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, possibly carbon neutral, synthetic gasoline?
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 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, 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 vast amounts of low cost energy.
Making synthetic hydrocarbons
Sasol's Fischer-Tropsch processes provides [1] two ways to do this. The reaction converts syngas to synthetic oil. Syngas is carbon monoxide 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. [2] 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.
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 we now use. There are already power lines from these plants that could be used to send power to them.
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 worse case numbers since all coal has some hydrogen.
Cost
At a penny a kWh, a MWh is $10. Therefore, the direct energy cost would be $13 per bbl and perhaps $2 for pumps and other equipment.
Coal ranges from $15/ton to $150/ton. Figured at $70/ton of carbon, synthetic oil would cost about $25/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/day (note 1) 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.) So before making a profit on the oil, a power plant converted into a coal to oil plant would make synthetic oil for upper limit of $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. Eventually (hundreds of years) 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 does reduce 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.
If a plant can buy penny kWh electricity, then on an industrial scale it should 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?
Next http://htyp.org/Penny_a_kW
(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. "