With 1 billion tons of biomass per year, the US could produce a lot of transportation fuel. Biomass gasification is a good way to get hydrogen, SNG, methanol, or ethanol from the forest and ag waste we have now. Anything to offset the use of fossil fuels, that is cost effective and CO2 neutral, should be considered.

Finally someone is talking about Biobutanol ... why doesn't anyone in the US government talk about it?

BP is a British company that is investing heavily in biobutanol, so this is at least in part a lobbying effort. However, in terms of fuel properties including energy denesity, butanol is much closer to gasoline than ethanol. Elevated fractions can be blended in by the refinery and the resulting fuel delivered in pipelines. Existing gasoline vehicles do not require fuel system retrofits to use such a blend. Note also that butanol derived from domestically produced coal was used successfully by the RAF in WW2.

The committee also has a valid point in arguing that CO2 emissions from aviation need to be addressed. Discouraging unneccessary flying through a mixture of fuel taxation, elimination of subsidies and financial integration of ground and air transportation providers would be a sensible short-term strategy. Also, more airports should Frankfurt's lead in using special tow vehicles to taxi aircraft between terminals and runways.

A longer-term approach would be the development of aircraft designs powered by piston engines running on liquid hydrogen and featuring detachable passenger/luggage compartments that can be pre-boarded prior to the aircraft's arrival. This may seem a strange mix of WW2 and space-age aerospace technologies but it may actually make sense for short-hop flights within Europe to and from long-distance hubs. To customers, speed in the air matters far less than total travel time.

- Piston engines with appropriate exhaust systems can operate much more quietly than jet engines can, making night-time flights feasible even for older airports located close to sprawling cities. Piston engines are also much cheaper to build, maintain and operate. The R&D effort to adapt them to hydrogen fuel is limited (cp. BMW's new 7h series; the company produced aircraft engines in WW2, hence the propellor logo).

- Hydrogen has a high energy density per unit of mass, which is critical in aviation applications. Note that while LH2 is far more expensive per unit of energy than refined kerosene, far less of it would have to be lifted into the sky. Also, cutting over to fuel cells and electrically driven propellors may become feasible and economically viable for small commercial aircraft even if it never will be for passenger cars, further reducing H2 consumption. Ironically, note that H2 is a byproduct of the production of biobutanol, which as discussed above would be a great substitute/additive for refined gasoline.

- The reduced cost of piston engines combined with the reduced mass of hydrogen fuel would compensate for the higher cost and weight of the detachable compartments, whether they are intended for passengers or for cargo. The chief attraction of such compartments would be reduced time on the ground, compensating for the lower airspeed to yield a TCO that is competitive with jet aircraft in the short-hop niche.

- Finally, it could be interesting to investigate take-off assist systems for specially adapted small commercial aircraft. On aircraft carriers, steam catapults are used to compensate for the short runway. In this context, the objectives would be reduced noise and on-board fuel requirements. Such a system could consist of a sled for the nose wheel, powered by a linear electric motor incorporated in the runway.

Interesting comments on aviation, Rafael, but I'm afraid you're wrong on a few counts.

Piston engined aircraft are not cheaper to run for commercial operators due to maintainance costs, hence all of the small/medium sized aircraft that operate commercially within European airspace that still have propellors run Gas Turbine Turbo prop engines.

There are practically no remaining piston engined aircraft in mainstream passenger transport.

Another good reason that piston airplanes are unpopular, is due to the costs of their fuels. 100 LL (Low Lead) avgas is MUCH more expensive that avtur (aviation turbine fuel aka kerosene).
As a result turbo prop aircraft are cheaper to run per rated Hp.

And the final nail in the coffin of piston planes, is the harsh NVH (noise, vibration, harshness) characteristics of the engines. Turbo props are much smoother and don't give the airframe destroying vibrations that piston engines emit.

Its unlikely that there will be a largescale return to piston aircraft. Plus small size turbo props continue to become more efficient and offer greater reliability, with less downtime.

