652

system with its DC/AC inverter, 16 % comes from a generator driven by the gas turbine, and the remaining 19% comes from the generator driven by the steam turbine. There is a 4% loss for pumps and blowers in the system. The overall efficiency of the hybrid power cycle is 65%.

EMISSIONS ~ A comparison of the NOx between a hybrid power cycle and a gas turbine combined cycle was made on the basis of equilibrium levels predicted from the burners in the two systems at their respective operating conditions. The results showed that the 20 MW hybrid power cycle is expected to generate 83% less NOx than a 20 MW gas turbine combined cycle.

The emission of sulfur dioxide (SOx) is expected to be only about 1% of the level from a gas turbine combined cycle because the fuel is desulfurized in the process (not shown on the simplified system schematic Figure 2). The contribution of carbon dioxide C02 to the atmosphere is expected to be about 24% lower than a gas turbine combined cycle due to the higher efficiency.

COST OF ELECTRICITY - The 30 year levelized cost of electricity for the 20 MW plant with a hybrid power cycle is estimated at 5.1 fi/kWh, without inflation, using methods recommended in EPRI TAG(4). This includes levelized plant cost of 1.4 (i/kWh, operating and maintenance (O&M) cost of 1.3 0/kWh, and levelized fuel cost of 2.4 0/kWh.

The 30 year levelized plant cost is based on overall capital cost of 1059 $/kW in 1995 dollars. This overall plant capital cost assumes 1000 $/kW for the fuel cell system. The capital cost for the gas turbine was estimated at 610 $/kW5 and the steam system at 1260 SkW(5). The cost of the heat exchanger which transfers heat to the gas turbine compressor exhaust was estimated at 9.5 $/kW compared to 53 $/kW in the 200 MW plant studies(1) based on its lower temperature and heat duty requirements.

The O&M cost includes the fuel cell system O&M cost projected by ERC at 0.8 fi/kWh including 5 year stack replacement. The combined O&M costs for the gas turbine and steam system is estimated at 0.5 0/kWh®. The levelized fuel cost of 2.4 0/kWh is based on a first year fUel cost of $ 3/MMBTU and a capacity factor of 0.91. The calculated levelizing factor is 1.374, an interest rate of 5.3%, no inflation and a fuel escalation rate of 2.5% per year.

COMPARISON WITH A 20 MW GAS TURBINE COMBINED CYCLE -- For perspective on the commercialization prospects for a 20 MW plant with a hybrid power cycle, a comparison was made with a 20 MW gas turbine combined cycle. The comparison addressed issues of performance and cost of electricity. The gas turbine combined cycle selected for the comparison is a commercially available model rated at 18.7 MW. This system is composed of a single gas turbine rated at about 13.4 MW and a 5.3 MW steam turbine. Published5 heat rate is 6870 BTU/kWh (49.7% LHV efficiency). The 30 year levelized cost of electricity for the 20 MW class combined cycle was estimated at 5.2 ¡i/kWh, without inflation, using EPRI TAG'4'. The 30 year levelized plant cost is based on published® cost of the commercially available model combined cycle, and estimates of installation and project cost, resulting in an estimated plant capital cost of 954 $/kW. The O&M cost3 of the combined cycle is in 1995 dollars. The levelized fuel cost of 3.1 fi/kWh is based on the same assumptions as used to estimate the fuel cost for the hybrid power cycle. A breakdown of cost of electricity is shown in Table 2 in comparison with the hybrid power cycle. As shown in Table 2, the hybrid power cycle COE fuel cost component at $3/MMBTU (S3.163/MMKJ) is significantly less than the fuel cost component for the combined cycle, off- setting the higher plant and O&M COE cost components in the hybrid power cycle. As first year fuel costs increase, the COE cost advantage of the hybrid system increases, as shown in Figure 3. The hybrid power cycle is competitive

Table2. with the combined cycle for 20 MW

HYBRID POWER CYCLE VS. installations in which the first year fuel cost is above $2.5/MMBTU.

TECHNICAL CHALLENGES -Near term hybrid power cycle commercialization in a 20 MW size, demonstrating efficiency of 65%, can be achieved with available heat exchanger technology and without anode recycle. For this near term application, attention must be directed to the integration of the fuel cell system with a gas turbine and steam system which have relatively low power output. In addition the gas turbine compressor air is heated external to the gas turbine and then returned to the gas turbine burner for supplementary heating before passing through the turbine. Gas turbine technology must be reviewed in detail with gas turbine suppliers and the gas turbine design modified to accommodate the hybrid power cycle integration requirements. In order to achieve the 72% efficiency promise of the hybrid power cycle, technology and development advancements are needed for Hybrid Cycle Provides Significant Advantage at heat exchangers capable of 1094°C and High Fuel Costs pressures up to 400 psig. In addition, an anode recycle at 650°C is required.

CONCLUSIONS — A 20MW hybrid power cycle for near term application using available technology, including a 760°C heat exchanger, with steam provided from the bottoming cycle steam system rather than anode recycle result in an estimated LHV efficiency of 65%. The NOx emissions are 83% lower than a 20 MW gas turbine combined cycle. The estimated cost of electricity for the near term 20 MW plant with a hybrid power cycle is 5.1 ji/kWh, which is competitive with a 20 MW combined cycle for installations where the fuel cost is above S2.5/MMBTU.

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