Investigation of Vacuum and Steam Treatments to Heat Treat and Sanitize Firewood-Grade Ash Logs and Ash Firewood

Abstract The goal of this project was to investigate the efficacy of vacuum–steam technology to sanitize low-quality ash logs and ash firewood. It is difficult to heat treat logs and firewood because of the relatively large cross-sectional dimension. Compared with hot air, steam has a greater heat capacity, and the condensation, without reducing the moisture content of wood, results in more efficient heat transfer. Also, the pressure gradient created by the vacuum accelerates heat transfer through the wood cross section. The vacuum–steam system consists of a vacuum source, a controlling device, a flexible container, and a steam generator. The white ash logs and firewood were harvested in Montgomery, Virginia. Ash log diameters ranged from 16.5 to 27.9 cm on the small end. The logs were cut into lengths of 1.82 m. After the vacuum was drawn to 300 or 500 mm Hg inside the container, steam was injected into the container. The steaming continued until 56°C was reached at the center of the lengths. The treatment time for all the logs varied from 5.5 to 14.5 hours, including a vacuum and a holding time of 30 minutes. The 1.82-m logs were cut into 40.6-cm-long bolts and then split into firewood, rarely larger than 15.2 cm on the wider side. The treatment time for firewood varied from 80 to 137 minutes, including a vacuum and a holding time at 56°C for 30 minutes at the core. There is no effect on quality, and the process can be tailored to different treatment capacities and is easily portable.