Test the Economics
A custom-designed and well-integrated steam/ethane power plant (Figure 4) could produce power at a favorable cost, especially if waste gas streams from boil-off of stored gas or gas treatment units were available. Table 2 shows that the hourly energy demand of a 1,000 tonnes per hour plant at a steam to methane ratio of 0.443 is 1,676,000 MJ. The fuel used that would be required in a typical direct-fired evaporation plant would be 892,000 MJ per hour. The difference, 784,000 MJ, represents the additional fuel used for 160 MW of power generation. With gas at $4.74/GJ ($5/million Btu), this fuel would cost $3,715, or $23.2 per MWh. If the market value of electric power is $50 per MWh, the hourly production is worth $8,000 and the savings are $4,285 per hour.
If the additional plant cost for the integrated regasification system were $2 million per MW of installed capacity, the payback period would be about 10 years, assuming 7,500 hours of operation per year. The plant cost is obviously an estimate, and actual costs depend heavily on the extent to which the generation and re-evaporation operations can be integrated and on the design parameters chosen for the steam cycle, but my calculations are conservative. No doubt the design and costs could be further optimized for a particular installation.
For the independently fired closed Brayton cycle (Figure 5) the only major heat loss is the flue gas — about 14% of the energy input (Figure 7). The marginal energy to be provided by combustion over that required for simple re-evaporation is calculated, as before, as 1,075 – 843 = 232/0.85 = 273 kJ. At a methane evaporation rate of 1,000 tonnes per hour, 273,000 MJ/h of fuel would be required to give a net power output of 54 MW, equivalent to a heat rate of 5.06 kJ per kWh (4,793 Btu per kWh). This corresponds to a fuel cost of 2.4 cents per kWh. The value of power over 7,500 annual hours of operation is $20.25 million against a fuel cost of $9.72 million.
7. Point the way. This Sankey diagram for a dual Brayton cycle regasifier plant shows how the fuel is consumed. The output and efficiency of the dual Brayton turbine system are comparable to those of a conventional gas/steam combined cycle, with no requirement for water or cooling system other than that provided by the methane re-evaporation process. Source: Battelle
For a generating plant costing $1,000 per kW, the simple payback would be a little over five years. The closed Brayton power cycle fuel options are more limited than for the Rankine power cycle, but the moderate heat exchanger temperatures give the Ranking cycle the flexibility to use fuels other than natural gas.
Using these same assumptions, a modern combustion turbine exhausts about 44.6% of its fuel input as waste heat, compared with 36.7% used for the revaporization process using a closed Brayton cycle (Figure 7). For an evaporation rate of 1,000 tonnes per hour at 843 kJ/kg, the energy converted to electric power would be 44.6/36.7 x 843 = 1,024,000 MJ per hour, which is equivalent to 284 MW. The additional hourly fuel cost is $6,186, and the value of the power produced is $14,200. At a cost of $1,000 per kW for the turbo-generator and 7,500 hours of operation per year, the payback period for the Brayton cycle option is also under five years, yet this option uses more than three times the amount of revaporized LNG to run the process.
--Donald Anson (ansond@battelle.org) is a research leader at Battelle specializin in energy system design and analysis.
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