Exergy analysis of combined simultaneous Liquid Natural Gas vaporization and Adsorbed Natural Gas cooling

doi:10.1016/j.fuel.2013.03.074

Fuel

Volume 111, September 2013, Pages 755-762

https://doi.org/10.1016/j.fuel.2013.03.074 Get rights and content

Highlights

•
LNG–ANG coupling is an effective method of LNG physical exergy utilization, especially in the case of LNG satellite gasification stations.
•
efficiency of LNG exergy utilization by LNG–ANG coupling can achieve up to 24%.
•
comparison with the most often used Ambient Air Vaporizer shows high superiority of LNG–ANG coupling.

Abstract

The liquefaction process of Natural Gas (NG) involves high energy consumption. Although this energy expenditure is partly offset by benefits in LNG transportation there is a need of improving the balance of the LNG process chain. Nowadays the cost of liquefaction of LNG oscillates between 0.45 and 0.55 kW h/kg when the thermodynamic minimum is 0.30 kW h/kg (assuming the composition is 100% methane). The proximity of these numbers means that the possibility of improving the liquefaction process is very limited. A different approach to the problem allows LNG as a source of exergy which can be utilized by combining LNG gasification with other processes to be considered. Such solutions can help optimize the economical balance of the overall LNG process chain. The paper proposes a novel idea of coupling the LNG regasification with the filling process of Adsorbed Natural Gas (ANG) tanks. Latent heat of LNG vaporization is directly used for the precooling of the ANG adsorption bed. This enables gas compressors to be avoided and improves the competitiveness of the ANG storage method which is an alternative to Compressed Natural Gas (CNG) storage and distribution. Exergy analysis presented in the article allows the proposed idea and conventional methods of Natural Gas regasification to be compared.

Introduction

Natural Gas is considered to be the most perspective energy source in forthcoming decades. It is recognized as one of the cleanest fossil fuels. The NG share in the global energy market shows a stable growing tendency. The International Energy Agency predicts a 25% share of NG in world primary energy sources in 2035 [1]. This prediction is even higher for countries with rich shale gas resources.

Natural Gas is usually transported from the wellhead to the processing plant and transferred to consumers through high pressure gas pipelines. Transport of NG from remote locations which are separated by large water reservoirs is conducted in its liquid form. Liquefied Natural Gas (LNG) has a high energy density (lower physical volume) around 600 times higher than when in gaseous form. Transportation of NG in its liquid form is also attractive for short time working boreholes, as is also the case with unconventional gas. The LNG share in overall NG turnover shows a stable growing tendency and is expected to exceed 25% soon [2]. Due to this observed increase there is a strong interest in improving the economical balance of the whole LNG process chain.

The LNG process chain is apart of the NG process chain (see Fig. 1) and consists of three steps: liquefaction, transportation and storage and regasification. Perfection in liquefaction technology seems to have achieved its limit approaching the thermodynamic minimum (latest patents indicate the energy of liquefaction as 0.35 kW h/kg of LNG [3] and even less [4]). Storage and transportation systems are very efficient and the only step of the LNG process chain with the potential of thermodynamic, and in consequence economic optimization, is regasification.

Section snippets

Exergy of LNG

Exergetic analysis can be considered as a convenient tool for the evaluation of different methods of LNG vaporization.

Exergy is the maximum amount of work that can be done by a subsystem as it approaches thermodynamic equilibrium with its surroundings by a sequence of reversible processes [5]. Equilibrium state means uniform temperature and pressure conditions as well as density, chemical composition, gravitational and electro-magnetic fields between the reference state (surrounding) and the

State of art in LNG gasification

Nowadays, methods of LNG exergy utilization are mostly based on units consuming heat energy from commonly accessible sources. A few proposals of practical utilization of LNG exergy exist but inconvenience in their implementation result in their mostly virtual applications till now.

