Combustion 2000 program to develop coal plant technology Future is bright for high efficiency, low emission coal-fired power systems

By Lawrence A. Ruth, US. Department of Energy, Pittsburgh Energy Technology Center, Massood Ramezan, Burns and Roe Services Corp., and Jon H. Ward, Science Applications International Corp.

Electric utilities consume nearly 800 mil- lion tons of coal annually. or about 8 5 8 of the coal used in the US. About 5 5 9 of all electricity generated in the U.S. is produced from coal. Although many elec- tric utilities now have excess generating capacity. this situation is expected to change during the 1990s.

According to the Energy Information Agency. the net effect of retiring aging power plants and adding new plants now under construction or due to come on-line by the mid-1990s will be to raise operable capacity to about 700.000 MW. This CB- pacity increase is equivalent to an annual growth rate of about 0 .59.

Electricity demand, however. is expect- ed to grow by about 2-39 annually during the 1990s. a rate comparable to projected rates of overall economic growth. Load growth like this will require 20.000 MW per year of new capacity-equivalent to building twenty 1000-MW power plants a year. Excess capacity is projected to dis- appear by 1995. and significant shortfalls between power demand and generating capacity will occur by the turn of the century.

Combustion 2000 program To meet demand, new plant construction will have to increase significantly during the late 1990s and early 21st century. And. because emissions regulations also are expected to tighten sharply over the next decade, this new capacity will be needed at a time when power producers may be under increased pressure to lower emksions of sulfur dioxide (SO,), nitro-

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gen oxides (NO,), and perhaps. air toxics and carbon dioxide (CO,).

The U.S. Department of Energy‘s Pitts- burgh Energy Technology Center (PETC) has initiated a program called Combustion 2000 to address the clean and efficient use of coal for power generation for the first decade of the 2lst century. The Coinbus- tion 2000 program was initiated to devel- op a technology base to improve environ- mental performance and thermal efficien- cy of future coal-fired power plants.

Two parallel. conlplementary engineer- ing development activities are expected to achieve these goals: the Low-Emission Boiler System (LEBS). and the High-

Performance Power System (HIPPS). Compared to other advanced power sys- tems now under development (gasification combined cycle. pressurized fluidized bed combustion. etc.), power systems devel- oped under the Combustion 2000 program will more closely resemble conventional coal-fired plants. This should make LEBS and HIPPS inore acceptable to utilities.

Low-Emission Boiler System Low-Emission Boiler System develop- ment rethinks coal-fired boiler design while using up-and-coming combustion and flue gas cleanup technologies to cre- ate a design base for new coal-fired boiler systems. Emission controls will be inte- grated with subsystems for combustion. steam generation, heat recovery and with other sections of the power system. The LEBS will use relatively near-term devel- oping technology to lower SO?. NO,, and particulate emissions. while keeping the cost of electricity comparable to that of conventional technology (e.g., a pulver- ized coal boiler with electrostatic precipi- tators and wet limestone desulfurization scrubbers).

The primary objectives for LEBS are to reduce emissions of SO? and NO, to one- third of that allowed under current New Source Performance Standards (NSPS). and pafticulates to one-half the regulated amount. Other objectives include reducing waste generation, producing usable by- products. improving ash disposability, re- ducing leaching, and increasing power plant thermal efficiency to 40%-42%.

Recent advances in combustion and flue gas cleanup technology will be incor- porated into LEBS design (See Low- Emission Boiler System candidate tech-

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er-one-fourth that of the current NSPS, and because of the higher efficiency, CO? emissions would be significantly reduced.

Flue gas desulfurization wastes will be virtually eliminated by using clean- up processes that generate usable by-products. One goal for the system is to achieve the performance objectives mentioned here while reducing electricity cost\ by 10% compared with current \ystems.

The leading HIPPS can- didate is a combined cycle system that uses ;1 gas tur- bine driven by a working fluid (air) separately heated in a novel high-temperature furnace. Figure I show5 a simplified schematic dia- gram of the HIPPS cycle In comparison, a diagram

nology). Table I outlines the performance of the candidate technologies. The DOE is currently supporting the development or demonstration of these and other technol- ogies.

Advanced electrostatic precipitators and improved fabric filters are candidates for removing more particulates from flue gas. Compared to current methods, either tech- nology also is capable of removing more of the fine particulates (under IO microns) that are present in the flue gas.

Cycle performance A typical net plant efficiency for a conventional steam cycle operating at 1800 psig with a primary superheat tem- perature of 1000 F and a 1000 F reheat temperature is approximately 34%-35%. However. for a 350-MW state-of-the-art plant operating at 4500 psig with I100 F primary superheat and 1100 F double re- heat temperatures. plant efficiency is esti- mated at 38%-39%. Improvements in low temperature heat recovery and environ- mental controls should increase an LEBS plant's efficiency enough to meet the 409-429 goal. Because more efficient plants use less fuel per unit o f electricity produced. an additional benefit of higher efficiency is lower emissions (including CO,) on a pound per kilowatt basis.

