Marine transport fuels from residual biomass and renewable electricity: Pathways for DME production from wheat straw gasification and water electrolysis

René Kofler

Research output: Book/ReportPh.D. thesis

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Abstract

A rapid transition towards an energy system with net zero greenhouse gas emissions is required to limit global warming to stay below 1.5 °C. The maritime transport sector is one of the hard-to-abate sectors, due to its reliance on energy-dense liquid fuels. Dimethyl ether (DME) has been identified as a promising alternative fuel for the shipping sector. DME (CH3OCH3) is a synthetic fuel produced from syngas (H2, CO and/or CO2) via one-stage or two-stage DME synthesis. The syngas can be produced from biomass gasification using residual biomass, like wheat straw, to ensure the sustainability of the produced fuel. Combining the production of DME via biomass gasification with electrolytic hydrogen from renewable electricity increases the fuel yield, carbon efficiency and energy efficiency of DME production plants.

This PhD thesis investigated the production of DME from wheat straw using the lowtemperature circulating fluidized bed (LT-CFB) gasifier integrated with electrolytic hydrogen production. The thesis is structured in the form of a paper collection containing five articles. In articles I – IV, 14 DME production plants based on wheat straw gasification in the LT-CFB gasifier combined with alkaline water electrolysis were designed and analyzed from a thermodynamic point of view, focusing on maximizing the energy, exergy, and carbon efficiency. The analyses were based on thermodynamic models implemented in the software Aspen Plus V11. In article V, a technoeconomic comparison of two wheat straw based DME production plants with two bamboo based DME production plants was conducted. On top of the thermodynamic modelling and economic analysis, an optimization model was used, optimizing the operation and size of the different plant units in an off-grid scenario.

Wheat straw gasification using the LT-CFB gasifier enabled efficient conversion, yielding biochar, which can be returned to the fields for carbon sequestration, soil amendment and potassium fertilizer, and a tar-laden gas. Two methods for tar removal were investigated. 1) Catalytic deoxygenation and condensation of tars yielded a high-quality bio-oil, while the carbon efficiency suffered from carbon losses from the catalytic upgrading process. 2) Partial oxidation (POX) for cracking and reforming of the tars made all the carbon in the gas available for DME synthesis. Plants combining the LT-CFB gasifier with POX and two-stage DME synthesis achieved the highest carbon and energy efficiencies, while having the highest electricity and hydrogen consumption. The use of a hydrogen quench after POX reduced the hydrogen consumption while increasing the overall electricity consumption. A comparison with an electricity production plant and a synthetic natural gas (SNG) production plant based on the LTCFB gasifier showed that carbon and energy efficiencies of the simplest DME production plant were lower than for the SNG production plant and higher than for the electricity production plant. By introducing the above-mentioned process steps, DME production and SNG production plants achieve similar efficiencies, but with a more complex system layout for the DME synthesis plant. The results from the technoeconomic comparison of wheat straw and bamboo based DMEsystems showed that variable instead of full load operation of the DME plants, reacting to the availability of the electricity, enabled a reduction of the levelized cost of fuel. Wheat straw based DME production systems achieved slightly higher levelized costs of fuel than bamboo based systems. A sensitivity analysis showed that under different cost scenarios, wheat straw based systems can achieve similar levelized cost of fuel as bamboo based plants.

This work showed that DME synthesis combining wheat straw gasification and water electrolysis can achieve high carbon and energy efficiencies. The additional production of biochar, process heat and district heating increase the relevance of these plants in a net zero energy system.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages320
ISBN (Electronic)978-87-7475-771-9
Publication statusPublished - 2023
SeriesDCAMM Special Report
NumberS244
ISSN0903-1685

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  • Thermochemical Biofuel Production

    Kofler, R. (PhD Student), Clausen, L. R. (Main Supervisor) & Ahrenfeldt, J. (Supervisor)

    01/09/201911/01/2024

    Project: PhD

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