Study Concludes That Best Way for a Biorefinery to Exploit Potential of Biomass Feedstock Is To Produce Ethanol, Furfural and FT-Diesel
18 March 2010
|Results obtained for the selected biorefinery systems. Credit: ACS, Cherubini and Strømman. Click to enlarge.|
Researchers at the Norwegian University of Science and Technology (NTNU) have developed a calculation procedure to estimate the maximum theoretical yields, as well as predictions of the conversion efficiencies in terms of mass, carbon, and energy efficiency, of selected biorefinery production chains.
Based on their calculations, they concluded that the best way to exploit all the potentials of a lignocellulosic biomass feedstock seems to be the production of ethanol from C6 polysaccharides, furfural from C5 polysaccharides, and FT-diesel from lignin. Combining the best feedstock with the most promising final products, their results show that up to 0.33 kg of bioethanol, 0.06 kg of furfural, and 0.17 kg of FT (Fischer-Tropsch)-diesel per kg of softwood can be produced and that mass, carbon, and energy conversion efficiencies of 56%, 70%, and 82%, respectively, can be achieved.
A paper on their study was published online 17 March in the ACS journal Energy & Fuels.
Francesco Cherubini and Anders Hammer Strømman selected lignocellulosic biomass as the raw material for their exercise. They calculated the carbon contents of lignocellulosic biomass components (cellulose, hemicellulose, and lignin) and products with the help of mathematical equations, and then modeled the chemical reactions for the conversion of feedstock to products using matrix algebra. The procedure determines the maximum amount of biofuels and/or biochemicals from biomass and the maximum mass, energy, and carbon conversion efficiency of the biorefinery pathway.
The team worked with three types of biorefinery systems:
Biofuel-oriented biorefinery, where the products are ethanol (from C5 and C6 polysaccharides) and FT-diesel (from lignin).
Chemical-oriented biorefinery, where the products are levulinic acid (from C6 polysaccharides), furfural (from C5 polysaccharides), and phenols (from lignin).
Biorefinery based on gasification of the entire feedstock to produce FT-fuels.
Among their findings:
The largest biofuel and biochemical yields are achieved with those feedstocks and components that have the lowest oxygen content and the highest carbon and hydrogen content, such as softwood. As a consequence, they note, the ethanol yield from C5/C6 polysaccharides is lower than that from the entire feedstock (which includes lignin), because sugars have a higher oxygen content.
The possibility to exploit lignin for biofuel production purposes strongly affects the biofuel yield. In fact, there is a great potential for the production of FT-diesel from lignin, since this is the substrate with the highest carbon content,with hydrogen acting as the limiting factor.
On a mass basis, ethanol has higher production potential than FT-diesel, but this margin strongly decreases if the energy content of the product is considered (ethanol has a heating value of 27 MJ/kg, whereas FT-diesel has a heating value of 42.7MJ/kg). The first-law energy efficiencies (defined as the ratio between the energy content of the products to the energy content of the feedstock) are ~90% for ethanol and ~85% for FT-diesel.
Even with the lowest mass efficiency (41%, versus 54% for the biofuel-oriented biorefinery), the gasification system produces a carbon-rich product that has a much higher heating value than that of ethanol, and, therefore, the resulting carbon and energy conversion efficiencies are higher than that for the biofuel-oriented biorefinery.
An overall interpretation of these theoretical results leads to some important remarks concerning the best way in which the three biomass components of a lignocellulosic feedstock should be exploited. Regarding C5 sugars, yields of furfural are higher than those of ethanol; furthermore, C5 polysaccharides conversion to ethanol still must face non-negligible technological constraints and effective yields are lower than those of the C6 polysaccharides. Hence, regarding the biomass sugar fraction, the best treatment pathways seem to be the conversion of C6 sugars to ethanol and C5 sugars to furfural, from which a wide spectrum of biofuels and chemicals can be synthesized.
Concerning the third component of a lignocellulosic biomass feedstock, the lignin, the best alternative seems to be the conversion to FT-diesel via gasification...Even if almost half of the mass of the feedstock is lost (as CO2), the final products still have 70% of the carbon and 82% of the energy content of the raw materials.
...the best way to exploit all the potentialities of a lignocellulosic biomass feedstock seems to be the production of ethanol from C6 polysaccharides, furfural from C5 polysaccharides, and FT-diesel from lignin.
—Cherubini and Strømman
Francesco Cherubini and Anders Hammer Strømman (2010) Production of Biofuels and Biochemicals from Lignocellulosic Biomass: Estimation of Maximum Theoretical Yields and Efficiencies Using Matrix Algebra. Energy Fuels, Article ASAP doi: 10.1021/ef901379s
I just wish we would get on with it. Carter talked about synthetic fuels 30 years ago and here we are still at the starting line.
Posted by: SJC | 18 March 2010 at 11:50 AM
The standard obstacles to biofuels seem to be high capital costs, low per-acre-per-year yields, high costs to ship feedstocks to refineries, and a business model that becomes much more or less competitive depending on the cost of feedstocks and the cost of fossil fuels.
Posted by: HealthyBreeze | 18 March 2010 at 01:50 PM
There's some interesting tech in solar assisted gasification, http://www.technologyreview.com/energy/24712/
The gasification route allows you to go straight to electric via gas turbines, ideal for balancing wind power.
