Characterization of biochar from lignocellulosic biomass

V okviru magistrskega dela smo želeli kritično ovrednotiti in preveriti možnosti za izboljšavo obstoječih metod za določanje specifične površine, kationske izmenjevalne kapacitete (CEC) ter vsebnosti policikličnih aromatskih ogljikovodikov (PAH) v biooglju. Ugotovitve smo predstavili na podlagi karakterizacije vzorcev biooglja, ki so bili pri različnih procesnih pogojih proizvedeni s pirolizo oz. uplinjanjem lesne in rastlinske lignocelulozne biomase. Specifično površino smo določili po modelu Brunauer-Emmett-Tellerjeve (BET) teorije. Želeno vrednost specifične površine, ki je navadno večja od 200 m2 g-1, je imela večina vzorcev biooglja iz lesne biomase. Največjo specifično površino (610,7 m2 g-1) je imelo biooglje, proizvedeno z uplinjanjem, pri bioogljih, proizvedenih s pirolizo, pa je na njeno velikost pomembno vplivala pirolizna temperatura. Velik vpliv je imela tudi izbira osnovne surovine, saj smo večjo specifično površino določili pri vzorcih iz lesne biomase. Določitev CEC smo najprej izvedli s tehniko centrifugiranja, nato pa pokazali, da je mogoča tudi izvedba s tehniko nučiranja, saj so bile masne izgube z vsakim nučiranjem le 1–2 %. Rezultati centrifugiranja in nučiranja so bili primerljivi predvsem pri določitvi CEC preko skupne vsote izmenljivih baznih kationov (CEC-BC) in sicer so za vzorce biooglja vrednosti CEC-BC segale od približno 0,73 do 77,72 meq/100 g. Največ sta k skupni vrednosti CEC prispevali vsebnosti baznih kationov Ca2+ in K+. Določanje PAH v biooglju je temeljilo na Soxhlet ekstrakciji s toluenom in sledečo GC-MS/FID analizo PAH v ekstraktih. Vsebnost PAH je bila največja v vzorcu biooglja, ki je bilo proizvedeno s procesom uplinjanja (1700,17 mgPAH kgSS-1), le en vzorec pa je imel vsebnost PAH nižjo od mejne vrednosti Evropskega standarda za biooglje, določene s 6 mgPAH kgSS-1. Izkazalo se je, da vsebujeta vzorca iz rastlinske biomase več različnih PAH kot vzorci iz lesne biomase. Izmed testiranih vzorcev se je za uporabo v kmetijstvu kot dodatek tlom najbolje izkazal vzorec biooglja Jakič bukev.The focus of the master’s thesis was to critically evaluate and examine the possibilities for improving existing methods for determination of specific surface area, cation exchange capacity (CEC) and the content of polycyclic aromatic hydrocarbons (PAH) in biochar. Findings presented are based on the characterization of biochar samples that were produced under different process conditions by pyrolysis and gasification of wooden and plant-based lignocellulosic biomass. Specific surface area was determined by Brunauer-Emmett-Teller (BET) theory. The majority of biochar samples from wooden biomass achieved the desired value of a specific area which is greater than 200 m2 g-1. Biochar produced by gasification had the largest specific surface area (610.7 m2 g-1), whereas the size of biochar produced by pyrolysis was significantly influenced by process temperature. The choice of basic raw material also had a great influence as we determined a larger specific area for samples made from wooden biomass. CEC determination was first performed using a centrifugation technique, but we showed that it is also possible to perform it by vacuum filtration, since the mass losses with each filtration were only 1–2 %. Results of both centrifugation and vacuum filtration were comparable mainly in the determination of CEC as the total sum of exchangeable base cations (CEC-BC), ranging from approximately 0.73 to 77.72 meq/100 g for biochar samples. Contents of base cations Ca2+ and K+ contributed the most to the total CEC value. Determination of PAH in biochar was based on Soxhlet extraction with toluene and subsequent GC-MS/FID analysis of PAH in the extracts. PAH content was the highest in the sample of biochar produced by gasification process (1700.17 mgPAH kgSS-1), only one sample, however, had a PAH content lower than the EBC standard limit set at 6 mgPAH kgSS-1. Samples from plant biomass were found to contain more different PAHs than samples from wooden biomass. Biochar from beech wood proved to be the most suitable of tested samples as a soil additive for agricultural use