Michael L. Swanson

Review of advances in combustion technology and biomass cofiring

Advances in combustion technology will be adopted only when they reduce cost and can be implemented with acceptable technical risk. Apart from technical risk, future decisions on new power plants will be principally influenced by trends in fuel cost, the efficiency and capital cost of new generating technologies, and environmental and regulatory policies including possible carbon taxes. The choice of fuel and generating technology for new power plants is influenced by an increasingly complex combination of interrelated factors: (1) current and future governmental polices on restructuring and deregulation of utilities, and environmental regulations that in the future could include taxes on carbon emissions; (2) macroeconomic factors such as proximity to load centers, electrical transmission lines, plant capital investment, delivered fuel cost, and fuel price stability; and (3) the state of development of new generating and environmental control technologies and the associated benefits and risks involved in their deployment, which are strongly related to fuel properties. This paper describes three advanced high-efficiency power systems for which the EERC has performed supporting research and development: (1) a coal-fired supercritical steam boiler with advanced emission controls; (2) an indirectly fired combined cycle using compressed air as the working fluid in a gas turbine (GT), fired either on coal alone or on coal and natural gas; and (3) two versions of a hybrid gasifier-pressurized fluidized-bed combustor (PFBC) system.

Long-Term Demonstration of Hydrogen Production from Coal at Elevated Temperatures Year 6 - Activity 1.12 - Development of a National Center for Hydrogen Technology

The Energy & Environmental Research Center (EERC) has continued the work of the National Center for Hydrogen Technology (NCHT) Program Year 6 Task 1.12 project to expose hydrogen separation membranes to coal-derived syngas. In this follow-on project, the EERC has exposed two membranes to coal-derived syngas produced in the pilot-scale transport reactor development unit (TRDU). Western Research Institute (WRI), with funding from the State of Wyoming Clean Coal Technology Program and the North Dakota Industrial Commission, contracted with the EERC to conduct testing of WRI’s coal-upgrading/gasification technology for subbituminous and lignite coals in the EERC’s TRDU. This gasifier fires nominally 200–500 lb/hour of fuel and is the pilot-scale version of the full-scale gasifier currently being constructed in Kemper County, Mississippi. A slipstream of the syngas was used to demonstrate warm-gas cleanup and hydrogen separation using membrane technology. Two membranes were exposed to coal-derived syngas, and the impact of coal-derived impurities was evaluated. This report summarizes the performance of WRI’s patent-pending coalupgrading/gasification technology in the EERC’s TRDU and presents the results of the warm-gas cleanup and hydrogen separation tests. Overall, the WRI coal-upgrading/gasification technology was shown to produce a syngas significantly lower in CO2 content and significantly higher in CO content than syngas produced from the raw fuels. Warm-gas cleanup technologies were shown to be capable of reducing sulfur in the syngas to 1 ppm. Each of the membranes tested was able to produce at least 2 lb/day of hydrogen from coal-derived syngas.

Advanced Gasification Mercury/Trace Metal Control with Monolith Traps

Three potential additives for controlling mercury emissions from syngas at temperatures ranging from 350 to 500 F (177 to 260 C) were developed. Current efforts are being directed at increasing the effective working temperature for these sorbents and also being able to either eliminate any potential mercury desorption or trying to engineer a trace metal removal system that can utilize the observed desorption process to repeatedly regenerate the same sorbent monolith for extended use. Project results also indicate that one of these same sorbents can also successfully be utilized for arsenic removal. Capture of the hydrogen selenide in the passivated tubing at elevated temperatures has resulted in limited results on the effective control of hydrogen selenide with these current sorbents, although lower-temperature results are promising. Preliminary economic analysis suggests that these Corning monoliths potentially could be more cost-effective than the conventional cold-gas (presulfided activated carbon beds) technology currently being utilized. Recent Hg-loading results might suggest that the annualized costs might be as high as 2.5 times the cost of the conventional technology. However, this annualized cost does not take into account the significantly improved thermal efficiency of any plant utilizing the warm-gas monolith technology currently being developed.

