Solid Fuel Generation from Urban Leaf Litter in Mixture with Grass Cuttings: Chemical Composition, Energetic Characteristics, and Impact of Preprocessing

Abstract

Urban biomass from green areas is a potential resource for bioenergy recovery, which is widely unused. Different types of organic material (e.g., grass, leaf litter) usually occur in mixtures due to common collecting practice. Forty samples of grass, leaf litter (genera: Acer, Quercus, Tilia), and mixtures of both, containing one third grass or leaf litter, were investigated to evaluate the effect of the “Integrated Generation of Solid Fuel and Biogas from Biomass” (IFBB) on material and energy fluxes as well as relevant characteristics of resulting energy carriers. IFBB divides biomass into a fiber-rich press cake and a highly digestible press fluid by mashing with subsequent pressing. Ensiling of samples was successful with pH values ranging from 4.2 in grass to 4.8 in pure Tilia samples. Concentration of most minerals with exception of Ca and Mg were higher in grass than in leaf litter silage. The IFBB treatment reduced the element concentration in the press cake independently from the substrate. Linear regression models revealed high influence of the initial concentration in silage on the concentration in the press cake. The lower heating value of the press cake was nearly constant (19 MJ kg−1 DMash free) independent from mixture. Methane yields from press fluid digestion ranged from 172 (mean of leaf litter samples) to 325 lN kg−1 VS (mixture of 33 % leaf litter—66 % grass). For an evaluation of the economic and ecological potential, models of the spatial and temporal occurrence of these biomasses need to be established.