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New Life Cycle Study Concludes That Biomass for Ethanol Is Not the Most Advantageous Energy and Emissions Use of the Feedstock

A new life cycle study assessing the benefit of cellulosic ethanol in the context of projected feedstock constraints concludes that in terms of reducing greenhouse emissions and fossil fuel dependency, more is lost than gained when prioritizing biomass or land for bioethanol, rather than for use in technology pathways involving heat and power production and/or biogas, or natural gas and electricity for transport. The study was published online in the journal Environmental Science & Technology on 4 October.

The study by researchers in Denmark begins with the conclusion that toward 2030, the amount of biomass which can become available for bioethanol or other energy uses will be physically and economically constrained, regardless of whether of global or a European perspective is applied. This implies that the use of biomass or land for bioethanol production will most likely happen at the expense of alternative uses.

Specifically, the study compares the use in transportation of cellulosic ethanol produced from whole-crop maize by fermentation in several process configurations to alternative use of the feedstock in heat and power applications fueled by coal or natural gas.

Among the scenarios investigated, the researchers found that using willow in CHP production combined with using electric cars for transport yields the highest GHG mitigation and reduction in oil dependency.

They also found that the optimal biogas scenario is for biogas to substitute for natural gas in heat and power production and to use the displaced natural gas to substitute for oil in the transport sector.

In terms of greenhouse gas emissions, the ethanol baseline scenarios provide by far the lowest net GHG mitigation compared to the alternative utilizations of land for energy purposes. For example, use of the land to produce willow used for combined heat and power in substitution for coal provides GHG mitigation more than twice and high, and even higher if combined with electric cars.

Even when using fodder byproduct as fuels, the ethanol scenarios are still compared to the alternatives. The researchers attributed the low net GHG mitigation in the ethanol scenarios to the considerable amounts of steam and electricity consumed in the process of converting biomass into ethanol, especially for pretreatment, hydrolysis, extract concentration, distillation, and drying processes.

Less energy is required for catalyzing anaerobic digestion in the biogas process, for the thermal gasification of willow, or for wood pellet manufacturing. Other factors contributing to the difference in GHG mitigation across the scenarios include the high CO2 content of coal, which results in large GHG mitigation when biomass is used to displace coal, and the high energy efficiency of electric motors compared to combustion engines.

The ethanol scenarios also provide a low net fossil fuel displacement compared to several of the alternative technology pathways. Up to 2.5 times as high oil savings can be obtained with the alternative energy crop utilization pathways, for example.

Overall, for the case presented, the reductions in GHG emissions and fossil fuel dependency, obtained by producing whole-crop maize for bioethanol production happens at the expense of other land/biomass utilizations, which would provide considerably larger reductions. Thus, for this technology case and perspective, more is lost than gained when prioritizing land/biomass for bioethanol.

This is mainly caused by the significant energy conversion losses in bioethanol production compared to use of biomass in the energy sector. The losses lie in the need for pretreatment (lignocellulosic based production), the relatively low fermentation yield of ethanol, the need to dry and further process the byproduct and residual unconverted matter in order to make use of them, and the need to separate ethanol and water, implying distillation in all known cases. Such losses are not present in alternative technologies, e.g., biomass conversion to electricity and/or heat by incineration or conversion to biogas.

As long as fermentation-based conversion of biomass to ethanol implies these losses, bioethanol will come out disadvantageous to the alternatives studied heres and this is the case for presently known bioethanol technologies including both starch and lignocellulose based production. Thus, the results question the assumed justification for lignocellulosic fermentation based bioethanol: instead of reductions in GHG emissions and fossil fuel dependency, net increases will much more likely be the outcome, when considering the alternative biomass/land utilizations deprived on behalf of bioethanol.

—Hedegaard et al. (2008)

Resources

  • Karsten Hedegaard, Kathrine A. Thyø, and Henrik Wenzel (2008) Life Cycle Assessment of an Advanced Bioethanol Technology in the Perspective of Constrained Biomass Availability. ASAP Environ. Sci. Technol., doi: 10.1021/es800358d

Comments

Jim

The charcoal-like stuff seems promising to me:

CNG Storage

The metal frameworks (also touted for Hydrogen) seem way too theoretical to me. Not sure they can make it in quantity any time soon.

I'm annoyed with how much effort has been made to store hydrogen which could've been directed to methane instead. The hydrogen proponents push their strategy, but when asked about NG, complain about the storage (and infrastructure!) problems of NG. Ridiculous.

Mark

Thanks Jim. Seems an elegantly simple alternative for avoiding SCUBA tank pressures or liquefacation.

Overall, this is pretty impressive too.
http://www.ecofuel-world-tour.com/9.0.html?&L=2

JackO

Interesting discussion. One thing that is often overlooked is that there are numerous valuable products that can be made from cellulosic biomass other than ethanol and methane. Our analysis has concluded that the best combined GHG and value creation use cellulose to gasify it to syngas and then produce any of the following three products: anhydrous ammonia, dimethyl ether (DME) or H2 gas. Done correctly, each of these can be made relatively cheaply. The ammonia replaces the use of NG in the making of virtually all of the US nitrogen fetilizer. DME can be used as direct replacement for diesel fuel in low pressure tanks and can even be used during a transition period in a dual-fuel scenario. H2 is still problematic because of transport, storage and distribution. its unfortunmate because we can already make H2 gas at $1.30/Kg.

I agree that in the near and medium terms, making fuels and products that are direct replacements for our existing processes and activities make a great deal of sense. And in most cases that still allows us to use biomass and create better products that can still flow through the same distribution systems. Our view is that we should focus biomass conversion toward: (1) nitrogen fertilizer, (2) DME for diesel replacement and (3) butanol for gasoline replacement. Burning biomass to make electricity is among the least beneficial uses of the material.

Engineer-Poet

Quoth JackO:

Our analysis has concluded that the best combined GHG and value creation use cellulose to gasify it to syngas and then produce any of the following three products: anhydrous ammonia, dimethyl ether (DME) or H2 gas.
H2 is the essential precursor to NH3, so those options are the same except for the last step.  DME for diesel fuel may be the best motor fuel, but I think you're wrong about value per se.  The consumption end of the system must also be considered; diesel engines will not remain the power supply of choice if the fuel is "high value" and thus expensive.
Burning biomass to make electricity is among the least beneficial uses of the material.
If the feedstock supply is limited, achieving the maximum field-to-wheels efficiency is probably going to yield the most value also.  Making electricity to run an electric vehicle (e.g. Smith Newton truck) is not only more efficient than conversion to liquid followed by ICE, the vehicle has a much broader selection of energy supplies.

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