Chemical derivatives of wood are the raw materials for a large number of other chemical and reprocessing industries. End uses are as diverse as liquid fuels, explosives, pharmaceuticals, food products and paints.
In addition, trees also produce bark, leaves, resins, sap with high carbohydrate levels, flowers, pollen and numerous other products which are also important to food, chemical and many other industries.
Feedstock For Liquid Fuels
Two important liquid fuels are derived from wood:
Methanol is recovered by a process known as gasification. Wood constituents are broken down to simple gases by heating wood to high temperatures in the absence of oxygen. The resulting carbon monoxide and hydrogen (together known as producer gas) are treated under pressure in the presence of certain copper-based catalysts, producing significant volumes of methanol. Methanol has potential as a liquid fuel, but is also used as feedstock for production of formaldehyde and other chemicals.
Ethanol is recovered from the polysaccharides in wood by acid hydrolysis. The acid releases the sugars (carbohydrates) in the wood, which are fermented using a yeast such as Saccharomyces to produce ethanol. The waste product of this process is lignin which may be used as a fuel, or further processed to provide binding agents.
It is interesting to note that for more than a decade automobile fuel in Brazil has been based on a 20% blend of ethanol with petrol (although there the ethanol is more often derived from sugar cane).
Wood as a Fuel
Around the world, wood is still the most commonly used source of fuel to provide warmth, and heat for cooking. Mostly, this wood is burnt in simple stoves although newer technology has considerably improved the efficiency of these.
In other cases electricity is generated using steam-driven generators and wood is the basic fuel. The wood is often the residue from some other wood processing activity (e.g. the slabwood and bark from sawmilling may be burnt to fuel wood dryers and drive generators).
Other Wood Chemicals
The pulp industry produces several different highly refined pulp products which are the basis for further chemical processing. These and some of their products are:
• Nitration Pulp - used in explosives, lacquers, printing
inks and rocket propellants.
• Lyocell Pulp - for premium texture fibres.
• Acetate Pulp - used for textile fibres, cigarette filters,
impact resistant plastics, photographic film and rigid
packaging.
• Viscose Pulp - for production of high strength cord (for
industrial belting, hoses and automobile tyres), textile
fibres cellophane and sausage coatings.
• Ether and Microcrystalline Pulps - a chemical
intermediate for further processing into pharmaceuticals,
food products, cosmetics, textile sizing, paints and
cements.
By-Products
Chemical wood pulping processes extract many other chemicals from wood - depending on the chemistry of the wood being pulped and the chemical process used. In the early stages of pulp digestion, volatile organic compounds may be 'flashed' off. High pressure and temperature steam released in bursts from the digester will carry substantial volumes of resin and turpentine compounds. These can be recovered by distillation and provide compounds used in preparation of paint, rosins, etc.
The black liquor produced during sulphite and kraft pulping contains significant quantities of resin acids, tall oil, complex sugars and other organic compounds. It also contains substantial quantities of the chemicals used in the pulp process, and the recovery of these is particularly important to the economics of the process.
Black liquor is often burnt to produce heat and electricity used in the pulp process. However, it may also be separated to recover organic chemicals. In both cases the products are re-circulated to ensure process salts are also returned to the pulping process.
Wood is probably the most versatile and useful material known to mankind, and in the presence of sunlight it is an almost endlessly renewable resource.