I lead the Green Process Engineering Group at the University of Trento, Italy. My current main research focus is on extraction and thermo-chemical conversion processes, in particular at high pressure (supercritical CO2 extraction, supercritical H2O gasification, hydrothermal carbonization) for the exploitation of organic waste and biomass: waste and biomass to energy and added-value compounds and materials. My research approach reflects my background and expertise, typical of the chemical and process engineering.
My research activity currently covers and covered different fields. During my my university final project (thesis) at the Imperial College of London (UK), I evaluated mechanical properties of polymeric materials at the nano-scale [A1].
During my PhD, I dealt with superficial characterization of liquid metals (Sn, Al, Si) interesting for the technological sector [A2, A3, A4, A5].
Nowadays, my research topics are in the fields of bioenergy, namely thermo-chemical conversion processes applied to biomass and organic waste, and food process engineering, as detailed below.
Biomass and waste to energy and materials/chemicals: thermo-chemical conversion processes
The research investigates the exploitation of biomass and waste for energy production through thermochemical conversion processes: traditional processes such as combustion, pyrolysis, air and steam gasification; and highly innovative processes such as gasification in supercritical water and hydrothermal carbonization. The activity encompasses both theoretical and experimental aspects.
In 2008 we developed a software tool capable to predict by-phase (gas-solid) equilibrium, heat of reactions and yields of the reaction products (syngas and char) of the traditional aforementioned thermo-chemical processes [A8].
In 2009 a bench-scale gasifier was realized and utilized for experimental runs with spruce pellets and sawdust [A11].
Grape marc, a residue of the vine-making process, was analysed in several aspects. In 2009 the substrate was tested in a bio-drying facility [A14]. In 2010, referring to the territory of the Autonomous Province of Trento as case-study, different grape marc energetic exploitation scenarios were analysed and modeled: a classic combustion process coupled to a steam cycle for cogeneration, an air gasification process and, finally, a steam gasification process with indirect heating [A15]. In 2012, pyrolysis tests of grape marc were performed and modeled [A20]: the analysis allowed a critical comparison of the most popular pyrolysis devolatilization models available in the literature.
Considering the knowledge gained in the field of supercritical technologies (see the section Food Engineering below) and energy processes based on thermochemical conversion, in 2010 I decided to initiate a new research line encompassing both these fields: supercritical water gasification (SCWG) of biomass. I personally believe that this process, highly innovative, has great potential.
Traditional gasification technologies have encountered a number of major difficulties hampering their development. First of all, the quality of the product gas is usually low, since it is contaminated by impurities like char and tar. Moreover, traditional gasification technologies require dry biomass, to avoid excessive drying costs. SCWG is a possible solution to these issues. Owing to the unique properties of water at supercritical state (i.e. temperature and pressure higher than 375 °C and 221 bar, respectively), the formation of tar and char is drastically reduced. Moreover, wet and low-quality biomass can be effectively converted into syngas, paving the way to the usage of large amounts of residual and waste materials. Actually, SCWG can be applied to both biomass with a high moisture content and waste waters having a significant content of organics dissolved: ideal candidates are sewage sludge, microalgae, high moisture agro-industrial waste, waste waters from paper mills.
There are several aspects of SCWG that are worthy of study and analysis. As far as theoretical aspects are concerned, we focused on thermodynamics [A18, D4, A38], reaction kinetics modeling [A22], and process design [A23]. Experimental activities were performed in a joint research with the Karlsruhe Institute of Technology (KIT) (Germany): the effect of the reactor material – stainless steel, inconel, ceramic – on the SCWG of biomass was investigated [A24, A51]; hydrochar was gasified in supercritical water [A29]; tests with a continuous SCWG reactor were performed to analyze the mutual effect of the main biomass constituents [A28, A33].
Since 2013, in parallel with the study of the gasification in supercritical water, I decided to start to work on the carbonization in subcritical water. This process, referred as hydrothermal carbonization (HTC), addresses exactly the same substrates as the SCWG: biomass and waste having a significant moisture content. HTC consists in treating the feedstock in a liquid water environment at moderate temperature (180-250 °C) and pressure (10-50 atmospheres) in order to transform it in a coal-like substrate. The substrate resulting from HTC, referred to as hydrochar, resembles – to some extent – the biochar from pyrolysis. HTC mimics the carbonization process that, in nature, transforms biological materials into coal during geological eras, with the difference that, under HTC conditions, the carbonization takes place in only a few hours. Hydrochar can find application is several sector:
- agronomy: as fertilizer and soil improver;
- bioenergy: as fuel vector (in the form of biochar pellets);
- raw material: biochar as adsorbent medium or as raw material for the production of activated carbon.
Given its quite mild operating conditions, the level of technology required for HTC is not very high: differently from SCWG, HTC is sufficiently simple to be implemented by SMEs.
