| Abstract | Odlagališta otpada predstavljaju ekološke, ekonomske i društvene izazove, prvenstveno zbog
onečišćenja nastalog od organskog otpada koji se raspada u samom odlagalištu. Razgradnjom
organskog otpada nastaje metan (CH4), staklenički plin koji doprinosi klimatskim promjenama.
Metan (CH4) se obično zbrinjava spaljivanjem čime ga se pretvara u ugljični dioksid (CO2) i
vodenu paru (H2O). Međutim, kada su razine metana ispod 20 %, spaljivanje na baklji postaje
neisplativo, što dovodi do njegovog izravnog ispuštanja u atmosferu.
U predmetnom istraživanju stoga je istražen potencijal fotokatalitičke oksidacije s ciljem samog
razvitka prikladne tehnologije za zaštitu zraka iznad odlagališta otpada. Ovaj proces koristi
Sunčevo zračenje i fotokatalizator, a pokazao se učinkovitim i isplativim za razgradnju
onečišćivala poput metana (CH₄) u odlagališnom plinu. Uz navedeno, razvijen je i matematički
model za računalnu simulaciju i dizajn budućih sustava za smanjenje emisija odlagališnih
plinova na samom izvoru.
Ispitivanje fotokatalitičke razgradnje na uzorku stvarnog metana (CH₄) s odlagališta neopasnog
otpada Totovec pokazalo je smanjenje metana (CH₄) za približno 20 %. Iako je smanjenje bilo
nešto niže od očekivanog, rezultati su potvrdili učinkovitost fotokatalitičke oksidacije, čak kada
je koncentracija metana nešto niža (< 10 %).
Matematički model otkrio je ključne čimbenike koji utječu na razgradnju metana (CH₄) u
prstenastom reaktoru, osobito važnost duljine cijevi. Iako je model bio učinkovit, upotreba alata
računalne dinamike fluida za simulaciju pokazala se izazovnom zbog potrebe za opsežnom
prilagodbom i stručnošću za ovu specifičnu primjenu. |
| Abstract (english) | Landfills are places intended for the disposal of solid waste and, as such, carry numerous
ecological, economic, and social problems. The decomposition of organic waste at landfills
produces methane (CH4), a greenhouse gas that is 25 times more potent than carbon dioxide
(CO2) in terms of heat retention, according to the Intergovernmental Panel on Climate Change
(IPCC). Accordingly, landfills represent one of the largest sources of methane (CH4) resulting
from human activity.
To reduce methane (CH4) emissions at landfills, landfill gas collection systems are often
installed to "capture" methane (CH4) and other gases emitted during the decomposition of
organic waste and transport them to a centralized facility for disposal. The most common
method of methane (CH4) disposal at landfills is flaring, which converts methane into carbon
dioxide (CO2) and water vapor (H2O). This practice is typical when the methane (CH4) content
in the landfill gas is at least 20%. When concentrations are lower, flaring becomes economically
unfeasible, leading to the release of methane into the atmosphere.
For this reason, the potential of photocatalytic oxidation was explored in this study on a real
landfill gas sample with lower methane (CH4) content. The goal of the study was to develop an
effective air protection technology for use at the source of landfill gas emissions. Photocatalytic
oxidation is a process that uses light-activated photocatalysts to create redox reactions that
degrade pollutants such as volatile organic compounds (VOCs) into harmless substances like
water and carbon dioxide (CO2).
The expected scientific contribution of this research is to confirm the effectiveness of solar
photocatalysis and demonstrate that it is a suitable technology for air protection above landfills,
which could potentially be used at the source of landfill gas emissions. Additionally, the
development of a mathematical model for the photocatalytic degradation of air pollutants will
provide the foundation for scaling up the process and applying it in real systems.
For the purpose of the research, the following four hypotheses were proposed: (1) Methane
(CH4), whose content in landfill gas composition is variable, can oxidize to carbon dioxide
(CO2) with the available amount of oxygen at the outlet from the landfill body; (2) Hydrogen
sulphide (H₂S), a component of landfill gases, is separated by the adsorption process from the
gas stream and remains on the photocatalytic material; (3) Suitable photocatalytic materials
based on titanium dioxide (TiO2) can be used for extended periods and regenerated if necessary
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before reuse; (4) An annular-type photocatalytic reactor represents a model system for the
simple upgrade of gas wells within the landfill body.
