As already mentioned, the older production methods for conversion of biomass to charcoal are slow processes and the yields are low. Several days are required to complete the process in earthen pits with seasoned wood, and the yields are only about 10 to 13% of the dry wood weight because most of the volatile organics leave the pyrolysis zone before carbonization occurs. But note that the coalification of biomass in nature is truly a long process compared to human-controlled processes. Coal is a product of the gradual, natural decompo­sition of cellulosic biomass, without free access to air, under the influence of pressure and temperature; it is formed through the successive stages of peat, lignite or brown coal, bituminous or soft coal, and anthracite or hard coal, characterized by increasing carbon content (с/. Fieser and Fieser, 1950).

As shown in Tables 8.6 and 8.7, the volatile matter in biomass as measured by the ASTM and TG procedures is much higher than the fixed carbon content, so to significantly increase charcoal yields, the volatiles must be carbonized as well. Closed reactors can be designed to keep the volatiles in the pyrolysis zone for longer periods and increase carbonization. The use of beehive kilns, for example, affords charcoal yields up to 35%, but the process still requires several days for completion (с/. Antal et al., 1996). The Ford Motor Company process in Badger-Stafford retorts was performed over 24-h cycles and the charcoal yields were about 27% (Table 8.8).

From a theoretical perspective, pure cellulose contains 44.4 wt % carbon, so the maximum theoretical yield of charcoal, assuming all of the cellulosics can be carbonized, is 44.4 wt %. But with dry wood chars containing about 60 to 70 wt % fixed carbon (с/. Table 8.6), the theoretical maximum yields of charcoal including volatile matter and ash from wood feedstocks then corre­spond to about 65 to 75% by weight of the dry wood. It is evident that if charcoal is the desired product, considerable process improvements should be possible.

A batch process that affords higher charcoal yields with biomass feedstocks over a relatively short reaction time has been developed (Antal et al, 1996). The biomass feedstock, usually logs, wood chips, or nutshells, is maintained under pressure up to about 0.7 MPa at typical temperatures of 450°C for pyrolysis times of 15 min to 2 h. The yields of dry charcoal have ranged from 42 to 62% as shown in Table 8.9. The PDU (process development unit) designed to demonstrate this technology employs a cylindrical steel canister covered by a lid. The canister is charged with biomass that is not predried and is placed within the pressure-tight steel vessel of the PDU, which for demonstration purposes was electrically heated. Startup involves heating the PDU to pyrolysis temperatures and then maintaining the PDU at that temperature. The steam formed from the contained moisture is released as needed to maintain the pressure at 0.7 MPa. Commercial systems will probably use the fuel gas emitted to supply heat. However, the results obtained with the PDU illustrate how the charcoal yields approach the theoretical limits, so the yields of other pyrolysis products may be too low to supply any needed fuel. The results from the PDU suggest that both the moisture in the biomass and the pressure can be manipulated to maximize charcoal yields. The vapors emitted during this batch process are kept in contact with the solid biomass undergoing pyrolysis at the internal pressure of the PDU. These conditions result in increased char and low tar yields at short reaction times compared to those that have been employed in most other processes.