Pyrolysis

Pyrolysis is the fundamental chemical reaction process that is the precursor of both the gasification and combustion of solid fuels, and is simply defined as the chemical changes occurring when heat is applied to a material in the absence of oxygen. The products of biomass pyrolysis include water, charcoal (carbonaceous solid), pyrolysis oils or tars, and permanent gases including methane, hydrogen, carbon monoxide, and carbon dioxide. The nature of the changes in pyrolysis depends on the material being pyrolyzed, the final temperature of the pyrolysis process, and the rate at which it is heated up. The pyrolysis pro­cess is a mildly endothermic reaction. The heat of vaporization of pure water is 2.26 KJ g-1 at 100 °C, while the chemical energy content of wood is only about 18.6 KJ g-1. Most of the energy obtained from biomass goes in moisture removal. This reinforces the facts that lower the moisture content, greater is the energy obtained.

As typical lignocellulosic biomass materials such as wood, straws, and stalks are poor heat conductors, management of the rate of heating requires that the size of the particles being heated be quite small. Otherwise, in massive materials such as logs, the heating rate is very slow, and this determines the yield of pyrolysis products. Depending on the thermal environment and the final temperature, pyrolysis will yield mainly char at low temperatures, <450 °C, when the heating rate is quite slow, and mainly gases at high temperatures, >800 °C, with rapid heating rates. An intermediate temperature and under relatively high heating rates, the main product is a liquid bio-oil, a relatively recent discovery, which is just being turned to commercial applications. There are 3 stages in the pyrolysis process: The first stage, prepyrolysis, occurs between 120 and 200 °C with a slight observed weight loss, when some internal rearrangements, such as bond breakage, the appearance of free radicals, and the formation of carbonyl groups take place, with a corresponding release of small amounts of water (H2O), carbon monoxide (CO), and CO2. The second stage is the main pyrolysis pro­cess, during which solid decomposition occurs, accompanied by a significant weight loss from the initially fed biomass. The last stage is the continuous char devolatilization caused by the further cleavage of C—H and C—O bonds.

In reacting chemical systems, the term severity is used to capture the idea that both the duration of heating and the final temperature influence the chemical products of pyrolysis. Very-low-severity treatments of short duration to a maximum temperature of about 250 °C are sometimes called torrefaction and result in a product that has lost some H2O and CO2 from pyrolysis while retaining almost all of the heat value. Traditional charcoaling is a medium-severity process, while the production of bio-oils is a short-duration high-severity process, which, if the duration at high temperature is maintained, will go all the way to gas and soot.

Depending on the reaction temperature and residence time, pyrolysis can be divided into fast pyrolysis, intermediate pyrolysis, and slow pyrolysis. Typically, fast pyrolysis has an extremely short residence time (~1 s); the reaction temperature is approximately 100 °C higher than that of slow pyrolysis (e. g. ^500 °C vs. ^400 °C). Short reaction times com­bined with an elevated temperature generally result in a higher yield of liquid product. A conventional moderate or slow pyrolysis process, with a relatively long vapor residence time and low heating rate, has been used to produce charcoal for thousands of years (Zhang et al., 2010).

Among the short residence-time processes (0.5-5 s) under development are vacuum pyrolysis at about 300-400 °C and 0.3 atm (U. of Sherbrooke, Canada), flash pyrolysis at about 500-650 °C and 1 atm (U. of Waterloo, Canada), hydropyrolysis in an atmosphere of hydrogen at about 500-600 °C and 5-6 atm (HYFLEX TM, IGT), and flash pyrolysis in atmospheres of hydrogen or methane at 600-1000 °C and 1-70 atm (Brookhaven National Laboratory). An interesting report of a relatively long residence time (10-15 min heat up, several hours at temperature) pyrolysis study at reduced pressures of 0.0004-0.004 atm and temperatures of 250-320 °C of wild cherry wood seems to contrast with the results of several reports on flash pyrolysis (http://journeytoforever. org/biofuel_library/liquefaction. html).

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