Page 330 - Synthetic Fuels Handbook

316 CHAPTER TEN

chemicals, starch binders, and wax should be burned only in open brick fireplaces.
The wax burns at too hot a temperature for metal stoves and chimneys. When using
manufactured logs in fireplaces, never crumble the burning log with tongs or poker.
Avoid using wood salvaged from poles, posts, and lumber that has been treated with
wood preservatives such as creosote or pentachlorophenol. These chemical compounds
may vaporize upon combustion and cause respiratory problems for those breathing the
fumes. Wet, green wood, or highly resinous wood should not be burned because of the
large amounts of wood tars, creosote, and wood extractives given off which can coat
chimney flues and cause serious chimney fires if ignited.

One by-product of wood burning is wood ash, which in moderate amounts is a fertil-
izer (mainly potash), contributing minerals, but is strongly alkaline as it contains sodium
hydroxide (lye). Wood ash can also be used to manufacture soap.
Smoke, containing water vapor, carbon dioxide, and other chemicals and aerosol par-
ticulates can be an irritating (and potentially dangerous) by-product of partially burnt wood
fuel. A major component of wood smoke is fine particles that may account for a large
portion of particulate air pollution in some regions. During cooler months, wood heating
accounts for as much as 60 percent of fine particles in Melbourne, Australia.
Slow combustion stoves increase efficiency of wood heaters burning logs, but also
increase particulate production. Low pollution/slow combustion stoves are a current area
of research. An alternative approach is to use pyrolysis to produce several useful biochemi-
cal by-products, and clean burning charcoal, or to burn fuel extremely quickly inside a large
thermal mass, such as a masonry heater. This has the effect of allowing the fuel to burn
completely without producing particulates while maintaining the efficiency of the system.
For example, wood-derived pyrolysis oil contains specific oxygenated compounds in
relatively large amounts (Phillips et al., 1990). A current comprehensive review focuses
on the recent developments in the wood/biomass pyrolysis and reports the characteristics
of the resulting bio-oils, which are the main products of fast wood pyrolysis. Sufficient
hydrogen added to the synthesis gas to convert all of the biomass carbon into methanol
carbon would more than double the methanol produced from the same biomass base
(Phillips et al., 1990).
Rapid heating and rapid quenching produce the intermediate pyrolysis liquid products,
which condense before further reactions break down higher-molecular-weight species into
gaseous products. High reaction rates minimize char formation. Under some conditions,
no char is formed. At higher fast pyrolysis temperatures, the major product is gas. Many
researchers have attempted to exploit the complex degradation mechanisms by conducting
pyrolysis in unusual environments.
Pyrolysis is the simplest and almost certainly the oldest method of processing a fuel in
order to produce a better one. Pyrolysis can also be carried out in the presence of a small
quantity of oxygen (gasification), water (steam gasification) or hydrogen (hydrogenation).
One of the most useful products is methane, which is a suitable fuel for electricity genera-
tion using high-efficiency gas turbines.
Cellulose and hemicelluloses form mainly volatile products on heating due to the
thermal cleavage of the sugar units. The lignin forms mainly char since it is not readily
cleaved to lower molecular weight fragments. The progressive increase in the pyrolysis
temperature of the wood led to the release of the volatiles thus forming a solid residue
that is different chemically from the original starting material. Cellulose and hemicel-
luloses initially break into compounds of lower molecular weight. This forms an “acti-
vated cellulose” which decomposes by two competitive reactions: one forming volatiles
(anhydrosugars) and the other forming char and gases. The thermal degradation of the
activated cellulose and hemicelluloses to form volatiles and char can be divided into
categories depending on the reaction temperature. Within a fire all these reactions take