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With global oil consumption of 5,220,000 barrels/day in 2010 (Source: Index Mundi), jet fuel represents about 2% of global emissions (Source: IPCC) and an opportunity for highly visible reduction in environmental impact reduction using synthetic fuels made from biomass or biojet fuel.

In North America, ASTM International produces standards for civilian jet fuel types. Standard ASTM D7566 Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons defines requirements for the use of biofuel. According to the standard, jet fuel can be mixed with up to “50% of Fischer-Tropsch hydroprocessed synthesized paraffinic kerosene”, that is: biojet fuel.

The Fischer-Tropsch (FT) process transforms a gas composed primarily of Hydrogen (H) and carbon monoxide (CO), into hydrocarbons and water. Hydrogen and Carbon-Monoxide are basic building blocks for synthesizing larger chemicals. FT synthesis assembles H and CO into Carbon (C) and Hydrogen blocks, and then assembles all the blocks together like beads on a string (Figure 1 and Figure 2).

fischer-tropsh1

Figure 1 – Fischer-Tropsch Process (Credits: Denis Poignonec)

To generate biofuel from biomass using FT synthesis, biomass must be turned into gas. This process is called gasification (Figure 1). It consists in heating biomass to a specific temperature (800-900 °C) in a controlled environment to break down the molecules into simple components. It results in a synthetic gas, or Syngas, that meets FT requirements (For more information on gasification process: Video 1 and Video 2).

biomass-biofuels

Figure 2 – From Biomass to Biofuel: gasification and Fischer-Tropsh process (Copiright Choren. Source Biofuelstep)

Amongst different biofuel feedstocks, switchgrass shows great potential. Switchgrass is a perennial warm season grass native to North America, found in remnant prairies, native grass pastures, and naturalized along roadsides. Grass plants can provide important habitats for wildlife, including game birds and other species threatened by the loss of tall grass prairie habitat associated with modern extensive agriculture. Where tilling agricultural practices have led to a historical decline in soil carbon stocks, perennial plants with a strong root system, such as switchgrass, have the potential to replenish carbon depleted soils with long terms soil carbon sequestration (Figure 2). Studies show that the sequestration rates resulting from switchgrass grown on carbon-depleted soils can range from -0.2 up to 0.57 Mg of C/acre/yr, depending on location. A safe estimate of 0.17 Mg of C/acre/year can be considered as an average sequestration rate for switchgrass (1).

switchgrass3

Figure 3 – Switchgrass benefits (Source Chariton Valley Biomass Project)

In terms of harvest, studies carried out in different regions of the United States show a switchgrass yield ranging from 2.3 up to almost 10 tons/acre/yr. Models estimate that on an agricultural scale (Figure 3), yields are likely to range from 3.2 to 5.8 tons/acre/yr, with an average of 4.6 tons/acre/yr (1). Interestingly, experiments show that very high levels of fertilization don’t necessarily guarantee increased biomass production.

culture4

Figure 4 – Switchgrass culture (source Warner Brothers Seed Company)

Not all fuels are created equal

Both the feedstock production and the fuel production process itself can vary significantly in terms of environmental impacts. Life Cycle Assessment (LCA) is a technique to assess the environmental aspects and potential impacts associated with a product, process, or service. It compiles an inventory of relevant energy and material inputs and environmental releases, and evaluates the potential environmental impacts associated with identified inputs and releases (Figure 5). It allows an “apples to apples” comparison and helps to make more informed decisions.

lca

Figure 5 – Life Cycle Stages (Source: EPA 1993)

To estimate the life cycle Greenhouse Gas emissions (GHG) of switchgrass-based FT jet fuel, we consider all inputs associated with switchgrass production: process fuels and electricity used in farming, fertilizer inputs, herbicide usage, feedstock and final fuel transportation, and energy consumption by the FT facility. Depending of the studies, these parameters are subject of great variations. For a switchgrass based biofuel, Table 1 average values can be considered for a LCA.

 

Table 1 – Cultivation and transport inputs for switchgrass (1)

Fuels (Btu/ton)

Diesel

107 533

Gasoline

0

Electricity

0

Nitrogen Fertilizer (g/ton)

7 701

Other Fertilizers (g/ton)

P2O5

1 236

K2O

2 488

Limestone

9 491

Herbicides

6.4

Transport

40 miles by truck in loads of 24 tons

FT Process Efficiency

45%

The results of a LCA for a switchgrass FT jet fuel show that the life cycle GHG emissions range from -0.02 to 0.2 times those of conventional jet fuel on average, with and without soil carbon sequestration, respectively (Table 2).

 

Table 2 – Life Cycle GHG Average Emissions with and without soil carbon sequestration (1)

Key Assumptions Without soil carbon sequestration With soil carbon sequestration
Life Cycle CO2 Emissions by Stage
Biomass Credit (gCO2/MJ)

-222.7

-222.7

Recovery of feedstock (gCO2/MJ)

6.4

6.4

Transportation of feedstock (gCO2/MJ)

0.6

0.6

Processing of feedstock to fuel (gCO2/MJ)

152.1

152.1

Transportation of jet fuel (gCO2/MJ)

0.5

0.5

Combustion CO2 (gCO2/MJ)

70.4

70.4

Land use change emissions (gCO2/MJ)

0

-19.8

GHG Emissions by Species
CO2 emissions (gCO2/MJ)

-63.1

-75.6

CH4 emissions (gCO2e/MJ)

0.2

0.2

N2O emissions (gCO2e/MJ)

10.3

10.3

Total GHG Emissions (gCO2e/MJ)

17.7

-2

Life Cycle GHG Emissions Relative to
Baseline Conventional Jet Fuel

0.2

-0.02

These results mean that on average, the life cycle GHG emissions for the FT jet fuel pathway with switchgrass are significantly lower (80%) than for a conventional jetfuel. When taking into account carbon sequestration, the negative emissions value means that the whole pathway becomes carbon negative. From planting, growing and harvesting switchgrass, to processing it into gas and jet fuel, and burning the jetfuel, the whole process can actually remove carbon from the atmosphere through enhancing carbon soil levels. This can have the additional benefit of improving soil quality for future agricultural uses.

In Germany, Choren Industries have developed a full scale gasification facility designed for 100% biomass feedstock. From the pilot plant in 1998, the company scaled up to industrial production levels of 18 million liters per year in 2002. As the first to operate a gasifier at such a large scale, Choren faced numerous technological challenges (More information: detailed article from a former employee). Due to technical issues, the plant was not viable. In early 2012, Linde Engineering Dresden acquired the gasification technology developed by Choren.

The most significant challenge for biojet fuels is not in developing viable alternative fuels that could reduce aviation’s GHG emissions. The technology exists. The challenge lies in developing and commercializing a large scale production of next generation of biomass feedstocks that could be grown in a sustainable manner. In 2012, the International Energy Agency reported that biofuel capacity has tripled since 2010 (Source: Biofuels Digest). With new projects continuously being announced in addition to those already operating (source: European Biofuels Technology Platform), the biofuel industry has willingness to face its greatest challenge head on. It gives biojet fuel a greener future

(1) Data sources: Stratton et al. 2010

 

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