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Reducing Carbon Footprint
- Anand Kumar, Principal Engineer
- Dattesh V Kondekar, Principal Engineer
- M K E Prasad, Sr Vice President
Process and Technology Technip KT India Limited
LNG imports and regasification is essential to meet energy demand of India. During regasification of LNG, ambient air and sea water are economical resources to meet heat requirements. This coupled with LNG cold energy integration by generating power will offer tremendous savings in fuel gas consumption and there by reduction in carbon footprint.

LNG import is one of the major options to India’s energy security. To meet our energy demand, the imported LNG needs to be regasified in LNG terminal to feed in to gas grid. Typically these regasification terminals consumes about 1% to 1.5% plant throughput as fuel to generate electricity to meet the power requirements. The main operating cost of LNG regasification terminal is the LNG Vaporization. Substantial heat duty (Approx. 800 KJ/Kg of LNG) is required for LNG vaporization and it is always a challenge to provide this energy in most optimum way to reduce carbon footprint. Simultaneous use of cold energy of LNG will substantially reduce the fuel requirement of terminal. It is challenge to designers & operators to conceive various ways to utilise this cold energy in terminal and or in adjacent industrial complexes.

LNG Vaporisation
LNG Vaporisation technology selection is based on available conditions of vaporisation medium like ambient air, sea water, natural gas etc. The suitability of type of vaporisation need to be studied for each case comprising the various vaporisation options like:
  • Submersible Combustion Vaporiser (SCV), where natural gas is used as a heat source.
  • Open R ack Vaporiser (ORV), where sea w ater is used as hea t source.
  • Shell & Tube Vaporiser (STV), where sea water is either directly or indirectly used as heat source.
  • Ambient Air Vaporiser (AAV), where ambient air is used directly as heat source.
  • Indirect Ambient Air Vaporiser (IAV) where ambient air is indirectly used as heat source through intermediate heating medium.
In this paper, discussions are limited to Ambient Air Vaporisers; those are generally suitable for tropical and sub-tropical climatic conditions.

Ambient Air LNG Vaporisers (AAV)
Ambient Air Vaporisers using Natural Convection of Air or Using Forced Draft Fan:
In this type of vaporiser, thermal energy of air is transferred to LNG through a series of surface heat exchangers, where the air travels down the heat exchanger. In natural convection type AAV, air flow is controlled by natural buoyancy of the cooled dense air, whereas in forced draft type AAV, air flow is regulated through forced draft fan mounted on top of vaporiser. Liquid LNG flows inside through the tubes and picks up heat for vaporisation from the air flowing on the outside of exchanger surface. Vaporiser is equipped with aluminium fins to increase the heat transfer area. Air Flow in forced draft vaporiser is approximately 1.7 times more than natural draft vaporiser, hence heat transfer area will be lower, and so fewer units are required. However, ice formation rate is more in forced draft type due to high velocity of air leading to more condensation of water from air on vaporiser surface.

Ambient Air Vaporisers is the best suited for tropical climate and is a proven technology. However, there are certain issues associated with this type of vaporiser, notably fog formation, icing of vaporiser surface towards the lower part and cold air recirculation. To overcome non availability of unit due to icing, spare vaporiser unit is normally provided and after fixed running hours (typical cycle of 4-8 hours) vaporiser in operation is taken out of service for defrosting. Operating expense of forced draft type is slightly more than natural draft type due to power consumption in Air Fans.

Indirect Ambient Air Vaporiser (IAV)
In this type of vaporiser, LNG is vaporised utilising ambient air with an intermediate heating fluid. Intermediate heating fluid used is generally glycol water. In IAV, glycol water is heated by ambient air using forced draft fans. This heated glycol water then transfers thermal energy to LNG in a shell & tube vaporiser (STV). Glycol water continuous circulation between IAV and STV is maintained by a dedicated glycol water pump. Transfer of thermal energy from ambient air to glycol water is a function of ambient design temperature and it is affected by other considerations such as cold air re-circulation. Generally large footprint is required for this type of vaporiser. IAV is well proven technology and also it is environment friendly technology as it offers low carbon footprint.

For plant location where ambient temperatures are very low for most part of the year, the applicability of IAV may not be attractive due to incremental cost associated with increased CAPEX (number of fans, air heater bays and supplemental SCV) and OPEX (incremental power consumption in air heater fans, glycol water pumps and fuel for supplemen tal SCV ).

Reverse Cooling Tower based LNG Vaporiser
Another indirect air type of vaporiser is “Reverse cooling tower or Heating tower”. In this type of vaporiser, ambient air heat is transferred to direct heating of cold water, then heated water is used to heat the glycol water, which in turn vaporises LNG in shell and tube vaporiser.

Heat transfer coefficient in heating tower depends upon water side resistance also unlike normal cooling tower where heat transfer coefficient largely depends upon air side resistance. This is due to condensation of water in heating tower. Heating tower is sized for design ambient temperature and humidity based on weather data analysis.

Heating tower type of vaporiser is also environment friendly as it consumes less energy and also it is proven technology.

