• Energy Research

Bioenergy is a promising solution for greenhouse gas (GHG) emissions mitigation. However, the emissions resulting from the different production stages must be quantified and evaluated. The life cycle assessment (LCA) method was used to compare and quantify the environmental burdens of three rice straw (RS) utilization scenarios for producing biogas, briquette fuel, and syngas. To our knowledge, this is the first study that applies the LCA approach to assess these three bioenergy scenarios in a single study where the main goal was to determine the most sustainable option. A total of 10 mid-point impact categories were investigated. The results indicated that the three scenarios achieved net positive energy and net negative GHG balances. The briquette fuel scenarios had the highest net energy balance (11,115 MJ/tonne dry RS), while the syngas scenario had the highest net GHG (-2,315 kg CO2-eq./tonne dry RS). Moreover, the syngas scenario was the most beneficial to the environment, achieving negative results in 9 out of the 10 impact categories; the largest was marine ecotoxicity (-853,897 kg 1,4-DB-eq./tonne dry RS). The biogas scenario achieved emission savings in 3 out of the 10 categories. Although the briquette fuel scenario had no negative values in the 10 categories, its overall contribution to environmental burdens was relatively low. Overall, the order of the three scenarios in terms of the most sustainable option is syngas > briquette fuel > biogas.

Abstract Bauxite residue (BR) is a highly alkaline type of solid waste generated by the aluminum industry that poses a significant environmental risk upon disposal. However, BR is abundant in metals, especially iron, that offer the desired catalytic activity for microwave pyrolysis. Thus, this study aimed to use BR as a low-cost microwave absorber and catalyst for biomass microwave pyrolysis to obtain higher quality bio-oil and biochar. The addition of BR to switchgrass, the representative biomass, did not facilitate microwave absorption because most of the iron in the BR was in the form of goethite and hematite. However, the addition of an efficient microwave-absorbing catalyst (e.g., K3PO4 or clinoptilolite) to the BR triggered synergistic effects, increasing the microwave heating rate by ~ 346% compared to K3PO4 or clinoptilolite alone, which was attributed to the reduction of hematite and goethite to maghemite and/or magnetite. The addition of 10% BR to a mixture of 10% K3PO4 and 10% bentonite further triggered synergistic effects that resulted in the highest microwave heating rate of 439 °C/min, which was a 211% increase compared to using 10% K3PO4 and 10% bentonite without BR, and doubled the Brunauer-Emmett-Teller (BET) surface area of the biochar, reduced the bio-oil acidity by up to 71% compared to that obtained using a single catalyst, and increased the alkylated phenols contents in the bio-oil by 339% compared to that produced without a catalyst. These results demonstrated that the synergistic effects of BR can only be triggered when mixed with another efficient microwave-absorbing catalyst.

Solid additives were used as a microwave absorber to improve the low microwave absorption rate of switchgrass going through pyrolysis, and as a catalyst to improve the bio-oil and biochar characteristics. The synergistic effects were manifested in the presence of a mixture of K3PO4 and clinoptilolite or bentonite compared with single catalyst, resulting in increased microwave absorption rate, and improved bio-oil and biochar quality. The sample of microwave heating switchgrass with 10wt.% K3PO4+10wt.% bentonite reached 400°C after 2.8min, compared with 28.8min through conventional heating, producing biochar with increase in BET surface area from 0.33m(2)/g to 76.3m(2)/g compared with conventional heating. Furthermore, water content of the bio-oil reduced from 22.7 to 15.0wt.% compared with biomass mixed with 20wt.% SiC, a chemically-inert microwave absorbing material used to increase microwave heating. Introducing catalysts showed a great potential for accelerating microwave heating and improving bio-oil and biochar qualities.

