Decarbonisation Technology - August 2023 Issue

Atmospheric pressure LNG in reservoir 95 is pumped to at least 1 , 500 psia in pump 90 The LNG is gasied in vapourising/condensing heat exchanger 10, then heated in exchangers 20, 30, 50 and 40 to above 500˚F The heated high-pressure gas produces power in turbine 65 as the uid expands to a pressure of about 550 psia The gas then ows through recuperator heat exchanger 50 and 60, returning heat to the cycle The gas is directed to the condensing side of heat exchanger 10 and re-condensed to an intermediate pressure LNG stream at about (540 psia/-130˚F) against the cold high - pressure LNG stream The intermediate pressure LNG is pumped to 1 , 500-2 , 000 psi in pump 96 and gasified in the ORC condenser 70 and recuperator 60 The heated gasied stream is directed to turbine 66 where power is generated as the gas expands down to the receiving gas pipeline pressure Over-running clutches allow system to provide ancillary electric s e rvices to the grid when not producing power

To ORC

GT EXH IN

High -t emperature turbine

High temperature 40

Gas turbine exhaust heat exchangers

High - temperature recuperator

Low - temperature turbine

Low temperature 30

GT EXH OUT

To gas pipeline

Warming heat exchanger

Low - temperature recuperator

Condensing/vapourising heat exchanger

ORC condenser

LNG reservoir

Intermediate - pressure to high - pressure LNG pump

Low - pressure to high - pressure LNG pump

Over-running clutch

Figure 2 Flow diagram explaining the Re-condensing Power Cycle for Regasification used as a CEGS system Courtesy: Just In-Time Energy

Table 1 shows the performance of a 250 MW CEGS system. It can produce 250 MW for five hours at peak times from Monday to Friday and recharge the LNG storage tank by operating the LNG plant for eight hours each night Monday to Thursday, followed by 56 hours of recharging from 10pm Friday until 6am Monday. The LNG plant and storage tank would be sized to also allow 48 hours of non-stop generation once every 60 days or for shorter times of generation more frequently. The system would return gas to the pipeline system during times of power generation at a rate of about 705,000

lb/hr (a flow rate equal to 400 MMSCFD) During recharging periods, the LNG plant would withdraw gas from a gas storage reservoir or a combination of pipeline and storage reservoir resources at a rate of about 264,000 lb/hr (equal to a flow rate of 150 MMSCFD). At peak times, the system would produce about 1.05 to 1.32 times the amount of electric energy used to produce the LNG (and any stored heat), thus moving this energy from off-peak to on-peak time at a fuel rate of about 4,500 BTU(LHV)/kW-hr. The difference between the charging flow rate of 264,000 lb/hr and the discharge rate

of 705,000 lb/hr is the increased deliverability provided by the system. This increased deliverability comes without requiring more reservoir cushion gas. This variance in charging and discharging gas flow rate can be taken advantage of by installing the system at a depleted oil or gas field- type gas reservoir. The low charging flow means less compression is needed to get the gas out of the gas reservoir, or possibly no

Receiving pipeline

Gas returned Energy ratio* Net power

Fuel rate,

to pipeline system, lb/hr

output, kWe BTU(LHV)/kW-hr

pressure, psia

1,000 1,000

702,100 693,000 705,100 709,800 702,550 717,750

1.00 1.25 1.05 1.31 0.99 1.29

245,000 246,420 262,500 263,500 257,000 256,930

4,700 5,400 4,520 5,200 4,500 5,330

800 800 600 600

Table 1 The performance of a combined electric and gas storage system Courtesy: Just In Time Energy Company Energy ratio equals kilowatt hours of electric energy produced per hour divided by the amount of kilowatt hours of excess renewableelectric energy required to produce the LNG and heat the molten salt per hour of power generation.