For biomass gasification, the process configuration to produce liquid fuels using FT synthesis is similar to that shown in Fig. 3.5 except that gasification involves an oxygen-steam blown fluid-bed gasifier with quench rather than the GE/Texaco entrained-flow gasifier used for coal gasification. In addition, sulfur removal requirements are much lower because of the very low sulfur content of biomass. Because of the dispersed nature of biomass supply, the annual amount of biomass available for any one plant site was assumed to be one million tonnes(dry)/year which results in a feed rate of 3,580 tonnes (AR)/day [2,370 tonnes(dry)/day]. Switchgrass was used for these evaluations, but the estimates for corn stover were essentially the same.
Because of the smaller scale, about one-tenth the size as for CTL, and the higher cost of biomass vs. coal, the product cost is estimated at $3.05/gge, and the breakeven crude oil price is $127/bbl with CO2 venting (Table 3.5). The scale-economics effect shows up in the cost per stream-day-barrel, which is over $144,000 vs. the larger CTL plant where it is less than $100,000 (Table 3.5). If CCS is added to the scheme, the fuel cost increases to $3.32/gge; and the crude oil breakeven price is $139/bbl. From a GHG perspective, the LC GHG emissions for biomass based FT
fuels/LC GHG emissions from equivalent petroleum-based fuels is -0.14 for the CO2 vented case, and the ratio is -1.35 in the CCS case (Table 3.5). With CO2 venting, the LC GHG emissions for the biomass FT fuels are slightly negative because of unconverted carbon in the char which is returned to the land and effectively stored there, offsetting fossil-based emissions associated with growing, harvesting and transporting the biomass. With CCS, the LC GHG emissions are highly negative because CO2 is being removed from the atmosphere with each growth cycle and geologically stored. If methanol/MTG is used, instead of FT, the economics are more favorable, and the LC GHG emissions are similar. The cost of CO avoided
' 2eq is about $20/tonne CO2eq for biomass to liquid fuels. It is higher because of the higher cost per stream-day-barrel of the smaller plant.
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