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Denmark should focus on converting straw to liquid biofuels – DTU research

Straw is the most widely available biomass in Denmark, and researchers, politicians, the agricultural sector, and the energy sector have therefore long discussed how to utilize straw optimally. New calculations from the Technical University of Denmark (DTU) indicate that there are both economic and environmental benefits from using straw for liquid biofuels for heavy transport instead of as a solid fuel for district heating (DH). Thus there should be a focus on developing this technology.

All of the four boiler houses at the Lisbjerg combined heat and power (CHP) facility in Aalborg, Denmark are clad in low maintenance glass enclosing the high rise part of the facility into two boxes. A new flue stack servicing all four boilers was built with the new straw-fired CHP from the left and the three existing three waste-to-energy lines from the right. New DTU research suggests that converting straw to liquid biofuels via thermal gasification might be the best long-term option.

Danish society has long discussed the optimal utilisation for straw, the most abundant annually reoccurring solid biomass. Is it by using it to produce combined heat and power (CHP), biogas, or to convert it into liquid biofuel? Researchers at DTU have now examined this question to determine which technology would be the best for utilization of straw for energy.

The overall conclusion from our analysis shows that the most favourable long-term solution is to convert straw into biofuel for heavy transport— in other words for shipping, transport of goods by road, and aviation. This applies from a technological, economic, and climate perspective, and we should therefore consider focusing more on the development of this technology track in Denmark, said Professor Marie Münster from DTU Management, who has headed the analysis project together with two of her PhD students, Giada Venturini and Amalia Pizarro-Alonso.

Three scenarios

The researchers have arrived at their conclusions by analysing a number of technologies in three scenarios. The findings have been accepted for publication in a paper in the September 15, 2019 edition of the journal “Applied Energy“. Titled “How to maximise the value of residual biomass resources: The case of straw in Denmark” the paper first only looks at the price development of the various technologies which can be used to convert straw into energy.

The prices are primarily based on figures from the Danish Energy Agency. In addition to the price development, the second scenario includes the fact that Denmark must be completely independent of fossil fuels by 2050, and the third scenario comprises the price development – become fossil-free while avoiding biomass imports in the future.

The Foulum biogas facility includes a complete 1.5 tonne per hour cereal straw briquetting line from C. F. Nielsen / Kinetic Biofuel. Research has shown that adding a small percentage pre-treated straw in the form of briquettes can significantly increase the yield of manure-based biogas plants.

The researchers note that biomass is not necessarily nor automatically carbon dioxide (CO2) neutral, and it, therefore, matters how biomass is used or obtained as an energy source.

Our own residual biomass—such as straw—will, therefore, become an increasingly valuable resource, and it is limited, so if we assume that we don’t need to import biomass in the future—and we also become independent of fossil fuels—it’s crucial that we know how to exploit our straw optimally. Here, our analyses point towards gasification of straw with subsequent conversion into liquid fuel as the most promising technology, Professor Münster said.

Benefits of thermal gasification of straw

Reduction of CO2 consumption via biomass in the transport sector has so far been done by first utilizing the biomass in CHP plants and then transferring the energy in the form of electricity for electric cars. However—according to the researchers’ calculations—it is more expedient to direct the biomass at heavy traffic, as it is uncertain whether electricity will become an economically viable alternative here.

In fact, the Danish transport sector has so far not managed to reduce its carbon emissions, and we should therefore already now strengthen the initiatives in this field so that we can use the biofuels that have the greatest effect on greenhouse gas (GHG) emissions.

Biofuels are energy carriers which can be used instead of—or combined with—conventional fuels. They are produced by converting biomass such as straw. The analysis shows that there are several benefits of biofuel gasification.

The reason why conversion of straw into biofuels through thermal gasification is the winning technology is that—despite more electric vehicles—we’ll still need liquid fuels for heavy transport, which accounts for 20 percent of both the EU’s and Denmark’s carbon emissions. At the same time, we can use the surplus heat from the gasification process for process heating and district heating. And the bio ash that is left when you’ve gasified straw can be spread on farmers’ fields as a substitute for phosphorus and potassium fertilizers—and it also constitutes a stable soil carbon store, Professor Münster explained.

Development of existing platforms

However, the conversion of straw into liquid biofuels via thermal gasification is a technology that still requires research and further development to become profitable, but—from a purely technical perspective—the solutions exist. Good results have been achieved at DTU Chemical Engineering, where researchers have succeeded in extracting large shares of the energy of straw as oil substitutes.

It is expected that the oxygen-blown Low-Temperature Circulating Fluidised Bed (LTCFB) gasifier with connected methanol synthesis can reach an efficiency of approximately 50 percent with straw. Added to this are other technological tracks at DTU Chemical Engineering where gasification and pyrolysis can be used to extract raw bio-oil from straw, which can be upgraded to fuel quality.

A round bale of cereal straw
According to DTU, approximately 54 petajoule of straw in Denmark can be converted into energy of which 23 percent is currently used as fuel for combined heat and power (CHP) plants while the rest is ploughed down. By coupling two different optimization models—Times-DKDK and Balmorel-Optiflow—Professor Marie Münster and PhD students Giada Venturini and Amalia Pizarro have calculated that, in the future, straw can be utilized optimally by being converted to liquid fuel for heavy transport via thermal gasification.

The current challenge is first and foremost to perform large-scale demonstration tests and to continue to increase the profitability of the technologies. Integration of electricity from wind turbines is among the areas being examined.

In addition to utilizing straw on the most efficient platform, thermal gasification can also create value by storing surplus electricity in biofuels. This concept integrates electrolysis in the process, and it can benefit from periodically low electricity prices, thereby stabilizing grid load and price development. Simulations have also shown that we can store approximately equal parts of biomass and electricity in biofuels and consequently more than double our biofuel yield. This will enable us to disseminate and store large parts of inexpensive wind energy in the energy system, explained Rasmus Østergaard Gadsbøll researcher and postdoc at DTU Chemical Engineering.

The researchers, therefore, agree that it is now only a matter of time before this method will be able to create great results in the conversion of biomass—including straw—to liquid fuels. And now that the new, extensive calculations even show that it is potentially the most optimal technology compared with others, there is no doubt that it will be an advantage for Denmark to speed up the process.

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