WTW GHG emissions of FTD with and without CCS for the two types of plants. Credit: The Click to enlarge.

They found that with the energy-based allocation method, fossil energy use of FTD is less than that of petroleum diesel, and GHG emissions of FTD could be close to zero or even less than zero with CCS when forest residue accounts for 55% or more of the total dry mass input to FTD plants. Without CCS, GHG emissions are reduced to a level equivalent to that from petroleum diesel plants when forest residue accounts for 61% of the total dry mass input. They also found that coproduct method selection is crucial for LCA results of FTD when a large amount of coproducts is produced.

The system boundary for the study is from wells to wheels (WTW), including a well-to-pump (WTP) stage covering the production and transportation of feedstock and the production, transportation, and distribution of fuel and a pump-to-wheel (PTW) stage covering vehicle operational activities.

In the FT process, solid feedstocks such as coal and biomass, are gasified to produce syngas, which is cleaned of CO₂ and sulfur compounds and then send to FT reactors where a catalyst is used to convert CO and hydrogen into the desired hydrocarbon products. The CO₂ may be vented or captured and sequestered (with the CCS technology). During FTD production, electricity could be produced from unconverted syngas, some of which could be exported to the electric grid as a coproduct. In addition to synthetic diesel FTD and electricity, FTD plants produce a mixture of hydrocarbons, the liquid portion of which is refined into finished FT diesel, naphtha (or gasoline).

There are two general designs for FTD production: recycling (RC) design and once-through (OT) design. In the RC design, unconverted syngas is recycled back for additional conversion, and the final tail gas is used for power generation. The OT design passes the syngas only once through a synthesis reactor and maximizes the power generation from the plant.

Total energy use for FTD production is more than 75% larger than that for petroleum diesel production, no matter how much forest residue is fed. Moreover, CCS incurs an additional energy penalty of 3-10%.

On the other hand, fossil energy use decreases linearly as the share of biomass increases. For example, when forest residue feedstock is increased to 55% by mass (CB55TL), fossil energy use is 96% (without the CCS) and 99% (with the CCS) of that of petroleum diesel. Moreover, since process fuel use in FT plants and FTD combustion during vehicle operation for the BTL pathways come from biomass, fossil energy use is reduced by 86% with BTL relative to that of petroleum diesel.

—Xie et al.

FTD from coal alone, without CCS, increases life-cycle GHG emissions by more 200%; with CCS, that drops to 5% or 29% depending on plant design.

Increasing forest residue share reduces GHG emission linearly because carbon in forest residue is from the atmosphere via photosynthesis during feedstock growth. For example, without CCS, GHG emissions are (1) reduced to a level equivalent to that from petroleum diesel when forest residue accounts for 61% of the total mass input to FT plants and (2) reduced by 77% compared to petroleum diesel with 100% biomass feed. With CCS, WTW GHG emissions of FTD can be almost zero when the forest residue share is 55%, while FTD from 100% biomass can achieve a 254% reduction in GHG emissions compared to petroleum diesel. The reduction in GHG emissions by CCS also increases as biomass share increases, even with consistent CCS ratios.

—Xie et al.

Total and fossil energy uses with different coproduct credit methods differed by 5-10% and 3-34% among the three methods (with two displaced electricity cases) for the same pathway, respectively.

The market-value-based allocation method is subject to the great variation in prices of different energy products. Energy-based allocation method is appropriate to use for FTD LCA, especially for pathways with small shares of FTD.

—Xie et al.

Resources

• Xiaomin Xie, Michael Wang, Jeongwoo Han (2011) Assessment of Fuel-Cycle Energy Use and Greenhouse Gas Emissions for Fischer-Tropsch Diesel from Coal and Cellulosic Biomass. Environmental Science & Technology Article ASAP doi: 10.1021/es1017703