University of Adelaide team exploring novel configuration for solar hybridized coal-to-liquids process
|Simplified flowsheet of the proposed solar hybridized coal- to-liquids (SCTL) process with the proposed solar hybridized dual fluidized bed (SDFB) gasifier. Credit: ACS, Guo et al. Click to enlarge.|
Researchers at the University of Adelaide (Australia) are proposing a novel configuration of a hybridized concentrated solar thermal (CST) dual fluidized bed (DFB) gasification process for Fischer–Tropsch liquids (FTL) fuels production. In their investigation of the process, reported in a paper in the ACS journal Energy & Fuels, they used lignite as the feedstock (Solar hybridized coal to liquids, SCTL), although the process could also be used with biomass.
Although fuel products produced via the Fischer-Tropsch process are high quality (free of sulfur, nitrogen and other contaminants found in petroleum-derived products), and coal is a plentiful and low-cost feedstock, the very high greenhouse gas emissions from coal-to-liquids production processes are a major barrier. As one approach to reducing the overall carbon intensity of FT fuels, there is growing interest in introducing concentrated solar power as a heat source into the gasification process.
Solar gasification is a process in which CST provides the heat to drive the endothermic gasification reactions, which displaces the partial combustion of the feedstock in conventional non-solar gasification. Hence, solar gasification has the potential both to reduce the CO2 emissions from the gasification process and to increase the raw syngas output from the gasifier per unit of feedstock. Solar gasification reactors have been widely studied over the past 30 years, with directly irradiated packed bed solar gasifiers for both coal and biomass among the first types to be assessed. The more recent of these reactors employ a SiC-coated graphite plate between the window and reaction zone to avoid direct contact between the reactants and the window, at the expense of a lower heat transfer rate. Packed bed gasifiers are relatively simple, robust, and cost-effective because they can tolerate a wide range of feedstock sizes and forms and do not require excess steam flows. Nevertheless, for feedstock with a significant ash content, their performance is limited by the buildup of ash on the top of the bed that inhibits heat and mass transfer through the bed and, in turn, the reaction rate and syngas productivity.
Entrained flow gasifiers perform well in heat and mass transfer, which increases significantly the syngas throughput, but imposes a strict requirement on the feedstock particle size due to the short residence time within the reactor.
As a third alternative, fluidized bed gasifiers offer the potential to resolve some of the heat and mass transfer limitations of the packed bed configurations and particle size sensitivity of entrained flow gasifiers. However, those employing direct irradiation to the bed, such as transparent glass tubes and top windowed devices, are also limited by contamination of the glass tube or window, while those with indirect irradiation are limited by poor heat transfer. Moreover, the single reactor solar hybridized gasifier also requires an air separation unit (ASU) for pure oxygen during periods of low solar irradiation to maintain the continuous operation of the whole FTL plant. However, the unsteady solar input leads to the unsteady operation of the ASU and the need for syngas storage to accommodate the unsteady syngas output, both of which add significantly to costs. Hence, it is desirable to seek alternative concepts with potential to address these challenges for the solar hybridized coal-to-liquids (SCTL) process.—Guo et al.
In the proposed University of Adelaide system, the solid bed material is the heat carrier to transfer the heat required by the gasification process from the combustion process and/or the solar receiver. The solar hybridized DFB (SDFB) gasifier offers the potential to store the solar thermal heat in the bed material as sensible heat in an additional storage tank.
A steady syngas output can be achieved by maintaining the constant temperature of the hot bed material to the gasification process through adjusting the additional fuel input to the combustion process according to the variation of the solar radiation variation.
The researchers simulated an SCTL process using a pseudodynamic model that assumes steady state operation at each time step for a one-year, hourly integrated solar insolation time series. They then investigated the annual energetic and environmental performance of this SCTL process as a function of the solar multiple (i.e., the heliostat field area relative to that required to meet the demand of the DFB gasifier at the point of peak solar thermal output); the bed material storage capacity; the assumed char conversion in the bubbling fluidized bed gasifier (BFBG); and the solar resource.
They found that solar energy can effectively be stored in the bed material to increase both the solar share and output while decreasing the CO2 emissions, with a commensurate increase in the heliostat field area.
For a solar multiple of 3 and bed material storage capacity of 16 h, the annual solar share is 21.8% and the annually averaged utilization factor of the heliostat field is 40.8%, assuming that the char conversion in the BFBG is 100%. However, the solar share is also found to be strongly dependent on the char conversion in the BFBG, so that the solar share decreases to zero as the conversion is decreased to 57%.
CO2 reduction. Among their findings was that the SCTL system can offer up to 39.4% reduction in CO2 emission relative to a conventional CTL conventional system with 16 hours of storage, even though this case requires a very large heliostat field (SM = 5). The theoretical limit is a 46.9% reduction, which requires more than 900 hours of storage.
Significant improvements are also possible with smaller fields and more realistic storage capacities, the authors found, although the carbon footprint still remains higher than the well-to-tank emissions for conventional petroleum diesel.
Peijun Guo, Philip J. van Eyk, Woei L. Saw, Peter J. Ashman, Graham J. Nathan, and Ellen B. Stechel (2015) “Performance Assessment of Fischer–Tropsch Liquid Fuels Production by Solar Hybridized Dual Fluidized Bed Gasification of Lignite” Energy & Fuels doi: 10.1021/acs.energyfuels.5b00007