This section discusses pyrolyzer design considerations in the production of liquid fuel and charcoal through pyrolysis.

3.7.1 Production of Liquid through Pyrolysis

Pyrolysis is one of several means of production of liquid fuel from biomass. The maximum yield of organic liquid (pyrolytic oil or bio-oil) from thermal decomposition may be increased to as high as 70% (dry weight) if the biomass is rapidly heated to an intermediate temperature and if a short residence time in the pyrolysis zone is allowed to reduce secondary reactions. Table 3.2 earlier in the chapter shows how heating rate, pyrolysis temperature, and residence time affect the nature of the pyrolysis product. These findings may be sum­marized as follows:

• A slower heating rate, a lower temperature, and a longer residence time maximize the yield of solid char.

• A higher heating rate, a higher temperature, and a shorter residence time maximize the gas yield.

• A higher heating rate, an intermediate temperature, and a shorter residence time maximize the liquid yield.

There is an optimum pyrolysis temperature for maximum liquid yield. The yield is highest at 500 °C and drops sharply above and below this temperature (Boukis et al., 2007). The residence time is generally in the range of 0.1 to 2.0 seconds. These values depend on several factors, including the type of biomass (Klass, 1998). We can use a kinetic model for a reasonable yield assessment. The one proposed by Liden et al. (1988) is successful in predicting pyrolysis liquid yields over a wide range of conditions.

Heat transfer is a major consideration in the design of a pyrolyzer. The heat balance for a typical pyrolyzer may be written as

[Heat released by char combustion] + [Heat in incoming stream] (3 ^)

= [Heat required for pyrolysis] + [Heat loss]

Assessing heat loss accurately is difficult before the unit is designed. So, for preliminary assessment, we can take this to be 10% of the heat in the incoming stream (Boukis et al., 2007, p. 1377).

Fast, or flash, pyrolysis is especially suitable for pyrolytic liquefaction of biomass. The product is a mixture of several hydrocarbons, which allows pro­duction of fuel and chemicals through appropriate refining methods. The heating value of the liquid produced is in the same range (15-19 MJ/kg) as that of the parent biomass. The pyrolytic liquid contains several water-soluble sugars and polysaccharide-derivative compounds and water-insoluble pyrolytic lignin.

Pyrolytic liquid contains a much higher amount of oxygen (~50%) than does most fuel oil. It is also heavier (specific gravity ~1.3) and more viscous. Unlike fuel oil, pyrolytic oil increases in viscosity with time because of polymerization. This oil is not self-igniting like fuel oil, and as such it cannot be blended with diesel for operating a diesel engine.

Pyrolytic oil is, however, a good source of some useful chemicals, like natural food flavoring, that can be extracted, leaving the remaining product for burning. Alternately, we can subject the pyrolytic oil to hydrocracking to produce gasoline and diesel.