The conversion of liquefied natural gas (LNG) to pipeline-quality gas requires large quantities of low-grade thermal energy that may be available from industrial waste streams, steam power plants, or ocean water at the point of discharging LNG from the tanker. Alternatively, heat may be provided by the combustion of LNG or another fuel. In either case, the large temperature differences between these heat sources and the temperature of the LNG can be used to operate an engine that will offset or eliminate the pumping or fuel costs incurred.
North American sources of natural gas continue to decline and, until recently, demand was steadily increasing. The difference between supply and demand was being met by increased imports of liquefied natural gas (LNG).
LNG is created when natural gas (NG) is liquefied by refrigeration at its source. The LNG is transported at near ambient pressure in large insulated tankers across an ocean and then transferred to on-shore or floating off-shore receiving stations with similarly insulated tanks. The liquid, at a temperature of about 110K (–260F), must then be pressurized to pipeline pressure, typically between 3.3 MPa and 10 MPa (500 psi and 1,500 psi), vaporized, and superheated to near ambient temperature before it can be added to an NG transportation or distribution pipeline.
The installation and operating costs of the LNG vaporizing system are major items in the final cost of NG. This article explores some practical and cost-effective combined heat and power (CHP) designs that will improve the energy efficiency of the LNG re-evaporation process.
The composition of NG varies with its source, resulting in slight variations in its thermodynamic properties. That said, NG’s methane content typically is 90% to 95% and, in this article, the analysis is based on the well-known properties of methane alone. The conclusions are qualitatively valid for commercial NG.
The temperature-enthalpy relationship for vaporizing methane is shown in Figure 1. At 3.4 MPa, subcritical methane remains in liquid form up to its boiling point, when it evaporates at constant temperature, and finally heats as a gas to near pipeline temperature. At 10.3 MPa supercritical conditions exist, and the transformation from liquid to gas is continuous with an increase in temperature, and there is no defined boiling point. The difference in the temperature profiles may affect the type of thermodynamic machine chosen for power generation, as discussed later.
1. Methane goes lower. The relationship between temperature and the heat content of methane is similar to that of water, yet the operating temperature range of methane is much lower. Note that the top line is for a constant pressure of 10.3 MPa (supercritical conditions) and the blue line shows the subcritical change of state at 3.4 MPa. Source: Battelle
Heat Transport via Low-Temperature Organic Rankine Cycle
In the ideal subcritical Rankine cycle, regardless of the working fluid selected, the working fluid is first pressurized and heated as a liquid, evaporated at constant temperature and pressure, superheated at constant pressure, and then expanded isentropically to generate work. The fluid is then condensed at constant temperature and pressure to its original state, and the process repeats.
Figure 2 shows a Rankine cycle (in red) superimposed on representations of the temperature and enthalpy changes of a seawater heat source (a→a’) and a heat sink consisting of a subcritical pressure liquid methane revaporization system (in blue). The physical processes for the subcritical case represented in Figure 2 are detailed in Table 1. Note that 81% of the heat input is conveyed to the methane via the exhaust stream.
2. Puzzle parts. Temperature-enthalpy plots for an ocean heat source, Rankine power cycle (shown in red) and liquid methane regasification process (shown in green). The working fluid in the Rankine cycle is assumed to be ethane. See Table 1 for data details. Source: Battelle
Table 1. Thermodynamic properties for state points shown in Figure 2. Source: Battelle
The conversion of liquefied natural gas (LNG) to pipeline-quality gas requires large quantities of low-grade thermal energy that may be available from industrial waste streams, steam power plants, or ocean water at the point of discharging LNG from the tanker. Alternatively, heat may be provided by the combustion of LNG or another fuel. In either case, the large temperature differences between these heat sources and the temperature of the LNG can be used to operate an engine that will offset or eliminate the pumping or fuel costs incurred.
You may recall from basic thermodynamics class that the classic heat engine requires a heat source and a heat sink at different temperatures in order to produce work. The key to understanding the proposed LNG revaporization processes that follow is that the temperature difference between the heat source (a→a’) and the heat sink supplied by the liquid methane (1→4) is sufficient to operate either a Rankine or a Brayton cycle heat engine. The similarity between the temperature-enthalpy profiles of the Rankine cycle and the methane evaporation process allows the cycle designer to marry those two processes. With the proper choice of the Rankine cycle working fluid, cycle parameters, including the relative mass flow rates of the different working fluids and power production, can be calculated.
In practice, at subcritical methane pressures it is possible to take advantage of the temperature plateaus in the boiling and condensation processes; for supercritical pressures it is probably necessary to use steam extraction between two or more turbine stages to make best use of the heat passing through the system.
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