Carbon intensity and lifecycle analysis
To make BIL hydrogen hub application and to evalue 45V tax credits, it requires a deep understanding of hydrogen carbon intensity through lifecycle analysis. With upstream emissions in the lifecycle GHG emissions, such as natural gas recovery and processing, it is important to check the carbon intensity by production technology and by feedstock.
Would any of types of hydrogen except green hydrogen qualify for 45V credits?
Currently, the most widely used method in the mass production of hydrogen is steam methane reforming (SMR), referred to as gray hydrogen. Through SMR, natural gas is leveraged as methane to produce hydrogen and carbon dioxide as the principal products. In Figure 2, the latest version of GREET model, GREET1 2021, shows that even with steam or electricity as co-products to generate displacement credits, gray hydrogen production carbon intensity is still close to 10kg CO2e/kg H2. The variation of methane leakage from 1% to 3% would easily increase the carbon intensity of gray hydrogen production by 1.4 kg CO2e/kg H2, or around 14%.
Combined with carbon capture and sequestration (CCS), hydrogen produced from SMR is called blue hydrogen. With the maximum 90% of CO2 capture rate, the carbon intensity of blue hydrogen could be reduced to approximately 3.2kg CO2e/kg H2. However, using the default GREET 2021 inputs, the capture rate could be no lower than 82% if it intends to maintain the carbon intensity below 4kg CO2e/kg H2. Coal gasification, coupled with CCS with a carbon capture rate up to 87%, could make the carbon intensity for hydrogen production close to 4kg CO2e/kg H2, but it is quite a stretch.
If the feedstock of SMR changes from fossil natural gas to renewable sources, the carbon intensity of hydrogen production could decrease significantly because of biogenic CO2 and/or avoided methane (CH4) emissions. For example, when landfill gas is used to feed SMR, the carbon intensity of hydrogen production would be reduced to less than 1kg or even close to 0.5 kg CO2e/kg H2, as shown in Figure 2. The important role of renewable natural gas to reduce carbon emissions has been demonstrated by California’s Low Carbon Fuel Standard (LCFS) program.
Last but not least, hydrogen from biomass gasification—and as a byproduct of chlorine plants with mass-based allocation method—can achieve carbon intensities of hydrogen production close to 1.5 kg CO2e/kg H2. The scale of this production is relatively small compared with other technologies.
What can we learn from California’s LCFS program?
California Air Resources Board (CARB) has certified over 80 hydrogen pathways both in gaseous and liquid phases and around 10 pathways are under public comment so far in 2022. Most feedstocks are fossil natural gas (NG), landfill gas (LFG), and dairy manure, as shown in Figure 3. CARB’s LCFS program provides valuable information about the impacts of renewable feedstock on the carbon intensity of hydrogen production.
Since the LCFS program focuses on hydrogen as transportation fuels and includes well-to-wheel GHG emissions, it is challenging to make apples-to-apples comparison with production carbon intensity without excluding emissions from transmission and distribution and other end uses. However, it is worth noting that the lifecycle GHG emission reduction occurs during the production phase when the feedstock is switched from fossil natural gas to landfill gas, and to dairy manure, with the reduction values on average as 29-53 gCO2e/MJ H2, and 326-344 gCO2e/MJ H2, respectively, or 3-6kg CO2e/kg H2 and 39-41kg CO2e/kg H2, respectively.
Emissions from other processes
There are approximately 1,600 miles of dedicated hydrogen pipelines in the United States. If gaseous hydrogen is piped from central plant to bulk terminal and to refueling stations with a total distance of 750 miles, emissions of 0.5 kg CO2e/kg H2 would be added using GREET 2021 default inputs. The emissions would be more than 10 times higher if it is transported by diesel trucks with gas tube trailers. Hydrogen can also be liquefied and transported with tanker trucks, which have larger payload with higher transportation efficiency, yet additional emissions from liquefaction and boil-off losses need to be considered as liquefaction of hydrogen is an energy intensive process.
The full lifecycle emissions vary depending on end uses—such as transportation, industry, and power generation. We will discuss each end use separately in forthcoming papers.