Lecture 8 GPL

Natural gas transportation

Increasing demand of Natural gas (NG) in many countries

NG has been prompted by a changing worldwide preference in power generation because of environmental concerns.

As a result, transport of natural gas over long distances has become very important

Due to the storage difficulties of NG, It also needs to be transported immediately to its destination after production from a reservoir

There are a number of options for transporting natural gas energy from oil and gas fields to market:

1) Pipelines

2) Liquefied natural gas (LNG)

3) Compressed natural gas (CNG)

4) Gas to solid (GTS)

5) Gas to power (GTP)

6) Gas to liquid (GTL)

7) Gas to commodity (GTC)

Pipelines Pipelines are a very convenient

method of transport , especially on land. However pipelines are not flexible as the gas will leave the source and arrive at its (one) destination.

If the pipeline has to be shut down, the production and receiving facilities and refinery often also have to be shut down because gas cannot be readily stored.

In the last decade, on average, over 12,000 miles per year of new gas pipelines have been completed most transnationally. Subsea pipelines are also becoming a viable option

Liquefied natural gas (LNG) Liquefied natural gas technology has been proven to be effective since

1970s.

LNG is the liquid form of natural gas. NG is cooled to approximately 162C, liquefies and has a volume approximately 1/600 that of gas at room temperature & weighs only about 45% as much as an equivalent amount of water.

LNG is a non-toxic, non-corrosive, colourless & odourless fuel

However, facilities for liquefying natural gas require complex machinery with moving parts and special refrigerated ships for transporting the liquefied natural gas to market.

Large cryogenic tanks are needed to store the liquefied natural gas; typically these may be 70 m in diameter, 45 m high and hold over 100,000 m3 of LNG. Refrigerated tanker is used to carry LNG that holds 135,000 m3 of LNG.

To make LNG thermodynamically effective and cost viable, larger market is required. LNG can also have significant boil off losses if stored for long time.

Liquefied natural gas (LNG)

Compressed natural gas(CNG)

NG can be transported in containers at high pressures, typically 1800 psig for a rich gas (e.g. ethane, propane, etc.) to roughly 3600 psig for a lean gas (mainly methane). NG at these pressures is termed compressed natural gas (CNG).

Compressed natural gas technology provides an effective way for shorter-distance transport of gas requires compressor and chillers but less expensive than liquefaction.

The technology is aimed at monetizing offshore reserves, which cannot be produced because of the unavailability of a pipeline or because the LNG option is very costly.

Technically, CNG is easy to be installed with lower requirements for facilities and infrastructure.

For distance up to 2000 kms, CNG is more economical than LNG.

Compressed natural gas (CNG) is used for vehicular transport as an alternative to conventional fuels (gasoline or diesel).

Gas to solid (GTS) Gas can be transported as a solid, with the solid being gas hydrate.

Natural gas hydrate is the product of mixing natural gas with liquid water to form a stable water crystalline ice-like substance.

GTS involves 3 stages: production + transportation + regasification

1. Natural gas hydrates can be formed purposely by mixing natural gas and water at 80 to 100 bar and 2 to 10C.

2. If the hydrate (slurry) is refrigerated to around 15C, it decomposes very slowly at atmospheric pressure so that the hydrate can be transported by ship to market in simple containers insulated to near-adiabatic conditions.

3. At the market, the slurry is melted back to gas and water by controlled warming for use after appropriate drying in electricity power generation stations or other requirements.

Lower cost than pipeline or LNG as it eliminates low temperature and necessity of compressing the gas

Hydrate deposits occur naturally and can be found several hundred meters thick and generally occur in two types of settings: (1) under Arctic permafrost (2) beneath the ocean floor.

Gas to Power

Currently, much of the transported gas destination is fuel for electricity generation.

Electricity generation at or near the NG reservoir source and transportation by cable to the destination(s) (GTP) is possible.

For instance, NG could be used as fuel for an offshore power plant, which would generate electricity for sale onshore or to other offshore customers.

Unfortunately, because installing high-power lines to reach the shoreline, GTP appears to be almost as expensive as pipelines, which defeats the purpose of an alternative less expensive solution.

There is significant energy loss from the cables along the long-distance transmission lines.

Currently GTP is used for getting energy from unpopulated areas like Alaska to more populated areas.

Gas to Liquids In gas to liquid (GTL) transport processes, the natural gas is

converted to a liquid, such as syncrude , methanol and ammonia, and is transported .

