Tag Archives: reformers

My latest invention: improved fuel cell reformer

Last week, I submitted a provisional patent application for an improved fuel reformer system to allow a fuel cell to operate on ordinary, liquid fuels, e.g. alcohol, gasoline, and JP-8 (diesel). I’m attaching the complete text of the description, below, but since it is not particularly user-friendly, I’d like to add a small, explanatory preface. What I’m proposing is shown in the diagram, following. I send a hydrogen-rich stream plus ordinary fuel and steam to the fuel cell, perhaps with a pre-reformer. My expectation that the fuel cell will not completely convert this material to CO2 and water vapor, even with the pre-reformer. Following the fuel cell, I then use a water-gas shift reactor to convert product CO and H2O to H2 and CO2 to increase the hydrogen content of the stream. I then use a semi-permeable membrane to extract the waste CO2 and water. I recirculate the hydrogen and the rest of the water back to the fuel cell to generate extra power, prevent coking, and promote steam reforming. I calculate the design should be able to operate at, perhaps 0.9 Volt per cell, and should nearly double the energy per gallon of fuel compared to ordinary diesel. Though use of pure hydrogen fuel would give better mileage, this design seems better for some applications. Please find the text following.

Use of a Water-Gas shift reactor and a CO2 extraction membrane to improve fuel utilization in a solid oxide fuel cell system.

Inventor: Dr. Robert E. Buxbaum, REB Research, 12851 Capital St, Oak Park, MI 48237; Patent Pending.

Solid oxide fuel cells (SOFCs) have improved over the last 10 years to the point that they are attractive options for electric power generation in automobiles, airplanes, and auxiliary power supplies. These cells operate at high temperatures and tolerate high concentrations of CO, hydrocarbons and limited concentrations of sulfur (H2S). SOFCs can operate on reformate gas and can perform limited degrees of hydrocarbon reforming too – something that is advantageous from the stand-point of fuel logistics: it’s far easier to transport a small volume of liquid fuel that it is a large volume of H2 gas. The main problem with in-situ reforming is the danger of coking the fuel cell, a problem that gets worse when reforming is attempted with the more–desirable, heavier fuels like gasoline and JP-8. To avoid coking the fuel cell, heavier fuels are typically reforming before hand in a separate reactor, typically by partial oxidation at auto-thermal conditions, a process that typically adds nitrogen and results in the inability to use the natural heat given off by the fuel cell. Steam reforming has been suggested as an option (Chick, 2011) but there is not enough heat released by the fuel cell alone to do it with the normal fuel cycles.

Another source of inefficiency in reformate-powered SOFC systems is basic to the use of carbon-containing fuels: the carbon tends to leave the fuel cell as CO instead of CO2. CO in the exhaust is undesirable from two perspectives: CO is toxic, and quite a bit of energy is wasted when the carbon leaves in this form. Normally, carbon can not leave as CO2 though, since CO is the more stable form at the high temperatures typical of SOFC operation. This patent provides solutions to all these problems through the use of a water-gas shift reactor and a CO2-extraction membrane. Find a drawing of a version of the process following.

RE. Buxbaum invention: A suggested fuel cycle to allow improved fuel reforming with a solid oxide fuel cell

RE. Buxbaum invention: A suggested fuel cycle to allow improved fuel reforming with a solid oxide fuel cell

As depicted in Figure 1, above, the fuel enters, is mixed with steam or partially boiled water, and heated in the rectifying heat exchanger. The hot steam + fuel mix then enters a steam reformer and perhaps a sulfur removal stage. This would be typical steam reforming except for a key difference: the heat for reforming comes (at least in part) from waste heat of the SOFC. Normally speaking there would not be enough heat, but in this system we add a recycle stream of H2-rich gas to the fuel cell. This stream, produced from waste CO in a water-gas shift reactor (the WGS) shown in Figure 1. This additional H2 adds to the heat generated by the SOFC and also adds to the amount of water in the SOFC. The net effect should be to reduce coking in the fuel cell while increasing the output voltage and providing enough heat for steam reforming. At least, that is the thought.

