klooiblog 2

Door mux op donderdag 01 februari 2007 19:57 - Reacties (1)
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Why fuel cell cars don't work - part 2

This is the second part of my blog series about hydrogen fuel cell cars, and why I think they don't work. If you haven't read the last part, I highly recommend it as it lays the foundations for understanding what exactly constitutes a fuel cell vehicle (FCV) and how it is different from battery electric vehicles (BEVs). Today I will talk about production, storage and transportation of hydrogen.

This is an extremely long, in-depth blog series, so I'll start by giving you a summary. This summary will exist at the top of every part of this series. If you're interested in the technical details, please do read on and make sure to come back for the next parts.

First of all, HFC cars are perceived to be a good bridge between fossil fuels and full electric because:
  • You can still fill up like you do with a gasoline or diesel powered car
  • The mileage you can get out of hydrogen is perceived to be more adequate than what you get from batteries
  • Hydrogen fuel cells are thought not to wear out as quickly as batteries (or conversely, batteries are thought to wear out very quickly)
  • Hydrogen as a fuel is perceived to be a relatively small infrastructural change from gasoline and diesel
  • Hydrogen is perceived as a cleaner solution than gasoline, diesel or natural gas
In reality,
  • You cannot fill up like you do with gasoline or diesel. It is actually pretty ridiculous how hard it is to fill up a HFC powered car
  • You won't even go 100 miles on current tech hydrogen tanks that are still safe to carry around in a car
  • Fuel cells wear out crazy fast and are hard to regenerate
  • Hydrogen as a fuel is incredibly hard to make and distribute with acceptably low losses
Additionally,
  • Hydrogen fuel cells have bad theoretical and practical efficiency
  • Hydrogen storage is inefficient, energetically, volumetrically and with respect to weight
  • HFCs require a shit ton of supporting systems, making them much more complicated and prone to failure than combustion or electric engines
  • There is no infrastructure for distributing or even making hydrogen in large quantities. There won't be for at least 20 or 30 years, even if we start building it like crazy today.
  • Hydrogen is actually pretty hard to make. It has a horrible well-to-wheel efficiency as a result.
  • Easy ways to get large quantities of hydrogen are not 'cleaner' than gasoline.
  • Efficient HFCs have very slow response times, meaning you again need additional systems to store energy for accelerating
  • Even though a HFC-powered car is essentially an electric car, you get none of the benefits like filling it up with your own power source, using it as a smart grid buffer, regenerating energy during braking, etc.
  • Battery electric cars will always be better in every way given the speed of technological developments past, present and future
http://tweakers.net/ext/f/tzkHPO7no9Z9UQ2oXsVdRAPX/full.jpg
All of this is written from the perspective of somebody who helped make these hydrogen-powered racing karts happen. You have to admit, that looks super awesome!

The math behind gas compression

One of the biggest problems with hydrogen is that it is certainly a great fuel judging from energy-to-weight ratio, but it is also a very non-dense gas. The energy density of gasoline is about 32MJ/L, whereas uncompressed hydrogen only stores 0.013MJ/L. In other words, about a 2500-fold discrepancy in volumetric energy density. This can only mean one thing: we need to compress this down to fit it inside a car. Or any vehicle.


Compressing hydrogen is lossy and hard
I very briefly touched upon hydrogen compression being lossy and causing all kinds of temperature effects, but why? Well, compressing and decompressing a gas - any gas - doesn't come for free. You need to put in a considerable amount of energy.

Intuitively, you need to put in effort (work) to compress a gas. That should be obvious; it's not something that happens automatically. This energy you put in effectively gets converted into heat. We have formulas for this process, which is called adiabatic compression:

P(1- γ) x Tγ = constant

with γ = 1.4054 for hydrogen (Adiabatic index of hydrogen)

where P is pressure (in Pascal) and T is absolute temperature (in Kelvin). So if we take hydrogen at 1 bar (100kPa) and room temperature (298K), this constant value that pops out of the formula is 27.54. Now we increase the pressure to 200 bars, what is the resulting temperature?

