Pyrolysis, biomass fixed carbon

One of the analyses not included in the compositional information presented here on biomass is the percentage of so-called fixed carbon. This subject will be discussed in Chapter 8 under pyrolysis because there is no fixed carbon as such in biomass. 

If biomass is subjected to the ASTM D 3172 procedure for determination of fixed carbon, chemical transformation of a portion of the organic carbon in biomass into carbonaceous material occurs as described here. All of the fixed carbon determined by the ASTM procedure is therefore generated by the analytical method. Furthermore, the amount of fixed carbon generated depends on the heating rate used to reach biomass pyrolysis temperatures and the time the sample is subjected to these temperatures. Nevertheless, such analyses are valuable for the development of thermal conversion processes for biomass feedstocks. But application of the ASTM procedures to biomass might more properly be called a method for determination of pyrolytic carbon or coking yields. In the petroleum industry, the Conradson carbon (ASTM D 189, differ-... 

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). 

The first unit operation in the production of waste or biomass derived activated carbon is pyrolysis, also known as baking or charring. In this process the material is heated in an essentially oxygen-free atmosphere to drive off the free moisture and volatiles. The material that remains is called char or fixed carbon. The char yield is dependent upon heating rate and in a study performed by Roberts et al. (1978) on the production of municipal solid waste derived activated carbon, the following... 

Biomass has a low fixed carbon/volatile matter ratio, favoring processes that maximize pyrolysis and cracking, producing higher CH4/H2 ratios, desirable for use in the most nearly commercial fuel cells. 

Given the specifics of the fast pyrolysis process in terms of feedstock requirements and process conditions, ie, fast heating and short residence time in reactor, it can be expected to yield biochar with a different set of properties compared to other conversion processes, such as slow pyrolysis or gasification. The short residence time can lead to incomplete charring of the biomass particle, as observed by Bruun et al. (2011,2012). This in turn leads to lower environmental stability of biochar, and therefore lower carbon sequestration potential. This is the case even when the biomass conversion during pyrolysis is apparently complete, as reported in Brewer et al. (2009). These authors observed lower stability of fast pyrolysis biochar, assessed based on fixed carbon content and aromaticity, compared to slow pyrolysis and gasification biochar produced from the same feedstock. 

Somewhat related is a process proposed and demonstrated on labscale by the University of Siegen (Germany). The process is called the (Herhof)-Integrierte Pyrolyse und Verbren-nung (IPV) process and is decribed in detail by Hamel et al.60 In this process, biomass is converted with high-temperature steam to pyrolysis gas in a fixed-bed reactor. The generated carbon from this reactor is led to a stationary FB combustor from which the hot ash is returned to the first-mentioned reactor. The ash works catalytically to reduce the tar content of the gas produced. The gas is further cleaned and conditioned using a scrubber and electrostatic filter from which the catch is returned to the FB combustor. 

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