Demineralization and improvement of fuel properties of miscanthus by applying acid pretreatment

Unatoč brzom razvoju drugih obnovljivih izvora, smatra se da će biomasa ostati najčešće korišteni obnovljivi izvor energije i u sljedećim desetljećima u EU. Biomasa se najčešće koristi za proces neposrednog izgaranja, nakon čega ostaju značajne količine pepela. Smatra se da je pepeo razlog nekih tehnoloških i ekoloških problema u sustavima za izgaranje te predstavlja nedostatak biomase kao energenta. Miscanthus x giganteus je poljoprivredna energetska kultura čiji uzgoj i uporaba u proizvodnji energije raste u posljednjem desetljeću. S pozicije njegovog iskorištenja u procesu neposrednog izgaranja i problematike same biomase u tom procesu, postoji potreba za pronalaskom rješenja smanjenja pepela s ciljem dobivanja učinkovitijeg procesa. Predtretman djelomičnog uklanjanja pepela prije izgaranja biomase jedan je od načina sprečavanja gore navedenih poteškoća do kojih dolazi primjenom procesa neposrednog izgaranja. Održivost, odnosno učinkovitost biomase u takvom obliku korištenja ovisi o nizu energetskih svojstava, među kojima su ogrjevna vrijednost te sadržaj pepela, fiksiranog ugljika, hlapivih tvari i koksa. Sadržaj koksa i visoka ogrjevna vrijednost poželjna su svojstva biomase. S druge strane, sadržaj hlapivih tvari i pepela negativno utječu na energetsku iskoristivost na način da svojim prisustvom smanjuju ogrjevnu vrijednost. Pepeo, kao jedno od najčešće istraživanih svojstava, je nepoželjan i zbog svog katalitičkog utjecaja na termičku razgradnju. Ciljevi ovog istraživanja bili su odrediti poboljšanje energetskih svojstava i smanjenje pepela (demineralizaciju) biomase miskantusa nakon kiselinskog predtretmana te odrediti potrebnu koncentraciju klorovodične kiseline, temperaturu i vrijeme trajanja hidrolize lignoceluloze u svrhu utvrđivanja optimalnog smanjenja pepela i povećanja energetskih svojstava biomase miskantusa. Istraživanje je bilo podijeljeno u četiri glavne faze, uključujući prikupljanje i pripremu uzoraka, zatim određivanje njenih energetskih svojstava, demineralizaciju biomase i dizajn eksperimenta te obradu i interpretaciju podataka. Istraživanje je pokazalo kako se demineralizacijom biomase značajno mijenjaju sva istraživana energetska svojstva, izuzev sadržaja silicija. Svi korišteni tretmani rezultirali su povoljnim promjenama, prvenstveno u pogledu smanjenja sadržaja pepela (do 56,07 %) te samim time sadržaja makro- i mikroelemenata. 18 energetskih svojstava biomase promijenjeno je utjecajem koncentracije HCl-a u otopini, 13 svojstava utjecajem vremena trajanja tretmana dok je temperatura bila značajni faktor kod promjene sadržaja željeza, celuloze te hemiceluloze. Značajni utjecaj međusobne interakcije koncentracije HCl-a i temperature zabilježen je kod promjene 5 svojstava, jednako kao i kod međusobne interkacije koncentracije HCl-a s vremenom trajanja tretmana. Interakcija temperature i vremena trajanja značajno je utjecala na smanjenje sadržaja natrija, dok je trostruka interakcija svih istraživanih parametara utjecala na promjenu sadržaja natrija, željeza te hemiceluloze. Korištenjem funkcije poželjnosti, optimizirani su eksperimentalni čimbenici s ciljem dobivanja biomase što poželjnijih energetskih svojstava. Takve rezultate moguće je postići korištenjem uvjeta s najkraćim vremenom trajanja (30 min), pri najvišoj koncentraciji HCl-a u otopini (0,05 %) te najnižoj temperaturi (25 °C).Introduction: The sustainability, i.e., efficiency of miscanthus biomass as feedstock for thermochemical conversion depends on a number of energetic properties, including calorific value and content of ash, fixed carbon, volatiles, and coke. The coke content and high calorific value are desirable properties of biomass. On the other hand, ash and volatile matter content have a negative impact on energy efficiency as they reduce the calorific value. Ash, one of the most studied properties, is also undesirable due to its catalytic effect on thermal decomposition, in terms of its' agglomeration, clinkering, and slag behavior. Demineralization, which is the hydrolytic pretreatment that removes part of the ash composition before biomass combustion, is one of the means to mitigate the above-mentioned difficulties encountered during combustion. Demineralization involves the removal of certain chemical elements from the biomass. The selection of washing solution depends on the elements or minerals that are present. Ash is mostly formed from the organic composition of the biomass, but to a lesser extent can be formed from inorganic composition or unbound liquids. Consequently, water is a suitable medium for dissolving and leaching alkali metals (e.g., K, Na, Cl) and their sulphates and chlorides. It is applicable to biomass with a high content of inorganic matter, since species with a lower ash content, such as miscanthus, have more alkali metals bound to the organic structure. The content of bound metal ions is reduced by pretreatment of the biomass with diluted acid, such as hydrochloric acid, because the acid changes the primary structure of the polymer, i.e. reduces the content of hemicellulose and cellulose, thereby releasing a certain amount of ions. In this case, the concentration of the acid in the solution used for the pretreatment is of utmost importance, as well as the duration of the pretreatment itself, since the degree of hydrolysis of the hemicellulose depends on it. In previous studies on different types of biomass, the acid solution has