High-Quality Solid Fuel Production from Leaf Litter of Urban Street Trees

Frank Hensgen,Daniel Leon Gomez,Michael Wachendorf,Lutz Bühle,Nodirjon Nurmatov

doi:10.3390/su8121249

Abstract

Leaf litter is a growing concern for cities. Due to adherent dirt such biomass is rarely utilized nowadays but may constitute a renewable energy source for communities or private households. Leaf litter from main roads, residential areas and city centres collected by the vacuum technique or the sweeper technique was sampled and analysed for ash content and chemical composition. Ash content of leaf litter collected by the sweeping technique was higher (21.6%–40.1% dry matter, DM) than in material collected by the vacuum technique (12.0%–21.7% DM). Leaf litter from residential areas had the highest contamination (21.7%–40.1% DM), followed by main roads (20.8%–26.2% DM) and city centres (12.0%–21.6% DM). Ash content was reduced by up to 60% with a washing treatment and reached values comparable to those achieved by multiple manual rinsing, which was conducted as a reference treatment. The chemical composition of washed leaf litter could be further improved by mechanical dehydration treatment due to a reduction in harmful elements for combustion such as K, Cl, Mg and S. Heating value of leaf litter increased and the risk of corrosion and ash melting during combustion were reduced.

Highlights

Trees are an increasing burden to city administrations, which have to dispose the autumn foliage
This paper aims to determine the effects of (i) provenience; (ii) collecting techniques; (iii) washing and (iv) mechanical dehydration on the ash and mineral concentration and the solid fuel quality in leaf litter, which was collected by the cleansing department of the city of Kassel through one full leaf litter fall period
Material collected in the city centre showed lower overall ash content and lower concentrations of specific minerals such as Ca, Mg, Na and P than the material collected in residential areas, but higher Cl concentrations

Summary

Introduction

Trees are an increasing burden to city administrations, which have to dispose the autumn foliage. Though combustion of leaf litter may be an efficient conversion process, ash concentration is a known problem, whether it is natural or caused by contamination due to collection, transport or storage. In addition to ash problems, several elements incorporated in non-woody biomass have severe negative effects if applied in the combustion process. It is mainly the higher proportions of potassium and magnesium in non-woody biomass that lead to problems of ash melting at lower temperatures, slagging and fouling, whereas the higher proportions of nitrogen and sulphur can lead to higher emissions (NOx, SO2), and the higher proportions of chlorine, sulphur and potassium lead to corrosion [12]. The relatively high ash concentration reduces the heating value of leaf litter

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