Tγ = constant / (P(1-γ)) = 25105

This yields T = 1351K

That's scorchingly hot. Actually, it is so hot that you can't just say that we have compressed 200 liters of hydrogen into the space of 1 liter; gas at higher temperatures in a confined space will have a higher pressure. So to find how much gas is actually in there, we need to use another formula in the adiabatic process:

PVγ = constant

at 25 degrees let's say we have 200 liters or 0.2 cubic meters. The constant then becomes:

100 x 103 x 0.21.4054 = 10415

At the resulting pressure of 200 bar:

10415 / (20 x 106) = V1.4054, so V = 4.61 liters

That's much larger than 1/200th of the original volume. 4.61 times as much, in fact. The reason for this is, well, hot gas actually takes up more space than cold gas. In order to 'fit' this amount of hydrogen into a 1-liter tank at 200 bar, we need to actively cool the gas as it is pressurized. Otherwise it will go in the tank 'hot', cool off and actually be much less than 200 liters unpressurized.

You can breathe now; you already survived 33% of the math in this blog post. Here is a really pretty 4K panoramic picture of the racing circuit in Aragon, Zaragoza where we raced in the summer of 2009 (click for 4K version).

http://tweakers.net/ext/f/nGflNfpnXXZmPuZWgtfrCVZB/full.jpg

Now to the energy issue: We know that 200 liters of hydrogen are about 16.53 grams and the specific heat (i.e. the amount of energy you need to put in it to get it to heat up by 1 K) is 14310 J/kgK, so this equates to 250kJ of heat energy we put into the system. Now comes the interesting part: that 200 liters of hydrogen initially contained 1.6MJ of energy. Just compressing it to 200 bars cost us 250kJ, which is 15.7% of the energy content of the hydrogen. Let this just sink in: just the act of compressing it requires a significant proportion of the energy actually contained in the hydrogen!

To make matters worse, when we decompress it inside a car to use it as an energy source, the exact same amount of energy needs to be added to the gas, otherwise it will enter the fuel cell way too cold and cause damage. This principle is how the superconducting magnets in nuclear magnetic resonance imaging (MRIs) are cooled. In a vehicle, we can use this cooling effect to our benefit though; because fuel cells are not that efficient, the ingoing cold hydrogen can be used to cool the stack or the waste heat from the fuel cell can be used to heat up the expanding gas. But this nevertheless poses a big problem with the total (well-to-wheel) efficiency of hydrogen. If you need to compress hydrogen more than once, for instance when you need to transport it with a tanker vehicle, then store it at a fill-up station, you are going to need a significant amount of additional energy.

We're just talking about compressing up to 200 bars, which is the limit for metal containers at the moment. If you would use composite (carbon fiber, etc.) or special lined steel containers, pressures of up to 700 bars are used. Again, realize that we are trying to put a 2500 times less dense fuel into our tank, we need to compress it as much as possible. This pressure is for instance used at the California quick-fill stations for the Honda FCX Clarity. At 700 bars, if we do the math, this compression energy loss amounts to 389kJ/L or 24.3%.

Just this compression energy loss isn't all. Three paragraphs ago i said that as you pressurize the gas it becomes hot and takes up more space. In order to fit inside a tank, you need to actively cool down the gas as it's pumped into the tank. Or, conversely, if you're transferring hydrogen from a high-pressure tank in a quick fill station to the lower-pressure tank inside a car, you need to continuously heat the gas, otherwise you risk overpressurizing as the gas heats up inside the tank to ambient temperatures. The compression and decompression cycles required for distribution make up a significant amount of the inefficiency in transporting hydrogen from place to place.

http://images.gizmag.com/hero/4334_280705110432.jpg

Also, with the pressures and precision involved, there is no way you can actually 'fill up' by yourself without a lot of technical assistance. A lot of engineering has gone into robotic hydrogen filling stations that make sure you don't risk blowback, leakages or freezing and/or burning your hands during the filling process. It's really a marvel of technology that (recently, since about 2008) Shell has been able to build these great human-operated filling stations using high tech filling nozzles. Talking about pressure...


Storing hydrogen is probably even harder
The central issue to this subchapter is the simple fact that hydrogen molecules are extremely tiny. So small, that they can fit through the holes between atoms and molecules of any solid material. Ever heard of gas-tight fittings? Not if you're talking about hydrogen. Every pipe, every tank, every container you make for storing hydrogen will slowly but surely leak hydrogen. Even the best materials will leak it pretty fast actually, and this of course has to do with the ginormous pressure differential between the outside world and the inside of the tank, as well as the need for hydrogen tanks to be somewhat portable, so wall thickness is bound to practical limits.

