Other Possibilities for Gasification-based Power Production: Solid Oxide Fuel Cells
Solid State Energy Conversion Alliance
A partnering of private industry, academia, and national laboratories, established and maintained by the DOE, the Solid State Energy Conversion Alliance (SECA) is tasked with the development and demonstration of coal-fueled central-station electric power generation technology based upon the solid oxide fuel cell (SOFC). The power plant resulting from this effort, integrating the SOFC and advanced coal gasification, is intended to meet key SECA objectives, namely, that the system be cost-competitive, generate electric power with high electric efficiency, provide for clean emissions and high-percentage carbon capture, and require low net water input. The SECA website has additional information and details on the SECA program, its participants, the SOFC technology, and the candidate power plant concepts being developed.
Gasification and the SOFC
The coal gasifier enables the SOFC to use coal as fuel, and there can be particularly good synergy between the coal gasification and SOFC technologies when they are teamed in a carbon-capturing electric power system. The syngas produced by a gasifier will typically contain hydrogen and carbon monoxide. Both gases can be used directly by an SOFC as fuel after conventional gas cleanup, and for the SOFC application, there is no specific need to shift the carbon monoxide to hydrogen and carbon dioxide before fuel cell entry.
Syngas from a coal gasifier can also contain methane, which could be converted in the fuel processing stream to hydrogen and carbon monoxide by conventional steam reforming. However, the SOFC generator module can be designed to do methane reformation internally, and even on-cell, getting moisture required for the process from anode off-gas recycle. Then, since reformation is endothermic, it could be purposely used by the module to assist with SOFC cooling, thereby lessening the need for fuel cell cooling air, reducing the power system cooling parasitic load, and tending to increase the system electric efficiency; the syngas methane content can be optimized by gasification process design and process-variable selection. Another benefit of designing the SOFC module for internal fuel reformation is that it will avoid the system need for dedicated reformation hardware outside the module, limiting system complexity and cost.
Another way SOFC module design can be used to significant advantage involves the requirement that the power system design must provide for the separation and capture of carbon dioxide produced. With an SOFC, the only required interaction of the anode and cathode reactant streams is the transfer of oxygen from cathode side (air) to anode side (fuel). All carbon, excepting the negligible amount in atmospheric air coming in at the cathode, will enter the module with fuel on the anode side, and it must exit the anode as carbon dioxide and, to a lesser extent, as any unreacted carbon monoxide. By designing the module to maintain the anode and cathode off-gas streams separated, dilution of that carbon-rich stream with atmospheric nitrogen from the cathode side will be precluded, and that design feature can be used to simplify immensely the carbon dioxide separation and capture process to be performed downstream.
To avoid reaction with nickel-bearing SOFC components, the sulfur content of the fuel stream entering the anode must be held to low levels, and would be routinely accomplished by conventional syngas sulfur cleanup and polishing. In addition to achieving the required SOFC materials protection, this process will result in power system sulfur oxide emissions that are essentially negligible.
Several coal gasifiers are available today, designed, manufactured, and offered for sale by commercial suppliers. These gasifiers are based on a variety of favored design concepts and approaches. As a result, they can differ in the composition of the syngas produced and the efficiency with which they convert coal energy content to syngas energy - a performance parameter typically termed cold gas efficiency. The gasifiers can also vary in their values of main operating parameters – e.g., process temperature, pressure, and demands for oxygen and steam. For the power system based upon the integration of coal gasification and SOFC technologies, these parameters, particularly cold gas efficiency and the oxygen and steam demands, will affect its electric efficiency.
Coal gasifiers are of three main types - Entrained Flow, Moving Bed, and Fluidized Bed. Click on the hypertext for details on the individual gasifier's characteristics.
Entrained flow gasifiers (e.g., GE Energy/Texaco, Shell, Conoco-Phillips/E-Gas, Siemens Sustec) could attract early interest for fuel cell power system applications because they are relatively well developed and are used in current IGCC power system designs and applications. Entrained flow gasification typically proceeds at relatively high process temperatures, requires oxygen input at relatively high rates, steam input at low to moderate rates, and it produces a syngas product with very small methane content, typically less than 1% (vol). Cold gas efficiencies for entrained flow gasification are usually in the circa 80% vicinity.
The moving-bed gasifier (e.g., Lurgi process) operates at moderate temperature levels, and with moderate oxygen and steam supply requirements. The cold gas efficiency achieved by this gasifier is higher, circa 90%, and its syngas product stream will have a methane content nominally in the 4-5% (vol) range. Fluidized-bed gasification (e.g., KBR Transport) proceeds with similar characteristics, but will exhibit a somewhat lower syngas methane content, typically in the 2-3% (vol) range.
