John P. Hurley

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.

Status of coal ash behavior research

The inorganic components contained in coal are converted to ash during combustion and gasification. The resulting ash can cause significant problems that include slag flow behavior, ash deposition, bed agglomeration, corrosion and erosion of system parts, fine particulate that is difficult to collect, and blinding of hot-gas cleanup filters. The type of problem and the degree of the problem are dependent upon coal composition, system operating conditions, and system design criteria. Much research has been conducted on ash formation and behavior in pulverized coal- and cyclone-fired utility boilers. Many of the mechanisms of transformations and ash deposition have been worked out, and predictive models have been developed. Ash behavior research in fluidized-bed combustion systems has been conducted to determine the effects of coal quality and operating conditions on bed agglomeration and deposition on heat-transfer and other surfaces. Ash can also influence gasification system operability. Problems such as maintaining slag flow and ash deposition have been encountered. In advanced power systems, hot-gas cleanup is a significant issue in which ash plays a significant role. The issues in hot-gas cleanup include ash deposition on filters, ash removal, and vapor-phase and ash-ceramic interactions. The chemical and physical characteristics of the ash also influence their ability to be collected in air pollution control devices.

The cause of surface tension increase with temperature in multicomponent aluminosilicates derived from coal-ash slags

An explanation is proposed for the increase of surface tension with temperature in multicomponent aluminosilicate systems such as those derived from coal-ash slags. Two major factors are considered: (1) depolymerization of aluminosilicates caused by rearrangements of intermediate structures in the surface layers, and (2) the increase in surface entropy caused by evaporation of some ash slag components. Electron spectroscopy for chemical analysis spectra were recorded for oxygen 1s photoelectrons on quenched bulk slags and on sessile drops to gain insight into the depolymerization of coal-ash slags with temperature. The tests performed on quenched bulk slags indicated replacement of bridging oxygen [Si-O] with non-bridging oxygen atoms [Si-O−] as a function of increasing temperature. Mössbauer spectra showed an increase in ferrous iron from 4% to 12% of total iron as temperature rose from 1400 °C to 1500 °C. The increase in non-bridging oxygens resulted from the reduction of tetrahedrally coordinated Fe3+ to octahedrally coordinated Fe2+. Also, the intensity of the non-bridging oxygen 1s photoelectron peak was higher when detected on the surface of a sessile drop than when detected from the bulk of the drop.

The role of physical factors in mass transport during sintering of coal ashes and deposit deformation near the temperature of glass transformation

Abstract The role of physical properties of melts such as viscosity, diffusion, and surface/interfacial tensions in sintering and deformation mechanisms of ash deposits above glass-transformation temperature is discussed. The differential thermal analysis (DTA) technique was applied to measure glass transformation and crystallization temperatures. Sintering of selected coal ashes was performed as a function of temperature in air. The mechanical properties of sintered ashes were measured below and above the glass-transformation temperature, T g . It was found that sintering propensities of amorphous ashes and superplastic-like deformation of deposits above T g depend on mass transport phenomena in the intergranular liquid phase.

Strength development at low temperatures in coal ash deposits

At temperatures below approximately 1900°F, ash particles formed in coal-fired energy systems are relatively hard and not prone to sticking to system surfaces. However, if the ash collects on a surface not exposed to a shearing gas flow such as the downstream side of a heat exchanger or the surface of a hot-gas filter, the deposit can develop enough strength over a period of minutes to days so that it becomes difficult to remove, in some cases growing to sizes that impede the flow of gas. This paper presents data from ongoing measurements of the significance of ash and gas composition, deposit temperature, and time on the rates of strength development in simulated low-temperature ash deposits. Preliminary results of surface composition and particle-size distribution analyses of the ash, including submicron material, are also presented to explain the possible mechanisms of strength development.

Coal-ash corrosion of monolithic silicon carbide-based refractories

Abstract Several silicon carbide-based monolithic refractories were subjected to static coal ash corrosion tests to determine corrosion mechanisms and rates. Two castable refractories with 75% and 85% SiC were exposed to two types of coal ash at temperatures from 1090°C to 1430°C. Several plastic refractories were exposed to a high-calcium coal ash at 1430°C for over 100 h. Optical microscopy and scanning electron microscopy with energy-dispersive X-ray analysis were used to determine the corrosion mechanisms and rates of these materials.

Hot-gas filter ash characterization

One of the key difficulties in the development of advanced pressurized fluidized-bed combustion (PFBC) and integrated gasification combined-cycle (IGCC) systems is the need to remove particulates from the gas stream at high temperatures and pressures. Research has revealed numerous cases of ash cake buildup on filter elements that has been difficult to remove using on-line jet pulsing. The objectives of this research are to: (1) determine the mechanisms by which a difficult-to-clean ash is formed and how it blinds or bridges hot-gas filters; (2) develop a method to determine the rate of blinding or bridging based on analyses of the feed coal and sorbent and on the operating conditions; and (3) provide suggestions for ways to prevent filter blinding and bridging by the troublesome ash. Four tasks are being performed: Task 1--field sampling and archive sample analysis; Task 2--laboratory-scale testing; Task 3--bench-scale testing; and Task 4--computer modeling. Results are presented from the first two tasks.

Corrosion of MA754 and MA956 in a Commercial Aluminum Melter

The University of North Dakota Energy & Environmental Research Center is working with Oak Ridge National Laboratory to test two oxide dispersion-strengthened alloys that could be used to construct very high-temperature heat recuperators for the aluminum-melting industry. For the initial tests, uncooled rings of MA754 and MA956 piping were exposed for 5½ months to gases leaving an aluminum melter furnace at 1200°–1290°C. The MA956 suffered spotty areas of severe corrosion and lost 25% of its weight. Scanning electron microscopy showed that there were small spots of alkali-rich corrosion products on the alloy surfaces, indicating the impact of droplets of fluxing agents. The corrosion products in these areas were mixed Fe, Cr, and Al oxides, which were depleted in Cr near the gas surface. However, Al concentrations in the remaining metal were typically between 3.5% and 4.0%, so there was a sufficient reservoir of Al remaining in the alloy to prevent simple breakaway corrosion which could have occurred if the Al were significantly depleted. The MA754 lost approximately 15% of its weight and showed void formation within 2 mm of the gas–metal surfaces. Within the porous area, the Cr had largely segregated into oxide precipitates up to 50 9m in diameter, leaving the remaining metal Ni-rich. Below the porous layer, the alloy composition was relatively unchanged. Remains of Na- and Al-rich particles that had impacted the surface sporadically were visible but had not obviously affected the surface scale as they had with the MA956.

Rates and Mechanisms of Strength Development in Low-Temperature Ash Deposits

At temperatures below approximately 1900°F, ash particles formed in coal-fired energy systems are relatively hard and not prone to sticking to system surfaces. However, if the ash collects on a surface not exposed to a shearing gas flow such as the downstream side of a heat exchanger or the surface of a hot-gas filter, the deposit can develop enough strength over periods of minutes to days so that it becomes difficult to remove, in some cases growing to sizes that impede the flow of gas. This paper presents data from ongoing measurements of the significance of ash and gas composition, deposit temperature, and time on the rates of strength development in simulated low-temperature ash deposits. Preliminary results of surface composition and particle-size distribution analyses of the ash, including submicron material, are also presented to explain the possible mechanisms of strength development.

Use of North Dakota lignite in advanced power systems

In order to develop critical data for Department of Energy (DOE) and private industry for advanced high-efficiency power systems using North Dakota lignite in pressurized gasification and combustion systems, tests were performed in bench-scale equipment at the Energy and Environmental Research Center (EERC). The primary objectives were to (1) determine the conversion levels for Center ND lignite under pressurized fluid-bed gasification conditions with sorbent addition as a function of temperature, (2) determine the sulfur capture using limestone or dolomite under gasification conditions giving 90% or higher carbon conversion, (3) evaluate char/coal conversion and sulfur capture in a pressurized fluid-bed combustor, (4) assess the potential for bed agglomeration under the preferred operating conditions for both systems.