Thermochemical potential of tall wheatgrass cultivated in heavy metal contaminated soils

Dedicated crops represent an important feedstock to decarbonise the energy sector and to meet the no net emissions of greenhouse gases by 2050. However, the greenhouse gas performance of biomass to energy can be negatively impacted by Indirect land use change (ILUC) effects. Consequently, cultivation of industrial crops on contaminated land is repeatedly suggested as an approach to minimize land use competition with food crops and land use change controversies. Therefore, this work aimed to study the effects of soils contaminated with heavy metals (namely Zn, Pb, Ni and Cd contaminated soils) on the thermochemical potential of tall wheatgrass, a perennial versatile crop, presenting bioenergy and phytoremediation potential. Results indicate that all contaminants, except cadmium, did not affect the yields, thus confirming the high tolerance of this plant to different stresses, including soils contaminated with heavy metals. Cadmium contamination reduced the yields by more than 25% (26%, low level of cadmium contamination, 4 mg/kg; 40%, high level of cadmium contamination, 8 mg/kg). Therefore, in those fields, a positive energy balance might not be achievable. Regarding the energetic potential of the biomass, results indicate that the contamination did not interfere with the HHV of tall wheatgrass, indicating that the biomass can be exploited for bioenergy. Considering biomass quality, results indicate that the ash and nitrogen content was similar for all the biomasses from control and contaminated soils. Understanding that tall wheatgrass ash and nitrogen content were not affected by contamination, which is a promising result, shows that for the tested contaminations, there will be no further load in ash residue and nitrogen emissions when using contaminated biomass in thermochemical purposes. The accumulation of potassium and heavy metals in the aerial biomass and its potential damage to pipes and furnaces was addressed also. The alkali index relates the amount of Na and K in the biomass per unit of energy with the probability of slagging and fouling formation through the thermochemical conversion of biomass. The calculated indexes for the crop, in control and contaminated pots, indicates that there is a high probability for the crop to cause slagging and fouling once results are higher than 0.34, even the biomass from control pots. An in depth analysis is being performed to certify these preliminary findings. Moreover, an initial analysis suggests that techniques such as fixed bed combustion heat (combustion), circulating fluidized bed for syngas production (gasification), and pyrolysis plus boiler for heat and steam; are good options to produce energy from tall wheatgrass cultivated in heavy metals contaminated soils. This choice reflects, their less restrictive usage requirements and the high level of potassium concentration in the biomass obtained in control and contaminated pots.