dollar a gallon gasoline

Demand for petroleum products is growing while existing fields are declining. If it is not already here, peak oil is not far off.

One way of engineering, especially for very large markets, is "design to cost."

What would it take to make dollar a gallon, carbon neutral, synthetic gasoline?

Why gasoline?

Hydrogen and gasoline can be thought of as energy storage media, like batteries but much higher performance. In the case of current gasoline the energy was stored in the hydrocarbons a long time ago. Unlike batteries, most of the reacting chemical (oxygen) comes from the air.

Hydrogen is widely considered a future fuel. It has serious drawback in that it either has to be stored liquid or under high pressure, or absorbed as in hydrides. All of these are low density.

The hydrocarbons that make up gasoline, diesel, jet fuel, etc. are energy dense liquids at normal temperatures and pressures. There is a vast technology base and infrastructure behind them.

As we run out of hydrocarbons (peak oil) some other primary energy source will have to replace oil. But if we have such an energy source, we can make hydrocarbons. All it takes is vast amounts of low cost energy.

How do you make synthetic hydrocarbons?

Sasol's processes [1] are two ways to do this. It costs a few dollars a barrel if you have syngas cheap enough. Syngas is carbon monoxide and hydrogen.

Most of the cost of syngas is in making hydrogen; the carbon could come from relatively small amount of coal, biomass or even heating limestone for CO2 and reducing that to CO with hydrogen.

The hydrogen comes from electrolysis of water. It takes 48 kWh to make a kg of hydrogen--about twice what you find in a gallon of gasoline (Gssoline is about 3.5 kg/gallon and about one part in seven is hydrogen).

If you can buy penny a kWh electricity, then on an industrial scale you can expect to make dollar a gallon synthetic gasoline.

Electricity, even in industrial quantities, is at least five times too expensive for this and we want *renewable* which makes solar the energy source of choice. Can we get solar power into this price range?

For 40 years, people have talked about solar power satellites. Based on an entire industry in space, it looks like they could deliver power for a penny a kWh or less. However, the complication of building up the industry and the long lead times are discouraging.

Done via a huge fleet of rockets hauling nearly a million tons a year to geosynchronous orbit, recent studies make a case for 5 cent per kWh, highly dependent on the cost of transportation into space at around $500/kg. The energy in rocket fuel is paid back in 40 days. It's the high cost of aerospace hardware that makes it expensive, not the energy.

A moving cable space elevator gets very close to the minimum energy to lift a kg, 14.6 kWh. The energy payback time is just over a day (30 hours). Alas, we don't have the nanotube cable yet, though progress is being made.

Laser propulsion isn't as well developed as rockets, and does not develop a lot of thrust but it is very efficient on propulsion mass while using a lot of laser power.

If we were willing to build an 8 GW laser (cost estimate $80 billion) we could use it and a low performance "pop up" rocket to 250 miles to push close to a million tons of construction materials out to GEO at a cost well under $100/kg. (The laser takes 15 minutes to accelerate the payload into orbit from a sub orbital trajectory.) The energy pay back is about two weeks.

This allows building power satellites for under $800/kW. The electricity could be sold for a penny a kWh or less and synthetic gasoline from the electricity sold for about a dollar a gallon.

On a crash program, we could be adding upwards of 500 GW of new power per year, in as little as seven years. That would be more than enough to compensate for the fall off in oil. Or as the military study concluded last year:

"Drill UP young man."

In the long run a *lot* of the SBSP energy needs to be converted to liquid fuels and we need to support a "dollar a gallon" figure for political and PR reasons.

I suspect the old landfills will be mined for the carbon and fed into plasma gasifiers to make syngas. That doesn't make nearly enough synthetic oil. Coal fired plants can be replaced with coal to oil syngas plants and the resultant synthetic oil pumped into the nearest crude oil pipeline. The water gas reaction, H2O + C--> H2 + CO takes 131 kj/mol, about 10 kj/g or 10 mj/kg. A kWh is 3.6 mj so the number is around 3 kWh/kg of coal or 3 MWh/t

A coal plant makes about 500 MWe from 250 ton/hr of coal so feeding it backwards (using 500-700 MW) would be close to balanced. You would get about a ton of oil out for a ton of coal in or around 6 barrels. So a former .5 GW coal plant run at 250 tons of coal per hour would make 1500 bbl/hr or 36,000 bbl/day. The power cost would be $180,000 per day or $5 per barrel for the power plus $10 a barrel for the coal (at $60/t) plus whatever capital cost it took. The 1.3 billion tons of coal the US burns in power plants plus about 450 GW would make about 21 million barrels of oil a day, completely replacing imports and domestic production. The same would approximately apply to the rest of the world.

Fischer-Tropsch process isn't that complicated, but I have seen a number of $30,000 per bbl/day (note 1) for an entire refinery being fed excess natural gas. The modified power plant doesn't need to make oxygen and while it could turn out local diesel, it would probably just put the whole output into a crude oil pipeline. If the power plant could be converted for ten percent of the above figure, then $3000/bbl/day is under $10/bbl for the capital cost. So before making a profit on the oil, this coal to oil converted power plant makes synthetic oil for $25 a barrel. Fed to existing refineries, the gasoline fraction from $25 oil would be under a dollar a gallon.

This process has to clean out all the sulfur out before the syngas goes into the reactor or the sulphur poisons the catalyst. The plants don't produce any carbon dioxide at the plant, it's all released from trains, aircraft, ships, trucks, farming tractors and personal transport. Eventually the plants will have to make do with biomass (turning all the carbon into liquid fuels), take carbon from limestone or even pull C02 from the air.

I am not saying that this will ever be done, it's just that a way out of the energy crisis seems to exist. Taking advantage of it is another question.

Keith Henson

(note 1) "Sasol's first international joint venture, a factory in Qatar that turns natural gas into liquid fuel, cost $1 billion, or about $30,000 per barrel of capacity. According to Sasol CEO Pat Davies, that's twice as much as a more conventional oil refinery costs. "