[ExI] Making synthetic fuel from trash and intermittent renewable energy

[Keith Henson]

Making synthetic fuel from trash and intermittent renewable energy

H. Keith Henson hkhenson at gmail.com

Abstract

This paper explores making synthetic fuel from trash and coal using
renewable energy. The key reaction, dating back to the 1850s, involves
heating carbon in steam to produce hydrogen and carbon monoxide. This
endothermic reaction requires heating, traditionally done by
alternately burning coke and injecting steam. Using intermittent
renewable electricity for heating is now feasible.

A metric ton of carbon requires 3.03 MWh of heat to produce 13.1 MWh
of syngas; a 4 to 1 energy gain. The gas can be stored, burned, or
converted into methane, jet fuel, or diesel. The water-gas shift
reaction can be used to increase the hydrogen at the expense of CO.
The resultant CO2 (about half) can be sorted out of the gas stream and
sequestered.

Following the water-gas shift, the Fischer-Tropsch (FT) process
converts syngas into hydrocarbons, with water as a byproduct.

A practical example involves using 9,000 tons of trash daily from the
Sylmar, CA landfill supplemented with coal, to produce syngas. The
project would need significant power and infrastructure, including a
3-GW vaporizer and new high-voltage DC lines.

The venture could generate over $600 million annually from the sale of
diesel, with costs for coal and power totaling $241 million. The
project addresses landfill overuse and methane leakage, and provides a
renewable energy solution for synthetic fuel production, though it
requires substantial investment and the development of a 3-GW
gasifier.

Background

History

In the early days of the Industrial Revolution, “gas works” made “town
gas” by heating coke (burning it) then shutting off the air and
blowing steam into the white-hot coke. This made CO and hydrogen. The
proposal here is to heat any carbon source in steam with renewable
power and then feed the syngas to a FT plant to make liquid fuels. It
takes 3 MWh to vaporize a ton of carbon in steam. (Making the steam
takes 0.33 MWh/ton of carbon.) This avoids burning the carbon to
provide process heat.

“Town gas is a more general term referring to manufactured gaseous
fuels produced for sale to consumers and municipalities.

The original coal gas was produced by the coal gasification reaction,”

https://en.wikipedia.org/wiki/Coal_gas check source

about 1/3rd of the way into the article.

“The problem of nitrogen dilution was overcome by the blue water gas
(BWG) process, developed in the 1850s by Sir William Siemens. The
incandescent fuel bed would be alternately blasted with air followed
by steam. The air reactions during the blow cycle are exothermic,
heating up the bed, while the steam reactions during the make cycle,
are endothermic and cool down the bed. The products from the air cycle
contain non-calorific nitrogen and are exhausted out the stack while
the products of the steam cycle are kept as blue water gas.

This gas is composed almost entirely of CO and H2, and burns with a
pale blue flame similar to natural gas.”

Chemistry

The chemical reaction upon which this depends is:

H2O + C → H2 + CO (ΔH = +131 kJ/mol)

18      12       2       28

“The reaction is endothermic, so the fuel must be continually
re-heated to maintain the reaction.“ (Wikipedia). Traditionally this
was done by alternately blowing air and steam through the hot coke,
burning a lot of the coke to drive the reaction. The idea of heating
the coke (or other carbon source) with electricity would not have made
economic sense at that time, even if someone had thought of it.

Carbon is 12 g/mol, 83.3 mol/kg; a kg would soak up 10900 kJ. A ton of
carbon evaporated in steam would need 10,900,000 kJ or 3.03 MW hours.

This would produce 1/6th of a ton of hydrogen with a combustion energy
content of 39.4 MWh/ton, about 6.57 MWh. The CO combustion energy is
10.1 MJ/kg. A ton of carbon produces 2,333 kg or about 6.55 MWh. The
reaction makes about 13.1 MWh of syngas from a ton of carbon and 3 MWh
of renewable electric power, an energy gain of over 4. Most of the
energy in the gas is from carbon sources such as trash or coal.

It’s an efficient way to use intermittent power, though. From this
point, the gas can be stored for winter, burned in a combustion
turbine, or made into methane, jet fuel, gasoline, or diesel.

Making Liquid Fuels

“The FT is a collection of chemical reactions that converts a mixture
of carbon monoxide and hydrogen, known as syngas, into liquid
hydrocarbons. These reactions occur in the presence of metal
catalysts, typically at temperatures of 150–300 °C (302–572 °F) and
pressures of one to several tens of atmospheres. The FT process is an
important reaction in both coal liquefaction and gas-to-liquids
technology for producing liquid hydrocarbons.[1]”

https://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process

The FT reaction is:

CO + 2H2 → (CH2)n + H2O. Half the CO in raw syngas must be converted
to hydrogen via the water-gas shift reaction.

CO + H2O → CO2 + H2. This CO2, about half, can be sorted out and stored.

Sasol has an FT plant in Qatar which makes 34,000 bbl/day of synthetic
diesel. It has run since 2007.

