Other Process Configurations

For clarity above, we have considered the main thermochemical routes and simplest cases to evaluate and illustrate the key issues in high-yield liquid fuel production. Design and cost numbers were developed for only one set of conditions to facilitate comparisons among processes (comparative economics). These numbers do not represent "business-case" economics. There are many other process configurations that could be considered using different biomass types, different coals, and different gasification technologies with a large number of engineering design and operational variants. Generally, the conclusions drawn from the above numbers would not change, although the exact numbers would depend on the details of each situation. However, a different process configuration that alters the perspective on the overall picture involves passing the synthesis gas once through the synthesis reactor (OT) and not recycling the unconverted synthesis gas back to the reactor to maximize liquid transportation fuels production. In the OT configuration, large quantities of unconverted synthesis gas plus low molecular weight hydrocarbons (fuel gas) are available for generating electricity in the combined-cycle power block, resulting in markedly increased electricity production [18-21]. For the CTL-OT case, the plant was designed to produce 50,000 bbl/day of liquid fuels, with higher coal feed rate. For the CBTL-OT case, the feed-rate of coal and biomass (one million tonnes of biomass (dry)/year), was the same as for the CBTL-RC configuration.

Because the electricity production in the OT case is about 35% (vs. 12% in the RC case) of the total product slate, the allocation of LC GHG emissions to the electricity portion significantly affects the LC GHG numbers for the liquid fuel. To more-clearly represent the carbon performance for the OT configurations, a global life-cycle Greenhouse Gas Index (GHGI) was defined as {the total LC GHG emissions for the synthetic route} divided by {the total LC GHG emissions reduction resulting from the displacement of products produced by the

Table 3.6 Cost and performance of once-through Fischer-Tropsch systems with increased electricity production [14, 19]

CTL-OT-V

CTL-OT-CCS

CBTL-OT-V

CBTL-OT-CCS

Coal feed, tonnes/

33,140

33,140

3,420

3,420

day (AR)

Biomass feed, tonnes/

0

0

3,580

3,580

day (AR)

Liquid FT products,

50,000

50,000

8,100

8,100

barrels per day

Power exported, kWe

1,745

1,470

315

276

Cost of fuels, $/gge

1.08

1.39

2.10

2.48

Crude oil equivalent

37

51

84

108

price, $/bbl

LC GGH ratio,

1.35

0.68

0.79

0.10

synthetic/current synthetic/current current route} (petroleum-derived gasoline and diesel and electricity from a supercritical PC plant venting CO2 {831 g CO2/kWe-h}, Table 2.4, Chap. 2) [19].

A 50,000 bpd CTL-FT-OT plant requires 33,140 tonnes of coal/day (increased from 26,700 tonnes/day) and exports an estimated 1,740 kWe of electricity to the grid, compared with 427 kWe for the CTL-FT-RC plant, a four-fold increase (Table 3.6 vs. Table 3.5). If the co-product electricity is priced at 6.0 0/kWe-h, the 2007 U. S. average electricity generating cost, the cost of the liquid transportation fuels produced is projected to be about $1.10/gge, with a breakeven crude oil price of about $37/bbl. The CTL-FT-OT plant is simpler, lacking all the recycle equipment, and the plant cost on a fixed coal feed rate basis would be about 10% less. For the 50,000 bpd CTL-FT-OT plant, the total plant cost is estimated at about $5.8 billion. Plant cost per tonne of feed coal is down, but on a stream-day-barrel basis it is up because less liquid fuel is produced per tonne of coal. The liquid fuels produced cost about 25% less than liquid transportation fuels produced in the recycle case. A combination of lower plant cost and improved FT heat utilization, involving integration of steam produced in the FT synthesis into the power island steam cycle leading to higher power generating efficiency, is responsible for this.

For the same CTL OT plant with CCS, the cost of the liquid transportation fuel produced is about $1.40/gge, and the breakeven crude oil price is about $50/bbl. The cost of CO2eq avoided for this case is $18/tonne. This is still lower than that for power generation from coal, which for IGCC is about twice that.

When coal (~63% on an energy basis) and biomass are fed to a smaller CBTL-OT plant (one million tonnes of biomass(dry)/year) producing about 8,100 bbl/day liquid transportation fuel, the total plant cost is about $1.3 billion for a CO2 venting plant and $1.4 billion for a CCS-based plant. The cost of the fuel produced increases to about $2.10 and $2.48/gge for a CO2 venting OT plant and for a CCS OT plant respectively when the CO2eq price is zero. The LC GHG emission rate for the CBTL-OT plant is about 80% of the LC GHG emission rate for these products as conventionally produced today, i.e., the GHGI is 0.8. For the CBTL-OT-CCS

case, the liquid transportation fuels are essentially zero carbon and the electricity exported is decarbonized on a LC GHG basis (Table 3.6). The CO2eq avoided cost is about $20/tonne CO2eq. If the MTG synthesis route was used rather than the FT synthesis route to produce liquid transportation fuel, the costs would be less; but the general conclusions would still hold. The product mix would be different5. Commercial product demand may be an important driver in process decisions in this situation.

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