dollar a gallon gasoline

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

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

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

Size of the energy problem

Over the next 30 years fossil fuel sources must be replaced. In round numbers we need an amount of energy about equal to the total current demand. That is about 15 TW. This requires 500 GW (half a TW) per year)of new power sources. Supplying this from power satellites, one of only two sources that scale to this size, requires lifting at least a million tonnes per year to GEO.

Why gasoline?

Hydrogen and gasoline are 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, one of the reacting chemicals (oxygen) comes from the air. Gasoline is 25 times as energy dense as lithium ion batteries.

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. "Liquid hydrogen has worse energy density by volume than hydrocarbon fuels such as gasoline by approximately a factor of four." [[1]]

The hydrocarbons that make up gasoline, diesel, jet fuel, etc. are energy dense liquids at normal temperatures and pressures. There is over 100 years of technology base and infrastructure behind using them.

As we run out of fossil hydrocarbons, some other primary energy source will have to replace oil. However, if we have such an energy source, we can make hydrocarbons. All it takes is very large amounts of low cost energy.

Making synthetic hydrocarbons

Sasol's [[2]] Fischer-Tropsch processes [3] provide two ways to do this. The reaction converts syngas[[4]] to synthetic oil[[5]]. Syngas is carbon monoxide [[6]] and hydrogen usually made by burning coal with limited oxygen and water.

Mining old landfills and feeding them into plasma gasifiers can also make syngas. [7] More carbon would come from coal, biomass or even separating CO2 from air and reducing it to carbon monoxide with hydrogen. The water gas reaction, H2O + C--> H2 + CO makes syngas. It's endothermic at 131 kj/mol, about 11 kJ/g or 11 MJ/kg. A kWh is 3.6 MJ so the reaction uses three kWh/kg of carbon or three MWh/t. The best way may be to heat coal in steam with an electric arc to the point the ash forms a melt.

Coal fired power plants will be idled in the first few years by space-based solar power. They are valuable rebuilt as coal to syngas plants or as coal to synthetic oil. They would pump the resultant synthetic oil into the nearest crude oil pipeline or the syngas into a re purposed gas pipeline. The hydrogen content makes this a bit questionable, but until the much safer natural gas displaced it in the 1950s, syngas with its poisonous carbon monoxide was distributed in iron pipes and used in homes.

The Fischer-Tropsch process needs twice that much H₂, 2H₂ + C0 --> (CH₂)x + H₂O. Making electrolytic hydrogen takes about 48 kWh/kg or 48 MWh/t. One sixth of a ton of hydrogen would take eight MWh, for 11 MWh/t of input carbon. This would result in 14/12th of a ton of oil or about 9.4 MWh/t or 1.3 MWh per barrel of oil. Processing 100 tons of carbon an hour, such a plant would draw 1100 MW and produce 730 bbl/hour or 17,500 bbl/day. This may sound like a lot, but it would take 20 converted power plants of this size to feed ExxonMobil's 350,000-bbl/day refinery at Beaumont, TX. It would take over a thousand of them to make the 20 million barrels per day of oil the US now uses. There are already power lines from these plants that could be used to send power to them from rectennas.

An alternative reaction is C + H2 --> (CH2)x where a ton of carbon and 1/6th ton of H2 are reacted with heat and pressure. This takes eight MWh/t of carbon, 6.9 MWh/ton of oil, or .95 MWh per bbl.

These are worst-case numbers since all coal has some hydrogen.

Recent work out of MIT has claimed to improved the efficiency of electrolysis. At 48 kWh/kg of hydrogen, the procsss is about 70% efficient. Improving it to 100% would reduce the power input to 33 kWh/kg and the energy per bbl by 0.2 MWh/bbl

Given this uncertainty, for rough analysis this page will use 1 MWh/bbl as the energy cost to make synthetic oil. At a penny a kWh, a MWh is $10.

Cost

Coal ranges from $15/ton to $150/ton. Figured at $70/ton of carbon, synthetic oil would cost about $20/bbl for energy and carbon plus the capital charge for the synthetic oil plants.

Sasol's most recent plant at Qatar cost $30,000 per bbl/day¹ for an oil synthesis unit fed with natural gas and the refinery to sort out the products. A modified power plant heating coal in steam with electricity should be less complicated and while it could turn out local diesel, it would probably just put the whole output into a crude oil pipeline to take advantage of existing refining infrastructure. If the power plant conversion cost $30,000/bbl/day, the capital cost would under $10/bbl (ten year write off at $3000/year, 300+ days/year.) Making a profit on the oil, a power plant converted into a coal to oil plant would make synthetic oil for perhaps $35 a barrel. Fed to existing refineries, gasoline from $35/bbl oil would be about a dollar a gallon.

Fischer-Tropsch process converts cleaned syngas to a mix of hydrocarbons. This process has to clean out all the sulfur out before the syngas goes into the reactor or the sulfur poisons the catalyst. The reactions don't produce any carbon dioxide at the plant; the trains, aircraft, ships, trucks, farming tractors and personal transport release the CO2. Sooner or later (depending on greenhouse gas considerations) the plants will have to make do with biomass (turning all the carbon into liquid fuels), or even pull C02 from the air. The process reduces carbon dioxide emissions by about half because the power plants no longer produce any.

If all 1.3 billion tons of coal per year the US burns in power plants became synthetic oil, the rate would about 120,000 t/hr of carbon or about 21 million barrels of synthetic oil per day, equal to current consumption. The conversion plants would draw 1300 GW. That is 2.6 years of power sat production at 500 GW/year.

Per the above, the formula for costing synthetic fuel is $20 for capital and carbon and $10 x cost of power in pennies per kWh. The cost of power is important, 10 cents per kWh electric power could be used to make $120/bbl oil. $120 oil refined into gasoline gives a cost of about $4 at the pump--which is about what gasoline cost in the summer of 2008 when oil was at $120/bbl.

If a plant can buy penny kWh electricity, then on an industrial scale it should make dollar a gallon synthetic gasoline out of coal.

Carbon neutral?

It takes about 100kWh/t to remove carbon dioxide from the air. See http://www.eurekalert.org/pub_releases/2008-09/uoc-cd092908.php (This is equal to 360 kWh/t of carbon.)

Experimentally these researchers have removed CO2 from air at a rate of 20 tons per year with a square meter of scrubber

Liquid fuels can be made out of carbon dioxide by n(CO2) + 3H2 --> (CH2)n + 2n H2O.

Electrolytic hydrogen requires 48 MWh/t currently. Recent results from MIT make it possible the energy might get down to 33MWh/t. The chemical equation above requires six t of H2 to 12t of carbon.

Taking (CH2)n as roughly diesel fuel, and using high efficiency electrolysis cells, the reaction requires 200 MWh for the hydrogen plus 4.3 MWh for the CO2 to make 14 tons of fuel. If a MWh is priced at $10, (1 cent/kWh) the energy cost would be $2000 for 14 t of fuel. At 7.33 bbl/t, 14 tons is 103 bbls of fuel. This makes the energy cost per bbl starting with CO2 about $20.

For 14 tons of fuel/day the scrubber area would be 880 square meters. . For 140 tons of fuel per day, the CO2 scrubber would require 8800 square meters of area. Built across the prevailing wind, and ten meters deep, it would be nearly a km in length.

Capital equipment has no exact current counterpart, but even including the CO2 capture might run about ten dollars a barrel per day capacity. That would produce $30/bbl synthetic oil and make possible dollar a gallon, carbon neutral fuel.

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?

Next: Penny a kWh

Footnotes

1. "Sasol's first international joint venture, a plant 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. "