Small scale attracts big interest

3 January 2019

Recent years have seen growing recognition that LNG (liquefied natural gas) – now very well established as an internationally traded commodity, with a significant global infrastructure in place – doesn’t always have to be large scale. Significant interest is developing in decentralised small-scale LNG technologies. By James Varley

Anew white paper from MAN Energy Solutions, Small-scale LNG, suggests that an important driver in making small-scale LNG feasible has been the establishment of large local hubs, “from which LNG is distributed via smaller carriers to decentralised locations” – the “hub and spoke” model, with sometimes a “‘milkship/milk run’ system to supply local small-scale LNG receiving terminals.”

Helped by the extensive supply infrastructure, LNG is ubiquitous, plentiful and the price has fallen dramatically. It’s a buyer’s market says Carsten Dommermuth of MAN ES. “In the past, the cheapest fuel you could have for an internal combustion (IC) engine was heavy fuel oil, but nowadays it is much cheaper to use LNG”, and the port facilities required for import, storage, regasifying and supply to power plants are now competitive at smaller scale.

At the same time LNG has the considerable added benefit of being a much cleaner fuel than heavy fuel oil (carbon content 200 gCO2/kWh (cf 278 gCO2/kWh for HFO) and essentially no soot or sulphur), significantly reducing environmental impacts.

It is also transportable (via ‘virtual pipeline’, eg LNG carrier in combination with rail and/or road tankers) to remote locations and islands, opening up the possibility of low emissions power generation for places currently served by coal, diesel or HFO fuelled plants.

LNG is compressed for transport at -162°C, the liquefaction reducing the volume by a factor of 600. Storage and transport of LNG requires no further refrigeration to maintain the liquid state, but one downside is that the energy needed for liquefaction amounts to about 10-25% of the calorific value of the gas, meaning that LNG only makes sense when transport distances are relatively long and, of course, no pipelines are to hand.

Why recips?

For small power plants (which, for MAN ES, means in the range 10 to 100 MW), particularly in electricity-only applications, reciprocating engines can claim a number of advantages over gas turbines, not least high simple cycle efficiency (about 46% for, say, a 14 MWe engine (even without two-stage turbocharging) vs typically around 34% for an industrial gas turbine of equivalent capacity) and excellent operational flexibility, with the capability to perform frequent starts/stops and fast ramping without maintenance penalty – very helpful for stabilising grids with a high proportion of intermittent renewables.

“We are therefore seeing a number of factors coming together that favour small- scale LNG and its use in IC engines”, says Carsten Dommermuth.

A good example is the North Mole project in Gibraltar, currently in the final stages of commissioning, with the first LNG delivery expected in January/February 2019.

The North Mole project consists of a terminal (being provided by Shell under a 2016 agreement with the government of Gibraltar) that will receive, store and regasify LNG for use in the new power plant.

The terminal includes a berth for a ‘small’ LNG carrier (length 117m) and five 1000 m3 double-wall stainless steel storage tanks. The LNG is transferred to the regasification unit as required by the power plant.

The regasification unit, which employs waste heat from the power plant for vapourisation, is operated by Gasnor, a 100% Shell owned subsidiary.

The 2 x 40 MW power plant, located next to the terminal, is being built for the Gibraltar Electricity Authority (Gibelec) by Bouygues Energies & Services SAS, which is acting as EPC contractor.

Gas turbines were considered but reciprocating engines were chosen for the electricity-only power plant, which consists of three 14.4 MWe MAN ES 14V51/60G gas engines and (for increased fuel flexibility) three 13.7MWe MAN ES 14V51/60DF dual fuel (gas/diesel) engines, with all six engines equipped with ABB generators. The engines are classified as medium speed, 500 rpm.

The gas engines employ the Otto cycle (with spark plugs) while the dual fuel engines use the Diesel cycle (no spark plug) and need a supply of diesel as pilot fuel (1%) for gas mixture ignition when operating on gas. A new diesel tank farm has been built between at the site to serve the new power plant.

It is envisaged that four engines will be operating at any one time, well able to meet the current maximum load of around 41 MWe, with two units in back-up mode.

The new plant will replace several existing but dated diesel engines and the end result will be significantly reduced emissions, not just of CO2 (thanks to the switch to LNG and state of the art engines) but also particulates, SO2 and NOx. NOx levels, already low within the engines thanks to optimised combustion, are further reduced with addition of SCR.

One stop shop

Looking beyond Gibraltar, the MAN ES white paper sees considerable potential for small-scale LNG, particularly on islands and in remote regions of the ASEAN countries, noting that reciprocating engine based generating capacity in the region has “an average age of 20 years and is mainly based on old diesel engines up to 7 MW.” Front runners in terms of potential small-scale LNG adoption are Indonesia and Philippines (to be the subject a future article).

With the 2016 acquisition of Cryo AB from Linde (now MAN Cryo), MAN ES finds itself in the position of being able to provide the regasification terminal as well as the power plant for future projects, creating a simplified ‘one stop shop’ for LNG-to-power that is likely to prove an attractive proposition for project developers. 

LNG storage tanks (white) at North Mole, power plant to the right

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