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allows for a better approximation of efficiency over the wide fluctuation in export steam load than would result from a single linear function. Figure 26-51 shows the results of this analysis.

The efficiency versus load curve shows thousand lbm (Mlbm) of steam per gallon of fuel over the normal range of daily steam loads. The cogeneration simulation uses the relationship, indicated by the solid line, to calculate the total fuel consumed within the boiler plant to meet any steam load not met by the cogeneration system.

The base case represents the operation of the existing facilities, while the other two cases represent the potential cogeneration configurations described below.

As the first step in configuring cogeneration plants for this site, the study team performed a simplified screening analysis of numerous potential systems and configuration options. A total of six potential system options passed the initial screening analysis. The study team then spoke with six manufacturers that produced these systems. In order to obtain the most appropriate cogeneration plant configurations for this project, they developed an outline of plant requirements and constraints that was sent to each of the

Fig. 26-51 Fuel-Efficiency vs. Steam Load Curves.

manufacturers, as follows:

1. Electrical:

— Total production of 10 to 15 MW using 2 to 4 prime movers (i.e., generator sets).

— Tie into existing switchgear facility at 13.8 kV using existing empty cubicle.

2. Thermal:

— Generate steam at 135 psig (10.3 bar) and discharge into header in boiler plant.

— Historical steam loads average roughly 90,000 lbm/h (40,800 kg/h), with minimum values of 20,000 to 30,000 lbm/h (9,000 to 13,600 kg/h).

— Natural gas available at 200 psig (14.8 bar) for 365 days per year; the facility has requested that the system not require a booster compressor.

4. Controls:

— System should track electrical load (prevent export to utility grid). Existing steam and electrical loads track each other well.

5. Physical:

— Existing cogeneration space includes two adjacent areas: a 60 ft (18 m) by 50 ft (15 m) area with no height limit and a 60 ft (18 m) by 80 ft (24 m) area with an 18 ft (5 m) height limit.

As a result of these specifications and discussions with suppliers, this list was narrowed to three prime mover configurations that would meet the above specification, including one reciprocating engine system and two gas turbine-based systems. This list was ultimately reduced through a second screening analysis to one for each prime mover technology by selecting the systems offering superior performance and lower capital and maintenance cost. Following are detailed descriptions of the two final alternatives.

Cogeneration System Operations Analysis

The study team performed a simulation of utility costs under three different plant configurations to determine which had the lowest overall operating cost.

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