What’s the best way to make clean hydrogen? Steam Methane Reforming (SMR) is the most common and cheapest way of producing hydrogen, but it also produces CO2 emissions. Capturing that CO2 is complex and costly. Schalk Cloete presents research on two new designs for “blue” hydrogen (blue = derived from natural gas/coal with carbon capture/CCS). He describes in detail Gas Switching Reforming (GSR) and Membrane-Assisted Autothermal Reforming (MA-ATR). His cost analysis says they both turn out only slightly more expensive than SMR, but they capture the CO2. In a world where there is no CO2 transport and storage infrastructure and therefore no CO2 market, there’s no reward for being slightly more expensive. So Cloete posits a very interesting approach to commercialising the new designs without having to wait for that to happen. Build the GSR and MA-ATR plants without the carbon capture fittings, making their process cheaper than SMR while delivering the same results. As CO2 infrastructure builds out – and carbon taxes rise – it’s a straightforward job to retrofit the CCS.
Hydrogen is currently enjoying an unprecedented surge in attention. Several countries and regions have issued formal hydrogen roadmaps, including the EU, the US, the UK, and Australia. In addition, the International Energy Agency has developed a comprehensive report assessing all elements of the hydrogen value chain.
One of the key findings of the report is that blue hydrogen (natural gas or coal with CCS) is likely to remain cheaper than green hydrogen (electrolysis from renewables) in several word regions. As shown below, this includes regions like Australia, Europe, Japan, the Middle East and the US. In China, India and North Africa, long-term blue and green hydrogen costs are very similar.
Blue Hydrogen cheaper than Green
However, there are two important considerations that could shift the balance further towards blue hydrogen: 1) the costs related to the intermittent nature of hydrogen production from renewables is not accounted for and 2) the potential of next generation CCS technologies to considerably reduce the cost of blue hydrogen is not considered. As a rough estimate, these considerations will move the green bars in the figure above up by about $0.4/kg and the yellow and grey bars down by about $0.4/kg, making blue hydrogen the lowest-cost long-term option in all regions.
This article will review two blue hydrogen technologies that can facilitate such cost reductions.
Chemical looping technology
The next generation of blue hydrogen technologies is based on the principle of chemical looping. This technology uses an oxygen carrier material (metal oxide powder) to transport oxygen from an air stream to a fuel stream, allowing for fuel combustion with integrated CO2 capture at almost no energy penalty.
For hydrogen production, chemical looping reforming can be employed. This technology works on the same principle, but the oxygen carrier also acts as a catalyst for the steam methane reforming reaction. In this way, the heat requirement of the endothermic (heat consuming) reforming reactions can be satisfied by burning waste fuel from the pressure swing adsorption (PSA) hydrogen separation unit with integrated CO2 capture.
One of the key practical challenges with the scale-up of chemical looping technology is the need to circulate the oxygen carrier material between two different reactors where it can be separately contacted with air and fuel. This is particularly challenging at the high pressures and temperatures required for high hydrogen production efficiency from natural gas reforming.
To address this challenge, we have been working on two concepts that keep the oxygen carrier material in a single reactor, eliminating this important scale-up challenge.
Gas Switching Reforming (GSR)
The first of these technologies, GSR, keeps the oxygen carrier material in a single reactor where it is periodically exposed to three different gas streams (as illustrated in the image below).
- An air stream that oxidises the oxygen carrier and releases heat, producing a hot depleted air stream with almost no CO2.
- A low-grade fuel stream from the PSA hydrogen separation unit that reduces the oxygen carrier once again, producing a stream of CO2 and H2O from which CO2 can easily be separated.
- A stream of natural gas and steam that is reformed to syngas (mainly H2 and CO) over the oxygen carrier that also acts as a catalyst.
This dynamic operation of the reactor requires a cluster of several coordinated reactors to operate as a steady state processing unit.
When deployed for hydrogen production, the GSR concept functions just like the fired tubular reformer used in conventional steam methane reforming that is responsible for the majority of global hydrogen production today. This process layout is illustrated below.
Membrane-Assisted Autothermal Reforming (MA-ATR)
The second alternative, MA-ATR, relies on hydrogen perm-selective membranes to continuously extract the hydrogen produced during the reforming reactions. These reactions are equilibrium limited (not all the fuel can be converted), but the continued extraction of the hydrogen product overcomes this equilibrium limitation to achieve higher fuel conversion at milder temperatures. In addition, direct extraction of pure hydrogen avoids the need for downstream water-gas shift and PSA units for extracting hydrogen from the syngas (as in the figure above).
This concept was first proposed as membrane-assisted chemical looping reforming, which faces the challenge of circulating an oxygen carrier material between two reactors at high temperatures and pressures. We have therefore simplified the concept to avoid the need for oxygen carrier circulation, using a conventional air separation unit, as shown below.
In the lower reactor regions, methane (CH4) is reformed to hydrogen (H2) over the nickel (Ni) catalyst, with the hydrogen continuously being extracted. Directly above the membranes, any fuel that slipped past the membranes is converted to CO2 and H2O by reducing the oxygen carrier material (NiO). In the upper reactor regions, high purity oxygen is injected to oxidise the oxygen carrier and release heat. This heat is then carried down to the membrane region by the good mixing in the fluidised bed reactor to supply the heat needed by the endothermic reforming reactions. Hence, separate high-purity streams of H2 and CO2 are produced from a single reactor.
As can be seen, hydrogen from GSR and MA-ATR is only slightly more expensive than conventional steam methane reforming (SMR) without CO2 capture. A key feature of both concepts is that they consume considerably less natural gas than SMR, but require significant electricity consumption. This fits well with a future scenario of clean electricity and scarcer natural gas resources. CO2 transport and storage is the most important “other” cost.
The CO2 avoidance cost is about €12/ton, implying that these technologies will slot in below the lower edge of the red bars (SMR with a CO2 price of $25/ton) in the figure from the IEA report at the top of this article.
Both concepts have been successfully demonstrated in the lab (GSR and two membrane concepts: MA-GSR and MA-CLR), showing their practical feasibility. Following these demonstrations and the promising techno-economic assessment results discussed above, we are now working on proposals for scale-up projects. The key elements to be demonstrated are the longevity of the oxygen carrier material, high temperature outlet valves (for GSR) and membranes (for MA-ATR) at larger scales.
Once these demonstrations are successfully completed, commercialisation can proceed rapidly. Both reactor concepts are attractively simple and unlikely to pose complications when going to commercial scales.
In addition, a promising business case exists where the plants can first be constructed without capturing CO2. In this case, the pressurised CO2 stream can be expanded for some extra power production and vented to the atmosphere, avoiding the need for CO2 compression, transport and storage. Without CO2 capture, both the GSR and MA-ATR concepts become cheaper than conventional SMR, minimising investment risk related to uncertain CO2 markets and CO2 infrastructure buildouts.
Then, once CO2 prices rise and CO2 transport and storage infrastructure gets established, these plants can easily be retrofitted for CO2 capture by simply adding a CO2 compression train.
Through this business model, we are hopeful that the GSR and MA-ATR technologies can accelerate the advent of the hydrogen economy, helping to decarbonise sectors other than electricity (which still account for 80% of global final energy consumption) at a reasonable cost.
Schalk Cloete is a Research Scientist at Sintef