Leakage rates can be calculated with Fick's law of diffusion:

J = D x p / t

Where J [mol/m^2s] is the mass flux of hydrogen, D [m2/s] is the diffusivity coefficient, p [mol/m3] is the molar density difference over the wall and t [m] is the wall thickness. Hydrogen has a molar weight of about 2g/mol and volumetric density of 89 g/m3, which makes for about 44.5 mol/m3. For best case steel, the diffusivity coefficient is about 10-7 at room temperature. This means that for a steel vessel with 1 square meter surface area, 1cm thick walls and 200 bar pressure difference, the diffusion rate is:

J = (1 x 10-7 x (200 x 44.5 / 0.01) = 0.089 [mol/s].

This is about 7500 mol/day or 15 kg/day - in other words, such a vessel would be empty long before the day ends. Again, let this sink in for a minute: a 1-cm thick, 1 square meter area steel tank that weighs as much as a human (77.5kg) will leak hydrogen so fast that it's unusable as a fuel tank.

The diffusivity becomes a lot higher at higher temperatures too, climbing about 2 orders of magnitude to D=10^-5 at 1000K. Why is that relevant? Let's think about distribution for a second. Turbulence and friction causes the gases in large natural gas pipelines to heat up considerably. Also, to save on additional cooling costs and power, currently gas pipelines pipe out hot gas and let the pipes themselves do the cooling. This is not feasible for hydrogen, as it would diffuse out so quickly. Really, any plain steel vessel is not usable for hydrogen storage or transmission. Worse still, all eligible metals have similar, often worse (aluminum) diffusion coefficients. Metals are not a great choice for hydrogen containers.

http://upload.wikimedia.org/wikipedia/commons/8/8f/Trans-Alaska_Pipeline_System_Luca_Galuzzi_2005.jpg

So why do I dwell on steel so much, can't we use something like polymer linings (~1 order of magnitude better)? The thing is, steel really is the only affordable option for long-haul pipelines. We already have really good oil and gas infrastructure in place, trillions of dollars worth. It's the wet dream of futurists to reuse this for the hydrogen economy. This is why some universities are doing studies into the exact loss rates of steel pipelines, as well as ways to increase the gas flow so that the actual losses become insignificant as compared to the amount of gas that is transported (which of course leads to the heat problem again). Because even if such a pipeline leaks a couple kg/s of gas; if it transports thousands of kilograms per second, that is an acceptable loss. Natural gas pipelines also lose an appreciable amount (about 0.5%) during transportation in these pipelines.

But, this is not actually practically feasible at the moment because one of the biggest hurdles towards hydrogen pipe infrastructure is that...


Hydrogen embrittlement makes logistics even harder!
A big problem with pipelines is a phenomenon called Hydrogen Embrittlement; as hydrogen diffuses through the container material it occasionally forms bonds with the iron atoms, transforming the iron into a different molecular structure and in the process creating additional internal stresses and even different crystal structures with additional boundaries or dislocations. Over a long period of time this lowers the maximum elastic stress and thus toughness of the steel. Contrary to LNG or oil pipelines, hydrogen pipelines need to either be significantly overengineered or replaced every 5-10 years because of this reduction in strength.

Actually, hydrogen embrittlement is not just a problem for steel, although it is by far most pronounced in steel. Aluminum and other metals as well as some polymers are adversely affected by hydrogen seeping into the microstructure. As it turns out, hydrogen fairly easily falls apart into single hydrogen atoms. Atomic hydrogen, much like any atomized material, is very reactive and will force itself into the microstructure of any material over time.


So if steel isn't usable, how do we store hydrogen?
http://tweakers.net/ext/f/Aqx4cuvL7PEIbUKRILk8a1eb/full.jpg
Hydrogen storage canisters on a hydrogen-powered folding bike

But that still leaves us with the question of how to store hydrogen in a car. Steel is not really a good option. Aluminium cartridges is how we did it at Formula Zero, but these also leaked at a rate of about 50% per week and only held about 100g of hydrogen. As it turns out, this is a fundamental problem. No materials exist with less than about 10^-9 diffusion rates, which would still mean your vessel is essentially empty after a month - or a year if you store it at lower pressures. This is even worse with more modern high-performance composite tanks that are much lighter than steel or aluminum for the same amount of storage, but have even worse leakage properties. So we have to look elsewhere.