SOFCs differ from proton conducting FCS, e.g. PEM FCs, in that the ion that moves is oxygen, not hydrogen. As a result, water produced in the fuel cell ends up in the hydrogen-rich stream and not in the oxygen stream. Having this additional water in the fuel stream of the SOFC can promote fuel reforming within the FC. This presents a difficulty in exhausting the waste water vapor in that a means must be found to separate it from un-combusted fuel. This is unlike the case with PEM FCs, where the waste water leaves with the exhaust air. Our main solution to exhausting the water is the use of a membrane and perhaps a knockout drum to extract it from un-combusted fuel gases.

Our solution to the problem of carbon leaving the SOFC as CO is to react this CO with waste H2O to convert it to CO2 and additional H2. This is done in a water gas shift reactor, the WGS above. We then extract the CO2 and remaining, unused water through a CO2- specific membrane and we recycle the H2 and unconverted CO back to the SOFC using a low temperature recycle blower. The design above was modified from one in a paper by PNNL; that paper had neither a WGS reactor nor a membrane. As a result it got much worse fuel conversion, and required a high temperature recycle blower.

Heat must be removed from the SOFC output to cool it to a temperature suitable for the WGS reactor. In the design shown, the heat is used to heat the fuel before feeding it to the SOFC – this is done in the Rectifying HX. More heat must be removed before the gas can go to the CO2 extractor membrane; this heat is used to boil water for the steam reforming reaction. Additional heat inputs and exhausts will be needed for startup and load tracking. A solution to temporary heat imbalances is to adjust the voltage at the SOFC. The lower the voltage the more heat will be available to radiate to the steam reformer. At steady state operation, a heat balance suggests we will be able to provide sufficient heat to the steam reformer if we produce electricity at between 0.9 and 1.0 Volts per cell. The WGS reactor allows us to convert virtually all the fuel to water and CO2, with hardly any CO output. This was not possible for any design in the PNNL study cited above.

The drawing above shows water recycle. This is not a necessary part of the cycle. What is necessary is some degree of cooling of the WGS output. Boiling recycle water is shown because it can be a logistic benefit in certain situations, e.g. where you can not remove the necessary CO2 without removing too much of the water in the membrane module, and in mobile military situations, where it’s a benefit to reduce the amount of material that must be carried. If water or fuel must be boiled, it is worthwhile to do so by cooling the output from the WGS reactor. Using this heat saves energy and helps protect the high-selectivity membranes. Cooling also extends the life of the recycle blower and allows the lower-temperature recycle blowers. Ideally the temperature is not lowered so much that water begins to condense. Condensed water tends to disturb gas flow through a membrane module. The gas temperatures necessary to keep water from condensing in the module is about 180°C given typical, expected operating pressures of about 10 atm. The alternative is the use of a water knockout and a pressure reducer to prevent water condensation in membranes operated at lower temperatures, about 50°C.

Extracting the water in a knockout drum separate from the CO2 extraction has the secondary advantage of making it easier to adjust the water content in the fuel-gas stream. The temperature of condensation can then be used to control the water content; alternately, a separate membrane can extract water ahead of the CO2, with water content controlled by adjusting the pressure of the liquid water in the exit stream.

Some description of the membrane is worthwhile at this point since a key aspect of this patent – perhaps the key aspect — is the use of a CO2-extraction membrane. It is this addition to the fuel cycle that allows us to use the WGS reactor effectively to reduce coking and increase efficiency. The first reasonably effective CO2 extraction membranes appeared only about 5 years ago. These are made of silicone polymers like dimethylsiloxane, e.g. the Polaris membrane from MTR Inc. We can hope that better membranes will be developed in the following years, but the Polaris membrane is a reasonably acceptable option and available today, its only major shortcoming being its low operating temperature, about 50°C. Current Polaris membranes show H2-CO2 selectivity about 30 and a CO2 permeance about 1000 Barrers; these permeances suggest that high operating pressures would be desirable, and the preferred operation pressure could be 300 psi (20 atm) or higher. To operate the membrane with a humid gas stream at high pressure and 50°C will require the removal of most of the water upstream of the membrane module. For this, I’ve included a water knockout, or steam trap, shown in Figure 1. I also include a pressure reduction valve before the membrane (shown as an X in Figure 1). The pressure reduction helps prevent water condensation in the membrane modules. Better membranes may be able to operate at higher temperatures where this type of water knockout is not needed.

