A role for Coal? Low-cost, negative emissions Blue Hydrogen from “MAWGS” Coal/Biomass co-gasification

Schalk Cloete summarises his co-authored study that explains how to make hydrogen at unbeatably low prices from coal/biomass co-gasification. Though the “blue” hydrogen process creates CO2, the self-contained plant using a membrane-assisted water-gas shift (MAWGS) reactor means 100% is captured easily. Better still, the use of biomass means the plant achieves negative emissions. The overall efficiency of the process is a very impressive 69%. The modelling predicts an attractive LCOH of €1.5/kg. That could even drop to an extremely low €1.06/kg if the development of the MAWGS technology exceeds expectations, and the hot water is used for district heating. If this process is proven it opens the door to a revival of coal in Europe and continued use of this abundant energy resource around the world. Only this time, coal will actually be helping the climate, says Cloete.

As the world’s energy security woes continue, we are forced to think more pragmatically about the energy transition. Energy security is critical for maintaining a healthy economy capable of bettering billions of lives around the world while simultaneously investing heavily in clean energy.

Europe is feeling the sting of the energy crisis particularly acutely. Natural gas prices have been at unheard-of levels for a full year and there is no end in sight. Although it’s easy to lay all the blame on Putin, the fact is that the crisis started well before the invasion of Ukraine. There was simply insufficient investment in gas supply infrastructure to counteract the phase-out of coal and German nuclear, exacerbated by a relatively cold 2020/2021 winter and a wind drought at the end of 2021.

Figure 1: Recent European natural gas price developments with a red “x” marking the invasion of Ukraine / SOURCE: TradingEconomics

Now, Europe and several other countries are turning back to coal, putting energy security before climate action. This trend illustrates the energy security value of coal which is far more uniformly distributed around the world than oil and gas. Energy-poor Europe, for example, has 300 years of proven reserves remaining at current consumption rates.

Obviously, we cannot keep burning coal in the conventional manner. But there is a solution that can help coal make up for much of the climate damage it has caused.

Negative-emission Blue Hydrogen

In a recent paper, we propose a new blue hydrogen process configuration based on coal/biomass co-gasification. The concept, illustrated in Figure 2 below, converts a blend of coal and biomass into hydrogen with 100% CO₂ capture and no other air pollution. Since 30% of the fuel input is biomass, this plant therefore achieves negative CO₂ emissions.

Figure 2: Block flow diagram of the proposed negative emission hydrogen plant. ASU = air separation unit, HGCU = hot gas clean-up, MAWGS = membrane-assisted water-gas shift.

Membrane-assisted water-gas shift (MAWGS) reactor

The key enabling technology in this novel plant configuration is the membrane-assisted water-gas shift (MAWGS) reactor. This technology has been of academic interest for decades, but recent commercial interest in blue hydrogen has led to several start-ups (e.g., Hydrogen Mem-Tech) commercialising the hydrogen perm-selective membranes at the heart of this concept.

In the MAWGS reactor, the equilibrium limited water-gas shift reaction, CO + H₂O <=> CO₂ + H₂, takes place over a conventional catalyst. However, the reactor also contains hydrogen perm-selective membranes that continuously extract the produced hydrogen, shifting the reaction equilibrium to produce more hydrogen and achieve near-complete conversion of CO to CO₂.

The CO₂-rich stream exiting the MAWGS reactor contains only small amounts of CO and H₂ that can be combusted with a small amount of pure oxygen from the air separation unit to yield a high-purity, pressurised CO₂ stream for transport and storage. Crucially, there is no place in this process configuration where CO₂ (or any air pollutants like NOx or SOx) can be vented to the atmosphere.

Co-gasification benefits: Coal + Biomass

Adding biomass to the feed of the reactor enhances the attractiveness of this concept. Like coal, biomass also offers good energy security, although the technical potential of sustainably produced biomass is limited. Higher biomass output is possible, but this involves land-use-change impacts that can cause food security issues and substantial indirect emissions from deforestation elsewhere.