I'm also skeptical of liquified H2 as an aviation fuel due to its crash characteristics. Kerosene is bad enought in a high velocity airframe breakup, but Liqufied H2 would be many times worse. First it would give flash burns to any people to come into contact with it, then it is much more likely to ignite given the slightest static spark causing a fairly huge and devastating fire. Then of course there's the ground handling issues and the fuel tank costs. I see Biomass to liquid fuels being a much better option.

Andy

Rafael,
I have always maintained that unless hydrogen can prove itself as a serious alternative for air travel (where its high energy: mass ratio means something) it cannot be considered viable for surface travel. Air travel would be the logical next step for hydrogen, after space travel.

Note that while LH2 is far more expensive per unit of energy than refined kerosene, far less of it would have to be lifted into the sky.
Not so fast, in the piston engine you are talking about, you would need roughly the same unit of energy for hydrogen and kerosene. So the cost thing remains. Or did you mean that you could save some fuel due to reduced fuel weight?

Ironically, note that H2 is a byproduct of the production of biobutanol,...
That does not sound right. Do you have a reference for that factoid? Also, do we know how the overall efficiency of butanol fermentation compares to ethanol ferementation?

Andy -

you're right to point out that piston engines play virtually no role in commercial aviation today, but your point about avgas is irrelvant in the context I presented since the fuel would be hydrogen. The primary reason for the use of turboprops is that airlines want aircraft to travel fast so they can move more passengers or cargo per day. Reducing wait times on the ground would eliminated the penalty associated with slower airspeed.

As for NVH, carmakers have figured out how to address that since piston engines were last used for anything larger than a Cessna. Plus, there is now a lot more expertise on aluminium crankcases, turbochargers, carbon fiber manifolds etc. Btw: maintenance on a jet engine or turboprop is anything but cheap.

Your point about fuel safety has merit, since kerosene is harder to ignite than hydrogen. However, by the time an aircraft crashes, fire is a very real hazard either way so I don't think it's an insurmountable issue. Btw, I strongly agree that biofuels, incl. BTL, are worth pursuing - for ground transportation.

An Engineer -

I was referring to the fuel mass that needs to be loaded onto the aircraft to achieve equivalent energy content. In addition, halving the airspeed cuts power requirements by a factor of 8 and energy requirements for the trip by a factor of 4. That, of course, holds true for any fuel but it's especially important for making hydrogen economically viable in aviation.

As for co-production of hydrogen, see e.g.

http://www.butanol.com/page6.html

Biobutanol has been around for a while, but the older processes all relied on bacteria that could not tolerate high concetrations on butanol in their growing medium. Those processes are inferior to ethanol fermentation, which mankind has been perfecting for millenia :-) However, new strains of bacteria and reactor designs are promising yields that are at least competitive; it's still early days yet, though, and ethanol from sugar and starch crops is viable at the industrial scale today.

Here's a relevant recent link regarding hydrogen as an aviation fuel (CH2G rather #than LH2 in this case, but still relevant). This Boeing project already incorporates a fuel cell rather than a modified reciprocating engine. Either way, ground-based take-off assist systems would permit the use of a smaller, lighter and cheaper power plant.

http://www.timesonline.co.uk/article/0,,2087-2330386,00.html

In addition, halving the airspeed cuts power requirements by a factor of 8 and energy requirements for the trip by a factor of 4.
How are you going to convince people to use slow hydrogen powered planes? That would be a serious disadvantage - potential deal killer.

Ground-based-take-off assist systems? Sounds a tad medieval to me...

As for biobutanol: We'd have to see. As I understand it, you need to sterilize the feedstock, to exclude the usual methane fermentation. That obviously add cost and reduce efficiency.

In general, I tend to think that a thermo-chemical conversion of waste to hydrocarbon (as opposed to biochemical conversion of hydrolysed waste to butanol) would be more robust, easier to control, cheaper and yield a more usefull product.

Rafael is flat wrong about the relationship between energy requirements and flight speed.  Required wing area for lift scales as 1/speed^2, making it roughly a wash for subsonic speeds.  Flying at high altitude (say, 35000 feet) creates roughly the same Reynolds number and fluid characteristics as flight at sea level and half the speed.

Rafael, if you're going to spout off you should do enough research to be sure you're not talking nonsense.