It seems likely that, no matter what improvements in membrane technology, the membrane will have to operate at pressures above about 6 atm, and likely above about 10 atm (upstream pressure) exhausting CO2 and water vapor to atmosphere. These high pressures are needed because the CO2 partial pressure in the fuel gas leaving the membrane module will have to be significantly higher than the CO2 exhaust pressure. Assuming a CO2 exhaust pressure of 0.7 atm or above and a desired 15% CO2 mol fraction in the fuel gas recycle, we can expect to need a minimum operating pressure of 4.7 atm at the membrane. Higher pressures, like 10 or 20 atm could be even more attractive.

In order to reform a carbon-based fuel, I expect the fuel cell to have to operate at 800°C or higher (Chick, 2011). Most fuels require high temperatures like this for reforming –methanol being a notable exception requiring only modest temperatures. If methanol is the fuel we will still want a rectifying heat exchanger, but it will be possible to put it after the Water-Gas Shift reactor, and it may be desirable for the reformer of this fuel to follow the fuel cell. When reforming sulfur-containing fuels, it is likely that a sulfur removal reactor will be needed. Several designs are available for this; I provide references to two below.

The overall system design I suggest should produce significantly more power per gm of carbon-based feed than the PNNL system (Chick, 2011). The combination of a rectifying heat exchange, a water gas reactor and CO2 extraction membrane recovers chemical energy that would otherwise be lost with the CO and H2 bleed steam. Further, the cooling stage allows the use of a lower temperature recycle pump with a fairly low compression ratio, likely 2 or less. The net result is to lower the pump cost and power drain. The fuel stream, shown in orange, is reheated without the use of a combustion pre-heater, another big advantage. While PNNL (Chick, 2011) has suggested an alternative route to recover most of the chemical energy through the use of a turbine power generator following the fuel cell, this design should have several advantages including greater reliability, and less noise.

Claims:

1.   A power-producing, fuel cell system including a solid oxide fuel cell (SOFC) where a fuel-containing output stream from the fuel cell goes to a regenerative heat exchanger followed by a water gas shift reactor followed by a membrane means to extract waste gases including carbon dioxide (CO2) formed in said reactor. Said reactor operating a temperatures between 200 and 450°C and the extracted carbon dioxide leaving at near ambient pressure; the non-extracted gases being recycled to the fuel cell.

Main References:

The most relevant reference here is “Solid Oxide Fuel Cell and Power System Development at PNNL” by Larry Chick, Pacific Northwest National Laboratory March 29, 2011: http://www.energy.gov/sites/prod/files/2014/03/f10/apu2011_9_chick.pdf. Also see US patent  8394544. it’s from the same authors and somewhat similar, though not as good and only for methane, a high-hydrogen fuel.

Robert E. Buxbaum, REB Research, May 11, 2015.

Hydrogen Cylinders versus Hydrogen Generators for Gas Chromatography

Hydrogen is an excellent cover gas for furnace brazing and electronic manufacture; it’s used as a carrier gas for gas chromatography or as a flame-detector gas, and it’s a generally interesting gas for chemical formation and alternate energy. If you are working in one of these fields you’ve got two maing options for sources of hydrogen: hydrogen cylinders and hydrogen generators with the maid difference being cost. Cylinder hydrogen is the more-commonly used for small demand applications, often aided by palladium membrane hydrogen purifiers if purity is an issue. Hydrogen generators are more generally used for larger -demand applications because they provide added safety, conveinience, and long-term savings. Having nothing better to do this evening, I thought I’d go through the benefits and drawbacks of each as applies to gas chromatography.

Point of use Cylinder Hydrogen Is Simple and Allows Easy Monitoring and Control. The smallest laboratories, those with one or two gas chromatographs, generally use a single hydrogen cylinder for each GC. This is called “point of use.” Each cylinder is typically belted to a wall and often fed into some type of hydrogen purifier (a getter or membrane). From there it supplies carrier and/or fuel gas to its application. When a cylinder is empty, the application is stopped, and the purifier is often stopped too (not necessary with membranes). A new cylinder switched in and, after a short break in period, the process is restarted. The biggest advantage here is simplicity; another advantage is the ease of pressure control and monitoring. Pressure is controlled by a regulator located right at the gas chromatograph. You can always check it and adjust it as needed. A main disadvantage is that the process has to stop whenever a cylinder needs switching.