Co-gasification with coal allows the limited amount of sustainably produced biomass at our disposal to be utilised at the maximum possible efficiency. The reason for the efficiency boost is that up to 30% biomass can be fed with coal into a high-temperature-high-pressure gasification process that enhances efficiency and avoids problems with tar formation. Pure biomass cannot be gasified at such high temperatures because the resulting molten ash is highly corrosive.

Thus, coal maximises the value we can gain from the available biomass supply while also greatly increasing the amount of hydrogen we can produce. Furthermore, coal offers security of supply for a large plant that benefits from economies of scale given that biomass availability can vary between seasons and years.

Techno-economic results

The proposed process achieves an unprecedented hydrogen production efficiency of 73% (conventional coal-to-hydrogen processes achieve about 60%). However, it does require electricity imports amounting to 4% of the heating value of fuel input, bringing the overall efficiency to a still very attractive 69%.

Economic results are shown in Figure 3 below where different plant configurations are compared. The reference plant uses a conventional water-gas shift reactor followed by a solvent CO₂ capture process and a pressure-swing adsorption unit for pure hydrogen recovery.

Figure 3: The levelised cost of hydrogen from a reference plant (Ref.) using conventional technology and three process configurations using the MAWGS technology. The process layout shown in Figure 2 is the MaxH2 configuration. [FOM = fixed operating and maintenance costs, VOM = variable operating and maintenance costs]

€1.5/kg LCOH

As shown, the inclusion of the MAWGS reactor in the MaxH₂ configuration reduces the hydrogen production cost by 16%. Substantial savings are observed in all cost components except for variable operating and maintenance (VOM) costs which include the cost of the aforementioned electricity imports at 60 €/MWh. Capital and fixed operating and maintenance (FOM) costs are reduced due to the enhanced efficiency and the process intensification offered by the MAWGS reactor (shift, CO₂ capture, and H₂ separation concentrated into a single unit), fuel costs are reduced due to the high process efficiency, and the CO₂ credit for biogenic CO₂ capture (50 €/ton) is increased because 100% of the produced CO₂ is captured and stored.

…or cheaper!

Even though a cost of 1.5 €/kg for negative-emission hydrogen is already highly attractive, further cost reductions are possible. In Figure 4 below, two extra cases are added, one where we removed the 30% contingency we added to the cost estimate of the MAWGS reactor due to its novelty and another where we also considered the potential of hot water (120 °C) exports for district heating (valued at 30 €/MWh). The district heating option is attractive because the plant has zero emissions and can therefore be built close enough to demand centres to establish a district heating network.

Figure 4: Further potential cost reductions from the MaxH2 configuration shown in Figure 3. [DH = district heating]

Finally, if the CO₂ credit for negative emissions is raised from 50 €/ton to 100 €/ton, the hydrogen production cost falls to 1.06 €/kg – a simply unbeatable cost for low-carbon hydrogen.

Conclusion

Large-scale production of blue hydrogen from co-gasification of coal and biomass can deliver carbon-free fuel to future low-carbon economies without compromise. It offers complete energy security, unbeatable costs, and negative CO₂ emissions.

The key obstacles to this technology include:

Successful scale-up of the MAWGS technology. The hydrogen perm-selective membranes are expected to be commercially available soon.
The availability of CO₂ transport and storage networks. Solid fuels produce large streams of CO₂, and these need to be stored on a large scale to minimise costs.
Strong anti-coal sentiment. The climate change community is heavily against any coal-related technology and may oppose even a technology as attractive as this one.

If these three challenges can be overcome, we could see a highly unexpected revival of coal in Europe and continued use of this abundant energy resource around the world. Only this time, coal will actually be helping the climate.

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Schalk Cloete is a research scientist studying different pathways for decoupling economic development from emissions and environmental degradation