## Researchers Propose Solar-Driven Biomass Gasification Pathway for Synthetic Fuel Production

##### 01 May 2009
 Schema of synfuel synthesis through solar-driven biomass gasification. Solar energy produces both heat for gasification and H2 via electrolysis. From Hertwich et al. (2009) Click to enlarge.

Researchers at the Norwegian University of Science and Technology (NTNU) are proposing a new process for producing synfuel from biomass using concentrating solar energy as its main energy source.

High temperature heat for biomass gasification is obtained from a molten-salt system in a solar concentrating tower. Hydrogen for reverse water gas shift reaction to avoid producing CO2 during the process is produced by electrolyzing water, driven by solar power.

The concept’s key feature is the use of high-temperature heat from a solar concentrating tower to drive the chemical process of converting biomass to a biofuel, obtaining a near-complete utilization of carbon atoms in the biomass.

The purpose of the concept, said Edgar G. Hertwich and Xiangping Zhang, is to obtain an easy to handle fuel with near-zero CO2 emission and reduced land-use requirements compared to first and second generation biofuels. A paper describing their proposal was published online 30 April in the ACS journal Environmental Science & Technology.

The researchers modeled the production of methanol, which can be used directly as transport fuel or as input for DME and FT-diesel synthesis. For comparison, they also modeled the production of methanol using only biomass as a fuel and also using coal as source of both carbon and energy. They assumed CO2 capture and storage of excess carbon produced during the fuel production for both of the alternative scenarios.

Transferring the high temperature heat from the solar concentrator to the gasifier and designing the available configuration of the gasifier are technological challenges. One of the feasible approaches is heating sand (Olivine, also the catalyst of gasification reaction) by hot molten salt from a solar concentrator tower, using a gasifier designed as a fluidized-bed of olivine particles.

—Hertwich and Zhang (2009)

Hot syngas from the gasifier is cooled and cleaned, then compressed to the reformer (methane reforming) and the reverse water gas shift (Re-WGS) reactor. Additional hydrogen from water electrolysis driven by solar power is fed into the shift reactor to convert CO2 to CO and adjust H2/CO ratio to satisfy the requirement of the synfuel production.

The steam produced by heat recovery steam generation (HRSG) can be used for gasification.

Characteristics of Three Scenarios with CO2 Capture and Compression
Solar-Driven Biomass GasificationBiomass-Fired Biomass Gasification with CO2 CaptureCoal Gasification with CO2Capture
Energy conversion efficiency (%) 60.9 42.0 36.5
Fuel productivity (kg fuel/100 kg resource) 121.0 39.9 62.2
Land area for biomass growth (m2/ton fuel/yr) 331 1,003 0
Land area for solar energy collection (m2/ton fuel/yr) 51.5 0 0
Fuel cycle atmospheric CO2 balance (gC/MJ fuel) 0 -32 25
Total cost estimate (2001$/GJ), inc. CO2 charge @$100/tC 7.5 8.9 10.8

The solar-driven third generation biofuel requires only 33% of the biomass input and 38% of total land as the second generation biofuel, while still exhibiting a CO2-neutral fuel cycle.

Little CO2 is produced during the process so that CO2 capture is not needed. Ninety percent of total carbon from biomass is converted to biofuel and emitted to the atmosphere after utilization as transportation fuel. Ten percent of carbon is oxidized and released to the environment.

The process is carbon neutral, with the important caveat that emissions due to land use change, harvesting, transport, and production of all required capital is not taken into account in this study.

—Hertwich and Zhang (2009)

Under the biomass-fired gasification scenario, 30% of the biomass is used to provide heat for biomass gasification and 20% is used to produced electricity and heat for the process and CO2 capture and compression. Only 30% of total carbon from the biomass feedstock is converted to fuel. With 50% carbon storage, the result is a carbon negative process, removing CO2 from the atmosphere.

Under the coal-to-liquids scenario, 60% of total carbon from the coal resource is captured and stored geologically, and 30% of total carbon is converted to transport fuel and then released to the atmosphere by utilization.

The solar driven process has higher initial capital costs; these, however, are offset by lower fuel costs.

Most of the 77 EJ of direct energy use in transportation in 2000 consisted of liquid fuels. Various scenario analyses assume a 2% growth rate, to 114 EJ in 2020. Producing 10% of this energy from second generation biofuels would require 44-130 million hectare (ha), assuming yields of 1-3 kg/m2y. In comparison, cropland today covers 1.5 billion ha.

Producing the same amount with third generation biofuels [solar-driven biomass gasification] requires 3 million ha for solar power and 22-51 million ha for biomass plantations. Given the concern of the effect of biofuels on land use, food production, and biodiversity impacts, reducing the land use by such a substantial amount offers a significant step forward.

