• Energy Research

This study evaluates the economic cost and sustainability of treating residual municipal solid waste (MSW) through five waste management scenarios. In the baseline scenario (Bsc), all waste was managed through landfilling, while in scenario 1 (Sc1) all waste was treated by incineration. Sc2 employed anaerobic digestion (AD) for food waste and landfilling, and Sc3 treated the waste through AD for food waste, incineration of combustible and plastic wastes, and landfilling. Sc4 treated the waste using AD, incineration, landfilling, and recycling of the plastic waste. The economic cost of waste management scenarios was estimated by calculating different economic variables, such as gate fees, including capital and operating costs, governmental incentives and levies, and also the potential of employed waste treatment technologies for resource recovery. The results revealed that Sc3 has the lowest economic cost of 238.1 mAUD/year, followed by Sc1 (261.9 mAUD/year), while Bsc proved to be the highest cost at 476.1 mAUD/year for MSW treatment. It was noticed that scenarios employing incineration had lower economic costs compared to Bsc and Sc2, mainly because incineration resulted in higher electricity generation and reduced greenhouse gas emissions. The sustainability assessment results confirmed that Sc3 had the lowest and Bcs the highest total economic cost and environmental damage.

The use of exoelectrogens in microbial fuel cells (MFCs) has given a wide berth to the addition of artificial electron shuttles/conduits as they have the molecular machinery to transfer the electrons exogenously to the electrode surface or to soluble or insoluble electron acceptors. Exoelectrogens transfer the electrons either directly to the electrode surface (via c-Cyts or pili) and/or mediate them by secreting electron shuttles such as, flavins or pyocyanin. Such microorganisms form electroactive biofilms on the electrode surface. They produce cyclopropane fatty acids and exopolysaccahride matrix to modify surface charge, which also provides favorable anchoring points for the retention of c-type cytochromes (c-Cyts). The longer subunit of PilA plays a vital role in cell attachment in the case of a well-known exoelectrogen Geobacter sulfurreducens during biofilm formation. G. sulfurreducens relies on flavin molecules for mediated electron transfer (MET) during initial biofilm formation and on c-Cyts and pili for the direct electron transfer (DET) during the later phase of biofilm formation. A new protein, cbcl inner membrane multiheme c-Cyt has been revealed in G. sulfurreducens that participates in the electron transfer when electron acceptor with low reduction potential (below 0.1 V) is used in the MFCs. On the other hand, inner membrane c-type cytochrome ImcH is involved in the reduction of electron acceptors exhibiting the potential above 0.1 V. Shewanella oneidensis, another exoelectrogen expresses CheA-3 histidine protein kinase for chemotactic responses to electron acceptors. S. oneidensis do not produce pili and utilizes flavin-cytochrome complexes to regulate the electron transfer to the electrode surfaces. The inherent electron transfer rates can be increased in order to improve the MFC performance. Such strategies as the anode surface modification with nanoparticles, expression of the genes for flavin biosynthesis pathway in the exoelectrogens, and chemical treatment of the microbial membrane have shown to increase the current outputs in the MFCs. This article provides the latest information about the exoelectrogens and molecular drivers involved in extracellular electron transfer (EET) mechanisms, and also summarizes the important characteristics of electroactive biofilms. It also highlights the different approaches that have been employed to facilitate the EET mechanisms and some uncommon exoelectrogens used in the MFCs recently.

Abstract Vegetation has successfully been used for phytoremediation of heavy metal(loid) contaminated soils. Previous works found that the metal(loid)-enriched biomass can be converted into biofuels through pyrolysis. However, the potential emission of metal(loid)s at higher pyrolysis temperatures, the leaching potential of minerals in chars, and the quality of the products needs further consideration. In this work, the metal(loid)-enriched biomass was engineered by pre-mixing with magnesium carbonate to study the effect on pyrolytic product properties and metal(loid) deportment. Heavy metal contaminated mangrove grown in a land contaminated with a lead–zinc smelter slags was used as the biomass. The biomass and magnesium carbonate mixture as the feedstock was subjected to pyrolysis at temperatures from 300 to 900 °C under the heating rate of 10 °C/min. Results showed that the feedstock mainly decomposed at temperatures between 176 and 575 °C. Amongst the 10 studied metal(loid)s in this work, most elements exhibited more than 70% of elemental recovery in chars at pyrolysis temperatures up to 700 °C. Pyrolysis also enhanced heavy metal stability in chars produced at temperatures above 300 °C. This study indicated that co-pyrolysis of heavy metal contaminated biomass with magnesium carbonate enabled the pyrolysis temperature up to 700 °C with minimal environmental risks, providing a safe and value-added way of phytoremediation residual management.

Slow pyrolysis of heavy-metal(loid)-contaminated Avicennia marina biomass obtained from phytoremediation was conducted to investigate the deportment of 12 heavy metal(loid)s in pyrolysis products (biochar, bio-oil, gas) at temperatures from 300 to 800 °C. The results indicated that different heavy metal(loid)s showed diverse volatilities, while all elements tended to transform into volatile products with the increase of pyrolysis temperature. Cd was found highly volatile, while Fe and Cu were non-volatile elements. The leaching analysis of biochars showed that pyrolysis was effective in reducing the mobility and bioavailability of the heavy metal(loid)s in biochars. Moreover, the risk assessment of biochars showed that the biochars derived from polluted biomass can be used as a potential soil amendment. Considering the energy consumption and risk of contaminant emission, pyrolysis temperatures of 400 to 500 °C were considered to be the optimum option for pyrolysis of this biomass.

Abstract The presence of oxygenated compounds in pyrolytic oil makes it highly acidic and unsuitable energy source for real-world applications. In-situ and ex-situ catalytic pyrolysis have been considered the most significant approaches to convert these oxygenated compounds into hydrocarbons or less oxygenated compounds, thereby increasing the carbon and hydrogen content in the bio-oil and improving its overall quality. A remarkable conversion of oxygenated compounds could also be achieved using a combined in-situ and ex-situ catalytic pyrolysis approach. Therefore, this study aimed to prepare Cu10%/zeolite and Ni10%/zeolite catalysts using a wet-impregnation method and investigate their potential for bio-oil upgrading in a combined in-situ and ex-situ catalytic pyrolysis mode and the results were compared with sole in-situ and ex-situ catalytic pyrolysis. In combined pyrolysis, Cu/zeolite was used in-situ and Ni/zeolite in ex-situ mode with four different catalyst to biomass (C/B) ratios (2, 3, 4 and 5). Interestingly, the results demonstrated that the combined pyrolysis with a C/B ratio of 5 achieved the highest deoxygenation activity (˜98%) and total hydrocarbon production (˜72%) as compared to sole in-situ (C/B ratio of 5) or ex-situ catalytic pyrolysis (C/B ratio of 3). It was further noticed that both the catalysts in sole in-situ pyrolysis promoted the formation of acids (˜28% by Cu/zeolite with C/B ratio of 5) in the bio-oil, but a negligible proportion of acids (˜1%) was obtained in sole ex-situ and combined pyrolysis mode. The major hydrocarbons detected in all the bio-oil samples were ethylidenecyclobutane, retene, fluorene, phenanthrene, and pyrene. The enhanced deoxygenation activity and hydrocarbon production by the catalysts can be attributed to the abundant acidic sites present inside the pores or on the surface of the catalysts that carried out major deoxygenation reactions, such as dehydration, decarboxylation, decarbonylation, aldol condensation, and aromatization. Overall, this study suggested that a combined in-situ and ex-situ catalytic pyrolysis approach could be advantageous for bio-oil upgrading as compared to sole in-situ or ex-situ catalytic pyrolysis mode.

Abstract Solar energy and biomass are the two major sources of renewable energy, which can be integrated to produce heat, power and transportation fuels, chemicals and biomaterials using pyrolysis. In this work, separate samples of chicken-litter waste and rice husk of different particle sizes (280 and 500 μm) were pyrolysed with a concentrated solar radiation to produce pyrolysis gases of high calorific value. Different operating parameters were investigated under the solar pyrolysis conditions. Heating rates from 10 to 500 °C/s and temperatures in the range of 800–1600 °C, generated from a lab-scale solar furnace with maximum power capacity of 1.5 kW, were applied. Temperature was found to have the highest effect, changing the gas yield from 10 to 39 wt%; decreasing the bio-oil and char yields from 48 to 41 wt % and 42 to 18 wt%, respectively as the temperature increased from 800 to 1600 °C. The highest specific energy content of the gas (7255 kJ/kg) was obtained with the 280 μm particle size chicken litter at 1600 °C. Overall, gases produced from solar assisted biomass pyrolysis have a high concentration of combustible products that could be directly used as fuels in engines or power plants.

The study assessed the environmental impacts of landfilling, anaerobic digestion and incineration technologies and investigated the effect of the replaced source of electricity on the environmental impacts of these waste to energy (WtE) technologies. Data published in the national pollutant inventories and ReCiPe impact assessment method were employed in this study. The study showed that electricity generation through incineration had the highest impacts on human health and ecosystems, followed by landfilling. Compared to the electricity of the Australian national grid, electricity generated from all three WtE technologies have a lower environmental impact. The results revealed that global warming and fine particulate matter formation with more than 97.6% contribution were the main impact factors for human health, while terrestrial acidification, global warming and ozone formation were contributing to more than 99% of the impacts to ecosystems. Global warming was the most impactful category on human health and ecosystems from incineration with over 85% contribution to both endpoint categories. Incineration revealed significantly higher avoided global warming impacts to human health and ecosystems than landfilling from the treatment of one tonne of solid waste by replacing electricity from brown coal, black coal or the Australian power grid. The growing share of renewable energy in the Australian power grid is expected to decrease the grid GHG emissions and the effect of the avoided impacts of replaced electricity. The results revealed that if the GHG emissions from the Australian power grid (757 kg CO2 eq/MWh) decrease to break-even point (621 kg CO2 eq/MWh), incineration loses the climate advantage over landfilling.

Abstract Utilisation of solar energy for thermochemical conversion of biomass can facilitate sustainable development of society by reducing greenhouse gas (GHG) emissions which originate from excessive use of conventional sources of energy. In this work, gas and liquid fuels obtained from solar assisted pyrolysis of chicken-litter waste were upgraded using CaO and char catalysts. The catalysts were loaded in the solar reactor in in-situ and ex-situ modes at different catalyst to biomass ratios. In both cases there was substantial decrease in CO2 accompanied by an increase in the formation of CO and H2 with temperature and catalyst to biomass ratio. The in-situ pyrolysis with 50% CaO loading exhibited maximum CO (63 wt%) and H2 (15 wt%) yields at 800 °C. Similarly, the in-situ pyrolysis with 50% char catalyst produced 60 wt% CO and 5 wt% H2. The addition of CaO exhibited considerable deoxygenation performance for the fatty acids. Minimum concentrations of fatty acids in the liquid product achieved with 50% CaO in the in-situ and ex-situ pyrolysis were 8% and 3%, respectively. On the other hand, the addition of char did not show significant deoxygenation difference for either the alcohols or fatty acids of the bio-oil compounds.