Assessment of Fischer−Tropsch liquid fuels production via solar hybridized dual fluidized bed gasification of solid fuels

Abstract

To mitigate the emissions from the widely studied and even applied coal to FT liquid (FTL) fuels systems, two kinds of promising renewable energy, biomass and solar energy, have been proposed and assessed as a partial or total substitute for coal feed. The concept of a solar hybridized FTL fuels production system has the potential to obtain higher productivity with lower greenhouse gas emissions, when compared with a conventional system. However, less attention has been paid to the comprehensive system analysis of this topic. Hence, the aim of the present thesis is to achieve the annual performance of the solar hybridized solid fuels to FTL fuels processes with novel configurations. A novel solar hybridized dual fluidized bed (SDFB) gasification process for FTL fuels production is proposed and investigated in the present thesis for cases with high reactivity solid fuels as the feedstock. The concept offers sensible thermal storage of the bed material and a process that delivers a constant production rate and quality of syngas despite solar variability. As a reference scenario for this concept, the proposed solar hybridized coal-to-liquids (SCTL) process is simulated for the case with lignite as the feedstock using a pseudo-dynamic model that assumes steady state operation at each time step for a one-year, hourly integrated solar insolation time series. For a solar multiple of 3 and bed material storage capacity of 16 h, the calculated annual solar share is 21.8%, assuming that the char conversion in the steam gasification process is 100%. However, the solar share is also found to be strongly dependent on the char conversion in the steam gasification process, so that the solar share is calculated to decrease to zero as the conversion is decreased to 57%. New configurations of the solar hybridized solid fuels (biomass and/or coal) to FTL fuels process are proposed and assessed, which are characterized with a novel SDFB gasifier with char separation, the incorporation of carbon capture and sequestration (CCS) and/or the use of FT reactor tail-gas recycle. Montana lignite and spruce wood have been chosen as the studied coal and biomass, respectively. Assessed using the pseudo-dynamic model, the annual solar share of the SCTL system can be increased from 12.2% to 20.3% by the addition of the char separation, for a char gasification conversion of 80%. To achieve well-to-wheel greenhouse gas emissions for FT liquid fuels parity with diesel derived from mineral crude oil, a biomass fraction of 58% is required for the studied non-solar coal and biomass-to-liquids system with a dual fluidized bed (DFB) gasifier. This biomass fraction can be reduced to 30% by the addition of carbon capture and sequestration and further reduced to 17% by the integration of solar energy with a solar multiple of 2.64 and a bed material storage capacity of 16 h. This reduction of the biomass fraction is very important given that biomass is typically more expensive than coal. As the biomass fraction is increased from 0% to 100%, the specific FT liquids output is decreased from 59.6% to 48.3% due to the increasing light hydrocarbons content. These two outputs (for biomass fractions of 0% and 100%, respectively) can both be increased to 71.5% and 70.9%, respectively, by integrating a tail-gas recycling configuration. Co-gasification of biomass with coal has the potential to further reduce the GHG emission from the SCTL systems, as discussed above. The application of biomass is usually limited by some properties (e.g., high moisture, low heating value and so on), which can be improved by torrefaction, as proved by previous work. Previous work also found that torrefaction can impact the bio-char gasification reactivity. In the present thesis, to better understand the influence of torrefaction on the bio-char gasification reactivity, further investigations were carried out on the char physicochemical characteristics that can influence the gasification reactivity, i.e., the char specific surface area, the char carbonaceous structure and the catalytic effect of inorganic matter in the char. The present experimental investigation showed that the influence of the torrefaction on the char gasification reactivity depended strongly on the biomass species and char preparation conditions. For a pyrolysis temperature of 800 ºC, the gasification reactivity of the chars from both the torrefied grape marc and the torrefied macroalgae were found to be lower than that of the chars from their corresponding raw fuels. This is mainly due to a lower specific surface area and a lower content of alkali metals (sodium and/or potassium) in the chars produced from both the torrefied grape marc and the torrefied macroalgae than for those chars produced from their corresponding raw fuels. However, the opposite influence of torrefaction was found for the macroalgae char when the pyrolysis temperature was increased to 1000 ºC. This is mainly due to a higher sodium concentration and a more amorphous carbonaceous structure for the torrefied macroalgae char than for the raw macroalgae char. In the present thesis, the process modelling results can be used for further economic analysis of the proposed novel configurations of solar hybridized coal and/or biomass to FTL fuels system via an SDFB gasifier. In addition, according to the experimental results of this study, the investigation of the influence of torrefaction on the bio-char characteristics can help to better understand the influence of torrefaction on the bio-char gasification reactivity.