Coal-to-Liquids - Efficiency and Performance

Comparing coal-to-liquid (CTL) transportation fuels to fuels from traditional production methods is a difficult undertaking because of the vastly diverse configuration options available for gasification processing. Traditionally, fuels like gasoline and diesel are refined from crude oil, a (comparatively) more uniform feedstock than coal —or, for that matter, biomass, petcoke, refinery waste, etc. Each potential feedstock, even between different coal ranks, has variable characteristics that can necessitate design changes (ash content, sulfur concentration, feed handling issues, etc.). Additionally, as a relatively new approach to producing liquid fuels, gasification has not had years of refinement to develop a “best,” most productive or cost-efficient approach, in part, due to the aforementioned flexibility in choosing feedstock, product, synthesis gas (syngas) cleaning and conditioning units. In fact, besides multiple gasifier options and configurations, the liquid fuel synthesis component itself can be approached multiple ways: methanol-to-gasoline or Fischer-Tropsch (FT) synthesis, to name a couple. Essentially, comparing petroleum-refinery produced transportation fuels and gasification-derived fuels requires design assumptions and limiting the scope of the comparison in order to come to meaningful conclusions.

Fischer-Tropsch Diesel from Coal and Biomass
A recent 2009 DOE/NETL report examines the economics of diesel produced via gasification and FT synthesis of coal and biomass. The study considers the life-cycle greenhouse gas (GHG) emissions, including carbon dioxide (CO₂), relative to petroleum-derived diesel. The study then develops a required selling price (RSP) for each design case:

• 100% coal feed (CTL).
• A mixture of coal and biomass (CBTL).
• 100% biomass (BTL).

The effect of a set CO₂ emission price (from $0 per metric ton CO2 equivalent [mtCO2eq] to beyond $138/mtCO2eq) or a low carbon fuel standard (LCFS) on the RSP is also investigated.

Study Limits and Assumptions
By limiting the scope of the study to diesel produced from coal and biomass by FT synthesis and comparing it to traditional diesel, the study is able to draw conclusions on the required diesel selling price—the price at which the CTL/CBTL/BTL plant would be economically feasible—under a variety of GHG emission scenarios. Please see the full report for details on the plant set-ups.

One conclusion reached in the study, which will make the rest of this discussion less complex, is that 100% biomass-to-diesel (BTL) is only preferred when carbon prices ($ per metric ton of CO₂ equivalent emitted) are very high, over $138/mtCO₂eq, which, for comparison purposes, would add over $1.70/gallon to the cost of petroleum diesel. This carbon price, therefore, seems unlikely. In fact, a survey of proposed carbon prices suggests a mean price point closer to $12/mtCO₂eq. The rest of this discussion will therefore focus only on CTL and CTL with a fraction of biomass co-fed: 8, 15, and 30 weight percent (weight fractions corresponding to 5, 10, and 20% of the total feedstock energy on an HHV basis, respectively).

Key Findings
A low carbon fuel standard (LCFS) would set the allowable amount of CO₂ or CO₂-equivalent emissions, typically relative to a petroleum baseline. This study analyzed the affect a LCFS would have on the feasibility of CTL and CBTL plants.

With no LCFS, different gasification-derived FT diesel plant designs are favored at different carbon prices ($ per metric ton CO₂ equivalent).

• With no carbon price or a very low one ($0-$5/mtCO2eq), a 100% coal feed with no CCS is favored and is feasible when crude oil is $84-$85/barrel (bbl).
• Remember: when carbon prices increase, the RSP of crude decreases because CTL/BTL diesels have negative GHG emissions relative to crude-derived diesel.
• Over a wide range of carbon prices ($5-$101), CTL with CCS is preferred and feasible with crude $47-$85/bbl.
• Above $101/mtCO₂eq, a feed with 15wt% biomass to a 100% biomass feed is preferred.
• At these carbon prices, however, the price of diesel would increase by more than $1.70/gallon, and is therefore unlikely to be implemented.

A 10% LCFS (10% under petroleum-derived diesel baseline) changes the preferred plant designs. In particular, CTL without CCS and CTL with “simple” CCS are unable to meet this standard.

• With carbon prices of $0-$18/mtCO2eq., CTL with aggressive CCS (over 95% capture) is preferred with crude from $89-$92/bbl.
• From $18-$58, a CBTL design with 8wt% biomass is preferred ($89-$83/bbl).
• From $58-$138, the percentage biomass is bumped up to 15wt% biomass feed ($83-$66/bbl).
• Again, however, at carbon prices that high, the price of diesel would be significantly raised.

Finally, with a 20% LCFS, CBTL with CCS is preferred. Over a carbon price range of $0-$58/mtCO2eq, 8wt% biomass is preferred at crude prices of $83-$93/bbl. With higher carbon prices ($58-$138), 15wt% is favorable and the RSP of crude drops ($66-$83/bbl).

Summary
While not strictly a performance or efficiency comparison, examining the economics of gasification-based diesel production as it relates to the price of crude allows for equivalent conclusions. A base comparison can be made assuming no carbon prices and no LCFS. In this case, CTL (with no CCS) must be sold for $2.49/gal to be feasible, which is equivalent to a crude oil price of $84/bbl.

If a price is attached to carbon emissions (as would be the case in several CO₂ regulation schemes), the required selling price decreases for all CTL and CBTL cases with carbon sequestration (due to these cases negative GHG emissions relative to a petroleum baseline).

The table below summarizes some key operational parameters for each design case (excluding purely biomass-to-liquids, which, as discussed above, would raise the price of diesel to potentially unacceptable levels—for a discussion of the omitted designs please consult the full report. The table also includes a summary of the RSP for diesel and crude oil and the GHG emissions for each design case relative to a petroleum-derived diesel baseline.

The report contains full data, as well as graphs that show the interplay between LCFS, carbon prices, and the various design cases and how they affect RSP. The primary findings under various scenarios have been described above and most point to the feasibility (performance and efficiency) of coal- and coal/biomass-to-diesel systems.

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