—Hertwich and Zhang (2009)

Resources

• Edgar G. Hertwich and Xiangping Zhang (2009) Concentrating-Solar Biomass Gasification Process for a 3rd Generation Biofuel. Environ. Sci. Technol., Article ASAP doi: 10.1021/es802853g

Lots of hectoring about the hectares of biomass. Of course if the wizards in wonderland would put a full press effort behind solving the problems with algal oil - we'd have no land issues to whine about.

That's a lot of effort to get a fuel which is only 20-28% efficient burning in an ICE. Wouldn't it be far better to just send the electricity to BEV?

Nice....on paper

Concentrated solar requires direct solar light only available with enough reliability in desert areas like Nevada or Mohave desert where there is no biomass...
inversely biomass is abundant where there is little direct solar light.

So just an intellectuel masturbation, but not a practical solution

It takes sun to grow the crops. If you are harvesting corn late summer and want to process the stalks, there would probably be enough sun. You can always store the biomass and let it dry until the sunny season.

You want to have the processing where the crops are grown. You can get concentrated solar thermal and concentrated PV, so electrolysis for more H2 to use with the extra carbon makes sense. For a capital payoff you would want to use the equipment 24/7 if you could, which is why they use molten salt.

HealthyBreeze

You might want to check your facts. Hydrogen, just like ethanol, has shown to be 43 to 48% in the thermal efficiency category. And that is without all the turbos and efficiency boosters used on diesels to get them to 40+%.

The interesting part of this study is that it recognizes that solar electricity is expensive while solar heat is cheap. Very cheap. As in the cheapest heat one can get.

With that said, the point regarding gasification is correct: the areas where you could get CST you couldn't get biomass. Thus the concept of a low temperature solar thermal assited biodigester makes more sense.

I think that they did the study to see how the numbers came out. They could recalculate based on the solar radiance numbers on the DOE site and others for states like Nebraska and so on. You can still get CST, just not as many hours per year.

Not necessarily. CST requires farily steady sunlight to prevent the salt from cooling and solidifying. If you are using oil as the medium then you don't have that problem but efficiencies and temperatures are lower.

The capital is in the solar CST. Lots of work has been done in Europe (Lurgi)and Canada (Dynamotive) on pyrolysis oils from biomass. These have up to half the energy density of crude fossil oil which means it is economic to ship them the long distances you need to bring the biomass to the sun.

How much heat are we talking about here? At what temperature does the process work?

The reason I ask is there may be other ways of using solar energy in places closer to the biomass.

"The salt melts at 430 F and is kept liquid at 550 F in an insulated cold storage tank."

This says to me that you keep the salt molten using a heat source, like natural gas. Even Kramer Junction in the Mojave is assisted by natural gas.

"The salt is them pumped to the top of the tower, where concentrated sunlight heats it in a receiver to 1050 F."

The should be more than high enough heat. Gasification runs at about 800f as I recall. So if you keep the salt molten with a heat source, clouds passing by should be no problem.

http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm

This technology would adapt quite well to the primary biomass producing region of the U.S., the Great Plains. Perennial grasses need lots of sunlight, and insolation in the plains region is high, even in winter, when cloudiness is low, especially in the Northern Plains. Although solar angle is low in winter, there's a tremendous amount of reflected sunlight bouncing off the snow in northern areas.

Gasification is an efficient way to deal with the problem of moving biomass bulk. Solar gasifiers can be located on-farm, so that biomass would be moved a very short distance, and resulting gas products could be moved efficiently by pipeline. Conversion of methane to methanol is also a possibility and easily accomplished.

The importance of portable biomass derived fuel is that it can be carbon neutral and can provide energy dense fuel for applications where battery technology is not very functional, such as in long distance, isotropic, non-rail cargo transport. Methane is also much more user-friendly at ambient temperatures than hydrogen.

"..so that biomass would be moved a very short distance, and resulting gas products could be moved efficiently by pipeline.."

Thank you Fred, this is what I have been saying. If we put the gasifiers every 10 x 10 miles through the farm belt, we can get enough biomass like corn stalks hauled less than 5 miles on average.

Most can be processed using solar and natural gas and bio methane from the plants can be put back into the pipelines. Combine that with some solar PV H2 and O2 to make even more fuel from the excess carbon, and/or return the bio char back to the land.

"If we put the gasifiers every 10 x 10 miles through the farm belt, we can get enough biomass like corn stalks hauled less than 5 miles on average."

In the 19th century, when railroads were being built out on the plains, towns were platted every seven miles along the track. The reason for this distance is that it gave even the most distant farmer the ability to make a round trip to the grain elevator in one day with a wagon and team. This was very well thought out by the planners of the day. I think we have the ability to do the same today.

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