COMBINING BIO-ENERGY PRODUCTION AND PHYTOEXTRACTION: EXAMINATION OF THE FATE OF HEAVY METALS OF VARIOUS CONVERSION ROUTES

摘要

About 700 km2 in the Campine area, located in the north-east of Belgium and east of The Netherlands, are contaminated by historic deposition of heavy metals, in particular Cd, Pb and Zn, caused by the pyrometallurgic processes from the non-ferro industry. Phytoremediation is a technique that involves the use of plants and their associated microorganisms for the stabilisation, degradation and/or removal of pollutants from the environment. Within this domain, phytoextraction, i.e. involving plants for removal of the heavy metals from the soil, is favoured over conventional remediation techniques in view of the surface contaminated. The crops, yet identified as suitable plants for phytoremediation in view of the soil characteristics and the climate in this area, are short rotation coppices of willow (Salix spp.), energy maize (Zea mays spp.) and rapeseed (Brassica napus L.).This study compares different options for bio-energy production from these crops contaminated with heavy metals. The bio-energy conversion routes from these crops studied are (i) thermo-chemical treatment of willow, i.e. combustion, gasification or pyrolysis; (ii) anaerobic fermentation of maize, in combination with thermo-chemical treatment of the digestate; (iii) biodiesel production from rapeseed, in combination with anaerobic fermentation of the rapeseed press cake and thermo-chemical treatment of the rapeseed straw, and (iv) biodiesel production from rapeseed, in combination with thermo-chemical treatment of both the rapeseed press cake and the rapeseed straw. For each of the conversion routes, the fate of the heavy metals is examined by making mass and energy balances of the different processing steps. From this analysis, critical operational parameters, favouring the heavy metals to concentrate in the mineral fraction of the converted biomass, are determined. This study allows to compare the conversion routes studied in heavy metal accumulation potential as well as in bio-energy production potential.