the largest contributor to emissions at pellet plants are the dryers themselves has been well understood, but in the past couple of years new large producers are learning about emissions in other parts of the process. In his presentation on post-dryer VOCs, Quentin Cannatella, senior consultant with Environmental Resources Management, shared the default wood pellet emission factors for softwood developed by the Georgia Department of Natural Resources. Volatile organic compounds (VOCs) from a dry hammermill are pegged at 2.5 pounds VOC per ton of softwood product. Pelletizer/pellet cooler emissions are set at 0.5 lb per ton without steam injection or extraction, and at 1.3 lb. VOC per ton with steam injection. There are also emissions to be considered with storage and handling, he added, which Georgia’s default values set at 0.4 lb VOC per ton. These are conservative estimates, he added, “but in the absence of site specific emission factors, this is a good starting point,” he said.

Pellet production facilities are not on the U.S. EPA’s “List of 28” major sources of emissions covered in the Prevention of Significant Deterioration (PSD) permitting program under the Clean Air Act, said Brad James, managing consultant with Trinity Consultants in his presentation of air permit requirements. Pellet producers need to keep the thresholds in mind, however. A plant will be considered a major source if it produces more than 250 tons per year of non greenhouse gas (GHG) PSD pollutants and more than 100,000 tons per years of GHG emissions. New construction or expansion planning needs to keep the thresholds in mind, and existing major sources doing upgrades may net-out of PSD review. One strategy being employed by producers learning they are out of compliance is to limit production to stay within the annual thresholds while working out plant design changes or installing control measures.

Understanding where VOCs are formed is key to designing systems to minimize or control them, Justin Price, principle at Evergreen Engineering, said in his webinar presentation. Knowing that VOC development is greatest at 160 degrees, one strategy is to optimize dryer operations so the woody material doesn’t reach that temperature until the very end of the drying process. He added that variations in material size and moisture content also impacts VOC development. More work needs to be done on other processes in pelletizing where the wood temperatures rise, including at the hammermills and pelletizing line. “There’s not a lot of research on the pellet mill process,” he said.

Frank DeSantis, vice president of engineering at Nestec Inc., closed out the webinar with a discussion of how regenerative oxidizers work, and the difference between regenerative thermal oxidizes and regenerative catalytic oxidizers.

The free webinar, sponsored by Megtec Inc., was recorded and will be available on the Biomass Magazine website within a few days after the event.