Pyrolysis, biomass charcoal yields

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 (c/. 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). 

Pyrolysis of biomass is divided into slow pyrolysis, which is well known to produce charcoal, for example, fast pyrolysis, which produces a high yield of liquid biofuels and other chemicals (Bridgwater, 2000) and flash pyrolysis. Slow pyrolysis (or carbonisation) requires low temperatures and very long residence time. In the carbonisation process the amount of char is maximised. 

Knowledge of the effects of various independent parameters such as biomass feedstock type and composition, reaction temperature and pressure, residence time, and catalysts on reaction rates, product selectivities, and product yields has led to development of advanced biomass pyrolysis processes. The accumulation of considerable experimental data on these parameters has resulted in advanced pyrolysis methods for the direct thermal conversion of biomass to liquid fuels and various chemicals in higher yields than those obtained by the traditional long-residence-time pyrolysis methods. Thermal conversion processes have also been developed for producing high yields of charcoals from biomass. 

Most of the literature in the past, including that by the author (13). reflected composition of tars produced as a byproduct of charcoal manufacture. These tars, from different biomass materials, exhibit like chemical compositions. The chars produced under similar carbonization conditions also exhibit similar physical and chemical properties (24). During the last decade or so, pyrolysis process research has confirmed that both the yield and chemical composition of pyrolysis oils are very dependent on reaction conditions (see e.g., 14-16.25.26). All biomass oils are not the same oil formation conditions determine oil composition. Similarly, it has been reported that different biomass feedstocks pyrolyzed under similar process conditions can give oil products with similarities in chemical composition (e.g., 27-30). 

Pyrolysis of biomass is carried out under inert atmosphere and forms, depending on the residence time and temperature, char, oil, and gas. Pyrolysis with long residence time at low temperamre (400°C) produces a black solid (charcoal), while fast pyrolysis at high temperarnre (500°C) favors the formation of a black liquor (bio-oil). The short contact times (<2s at ca. 500" C) thus maximize the liquid yield. Fast pyrolysis is preferred by the chemical industry because of the relative ease of handling liquids. However, bio-oil produced by pyrolysis of bulk biomass contains more than 400 different components like carboxylic acids, ketones, aldehydes, sugars, furans, (substituted) phenols, aromatics, and tar (Table 1). Separation of useful chemicals from this complex pool is very difficult. As an alternative, pyrolysis can also be used as a first step for generating heat or electricity, followed by combusting the pyrolytic products. Excellent papers and reviews that describe fast pyrolysis in more detail are available [27-32]. 

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