Evaluating industrial drying of cellulosic feedstock for bioenergy: A systems approach

A large portion of herbaceous and woody biomass must be dried following harvest. Natural field drying is possible if the weather cooperates. Mechanical drying is a certain way of reducing the moisture content of biomass. This paper presents an engineering analysis applied to drying of 10 Mg h^-1 (exit mass flow) of biomass with an initial moisture content ranging from 25% to 70% (wet mass basis) down to 10% exit moisture content. The requirement for hog fuel to supply heat to the dryer increases from 0.5 dry Mg to 3.8 dry Mg h^-1 with the increased initial moisture of biomass. The capital cost for the entire drying system including equipment for biomass size reduction, pollution control, dryer, and biomass combustor sums up to more than $4.7 million. The operating cost (electricity, labor, repair, and maintenance) minus fuel cost for the dryer alone amount to 4.05 Mg^-1 of dried biomass. For 50% moisture content biomass, the cost of fuel to heat the drying air is $7.41/ dry ton of biomass for a total $11.46 per dry ton at 10% moisture content. The fuel cost ranges from a low of $2.21 to a high of $18.54 for a biomass at an initial moisture content of 25% to 75%, respectively. This wide range in fuel cost indicates the extreme sensitivity of the drying cost to initial moisture content of biomass and to ambient air humidity and temperature and highlights the significance of field drying for a cost effective drying operation.