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The Cogeneration Plant and Utility Dist. System

PresentationVanderbilt University Combined Heat and Power Plant - A history of sustainable progress

Assessing Utility Systems and Related Costs

In 1985, Vanderbilt University Plant Operations department conducted an in-depth analysis of the condition of the University facilities, including all of the major utilities supply systems and subsystems. The analysis resulted in a plan for improvements in the stand-by capacity of the steam plant; an increase in the reliability of both steam and electric generation; a reduction of expenses through improved efficiency; and opportunities for cost containment. Paramount to the success of the plan was the requirement that the Plant Operations facility be non-intrusive within the 333 acres of contiguous property and 151 buildings.

At the time, the basic mission statement of the Vanderbilt Steam Plant was to provide for the generation and distribution of steam at required conditions for a variety of campus needs. A five part plan to support that mission was developed, as follows:

  1. To direct Plant Operations operating and maintenance expenditures to meet institutional goals
  2. To initiate programs for facility modernization and cycle maintenance
  3. To implement cost containment opportunities
  4. To retain flexibility in facilities budgeting and spending
  5. To undertake what is affordable without further maintenance deferrals

 

The principle strategies involved with this project were:

  • The cost of financing the project must not impact any departmental operating budget through increased operating cost, or divert funds that could be applied to the academic or institutional programs. The total project cost must be self-liquidating.
  • The project must provide for emergency power to allow the plant to operate in the event of isolation from the utility.
  • The project must provide fuel flexibility to insure a reliable source of energy both immediately and in the future, as determined by cost of varying fuels.
  • The project must provide Vanderbilt with sufficient environmental permitting flexibility to avoid large future capital expenditures due to regulatory changes.
  • The project must eliminate the accumulated backlog of deferred maintenance while maximizing the use of existing space.

 

On April 1, 1986, a $10,600,000 proposal was submitted to the Board of Trust for the installation of the following components:

  • One 200,000 lb/hr boiler rated at 650 PBIG/ 750°F, with coal as the primary fuel. The boiler would also be supplied with a gas/oil burner.
  • One 6.5 Megawatt back-pressure steam turbine-generator for cogeneration of electricity as a by-product of campus steam use.
  • A rock-bored tunnel connecting the main campus with the adjacent Peabody campus which merged with Vanderbilt University in the summer of 1979. This new interconnect would allow for the retirement of plant equipment over 60 years old on the Peabody campus.

 

The financial plan recommended a 20 year amortization period, using funds from a portion of a 1986 institutional bond issue of $150,000,000. The debt service would be paid by the Plant Operations Department from revenues associated with cogenerated electricity. In addition, the retirement of the Peabody steam plant would reduce $450,000 in annual operating expense, and allow for the reallocation of that capital to other program needs. Retiring the Peabody plant had the additional benefit of avoiding substantial capital expenditures required to modernize the plant. The plan also provided a Depreciation Reserve, allowing replacement of the plant’s major components at the end of their useful life.

The Board of Trust approved the project, and a Request for Proposal (RFP) was developed for a design-build team project which was offered to three qualified contractor/engineer teams on June 26, 1986, with an August 26, 1986 bid date.

A June 18, 1986 Notice of Proposed Rulemaking published by the Environmental Protection Agency required an extension to the bid date to allow Vanderbilt to assess the potential impact of this ruling. The ruling's affect on the original proposal would require the installation of S02 scrubbers, or a change in the scope of work to include fluidized bed technology. Vanderbilt’s assessment of those alternatives indicated several risks due to uncertainties with classification of the effluent from the scrubbers, a continued emergence in the fluidized bed technology, and substantial increases in both capital cost and operating cost over the life of the plant. A careful evaluation of the EPA regulations indicated that three boilers, each rated at less than 100,000,000 BTUH input, could be permitted without the need for scrubbing equipment, and that the original budget of $10,600,000 was still adequate for the project. On that basis, a change in the original scope of supply was published as an addendum to the RFP, and bids were opened on September 16, 1986.

