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Before specifying a gas turbine for that nextcombined-cycle plant, take a closer look at

diesels

Diesels in

combined cycle

By Steven E. Kuehn, Senior Editor

Before specifying a gas turbine for that next combined-cycle plant, take a closer look at diesels

Ironically, the role of the piston in power production seems to be expanding. The irony lies in the factthat technology developed at the turn of the last century is poised to take on an even greater portion ofelectric power generation at the turn of this century, despite advances in other, more contemporaryprime movers such as gas turbines.

Why diesel?

In their paper ODiesel Combined Cycles Using Fired Boilers,O (POWER-GEN Americas ?94, Orlando,Fla.) authors Thomas Davis, Sandwell Energy, and

F. Mack Shelor, Wartsila Diesel U.S., presented a compelling case why the diesel engine is a veryattractive choice for

producing power in the combined-cycle configuration.

Davis and Shelor said the power industry is fast becoming aware of the pitfalls of designing powerplants that rely solely on a premium fuel such as natural gas. Even though environmental regulationspush plant developers towards gas turbines, there are drawbacks to consider. For example, gas turbinepower plants often have to be built around available turbine frame sizes with a more narrow range of

Page 1 of 6Before specifying a gas turbine for that next combined-cycle plant, take a closer look at di...

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application than other prime movers. The authors claim that these facilities often have to operate atefficiencies far lower than originally anticipated because of extreme dispatching requirements thatappear after the plant has been built. Moreover, some projects just won?t be built because the long-termfuel economics of the gas turbine combined cycle can make the choice unattractive.

The challenge

Power plant developers are now faced with a challenge: How does one design a power plant that is asclean and efficient as natural gas-fired plant, uses proven components, has better wide-rangeperformance and dispatching characteristics than gas turbines, and is more economically competitivethan integrated coal gasification combined cycle technology? Diesels in a combined cycle (DCC), that?show.

Their position rests on the fact that the medium-speed diesel is already one of the most efficient simplecycle sources of electricity, especially with lower grade fuels. Large units, said Davis and Shelor, haveheat-rate efficiencies as high as 45 percent, equating to a heat rate of 7,580 Btu/kWhr, and no otherpower production prime mover can match this efficiency. Diesels also offer designers fuel flexibility andcan burn an extreme variety of fuels without sacrificing many of its positive operating attributes.

Diesels are the first building block in a highly efficient combined cycle system that relies on the hot gasand oxygen in the diesel?s exhaust to combust either natural gas, light distillate oil, heavy oil or coal, ina boiler. Although the concept of recovering diesel exhaust heat and cooling water heat is not new, saidDavis and Shelor, using diesels to help fire a boiler is. By using a fired boiler, steam can be generated atsufficient temperature and pressure to operate a Rankine steam cycle efficiently.

Diesel combined-cycle plants can be configured in much the same way a gas turbine plant would be.However, the diesel combined-cycle scheme requires supplemental firing to generate appropriate steamconditions. The most efficient cycle, therefore, would not be achieved until combustion air andsupplemental fuel are minimized to levels that satisfy steam conditions, steam generation and powergeneration constraints.

Tailor made

Flexibility is the key to the DCC, and it can be tailored to meet specific steam and power needs. Davisand Shelor detailed several of the possible configurations in their paper. In a simple natural gas-firedscheme, supplemental combustion air is not required and the burner operates totally on the oxygencontained in the diesel?s exhaust and the burner?s fuel. Boiler temperature is controlled by the amountfuel gas added to the exhaust flow and is similar to firing a duct burner with supplemental fuel in a gasturbine combined cycle. However, unlike the gas turbine, the diesel cycle will always require somesupplemental fuel to boost exhaust temperatures high enough to attain efficient steam conditions.

When heavier supplemental fuels are specified, some supplemental combustion air is required to attainstable combustion. When air is added, however, burner exit gas temperatures become too high foroptimal heat recovery in the boiler. To dampen gas temperatures, a burner bypass system is fitted. Thebypass redirects a portion of the exhaust gas so it can be injected into the boiler as overfire air. Thisallows sufficient oxygen to support stable lower-grade fuel combustion in the diesel exhaust streamwhile bringing down overall gas temperatures entering the superheater to a point just high enough toovercome boiler pinch points?the minimum differential between the gas temperature and the boilersaturated water temperature. Remember, for any given set of steam conditions and exhaust heatconditions, as the temperature differential increases, combined-cycle plant output will decrease and heatrate will increase.



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Controlling the firing rate and burner bypass flow are features not commonly found on gas turbinecombined-cycle plants. One plus of this feature is that

it allows a wide range of steam power demands to be met for applications such as industrialcogeneration where steam and power demand swings widely and independently.