Two options are in use right now. The first is to liquify hydrogen. Hydrogen boils at 20 K - very near absolute zero - so if you cool it down to below that temperature you can carry it around practically unpressurized in liquid form. This is how most long-term storage containers worked for many decades, like NASA's Space Shuttle. The downside to this is that you need really good thermal insulation as well as some way to keep refrigerating the hydrogen tank. This is, as you might imagine, really hard. Not necessarily because insulation and refrigeration is particularly difficult, but because you can't make the tank a closed, isolated chamber like a thermos. It needs an entry/exit point that can resist high pressure and very large temperature differentials, and that is almost only possible with ceramics and metals - which conduct heat way too well. So there has to be a constant power source attached to your car, or a large battery, that can make sure that the hydrogen doesn't boil off when you are not using your car. Worse still: if something goes wrong and your unattended car loses power, the hydrogen starts boiling, pressurizing the tank and causing an explosion. So for maximum security you need either a safety valve or a tank that can still withstand these super-high pressures. This is one of those fundamental problems that make hydrogen storage inherently unsafe, whereas gasoline and battery storage can be inherently safe.

The other method is a combination of pressure, adsorption and high-tech interior pressure vessel liners. Hydrogen in gaseous form has its molecules very far from each other; pushing them together to increase the density creates a high pressure. However, some materials like activated carbon and metal hydrides can form weak but relatively stable bonds with hydrogen, causing the hydrogen molecules to stick to the surface of the liner in a very compact fashion. You still need some pressure to keep the hydrogen adsorbed - and when you release the pressure (or heat the liner), the hydrogen removes itself from the surface again - but for the same density we are talking an order of magnitude lower pressures. The downside to this method is of course the fact that any adsorbant is pretty heavy, because hydrogen is such a small atom compared to all the others and activated carbon can only adsorb at most one hydrogen molecule per carbon atom. This makes current vehicle-class adsorption hydrogen tanks about the same weight as pressure tanks for the same amount of fuel. A couple hundred kilograms of tank to store 5kg of hydrogen.

http://world.honda.com/FuelCell/FCX/tank/images/09.gif
The pressure tank of a modern Honda FCX Clarity (2011+ models)

This is one of the reasons why the newest generation of Honda FCX Clarity still uses pressurized hydrogen gas tanks; at a pressure of about 350 bars.

Making hydrogen? Downright impossible

Now comes by far the most controversial part, and the one that I think gets most overlooked because it is not actually happening on a large scale at all yet. Making hydrogen is a very environmentally damaging process and although 100% clean methods exist, they will take too long to set up and are currently too expensive to make hydrogen cars affordable in the foreseeable future. Compare making hydrogen to making ethanol fuels; even though ethanol as a fuel is very 'clean' compared to other vehicle fuels, you need an unsustainable amount of cornfields to produce bioethanol. Or you need to extract it from oil, which defeats the purpose (and is less efficient and more polluting than just extracting gasoline and diesel from oil and using that).

Roughly the same problem exists with hydrogen production. There are very roughly speaking four major methods of hydrogen production:

Hydrogen as a by-product
At the moment, if you buy a hydrogen canister from Linde or DOW or some other chemical company, you get hydrogen that is most likely not actually produced with the intention of making hydrogen. Hydrogen is a common by-product from a bunch of chemical reactions used to make everyday chemicals; ammonia, nitrogen oxide, carbon dioxide for carbonation of drinks, alkali metal production, etc. etc. . Some continuous process chemical plants that make these substances are essentially already setup to produce hydrogen; at some point in the process there is an easy way to tap off some very pure hydrogen. For a while, this was actually very cheap to come by as supply was high (especially with the relatively large hydrogen flows in ammonia plants - a big component of fertilizer) and demand was low. The downside is of course that this isn't sustainable as a production method for a true hydrogen economy. Besides, the energy efficiency is very low if you really only look at the amount of hydrogen produced. Lastly, especially in the case of ammonia production, the source of the hydrogen is actually natural gas which means this process is always worse (energetically) than standard steam reforming (I'll get into that a couple subchapters down).