High-Performance Power System A 1 though L EB S de ve 1 opme t i t w i I I be challenging. the second part of the Coni- bustion 2000 program may be more so. The goal of the High-Performance Power System is to use advanced technology to achieve a thermal efficiency of 47% or greater. Moreover, the environmental per- formance objectives of HlPPS are even more stringent. For example. SO,. NO,. and particulate emissions would be Iow-

of a coal-fired Rankine cy- cle is shown in Figure 2 . As with other high-performance systems, the HIPPS cy- cle can provide high thermal efficiency and reduced CO, emissions. Moreover, with HIPPS, the technological risk ap- pears to be relatively low. Except for advanced flue gas cleanup technology needed to achieve very low emissions, the only other required new technology is the high-temperature furnace.

Unlike other advanced power plant con- cepts, HIPPS has attributes very similar to current, traditional power plant designs. A gasifier is not required, nor is equipment needed to clean the hot fuel gas before the turbine.

System conceptual design The Pittsburgh Energy Technology Center developed the cycle presented in Figure I

as an illustrative example and case study. This particular HIPPS combined-cycle power plant consists of a Brayton topping cycle that uses air as the working fluid, and a nonreheat Rankine steam bottoming cycle.

In this system, coal is bumed in a high- temperature furnace, where the Brayton cycle air is indirectly heated and steam is superheated. A supplemental clean fuel, e.g., natural gas, is used to boost the temperature of the air leaving the furnace to the turbine inlet temperature. This type of supplemental heating is required be- cause it would be difficult to design a furnace capable of heating air to modern turbine inlet temperatures of 2300 F or greater. Advanced versions of HIPPS would eliminate supplemental heating.

The hot, pressurized air is expanded in the turbine to generate electricity. Conse- quently, heat from the turbine exhaust air and the furnace flue gases is recovered to raise steam for the Rankine cycle.

The high-temperature furnace, a critical feature of HIPPS, is used to heat the clean working fluid (air) to temperatures ap- proaching turbine inlet temperatures. Sim- ilar to a conventional boiler, the HIPPs furnace will contain radiant and convec- tive sections. However, it will be even more critical to closely control and match the combustion and heat transfer processes because the heat transfer sur- faces will be at a higher temperature than in a boiler.

The design requires heat transfer surfaces unlike those used in conventional boilers or fumaces. Advanced metal alloys and/or ce- ramics will be needed. Heat transfer geom- etries other than tubes are a possibility. Temperature-induced stresses may be lower and fabrication and joining may be easier with non-tubular shapes, such as flat plates or channels. (Note: Channel configurations were employed in ceramic recuperators marketed a decade ago.)

Fortunatelv. such heat transfer materi- to the furnace as preheated combustion (FGD) system and mixes with the unrecv- als will be under lower mechanical stress because they will contain air under the relatively low pressure of the turbine inlet (about 150-200 psi).

After expansion in the turbine, the ex- haust enters a conventional heat recovery steam generator (HRSG) where it boils and partially superheats the feedwater. A portion of the turbine exhaust is diverted

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air. The exhaust tha; is not recycled to the furnace is cooled in the HRSG economiz- ers.

In the furnace, heat is transferred to Brayton cycle air and to steam. Additional heat is transferred to the feedwater from the flue gas leaving the furnace in the HRSG economizers. The flue gas then passes through a flue gas desulfurization

cled gas turbine exhaust before being vented to the atmosphere.

To help compare this system with mod- ern coal-fired plants, cycle analyses have assumed a conventional wet limestone FGD system. However, as mentioned. HIPPS will include an advanced flue gas cleanup system.

The steam generated in the HRSG with

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References:

NOX-ROX BOX Flue Cas Cleanup Demonstration Project. DOE/FE- 0145. Washington, D.C.: Govern- ment Printing Office, November 1989. 6 . US. Department of Energy, Office of Clean Coal Technology. Compre- hensive Report to Congress, Clean Coal Technology Program: Demon- stration of Innovative Applications of Technology for the CT- 12 1 F C D Pro- cess. DOE/F E-0 1 58. Wash i ngton, D.C.: Government Printing Office, February 1990. 7. U S . DOE, Office of Clean Coal Technology. Comprehensive Report to Congress, Clean Coal Technology Program: IO-MW Demonstration of Cas Suspension Absorption. DOE/FE- 01 98P. Washington, D.C.: Govern- ment Printing Office, July 1990.

heat from the flue p s ant1 tui-bine exhaw is superheated in the furnace to turhin throttle conditions of I150 Fi13.50 psig. Although this temperature is above the 1000 F to I100 F range typical of todiiy's steam cycles. i t is well \\ i t h i n the capabil- ities of systems expected to be deployed in the next 10-15 years.

