A review of the state-of-the-art in solar-driven gasification processes with carbonaceous materials
Introduction
Each hour, approximately 4.3 ⋅ 1020 J of energy from the sun impinge directly upon the earth’s surface (Lewis and Nocera, 2006), providing the most abundant and exploitable renewable energy resource available. However, the sunlight arrives in a relatively dilute form, and it is intermittent and unequally distributed. These barriers can be overcome by concentrating and storing sunlight in a chemical form [i.e., solar fuels (Perkins and Weimer, 2009, Piatkowski et al., 2011, Steinfeld and Meier, 2004)]. Solar fuels such as H2 and CO can be used to greatly alleviate worldwide dependency on fossil fuels. The combination of CO, CO2, and H2 constitutes synthesis gas (syngas): The precursor to liquid hydrocarbon fuels via Fischer-Tropsch synthesis (Corporan et al., 2007, Dry, 2002, Swanson et al., 2010) or other known catalytic routes required to drive transportation and power sectors. Syngas can also be directly combusted in a combined cycle or converted completely to H2 via a water-gas shift reaction. Solar fuels can be produced in uninhabited desert regions with optimal solar irradiation and transported to population centers. Solar facilities like power towers with secondary concentrators coupled to arrays of heliostat fields and parabolic dishes are capable of concentrating solar irradiation in excess of 1000 suns (where 1 sun = 1 kW⋅m−2), optimal for driving solar thermochemical processes and cycles that produce syngas. Several thermochemical pathways have been proposed and investigated to produce syngas by harnessing the power of the sun (Steinfeld, 2005).
Of special interest are solar technologies aimed at the gasification of carbonaceous feedstock (Perkins and Weimer, 2009, Piatkowski et al., 2011) with steam and/or CO2, where the process heat needed to drive the reaction is derived from concentrated solar irradiation (i.e., solar-driven allothermal gasification). The carbonaceous feedstock is upgraded in calorific content equal to the enthalpy change of the endothermic reaction, which results in the net storage of solar energy in a chemical form. The carbonaceous feedstock is transformed into a fuel with a broader range of more efficient applications. This is an attractive alternative to conventional autothermal gasification processes where pure streams of O2 are introduced into the system to combust a portion of the feedstock (i.e., oxy-combustion) for process heat to drive the reactions, thereby, lowering the overall energy content of the products. Solar-driven gasification processes are also relatively free of combustion products and tars that are associated with autothermal gasification.
The overall process is schematically depicted in Fig. 1, which shows the inputs of concentrated solar irradiation coupled to an array of carbonaceous feedstocks and steam and/or CO2 to produce syngas. The resulting syngas can be directly combusted, shifted to H2, or transformed into a liquid hydrocarbon for powering the transportation sector.
Solar-driven gasification for an array of carbonaceous feedstocks, both biomass and solid fossil fuels, has been performed to analyze theoretical thermodynamic performance of different processes, to determine reaction mechanisms and associated kinetic parameters, and to develop solar thermochemical reactor technologies (i.e., solar gasifiers) adapted to specific carbonaceous feedstocks for optimal performance over a range of scales. These results are contained in numerous publications that report work performed over many years. Our aim is to consolidate the principal findings and provide a comprehensive summary of the work to date in the area of solar-driven gasification.
Section snippets
Economic and thermodynamic analyses
The framework for comparing solar gasification to conventional gasification both with biomass was proposed in an economic assessment where four different solar routes compared to the conventional gasification route (Nickerson et al., 2015). Solar gasification was competitive with conventional gasification for certain scenarios. Due to the relatively low cost of natural gas in the United States in 2012, it was difficult for solar gasification to be cost effective without subsidies. However,
Kinetic analyses
Accurate determination of chemical kinetics is vital for designing and optimizing solar gasifiers that are customized for different feedstocks with a range of size distributions. The identification of the rate limiting mechanism(s) and determination of kinetic parameters is essential for designing and modeling efficient solar gasifiers. Pyrolysis represents an important reaction mechanism to consider in gasification processes as volatiles diffuse through pores and release tars. For a range of
Solar gasifiers
Solar thermochemical reactors (i.e., solar gasifiers) have been designed for operation over a range of temperatures to accommodate a range of applications (Puig-Arnavat et al., 2013). At the high temperature required to drive the process, there is very little, if any, opportunity to take advantage of spectrally selective materials to maximize absorptions in the solar spectrum while minimizing emissions to environment as can be done at lower temperatures (Cao et al., 2014). Cavity-type reactors
Summary and conclusions
We have examined the state of the research currently being pursued to examine the potential impact of using concentrated solar irradiation as a primary heat source to drive allothermal gasification processes with steam and CO2. From this work, the potential to transform carbonaceous feedstock into a range of fuels is highlighted, and research in the areas of economic, thermodynamic, and kinetic analyses provide a platform for moving the research forward. These results have guided the
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