Researchers Estimate “Sun-to-Fuel” Fuel Yields from Different Biomass-based Processes, Propose New Process to Deliver Higher Yield
|Estimated values of the overall annual biofuel yield from 1 m2 of land area with annual solar incident energy of 6307 MJ/ m2·y. Credit: ACS, Singh et al. Click to enlarge.|
A team from Purdue University has estimated the “sun-to-fuel” (S2F) yield for liquid hydrocarbon fuel via different biomass pathways and found that the S2F yield for self-contained processes that mainly rely on the biomass to supply all the energy need is “quite low”. Even at a high biomass collection rate of 6.25 kg/m2·y, only ~1.16% of the solar energy is estimated to be recovered as liquid fuel.
The researchers also found that the S2F yield can potentially be increased by a factor of 1.5-3 when all the available land area is not used to grow dedicated fuel crop but the solar energy falling on a portion of the land area is harnessed as hydrogen which is then used in novel augmented biomass conversion processes to increase biomass carbon yield as liquid fuel.
In a paper on their work published 7 June in the ACS journal Environmental Science & Technology, they propose a novel standalone H2Bioil-B process, in which a portion of the biomass is gasified to provide H2 for the fast-hydropyrolysis/hydrodeoxygenation of the remaining biomass, as a means to increase the yield.
Due to the relatively low efficiencies for growing dedicated biomass for transportation fuel and the limited availability of SAW [sustainably available waste] biomass, there is a need to critically examine the amount of liquid fuel that can be produced from a given quantity of biomass. First, we need to understand the potential of self-contained process where biomass is the main source of energy and liquid fuel production is maximized. Second, we need to develop augmented processes that will synergistically use supplemental energy available at much higher efficiencies to increase liquid fuel production. This will provide us the potential to efficiently increase liquid fuel production from a given quantity of biomass and thereby an assessment of the degree to which biomass can play a role in a transportation fuel infrastructure.
—Singh et al.
Two main contributing factors to the low S2F yield for self-contained process are:
- The relatively low recovery of solar energy as biomass energy; and
- The fact that less than 50% of the biomass carbon recovery in the liquid fuel is from self-contained processes.
Augmented processes are estimated to have higher S2F yield because hydrogen is harvested from solar energy with a much higher efficiency than biomass.
The energy content on a per carbon basis for biomass sources such as Switchgrass, Poplar, and sugars is only two thirds of the energy content of the molecules composing gasoline. This means that when biomass carbon molecules are upgraded to a high energy density liquid fuel such as gasoline, even for a 100% energy efficient conversion process, about one-third of the biomass carbon atoms will be rejected as low energy molecules such as CO2.
With reasonably optimistic process conversion efficiencies, we find...that roughly only half of the biomass carbon is recovered as high energy density liquid fuel. Most likely, the lost carbon will be in its low-energy state as CO2. Considering the fact that solar energy to biomass carbon is a low-efficiency process, it is attractive to find processes that will economically either reduce or eliminate the loss of condensed biomass carbon as CO2 during the biomass to liquid fuel conversion process.
It is possible to envision thermochemical as well as biochemical conversion processes using supplementary sources of energy such as heat, H2, or electricity to increase production of high energy density liquid fuel per ton of biomass. We have focused here on thermochemical routes. It is worth noting that for self-contained biomass conversion processes both biochemical as well as gasification/FTD processes were found to provide similar yields. Therefore, we expect that the yields calculated from thermochemical processes using supplemental energy should provide good target estimates for the corresponding non-thermochemical processes as well.
—Singh et al.
|H2Bioil-B process where a portion of the feed biomass (32-42%) is fed to the gasification zone to provide H2 for fast-hydropyrolysis and HDO of the remaining biomass fed to the hydropyrolysis zone. Credit: ACS, Singh et al. Click to enlarge.|
In the proposed H2Bioil-B process, depending on the efficiency of gasification section, 32-42% of the total biomass is gasified to produce a syngas which is sufficient to hydropyrolyze and hydrodeoxygenate the remaining fraction of the biomass that is directly fed to the hydropyrolysis zone.
The hot gas from the gasifier is directly injected in the pyrolyzer zone. If needed, the temperature of the exhaust gas prior to its injection in the pyrolyzer zone may be adjusted. Also, if required, a hot or a cold recycle stream may be injected between the gasifier and the pyrolyzer zone to provide better temperature control in the pyrolyzer section of the reactor.
The researchers found that while a process such as H2CAR [hybrid hydrogen-carbon process], based on gasification/FT chemistry, can recover nearly 100% biomass carbon, it would also need approximately 0.33 kg H2/L oil produced. On the other hand, fast hydropyrolysis/ HDO-based H2Bioil has a potential to recover ~70% biomass carbon with 0.11 kg H2/L oil.
They estimated the H2Bio-B process is estimated to be able to produce 125-146 ethanol gallon equivalents (ege)/ton of biomass of high energy density oil. The augmented version of fast-hydropyrolysis/hydrodeoxygenation, where H2 is generated from a nonbiomass energy source, is estimated to provide liquid fuel yields as high as 215 ege/ton of biomass.
This H2Bioil-B process, after successful experimental demonstration, could result in a high energy density liquid fuel yield that is greater than other known self-contained processes.
—Singh et al.
Navneet R. Singh, W. Nicholas Delgass, Fabio H. Ribeiro and Rakesh Agrawal (2010) Estimation of Liquid Fuel Yields from Biomass. Environ. Sci. Technol., Article ASAP doi: 10.1021/es100316z