Carbonization is a slow pyrolysis process, in which the production of charcoal or char is the primary goal. It is the oldest form of pyrolysis, in use for thousands of years. The biomass is heated slowly in the absence of oxygen to a relatively low temperature (~400 °C) over an extended period of time, which in ancient times ran for several days to maximize the char formation. Figure 3.2 is a sketch of a typical beehive oven in which large logs were stacked and covered by a clay wall. A small fire at the bottom provided the required heat, which essentially stayed in the well-insulated closed chamber. Carboniza­tion allows adequate time for the condensable vapor to be converted into char and noncondensable gases.

Conventional pyrolysis involves all three types of pyrolysis product (gas, liquid, and char). As such, it heats the biomass at a moderate rate to a moderate temperature (~600 °C). The product residence time is on the order of minutes.

Fast Pyrolysis

The primary goal of fast pyrolysis is to maximize the production of liquid or bio-oil. The biomass is heated so rapidly that it reaches the peak (pyrolysis)

temperature before it decomposes. The heating rate can be as high as 1000 to 10,000 °C/s, but the peak temperature should be below 650 °C if bio-oil is the product of interest. However, the peak temperature can be up to 1000 °C if the production of gas is of primary interest. Fluidized beds similar to the one shown in Figures 3.5 and 3.9(a) and (b) (see p. 82), may be used for fast pyrolysis.

Four important features of the fast pyrolysis process that help increase the liquid yield are: (1) very high heating rate, (2) reaction temperature within the range of 425 to 600 °C, (3) short residence time (< 3 s) of vapor in the reactor, and (4) rapid quenching of the product gas.

Flash Pyrolysis

In flash pyrolysis biomass is heated rapidly in the absence of oxygen to a rela­tively modest temperature range of 450 to 600 °C. The product, containing condensable and noncondensable gas, leaves the pyrolyzer within a short resi­dence time of 30 to 1500 ms (Bridgwater, 1999). Upon cooling, the condens­able vapor is then condensed into a liquid fuel known as bio-oil. Such an operation increases the liquid yield while reducing the char production. A typical yield of bio-oil in flash pyrolysis is 70 to 75% of the total pyrolysis product.