Of particular interest to the SECA program, and to an SOFC-based integrated gasification fuel cell (IGFC) power system, is catalytic coal gasification. This process experienced development in the 1980s for synthetic natural gas production purposes. However, it has not been pursued extensively for coal-fueled power system or IGFC application, and is presently not available commercially. Compared to conventional gasification summarized above, a catalytic gasifier would require less oxygen input, run at a lower process temperature, and produce a syngas stream with a higher methane concentration [15-30% (vol)], in addition to hydrogen and carbon monoxide. With its lower operating temperature, a relatively high cold gas efficiency, at least 90%, is projected for catalytic gasification, and this characteristic, as well as the process need for less oxygen input, would directly support high-efficiency IGFC power system operation. A key link between catalytic coal gasification and high-efficiency IGFC operation is the high methane content of its syngas; as noted above, methane reformation within the SOFC generator module can be used, by design, to assist module cooling, and can thereby lead to reductions in the parasitic power demand associated with cooling air supply.
An SOFC electric power generator in an IGFC system could conceivably be fueled with syngas supplied by any of the available conventional coal gasifiers, and DOE/NETL studies indicate that power system electric efficiencies in the 45-50% range (net AC/coal HHV, and including the parasitic-power effect of gas compression required for CO2 sequestration) are achievable, depending on whether the power system uses atmospheric-pressure or pressurized SOFC generator modules. However, from the efficiency viewpoint particularly, the preferred coal gasification approach for the application is catalytic. Using that technology, IGFC system efficiencies in 56-60% range are projected, again depending on SOFC module pressurization.
Typical major components of an IGFC system, this one centered on an SOFC generator running at atmospheric pressure, are identified in the simplified cycle diagram of Figure 1. The system fuel, coal, is processed to syngas by the gasifier, which is then supplied to the SOFC generator after cleanup and the syngas pressure is dropped to the SOFC generator operating pressure. The syngas pressure reduction step is accomplished in this system concept by an expander/generator, which thereby produces part of the cycle's gross power generation. Oxygen for the coal gasification process is provided by a conventional air separation unit, and steam for the gasifier is raised by power system heat and recycled water. Note that the SOFC generator is configured to maintain the anode and cathode off-gas streams separated, and the anode off-gas, which contains some electrochemically-unreacted hydrogen and carbon monoxide, is combusted to completion at the oxy-combustor. Maintaining the off-gas streams separated restricts the large atmospheric-nitrogen component to the cathode side, and simplifies the CO2 capture process to one involving anode off-gas cooling, water-vapor condensation, CO2 drying, and CO2compression. Heat recovered from the anode-side process can be used by a power-generating bottoming cycle. On the cathode side, process air for the SOFC electrochemical process and for generator cooling is provided by an air blower; heat can be recovered from the hot cathode off-gas stream to preheat the process air as needed, and for the generation of additional power generation, if it is economical to do so.
|Figure 1. IGFC power system cycle schematic, with atmospheric-pressure SOFC.
(click to enlarge)
Due to the inherently efficient SOFC, and to using recovered SOFC exhaust heat to generate additional electric power, an IGFC system is capable of operating at a high electric efficiency that significantly exceeds those associated with conventional pulverized coal (PC) and integrated gasification combined cycle (IGCC) power systems. IGFC efficiency margins considered achievable, based upon NETL comparative studies of advanced power systems, are apparent in Table 1.
The power system cycle for the IGFC pressurized-SOFC case is basically similar to the atmospheric-pressure cycle depicted in Figure 1, but it would run the SOFC at elevated pressure, achieving an SOFC voltage boost, and would replace the cathode-side process-air blower with a compressor. In the baseline pressurized-system configuration, an expander/generator would be installed in the cathode off-gas stream to reduce gas pressures and generate additional AC power. Optionally, an expander/generator set could also be placed in the anode off-gas stream, just downstream of the oxy-combustor, and ahead of off-gas heat recovery.
In addition to high power system efficiencies, the NETL studies also project significant IGFC system power plant capital cost, cost of electricity, and net water usage advantages. The SECA website provides additional system cycle and NETL study detail.
Summarizing, IGFC electric power systems are projected to be effective in meeting SECA program objectives, particularly if they integrate catalytic coal gasification with SOFC generator module designs that separate anode and cathode off-gas streams, and feature methane reformation-augmented SOFC cooling. Studies indicate that by this approach cost-effective power systems can be developed that will operate cleanly with very high electric efficiencies, while providing for high levels of carbon capture, and requiring low net water input.
Table 1. Power System Efficiency Estimates.
* The efficiency estimates include the effects of AC parasitic power loads due to CO2 compression.