DEVELOPMENT OF AN ADHESIVE CANDLE FILTER SAFEGUARD DEVICE

In order to reach the highest possible efficiencies in a coal-fired turbine-based power system, the turbine should be directly fired with the products of coal conversion. Two main types of systems employ these turbines: those based on pressurized fluidized-bed combustors and those based on integrated gasification combined cycles. In both systems, suspended particulates must be cleaned from the gas stream before it enters the turbine so as to prevent fouling and erosion of the turbine blades. To produce the cleanest gas, barrier filters are being developed and are in use in several facilities. Barrier filters are composed of porous, high-temperature materials that allow the hot gas to pass but collect the particulates on the surface. The three main configurations of the barrier filters are candle, cross-flow, and tube filters. Both candle and tube filters have been tested extensively. They are composed of coarsely porous ceramic that serves as a structural support, overlain with a thin, microporous ceramic layer on the dirty gas side that serves as the primary filter surface. They are highly efficient at removing particulate matter from the gas stream and, because of their ceramic construction, are resistant to gas and ash corrosion. However, ceramics are brittle and individual elements can fail, allowing particulates to pass through the hole left by the filter element and erode the turbine. Preventing all failure of individual ceramic filter elements is not possible at the present state of development of the technology. Therefore, safeguard devices (SGDs) must be employed to prevent the particulates streaming through occasional broken filters from reaching the turbine. However, the SGD must allow for the free passage of gas when it is not activated. Upon breaking of a filter, the SGD must either mechanically close or quickly plug with filter dust to prevent additional dust from reaching the turbine. Production of a dependable rapidly closing autonomous mechanical device at high temperatures in a dusty gas stream is difficult because of problems with materials corrosion, dust leakage, and detection of filter failure. Therefore, the Energy & Environmental Research Center is using its knowledge of the factors that make filter dust sticky at gas filtration temperatures to make a simple and inexpensive SGD that employs an adhesive yet thermodynamically stable coating on a highly porous ceramic substrate. The SGDs are placed on top of individual candle filters at the filtered gas exit. Upon failure of the filter, the dirty gas flows through the SGD where the adhesive surface rapidly and permanently traps dust particles, causing the device to plug and prevent the dust from reaching the turbine.

ADVANCED POWER SYSTEMS ASH BEHAVIOR IN POWER SYSTEMS

The overall goal of this initiative is to develop fundamental knowledge of ash behavior in power systems for the purpose of increasing power production efficiency, reducing operation and maintenance costs, and reducing greenhouse gas emissions into the atmosphere. The specific objectives of this initiative focus primarily on ash behavior related to advanced power systems and include the following: � Determine the current status of the fundamental ash interactions and deposition formation mechanisms as already reported through previous or ongoing projects at the EERC or in the literature. � Determine sintering mechanisms for temperatures and particle compositions that are less well known and remain for the most part undetermined. � Identify the relationship between the temperature of critical viscosity (T cv ) as measured in a viscometer and the crystallization occurring in the melt. � Perform a literature search on the use of heated-stage microscopy (HSM) for examining in situ ash-sintering phenomena and then validate the use of HSM in the determination of viscosity in spherical ash particles. � Ascertain the formation and stability of specific mineral or amorphous phases in deposits typical of advanced power systems. � Evaluate corrosion for alloys being used in supercritical combustion systems.

Task 6.5 - Gas Separation and Hot-Gas Cleanup

Catalytic gasification of coal to produce H{sub 2}- and CH{sub 4}-rich gases for consumption in molten carbonate fuel cells is currently under development; however, to optimize the fuel cell performance and extend its operating life, it is desired to separate as much of the inerts (i.e., CO{sub 2} and N{sub 2}) and impurities (i.e., H{sub 2}S and NH{sub 3}) as possible from the fuel gas before they enter the fuel cell. In addition, the economics of the integrated gasification combined cycle (IGCC) can be improved by separating as much of the hydrogen as possible from the fuel, since hydrogen is a high-value product. One process currently under development by the Energy & Environmental Research Center (EERC) for accomplishing this gas separation and hot-gas cleanup involves gas separation membranes. These membranes are operated at temperatures as high as 800 C and pressures up to 300 psig. Some of these membranes can have very small pores (30-50 {angstrom}), which inefficiently separate the undesired gases by operating in the Knudsen diffusion region of mass transport. Other membranes with smaller pore sizes (<5 {angstrom}) operate in the molecular sieving region of mass transport phenomena, Dissolution of atomic hydrogen into thin metallic membranes made of platinum and palladium alloys is also being developed. Technological and economic issues that must be resolved before gas separation membranes are commercially viable include improved gas separation efficiency, membrane optimization, sealing of membranes in pressure vessels, high burst strength of the ceramic material, pore thermal stability, and material chemical stability. Hydrogen separation is dependent on the temperature, pressure, pressure ratio across the membrane, and ratio of permeate flow to total flow. For gas separation under Knudsen diffusion, increasing feed pressure and pressure ratio across the membrane should increase gas permeability; decreasing the temperature and the permeate-to-total flow ratio should also increase gas permeability. In the molecular sieving regime of mass transport, the inlet pressure and pressure ratio should have no effect on gas permeability, while increasing temperature should increase permeability.