System performnnce model in^ was per- formed using the ASPEN PLUS Flow-

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sheet Simulator. The sys- tem was designed for a net plant output of 100 MW to realize moderate economies of scale i n the pumps, compressors. and turbines. (Note: A 100-MW plant was selected for this case study. but i t is not implied tha t commercial plants must be this size.) To give realistic estimates of sys- tem performance, numer- ous parameters. such as minimum approach tem- peratures and turbine effi- ciencies, were chosen to be comparable to those of conventional fossil power plants. A cycle efficiency of 47.2% is predicted for the 2300 F gas turbine base case.

System sensitivity Figure 3 shows the sys- tem's sensitivity to gas tur- bine pressure ratio and inlet temperature. The variation in supplemental fuel use is shown in Ta- ble 2 for gas turbine inlet temperatures of 2300. 2400. and 2500 F. and furnace exit air temperatures of 1800 F and 2000 F.

At a given turbine inlet temperature. the system's efficiency is relatively insen- sitive to small changes i n pressure ratio and furnace exit air temperature. Howev- er. system efficiency improves with in- creasing gas turbine temperature. assum- ing that the temperature increase is due to improved turbine materials or cooling methods. and not to increased cooling Rows.

Supplemental fuel use also rises with increasing turbine inlet temperature if the temperature of the air exiting the furnace remains constant. Parametric studies show

an increase in efficiency of about 0.75% per 100 F increase in gas turbine inlet temperature. and about 0.6% per 100 F increase in steam temperature. The inlet temperatures of stationary gas turbines are currently less than aeroderivative tur- bines. but are increasing by an average of 20-25 F per year. Higher inlet temperature (2500 F) gas turbines seem likely to be- come available by the year 2000.

Table 3 lists the power production. con- sumption, and key characteristics of the plant for the base case operating condi- tion. Table 4 shows the sensitivity of net plant efficiency to small variations in component and process parameters. Ini- provements in turbine inlet temperature have significant benefits. as do conipres-

si (1 030 kPa) to maintain constant steam quality

ntroplc efficiency causes the steam quality in the steam turbine exhaust

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, . sor and steam turbine efficiencies. Note that a nonreheat steam bottoming cycle was assumed for the base case. Use of a reheat cycle may provide a significant efficiency improvement. but the costs vs efficiency improvement need to be exam- ined to determine if that arrangement is practical and economical.

Another HIPPS program goal is to re- duce SO2 and NO, emissions to one- fourth or less of current NSPS levels. These reductions may be achieved using the combustion and flue gas cleanup tech- nologies described for the LEBS program. Because of the improved plant efficiency of the HIPPS cycle, emissions of SO, and CO? (on a IblkWhr basis) could reach up to 26% less than for a conventional, 35% efficient, pulverized coal-fired plant equipped with the same FGC technology. This improvement would be even greater if natural gas was the supplemental fuel, which contributes no sulfur and less CO?.

Program approach /

Major system development efforts are re- quired for both LEBS and HIPPS. For the Combustion 2000 program to be success- ful, all major plant subsystems (e.g., coal preparation. combustion. heat transfer,

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emission control, etc.) need to be integrated. The new plants need to be designed from the ground up with the new technologies in mind. Therefore, when re- questing proposals, DOE encouraged participants to form interdisciplinary teams to ensure expertise in a broad range of technical areas.

Both LEBS and HIPPS are multi-phase activities, proceeding from concept definition and preliminary R&D through engineering development and testing of prototypes and subsystems to construction and opera- tion of proof-of-concept plants.

Awards to three industry teams for the entire four- phase Low-Emissions Boil- er project were made in August and September 1992. These teams are led by ABB Com- bustion Engineering (Windsor, Conn.), Babcock & Wilcox (Alliance, Ohio) and

Commercial readines?

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Riley Stoker Corp. (Worcester, Mass.). Phase I includes system analysis and R&D plan formulation, component defini- tion, and the preliminary design of a com- mercial generating unit.

The HIPPS development project has three phases. Awards to two industry teams to conduct Phase I were made in September 1991 and February 1992. These teams are headed by Foster Wheeler Development Corp. (Livingston, N.J.), and United Technologies Research Center (East Hartford, Conn.). Phase I will last two years and will include con- cept definition, engineering analysis, modeling, experimental work, and the preliminary design of a commercial-scale plant. Phase I1 will be initiated with re- quests for proposals that will be issued in the fall of 1993 and will be open to both existing team members and new partici- pants. The anticipated project schedules are shown in Figure 4. END

AUTHORS Lawrence A. Ruth is the Director of the Coal Utilization Division at the U.S. De- partment of Energy. He holds BS and MS degrees in chemical engineering from the City College of New York and a PhD de- gree in chemical engineering from the City University of New York.

Massood Ramezan is a senior engineer with Bums and Roe Services Corp. He holds BS, MS and PhD degrees in me- chanical engineering from West Virginia University.

Jon H. Ward is a research engineer with Science Applications International Corp. He holds a BS degree in mechanical engi- neering from Virginia Polytechnic Insti- tute and State University and an MS de- gree in mechanical engineering from Massachusetts Institute of Technology.

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