The College of New Jersey owns and operates a cogeneration plant utilizing one dual fuel Solar Turbines, Inc. Taurus turbine. Electricity is potentially the most costly component of The College’s utilities bill, but is significantly offset by onsite power generation. Total cost savings from combined heat and power for FY 2007 was $3,500,000; the electric cost savings component is $2,500,000, and the steam cost savings component is $1,000,000 (See below Economics of Cogeneration). The College currently generates approximately 90% of its electric consumption, with the remainder supplied by PSE&G.
The cogeneration plant consists of one 5.2 MW Solar Turbine dual fuel turbine (natural gas and #2 low sulfur oil); one 6 MW Ideal generator; one Davis duct burner; one ERI heat recovery steam generator (HRSG) with a capacity of 25,000 PPH unfired and 40,000 PPH fired; and two Gardner Denver natural gas reciprocating compressors with lead/lag controls. The cogeneration plant utilizes a Wonder Ware data highway with Allen-Bradley programmable logic controls (PLC’s) platform.
The turbine converts chemical energy from the fuel source into mechanical energy. Unlike four-stroke car engines with reciprocating pistons, which use the four combustion processes, combustion; expansion; compression; and exhaust separately, a turbine operates on the Brayton Cycle, which accomplishes the four combustion processes simultaneously.
The Brayton Cycle
In the Brayton cycle, a turbine is injected with fuel through fuel injectors. The fuel is combusted, resulting in high temperature in the combustion chamber.
The hot gas in the combustion chamber expands and does work on the multi-stage turbine blades, causing the main shaft to rotate. The internal combustion temperature of the combustion chamber is nominally 1,250 degrees F. The main shaft rotates at approximately 15,000 RPM. The rotation of the main shaft produces mechanical energy to drive the turbine’s multi-stage compressor (see below), and accounts for approximately 70% of the total mechanical energy. Approximately 30% of the total mechanical energy due to rotation of the shaft is converted to electrical energy by driving a 4,160 volt, 5.2 MW generator.
Rotation of the main shaft provides the mechanical energy required to drive the multi-stage compressor blades. This is a vital part of the Brayton cycle, because rotation of the compressor blades provides compressed air flow through the turbine to feed the combustion process.
The exhaust gases exit the turbine at a temperature of nearly 1,000 degrees F and are directed to the HRSG, which extracts the thermal energy from the hot exhaust for the production of heat (steam).
Economics of Cogeneration
The purpose of cogeneration is to generate electricity and produce heat or steam via recovery of the turbine’s waste heat. This process is known as “Combined Heat and Power” and has saved The College several millions of dollars per year in energy costs. The plant produces nominally 5.2 MW of electricity, 25,000 pounds per hour (PPH) of heat-recovered steam, and 15,000 PPH of steam by supplemental firing with the duct burner.
The economic benefit of cogeneration is determined by comparing Central Utilities Plant, fuel and utilities operating costs using two operating scenarios: (1) assume operation of the cogeneration plant, which utilizes combined heat and power, versus (2) assume shut down of cogeneration plant and produce steam by the dual fuel, conventional boilers, and purchase power from the utility grid. The economic analysis would utilize unit cost of tariff Cogeneration Interruptible Gas (CIG), non-firm tariff Market Price Service Gas (MPSG) boiler gas, unit cost of fuel oil, and cost per KW and KWH for electricity purchased from the utility.
The total net economic benefit of cogeneration in FY2007 was $3,500,000.
Electric Cost Savings Component
The College consumed 41,700,000 KWH in FY 2007. Should The College have chosen to do so, the aggregated (including demand cost) cost to purchase total campus electric requirements would have been $7,800,000 or $0.19 per KWH. The cogeneration plant generates 37,000,000 KWH at a total CIG gas cost of $4,600,000, or $0.124 per KWH. Therefore, the net cost benefit of combined heat and power utilizing the cogeneration plant favorably offset the cost of purchased electric to $2,500,000 in FY 2007.
Steam Cost Savings Component
The operation of the cogeneration plant produces 25,000 PPH of waste (free) steam. By firing the duct burner, 15,000 PPH of very inexpensive steam is produced because of the high efficiency of the duct burner and the relatively inexpensive cost of cogeneration gas tariff CIG. Duct burner steam savings were calculated using a 10% differential in efficiency between the duct burner and a standard boiler, and the unit cost difference between boiler gas and the CIG gas. This difference in cost is important because the duct burner is fired using CIG gas and displaces consumption of boiler gas pound for pound of steam. Tariff CIG gas is much less expensive than the tariff gas used in a conventional boiler. Net steam cost savings of $1,000,000 were realized by operating the cogeneration plant HRSG and duct burner.