Hydrogen from electrolysis

This is an awesome video. Not relevant to the text, but worth viewing anyway

The big promise of hydrogen is electrolysis from water. It is very easy to do; just put two metal plates in water, add a little bit of salt to make the water conduct reasonably well, put a current through the plates and hey presto, you're producing oxygen and hydrogen from water. This is the exact reverse reaction from a hydrogen fuel cell. It's even so exact, that a perfect electolysis setup is exactly as efficient as a perfect (cold cathode) fuel cell - roughly 80%. In practice, of course, things get a lot harder.

Plain electrolysis as you might have done in high school is out of the question of course. Most plate materials oxidize within minutes, especially on the oxygen side. Efficiency is also really, really bad. This has a lot to do with the fact that it's very hard to move those ions around in water at a reasonable speed. Most of the energy you put into the water gets lost as ions formed at one electrode bounce their way through the water towards the other side (high effective internal resistance). This also limits the volumetric/gravimetric efficiency of such a setup.

Another big problem with electrolysis from water is, like in fuel cells, the effect of electrode poisoning. Any contaminants in the water will most certainly react with either the hydrogen or oxygen - or both - to eventually form a thin and slimy material on the electrodes, which further reduces the speed of electrolysis.

Don't worry too much though; these problems aren't something that really stops anyone from using electrolysis. People have found ways around this; by using various better electrode materials and catalysts (platinum group materials work best), higher temperatures, higher pressures and additives that make sure that any contaminants present go away from the electrodes to some other collection point. Electrolysis is a fairly well-understood process. We have been doing it for ages now.

There have even been developments to reduce or eliminate the required subsequent pressurization of the hydrogen, with fancy high-pressure electrolysis systems or by carrying the reaction components away in special high-pressure wicks. As we've calculated before, this can improve the total cycle efficiency by up to 25% at 700 bar. Unfortunately, as far as I know there has been close to zero development on this technology since I started being technically interested in fuel cells and the highest pressure HP electrolyzers still only operate at about 20-80 bar, negating most efficiency gains because of some inherent downsides to high pressure electrolysis (like the hydrogen and oxygen permeating through the proton exchange membrane, causing them to mix instead of stay neatly separated). Practical electrolyzers at the moment get about 25% efficiency for fuel cell grade hydrogen.

http://my.voyager.net/~jrrandall/BublGen.gif
Images like this simply don't work. Nope.

Still, it's very important to grasp that hydrogen from electrolysis isn't as clear-cut a solution for hydrogen production as it's sometimes portrayed. It is actually pretty expensive, not necessarily efficient and it is fairly preposterous to for instance claim (like I've seen many times) that you can 'essentially just put the current from an offshore wind turbine into the sea water and catch the bubbles'. That sounds awesome and if it were really that simple it would be a good production method (invisible power generation as viewed from land, can easily meet the demand, etc.). But you actually have to do extensive pre-treatment of the water before it can be electrolyzed, and the electrolysis doesn't 'just work' on any input, you need a big power conversion platform and hydrogen electrolysis plant as well - adding quite significant cost. This makes electrolysis typically by far the most expensive and resource-intensive method, though it's the cleanest and most sustainable truly scalable method in the end.

Hydrogen from natural gas
http://www.tokyo-gas.co.jp/techno/challenge/img/014-1_e.jpg
The methane gas reforming process

The most promising technique for generating hydrogen currently is steam reformed natural gas. If you heat up water and natural gas to almost pyrolysis temperatures (in excess of 1000K), the natural gas naturally reacts with the water to produce carbon dioxide and hydrogen gas. These gases have wildly different densities, so the mixture quickly stratifies and very pure hydrogen gas can be scooped off the top. With a controlled inflow of gas, water and a controlled removal of carbon dioxide and hydrogen, this process can be made into an extremely fast continuous reaction. This is helped by certain catalysts. Theoretically steam reforming of methane and other n-anes (ethane, propane, butane, etc.) can be done at acceptable speeds with about a refrigerator-sized machine. Because of the long start-up time (the machine needs to heat up before every use) it's not quite possible to put a steam reforming machine into a car, but it is certainly possible to use them at fuel stations. This eliminates the need for (leaky) hydrogen pipelines and storage tanks. From a practical point of view, steam reforming is awesome. The wikipedia article still speaks of the need for complicated carbon monoxide filtering, but this is a solved problem. Multiple companies have announced or are already selling these small-scale hydrogen plants. And even though we're probably past peak oil, there is still a crapton of methane gas in the earth's crust to help us move around for the next couple of decades.

