The present work discussed below is about the energy integration of LNG import terminals to adjacent power plants with the aim of generating mutual profits for both systems. There are opportunities for performance enhancement and clever use of the residual non-workable energy from the power plant side into the regasification process, as well as the regasification process lowering the reservoir temperature level for heat rejection, working as a virtual dead state.
The combined cycle gas turbine (CCGT) plant is a very efficient assembly of a gas turbine (GT) to a Rankine cycle, whose goal is to prioritize the electrical output conversion. Flue gases are rejected by the GT allowing production of steam that drives the steam turbine (ST) of a coupled Rankine cycle. Overall performances superior to 65% are not unusual, and explain the huge number of power plants based mainly on natural gas, a suitable and convenient fuel to this kind of application.
A conventional CCGT power plant was thermodynamically modelled and simulated under steady state conditions, and results were taken as a base line to examine the advantages of its integration to an LNG regasification process. The schematic view of the CCGT plant is shown in Figure 1, displaying the two integration alternatives.
In the conventional CCGT power plant, NG is burned at the NG Combustor of the GT with air at atmospheric conditions, captured from the environment (see Air Inlet Conventional). Right after the conversion into workable power, flue gases from the discharge of the Gas Turbine are directed to the Heat Recovery Steam Generator (HRSG), which drives the Rankine cycle, with a secondary conversion of energy. The rejected energy streams of the CCGT system are flue gases released at the Stack and heat at low temperature at the Steam Condenser.
The two proposed LNG to CCGT coupling alternatives promoted the fuel regasification by recovering available heat streams, avoiding auxiliary output for the regasification process at high rates. From the power plant point of view, the coupling brought performance enhancement.
Fig. 1. Scheme of the conventional CCGT power plant and LNG regasification alternatives
The first integration alternative was built around the GT, where the LNG exchanged heat to the turbine air inlet, replaced to Air Inlet-Alternative 1. A first heat exchanger was placed to act as the LNG evaporator, and a second one as an intercooler. Gains came from the rising of air density due to a temperature drop, and though the increase on the volumetric performance of the GT air compressor. The intercooler is a standard exchanger for GTs, and was forced to operate at lower temperatures, and again bringing better performance to the engine.
The second integration alternative added to this last one an extra coupling around the vapour cycle condenser. Heat integration started at the condenser unit, where LNG changed phase from liquid to gas, while the cycle working fluid was condensed at a lower temperature level than the regular dead state. Air intake was changed to a NG Superheater (Air Inlet-Alternative 2) and is cooled down with the aid of a stream of NG at low temperature.
Both alternatives were able to deliver a surplus of regasified NG to be injected to any distribution pipeline, besides the amount of fuel that is consumed by the power plant, as a result of the heat rejected by the system.
All systems were simulated at steady state regime, and LNG was assumed to be pure methane, for the sake of simplicity. The gas turbine was set to produce a fixed electrical output of 30 MW, allowing the Rankine vapour cycle and all other systems to deliver a variable output. Figure 2 shows energy flows for each of the cycles.
Fig. 2. Control volume of the conventional CCGT power plant and LNG regasification integration alternatives
The energetic performance of the reference CCGT plant was computed by the ratio of net electrical output to NG input. Efficiency of the 2 integrated alternatives with LNG was enhanced by two main factors: specific gains in electrical output from the power plant side, and avoided energy for LNG regasification, from the supply chain point of view. Main results are displayed on the next table.
The gas turbine was set to deliver a fixed electrical output (30 MW), leaving the Rankine cycle free to reach a net output WRankine according to the changes in the
amount of flue gases produced by the topping machine. The highest CCGT electrical net output was obtained for the reference cycle, without the coupling to the LNG heat recovery. This can be seen as a contradiction at a first glance, but it came from the fact that the integrated cycles demanded less fuel to deliver the fixed output at the gas turbine as compared to the reference one, which consumed the highest amount of fuel and therefore produced more flue gases.
This behavior makes sense by observing the net electrical efficiency, which showed higher performances for the integration strategies, as the fuel consumption dropped for both GT and Rankine cycles working at a lower environmental temperature. The overall energetic efficiency went from approximately 50% for the reference cycle up to 86% when considering both the performance gains of the power cycles and the avoided energy of the LNG regasification process. Heat exchange process was able to deliver 6.6 times more natural gas to the pipeline than the power system consumed amount in alternative 1, and about 21.7 times in alternative 2
Table 1. Main system outputs for the reference CCGT and the two proposed integration alternatives depicted in Figure 1, for a fixed gas turbine electrical output of 30MW
The EROI of a power plant is the ratio of usable energy returned by the plant along its lifetime to the overall invested energy needed to make this energy usable. The returned energy is the product of average power P to the assessed elapsed time t. Invested energy has a fixed part for construction and deconstruction Efix, and a variable time dependent amount PI, that stands for maintenance and fuel provisioning.
EROI=(P t)/(E_fix+P_I t)
In the present work, the focus was on the EROI gain of the proposed integrations. The energy required for provisioning fuel for the three simulated plant scenarios was considered the same. Although the reference plant needs an additional regasification system, the energy consumption of the regasification process with open rack vaporizers is approximately 28.8 kJ/kg for driving seawatercirculating pumps. This number is negligible when compared to the energy consumption of the liquefaction process (1800 kJ/kg).
As PI was considered the same in all scenarios, the EROI gain of the proposed integrations will be the power output enhancement, expressed by the specific power output, displayed in the previous table in kW/ Nm3. Thus, integration 1 and 2 lead to a 13% and 19% gains in the EROI compared to the same parameter of the reference plant. This is an important parameter in investment decision in the energy sector.
In conclusion, both integration alternatives lead to an electrical efficiency enhancement when compared to the non-integrated cycle: from 49% for the reference case to 56% for alternative 1 and 58% for alternative 2, a gain of 6.32% and 9.09%, respectively. When considering the overall performance, which includes the thermal energy for LNG regasification, alternative 1 reached 63% and alternative 2 reached 86% considering a 1st law efficiency analysis. The energy return on investment of each alternative was also enhanced by 13% and 19%, respectively.
Diogo Angelo Stradioto
APS Engenharia, Porto Alegre, Brazil,
Marina Fonseca Seelig
Paulo Smith Schneider*
Department of Mechanical Engineering
Porto Alegre, Brazil