Thus, in 2014 we designed and built in-house at UNITN a lab-scale HTC reactor (V: 50 mL) and preliminary experimental results were performed utilizing as feedstock vine-making residues [A27]. HTC became shortly thereafter my main research topic. We tested HTC of municipal solid waste – namely, off-specification compost [A35] and OFMSW [A58] – and agro and agro-industrial waste: grape marc [A43, A57] olive pomace, olive trimmings and olive mill waste water [A45, A49, A61] and cactus [A52]. Our experimental findings underlined the importance of (and the conditions for) the formation of secondary char, due to the re-condensation/polymerization in the liquid phase of molecules previously dissolved from the solid feedstock. A whole HTC process, from raw biomass to pelletized hydrochar, was designed, modeled and simulated, performing energy and cost analyses [A48]. The HTC reaction kinetics was investigated and modeled [A40, A60], demonstrating that the prevailing path is the solid-solid reaction converting the biomass in primary char, and that the secondary char formation becomes of relevance at temperature higher than 220 °C. In 2017, we designed and constructed in-house a bench-scale HTC reactor: 2 L in volume, design temperature and pressure equal to 300 °C and 140 bar, respectively [A59]. Benefitting from national and international collaborations, several research activities were performed. An integrated HTC-anaerobic digestion process was tested to valorize spent-coffee [A53]. Activated carbon was produced from grape seeds by a two-stage process: HTC followed by KOH activation [A54]. Activated carbons were also produced starting from a pyrolytic treatment of municipal solid waste [A56]. Hyperspectral imaging was tested as a quick alterative to determine the polysaccharide contents of biomass and hydrochars [A55].
At present, I have collaborations regarding HTC with the Boston University and the Cornell University (U.S.), the KIT and the Hohenheim University (Germany), the CNRS of Albi (France), the Aalto University (Finland), the Gadjah Mada University (Indonesia), plus several others in Italy.
Food Engineering: supercritical fluid extraction
Since my arrival in 2004, I started a new research line at the University of Trento intended at valorizing food-industry by-products, paying particular attention at the by-products of the wine-making industry. Actually, in Trentino wine and apples represent the key-products of the agro-industrial sector.
The focus is on the supercritical extraction: high pressure CO2 is utilized as solvent to extract from various substrates (grape seeds, various vegetable seeds, grape marc, officinal plants, fish powders, by-products from agro-industrial processes) valuable compounds for the food, cosmetic and pharmaceutical sector (grape seed oil, vegetable seed oil, natural antioxidants, omega-3 rich oil). The technology offers interesting advantages: CO2, differently from common solvents, is not toxic, not flammable and environmentally friendly. The drawbacks of the technology lay in the high pressure necessary for the extraction (hundreds of bar). This makes the cost for the supercritical extraction plant greater than the cost for standard extraction equipments, while the operating costs result, generally, similar. The supercritical extraction process is (rarely) industrially adopted for the better quality of the extracted compounds and, all in all, when the extracted substances have a significant added value. In Italy there is a single large industrial supercritical CO2 extraction plant (plant Lavazza for the decaffeination of coffee) plus many others at a smaller scale. In some other countries the technology finds a larger application.
At UniTN, I realized a new small laboratory – the “Biomass Laboratory” – to host the experimental research activities centered on Food Engineering (now the lab hosts also bioenergy equipment such as HTC reactors and others). Using the small loans gradually obtained, I acquired, next to typical lab equipment, a supercritical extractor that in time has been used and implemented for the different tests, then a Soxhlet extractor for using common organic solvents, a small mechanical extractor, a rotavapor and a gas chromatograph with flame ionization detector.
With regard to the experimental work, I dealt with the supercritical extraction of oil from grape seeds [A7, A13, A31, A34, A46], sunflower seeds [A12] and Jatropha seeds [A32], of polyphenolic compounds with antioxidant properties from grape seeds and skins [A13, A19, A46], of omega-3 rich oil from trout processing waste [A21]. By modifying the supercritical CO2 apparatus, extraction of polyphenols from grape skins and defatted grape seeds was performed using subcritical water [A37].
The experimental work is complemented by modeling activities of phenomena and processes. I think my most important scientific contribution to the sector is the development of theoretical tools to predict the course of a supercritical extraction and their critical evaluation. In 2007, I published [A6] concerning the effect of model uncertainties on predictions and, considering experimental tests by me and by other authors, I proposed a simple model capable to explain some discrepancies found in the literature and relevant to experimentally measured solubility value of oil in CO2 [A7]. In 2008, I questioned a simplifying hypothesis adopted by the models available in the literature [A9]: the implications of assuming all the particles to be extracted sharing the same “mean” dimension versus accounting for the real particle size distribution. In 2009, I proposed an innovative model based on the vegetable structure of the seeds [A10, A12], model that was carefully compared with others available in the literature, showing its greater potentialities [D3, A41]. In 2010, I published on supercritical extraction of seed oil at industrial-scale, designing the plant and the process for a case study and performing an economic feasibility analysis [A16]. The same year, I published on the effect of the solvent fluid-dynamics in a supercritical extractor [A17]. The effect of process parameters on the supercritical CO2 extraction kinetics of seed oil was deeply investigated with both experimental and modeling approaches [A34]. Both approaches were also used concerning the solubility of seed oil in supercritical CO2 [A44].
Together with a company located in Trentino dealing with trout-breeding, processing and selling, I evaluated opportunities for the exploitation of trout processing waste (viscera, bones, skins, heads) as a source of omega-3 rich oil and proteins. We started in 2011 analyzing the supercritical CO2 extraction of trout viscera, bones, skins, and the quality of the trout oil [A21]. In 2014, we designed a plant and performed an economic feasibility study relevant to the enrichment in omega-3 of trout oil derivatives. The enrichment processes was based on the utilization of a fractionating column with supercritical CO2 as fractionation medium [A26]. In 2017, the analysis was further extended with a global bio-refinery approach considering also the recovery step for oil and protein, and the ethanol oil transesterification step [A50].
In general, the focus is on the extraction/fractionation of bioactive substances, such as the omega-3 from fish oil [A21, A26, A50] and the antioxidants contained in grape marc and its constituents, seeds and skins [A13, A19, A31, A37, A39, A46].
Considering the expertise in the field, I was invited to write a couple of review articles: