Miscanthus for contaminated and marginal sites
In August, we completed the three-year MISCOMAR+ research project, financed by NCBR as part of the third call for proposals for the ERA-Net Cofund FACCE SURPLUS. Aberystwyth University (UK) was the coordinator of the European consortium, and the Institute for Ecology of Industrial Areas from Katowice was the coordinator of the Polish consortium. CBI Pro-Akademia was one of the partners working in the task dealing with the valorization of Miscanthus grown on marginal soils contaminated with heavy metals, primarily cadmium, lead and zinc.
As part of the project, we conducted a series of gasification tests at the Delft University of Technology, at the Process & Energy Department in a bubble fluidized bed with an innovative concept of indirect heat supply for gasification reactions. The aim of the tests was to determine which gasification products (gas, fly ash, char or maybe bed material) receive the above-mentioned heavy metals during the process and whether there is a relationship between the destination and the process parameters, primarily temperature. The tests were performed at three different bed temperatures: 700, 750 and 800°C.
The results of the study showed that the content of heavy metals in miscanthus grown on marginal land does not exclude valorization by gasification. During this process, heavy metals accumulate mainly on solids, i.e. char, bed material and (fly) ash. Trace amounts of heavy metals are also found in the gas, but their content will be reduced in the final gas treatment before the gas engine or similar application. One of the most important information obtained is the fact that the content of heavy metals in char produced during gasification is within the legal limits (standards) for fertilizers used in agriculture.
In addition to gasification tests, we also carried out simulations based on Gibbs free energy minimization to identify possible pathways of separation of heavy metals from char, as well as laboratory-scale char leaching tests. The simulation results showed that it would theoretically be possible to separate different elements, i.e. heavy metals and others, by using a dedicated removal sequence. For example, using (simulating) a leaching sequence with water, citric acid, acetic acid, concentrated hydrochloric acid and dilute hydrochloric acid, successively the separation of the ions of potassium, silicon and sodium, magnesium, cadmium, zinc, aluminum, calcium and phosphates, and finally lead could be achieved. The results of simulation and leaching tests obtained here should be treated as preliminary studies and further experimental work should be carried out in order to clarify and confirm them.
To get a picture of the recovery potential, we also developed an example scenario for a specific energy application using Miscanthus biomass. Examples include a decentralized installation producing electricity via the biomass gasification process and a cogeneration system with an electrical capacity of 1 MW, operating for 8,000 hours per year. The estimated area needed to cultivate Miscanthus would be 434 ha. The total metal recovery potential would range from 5.8 kg per year for cadmium to 1261 kg for zinc, which especially in the latter case is already not negligible. Fractionation and recovery of individual elements or their compounds is one of the main assumptions of the circular economy, and is one of the six "R's" of sustainable development: Recycle. Future studies should consider the environmental cost in terms of reagent consumption, energy and potential new waste, against the benefits of waste stream management, recovery of raw materials that would otherwise have to be extracted from the Earth, and even the potential for environmental clean-up, e.g. through crops specifically used for phytomining.