Rotary engine GTL alternative to reliquefaction
Korean technology solutions provider Marine Radio Company Ltd. (MRC) is developing a small-scale rotary engine gas-to-liquid solution as a potential alternative to reliquefaction systems for LNG carriers, according to a presentation at the Kormarine conference in Busan in October 2019.
The solution produces synthetic gas from boil-off gas, after passing the BOG through a compressor and then through a rotary engine. The rotary engine stage will generate power that can be consumed elsewhere. The exhaust gases from the rotary engine are then treated in a water gas shift reaction, and passed through a heat exchanger before being processed in a Fischer-Tropsch reactor to produce synthetic hydrocarbon fuels. These fuels include e-diesel, e-methanol, e-LNG or e-propane.
MRC is also investigating the possibility of producing hydrogen by directly processing the waste gases from the rotary engine after the water gas shift reaction through an H2 membrane.
The solution was developed and patented by biofuels-focused Canadian alternative energy developer, Epiphany Energy, and has been licensed to MRC for marine applications. The system was originally designed with one eye on land-based carbon-intensive environments, such as commercial agriculture.
However, maritime applications did not MRC saw commercial applicability as a less capital-intensive alternative to reverse Brayton cycle-based reliquefaction systems for LNG carriers. The system is robust, compact and does not require external energy to operate, offering significantly lower opex costs than existing systems.
The use of a rotary (or Wankel) engine as an auto-reformer offers particular advantages in MRC’s design. A rotary engine has up to 270 degrees of eccentric shaft “rotation” in its spinning triangle layout, compared with the 180 degrees of crankshaft rotation per stroke, or 720 degrees per combustion cycle, attained in a reciprocating four-stroke piston engine.
By prolonging the Wankel compression and combustion strokes, and shortening the exhaust and intake strokes, the combustion phase can be extended to up to one-third of the combustion cycle. This extends the partial oxidation cycle, lowering the proportion of unburnt fuel, and raising the proportion of hydrogen and carbon monoxide (CO) in the exhaust gas stream.
The Wankel engine offers a number of other advantages: the speed of rotation in a rotary engine combustion chamber generates significantly more turbulence in the air-fuel mixture than in Diesel-cycle combustion engines, which also tends to encourage oxidation reactions to complete.
The turbulent mixing within the combustion chamber also permits the engine to accommodate fuels with lower calorific values, which makes it highly suitable for combusting BOG streams. The engine has a relatively wide operating window, reducing its susceptibility to knocking.
The higher level of CO and CO2 emissions produced by four-stroke Otto cycle rotary engine is an inherent advantage for the system as a reformer. MRC notes that around CO accounts for around 15% of the exhaust gases at the manifold, while H2 exceeds 26%. N2 is around 48%.
The engine has a lower compression ratio and consequently lower combustion temperatures, with a peak temperature of 850 degrees centigrade and a peak pressure of 45 bar, while the possibility of recycling tail gas captured after the Fischer-Tropsch reactor would permit dilution of the fuel-air mix, reducing combustion chamber temperatures further.
The Wankel cycle engine has a relatively compact footprint compared with conventional Otto cycle or Diesel cycle engines, and does not require cams, valves or reciprocating parts in its simple design.
As previously noted, the system also has the potential to have lower operating costs, as the rotary engine generates energy. A key objective during the development process will be to demonstrate the reliability of the GTL system over extended periods in a marine environment, although similar designs have been deployed for onshore applications.
After the Fischer-Tropsch process, the system offers the alternative of producing a synthetic diesel or other liquid hydrocarbon, and either capture or choose not to capture wax produced during the process.
The diesel-only option is likely to be more attractive to potential customers, as capturing wax from the Fischer Tropsch process penalises diesel fuel production by around 50%, and also lowers the electricity generated from the process by around 40%.
Epiphany Energy Corp estimated that a rotary engine GTL solution processing 16 tonnes/day of pure methane could generate 330kW and produce up to 8 tonnes of diesel per day. MRC notes that tests on the synthetic diesel produced indicated the fuel produced lower greenhouse gas emissions than comparable fossil-based diesel, with a particular 50% reduction in particulate matter (PM) emissions.
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