Implementing the Plan

The selected design-build team was Stanley Jones Mechanical Contractors, and I. C. Thomasson Engineers. As a result of the required EPA change, and through the ingenuity of the design-build team, the project ultimately included several enhancements, including the following:

1.)  Three 70,000 lb/hr coal-fired boilers, with two of these units equipped with gas/oil burners
2.)  One 1,175 ton coal silo with a totally new coal handling system
3.)  Modernization of the existing ash handling system, integrated with an existing reverse air bag-house
4.)  Addition of a motor for a 500 HP Induced Draft fan
5.)  Addition of a truck scale for weighing coal deliveries
6.)  A new electronic combustion control system using fully redundant digital Moore 352 subsystem controllers

In 1988 Vanderbilt installed the first cogeneration system consisting of a 7,000 kW steam turbine driven synchronous generator which is connected to the 4,160-volt distribution system. In 1989, a second steam turbine generator was added to the 13,800-volt system and is rated at 4,500 kw.

The first generator, TG 1, is referred to as a backpressure system since the generation produced is a by-product of the steam distributed to the entire Vanderbilt Campus and Medical Center. The second steam turbine generator, TG 2, is a condensing turbine and produces electricity independent of process steam loads. One of the principle design requirements was to provide fuel flexibility for the production of steam at the plant. This plant can use coal or natural gas to generate steam, allowing for the on-site production of electricity regardless of which fuel is selected.

As described, load capacity of TG 1 is a function of steam demand. For every 27 lbs. of steam that flows through the main steam pipe leaving the Power Plant, 1 kw is produced from TG 1. Over the last 20 years steam demand has grown from approximately 150,000 lbs. per hour on peak to over 290,000 lbs per hour.

The load capacity of TG 2 is a function of demand set point. Previous to the implementation of the Limited Interruptible Power Contract, (LIP), with TV A and NES, this unit was dispatched to control at full load during the peak demand period during normal business hours and reduced to minimum load at night and during weekends. The LIP contract provided electricity at a reduced total cost and resulted in a change of operating philosophy for TG 2. This unit now operates at a minimum load (500 kw) unless needed for demand control during a loss of other generating equipment or during a LIP interruption.

Daily Operation of the Steam Plant

Eight, and sometimes nine, truckloads of West Virginia low sulfur coal with an HHV of 13,500 Btu per pound are trucked to the campus daily. These loads, approximating twenty-five tons each, are weighed and off-loaded onto a receiving grate above an auger and then fed by the auger to the top of a seventy-four feet tall, thirty-six feet diameter silo with an eleven hundred ton capacity. When full the silo has a five day winter “turn cycle.”

The coal is automatically transferred twice daily from the silo to an eighty ton day bunker, from which it is distributed two-hundred pounds at a time to each of three Detroit Stoker spreader stokers, and burned. Combustion controls are Moore single loop digital electronic with oxygen trim. The heat of combustion is transferred in three Henry Vogt Company two-drum, natural circulation, tube and tile boilers to generate steam. Each boiler is rated at 70,000 pounds per hour, with steam conditions of 650 PSIG, and 750°FTT.

Helping to maintain the “invisible” image of the power house is a Zurn baghouse, which was installed in 1980. This baghouse, consisting of eight modules, and 1165 individual 8" bags, has a rated efficiency of 99.5 percent, and maintains opacity at less than ten percent. Emissions from the 215 feet tall masonry stack have always been within EPA permit limits.

Following combustion, both bottom and fly ash are transferred by a Detroit vacuum ash system with an Ashtec ash conditioner to an eighty ton capacity ash storage silo. Ash is used as a feedstock ingredient in the manufacture of cement mix.

Two of the three boilers are equipped with Coen sidewall burners capable of firing either No.2 oil or natural gas, in addition to coal, to provide Vanderbilt some insulation against changes in fuel supply markets.

Steam from the boilers at 650 PS1G and 750°FTr is supplied to a single stage, impulse type steam turbine designed and manufactured by Murray Turbomachinery Corporation of Burlington, Iowa as part of a complete packaged turbine-generator. The turbine acts as a pressure reducing valve (PRV) while simultaneously producing electrical power. Exhaust steam from the back pressure turbine is directed to the campus steam distribution system.