Design Consideration of Ambient Air Vaporiser
Ambient air thermal energy is utilised for LNG vaporisation in heat exchanger either through natural convection of air or forced draft or through intermediate fluid with forced draft. Heat transfer in all ambient air type of vaporisers depends upon the ambient air temperature in contact with heat exchanger and humidity. Air exchangers are normally designed for ambient temperature such that site ambient temperatures are greater than design ambient temperature for more than 90 per cent of the time in a year. An ambient temperature – time occurrence cur ve for a typical tropical site is given below.

From above Figure 1, we can infer for this typical tropical site that the design point of air exchanger can be around 20 deg C as 90 per c ent of time in a year the ambient temperature is above that temperature. And for a period (~ 10 per cent) when ambient temperature is less than 20 deg C, either supplementary vaporiser like SCV can be provided or alternatively plant throughput can be reduced. Based on such curve, design point of air exchanger can be determined for any site climatic conditions. It is to be noted that lower the design temperature, larger the air heat exchanger size for given temperature approach, duty and flow-rate of fluid.

Procedure for study of ambient air based Vaporisation system can be described as given below:
  • Estimate required LNG Vaporisation Duty.
  • Collect data: historical ambient air condition (temperature & humidity) of site (daily maximum & minimum).
  • Use CFD to determine actual ambient temperature in contact with air heat exchanger for given equipment layout and optimise layout to get the best performance of air heat exchanger.
  • Based on actual ambient temperature, determine suitability of air heat exchanger for worst (winter) condition. Then determine the requirement of supplementary vaporiser to meet total vaporisation duty.
Differential heat duty (for winter months) can be achieved by installing submerged combustion vaporiser (SCV). However, requirement of SCV needs to be studied on case by case basis like for plants located in tropical climate, additional heater may not be required or if winter duration is very short, then plant load can be reduced (if feasible).

Indirect Ambient Air Vaporiser with Rankine Cycle to Generate Power:
As discussed in previous sections using ambient air as a heating medium reduces the requirement of fuel consumption in LNG regasification terminals. In order to further reduce the energy consumption to improve carbon credit of regasification terminals, the innovative utilisation of cold energy need to be looked into.

One of the schemes to utilise LNG cold energy is to generate electrical power using the concept of Rankine cycle with propane as intermediate fluid, which is described in Figure 2.

LNG is vaporised in propane condenser where cold energy of LNG is transferred to vapour propane to condense it. Vaporised LNG is superheated to required send out conditions in glycol water (GW) - LNG heater. Liquid propane from propane condenser is transferred to propane vaporiser through pump. Heat of vaporisation of propane is provided through air heaters using glycol water loop. In air heater, glycol water is heated by ambient air through forced draft fan mounted on top of air heater. Glycol water then passes this thermal energy to propane for vaporisation. Vaporised propane is then expanded in Expander which utilises the useful energy to produce power through generator.

Power generation in Expander is a function of propane vapour inlet pressure, which depends on glycol water temperature, which in turn depends upon ambient air temperature. As the ambient temperature increases, power generation through expander also increases.

Intermediate fluid is selected such that its condensation temperature at low pressure is suitable to vaporise LNG and also it shall be vaporised at near ambient temperature conditions at reasonably high pressures for expander operation. Propane nearly fits to this criterion. This scheme with propane Expander generates sufficient power for normal operation of standalone Re-gasification facility. Expanders are normally highly reliable equipment and its service is also very clean, hence no shutdown is envisaged other than regular maintenance which can be supported by state grid.

Air heater, which is provider of heat to system to be sized for optimum ambient temperature and humidity considering detail analysis of weather of site as discussed in earlier section.

Advantages of this cold energy integrated vaporisation scheme are:
  • Meets plant power requirement through cold energy recovery hence very low operating expenses
  • Best suited for tropical climate however can be tailored for other climatic condition also
  • LNG Vaporisation is possible even if Expander is out of service by operating Bypass JT Valve
  • Cold energy recovery for power generation is a proven technology and coupled with Ambient air heater, it is economical and environment friendly
  • Negligible greenhouse gas emission i.e . low carbon foot pr int. Even though CAPEX of this scheme is high, the pa yback time is v ery attractive.

Case Study of Indirect Ambient Air Vaporiser Using LNG Cold Energy to Generate Power
A comparative study of “Indirect Ambient Air vaporizer (IAV) with cold energy integration to generate power” with “IAV without cold energy integration” for a typical 5 MMTPA LNG Regasification plant is carried out. This study showed that IAV with cold energy integration can generate enough power to meet all its normal power requirements, in turn offering significant savings in energy consumption. A comparison of indirect air ambient vaporizer with and without cold energy integration is given in Table1.

Conclusion
To meet India’s energy appetite, LNG import / regasification is one of the immediate options. Ambient air and sea water are most economical sources to meet the heat requirement of Regasification. This coupled with LNG cold energy integration by generating power will offer tremendous savings in fuel gas consumption and there by reduction in carbon footprint.