So what's the downside? Well... obviously, it still outputs carbon monoxide. A lot of it, actually. Also, the gas still needs to be compressed. If we ignore the energy cost of pumping up, refining and transporting the natural gas, as well as startup energy cost of the machine, the best case cycle efficiency of steam reformed hydrogen looks a bit like this:
  • Steam reforming process: about 70% efficiency (see below)
  • Compression to 200 bar: 85% efficiency
  • Decompression into the fuel cell: 95% efficiency using decompression cooling for the fuel cell
  • Fuel cell efficiency: 60%
This makes for a total methane-to-fuel cell efficiency of 34%. At a currently typical 125Wh/km, this means we require about 369Wh/km from the natural gas. Natural gas produces 1 mole of CO2 per mole of gas, and one mole of gas contains 891kJ of energy, or 164Wh. In other words, per kilometer we produce 2.25 moles of CO2, or 99g. that is exactly the same as a compact petrol or diesel powered car. Environmentally, you win absolutely nothing with this approach. And this calculation is fairly positively biased; I'm ignoring most other sources of inefficiency in this chain, and I'm assuming a state of the art electrical drivetrain.

Also, methane is the best possible situation: methane contains 1 carbon atom for 4 hydrogen atoms. The less carbon, the better. Any other fuel contains relatively more carbon, and thus will yield worse CO2 emissions.

As for the efficiency of any chemical process, in general it's very easy to analyze this; you simply take the enthalpy of formation for all molecules on both sides of the reaction and subtract them from each other. For instance, if we take steam reforming:

CH4 + 2 H2O <=> CO2 + 4 H2

Note that this is the reduced version of the steam reforming process; in reality this is a two-step process with carbon monoxide being involved somewhere. Now, the enthalpy of formation for these molecules is:
  1. CH4 (gas): -74.87 kJ/mol
  2. H2O (gas): -241.818 kJ/mol
  3. CO2 (gas): -393.509 kJ/mol
  4. H2 (gas): 0kJ/mol
Doing the math, this yields a difference of 164.997 kJ/mol of CH4 converted into 4 moles of H2, or 29.5% more energy on the right hand side compared to the left side. This means the reaction is endothermic: you need to *add* energy to keep it going.

This is not necessarily the entire story, though; you can also look at it from the point of view of heating value. The HHV of methane is 889kJ/mol, and for hydrogen is 286kJ/mol. But, we make 4 moles of hydrogen for each mole of methane, which means that the actual usable energy - ignoring the fact that we need to heat up and cool down some water in the process - goes up! We extract 1144kJ worth of hydrogen fuel from just 889kJ worth of methane fuel. Of course, this energy needs to come from somewhere and this is exactly the reason why the reaction is endothermic. So far so good, right?

Well, the higher heating value isn't necessarily a good measure for the amount of actual energy you can recover from both methane and hydrogen gas. In practice, the lower heating value - which takes into account that you need to heat up and/or cool down the water consumed/produced by a reaction - is a better approximation of what you can actually expect to get. The lower heating value of natural gas and hydrogen are 801 and 242kJ/mol respectively, yielding about 121% actual usable fuel efficiency from this process, for 129.5% energy consumption - or a total theoretical maximum process efficiency of 93.4%.

This is roughly how you do these calculations. In practice, the reason for the much lower than theoretically possible efficiency of steam reforming is the fact that this is actually a two-step process. A lot of energy is lost in the carbon monoxide reaction, reducing the theoretical maximum to about 85%. I'll leave the math to this as an exercise to the reader.

http://tweakers.net/ext/f/MQk2vukjfj7EPdT4ZVGfa7Vn/full.jpg
A rough illustration of the effectiveness of hydrogen from the gas reforming process. I'd say this is NOT an effective way to win the race

Hydrogen from plant matter, coal and other heavy fuels
There are other ways to do steam reforming though, and another big possibility is to use more abundant fossil fuels as well as biofuels. But... if you've read the previous chapter already, you'll be asking: how is this better? Short answer: with heavier fuels we can do carbon stripping and carbon capture.