been shown to be effective in dissolving and removing carbonates, sulphates and alkali chlorides. Materials and methods: In the 2020 spring harvest, three Miscanthus plants were collected from each of the four replicates and then homogenized. The collected biomass was ground and sieved to the fraction of 250 μm - 1000 μm for the purpose of further analyzes. The untreated biomass was characterized by proximate analysis (ash, volatiles, coke and fixed carbon), final analysis (C, H, N, S, O), determination of heating values (both higher and lower), mineral composition analysis of major (potassium, calcium, sodium and magnesium) and minor elements (copper, iron, manganese and zinc), and structural properties analysis (cellulose, hemicellulose and lignin). Demineralization was carried out by adding 20 g of the sample to 400 mL of HCl solution, the ratio of sample to HCl solution being 1:20. The pretreatment was carried out under constant stirring at temperatures ranging from 25°C to 80°C and a mixing rate of 2.5 Hz. The temperatures used in the pretreatment were room temperature, which is the temperature at which energy consumption is not usually required, and elevated temperature (up to 80°C). The duration ranged from 30 to 180 minutes. The HCl concentrations used for hydrolysis were 0.00%, 0.025%, and 0.05%, i.e., up to the concentration that has shown a significant effect on the degree of demineralization in previous studies. In order to evaluate the effects of these factors on the achievement of the optimal degree of demineralization and the energetic properties of the biomass, a laboratory experiment was planned according to a full factorial design (23), using midpoints for each factor. Such an experimental design allowed the effects of all factors, as well as their interactions, to be studied in all combinations of their values. Each combination was repeated twice, and the midpoint was repeated six times. After treatment, the samples were analyzed in the same way as the untreated biomass. After the results of the laboratory analyzes were collected, the influence of each factor on the degree of demineralization and energetic properties was determined. Results and conclusions: The energetic characterization of the untreated biomass allowed a careful and detailed verification of any changes caused by further treatments, but also ensured that the biomass used was consistent in terms of composition with standard values and the results of previous studies, especially in a comparable agroclimatic area. Regarding the physicochemical properties of the treated biomass, all treatments successfully reduced the ash content (by 20.88 % to 56.07 %), with a significant influence of the concentration of acid in the solution and the duration of the treatment. All other physicochemical properties experienced an increase or decrease in their content depending on the treatment. HCl concentration and duration of treatment significantly affected all properties, while the interaction between HCl concentration and temperature affected the content of coke, solid carbon and volatiles. All treatments, except T1 and T3, decreased the higher heating value of biomass by a maximum of 2.83 % and the lower heating value by 3.19 %. For both properties, the influence of HCl concentration in the solution and the duration of the process were found to be factors with significant influence. The carbon and hydrogen contents were successfully increased by all treatments. For carbon content, HCl concentration and duration had a significant influence on the degree of increase, while for hydrogen content, the concentration was influenced by the interaction between concentration and duration. The interaction between HCl concentration and duration significantly reduced nitrogen content, which was also reduced in all other treatments. Sulfur content was increased and decreased, depending on the effect of treatment duration. The oxygen content was also increased or decreased, but depending on the HCl concentration in the solution and the duration of the treatment. The content of all the major elements studied was successfully reduced with all the treatments, and the concentration of HCl in the solution was found to be an important factor. A significant effect of the interaction between concentration and temperature was also found for potassium content, while for sodium content the effect of a threefold interaction of HCl concentration, duration and temperature was found. As for trace elements, all applied treatments resulted in successful reduction of micronutrient content. The decrease in iron content was significantly influenced by duration as well as by the triple interaction of all factors. HCl concentration led to a decrease in manganese and zinc content, while for zinc the influence of temperature was also observed. Lignocellulosic composition, i.e. cellulose, hemicellulose and lignin content, was increased or decreased depending on the treatment. Treatment temperature was found to be a significant factor in changing cellulose content, while lignin content depended on the interaction between HCl concentration and duration. Hemicellulose content was significantly affected by all factors studied, both individually and in all mutual interactions. Statistical optimization of the demineralization process showed that the most desirable results could be obtained by using the conditions with the shortest duration (30 min), the highest concentration (HCl, 0.05 %) and the lowest temperature (25 °C)