Abstract
In the past, many studies have demonstrated different Greenhouse gas (GHG) impacts from the use of LNG. In theory, using emission coefficients in combustion only, liquified natural gas (LNG) results in about 25% lower GHG emissions than diesel (MGO) or bunker oil (HFO). However, larger well to tank (WTT) emissions for the LNG supply chain as well as un-combusted methane (CH4) from the ship's engine might more than nullify any GHG gains.
A recent study performed for SEA\LNG and SGMF by the company Thinkstep reports that if the whole world fleet shifted from traditional fuels to LNG, the maritime greenhouse gas (GHG) emissions could be reduced by around 15%. This measured on a well to wake basis (WTW). A key question to ask when such large reduction figures are presented is how they got there. To understand the assumptions, hypotheses and limitations, I created a simple spread-sheet model. I find that the favourable result for LNG comes as a result of assumptions employed when Thinkstep calculate well to tank emissions (WTT); their assumption of higher thermal efficiency for LNG than for diesel in the (engine) combustion process; and their low amounts of un-combusted methane in the exhaust gas from the ship's engines.
For the Well to tank (WTT) calculations, Thinkstep have used: LNG 18.5 Gram of CO2 per MJ; MGO 14.4 Gram CO2 per MJ; HFO 13.5 Gram CO2 per MJ. The LNG and the MGO figures are in line with previously published figures both in magnitude and in relative difference. In contrast, the HFO figure is high if we consider HFO to be ‘the bottom of the barrel’ and the waste from the refinery process. Since the 2020 Sulphur cap was first introduced in 2008, we have consistently been reminded that desulfurizing residual fuel oils implies cost and complexity similar to conversion from residual to distillate. This means that conversion from HFO to diesel costs up to 10 % - 15% of the energy content in the HFO. With new modern refineries set up to convert all crude into higher priced products, HFO will hence from 2020 come only from existing refineries where it’s share of the energy consumption is next to zero. If we acknowledge the lower energy consumption in delivering HFO and deduct the refinery part from the Thinkstep figures, we get 9.6 gram of CO2 eq. per MJ for HFO, rather than 13.5.
A recent study performed for SEA\LNG and SGMF by the company Thinkstep reports that if the whole world fleet shifted from traditional fuels to LNG, the maritime greenhouse gas (GHG) emissions could be reduced by around 15%. This measured on a well to wake basis (WTW). A key question to ask when such large reduction figures are presented is how they got there. To understand the assumptions, hypotheses and limitations, I created a simple spread-sheet model. I find that the favourable result for LNG comes as a result of assumptions employed when Thinkstep calculate well to tank emissions (WTT); their assumption of higher thermal efficiency for LNG than for diesel in the (engine) combustion process; and their low amounts of un-combusted methane in the exhaust gas from the ship's engines.
For the Well to tank (WTT) calculations, Thinkstep have used: LNG 18.5 Gram of CO2 per MJ; MGO 14.4 Gram CO2 per MJ; HFO 13.5 Gram CO2 per MJ. The LNG and the MGO figures are in line with previously published figures both in magnitude and in relative difference. In contrast, the HFO figure is high if we consider HFO to be ‘the bottom of the barrel’ and the waste from the refinery process. Since the 2020 Sulphur cap was first introduced in 2008, we have consistently been reminded that desulfurizing residual fuel oils implies cost and complexity similar to conversion from residual to distillate. This means that conversion from HFO to diesel costs up to 10 % - 15% of the energy content in the HFO. With new modern refineries set up to convert all crude into higher priced products, HFO will hence from 2020 come only from existing refineries where it’s share of the energy consumption is next to zero. If we acknowledge the lower energy consumption in delivering HFO and deduct the refinery part from the Thinkstep figures, we get 9.6 gram of CO2 eq. per MJ for HFO, rather than 13.5.