Abstract
This study examines the GHG emissions associated with producing bio-hydrocarbons via fast pyrolysis of Miscanthus. The feedstock is then upgraded to bio-oil products via hydroprocessing and zeolite cracking. Inventory data for this study were obtained from current commercial cultivation practices of Miscanthus in the UK and state-of-the-art process models developed in Aspen Plus®. The system boundary considered spans from the cultivation of Miscanthus to conversion of the pyrolysis-derived bio-oil into bio-hydrocarbons up to the refinery gate. The Miscanthus cultivation subsystem considers three scenarios for soil organic carbon (SOC) sequestration rates. These were assumed as follows: (i) excluding (SOC), (ii) low SOC and (iii) high (SOC) for best and worst cases. Overall, Miscanthus cultivation contributed moderate to negative values to GHG emissions, from analysis of excluding SOC to high SOC scenarios. Furthermore, the rate of SOC in the Miscanthus cultivation subsystem has significant effects on total GHG emissions. Where SOC is excluded, the fast pyrolysis subsystem shows the highest positive contribution to GHG emissions, while the credit for exported electricity was the main ‘negative’ GHG emission contributor for both upgrading pathways. Comparison between the bio-hydrocarbons produced from the two upgrading routes and fossil fuels indicates GHG emission savings between 68% and 87%. Sensitivity analysis reveals that bio-hydrocarbon yield and nitrogen gas feed to the fast pyrolysis reactor are the main parameters that influence the total GHG emissions for both pathways.
Highlights
Concern over global climate change due to increased anthropogenic greenhouse gas (GHG) emissions has prompted global action to limit the rise in global average temperature to 1.5 °C above pre-industrial levels [1]
Renewable Energy Directive (RED) specifies that allocation between coproducts should be performed via energy content in terms of lower heating value (LHV) and states that ‘‘GHG emission saving associated with excess electricity is equal to the amount of greenhouse gas that would be emitted when an equal amount of electricity is generated in a power plant using the same fuel as the cogeneration unit” [3]
The rate of soil organic carbon (SOC) had a pronounced effect on emissions allocated to the Miscanthus cultivation subsystem and showed no impact on emissions assigned to the other subsystems in both the best and worst SOC cases
Summary
Concern over global climate change due to increased anthropogenic greenhouse gas (GHG) emissions has prompted global action to limit the rise in global average temperature to 1.5 °C above pre-industrial levels [1]. CO2 emissions attributed to fossil fuel combustion and industrial processes constitute 65% of total anthropogenic GHG emissions and are primary contributors. 27% of global transport fuel supply by 2050, with the aim of cutting CO2 emissions by 2.1 Gt CO2 eq per annum [2]. As part of the commitment to cut global GHG emissions, the EU has set a target to produce at least 10% of the energy used in the transport sector from renewable sources by 2020 [3]. In 2012, biofuels from food sources constituted 4.5% of road transport fuel supply in the EU. In the UK, road transport accounts for about 20% of total GHG emission, it is targeted for decarbonisation [5]
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