SET-Laub: utilization of leaves by combustion

Municipalities should be able to produce a fuel for their own energy recovery or for marketing to larger biomass furnaces. The SET-Laub project sets the framework for a recycling cycle of leaves within municipal and regional boundaries. In this context, the storage of leaf fuel over an extended period of time should enable cost-effective, year-round heat supply.

Leaves are collected at the muncipal level in the course of street cleaning, park maintenance or organic waste collection and subsequently composted. Due to the sometimes poor rotting properties and at the same time relevant heating values, the energetic use of leaves is particularly interesting for municipalities. But what influence does leaf (co-)combustion have on the resulting emissions, and how should a leaf fuel be provided? In the SET-Laub project, the use of leaves for heat provision is analyzed energetically, economically and ecologically.

Fraunhofer UMSICHT compares biomass potential, energy and material balance of the conversion process, economic efficiency and environmental effects to the classic fuel wood (as wood chips). The existing leaf potentials and the supply chains are examined to ensure a high transferability.

Leaf combustion

Various mixture proportions of different foliage fractions were thermally tested with a conventional woody fuel in an incinerator (400 kW). During combustion, the Fraunhofer researchers quantified possible environmental impacts (dust, NOx, CO, organic and inorganic substances) in order to assess their use in combustion plants. Also investigated was the influence of collection location (street leaves, park maintenance, etc.), exposure duration, and storage type (ensiling and balling, briquettes) on the emission behavior of the potential fuel.

In the fall of 2016, a total of 25 big bags of leaves were collected from different locations as part of a muncipal leaf collection to analyze the leaf samples with respect to combustion-relevant parameters (water content, ash content, volatile content, elemental analysis, heating value/calorific value, heavy metals, ash melting point). Depending on the fraction, ash contents between 17 and 44 percent were determined. A sieving at 4 mm mesh size showed a quite good sievability of the ash fractions. Overall, the leaf briquettes showed good combustion properties. With the exception of the total dust parameter, the emission values were below the TA Luft limits. Although the NOx concentrations were twice to three times higher for the hardwood fuels than for the wood fuel, they were still below the corresponding limit value. Other emissions (dioxins and furans, BaP, CO, total C) were also unremarkable.