Carbon stripping is a slight modification to the steam reforming process. Methane is the ideal input to steam reforming, because it has relatively uncomplicated chemistry and it's 'easy' to control the process such that very little undesired compounds get created. When dealing with larger carbohydrates - for instance biodiesel typically has a mixture of molecules 12-20 carbon atoms in length. The different molecules have different optimum temperatures, pressures and partial pressures for steam reforming (or any chemical reaction, really). The solution? Break down complex hydrocarbons to methane, and then do steam reforming. This is called carbon stripping. In general, the chemical reaction is:

CnHm => n CH4

Of course, this doesn't always fit. Actually, it rarely fits. There are very few if any pure hydrocarbons larger than methane that have exactly 4 times as many hydrogen atoms as carbon atoms, so like in steam reforming, a bit of H2O is added on the left side to 'add' hydrogen in the mix, and unbalanced carbon is ejected as CO and CO2. The methane is then reprocessed in classic steam reforming.

This is a great process because most heavier hydrocarbons are incredibly rich in hydrogen; for both weight and volume, classic liquid fuels like gasoline and diesel have more hydrogen atoms than liquefied hydrogen by itself. Also, this allows for production of hydrogen from any hydrocarbon: coal, oil but short-cycle fuels as well, e.g. wood, waste ecological matter, even sewage. Of course, at some point you start to run into the issue of energy balance. As you decrease the amount of energy in the source matter, you have to add more and more energy to 'get the hydrogen out', until you actually have to add more energy than the heating value of the resulting hydrogen. This is also the answer for everybody who has been asking 'well, why don't you just steam reform pure water?'. Apart from the fact that it doesn't work chemically, it would cost much more energy to get hydrogen from water than other methods. By the way, steam reforming water into hydrogen would technically probably be pyrolysis? Not sure about that one. Maybe someone in the comments can help me out with that one.

Carbon stripping is a good way to get one unified fuel type (hydrogen) from any input fuel, and one of the big underlying lures of the hydrogen economy. Much like electricity, hydrogen is a big common denominator for all chemical energy sources. With steam reforming, we can make use of that fact. But, like with everything I have talked about before, the issue of efficiency is a big one. Carbon stripping is an energy intensive process that always yields fundamentally lower total process efficiency than just steam reforming from methane. To the point where at the moment, no hydrogen producer can beat the efficiency of a well-tuned Atkinson cycle internal combustion energy just in fuel to hydrogen conversion. In other words: even just production of hydrogen in this way is - at least with current technology - less efficient than running cars directly on gasoline or diesel. Also, the carbon emission argument still stands, but with the use of short-cycle fuels (plant matter that absorbs the same amount of carbon from the atmosphere as it grows) this may be said to be a moot point.

And then there is Carbon capture. Carbon capture in general means that you don't release CO2 emissions into the air, but you find some other place for it. For instance pressurizing it into empty gas or oil wells, using it to grow plants under a very carbon rich atmosphere or using it directly for other chemical processes. Of course this can be done for any process that generates gaseous carbon emissions, but it is especially well-suited to large stationary plants. It wouldn't be feasible to add a big exhaust gas bag to the tailpipe of your car. Economies of scale and the ability to position a hydrogen production plant right on top of a suitable carbon capture location make this an attractive option. It is quite expensive though, and the cost is one of the big reasons for the failure to find investors for two big carbon capture projects in the Rotterdam and Delfzijl.

But what about bacteria? Algae? I've read so much about that recently!
That's right, Jay.*

Conclusions and next up...!

http://tweakers.net/ext/f/a50ZCrefYlyE0a0XzyDWh2Pi/full.jpg

In part one we took a look at how fuel cell vehicles work, now we've looked at a lot of the math behind logistics of hydrogen. By now it should be clear that hydrogen is a very complicated fuel to use for vehicles and even if we would like to use it on a large scale, it would require a complete retooling of our existing infrastructure. But there are other ways around all of this. In the next part I will touch upon alternative fuels for fuel cells and alternative fuel cell types.

*I'm being quite dismissive here, mostly because yes: these methods do exist in laboratories but also no: these methods won't be available in a any kind of short term and have very little backing. Most of what you hear about them in media is just a cry for attention and money to develop these fabrication methods more. Therefore, I'm obviously not in a position to critique or analyze them, but I also feel like it's doing them a disservice to put them on the same level as established, scalable methods of hydrogen production