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Researchers increase carbon efficiency of BtL process from 38 to >90% by adding hydrogen from renewable sources; 2.4x more fuel produced

Researchers in Norway report that the carbon efficiency of a conventional Biomass-to-Liquid (BtL) process can be increased from 38 to more than 90% by adding hydrogen from renewable energy sources. The increased carbon efficiency is possible because the water gas shift reaction is avoided and instead a reversed water gas shift is introduced to convert CO2 to CO.

This means that the amount of fuel can be increased by a factor of 2.4 with the same amount of biomass. In addition, the emitted amount of CO2 per produced unit can be reduced by a factor of 16.

They also determined that there is a near linear relationship between added hydrogen and production of surplus FT-products—meaning that maximum hydrogen addition is favorable.

Based on their evaluation of process technology and economics, the researchers concluded that converting excess renewable electric power to advanced biofuels in a power and biomass to Fischer-Tropsch liquid fuels (PBtL) plant is a sensible way of storing energy as a fuel with a relatively high energy density. Their open-access paper is published in the journal Fuel.

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Energy flow of the staged PBtL concept. The steam produced from the FT reactors (FT cooling) is energy that partly can be utilized.

In their process, hydrogen is produced through high temperature steam electrolysis in a solid oxide electrolysis cell (SOEC), with high temperature steam generated from the hot syngas from the Fischer-Tropsch biomass-to-liquids process. The oxygen produced from the SOEC is sufficient as oxidant in the gasifier, thereby eliminating the need for a cryogenic air separation unit.

The use of high-temperature electrolysis allows some of the required energy to be supplied by available heat.

The required electrical power for the extra production is estimated to be 11.6 kWh per liter syncrude (C5+). By operating the SOEC iso-thermally close to 850 °C the electric energy may be reduced to 9.5 kWh per liter—close to the energy density of jet fuel.

With an electrical power price of 0.05 $/kWh and with SOEC investment cost of the 1000 $/kW(el), the levelized cost of producing advanced biofuel with the PBtL concept is $1.70/liter ($6.40/gallons US)—approximately 30% lower than for conventional BtL.

Resources

  • M. Hillestad, M. Ostadi, G.d. Alamo Serrano, E. Rytter, B. Austbø, J.G. Pharoah, O.S. Burheim (2018) “Improving carbon efficiency and profitability of the biomass to liquid process with hydrogen from renewable power,” Fuel, Volume 234, Pages 1431-1451 doi: 10.1016/j.fuel.2018.08.004

Comments

Engineer-Poet

About time someone got a clue about this.  Fixed carbon has always been the limiting factor in biofuels production; any process which wastes it as CO2 is throwing away the most precious part of the feedstock.

But at $6.40/gallon production cost, I don't think this scheme is going anywhere.

SJC

I posted years ago, add more hydrogen you get more fuel.

Keith D. Patch

I have no idea where the authors are going to find a solid oxide electrolysis cell (SOEC) investment cost of 1000 $/kWe.

However, I agree that utilizing green hydrogen is a great way to improve the technoeconomic viability of any X-to-fuels process.

Best,

--Keith
@KeithDPatch

Engineer-Poet

Looking more closely at the numbers, this scheme requires about 44 kWh per gallon of product.  That is roughly what it takes to produce one GGe of hydrogen (1 kg), and the hydrogen can be used much more efficiently.

As electricity it can be used far more efficiently still.  43.9 kWh will send a Tesla Model S 115 miles, or a Chevy Bolt 188 miles.  One gallon of synthetic gasoline will drive a Prius some 50 miles or so.

Note that this is WITH the assistance of the embodied energy of the biomass!  Any practical scheme has to do MUCH better than this.

Peter_XX

This concept was a well-known in Sweden already at least 15 years ago from work by Chemrec and others. If you synthetize "hydrogen-rich" fuels, such as e.g. methanol or DME, instead of hydrocarbon fuels (gasoline and diesel fuels), you need – or can utilize – even more hydrogen. (For various reasons, Chemrec did not succeed commercially but this is a completely different story.)

In a situation where the refinery needs to shift the balance from gasoline to diesel fuel, you also need more hydrogen. This is the current situation in Europe. Perhaps this will change if (?) market penetration of diesel cars decreases. However, the most energy-efficient solution would be increasing the share of diesel cars, particularly now when we already know that the NOx problem has been solved. Well, at least this problem will be solved no later than September 1 this year, when the new Euro 6d-TEM regulation is enforced. In a scenario of increased use of diesel fuel, more hydrogen would be needed in the refineries.

From an energy efficiency perspective, it makes more sense to use any non-fossil hydrogen in (fossil) refineries and biorefineries rather than opting for direct use of hydrogen in vehicles. If such hydrogen could be produced cost-effectively, there would, for sure, be a big market waiting without any need for change in the fuel supply chain. However, I think there will be some problems with production cost…

gryf

There are several "flaws" in this study that everyone has pointed out.
The use of Hydrogen (or Hydrotreating) is a standard practice in the petrochemical industry for fuel upgrading or removing impurities, e.g. Sulfur or heavy metals. Though this Hydrogen comes from Steam Methane Reforming not renewable sources.
Also, why use SOEC for Electrolysis? Maybe this has something to do with SINTEF research where one of the authors works. Why not include Zero-Gap Alkaline Water Electrolysis? Nel Hydrogen (a Norwegian company) and Thyssen Krupp already have production scale equipment used in the Ammonia Industry and probably is cheaper.
BTW studies in the Ammonia Industry say if Renewable Electricity is to be competitive it needs to cost $25/MWh.

Engineer-Poet

SOEC has the advantage that the waste heat from the electrolysis step is at a high enough temperature to help drive gasification and the reverse water gas shift.  Presumably the energetics would be much worse without this.

gryf

E-P you are correct about the use of the waste heat.
After reading the article in more depth and one of the references (Bernical et al. [4] ) there are references to alkaline water electrolysis, e.g. "Compared to alkaline water electrolysis, high temperature steam electrolysis requires less electrical power. "
Also, in Section 3.4. Modeling the Solid Oxide Electrolysis Cell
"excess heat from the gasifier is available and can be used to heat steam to temperatures well above 1000 °C. When feeding this hot steam to an SOEC . . . allows for electric input that is lower than what is needed at room temperature.".
The article probably more importantly points out that the cost of producing advanced biofuel with the PBtL concept even with Hydrogen upgrading would still be very high compared to current fuel prices.

Peter_XX

In a refinery where high yield of gasoline is the objective, there is a surplus of hydrogen, even though some of it is used for hydrogenation and desulfurization. However, in European refineries, where high yield of diesel fuel is the objective, there is lack of hydrogen. As correctly mentioned by gryf, this hydrogen is mostly made from methane but this is from fossil CNG/LNG, so there would be some scope of replacing this hydrogen. Efficiency in a Btl refinery could be greatly increased if hydrogen from another source would be available.

Interesting to find that work on DME/MeOH is conducted:
http://www.greencarcongress.com/2018/08/20180814-meohdme.html

Engineer-Poet

Net producers of hydrogen, Peter?  Do you have a source for this?  I am very skeptical, given the hydrogen required to turn today's heavy crudes into light products like gasoline.

If gasoline production was a net producer of hydrogen there wouldn't be any great reason to move refining to places where natural gas (hydrogen) is cheap, but that is exactly what we see.

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