The dry distillation of hardwood was commercial technology and was quite common up until the early 1900s (Riegel, 1933). In the older industrial plants,

“Adapted from Grover (1989). Thermograms were obtained with powdered samples. The heating rate was 4°C/min in an atmosphere of nitrogen flowing at a rate of 0.3 L/min. The devolatilization range is the temperature at the beginning and end of devolatilization. The volatiles emitted at 320-500°C are the wt % of the original sample.

hardwood logs were placed on steel buggies and pushed into large, horizontal steel retorts. The doors were closed and external heating was supplied with gas or oil, often within a brick enclosure. Openings were provided for removal of volatiles. Heating was rapid for a few hours until the process became exothermic, and then the external heating was reduced and increased again when needed to complete the distillation. The distillation cycle was about 24 h including a 2-h cooling period. The buggies were then cooled for a few days in special airtight cooling chambers. The dry distillation of softwoods to obtain turpentine was carried out in smaller horizontal retorts in which wood chips were placed without buggies. The average product yields per cord of seasoned hardwood from typical commercial pyrolysis processes were about 1025 kg (950 L) of pyroligneous acid containing 7% acetic acid or equivalent, 4% crude methanol and acetone, 9% tar and oik, and 80% wa­ter; 454 kg of charcoal; and 212 m3 of fuel gas having a heating value of 9.3 to 11.2 MJ/m3 (Lowenheim and Moran, 1975). The pyroligneous acid is allowed to settle to remove insoluble tar and the clear decanted liquor is subjected to extraction or distillation or both to separate acetic acid from methanol and tar. For each tonne of acetic acid produced, about 625 to 800 L of crude methanol is recovered.

Until about the 1930s, when they were displaced by other processes that utilized fossil feedstocks, wood pyrolysis processes were used in industrialized countries for the manufacture of several chemicals and products. One example of this practice is the distillation plant operated by the Ford Motor Company using feedstock of hogged scrapwood from the automobile body plant (Riegel, 1933). A flow schematic for this plant is shown in Fig. 8.3. Vertical, cylindrical, steel retorts 3 m wide by 12 m high with an inside refractory wall 0.46 m thick were used. The hogged wood was dried to a moisture content of 0.5 wt % and consisted of 70% maple, 25% birch, and 5% ash, elm, and oak. Plant capacity was 363 t/day of scrap wood. The retorts were operated continuously for 2-week periods and the heat was supplied entirely by the exothermic pyrolysis reactions or the pyrolysis gas. At startup, the gas was employed to raise the temperature to 540°C, and then external heat was not needed. The average temperatures were 515°C in the center of the retort and 255°C near the bottom. The charcoal was discharged at the bottom of the retorts, cooled, screened, and briquetted. The pyroligneous acid was recovered from the overhead and the pyrolysis gas was used as boiler fuel except for the fuel used on startup of the retorts. The pyroligneous acid was distilled in batch units to remove dissolved tar, and the overhead was then fractionated in other distillation units. The product yields from this plant are shown in Table 8.8. The char, tar, and pitch yields are considerably higher than the yields of chemicals. It is somewhat surprising to note, since wood was a primary feed­stock for the manufacture of methanol (wood alcohol) before its displacement

Product Yield per tonne of dry wood

aAdapted from Riegei (1933). The average heating value of the dry gas was 11.39 MJ/m3 (n). The average composition of the gas in mol % from the retorts was H2> 2.2; CO, 23.4; C02, 37.9; CH4) 16.8; C„Hm, 1.2; 02, 2.4; N2) 16.0. The esters were produced from intermediate acid and product methanol or external ethanol.

by natural gas, that the methanol yield, including that used to esterify the acids formed in the process, is much less than the yields of the other pyrolysis products. In the Ford plant, the acetic acid was converted to esters, since they and not the free acid were needed in other automobile manufacturing operations. Flowever, other companies produced the free acid from the pyrolig­neous acid by direct solvent extraction. The Brewster process used isopropyl ether as the solvent and the Suida process used a high-boiling wood oil fraction from the pyrolysis plant as the solvent (Riegel, 1933). Eventually, these and other pyrolysis processes were phased out, as they were replaced by synthetic methods based on fossil feedstocks.