For each cycle, according to Davis and Shelor, there is an optimum point of bypass flow where both the

steam conditions and steam flow are achieved at maximum efficiency. As the efficiency of heat recoveryincreases, the amount of bypass flow will increase. OIn general, single pressure non-reheat cycles willhave optimum efficiency at lower bypass flows such as 0 percent to 50 percent of the total diesel exhaustflow,O the authors explained, and that Osingle pressure cycles will optimize at approximately 50 percentto 70 percent bypass flow.O

Natural gas diesels

Natural gas-fired diesels are an established technology, have been available for years and have achievedefficiencies as high as 45 percent in the simple cycle. In the case of a DCC, a critical consideration forburner design is the level of oxygen in the exhaust gas leaving the engine. According to Davis and

Shelor, large, medium-speed, four-stroke diesels injecting natural gas directly into the cylinder will havean exhaust gas oxygen level between 12.5 percent and 13 percent, and that is plenty to fire thesupplemental natural gas.

Emissions from a natural-gas fired DCC are low to begin with and NOx reduction is an inherent part ofthe process. The combustion of the exhaust gas reduces NOx in two ways: first by reburning and secondby dilution. This reduces NOx levels by as much as 50 percent to 70 percent when compared to the usuallevels found in the diesel?s exhaust. It is likely small-scale DCCs will not require any NOx reduction,but larger units might need selective catalytic reduction (SCR) to meet very stringent controlregulations.

Heavy fuel oil

Although combustion turbines are limited when it comes to fuel choice, diesels are not. Diesels havebeen designed to burn all grades of heavy fuel oil (HFO). Davis and Shelor noted that refiners arefinding a weak market for HFO and that is favoring long term fuel contracts because high-sulfur HFO isin abundant supply.

Unfortunately, a diesel?s NOx and SO2 emissions are relatively high when operating in a simple-cyclearrangement. Several developments, including injector design, combustion chamber and piston design,and electronic engine controls, are being applied to help reduce emissions. After treatment strategiesincluding SCR and scrubbers have also proven effective with high-sulfur HFO.

The DCC system takes advantage of these technology improvements and adds the effect of dilution fromreburning to the equation, but there are some noteworthy differences, said Davis and Shelor. When HFOis used as a supplemental fuel to fire the boiler, special burners that supply a small amount of extra airare needed to achieve complete combustion. The increased oxygen allows more fuel to be burned andincreases the effect of dilution. Babcock & Wilcox recently developed a proprietary burner design forDCC boilers optimized to maintain low oxygen levels in the windbox. Pilot testing was completed in1994 and proved the unit was capable of stable and efficient combustion at the low oxygen levelsneeded to achieve optimum DCC performance.



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Diesel and coal

Public Law 102-154 provides funds to the U.S. Department of Energy (DOE) to conduct cost-sharedClean Coal Technology (CCT) projects for the design, construction and operation of facilities that O. . .shall advance significantly the efficiency and environmental performance of coal-using technologies andbe applicable to either new or existing facilities.O According to the executive summary at the beginningof the OComprehensive Report to Congress Clean Coal Technology Program, Coal Diesel Combined

Cycle Project,O this act, together with Public Law 101-512, made a total of $600 million available forthe fifth round of general requests for proposals under the CCT program.

Among the 24 proposals received by the DOE, five were selected for funding. One of the five was a coaldiesel combined-cycle (CDCC) project proposed by a team consisting of Eaton Utilities Commission,Cooper-Bessemer Recip-rocating Products Div., Cooper Industries Inc., and Arthur D. Little Inc., withadditional support from the Ohio Coal Development Office.

The DOE will provide funding assistance for the design, construction and operation of a 90-ton-per-day14-MWe, diesel engine-based, combined-cycle demonstration plant using coal water fuels (CWF). Theplant will be located at a power generation facility at Easton Utilities Commission?s Plant No. 2 in

Easton, Md. The project, including the demonstration phase, is set to last 79 months and will cost $38.3million. According to the report, the DOE will fund 50 percent of the project and with Arthur D. Littleacting as the prime contractor.

Demonstrating the concept

The CDCC project will demonstrate an advanced CDCC system based on two Cooper-Bessemer 20-cylinder diesel engines. The demonstration is designed to provide critical data on the performance,reliability and component life information for all major subsystems, including the CWF metering andinjection system, medium speed diesels, lube oil systems, exhaust cyclones, turbochargers, heat recoverysteam boilers, steam turbines and exhaust emissions aftertreatment. The plant will be installed as a two-diesel extension of the existing 25-MWe generating plant and a total of 6,000 engine hours of testing isplanned using CWF fuel.

