Regenerable Sorbents for Hot Fuel Gas Desulfurization for Fuel Cell Applications
Over the past two decades, significant effort has been devoted by the DOE, academic groups, and industry to develop regenerable sorbents for the desulfurization of fuel gas streams (particularly coal-derived) at high temperatures. To date, no bulk oxide or mixed oxide sorbent has been found that achieves the desired criteria for high reactivity, efficient H2S removal and regenerability in cyclic operation. Desulfurization of fuel gases to the lowest possible levels is critical from an environmental and commercial standpoint because of the polluting impact of and governmental restrictions on the amount of sulfur released into the atmosphere from the combustion of coal.
Desulfurization is as essential for the efficient, long term operation of fuel cells. Fuel cells continue to undergo rapid development for both stationary and transportation applications. Low-sulfur diesel, fuel oils and hydrogen are being contemplated for use with fuel cells domestically. For even low-sulfur fuels, however, there remains a pressing need for a sorbent unit capable of effectively removing H2S upstream of the fuel cell anode to prevent sulfur poisoning and performance degradation. Moreover, for high temperature fuel cells, such as solid oxide fuel cells (SOFC), a sorbent is needed with the additional capability to reliably perform cyclic operations with high structural stability at temperatures exceeding 600°C.
Professor Maria Stephanopoulos and colleagues at Tufts University have developed a solution to these challenging problems. They have developed a novel regenerative cerium and lanthanum oxide based sorbent technology that employs a swing-adsorption/desorption process, whereby anode offgas is used to sweep desorbed H2S to the burner that supplies heat to the fuel processor. This technology promises:
- excellent sulfur removal efficiency (to <1ppm H2S);
- very fast adsorption of H2S under high space velocities, allowing only the surface of the sorbent to sulfide;
- regeneration (desorption of H2S) in various gas streams at times comparable to the adsorption times;
- a single, onboard sorber/regenerator pair configuration, enabling both small (<1L / 5kW) and large scale fuel cell power plants;
- off-site regeneration design configurations through the use of removable sorbent cartridges;
- a reversible process, with no irreversible structural complications, enabling long cycle lifetimes;
- flexibility in regeneration (gas type and temperature);
- good reactivity and stability at temperatures ranging 650-800 °C.
Desulfurization for Portable Auxiliary Power Units (APUs), Fuel Cells
Commercial prospects for using fuel cells as APUs are increasing. In the transportation sector, APUs can serve as range extenders for cars and small power/heating modules for trucks and refrigeration units. Employing APUs in large trucks to produce electricity and cabin heat instead of employing engine idling, for example, can reduce CO2 emissions of 20-30 tons per year per truck according to a recent DOE calculation. At efficiencies approaching 50%, such APUs could dramatically reduce fuel consumption and cost as well. Common diesel (DF-2) fuel, however, contains impurities, particularly sulfur, that can poison the APU. The Tufts regenerative sorbent technology enables the creation of compact, commercially feasible systems for both onboard and removable cartridge based removal of the sulfur upstream of the APU.
Additional portable APU markets that may benefit from onboard or removable cartridge based desulfurization include:
- robust, fuel source tolerant portable electronics (mobile communications, computers, soldier power sources/equipment)
- portable generators (home, light industry)
- passenger transportation (bicycles, scooters, autos)
- marine and underwater (boats, submarines)
Desulfurization for Stationary Fuel Processing/Electricity Generation/Hydrogen Production
Global reserves of fossil fuels are becoming increasingly “sour” with sulfur. The Tufts regenerable sorbent technology may be employed by companies and utilities in the large gas processing market, offering the refining, pipeline and terminal industries significant commercial and operational benefits to meet existing and future clean fuels regulations.
This technology is particularly well suited to reducing pollutant emissions from coal gasification. Coal offers strong potential as an important fuel for traditional power generation in the US due to its secure supply and broad availability with large domestic reserves. The importance of coal is recognized by the government through the Clean Coal Initiative. By providing for an improved desulfurization treatment of coal gasification syngas at the desired elevated temperatures, this technology promises to improve overall efficiencies to meet Clean Coal Initiative goals.
At the same time, this technology also serves to facilitate the generation of hydrogen to power fuel cells, as hydrogen is a major component of syngas. Desulfurization of the hydrogen allows for the production of ultra-pure hydrogen with only trace amounts of residual sulfur.
To date, development of the Tufts regenerable sorbent technology has been funded by the Army Research Laboratory Collaborative Technology Alliance (CTA) on Power and Energy and the NASA Glenn Research Center. Given the source of research funding, initial focus has been centered on ensuring the flexibility of design, fuel and size parameters to meet key requirements for intended military power applications.
US Patent No. 7,871,459, Issued January 18, 2011.
Additional patent pending technologies are also available from Tufts University that may prove to be highly complementary:
- Substantially Reduced Metal Catalysts for Fuel Cells (T001234)
- Improved Stability Reduced Metal Catalysts for Fuel Cells (T001335)
Tufts University seeks to enter into licensing relationships with industry or partner with an entrepreneurial individual or team to explore the potential to develop a startup company.