Unlike renewable energy sources, burning fossil fuels has severe environmental impacts, such as greenhouse gas (GHG) emissions and climate change. Therefore, this study was conducted to assess and compare the environmental impacts of three biogas utilization scenarios for energy production. The life cycle assessment (LCA) method was used to compare (i) biogas combustion in combined heat and power (CHP) unit, (ii) biogas burning in a steam boiler, and (iii) biogas upgrading using pressure swing adsorption (PSA) unit to determine the most sustainable option. The results revealed that the upgrading scenario was the best option, achieving emission savings in 8 out of 10 investigated impact categories. Among them, the emission saving was the highest in the marine aquatic ecotoxicity category (-4276.97 kg 1,4-DB eq./MJ). The CHP scenario was the second-best option, followed by the boiler scenario (worst option), and both had the most beneficial performance in the ozone depletion potential category with 6.29E-08 and 9.88E-08 kg CFC-11-eq./MJ, respectively. The environmental burdens of the boiler scenario were the highest in the marine aquatic ecotoxicity category (248.92 kg 1,4-DB eq./MJ). Although the CHP and boiler scenarios contributed to environmental burdens in all impact categories, they achieved beneficial performances compared to fossil fuel-based systems.

Enormous annual sewage sludge (SS) volumes pose global environmental challenges owing to contamination and significant greenhouse gas emissions. Here, we investigated the economic viability of co-pyrolyzing SS and biomass waste to produce biofuels (bio-oil and gas) and biochar. Net present worth (NPW) analysis, the sale product break-even price, and sludge handling price (SHP) were used to determine the profitability of co-pyrolysis compared with SS pyrolysis alone and conventional treatment methods. In this study, the sale prices of biochar based on quality (i.e., stability, carbon sequestration effectiveness, and heavy metal content) were estimated to be 2.24, 1.44, and 0.98 CAD/kg for high-, medium-, and low-grade biochar. The bio-oil prices, estimated based on the higher heating values of bio-oil and diesel, ranged from 0.80 to 1.22 CAD/kg. Sawdust (SD) and wheat straw (WS) were the chosen co-pyrolysis feedstocks, with four mixing ratios (20, 40, 60, and 80 wt%). Economically, SD (40 wt% mixing ratio) co-pyrolysis achieved the best performance, with a maximum NPW of 8.71 million CAD. SD single and co-pyrolysis were the only profitable scenarios. Moreover, SS single pyrolysis and WS co-pyrolysis exhibited higher profitability than conventional SS treatment methods, with SHPs of 65 and 40 CAD/1000 kg dry sludge, respectively. Sensitivity analysis highlighted the dependence of economic performance on biochar and bio-oil market value. This study offers the first economic analysis of this approach and enhances our understanding of the potential of co-pyrolysis for biofuel and biochar production, providing innovative solutions for the environmental challenges of SS disposal.

Abstract Thermogravimetric analysis was performed to differentiate between the thermal and thermo-catalytic decomposition behaviour and kinetics of switchgrass (SG) pyrolysis and catalytic pyrolysis using bentonite or clinoptilolite. Silica sand was tested as an inert material with a thermal conductivity close to bentonite. Using a three-parallel reaction mechanism, it is shown that bentonite and clinoptilolite significantly increased the conversion percentage of all three pseudo-components, while the major difference was found for pseudo-lignin. Silica sand significantly increased the conversion of pseudo-hemicellulose only. Addition of catalysts increased the conversion of pseudo-lignin by 43 and 36% compared to SG and sand, respectively. The lowest activation energy value of pseudo-lignin was found for 30Bento which was lower by 29 and 25% than SG and sand, respectively. Based on subsequent study using microwave catalytic pyrolysis, it was observed that lignin decomposition was promoted by clinoptilolite and the phenolic compounds particularly the alkylated phenols in bio-oil product were increased by 49% higher than SG, which are primarily derived from lignin. The results obtained from the two-step kinetic model confirmed the speculation based on one-step 3-components kinetics that catalysts are mostly responsible for promoting the catalytic cracking of vapours released from lignin decomposition at high pyrolysis temperatures.