Παραγωγή Βιοαερίου από Αναερόβια Χώνευση Απορριμμάτων

Abstract

Η αναερόβια χώνευση είναι μια διεργασία που χρησιμοποιείται ευρέως για την επεξεργασία ιλύος σε εγκαταστάσεις επεξεργασίας λυμάτων, καθώς και για την επεξεργασία ζωϊκών αποβλήτων, γεωργικών υπολειμμάτων και ενεργειακών καλλιεργειών κυρίως σε χώρες της κεντρικής και βόρειας Ευρώπης.

Στην παρούσα μεταπτυχιακή εργασία, έγινε αρχικά μια βιβλιογραφική αναζήτηση και συγκέντρωση πληροφοριών σχετικά με τη χρήση και τις προοπτικές της αναερόβιας χώνευσης ως μεθόδου επεξεργασίας των αστικών στερεών απορριμμάτων, που αφορούσε τις θεωρητικές, περιβαλλοντικές, τεχνικο-οικονομικές και τεχνολογικές πτυχές της. Συγκεντρώθηκαν πληροφορίες που αφορούν τα βιολογικά στάδια της αναερόβιας χώνευσης και τους παράγοντες που τα επηρεάζουν, όπως επίσης και το πώς αυτά λαμβάνονται υπόψη στις εγκαταστάσεις επεξεργασίας. Συγκεντρώθηκαν ακόμη πληροφορίες που αφορούν τα προτεινόμενα υποστρώματα για χρήση, τα οφέλη και τα μειονεκτήματα της διαδικασίας για την επεξεργασία των απορριμμάτων, καθώς και τις εμπορικές τεχνολογίες που έχουν αναπτυχθεί για την κατασκευή τέτοιων μονάδων.

Στη συνέχεια πραγματοποιήθηκαν πέντε πειραματικοί κύκλοι αναερόβιας χώνευσης σε κλίμακα εργαστηρίου, με στόχο την εκτίμηση της έκλυσης βιοαερίου από μηχανικά διαχωρισμένο οργανικό κλάσμα απορριμμάτων. Η πειραματική μέθοδος που χρησιμοποιήθηκε βασίστηκε στη χρήση του συστήματος OxiTop, το οποίο καθιστά δυνατή τη μέτρηση της πίεσης του βιοαερίου σε αεροστεγώς κλεισμένα μπουκάλια χώνευσης. Οι φορτίσεις των δειγμάτων που χρησιμοποιήθηκαν ήταν 1gVS/L, 2gVS/L, 5gVS/L και 10gVS/L. Τα αποτελέσματα των μετρήσεων που προέκυψαν χρησιμοποιήθηκαν για τον προσδιορισμό θεωρητικών καμπυλών μέσω της απλής εκθετικής εξίσωσης και της εξίσωσης Gompertz, οι οποίες περιγράφουν τη μεταβολή του όγκου του βιοαερίου σε κάθε δείγμα, και με βάση αυτές έγιναν οι υπολογισμοί της έκλυσης βιοαερίου.

Τέλος, με τη βοήθεια υπολογιστικού προγράμματος και κάνοντας χρήση των πειραματικών αποτελεσμάτων και τιμών από τη βιβλιογραφία, έγινε μια μικρή τεχνικοοικονομική μελέτη μιας μονάδας αναερόβιας χώνευσης απορριμμάτων και ιλύος. Παράλληλα, εξετάστηκε η χρήση γρασιδιού σαν επιπλέον υπόστρωμα στην προσπάθεια αύξησης της απόδοσης της εγκατάστασης, και διερευνήθηκε η βιωσιμότητα και η αυτοδυναμία της.

Anaerobic digestion is a biological conversion process, where biomass is ultimately converted to biogas, in the absence of oxidizing factors. This process is widely used for sludge treatment at wastewater treatment plants, while treatment of animal manure, agricultural residues and energy crops is also practiced mostly in central and northern Europe. This postgraduate thesis contains at first a detailed bibliographic survey of the anaerobic biological process, along with its application on municipal solid waste treatment. Following the theoretical data, five lab-scale sessions of experiments took place in order to estimate the biogas yield of mechanically-sorted organic fraction of municipal solid waste. Finally, using an excel-based computer program along with experimental values and values from the literature, a preliminary techno-economic study of a treatment plant was investigated in terms of technical and economic feasibility.

Literature review of anaerobic digestion

As stated in the introduction, anaerobic digestion is a biological process which entails the conversion of biodegradable organic matter -through a series of stages- to biogas, with the absence of oxygen. The decomposition of organic matter occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Anaerobic bacteria break down complex organic matter into monomers, which are then converted into intermediate byproducts to ultimately be converted to methane and carbon dioxide, the basic components of biogas. The major factors affecting anaerobic bacteria and therefore the stages of the process are temperature, pH, concentration of volatile fatty acids (VFA) and ammonia, nutrients and toxic substances. Suitable feedstock substrates for this procedure include animal manure, agricultural residues, energy crops, sewage sludge and the organic fraction of municipal solid waste. Finally, the two products of anaerobic digestion, biogas and digestate, can both be exploited in order to be profitable. Biogas can be used for energy production through CHP engines, and digestate can be processed and used as soil conditioner for agricultural purposes, depending on quality standards.