In the first step, methane is mixed with steam and converted to syngas or synthetic gas (mixtures of carbon monoxide and hydrogen) using suitable catalyst technology

The syngas is then converted into a liquid using a Fischer-Tropsch process (in the presence of a catalyst) or an oxygenation method (mixing syngas with oxygen in the presence of a suitable catalyst).

The produced liquid can be a fuel, usually a clean-burning motor fuel (syncrude) or lubricant, or ammonia or methanol or some precursor for plastics manufacture.

Why choose GTL? 1. Utilisation of natural gas in stranded locations

2. Utilisation of associated gas

3. Synthesis of environmental friendly fuels

4. Life extension of oil pipelines

5. Product upgrading

Gas to Commodity

Commodities such as aluminium, glass, bricks, cement, and iron bars all require large quantities of energy in their making.

In the gas-to-commodity (GTC) concept, the gas is converted to thermal or electrical power, which is then used in the production of the commodity, which is then sold on the open market.

The gas energy is transported via the commodity, but there are many market risks, which should be fully assessed.

The cost of a GTC plant is very high and raw materials for conversion to commodities, e.g., bauxite, silica sand, and limestone, may be difficult to import to sites with reliability.

Therefore, much thought has to be given before embarking on the project(s) and monetizing the gas by this route

Weighing the options for gas transportation

Pipelines - for shorter distance, pipeline is economically more viable, onshore pipelines are easy to install & technologically less complicated, only requires pipeline and compressor stations. However geographical issues (offshore) and political stability can cause issues

CNG & GTS - Where pipelines are not possible, CNG and GTS (hydrate) is cost effective compared to LNG this is because they are more simple with less capital cost requirement. Most effective for shorter distance and smaller volumes.

LNG - efficient technology for long distance transport, cost efficient for long distance transport & large quantities, LNG is a better option for distance above 2000 km

GTL - natural gas is converted directly to liquid products such as methanol wide group of potential products can be derived using this method, requires a complex chemical plant for various products

Weighing the options for gas transportation

Pipeline capacity design The basic concepts involved in pipeline capacity design are shown in

this figure:

The supply sources of natural gas imported into a pipeline could be from another pipeline, LNG, gas processing plants, and gas gathering systems.

Gas then goes through a long-distance trunk-line and eventually reaches the consuming markets.

During the nonheating season (springsummer), excess gas goes to LNG peaking facilities and underground natural gas storage .

During the heating season (winter) or peak period, additional gas is supplied into the pipeline transmission system to meet the demand from the customers.

This pattern will be changed in the future because of two new issues:

1) much larger LNG imports

2) the increasing use of natural gas for electricity generation

Pipeline design means proper size, appropriate distance between compression stations, and adequate compressor sizes

Pipeline throughput depends on pipeline diameter and the operating pressure; taking into account the length of the pipeline and the land.

Onshore pipeline operating pressure: 700 to 1,100 psi. For Offshore pipelines: 1,400 to 2,100 psi

(depending on the material and the age of the pipeline)

Pipeline size

Pressure calculation in pipeline

D L q

P1 P2

After gas processing, the gas in the transport line is purely methane, a single phase compressible fluid.

The calculations use the average values of Z, T and for the entire pipeline

The calculations are for horizontal pipelines with the assumption that kinetic energy pressure drop can be neglected (as flow rate is not very high)

=

. .

.

. +

.

.

Where is pipeline relative roughness (constant) Reynolds number = .

Where is viscosity in cp D is pipeline diameter in inches

= (. )

+

Where: P1 and P2 are pressure in psi is gas gravity

is average Z-factor is average temperature in Rankin D is diameter in inches q is flow rate in Mscf/d L is length in ft is friction factor

Compression

The pressure of natural gas flowing through a pipeline decreases along the distance because of friction pressure drop.

Therefore, compressors are needed to ensure that the natural gas gets to the destination with sufficient pressure along the path and outlet.

In pipeline network, compressor stations are usually placed between 50 and 100 miles apart.

The average horsepower per compression station is about 14,000, and this can move about 700 MMcf/d of natural gas.

Two types of compressors are used: reciprocating and turbine engines.

Besides compressors, there are other components in a compressor station. These include scrubbers and filters.

Although gas is treated before entering the transportation pipelines, liquid may still condense and accumulate in the pipelines during the transportation process.

Horsepower (hp or HP) is the work done over a period of time. One hp equals 33,000 ft-lb/min, or 746 watts, or 75kg-m/s.

It is commonly calculated be this expression, Work (W) is

Where k equals Cp/Cv at constant pressure and volume respectively

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Lecture 8 GPL