Turbine control is provided by a Woodward Dg505 governor configured for exhaust pressure (ranging from 60 to 125 PSIG) control, and KW limiting. The turbine has a 10" inlet with multiple (5) valves, bar-lift design, controlled by the governor through a pneumatic actuator.

Turbine operating speed, optimized for maximum efficiency, is 9807 RPM. This operating speed can be reduced to the 1800 RPM 4 pole generator speed through a Lufkin Industries speed reduction gear. The reduction gear design includes a rigid cast iron housing and double helical thru-hardened precision hobbed and finish lapped gearing.

The synchronous electrical generator, manufactured by Ideal Electric Company, will produce 6850 KW at 4160 V, 60 Hz, 3 phase, when the turbine is provided with 200,000 pounds per hour of steam at 650 PSIG and 750°FTI: The generator enclosure is totally enclosed, water-to-air cooled (TEWAC). Cooling water for the generator heat exchanger and the oil coolers for the turbine-generator is provided by a dedicated Baltimore Aircoil cooling tower.

A second steam turbine-generator, also manufactured by Murray Turbomachinery, allows the boilers to continue operation at full load, the most efficient point, as thermal load requirements are reduced. Steam not required for campus use is routed around the backpressure turbine, and supplied at boiler outlet conditions to the condensing turbine. This second steam turbine is a nine stage unit, impulse type, with a Curtis first stage, and operates at 4755 RPM. Turbine control is provided by a Woodward DG 505 governor configured for inlet pressure control and real power sensing. The single valve inlet is controlled by the governor through a pneumatic actuator.

Turbine operating speed is reduced to the 4 pole generator speed (1800 RPM) through a speed reduction gear manufactured by Lufkin. This speed reducer, like the reducer for TG # 1, includes thru-hardened precision hobbed and finish lapped gearing.

The electrical generator, manufactured by Ideal, will produce 4500 KW when the turbine is supplied with 47,500 pounds per hour of design condition steam, condensing to 3.0 inches of mercury absolute (HgA).

Steam is exhausted upward from the second turbine, through a horizontal 36” line, and downward into a Graham shell-and-tube surface condenser. This two pass tube-side, one pass shell-side heat exchanger is designed to condense 49,000 pounds per hour at 2-1/2" HgA when provided with 5200 GPM of cooling water. Following an isothermal change in phase within the condenser, the condensate is pumped by one of two 100 percent capacity condensate pumps to the de-aerator. The pumps are driven by 7-1/2 HP, 1750 RPM induction motors.

Cooling water for the condenser is provided by a four cell, Baltimore Aircoil cooling tower. The cooling tower, with an eleven hundred gallon sump, is mounted on the roof of the building housing three Ingersol Rand cooling water pumps.

The pumps are provided with mechanical seals, and are driven by GE 50 HP TEFC electric motors at 1775 RPM.

The deaerator is provided with 10 PSIG steam from a low pressure header to remove oxygen from the condensate, and to provide 230'F feedwater to the economizer. With the fuel burned, the 230'F precludes cold-end corrosion on the fin-side of the economizer. The feedwater is pumped to the economizers by three KSB pumps, each rate at 250 GI'M and 2,110 feet of head.

One economizer is provided for each of the three boilers. Manufactured by Eco, Inc., Tulsa, OK, the economizers are finned-tube type, two fins per inch, with 2" O. D. tubes. The boiler feedwater is heated from 230°F to 400°F by 580°F stack gas, which is cooled to 378°F. The 400°F feedwater is then fed into the steam drum of the Henry Vogt boilers, and the cycle is repeated.

Assessment of the 1988 Project

The five part plan to provide for the generation and distribution of steam at required conditions for a variety of campus needs had been materialized. By most measures, the project was a success. Vanderbilt had provided itself with self�? liquidating capital investments and insulation against changes in fuel supply markets by maintaining fuel flexibility.

