The objectives of the work were to produce pyrolysis vapours for catalytic treatment as a parallel study to the ablative pyrolysis work being performed at NREL using the same principle of contacting biomass with a heated surface under conditions of high relative motion and applied pressure. The work was fundamental in assessing the requirements for ablation and preliminary yield data was obtained. The work is significant in establishing some of the basic work on ablative pyrolysis.

5.4.2 Description

Reed and Cowdrey constructed a "heat flux concentrator" initially to investigate ablative pyrolysis comprised of a drill press which forced and rotated wood dowels into a 1.2 cm diameter tapered hole in a heated copper block (6). The vapours produced emerged through 12 holes in the bottom of the block and were condensed and collected in traps and a gas burette. The temperature range used was 500-700°C and total liquid yields of over 50% on an as fed basis were obtained. The forced contact of the wood with the concentrator however caused the holes to be plugged after about 10g of wood was fed.

A "pyrolysis mill" was then constructed as shown in Figure 5.4 (7,8,9,10,11). This second reactor was designed using the principles of a conventional mill for grain (12). The pyrolysis mill consists of a stationary upper "stone" and a rotating lower "stone" which are both of copper and in contact with heaters. The lower plate is rotated at speeds up to 80 rpm, the pressure being controlled with a spring. The reactor wall is heated to 300-400°C to prevent pyrolysis vapour condensation inside the reactor. Wood particles enter the reactor through the upper plate and the vapours escape through the plate gap and then to a series of four liquid traps. Char and ash accumulate inside the reactor. Reed estimated the average heat flux

in the first reactor to be 6.8 W/cm^ and 5.08 W/cm^ in the second reactor (13).

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Some results for the contact pyrolysis mill are presented in Table 5.6. Using this reactor system, total liquid yields of up to 54 % of liquid based on dry feed have been achieved. The only feedstock tested was bone dry wood. Feed rates of up to 0.2 kg/h were achieved with run times up to 1.5 hours.

One of the main problems was the escape of the pyrolysis vapours through the plates due to plugging of the holes in the disc. Unreacted feed and char remains in the hot reactor environment where it undergoes more conventional pyrolysis. Scale up of this system could be difficult. No vapour residence time values are quoted. Cowdrey did not take into account the variation of the wood decomposition temperature with heat flux and reactor heated surface temperature as discussed by Lede in his design study (14). The concept is interesting, but the problem with using disks is that variable particle size causes processing problems due to disk spacing. Small particles will, therefore, not be under applied pressure and will more slowly carbonise on the hot plates. Relative motion between particles and disk cannot be maintained as the particles tend to stick to one surface and not move as intended. Ribbed disks did not solve the problem. No further work has been carried out or is planned and the reactor has been dismantled (15).