The various downstream uses of syngas require that most of the contaminants present in raw syngas be removed to very low levels prior to use. Many of these contaminants can contribute to erosion, corrosion, and loss of strength in gas turbine components, and can act as poisons to the catalysts often used in syngas conversion and utilization processes. These same contaminants include or result in regulated air pollutants such as SOx, NOx, particulates, and mercury and other trace metals, which must be removed to increasingly low levels to meet stringent regulatory limits on air emissions.
Conventional methods for removing sulfur and other contaminants from syngas typically rely on chemical or physical absorption processes operating at low temperatures. However, after contaminant removal, the gas has to be reheated prior to its use in a gas turbine or other chemical synthesis process; in the case of downstream hydrogen production, additional steam needs to be added back to the syngas. These process swings adversely impact the plant's thermal efficiency and cost. Techno-economic analysis shows that gas-cleaning processes amenable to higher operating temperatures could significantly reduce this efficiency loss and improve the gasification plant's commercial viability. It is also critical that, while improving efficiency and reducing cost, the gas cleaning removes a wide variety of coal contaminants (including hydrogen sulfide, ammonia, hydrogen chloride, and carbonyl sulfide, as well as various forms of trace metals, including arsenic, mercury, selenium, and cadmium) to extremely low levels. Accordingly, the R&D approach in this area focuses on the development of high-efficiency processes that operate at moderate to high temperatures and provide multi-contaminant control to meet the highest environmental standards.
||RTI pilot-scale unit at Eastman Chemical Company gasifier. (Eastman Company)
High-temperature Syngas Cleanup Technology
Research Triangle Institute (RTI) is leading a project at Tampa Electric Company's (TECO) 250-megawatt (MW) Polk Power Station – an integrated gasification combined cycle (IGCC) plant located near Tampa, Florida. The project involves development of the High Temperature Desulfurization Process, which is a sorbent-based technology operating at relatively high syngas temperatures for removing hydrogen sulfide and carbonyl sulfide from syngas, down to a total sulfur down to less than one part per million, from a slip stream of coal-derived syngas at elevated temperature produced by the utility's coal gasifier. Scale-up for a 50-MWe demonstration size unit is underway. Pure, marketable solid sulfur will be created from the sulfur captured. Carbon dioxide will be captured using an activated MDEA system, with the intention of accelerating commercial deployment of these technologies for gasification of coal and petroleum coke. However, RTI's system (referred to as a warm syngas cleaning system) is capable of achieving very high levels of sulfur removal in syngas produced from the gasification of high-sulfur fuels. The activated MDEA process can then be applied to the low-sulfur syngas stream to capture carbon dioxide for sequestration, and, simultaneously, remove any trace sulfur from the syngas.
Warm Gas Multi-Contaminant Removal System
Two warm gas multi-contaminant removal system projects are in development and are designed to be used after the bulk warm gas sulfur removal step. NETL's ORD Warm Gas Cleanup project targets suitable levels of trace contaminant capture from syngas, essentially by developing sorbents capable of removing EPA designated toxic trace contaminants (mercury, arsenic, selenium, phosphorus, antimony, and cadmium) from high temperature syngas (up to 550°F). Focus is on testing and developing palladium sorbents for the capture of the trace metals. Similarly, TDA Research, Inc. is working to remove anhydrous ammonia (NH3), mercury (Hg), and trace contaminants from coal- and coal/biomass-derived syngas using a high-capacity, low-cost sorbent.
Hydrogen is often the desired product of the gasification process, given its importance as primary feedstock for fuels synthesis, fertilizer and chemicals synthesis, or power generation in 90% CO2 capture scenarios. In this case, inexpensive post-gasification separation of hydrogen from CO2 following (or along with) the shifting of gas composition is needed. For effective integration with advanced gasification technologies, and to realize the full advantages of high-temperature gas cleaning technologies, hydrogen and CO2 separation must be accomplished at high process temperatures. High temperature operation also offers the possibility of enhancing the water-gas-shift process through integration with advanced membranes operating at similar temperatures. Technologies that are capable of producing both hydrogen and CO2 at high pressure can avoid significant recompression costs that would further enhance plant economics, particularly in the case of carbon storage which requires very high compression of the CO2.
The hydrogen transport membrane, which uses metal or metal alloy materials with surface exchange catalysts to separate hydrogen from CO2, is being aggressively developed. Several projects have developed hydrogen membranes that have achieved fluxes and hydrogen purity high enough to encourage continued development of this cutting edge technology. These technologies operate at higher process temperatures designed to integrate at increased efficiency with advanced warm syngas cleanup technologies. This also offers the possibility of enhancing water gas shift through integration with advanced membranes, since both processes operate at similar temperatures.
The primary technical challenges for membrane-based technologies include optimization of the composition and microstructure of membrane materials, development of thin defect-free membrane films for enhancing flux, development of robust seals, ability to accommodate contaminants in the syngas, and operation at high-permeate pressures.
Praxair and Worcester Polytechnic Institute are developing an integrated, cost-effective hydrogen production and separation process that employs palladium and palladium-alloy membranes.
The Eltron Hydrogen Transport Membrane technology under development uses metal or metal alloy materials for separating hydrogen from carbon dioxide. The carbon dioxide remains on the feed side of the membrane, and therefore also remains at high pressure so compression costs for sequestration are reduced. The operating temperature of 535-825°F for these novel membranes is compatible with emerging warm gas-cleaning technology, enabling even better thermal efficiency and process economics for future coal-to-hydrogen plants using both technologies.
||Water-Gas Shift Vessel at the National Carbon Capture Center
Reduction of Steam Use in Water-Gas Shift (WGS)
One of the efforts at the National Carbon Capture Center at the Power Systems Development Facility involves testing of improved WGS catalysts. Results and analyses support reduction of steam-to-carbon monoxide ratios, thereby reducing steam requirement for IGCC cycles, which in turn will reduce the cost of electricity with CO2 capture.
Long-term candle filter tests (Transport Gasifier)
Researchers at the National Carbon Capture Center at the Power Systems Development Facility are utilizing the transport gasifier for testing and evaluating a particulate control device (PCD) in ongoing, long-term operation and filter element testing using a variety of fuels including coal/biomass mixtures to characterize the performance of the different technology units, their integration, and balance-of-plant processes.
Advanced CO2 Capture Technology for Low-Rank Coal IGCC Systems
Researchers are demonstrating the technical and economic viability of a new IGCC power plant designed to efficiently process low-rank coals.
Low-cost, Environmental Friendly Thermal Storage for CO2 Sequestration
Creare, Inc. developed the Combined Thermal and CO2 Storage System. This can be regarded as a syngas processing technology, in that CO2-containing syngas can be processed through the system to remove the CO2by reacting with CaO, which can be regenerated and used again.
As part of the support for the Syngas Processing key technology, studies are being conducted to provide unbiased comparisons of competing technologies, determine the best way to integrate process technology steps, and predict the economic and environmental impacts of successful development.
Recently Completed Projects
Other key technologies within Gasification Systems include the following: