Hot-gas cleanup for coal-based gas turbines.
THE AVAILABILITY OF reliable low-cost electricity is the cornerstone for the United States to compete in the world market and to maintain its employment opportunities and standard of living. The U.S. Department of Energy (DOE) predicts total consumption of electricity in the United States will rise from 2.7 trillion kilowatt-hours in 1990 to 3.5 trillion in 2010. Although energy sources are diversifying, fossil fuel still provides 90 percent of the nation's energy. Coal is the country's most abundant fossil-fuel resource and the source of 56 percent of its electricity.
Historically, coal has been the fuel of choice because of its availability and low cost. A new generation of technology has enabled combining the use of coal and protection of the environment. Such new ultraclean high-efficiency systems virtually eliminate the pollutants associated with coal-fired plants built before the 1970s.
Integrated gasification combined-cycle (IGCC) and pressurized fluidized-bed combustion (PFBC) are two examples of types of new-generation advanced coal-fired power plants. In these plants, coal-derived gases are cleaned prior to or during combustion at elevated pressures and temperatures, which makes the compatible with gas turbine power generation systems. Such gas turbine systems will achieve exceptional efficiencies by the end of the decade, assisted by a government-sponsored advanced turbine system (ATS) initiative.
In commercially available IGCC systems, superior environmental performance has already been achieved. With the adaptation of hot-gas cleanup, large-scale demonstration plants will achieve 45 percent efficiency by the turn of the century. IGCC plant efficiencies will climb to 52 percent and greater as advanced turbine systems are incorporated.
In an IGCC system, fuel gas, which is composed of hydrogen and carbon oxides, i generated in a gasifier by coal reacting with steam and air or oxygen. The pressurized fuel gas is then cleaned and fed to a high-efficiency combustion ga turbine/generator. The hot turbine exhaust gas produces steam to drive a steam turbine/generator.
Coal is converted into a gaseous fuel, which, when cleaned, is comparable to natural gas. More than 99 percent of the coal's sulfur and particulate contaminants are removed from the gaseous fuel by gas cleanup processes before being burned in the gas turbine.
In the PFBC process, jets of air in a fluidized bed suspend a mixture of coal and limestone or dolomite during combustion, converting the mixture into a suspension of red-hot particles that flows like a fluid. The limestone captures sulfur oxides that are released by the burning coal.
Operating the combustor at higher than atmospheric pressure increases the power produced by the combined-cycle system. The gas stream from the combusted coal drives a gas turbine, and the steam generated from the heat in the fluidized be drives a steam turbine. To further improve plant efficiency and to lower emissions, hot-gas cleanup and partial gasification are added, resulting in a plant with greater than 50 percent efficiency.
For IGCC systems, hot-gas cleanup is a means of controlling particulate and gaseous species in an environment of high temperatures [ranging from 800 [degrees] to 1200 [degrees] F (425 [degrees] to 650 [degrees] C) for gases and 1200 [degrees] F (650 [degrees] C) for particulate matter], and of system pressures ranging from 150 to 350 psi (10 to 20 atmospheres). For PFBC systems, hot-gas cleanup refers to the control of particles at like pressures, but in a higher temperature range, 1500 [degrees] to 1600 [degrees] F (815 [degrees] to 870 [degrees] C). The gaseous species of primary concern include sulfur and nitrogen, which, when combusted, are oxidized to sulfur and nitrogen oxides.
The benefits of cleaning coal gas at high temperatures and pressures derive fro the combined-cycle concept, wherein temperatures and pressures of the gasifier/combustor and turbine are matched. This approach allows the gasifier and cleanup system temperatures and pressures to be compatible with the turbine system while simultaneously meeting fuel specifications. Hot cleanup operation at elevated pressures greatly reduces the volume of gases that require processing. Costs and environmental concerns are minimized through this temperature and pressure compatibility.
Hot-gas cleanup represents a trade-off of the capital and operating costs of high-temperature and -pressure equipment versus those of a cold cleanup system. Typical cold cleaning systems require an elaborate wastewater handling cycle an heat exchangers to cool the process gases and later reheat the same gases.
Hot-gas cleanup improves overall system efficiency by as much as three percentage points. Higher efficiency means that smaller amounts of coal can be burned to produce more power. Lower plant costs and less pollutant emissions ca be realized.
The hot-gas cleanup program is driven by the need to develop feasible technologies for controlling or removing contaminants from coal-derived gases. Hot-gas cleanup systems must meet acceptable fuel specifications, exceed environmental standards, show economic superiority, and lower technological risk.
Unlike the combustion of clean fuels, such as distillate oil and natural gas, coal combustion and gasification can produce significant amounts of particulate and release high levels of sulfur, nitrogen, and alkali. Coal-based power generation systems require efficient hot-gas cleanup systems to protect power generation equipment (the gas turbine) and to control emissions.
Rotating and stationary airfoils in the hot-gas path are the parts of the gas turbine most sensitive to contaminants. To protect these airfoils, hot-gas cleanup systems for IGCC and PFBC power generation systems must clean the raw gas to certain levels. There are three enemies of the turbine hot-gas path:
* Deposition: accumulation of small particulates on airfoil surfaces. If the total number of particles is too high, airfoil surfaces must be cleaned more frequently.
* Erosion: rapid wear of turbine airfoils due to particulates. If too many larger particles enter the turbine, erosion will be heavy, requiring the manufacturer to lower estimated blade lives.
* Corrosion: rapid chemical attack of airfoil materials after breakdown of surface coatings. When sulfur and alkalis enter the gas turbine, alkali sulfate will form and then condense on turbine blading. This results in rapid surface wastage of blading alloys.
Turbine manufacturers have their own guidelines for preventing excessive hot-ga path deterioration; however, the table on page 74 shows generally accepted limits.
WHAT IS AN ATS?
An ATS is a more efficient and lower-cost advanced-cycle power generation syste that will run on natural gas and can burn coal-derived fuel.
Continuing improvements have occurred in turbine efficiency, reliability, availability, and maintainability, as well as in reducing emissions. New gas turbine system developments in the near future will deliver efficiencies in the mid-50 percent range. These systems feature dry low-N[O.sub.x] combustion systems, with guaranteed nitrogen oxide levels of 25 parts per million or lower
ATSs feature new thermodynamic cycles and advances in materials science. The objective of the ATS for utility applications is to exceed 60 percent plant efficiency (lower-heating-value basis for a natural gas-fired utility system) while reducing the unit cost of electricity and lowering pollutant emissions. Parallel efforts in hot-gas cleanup systems will ensure that coal-derived fuel gases will be sufficiently clean to meet gas turbine requirements.
As the gap between the cost of natural gas and coal grows, advanced coal-based IGCC and PFBC plants will become common, largely due to advantages gained from ATSs. In addition, many natural gas-fired ATS plants may be repowered as IGCC o PFBC plants.
Research on the high-temperature and -pressure control of sulfur species focuse primarily on gas-gas separation via absorption processes. Sorbents made of mixed-metal oxides offer the advantages of regenerability. The sorbents are predominantly composed of zinc and are made into media that can be used in fixed-bed, moving-bed, fluidized-bed, or transport reactor configurations.
Currently, research is focusing on the sorbent zinc titanate. Its general reaction chemistry is:
Absorption: ZnOTiO2+[H.sub.2]S [right arrow] ZnS+Ti[O.sub.2]+[H.sup.2]O.
Regeneration: ZnS+Ti[O.sub.2] [right arrow] ZnO+Ti[O.sub.2]+S[O.sub.2].
Material costs to produce the various zinc titanates for research are in the vicinity of $8 per pound. As demand grows and improved manufacturing methods surface, such costs are projected to be reduced to less than $3 per pound after the year 2000.
Advanced [H.sub.2]S absorption efforts at the GE Corporate Research and Development Center's integrated gasifier/hot desulfurizer plant in Schenectady, N.Y., have focused on developing zinc titanate sorbent formulations for a moving-bed reactor. Engineers have demonstrated a manufacturing technique that yields zinc titanate pellets with the necessary mechanical strength to withstan the slow-moving bed. Doping the titanate with a molybdenum oxide additive minimizes internal pellet stresses induced by chemical reactions. A notable accomplishment is that sulfur emissions can be consistently controlled to below 50 parts per million at pilot-plant scale.
Z-sorb, a proprietary sorbent developed by Phillips Petroleum of Bartlesville, Okla., has also exhibited superior sulfur absorption in lab-scale tests and is being readied for pilot-scale testing in the GE hot desulfurizer plant. Z-sorb has been tested for use in both GE moving-bed and the fixed-bed configurations.
For the fluidized-bed configuration, activities by the Research Triangle Institute in Research Triangle Park, N.C., have centered on developing new and/or commercially available sorbent compositions and fabrication methods that enhance long-term chemical reactivity and mechanical strength. Work is being conducted in a bench-scale reactor for use in pilot-plant tests. A major achievement has been the successful completion of a 100-cycle test using a commercially made 200-pound batch of zinc titanate sorbent. The average sulfur capacity was enhanced, and an order-of-magnitude improvement in attrition resistance was reached.
A hot-gas desulfurization process development unit is being designed by the U.S Department of Energy's Morgantown Energy Technology Center in an attempt to collect data on concept feasibility, process performance, engineering problems, and scale-up questions that relate to fluidized-bed and transport reactor configurations. It is expected to begin operating late next year.
In addition to advancing [H.sub.2]S absorption, methods for treatment of off-gases from the [H.sub.2]S absorption process have resulted in the development of a direct sulfur recovery process. Laboratory-scale investigation have demonstrated up to 99 percent conversion of a sorbent regeneration off-gas to elemental sulfur. A self-contained skid-mounted mobile unit for bench-scale testing has been built. Plans are to conduct integrated testing with a 10-inch fluid-bed gasifier at the METC later this year, followed by pilot-scale testing next year.
HOT-GAS PARTICULATE REMOVAL
Numerous methods for achieving hot-gas particulate removal at high temperature and pressure are emerging. Government-sponsored research and development have been largely responsible for a rapid progression of several particulate control concepts that have great commercial potential. The pilot-scale filter systems being evaluated at the DOE's Power System Development Facility located near Wilsonville, Ala., are discussed. These and other concepts will be proved at a commercial scale in a number of clean coal technology demonstration projects.
Particulate removal for PFBC and IGCC applications differ in particle morpholog and process conditions. In the PFBC application, two particulate-removal system are required: one for the combustor and one for the partial gasification process. In the combustion application, particulate loading will range between 5000 and 12,000 parts per million by weight, with a mean particle size between TABULAR DATA OMITTED 7 and 10 micrometers. Particulate evolving from IGCC and partial gasification of advanced PFBC systems vary dramatically in characteristics. IGCC-produced particles or char tend to be smaller (2 to 5 micrometers), less spherical, and less cohesive than PFBC-produced dust. The smaller nonspherical particles produce a higher resistance to flow when collected on a filter. The lack of cohesive forces between the collected particles can produce reentrainment during filter cleaning.
IGCC filter system particulate loading varies widely with gasifier type and cha recycle arrangements but generally range from 1000 to 4000 parts per million weight. In IGCC systems, the operating pressures can be two to four times highe than the PFBC operating pressures, resulting in higher pressure requirements fo filter cleaning blow-back gas. IGCC filters will typically be smaller than PFBC filters per unit of energy produced due to the higher system pressures and greater heating value of the gasification-derived fuel gas. In IGCC applications, filtration temperatures are usually lower, 1000 [degrees] to 1200 [degrees] F (595 [degrees] to 650 [degrees] C). The chemically reduced and lower-temperature conditions of IGCC systems are not as detrimental to the ceramic filter materials when compared to the higher PFBC temperatures of 1500 [degrees] to 1600 [degrees] F (815 [degrees] to 870 [degrees] C).
One of the filter systems selected for pilot testing is the granular-bed filter from Combustion Power Co. Inc. of Menlo Park, Calif. In this concept, particle-laden inlet gas is discharged into a slowly moving bed of granular material.
The dirty inlet gas is distributed at several locations below the fixed surface of the bed. The 3-millimeter solid sphere granules, made of high-temperature ceramic, flow out by gravity through the vessel bottom. While the bed of granules moves down, the dirty inlet gas flows up past the bed material. The intimate contact of particle-laden gas and the granular bed facilitate particle capture through inertial impaction. Cleaned process gas then flows from the surface of the bed and leaves the vessel. The dirty granular bed material is pneumatically conveyed, cleaned, and returned to the filter vessel. The conveyance gas is cleaned through a conventional high-pressure baghouse to remove the collected dust. Prior test results at PFBC conditions have produced particle capture efficiencies that have averaged above 98 percent and as high a 99.8 percent.
Additionally, two candle filter systems are planned for pilot-scale testing at the Power System Development Facility. One system, from the Westinghouse Electric Corp. of Pittsburgh, arranges the candles vertically in cluster assemblies suspended from a tube sheet within a pressure vessel. The other system, from Industrial Filter & Pump Manufacturing Co. of Cicero, Ill., is designed with candle filters suspended from a single ceramic tube sheet, which is contained in a pressure vessel.
In general, candle filter elements are porous 1.5-meter-long ceramic tubes with a 60-millimeter outside diameter and 10-millimeter wall thickness. One end of the tube is closed, and the other end is flared and sealed to a tube sheet. Candle filters perform particle capture by dust cake filtration with collection efficiencies greater than 99 percent.
In the Westinghouse concept, dirty process gas enters the vessel and passes through the candle filter surface, where a dust cake is formed. Cleaned process gas is conveyed from the inside of the filter to a plenum above the tube sheet and to the vessel exit piping. Filter cleaning or removal of the dust cake is accomplished by pulsed reverse gas flow originating from a high-pressure gas reservoir.
Hot-gas cleanup is being demonstrated at a commercial scale in nine of the projects in the Clean Coal Technology program. The nine projects are at ABB Combustion Engineering Inc. of Springfield, Ill.; Air Products and Chemicals Inc. of Calvert City, Ky.; Appalachian Power Co. of New Haven, W. Va.; Dairylan Power Cooperative/Midwest Power System Inc. of Pleasant Hill, Iowa; Ohio Power Co. of Brilliant; Sierra Pacific Power Co. of Reno, Nev.; TAMCO Power Partners of Coeburn, Va.; Tampa Electric Co. of Lakeland, Fla.; and Wabash River Project of West Terre Haute, Ind. The entire Clean Coal Technology program is funded by nearly $2.5 billion of federal government money and more than $4 billion from private companies. The Clean Coal Technology program is one of the nation's largest investments in environmental technology.
By early 2000, Clean Coal Technology plant operations will verify performance projections for as many as 11 IGCC and PFBC projects. The nine projects featuring some aspect of hot-gas cleanup are valued at a total of more than $3 billion.
In conclusion, hot-gas cleanup offers an important contribution for coal to economically meet a stiff environmental challenge. Clearly, the promise of hot-gas cleanup is that it provides an economic balance between highest plant efficiency and superior environmental emissions control for advanced coal-based power generation.
What energy crisis?
GASOLINE PRICES ARE currently stable at levels comparable in real terms to thos of the 1960s. The severe winter of 1994 led to no widespread fuel oil shortages or failures in electricity distribution. Does all of this mean that the energy crisis no longer exists? Can we at last breathe easily and regard the seemingly intractable energy problem to be a thing of the past? Hardly. Energy remains a multifaceted societal issue.
This nation's prosperity rests to a large extent on relatively low-cost and plentiful electrical power. However, several factors significantly impact the future cost and availability of electricity. The ages of existing plants and projections of electric power demand, for example, point to the need for new generating capacity early in the next century. Legislative mandates for nonpolluting electrically powered automobiles is but one aspect of this increasing demand. Environmental concerns associated with electric generation--the greenhouse effect and acid rain--require new power plants to be very clean relative to those of earlier decades. Socioeconomic and geopolitical factors also make it desirable to use indigenous U.S. energy resources: coal, natural gas, geothermal reservoirs, and hydropower.
Another imperative is that further strides be made in the thrifty end use of electricity. As a decade or more is typically required to bring a technology that has proven itself workable into a first commercial embodiment, U.S. prosperity in the early 2000s rests on today's efforts. The three articles beginning on page 70 relate to current energy system developments that may well have considerable impact in the years ahead.
Within ASME, the Advanced Energy Systems Division provides a focus for individuals and organizations dedicated to the competitiveness of the United States in the critical energy systems arena.
MICHAEL J. MORAN Department of Mechanical Engineering The Ohio State University Columbus, Ohio
Executive Committee Member ASME Advanced Energy Systems Division
Daniel C. Cicero, Richard A. Dennis, and Donald W. Geiling are project managers and Dale K. Schmidt is a product manager in the U.S. Department of Energy's Morgantown Energy Technology Center in Morgantown, W. Va.
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|Title Annotation:||Advanced Energy Systems; includes related article|
|Author:||Cicero, Daniel C.; Dennis, Richard A.; Geiling, Donald W.; Schmidt, Dale K.; Moran, Michael J.|
|Date:||Sep 1, 1994|
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