Cogeneration

Bucknell converted from coal fired boilers to a cogeneration plant in 1998. The cogeneration plant provides the campus with both heat (steam) and electricity year round. For a detailed description of cogeneration and Bucknell's conversion see the following PDF and Bucknell World article.

Bucknell's Cogeneration Power Plant.pdf


Environmental Affects

After 1997, Bucknell University produced total emissions of 37,756 Metric Tons of carbon dioxide equivalents (MTeCO2). Prior 1997, total greenhouse gas emissions were greater than 60,000 MTeCO2. The difference from FY 1996 to FY 1997 is due to the phase in of the co-generation plant to replace the coal power plant as well as reducing the amount of electricity purchased. (Green House Gas Inventory, Christine Kassab '09) The new plant produces 75% to 99% lower emissions of pollutants such as Sulfur Dioxide, Nitrogen Oxides, Carbon Monoxide and Particulates than the old plant.

Equipment and Orientation

The current operation equipment in the power house consists of

The main operating orientation utilizes only the 5 MW dual fuel turbine (natural gas first, oil second) which generates both thermal energy into the HRSG and electricity. The two water tube boilers act to provide supplementary and backup thermal energy. This line-up provides Bucknell with 140,000 pph firm steam generating capacity thus assuring the campus with plenty of future and backup capacity.

A Sankey diagram of the primary cogeneration line-up can be seen below:


Control System

The Supervisory Control and Data Aquisition (SCADA) system is made up of a network of Allen Bradley Programmable Logic Controllers (PLC). Each PLC is dedicated to a particular piece of equipment such as the Gas Turbine, Boiler Management System, Water Softener, etc. The PLC’s each have a logic program that controls the operation of its equipment via input and output devices. For example input devices like, push buttons, limit switches, temperature probes, flow transmitters, etc and output devices, like, motor starters, valve controllers, indicator lights and alarm horns are all electrically connected to the PLC.

The PLC network in turn connects to a network of 4 operator workstations located in a control room environment. These workstations communicate with each other via Ethernet on a Windows 2000 platform. Each workstation is running an industrial automation software package called “Citect”. This software allows the plant operators to monitor the operations via graphical animations of the plant. Operators can control set points, start and stop equipment, view and respond to alarms, as well as view real time and Historical data. Redundancy is also a very important aspect of plant operation and is built into the SCADA system at critical need areas.

Screen captures of the controls with accompanying discriptions can be seen in the following PDF:

Plant Controls.pdf


Operation

The various steam load demands of the campus system result from unscheduled and unanticipated events. These loads and their general amplitudes result from operations ranging from food preparation, steam heating of buildings for general comfort as well as thermal energy to steam absorbers located throughout the University.

The response of the power plant must be accomplished in time and in the correct amount to continuously maintain steam flow adequacy (pounds per hour) as well as simultaneously supplying a constant supply pressure (pounds per square inch).

The intended mandate for the power plant then must focus on matching just the right amount of fuel into the boilers to maintain the capacity (a variable from 0 to 70,000 pounds per hour) and supply pressure requirements (a fixed pressure at approximately 13 psig).

Steady state operation of the steam system results in trending of the pressure reading of the send out at the power plant. With a normal set point of 13 psig, any divergence or change above or below this reading will immediately signal the response of the energy systems; decay in pressure indicates a higher usage rate. Similarly a rise in pressure indicates the plant is producing too much thermal energy and it needs to be lowered.

The “reflection” of a rising or lowering steam pressure on the steam turbine exhaust location provides the first controlled increase or decrease in the steam flow. Assuming for instance the need for more campus heat has resulted in a lowered pressure reading, this signal then is fed to the governor valve of the steam turbine which increases the rate of steam flowing through the turbine thus producing or restoring the pressure reading to the original set point value. Side effects of correction of the steam flow rate results in no change in speed but rather a higher electrical output from the steam turbine generator. (The controller for this first stage of pressure/flow control is the digital governor.)

Since the back pressure controlled steam turbo generator system has made an instant rate change through the 200 psig governor valve, the next process variable to be corrected must be the steam boiler system. The offset condition in the case illustrated will be a corresponding pressure decrease in the boiler steam drum which is kept at 200 psig. Now the boiler master controller sees and interprets the decrease in steam boiler pressure and increases the firing rate of the boiler through the opening of the fuel valve (natural gas or oil in the case of the two package boilers or natural gas fuel valve in the case of HRSG steam production). The compensating change in boiler firing rate produces an increase in the steam drum pressure by producing more steam.

For purposes of simplicity the thermal balancing can be accomplished in any single boiler or boiler group through the boiler master controller whose design allows different boilers to be operated in parallel without focusing the full correction of steam flow to a single boiler. It should be noted however that in normal plant steam and electric production modes the only boiler that operates is the HRSG which uses gas turbine exhaust to achieve 0 to 25,000 pounds per hour of steam output with no additional natural gas applied at the internal spud burner system. Steam demands above 25,000 lbs/hour from the campus are controlled through the modulation of the HRSG natural gas valve.

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