Impact of aggregate moisture on dryer drum energy demand in asphalt mixture production: Field trial and model validation

Abstract

Drying and heating of mineral aggregates is the most energy-intensive process in asphalt mixture production, with aggregate moisture as a key parameter. This study investigates the impact of aggregate moisture on dryer drum energy consumption based on field trials conducted at a batch asphalt mixing plant. Two limestone aggregate fractions, exhibiting different moisture contents, were dried and heated under controlled operating conditions and characterized by laboratory analyses. Aggregate moisture ranged from 1.46 to 9.83 m%, with the 0/2 fraction exhibiting nearly twice the moisture content of the 0/16 fraction. This observation was corroborated by laboratory tests demonstrating that particle size fractions below 2 mm disproportionately increase water absorption with decreasing particle size. The specific energy demand of drying and heating aggregates ranged from 80.07 to 139.75 kWh/t. Assuming a linear relationship between relative energy demand and aggregate moisture content, an increase of 1 m% in moisture corresponds to an additional energy demand of 6.86 kWh/t of asphalt mixture. Based on a life cycle assessment (LCA) of an asphalt concrete (AC) base layer, this increase results in a global warming potential (GWP) of 1.87 kg CO₂-eq/t asphalt mixture when using natural gas and 2.52 kg CO₂-eq/t when using heating oil. From an economic perspective, a reduction of aggregate moisture by 1 m% yields potential cost savings ranging from 0.16 to 0.58 €/t of asphalt mixture. Comparison with a commonly applied thermodynamic model yielded comparable theoretical energy demand, underscoring the necessity of accounting for casing losses in the energy balance.