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The unit basically consists of three packaged skids which require minimum field assembly at the customer's site. The fuel cell skid includes two 300-cell stacks, two IHI plate reformers, and a housing vessel. The reformers convert natural gas into hydrogen and carbon monoxide as the stack fuel feed.

The process skid contains a heat recovery steam generator (HRSG), a turbogenerator, two nitrogen botdes, two desulfurization bottles, six demineralizer bottles, and two BFW pumps. The turbogenerator supplies compressed air as the stack oxidant feed and generates additional power by expanding the stack hot exhaust gas. A high frequency generator which rotates at the same high speed (40,000 rpm) as the turbine-compressor is used. It produces about 10% of the total power output. The expanded gas from the turbogenerator flows to the HRSG where waste heat is recovered to: (1) preheat the compressed air feed to the stacks, (2) preheat desulfurized natural gas from the desulfurizers, and (3) generate steam for the reformers. The HRSG is equipped with an auxiliary burner and the necessary burner control center to provide startup heat for the fuel cell unit. The BFW pumps deliver boiler feed water from the demineralizers to the HRSG. The nitrogen bottles provides purge gas during startup and shutdown and pressurized gas to operate the stack bellows. Most of process pipes are located on top of the HRSG to minimize plot area and pipe support structure requirements.

The electrical skid contains a power conditioning unit and a system control unit. The power conditioning unit has an inverter to convert the stack DC power into AC power. It also has a rectifier-inverter to adjust the turbogenerator high frequency power to standard 60 cycles. The system control unit is a simple industrial-size PC-based system geared for unattended operation of the fuel cell unit.

Each skid is sized within the height, length, and width limits for shipping. The unit is designed with maintenance in mind. Equipment requiring frequent service or replacement, such as the turbogenerator, nitrogen bottles, desulfurizer bottles, demineralizer bottles, and BFW pumps on the process skid, is easily accessible. The fuel cell vessel is oriented so that one vessel head does not face other skids and can be freely lifted open to pull out the stacks and reformers for repairs and other services. The only utility required by the market entry unit is 2 gpm raw water. No cooling water, nitrogen (for normal operation), boiler feed water, and compressed air are consumed.

Plant Performance

The energy conversion of the natural gas feed at full load operation and 60 F design ambient temperature is shown in Figure 2. The overall electric efficiency is very high, 54.4% based on HHV and 60% based on LHV. These efficiencies take into account conversion losses in the power conditioning unit. Because the current design is geared toward maximum power production, the

HRSG flue gas temperature is hot enough only for (┬╗generation of hot water. The recoverable heat corresponding to a 100 F final flue gas temperature is 28% of the total energy in the natural gas feed. The total thermal efficiency, including both power and hot water generation, is 82% on a HHV basis.

In the fuel cell unit, only the boiler feedwater pumps and control valve motors require minimal power. The total required auxiliary power is negligible and is one of the major reasons that the overall electric efficiency is so high.

Figure 3 shows the fuel cell unit is capable of turning down to approximately 30% load and still maintaining a reasonable electric efficiency close to 35% at this minimum load. The corresponding cogeneration efficiency is 67%. The amount of energy available for hot water production, which is the difference between the electric and cogeneration efficiencies, actually increases as the load is reduced. The fuel cell unit is sized for 60 F temperature. At both lower

Figur* 3

Part Load Performance

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Figur* 3

Part Load Performance

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