Investigation of municipal solid waste (MSW) as feedstock

MSW has been identified as a heterogeneous material of which the composition varies widely. The most common MSW treatment methods to date include landfill, incineration, composting and recycling. Anaerobic digestion of MSW deals with only one fraction of the waste, the organic fraction. The quality of this fraction is influenced not only by the sorting system but also by various methods used for its separation. The advantages of AD treatment include mostly energy and environmental benefits, while drawbacks include mostly cost issues and the fact that it’s a fraction treatment only, instead of a whole waste treatment. The decision to implement AD of MSW rather than alternative technologies depends on a number of factors which should be taken into account beforehand. Anaerobic digestion plants classification can be based on temperature of operation (“mesophilic” or “thermophilic”), total solids concentration in the digester (“wet” or “dry”) and one stage or multi-stage depending on the number of digesters used. The experimental process and sessions A major part of this postgraduate thesis includes five lab-scale experimental sessions in order to estimate the biogas yield of mechanically-sorted organic fraction of MSW. In order to achieve this, batch tests were conducted using the OxiTop® system for biogas pressure measurement. Digested sewage sludge was used as inoculum in the samples, which were fed with pre-digested sludge or organic fraction of MSW. Organic load ranged from 1 gVS/L to 10 gVS/L. Blank samples containing only digested sludge were used in every session. All samples were held in mesophilic conditions with the aid of a water-bath lab device. The first experimental session included pre-digested sewage sludge as feedstock at 2 gVS/L. During this session, the biogas methane percentage was also estimated with the help of a gas chromatograph apparatus. The second and third experimental sessions included both sludge-fed and organic fraction-fed samples with loadings of 2 gVS/L and 1 gVS/L respectively. Finally, the fourth and fifth experimental sessions included only organic fraction-fed samples at 1 gVS/L, 2 gVS/L, 5gVS/L and 10 gVS/L respectively.

Experimental results and mathematical modeling

Using the OxiTop® system measurements for each sample, biogas pressure graphs were created. In order to estimate the theoretical biogas yield, a mathematical model was necessary based on curve-fitting on the measurement graphs. The equations used in order to achieve this are the substrate-limited exponential equation and the Gompertz equation In order to determine the biogas yield of the feedstock, the ideal gas equation was used using the maximum biogas pressure value from a mathematical modeling. Biogas yield of samples fed with pre-digested sewage sludge ranged from 0,412 to 0,518 L/gVS, while biogas yield of samples fed with organic fraction of MSW ranged from 0,285 - 0,500 L/gVS.

Preliminary AD plant design and investigation

Finally, using the excel-based computer program “The Anaerobic Digestion Economic Assessment Tool version 2.0” by NNFCC, a preliminary techno-economic study of an AD plant was carried out. Gas yields of the different subtrates used and solid loadings of the digester were taken from the results of the experimental study. Other necessary data were taken from the literature. The first scenario examined the feasibility of an anaerobic digestion plant using MSW and sewage sludge produced from a population of 10.000 as substrates. In order to increase the plant’s efficiency, the second scenario included grass silage as an additional substrate, while keeping the same total solids concentration and retention time in the digester. Annual profit and loss calculations were held for both scenarios in order to investigate the plant’s feasibility. The second scenario showed profit in comparison with the first scenario, provided that the digestate was exploited along with the biogas.

Summary and conclusions

Anaerobic digestion is a municipal solid waste treatment method that produces energy instead of requiring energy like most common waste treatment methods. Therefore, a technoeconomic study is necessary to determine the most appropriate treatment method, depending on the parameters in each case. A critical point of the AD treatment efficiency is the quality of the organic fraction and consequently the quality of the digestate, in order to be used as a soil conditioner. The experimental part of this postgraduate thesis showed a good reproducibility of results on samples with the same organic load, and an (expected) increase of biogas production in samples with higher organic loads. Biogas yields ranged from 0,41 to 0,50 L/gVS for organic loads of 1 gVS/L and 2 gVS/L, and from 0,285 to 0,36 L/gVS for organic loads of 5 gVS/L and 10 gVS/L. In similar level ranged the biogas yield of pre-digested sewage sludge, 0,50 L/gVS. Comparison of the two equations used for curve fitting, showed that both gave similar results regarding theoretical gas yields. The main difference was in the concave of the gas production curve. In conclusion, the preliminary AD plant design scenarios showed that the addition of the grass silage substrate was essential for increased plant efficiency, in order to be profitable. The feasibility of the plant is directly influenced by the potential use of the digestate. Failure to exploit the plant’s digestate showed no annual profit in both scenarios.