The goal of cost containment was achieved through reduction of electrical costs through self generated electricity, and reduction in the previous electrical consumption base through the retirement of inefficient and depreciated centrifugal chillers. Space for a central absorption chilled water plant became available by retiring the old, inefficient steam plant, and the new rock bored tunnel provided access to newly remodeled buildings on the Peabody campus. Capacity for stand-by capacity was improved and a substantial portion of its deferred maintenance backlog was retired. Potential risks with environmental problems associated with coal and the fluorocarbon issue were avoided.

All of these benefits materialized from the single objective of providing a continuous and reliable source of steam, even with a loss of electrical service.

Stand-by Emergency Capacity

In 1995 Vanderbilt installed two diesel-engine generators each rated at 2,750 kw. They are located on the Peabody Campus and are electrically connected to the main 13.8 kv substation through a rock-bored tunnel that runs from the Peabody Campus to a previously existing walk-thru tunnel system at the Stevenson Chemistry building. From this location, high voltage cable is routed to the main substation through the pre-existing tunnel system to Bus #2 in the main substation.

These units are operated on a stand-by schedule and are used during emergency conditions such as loss of NES service or for kw reductions required by a LIP interruption. If a power loss is experienced at the main substation, both units are designed to crank, come to synchronous speed, produce voltage and connect to the Peabody Campus system.

1998 Co-Generation Plant Expansion Driven by Demand

The plant built in 1988 continues to serve the University and Medical Center reliably and efficiently today. But in 1998, a decade after the initial plant expansion project, demand for steam and electric was outpacing existing generation and distribution systems primarily due to construction of new buildings on campus and in the Medical Center.

A second study was undertaken, again enlisting the expertise of I.C. Thomasson and Associates to conduct a comparative analysis of options, including a construction of a separate generation facility and expansion of the existing co-generation plant. The project plan was presented to Vanderbilt���s Board of Trust and the financing of a $25mm expansion of the exiting facility was approved.

In 2000, after construction of a building to house the units, two natural gas-fired combustion turbine generators each rated at 5,000 kw at ISO conditions were installed. The exhaust heat off of each of these turbines goes into a HRSG (Heat Recovery Steam Generator) boiler rated at 100,000 lbs steam per hour at 125 PSI. The exhaust heat off the turbines can produce 30,000 lbs of steam per hour. Duct burners on each boiler can be used to produce extra steam if needed. These units (located adjacent to the original Power Plant) are electrically connected to the 13.8 kv system Bus 3 in the main substation. One of the critical capabilities of these units is that, although they normally operate in parallel with the NES system, they can be operated independently without connection to the NES system.

Also in 2000, Vanderbilt constructed a 4000 foot rock-bored tunnel to supply the Children’s Hospital with steam through a 14" steam line and 13,800 volts of electrical power.

Current Cogeneration Plant Capacity

Steam Generation    
Primary Steam Generation: Rated Capacity: Fuel:
1988 Steam Plant:    
Boiler # 7 (650 psig/7500F) 70,000 pph Coal or Natural Gas
Boiler # 8 (650 psigI7500F) 70,000 pph Coal or Natural Gas
Boiler # 9 (650 psigI750°F) 70,000 pph Coal
Primary Steam Generation: Rated Capacity: Fuel:
Cogeneration 2000 Facility:    
HRSG #1 (125 psig/416°F) 100,000 pph Natural Gas
HRSG #2 (125 psig/416°F) 100,000 pph Natural Gas
Total-Both Plants: 410,000 pph  
Reserve Steam Generation: Rated Capacity: Fuel:
Existing Steam Plant:    
Boiler #5 (125 psig Sat.): 100,000 pph Coal
  70,000 pph Natural Gas
     
Electric Generation    
1988 Steam Plant: Rated Capacity:  
Steam Turbine Generator #1 7,000 KW  
Steam Turbine Generator #2 4,000 KW  
Stand-by Generation: Rated Capacity:  
Diesel Engine/Generator #3 2,000 KW  
Diesel Engine/Generator #4 2,000 KW  
Cogeneration 2000 Facility: Rated Capacity:  
Gas Turbine/Generator #1 5,000 KW  
Gas Turbine/Generator #2 5,000 KW  
Total-All Plants: 25,000 KW