Reduction of Organic Emissions by Using a Multistage Drying System for Wood-Based Biomasses

Abstract

Organic emissions during the thermal drying process are strongly dependent on the drying temperature. In the traditional single stage drying system, the inlet temperature of the drying air has to be relatively high in order to keep the airflow for drying small. In the multistage drying system, the drying airflow is heated up again after the first drying stage with higher moisture content, and then again after the second, and subsequent drying stages. In this method, the drying temperatures are limited in all stages to acceptable low levels, and only the moisture content of the drying air is increasing from one stage to another. As a result the multistage drying system has a lower drying temperature. We have studied the dependence of the organic emissions on the drying temperature, and present the results from drying units operating at temperatures of 100–200°C and below 100°C. The results are compared to previous measurements found in the literature. The estimates for the emissions at higher drying temperatures are derived from the literature values.

Keywords:

• Biomass drying
• Combined heat and power production
• Multistage drying system
• Organic emissions

Acknowledgments

This work was financially supported by Technology Development Centre in Finland (TEKES), Pohjolan Voima, Stora Enso, and Vapo.