The potential methods of LNG vaporization are:

I.
Vaporization by using an ambient heat source like sea or river water or air. Examples of systems: Open Rack Vaporizers, and Ambient Air Vaporizers.
Recently the most

Exergy efficiency of an ambient air vaporization unit (AAV)

An Ambient Air Vaporizer is the most common type of LNG vaporizer. Due to this fact the analysis of an AAV is presented in this paper. As a common unit, used for continuous and periodical (LNG satellite stations) vaporization, an AAV is the best reference case study for the method proposed in the article.

An AAV is a simple unit. It receives heat energy for the vaporization process directly from ambient air. Resulting cooling power is usually not utilized in spite of its potential applications

Adsorbed Natural Gas

Adsorbed Natural Gas (ANG) is a method of Natural Gas (NG) storage. The concept relies on storage of light hydrocarbon molecules (mainly methane) onto a surface of solid porous material thanks to adsorption phenomenon. With the correct choice of adsorbent material the pressure benefits are achieved. The ANG system is designed to work under pressure up to 35 bar as optimal. It can store the same amounts of NG as the 160 bar CNG tank with the same tank volume [20]. Due to the fact that a 1 bar ANG

Coupling of ANG and LNG gasification general idea

Most of the discussed methods of LNG exergy utilization are focused on large scale working vaporization units which have existed for a long time. However, there are small satellite LNG gasification stations which work non-continuously. Exergy losses generated by them are usually neglected as it seems there is no alternative solution for them. As periodically working units they demand coupling with other periodical processes to utilize released exergy.

A proposed solution is the coupling of LNG

Pressure benefits

The adsorption process depends strongly on temperature. The lower the temperature of the adsorbent the lower the gas pressure needed to achieve the same uptake. In case the adsorbent temperature significantly reduces in relation to the environment temperature, the demanded pressure can drop from tens to a few bar. This results without the necessity of using a gas compressor. After the filling process is finished and the cut-off valve closed the desorption process occurs due to heat transfer

Exergy analysis of LNG–ANG system

Exergy analysis of a non-continuous process was carried out for the finished portion of exergy which flows in and out of the system and also for the exergy which is lost during the process. Calculations were done for two variants:

(1)
An insulated system with no heat leak from the ambient. Latent heat of vaporization is used for adsorbent precooling and compensation of the heat of adsorption.
(2)
A system with heat leak from the environment assumed as 15% of the total energy balance. The requirement

Results and conclusions

Coupling of Liquefied Natural Gas gasification and Adsorbed Natural Gas filling processes is an effective and perspective method of Liquefied Natural Gas exergy utilization. Depending on the vaporization pressure and target temperature of the adsorbent the overall exergy efficiency can reach 24%. Commonly used Ambient Air Vaporizer units obtain total efficiency in exergy utilization up to 3% (15 bar vaporization unit) and 1.6% (40 bar vaporization unit) for the temperature of air: 280–260 K.

Acknowledgements

This work has been supported by the National Centre for Research and Development, as Strategic Project PS/E/2/66420/10 Advanced Technologies for Energy Generation: Oxy-combustion technology for PC and FBC boilers with CO₂ capture and statutory funds from Ministry for Science and Higher Education (S-10057/I-2201).

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Citation Excerpt :
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Exergy analysis of combined simultaneous Liquid Natural Gas vaporization and Adsorbed Natural Gas cooling

Highlights

Abstract

Introduction

Section snippets

Exergy of LNG

State of art in LNG gasification

Exergy efficiency of an ambient air vaporization unit (AAV)

Adsorbed Natural Gas

Coupling of ANG and LNG gasification general idea

Pressure benefits

Exergy analysis of LNG–ANG system

Results and conclusions

Acknowledgements

Energy

Appl Energy

Dictionary of Energy

A brief commented history of exergy from the beginnings to 2004

Int J Thermodyn

Exergy method, technical and ecological applications

Magnetic systems depending on three or two variables; thermodynamic analysis and some existing and possible applications

Energy Convers Manage

Design optimization of single mixed refrigerant natural gas liquefaction process using the particle swarm paradigm with nonlinear constraints

Energy

An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes

Energy