Multi-cylinder Systems Provide Fewer interruptions in Gas Supply. Larger laboratories with multiple GCs tend to use multiple hydrogen cylinders with complex switchover systems, or hydrogen generators. When multiple cylinders are used, they are typically racked together and connected to a manifold and a purifier. Tanks are emptied in series so that there is no disruption. When each take empties, the hydrogen tank is switched automatically or manually to maintain the flow and pressure. One problem with this is that the pressure does not typically stay constant as the cylinders switch since each has its own regulator and all will be set slightly differently. As the hydrogen cylinders have separate regulators, there can be pressure changes during cylinder switches; and, as the packs are located further from the GC there is a tendency for the pressure to vary as the flow varies.

Another issue with cylinder packs is that purity can suffer as there is more room for leaks and degassing in the line. This can be solved by point-of-use purifiers installed in the hydrogen lines just prior to the GC or other application.

A final issue with cylinder packs is safety: with so many cylinders, there is a lot of potential for really disastrous leaks and fires: one leak can empty many cylinders and there is no likely room that is big enough to disperse that hydrogen quickly enough. The potential is made greater since the cylinder packs are often located at a distance from where the experiments (and people) are. Maintenence becomes an issue too since the manifolds and automatic switches become complicated quickly. The hydrogen is under great pressure, and even if fires are avoided, a pressure release can be deadly. Manifolds are complex enough that they generally require a trained technician to trouble-shoot any problems; it can also take an expert to handle multiple cylinder changes to minimize contamination and pressure variation.

A main advantage of hydrogen generators is that it avoids cylinder changes; it’s also somewhat safer and saves money for larger users. Changing cylinders can be difficult and time consuming as mentioned above; hydrogen bottles must be monitored to check that gas does not run out, and you’ve got to make sure that cylinders don’t fall (especially on you), and that leaks don’t arise, and that explosive hydrogen does not escape. Much of this is alleviated with a hydrogen generator. One can have a very large tank of water or methanol — far larger than any reasonably safe gas tank, so running out is less of a problem. In some systems, the water can come from municipal pipes so there is almost no chance of running out.

Safety is provided by limiting the output of the generator to the amount the room will vent. Thus, a room with 100 ft3 of air circulation can host a hydrogen generator of up to 4.5 scfh output (about 2 slpm) with no fear of reaching explosive limits. Further, unlike cylinders, most hydrogen generators can be fitted with alarm features to alert the user to operating problems, and most have automatic shut down capabilities that trigger if the unit malfunctions. All of these factors contribute greatly to the overall safety of in the lab.

Another advantage is that methanol and water are a lot cheaper than hydrogen and there is no switchover system, cylinder rental, and less manpower need (cylinder rental cost is often greater than the cost of gas). The first cost of the generator is typically on the order of $10,000, similar to the cost of a manifold switchover system and a hydrogen purifier.

The Source Options for High purity hydrogen generators are electrolysis and methanol reformer generators. These are virtually the only continuous use hydrogen generators. They are both available in outputs from 150 ccm to 50 slpm, i.e. enough to supply single or multiple GC’s (also used for modest-sized braze furnaces, IC tool production, and laboratory-scale fuel cell testing). All hydrogen generators provide continuous hydrogen outputs as feed water or methanol is provided upstream of the hydrogen output, and they all offer safety advantages. They all take less space than the cylinders and avoid the leaks and impurity spikes that arise when cylinders are switched.

In Electrolytic Hydrogen generators Purified water, either purchased separately, or purified on-site is mixed with an electrolyte, generally KOH, and converted to hydrogen and oxygen by the electrolytic reaction H2O –> H2 + ½ O2.  As the hydrogen produced is generally “wet”, containing water vapor, the hydrogen is then purified by use of a desiccant, or by passage through a metal membrane purifier. Desiccants are cheaper, but the gas is at best 99.9% pure, good enough to feed FIDs, but not good enough to be used as a carrier gas, or for chemical production. Over time desiccants wear out; they require constant monitoring and changing as they become filled with water vapor. Often electrolytic hydrogen generators also require the addition of a caustic electrolyte solution as caustic can leak out, or leave by corrosion mechanisms.

