The Operational Objective 2 (OO-02) will consider first the possibility to perform an energy recovery from different sludge, produced by wastewater treatment plants of the tanning district, using the technique of Anaerobic Digestion. This choice is the preferred route for the energy recovery also from other types of organic wastes, selected in the OO-01 of the present project, both using such wastes alone either in mixture with the sludge above. The selected matrices will have to be pretreated and acclimatized to the digestion, these two phases will be object of study and experimentation in order to find the best compromise between performance and operating costs particularly with regard to anaerobic digestion products. Among the activities of the OO-02, in fact, in addition to the tuning of the desulfurization process of the biogas produced, the conditions of the anaerobic digestion process will be also analyzed in order to optimize the characteristics of the final digestate as a function of an additional energy recovery (gasification) and material recovery (OO-03). The best operating conditions for the anaerobic digestion will also be identified by chemometric techniques for the extraction of information from experimental data (dataset).
Two pilot mixed anaerobic reactors (5 m3, each), made of polypropylene (PP), will be placed at the Cuoiodepur and Aquarno plants in order to investigate the anaerobic process under different operating conditions to meet the specific needs of the digestion process and co-digestion. The reactors will be equipped to ensure proper operation, to monitor and control the operating parameters of the anaerobic digestion process. Two innovative processes will be investigated for the removal of hydrogen sulphide (H2S) present in the biogas and nitrogen in the anaerobic digestion supernatant:
Process 1 Removal of the two pollutants separately;
Process 2 Biological oxidation of nitrogen to nitrate and subsequent use of this product in a biotrickling filter for the oxidation of H2S in the biogas.
Lab-scale tests will be conducted in order to calibrate the mathematical models that will be used to select the best process solution suited to the characteristics of the waste to be treated.
The digestate produced in the pilot digesters will be filtrated and thermally dried up to a moisture content of 20-30 wt % before sending it to the gasification process to complete the energy recovery. The various types of dried digestate will be characterized also in terms of proximate/ultimate analyses and energy content (Low Heating Value).
Gasification tests will be performed on a fixed-bed gasifier of a pilot-scale plant operating at atmospheric pressure, provided by the partner ITALPROGETTI. The plant has a potential of about 5 kg/h of digestate with 20-30% moisture. This size is significant for subsequent scale-up to commercial size plant.
It mainly comprises a solid feeding system, an updraft/downdraft gasifier, and a gas cleaning-up system. The gasifier is a stainless steel cylindrical reactor with an internal diameter of 165 mm which can operate in updraft or downdraft configuration. The solid bed rests on a fixed grate equipped with an automatic ash removal system. The temperature profile inside the gasifier is measured by 8 Ktype thermocouples. The product gas flows through wet scrubber systems that work in series to remove dust and tars. The cleaned gas passes through a fan which is utilized to draw the gas from the gasifier and, after that, part of the cleaned gas is sent to the analysis section and the rest is burned in a flare. The main plant parameters were continuously monitored and controlled remotely through a touch screen control panel.
Tests will be performed continuously for a number of hours (about 8h) need to verify the stability of the process of gasification and were performed varying the following parameters functional/operational:
- Reactor configuration (updraft and downdraft)
- Digestate type
- Moisture content of digestate
- Digestate flow rate
- Air/digestate ratio to the gasifier
ITALPROGETTI will ensure the maintenance of the plant components. Structural and functional modifications to the plant will be made by ITALPROGETTI after consultation/discussion with the partner DICI.
For each feedstock used, the solid residues accumulated under the grate during each gasification test will be discharged, weighted and analyzed by the partners DICI and ARCHA in duplicate in terms of proximate/ultimate analysis, content of heavy metals and their leachability in order to identify potential reuses (cement / bricks, compost, fertilizers, etc.) and/or disposal methods in accordance with European regulations. The wastewaters discharged from the scrubber systems at the end of the test will be discharged and analyzed by DICI and ARCHA to identify the necessary treatments for their recycle in the syngas cleaning systems.
On the basis of experimental results obtained, the gasification performance will be evaluated in terms of the dry gas yield, lower heating value of the dry product gas (LHV), cold gas efficiency (CGE), feedstock carbon distribution in the different gasification products (gas, condensate and solid) and product yields.
Economic evaluations will be also carried out related to the integration between the energy consumption of thermal drying phase of the digestate, necessary for the gasification phase, and the same gasification, thus limiting the consumption of natural gas or other fossil fuel with consequent economic benefits.
The pilot anaerobic digesters, desulfurization and gasification processes will be designed on the basis of experience gained from laboratory tests through techniques of Experimental Design in order to carry out tests with the highest level of information.
The aim of the project is to develop an integrated strategy for energy recovery of these wastes in which:
- A first portion of energy, in the form of biogas, is recovered by anaerobic digestion;
- A second part of energy (from the bio-digestate) is recovered in form of gaseous mixture of CO, H2, CH4 by gasification, identifying an "optimum point" for which the conversion of the first process (anaerobic digestion) maximizes the energy efficiency of the entire process.
Further in order to obtain high energy recovery efficiency of the whole integrated process, the following aspects will be developed:
- Use of pre-treatments of the sludge to accelerate the subsequent anaerobic digestion;
- An innovative design of the anaerobic digesters according to the characteristics of the sludge to be treated;
- An innovative system of desulphurisation of the biogas which, at the same time, realizes the denitrification of supernatant liquid.