The first phase of research included the selection of photocatalytic material based on previous
research on commercial photocatalysts. Research has shown that titanium dioxide (TiO2) has
the most effective photoactivity, the highest stability and the lowest costs, which is why it was
chosen as the photocatalyst for conducting research. In the research itself, the photocatalyst was
applied to a mesh of glass fibres.
The next step in the research was the assembly of a laboratory photocatalytic reactor.
Photocatalytic reactors for air treatment require appropriate design with the aim of achieving
high quantum yields for long operating times with minimal costs. In order to achieve the above,
it is necessary to choose the appropriate configuration of the reactor and the source of UV
radiation in addition to the photocatalyst. In the test, an annular type of reactor was used, which
consists of two glass, concentric cylinders, between which the reaction mixture flows. The light
source (UV lamp) is placed in the inner cylinder, parallel to the tube. To simulate solar
radiation, Narva BIO vital® LT T5 24W / 958, UV lamp with the length of 549 mm was used.
The lamp in question emits radiation with a characteristic part of the UV-A and UV-B spectrum
(λ > 315 – 400 nm).
As a final step, the photocatalytic reactor was modelled using Computational Fluid Dynamics
(CFD). The computer program COMSOL Multiphysics was used for data visualization. In
addition to visualization, the mentioned computer program also enabled the computer
simulation of methane (CH4) decomposition in the test reactor using the estimated kinetic
parameters. Computer simulations were performed for different cases in order to be able to
compare the results obtained by the research with those obtained in the computer simulation.
The purpose of the above was to check the accuracy of the computer simulation, which will
ultimately enable further optimization of the test system and the adaptation of future potential
air protection systems above the waste disposal site.
Regarding the results themselves, based on which the proposed hypotheses were confirmed, the
results of the photocatalytic oxidation of methane (CH4) on a real gas sample from the
"Totovec" non-hazardous waste landfill showed an average reduction in methane (CH4)
concentration of 19.91 % across three measurements, specifically 21.59 % in the first
measurement, 18.15 % in the second, and 20.00 % in the third measurement. The
aforementioned confirmed the effectiveness of photocatalytic oxidation as methane (CH4) was
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oxidized to carbon monoxide (CO) and carbon dioxide (CO2). This was confirmed by observing
the decomposition products, evidenced by a 30 % increase in carbon monoxide (CO) levels, as
CO concentrations rose from 2 to 3 ppm in the first and second tests and from 3 ppm to 4 ppm
in the third test.
Regarding the second hypothesis, the presence of hydrogen sulphide (H2S) in the landfill gas
samples was low (< 100 ppm) due to the prior separation of food waste at the source (the
landfill), and no significant difference in methane (CH4) degradation efficiency was observed
between samples with and without hydrogen sulphide (H2S) during the testing.
The third hypothesis was confirmed by using a fiberglass mesh in the research. The fiberglass
mesh served as a carrier for the photocatalyst, titanium dioxide (TiO2), and enabled efficient
photocatalytic oxidation due to its large surface area and porous structure, as supported by the
results of the photocatalytic oxidation during the study. This mesh had previously proven to be
a very good solution, as it could be regenerated after use through thermal treatment, thereby
extending its usage time. Thermal regeneration of the fiberglass mesh was effective due to the
high thermal stability of titanium dioxide (TiO2), allowing the removal of organic pollutants
through thermal oxidation. This kept the surface clean and ready for the further adsorption of
pollutants for additional photocatalytic degradation.
Concerning the fourth hypothesis, the annular-type photocatalytic reactor represents a model
system for the simple upgrade of gas wells at landfills due to its efficient design. The ring shape
allows for a larger surface area for the photocatalyst, promoting effective interaction between
the landfill gases and the used catalyst, as demonstrated in the model simulation created in the
COMSOL Multiphysics software. This design, being of a ring type, facilitates integration into
existing landfill gas collection systems, which are cylindrical in shape and thus require minimal
modifications to the gas wells. |