Development

The only part of this proposal that does not already exist at scale is
the electrically heated gasifier. There is no reason it would be very
expensive per ton of capacity, certainly much less than
platinum-containing electrolytic cells used to make hydrogen.

One problem with using trash as a carbon source is that we don’t make
enough of it. Can we collect enough biomass? Possibly. It would reduce
the cost of waste collection if biomass were collected with the trash.

Producing hydrogen

Optimized for hydrogen production, a ton of carbon can make 1/3rd of a
ton of hydrogen at an energy cost of 3.3 MWh. A ton of hydrogen would
take 10 MWh to make. At $20/MWh, the cost would be $200/ton or 20
cents per kg. To this must be added the capital cost of the plant and
the disposal cost of the CO2, but even so, it should come in less than
the $1.50/kg cost of gray hydrogen. Electrolytic hydrogen takes 50
MWh/ton to make, 5 times as much energy, and requires expensive
platinum.

If the cost of a 9,000-ton per day plant is a billion dollars, written
off in 5 years, the yearly production of hydrogen would be $1B/(3,000
t/d x 365 d/y x 5 y) or around $180/ton. That about doubles the cost
of hydrogen to 40 cents per kg, which is still a bargain.

A back-of-the-envelope example

The closest landfill to where I live (Sylmar, CA) gets 9,000 tons of
trash per day. Call it 4,000 tons/day of carbon. An installation
half the size of Sasol’s Oryx plant (which makes 17,000 bbl/day) would
need about 8,500 tons of carbon (half lost to making hydrogen), so it
would need ~4,500 tons of coal per day in addition. That is around 45
rail cars per day which is a modest amount, and there is a nearby rail
line.

Vaporizing this amount of carbon would be 8,500 t/24h x 3 MWh/t, a
little over a GW. If the peak load (when renewable power is available)
were 3 times the average, the vaporizer would use 3 GW. That just
happens to be the capacity of the nearby Sylmar converter station, so
the power lines could handle it.

It is about 40 miles from the landfill to a Chevron refinery where the
syngas could be processed into synthetic jet fuel and diesel. There
are several old oil fields along the route. It would take effort to
decide if the oil fields were suitable to store a buffer of syngas,
but they probably are.

Rough Economic Analysis

Income at $100/bbl ($2.40/gal) for diesel, the gross annual sales of
this venture would be 17,000 bbl/day x 365 days/year x $100/bbl or
~$620 M/year.

Figuring cost, the trash is free, and the coal is $66 million (at
$40/ton). The power would cost 3,000 MW x 365 days/year x 8 hours/day
x $20/MWh or $175 million per year. (The least expensive PV is $13.50
per MWh.) This leaves $405 million gross income per year.  Maintenance
and labor might reduce this to $250 M. For a 5-year return on capital,
the project could cost up to $1.25 B. The Sasol plant cost a billion
dollars, but that included a refinery. I wonder if there is an unused
pipeline close to the 405 freeway. If not, pipelines are around $8
million a mile.

Research and Development

The one part of this project that does not exist at scale is a 3 GW
vaporizer. That’s an awful lot of power but not unprecedented for
industrial processes. A blast furnace for iron production ranges from
1 to 5 GW, most of it from the combustion of coke. Arc furnaces for
mini steel mills are much smaller, typically 50 MW (they are melting,
not reducing the iron). Arc is probably not the right approach to heat
trash or coal. Induction heating might be better, though this is 60
times more power than any existing induction furnace. A complicating
factor for induction heating is that the furnace shell can’t be a
conductor. How much hoop stress would be needed to contain the
pressure needs to be calculated. It should not be much worse than
water.

Figuring trash at a density of one, and a holding time of a day, the
interior volume of the furnace would be 9,000 cubic meters. The
largest blast furnace in the world is 6,000 cubic meters. If the
furnace were a 45-meter-tall cylinder, it would be a little over 16 m
in diameter.

Gas Flow

The flow of syngas would be 1/6 ton of H2 per ton of carbon. Hydrogen
is 500 mol/kg, 500 k mol/ton; 1/6 ton would be 83.3 k mol. At STP, a
mol of a gas fills 22.4 l, 1/6 ton would have a volume of 1867 kl. The
CO volume is equal, so 3,733 cubic meters of gas flow per hour per ton
of carbon vaporized. For a peak of 1,000 tons per hour, the gas flow
would be 3,733,000 cubic meters per hour. The proposed furnace is 200
square meters in area making the upward gas flow around 3,733,000
m3/200 m2 per hour or ~5 m/s (11 mph). That does not include the
pyrolysis gas or the water vapor from drying out the trash. This is
probably not too fast to lift the trash, but further research is
needed.

Vaporizing 8,500 tons of carbon per day would need about 12,800 tons
of steam or 531 tons per hour or 0.147 ton/s or 147 kg of steam per
second. Assuming water at 100 degrees C, and 2,257 kJ/kg to boil, it
would take 331,779 kJ/sec to boil the water or 332 MW, or about ten
percent of the power input to vaporize the carbon in steam. How the
steam would be generated and introduced is TBD. (The newly made syngas
has to be cooled, and it is hotter than 100 C.) Steam use will be
three times higher in peak production periods.