In Reformer-based hydrogen generators a methanol-water mix is pumped to about 300 psi and heated to about 350 °C. It is then sent over a catalyst where it is converted to a hydrogen-containing gas-mix by the reaction CH3OH + H2O –> 3H2 + CO2. Pure hydrogen is extracted from the gas mix by passing it through a membrane, either within the reactor (a membrane reactor), or by use of a membrane purifier external to the reactor.

Both systems provide continuous gas supply of high purity gas. The need to change and store cylinders is eliminated, saving time and cost. One adds water or methanol-water as needed, and hydrogen is produced as long as there is electricity in the lab. Eliminating cylinder changeouts reduces downtime and minimizes the potential for air contamination.

Consistent gas purity is enhanced further because hydrogen generators often contain metal membranes. Hydrogen is delivered at  99.9999% purity, and remains constant over time. This consistent purity provides reliability for the GC system. Electrolysis systems with only a desiccant to remove water vapor from the hydrogen should be used only where high hydrogen purity less important than high hydrogen pressure. Even with a fresh cartridge, desiccant-purified gas never exceeds 99.9% and this purity decreases with time as the desiccant wears out; if purity is an issue add a membrane purifier, or use a methanol reformer.

Single cylinders are quite compact; where many cylinders would be needed space saving favors use of a generator. The relatively small size of hydrogen generators allows them to be conveniently located on the lab bench; they consume a lot of valuable lab and storage space than multiple cylinders. Related to space savings is zoning. Once you have many cylinders, you begin to run into zoning issues regarding how close your laboratory can be to bus stops, churches, and children. Zoning can limit distances to 500 feet, or 1/10 mile.

Short term cost savings favor cylinders; long term and large outputs favor generators. Hydrogen in cylinders is fairly expensive, the more so when cylinder rental is included. In Detroit, where we are, hydrogen costs about $70 each cylinder low low-purity gas, or $200 for high purity gas. Each cylinder contains 135 scf of gas. If you use 1/10 cylinder per day, you will find you’re spending about $7,300 per year on hydrogen gas, with another $1000 spent on cylinder rental and delivery. This is about the cost of a comparable hydrogen generator plus the water or methanol and electricity run it. If you use significantly less hydrogen you save money with cylinders, if you use more there is significant savings with a generator.

Most hydrogen generators have delivery pressure limitations compared to cylinders. Cylinders have no problem supplying hydrogen at 200 psi or greater pressures. By contrast, generators are limited to only the 60-150 psig range only. This pressure limitation is not likely to be a problem, even for GCs that need higher pressure gas or when the generator must be located far from the  instruments, but you have to be aware of the issue when buying the generator. Electrolysis systems that use caustic provide the highest pressures, but they tend to be the most expensive, and least safe as the operate hot and caustic can drip out. Fuel cell generators and reformers provide lower pressure gas (90 psi maximum, typically), but they are safer. In general generators should be located close to the instruments to minimize supply line pressure drop. If necessary it can pay to use cylinders and generators or several generators to provide a range of delivery pressures and a shorter distance between the hydrogen generator and the application.

Click here for the prices of REB Research hydrogen generators. By comparison, I’ve attached prices for electrolysis-based hydrogen generators here (it’s 2007 data; please check the company yourself for current prices). Finally, the price of membrane purifiers is listed here.

Maintenance required for optimal performance. Often electrolytic hydrogen generators require the addition of a caustic electrolyte solution; desiccant purified gas will require the monitoring and changing of desiccant cartridges to remove residual moisture from the hydrogen. Palladium membrane purifiers systems, and reformer systems need replacement thermocouples and heaters every few years. Understanding the required operating and maintenance procedures is an important part of making an informed decision.

Conclusion:

Cylinder hydrogen supplies are the simplest sources for labs but present a safety, cost, and handling concerns, particularly associated with cylinder change-outs. Generators tend to be more up-front expensive than cylinders but offer safety benefits as well as benefits of continuous supply and consistent purity. They are particularly attractive alternative for larger labs where large hydrogen supply can present larger safety risks, and larger operating costs.