The CDCC is expected to attain efficiencies of 48 percent lower heating value for the demonstration.

Based on contemporary stationary reciprocating engine technology, the CDCC relies on a recentlydeveloped process that allows coal to be burned much like heavy fuel oil. The basic layout of the plantconsists of CWF preparation, two diesel engines, a combined-cycle power generation block and anemissions control subsystems (Figure 1).

The report states that Ohio No. 4, 5 and 6 bituminous coals will be mined at Sugar Creek, Ohio, andcleaned to 2 percent ash content. The CWF will be processed near the mine site and transported to the

plant via 6,500-gallon tank trucks.

Specifically, the prime movers for the project are two Cooper-Bessemer Model LSV-20 engines. The20-cylinder 4-stroke diesels (15.5-inch bore by 22-inch stroke) are rated at 400 rpm and 208 psi brakemean effective pressure. Each engine will be coupled to a 6.3-MWe generator and will consume 7,228pounds-per-hour (pph) of CWF and another 84 pph of diesel pilot fuel. Each cylinder is fitted with aCWF injector designed with 18 orifices to ensure thorough combustion of the fuel/air mix. To combustthe coal fuel efficiently, a small portion of diesel fuel is ignited first in a pilot combustion chamberadjacent to the primary combustion chamber.



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To protect the turbochargers, the system relies on cyclones designed to remove 80 percent of 20-micrometer size particles and 50 percent of 5-micrometer size particles from the exhaust gas flow.Cleaned gas flows to the turbochargers while solids are removed from the under flow by a rotary valve.

Exhaust from the turbochargers then flows through a heat recovery boiler which makes steam to spin thesteam turbine. To control emissions (Table 1), an integrated process involving several subsystems isused and includes in-cylinder NOx reduction, the cyclones, a selective catalytic reduction reactors for

each unit, sorbent injection, the baghouse and a flue gas sampling system.

100-MW DCC

In a recent paper titled O100-MW Diesel Combined Cycle Power Plant,O Anders Ahnger, WartsilaDiesel, detailed the conceptual design of a large-scale DCC plant (Figure 2). The 100-MW plant is basedon six Wartsila Vasa 18V46 engines and a steam turbine. Each of the six generators output 15.8 MWeand the steam turbine is rated at 8 MW to 11 MWe depending on the fuel, either natural gas or heavy oil.

The plant is divided into two three-by-three separate engine rooms with a middle section including allmajor common equipment. The middle section includes all the electrical systems, high and low voltage,

the central cooling system and water treatment.

According to Ahnger, careful consideration was given to the noise aspects of the plant and it can meeteven the most stringent regulations. The design value for the plant is 45 decibels at approximately 300feet from the plant. To achieve this, each diesel is enclosed in a separate room and sound-absorbingmaterials are used in

the walls.

Special attention was also paid to the logistical aspects of the plant. Because some of the engine sparecan weigh more than a ton, the plant is fitted with small local cranes, separate cranes in each engineroom and cranes strategically located to move spares and equipment through the plant.

The plant can be fully automated depending on the operators needs and local labor costs usingprogrammable logic controllers. The system covers engines, the steam turbine, the electrical system andemissions controls.

Combined-cycle considerations

Because diesels in the 15-MW to 16-MW range already have gross efficiencies of 45 percent to 46percent, 50 percent efficiencies in the combined cycle are easily attained. For the 100-MW concept, thecombined cycle is based on an ordinary steam cycle. The available engine heat sources, i.gif., exhaustheat and heat rejected to the cooling system, are used to produce steam. Each engine is fitted with an

exhaust gas boiler which is equipped with economizers, evaporator and superheater sections, andindividual steam drums and controls. Optimal steam pressures for the condensing turbine?s cycle are176 psi for heavy oil operation and 120 psi for natural gas operation. For back pressure applications,higher steam pressures are required and would call for a fired boiler arrangement similar to the onediscussed previously in this article.

There is plenty of evidence that applying reciprocating engine technology

in a combined cycle can offer power producers efficiencies and operational flexibility. Indeed, gas



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turbines also poses their fair share of these same attributes, but DCC plants may have an edge when itcomes to part-load conditions and when natural gas fuel supplies are questionable. Although notaddressed here, maintenance and reliability issues come into play, particularly when poor quality fuelsare used, and a close analysis of associated costs is advised. Nevertheless, DCC is a well-developedconcept and may be an ideal way to solve often conflicting design requirements. END

NOX reductionb (%) 90

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