Posted by: 3PeaceSweet | 18 March 2010 at 02:54 PM
Seems like a well thought out study that will be of real value going forward. SJC, even though it appears like there is little progress - it is far greater than that. Not only have we made progress with technology, there is a distinct change in attitude toward alternative energy resources on the political front. BOTH left and right have reason to embrace biofuels, at least in the U.S. the left for jobs, environment, the right, to strengthen national security.
Biofuels are just one solution to the energy independence goal. There are many others but this one is viable now and for the future use of domestic liquid fuels.
Posted by: sulleny | 18 March 2010 at 03:51 PM
"I just wish we would get on with it. Carter talked about synthetic fuels 30 years ago and here we are still at the starting line.
Posted by: SJC | March 18, 2010 at 11:50 AM"
Ditto, and don't forget the years of fed grants..
Posted by: kelly | 18 March 2010 at 04:10 PM
Biofuels in the current stock of vehicles is a bit of a dead end. But when you combine it with PHEV-20 type vehicle which will reduce fuel consumption by 70% your biofuels will go a lot further (literally & metaphorically)
15,000 miles @ 30mpg = 500 gallons / year
7,500 electric miles + 7,500 miles @ 50mpg = 150 gallons / year
If you can only realistically produce 50 gallons of biofuels per car (number for illustrative purposes only!) it would only amount to 10% of the first vehicles fuel consumption, but a third of the PHEV's liquid fuel requirements.
Posted by: 3PeaceSweet | 18 March 2010 at 04:17 PM
"..don't forget the years of fed grants.."
Remember the billions in tax breaks for the oil companies that far outweigh any grants given for renewable energy.
Posted by: SJC | 18 March 2010 at 09:03 PM
And remember the trillions to pay for wars and defence because the west made itself unpopular and enriched dictators due to oil-politics.
Posted by: Alain | 19 March 2010 at 02:50 AM
The 82% conversion efficiency figure for biomass to hydrocarbons via gasification/FT synthesis which they give, and which by the way I calculated a while ago even before these scientists, does not apply to lignin only. C5, C6 sugars, and any kind of biomass material can be converted to hydrocarbons with 82-83% theoretical thermal efficiency via gasification/FT synthesis. The real question is what is the highest efficiency that can practically be achieved. C6 sugars that can be fermented, like starch, can be converted to ethanol with 96-97% thermal efficiency, so brewing still remains a very efficient conversion technology.
Posted by: black ice | 19 March 2010 at 05:03 AM
Aren't there huge variables with FT conversion, like moisture content of the biomass? It takes energy to heat the moisture, but there is no usable energy in the H20, right? There's also the question of how much pollution is released in the process of heating biomass for FT?
Posted by: HealthyBreeze | 19 March 2010 at 08:24 AM
This figure (82%) is deducted from thermochemical equations which assume that water is in gaseous state; it does not account for any heats of evaporation. Likewise, energy needed for moving stuff around, fans, compressors etc is also not accounted for.
Indeed, in real life the biomass has to be dried, and water needs to be evaporated to make steam for the gasification reaction. There will be substantial heat losses from the gasification reactor. Electricity is needed to power equipment, and to prepare the feedstock. All of this reduces efficiency. The good thing is that a very substantial fraction of the energy can be recovered as FT reaction heat that has the potential to cover the electricity requirements of the plant. Biomass can be air dried to a very low moisture content. I think that 60% efficiency is easily achievable by streamlined plant design, and possibly even more, up to 70%. with a 60 to 70% efficiency, 4 lbs of dry biomass gives 1 lb of hydrocarbons.
As for the pollution, I think that it is no greater than from a biomass burning power station or boiler.
Posted by: black ice | 19 March 2010 at 02:13 PM
Thanks for the solar thermal link. I have said for years that this was a good way to get the heat for gasification. Many said that it was not and if we all listened to them we would be miles behind the starting line today.
Posted by: SJC | 19 March 2010 at 03:17 PM
I read the solar gasification article, interesting from the theoretical point, but I think there will be big challenges as the gentlemen involved with the project already pointed out. There is not enough sunshine in wet and cool areas where biomass grows best for this to work, and even in the most favorable locations such as the desert it can't function more than 10 hours a day. And then there is another problem - what to do with FT reactor tail gas. Maybe it can be stored and used to heat the reactors overnight when there is no sun...
Posted by: black ice | 20 March 2010 at 09:31 AM
what I wanted to add - I think that solar should be used for drying the biomass feedstock, not the actual gasification.
Posted by: black ice | 20 March 2010 at 09:34 AM
There are way of storing the heat using molten salts. Even if you can only gasify part of the day, that just lengthens the payback period, but we get a much better outcome. It is time that we look beyond the profit and start looking for the good.
Posted by: SJC | 21 March 2010 at 01:00 PM
No, I have nothing against solar gasification. These guys have a very great idea. Because it is an endothermic process and therefore needs heat input, gasification is one process that can truly benefit from zero-carbon solar heat. Solar can provide at least some of the heat requirements, if not all.
I can see the technology being applied in sunny desert areas to gasify local coal, shale, and shipped biomass for fuels production, using seawater for the process, by the way.
Posted by: black ice | 22 March 2010 at 06:02 AM