Guest Editorial Biomass Drying: Areas for Future R&D Needs and Sustainable Energy Development

The energy supply and demand data shows that the world annual consumption of energy was 447.1 quadrillion Btu in 2004 (1.31 10 kWh or 11.27 billion tons of oil equivalent). Eighty-six percent of this was from fossil fuels, including oil, gas, and coal, which produced 27.2 billion metric tons of carbon dioxide to the atmosphere. The heavy reliance on the fossil fuels has already caused serious consequences such as global warming and potential energy crisis. Therefore, finding alternative and sustainable energy resources has been a critical issue in the world to ensure the future energy supply and to reduce the greenhouse gas emissions. Biomass has been recognized as one of the most promising sources for the future fuels and the future energy. Biomass, in the forms of wood chips, sawdust, bagasse, grass, and agricultural residues, is generated from trees, agricultural crops, or purpose-planted coppices, which absorb carbon dioxide needed for photosynthesis during growing. In this way, the whole system, from feedstock growing through energy processing to energy consumption, is carbon neutral. In order to increase the energy efficiency, improve the energy quality, and reduce emissions in the energy conversion, drying of biomass to the required moisture content is important in the development of the alternative energy systems. In a boiler, the energy efficiency can be increased by 5–15% with the increased combustion temperature and the reduced flue gas temperature by using the dried instead of wet biomass. In gasification and pyrolysis, biomass needs to be dried to even lower moisture content, from 2 to 10%, to reduce the water content in the energy products. However, there are issues in biomass drying: energy efficiency, environmental impacts, risk of fire, and costs. Dryers for biomass drying can be rotary dryer, barndrying, packed moving bed dryer (conveyor dryer), and pneumatic dryer. Because biomass is combustible, with an auto-ignition temperature of 260–288 C, the risk of fire needs particular consideration. This potential risk could become a disaster when the moisture content of biomass is lower than a critical value and the drying temperature is high. In this regard, appropriate dryer control based on accurate prediction or detection of the moisture content and material temperature can optimize the drying conditions without triggering the fire risk. The mathematical modeling of the drying process can offer a powerful tool to predict the moisture content and material temperature with known drying conditions and material characteristics. The environmental impact with biomass drying is another issue that needs to be controlled carefully. The emission of resins and other organic acids can cause ‘‘blue haze,’’ a common phenomenon due to the condensation of these emissions. Emissions of nitrous oxide (NOx), formaldehyde (HCHO), and particulate matters can also damage the reputation of biomass. Therefore, understanding of the components and concentrations of the emission and the influence of the drying conditions are helpful to mitigate these environmental impacts. In the development of the biomass drying technology, multistage drying, exhaust air recycling, and heat recovery are being explored as well as the optimization of the drying conditions. More importantly, superheated steam drying has attracted great interests in order to prevent the risk of the fire and to increase the energy efficiency of the drying process. Considering the potential contribution of biomass to the future world energy supply and benefits with the improved drying, biomass drying will attract more interests both from the academic world and from industry in the coming years.