Gas Cleanup

The bottom of the induction furnace will have a pool of slag which
must be drained off from time to time. The gas flow would be up
through 40 meters of trash. This should remove most of the pyrolysis
products (smoke, bio oil) and recycle the carbon down where it can be
vaporized. The remainder will be cleaned up with a heated catalytic
grid or alternately a plasma torch. The steam content of the gas
stream will need to be controlled.

One safety issue is apparent. If the syngas is going to be used to
make liquid fuels, about 1/3 would be CO. A leak of the magnitude of
the Aliso Canyon leak could kill a large number of people from the CO.

https://en.wikipedia.org/wiki/Aliso_Canyon_gas_leak

This will have to be addressed. Leaking syngas could be burned.

Another consideration is that considerable energy is released in the
FT reaction. The possibility of this happening in a storage reservoir
needs to be addressed.

Funding a Study

Modern warfare is completely dependent on jet fuel and diesel.
However, we live at the tail end of the fossil fuels era. While
supplies are currently adequate, this will not be true in the long
term. The US DoD had an experimental project that made 11 gallons of
diesel from a ton of trash. This method, using renewable energy for
heat, would make about 80 gallons per ton of trash supplemented
withcoal.

The proposal that might be presented to DARPA is to use trash and coal
heated by renewable power in steam to make syngas. The syngas can be
turned into jet fuel and diesel by FT plants.

There are lots of engineering and economic problems to solve, but the
point is that intermittent renewable energy can be used to make
synthetic fuel to replace that which is made from oil.

The induction-heated vaporizer is as big as the largest blast furnace.

It would take 8-10 of these to make the Los Angeles trash into diesel.
At 5 square km/GW for PV, 15 km2 for one of them, 150 km2 for enough
power to make diesel out of all the Los Angeles trash. The existing
Pacific Intertie is 3 GW, so to get the power into the vaporizers ould
take ~10 new high-voltage DC lines.

It’s a huge job, but possible with existing technology.

I don’t know if it can it be done before existing technology is obsolete.

Nanotechnology would probably allow a home fuel machine.

Environmental Considerations

Another problem this solves is that the US has been overrun by
landfills. The EU and China use incinerators that could be replaced by
this method to make syngas.

This proposal would stop landfill leakage of methane. Landfill
leakage is a substantial source of methane.

The process would eliminate persistent chemicals and drugs if sewer
digester sludge were included in the feed.

This process eliminates plastics of all kinds. Unlike incinerators,
it releases no dioxins into the environment from PVC.

Harvested brush could be used in place of coal.

Energy Considerations

A problem with renewables is that the grid cannot absorb them when the
load is smaller than the supply, leading to curtailment.

This is wasteful, but using this energy to make hydrogen is too
expensive. The electrolyzers are expensive largely due to the
platinum in them and using them less than all the time increases the
effective capital cost. (Any capital equipment used ¼ of the time
increases the capital cost by a factor of 4.)

This proposal would purposely install much more renewable power than
the grid could absorb and use all the power in excess of grid need to
make fuel.

Objections

> Trash is “not a resource.”

It is a source of carbon, though. If you have lots of excess
renewable power and a low-cost source of carbon, you can make diesel
for around $20/bbl. The big problem is that we don’t make enough
trash.

Converting syngas to jet fuel or diesel is around 75 percent
efficient. It is well understood. The Sasol plant in Qatar has been
operating since 2007 and a previous version was supplying much of the
fuel for South Africa during the apartheid era.

> What are the specific challenges in scaling up this process to a commercially viable level, especially in terms of integrating intermittent renewable energy?

The one piece that does not exist at scale is the electrically heated vaporizer.

> What is the most efficient way to store and transport the syngas produced by this reaction?

The only economical way I am aware of for storage is an empty gas or
oil field. Transport is by pipeline.

> How do you plan to address the long-term sustainability of this process, especially in terms of CO2 emissions and air capture?

If you are making hydrogen from coal this way, you can capture and
store all the CO2. In making fuel, half the carbon is in the fuel and
is released when the fuel is used. If half the carbon comes from
biomass or trash, the accounting is more complicated. The fuel might
be rated as carbon neutral. Eventually, all the carbon will have to
come from biomass or be taken out of the air if humans are still using
hydrocarbon fuels. None of these will be problems after nanotechnology
comes along, but who knows how long that will take?

>What are the potential costs and environmental trade-offs compared to other forms of renewable energy storage or synthetic fuel production?

There isn’t any storage out there for renewable energy that scales to
seasons. Such synthetic fuel production that exists is many times as
expensive as oil.

All the trash that Los Angeles produces plus about half that much in
coal would supply the US military with fuel.

[This is available as a Word file if anyone wants it.]
Keith