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Solving carbon emissions with LNG

LNG Industry,


Rob Sherwood is a member of the Institution of Gas Engineers & Managers (IGEM) Young Persons Network (YPN) and his paper featured in the Young Person Paper Competition which took place in Derbyshire earlier this year. IGEM is a professional membership body which is aimed at continuing professional development for all its members. The YPN is a vehicle for supporting the education, training and professional development of younger members through a mixture of events, competitions and networking opportunities.

The task to reduce carbon emissions cannot be solved with one simple solution. With demand for energy forecast to remain at current levels and local fuel reserves depleting we are forced to look to new innovative technologies for the answers.

Reducing the physical amount of CO2 released (via renewable generation and carbon capture) will help, although we should also focus on identifying new ways to increase efficiency across all industries, and the gas industry could lead the way.

Energy in the LNG industry

The global LNG industry is energy intensive; energy is consumed or converted at each stage of the process, this means there is a lot of scope for increasing efficiency.

 Figure 1. A flow diagram of the global LNG process.

We can track 1 tonne of natural gas from Qatar’s North Field to the Grain LNG importation terminal. It starts its journey as gas under the Persian Gulf and ends as gas in the UK transmission system, the energy needed for this journey is:

  MWh per tonne LNG 
Liquefaction 0.486 
Shipping (6558 miles) 0.287
Regasification 0.262
Total 1.350

This is a conservative estimate as there has been no account for the drilling, cleaning, storage, and any gas shrinkage (losses) that occur during the whole process. Yet the energy required is still equivalent to the typical household consumption for nearly a month.

So why is all this energy used in order to transport gas? This is because 1 tonne of gas, when burnt, produces 15.11 MWh, this is equivalent to nearly a years worth of energy for the average household based on a family of four, consuming 50kWh of energy per day.

Energy consumption at an LNG terminal

The re-gasification of 1 tonne of natural gas uses about 0.262 MWh, this is less than what is needed to liquefy it but is nonetheless still a significant contribution to the energy use of the process. The energy use at an LNG terminal can be split into two parts:

Heating energy:   0.243 MWh

Electrical energy: 0.019 MWh

The heating energy is the energy required to heat the LNG from -160 °C back to about 10 °C. This is provided by burning fuel (a fraction of the stored LNG) in a submerged combustion vapouriser (SCV). The electrical energy comes from:

kWh per tonne LNG
In-tank pump 2
Ex-tank pump 8
Vaporiser fan 3
Boil-off gas compressor 6
Total 19 (or 0.019 MWh)

The hot water pipe (using waste heat from E.ON’s Grain power station) should effectively mean the heating energy part of the re-gasification process will be carbon neutral (as the water is waste). The opportunity then for Grain LNG to further increase efficiency comes from the source of electrical energy used; if it can be generated from on-site waste then this should reduce the amount generated from power stations and hence reduce CO2.

Principles of generation

In all types of large-scale power generation, energy needs to be removed from the motive fluid. This is because they work on a closed cycle; removing the low grade energy at the end ensures more input heat energy is converted to electricity in the other half of the cycle. This removed energy usually goes to waste.

The key mechanism required in a closed cycle power station is a source of heat (usually fuel combustion) and a source of cold (usually sea or river water).

When the motive fluid (water) is heated, it gains temperature and pressure and eventually evaporates into steam. Some, but not all, of this temperature and pressure is then converted to electricity. The low pressure steam must then be re-condensed into water (as steam cannot be repressurised without using more energy than you can get back out). This cooling stage is crucial in ensuring the efficiency of the power station.

Therefore, in principle, all that is needed to generate power is a source of heat, a source of cold and a fluid that has a liquid/vapour phase change in between these two temperatures. Essentially, it doesn’t matter what the temperature of the hot and cold sources are as long as there is sufficient difference between the two.

Grain LNG as the most efficient LNG terminal?

At Grain LNG there is a source of heat with the hot water pipe, this can provide water at about 40 °C. There is also an abundant source of cold with the LNG kept at about

-160 °C. So just by using the existing resources at the site, Grain LNG has a potential 200 °C temperature gradient from which it is possible to generate power.

Also, unlike most power stations, the energy removed from the fluid by the cold source (LNG) will be used in the re-gasification process, thus ensuring maximum efficiency.

What would this generator look like?

This type of generation using low grade heat works on the Organic Rankine Cycle (ORC) and has been used for geothermal power for several decades. The best motive fluid with a liquid/vapour phase change between -160 °C and 40 °C is propane (phase change at -42 °C).

Figure 2. The line diagram of the ORC, the modular unit is shown within the dotted line. Propane is the motive fluid (green line).

The energy generated from this system is proportional to the LNG running through the terminal, this works out at ~0.0186 MWh per tonne of LNG. This is almost exactly equivalent to the 0.019 MWh of electricity used in vaporising 1 tonne of LNG.

Figure 3. Figure 3 A waste heat recovery system close to Las Vegas, US generates 7.5MW using the Organic Rankine Cycle and waste heat from gas-fired compressors.

Therefore Grain LNG has the potential to significantly reduce the emissions associated with electricity consumption and thus the overall emissions with the installation of this technology.

An Organic Rankine Cycle generator at Grain LNG will be a further step in increasing the efficiency of the terminal. It also sets a new standard for energy innovation and should encourage all businesses to think again about their current energy consumption and what they can do to help reduce CO2 emissions.

Read the article online at: https://www.lngindustry.com/small-scale-lng/29102012/solving-carbon-emissions-with-lng_193/


 

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