A detailed comparative economic analysis of the production costs of fuel oil and synthetic gasoline and diesel fuel has been performed for direct liquefaction of biomass (Elliott et ah, 1990). To bring this analysis to today’s dollars, adjustments should be made to account for inflation and other factors. But since the treatment is a comparative analysis, the differences should remain the same. The basic parameters used for this analysis are listed in Table 8.14.

Three processing steps were involved in the assessment: liquefaction of wood — chip feedstock to a primary crude oil, catalytic hydrotreatment of the crude oil to a deoxygenated product oil, and refining of the product oil to gasoline or diesel fuel. Atmospheric flash pyrolysis (cf. Scott and Piskorz, 1983; O’Neil, Kovac, and Gorton, 1990) and pressurized solvent liquefaction (с/. Appell et al, 1975; Thigpen and Berry, 1982) were analyzed. Two versions of each process were used—one based on the technology as developed (present tech­nology) and one based on anticipated future improvements (potential tech­nology).

The atmospheric flash pyrolysis process for the present technology case is illustrated in the accompanying flowsheet (Fig. 8.5). One-millimeter particle size wood fibers are rapidly pyrolyzed in a fluidized-bed reactor at 500°C to vapors and char. The condensed vapors form the primary oil product, which contains approximately 39% oxygen on a dry basis. The second and third steps are not shown in Fig. 8.5. In the second step, the primary oil is upgraded in a two-stage catalytic hydrotreatment process using a conventional sulfided cobalt/molybdenum-on-alumina petroleum hydrotreatment catalyst. In the third step, the upgraded product is subjected to distillation, hydrodeoxygen­ation of the light fraction, hydrocracking of the heavy fraction, catalytic reform­ing of the gasoline fraction, and steam reforming of the hydrocarbon gas product to produce hydrogen for the process. A gasoline and diesel product slate is produced. For the potential technology case, pyrolysis takes place in a circulating fluidized-bed reactor, which has the advantage of much greater throughput than the present technology case. An advanced three-stage catalytic hydrotreatment is used in the second step and is assumed to yield a high- octane gasoline requiring no further processing other than fractionation.

The present technology case for pressurized solvent liquefaction is illus­trated in Fig. 8.6. Wood chips are ground to less than 0.5 mm and mixed with recycled wood-derived oil. A sodium carbonate solution and synthesis gas are added to the slurry prior to preheating. Liquefaction takes place in a tubular, upflow reactor at 350°C, 20.5 MPa, and a 20-min residence time. Gas is flashed from the reactor effluent. A portion of the liquid is recycled. Water is separated from the primary oil and is treated before discharge. The synthesis gas is obtained from a portion of the feedstock, which is gasified in an oxygen-blown gasifier. The product oil is upgraded and refined in a manner similar to the flash pyrolysis oil. The potential case for this process uses an extruder feeder to feed high concentration wood slurries. The oil phase of the slurry consists of recycled vacuum distillate bottoms. Superheated steam is added to the reactor to provide the reactor heat requirement. Sodium carbonate is assumed to be recycled entirely in the distillate bottoms, and no reducing gas is added to the reactor. The liquid product stream is separated into a distillate product and recycled bottoms in a vacuum distillation tower. Catalytic hydrotreatment

Atmospheric flash
pyrolysis

“Adapted from Elliott et al. (1990). The product energy value is the average U. S. spot market price from 1977 to 1987 for comparable petroleum-based liquid fuel. The average price of U. S. crude oil at the wellhead in 1987 was S2.61/GJ ($15.40/bbl).

is used to upgrade the primary oil as in the present technology case. The upgraded product is assumed to be a high-octane gasoline requiring no further processing other than fractionation.

Detailed estimates were prepared of the capital and operating costs of each of the four processes, which were designed around the results obtained from laboratory, PDU, and pilot-scale tests. A summary of these costs is presented in Table 8.15. The atmospheric flash pyrolysis process is clearly more economi­cal than the pressurized solvent liquefaction process for production of similar products. Using the average U. S. spot market price for fuel oil in the period 1977 to 1987, the ratio of primary oil cost to energy value is about 1.4 at a green wood chip price of $30/t for the present technology case for atmospheric flash pyrolysis. This means that the product is 40% more costly than comparable petroleum fuel. The potential technology case for atmospheric flash pyrolysis is the only process design of the four analyzed that appears capable of producing primary oil product competitive with comparable petroleum fuel. On the basis of sensitivity studies, each process appears to be more sensitive to feedstock cost than to capital cost. Although thermochemical conversion processes are generally capital intensive, the range of capital costs examined had little effect on the final product cost. However, sensitivity analysis of finished product cost and feedstock price showed that when the green wood chip price is $10/t, the ratio is less than 0.9.

It was concluded from this assessment that the most promising process for gasoline production by direct liquefaction of biomass is atmospheric flash pyrolysis. The high-pressure process may have the same future potential, but the uncertainties are much greater.