An academic paper has found that blue hydrogen could emit more than gas, although other experts have argued it is based on a worst-case scenario.
The paper finds blue hydrogen CO2 emissions are only 9-12pc less than for grey hydrogen. And when including methane, the greenhouse gas footprint is 20pc higher than burning natural gas or coal for heat and 60pc higher than burning diesel oil, it said.
The paper assumes a 3.5pc emission rate of methane from natural gas, and uses the 20-year global warming potential. It also assumes blue hydrogen is made using steam methane reforming (SMR) with a carbon capture rate of 85pc for the SMR process, and just 65pc of CO2 in the flue gas.
Only two facilities that produce blue hydrogen are in commercial operation, meaning data is limited. Data from Shell's Alberta facility show a mean efficiency of 79pc from the SMR process, with daily rates varying from 53pc to 90pc, the paper said. It did not provide data from the Air Products facility in Texas.
The existing blue hydrogen facilities only capture CO2 from the SMR process itself, the paper said. But natural gas must be combusted to create high heat and pressure to drive the SMR process, and they are not capturing CO2 from the resulting flue gas.
The CO2 is more dilute in this stage, and the paper assumes just 65pc of this flue gas can be captured.
As a result, it finds emissions of 5.8g CO2 per MJ of hydrogen from the SMR process, and an additional 11.1g CO2/MJ from the flue gas. It assumes methane emissions of 42.1g CO2e/MJ and 35.3g CO2e/MJ for both processes.
It adds another 8.2g CO2/MJ for the power needed for the SMR process, and 8.1g CO2/MJ for the power needed to capture flue gas.
The paper represents a case where blue hydrogen "is done really badly and without any sensible regulations", David Joffe, head of carbon budgets at the UK's climate change committee (CCC) said. The CCC advises the UK government on its climate targets.
Well-regulated and monitored gas production "can achieve much lower methane leakage than 3.5pc", Joffe said. And authothermal (ATR) reforming is expected to achieve much higher capture rates than SMR. The CCC assumes 95pc with ATR, while tech providers claim 98pc, Joffe said.
The use of a 20-year timescale overstates the impact of methane and understates the impact of carbon dioxide on global warming, Joffe said. Methane is a much more powerful greenhouse gas than CO2, but has a much shorter lifetime in the atmosphere.
The CCC estimates that blue hydrogen could save emissions of up to 85pc compared with normal natural gas, using a 100-year global warming potential. But this is not guaranteed and the CCC has recommended to the government that savings of this order be demonstrated as a priority, Joffe said.
The paper was published in academic journal Energy Science and Engineering, and was funded by the Park Foundation and Cornell University.
