Gasification is a chemical conversion process that converts carbonaceous materials and steam into syngas, a mixture of hydrogen and carbon monoxide that can be used downstream for power generation or converted into higher value-added products, such as transportation fuels. However, this conversion process requires energy, which is typically provided by the combustion of a fuel that releases carbon dioxide. Hybridizing the gasification process with solar energy can reduce the amount of combustion fuel required to provide the process heat, thereby reducing carbon dioxide emissions from the gasification process. Furthermore, fuel-flexible gasification reactor technologies enable low carbon-emitting fuels to be used in the gasification process to further lower carbon dioxide emissions.
Under a Department of Energy (DOE) funded program, Advanced Cooling Technologies, Inc. (ACT) developed a fuel-flexible hybrid solar coal gasification reactor that not only utilizes solar energy to reduce the process energy requirements but enabled low carbon-emitting fuels, such as natural gas, to be used as the combustion fuels, thus further reducing carbon emissions. This gasification reactor technology was developed from a high-temperature annular heat pipe with an integrated fluidized bed gasification reactor, as shown in Figure 1.
This allothermal reactor technology (i.e. heat provided externally from the gasification reactor) enables any thermodynamically favorable fuel to be combusted in the bottom half of the reactor and transfer heat to the carbonaceous material and steam in the gasification zone. Additionally, the annular heat pipe reduces temperature distributions in the fluidized bed producing more consistent conversion. This is demonstrated by the less than 2°C axial temperature distribution of the annular heat pipe reactor at steady state conditions, shown in Figure 2.
After fabrication, the gasification reactor was placed into a test apparatus and activated charcoal was fluidized with inert argon fluidization gas and steam at a temperature of 900°C. The conversion of steam and activated charcoal to hydrogen and carbon dioxide was evaluated by gas chromatography at steam residence times varying from 1 second to 5 seconds. As shown in Figure 3, the conversion of hydrogen and carbon monoxide is directly proportional to the residence time of the steam in the carbon bed reaching an 88% conversion at the maximum residence time evaluated.
This reactor configuration demonstrated that an all thermal gasification reactor linked thermally through an annular high-temperature heat pipe was feasible, thereby providing fuel flexibility to the gasification process. Additionally, creating the steam required for the gasification reactor is an energy-intensive process requiring the combustion of fuels to convert liquid water into steam. However, using solar energy to produce the steam reduces the fuel requirements of the overall gasification process, and reduces carbon emissions. Thus, combining the use of solar energy to reduce the energy required for steam generation and the fuel-flexibility of the annular heat pipe with an integrated fluidized bed gasification reactor produces syngas in a low-carbon emission process.