Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Subject "Acacia feedstock"

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    Life cycle assessment and economic analysis of carbon sequestration through pyrolysis of invasive alien plants.
    (Stellenbosch : Stellenbosch University, 2024-03) van Schoor, Annalie Ula-Caye; Görgens, Johann Ferdinand; Stafford, William; Stellenbosch University. Faculty of Engineering. Dept. of Chemical Engineering. Process Engineering.
    ENGLISH ABSTRACT: Identifying feasible solutions that can help to reduce greenhouse gas emissions are essential in responding to the urgent need to mitigate climate change. Carbon capture and sequestration (CCS) has been identified as a solution, by mitigating the release of CO2 into the atmosphere. Two specific CCS technologies were considered in this study: biochar production through slow pyrolysis of invasive alien plants (IAPs), and carbon capture from an existing coal power station’s offgas (retrofitment). Furthermore, using IAPs to produce biochar has added environmental benefits, such as increasing biodiversity, and improving water resources. The main aim of the project was to compare the techno-economic feasibility and environmental impacts of carbon sequestration through biochar-from-IAPs-pyrolysis, compared to CCS from a coal power station. Three alternative pyrolysis technologies for industrial production of biochar from IAPs were considered: namely- a semi-batch retort system (SBR), a vertical continuous retort (VR) and an indirect retort carbonizer (IR). Both the SBR and VR system co-produced charcoal-for-energy as well as biochar-for-sequestration, while the IR system produced only biochar-for-sequestration. Process simulations were conducted in AspenPlus® using published and experimental data. The char yields were 31.25 wt.%, 27.93 wt.%, and 29 wt.% for SBR, VR and IR pyrolysis technologies, respectively. Although the pyrolysis systems were able to generate sufficient energy to be selfsufficient by burning all pyrolysis vapours, significant thermal energy was not recovered. The net energy efficiencies were 94.9%, 73.8% and 80.7% for the SBR, VR and IR systems. Moreover, the carbon capture system was highly energy-intensive and resulted in a 10% power plant efficiency drop. A Life Cycle Assessment was conducted in SimaPro® to determine the environmental impacts associated with each technology. The net Global Warming Potential (GWP) for sequestering 1 kg of carbon for the SBR, VR, IR and power-station-CC were -3.48 kg CO2e, -3.35 kg CO2e, -3.41 kg CO2e and -3.08 kg CO2e. The pyrolysis technologies therefore had a lower GHG emissions than the powerstation-CC and had lower impacts in several other environmental impact categories, such as acidification, eutrophication, and fossil fuel depletion. Economic feasibilities were evaluated to calculate the minimum selling price (MSP) to achieve a nominal internal rate of return (IRR) of 20%. The lowest MSP was US$972/MT for biochar from the IR system, indicating that larger biochar production was essential to improve profitability. Comparatively, the biochar MSP was US$2250/MT from the VR and US$1186/MT from the SBR; demonstrating financial benefits of co-producing charcoal and biochar from one technology. The monetisation of carbon sequestered through sales in certified carbon credits, had minor impacts on the profitability (2-5%) but it was essentially to improve the desirability of biochar to buyers. The highest cost of CO2 sequestering (US$1071/MT CO2e) for the VR, while the lowest (US$468/MT CO2e) was with the IR – demonstrating the importance of biochar technology selection for better profitability. The IR and SBR process was more cost efficient for carbon sequestration than the powerstation-CC that had a CO2 cost of US$678/MT CO2e. Therefore, opportunities exist to implement the production of biochar as a successful means of carbon sequestration, as it presents both environmental and economic benefits in comparison to the power-station-CC.